Viscose fiber is the main kind of regenerated cellulose fiber, which is made by extracting pure cellulose from cellulose raw materials (such as cotton linter, wood, reed, bagasse, etc.) that can not be directly woven, preparing a viscous spinning solution after being treated with caustic soda and carbon disulfide, and then wet spinning.
1. Morphological structure of viscose fiber
Under the microscope, the viscose fiber is a straight cylinder in the longitudinal direction, and its cross section is irregular and serrated. The cross-sectional structure of viscose fiber is uneven, which consists of an outer layer (cortex) and an inner layer (core). The crystallinity and orientation of the cortex are high, and the structural compactness is higher than that of the core. The crystallinity and orientation of the core layer are low, and the structure is loose.
In the production process, viscose fiber has been washed, impurity removed and bleached, and natural pigments, ash, grease and waxy substances have been removed. It is a relatively pure fiber with much lower impurity content than natural cellulose fiber.
2. Chemical structure and supramolecular structure of viscose fiber
The chemical composition of viscose fiber is the same as that of cotton fiber, and the complete hydrolysate is β-D- glucose. However, the polymerization degree of viscose fiber is much lower than that of cotton. The polymerization degree of cotton is thousands or even tens of thousands, while that of ordinary viscose fiber is only 300,400, and that of viscose fiber with high wet modulus, such as "Fuqiang fiber", is 500,600. Viscose fiber macromolecules expose more hydroxyl and aldehyde groups than cotton fibers, and have high hygroscopicity, and the standard moisture regain reaches 12%.
From the supramolecular structure, viscose fiber is also a partially crystalline polymer, but amorphous region is higher than cotton, with a lower crystallinity of 30%-40% and coarse grain size. The orientation degree of viscose fiber is also low, but it can be improved with the increase of drawing degree in production. Under the condition of low drawing, the orientation degree is 0.54; under the condition of high drawing, the orientation degree can reach 0.88. With a certain degree of polymerization, the higher the degree of orientation, the higher the fiber strength.
3. Performance of viscose fiber
Compared with natural cellulose fibers such as cotton and hemp, viscose fibers have great differences in properties because of their different polymerization degree, aggregated structure (supramolecular structure) and morphological structure.
The wet strength of ordinary viscose fiber is only about half of the dry strength. This is because viscose fiber has a low degree of polymerization and orientation and a large amorphous area. After water molecules enter amorphous region, the intermolecular force is further weakened, causing molecular chains to slip and break easily. Therefore, low tension or loose processing should be adopted in dyeing and finishing.
Like other cellulose fibers, viscose fibers are sensitive to acids and oxidants. However, viscose fiber has loose structure, low degree of polymerization, crystallinity and orientation, more voids and internal surface area, and more exposed hydroxyl groups than cotton, so its chemical activity and sensitivity to acids and oxidants are greater than cotton. The stability of viscose fiber to alkali is much worse than that of cotton and mercerized cotton. Under the action of concentrated caustic soda, viscose fiber will swell and even dissolve violently, which will make the fiber lose weight and reduce its mechanical properties. Therefore, concentrated alkali should be used as little as possible in dyeing and finishing.
Because viscose fiber has more amorphous region and looser supramolecular structure than cotton and mercerized cotton, it has high hygroscopicity, and its adsorption capacity for dyes and chemical reagents is greater than that of cotton and mercerized cotton, and its adsorption capacity is as follows: viscose fiber >; Mercerized cotton >; Cotton.
The dyeing performance of viscose fiber is similar to that of cotton. Although viscose fiber absorbs more dyes than cotton, it has a skin-core structure and a compact cortex structure, which will hinder the absorption and diffusion of dyes. The core structure is loose and the adsorption capacity of dyes is high. Therefore, when dyed at low temperature for a short time, the color of viscose fiber is lighter than cotton, and it is easy to cause uneven dyeing. When dyed at high temperature for a long time, the color is deeper than cotton.
Second, high wet modulus viscose fiber
Ordinary viscose fiber swells violently in wet state, the breaking strength is obviously reduced, the wet modulus is very small, and it has a large elongation under a small load. When the fabric is washed, it is easy to deform under the action of rubbing force, and it shrinks violently after drying, and its size is very unstable. Moreover, the alkali resistance is poor, and the blended fabric with cotton can not be mercerized. Wet processing must be loose. If it is carried out under tension, the fabric will stretch greatly.
In order to overcome the above shortcomings of ordinary viscose fibers, people have developed viscose fibers with high wet modulus, which have high strength, low elongation, low expansion and high wet modulus, and are called the second generation viscose fibers.
The varieties of high wet modulus viscose fiber mainly include Fuqiang fiber and Modal fiber, and their main properties are shown in Table 1
Table 1 Comparison of properties of cellulose fibers taken.
1. Fuqiang fiber
Fuqiang fiber is made of high-quality slurry raw materials, and keeps the fibril structure in natural fibers as much as possible. It is made by fully stretching during spinning, and has the characteristics of high dry and warm strength, low elongation, high mixed modulus and good stability to alkali.
The polymerization degree of Fuqiang fiber is generally 500~600, which is higher than that of ordinary viscose fiber. The product degree and orientation degree are the highest among the existing viscose fiber varieties, and the crystal grains are also the largest, with high crystallinity and orientation. The fiber structure is compact, the intermolecular force is large, and the dry and wet strength, transverse swelling degree, elastic modulus and luster of the fiber are also high, but the elongation at break, longitudinal shadow degree, dyeing performance and hook strength will be reduced.
The cross-section of Fuqiang fiber is different from that of ordinary viscose fiber, and it is a smooth round or nearly round all-core structure. Fuqiang fiber, similar to cotton fiber, has fibrillar structure arranged at a certain angle with the fiber axis, but ordinary viscose fiber does not have this special structure, so Fuqiang fiber has "fibrillation phenomenon", which is easy to make the fiber hairiness and reduce the wear resistance and dyeing brightness.
The breaking strength of Fuqiang fiber in dry state is much higher than that of ordinary viscose fiber, and better than that of cotton fiber. The loss of wet breaking strength is less than 30%. Because Fuqiang fiber has higher dry and wet breaking strength and higher wet modulus and lower dry and wet elongation, the fabric has better dimensional stability, wrinkle resistance, less deformation after washing, and the dyeing performance of Fuqiang fiber is similar to that of ordinary viscose fiber.
Fuqiang fiber has high stability to alkali solution, and its solubility is 9% in NaOH solution of10% at 20℃, while that of ordinary viscose fiber is as high as 50%. When treated with NaOH solution with concentration of 5%, Fuqiang fiber can almost maintain its original strength and has little deformation. Because Fuqiang fiber has high stability to alkali liquor, its blended fabric with cotton can be mercerized.
Modal fiber is a new generation of cellulose fiber produced by Lenzing Company in Austria, which is improved on the basis of Fuqiang fiber. Its basic structure is similar to Fuqiang fiber, and its spinning process has less environmental pollution than Fuqiang fiber and ordinary viscose fiber. Modal fiber has better thousand and wet strength, wet modulus and shrinkage than ordinary viscose fiber, and its dry and wet strength is 25-30% higher than that of ordinary viscose fiber. In wet state, it has low swelling degree and is soft as cotton fiber. However, Modal fiber products have poor crease resistance, so the finished products need resin crease-resistant finishing.
Iii. Lyoce 1 1 fiber
L vocal is a new type of regenerated cellulose fiber formed by directly dissolving cellulose slurry in organic solvent N- methylmorpholine -N oxide (NMMO). Its raw materials are beech, eucalyptus or coniferous wood pulp, and the recovery rate of organic solvent NMMO is over 99%. The production process is pollution-free. The trade name of Lyocell fiber produced by Acordis Company imported from China is "Tencel".
Lyocell dimension has excellent properties, including the natural comfort of cotton fiber, the drape and bright color of viscose fiber, the high strength of synthetic fiber, and the soft feel and elegant luster like silk.
Lyocell fiber includes filament and short fiber, and short fiber can be divided into ordinary type (uncrosslinked type) and cross-linked type, the former is like Lyocell, and the latter is like Lyocell A 100.
Ordinary Lyocell fiber has obvious fibrillation phenomenon. By using the easy fibrillation property of ordinary Lyocell fiber, the fabric can be processed into peach skin style, but it must be processed into smooth style through multiple dyeing and finishing processes to meet the requirements. The cross-linked Lyocell fiber needs much less dyeing and finishing procedures when it is processed into a smooth style, and it is not easy to pilling during wearing.
The chemical structure of Lyocell fiber is the same as that of cotton and hemp, and the polymerization degree is generally 500 550, which is higher than that of ordinary viscose fiber (250 300), and the molecular weight distribution is also more concentrated than that of viscose fiber. Crosslinked Lyocell fiber has a certain amount of cross-linking between macromolecules besides the macromolecular chain composed of β-D- glucose residues.
The cross-sectional shape of Lyocell fiber is different from that of ordinary viscose fiber and cotton, which is oval or nearly round, with smooth surface and curly appearance. Lyocell fiber has a certain degree of sheath-core structure.
Lyocell fiber has high dry and wet strength, high initial modulus, low shrinkage in water, good dimensional stability, high moisture absorption and swelling, and outstanding fibrillation characteristics.
(1) Physical and mechanical properties Lyocell fiber's dry and wet strength is obviously higher than that of cotton and other regenerated cellulose fibers. After moisture absorption, its strength is reduced, but it can still maintain 80% of its dry strength, which is much higher than that of other regenerated cellulose fibers. Therefore, Lyocell fiber can undergo severe mechanical treatment and water treatment during wet processing without damaging the fabric quality. Lyocell fiber can still maintain a high modulus in wet state, which can ensure that the fiber can be processed under wet or wet conditions.
The swelling ratio of Lyocell fiber in water is about 1.4 times, which increases the contact area between fibers, increases the surface friction resistance, and makes it difficult for fibers to move relative to each other, resulting in tight and stiff fabric structure after meeting water, which is easy to cause wrinkles and scratches during wet processing, and a large number of hairiness is generated due to the friction between fabrics or between fabrics and machinery. High transverse swelling ratio will bring great harm to wet processing of fabrics.
(2) Fibrillation characteristics Fibrillation means that the fibers peel off finer microfibers layer by layer along the axial direction, which is a unique structural feature of fibers with fibrillar structure. Different fibers have different degrees of fibrillation due to different chemical structures and aggregate structures, and the degree of fibrillation of Lyocell fiber is much more serious than other regenerated cellulose fibers.
In water, ordinary Lyocell fiber has a much greater swelling degree in the radial direction than in the axial direction, and has a higher wet rigidity. At this time, if the fiber is repeatedly subjected to mechanical friction, the surface of the fiber splits layer by layer along the fiber length direction to obtain finer fine fibers (diameter 1~4μm), one end of which is fixed on the fiber body, and the other end is exposed on the fiber surface, forming many tiny fuzz. In the case of extreme fibrillation, these fibrils will intertwine with each other to form balls.
The fibrillation of Lyocell fiber has both advantages and disadvantages. The advantage is that the fabric can obtain peach skin style by using the fibrillation characteristics of fiber. On the negative side, when Lyocell f-dimensional fabric is wet-treated, the primary fibrillation proceeds quickly, which makes the fabric have a furry appearance. Moreover, the fabric with incomplete fibrillation will bring a lot of troubles to the subsequent dyeing, finishing and even clothing washing. Crosslinked Lyocell fibers (such as Tencel A 100) can prevent fibrillation, and dyeing and finishing can also prevent fibrillation or remove the generated fibrils.
(3) Dyeing properties Lyocell fiber, like cotton and viscose fiber, can be dyed with reactive dyes, direct dyes, sulfide dyes, vat dyes, etc., and reactive dyes are often the main dyes. However, the morphological structure, aggregated structure, physical and mechanical properties, sensitivity to chemicals, fibrillation properties and so on of Lyocell fiber are not exactly the same as those of other cellulose fibers such as cotton, hemp and viscose fiber, so the dyes are different for Lyocell fiber. Generally speaking, under the condition of the same or similar fabric specifications, the dye uptake, fixation rate and dyeing depth of Lyocell yarn or fabric are obviously higher than viscose and cotton fiber, which has been confirmed by the test results and production practice of many dye and printing and dyeing manufacturers.