| Sumario: | Oxygen delignified Kraft pulps from radiata pine were treated with xylanase in the presence of a quelating agent prior to their chemical bleaching with the sequences WZE(OP)P, E(P)WZP and E(OP)WZP. Enzymes tested included Cartazyme from Sandoz, Ecopulp from Alko-ICI, Irgazyme 40 from Ciba-Genencor and Pulpzyme HB from Novo Nordisk. A positive effect of these xylanases was observed with the sequence XQ-E(P)WZP, which yielded pulps with a final brightness of 85-86% ISO. Application of the sequence XQ-E(OP)WZP led to pulps with brightness within the target value of 90% ISO, although enzyme effect in this case was less evident. In turn, enzymes were ineffective in the sequence X(Q)-WZE(OP)P. These results emphasize the need to design a proper chemical sequence to optimize biobleaching. In most experiments, beatability of enzyme-treated pulps was slightly affected, requiring up to 15% more revolutions in the PFI mill than the controls to reach the same tensile index.
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