Urethane and polyurethane adhesives are used on both porous and nonporous substrates like wood, metal, glass, and plastic. They offer extremely strong bonds and are available as single-component adhesives or two-part systems. Urethane and polyurethane adhesives are water resistant, with some compounds offering high resistance to heat. Single-component adhesives cure by reacting to moisture in the air. Two-part systems cure rapidly and are used in low-humidity environments because they do not require moisture to cure.