As a core component of masks, melt-blown cloth has become the tightest material in the current mask production chain. Medical-surgical masks usually consist of three layers of non-woven fabric, the middle layer plays the most important filtering and blocking role, and the material used is a melt-blown non-woven fabric.
melt-blown cloth may not seem special, but the production process is very complex. automation melt-blown fabric mask extrusion lineproduction technology can be summarized as three major links: first, naphtha is extracted from oil, then chemically processed into polypropylene, and finally melt-blown to make cloth.
Selection of raw materials
Polypropylene (PP) is easy to form into filaments, and can become quite fine fibers (2~3 μm in diameter) with a high melting index (MFI≥1500g/10min). Although the resulting melt-blown cloth has a much larger gap than the size of the novel coronavirus (around 0.1 μm), it still has a strong ability to filter viruses in the environment.
Because, first, the virus cannot exist independently, mainly through secretions and droplets when sneezing, the size of the droplets is about 5 μm. Second, the melt-blown cloth is essentially a fiber filter, and droplets containing the virus are electrostatically adsorbed on the surface when they are close to the melt-blown cloth and cannot pass through.
The production process of melt-blown cloth is mainly divided into three steps: melt extrusion, melt-blown process, and electret treatment.
The melt-blown fabric mask extrusion line is used to melt polymer particles to melt die assemblies, including polymer dispensing systems, die systems, drawn hot airline channels, and heated insulation elements, and are extruded by the injection holes of the die. The spinneret diameter of the melt-blown process is much smaller than that used in the inner and outer spun bond fabrics because the melt-blown cloth fiber diameter is only one-tenth of the spun bond cloth fiber.
In order to achieve the required fineness, in addition to relying on smaller spinnerets, the melt fibers need to be stretched by high-speed hot air flows on both sides while the spinneret is extruded. In addition, the room temperature air on both sides is mixed with the hot flow air, so that the stretched melt can be cooled and solidified.
The microfiber, which has been stretched and cooled, is blown to the condensing curtain under the action of the stretching airflow. The lower part of the curtain is equipped with a vacuum suction device, which can adsorb the fiber on the condensing curtain and bond the fiber to the net using its own waste heat.
Finally, it is essential that the electret treatment is that the emitting electrode on the spinning line imparts a lasting electrostatic charge to the melt-blown formed fibers, giving the melt-blown cloth an additional electrostatic adsorption effect without hindering breathing. The non-woven fabric produced by the melt-blown method has 35% filtration performance, and the filterability can be increased to 95% after electret treatment.
Factors affecting the quality of melt-blown cloth
1. MFI for polymer raw materials
melt-blown cloth, as the best barrier for masks, is an extremely fine material with many crisscrossed microfibers stacked in random directions. Taking PP as an example, the higher the MFI, the thinner the wire drawn during melt-blown processing, the better the filtration performance.
2. Hot air velocity
At the same temperature, screw speed, and receiving distance (DCD), the faster the hot airspeed, the smaller the fiber diameter, the softer the non-woven feel, the more tangled the fiber, the more dense and smooth the fiber mesh, and the higher the strength.
3. Hot air-jet angle
The angle of hot air injection mainly affects the tensile effect and fiber morphology. Smaller angles cause the thin stream to form parallel fiber bundles, resulting in poor nonwovens uniformity. If the angle tends to 90°, a highly dispersed and turbulent airflow will be generated, which facilitates the irregular distribution of the fibers on the condensing curtain, and the resulting melt-blown cloth has good anisotropic properties.
4. Receiving distance (DCD)
Excessive acceptance distances can lead to loss of longitudinal and transverse strength, bending strength, fluffy feel of non-woven fabrics, and reduced filtration efficiency and filtration resistance in melt-blown processes.
5. Screw extrusion speed
Under the condition of constant temperature, the screw extrusion rate should be kept within a certain range. Before a critical point, the faster the extrusion speed, the higher the amount of melt-blown cloth, the greater the strength; beyond this threshold, the strength of the melt-blown cloth decreases, especially when MFI > 1000, it is more obvious, probably because the extrusion rate is too high, the silk strips are not drawn enough and the silk is severe so that the cloth bonding fibers are reduced and the strength of the melt-blown cloth decreases.