Influence of ester diluents and chain extension on polyurethane viscosities

Polyurethanes are often used as adhesives in laminated packaging, where viscosity is critical. Poly diethylene glycol adipate macrodiol was chain-extended with hexamethylene diisocyanate at 1:0.3 mol [1] to produce a polyurethane prepolymer [2].  Then it was diluted with ethyl acetate 1:1 (w/w) to produce the first adhesive component.  As the second adhesive component, aromatic tri-isocyanate CAS 53317-61-6 was used at 1.4 mol excess to assure eventual gelation.

Right after mixing the blend viscosity η was periodically measured per ASTM D445. The narrow capillary of the Cannon-Fenske viscometer minimized the vaporization of ethyl acetate. Polymerization-induced thickening was gradual, Fig. 1, with stronger non-linearity observed at 50°C than 25°C.  Catalytic effects of the ester linkage in ethyl acetate could be responsible for the acceleration of the carbamate formation, since chemical reactivity generally intensifies at higher temperatures. Eventually, the adhesive formulation was fortified with nanoparticles and successfully used to laminate multilayer plastic films and Al foils.