Water vapor transmission characteristics

The water vapor transmission characteristics of an SBR latex modified mortar are twice as good as those of an unmodified mortar. The water vapor transmission rate, as measured in grains/square foot/hour/inches mercury is as follows ... 

This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and quick-dry paints are most susceptible to the problem. Their use should be avoided unless the absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quahty. 

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). 

Rubber Property-Vapor Transmission of Volatile Liquids Oetermining Gas Permeability Characteristics of Plastic Film and Sheeting Water Vapor Transmission of Organic Coating Films... 

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. 

It is really interesting that the water vapor transmission rate is the lowest of the commonly used biodegradable polymers this characteristic is truly relevant, for example, in packaging applications. 

Although general-purpose films are available, they can be varied in composition to meet specific requirements. Such special formations may be superior to standard films in flame resistance, heat sealing characteristics, water absorption, cold resistance, water vapor transmission, light or solvent resistance, etc. 

As mentioned above, heat and mass transfer in textile materials is a complex phenomenon that includes several mechanisms. Textile material properties significantly influence these mechanisms. Several researchers studied the effect of these properties on heat and mass transfers at three different levels (1) the microscopic level (chemical composition, morphological characteristics, fineness, cross-section, porosity, and water content of the component fibers), (2) the mesoscopic level (yam structure and properties), and (3) the macroscopic level (the fabric s physical and stmctural characteristics and finishing treatments) [3,22,23]. Thus, in the following section, heat transfer properties, such as thermal conductivity, thermal resistance, thermal absorptivity, and thermal emissivity and mass transfer properties, such as water vapor transmission and liquid water transmission, will be defined at fiber, yam, and fabric levels. 

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. 

The three main characteristics of the common packaging films, viz. the water vapor transmission rate, oxygen permeability and CO permeabihty are given in Table 3. 

It is common in the industry to report the barrier values as water vapor transmission rate or gas transmission rates rather than as permeability coefficient. This practice, although apparently practical, is in effect confusing and misleading, since it does not allow easy comparison of the barrier characteristics of polymers measured at different conditions or in different countries. This is even more confusing when different units are used. The use of permeability coefficient should be encouraged as common practice in the industry. 

An alternative method for evaluating the water vapor transmission rate of a flexible material is to use it as a barrier separating two chambers, one at high humidity and the other at low humidity. The rate of water vapor permeation is determined by measuring the increase in the concentration of water vapor in the low humidity chamber. Both absolute and comparative methods exist for calculating the rise in water vapor concentration, an infrared absorbance technique being commonly used. In comparative methods the diffusion cell is calibrated with a film of known barrier characteristics. The water vapor transmission rate is calculated as the mass of water permeating the barrier as a function of time and area. 

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