Permeability of gases

The permeation rate of PP with different gases is compared to other polymers in Table 22. It can be seen that the resistance to permeability of gases improves with orientation. In addition, PP offers good resistant to permeability of gases which is comparable to HDPE but significantly better than LDPE. 

Those readers who wish further knowledge are referred to monographs [232-243, 261] which are excellent sources. 

The characteristics of gel membranes (hydrophilic membrane) depend strongly on the measurement environment. Therefore, in order to measure the permeability of a gas through a gel membrane, it is important to develop a technique in which measurements can be made at a swollen state in the presence of water. 

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Rogers, C.E., Stannett, V., and Szwarc, M., Permeability valves permeability of gases and vapors through composite membranes, Ind. Engr. Chem., 49,1933 (1957). 

Hollow fibers have been tried as liquid-core fibers in spectrophotometry [118] and porous fibers have been well adapted for gas measurements [119,120]. The latter are made of porous polymers, which can trap indicators or reagents in the matrix with a very high permeability of gases and liquids. For liquid applications, porous silica gel has also been used [121],... 

Figure 2.39 Molecular-weight-normalized permeability of gases through Vycor microp-orous glass membranes [60], Reprinted from Techniques of Chemistry, Vol. VII, Membranes in Separations, S.T. Hwang and K. Kammermeyer A. Weissberger (ed.) Copyright 1975. This material is used by permission of John Wiley Sons, Inc.

The permeability of gases through membranes is most commonly measured in Barrer, defined as 10-10 cm3(STP)/cm2 s cmHg and named after R.M. Barrer, a pioneer in gas permeability measurements. The term ji/ pio — pit), best called the pressure-normalized flux or permeance, is often measured in terms of gas permeation units (gpu), where 1 gpu is defined as 10 6 cm3(STP)/cm2 s cmHg. Occasional academic purists insist on writing permeability in terms of mol m/m2 s Pa (1 Barrer = 0.33 x 10-15 mol m/m2 s Pa), but fortunately this has not caught on. 

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. 

Further applications to new fields can be found in the work of P. Langevin On the Recombination of Electrically Dissociated Gases (ThSse, Paris 1902) and On the Magnetic Permeability of Gases (/. d. Phye., 4 [1905], 678). 

Patil, G.S., Bora, M. and Dutta, N.N. (1995). Empirical Correlations for Prediction of Permeability of Gases Liquids Through Polymers. J.Memb.ScL, 101,145-152. 

Permeability of gases and vapours through plastic involves absorption, followed by diffusion, followed by evaporation and desorption from the other face. Permeability is greatest with amorphous plastics where diffusion occurs via the spaces between the moving mass of molecular chains. Crystalline plastics or those with crystalline regions present a greater barrier to diffusion. With thinner materials where pinholes or micropores occur, diffusion may occur via these small holes. Various other factors influence permeation, including ... 

The permeability of gases and vapors in a flawless polymer matrix is well established by the solution-diffusion principle in which permeability, P, is given by the product of solubility, s, and the diffusivity, D, i.e., P = sD. 

As the molecular size of most gases is much smaller than any scale of structure expected in polymer blend morphology, diffusion and permeability of gases can be employed to determine the phase behavior of a polymer blend. Therefore, the study of transport phenomena in blends would be motivated not only by the requirements of producing improved barrier materials but also by the continuous interest in the nature and characterization of polymer blend morphology. 

Since the solubility of various gases in ILs varies widely, they may be uniquely suited for use as solvents for gas separations [97]. Since they are non-volatile, they cannot evaporate to cause contamination of the gas stream. This is important when selective solvents are used in conventional absorbers, or when they are used in supported liquid membranes. For conventional absorbers, the ability to separate one gas from another depends entirely on the relative solubilities (ratio of Henry s law constants) of the gases. In addition, ILs are particularly promising for supported liquid membranes because they have the potential to be incredibly stable. Supported liquid membranes that incorporate conventional liquids eventually deteriorate because the liquid slowly evaporates. Moreover, this finite evaporation rate limits how thin one can make the membrane. This means that the net flux through the membrane is decreased. These problems could be eliminated with a non-volatile liquid. In the absence of facilitated transport (e.g., complexation of CO2 with amines to form carbamates), the permeability of gases through supported liquid membranes depends on both their solubility and diffusivity. The flux of one gas relative to the other can be estimated using a simplified solution-diffusion model ... 

Permeability of gases and vapors through a film is an important consideration in many applications of polymers. A high permeability is sometimes desirable. For example, in fruit-packaging applications of plastics film it is desirable to have high permeability of carbon dioxide. On the other hand, for making inner tube and tubeless tires, or in a child s balloon, the polymer used must have low air permeability. 

For semi-crystalline polymers above Tg, the permeability is proportional to the nth power of the amorphous volume fraction n lies between 1.2 and 2. The gas must diffuse between the lamellar crystals, and the detailed morphology depends on the polymerisation route, thermal history and whether orientation is present. The permeability of gases of molecular weight M is approximately inversely proportional to y/M. However, Table 11.2 shows that the ratio of CO2 to O2 permeability in glassy polymers is higher than in semi-crystalline polymers. 

However, within a series of monovalent cations with increasing ionic radius, such as Li+,K+,Cs+, the permeability decreases in this order for nonpolar gases. The same is true for earth alkaline cations in the series Mg +,Mg +,Ca +. Therefore, it is concluded that both the size of the cation plays a role in increasing the void volume, and the change in polarity is another factor for the performance. In brominated PPE, the permeability of gases increases with the degree of bromination. °... 

The difference in relative permeability of gases through the membranes offers an attractive way for the enrichment of industrial gases. The transport properties of N and O2 through porous polystyrene materials were found to be much higher than through those prepared by bulk polymerization [43]. 

Figure 12.23. Minimum permeability of gases through a polymer filled with plates of different L/W ratio oriented parallel to the surface of the film, (a) LIW 1 (cubes) (b) L/W(c) (2 x 2 x i plates) ...

Suchdeo, S.R. and J.S. Schultz. "The permeability of gases through reacting solutions the carbon dioxide-bicarbonate membrane system." Chemical Engineering Science 29 (1974) 13-23. 

L. Ferguson, P. Scovazzo, Solubility and permeability of gases in phosphonium-based room temperature ionic liquids data and correlations, Ind. Eng. Chem. Res., 46,1369-1374 (2007). 

Table 17-2. Permeabilities of gases in various membranes cm (STP)cm/[cm. s.cm Hg] x 10 Reference Code N = Nakagawa, 1992 DR = Drioli and Romano, 2001 G = Geankoplis, 2003...

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