Effects of molecular weight of urea–formaldehyde resins on wettability and adhesion at wood surface, interphase, and plywood

Various factors, such as molecular weight (MW), reactivity, wettability, surface roughness, surface adhesion, interphase adhesion, and cohesion of adhesives, affect the adhesion strength of urea–formaldehyde (UF) resins when bonding wood. This study was conducted to understand the MW effects of UF resins on wettability and adhesion at wood surface, interphase, and plywood. UF resins with two formaldehyde/urea (F/U) molar ratios (1.0 and 1.6) and three MW levels such as low-MW (L-MW), middle-MW (M-MW), and high-MW (H-MW) were evaluated for their wettability and adhesion. As the MW increases, the surface tension of 1.0 UF resins slightly increased, while that of 1.6 UF resins slightly decreased. The contact angle of UF resins on earlywood and latewood either from sapwood or heartwood (Radiata pine: Pinus radiata D. Don) before and after the extractive removal was dependent on the wood surface roughness rather than on the MWs of resins, showing a weak negative linear relationship with the surface roughness. The maximum surface adhesion force of 1.0 UF resins was greater at latewood than earlywood from sapwood before the extraction, but it decreased after the extractive removal. 1.0 UF resins showed the highest interphase adhesion at H-MW level, whereas 1.6 UF resins gave the strongest interphase adhesion at M-MW level although 1.6 UF resins had greater interphase adhesion than those of 1.0 UF resins for sapwood and heartwood. However, tensile shear strength of plywood did not proportionately relate to the wettability, surface adhesion, and interphase adhesion of UF resins. These results suggest that the MW, wettability, and surface adhesion of UF resins are indirectly related to interphase adhesion and adhesion strength in wood bonding.