Recent applications of gel permeation chromatography in the paint industry: Part 2

The interaction between formaldehyde and urea or thiourea has been reported by Richard and Gourdenne who used G.P.C. columns packed with reticular polystyrene gel of pore sizes 3×104, 3×103, 103, 500, 200, 100, 60 and 60 A and which were eluted with N, N‐dimethylformamide at 50°C. These authors prepared precursors such as monomethylolurea and N, N‐dimethylolurea in aqueous base at 4°C which prevented their autocondensation. These materials were positively identified by proton nuclear magnetic resonance spectroscopy, but they were not resolved from one another nor from urea itself on the G.P.C. columns on account of the fact that the strong solvation by the eluent dimethyl‐formamide was not affected by the substitution of one or two methylol groups into the urea molecule. Using an acid solution at room temperature, it was found that the reaction between formaldehyde and urea or disubstituted urea produced substances such as methylene diurea and methylene di (N, N‐dimethylurea) which, respectively, eluted before and after urea itself. When urea or thiourea was reacted with formaldehyde at a molar ratio of 1:1.8 in a 30% aqueous solution of pH 8 at 95°C, it was shown by nuclear magnetic resonance spectroscopy that the individual urea residues were linked by ether bridges only and that methylenic linkages were totally absent. The degree of cross‐linking of the products was expressed in terms of the ratio of the number of protons included in these bridges to the total number of protons in the various methylene groups as determined by nuclear magnetic resonance spectroscopy. Five samples were taken from such a reaction between urea and formaldehyde. Their G.P.C. curves were obtained and they were shown to display a wider molecular size distribution as the calculated degree of cross‐linking increased. The initial sample was found to be comprised of a mixture of mono‐ and dimethylolurea, but the samples taken later on during the reaction were more highly polymerised and could not be assigned individual molecular structures. A similar conclusion was reached using a mixture of thiourea and formaldehyde in the preparation of a resin.