Study On Lyocell Fiber From A Cheap Pulp With High Hemicellulose Content

Lyocell fiber has attracted great attention in the textile field since it was developed in the mid-1970s, because Lyocell fiber technology is a relatively simple and environmentally friendly way to produce regenerated cellulose fiber compared to the conventional viscose process. However, the development of Lyocell fiber is relatively slow due to its high price. Therefore, it is crucial to reduce the cost of Lyocell process for the further development.It is reported that the pulp material containing more than 90% alpha-cellulose is called high alpha-cellulose pulp and the pulp material containing around 20% hemicellulose is called high hemicellulose pulp. In the conventional viscose process, only high alpha-cellulose pulp can be used as raw material in order to obtain a high yield of viscose fiber. Up to now, Lyocell fiber process still follows the conventional viscose process using this high alpha-cellulose pulp material. However, the cost of high alpha-cellulose pulp is higher than that of high hemicellulose pulp. It is significant in industry if a cheap pulp containing high hemicellulose can be used to produce Lyocell fiber since using a cheap pulp can reduce the cost in certain degree. The aim of this paper is to investigate the feasibility of using the cheap pulp with high hemicellulose content to produce Lyocell fiber.Firstly, the yield of Lyocell fiber from high hemicellulose pulp and high alpha-cellulose pulp was measured. It was found that the yield of them was approximately equal. That means the yield of Lyocell fiber is not dependent on the alpha-cellulose content in pulp material.Secondly, the dissolvability, spinnablility and stablity of high hemicellulose pulp in N-methlymorpholine-N-oxide (NMMO) aqueous solution were studied. The results showed that the dissolvability and spinnablity of high hemicellulose dope were better than those of high alpha-cellulose dope. Although the stablity of high hemicellulose dope decreased slightly, it can be solved by using more hydrogen peroxide solution (H₂O₂) to purify and recycle NMMO.Thirdly, the molecular weight (MW) and molecular weight distribution (MWD) of high hemicellulose pulp was measured since it was important for Lyocell fiber process to choose appropriate pulp material. The gel permeation chromatograghy (GPC) method has been widely used and accepted for measuring the MW and MWD for polymers. However, the method has its limitations. The key of this method is to find the suitable solvents to dissolve the polymer well. But cellulose can not be dissolved in most of the organic solvents because of inter- and intro- hydrogen bonding of the cellulose chains. Lithium chloride / N, N-dimethlylacetamide (LiCl/DMAc) can dissolve cellulose, but the dissolution process is very complicated, which includes pre-activation, solvent exchange, swelling and dissolution. Furthermore, more attention must be paid to each step, and cellulose has to be dissolved for 5-10 days according to the type of cellulose pulp. Therefore, it is necessary to develop a new way to measure MW and MWD. In this work, rheology method was selected to measure MW and MWD of cellulose.Moreover, during the prediction of MW and MWD of cellulose using rheology method, the influence of cellulose concentration in NMMO·H₂O solution on the measured result was investigated. It is shown that the results of the rheology method and the GPC method are comparable. It is also found the concentration of cellulose in NMMO·H₂O solution affected the calculated results when rheology method was applied. But there is no significant influence of cellulose concentration on the calculated data when the cellulose concentration is high enough. That is to say, when rheology method was applied to prediction of MW and MWD of cellulose, high concentration of cellulose in NMMO·H₂O solution has to be used in order to get a reliable and stable data. In addition, the rheology method is simple and fast. Therefore it is a useful and easy way to analyze the MW and MWD of cellulose in the fiber industry.Fourthly, the structure and properties of Lyocell fibers from high hemicellulose pulp and high alpha-cellulose pulp were compared in order to investigate the influence of hemicellulose on the quality of Lyocell fiber.It was found that the mechanical properties of high hemicellulose Lyocell fiber would decrease slightly, but a higher hemicellulose concertration spinning solution could be processed, which would increase the mechanical properties and the efficiency of the fiber process. Meanwhile, the fibrillation resistance and dyeing properties of high hemicellulose Lyocell fiber could also be improved.In addition, compared to high alpha-cellulose Lyocell fiber, high hemicellulose Lyocell fiber has slightly higher orientation, lower crystallinity, smaller fibril aggregation size and more stable structure.Those findings showed that the properties of high hemicellulose Lyocell fiber could be comparable to those of high alpha-cellulose Lyocell fiber. Therefore, it is feasible that this cheap pulp containing high hemicellulose could be used to produce Lyocell fiber.Here, it is well known that Lyocell fiber technology is a close-cycled processing. Cellulose pulp is directly dissolved in NMMO·H₂O solution to form spinning solution. After that, filament (ejected spinning solution) is precipitated from coagulation bath and regenerated into Lyocell fiber. However, during industrial production, some oligosaccharides would be dissolved and accumulated in coagulation bath in this process. It is possible that this phenomenon would influence formation of fiber and affect the properties of obtained fiber and the process of solvent recovery. Therefore, these problems were also investigated in the paper.The research results showed the monosaccharide composition of Lyocell fibers and their raw material pulps was approximately equal. A sugar mixture was made according to the monosaccharide composition of Lyocell fiber and the method of measuring the total sugar mixture content in coagulation bath was established. It was found that the saturation point of sugar mixture dissolved in coagulation bath (9%, w/w, NMMO aqueous solution) at 0°C was about 32%. Besides, the influence of content of oligosaccharides in coagulation bath on the fiber formation and properties was investigated. Further work is being carried out to study on the influence of accumulated sugar in coagulation bath on solvent recovery.