Water vapour diffusivity

Dyrstad, K., Veggeland, J., and Thomassen, C. (1999), A multivariate method to predict the water vapour diffusion rate through polypropylene packaging, Int. J. Pharm., 188,... 

The kinetics of actual adsorption of water on the sites of alumina is very fast. However, a substantial resistance to mass transport can be exhibited by the finite diffusivity of water molecules from the external gas phase to the adsorption sites through the porous network of the adsorbent particle. Diffusion of water vapour (molecular and Knudsen) through the pores of the alumina particle as well as the surface diffusion of adsorbed water on the pore walls [ 11-13] can contribute to the overall transport process. The presence of other non-adsorbing or adsorbing components can significantly influence both pore and surface diffusivity values for water. Table 3 shows a family of water vapour diffusivity data on Rhone-Poulenc grade A alumina in presence of N2 and He as carrier gases at a total gas pressure of 1.0 atmosphere. The water isotherm has a type IV shape [ 9,11]. Pore diffusion... 

A different approach to the CV generation was utilized by Tao and Miyazaki (1991), who reduced the sample with borotetrahydride in a continuous flow system, and used porous PTFE tubing as a gas-liquid separator. Disturbing gaseous hydrogen and water vapour, diffusing together with Hg(0) and metal hydrides, were removed in a hollow fiber... 

Oxygen and water vapour diffusion coefficients in the GDL coincide. 

When adding a coating to breathable membranes, the coating must also be breathable, i.e. water repellent while permitting water vapour to penetrate both membrane and coating. This property is produced by microporosity or by water vapour diffusion and results in improved climate conditions within the enclosed space. 

A very important concern for workers wearing thermal protective clothing is its often detrimental effect on the wearer s comfort. Besides being potentially cumbersome, in order to protect sufficiently, the material in a garment must be substantial and is likely to add to the worker s thermal load. And for steam and hot water protection, the material s breathability or water vapour diffusion is normally compromised. The search for protective materials that will also provide some level of comfort is longstanding and ongoing. 

In the Belgian design guides, four vapour retarding classes have been developed, depending on the resistance to water vapour diffusion of a vapour retarder (expressed as the diffusion thickness or equivalent air layer thickness pd). Table 4.2 gives the boundaries of the vapour retarding classes, and some examples of materials. 

In general, cement is less likely to be affected by storage according as it is more finely ground. This may appear somewhat surprising, but the reason is that in finer cement the average radius of the pores or voids between the cement particles is smaller, so that water vapour diffuses less easily through the bed of cement. 

KAL Kalaouzis, P.J. and Demertzis, P.G., Water sorption and water vapour diffusion in food-grade plastics packaging materials effect of a polymeric plasticizer, Packag. Technol. Sci., 5, 133, 1992. 

Figure 6. Dependence of water vapour diffusion log Do (Do = zero concentration diffusion coefficient) on the clay content of microcomposites, intercalated nanocomposites, and exfoliated nanocomposites.

Measurement of water-vapour diffusion, solubility and permeability requires special techniques and care because of the unique nature of water. A number of techniques have been developed for the routine measurement of the water-vapour permeability of polymer membranes. " More accurate measurements may be obtained by using the techniques described for gases however, care must be taken to avoid the possibility of water sorbing or desorbing on to or from glass surfaces. With due caution, steady-state permeation rates may be measured in this fashion, but permeation lag-times are unreliable. 

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