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An unusual increase in stress due to volume shrinkage under constant constraint conditions was observed when a Caytur 21C extended Adiprene polyurethane specimen was tested at 121 °C (250°F) and 1 percent constant strain. The stress first decreased for 2 h and then actually increased until the test had progressed for about 40 h, when it again started to decrease. This stress enhancement was attributed to contraction of the hard segments brought about at the relatively high test temperature by conversion of branching biuret linkages to linear urea linkages in the presence of free amine. The resulting increase in density causes shrinkage in volume, thus produces stress buildup. After 40 h, the usual stress relaxation process becomes dominant again. This phenomenon was found to be closely associated to Caytur 21 and to depend strongly on the temperature and strain conditions. However, it was insensitive to isocyanate (NCO) content and Caytur amount.
negative stress relaxation, stress enhancement, stress buildup, stress relaxation, heat treatment, urea linkage, biuret linkage, free amine
Senior research scientist, Central Research Laboratories, The Firestone Tire and Rubber Co., Akron, Ohio