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Measurement of electrical conductivity in carbon black filled polymers

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Measurement of electrical conductivity in carbon black filled polymers

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Content MEASUREMENT OF ELECTRICAL CONDUCTIVITY
IN
CARBON BLACK FILLED POLYMERS
by
Abhay D .Sawant
A Thesis Presented to the
FACULTY OF THE SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE IN CHEMICAL ENGINEERING
August 198 9
UMI Number: EP41831
All rights reserved
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This thesis, w ritten by
Abhay D. Sawant
under the guidance o fh ls F a c u lty Committee
and approved by a ll its members, has been
presented to and accepted by the School of
Engineering in p a rtia l fu lfillm e n t of the re
quirements fo r the degree of
Master of Science
in Chemical Engineering
August 4, 1989
Date ...........................
Faculty Committee
ACKNOWLEDGEMENTS
The author is deeply grateful to Professor
Ronald Salovey for his unsparing support in the
provisions for the laboratory work and continual
discussions to encourage the independent study.
He would like to thank Dr. K. S. Shing and Dr.
Ronald Minet for serving on his guidance committee.
TABLE OF CONTENTS
Acknowledgements ............................. ii
List of Figures .............................. ^
1. INTRODUCTION ................................. 1
2. LITERATURE SURVEY ........................... 3
2.1. Carbon Black Morphology ............... 3
2.2. Modes of Conduction ................... 11
2.3. Percolation Threshold Theories ....... 15
3. EXPERIMENTAL DETAILS ........................ 19
3.1. Materials .............................. 19
3.2. Sample Preparation ..................... 20
3.3. Experimental Setup ..................... 21
4. RESULTS ...................................... 23
5. DISCUSSION .... 37
6. CONCLUSIONS .............................. 4 0
REFERENCES .................................... 43
iii
LIST OF FIGURES
Figure 1: Plot of Volume Resistivity Versus
Temperature for Polystrene and 5%
Carbon Black......................... 24
Figure 2: Plot of Volume Resistivity Versus
Temperature for Polystrene and 15%
Carbon Black......................... 25
Figure 3: Plot of Volume Resistivity Versus
Temperature for Polystrene and 20%
Carbon Black......................... 27
Figure 4: Plot of Volume Resistivity Versus
Temperature for Polystrene and 25%
Carbon Black......................... 28
Figure 5: Plot of Volume Resistivity Versus
Temperature for Polystrene and 30%
Carbon Black......................... 29
Figure 6: Plot of Volume Resistivity Versus
Carbon Black Concentration for
Polystrene........................... 31
Figure 7: Plot of Volume Resistivity Versus
Temperature for Polyvinyl Chloride
and 5% Carbon Black................. 32
iv
Figure 8: Plot of Volume Resistivity Versus
Temperature for Polyvinyl Chloride
Carbon Black Concentrations of 15%,
25%, and 30% ........................ 33
Figure 9: Plot of Volume Resistivity Versus
Temperature for Polyvinyl Chloride
and 25% Carbon Black................ 34
Figure 10: Plot of Volume Resistivity Versus
Carbon Black Concentration for
Polyvinyl Chloride.................. 35
v
CHAPTER 1
INTRODUCTION
Electrical conductivity of carbon black filled
polymers finds applications in the industry as
antistatic agents, for wire and cable sheathing and to
shield against electromagnetic interference (EMI) and
radio frequency interference (RFI) [1]. Polymers are,
ordinarily, insulators to which conductivity is
imparted by the addition of a fillers of high
intrinsic conductivity, such as carbon black
particles, metallic fillers and carbon fibers.
Steady shear behavior of pure and carbon black
filled styrene-butyl methacrylate copolymers used as
toners in the electrophotgraphic process were studied
by K. Lakdawala [2] in this laboratory. In these
carbon black-polymer composites, the viscosity becomes
unbounded at low shear stress. This is referred to as
yielding and the stress as yield stress . It was
proposed by Lakdawala et al . [3,4] that the yield
phenomenon arises from the formation of an independent
and continuous carbon black network at low shear
rates.
1
The purpose of this project is to examine the
formation of an independent and continuous carbon
black network at elevated temperatures within the
polymer matrix by electrical conductivity
measurements.
2
CHAPTER 2
LITERATURE SURVEY
2.1. Carbon Black Morphology
Carbon black was initially made by the Channel
process; today nearly 97% of all the blacks are
created from oil by the furnace process [5,6}.The
primary source for feedstock is refinery heavy bottom
oils. In this process residual refining oil is
injected into a natural gas and air mixture at
temperatures of 1200 °C to 1800 °C. The rate of
injection into the combustion zone affects the carbon
black grade produced. The burning gas generates a very
hot, turbulent atmosphere for cracking the feedstock
oil, the polyaromatic molecules pyrolyze readily to
form carbon black. The reactions by which the aromatic
feedstock is converted to elemental carbon are
complex. Spherical carbonaceous particles are formed
which become increasingly viscous. The fusion of
particles still in liquid-like state produces
aggregates or primary structures of acniform
morphology [7],Within each aggregate the carbon atoms
are arranged in imperfect graphite • layers. These
aggregates are the basic structural units in carbon
blacks, and the aggregate morphology plays a key role
in the electrical conductivity of carbon black-polymer
composites. Downstream of the combusion zone, a water
quench is used to rapidly reduce the temperature and
terminate the reaction. The 'smoke' that exits is
filtered to yield fluffy carbon black. At this stage,
the primary properties of particle size, surface area,
and primary structure have already been established.
These aggregates have a tendency to join by van der
Waals' forces of attraction into loosely bound
agglomerates during subsequent processing operations
[7,8,9]. This agglomeration is called the secondary or
transient structure because the agglomerates can be
easily and reversibly broken up by shear or thermal
disturbance into the original aggregates [6].
Various combinations of the primary properties
are obtained by process changes, such as feedstock
flow rate, gas rate, air rate, and quench position.
Particle size is principally controlled by feedstock
rates and temperatures. A higher combustion ratio
increases the temperature and reduces the particle
size as well as the yield. Within limits, surface area
can be controlled independently of either particle
size, or structure by changes in reactor dimensions
and temperatures. Structure is more dependent on the
manner of feedstock injection into the reactor.
Structure can also be lowered with additives, usually
small quantities of alkali metal salts.
The particle size, surface area and structure
are used to characterize these aggregates, and, also,
influence the degree of electrical conductivity which
the carbon black imparts to the composite [8,10].The
only direct method for measuring the particle size and
particle size distribution is provided by electron
micrographs. The average diameter of the ultimate
particles of rubber-grade furnace blacks ranges from
about 19 to 95 nm as determined by direct measurements
from electron micrographs [6]. This method is
extremely time-consuming, therefore other tests like
the tint strength test (ASTM D3265) are used. In the
tint strength test, a carbon black sample is mixed
with zinc oxide and a soyabean oil epoxide to produce
a black or grey paste. This paste is then spread to
produce a suitable surface for measuring the
reflectance of the mixture with a photoelectric
reflectance meter. This reflectance is than compared
with the reflectance of paste containing the Industry
Tint Reference Black (ITRB) prepared in the same
manner. The tint test is affected by the structure as
well as by the particle size of the black. For a given
particle size, higher-structure blacks have lower
tinting strengths. An average particle size can be
estimated from statistical equations that relate tint
strength and structure to particle size as measured
from electron micrographs[11]. A smaller particle size
promotes interaggregate interaction and lessens the
effect of viscous drag of the polymer on the aggregate
during the dispersion process [1].
Surface area plays an important role in the
carbon black-polymer composite as it determines of
the amount of interaction between the polymer and
carbon black. Surface area describes the size and
porosity of the individual particles in the aggregate.
The surface area is inversely related to the particle
size therefore higher surface area grades of carbon
black consist of smaller particles and have more
aggregates per unit mass than low surface area grades
of carbon black. A procedure to determine the surface
area of particulate matter was developed by Brunauer,
Emmett, and Teller (BET) in 1938 [30]. The BET method
is based on the adsorption of a gas, usually nitrogen,
on the surface. A simpler one-point adsorption
technique (ASTM D3037) has become a standard technique
for measuring carbon black surface areas. The surface
area of carbon blacks can also be calculated from
measurements made on electron micrographs. Some other
methods like adsorption of iodine (ASTM D1510) and
CTAB (cetyl trimethyl ammonium bromide) (ASTM D3765)
from solution are also used to obtain a measure of the
surface area of carbon blacks. The CTAB surface area
is influenced less by porosity than by the nitrogen
surface area, because the CTAB molecule is larger than
the nitrogen molecule. Therefore, the BET method does
not give accurate results for the surface area of
porous blacks.The difference between the BET and CTAB
methods for porous carbon blacks is means of
characterizing porosity. Most grades of furnace blacks
with surface area below 130 m^/g are free of
microporosity (pores < 20 °A diameter) [8]. Using the
method of adsorption of iodine the surface area is
measured as iodine number- the milligrams of iodine
per gram of carbon black. Iodine number has been found
to be usually within 10% of the nitrogen adsorption
method [12]. A higher surface area carbon black
aggregate denotes an overall smaller particle size
with a larger number of aggregates per unit mass of
the polymer, hence smaller interaggregate distances
with increased conductivity [13].
The primary structure is the degree to which
carbon particles are bound together in aggregates and
secondary structure is the agglomeration of aggregates
due to Van der Waal's forces. Aggregation contributes
to nearly all of the structure-related effects
observed in carbon black-polymer composite, while
agglomeration is easily destroyed during mixing and
has only minor effects on compounded carbon black-
polymer composite. A low-structure black may have on
an average of 30 particles per aggregate, whereas a
high-structure black may average up to 200 particles
per aggregate [12,14].Due to the loose packing of the
aggregates, the void volume between the aggregates
per unit weight of carbon black increases with the
number of particles per aggregate. Structure
measurements are based on measuring these voids by
filling them with a liquid, as in the DBP (dibutyl
pthallate) absorption test (ASTM D2414), or by
measuring the compressibility, as in the void volume
test. In the void volume test, a measured quantity of
carbon black is compressed at a constant pressure. The
specific volume of the compressed pellet is calculated
from its dimensions and then the volume occupied by
the carbon is subtracted to obtain the volume of
voids. The DBP test is influenced by the amount of
8
work done on the carbon black in the pelletizer. The
DBP value of most carbon blacks ranges from 59 to 140
cm-5/100 gm [12].The compressed DBP test (ASTM D3493)
and void volume test, give results that are less
influenced by the amount of pelletizer work [15] . A
procedure to characterize the aggregate in terms of
length, width, and shape is described in ASTM D3849.
Measurements obtained from electron micrographs have
also been used to characterize aggregate morphology in
terms of shape, size, and bulkiness.
In a high structure carbon black the aggregates
are highly branched clusters of particles which
increase aggregate-to-aggregate contact and therefore
impart higher electrical conductivity than in the low
structure carbon black [1] .
The chemical nature of a carbon black surface is
variable. There is evidence of the presence of
atleast four oxygen-containing groups on the surface:
carboxyl, phenol, quinone, and lactone [6]. The
surfaces also differ in adsorptive capacity and in the
distribution of sites of high energy. Norman [7] has
reported that the surface chemistry of carbon black
also infuences the formation of aggregates. The
functional groups can reduce the electrical
conductivity of the composite by trapping electrons at
9
the surface, raising the potential barrier across the
gap, forming an insulating coating around the carbon
black particles or by affecting the dispersion [8].
Carbon black must be adequately dispersed in
order to determine its effect on the properties of the
composite. In the early stages the molten polymer is
forced into the interstices of the agglomerates and
aggregates. When all voids are filled within the
polymer, the black is considered 'incorporated'. On
further mixing, deagglomeration takes place, where the
carbon black agglomerates are pulled apart by shearing
forces. In the final stage the individual aggregates
are distributed throughout the polymer randomly. In
the stages of agglomerate and aggregate dispersion,
there is a continuous rise in electrical resistivity
due to increase in interaggregate gaps [16,17].
Small-particle-size (high surface area) carbon
blacks are difficult to 'incorporate'. High-structure
blacks contain more void volume to be penetrated by
the polymer and thus take longer to be incorporated
but once incorporated, they disperse more rapidly than
lower-structure blacks [6] . In a composite, the
second-power peak of a mixing energy profile roughly
identifies the point of incorporation. Under certain
mixing conditions, carbon black can become
111
concentrated at the interface between two incompatible
polymers, this has the effect of increasing the number
of contact points and decreasing the inaggregate gap
width.
2 . 2.Modes of Conduction
Polymers are inherently insulating, with volume
resistivity in the range of 10-^ to 10^-® ohm-cm [18] .
The reason for their insulative properties is the
filled valence band of molecules and the large energy
gap between the valence and the conduction band. To
this inherently insulating polymer, conductivity is
imparted by addition of carbon black. At low loadings
of carbon black the resistivity of the composite is
essentially the same as that of the pure polymer. As
the loading is increased, a percolation threshold of
critical loading is reached where the electrical
resistivity starts to decrease rapidly as a function
of the loading. The entire region of decreasing
electrical resitivity is called the percolation
region. In the percolation region, the resistance to
passage of electrons from one carbon black aggregate
to another is due to a sandwiched layer of insulating
polymer between adjacent carbon black aggregates. The
gap width between the aggregates is approximately 15-
~ 11
100°A. Regarding the physical processes involved in
the conduction in carbon black-polymer composites,
various mechanisms have been proposed. It appears that
many physical processes can be involved and the
dominant process depends upon the ingredients in the
composite and the conditions of measurement.
Many studies [18-24] have shown that the
electrical conductivity of carbon black-polymer
composite does not increase linearly with the volume
fraction of carbon black present. In the percolation
region, the electrical conductivity is highly
sensitive to even minute changes in the carbon black
loading. Furthermore, electron micrographs of carbon
black-polymer composite show that, even at loadings
which are insufficient to form a continuously
connected network throughout the sample, there is a
measureable electrical conductivity. Polley and
Boonstra [25] proposed an electron hopping mechanism,
where the electrons jump across this gap, with an
exponential dependence on gap width as in Frenkel's
theory of contact resistance, van Beek and van Pul
[21,22] proposed that electron passage in these
systems is due to tunneling, regarded as a special
case of internal field emission. Their evidence for
12
tunneling was nonlinear current-voltage
characteristics of carbon black-polymer composites.
The phenomenon of tunneling is purely a quantum
mechanical one having no counterpart in classical
mechanics [2 6] . According to the tunneling model of
conduction, an electron may travel along a continuous
carbon pathway and must tunnel across any gap it
encounters to reach the next conducting pathway. The
tunneling current across the gaps [23] is given by
• r \ ■ 7IXW
U(e) = D0expl-- (M _ el < e0 (1)
where
j (8) = tunneling current
jg = limiting tunneling current
e = gap electric field
m — rest mass of electron
Vg = barrier potential
X = (2mVo/h.2) i/2
Eq =4Vo/ew
The exponential dependence of tunneling current
on the gap width shows that the highest conducting
carbon black-polymer composite will be made up of
chains of carbon black separated by small
13
interaggregate gaps, with virtually no conduction
between aggregates which are separated by somewhat
larger gaps.
Further evidence for the role of tunneling in
electrical conduction comes from the temperature
dependence of the resistivity at low applied electric
fields where the material is ohmic at low electric
potentials and the current-voltage characteristics are
linear. From Eq. (1), there is another source of
electric field other than applied voltage and that is
the thermally generated electric field across the
gap. Thermal fluctuations in the material give rise to
voltage fluctuations across the gap, of root mean
square magnitude (kT/C)1/^ where k is the Boltzmann
constant and C is the capacitance of the junction
formed by the small regions of carbon black aggregates
of area on either side of the gap. In carbon black
composites, C is small, therefore the voltage
fluctuations are of an appreciable magnitude. It has
been shown [23] that the tunneling conductance of a
junction is given by
14
G = O0 exp ( 2)
where
(2 a)
7C%wk
wA
87C
(2b)
u (2 c)
Ti and Tq are the limiting temperatures above and
below which conductivity becomes independent of
temperature. Thus, thermally induced voltage
fluctuations can be used as evidence in support of the
role of electron tunneling in conductance through
carbon black-polymer composites [8].
2.3. Percolation Threshold Theories
A macroscopic specimen of a composite contains a
large number of aggregates separated by many gaps of
different widths and potential barriers. In order for
conduction to take place through the specimen, there
must be pathways of sufficiently low resistivity which
extend throughout the entire specimen. Aggregates
which are isolated, or which are in the form of
agglomerates which are isolated from neighbouring
aggregates or agglomerates by gaps of high
resistivity, do not contribute to the conductance. In
a carbon black-polymer composite there exists a
15
resistivity, do not contribute to the conductance. In
a carbon black-polymer composite there exists a
critical volume percentage of carbon black above which
the composite changes from an insulator to a
conductor. This critical filler content is known as
the percolation threshold [1] . Percolation theory
predicts the percolation threshold and explains the
arrangement of aggregates and gaps and the effect of
this arrangement on conductivity. It is a statistical
treatment which has been applied to many processes
such as diffusion and heat transfer and, in polymer
chemistry, to chain branching and gelation.
In the percolation theory, as developed by
Kirkpatrick [27], a composite is regarded as a
lattice of conductive sites joined by resistive bonds.
The sites have zero resistivity and the bonds have a
finite resistivity; while in the absence of a bond,
adjacent sites are separated by a medium of infinite
resistivity. By Monte Carlo techniques, bonds or sites
are removed at random and the conductivity of the
lattice is calculated. The removal of bonds is carried
out under the 'bond percolation model' whereas the
random selection of sites is referred to as the 'site
percolation model'. On a cubic lattice, the bond
treatment gives a percolation threshold or critical
volume fraction of 0.25, while the site treatment
gives a critical volume of 0.31. In a carbon black-
polymer composite although the individual carbon black
particles may be spherical, their aggregation makes
the electrical behaviour similar to systems containing
randomly arranged filamentary conductors. Landauer
[28] has discussed the role of the shape of conducting
elements in conductor-insulator ccomposites, and his
conclusions apply to carbon-polymer composites. He
concluded that a filamentary conductor will reach
percolation threshold at which a continuous conduction
pathway is established throughout the sample at a
lower volume percent of conductor than for spherical
conducting elements.Thus the actual value of the
percolation threshold depends on the aspect ratio of
the conducting elements and their arrangement in the
lattice [27] .
At the percolation threshold, these treatments
predict a power law dependence of electrical
conductivity on the concentration above the
percolation threshold given by the following
relationship,
a = a0 (p-pc)b (3)
17
where p = probability that a site or a
bond is present.
pc = critical or threshold
probability.
The coefficient in eqn.(3) has been calculated to be
1.65 [27] .
A different approach to theoretically explain
percolation threshold is made by the effective
medium theory. A network of resistors of randomly
distributed resistivity can be simulated by an
effective medium, defined as one in which the total
field inside the medium is equal to the external
field [27] . By knowing the field far from a given
resistor junction, one can calculate the current which
will generate the same field at that junction as the
external field.This is done by application of
electrical circuit theory to the network.
A simple form of bond resistivity distributions
agrees well with the bond percolation theory at high
bond occupancy. Sherman [2 9] reported that the
percolation theory is most suitable in the vicinity of
the threshold, while the effective medium theories
are regarded as more applicable away from the
threshold.
CHAPTER 3
EXPERIMENTAL DETAILS
. 3 , ^ - 1 . . MaLaiJLala
The polystyrene (PS) was obtained from Dow
Chemical Company of Midland, Michigan. The polyvinyl
chloride (PVC) was obtained from Poly Sciences Inc. .
Gel permeation chromatography, calibrated with
polystyrene standards, yielded weight-average
molecular weights of PS and PVC as 250-300K and 11,700
respectively. The glass transition temperatures of PS
and PVC measured by differential scanning calorimetry
(Perkin Elmer DSC-4) were 104°C and 82°C ,
respectively.
The carbon blacks designated as Raven 410 and
Raven 7000 respectively, were purchased from City
Service Columbia Chemicals. The products were made by
the furnace process and were available in powder or in
bead form. The surface areas of the carbon blacks,
from nitrogen adsorption studies were reported to be
24 m^/g and 625 m^/g, respectively.
19
3.X.2. sample grgpar. atio. x i
Polystyrene beads and carbon black were weighed
separately to achieve the desired loading. Mixing was
carried out in an internal mixer (Brabender
Plasticorder) at 150 °C at a speed of 50 rev/min for
the first 7.5 min and at a speed of 100 rev/min for
the next 7.5 min. The samples were then compression
moulded in a Preco Hydraulic Press at 150 °C under 15
x 107 Pa platen pressure for 10 min. Circular discs 8
cm in diameter and approximately 1.3 mm thick were
obtained. For polyvinyl chloride the same procedure
was followed, but dry blending of polyvinyl chloride
powder and carbon black was carried out at room
temperature instead of 150 °C as for polystyrene.
These samples were compression moulded as before to
form circular samples 5.4 cm in diameter and
approximately 1.9 mm thick.
These samples were vapour deposited with silver
to form the electrodes in an Edwards Vacuum
Evaporator. The connecting wires were attached using
an adhesive silver-epoxy paint purchased from Dupont
Company.
20
3.3. Experimental Setup
The experimental setup consisted of a cell for
the measurement of electrical conductivity by the two
probe method. The aluminium cell had a teflon plate in
which the sample was placed. A K-type thermocouple
probe was embedded (close to the sample) in the teflon
plate to accurately measure the temperature using a
Keithley Thermometer . The connector wires from the
electrodes were soldered to the BNC connectors screwed
to aluminium sleeves at either end of the cell. The
negative terminals of the bnc connectors were earthed
to the exterior of the cell by an aluminium sleeve.
This shielded the cell from stray electric fields
which could affect the measurements. A voltage of 0.1
V was applied to the ends of the sample and the
current flowing was measured by a high impedence
Keithley Electrometer 617 with a built-in voltage
source. For measuring the temperature variation of
electrical resistivity of the carbon black-polymer
composite, the cell was placed in an environmental
chamber. From the voltage and current the resistance
of the sample was internally computed by the
electrometer when placed in v/l mode. The sample was
heated at a constant rate of about 2 °C/min and the
temperature was recorded at regular intervals by a
strip chart recorder and the resistance by the
electrometer.After completion of the heating
experiment the sample was cooled at about 4 °C/min and
similarly temperature and resistance was recorded as a
function of time. At the conclusion of the experiment,
from the sample dimensions the volume, resistivity was
calculated [30]. These data were then transformed to
semilogarithmic plots of temperature and volume
resistivity which are presented in the results.
22
CHAPTER 4
RESULTS
The volume resistivity of polystyrene containing
5, 15, 20, and 30% by weight of carbon black of
surface area 24 /g were measured at various
temperatures between 25 and 230 °C. The volume
resistivity plotted as a function of temperature for
PS containing 5% carbon black is shown in fig. 1. On
initial heating the volume resistivity is almost
constant till 150 °C, then an increase in volume
resistivity commences and reaches a maximum at 180 °C.
The decrease in volume resistivity is about three
orders of magnitude. During cooling the resistivity is
fairly constant with a slight peak at 100 °C. Upon
reheating, there is an increase in volume resistivity
but the maximum is much smaller than the first
heating. The repeated cooling curve is fairly close to
that of the first cycle.
The volume resistivity is plotted as a function
of temperature for PS containing 15% carbon black is
shown in fig. 2. The heating curve shows a smaller and
more gradual peak. The decrease in volume resistivity
is about nine orders of magnitude.The maximum is
23
VOLUM E RESISTIVITY (OHM-CM)
DOW STYRON + 5% CARBON BLACK
* “ HEATING
p - REHEATING
+ - COOLING
x-RECOOLING
0 50 100 150 200 250
TEMPERATURE (CELSIUS)
Figure 1: Plot of Volume Resistivity Versus Temperature for
Polystyrene and 5% Carbon Black.
to
> £ = ■
DOW STYRON + 15% CARBON BLACK
* - HEATING
o - REHEATING
+ - COOLING;
x - RECOOLING
10"
1 0 10
oo
(-H
C O
100 120 140 160 180 200 220
TEMPERATURE (CELSIUS)
Figure 2: Plot of Volume Resistivity Versus Temperature for
Polystyrene and 15% Carbon Black.
almost absent during reheating. The cooling and
recooling curves are almost identical.
The volume resistivity is plotted as a function
of temperature for PS + 20% CB system (fig. 3) .
During the first heating cycle the volume resistivity
increases as the temperature increases above the glass
transition temperature (104 °C) but as the temperature
increases further the volume resistivity passes
through a maximum and then drops rapidly by seven
orders of magnitude over 50 °C temperature range.
The data for 25% carbon black sample is shown in
fig. 4. In this case the decrease in volume
resistivity is eight orders of magnitude. The volume
resistivity does not regain the high resistivity
observed during first heating.
The volume resistivity of 30% carbon black
sample was measured to be 1 x 107 ohm-cm at 25 °C
(fig.5). Higher concentrations of carbon black filler
produce lower resistivities at room temperature. There
is hardly any change in the cooling curves.
In all of the above cases an interesting
phenomenon observed on completion of the heating
cycle, was a significant hystersis during cooling.
During the first cooling the volume resistivity shows
a peak at the glass transition temperature. The volume
26
V O LUM E RESISTIVITY (OHM-CM)
DOW STYRON + 20% CARBON BLACK
♦-HEATING
o - REHEATING
+ - COOLING
x - RECOOLING
100 150 200 250
TEMPERATURE (CELSIUS)
Figure 3: Plot of Volume Resistivity Versus Temperature for
Polystyrene and 20% Carbon Black.
to
VO LUM E RESISTIVITY (OHM-CM)
DOW STYRON + 25% CARBON BLACK
* - HEATING
\ o - REHEATING
\ + - COOLING
\ x-RECOOLING
0 50 100 150 200 250
TEMPERATURE (CELSIUS)
Figure 4: Plot of Volume Resistivity Versus Temperature for
Polystyrene and 25% Carbon Black.
DOW STYRON + 30% CARBON BLACK
o - REHEATING
+ - COOLING
x - RECOOLING
00
200 250 100 150
TEMPERATURE (CELSIUS)
Figure 5: Plot of Volume Resistivity Versus Temperature for
Polystyrene and 30% Carbon Black.
resistivity did not regain its high value of the first
heating cycle on cooling. Furthermore, the effects of
hystersis diminished on subsequent thermal cycling.
The hystersis phenomenon becomes more prominent as one
approaches the percolation threshold.
The volume resistivity @ 25 °C was plotted as a
function of the loadings of carbon black to obtain the
percolation curve (fig. 6) for polystyrene and carbon
black, 24m^/g system.
The volume resistivity is plotted as a function
of temperature for polyvinyl chloride and carbon black
system for different loadings of carbon black and for
different surface areas (figs. 7-10 ) . For pure PVC
(fig.7), the volume resistivity of the sample
decreased rapidly at the glass transition temperature
(87°C). For PVC with 5% loading of carbon black (24
m^/g), the curve is similar to that of pure PVC but
the temperature at which the rapid decline in volume
resistivity commences is higher than the glass
transition temperature of the polymer.
The volume resistivity is plotted as a function
of temperature for PVC with 5% carbon black (625
m^/g) system in fig.7. The lower surface area carbon
black, 24m2/g, was replaced by a higher surface area
filler, 625m2/g, to study the effects of surface area
30
PERCOLATION CURVE FOR PS + CB, 24m2/g
—
O
O 108
>
% LOADING
Figure 6: Plot of Volume Resistivity Versus Carbon Black
uj Concentration for Polystyrene.
V O LUM E RESISTIVITY (OHM-CM)
PVC + 5% CB, 24m2/g
PVC + 5% CB! 625m^g
10 >4
100 120 140 160 180 200 220
TEMPERATURE (CELSIUS)
w Figure 7: Plot of Volume Resistivity Versus Temperature for
f ' J Polyvinyl Chloride and 5% Carbon Black.
* - PVC + 15% CB
o - PVC + 25% CB
x - PVC + 30% CB
0 50 100 150 200 250
TEMPERATURE (CELSIUS)
Figure 8: Plot of Volume Resistivity Versus Temperature for
Polyvinyl Chloride and Carbon Black with Concentrations
°M.5%, 2 5 % _ a n d _ 3 Q % . _ --------------------------
VOLUME RESISTIVITY (OHM-CM)
10*
o - PVC + 25% CB, 24m2/g
x - PVC + 25% CB, 625m2/g
0 50 100 200 250 150
TEMPERATURE (CELSIUS)
Figure 9: Plot of Volume Resistivity Versus Temperature of
Polyvinyl Chloride and 25% Carbon Black.
u >
l f c >
r
i
i
i
PERCOLATION CURVE FOR PVC + CB, 24m2/g
2
u
i
% LOADING
Ficnore 10: Plot of Volume Eesistivity Versus Carbon Black
Concentration for Polyvinyl Chloride.
on volume resistivity and the agglomeration mechanism
of the filler. The 625m2/g carbon black loaded sample
showed a lower volume resistivity at 25 °C than for
24m^/g carbon black in PVC.
For 15%, 25%, and 30% loadings of carbon black
(fig. 8), the volume resistivity is initially constant
and then there is a sudden increase at the melting
point (182 °C) of about three orders of magnitude. We
observe that for higher loadings the electrical
resistivity attains a low value corresponding to the
intrinsic conductivity of the carbon black.
The volume resistivity is plotted as a function
of temperature for PVC + 25% CB system is shown in
fig. 9.
The volume resistivity @ 25 °C was plotted as a
function of the loadings of carbon black to obtain the
percolation curve (fig. 10) for polyvinyl chloride and
carbon black, 24m2/g system.
CHAPTER 5
DISCUSSION
The purpose of this project was to examine the
formation of an independent and continuous carbon
black network at elevated temperatures within the
polymer matrix by electrical conductivity
measurements. The exsistence of a carbon black network
has been proposed as the cause of a yield phenomenon
in steady shear rheology and in oscillating shear
observed at low shear rates [2,3,4] . In what
follows,evidence for the formation of an independent
carbon black network, as inferred from the
experimental results already presented, will be shown.
The results in fig. 4 are an example of evidence
for the exsistence of a carbon black network. The
volume resistivity of the system is initially at a
high value of 5 x 1011 ohm-cm and rise gradually
till 160 °C and then drops by eight orders of
magnitude. The final volume resistivity is about 105
ohm-cm, which is quite low as compared to that in
dispersed state. This is indicative of the formation
of a continuous conductive carbon black network. The
viscosity of the polystrene matrix below 160 °C is not
low enough to allow the movement of carbon black
37
aggregates. Due to addition of carbon black the
viscosity of the system increased and temperatures
higher than the glass transition temperatures were
needed to acheive agglomeration of the aggregates.
The positive temperature coefficient (PTC)
phenomenon (figs. 1-5), i.e. the initial rise in the
volume resistivity with increasing temperature, can be
explained by taking into account the volume expansion
of the polymer. The coefficient of volume expansion of
the polymer is much greater than that of carbon black,
which causes the distance of separation between the
aggregates to be increased further resulting in an
increase in volume resistivity with an increase in
temperature.
From figs 4 and 5 we observe that when the
network is formed during the first heating, on further
thermal cycles the network is not significantly
disrupted by segmental motion indicating the
independent nature of the network. At high carbon
black concentrations a large number of aggregates are
in contact with each other, providing an extensive
network and hence an extensive conducting path. The
network is so extensive that it cannot be disrupted by
segmental motion and lack of PTC phenomenon on thermal
cycling.
Surface area plays an important role in
conductivity of the carbon black-polymer composite.
The higher the surface area, the greater the number of
aggregates per given weight fraction of the filler.
This leads to greater aggregate-aggregate interaction
and lower the conductivity of the composite (Figs. 7
and 9) .
From fig.6 we conclude that the percolation
threshold of polystrene and carbon black of surface
area, 24m^/g system is at 25%. As the concentration of
filler in the polymer increases, a threshold is
reached where a small addition of the filler causes a
large drop in volume resistivity as more conductive
paths are established. Similarly, from fig.10 we
conclude that the percolation threshold of polyvinyl
chloride and carbon black of surface area, 24m2/g
system is between 5%-15%.
CHAPTER 6
CONCLUSIONS
The formation of a irreversible carbon black
network was suggested by K. Lakdawala et al. [2,10] in
carbon filled polymer systems. They had hypotheised
that the yield phenomenon observed in steady shear
experiments for filled polymer systems was possibly
due to network formation which causes the viscosity of
the system to become unbounded. The above results
confirms their hypothesis. The formation of a
continuous carbon black network due to agglomeration
of the carbon black aggregates is concluded from the
rapid decrease in volume resistivity over a very small
temperature range. Temperatures higher than the glass
transition temperatures were needed to lower the
vehicle viscosity and facilitate the movement of
carbon black aggregates to form a network. For PS +
20% CB system this temperature is around 180 °C ,
substantially higher than the glass transition
temperature of PS.
Network formation was observed to be an
irreversible process is indicated by the volume
resistivity hystersis. The volume resistivity after a
40
sharp decrease on the first heating cycle remained at
a low value on sucessive thermal cyclings.
The positive temperature coefficient (PTC) i.e.
an increase in resistivity with temperature was
observed, which is attributed to the thermal expansion
of the polymer. The polymer has a much higher
coefficient of expansion as compared to the filler,
therefore as the temperature increases above the
glass transition temperature the increase in volume of
the polymer is much higher than that of the filler, so
the distance of separation between the aggregates
increases further causing an increase in volume
resistivity. As we increase the temperature further
the vehicle viscosity decreases and the flocculation
of the aggregates takes place to form a conductive
carbon black network and the volume resistivity drops
rapidly.
For higher loadings there is a decrease in
interaggregate gaps and the influence of temperature
on the electrical resistivity of the system is very
little as compared to that at low loadings where there
is a drop of seven orders of magnitude (for PVC + 5%
CB system). The insensitivity to temperature suggests
that the mechanism of conduction at high loadings is
'through going chains' with physical contact between
41
the carbon black aggregates. At low loadings, the
dominant mechanism of conduction is either tunneling
through the gaps, assisted by thermal fluctuations, or
thermal activation of electrons over the potential
barrier of the gap.
For the PVC + CB system, the effect of surface
area of the carbon black was studied. Increasing the
surface area from 24 m2/g to 625 m2/g enhanced network
formation by increasing the number of aggregates per
given weight fraction of the filler, increasing the
aggregate interaction due to smaller interaggregate
gaps and providing more conductive paths.
REFERENCES
[1] A. Medalia, Rubber Chem. Technol., 59, 432
(1986) .
[2] K. Lakdawala, Doctoral Dissertation, University
of Southern California, 1986.
[3] K. Lakdawala and R. Salovey, Polym. Eng. Sci.,
27, 1043 (1987).
[4] K. Lakdawala and R. Salovey, Polym. Eng. Sci.,
27, 1035 (1987) .
[5] E. M. Dannenberg, Encyl. of Chem. Technol., 3rd.
Edn. vol. 4 , Wiley Publ..
[6] M. Morton, 'Rubber Technology' , Von
Nostrand Reinhold Company, New York,
3rd Edn, 1987.
[7] R. H. Norman, ' Conductive Rubber and
Plastics', Applied Science
Publishers, London, 1970.
[8] E. K. Sichel, 'Carbon Black-Polymer
Composites', Marcel Dekker, New York,
1982 .
[9] A. Voet, Rubber Chem. Technol., 54, 42
(1980) .
43
[10] K. Lakdawala and R. Salovey, Polym.
Eng. Sci., 25, 797 (1985).
[11]-N. L. Smith, Rubber World, 162(2),
51(adv.), (1970).
[12] A. I. Medalia,Rubber Chem. Technol., 51,
437 (1970) .
[13] D. J. Sommers, Polym. Plast. Technol.,
230, 83 (1984) .
[14] A. I. Medalia,Rubber Chem. Technol., 45,
1171 (1972) .
[15] A. C. Patel and W. A. Brown, ' Carbon
Black ', Rubber Division, ACS
Lecture Series (1984).
[16] B. B. Boonstra and A. I. Medalia, Rubber
Chem. Technol.,36, 115 (1963) .
[17] B. B. Boonstra, Rubber Chem. Technol.,
50,194 (1977).
[18] A. Manson and L. H. Sperling, 'Polymer
Blends and Composites', Plenum
Press, New York, 1976.
[19] W. F. Verhelst, et. al., Rubber Chem.
Technol., 50, 735 (1977).
[20] E. O. Forster, IEEE Trans. Power Appar.
Sys., PAS-90, 913 (1971) .
44
[21]
[22]
[23]
[24]
[25]
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[27]
[28]
[29]
L. K. H. van Beek and B. I. C. F. van
Pul, J. Appl. Polym. Sci., 6,651
(1962) .
L. K. H. van Beek and B. I. C. F. van
Pul, Carbon, 2, 121 (1964).
P. Sheng, E. K. Sichel, and J. I.
Gittleman, Phys. Rev. Lett., 40, 1197
(1978) .
P. Sheng, E. K. Sichel, and J. I.
Gittleman, Phys. Rev. B, 18, 5712
(1978) .
M. H. Polley and B. B. Boonstra, Rubber
Chem. Technol., 30, 170 (1957).
L. I. Schiff, ' Quantum Mechanics ', 3rd
Edn., McGraw-Hill Book Company, New
York, 1968.
S. Kirkpatrick, Rev. Mod. Phys., 45, 574
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R. Landeuer,. ' Electrical Transport and
Optical Properties in Homogenous
Media',American Institute of Physics,
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[30]
K. N. Mathes, ' Encyl. of Polymer Science and
Engineering1, John Wiley and Sons, New York,
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46

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Asset Metadata

Creator Sawant, Abhay D. (author)

Core Title Measurement of electrical conductivity in carbon black filled polymers

Degree Master of Science

Degree Program Chemical Engineering

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

Tag chemistry, polymer,engineering, chemical,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Salovey, Ronald (committee chair), Minet, Ronald G. (committee member), Shing, Katherine S. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c20-316014

Unique identifier UC11260039

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