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How to improve the filtration efficiency level of meltblown cloth?
Meltblown non-woven fabric is the core material of medical masks, and its filtration efficiency directly affects the protective effect of the mask. There are many factors affecting the filtration performance of meltblown cloth, such as fiber linear density, structure, thickness and density of the fiber web. However, as the air filter material of the mask, if the material is too tight, the pores are too small, and the breathing resistance is too large, the user cannot inhale air smoothly, and the mask loses its use value. This requires the filter material not only to improve its filtration efficiency, but also to reduce its breathing resistance as much as possible, and breathing resistance and filtration efficiency are a pair of contradictions. The electrostatic electret treatment process is the best way to solve the contradiction between breathing resistance and filtration efficiency. Filtration mechanism of melt-blown non-woven fabrics Among the filtration mechanisms of melt-blown filter materials, the generally recognized mechanisms mainly include Brownian diffusion, interception, inertial collision, gravitational sedimentation and electrostatic adsorption. Since the first four principles are all mechanical barriers, the filtration mechanism of meltblown cloth can be simply summarized as mechanical barrier and electrostatic adsorption. The average fiber diameter of the mechanical barrier polypropylene meltblown cloth is 2 to 5μm, the particle size in the air is greater than 5μThe droplets of m can be blocked by the meltblown cloth. When the particle diameter is less than 3μWhen m, the fibers in the melt-blown cloth are randomly arranged and the interlayers are crossed and formed to form a fiber filter layer with multiple curved channels. When the particles pass through various types of curved channels or paths, the particles are mechanically filtered by van der Waals force. adsorbed on the fiber surface. When the particle size and airflow velocity are both large, the airflow approaching the filter material will flow around due to obstruction, and the particles will collide with the fibers directly due to inertial action and be captured. When the particle size is very small and the flow rate is very low, the particles impinge on the fibers and are captured by the diffusion caused by Brownian motion. Electrostatic adsorption Electrostatic adsorption refers to the capture of particles by the Coulomb force of the charged fibers (electrets) when the fibers of the filter material are charged. When dust, bacteria, viruses and other particles pass through the filter material, the electrostatic force can not only effectively attract the charged particles, but also capture the induced polarized neutral particles with the electrostatic induction effect. As the electrostatic potential increases, the electrostatic adsorption effect is stronger. Since the filtration efficiency of ordinary melt-blown non-woven fabrics is less than 70%, the electrostatic electret process is not enough to rely solely on the mechanical blocking effect of the three-dimensional aggregates of fibers with fine fibers, small voids and high porosity of melt-blown ultra-fine fibers. Meltblown filter materials generally add electrostatic charge effect to meltblown cloth through electrostatic electret process, and use electrostatic method to improve filtration efficiency, so that the filtration efficiency can reach 99.9% to 99.99%, and a very thin layer can achieve The expected standard, breathing resistance is still small. The current electrostatic electret methods mainly include electrospinning method, corona discharge method, triboelectric method, thermal polarization method, low-energy electron beam bombardment method, etc. Among them, corona discharge method is the best electrostatic electret method at present. The corona discharge method is to charge the melt-blown material by one or more sets of corona discharges through a needle electrode of an electrostatic generator (voltage is generally 5-10KV) before the melt-blown fiber web is wound, and the needle tip is applied when high voltage is applied. The air below produces corona ionization, resulting in local breakdown discharge. The carriers are deposited on the surface of the melt-blown cloth by the action of the electric field. Become an electret filter material. Increasing the charge on the surface of the melt-blown cloth can be obtained by the electret treatment process of the corona discharge method, but in order for this electrostatic storage to not decay, the composition and structure of the melt-blown electret material must be conducive to charge retention. One way to improve the charge storage capacity of electret materials can be through the introduction of additives with charge storage properties to generate charge traps to capture charges. Therefore, compared with ordinary melt-blown production lines, the production of melt-blown production lines for air filter materials requires a high-voltage electrostatic discharge device to be added to the production line, and electret masterbatches, such as tourmaline, need to be added to the production raw material polypropylene (PP). particle.
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