Permeability water vapor

Permeability is defined as the product of the diffusion constant and the solubility coefficient. Water vapor permeability (WVP] is defined as the time rate of water vapor transmission through unit area of flat material with a definite unit thickness induced by unit vapor pressure difference between two specific surfaces, and under indicated temperature and humidity conditions. (ASTM E 96, Standard Test Method for Water Vapor Transmission of Materials]. 

Bourtoom and Chinnan (2008] studied the WVP of rice starch-chitosan blend films with different type and concentrations of plasticizer. They observed that the WVP increased from 5.45 to 8.68 gmm/m day kPafor sorbitol 14.53 to 28.73 gmm/m day kPa for glycerol and finally from 14.70 to 19.10 g mm/m day kPa for polyethylene glycol. This tendency was due to possible structural modifications of the polymer network and also because the network could become less dense as a result of increasing the mobility of the polymeric chains or by the free volume of the film. Also, the increase of WVP could be related to the hydrophilicity of the plasticizers because the presence of this material increases the concentration of polar residues in hydrocolloid based film 

The material to be tested is fastened over the mouth of a dish that contains either water or a desiccant. This assembly is placed in an environment of constant humidity and temperature. The gain or loss in weight of 

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. 

The problem of permeability exists whenever a plastic material is exposed to vapor, moisture, or liquids. Typical cases are electrical batteries, instruments, components installed underground, encapsulated electrical components, food packaging, and various fluid-material containers. In these cases, a plastic material is called upon to form a barrier either to minimize loss of vapor or fluid or to prevent the entrance of vapor or fluid into a product. From the designers viewpoint, the tolerable amount of permeation established by test under conditions of usage with a prototype product of correct shape and material is the only direct answer. 

The use of correct shrinkage information is very important, not only for having the desired proportions of a product, but also for functional purposes. The shrinkage data can be shown in a range of two values. The lower figure is intended to apply to thin parts, 

Tolerance Where very close tolerances are involved, preparing a prototype of the full size product may be necessary to establish critical dimensions. If this step is not practi-tal, it may be necessary to test a mold or die during various stages of cavity or die opening manufacture with allowances for correction in order to determine the exact shrinkage needed. 

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Significant piopeities of insulation (Table 1) include tliermal conductivity, fite resistance, and ntiniinal production of toxic gases primarily during combustion. Other criteria include water-vapor permeability, resistance to water absorption, and dimensional stability over prolonged periods of submission to extreme environments. 

Water-Vapor Permeability. Water-vapor permeabiUty depends on the polymer used for the coating layer and its stmcture. Vinyl-coated fabrics have Httie water-vapor permeabiUty due to the coating layer. Although polyurethane polymer is water-vapor permeable, urethane-coated fabrics also have low permeabiUty values due to their soHd layer stmcture. On the other hand, man-made leathers have good permeabiUty values as high as that of leather due to their porous layer stmcture. The permeabiUty of grain-type is lower than that of suede-type, influenced by finishing method. 

To measure gas and water vapor permeability, a film sample is mounted between two chambers of a permeability cell. One chamber holds the gas or vapor to be used as the permeant. The permeant then diffuses through the film into a second chamber, where a detection method such as infrared spectroscopy, a manometric, gravimetric, or coulometric method isotopic counting or gas-liquid chromatography provides a quantitative measurement (2). Die measurement depends on the specific permeant and the sensitivity required. 

Cellulosic plastics are usually used for the membrane, but any water vapor permeable material is a good possibility, provided the film has good mechanical properties. 

Multilayered materials owe their properties and behavior to the properties of and the interactions between the components (5). Each of the two or more components contributes its particular property to the total performance of the multilayered material. For example, in Pouch 1, Table II, the aluminum foil provides high oxygen and water vapor permeability resistance, poly (ethylene terephthalate) provides structural strength and stiffness, and the ethylene-butene copolymer provides a heat sealable layer. If the components of the multilayered materials interact then the whole would be something different than the sum of its parts. In other words, the properties of the components of the multilayered materials are not independent of one another but rather are interdependent. 

PVdC-coated PVC increases the sheet s water-vapor permeability resistance by a factor of 5-10. The coating is applied at different loadings per square meter of PVC sheet. The coating is applied on one side and usually faces the product and lidding material. Stability studies will establish whether interactions with the product are taking place. This is usually indicated by discoloration of the blister or the product. 

Laminated sheets made from PVC bonded to chlorotrifluoroethylene (CTFE) have the lowest water-vapor permeability of all the materials used in blister packaging. Compared to 250 pm PVC sheet, a lamination of 200 pm PVC-19 pm CTFE (8 mL PVC-0.8 mL CTFE) has a 15-fold lower water-vapor permeability. There are many grades of CTFE available with more or less barrier and ease of formability. Figure 1 shows the difference between flat sheet and formed blister moisture permeabilities. 

Nylon, aluminum, PVC (OPA-aluminum-PVC) composites offer functional alternatives to traditional ther-moformed materials. With a laminate structure of 25 pm OPA, 45 pm aluminum, with 60 pm PVC (1 mL OPA 1.8 mL aluminum-2.4 mL PVC), it is possible to almost completely eliminate water-vapor permeability [25]. 

Park SK, Rhee CO, Bae DH and Hettiarachchy NS. 2001. Mechanical properties and water-vapor permeability of soy-protein films affected by calcium salts and glucono-D-lactone. J Agric Food Chem 49(5) 2308-2312. 

Example ps at the sublimation front is 0.937 mbar (-21 °C) (see example in Table 1.9), in the chamber a pU20 = 0.31 mbar has been measured, resulting in a pressure difference of approx. 0.6 mbar. With these data, the water vapor permeability blp = 1.1 10 2 kg/h m mbar is calculated. With this data known, it is possible to calculate dp for different conditions, if the mass of frozen water miCL, the time /MD, the thickness (d) and the surface (F) are known. This dp depends from the amount vapor transported and thereby from the heat transfer (Table 1.9). In the examples given it changes between 0.17 mbar in a slow drying process (6 h) to 0.6 mbar for a shorter drying time, 2.5 h. 

PVA films have high water-vapor permeability (water-vapor-transmission coefficient PH2o is 270 g 0.1 mm/10 h m2 cm-Hg) [294] that increases rapidly with relative humidity and with decrease in the hydrolysis degree. 

Only water vapor permeable films are comfortable to wear... 

Figure 15.4 Water vapor permeability behavior of TPS/clay hybrids of different kinds of clays at 24°C. (Reproduced with permission from Park et al., 2002, Wiley-VCH Verlag GmbH Co. KGaA).

Bertuzzi, M. A., Castro Vidaurrea, E. F., Armadaa, M., Gottifredia, J. C. (2007). Water vapor permeability of edible starch based films. Journal of Food Engineerings 80s 972-978. 

Tarvainen et al. (2002) studied the film-forming ability of starch acetate (DS 2.8) and the effect of commotfly used plasticizers on the physical properties of starch acetate films. The properties were compared with ethylcellulose films. Mechanical studies, water vapor and drug permeability tests, and thermal analysis by differential scarming calorimetry (DSC) were used to characterize the film-forming ability of starch acetate and efficiency of tested plasticizers. Starch acetate films were foimd to be tougher and stronger than ethylcellulose films at the same plasticizer concentration. Also, in most cases, the water vapor permeability of starch acetate... 

Another advantage of nitrile rubbers that contain PVC combinations is that they are non toxic and not influenced by mineral oils. Water vapor permeability is also good. 

Water vapor permeable to prevent maceration and promote wound healing... 

The observations in Sect. 2.6.1 clearly apply to the barrier effects in Fig. 2.62 especially with regard to the moisture or water vapor permeability. The permeability of water vapor for polyvinyl alcohol is unacceptable compared to acceptable levels in polyvinyl acetate, polybutyral and cyanoacrylate. A high level of moisture... 

Table 2 Overview About Different Permeabilities of Different Film Coatings (Water Vapor Permeability) [73]... 

The rubbery character and particular physical and chemical properties of polyisobutene stem from its paraffinic origins. Its outstanding properties are its low glass transition temperature, very low water vapor permeability and resistance to many chemicals. At room temperature polyisobutene is resistant to dilute and concentrated mineral acids and bases, as well as hydrogen peroxide. 

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. 

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