Methods to improve the quality of meltblown non-woven fabrics
Melt blowing is a method that relies on high-temperature, high-speed airflow to blow the polymer melt to make it quickly stretched to prepare fibers. After the polymer chips are heated and pressurized by a screw extruder into a molten state, they reach the spinneret hole at the front of the nozzle through the melt distribution channel, and are stretched by two convergent high-speed and high-temperature air streams after extrusion to make them thin . The thinned fiber is cooled and solidified on the mesh device to form a melt-blown non-woven fabric.
Continuous meltblown nonwovens production technology has experienced more than 20 years of development in our country. The application fields have expanded from the initial battery separators, filter materials, oil-absorbing materials and thermal insulation materials to medical, sanitation, health care, protection and other fields. Its production technology has also developed from a single meltblown production to a composite direction. Among them, the meltblown composite material processed by electrostatic electret has the characteristics of low initial resistance, large dust holding capacity, and high filtration efficiency. It can be widely used in electronic manufacturing, food, beverage, chemical industry, airport, hotel and other places. Purification treatment and medical high-performance masks, industrial and civil dust collector filter bags.
Polypropylene melt blown non-woven fabric (a kind of ultra-fine electrostatic fiber cloth that can capture dust), the pore size and thickness of the fiber and other factors affect the filtering effect. Particles of different diameters are filtered through different principles, such as particle volume, impact, and diffusion principles that result in being blocked by fibers, and some particles are filtered by electrostatic fibers through the principle of electrostatic attraction. The filtration efficiency test is carried out under the particle size specified in the standard, and different standards will use different sizes of particles for testing. BFE often uses bacterial aerosol particles with an average particle diameter of 3 μm, while PFE generally uses sodium chloride particles with a diameter of 0.075 μm. In terms of filtering effect alone, PFE has a higher effect than BFE.
In the standard test of KN95 masks, particles with an aerodynamic diameter of 0.3μm are used as the test object, because particles larger or smaller than this diameter are more likely to be intercepted by filter fibers, while particles of intermediate size such as 0.3μm are more difficult to filter. Although the virus is small in size, it cannot be transmitted in the air alone. It needs droplets and droplet nuclei as carriers to be dispersed in the air, so it can be easily filtered out.
The core of meltblown cloth technology is: under the premise of reducing the respiratory resistance to the minimum, it can also achieve high efficiency filtration, especially for meltblown cloths above N95, VFE grade meltblown cloth to electret masterbatch formula, performance of meltblown materials, The spinning effect of the meltblown line, especially the addition of electret masterbatch, will affect the thickness of the spinning fiber and the uniformity of the web. Low resistance and high efficiency are the core technology.
Factors affecting the quality of meltblown cloth
MFI of polymer raw materials
As the best barrier layer for masks, melt-blown cloth is a very 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 meltblown processing and the better the filtration performance.
Hot air jet angle
The hot air jet angle mainly affects the stretching effect and fiber shape. 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 airflow will be generated, which is beneficial to the random distribution of fibers on the condensed mesh curtain, and the final meltblown fabric has good anisotropic properties.
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 melt blown cloth On the contrary, the strength of the fabric decreases, especially when the MFI>1000 is more obvious. It may be because the extrusion rate is too high, which leads to insufficient drafting of the filaments and serious merging, which reduces the bonding fibers on the cloth surface and reduces the strength of the meltblown fabric.
Hot air velocity and temperature
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 and smoother fiber web. Strength increases.
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 will cause the filtration efficiency and filtration resistance to decrease in the meltblown process.
Meltblown die head (hard index)
Mold material and process temperature setting. Use some low-end mold steel instead, there will be some small cracks invisible to the eye during use, the aperture processing is rough, the accuracy is poor, and the machine is directly on the machine without polishing. It causes uneven spinning, poor toughness, different spinning thickness, and easy crystallization.
Net bottom suction
Process parameters such as air volume and air pressure of the bottom of the net
The mesh curtain speed is slow, the meltblown cloth weight is higher, and the filtration efficiency is higher. The opposite is true.
Electret voltage, electret time, electret molybdenum wire distance, electret environment humidity and other aspects will affect the filtration quality and other parameters.