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Comparison of Methods of Examining the Scorching of Rubber Stocks

1945; American Chemical Society; Volume: 18; Issue: 2 Linguagem: Inglês

10.5254/1.3546747

1943-4804

J. F. Morley, J. R. Scott, W. H. Willott,

Polymer Nanocomposites and Properties

Abstract A comparison has been made between the plasticity and solubility methods of examining the scorching of accelerated rubber stocks; trials with a third method, based on the gelling of a solution of the stock, were unsuccessful. In a comparison using 10 different stocks the plasticity and solubility methods arranged these in practically the same order as regards tendency to scorch, showing that the two methods measure essentially the same property. The plasticity method usually detects an earlier stage of scorching than the solubility method using benzene as the solvent. Attempts to improve the sensitiveness of the latter method by using a less powerful solvent did not give promising results, as petroleum ether and diethyl ether behaved very similarly to benzene, while cyclohexanone did not readily dissolve the unscorched stock. The solubility method is unsuitable for stocks containing considerable percentages of gas black, e.g., tire tread stocks, and presumably also for those containing other strongly reinforcing fillers, because the unscorched stock does not dissolve in benzene. In determining the period for which a stock can be kept at a given temperature, or the temperature at which it can be kept for a given period, without practically serious scorching taking place, the plasticity method is to be preferred on account of its greater sensitiveness. It has the further advantage of giving a quantitative measure of the degree of scorching. However, for purely comparative purposes (but excluding stocks of the type referred to in 4 above) the solubility method is just as reliable, and has the advantage of being much simpler and quicker. By this method, also, it would usually be possible to decide whether any given piece of stock were scorched or not, whereas by the plasticity method this could not be done unless an unscorched piece of the same stock was available for comparison. In using the parallel-plate plastimeter for scorching tests, a relative long period of compression (10 minutes or more) gives the most sensitive indication of the change in plasticity produced by scorching. In the stocks examined the rate of scorching is, on the average, approximately doubled by a temperature rise of 10° C.