Fiber quality of liquid packaging board after hydropulping

Abstract This study aimed to investigate fiber quality in recycled papers produced from liquid packaging board (LPB) using hydropulping combined with pretreatment consisting of perforation and 0.2 bar hydraulic pressure. The pretreatment accelerated flake reduction and improved fiber disintegration while maintaining low energy consumption. Under moderate pulping conditions (18 kWh/t), a pulp yield of 81.1% was achieved after screening with a 0.35 mm slot-size screen, which was 9% higher than without pretreatment. Considering an initial plastic content of approximately 11 wt% in LPB, the 0.35 mm screening removed approximately 92–94% of the plastic from the accept fraction. Improved deflaking increased fiber swelling and enhanced the mechanical properties of the recycled papers. High pulping intensity further reduced flake content but increased plastic contamination in the accept fraction. Microplastics were detected in all recycled papers, indicating that plastic contamination was not completely avoided during hydropulping under the investigated conditions. Chemical analyses showed that AKD sizing contributed to the hydrophobic character of the carton fibers but was largely removed during pulping and did not provide hydrophobicity to recycled papers, whereas the presence of nitrogen-containing compounds suggested a wet-strength resin (PAE), which likely contributes to the resistance of the carton structure to disintegration. Flotation-based separation of hydropulper reject was effective for separating fiber flakes from plastic; however, the flakes require additional deflaking before they can be recirculated to the accept fraction to improve yield. After flotation, the plastic fraction contained approximately 94% clean plastic after washing, indicating strong potential for further recycling.