Improving sizing performance of middle layer of liquid packaging board containing high-yield pulp

As the important part of liquid packaging board, the middle layer is generally produced from a mixed pulp furnish by unbleached softwood kraft pulp and high-yield pulp (HYP). However, the addition of HYP would bring unsatisfactory properties (e.g. hydrophilicity) to liquid packaging board. Herein, in this study, the sizing performance of middle layer of liquid packaging board was evaluated by varying the kraft pulp to HYP ratios of middle layer, the dosage of sizing agent and cationic additives etc. It showed that the edge penetration value and Cobb value of middle layer illustrated a better liquid repellence with the addition of sizing agent. The improved sizing performance of the middle layer of liquid packaging board can be ascribed to the excellent performance of the added cationic additive (polyethyleneimine (PEI), alums) for trapping anionic trash via neutralization/flocculation effects. In addition, a binary cationic additive system (Al-2(SO4)(3)-PEI) was proposed to study the improvement on the sizing performance. It was found that a desirable liquid repellence ability can be successfully achieved at a binary additive system containing 1.5% of Al-2(SO4)(3) and 0.2% of PEI, even when dosage of rosin sizing agent was reduced to 0.5%. The study put forwards a practical approach to improve the sizing performance, especially the liquid repellence ability of liquid packaging board when containing HYP.