Heterogeneous crosslinking of waterborne two-component polyurethanes (WB 2K-PUR); stratification processes and the role of water

Stoichiometric imbalance and crosslinking conditions during film formation of waterborne two-component polyurethanes (WB 2K-PUR) play a significant role in the development of material properties. Changing isocyanate-to-hydroxyl (NCO:OH) ratios from 1.0 to 2.2 over a range of humidities significantly affects film morphology, and these studies show that while films with higher NCO:OH exhibit increased Tg and surface roughness when crosslinked at high RH, a Tg decrease is observed at elevated RH. Higher RH conditions not only result in increased urea and decreased urethane content, but also facilitate enrichment of poly(ethylene glycol) (PEG) functionality near the film-air (F-A) interface due to stratification of PEG-modified polyisocyanate crosslinkers. Reaction-induced stratification also occurs during film formation resulting in the Tg differences between F-A and F-S interfaces: namely Tg,F-A>Tg,F-S at 32 and 52%RH, whereas Tg,F-A≤Tg,F-S at 75%RH, as determined by interfacial micro thermal analysis (μTA). This behavior is attributed to concentration gradients of water during film formation and their corresponding effects on isocyanate hydrolysis reactions in conjunction with PEG stratification near the F-A interface. Furthermore, excessive stoichiometric NCO:OH imbalance results in the formation of microscopic 'hills' and 'valleys' on the F-A surface having Tg differences of 6 °C. These phenomenological processes are incorporated into a model describing WB 2K-PUR film formation as a function of crosslinking conditions and resulting morphological features.