Development of electret technology for melt blown cloth
The core of the mask is melt blown cloth. Besides the mechanical properties of melt blown cloth, the most important thing is electrostatic electret. Because there is no melt blown material treated by electrostatic electret technology, the filtration efficiency is only (10-40)%, and 99% virus and aerosol can not be filtered. The electrostatic electret, on the other hand, has an obvious watershed, which distinguishes the front and rear echelons
First tier (electret) - high efficiency and low resistance materials (high end)
At the same time, the material meeting high efficiency (99% +) and low resistance is (20-35) g / m2 water electret electrostatic melt blown material, which is the main material of modern advanced N95 mask. Because of the high electric field, there is less material to stack, and the N95 efficiency can be achieved when the material is (20-35) g / m2. At this time, because the material is thin, the respiratory resistance is very small. The best material can even reach 20g / m2, and the electret has a surface electric field of 15000 volts. This material adopts water electret process to ensure that the electret on polypropylene is permanent.
Tier 2 (corona electrostatic) - high efficiency and high resistance materials (low end)
(45-60) g / m2 corona hot electret electrostatic melt blown material, which is now the main material for general medical masks and some low-end N95 masks. Because the material selected is polypropylene heating plus corona belt point, the formation is not permanent static electret, less charge, uneven distribution, so N95 mask material, need to stack more polypropylene wire in order to achieve the qualified filtration efficiency. So the amount of polypropylene needed to stack is more than twice that of the former. This leads to two problems
First, the hot electret technology is used, which has less electric charge, unstable electric field, and will be naturally de electrified in the process of placing (without exception);
Second, with so many materials stacked, the respiratory resistance is high.
1、 Lifetime of different electret methods
The electrostatic electret of the material is permanent and the shelf life is basically 10 years; the strategic reserve materials of electrostatic melt blown in some European Union countries are rotated in the warehouse for more than 7 years;
Hot electret electrostatic melt blown technology, the static electricity of the material is unstable and non permanent.
The spread of the epidemic has led to higher and higher standards for masks all over the world. Water electret can achieve high efficiency and low resistance. However, many people think that the technology is not mature and stable. In fact, the water electret technology has a history of more than 20 years and has gone through five generations.
2、 Development history of water electret
As early as 1996, the first generation of water electret was born in China. The original way can be said to be the most primitive method. The effect of filtration can be 99% by using high-pressure nozzle to spray melt blown cloth. However, the technology was not mature at that time, and the treatment of cloth was not uniform enough, which was easy to cause damage to the base cloth, and the application field was not widely known.
The second generation product adopts rolling jet, and the base cloth is electrified by centrifugal force. However, this method consumes a lot of energy and is not conducive to large-scale production.
The third generation of water electret adopts spray nozzle and vacuum suction, which can achieve effective electret effect. It has been applied by some enterprises up to now, but the technology is not mature enough. The production of 1.6m equipment is about 600kg per day.
The fourth generation of water electret product is developed in recent years. It uses combined electret liquid to make melt blown cloth electrified by ultrasonic technology. This method has strong applicability and can electret long and short fibers and mixed electret. Moreover, the mechanical force of the ultrasonic sputtering charging device is small, and it is not easy to damage the electret with thin thickness and low strength. However, due to the limited ultrasonic power and small mechanical force, the liquid is not saturated enough to electret.
The fifth generation of water electret products (the latest generation) is completed with the cooperation of multipole jet electret solution and negative pressure. It can make the matrix material quickly and fully wetted, and can make the matrix fully infiltrate in the thickness direction, and produce better electret effect. The daily output of 1.6m equipment is about 1.2 tons.
Electret solution needs special treatment, and some electret masterbatches are not suitable for water electret charging. All electrets are not suitable masterbatch for water electret.
3、 Principle and advantage of water electret electrification
Water electret is different from traditional corona electret in electret mode. The melt blown cloth passes through the water electret equipment and is cooled by cold water at first. The pure water spurts the melt blown cloth through a certain angle, pressure and velocity. The static electricity is generated by the friction between the water and the cloth. Then, the conductive materials in the electret masterbatch are transported to tourmaline or zircon to achieve electrostatic residence.
The advantage of the electrostatic charge is that the electrostatic charge is placed on the surface of the cloth, and the electrostatic charge is left on the surface of the cloth.
The second process of water electret is a drying and electricity locking process for melt blown cloth. The drying process uses the flow of hot air, and uses the increase of surface temperature and air flow to dry. In this process, it faces a high temperature, a high humidity and an air flow. This process is actually a working principle of electrostatic removal equipment, such as If it is not the electricity in the electret, it will be taken away. In the second process, a part of the electricity on the surface of melt blown cloth is eliminated, which is called virtual electricity.
After these two processes, the melt blown cloth leaves the electricity slowly released by the electrical stone powder or zircon, rather than the virtual electricity on the cloth surface. At this time, the filtration efficiency detected is similar to that after aging, without virtual electricity. This kind of electricity is determined by the physical and chemical properties of the storage medium and will not lose power quickly.
One of the simplest methods to test power-off is to put the conventional corona electrostatic cloth and the water electret corona cloth in the 70 ° oven. Generally, there will be significant changes in four hours. The efficiency of the corona charged fabric decreases significantly, while the water electret cloth has almost no attenuation.
These high-end technologies were first held by large foreign companies. In Japan, the initial price of such a mature (melt blown + water electret) synchronous production line was as high as $3 million. In 2018, Peter Cai, a Chinese scientist who is known as the father of N95, developed a charging method with hydraulic friction. This treatment can improve the filtration efficiency of melt blown cloth by 20 times, and the charging is full and stable. In the past two years, some large domestic textile enterprises have introduced foreign technology and gradually put on-line water electret equipment. Large factories quietly produce high-end meltblown fabrics, which are expensive and do not want to lose core technology. After the outbreak of Xinguan epidemic, the demand for masks at home and abroad has increased significantly, and high-end melt blown fabrics are still in short supply.
As early as before the epidemic, the market price of FFP2 meltblown cloth was not cheap. At present, the price of fpp2 meltblown cloth in small factories is (250000-350000) / T. The price of ffp3 meltblown fabric is about 500000 / T, and the high-end part of a few large factories is as high as 700000-800000 / T. The reason why the price is expensive is mainly due to the high cost of water electret equipment, limited production capacity and large daily energy consumption. At the same time, the long-term electrified melt blown fabric is also a national strategic reserve material, which is difficult to find in the mass market. Recently, there have been a number of manufacturers of water electret equipment in China, and the product quality and output are also uneven. The spunlace process requires exquisite technology, and the drying and locking power must be controlled reasonably to produce high-efficiency and low resistance electret cloth.