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Cite this document
Oven-aging tests are now performed by many plastics laboratories, and several oven methods have been standardized. However, when these methods are applied to polypropylene, interlaboratory reproducibility is usually very poor. Even within one laboratory, results may vary by as much as 40 per cent. The causes of variability have not been well understood nor have the means been available to obtain satisfactory precision. There is some concern as to the significance of the test even if acceptable precision were attained. The work described in this paper was designed to answer some of these questions. These studies have shown that variability is a function of oven design and air flow, as well as the preparation method and dimensions of sample. Control of these variables can lead to greatly improved precision. Several other sources of potential variability were shown to be of little significance in the test. Molecular weight determinations showed that in oven tests at 150 C (302 F), molecular weight decreases gradually from the start, even though no visual changes can be detected during this period. The first visual change is crazing of the sample, which occurs just before a rapid decrease in molecular weight. Thus, oven-aging time records the onset of complete failure and is, therefore, a highly significant property of the material. As a result of these studies, most of the variables and their effects appear to be understood, and further round-robin tests can be planned if there is sufficient interest.
Forsman, J. P.
Enjay Laboratories, Linden, N. J.