Today, there are many friends who come to consult about melt blown pp. they say that the raw materials of PP are OK, the machine runs normally, and the produced melt blown cloth can't reach 95 standard after being tested by a professional testing agency. Why?
It seems that the melt blown cloth is not special, but the production process is very complex. The production process can be summarized as three links: first, naphtha is extracted from petroleum, then it is chemically processed into polypropylene, and finally it is made into cloth by melt blown technology. Here is the last link.
Selection of raw materials
Polypropylene (PP) is easy to be filamented, and can become a fairly fine fiber (diameter 2-3 μ m) with a high melt index (MFI ≥ 1500g / 10min). Although the gap of novel coronavirus produced is much larger than that of the new coronavirus size (about 0.1 M), its ability to filter viruses in the environment is still strong.
Because the virus can not exist independently, and its transmission way mainly includes secretion and sneezing droplets, the size of which is about 5 μ m; the melt blown cloth is essentially a kind of fiber filter, and the droplets containing the virus will also be adsorbed on the surface by static electricity after they are close to the melt blown cloth, unable to pass through.
The diameter of spinneret hole in melt blown process is much smaller than that of spinneret hole used in inner and outer spunbond, because the diameter of spunbond fiber is only one tenth of that of spunbond fiber. In order to achieve the required fineness, in addition to relying on smaller spinneret holes, it is also necessary to extrude the spinneret holes at the same time so that the melt fibers are drawn by the high-speed hot air flow on both sides. In addition, the room temperature air on both sides is mixed with hot air, which can make the drawn melt cool and solidify.
The ultra-fine fiber, which is drawn and cooled and solidified, is blown to the condensing screen under the action of draft air flow. There is a vacuum suction device at the lower part of the condensing screen, which can adsorb the fiber on the condensing screen and make use of its own waste heat to glue the synthetic screen.
Last but not least, the electret treatment is very important. The emitter on the spinning line can make the melt blown fiber with a permanent static charge, so that the melt blown fabric has an additional electrostatic adsorption effect without hindering breathing. The non-woven fabric produced by melt blown process has 35% filtration performance, and the filtration performance can be increased to 95% after electret treatment.
Factors affecting the quality of melt blown cloth
MFI of polymer raw materials
As the best barrier layer of the mask, the melt blown cloth is a very fine material, which is made of many crisscross microfibres stacked in random direction. Taking PP as an example, the higher MFI is, the finer the wire drawn out during melt blown processing, and the better the filtration performance.
Hot air injection angle
The hot air injection angle mainly affects the drawing effect and fiber morphology. The smaller the angle, the parallel fiber bundle will be formed, which will lead to the poor uniformity of the non-woven fabric. If the angle tends to 90 °, a highly dispersed and turbulent air flow will be generated, which is conducive to the random distribution of fibers on the screen, and the anisotropic property of the final melt blown cloth is good.
Screw extrusion speed
Under the condition of constant temperature, the extrusion rate of screw should be kept in a certain range: before a certain critical point, the faster the extrusion speed is, the higher the quantity of melt blown cloth is, and the greater the strength is; when the critical value is exceeded, the strength of melt blown cloth will decrease, especially when MFI > 1000, which is more obvious. It may be because the high extrusion rate leads to the insufficient draft of wire rod and the serious silk, so the cloth surface is sticky The results show that the strength of the melt blown fabric is decreased.
Hot air velocity
At the same temperature, screw speed and receiving distance (DCD), the faster the hot air speed, the smaller the fiber diameter, the softer the hand feel of the non-woven fabric, and the more tangled the fibers, resulting in the denser, smoother and higher strength of the fiber mesh.
Receiving distance (DCD)
Too long acceptance distance will lead to the decrease of longitudinal and transverse strength, bending strength, fluffy hand feeling of non-woven fabric, and the decrease of filtration efficiency and resistance in melt blown process.
Melt blown die head
Many spinneret die materials on the market are not standard materials. Some low-end die steels are used instead. During the use process, there will be some small cracks that can not be seen by the eyes. The hole diameter processing is rough and the accuracy is poor. It is directly used without polishing. It causes uneven spinneret, poor toughness, different spinneret thickness and easy to produce crystallization.