Fremgangsmåde til fremstilling af blandinger af diphenylmethan-diisocyanater og polyphenyl- polymethylen-polyisocyanater med et formindsket iodfarvetal

摘要

The invention relates to a process for the preparation of mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates (crude MDI) having a reduced iodine colour value by reaction of corresponding mixtures of diphenylmethanediamines and polyphenylpolymethylenepolyamines with phosgene in the presence of at least one inert organic solvent at elevated temperature, in which, after phosgenation of the reaction mixture is complete, low-molecular-weight alkanols, polyhydric alcohols, preferably di- to octahydric alcohols having a molecular weight of 60 to 350, or mixtures thereof are incorporated in an effective amount, advantageously in an amount of 0.01 to 5% by weight, relative to the crude MDI weight, the phosgene and the inert organic solvent are then separated off, 0 to 5% by weight, relative to the crude MDI weight, of at least one phenol-based antioxidant and/or aryl phosphite are added to the reaction product, and the reaction mixture is subjected to a heat treatment.