Influence of the free isocyanate content in the adhesive properties of reactive trifunctional polyether urethane quasi-prepolymers

The effect of the free isocyanate (NCO) content on the adhesive properties of urethane quasi-prepolymers was studied. Polyether-based isocyanate ended quasi-prepolymers containing between 13 and 21 wt% NCO were prepared by reacting a trifunctional polypropyleneglycol with polymeric MDI. The quasi-prepolymers were characterized by infrared spectroscopy (IR), proton nuclear magnetic resonance ( 1H-NMR) spectroscopy, Brookfield viscosity, and differential scanning calorimetry (DSC). Immediate adhesion of quasi-prepolymers was studied by means of a probe test performed with a texture analyzer. Adhesion properties of the moisture-cured quasi-prepolymers have been obtained from single lap shear tests of vulcanized styrene–butadiene (R1) rubber/cured quasi-prepolymer joints. The increase in the free isocyanate content produced quasi-prepolymers with lower average chain length and less intermolecular interactions between polymer chains. Due to the presence of free isocyanate molecules, lower Brookfield viscosity values and lower glass transition temperatures were obtained by increasing the free isocyanate content in the quasi-prepolymers. The lower the free NCO content, the higher the cohesion and the immediate adhesion of the quasi-prepolymers because of the increase in the average molecular weight. During debonding, fibrillation was observed in the quasi-prepolymers with free NCO content lower than 20 wt%. Single lap shear strength values of R1 rubber/moisture cured quasi-prepolymers joints showed a particular trend as a function of the increase in the NCO content, which can be ascribed to the opposite trends in the cohesion and adhesion properties of the cured quasi-prepolymers.