Preparation Of High-grade Bamboo Dissolving Pulp And Its Application For The Synthesis Of Cellulose Acetate

Cellulose triacetate(CTA)can be used for cellulose acetate(CA)with a degree of substation(DS)above 2.9,which is obtained through acetylation for all three hydroxyl groups of anhydroglucose unit in cellulose.Cellulose diacetate(CDA)with a DS of 2.4-2.6 can be obtained by hydrolysis for CTA.The field of application for CTA includes photographic film base,plastics,textile fiber,membrane materials,and CDA can be used as cigarette filters and textile fiber.Original materials used for preparing cellulose acetate are generally high quality celluloses with an a-cellulose content of more than 95%,therefore,cellulose materials capable of being used for preparing CA are usually cotton linters,as well as high-grade wood dissolving pulps,which are applied in commerce widely.Bamboo has the advantages of low cost and regeneration,but there is no report about bamboo pulps used in CA preparation.High-grade bamboo dissolving pulp for CA(high-grade bamboo pulp)with a higher a-cellulose content,degree of polymerization(DP)and brightness was prepared in the study through the process flows of acidic prehydrolysis,twice sulphate cook with moderate temperature,and bleaching using pretreatment of xylanase and DMD,and the optimal conditions in these flows was determined.The investigations showed that suitable enhancement of sulfidity can improve selectivity of delignify in cooking and avoid degradation of carbohydrate.The increment of brightness in the stage of xylanase and DMD treatment occupied for 15.29%of the total of the increment in whole bleaching process;the decrease of Kappa number occupied for 37.87%of the total of decrease in whole bleaching process;while the decrease of intrinsic viscosity only occupied for 13.77%the total of decrease in whole bleaching process.Therefore,the two treatments can reduce cellulose degradation,enhance delignification,and improve the brightness of bamboo pulp.Hydrolytic action of xylanase for hemicellulose was demonstrated by FTIR,and X-ray diffraction proved the delignification of DMD for bamboo pulp because of complete disappearance of the diffraction peak at 2θof 29.4(?) attributed to inorganic constituents after DMD treatment.The change of chemical compositions on the surface of bamboo pulps in each stage of bleaching was analyzed in detail by monitoring the ratio of O/C and the contents of four components of C1s indicative of C atoms with 0,1,2 or 3 bonds to neighboring O atom(s).Extractive content on the surface of pulp increased markedly during the stage of H₂O₂ bleaching, and the surface coverage of extractives for the pulp of final bleaching was more than that of lignin.DMD treatment resulted in the decrease of crystallinity and crystallite size of bamboo pulp during bleaching;however,they increased in other stage of bleaching,and on the whole crystallinity and crystallite size of pulp after bleaching were more than these before bleaching.This entirely removing of lignin and hemicellulose in high-grade bamboo pulp can be seen demonstrated by SEM observation,because of full of grooves,splits, apertures,filaments and fibrils in surfaces of the pulps,and large numbers of apertures and cracks with uneven sizes distributing over the lengthwise section.As compared with high-grade hardwood pulp used for preparing CA,in spite of higher contents of xylan,ash and extractive in high-grade bamboo pulp,its physical and chemical properties came up to the requirement for acetylation;however,the higher crystallinity and crystallite size,as well as more hydrogen bond can affect its reactivity of acetylation.The influence of temperature,time and catalyst to acetylation of high-grade bamboo pulp was investigated,and found that the increases of both temperature and catalyst could raise the DS of CA,but increasing temperature is more prominent for the decrease of the DP.Therefore,a lower temperature(50℃)and a higher catalyst dosage were adopted in acetylation of high-grade bamboo pulp.Among various pretreatment modes,it was optimal for the activation of the pulp to use acetic acid and 1%sulfuric acid because reactive time of acetylation could be reduced while CA prepared maintained a higher DP.Some insoluble residues remained in acetylation medium of high-grade bamboo pulp,which resulted in the decrease of the transmittance of reaction solution and the brightness of CTA prepared. Characterization of the insoluble residue indicated that its formation was associated with XDA,which has poor solubility in acetylation medium,An addition of good solvent of XDA such as 1,2-dichloroethane to acetylation medium of high-grade bamboo pulp could release the aggregation of XDA and CTA and dissolve the insoluble residue,which led to the improves of the properties of CTA prepared such as brightness,crystallinity and thermal stability.However,measuring the properties such brightness,DP and particles in acetone for CDA prepared by acetylation and subsequent hydrolysis for high-grade bamboo pulp,indicated that it had high quality and had access to commercial CDA for cigarette filter because of a lower content of xylan in high-grade bamboo pulp(3.7%),most of which can be hydrolyzed by sulfuric acid catalysis.TG analysis indicated that thermal stability of CTA was more than that of CDA and the existence of acetyl group could improve thermal property of CA.The two peaks corresponding to crystallization and melting overlapped in DTA curve of CTA,while owing to less perfect and smaller crystallites in CDA,more facile motility for the molecule chains or chain segment within its crystalline region resulted in the melting peak appearing in a lower temperature,and the interval between the melting and decomposition temperature enlarged,accordingly;on the other hand as compared with CTA,the peak corresponding to gamma transition for CDA was more conspicuous and Tg appeared in a higher temperature.CDA prepared from high-grade bamboo pulp could be spinned favorably,and tensile properties of the CDA fibers spinned indicated of suitability for cigarette filter. DMA of the fibers demonstrated that a-relaxation was related to the glass-to-rubber transition;theβ^* shoulder was due to loss of moisture;theβrelaxation is tentatively assigned to local motions of the main chain(glucose unit);the low-temperatureγrelaxation was humidity-dependent.The study on degradation of CDA fibers showed that weight loss of CA fibers immersed in NaOH solution chiefly depended on acetylation,and alkaline treatment promoted the degradation of CA fibers in cellulase solution by reason of deacetylation,especially when DS of CA fibers reached 0.8,cellulase degradation increased most markedly.¹NMR spectra suggested that only in a lower NaOH concentration(=0.25 N)deacetylation reaction was affected by the reactivities of ester groups at position 2,3 and 6 in anhydroglucose unit,but did not follow the theoretical trend in the three positions.Moreover,the DS for polymer molecules in CA fibers were dispersive after alkaline treatment in heterogeneous condition and the DS of the product increased during sequent cellulase degradation.This was also demonstrated by the result of IR analysis and X-ray diffraction.