Tuning the properties of soy protein isolate-based adhesive using various sustainable additives

This work investigates the adhesive property of Soy Protein Isolate (SPI) polymer solution by studying mechanical properties of composites formed using waste wood granules and SPI solutions. To improve the adhesive strength of SPI solution, Carboxymethyl Cellulose Sodium (NaCMC) was mixed (in the weight ratios of 9:1 and 8:2) due to its strong gel formation capabilities. The adhesive performance of these composites was further investigated in the presence and absence of non-toxic additives, including sorbitol (SOR) and stearic acid (SA). These additives were expected to enhance the strength of the SPI-based adhesive through the formation of strong interactions between the functional groups of the molecules. The wood/adhesive composite showed a better (than the untreated wood) mechanical strength with the increased concentration of NaCMC. The un-modified SPI/NaCMC(S2) composite with the weight ratio of 8:2 showed higher bending strength of 34.8 % and 186.2 % when compared to the SPI/NaCMC composites of the same ratio having sorbitol (SS2) and stearic acid (SSA2%-2) respectively. But at the same time, bending strain decreased due to the lack of flexibility in the SPI/NaCMC (S2)(0.667 %) composite when compared to sorbitol (SS2)(1.46 %) and stearic acid (SSA2%-2) (1.76 %)-based adhesives. However, the adhesive strength of the SPI/NaCMC-based composites increased with increase in the concentration of NaCMC verified by the field emission scanning electron microscopic (FE-SEM) images that confirmed the relatively smooth fracture surface of the composites. As a result, this work indicates the possibility of the S2 polymer solution being used as an effective bio-adhesive in the place of toxic adhesives in the market.