Hexamethylene diisocyanate
Hexamethylene diisocyanate is the aliphatic building block behind the trimers and biurets that make UV-stable polyurethane and polyaspartic topcoats possible.
HDI is a linear aliphatic diisocyanate: a six-carbon chain with an NCO group at each end (molecular weight about 168.2 g/mol, 50.0% NCO). It is a colorless liquid with a higher vapor pressure than MDI.
Because monomeric HDI is volatile and hazardous to handle in open application, it is converted into low-monomer polyisocyanates, mainly the HDI isocyanurate (trimer) and HDI biuret, before reaching coatings. Those oligomers are the hardeners in 2K aliphatic polyurethane topcoats and in polyaspartic systems.
With no aromatic ring next to its NCO groups, HDI-based coatings do not form the colored photo-oxidation products typical of MDI or TDI, which is why they hold gloss and color outdoors. Aliphatic NCO groups are less reactive than aromatic ones, so 2K polyurethanes based on HDI oligomers usually rely on catalysts.
HDI monomer is a strong respiratory and skin sensitizer. Commercial HDI polyisocyanates are supplied with very low residual monomer, but the oligomers themselves remain sensitizers and aerosols must be controlled during spraying.
Coatings use HDI oligomers such as trimers and biurets with very low residual monomer. Check the SDS for the stated monomer content of a specific product.
HDI has no aromatic ring, so it does not form the light-absorbing, colored oxidation products that cause aromatic polyurethanes and polyureas to yellow.
