Polyurethane (PU) is frequently used in safety shoes as it offers several positive properties.
- Very light
- Very flexible
- Very good shock-absorbing properties
- Highly abrasion-resistant
- Good anti-slip properties
All of this has a very beneficial effect on the wearer’s natural movement patterns and when it comes to the product life of the sole as regards mechanical influences.Above all, the PU sole material is subject to a natural ageing process – hydrolysis.
PU consists of long polymer chains that are gradually split apart due to the effect of moisture. As a consequence, PU loses flexibility over time and gradually becomes brittle.Ethylene vinyl acetate (EVA), which is well-known from the sports shoe industry, is not subject to hydrolysis, but it does present one disadvantage: it shrinks more quickly when exposed to constant loads like walking and running, and so it no longer returns to its original shape.
Polyester shoe sole systems are much easier to formulate and process than their polyether counterparts. The major difficulty is associated with the shrinkage which might be encountered during the processing of polyether systems especially when moulding thick parts. This is of course due to the presence, of an unreactive blowing agent in the formulation in addition to the CO2 generated from the reaction of water with the isocyanate. Polyester shoe sole systems expand, only, under the influence of carbon dioxide. The formulations do not contain unreactive blowing agents. The two, major components of polyester footwear systems are the isocyanate prepolymer and the polyol blend