The newest and fastest growing category of hot melt adhesives is reactive urethanes, designed for high impact, toughened applications. Introduced in the early 1990’s, reactive urethanes are a new, hybrid chemistry. These adhesives are the only thermosetting hot melt adhesives that combine the advantages of traditional thermoplastic technology with the high performance characteristics of reactive adhesive chemistries (Figure 1). Reactive urethanes begin to polymerize as they cool, but also undergo a secondary moisture cure that causes the adhesive to crosslink, forming a new polyurethane polymer that provides higher strength and flexibility than traditional hot melts.
 
  Applied as a drop or a thin bead, high viscosity reactive urethanes set quickly, becoming structurally rigid in minutes. This rapid solidification allows the on-line manufacturing process to continue uninterrupted, while secondary moisture cure continues for up to 24 hours. Reactive urethane hot melts provide unmatched performance capabilities including excellent solvent and chemical resistance, flexibility, and bond versatility.  Because they are thermoset adhesives, reactive urethane hot melts offer lower initial green strength and can be formulated to have extended open times in excess of three minutes if necessary. These extended open times allow reactive urethane hot melts to be applied using a variety of methods including roll coating, swirl coating, spraying or standard adhesive dispensing.
 
  Reactive urethanes are suitable for use on thermally sensitive substrates, as their application temperature is only 250°F, substantially lower than the 375° to 450°F application temperatures of traditional hot melts. Unique for this category of adhesive, reactive urethane adhesives are heat resistant and can be used in heat sensitive end use applications, as well as for bonding appliance housings and cabinets, bonding thermal and sound insulation panels in dishwashers and dryers, and adhering labels, nameplates, touchpads, and vinyl gaskets.