Mechanical properties of high density cellular polyurethanes

The use of cellular polyurethanes either as microporous foams in shoe upper materials (poromerics) or in closed cell form as soling materials has increased rapidly during the last few years in the footwear industry. Compared with vulcanised rubbers, these materials have high strength over an extended temperature range, high set and good resistance to cut growth. The research now presented.has been concerned with determining the reasons for these mechanical properties of polyurethane as compared with compounded conventional vulcanised rubbers. The supplementary contribution to the thesis presents a review of earlier work on the strength and reinforcement of crystalline, amorphous and filled vulcanised rubbers and includes a short section on the effect of crosslinking on ultimate failure properties of natural rubber. This study has been extended by the author into the effect of chain branching in polyurethane elastomers on the failure properties. The viscoelastic properties of natural and artificial leathers are also discussed in order to demonstrate similarities between the different materials and show how a cellular polyurethane sheet has hysteresial properties similar to those of a natural material of fibrous structure. It is shown that a cubical lattice model can be applied to explain the differences between such mechanical properties as modulus, tensile and tear strength of a cellular polyurethane and the corresponding solid material of the same polymer. This model, which has previously been applied to the mechanical properties of a natural rubber latex foam, indicates that the strength of cellular polyurethanes is due to the very high strength of the solid material. An extensive investigation into the effect of time and temperature on the tensile properties of cellular and solid polyurethanes is presented in order to show that polyurethanes of the type used in poromerics have a very broad relaxation spectrum extending over 18 decades of time. Because of this response to deformation, the failure properties remain fairly constant over the temperature range from 21 – 160°c. Above 160°c, the tensile properties fall quite markedly. Stress softening in these polyurethanes is very high and can only be reversed by heating to temperatures above 160°c. The cut growth and fatigue properties of cellular and solid polyurethanes are considered. Following a brief review of the investigation on cut growth and fatigue of vulcanised rubbers involving the use of tearing energy theory, it is shown that cut growth and hysteresis properties of vulcanised .rubbers can be correlated. The lower limit of tearing energy (To) below which no cut growth takes place in the absence of chemical effects is found to be higher for polyurethanes than for vulcanised rubbers. Fatigue failure of cellular polyurethanes is found to be due to cut growth from the largest pore in the sample. These data are also compared with measurements on other two phase elastomer systems such as styrene butadiene copolymer vulcanisates with high styrene content and polystyrene-polybutadiene thermoplastic rubbers. From an extensive review of the literature on the structure of polyurethane elastomers, it is deduced that polyurethanes of the type used in poromerics consist of a segmented structure of long polyester chains connected to very minute (25Å) hard urethane segments. The cohesion of the hard segments is primarily due to hydrogen bonding and other physical forces. It is concluded that the high strength, good cut growth resistance and broad relaxation spectrum of polyurethanes are due to the reinforcement given by the hard urethane segments which act as well dispersed minute filler particles in the polyester rubber matrix. The hydrogen bonding between the hard segments dissociates at approximately 170°c so giving a degree of thermoplasticity which produces a very high permanent set. An appendix discusses some of the practical applications in the footwear industry of the work presented, such as forming of poromerics, tearing from stitch-holes and flex cracking of solings