Process for the preparation of polyisocyanates containing isocyanurate groups and the use thereof

A process for the preparation of clear and substantially colorless or slightly colored, solvent-free polyisocyanates containing isocyanurate groups, having a viscosity of less than about 10,000 mPas at 25.degree. C. and a monomeric diisocyanate content of less than about 3% by weight, comprising partially trimerizing the isocyanate groups of hexamethylene diisocyanate in the presence of at least one quaternary ammonium hydroxide as catalyst and subsequently separating excess, unreacted diisocyanate and solvent which may be used, said process characterized in that an ammonium hydroxide which is hydroxyalkyl substituted at the nitrogen atom is used as the quaternary ammonium hydroxide.