ARE THE PULP FIBER WALL SURFACE LAYERS THE MOST RESISTANT ONES TOWARDS BLEACHING?

Abstract

The residues of the wood cell wall compound middle lamella affect the composition of the relevant pulp fiber surface layers and influence the fibers’ bleachability. The objective of the present work was to separate the eucalyptus kraft and birch organosolv pulp fiber wall surface layers by hydromechanical peeling and to proceed with enzyme boosted bleaching of the separated fiber wall layers. The initial content of lignin and heteroaromatic compounds (“false” lignin) was determined by chemical methods and UV-spectra. The separated fiber wall surface layers representing the residues of the primary wall P and outer layer S1 of the secondary wall, as well as the main part of the secondary wall were exposed to the bleaching sequence peroxide-xylanase treatment-alkaline extraction-peroxide (P1-X-E-P2). Brightness measurements revealed significant distinctions between the preparations. The final brightness of the main part of eucalyptus kraft pulp fibers reached 67%, while the brightness of the surface layers attained only 50% ISO. Similar results were obtained for birch organosolv pulp. It was concluded that the main reason for the described phenomena is the discordant chemical composition of the different fiber wall layers.

Keywords

• Bleaching
• Pulp fibers
• Surface layers
• Hydromechanical peeling
• Eucalyptus
• Birch