Effect of Moisture Variations on Curing Rate of GR-S

Abstract

Abstract Variations which are beyond experimental error have occurred in the rate of curing of GR-S control and production lot samples tested in this laboratory. It is believed that other laboratories in the synthetic rubber industry have encountered similar variations. Changes in the moisture content of the compounded stock at the time of curing have been suspected of causing the fluctuations in the rate of curing. Several papers on the effect of moisture on the rate of curing of GR-S have dealt largely with relatively high moisture contents. However, this investigation was undertaken to correlate the 300 per cent modulus with the moisture contained in the stock at the time of cure. Rupert and Gage observed that, in testing a large master batch of GR-S at intervals, variations in the rate of curing and modulus values beyond experimental error occurred without any apparent relation to time of standing. The same authors have shown that both GR-S raw polymer and compounded stock absorbs or adsorbs moisture from the air, the amount depending on the humidity of the air. Braendle and Wiegand have shown that the moisture content of rubber-grade carbon black during storage can vary from 0 to 4 per cent. Samples of black taken in this laboratory between December 1946 and March 1947 varied from 0.2 to 2.5 per cent moisture. Under the same conditions at which the black contained 1.0 per cent moisture, the other pigments (sulfur, zinc oxide and mercaptobenzothiazole) each contained roughly 0.7 per cent moisture. Two previous papers have shown that the moisture retained in a mixed batch varies in proportion to, but is less than, the moisture added to dry pigments. Both authors, however, used relatively inaccurate means to determine the water retained, and were interested in higher moisture contents than mixed batches would normally contain.