What is Moisture-cured polyurethane
Moisture-cured polyurethane is one-part polyurethane that its cure is initiatedly environmental moisture. The moisture-curable polyurethane is mainly consists of isocyanate-terminated pre-polymer. Various types of pre-polymer can be used to provide required property. For instance, isocyanate-terminated polyether polyols are used to provide good flexibility due to their low glass transition temperature. Combining soft segment, such as polyether, and hard segment, such as polyurea, provides good hardness and flexibility of coatings. Moreover, the properties are also controlled by selecting types of isocyanates to incorporate with pre-polymer.
Two main types of isocyanates are aromatic isocyanate and aliphatic isocyanate. Aromatic isocyanate has high reactivity. However, it has poor exterior durability and severe discoloration. Some examples of aromatic isocyanates are toluene diisocyanate(TDI) and 4,4’diphenylmethane diisocyanate(MDI). On the other hand, aliphatic isocyanate, such as, isophorone diisocyanate (IPDI), offers excellent weatherability and color retention; nevertheless, the reactivity of aliphatic isocyanate is low, so some catalysts may be required. Therefore, types of isocyanate are important to achieve desirable property. Furthermore, additives, solvents, pigments, etc. can be added based on application. However, raw materials for moisture-cured polyurethanes must be controlled to be moisture-free in order to obtain good storage stability and film property.
The other advantage of moisture-curable polyurethane is that it is one component. Therefore, it is easy to use since no proper mixing ratio is required, compared to two-component coatings. The moisture-cured PU is crosslinked by the reaction of Isocyanate-terminated pre-polymer and water in the air, producing amines and small amount of carbon dioxide. Finally, the reaction of amines and the rest of isocyanate-terminated pre-polymer takes place which, forms urea linkage.