Optimizing the brightness and mechanical strength of tissue paper made of deinked pulp using isolated soy protein and chitosan by using response surface methodology

The lower mechanical properties of paper made from recycled fiber is due to their inherent properties of the used fibers. In order to improve strengths, various polymers such as cationic polymer are used, while today the global trend is the utilization of natural and renewable materials. In this study, two biopolymers including chitosan (Ch), isolated soy protein (ISP) along with glutaraldehyde (GA) were used to optimize the mechanical properties of the tissue paper made from the de-inked fibers. For this purpose, at the first chitosan solution with loading level of 0% to 2%, then glutaraldehyde with loading level of 2% to 6%, and finally isolated soy protein with 1% to 3% levels were added to the pulp suspension. The properties of handsheets were measured by standard test methods and the results were modeled using Design Expert.10 software and response surface method (RSM). The results and obtained models showed that addition of these additives did not make a significant difference on brightness of the hand sheet paper compared with the control sample. However, the addition of these additives increased the dry and wet tensile and tear strengths, which in the optimal addition level the increasing levels for mentioned properties were 145%, 35% and 70%, respectively, compared with the control sample. The increasing rate of tensile and tear resistance were higher than increasing rate obtained by conventional dry strength agents, such as cationic starch.