Construction of high-bio-content photocurable soybean-based urethane methacrylates and application for wood coatings

The aim of this research is to create photocurable urethane methacrylates based on soybeans as a more bio-rich and environmentally friendly substitute for fossil-based, non-renewable raw materials in conventional wood treatments. Considering this aim, soybean-based urethane methacrylate oligomers were synthesized through a three-step-reaction process; at first, epoxidized soybean oil (ESBO) was reacted with a bio-acid, itaconic acid (IA), to obtain IA modified ESBO oligomer (I-ESBO). In the second stage, methylene diphenyl diisocyanate (MDI) and 2-hydroxyethyl methacrylate was reacted to form MDI-HEMA adduct. In the third step, I-ESBO oligomer and MDI-HEMA adduct were reacted in order to obtain a high bio-content, photocurable oligomer, denoted as UA-I-ESBO, having urethane methacrylate (UA) groups on its backbone. All the synthesized chemical species were characterized by Fourier transform infrared (FTIR) and proton nuclear magnetic resonance ( 1 H NMR) spectroscopies. Wood coating formulations were prepared by using the synthesized UA-I-ESBO oligomer and various types of reactive diluents. The UV-cured free films of coating formulations were characterized by means of physiochemical, mechanical, and thermal properties. Coating performance of the formulations was evaluated on coated UV-cured wood-based panels by using Taber abrasion, cross-cut adhesion, and water immersion tests. Overall results indicate that the number of functional groups in the coating formulation directly influences crosslinking density and, consequently, the mechanical performance of the coating layer. Moreover, the proposed high bio-content photocurable formulation, with a bio-mass content of 41% on average, can be considered an environmentally friendly alternative to conventional wood coatings due to its potential to replace fossil-derived raw materials and its solvent-, water-, and heat-free photocuring process.