The N in N95 masks represents not resistant to oil, that is, not resistant to oil; The number represents the filtration efficiency when tested with 0.3 micron particles, and 95 means it can filter out at least 95% of small particles such as influenza virus, dust, pollen, haze, and smoke. Similar to medical surgical masks, the main structure of N95 masks consists of three parts: a surface moisture-proof layer, a middle filtering and adsorption layer, and an inner skin layer. The raw material used is high molecular weight polypropylene meltblown fabric. Since they are all meltblown fabrics, what are the reasons for the filtration efficiency not meeting the standard?
Reasons for the substandard filtration efficiency of mask meltblown fabric
The filtration performance of meltblown non-woven fabric itself is actually only below 70%. It is not enough to rely solely on the mechanical barrier effect of the three-dimensional fiber aggregates of meltblown ultrafine fibers with fine fibers, small voids, and high porosity. Otherwise, simply increasing the weight and thickness of the material will greatly increase the filtration resistance. So melt blown filter materials generally add electrostatic charges to the melt blown fabric through the process of electrostatic polarization, using electrostatic methods to improve filtration efficiency, which can reach 99.9% to 99.99%. That is to say, reaching N95 standard or above.
Principle of melt blown fabric fiber filtration
The melt blown fabric used for N95 standard masks mainly captures particles through a dual effect of mechanical barrier and electrostatic adsorption. The mechanical barrier effect is closely related to the structure and properties of the material: when the meltblown fabric is charged by corona with a voltage of several hundred to several thousand volts, the fibers diffuse into a network of pores due to the electrostatic repulsion, and the size between fibers is much larger than that of dust, thus forming an open structure. When dust passes through the melt blown filter material, the electrostatic effect not only effectively attracts charged dust particles, but also captures polarized neutral particles through electrostatic induction effect. The higher the electrostatic potential of the material, the higher the charge density of the material, the more point charges it carries, and the stronger the electrostatic effect. Corona discharge can greatly improve the filtration performance of polypropylene meltblown fabric. Adding tourmaline particles can effectively improve the polarizability, increase filtration efficiency, reduce filtration resistance, increase fiber surface charge density, and enhance the charge storage capacity of the fiber web.
Adding 6% tourmaline to the electrode has a better overall effect. Too many polarizable materials can actually increase the movement and neutralization of charge carriers. Electrified masterbatch should have nanometer or micro nanometer scale size and uniformity. Good polar masterbatch can improve spinning performance without affecting the nozzle, enhance filtration efficiency, resist electrostatic degradation, reduce air resistance, increase the density and depth of charge capture, increase the probability of more charges being trapped in the fiber aggregates, and keep the captured charges in a lower energy state, making it difficult to escape from charge carrier traps or be neutralized, thus slowing down degradation.
Melt blown electrostatic polarization process
The process of melt blown electrostatic discharge involves adding inorganic materials such as tourmaline, silicon dioxide, and zirconium phosphate to PP polypropylene polymer in advance. Then, before rolling the fabric, the melt blown material is charged by one or more sets of corona discharges using a needle shaped electrode voltage of 35-50KV generated by an electrostatic generator. When high voltage is applied, the air below the needle tip produces corona ionization, resulting in local breakdown discharge. The charge carriers are deposited on the surface of the melt blown fabric through the action of the electric field, and some of them will be trapped by the trap of the stationary mother particles, making the melt blown fabric a filter material for the electrode. The voltage during this corona process is slightly lower compared to a discharge with a high voltage of around 200Kv, resulting in less ozone production. The effect of charging distance and charging voltage is counterproductive. As the charging distance increases, the amount of charge captured by the material decreases.
Electrified meltblown fabric is required
1. One set of melt blown equipment
2. Electrified masterbatch
3. Four sets of high-voltage electrostatic discharge devices
4. Cutting equipment
Meltblown fabric should be stored moisture-proof and waterproof
Under normal temperature and humidity conditions, PP melt blown polarizable materials have excellent charge storage stability. However, when the sample is in a high humidity environment, a large amount of charge loss occurs due to the compensation effect of polar groups in water molecules and anisotropic particles in the atmosphere on the charges on the fibers. The charge decreases with increasing humidity and becomes faster. Therefore, during transportation and storage, the meltblown fabric must be kept moisture-proof and avoid contact with high humidity environments. If it is not stored properly, the masks produced will still be difficult to meet the standards.
Dongguan Liansheng Non woven Technology Co., Ltd. was established in May 2020. It is a large-scale non-woven fabric production enterprise integrating research and development, production, and sales. It can produce various colors of PP spunbond non-woven fabrics with a width of less than 3.2 meters from 9 grams to 300 grams.
Post time: Oct-27-2024