natural cellulose fiber
It is composed of cellulose macromolecules and is derived from fibers obtained from seeds, fruits, stems, leaves and other parts of plants, such as cotton fiber, kapok fiber, flax fiber, ramie fiber, hemp fiber, jute fiber, etc.

regenerated cellulose fiber
Trees and grasses also contain a large amount of cellulose polymers, but they do not exist in the form of long fibers and cannot be used directly as fibers. The chemical structure obtained after a series of chemical and physical processing of these cellulose-rich natural plant materials is still cellulose, but its relative molecular weight, distribution and supramolecular structure have changed. Such fibers are regenerated cellulose fibers.
The starting raw material of regenerated cellulose fiber is cellulose. The cellobiose structural unit in the fiber structure contains a large number of hydrophilic groups - hydroxyl groups. This is the reason why cellulose fibers, including natural cellulose fibers, have good hygroscopicity.

Source of raw materials for regenerated cellulose fiber
Almost all natural plants rich in cellulose can be used as raw materials for regenerated cellulose fibers and semi-synthetic fibers, such as cotton lint, cotton stalk bark, trunks and branches of coniferous and broad-leaved trees, bamboo stems, sugarcane residues, reed stems, hemp, flax, ramie, etc.

Types of regenerated cellulose fibers
Regenerated cellulose fibers include: viscose, Modal®, Lyocell (trade name: Tencel®), hemp straw core viscose, bamboo (pulp) fiber, cellulose acetate fiber, etc.

Ordinary viscose fiber
Ordinary viscose fibers are mostly made of cotton linters, commonly known as "artificial cotton" and "rayon".
The production process is to use chemical refining to remove impurities in the raw materials to make pulp. After alkali treatment, carbon disulfide and alkali cellulose are sulfonated to form a spinning solution. Finally, a wet spinning process is used to squeeze the thin stream of spinning solution ejected from the spinneret into a coagulation bath composed of sulfuric acid/sodium sulfate/zinc sulfate, etc., and regenerate into cellulose to solidify.
The unevenness of the complex molding process will cause the cross-section of conventional viscose fibers to be round or irregular, with holes inside and irregular grooves in the longitudinal direction.

Viscose has excellent hygroscopicity and dyeability, but its modulus and strength are low, especially the wet strength.

modal fiber
Modal fiber is the trade name of high wet modulus viscose fiber. The difference between it and ordinary viscose fiber is that Modal fiber improves the shortcomings of low strength and low modulus of ordinary viscose fiber in the wet state. It also has higher strength and modulus in the wet state, so it is often called high wet modulus viscose fiber. Similar products from different fiber manufacturers also have different names, such as Fuqiang Fiber, Tiger Kapok, and Newal®.

Modal production process
Compared with ordinary viscose fiber, the main differences in its production process are:
(1) Cellulose should have a high average degree of polymerization (about 450);
(2) The prepared spinning solution has a higher concentration;
(3) Prepare appropriate coagulation bath composition (such as increasing the content of zinc sulfate), and reduce the coagulation bath temperature to slow down the molding speed.

Modal fiber properties
The structure of the inner and outer layers of the fiber is relatively uniform. The sheath-core structure of the fiber cross-section is not as obvious as that of ordinary viscose fibers. The cross-sectional shape tends to be round or waist-round, and the longitudinal direction is smoother. It has higher strength and modulus in the wet state, and has excellent hygroscopic properties.

Lyocell fiber
Lyocell fiber was invented by the British Cotauer Company and later produced by the Swiss Lenzing Company. The trade name is Tencel®. The trade name in my country adopts its homophonic "Tencel®".

Lyocell production process
Lyocell uses N-methyl morpholine oxide (NMMO) aqueous solution as the solvent, which can directly dissolve cellulose pulp into the spinning solution, and then uses wet spinning or dry-wet spinning methods to shape the fiber using a certain concentration of NMMO-H2O solution as the coagulation bath, and then the nascent fiber is stretched, washed, oiled, and dried.
Compared with the ordinary viscose fiber production method, the biggest advantage of this spinning method is that NMMO can directly dissolve cellulose pulp, the production process of spinning stock solution can be greatly simplified, and the recovery rate of NMMO can reach 98%, and the production process hardly pollutes the environment.

Lyocell fiber properties
The morphological structure is completely different from ordinary viscose. The cross-sectional structure is uniform and round, and there is no skin or core layer. The longitudinal surface is smooth and has no grooves. It has better mechanical properties than viscose fiber.