Extending the limits of using chemithermomechanical pulp by combining lignin microparticles and hot-pressing technology

A promising alternative to extend the limits of chemithermomechanical pulps (CTMPs) has been proposed to produce extremely resistant waterproof paper for use in sustainable packaging products, replacing plastics. The synergies between the incorporation of lignin microparticles (LMPs) in a pulp furnish (mass), with retention agents and hot-pressing technology, have been successfully tested in CTMP paper sheets. The addition of LMPs as a wet-strength agent, combined with the high temperature produced in hot-pressing, allows the softening of the LMPs to enhance mechanical properties. Two retention agents, cationic starch (CS) and chitosan (CH), have been studied to ensure the retention of the LMPs. Results show that both CS and CH remarkably improve the wet tensile index while maintaining or slightly increasing the dry tensile index. Regarding hot-pressing, three seconds of pressing is enough to achieve these properties, and some moisture (20%) in the sheets prior to pressing favors the wet strength. CH is not only the most promising retention agent, but it also significantly increases the wet tensile index (around 50 kNm/kg), maintaining more than 65% of the dry tensile index, creating an extremely resistant waterproof paper. Additionally, LMPs increase the short-span compression test (SCT) index by 180% and reduce air permeability 16 times compared to the untreated CTMP paper.