Further Information - Safegard
|Further Information - Safegard..... "The Bellows Effect"|
“The Bellows Effect”... why a garment made from fabric with a superior particle filtration but low air permeability might not offer the best protection against fine dust particles...
The principal of a set of bellows goes back to ancient times of the early Chinese, Roman and Greek civilizations; fill a flexible chamber with air then apply pressure to force the air out through an aperture. It is also an unavoidable consequence of making a coverall made from fabric with low breathability and may be important in protection against dust particles
Microporous film fabric such as that used in MicroMAX, along with flashspun polyethylene fabric both have very low air permeability. When a user wears such a coverall, normal movements such as walking creates the same effect as pumping the handles on a set of bellows: changes in pressure result inside the suit as the air moves around. You can see and feel this when the legs of a garment “balloon” whilst walking. The air cannot pass through the fabric so will choose the path of least resistance - between the teeth of the zip and other connections such as the face opening, the neck, the cuffs and the ankles – even the stitch holes on seams – especially if seams have been subjected to stress causing the holes to open slightly.
The result can be a constant flow of air, backwards and forwards through such openings. This is less critical in liquid protection as liquids tend to be directional. However, in fine dust particle protection it might be critical. Such particles will tend to float freely in the atmosphere and travel with any air-current. If such currents are through seam holes or openings into a garment then the dust will follow. Thus the “Bellows Effect” can result in the active “sucking” of particles into the garment. A real world experience of this is in carbon black production; carbon black is an extremely fine dust used as a filler in rubber products. Users of non-breathable disposable coveralls wearing a white shirt beneath may notice a line of black dots along the shirt sleeve on removal of the coverall – the result of the dust being sucked through the seam holes during use… one of the more obvious results of “The Bellows Effect".Consequences
- In fine particle protection a coverall made with a low breathability fabric, even if the fabric itself has a superior particle filtration, may not offer the best protection unless the zip, face opening, wrists and ankles are taped up and sealed because the Bellows Effect can result in dust being drawn into the garment. (Type 5 finished garment tests are generally conducted with such taping on garments precisely for this reason.
- A breathable fabric such as Safegard does not create a bellows effect because the air can easily pass through the fabric. Thus provided the particle filtration of the fabric is sufficient for the dust particle being protected against, Safegard is likely to offer better protection in normal circumstances than any non-breathable fabric that has superior dust filtration.
- “The Bellows Effect” can have a positive effect in terms of comfort: in Lakeland’s Cool Suits the low breathability of the main garment fabric creates a movement of air through the breathable panel set in the rear – which is precisely what is required to help keep the wearer cool and comfortable!