You don’t have to be a scientist to appreciate the importance of filtration in our world today. Face masks and air filtration systems have been employed for protection against a pathogen in a pandemic and against harmful particulates from a forest fire or pollution. The filtration action by face masks and other filtration media might be achieved by a combination of various factors, one of which is the size exclusion effect or sieving action. This simply means the things you do not want (e.g. aerosols, dust particles) should be too big to pass through the openings (pore channels) in the filter media. Today we will take a deeper dive into characterization of the size of pore channels in filter media.
Are the Sizes of All “Pores” Equally Important In a Filter Media?
Perhaps not! Imagine driving a car through a tunnel; you would only be able to get through if the tunnel is bigger than your car throughout its path. A motorcycle should have no problems going through a tunnel of the same size, but the same might not be said about a truck. Through-pores are like these tunnels. They are opening channels that link one end of the filter media to the opposite end. For filter medias, the sizes of these through-pores are likely your primary concern when it comes to the sieving action.
Other openings might include open pores, blind pores, and closed pores. While the sizes of these pores may not directly impact the size exclusion effect, presence of these pores may impact the overall filtration process in other ways.
How are the Through-Pores Sizes Characterized?
For sieving action, the size of the largest particulates that can pass through the filter media depends on the size of the largest through-pore. This may be by design, or a flaw in certain specimens that causes a performance failure. To test for the largest through-pore sizes, bubble point testing has long been employed in the membranes industry. As the name suggests, bubble point testing typically involves submerging the wetted filter media of interest under a liquid (typically water or alcohol), pushing air through the media with a pressure gauge, and noting the pressure at which a stream of bubble begins to emerge. This pressure is then reported as the bubble point.
The capillary flow porometry (CFP) technique not only helps to minimize the subjectivity associated with the traditional bubble point testing as described above, but it can also provide additional information. Analysis is typically conducted by wetting a filter media with a chosen fluid, enclosing it in a sample holder, applying increasing air pressure from one end of the filter media, and measuring the resulting air flow. At first, there should be no flow, since all the pores are occupied by the wetting fluid. As the pressure increases, more fluid is forced out of smaller and smaller pores, resulting in higher air flow through the specimen. The pressure is increased until all pores are emptied. In a typical analysis, this wet analysis is immediately followed by a dry analysis, i.e. the same range of air pressure is increasingly applied to the dry specimen.
Through this process, a more comprehensive pore size distribution including the maximum, mean and minimum pore size can be provided, rather than just the bubble point. If the thickness of the filter media is known, % porosity can also be estimated. Additionally, permeability of air or other non-corrosive liquid through the filter media can be determined by this technique as well.
What Does the CFP Pore Size Distribution Data Look Like?
Below is an example of a wet and dry flow curves obtained from the CFP analysis. Each pressure point is correlated to a pore diameter using the Washburn equation, assuming cylindrical pore geometry and applying known properties of the wetting fluid. The pressure at which the resulting flow can be detected reflects the bubble point, from which the maximum pore size is calculated. The midpoint of the increase in the wet curve reflects the mean pore diameter, while the minimum pore size is calculated from the point where the wet and dry curves meet or become parallel. The overall pore size distribution is reflected in the % differential flow curve. Tabular data as well as summary of the most critical outputs are also provided. See a sample report.
What Type of Samples Can be Characterized by Capillary Flow Porometry (CFP)?
The CFP instrument is typically set up to analyze relatively flat samples such as membranes and discs, though hollow fibers can also be readily analyzed. Other sample configurations, such as filter cartridges, might be feasible but require custom accessories. The standard sample holder is 25 mm in diameter, however samples up to 50 mm in diameter can be accommodated. A wide variety of sample types can be analyzed, from fabrics to fuel cell separators, as long as the chosen fluid can completely wet the material.
What Types of Wetting Fluids Can be Used?
Porofil, a fluorocarbon, is the most widely used wetting fluid for this analysis. It could wet most materials, even PTFE, and does not readily evaporate from the pores without applied pressure. Additionally, it typically does not cause unwanted chemical or physical interaction with the specimen such as dissolution or swelling. Other fluids may also be used; however, their properties should be carefully considered. Example of wetting fluids include the following.
|Excellent universal wetting fluid
|Same as above but with lower vapor pressure
|Good for bubble point only as it has high vapor pressure
|Good for bubble point only as it has high vapor pressure
|Good, but limited wetting capability
|Good, with virtually zero vapor pressure
|Good for hydrophilic materials
How Can Particle Technology Labs Help?
Aside from guiding you through the CFP analysis of your through-pores as outlined above, the experts at PTL would also be happy to discuss how other techniques might be of interest for your filter media including gas physisorption, mercury intrusion porosimetry (MIP), Scanning Electron Microscopy (SEM), and dynamic water vapor sorption (DVS). Contact us for consultation today!
By Chorthip Peeraphatdit – Particle Characterization Chemist IV / Team Leader