COVID-19: Mask’s Filter and Its Electret Treatment
COVID-19: Mask’s Filter and Its Electret Treatment
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COVID-19: Mask’s Filter and Its electret treatment. For medical masks, the electret meltblown cloth with electret masterbatch and high-voltage piezoelectric treatment is the core part of the mask to filter viruses.
Only after the electret treatment has a lot of charges on the meltblown cloth is the same The charged 0.3 micron non-oily small particles have an efficient adsorption effect, usually more than 95%. Some manufacturers also use two layers of 25-28 grams of electret treated melt blown cloth to achieve 50-60 grams of electret melting The effect of spraying.
According to the appropriate ratio, mix with polypropylene resin uniformly or adjust the ratio with master batch feed metering device.
After stable production, the non-woven fabric passes through the designed electret equipment tunnel to charge the non-woven fabric.
There are several types of charging power supplies: filtered air electrostatic field generation power supply, plasma electric field generation power supply, and pulse power plant generation power supply.
Finally, the effect of electret is related to polypropylene meltblown raw materials, production process, fiber size, weight, electret equipment, electret power supply voltage, etc. It can be used in batches only after the actual trial test is qualified.
China is the world’s largest producer and exporter of masks
At the beginning of 2020, the COVID-19 virus is like a sudden storm sweeping all countries around the world. Epidemic prevention has become our top priority. The COVID-19 virus is very contagious. Wearing a mask is not one of the most effective ways to prevent virus transmission.
In 2019, our daily output of masks in China was 20 million, and the total number of masks worldwide was 40 million. We accounted for 50% of the global production capacity. Today, the production volume of our masks has reached more than 200 million per day.
The surge in demand for masks has caused a blowout growth in our mask industry chain. Various companies in various industries have entered such a field of mask production. inside.
I believe that many people may not know what meltblown cloth is, what is electret processing and what is electret masterbatch before 2020, because everyone does not pay much attention to masks. With the rapid growth of the mask industry, many new friends have entered the field of mask production.
Judging from the current situation of the epidemic, people’s awareness of epidemic prevention and protection has increased after the epidemic, and our mask industry may also be a long-term and long-term development process, in order to enable our masks and related fields to be more stable, orderly and high-quality.
For development, we need to use a professional and scientific attitude to understand and study it.
Classification of masks:
- First, take a look at the classification of masks. Masks are divided into ordinary gauze masks, daily-use protective masks, industrial dust masks and medical masks. Ordinary gauze masks have better cold and warm air permeability and can block some dust.
- Daily-use protective masks mainly prevent PM2.5, pollen, etc., industrial dust masks mainly isolate dust and other things in the industrial production process, and play a protective role in occupational protection.
- In the mask classification, only medical surgical masks and medical protective masks in medical masks can effectively block bacteria and viruses in the air.
China’s technical requirements for medical surgical masks
According to China’s technical requirements for medical surgical masks, there are mainly several aspects:
- the air resistance of the mask should be small. In other words, we will not breathe out after wearing masks, and the breathability is better;
- filtration of bacteria. The bacteria filtration rate should be high, so that bacteria cannot enter our body;
- timeliness and longer validity period. That is, we must be able to maintain the indicator of bacterial filtration rate for a long time. These three technical points are closely related to electret, electret processing and electret masterbatch.
What is electret and electret treatment?
The electret is a kind of dielectric that can store charge for a long time. Our polypropylene fiber is a typical electret material, which is very suitable as our filter material.
The polypropylene material is melt-blown into melt-blown cloth, which is the core filter layer in the mask. After the meltblown cloth is made, the meltblown cloth will be electret treated, and static electricity will be applied to the meltblown cloth to charge the surface of the fiber, thereby being able to capture particles in the air and improve the filtration efficiency.
Electret processing is the process of adding more charge to our electret.
There must be electret equipment for electret treatment, which is our electret or electrostatic generator. Large-scale meltblown machines, whose width is more than 1 m2, will use electret machines to perform electrical treatment on them.
The general voltage is between 60,000 and 120,000 volts. For some small meltblown equipment, such as 65 machines, we will use some 50 to 60 KV electrostatic generators to conduct electrical treatment on them.
Filter material and filter structure
Next, let’s look at our filter material. What does it look like? What is its structure?
Let’s take a look at this picture. This is a schematic diagram of the structure of our melt spray cloth under a high-power microscope.
As you can see in this picture, the melt blown cloth is a kind of multi-layer crisscross network structure. The vertical and horizontal of this network structure is a random and disordered arrangement, which makes the fiber web in the middle of the meltblown cloth have a large number of tiny voids.
At the same time, this random distribution makes the air of the melt blown cloth a kind of multi-curved channel when it enters. The twists and turns are not straight up and down.
This feature causes the particulate matter and melt in our air. The probability of the sprayed fiber colliding and being blocked is greatly improved.
Meltblown fiber filtration mechanism
What exactly is the filtering mechanism of meltblown fibers to filter particulate matter in the air? There are two main methods for filtering particulate matter in the air. The first is mechanical barrier, and the second is electrostatic adsorption.
The so-called mechanical barrier is to use the Brownian motion inertial collision of the particles to intercept and intercept the particles mechanically. However, this method has a very poor filtering effect for such particles with a particle size of less than one micron.
The particle size of a typical virus is between 0.02 and 0.2 microns, and the particle size of the new coronavirus is 0.1 microns. Obviously, the use of mechanical barriers cannot stop these tiny particles.
In order for this virus and bacteria to be intercepted by the mask, a second filtering method is required, that is, electrostatic adsorption.
Bacteria and viruses are charged on the surface. Electrostatic adsorption is the use of Coulomb force to adsorb viruses and bacteria through the charge on the mask, which can effectively prevent such small particles from entering the human body. Masks can effectively block bacteria and viruses, mainly relying on electrostatic adsorption.
What kind of material is suitable for melt blown cloth?
We have just mentioned that polypropylene material is melt-blown to form a kind of ultra-fine fiber, so it needs to have high fluidity, and the melt-blown material has a melt index of more than 1500 before it can be melt-blown. The silk will be thinner.
Some time ago, the meltblown material was very tight and the price was very high. Some friends might choose some materials with low melting index, such as S2040 and so on, for meltblown. Then you can imagine that the melt blown The quality of the spray material must not meet the standard requirements.
Because the lower the molecular weight of the polymer, the higher the melt index and the lower the viscosity of the melt. Such materials are more suitable for meltblown. The meltblown cloth produced by meltblown is of better quality and can adsorb viruses!
Production Process
There are two main production processes for meltblown materials. The first is an integrated polymerization reaction catalyzed by metallocene. This method is a one-step material made by polymerization of PP (polypropylene) monomers, and its molecular weight is controlled to meet the technical parameter requirements.
The meltblown material polymerized by this method has a relatively narrow molecular weight distribution, and a relatively small and uniform melt index fluctuation.
Another process method is to use peroxide to break and degrade some low-melting-point polypropylene materials, making it a high-fluidity melt-blown material.
Compared with polymeric meltblown materials, meltblown materials made of peroxide have a wider molecular weight distribution, its crystallization and reaction may be insufficient, the smell is also relatively large, the fluctuation of the melt index is also relatively large, and it is resistant to aging. Performance is also relatively poor.
Reflected in our melt-blown process, there will be die plugging. The melt-blown cloth has a strong smell, is brittle, and has poor aging resistance. The yarn may be uneven in thickness.
The above picture is the anti-aging comparison experiment of meltblown cloth made by polymerization method and peroxide degradation method. Sample 1 is melt blown material made by peroxide degradation method, and sample 2 is melt blown material made by polymerization method.
Both of these materials are foreign big-name products that I choose. Even such a big-name product can see that there is still a big difference between our peroxide degradation method and polymerization method to produce melt-blown cloth. You can see through the test conclusions. The former is obviously inferior to the latter in aging resistance.
What is N95 mask?
Under the specified conditions, the aerosol filtration rate of Staphylococcus aureus reaches more than 95%. Only masks that meet this standard can effectively protect against bacteria and viruses. This is the origin of the concept of N95 masks.
How can we achieve a bacterial filtration rate of 95% or even more than 99%? Then you need to use a good meltblown material, preferably the meltblown material produced by the polymerization method, because it sprays finer and more uniform filaments, so that the meltblown fiber has a larger specific surface area to absorb more charges.
In addition, good melt-blown equipment and melt-blown process are also needed, and at the same time there is enough voltage to apply static electricity.
So with good meltblown materials, meltblown equipment, and meltblown technology, can N95 masks be made? The answer is not complete, because the charge on organic materials decays very quickly, and its timeliness is very short!
The meltblown material that was just made passed the inspection, but it failed to meet the standard after a period of time. At this time, electret masterbatch is needed!
The role of electret masterbatch
When the meltblown material is subjected to electret treatment, without the participation of the electret masterbatch, the electric charge on the meltblown fiber presents an irregular arrangement and distribution, so its electric field vector is very low, and the charge initiator in the electret masterbatch participates Under the premise of, the electric field vector will be significantly improved, and a stable electric field voltage can be formed for a long time. So we say that the electret masterbatch is the “battery” of meltblown cloth.
Some friends may ask whether the meltblown cloth after adding the additive masterbatch can achieve 95% or even higher filtration efficiency?
This question is not comprehensive enough. Because we just said that making a good high-quality meltblown cloth is related to the material, meltblown process, voltage, etc., the achievement of the N95 standard is not necessarily related to the electret masterbatch.
The biggest role of electret masterbatch is to help us to store the electric charge of this kind of melt blown cloth that reaches the N95 standard and extend its validity period! For example, our country has a technical standard requirement for new energy vehicles, the cruising range, and it needs an efficient and long-lasting battery to continuously provide electricity to meet the requirements.
Electreting the meltblown cloth is equivalent to charging the meltblown cloth to store more electric charge. The electret masterbatch is our meltblown cloth battery!
Japan’s N95 masks are valid for up to three years! Therefore, it requires high-quality electret masterbatch to give him such an ability to maintain such a good bacterial barrier efficiency for three years.
Organic electret masterbatch and inorganic electret masterbatch
The core material of electret masterbatches is the charge initiator. According to the different types of charge initiators, we can divide the electret masterbatches on the market into two categories: one is organic electret masterbatches, and the other is Inorganic electret masterbatch.
The organic electret masterbatch is transparent in appearance, looks very beautiful, and the filter performance is very good in all aspects, but it also has some shortcomings that the electrostatic field is extremely unstable, which will cause this rate-effective instability.
The attenuation of the charge on organic materials will also cause this rate-effective attenuation. So far, no charge initiator of organic materials has been found that can store the charge for more than three months without attenuation!
When the inorganic charge initiator reaches 1% in the final product, its charge storage capacity can reach more than 12 months without decay, so inorganic electret masterbatch is still recognized as the most effective product.
So what are the main ingredients in the inorganic electret masterbatch? Its charge initiator is tourmaline powder!
Tourmaline
Tourmaline is the general name of the tourmaline group of minerals. It is a cyclic silicate mineral. We often hear lithium tourmaline, aluminum tourmaline, iron tourmaline and so on.
Tourmaline has the characteristics of self-discharge, preheat discharge, negative ion release, far infrared and so on. Because tourmaline changes in ambient temperature and pressure, the charged particles in its crystal structure will shift, and the centers of positive and negative charges will separate, causing the two poles of tourmaline to form an electrostatic field, that is, with polarity. .
Due to the difference in the crystal structure of tourmaline, as well as the different atoms and ions contained in it, its physical properties will change greatly. Different tourmalines have completely different physical properties!
Many electret masterbatches are made of tourmaline, but different electret masterbatches differ greatly in their ability to store electric charges. Because different tourmalines have different components, the formulas in the production process are also different! It is not that the electret masterbatch made of tourmaline is the same!
To give an example, many substances are made of carbon atoms, such as graphite, carbon black, diamond, etc. Because of the difference in structure, the effect is completely different, and the price will also vary greatly. The excellent physical properties of tourmaline powder will increase with the decrease of its powder particles. This is what we call the small size effect! Therefore, we will desperately want to make tourmaline powder particles smaller.
But there is a problem here, because there is still a kind of van der Waals force between the molecules. When you make tourmaline powder very small, the tiny powder particles will attract each other and gather together again. At the same time, more small particles gather together to form a large agglomerate, which will cause uneven electric charge formed on the surface of the fiber when reflected on the material. Because the tourmaline is unevenly dispersed, we have to carry out the tourmaline powder Surface modification.
The process of surface modification of tourmaline powder
First, we use jet mill to crush it to nanometer size.
At the same time, in order to reduce the adsorption on the surface of small-size powder, we have to organically coat the surface of the particles, so the production process of electret masterbatch is how to deal with this kind of powder.
Then, through the extrusion equipment, the nanometer tourmaline powder is evenly dispersed on the carrier material, so that our meltblown fiber can be uniformly attached to the tourmaline powder to release the stored charge, and the meltblown material has strong electrostatic adsorption It can effectively filter viruses and bacteria.
Finally, after the production of the electret-treated meltblown cloth is completed, its surface is long-term charged. But because the moisture in our environment will reduce the charge and cause the attenuation of the filtering effect, a good meltblown cloth and a good mask must have a better storage method and a good transportation method.
It is recommended to vacuum seal the packaging after production to prevent moisture invasion, so that the meltblown cloth and mask can maintain the adsorption of bacteria and viruses for a longer time.
(source:internet, reference only)
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