Preparation And Application Of Nanocellulose Fibers Under NaOH/Urea System

The nanocellulose fibers extracted from plant fibers is a polymer material with great application prospects in the fields of medicine,reinforced materials,and transparent films,and it is also one of the global research hotspots.At present,the preparation of nanocellulose fibers is still in the primary stage of large-scale production.Existing processes have the disadvantages of low yield,corrosive solvents,and high cost.Therefore,a simple,low-cost and relatively environmentally-friendly preparation method has important implications for the large-scale production of nanocellulose fibers.In this dissertation,a new method for preparation of nanocellulose fibers was proposed.The cellulose was subjected to low-temperature pretreatment using NaOH/Urea solution system.The morphology and properties of fully-swollen fibers under this system were studied and analysed in depth.The pretreated sample was mechanically treated with a plant nano-pulper,and finally the microfibrillated ellulose(MFC)was pretreated by NaOH/Urea solution.Meanwhile,two kinds of nanocellulose fibers HNC and TNC were prepared by pretreatment of cellulose with concentrated sulfuric acid and TEMPO combined with high pressure homogenous mechanical treatment.The microstructure,crystallinity,thermal stability,and particle size of the three nanocellulose fibers MFC,HNC and TNC were comprehensively compared and characterized.Using MFC and PVA as raw materials,the MFC/PVA composite membrane material was prepared by physical copolymerization and mixing method.The effect of nanocellulose fibers as reinforcing agent on the microstructure,mechanical properties,thermal stability and water resistance of the composite membrane was discussed.The main findings are as follows:Based on the NaOH/Urea solution system can produce a significant swelling effect on the fiber,is an effective pretreatment method.The water-preservation value of the fiber after pretreatment at-4? increased to 192%.With the decrease of the pretreatment temperature,the devitrification of the fiber became more obvious,and the fiber length became shorter and the fine fiber content decreased.The pretreated fibers can be seen clearly along the longitudinal direction of the fiber in the micro-fiber loosening layer structure,the fiber surface appears uneven,gully cracks and other features.Nanocellulose fibers maintains cellulose Istructure,and the crystallinity decreases to some extent.The width is about 10 to 100 nm,and the length is from a few hundred nanometers to several micrometers.The degradation of nanocellulose fibers began at 223?,and the maximum degradation temperature was 338?.Acid hydrolysis and TEMPO oxidation pretreatment combined with mechanical treatment of nanocellulose fibers HNC and TNC,the yield was 31.3%,63.4%.The crystallinity of HNC and TNC were 80.11% and 74.75%,all of which maintained the cellulose I structure.The short rod-shaped HNC has a diameter between 20-40 nm and an average length of 150-300 nm,and begins to degrade at 225?.The maximum degradation temperature is 314?.The average length of the slender rod-shaped TNC increased significantly,showing a larger aspect ratio,and began to degrade at 220?.The maximum degradation temperature was 336?.Zeta potential analysis showed that HNC and TNC potentials were-53.6mV and-43.2mV,respectively,and the former had better dispersibility.The MFC/PVA composite membranes with different MFC content were prepared by physical blending.The morphology analysis showed that MFC and PVA kept good compatibility when the amount of MFC added was less than 6%.The contact angles of PVA film and MFC film are 58.7° and 46.8°,respectively,and shows that the hydrogen bonding force generated between PVA and MFC due to hydroxyl groups in the molecular chain.Thermal stability analysis showed that with the addition of MFC,the thermal stability of composite film slowly increased.The mechanical analysis showed that when the addition amount of MFC increased to 10%,the maximum elastic modulus of the composite film is2387 Mpa,which is 3.18 times higher than that of pure PVA.With the MFC addition amount increased from 0% to 10%,the change trend of the tensile strength of the composite film first becomes larger and then decreased.When 8% of MFC was used,the best tensile strength was55.34 Mpa.The swelling law showed that the swelling degree of the composite membrane gradually decreases with the increase of MFC content,and the composite membrane shows better water resistance.When the water/alcohol ratio is 1:9 and MFC addition amount is 8%,the swelling ratio tends to reach 25.60%.When the water/alcohol ratio is 1:3,the swelling ratio tends to reach 118% when the MFC addition amount is 10%.