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Special conditioning is often required in the handling of adhesives and the processing of plastics. Tests to determine these conditions are straightforward and offer no particular problem to technologists. This phase is not subject to standardization. Conditioning for and during testing of adhesives and plastics is necessary in the evaluation of the results and to obtain comparable results. The conditions are fairly well standardized and correlated as evidenced by the work of the ASTM and the Federal Specifications Technical Committees. Further standardization is needed in accelerated service tests; some of this work, particularly on adhesives, is under way. The principal faults in these tests are (a) variability in service conditions, (b) inaccurate knowledge of service conditions, (c) distortions caused by using more strenuous laboratory test conditions to obtain acceleration, and (d) differences in concentration of degradation products in the materials. With factors such as these, a high degree of correlation between accelerated laboratory tests and behavior in service is not to be expected except in rare instances, although reasonably good correlation has been obtained in some cases. Experience in interpreting the results of the accelerated laboratory tests is one key to the problem. Accelerated weathering testing is in an unsatisfactory state. The degree of correlation between laboratory tests and behavior on exposure is far from satisfactory. This lack of correlation is attributed to (a) the great variability in outdoor weather, (b) variability in laboratory accelerated weathering test equipment, (c) distortions caused by using greater light intensities, higher temperatures, different moisture conditions, and more prolonged periods of these conditions to obtain acceleration in the laboratory test, and (d) the resulting increased concentration of degradation products in the adhesive and plastic.
Reinhart, Frank W.
Chief, National Bureau of Standards, Washington, D. C.