Viscoelastic behavior of flexible slabstock polyurethane foams: Dependence on temperature and relative humidity. I. Tensile and compression stress (load) relaxation

The relaxation behavior of the load in compression and the stress in tension was monitored at constant temperature and/or relatively humidity for a set of four slabstock foams with varying hard-segment content as well as two of the compression molded plaques of these foams. The majority of the compression relaxation tests were done at a 65% strain level in order to be consistent with the common ILD test. The tensile stress relaxation tests were performed at a 25% strain level. Over the 3-h testing period, a linear relationship between the log of compressive load or the log of tensile stress versus log time is observed for most testing conditions. For linear behavior, the values of the slope or the load/stress decay rate are comparable in both the tension and compression modes with the values being slightly higher in magnitude for the compression mode. These rates of decay are in the range of −2.2 × 10 −2 to −1.7 × 10 −2 for a 21 wt % hard-segment foam and −3.2 × 10−2 to −2.4 × 10−2 for a 34 wt % hard-segment foam. Increasing %RH at a given temperature does bring about a steady decrease in the initial load or initial stress as well as a slight increase in the rate of relaxation. The effect of temperature on the relaxation behavior is most significant at temperatures near 125°C and above. The FTIR thermal analysis of the plaques indicates that this significant increase is due to additional hydrogen bond disruption and possible chain scission taking place in the urea and urethane linkages that are principally present in the hard segment regions. The relaxation behavior in both tension and compression is believed to be mostly independent of the cellular texture of the foam at the strain levels given above. This conclusion is based on the similar relaxation behavior between the plaques and the foams. © 1994 John Wiley & Sons, Inc.