Synthesis and Characterization of Castor Oil-Based UV-Curable Water-Reducible Polyurethane Dispersionsfor Wood Coatings

摘要

A water-reducible UV-cured (WUV) coating for wood finishing was investigated in this study. First; modified castor oil (CO; MCO) was obtained by transesterification of glycerin and CO, and urethane acrylate oligomers were further synthesized from MCO,2,2-dimethylpropionic acid, hydroxyethyl methacrylate (HEMA), and hexylene diisocyanate (HDI). The urethane acrylate oligomer had a number average molecular weight (Mn) of 1604 g mol"' and a weight average molecular weight (Mw) of 8086 g mol"1. Finally,the oligomer was neutralized with triethylamine and dispersed in water. An acrylated polyurethane dispersion (APUD) with a slightly opalescent appearance was obtained by removing the acetone. The solid content, pH value, Gardner color value and viscosity of the APUD were 33.7%, 8.34, 1.7, and 3470 cps, respectively. WUV coatings were formulated by mixing the APUD with 1, 3, or 5 wt% of a photoinitiator (Irgacure 2959). The film performances of WUV coatings indicated that the coating containing 3 wt% photoinitiator showed balanced film properties and superior adhesion and flexibility, but low hardness. To improve the hardness, the HDI was partially replaced with isophorone diisocyanate (IPDI), and results showed that all films showed excellent lightfastness, but the WUV coating with an HDI/IPDI molar ratio of 0/1 had the highest gloss transition temperature and onset temperature in the early pyrolysis stage. However, considering the balance of various film properties, the WUV coating with an HDI/IPDImolar ratio of 1/1 had superior film hardness, gloss, and flexibility, and should be applied for top coating of wood finishing.