Features of changes in the electrical resistance of mixtures of crystallizing polymers with carbon black upon heating

Objectives. The effects of positive and negative temperature coefficients (PTC and NTC, respectively) in carbon black-filled conductive polymer composites based on high-density polyethylene grade 277-73 and polypropylene grade 01050 were investigated. Carbon black electrically conducting grade OMCARB C-140 (UM-76) was used as the filler.

Methods. To study the electrical characteristics of the compositions, plates were pressed with brass contact electrodes at the plate ends to simulate polymer heaters. The electrical resistance of the samples was evaluated using an ohmmeter DT9208A (RESANTA, Latvia). Tests at elevated temperatures were carried out in an SNOL 3.5 heat chamber (NPF TherMIX, Russia) with a heating rate of ~3℃/min. The crystallinity of the samples during heating was assessed by differential scanning calorimetry on a DSC 204F1 Phoenix device (NETZSCH, Germany) with a heating rate of 3℃/min.

Results. The complex PTC and NTC mechanisms in mixed polymer compositions are not solely related to thermal expansion and melting of the polymer. While changes in the electrical resistance of carbon-filled polymer composites are associated with the presence of crystalline regions with defects, the destruction of the conductive channels occurs at the earliest stages of polymer melting due to the formation of expanding amorphous “microdroplets” of the hot melt. For a carbon-filled, electrically conductive mixture of polyethylene and polypropylene, the magnitude and nature of the change in the peak temperature of the PTC depends on the melting onset temperature of the lowest-melting phase of polyethylene. At the same time, the heterogeneity of the mixtures of crystallizing polymers with technical carbon increases the thermal stability of the material by expanding the PTC zone into the melting region of the higher-melting phase of polypropylene. When comparing electrically conductive compositions of polymers with different melting points and carbon black, the low-melting polymer determines the temperature of self-regulation and the nature of PTC, while the high-melting polymer shifts the jump in electrical conductivity to the region of elevated temperatures.

Conclusions. The activation energies of carbon-filled mixtures of polyethylene and polypropylene, which are weakly dependent on the mixing method, are approximately 44 ± 3 kJ/mol. The obtained values are consistent with the activation energy values for the viscous melt flow process. The method of mixing the components in mixtures of carbon-filled compositions based on crystallizing polymers was found to have little effect on PTC. The use of carbon-filled polymer compositions with a mixed matrix of polyethylene and polypropylene allows for the regulation of the intensity of PTC and NTC.

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