The aim of this study was to evaluate how the pulp bleachability ability is correlated with the wood chemical characteristics, the cooking conditions, the kappa number of the brown pulp and the bleaching sequences, based on the pulp bleached brightness, viscosity, bleaching chemical consumption and bleachability. The eucalyptus wood (A, B, and C), appropriately characterized as the chemical constituents (holocellulose content, insoluble and soluble lignin content, total extractive content, ash and insoluble ash in hydrochloric acid) were used for pulp production by modified kraft cooking, using two different pulping conditions: Condition I (PI) - 16.5% effective alkali, 30% sulfide, and 155ºC cooking temperature; and Condition II (PII) - 17.5% effective alkali, 32% sulfide, and 147ºC cooking temperature. Both conditions were performed to kappa number 15.5 ± 0.5 and 18.0 ± 0.5. The pulping parameters and pulp properties evaluated were: screened yield, viscosity, brightness, hexenuronic acid content, and index k/corrected kappa (ratio between the light absorption rate, a brightness formula term, and the corrected kappa number, disregarding the hexenuronic acid contribution). Each one of the pulps was bleached, to achieve 90 ± 1% ISO brightness, following three pre-defined bleaching sequences: 1 Sequence - OA(ZE)DP; 2 Sequence - OA/D(EOP)DP; and 3 Sequence - OD(EOP)DP. The pulps were investigated for brightness, viscosity, bleaching chemical consumption and bleachability. The bleachability was affected by the wood. Furthermore, the bleaching abilities were affected by the cooking condition and the brown pulp kappa number. The 1 and 2 sequences were satisfactory to achieve the required brightness (90 ± 1% ISO) unlike to the verified to the 3 sequence. The best bleachabilities were obtained from the brown pulps with higher kappa number 18.0 ± 0.5. It was verified close correlation between bleaching ability and index k/corrected kappa, which indicated the possibility of using this property of unbleached pulp for predicting pulp bleachability.
bleachability; index k/corrected kappa; brightness; viscosity