Mw. Urban et al., Interfacial studies of crosslinked urethanes: Part III. Structure-propertyrelationships in polyester waterborne polyurethanes, J COAT TECH, 71(888), 1999, pp. 75-85
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.