Studies On Coagulation Conditions Of Cellulose Membrane And Fiber In NaOH/Urea Aqueous Solution

With the decreasing of the reserved petroleum and the pollution caused by the oil-based products, it is urgent and promising to develop the natural polymer as supplement for the non-degraded synthetic polymers. This novel polymer material system will be one of the important items of polymer in the new century. Cellulose is the most abundant natural polymer, with a huge annual production of 200 billion ton. It has a unique reaction and molecular characters, such as non-toxicity, safety, biodegradability, biocompatibility, hydrophilicity, chemical stability, higher mechanical strength, recoverability, high reproducibility and low cost. Nowadays, the cellulosic products based on the cellulose materials such as cellulose membranes and fibers have been widely developed a series of industry applications. Usually, each application demands specific requirements on the structure and physical properties of the yielding cellulosic products, which strongly depend on the coagulant composition and coagulation conditions during the coagulation/regeneration process. Our laboratory has exploited a new solvent -7.5 wt% NaOH/ 11 wt% urea aqueous solution, which could completely dissolve the cellulose to obtain transparent cellulose solution when the solvent was precooled to -10 °C. The main contents of this thesis include the preparation of the regenerated cellulose membranes and fibers by wet shaping method and investigation the effects of coagulation conditions on their structure, morphology and physical properties.The creative points of this work are as follows. (1) A series of regenerated cellulose films are prepared from cellulose in 7.5 wt% NaOH/ 11 wt% urea aqueous solution. The uniform design method was used successfully to evaluate the optimal coagulant and coagulation conditions of RC films; (2) The effects of coagulants on the structure, water permeation and mean pore size of the porous membranes were clarified; (3) The effects of coagulation conditions and wet spinning method on the cross-section shape, structure and physical properties of novel fibers were investigated systematically.The main contents and conclusions in this thesis are divided into the following parts. Effects of coagulant components and coagulation conditions on structure and properties of regenerated cellulose (RC) films prepared from cellulose in 7.5 wt% NaOH/11 wt% urea aqueous solution were investigated by ~13C NMR, X-ray diffraction, scanning electron micrograph, tensile testing, etc. The uniform design method based on theoretic accomplishments in number-theoretic method was used to optimize the coagulationconditions of H2SO4 aqueous solution with various concentrations (from 1 to 15 wt%), time (from 1 to 15 min) and temperature (from 25 to 55 °C). Moreover, a series of RC films coagulated, with 5 wt% H2SO4/Na2SO4, Na2SO4, HOAc and (NH4)2SO4 respectively under different concentration (from 1 to 20 wt%) and time (from 1 to 20 min) at 25 °C were also investigated. The results indicated that the optimal coagulant concentration and coagulation time of the RC films are 5 wt% H2SO4 for 5 min, 5 wt% H2SO4/5 wt% Na2SO4 for 5 min, 5 wt% Na2SO4 for 15 min, 3 wt% HOAc for 5 min and 5 wt% (NH4)2SO4 for 3 min at 25 °C. The films coagulated at relatively low temperature possessed better mechanical properties than that of relatively high temperature. The values of (7b and Sb of the RC films are better than those of commercially available cellophane in the wide range of coagulant and coagulation conditions. The optimal coagulant is 5 wt% H2SO4/Na2SO4 aqueous solution, and the coagulated films possessed more homogeneous structure and relatively better structure and properties on the whole. All of the RC films prepared from various coagulants exhibited good light transmittance. The results from XRD, SEM, optical transmittance and tensile testing indicated the counter-diffusion between solvent in the cellulose solution and coagulant determined the structure and properties of the RC films. The coagulation mechanism can be described as a two-phase separation, namely cellulose-rich phase in gel and cellulose-poor phase in solution.A series of regenerated cellulose membranes was prepared from cellulose in NaOH/urea aqueous solution by coagulating with various coagulants including H2SO4, HOAc, H2SO4/Na2SO4, Na2SO4, (NH4)2SO4, H2O, C2H5OH and (CH3)2CO, respectively. The morphology, water permeability and physical properties of the membranes were investigated by scanning electron micrograph (SEM), atom force micrograph (AFM), flow rate method, tensile test, etc. All of the RC membranes prepared by coagulating with various coagulants showed the homogeneous porous structure in the free surface. The membranes coagulated from acidic aqueous solution displayed the relatively small pore size and narrow distribution than those of membranes coagulated from pure water and salt, whereas those coagulated with the organic coagulation bath possessed the large pore size and wide distribution. It was noted that the membranes in the cross-section exhibited the slightly smaller pore size and narrower distribution than that of the free surface, indicating a homogeneous porous structure and relatively high content of open pores in the cellulose matrix. The membranes prepared from acidic and pure salt aqueous solution showed the better physical properties, however, the surface defection, inhomogeneous structure and relatively large pore size of the RC-HO, RC-AC and RC-ET membranes resulted in adecrease of tensile strength and optical transparency. The mean pore sizes (2rf) of the membranes determined by the flow method were obviously smaller than 2rc and 2ra from SEM and AFM. Interestingly, the mean pore sizes and water permeability of the membranes were much larger than those of membranes from cuoxam and viscose, providing more wide application for separation technique filed. The membrane formation process was mainly related to the diffusion between nonsolvent and solvent, which lead to the two-phase separation of the cellulose rich-phase in gel and lean-phase in solution. This work provides promising potential way to prepare different pore size and distribution and physical properties porous membrane materials by choosing the proper coagulants.Effects of coagulation conditions and wet spinning method on structure and properties of regenerated cellulose (RC) fibers prepared from cellulose in 7.5 wt% NaOH/11 wt% urea aqueous solution on the pilot scale spinning machine were investigated by tensile testing, optical microscopy, scanning electron micrograph and X-ray diffraction. H2SO4/Na2SO4 aqueous solution was adopted as the first coagulation bath and H2SO4 aqueous solution as the second coagulation bath on the basis of the laboratory scale wet spinning experiment. The fibers wet spun from the two-stage coagulation obviously show the better mechanical properties than that from the one-stage coagulation, which lie in the range of 1.15-2.00 cN/dtex for tensile strength and 9.2-13.4 % for elongation at break, respectively. The optimal coagulation conditions for two-stage coagulation are 10 wt.-%H2SO4/15 wt.-%Na2SO4 for the first coagulation bath and 5 wt.-%H2SO4 for the second bath or 5 wt.-%H2SO4/15 wt.-%Na2SO4 for the first coagulation bath and 10 wt.-%H2SO4 for the second bath. Moreover, the lower temperature or longer coagulation time is more favorable for fibers with better mechanical properties compared with that of relatively high temperature or less coagulation time. The results from optical microscopy and SEM indicated that the novel fibers shows the smooth surface and circular cross-sectional shapes in the wide range of the coagulation conditions and different wet spinning method, similar to the natural silk, which is considerable different from the viscose rayon with lobulate cross-sections. Therefore, this work provides the valuable coagulation conditions data, and suggested the improvement of the pilot scale spinning machine and coagulation conditions could further increase the tensile strength of the wet-spinning novel fibers.The results of this foundation research mentioned above revealed the coagulation conditions, structure and physical properties of regenerated cellulose membranes and fibers prepared in NaOH/urea aqueous solution. It not only had academic significance andprospect of application, but also provided important scientific data for the industrial exploration and application of the cellulosic products.