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During sequential bleaching operations, pulp fiber properties are gradually changed due to mechanical and chemical treatments. In this study, the correlations between pulp or fiber properties such as kappa number, viscosity, total charge, fiber length, and zero-span tensile strength as well as Scott bond of elemental chlorine free (ECF) bleached softwood kraft pulps was investigated. The influence of zero-span tensile strength and Scott bond on tensile and tear strength was also discussed. The Scott bond and zero-span tensile strength showed a strong logarithmic correlation with pulp kappa number and pulp viscosity, while the regression coefficient for Scott bond was negative. An overall deterioration of paper tensile and tear strength from pulps whether beaten or not were observed along the multi-stage ECF bleaching operations. Changing contributions to sheet tensile or tear strength could be mostly attributed to changes in zero-span tensile strength rather than Scott bond during ECF bleaching.

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The influence of atmospheric high consistency refining (AHCR) on the properties of bleached chemical reed pulp was investigated. Fiber quality, water retention value (WRV), dynamic drainage, and physical properties of handsheets were determined. The results showed that compared with low consistency refining (LCR), AHCR maintained reed pulp fiber length, had lower fines generation, produced more fiber curl and kink, and improved WRV and dynamic drainage. Compared with LCR pulp, the tear index, folding strength, and tensile energy absorption (TEA) of AHCR pulp were increased, while tensile index was maintained at the same value. A mill trial was performed to demonstrate the benefits of using AHCR, which was to improve machine runnability and to enhance the performance of the paper made from reed pulp.

The totally chlorine free (TCF) bleaching was studied for wheat straw soda-anthraquinone (AQ) pulp with a low kappa number of 8.3 using the sequence of OQPO and OQPOPO. It was found that the oxygen delignification on wheat straw pulp was still effective despite its low initial kappa number. A delignification extent of 42% and an ISO brightness increase of 10.9% were achieved. In the OQPO sequence, an ISO brightness ceiling of 87.2% was achieved as the hydrogen peroxide (H2O2) charge was increased from 2% to 6% in the PO stage. In the OQPOPO sequence, 2% H2O2 was added in the second PO stage and the pulp brightness exceeded 88% with a total charge of 4% H2O2 in the POPO bleaching. Higher brightness values of 89.5% and 90.1% were obtained under the total charges of 6% and 8% H2O2, respectively. Compared to the OQPO sequence, the higher brightness and equal viscosity of the bleached pulps were obtained under the same total charge of H2O2 in the OQPOPO sequence. The bleached pulps with high ISO brightness values (greater than 88%) retained acceptable strength properties. A slight decrease of fiber length occurred as the bleaching sequences proceeded, while the fiber width and curl index somewhat increased.