Coupling laccase/PHB and Ca2+ treatment enable high-strength straw chemi-mechanical pulp

Non-wood fibers (e.g., straw) show a large volume in China, which effectively offsets the shortage issue of the forest resource, especially as raw material in the pulping and papermaking industry. The chemi-mechanical pulp (CMP) from the straw is significantly developed due to the advantages of high pulp yield, mechanical strength, and low cost. While the large-amount lignin exposed the pulp fiber surface of straw CMP negatively affects the bonding between its fibers, thus restricting the further improvement of its physical properties. Herein, we developed an environmentally friendly strategy of coupling the laccase/PHB and Ca2+ treatments to modify the chemical and structural properties of fibers toward the high-strength straw CMP. The laccase/PHB treatment removes the lignin of the straw CMP with a Kappa number decrease from 108.28 of the original sample to 75.65 with an 8 u/g dosage of laccase at a 3% PHB system. The removal of lignin with a stiff property also modifies the fiber morphologies of straw CMP, including increasing the measured fiber curl index from 8.0% to 8.5%, and the kink angle from 129° to 137°. Simultaneously, the introduced Ca2+ triggers the strong bonding with the anionic groups of straw CMP fibers, which creates enough, multiple bonds in this fiber system. The coupling laccase/PHB and Ca2+ treatments boost the mechanical strength of the straw CMP, more specifically, the tearing index, tensile index, and bursting index increase from 3.51 mN•m2/g, 20.01 N•m/g, and 1.57 KPa•m2/g of original pulp to 4.56 mN•m2/g, 29.80 N•m/g, and 2.26 KPa•m2/g, respectively. This integrated strategy of laccase/PHB and Ca2+ treatment provides a valuable guideline for improving the strength performance of plant fibers, beyond the straw CMP.