Origin of Impurities Formed in a Polyurethane Production Chain. Part 2: A Route to the Formation of Colored Impurities

The quality of methylene diphenyl diisocyanate (MDI) products, which are valuable feedstocks in the industrial manufacture of polyurethanes, can be compromised by the presence of color, presumed to arise from trace impurities. One undesired branch in the synthesis chain originates with phosgenation of diaryl ureas, formed from reactions between aryl isocyanates and polyamine precursors. Subsequent key steps include, (i) breakdown of the primary compounds, substituted chloroformamidine-<i>N</i>-carbonyl chlorides (CCC), to give aryl isocyanide dichlorides, ArNCCl₂, (ii) an apparent equilibrium connecting CCC with aryl carbodiimides, and (iii) the thermolysis of ArNCCl₂ in the presence of MDI. Color formation is associated directly with the last process; it involves several events, including HCl elimination from reaction of ArNCCl₂ and MDI, formation of carbon-centered radicals, and a contribution from oxidation at the methylene bridge