Prediction of the content of NCO-groups in urethane prepolymers based on oligomeric diols with unidentified molecular weight
An effective and easy to use method for estimating the molecular weight of starting oligomeric diols used for the synthesis of polyurethanes is developed. The prepolymers - linear oligomers with terminal isocyanate groups - are first synthesized as an intermediate product. To synthesize such prepolymers used in the free-casting manufacture of polyurethane and polyurethane urea compositions, oligodiol is reacted with an excess of diisocyanate, the ratio of NCO/OH groups being equal to or slightly higher than 2. This is very important for the second stage of synthesis when the prepolymer interacts with hardeners usually containing amine and hydroxyl groups. When the ratio NCO/OH ≫ 2 the resulting prepolymer contains a large amount of low molecular weight diisocyanate with a much higher activity compared to the prepolymer. As a result, the potlife of polyurethane and polyurethane urea compositions can decrease to an unacceptable level. On the other hand, when the ratio NCO/OH < 2, the prepolymers with high molecular weight and relatively high viscosity can occur, which also complicates the processing of the aforementioned compositions. Maintaining the desired value of the ratio in the synthesis of prepolymers necessitates precise data on the molecular weight of the oligomeric diols used in the synthesis. Current methods for determining the molecular weight of oligomeric diols, based on the analysis of the content of hydroxyl groups have a number of disadvantages, namely the duration and complexity of the procedure and the high cost of the reagents. The developed method is based on conducting advanced syntheses of prepolymers with predetermined ratios between the initial oligomeric diols and diisocyanates. The equations linking the expected content of isocyanate groups in the prepolymer with the weight amounts of the starting components are derived. Testing of the method was carried out using ligoester and oligobutadiene isoprendiol as initial oligomeric diols.