Siloxane-modified waterborne UV-curable polyurethane acrylate coatings: Chemorheology and viscoelastic analyses

Abstract

Abstract The effects of molecular weight of PDMS on chemorheology and viscoelastic behavior of siloxane modified waterborne UV-curable polyurethane acrylate dispersions (Si-WUPAs) were assessed via a photo rheometer as a versatile tool for in situ observation of chemical and mechanical characteristics during photopolymerization reaction. The roles of PDMS soft segment and its Mw on the rheological behavior were examined followed by exploring the flow curves of Si-WUPA system. The viscosity of Si-WUPA samples decreased with increasing shear rate, showing a pseudoplastic behavior for high-molecular-weight PDMS. However, the unmodified sample and specimens containing dual soft segments of PTMG and low Mw of PDMS revealed a Newtonian behavior. The curing process was accomplished using a photoinitiator to induce polymerization of acrylate double bonds at 40 °C and 60 °C, signifying a-three-step curing process. Enhanced drying time and gelation points were observed upon increasing Mw and content of PDMS. Correspondingly, the storage modulus diminished due to incomplete curing because of contribution of more flexible PDMS chains to reaction. Thermomechanical properties of UV-cured Si-WUPAs films were systematically studied via DMTA, revealing two different behaviors depending on the Mw of PDMS. Si-WPUAs systems showed enlarged width of tanδ peak in half of heights unconditionally, suggesting increased incompatibility and non-uniformity of the final film caused by micro-phase separation of PDMS chains. Mechanical properties were also inspected by tensile test confirming the DMTA results. Tensile isotherms of cured Si-WPUA specimens were remarkably sensitive to both Mw and the content of PDMS.