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Staged process for obsolete propellant disposal: polymer-matrix composite waste management

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Staged process for obsolete propellant disposal: polymer-matrix composite waste management

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Content STAGED PROCESS FOR OBSOLETE PROPELLANT DISPOSAL POLYMER MATRIX COMPOSITE WASTE MANAGEMENT by Jer-Yuan Shiu A Dissertation Presented to be the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirement for the Degree DOCTOR OF PHILOSOPHY (Environmental Engineering) February 1997 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES. CALIFORNIA 90007 This dissertation, written by Jer-Yuan Shiu under the direction of h..?...... Dissertation Committee, and. approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillment of re quirements for the degree of DOCTOR OF PHILOSOPHY ■ ■ ■ ■ ■ r r r ... ' DtSK o f Graduate Studies D ate Apx.il...1 C L ..1 9 .2 .7 . DISSERT ATIOhLCOMMi : e R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. ACKNOWLEDGMENTS The author is extremely grateful to my advisor, Professor Teh Fu Yen, for his support, guidance and encouragement throughout this work. I also would like to thank my committee members, Professor Massoud Pirbazari and Professor George V. Chilingar, for their assistance and valuable comments. Appreciation for inspiration throughout the research period is extended to my colleagues in Dr. Yen’s Energy Group, particularly Iris Yang, Robert Chang and Sung Hyun Kwon for their participation in this research. Also many thanks are given to my associates, Dr. Louis Lian, Jau Ren Chen, Kai Dunn, Dawood Momeni, Steve Lu. Steven Tu, Derek Chitwood, Dr. Badri Badriyha, and Robert Bueche. A special thanks is extended to Dr. Donald D. Tzeng at United Technologies Chemical Systems for his financial support of this research project, as providing of the research materials. Financial support of scholarship program provided by the Division of Biosystems and Process Sciences at Brookhaven National Research Laboratory is also acknowledged. Finally, I wish to express my deep gratitude to my parents, and particularly to my wife Pei-Jung and daughter Ellen, for their patience and support. I delicate this thesis to them. ii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. TABLE OF CONTENTS Cage ACKNOWLEDGMENTS ii LIST OF FIGURES vi LIST OF TABLES x ABSTRACT xi CHAPTER 1 INTRODUCTION I 1.1 Background 1 1.2 Significance of the Problems 5 1.3 Research Objectives 9 1.4 Approaches to the Problems 12 2 DESCRIPTION OF COMPOSITE PROPELLANTS 14 2.1 Polymer Composites 16 2.1.1 Polymer Matrix 18 2.1.2 Fillers 24 2.2 Solid Propellants 26 2.2.1 Energetics 29 2.2.2 Metallic Fuel 30 2.2.3 Polymer Binders 32 2.3 Polyurethanes 34 2.3.1 Polyols 38 2.3.2 Isocyanates 40 3 TECHNICAL BACKGROUND 42 3.1 Overview 42 3.2 Matrix Modification 44 3.2.1 Solvent Swelling 44 3.2.2 Fragmentation 48 iii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 3.3 Ultrasound Irradiation 51 3.3.1 Cavitation 54 3.3.2 Emulsion Systems 61 3.4 Biodegradation 64 3.4.1 Fungal Degradation 66 3.4.2 Reversed Micelle System 68 4 STAGED PROCESS FOR COMPOSITE PROPELLANT 72 4.1 Introduction 72 4.2 Experimental 74 4.2.1 Samples 74 4.2.2 Procedures 76 4.2.3 Analytical Methods 80 4.3 Results and Discussions 83 4.3.1 Separation Efficiency 83 4.3.2 Polymer Degradation 98 4.3.2.1 Ultrasonic Degradation 98 4.3.2.2 Fungal Degradation 102 4.3.3 Biodegradation of Energetics 109 4.4 Conclusions 110 5 DEPOLYMERIZATION OF POLYMER MATRIX 117 5.1 Introduction 117 5.2 Solubility Parameter Spectra 117 5.2.1 Solvent Matching 119 5.2.2 Data Interpretation 126 5.2.3 Conclusions 136 5.3 Ultrasonic Degradation 137 5.3.1 Theoretical Background 138 5.3.2 Kinetic Investigation 148 iv R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 5.3.3 Conclusions 163 6 APPLICATIONS FOR SCRAP TIRES RECYCLING 164 6.1 The Scrap Tires Situation 164 6.1.1 Environmental Impacts 164 6.1.2 Current Disposal Practices 165 6.2 Properties of Tires 171 6.3 Recycling Technologies 175 6.3.1 Mechanical Methods 177 6.3.2 Devulcanization 179 6.4 Experiments 181 6.4.1 Objectives 181 6.4.2 Procedures 182 6.4.3 Results and Discussions 184 6.5 Conclusions 191 7 REMARKS AND FUTURE WORK 197 REFERENCES 202 v R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. LIST OF FIGURES Pages Figure 2.1 Classes of composites based on the geometrical morphology. 17 Figure 2.2 Classification of polymers. 20 Figure 2.3 Schematic representation of (a) thermoplastic and (b) 21 thermosetting polymers. Figure 2.4 Propellant processing - basic quick mix process. 28 Figure 2.5 Current generation of military explosives. 31 Figure 2.6 Polyurethanes are used for a variety of applications. 35 Figure 2.7 Chemical structure of IPDI and prepolymer (R-45M) 37 Figure 3.1 Swelling (jQ) o f natural rubber as a function of solvent 49 Hildebrand parameter (‘ 8): a, pure gum; b, tire tread. Figure 3.2 Effect of sonication on the molecular weight of polystyrene 53 in toluene. Figure 3.3 Effect of cavitation on ultrasound degradation of polystyrene 56 in toluene, viscosity of solution, 8, given by flow time in the viscometer is plotted against irradiation time, t. (1), pure toluene; (2), solution with air present; (3), degassed solution. Figure 3.4 Adsorption of emulsion molecules at an interface. 63 Figure 3.5 Revered micelle system in biodegradation. 70 Figure 4.1 Swelling effect of effective solvents on propellant, initial 85 4x4x4 cube. Figure 4.2 Matrix of composite propellant (50X): (a) original, (b) swollen. 87 Figure 4.3 Fragmentation of swollen propellant with centrifugal force 88 at 500 rpm. Figure 4.4 Sonication reactor with commercial titanium hom. 90 vi R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.5 Figure 4.6 Figure 4.7 Figure 4.8 Figure 4.9 Figure 4.10 Figure 4.11 Figure 4.12 Figure 4.13 Figure 4.14 Figure 4.15 Figure 4.16 Figure 5.1 Figure 5.2 The solubility increase o f polymer matrix verse duration of 91 ultrasound irradiation. The residue of propellant verse benzoyl peroxide concentration. 92 under same sonication condition (40 W/cm2 , 20 min). The residue of propellant verse nonionic surfactant (1 -Nonanol) 95 concentration under same sonication condition (40 W/cm2 , 20 min). The separation of metallic fuel and polymer matrix after 96 ultrasonication Degraded polymer was dissolved in the solvent leaving metallic fuel at the bottom. The separation results after matrix modification and ultrasound 97 irradiation. Color test for polyurethane solutions after different duration of 100 ultrasound irradiation. Relationship between intrinsic viscosity (tj) and average 103 molecular weight (M) o f prepolymer (R-45M). UV spectra of polyurethane solution: (a) before and (b) after 104 fungal degradation. Color test for polyurethane solution: (a) before and (b) after 106 fungal degradation. Molecular weight change of polyurethane after biodegradation 108 with white rot fungi enzyme through reversed micelle system. UV spectra of HMX in diluted activated sludge solution: (a) initial, 111 (b) after 25-day incubation, and (c) background. Schematic diagram of the staged process for composite 114 propellants disposal. Solvent Hildebrand parameter (8,) verse swelling effect (Q) 121 for natural rubber: (a) pure gum; (b) tire tread. Spatial representation in Hansen space of two-component 125 solvent mixture - polymer interaction. vii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5.3 Figure 5.4 Figure 5.5 Figure 5.6 Figure 5.7 Figure 5.8 Figure 5.9 Figure 5.10 Figure 5.11 Figure 5.12 Figure 5.13 Figure 5.14 Figure 5.15 Hildebrand parameter (5t) - hydrogen bonding parameter (yc ) 127 solubility map for polyurethane based propellant. Solubility matching (case one) with a two-component mixture 128 for polyurethane based propellant. Solubility matching (case two) with a two-component mixture 130 for polyurethane based propellant. Hansen parameters 8d - 8P solubility map for polyurethane- 131 based propellant. Hansen parameters 8d - 8h solubility map for polyurethane- 132 based propellant. Hansen parameters 8P - 8h solubility map for polyurethane- 133 based propellant. Hansen parameters S d - 8a solubility map for polyurethane- 134 based propellant. Representation of a Hansen parameter solubility sphere with 13 5 radius of interaction 'R and projections on three axial planes. Schematic representation (a) polymer molecules in dilute 139 solution; (b) the stretching out of part of a polymer chain by solvent movement around collapsing cavitation bubble. First -order rate plots taken from gpc curves for ultrasonic 140 degraded polystyrene; (curve) 10°DP, and 103 k (min'1 ), receptively: (a) 4.5, 14.3; (b) 5.0, 24.7; (c) 5.5, 32.6; (d) 6.0, 37.5; (e) 6.5, 45.4; (f) 7.0, 53.0; (g) 7.5, 64.0; (h) 8.0, 93.1; a) 9.0, 97.4. Schematic representation of the crosslinked polymer matrix. 143 Example of combination of the statistical and kinetic 146 methods for network formation. The degradation mechanism of coal in continuous-mixture 147 kinetic model. viii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5.16 Figure 5.17 Figure 5.18 Figure 5.19 Figure 5.20 Figure 5.21 Figure 5.22 Figure 6.1 Figure 6.2 Figure 6.3 Figure 6.4 Figure 6.5 Figure 6.6 Figure 6.7 Solubility of polyurethane matrix in propellant verse the sonication duration (power intensity = 40 W/cm2 , sample concentration is lg/50 ml). Solubility of polyurethane matrix in propellant verse the sonication duration (power intensity = 80 W/cm2 , sample concentration is lg/50 ml). Effect of ultrasonic intensity on the degradation of polystyrene: (1) 4.89 W/cm2 ; (2) 9.58 W/cm2 ; (3) 12.5 W/cm2; (4) 15.8 W/cm2 . Solubility of polyurethane matrix in propellant verse the benzoyl peroxide concentration (power intensity = 40 W/cm2 , duration is 20 min, sample concentration is lg/50 ml). Calibration curve of molecular weight based on standards of polystyrene. GPC of propellant solution after different duration of ultrasonication. Schematic representation of the polymer matrix fragments. Scrap tire use and disposal. Discard tires in California 1992. Cross section of a high-performance passenger tire. Volume change of crumb rubber in chloroform. IR spectra of multipolymer mixture of the devulcanized tire after the staged process. Schematic flowchart of the treatment process for scrap tire. Firestone Tire & Rubber Co. process for recovery from polyurethane scrap. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. LIST OF TABLES Pages Table 2.1 Shipments of composites in the past. 15 Table 2.2 Fillers for polymers. 25 Table 2.3 Characteristics of polyols. 39 Table 3.1 Factors that affect the formation of cavitation. 57 Table 4.1 Inert propellant composition. 75 Table 4.2 Results of propellant swelling with various solvents and 84 their solubility parameters. Table 4.3 Type and name of selected surfactants. 93 Table 4.4 The UV absorption of polyurethane solutions after different 99 sonication durations. Table 4.5 LMWPLT viscosity measurement. 107 Table 5.1 Rates constants for benzoyl peroxide degradation on 155 polystyrene. Table 5.2 The average molecular weight of polyurethane matrix after 161 ultrasonic degradation. Table 5.3 The average molecular weight of degraded polyurethane 162 network based on central breakage theory and specific degradation assumption. Table 6.1 General composition of passenger tires. 174 Table 6.2 Development of new recycling technologies. 176 Table 6.3 Swelling effects of various solvents on passenger tire. 186 x R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. ABSTRACT In order to find a safe, permanent, and environmentally sound method for the disposal of polyurethane-based propellants, this research utilized matrix modification, ultrasound irradiation and biological technologies to develop a feasible staged process to separate the useful components from the obsolete composite propellants and to degrade or completely decompose the synthetic cross-linked polymer matrices into low molecule-weight oligomers. In the initial stage, matrix modification and ultrasound irradiation techniques were used to separate the energetic solids and metallic fuel from the solid propellants. The propellant specimen was first comminuted in an appropriate swelling agent to relax the crosslinked matrix and then fragmented with centrifugal plus hydraulic forces. Most of the energetic solids were ejected and collected after this process. Subsequently an ultrasonic depolymerization with the aid of a free radical initiator was performed under the induced cavitation environment. The fully swollen propellant fragments was degraded via free radical oxidation within the locally high temperature and high pressure cavitation. Preliminary results indicated that the high molecule-weight polyurethane fragments had been broken down into soluble lower molecule-weight oligomers. The metallic fuel was then able to be separated from the partially degraded polyurethane binders. Finally, the biodegradation process with fungal species capable of degrading the low polymerized polyurethane was examined and demonstrated to serve as the xi R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. polishing stage to prevent the synthetic polymers in the waste stream from generating more pollution. The polymers were partially degraded into simple phenolic compounds by the enzyme extraction of white rot fungi. The bulk solution will then be carried into a separation tank to break up the mixture of the solvent-water system. The waste stream after this stage will then be discharged into the traditional sewer system. The waste stream will be treated in a conventional biological wastewater treatment plant to completely degrade the potential pollutants into harmless substances. The schematic flow diagram of the proposed staged process is outlined in Chapter Four. The final goal is to deliver a complete operational scheme to minimize typical composite propellants wastes and other polymer matrix composite (PMC) wastes intended for landfill disposal and to reuse the separated constituents as raw materials for various applications. Due to the extreme low degree of polymerization, the obsolete composite propellants are especially suitable for this staged process because the integrity of the polymer matrix will become very fragile after solvent relaxation. For most chemical tough PMC materials, the swollen network may not become as weak as that of the propellant, but their physical properties will deteriorate significantly after similar staged process. Therefore, the same treatment principles can be applied to other PMC materials. Among the PMC wastes, scrape tires were selected for the same investigation because of their environmental impacts which have posed serious wastes management xii R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. problems. Although there are several disposal practices currently in existence, no single one of them is capable of completely recycling scrap tiers without creating additional pollution. One of the most widely used disposal method is to process whole tires into crumb rubber through mechanical means. But these devulcanized rubber can only be used for limited applications such as floor mat because of their low quality. Only through devulcanization can the virgin properties of the tires be reclaimed and be manufactured into new products with competitive high quality. The staged process was able to devulcanize the tire rubber by cleaving the sulfur bonds. Unlike other disposal methods, the end products through this process, i.e., the separated filler materials, have significant potential for reuse. The staged process is a recycle process which will not only reduce the quantity of the tire wastes in stockpiles but will also reuse part or all of the separated materials. There will not be end use wastes that need to be disposed of. As the natural resources available to use is but limited and consumption of raw materials continues to grow, this staged process provide a significant and economical solution for two of the most challenging solid wastes disposal problems. These PMCs need not become useless wastes. With proper and efficient treatment, they can be recycled into useful products. And since both the obsolete propellants and scrap tires are of such substantial quantities, it is worth to take a different point of view on them, and try to turn wastes into valuable resources. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 1 INTRODUCTION The hazardous waste management problem is hardly new and in feet waste disposal problems have plagued mankind since introduction of even the most rudimentary attempts at manufacturing. As it is, public health and environmental problems associated with the disposal and treatment of hazardous waste continue to challenge society in ever-increasing dimensions. As the generation of waste is unavoidable and as the demand for a clean, safe environment grows, the clash of these two conflicts must be dealt with. The task of resource conservation and waste management becomes even more critical as the world population continues to grow while the natural resources available to us is but limited. Reduction and minimization of hazardous waste sources and better management of the wastes will, undoubtedly, remain a top priority for industry and commerce, government, as well as for the public for generations to come. 1.1 Background Problems resulting from the mismanagement of hazardous wastes can be described in a number of ways. One approach in identifying hazardous waste problems is to trace the problems back to their historical, regulatory background. The history of waste management in the United States essentially parallels the country’s progress in economic development. The U.S. began its economic surge with the industrial revolution in the latter part of the 1 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 19th century. Rapid growth continued through the early 20th century, fueled by the defense demands of World War II. After the war, the nation’s revitalized industrial emphasis shifted from armaments to the production of consumer goods. The production of both military and consumer goods generated huge quantities of municipal refuse and industrial waste. As early efforts at hazardous waste management were lacking, often ranging from ineffective to non-existent, not only were hazardous waste dumped onto the ground, into ditches, and in trash dumps, industrial wastes were passing through public owned treatment plants with inadequate treatment. Moreover, garbage generated by Americans in their daily lives made its way into the environment, posing a serious threat to public health and the ecological system. Uncontrolled releases continued through 1970s. EPA estimated hazardous waste generation at 20 to 50 million tons annually in 1977, adding the estimate that no more than 10% was disposed of in a manner that was safe to the environment (Council on Environmental Quality, 1977). By 1979, the EPA estimate of hazardous waste generation was 51 million tons per year (Council on Environmental Quality, 1979). This situation prompted Congress to pass the Solid Waste Disposal Act of 1965, the first federal government attempt to improve solid waste disposal practice. But in spite of intensified regulatory constrains, generation of hazardous waste continued to increase. Mounting concerns for human health and environmental protection led to amendment of the Act with the 1970 Resources Recovery Act. These were the predecessors to the Resource Conversion and Recovery Act (RCRA) of 1976, the major legislation attempted to manage 2 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. hazardous waste by imposing regulatory requirements upon generators and transporters of hazardous wastes, as well as treatment, storage, and disposal facilities. In spite of the development and following enforcement of these environmental laws, they had little effect on federal government activities since federal government agencies were largely unfettered by these laws. Not until 1986 were federal agencies brought under the Superfiind rules. In the meanwhile, the Departments of Energy and Defense hid their hazardous waste practices behind the national security curtain, and the true picture of these practices is only recently beginning to emerge. There is generally a basic misconception in the minds of many that hazardous waste generation is, if not solely, predominately the result of industrial manufacturing and processing operations. In feet, this is absolutely incorrect. Hazardous wastes are generated from a number of public and private sector sources, some quite obvious and some, despite the potential hazards they generate, often totally unrecognized. As mentioned above, in recent years, several federal agencies, namely, Departments of Energy and Defense, have been shown to be among the worst offenders of hazardous waste management statues, regulations, and policies. In the name of security, very little about the militaiy’s share of pollution has been made public. Most people tend to associate toxic waste emission with civil industrial giants like Union Carbide, Exxon, Du Pont and so on. In feet, the Pentagon produces well over a ton of toxic wastes every minute, an early output that some contend is greater than that of the top five U.S. chemical companies combined (Shiftman, 1992). 3 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. To understand the scope of this problem, one has to remember that Department of Defense itself is a vast industrial enterprise. To begin with, every year the Pentagon purchase million barrels of fuel oil - a toxic material itself Through its daily routine operations, the military generates large amount of hazardous byproducts. Huge quantities of toxic wastes were produced every day just to maintain the military’s collection of vehicles, tanks, plants, ships, and missiles, not to mention the hazardous wastes left behind by the testing, production, and storage of ammunition and various weapons. According to a 1996 Department of Defense release, military installations around the country released 11.4 million pounds of toxic chemicals during 1993 alone (Leutwyler, 1996). Another Pentagon’s study indicates that dangerous hazardous wastes, stored or disposed of improperly at virtually every U.S. military installation in every state, may currently contaminate more than 20,000 sites on land currently or formerly' owned by the U.S. (Shulman, 1992). At these locations, millions of toxic wastes have fouled thousands of square miles of soil and polluted the air and groundwater in communities across the country and hundreds of bases overseas. In addition to the contaminated military sites, there are the unexploded ordnance and the hazardous materials they contain. They pose a largely unregulated environmental problem which presents many of the same kinds of long-term, chronic ecological threats as do other toxic wastes. Whatever their designation, the danger of buried ordnance is fatally evident. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 1.2 Significance of the Problems As we know, various weapon systems, including missiles, rockets, nuclear weapons, and even some space applications, were extensively developed by the U.S. military during the Cold War. Unaware of the environmental impacts of such developments at the time, the military had inadvertently created a whole array of ecological problems which have since evolved into long-term environmental issues. And overshadowed by the Cold War’s commentary external peril, U.S. military’s legacy of environmental contamination was largely unmonitored and unaccounted for. With the end of the Cold War, United States and nations around the world starts to engage in arms reductions on an unprecedented scale. U.S. military is faced for the first time with the need to manage the dismantlement of vast numbers of “excess” weapons, including nuclear warheads and the fissile materials they contain. Tens of thousands of weapons will no longer be needed for military purposes. The task of managing this reversal of the arms competition is complex, costly, and long-term. Among the hazardous materials used to manufacture weapons, the obsolete composite propellants are one of the most critical and pressing issue since the risk of not destroying them increases as a result of weapon decay and the leaking toxins. Moreover, management of propellant disposal has mostly been unregulated until recently. The dangers involved in disposing obsolete composite propellants is quite obvious since they are noted for their high energy and thus highly explosive. The focus of this research is on the solid propellants cast in missiles and rocket boosters. They pose special 5 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. problems since as time passes by, they get older and older, it becomes less and less likely that they can be put to any beneficial use. One traditional approach to the disposal of obsolete propellants was the open-pit burning. Essentially, propellant waste was destroyed by blowing it up or burning it. During combustion, however, it generates tremendous amount of exhaust gases, including NOx and halogenated gases, that results in secondary air and groundwater pollution. This method has long been outlawed as environmental regulations instituted along the years become more stringent. But it did not prevent some companies from continuing this illegal practice as many defense contractors found themselves stuck with tons of explosives left behind after decades of defense contracts. The July 26, 1994 blast that killed two Rockwell scientists was a result of such an illegal disposal practice (Pols et aL, 1996). The blast occurred in Rockwell International’s Rocketdyne test facility in Santa Susana, 35 miles northwest of Los Angeles, when five scientists were mixing explosive chemicals. It killed two Rockwell physicists instantly and badly burned a third. Rockwell had maintained that the scientists were conducting legitimate scientific experiments. Not until early 1996 did the firm plead guilty to three federal felony accounts of illegal hazardous waste disposal and conceded the deaths were the result of illegal practice - the scientists were disposing of triaminoguanidine nitrate, or TAGN, a highly volatile chemical used in gun propellants. Prior to the 1994 blast, Rockwell has been implicated in numerous pollution violations over the years (Schine, 1995). However, nowhere has Rockwell had more 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. persistent proBfems than at its Rocketdyne test facility. In 1989, an energy survey found widespread nuclear contamination. In 1990, California’s Environmental protection Agency fined the company a total of $280,000 for hazardous waste violations, including burning toxic solvents in open barrels and improperly storing excessive propellant. More recently in 1992, the state fined Rockwell another $650,000 for similar problems at Rocketdyne and other facilities. 1994’s fetal accident led Rockwell to agree in a plea bargain worked out with federal prosecutors to pay an unprecedented $6.5 million fine, the largest fine ever assessed in a hazardous waste case in the state of California, according to the U.S. district judge in charge of the case. During the 21-months government investigation, it became clear that Rockwell had a nearly 30-year history of destroying explosive waste by blowing it up without opposition from government agencies (Bernstein and Levin, 1995). State documents indicated that propellant and other explosives were being detonated or burned at Santa Susana as early as the 1960s. Burning was common throughout the defense and aerospace industries at that time, and the practice was defended as safer than transporting explosive waste on streets and highways. Although increasing environmental oversight made them illegal, open burning and detonation were so routine that some companies explained the procedures in safety manuals. Disposing of explosive wastes had become a chronic headache for many companies, especially defense contractors who have inevitably accumulated explosive 7 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. stockpiles in thousands of pounds. And as the years have gone by, there are fewer and fewer places where the materials can go where it can be taken safely. The other approach to obsolete propellant disposal is storage. Traditionally, the military has stockpiled old rockets in storage sites. But since energetic ingredients are the major components of solid propellants, it should be expected that long-term storage might lead to deterioration. The energetic ingredients may undergo various interactions with each other or with air. Not surprisingly, such chemical interactions will significantly affect the stability of the propellants. This past February, Time magazine reported that the Army discovered that 1,500 of the 300,000 M-55s, manufactured between 1961 and 1966 and stored in five of its chemical-weapons storage sites, had external leaks, and some 10,000 more may be leaking internally (Thompson, 1996). Sarin, the deadly poison that was packed into the nose cone, corroded the aluminum casing. And sarin leaking into the rear chamber accelerates the decay of the stabilizing agent that prevents the rocket fuel from “auto-ignition.” Should such ignition occur, it could detonate other weapons stored on the sites. The report also pointed out that because there was no safe way to dismantle the rocket, the deadly nerve agent and the rocket fuel had been locked in a slowly rotting shell for more than 30 years. Storage is obviously not a permanent solution for the disposal of solid propellants. The incidents discussed in this section are but two of a number of cases that have not only galvanized public opinion, they have also reflected the importance and urgency of the solid propellant waste problems. Finding a safe and permanent method for solid 8 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. propellant disposal is obviously a critical issue, and it has become even more pressing with the demilitarization of missiles and rockets following the end of the Cold War (Shiu et aL, 1993). 13 Research Objectives The target materials for this research, the solid composite propellants, are not only hazardous wastes under RCRA regulations, they can also be classified as polymer matrix composite (PMC) wastes. As PMC wastes, current solid propellants essentially have a typical composition of two phase constituents: polymer matrix and fillers (Le., energetic solids and metallic fuel). Although fillers usually account for more than eighty (80) percent in current propellant formulation, the feet that the three-dimensional polymer matrix is extremely resistant to any decomposition reactions complicates the research. The nature of these components will be discussed in details in Chapter Two. Due to their explosive nature, solid propellants cannot be processed safely through incineration or landfill disposal, the two common methods in handling hazardous wastes. It is self-evident that the safety issue has to be a primary consideration in developing any disposal process for polymer matrix composite wastes. Currently, several techniques are being actively pursued by different research groups, including aqueous maceration and extraction of propellant by Tbiokol, cryogenic washout by General Atomic, and ammonia extraction at supercritical or near-supercritical R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. conditions by Hercules. In addition, alternative methods leading to a milder recovery process are also being researched (Shiu et al, 1995). The primary goal in developing an optimal technique for the management and disposition of obsolete solid propellants can be divided into three main objectives: Separation of Filler Components As noted, most of the advanced solid propellants essentially contain three major components: energetics, metallic fuel, and polymer matrix. In order to maintain the high performance of the composite, it is very important from the macro point of view for the energetic solids and metallic fuel to uniformly dispersed inside the polymer matrix (network) but without any chemical or significant bonding. Since the two major filler components, energetics and metallic fuel, usually occupy more than 80% in the entire propellant composition, the best strategy to handle composite propellants will be to retrieve their useful components by removing the polymer binders. If the separation process is satisfactory, not only could the volume of the wastes be significantly reduced, the separated valuable components could also be subject to reuse. Degradation of Polymer Matrix The bulk form of polymer matrix composite (PMC) materials such as the solid composite propellants is based on the structure formed by polymers. Various types of polymers can be used, both natural and synthetic, but synthetic polymers completely dominate the markets. 10 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. It is well known and understood that these man-made substances, such as plastics and rubbers, are usually manufactured for environmentally durable and will last for a long time with little deterioration if any at all Because of this durable nature, one major task for this research is to cleave and degrade the 3-dimensional network formed by the polyurethane binders. Although only 10% to 15% of the propellant is composed of the polyurethane binders, they still need to be degraded or recycled since these materials are not degradable in natural environment. Recycle of PMC Wastes In addition to developing a disposal method for separating the fillers from the polymer matrix in solid propellants, this research can also be considered as a recycling method for PMC wastes. Based on the fundamental characteristics of PMC, the constituents remain their individual identity in such a way that they can be physically identified. In other words, the fillers are only physically trapped inside the polymer matrix without chemical bonding among them. Therefore, the separated filler materials, including the polymer portion, are expected to be recycled or reused. In recent years, new PMC articles have continued to be invented and manufactured in order to meet the demands for high performance materials. The quantity of PMC applied in a wide range of fields have been rapidly growing, thus creating a huge waste problem after they are used. One obvious example is the scrap tires which are generated in tens of millions each year in California alone and undoubtedly many more all over United States. 11 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Lacking an efficient disposal technique, most of the PMC wastes end up in stockpiles and landfills with little treatment. In applying the process developed by this research to PMC wastes, it is expected considerable amounts of the wastes accumulated can be minimized and recycled. 1.4 Approaches to the Problems To accomplish the three objectives proposed in the previous section, two major tasks have to be accomplished. The first task is to separate the three major components, energetics, polymer binders, and metallic fuel; the second, to degrade the high molecular weight polymers into low molecular weight oligomers or environmental acceptable wastes. Existing technologies for solid composite propellants disposal generally include either physical, chemical or biological method. Based on the literature research, many technologies did show certain degree of potential but no one method can completely process the propellant wastes. Only through the staged process integrating all three techniques can the quantity of PMC wastes be minimized, and certain, if not total, portion of the wastes can be recycled back into raw materials without causing pollution. The techniques integrated in the staged process developed in this research are: • Matrix Modification • Ultrasound Irradiation • Biodegradation. 12 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Details o f these technologies will be described and illustrated in Chapter Three, including both theoretical background and applications. 13 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 2 DESCRIPTION OF COMPOSITE PROPELLANTS Composite materials have been used for thousands o f years. Early humans used mud to make bricks for construction purpose, straw, as well as fiber, were mixed inside as reinforcing material which forms the composites. The use o f alloy has also been found quit early in history. Copper alloyed with zinc to form bronze which would be stronger than pure metal. It was then used to make containers and weapons. Nowadays, new composites are continuously invented and manufactured to fulfill the needs for high performance materials. Modem composites have very wide applications in different areas. They are used in transportation, construction, electrical and electronics appliances, business equipment, aircraft and aerospace components, and in a number of other areas. The demand for composite materials has been getting higher and higher. Table 2.1 indicates that the shipment of composites in the U.S. was already over 2.2 billion pounds in 1987, and with average 3.6 % increase per year to 3.2 billion pounds in 1995 (Storck, 1996). Because composites are made for high performance and environmental durable, it is very difficult to recycle them once they are used and become wastes. They usually end up in landfills and create environmental problems. How to minimize or recycle this type of wastes, therefore, would remain a major challenge for mankind now as well as in the future. 14 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C/3 C3 Q, .s C/3 ♦ -* ’55 1 o u C m 0 c n C 1 o , 2 C /3 (N _U x > CO H o s O u ~ i O n O n s ON c n O n O n O n O n O ON O n O n 00 O n 0 0 0 0 O n r* * 0 0 O n N O 00 O n U - 4 o c o - 4 .4 % 2.2 3 .5 2.7 3.3 5.0 01 5.4 2.9 3 . 6 % 23.6 0 0 645.0 195.3 402.4 09ZZ 374.4 994.1 0 0 o ' 5 ? n p n 23.7 9991 629.3 O 0 0 394.4 314.8 372.0 981.8 106.7 P O p p -T 24.2 160.7 596.9 0 0 376.3 299.3 363.5 945.6 0 0 o i o rT 25.4 147.5 530.0 165.7 352.0 274.9 319.3 822.1 89.3 N N O fS n 32.3 143.2 483.0 162.2 332.3 260.0 304.4 o o r * * 83.4 0 0 © i n i n fS r- 0 0 135.2 420.0 148.7 355.0 231.1 275.0 682.2 73.8 r - i n n P S 39.0 153.0 468.0 165.0 350.0 241.0 375.0 705.0 79.0 o i n r - i n N ,2 '5 o o n £ 15 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 2.1 Polymer Composites It is obvious and well understood that two or more materials judiciously combined will perform differently and often become more efficient than the materials function by themselves. Although this simple concept offers a useful and even revolutionary way of thinking about the application o f materials, the actual range o f possibilities, so important for their use, is completely obscured by this concept. It is, therefore, worthwhile examining the classification o f composites which at the same time illustrates geometrical morphology or arrangement o f filler and polymer relative to each other. It is necessary to understand the fundamental descriptions and properties o f these advanced materials. In principle, composites can be constructed out of any combination of two or more materials, whether metallic, organic, or inorganic. Although the possible material combination in composites are virtually unlimited, the constituent forms are more restricted. The matrix is the body constituent, serving to enclose the composite and give it its buck form. So far, little has been said about the nature o f the polymer composites other than indicating that they are mixed systems o f polymers and fillers. There is no universally accepted definition of polymer composites. Working definition of composite material usually takes into account both the structural form and composition of the material constituents. One can classify the composites based on the geometrical morphology, as shown in Figure 2.1. The composite matrix can be a 16 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. FIB ER COM POSITE PARTICULATE COMPOSITE LAM INAR CO M PO SITE FLA K E COM POSITE FILLED COM POSITE Figure 2.1 Classes of composites based on the geometrical morphology. (Source: Schwartz, M. M., Composite Materials Handbook, 1992). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. polymer, a metal, or a ceramic. For the purpose o f illustration, composites can be categorized into organic (polymer) matrixes based composites and inorganic matrixes based composites. Since the polymer matrix composites (PMC) dominate the market for composites production and generally have caused more serious environmental problems than other kinds of composites, they are carefully examined in the followings for the purpose o f this research (Sheldon, 1982). Polymer matrix composites are, as identified by their name, essentially materials of two separated origins which have been physically produced by dispersing one phase (filler) in a continuous matrix (polymer). In most cases, the matrix provides the framework, and the filler provides the desired engineering and specific functional properties. In order to obtain the optimum properties in the composites, the two materials must be compatible and not react in a way that would degrade or destroy their inherent properties. Thus, it is important for the matrix and filler materials to exist as two separate constituents (Schwartz, 1992). 2.1.1 Polymer Matrix In order to appreciate in which way and to what extent the properties of polymer composites different from those of the parent polymers, it is necessary to understand the fundamental nature o f polymers. In addition, this nature may depend upon the way the polymer was made in the first place. Different methods in principle being able to produce 18 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. material o f essentially the same chemical nature but having different physical properties, it is important to consider the details of the actual synthesis of polymers. Various types of polymers , both natural and synthetic, have been used in manufacturing composite materials. There are three major classes of polymers formed (Figure 2.2): First, the thermoplastic polymers which undergo softening or melting on heating and therefore are amenable to liquid-flow shaping technique; Second, the thermosetting polymers in which primary bond chains are formed between the main chains o f the polymer, thus putting restrictions on any possible thermal-forming operations following initial cure (Figure 2.3); and Third, the elastomers which exist as both thermoplastics and thermosets (Schwartz, 1992). Thermoplastics Thermoplastic matrix system, commonly known as plastics, are linear or branched polymers which can be melted upon the application of heat. They can be molded and remolded into virtually any shape using processing technologies such as injection molding and extrusion. Thermoplastic matrixes are emerging as important materials for composite applications because o f their improved fracture toughness over thermoset materials and their potential for much lower costs in the manufacturing of the finished composite. Generally, thermoplastics do not crystallize easily upon cooling to the solid state because this requires considerable ordering of the highly coiled and entangled macromolecules present in the liquid state. Those which do crystallize invariably do not 19 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. Polymers I I Thermoplastics Elastomers Thermosets Crystalline Amorphous Figure 2.2 Classification of polymers (Source: Young R. J. and Lovell, P. A., Introduction to Polymers, 1991). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. (a) CROSS-LINKS Figure 2.3 Schematic representation of (a) thermoplastic and (b) thermosetting Polymers. (Source: Schwartz, M. M., Composite Materials Handbook, 1992). 21 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases o f such polymers are characterized by their melting temperature (Tm). Many thermoplastics are, however, completely amorphous and incapable of crystallization, even upon annealing. Amorphous polymer (and amorphous phases o f semi-crystalline polymers) are characterized by their glass transition temperature (Tg), the temperature at which they transform abruptly from glass, Le., hard, state to the rubbery, Le., soft, state (Young and Lovell, 1991). Thermosets Thermoset matrix systems dominate the composite industries because of their reactive nature. The reasons begin with monomers or oligomers state characterized by quit low viscosity (Le., high flow). This allows real impregnation of solids into composite form and a mean of achieving high strength, high stiffness crosslinked network in the cured parts. Initially, the viscosity of these resins is low. However, these materials were manufactured through chemical reactions which crosslink the polymer chains, and thus connect the entire matrix together in a three-dimensional network (curing). Thermosets, because of their 3-dimensional crosslinked structure, tend to have high dimensional stability, high temperature resistance, and good resistance to solvents. They are very difficult to recycle without destroying the network. Unlike linear and branched polymers, network polymers do not melt upon heating and will not dissolve, though they may swell considerably in compatible solvents. 22 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Elastomers Elastomers are crosslinked rubbery polymers (Le. rubbery networks) that can be stretched to high extensions then rapidly recover their original dimensions when the applied stress is released. This extremely important and useful property is a reflection o f their molecular structure in which the network is o f low crosslink density. The rubbery polymer chains become extended upon deformation but are prevented from permanent flow by the crosslinks, and driven by entropy, spring back to their original positions upon removal of the stress. In other words, the presence o f the polymer long chains is a necessary condition for rubberlike elasticity. In addition, for any rubberlike behavior to occur, the system must also possess sufficient internal mobility so that, during deformation and during recovery, the required rearrangement of chain configuration can occur. Internal mobility allowing segmental motions of the chain elements alone, however, is not sufficient to assure genuine rubberlike behavior, a permanent of structure is also required. To achieve permanence, cross-linkages which join the chains into a network o f “infinite” extent is usually inserted. Chains bound at either end to the network cannot dissipate any orientation induced by deformation except when the initial macroscopic dimensions of the sample is restored, whereas separate linear molecules could dissipate through spontaneous rearrangements of their configurations. Therefore, the existence of a permanent network structure is the key to the main characteristics of elastomers (Flory, 1953). 23 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 2.1.2 Fillers A wide range o f fillers, including pigments and other additives, are used in polymeric systems formulation. Many of these find applications in other technologies and their availability owes itself not so much to the demands of the polymer industry, as to, for example, paper making, cosmetics, the steel industry, ceramics manufacture, paint making, etc. Supplementary demand for new types o f fillers have been growing considerably because, on the one hand, the facility of many polymers tends to accommodate otherwise surplus materials without undue deterioration in properties or even to upgrade behavior, and, on the other, they become competitive with other structural material such as metals, when they are filled with certain fibrous substances. Examples of some, but not limited to, of these are shown in Table 2.2. The range can be subdivided in various ways, for convenience this section will first group them into particulate and fibrous fillers. Clearly the former will embrace not only fillers o f a regular shape such as spheres but also many o f irregular shape possibly having extensive convolution and porosity in addition. As previously noted, both the matrix and filler exist as two separate constituents that do not alloy and, except for a bonding action, do not combine chemically to any significant extent. Particle size has an important influence on properties either by disruption flow patterns in fabrication or subsequent deformation processes, or, more importantly, through an effect on the interfacial contact area between resin and filler. Some knowledge of particle size distribution is of importance in product design. This aspect has 24 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Table 2.2 Fillers for polymers. Particulate Fibrous Organic Inorganic Organic Inorganic Woodflour Glass Cellulose Whiskers Cork Calcium carbonate Wool Asbestos Nutshell Alumina Carbon / graphite Glass Starch Beryllium oxide Aramid fibre Mineral wool Polymers Iron oxide Nylons Calcium sulphate Carbon Magnesia Polyester Potassium titanate Protein Magnesium carbonate Titanium dioxide Zinc oxide Metal powder Silica Silicates Barium ferrite Barium sulphate Molybdenum disulphide Silicon carbide Potassium titanate Clays Boron Alumina Metals Sodium aluminium hydroxy carbonate K ) Source: Sheldon, R. P., Composite Polymeric Materials, 1982. been of growing concern in recent years as technologists have attempted to incorporate increasing amount of relatively cheap filler into polymers to extend their use or specifically to improve properties. 2.2 Solid Propellants Solid propellants were first used in China for military purposes in the early 11th century. Essentially the same composition, a mixture of loose powder containing sulfur, nitrate salts, and carbon (charcoal), was used until relatively recently. This kind of composition could not be developed for large diameter (high thrust) motors due to its unreliable ballistic properties. And although later developments in double base technology made it possible to consolidate the loose powders into homogeneous forms or grains, solid propellants were still limited to small diameter motors. Compression molded propellants were not developed until it was discovered that rubber could be mixed with an oxidant to form a strong, well-consolidated mass under elevated temperature and pressure. By varying this technique, high thrust and complex geometry motors were constructed and satisfactorily fired. But it was after the development of cast composite propellants that large diameter solid grains suitable for first- stage ballistic missiles or space boosters were manufactured (Gould, 1966). Modem propellant manufacture embodies all aspects of chemistry. Crosslinked polymer networks synthesized during polymerization in the presence of the propellant ingredients and often with the combustion chamber of the rocket motor as the reaction 26 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. vessel have been the subject of extensive research in the past few decades as a direct result o f federal support. These researches mainly focused on the requirements of energy, physical properties, and processing characteristics o f propellants (Arendale, 1969). Due to the high acceleration of the projectile longitudinal strains developed in the bore of the grain and extreme temperature variation during ballistic projection, the propellant must therefore have a high flexibility and ultimate tensile strain limit and thermodynamic stability. The unbumed propellant itself acts as heat shield between the flame and chamber walL The price of this design is higher flexibility demanded from the propellant grain, which has to be maintained over an extended temperature range. An example for this are the air-to-air missiles positioned under the wings of supersonic aircraft. Since steel wall and propellant grain have different thermal coefficients of expansion, strains are included which might lead easily to cracking of the grain with disastrous consequences upon ignition. Mechanical characteristics mainly depend on the binder but also on the particle size and on the adhesive between particles and binders. The polymers, often with plastizers, not only provide the matrix to contain other ingredients, but also participate in the combustion serving as a fuel for suspended energetic particles (Gould, 1969). Having established the satisfactory or within- specification ballistics and mechanical properties of a new propellant system, a manufacturer is then faced with a reasonably limited choice of manufacturing methods. The general composite propellant manufacturing system is shown in Figure 2.4. It is a standard flow diagram of the steps involved in propellant manufacturing ranging from preparation of binder and energetic ingredients, fuel 27 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. solids feeder cross-linker feeder prepolymer feeder metering pump energetics disperser clear carrier recycle solids feeder separator jet m ixer propellant to vacuum casting aluminum disperser to 00 Figure 2.4 Propellant Processing - Basic quick mix process. {Source: Schwartz, M. M., Composite Materials Handbook, 1992). preparation, propellant mixing, to casting and curing. Noting that processability is an important factor in propellant formulation and processability determination is a function of the rheology of the propellant. The viscosity of composite propellant slurry is an important parameter to the processability and castabOity. It is affected not only by the binder, but also by the size distribution, shape, and surface properties of solid fillers in composite propellant. As any polymer matrix composites, solid propellants, in their most current formulation, consist of a main structure o f a composite of a 3-dimensional cross-linked network. Regardless of type, all propellants contain binders that can be cured through reactions of hydroxyl or carboxyl groups. The backbone of the polymer matrix can be polyether, polyester, or polyurethane. Up to 90% (by weight) of the composites are solid components which include fuel (aluminum powder) and energetics such as nitrates and perchlorates, e.g., ammonium perchlorates (AP). These materials are trapped in the cross- linked network without chemical bonds. In some instances, the inert portions of the coloring matter and other plasticizers such as dioctyl adipate (DOA) are added to adjust the physical properties. The three major components, energetics, metallic fuel, and polymer binders, are described in the followings. 2.2.1 Energetics The chemicals used to manufacture propellants are mainly developed by government laboratories and the defense industry, and are little known outside of these communities. 29 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Their research usually focuses on developing new energetics and propellants for military and space applications. Research literature on propellant studies have shown new development on energetics either invented or updated and the formulations usually contain nitrogen, fluorine, or chlorine. Ammonia perchlorate (AP) is currently the predominant energetic used in a variety o f propulsion systems. The other widely used energetics for propellants are HMX (tetranitrotetraazacyclooctane), RDX (trinitrohexahydrotriazine), TNT (trinitrotoluene). (Figure 2.5) They represent the current generation of military explosives. Among the four, TNT is best known to the public. Although TNT is still widely used by the military, its explosive power is lower than that of HMX and RDX, and its manufacturing process causes serious environmental problems (Stu, 1994). The combination o f HTPB/AP is the current composite propellant composition that gives high performance, long service life and low cost. The nature of the fuel-binder which allows high concentration of solid filler and good anti-aging characteristics has greatly extended the solid rocket technology. Therefore, the addition of a bonding agent is essential to ensure the inproved interaction at the binder-filler and to have best mechanical property behavior. 2.2.2 Metallic Fuel A major advance in propellant technology occurred when it was discovered that metallic fuels could be incorporated into the binder-energetic mixture to give energy as well as 30 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. V e 'i •c ■ a 'C I i s 1 0 3 X 0 > X // o / > \ — O z— o // 0 > e ‘3 1 v 1 § \/ 4 > c « 3 •/ o— z K K 0=2 * \ o 2 o z 31 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. F ig u re 2 . 5 Current generation o f m ilitary explosives higher density without affecting the mechanical properties o f the propellants. The specific inpulse of propellants can be substantially increased by incorporating a metal, such as aluminum or magnesium. The metallic additive can be considered to be oxidized by the stream, and therefore it does not require an additional oxidant. In the case where aluminum is added to a hydrocarbon-ammonium perchlorate system, the combustion proceeds as follows: NH4CIO4 + (CH2 )„ + 2A1 ~ > 1/2 N2 + CO + 5/2 H 2 + A 1 2 0 3 + HC1 [2.1] Scurlock et aL (1963) have investigated the effect of aluminum content on the PVC- based propellants and found that the maximum specific inpulse foils in the range o f 18 to 2 0 % aluminum. They also found that the burning rate increased with the addition o f finely divided aluminum, particularly if the aluminum used is spherical, in contrast to the shape of ground AP. 2.2.3 Polymer Binders Solid propellants utilize polymers, often with plasticizers as matrixes, to contain other ingredients participating in the combustion. The polymer-plasticizer combination provides the mechanical properties that allow the propellants to be processed into the desired shape and dimensions. The cast composite procedure calls for a liquid fuel to be mixed with a solid oxidizer. When the solids are thoroughly dispersed, the semisolid, workable "batter" 32 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. can then be cast into a rocket motor cavity. By cooling or by controlling the chemical reactions within the fuel, the mixture will set up or cure to a solid. The liquid fuel thus becomes a binder, Le., a component which performs two functions: that of imparting good mechanical properties to the propellants, and that of being burned as fueL Current binder candidates are most likely to be materials typified by low glass- transition temperatures ranging well below -50° F. and high softening temperatures, above 200° F. Typical polymers may include polyisobutylene, polysulfide, polybutadiene, etc. The materials are generally solids of high molecular weight. To be processable, the starting binder ingredients must be liquid so that the oxidizer crystals and powdered metallic fuels can be dispersed. In practice, the mechanical properties of a solid propellant improve as the ratio of binder to energetics increases. However, in most systems peak energetics occur at the level of 9-11% binder by weight, whereas minimum acceptable physical properties are first achieved at the 14-16% level The importance o f reliable mechanical properties can be illustrated by the feet that most operational systems accept the sacrifice in energy and operate at the 14-16% binder level (Scurlock et a l, 1963). To meet the needs o f higher energy and precise mechanical requirements, the polyurethane-based propellants were developed around the mid-1950s, and became the representative of the advanced types of propellants. Polyurethane propellants derive their name from the rubbery matrix which is formed through polyurethane polymerization. Since 33 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the most important task in this research is to cleave the polyurethane binders o f the propellant, it is necessary to understand the characteristics of polyurethanes. 2 3 Polyurethanes Polyurethanes (PU) have applications in various areas and their applications continue to expand. A consideration of particular properties of certain grades of polyurethanes and the way in which these compounds are used will serve to demonstrate their versatility in such forms as foams, rigid polyurethanes, elastomers, adhesives, binders, coatings, and paints. Major areas in which they have been extensively used include automotive components, furniture, construction, thermal insulation, and footwear. (Figure 2.6) All polyurethanes are based on the exothermic reaction o f polyisocyanates with polyol molecules which contain hydroxyl groups. In principle any diol/triol mixture which reacts with a diisocyanate will yield a network. However, to be useful as propellant binder, additional requirements must be met. The most important requirements are low cure shrinkage, low reaction exotherm, rubbery characteristics down to arctic temperatures, good aging stability, and ease o f handling during propellant manufacture. The urethane reaction is particularly useful for solid propellant applications because of its quantitative nature, convenient rate which can be adjusted by proper choice o f additives, and the availability of many suitable hydroxyl compounds which permit the tailoring of propellant mechanical properties. A urethane group is formed by the chemical reaction between a polyol and an isocyanate: 34 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figures I-l :o 1-3 Polyurethanes are used for a variety o f applications 1. M etal faced building panels 2. Shoe and boot soles 3. C ar exterior panels 4. C ar seating 5. H ousings for electronic equipm ent 6. B uoyancy in boats 7. R e frig e ra to r insulation 3. S tru c iu ra l foam fu rn itu re 1 Figure 2.6 Polyurethanes are used for a variety of applications. (Source: Woods, G., The ICI Polyurethanes Book, 1990). 35 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. n HO-R-OH + nNCO-R'-OCN - > -[-C-R-CO-NH-R'-NH-]„- [2.2] Polyol Diisocyanate Polyurethane Baker and Gaunt (1949) proposed that the reaction is initiated through the attack of an alkoxide ion on the carbon atom o f the isocyanate group: (-) (+) (-) (+) (-) R—N “ C “ O + H ---O R ♦=* RN— C = 0 —* • RNHCOOR [2.3] < i • • i • ■ i H - ---O— R Thus, any substitution of the isocyanate molecule which renders this carbon atom more positive should increase the reactivity o f the isocyanate. This was verified by Baker and Holdsworth (1947), who showed that reactivity greatly increased in the following order: The main structure of the polyurethane-based propellants is a cross-linking polymer matrix which physically traps the energetics and metallic fuel together to form a homogeneous composite. Polyurethane propellants derive their name from a rubbery matrix which belongs to the formal class o f polyurethane (PU) elastomers, especially those containing hydroxyl-terminated polybutadiene (HTPB) (Gould, 1969; Woods, 1990; Sorenson and Campbell, 1962). THPB binder is hydroxylated polymeric material and, as such, is curable using isocyanates. To reduce the moisture sensitivity of the curing reaction, isophorone diisocyanate (EPDI) was selected as the primary curing agent (Figure 2.7). To H 36 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. r t : g x? e .2 C B CO CO Q P i t o I Pi \ g 3 g e l i s u * 1 o / u II T o B 37 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. F ig u re 2 . 7 Chemical structure o f IPD I a n d prepolymer (R-45M) achieve sufficient mechanical properties o f the propellant, the curing ratio (NCO/OH) usually rangs form 0.85:1 up to 0.95:1. 23.1 Polyols Some 90 percent of the polyols used in making polyurethanes are polyethers with terminal hydroxyl groups. Hydroxyl-terminated polyethers are also used to obtain polyurethane with special properties. Factors in achieving these properties include the choice of polyol, especially with regard to the size and flexibility o f its molecular structure and its functionality control, and to a large extent the degree of cross-linking achieved in the polymer that is formed in the reaction with the polyisocyanate (Fu et aL, 1985). The hydroxyl-terminated polybutadienes (HTPB) are the only prepolymers which have been specifically prepared for propellant binders. They combine the high specific impulse of the well-proved carboxyl-terminated polybutadiene with the clean, stoichiometric urethane reaction, yielding propellants with unsurpassed mechanical properties. According to Cohen and Siegmann's investigation on the structure-property relationship of HTPB- based polyurethanes, polyurethane-based matrixes are superior to the common polyester- and polyether-based matrixes. The former has lower water permeability, better stability in a moist atmosphere, higher electrical insulation, and lower glass-transition temperature (Tg) (Cohen et aL, 1987). The characteristics of the polyols in making the two principal classes of polyurethane, flexible and rigid, are shown in Table 2.3. The “hydroxyl value” is used as a 38 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 2.3 Characteristics of polyols. Characteristics Flexible foams and elastomers Rigid foams/solids and stiff coatings Molecular weight range 1,000 to 6,500 400 to 1,200 Functionality range 2.0 to 3.0 3.0 to 8.0 Hydroxyl value range 28 to 160 250 to 1,000 (mg KOH/g) Source: Woods, G., The ICI Polyurethanes Book, 1990. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. measure o f the concentration o f isocyanate-reactive hydroxyl groups per weight of the polyol and is expressed as mg KOH/g. Polyols sold for use in polyurethanes are invariably characterized by hydroxyl value as this is convenient for the calculation of stoichiomatric formulation. The measured hydroxyl value of a polyol is related to its molecular weight and functionality: 56.1 x functionality Hydroxyl value (mg KOH/g) = -----------------------------x 1000 [2.4] molecular weight 2.3.2 Isocyanates The other major method of controlling the properties of the final polyurethane is by varying the types of isocyanates used. Isocyanates may be modified in many ways to give products different physical and chemical properties. Although aromatic isocyanates (TDI and MDI) are most commonly used in commercial applications, the aliphatic isocyanates are more widely used as propellant binders for military applications. In general, the full-cured elastomers are tough, abrasion-resistant materials with a high mechanical resistance to many solvents and chemicals, with the exception of strong based acids, oxidizing agents, and a few strong polar solvents. The polyurethane elastomers based on aliphatic diisocyanates, such as l-isocyanato-3-isocyanatomethyl- 3,5,5- trimethylcyclohexane (IPDI) and 1,6-diisocyanatohexane (HDI), are also highly resistant to moisture sensitivity, weathering and discoloration by light. 40 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The considerable growth o f the polyurethane industry has resulted in an equally considerable increase in the problem o f eliminating and reusing polyurethane wastes. Although a market has been found for chips o f soft polyurethane foam wastes by bonding together the chips to form composite materials, it is nevertheless possible to use only a limited quantity of soft-foam material in this way. Unfortunately, the same is true for the utilization of wastes from semihard and hard polyurethane foams or elastomer granulates. Accordingly, large quantities o f polyurethane wastes from the manufacture of hardened soft foam and elastomers must be dumped in waste collecting areas or destroyed in incinerators. Unfortunately, this gives rise to considerable ecological, technical, and economic problems because of the low specific gravity and, hence, large volume o f wastes. 41 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 3 TECHNICAL BACKGROUND Major methods employed by current polymer wastes recycling technologies include processing the materials through separation, degradation, and purification. Each method has its merits and limitations, which is examined in this chapter in terms of its effectiveness in treating the specific polymer matrix composite (PMC) wastes and how it is incorporated in this research. 3.1 Overview A polymer matrix can be broken down in a number of ways, both directly by the action of aggressive chemicals, and indirectly through stress-producing mechanisms. They include thermal, radiation, mechanical, and biological degradation. In practice, two or more techniques may be combined to create coupled breakdown environments. To manage and recover the targeted wastes of the obsolete propellants, two major tasks have to be accomplished. The first task is to separate the three major components, energetics, polymer binders, and metallic fuel; Secondly, the high molecular weight polymers need to be degraded into low molecular weight oligomer or monomers. The two tasks can be accomplished by using the techniques of matrix modification, ultrasound irradiation and biodegradation. 42 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Matrix modification has been successfully used in separating naturally occurring composite systems such as oil shale or coal into useful fractions (Sadeghi et al., 1982). Through solvent swelling, the fragmentation of cross-linked systems such as asphalt has also been accomplished in several studies (Chan and Yen, 1981; Lee et al., 1988). In addition, as shown by Young's modules and other mechanical methods, the physical integrity of the composites has been destroyed. For a number of years, ultrasonic wave has been utilized as a method for tar sand separation to aid in the recovery of bitumens (Sadeghi et al., 1990). It has also been used for heavy oil upgrading to crack the high molecular weight asphaltene into low molecular weight oil (Sadeghi et al., 1994), as well as for the dechlorination of trihalocarbons in water (Chen et al., 1990). The results have been significant and may develop into promising petroleum recovery techniques. Plastics or elastomer composites are usually difficult to degrade through biological means. However, a number of recent studies have indicated that polyurethanes are susceptible to microbial degradation (Filip, 1992; Shuttleworth and Seal, 1986; Pathirana and Seal, 1984; Darby & Kaplan, 1968). White rot fungi have been used to degrade a wide range of synthetic materials, including polycyclic aromatic compounds, chlorinated aromatic compounds, pesticides, dyes, munitions and lignin (Valli & Brock, 1991; Smith & Gold, 1979). Their high oxidizing capability is effective in degrading various compounds through enzymatic reactions (Barr and Aust, 1994). 43 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Clearly, the techniques of matrix modification, ultrasonic degradation and biodegradation are effective to various degrees in treating the various components in obsolete propellants. This study, therefore, will combine all three techniques in its approach to the treatment of the propellant wastes. 3.2 Matrix Modification One of the most critical factors in developing recycling technologies is how to minimize the size of the target wastes. As noted, the polymer matrix composites (PMC) are difficult for size reduction without a lot of force and special machinery because they possess the elastomer characteristics of durability as a result of their network structure. This crosslinks structure makes the PMC materials very durable and can resist acid, base, and even organic solvents. Rather than directly attacking the tuff integrity in the structure, this research uses the matrix modification technique prior to the treatment process which should be more economical and more efficient than the traditional mechanical approach for size reduction. The two major effects caused by the matrix modification techniques include solvent swelling and fragmentation. 3.2.1 Solvent Swelling The chemical linkages for a linear chain of ho mo polymers in an effective solvent system usually can be easily broken by stirring, resulting in an uniform polymer solution; and the bond breakage can often be detected by viscosity reduction. For the three- 44 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. dimensional cross-linked network in the polymer materials, however, this approach is often ineffective. However, the elastomers can be swollen and expanded to a significant degree under proper solvent systems. The introduction of an effective organic solvent to a cross-linked system can cause the network to expand. The choice of the solvent is of paramount importance because ineffective solvents will cause the polymer molecule to adopt a tight configuration as opposed to the desired expanded coiled configuration occurred under the appropriate solvent systems. Solvent swelling is often used to determine the degree of cross-linking in polyurethane elastomers. In many chemical comminations, this expansion can exceed the internal force holding the network or crystallite system together, thus causing its dissolution. The dilation of a brittle solid can be expressed by where S is the strain energy per unit volume, v is the Poisson ratio, G is the shear modules, and V is the volume. The fracture of a brittle solid can be expressed as [3.1] F [3.2] 2 E 45 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. where F is the fracture energy per unit volume, cr is the tensile stress, and E is the bulk modules. Comminations will occur when S > F. The swollen network can be broken in order to cause the linkage or the bonds to rupture. The degree of solubility is affected by the attractive and repulsive forces within and around the molecules in solution. The intermolecular forces which affect the solubility include columbic interactions, electrostatic forces, hydrogen bonding, London forces, chemical bonding, and metallic bonding. The dissolution process occur when solute contact with solvent, and when all the forces of attraction and repulsion reach a state of equilibrium (Lin, 1992). Based on the statistical mechanical principles, the internal energy of the substrates should be matched by the internal energy of the invading solvent. Interpreting this in another way, the selection of the solvent must be based on the concept of Hildebrand solubility parameters (Weinberg and Yen, 1981). In many matrix modification methods, a total dissolution caused by the solvents introduced has occurred. Solubility Parameters The solubility parameter principle provides a reasonable guideline and has been widely used as an indicator to determine the compatibility (i.e. miscibility) between the polymers and the solvents systems. This principle has been applied, for example, to the selection of a solvent for dissolving a particular polymer, to determine the specification 46 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. of an elastomer which will not swell (i.e. absorb liquid) in situations known to involve contact with certain liquids, and to predict of possible environmental crazing of a polymer during service. Polymer matrices can be penetrated and swollen by the solvent molecules. Both concentration gradient controlled diffusion and relaxation controlled swelling contribute to the rate and extent of the penetration sorption. Solvent sorption can induce different levels of local swelling depending on the amount of penetration and on the intensity of the osmotic tensions generated by the solvent-polymer pair. The physical phenomena that simultaneously occur are dissolution, diffusion, swelling, and relaxation, together with deformation and stress buildup in the matrix (Apicella et al., 1992). Interaction between the polymer and the solvent systems can be described by the solubility principle “like dissolves like”. In other words, the Hildebrand solubility parameters of the polymer should match with those of the solvent to reach the maximum expansion or dissolution. Hildebrand parameter ranges for certain polymers may be determined by observing their dissolution behaviors, degree of swelling, or other polymer properties in a “spectrum” of solvents with known Hildebrand parameters. Based on test method employed in the ASTM (D3132-72), the polymer solubility parameter ranges for the polymer can also be determined by adding the selected solvent to a 1 or 2 g sample of solid polymer in a test tube so that the contents of the solids in the final solution are appropriate for the expected use. The mixture may be warmed and stirred to 47 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. increase the rate of dissolution, but it is cooled and observed at room temperature, the polymer being judged insoluble if there are gel particles or cloudiness present. By successive choices, upper and lower pairs of the two adjacent solvents are found, one of which dissolves the polymer and one of which does not, thus defining the solubility range in each hydrogen bonding class. The results can be presented by the multiple regression method. Closely related to the solubility of the polymer in a solvent is the corresponding solubility of the solvent. A method which may be applied to polymers that are crosslinking (or those crosslinked before testing through irradiation) assumes that the degree of interaction with a solvent and the swelling of a polymer reach its maximum when the cohesion parameters match. A slightly crosslinked polymer is exposed to a series of solvents in the form of either liquids or vapors. The polymer is swollen (but does not dissolve because of the crosslinks) and the extent of the swelling can be plotted against the Hildebrand parameters of the solvents. Figure 3.1 shows an example of swelling with unspecific volume of solvent imbibed as a function of solvent 8 values (Barton, 1983). Detailed analyses and kinetics will be further discussed in Chapter 5. 3.2.2 Fragmentation Polymers can be penetrated and swollen by small molecules. Both concentration gradient controlled diffusion and relaxation controlled swelling contribute to the rate 48 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 5/MPa “ Figure 3.1 Swelling (jQ) of natural rubber as a function of solvent Hildebrand parameter (*8): a, pure gum; b, tire tread, (after Mark, H. F. et al, 1950) 49 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. and extent of pentrant sorption. The physical phenomena that simultaneously occur are dissolution, diffusion, swelling, and relaxation, together with deformation and stress buildup in the matrix. A wide variety of effects can be therefore exhibited depending on the polymer-pentrant system. The initially surface localized swelling tensions may promote, according to the mechanical craze and yielding resistance of the polymer. Under the influence of penetrant, it can be observed that polymer matrix may easily fracture in a fragile manner rather exhibiting elasticity and normal ductile M ure (environmental stress cracking). The basic phenomena responsible for this embritlement are the osmotic tensions generated by the penetrated sorption reduce the strength capability of the material. The most obvious effects of a solvent or a wetting agent on the mechanical properties of a polymer is its deterioration during use. Such phenomena are generally described as the environmental stress cracking, the failure of the material by breaking when exposed to mechanical stresses in the medium; or environmental crazing, the failure of the material by the development of a multitude of fine cracks when exposed to the medium, even as a result of internal stresses in the absence of applied mechanical stress. These effects are particularly significant for glassy polymers, but less so for semicrystalline polymers. They are also closely related to the dissolution and swelling of the polymer caused by the solvent as described above. 50 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. As one paramount consideration for all degradation methods is the size of the samples, in the case of rubbery matrixes, size reduction is usually achieved by cutting samples as finely as possible with sharp scissors, or by putting the sample through a rubber mill. This mechanical fragmentation method, however, cannot be used for the propellant samples used in this study due to their explosive nature. For propellant samples, the optimum size has to be achieved by solvent relaxation. After swelling, the samples become very fragile and the entire structure loosened, the propellant samples have been fragmented and their sized reduced significantly. To create the finest possible sample sizes is crucial for this research, it serves two purposes: First, to increase the recovery rate of the solids which are physically trapped in the matrix; and secondly, to increase the contact surface for further degradation. 3.3 Ultrasound Irradiation The study on the reactions initiated by ultrasonic energy began around 50 years ago in this country. Since then, the mechanism of acoustic-chemical reactions remains poorly understood, thus limiting their practical applications. Yet, in recent years, some considerable advances have been made in the technological generation of newer, higher intensity ultrasonic generators. Advances in ultrasound technology are also becoming more successful in enhancing chemical processing operations by increasing the rates of individual transport phenomena involved in the overall operation. 51 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Ultrasonic energy may activate various mechanisms to promote the degradation of the macro molecular polymers. Schmid and Rommel (1939) found an irreversible and permanent reduction in the viscosity of solutions of polystyrene, polyacrylates, and nitrocellulose after exposure to ultrasounds. Chandra and co-workers (1966) reached similar conclusions in their investigation on the degradation of natural rubber in petroleum ether, cyclohexane and toluene. These early works noted that the initial decrease in viscosity was quite rapid but it slowed down with time and reached a limited value below which no further reduction occurred. As in Figure 3.2 (Price, 1990), two effects have generally been accepted as the main characteristics of ultrasonic degradation, namely, the existence of a limited degree of polymerization and the more rapid degradation rate for higher molecular weights. The applications of ultrasound are those which produce changes through the wave propagation. Ultrasonic energy activates various mechanisms to promote the changes. The three major ultrasound mechanisms are: the hydrodynamic shear forces created by acoustic microstreaming sufficient to cleave very large macromolecules; the acoustic streaming with its associated shearing forces to enhance solute dispersion in solution, and the generation of bubbles by cavitation which are instantly collapsed with local high temperature and high pressure to form free radicals for polymerization or depolymerization in solution (Suslick, 1988). In terms of the theoretical understanding of how these mechanisms work, there have been several theories developed. In the early stage of development, it was 52 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. INITIAL MOLECULAR WEIGHT 9 6 0 0 0 0 2 6 0 0 0 0 1 2 7 0 0 0 Sonication time (m in) Figure 3.2 Effect of sonication on the molecular weight of polystyrene in toluene, (after Price G. J., 1990) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. suggested that chain breakage is caused by friction forces raised between the fast moving solvent molecules and the slower moving polymer molecules. Later, Weissler (1950) investigated the effects of the cavitation on the ultrasonic degradation of polystyrene in toluene, proving that only under the presence of a cavitational environment can degradation occur, which does not happen in pure toluene or a degassed solution. The cavitation generated under the ultrasound irradiation has since then been recognized as an indispensable factor for the ultrasonic degradation of macromolecules (Mason, 1990). 3.3.1 Cavitation The simplest example for cavitation and perhaps the easiest to observe was the formation of bubbles in liquids supersaturated with gas. It is quit familiar to anyone who has ever opened or poured a carbonated beverage. Generally, cavitation was referred to as the formation and the subsequent dynamic cycle of bubbles in liquids including water, organic solvent, biological fluids, liquid helium, and molten metals, as well as many other fluids. It may be oriented by hydrodynamics, thermal and acoustic effects. The emphasis for this research is on the acoustic cavitation caused by the ultrasonic radiation. Cavitation is produced by the alternating patterns of compression and rarefaction with high and low pressure points - generated during sound waves in half cycles. As the liquid is stretched beyond its strength during rarefaction, these cavities 54 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. flow from microscopic nuclei. During the subsequent compression phase, they implode violently. This phenomenon occurs at a rate proportional to the applied ultrasound frequency, which can range from the ultrasonic threshold up to a practical limit of about 100 kHz. Though cavitation vacuities are extremely small and release only minute amount of energy individually, the cumulative effects of millions of implosions per second can be intense (Mason and Lorimer, 1988). Figure 3.3 clearly demonstrated that the degradation have been due to the effects of cavitation (Weissler, 1950). For the most widely used commercial instruments, the 20 kHz ultrasonic units, the period per cycle is 5.0x10 * 5 second. The collision of water molecules happens in less than one quarter of the cycle. Thus, the collision time should have a magnitude of about 1.250x1 O '5 second. The high temperature and pressure are the results of this collision. (Lin, 1992) Acoustic cavitation refer to the formation and activity of small gas or vapor- filled cavities (bubbles) in liquids exposed to ultrasound. By the formation, growth and collapse of microscopic gas bubbles, the low-energy density of a sound field is converted into the high-energy density characteristic of the interior and surroundings of a collapsing bubble. There are several factors that affect the cavitation, as in Table 4.1 (Okuyama and Hirose, 1963). By varying these factors, the degradation rate of polymers can be either enhanced or retarded. Results from several investigations regarding the effects of these factors have provided a solid foundation for the ultrasonic degradation for various polymers. 55 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. V isco sity (sec) 0 60 120 180 Time (min) Figure 3.3 Effect of cavitation on ultrasound degradation of polystyrene in toluene. viscosity of solution, 8, given by flow time in the viscometer is plotted against irradiation time, t. (1), pure toluene; (2), solution with air present; (3), degassed solution, (after Weissler, A. J., 1950) 56 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 3.1 Factors that affect the formation of cavitation. Factor Favor condition Physical properties of the solvent High vapor pressure Reaction temperature Lower Irradiation frequency Depend on the polymer The presence of dissolved gases Saturated with monoatomic or diatomic gas Cleanliness of the reaction system Higher Reaction overpressure Higher Irradiation power Higher (after Okuyama and Hirose, 1963) 57 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. There are two types of cavitational bubbles, stable and transient. Stable bubbles oscillate about an equilibrium size for a relatively long period of time, often for many acoustic cycles. The oscillation causes great disruption and movement of the adjacent liquid molecules which are responsible for many mechanical effects associated with cavitation. Transient bubbles exist usually only for less than one acoustic cycle. They generally involve the exponential rise of the radius of the bubble to at least twice its original value followed by a very rapid collapse. Thus , there are several effects on surrounding molecules. These effects have been estimated to produce extremely high temperature and pressure, the effects of which can cause both mechanical process and chemical changes such as bond cleavage to form radical species. Both the mechanical main chain degradation of the macromolecules and the redox reactions on dissolved substances occur under these circumstances. The cleavage of a covalent bond in a macro molecular structure can occur in two ways (Price, 1990): R-R — > R~ + R' Heterolytical Cleavage [3.3] R-R~>2R- Mono lyrical Cleavage [3.4] Based on the studies of Henglein (1954 & 1955), the production of macromolecular radicals is perhaps the most common breakdown mechanism and has been observed for many carbon skeleton polymers. The high molecular weight polymer can be ruptured into two lower molecular weight radicals under ultrasound induced cavitational environment. Thus, the reaction of ions is insignificant and can be excluded from this discussion. 58 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Free Radical Reactions In a natural environment, rubber and rubber-like materials will undergo deterioration. Likewise, polyurethanes also deteriorate under some circumstances. This deterioration is subject to free radical oxidation. Free radicals may be defined as species having one or more unpaired electrons. Due to their insatiability, free radicals are quite active and tend to react with each other by forming chemical bonds and releasing energy, thus changing from a high energy state to a low energy state. Watson (1953) explained this process in four steps: R-R — > R- + R- : mechanical rupture into free radical [3.5] r. + r. „> r.r : radical recombination [3.6] r. + o 2 ~> R 02 - : radical termination [3.7] R 02- — > -->stable molecule : breakdown stabilization [3.8] Free radicals can be formed through thermolytic reactions, photolytic reactions, high energy radiation and redox reactions. According to the Rice mechanism, free radical oxidation occurs in the order of initiation, propagation and termination. To accelerate the free radical oxidation of the polymer matrix, ultrasonication is one of the most effective methods. Under ultrasound irradiation, free radicals can be generated rapidly and economically. The degradation of the macromolecular polymer occurs through the free radical reactions in the cavitation environment with instantaneously high pressure and high 59 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. temperature created by the ultrasound irradiation. The mechanism of polyurethane degradation under ultrasonication can be described as: R-R’ R- + R’- : mechanical rupture into free radical [3.9] R. + R’. „> R-R’ : radical recombination [3.10] r-r’ - > r- + r’- : small free radical generation [3.11] r. + r- — > Rr : radical termination [3.12] Rr — > — > continuous breakdown to a smaller molecule [4.13] The generation of free radicals in the polymer system investigated, may occur in several ways under ultrasonication. Generally, two free radical compounds (R* and R’ ) with lower molecular weight are generated by the bond cleavages of the macromolecules (RR’). The small free radicals may either come from the small polymer molecules or free radical initiators, such as hydrogen peroxide and benzoyl peroxide, by ultrasonic irradiation. In addition, when an polymer radical reacts with a hydroxyl radical, a termination reaction occurs and forms a new lower molecular weight polymer. Following the same pattern, the macromolecular polymers will be continuously degraded until a certain limit of the molecular weight is reached. Macromolecular polymer fragments can be depolymerized under ultrasound induced cavitation with the aid of free radical initiator in the organic solvent system. In fact, the hydroxyl free radical which is known as the most reactive free radical can be economically generated by adding water or hydrogen peroxide to the ultrasound irradiation process. The emulsion system is thus formed by mixing aqueous liquid into 60 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the polymer solution. Furthermore, emulsifying agents such as surfactants (surface- active agents) are usually used to stabilize the dispersion o f the two insoluble liquids of water and the polymer solution. 3.3.2 Emulsion Systems As indicated in several researches (Lian, 1993; Lin, 1992), the interfacial reaction between phases (emulsion system) may occur faster that that in the mono-phase. The added aqueous liquid will form numerous micelles, a discontinuous phase, which will tremendously increase the chance of contact. Experimentally, when a reaction occurs in the micelle cage, the collision probability is increased 2x10 fold (Bakalik and Thomas, 1977). An emulsion is usually a result of the dispersion of two mutually insoluble liquids. It usually contains emulsifying agents such as surfactants and surface-active agents that stabilize the dispersion. The dispersion fluid is called the internal or discontinuous phase, and the dispersing medium is called the external or continuous phase. The particle size of the emulsion determines the stability of the emulsion system and its appearance, and is a function of the nature of the phases, the quantity and the type of surfactant, and the processing techniques. The most thermodynamically stable energy configuration in an emulsified droplet is spherical. An effective selection of the type and the amount of emulsifying agent can provide sufficient thermodynamic energy 61 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. to maintain the dispersion of the particles in the internal phase. Normally, increasing the amount of surfactants in the emulsion will improve the hydrophilic-lipophilic balance and decrease the particle size o f the emulsion. The surfactants are amphipathic molecules that contain both distinct hydrophobic and hydrophilic regions. The polymer forms either the Hartley micelles or the reversed micelles from its interaction with the surfactants. During the rearrangement of the molecules in these micelles, functional groups are likely to orient toward the aqueous phase, which greatly increases the free radical reaction rate (Lian, 1993). A micelle is a small aggregation of surfactant molecules in the emulsion. These molecules orient toward the lipophilic end of the surfactants in the oil phase but they orient toward the hydrophilic end in the aqueous phase (Figure 3.4). The initial small amount of surfactants added to the bulk of pure water will stabilize and disperse. As more surfactants are added to the system, however, a dynamic equilibrium is formed between micelles and the solubilized surfactants. This equilibrium prevails until more surfactants are added to form micelles. Generally, we call this point critical micelle » concentration (CMC). A properly selected surfactant that produces the lowest critical micelle concentration tends to produce the smallest emulsion particles and the most stable emulsion as well. It has demonstrated that micelle is capable of trapping and transporting radicals, and vesicles are apt to orient the conformation of large molecules. Through 62 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 3 .4 Adsorption o f emulsion molecules a t a n interface, (after Wright a n d Merten, 1959) membrane-mimetic agents, the probability of collision is greatly increased, which in turn enhances the reaction rate. The ultrasonic energy is exerted in a microenvironment to the locations where it is needed, without heating up the entire volume of solution. Hence, this surface extraction process is very efficient for energy consumption during the degrading the macromolecules. 3.4 Biodegradation In recent years, microorganisms have been widely used in research on pollutants degradation since biodegradation is one of the most economical and thorough treatment process. Basically, degradation means loss of properties. In the case of a polymeric material, this loss of properties can occur due to changes in the assemblies of the macromolecules which form the material or due to the breaking of the macromolecules or both. In the situation when the material is degraded but not its molecules, the formed fragments are formed small enough for the purpose of degradation but their chemical compositions are still similar to that of the original material. A piece of disintegrated polymeric material or materials made of macromolecules dissolved in aqueous medium are examples of this kind of degradation. Another kind of degradation causes both the material and its macromolecules to break down. The breakdown of the macromolecules can be triggered either by a 64 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. chemical process, such as hydrolysis or oxidation, or by biological agents, such as microorganisms or released enzymes, or both (Vert et al, 1992). Use of microorganisms or released enzymes to degrade plastics and polymers has been intensively investigated in the past few years. In the research conducted by Seal (1988), the biodegradability of both natural and synthetic polymers, such as cellulose, starch, natural rubber, synthetic rubbers (polyisoprenes, polyisobutadiene, styrene-butadiene rubber, acrylonitrile-butadiene, polycholroprenes, polyurethanes, polysulphide rubbers, silicone rubbers), polyamides polyethylenes, polystyrene, and several carbon-carbon polymers, have been studied and have shown great potentials. Degradation by-products resulting from a simple chemical attack can be dispersed in the living environment without further interactions. It can also be inserted into the biological cycles to be turned to water and carbon dioxide or to be assimilated and thus contribute to the formation of the new living matter. The biodegradation techniques have been adopted by both government and private industries. The five fungi cultures used by the ASTM method (G21) for determining the resistance of synthetic polymeric material to biodegradation are Apergillus rtiger, Penicillium funiculosum, Chaetomium globosum, Gliocladium virens, and Aureobasidium pullulans. Klausmeier et al. (1961) have studied the effects of the biodegradation on a variety of polyurethanes such as diisocyanates and hydroxylated compounds including glycols, polyethers, plotesters. They have concluded that, when compared to other 65 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. polymer systems, their potential for biodegradation is in the order of polyurethanes > polyesters > polyethylenes. The traditional acclimation method is used to determine the efficiency of the various fungi in degrading polymers. It is done by inoculating the spore suspension on thin-surface polymer specimens which are then observed after incubation for a certain period of time. Research has shown that, through metabolic activities, the fungi species are more effective in degrading the synthetic materials than the bacteria species. 3.4.1 Fungal Degradation Fungi are generally regarded as strict obligate chemoheterotrophs. They are unable to photosynthesize, and therefore need energy-rich substrates to meet both their energy and biomass requirements. Fungi produce a wide variety of extracellular enzymes, mainly oxidase of hydrolases, and can utilize most naturally occurring organic substrates. Other carbon sources used by fungi include alcohol, hydrocarbons, glycerol and starch (Schoemaker, 1990). Darby and Kalpan (1968) also studied the fungal susceptibility of one hundred laboratory-synthesized polyurethanes prepared by reacting four types diisocyanates and 25 diols, with six fungal species. They found that all polyester polyurethanes are highly susceptible to fungal degradation, but polyether polyurethanes have the highest resistance level. 66 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Filip (1992) used two fungal species, Aspergilus niger and Cladosporitm herbarum, indigenous to soil and surface water in their study on deterioration of synthetic polymers. They found that both species were effective in deteriorating a polyether based polyurethane. The fungi obviously utilized isocyanate from the polymer molecules as a nutrient source. From the interpretation of the infrared and E R . spectra, they deduced the sequence for the microbial decomposition of polyurethane as: (1) decomposition of the remaining free isocyanate groups; (2) splitting of the urea and amide groups; (3) breaking off of the urethane groups; and (4) a cleavage of the rings of the isocyanuric acid units. Patheirana and Seal (1984) in a series of studies on the biodeterioration of polyurethane with enzymes produced by fungi had also shown very promising results. A single fungus culture may produce various enzymes to accommodate whatever nutrient available; therefore, these enzymes decide the biodegradability of fungal species on different polymers. They investigated the activities of three enzymes, protease, esterase and unease, produced by four fungal species under the effects of pH and media constituents and found that the unease works better in acidic conditions in most cases. The ability of white rot fungi to degrade an extremely diverse range of very persistent or toxic environmental pollutants is also of interest to many researchers. Similar to polyurethanes, lignin consists of a highly complex three-dimensional structure. White rot fungi (Phanerochaete chrysosporium) is capable of degrading 67 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. lignin effectively through nonspecific attacks; therefore, it is reasonable to believe that the polyurethanes can be degraded by this species. Phanerochaete chrysosporium appears to produce oxidizing agents such as superoxide anion(0 2 "), hydrogen peroxide (H2O2), hydroxyl radicals (-OH), and singled oxygen (O2) in response to low levels of key sources of carbon, nitrogen or sulfur nutrients. They break the bonds between subunits and bring about a gradual depolymerization of lignin (Atlas and Bartha, 1993). The selective acclimation from low molecular weight polyurethanes to high molecular weight polyurethanes by controlling the nutrient supply shall be the primary approach in studying the degradability for various fungal species. In addition, by controlling the physical conditions such as temperature and pH, or by adding chemicals to stimulate the production of enzymes, the growth conditions for fungi can be optimized to enhance their ability to degrade polyurethanes. 3.4.2 Reversed Micelle System It is known that all biodegradation reactions happen on the interfacial area between polymers and microorganisms. In most studies, the degradation of the polyurethane films or grains was very slow and could not satisfactorily degrade the polymers within a reasonable amount of time, though the degradation activity did occur. Dissolving or pulverizing the polymers in good solvents may increase the contact area for further degradation, but complying the solvent solution of polymers with the aqueous solution of microorganisms without retarding their degradability is difficult to achieve. 68 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. During the last decade, there has been a rapid advance in the understanding and application of biochemical processes in many industries. The application of biocatalysts ( whole cell, purified enzymes and cell organelles) in organic media is growing due to the minimal denaturation o f biocatalysts and the possibility o f water-insoluble substrates (Lee, 1990). In many chemical reactions the transformation of substrates to products is accelerated with a biocatalyst favoring it over other possible reactions. In recent years, interest in multi-phase processes, using two immiscible solvents, has been noted. Because there is no strong polar character in a water-miscible organic medium, this system is less suitable for application of high water-insoluble compounds. In addition, enzyme activity often loses its activity when the percentage of water-miscible organic medium exceeds 10 - 40% (Hilhorst, 1984). In a two-phase system composed of water and an organic medium, water contains enzymes or cells, and the organic medium contains hydrophobic substrates. Lilly (1982) has conduct experiment with biocatalysis in a variety of water- immisible organic solvent systems, and the enhancement effects have been observed. A reverse mice liar medium is a multi-phase system that has tiny water 'pools' formed by surfactants in a waster-immiscible organic phase (Figure 3.5). Enzymes and microbial cells can be included into reverse micelles and maintained with preserved activity, which micelles possessed functions as microreactors. The following characteristics of the multiphase system such as the reverse micelle has led to great interest: 69 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Oragnic phase containing polymer Aqueos phase containing enzyme or immobilized catalyst Figure 3.5 Revered micelle system in biodegradation, (after Lilly, 1982) o a. Substrates that are poorly soluble in water are easily facilitated in the system. b. Reaction equilibrium may be shifted. c. Substrate or product inhibition can be reduced because of the diffusion effects. Lee and Yen (1990) successfully used a reversed micelle system to remove sulfur from coal. The multiphase biocatalysis process which consists o f an organic medium, a surfactant and an aqueous phase that contains microorganism or its cell-free enzyme extract has been successful in removing sulfur from bitumen coal. Generally, the following parameters are important for the reverse micelle engineering process: a. W0 = [H2O] / [surfactant]; b. organic medium choice; c. surfactant type. In addition, other factors such ionic strength, temperature, pH and agitation may also affect the activity o f biocatalysts in micelles. The preliminary experiments and procedures conducted by this study in determining the feasibility o f these techniques for polymer matrix degradation will be described in next chapter. 71 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 4 STAGED PROCESS FOR COMPOSITE PROPELLANT 4.1 Introduction For several decades composite propellants have been extensively used for military purposes, including missiles, rockets, and some space applications. It is now quite known that advanced propellants not only have high explosive energy to propel large diameter motors, but also have high stability under combat conditions. However, when these highly explosive materials become obsolete, it is very difficult to find a proper method for their disposal. In the past, open buming/detonation was the only method for the disposal of composite propellants. Because of stringent environmental regulations instituted as a result of growing environmental concerns, this traditional approach is no longer available. In addition, detonation of obsolete propellants involves tremendous dangers as manifested in several accidents reported which all result in either deaths or serious injuries (Berstein and Levin, 1995). Since energetic ingredients are the essential components of solid propellants, it is expected that they will undergo unfavorable deterioration if stored for a long period of time. The deterioration is generally caused by the interactions of various ingredients. This will significantly affect the propellant stability. Therefore, transportation to landfills is obviously not the solution for the propellants disposal problems. 72 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. With no effective disposal method, large amount of this kind o f hazardous waste has been accumulated for years. Finding a safe and permanent method for the disposal of obsolete composite propellant is a critical issue for both government and private industries. This urgency has become even more pressing with the demilitarization of missiles and rockets as a result o f the end of the Cold War. The Resources Conservation and Recovery Act (RCRA) and EPA regulations have further prompted the study o f the recovery process for spent propellants. As the key components o f the polymer composite, the inorganic solids (energetic and metallic fuel) are trapped in the cross-linked network without chemical bonds. Once the polymer matrix network could be destroyed and removed, these valuable inorganic materials can be extracted for reuse. These hazardous propellant wastes will no longer pose a serious disposal problem but can be recycled or reused. As the main objective of this research is to develop a safe and permanent method for the disposal of polyurethane-based propellants, the techniques o f matrix modification, ultrasonic degradation and biodegradation techniques were utilized to (1) develop a feasible process to minimize the stock pile of obsolete propellants. (2) separate the useful solids from the composite propellants. (3) degrade the synthetic polymer binders in the waste stream. 73 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4.2 Experimental 4.2.1 Samples The formula for the samples used in this research was similar to the solid propellants used in the Titan ballistic missiles. But instead o f using such highly explosive propellants, inert samples were synthesized and used in experiments. In the inert propellants, the energetics, AP or HMX, were excluded and replaced with a combination o f salts (sodium chloride and ammonium sulfate) with similar densities. The composition o f the inert sample is shown in Table 4.1. The HTPB/EPDI based polyurethane matrix forms the main structure o f the propellant, and aziridine is a crosslinking agent which further entraps inorganic salts into the organic environment. In the formulation, the organic, inorganic, and metallic concentrations are 14.9%, 53.8%, and 31.3% respectively. To eliminate the interference of the fillings in the inert propellant during the investigation, a fully cured one-shot polyurethane gum stock and lower molecular weight polyurethane (LMWPU) were synthesized with 3% plasticizer, and a HTPB to IPDI ratio equal to 9:1 was used for the polymer degradation experiments. The polyol used was HTPB (R-45M, OHV=0.72), provided by Elf Atochem North American Inc. The isocyanate used was IPDI (trifunctional aliphatic) from Aldrich. Different degree of polymerization o f LMWPU were synthesized by terminating the reaction with ethyl alcohol at different length o f time: 5, 10, 20, 30 minutes. Subsequently these LMWPUs were washed with ethyl alcohol twice to remove the monomer. 74 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 4.1 Inert propellant composition Ingredient Percentage (W/W) Sodium chloride 35.1 Aluminum powder, MD-10 35.1 Ammonium sulfite 18.7 Hydroxyl terminated polybutadiene (HTPB) 14.4 and isophorone diisocyanate (IPDI) Organic modifiers 0.5 100.0 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4.2.2 Procedures The initial phase of this research was to conduct experiments for the separation o f the three major components o f propellants: inorganics, metallic fuel, organics. The techniques of matrix modification and ultrasound irradiation were examined. It is expected that the goals o f this research can be achieved by either one single method or a combined process. After the three components are successfully separated, the other equally important task is to prevent the waste stream from additional pollution. Therefore, the recalcitrant substance in the waste stream, mainly low molecular weight or partially degraded polyurethanes, needs to be further degraded before discharge. The efficiency of degradation losing enhanced ultrasonic degradation and biodegradation techniques were also performed. Matrix Modification Mechanical fragmentation such as cutting with sharp scissors or grinding mill, the traditional approach to the size reduction of polymer matrix composites (PMC) wastes, such as rubber tires and rubbery articles, is obviously unsuitable for propellants because of their highly explosive nature. In this research, matrix modification technique which can be described in two stages, that of solvent swelling and centrifugal/hydraulic fragmentation, was first utilized to breakdown the propellant sample. The introduction of organic solvent to a crosslinking composite can cause the crosslink network to expand, as well as significantly reduces its physical strength. The integrity of the polymer matrix after swelling will become very fragile due to embrittlement, 76 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. and the mass o f fully swollen sample can be easily ruptured into small fragments by mild shearing force. Based on statistical mechanical principles, the internal energy of substrates should be matched by the internal energy of the invading solvent for maximum swelling effects. Interpreting this in another way, the selection of the suitable solvent must be based on the concept o f solubility parameters. Therefore, to find a suitable for the polyurethane-based propellants, the swelling experiments were conduct with a wide range spectrum o f solvents based on the solubility parameter principles and ASTM guidelines. Propellant samples and polyurethanes gum stock, one gram respectively, were cut into 4x4x4 mm uniform size cubes and submerged in 20 ml of various solvents in 50 ml beakers, and change o f each sample volume was measured daily for one week. Since the size o f the sample is the usually most critical factor in any kind of process, the fragmentation process was initiated first. After being fully swollen in the appropriate solvent, the samples were expected to become fragile and the entire structure loosened. By forcing these fragile samples through multi-size screens or membranes with centrifugal or hydraulic pressure, the size of the swollen sample could be significantly reduced. Ultrasound Irradiation As noted, the rubber and rubbery-like materials including polyurethanes may undergo deteriorate under normal conditions due to free radical oxidation Free radicals may be defined as species having one or more unpaired electrons. Due to their instability, free 77 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. radicals are quite active and tend to react with each other by forming chemical bonds. Free radicals can be formed through thermo lytic reactions, photo lytic reactions, high energy reactions and redox reactions. The ultrasound irradiation process was performed immediately following the fragmentation process in order to further separate the metallic fuel from the polyurethane binders. First, the effect of duration was examined, the propellant solutions were subject to different duration of ultrasound irradiation, and the solubility o f propellant was measured. Later, the enhancement with the aid o f a free radical initiator which would provide an oxidation degradation environment was also conducted. From previous experiments, the optimal conditions of the ultrasonication in monophase or emulsion system with the aid o f different surfactants were used for overall efficiency. Through this process, the polyurethane fragments were further degraded and became soluble in the solvent, the insoluble metallic fuel was precipitated and collected at the bottom of reactor. Detail examination o f the polyurethane degradation under ultrasound irradiation was performed and presented in Chapter Five (5). The purpose for this experiment was to determine the appropriate parameters for scale-up preparation, and the limitation of ultrasonic degradation (Mi,m ). 78 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Biodegradation Recently microorganism have been widely used in researches on pollutant degradation, because biodegradation in most cases has been both economical and highly efficient. After the energetics and metallic fuel are separated and recovered, the partially degraded polymer needs to be further degraded into harmless substances or mineralized before being discharged. To complete the treatment of waste stream, biodegradation technique was used as a polishing stage (Yang et al., 1992). Two fungi groups, Aspergillus niger (ATCC # 9642) and Chaetomium globosum (ATCC #6205), both been reported effective in degrading polyurethane (Filip, 1992; Darby & Kaplan, 1968), were used for experiments. To acclimate both fungi species to polyurethane, just a small amount o f LMWPUs (5-minute polymerization) was spread on the inoculated agar plate initially. The fungi were expected to utilize the polyurethane as the only available and good carbon nutrient source for growth. Then the growing fungi would be transferred to another plate with a slightly higher molecular weight PU. After repeating inoculation several times, the ability o f fungi to degrade polyurethane should be observed. Enhanced biodegradation for PU through a multiphase media, containing an organic medium (solvent), a surfactant, and an aqueous phase containing fimgi or their cell-free enzyme extract, was attempted. Due to the same hydrophobic property o f both PU and solvent, the aqueous solution would form reverse micelles suspended inside the polymer solutioa The fungi or their enzyme then could degrade the PU fragments through the 79 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. interfacial contact Another powerful fungi specie, white rot fungi (Barr & Aust, 1994) , was used in this enhancement experiment. The crude extracellular enzymes (peroxidase) produced by white rot fungi (Phanerochaete chrysosporium) were contained in the reverse micelles, and reacted with LMWPU in the solvent through the interfacial contact. 4.2.3 Analytical Methods To analyze the recovery rate and the degradation results, several methods were employed. The followings are brief descriptions of the methods used: Dry Weight Measurement The three separated fractions, solvent soluble, water soluble, and residue, were measured by dry weight to determine the process efficiency. It was assumed that the polymer binders should be extracted by solvent, the inorganic energetics should be extracted by water, and the metallic fuel should be the only content in residue. The three separated fractions (Le. solvent soluble, water soluble and residue), were measured by dry weight to determine the process efficiency and mass balance control It was assumed that the polymer binders and organic additives were extracted by solvent, the inorganic energetics were extracted by water, and the residue was only metallic fuel 80 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. UV/Visible Spectrophotometry The processed propellant solutions were analyzed with a UV/Visible spectrophotometer. Based on previous studies (Allen et aL, 1958; Henglein, 1954 & 55), the ultrasonic degradation of polymer is mainly a result o f the cleavage o f the C-C bond of long chain polyol backbone rather than that of the urethane bonds. Therefore, other than through the detection of possible degraded products after the process, the degradation efficiency may be monitored by the existence o f urethane groups in the solution. Colorimetry The color indicator ( /wlimethylaminobezaldehyde), which can react with resin containing urethane groups to form yellowish color, was used to double confirm the ultrasonic degradation results. Since the 3-dimensional cross-link network structure was believed to be degraded into lower molecular weight oligomers, and the intensity of color developed in the solution should be proportional to the quantity of the urethanes groups. But only a qualitative determination can be made via this color test (Swann, 1958). Viscometry Molecular weight determination by viscometry is principally based on the feet that the viscosity of the polymer solution, q, is generally larger than that of the solvent, qs , and depends on the molecular weight of the polymer (assuming that concentration, temperature, and shear rate are kept constants). Between the intrinsic viscosity [q] of the linear polymer 81 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. solution and molecular weight o f the following semi-empirical relation, the Mark-Houwink- Sakurada (MHS) equation, which holds over a very large range of molecular weights: [t|] = K m NT [4.1] where "K" and "a" are empirical constants with characteristics related to both the polymer itself and the solvent at constant temperature, through this equation, the viscosity-average molecular weight, Mv , of a polydisperse polymer is obtained. Therefore, the intrinsic viscosity which is proportional to the molecular weight can be used to monitor the degree of degradation. (Philip, 1975) Gel Permeation Chromatography (GPC) This is a routine analysis for polymer molecular weight determination. It was recognized that polydisperse systems are separated in a GPC column according to the molecular size in solution rather than the molecular weight. It was observed that larger molecules have lower retention volumes in the column than smaller ones. It was also observed that the relationship between molecular size and retention volume is in most cases linear in a semilog plot. A generally accepted model is as follows: smaller molecules are able to diffuse in and out of the pores in the beads more readily than larger ones. This effectively increase the retention volume as molecular size decreases. Hence, a relationship between retention volume and molecular weight distribution exists. Through the selective pores in the 82 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. separation column, the molecular weight distribution (MWD) could be obtained to monitor the degradation. 43 Results and Discussions 4.3.1 Separation Efficiency Based on the results o f the swelling experiments (Table 4.2) with various solvents, toluene, THF, benzene and chloroform caused the greatest swelling effects on the polyurethane network. With effective solvents, the samples swelled to ten times their initial size within five (5) days, as indicated in Figure 4.1. Propellant sample showed better swelling result due to its lower degree o f polymerization than the fully cured gum stock. After that period of time, however, there was no notable volume change. But a propellant sample which had been submerged in chloroform for more than one month did yield a volume about twenty times o f the original. This illustrated that creep swelling would make significant difference in the long run. Swelling of a network will always exert strain on the whole system. Doubtlessly the binder-filler interaction will be decreased until the filler particles are rejected. Experiments conducted in this research will show the evidence of filler separation, which is the major objective of this study. The compatibility o f the solubility parameters of the polymer network and those o f the solvents should be matched. The solubility parameter relations will be further analyzed and discussed in Chapter five (5). Through detailed analysis, the ideal solvent which may be effective and environmentally acceptable should be delivered. 83 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 4.2 Results o f propellant swelling with various solvents and their solubility parameters* Solvent Hildebrand and Hansen parameter Result (8/M Pa,y2) 8d 8 p S o S t Heptane 15.3 0.0 0.0 15.3 . Octane 15.4 0.0 0.0 15.4 - Cyclohexane 16.5 3.1 0.0 16.8 - Ethyl acetate 13.4 8.6 8.9 18.2 ++ Toluene 16.4 8.0 1.6 18.3 ++ Tetrahydrofuran 13.3 11.0 6.7 18.5 ++ Benzene 16.1 8.6 4.1 18.7 ++ Chloroform 11.0 13.7 6.3 18.7 + Chlorobenzene 17.4 9.4 0.0 18.7 - Acetone 13.0 9.8 11.0 19.7 - Dioxane 16.3 10.1 7.0 20.7 - Pyridine 17.6 10.1 7.7 21.7 + Methanol 11.6 13.0 24.0 29.7 - (*: Barton, A. F. M., 1983 & 1990) 84 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Chloroform / T H F Toluene 0 1 2 3 4 5 6 Tim e (days) Figure 4.1 Swelling effect of effective solvents on propellant, initial 4x4x4 cube. 85 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Energetic Solids Examined under the microscopy, the swollen propellant specimen exhibit numerous pockets loosely containing energetic solids. Figure 4.2 obviously indicated that the energetic solids were no longer bonded by the polymer matrix which became very porous with numerous pockets. The polymer matrix became very fragile and easily broken by cleaving through these pockets. The fragile, swollen sample was then subjected to fragmentation by centrifugal/hydraulic means. After being forced through the fine wire screen at 500 rpm centrifuge, the sample was reduced to smaller than one (1) mm and the energetic solids were released due to the destruction of pockets (Figure 4.3). The energetic was water extracted and then weighted after had been dried up overnight in the vacuum oven. The results indicated that more than 90% o f the inorganic salt was obtained. Although, most of the targeted energetic solids trapped inside were separated, the aluminum powder (metallic fuel) was found to remain tightly combined with the polymer fragments due to smaller particle size and stronger adhesive force with polymer. Metallic Fuel The ultrasound irradiation was conducted immediately following the matrix modification process in order to further separate the metallic fuel from the polyurethane binder. The device used was a model VC-50 ultrasonic probe (Sonics & Materials) operating at 20 kHz 86 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.2 Matrix of composite propellant (50X): (a) original, (b) swollen. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.3 Fragmentation of swollen propellant with centrifugal force at 500 rpm. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. and 10 W output in a 50 ml beaker contains 20 ml chloroform with 0.2 gram propellant sample which had been fully swollen and fragmented (Figure 4.4). The propellant solutions were first subject to 10,20,30,40 and 50 min. ultrasound irradiation, the solubility were determined by filtering out the insoluble portions and being allowed to dry up overnight in the vacuum oven. Figure 4.5 shows that the solubility of propellant (polyurethane matrix) increased with the duration of sonication, the effect of ultrasound irradiation was proven capable of degradation the insoluble polymer in to soluble. Subsequently, the enhancement of uhrasonication with two most commonly used free radical initiators, benzoyl peroxide (solvent soluble) and hydrogen peroxide (water soluble), were examined respectively. In monophase system, each 20 ml propellant solution was added with a different amount o f benzoyl peroxide, and then sonicated for 20 minutes. The insoluble portion of the sample was determined after the process. The results as shown in Figure 4.6 indicated that the solubility of PU also increased with the amount of benzoyl peroxide added. For emulsion system, the selection of suitable surfactant was first performed. Three different types of surfactants (Table 4.3) were added in the polymer solutions with water. The results indicated that nonionic surfactant ( Tween 80 and 1-Nonanol) could form more suitable emulsion system than the cationic and anionic ones. Later, various amount of hydrogen peroxide was added then sonicated for 20 minutes, the results showed that the effect of emulsion system did better than the monophase system, however, the residue was 89 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Titanium Horn Teflon Collar with O-Rings Gas Inlet/Outlet • / • * /Cooling.*/ •Both •».//• #* r ii ^r^Gloss Cell .•r*ey, t ' ? ,\U A v V Sam ple / " Figure 4.4 Sonication reactor with commercial titanium horn 90 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 18 -- 16 - _ 1 4 " g 12-- g 1 0 '' W 8 -- M g « • ' * < - ■ Vi 2 -- /• ■ N S © 55 O N - < Blank PU Propellant 0 10 20 30 40 50 SONICATION TIME (min) Figure 4.5 The solubility increase of polymer matrix verse duration of ultrasound irradiatioa 91 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. R esidue (%) 45 Benzoyl peroxide (%, WT V ) Figure 4.6 The residue of propellant verse benzoyl peroxide concentration, under same sonication condition (40 W/cm2 , 20 minutes) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 4.3 Type and name o f selected surfactants. Type Name cationic • Dowex-2 • dodecyltrimetbylammonium bromide anionic • sodium lignosulfonate • 1 -octanesulfonate acid nonionic • Tween 80 • 1-Nonanol R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. always less than 35% due to the incomplete separation of two phases which had some metallic fuel trapped in the thin emulsion layer. Although hydrogen peroxide was the most effective oxidant and did show the effect, due to the difficult separation of emulsion, the measurement of separated three fractions had been various and inconclusive. However, the aid of nonionic surfactant did show enhancement on solubility which may due to the wetting effect on the polymer fragments (Figure 4.7). Therefore, only monophase system was used in the reaction system for overall efficiency determination. Up to this stage, most of the organic constituents were dissolved in the solvent, silver colored powder was observed precipitated at the bottom, as in Figure 4.8. The residue, as expected, contained more than 80% aluminum based on its chemical analysis. Overall Efficiency Based on the previous experiments, the solutions with polymer fragments were subject to 20 minutes o f ultrasound irradiation with the aid o f 2% benzoyl peroxide, and the soluble PU in the solvent were measured after the insoluble portions were filtered out and had been dried up overnight in a vacuum ovea The separation results after matrix modification and ultrasound irradiation at different runs (solvents) are shown as in Figure 4.9. Compared with the original formula, the separation efficiency of energetics was close to 90% (47% versus 53.8%), which was quite satisfactory. An average 45% of sample was collected as residue which contains majority of aluminum It is believed that both recovered solids may be reused after going 94 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. RESIDUE (%) 45 40 35 30 25 20 15 10 5 0 Figure 4.7 The residue o f propellant verse nonionic surfactant (1-NonanoI) concentration under same sonication condition (40 W/cm2 , 20 minutes) Surfactant concentration (%, V/V) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.8 The separation of metallic fuel and polymer matrix after uhrasonication. Degraded polymer was dissolved in the solvent leaving metallic fuel at the bottom. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. e ' e n O < L U U oi tu a. O 5 £ ■ organic ■ inorganic □ residue Original Formula Toluene Chloroform THF Figure 4.9 The separation results after matrix modification and ultrasound irradiation. (0.2 gram propellant in 20 ml solvent with 2% benzoyl peroxide after sonicated 20 minute sonication) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. through further simple purifying process. The separated solvent solution was further analyzed with GPC, the results indicated the molecular weight of the polymer after the treatment is less than 10,000. 4.3.2 Polymer Degradation To determine the capability of the ultrasound irradiation and biodegradation techniques, fully cured gum stock and different degree o f polymerization of polyurethane were used. It is expected that the polymer degradation was mainly achieved by ultrasonication, and then further completely degradation with fungal degradation. 4.3.2.1 Ultrasonic Degradation The ultrasonic degradation of polyurethane was investigated with the gum stock by the matrix modification and ultrasound irradiation processes. The results from UV spectra (Table 4.4) showed that the characteristic peak height of polyurethane (at about 243 nm) increased with the duration of sonication. This indicated that the integrity of the polyurethane network had been destroyed and had become soluble in the solvent. The color test results also confirmed this conclusion (Figure 4.10). Molecular Weight Change The molecular weight of the polymer segments dissolve in the solvent was determined with viscometry. The viscometry was based on the HMS equation (Equation 4.1), where " K m " 98 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 4.4 The UV absorption o f polyurethane solutions after different sonication durations Ultrasonication Duration (min.) Absorbency at 243 nm 0 0.31 10 0.44 20 0.50 30 0.56 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.10 Color test for polyurethane solutions after different duration of ultrasound irradiation R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. and "a" are empirical constants with characteristics related to both the polymer itself and the solvent. For polyurethane, the constant “a” is larger than 0.5, and most literature (Allen et aL, 1964; Mark & Sung, 1982) indicated a range from 0.7 to 0.8. As the intrinsic viscosity measurement is an indirect assessment for molecular weight determination, and needed to extrapolated from the plotting of the viscosity of polymer solution verse the different concentrations. The Osward viscometer was used in the experiment to measure the viscosity based on the relationship below: q=Bpt [4.2] where B is the apparatus constant, p is the density of the solution, t is the time for solution to pass two marks on the viscometer. By using pure water, which density is known as 0.9987 g/ml at 25° C, and the time (t) is measured as 31.11 sec, the B constant was determined as 0.0684 cp.cm3 /g.sec. The viscosity of solvent (chloroform), r|s , was calculated as 3.43 as t measured at 33.8 seconds. For determination of the constants in HMS equation, the prepolymer HTPB (R- 45M), which has known average molecular weight of4 000, was diluted with chloroform to different concentrations and measured the viscosity. Based on the By plotting the data which is ideal at 1.1 < r\r < 1.5 range, "Hsp "Hr [q] = lim = lim [4.3] C — * 0 q C — * 0 q where qr = q /q s relative viscosity qsp - qr -1 specific viscosity 101 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The intrinsic viscosity [rj] of R-45M was obtained to be 0.14. by extrapolated and intersected with the vertical axis (C = 0) as in the Figure 4.11. Plug in the intrinsic viscosity of prepolymer R-45M into the HMS equation, the value K = 3.36E-4 after calculation by assuming a = 0.75, therefore, the Equation. 4.1 will become: [q] = 3.36x 104 M0 '7 5 [4.4] The equation 4.4 was used to approximately calculate the molecular weight of polymer in the solution after different treatments throughout this study. The molecular weight change of lower polymerized PU which could completely dissolved in solvent to form homogeneous solution after different duration of ultrasound irradiation (Figure 4.8), which indicated the molecular weight of the polymer decrease with the duration of the ultrasound irradiation. It demonstrated that the polymer was degraded under the ultrasonication. 4.3.2.2 Fungal Degradation For the feasibility of using fungi to degrade the polymer binders remaining in the solution, the low molecular weight polyurethane (LMWPU) samples were incubated with two fungi species, Aspergillus niger and Chaetomium golbosum, for one week. UV spectra (Figure 4.12) clearly revealed a decrease of polyurethane peak and an increase of phenolic compound peak at 270 nm. The results indicated that these lower molecular weight polyurethane (LMWPU) in the solution were able to be further degraded into simple phenolic compounds by fungi species through this polishing stage. 102 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. V i s c o s i t y 0 . 1 6 0 .1 5 6 - 0 .1 5 4 0 .1 5 2 - 0 . 1 5 - 0 .1 4 6 - 0 .1 4 4 0 .1 4 2 - In rjJC 0. 14 0 .1 3 8 - 0 .1 3 6 - 0 .1 3 4 0 0 . 4 0.8 1.2 1.6 2 2 . 4 C. g / 1 0 0 c . c . Figure 4.11 Relationship between intrinsic viscosity ( tj) and average molecular weight (M) of prepolymer (R-45M) 103 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 1.6 1.4 1.0 >260 Figure 4.12 UV spectra of polyurethane solution; (a) before and (b) after fungal degradation, (after Yang, 1992) 104 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The colorimetry method was used to reconfirm the conclusion. The intensity of yellowish color of the polymer solution due to the reaction of urethane group were increased with the duration of ultrasound irradiation (Figure 4.13). The viscosity measurement of LMWPU samples for the molecular weight determination was operated at 25°C and chloroform was chosen as the extraction solvent. The results for LMWPU samples after fungal degradation are shown in Table 4.5. The decrease of intrinsic viscosity of LMWPU samples after fongi attack can indicate the degradation of PU into smaller and simple molecules with lower molecular weight. The molecular weight for the sample was calculated with Equation. 4.4, and the initial molecular weight of LMWPU was determined to be over 50,000, which is much high than the one of polymer after ultrasonic degradation. Therefore, the partially degraded polymer in the waste stream after the separation process (matrix modification and ultrasound irradiation) was able to be degraded into simple phenolic compound in the fungal degradation (polish stage). Reversed Micelle System Several reversed micelle systems consisting one liter organic phase (solvent and polymer), 2% aqueous phase (crude enzyme of Phanerochaete chrysosporium), and 0.7 g surfactant (Tween 80) was prepared and inoculated for one week. Each reaction system was terminated at different period (day), and the molecular weight of PU was measured by viscometry. The results, as shown in Figure 4.14, indicated that the LMWPU with initial 105 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.13 Color test for polyurethane solution: (a) before and (b) after fungal degradation, (after Yang, 1992) 1 0 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 4.5 LMWPU* viscosity measurement Treatment Intrinsic viscosity fo] Molecular weight' none 1.16 50,000 A. niger3 0.372 10,000 C. globosumb 0.461 15,000 a. in chloroform (after Yang, 1992) b. agar plate inoculation c. calculated with Eq. 4.4 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Molecular Veight (g/m ole) 7000 6000 5000 4000 3000 2000 1000 0 1 2 3 5 4 6 7 Time (days) Figure 4.14 Molecular weight change of polyurethane after biodegradation with white rot fungi enzyme through reversed micelle system, (after Chang, 1995) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. molecular weight of 8,000 was degraded down to less than 3,000. The degradation rate was obviously greater than the one on solid media. (Yen et al, 1995) 4.3.3 Biodegradation of Energetics In addition to the recalcitrant polymer binders, the other potential pollutant in the waste stream will be the residue of the energetic solids. As noted in Chapter Two, the modem generation o f energetics used for advanced propellant manufacturing are ammonia perchlorate (AP), tetranitrotetraazacyclooctane (HMX) and trinitrohexahydrotriazine (RDX). Although AP is the predominant energetic material and can be used as fertilizer for agricultural purposes, the HMX and RDX are more resistant to biodegradation and may cause potential impact when directly released into the environment. Most of the energetics contain nitro groups and can be utilized by organisms as energy sources. However, RDX and HMX containing organo-nitro groups can only be biodegraded through denitrification under anaerobic conditions. The physical and chemical structure of RDX and HMX are quite similar (Figure 2.3). Therefore, biodegradability test of HMX was conducted as part of the polishing stage. The microorganisms for this experiment were obtained from the Hyperian wastewater treatment plant. A set of reactors was prepared. Each reactor contained 0.05 grams of HMX and 100 ml of diluted sludge solution (Le., 500 ppm of HMX), and flushed with inert gas to remove the oxygen in the head space before sealing. One reactor each day of the experiment was opened and the solids portion was removed through the centrifuge 109 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. method. The solution was then analyzed with a Beckman Model 25 UV spectrophotometer. To prevent the possibility of absorption of suspended solids, a control (with dead culture) was also prepared as a background. As shown in Figure 4.15, the results indicated that the concentration of HMX decreased from 500 ppm to non-detectable level within two week (Yen et al, 1995). This clearly indicated that the biological wastewater treatment is capable of treating trace amounts of energetic solids in the waste stream after the proposed staged process. 4.4 Conclusions The preliminary results indicate that the techniques of matrix modification, ultrasonic degradation and biodegradation are, in different aspects, effective in handling polyurethane- based propellants in an environmentally safe and stable manner. Two major tasks were accomplished when these techniques were applied in consecutive stages. One is the successful separation of the two recyclable filler components (Le., energetics and metallic fuel) from polymer, by destroying the 3-dimensional polymer network; the other is the degradation of the potential pollutants, polymer binders and energetic solids. The matrix modification is able to relax the polymer network with effective solvent then fragment the three dimensional structure to accomplish the size reductioa As noted, the propellants are not suitable for the traditional grinding process, the matrix modification technique is not only safe but also save tremendous energy. 110 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4.15 UV spectra of HMX in diluted activated sludge solution: (a) initial, (b) after 25-day incubation, and (c) background, (after Kwon, 1995) 111 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The ultrasonic irradiation is effective in breaking the polymer fragments into lower molecular weight oligomers. Since the polymer matrix is the most difficult element to dispose among PMC wastes, the success of destroying the polymer network with the ultrasound irradiation is the major determinant. The separation of composite propellant through matrix modification and ultrasonic degradation is proven to be feasible. The 3-dimensional crosslinking network structure was degraded into lower molecular weight oligomers through the staged procedures. Valuable components (inorganic solids) were separated from the polyurethane binder. The results showed that more than 90% of the inorganic solids (energetic and metallic fuel) in the composite propellants has been separated for possible reuse. In other words, the quantity of waste has been reduced to less than 20%. Several other treatment processes have been proposed recently, among them, cryogenic washout and ammonia extraction under supercritical and near-supercritical conditions. In comparison, the staged process operating under normal temperature and atmosphere pressure is more economical and may possess fewer risks. The polishing treatment via biodegradation is apt to remove the pollutants in the waste stream. There are two major pollutants in the waste stream, polymer binders and energetic solids. The polymer binders, Le., polyurethane, in this research, were degraded into simple phenolic compounds which can then be handled by traditional waste water treatment plant. Meanwhile, the major energetic materials, such as HMX and RDX have also been completely degraded by activated sludge. After the ultrasonic degradation, the 112 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. major pollutant, the polymer binders in the waste stream could be further degraded into simple phenolic compounds by fungi activity to prevent further pollution. Clearly, the disposal problem of propellants can be resolved through a combination of size reduction, polymer matrix degradation, separation, and polishing treatment in consecutive stages. Therefore, a feasible staged process which integrated these techniques is proposed. The schematic representation of the staged process is shown in Figure 4.16. There are mainly three stages in the proposed process, the first stage is size reduction which is accomplished by matrix modification; the second stage is polymer matrix destruction and separation which are accomplished by ultrasound irradiation; the third is the polishing stage which is accomplished by biodegradation. In the initial stage, matrix modification and ultrasound irradiation techniques were used to separate the energetic solids and metallic fuel from the solid propellants. The propellant specimen was first comminuted in an appropriate swelling agent to relax the crosslinked matrix and then fragmented with centrifugal plus hydraulic forces. Most of the energetic solids were ejected and collected after this process. Subsequently an ultrasonic depolymerization with the aid of a free radical initiator was performed under the induced cavitation environment. The fully swollen propellant fragments was degraded via free radical oxidation within the locally high temperature and high pressure cavitation. Preliminary results indicated that the high molecule-weight polyurethane fragments had been broken down into soluble lower 113 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. propellant solvent I i energetics free radical initiator surfactant solvent water metallic fuel discharge solvent / water separation fragmentation swelling ultrasonication emulsion system biodegradation polyurethane reversed micelle system biological wastewater treatment Figure 4.16 Schematic diagram of the staged process for composite propellants disposal. 114 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. molecule-weight oligomers. The metallic fuel was then able to be separated from the partially degraded polyurethane binders. Finally, the biodegradation process with fungal species capable of degrading the low polymerized polyurethane was examined and demonstrated to serve as the polishing stage to prevent the synthetic polymers in the waste stream from generating more pollution. The polymers were partially degraded into simple phenolic compounds by the enzyme extraction of white rot fungi. The bulk solution will then be carried into a separation tank to break up the mixture of the solvent-water system. Since the volatility of the solvent is higher than that of the water, the separation can be achieved at the elevated temperature. The waste stream after this stage will then be discharged into the traditional sewer system. The waste stream will be treated in a conventional biological wastewater treatment plant to completely degrade the potential pollutants into harmless substances. Due to the extreme low degree of polymerization, the obsolete composite propellants are especially suitable for this staged process because the integrity of the polymer matrix will become very fragile after solvent relaxation. For most chemically tough PMC materials, the swollen network may not become as weak as that of the propellant, but their physical properties will deteriorate significantly after similar staged process. Therefore, the same treatment principles can be applied to other PMC materials. 115 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Unlike other disposal methods, the end products through this process, i.e., the separated filler materials, have significant potential for reuse. The staged process is a recycle process which will not only reduce the quantity of the tire wastes in stockpiles but will also reuse part or all of the separated materials. There will be no end use wastes that need to be disposed of. As the natural resources available to use is but limited and consumption of raw materials continues to grow, this staged process will provide a significant and economical solution for two of the most challenging solid wastes disposal problems. These PMCs need not become useless wastes. With proper and efficient treatment, they can be recycled into useful products. And since both the obsolete propellants and scrap tires are of such substantial quantities, it is worth to take a different point of view on them, and try to turn wastes into valuable resources. 11 6 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTERS DEPOLYMERIZATION OF POLYMER MATRIX 5.1 Introduction The main structure of all polymer matrix composites (PMCs) is the crosslinked network, the fillers are evenly dispersed to form macroscopic homogeneous materials. To separate the different phases constituents, it is essential to depolymerize the polymer matrix. In the staged process, this task is achieved through matrix modification and ultrasound irradiation subsequently. To obtain the most economical and effective conditions and for scale-up applications, it is necessary to further investigate these two depolymerization processes through kinetics studies. 5.2 Solubility Parameter Spectra The solubility parameter spectra of solvents with gradually increasing solubility parameter values have been commonly employed in polymer research to determine the solubility characteristics and the swelling (physical aging) extent of crosslinked polymer systems. The crosslinked polymers usually do not completely dissolve in the solvent systems. Instead, solvent is imbibed by the crosslinked polymer and causes it to swell. Materials with low degree of polymerization, such as composite propellants, can be swollen to a significant extent that the reduction of projected size can be easily achieved. The 117 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. solubility parameter of the solvent which causes maximum swelling is defined as the solubility parameter of the crosslinked material. There are strong attractive or cohesive forces existing between the molecules and considerable potential energy in the condensed phases (solids, liquids, or solutions) than in the vapor phase. The stabilizing or cohesive effect in condensed phases can be expressed in terms of the cohesive pressure which is dimensionally identical with the cohesive energy density (cohesive energy per unit volume). The basic concept of the solubility parameter comes from the assumption that there is a correlation between the cohesive energy density and mutual miscibility (Weinberg & Yen, 1980). The cohesive energy density, C, is defined as : C = -U/V [5.1] where U is the energy change for complete isothermal vaporization of the saturated liquid to the ideal gas state and V is the molar volume of the liquid. The solubility parameter, 8, is defined by Hildebrand & Scott (1950) as the positive square root of the molecular cohesive energy density (Barton, 1975): 8 = C1 / 2 = (-U/V)i/2 [5.2] The description of “solubility parameter” and Equation 5.2 suggest a close link between the phenomenon of “solubility” or “miscibility” and that of “cohesion” or “vaporization”. This can be appreciated by considering what happens in a mixing process: the “like” molecules of each component in a mixture are separated from one 118 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. another by what amounts to an infinite distance, comparable in some aspects to what happens in the vaporization process. The term “solubility parameter” is too restrictive for a quantity which may be used to correlate such a wide range of physical and chemical properties and the term “cohesive parameter” refers to the group of parameters of dimension (pressure)1 7 2 which include the Hildebrand parameter defined in Equation 5.2. The Hildebrand parameter has sometimes denoted the “total” cohesion parameter, 5t, because there is a variety of “partial” cohesion parameters, but the subscript “t” is usually omitted if this can be done without ambiguity. 5.2.1 Solvent Matching From the thermodynamic point of view, the Gibbs free energy of mixing of the system, AGm , can be expressed as Equation 5.3. When a material is dissolving in a solvent, the Gibbs free energy of the process must be negative. AGm = AHm - TASm [5.3] Since the entropy changes, ASm , is always positive, the heat of mixing, AHm , determines whether dissolution occurs or not. Only when the heat of mixing is small enough, the dissolution will take place. Although there is no exact formula for the molar heat of mixing, the Hildebrand-Scott equation represents the total heat of mixing fairly well. According to the Hildebrand-Scott theory (Mellan, 1968; Barton, 1983), the enthalpy of mixing is given by 119 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. A H m = Vm(5i - 8 2 ) 2 <|>1 « j >2 [5.4] where Vm is the molecular volume of the mixture, and ( j > t, <|»2 are the volume fractions and 8 1 , 8 2 are the solubility parameters of the solvent and solute respectively. Therefore, when 8 1 is close to 8 2 , the heat of mixing will be either small or zero and the free energy change will be negative. The above discussion holds for non-polar liquids. It has been proved to be true in practice that the same principle holds for polar liquids and solids (Barton, 1975 & 1983). However, the relationship produced is a simple one and is easy to use as a rough guideline. Hildebrand Parameter and Hydrogen Bonding In general, the solubility parameter is considered as total value (8t) only. Polymer solubility parameter ranges may be determined experimentally by observation of their dissolution behavior, degree of swelling, or other polymer properties in a spectrum of solvents with known Hildebrand parameters (Figure 5.1). Specific effects such as hydrogen bonding and charge transfer interactions can lead to negative AHm but are not taken into account by Equation 5.4. A separate qualitative judgment must be made to predict their effects upon miscibility. In feet, for the determination of the one-component solubility parameter (5,) of polymer or resin, the range of the solubility parameter are affected significantly by the hydrogen bonding capacity of the solvents. 120 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. C D 0, V) a z < cc C O U i □ X ° m — sj d 121 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .1 Solubility spectrum o f th e dried untreated coal. Ws i s th e sw ollen volum e a n d W0 i s th e initial volume, (after Yen, 1980) Burrell (1975) was the first to deal with hydrogen bonding (yc ) in solubility parameters. Solvents are divided into three classes according to their hydrogen bonding capability: 1 . poor- such as hydrocarbons and chlorinated hydrocarbons. 2 . moderate - such as ketones, esters, and ethers. 3. strong - such as alcohols, amines, acids, and amides. Although more complicated theories have been developed, this classification is used in this work as rough estimate of the hydrogen bonding effect. To be accordant, only solvents with the same level of hydrogen bonding capacity for the solvents, or only poor hydrogen bonding solvents are selected. As employed in the ASTM method, mixed solvents allow small 8 increments (typically 0.5 MPaI/2 ) to be made for polymer characterization. In this study, the ideal solvent mixture is expected to be derived from the data analysis and its solubility parameters are determined by the following equation (Mellan, 1968): 5m = 5 i< j> i + 8 2 < J >2 [5-5] where 8 m , 8 i, and 8 2 are the solubility of the mixture and the two solvents respectively, and (j)i and < J >2 are the volume fractions of the correspondent solvents. In the case of polymers with more than one type of functional group, and particularly for copolymers and polymer blends, more than one maximum may be observed in the Hildebrand parameter swelling spectrum. Some claimed that three or four distinct peaks can be identified, but the scatter resulting from the imprecision of using a single 122 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. cohesion parameter for a solvent undergoing many types of interaction indicates that in these circumstances this approach is of limited value. Three Dimensional Hansen Parameters Instead of retying on a simple swelling-Hildebrand parameter curve, more complicated theories have been developed. Hansen (1967) proposed an extension of the Hildebrand parameter method to polar and hydrogen bonding systems. It was assumed that dispersion (d), polar (p), and hydrogen (h) bonding parameters were valid simultaneously, related by Equation 5.7 with values of each component being empirically on the basis of many experimental observation. As mentioned earlier, molecular cohesive energy (-U) arises from several sources. Hansen and Skaarup (1967) have classified cohesive energy into three categories, based on the energy contribution: (1) -Ud for non-polar or dispersion interactions, (2) -Up for polar interactions, and (3) -ty, for hydrogen bonding or similar specific interactions. At the same time, the solubility parameter can be expressed in three different terms, according to these contributions. The three components proposed by Hansen are not arithmetically additives, but serve as vectors along orthogonal axes. The end point of the radius vector thus represents -U = (-Ud ) + (-Up) + (-Uh) ^ = 5d2 + 6 p 2 + S h2 [5.6] or [5.7] 123 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Hildebrand’s solubility parameter in the three-dimensional space, and this approach is particularly useful for the prediction of polymer swelling in solvent mixtures. Froehling (1976) calculated the components of the solubility parameter of a mixture of two solvents. The mixture, m, of solvent a and b, the resultant of the three sets of components due to dispersion, polar, and hydrogen bonding forces is the endpoint of the radius vector when they are combined by means of equations m 5x= a f 5 x + V 6 x x = d,p,h [5.8] The composition of the mixture to give the maximum swelling of a polymer, p, corresponds to a minimum distance between the coordinates of the polymer (point P in Figure 5.2) and of the mixture (point M) and this occurs at a mixture such that PM and AB are perpendicular. Since PM and AB are orthogonal, the scalar or dot product of the vectors represented by these lines vanish, so S (m 5x - p 8*)( b 5x - a 5x) = 0 [5.9] For the line AB, we have: Ole 3e me 8e me 8e Od“ Od O p “ O p Oh - Oh be ae be ae be ae O d -O d Op - Op Oh - Oh Combining expressions [5.10] and [5.11] yields a 5d L ( % - % ) ( % - % ) m 8d = a8d + b 8d --------------------------------- [5.11] I ( b 8x-a S x )2 124 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5.2 Spatial representation in Hansen space of two-component solvent mixture - polymer interaction, (after Rigbi, 1978.) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. and substitution into Equation 5.1 gives I ( p 8*-a 8x ) ( b 8x- I ,8x ) = 1 - % = 1---------------------------------------------------- [5.12] l ( W The length of the line PM may be calculated readily from the coordinates m 8x and p 5x, this being an inverse measure of the interaction between the solvent mixture and the polymer. 5.2.2 Data Interpretation A variation on the procedure of swelling in pure liquids is the use of mixed solvents, with effective solubility parameters taken as the means of volume fraction of the individual liquid Hildebrand parameters. The maximum fells in the same range of Hildebrand parameter values as those estimated by other methods, but the effect of specific interactions can be significant, especial the hydrogen bonding. Figure 5.3 showed the effect of Hildebrand parameter (5t) and hydrogen bonding (yc ) index for polyurethane based propellant sample based on the swelling results in Chapter four. The effective solvent clearly formed an irregular area at 0 <yc < 7 and 15 < S t < 20. The more precise range could be set through solubility matching. There are two cases in matching the solubility of polymer with solvent mixture. In Case One (Figure 5.4), the mixture (M) was formed by one effective solvent (A) mixed with one (or more) inert solvent (nonsolvent, B). the fraction ratio (a < t > / % = AM/BM) may be justified within the solvent range. Based on the ASTM test method, the selected solvent (A) is added to a 1 or 2 g sample of solid polymer in a test tube such that the final solution 126 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. o 1 ) o o o o o o o o o o o o o o o o o oO -*> 1 1 I V < 5 n ) o < > ^ ' 0 0 ------ t - ----------- o < ► 1 > a o < < ) ) o o < ? o m m < N o C N <n ■ n m (J 8. 0 b < o. T 3 % 2 u § •e H 1 o o. U « s x> _3 O < n O X* W CN C O U c 3 a o o .5 -a c j8 § 0 0 s 1 I < D '"S' > c O ’ ■ " w O « - < u « « » ® i-f o . ® •a 5 S ® 2 -o x o * > E 2 s « cn < n < u o o a. 5 ' • co o E w 127 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. o o IT) CN o CN CO O 128 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .4 Solubility matching (case one) w ith a two-component mixture fo r polyurethane based propellant. ( o : poor; a : moderate; • : effective) has about the correct solids content for the expected use. By successive choices, the effective range between these two solvents could be found. In Case Two (Figure 5.5), theoretically the mixture (M) can also be formed with two or more mutually soluble nonsolvents (C and D). As long as it is within the solvent range, the mixture should be effective, and may reach the maximum swelling by successive trials. The three-dimension Hansen parameters are difficult to express, therefore, they were usually projected on three orthogonal plants ( 5d -8p , 8d -8h , and 8p-5h). In three individual projections (Figure 5.6 to Figure 5.8), only slight correlation could be observed from the scattered data but the 8p-8h plotting inFigure 5.8 which showed clearer trend. To enhance the dimensional accuracy, the “polar solubility parameter,” 8 „ is based on 8a2 = V + Sh2 [5.13] Therefore, Figure 5.9 was plotted with calculated 8 a verse 8 d , and the trend was more prominent than the three previous components. The irregular range could be approximately drawn, however, there is no sharp distinction between “solvent” and “nonsolvent” based on partial solubility parameters of many common solvents. Based on Equation 5.7, the range of solvent should be on the sphere with radius of total solubility parameter. Ideally, each solute, j, was characterized by a set of Hansen parameters (defining the center of the solubility sphere which could be used in Equation. 5.7 and interaction radius J R (Figure 5.10). For solubility to be predicted, J R has to be less than 129 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. in cs o < N t o m < p in m O 130 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .5 Solubility matching (case two) w ith a two-component mixture fo r polyurethane based propellant. ( o : poor; a : moderate; • : effective) CN CN O CN 00 c o o o o o o • < 1 CL CO © CN 00 *0 - CO V O 131 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .6 H ansen parameters 8 d - 8 P solubility m a p fo r polyurethane-based propellant. ( o : poor; a : moderate; • : effective) © CN o o| * o o oo < ► 0 - co 1 0 -C CO CN CN © CN 00 'O - C O < 0 132 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .7 H ansen parameters 8 < j - 8 h solubility m a p fo r polyurethane-based propellant. ( o : poor; a : moderate; • : effective) c c o 1 o o V o t 0 c o 8 ° < • O ▼ o o 8 o o •c < < • • o CN wo O CN VO C l CO w o ■ JC C O 133 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .8 H ansen parameters 8 P - 8 h solubility m a p fo r polyurethane-based propellant. ( o : poor; a : moderate; • : effective) in C N - 9 0 - O 0 O O o < N <n m c d CO C S n o (N 00 2 “ cO V O >n i 134 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5 .9 Hansen parameters 8 d - 8 a solubility m a p fo r polyurethane-based propellant. ( o : poor; a : moderate; • : effective) 6 d M Figure 5.10 Representation of a Hansen parameter solubility sphere with radius of interaction ‘ R and projections on three axial planes. (after Beerbower and Dickey, 1969) 135 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the distance i j R of the solvent at position ('8d , ‘ Sp, l8d ) from the center of the sphere of solubility (J 8 d , J 8 p , J 8 d ): i J R = [4 (‘ 8d- J 8d )2 + C 8p - j8p)2 + ('8h-J 8h)2]I /2 [5.14] The doubling of the 8d scale, leading to the factor “4” in the first term, was intended to make the “volume of solubility” approximately spherical, although this may not be necessary, as discussed in below. Similarly, the more precise area could be decided through matching the solubility of polymer in two ways. Ideally, the set of Hansen parameters of the polymer (the approximate center of the range) and the minimum value of effective radius J R could be determined. 5.23 Conclusions Several conclusions can be drawn from the above results: 1. From the swelling experiments, it is believed that for maximum swelling to occur, the solvent must interact with the polymer matrix in the same corresponding solubility parameters. The Hildebrand parameter and hydrogen bonding capability did provide a soluble range of polyurethane, but the projections of the three-component Hansen parameters were inconclusive. 2. Hydrogen bonding is an unsymmetrical interaction, involving a donor and an acceptor. Strong hydrogen bonding solvents appear to compete with specific sites in both . The solvents with poor hydrogen bonding capability interact mainly through physical forces. For the moderate hydrogen bonding solvents there may be both kinds of interaction. Since the 136 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. goal here is to swell the crosslinked polyurethane network, it appears that a solvent mixture of poor hydrogen bonding capability should be selected. 3. The solubility parameters of many polymers have been characterized to provide a useful data base. Through rapid computer fitting, the calculation of the most efficient mixtures of two or more component solvents for polymers and resins could be developed. Various combinations can be examined and tested experimentally to yield more economical and comprehensive results. 5.3 Ultrasonic Degradation The use of ultrasound on polymers in solution have been studied for quite some time. In the 1980s there had been a veritable explosion in the number of reactions of organic compounds take place in organic solvents under the influence of ultrasounds. The degradation of polymer molecules by the action of heat, light, ultrasonic radiation and various reagents has proven to be a subject of great practical importance and of equal theoretical interest (Mason & Lorimer, 1988). While many studies on polymer degradation have appeared, they have often encountered difficulties in characterizing the polymeric products. The mechanism of polymer degradation under ultrasound irradiation tends to be inconclusive as shown in numerous investigations done in the past (Price, 1990). The study of degradation kinetics for ultrasonic degradation is indeed challenging since the polymer undergoes a series of parallel reactions and becomes a 137 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. mixture of various chain length polymers. Thus, there are a number of rate constants that need to be determined for individual reactions, which is extremely difficult to accomplish, especially for crosslinked polymers. 5.3.1 Theoretical Background Most of the investigations for polymer degradation were focused on the molecular weight distribution of completely dissolvable macromolecules, which were mainly long linear chain or branched polymers without crosslinks (Figure 5.11). One of the first report on the effects of ultrasound degradation for several natural polymers was concluded by the reduction in solution viscosity. This result was then challenged by the works that showed viscosity was also reduced due to the thixotropic effects. However, the presence of bond breakage was firmly established by the works of polymer distribution in later researches. While the studies of ultrasonic degradation are many, there are no agreement to date on the theory to be applied to the process nor any overwhelming experimental evidence that would allow one to decide among the various theories or to describe a better one. Experimentally, it is observed that the rate of disappearance of a long chain polymer with a given degree of polymerization (DP) is first order as shown in Figure 5.12 (Smith & Temple, 1968). The reaction order is more complex, however, since the rate constant is a function of the gross polymer concentration and the chain length of the species undergoing degradation. 138 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5.11 Schematic representation (a) polymer molecules in dilute solution; (b) the stretching out of part of a polymer chain by solvent movement around collapsing cavitation bubble, (after Okuyama and Hirose, 1963) 139 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. o X z o H < < r u. H X O 3 0 3 0 1 0 TIME (MINUTES) Figure 5.12 First -order rate plots taken from gpc curves for ultrasonic degraded polystyrene; (curve) 10°DP, and 103 k (min'1 ), receptively: (a) 4.5, 14.3; (b) 5.0, 24.7; (c) 5.5, 32.6; (d) 6.0, 37.5; (e) 6.5, 45.4; (f) 7.0, 53.0; (g) 7.5, 64.0; (h) 8.0, 93.1; (I) 9.0, 97.4. (after Smith and Temple, 1968) 140 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Basedow and Ebert (1977) used both the concentration of each species and the overall molecular weight distribution versus the duration of sonication as a way of monitoring degradation. From a given molecular weight distribution, both the concentration of each species and the overall molecular weight change could be monitored with time. In the Jellinek and White (1951) model all bonds in molecules greater than D Piim are equally apt to break at random. Based on this assumption, the rate equations was derived as dxt/dt = 0 for Pt < P iim and [5.15] d^/dt = kn,(Pr 1) for Pt > P u n , where dx/dt is the rate of degradation, x is the number of chain breakages per unit volume, nt is the number of molecules with length Pt, and k is the rate constant which is assumed to be independent only on the system and experiment condition and not on Pt or n,. Ovenall et al. (1958) assumed that bonds with DP|jn/2 units from either end of the chain would not break, while an equal probability existed for ruptured along the center segment. Similarly, Gooberman and Lamb (1960) also determined the distribution of material for degraded polystyrene sample and concluded that rupture preferentially occurred near the center of the molecules. The kinetics for ultrasonic degradation of linear or branched polymers generally are widely studied. Based on several studies (Mostafa, 1958; Schmid, et al 1956), 141 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. initially narrow polydispersity fractions became broader after degradation before narrowing as M i,m at long sonication times, and the bond breakages had a ‘most probable’ distribution, although some studies also indicated non-random degradation at the center of the polymer backbone. Two of the most commonly accepted concepts are that (1) The rate of degradation decreased with decreasing molecular weight of polymer, and (2) There is a limiting molecular weight (Miim ) below which no further cleavage occurs. For network (crosslinked) polymers (Figure 5.13), the degradation mechanism are quite different from studies on linear or branched polymers. As noted, the crosslinked polymer matrices were not completely dissolved in the initial condition. The analysis of the molecular weight change might be more complicated due to the inconsistency of the bulk solution since the content of polymer might vary when the newly degraded polymer continuously dissolved into the solution throughout the process. Since the depolymerization is the reverse process of polymerization, it is necessary to understand how the polymeric structure formed. The network formation theories can be divided into two groups depending on the way they generated the molecules: (a) statistical theories in which branched and crosslinked structures are generated from monomer units or larger structural fragments occurring in different reaction states. The reaction state is characterized by the number and type of reacted functional groups and type of bonds by which they are bound to neighboring units; (b) 142 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 5.13 Schematic representation of the crosslinked polymer matrix. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. kinetic theories in which the branching process is described by an infinite set of kinetic differential equations by which other than chemical kinetic controls can be approximated. The reaction can occur between any pair of molecules by which a larger molecule is formed. The reaction rate is proportional to the product of the numbers of unreacted functional groups in the respective reaction partners. The difference between the statistical and kinetic methods is given by the fact that the statistical generation is always a Markovian distribution, e.g. in case of a bifimctional monomer the most probable or pseudo-most probable distributions. Kinetic generation is described by deterministic deferential equations. Although the individual addition steps can be Markovian, the resulting distribution can be non- Markovian. To preserve the simplicity of the statistical generation from units without violation of the corrections resulting from the possible non-Markovian character of the distribution, the fact has been employed that connections between groups of independent reactivity can be cut and reformed again at random. It means that the structure generated from original polyfunctional units by the kinetic method will be identical with that generated in the following three steps: ( 1 ) cutting the connections between groups of independent reactivity and labeling the points of cut. The distribution of units which results will be of lower functionality; (2 ) applying the kinetic method to this new distribution. What results is a distribution of branched fragments; these fragments still carry the labeled point of cut; and (3) recombining the labeled point of cut using the statistical methods (Dusek and MacKnight, 1988). 144 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The application of this method is visualized using the example of a tricomponents system composed of a telechelic polymer with end groups of independent reactivity, a trifimctional crosslinking agent and a bifunctional chain extender where the latter two compounds exhibit a substitution effect on the reactivity of groups (Figure 5.14). The fragments methods was recently applied to initialed crosslinking of a tetrafimctional monomer, to the treatment of postetherification in expoxy-amine curing, and to the additional crosslinking by side reactions in the formation of polyurethanes. The depolymerization may follow the same method but in inverted way. Wang et al. (1994) in their studies on the kinetics for degradation of coal, which is believed to be composed of crosslinked polyaromatic units, interpreted their results by a mathematical model based on the continuous kinetics for specific and random degradation processes (Figure 5.15). In random degradation, which generally occurs in long chain polymers degradation, the main chain (backbone) of polymer was broken down to form a smooth product MWDs that cover a wide range o f MW. In specific (erosive) degradation, the chain scissions are responsible for forming small molecular weight oligomers, such as monomers, dimers, and trimers. Both processes are important in degrading the large MW fragments in the polymer. 145 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. INITIAL SY STEM a 3CUO telechelic polymer O (independent re a c ti vity of a 1 a ♦ b -» a b o '. b - * a 'b b CL i0 b a ' a ’ O— O 1 s t STEP: cu ttin g a O ^ O o x a b b x a b a ’ a ' T » O----- O 2nd STEP ^ kinetic m ethod (build-up of c lu sters] b y X L -X b % / ' (distribution) b a ' 3rd STEP sta tistic a l m ethod (recombination of x ] X. ]_fxx] x x J ^ Y b a* Figure 5.14 Example of combination of the statistical and kinetic methods for network formation, (after Dusek, 1987) 146 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. specific degradation monomers specific degradation dimers Coal specific degradation trimers random degradation random degradation mixture Figure 5.15 The degradation mechanism of coal in continuous-mixture kinetic model, (after Wang, 1994) 147 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 5.3.2. Kinetic Investigation In this research, the aim of studying the kinetics of the degradation is to characterize the depolymerization reaction in terms of a rate constant, and the maximum degradation of polymers (Mum ) could be achieved through the sonication process, which might be used in scale-up practice and other applications. As indicated in Table 3.1, there are several factors may affect the rate of degradation in addition to the duration of irradiation. Generally, the power density and duration were the two major factors that need to be controlled in practice. Therefore, an optimal condition that is both economical and timely under ambient temperature and pressure may be derived from the kinetic study. Solubility of Polymer Matrices From the separation point of view, how to effectively remove the polymer from PMC materials is by for the most crucial issue. The kinetic study here is to examine the power intensity with or without the free radical initiator (benzoyl peroxide) affecting the solubility of the polyurethane matrix of the propellant. All the propellant sample solutions (one gram propellant with 50 ml solvent, 2 % W/V) first went through the matrix modification process, the size could be uniformly reduced to 1 mm after passing the wire screens, and most of the inorganic solids were removed. Then the buck solution was solicited under required conditions. To avoid incomplete separation of metallic fuel and polymer, only single phase reaction 148 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. systems were examined. The monophase system consisting only propellant sample in an effective solvent (THF) was irradiated with different duration of ultrasound with or without the aid of solvent soluble free radical initiator (benzoyl peroxide). Ultrasonic energy was provided by the Sonic & Material probe operating at 20 kHz and with a maximum output at 600 W. The effects of ultrasound irradiation was first investigated in terms of power intensity and the duration. Figures 5.16 and 5.17 showed the solubility increased with both power density and duration. The results indicated that the solubility (y) of polymer under ultrasound irradiation might be described as y = Y i (1 - e'kt) + y0 [5.17] where y0 is the initial solubility of the polymer matrix under the effect of solvent (Apukhtina, 1966); yi (= 1 - yo) is the insoluble portion; and k is the rate constant at specific power intensity (I). Therefore, yi initially has the value of zero then gradually increased under the effects of ultrasound irradiation. The data has indicated that yi fits the pseudo first order quit well. For best fitting, k was around 0.025 when I = 40 w/cm2 and was around 0.04 when I = 80 w/cm2 . These data were based on the 2 % W/V concentration. Jellinek (1959) had performed the kinetic studies for the ultrasonic degradation of polystyrene, and found that the rate constant was a linear function of power intensity as shown in Figure 5.18. A similar increase in k with increased ultrasonic power has 149 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 100 90 - 80 - 70 •- 60 -- ultrasound effect 50 -- J3 o C /3 30 -- solvent effect 20 -■ 10 -- 40 5 0 0 30 10 20 Time (min) Figure 5.16 Solubility of polyurethane matrix in propellant verse the sonication duration (power intensity = 40 W/cm2 , sample concentration is lg/50 ml) 150 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Solubility (%) 1 0 0 90 -- 80 - 70 -- 60 -- ultrasound effect 50 -■ 40 30 -- solvent effect 20 -- 10 -- 40 50 0 20 30 10 Time (min) Figure 5.17 Solubility of polyurethane matrix in propellant verse the sonication duration (power intensity = 80 W/cm2 , sample concentration is lg/50 ml) 151 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Ultrasound Intensity (Wcnf*) Figure 5.18 Effect of ultrasonic intensity on the degradation of polystyrene: ( 1 ) 4.89 W/cm2; (2) 9.58 W/cm2; (3) 12.5 W/cm2 ; (4) 15.8 W/cm2. (after Jellinek, 1959) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. also been found for a wide range of polymers (Shaw & Rodrigaez, 1967; Szorek, 1979). Although kinetics for this research can be very complicated which may be affected by numerous conditions, such as degree of polymerization, type of the solvent, reactor volume, degree of concentration, and tip area of ultrasonic horn, etc., it is safe to say that power intensity will increase the solubility of polymer matrices, which might follow pseudo fist order reaction. Since the ultrasonic degradation of polymers is due to the fiee radical oxidation reaction, the addition of fiee radical initiator should enhance the rate of degradatioa Accordingly, experiments with the aid of benzoyl peroxide (BZO) 2 were also conducted under the same sonication condition (40 W/cm2 , 20 minutes). Figure 5.19 showed that the solubility of polymer increased with the concentration of benzoyl peroxide in near linear relationship. However, huge dosage might not be economically effective. Smith and Temple (1968), in their studies of polymer degradation by benzoyl peroxide, also concluded that the rate constant increased with the concentration of benzoyl peroxide as shown in Table 5.1. The relation between the rate constant and the concentration was also near linear. Change of Molecular Weight The solubility of solid polymer fragments could reveal the effectiveness of the process in macro scale. Once the degraded products became soluble in the solution, a detailed 153 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 100 90 . ■ 80 ■ ^ 701 n P 1 b 60 > v ~ 50 IS 2 40 o ^ 30 20 10 • 0 0 1 2 3 4 5 Benzoyl peroxide concentration (% W/V) Figure 5.19 Solubility of polyurethane matrix in propellant verse the benzoyl peroxide concentration (power intensity = 40 W/cm2 , , duration is 20 min, sample concentration is lg/50 ml). 154 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. Table 5.1 Rates constants for benzoyl peroxide degradation on polystyrene [Peroxide] g/ 1 0 0 ml 1 0 3 k min 1 Remarks 1 . 0 0 0.057 toluene 2 . 0 2 0.16 benzene 2 . 0 1 0 . 1 2 benzene 4.00 0.28 benzene 4.00 0.27 benzene 6 . 0 0 0.39 benzene “ All samples at 1.0 g/100 ml. (after Smith & Temple, 1968) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. picture of what occurs during the ultrasonic degradation could be obtained through the instrumental analysis. The development o f gel permeation chromatography has provided the polymer chemist a valuable tool for arriving at rapid and accurate molecular weight distribution, and a deeper insight of the process. The MWDs of the polymer solution, based on the polystyrene molecular-weight standards, were determined by using a Water L. C. system including the following equipments: 1. Water Asso. Model 6000A Solvent Delivery System 2. Water Asso. Model 440 Absorbency Detector (254nm) 3. Hewlett Packer recorder Two u-styragel columns (100A, 7 mm ID x 30 cm length, = 3000 plates each) were connected in series. HPLC reagent-graded tetrahydrofuran (THF, Aldrich) was used as mobile phase. The injected sample volume was lOOul The wavelength of 254 nm was selected in the experiments. The samples were filtered across a 0.45 um millipore filter, and the flow rate was adjusted to 1 . 0 ml/min. Polystyrene standard containing five different molecular weight (range 10 6 to 104 ) was dissolved in THE for peak-position calibration as rough reference for the experiment. Figure 5.20 showed the relationship of elution time and molecule weight of polymer. For molecular weight, the limit of the column separation was as high as 1 0 ,0 0 0 , anything lower than this cannot be monitored. However, the molecular less than 50,000 was considered small enough for biodegradation. 156 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 9 -- 8 -- ^ 7 - ^ 6 - t 8 " M 4 - ^ 3 - 2 13 15 9 11 5 7 Time (min) Figure 5.20 Calibration curve of molecular weight (M.W.), based on standards of polystyrene. 157 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Coupling with the experiments for solubility, the processed solutions were analyzed in different treatment conditions. The molecular weight changes regarding each factor were monitored by gel permeation chromatography (GPC). Figure 5.21 showed no molecular weigh polymer higher than 1 0 , 0 0 0 was detected during the entire process as the solubility increased. The results indicated that the degradation mainly took place at the surface of the polymer matrix fragments through specific (erosive) scissions rather than through random scission, since the GPC showed the molecular weight distribution dropped rapidly to less than 1 0 , 0 0 0 and could not be separated. From the skeletal structure of the network (crosslinked) polymer, the above results might be reasonably explained. As shown in Figure 5.22, the ultrasound irradiation can only affect the surface of crosslinked polymers fragments, and could not reach beneath the surface since the fragments were not fixed but quit mobile, Le., moving and spinning. This might result in gradual size-reduction and degraded products with low molecular weight, such as monomers, dimers, and trimers, as concluded by Wang (1994). By viscometry method, the final products of the treated solution containing average molecular weigh was obtained. Table 5.2 summarized the results under various conditions. The degraded products might include single unit of urethane segment with broken chain of prepolymer on both end ( ), or two units ( ) , even three units( -•—•— ), but the probability of generation of over three units was very low. As shown in Table 5.3, the degraded polymers had molecular weight approximately around 8 , 0 0 0 which could be explained by a combination of the central breakage theory and Wang’s model. The UV 158 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. u r » CKi 30 min. ur» 1 0 min. 0 mim. Figure 5.21 GPC of propellant solution after different duration of ultrasonication. 159 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. i n a c c e s s i b l e c o r e Figure 5.22 Schematic representation of the polymer matrix fragments, (after Dusek, 1982) 160 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 5.2 The average molecular weight o f polyurethane matrix after ultrasonic degradation Sonication condition ---------------------------------------------- Molecular weight* of power intensity duration degraded oligmer mixture (W/cm2 )__________(min)_________________________________ 0 4,000 1 0 6,400 2 0 5,900 30 7,800 40 6,900 50 8,700 0 3,200 1 0 4,800 2 0 5,900 30 7,300 40 6,400 50 7,800 * measured by visometry. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 5.3 The average molecular weight of degraded polyurethane network based on central breakage theory and specific degradation assumption Degraded product Approximate Molecular weight single unit 4,000 two unit 8 , 0 0 0 three unit 1 2 , 0 0 0 average 8 , 0 0 0 Note: prepolymer (—) has average molecular weight around 4,000 and urethane bond (•) was neglected R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. spectra of polyurethane-based propellant treatment in Chapter Four has supported this conclusion, which indicated that C-C backbone cleavage rather than urethane bond was the major reason for the polyurethane network degradation. S 3 3 Conclusions Under the effects of ultrasound, the solubility of polyurethane network increased with the ultrasonic power intensity in pseudo first order relationship, and also increased with the concentration of benzoyl peroxide in near linear relationship. For polyurethane-based propellant (crosslinked polymer network), the depolymerization was accomplished through specific (erosive) degradation mechanism when irradiated with ultrasound. The polymers segments were gradually reduced to small molecular weight molecules which can then be dissolved in the solvent. 163 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 6 APPLICATIONS FOR SCRAP TIRES RECYCLING 6.1 The Scrap Tires Situation The largest problem area for managing the polymer matrix composite (PMC) wastes is the scrap tire disposal. Scrap tires have long been a major source of solid wastes. More than two billion used tires have already been stockpiled in the United States. And of the additional 285 million tires discarded each year, only about 100 million are reused or recycled, leaving the remaining 185 million tires for landfills, stockpiles, or illegal dumps across the country. (May, 1995) Such large quantify of wastes that are highly resistant to natural corrosion will continue to pose a potential threat to public health and the environment for as long as they exist. 6.1.1 Environmental Impacts Like other synthetic materials, rubber tires could take centuries to degrade naturally. Today’s radial tires last twice as long as bias-ply ones made twenty years ago since they are built for durability. And despite the fact that scrap tires constitute only slightly more than one percent by weight of the total municipal solid waste, they present a special disposal and recycling problem because of their size, shape, physical and chemical properties. 164 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Tires are difficult for landfill disposal. Due to their lighter density, whole tires tend to rise to the surface of a landfill, sometimes shooting upward with tremendous force. As a result, tires may penetrate the final cover following the closure of the landfill. As the landfill operators have raised the fees for accepting whole tires (or refused to accept at all), numerous large illegal stockpiles have developed. Significant environmental risks also come from improperly managed stockpiles on vacant lots, roadside, or riverbeds throughout the United States. These tires harbor stagnant water, and provide an excellent breeding grounds for disease-carrying mosquitoes and rats. Tire fires are another major hazard, since tires are highly flammable especially when stockpiled. It is extremely difficult to extinguish tire fires, because a large amount of heat will be generated during combustion, and virtually every tire in the pile has access to air. Open burning has been outlawed since uncontrolled combustion of tire piles generates smoke (carbonaceous particulate) and toxic air pollutants, including benzene and polynuclear aromatic hydrocarbons. The intense heat leads to generation of a pyrolytic oil that becomes mixed with the water used to fight the fire. The oil may then contaminate surrounding soil, surface waters, and ground water. (United State Congress, 1990) 6.1.2 Current Disposal Practices Like addressing any solid waste issue, any solution for scrap tires must examine their various sources. Almost 90% are returned by the consumer to thousands of dealers 165 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. across the country when wore out tires are replaced with new ones. Currently there are several alternatives dealing with the tire situation (Figure 6.1). Beside the fundamental source reduction, they include reuse and retreading, various recycling practices, use as fuel, and, lastly, landfill disposal. For example, Figure 6.2 illustrates the recovery and disposal of discard tires in California. (California Integrated Waste Management Board, 1992) The first most desirable method is source reduction since it aims to reduce the amount of waste generated at the source and every automobile owner can participate. Source reduction can be accomplished through better tire care, minimized driving through increase use of public transportation and carpooling, and reduced tire misuse such as improper inflation and excessive speeds. Although these techniques for source reduction are currently in practice, they are not being used to their full potential. The next best alternative is to reuse the tire. Reuse of tires postpones sales of new tires and lower waste tire generation rates. But whether used tires can be resold or reused often depends on the amount of legal treat remaining. Many of the used tires that could be reused are often retreated. Retreading is, of course, a cheap, low-energy way to put retired tires back to work. It decreases the number of waste tires that will require eventual disposal by reusing the tire casing. The casings are then used for miscellaneous products such as muffler hangers and snow blower paddles. Generally 30 to 50 percent less expensive than new ones, retreaded tires also consume less energy during the manufacturing process. Unfortunately, the number of retreaded tires sold on 166 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Landfill Stockpile Reuse, Retread & Exported Disposal Used Tires Waste Tires Figure 6 .1 Scrap tires use and disposal. (source: California Integrated Waste Management Board, 1992) Recycling Tire-Derived Fuel Mechanical Reclaim Devulcanization Alternatives to Disposal 7.4% Retread 3.9% Other 3.9% Reused 4.6% Exported 16.7% Energy Production 4.6% 58.9% Landfilled Stockpiled Figure 6.2 Discard tires in California 1992 (source: California Integrated Waste Management Board, 1992) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. a national level has declined due in part to inexpensive domestic and imported new tires. Moreover, this alternative, though an important primary method for reducing the number of waste tires generated and required disposal, plays only a small part of the annual take-off. (Lin, 1993; Serumgard & Eastman, 1995) Currently energy recovery appears the best solution for the tire dilemma. As fuel, each pound of tire provides about 15,000 Btu per pound in average, higher than the same weight of bituminous coal which produces about 12,000 Btu. In addition to the high heating value, tires can be less polluting since the have lower emissions of carbon dioxide, and they are often less expensive than coal. Cement industries, paper manufacturers, electric-utility power plants, and other industrial processes in the United States have been using tires as a supplemental fuel in coal-fired plants since the 1970s and the potential in this area appear large especially for combustion process facilities. (Kearny, 1990) A big advantage of using scrap tires in cement kilns is that there is no lingering solid waste disposal problems. The tire is completely consumed because the leftover tire ash, which is essentially unusable in other processes, becomes part of the cement and the iron oxide from the steel in tires improves the cements strength. There are, however, limits to burning tires as fuel. In the case of cement kilns, only a limited number of utilities in the United States have the types of boilers that are needed to bum tires. Another limit is that when tires exceed 30 percent of kiln fuel, their chemistry 169 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. adversely alters the cement’s curing process. (California Integrated Waste Management Board, 1992) Pyrolysis is also a very common thermal treatment process, which broke the organic chemical bonds of tire under the occurrence of combustion in the absence of oxygen. In this process, scrap tires are decomposed into three recoverable fractions such as low grade carbon black, gas, energy powering oil, and steel. However, the economical efficiency is still unclear and not widely used. Another option for long term resolution of tire disposal is to recycle them. Avoiding completely relay on limited techniques and recognizing tires as one of the most recyclable materials, many recycling technologies are continuously researched by both government agencies and private sectors. The major tire recycling programs are mechanical reclaim and devulcanization. Shredding and grinding the tires to their major components (i.e., rubber, steel wires, and threads) may accomplished by mechanical methods. The rubber can then be used to create artificial reefs, dock bumpers, planters, swings, and even energy-efficient house walls. Tires also can be sliced, chopped, shredded, chipped for various uses. Crumb rubber, for instance, can be combined with glue to make floor mats, bumper guards and other products. Rubberized asphalt, i.e., asphalt mixed with crumb rubber, has been used for flooring, surfacing, and paving roadways to increase durability (Smith, 1995). Devulcanization provides an ultimate solution way of reclaiming part of tire rubber’s virgin properties by breaking the sulfur bonds, the result is devulcanized 170 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. rubber which has regained some of its original characteristics and can be used to make competitive quality products as from virgin rubber (Okuda & Hatano, 1987). Further details of the tire rubber recycling technology are covered in section 6.3. Of the alternatives currently available for scrap tires disposal, landfilling has traditionally been the accepted method. But landfills are increasingly no longer accepting whole scrap tires for burial because, as described in section 6 . 1 . 1 , whole tires trap methane gas, breed mosquitoes, and bum uncontrollably when they catch fire. Whole tires also consume considerable amounts of landfill space since they are not easily compacted nor do they decompose. Ultimately, no single disposal practice in existence can handle the sheer volume of the stockpiled tires. A combination of the various tire recycling, reuse, and recovery technologies has to be examined to find solutions for the waste tire dilemma. While considering the scrap tires as one of the most recyclable products, the waste management industries are developing several methods to transform this abundant and otherwise unusable surplus into new products. 6.2 Properties of Tires It is important to understand the properties of tires and the process by which they are made and why it is difficult to reverse this process before we examine the processes that convert tires into recycled rubber. The tire represents the rubber article with the most complicated construction. Construction and materials are the outcome of years of 171 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. development and experience. In terms of vehicle applications, tires may be roughly described in three categories: light-duty passenger tires, heavy-duty truck tires, and specialty tires. Tires for passenger vehicles and light trucks typically weight from 12 to 30 pounds. Larger tires such as those for heavy-duty trucks and buses, farm and agriculture tractors, and industrial tires can weight from 35 pounds to several hundred pounds. Finally there are the specialty tires, ranging from aircraft tires to tires used on construction equipment and military vehicles. Regardless of their types, tires generally have the same components. Essentially, a tire is a cord and rubber composite. Tires have plies of reinforcing cords extending transversely from bead to bead, on top of which is a belt located below the tread. The belt cords have low extensibility and are made of steel and fabric depending on the tire application. Figure 6.3 illustrates the key components of a steel belted radial tire. (Mark et al., 1994) A variety of materials have been used in manufacturing tires although different tire manufacturer may apply different rubber compounding formula. They typically combine several types of natural and synthetic rubbers, carbon black, and various extenders and anti-oxidants. General formulation of passenger tire is shown in Table 6 .1. Usually 25-30 percent of the rubber used for modem radial tires is natural rubber, with the balance being synthetic rubbers. In general, the main structure of all tires is a composite of a three-dimensional crosslinked polymer network with carbon black and 172 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. TREAD BELT WEDGE PLY TURN UP CHAFER BEADS SIDEWALL NYLON OVERLAYS STEEL BELTS BODY PLIES APEX TOE-GUARD Figure 6.3 Cross section of a high-performance passenger tire, (after Mark J. E. et al., 1994) - j Table 6.1 General composition o f passenger tires Ingredient Weight (%) natural rubber 14 synthetic rubber 27 steel bead 1 0 carbon black 28 sulfur and zinc oxide 3 extended oil 1 0 synthetic fabric 4 antioxidant 4 Source: Goodyear Tire Company, 1996. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. reinforcing materials. The process of crosslinking polymer by sulfur or zinc oxide is called vulcanization. Tires cannot be made without vulcanization. Unvulcanized rubber is not very strong, does not maintain its shape and is very sticky. Vulcanization is the process of chemically producing network junctures by the insertion of crosslinks between polymer chains. Through vulcanization, sulfur is added to natural rubber to give it greater strength and resiliency. As a result, the rubber used in manufacturing tires is inert chemically and hard to break down. This is why wom-out tires are difficult to recycle. The vulcanization process is usually carried out by heating the rubber and mixing it with vulcanizing agents in a mold under pressure. In the curing or vulcanization presses, the compound flows into the mold shape to give a design to the tread and the desired thickness to the sidewall. Vulcanized rubber has a three dimensional chemical network which in infusible and up to now insoluble. The presence of this network makes rubber and thus tires very resistant to a variety of recycling treatment techniques. (Hofmann, 1989; Barlow, 1993) 6.3 Recycling Technologies In terms of the technical aspects of recycling vulcanized tire rubber, there have been numerous research studies published, some requiring devulcanization and some reusing the tire in its vulcanized form. As identified by Pett et al. (1995) new technologies (Table 6.2) that need to be explored to supplement those in existence include 175 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Tabic 6.2 Development o f new recycling technologies Recycling technologies devulcanization deploymerization multi-material coupling agents waste material as compounding ingredients rapid, easy material definition low cost recovery and separation of rubber R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. improvements in the technologies for breaking of crosslinks, i.e., devulcanization; breaking of backbone chains, i.e., depolymerization; and chemical modification. Currently there are several options for scrap tire rubber processing, they can be described in two categories: mechanical methods and devulcanization techniques. 6.3.1 Mechanical Methods Basic mechanical recycling process for scrap tires involves size reduction through either the ambient grinding or the cryogenic processing. Ambient Grinding vs Cryogenic Processing There are two general mechanical processing technologies available: ambient or cryogenic. The main difference lies in the temperature at which the two processes are performed. Ambient grinding is conducted at room temperature and produces rubber particles with a rough exterior surface. A tire shredder has two sets of counter-rotating steel shafts driven by either a diesel engine or electric motor. Both shafts possess cutting blades that are positioned opposing each other. After the whole tires are placed on the automatic conveyance system, they are carried and dropped into the cutting blades. While being pulled through the blades, the tires are shredded. Materials from a previous pass of the cutting blades will be sorted and routed back for further processing by classifiers which have screens that can pass or collect materials depending on the size. This continues until the material is reduced to the desired size. 177 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Through shredders, classifiers, and other facilities such as the magnets to remove metal pieces and mills, whole tires can be granulated into particles of rubber from one-eighth inch to about one-half inch in size while the non-rubber particles such as polyether fillers and steel are separated from the tire rubber. Cryogenic processing is performed at a temperature below the glass transition temperature. It uses liquid nitrogen to super cool the tire rubber prior to grinding. The cooled rubber becomes extremely brittle and easier to be ground (or cracked) into fine and ultrafine powder form. As a result, cryogenically produced ground rubber is typically smaller and finer than ambiently processed rubber. However, cryogenically produced ground rubber tends to be more expensive than ambiently produced ground rubber. In either process, steel and fabric can be separated from ground rubber by magnetic and gravity separators. Product Applications Scrap tires may be processed into various-sized particles. Size reduction greatly enhances the disposal options since shredded tires are typically cheaper to transport than whole tires. It also increases the options available for finding secondary uses for the tires. The shredding of a whole tire is usually accomplished by breaking down the tire into 2-inches or 1.5-inches long strips by a shredder. Shredded tires can be used as road base, carpet underlay, parking curbs, railroad crossing beds, and landfill cover. 178 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Crumb rubber, usually the end products of the mechanical processing methods, can be reused as a filler in a variety of products, including pneumatic tires, friction material-brake pads, molded rubber goods, bound rubber goods, athletic surfaces and playgrounds, rubber plastic compounds, tire/bias ply and retread stocks, tire interlinear, crack sealer/asphalt, asphalt rubber binder, etc. Unfortunately, the market for these vulcanized recyclates represents only a small part of the annual generation and is slow to expand. However, crumb rubber also can be further processed to reclaim part of rubber’s virgin properties by devulcanization, or it can be transformed into raw monomers by chemical reclaim. 6.3.2 Devulcanization As discussed in section 6.2, when the rubber for tires is produced, sulfur is added to the rubber through the process of vulcanization to strengthen it and ensure longer product life. Even after basic mechanical processing such as grinding and shredding, the potential use for crumb rubbers is limited by the fact that the rubbers after vulcanization become quite inert and can only be used as fillers for thermoplastic application, the market of which is, after all, relatively small. Therefore, to develop an effective technology to reverse the vulcanization process is the key to any research looking for economically and environmentally sound uses for the rubber used in tires, because the devulcanized rubber has high 179 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. substitutability for nature or synthetic rubber. The advantages of devulcanized rubber can be summarized as: (Far West Regional Technology Transfer Center, 1995) • A short breakdown and mix time • Low power consumption during breakdown and mixing • Controllable mixing, calendering and extrusion temperatures • Fast uniform calendering and extrusion • Improved building tack • Improved green strength and firming of uncured stocks • Reduced swelling and shrinkage during extrusion Devulcanization in a sulfur cured rubber can be defined as “the process of cleaving, totally or partially, the poly-, di- and monosulfide crosslinks which were formed during the initial vulcanization.” (ASTM D 1566) Devulcanization process generally falls into four categories: m microwave, ultrasound, chemical and biological method. Microwave and ultrasound use energy waves to excite and break the sulfur bonds. Chemical methods use chemical reactants to break the same sulfur bonds. Finally, biological methods employ sulfur-eating bacteria to break the links. The treatment of cleavage of crosslinks by microwave curing and biological methods is outside the scope of this research which focuses on a staged process combining the mechanical method of size reduction and devulcanization through ultrasound and chemical methods. An excellent overview of the various chemical methods for devulcanizing rubber is provided by Warner who in his 1995 paper reviewed an extensive list of the chemical probes that were researched and proven efficient. The efficient chemicals cited include triphenyl phosphine, thiol-amine reagent, dithiothreitol, lithium aluminum hydride, and so on. 180 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. Research on devulcanization of rubber with ultrasonic waves has long been discussed in technical literature, and several significant achievements were report recently. Okuda and Hatano successfully accomplished devulcanization by subjecting a natural rubber vulcanizate to ultrasonic waves, and then revulcanized it, which was found to have similar properties of the original vulcanizate. Further more, Isayev (1993) of University of Akron has been granted a U.S. patent for devulcanizing rubber with ultrasonics. 6.4 Experiments Based on the previous experience and reviewed papers, the devulcanization experiments were conducted with similar protocol developed for composite propellants. Matrix modification was still employed for sample size reduction. Subsequently, a combination of mechanical and ultrasonic devulcanization techniques was performed, though in the case of rubber, the depolymerization should causing by the cleavage the crosslink bonds associated with the three-dimensional network . 6.4.1 Objectives The goal of this research is to develop a economical process for recycling tire rubber based on the staged process developed for propellant disposal. Vulcanized rubber, having a three-dimensional structure as composite propellant, cannot flow in the 181 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. manufacturing processes, thus limiting the optimal reuse of tires in recycling programs. How to break this crosslinked network is the focus of this study. As our goal is to fully recycle the vulcanized elastomers in an environmentally sound manner in order to mitigate the used tires disposal problem, separation of the scrap tire into major original components with high quality, mainly the rubber and carbon black, for manufacturing new products is another key objective. 6.4.2 Procedures The samples used in the experiments were obtained from a scrap passenger tire made by Dunlop, and commercial crumb rubber of a variety of mesh sizes which were obtained from two scrap tires companies. The exact composition of these samples is hard to determine without first-hand knowledge of the exact tire properties and tread wear. However, the sidewall of the Dunlop tire is expected to closely resemble standard tire composition. The content of sulfur and carbon black in the sidewall specimens were determined, through chemical experimentation, with 2-4% sulfur and 30-35% carbon black which are very close to the general tire formulation described in Table 6.1. The experiments for recovery of these two components was compared to the results obtained from the sidewall as a matter of reference. 182 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Matrix Modification The sidewall portion of the scrap tire was cut into one centimeter squares with the original quarter-inch thickness. These tire squares and three different size crumb rubber samples (#120, #40, #10) were used for swelling experiments. The swelling experiments were performed with 2 grams samples submerged in a spectra of solvents based on the solubility principle and ASTM guideline, and the volume change was measured daily. The fully swollen samples were expected to be more fragile due to the embrittlement. However, the fragmentation process with grinding device was performed consequently to reduce the size as fine as possible. Chemical Devulcanization Devulcanization with lithium aluminum hydride, which is less toxic than other chemical reagents used, at elevated temperature was conduct with the fully swollen sidewall sample; Two gram well fragmented sample and 0.5 gram lithium aluminum hydride was refluxed with the effective solvent (at its boil point temperature), and the solubility change of sample was observed. The majority of the solid rubber sample was expected to become soluble within certain reaction time. Later, the enhancement with ultrasonic waves on the chemical devulcanization was examined, the reaction time for completely dissolution of rubber matrix should be 183 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. greatly reduced. The ultrasound device used is the US-600 model operating at 400 W output and 20 kHz. After the polymer matrix and organic constituents were dissolved, the reinforcing materials such as the steel and fabrics could be easily removed. But for determination of separation efficiency, organic portion and carbon black might serve as better index. Therefore, the weights of the two separated portions were measured after removing the solvents through overnight vacuum in order to determine the mass balance control and separation efficiency. The degree of devulcanization was determined by the sulfur contained in the solvents after the chemical and ultrasonic combined process. ASTM D3177-89 for determining sulfur content in Coal and Coke was modified and utilized for this purpose. 6.4.3 Results and Discussions The results can be examined from three aspects: the swollen efficiency of the solvents; the separation results; and the degree of devulcanization. Solvents Efficiency Due to the high degree of polymerization in tire rubber, the swollen efficiency for the tire rubber was as expected to lower than that for the propellants. Surprisingly, the rubber chip and crumb rubber behaved quite differently. 184 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. For rubber chip, the results o f swelling experiment were summarized as in Table 6.3. Although the volume changes vary with different solvents, overall the solvents which have Hildebrand solubility parameter around 19 MPa' l/2 are most efficient in processing the sidewall rubber of tires. The average volume of the specimen swollen after submerged in the effective solvents is around 4 to 5 times of the original volumes within first three days, then no noticeable change after that. For crumb rubber, much less significant results were exhibited under the same swelling condition. As shown in Figure 6.4, the volume of the crumb rubber increased no more than approximately two times no matter how long they stayed in the most effective solvent (chloroform), and the finer the particle size the less swelling effects. This suggests that chemical change has taken place during the crumbing process, possibly as a result of the introduction of heat and pressure which have changed the density and makeup of the rubber, the finer particle went through more grinding process and became more inert. This is clearly a support for this processes using matrix modification prior chemical devulcanization, the shredded tire strips would be the best source rather than the mechanical ground rubber, even the costly cryogenic crumb rubber. Since, though the swelling effect may not be considered significant, the physical properties, such as tensile strength and elongation, however, were greatly reduced. The samples also became very brittle and easily broken down to less than 2 mm by simple grinding device. The benefit of matrix modification not only saved the energy for size reduction, 185 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Table 6.3 Swelling effects of various solvents on passenger tire Solvent Solubility parameter* ( 6 / MPal/2 ) Volume change ( Q = V/V initial) hexane 14.9 1.5 dimethylamine 16.1 2 . 2 cyclehexane 16.8 2.5 xylene 18.1 3.0 toluene 18.2 3.0 tetrahydrofuran 18.5 3.3 chloroform 18.7 4.5 benzene 18.8 3.0 acetone 19.1 3.0 o-dichlorobenzene 20.5 3.0 methanol 29.7 1 . 1 * Reference: Barton A. F. M., 1983. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. o a S < D O) C (Q £ O < D E 3 O > 2.5 # 1 0 2 #40 1.5 1 #120 0.5 0 4 5 6 7 1 Time (day) Figure 6.4 Volume change of three different size crumb rubber in chloroform R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. but also prevented the rubber becoming inert resulting from the induction of high heat during the grinding process. Separation Efficiency The swollen tire fragments after matrix modification process and chemical reagent (lithium aluminum hydride) were mixed and then refluxed with o>dichlorobenzene (which has high boiling point at 179 °C). It took 72 hours of reaction, the solid state rubber matrix of the tire sample became soluble. When ultrasonic waves were applied for one hour, the reaction time for complete dissolution was greatly reduced to approximately one day. After the polymer matrix was dissolved in the solvents, the insoluble portions, steel beads and the fabric, were very easy to be removed and separated. What remains in the solution was a mixture including carbon black, polymer and extended oils, which need further separation. The carbon black is the target material for recycle. Since the size of the carbon black used in tires is extremely fine, regular filtration processes was not able to separation it from the solution. Unable to effectively separating carbon black from the bulk solution (250 ml) and assuming it is homogeneously dispersed, only portion of the solution (50 ml) was used to determine the amount of carbon black with the aid of centrifuge and Celite enhanced filtration. 188 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The use of centrifuge was proven unsuccessful for the carbon black separation even up to 800 rpm. But the Celite enhanced filtration had some effect. Celite was used to form a cake on top of the filter paper, which was then used to filter the carbon black. After repeated filtrations, clearer polymer solution was obtained. The result indicated the carbon black less than 25% of tire sample weight was filtered, which was quit close to the formulation (28%). Unfortunately, this technique was not suitable for large scale application, it would be the major area where future research is needed. The filtrate was examined with an IR spectrophotometer. Although the spectra of multipolymer mixture was difficult to identify (Figure 6.5), the characteristics of the pattern of the peaks in the natural rubber were observed. For the separation of rubber and oil, the problem was even more difficult. The rough concentration of polymer and oil was not able to be determined due too the unsaticifatory separation. However, after precipitating the polymer in the filtrate in methyl alcohol, the transparent natural rubber resins were observed at the bottom of the reactor when alcohol was added. Degree of Devulcanization It was believed that the devulcanization has been accomplished as the polymer matrix was dissolved in solution after the process. Therefore, after the insoluble (including steel beads and the fabrics) were removed, the solution was analyzed with ASTM (D3177-89) method to determine the sulfur content released from the sample through 189 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. o o 0 1 « o o o o 0 1 O o at z • < » « o s o n a < o o o T o o o o 3800.0 3000.0 - - I -------- aeoo. o 1. 800 . o W A V E N U M B ER 8 1400.0 (CM—1) l o g o . Figure 6.5 IR spectrum of multipolymer mixture of the devulcanized tire after the staged process. 190 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the process. The sulfur content in the solution was first converted into sulfate ion, then precipitate as barium sulfate form. Form the dry weight of barium sulfate obtained, the sulfur content could be calculated. The results indicated more than 90% (2.8% versus 3%) of the sulfur originally bonded with the rubber material was released in organic forms or inorganic complexes. Contrast to the depolymerization mechanism of composite propellant, the poly-, di- and monosulfide crosslinks which were formed during the initial vulcanization are weaker than the backbone bonds (Isayev, 1993). The devulcanization of rubber article in this process was believed mainly due to the cleavage of carbon-sulfur (C-S) and sulfur-sulfur (S-S), rather than carbon-carbon (C-C) bonds. Of the recycling technologies developed by this research, devulcanization has been quit effective and complete through the chemical and ultrasonic combined process. The resulting devulcanized rubber is typical a mixture of polymers which is consistent as the original polymers that are used in the virgin constitutes of the tire. 6.5 Conclusions California has more registered vehicles than any other state, thus has the largest inventory of used tires. It has been estimated about 33 million waste tires exist, posing a health and safety risk, and every year more than 28 million used tired will be discarded. Under the California Tire Recycling Act of 1990 (AB 1843), a Tire Recycling Program and the California Tire Recycling Management Fund are created. 191 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. The California Integrated Waste Management Board has been authorized to fund research and business development projects $ 1 million every fiscal year since 1992. Despite the claiming of success in using scrap tire as fuel supplement, it is doubtless the importance of the research in the recycling technologies, especially the devulcanization. Based on the results discussed in 6.4, the developed staged process, which is used as a minimization and recovery method to separate the various ingredients in the polymer matrix composites as discussed in the previous chapters, also appears to be a promising technology in delivering quality recycled rubber. From the previous results, the effective solvent was selected as the process medium, and the samples were then treated with the proposed process in Figure 6.6. The procedures in separating the various ingredients in tire rubber were the same as those used for propellants in Chapter 4. The developed staged process has succeeded in dissolving the sidewall of scrap tires and useful components such as the carbon black has been recovered. Rubber in the tire has also been extracted and the reinforcing materials separated. Since the carbon-sulfur (C-S) and sulfur-sulfur (S-S) bonds in the vulcanized rubber have been broken down through the ultrasonics, the treated rubber become pliable and can be reused in a manner similar to that of the uncured rubber used in tire manufacturing. The staged process developed in this research, therefore, may provide one more reuse and recycle option for waste tire rubber. 192 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. tire strip solvent I I chemical steel fabric solvent carbon black polymer residue ultrasonication carbon black separation polymer precipitation solvent removal matrix modification Figure 6.6 Schematic flowchart of staged process for scrap tire disposal 193 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. To make the staged process more realistic and feasible for commercial allocation, additional works are needed in the fixture: Carbon Black Recovery Carbon black is the major filler for rubber, and the most potential reusable material from the scrap, except the polymers. How to separate the carbon black from the mixed solution will be the major task next. The Celite enhanced filtration is not suitable for large scale application. The patented process of Firestone Co., (Figure 6.7), may be the solution and worth trying. Sulfur Removal Devulcanization is the ultimate form of recycling the scrap rubber, as the rubber is transformed back into crude oil-like monomers. During the reclaiming process, chemicals are added to tire crumb to break down the polymer in the rubber material into feed stock-grade monomers. The resulting monomers then become raw materials for making virgin synthetic rubber. This process provides an economical way of reclaiming part of the tire rubber virgin properties by breaking the sulfiur bonds that were introduced when virgin rubber was vulcanized into the tire manufacturing process. However, the sulfiir content need to be removed before the polymer mixture could be utilized in any application. As discussed, lots of methods can be utilized to 194 R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. STEAM CARBON DIOXIDE H2 0 SOLVENT (TOLUENE) CASEOUS H CL WATER SOLVENT N aO H SOLVENT (T H F ) SOLVENT N aC l FILTER FILTER AMINE FILTER S C R A P RUBBER AMINE HCl SALT REMOVE SO LV EN T AQUEOUS AMINE HCL SALT AQUEOUS AMINE PLUS NaCL STRIP OFF WATER STRIP TO REMOVE SOLVENT POLYETHER DIOL POLYETHER DIOL SOLUTION POLYETHER DIOL PLUS AMINE POLYURETHANE AND RUBBER SCRAP POLYETHER DIOL PLU S AMINE Figure 6.7 Firestone Tire & Rubber Co. process for recovery from polyurethane scrap. (Source: U.S. Patent 4,035,314) R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. precipitate the sulfur into inorganic salts, such as calcium sulfate, magnesium sulfate etc. These sulfate salts are also valuable material which may help reduction the capital of this process. Repolymerization One major barrier to the waste tires markets is the quality of the reclaimed rubber. In tire manufacturing, for example, the percentage of recycled rubber that can be “mixed- in” with virgin rubber to make passenger tires depends on the degree of devulcanization and purity of the reclaimed rubber. Very little, if any at all, reclaimed rubber can be used in today’s radial tires because reclaimed rubbers are often only partially devulcanized and thus their qualities are generally lower than those of virgin rubbers. Since the rubber reclaimed by this study is completely devulcanzied and virgin rubber properties such as elasticity have been recovered, the reclaimed rubber should be easier to blend with and be incorporated at substantial levels with other virgin materials for the manufacture of new products, including tires, than the long established reclaimed rubbers. 196 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 7 REMARKS AND FUTURE WORKS The objective of this research is to develop a feasible method to minimize the existing stock piles of obsolete composite propellant wastes. Because the composites contain more than one property constituent, single treatment technique is not effective in handling such complex materials. This research adopts the same principles used in many wastewater treatment plants which integrate several operations to accomplish the task. Therefore, a staged process which combine matrix modification, ultrasound irradiation, and biodegradation operations was examined and demonstrated to be effective in separating the composite propellants back to their major components ready for reuse or recycle. The same protocol also provides a feasible alternative disposal method for polymer matrix composites (PMC) which usually end up in landfills due to the lack of economical treatment techniques. In this research, scrape tires, one major source for composite wastes which has increasing environmental impact, were also able to be devulcanized and separated into their original ingredients. It has significant implications for solid waste management. This staged process provides a feasible treatment and recycling method for two of the most challenging solid wastes, namely the obsolete composite propellants and scrap tires. The following summarized the important points which are based on the work presented in this dissertation. 197 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 1. Due to the extreme low degree of polymerization, the obsolete composite propellants are especially suitable for this staged process because the integrity of the polymer matrix will become very fragile after solvent relaxation. For most PMC materials, the swollen network may not be as weak as that of the propellant but with significantly decreased physical properties after the staged process. Therefore, the same treatment principles are applicable. 2. Unlike other disposal methods, the end products (separated filler materials) from this process have great potential for recycling. After the separation process, the filler materials which are usually very inert (such as fiber and carbon black) and only physically trapped inside the three-dimensional polymer matrix may be put back to feed stock as raw materials to manufacture new products. 3. The polymer matrix providing the integrity of PMC material is mainly destroyed by the matrix modification and the ultrasound irradiation process consecutively. The efficiency depends on the degree of polymerization (DOP) of the polymer matrix. Lower DOP will lead to lower treatment cost since the matrix modification will reduce the physical property (strength) more significantly and save the energy needed in the ultrasound irradiation. For high DOP composites, such as the scrape tires, the matrix modification process still shows certain effects, but more powerful ultrasound output (or duration) is needed. 198 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4. For the ultrasonic degradation process, power density is increased in order to enhance the degradation results. Although initially degraded rapidly, the macromolecular polymers eventually reach certain limitation rather than mineralized completely. To be more economical, other enhancement techniques were also studied in this research. For example, the emulsion system which has been utilized in upgrading asphalt was also proven more effective than the mono-phase system. 5. Although synthetic polymers are usually recalcitrant, the high molecular weight polymers can be completely degraded into simple substance by using ultrasonic degradation and biodegradation consecutively. Both processes have their limitations. The ultrasonic process can degrade the polymers to certain limit of molecular weight, and the biodegradation process to certain high molecular weight. Therefore, after the ultrasonic degradation process when the molecular weights of the polymers become lower than the capability of biodegradation, complete degradation can be achieved. This is the key advantage of the staged process. There is much additional work to be done to provide wider applications to the work described in this dissertation. The followings are suggested for future work: Reuse of the Degraded Polymer During the staged process, the three-dimensional polymer network of the composite propellants was destroyed into low molecular weight oligomers first by the matrix 199 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. modification and ultrasound irradiation processes then further degraded into simple or nontoxic substance by the biodegradation process. Although the low molecular weight oligomers can theoretically be reused as raw materials in the feed stock to manufacture new polymeric products, further studies are needed. Several reports have shown potential in reusing the devulcanized polymer to make products with competitive high quality. Okuda & Hatano (1987) has a pattern (JP 62,121,741) which used devulcanized rubber to manufacture tires with similar performance as the new ones. Romine (1996) used partially desulfurized rubber to manufacture shoe sole which was proven more durable than those made from vulcanized crumb rubber combined with thermoplastic rubber. Environmentally Sound Solvent Mixture For composites made of more than one type of polymer, the solubility fragmentation technique is more effective in separating the polymers. Traditionally, different solubility characteristics have been used to segregate multi-type polymer mixture. Due to the properties of the composite materials, the polymer matrix is usually hard to process without solvent relaxation. Since the effectiveness of the solvent system varies with the polymer used in the composition, the concept of the solubility parameter provides the basic guideline for finding an ideal medium for the process. Because the solubility parameter of most polymers such as benzene, toluene, and chloroform are usually organic and toxic, an ideal solvent mixture that can process 200 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. the polymer but less toxic or nontoxic is needed and should be studied based on the solubility parameter principle in order to make it environmentally acceptable. Extensive Use of White Rot Fungi The biodegradation of plastic and rubber wastes has been extensively studied. Most of the researches use single phase medium such as soil and water. Through the enhancement via the reversed micelle system, the biodegradability of the hydrophobic pollutants may be greatly increased. Since the white rot fungi is so effective in degrading a wide range of recalcitrant pollutants, it stands to reason that the trace amount of the pollutants in the waste stream can be completely degraded through the reversed micelle system. 2 0 1 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. REFERENCES Aitken, M.D.; Venkatadri R.; and Irvine, L.R. 1989. Oxidation of phenolic pollutants by a lignin degrading enzyme from the white rot fungus Phcmerochaete chrysosporium. Water Resources 23(4):443-450. Allen, G.B.; Booth, C.; and Jones, M.M 1964. Polypropylene Oxide I: An Intrinsic Viscosity / Molecular weight Relationship. Polymer, 5:195-199. Allen, P.E.M.; Burnett, G.M.; Hastings, G.W.; Melville, H.W.; and Ovenall, D.W. 1958. 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Creator Shiu, Jer-Yuan (author)

Core Title Staged process for obsolete propellant disposal: polymer-matrix composite waste management

School Graduate School

Degree Doctor of Philosophy

Degree Program Environmental Engineering

Degree Conferral Date 1997-02

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag engineering, civil,engineering, environmental,OAI-PMH Harvest

Language English

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c17-276889

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