MDI flakes are melted and
feeded to a vessel (or melted in the vessel) and heated up to 50 – 60 OC. fresh MDI
gives a clear, colourless liquid, a slight turbidity because of beginning dimerization of
MDI may be accepted. Voranol at room temperature is added stirring during
approximately 30 minutes, when 5kg of prepolymer is produced. The reaction is
exothermic, but the reactivity of Voranol is very low, therefore crystallization of the
MDI may occur in case the content of the vessel cools down under 40 OC. the mixture
is heated up to maximum 80 OC and held at this temperature for 3 hours. After 3
hours, the NCO-content should be stable. The result is a clear, colourless or slightly
yellow, viscous prepolymer. At the end of the experiments, quality control tests were
performed to determine the viscosity and NCO-content of the product.