Talking about the production technology of non-woven fabric manufacturer's melt blown cloth

As the core component of masks, meltblown cloth has become the most demanding material in the current mask production chain.


Medical surgical masks (structure as shown in Figure 1) are usually composed of three layers of non-woven fabrics, the middle layer plays the most important role of filtering and barrier, and the material used is melt-blown non-woven fabric.





Melt blown cloth looks nothing special, but the production process is very complicated. The production process can be summarized into three steps: firstly extract naphtha from petroleum, then chemically process it into polypropylene, and finally undergo melt-blown process The general process is shown in Figure 2. The last link is mainly introduced here.





One, the choice of raw materials


Polypropylene (PP) is easy to form filaments, and when the melt index is high (MFI≥1500g/10min), it can become a fairly thin fiber (diameter of 2~3 μm). As shown in Figure 3, although the gap of the meltblown fabric is much larger than the size of the new coronavirus (about 0.1 μm), its ability to filter viruses in the environment is still very strong.


Because, first, the virus cannot exist independently, and its main transmission routes are secretions and droplets when sneezing, the size of the droplets is about 5 μm; second, the meltblown cloth is essentially a fiber filter When the virus-containing droplets are close to the meltblown cloth, they will also be electrostatically adsorbed on the surface and cannot penetrate.





2. Production process


The production process of melt blown cloth is mainly divided into three steps: melt extrusion, melt blown process and electret treatment. The specific flow diagram is shown in Figure 4.








Relying on the screw extruder to melt the polymer particles to the molten die assembly, including the polymer distribution system, the die system, the drafting hot air pipeline channel and the heating and heat preservation elements, and the spinning of the die Hole extrusion (Figure 5a, Figure 5b). The diameter of the spinneret holes in the meltblown process is much smaller than the spinneret holes used in the inner and outer layers of spunbonded fabric, because the diameter of the meltblown fiber is only one tenth of that of the spunbonded fabric.


In order to achieve the required fineness, in addition to relying on smaller spinneret holes, the melt fibers need to be drawn by high-speed hot air streams on both sides at the same time as the spinneret holes are extruded (Figure 5c). The room temperature air on both sides is mixed with hot air flow, which can cool and solidify the attenuated melt.


The superfine fibers that have been drawn and solidified by cooling are blown to the condensation net curtain under the action of the drafting airflow. There is a vacuum suction device at the bottom of the condensation net curtain, which can adsorb the fibers on the condensation net curtain and use their own waste heat to bond together. network.


Finally, the most important thing is the electret treatment. The emitter electrode on the spinning line can make the melt-blown fiber have a long-lasting static charge, so that the melt-blown cloth has additional static electricity without hindering breathing. Adsorption effect. The non-woven fabric produced by the meltblown method has 35% filtration performance, and the filtration performance can be increased to 95% after electret treatment.





Three, factors affecting the quality of melt blown cloth


1. MFI of polymer raw materials


Melt blown cloth, as the best barrier layer for masks, is an extremely fine material, with many crisscross microfibers stacked in random directions. Take PP as an example. The higher the MFI, the finer the filaments drawn during melt-blown processing and the better the filtration performance.


2. Hot air velocity


Under the conditions of the same temperature, screw speed and receiving distance (DCD), the faster the hot air speed, the smaller the fiber diameter, the softer the hand of the non-woven fabric and the more fiber entanglement, resulting in a denser fiber web And it is smooth, and its strength is improved.


3. Hot air jet angle


The hot air jet angle (see Figure 5a) mainly affects the stretching effect and fiber morphology. The smaller the angle will promote the formation of parallel fiber bundles, resulting in 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 beneficial to the random distribution of fibers on the condensed mesh curtain, and the finally obtained meltblown cloth has good anisotropic properties.


4. Receiving distance (DCD)


Too long a receiving distance will cause the longitudinal and transverse strength and bending strength to decrease, the non-woven fabric feels fluffy, and it will cause the filtration efficiency and filtration resistance to decrease in the melt-blown process.


5. Screw extrusion speed


In the case of constant temperature, the screw extrusion rate should be maintained within a certain range:


Before a certain critical point, the faster the extrusion speed, the higher the quantitative and the greater the strength of the melt-blown cloth; when the critical value is exceeded, the strength of the melt-blown cloth decreases, especially when MFI>1000 , It may be because the extrusion rate is too high, which leads to insufficient drafting of the thread and serious merging, which reduces the bonding fiber on the cloth surface and the strength of the meltblown cloth.