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Abstract
The reaction of bis (4-isocyanatophenyl) methane (MDI) with a diol mixture of α, ω - dihydroxypoly (ethylene adipate) (E, Mn=2000) and ethylene glycol (EG) (molar ratio=1 2) were studied to determine the effects of the NCO/OH molar ratio of polyurethane formation in N, N-dimethylformamide (DMF) on the concentrations of allophanate linkages and viscosity retention of an essentially linear polyurethane solution used industrially for coatings. Even slight increase in NCO/OH molar ratio increased the concentration of allophanate linkages in polyurethanes. Viscosity retention of polyurethane solution following a stability test decreased with increase in the concentrations of allophanate linkages in polyurethanes. Polyurethanes containing less than 5 × 10-3 mmol· g-1 allophanate linkages were shown to exhibit more than 90 % viscosity retention after a stability test at 70°C for 30 h which was required to the polyu rethane solution used for coatings.
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