Study On The Biobleaching Technology And Mechanism Of Salix Psammophila Chemical Pulp

The biological structure, fiber morphology and pulping properties of Salixpsammophila were investigated in the present study. The effects of biobleaching on thefiber morphology, bleaching properties and physical strength properties of Salixpsammophila chemical pulp were studied, and the biobleaching mechanism wasresearched by analyzing the structural changes of residual lignin in pulp.Firstly, the chemical composition of Salix psammophila were analyzed, and thestructure of biological tissues, cell shape, fiber morphology of Salix psammophila werestudied with light microscope（LM）and fiber quality analyzer（FQA）.The cellulose content of Salix psammophila is as high as47.74%, and lignincontent is low, which similar to that of Populus tomentosa and wheat straw. Thegrowth ring boundaries of Salix psammophila are distinct. Its xylem ray is developedand the vessels divide into single-hole vessel, double-hole vessel and treble-hole vessel.For Salix psammophila, fiber length is short, and the mean value is only about0.52mm.However, the length-width ratio of fiber is35, the ratio of thickness of cell-wall toin-diameter of fiber cell is0.47, the distribution of fiber length and width are veryuniform, and the fines content is low. Therefore, Salix psammophila can be used inpulp and papermaking industrty as a kind of excellent fibrous materials.Secondly, the kraft pulp（KP）of Salix psammophila was pretreated by xylanase,and the effects of pretreatment on bleaching properties and paper properties wereresearched in this study. The experimental results indicated that the suitable conditionsfor xylanase pretreatment as follows: the temperature was50℃, pH value was7, pulpconcentration was8%, treatment time was60min, and enzyme dosage was10IU·（gdry）^-1. The properties of kraft pulp were improved by treatment with xylanase,including the mean length of fiber was increased, the fines content was decreased,brightness and physcial strength properties were improved after xylanase pretreatment.Generally, brightness and physcial strength properties were improved, and scatteringcoefficient, absorption coefficient and opacity were decreased with the increase ofenzyme dosage. However, brightness and physcial properties were decreased, andscattering coefficient, absorption coefficient and opacity were increased when thedosage of enzyme beyond10IU·（g dry）^-1. The analysis results of scanning electronmicroscope（SEM）, X-ray diffraction（XRD）and fourier transform infrared spectroscopy（FTIR） showed that the fiber was softened and swelled, and the cristallinity of cellulose was inceased after treatment with xylanase, which is beneficial to theimprovement of pulp properties.Finally, residual lignin samples were extracted from the pulps before and afterxylanase pretreatment, and the qualitative and quantitative analysis was carried on thefunctional groups of these samples with Fourier Transfor Infrared Spectroscopy （FTIR）,phosphorus Nuclear Magnetic Resonance(³¹P-NMR)techniques. And, furthermore, theeffects of xylanase treatment on the molecular weight distributions of residual ligninwere studied by using gel permeation chromatography coupled with laser lightscattering technology（GPC-LS-RI）. The analysis results showed that the Klason lignin,UV lignin and total content of lignin in pulps were reduced after xylanase treatment.The contents of aliphatic hydroxyl（A-OH）, total phenolic hydroxyl（T-OH） andcarboxyl （-COOH） in the residual lignin extracting form unbleached and bleached pulpof Salix psammophila were determined, and all of them increased after xylanasetreatment. During DP bleaching process, the contents of aliphatic hydroxyl （A-OH）and carboxyl （-COOH） in the residual lignin increased, and the content of totalphenolic hydroxyl （T-OH） decreased significantly. In addition, the average molecularweight of the residual lignin reduced, lignin polydispersity was improved and themolecular weight distribution of lignin was widened after xylanase pretreatment.