Melamine-Based Polyols for Rigid Polyurethanes

Melamine is a very thermoresistant aromatic heterocyclic compound, with three -NH2 groups, which makes it very attractive for use as a starter for polyol synthesis. Unfortunately, melamine is very difficult to directly alkoxylate with PO or EO. This difficulty is because of the amidic structure (melamine is the amide of cyanuric acid) and because of the tautomeric forms (characteristic to all amides) [31]  

Melamine is soluble only in water and has low solubility in dimethylsulfoxide (DMSO) and in other aprotic dipolar solvents (9% at 120 °C), in glycerol or ethylene glycol (10% at 140 °C). In the majority of other usual solvents it is insoluble. Kucharski and Lubczak discovered a new class of reactive solvents for melamine [36] poly (hydroxymethyl) derivatives of cyclohexanone, acetone, nitromethane which are able to dissolve 50-60% melamine. Melamine can be totally propoxylated or ethoxylated at lower temperatures (70-90 °C), in aprotic dipolar solvents (for example DMSO, dimethylformamide, N-methyl pyrrolidone and so on), in the presence of quaternary ammonium hydroxides as catalysts [for example tetrabutyl ammonium hydroxide (TBAH)], at a low reaction rate (reaction 15.35), for a very long reaction time (40-50 hours) [31, 37]. The resulting hexafunctional polyols give very thermostable rigid PU (up 200 °C). 

The very long reaction time, the necessity of recycling an expensive solvent, and the high cost of the catalyst, mean that this synthesis method is not applied in practice. 

A useful synthetic variant to melamine-based polyols is to alkoxylate the condensates of melamine with carbonyl compounds. 

by the reaction of melamine with formaldehyde and diethanolamine, a melamine-based Mannich base (reaction 15.36), is obtained quantitatively in 1-2 hours at 60-70 °C. The resulting Mannich base is a hexafunctional starter [10, 21, 31]  

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