Synthesis, rheological, and thermal properties of waterborne hyperbranched polyurethane acrylate dispersions for UV curable coatings

Abstract

A series of waterborne hyperbranched polyurethane acrylate (WHUAs) ionomers used for ultraviolet curable waterborne coatings were synthesized. The average particle size of aqueous dispersion ranged between 48.2 and 75.3 nm at 0.05% concentration determined by laser light scattering. The effects of end group of WHUAs on rheological properties were investigated. WHUAs have much lower viscosity than EB2002, commercial linear waterborne polyurethane acrylate. Moreover, the glass transition temperature (T_g) evaluated by differential scanning calorimetry of samples showed that the influence of end capping by hard segment consisting of toluene diisocyanate–hydroxyethyl acrylate is significant due to the increase of crosslink density. All cured WHUA have higher glass transition temperatures than those of cured EB2002. The results of thermogravimetric analysis for cured WHUA films indicated good thermal stability with no appreciable weight loss until 200°C, and that an increase in the hard segment content provoked the increases in thermal degradation temperature. The activation energies were calculated by Flynn–Wall method to be 91.3, 114.3, and 139.7 kJ mol⁻¹ for cured WHUA62, WHUA44, and WHUA26, with the individual ratios of 6:2, 4:4, and 2:6 for salt-like group to double bond at the terminals, compared with 81.1 kJ mol⁻¹ of EB2002 in N₂ atmosphere, respectively.