Polyurethane (PU) hardener is polyisocyanates hardener that reacts with the polyol resin to form polyurethane system. When these two liquid parts are mixed, they will react completely. Chemical bonds called crosslinks are formed between them and they are cured into a solid form. The number and type of functional groups and structure of resin and hardener dramatically impact the reaction speed and the cured performance.

Isocyanates are very effective hardeners. They are versatile and can react with hydroxyl functional groups to form polyurethane chains. There are two main classes of polyisocyanates: aromatic isocyanate and aliphatic isocyanate. The example of aromatic isocyanates is toluene diisocyanates (TDI), while aliphatic isocyanates is hexane diisocyanate (HDI).

Aromatic isocyanates are often used to produce polyurethane systems that is suitable in indoor applications, such as primers, foam, elastomers, adhesives and coatings. Aromatic isocyanates can react with polyols extremely fast. However, aromatic isocyanates provide relatively poor resistance to UV light and tend to increasing in yellowness over time. Therefore, aromatic isocyanates are not used in coatings applications where sunlight is particularly harsh

Aliphatic isocyanate are used predominately in coating applications because they produce polyurethanes with excellent UV resistance and exterior durability. In comparison to aromatic isocyanates the aliphatic isocyanates are slower in their reaction with polyols. For crosslinking of coatings polymers of diisocyanates, such as the trimers are used. HDI is the most often used to form color-stable polyurethane coatings, that can significantly enhance a products appearance, lengthen its lifespan and offer high abrasion resistance.