Abstract: Disclosed herein are liquid polyisocyanate mixtures containing isocyanurate groups and having an NCO content of 15 to 30% by weight obtainable by partial trimerization of the isocyanate groups of polyisocyanate mixtures of the diphenyl methane series containing 80 to 100% by weight diisocyanatodiphenyl methane isomers and 0 to 20% by weight higher than difunctional polyisocyanates of the diphenyl methane series, 40 to 80% by weight of the diisocyanatodiphenyl methane isomers consisting of 4,4'-diisocyanatodiphenyl methane, 20 to 60% by weight of 2,4'-diisocyanatodiphenyl methane and 0 to 8% by weight of 2,2'-diisocyanatodiphenyl methane and the percentages shown adding up to 100.

Abstract: A new process for the production of very pure 4,4'-diisocyanatodiphenylmethane is disclosed, in which diisocyanatodiphenylmethane isomers which are obtained by distillation from the phosgenation products of aniline/formaldehyde condensates are initially freed from 2,2'- and 2,4'-isomers under certain distillation conditions and are then further worked-up by distillation, in that in a first final stage, from 50 to 90%, by weight of the diisocyanate freed from 2,2'- and 2,4'-isomers is isolated as the head product in the form of pure 4,4'-diisocyanatodiphenylmethane and in a second final stage, another quantity of pure 4,4'-diisocyanatodiphenylmethane is separated as the head product from the distillation of the sump of the first final stage.