In the manufacture of non-pneumatic tires, there is often a need to bond non-polar components, such as cured diene-rubber components, to polar thermoset components, such as structural components formed from various polar thermoset materials. As the skilled person appreciates, the ability to bond polar elements to non-polar elements is frustrated by the surface tension that exists between these elements. Where the composites formed by bonding non-polar elements to polar elements undergo mechanical and dynamic stresses, the incompatibility between the elements can facilitate failure of the bond.
Attempts have been made to bond polar thermoset elements, such as polyurethane structural tire components, to non-polar tire elements, such as tire treads. For example, U.S. Pat. No. 4,942,093 proposes the use of a thermosetting adhesive composition including a bismaleimide compound and an isocyanate-terminated rubber to bond diene rubber to the polyurethane. The adhesive composition is applied to either the polyurethane substrate or an uncured rubber element (i.e. a non-polar element), and upon heating, the adhesive crosslinks with the uncured rubber while the uncured rubber is simultaneously crosslinked. It is suggested that the curing step occurs at temperatures of from 120 to 175° C. and pressures from 50 to about 10,000 psi.
U.S. Pat. No. 8,623,169 proposes that cured polyurethane can be adhered to uncured diene-based elastomer by using an adhesive system that includes a specific primer adhesive and a secondary adhesive layer. The adhesive primer composition includes a polyisocyanate compound and a vinyl ester or an unsaturated polyester resin. The secondary adhesive is compatible with the adhesive primer and may include phenolic-based glues or diene glues. The diene glues include a crosslinking agent, such as a polyester or vinyl ester resin, while the elastomer is preferably a polyvinylpyridine/styrene/butadiene elastomer. The composites are press cured, for example, at temperatures of 165° C. and 20 bar.