Patent ID: 7230130

Claim:
A process for preparing polyisocyanates by phosgenation of methylenedianiline, prepared by reacting aniline with formaldehyde in the presence of acid catalysts, which comprises, in a semicontinuous process, introducing aniline with or without acid catalysts, feeding formaldehyde with or without acid catalyst through a mixing element into a circuit in which aniline with or without acid catalyst and with or without previously added formaldehyde is circulated and, after feeding in at least 50% of the total amount of formaldehyde to be fed in, heating the reaction mixture to a temperature above 75° C., wherein the phosgenation is carried out at temperatures of from 70 to 120° C. and at a pressure of from 0.8 to 5 bar via a two-step reaction in the presence of at least one inert organic solvent, the first phosgenation step being carried out in a static mixer and the second phosgenation step being carried out in a dwell-time apparatus wherein in the dwell-time apparatus the mass ratios of phosgene to hydrogen chloride being at the same time 10-30:1 in the liquid phase and 1-10:1 in the gas phase.