| 摘要 |
A process for purifying and reducing the acidity of a polymethylene polyphenylpolyisocyanate mixture wherein the mixture is subjected to a distillation step, the distillation step being conducted by passing a flowing stream of the mixture through a column having a thin-film evaporator mounted therewith, whereby the solvent and impurities are taken overhead, and then the distillation residue of polymethylene polyphenylpolyisocyanate is allowed to cool, and comprising maintaining the distillation residue of the polymethylene polyphenylpolyisocyanate at a temperature of 190 DEG C. to 250 DEG C. for a time period of from 1 to 60 minutes and then recirculating continuously a portion of the distillation residue to the distillation column for admixture with the flowing feed stream of crude polymethylene polyphenylpolyisocyanate. The acid level of the polymethylene polyphenylpolyisocyanate mixture treated by the improved process of the invention is substantially reduced without adversely affecting the isocyanate equivalent weight, viscosity or other physical and chemical characteristics of the product. |
