Change of electrical characteristics of rubber in the process of “swelling–deswelling”

Abstract

Objectives. The main indicator that determines electrical conductivity of rubbers is specific volumetric electrical resistance (ρv ). The purpose of this work is to investigate changes in this indicator during swelling and deswelling of electrically conductive rubbers. When considering the swelling process of rubbers in liquid media at a molecular level, an analogy of this process with mechanical deformation of the material is drawn and common features and differences of these processes are revealed.Methods. For rubber compositions based on paraffinate and alkyl sulfonate nitrile butadiene rubbers, the degree of their swelling and the change in linear dimensions in heptane and in gasoline grades 80, 92, and 95 were determined. The ρ v value was determined by a potentiometric method: the initial value was measured after temperature control of rubbers for 1 h at 120°C, and the second measurement was carried out after these rubbers were swollen in the solvents for 48 h, followed by drying at 20°C to a constant weight and repeated temperature control under the same conditions. Using an IR Fourier spectrometer, spectra of the solvents used were obtained before and after identification of the investigated rubber samples in them.Results. It was shown that the type of rubber and solvent used influence the degree of rubber swelling. Rubber compositions based on natural rubbers with a large amount of attached acrylonitrile, obtained in the presence of an alkyl sulfonate emulsifier, have the highest resistance to swelling. The effect of the used solvent on the change in the degree of swelling is determined by its affinity for rubber and the presence of polar additives that increase the octane number of gasoline. It was established that the linear change of the samples upon swelling in the indicated solvents varies according to the length and thickness of the samples. Results show that depending on the type of rubber used and the degree of its filling, the described rubber processing technology leads to a decrease in the ρ v value by 2 to 20 times. The greatest effect of ρ v reduction is observed in low-filled rubber compositions based on paraffinate nitrile rubbers. The spectra of the frustrated total internal reflection of the solvents after their interaction with the studied rubbers show that particulate extraction of dibutyl phthalate, which was used as a plasticizer in rubber compounding, takes place as a result of rubber swelling.Conclusions. The proposed method of rubber processing reduces the ρ v value by removing dibutyl phthalate from the studied rubbers and forming a more developed carbon–elastomer structure. Furthermore, it solves the problem of the negative effect of the plasticizer on the ρ v value of rubber without excluding it from the rubber composition.