Effect of long-term compression on rigid polymer foams
The sponsors of this project have been using the rigid heavily-crosslinked polyurethane foam detailed in this study for load-bearing applications. One of the main requirements of this material is that it must possess excellent recovery properties following extensive compressive periods over several years. For such long loading regimes, there is need for detailed understanding of the compressive behaviour of this material, and its subsequent recovery upon release. More recently, there has been a growing interest in replacing the polyurethane foam with an alternative cellular plastic that possesses similar, if not identical, compressive recovery and behaviour. Attention was focused on the other primary polymer contingent, a polyethylene foam. A polyimide foam was also considered as it was already being used in applications similar to those of the polyurethane foam. The structures of the foams were investigated by means of Differential Scanning Calorimetry, Scanning Electron Microscopy and Image Analysis. The deformation mechanisms that occur during the application of a compressive force were examined visually via a scanning electron microscope compression rig. The mechanical analysis involved stress-strain testing whereby three stages of compression were identified (‘linear elastic, stress plateau and densification’), as described in the literature. Quadratic relationships were found to exist between the foam density and the ‘elastic modulus, plateau modulus and the compressive strength’ respectively. Such relationships had previously been found to exist in the literature, but not for the rigid variety of foam at such a broad range of densities. Further analysis included a detailed study of the recovery of the polyurethane (100 kg m[sup]-3 to 800 kg m[sup]-3) foams, a lightly-crosslinked polyethylene foam and a non-crosslinked polyimide foam. The foam samples were compressed by strains which spanned their linear elastic and stress plateau regimes i.e. by 2.5% to 35% for periods ranging from 3 days up to one year at ambient temperature. This analysis was also undertaken at elevated temperature as a means of accelerating the ageing process. Recovery of all of the samples was monitored for a minimum of 100 days at ambient temperature following release. Recovery of all of the foams tested was found to occur in two stages; an initial rapid recovery within the first day following release followed by a much slower recovery phase over a period of approximately 100 days. The initial rapid recovery was attributed to the recovery of the bulk polymer whilst the recovery of the cellular structure was associated with the ensuing slower recovery phase. In addition, recovery of the foams was found to be dependent more upon the compressive strain than on other parameters, such as compressive period and foam density. For compressive periods exceeding two weeks, recovery is almost independent of the latter parameters.