Barrier coatings on dry-formed pulp with lignin, stearic acid, and combinations thereof

Abstract

Barrier coatings on dry formed pulp were studied in this article, which were derived from Kraft lignin, stearic acid, and combinations thereof. Coating layers were applied by spray-coating with a solution of lignin or stearoyl chloride and subsequent heat treatment. Alternatively, lignin was esterified with stearoyl chloride on beforehand or combinations of lignin and stearoyl chloride on the air-laid mat were done. Since the treatments were applied prior to thermopressing, the coating agents permeated the top layers of each substrate. As our results show, coatings with lignin could improve the tensile strength and stiffness of the substrate. Grafting with stearic acid, on the other hand, affected the tensile properties negatively, which was argued to arise from worse binding of the cellulose fibers and degradation due to the presence of acid moieties. All treatments improved the barrier properties, as noted by a reduction in air permeation and water-vapor transmission rate (WVTR). The effect of spray coated lignin on WVTR was best, albeit showing less effect on the water absorption as measured by COBB₁₈₀₀. Stearic acid grafting yielded the opposite trend, i.e., reducing water absorption to a greater extent, while affecting WVTR less. Combinations of stearoyl chloride and lignin showed synergies and additive effects to some extent. In conclusion, various treatments for dry formed fibers were implemented and tested, which may promote the development of new barrier solutions for cellulose-based materials.