Abstract
There are in use today several methods for determining the scorching point of rubber and synthetic compounds. Some compounders use the Williams plastometer method, while others press-cure a thin slab of compound for increasing periods of time at 220° F or thereabouts until an examination of the slab shows that vulcanization has taken place. Another method consists of heating several pieces of the compound in an oven and removing at constant intervals until the compound scorches. The test-pieces in the latter method are quite often sheeted out on a mill to determine if any vulcanization has taken place. These methods all give good indications of the scorching point; however, each one is a rather long and tedious test. The Mooney plastometer scorch test cuts down the time considerably and gives a picture of the scorching rate, when the plasticity is plotted against the time. It is necessary, as a rule, to run this test only 10 or 12 minutes at 250° F to determine just how scorchy a compound really is, while the other methods cited sometimes require an hour or more to determine the scorching characteristics. It is desirable also to express the results of any test in the form of a graph or curve, so that one may make comparisons more easily without reviewing a lot of data. The Mooney scorch test gives results which can be plotted in the form of a curve, which also shows some very interesting facts aside from the scorching characteristics. The plasticity of the compound, the point at which vulcanization begins, and the rate of vulcanization after it begins, are shown. Summarizing the advantages of this test in a few words, one can say that it affords a quick, convenient method for determining scorching characteristics, and that it gives a clearer and more informative picture of scorching than other methods used today. Just a few illustrations demonstrating the use of the Mooney plastometer as an instrument for determining scorch have been given in this report. They do, however, point out the value of this test in the following ways: (1) The Mooney scorch test gives a quick method for studying the effect of various accelerators and accelerator combinations on the scorching characteristics of rubber compounds. It can also be used to show the effect of a retarder with various accelerators, and the effect of any other compounding change which might have a bearing on the scorching characteristics. (2) The Mooney scorch test may be used to show the relationship between scorching and rate of curing. As has been pointed out, there appears to be a definite relationship; however, before any general statements can be made, a considerable amount of work would be necessary. In the particular example cited here (Curve Sheet No. 2), one could visualize, from the shape of the Mooney scorch curves, the rate of curing of these compounds from the first signs of vulcanization to the peak cure.
Error bounds for the numerical evaluation of Legendre polynomials by a three-term recurrence
