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Chemical depolymerization of amine-epoxy cured carbon fiber reinforced polymer composites and their re-use
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Chemical depolymerization of amine-epoxy cured carbon fiber reinforced polymer composites and their re-use
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Content
Chemical Depolymerization of Amine-Epoxy Cured Carbon Fiber Reinforced
Polymer Composites and their Re-Use
By
Carlos Navarro
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
in Partial Fulfilment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
December 2022
© Copyrights by Carlos Navarro, 2022. All rights Reserved
ii
Acknowledgments
A Ph.D. is not easy but it has been an incredibly rewarding experience thanks to the breadth
of unique people I have crossed paths with. First and foremost, I want to express my sincere thanks
and appreciation to my advisor, Travis Williams, who has been a remarkable mentor and friend
and expanded my horizons of what is possible. He and his wife, Sarah, have shown me kindness
from my first steps onto the USC campus and it is for that reason I joined this university and this
research group.
I would like to express my gratitude to my committee members: Professors Steve Nutt,
Jahan Dawlaty, Mike Inkpen, Megan Fieser, and Ralf Haiges for their support and feedback.
Thank you to all the Williams group members I had the opportunity to bump shoulders
with in lab: Van Do, Valery Cherepakhin, Ivan Deminates, Zhiyao Lu, Anju Nalikezhathu, Long
Zhang, Yuhao Chen, Justin Lim, Alexander Maertens, Adriane Tam, AJ Chavez, Lisa Kam,
Katelyn Michaels, and Cassondra Giffin. Time always flew by at work with such exciting and
interesting folks to share time with. An extra special thank you to Zehan Yu and Yijia Ma – the
best collaborators anyone could ask to work with.
A special thank you to the staff of the chemistry department at USC for all their immense
work: Michele Dea, Magnolia Benitez, Allan Kershaw, Frank Devlin, Michael Nonezyan, David
Hunter, Carole Phillips, Jessy May, Surya Prakash, and Robert Aniszfeld. Thank you as well to
the lovely USC Wrigley Institute for Environmental Studies for their programming which set the
foundation for my company.
Thank you to all the LJS friends who kept me company through the past five years and
ensured there was never a dull moment, especially Keying Chen, Bryce Tappan, and Josh Feng. A
iii
special thank you to a new friend I have made as well, Kimberley Crawford, who I cannot wait to
spend more time with.
A final, heartfelt thank you to my family for their love and support: my mom and dad, my
brother, my favorite aunt, my partner Kapena and his mom and grandparents. Your warmth and
energies helped maintain my sanity. I miss you dad.
iv
Table of Contents
Introduction
Acknowledgements……………………………………………..………………………………. ii
List of Figures………...………………………………………………………………………… vii
List of Schemes..……………………………………………………………………...………… x
Abbreviations…………………..…………………………………………………………..…… xii
Abstract…………………………………………………………………………………………. xi
Chapter 1. A Structural Chemistry Look at Composites Recycling
1.1. Introduction………………………………………………………………………………… 1
1.2. Physical Recycling Methods……………………………………………………………….. 5
1.3. High-Pressure Decomposition……………………………………………………………… 7
1.4. Atmospheric Pressure Decomposition……………………………………………………... 9
1.5. Thermoset Matrices with Inherent Recyclability…………………………………………... 13
1.6. Overview…………………………………………………………………………………… 16
1.7. References………………………………………………………………………………….. 18
Chapter 2. Mechanism and Catalysis of Oxidative Degradation of
Fiber-Reinforced Epoxy Composites
2.1. Introduction……………………………………………………..…..………..……………...23
2.1.1. Composite Materials Recycling…………………………………………………….. 23
2.1.2. Known Methods…………………………………………………………………….. 24
2.1.3. A Role for Catalysis………………………………………………………………… 26
v
2.2. Results and Discussion
2.2.1. Digestion of Small Molecule Matrix Models………………………………………. 27
2.2.2. Digestion of Amine-Cured Matrix………………………………………………….. 29
2.2.3. Digestion of Composite Materials…………………………………………………... 30
2.3. Conclusion………………………………………………………………………………….. 31
2.4. Experimental Section
2.4.1. Materials and Methods……………………………………………………………… 33
2.4.2. Resin Preparation…………………………………………………………………… 33
2.4.3. Synthesis of 2.6a……………………………………………………………………. 34
2.4.4. Synthesis of 2.6b……………………………………………………………………. 36
2.4.5. Monomer and Matrix Digestion…………………………………………………….. 38
2.5. References………………………………………………………………………………….. 41
Chapter 3. Catalytic, Aerobic Depolymerization of Epoxy
Thermoset Composites
3.1. Introduction………………………………………………………………………………… 43
3.2. Results and Discussion……………………………………………………………………... 45
3.3. Conclusion…………………………………………………………………………………. 54
3.4. Experimental Section
3.4.1. Materials and Methods……………………………………………………………… 54
3.4.2. Synthesis of 3.1……………………………………………………………………... 54
3.4.3. Catalyst Screening Studies………………………………………………………….. 56
3.4.4. Preparation and Degradation of Lightly Crosslinked Amine-Epoxy Polymer
Blocks………………………………………………………………………………………. 57
3.4.5. Conversion of 3.4 to 3.5…………………………………………………………….. 59
vi
3.4.6. Preparation and Degradation of Fully Crosslinked Amine-Epoxy CFRP Panels….. 61
3.4.7. Preparation and Recycling of Partially-Cured Pre-Preg Scrap……………………... 62
3.4.8. Life Cycle Primary Energy Consumption Study……………………………………. 63
3.5. References………………………………………………………………………………….. 67
Chapter 4. Incorporating Singlet Oxygen into CFRP Recycling &
Manufacturing
4.1. Introduction………………………………………………………………………………… 70
4.2. Results and Discussion
4.2.1.
1
O
2
Degradation of Molecular Models……………………………………………… 71
4.2.2. Tracking Movement of
1
O
2
Within Polymers ……………………………………… 74
4.2.3.
1
O
2
Recycling of CFRP Panels……………………………………………………… 77
4.3. Conclusion…………………………………………………………………………………. 79
4.4. Experimental Section
4.4.1. Materials and Methods……………………………………………………………… 80
4.4.2. General Procedure for
3
O
2
and
1
O
2
Degradation Reactions…………………………. 81
4.4.3. Synthesis of 4.5…...………………………………………………………………… 82
4.5. References………………………………………………………………………………….. 84
vii
List of Figures
Figure 1.1. Physical and chemical recycling processes for waste CFRPs and their recyclates… 2
Figure 1.2. (A) Commercial mechanical recycling instrument; (B) High voltage fragmentation
instrument; (C) Glass fibers recovered using high voltage fragmentation; (D) Glass fibers
recovered from mechanical recycling.………………………………………………………….. 6
Figure 1.3. A generalized diagram demonstrating the two pathways by which new bonds are
formed between different functional groups in a CAN…………………………………………. 14
Figure 1.4. Structure of a Cleavamine curing agent containing a central acetal group and the
reaction of its acid hydrolysis into a ketone and two amino alcohols…………………………... 15
Figure 1.5. The reversibility of the Diels-Alder cycloaddition reaction between multi-diene 1
and multi-dienophile 2 allows the resulting polymer to repair cracks without requiring
additional reagents to catalysts………………………………………………………………….. 16
Figure 2.1. Effect of catalyst on 2/5 2.1:2.2 amine-cured epoxy. MTO = MeReO
3
, VC =
ascorbic acid..…………………………………………………………………………………… 29
Figure 2.2. SEM images of carbon fibers that are (left) virgin, (center) recovered by
solvolysis, and (right) recovered by oxidative degradation.…………………………………… 31
Figure 2.3.
1
H NMR spectrum of compound 2.6a at 25 °C in CDCl
3
.……………………….… 35
Figure 2.4.
13
C NMR spectrum of compound 2.6a at 25 °C in CDCl
3
.…….…………………… 35
Figure 2.5.
1
H NMR spectrum of compound 2.6b at 25 °C in CDCl
3
.………………………….. 37
Figure 2.6.
13
C NMR spectrum of compound 2.6b at 25 °C in CDCl
3
………………………….. 37
Figure 2.7.
Stacked
1
H NMR spectra for the decomposition of 2.6a at 40
o
C in the presence of
hydrogen peroxide and acetic acid-d
4
. Time interval is 20.0 min………………………………. 39
Figure 2.8.
Stacked
1
H NMR spectra for the decomposition of 2.6b at 40
o
C in the presence of
hydrogen peroxide and acetic acid-d
4
. Time interval is 20.0 min………………………………. 40
Figure 3.1.
Recovered fibers after 24 h DMSO wash from (a) room temperature aged
viii
pre-pregs, (b) 110 ºC cured pre-pregs, with visible matrix residues, (c) 110 ºC cured
pre-pregs from (b) treated with aerobic conditions after 1 week, removing residual matrix…… 52
Figure 3.2. Primary energy analysis of the three most employed CFRP recycling methods…… 53
Figure 3.3.
1
H NMR spectrum of compound 3.1 at 25 °C in CDCl
3
……………………………. 56
Figure 3.4.
1
13 NMR spectrum of compound 3.1 at 25 °C in CDCl
3
…………………………… 56
Figure 3.5.
1
H NMR spectrum of compound 3.4 at 25 °C in CDCl
3
……………………………. 58
Figure 3.6.
13
C NMR spectrum of compound 3.4 at 25 °C in CDCl
3
…………………………… 59
Figure 3.7.
1
H NMR spectrum of compound 3.5 at 25 °C in CDCl
3
……………………………. 60
Figure 3.8.
13
C NMR spectrum of compound 3.5 at 25 °C in CDCl
3
…………………………… 60
Figure 3.9.
Glass transition temperature of pre-pregs aged at room temperature, cured at
110 °C for 3 h, and cured at 120 °C for 3 h…………………………………………………….. 62
Figure 3.10. Processes and material flows used to model primary energy consumption. Use
phase is the same for all pathways and not explicitly modeled. We assume there is no energy
generated from landfill gas derived from CFRP………………………………………………... 64
Figure 4.1. Structures of key compounds studied within this chapter………………………….. 72
Figure 4.2.
1
H NMR kinetics demonstrating degradation rates of 4.4 under triplet and singlet
oxygen at 60 ºC and 100 ºC…………………………………………………………………….. 73
Figure 4.3. Tentative structural assignments of mass signals identified from LC-QTOF from
the aerobic degradation of 4.4…………………………………………………………………... 73
Figure 4.4. Fluorescence signal comparison of 4.3, 4.5, and 4.6 after excitation at 394 nm…… 75
Figure 4.5. (Left) Cross-sectional images of partially cross-linked resin cubes spiked with 4.5
in acetic acid with rose bengal at 100 ºC under varied reaction conditions: (left column) 1 atm
O
2
with 495 nm light; (center column) 1 atm O
2
with no light; (right column) 1 atm N
2
with 495
light. (Right) Sample FLIM of a resin cube at t = 0 min illustrating separation of compounds
based on average fluorescence lifetime. The right corner of the arc has brief fluorescence
ix
lifetimes, while long phosphorescence lifetimes are in the left corner of the arc. A
color-spectrum bar shades the cross-sectional images on the left based on each individual
pixel’s fluorescence lifetime……………………………………………………………………. 75
Figure 4.6. A customized Parr reactor for pressurized photochemistry featuring an LED……... 78
Figure 4.7. Degradation of 60% 4.1: 4.2 CFRP panels under
3
O
2
(left) and
1
O
2
(right) for 15
hours…………………………………………………………………………………………….. 78
Figure 4.8.
1
H NMR spectra of the E/Z mixture of 4.5…………………………………………. 84
x
List of Schemes
Scheme 1.1. Bonds within amine-cured epoxies which have been experimentally identified as
cleavage sites when treated with supercritical water and alcohols. …………….……………… 8
Scheme 1.2. Hitachi’s transesterification conditions for depolymerization of acid-anhydride
linked FRP resins..……………………………………………………………………………… 11
Scheme 1.3. Matrices cross-linked at nitrogen can be cleaved by oxidizing it to a hydrolyzable
imine though (top) hydride abstraction in benzoxazine using high valent metal-oxo species,
or (bottom) oxygen atom transfer under acidic conditions using hydrogen peroxide to form
N-oxides………………………………………………………………………………………… 12
Scheme 2.1. Mechanism for the cleavage of acid-anhydride cured polymers in the Hitachi
benzyl alcohol / trisodium phosphate conditions.………………………………………….…… 25
Scheme 2.2. Synthesis of a common amine-cured matrix using 3,3’-diaminodiphenylsulfone
(DDS), 2.1, and the diglycidyl ether of bisphenol A (DGEBA), 2.2…………………………… 26
Scheme 2.3. Proposed mechanism for the cleavage of amine-cured FRP matrices under
oxidative, acidic peroxide conditions…………………………………………………………… 27
Scheme 2.4. Degradation of Me
4
DDS and Bu
2
DDS in the presence of acetic acid and hydrogen
peroxide…………………………………………………………………………………………. 28
Scheme 3.1. Reaction conditions and results of (top) our previously reported recycling
process and (bottom) our new aerobic depolymerization process……………………………… 45
Scheme 3.2. Reaction conditions and products of the aerobic digestion of a fully cured amine-
epoxy carbon fiber composite…………………………………………………………………... 49
Scheme 3.3. Conversion of recovered organics into commercial chemical bisphenol A through
a facile one-step reaction with quantitative yields……………………………………………… 50
Scheme 3.4. Preparation of 3.1………………………………………………………………….. 54
Scheme 3.5. Degradation of a mildly crosslinked resin block………………………………….. 57
xi
Scheme 3.6. Conversion of 3.4 to Bisphenol A (3.5)…………………………………………… 59
Scheme 3.7. Aerobic recycling of a fully-cured CFRP panel…………………………………... 61
Scheme 4.1. Proposed structure and degradation of resin monomers modified with
1
O
2
-labile
functional groups for facile end-of-life recycling………………………………………………. 79
Scheme 4.2. Synthesis of 4.5…………………………………………………………………… 82
xii
Abbreviations
CFRP: carbon fiber reinforced polymer
CAN: covalent adaptable networks
HVF: high voltage fragmentation
FBP: fluidized bed process
DGEBA: diglycidyl ether of bisphenol A
OAT: oxygen atom transfer
3,3’-DDS: 3,3’-diaminodiphenyl sulfone
MTO: methyltrioxorhenium
VC: ascorbic acid
Me
4
DDS: N,N,N’,N’-tetramethyl-3,3’-diaminodiphenyl sulfone
SEM: scanning electron microscopy
T
g
: glass transition temperature
xiii
Abstract
CFRPs are a class of structural materials globally utilized for their high strength-to-weight
ratios, providing great performance and environmental benefits during their service lives.
However, there are no effective ways to recycle end-of-life CFRP waste or its manufacturing scrap
despite years of awareness of this engineering problem (Chapter 1). By synthesizing and studying
small molecule analogs of the thermoset polymer, we identified the intermediate steps of how
acidic peroxide cleaves the key C—N bonds formed during the curing reaction (Chapter 2).
Incorporating manganese and aluminum catalysts as oxygen-atom transfer and Lewis acid
catalysts, respectively, allowed pressurized oxygen gas to act as the terminal oxidant instead of
hydrogen peroxide (Chapter 3). This recycling reaction yields carbon fibers still preserved in their
original architecture, substantially undamaged and cleaned of polymer residues, but presents too
many safety hazards to be executed at industrially relevant scales. We have explored photo-
generated singlet oxygen to affect similar oxidations at reduced pressures to increase safety, with
exciting preliminary results (Chapter 4). Similarly, we have prepared resin monomers synthesized
with singlet oxygen-labile functional groups with the goal of preparing easily recyclable CFRPs
in the future, without compromising their performance or properties.
1
Chapter 1. A Structural Chemistry Look at Composites Recycling
1.1. Introduction
This chapter mostly duplicates a manuscript published in Material Horizons alongside co-
authors Cassondra R. Giffin, Boyang Zhang, Zehan Yu, Steven R. Nutt b and Travis J. Williams.
1
Fiber-reinforced polymer (FRP) composites are structural materials that offer higher
specific properties, longer life, and improved efficiency compared to traditional structural metals.
2
FRPs are now commonly used in aerospace, wind turbine, marine, and sporting goods applications,
with emerging large-scale use in the automotive industry and some civil engineering applications.
Most of the polymer matrices used in these materials are thermosets, frequently epoxies, and
undergo polymerization in the manufacturing process to cure the resin from a viscous liquid into
a stiff, glassy solid. The irreversibility of this process makes recycling FRPs challenging. The
absence of a sustainable recycling pathway is an increasingly urgent problem impeding wider
adoption of these materials.
FRP composites have become a primary structural material in the latest generation of
commercial aircraft because they are lighter, more resistant to fatigue and corrosion, and reduce
fuel consumption and maintenance when compared to structural metals. Uses include fuselage
sections, wings, and control surfaces in aircraft such as the Boeing 787 Dreamliner and Airbus
A350 XWB, which consist of more than 50% composite parts by weight. FRPs are also important
in the wind energy industry: glass fiber-reinforced polymers are currently the primary structural
material in wind turbine blades. The wind industry is motivated to transition to carbon fiber for
manufacturing larger turbines. With blade lengths continuing to grow, carbon fiber is used
2
selectively in spar caps to provide the stiffness required to prevent column collisions under gust
loads. Composites are also widely seen in high performance sporting goods, ranging from marine
vessels to racing bicycles, golf shafts, skis, and hockey sticks. This range of applications is poised
to expand. FRPs have also emerged as potential replacements for structural metals in high-volume
automotive applications. Particularly, the BMW i3 and 7 series BEVs (battery electric vehicles)
use FRPs to reduce weight and extend range, which help meet stringent standards for greenhouse
gas emissions. In addition, FRPs are used for structural retrofitting, power transmission lines, and
modular housing. While expanding use of FRPs reduces adverse environmental impact,
particularly arising from fuel savings, these benefits are mitigated by the problem of FRP waste
disposal.
Figure 1.1. Physical (top) and chemical (bottom) recycling processes for waste CFRPs and
their recyclates.
There are currently no approaches for recycling end-of-life FRP composite products that
can keep up with the volume of waste. Existing methods focus on recovering fibers by either (1)
shredding the FRP and downcycling it to be an additive in materials like reinforced concrete or (2)
3
pyrolyzing or dissolving the polymer matrix. These processes (Figure 1.1, top) destroy the matrix
and can damage the fibres, thus reducing their length, strength, and stiffness. This converts aligned
fiber beds into lower-value random short-fiber mats.
Three decades of work in FRP composite recycling have not resulted in a tenable solution
to the deconstruction and recycling of these materials, but methods are now beginning to appear
that specifically target the chemical vulnerabilities of certain composite matrices (Figure 1.1,
bottom). Chemical recycling methods are diverse, ranging from using the unique properties of
supercritical solvents to separate polymer matrices from their reinforcing fibers, to chemical
reactions tailored for selective bond cleavage. Through these advancements, new products such as
near-virgin quality fibers and useful organic small molecule and polymeric recyclates are being
recovered for the first time.
Another emerging strategy for managing CFRP lifecycles is to build the deconstruction
plan into the matrix at the point of its original manufacture. Popular strategies, such as vitrimers
or covalent adaptable networks (CAN), function based on reversible, dynamic covalent bond
exchange processes available with reactions like transesterification, Diels-Alder cycloadditions,
reversible radical processes, and olefin metathesis.
3
These materials can act as conventional
thermosets under normal operating conditions, but when the bond exchange dynamics are fast
relative to the duration of an external stimulus, such as heat, the polymer acts like a thermoplastic
with high malleability and reprocessability.
4,5
While such materials are not well-suited to the
manufacture of components for heat-sensitive applications, this strategy can simplify matrix
recycling and undoubtedly will be useful in some select applications. Since it’s hard to imagine
4
that we would fly in an airplane that is designed to deconstruct when it is heated, expanded efforts
to recycle matrices found in current composites are clearly necessary.
Fiber-reinforced polymer composites are ‘‘an important cross-cutting technology’’ for
U.S. manufacturing,
6
with applications in transportation (aerospace, automotive), power
generation, and infrastructure. The global composites industry is experiencing ca. 8% annual
growth, leading up to a total volume of $131B in 2024. Composites are emerging in automotive
manufacturing; 45 billion pounds of composite materials were sold into the industry in 2019.
7
Aerospace growth is predicted to continue across several application areas, and wind energy
(blades) is slated for similar growth.
8
By contrast to the industry’s rapid growth, the present state of composites recycling is
woeful: a mere 1% of CF (estimated) is recovered and reused as of 2020. This stems from key
gaps in recycling technology: current recycling approaches cannot recover the resin or retain the
fiber architecture/alignment/ continuity, relegating the reclaimed fiber to downcycling pathways.
Leaders in the aerospace and automotive industries tell us that the absence of a reasonable
recycling pathway for these materials at the end of their lives is a key factor impeding their more
broad use.
9,10
This creates strong push to develop new technology, and we see few firms in this
space
This Chapter provides an overview of methods that are currently in use in FRP recycling,
then show how the design of methods to target specific polymerization linkages in thermoset
matrices can enable new progress in this area, ultimately to enable new chemical methods with
which to approach the FRP recycling problem.
5
1.2. Physical Recycling Methods
Physical methods for CFRP recycling rely on size-reduction strategies like pulverization,
where composite waste is mechanically shredded into pellets and added into new composites or
cement as structural filler.
11
There is merit in this approach: composite additives in concrete are
advantageous, because its epoxy matrix improves bonding with the concrete, imparting improved
ductility, load-carrying capacity, and fracture toughness.
12
Although pulverization fully re-uses the
waste composite, the value obtained by using CFRP material as an additive is minute compared to
the initial financial and energetic costs used to manufacture the starting fibers. Carbon fibers are
estimated to cost up to $30 USD/lb and require up to 75 kWh/lb to produce, while reinforced
concrete costs about $0.17 USD/lb.
13,14
While this approach preserves some value, much value
remains to be recovered if the carbon fiber weaves could be preserved for high-quality reuse.
There’s environmental merit in this technology because composites that are shredded and
used as building materials do not go immediately to landfill. Globally, < 1 % of composite waste
is recycled, and even if it is significantly downcycled into a single-use material like concrete, some
of its in-service life can be preserved and landfill impact can be reduced. As more composite waste
is recycled, presumably using increasingly value-preserving technologies, this downcycling
method will retain value, because both the volume of unutilized waste is large, and downcycling
by shredding can impart a second life to materials that will remain refractory to modern methods.
Alternative strategies to pulverization have been developed that can separate matrix from
carbon fiber. One example is high voltage fragmentation (HVF), a method in which the composite
is immersed in water and repeatedly pulsed with electrical discharge (Figure 1.2). This creates
extreme temperatures and pressures at the composite surface that disintegrate the matrix.
15
6
However, the treatment time necessary to recover fibers without residue is too long to be practical
and reduces the average fiber length.
15
In an alternative to pyrolysis, fluidised bed processes (FBPs)
were developed where hot air passes through a silica bed containing shredded composites to
remove the matrix from the fiber.
2
The matrix particles are carried away by the air stream to a
separate chamber for destruction at 1000 °C.
2
FBPs have the same limitations as HVF in that the
matrix is destroyed, and the fibers are disordered, which highlights two key opportunities for
improving composite recycling.
Figure 1.2. (A) Commercial mechanical recycling instrument; (B) High voltage
fragmentation instrument; (C) Glass fibers recovered using high voltage fragmentation; (D)
Glass fibers recovered from mechanical recycling.
9
Generally, these methods are analogous to pyrolysis in that the polymer matrix is discarded and
the fibers are downgraded, sacrificing fiber continuity and fiber architecture. Depending on the
approach, however, much higher quality fibers can be retained and re-manufactured into higher-
value products, such as moulding compounds, than are possible with pyrolysis recyclates or
reinforced concrete.
7
1.3. High-Pressure Decomposition
High-pressure methods for recycling composites typically rely on a solvent system,
sometimes with an acid or base reagent, that is heated and pressurized to become supercritical.
Supercritical fluids have unusual properties, including low viscosity, high diffusivity, and
increased solvation strength, and these qualities are appealing for composite recycling, because
they better permeate the material and accelerate matrix dissolution.
16
Supercritical solvents can
facilitate bond cleavage within polyester and amine-cured composites that are inert under other
conditions. Thus, high-pressure recycling has become an important area of investigation, allowing
recovery of carbon fibers with excellent physical properties using inexpensive, non-toxic, and
recyclable reagents.
17
Because of the diversity of reagents and conditions used in this general
strategy, the quality of the fibers varies widely. More chemically mild approaches enable recovery
of relatively undamaged fibers, as is the case in hydrolysis of polyester-based matrices. Other
conditions that require strongly oxidative or corrosive conditions can damage or cleave fibers and
downgrade them to applications such as bulk molding compounds. This highlights the general
conundrum of aerospace-grade resins: the conditions required to disassemble more robust
thermoset polymers used in more demanding applications also tend to degrade their imbedded
fibers.
Common solvents used in supercritical CFRP recycling include water, short chain alcohols
and ketones, or a mixture of these. Mixed solvent systems have the benefit of reducing the
supercritical temperature and pressure thresholds and increasing the matrix by-product solubility.
9
Supercritical solutions of water have successfully removed > 95% of amine-cured epoxy resins
from fibers in as little as 15 minutes, yielding derivatized monomers like methylenedianilines and
8
biphenyldiamines.
18
Based on by-product analysis, these conditions appear to target crosslinked
C-N bonds and secondary alcohols, and the observation of diaminobenzophenone species implies
there is an oxidant, likely oxygen gas, performing C-H and oxidations (Scheme 1.1).
18,19
Other
solutions like 80% acetone in water, 20% butanone in water, or neat propylene glycol have also
been effectively used for dissolving amine- and anhydride-cured epoxy matrices.
20–22
Scheme 1.1. Bonds within amine-cured epoxies which have been experimentally identified as
cleavage sites when treated with supercritical water and alcohols.
8
A variety of chemical additives have been studied to accelerate resin dissolution, frequently
by changing the dissolution mechanism. Hydroxide salts added to supercritical alcohol solutions
remove 70% more matrix from amine-cured composites than control experiments.
23
The molecular
mechanism for this is not obvious. The same process has been adapted from a batch reaction to a
semi-continuous process that enables recovery of resin-free fibers at conditions 100 °C less forcing
and 10 MPa less forcing than those of the batch process. This is realized by reducing mass transfer
limitations, showcasing the impact of clever reactor design on CFRP recycling.
23
Liu et al. added
9
phenol and potassium hydroxide to supercritical water, and observed improvement of more than
80 wt% in resin removal. They attributed this success to phenxoyl-based free-radical reactions.
24
Adding ionic liquids to ethylene glycol shifts the product distribution away from long-chain
oligomers to monomers in polyester matrix transesterification processes.
25
Despite the benefits of
chemical additives, scaling up high-pressure recycling is inherently difficult due to the high energy
demand, safety considerations, and need for specialized reactors.
26
1.4. Atmospheric Pressure Decomposition
The introduction of mild conditions selectively to depolymerize FRP matrices opens new
possibilities in FRP recycling, because such conditions enable fiber recovery with less damage,
sometimes even retaining their original weave, and fine chemicals that form from an orderly
deconstruction of the composite matrix. Chemical recycling methods that have been developed
near ambient pressure also tend to be safer and easier to implement at an industrial scale than high-
pressure methods, increasing their likelihood of adoption.
27
However, without the brute force
provided by supercritical temperatures and pressures, these reactions must rely on grace and design
to select chemical reagents to cleave crosslinking bonds in the matrix: this fundamentally changes
FRP recycling from an engineering problem to a chemistry problem.
Designing systems for CFRP recycling that target specific features of the network of their
thermoset polymers at mild conditions requires expertise in basic reaction chemistry, which has
not previously been a focus of the composites recycling community. As new chemistries are
introduced; however, milder and more delicate conditions are emerging. As a result, we are seeing
cases now where polymers and fine chemicals can be collected while simultaneously recovering
fibers. Unfortunately, any recycling approach based on a specific polymer structure or formulation
10
will probably be limited in use to CFRPs containing that same (or analogous) linking chemistry.
Thus, the problem becomes complex, and the continued relevance for bulk physical methods is
highlighted as one considers the problem of unknown or mixed composite waste streams.
When engineering these new chemical processes, conditions used should be inexpensive,
selective, safe to handle on large scale, and yield only benign by-products. It is likely that a reason
that the community has seen so few is that process minded thinkers have set aside possible but
impractical methods that could homogenize thermoset matrices, but are irrelevant because of
reagent cost, safety, or other practical concerns. Some emerging examples of successful chemical
methods follow.
The crosslinking polyester bonds in anhydride-cured epoxy composites are susceptible to
acid- and base-catalyzed transesterifications, which has been exploited to great success. Strong
acids like p-toluenesulfonic acid in acetic anhydride solution can homogenize polyester resin
blocks at temperatures as low as 80 °C, though other bonds, such as the quaternary carbon in
bisphenol A moieties, are cleaved as well.
28
Short-chain alcohol solutions containing Lewis bases
like hydroxides or amines degrade these matrices in as little as 90 min.
29–31
Derivatized monomers
and high-quality recycled carbon fibers for re-use are usually recovered from these reactions, as
Hitachi Chemical demonstrated.
32,33
The inherent lability of the ester, the linker formed upon
curing anhydride-cured epoxy resin system, is a feature of these methods that contributes to their
success: the lability of the ester serves almost like an engineered strategy for polymer disassembly
(Scheme 1.2). Unfortunately, programming this or any other designed vulnerability into the CFRP
matrix will limit the material’s use cases, because it will have a known vulnerability. There are,
11
however, exceptions, such as efforts to incorporate cleavable amines into epoxies that facilitate
deconstruction.
34
Scheme 1.2. Hitachi’s transesterification conditions for depolymerization of acid-anhydride
linked FRP resins.
27
Unlike esters, or even amides, that are amenable to acidic or alkaline solvolysis, epoxy
thermosets do not contain labile bonds that enable facile disassembly. The carbon – nitrogen bonds
that impart stability and rigidity to amine-cured epoxies are comparatively stable to acids and
bases, so researchers have used more forcing conditions to realize less complete dissolution than
is realized with anhydride-cured matrices. For example, an amine-cured resin block must be
soaked in nitric acid for 21 days for degradation to occur.
35
Solutions containing excess Lewis
acids, like zinc or aluminum chlorides, have been used successfully to digest composites, however,
the molecular mechanism of these degradation reactions remains unclear.
36,37
One group suggests
that aluminum coordination to Lewis basic sites in the CFRP matrix creates a leaving group, but
this seems uncertain, as aluminum salts other than the chloride fail to catalyze the reaction.
37
Basic
12
conditions are similarly forceful and unsuccessful: imidazole-cured epoxy novolac matrices
require 2 h in molten potassium hydroxide to homogenize.
38
If the composite is swelled in nitric
acid first, solutions of potassium hydroxide in poly(ethylene glycol) can eventually fully degrade
the resin.
39
Scheme 1.3. Matrices cross-linked at nitrogen can be cleaved by oxidizing it to an
hydrolyzable imine though (top) hydride abstraction in benzoxazine using high valent metal-
oxo species, or (bottom) oxygen atom transfer under acidic conditions using hydrogen
peroxide to form N-oxides.
Alternative chemical strategies are needed to recycle amine-cured epoxy composites
mildly and selectively. Their carbon – nitrogen bond presents a potential target for selective
cleavage, possibly by conversion to an N-oxide or iminium cation, as the latter is readily cleaved
by water.
40
An analogous strategy was successfully utilized in depolymerizing benzoxazine-epoxy
composites by abstracting a hydride atom alpha to an aniline nitrogen center using a high valent
ruthenium species (Scheme 1.3, top).
41
Another method is to use an inexpensive peroxide, like
hydrogen peroxide, as an oxygen atom transfer agent to form an amine oxide, or N-oxide (Scheme
13
1.3, bottom).
42,43
We have previously reported the formation of imines from N-oxides via NMR
spectroscopy in our degradation studies of small molecule analogues of amine-cured epoxies.
44
Through this molecular study, we argue that other successful reports of amine-cured epoxy resin
degradation involving oxidants like hydrogen peroxide misattribute the mechanism to a series of
hydroxyl radical-based processes.
45–47
Identifying the most-likely mechanism occurring during
these degradation reactions is key to build on this knowledge as the community develops an
optimal process for depolymerizing amine-cured epoxies.
There seems to be great promise in this general strategy of designing processes for selective
depolymerization of CFRP thermoset matrices, but there are intrinsic limits to the approach.
Structure-dependent recycling strategies will almost certainly be ineffective on polymers for which
they were not designed. Using such a method would require the waste processor to know the basic
composition of the waste stream. Further, the practical limitations of safety and cost must be
superimposed on any of these methods before they can be scaled. Reagents in this literature such
as nitric acid, hydrogen peroxide, certain expensive solvents, and even O
2
itself impose cost and
safety limits on the deployment of such processes. Much chemistry and engineering remains to be
done to overcome these hurdles.
1.5. Thermoset Matrices with Inherent Recyclability
Developing thermoset matrices with inherent recyclability, which allows them to be easily
removed or recycled, is another attractive approach to composite recycling. Introducing labile
chemical bonds and CANs (covalent adaptable networks) are two popular strategies to achieve
this. By introducing degradable chemical bonds (such as the ester links), thermoset matrices can
become recyclable when exposed to external stimuli, such as temperature, chemicals, or
14
photolysis.
48
A review of potential recyclable thermosets for structural applications and limitations
thereof has appeared recently.
48
A key problem of using degradable chemical bonds to modify
thermoset matrices to increase recyclability is that the matrix structure is destroyed after
degradation, which eliminates the value of the recyclates. CANs overcome this problem, as they
retain the overall structure of the matrix after recycling. CANs can be categorized into two
subgroups - dissociative and associative - based on the chemical mechanisms involved: original
chemical bonds are broken either before or after the formation of new chemical bonds (Figure 1.3).
Figure 1.3. A generalized diagram demonstrating the two pathways by which new bonds are
formed between different functional groups in a CAN.
Introducing thermally triggered degradable chemical bonds can reduce the degradation
temperature. For example, by adding thermally cleavable carbamate bonds, redesigned
cycloaliphatic diepoxides were decomposed between 200-300 ℃, whereas commercial
cycloaliphatic diepoxides are normally stable up to 350 ℃.
49
Ester bonds are also frequently used
to increase thermal reworkability. One example features the use of hyperbranched polyaminoester
(PAE) with secondary alkyl esters as an additive to modify a conventional epoxy resin (DGEBA).
The additive reduced the decomposition temperature up to 100 ℃.
50
15
Some recyclable thermoset CANs can be triggered by specific chemical reagents. Early
work on associative CANs utilized photoinitiated radical chain transfer reactions.
51
More recent
examples include a proprietary epoxy resin system called “Cleavamine,” which contains acid-
labile formyl and acetal groups (Figure 1.4).
52
This recyclable resin exhibits thermal and
mechanical properties similar to non-recyclable resins, and this resin is easily degraded in an acidic
environment.
34
In this mechanism an ether group is protonated, leaves as an alcohol, and is replaced
by water to form a hemiacetal. The second ether group is similarly protonated and leaves, yielding
a ketone.
Figure 1.4. Structure of a Cleavamine curing agent containing a central acetal group and the
reaction of its acid hydrolysis into a ketone and two amino alcohols.
33
A second example is based on the Diels-Alder reaction, combining multi-furan and multi-
maleimide polymers to form a novel, dynamic material (Figure 1.5).
53
The polymer displayed
mechanical properties similar to commercial epoxy resins with the added advantage of self-
healing. Structural failure was remediable after thermal treatment at 120-150 ℃ for about two
hours.
Figure 1.5. The reversibility of the Diels-Alder cycloaddition reaction between multi-diene 1
and multi-dienophile 2 allows the resulting polymer to repair cracks without requiring
additional reagents to catalysts.
52
16
Leibler and co-workers undertook pioneering work based on associative CANs, which
involved a new group of material called vitrimers.
54
Via transesterification reactions with proper
catalysts, the viscosity of these vitrimers decreased only slightly with increasing temperature,
whereas viscosity of conventional polymers changes rapidly near the glass transition temperature.
1.6. Overview
Physical methods to reclaim value from composite materials have created an avenue to
deflect composite waste from landfill and re-insert waste materials into consumer products.
However, the downcycling of value in such processes has limited their commercialization such
that these methods reclaim only a small portion of composite waste. A second generation of
approaches is now emerging in which chemical strategies are being designed selectively to
deconstruct the bonds that hold together composite thermoset matrices. Such strategies, along with
new composite resin technology in which cleavable bonds are built into the polymer when it is
first produced, both move the composite materials industry toward sustainability and highlight the
vital role that synthetic chemistry must play in enabling fundamental advancement in composites
engineering.
C O O
O
O
4
N
O
O
N
3
O
N
O
O
N
O
O
O
1
2
1
2
17
The present landscape for composite recycling includes multiple approaches, each with
intrinsic drawbacks and advantages, providing fertile ground for opinion and speculation about a
path forward. A practical criterion for comparing recycling approaches should weigh both
sustainability and economic factors. Clearly, any approach that destroys the fiber architecture and
continuity must either (a) include realignment of fibers and the attendant cost, or (b) be considered
a downcycling approach. Downcycling to lower-performance products reclaims fibers for insertion
into a second application but postpones recycling to the end of the second service life. Such
approaches also sacrifice the matrix, a highly engineered material of value not unlike that of the
fibers. While such approaches do not require new technology and may thus be economically
viable, they are not preferred as a long-term solution. Chemical approaches that can reclaim both
fibers and matrix components offer a superior solution, provided they operate at acceptable rates
and costs, do not create additional recycling problems, and selectively cleave the matrix polymers
such that high-value molecular components are retained. Chemical solutions avoid mechanical
shredding and retain fiber architecture, avoiding major downstream costs. However, the challenge
with chemical processes lies in finding a scalable solution that requires only relatively mild
conditions.
Finally, all approaches face a major issue – lack of market pull – and until this issue is
addressed, progress is likely to be modest in the foreseeable future. Eventually, the waste disposal
problem will become so severe that legislation will be introduced to pressure consumers and
producers to adopt more sustainable practices.
18
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23
Chapter 2. Mechanism and Catalysis of Oxidative Degradation of
Fiber-Reinforced Epoxy Composites
2.1. Introduction
2.1.1. Composite Materials Recycling
This chapter mostly duplicates a manuscript published in Topics in Catalysis alongside co-
authors Elyse A. Kedzie, Yijia Ma, Katelyn H. Michael, Steven R. Nutt & Travis J. Williams.
1
Carbon fiber-reinforced polymer (CFRP) composites are structural materials that offer
higher specific mechanical properties, longer service life, and improved efficiency and versatility
compared to traditional structural metals.
2–4
FRPs contain long, continuous fibers (e.g., carbon,
glass, aramid) encased within a thermoset polymer (e.g. epoxy) matrix (thermoplastic matrices are
also used). The thermoset matrix is formed through an irreversible cure reaction of a resin,
typically converting it from a viscous liquid to a stiff, glassy solid, whether the resin is infused
into the fiber bed as liquid or pre-impregnated with the fibers in a prepreg sheet material. CFRPs
are commonly used in applications including aerospace, wind turbines, and marine products, with
emerging large-scale use in automotive manufacturing and civil engineering. This growth in the
composites market creates a major sustainability problem, as there is no efficient strategy for
recycling or reusing composite material waste at the end of the life of the material. This leads to
millions of tons of non-degradable waste and im-pedes the even wider adoption of this class of
materials in large-scale manufacturing. This issue is compounded by the inefficiency of CFRP
manufacturing methods. For example, 10% to 30% of purchased prepreg material is typically
scrapped in processing, and no adequate approaches exist for reusing production scrap.
5
24
Furthermore, environmentally conscious public policies, such as the European end-of-life vehicle
directive (2000/53/EC), which dictates that new vehicles must be at least 85% recyclable by
weight, apply additional pressure to solve recycling of scrap and end-of-life composite materials
to enable the industry’s continued growth.
6
Three decades of work in FRP composite recycling have not resulted in a tenable solution
to the recycling of composite waste and scrap.
7
Although both the embedded fibers and the
polymer resin are highly engineered materials with substantial value, current recovery research
emphasizes reclaiming only the fibers by thermal or chemical means.
8
Thermal degradation
(pyrolysis) involves high temperatures to decompose the polymer matrix. While effective, it is
energy intensive and can degrade the fibers and produce undesirable (residual) solid and gaseous
byproducts. Solvolysis methods dissolve the matrix in one or more solvents at near- or super-
critical conditions. This allows for recovery of resin components for some processes, but
crosslinked polymeric materials are very difficult to process in solution.
2.1.2. Known Methods
Hitachi Chemical illustrated an excellent example of monomer recovery in the
depolymerization of a composite matrix by using alkoxide conditions to realize solvolysis of a
polyester-based composite matrix (Scheme 2.1).
9
This type of depolymerization may ultimately
enable the recovery of useful small molecules from the crosslinked polymer, which would preserve
partial end value for the first time.
10
The chemical mechanism for this reaction involves a simple
transesterification: a potassium alkoxide adds to a linking ester group in the matrix, thus cleaving
the polymer and protecting the carboxylate group as a benzyl ester. Benzyl alcohol is a convenient
25
solvent for this reaction, because it both enables intercalation of the reagents into the matrix and
provides the requisite alkoxide nucleophile.
Scheme 2.1. Mechanism for the cleavage of acid-anhydride cured polymers in the Hitachi
benzyl alcohol / trisodium phosphate conditions.
An important aspect of the Hitachi approach is that the mechanism of depolymerization is
specific to matrices that are linked through ester groups. The overwhelming majority of high-
performance composite materials currently in use are thermoset resins based on aniline/epoxide
curing chemistry. Curing such resins results in alkylated aniline linkages that are largely inert to
alkoxide conditions. Thus, orderly deconstruction of epoxy-based matrices is a much more
challenging problem that that of polyesters, because epoxies lack a linkage that is vulnerable to
solvolysis. To demonstrate this point, we prepared epoxy matrix 3 (Scheme 2.2) and applied
Hitachi’s conditions to its depolymerization. The reaction occurred much more slowly than for the
polyester matrix, because the transesterification mechanism is not available for these amine-cured
systems. To combat this problem, conditions have appeared in the literature involving acidic
hydrogen peroxide solutions for degradation of amine-linked epoxy matrices.
2
O
O
O
O O
O
O
BnOH Na
3
PO
4
Bn- = C
6
H
5
CH
2
-
BnO
O
OBn
O HO
O
O
OH
BnO
Mechanism:
O
O
BnO
O
O
BnO
fragments
26
Scheme 2.2. Synthesis of a common amine-cured matrix using 3,3’-diaminodiphenylsulfone
(DDS), 2.1, and the diglycidyl ether of bisphenol A (DGEBA), 2.2.
2.1.3. A Role for Catalysis
New methods must be developed to target the specific linking groups present in amine-
cured epoxies if these processes are to be employed efficiently on an industrial scale. Herein, we
show a mechanistic path for the depolymerization of amine-cured epoxies under oxidative acidic
peroxide conditions. We propose this mechanism proceeds by oxygen atom transfer (OAT),
followed by imine formation and imine hydrolysis (Scheme 2.3). It stands to reason, therefore, that
catalysts capable of accelerating either the O-atom transfer or elimination step (or both) could
accelerate the degradation of these composite matrices. Ultimately, validating (or refuting) this
mechanism and understanding the kinetics of the OAT reaction could help in identifying
conditions that enable the use of an oxygen atom source that is more sustainable than peroxide.
S
O
H
2
N
O O
O
NH
2
+
O
O
Δ 1 DDS
2
S
O O
N
OH
O
N
O
OH
O
n
3
High molecular weight, crosslinked polymer matrix
A bi-directional bis(epoxide)
resin curing
27
Scheme 2.3. Proposed mechanism for the cleavage of amine-cured FRP matrices under
oxidative, acidic peroxide conditions.
2.2. Results and Discussion
2.2.1. Digestion of Small Molecule Matrix Models
Small molecule models of amine-cured epoxy matrices, in which the linking glycerol
fragments within the matrix were modeled respectively as methyl (2.6a) and n-butyl (2.6b) groups,
were synthesized and digested under acidic peroxide conditions (Scheme 2.4). NMR kinetics
studies on Me
4
DDS (2.6a) revealed rapid oxygen atom transfer to form N-oxide intermediates (2.6)
within minutes, proceeding to 50% conversion in ca. 1.5 hr at 40 °C. Prolonged heating of
Me
4
DDS-N, N’-dioxide (2.7a) at 80 °C for 12 hr resulted in degradation of ca. 60% of the dioxide.
Dioxide intermediate 2.7a formed fully before any degradation product from this material could
be formed. Thus, O-atom transfer is much faster than elimination in this sequence. Dioxide 2.7a
appears to convert slowly to acetoxymethanol-d
4
(2.10a), which would form by elimination of
2.8a’s N-oxide, and a DDS derivative that can go on to further oxidation. To accelerate the
conversion of 2.7a to cleavage products, we screened a panel of oxophilic Lewis acid catalysts.
S
O O
N
OH
O
O atom
transfer
S
O O
N
OH
O
BnO
No C=O
means
no rxn
O
BnOH
X
Na
3
PO
4
Y
Y O
+ H
+
-H
2
O
S
O O
N
OH
O
S
O O
NH
2
OH
O O
+
imine hydrolysis
4 5
3
28
Among an initial screen of sulfuric acid, gadolinium(III) triflate, yttrium(III) triflate, gallium(III)
triflate, scandium(III) chloride, triphenylborane, and trifluoroborane, only ScCl
3
enabled full
conversion of 2.7a to decomposition products in 12 hours at 80 °C. Safety note: no transition metals
with populated d orbitals (e.g. FeCl
3
, a d
5
metal) can be used in this experiment. Such species will
catalyze rapid decomposition of hydrogen peroxide, which could create an explosive oxygen/fuel
mixture when combined with acetic acid vapor. Scandium(III) and trivalent lanthanides are d
0
metals and can thus be used safely under these conditions.
Scheme 2.4. Degradation of Me
4
DDS and Bu
2
DDS in the presence of acetic acid and hydrogen
peroxide.
A dibutylated analog of DDS (2.6b, Bu
2
DDS) behaves analogously to 2.6a. This extended
model system is used to probe the cleavage sequence available to singly alkylated DDS anilines,
whether formed as such upon resin curing or generated following the first dealkylation of a fully-
alkylated aniline. Use of the butyl group as a model of the polymer was also chosen to facilitate
identification of the C
4
degradation products. We observe that under conditions analogous to the
oxidation of 2.6a, this system undergoes initial conversion to nitrone intermediate 2.7b somewhat
slower than oxidation of 2.6a, compare 3.8 versus 1.5 hours to reach 50% conversion, respectively.
Cleavage of 2.7b proceeds to yield products 2.9b and 2.10b; the former (major) species was
identified by appropriate patterns in the
1
H NMR and phase-gated HSQC spectra appearing at ∂ =
S N
O O
N
30% H
2
O
2
aq
CD
3
CO
2
D
40
o
C
S NMe
2
O O
N
O DO
Me
H
6a: R, R’ = Me, Me
4
DDS
6b: R = nBu, R’ = H, Bu
2
DDS
S Me
2
N
O O
NMe
2
O O
8a
S N
O O
H
N
9a: R’ = H
9b: R’ = nPr
further oxidation
R'
O
H R' H
DO OAc-d
3
10a: R’ = H
10b: R’ = nPr
+
R’
R
R’
R
S N
O O
H
N
O
7a
7b
29
5.1 (
1
H) and 101 (
13
C) ppm. No evidence of olefin formation was observed. Unlike its
tetramethylated congener, cleavage of 2.7b proceeded at a rate like the oxidation of 2.6b: only
modest portions of 2.7b (< 10%) accumulated in the NMR experiment.
11–13
Thus, in this system, it
is not obvious which step of the sequence should be the subject of catalyst development. In the
overall sequence of degradation of a fully alkylated DDS, it appears that elimination of the initially
formed N-oxide intermediate is the slowest step.
2.2.2. Digestion of Amine-Cured Matrix
Based on the proposed mechanism, two possible rate-limiting steps in the digestion are O-
atom transfer and elimination. Catalysts with activity for each of these respective steps were
screened for their effect on epoxy matrix dissolution time. Methyltrioxorhenium (MTO) and
ascorbic acid (VC) were applied as OAT catalysts while Lewis acids scandium trichloride (ScCl
3
)
and aluminum trichloride (AlCl
3
, a ScCl
3
analog that appears elsewhere in the composites
degradation literature) were tested as elimination catalysts.
14–16
Figure 2.1. Effect of catalyst on 2/5 2.1:2.2 amine-cured epoxy. MTO = MeReO
3
, VC =
ascorbic acid.
Figure 2.1 shows that neither MTO nor ascorbic acid modulate the rate of polymer
degradation beyond measurement error. This is consistent with the behavior of our Me
4
DDS
30
model, because we find that 2.6a undergoes O-atom transfer rapidly relative to elimination of 2.7a.
For the elimination catalysts, the effectiveness of AlCl
3
is comparable to the background reaction.
ScCl
3
reduced dissolution time significantly compared to background, from which we infer that in
the present polymerized matrix, elimination, rather than OAT, is rate-limiting.
2.2.3. Digestion of Composite Materials
We next applied our scandium-based conditions to the degradation of carbon fiber-
reinforced epoxy panels. Because of the stiff nature of the fibers, it is difficult to determine
precisely the time at which matrix is completely degraded away from its imbedded fibers, so
experiments with and without scandium were performed through the same time duration.
Degradation of FRP panels based on resin monomers 2.1 and 2.2 (1:1 ratio) under peroxide/
acetic acid conditions allowed for recovery of clean carbon fibers. SEM images (Figure 2.2)
revealed that fibers isolated from the reaction conditions are unaffected and remain in pristine
condition. This finding is significant, as these carbon fibers, highly engineered materials of
substantial value that drive both the cost and the energy demand of production,\ can be recovered
and repurposed without resorting to pyrolysis.
8,17,18
While in these experiments we recovered fibers
in disordered orientations, which would limit their direct usability to applications appropriate for
short fiber mats, we are working to develop conditions to remove the matrix without disrupting
the fiber weave.
31
Figure 2.2. SEM images of carbon fibers that are (left) virgin, (center) recovered by
solvolysis, and (right) recovered by oxidative degradation.
Mass spectral studies of the panel degradation solution by GC/MS and MALDI MS did not
reveal significant portions of any expected monomer (e.g., bisphenol A, DDS, or a derivative
thereof). This is unsurprising, because we observe by NMR, that peroxide/ acetic acid conditions
destroy these aromatic moieties in less than 12 hours at 80 °C in our model studies. Thus, as we
expect, most of these materials should be destroyed quickly at the composite panel decomposition
conditions, which require minimally 6 hours at a temperature of 110 °C. Identifying and
accelerating the rate-limiting step in this catalysis is key for applying milder digestion conditions,
ultimately to realize successful monomer re-isolation.
2.3. Conclusion
CFRP composites are commonly employed in a range of different fields due to their light
weight and versatility, but their expanded and continued use is restricted by the lack of standard
procedure for the deconstruction and reuse of these materials. Prior work that has enabled recovery
32
of resin monomers in this field is limited to Hitachi’s solvolysis conditions for acid anhydride-
linked epoxies. The Hitachi chemistry relies on the ester-based chemistry around which that resin
is designed. Ester solvolysis conditions should not affect degradation of the more common amine-
cured epoxies, since these structures lack a solvolysis-labile functionality. Some peroxide-based
conditions are known to degrade these epoxy-based systems, but there is no prior explanation of
how this chemistry might work.
We report the first investigation into the molecular mechanism of the digestion of amine-
cured epoxy CFRP composite materials under acidic peroxide conditions. We have shown that
dissolution times of these matrices can be affected by treatment with acetic acid and 30% hydrogen
peroxide, reaching completion in as few as 6 hours. Furthermore, we have shown that this reaction
time can be accelerated by scandium(III) chloride, which is functioning as an elimination catalyst
in a small-molecule model system. We show that these reaction conditions are too oxidative for
small molecule re-isolation, because our monomers are not stable to the acidic, oxidative
conditions. However, we demonstrate that high-value, pristine carbon fibers can be recovered from
digested CFRP materials. Our investigation reveals that the digestion mechanism proceeds by an
O-atom transfer reaction, in which an N-oxide or nitrone species forms, followed by a sequence to
break a key polymeric C—N linkage. We find that oxygen atom transfer is rapid relative to
elimination of the initially-formed N-oxide intermediate, which presages the possibility of using a
less aggressive oxygen atom source, perhaps air itself, for this step.
Ongoing work in this area should focus on (1) identification of conditions to affect the
requisite oxidation step of the degradation sequence in a sustainable, environmentally benign way
that preserves the aromatic moieties present in the composite resin monomers, and (2) acceleration
of the requisite downstream elimination/cleavage events. Identification of such conditions will
33
enable both large-scale implementation of oxidative depolymerization of epoxy composites and
re-isolation of high-value monomers for resin recycling, thereby opening a route to recycling of
both fibers and matrices.
2.4. Experimental Section
2.4.1. Materials and Methods
All NMR spectra were acquired on a Varian VNMRS 500 or 600 spectrometer and
processed using MestreNova. All chemical shifts are reported in units of ppm and referenced to
the residual
1
H or
13
C solvent peak and line-listed according to (s) singlet, (d) doublet, (t) triplet,
(dd) double doublet, etc. Degradation studies were performed in 8” J-young tubes (Wilmad or
Norell) with Teflon valve plugs to avoid possible loss of solvent or products at elevated
temperatures. GC/MS data was acquired on an Agilent 5973/HP 6900 instrument, MALDI
chromatograms were obtained from a Bruker Autoflex Speed MALDI Mass Spectrometer and IR
data was acquired on a Nicolet iS5 FTIR spectrometer.
2.4.2. Resin Preparation
The resin monomers used for the amine-cured epoxy resins DDS (2.1) and DGEBA (2.2)
were acquired from Huntsman Corporation. For degradation studies, matrices were prepared from
resins using a 2/5 amine/epoxy molar ratio and were mixed at room temperature in clean aluminum
cans until fully homogenized. The mixture was then heated to 120 °C in a pre-heated convection
oven to further improve the mixing quality, yielding a clear homogenous mixture.
34
2.4.3. Synthesis of 2.6a.
N, N, N’, N’-Tetramethyl-3,3’-DDS (Me
4
DDS) was prepared by combining 1.01 g (4.06
mmol, 1 equiv) DDS, 2.50 g (83.3 mmol, 20.5 equiv) paraformaldehyde and 50 mL acetic acid in
a 250 mL round bottom flask. The flask was purged with N
2
, and 2.60 g (41.4 mmol, 10.2 equiv)
sodium cyanoborohydride was added. The flask was stirred at 60 °C for 22 hours. The resulting
cloudy white solution was basified to pH 11 with 25% sodium hydroxide, extracted with 75 mL
portions of dichloromethane, dried over anhydrous sodium sulfate, and concentrated to yield 288
mg (23%) of product.
1
H NMR (500 MHz, Chloroform-d) δ 7.29 (appt, J = 8.0 Hz, 2H), 7.25 (dd, J = 2.6, 1.8 Hz, 2H),
7.20 (dt, J = 7.7, 1.2 Hz, 2H), 6.83 – 6.80 (m, 2H), 3.00 (s, 12H).
13
C NMR (151 MHz, Chloroform-d) δ 150.54, 142.45, 129.71, 116.06, 114.90, 110.24, 40.32.
FTIR n
max
/cm
-1
3430 (OH, water), 2894, 1416, 1360 (CH
3
), 1296, 1066 (S=O).
mp 129.2 – 130.8 °C
GC/MS for C
16
H
20
N
2
O
2
S: calc’d 304.1, found 304.1 g/mol.
35
Figure 2.3.
1
H NMR spectrum of compound 2.6a at 25 °C in CDCl
3
.
Figure 2.4.
13
C NMR spectrum of compound 2.6a at 25 °C in CDCl
3
.
36
2.4.4. Synthesis of 2.6b.
N, N’-Dibutyl DDS (Bu
2
DDS) was prepared by mixing 0.50 g (2.01 mmol, 1 equiv) DDS,
1.40 g (10.2 mmol, 5.1 equiv) 1-bromobutane and 0.80 g (10.1 mmol, 5.0 equiv) pyridine in 15
mL tetrahydrofuran. The solution was allowed to react at 60 °C for 48 hours, giving an orange
solution with a dark-orange oil. The mixture was concentrated and extracted with 15 mL water
and three 20 mL portions of ethyl acetate. The combined organic layers were washed with 20 mL
of brine, dried over anhydrous sodium sulfite and concentrated. The product was isolated by silica
column chromatography using a gradient of hexane – ethyl acetate (100:0 – 20:80), lyophilized
with benzene to yield 25 mg (3%) of pristine, white solid. The balance of the material remained in
mixed chromatography fractions.
1
H NMR (500 MHz, Chloroform-d) δ 7.23 (appt, J = 7.9 Hz, 2H), 7.18 (dt, J = 7.7, 1.7, 0.8 Hz,
2H), 7.10 (t, J = 2.1 Hz, 2H), 6.70 (dd, J = 8.1, 2.5, 1.0 Hz, 2H), 3.84 (apps, 2H), 3.11 (appq, J =
6.3 Hz, 4H), 1.63 – 1.55 (m, 4H), 1.47 – 1.37 (m, 4H), 0.98 – 0.93 (m, 6H)
13
C NMR (126 MHz, Chloroform-d) δ 149.04, 142.67, 130.02, 116.85, 115.87, 110.69, 43.61,
31.54, 20.37, 14.01.
FTIR n
max
/cm
-1
3391 (2° N-H), 2954, 1471 (CH
3
), 726 (CH
2
), 1337, 1149 (S=O)
mp 125.0 - 128.0 °C
MALDI-MS for C
20
H
28
N
2
O
2
S: calc’d 360.2, found 360.5 g/mol.
37
Figure 2.5.
1
H NMR spectrum of compound 2.6b at 25 °C in CDCl
3
.
Figure 2.6.
13
C NMR spectrum of compound 2.6b at 25 °C in CDCl
3
.
38
2.4.5. Monomer and Matrix Digestion
Representative digestion experiments were done by reacting 1.00 g prepared (cured)
polymer matrix in a 1 L flask with 60.0 mL glacial acetic acid (EMD Millipore) and 10.0 mL 30%
aqueous hydrogen peroxide (EMD Millipore). The resulting mixture was refluxed (110 °C bath)
with additional 5.00 mL hydrogen peroxide solution portions added every hour. These reaction
conditions were also applied to a ca. 5 g sample of an FRP panel based on resin monomers 2.1 and
2.2. The panel was an 8-ply composite laminate, fabricated using 2 ´ 2 twill weave carbon fiber
fabrics (3K, 193 g/m
2
, FibreGlast,) and a 1/1 amine/epoxy stoichiometric ratio matrix. Final
polymer content was 45 ± 2 wt%. The cured laminates were cut into 100 ´ 20 mm coupons and
subjected to digestion. Carbon fibers from the reaction mixture were separated and examined by
SEM. The product mixture was screened for monomeric materials by GC/MS and MALDI mass
spectroscopy, using sodium dihydroxybenzoate as the MALDI matrix.
The degradation of Me
4
DDS was studied by placing 5.1 mg (17 µmol, 1 equiv) Me
4
DDS,
700 µL acetic acid-d
4
(Cambridge Isotope Labs) and 50 µL 30% hydrogen peroxide (490 µmol,
29 equiv) into a J-young NMR tube and letting react in an overnight VT NMR experiment at 40
°C.
39
Figure 2.7. Stacked
1
H NMR Spectra for the Decomposition of 2.6a at 40
o
C in the presence
of Hydrogen Peroxide and Acetic Acid-d
4
. Time interval is 20.0 min.
The degradation of Bu
2
DDS was observed by placing 7.9 mg (22 µmol, 1 equiv) Me
4
DDS,
700 µL acetic acid-d
4
and 50 µL 30% hydrogen peroxide (490 µmol, 22 equiv) into a J-young
NMR tube and letting react in an overnight VT NMR experiment at 40 °C.
40
Figure 2.8. Stacked
1
H NMR Spectra for the Decomposition of 2.6b at 40 ºC in the presence
of Hydrogen Peroxide and Acetic Acid-d
4
. Time interval is 10.7 min.
The dissolution rate of amine-cured epoxies was shown to be dependent on both the
chemical reaction rate and the diffusion rate of the reagents through the material. To accelerate the
diffusion limit, 1.00 g of the epoxy sample was placed in 100 mL acetic acid at 110 °C for 30 min;
then 1 mol% selected catalyst and 2 mL 30% hydrogen peroxide were added. Epoxy dissolution
time was defined as the time for the epoxy to completely dissolve into solution, determined
visually. The products from these reactions were analyzed by GC/MS and MALDI mass
spectrometry.
41
2.5. References
(1) Navarro, C. A.; Kedzie, E. A.; Ma, Y.; Michael, K. H.; Nutt, S. R.; Williams, T. J.
Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy
Composites. Top Catal 2018, 61 (7–8). https://doi.org/10.1007/s11244-018-0917-2.
(2) Xu, P.; Li, J.; Ding, J. Chemical Recycling of Carbon Fibre / Epoxy Composites in a Mixed
Solution of Peroxide Hydrogen and N , N-Dimethylformamide. Compos Sci Technol 2013,
82, 54–59. https://doi.org/10.1016/j.compscitech.2013.04.002.
(3) Das, M.; Varughese, S. A Novel Sonochemical Approach for Enhanced Recovery of Carbon
Fiber from CFRP Waste Using Mild Acid − Peroxide Mixture. 2016.
https://doi.org/10.1021/acssuschemeng.5b01497.
(4) Campbell, F. Manufacturing Technology for Aerospace Structural Materials, 1st ed.;
Elsevier, 2006.
(5) Asmatulu, E.; Twomey, J.; Overcash, M. Recycling of Fiber-Reinforced Composites and
Direct Structural Composite Recycling Concept. J Compos Mater 2014, 48 (5), 593–608.
https://doi.org/10.1177/0021998313476325.
(6) Directive 2000/53/EC of the European Parliament and of the Council; European Union,
2000.
(7) Oliveux, G.; Dandy, L. O.; Leeke, G. A. Current Status of Recycling of Fibre Reinforced
Polymers: Review of Technologies, Reuse and Resulting Properties. Prog Mater Sci 2015,
72, 61–99. https://doi.org/10.1016/j.pmatsci.2015.01.004.
(8) Pimenta, S.; Pinho, S. T. Recycling Carbon Fibre Reinforced Polymers for Structural
Applications: Technology Review and Market Outlook. Waste Management 2011, 31 (2),
378–392. https://doi.org/10.1016/j.wasman.2010.09.019.
(9) Nakagawa, M.; Kuriya, H.; Shibata, K. Characterization of CFRP Using Recovered Carbon
Fibers From Waste CFRP. 5th International Symposium on Fiber Recycling (ISFR) 2009,
241–244.
(10) Maekawa, K.; Shibata, K.; Kuriya, H.; Nakagawa, M. Proceedings of the 60th Society of
Polymer Science Japan Annual Meeting; 2011; p 6.
(11) Gella, C.; Ferrer, È.; Alibés, R.; Busqué, F.; de March, P.; Figueredo, M.; Font, J. A Metal-
Free General Procedure for Oxidation of Secondary Amines to Nitrones. J Org Chem 2009,
74 (16), 6365–6367. https://doi.org/10.1021/jo901108u.
(12) Murray, R. W.; Iyanar, K.; Chen, J.; Wearing, J. T. Synthesis of Nitrones Using the
Methyltrioxorhenium/Hydrogen Peroxide System. J Org Chem 1996, 61 (23), 8099–8102.
https://doi.org/10.1021/jo961252e.
42
(13) Aurich, H. G.; Franzke, M.; Kesselheim, H. P. Aromatic Solvent-Induced Shifts in the 1H-
NMR Spectra of Nitrones. Tetrahedron 1992, 48 (4), 663–668.
https://doi.org/https://doi.org/10.1016/S0040-4020(01)88126-6.
(14) Hudson, A.; Betz, D.; Kühn, F. E.; Jiménez-Alemán, G. H.; Boland, W.
Methyltrioxorhenium. In Encyclopedia of Reagents for Organic Synthesis; John Wiley &
Sons, Ltd: Chichester, UK, 2013. https://doi.org/10.1002/047084289X.rn00017.pub3.
(15) Wang, Y.; Cui, X.; Ge, H.; Yang, Y.; Wang, Y.; Zhang, C.; Li, J.; Deng, T.; Qin, Z.; Hou,
X. Chemical Recycling of Carbon Fiber Reinforced Epoxy Resin Composites via Selective
Cleavage of the Carbon–Nitrogen Bond. ACS Sustain Chem Eng 2015, 3 (12), 3332–3337.
https://doi.org/10.1021/acssuschemeng.5b00949.
(16) Liu, T.; Zhang, M.; Guo, X.; Liu, C.; Liu, T.; Xin, J.; Zhang, J. Mild Chemical Recycling
of Aerospace Fiber/Epoxy Composite Wastes and Utilization of the Decomposed Resin.
Polym Degrad Stab 2017, 139, 20–27.
https://doi.org/10.1016/j.polymdegradstab.2017.03.017.
(17) Witik, R. A.; Gaille, F.; Teuscher, R.; Ringwald, H.; Michaud, V.; Månson, J.-A. E.
Economic and Environmental Assessment of Alternative Production Methods for
Composite Aircraft Components. J Clean Prod 2012, 29–30, 91–102.
https://doi.org/https://doi.org/10.1016/j.jclepro.2012.02.028.
(18) Witik, R. A.; Payet, J.; Michaud, V.; Ludwig, C.; Månson, J.-A. E. Assessing the Life Cycle
Costs and Environmental Performance of Lightweight Materials in Automobile
Applications. Compos Part A Appl Sci Manuf 2011, 42 (11), 1694–1709.
https://doi.org/https://doi.org/10.1016/j.compositesa.2011.07.024.
43
Chapter 3. Catalytic, Aerobic Depolymerization of Epoxy Thermoset
Composites
3.1. Introduction
This chapter mostly duplicates a manuscript published in Green Chemistry alongside co-
authors Yijia Ma, Katelyn H. Michael, Hanna M. Breunig, Steven R. Nutt and Travis J.
Williams.
1
Carbon fiber-reinforced polymers (CFRPs) are composite materials composed of long,
continuous strands of highly engineered carbon fibers (CFs) embedded within a thermoset polymer
matrix. These materials have superior strength-to-weight ratios and longer service lives than
conventional structural metals.
2
FRPs are thus widely used in aircraft, wind turbine blades, and
recreational equipment, with global consumption exceeding 128.5 kilotons in 2018.
3
The
irreversible curing process of the amine-linked epoxy thermoset matrices prevents component
separation or recycling via heating or remolding, as is possible with some thermoplastics.
Currently, there is no effective recycling method for end-of-life CFRP waste, nor is there a means
of recycling the ca. 30% of material discarded as scrap during manufacturing of many CFRP
composite products.
4
Landfilling is widespread practice.
A small fraction of CFRP waste is recycled, generally by pyrolysis or solvolysis, although
reports of chemical depolymerization are beginning to appear.
5
Pyrolysis downcycles composites
by heating to temperatures up to 700 °C, thus destroying the matrix and leaving low value,
disordered, short fiber mats. Solvolysis methods aim to dissolve the thermoset matrix, which
becomes plausible only at or near supercritical conditions. In some limited cases, this leaves CFs
for collection and repurposing, although typically with damage to the physical properties of the
44
fibers. Chemical depolymerization methods attempt to disassemble the matrix into constituent
monomers for re-use, freeing the CFs for subsequent reprocessing with little downcycling, if any.
Of these methods, both pyrolysis and solvolysis are energetically demanding, eliminate the value
of the matrix, and downcycle the CFs, thus limiting the value of the recyclates and capping (or
eliminating) the value available to support the cost of remanufacturing the material. Chemical
depolymerization can preserve ordered, long CF materials if mild conditions can be realized for
the breadth of chemical motifs that pervade the CFRP matrix systems in commercial use.
Alternative strategies to improve CFRP recyclability utilize resins containing reversible,
dynamic covalent bond exchange processes such as vitrimers or covalent adaptable networks. This
allows them to act as conventional thermosets under normal operating conditions, but may be
reprocessed like a thermoplastic when exposed to a stress, like heat, for extended periods of time.
6
Though clever and beneficial, these composites cannot be used for high-performance applications,
so there is still a grave need to recycle matrices found in current composites.
Strategies for chemical depolymerization of CFRPs commonly rely on strongly acidic, basic,
or oxidative conditions to cleave the matrix.
2,5
With few molecular-level studies known for these
reactions, our group determined a mechanism for peroxide-mediated decomposition of epoxy-
based CFRPs (Scheme 3.1, top).
7
We found that peroxide utilizes the very C—N bond that is the
basis of the thermoset polymerization reaction as the target for attack, initially by a fast oxidation
of the nitrogen, then through a slower, catalytic solvolysis of the polymer. CFs recovered from the
reaction appeared undamaged by the oxidative and acidic conditions of the process, although the
polymer matrix was destroyed in the reaction. The latter is a common feature of all methods that
we have encountered for recovery of CFs from amine-linked epoxy composites.
45
Scheme 3.1. Reaction conditions and results of (top) our previously reported recycling
process and (bottom) our new aerobic depolymerization process.
3.2. Results and Discussion
We report here the first aerobic, catalytic process to depolymerize epoxy CFRP materials
that enables both recovery of ordered carbon fibers and matrix materials (Scheme 3.1, bottom).
The new method uses catalytic aerobic oxidation to replace the peroxide of the prior system, thus
both obviating the challenges of hydrogen peroxide and supercritical solvent, while simultaneously
reducing the oxidative potential of the system such that matrix recyclate materials can be collected.
The mild nature of the reaction balances the need to intercalate reagents and catalysts into very
rigid CFRP materials (glass transition temperature, T
g
> ca. 180
o
C) while simultaneously
preserving the integrity, continuity, and order of the carbon fibers. It is moreover the first method
we know that enables recovery of woven fiber materials intact and preserves value from amine-
linked epoxy thermoset polymers.
46
Table 3.1. Summary of catalyst screening for small molecule demethylation (conversion
calculated by
1
H NMR).
Our survey of literature methods for cleavage of amine-linked epoxy thermosets revealed that
most involve oxygen, frequently added inadvertently, which we inferred was likely accessing a
mechanism analogous what we found for hydrogen peroxide.
8–10
With that insight, we set about to
design a process based on catalytic aerobic oxidation. Discovery of our process commenced with
identification of MnCl
2
and AlCl
3
, respectively as catalysts for the process’s oxidative and
solvolytic steps. We identified these by screening catalysts against aerobic demethylation of a
small molecule matrix analog, tetra-N-methyl-3,3’-diaminodiphenylsulfone (3.1). With some
experimentation, we found that superior results were obtained with manganese and ruthenium
among various chloride salts among various chloride salts of Fe, Mn, Cu, Co, and Ru that we
screened as possible oxidation catalysts. Table 3.1 shows representative rates of aerobic
dealkylation of our model compound in the presence of air and AlCl
3
with these metal salts.
Addition of salen or phenanthroline did not significantly improve performance, vide infra. Two
Lewis acids were identified in our prior studies for solvolysis of oxidized polymer, ScCl
3
and
AlCl
3
, and either sufficed to degraded the model compound within 17 hours in presence of MnCl
2
or RuCl
3
.
7
MnCl
2
and AlCl
3
were selected for further study due to their low cost and toxicity, and
because they are naturally abundant.
NMR data collected from degradation of tetra-N-methyl-3,3’-diaminodiphenylsulfone (
1
H and
COSY) showed no signals related to iminic CH
2
protons when the reaction was conducted with
47
O
2
/MnCl
2
in place of peroxide. This suggests that aerobic degradation proceeds through a
somewhat different path than was observed with hydrogen peroxide. While this mechanism
remains unknown, control reactions on the aerobic conditions show that both catalysts are
essential. A trial conducted without manganese in presence of air shows slow color change from
clear to brown, which is slowed when the headspace is flushed with an inert gas, indicating 1 is
prone to aerobic oxidation, but in the absence of either catalyst, no cleavage of the molecule is
observed.
To observe whether data collected from our small molecule model reactions translates to
thermoset polymer degradation, we prepared fiber-free matrix blocks from 3,3’-
diaminodiphenylsulfone (3,3’-DDS, 3.2) and the diglycidyl ether of bisphenol A (DGEBA, 3.3),
which are common monomers used in the manufacture of aerospace pre-impregnated (pre-preg)
fiber fabrics. The monomers were blended in a 2:5 ratio, which gives a lightly crosslinked solid
thermoset material with a T
g
of 51 °C (see Experimental Section for preparative details).
Upon attempting to disassemble these neat polymer samples, we found that a pre-treatment
step with an appropriate solvent for the polymer structure is essential to labializing the polymer.
Thus, these blocks were pre-treated in benzyl alcohol at 110 °C for 4 h before catalysts were
applied. This is known to weaken the adhesion between plies of CFRP materials and introduce
sites for reagents to intercalate.
11
In fact, we have observed that metal salt transport through CFRPs
seems to be facilitated by the channels of the fibers and inhibited by continuous polymer layers,
even in a system with no pre-treatment step.
12
In the present case, polymer degradation reactions
in acetic acid do not degraded the matrix, even at reaction temperatures above its T
g
, without benzyl
alcohol pre-treatment.
48
Once pre-treated, matrix block samples were homogenized overnight using MnCl
2
and either
ScCl
3
or AlCl
3
, with a continuous stream of air at reflux. While we examined ligated species, eg.
Mn(phen)
2
Cl
2
, these did not enable reaction.
13
Whilst this was effective with our small molecule
model, we conclude that metal salts intercalate the swollen matrix sufficiently for reaction, but
coordination complexes do not. Both catalysts are required: when either the oxidation or solvolysis
catalyst is absent, just over half of the sample was degraded. This is difficult to quantify whereas
solvent swelling cannot easily be separated from polymer homogenization. As expected, control
experiments without oxygen supply failed to degrade the material.
We demonstrated our overall process by manufacturing 4-ply, fully crosslinked amine-epoxy
composite panels of size 4 cm x 1.7 cm by curing 3.2 and 3.3 to a final T
g
of 160 °C (1:3 weight
ratio, see Experimental Section for preparative details). We chose to begin with panels prepared
in-house, because the polymer structures of existing commercial materials are proprietary, and we
wanted to be sure of the structure of our panels in the experiment.
Our fully cured composite panels, like those found in end-of-life CFRP waste, have higher T
g
s
than our neat polymer materials, thus making reagent and catalyst intercalation more challenging
with the composite panels. To mitigate this, we begin our process (Scheme 3.2) by swelling the
panels in benzyl alcohol for 4 hours at 200 °C, thus converting the rigid matrix to a pliable
consistency. After pre-treatment in benzyl alcohol, applying 10 atm of O
2
at 180 °C in acetic acid
with MnCl
2
and AlCl
3
for 43 h fully removes the matrix, leaving behind clean, woven fibers. The
resulting solution can be neutralized to yield a brown precipitate. See Experimental Section for
procedural details.
49
Scheme 3.2. Reaction conditions and products of the aerobic digestion of a fully cured
amine-epoxy carbon fiber composite.
The overall mass balance of recovered recyclates is very good. Typical commercial aerospace
CFRP pre-preg materials have ca. 65-70 wt% carbon fibers. In our model system, we recover 62%
of the mass of our composite as the woven fiber sheets, which represents quantitative recovery of
the fibers. An additional 11 wt% of the overall composite is recovered as organic materials that
are products of the thermoset matrix (vide infra), giving an overall 73% minimal mass efficiency
for the process. The remaining mass comprises small molecules in the digest solution. We are
currently optimizing our process to reclaim a higher portion of these organics.
The recovered carbon fiber fabric weaves represent a new class of recycled CF. Unlike other
reclaimed carbon fiber materials whose fiber lengths are difficult to control, our fiber length is
generally preserved, preventing its conversion into lower-value random short-fiber mats as are
recovered from pyrolysis processes.
14,15
The fibers remain woven in their original architecture,
which has very high value as carbon fiber weaving during manufacturing is an energy-intensive
50
process.
16
We are not aware of other methods for CFRP degradation that preserve the order of the
imbedded carbon fiber fabric.
Scheme 3.3. Conversion of recovered organics into commercial chemical bisphenol A
through a facile one-step reaction with quantitative yields.
As mentioned above, materials from both DDS and bisphenol A can be recovered from the
polymer digest solutions (Scheme 3.3). Upon neutralization of the crude digest solution, a brown
precipitate is obtained. Combustion analysis data show that this comprises the DDS monomer from
the original polymer matrix (Found: C, 60.99; H, 5.87; N, 1.54; S, 1.55). This is best characterized
as an oxidation polymer of DDS formed in situ with the degradation of the original thermoset.
17
Metal catalysts are known to effect this manner of aerobic coupling of anilines to form new N-N
and N=N bonds, which appears to be the case here.
18–20
The structure can be confirmed by Raman
and combustion analyses, particularly by a 2:1 ratio of nitrogen-to-sulfur in the latter. We have
previously characterized this material from peroxide oxidation of analogous materials, and we
have showed its unique utility as an accelerator for curing anhydride-based resins (Scheme 3.1,
bottom right) used in applications such as automotive manufacturing.
17
Thus, our recovered DDS
material can be upcycled into new composite materials through this path. The neutralized digestion
solution from aerobic degradation contains the derivatized monomer recovered from bisphenol A,
51
which is isolated as its bis(diacetylglyceryl) ether (3.4). This material can be converted
quantitatively to bisphenol A (3.5) by a neat reaction with KOH at 150 °C.
Composite product manufacturing is a characteristically inefficient process, with as much as
30% of pre-preg sheets being lost as cutting scrap.
21
This scrap is typically landfilled, much like
end-of-life CFRPs, even though it is not fully cured. Such materials cure slowly at ambient
temperature, so this waste stream represents a chemically analogous problem to that of recycling
end-of-life CFRPs, but with a much lower T
g
point. Thus, we applied our recycling process to
partially cured pre-preg scrap. Three samples of 1-ply commercial Cytec 5320-1/8HS pre-preg
were partially cured to varying degrees, representing different cure stages seen in pre-preg scrap
waste streams. We find that a simple DMSO wash is sufficient to recover clean carbon fiber tows
from ambient temperature-aged pre-pregs (Figure 3.1a). The same conditions remove most of the
matrix from partially cured pre-pregs of T
g
= 50 °C (Figure 3.1b). Treating these partially cleaned
fibers with MnCl
2
/AlCl
3
conditions in refluxing acetic acid with an air sparger (ambient pressure)
gave clean fibers after one week (Figure 3.1c). These data show us that our method can be applied
to a commercial aerospace pre-preg, even at ambient O
2
pressure.
52
Figure 3.1. Recovered fibers after 24 h DMSO wash from (a) room temperature aged pre-
pregs, (b) 110 ºC cured pre-pregs, with visible matrix residues, (c) 110 ºC cured pre-pregs
from (b) treated with aerobic conditions after 1 week, removing residual matrix.
Lowering the high energy intensity of CFRPs is a significant challenge facing its expanded use as
a structural material. To understand the viability of our proposed chemical upcycling process, we
estimated life-cycle primary energy use and compared it against the primary energy use of
landfilling and pyrolysis end-of-life pathways. We assumed landfilling recovers negligible energy
from the CFRP, while pyrolysis incurs a 10% energy penalty of manufacturing virgin carbon fiber
to recover 82% of the carbon fiber as short, disordered fibers which can be recycled only once
through this pathway due to degradation.
22
While it is unclear to what extent these recycled carbon
fiber will displace virgin, we assume a 50% offset from this recycling pathway. Chemically
recycling CFRPs requires an estimated 20% energy intensity of manufacturing virgin CF and resin
to recover 100% of the carbon fiber as sheets, offsetting 60% of virgin CF and 29% of virgin resin.
As the fibers are recovered still woven rather than disordered fibers, we adjusted the energy
intensity of compression molding by subtracting the energy intensity of sheet molding.
53
Figure 3.2. Primary energy analysis of the three most employed CFRP recycling methods.
The results of this study are summarized in Figure 3.2 (see 3.4.8 for additional
information). A single loop pyrolysis pathway lowers the total embodied energy of CFRP by 45%,
while cutting the CFRP manufacturing process energy use by 54% due to reduced virgin material
production. The pyrolysis process also adds a 4% energy penalty to the total manufacturing process
that uses a mix of virgin and recycled fibers. In comparison, single loop chemical recycling lowers
the embodied energy of CFRP by 56% at 2097 MJ/kg, 20% lower than that of pyrolysis. This
chemical upcycling adds a 6% energy penalty to the total manufacturing process which uses a mix
of virgin and recycled carbon fiber sheets with resin. It is worth noting these values detail a single
recycling loop. This represents the theoretical limit for once-through pyrolysis, but as the detailed
chemical recycling process preserves the native architecture of the carbon fiber sheet, it
realistically may proceed through several recycling loops and provide greater energy reductions
than what was estimated here.
54
3.3. Conclusion
In sum, we report the first catalytic, aerobic conditions for depolymerization of amine-
linked epoxy thermoset matrix polymers found in high-performance CFRPs. Simple metal salts,
MnCl
2
and AlCl
3
, catalyze the matrix oxidation and solvolysis, respectively, ultimately cleaving
the crosslinking C—N bonds, leaving undamaged carbon fiber sheets. Unlike more forcing
conditions for CFRP degradation, this approach enables collection of organic materials from the
polymer matrix, which are suitable for further upcycling. This technology has valuable
applications in reducing the accumulating waste from pre-preg scrap and end-of-life CFRPs.
3.4. Experimental Section
3.4.1. Materials and Methods
All NMR spectra were acquired on a Varian VNMRS 500 or 600 spectrometer and
processed using MestreNova. All chemical shifts are reported in units of ppm and referenced to
the residual
1
H or
13
C solvent peak and line-listed according to (s) singlet, (d) doublet, (t) triplet,
(dd) double doublet, etc. High pressure degradations of composite panels were performed within
a Series 4750 General Purpose 125 mL reactor from the Parr Instrument Company. GC/MS data
was acquired on an Agilent 5973/HP 6900 instrument.
3.4.2. Synthesis of 3.1
Scheme 3.4. Preparation of 3.1.
55
Compound 3.1 was prepared by charging a 50 mL round bottom flask with a magnetic stir
bar, 1.00 g 3,3’-diaminodiphenyl sulfone (3.2, 4.03 mmol, 1 equiv.), 2.50 g paraformaldehyde
(83.2 mmol, 21 equiv.) and 50 mL glacial acetic acid. The flask was sealed with a stopper and
purged under N
2
for 20 mins. Under inert gas, 2.60 g sodium cyanoborohydride (41.3 mmol, 10.2
equiv.) was added in portions with stirring, resealed, and heated to 60 °C for 16 h. Afterwards, the
reaction was neutralized using aqueous NaOH, then extracted with three portions of
dichloromethane. The combined organic fractions were rinsed with brine and dried over Na
2
SO
4
before concentrated on a rotary evaporator. The crude product was dry-loaded onto silica and
purified through column chromatography using a hexanes – ethyl acetate gradient (100:0 – 20:80),
and the isolated product was recrystallized from a 75% solution of methanol in water, 0.420 g.
1
H NMR (500 MHz, Chloroform-d) δ 7.29 (appt, J = 8.0 Hz, 2H), 7.25 (dd, J = 2.6, 1.8 Hz, 2H),
7.20 (dt, J = 7.7, 1.2 Hz, 2H), 6.83 – 6.80 (m, 2H), 3.00 (s, 12H).
13
C NMR (500 MHz, Chloroform-d) δ 150.54, 142.45, 129.71, 116.06, 114.90, 110.24, 40.32.
FTIR n
max
/cm
-1
3430 (OH, water), 2894, 1416, 1360 (CH
3
), 1296, 1066 (S=O).
mp 129.3 – 130.8 °C
GC/MS for C
16
H
20
N
2
O
2
S: calc’d 304.1, found 304.1 g/mol.
56
Figure 3.3.
1
H NMR spectrum of compound 3.1 at 25 °C in CDCl
3
.
Figure 3.4.
13
C NMR spectrum of compound 3.1 at 25 °C in CDCl
3
.
3.4.3. Catalyst Screening Studies
A series of catalysts and ligands were screened for aerobic demethylation of 3.1. To an 8-
dram vial, 50.0 mg 3.1 (0.164 mmol, 1 equiv.), 3.4 mg RuCl
3
(0.0164 mmol, 0.1 equiv.), 1 mL of
a 0.0164 M solution of AlCl
3
in glacial acetic acid (0.0164 mmol, 0.1 equiv.), and an additional 2
mL glacial acetic acid (3 mL final volume). The open vial was heated in an oil bath at 50 °C for
17 h. After, a 0.3 mL aliquot was removed and placed into an NMR tube containing 0.7 mL
57
chloroform-d and 3.0 µL of 1,2-dichloroethane (0.0379 mmol, 0.23 equiv.). From its
1
H NMR
spectra, the intensity of the aromatic and methyl protons of 3.1 were compared to the internal
standard 1,2-dichloroethane to calculate the amount of 3.1 remaining. This process was repeated
for other metal salts: FeCl
2
, MnCl
2
, CuCl, and CoCl
2
. Some trials added an extra 10 mol% of 1,10-
phenanthroline or N,N’-ethylenebis(salicylimine).
3.4.4. Preparation and Degradation of Lightly Crosslinked Amine-
Epoxy Polymer Blocks
Polymer cubes were prepared by mixing 3,3’-diaminodiphenylsulfone with diglycidylether
of bisphenol A in a 2:5 molar ratio (T
g
= 51 °C) and heated within a 120 °C oven to yield a clear,
homogeneous mixture. The resin was cured via a cycle consisting of a 1.5 °C/min ramp rate to
250 °C, and a dwell for 30 min.
Scheme 3.5. Degradation of a mildly crosslinked resin block.
A 2.00 g resin disk of 5 cm diameter and 1 mm thickness was cut into strips and, optionally,
pretreated by submerging and heating in benzyl alcohol at 110 °C for 4 h. The sample was then
added to a three-neck 500 mL round bottom flask fitted with a reflux condenser, thermometer, a
magnetic stir bar, and an air sparger. To the flask was added 150 mL glacial acetic acid and 0.200
g of an oxidation catalyst (MnCl
2
or Mn(phen)
2
Cl
2
) and solvolysis catalyst (AlCl
3
or ScCl
3
). The
reaction was heated to reflux with stiring and air bubbled into the solution for 1 day.
58
Reactions where the combination of catalysts homogenized the resin (MnCl
2
with AlCl
3
or
ScCl
3
), the reaction solvent was neutralized with an aqueous solution of NaOH and extracted three
times with ethyl acetate. The combined organic fractions were rinsed with brine, dried over
Na
2
SO
4
, and concentrated via rotary evaporator. The remaining oil was dry-loaded onto silica and
purified via column chromatography using a hexanes – ethyl acetate gradient (100:0 – 20:80) to
recover 0.021 mg of bisphenol A bis(diacetylglyceryl) ether (3.4). The material was also separately
synthesized following reported methods and characterization is in agreement with the literature.
23
1
H NMR (400 MHz, Chloroform-d) δ 7.16 – 7.09 (m, 4H), 6.82 – 6.76 (m, 4H), 5.35 (dtdd, J =
6.1, 5.1, 3.9, 0.9 Hz, 2H), 4.42 (ddd, J = 12.0, 4.0, 1.0 Hz, 2H), 4.28 (ddd, J = 12.0, 6.0, 1.0 Hz,
2H), 4.09 (dd, J = 5.1, 1.0 Hz, 4H), 2.08 (dd, J = 8.7, 1.0 Hz, 12H), 1.62 (s, 6H).
13
C NMR (101 MHz, Chloroform-d) δ 170.61, 170.29, 156.11, 143.77, 127.78, 113.96, 69.77,
65.96, 62.55, 41.74, 30.98, 20.96, 20.75.
Figure 3.5.
1
H NMR spectrum of compound 3.4 at 25 °C in CDCl
3
.
59
Figure 3.6.
13
C NMR spectrum of compound 3.4 at 25 °C in CDCl
3
.
3.4.5. Conversion of 3.4 to 3.5
Scheme 3.6. Conversion of 3.4 to Bisphenol A (3.5).
0.100 g of 3.4 (0.184 mmol, 1 equiv.), 0.103 g KOH (1.84 mmol, 10 equiv.), and a stir bar
was added to a 25 mL round bottom flask. The flask was sealed with a rubber septum, purged with
N
2,
and kept under an N
2
atmosphere. The flask was heated with stirring at 160 °C for 2 h, after
which the reaction was dry-loaded onto silica and purified via column chromatography using a
gradient of hexanes – ethyl acetate (100:0 – 20:80), exclusively yielding 0.041 g of 3.5 (98%).
60
1
H NMR (400 MHz, Chloroform-d) δ 7.16 – 7.03 (m, 4H), 6.81 – 6.64 (m, 4H), 4.75 (s, 2H), 1.62
(s, 8H).
13
C NMR (101 MHz, Chloroform-d) δ 153.27, 143.27, 127.91, 114.68, 41.67, 31.05.
Figure 3.7.
1
H NMR spectrum of compound 3.5 at 25 °C in CDCl
3
.
Figure 3.8.
13
C NMR spectrum of compound 3.5 at 25 °C in CDCl
3
.
61
3.4.6. Preparation and Degradation of Fully Crosslinked Amine-
Epoxy CFRP Panels
Completely cured amine-epoxy panels were prepared by mixing 3,3‘-
diaminodiphenylsulfone and diglycidylether of bisphenol A in a 1:3 weight ratio and cured using
a cure cycle consisting of a 1.5 °C/min ramp rate, a 120 °C dwell of 3 h, another 1.5 °C/min ramp
rate, and a post cure at 180 °C for 3 h.
Scheme 3.7. Aerobic recycling of a fully-cured CFRP panel.
A 0.974 g 4-ply composite laminate, prepared in-house as previously described, was pre-
treated by placing it in a 100 mL round bottom flask and adding benzyl alcohol until it is fully
submerged (~ 40 mL). The flask was heated at 200 °C for 4 h, then the flask was removed and let
cool. The softened composite was recovered from the flask, pat-dried, and rinsed with acetone.
The pre-treated composite was then placed in a 125 mL Parr reactor with a stir bar, 80 mL acetic
acid and 5 wt% MnCl
2
and AlCl
3
. The reactor was purged with O
2
gas three times, then charged
with 10 atm O
2
. The vessel was left in an oil bath with magnetic stirring for 43 h. Afterwards, the
reactor was removed from the oil bath and let cool to room temperature. Any remaining headspace
pressure was vented, and the reactor was opened. The reaction solvent was poured into an
Erlenmeyer flask, while woven fibers were removed using tweezers, washed with water and
acetone, and air-dried (597 mg).
62
The reaction solvent was neutralized with aqueous sodium hydroxide and extracted with
three portions of ethyl acetate. The combined portions of ethyl acetate were washed with brine,
dried over Na
2
SO
4
, and concentrated over rotary evaporator to yield 0.010 mg of an orange oil.
3.4.7. Preparation and Recycling of Partially-Cured Pre-Preg Scrap
Three types of pre-preg (eight-harness satin carbon fiber fabric T650-35 3K with
toughened epoxy resin Cycom 5320-1, 36% resin by weight) were prepared: room temperature
aged, where the samples were left outside at room temperature for 20 days, and two degrees of
partially cured pre-pregs, prepared by heating the pre-preg sample in an oven at 110 °C or 120 °C
for 3 h. Modulated differential scanning calorimetry was used to determine the final T
g
of the pre-
pregs (Figure S7), revealing pre-preg aged at room temperature had a T
g
of 42 °C, while those
cured at 110 °C and 120 °C had T
g
s of 50 °C and 87 °C, respectfully.
Figure 3.9. Glass transition temperature of pre-pregs aged at room temperature, cured at
110 °C for 3 h, and cured at 120 °C for 3 h.
Solvent washes were conducted by partially submerging the pre-preg into a crystallizing
dish containing DMSO for a day. Undissolved epoxy matrices were recovered from solution via
63
filtration. Clean carbon fiber fabrics were recovered from room temperature-aged samples, as well
as 14 wt% of epoxy matrix. 110 °C cured pre-pregs contained fibers with matrix residues still
present, as well as 22 wt% of undissolved epoxy matrix. The fabric-containing residue was treated
with the aerobic recycling conditions by reacting it with 1 wt% MnCl
2
and AlCl
3
submerged in
glacial acetic acid in round bottom for 7 days, after which clean fiber tows were recovered. For
pre-pregs cured at 120 °C, there was almost no change after DMSO wash and only 1 wt% of
undissolved epoxy was recovered. The pre-preg was similarly treated with the aerobic degradation
reaction, reacting it with 1 wt% MnCl
2
and AlCl
3
, submerged in glacial acetic acid within a round
bottom flask for 7 days. Residue-free and soft fabrics were not recovered, though the material was
noticeably softer.
3.4.8. Life Cycle Primary Energy Consumption Study
Lowering the high energy intensity of CFRP is a significant challenge facing its expanded
use as a structural material. In this analysis we estimate life-cycle primary energy consumption for
our proposed recycling process and compare it with landfilling and pyrolysis end-of-life pathways.
Values for pyrolysis and chemical recycling are provided for a single recycling loop, representing
the theoretical limit for once-through pyrolysis. As chemical recycling preserves carbon fibers in
the carbon fiber sheet and can potentially offer indefinite looping, the manufacturing of virgin
carbon fiber is expected to reduce beyond what is estimated in this study. While a complete life-
cycle assessment is outside the scope of this study, we note the importance of estimating raw
material consumption, greenhouse gas emissions and other air pollutant emissions and their
associated impacts in a future study.
On-site and primary energy intensities (Btu/lb) to produce virgin carbon fibers and resin
(epoxy thermoset), and for compression molding were taken from the DOE 2017 Bandwidth study
64
on CFRP manufacturing.
16
Values were based on 2010 production data with primary energy values
reflecting 32.3% energy losses from offsite electricity generation and transmission. Manufacturing
processes are shown in Figure 3.10. Mass flows were estimated assuming the same material losses
between sub-processes as reported in Das 2011, and by adjusting the carbon fiber weight content
from 50% to 65% to better represent the material in this study.
24
Figure 3.10. Processes and material flows used to model primary energy consumption. Use
phase is the same for all pathways and not explicitly modeled. We assume there is no energy
generated from landfill gas derived from CFRP.
In the case where used CFRP is sent to a landfill, we assumed materials were transported
100 km prior to shredding, and then transported 300 km to a landfill. Transportation distances are
assumed from Witik et al., 2013.
22
For truck transportation of materials, we assume a class 7 truck
weighing 28,000 pounds carries 75% of the remaining maximum allowed truck-weight in
California (80,000 pounds), generating a payload of 39,000 pounds per truck of material. We use
the EPA weight-based fuel consumption factor of 11.3 gallons/1000 ton-miles and a diesel energy
density of 38.6 MJ/L. The energy intensity of the shredder was estimated from a commercial large
single-shaft shredder capable of processing up to 20,000 lb/hr with an average loading rate of 30%.
We believe our estimate of 0.1 MJ/kg direct energy is reasonable given Witik et al. 2013 used a
65
value of 0.0025 MJ/kg, but energy for size reduction of PET plastics has been reported at values
ranging from 1.7 to 5.2 MJ/kg primary energy. Shredders were assumed to operate on electricity
and were adjusted for upstream losses. We use a scale up factor of 1.2 for diesel consumption to
reflect upstream processes associated with diesel production.
In the case where used CFRP is sent to a pyrolysis facility, we assumed materials were
transported 1000 km prior to shredding and pyrolysis, followed by the transport of ash 300 km to
a landfill, and the transport of recovered carbon fiber 1000 km for recycling. Transport and size
reduction are modeled using the same assumptions as done for the landfill case. Pyrolysis is
assumed to generate energy that will offset both heating oil and electricity for onsite use with no
transmission losses. The energy penalty of pyrolysis of CFRP is not easily known, and as such, we
assume an energy penalty equal to 10% of manufacturing virgin carbon fiber, as was assumed in
Witik et al. 2013. Primary energy from the process is assumed using the same fuel to electricity
ratio and upstream generation and transmission energy losses as primary manufacturing processes.
The process is estimated to recover 82% of carbon fiber, in a short fiber form, and we assumed
carbon fiber can only be recycled one time through this pathway due to the degradation of the
fibers. While it is unclear to what extent these carbon fibers will replace virgin carbon fiber, we
assumed a 50% offset resulting from this recycling pathway. The production of resin and the
compression molding sub-processes remain unchanged. We assume 8% of the resin is ash, which
is sent to landfill.
In the case where used CFRP is sent to a chemical recycling facility based on the process
presented in this study, we assumed materials were transported 1000 km prior to hydrolysis and
polymerization, followed by the transport of residuals 200 km to a landfill, and the transport of
recovered carbon fiber sheets and resin 1000 km for recycling.
66
As industrial scale data is not available, we assume an energy intensity equal to 20% of
manufacturing virgin carbon fibers and resin, as is reported for a commercial chemical recycling
process. Primary energy from the process is assumed using the same fuel to electricity ratio and
upstream generation and transmission energy losses as primary manufacturing processes. Lab
scale experiments presented in this study were used to determine mass flows for these life cycle
stages. The process is estimated to recover 100% of carbon fiber, in a sheet form, and we assumed
a 60% offset of virgin carbon fiber, and 29% offset of virgin resin. We adjusted the energy intensity
of compression molding to reflect the use of carbon fiber sheets rather than fibers by subtracting
the energy intensity of sheet molding.
We find that virgin CFRP has an embodied energy of 4739 MJ/kg, with negligible energy
associated with its landfilling (Figure 3.2). A single loop pyrolysis pathway lowers the embodied
energy of CFRP by 45%, with the pyrolysis process cutting the CFRP manufacturing process
energy consumption by 54% due to reduced virgin material production. The pyrolysis process adds
a 4% energy penalty to the new manufacturing process that uses a mix of virgin and recycled
fibers. In comparison, the single loop upcycling pathway lowers the embodied energy of CFRP by
56% at 2097 MJ/kg, 20% lower than CFRP from single loop pyrolysis. The upcycling process
adds a 6% energy penalty to the new manufacturing process that uses a mix of virgin carbon fiber
and resin and recycled carbon fiber sheets and resin.
Key uncertainties remaining include the true energy intensity of pyrolysis and upcycling.
Additionally, the number of recycling cycles that both pyrolysis and upcycling can offer will play
a major role on the theoretical limit of energy reduction that upcycling can offer. Assuming
thousands of cycles of upcycling drives the production of virgin materials to negligible levels,
would drive CFRP to an embodied energy intensity of 142 MJ/kg (95% reduction). More
67
realistically, some virgin materials will be necessary to ensure the quality of product. Finally, the
potential coupling of upcycling and pyrolysis as an end-of-life opportunity to generate energy from
the upcycling residues would be important to evaluate in a future analysis.
3.5. References
(1) Navarro, C. A.; Ma, Y.; Michael, K. H.; Breunig, H. M.; Nutt, S. R.; Williams, T. J.
Catalytic, Aerobic Depolymerization of Epoxy Thermoset Composites. Green Chemistry
2021, 23 (17), 6356–6360. https://doi.org/10.1039/D1GC01970H.
(2) Karuppannan Gopalraj, S.; Kärki, T. A Study to Investigate the Mechanical Properties of
Recycled Carbon Fibre/Glass Fibre-Reinforced Epoxy Composites Using a Novel Thermal
Recycling Process. Processes 2020, 8 (8), 954. https://doi.org/10.3390/pr8080954.
(3) Sauer, M. Composites Market Report 2019; 2019.
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Manufacturing Waste Flows through Process Mapping. J Clean Prod 2015, 91, 251–261.
https://doi.org/https://doi.org/10.1016/j.jclepro.2014.12.033.
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Chemistry Look at Composites Recycling. Mater Horiz 2020, 7 (10).
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(6) Kloxin, C. J.; Bowman, C. N. Covalent Adaptable Networks: Smart, Reconfigurable and
Responsive Network Systems. Chem Soc Rev 2013, 42 (17), 7161–7173.
https://doi.org/10.1039/c3cs60046g.
(7) Navarro, C. A.; Kedzie, E. A.; Ma, Y.; Michael, K. H.; Nutt, S. R.; Williams, T. J.
Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy
Composites. Top Catal 2018, 61 (7–8). https://doi.org/10.1007/s11244-018-0917-2.
(8) Huan, X.; Wu, T.; Yan, J.; Jia, X.; Zu, L.; Sui, G.; Yang, X. Phosphoric Acid Derived
Efficient Reclaimation of Carbon Fibre for Re-Manufacturing High Performance Epoxy
Composites Reinforced by Highly-Aligned Mat with Optimized Layup. Compos B Eng
2021, 211, 108656. https://doi.org/https://doi.org/10.1016/j.compositesb.2021.108656.
(9) Liu, T.; Zhang, M.; Guo, X.; Liu, C.; Liu, T.; Xin, J.; Zhang, J. Mild Chemical Recycling
of Aerospace Fiber/Epoxy Composite Wastes and Utilization of the Decomposed Resin.
Polym Degrad Stab 2017, 139, 20–27.
https://doi.org/10.1016/j.polymdegradstab.2017.03.017.
(10) Wang, Y.; Cui, X.; Ge, H.; Yang, Y.; Wang, Y.; Zhang, C.; Li, J.; Deng, T.; Qin, Z.; Hou,
X. Chemical Recycling of Carbon Fiber Reinforced Epoxy Resin Composites via Selective
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Cleavage of the Carbon–Nitrogen Bond. ACS Sustain Chem Eng 2015, 3 (12), 3332–3337.
https://doi.org/10.1021/acssuschemeng.5b00949.
(11) Nandi, S.; Winter, H. H. Swelling Behavior of Partially Cross-Linked Polymers: A Ternary
System. Macromolecules 2005, 38 (10), 4447–4455. https://doi.org/10.1021/ma048335e.
(12) Lo, J. N.; Nutt, S. R.; Williams, T. J. Recycling Benzoxazine–Epoxy Composites via
Catalytic Oxidation. ACS Sustain Chem Eng 2018, 6, 7227–7231.
https://doi.org/10.1021/acssuschemeng.8b01790.
(13) McCann, S.; McCann, M.; Casey, M. T.; Jackman, M.; Devereux, M.; McKee, V. Syntheses
and X-Ray Crystal Structures of Cis-[Mn(Bipy)2Cl2] · 2H2O · EtOH and Cis-
[Mn(Phen)2Cl2] (Bipy = 2,2′-Bipyridine; Phen = 1,10-Phenanthroline); Catalysts for the
Disproportionation of Hydrogen Peroxide. Inorganica Chim Acta 1998, 279 (1), 24–29.
https://doi.org/https://doi.org/10.1016/S0020-1693(98)00031-0.
(14) Fernández, A.; González, C.; Cayumil, F. A. L. E.-R. K. E.-R. Characterization of Carbon
Fibers Recovered by Pyrolysis of Cured Prepregs and Their Reuse in New Composites. In
Recent Developments in the Field of Carbon Fibers; Lopes, C. S., Ed.; IntechOpen: Rijeka,
2018; p Ch. 7. https://doi.org/10.5772/intechopen.74281.
(15) Oliveux, G.; Dandy, L. O.; Leeke, G. A. Current Status of Recycling of Fibre Reinforced
Polymers: Review of Technologies, Reuse and Resulting Properties. Prog Mater Sci 2015,
72, 61–99. https://doi.org/10.1016/j.pmatsci.2015.01.004.
(16) Bandwidth Study on Energy Use and Potential Energy Saving Opportunities in U.S. Carbon
Fiber Reinforced Polymer Manufacturing; 2017.
(17) Ma, Y.; Navarro, C. A.; Williams, T. J.; Nutt, S. R. Recovery and Reuse of Acid Digested
Amine/Epoxy-Based Composite Matrices. Polym Degrad Stab 2020, 175.
https://doi.org/10.1016/j.polymdegradstab.2020.109125.
(18) Ryan, M. C.; Martinelli, J. R.; Stahl, S. S. Cu-Catalyzed Aerobic Oxidative N–N Coupling
of Carbazoles and Diarylamines Including Selective Cross-Coupling. J Am Chem Soc 2018,
140 (29), 9074–9077. https://doi.org/10.1021/jacs.8b05245.
(19) Fritsche, R. F.; Theumer, G.; Kataeva, O.; Knölker, H.-J. Iron-Catalyzed Oxidative C−C
and N−N Coupling of Diarylamines and Synthesis of Spiroacridines. Angewandte Chemie
International Edition 2017, 56 (2), 549–553.
https://doi.org/https://doi.org/10.1002/anie.201610168.
(20) Wheeler, O. H.; Gonzalez, D. Oxidation of Primary Aromatic Amines with Manganese
Dioxide. Tetrahedron 1964, 20 (2), 189–193. https://doi.org/https://doi.org/10.1016/S0040-
4020(01)93207-7.
69
(21) Asmatulu, E.; Overcash, M.; Twomey, J. Recycling of Aircraft : State of the Art in 2011.
2013, 2013.
(22) Witik, R. A.; Teuscher, R.; Michaud, V.; Ludwig, C.; Månson, J.-A. E. Carbon Fibre
Reinforced Composite Waste: An Environmental Assessment of Recycling, Energy
Recovery and Landfilling. Compos Part A Appl Sci Manuf 2013, 49, 89–99.
https://doi.org/https://doi.org/10.1016/j.compositesa.2013.02.009.
(23) Glazer, J. Monolayer Studies of Some Ethoxylin Resin Adhesives and Related Compounds.
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https://doi.org/https://doi.org/10.1002/pol.1954.120137004.
(24) Das, S. Life Cycle Assessment of Carbon Fiber-Reinforced Polymer Composites. Int J Life
Cycle Assess 2011, 16 (3), 268–282. https://doi.org/10.1007/s11367-011-0264-z.
70
Chapter 4. Incorporating Singlet Oxygen into CFRP Recycling &
Manufacturing
4.1. Introduction
Carbon fiber reinforced polymer (CFRP) composites continue to gain traction and
dominate as the premier structural material for manufacturing in industries where weight reduction
is crucial. Global demand for CFRPs in 2018 reached 128.5 kilotons, with the bulk of the material
utilized in aerospace, wind turbine blades, and recreational consumer equipment.
1
Despite their
popularity, CFRP recycling is difficult due to the irreversible curing process of the amine-epoxy
thermoset matrix which binds and maintains the alignment of the carbon fiber reinforcement.
Ongoing research into effective composites recycling emphasize recovering the carbon fibers with
as little disruption to its length, weave architecture, and surface chemistry to maximize their re-
use potential in remanufactured composites.
2
Diverse reaction conditions for chemical recycling of CFRPs have been reported, applying
acidic, basic, or oxidative conditions. Our group has previously proposed a mechanism detailing
the molecular events which degrade amine-cured epoxy CFRPs under acidic peroxide conditions.
From these conditions we recover carbon fibers, chemically undamaged, while the polymer is
over-oxidized and degraded into small, low-value organic compounds.
3
We built on this
foundation while reducing the hazards associated with scaling this technology to industrial levels
by exchanging peroxide with aerobic oxygen as the terminal oxidant and added manganese and
aluminum catalysts, respectively to aid with the oxidation and elimination steps.
4
However, this
process introduces a new limitation in long reaction times of 44 hours. Here, we discuss a
modification to the aerobic recycling process where we find photo-chemically generated singlet
71
oxygen (
1
O
2
) can degrade our small molecule model of amine-cured polymer matrices four times
as quickly. We also report a diffusion experiment tracking permeation of O
2
through a solid
polymer structure using fluorescence microscopy, as well as an early concept of a potentially new
class of
1
O
2
labile thermoset resin additives for facile recycling after their end-of-life.
4.2. Results and Discussion
4.2.1.
1
O
2
Degradation of Molecular Models
Preliminary degradation experiments probing the efficacy of
1
O
2
for CFRP degradation
began by studying fully cured composites prepared from the bi-directional, bis-aniline curing agent
3,3’-diaminodiphenylsulfone (3,3’-DDS, Figure 4.1, 4.1) and the bi-directional epoxide diglycidyl
ether of bisphenol A (Figure 4.1, 4.2). Rose bengal (Figure 4.1, 4.3) and 495 nm light from blue
LEDs were used photochemically to generate
1
O
2
. Two parallel CFRP degradation experiments
were conducted in benzyl alcohol with a 7-fold excess (relative to 4.1) of K
3
PO
4
under an oxygen
gas atmosphere at 180 ºC, except one experiment contained a catalytic amount of the
photosensitizer rose bengal (relative to 4.1) and was exposed to a blue LED, while the other
excluded light. Excitingly, after 40 h we saw the sample treated with
1
O
2
separated into its
individual plies in solution, still stiff with resin, while the sample treated with
3
O
2
persisted as a
laminate until the plies were physically separated using tweezers. This suggests that the solvent
lifetime of
1
O
2
may be long enough for a reaction to occur with the thermoset polymer at
temperatures above the material’s glass transition temperature (T
g
). Crossing this temperature
threshold is key for reagents to intercalate throughout the composite and enable degradative
chemical reactions both from the outside and within. Motivated by these results, we applied
1
O
2
conditions to our small molecule analog of the thermoset polymer, a tetra-N-methylated version of
3,3’-DDS (Figure 4.1, 4.4), to gain mechanistic insights of the cleavage pathway.
72
Figure 4.1. Structures of key compounds studied within this chapter.
We analyzed the molecular events that enable polymer degradation through the study of
4.4 as a homogeneous model system. We found significant degradation rate and reactivity
advantages afford by
1
O
2
on 4.4 in acetic acid compared to identical conditions under
3
O
2
. The
more active
1
O
2
consumed the N-methyl
1
H NMR singlet much more rapidly compared to
3
O
2
across several temperatures: at 60 ºC,
1
O
2
degraded half of 4.4 within 5 h, while
3
O
2
failed to
degrade even 5%. At 100 ºC,
3
O
2
degraded 96% of 4.4 in 4 h, while
1
O
2
reached these levels after
just 1 h.
1
H NMR reaction monitoring does not show clear conversion of 4.4 to another discrete
compound, but rather a variety of different compounds whose signals fluctuate over the time
course, suggesting several active degradation pathways.
S
O
O
NH
2
H
2
N
3,3’-diaminodiphenyl sulfone
(3,3’-DDS, 4.1)
bi-directional curing agent
O O
O O
diglycidyl ether of bisphenol A (4.2)
bi-directional epoxide
Na
+
Na
+
O
O
O
-
O
-
O
I
I I
I
Cl
Cl
Cl
Cl
rose bengal (4.3)
singlet oxygen photosensitizer
S
O
O
N N
Me
4
DDS (4.4)
small molecule polymer analog
O
trans-1-(2’-methoxyvinyl)pyrene (4.5)
singlet oxygen sensor
O
1-pyrenecarboxaldehyde (4.6)
tripped singlet oxygen sensor
73
Figure 4.2.
1
H NMR kinetics demonstrating degradation rates of 4.4 under triplet and singlet
oxygen at 60 ºC and 100 ºC.
Figure 4.3. Tentative structural assignments of mass signals identified from LC-QTOF from
the aerobic degradation of 4.4.
Both reactions follow first-order kinetics and share similar mass fragmentation patterns
after analysis of aliquots by liquid chromatography – quadrupole time of flight mass spectrometry
(LC-QTOF). Mass fragments corresponding to the sequential dealkylation of 4.4 were identified
in both reactions, indicating cleavage of the key C – N bonds formed during the resin curing stage.
Assignments of the 307 m/z and 321 m/z peaks as hydroxylated species are supported by
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200 250 300
[MeDDS]/[MeDDS]initial
Time (min)
3O2 100 C
1O2 100 C
3O2 60 C
1O2 60 C
74
agreement with the isotopic ratios of the [M+1], [M+2], and [M+3] ion peaks. These species can
be rationalized as the products formed from the thermal scission of the hydroperoxide O – O bond
formed after addition of oxygen.
5
Larger mass signals were also observed that were apparently
dimeric, which we suspect are hydrazine-functionalized dimers that we have identified
previously.
4
Previous studies on dye-sensitized photo-oxidation of aliphatic amines have identified
at least two available chemical mechanisms: amine oxidation by
1
O
2
and radical amine species
formation by the photo-excited dye and
3
O
2
.
6
We suspect the same is true here, especially as the
reaction rate is significantly reduced under an inert atmosphere or at reflux, when O
2
gas
availability is low, or in the presence of a radical inhibitor like butylated hydroxytoluene.
4.2.2. Tracking Movement of
1
O
2
Within Polymers
Permeation and intercalation of gases into thermoset polymers is an under-studied area of
research. To better understand the movement of O
2
through these amine-cured epoxy polymers,
we synthesized a series of partially crosslinked resin cubes spiked with a known molecular probe
selective for detecting
1
O
2
, 1-(2’-methoxyvinyl)pyrene (Figure 4.1, 4.5). Exposure of 4.5 to
1
O
2
initiates a known [2+2] cycloaddition reaction which collapses to form 1-pyrenecarboxaldehyde,
4.6 (Figure 4.1). Comparing fluorescence data of 4.5, 4.6, and 4.3 in glacial acetic acid shows a
unique signal from 4.6 at 468 nm after excitation at 394 nm, providing a useful chemical proxy for
detecting and visualizing the presence of
1
O
2
through the internal area of polymer sample. We
prepared a series of 1 cm
3
resin cubes composed of a 40% ratio of 4.1: 4.2 (T
g
= 60 ºC), spiked
with nanomole quantities of 4.5, and monitored the cube internal cross-section via confocal
fluorescence spectroscopy and tracked fluorescence signals using fluorescence lifetime imaging
(FLIM). FLIM produces a color-coded heat-map contrasting the different fluorescence lifetimes
75
present, providing a visual tool for tracking the quantity of 6 present in the cube based on how the
loci of the fluorescence signal shifts (Figure 4.5).
Figure 4.4. Fluorescence signal comparison of 4.3, 4.5, and 4.6 after excitation at 394 nm.
Figure 4.5. (Left) Cross-sectional images of partially cross-linked resin cubes spiked with 4.5
in acetic acid with rose bengal at 100 ºC under varied reaction conditions: (left column) 1
atm O
2
with 495 nm light; (center column) 1 atm O
2
with no light; (right column) 1 atm N
2
with 495 light. (Right) Sample FLIM of a resin cube at t = 0 min illustrating separation of
compounds based on average fluorescence lifetime. The right corner of the arc has brief
fluorescence lifetimes, while long phosphorescence lifetimes are in the left corner of the arc.
A color-spectrum bar shades the cross-sectional images on the left based on each individual
pixel’s fluorescence lifetime.
0
100
200
300
400
500
600
700
800
900
1000
300 400 500 600 700 800 900
Intensity
Emission Wavelength (nm)
4.5
4.6
4.3
76
This experiment yielded many interesting results. Some of them were expected: cube
degradation rate increases as the gas headspace changes from N
2
, to
3
O
2
,
1
O
2
. We expected to would
witness a gradual signal increase starting from the cube exterior toward the interior as O
2
permeates
the cube, but interestingly, we see a broad increase in fluorescence throughout the entire cube
within an hour of reaction. This proliferates throughout the whole material within 3 hours. Given
that we see that rose bengal does not intercalate the resin cube (cross-section does not show the
characteristic rose bengal stain) and
1
O
2
should have a solution lifetime on the order of milliseconds
under these conditions, the polymer itself must also be acting as a photosensitizer. Preliminary
experiments with catalytic amounts of 4.1 with known
1
O
2
detector 1,3-diphenylisobenzofuran
under an O
2
atmosphere does show
1
O
2
generation, which is in good agreement with reported
instances of the para-isomer of 4.1 acting as a photo-irritant on skin products.
7
Future experiments
will reveal how efficient polymers based on 4.1 and 4.2 are as photosensitizers for generation of
1
O
2
, but these results are promising as they suggest that the polymer itself may aid in generating
reactive oxygen species with UV light. Regardless, this indicates that gas intercalation through
rigid thermosets, and thus their composites, is easier than that of metals and organic compounds,
as seen in our group’s previous experience tracking diffusion of cerium through composite plies.
8
We also see that
1
O
2
preferentially exploits any surface or internal bubble defects in the solid
polymer thus facilitating polymer degradation around defects. Cubes treated under
3
O
2
conditions
(Figure 4.5, center column) showed a gradually stronger fluorescence signal for 4.6, suggesting
either background generation of
1
O
2
by residual visible light or a side-reaction which that 4.5.
Either way, the formation of 4.6 under these conditions is much slower than seen under deliberate
1
O
2
conditions (Figure 4.5, left column). The cube treated under an N
2
atmosphere (Figure 4.5,
right column) features a consistent strong green signal for 4.6 after an hour and persisted at that
77
level. Residual dissolved O
2
present in the solvent may have been photoactivated and consumed,
reacting with the sensor until it was fully consumed. The N
2
-treated cube persisted virtually
unreacted under these conditions, emphasizing the necessity of an oxidant for degradation.
4.2.3.
1
O
2
Recycling of CFRP Panels
Encouraged by these results, we conducted some preliminary degradation experiments
utilizing a modified pressure reactor with a customized LED light insert to provide the necessary
photon to generate
1
O
2
while still applying elevated pressures. Limited by the operational
temperature of the LED probe, we degraded two identical carbon fiber composites based on a 60%
ratio of 4.1: 4.2 (T
g
= 100 ºC) in acetic acid for 15 h: one under 6 atm O
2
and the other under a
mixed pressure of 4 atm N
2
and 2 atm O
2
with rose bengal photosensitizer. We find that with
1
O
2
we can induce effective resin removal without an elimination catalyst faster than with
3
O
2
conditions featuring three times the pressure (Figure 4.7), but more work is needed to achieve full
resin removal. We know additional design and engineering work is necessary to prolong the
performance and life of the LED under these oxidative and corrosive conditions, but the safety and
risk reduction benefits of applying lower oxygen pressures is more valuable.
78
Figure 4.6. A customized Parr reactor for pressurized photochemistry featuring an LED.
Figure 4.7. Degradation of 60% 4.1: 4.2 CFRP panels under
3
O
2
(left) and
1
O
2
(right) for 15
hours.
After these experiments with
1
O
2
-mediated degradation, we thought about the expanded
popularity and commercialization of ‘disconnectable’ composite matrix systems based on
reversible chemical reactions (e.g. Diels-Alder, imine, acetal, or aminal hydrolysis) enabled by
heat or acid.
9
Widespread adoption of these resins in automobile manufacturing (cars, trucks,
planes, etc) has been limited, because these vehicles often encounter these recycling triggers of
high temperatures or environmental acids which risk warping, deforming, or degrading the
79
composite. Natural generation of
1
O
2
, however, is comparably much rarer for a vehicle to encounter
during their service life, so we have ideated a series of di-amine and di-epoxide resin monomers
that have
1
O
2
-labile functional groups and allow for cleavage, or conversion to a new, easily
cleavable polymer (Scheme 4.1). Furans like 4.7 are known to undergo addition reactions with
1
O
2
which result in the formation of an unsaturated diester which are easily cleaved under known
transesterification conditions. Electron-rich olefins such as 4.8 may similarly react with
1
O
2
and
cleave the olefin to form two new carbonyl species.
10
Including these di-epoxides or di-amines as
drop-in replacements for 1 – 10 mol% of the commonly used 4.1 or 4.2 can yield a new resin
system which are either degraded under
1
O
2
or converted to more easily recyclable chemical
motifs. Future work on this project is underway by Justin Lim, who is actively working on the
synthesis and purification of 4.7 and 4.8. Once purified, Lim will investigate what effect these
additives have on the final polymer’s physical properties and assess if their incorporation improves
their degradation rate under
1
O
2
conditions.
Scheme 4.1. Proposed structure and degradation of resin monomers modified with
1
O
2
-labile
functional groups for facile end-of-life recycling.
4.3. Conclusion
Aerobic recycling of amine-epoxy thermoset carbon fiber composites allows for recovery
of clean and ordered carbon fiber weaves and organic byproducts from the polymer for use in
O
O O
O
O
NH
2
H
2
N
1
O
2
cycloaddition
4.7
4.8
O O
O
O
O O
O
H
2
N
NH
2
O
modified diepoxide
and diamine resin monomers
cleaved or easily cleavable
resin monomers
80
casting new resins. This chemical depolymerization strategy offers a significant increase in
recyclate value compared to what is reclaimed from existing pyrolysis methods, but this strategy
will be difficult to scale, as it relies on elevated oxygen pressures (10 atm) for oxidation. We have
preliminary evidence indicating photochemically generated
1
O
2
can enable similar degradation of
amine-epoxy thermosets under reduced pressures (~ 2 atm), improving the safety and scalability
of this process. Additional work is underway to evaluate the efficacy of
1
O
2
for degrading
thermoset polymers and what effect they may have on the surface chemistry of the recovered
fibers. Heartened by these results, we also envision a new class of resin monomer additives which
contain
1
O
2
-labile functional groups, providing a chemical handle for easier end-of-life recycling
under our reported conditions.
4.4. Experimental Section
4.4.1. Materials and Methods
All NMR spectra were acquired on a Varian VNMRS 500 or 600 spectrometer and
processed using MestreNova. All chemical shifts are reported in units of ppm and referenced to
the residual
1
H or
13
C solvent peak and line-listed according to (s) singlet, (d) doublet, (t) triplet,
(dd) double doublet, etc. High pressure degradations of composite panels were performed within
a custom-built Series 4760 250 mL reactor from the Parr Instrument Company containing a blue
Cree XP-E LED soldered onto electrical contacts and powered by a DC power supply at 2.5 V.
LC/MS data was acquired on an Agilent 6545 LC/Q-TOF instrument under a solvent bypass
method of 100% water for 3 minutes. All fluorescence microscopy data was collected using a
LEICA SP-8X microscope with a multiphoton laser attachment.
81
4.4.2. General Procedure for
3
O
2
and
1
O
2
Degradation Reactions
Small molecule
3
O
2
degradation studies of 4.4 were conducted by mixing 50 mg (0.16
mmol) of 4.4 with 3 mL acetic acid, 17 μL (1 equiv.) of 1,1,2,2-tetrachloroethane (NMR internal
standard), and a magnetic stir bar in a 10 mL round bottom flask. The flask was sealed with a
rubber septum tied with an aluminum wire and maintained under an O
2
environment with a
balloon. Reaction progress was monitored by removing 0.2 mL aliquots via syringe, diluting with
0.5 mL d-MeCN, and analyzing by
1
H NMR and LC-QTOF.
1
O
2
degradation studies were similarly
conducted except 7 mg (0.008 mmol, 5 mol %) rose bengal was included with the reagents and a
blue LED was shone on the flask at 2.5 V.
Partially cured composite panels were prepared as previously reported at a 60% ratio of
amine:epoxy.
3
Panels were placed in a modified 250 mL Parr reactor and sufficient acetic acid was
added until they were submerged, ca. 140 mL, along with a magnetic stir bar. For
3
O
2
degradation
studies, the Parr was then sealed, charged with 6 atm O
2
, and heated in an oil bath at 120 ºC for 15
h.
1
O
2
studies were performed similarly except 50 mg rose bengal was added, the head attachment
of the modified Parr was replaced with a blue LED connected to a DC power supply at 2.5 V, and
the Parr was charged with 2 atm O
2
and 4 atm N
2
. After 15 h, composite panels were removed,
washed with water and acetone, then let air dry.
Resin cubes of volumes 1 cm
3
each containing a 40% ratio of amine:epoxy resin were
prepared with 0.85 mM 4.5 by preparing a solution of ~2 mg 4.5 in dichloromethane, placing it
onto an aluminum tray, and evaporating the solvent under a stream of N
2
. The resin monomers
were mixed in the tray and cured as previously reported.
3
Resin cubes were placed in a 250 mL
round bottom flask containing a magnetic stir bar, 120 mg of the photosensitizer rose bengal, and
100 mL acetic acid or enough submerge the cube. The flask was sealed with a stopper, purged,
82
and maintained under an O
2
pressure via balloon. The flask was then heated to 100
ºC while stirring
in the presence of a blue LED for photoactivation of rose bengal. Cubes were removed at regular
intervals of 30 min to track kinetics of
1
O
2
intercalation until all cubes were removed, 180 min
total. This process was repeated twice more, except one set was treated under N
2
rather than O
2
,
and another set was treated in absence of the blue LED light and the flask was wrapped in
aluminum foil to exclude as much light as possible. Cubes were wiped with a paper towel and
imaged via confocal fluorescence microscopy at an internal depth of approximately 5 mm, or the
next nearest distance absent significant bubble defects.
4.4.3. Synthesis of 4.5
Scheme 4.2. Synthesis of 4.5.
Vinyl ether 4.5 was prepared according to Thompson, et al.
11
A 10 mL round bottom flask
was prepared with 3 mL dry tetrahydrofuran (THF) and 229.0 mg pyrene-1-carbaldehyde (1.00
mmol, 0.91 equiv.) swirled together until homogeneous. The 10 mL flask was then purged with
N
2
and protected from light. A 25 mL round bottom flask was prepared with 379.8 mg
(methoxymethyl)triphenyl phosphonium chloride (1.10 mmol, 1 equiv., Acros Organics), THF (5
mL), and a magnetic stir bar. The flask was sealed with a stopper and purged with N
2
. The flask
O
O
1. n-BuLi in hexanes, THF,
N
2
(1 atm), - 25 ºC
P
+
O
Cl
-
2.
in THF, - 25 ºC - room temp.
31%
4.9
4.6 4.5
83
was then placed on a stir plate and in a cold bath consisting of a 7:3 ratio of water to methanol and
dry ice so that 1.10 mmol of n-butyllithium in hexanes could be added dropwise at approximately
-25
o
C while stirring. The flask was left to stir for 20 minutes while still submerged in the ice bath.
Afterward, the contents of the 10 mL flask (aldehyde in THF) were added slowly via syringe into
the 25 mL flask while continuing to stir and cool. The flask was then protected from light and left
to stir overnight, gradually coming to room temperature as the ice bath equilibrated with the room.
Thereafter, the phosphonium byproduct was removed via extraction using saturated ammonium
chloride and diethyl ether. The organic layer was then dried using MgSO
4
and concentrated in
vacuo. The resulting oily residue was chromatographed as a liquid through normal phase column
chromatography using a hexane - diethyl ether gradient (100:0 - 0:100). The product was
concentrated in vacuo and purged with N
2
to be stored in a desiccator protected from light at -20
ºC.
1
H NMR and
13
C NMR agreed with reported spectra but our product was obtained as a 1.4:1
mixture of E/Z isomers. For our purposes, resolution of isomers was unnecessary, and the material
was used as is.
1
H NMR (400 MHz, CD
3
CN) δ 8.61 (d, J = 8.1 Hz, 1H), 8.42 (dd, J = 13.6, 9.3 Hz, 2H), 8.26 –
8.02 (m, 10H), 7.35 (d, J = 12.7 Hz, 1H), 6.86 (d, J = 12.7 Hz, 1H), 6.62 (d, J = 7.2 Hz, 1H), 6.23
(d, J = 7.1 Hz, 1H), 3.89 (s, 3H).
84
Figure 4.8.
1
H NMR spectra of the E/Z mixture of 4.5.
4.5. References
(1) Sauer, M. Composites Market Report 2019; 2019.
(2) Rybicka, J.; Tiwari, A.; Alvarez Del Campo, P.; Howarth, J. Capturing Composites
Manufacturing Waste Flows through Process Mapping. J Clean Prod 2015, 91, 251–261.
https://doi.org/https://doi.org/10.1016/j.jclepro.2014.12.033.
(3) Navarro, C. A.; Kedzie, E. A.; Ma, Y.; Michael, K. H.; Nutt, S. R.; Williams, T. J.
Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy
Composites. Top Catal 2018, 61 (7–8). https://doi.org/10.1007/s11244-018-0917-2.
(4) Navarro, C. A.; Ma, Y.; Michael, K. H.; Breunig, H. M.; Nutt, S. R.; Williams, T. J.
Catalytic, Aerobic Depolymerization of Epoxy Thermoset Composites. Green Chemistry
2021, 23 (17), 6356–6360. https://doi.org/10.1039/D1GC01970H.
85
(5) Zeinali, N.; Oluwoye, I.; Altarawneh, M. K.; Almatarneh, M. H.; Dlugogorski, B. Z.
Probing the Reactivity of Singlet Oxygen with Cyclic Monoterpenes. ACS Omega 2019, 4
(9), 14040–14048. https://doi.org/10.1021/acsomega.9b01825.
(6) Davidson, R. S.; Trethewey, K. R. The Mechanism of the Dye-Sensitised Photo-
Oxygenation of Amines. J Chem Soc Chem Commun 1975, No. 16, 674–675.
https://doi.org/10.1039/C39750000674.
(7) Albuquerque, R. v; Malcher, N. S.; Amado, L. L.; Coleman, M. D.; dos Santos, D. C.;
Borges, R. Sa.; Valente, S. A. S.; Valente, V. C.; Monteiro, M. C. In Vitro Protective Effect
and Antioxidant Mechanism of Resveratrol Induced by Dapsone Hydroxylamine in Human
Cells. PLoS One 2015, 10 (8), e0134768-.
(8) Lo, J. N.; Nutt, S. R.; Williams, T. J. Recycling Benzoxazine–Epoxy Composites via
Catalytic Oxidation. ACS Sustain Chem Eng 2018, 6, 7227–7231.
https://doi.org/10.1021/acssuschemeng.8b01790.
(9) Navarro, C. A.; Giffin, C. R.; Zhang, B.; Yu, Z.; Nutt, S. R.; Williams, T. J. A Structural
Chemistry Look at Composites Recycling. Mater Horiz 2020, 7 (10).
https://doi.org/10.1039/d0mh01085e.
(10) Foote, C. S. Mechanism of Addition of Singlet Oxygen to Olefins and Other Substrates.
Pure and Applied Chemistry 1971, 27 (4), 635–646.
https://doi.org/doi:10.1351/pac197127040635.
(11) Thompson, Ambler.; Canella, K. A.; Lever, J. R.; Miura, Kyo.; Posner, G. H.; Seliger, H.
H. Chemiluminescence Mechanism and Quantum Yield of Synthetic Vinylpyrene Analogs
of Benzo[a]Pyrene-7,8-Dihydrodiol. J Am Chem Soc 1986, 108 (15), 4498–4504.
https://doi.org/10.1021/ja00275a040.
Abstract (if available)
Abstract
CFRPs are a class of structural materials globally utilized for their high strength-to-weight ratios, providing great performance and environmental benefits during their service lives. However, there are no effective ways to recycle end-of-life CFRP waste or its manufacturing scrap despite years of awareness of this engineering problem (Chapter 1). By synthesizing and studying small molecule analogs of the thermoset polymer, we identified the intermediate steps of how acidic peroxide cleaves the key C—N bonds formed during the curing reaction (Chapter 2). Incorporating manganese and aluminum catalysts as oxygen-atom transfer and Lewis acid catalysts, respectively, allowed pressurized oxygen gas to act as the terminal oxidant instead of hydrogen peroxide (Chapter 3). This recycling reaction yields carbon fibers still preserved in their original architecture, substantially undamaged and cleaned of polymer residues, but presents too many safety hazards to be executed at industrially relevant scales. We have explored photo-generated singlet oxygen to affect similar oxidations at reduced pressures to increase safety, with exciting preliminary results (Chapter 4). Similarly, we have prepared resin monomers synthesized with singlet oxygen-labile functional groups with the goal of preparing easily recyclable CFRPs in the future, without compromising their performance or properties.
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Creator
Navarro, Carlos
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Core Title
Chemical depolymerization of amine-epoxy cured carbon fiber reinforced polymer composites and their re-use
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Degree Conferral Date
2022-12
Publication Date
11/11/2024
Defense Date
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Tags
aerobic
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