THERMOPLASTIC POLYMER COMPOSITES WITH PYRORESISTIVE PROPERTIES AND THEIR APPLICATIONS FOR PLASTIC WELDING

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Abstract
Pyroresistive composites based on high-density polyethylene (HDPE) and polyvinyl chloride (PVC) were studied, which contained two types of filler, carbon black (C) or chopped carbon fibers (CC) in an amount of 8 vol.%. It was shown that when an electric voltage is applied to a composite sample, Joule heat is released, which heats the composite to an equilibrium temperature Te. The dependence of the equilibrium temperature on the voltage U is described by the quadratic equation Te = T0 + aU2, and on the electric power P supplied to the sample by the linear equation Te = T0 + b·P, where T0 is the initial temperature, a, b are the coefficients. However, when the voltage is increased above certain values, a deviation from the quadratic law is observed, and the achievable temperature Te is significantly lower than that calculated by the equation. The reason for the deviations is the manifestation of the PTC effect (positive temperature coefficient of resistance), as a result of which an increase in temperature due to an increase in voltage causes thermal expansion of the polymer matrix, which leads to the destruction of the conductive chains of the filler and an increase in the resistance of the sample. A comparison of the curves for composites based on HDPE and PVC demonstrates a significantly higher level of influence of the PTC effect for composites based on PE, and also a higher influence for composites containing carbon fibers compared to carbon black. A comparative calculation of the amount of energy Q required to heat a composite sample to Te and the amount of Joule thermal energy E released in the sample when electrical power is supplied to it showed that the energy Q is 20% of the Joule thermal energy E released in the sample, i.e. the main part of the Joule thermal energy (80%) is dissipated to the environment. The value of the E/Q ratio depends on the intensity of exchange with the environment, as well as on the geometry of the sample. The possibilities of contact welding of plastics using a heating element (HE) made of pyroresistive material are shown. This method demonstrated the maximum adhesive strength of the welded joint, which is comparable to the strength of the welded polymers, both for HDPE and PVC, and is promising for use in various industries.

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