Water vapor transmission rate WVTR

The water-vapor transmission rate (WVTR) is another descriptor of barrier polymers. Strictly, it is not a permeabihty coefficient. The dimensions are quantity times thickness in the numerator and area times a time interval in the denominator. These dimensions do not have a pressure dimension in the denominator as does the permeabihty. Common commercial units for WVTR are (gmil)/(100 in. d). Table 2 contains conversion factors for several common units for WVTR. This text uses the preferred nmol/(m-s). The WVTR describes the rate that water molecules move through a film when one side has a humid environment and the other side is dry. The WVTR is a strong function of temperature because both the water content of the air and the permeabihty are direcdy related to temperature. Eor the WVTR to be useful, the water-vapor pressure difference for the value must be reported. Both these facts are recognized by specifying the relative humidity and temperature for the WVTR value. This enables the user to calculate the water-vapor pressure difference. Eor example, the common conditions are 90% relative humidity (rh) at 37.8°C, which means the pressure difference is 5.89 kPa (44 mm Hg). 

Water Transport. Two methods of measuring water-vapor transmission rates (WVTR) ate commonly used. The newer method uses a Permatran-W (Modem Controls, Inc.). In this method a film sample is clamped over a saturated salt solution, which generates the desired humidity. Dry air sweeps past the other side of the film and past an infrared detector, which measures the water concentration in the gas. For a caUbrated flow rate of air, the rate of water addition can be calculated from the observed concentration in the sweep gas. From the steady-state rate, the WVTR can be calculated. In principle, the diffusion coefficient could be deterrnined by the method outlined in the previous section. However, only the steady-state region of the response is serviceable. Many different salt solutions can be used to make measurements at selected humidity differences however, in practice,... 

Water vapor imbibition, polymer precipitation by, 75 807-808 Water vapor transmission rate (WVTR) for VDC copolymers, 25 709-710 Water-vapor transmission rate (WVTR), barrier polymers, 3 375, 387-388 selected polymers, 3 387t Waterwall furnaces, 72 319, 320, 326 Water washing, in soap making, 22 735 Waterways... 

The point-flex test developed in this work was not considered an adequate substitute for the MIL-B-131B Performance Test. This test, like the Water Vapor Transmission Rate (WVTR) test after low-temp flexing at —20°F separates good from poor materials in the same manner as the spec test, bet does not rate barrier materials In the same order of quality... 

Water Transport. Two methods of measuring water-vapor transmission rates (WVTR) are commonly used. The newer method uses a Permatran-W (Modern Controls, Inc.). The other method is the ASTM cup method. 

Table 2.8 Water vapor transmission rate (WVTR) of barrier dressings at 37°C, ASTM D 1653-03...

A gravimetric method is often used for the measurement of water vapor permeability. The water vapor transmission rate (WVTR) according to this method is the amount of water vapor in g that permeates in 24 hours under standard test conditions (temperature, air relative humidity, water vapor sorbant) through aim2 sample surface area. The method measure a flux and not a permeability coefficient. 

Water-Vapor-Transmission Rate (WVTR) of Rigid Ceilnlar Piastics ASTM E 96 (TM)... 

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. 

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

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 gas transmission rate, GTR, is the quantity of permeant flowing per unit area per unit time. The water vapor transmission rate, WVTR, is a GTR defined specifically for water vapor. The two together are permeant transmission rates, identified as F in Fig. 14.3. 

Desiccant Method A desiccant covered by the film to be tested and placed in a humid chamber. Moisture from the chamber permeates the film and is picked up by the desiccant. After a measured period of time the test dish is reweighed and the water vapor transmission rate (WVTR) of the material is calculated. 

Water Vapor Transmission Rate (WVTR) Through Plastic Film and Sheeting Using a Modulated Infrared Sensor. Primary Film Test Method ASTM F1249. A procedure for determining the WVTR through flexible barrier materials, film, and sheet up to 3 mm in thickness is known as ASTM FI249. 

Barrier Properties Exact Plastomers create controllable gas-permeable films for modified atmospheres, as well as gas-permeable films and packaging for such items as fresh vegetables, fhiits, flowers, and a growing range of other horticultural products. Users can select carbon dioxide and oxygen transmission rates (OTR) that meet their needs, with low water vapor transmission rates (WVTR) and high levels of clarity and gloss. " ... 

Water vapor transmission rate (WVTR) gm day Test temperature = 311K relative humidity = 90% butene-1 density range = 0.900-0.910 g cm 15.5-19.5 (38)... 

Diffusion coefficients and solubility coefficients are the fimdamental parameters that control mass transfer in packaging systems. In practice, however, the permeability P is the most important of the permeation properties. Characteristic values for the permeability are the transmission rates (oxygen transmission rate, OTR water vapor transmission rate, WVTR). They indicate the amount of a given substance that passes a unit surface area of film per unit time, depending on the partial pressure gradient for a given temperature, the relative humidity and the given thickness of the film. 

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