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Study of electrical phenomena in carbon black-filled HDPE composite.

INTRODUCTION

The electrical conductivity of carbon black-filled polymer composites has been the subject of both theoretical and experimental interest for a long time because of various industrial applications, such as self regulating heaters, circuit protection devices, anti-static protectors, and electromagnetic interference shields, (1-4). The major research topics in this field have included explanations of the electrical phenomena, which involve percolation, positive temperature coefficient (PTC), and negative temperature coefficient (NTC) of electrical resistivity (5-14).

Through a review of electrical conduction in carbon black composites, Medalia explained that the percolation is due to the tunneling of electrons and the conductivity is controlled by the gaps between carbon black aggregates (8). For the mechanism of PTC, Kohlor (9) and Ohe (10) suggested that the PTC phenomenon is caused by the thermal expansion, and the resistivity increase is due to the widening of the gaps between carbon black aggregates and the change of the uniformity of the gaps, respectively. Also, Meyer supposed that thin crystalline forms of polymer are much better conductors than amorphous films, which appear after crystalline melting (11, 12). In the case of NTC, it was reported by Narkis et al. and Tang et al. that the resistivity decrease is due to the reagglomeration of carbon black aggregates (13, 14). However, because these phenomena cannot be observed by electron microscopy and other techniques, there has been difficulty in finding a comprehensible explanation with experimental evidence for the phenomena, and thus, most of the mechanisms still remain controversial. Narkis et al. (15) and Kubat et al. (16) showed the temperature dependence of the real part of the relative permittivity and the loss factor through the study of AC electrical properties in carbon black-filled polymer composites. However, these results were not explained in relation to PTC and NTC phenomena.

In this paper, the measurement of admittance and the phase angle was performed to suggest the experimental evidence for identifying such phenomena as percolation, PTC, and NTC. For the study, the variations of admittance and phase angle in the carbon black-HDPE system were observed during the occurrence of the phenomena, and the relationship between these electrical properties and the changes of carbon black morphologoy was investigated.

EXPERIMENTAL

Material and Sample Preparations

High-density polyethylene (HDPE, from Union Carbide co.) with a density of 0.0945 kg/[m.sup.2] was used as a matrix polymer. The HDPE has a weight average molecular weight (Mw) of 150,000 and a polydispersity index of 8.5. Vulcan XC-72 (Cabot co.) was used for the conductive carbon black. The carbon black had average particle size of 30 nm, [N.sub.2] surface area of 0.254 [m.sup.2]/kg and DBP absorption of 17.8 cc/kg.

Composites of HDPE and carbon black were prepared by using two roll mill. The mixing was performed for 5 min for the batch of each carbon black concentration at the roll surface temperature of 150 [degrees] C. The compounds were preheated for 15 min without pressure and then compression molded for 5 min at 180 [degrees] C with a specially designed mold. The samples were 200 mm in length, 10 mm in width, and 1.5 mm thick. Two nickel-plated copper wires (7 strands, 0.45 mm thickness per each strand) were used for electrodes; these were inserted at each side of the sample strips when the samples were molded. Also, the samples were jacketed with polymeric tubes to maintain the dimensions when the samples were exposed at over the crystalline melting temperature of the matrix polymer. Annealing of the samples was conducted at 140 [degrees] C for 24 hr and the rate of heating and cooling was 1 [degree] C/min.

Measurement of Electrical Properties

Admittance and phase angle in the AC field were measured with an LF Impedance Analyzer (Hewlett Packard Model 4192A) in the frequency range of 0.01 kHz to 10 Mhz; the values of admittance were converted and expressed in those per unit cube. The measurement, except the experiments of frequency dependence, was performed at the frequency of 100 kHz because it covers the whole measuring range. The samples were placed in the chamber to measure the electrical properties at various temperatures; two coaxial cables were used for connection between the samples in the chamber and the impedance analyzer. A computer system with a GPIB card was adapted for automatic data acquisition. The properties as a function of carbon black concentration and frequency were measured at a temperature of 25 [degrees] C. Also DSC (DuPont Model 2910) and Gel Permeation Chromatography (Waters Model 150C) were used for characterization of the polymer matrix.

RESULTS AND DISCUSSION

Resistor-Capacitor Circuit Model

In general, carbon black has been thought to exist as a form of agglomerates, which comprise aggregates fused together in chains or clusters by spherical particles. The electrical conductance in the carbon black-polymer composites depends on the distance between aggregates; hence, the composites can be regarded as the system composed of random arrays of closely spaced conductors dispersed in an insulating polymer matrix. Kawamoto suggested the simple resistor-capacitor (RC) circuit model, which is shown in Fig. la, in order to explain the electrical properties for the carbon black-polymer system (17). In the model, the two possible electrical current components, competing with each other in a contact region between carbon black aggregates, are supposed; one is a current through an internal contact resistor [R.sub.c], and the other is a current through a contact capacitor [C.sub.c]. [R.sub.a] represents the resistance within the carbon black aggregate. If an equivalent single parallel RC circuit model, as shown in Fig. lb, is adapted to simplify the composite in which there are multiple RC circuits, the current components in the composite can be analyzed by an impedance analyzer. In the experimental measurement, the net admittance through resistor and capacitor in the single parallel circuit is given by

Y = G + jB (1)

Phase Angle ([Theta]) = [tan.sup.-1](B/G) (2)

where the conductance (G), which is the real part of the admittance, is due to the current through the contact resistor, and the susceptance (B), which is the imaginary part of the admittance, is due to the current through the contact capacitor. The phase angle ([Theta]) means the phase difference between two current components. From the relationship of conductance, susceptance, admittance, and phase angle, it can be found that the measurement of these electrical properties not only gives the electrical response itself, but also could provide effective information about carbon black distribution in the composites. The following shows how the electrical phenomena in the carbon black-HDPE system can be explained in relation to the morphological change within the composite by using this experimental technique.

Percolation and Annealing

Figures 2a and 2b show admittance and phase angle, respectively, as a function of carbon black concentration for non-annealed and annealed HDPE samples. It is well known that the mixing process in the carbon black-polymer system leads the agglomerates to break into the aggregates (or smaller agglomerates than in the dry state before mixing) and lowers the conductance (17). In this study, annealing was employed to recover the structure of agglomerates and to get reproducible data.

Figure 2a shows a typical curve of admittance versus carbon black concentration. As expected, both non-annealed and annealed samples exhibit a steep increase in admittance at their percolation threshold and maintain high values after percolation. The variation of phase angle with an increase in carbon black concentration for both samples is shown in Fig. 2b. In the Figure, the values of phase angle for two kinds of samples, which are high at the low concentrations, decrease abruptly at their percolation threshold and become low after the percolation region. These results can be explained again on the basis of the relationship of the electrical properties and the morphological change. That is, if the composite has a high value of the phase angle, over 90 [degrees] with a low value of admittance as shown in the low level of carbon black concentrations, it can be explained that most of the current flows through only the contact capacitors of the composite, and there exist few direct contacts between carbon black aggregates. Alternatively, if the composite has a low value of phase angle, near with a high value of admittance as shown in the high level of carbon black concentrations, it can be said that the current mainly flows through the contact resistors within the composite and the conduction networks between carbon black aggregates are well formed. And, from the rapid change of admittance and phase angle in the percolation region, it can be presumed that the distribution of carbon black in the composite changes with carbon black concentration. Figures 2a and 2b also show the effect of annealing on the electrical properties, and it can be seen that the percolation threshold for annealed samples is shifted to a low concentration by about 2-3 phr as compared with non-annealed samples. The effect of annealing is especially remarkable with the composites in the percolation region, and in the case of the sample filled with 20 phr, the admittance increases from 1.1x[10.sup.-5] [[Omega].sup.-1][cm.sup.-1] to 7.1x[10.sup.-4] [[Omega].sup.-1][cm.sup.-1].

In Fig. 3, the conductance and susceptance as a function of carbon black concentration for annealed composites are plotted. In the Figure, as the level of concentration is low, the values of susceptance are higher than those of conductance, implying that the current mainly flows through the contact capacitors. As the level of concentration is high, the values of conductance are higher than those of susceptance, implying that the current mainly flows through the contact resistors. And the reverse is occurring at the percolation region. In view of these results, it can be confirmed again that the plotting of the phase angle, which means the ratio of susceptance and conductance, is useful for describing morphological changes in carbon black-filled composites. On the other hand, in Fig. 3, as the carbon black concentration is increased, the susceptance increases to the concentration of about 20 phr and then decreases. The assumption is that the increase is due to the reduction of the distance of carbon black aggregates, and the decrease is due to the increase of direct contact between them.

Figures 4a and 4b describe how the distribution changes of carbon blacks within the composites occur during annealing. For the experiments, two nonannealed samples filled with 16 phr and 18 phr carbon black were selected, and the distribution changes were observed by measuring the admittance and phase angle.

A three-stage explanation is possible for the results shown in Fig. 4a. In the first stage, the values of admittance for two samples decrease at the beginning of the heating process and then begin to increase steeply. In the second stage, at which the temperature is maintained at 140 [degrees] C, the steep increase of the admittance in the first stage continues, and then the slope gradually diminishes. In the third stage, the values of admittance decrease at the beginning of the cooling process and increase again. It can also be seen that while the annealing effect with the 16 phr sample is small, the 18 phr sample is effectively annealed and the value of admittance changes from 1.4x[10.sup.-6] [[Omega].sup.-1][cm.sup.-1] to 3.2x[10.sup.-5][[Omega].sup.-1][cm.sup.-1].

In Fig. 4b, the values of phase angle for two samples are plotted during the process of annealing. The changes of phase angle at the first and the third stage can be explained with the DSC thermogram in Fig. 5 which shows the thermal behavior of the matrix polymer (HDPE) during the heating and cooling process. In the Figure, the crystalline melting and the crystallization for the matrix are observed at 130 [degrees] C ([T.sub.m]) and 118 [degrees] C ([T.sub.c]), respectively. From Figs. 4b and 5, one can assume that the abrupt increase of the phase angle that follows after the decrease at the beginning of the heating process is due to the crystalline melting of the matrix, and the abrupt decrease at the end of the cooling process is due to the crystallization of the matrix. In the second stage, the values of phase angle sharply decrease, and low phase angles are maintained with increasing annealing time, which means that the reagglomeration between carbon black aggregates is increased within the composite. This result confirms that there exists a driving force for the reagglomeration between carbon black aggregates, and this force is likely attributed to Van der Waals interaction between carbon black aggregates activated by molecular motion in the polymer melt (18).

Frequency Dependence of Carbon Black Networks

Figures 6a and 6b show the frequency dependence on admittance and phase angle, respectively, for five annealed samples with different carbon black concentrations in the percolation range. In the Figures, both admittance and phase angle increase with increasing frequency, and the frequency dependence on admittance and phase angle decreases with increasing carbon black concentration. As the frequency is swept from 0.01 kHz to 10 MHz with the 16 phr filled sample, the admittance increases from 1.6x[10.sup.-7] [[ohms].sup.1][cm.sub.-1] to 1.1x[10.sup.-4] [ohms].sup.-1][cm.sup.-1], and the phase angle increases from 1.7 [degrees] to 85.5 [degrees]. On the other hand, with the 20 phr filled sample, the admittance increases from 7.0x[10.sup.-4] [[ohms].sup.-1][cm.sup.-1] to 1.2x[10.sup.-3] [[ohms].sup.-1][cm.sup.-1] and the phase angle increases from 0.04 [degrees] to 27.5 [degrees] as the frequency is swept. A little frequency dependence observed at high concentrations can be explained with the model shown in Fig. 1. That is, as the concentration is increased within the composite, the amount of direct contacts between carbon black aggregates is increased, and consequently, the component of contact capacitors that has a strong dependence on frequency is decreased.

PTC and NTC

Figures 7a and 7b show the variation of the admittance and the phase angle for the annealed 20 phr carbon black filled composite during the heating and cooling process, respectively.

In Fig. 7a, it can be seen that the admittance of the sample decreases in response to the increase of temperature to below the [T.sub.m] of HDPE (PTC), and then steeply increases with a temperature rise over [T.sub.m] (NTC). Also, the Figure shows that the phase angle increases with temperature and reaches a maximum value at around [T.sub.m]. After [T.sub.m], the phase angle abruptly decreases, almost reaching room temperature. It can be inferred that the conduction networks within the composite are broken with the temperature reaching [T.sub.m] and then are reformed at over [T.sub.m]. Therefore, these results could be regarded as experimental evidence that the PTC phenomenon is due to the deagglomeration or the breakage of conduction networks. On the other hand, with this result only, it cannot be proved that the resistivity increase in PTC phenomena is caused either by simple enlargement of the gaps between carbon black particles [proposed by Kohler (9)] or by the change of the uniformity of the gaps [proposed by Ohe (10)] or others. However, the theory that the crystalline melting of the matrix results in a carbon black distribution change and breakage of the conduction networks mainly formed at amorphous area in the polymer matrix, which was proposed by Narkis (13) and Tang (14), might be proper to describe the PTC phenomena in the carbon black-crystalline polymer system. Also, we observed the regeneration of the conduction networks over [T.sub.m] and this confirmed that the mechanism of the NTC phenomenon is due to the reagglomeration or the networking induced by the attraction (such as Van der Waals interaction) between carbon black aggregates (13, 14).

In Fig. 7b, it can be seen that the admittance decreases to almost the crystallization temperature ([T.sub.c]) of HDPE, and then gradually increases during the cooling process. In the case of the phase angle, the value keeps decreasing after [T.sub.c] with a temperature decrease. These results mean that the conduction networks are broken in the beginning of crystallization of HDPE, and then are reformed (reagglomeration) during the crystallization. It is probable that the breakage of carbon black networks near [T.sub.c] is due to the nucleation, and that reformation of the networks below [T.sub.c] is due to the gathering of carbon blacks in the interphase of lamella fibrils or spherulites by sweeping out from the crystals of HDPE (13).

CONCLUSION

The behaviors of percolation, PTC, and NTC in carbon black-filled HDPE composites are explained on the basis of single parallel RC circuit model by measuring of admittance and phase angle during the occurrence of the phenomena. The morphological change of carbon black in the the percolation region and the experimental evidence were suggested to clarify the theory that the PTC phenomenon is due to the deagglomeration or the breakage of conduction networks, and the NTC phenomenon is due to the reagglomeration or the networking of carbon black aggregates. Also, it was seen that admittance and phase angle increase with increasing frequency, and the frequency dependence on admittance and phase angle decreases with increasing carbon black concentration. It can be explained that a little frequency dependence observed at a high concentration of carbon black is due to the decrease of the components of the contact capacitor that have a strong dependence on frequency. Further, the annealing technique adapted in this study was effective in increasing the admittance of the composite, and it is assumed that this increase is due to the reagglomeration of carbon black aggregates.

ACKNOWLEDGMENT

The authors acknowledge the financial support of the Korea Research Foundation made in the 1996 program year.

REFERENCES

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14. H. Tang, J. Piao, X. Chen, Y. Luo, and S. Li, J. Appl. Polym. Sci., 48, 1795 (1993).

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18. N. Probost et al., Carbon Black, J.B. Donnet et al., eds., Marcel Dekker, New York (1993).
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Author:Lee, Gun Joo; Suh, Kyung Do; Im, Seung Soon
Publication:Polymer Engineering and Science
Date:Mar 1, 1998
Words:3248
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