HDI biuret oligomer
The HDI biuret was one of the first commercial low-monomer aliphatic polyisocyanates, and it remains a hardener of choice for flexible, light-stable polyurethane enamels and clearcoats.
HDI biuret is an oligomer built from three molecules of hexamethylene diisocyanate (HDI) linked through a biuret group, giving a molecule with three free NCO groups (molecular weight about 478.6 g/mol for the idealized structure). Commercial grades also contain higher oligomers, have an NCO content of roughly 22–23% and contain very little residual HDI monomer, typically well below 0.5%.
HDI biuret is the A-side hardener in two-component aliphatic polyurethane coatings, crosslinking acrylic and polyester polyols to give tough, glossy films that resist yellowing and chalking. Typical uses include transportation and refinish topcoats, industrial maintenance enamels, aerospace and clearcoats over aromatic primers.
The biuret structure forms when HDI reacts with a small amount of water: some NCO groups hydrolyze to amines, which react with more HDI to form ureas, and those ureas react with further HDI to form the biuret. The result is a trifunctional (or higher) aliphatic polyisocyanate.
Although residual monomer is very low, HDI biuret is still an isocyanate: spray mists can cause respiratory sensitization, and it may cause skin sensitization. Spray application requires suitable respiratory protection (supplied air is common), skin and eye protection, and good ventilation. In the EU, diisocyanate training rules apply to products with more than 0.1% monomer.
Store tightly sealed under dry nitrogen; moisture causes gelling, cloudiness and CO₂ pressure.
Both are low-monomer, trifunctional HDI oligomers. The trimer (isocyanurate) usually has lower viscosity and better thermal stability; the biuret is known for flexibility and has a long track record in topcoats. Many formulators choose by viscosity, pot life and film performance testing.
HDI is aliphatic, so the cured polyurethane contains no aromatic rings that can oxidize into yellow, light-absorbing structures. That is why aliphatic hardeners are used for topcoats.
Not without controls. Low monomer content reduces vapor hazards, but spray mists of polyisocyanates can still cause respiratory sensitization. Follow the SDS, which typically calls for supplied-air or other suitable respiratory protection during spraying.
Use the NCO content of the hardener and the OH number of the polyol to match isocyanate and hydroxyl equivalents, usually with a small NCO excess. The mix-ratio guide walks through the arithmetic.
