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Investigations of some dielectric properties of natural rubber Holroyd, Louis Vincent

Abstract

This paper deals with the problem of measuring the dielectric properties of rubber under various conditions of stress and temperature over a broad range of frequencies. A discussion of the more modern theory of dielectrica is given and a survey made on the mechanical and electrical properties of natural and synthetic rubbers. Values of dielectric constant and dissipation factor are presented for natural rubber gum stock (2% sulphur) unstretched and stretched 300% at 25°C and unstretched and stretched 300% at 68°C over the range of frequencies 10² to 10⁷ c.p.s. The dielectric constant decreases and the loss peak shifts to higher frequencies when the temperature is raised. Stretching the rubber at 25°C lowers the dielectric constant and increases the losses. The changes upon stretching may be due to orientation and crystallization of the molecular dipole components of the rubber. Heating the stretched rubber to 68°C seems to increase the dielectric constant and to lower the loss peak. This may be explained by the assumption that the higher temperature lowers the degree of orientation and melts the crystallites.

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