Process for the preparation of polyisocyanates of the diphenylmethane series

摘要

Process for preparing mixtures comprising diphenylmethane diisocyantes and polyphenylpolymethylene polyisocyanates having a higher HunterLab color (L) number by staged reaction of the corresponding mixtures of diphenylmethanediamines and polyphenylpolymethylenepolyamines with phosgene in the presence of at least one solvent whereby in a first stage the corresponding carbamoyl chlorides and amine hydrochlorides are formed and whereby in a subsequent stage the residual carbamoyl chlorides are dissociated into the corresponding polyisocyanates and hydrogen chloride and amine hydrochlorides are phosgenated to form ultimately the corresponding polyisocyanates in which some amine hydrochlorides remain unreacted at the point where residual excess phosgene is removed from the reaction mixture.