Influence of the hard-to-soft segment ratio on the adhesion of water-borne polyurethane adhesive

Three water-borne polyurethane dispersions were synthesised by the pre-polymer mixing process. Different hard segment contents in the polyurethanes were obtained by varying the diisocyanate/macroglycol (NCO/OH) molar ratio. A decrease in the NCO/OH ratio produced an increase in the mean particle size and in the pre-polymer viscosity, as well as a decrease in the molecular weight of the polyurethane. On the other hand, the lower the NCO/OH ratio, the more crystalline the polyurethane and the lower the resistance to flow at high temperature. Lower NCO/OH ratios improved the thermal degradation stability of the polyurethane. Finally, a high initial adhesion to PVC was obtained in all joints produced with the aqueous polyurethane dispersions and the final adhesion increased as the NCO/OH ratio in the water-borne polyurethane adhesive decreased.