Polymer Composites Prepared by Compression Molding of a Mixture of Carbon Black and Nylon 6 Powder

GABRIEL PINTO,” CIPFUANO L6PEZGONZhEZ,2 and ANA J I m N E Z - m N 3

JLkpartamento de Ingenieria Quimka Industrial y del Medio Ambiente E l S I Indmtriales, Unwersidad PoZitkcnica de Madrid

Madrid , Spain

=Facultad de Ingenieria Mechica Universidd P o n t i i B0lwcuiun.a

Medellin, Colombia

3Departamento de Ingenieria Electrcinica ElSI Telecomunicacibn

Unwersidad Po&cnica de Madrid Ciudad Uniuersitaria, Madr id Spain

The results of an experimental study on the effect of processing variables and filler concentration on the electrical resistivity of conductive composites based on nylon 6 filled with carbon black are reported. A typical percolation behavior in the effect of electroconductive filler content on the resistivity was found. The electrical resistivity of the composites is > 10l2 ohm.cm unless the carbon black content reaches the percolation threshold at -9 w t o ? , beyond which the resistivity de- creases markedly by as much as twelve orders of magnitude. Two parameters of molding process-temperature and time-were shown to have a notable effect on the resistivity of composites, whereas pressure has no influence on this property in the pressure range considered. There is no sharp variation in the density due to the onset of percolation, and the hardness of samples is not influenced by the presence of the filler.

lNTRODUCTION ously with increasing electroconductive filler content,

onducting or semiconducting polymer composites C have been studied extensively because of their numerous high technological electrical and electronic applications in a variety of areas such as self regulat- ing heaters, over-current and over-temperature pro- tection devices and matemls * for elechmagnetic/mdiofie- quency interference (EMI/RFI) shielding in electronic

Conductive polymer composites are prepared by different techniques and with different materials ( 1 1-20). Specifically, the electrical conductivity of carbon black-filled polymer composites have been the subject of both theoretical and experimental in- terest (8, 21- 26).

It is generally known that the electrical resistivity for polymer composites does not increase continu-

devices (1-10).

r0 whom correspondence should be addressed.

but there is a critical-composition (percolation concen- tration) at which the resistivity drops magnitudes from the insulating range to values in the semicon- ductive or metallic range (4, 18). For efficiency, in order to decrease the difficulty of the process and eco- nomic costs, the amount of the conductive phase for achieving materials with high conductivity should be usually as small as possible. A huge number of differ- ent statistical, geometric and thermodynamic models have been proposed for the estimation of the conduc- tivity (or inverse resistiviQ) /filler concentration curves (2, 22, 25, 27-35). An interesting review of the differ- ent theories was given by Lux (36).

In this paper we report a study on the influence of filler concentration on the electrical resistivity of com- posites produced by hot compaction by means of the compression molding of a mixture of carbon black and nylon 6 powder. These data, along with our previ- ously reported data (37) on the influence of filler con- centration on the resistivity of composites made of

804 POLYMER COMPOSlTfS, DECEMBER 7999, Vol. 20, No. 6

Polymer Composites Prepared by Compression Molding

copper powder embedded in nylon 6, may be helpful in developing theoretical models to better understand the variation of electrical properties of such materials.

We recognize that there are several important pro- cessing parameters for this system such as tempera- ture, pressure and time that control the electrical properties of the composites. The effects of these pro- cessing parameters on the volume resistivity of the conductive composites will be also discussed.

For checking whether the percolation phenomenon can be also seen in other characteristic parameters, and in order to complete the characterization of these materials, the density and hardness of the compos- ites, as example of properties, have been investigated.

EXPERIMENTAL M a t d d .

A commercial grade nylon 6 (used mamly for manu- facturing of fishing nets) supplied in the form of pow- der by Poliseda S.L. was used as matrix polymer. The nylon 6 had a weight average molecular weight (M,) of 24,500, a density of 1.13 g/cm3, a glass transition temperature of -5O"C, a melting temperature of -225°C and electrical resistivity of around 1013 ohm . cm. The cumulative size distribution of the nylon 6 powder was reported in previous works (37.38).

The electrical conducting filler used was carbon black. delivered by Quimipur. This carbon black had average particle size of 50 nm, dibutyl phtalate ab- sorption of 72 cm3/ 100 g, density of 1.85 g/cm3, and electrical resistivity, taken as the tabulated value for the graphite (39) of the order of lC3 ohm cm.

Sample Prepamtione Carbon black filled nylon 6 composites were fabri-

cated by mixing the polymer and the filler powders for 2 h in an internal mixer, followed by compression molding in a specifically designed mold with three cavities of 30 mm diameter and 3 mm thickness each one. In order to evaluate the influence of processing parameters on the properties of samples, the three more important molding parameters were vaned. Pressures ranging from 10.0 to 24.0 MPa and temper- atures ranging from 200 to 230°C were used, for a time of molding ranging from 5 to 30 min. Samples with mer contents in the range 0 to 40 wt?! were pre- pared. Sample thickness (necessary for the calcula- tion of resistivity) was determined using a micrometer, to an accuracy of 0.01 mm. Thickness measurements were taken at five locations and averaged.

Compoaits Chuactcdzation Technlquea The electrical resistivity was measured using a two-

point arrangement. Three specimens of each composi- tion were tested, taking four data points on each sam- ple. From this, the medium value of the twelve measurements was determined. For ensuring a good electrical contact the surfaces of samples were pol- ished with a sandpaper and silver leaf was placed be- tween the electrodes and the samples, by means of silver paint.

Measurements of volume electrical resistance higher than lo3 ohm were made using a megohmeter (Quad- tech model 1865). Measurements of low resistance were made using a digital multimeter (Leader model 856). A constant voltage of 100 V was supplied to the samples and the resistance of the samples was meas- ured after one minute, using a test cycle consisting of 20 s charge, 20 s dwell, 20 s measure, and 20 s dis- charge. Before starting a new test, the electrodes were short-circuited for 5 min to eliminate any effect of the previous electrification. The procedure used to esti- mate the volume resistivity, p, from resistance, in the present study was similar to those reported earlier (37, 40). Significant differences among different speci- mens with the Same composition were not apparent, which is a guarantee of homogeneity in the manufac- ture process. The reproducibility of values of p was better than one order of magnitude.

The density of the composites were measured by difference of weight in the air or with the sample im- mersed in a liquid, at 23"C, using a Mettler AJ 100 balance equipped with a density determination kit. The density was determined in accordance with ASI'M D 792-9 1, using ethanol of 96% purity as the liquid of known density.

The hardness of the samples was determined at 23°C using a durotronic model 1000 Shore D hard- ness tester, in accord with ASTM D 2240-68. Five data points were taken on each samples, and no dif- ference was found between hardness measurements on both faces of each specimen.

RESULTS AND DISCUSSION

The dependence of electrically resistive composites on filler fraction is shown in Fig. 1, on a logarithmic scale to emphasize the drastic decrease in resistivity at a certain filler concentration. The mixtures show a resistivity in accord with plausible percolation consid- erations, i.e., resistivity of composites decreases by much as twelve orders of magnitude for a given range of filler content, as pointed out by Fowler (41) for simi- lar samples prepared with carbon filler in a nylon 6/6 matrix.

The value of composite resistivity found in the re- gion 0 to -9 wP! is only slightly lower than the resis- tivity of host polymer. This value is in accord with Litman and Fowler (42), who suggested that the gen- eral carbon black critical concentration range is be- tween 4 and 10 wt?!, depending on resin matrix, pro- cessing techniques and conditions, and other variables. Beyond the critical filler fraction (-9 wt?!) resistivity drops by several orders of magnitude, owing to the formation of a three-dimensional infinite cluster or continuous network of the filler phase within the poly- mer matrix, as pointed out many times (43-46). It is appropriate to emphasize that this network need not present actual physical contact between neighboring particles, but does mean that they are at least within close enough proximity for electron jump to occur, as noted by Bigg (5). For compositions higher than that

805 POLYMER COMPOSITES, DECEMBER 19S, Vol. 20, No. 6

12 24 36

Filler content ( wt-% ) t'kJ. 1 . Variation of the volume electrical resistivities of nylon 6/carbon black composites with weightfraction of carbon black The processing conditions were 21 5°C. 20.0 MPa and 15 min

n I

E - 20 E 0

f 0

h >

W

c) .I

.I * .= 10

I I

210 230

Processing temperature ( "C ) FYg. 2. Plot of resistivity as afunction of processing tempera- ture at aprocessing time of 15 min and a carbon black con- centration of 32 wt%.

of a critical filler fraction, there are conductive contin- uos chains of conducting filler particles and noncon- ductive deadlock branches. For these compositions, the higher the filler fraction, the lower the composite resistivity, p. because of the increasing conductive network density of the filler phase. After the sharp drop in resistivity, this magnitude drops more slowly

with the increasing of filler fraction. The reason for this is that as filler content reaches a certain value, almost all filler particles become part of conductive chains and conductive clusters get a more nearly per- fect structure.

The described overall dependence of the electrical resistivity on the fdler concentration is similar to that observed by many workers on conductive polymer composites filled with carbon black (24,25). As mentioned in the previous sections, the process-

ing parameters to be considered for this system are temperature, pressure, and time. Our results coincide in general with those reported previously by Chan et aL (24), who studied the electrical properties of polymer composites prepared by sintering mixtures of carbon black and ultra high molecular weight polyethylene powders. In this way, the processing pressure has no significant effect on the resistivity in the pressure range considered (from 10.0 to 24.0 MPa). However, temperature and time were identified as parameters that can have a notable effect on the resistivity.

Flgure 2 shows a plot of the resistivity of the materi- als prepared at a pressure of 20.0 Mpa with a pro- cessing time of 15 min and a carbon black concentra- tion of 32 wt% as a function of temperature. The resistivity of the composite materials increases with temperature. This is believed, according to Chan et aL (24). to be caused by a higher degree of intermixing between the carbon black and polymer partices in the interfacial regions as a result of the lower viscosity of the polymer at the higher temperatures, and therefore clusters break up.

A plot of the resistivity as a function of processing time at a pressure of 20.0 MPa, a processing tempera- ture of 215"C, and a carbon black concentration of 32 wt% is shown in Flg. 3. The resistivity decreases with increasing of processing time. This can be caused by the fact that when the materials are being compacted, solid conductive channels of carbon black start to form: and thus, the resistivity decreases. Chan et aL (24) found that the resistivity decreased with process- ing time at the beginning and increased after approxi- mately 15 min of compaction. They suggested that the minimum resistivity was reached when the compaction process was completed, and they considered that a further increase in processing time promotes the in- terfacial mixing of the carbon black channels. In our case, this minimum was not found, but probably, based on data provided by Chan et aL (24), it could appear for times of processing higher than that con- sidered in the range studied.

Figure 4 illustrates the effect of varying carbon black concentration on the density of materials, using the same processing conditions that the referred in Rg. 1. It is seen in Rg. 4 that the density values in- crease continuously with increased carbon black con- tent, thus not showing any indication of percolation transition. The linear relationship was expected based on the knowledge that the Rule of Mixtures (39) gen- erally gives good density prediction.

806 POLYMER COMPOSITES, DECEMBER 7999, Vol. 20, No. 6

Polgmer Composites Prepared by Compression Molding

30

10

A

10 20 30

Processing time ( min ) Q. 3. Plot of resistiuity as afunction of processing time at a processing temperature of 215°C and a carbon black concen- tration of 32 wt96.

1 .os

1.04

12 24 36

Filler content ( wt-Yo ) Hg. 4. Measured density and calculated denslty/jiller con- centration cluves for an o w r d u~lume of voids acalunts for .......: 1096, -------: 15%. - . - .: 2096. processing m&mm were 215°C. 20.0 Mpa and 15 min.

Figure 4 shows also the calculated concentration curves for the systems investigated, assuming differ- ent extent of void fraction in the samples, calculated as explained in previous works (37, 38). where more detail was included. The data shown in Fig. 4 reflect that the measured densities all fall into the range of fraction of voids in volume from 0.10 to 0.20. This fraction increases with filler content. This effect can be explained by the fact that, as explained by Verhelst et aL. (47) and Dannenberg (48). carbon black rarely exists as individual spheroidal particles. Instead, par-

ticles exist in coalesced aggregates forming domains or nodules, consisting of concentric, imperfect graphitic layer planes, with imperfections and holes, where the molten polymer is unaccessible given its viscosity. With an increase of filler content, the concentration of such holes in the border of carbon black nodules in- creases, and this can explain the increasing of overall volume of voids. Figures 5 and 6 show the effect of processing tem-

pemture and processing time, respectively, on the den- sity of materials. The increase of both parameters promotes an increase in density, due to the higher ac- cessibility of molten polymer to the surface of carbon black particles. The difference of density and thus the fraction of voids seem not to highly influence in elec- trical resistivity, probably because of the higher influ- ence of dimensions of the carbon black channels, as explained in previous paragraphs.

The Shore D hardness of samples was approxi- mately constant with filler concentration, within the range from 68 to 72 unities.

CONCLUSIONS

In this article we have described an experimental study on the effects of the filler content and process- ing parameters on the electrical resistivity, density, and hardness of composites of carbon black embed- ded in nylon 6 prepared by compression molding. From the results obtained, the following conclusions can be drawn:

1. The electrical resistivity of composites decreases as much as twelve orders of magnitude for a given range of filler concentration, showing a percola- tion transition.

2. The percolation threshold value is approximately 9 wt?h of carbon black.

3. Time and temperature are two processing pa- rameters that have a notable effect on the resist- ivity of materials, but the processing pressure has been shown to be an unimportant parameter in the pressure range considered (10.0 to 24.0 ma).

4. There is no distinct break in the density/filler concentration relationship. From density meas- urements it can be observed that fi-action of voids increases with filler concentration.

with filler concentration. 5. The Shore D hardness is approximately constant

ACKNOWLEDGMENTS The authors would like to thank to Angel L. Fuerte

from POLISEDA S.L. for preparing the powder of nylon 6, to Jose Vicente Alonso for technical assist- ance in the design of several equipments used in this work, and to the reviewers for their valuable sugges- tions. The support provided by the Agencia Espaiiola de Cooperacion Internacional (AECI) of Spain and by

POLYMER COMPOSITES, DECEMBER 7999, Vol. 20, No. 6 807

Gabriel Pinto, Cipriano Lbpez-Gonzdez, and AM Jirrknez-Martin

h c, .I

3 Q

1.07 1 L

210 230

Processing temperature ( “C ) Fig. 5. plot of density as afunction of processing temperature at a processing time of 15 rnin and a carbon black concentra- tion of 32 wt%.

1.10

1.07

10 20 30

Processing time ( min ) Fig. 6. Plot of density as afunction of processing time at a processing temperafure of 21 5°C and a carbon black concen- tration of 32 wt%.

the Universidad Pontificia Bolivariana of Colombia, by one of us (C. Upez -Godez ) , under Project Coop- eracion Universitarja/AL.E. 98 No. 640 is also highly appreciated.

REFERENCES 1. M. Narkis, A. Ram. and F. Flasher, Pdym Eng. Sci., 18,

2. S. K. Bhattacharya and A. C. D. Chaklader. Polyym

3. D. A. Seanor. in Electrical properlies of Polymers, Aca-

4. S . K. Bhattacharya. in Metal Filled Polymers, Dekker,

5. D. M. Bigg, Polym Compos., 8, 1 (1987). 6. T. Kimura and S . Yasuda, Polymer, 29, 524 (1988). 7. J. Delmonte, in Metal/pdymer CorrtpHtes, Van Nostrand

649 (1978).

plast. ~ e c h n ~ ~ ~ng.. is, 21 (1982).

demic Press, New York (1982).

New York (1986).

Reinhold, New York (1990).

8.

9.

10.

11.

12.

13. 14. 15.

16.

17.

18.

19.

20. 21.

22.

23.

24,

25.

P. Lafuente, A. Fontecha, J. M. Diaz, and A. Munoz- E S ~ O M . Rev. Wt. Modem. 447,257 (1993). M. Stoessl, Powconversion & Intelligent Motors. 19, 50 (1993). T. Kimura. Y. Asano. and S. Yasuda, Polymer, 37, 2981 ( 1996). G. E. Wneck, J. C. W. Chien, F. E. Karasz, and C. P. Lillya, Polym Commun, 20, 143 (1979). T. A. Skotheim, in Handbook of Conducting Polymers, Dekker. New York (1986). M. Thakur, Macromdecules , 21, 661 (1988). G. Porta and L. Taylor, J. Mater. Res.. 3. 21 1 (1988). J. M. Margolis, in Conductive Polymers and Plastics, Chapman and Hall, New York (1989). L. Nicodemo, L. Nicolais, G. Romeo, and F. Scafora, Polyrn Eng. Sci. 18,293 (1978). R B. Seymour, in Conducting Polymers, Plenum Press, New York (1981). E. K. Sichel. in Carbon Black Polymer Composite, Dekker. New York (1982). S. Rhadhalaishnan and D. R Saint, J. Mater. W, 28, 5950 (1991). R. F. Die . Rev. Pltrstc. M o d e m , 434,201 (1992). F. J. Baltii-Calleja. T. A. Ezquerra, D. R Rueda. and J. Alonso-L6pez. J. Mater. Sci Lett, S, 165 (1984). M. Sumita, H. Abe, H. Kayaki. and K. Miyasaka, J. M a c r o m d Sci., B25, 171 (1986). E. P. Mamunya, V. F. Shumiskii. and E. V. Lebedev, Polym Sci. B, 38, 835 (1994). C. Chan, C. Cheng, and M. M. F. Yuen, Po& Eng. Sci, 37, 1127 (1997). E. P. Mamunya, V. V. Davidenko, and E. V. Lebedev, Corn. Interfaces. 4. 169 I19971.

26. G. i. Lee &d k. D. Suh, Polym. Eng. Sci., 38, 471

27. C. Rajagopal and M. Satyam. J. AppL Phys., 49, 5536

28. M. A. Berger and R. L. McCullough, C o n Sci T e c h L .

29. B. Wessling and H. Volk, Synth Meth., 18,671 (1987). 30. K. Yoshida, J. Phys. Soc. J p n . SS, 4087 (1990). 31. A. Ahmed and F. R. Jones, J . Mater. Sci, 25, 4933

32. M. Sumita, K. Sakata, S. Asai, K. Miyasaka, and H.

33. S. De Bondt, L. Froyen, and A. Deruyttere, J. Mater.

34. E. P. Mamunya, V. V. Davidenko, and E. V. Lebedev,

35. X. B. Chen, J. Devaux, J. P. Issi. and D. Billaud, Polym

36. F. Lux, J. Mater. Sci., 28, 285 (1993). 37. A. Larena and G. F’inb, Polym Comp., 18.536 (1995). 38. A. Jimt5nez-Martin. MSc thesis, Universidad PoliWnica

39. W. F. Smith, in Principles of Materials Science and

40. G. pinto, J. Chem Educ., 78, 683 (1996). 41. N. Fowler, MSc thesis, University of Lowell (1981). 42. A. M. Litman and N. E. Fowler, in Engineered Materials

Handbook, VoL 2, Engineering Plastics, ASM Interna- tional, Metals Park, Ohio (1988).

43. H. Scher and R. Zallen. J. Chem. Phys. , 53. 3759 ( 1979).

44. A. Malliaris and D. T. Turner, J. AppL Phys.. 42, 614 (1971).

45. F. Bueche, J. Plym Sci.: Polym Phys. E., 11, 1319 (1973).

46. J. Jan txn , J. AppL Phys.. 48, 966 (1975). 47. W. F. Verhelst. K .G. Wolthius, A. Voet, P. Ehrburger. and

J. B. Donnet, Rubber C h e n Technd. 50, 735 (1977). 48. E. L. Dannenberg. in Encyclopedia of Composite Mate-

rials and Components, M. Grayson, ed., Wiley, New York ( 1983).

(1998).

(1978).

22. 3 (1985).

(1990).

Nakagawa, Polym BulL, 25,256 (1991).

Sci., 27. 319 (1995).

Polym Camp., 16. 319 (1995).

Eng. Sci, 35,637 (1995).

de Madrid, Madrid (1998).

Engineering. McGraw-Hill, New York (1992).

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