Study On The Stability Of Dope-dyed Lyocell System Based On The Interaction Of Components In The Solution

Lyocell technology,using aqueous N-methylmorphline-N-oxide(NMMO)solution as solvent to prepare regenerated cellulose fiber,was known as the most promising cellulose regeneration process.With the technology and the capacity development of the Lyocell fiber,functionalized and differential Lyocell products will become a new research focus.Dope-dyed Lyocell fiber has the advantages of short process,less environmental pollution,and high color fastness.However,the effect of pigment on the stability of NMMO and Lyocell process is the main problem in actual production.Therefore,molecular dynamics simulation and experimental method were used to study the interaction of the components in dope-dyed Lyocell system and the stability of the Lyocell process,which is meaningful for pigment selection and optimization of the dope-dyed Lyocell technology.Firstly,carbon black(CB),titanium dioxide(TiO₂),iron oxide(Fe₂O₃),pigment red 255(P.R.255),copper phthalocyanine(CPC)and indoanthraquinone(IDA)were selected as the pigments,which represent nano carbon materials,inorganic oxides and organic pigments,respectively.The variation of N-O,C-N bond length in NMMO molecules with pigments was calculated by molecular dynamics simulation.The results showed that the equilibrium bond length of N-O bond was obviously affected by the pigment of Fe₂O₃ and CPC,indicating that these pigments may have influence on stability of NMMO molecular structure.With the consideration of the simulation results and practicability,TiO₂,CB and P.R.255,which have little influence on the structure stability of NMMO,were selected as the pigments for further study.TiO₂ is the representative inorganic oxide pigment.The particle size distribution test and sedimentation test showed that aqueous NMMO solution can naturally disperse TiO₂.In order to understand the self-dispersion of TiO₂ in aqueous NMMO solution,a simplified structural model of H₂O,NMMO and the aqueous solutions on the crystal surface of rutile TiO₂ was built.Interaction between the TiO₂ crystal surface and aqueous NMMO solutions with different NMMO concentrations was studied by molecular dynamics simulation.Ordered adsorption molecular layers contained H₂O and NMMO were observed on the TiO₂ crystal surface.High concentration of aqueous NMMO solution can increase the stability of the interfacial adsorption molecular layers.The two oxygen atoms with different polarity in NMMO molecules connect the TiO₂surface and the solvent molecules in solution through electrostatic interaction.Although TiO₂ had strong average electrostatic interaction energy with H₂O,it was not enough to make TiO₂ particles disperse stably in water.50%aqueous NMMO solution(mass ratio)can reduce the average particle size of TiO₂ to about 350 nm according to the nano size analyzer result.The electrostatic interaction can also affect the rheological properties of Lyocell solution.Rheological tests showed that the viscosity of the Lyocell solution increased first and then decreased with the increase of TiO₂ content.The increase of water content in the system can weaken the influence of TiO₂ on the viscosity of the solution.The statistical results of bond length and UV spectra showed that the electrostatic interaction did not affect the chemical stability of the recovered NMMO.Nano CB with different surface groups(amino group and carboxyl group)was selected to prepare black Lyocell fiber.The simulated results showed that there was van der Waals interaction between the weak polar part of NMMO and Unmodified-CB surface,but the interaction energy was far less than the electrostatic interaction energy between TiO₂ and NMMO.Therefore,CB was easy to agglomerate in aqueous NMMO solution.Furthermore,COOH-CB and NH₂-CB surfaces were built by introducing carboxyl group and amino group on the CB surface.The COOH-CB surface can improve the affinity to water;however,the strong electrostatic interaction and hydrogen bond make N-O bond length of NMMO increased obviously.The electrostatic interaction energy between NH₂-CB surface and NMMO was lower,which ensured the dispersion and had little effect on the chemical stability of NMMO at the same time.Results of UV spectra were consistent with the simulation results,and the effect of CB 3#on the solvent recovery was not obvious.P.R.255(3,6-diphenyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione)was selected for the preparation of red Lyocell fiber.The interaction between P.R.255 surface and aqueous NMMO solutions with different concentrations was studied by molecular dynamics simulation.The results showed that the hydrophilic functional groups(carbonyl and N-H bonds)and the hydrophobic parts(C-H bonds)of P.R.255 can form hydrogen bonds with H₂O molecules and van der Waals interactions with NMMO molecules respectively.When the NMMO concentration in solution was 10%,the total interaction energy between P.R.255 and aqueous NMMO solution come to the maximum.The average particle size of P.R.255 in 10%aqueous NMMO solution was about 170 nm according to the nano size analyzer result.However,for P.R.255,small particle size led to the migration of pigment in the solidification process becauss of weak interaction between cellulose and pigment.Therefore,Lyocell solution was prepared by dispersing P.R.255 in water and then mixed with 50%aqueous NMMO solution.Finally,Dope-dyed Lyocell fibers with the content of 1%TiO₂,CB3#,P.R.255 were prepared and the strength of the fiber was up to 4.39 c N/dtex.