Synthesis of a novel waterborne diethanolamine-modified lignin/polyurethane emulsion and its application as an adhesive for polypropylene decorative film on wood-based panels

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 DEA-modified 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.