The spinning fluid is continuously, quantitatively and evenly extruded from the capillary holes of the spinneret or spinneret with a spinning pump (or metering pump) to form a liquid stream, and then in air, water or The process of solidification into filaments in a coagulation bath is called spinning or fiber forming.
The newly spun filaments are called primary fibers. Spinning is a key process in the production of chemical fibers. Changing the spinning process conditions can adjust the structure of the fiber within a wide range, thereby correspondingly changing the physical and mechanical properties of the resulting fiber.
According to the different properties of fiber-forming polymers, the spinning methods of chemical fibers mainly include melt spinning and melt spinning. In addition, there are Special or unconventional spinning methods. Among them, according to the different coagulation methods, the melt spinning method is divided into two types: wet spinning and dry spinning. In the production of chemical fibers, most of them are produced by melt spinning, followed by wet spinning, and only a small amount are produced by dry or other unconventional spinning methods.
1 Melt spinning method
Also known as melt spinning , referred to as melt spinning. It is a chemical fiber spinning method in which the polymer is heated and melted, extruded through a spinneret, and cooled and solidified in the air to form fibers. Polymers used for melt spinning must be able to melt into a viscous flow state without significant decomposition. Polyester fibers, polyamide fibers and polypropylene fibers can all be produced by melt spinning.
Features:
The main features of the melt spinning method are The spinning speed is high (1000~7000m/min), no solvents, precipitants and their recovery and circulation systems are needed. The equipment is simple and the process is short. It is an economical, convenient and efficient forming method. However, the number of spinneret holes is relatively small.
Process flow:
1. Preparation of spinning melt – continuous polymerization worth melt The award-winning pre-crystallized and dried fiber-forming polymer slices are added from the polymer hopper, and the screw extruder is heated in sections according to the requirements to melt, mix, meter and extrude. After extrusion, the spinning box The elbow between the bodies is fed into the melt metering pump.
2. The melt is quantitatively extruded through the small hole of the spinneret through the metering pump in the spinning box to form a thin stream of melt.
3. After the thin stream of melt enters, it is cooled, solidified and formed into primary fibers at a lower temperature and in a cooling blowing environment.
4. The virgin fiber is then oiled, networked and wound into a cylinder.
5. This winding yarn is then subjected to subsequent drawing-heat setting and other secondary molding processes to obtain a finished fiber with applicability.
Classification:
Melt spinning method According to the melt preparation process, it is divided into direct spinning and slice spinning. The process of directly feeding the polymerized polymer melt into a metering pump for metering, extrusion and spinning becomes the direct spinning method; while the polymer pellets undergo pre-crystallization, drying and other necessary pre-spinning preparations, and then the Matsuri screw The technology of extruder melt spinning is called chip spinning. Direct spinning technology is often used in large-scale industrial production, which is beneficial to reducing production costs. However, it is difficult to produce differentiated fiber varieties, and only slight changes can be made in linear density and fiber cross-sectional shape. The chip spinning method is more flexible, easy to change varieties, and produces small batches of differentiated fibers with high added value.
2 Solution spinning method
Solution spinning is the process of fiber spinning The polymer is dissolved in a certain solvent to prepare a spinning solution with a suitable concentration. The spinning solution is then spit out from tiny holes into a coagulation bath or hot gas, and the polymer is precipitated into solid filaments. After stretching – shaping – washing – drying and waiting for processing, the finished fiber can be obtained.
Obviously, the solution spinning production process is more complicated than melt spinning. However, for some products that have not yet been melted, For polymers that decompose, you can only choose this spinning forming technology. Solution spinning can be divided into wet spinning, dry spinning, and dry and wet spinning.
1 Wet spinning
Wet spinning is referred to as wet spinning Spinning. A chemical fiber spinning method in which a polymer is dissolved in a solvent and a thin stream is ejected through a spinneret into a coagulation bath to form fibers. Fiber-forming polymers suitable for wet spinning have a decomposition temperature lower than the melting point or are easily discolored when heated, and can be dissolved in appropriate solvents. Synthetic fibers such as polyacrylonitrile fiber and polyvinyl alcohol fiber and man-made fiber varieties such as viscose fiber and cupro fiber are produced by wet spinning. Most of the fiber cross-sections obtained by wet spinning are non-circular and have an obvious sheath-core structure, which is mainly caused by the solidification of the coagulation liquid.
Features:
The speed of wet spinning is low, and The number of holes in the spinneret is larger than that in melt spinning, and the process flow is complex, the investment times are large, and the production cost is high. Generally, in the production of short fibers, multi-hole spinnerets or stepped spinneret holes can be used to improve production capacity. Thus making up for the shortcoming of low spinning speed.
Process flow:
1. Prepare the spinning stock solution.
2. Press the stock solution out of the spinneret to form a thin stream.
3. The thin stream of raw liquid solidifies into virgin fibers.
4. The virgin fibers are packaged or directly processed.
2 Dry spinning
Dry spinning is a type of solution spinning. If fiber-forming polymers can be found A solvent with a low boiling point and good solubility is used to make a spinning solution. At this time, the spinning solution can be spit out from the tiny holes and enter the heated gas. The solvent in the spinning solution evaporates and the polymer filaments gradually After coagulation, stretching, shaping, washing, drying and other post-processing processes, the finished fiber can be obtained. Acrylic, spandex, chlorine and vinylon all adopt dry spinning processes.
Features:
Dry spinning has continuous production and spinning speed It has the advantages of high efficiency, large output, and less pollution; the fiber quality, chemical resistance, and dyeing performance are better than wet-spun fibers. However, dry-process fibers have poor chlorine resistance, are technically difficult, require solvent recovery, and have relatively high production costs.
Dry spinning & wet spinning
Concentration ratio of spinning solution The wet method is high, generally up to 18%~45%, and the corresponding viscosity is also high. It can withstand greater spinneret stretching (2~7 times) than wet spinning, and can easily produce finer fibers than wet spinning.
The mechanical resistance of the thread on the spinning line is much smaller than that of wet spinning, and the spinning speed is higher than that of wet spinning. However, due to the limitation of the solvent evaporation speed, the dry spinning speed is higher than that of wet spinning. Melt spinning is low.
The number of spinneret holes is much less than that of wet spinning. This is because the dry curing method is slow and the filaments are easy to adhere before curing. Generally, dry spinning short fibers are sprayed The number of holes is about 1200, while the number of holes in wet-spun short fibers is as high as tens of thousands. Therefore, the production capacity of a single spinning position of dry spinning is much lower than that of wet spinning. Dry spinning is generally suitable for producing filaments.
Process flow:
3 Dry-wet spinning
Dry-wet spinning is a type of solution spinning, also known as dry-jet wet spinning. The combination of the characteristics of spinning and dry spinning is particularly suitable for the molding and processing of liquid crystal polymers, so it is often called liquid crystal spinning.
That is, the fiber-forming polymer is dissolved in a certain solvent to prepare a spinning solution with an appropriate concentration, and then the spinning solution is spit out from a tiny hole. There is a short air sandwich, where the resistance of the filament is small, and the polymer in the liquid crystal state is conducive to high orientation under high-strength stretching conditions, and then the filament enters the low-temperature coagulation bath to complete solidification and molding, and The liquid crystal macromolecules are placed in a highly ordered frozen liquid crystal state, and the finished fiber has mechanical properties of high strength and high modulus.
At present, wet and dry spinning has been used in polyacrylonitrile fiber, polylactic acid fiber, chitosan fiber, dibutyryl chitin, polyvinyl chloride fiber, aromatic polyester fiber, etc. It is used in the preparation of fibers such as amide fiber and polybenzimidazole fiber.
Features:
Dry and wet spinning can spin high-viscosity spinning solution, thereby reducing solvent recovery and unit consumption. At the same time, its forming speed is high, and the resulting fiber has a uniform structure, a metallic round cross-section, and even strength and elasticity. Improved, dyeability and color are better. A major disadvantage of wet and dry spinning is that after the thin stream of spinning dope breaks, the dope can easily flow along the spinneret. This means that if a single filament breaks during porous spinning, it is likely that the dope will overflow and the dope will flow. Causes other filaments to break, thereby destroying the continuity of the spinning process.
Process flow:
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