Application of acetylated nanocrystalline cellulose in modified soy-based adhesive for plywood

Aimed to develop a formaldehyde-free adhesive that can compete with conventional adhesives in wood industry, soy flour (SF), an abundant, environmentally friendly and renewable material, was used. Using sodium hydroxide (NaOH), polyethylene imine (PEI) and acetylated nanocrystalline cellulose (NCC) at two levels (one and three percent by weight of the dry weight of SF / PEI) as modifying agents, different formulations were prepared. Modifying agents help to improvement of soy adhesive properties. Physical properties of synthesized adhesives such as the solid content, pH, gel time and viscosity of adhesives were measured. Beech (Fagus orientaleis) plywood samples were made by each formulation and their water resistance, dry and wet shear strength were measured in accordance with related standards. The results were compared with those of PF samples. Adhesive structural changes were evaluated with FTIR spectroscopy. With evaluation of acetylated NCC, modification of hydroxyl groups and replacing them to ester groups confirmed by FTIR spectroscopy, and the DS determined by titration was 0.34. X-ray analysis showed that the acetylation process reduced the crystallinity. Solid content and viscosity of adhesives were increased by adding modifying materials. Plywood samples with formulation of SF/PEI/NaOH/NCC in both two levels of acetylated NCC had shear strength and water resistance similar to PF adhesive. FTIR analysis indicated that NaOH denatures SF; amino groups buried inside the compact protein structure become available. Compared to SF/NaOH, SF/PEI/NaOH showed small shifts in absorbance peaks. Primary and secondary amines in SF/PEI/NaOH diminished or disappeared. Considering functional groups of SF and amino groups of PEI which have potential to react hydroxyl and carbonyl groups of acetylated NCC, hydrogen bonds within adhesive component developed.