Solventborne polyurethane adhesives have been used for over 50 years. They have a long history of producing high-strength, durable bonds, and can be used on a range of substrates. As industrial hygiene concerns in the workplace have grown, an effort has been made to eliminate solvents when possible. Polyurethane dispersions have been developed that have excellent adhesion performance and effectively address the safety aspects raised by the use of solvents.
The backbone of a polyurethane dispersion will have the same basic composition as the solvent-based polymers, but will contain hydrophilic groups that allow the polymer to be dispersed in water. The dispersed particles consist of a hydrophobic core surrounded by a hydrophilic shell composed of ionic groups and/or long-chain hydrophilic nonionic groups. In contrast to other types of dispersions, polyurethane dispersions most often contain only internal emulsifying groups, meaning they are built into the polymer backbone. Other dispersions, such as polychloroprenes, are stabilized by the addition of external emulsifiers. Ionic or nonionic surfactants that are not chemically attached to the polyurethane polymer could migrate out of the adhesive and lead to bond failures.