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A novel method to prepare high-resistivity polymer composites.

INTRODUCTION

Conductive polymer composites have been used in resistors, heaters, circuit protection devices, and antistatic materials. The conventional method of preparing conductive polymer composites is to disperse a conductive filler such as carbon black, graphite, a metal, a metal oxide, or a ceramic in a polymer. This conventional method can be used to produce a wide variety of products. However, for many combinations of polymeric matrix and conductive filler, it is extremely difficult to obtain reproducible results in some resistivity ranges of interest. The reason for this is that the loading curve, as shown in Fig. 1, has a very short relatively flat upper portion corresponding to the resistivity of the polymer, and then it falls steeply as the concentration of the filler increases. As the filler concentration increases beyond the percolation threshold, the sharp decrease in the resistivity is due to the combined effect of an increase in the number of conductive paths and a decrease in the interparticle distances. As the filler concentration further increases, the resistivity flattens out as the resistivity of the conductive polymer composite approaches the resistivity of the filler itself. Important factors which affect the shape of the loading curves include particle size and morphology of the filler.

For many applications, such as self-regulating heaters (1), antistatic products, shielding materials, and resistors, the desired resistivity often falls on the steep portion of the loading curve. Consequently, the resistivity of the product can change very significantly if there are small changes in the process conditions or the starting materials. It is highly desirable to have a conductive polymer composite which has a loading curve with a shape that is different from a conventional one. In particular, the steep part of the curve can be modified so that the resistivity changes more gradually with the concentration of the conductive filler, as shown in Fig. 1.

We have developed a novel method for preparing high-resistivity conductive polymer composites with the desirable loading-curve characteristics (2, 3). In this method, the traditional conductive filler such as carbon black or a metal powder is replaced by a conductive composite filler. The precursor to the composite filler is a conventional conductive polymer composite which is prepared by mixing a polymer and a conductive filler. The resulting compound, which is crosslinked to a certain level, is then ground into a fine powder which we refer to as the composite filler. The composite filler is blended with another polymer to produce a material that we refer to as the conductive particulate composite. By using the composite filler instead of carbon black or a metal powder, the steep part of a typical loading curve can be modified so that the resistivity changes more gradually with the concentration of the filler. Because of this method, high-resistivity positive-temperature-coefficient (PTC) materials can be produced consistently with the targeted resistivity. In certain ways, this method is analogous to the dispersion of carbon black in a polymer blend system having a discrete phase and a continuous phase with the carbon black distributing predominantly in the discrete phase (4).

METHOD OF PREPARATION

A conductive compound was prepared by mixing 56 wt% high-density polyethylene (Marlex 50100, from Phillips Petroleum), 43 wt% carbon black (Statex G, from Colombian Chemicals), and 1 wt% antioxidant in a Banbury mixer. The compound was extruded into strands through a die and was irradiated to doses ranging from 5 to 80 Mrad using a 1-MeV electron beam. The strands were then cryogenically pulverized until all particles (composite filler) were smaller than 250 microns, as shown in Fig. 2.

Slabs of these compounds were prepared for testing by compression molding before the compounds were irradiated. Then the slabs were irradiated with different beam doses. The [M.sub.100] (the modulus value required to stretch the samples at 150 [degrees] C to 100% of its original length) was measured on the compression-molded slabs. The resistivity of the 80-Mrad composite filler was measured to be approximately 10 ohm-cm at an electric field strength of 0.7 V/cm.

For each different irradiation dose, 67.5 wt% of the composite filler was tumble-blended with 32.5 wt% of high-density polyethylene powder (FA750, from USI Chemicals). The blends were extruded into 0.030 by 3.0 inch tapes. One-mil-thick electrodeposited copper electrodes were laminated on the opposite sides of the tapes. Resistance values of these tapes were measured at 23 [degrees] C and at various field strengths, using a pulse technique with a pulse duration of 100 microseconds and a repetition rate of one per second. This technique cannot be applied to measure the resistivity of highly conductive composites with high voltages because of the limitation of current in the power supply. The resistivity values for low-resistivity materials were measured with an ohmmeter (applied voltage = 50 milli Vs) at 23 [degrees] C. Sections of the extruded tapes were cooled in liquid nitrogen and cryosectioned with a microtome to give samples less than 1000-[angstrom] thick. These samples were studied with a scanning transmission electron microscope (STEM).

Composite fillers were also prepared by chemical crosslinking. A mixture consisting of 56 wt% Marlex 50100, 43% Statex G, and 1% antioxidant was prepared. The mixture was added to a Brabender mixer with 5% by weight of the mixture of Luperco 130XL (a chemical-crosslinking agent containing peroxide available from Elf Atochem). This mixture was cross-linked by heating at 200 [degrees] C for 30 minutes and was then ground to produce particles smaller than 250 microns. The resistivity of this material was measured to be approximately [10.sup.4] ohm-cm at an electric field strength of 1300 V/cm. Using the method described above, conductive particulate composites were prepared and their resistivity values were measured.

Loading curves were produced by measuring the resistivity of the conductive particulate composites which had been prepared with different amounts of 80-Mrad composite filler and with different amounts of the chemical-cross-linked composite filler in high-density polyethylene (FA750). In addition, a conventional loading curve was produced by measuring the resistivity of conventional conductive polymer composites which were prepared by mixing different amounts of carbon black (Statex G) in high-density polyethylene (FA750).

RESULTS AND DISCUSSION

In the preparation of conductive particulate composites, a number of parameters such as composite filler size, composite filler size distribution, degree of crosslinking, degree of composite filler loading, and degree of composite filler dispersion have strong influence on the electrical properties and the shape of the loading curves.

Effects of Composite Filler Size

The average particle size and the particle size distribution would naturally be considered to be the critical parameters in controlling the resistivity of the conductive polymer composites. To study the effects of the composite filler size on the resistivity of the conductive particulate composites, the composite filler was separated into different size fractions by sieving. These size fractions were then compounded in the same proportion with the high-density polyethylene (FA750) - 67.5 wt% filler and 32.5 wt% high-density polyethylene. The compounds were extruded into tapes. The resistivity values of these materials were measured at different electric field strengths as shown in Fig. 3. The results clearly indicate that the resistivity decreases as the size of the composite fillers increases. However, the resistivities of particulate composites prepared with composite fillers of different sizes show the same field dependence, as shown in Fig. 4. This implies that the composite filler size affects only the resistivity of the particulate composites but not their electric-field dependence.

Effects of Crosslinking Density

Figure 5 shows the resistivity of the high-resistivity conductive particulate polymers prepared with the composite fillers which had been beamed to different doses. The crosslinking density was characterized by measuring the moduli of the composite filler materials at 150 [degrees] C. Figure 6 shows the [M.sub.100] of the composite filler materials as function of beam dose. Clearly, the resistivity starts to attain a relatively constant value after the composite filler compound was beamed to above 40 Mrad, while the modulus of the composite filler compounds keeps increasing and starts to level off only at beam doses above 120 Mrad. Figure 7 shows a series of STEM pictures of the particulate composites containing the composite fillers that had been beamed to different doses. At 5 Mrad, the carbon black is uniformly dispersed in the polymer matrix and, hence, we cannot differentiate the composite filler and the polyethylene matrix. At 15 Mrad, boundaries between the composite filler and the polymer matrix start to appear even though a substantial amount of carbon black still leaches out to the polymer matrix. Between 25 and 40 Mrad, clear boundaries between the composite filler and the polymer matrix appear, and the amount of carbon black which has been leached out to the polymer matrix reduces to a very small amount. Well-defined domains of the composite filler are observed. This observation correlates well with the results shown in Fig. 5. The region where the resistivity changes rapidly is caused by the leaching out of the carbon black from the composite filler into the polymer matrix. At low beam doses, this method produces a conductive particulate composite which is not that much different from a conventional conductive polymer composite. As the beam dose increases, the composite filler stays as separate domains from the polymer matrix because the increase in the modulus of the composite filler materials helps to retain the carbon black inside the composite filler during blending. This leads to the appearance of a relatively fiat region (above 40 Mrad) in the loading curve.

Loading Curves of Particulate Conductive Composite Materials

Figure 8 shows the loading curves of the conductive particulate composites prepared with the 80-Mrad composite filler and with the chemical-crosslinked composite filler and of a conventional conductive polymer composite. Comparison of these loading curves shows that the resistivity of the particulate composites changes more gradually as a function of the idler concentration.

The resistivity of the composite filler is undoubtedly an important factor which affects the shape of the loading curve and the resistivity of the particulate composites. In this study, the resistivity of the composite fillers varies from 10 to [10.sup.4] ohm-cm, which is substantially higher than that of a typical carbon black ([10.sup.-2] ohm-cm). The resistivity of the composite filler prepared by chemical crosslinking (approximately [10.sup.4] ohm-cm) is about a thousand times that of the composite filler prepared by electron beam radiation (approximately 10 ohm-cm) even though the composite fillers prepared by the two methods have the same carbon black concentration. The reason for this difference is that the crosslinking introduced by electron beaming and by a chemical-crosslinking agent is different. Radiation-induced crosslinks occur only in the amorphous regions, whereas chemical crosslinking, which occurs at temperatures above the melting point of the polymer, is more homogeneous (5-7). The results shown in Fig. 8 reveal that the resistivity of the composite fillers indeed has a strong influence on the resistivity of the particulate composites - the resistivity of the particulate composites increases as the resistivity of the composite fillers increases. In addition, the composite fillers have a very low level of structure [ILLUSTRATION FOR FIGURE 1 OMITTED] as compared with most conductive blacks. The morphology of the filler particles can affect the contact areas between the filler particles. The combination of these two factors is likely to be the driving force for the shape change in the loading curve.

The loading curves for the particulate composites prepared with the electron-beam-crosslinked composite filler were measured at two electric field strengths. The results clearly show that the loading curve shifts to lower resistivity values at high electric field strengths. The field-dependent resistivity is observed not only at room temperature but also at high temperatures up to 135 [degrees] C as shown in Fig. 9. This information is very important in the design of products for high-voltage applications because the resistance of the products will depend on the thickness of the particulate composite and the applied voltage.

The PTC effect observed in the particulate composites is very interesting because it is unknown at this time whether this PTC effect is caused by the melting of the crystallites in the polymer matrix, or the melting of the crystallites in the polymer, of the composite filler, or their combination.

CONCLUSIONS

The composite filler method has been demonstrated to be a viable means for preparing high-resistivity particulate composites reproducibly. In this study, the results show the change in the shape of the loading curves is controlled by the resistivity, the shape, and the size of the composite filler. Crosslinking of the composite filler can be done by electron beaming or through the use of a chemical-crosslinking agent. The resistivity of the conductive particulate composites is shown to be electric field dependent - the resistivity decreases as the electric field strength increases.

ACKNOWLEDGMENTS

The author wishes to express his gratitude to Raychem Corporation for allowing publication of this paper which is based on the materials developed in Raychem Corporation and reported in Raychem patents: U.S. Patent 4,866,452 (P. Barma and C.-M. Chan), U.S. Patent 5,106,538 (P. Barma, C.-M. Chan, M. Mohebban, N. Rosenweig, and N. Kurjakatko), and U.S. Patent 5,106,540 (P. Barma, C.-M. Chan, M. Mohebban, and N. Rosenzweig).

REFERENCES

1. P. Barma and C.-M. Chan, U.S. Patent 4,866,452 (Sept. 12, 1989).

2. P. Barma, C.-M. Chan, M. Mohebban, and N. Rosenweig, U.S. Patent 5,106,540.

3. P. Barma, C.-M. Chan, M. Mohebban, N. Rosenweig, and E. Kurjatko, U.S. Patent 5,106,538.

4. M. Sumita, K. Sakata, Y. Hayakawa, S. Asai, K. Miyasaka, and M. Tanemura, J. Colloid Polym. Sci., 270, 134 (1992).

5. D. J. Dijkstra, W. Hoogsteen, and A. J. Pennings, Polymer, 30, 866 (1989).

6. Y. H. Kao and P. J. Philips, Polymer, 27, 638 (1986).

7. G. Gielenz and B. J. Jungnickel, J. Colloid Polym. Sci., 260, 742 (1982).
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Author:Chan, C.-M.
Publication:Polymer Engineering and Science
Date:Feb 1, 1996
Words:2321
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