Regioselective surface esterification of softwood mechanical pulp fines as hydrophilic paper strength additives

The fines fraction in mechanical pulp, produced during wood processing can impact the mechanical properties of the resulting paper positively or negatively, making them potential additives for paper strength improvement at the right loading level. To enhance fines' effectiveness as paper strength additives that have heterogeneous surface chemistries, this work employed a regioselective surface esterification method conducted under mild conditions in acetone, with the addition of succinic anhydride and imidazole. This approach converted surface hydroxyl groups of fines into carboxylic acid groups, resulting in a final content of near 0.5 mmol g −1 . This modification increased fines' hydrophilicity, as evidenced by enhanced moisture absorption capacity in dynamic vapor sorption analysis. These fines with additional functionality strengthened intermolecular bonding, leading to over 150% enhancement in tensile strength and Young's modulus when cast into thin films. Fines were substituted at 10–20 wt.% concentration to the fiber fraction of softwood thermomechanical pulp (TMP) and bleached chemi-thermomechanical pulp (BCTMP) forming handsheets. It was observed that fines modified with succinic anhydride and imidazole were more effective in enhancing paper strength, including tensile (up to 30%), burst (up to 39%) and tear (up to 4%) indices. This improvement was likely due to enhanced intermolecular bonding between modified fines and the fiber surfaces. By modifying mechanical pulp fines with succinic anhydride and imidazole, we have converted a potential heterogeneous waste product from the industry into a valuable by-product to enhance paper performance.