A sudden outbreak of the virus, so that antivirus masks become a hot topic for a while. There are many types of masks on the market, among which surgical masks and N95 grade masks can provide good anti-virus effect. So how does it work?
Surgical masks are generally divided into three layers: (inner layer) hygroscopic layer, (middle layer) core filter layer and (outer layer) water blocking layer.
Compared with surgical masks, N95 masks are optimized in structure and have more layers in the core filtration layer. In addition, manufacturers such as 3M and Honeywell enhance the filtration capacity of the fiber through electrostatic treatment, so the protective performance of masks is better.
The key to the effectiveness of these two masks lies in the fact that their core filtration layer is composed of a layer of melt-blown nonwoven fabric, which is a film made of many crisscross fibers in random directions and made of polypropylene. The diameter of the fibers ranges from 0.5 to 10μm.The scanning electron microscope image is as follows.
As can be seen from the scanning electron microscope, the gaps between the fibers of melt-blown nonwovens are still large, so how does it filter out viruses from the environment? Although novel coronavirus is small in size, about 100 nm, the virus cannot exist independently, and its transmission route is mainly secretions and sneezing droplets, and the size of droplets is about 5 nm. Melt-blown nonwovens can be filtered through a variety of filtration mechanisms, such as droplets, and fiber voids act as a sieve, but this is only one of the most basic interactions.
Depending on the size of the droplets and the airflow velocity, the fiber can capture virus-containing droplets through a variety of mechanisms:
· inertial collision -- when a large particle with large inertia encounters the filter fiber, it cannot change direction due to its inertia, so it will hit the fiber and attach to the fiber.
· interception -- when particles move along the airflow and happen to come into contact with the fiber surface, medium sized particles that are easy to move along the airflow come into contact with the filter fiber and are intercepted by the fiber.
· diffusion -- when the particle size is extremely small, it is controlled by the movement of random particles (Brownian motion of small particles), the direction of movement is separated from the direction of the airflow, and it is absorbed when it touches the fiber.
· electrostatic attraction -- when there is a static charge on the fiber, the electrostatic charge on the fiber will produce electrostatic adhesion to the particle.
For these interactions to occur, the diameter of the fiber is required to be as small as possible, while the diameter of the fiber in the melt-blown nonwovens is between 0.5 and 10 m, with large specific surface area, compact arrangement and good filtration performance. In both surgical masks and N95 masks, fused spray cloth is used as the core filter layer, so it has a good filtering effect on droplets or aerogel containing virus.
Due to the large number of layers of melt-blown nonwovens used in N95 masks, excessive consumption of N95 masks will consume melt-blown nonwovens resources too quickly, causing a more serious shortage of masks. According to the relevant production plant, the price of melt-blown nonwovens is rising and raw materials are in short supply. Therefore, we also call on readers to follow the recommendation of academician zhong Nanshan and use surgical masks in daily life, so as to reduce the consumption of N95 masks and leave the production resources to the medical staff in need and the compatriots in the affected areas.
(source: fenner electron microscope)
