Interfacial studies of crosslinked urethanes: Part III. Structure-propertyrelationships in polyester waterborne polyurethanes

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 reacti ons leading to the formation of polyurethane and polyurea. Concentration le vels of unreacted isocyanate (NCO) are greater at the film-air (F-A) interf ace than the film-substrate (F-S) interface. In contrast to the previous st udies on polyacrylate emulsion urethanes, no stratification was detected be tween 0.65 to 1.14 mu m near the F-A and F-s interfaces. This behavior is a ttributed to equivalent weight differences, 3100 g/eq for polyacrylate and 1140 g/eq for polyester. Solvent evaporation experiments show that approxim ately 10% of the initial water concentration remains in the film for extend ed 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 amoun t of shear induced on the shear thinning waterborne urethanes, which subseq uently 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, stor age modulus, crosslink density, and film hardness.