| 摘要 |
In the process of producing polyisocyanates by (a) condensing an alkyl-N-phenylcarbamate having 1 to 3 carbons in the alkyl moiety with formaldehyde, paraformaldehyde a formaldehyde forming compound such as trioxane in the presence of an acid to produce a condensate containing a mixture of diphenylmethane dicarbamates and polymethylene polyphenyl carbamates with by-product N-benzyl compounds, rearranging said N-benzyl compounds in said condensate with acid catalyst to obtain a pyrolysis feed mixture containing a mixture of diphenylmethane dicarbamates and polymethylene polyphenyl carbamates with by-product amine and amine salts, and (b) thermally decomposing the carbamate moieties in the pyrolysis feed mixture to isocyanate moieties to produce a polyisocyanate mixture of diphenylmethane diisocyanates and polymethylene polyphenyl isocyanates, the improvement comprises increasing the percent isocyanate content of said polyisocyanates by prior to step (b) removing the amine and amine salt by-products by extracting the pyrolysis feed mixture with a solution of an alkanol having 1 to 4 carbon atoms in the alkyl moiety and aqueous hydrochloric acid, and then removing the alcohol-hydrochloric acid extract. |
