In this study, the inverse relationship between lifetime and secondary creep rate was verified suggesting that shorter duration tests can be used to evaluate lifetimes. Also, the variability in creep rate was significantly lower than previous studies suggesting that using enhanced testing facilities could reduce required number of repetitions. A novel finding was an observed transition point in applied loading level where cyclic humidity conditions had the highest creep rates at low loads and constant high humidity conditions had the highest creep rate at high loads. Finally, it was determined that a standard moisture cycling history for a standard test protocol will be difficult to determine and the moisture sorption behaviour of each box type must be accounted for.

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The effect of fibre length and fibre aspect ratio on the reinforcement of soybean-based polyurethane foams was investigated. Micro-crystalline cellulose fibres and 260-µm long glass fibres embedded inside polyurethane foams were studied separately. Using X-ray tomography, it was determined that short micro-crystalline cellulose fibres were found solely embedded within the cell struts of the polyurethane foam. The cell struts were reinforced by the micro-crystalline cellulose fibres based on composite theory. An attempt was made to predict the reinforcement by using existing micro-mechanical models including the rule of mixtures and shear-lag theory. The overall foam compressive modulus increased based on the reinforcement of the cell struts and correlated with the foam mechanics model developed by Gibson and Ashby. The intermediate length 260-µm long glass fibres were found to span cells in polyurethane foam and were not embedded within the cell struts. These glass fibres did not contact each other. The reinforcement performance of the intermediate length glass fibres was found to be worse than the short micro-crystalline cellulose fibres. Therefore, these intermediate length fibres that span cells should be avoided for use in reinforcement of soybean-based polyurethane foams.

Polylactic acid films were foamed through a solid-state process using liquid carbon dioxide as a physical blowing agent. The foamed films were stretched to understand the effect of cell strut alignment on elastic modulus and yield stress normalised with density. Through stretching, cell strut alignment was achieved and verified with scanning electron microscopy. Aligning the cell struts improved the elastic modulus by a factor of 4 and the yield stress by a factor of 5 while decreasing the strain percentage at break. Aligned cell struts axially strain during tensile deformation which requires more force than unaligned cell struts which bend at initial deformations. The flexographic printability and surface topography of the foamed polylactic acid films was also analysed. The temperature used to impregnate carbon dioxide into the polylactic acid films was varied to understand its significance on printability and surface topography. Samples impregnated at ambient temperatures had smooth surface topography and a shiny lustre and good printability, whereas, samples impregnated at sub-ambient temperatures had a dull and rough texture and poor printability. Also, the carbon dioxide content had an effect on the surface topography and printability.

Introducing glass fibre to polyurethane foams increases the foam stiffness without raising the isocyanate content. This allows glass fibre reinforced polyurethane foams to be used in structural applications. Glass fibre reinforced polyurethane foams can be manufactured on a large scale using a chopper gun spray system. The glass fibre is commonly supplied as a roving (long strands of fibre wound into a spool) and the chopper gun breaks the roving into equal length pieces, which delivers a stream of glass fibre at a constant mass flow rate. Residual natural fibre is cost-effective, abundant and renewable making it an ideal candidate to replace non-renewable glass fibre in reinforcing polyurethane foam. However, residual natural fibre are often supplied as loose tufts and require multiple steps to be made into roving. In this study, a novel concept was developed that can meter natural fibre at a constant mass flow rate. The concept has the potential for developing natural fibre reinforced polyurethane foams on a large scale, with uniform fibre dispersion and high fibre volume fractions. The concept is verified through mathematical simulations and a prototype.