Many people know that the core of surgical masks and N95 masks is the middle layer – meltblown cotton.
If you still don’t know, let’s briefly review it first. Surgical masks are divided into three layers, with the outer two layers being spunbond non-woven fabric and the middle layer being meltblown cotton. Whether it is spunbond non-woven fabric or meltblown cotton, they are not made of cotton, but of plastic polypropylene (PP).
Behnam Pourdeyhimi, deputy director of the Institute of Non woven Materials at North Carolina State University and a professor of materials science, explained that the front and back layers of non-woven fabric on surgical masks do not have the ability to filter microorganisms. They can only block liquid droplets, and only the middle layer of meltblown cotton has the function of filtering bacteria.
The filtering function of melt blown non-woven fabric.
In fact, the filtration efficiency (FE) of fibers is determined by their average diameter and packing density. The smaller the fiber diameter, the higher the filtration efficiency.
The diameter of melt blown cotton finished fibers is approximately between 0.5-10 microns, while the diameter of spunbond layer fibers is around 20 microns. Due to the ultrafine fibers, melt blown cotton has a large surface area and can adsorb various micro particles. What’s even more impressive is that meltblown cotton is relatively breathable, making it a good material for making mask filters, while spunbond non-woven fabric is not.
Let’s take a look at the manufacturing process of these two types of non-woven fabrics.
When making spunbond non-woven fabric, polypropylene is melted and pulled into silk, which then forms a mesh——Compared to spunbond nonwoven fabrics, meltblown cotton has much more advanced technology, and in fact, meltblown technology is currently the only technology used for large-scale production of micron sized fibers.
The manufacturing process of melt blown cotton
The machine can generate high-speed hot air flow, which will spray molten polypropylene from the extremely small opening melt jet nozzle, with the same effect as spray
Misty ultrafine fibers gather on rollers or plates to form melt blown non-woven fabrics – in fact, the inspiration for melt blown technology comes from nature. You probably don’t know that nature also produces meltblown materials. There are often some strange looking wigs near volcanic craters, which are Pele’s hair, which is made of basaltic magma blown by the hot wind of the volcano.
In the 1950s, the US Naval Research Laboratory (NRL) first used meltblown technology to manufacture fibers for filtering radioactive materials. Nowadays, melt blown technology is not only used to manufacture filter materials for filtering water and gas, but also for manufacturing industrial insulation materials such as mineral wool. However, the filtration efficiency of meltblown cotton itself is only about 25%. How did the 95% filtration efficiency of N95 masks come about?
This is a crucial step in the manufacturing process of medical meltblown cotton – electrostatic polarization treatment.
It’s like this, as we just mentioned, the filtration efficiency of masks is related to their diameter and filling density. However, if woven too tightly, the mask will not be breathable and the wearer will feel uncomfortable. If electrostatic polarization treatment is not done, the filtration efficiency of melt blown fabric that can make people feel less suffocated is only 25%.
How can we improve breathability while ensuring filtration efficiency?
In 1995, engineering scientist Peter P. Tsai from the University of Tennessee came up with the idea of electrostatic precipitation technology used in industrial filtration.
In industry (such as factory chimneys), engineers use an electric field to charge particles and then use the power grid to suck them up in order to filter out extremely small particles.
Using electrostatic precipitator technology to filter air
Inspired by this technology, many people have attempted to electrify plastic fibers, but have not been successful. But Cai Bingyi did it. He invented a method to charge plastic, ionizing the air and electrostatically charging the meltblown fabric, turning it into an electret, a permanently charged material similar to Pikachu.
After being transformed into Pikachu, a layer of Pikachu melt blown cloth can not only reach 10 layers without electricity, but also attract particles with a diameter of about 100 nm, such as COVID-19.
It can be said that with Cai Bingyi’s technology, N95 masks were created. Billions of people’s lives around the world are protected by this technology.
Coincidentally, Cai Bingyi’s electrostatic charging technique happens to be called the corona electrostatic charging, which is the same type of corona as the coronavirus, but here corona means corona.
After watching the manufacturing process of medical grade melt blown cotton, you will understand its technical difficulty. In fact, the most difficult part of the manufacturing process of meltblown cotton may be the mechanical manufacturing of meltblown cotton.
In March of this year, Markus M ü ller, the sales director of Reicol, a German supplier of meltblown machinery, stated in an interview with NPR that in order to ensure that the fibers are fine and of stable quality, meltblown machines require high precision and are difficult to manufacture. The production and assembly time of a machine is at least 5-6 months, and the price of each machine can reach $4 million. However, many machines on the market have uneven quality levels.
Hills, Inc. in Florida is one of the few manufacturers in the world that can manufacture meltblown cotton equipment nozzles. Timothy Robson, the company’s R&D manager, also stated that meltblown cotton equipment has a high level of technological content.
Although China’s annual production of masks accounts for about 50% of the world’s total, making it the largest producer and exporter of masks, according to data from the China Industrial Textile Industry Association in February, the national production of meltblown non-woven fabrics is less than 100000 tons per year, indicating a significant shortage of meltblown non-woven fabrics.
Considering the price and delivery time of meltblown fabric manufacturing machinery, small businesses are unlikely to produce large quantities of qualified meltblown cotton in a short period of time.
How to determine if the purchased mask is qualified and made of melt blown cotton?
The method is actually very simple, take three steps.
Firstly, because the outer layer of spunbond non-woven fabric in sandwich cookies has waterproof properties, qualified medical masks should be waterproof. If they are not waterproof, how can they filter the droplets sprayed from the mouth? You can try pouring some water on it like this big brother.
Secondly, polypropylene is not easy to catch fire and is prone to melting when exposed to heat, so melt blown cotton will not burn. If baked with a lighter, the meltblown cotton will roll up and fall off, but it will not catch fire. In other words, if the middle layer of the mask you buy catches fire when baked with a lighter, it is definitely a fake.
Thirdly, medical meltblown cotton is Pikachu, which has static electricity, so it can pick up small pieces of paper.
Of course, if you need to use the same mask multiple times, the inventor of N95, Cai Bingyi, also has disinfection suggestions.
On March 25th of this year, Cai Bingyi stated on the website of the University of Tennessee that the electrostatic polarization effect of medical masks and N95 masks is very stable. Even if the masks are disinfected with hot air at 70 degrees Celsius for 30 minutes, it does not affect the polarization properties of the masks. However, alcohol will carry away the charge of the meltblown fabric, so do not disinfect the mask with alcohol.
By the way, due to the strong absorption, barrier, filtration, and leakage prevention skills of meltblown cotton, many female products and diapers are also made with it. Kimberly Clark was the first to apply for relevant patents.
Dongguan Liansheng Non woven Technology Co., Ltd. was established in May 2020. It is a large-scale non-woven fabric production enterprise integrating research and development, production, and sales. It can produce various colors of PP spunbond non-woven fabrics with a width of less than 3.2 meters from 9 grams to 300 grams.
Post time: Oct-26-2024