Study on Crystallization of Masticated Natural Rubber by Stress Relaxation Method

Abstract

The effect of mastication on the crystallization of rubberlike polymer in the elongated state was studied by the stress relaxation method at low temperature.
Pale crepe of natural rubber was selected as the sample of this experiment. This sample (3±0.01g) was masticated in the internal mixer at 20°C in the oxygen atmosphere (in air). The mastication time was varied between 0.5∼5min. The masticated rubber was sheeted by the open roll, and the test strips (with breadth 1cm, length 3cm, thickness ca. 1.5∼2mm) were cut out from this sheet.
The stress relaxation of these samples was studied by relaxometer at -23, -12, -2°C. For the comparison, the data at 50, 70°C were also taken. For the period from the attachment of the sample on the cramp to the start of the measurement, it is naturally assumed for the sample to be crystallized. This crystallinity for this period, however, is considered to be negligible for the long measurement time.
The tendency of the stress relaxation curves of the masticated rubber was independent of the relaxation temperature. That is, the more heavily the sample was masticated, the lower the initial stress was and the higher the relaxation rate was (Fig. 1 and Fig. 2). This tendency was not changed in the elongation between 15∼150%.
The effect of the variation of temperature on the stress relaxation was studied for the sample masticated for the same time. The higher the temperature of the measurement was, the lower the initial stress was, and the slower the relaxation rate was (Fig. 3 and Fig. 4).
From these results, the factor due to the crystallization and the factor due to the intermolecular folw are thought to effect the relaxation phenomena of these masticated rubbers at low temperature. The crystallization of the masticated rubber seems to be remarkably interfered with by the molecular chain ends which was produced by the chain scission of molecules in mastication.