1.2 Polyurethane wheel

A polyurethane wheel covering consists of an elastomer obtained exclusively from the synthesis of raw materials. Polyurethanes are chemical compounds obtained from a polymerisation reaction triggered by mixing two components, belonging to two different families of compounds (Di-Isocyanates and Polyalcohols), that were previously heated to temperatures that keep them in the liquid state with relatively low viscosity. In general, elastomer polyurethanes do not contain any additional mineral loads. The reactive mix is cast or injected into heated moulds containing the metal or plastic centres. Thanks to the temperature of the mould and of the wheel centre body, the polymerisation reaction can be completed inside the polyurethane, while the polyurethan is chemically linked to any adhesive that may be present on the surface of the wheel centre body.

• Mould-on polyurethane is no longer fusible, has good elasticity characteristics in addition to medium-high hardness and compression and traction strength.

• Injected polyurethane is fusible even after moulding; in general, it has inferior elasticity characteristics but superior hardness with respect to mould-on polyurethane.

The following are some of the main physical-mechanical characteristics of polyurethane (for the definition of each characteristic see the standards indicated next to that parameter):

• hardness UNI EN ISO 868:1999; ASTM D 2240-2004
• specific density UNI 7092:1972; ISO 2781:1988
• impact strength UNI 7716:2000; ISO 4662:1986
• abrasion loss UNI 9185:1988; DIN 53516:1987
• ultimate tensile strength UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998
• ultimate elongation UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998
• tearing resistance UNI 4914:1987; ASTM D 624b-2000
• compression set UNI ISO 815:2001