Process for the continuous thermal decomposition of esters of carbamic acids and use of mixtures of esters of carbamic acids and isocyanates obtained by this process for the preparation of isocyanates

摘要

A carbamic acid ester corresponding to the formula R1-NH-CO-OR2 is thermally cleaved to form isocyanate R1-NCO and alcohol R2-OH fractions. This cleavage is accomplished by boiling the carbamic acid ester, condensing the vapor given off in a first fractionation column, and condensing the vapor from the first fractionation column in a second fractionation column. The boiling of the carbamic acid ester is carried out in a manner such that the average dwell time in the reaction vessel is from 1 to 20 hours, the temperature is from 160 DEG to 260 DEG C. and the pressure is from 0.001 to 2 bar. The isocyanate fraction obtained by this cleavage process may be used as a starting material for a transurethanation reaction in which a lower boiling isocyanate R3-NCO is produced. The radicals R1, R2, and R3 are defined herein.