Synthesis of the UV-curable waterborne polyurethane and its application in advanced water-based varnish

Purpose This paper aims to prepare a kind of the UV-curable waterborne polyurethane (UVWPU) and its application in volatility organic compound (VOC)-free advanced water-based varnish. Design/methodology/approach In this paper, a series of UVWPU systems with emulsion stability and excellent film properties were prepared by internal emulsification was obtained from polycarbonate glycol, isophorone diisocyanate and dimethylolpropionic acid as raw materials; acryloyloxy group is introduced through hydroxyethyl methacrylate (HEMA) or pentaerythritol triacrylate (PETA) to obtain photosensitive performance, then neutralized with triethylamine and dispersed with water. A photoinitiator was added to UVWPU, and the coating film was cured by ultraviolet light and the properties were tested. Findings The results show that when the mass ratio of –NCO and –OH was 2.0, the content of dimethylolpropionic acid is 5.5% and the molar ratio of HEMA to PETA is 3.5, solid content of the emulsion is 40% and UVWPU had excellent dispersion performance in water and storage stability. When the photoinitiator is added to the UVWPU emulsion with a mass fraction of 5%, the light curing duration time is 60 s, water absorption of cured film was 8.8%, the adhesion was 0 level, pencil hardness reached 4 H and impact strength is 50 kg·cm, the cured coatings exhibited not only greater hardness, excellent adhesion and excellent mechanical properties but also good thermal stability. The authors successfully obtained a high-performance UVWPU and can meet the requirements of advanced water-based varnish. Research limitations/implications Because of the limitation of experimental conditions, some performances have not been tested. Therefore, researchers are encouraged to conduct further tests. Practical implications The paper established a method to synthesize UVWPU with green surfactants can be used in the coatings, adhesives, finishing agents and so on. Originality/value As an excellent surface treatment material, UVWPU is widely used in wood, construction industry, automobile coating and other fields. The paper provides a method of preparing the UVWPU. A photoinitiator was added to UVWPU, and the coating film was cured by ultraviolet light and the film is uniform and endowed with remarkable heat resistance, high gloss, good fullness and to meet the requirements of VOC-free advanced water-based varnish.