Permeability to water vapor

BiaxiaHy orieated PPS film is transpareat and nearly colorless. It has low permeability to water vapor, carbon dioxide, and oxygen. PPS film has a low coefficient of hygroscopic expansion and a low dissipation factor, making it a candidate material for information storage devices and for thin-film capacitors. Chemical and thermal stability of PPS film derives from inherent resia properties. PPS films exposed to tolueae or chloroform for 8 weeks retaia 75% of theh original streagth. The UL temperature iadex rating of PPS film is 160°C for mechanical appHcatioas and 180°C for electrical appHcations. Table 9 summarizes the properties of PPS film. 

Makower and Nielsen 21) have described another method that can be used to alter the texture of the dehydrated material in order to increase its permeability to water vapor and to shorten the time required for removal of water. The method, called the lyophilization procedure, involves the following steps ... 

Furthermore, the transplants are often packed in aluminum boxes, which are sealed by sterile filters permeable to water vapor. Even if the box can be designed with a negligible resistance to water vapor flow, the heat transfer is substantially reduced. 

Subramaniam, 1988]. Hydrochlorination, usually carried out at about 10°C, proceeds by electrophilic addition to give the Markownikoff product with chlorine on the tertiary carbon (Eq. 9-33) [Golub and Heller, 1964 Tran and Prud homme, 1977]. Some cyclization of the intermediate carbocation (XXVI) also takes place (Sec. 9-7). The product, referred to as rubber hydrochloride, has low permeability to water vapor and is resistant to many aqueous solutions (hut not bases or oxidizing acids). Applications include packaging film laminates with metal foils, paper, and cellulose films, although it has been largely replaced by cheaper packaging materials such as polyethylene. 

The rate of transmission of gases and vapors through polymeric films varies with the structure of both the diffusate molecule and the polymer. Polymers with polar groups, such as cellulose and cellulose acetate, are permeable to water vapor, but polymeric hydrocarbons, such as PIB, are essentially impervious to water vapor. 

Exemplary water-repellent treatments for masonry surfaces include metal stearates, oils, waxes, acrylates (both polymers and monomers), silicones (solvent-based and emulsion), siliconates, silanes and, fluorochemicals. In contrast, to waterproofing coatings, water-repellent coatings, because they are permeable to water vapor, do not trap moisture and, therefore they can reduce spalling. In addition most water-repellent coatings do not alter the appearance of a porous masonry. 

Methods based on the observation of a sudden change in properties in a series of PVCs (e.g., ion permeability, permeability to water vapor, color, gloss stability after coating with a silk-luster paint)... 

Another approach to radiation loss reduction might be the alteration of the salt water surface in some manner to lower its emissivity for thermal radiation. If a transparent thin liquid film or porous solid film of low thermal emissivity, permeable to water vapor, could be floated on the salt water, solar energy could continue to be absorbed on the basin bottom, water would vaporize, but thermal radiation loss would be reduced. Whether materials with these properties can be found and successfully utilized remains to be seen. 

Special properties of polyvinyl acetate are the very high hydrogen bonding and adherence to moist surfaces, and the self-plastization by absorption of less than 10% by weight of water. The absorption of water is sufficient to make the polyvinyl acetate flexible with natural body movement, pliable on moist tissue, permeable to water vapor and air, but not permeable to bacteria. Therefore, copolymers comprising polyvinyl acetate were synthesized for nonaqueous barrier dressings. Monomers were included to lightly crosslink the polyvinyl acetate. 

The last conclusion must be qualified because Parylene-C exhibits an exponential rise in the permeability to gases with a linear increase in temperature (10). The permeability to water vapor, for example, is increased by a... 

An important property of a PVAc film is its permeability to water vapor. The permeability to saturated water vapor at 40°C is 2.1 g/(h-m2) for a film 0.025-mm thick. This allows the film to be laid down on a damp surface with trapped moisture gradually passing through the film without lifting or blistering it. 

Higuchi, T., and Aguiar, A. (1959). A study of permeability to water vapor of fats, waxes and other enteric coating material. J. American Pharmaceutical Assoc. 48(10), 574-583. 

These membranes are used for the ultrafiltration of biological liquids (blood and urine), for hemodialysis in an artificial kidneys and for hemooxygenation in an artifidal lung The extraordinarily hi water permeability allows us to use the polyionic complexes as the additives to usual film-forming resins for preparing materials with a high coefficient of permeability to water vapor ... 

I4-.44C Consider a tank that contains moist air at 3 atm and whose walls are permeable to water vapor. The surrouriding air at 1 atm pressure also contains some moisture. Is it possible for the water vapor to flow into the tank from surroundings Explain. 

Water resistance is an important factor in concrete and masonry constmetion for the safety, health, and comfort of building occupants (see Cement). Several texts on concrete constmetion describe the methods for obtaining water resistance (72—76). The term waterproof describes concrete and masonry that is completely impervious to water and its vapor, whether or not the water is imder pressure. Waterproof constmetion involves the use of some type of barrier that covers aU surface pores or capillaries. Water repellent describes concrete or masonry that repels water without significantly reduced permeability to water vapor. In this discussion, concrete and masonry are used synonymously. 

Polyvinylidene chloride is primarily extruded as a film that has low permeability to water vapor and air and is therefore used as the familiar clinging plastic food wrap, Saran Wrap. 

Vapor permeable adhesive films are of use in those wounds in which granulation tissue is established and wound exudate is declining. These products were developed as materials that would, in part, mimic the performance of skin. The resultant products were transparent, synthetic adhesive films generically described as vapor permeable adhesive membranes, and comprised of transparent polyurethane or other synthetic films of low reflectance, evenly coated on one side with a synthetic adhesive mass. The adhesive is cohesive and inactivated by contact with moisture and will not therefore stick to moist skin or the wound bed. The films are permeable to water vapor, oxygen, and carbon dioxide but occlusive to water and bacteria and have highly elastomeric and extensible properties. They are conformable, resistant to shear and tear, sterile and particle free. 

Crosslinking addition of a calcium salt, such as calcium citrate or calcium chloride, causes crosslinking of the alginic acid polymer resulting in an apparent increase in molecular weight. Films crosslinked with triphosphate (tripolyphosphate) and calcium chloride were found to be insoluble but permeable to water vapor. Drug permeability varies with pH and the extent of crosslinking. ... 

The fact that silicone coatings are able to prevent silver migration has, of course, been known for over ten years. But the mechanism for this has not been understood. Silicones, as a general polymer class, have a very high permeability to water vapor. Hence, silicones cannot be considered as "barrier coatings" which function by excluding water from the region between the conductors. 

Paradoxically, softener-treated cotton also exhibits an improved permeability to water vapor, leading to an improved comfort in wear. 

As already mentioned, softener-treated cotton items exhibit enhanced permeability to water vapor. That paradoxical effect is due to the reduction of the water... 

Halar fluoropolymer has low permeability to water vapor and various other gases. Water vapor permeability measured at 38°C and 90% RH was found to be 0.15 g mil/100 in 24 hrs. 

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