Permeability water vapor transmission rate

When determining the permeability of films to water vapor, we seal a desiccant into a small cup with the polymer covering the opening. We weigh the cup before placing it in an oven at controlled temperature and humidity. After a given period of time we remove it and weigh it a second time. We calculate the film s water vapor transmission rate based on the area of the cup s mouth and the time that it was in the oven. 

The effect of plasticizers and temperature on the permeability of small molecules in a typical vinylidene chloride copolymer has been studied thoroughly. The oxygen permeability doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeability are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a litde more complicated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeability and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeability doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

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. 

Moisture Protection of SEPIFILM LP Water vapor transmission rates were measured on free films, including titanium dioxide (Figure 27). SEPIFILM LP shows significantly lower moisture permeability compared to regular or PVA-based coating formulations. Removal of titanium dioxide (SEPIFILM LP clear) improves moisture resistance. 

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. 

The water-vapor transmission rate (WVTR) is another descriptor of barrier polymers. Strictly, it is not a permeability 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 permeability. Common commercial units for WVTR are (gmil)/(100 in.2d). Table 2 contains conversion factors for several common units for WVTR. This text uses the preferred nmol/(ms). The WWTR 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 permeability are directly related to temperature. For 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. For 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). 

Figure 4. Water Permeability for Polypropylene and Polycarbonate. Polypropylene measurements were made on 9.5 mil extruded sheet polycarbonate measurements were made on 10 and 20 mil extruded sheet. Note that permeability has water pressure in the denominator which is distinctly different from a WVTR (water vapor transmission rate).

Figure 15.12. Oxygen permeability and water vapor transmission rate (WVT) behavior of PA-6/Amorphous PA-6I/6T blends at 95% RH.

The permeability coefficient, P, combines the effects of the diffusion and solubility coefficients. The barrier characteristics of a polymer are commonly associated with its permeability coefficient values. The well-known relationship P = DS holds when D is concentration independent and S follows Henry s law. Standard methods for measuring the permeability of organic compounds are not yet available. ASTM E96 describes a method for measuring the water vapor transmission rate. ASTM D1434 describes a method for the determination of oxygen 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. 

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). 

Two different definitions of water vapor permeability are used in the BS and ASTM standards, and this can cause confusion. BS 3177 and BS 2782 define the water vapor transmission rate as the mass transfer rate of water vapor per unit area (g m 24h) (see Section 1). but they call it "permeability," When the material is homogeneous and the transfer rate is inversely proportional to thickness. BS 3177 defines an "equivalent permeability" as the "permeability" multiplied by the thickness of the test piece in thousandths of an inch (g mil m 24h). 

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 water vapor permeability, WVP, is calculated based on the water vapor transmission rate ... 

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 A measure of water vapor (moisture) permeability of a barrier wall such as plastic film. Vapor transmission rate, VTR, is a eoeffi-cient in modified Pick s first law that states that the... 

In general, cellulosics are hard, stiff materials which can be compounded with plasticizers to improve their flow, or ease of processing, and to improve the flexibility of finished moldings. They are extremely tough plastics and are reasonable electrical insulators but have high gas permeability and medium water vapor transmission rates. 

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