Wet spinning
One of the main spinning methods of chemical fiber, referred to as wet spinning. Wet spinning includes the steps of: (1) preparing a spinning dope; (2) extruding the raw solution from the spinning orifice to form a fine stream; (3) solidifying the fine stream into a nascent fiber; (4) nascent fiber wrapping or directly Post-processing.
The fiber-forming high polymer is dissolved in a suitable solvent to obtain a solution having a certain composition, a certain viscosity, and good spinnability, and is called a spinning dope. The spinning dope can also be obtained directly from homogeneous solution polymerization. The polymer swells before dissolution, that is, the solvent first penetrates into the interior of the polymer, so that the distance between the macromolecules increases, and then dissolves to form a uniform solution. The entire process takes a long time and the speed of the swelling process has an important effect on the dissolution rate. Before the spinning, the polymer solution must be subjected to a pre-spinning preparation process such as mixing, filtration and defoaming, so that the properties of the spinning dope are uniform, the gel pieces and impurities entrained therein are removed, and the bubbles in the liquid are removed. . In the production of viscose fibers, the pre-spinning preparation also includes a ripening process to impart the necessary spinnability to the viscose.
The spinning dope is sent to the spinning machine by a circulating pipe, metered by a metering pump, and then passed through a candle filter and a connecting pipe to enter the spinneret (cap). The spinneret is typically made of an alloy of gold and platinum or a tantalum alloy. A plurality of perforations are regularly distributed on the spinneret with a pore diameter of 0.05 to 0.08 mm. The fine liquid flowing out from the orifice of the spinning hole enters the coagulation bath, and the solvent in the fine flow of the raw liquid diffuses into the coagulation bath, and the coagulant permeates into the fine flow, so that the fine flow of the raw liquid reaches a critical concentration, and precipitates in the coagulation bath to form a fiber. . Diffusion and solidification in wet spinning are physicochemical processes, but some chemical fibers undergo chemical changes simultaneously during the wet spinning process. For example, viscose fiber, cellulose yellow acid sodium is decomposed into cellulose regenerated fibers.
The wet spinning speed (referred to as the first godet speed of the virgin fiber) is limited by the double diffusion speed of the solvent and the coagulant and the fluid resistance of the coagulation bath, and thus is much lower than the melt spinning speed. The ratio of the linear velocity of the first godet to the extrusion speed of the spinning dope is referred to as the spinneret draw ratio. The wet spinning draw ratio is generally negative, zero, or a small positive value in order to improve the stability of the forming process.
The spinning dope is an elastic body that is both viscous and elastic. When the raw liquid is pressed out from the spinning hole, there is a hole swelling effect (Barnes effect), so that the diameter of the extrusion fine flow is larger than the diameter of the spinning hole. In the wet spinning process, the expansion ratio is generally 1-2.
Under the tensile force of the first godet, the extrusion stream gradually tapers after crossing the maximum diameter, and the refining process continues until the stock solution is completely solidified. The change in the diameter of the fine stream in the wet spinning is not only the result of the tensile deformation, but also related to the mass transfer process. A section of the spinning process from the spinneret to the solidification point is the fiber forming zone, which is a critical area for the formation of the fibrous structure.
Wet-spun virgin fibers are in a swollen state due to the large amount of liquid contained therein. The macromolecules have great mobility and low degree of orientation, and their morphological structure is closely related to the spinning process conditions. By selecting and controlling the spinning process conditions, fibers of different cross-sectional shapes or special capillary structure and special properties can be produced. Wet spinning has a variety of different forming methods, and the spinning machine also has a variety of different structures. For example, there are single bath method or double bath method, there are deep bath method or shallow bath method, funnel forming or tube forming.
The winding device of the wet filament spinning machine has a centrifugal tank type or a bobbin type. When the short fibers are spun, a spinning after-treatment machine is usually used, and the nascent fibers after forming the respective spinning portions are bundled into a bundle and continuously subjected to post-treatment.
Wet spinning not only requires a wide variety of bulk liquid preparation and pre-spinning preparation equipment, but also has coagulation bath, circulation and recycling equipment. The process is complicated, the plant construction and equipment investment costs are large, the spinning speed is low, so the cost Higher. When manufacturing segmented fibers, tens of thousands of holes of spinnerets or cartridge spinnerets can be used to increase productivity. Generally, only synthetic fibers which cannot be melted by melt, such as polyacrylonitrile fibers and polyvinyl alcohol fibers, are suitable for wet spinning of staple fibers and filament bundles with a polymer solution.
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