Soy Protein-Based Polyelectrolyte Complexes as Biobased Wood Fiber Dry Strength Agents

Soy protein flour, isolate, and their derivatives were investigated to enhance the dry strength properties of paper. The soy systems were applied in aqueous suspensions consisting of lignin-free and recycled fibers to investigate their adsorption and interaction effects. Experiments using soy flour, soy protein isolate and its hydrolysates, cationized soy flour, and dual systems consisting of soy flour combined with cationic starch or chitosan were pursued. Improved paper strength was obtained when soy protein flour was utilized in combination with conventional treatments based on cationic polymers. For example, increases in ultimate tensile and compressive strength of lignin-free fiber paper of 23% and 10%, respectively, were measured when dual systems consisting of soy flour and cationic starch were applied relative to the fibers with no additive. In the case of lignin-containing recycled fibers, improvements of 52% and 56%, respectively, were obtained for a soy flour-chitosan dual system (compared to fibers without additive). The results confirm that an opportunity exists to valorize residual soy products that are inexpensive, environmentally friendly, and offer an alternative sustainable option for paper performance enhancements relative to conventional (nonsustainable) dry strength additives.