Synthesis and characterization of a novel tri-functional bio-based methacrylate prepolymer from castor oil and its application in UV-curable coatings

A novel tri-functional castor oil-based methacrylate prepolymer (MCOG) was developed by mercapto castor oil (MCO) and glycidyl methacrylate (GMA) in this study through a two-step method. Primarily, the mercapto castor oil (MCO) was prepared through thiol-ene reaction of castor oil and mercaptoacetic acid, and subsequently an esterification reaction based on above product and glycidyl methacrylate (GMA) was conducted. FT-IR and H-1 NMR spectra collectively revealed that MCOG was successfully synthesized. As a dominant component, MCOG was formulated with other additives including a reactive diluent pentaerythritol tri-acrylate (PETA), a photoinitiator (TPO) and another self-made polyurethane acylate prepolymers (B-215) and cured under UV irradiations. UV-curing kinetics of the MCOG/PETA/B-215 film was detailly investigated by tracing the ATR-IR spectroscopy and gel content and it demonstrated that the cured films had more than 95% C=C double bond conversion. Moreover, dynamic mechanical property, thermal stability and mechanical property of the cured films were respectively determined by dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA) and tensile measurement. DMA testified that with the increasing of B-215 content, storage modulus, crosslinking density of the cured films dropped, whereas the corresponding glass transition temperatures (Tg) increased. TGA showed that the maximum thermal decomposition temperatures were all above 460 degrees C. Tensile tests indicated that the better tensile strength was obtained with the content of 20% of the B-215 in the cured film. Additionally, transparency and flexibility of the cured films were respectively analyzed by digital image and scanning electron microscopy (SEM). Besides, these cured films displayed high pencil hardness (5H-6H) and excellent resistance to acidic and alkali solutions.