DEVELOPMENT OF NOVEL BINARY AND TERNARY
CONDUCTIVE COMPOSITES BASED ON POLYETHYLENE,
LOW MELTING POINT METAL ALLOY AND CARBON
BLACK
Edward Bormashenko, Semion Sutovski, Roman Pogreb, Avigdor
Sheshnev, Alexander Shulzinger
The College of Judea and Samaria, Ariel, 44837, Israel
Mark Levin, Avital Westfrid
Bar Ilan University, Department of Physics, Ramat Gan, Israel
Zahava Barkay and Abraham Katzir
Tel-Aviv University, School of Physics and Astronomy, Tel Aviv,
66978, Israel
ABSTRACT
This work presents the novel binary and ternary composite materials based on a low-density
polyethylene, metal alloy, which have a melting point close to those of polyethylene and carbon black, obtained
under extrusion processing. The process presented in the paper allowed production of the composite with up to
25 wt.% of the alloy dispersed uniformly in the polymer matrix. The structure of the composite films was studied
with a scanning electron microscope. Possibility of obtaining oriented structures was demonstrated. Conductivity
of the composite was investigated and non-ohmic behavior of the ternary composites was revealed. Temperature
dependence of the thermal capacity was studied. The IR (infrared) spectra of the composites were studied using
an FTIR (Fourier-transform infrared) spectrophotometer. It was shown that processing polyethylene in the
presence of a low-melting-point alloy causes changes in the structure of polyethylene. The presence of alloy
caused intensive oxidation of carbon black in the ternary composite as well.
1. INTRODUCTION
Conductive polymers have been the theme of much investigation recently not only from fundamental
scientific interest but also from a practical point of view of growing applications in electromagnetic shielding,
corrosion inhibition, electrical self-heating, microelectronics, etc (1-3). The authors propose a new approach to
the design of conductive polymers: extrusion mixing of polymers with metals which demonstrate a melting point
close to that of the polymer matrix. The addition of conductive fillers to polymers, such as metallic powders or
carbon black is well-known and it has been successfully applied in many electrical and shielding applications.
Olivero, Cheng and Radford studied the electrical properties of epoxy resins filled with metallized ceramic
microballoons (4-6). It was shown that ultra-lightweight conductive materials could be created using such
metallized microballoons. Polyurethane composites filled with alumina fibers were studied by Lu and Xu (7).
The prevailing approach to the changing of electrical properties of traditional polymers is filling the
polymer with carbon black (8). Electrical properties of carbon-filled polymer composites were studied
thoroughly by the group led by Professor Narkis (9-10). It is recognized that such composites are quite
competitive with intrinsic conductive polymers. Positive temperature coefficients and non-linear effects inherent
for carbon-black-polymer composites were studied by Heaney and Adriansee (11-13). Experimental data
obtained by different groups were explained within the framework of inner-particle tunneling conduction (14)
and framework of classical percolation theory (15). However these basic explanations are not compatible with
each other conceptually, and novel percolation-tunneling model was proposed by Balberg (16). According to
Balberg the conductivity of carbon black-polymer composites was found to depend on the structure of the carbon
black particles.
Ternary composites, which contain polymer matrix, metal particles and carbon black were studied
thoroughly by El-Tantawy and Narkis recently (17-18). The authors propose a new approach to the filling of
thermoplastic polymers with metal particles: to mix the polymer with metal alloy with a melting point which is
close to that of polymer material, using traditional extrusion equipment. The authors studied similar composites
when low-melting-point chalcogenide glasses were introduced in the middle density polyethylene matrix (19). It
was shown that very different structures could be obtained in such systems: from spherical particles dispersed
uniformly in the polymer matrix up to strictly oriented “fiberlike” structures, by varying the extrusion
parameters. The authors studied properties of obtained composites using different experimental methods,
including IR spectroscopy carried out with an FTIR spectrometer. Fourier transform spectra of polymers
(including those filled by carbon black) are discussed in (20). The catalytic activity of metals when processed in
conjunction with polymers was discussed in detail in (21).
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2. EXPERIMENTAL
2.1 Materials and Technology of Preparing of Composite
The base in this study is a LDPE/low-melting-point metal alloy composite. The low-melting-point
metal alloy contained: Sn – 45 (weight) %, Zn –20%, Cd-35 %. The melting point of the alloy is 160° C.
Samples of low-density polyethylene with very different viscosities: Ipethene 320 and Ipethene 4203 were
supplied by Carmel Olefinim LTD. Physical properties of polymers: Ipethene 320 - density 0.920 g/cm3, melt
flow index = 2.0 g/10 min (ISO 1183, t =190 ºC), Ipethene 4203 - density 0.920 g/cm3, melt flow index = 0.2
g/10 min. The process of preparing the composite included the following main stages (see Fig.1): 1. preliminary
mixing of the granulated metal alloy (the average size of the granules – 1-3 mm) with polymer, using a singlescrew extruder (diameter of the screw – 32 mm, L/D =25), under the temperature regime T 1 = 160 °C, T2 = 170
°C, T3 = 180 °C, T4 = 200 °C (temperatures are given according to heating zones); 2. pelletizing; 3. extrusion of
the film using a Randcastle Mictrotruder equipped with mixing zone, cast film die and drawing unit (screw
diameter 12.5 mm, L/D ratio = 25).
We processed 3 types of the binary composite, which contained 5, 10 and 15 percent (by weight) of the
metal alloy respectively. Composite films were obtained with a thickness of 50-300 μm by varying the drawing
speed in the range 20-120 cm/min. We processed ternary composites, which contained LDPE, low-melting-point
metal alloy and carbon black as well. Carbon black was introduced at the stage of the film extrusion. The ternary
composites were processed under the foregoing temperature regime. Maximal weight concentration of carbon
black was 2%. In spite of the high percentage of filling, the processability of the composite is satisfactory. In
order to obtain more thin films with the thickness 30-50 μm necessary for IR spectroscopy procedures, we
applied a high-precision laboratory hydraulic press, equipped with a heated platens Specac P/N 15515. The
pressing of thin films was carried out under the temperature 180 ºC and pressure 510 5 Pa.
2.2 Electronic Microscopy Study of the Structure of the Composite
Fig. 2 presents the SEM image of the typical structure obtained when the blend, which contained 15% of
metal alloy and 85% of high-viscosity LDPE (Ipethene 320) was processed into film of 100 μm in thickness. It
can be seen that the alloy could be dispersed in polymer with high homogeneity; very similar structures were
obtained when 5% and 10% composites were processed. Particles of alloy which have mainly spherical form
with a 1-5 μm average diameter were dispersed uniformly in the LDPE matrix. Fig. 3a-3b present the SEM
images of the ternary composite, which contained 83% LDPE (Ipetnene 320), 15% alloy and 2% of carbon
black. It could be concluded from the comparison of Figures 3a-3b that the distribution of carbon black in the
sample is uniform, with the exception of regions filled by metal alloy. Very different structures were formed,
when low-viscosity polyethylene (Ipethene 4203) was used a matrix material. Lengthened particles of metal
alloy, oriented in the film stretching direction were obtained (Fig 4).
2. 3. Composite Conductivity Study
The conductivity of the composite films was measured. Conductivity along and crosswise to the
drawing direction was taken. No anisotropy of the conductivity was revealed. It was established that conductivity
depends very slightly on the thickness of the film. The results of conductivity measurements are summarized in
Table 1.
It is clear from the data presented that in spite of the obvious tendency of decreasing specific resistivity
as the concentration of the alloy increases, the percolation threshold is not yet achieved. It can be seen that
introduction of carbon black in the composite does not cause changes in the resistivity. It can be concluded that
“sub-networks” of alloy and carbon black don’t interact but rather work independently. The second reason,
which hinders the influence of carbon black on the conductivity is the oxidation of carbon black which would be
discussed below. Therefore, the authors continue their study in two main directions: (1) improvement of the
preliminary mixing of components using twin-screw extruders, (2) increasing the carbon black proportion in the
blend which can serve as an intervening carrier of the electrical charge between metal alloy particles. Such
ternary composites as LDPE/low-melting-point metal alloy/carbon black demonstrated good processability when
processed as described in Fig.1, and we believe that threshold concentrations of components which will cause
dramatic change in the conductivity of the composite are achievable.
We studied the current-voltage dependences for the binary and ternary composites. The films three
centimeters square were exposed to growing voltage and current was measured. Typical current-voltage plot for
the binary composites is presented in Fig. 5. All binary composites demonstrated linear current-voltage
relationships. The ternary composites demonstrated more complicated behavior. All ternary composites
presented non-linear initial portion of the curve (see Fig. 6). After exceeding of some threshold value of the
voltage the behavior of the relationship changes to ohmic. This phenomenon (switching from low to high
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resistance in carbon black polymer composites) was discussed by Balberg (15), and it is related to the reversible
heating-induced-swelling of polymer layer between adjacent carbon black particles.
2.3 Study of temperature dependence of thermal capacity of the composite
Thermal capacity of the composite was studied using Mettler Toledo Star scanning differential
calorimeter. Thin film samples for calorimetry were obtained, using pressing procedure as described in
paragraph 2.1. Fig. 7 presents the temperature dependence of the thermal capacity of the binary composite,
normalized to the maximal value of thermal capacity (speed of the heating was 10K/min). Two peaks: 110°C
inherent for LDPE, and 160 °C inherent for metal alloy could be recognized from the experimental data. Pure
polyethylene demonstrated peak on the temperature dependence of thermal capacity at 115-120 °C. We related
the effect to the changes in the structure of LDPE, which took place after extrusion in the presence of the alloy.
These changes will be discussed below.
2.4 Study of the infrared spectra of the composites
The IR absorbance spectra of the composite were studied with Fourier transform infrared spectrometer
(FTIR) Bruker Vector 22. It could be recognized from the spectral data (Fig.8) that the composite, which
contained metal alloy demonstrated three new absorption “dips” which are not typical for pure polyethylene: at
1020 cm-1, 1235 cm-1 and 1723 cm-1. All these dips are relatively weak on the spectrum of the binary composite,
but we can see that on the spectrum of the ternary composite they are strengthened very significantly. We
propose this interpretation of the spectral data: the dip at 1723 cm-1 is inherent for the absorption of the C=0
group (usually very strong peak, caused by stretching vibrations of the C=O group); the strong dip at 1235 cm-1
and the weak dip at 1020 cm-1 are characteristic dips of the C-O-C group. We concluded that extrusion
processing of polyethylene in the presence of metal alloy caused strong thermal oxidation of LDPE; the
influence of metals on the LDPE degradation process was studied in (20). It is reasonable to suggest that the
reactions described in (20) take place as follows:
O2
~CH2-CH2-CH2-CH2~
alloy
~CH2-C-CH2-CH2~ + H2O
║
O
On the other hand, the alloy accelerates the crosslinking process in the polymer, which takes place
under high temperatures in the extruder, and crosslinked structures described in (15) are formed as follows:
~CH2-CH2-C-CH2~
O
~CH2-CH2-C-CH2~
When we introduced carbon black in the formulation, the dips inherent for C=O and C-O-C groups were
strengthened, indicating that the alloy catalyzed the oxidation of carbon under the high temperatures of the
extrusion processing. The processes of thermal oxidation and cross-linking will change the structure and the
mechanical properties of the polyethylene matrix. The mechanical properties of the composite are under
investigation now.
One more interesting feature could be recognized from the analysis of IR spectra: the presence of metal
alloy particles in the polyethylene suppresses the interference typical for the FTIR spectra of thin polymer films.
The explanation of this fact is simple: multiple reflection and scattering of the light on the metal particles
decrease the interference phenomenon, so that weak absorption dips which could not be distinguished on the
spectrum of the pure LDPE could be identified on the spectrum of the composite film.
CONCLUSIONS
Our work has demonstrated that LDPE films with high concentrations of low-melting-point metal
alloys could be obtained using traditional extrusion equipment. Ternary composites, which contained LDPE, low
melting point metal alloy and carbon black, were obtained as well. The SEM study of the structure of the
composite films indicated good homogeneity of the samples. It was revealed that the viscosity of the matrix
polymer determines structure of obtained composite film. The conductivity of the composite was studied, and in
spite of the fact that the percolation threshold was not achieved, the authors suppose that the composite could be
a potential for various industrial applications (shield material, etc) in combination with carbon black and other
materials used as conductive agents. It has to be emphasized that the composite could be processed by traditional
4- 14
methods, including extrusion and injection molding The thermal dependence of the thermal capacity of the
composite was studied. The IR absorbance spectrum of the binary and ternary composite was studied with an
FTIR spectrometer. It was established that the presence of the alloy caused accelerated oxidation and
crosslinking of polyethylene under extrusion processing.
ACKNOWLEDGEMENTS
This work was supported by the Israel Ministry of Science, Culture and Sport (Project No. 1461-2-00)
and Israel Ministry of Absorption The authors are thankful to Professor Moshe Narkis and Dr. Ester Segal for
the assistance in the measurement of conductivity of the ternary composites.
REFERENCES
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Basel, (1998).
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(1998).
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8. Y. H. Hou, M. Q. Zhang, M. Z. Rong, G. Yu, H. M. Zeng, Journal of Applied Polymer
Science, 84, 2768, (2002).
9. M. Narkis, A. Ram, Z. Stein, Polymer Engineering and Science, 21, No.16, 1049 (1981).
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Michels, J. C. M. Brokken-Zijp, Synthetic Metals, 84, 871 (1997).
13. L. J. Adriaanse, I.-P. Faneyte, H. C. F. Martens, J. A. Reedijk, H. B. Brom, M. A. J.
Michels, J. C. M. Brokken-Zijp, Synthetic Metals, 84, 929 (1997).
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16. I. Balberg, Carbon, 40, 139 (2002).
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Table 1
Conductivity of composite films
Formulation of the composite
100% LDPE Ipethene 320
5% alloy + 95% LDPE Ipethene 320
10% alloy + 90% LDPE Ipethene 320
15%alloy + 85% LDPE Ipethene 320
15% alloy + 1% carbon black + 84%LDPE
Ipethene 320
15% alloy + 2% carbon black + 83%LDPE
Ipethene 320
20% alloy + 80% LDPE Ipethene 320
25% alloy + 75% LDPE Ipethene 4202
Specific Resistivity, Ωcm
1017
1013
1.5÷3·1012
1÷5·1010
1÷5·1010
1÷5·1010
7·109-2·1010
106-107
low melting
point alloy
mixing extruder
extruded
blend
pelletizer
granules of the
polymer/alloy
blend
extruder
composite film
drawing unit
cast film die
Fig.1. Technological process of preparing the LDPE/low melting point alloy composite films
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Fig.2. SEM image of the typical structure of the composite, based on high-viscosity LDPE
and low melting point alloy (15% alloy + 85% LDPE Ipethene 320, the thickness of the
sample - 100 μm). The white shapes are alloy particles and the background is polymer.
50μm
50 μm
a
b
Fig. 3. SEM image of the structure of the ternary composite 15% alloy + 2% carbon black
+83% LDPE. Ipethene 320 – image of the carbon distribution in the sample; the black shapes
are particles of alloy. b - backscattering SEM image of the same sample; the white shapes are
alloy particles and the background is polymer.
4- 17
Fig. 4. Optical microscopy image of the typical structure of the composite, based on lowviscosity LDPE and low melting point alloy (15% alloy + 85% LDPE Ipethene 4203, the
thickness of the sample - 100 μm). The black shapes are alloy particles and the background is
polymer.
4- 18
1.6E-11
1.4E-11
1.2E-11
I,A
1E-11
8E-12
6E-12
4E-12
2E-12
0
0
5
10
15
20
25
30
35
40
45
50
U,V
Fig. 5. Current-voltage dependence for the binary composite (85% LDPE+15% alloy). The
thickness of the sample - 135 μm.
2.5E-12
2E-12
I,A
1.5E-12
1E-12
5E-13
0
0
10
20
30
40
50
60
70
U,V
Fig. 6. Current-voltage dependence for the ternary composite (83% LDPE+15% alloy+2%
carbon black). The thickness of the sample - 200 μm.
4- 19
1
0.9
0.8
0.7
Cp
/Cmax
0.6
0.5
0.4
0.3
0.2
0.1
0
20
30
40
50
60
70
80
90 100 110 120 130 140 150 160 170 180 190 200
o
t, C
Fig. 7. Temperature dependence of the thermal capacity (a.u.) of the composite which
contained 15% alloy and 85% LDPE.
Fig. 8. Infrared transmittance spectra of pure LDPE, 15% alloy +85% LDPE composite and
ternary composite: 15% alloy + 2% carbon black + 83% LDPE
4- 20