The effect of free isocyanate content on properties of one component urethane adhesive

One-component polyurethane adhesives were prepared from difunctional linear polyether polyol and two types of isocyanates in order to achieve excellent adhesion to various substrates. The structure of the prepolymer was varied by using different stoichiometric excesses of isocyanate in the range between 5 and 24 wt%. The simulation of synthesis between polyol and two isocyanates with 15 wt% of the NCO groups was performed on a rheometer in order to determine the optimal reaction time. The average molecular weight, determined by GPC, and Brookfield viscosity of the prepolymers decreased with increasing amounts of free isocyanate (NCO) concentration. The structure of the prepolymers as well as the type and amount of side reactions as a function of the free NCO was determined by NMR analysis. The cross-linking, arising from allophanate linkages, was more pronounced at higher amounts of free NCO. Thermal properties of synthesized urethane prepolymers were investigated by DSC and TGA techniques. The glass transition temperatures (Tg) of the prepolymers depend on the type of isocyanate used for the synthesis. Adhesion strength was also tested. Maximum tensile strength was observed at 15 wt% of NCO for both series of prepolymers.