Improving Bleachability of Thermomechanical Pulp by Ozone–Hydrogen Peroxide System

Thermomechanical pulp (TMP) was bleached with an ozone–hydrogen peroxide system, with the addition of hydrogen peroxide and ethylene glycol to enhance bleaching efficacy. The ISO brightness of TMP increased proportionally with higher concentrations of hydrogen peroxide across all sequences. Notably, TMP treated with the ozone and hydrogen peroxide sequence (ZP sequence) exhibited ISO brightness levels comparable to those of TMP bleached with hydrogen peroxide alone. However, the addition of hydrogen peroxide during the Z stage of the pZP sequence resulted in a 2.4% increase in ISO brightness and enhanced lignin solubility. A brief alkaline extraction step in the pZEP sequence led to a significant 4.3% increase in ISO brightness, though this improvement was accompanied by a yield loss due to removal of lignin in the E stage. Furthermore, the addition of polyethylene glycol alleviated yield loss by enhancing the selectivity of ozone bleaching, while effectively maintaining a stable ISO brightness.