Kinetics of UV-curing of waterborne polyurethane acrylate dendrimer

Abstract

In this study, a dendrimer with 19 sites was modified to form a waterborne oligomer with hydrophilic sites and polyurethane acrylate (PUA) sites. Two PU systems hexamethylene diisocyanate (HDI) and methylene-bis (4-cyclohexylisocyanate) (HMDI) were used; each of them had 5, 9, and 13 PUA sites, respectively. Experimental results revealed that the HDI system was more thermally stable than the HMDI system. The HMDI system, however, exhibited better hardness than the HDI system, even though the final curing conversion of HMDI was low. Additionally, more PUA sites of a dendrimer were associated with greater hardness of the cured resin. In a UV-curing study, simulations revealed that the autocatalysis model could describe the UV-curing mechanism of the PUA-dendrimer system. Both the reaction rate constant k and the final conversion α were highest at a concentration of the photoinitiator of approximately 3 wt% for both HDI and HMDI systems. The reaction rates of both systems were highest when the number of PUA sites was nine. The final conversion of both the systems, however, was optimal when the number of PUA sites was 13. The final conversion in the acrylic reaction increased with temperature up to 80 °C for both the systems, owing to an increase in the ratio of potentially active sites against nonactive sites, according to Arrhenius theory. The rate constant, k, however, was optimal at 60 °C because a higher temperature damaged the photoinitiator. Finally and most importantly, HMDI always had a lower rate constant k and final conversion α than HDI under similar conditions in studies that were conducted to examine the effects of photoinitiator concentration, number of PUA sites, and reaction temperature, revealing that the steric hindrance by the cyclohexane in HMDI negatively influenced its curing kinetics.