Membrane bubble point

The apparatus used to measure membrane bubble points is shown in its simplest form in Figure 7.7 [4], Bubble point measurements are subjective, and different operators can obtain different results. Nonetheless the test is quick and simple and is widely used as a manufacturing quality control technique. Bubble point measurements are also used to measure the integrity of filters used in critical pharmaceutical or biological operations. 

Figure 7.8 shows typical results comparing microbial challenge tests using 0.22-pm P. diminuta with membrane bubble points for a series of related membranes [6], In these tests at a microbial reduction factor of 108-109, the membrane has a bubble point pressure of only 40 psig, far below the theoretical... 

The bubble point measurement is recognized as an ASTM procedure (F316-80 [26] and E128-61 [27]). This technique allows the determination of pore diameters and the presence of defects in the membrane. Bubble point is based on Jurin s law. If a porous membrane is impregnated with a liquid (e.g., water, alcohol) each pore has a meniscus of condensate at the gas-liquid interface which opposes the flux of gas. To unblock the pores a pressure Ap must be applied. According to the Jurin s law, the smaller the pores, the higher the pressure required... 

Microfiltration membranes are characterized by bubble point and pore size distribution whereas the UF membranes are typically described by their molecular weight cutoff (MWCO) value. The bubble point pressure relates to the largest pore opening in the membrane layer. This is measured with the help of a bubble point apparatus.t Jt l The average pore diameter of a MF membrane is determined by measuring the pressure at which a steady stream of bubbles is observed. For MF membranes, bubble point pressures vary depending on the pore diameter and nature of membrane material (e.g., hydrophobic or hydrophilic). For example, bubble point values for 0.1 to 0.8 pm pore diameter membranes are reported to vary from 1 bar (equals about... 

Membrane Characterization The two important characteristics of a UF membrane are its permeability and its retention characteristics. Ultrafiltration membranes contain pores too small to be tested by bubble point. Direc t microscopic observation of the surface is difficult and unreliable. The pores, especially the smaller ones, usually close when samples are dried for the electron microscope. Critical-point drying of a membrane (replacing the water with a flmd which can be removed at its critical point) is utihzed even though this procedure has complications of its own it has been used to produce a Few good pictures. 

The physical characterisation of membrane structure is important if the correct membrane is to be selected for a given application. The pore structure of microfiltration membranes is relatively easy to characterise, SEM and AFM being the most convenient method and allowing three-dimensional structure of the membrane to be determined. Other techniques such as the bubble point, mercury intrusion or permeability methods use measurements of the permeability of membranes to fluids. Both the maximum pore size and the pore size distribution may be determined.1315 A parameter often quoted in manufacturer s literature is the nominal... 

Bubble Point Large areas of microfiltration membrane can be tested and verified by a bubble test. Pores of the membrane are filled with liquid, then a gas is forced against the face of the membrane. The Young-Laplace equation, AF = (4y cos Q)/d, relates the pressure required to force a bubble through a pore to its radius, and the interfacial surface tension between the penetrating gas and the liquid in the membrane pore, y is the surface tension (N/m), d is the pore diameter (m), and P is transmembrane pressure (Pa). 0 is the liquid-solid contact angle. For a fluid wetting the membrane perfectly, cos 0 = 1. 

A commonly used simple method for determining if there are any cracks or pinholes in microporous membranes is the so-caUed bubble point test. It has been used by many organic membrane manufacturers and users alike and is also being adopted by some inorganic membrane manufacturers. The method utilizes the Washburn equation... 

Membrane Initial Bubble Point (psi) Challenge Level (CFU/47 mm disk) 1st 2nd 3rd Final Bubble Point (psi) 1st 72 hr. Results 2nd 3rd... 

Bubble Point Constancy. Although the exact relationship between the bubble point and the "pore size" of a microfiltration membrane is a matter of dispute (11, 12, 13, 14), nevertheless, it remains the quickest and most convenient means for demonstrating the continuing integrity of a membrane filtration system. It is consequently important that the bubble point be both reproducible (within a given range) and constant. It was, therefore, of considerable interest to discover that the bubble points of both conventional and poly(vinylidene fluoride) membranes increased with immersion time in deionized water whereas those of Tyrann-M/E and polyamide remained essentially constant (Figure 6). 

Figure 6. Bubble point vs. duration of immersion for various 0.45/iM membranes 1 = conventional 2 = Tyrann-M/E 3 = PVF 4 = polyamide.

Table V. Effects of Flexing and Autoclaving Upon the Bubble Points of Water-Wet 0.45 /iM Membranes...

The so-called bubble point of a membrane - a measure ofthe membrane pore size - can be determined by using standard apparatus. When determining the bubble point of small, disk-shaped membrane samples (47 mm in diameter), the membrane is supported from above by a screen. The disk is then flooded with a liquid, so that a pool of liquid is left on top. Air is then slowly introduced from below, and the pressure increased in a stepwise manner. When the first steady stream of bubbles to emerge from the membrane is observed, that pressure is termed the bubble point. ... 

The membrane pore size can be calculated from the measured bubble point Pj, by using the dimensionally consistent Equation 10.9. This is based on a simphstic model (Figure 10.6) that equates the air pressure in the cyhndrical pore to the cosine vector of the surface tension force along the pore surface [6] ... 

The degree of removal of microbes of a certain size by a membrane is normally expressed by the reduction ratio, R. For example, if a membrane of a certain pore size is fed 10 microbes per cm- and it stops them all except one, the value of log reduction ratio log R is 7. It has been shown [7] that a log-log plot of R (ordinate) against the bubble points (abscissa) of a series of membranes will produce a straight line with a slope of 2. 

Aseptic Lltration is necessary for parenteral formulations. Because both lipids and the structure of liposomes are unstable at high temperatures, conventional terminal steam sterilization is not suitable for liposome formulations. Thus, the membrane aseptic Lltration is the most reliable method for sterilizing liposome formulations. Since the possibility exists for the membrane being defective, it is advisable to test the integrity of the assembled unit by carrying out a bubble-point test. This test... 

The bubble point test is a popular single-point physical integrity test for disc filter membranes based on Eq. (21). A fdter medium is wetted with a liquid, and test gas pressure is slowly raised until a steady stream of bubbles appears from a tube or hose attached to the downstream side of the filter and immersed in water (Fig. 9). The pressure at which the bubbles first appear is recorded as the bubble point and is related to the largest pores in the fdter medium. A pore size can be calculated from Eq. (21) however, it must be realized that the bubble point test does not measure the actual pore size, but only allows correla-... 

The bubble point test, while popular, has some deficiencies that must be realized. First, there is variation in the operator detection of the test end point that is, the first appearance of gas bubbles rising in the liquid. Some operators are able to see smaller bubbles than others. In a recent study, a panel of seven observers recorded the initial detection of a steady stream of air bubbles rising from a capillary held under water as the air pressure was gradually increased. The observers, who had received different degrees of training, identified the simulated bubble point as occurring at air flows of 5 to 50 mL/min corresponding to air pressures of 34 and 38 psi, respectively, for a 90-mm disc filter membrane [56]. 

With large-surface-area membrane filter media, the interpretation of the true bubble point can be further complicated because of the diffusion of the test gas through the media. Because the filter media are more than 70% void space, a liquid-wetted membrane is virtually a thin film of liquid across which a test gas will diffuse, governed by Fick s law. 

Bechhold prepares collodion membranes of graded pore size, measures bubble points and uses the term ultrafilter -1907... 

Figure 7.4 Membrane pore diameter from bubble point measurements versus Bacillus prodigiosus concentration [1], Reprinted from W.J. Elford, The Principles of Ultrafiltration as Applied in Biological Studies, Proc. R. Soc. London, Ser. B 112, 384 (1933) with permission from The Royal Society, London, UK...

The bubble point test is simple, quick and reliable and is by far the most widely used method of characterizing microfiltration membranes. The membrane is first wetted with a suitable liquid, usually water for hydrophilic membranes and methanol for hydrophobic membranes. The membrane is then placed in a holder with a layer of liquid on the top surface. Air is fed to the bottom of the membrane, and the pressure is slowly increased until the first continuous string of air bubbles at the membrane surface is observed. This pressure is called the bubble point pressure and is a characteristic measure of the diameter of the largest pore in the membrane. Obtaining reliable and consistent results with the bubble point test requires care. It is essential, for example, that the membrane be completely wetted with the test liquid this may be difficult to determine. Because this test is so widely used by microfiltration membrane manufacturers, a great deal of work has been devoted to developing a reliable test procedure to address this and other issues. The use of this test is reviewed in Meltzer s book [3],... 

The bubble point pressure can be related to the membrane pore diameter, r, by the equation... 

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