The use of UV curing epoxy-acrylate and urethane-acrylate coatings for improving the weathering resistance of wood

One of the most effective methods to protect wood against weathering is the use of coating. Among different coatings, light-curing ones have been attracted much attention due to less investment, low energy consumption, low cost, high solid content and solvent-free, high speed curing and yield, etc. In this study, the effect of two transparent UV-curing coatings, including epoxy acrylate and urethane acrylate on the weathering resistance of spruce wood was investigated.

200 mm-thick coating layers were applied on the surface of wood using a film applicator followed by UV-curing process of epoxy acrylate and urethane acrylate each for 5 and 40 seconds, respectively. Then, the coated specimens and control one were exposed to 6 months weathering. Finally, the effect of weathering on the specimens were tested using ATR-FTIR, stereo microscope, SEM, calorimetry, roughness, water-droplet contact angle and cross-cut adhesion.

Results showed that the color of uncoated wood specimens changed and their roughness increased due to weathering. Many surface cracks occurred in the uncoated wood after weathering. Many cracks were also developed on the surface of urethane-acrylate coating, while no surface cracks were observed in the epoxy-acrylate coating. The discoloration level caused by weathering was higher in the control sample compared to that of the coated samples. The most resistant coating to the weathering discoloration was epoxy acrylate. The contact angle of weathered samples was affected by the surface cracks and roughness values in addition to changes on the surface chemistry. ATR-FTIR results showed a reduction in the intensity of peaks at wavenumbers of 1740 cm-1 (vibration of C=O) and 1504 and 1595 cm-1 (vibration of C=C in aromatic ring of lignin) which indicates the degradation of hemicelluloses and formation of new carbonyl compounds and lignin degradation in specimens due to the weathering. Reduction in the intensity of peaks at wavenumbers of 1510 cm-1 (vibration of CO-NH) and 1248 cm-1 (vibration of C-O-C) in wood samples coated by urethane acrylate showed the cleavage of urethane chains due to UV radiation. Results also showed that urethane acrylate had lower adhesion strength than epoxy acrylate, and the strength was reduced for both coatings as the result of weathering.

In general, both epoxy-acrylate and urethane acrylate coatings increased the resistance of wood against weathering. However, epoxy acrylate had much better performance compared to urethane acrylate. The adhesion strength and color of the epoxy-acrylate coating with no surface cracks were less affected by the weathering.