After the novel coronavirus epidemic, more and more people have realized the important role of masks. So, these scientific knowledge about masks.Do you know?
How to choose a mask?
In terms of design, if ranked according to the priority of the wearer’s own protective ability (from high to low): N95 masks>surgical masks>ordinary medical masks>ordinary cotton masks.
For the novel coronavirus infected pneumonia, medical surgical masks and masks with filtration of non oily particles greater than or equal to 95%, such as N95, KN95, DS2, FFP2, have obvious blocking effect.
Classification of medical masks
At present, medical masks in China are mainly divided into three types: medical protective masks with the highest protection level, medical surgical masks commonly used in invasive operating environments such as operating rooms, and ordinary level disposable medical masks.
The material of medical masks
We usually say that masks are made of non-woven fabric material, which is a non-woven fabric compared to textile fabric. It is composed of oriented or random fibers. Specifically for masks, all of their raw materials are polypropylene (PP), and medical masks generally have a multi-layer structure, commonly referred to as SMS structure.
Chemical knowledge
Polypropylene, also known as PP, is a colorless, odorless, non-toxic, and semi transparent solid substance formed by the polymerization of propylene. The molecular formula is – [CH2CH (CH3)] n -. Polypropylene is widely used in the production of fiber products such as clothing and blankets, medical equipment, automobiles, bicycles, parts, conveying pipelines, chemical containers, and is also used in food and drug packaging.
From the perspective of mask materials, polypropylene high melting point non-woven fabric special material has become the best choice, producing polypropylene products with a melt mass flow rate of 33-41g/min, meeting the standard of sanitary polypropylene non-woven fabric.
The non-woven fabric produced from polypropylene non-woven fabric special material can be used for disposable surgical gowns, sheets, masks, covers, liquid absorbent pads and other medical and health products. Among them, non-woven masks are made of two layers of fiber non-woven fabric specially used for medical and health purposes, with an additional layer of filter spray cloth with a filtration and antibacterial effect of over 99.999% added in the middle, which is welded by ultrasonic waves.
Anti virus medical mask
The masks that can provide virus protection mainly include medical surgical masks and N95 masks. According to the national standard YY 0469-2004 “Technical Requirements for Medical Surgical Masks”, the important technical indicators that medical surgical masks must meet include filtration efficiency, bacterial filtration efficiency, and respiratory resistance:
Filtration efficiency: Under the condition of air flow rate (30 ± 2) L/min, the filtration efficiency of sodium chloride aerosol with a median diameter of (0.24 ± 0.06) μ m in aerodynamics is not less than 30%;
Bacterial filtration efficiency: Under specified conditions, the filtration efficiency for Staphylococcus aureus aerosols with an average particle diameter of (3 ± 0.3) μ m shall not be less than 95%;
Respiratory resistance: Under the condition of filtration efficiency flow rate, the inspiratory resistance does not exceed 49Pa and the expiratory resistance does not exceed 29.4Pa.
The second criterion for ensuring bacterial filtration efficiency is that the filtration efficiency of Staphylococcus aureus bacterial aerosols should not be less than 95%, which is the origin of the N95 concept. Therefore, although N95 masks are not medical masks, they meet the standard of 95% filtration efficiency and can better fit the human face, so they can also play a good role in virus prevention.
Melt blown non-woven fabric
The main material that brings virus filtering effect to these two types of masks is the extremely fine and electrostatic inner layer filter cloth – meltblown non-woven fabric.
The main material of melt blown non-woven fabric is polypropylene, which is an ultra-fine electrostatic fiber cloth that can capture dust. When droplets containing the pneumonia virus approach the meltblown non-woven fabric, they will be electrostatically adsorbed on the surface of the non-woven fabric and cannot pass through.
This is the principle of this material isolating bacteria. After being captured by ultrafine electrostatic fibers, dust is extremely difficult to detach due to cleaning, and washing with water can also damage the electrostatic suction ability. Therefore, this type of mask can only be used once. Suitable levels for melt blown filtration of flat masks include: ordinary level, BFE95 (95% filtration efficiency), BFE99 (99% filtration efficiency), VFE95 (99% filtration efficiency), PFE95 (99% filtration efficiency), KN90 (90% filtration efficiency).
Specific composition
Medical surgical masks are generally made of three layers of non-woven fabric. The material is spunbond non-woven fabric+meltblown non-woven fabric+spunbond non woven fabric. Short fibers can also be used in one layer to improve skin texture, namely ES hot-rolled non-woven fabric+meltblown non-woven fabric+spunbond non-woven fabric. The outer layer of the mask is designed to prevent droplets, the middle layer is filtered, and the memory absorbs moisture. Meltblown fabrics are generally selected to weigh 20 grams.
The N95 cup type mask is composed of needle punched cotton, meltblown fabric, and non-woven fabric. The meltblown fabric usually weighs 40 grams or even more, and with the thickness of needle punched cotton, it looks thicker than flat masks in appearance, and its protective effect can reach at least 95%.
Several layers of masks are not specified in the national standard GB/T 32610 for masks. If it is a medical mask, it should have at least 3 layers, which is what we call SMS (2 layers of S layer and 1 layer of M layer). Currently, the highest number of layers in China is 5, which is SMMMS (2 layers of S layer and 3 layers of M layer). It is not difficult to make masks, but it is difficult to make SMMMS cloth. The price of an imported non-woven fabric equipment is over 100 million yuan.
The S here represents the spunbond layer, which has a relatively coarse fiber diameter of around 20 micrometers (μ m). The two-layer S spunbond layer mainly supports the entire non-woven fabric structure and does not have a significant effect on barrier properties.
The most important layer inside a mask is the barrier layer or meltblown layer M. The fiber diameter of the meltblown layer is relatively thin, around 2 micrometers (μ m), so it is only one tenth of the diameter of the spunbond layer. This plays a crucial role in preventing bacteria and blood from penetrating.
If there are too many S spunbond layers, the mask will become harder, while if there are too many M meltblown layers, breathing will be more difficult. Therefore, the difficulty of breathing in the mask can be used to judge its isolation effect. The more difficult it is to breathe, the better the isolation effect. However, if the M layer becomes a thin film, it is basically not breathable, and viruses are blocked, but people cannot breathe either. So, this is also a technical issue.
To better illustrate the issue, we will compare the spunbond layer S fiber, meltblown layer M fiber, and hair in the following figure. For hair with a diameter of 1/3, it is close to the spunbond layer fiber, while for hair with a diameter of 1/30, it is close to the meltblown layer M fiber. Of course, researchers are still developing finer fibers to ensure better antibacterial and barrier properties.
As mentioned earlier, the finer the M layer, the more it can block the entry of small particles such as bacteria. For example, N95 refers to the ability to block 95% of small particles (0.3 microns) under normal conditions. According to the national standard GB/T 19083 for medical protective masks, the filtration efficiency of the mask for non oily particles meets the requirements in the table below at a gas flow rate of 85L/min
Table 1: Filtering Levels of Medical Protective Masks
From the above explanation, N95 is actually a 5-layer mask made of polypropylene non-woven fabric SMMMS that can filter 95% of fine particles.
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-18-2024