The filter layer in most disposable masks and many air filters is meltblown polypropylene: a web of very fine fibres. Fine fibres alone catch particles mechanically, but a thin web that relied only on mechanical capture would either let too many sub-micron particles through or need to be so dense that it would be hard to breathe through. The answer is an electret — fabric that carries a long-lasting electrostatic charge, so fibres also attract and hold particles electrostatically.
How Corona Electret Charging Works
On a meltblown line, the web passes beneath rows of sharp needles held at a high DC voltage. The intense field at each needle tip ionises the air, a corona discharge, and the field drives those ions onto the moving fibres. Part of the charge is trapped in the polymer, and electret masterbatch additives in the polypropylene help it stay there for the product's shelf life.
LISUN's system has three parts:
- High-voltage DC supply — the ESD-RPB, rated 30 kV/5 mA or 60 kV/2.5 mA, in positive or negative polarity.
- Corona bars — ESD-BAR pin-type bars with tungsten needles in 800, 1200, 1600 and 2000 mm effective widths. LISUN builds in anti-arcing and current-stabilising circuits so that every needle delivers an even charge across the web.
- Racks — ESD-RACK stainless frames that guide the web over alumina rollers and hold up to four charging units, installed before the winder.
Despite the "ESD" in the model names, these are charging devices for production, not ESD immunity test equipment.
What Determines Charging Quality
- Even charge across the width — needle spacing, bar height and current regulation all matter. Uneven charging shows up as filtration efficiency varying across the roll.
- Material — the polymer grade and electret additive control how much charge is trapped and how long it lasts.
- Environment — humidity and fibre temperature at the charging point affect results.
Because the charge cannot be seen, filtration efficiency testing is how the process is controlled.
Measuring Particulate Filtration Efficiency
A PFE tester generates a defined test aerosol and draws it through a fixed area of the sample at a set flow rate. Upstream and downstream particle concentrations give the efficiency, and a differential pressure sensor gives airflow resistance (breathability) at the same time.
Respirator standards use two aerosols:
- Sodium chloride (NaCl) for filters rated against non-oily particles (for example KN classes and N-type respirators).
- Oil aerosol — DOP or equivalents such as DEHS or paraffin oil — for filters rated against oily particles (KP, R and P types).
The LISUN GLE-20 has fully separate salt and oil aerosol paths so that one test cannot contaminate the other. The supplier specifies a 100 cm² test area, a flow range of 10–100 L/min, an efficiency range of 0.001–99.999%, 0–1000 Pa resistance measurement, and count median diameters of 0.075 ±0.02 µm (NaCl) and 0.185 ±0.02 µm (oil), as the respirator test standards specify. Its listed standards include EN 149, EN 143, GB 2626, GB 19083, ASTM F2299 and EN 14683.
From Media to Mask
A good filter medium is necessary but not sufficient. A respirator on a face only protects if it seals, which is why tight-fitting respirators are fit tested on each wearer. Quantitative fit testers such as the AccuFIT 9000 compare particle concentrations inside and outside the facepiece during a set of exercises.
Frequently Asked Questions
Does the electret generator measure efficiency? No. It only charges the fabric. Efficiency is measured separately with a PFE tester.
Why test at a specific particle size? Filters are least efficient for particles of a few tenths of a micron, too large for diffusion to capture them well and too small for interception and impaction. Test aerosols are chosen near this most-penetrating size to give a conservative result.