Filter principle of melt blown cloth
The main material of melt blown non-woven fabric is polypropylene, which is a kind of ultra-fine electrostatic fiber cloth and can catch dust. When the droplets containing pneumonia virus are close to the melt blown non-woven fabric, they will be electrostatic adsorbed on the surface of the non-woven fabric and cannot be penetrated. This is how the material insulates against germs. When the dust is caught by the ultra-fine electrostatic fiber, it is not easy to be separated by cleaning, and water washing will also destroy the electrostatic dust collection ability. So the mask can only be used once.
For medical masks, this layer is the core part of the electret melt blown cloth with electret masterbatch and electret high voltage electrical treatment. Only after electret treatment, many charges on the melt blown cloth can effectively adsorb 0.3 μ m non oily small particles with the same charge, which is usually required to be more than 95%. In fact, only professional instruments can test this layer. Some manufacturers also use two kinds of materials Layer 25-28g electret treated melt blown cloth to achieve the effect of 50-60g electret melt blown cloth!
Types of electret Masterbatch
There are generally three types of electret masterbatch on the market at present: carbide production, gas silicon production and nitrogenous chemical fatty acids. Their appearance, characteristics and addition amount (for reference only):
Basic composition of electret Masterbatch
In the process of melt blown nonwovens spinning, adding 2% - 5% additives can effectively increase the density and depth of the charge trapping energy well in the melt blown nonwovens, so as to improve the comprehensive filtration efficiency and heat attenuation resistance of the melt blown nonwovens.
Basic parameters of electret masterbatch: 1. Appearance: cylindrical uniform particles. 2. Bulk density: 0.50-0.80g/cm. 3. Color: white or light yellow 4. Dosage: 2.0-5.0%
Why is electret masterbatch added to melt blown cloth?
In addition to mechanical barrier, mask filtration also has electrostatic adsorption. Fiber filter material is a kind of common material to deal with pollution and purify air and water. The mechanism of dust collection mainly depends on the mechanical blocking effects such as Brownian diffusion, interception, inertial collision, direct interception, etc., but the mechanical blocking effect is very poor for the particle whose particle size is less than 1 μ m, which can not play a purification role.
In addition to the original mechanical barrier effect, in the process of air filtration, electrostatic adsorption is added, and charged particles in the gas phase are directly attracted and captured by Coulomb force, or neutral particles are induced to generate polarity and then captured, which can more effectively filter sub particles in the gas carrier phase, greatly enhancing the filtration efficiency, while air resistance will not increase. This is the so-called high efficiency and low resistance filter material. Electret materials have this property.
In order to make this electrostatic storage, the material composition and structure are required to make the electret conducive to charge retention. In the process of electrostatic storage and charge retention, the addition of electret masterbatch plays an important role.
Adding a small amount of electret masterbatch can increase the density and depth of the charge trapping energy well, effectively release negative ions and store charges, and improve the comprehensive filtration efficiency and heat attenuation resistance of the melt blown non-woven fabric. Under the condition of the same fiber fineness and gram weight, it can reduce the resistance of non-woven products and improve the filtration efficiency of non-woven products.
The electret master batch with excellent performance and stable quality is an indispensable material for the production of electret melt blown cloth. It has the function of long-term generation of electric ion storage charge. Under the action of initiator, Mg Li tourmaline can permanently release electric charge and air anion, increase the electrostatic absorption of mask and the ability of negative ion sterilization, and effectively block the droplets, dust, virus, etc. below micron.
When the electret is heated and pressurized, the discharge characteristics of Mg Li tourmaline will be further strengthened, and the effect will be doubled when the temperature is increased by 10 degrees, and the finer the fineness is, the more static electricity will be generated, and the higher the voltage will be.
Electret is a kind of dielectric material with long-term charge storage function, which has the advantages of high efficiency, low flow resistance, antibacterial, energy saving, etc. on the basis of ensuring the physical collision and barrier effect of conventional filter material, the electrostatic adsorption effect is increased. The electret treatment makes the filter material fiber with electric charge. Combined with the characteristics of dense melt blown microfiber material, a large number of electrodes are formed between the charged fibers. The charged fibers can not only attract most of the charged particles in the environment like magnets, but also polarize some of the uncharged particles, so as to absorb some pollutants with smaller particle size, even viruses of nanometer scale The substance can also be electrostatic adsorption or charge exclusion.
The results show that the electret effect of the melt blown cloth is greatly improved by adding electret masterbatch, and its surface charge density, filtration performance and antibacterial property are all significantly improved.
Corona charging is the most widely used method to produce electret filter materials in industrial production. The corona discharge which causes the partial breakdown of air by a uniform electric field produces the ion beam to bombard the dielectric and deposit the ion charge in the dielectric, then the material will be charged.
Put the prepared melt blown non-woven fabric on the electrode plate → align multiple rows of corona needle tips with the sample → turn on the high-voltage electrostatic generator to increase the needle voltage → reach a certain voltage → the air near the electrode needle produces partial corona discharge → release electrons or ions → capture the same charge with the discharge needle polarity on the sample surface under the electrode → make the polypropylene melt blown fiber charged.
At present, the development of electret masterbatch is only produced by a few enterprises in China, many of which are used by melt blown cloth factories themselves. With a large number of melt blown cloth put into production in China, and the production of mask, protection and other melt blown cloth, the market of electret masterbatch is in a state of shortage at present.
What are the factors that affect the electret performance of the melt blown cloth?
Firstly, it should be observed from two aspects: material structure and electret process conditions
Influence of material structure and properties on electret performance
1.crystallinity of polypropylene
Crystallinity is an important parameter of crystalline polymer, which has an important influence on the physical and mechanical properties of materials. The factors that affect the crystallinity include the regularity of polypropylene, nucleating agent, etc., but the regularity is the main factor. The higher the regularity and crystallinity of the material, the better the stability of space charge and the stronger the crystallization ability.
Figure 1: XRD diffraction pattern of polypropylene
It has been shown that nucleating agent has an obvious effect on the crystallization behavior of PP. Generally speaking, nucleating agent can improve the crystallization temperature of PP, increase its crystallinity and accelerate its crystallization rate. Therefore, the effect of nucleating agent is generally evaluated by the increase of crystallization temperature. The more the crystallization temperature of PP increases, the more effective the nucleating agent will be. When PP is nucleated and crystallized at high temperature as a nucleating agent, the stability of space charge can be improved to a certain extent.
Fiber fineness is one of the important parameters to determine the filtration efficiency and pressure loss of filter materials. When the quantity of filter material is the same, the finer the fiber, the larger the surface area of filter material, the larger the filtration coefficient and the better the adsorption effect.
Table 1: relationship between average fiber diameter and initial filtration performance of melt blown nonwovens
It can be seen from the table that the smaller the diameter of the fiber is, the higher the initial filtration efficiency and the higher the initial surface electrostatic potential are when other parameters of the fiber mesh remain unchanged. In addition, because the surface charge intensity of the material is directly proportional to the filtration efficiency, when the surface charge of the material is more, it is easy to capture more charged particles, which is beneficial to the filtration efficiency. However, when the fiber diameter is large, the corresponding filtration resistance is small, which is beneficial to the development of high-efficiency and low resistance filtration materials.
In the actual storage process, the filtration efficiency of the material will change with time, showing a downward trend. Compared with the samples with large fiber diameter and small fiber diameter, the filtration efficiency is initially high. With the extension of storage time, the filtration efficiency of all samples will decline with time. For the samples with small diameter, the percentage of filtration efficiency decline is small, and the retention rate of filtration efficiency is small It is high, so the filtration efficiency is maintained for a long time.
Figure 2: surface potential attenuation of samples with different average fiber diameters
3.hole diameter of material
The pore diameter of the filter material is an important parameter to determine the filtering accuracy of the material and the effect of surface filtration. The filtration function of porous filter material is realized through its internal microporous channel, so the filtration precision of melt blown material depends on its microporous size to a great extent.
Table 2: relationship between fiber pore size and sample initial filtration efficiency and initial surface electrostatic potential.
It can also be seen from the table that the pore size has little effect on the initial surface electrostatic potential of the material. Basically concentrated at about 6.5kv. However, the initial filtration efficiency of samples with small pore size is higher. With the extension of storage time, the filtration efficiency of the material gradually decreases with time, and finally it is stable at a certain value. So to some extent, the materials with small pore size have better filtration performance and electret performance.
Effect of electret process conditions on electret performance
Generally, corona charge is used for electret treatment of melt blown polypropylene nonwovens. Charging voltage, charging time, charging distance and environmental humidity have great influence on the stability of static charge of air filter materials.
Among them, the charging voltage has the most obvious effect on the sample's electret effect, followed by the environmental humidity and charging distance, and the charging time has the least effect on the sample's electret effect.
With the increase of charging voltage, the filtration efficiency of melt blown electret nonwovens increases. This is because with the increase of the charging voltage, the electret charge on the surface of the sample increases, and the adsorption on the particles increases, thus improving the filtration efficiency.
With the increase of charging time, the filtration efficiency of the melt blown electret material increases, and its influence law is similar to that of charging voltage. This is because the more charging time, the more charge captured on the sample surface, the more particles can be adsorbed, so the better the filtration efficiency.
With the increase of charging distance, the filtration efficiency of melt blown electret decreases. This is because the larger the charging distance, the less the charge captured on the sample surface, and the less the adsorbable particles, so the filtration efficiency will be reduced.
Theoretically, when the melt blown nonwovens are stored in the atmosphere, the environmental temperature has no obvious effect on the charge storage life of the materials. But on the whole, whether the filtration efficiency or the surface electrostatic potential of the material, there will be attenuation phenomenon with the extension of time, which is not good for the use of the material.
For example, the melt blown material in the mask is usually placed for a long time after production. During storage, the filtration and electret performance of the material will decline with time. The worst case is that the material has failed in use and has little filtration effect.
For the melt blown electret filter material, the ideal state is that the filter performance and electret performance of the material remain at a certain stable level, but the environmental conditions have a certain impact on the filter performance and electret performance of the material, especially the charge storage stability.
Methods to improve the electret durability of melt blown materials
In material selection:
We can start from improving the crystallinity of the material, such as selecting polypropylene with high isotacticity as the raw material, or doping the material to form microcrystalline structure. At the same time, it can improve the density of the material, such as reducing the fiber fineness, increasing the thickness of the material, increasing the porosity and so on.
In electret process:
The charge storage stability of materials can be improved to a certain extent by using high-voltage charging, reducing charging distance, extending charging time properly or adopting high-temperature charging, and heat treatment after electret. At the same time, attention should be paid to the sealing and preservation of the materials to avoid direct contact with the humid environment, which is conducive to maintaining the charge stability of the materials to a certain extent.