Elongation Hysteresis of Hevea and Synthetic Elastomers

A hysteresis test has been developed which measures directly the energy loss per cycle of elongation. Six samples are stretched, like the spokes of a wheel, between a small disk and a large disk. The large disk is motor driven, and the small disk, mounted on a ball bearing, is displaced eccentrically with respect to the larger disk. In the elongation cycle which each sample undergoes with each revolution, the hysteresis causes an average force on the small disk which is normal to the eccentric displacement and is proportional to the energy loss per cycle. A lever and balance system makes it possible to measure this force continuously while the machine is running.Measurements were made on gum stocks of Hevea and five commercial synthetic elastomers. Testing conditions included temperatures, T, of 0°, 35°, 70°, and 105 °C., frequencies, f, of 5 and 50 c.p.s., mean elongations, e, of 50, 100, and 200%, and elongation amplitude, A, 25%. Absolute hysteresis, H, (loss/cycle-(amplitude) 2 ) at first decreases with increasing e and then flattens or goes through a minimum. Hysteresis decreases with increasing T. The temperature coefficient between 0° and 35 °C. is very large for Hycar. Hysteresis usually increases with f, but only slightly in most cases; and Hevea at e = 400% shows a reversal of frequency effect, which is presumably associated with crystallization effects.Under the same test conditions, the hysteresis of the synthetics is 10 to 20 times that of Hevea.When the hysteresis data for all synthetic stocks at various temperatures are plotted against T–T s , T s being second order transition temperature of the respective elastomer, the data lie within a rather narrow band. From this it is concluded that the hysteresis of synthetic elastomers arises from the same molecular forces which cause hardening and embrittlement at and below T s .