RU2248552C1 - Method of determination of maximum size of membrane pores by bubble - Google Patents

Abstract

FIELD: control instruments; measurement technology; chemical, medical, electronic and other industries.

SUBSTANCE: proposed method may be used for check of structural properties of porous thin films. Membrane impregnated preliminarily in liquid is placed in cell and is filled with liquid; then, gas is fed to cell and its pressure is measured. Cell with membrane is placed in vessel. Cell and vessel are hydraulically interconnected. Nondestructive pressure differential is created on membrane by discharging liquid from vessel. Gas in form of vapor of the same liquid is fed to vessel; pressure of vapor in vessel is measured continuously. Pressure above membrane at which pressure differential disappears is noted. Coefficient of surface tension is determined by pressure and then maximum size of membrane pores is calculated by Laplace formula.

EFFECT: enhanced efficiency of testing due to avoidance of break of membranes; extended field of application.

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Description

The invention relates to measuring and experimental equipment. It can be used to control the structural properties of porous thin films in the chemical, medical, electronic and other industries.

A known method for the study of porous materials, based on a comparison of the maximum flow rate during evaporation of a liquid from the surface of a porous material, with a flow rate in the presence of a difference in hydrostatic pressures. The method allows you to calculate the average radius of the capillaries, while the measurements obtained are indirect. The measurement error depends on the state of the porous body. With large holes, the error will be very high. The method does not allow to determine the size of the maximum pores. (A.S. USSR No. 524110, IPC G 01 N 11/00, publ. 05.08.76, bull. No. 29)

The closest in technical essence is the method for determining the maximum pore size by the bubble method (GOST R 50516-93. Polymeric membranes, method for determining the bubble point of flat membranes) A thin porous membrane is soaked in a liquid. Clamp in the cell. Fill the cell with liquid so that the surface of the liquid completely covers the surface of the porous membrane. Gas is introduced into the cell under the membrane. Continuously measure the gas pressure. The maximum pore size is calculated from the pressure at which the first gas bubble passes through the membrane. The method is simple and effective, but practical use is limited by the strength of the membranes. So for the study of thin films with pore sizes of the order of 0.5-0.05 microns, the pressure drop across the membrane leads to stresses that destroy the material. The use of substrates leads to a distortion of the measurement results. Application of the method in the study of track membranes leads to the destruction of the latter during the test.

The aim of the invention is to increase the efficiency of tests by preventing the destruction of membranes and expanding the range of thicknesses of films on which measurements can be made by the proposed method.

This goal is achieved by the fact that the membrane, previously impregnated in a liquid, is installed in the cell, filled with liquid, gas is supplied to the cell and its pressure is measured. The cell with the membrane is placed in a vessel. The cell and the vessel are hydraulically interconnected. A non-destructive pressure drop is created on the membrane by pumping liquid from the vessel. A gas is supplied to the vessel, in which vapors of the same liquid are used, and the vapor pressure in the vessel is constantly measured. The pressure above the membrane is fixed at which the pressure drop across it disappears. The pressure determines the value of the surface tension coefficient of the liquid, then the maximum pore size of the membrane is calculated using the Laplace formula.

The creation of a pressure drop across the membrane by pumping out the liquid helps to ensure its integrity and thereby minimize membrane loss due to rupture during testing.

The pressurization of a vessel with a cell in pairs of the same liquid will lead to condensation of vapor on the membrane surface. In this case, the temperature of the liquid will increase. The supply of heat from steam to the membrane surface with a moistened liquid occurs by condensation. Heat is removed from the membrane to the liquid by thermal conductivity. The boundary conditions are determined by the criterion Bi = a · d / l, where

Bi - criterion of regional similarity (Biot criterion);

a is the heat transfer coefficient during condensation;

l is the coefficient of thermal conductivity;

d is the characteristic size (cell diameter).

With Bi tending to infinity (practically at Bi> 100), the surface temperature of the membrane will be equal to the vapor temperature above the membrane (Theoretical Foundations of Heat Engineering. Thermotechnical Experiment. Handbook edited by V. A. Grigoriev and V. M. Zorin, book 2, M ., Energoatomizdat, 1988, pp. 197-198). For a cell with a diameter of 50 mm and a Freon 12 liquid, the criterion Bi = 1000. The surface temperature of the membrane with wetted liquid can be determined by the vapor pressure above the membrane. As a result of heating the liquid, the surface tension decreases, as a result of which vapor breaks under the membrane, and the pressure drop across the membrane disappears.

The method eliminates visual inspection, thereby increasing the accuracy of determining the maximum pore size of the membrane by the bubble method. In this way, membranes of any porosity and any thickness can be tested without destroying the membranes during determination.

The principle of operation of the proposed method can be explained using the drawing. The porous membrane 1 is pre-soaked in a liquid. They are placed in cell 2. Cell 2 with the membrane 1 is installed in a sealed vessel 3. At the bottom of the cell 2 there are holes that hydraulically connect the cavity under the membrane 1 with the vessel 3. The vessel 3 is filled with liquid to the level that covers the cell 2 completely. Then gradually begin to pump out the liquid from the vessel 3. At the same time, the moistened membrane 1 retains the liquid due to the action of capillary forces, so there is a difference in liquid levels in the cell 2 and the vessel 3, which is fixed by a differential pressure sensor. When you reach the specified pressure drop across the membrane 1, the pumping liquid stops. In a vessel 3 in the area above the membrane 1 under a certain excess pressure serves vapor of the same liquid. In this case, the vapor pressure in the vessel 3 is continuously measured. The pressure in the vessel 3 is gradually increasing. A thin layer of liquid appears on the surface of the membrane 1, the temperature of which increases with increasing pressure in the vessel 3. Due to the fact that the heat transfer to the surface of the membrane 1 by condensation is many times higher than the heat transfer from the membrane 1 to the liquid, the surface temperature of the membrane 1 acquires a value equal to the temperature of the surrounding vapor and can be determined by the pressure in the vessel 3 using the equation of state saturated vapors. With increasing temperature of the membrane 1, the surface tension of the wetting liquid monotonously decreases. At some point in time, the pressure drop across the membrane 1 exceeds the capillary holding capacity. Under the membrane 1 pairs break through, while the pressure drop across the membrane 1 disappears. The pressure in the vessel 3 is fixed at which the pressure drop across the membrane 1 disappears. From this pressure, the value of the surface tension coefficient of the liquid is determined from the saturated vapor curve. Further, according to the Laplace formula, the value of the maximum pore size of the membrane is calculated.

Example:

At the Keldysh Center, the maximum pore size of a fluoropolymer film membrane was measured. Freon 12 was used as a working fluid. At an initial pressure of 5.4 bar in the vessel, a pressure drop of 7.5 kPa was created on the membrane. With an increase in the pressure of freon in the vessel to a value of 7.52 bar, the pressure drop disappeared. The surface tension coefficient σ = 7.5 · 10 ^-3 N / m was found from the pressure and the maximum pore size d = 1.0 μm was calculated using the Laplace formula.

Claims (1)

The method of determining the maximum pore size of the membrane by the bubble method, namely, that the membrane pre-impregnated in a liquid is installed in the cell, the cell is filled with liquid, gas is supplied to it and its pressure is measured, characterized in that the cell is placed in a vessel, while the cell and the vessel are hydraulically interconnected, after which, pumping out the liquid, they create a pressure differential across the membrane that does not destroy the membrane; gas is supplied to the vessel, which is not filled with liquid, using vapor of the same liquid as continuously measuring the pressure above the surface of the membrane and the pressure drop across the membrane is fixed over the pressure surface of the membrane, wherein the membrane pressure difference disappears on the determined pressure value of the liquid surface tension coefficient is then calculated according to Laplace's formula maximum value of the size of the membrane pores.