SPIE Proceedings, 4695, 2002, Development of novel composite based on polyethylene and low melting point metal alloy

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1 SPIE Proceedings, 4695, 2002, Development of novel composite based on polyethylene and low melting point metal alloy Edward Bormashenko *a, Semion Sutovski a, Roman Pogreb a, Avigdor Sheshnev a, Alexander Shulzinger a, Mark Levin b, Zahava Barkay c & Abraham Katzir c a The College of Judea and Samaria, Ariel, 44837, Israel b Tel-Aviv University, School of Physics and Astronomy, Tel Aviv, 66978, Israel c Bar Ilan University, Department of Physics, Ramat Gan, Israel ABSTRACT This work presents the novel composite material based on a polymer (LDPE) and metal alloy, which have a close melting point. The process of blending the main components of the composite was developed using the extrusion mixing. The process of extrusion of thin ( m) films based on the composite was worked out using an extruder equipped with a cast film die. This process allowed production of the binary composite, with up to 15 wt.% of the alloy dispersed uniformly in the polymer matrix. It was shown that carbon black is highly compatible with the basic components and could be dispersed uniformly in the obtained films. The structure of the composite films was studied with a scanning electron microscope. Physical properties of the binary and ternary composites were investigated, including conductivity and temperature dependence of the thermal capacity. Non-ohmic behavior of the ternary composites was revealed. The IR spectra of the composites were studied using an FTIR 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. Keywords: composite, polyethylene, alloy, low melting point, conductivity, carbon black, infrared spectrum 1. INTRODUCTION The development of novel conductive polymers and composites is crucial for future progress in the field of electroactive polymers (EAP). 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 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 (1-3). 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 (4). Bar-Cohen indicated that polyurethane seems to be a good candidate for EAP actuator applications, because it produces large displacement under electroactivation (5). The prevailing approach to the changing of electrical properties of traditional polymers is filling the polymer with carbon black. Electrical properties of carbon-filled polymer composites were studied thoroughly by the group led by Professor Narkis (6-7). 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 (8-9). Experimental data obtained by different groups were explained within the framework of inner-particle tunneling conduction (10) and framework of classical percolation theory (11). However these basic explanations are not compatible with each other conceptually, and novel percolation-tunneling model was proposed by Balberg (12). According to Balberg the conductivity of carbon black-polymer composites was found to depend on the structure of the carbon black particles. 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 (13). It was shown that very different structures could be obtained in such systems: 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

2 (including those filled by carbon black) are discussed in (14). The catalytic activity of metals when processed in conjunction with polymers was discussed in detail in (15). 2. EXPERIMENTAL RESULTS 2.1 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%, Cdsupplied by Carmel Olefinim (density g/cm 3, melt flow index = 2.0 g/10 min (ISO 1183)). 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 single-screw extruder (diameter of the screw 32 mm, L/D =25), under the temperature regime T 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). low melting point alloy mixing extruder extruded blend pelletizer granules of the polymer/alloy blend composite film drawing unit extruder cast film die Fig.1 Technological process of preparing the LDPE/low melting point alloy composite films 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 m by varying the drawing speed in the range 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 m necessary for IR spectroscopy procedures, we applied a high-precision laboratory hydraulic press, equipped with a heated platens Specac P/N The pressing of thin films was carried out under the 5 Pa.

3 2.2 Electronic Microscopy Study of the Structure of the Composite Fig.2 SEM image of the typical structure of the composite (15% alloy + 85% LDPE, the thickness of the sample m). The white shapes are alloy particles and the background is polymer. 50m a 50 m b Fig. 3 SEM image of the structure of the ternary composite 15% alloy + 2% carbon black +8 3% LDPE. 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. Fig. 2 presents the SEM image of the typical structure obtained when the blend which contained 15% of metal alloy was processed into film of 100 m 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

4 alloy which have mainly spherical form with a 1-5 m average diameter were dispersed uniformly in the LDPE matrix. Fig. 3a-3b present the SEM images of the ternary composite, which contained 83% LDPE, 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. 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. Table 1 Conductivity of composite films Formulation of the composite Specific Resistivity, cm 100% LDPE % alloy + 95% LDPE % alloy + 90% LDPE 12 15%alloy + 85% LDPE 10 15% alloy + 1% carbon black + 84%LDPE 10 15% alloy + 2% carbon black + 83%LDPE 10 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 - 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) increasing the metal proportion in the composite, (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. 1.6E E E-11 1E-11 I,A 8E-12 6E-12 4E-12 2E U,V Fig. 4. Current-voltage dependence for the binary composite (85% LDPE+15% alloy). The thickness of the sample m.

5 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. 4. 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. 5). After exceeding of some threshold value of the voltage the behavior of the relationship changes to ohmic. This phenomenon (switching from low to high resistance in carbon black polymer composites) was discussed by Balberg (12), and it is related to the reversible heating-induced-swelling of polymer layer between adjacent carbon black particles. 2.5E-12 2E E-12 I,A 1E-12 5E U,V Fig. 5 Current-voltage dependence for the ternary composite (83% LDPE+15% alloy+2% carbon black). The thickness of the sample m. 2.3 Study of temperature dependence of thermal capacity of the composite Fig.6. Temperature dependence of the thermal capacity of the composite which contained 15% alloy and 85% LDPE Cp / Cmax t, o C

6 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. 6 presents the temperature dependence of the thermal capacity of the binary composite, normalized to the maximal value metal alloy could be recognized from the experimental data. Pure polyethylene demonstrated peak on the temperature dependence of thermal capacity at 115- 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. Fig. 5. Infrared transmittance spectra of pure LDPE, 15% alloy +85% LDPE composite and ternary composite: 15% alloy + 2% carbon black + 83% LDPE It could be recognized from the spectral data that the composite, which contained metal alloy demonstrated -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 (15). It is reasonable to suggest that the reactions described in (15) take place as follows: ~CH 2 -CH 2 -CH 2 -CH 2 ~ O 2 alloy ~CH 2 -C-CH 2 -CH 2 ~ + H 2 O 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:

7 ~CH 2 -CH 2 -C-CH 2 ~ O ~CH 2 -CH 2 -C-CH 2 ~ 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. 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 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 ) 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 - Journal of Advanced Materials, 31, No.1, pp.42-50, , No.1, pp.51-55, Journal of Testing and Evaluation, 21, No.5, pp , Journal of Applied Polymer Science, 65, Issue 13, pp , Low-mass muscle actuators using electroactive SPIE Proceedings, 3324, pp , Narkis M., Ram A., Stein Z., Electrical Properties of Carbon Black Filled Crosslinked Polyethylene, Polymer Engineering and Science, 21, No.16, pp , Narkis M., Zilberman M, Siegman A., On the Curiosity of Electrically Conductive Melt Mixed Doped Polyaniline/Polymer Blends versus Carbon-Black/Polymer Composites, J. Polym. Adv. Technol., 8, p. 525, Heaney M.B., Electrical Transport Measurements of a Carbon-Black-Polymer Composite, Physica A, 241, pp , 1997.

8 9. Adriaanse L.J., and oth. Non-Linear Transport in Disordered Carbon-Black/Polymer Composites, Synthetic Metals, 84, pp , Sichel E.K., Gittelman J.I., Sheng P., Transport properties of the material carbon polyvinylchloride, Physical Review B, 18 (10), pp , Reboul J.P., Mausalli G., About some DC conduction processes in carbon black filled polymers, International Journal of Polymeric Materials, 5(5), pp , Balberg I. A comprehensive picture of the electrical phenomena in carbon black-polymer composites, Carbon, 40, pp , Bormashenko Ed., Sutovsky S., Pogreb R., New Oriented Polymer/Thermoplastic Glass Composites for IR Optics, Advanced Engineering Materials, 2, No.10, pp , Coleman M.M., Painter P.C., Fourier Transform Infrared Spectra of Macromolecules, in Applications of Polymer Spectroscopy, edited by Edward G. Brame, JR., p. 135, Academic Press, New York, San Francisco, London, Kirillova E., Shulgina E., Ageing and stabilization of thermoplastic polymers, Chemistry, Leningrad, * Corresponding author, address: The College of Judea and Samaria, The Research Institute, Laboratory of Polymer and Composite Materials, 44837, Ariel, Israel, Tel: , Fax: polytris@netvision.net.il