Surgical masks are typically made of nonwoven polypropylene fabric and mostly consist of three layers. The first and third layer are generally spunbond, while the middle layer is melt-blown. This composition is commonly referred to as SMS technology (Spunbond / Meltblown / Spunbond).
The density of the first and third layer is usually 20 gsm (gram per m²), while the middle layer is 25 gsm. The reason for using nonwoven fabric is mostly because of the higher air permeability, the higher bacterial filtration efficiency and the lower manufacturing cost compared to woven fabrics. The same goes for the respirator, where it’s common to have three or four layers. As with the surgical masks, the outer layers are made of nonwoven polypropylene, while the middle layer is made of melt-blown nonwoven polypropylene. Together, the fibers form a multi-layered web where the spaces between the fibers allow for breathability.
The POROLUX™ 100 is a gas liquid porometer, based on the ‘pressure scan’ method. This is a fast, yet reproducible method whereby air pressure is continually increased while the resulting flow rates are recorded simultaneously. The pressure scan method is preferred for environments where simplicity, speed and reproducibility are the main requirements. Therefore, the POROLUX™ 100 is the right choice for quality control and assurance.
First, the sample is impregnated with an inert, nontoxic wetting liquid. Next, the sample is secured in the POROLUX™ 100 where an inert gas (e.g. nitrogen) is used to displace the liquid out of the porous fiber network. This ‘wet run’ results in a ‘wet curve’ which represents the measured gas flow through the sample against the applied pressure (inversely proportional to the pore size).
Apart from the impregnation with liquid, the same method as above is used for a dry sample (‘dry run’) measurement. The ‘half-dry curve’ is obtained by dividing the flow values of the dry curve by 2. This curve is also plotted against the applied pressure in the same graphic. From the wet curve, dry curve and the half-dry curve data, information about the porous network can be obtained.
The maximum operating pressure of the POROLUX™ 100 is 1.5 bars (22 psi), which permits measuring pore sizes down to 0.4 μm. The determination of the first bubble point (or maximum pore size) – hereafter referred to as “FBP” – is based on the test method described in the ASTM F316-03 standard.
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