Compression recovery of rigid polymer foams following confinement at elevated temperature
Cellular materials are all around us. They can be found in nature as in bone, wood, leaves and even in our food. In the last fifty years, man has produced many synthetic cellular materials: firstly with polymeric foams and more recently with foamed metals, ceramics and glass. Polymer foams are used in a variety of applications ranging from coffee mugs to the feet of the Apollo Lunar Module, for which they were used as shock absorbers. This project was aimed at understanding the recovery from long-term compression of rigid polymer foams. Understanding the dynamics involved in the recovery process of foams is very important, especially in the automotive industry where it determines safety of the driver, passengers and pedestrians, for example, in car bumpers. In this study, foam samples were compressed by strains which spanned their linear elastic and stress plateau regions, i.e. 2.5% - 35% for one month at various temperatures. Recovery occurred in two stages, designated phase 1 and phase 2. Phase 1 is the initial recovery, which dominates the full recovery process and is complete within hours or days. Phase 2 is a lesser recovery occurring over a much longer period of approximately 100 days. The initial recovery is associated with the polymer itself, whilst phase 2 recovery is associated with the cellular structure. Recovery of all samples was monitored for a minimum of 100 days at ambient temperature. Tests were also carried out to see how the environmental surroundings affect the polymer recovery. The different polymer foams which were investigated were: • Polyethylene • Polyetherimide • Polyurethane • Polysulphone The polymers tested all showed very different responses to the changes in temperature. All polymers investigated at different compressive strains demonstrated reproducible Arrhenius plot slopes under different conditions and hence a reasonably reproducible set of values of recovery process. Analyses were based on the final total recovery of the thickness as the most reliable parameter of recovery. It has been demonstrated that the mechanism of polymer deformation and recovery probably does not involve chain scission but backbone vibration; that the best parameter for characterising the recovery process is the final total dimensional recovery of the sample; and that subtle environmental changes have a large effect on the recovery from compression, although temperature and humidity are not responsible.