The most important product group within the family of aliphatic di- and polyisocyanates are the aliphatic polyisocyanates. These derivatives are pre-polymerized diisocyanate monomers of medium molecular weight still bearing unreacted isocyanate-(NCO-) groups. The prepolymerization is performed to overcome the hazardous potential of the diisocyanate monomers, to achieve a low vapour pressure of the isocyanate compound and also to have benefits in the final application, i.e. to reduce the time for complete formation of the highly crosslinked polyurethane network.
Typical reactions to generate polyisocyanates – mainly for the application in coatings - are homooligomerisation reactions like trimerisation (isocyanurate formation), dimerisation (uretdione formation) and biuret formation (reaction of urea of a diisocyanate with excessive diisocyanate). These reactions are quenched at certain conversion rates and the excess of diisocyanate monomer is removed by a distillation process. Therefore, such products typically contain less than 0.5 % of diisocyanate monomer. Due to the physical nature of the products and the requirements in the coatings applications, the polyisocyanates are often supplied as a solution in organic solvents.
The basic structures of the most important polyisocyanates are shown below. Commercial products always contains significant amounts of higher oligomers.
isocyanurate / trimer
The market of those polyisocyanates for coating applications is dominated by the biuret- and trimer-polyisocyanates of HDI. To some extend the trimer of IPDI is used as well. These materials are the standard crosslinkers in light-stable 2-component or 2-pack polyurethane coatings.
Dimers are thermally not stable, splitting back into the original NCO-groups. Starting temperature and rate of the cleaving reaction depends mainly on the structure of the basic diisocyanate.
The adduct formation with oligo- and polymeric di- and triols is another oligomerisation technique. The resulting polyisocyanates are used, besides the coatings area, in the field of industrial adhesives, sealants and elastomers. Depending on the molecular weight of the polyol as well as the ratio of diisocyanate and polyol a content of lower than 2 pbw. of residual aliphatic diisocyanate monomer in the final product can be achieved without applying a distillation step. With distillation step the free monomer content can be reduced to below 0.5 %.