VISCOELASTIC BEHAVIOR OF FLEXIBLE SLABSTOCK POLYURETHANE FOAMS AS A FUNCTION OF TEMPERATURE AND RELATIVE-HUMIDITY .2. COMPRESSIVE CREEP-BEHAVIOR

The compression creep behavior was monitored at constant temperature and/or relative humidity for two slabstock foams with different hard-segment content. The tests were performed by applying a constant load (free falling weight) and then monitoring the strain as a function of time over a 3-h time period. A near linear relationship is obtained for linear strain versus log time after a short induction period for both foams and at most conditions studied (except at temperatures near and above 125-degrees-C). The slope of this relationship or the initial creep rate is dependent on the initial strain level, especially in the range of 10-60% deformation. This dependence is believed to be related to the cellular struts buckling within this range of strain. At deformations greater than 60% and less than 10%, the solid portion of the foam is thought to control the compressive creep behavior in contrast to the cellular texture. Increasing relative humidity does cause a greater amount of creep to occur and is believed to be a result of water acting as a plasticizer. For low humidities increasing the temperature from 30 to 85-degrees-C, a decrease in the rate of creep is observed at a 65% initial deformation. At 125-degrees-C, an increase in the creep rate is seen and is believed to be related to chemical as well as additional structural changes taking place in the solid portion of the foams. The creep rate is higher for the higher hard-segment foam (34 wt%) than that of the lower (21 wt%) at all of the conditions studied and for the same initial deformation level. This difference is principally attributed to the greater amount of hydrogen bonds available for disruption in the higher hard-segment foam. (C) 1994 John Wiley & Sons, Inc.