Common test method for membrane porosity - Database & Sql Blog Articles

Introduction to Common Test Methods for Membrane Porosity (Bethes Instruments) Porosity is a key parameter used in the study and application of membrane materials, including membranes and hollow fiber membranes. It is typically defined as the percentage of pore volume relative to the apparent volume of the membrane: ε = V_pore / V_membrane_apparent. However, the term "pore" here refers specifically to through-pores—channels that allow fluid to pass through. Researchers often use porosity to evaluate filtration performance, permeability, and separation efficiency. Unfortunately, the current commonly used definition of porosity does not always align with actual through-hole pores. This discrepancy can lead to misleading conclusions. For instance, a membrane with high porosity might not necessarily exhibit better filtration performance or higher permeability. Similarly, a membrane with zero through-pore area could still show measurable porosity due to surface features like convexities, concavities, or blind holes. Membrane pore size analyzers based on the bubble pressure method are designed to measure pore throat sizes, not the true pore volumes. These instruments calculate pore area based on throat dimensions, which results in lower porosity values compared to traditional methods. Only when the membrane has ideal cylindrical pores—where the throat and opening are uniform and no irregular structures exist—can the measured porosity closely match conventional results. In practice, such ideal conditions are rare. Here are some common methods used to determine membrane porosity: **Method 1: Weighing Method (Wet or Immersion Method)** This method involves measuring the weight change of a membrane before and after it is wetted with a suitable liquid, such as water. The pore volume is calculated from this weight difference. The skeleton volume of the membrane is determined using its dry mass and material density. Porosity is then calculated as: ε = V_pore / V_membrane_apparent = V_pore / (V_pore + V_skeleton) **Method 2: Density Method (Dry or Volume Method)** Using the densities of the membrane material and the apparent density of the membrane, porosity can be calculated. The formula is: ε = (ρ_apparent - ρ_material) / ρ_apparent Where ρ_apparent is derived from the membrane's mass and apparent volume. **Method 3: Gas Adsorption Method** This technique uses low-temperature nitrogen adsorption to determine pore volume. It is effective for pores smaller than 200 nm but cannot characterize larger pores, making it unsuitable for many types of membranes. **Method 4: Mercury Intrusion Method** In this method, mercury is forced into the membrane’s pores under pressure, allowing the measurement of pore volume and size. However, this method requires high pressure to penetrate micropores, and it is more suited for rigid materials. Most membranes are elastic, and they may deform or collapse during the process, leading to significant errors. It’s important to note that the porosity values obtained from these methods do not always represent true through-pores. Instead, they may include non-connected voids, surface irregularities, or closed pores. As a result, comparing porosity data between different membranes may not accurately reflect their filtration or separation capabilities. Researchers should be aware of this limitation when interpreting results and designing experiments.

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