The basis of mask is meltblown fabric. In addition to mechanical properties of meltblown fabric, electrostatic electret is most important. Since there is no meltblown material processed by electrostatic electret technology, filtration efficiency is no more than (10-40)%, and it is not possible to filter more than 99% of viruses and aerosols. As for electrostatic electret, watershed is obvious here, along which front and rear echelons are distinguished:

First echelon (water electret electrostatic) - high-performance and low-resistance materials (high-end)

The material that has both high efficiency (99%+) and low resistance is a (20-35)g/m² melt-blown water-electret electrostatic material, which is main material of today's advanced N95 masks. Because electric field is strong, less materials need to be laid, and N95 level efficiency can be achieved when material is (20-35)g/m. At this time, because material is thin, breathing resistance is very small. The best material can even reach 20 g/m², and electrostatic electret has a surface electric field of 15,000 volts. This material uses a water electret process to ensure that electret on polypropylene is permanent.

Second tier (corona of static electricity) - material with high efficiency and high resistance (low level)

(45-60)g/m² Meltblown corona thermoelectret electrostatic material, which is main material selected for making general medical masks and some low cost N95 masks. Since material is made of polypropylene with corona points when heated, what is formed is not a permanent electrostatic electret, with less charge and uneven distribution. Therefore, more polypropylene wires must be laid in manufacture of N95 mask materials in order to achieve standard filtration efficiency. Therefore, amount of polypropylene laying required is more than twice amount of first laying. This raises two issues:

Firstly, it uses thermoelectric technology, which has a lower charge, an unstable electric field, and naturally loses power when placed (all without exception);

Secondly, when laying a large amount of materials, breathing resistance is high.

1. Life of various electret methods

Water-electret-electrostatic melt blowing technology, material's electrostatic electret is permanent, and shelf life is basically 10 years; materials of strategic reserve of electrostatic melt blowing in some EU countries onlyhave been in stock for more than 7 years. ;

Thermoelectrostatic melt blowing technology, static electricity of material is unstable and unstable.

The spread of epidemic has led to fact that standards for masks around world have become higher and higher. Water electret can provide high efficiency and low resistance, but many people consider this technology not mature enough. In fact, water electret technology has been in use for more than 20 years. Years of history, before and after five generations.

Second, history of development of water electret

Back in 1996, first generation of water electrets appeared in China. We can say that original method was most primitive. Using a high-pressure nozzle to spray meltblown fabric can achieve a filtration effect. 99%. But at that time, technology was immature, fabric processing was not uniform enough, it is easy to damage base fabric, and application area was not wide enough, so it was not known to general public.

The second generation product uses jet blasting, which uses centrifugal force to electrify base fabric, but this method consumes a lot of energy, is not suitable for large-scale production, and is not very practical.

Third generation water electret uses nozzle atomization and vacuum suction to achieve an efficient electret effect. So far, it has been used by some companies, but process is not mature enough, and characteristic is that yield is not high. The daily productivity of 1.6M equipment is 600 kg vol.

In recent years, fourth generation water-electret product has been developed. It uses combined electret liquid to electrify meltblown fabric with ultrasonic technology. This method has a wide application and can electrify long and short fibers as well as mixed electrets. , and mechanical force of ultrasonic atomization charger is small, and it is not easy to damage electret with thin thickness and low strength. However, due to limited power of ultrasound and small mechanical force, liquid does not penetrate electret sufficiently.

The fifth generation water electret product (the latest generation) is made by interaction of multi-electrode atomized electret liquid and negative pressure, which can ensure that matrix material is quickly and completely wetted, and also make matrix in thickness direction Fully penetrate surface, resulting in a better static effect electricity. The productivity of 1.6M equipment is about 1.2 tons per day.

Electret liquid requires special processing, and there are requirements for electret masterbatch, some electret masterbatch is not suitable for charging with electret water. Not all expensive electretsare suitable masterbatches for aqueous electrets.

Third, principle and advantages of water electret charging

The water electret differs significantly from traditional corona electret in electret mode. After meltblown fabric passes through water electret equipment, it is first subjected to cold water cooling, and pure water passes through a certain angle, pressure and flow rate to spray meltblown fabric, and friction between water and fabric generates static electricity, and then electret masterbatch. Transferring conductive material to tourmaline or zircon to achieve electrostatic stay.

The advantage of water electret lies in its charging process, which is carried out in an environment with high humidity. The static electricity that remains on fabric is also electricity that is actually in fabric. Static electricity on surface of fabric is completely absorbed by water.

The second process of water electret is drying and fixing melt-blown fabric. The drying process uses hot airflow to dry by raising surface temperature and increasing surface airflow. This process encounters high temperature, high humidity and air flow. This process is actually working principle of static electricity removal equipment. If it is not electricity in electret masterbatch, it will be removed. The second process removes some of electricity on surface of meltblown, i.e. virtual electricity.

After these two processes, meltblown fabric leaves behind electricity slowly released by tourmaline powder or zircon, rather than virtual electricity on surface of fabric. The filtration efficiency found at this time is already similar to that after aging. No false electricity. This kind of electricity is determined by physical and chemical properties of carrier, and it does not lose power quickly.

One of easiest ways to test for a power outage is to place a regular ESD corona cloth and a water electret cloth in a 70° oven. As a rule, in four hours there will be a significant change. The efficiency obviously drops, but fabric of water electret has almost no attenuation.

The earliest advanced technologies were in hands of large foreign companies. In Japan, starting price of such a mature synchronous production line (melt blowing + water electret) was more than 3 million US dollars. In 2018, Peter Kai, a Chinese scientist known as father of N95, developed a hydraulic friction electrification charging method. This treatment can improve filtering efficiency of meltblown fabric by 20 times, and charging is complete and stable. Over past two years, some major domestic textileLarge enterprises introduced foreign technologies and gradually launched water-electric equipment. Large factories silently produce high-quality melt-blown fabric, which is expensive and does not want core technology to be lost. After outbreak of new corona epidemic, demand for masks at home and abroad has risen sharply, and high-quality melt-blown fabrics are still in short supply.

Before epidemic, market price of FFP2 blown fabric was high. At present, price of FPP2 blown fabric in small factories is (25-35) ten thousand per ton. The price of FFP3 blown fabric is about 500,000 per ton, and elite part of a small number of large factories reaches 700,000-800,000 per ton. The reason for high price is mainly due to high cost of water-electric equipment, limited production capacity and high daily energy consumption. At same time, long-term charged blown fabric, which is difficult to find in mass market, is also used as a national strategic reserve material. Recently, there have been several manufacturers of water electret equipment in China, and product quality and performance are also uneven. The spunlace process requires exquisite workmanship, and at same time, electricity for drying and blocking must be intelligently controlled to produce high-efficiency and low resistance water electret fabrics. .