The investigation of a side reaction leading to colour formation in a polyurethane production chain

In the industrial synthesis of 4,4’-methylene diphenyl diisocyanate (MDI), an unwanted side reaction between the product and the starting material, 4,4’-methylene dianiline, can lead to the formation of ureas. It has been postulated these ureas undergo further reaction with phosgene to produce a precursor to chlorine radicals, which could then attack the MDI backbone forming conjugated systems that would promote colour in the final products. To investigate this process model compounds including 4-benzylaniline (4-BA) and 1,3-diphenylurea were used as starting materials. The reactions carried out showed the phosgenation of the urea forms a chloroformamidine-N-carbonyl chloride (CCC) which upon heating > 303 K can break down to form an isocyanide dichloride (ID). Conventional synthesis routes were used to gain high yields of p-tolyl and phenyl isocyanide dichlorides in order to analyse the compounds. It was found that upon heating to 453 K or irradiating the isocyanide dichlorides in the process solvent (chlorobenzene) coloured solutions were formed; with the presence of MDI and oxygen increasing the intensity of the colouration. Electron paramagnetic resonance spectroscopy was used to gain information on the use of isocyanide dichlorides as a source of chlorine radicals. Using N-tert-butyl--phenylnitrone (PBN) as a spin trap, an 8 line spectra relating to the chlorine adduct was measured confirming the production of Cl•. Throughout the project side reactions involving the formation of carbodiimide from CCC and a secondary route for the phosgenation of the urea to the isocyanate have been investigated and are presented within a global reaction scheme. It was also found the ureas were only partially soluble in the process solvent leading to research into the structure of three different urea molecules and the proposal of a modified reaction scheme