As the core component of masks, melt spray cloth has become the most demanding material in the current mask production chain. Surgical masks (usually composed of three layers of non-woven cloth, the middle layer plays the most important role in filtering and blocking, the material used is melt spraying non-woven cloth.
Polypropylene (PP) is easily filament able and can be made into fairly fine fibers (2~3 min diameter) with a high melting index (MFI≥1500g/10min). As shown in Figure 3, although the melt-jet gap produced is much larger than the novel Coronavirus size (about 0.1 m), it still has strong virus filtering capability in the environment. First, the virus cannot exist independently and is spread by secretions and sneezing droplets of about 5 m in size. Second, molten spray cloth is essentially a fiber filter. Droplets containing viruses will also be attracted to the surface by static electricity when they get close to molten spray cloth and cannot pass through.
The production technology of melting spraying cloth is mainly divided into three steps: melting extrusion, melting spraying, and stationary treatment.
By means of a screw extruder, the polymer particles are fused to the fused mold head assembly, including the polymer distribution system, the mold head system, the draft hot air pipeline channel, and the heating element, and are extruded from the spinneret hole of the mold head. The diameter of the spinneret hole in the melt spraying process is much smaller than that of spinneret hole in inner and outer layers because the diameter of melt spraying fabric is only one-tenth of that of spun-bonded fabric. In order to achieve the desired fineness, in addition to relying on smaller spinneret holes, the melt fibers need to be drawn by high-speed hot airflow on both sides while the spinneret holes are extruded. In addition, the room temperature air on both sides is mixed with the hot airflow, so that the drawn melt can be cooled and solidified. The ultrafine fiber through drafting and cooling and curing is blown to the setting screen curtain under the action of draft airflow. The lower part of the setting screen curtain has a vacuum suction device, which can adsorb the fiber on the setting screen curtain and use its own waste heat to bond into the net. Finally, it is critical to electropole treatment. The emitting electrode on the spinning line enables the molten spray forming fiber to have a long-lasting electrostatic charge, thus enabling the molten spray fabric to have an additional electrostatic adsorption effect without obstructing respiration. The non-woven fabric produced by melting spray has a 35% filtration property, and the filtration property can be increased to 95% after electret treatment.
1. MFI of polymer materials
As the best barrier layer for masks, the melt-jet cloth is an extremely fine material, which consists of a lot of criss-cross ultrafine fibers stacked in random directions. Take PP as an example, the higher the MFI is, the finer the wire is drawn during melting and spraying processing, and the better the filtration performance is.
2. Thermal flow speed
At the same temperature, screw speed, and receiving distance (DCD), the faster the hot airspeed is, the smaller the fiber diameter is, the softer the feel of the non-woven cloth is gradually, and the more the fiber tangles are, which leads to the more compact and smooth fiber net and the higher the strength.
3. Hot air injection Angle
Hot air injection Angle mainly affects the tensile effect and fiber morphology. The smaller Angle will cause the small stream to form a parallel fiber bundle, which will lead to the poor uniformity of non-woven fabric. If the Angle tends to 90°, highly dispersed, and turbulent airflow will be generated, which is conducive to the random distribution of fibers on the coagulation screen curtain, and the final anisotropy performance of melt spray cloth is good.
4. Reception distance (DCD)
Excessively long acceptance distance will lead to the decrease of longitudinal and longitudinal strength and bending strength, and the fluffy feel of non-woven fabrics, which will lead to the decrease of filtration efficiency and filtration resistance in the melting and spraying process.
5. Extrusion speed of the screw
Under the condition of constant temperature, the extrusion rate of the screw should be kept within a certain range:
Before a certain critical point, the faster the extrusion speed, the higher the amount of molten spray, the greater the strength; When the critical value is exceeded, the strength of melt spraying cloth decreases instead, especially at MFI > 1000. This may be because the extruding rate is too high, which leads to insufficient wire drawing and serious wire so that the cloth surface bonding fiber decreases, and the strength of melt spraying cloth decreases.