Some Practical Aspects of Rubber Evaluation

In various papers, published in the past three years, E. C. Zimmerman and the writer have made references to the variability of crude rubber as determined by the properties of vulcanized compounds of various types. We were, I believe, the first to point out the chief stumbling block in the way of establishing a standard test formula for evaluating crude rubber, namely the variation in both quality and rate of cure produced by curing the same rubber with different curing agents. At the outset we were confronted with difficulties which resulted from a deplorable lack of standardization of rubber testing methods, and more specifically from a lack of agreement as to the proper way to select comparable cures. Indeed, it is obvious that these drawbacks have been among the major causes for lack of correlation of available data on the subject. We have stressed the importance of considering those properties of the vulcanizate which are reflected in the performance of the finished product, and have stated our objections to many of the popular criteria, such as “slope,” tensile product, tensile, and coefficient of vulcanization. We had concluded that aging should be the chief criterion of best technical cure. Accelerated age tests cannot be relied upon for comparison of different mixes, but experience has shown that for lightly loaded mixes hand tear is a reliable method of fixing the best aging cure. With this as a means of selecting the time of cure, the quality was studied by comparing the stiffness of the stress-strain curve at best cure. Later, in a paper on acceleration classification, if was shown that these conclusions might properly be modified when dealing with loaded mixes. Here the cures, as selected by hand tear and by maximum, tensile product, were in substantial agreement, except in the case of non-accelerated stocks.