Polyurethane shoe sole systems have been in the market for more than half a century.
The systems fall in two categories.
A -Polyether systems, which result in soles with excellent surface finish. They are used in street shoes and they are very resistant to hydrolysis.
B -Polyester systems, which result in low density soles with excellent resistance to abrasion. They are used as interlayers as well as soles of sport shoes, especially tennis shoes.
To the more adventurous development minded people a third category can be introduced. It consists of a hybrid of polyether and polyester systems.
The basics of PU formulations require that soles are the reaction products of an isocyanate prepolymer and a polyol system. Their hardness is imparted by the precipitation of hard segments in the polymer matrix and their density is the result of the evolution of CO2 generated from the reaction of the isocyanate and water as well as because of the presence of small amounts of a blowing agent in the mix, as it is the case of polyether systems.
R-NCO + H20 --------> RNH-COOH---------> RNH2 + CO2