Soy Proteins As a Sustainable Solution to Strengthen Recycled Paper and Reduce Deposition of Hydrophobic Contaminants in Papermaking: A Bench and Pilot-Plant Study

Hydrophobic contaminants (stickies) incorporated with recycled fibers cause severe papermaking processing and product quality problems, which lead to low runnability and increased production cost. Stickies negatively affect paper strength and many other properties. In this work, we propose a sustainable approach by the application of soy protein isolate (SPI), soy flour (SF), and soybean lipoxygenase (LOX) as agents to combat hydrophobic contaminants. Tests at the bench and pilot-plant scales and under conditions similar to industrial operations demonstrated the reduction of associated challenges and the improvement of a paper’s dry strength. The soy agents were applied to aqueous dispersions of lignin-free recycled fibers (dosage levels of 1–2% based on the fiber dry weight), which contained additives typically used in papermaking (fillers, sizing agent, and others). Talc, a common detackifier, was applied in similar systems that were used as reference. The proteins were added under both high and low shear conditions, and their effect in reducing paper tackiness and increasing internal bonding was confirmed. The maximum reduction in tacky particles count was achieved with SF (2% addition level based on fiber) under mild agitation and 30 min contact time. Remarkably, synergistic effects on the fiber electrostatic charges and paper porosity and formation were observed upon the addition of the soy proteins. The obtained results indicate that residual soy products represent an inexpensive, sustainable, and environmentally benign solution to enhance papermaking performance relative to conventional and more expensive agents that are in current use.