PROCESS FOR THE PREPARATION OF POLYIROCYANATES OF THE DIPHENYLMETHANE SERIES

A process is provided for preparing mixtures comprising diphenyl-methane diisocyanates and polyphenylpolymethylene polyisocyanates having a higher HunterLab colour (L) number by reaction of the corresponding mixtures comprising diphenylmethanediamines and poly-phenylpoly-methylene-poly-amines with phosgene in the presence of at least one solvent, where in the staged work-up subsequent to the phosgenation reaction, the level of chlorine-containing impurities is reduced by the so-called heat treatment/dechlorination process to a specifically defined level before the reaction mixture is subjected to temperatures above a temperature specifically defined. Provided that this level of impurities is not exceeded, very little or no further colour deterioration occurs when the product stream is heated to higher temperatures in subsequent processing steps.