To improve the surface bonding strength of polypropylene (PP) decorative films on wood-based panels, one kind of lignin, namely sodium lignosulfonate (SL) filled waterborne polyurethane (WPU) composites were used as adhesives on the back of the PP decorative films, and the SL was modified by the addition of two alternative types of silane that contain either ethylene-acyloxy or epoxy groups. The PP decorative films were hot-press onto plywood using the different types of SL modified WPU as an adhesive. The SL and SL modified WPU were characterized comprehensively. The surface properties the PP decorative films on plywood surfaces were tested. The results showed that both types of silane can be grafted on the SL between hydrolyzed Si OH groups and OH groups. The long organic chain of silane can physically twist to the WPU, where self-polymerization of the silane helps to form network structures. As a result, the surface bonding strength of PP decorative films on plywood surfaces largely improved with SL filled WPU adhesives and no detachment occurred in boiling water. Compared with the ethylene-acyloxy silane, the epoxy silane performed better which was associated with the reactions between the epoxy groups and free water.
Incorporation of short wood fillers such as wood flour (WF) into polypropylene (PP) often results in a marked reduction of toughness, which is one of the main shortcomings for WF/PP composites. This research reports a facile approach to achieve toughening of WF/PP composites via introducing self-assembling β-nucleating agents into PP matrix. The effect of two kinds of nucleating agents, an aryl amide derivative (TMB5) and a rare earth complex (WBG II), at varying concentrations on the crystallization and mechanical properties of WF/PP composites was comparatively investigated. The results showed that both nucleating agents were highly effective in inducing β-crystal for WF/PP, with β-crystal content (kβ) value reaching 0.8 at 0.05 wt% nucleating agent concentration. The incorporation of TMB or WBG significantly decreased the spherulite size, increased the crystallization temperature and accelerated the crystallization process of WF/PP. As a result of PP crystalline modification, the toughness of composites was significantly improved. Through introducing 0.3 wt% TMB or WBG, the notched impact strength and strain at break of WF/PP increased by approximately 28% and 40%, respectively. Comparatively, although WF/PP-WBG had slightly higher Kβ value than WF/PP-TMB at the same concentration, WF/PP/TMB exhibited more uniform crystalline morphology with smaller spherulites. Furthermore, the tensile strength and modulus of WF/PP-TMB were higher than WF/PP-WBG. This matrix crystalline modification strategy provides a promising route to prepare wood filler/thermoplastic composites with improved toughness and accelerated crystallization.
To improve the surface bonding strength of polypropylene (PP), decorative films on wood panels, sodium lignosulfonate (SL) was modified with diethanolamine (DEA) to graft two long hydrocarbon chains with end hydroxyl groups on the molecule. The product reacted with waterborne polyurethane (WPU) monomers to form reinforced emulsions. The novel emulsion was used as adhesives on the back of PP decorative films and compared to the unmodified SL. Different contents (5 wt.% and 10 wt.%) of modified SL content were considered in the dry WPU. The PP decorative films were hot-pressed onto plywood using different kinds of WPU. Results showed that two long hydroxyl groups were grafted on the SL molecule after DEA modification, which reacted with the WPU monomers to form reinforced WPU emulsions. The DEAmodified SL showed better dispersibility compared to pure SL, avoiding aggregation. Owing to their heat resistance and high stiffness, the DEA-modified SL/WPU emulsions were superior in thermal properties and surface bonding strength.
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