Dissolution And Spinning Process Of Cellulose-Phosphoric Acid/Poly Phosphoric Acid

With the decrease of petroleum resource,cellulose fibers have attracted much attention because of their abundant and renewable raw materials.In this dissertation, we explored a new manufacturing technique for fabricating high performance cellulose fiber.Cellulose was dissolved in the compound solvent of phosphoric/ polyphosphoric acid and formed liquid crystal solution,and then the cellulose fiber could be fabricated from this liquid crystal solution.The needed mixing time of compound solvent,the clearing point of different cellulose solutions,and the phase transition behavior of the liquid crystal solution were studied.The influence of temperature,cellulose concentration,P₂O₅ concentration in the compound solvent on the dynamic rheological properties of the liquid crystal solution was explored.The effects of spinning temperature,jet stretching ratio and the air gap length on the spinning properties were discussed and the spinning conditions were determined.The structure and properties of resultant cellulose fiber were analyzed in details.The needed mixing time of compound solvent was confirmed to 90min through UV-visible spectrophotometer.The liquid crystal behavior of cellulose-phosphoric/ polyphosphoric acid solutions were observed with the aid of a polarization microscope and the clearing point of the liquid crystal solution was recorded,the clearing point of the liquid crystal solution increases with increasing cellulose concentration and the degree of polymerization of cellulose.The thermal behavior of the liquid solution was analyzed by differential scanning calorimetry(DSC).During heating process,two endothermal peaks appeared on the DSC curves of the solution, one was the melting peak of cellulose crystals in the compound solvent which was induced by supercooling,the other was the phase transition peak of the solution from anisotropic phase to an isotropic one.Both peak temperatures increased with increasing cellulose concentration,while the phase transition enthalpy showed a minimum value,which suggested that there were most anisotropic phase in the liquid crystal solution when the cellulose concentration was 16%.The dynamic rheological experiment of cellulose/ phosphoric acid/ polyphosphoric acid solution was done.The liquid crystal solution represented elastic behavior and low dissipation.As temperature increased,the solution viscosity firstly droped,then level off phenomenon of liquid crystal was presented,after that, viscosity droped because of serious thermal degradation of cellulose- phosphoric/ polyphosphoric acid solution in high temperature;With the increase of cellulose content,isotropic phases turned into anisotropy phases gradually,which resulted the solution viscosity droped to the minimum point then rising with the cellulose content increasing.The viscosity of solution reached to the minimum point when the cellulose content was about 15%;Viscosity of liquid crystal solution droped as the content of P₂O₅ increasing,the most appropriate content of P₂O₅ was 74%;The elastic modulus and viscosity of liquid crystal solution with high degree of polymerization were higher than the one with low degree of polymerization.A spinning process was developed by introducing twin screw extruder into dry-wet spinning technology.The appropriate process conditions and technological parameters were explored and technical parameters were confirmed.The results showed:The mechanical performance of fiber was improved when the content of P₂O₅ or cellulose was increased;Rising solution temperature of spinning could improve the fluidity of solution,and result in larger jet stretching ratio which could improve mechanical performance of fiber;Denier of fiber minished and mechanical performance of fiber was improved when jet stretching ratio was increased;The best air gap length in spinning was about 4.9cm.At last,the surface and section structure,the crystal structure,the tropism structure and the heat-resistant structure of cellulose fiber were analysed by scanning electronic microscope(SEM),X-ray diffraction analysis(XRD),velocity of sound orientation and thermal gravimertic analysis(TGA),as a result:The surface of fiber was smooth,and the section was close-grained,only few small holes was observed;In the process of cellulose dissolving in the mixed acid solvent and making the solution into fiber,the structure of cellulose unit cell changed celluloseâ… into celluloseâ…¡,the crystallity and degree of orientation of cellulose were increased along with the increasing of jet stretching ratio;The thermal weightlessness step appeared at 185～250℃,when thermal decomposition of cellulose occurred acutely.Evaluated dynamics parameters of thermolysis with both Friedman method and Kissinger method.The originality of this study has been reflected from the following perspectives: (1) The needed mixing time of compound solvent was confirmed to 90min through UV-visible spectrophotometer;(2) The special phase transition behavior of cellulose/ phosphoric acid/ polyphosphoric acid liquid crystal solution was presented through DSC and dynamic rheological experiment.