There are at least five filtration mechanisms: diffusion, inertia, interception, gravity and electrostatic.
Diffusion, the smaller the size of the particles and the lower the velocity of the airflow, the more pronounced the diffusion effect. Generally, particles below 0.1μm are filtered out by diffusion by depositing them on the surface of the fibres. Particles larger than 0.3μm are difficult to filter by diffusion.
Inertia, the larger the particle size, the greater the air velocity, the easier it is to collide with the fibres due to inertia.
Interception is mainly the result of particles being intercepted by the complex arrangement of fibres under the influence of van der Waals forces.
Gravity, it is generally accepted that particles larger than 0.5 μm in diameter are deposited on the fibres mainly by gravity.
Electrostatics, fibres can be charged by friction, electret or other means, and particles may also be charged. "Homogeneous charges attract and opposites repel". This is present between particles and particles and between particles and fibres. By electrostatic attraction, the particles can be filtered.
Standards for eliminating the effects of static electricity
Air filtration materials such as meltblown fabric and electrostatic cotton are electrically charged by means of water electret and electric electret in order to obtain high efficiency. Materials such as glass fibre and PTFE, and filters made of them, can also become electrically charged for various reasons. These electrostatic charges are not completely reliable and will decay over time and with the environment changes, thus affecting the filtration efficiency of the masks and filters. Therefore, the effect of static electricity on the filtration efficiency of filter materials, masks and filters needs to be examined.
For air filters, direct electrostatic removal is used, followed by conventional testing to assess the worst-case condition of the filter media and filter in use; for masks, efficiency after loading is needed, and changes in efficiency are evaluated by continuous cumulative loading with salt or oil aerosol.
1) Relevant contents in air filter standards
ISO 16890-4Air filters for general ventilation -Part 4:Conditioning method to determine the minimum fractional test efficiency
EN 779Particulate air filters for general ventilation-Determination of the filtration performance
JIS B 9908 Test method of air filter units for ventilation and electric air cleaners for ventilation
ANSI/ASHRAE 52.2Method of testing general ventilation air - cleaning devices for removal efficiency by particle size
ISO 29461-1Air intake filter systems for rotary machinery-Test methods- Part 1: Static filter elements
ISO/TS 21220Particulate air filters for general ventilation - Determination of filtration performance
ISO 16890 claims that it is a 24h IPA vapour under the most stringent conditions for the removal of static electricity, however, the standard also states that, the procedure described here indirectly but quantitatively shows the extent of the electrostatic charge effect on the initial performance on a full size filter. It indicates the level of efficiency obtainable with the charge effect removed (or minimized by IPA vapour conditioning) and with no increase in mechanical efficiency. It should not be assumed that the measured conditioned ("discharged") efficiency always represents real life behaviour. Exposure to some types of challenge, such as combustion particles, fine particles or oil mist in service may affect the action of these electric charges so that the initial efficiency may drop substantially after an initial period of service. This drop in the fractional efficiency can be reduced by a slight increase in mechanical efficiency from the collection of particles in the filtration media.It can be seen that most of the methods of discharging use IPA vapouror or immersed in IPA.The particulate removal efficiency of the filter element is measured as a function of the particle size in the range 0,3 μm to 10 μm of the unloaded and unconditioned filter element as per the procedures defined inISO 16890-2.After the initial particulate removal efficiency testing, the air filter element is conditionedaccording to the procedures defined in this part of ISO 16890 and the particulate removal efficiency is repeated on the conditioned filter element.
2) Requirements in mask standards
The American standard NIOSH 42 CFR PART 84 requires an aerosol mass load of 200mg to test the change in filtration efficiency during loading.
The European standard EN 149 requires that the aerosol be sprayed for 3min and then the filtration efficiency is tested for 30s.
Clearly, for masks, the American standard and the European standard have different regulations. In terms of testing, the different standards have specific requirements for equipment. The American standard so specifies that the TSI is written into the standard.
Korea follows the European standard system and China follows the American standard system. It is also just a matter of who is more closely associated with a country's mask development process and has nothing to do with which standard is more suitable for the face shape of Asian countries. Only Japan, with its own specific standards, has a different system to anyone else.
Conclusion
Compare the two attitudes towards "unreliable static electricity" shows that a more rigorous evaluation method has been adopted for air filters.
Some expert in the industry have conducted the experiment on melt blown non-woven fabric: loading test according to the NIOSH standard; treated with ethanol; treated with iso-propanol. Test the filter efficiency after each treatment, and found that the filter efficiency has hardly decreased after loading; and limited reduction in filter efficiency after treatment with alcohol; the efficiency decreased a lot after treatment with isopropanol.
The electrostatic discharging method in air filter is more stringent, but it looks more like a recommendation. While, the loading test specified in the mask standard is mandatory.
Also, most of the standards that have provisions for static discharging are for filters for general ventilation, and typically the filter media used are mostly glass fibre, PTFE and filter paper, which carry a limited amount of static electricity and may have a limited effect on filter efficiency before and after discharging. Currently, more companies that make melt blown fabrics are also developing melt blown fabric composites medium for filters. The effect of static electricity on these filter efficiency can be even greater, and it is worthwhile to pay attention to how to evaluate the impact of static electricity!
