A few months ago, when there was the shortage of medical masks, I decided I wanted to create a DIY mask to work as a replacement for the medical masks in the market. But in order to do that, I had to understand the structure of masks, so it required a lot of researching. As someone who had minimum knowledge on viruses and bacteria it was a very difficult process for me. And there were a lot of scientific readings I did, but to keep things brief, based on what I found, medical masks (or for fine dust) consists of three layers: the exterior, filter and the lining in the interior that touches the mouth.
The outer layer of the medical mask is mainly made of a non-woven fabric that protects against splashes and large dust. And since the inner layer directly touches the skin, it is made of medical non-woven fabric with a soft texture. These fabrics also differ from the ones used in normal masks; the medical masks can catch small particles better because it uses a randomly entangled non-woven fibrous tissue instead of the usual fibrous tissue that intersects at right angles. But, most of the fine particles are caught in the filter layer and not by the cloth. Depending on the type of mask, 1 to 3 layers of filters are between the exterior and the interior cloth. The role of the filter, is to filter out ultrafine particles with a size of 0.1-1 µm that cannot be caught by the outer cloth. To catch a lot of fine particles without a filter, the cloth needs to be very dense, but if it is too dense, it becomes too hard to breathe. So, to keep the mask breathable you need to use a powerful filter that’s also easy to breath through.
This powerful filter is a filter called Melt Blown. This filter is created by dissolving then blowing Poly Propylene which creates a nonwoven sheet from the randomly deposited fibers. The function to generate static electricity is also added onto the sheet, which will help filter out ultra-fine particles. To do this, the filter is electrostatically treated with an ultra-high voltage current of more than 25,000 volts. Static electricity is stored in the process of sending and receiving electrons through friction, and this stored electricity is stopped and will only react when it touches the dielectric. And since most fine particles are polarized, they will stick to the filter through the pushing and pulling of negative and positive forces, like how opposite poles of magnets stick to each other. But since the electrostatic is in a stopped state, the person wearing the mask won’t feel the electricity at all. Basically, it is an advanced technique that creates a kind of surface tension and confines the static electricity from escaping.
Therefore, the most important part of MB filter is the applying of the electrostatic treatment. The quality of the filter depends on how uniformly high-voltage static electricity can be applied to the textile tissue and how long the static electricity on the textile tissue can be stably maintained for a long time.
An MB filter mask can usually last around a single day of usage. After one day of use, it should discard as it cannot be washed or cleaned. Washing the mask will destroy the agglomeration of fiber tissues and the ability to create static electricity since under wet conditions the electrostatic ability disappears. However, this does not mean it can’t withstand the moisture from the human body during use. But, to get maximum performance from the filter, it is not recommended to reuse it, soak it in water or use it for an extended period of time (more than a day).
As the MB filter plays a critical role in the mask, when purchasing a mask, you should first check any information written about the filter before purchasing it. In the next article, we will post information on the mask ratings that may be helpful when purchasing a mask. If anybody has more information about the ratings, the filters or possibly even explain the topic better, please comment below!
By Sue Kim