Wood Bio-Adhesives Made by Polymerizing Oxidized Starch with Deep Eutectic Solvent-Modified Lignin

In the present work, a new bio-sourced adhesive system based on deep eutectic solvent-modified lignin and oxidized starch (OSTL) resin is presented. For this purpose, unmodified and choline chloride–Zinc chloride (ChCl–ZnCl2) deep eutectic solvent modified lignin at different contents (10%, 20%, and 30%) were used to prepare the OSTL resin. Ammonium persulfate (APS) was the oxidizer employed for the oxidation of starch, and urea was used as a low cost and effective crosslinker agent in the OSTL resin. FTIR analysis indicated that the content of carboxyl and carbonyl groups changed after the curing of the OSTL resin compared to oxidized starch (OST). DSC analysis indicated that the curing temperature of the OSTL resin containing DES-modified lignin was lower than that for unmodified lignin. Also, greater dimensional stability and mechanical strength could be achieved by increasing the amount of DES-treated lignin in the OSTL wood adhesive from 10 to 30 wt%. Based on the findings of this research, the physical and mechanical properties of the particleboard panels bonded with this type of bio-adhesive were acceptable according to the relevant standards. Additionally, urea can thus be used as a good cross-linker, not only to crosslink just OST, but also to connect DES-modified lignin and oxidized starch molecules. Under the conditions used, particleboards bonded with an oxidized starch–urea–pristine lignin adhesive presented decreasing internal bond (IB) strength with an increasing proportion of lignin. Conversely, when the same adhesive using DES-modified lignin was used, the internal bond (IB) strength improved with the increasing proportion of DES-modified lignin. At 30% proportions of lignin, the oxidized starch–urea–DES-modified lignin presented a 27% improvement in strength. Finally, it can be noted that this work brings a new insight to the development and application of lignin-based bio-adhesives to bond wood-based panels.