Improving strength properties of recycled and virgin pulp mixtures with dry strength agents

Abstract

In the paper industry, the increasing use of recycled fibers as a substitute for virgin fibers has resulted in decreased strength properties. In this study, old corrugated container fibers were added in different ratios to virgin kraft fibers, and the effects on paper properties were investigated. Furthermore, 0.75% cationic starch and 0.75% Luredur® with respect to oven‐dry fibers were added separately to old corrugated container fiber‐containing/fiber‐free suspensions, and the effects on handsheet strength properties were determined. Starch is the oldest and most common additive for improving the strength of paper. Luredur®, a cationic polymer‐based vinyl amine and N‐vinylformamide, is used for the same purpose in papermaking. Results indicated that, except for the tear index, old corrugated container fiber addition decreased handsheet strength properties in the agent‐free samples. The cationic starch addition was more efficient than the Luredur® with regard to tensile index, stretch, tensile energy absorption, and burst index. However, the effect of Luredur® on the tear index was more pronounced compared to that of the cationic starch. Consequently, comparable handsheets strength was achieved in the 100% virgin and dry strength additive‐free fibers with the addition of 0.75% cationic starch to a suspension containing 20% old corrugated containers and 80% virgin fiber.