Vapor permeance

Typical vapor permeance of common building materials (from ASHRAE,... 

Covers water vapor transmission rate (WVTR), water vapor permeance, and water vapor permeability. 

Cuq, B., Gontard, N., Cuq, J.L., and Guilbert, S. Functional properties of myfibrilar protein based biopackaging as affect by film thickness, /. Food Sci., 61,580,1996. Donhowe, l.G. and Fennema, O. The effect of relative humidity on water vapor permeance of lipid-hydrocolloid bilayer films, JOACS, 69, 1081, 1992. 

BS 4370, Part 2, Method 8 is related to ISO 1663. 1981 [25] and describes the determination of water vapor transmission rate (pg/(nr s)), water vapor permeance (ng/(m s Pa)) and water vapor permeability (ng/(m s Pa) for rigid cellular materials that have thicknesses of between 10 mm and 70 mm. The first two properties are specific to the specimen thickness, whereas the permeability is a property of the material if this is homogeneous. A table is included in this British Standard giving conversion factors for all three properties. The method described is suitable for materials having a water vapor transmission rate of between 200 to 3000 pg/(m" . s). 

More information on P, S, and D of water vapor of PLA and some conventional petroleum-based polymers at 23°C is summarized in Table 12.5. Table 12.6 provides data for water vapor permeance and permeability at 25 and 38°C. PLA has lower water vapor barrier properties than equivalent petroleum-based polymers, and it can be included in the group of low-barrier polymers following the classification of Salame and Steingiser [40]. In this classification, a high-barrier polymer has a water vapor permeation of not more than... 

Butt TK. Water resistance and vapor permeance of weather resistance barriers. J ASTM Int 2005 2 1. 

The rate of water vapor transmission (WVT) is defined as the time rate of water vapor flow of a body between two specified parallel surfaces normal to the surfaces, under steady conditions through unit area, under conditions of test. Water vapor permeance is defined as the ratio of the WVT of a body between two specified parallel surfaces to the vapor pressure difference between the two surfaces. An accepted unit of permeance is perm. Water vapor permeability is defined as the product of permeance and thickness. The unit for permeability is perm-in. 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

Figure 8.5 Comparison of (a) crossflow, (b) counterflow and (c) counterflow sweep module performance for the separation of water vapor from air. Membrane water/air selectivity = 100, water permeance = 1000 gpu.

Thermal insulation is the second largest application of cellular polymers, and the largest application for the rigid materials, because of their thermal conductivity, ease of application, cost, moisture absorption, and water vapor transmission (or permeance). 

The reciprocal of permeance is called (unit) vapor resistance and is expressed as... 

Reconsider Ihob. 14-62. In order to reduce the migration of water vapor llirough the wall, it is proposed to use a0.051-nim-thickpolyethylene film with a permeance of 9.1 X 10"kg/s m Pa. Determine the amount of water vapor that will diffuse tlirough the wall in this case during a 24-h period. 

Consider a brick house that is maintained at 20°C and 60 percent relative humidity at a location where the atmospheric pressure is 85 kPa. The walls of the house are made of 20-cm-thick brick whose permeance is 23 X 10 kg/s m -Pa. Taking the vapor pre,ssure al the outer side of the wallboard to be 7.CTO, determine the maximum amount of water vapor that will diffuse through a 3-m X 5-ra section of a wall during a 24-h period. 

Both of these methods may be used to calculate permeability, permeance, and water vapor transmission rate (WVTR). ASTM C 355, formerly used for rigid foams, has been discontinued and replaced by ASTM E 96. 

Pervaporation membranes are commonly subject to swelling. This has the effect of increasing the permeance as the concentration of permeating component increases. In addition, permeance generally increases with temperature. Hence, Joi is not a constant, but can be expressed as a function of temperature and of the partial vapor pressure of the permeating component, i.e.,... 

Their laboratory PVMR consisted of a reservoir in which the reactants were placed together with Nafion pellets, which acted as the catalyst. The liquid in the reservoir was continuously recirculated through the membrane tube, which was placed externally to the reactor. The membrane, itself, was also shown to be catalytic. A flow of inert gas (rather than vacuum) was used to remove the vapors and water from the membrane permeate. For the methanol esterification reaction the improvement in yield was modest (final conversion 77 % vs. 73 % corresponding to equilibrium), because the membrane was not very permselective towards the reaction products. Significant improvements, on the other hand, were observed with the butanol reaction (final conversion 95 % vs. 70 % corresponding to equilibrium), as the membrane is more permselective towards the products of this reaction. Exchanging the acidic protons in the Nafion membranes with cesium ions significantly improved the permselectivity, but also reduced membrane permeance. 

Permeance - A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury. 

Random copolymers show good chemical resistance to acids, alkalis, alcohols, and to low-boiling hydrocarbons, but are not suitable for packaging aromatic hydrocarbons. Moisture-barrier properties are good. For example at 38 C (100 F) and 90% RH, permeance to water vapor for one such copolymer is 235 g pm/m 24 h. 

In addition to the permeability coefficient, other parameters are also used to express the barrier characteristics of plastic materials. These include permeance (R), gas transmission rate (GTR), water vapor transmission rate (WVTR), and thickness normalized flow. The relationship between these parameters is shown in Fig. 14.3. 

The ASTM standards adopt definitions that are consistent w ith the equivalent definitions for gas transmission. Water vapor transmission rate is the mass transfer rate of water vapor per unit area (g nr 24h). Permeance is the ratio of the water vapor transmission rate to the difference in vapor pressure between the surfaces of the test piece measured in mm of mercury this unit is known as the metric perm (g nr 24h mmHg). This is equivalent to the gas transmission rate. Permeability is the product of the permeance and the thickness of the test piece, assuming that the permeance is inversely proportional to thickness for homogeneous materials this unit is known as the perm-centimetre (g cm nr 24h mmHg). Since the adoption of SI units, the water vapor permeability may also be expressed in the units of microgram meter per newton hour (pgm N h or pgm m Pa h ). 

The WVTR, the permeance of the film to water vapor, and the water vapor permeability coefficient may be determined from this test. 

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