Natural castor oil based 2-package waterborne polyurethane wood coatings

Abstract

The effects of four kinds of hardener on the properties of castor oil (CO) based 2-package waterborne polyurethane (2K-WPU) wood coatings were examined. Modified castor oil (MCO) was prepared by transesterification of glycerol and CO at the molar ratio of 2.0. The waterborne polyurethane-dispersed polyol (PUDp), one component of the 2K-WPU, was synthesized from MCO, dimethylol propionic acid (DMPA) and isophorone diisocyanate (IPDI) by the acetone process to provide a prepolymer with a carboxyl and hydroxyl groups. Then the prepolymer was neutralized by triethylamine (TEA) and dispersed into water. After vacuum distillation to remove acetone, the PUDp was obtained and then mixed with four different hardeners: IPDI, hexamethylene diisocyanate (HDI), polyethylene glycol (PEG) modified PIPDI (polymeric IPDI) and PEG-modified PHDI (polymeric HDI). The NCO/OH molar ratio of 1.5 was used and a 2K-WPU coating was obtained. The results showed that the film of the 2K-WPU coatings obtained from IPDI hardener had excellent gloss and hardness. On the contrary, the film containing PEG-modified PIPDI hardener (PEG-PIPDI) had lower hardness and gloss but higher tensile strength. The film containing PEG-modified PHDI hardener (PEG-PHDI) showed the best elongation at break, abrasion resistance and impact resistance, though it had the worst hardness. The film with HDI hardener had the best hardness and highest tensile strength and superior water resistance among all the films with different hardeners, and it was suitable for wood coatings.