Study On Preparation, Structure And Properties Of Regenerated Cellulose Fibers From A Novel Solvent System

Cellulose is the most abundant natural polymer on Earth. Its annual natural "production" is estimated as 100 billions tons per year. Cellulose is also a biodegradable polysaccharide; it is decomposed easily by fungi and soil bacteria. Furthermore, it is a renewable natural resource, reproducing itself through natural cycle by using sunlight, which is an infinite, free and clean source of energy. In the recent years, a strong consumer demand for biodegradable and eco-friendly products has appeared in order to increase health and life quality. On the other hand, oil price increase and its scarcity in the future are accelerating the development of new biodegradable products emerging from the research on biomaterials. The annual natural polysaccharide production is a thousand times more important than synthetic polymer production based on oil resource. Therefore, polysaccharides and especially cellulose are a very appealing alternative for substituting synthetic polymers. Biopolymers produced from the largest chemical reactor, Nature, are one of the most promising materials of tomorrow.However, cellulose cannot be melted or dissolved in ordinary solvents because of strong intra- and intermolecular hydrogen bonds. The major problem of cellulose is that it is degrading before melting. Consequently, to process cellulose and make fibers, films, filler, nonwoven materials or 3D objects, cellulose has to be either dissolved in direct solvents or derivatised and then processed.Although there are a number of approaches to produce regenerated cellulose, such as viscose rayon, cuprammonium cellulose, Lyocell fibers, the market is shrinking due to the environmental and economic feasibility concerns of these methods. Some other processes, such as using organosolvents and ionic liquid can also dissolve cellulose, but the high cost and organic solvent recovery problems hinder their further applications in a large scale. For these reasons, cellulosic materials are regarded as un-moldable materials. Because of the un-moldable identity, wood and cotton fibers are difficult to be refabricated as other thermosetting and thermoplastic polymers. Therefore, fundamental understanding of the cellulose dissolution chemistry in aqueous solution is particularly interested by cellulose chemists. If an effective, economic and environmentally friendly cellulose dissolution method can be developed, a new platform for producing moldable cellulosic intermediate materials will be created providing new opportunities for using cellulosic materials as a renewable and sustainable engineering polymers. Our laboratory has exploited a new solvent- 8 wt% NaOH/6.5 wt% CS(NH2)2/8 wt% CO(NH2)2 aqueous solution, which could completely dissolve the cellulose to obtain transparent cellulose solution when the solvent was precooled to-10℃. The novel solvent system has many advantages:the solvent is low toxicity and low cost; untreated cellulose was dissolved directly; the novel solvent is a direct solvent, and no derivative was produced during dissolution and regeneration; no obvious degradation of cellulose occurs; cellulose can be dissolved rapidly; the technology is very simple, and it only takes 2-3 hours to prepare cellulose fibers; the regnerated cellulose fibers and films can be directly produced on the viscose machines after some minor revision. In addition, the new solvent is more powerful in dissolving cellulose, and can be used to prepare more stable spinning solutions containing higher concentrations of cellulose than other solvent of NaOH systems. The novel technology is promising to replace viscose technology.This work has two different goals, the first is the fundamental understanding of the behaviour of the mixing of cellulose in NaOH complex aqueous solutions, and the second is to apply this knowledge to the manufacturing of membranes and fibers. Our focus in this study is in the dissolution process of cellulose in this solvent at low temperatures and possible mechanism that is involved in low temperature dissolution of cellulose in this NaOH complex system which is still far from being completely understood, the improvement of its solubility, the rheological properties and gelation phenomenon, the kinetics of cellulose regeneration from this solvent and finally the application of this novel NaOH solvent system (preparation of cellulose fiber and membrane).The dissertation is divided into seven chapters accordingly.The first chapter deals with the literature review on cellulose, its processing and the preparation of regenerated cellulose materials. First the current situation of the usage of the cellulose resource, the restraining factor of cellulose large-scale industrial use. Then the chemical structure of cellulose, its organisation in microfibrils and its crystalline form are presented. Finally the cellulose processing, including dissolving and regenerating cellulose from different solvents. The last part illustrates the goal of this thesis and the issues to be solved.The second chapter presents the preparation of cellulose- NaOH complex solutions. Dissolution of a number of cellulose samples in these aqueous solvent was investigated with respect to the influence of solvent environment (weight ratio of NaOH), precooled temperature, dissolving method and stirring rate. The process of direct dissolution of cellulose was observed using a polarized light microscope (PLM) because of the high crystallinity of cotton linters. A procedure for dissolving cellulose was developed and optimized, the cellulose solubility was enhanced by adopting the two steps dissolution method. The activation energy of dissolution (E.s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The overall process of cellulose dissolution was exothermic and was favored by lower temperature (especially under-10℃). The swelling and dissolution process of varied cellulose in NaOH aqueous solution systems (NaOH/H2O, NaOH /CS(NH2)2/H2O, NaOH/CO(NH2)2/H2O and NaOH/CS(NH2)2/CO(NH2)2/H2O) as well as the dependence of swelling and dissolution behavior of cellulose fibers on the quality of the solvent were investigated detailedly. The extraction of pure cellulose from rice straw and the complete dissolution of the result cellulose in the new solvent was also studied for the first time. As the result, the NaOH/CS(NH2)2/CO(NH2)2/H2O was proved to be more powerful than NaOH/CS(NH2)2/H2O and NaOH/CO(NH2)2/H2O and the novel solvent does not degrade cellulose even after storage times of up to 1 month.The third chapter presents the mechanism of cellulose dissolving in NaOH complex systems. 13C-NMR and Solid-state 13C-NMR spectrum proved that NaOH, CS(NH2)2, and CO(NH2)2 were bound to cellulose molecules, which brought cellulose molecules into aqueous solution to a certain extent and prevented cellulose macromolecules from associating. The result from scanning electron microscopy (SEM), transmission electron microscope(TEM), differential scanning calorimeter at low temperature (DSC), surface area and porosity analyzer (B.E.T) and wide-angle X-ray diffraction (WAXD) experiments explained the mechanism of cellulose dissolving in NaOH/CS(NH2)2/CO(NH2)2 aqueous solution for the first time, it revealed that the Na-cellulose complex and the hydration of alkali ions formation are the key factors to the dissolving mechanism. The role of CO(NH2)2 and CS(NH2)2 are believed the donor and acceptor of hydrogenbonding when hydrated, which connect to the hydroxyl groups in cellulose and prevent the regeneration of cellulose through the inter- and intra-chains association. The synergic interactions between the solvent compositions play a key role to dissolve the cellulose aggregates. The relatively strong intermolecular interaction between cellulose, NaOH, CS(NH2)2 and CO(NH2)2 occurred, and the NaOH was bound to cellulose directly and the CO(NH2)2 and CS(NH2)2 acted as a layer outside that prevents the association of cellulose macromolecules and lead to the dispersion of cellulose chains in NaOH/CS(NH2)2/CO(NH2)2 aqueous solution. While the aggregation of the molecular in solvent occurred under ambient temperature would cause the decrease of dissolving power.The fouth chapter presents the rheological properties of cellulose- NaOH complex solutions. A brief review of literature describes the structure and the properties of cellulose-NaOH aqueous solution and its unique gelation properties under high and low temperature. Then the experimental results obtained are presented and discussed. The steady state flow of the cellulose-NaOH-water solution under shear stress is examined, especially at high shear rate (according to actual spinning process), were investigated systematically, including the dependence of the zero shear viscosity, activation energy, non-Newtonian Index and structural viscosity index on cellulose concentration and temperature. On the basis of data from the steady-shear flow test, two critical overlap (C and C**) of the cellulose solution were determined, respectively, to be 2.1 wt%, and 5.0 wt%. The critical concentration C* and C** delimitate three different states of the cellulose solution:partly entanglement semidiluted solution, semidiluted tangled network and concentrated solutions (network with associates and regions of entanglement). Experimental conditions like temperature, time and content of cellulose on gelation of solutions was investigated. The rheological experiments help to determine how to get suitable cellulose spinning dope with best flow ablility and viscosity stability. The results are favorable for predicting the spinnability of the cellulose-NaOH complex solution spinning solution and are particularly important before using this effective solvent system for the spinning process.Chapter fifth, is devoted to the kinetics of cellulose regeneration from cellulose-NaOH complex gels immersed in a varied nonsolvent bath. The influence of cellulose concentration, of the nature of the non solvent bath and its concentration, temperature is examined. By building a spinning model, the influence of precipitant compositions and concentration, coagulation time and temperature, and cellulose concentrations on coagulation rate has been demonstrated by examining the thickness and surface morphology of the coagulated layer, which is important for understanding and controlling the process of cellulose shaping from aqueous NaOH/CS(NH2)2/CO(NH2)2 solutions. The kinetics of diffusion-controlled chemical reactions is regarded as the mechanism of the coagulation process. The coagulation process includes diffusion on the contact surface of the polymer solution with the precipitant and of the chemical reaction between the acid and the alkali.Chapter sixth is dedicated to the preparation of novel regenerated cellulose fiber from the novel solvent system. The optimal coagulation conditions were determined for the spinning of cellulose fibers. The effects of coagulation conditions on structure formation and tensile properties of the regenerated cellulose fibers were investigated by tensile tester, wide-angle X-ray diffraction (WAXD), Fourier Transform Infrared Spectroscopy (FT1R), polarized light microscope (PLM) and scanning electron microscope (SEM). The results show that the tensile properties of the regenerated fibers change with the coagulation concentration, bath temperature, coagulation time, as well as the jet stretch ratio and drawing ration. Cellulose fibers prepared under optimal condition showed excellent mechanical properties compared to commercialized cellulose fibers such as Lyocell and rayon fibers. The prepared novel fibers have a typical structure of celluloseⅡ, a circular cross-sections and homogeneous morphological structure. The aggregation structure, morphostructure and processing relationship of regenerated cellulose fibers at different wet spinning processes were also studied. The crystallinity, crystal size and orientation for the regenerated cellulose fibers changed under different spinning conditions. In addition, this work provides a valuable coagulation conditions data (drawing the as-spun fiber in the coagulation bath at certain time), 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.Chapter seventh focuses on the preparation of porous cellulose materials via pre-gelation approach. By taking advantage of this unique gelation phenomenon of cellulose solution, the cellulose hydrated membranes in NaOH/CS(NH2)2/CO(NH2)2 aqueous solvent system were prepared successfully via pre-gelation processing.The morphology and structure of the hydrated membranes through general solution casting and pre-gelation approaches were studied by scanning electron microscopy (SEM) and wide-angle X-ray diffractometry (WAXD), differential scanning calorimeter (DSC) and Fourier Transform Infrared Spectroscopy (FTIR). The kinetics of the formation of the cellulose gel membranes under low and high temperature and their resulting membrane was proposed for the first time. The results indicate that the general solution casted membranes were packed densely while the pre-gel membranes possessed a mesh structure because of random junction of the intra- and inter-molecules when solution was gel. Both membranes showed typical celluloseⅡcrystallinity and exhibited similar properties. These membranes can be utilized for membrane separation technologies such as dialysis and ultrafiltration and we expect that the method developed in this study could serve as a controllable way to fabricate such cellulose hydrated membranes with controlled structure. The porous cellulose sponge was also prepared successfully through this pre-gelation method.In a summary, we have gained new insight into the dissolution process, rheological properties, dissolving mechanism of cellulose in NaOH/CS(NH2)2/CO(NH2)2 aqueous solution. Moreover, coagulation conditions, structure and physical properties of regenerated cellulose membranes and fibers prepared in this new solvent system. It not only had academics significance and prospect of exploration application, but also provided imports and application of the cellulosic products. It may offer an alternative route to replace more hazardous existing methods for the production of regenerated cellulose fibers.