| 摘要 |
A PROCESS FOR THE PREPARATION OF POLYISOCYANATES The present invention relates to a process for the preparation of a polyisocyanate comprising (a) thermally decomposing in a tube reactor a solution of an N-substantially carbamic acid ester corresponding to said polyisocyanate in a solvent used as decomposition medium, wherein said solvent (i) is capable of dissolving the carbamic acid ester, (ii) is stable at the decomposition temperature and chemically inert towards the carbamic acid esters and the polyisocyanate product, (iii) can be distilled without decomposing under the conditions of decomposition of carbamic acid esters, and (iv) has at least one miscibility gap with an extracting agent used according to the extraction step (c), said solutions being carried along the internal wall of reactor; (b) separating the gaseous materials formed in the tube reactor by fractional condensation into a fraction I comprised mainly of the alcohol by-product and a fraction II comprised mainly of polyisocyanates, isocyanatourethanes, unreacted carbamic acid ester, and the solvent used in step (a); (c) extracting the polyisocyanate from said fraction II with an extracting agent that is at least partly immiscible with the decomposition medium and is a solvent for the polyisocyanate, and optionally distilling the resultant solution of the polyisocyanate in the extracting agent; and (d) recycling the portion of fraction II remaining after the polyisocyanate is extracted. Le A 26 498 |
