A parametric study was made of the stress relaxation of linear polyethylene in air and a stress cracking environment, Igepal. Breaking time is shown to be a function of reduced variables of time, temperature, and molecular weight. Acreleration of tests for failure time can be achieved if the test temperature is raised, since for 0.96 density P.E. over the range of 50–110°C no mechanism change occurs. Stress relaxation measurements are shown to give results similar to the bent strip stress cracking test (ASTM D 1693-60T). This latter test can also be accelerated 80 times for 0.96 density polyethylene and 1500 times for 0.95 density polyethylene. A slight modification of the test procedure is required to obtain meaningful results. Some effects of strain, thickness, and presoaking are also reported.
An apparatus to measure tensile creep of soft polymeric materials has been designed and tested. This servo-controlled device permits recording the length of a plastic sample as a function of time from approximately 1 sec after loading. Length changes up to 20 cm can be determined to a precision of ±0.0012 cm. The instrument is stable to ±0.0012 cm over periods of several hours.
Reduced variables of time-temperature and molecular weight are used to describe the stress relaxation failure of 0.95 density polyethylene in a stress cracking agent, Igepal CO-630. This method was previously shown to work for 0.96 density linear polyethylene. The 0.95 density polyethylene has an apparent activation energy of 38 kcal for stress cracking. This is appreciably higher than the 22 kcal found for 0.96 density linear polyethylene. A much higher dependence on molecular weight was also found. The 0.95 density polyethylene has a lifetime that is proportional to Mv5.8 while the 0.96 density polyethylene has a lifetime proportional to Mv3.4. Some data are also shown which indicate a difference in the sensitivity to strain for the two materials. This can lead to appreciably longer lifetimes for the 0.95 density polyethylene at low strain.
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