Test Methods and Comparative Results for Face Masks

Since the start of this year, coronavirus outbreaks have led to the development of treatment methods in various parts of the world. Until definitive therapeutic drugs or vaccines are discovered through research, the only measure we can take to protect ourselves from the outbreak is self-protection.

Face masks are at the forefront of these measures. During the pandemic, we all became familiar with the types of face masks and their protective values. But how are these masks characterized? In this article, we will examine how face masks are characterized and compare 3-layer surgical masks (shown on the right in the image below) with single-layer, molded cup or conical masks (shown on the left in the image below). The examination of results does not aim to define virus or bacterial filtration efficiency. Surgical masks consist of 3 layers. They are designed to prevent particles generated by the wearer's breathing from reaching the environment. The outermost layer is made of polypropylene material in a nonwoven hydrophobic structure, usually containing large pores to trap droplets. The middle layer is made of non-woven, melt-blown polypropylene material with small pores that performs the actual filtration process. The inner layer, also non-woven with large pores, aims to provide comfort to the user and prevent perspiration and saliva from passing to the middle layer. The pore size distribution of these three layers, particularly the pore size distribution in the middle layer, determines the mask's filtration efficiency. Small pores physically prevent large particles from passing through by filtering them. Due to the non-woven structure, the tortuosity of this porous structure increases, which in turn increases filtration efficiency. Surgical masks provide some protection to the wearer, such as healthcare workers, against airborne pathogens, but they are not intended to function with respiratory mask efficiency since they can loosen [1]. However, it has been found that surgical face masks significantly reduce the detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, leading to the conclusion that surgical face masks can prevent the transmission of coronavirus and influenza virus [2] Single-layer, molded cup or conical masks are generally designed to protect users from dust in applications such as agriculture, forestry, and construction.

Characterization Methods 1) Capillary Flow Porometry (CFP)

This method is used to define the pore size distribution, which is related to the size of particles that will or will not pass through the material. In this method, a filter sample is wetted with a liquid of known surface tension and the aim is to fill all pores with this liquid. Subsequently, the sample is exposed to controlled increasing air pressure. Initially, there is no air flow. However, when pressure rises above a certain level (this pressure level depends on certain physical forces created between the liquid used and the filter material), the liquid in the pores empties and air flow occurs. The Washburn equation describes the relationship between the pressure at which pores empty and the pore diameter: D: Pore diameter P: Pressure ϒ: Surface tension between air and the liquid filling the pore Θ: Contact angle between the liquid filling the pore and the filter material k: Pore shape factor. As a result, we obtain the size distribution of permeable pores in the material, that is, pores with both ends open. With the resulting graph, we reach conclusions about the minimum diameter, average diameter, or maximum diameter of the permeable pores in the filter material. However, this method does not provide information about the volume of pores in the material, that is, we cannot know whether pores of a certain diameter are more or less abundant. For this, we need another measurement technique.

2) Mercury Intrusion Porosimetry (MIP)

The mercury porosimetry method is also based on the Washburn equation, but this time instead of measuring at which pressure point the liquid filling the pores overcomes the physical forces between it and the material and empties the pores, the opposite approach is taken: a liquid with high surface energy such as mercury is used to measure at which pressure point this liquid fills the pores. In this method, as pressure is controlled and increased, the mercury level is measured, and consequently, the amount of mercury filling the pores is known, and using this information, the total pore volume and pore size distribution in the sample are determined. The most significant difference between these two methods is that only the first method, CFP, distinguishes permeable pores with both ends open. The result obtained in mercury porosimetry also includes other pores in the material, such as blind pores with one end open, and other surface irregularities. Therefore, while both pieces of equipment are necessary for research and development studies, CFP alone is sufficient for quality control work.

Experimental Conditions

In the Quantachrome central laboratory, a 3-layer surgical mask designated as Mask 1 and a single-layer molded mask designated as Mask 2 were tested using CFP and MIP methods and the results were compared. An Anton Paar Porometer 3G zh instrument was used for CFP analysis. For Mask 1, the 3 layers were analyzed both separately and together and the results were compared. Mask 2 was used as is. Samples of 25 cm diameter were cut for analysis, completely wetted with Porofil and a standard sample holder was used. Total analysis time was approximately 30 minutes. [caption id="attachment_103431" align="aligncenter"] Table 1: Analysis Parameters – For Porofil[/caption] For MIP analysis, an Anton Paar PoreMaster instrument was used. A 0.024 g sample was cut from Mask 1 and placed in a standard 0.5 cm³ sample cell. A 0.0391 g sample was cut from Mask 2. Both samples were first placed in low pressure stations and pressure was increased to 50 psi for measurement. Subsequently, they were placed in the high pressure station and pressure was increased to 60,000 psi for measurement. Total analysis time is approximately 30 minutes. [caption id="attachment_103433" align="aligncenter"] Table 2: Analysis Parameters – For Mercury[/caption]

Results

When examining the CFP analysis results obtained using Mask 1 and Mask 2 as is, the difference is easily apparent. As seen in Graph 1 and Table 3, Mask 1 contains permeable pores of much smaller diameter compared to Mask 2. [caption id="attachment_103435" align="aligncenter"] Graph 1: Comparison of pore size distribution for Mask 1 (red and pink graphs) and Mask 2 (black and grey graphs)[/caption] [caption id="attachment_103436" align="aligncenter"] Table 3: CFP Analysis results – Both-ends-open pores were measured[/caption]   When Mask 1's layers were analyzed separately using the CFP method, results were obtained as shown in Table 4. As expected, the smallest pores are in the middle layer and the middle layer dominates the mask's filtration effect. In addition, we observe an interesting result when examining Tables 3 and 4. When Mask 1 is analyzed as a whole, it has smaller pores than the pore size in the middle layer. This is because the overlapping layers increase pore tortuosity, which in turn reduces pore size. [caption id="attachment_103437" align="aligncenter"] Table 4: CFP Analysis results – Both-ends-open pores in all 3 layers of Mask 1 were measured separately[/caption]   When comparing the mercury porosimetry results for both masks, we obtain results similar to CFP analysis; it is seen that Mask 1 has the smallest pores (see Table 5). Both masks have pores larger than 300 µm in diameter and the same average pore diameter, but Mask 1 has much smaller pores. Pores smaller than 20 µm found in Mask 1 are not present in Mask 2 (see Graph 2). A striking result is the comparison of mercury volume filling the pores. The pore volume in Mask 1 is nearly twice the pore volume in Mask 2. With this result, we can say that Mask 1 has both smaller pores and a greater volume of pores. [caption id="attachment_103438" align="aligncenter"] Table 5: Mercury Porosimetry Analysis results – Comparison of pore measurement in Mask 1 and Mask 2[/caption] [caption id="attachment_103439" align="aligncenter"] Graph 2: Mercury Porosimetry dV/dlogD vs pore diameter graph for Mask 1 (red) and Mask 2 (grey)[/caption]

Evaluation

CFP and MIP methods have been demonstrated to be useful techniques for characterizing different fabric materials used in personal protective equipment manufacturing and revealing their differences. The CFP method was able to clearly show the differences in the smallest pores and pore size distribution in both masks. MIP additionally provided information about total pore volume and smaller pores capable of capturing particles that do not travel directly through a filter layer. References 1. T. Greenhalgh et al, (March, 2020) Oxford COVID-19 Evidence Service Team 2. N.H.L. Leung, et al. Nat Med (2020). doi.org/10.1038/s41591-020-0843-2   Seda Giray Application Specialist Anton Paar Türkiye