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Electrical conductivity of poly(vinyl chloride) plastisol-short carbon fiber composite.

INTRODUCTION

There has been recent interest in using carbon fiber to produce electrically conductive polymers. Martinsson et al. (1) studied anisotropic conductive thermoplastics filled with electrically conductive fillers, including carbon fibers, processed by extrusion and injection molding. Calleja et al. (2) studied electrical conductivity of high density polyethylene-carbon fiber composites mixed with carbon black. Li et al. (3) reported electrical and mechanical properties of conductive polyethersulfone composites containing carbon fibers processed by hot-pressing. Ramadin et al. (4) reported the electrical properties of laminated epoxy-carbon fiber composites.

Jana et al. (5, 6) studied electrically conductive polychloroprene rubber filled with short carbon fibers processed by the cement mixing and the mill mixing methods. The average fiber aspect ratio (L/D), which was initially 800, was reduced to [approximately]25 and 100 in the mill and cement mixing processes (5). According to Jana et al. (6), higher fiber aspect ratio produces composites with higher electrical conductivity.

This paper reports an electrically conductive network formed in PVC plastisol-carbon fiber composites with very low concentrations of short carbon fibers. PVC plastisol is a suspension of small particles of PVC resin in a liquid plasticizer. Here the short carbon fibers were mixed with the PVC plastisol by mechanical stirring to form a paste. Then the paste was further plasticized under static conditions.

PVC plastisol-carbon fiber composites differ from other short carbon fiber-filled polymer systems that have been reported (1-6) in that the short carbon fibers retain their initial lengths. In this paper the effects of concentration and initial length of the carbon fibers on the electrical resistivity of the conductive network of PVC plastisol-carbon fiber composites were studied. The surface resistivity of the composite samples prepared by the plastisol method was also compared with samples prepared by the mill mixing method.

EXPERIMENTAL

Materials Used

Micro-suspension PVC resin (type PSM-31) manufactured by Shenyang Chemical Industry Co. (China) was used in the plastisol. Resin PSM-31 is of moderate degree of polymerization and low viscosity in the plastisol. Long carbon fibers (medium strength, 0.007 mm diameter, based on polyacrylonitrile (PAN) fibers) were obtained from Liaoyuan Petrochemical Works (China). The long carbon fibers were cut to form short carbon fibers with specific lengths. A commercial grade suspension PVC resin (moderate degree of polymerization) was used to prepare samples by the mill mixing method. Other compounding ingredients such as di-octyl phthalate (DOP), barium-cadmium-zinc liquid stabilizer were also commercial grade.

Preparation of the Samples

Plastisol Method

The PSM-31 PVC resin was mixed with DOP and the stabilizer, to form a PVC plastisol. The short carbon fibers were then dispersed into the plastisol by stirring. The lengths of the carbon fiber were 1.2, 2.0, 3.0, and 5.0 min. The fiber-filled paste was coated on a steel plate, and then the plate was put into an oven for 10 min at 180 [degrees] C to melt the plastisol. Sheets of the composites, 1 mm thick, were prepared by this static processing method. A key aspect of this method is that the fibers were not broken.

The samples prepared by the plastisol method had two different surfaces: the surface in contact with air during processing (called the front surface) and the surface in contact with the steel plate (called the reverse surface).

Mill Mixing Method

A two-roll mill was also used to prepare samples. Short carbon fibers with 5.0 mm length were mixed with PVC resin, DOP, and stabilizer. The mixture was melted in a two-roll mill at 160 [degrees] C for 5 min to form 0.8 mm thick sheets.

Measurement of the Electrical Resistivity

The samples were cut into 10 x 10 cm squares with a paper cutting machine prior to measuring the volume resistivity. Each square had four edges, two run parallel to the direction for the coating process and two run normal to the coating direction. As a result of cutting the samples, the tips of the carbon fiber were exposed on the edges of the samples.

A high resistance meter (Shanghai Electrical Meter Factory, China, ZC-46A) was used for measuring the surface resistivity. An amp-volt-ohm meter (Univolt, Japan, DT-830), which was also called avometer, was used for measuring the volume resistivity of the conductive network of the samples prepared by the plastisol method. For this measurement, the avometer was used as an ohmmeter.

All of the samples for electrical measurements were treated at [approximately]23 [degrees] C and relative humidity of 50 [+ or -] 5%, for 24 h, before the measurement.

For measuring the volume resistivity of the conductive network of the samples prepared by the plastisol method, a sample with 10 cm length and 1 mm thickness was placed on an insulated plate. The two probes of the avometer were then brought into contact with the two opposite edges of the sample. The probes directly contacted the tips of the carbon fiber exposed on the edges. As a result, the resistivity data thus obtained expressed the conductivity of the carbon fiber network in the samples.

The volume resistivity of the conductive network was tested in two ways: by contacting the probes to opposite edges parallel to the coating direction and contacting the probes normal to the coating direction.

The surface resistivity data were tested by the general method. For the samples of the plastisol method, the surface resistivity of both the front and reverse surfaces was tested. The surface resistivity of the samples prepared by the plastisol method was compared with the samples by the mill mixing method.

Observation of the Morphology

The morphology of short carbon fiber in the samples was observed with an optical microscope.

RESULTS AND DISCUSSION

Effects of Carbon Fiber Length and Concentration on the Volume Resistivity of the Conductive Network of Carbon Fiber in the Samples Prepared by the Plastisol Method

The dependence of the volume resistivity of the conductive network of carbon fiber on the initial length of carbon fiber for the samples prepared by the plastisol method is shown in Table 1. The concentration of carbon fiber in the samples was 2 phr. The other materials in the composite were PVC (PSM-31, 100 phr), DOP (60 phr), and a stabilizer (4 phr). The resistivity was measured in two directions. Direction A ran parallel to the coating direction of the samples (i.e. the two probes of the avometer were contacted with two opposite edges that were normal to the coating direction) and direction B was normal to the processing direction.

As shown in Table 1, for a carbon fiber length of 1.2 mm (L/D = 171), the resistivity was higher than 2 x [10.sup.4][Omega]-m. When the initial length of the carbon fibers was 2.0 mm (L/D = 285), the resistivity dropped to 9.6 x [10.sup.-1] [Omega]-m for direction A and 12.6 x [10.sup.-1] [Omega]-m for direction B. Increasing the length of carbon fiber from 2.0 mm to 5.0 mm resulted in only a small reduction of the resistivity.

It is evident that there is a conductive network in the samples of PVC plastisol-carbon fiber composites when the initial length of the carbon fiber was longer than 2.0 mm (L/D = 285). The conductive network showed excellent conductivity, although the concentration of carbon fiber was very low, i.e., 2 phr. According to these results, the initial length of the carbon fiber plays an important role in the conductivity of the network. It seems that there is a critical fiber [TABULAR DATA FOR TABLE 1 OMITTED] length in the range of 1.2 to 2.0 mm to achieve conductivity with a fiber concentration of 2.0 phr.

The dependence of the volume resistivity of the conductive network of carbon fiber on the concentration of the fiber in the samples prepared by the plastisol method is shown in Table 2. The concentrations of carbon fiber were 0.5, 1.0, 1.5, 2.0, and 4.0 phr. The composite composition was otherwise the same as for the samples shown in Table 1. The initial length of carbon fiber was 3.0 mm (L/D = 428). When the concentration of carbon fibers increased from 0.5 to 1.0 phr, the resistivity dropped sharply from 4.3 x [10.sup.5] to 5.1 x [10.sup.0] [Omega]-m for direction A. When the concentration was 4.0 phr, the resistivity dropped to 7.3 x [10.sup.-2] [Omega]-m.

The concentration where a sharp transition of electrical performance occurs from insulative to conductive represents the critical concentration. For the samples prepared by plastisol method, the critical concentration of the network of carbon fiber occurred in the range of 0.5 to 1.0 phr (see Table 2). This is a very low critical concentration for a carbon fiber-filled polymer composite.

According to the data shown in Table 1 and 2, the resistivity in direction A was always lower than in direction B. It is well known that short fibers in a fluid will preferentially orient in the flow direction, which for this coating process was direction A. As a result, the resistivity in the direction A was always lower than the direction B because of the better fiber orientation in direction A.

The Surface Resistivity of the Samples Prepared by the Plastisol Method

The dependence of the surface resistivity of PVC plastisol-carbon fiber composite on the initial length of the fiber is shown in Table 3. The compositions of the samples were the same as the samples in Table 1.

The surface resistivity was tested on the two different surfaces: the front and the reverse surfaces. The surface resistivity of the front surface of the samples dropped sharply from 3.9 x [10.sup.8] [Omega] to 9.0 x [10.sup.3] [Omega], when the initial length of the carbon fibers increased from 1.2 to 2.0 mm. For the sample with an initial fiber length of 5.0 mm, the resistivity of the front surface dropped to 5.9 x [10.sup.2] [Omega].
Table 2. Effects of the Concentration of Carbon Fiber on Volume
Resistivity of the Network of Carbon Fiber in the Samples Prepared
by the Plastisol Method.

(Initial length of carbon fiber 3.0 mm)

 Volume Resistivity of
 the Network ([Omega]-m)
Concentration Initial
of Carbon Aspect Direction Direction
Fiber (phr) Ratio (L/D) A B

0.5 428 4.3 x [10.sup.5] 5.5 x [10.sup.5]
1.0 428 5.1 x [10.sup.0] 6.6 x [10.sup.0]
1.5 428 1.0 x [10.sup.0] 2.4 x [10.sup.0]
2.0 428 5.1 x [10.sup-1] 9.3 x [10.sup.-1]
4.0 428 7.3 x [10.sup.-2] 7.9 x [10.sup.-2]
Table 3. Effects of the Initial Length of Carbon Fiber on Surface
Resistivity of the Samples Prepared by the Plastisol Method.

(Carbon fiber concentration 2.0 phr)

 Surface Resistivity ([Omega])
Initial Length Initial
of Carbon Aspect Front Reverse
Fiber (mm) Ratio (L/D) Surface Surface

1.2 171 3.9 x [10.sup.8] 1.8 x [10.sup.11]
2.0 285 9.0 X [10.sup.3] 5.7 X [10.sup.9]
3.0 428 8.9 x [10.sup.3] 4.8 x [10.sup.9]
5.0 714 5.9 x [10.sup.2] 2.9 x [10.sup.9]


It has to be noted that the volume resistivity of the network of carbon fiber also exhibited a sharp reduction in the length range of 1.2 to 2.0 mm, which is the critical fiber length of the network.

The surface resistivity of the reverse surface was much higher than that of the front surface (see Table 3). There was not a sharp reduction for the resistivity of the reverse surface. The difference in the resistivity between the reverse and front surfaces was caused by an accumulation of short carbon fibers at the front surface. Observation of the morphology showed that the concentration of carbon fiber near the front surface was higher than near the reverse surface.

The effect of carbon fiber concentration on the surface resistivity is shown in Table 4. The concentrations of the fiber were 0.5, 1.0, 2.0, and 4.0 phr. The composite formulations were the same as for the samples in Table 1. When the concentration increased from 0 to 0.5 phr, the surface resistivity of the front surface was reduced from 1.4 x [10.sup.12] [Omega] to 2.9 x [10.sup.10] [Omega]. A sharp reduction of the surface resistivity of the front surface occurred in the range of 0.5 to 1.0 phr, in which the surface resistivity dropped to 2.2 x [10.sup.4] [Omega]. That concentration range is the critical concentration of the network of carbon fiber (see Table 2).

Excellent antistatic properties were obtained for the sheets prepared from the PVC plastisol-carbon fiber composites at a very low concentration of carbon fiber, ca. 1.0 phr.

The Surface Resistivity of the Samples Prepared by the Mill Mixing Method

The effects of the carbon fiber concentration on the surface resistivity of the samples prepared by the mill mixing method are shown in Table 5. The fiber concentration in the mill mixed samples were 3.0, 5.0, 10.0, and 20.0 phr. The initial length of the fibers was 5.0 mm. The formulations contained PVC (100 phr), DOP (40 phr), and stabilizer (4 phr).
Table 4. Effects of the Concentration of Carbon Fiber on Surface
Resistivity of the Samples Prepared by the Plastisol Method.

(Initial length of carbon fiber 3.0 mm)

 Surface Resistivity ([Omega])
Concentration Initial
of Carbon Aspect Front Reverse
Fiber (phr) Ratio (L/D) Surface Surface

0 - 1.4 x [10.sup.12] 1.5 x [10.sup.12]
0.5 428 2.9 x [10.sup.10] 6.6 x [10.sup.10]
1.0 428 2.2 x [10.sup.4] 1.9 x [10.sup.9]
2.0 428 8.9 x [10.sup.3] 4.8 x [10.sup.9]
4.0 428 2.2 x [10.sup.3] 1.1 x [10.sup.10]


Although the concentrations of carbon fiber in the mill mixed samples were much higher than in the plastisol samples, there was no sharp reduction of the surface resistivity in Table 5.

The Morphology of Carbon Fiber in the Composite Samples

The morphology of the carbon fibers in the samples prepared by the mill mixing method and the plastisol method was observed with an optical microscope. The photomicrographs of the samples prepared by the two methods are shown in Fig. 1.

Figure 1a shows that the carbon fibers in the mill mixed samples were broken. The average length of the broken fibers was [approximately]0.2 mm (L/D = 28). Since the initial length of the fiber in the mill mixed samples was 5.0 mm (L/D = 714), it is evident that there was a considerable breakage of the fibers during the mill mixing. The fiber length distribution of the mill mixed samples was broad, as shown in Fig. la. It was also found that the fibers were highly orientated in the mill mixed samples.

The morphology of carbon fibers in the plastisol samples is shown in Fig. lb. The fibers mainly retained their initial length without obvious breakage. The fiber length distribution, which was determined by observation of the morphology, was narrow. The fibers in the plastisol samples were only partially orientated.

The fiber breakage and orientation that occurred during mill mixing explains the different resistivity behavior of the mill mixed and plastisol samples. A network, with excellent conductivity, was easily formed in the plastisol samples at very low concentrations of fiber ([approximately]1.0 phr), because the fiber retained its initial length. In the mill mixed samples, a network of the fiber does not form, because of the reduced fiber length and high orientation. The partial orientation of the fibers in the plastisol samples causes the difference in the resistivity between the directions A and B (see Table 1 and 2).

The morphology observation also showed that the concentration of carbon fiber in the area near the front surface was higher than that near the reverse surface. This explains why the resistivity of the front surface was lower than the reverse surface (see Table 3 and 4) in the samples prepared by the plastisol method.
Table 5. The Surface Resistivity of the Samples Prepared by Mill
Mixing Method.

(Initial length of carbon fiber 5.0 mm)

Concentration of Carbon
Fiber (phr) Surface Resistivity ([Omega])

3.0 4.0 x [10.sup.12]
5.0 1.2 x [10.sup.12]
10.0 1.1 x [10.sup.12]
20.0 9.7 x [10.sup.11]


CONCLUSIONS

1. The short carbon fibers in PVC plastisol-carbon fiber composite essentially retained their initial lengths during the preparation of the samples. A network of carbon fiber with high aspect ratio was formed at low concentration of the fiber.

2. The network of carbon fiber in the plastisol samples with fiber concentration of 2.0 phr shows excellent electrical conductivity when the initial length of the fiber is longer than 2.0 mm (L/D = 285).

3. The critical concentration of carbon fiber where a sharp transition of electrical properties occurs from insulative to conductive, for the fiber network in plastisol-carbon fiber composite, occurred in the range of 0.5 to 1.0 phr when the initial fiber length was 3 mm (L/D = 428).

4. The sheets of PVC plastisol-carbon fiber composites showed excellent antistatic property. The critical concentration of carbon fiber for surface resistivity of the front surface of the samples was the same as for the resistivity of the fiber network (i.e. 0.5 to 1.0 phr). The resistivity of the reverse surface was higher than that of the front surface, because the concentration of carbon fiber in the area near the front surface was higher than that near the reverse surface.

5. It is difficult for the samples prepared by the mill mixing method to form a conductive network because the carbon fibers in the samples were broken and orientated.

REFERENCES

1. J. Martinsson and J. L. White, Polym. Compos., 7, 302 (1986).

2. F. J. B. Calleja, R. K. Bayer, and T. A. Ezquerra, J. Mater. Sci., 23, 1411 (1988).

3. L. Li and D. D. L. Chung, Composites, 25, 215 (1994).

4. Y. Ramadin, S. A. Jawad, S. M. Musameh, M. Ahmad, A. M. Zihlif, A. Paesano, E. Martuscelli, and G. Ragosta, Polym. Intern., 34, 145 (1994).

5. P. B. Jana, S. Chaudhuri, A. K. Pal, and S. K. De, Polym. Eng. Sci., 32, 448 (1992).

6. P. B. Jana and A. K. Mallick, J. Elast. Plast., 26, 58 (1994).
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Author:Guoquan, Wang; Peng, Zeng
Publication:Polymer Engineering and Science
Date:Jan 1, 1997
Words:3162
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