The secret of spinning

Recently, a friend called to consult the spinning test business, feel that non-spinning (chemical fiber) professional friends, spinning as a very simple thing, at the beginning of the exchange will cause a lot of misunderstanding, in view of this, through this article, give you talk about some common sense in chemical fiber.

The first thing to mention is that some friends think of spinning too simple, using the idea of injection molding or even blow molding to guess spinning, which is a misunderstanding.

Spinning can be said to be the most complex and technically demanding in the post-processing of polymer materials. For example, there are professional textile universities, such as Donghua University (formerly known as China Textile University); There is also a special chemical fiber professional, have not heard which university set what blowing film, injection molding and other professional right? And there is even a special chemical fiber machinery specialty. The spinning equipment is also very complex, the requirements are also very high, such as spinning components, spinneret, drafting, winding equipment, etc., the processing accuracy is very high; And some production lines can reach hundreds of meters long, here will not be detailed.

Moreover, spinning has the most demanding requirements on raw materials. In general, the molecular weight of raw materials is too low or too high, and the molecular weight distribution can not be too wide. The requirements for raw materials for blowing film and injection molding are much lower. Especially injection molding, low molecular weight can also be obtained, but the mechanical properties of injection molding parts are lower.

According to the manufacturing method and the characteristics of the fiber, there are many types of spinning, such as melting method, solution method spinning; Conventional spinning, melt-jet spinning, electrostatic spinning, etc. Due to my limited technical background, a variety of spinning technology is only a little knowledge, so this article is only a rough introduction.

Melt spinning is the melting of polymer materials to spin, which is characterized by low production cost and high efficiency. In terms of production scale, melt spinning is the mainstream, including polyester filament, staple fiber, nylon, polypropylene filament, nylon monofilament and so on. Among them, filaments are mainly DTY and FDY filaments according to their use.

Corner of the POY workshop

One step FDY workshop corner

Corner of DTY workshop

Solution spinning is the spinning of polymer materials that need to have fluidity with the help of solvents. Because it contains solvents, the production cost is relatively high, the efficiency is low, and some will also have waste gas or waste liquid. Solution spinning is suitable for the production of some special fibers, such as the recognized three high-performance fibers (carbon fiber, aramid, high-strength polyethylene fiber) are made through solution spinning.

Wet spinning machine

Corner of the wet spinning workshop

Melt spinning and solution spinning are conventional spinning methods. The fibers produced by melt spinning and solution spinning are mainly measured by strength, densification (linear density), number of holes in spinneret and dry strip. In general, the strength of the silk is in the range of 4 ~ 45g/d, the compound filament size is in the range of 50 ~ 3000d, and the monofilament size is in the range of 0.3 ~ 10d.

The filament mentioned above refers to the compound filament, which is actually a bundle of silk, which is composed of a number of very fine filaments, and the diameter of these filaments is generally in 10 ~ 30um (1 ~ 2d). And there is a kind of silk is monofilament, a wire can be used, the most common is nylon fishing line, a fishing line is a monofilament, generally in the diameter of 0.1 ~ 0.5mm. Monofilament is mostly manufactured by melt spinning method, such as PCL, PPDO fishbone line, which is now popular in medicine, is processed by melt spinning monofilament; There are also more cutting-edge PLA human implants, which are also made of melt spun monofilament.

With the development and needs of technology, there have been some new spinning technologies, such as melt-jet spinning and electrostatic spinning. In fact, melt-jet spinning and electrospinning can not make silk, but non-woven fabric.

The bottom of the melt-blown spinning spinneret is shaped like a long plate with sharp corners, and there is only a row of spinneret holes at the sharp corners. The outer side of the spinneret is provided with a gap parallel to the sharp Angle. When spinning, a high speed air flow is emitted from the gap, and the polymer material melt flowing from the spinneret is blown into a fine filament and blown to the receiving device below. By adjusting the moving speed of the receiving device, a non-woven fabric made of fine filaments can be obtained. For example, PP non-woven fabric used as the middle layer of masks is produced by melt-jet spinning method. The non-woven fabric produced by this method is generally of low strength and cannot be used as a clothing fabric. However, the diameter of the melt-blown method is smaller, which can reach less than 5um, which is very suitable for use as a filter material, which is also the reason why the middle layer of the mask uses PP melt-blown non-woven fabric.

Corner of melt-blown spinning workshop

Small melt-jet spinning machine

Electrostatic spinning is the use of high voltage electrostatic field (more than 10,000 volts), from the charged spinning droplets extracted very fine silk, collected in the positive terminal end, is also the form of non-woven fabric, its strength is also very low, but the diameter of the silk is smaller, can reach the nanometer level, with very good adsorption and filtration effect. Based on this, electrostatic spinning has very high requirements for dust prevention, otherwise the dust adsorbed by the obtained product will be a lot, affecting its test and use. However, electrospinning is subject to a very low yield and is still in the experimental stage, and large-scale production has not yet occurred. In general, electrospinning is easier to achieve using the solution method than the melting method.