Suzhou Zhanheng intelligent sharing: Several spinning methods commonly used in chemical fibers

The spinning fluid is continuously, quantitatively and uniformly extruded from the capillary pores of the spinneret or spinneret by a spinning pump (or metering pump), and then solidified into strips in air, water or coagulation bath. The process is called spinning or fiber forming.

Freshly spun silk strips are called primary fibers. Spinning is a key process in the process of chemical fiber production. Changing the spinning process conditions can adjust the structure of the fiber in a large range, so as to change the physical and mechanical properties of the fiber.

According to the different properties of fiber forming polymers, the spinning methods of chemical fibers mainly include melt spinning method and melt spinning method, in addition, there are special or unconventional spinning methods. Among them, according to the different solidification methods, melt spinning is divided into wet spinning and dry spinning two kinds. 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.

01 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 the spinneret hole, and cooled and solidified in the air to form fibers. The polymer 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 be produced by melt spinning.

Features:

The main characteristics of melt spinning method are high spinning speed (1000~7000m/min), no solvent and precipitator and its recovery, circulation system, simple equipment, short process flow, is an economic, convenient and efficient forming method. But the number of spinneret holes is relatively small.

Process flow:

1. Spinning melt preparation -- continuous polymerization worthy of melt awards after pre-crystallization and drying, the fiber forming polymer slices are added from the polymer bucket, and the screw extruder heated in accordance with the requirements is melted, mixed, measured and extruded successively, and sent into the melt metering pump through the bend pipe between the spinning box of the extrusion opportunity.

2. The melt is quantitatively extruded through the small hole with spinneret head to form a trickle of melt through the metering pump in the spinning box.

3. After the melt is used, it is cooled, solidified and formed into the primary fiber under low temperature and cooling air environment.

4. The primary fiber is then oiled and networked and wound into a cylinder.

5. This winding wire is then processed by subsequent stretch-heat setting and other secondary forming processes to produce a suitable finished fiber

Categories:

According to the melt preparation process, melt spinning is divided into direct spinning and slice spinning. The process of directly feeding the polymerized polymer melt into the metering pump and extruding for spinning is called direct spinning method. The technology of melt spinning of the polymer through pre-crystallization, drying and other necessary preparation before spinning is called slice spinning method. Direct spinning technology is often used in large-scale industrial production, which is conducive to reducing production costs, but it is difficult to produce differential fiber varieties, and only slight changes can be made in online density and fiber cross section shape. The slicing and spinning method is more flexible, easy to change varieties, and produces small batch and high value-added differential fibers.

02 Solution spinning method

Solution spinning is to dissolve the fiber forming polymer in a solvent, prepare a spinning solution with appropriate concentration, and then the spinning solution from the tiny hole into the solidification bath or hot gas, the polymer precipitation into a solid filament, after stretching - shaping - washing - drying waiting for the treatment process can get the finished fiber

Obviously, the solution spinning production process is more complex than melt spinning, however, for some polymers that have not yet been melted and decomposed, you can only choose this spinning forming technology, solution spinning and wet spinning, dry spinning, dry and wet spinning.

01 Wet spinning

Wet spinning is referred to as wet spinning. Chemical fiber spinning process in which polymers are dissolved in solvent and sprayed through spinneret holes into a coagulation bath to form fibers. Fiber forming polymers suitable for wet spinning, whose decomposition temperature is lower than the melting point or easily discoloration when heated, and can be dissolved in appropriate solvents. Synthetic fibers such as polyacrylonitrile fiber and polyvinyl alcohol fiber and artificial fibers such as viscose fiber and copper ammonia fiber are produced by wet spinning. The cross sections of the fibers obtained by wet spinning are mostly non-circular and have obvious skin core structure, which is mainly caused by the solidification of the solidifying liquid.

Features:

The speed of wet spinning is low, and the number of holes of spinneret is more than that of melt spinning, the process flow is complicated, the casting time is large, and the production cost is high. In general, in the production of short fibers, porous spinnerets or graded spinnerets can be used to improve the production capacity, so as to make up for the defects of low spinning speed.

Process flow:

1. Prepare spinning stock solution.

2. Press the stock solution out of the spinneret hole to form a trickle.

3. The raw liquid is solidified into the primary fiber.

4. New fiber packaging or direct post-processing.

02 Dry spinning

Dry spinning is a kind of solution spinning, if the fiber forming polymer can find a low boiling point, solubility and good solvent to make spinning solution, at this time the spinning solution can be spit out from the tiny hole into the heated gas, the solvent in the spinning solution volatilizes, the polymer filament gradually solidified, after stretching, setting, washing and drying and other post-treatment process can get the finished fiber. Acrylic fiber, spandex, chlorylon and vinylon are all spun by dry spinning process.

Features:

Dry spinning has the advantages of continuous production, high spinning speed, large yield and less pollution. The quality, chemical resistance and dyeing properties of the fiber are better than those of the wet-spun fiber. However, the chlorine resistance of dry fiber is poor, the technical difficulty is greater, the solvent recovery is required, and the production cost is relatively high.

Dry spinning & wet spinning

The concentration of the spinning solution is higher than that of the wet method, generally up to 18%~45%, and the corresponding viscosity is also high, and it can withstand the stretching of the spinneret larger than that of the wet spinning (2~7 times), and it is easy to make finer fibers than the wet spinning.

The mechanical resistance of the spinning line is much lower than that of wet spinning, and the spinning speed is higher than that of wet spinning, but due to the limitation of solvent evaporation speed, the dry spinning speed is lower than that of melt spinning.

The hole number of spinneret head is much less than that of wet spinning, because dry curing is slow and the strips are easy to stick before curing. Generally, the number of spinning holes of dry spun short fibers is about 1200 holes, while the number of holes of wet spun short fibers is as high as tens of thousands of holes, so the production capacity of a single spinning point of dry method is much lower than that of wet spinning, and dry spinning is generally suitable for the production of filament.

03 Wet and dry spinning

Wet and dry spinning is a kind of solution spinning, also known as dry spray wet spinning method, which combines the characteristics of wet spinning and dry spinning, especially suitable for the forming and processing of liquid crystal polymers, so it is also often called liquid crystal spinning.

The spinning solution with suitable concentration is prepared by dissolving the fiber forming polymer in some solvent, and then the spinning solution is spit out from the tiny hole, first through a short air interlayer, where the polymer in the liquid crystal state is conducive to high orientation under the condition of high tensile strength due to the low resistance of the filament strip, and then the filament strip enters the freezing bath at low temperature to complete curing and forming. The liquid crystal macromolecules are placed in a highly ordered frozen liquid crystal state, and the finished fiber obtained has high strength and high modulus mechanical properties.

At present, wet-dry spinning has been applied in the preparation of polyacrylonitrile fiber, polylactic acid fiber, chitosan fiber, dibutyl chitin, polyvinyl chloride fiber, aromatic polyamide fiber, polybenzimidazole fiber and other fibers.

Features:

Dry and wet spinning method can spin high viscosity spinning stock solution, thus reducing solvent recovery and single consumption, at the same time, its forming speed is higher, the obtained fiber structure is uniform, the cross section metal round, strength and elasticity are improved, dyeing and color is better. One of the main disadvantages of wet and dry spinning is that after the spinning stock liquid is broken, the stock liquid is very easy to flood along the spinneret, which means that if a single filament breaks during the porous spinning process, it is likely to break other filaments because of the stock liquid flooding, thus breaking the continuity of the spinning process.