Polyisocyanates / Isocyanate Prepolymers

Properties and Applications

In many industrial applications monomeric isocyanates are unsuitable due to health concerns. For this reason, higher molecular weight polyisocyanate resins such as isocyanate functional prepolymers, isocyanate-alcohol adducts and isocyanurate trimers are preferred because these compounds have a much lower or negligible vapor pressure and a very low level of monomeric isocyanates.

Both aromatic and aliphatic polyisocynantes are commercially available. The former are considerably more reactive and are typically much cheaper and used on a much larger scale. However, urethanes derived from aromatic isocyanates have a tendency to yellow when exposed to UV light and show a relatively rapid loss of gloss on weathering whereas aliphatic isocyantes have excellent UV resistance and better gloss retention.

Polyisocyantes are typically cured by coreactants containing reactive hydrogen such as amines and alcohols. Hydroxyl-based oligomers (polyols) are mainly used including hydoxylated polyethers and polyesters and hydroxylfunctional polyacrylates. Other possible isocyanate coreactants include carboxylic acids, ureas, urethanes and epoxies.

Polyisocyanates can be used to make coatings, adhesives and sealants that either cure at ambient temperature or at moderately elevated temperatures. They are typically used as two-component formulations since polyisocyantes readily react with alcohols and amines. However, one component formulations are also possible when the isocyanate groups are blocked by compounds such as caprolactam, oximes, malonates or dimethylpyrazole. At room temperature, blocked polyisocyanates do not react with polyols whereas at elevated temperatures the blocked polyisocyanate releases the blocking agent and becomes reactive.

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