Cyclic deformation of polychloroprene rubber/carbon black composites

11 September 2017
Deepak Sethi, Ranvijai Ram, and Dipak Khastgir
The electrical and mechanical properties of conducting samples, containing various types and concentrations of carbon black fillers, were examined after being subjected to different flexing modes.

In general, polymers act as insulators and do not conduct electricity. There is an ever-growing need, however, for electrically conducting polymers in various applications. They could be used, for example, for electromagnetic interference shielding materials, static electric charge disssipation, rubber contact switches, pressure-sensitive sensors and semiconducting materials in high-voltage cables, and circuit components in microelectronics.1–6

Such electrically conducting rubber composites are typically prepared by dispersing conducting fillers—including carbon black, carbon fibers, carbon nanotubes, graphene, coke, metal oxides (e.g., alumina or mica), and non-oxidizing metal powders (e.g., silver or nickel)—in the rubber matrix.7–9 Among these different types of conductive additives, carbon black is the most widely used because it is inexpensive, easily processed, and provides good reinforcement to rubber matrices.10, 11 The electrical conductivity of the resultant rubber composites—e.g., with a polychloroprene rubber (CR) matrix—is influenced by the type and amount of carbon black, as well as by its degree of dispersion within the rubber matrix and by filler–matrix interfacial effects.12–14 As the elastomeric composites are suitable for dynamic applications (i.e., when they are exposed to variations in temperature and other parameters), it is important to understand how their electrical and mechanical characteristics respond to such cyclic deformations. Although the electrical properties of CR/carbon black composites have previously been studied,15–17 the effect of cyclic deformations and temperature changes on the electrical and mechanical characteristics of the materials have not yet been investigated.

In this work,18 we have therefore conducted a series of experiments to examine how the electrical and dynamic mechanical properties of CR/carbon black composites are influenced by a variety of mechanical deformations (e.g., cyclic bending and compressive flexing) and temperature changes. To fabricate our samples, we used a CR matrix and a number of carbon black types as fillers (in different concentrations). In particular, we used intermediate super-abrasion furnace (N220), high-abrasion furnace (N330), semi-reinforcing furnace (N770), and conducting (CCB) carbon black fillers. We performed dry mixing in a two-roll mill to disperse the fillers within the CR matrix.

To assess the electrical conductivity of our samples, we conducted both AC and DC conductivity tests.18 Our AC conductivity results (see Figure 1) indicate that the conductivity of the CR matrix is very low, whereas that of carbon black is much higher. We find that at low carbon-black loadings, the conductivity of the composites is essentially that of CR, and that the conductivity initially increases only marginally with increased filler content (because the samples do not yet contain a conductive network). We observe a sharp increase in conductivity, however, at a critical concentration—the percolation threshold—when a conducting path forms. The rate of increased conductivity is greatest for our CCB-filled composites, followed by the N220, N330, and then N770 samples. This implies that the variations in conductivity mainly arise because of differences in the structure and particle size of the carbon black particles.

Measured AC conductivity (at a frequency of 1kHz) of chloroprene rubber (CR)/carbon black composites for a range of carbon black loadings and types. The critical filler concentration (percolation threshold) occurs at 25, 30, 40, and 80 parts per hundred rubber (phr) for the conducting (CCB), super-abrasion furnace (N220), high-abrasion furnace (N330), and semi-reinforcing furnace (N770) carbon black samples, respectively, and thus decreases with increasing surface area of the filler particles.

We also measured the effect of both bend and compressive flexing on the DC conductivity of our composite samples (see Figure 2). When the samples were subjected to repeated flexure under bending, we find—see Figure 2(a) and (b)—that the conductivity initially (for 1000 cycles) decreased abruptly, and then decreased at a slower rate. Moreover, after 2000 cycles the change in conductivity was negligible with additional flexing cycles. In contrast, our compressive flexing results—see Figure 2(c)—indicate that the relative conductivity of the samples at first (up to 9000 cycles) decreases rapidly and then decreases at a steadier rate. Finally, after 18,000 cycles, the relative conductivity reaches a plateau. Since this rate of compressive flexing is very fast compared with the bend flexing, the conducting networks in the samples break down at a much faster rate. Accordingly, the decrease in relative conductivity we observe is higher during compressive flexing than during bend flexing (although the absolute decrease in DC conductivity is similar for both sets of flexing measurements).

(a) Absolute DC conductivity (on a log scale) of the CR/carbon black composite samples during bend flexing tests, as a function of the number of flex cycles. The relative DC conductivity of the samples during (b) bend flexing and (c) compressive flexing tests are also shown. σ: Conductivity. σ0: Initial conductivity.

From our compressive flexing results (see Figure 3) we also see that there is a dramatic increase in the buildup of heat during the first 9000 flex cycles. Following this jump, we measured only a small, and then stabilizing (after 45,000 cycles), increase in the temperature of the samples. These changes in the level of heat buildup with flexing cycles is also a consequence of the formation and breakdown of conducting network structures in the samples (i.e., between the carbon black fillers and the polymer matrix). We find that the buildup is highest for the N200 sample, followed by the N330, N770, and then the CCB samples.

Heat buildup within the composite samples as a function of the number of compressive flex cycles.

The relationship between dynamic strain and storage modulus of our composites, as a function of filler concentration, is illustrated in Figure 4. We observe that the storage modulus values of the samples are highest for low dynamic strain amplitudes (i.e., about 5×10−4) and that there is a sharp decrease in the storage modulus toward the maximum dynamic strains (about 22×10−2). In addition, our results suggest that the formation of carbon black aggregates (and thus a carbon black–polymer chain network) is an important factor in determining the storage modulus value. Indeed, these networks are also responsible for imparting conductivity to the insulating polymer matrix of the samples. The conducting continuous networks will form when the average gap between the filler-particle aggregates in the matrix is less than or equal to 10Å.

Measured storage modulus, as a function of dynamic strain amplitude, for the (a) CCB (10, 20, 30, and 50phr), (b) N220 (10, 30, 50, and 70phr), (c) N330 (10, 30, 50, and 70phr, and (d) N770 (10, 30, 50, 70, 90, and 110phr) composite samples. For comparison, the results for the pure CR matrix (gum) are given in each case.

In summary, we have experimentally tested the dynamic electric and mechanical properties of elastomeric CR/carbon black composites. For instance, we subjected the composites to cycles of different flexing modes and observed a decrease in conductivity and dynamic modulus with increased flexing. We also found that the amount of heat buildup within the samples increased with increasing numbers of flexing cycles. Most of our experimental results can be explained by the formation and destruction of conducting networks of filler particles within the polymer matrix of the samples. In our future work we plan to investigate the effect of different elastomer matrices and types of carbon fillers on the mechanical and electrical properties of composites, under repeated deformation and relaxation conditions.


Deepak Sethi
Rubber Technology Centre, Indian Institute of Technology (IIT)

Deepak Sethi obtained his BE (mechanical engineering) from the CR State College of Engineering, India, and his MTech (rubber technology) from the IIT. Since 1997 he has been working as an aeronautical engineer in the Indian Air Force.

Ranvijai Ram
Rubber Technology Centre, Indian Institute of Technology (IIT)

Ranvijai Ram obtained his BSc from Veer Bahadur Singh Purvanchal University in 2000 and his MSc from Chaudhary Charan Singh University in 2006 (both in India). He has has an MTech from the IIT (obtained in 2012), where he has been a senior research fellow since 2013. His research is focused on conductive composites and polymer nanocomposites.

Dipak Khastgir
Rubber Technology Centre, Indian Institute of Technology (IIT)


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DOI:  10.2417/spepro.006958