Coating And Weathering Performance Of Wood Modified With N-hydroxymethyl Resin

Modification with N-hydroxymethyl resin, DMDHEU, can effectively improve the physical and mechanical properties of wood. Compared to the untreated wood, water absorption of the modified wood was decreased, while both the dimensional stability and glue strength were improved. In addition, DMDHEU modification showed significant effects on the coating and aging performance of wood, thereby influencing the practical application of the resulting wood products.This study focused on the effects of treatment of poplar wood with DMDHEU on the coating and aging properties. Coating parameters included the surface contact angle, film thickness, drying rate, adhesion strength and abrasive resistance, while aging properties were characterized by artificial QUV test. The main results were given below:1) Wood treated with DMDHEU caused an increase in volume and weight, indicating that DMDHEU is incorporated into the wood microstructure due to its small molecular weight and good permeability of wood.2) The water exhibited a contact angle of80°on the wood surface, which is comparable to acrylic. Acrylic transparent topcoat had a greater contact angle than water and water-based primer. Compared with the transparent acrylic paint and water-based primer, waterborne acrylic white and brown coatings displayed a lower initial contact angle. Solvent-based polyurethane paint was more viscous than water-based acrylic ones. DMDHEU treatment slightly improved the wettability of wood surface for the waterborne acrylic paint and the contact angle was reduced up to30%. Modification did not influence the contact angle of solvent-based polyurethane paints.3) Modification caused a negative effect on the surface dry rate of transparent water-borne acrylic coatings but without any substantial effects on the other three coatings. The transparent water-borne acrylic coatings exhibited the longest full dry time on both the untreated and treated wood, while the other three coatings had similar full dry time. Compared with the untreated wood, treatments with DMDHEU hardly influenced the full drying time of coatings. Solvent-based polyurethane paint exhibited a greater adhesion with the surfaces of both the untreated and treated wood. In general, wood modified with DMDHEU did not substantially influence the drying rate of coatings used.4) The blocking effects between paint films was minor reduced due to wood modification and mainly depended on the types of paint used. 5) Wood coated with water-based acrylic coatings was less abrasion proof than wood coated with solvent-based polyurethane paints. Modification with DMDHEU enhanced the abrasion of wood substrate and also the resulting coated wood.6) DMDHEU treatment facilitated to improve the resistance against the photo-degradation of lignin and hemicelluloses during artificial aging process.7) Wood coated with stain coatings was easier to change its surface color due to penetration of UV light and breakdown of lignin. For the polyurethane paints, the surface erosion was considerably reduced because UV light cannot penetrate the film to damage the wood, thereby maintaining the interfacial adhesion between wood and coating film.In general, modification with N-hydroxymethyl does not cause any negative impact on the coating performance of wood.