Water vapor permeability, various

Water vapor permeability (WVP) for starch-based films plasticized with different contents (a) and types (b) of binary polyol mixtures at various relative humidities at 25°C. 

Halar fluoropolymer has low permeability to water vapor and various other gases. Water vapor permeability measured at 38°C and 90% RH was found to be 0.15 g mil/100 in 24 hrs. 

Table 7-4. Permeability Coefficients of Gases and Water Vapor in Various Polymers at 20° C. The Gas Passage, Am, Is Measured in cmi. Barex Is a Graft Copolymer of Acrylonitrile Methyl Acrylate on Nitrile Rubber. Lopac Is a Copolymer of Methacrylonitrile and Styrene.

Additionally, the favorable comparison of these theoretical predictions with experimental data (Figure 2A0d) gives some credibility to the conclusions above. In Figure l.lOd we plot experimental water vapor permeabilities of various solvent cast nanocomposite films. The experimental behavior follows closely the theoretical trend and is enclosed between the response of exfoliated systems (especially for low filler loadings) and that of intercalated systems (for moderate and higher loadings). This reflects the same effective filler aspect ratio... 

FIGURE 2.10 Theoretical predictions based on path tortuosity [eq. (2.9)], as a function of (a) filler aspect ratio a = 1 to 1000 (b) filler aspect ratio and alignment (5 = 1 perfect smectic alignment—dashed lines S = 0 random orientation —solid lines) (c) filler aspect ratio for a constant volume fraction (pv = 5%. (d) Comparison of the same theoretical predictions (parameters as indicated) with experimental values for water vapor permeabilities in various polymer-montmorillonite nanocomposites. (From Refs. 39-41.)... 

Film forming materials, plasticizer type and amount and other additives affect film properties differently. Table 1 and 2 show the water vapor permeability (WVP) and oxygen permeability (OP) of different polysaccharide-, protein-, composite- and frait and vegetable-based films with various plasticizers using different processes. Comparisons among them are difficult because of different film compositions, test conditions (RH, temperature), methodology, and number of replications were used in each woik. 

TABLE 1 Water vapor permeability (WVP), oxygen permeability (OP), and mechanical properties (tensile strength, TS elastic modulus, EM and % elongation, %E) of various edible films from wet process (solution casting)... 

Table III. Water vapor permeability mol.m .s .Pa ) of various protein-based films and comparison with some synthetic films and biodegradable films (5).

Halar fluoropolymer has low permeability to water vapor and various other gases. Water vapor... 

Table 8. Comparison of the Permeabilities of Various Polymers to Water Vapor ...

There are substantial differences in the rates at which water vapor and other gases can permeate different plastics. For instance, PE is a good barrier for moisture or water vapor, but other gases can permeate it rather readily. Nylon, on the other hand, is a poor barrier to water vapor but a good one to other vapors. The permeability of plastic films is reported in various units, often in grams or cubic centimeters of gas per 100 in.2 per mil of thickness (0.001 in.) of film per twenty-four hours. The transmission rates are influenced by such different factors, as pressure and temperature differentials on opposite sides of the film. 

It should be recognized that all plastic materials over a time period allow a certain amount of water vapor, organic gas, or liquid to permeate the thickness of the material. It is only a matter of degree of permeation between various materials used as barriers against vapors and gases. It has been found that the permeability coefficient is a function of the solubility coefficient and diffusion coefficient. The process of permeation is explained as the solution of the vapor into the incoming surface of the barrier, followed by diffusion through the barrier thickness, and evaporation on the exit side. 

Blankt has reported the permeability (in cm3 of gas s cm 2 surface) of various spread monolayers to water vapor at 25 °C. For several different / X-type compounds at different it values, the permeabilities are as follows ... 

Figure 2.60 Permeability of water vapor as a function of temperature through various polymer films.

P. H. List, G. Kaosio, Water vapor and oxygen permeability of various tablet coatings, Acta Pharm Techn 28(1) 1982. 

For example, consider a soft drink or soda bottle. What are the requirements Cheap Strong enough to contain up to 2L of soda. Optically clear. And very important, a barrier for 0O2, because carbonated beverages taste flat once the concentration of carbon dioxide falls too low. Table 8-1 lists the permeability of various polymers to C02 as well as to oxygen and water vapor. 

The insoluble cellulose derivatives utilized for permeation control of various species (e.g. oxygen and water vapor transport in coated pharmaceuticals, contact lenses, packaging, or water and solute transport through semi-permeable membranes in reverse osmosis, as well as drug release from reservoir systems) differ considerably in their permeability characteristics according to the type and extent of substitution, as well as their molar mass. However, very few comparative data are available from the literature on the polymers actually used in biological applications. Recently, new results have been published. Thus, Sprockel et al. [142] determined the water vapor transmission through various CA, CAT, CAB and CAPr films at different relative humidities (Table 22). 

A comparison of moisture vapor permeation through various polymers can be seen in Table 3.5. Notice that PCTFE is only second to FEP and both are among the most resistant plastics to water vapor permeation. Permeability data can be found in Appendixes I through IV. 

An eqnation has been derived relating the effective diffusivity of porous foodstuffs to various physical properties such as molecular weight, bulk density, vapor space permeability, water activity as a function of material moisture content, water vapor pressure, thermal conductivity, heat of sorption, and tanperature [80]. A predictive model has been proposed to obtain effective diffusivities in cellular foods. The method requires data for composition, binary molecular diffusivities, densities, membrane and cell wall permeabilities, molecular weights, and water viscosity and molar volume [81]. The effect of moisture upon the effective diffusivity is taken into account via the binding energy of sorption in an equation suggested in Ref. [77]. 

---