As the core component of masks, meltblown cloth has become the most scarce material in the current mask production chain.
Medical surgical masks (structure shown in Figure 1) are usually composed of three layers of non-woven fabrics, the middle layer plays the most important role in filtering and blocking, and the material used is melt-blown non-woven fabrics.
Melt-blown cloth does not seem to be special, but the production process is very complicated. It is chemically processed into polypropylene, and finally made into a cloth through the melt-blown process. The general process is shown in Figure 2. The last part is mainly introduced here.
First, the selection of raw materials
Polypropylene (PP) is easy to be filamented, and in the case of high melt index (MFI≥1500g/10min) It can become quite thin fibers (2~3 μm in diameter). As shown in Figure 3, although the voids of the prepared meltblown cloth are much larger than the size of the new coronavirus (about 0.1 μm), its filtering ability for viruses in the environment is still very strong.
Because, first, the virus cannot exist independently, and its transmission routes are mainly secretions and droplets when sneezing, and the size of the droplets is about 5 μm; second, the meltblown cloth is essentially A kind of fiber filter, after the droplets containing virus are close to the melt-blown cloth, they will also be electrostatically adsorbed on the surface and cannot pass through.
II. Production process
The production process of meltblown cloth is mainly divided into three steps: melt extrusion, meltblown process and electret treatment. The specific flow chart is shown in Figure 4.
Rely on the screw extruder to melt the polymer particles and send them to the molten die assembly, including the polymer distribution system, the die system, the pulling The hot air line channel and heating insulation element are extended and extruded from the spinneret hole of the die (Fig. 5a, Fig. 5b). The diameter of the spinneret holes in the melt-blown process is much smaller than the spinneret holes used in the inner and outer layers of the spunbond fabric, because the fiber diameter of the melt-blown fabric is only one tenth of that of the spunbond fabric.
In order to achieve the required fineness, in addition to relying on smaller spinneret holes, it is also necessary to draw the melt fibers by high-speed hot air flow on both sides while extruding the spinneret holes (Fig. 5c), in addition, the room temperature air on both sides is mixed with hot air, which can cool and solidify the thinned melt.
The ultra-fine fibers that have been drawn and cooled and solidified are blown to the condensation screen curtain under the action of the drafting air flow. There is a vacuum suction device at the lower part of the condensation screen curtain, which can adsorb the fibers on the Self-waste heat bonded into a mesh.
Finally, the electret treatment is crucial. The emitter electrode on the spinning line can make the melt-blown fibers have a lasting electrostatic charge, so that the melt-blown cloth can be breathed without hindrance. It has an additional electrostatic adsorption effect. The non-woven fabric produced by the melt-blown method itself has a filtration performance of 35%, and the filterability can be increased to 95% after electret treatment.
III. Factors affecting the quality of melt-blown cloth
1. MFI of polymer raw materials
melt-blown cloth is the most suitable for masks The best barrier layer is a very fine material, which is made up of many criss-cross microfibers stacked in random directions. Taking 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 same conditions of temperature, screw speed and receiving distance (DCD), the faster the hot air velocity, the smaller the fiber diameter and the better the non-woven feel. Gradually softening, the more fibers become entangled, resulting in a denser and smoother web with increased strength.
3. Hot air injection angle
The hot air injection angle (see Figure 5a) mainly affects the stretching effect and fiber morphology. Smaller angles encourage the thin streams to form parallel fiber bundles, resulting in poor nonwoven uniformity. If the angle tends to 90°, a highly dispersed and turbulent airflow will be generated, which is conducive to the random distribution of fibers on the condensed mesh curtain, and the final meltblown cloth has good anisotropic performance.
4. Receiving distance (DCD)
Excessive receiving distance will lead to the decrease of vertical and horizontal strength and bending strength, and the non-woven fabric feels fluffy, which will lead to filtration in the melt-blown process. Efficiency and filtration resistance drop.
5. Screw extrusion speed
Under the condition of constant temperature, the screw extrusion rate should be kept within a certain range:
Before a certain critical point , the faster the extrusion speed, the higher the quantitative melt-blown cloth, and the greater the strength; when the critical value is exceeded, the strength of the melt-blown cloth decreases, especially when MFI>1000, which may be because the extrusion rate is too high. As a result, the drafting of the filaments is insufficient, and the filaments are serious, so that the bonding fibers on the cloth surface are reduced, and the strength of the melt-blown cloth is reduced.
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