Dissolution Of Cellulose In Alkali Complex Solvent And Its Spinning Properties

Cellulose is a kind of renewable and friendly environment material. But cellulose has not reached its potential applications because it cannot be melted to fabricate into a desired form or to be dissolved in a common solvent,attributing to the strong inter- and intra- molecular hydrogen bonding.Though wet-spinning for cellulose have a long history,there are many problems for rayon,cuprammonium and Lyocell,including environmental pollution and high cost.In this paper,cellulose was dissolved in NaOH/Thiourea/Urea/H₂O system and for wet-spinning.The dissolution of cellulose in this complex solvent system was further observed.The rheological properties of cellulose solution were studied. Also the relationship between properties of regenerated cellulose fibers and spinning conditions was investigated.The mechanics of coagulate process between wet-spinning was preliminarily discussed.Main studies and results are summarized as followed:The process of dissolution for cellulose in alkali complex solvent system at different temperature was observed by optical microscope.The results show cellulose was starting to dissolve at 0℃.As the temperature decreasing,cellulose was gradually dissolved.And cellulose was dissolved completely until -8℃.There was no peak for cellulose derivatives in ¹³C-NMR spectra of cellulose in the solution,indicating an absence of derivatives.It could be concluded that NaOH/Thiourea/Urea/H₂O is a direct solvent for cellulose.The rheological properties showed that all the solutions were pseudoplastic fluid.Under the same solid content,the shearing stress and structure index decrease with increasing the temperature.Viscosity flow activation energy changed little with the increasing of the concentration of cellulose solution.We got the optimization coagulate conditions and draw conditions during wet-spinning by uniform design.The tensile strength is 1.29 cN/dtex when coagulation is 10 wt%H₂SO₄/12.5 wt%Na₂SO₄ aqueous solution,coagulate temperature is 15℃,minus stretch ratio is -30%.XRD shows the regenerated cellulose fibers have a typical structure of celluloseⅡ,and crystallinity is a little lower than this of the raw cellulose pulp but higher than this of the rayon fiber.Optical microscope and SEM pictures show that the regenerated cellulose fibers have a circular cross-sections and homogeneous morphological structure different from leaf shape for rayon fibers.This is due to different coagulated mechanics.As coagulation temperature rised up,the coagulation rate and mass transfer rate difference△k increased quickly.The coagulation mechanics during wet-spinning can be described by two-phase separation,namely a cellulose-rich phase in the coagulated layer and a cellulose-poor phase in uncoagulated layer.The model experiment was used for testing△k.The results show that coagulation process has a relationship with the concentrate of cellulose solution, coagulation nature,concentrate of coagulation bath,coagulating time and molecular volume of coagulant.The ratio of the congulation thickness to the square root of congulating time(ε/t^1/2) decided the coagulation rate, which changed with the alteration of concentrate of coagulation bath and molecular volume of coagulant.Mass transfering rate difference△k between cellulose solution and coagulation bath increases with the increase of coagulation concentration and prolonged coagulating time.As concentration of cellulose solution increases from 4 wt%to 6 wt%, coagulating rate increases,but△k decreases slightly.Minus stretch ratio and draw ratio have a great impact on the structure and properties of regenerated cellulose fibers in the spinning process.Cellulose solution expanded in the extrusion causes actual minus stretch ratio higher than apparent minus draw ratio.This will damage spinning process and reduce quality of the fibers.When expanded ratio B₀ increases,maximum spinning rate v_Lmax decreases.It can be concluded that the spinnability for cellulose solution is bad.Though varied actual flowing rate(v_f),relaxation time of cellulose solution(τ),Young modulus(Xe) and breaking strength(S^*),it can elevate effectively spinning rate and enhance increase stability in spinning process.As the draw ratio rises,the tensile strength of the prepared fibers increases, while the elongation at break decreases.