Effect of Strain on Rate of Crystallization of Natural Rubber

Abstract

Abstract The crystallization of vulcanized natural rubber under strain gives this polymer a strength which is not present in a noncrystallizing rubber. However, if the rubber is stretched very rapidly the strength falls suggesting that a finite time is necessary for the crystallization to build up. An apparatus was constructed to enable changes in crystallinity to be detected over a time range down to hundredths of a second after the rubber had been stretched. The method consisted in extending the rubber very rapidly by passing a long strip between two sets of rolls, the second of which was rotating at a greater speed than the first. At any chosen point between the rolls there was a sample of rubber which had been stretched for a fixed time. By moving between the sets of rolls one moved along the time scale after stretching. The crystallinity of the rubber was detected by an x-ray technique. The results showed an initial period (which varied from one twentieth sec for an extension ratio of 5 to 20 sec for an extension ratio of 3.6) in which no crystallization was observed. This was followed by a gradual rise in crystallinity. The level of crystallinity was still increasing after periods of a week after stretching.