HDI Trimer

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HDI trimer is one of the most common HDI homopolymers. It is a triisocyanate containing an isocyanurate heterocyclic structure.

Its molecular formula is C24H36N6O6, and its molecular weight is 504.6. The CAS number is 3779-63-3, and the EINECS number is 223-242-0. HDI trimer belongs to HDI homopolymers, and the general CAS number for HDI homopolymers is 28182-81-2.

Structural formula:

Physical and Chemical Properties

HDI trimer is a medium-viscosity, light yellow, transparent liquid that can be diluted with organic solvents.

The theoretical NCO mass fraction of HDI trimer is 25%, but due to side reactions during the trimerization process, the product generally contains a small amount of HDI oligomers (molecules containing more than two isocyanurate rings). The average molecular weight is higher than the theoretical value, and the average functionality is usually between 3 and 4. The NCO mass fraction is generally around 22%.

Characteristics and Uses

Other types of diisocyanate trimers are solids, but HDI trimer is a liquid, allowing for the preparation of solvent-free or high-solids coatings without the need for solvents.

HDI trimer has superior performance compared to HDI biuret, as evidenced by: the lower viscosity of HDI trimer polyisocyanate compared to biuret, which allows for less solvent use and the preparation of high-solids products, reducing air pollution and benefiting environmental protection; the isocyanurate ring of HDI trimer is very stable and does not easily deteriorate, with little change in viscosity after storage; the light resistance of HDI trimer products is higher than that of biuret; the service life of HDI trimer is longer than that of biuret; and the hardness of HDI trimer products is high, while the toughness and adhesion are similar to those of biuret.

The reactivity of HDI trimer is lower than that of aromatic polyisocyanates but higher than that of cycloaliphatic polyisocyanates. If necessary, zinc octoate, organotin, or organobismuth catalysts can be added.

HDI trimer is mainly used as a crosslinking agent for non-yellowing two-component polyurethane coatings and adhesives. The coating resin can be a hydroxyl-containing polyester, acrylate, or medium-to-short oil alkyd resin. It can be used in combination with IPDI trimer to achieve good drying properties, surface hardness, pot life, and resistance to environmental chemical etching. Typical applications include automotive paints, refinish paints, wood coatings, industrial coatings, and plastic coatings.

When using, it can be diluted with urethane-grade solvents to a solid content of 40%.