Interfacial Studies of Crosslinked Urethanes: Part III. Structure-Property Relationships in Polyester Waterborne Polyurethanes

By M.W. Urban, C.L. Allison, C.C. Finch, B.A. Tatro

These studies examine crosslinking reactions of polyurethanes (PURs) using attenuated total reflectance Fourier-transform (ATR FTIR) spectroscopy and show that higher relative humidity (RH) accelerates the crosslinking reactions leading to the formation of polyurethane and polyurea. Concentration levels of unreacted isocyanate (NCO) are greater at the film-air (F-A) interface than the film-substrate (F-S) interface.

In contrast to the previous studies on polyacrylate emulsion urethanes, no stratification was detected between 0.65 to 1.14 µm near the F-A and F-S interfaces. This behavior is attributed to equivalent weight differences, 3100 g/eq for polyacrylate and 1140 g/eq for polyester.

Solvent evaporation experiments show that approximately 10% of the initial water concentration remains in the film for extended periods of time, resulting in reactions leading to the formation of urea near the F-S interface. PUR film formation occurs in two stages, a solvent vapor pressure controlled stage, followed by a diffusion controlled stage.

The duration of each stage depends on several factors, including the amount of shear induced on the shear thinning waterborne urethanes, which subsequently affects the exposure of isocyanate aggregates to water. Increased RH significantly affects structure-property relationships of water-borne PURs due to urea formation, which alters the glass transition temperature, storage modulus, crosslink density, and film hardness.