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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, Yelena Bormashenko, Mark Levin and Avital Westfrid Journal of Thermoplastic Composite Materials 2004; 17; 245 DOI: 10.1177/0892705704041160 The online version of this article can be found at: http://jtc.sagepub.com/cgi/content/abstract/17/3/245

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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 AND YELENA BORMASHENKO The College of Judea and Samaria Ariel, 44837, Israel MARK LEVIN AND AVITAL WESTFRID Department of Physics Bar Ilan University Ramat Gan, 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. Oriented structures were obtained when high-viscosity polyethylene was used as a matrix material. Resistivity of the composite was investigated and nonohmic 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. The presence of the alloy caused intensive oxidation of carbon black in the ternary composites as well. KEY WORDS: composite, polyethylene, metal alloy, low melting point, carbon black, extrusion, orientated structures, conductivity, nonohmic, infrared spectra.

INTRODUCTION

C

been the theme of much investigation recently, not only from fundamental scientific interest but also from a

ONDUCTIVE POLYMERS HAVE

*Author to whom correspondence should be addressed. E-mail: [email protected], [email protected]

Journal of THERMOPLASTIC COMPOSITE MATERIALS, Vol. 17—May 2004 0892-7057/04/03 0245–13 $10.00/0 DOI: 10.1177/0892705704041160 ß 2004 Sage Publications


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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 has been successfully applied in many electrical and shielding applications. Olivero et al. 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 changing the electrical properties of traditional polymers is to fill the polymer with carbon black [8]. Electrical properties of carbon-filled polymer composites were studied thoroughly by the group led by Narkis et al. [9,10]. It is recognized that such composites are quite competitive with intrinsic conductive polymers. Positive temperature coefficients and nonlinear 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 the framework of classical percolation theory [15]. However, these basic explanations are not compatible with each other conceptually, and a 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 a polymer matrix, metal particles, and carbon black, were studied thoroughly by El-Tantawy and Narkis recently [17,18]. Our group proposes 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 lowmelting-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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Development of Conductive Composites

EXPERIMENTAL Materials and Technology of Preparing the Composite The base in this study is an 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, resistivity, ¼ 7.34 106 cm. Samples of low-density polyethylene with very different viscosities (Ipethene 320 and Ipethene 4203) were supplied by Carmel Olefinim Ltd. The physical properties of the polymers: Ipethene 320 (lowviscosity LDPE) – density 0.920 g/cm3, melt flow index ¼ 2.0 g/10 min (ISO 1183, t ¼ 190 C); Ipethene 4203 (high-viscosity LDPE) – density 0.920 g/ cm3, melt flow index ¼ 0.2 g/10 min. The process of preparing the composite included the following main stages (see Figure 1): (1) preliminary mixing of the granulated metal alloy (the average size of the granules – 1–3 mm) with polymer, using a single-screw extruder (diameter of the screw – 32 mm,

low melting point alloy

polymer

mixing extruder extruded blend pelletizer granules of the polymer/alloy blend extruder drawing unit composite film cast film die Figure 1. Technological process of preparing the LDPE/low-melting-point alloy composite films.


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L/D ¼ 25), under the temperature regime T 1 ¼ 160 C, T 2 ¼ 170 C, T3 ¼ 180 C, T4 ¼ 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 ¼ 25). The extrusion of the film was performed under the foregoing temperature regime. We processed 6 types of the binary composite, which contained 5, 10, 15, 20, 25, and 30% (by weight) of the metal alloy respectively. Composite films were obtained with a thickness of 50–300 mm 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 temperature regime given above. 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 thinner films with the thickness of 30–50 mm 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 C and pressure 5 105 Pa. Study of Temperature Dependence of the Thermal Capacity of the Composite The thermal capacity of the composite was studied using a Mettler Toledo Star scanning differential calorimeter. Thin film samples for calorimetry were obtained, using a pressing procedure as described in ‘‘Materials and Technology of Preparing the Composite’’. Figure 2 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 10 K/min). Two peaks: Ts ¼ 110 C (inherent for LDPE), and Tm ¼ 160 C (the melting point of metal alloy) could be recognized from the experimental data. It is readily seen that the composite has been processed under the temperature Tpr > Tm > Ts so that the polymer and metal alloy were in liquid state under processing. This fact allowed the successful dispersing of metal alloy particles in the polymer matrix and formation of oriented structures as discussed below. Optical and Electronic Microscopy Study of the Structure of the Composite Figure 3 presents the SEM image of the typical structure obtained when the blend, which contained 15% metal alloy and 85% low-viscosity LDPE (Ipethene 320), was processed into film of 100 mm in thickness. It can be seen


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Development of Conductive Composites 1 0.9 0.8 0.7 0.6

Cp

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 Figure 2. Temperature dependence of the thermal capacity (a.u.) of the composite which contained 15% alloy and 85% LDPE.

Figure 3. SEM image of the typical structure of the composite, based on low-viscosity LDPE and low-melting-point alloy (15% alloy þ 85% LDPE Ipethene 320, the thickness of the sample – 100 m). The white shapes are alloy particles and the background is polymer.

that the alloy could be dispersed in polymer with high homogeneity; very similar structures were obtained when 5, 10, 15, 25, and 30% composites were processed. Particles of alloy, which have mainly spherical form with a 1–5 mm average diameter, were dispersed uniformly in the LDPE matrix. Single-screw extrusion of the composite yielded a very narrow size distribution of metal particles dispersed in the polymer matrix displayed in


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E. BORMASHENKO ET AL. 60. 0 56. 0

f, %

52. 0 48. 0 44. 0 40. 0 36. 0 32. 0 28. 0 24. 0 20. 0 16. 0 12. 0 8. 0 4. 0 0. 0 0. 0

(a)

5.0

10.0

15.0

20.0

25.0

30.0

35.0 40.0 D,mkm

(b)

Figure 4. Size distribution of metal particles for binary composites: (a) – composition 30% alloy þ 70% LDPE Ipethene 320, (b) – composition 15% alloy þ 85% LDPE Ipethene 320.

Figure 4(a) and (b). Sized distribution of particles was studied using an Olympus microscope, equipped with CCD camera and SIAMS-600 software, which allowed automatic treatment of speckled images. It could be concluded that the percentage of metal alloy has only a slight influence on the size distribution of metal particles, so aggregation of metal alloy was not observed (up to 30% metal content). Very different structures were formed when high-viscosity polyethylene (Ipethene 4203) was used as a matrix material. Lengthened particles of metal alloy, oriented in the direction of the film stretching, were obtained (Figure 5). Figure 6(a) and (b) present the SEM images of the ternary composite, which contained 83% LDPE (Ipetnene 320), 15% alloy and 2% carbon black. It could be concluded from the comparison of Figure 6(a) and (b) that the distribution of carbon black in the sample is uniform, with the exception of regions filled by metal alloy. Composite Resistivity Study The resistivity of the composite films was measured by the use of setup presented in Figure 7. Silver contacts were applied to the sample using quick drying silver paint, supplied by Stansted Essex Co. The size of samples was 20 20 mm, the distance between contacts was 12 mm. Resistivity along and crosswise to the drawing direction was taken. No anisotropy of the resistivity was revealed. It was established that conductivity depends very slightly on the thickness of the film. The results of resistivity measurements are summarized in Table 1.


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100 µm

Figure 5. Optical microscopy image of the typical structure of the composite, based on high-viscosity LDPE and low-melting-point alloy (15% alloy þ 85% LDPE Ipethene 4203, the thickness of the sample – 100 m). The dark shapes are alloy particles and the background is polymer (image obtained using CCD camera).

50 µm (a)

50 µm (b)

Figure 6. SEM image of the structure of the ternary composite 15% alloy þ 2% carbon black þ 83% LDPE Ipethene 320. (a) – 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.


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EM shielded box

Computer Programmable Voltage Source Keithley 230

contacts

Electrometer/High Resistance Meter Keithley 6517 Sample Figure 7. Setup used for resistivity measurements.

Table 1. Resistivity (at room temperature) of composite films. Formulation of the Composite 100% LDPE Ipethene 320 5% alloy þ 95% LDPE Ipethene 320 10% alloy þ 90% LDPE Ipethene 320 15% alloy þ 85% LDPE Ipethene 320 15% alloy þ 1% carbon black þ 84%LDPE Ipethene 320 15% alloy þ 2% carbon black þ 83%LDPE Ipethene 320 20% alloy þ 80% LDPE Ipethene 320 25% alloy þ 75% LDPE Ipethene 4203 30% alloy þ 70% Ipethene 4203

Resistivity, X cm 1017 1013 1.5 3 1012 1 5 1010 1 5 1010 1 5 1010 7109 2 1010 109 5 109 108

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 do not interact but rather work independently. The second reason hindering the influence of carbon black on the conductivity is the oxidation of carbon black, which will 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, serving 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 Figure 1, and we believe that threshold


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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 3 cm2 were exposed to growing voltage, and the current was measured. A typical current–voltage plot for the binary composites is presented in Figure 8. All binary composites demonstrated linear current–voltage relationships. The ternary composites demonstrated more complex behavior. All ternary composites presented a nonlinear initial portion of the curve (see Figure 9). After exceeding some threshold value of the voltage, the behavior of the relationship changes to ohmic. 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 Figure 8. Current–voltage dependence for the binary composite (85% LDPE þ 15% alloy). The thickness of the sample – 135 m. 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 Figure 9. Current–voltage dependence for the ternary composite (83% LDPE þ 15% alloy þ 2% carbon black). The thickness of the sample – 200 m.


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This phenomenon (switching from low to high resistance in carbon black polymer composites) was discussed by Balberg [16], and it is related to the reversible heating-induced swelling of the polymer layer between adjacent carbon black particles. Study of the Infrared Spectra of the Composites The IR transmittance spectra of the composite were studied with a Fourier transform infrared spectrometer (FTIR) Bruker Vector 22. It could be recognized from the spectral data (Figure 10) that the composite which contained metal alloy demonstrated three new absorption ‘‘dips’’ which are not typical for pure polyethylene: at 1020, 1235, and 1723 cm1. 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 cm1 is inherent for the absorption of the C¼O group (usually a very strong peak, caused by stretching vibrations of the C¼O group); the strong dip at 1235 cm1 and the weak dip at 1020 cm1 are characteristic dips of the COC 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 [21].

Figure 10. Infrared transmittance spectra of pure LDPE, binary composite (15% alloy þ85% LDPE) and ternary composite (15% alloy þ 2% carbon black þ 83% LDPE).


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It is reasonable to suggest that the reactions described in [21] take place as follows: O2

–CH2–CH2–CH2–CH2–

–CH2–C–CH2–CH2– + H2O alloy 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 [21] are formed as follows: –CH2–CH2–C–CH2– O –CH2–CH2–C–CH2–

When we introduced carbon black into the formulation, the dips inherent for C¼O and COC 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 crosslinking 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 the structure of the obtained composite film. The resistivity of the composite


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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 must be emphasized that the composite could be processed by traditional methods, including extrusion and injection molding. The thermal dependence of the thermal capacity of the composite was studied. The IR transmittance spectra of the binary and ternary composite were studied with an FTIR spectrometer. It was established that the presence of the alloy caused accelerated oxidation and crosslinking of polyethylene under extrusion processing. ACKNOWLEDGMENTS This work was supported by the Israel Ministry of Science, Culture and Sport (Project No. 1461-2-00) and the Israel Ministry of Absorption. The authors are thankful to Professor Moshe Narkis and Dr. Esther Segal for their assistance in the measurement of conductivity of the ternary composites, and to Dr. Oleg Stanevsky for his help in the preparation of this paper. REFERENCES 1. Angelopoulos, M. (1998). Conducting Polymers in Microelectronics, (Chap. 32). In: Scotheim, T.A., Elsenbaumer, R.L. and Reynolds, J.B. (eds), Handbook of Conducting Polymers, pp. 921–945, Marcel Dekker Inc., New York, Basel. 2. Lu, W-K., Basak, S.R.L. and Elsenbaumer, R.L. (1998). Corrosion Inhibition of Metals by Conductive Polymers, (Chap.31). In: Scotheim, T.A., Elsenbaumer, R.L. and Reynolds, J.B. (eds), Handbook of Conducting Polymers, pp. 881–920, Marcel Dekker Inc., New York, Basel. 3. Kuhn, H.H. and Child, A.D. (1998). Electrically Conducting textiles, (Chap. 35), In: Scotheim, T.A., Elsenbaumer, R.L. and Reynolds, J.B. (eds), Handbook of Conducting Polymers, pp. 993–1013, Marcel Dekker Inc., New York, Basel. 4. Olivero, D.A. and Radford, D.W. (1999). Non-contact Percolation and Conductivity in Microsphere Composites, Journal of Advanced Materials, 31(1): 42–50. 5. Olivero, D.A. and Radford, D.W. (1997). Integrating EMI Shielding into Composite Structures, SAMPE Journal, 33(1): 51. 6. Radford, D.W. and Cheng, B.C. (1993). Metallized Microballon Filled Composite EMI Shielding Materials, Journal of Testing and Evaluation, 21(5): 396–401. 7. Lu, X. and Xu, G. (1997). Thermally Conductive Polymer Composites for Electronic Packaging, Journal of Applied Polymer Science, 65(13): 2733–2738. 8. Hou, Y.H., Zhang, M.Q., Rong, M.Z., Yu, G. and Zeng, H.M. (2002). Improvement of Conductive Network Quality in Carbon Black-filled Polymer Blends, Journal of Applied Polymer Science, 84: 2768–2774.



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