The structure of polyurethane can be complex and diverse containing "hard" and "soft" segments, which contribute to the balance between rigid and elastomeric properties. Polyurethanes are synthesised by two methods namely, prepolymer method and one- shot method. In prepolymer method, the polyisocyanate and polyol are reacted to form an intermediate polymer called "prepolymer". It is then converted into final high molecular weight polymer by further reaction with diol or diamine chain extender.