Question: What is the difference between a polyurethane adhesive and a polyurea adhesive?
Answer: Polyurethanes and polyureas both cure to systems that can vary from rigid to very flexible solids in their final properties. The two are quite similar, but with some obvious differences. While polyurethanes have been used for many years as adhesives and sealants, polyureas are relatively new to the industry. From a chemical standpoint, a polyurethane is made from the reaction of an isocyanate with a polyol, whereas a polyurea is formed from an isocyanate reacting with a multifunctional amine. It is also possible to make so-called “hybrid” systems, in which the isocyanate is reacted with a mixture of hydroxyl and amino groups.
The most important difference is that the polyurea reaction is much faster than the polyurethane one, and the systems can gel within a few seconds after mixing. Polyureas have been used very successfully in the coatings industry, where the two components are mixed using plural spray equipment; polyurea adhesives, however, are relatively new.
One issue has been that the adhesives gel so quickly that that the liquid does not have time to spread and wet the bonding surface. In addition, heat-sensitive substrates can be damaged by the strong exotherm generated by the fast curing. However, slowing down the curing is possible and has led to successful applications (e.g., high-speed wood bonding). In addition, both polyurea adhesives and sealants are available commercially. Polyurea adhesives also lead to new bonding possibilities where they can be used like a spot weld, and there is a technique possible where parts are preassembled and the adhesives is injected into the bond line through preformed grooves.
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