HDI isocyanurate (trimer)
The most widely used hardener for UV-stable two-component polyurethane and polyaspartic coatings, prized for gloss and color retention.
The HDI trimer, or HDI isocyanurate, forms when three HDI molecules join through their NCO groups into a six-membered isocyanurate ring, leaving three free NCO groups on the arms. Commercial products are mixtures of the trimer with some higher oligomers, supplied at about 21–23% NCO with very low residual HDI monomer.
HDI trimer is the go-to A-side for:
Its functionality of three or more builds a crosslinked network, while the stable isocyanurate ring adds heat and weathering resistance. At about 21.8% NCO the equivalent weight is roughly 193 g/eq. Standard grades are fairly viscous, so low-viscosity versions are offered for high-solids and low-VOC formulas.
Although residual monomer is low, HDI polyisocyanates are still respiratory and skin sensitizers, particularly as spray mist. Keep containers tightly closed; moisture raises viscosity and can form solids.
It is an aliphatic, multifunctional isocyanate that reacts with aspartic esters at a controllable rate, producing hard, crosslinked, UV-stable films.
Both are HDI polyisocyanates used in light-stable coatings. Trimers generally offer lower viscosity and better heat stability; biurets are valued for flexibility. Formulators choose based on the full system.
