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The Important Role of Electret Electrostatic Treatment In Surgical Masks

2020-02-19


The Important Role of Electret Electrostatic Treatment In Surgical Masks

Since the outbreak of the new coronavirus, masks have become a necessity.The current fight against the new coronavirus outbreak is in a state of seesaw, and it is expected to take some time, and masks will be a hot item for some time to come.

Disposable surgical masks and N95 masks   

      


During this period of time, the whole people also carried out a popular science on the knowledge of masks.Know what is called N95 mask, what is called surgical mask, know the mask is made by PP non-woven fabric......

Surgical mask construction  

      


Also know that in this case, normal gauze masks, even activated charcoal masks, are not effective against the virus.Because although cloth can block large particles of dust, but for less than a certain diameter can not stop.

Masks fibers  

      


A common surgical mask has a three-layer structure. When wearing the mask, the white side should be turned inwards and the side with the layering should be turned upward. Firstly, the two straps of the lower edge should be tied behind the neck and pulled tight to make the lower part of the mask reach the base of the chin.Pull up the upper edge of the mask, cover the mouth and nose with the mask, pull the belt behind the ear and tie it tightly to the head, do not tie it to the ear.Press the wire at the base of the nose above the mask with your index fingers on both hands to make it close to the skin of the nose. Then move your index fingers to the side to make the mask close to the skin of the face.These three layers of non-woven fabric have their own characteristics, the most outer layer, the most inner layer is the spunbonded non-woven fabric, can be done as a support, the blue part of waterproof, white part of better moisture absorption.The filter layer in the middle is melt-blown non-woven fabric, which is the most important layer. The fiber diameter of melt-blown layer is relatively thin, about 2 microns (diameter m), so it is only one tenth of the diameter of spun-bonded layer, which is crucial for preventing bacteria and blood penetration.Sometimes, of course, the number of layers will increase in order to increase the effect.The N95 mask has a similar structure, but the non-woven booker is heavier and thicker.And the N95 mask is pre-formed to fit the face shape better.

Relation curve between electret voltage and filtration efficiency  

      

At first thought, a little strange, just like the dishcloth we use in the kitchen, always good with cotton, because the absorption of cotton cloth is good, and PP is plastic, plastic cloth to do dishcloth is always useless.Then why, PP do non-woven cloth, made into a mask, filtration effect so good?

When we were kids, we played games where we could suck up scraps of paper by rubbing a pen on our hair or clothes.Why is that?This is when the insulator generates static electricity during friction, allowing it to absorb other objects.

The reason for the good adsorption effect of surgical masks is that the non-woven fabric of the melt-blown layer is treated with static electricity, and the technology can keep the static electricity for a long time, which is electret static electricity.PP is an insulator and in plastics is a non-polar material that produces static electricity.

In 1976, j. Van Turnhout et al. made use of polypropylene film cut into small strips, and processed the charged strips into cringed state to form electret fibers, which were then made into cloth, which could be made into filter materials.These filter materials can not only use cloth mechanical filtration, but also use electrostatic physical adsorption, for the solid particles in the air, bacteria, viruses and other sundry filtration effect greatly improved.

At present, the main electret methods are electrospinning, corona discharge, triboelectric, thermal polarization, low energy electron beam bombardment and so on.Corona discharge method is a common electrostatic electret method.

The relationship between electret voltage and filtration efficiency has been studied using melt-blown non-woven fabrics.At present, the commonly used electret voltage is between 30 and 60KV.

By the way, masks are so tight these days that there are all kinds of ways to talk about reusing them online.Knowing the principle above, we know the method of recycling.First, you can't wash it with water or any other liquid.The new coronavirus is afraid of heat, and it will be over after 30 minutes at 56℃. You can add heat to the mask, but the temperature should not be too high.The melting point of PP is 160 ℃, deforming around 120℃, so it can be treated at 80-100℃ for a period of time, killing the virus.Some people say that in the microwave, PP materials are polar, microwave doesn't work, we have a lot of the microwave can use of lunch box, is PP), but does not guarantee that mask are metal or other polar materials, in the microwave will accident caused by the temperature rise (we heat the food in the microwave oven, it is because of material in the food are basically polar, under the high frequency microwave, polar material back and forth movement result in higher temperature to heat).Of course, it is not recommended to reuse too many times, because every time you use, static electricity will consume a part, consumed, electrostatic adsorption is not effective, filtration efficiency will be greatly reduced!       

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