Membrane macrovoid

One specific advantage of this process is that the dense polymer layer firmly adheres to a porous support offering the ability to withstand high pressure. Hollow fiber membranes utilized in gas separation need to withstand continuous pressures up to and exceeding 1000 psi. For gas separation, it is desirable to have a continuous porous stmrture as the support without macrovoids, which can often be produced if the phase inversion variables (such as polymer solution concentration) are not optimized. For ultra- and microfiltration membranes, macrovoids often exist but do not critically restrain the... 

The presence of macrovoids in hoUow-fiber membranes is a serious drawback since it increases the fragUity of the fiber and limits its abUity to withstand hydrauhc pressures. Such fibers have lower elongation and tensile strength. 

Hollow fiber membrane(s), 70 766 76 1-31 additional types of, 76 24 advantages of, 76 3 categories of, 76 2-3 in desalination, 76 22 development of, 76 1 extractors, 70 787 fiber treatment for, 76 12-18 future prospects for, 76 26-28 glass and inorganic, 76 23-24 handling and unit assembly of, 76 15-18 interpenetrated wall matrix in, 76 15 low pressure, 76 24-26 macrovoids in, 76 12 materials associated with, 76 18-24 melt spinning of, 76 9-10... 

The development of the Loeb-Sourlrajan asymmetric cellulose acetate membrane (1) has been followed by numerous attempts to obtain a similar membrane configuration from virtually any available polymer. The presumably simplistic structure of this cellulose acetate membrane - a dense, ultrathln skin resting on a porous structure - has been investigated by transmission and scanning electron microscopy since the 1960s (2,3). The discovery of macrovoids ( ), a nodular intermediate layer, and a bottom skin have contributed to the question of the mechanism by which a polymer solution is coagulated to yield an asymmetric membrane. 

Figure 16. Reverse osmosis membrane exhibiting three types of macrovoids large (a), medium (b), and small (c). (A) Before testing and (B) after exposure to 13, atm hydraulic pressure. A longitudinal crack in the skin is designated by the per-...

Figure 17. Cross-section of a reverse osmosis membrane with dense-walled macrovoids...

Thorough analysis and evaluation of membrane morphology is mandatory for the understanding of transport phenomena in membranes, and es pecially for those with rather complex structures, as described in the present manuscript. Each single membrane can be viewed perhaps as a "black box" when operating in a certain well-defined system. Yet, any deduction on transport mechanism that is based solely on transport data is highly speculative. For example, the presence of a double skin, macrovoids, the densifica-tion of the nodular layer, and other items described herein cannot be predicted by the analysis of transport data. But they can be identified, and can be very supportive to "whoever dares to look into the black box."... 

Numerous asymmetric membranes were prepared under various conditions and their cross-section was examined by SEM. Typical of the results are those shown in Figure 5 for the membrane cast from 70 30 THF-formamide and gelled in IPA. Close inspection of Figure 5 reveals a thin, relatively dense skin supported by a microporous layer. The support layer contains macrovoids, the cause of which is presently under investigation. 

Figure 2.4 SEM micrograph of a cross-section of a hollow-fiber dialysis membrane (Polyflux, Cambro) with an anisotropic structure and macrovoids in the support layer (left), and details of the inner porous separation layer in two different magnifications (right reprinted from [12], with permission from Wiley-VCH, 2003).

Microfiltration and UF membranes can be asymmetric, with a denser side and a more open side, or uniform without macrovoids (See Figure 16.3). The open area behind the denser surface in an asymmetric design means there is less resistance to water permeating the membrane. Operating pressure can be lower and the membrane systems can be more productive. The limitation of the asymmetric design is that the material, predominately used in the hollow fiber configuration, is not as strong as the uniform cross section. 

A partial list of commonly encountered structural irregularities in phase Inversion membranes includes irregular gelation, wavemarks, macrovoids and blushing. 

In preparing membranes via the phase inversion process for applications in pressure-driven processes, the formation of macrovoids should be avoided completely. These finger-like pores of the type present in the substructure of membranes (b) and (c) of Fig. 3.6-1, severely Hmit the compaction resistance of the membrane. Membranes with a sponge-Hke structure (Fig. 3.6-la) are to be preferred. 

Figure 15.8 SEM image of a PANI membrane prepared by the phase inversion technique with the appearance of macrovoids. (Reprinted with permission from Advanced Functional Materials, High-performance, monolithic polyaniline electrochemical actuators by j.-M. Sansinena, J. Gao and H.-L. Wang, H.-L., 13, 9, 703-709. Copyright (2003) Wiley-VCH)...

In addition, the high affinity between solvent and nonsolvent (low /12) causes the instantaneous demixing. In the case of an instantaneous process, the governing factor is the concentration gradient, which diverges in the direction in which diffusion occurs and causing asymmetric structures and macrovoids. Macrovoids consist in poms that could have sizes similar to the thickness of the membrane. 

Finally it has to be mentioned that macrovoids cause weaknesses in the membrane and therefore, normally they are undesirable." ... 

In the previous works, performed with membranes, different manbrane morphologies have been obtained depending on the relative humidity and time of exposure of the polymeric solution to the vapor. Under none of the conditions assessed was there evidence of macrovoids formation when separation is induced by vapor. - ... 

This is an advantage of the technique, because as noted above, these macrovoids can cause weaknesses in the membrane. 

Barzin, J. and B. Sadatnia. Correlation between macrovoid formation and the ternary phase diagram for polyethersulfone membranes prepared from two nearly similar solvents. Journal of Membrane Science 325(1) (2008) 92-97. 

Fig. 3.4 Nonisotropic nucleus growth during macrovoid formation in membranes.

L. Zeman, T. Fraser, Formation of air-cast cellulose acetate membranes. Part I. Study of macrovoid formation. Journal of Membrane Science 84 (1993) 93. 

S.A. McKelvey, W. Koros, Phase separation, vitrification and the manifestation of macrovoids in polymeric asymmetric membranes. Journal of Membrane Science 112 (1996) 29. 

C.A. Smolders, A. J. Reuvers, R.M. Boom, I.M. Wienk, Microstructures in phase-inversion membranes. Part 1. Formation of macrovoids. Joumo of Membrane Science 73 (1992) 259. 

Figure 1.2 Electron micrograph ofa UF asymmetric membrane showing the skin layer on the top (feed side) and macrovoids in the interior.

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