The application of totally chlorine-free bleaching (TCF) is well recognised for its environmental compatibility. However, its application is restricted in comparison to a chlorine-based bleaching sequence. TCF bleaching produces pulp with relatively lower brightness and strength properties. Oxygen delignification (O-stage) is commonly used as a first bleaching stage; therefore its selectivity influences TCF bleaching performance. In this study, the selectivity of the O-stage and hydrogen peroxidereinforced O-stage (Op-stage) on tropical hardwood kraft pulp were 0.53 and 0.71, with 37.8% and 55.5% kappa number (Kn) reductions, respectively. The addition of photo pre-treatment prior to the O-stage and Op-stage improved the Kn reduction to 53.7% and 59.2%; consequently the selectivity was enhanced to 1.0 and 0.73, respectively. The Fourier transform infrared spectra showed that photo pre-treatments improved the selectivity by protecting the cellulose from carbonyl-induced degradation. This protective effect was more prominent in the O-stage than the Op-stage. However, the combination of peracetic acid and photo treatment did not further increase bleaching selectivity in either stage. In short, blue light and UV irradiation treatment with or without the prior addition of peracetic acid to ordinary or hydrogen peroxidereinforced oxygen delignified pulp displayed a similar bleaching effect on the resultant pulp.

In this study, the bleachability of commercial mixed tropical hardwood brown kraft pulp by oxygen delignification (O stage) was examined. It was found that the effective reduction of kappa number was limited to about 35%, and the pulp viscosity was 20.3 cP with a selectivity less than 0.60 and ISO brightness of ca. 43%. The selectivity and pulp brightness of the O stage were improved by adding H 2 O 2 (O P stage) because it decreased the kappa number to a greater extent. However, the addition of hydrogen peroxide caused more serious cellulose degradation. In order to minimize the drop of pulp viscosity during the O P stage, a small amount (0.04%) of anthraquinone (AQ) was added. The results showed that the AQ-aided O P stage was capable of preventing cellulose degradation and thus improved the bleaching selectivity about 60%, in comparison to the ordinary O stage. Moreover, the AQ-O P pulps retained significantly less hexenuronic acid than the pulps from O and O P stages.