Considerations on the Selection of Polyurethane Systems for Midsole Wedges in Athletic Footwear Application

Abstract

Most developments of high performance polyurethane foam systems for manufacturing sport shoe midsole wedges have concentrated on reducing the density while maintaining physical properties. In the drive to reduce the density of the midsole material it has become apparent that maximizing any one physical property can result in the deficiency of another critical parameter. Increasing the hardness of a soling material, for example, may be needed to increase the load capacity and potentially result in improved cushioning. However, if the system is not properly modified taking into account some basic principles of polyurethane elastomers, the resulting foam can have lower resiliency and tear strength. Therefore, it is important to understand the interrelationships of properties to ensure both the durability and performance of a sport shoe. Tear strength, tensile strength and elongation are specified to ensure the strength and durability, whereas pendulum rebound, skin hardness and resiliency are specified to ensure the performance and comfort of the shoe. This paper describes in detail the physical property balance for midsole wedge materials that satisfies typical hardness specifications of sport shoe manufacturers. It advocates using compression hardness instead of indentation or skin hardness to specify the load bearing characteristics of midsole materials. This paper also shows that the physical properties of a system vary as a function of the processing conditions, as well as how the measurement of these properties are affected by the testing environment. Using this information, the sport shoe designer can engineer not only the right product, but also the right process to guarantee the maximum performance and durability of a shoe.

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