The synthesis of polyurethane adhesives is based on the unique chemical properties of isocyanates. Isocyanate is a compound containing an isocyanate group (-NCO) in the molecule. This group has a highly unsaturated bond structure with overlapping double bonds and can react with various compounds containing active hydrogen. In the field of polyurethane adhesives, isocyanates containing 2 or more -NCO characteristic groups are mainly used. According to whether the product is yellowing under light, polyurethane adhesives are divided into general isocyanate polyurethane adhesives and yellowing resistant isocyanate polyurethane adhesives.
General-purpose isocyanates, that is, aromatic isocyanates, are currently the most widely used isocyanates in the polyurethane industry. The methylene group connected to the benzene ring in the structure is easily oxidized to form quinone chromophores, which leads to yellowing of the material. Commonly used general-purpose isocyanates include TDI, MDI and polymethyl polyphenyl isocyanate (PAPI). TDI is liquid at room temperature and easy to use. It is the first isocyanate used in the polyurethane industry.
In order to improve the yellowing of polyurethane materials caused by universal isocyanates, in addition to the use of related additives, the generation of chromophores with benzene ring conjugated quinone structure should be avoided. For this reason, researchers have developed many yellowing-resistant isocyanates: such as xylylene diisocyanate (XDI), hexamethylene diisocyanate (HDI), isophorone isocyanate (IPDI) and so on.
