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Factors affecting the attenuation of polypropylene electret non-woven fabrics

by:XINYU Non-woven     2022-01-20

(1) Factors affecting the electret effect

The polypropylene meltblown non-woven fabric was electretized by the corona electret method. Generally, the surface electrostatic potential of the meltblown nonwoven filter material increases with the increase of charging voltage and charging time, and decreases with the increase of charging distance and ambient humidity. Among them, the charging voltage has the most obvious effect on the electret effect of the sample, followed by the environmental humidity and charging distance, and the charging time has the least effect on the electret effect of the sample. The charging voltage, charging distance and charging time have little effect on the electret stability, while the environmental humidity has a greater impact on the electret stability, and the greater the environmental humidity, the worse the electret stability, and the more the charge decays.

With the increase of the electret voltage, the electrostatic potential on the surface of the melt-blown cloth increases, indicating that with the increase of the voltage, the energy of the generated carriers increases. In addition to being captured by the surface traps deposited on the surface of the meltblown nonwoven, it can also pass through the pores of the meltblown cloth to reach a certain depth of the surface and be captured by various traps inside the meltblown cloth, so the surface of the meltblown nonwoven The charge increases as the electret voltage increases. However, as the voltage continues to increase, the surface electrostatic potential of the meltblown cloth decreases. The reason may be that after the voltage continues to increase, the energy of generating carriers increases to a certain extent, and a slight breakdown occurs. That is, in addition to being captured, ions with high energy are no longer captured by the meltblown cloth, but directly penetrate it and be guided away, so that the surface electrostatic potential of the meltblown nonwoven fabric is reduced.

The electret spacing is one of the important factors affecting the electret effect. If the electret spacing is too small, it is easy to break down the meltblown nonwoven fabric quickly. However, the electret spacing should not be too large. The larger the electret distance, the worse the electret effect. Therefore, an appropriate electret spacing should be selected.

As the electret time increases, the surface potential of the material increases. This is because the longer the electret time is, the more charges are deposited on the surface of the non-woven sample, and the potential of the electret surface increases. However, as the time continues to increase, the breakdown phenomenon of the melt-blown cloth under the action of the ion beam for a long time is more serious, resulting in a decrease in the surface charge.

With the increase of the heat treatment temperature, the surface electrostatic potential of the non-woven fabric increases, but when it increases to a certain value, the surface electrostatic potential of the non-woven fabric shows a downward trend. The reason is analyzed that heat treatment can transform the crystal form of melt-blown polypropylene fibers.

(2) Influence of electret on filtration efficiency

Fiber diameter and areal density have a direct effect on the dust filtration efficiency of PP meltblown nonwovens. The decrease of , and the increase of areal density, the filtration efficiency of the material is obviously improved. The effects of fiber diameter and areal density on filtration efficiency are complementary to a certain extent, that is, increasing the areal density of the product can appropriately compensate for the decrease in filtration efficiency caused by the increase in the average fiber diameter. The decrease in fiber diameter and the increase in areal density will result in an increase in air permeation resistance, and at the same time will lead to a decrease in production efficiency and an increase in production cost.

Electret treatment is a solution to the contradiction between breathing resistance and filtration efficiency. Compared with the non-electret meltblown filter material, the filtration performance of the electret filter material changed dramatically, and with the increase of the electret voltage, the filtration efficiency showed an upward trend. The electret process increases the number of charges accumulated on the surface of the filter material in the same time, the surface potential becomes higher, and the surface of the filter material forms a more open and open structure, which enhances the capture effect of dust. With the increase of the electret voltage, the charge density on the surface of the filter material increases, and the adsorption and polarization of particles increases, so the filtration efficiency increases with the increase of the electret voltage. The filtration efficiency of the meltblown material after electret treatment is obviously improved, but its breathing resistance changes very little. This is because the electret treatment only makes the meltblown fibers electrostatically charged, and has no effect on the fiber structure of the meltblown material, so it has little effect on the breathing resistance.

(3) Influence of environmental factors on electret non-woven fabrics

Polypropylene melt-blown non-woven fabric electret air filter material treated with different solvents After soaking (water, formaldehyde, ethanol, isopropanol and acetone), the filtration efficiency of the samples decreased to varying degrees. The filtration efficiency was basically unchanged when soaked with water, and decreased less when soaked with formaldehyde and ethanol. Filtration efficiency dropped by almost half during soaking. Solvent soaking did not significantly change the apparent structure of the fibers, and the decrease in filtration efficiency was not caused by the change in resistance. It is caused by the influence of the solvent on the charge storage capacity of the material through the swelling effect. Water and formaldehyde immersion has little effect on the chargeability of the material, and the immersion of isopropanol and acetone has a greater impact, especially acetone. The charge is almost completely eliminated.

The immersion of common aqueous disinfectants will not cause significant attenuation of electret charge in the material, so it does not The filtration performance of the material will not be greatly reduced (compared to pure alcohol, 75% medical alcohol will not cause a significant impact), but the filtration performance of the material will be greatly reduced by common cleaning solutions, which may be due to the surfactant in common cleaning solutions. The strong interaction between the hydrophobic group and the polypropylene molecule leads to the decay of the electret charge.

Under normal temperature and humidity conditions, the PP melt-blown electret filter material has very good charge storage stability, but when the sample is stored in a high humidity environment, due to the water The polar groups in the molecule and the anisotropic ions in the atmosphere compensate for the charge on the fiber, resulting in a large loss of charge. Therefore, environmental humidity is an important factor affecting the charge storage stability of PP melt-blown electret filter material at room temperature. The surface electrostatic potential of the sample gradually decreases with the increase of ambient humidity, and the higher the humidity, the faster the downward trend.


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