For the preparation of polyisocyanate crosslinkers from monomeric diisocyanates, urethane formation, allophanate formation and biuret formation as well as “dimerization” and "trimerization" are of particular industrial importance.
A prominent example of urethane formation as a modification reaction is the very first representative of polyisocyanates for coatings, Desmodur® L (L here stands for “Lack”, German for coating). It is obtained from an aromatic diisocyanate, TDI. The coatings that can be obtained from this product have very good mechanical and chemical resistance properties, but, due to their limited light fastness, they are restricted to select applications, e.g. indoor coating systems.
By contrast, paints and coatings based on aliphatic polyisocyanates have excellent weather and light stability and can be used universally. The first representative of this class of polyisocyanates was Desmodur® N (N stands for "non-yellowing"). Probably the most important polyisocyanates today for weather-resistant polyurethane coatings are HDI-biuret (e.g. Desmodur® N 75 BA) and HDI isocyanurate (e.g. Desmodur® N 3390 BA) (Fig. 5).