Study On Preparation And Coagulation Characteristics Of Spinning Solution For NMMO Method Cellulose Fiber

In order to prepare high performance cellulose fibers,this thesis discusses the significance of cellulose pretreatment in preparation of cellulose fiber spinning solution,studies the rheological properties and spinnability of spinning solution.A solution was proposed on the basis of analyzing the mechanism of side reactions in spinning solution.By means of calculating diffusion coefficients in cellulose coagulation,the correlation between coagulation parameters and fiber structure properties were explored.These researches provide theoretical guidance for the preparation of high-quality cellulose fiber.HAAKE rheometer,polarizing microscope,refractometer,ultraviolet visible spectrometer,X-ray diffraction instrument,field emission sweep mirror,Fourier transform infrared spectroscopy,high efficiency liquid chromatography,differential scanning calorimetry,small angle X-ray scattering,fiber elongation meter,fiber fineness meter and other analytical techniques were used to tracking study concentration of NMMO,cellulose dissolution and spinning process.The preparation process of cellulose fiber has been studied in depth.In this thesis,the NMMO aqueous solution with different contents was prepared by low pressure concentration,the corresponding equation of NMMO content and refractive index in NMMO aqueous solution was established.The process of cellulose pretreated by NMMO aqueous solution was tracked by polarizing microscope with CCD and the result showed that solution with below 78wt.%of NMMO couldn't lead to cellulose dissolution and change in chemical crystal structure while the crystallization reduced and a bubble-like structure in amorphous areas formed.This is very beneficial to improve the subsequent dissolution of cellulose and improve homogeneity of spinning solution.On the basis of pretreatment,the dissolution process of pretreated cellulose in high concentration NMMO aqueous solution was also tracked,and influences of cellulosic nature,content and temperature on rheological properties of spinning solution were analyzed by rheometer.During cellulose dissolution,the viscosity of the system first decreased with the dispersion of the pulp,then increased significantly with the dissolution of cellulose,and finally stabilized.The system temperature decreased significantly in the initial stage with the removal of a large amount of water,and then recovered to stability because the water was no longer removed.Both the increased temperature and the stirring speed can promote dissolution,and the temperature should be raised appropriately in the initial stage of dissolution to facilitate the rapid removal of water.Rheological character study on spinning solutions showed that the higher the cellulose content and the higher the polymerization,the higher the viscosity of the cellulose/NMMO/H2O.Viscous flow activating energy of spinning solution with a polymerization of 742 is high,which was 62.7 kJ/mol,which shows that the viscosity of spinning solution is sensitive to temperature.Cellulose content has little effect on the activation of the viscosity,but the higher the polymerization of cellulose,the higher the activation of the viscosity.Cellulose fiber spinning solution is a shear-thinning fluid,and the higher the shear rate,the smaller the viscosity activation energy.The low molecular weight and higher temperature are favorable to increase the critical shear rate.When the molecular weight and the concentration are higher,the degree of structuralization is higher,the non-Newtonian is stronger,the physical stability is worse,the non-Newtonian index is lower,the elastic modulus is higher,and the proposed elastic network structure is denser.In order to solve the problem of side reaction in spinning solution and ensure fiber quality,the mechanism of side reactions in NMMO and spinning solution were studied.NMMO decomposes under conditions of acidic,high temperature and transition metal ions.Protonation reduces the energy of N-O key energy and is prone to fracture under acidic conditions;catalysis of Fe3+ alone is mild while Cu2+can cause severe decomposition of NMMO.A combination of factors also leades to significant decomposition of NMMO.Therefore,NMMO should be stored at low concentrations,at low temperatures,in alkaline conditions,and without contact with transition metal ions such as Fe3+,Cu2+,etc.In spinning solution,high temperature causes decomposition of NMMO and the breakage of cellulose macromolecules.Under acidic conditions,protonation of H+makes NMMO decompose and produce free radicals which could attack ?-1,4 glycoside bonds to break the cellulose macromolecules chain.Reductive terminal carbonyl groups of cellulose,which could react with NMMO,falls off one by one in strong alkaline solution.The redox reaction produces a large amount of colored substances.Small molecule reduced sugars in the system have a similar effect.The presence of Fe3+and Cu2+ greatly reduces the thermostability of spinning solution,by leading to degradation of cellulose and decomposition of NMMO,respectively.Fe3+can't react with NMMO by itself,and needs to be reduced by reducing sugars in the system to a low valence state that could react with NMMO to produce free radicals whose attack on cellulose leads to cellulose degradation.Cu2+ itself can react with NMMO to generate amine radicals and copper-oxygen radical complexes,which can continue to react between themselves,reducing the number of free radicals that attack cellulose.The absorption peak of the side reaction product catalyzed by Fe3+ was located at 270nm,and the absorption peak of the side reaction product catalyzed by Cu2+ was located at 315nm.When the degree of side reaction was high,a new absorption peak appeared at 465nm.Increasing the pH to about 10 can improve the thermal stability of the spinning solution.Propyl gallate improves the thermal stability of the system by capturing free radicals generated by side reactions in the system,while it is oxidized to quinone derivatives with high conjugated cage-types structure and darker color.which has a strong absorption at 275nm.Hydroxylamine can capture free radicals in the system,and can reduce the catalytic ability of metal ions by forming complexes,and its ability to reduce colored substances can be reduced the chromaticity of the system,but its own reducibility and the nature of gas generated by decomposition limit its addition.Making full use of the advantages of various stabilizers,by increasing the pH.adding propyl gallate and hydroxylamine,the thermostability of the spinning solution can be effectively improved,and the color of the system can be reduced.Effective measures are to improve the thermostability of spinning solution:starting from the raw materials,increase the content of ?-cellulose,reduce the content of small molecular reducing sugars and metal ions,increase the dissolution efficiency of cellulose through pretreatment,adjust the pH of the system,and add propyl gallate and hydroxylamine.In order to study the double diffusion process of the spinning solution in the coagulation bath,this thesis used polarizing microscope to record the boundary movement of the molten and crystalline spinning solution circular flakes in NMMO/H2O solution,and established boundary movement model.Fick's law was used to derive and solve the diffusion equation of the spinning solution in NMMO/H2O coagulation bath using the separation variable method and the Bessel function.The diffusion coefficients of NMMO and H2O were calculated respectively,and influences of cellulose content,degree of polymerization and coagulation conditions on diffusion coefficient were discussed.The research results showed that the increase of cellulose content causes the diffusion coefficient of NMMO and H2O in molten spinning and the diffusion coefficient of NMMO in the crystalline spinning to be significantly reduced,while has little effect on diffusion coefficient of H2O in crystalline spinning.The degree of cellulose polymerization also has a significant effect on the diffusion coefficient in the molten spinning solution,but has little effect on the diffusion coefficient in crystalline spinning solution.Increase of temperature of coagulation bath is beneficial to both NMMO and H2O diffusion in the two states of spinning solution,and the increase of the NMMO concentration in the coagulation bath causes an insignificant decrease in the diffusion coefficient.In spinning process,the diffusion process could be divided into two stages.In the initial stage of diffusion,the diffusion is relatively fast,and the rapid diffusion stage is completed within 0.5s.This stage is mainly the formation of hydrogen bonds and the surface dense layer.In the later stage of diffusion,the diffusion speed is significantly reduced,and the diffusion coefficient is about onefifth of the rapid diffusion stage.The cellulose polymerization degree,solid content,coagulation bath conditions,etc.have basically the same influence on the diffusion during the spinning process.In order to explore the effects of spinning solution properties and spinning conditions on spinning and fiber properties,the effects of spinning solution properties and spinning process conditions on fiber properties were studied.Studies have found that increasing the cellulose content and degree of polymerization can improve the mechanical properties of the fiber to a certain extent,but with the decline in spinnability,spinning discontinuity and other phenomena affect the fiber performance.By adding about 10%of high degree of polymerization pulp to low degree of polymerization pulp,spinnability can be improved,and at the same time,cellulose fibers with excellent properties can be obtained.The crystallinity of cellulose fiber is about 63%.Increasing the concentration of the coagulation bath or lowering the temperature of the coagulation bath will help to obtain fibers with outstanding mechanical properties;increase the air gap length,increase the spinning speed and draft ratio,and obtain cellulose fiber with high orientation and high mechanical properties.