Study On The Solubility Of Dissolving Pulp For Lyocell Fiber

China is the country with the highest output of paper and paper products in the world.In2020,the output of machine-made paper and paperboard products exceeded 127 million tons.However,with the spread of global trade frictions and the intensification of homogenization competition in the paper industry,a large number of paper-making enterprises are facing the dilemma of continuous shrinking economic effects or even losses,and urgently need to upgrade and transform their products and technologies.Lyocell fiber is a new type of regenerated cellulose fiber prepared by dry-jet wet spinning with N-methylmorpholine-N-oxide(NMMO)as solvent.Different from viscose fiber,the production process of Lyocell fiber is green and environmentally friendly.The solvent is 99%recyclable,and the performance of the fiber is also better than that of viscose fiber.Compared with viscose fiber,dissolving pulp for Lyocell fiber has stricter requirements on ?-cellulose content,degree of polymerization(DP),metal ion content and ash content.At present,most of the dissolving pulp used in the production of Lyocell fiber in China needs to be imported,and domestic dissolving pulp cannot fully satisfy the modern Lyocell fiber production process in terms of solubility and spinnability.Therefore,it is of great theoretical significance to clarify the differences in the dissolution process of various type of dissolving pulp in NMMO and to explore the main factors affecting the dissolution properties of dissolving pulp.At the same time,by selecting a suitable post-processing process,the method of obtaining dissolving pulp for Lyocell fiber from paper pulp refining is discussed,which provides new ideas and references for the transformation and upgrading of papermaking enterprises.In this thesis,four commercial dissolving pulps are used as raw materials,and the dissolution process and dissolution state differences in the NMMO solution were observed through a polarizing microscope.Then ?-cellulose content in the four kinds of dissolving pulp,degree of polymerization,crystallinity,metal ion content,ash content and fiber morphology are comprehensively analyzed and investigated.Combining with the analysis of laser scanning confocal microscope(LSCM),the changes of residual lignin in different dissolving pulps and the influence of various indicators on its solubility in NMMO were investigated.The results show that the residual lignin inside the fiber and the lignin adsorbed on the fiber surface are the most critical factors affecting the solubility of dissolving pulp in NMMO.At the same time,metal ions will also have a certain impact on the dissolution performance.In order to verify the influence of lignin on the dissolution performance of dissolving pulp in NMMO,a dissolving pulp with excellent dissolving performance was selected for the deposition and adsorption of alkali lignin and acid lignin,and LSCM was used to observe the lignin on the fiber.And the solubility of the dissolved pulp with deposited and adsorbed lignin showed that alkali lignin is more likely to deposit and adsorb on the fiber.The fluorescence intensity of the alkali lignin-adsorbed fiber rose rapidly from 0.44 to 15.56,and dropped to 1.16 after washing.It is difficult to disperse and swell the fibers with a small amount of lignin in the NMMO,and the system will be gelled after 45 minutes of dissolution,and the dissolution process will stop.In addition,fibers that adsorb 0.001 M iron and copper ions will be separated from NMMO after dissolution of 30 minutes.On the basis of the above results,domestic softwood pulp was used as raw material,and the pulp was extracted firstly at 30 °C and an alkali concentration of 12 wt% for 1 hour.Then it was bleached for 1 hour at 90 °C and a hydrogen peroxide concentration of 0.05 wt%,and finally treated with 12 wt% acetic acid at 100 °C for 2 hours.With the three steps,a dissolving pulp for Lyocell fibers with good dissolving properties was obtained successfully.The main parameters of the dissolving pulp are as follows: ?-cellulose 96.12%,ash content 0.07%,iron ion 3.47 mg/kg,whiteness 88.89%,fiber crystallinity 78.29%,and cellulose polymerization degree 653.