Humidity permeability

PERMANENT GASES Table 3 lists the permeabilities of oxygen [7782-44-7] nitrogen [7727-37-9] and carbon dioxide [124-38-9] for selected barrier and nonbarrier polymers at 20°C and 75% rh. The effect of temperature and humidity are discussed later. For many polymers the permeabihties of nitrogen, oxygen, and carbon dioxide are in the ratio 1 4 14. 

Permeation in the vinyUdene chloride copolymer and the polyolefins is not affected by humidity the permeability and diffusion coefficient in the ethylene—vinyl alcohol copolymer can be as much as 1000 times greater with high humidity (14—17). 

As is to be expected, the table shows that as the humidity is increased, causing swelling and an increase in the interchain separation, so the oxygen permeability increases. Also, as expected, the percentage increase is greater the higher the vinyl alcohol content. 

Most of the above can be made into plastic films - primarily used for wrapping. Film properties vary widely from permeable for food to impermeable to preserve dryness. Paper, treated or untreated, has been used for many years as a covering film, but has low strength when wet and is difficult to make transparent. In the 1920s, the transparency of cellophane revolutionized wrap. It is regenerated cellulose, like viscose rayon, except it is extruded in sheet instead of fiber, unfortunately it is sensitive to water and humidity. 

C °C/75%RH 5%. For liquid products stored in semi-permeable containers subject to water loss, exposure to lower humidities, e.g., 25° 2°C/... 

Humidity and temperature also affect permeability. It has long been known that skin hydration, however brought about, increases skin permeability. Occlusive... 

Permeability of an FML is evaluated using the Water Vapor Transmission test.28 A sample of the membrane is placed on top of a small aluminum cup containing a small amount of water. The cup is then placed in a controlled humidity and temperature chamber. The humidity in the chamber is typically 20% relative humidity, while the humidity in the cup is 100%. Thus, a concentration gradient is set up across the membrane. Moisture diffuses through the membrane, and with time the liquid level in the cup is reduced. The rate at which moisture is moving through the membrane is measured. From that rate, the permeability of the membrane is calculated with the simple diffusion equation (Fick s first law). It is important to remember that even if a liner is installed correctly with no holes, penetrations, punctures, or defects, liquid will still diffuse through the membrane. 

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 gas permeability of the constructive materials after treatment by chemicals was examined. Influence of types of concrete, cement and gypsum, preliminary treatment by different chemicals, time between treatments by organic compounds, humidity of constructive materials, time between preparation of chemicals and treatment of materials (aging of chemicals), time between treatment of materials and testing (aging of treated materials) were investigated. 

The physical state of the skin is considerable affected by external factors such as relative humidity, temperature, and air movement at the skin surface. If this contact is broken (for example, by external applications of ointments or creams), it is reasonable to assume that the new skin will change in some way, sometimes to an extent that creates new conditions of permeability for the test material. This would be the case, for example, if the stratum comeum becomes more hydrated than normal due to the topical delivery form. Temperature might also have an effect, as is the case when any constituents of the vehicle affect the inner structure of the skin through interactions with endogenous skin substances. Often several of these processes occur together. 

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. 

The gas permeability increases with increase the relative humidity, with decrease hydrolysis degree of PVA, with increase temperature, and tends to decrease sharply as the degree of ciystallinity increases. The decrease in ciystallinity and the decrease in Tg would be expected to increase the gas permeability. 

The use of buccal cell cultures for assessing the permeability of the buccal mucosa has attracted recent attention (see Chap. 7 for a more extensive summary). In order to culture buccal epithelial cells, the cells must be harvested from an appropriate source and cultured under specific conditions using an appropriate growth medium, temperature, and humidity [46], Cell cultures have been successfully grown from hamster cheek pouch. These cultured cells, however, did not differentiate to form a complete keratinized surface as seen in the normal hamster cheek pouch, and they consequently displayed a greater permeability to compounds when compared with keratinized hamster cheek pouch mucosa [134], Therefore, the cultured hamster cheek cells more closely mimicked the human buccal mucosa due to their lack of keratinization, and so this may be an appropriate model for predicting permeability through the human buccal mucosa. 

The WVTR through the container is determined by the container wall thickness, the permeability of the material and the difference between the external and internal relative humidity environments. Waterman et al. [99] determined the theoretical rate of water permeation through a standard 60-cc bottle when stored at 40°C/75% RH. This equated to an uptake of 1 mg of water per day. They commented that even if the product had been packed under low water vapour conditions the relative humidity conditions within the container would be equate to 50% RH within 1 day. The WVTRs (see Table 2.8) for some common packaging materials were reported by Waterman et al. [99]. 

Some evidence indicates that long-term use of topical antimicrobial agents may alter skin flora. Water content, humidity, pH, intracellular lipids, and rates of shedding help retain the protective barrier properties of the skin. When the barrier is compromised (e.g., by hand hygiene practices such as scrubbing), skin dryness, irritation, cracking, and other problems may result. Although the palmar surface of the hand has twice as many cell layers and the cells are >30 times thicker than on the rest of the skin, palms are quite permeable to water. 

Soil biofilters are relatively large compared to filters using other media since soil pores are smaller and compounds have low permeability in soil. Soil biofilters also have limited depths due to problems associated with maintaining humidity in soil and minimizing pressure drop. Furthermore, soil sorption capacity is limited and residual contaminants are vented immediately to the atmosphere. 

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