- Email: [email protected]
Electrical resistance measurement of conductive network in short carbon fibre-polymer composites
Polymer Testing16 (1997)277-286
PII:
Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0142-9418/97/$17.00
SO142-9418(96)00049-9
ELSEVIER
TEST METHOD Electrical Resistance Measurement of Conductive Network in Short Carbon Fibre-Polymer Composites Wang Guoquan Department
of Polymer Science, Beijing University of Chemical Technology, People’s Republic of China (Received
1 August 1996; accepted
11 September
Beijing 100029,
1996)
ABSTRACT The volume resistivity of the conductive network of short carbon fibre-filled polymer composites was tested by three methods, i.e. the direct contact method, the steel needle
electrode
method, and the potential difference
method. All of the three
methods were designed to decrease the contact resistance between the probes and the jibre in the network. The data obtained by the potential difference method were lower than both of the other two methods. The results from the direct contact method and the steel needle electrode method are similar for the critical concentration and critical length of the carbon fibre in the conductive network. 0 Science Ltd.
1997 Elsevier
1 INTRODUCTION It has recently been of interest to use conductive fillers whilst making electrically conductive polymers. The conductive fillers include aluminum, copper, or nickel powder, metal-coated glass fibres, and carbon black. More recently, the use of carbon fibres in making conductive polymers has been of great interest, although carbon black is the most widely used filler. For studies of conductive filler-filled polymers, a percolation theory has been used to characterize conduction in the composites.’ The theory implies a network of the filler for conduction. Recently, the percolation theory has been utilized to explain the conductive behavior of carbon fibre-filled polymer systems. Jana et al. reported the factors controlling the formation of the conductive network in the short carbon fibre-reinforced polychloroprene rubber.’ 277
278
W. Guoquan
To test the conductivity of the conductive fibre-filled composites, the four probe technique was mainly used. Li et al. have studied the electrical resistivity of conductive polyethersulfone composites containing carbon fibre, nickel fibre or stainless fibre by using a four probe method.3 Jana et al. reported the electrical conductivity of short carbon fibre-filled polychloroprene measured by a four probe technique.* The conductive behavior of the fibre network in the composites is different from the bulk of the composites and therefore needs a specific technique for measurement of the conductivity of the network, especially a specific probe system. To test the resistivity of the network, the probes have to be directly connected with the network, so that the contact resistance between the probes and the network may be decreased. For measurement of the effectiveness of electromagnetic interference shielding of conductive polyethersulfone composites, Li et al. applied a conductive silver paint to the surface of the sample in order to obtain a continuous metallic contact between the sample and the steel shielding tester chamber. 3 Special electrode systems were also used in metal-filled polymer composites. Schruben et al. designed a electrode system resembling a sandwich to obtain a fine contact with the samples of metalfilled polymer composites.4 This paper reports three methods: the direct contact method, the steel needle electrode method, and the potential difference method, which are designed especially for measurement of the volume resistivity of the conductive network in short carbon fibre-filled polymer composites. All of the three methods are based on designing a probe system which can decrease the contact resistance between the probes and the network in the composites. The third method is similar to the four probe method. The polymer bases used in this work were poly(viny1 chloride) (PVC) plastisol, ethylene-vinyl acetate copolymer (EVA), and polychloroprene rubber. The effects of the initial length and concentration of short carbon fibre on the resistivity of the fibre network in the fibre-filled PVC plastisol were measured using all three methods. The differences between the three methods are discussed.
2 EXPERIMENTAL 2.1 Materials The polymers used in this work were PVC, EVA, and polychloroprene. A micro-suspension PVC resin (type PSM-31) manufactured by Shenyang Chemical Industry Co. (China) was mixed with dioctyl phthalate (DOP) and barium-cadmium-zinc liquid stabilizer, forming the PVC plastisol. The plasti-
Measurement of conductive network in composites
279
sol was then mixed with the short carbon fibre, forming a carbon fibre-filled paste. Other polymeric materials were EVA emulsion and polychloroprene solution, which were also fluids and therefore easy to mix with the short carbon fibre. The EVA emulsion, with 50% solid content, was obtained from Beijing Organic Chemical Works (China). The polychloroprene solution (40% solid content) was obtained from Yichun Chemical Works (China). Long carbon fibre (middle strength, 0.007 mm diameter), manufactured by Liaoyuan Petrochemical Works (China), was based on polyacrylonitrile (PAN) fibre. The long carbon fibre was cut forming a short carbon fibre with specific length. 2.2 Preparation of the samples For preparation of the samples of carbon fibre-PVC plastisol composites, the short carbon fibre with specific length was put into the PVC plastisol, and then the fibre was dispersed into the plastisol by stirring. The lengths of the carbon fibre were 1.2, 2.0, 3.0, and 5.0 mm. The fibre-filled paste was coated onto a steel plate and then the plate was put in an oven for 10 min at 180°C to plastify the plastisol. The sheets of the composites, with 1 mm thickness, were prepared by the static processing mentioned above, without breaking the carbon fibre . The samples of carbon fibre-EVA composite and carbon fibre-polychloroprene composite were prepared by dispersing the short carbon fibre, respectively, into the EVA emulsion and the polychloroprene solution, forming the fibre-filled pastes. The pastes were coated on the molds, and then dried at room temperature. 2.3 Measurement of the electrical resistivity In this work, the volume resistivity of the conductive network of short carbon fibre-polymer composites was measured. Before measuring, the samples were cut into squares with 10 cm side length using a paper cutting machine. As a result of cutting the samples, the tips of the carbon fibre were exposed on the edges of the samples. Three methods were used in this work to determine the volume resistivity of the conductive network: the direct contact method, steel needle electrode method, and potential difference method. In all of the three methods, the electrical probes were directly brought into contact with the carbon fibre in the samples.
280
W. Guoquan
2.3.1 Direct contact method In this method, a sheet of short carbon fibre-polymer composite was put on an insulated plate. For measurements of high resistance (higher than lo%), a high resistance meter (Shanghai Electrical Meter Factory, China, ZC-46A) was used. An avometer (Univolt, Japan, DT-830) was used for measurement of low resistance (lower than lo5 Cn). The tests were performed at 23+2”C. The two probes of the high resistance meter (or the avometer) were brought into contact with two opposite edges of the sample, and directly contacted with the tips of carbon fibre exposed on the edges. Hence, the resistance data thus obtained can represent the conductivity of the carbon fibre network in the samples. The calculation of volume resistivity depends on the following formula:
where t = the distance of the two opposite edges of the sample, i.e. 10 cm. R = the resistance data tested by the method mentioned above. A = the area of an edge of the sample, i.e. 1 cm’. pV= the volume resistivity of the conductive network of the short carbon fibre. 2.3.2 Steel needle electrode method It is evident that the direct contact method mentioned above is a rough measurement method. Another method, called the steel needle electrode method, was designed to improve the direct contact method by using a special electrode system. The steel needle electrode system is shown in Fig. 1. Nine
-
b’
-
c’
-d Sample
-
e’
-f
Fig. 1. Steel needle electrode
-
g’
-
h’
-
i’
system.
Measurement of conductive network in composites
281
pairs of steel needles are pricked into the two opposite edges of the sample, and therefore the needles are directly and steadily connected with the carbon fibre in the conductive network. The distance between two adjacent needles is 1 cm. The depth of the needles pricked into the samples is about 3 mm. The apparatus for measurement of the resistance are the same as the direct contact method. 2.3.3 Potential difference method The steel needle electrode method is a modification of the direct contact method with an improved electrode system. However, the resistance data obtained by the steel needle electrode system may also have some errors caused by the contact resistance between the needles and the samples. The potential difference method was designed to eliminate the influence of the contact resistance on the test data. The electrode system of the potential difference method is shown in Fig. 2. The essential principle of the potential difference method used in this work is similar to IS0 3915 and IS0 1853. The measurement system in IS0 3915 (or IS0 1853), which is suitable for carbon black-filled polymer systems, is a four pole method designed to eliminate the errors caused by the contact resistance. The potential difference method used in this work is also a four poles method, but the electrode system is different from IS0 3915, so as to test the resistance of the conductive network of the carbon fibre. As shown in Fig. 2, the power supply poles are 9 pairs of steel needles. The needles at the same edge of the sample are connected to each other by
Potential difference
Fig. 2. The apparatus for the potential difference method.
282
W. Guo9uan
the wires, forming two opposite poles. The current passed through the sample is tested by an ammeter. The potential difference probes are two steel needles which are pricked into the surface of the sample. The distance between the two needles is 2 cm. In such a electrode system, both of the power supply poles and the potential difference probes are directly connected with the carbon fibre in the conductive network. The potential difference between the two probes is tested by a voltmeter (the internal resistance of the voltmeter is higher than lo6 a). The apparatus shown in Fig. 2 is designed for measurement of the samples with resistance data less than lo3 IR. In other words, the potential difference method is only suitable for measurement of low resistance samples. The calculation of volume resistivity using the potential difference method depends on the following formula:
where U = the potential difference data. I= the current passed through the sample. A = the area of an edge of the sample, i.e. 1 cm2. d = the distance between the two potential difference probes, i.e. 2 cm.
3 RESULTS 3.1 Comparison of the volume resistivity tested by the direct contact method and the steel needle electrode method with different numbers of the needle pairs The volume resistivity of three samples of carbon fibre-filled PVC plastisol was tested by the direct contact method and the steel needle electrode method. The formula of the three samples is shown in Table 1. The results are shown in Table 2. During the measurements using the steel needle electrode method, the numbers of the needle pairs are increased from 1 to 9. For measurement by one pair of needles, two opposite needles (e.g. a and a’, b and b’, etc) are used to test the resistance data. The measurement is performed nine times with different needle pairs, and an average resistance taken. For the measurement using two pairs of needles, two needles at the same side (e.g. a-b, a-c, c-d, a’-b’, etc) are connected to each other by the wires, forming the two needles
Measurement of conductive network in composites
The compounding
283
TABLE 1 ingredient of the three samples Compositions (phr)
Sample I
Sample 2
Sample 3
100 60 4 4 3
100 60 4 1.5 3
100 60 4 2 5
PVC DOP Stabilizer Carbon fibre Initial length of carbon fibre (mm)
TABLE 2 The volume resistivity of the three samples tested by the direct contact method and the steel needle electrode method Testing method
Log resistivity (0 cm) Sample I
Sample 2
Sample 3
Direct contact
0.89
2.37
1.39
Steel needle electrode: Number of needle pairs 1 2 3 4 5 6 7 8 9
0.86 0.56 0.38 0.32 0.25 0.20 0.17 0.15 0.15
2.30 2.20 2.08 1.95 1.84 1.82 1.81 1.79 1.79
1.34 1.08 0.97 0.86 0.73 0.65 0.62 0.58 0.58
electrode system. Then the measurement is performed with the two needles system eight times. Then the resistance data are tested by using the three needles electrode system, the four needles system, up to a nine needles system. As shown in Table 2, the resistivity data tested using the direct contact method are close to the one needle electrode system. The resistivity data decreases with increasing of numbers of the needles in the electrode systems. According to the results shown in Table 2, nine needles are enough for the steel needle electrode method. In fact, when the numbers of the needles of the needles are higher than eight, there is no decrease in the resistivity data.
W. Guoquan
284
3.2 Comparison of the three methods on the samples of carbon fibrefilled PVC plastisol composites The effects of concentration of short carbon fibres on the volume resistivity of the conductive network in the samples of the carbon fibre-filled PVC plastisol composites were tested by all three methods. The electrode system used in the steel needle electrode method was the nine needles system. The results are shown in Table 3. The initial length of carbon fibre is 3.0 mm. Other materials are the same as in Table 1. As shown in Table 3, the data tested using the potential difference method are less than both the direct contact method and steel needle electrode method, because the influence of the contact resistance on the test data is eliminated by using the potential difference method. The volume resistivity of the conductive network decreases sharply, as shown in the data tested using both the direct contact method and steel needle electrode method at a concentration range of OS-l.0 phr. This is the critical concentration of carbon fibres for the conductive network. The direct contact method is indeed a rough measurement method, as well as a rapid test method. However, the method can be used to test the critical concentration of carbon fibres in the conductive network with a result similar to the steel needle electrode method. The initial length of the carbon fibre, as well as the concentration of the fibre, plays an important role in the conductivity of the conductive network. The results are shown in Table 4. The concentration of the fibre is 2 phr. Other materials are the same as in Table 1. As shown in Table 4, the volume resistivity, tested by both the direct contact TABLE 3 Effects of the concentration of carbon fibre on the volume resistivity of the conductive network in carbon fibre-PVC plastisol composites tested by the direct contact method (method A), the steel needle electrode method (method B), and the potential difference method (method C) Concentration of carbon jibre (phr)
0.5
1.0 1.5 2.0 4.0
Log volume resistivity (0 cm)
Method A
Testing methods Method B
Method C
7.73 2.89 2.37 1.96 0.89
6.88 2.35 1.79 1.45 0.15
2.05 1.30 0.66 0.08
Measurement
of conductive
network in composites
285
TABLE 4 Effects of the initial length of carbon fibre on the volume resistivity of the conductive network in carbon fibre-PVC plastisol composites tasted by the three methods Initial length of carbon jibre (mm)
1.2 2.0 3.0 5.0
Log volume resistivity (0 cm)
Method A
Testing methods Method B
Method C
6.30 2.10 1.96 1.39
5.50 1.72 1.34 0.58
1.25 0.85 0.25
method and steel needle electrode method, has a sharp decrease at a length range of between 1.2 and 2.0 mm. This is a critical length of carbon fibre for the resistivity of the conductive network. 3.2.1 Comparison of the three methods used in the samples of carbon jibre-filled EVA and polychloroprene The volume resistivity of the conductive network of carbon fibre in the samples of the fibre-filled EVA and polychloroprene was tested by all three methods. The results are shown in Table 5. The concentration of the fibre is 2 phr, with an initial length of 3.0 mm. As shown in Table 5, the data obtained by using the potential difference method is lower than both of the other two methods; similar to the results in the carbon fibre-filled PVC plastisol.
TABLE 5 The volume resistivity of the conductive network in carbon fibre-filled EVA and polychloroprene Polymer-matrix
EVA Polychloroprene
Log volume resistivity (R cm)
Method A
Testing methods Method B
Method C
2.30 1.30
2.04 1.08
0.45 0.30
286
VV.Guoquan
4 CONCLUSIONS Three methods, the direct contact method, the steel needle electrode method, and the potential difference method, were used in this work to test the volume resistivity of the conductive network of carbon fibre-filled polymer composites. All of the three methods were designed to decrease the contact resistance between the probes and the fibre in the network. The following observations are made: (1) For the steel needle electrode method, when the numbers of the needle pairs are higher than 8, the resistance data are not decreased, and therefore 9 pairs of needles are enough for such a electrode system. (2) The resistivity data tested using the potential difference method are less than both of the other two methods, because the influence of the contact resistance on the test data is eliminated by using the potential difference method. However, the potential difference method is not suitable for measuring high resistivity. (3) The volume resistivity of the conductive network in the samples of carbon fibre-filled PVC plastisol composites has a sharp decrease at the fibre concentration range of OS-l.0 phr. This critical concentration tested by the direct contact method is similar to the data tested by the steel needle electrode method. The data of the two methods mentioned above are also similar in measurement to the critical length of the fibre. (4) To test the samples of carbon fibre-filled EVA and polychloroprene, the data obtained by the potential difference method are lower than both the other methods, which is similar to the results in the samples of the fibre-filled PVC plastisol.
REFERENCES 1. Lee, B., Electrically conductive polymer composites and blends. Polymer Engineering and Science, 1992, 32, 3642. 2. Jana, P. B., Chaudhuri, S., Pal, A. K. and De, S. K., Electrical conductivity of
short carbon fibre-reinforced polychloroprene rubber and mechanics of conduction. Polymer Engineering and Science, 1992, 32, 448-456. 3. Li, L. and Chung, D. D. L., Electrical and mechanical properties of electrically conductive polyethersulfone composites. Composites, 1994, 25, 215-224. 4. Yang, L. and S&r&en, D. L., Electrical resistivity behavior of mold-cast metalfilled polymer composites. Polymer Engineering and Science, 1994, 34, 11091114.
PII:
Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0142-9418/97/$17.00
SO142-9418(96)00049-9
ELSEVIER
TEST METHOD Electrical Resistance Measurement of Conductive Network in Short Carbon Fibre-Polymer Composites Wang Guoquan Department
of Polymer Science, Beijing University of Chemical Technology, People’s Republic of China (Received
1 August 1996; accepted
11 September
Beijing 100029,
1996)
ABSTRACT The volume resistivity of the conductive network of short carbon fibre-filled polymer composites was tested by three methods, i.e. the direct contact method, the steel needle
electrode
method, and the potential difference
method. All of the three
methods were designed to decrease the contact resistance between the probes and the jibre in the network. The data obtained by the potential difference method were lower than both of the other two methods. The results from the direct contact method and the steel needle electrode method are similar for the critical concentration and critical length of the carbon fibre in the conductive network. 0 Science Ltd.
1997 Elsevier
1 INTRODUCTION It has recently been of interest to use conductive fillers whilst making electrically conductive polymers. The conductive fillers include aluminum, copper, or nickel powder, metal-coated glass fibres, and carbon black. More recently, the use of carbon fibres in making conductive polymers has been of great interest, although carbon black is the most widely used filler. For studies of conductive filler-filled polymers, a percolation theory has been used to characterize conduction in the composites.’ The theory implies a network of the filler for conduction. Recently, the percolation theory has been utilized to explain the conductive behavior of carbon fibre-filled polymer systems. Jana et al. reported the factors controlling the formation of the conductive network in the short carbon fibre-reinforced polychloroprene rubber.’ 277
278
W. Guoquan
To test the conductivity of the conductive fibre-filled composites, the four probe technique was mainly used. Li et al. have studied the electrical resistivity of conductive polyethersulfone composites containing carbon fibre, nickel fibre or stainless fibre by using a four probe method.3 Jana et al. reported the electrical conductivity of short carbon fibre-filled polychloroprene measured by a four probe technique.* The conductive behavior of the fibre network in the composites is different from the bulk of the composites and therefore needs a specific technique for measurement of the conductivity of the network, especially a specific probe system. To test the resistivity of the network, the probes have to be directly connected with the network, so that the contact resistance between the probes and the network may be decreased. For measurement of the effectiveness of electromagnetic interference shielding of conductive polyethersulfone composites, Li et al. applied a conductive silver paint to the surface of the sample in order to obtain a continuous metallic contact between the sample and the steel shielding tester chamber. 3 Special electrode systems were also used in metal-filled polymer composites. Schruben et al. designed a electrode system resembling a sandwich to obtain a fine contact with the samples of metalfilled polymer composites.4 This paper reports three methods: the direct contact method, the steel needle electrode method, and the potential difference method, which are designed especially for measurement of the volume resistivity of the conductive network in short carbon fibre-filled polymer composites. All of the three methods are based on designing a probe system which can decrease the contact resistance between the probes and the network in the composites. The third method is similar to the four probe method. The polymer bases used in this work were poly(viny1 chloride) (PVC) plastisol, ethylene-vinyl acetate copolymer (EVA), and polychloroprene rubber. The effects of the initial length and concentration of short carbon fibre on the resistivity of the fibre network in the fibre-filled PVC plastisol were measured using all three methods. The differences between the three methods are discussed.
2 EXPERIMENTAL 2.1 Materials The polymers used in this work were PVC, EVA, and polychloroprene. A micro-suspension PVC resin (type PSM-31) manufactured by Shenyang Chemical Industry Co. (China) was mixed with dioctyl phthalate (DOP) and barium-cadmium-zinc liquid stabilizer, forming the PVC plastisol. The plasti-
Measurement of conductive network in composites
279
sol was then mixed with the short carbon fibre, forming a carbon fibre-filled paste. Other polymeric materials were EVA emulsion and polychloroprene solution, which were also fluids and therefore easy to mix with the short carbon fibre. The EVA emulsion, with 50% solid content, was obtained from Beijing Organic Chemical Works (China). The polychloroprene solution (40% solid content) was obtained from Yichun Chemical Works (China). Long carbon fibre (middle strength, 0.007 mm diameter), manufactured by Liaoyuan Petrochemical Works (China), was based on polyacrylonitrile (PAN) fibre. The long carbon fibre was cut forming a short carbon fibre with specific length. 2.2 Preparation of the samples For preparation of the samples of carbon fibre-PVC plastisol composites, the short carbon fibre with specific length was put into the PVC plastisol, and then the fibre was dispersed into the plastisol by stirring. The lengths of the carbon fibre were 1.2, 2.0, 3.0, and 5.0 mm. The fibre-filled paste was coated onto a steel plate and then the plate was put in an oven for 10 min at 180°C to plastify the plastisol. The sheets of the composites, with 1 mm thickness, were prepared by the static processing mentioned above, without breaking the carbon fibre . The samples of carbon fibre-EVA composite and carbon fibre-polychloroprene composite were prepared by dispersing the short carbon fibre, respectively, into the EVA emulsion and the polychloroprene solution, forming the fibre-filled pastes. The pastes were coated on the molds, and then dried at room temperature. 2.3 Measurement of the electrical resistivity In this work, the volume resistivity of the conductive network of short carbon fibre-polymer composites was measured. Before measuring, the samples were cut into squares with 10 cm side length using a paper cutting machine. As a result of cutting the samples, the tips of the carbon fibre were exposed on the edges of the samples. Three methods were used in this work to determine the volume resistivity of the conductive network: the direct contact method, steel needle electrode method, and potential difference method. In all of the three methods, the electrical probes were directly brought into contact with the carbon fibre in the samples.
280
W. Guoquan
2.3.1 Direct contact method In this method, a sheet of short carbon fibre-polymer composite was put on an insulated plate. For measurements of high resistance (higher than lo%), a high resistance meter (Shanghai Electrical Meter Factory, China, ZC-46A) was used. An avometer (Univolt, Japan, DT-830) was used for measurement of low resistance (lower than lo5 Cn). The tests were performed at 23+2”C. The two probes of the high resistance meter (or the avometer) were brought into contact with two opposite edges of the sample, and directly contacted with the tips of carbon fibre exposed on the edges. Hence, the resistance data thus obtained can represent the conductivity of the carbon fibre network in the samples. The calculation of volume resistivity depends on the following formula:
where t = the distance of the two opposite edges of the sample, i.e. 10 cm. R = the resistance data tested by the method mentioned above. A = the area of an edge of the sample, i.e. 1 cm’. pV= the volume resistivity of the conductive network of the short carbon fibre. 2.3.2 Steel needle electrode method It is evident that the direct contact method mentioned above is a rough measurement method. Another method, called the steel needle electrode method, was designed to improve the direct contact method by using a special electrode system. The steel needle electrode system is shown in Fig. 1. Nine
-
b’
-
c’
-d Sample
-
e’
-f
Fig. 1. Steel needle electrode
-
g’
-
h’
-
i’
system.
Measurement of conductive network in composites
281
pairs of steel needles are pricked into the two opposite edges of the sample, and therefore the needles are directly and steadily connected with the carbon fibre in the conductive network. The distance between two adjacent needles is 1 cm. The depth of the needles pricked into the samples is about 3 mm. The apparatus for measurement of the resistance are the same as the direct contact method. 2.3.3 Potential difference method The steel needle electrode method is a modification of the direct contact method with an improved electrode system. However, the resistance data obtained by the steel needle electrode system may also have some errors caused by the contact resistance between the needles and the samples. The potential difference method was designed to eliminate the influence of the contact resistance on the test data. The electrode system of the potential difference method is shown in Fig. 2. The essential principle of the potential difference method used in this work is similar to IS0 3915 and IS0 1853. The measurement system in IS0 3915 (or IS0 1853), which is suitable for carbon black-filled polymer systems, is a four pole method designed to eliminate the errors caused by the contact resistance. The potential difference method used in this work is also a four poles method, but the electrode system is different from IS0 3915, so as to test the resistance of the conductive network of the carbon fibre. As shown in Fig. 2, the power supply poles are 9 pairs of steel needles. The needles at the same edge of the sample are connected to each other by
Potential difference
Fig. 2. The apparatus for the potential difference method.
282
W. Guo9uan
the wires, forming two opposite poles. The current passed through the sample is tested by an ammeter. The potential difference probes are two steel needles which are pricked into the surface of the sample. The distance between the two needles is 2 cm. In such a electrode system, both of the power supply poles and the potential difference probes are directly connected with the carbon fibre in the conductive network. The potential difference between the two probes is tested by a voltmeter (the internal resistance of the voltmeter is higher than lo6 a). The apparatus shown in Fig. 2 is designed for measurement of the samples with resistance data less than lo3 IR. In other words, the potential difference method is only suitable for measurement of low resistance samples. The calculation of volume resistivity using the potential difference method depends on the following formula:
where U = the potential difference data. I= the current passed through the sample. A = the area of an edge of the sample, i.e. 1 cm2. d = the distance between the two potential difference probes, i.e. 2 cm.
3 RESULTS 3.1 Comparison of the volume resistivity tested by the direct contact method and the steel needle electrode method with different numbers of the needle pairs The volume resistivity of three samples of carbon fibre-filled PVC plastisol was tested by the direct contact method and the steel needle electrode method. The formula of the three samples is shown in Table 1. The results are shown in Table 2. During the measurements using the steel needle electrode method, the numbers of the needle pairs are increased from 1 to 9. For measurement by one pair of needles, two opposite needles (e.g. a and a’, b and b’, etc) are used to test the resistance data. The measurement is performed nine times with different needle pairs, and an average resistance taken. For the measurement using two pairs of needles, two needles at the same side (e.g. a-b, a-c, c-d, a’-b’, etc) are connected to each other by the wires, forming the two needles
Measurement of conductive network in composites
The compounding
283
TABLE 1 ingredient of the three samples Compositions (phr)
Sample I
Sample 2
Sample 3
100 60 4 4 3
100 60 4 1.5 3
100 60 4 2 5
PVC DOP Stabilizer Carbon fibre Initial length of carbon fibre (mm)
TABLE 2 The volume resistivity of the three samples tested by the direct contact method and the steel needle electrode method Testing method
Log resistivity (0 cm) Sample I
Sample 2
Sample 3
Direct contact
0.89
2.37
1.39
Steel needle electrode: Number of needle pairs 1 2 3 4 5 6 7 8 9
0.86 0.56 0.38 0.32 0.25 0.20 0.17 0.15 0.15
2.30 2.20 2.08 1.95 1.84 1.82 1.81 1.79 1.79
1.34 1.08 0.97 0.86 0.73 0.65 0.62 0.58 0.58
electrode system. Then the measurement is performed with the two needles system eight times. Then the resistance data are tested by using the three needles electrode system, the four needles system, up to a nine needles system. As shown in Table 2, the resistivity data tested using the direct contact method are close to the one needle electrode system. The resistivity data decreases with increasing of numbers of the needles in the electrode systems. According to the results shown in Table 2, nine needles are enough for the steel needle electrode method. In fact, when the numbers of the needles of the needles are higher than eight, there is no decrease in the resistivity data.
W. Guoquan
284
3.2 Comparison of the three methods on the samples of carbon fibrefilled PVC plastisol composites The effects of concentration of short carbon fibres on the volume resistivity of the conductive network in the samples of the carbon fibre-filled PVC plastisol composites were tested by all three methods. The electrode system used in the steel needle electrode method was the nine needles system. The results are shown in Table 3. The initial length of carbon fibre is 3.0 mm. Other materials are the same as in Table 1. As shown in Table 3, the data tested using the potential difference method are less than both the direct contact method and steel needle electrode method, because the influence of the contact resistance on the test data is eliminated by using the potential difference method. The volume resistivity of the conductive network decreases sharply, as shown in the data tested using both the direct contact method and steel needle electrode method at a concentration range of OS-l.0 phr. This is the critical concentration of carbon fibres for the conductive network. The direct contact method is indeed a rough measurement method, as well as a rapid test method. However, the method can be used to test the critical concentration of carbon fibres in the conductive network with a result similar to the steel needle electrode method. The initial length of the carbon fibre, as well as the concentration of the fibre, plays an important role in the conductivity of the conductive network. The results are shown in Table 4. The concentration of the fibre is 2 phr. Other materials are the same as in Table 1. As shown in Table 4, the volume resistivity, tested by both the direct contact TABLE 3 Effects of the concentration of carbon fibre on the volume resistivity of the conductive network in carbon fibre-PVC plastisol composites tested by the direct contact method (method A), the steel needle electrode method (method B), and the potential difference method (method C) Concentration of carbon jibre (phr)
0.5
1.0 1.5 2.0 4.0
Log volume resistivity (0 cm)
Method A
Testing methods Method B
Method C
7.73 2.89 2.37 1.96 0.89
6.88 2.35 1.79 1.45 0.15
2.05 1.30 0.66 0.08
Measurement
of conductive
network in composites
285
TABLE 4 Effects of the initial length of carbon fibre on the volume resistivity of the conductive network in carbon fibre-PVC plastisol composites tasted by the three methods Initial length of carbon jibre (mm)
1.2 2.0 3.0 5.0
Log volume resistivity (0 cm)
Method A
Testing methods Method B
Method C
6.30 2.10 1.96 1.39
5.50 1.72 1.34 0.58
1.25 0.85 0.25
method and steel needle electrode method, has a sharp decrease at a length range of between 1.2 and 2.0 mm. This is a critical length of carbon fibre for the resistivity of the conductive network. 3.2.1 Comparison of the three methods used in the samples of carbon jibre-filled EVA and polychloroprene The volume resistivity of the conductive network of carbon fibre in the samples of the fibre-filled EVA and polychloroprene was tested by all three methods. The results are shown in Table 5. The concentration of the fibre is 2 phr, with an initial length of 3.0 mm. As shown in Table 5, the data obtained by using the potential difference method is lower than both of the other two methods; similar to the results in the carbon fibre-filled PVC plastisol.
TABLE 5 The volume resistivity of the conductive network in carbon fibre-filled EVA and polychloroprene Polymer-matrix
EVA Polychloroprene
Log volume resistivity (R cm)
Method A
Testing methods Method B
Method C
2.30 1.30
2.04 1.08
0.45 0.30
286
VV.Guoquan
4 CONCLUSIONS Three methods, the direct contact method, the steel needle electrode method, and the potential difference method, were used in this work to test the volume resistivity of the conductive network of carbon fibre-filled polymer composites. All of the three methods were designed to decrease the contact resistance between the probes and the fibre in the network. The following observations are made: (1) For the steel needle electrode method, when the numbers of the needle pairs are higher than 8, the resistance data are not decreased, and therefore 9 pairs of needles are enough for such a electrode system. (2) The resistivity data tested using the potential difference method are less than both of the other two methods, because the influence of the contact resistance on the test data is eliminated by using the potential difference method. However, the potential difference method is not suitable for measuring high resistivity. (3) The volume resistivity of the conductive network in the samples of carbon fibre-filled PVC plastisol composites has a sharp decrease at the fibre concentration range of OS-l.0 phr. This critical concentration tested by the direct contact method is similar to the data tested by the steel needle electrode method. The data of the two methods mentioned above are also similar in measurement to the critical length of the fibre. (4) To test the samples of carbon fibre-filled EVA and polychloroprene, the data obtained by the potential difference method are lower than both the other methods, which is similar to the results in the samples of the fibre-filled PVC plastisol.
REFERENCES 1. Lee, B., Electrically conductive polymer composites and blends. Polymer Engineering and Science, 1992, 32, 3642. 2. Jana, P. B., Chaudhuri, S., Pal, A. K. and De, S. K., Electrical conductivity of
short carbon fibre-reinforced polychloroprene rubber and mechanics of conduction. Polymer Engineering and Science, 1992, 32, 448-456. 3. Li, L. and Chung, D. D. L., Electrical and mechanical properties of electrically conductive polyethersulfone composites. Composites, 1994, 25, 215-224. 4. Yang, L. and S&r&en, D. L., Electrical resistivity behavior of mold-cast metalfilled polymer composites. Polymer Engineering and Science, 1994, 34, 11091114.