Medical surgical masks are usually composed of three layers of non-woven fabric, with the middle layer playing the most important filtering and blocking function. The material used is meltblown non-woven fabric. The production process of melt blown fabric involves producing and processing polypropylene, which is then processed into a cloth like material through the melt blown process.



1、 Selection of raw materials



Polypropylene (PP) is easy to form into fibers and can become very fine fibers (with a diameter of 2-3) with a high melt index (MFI ≥ 1500g/10min) μ m) , although the gap of the prepared melt blown cloth is smaller than that of the novel coronavirus (0.1 μ M is much larger, but its ability to filter viruses in the environment is still very strong. It is because, first of all, the virus cannot exist alone, and its specific transmission routes include secretions and sneezing droplets, which are in the size of 5 μ About m; Secondly, meltblown fabric is essentially a type of fiber filter, and virus rich droplets that come close to the meltblown fabric will also be electrostatic adsorbed on the surface and cannot pass through.



2、 Processing technology



The processing process of melt blown fabric is specifically divided into three steps: melt extrusion, melt blown process, and electret treatment. By using a screw extruder, polymer particles are melted and sent to the molten die assembly, which includes a polymer distribution system, die system, draft hot air pipeline channels, and heating insulation elements, and are extruded through the spinneret holes of the die. The diameter of the spinneret hole in the melt blown process is much lower than that used in the inner and outer layers of spunbonded fabric, because the fiber diameter of the melt blown fabric is only 1/10 of that of the spunbonded fabric fiber. In order to achieve the required fineness, in addition to using smaller spinneret holes, it is also necessary to extrude the spinneret holes while allowing the melt fibers to be stretched by high-speed hot air flow on both sides. In addition, room temperature air on both sides is mixed with hot air flow, which can cool and solidify the stretched melt into shape. The ultrafine fibers that have been drawn and cooled and solidified are blown towards the condensation screen curtain under the function of draft airflow. There is a vacuum suction device at the bottom of the condensation screen curtain, which can adsorb the fibers onto the condensation screen curtain and use its own waste heat to adhesive the synthetic mesh. Finally, it is particularly important to perform electret treatment. The emission electrode on the spinning line can cause the meltblown fibers to carry a persistent static charge, thereby enabling the meltblown fabric to have additional electrostatic adsorption effect without hindering breathing. The non-woven fabric produced and processed by melt blown method itself has 35% filtration characteristics, and after being subjected to electret treatment, its filtration performance can be improved to 95%.



3、 Factors affecting the quality of melt blown fabric



1. MFI of polymer raw materials


Melt blown fabric, as the best barrier layer for masks, is an extremely fine material with many crisscrossing ultrafine fibers stacked in random directions inside. Taking PP as an example, the higher the MFI, the finer the wire pulled out during melt blown processing, and the better the filtration characteristics.


2. Hot gas flow rate


At the same temperature, screw speed, and receiving distance (DCD), the faster the hot air rate, the smaller the fiber diameter, and the softer the feel of the non-woven fabric. The more tangled the fibers, the denser and smoother the fiber mesh, and the higher the strength.


3. Hot air injection angle


The angle of hot air injection mainly affects the stretching effect and fiber morphology. Reducing the angle will promote the generation of parallel fiber bundles by fine flow, resulting in poor uniformity of non-woven fabrics. If the angle tends towards 90 °, a highly dispersed and turbulent airflow will be generated, which is beneficial for the random distribution of fibers on the condensing mesh curtain, resulting in good anisotropic properties of the melt blown fabric.


4. Receiving distance (DCD)


Too long acceptance distance will reduce the longitudinal and transverse strength and bending strength, and the non-woven fabric will feel fluffy. In the melt blown process, it will reduce the filtration efficiency and resistance.


5. Screw extrusion rate


Under constant temperature, the screw extrusion rate should be controlled within a certain range. Before a certain critical point, the faster the extrusion rate, the higher the quantity and strength of the melt blown fabric; When the critical value is exceeded, the strength of the melt blown fabric decreases inversely, especially when the MFI is greater than 1000, or due to the excessive extrusion rate, the wire rod draft is not sufficient and the wire is severely mixed, resulting in a decrease in the fiber bonding on the fabric surface and a decrease in the strength of the melt blown fabric.