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

Introduction to Common Test Methods for Membrane Porosity (Bethes Instruments)
Porosity is a commonly used important index in the application and research of membrane materials such as membranes and hollow fiber membranes. Porosity is generally defined as the percentage of the pore volume in the porous membrane as a percentage of the apparent volume of the membrane, ie: ε = V pore / V membrane appearance.
The pores are the transport channels of the fluid, and the "pore" here should be referred to as "through-hole pores". Usually researchers hope to use this parameter to evaluate the filtration performance, permeability and separation capacity of the membrane. However, due to definitions and limitations of test methods, the "porosity" in the parameter "porosity" which is often seen and commonly used at present is not referred to as "through-hole pore", so this definition The porosity does not constitute a positive correlation with the filtration performance, permeability, and separation ability of the membrane. That is to say, if the porosity is large, the filtration performance is not necessarily good; the permeability is zero, and the porosity is not necessarily zero.
For the membrane pore size analyzer with the principle of bubble pressure method, if the pores on the membrane are not ideal cylindrical pores, it can not be used to analyze the porosity, because the pore size distribution of the instrument tested by the principle is the size of the pore throat. information. The pore area is calculated by the size of the through-hole throat, and the porosity is calculated according to the appearance of ε = V / V film = S hole / S film. This value is much smaller than the pores obtained by the conventional methods. rate. Only when the pores of the membrane are ideal cylindrical pores, that is, the pore throat and the orifice are the same size and there are no other convex and concave, slit structures, the porosity obtained from the pore size of the through pores is the porosity obtained by the conventional methods. Close (this situation is almost non-existent in practice).

Here are a few common test methods:
Method 1: Weighing method (wet method, immersion method)
Principle: The pore volume V pore of the membrane is determined according to the change of the weight of the membrane before and after wetting a suitable liquid (such as water); the skeleton volume of the membrane V membrane skeleton can be obtained by the density of the membrane raw material and the dry film weight; Membrane porosity:
ε = V hole / V film appearance = V hole / (V hole + V film skeleton)

Method 2: Density method (dry method, volume method)
Principle: See the following formula derivation, so only the density of the film material ρ film material and the apparent density of the film ρ film appearance, the porosity ε can be calculated. The apparent density ρ film appearance can be obtained from the appearance volume and quality.
ε = V hole / V film appearance = (V film appearance - V film skeleton) / V film appearance = (ρ film appearance - ρ film material) / ρ film appearance

Method 3: Principle of gas adsorption method: The pore volume is obtained according to low temperature nitrogen adsorption, thereby obtaining porosity. This method can only obtain the pore volume of pore structure with a size below 200 nm, and can not characterize the pores above 200 nm, which is not suitable for a large number of membranes.

Method 4: The principle of mercury intrusion method: According to the principle of mercury intrusion method, the mercury is pressed into the “pores” of various structures of the membrane by pressure, and the pore volume and size data of the membrane are obtained according to the pressure and volume of mercury injection; The disadvantage is that the pressure required to press mercury into the micropores is relatively large. This method is more suitable for analyzing rigid materials. For most membrane materials, it is elastic material, which is prone to deformation or “collapse” during the process of injecting mercury, resulting in Large error.

For the above four methods of porosity, the “porosity” in the obtained porosity data is not “through-hole pore”, nor is it “through-hole pore throat”; if it is not “through-hole pore”, then this “ Porosity does not meet the researchers' desire to evaluate filtration performance, permeability, and separation capabilities. For example: A film through hole is zero, the surface "convex, concave, closed, blind hole" and other structures form a porosity of 40%; B film porosity is 20% and there are through holes; then, we can not rely on the pore Rate data compares the filtration performance of the two membranes. This point needs attention in research and application.