Abstract

This paper is a description of some studies of the phenomenon variously described in the literature as heat embrittlement, heat tenderness, heat shortening, etc. For purposes of discussion the term heat embrittlement will be employed in a general sense to denote the change in breaking elongation resulting from a change in test conditions, i.e., test temperature, cure, or aging. Inasmuch as the expression heat embrittlement is purely qualitative, no definite experimental procedure for its determination is implied, such as is implied, for instance, in the term ultimate tensile strength. Such lack of definiteness in terminology often leads to considerable confusion and the development of widely different test techniques for measuring the same phenomenon. Illustrative of the latter point are two heat embrittlement tests, one introduced by W. B. Wiegand and the other by R. B. Hobson. The first method is based on a measurement of the length of time which a stretched T-50 strip will withstand contact with an iron heated to 400° F without breaking. In the second method, samples of raw polymers are air-aged for specified periods of time, following which benzene solubilities are determined. The amount of residue remaining after extraction is employed as a measure of the heat embrittlement of the material. With the exception of Hobson's method, there appeared to be general acceptance of the idea that heat embrittlement was correlated in some manner with breaking elongation. It was accordingly decided in this study to measure breaking elongations directly, under various test conditions, and then to express the results in terms of heat embrittlement indices, each of which is defined mathematically. Although these definitions were originally derived on purely empirical bases, they appear to yield results capable of unique and useful interpretations, as will be pointed out later.