Permeation of oxygen

The outer packaging must protect the tea from light and moisture absorption. Polypropylene or coated ceUophane outer wraps for paper board tea packages provide a barrier to loss of tea aroma and retard permeation of oxygen and foreign flavors. Low temperature improves storage stabiHty. Properly packaged and stored teas retain acceptable flavor for about a year. 

Carbon Dioxide Transport. Measuring the permeation of carbon dioxide occurs far less often than measuring the permeation of oxygen or water. A variety of methods ate used however, the simplest method uses the Permatran-C instmment (Modem Controls, Inc.). In this method, air is circulated past a test film in a loop that includes an infrared detector. Carbon dioxide is appHed to the other side of the film. AH the carbon dioxide that permeates through the film is captured in the loop. As the experiment progresses, the carbon dioxide concentration increases. First, there is a transient period before the steady-state rate is achieved. The steady-state rate is achieved when the concentration of carbon dioxide increases at a constant rate. This rate is used to calculate the permeabiUty. Figure 18 shows how the diffusion coefficient can be deterrnined in this type of experiment. The time lag is substituted into equation 21. The solubiUty coefficient can be calculated with equation 2. 

Commonly used cells use Pt/C>2 or Pt/air as reference electrodes. At very low partial pressures of oxygen, care must be taken to avoid direct permeation of oxygen through stabilized zirconia from the air (or reference electrode) [74, 75]. The effect may be avoided by applying reference electrodes with activity near that observed at the working electrode. A well-defined buffer system like a metal-metal... 

It is known that most metals possess lower gas permeability than plastics by 6-8 orders of magnitude. An unbreakable and lightweight thin stainless steel foil substrate has been used for flexible OLEDs [75]. Therefore, a several micrometers thick metal layer can serve as a highly effective barrier to minimize the permeation of oxygen and moisture. Hence, the... 

The copolymer of isobutylene with a few percent isoprenc (butyl rubber) can be cured to produce an ozone-resistant elastomer with low permeability to oxygen and nitrogen. Butyl rubber has a Tt of — 70 C a refractive index of 1.5081, and a coefficient of linear expansion of 5.7 X 10 cm/cxn C. Chloro and bromo butyl rubber are more resistant to the permeation of oxygen and nitrogen than butyl rubber. 

Carbon Dioxide Transport. Measuring the permeation of carbon dioxide occurs far less often than measuring the permeation of oxygen or water. A variety of methods arc used however, the simplest method uses the l crmatran-C instrument (Modern Controls. Inc.). 

Figure 11.1 Schematic examples of passive diffusion, facilitated transport and coupled transport. The facilitated transport example shows permeation of oxygen across a membrane using hemoglobin as the carrier agent. The coupled transport example shows permeation of copper and hydrogen ions across a membrane using a reactive oxime as the carrier agent...

These calculated values are comparable to the experimental values given in Table 9-2 for permeation of oxygen and water through PS. 

Figure 8.25 Permeation of oxygen through several polymer films. This illustrates the diversity of the properties of available polymers and shows the relatively high permeability of polydimethylsiloxane, a common component of prolonged

W.A. Fisher, Permeation of oxygen and partial electronic conductivities in solid oxide electrolytes, in W. van Gool (Ed.), Fast Ion Transport in Solids. Elsevier, New York, 1973, pp. 503-512.. 

Reaction of 1-butene and permeation of oxygen through the membrane... 

Finally, the possibility of product migration into the pack (e.g. loss of preservatives) coupled with permeation of oxygen, carbon dioxide and moisture must also be consid ered as a factor which could cause changes in the migratory nature of constituents in the plastic. 

Structures composed of an ITM dense layer and the contiguous support are called wafers. Several wafers, when joined together and placed inside a pressure vessel forming a separation device, are the ITM air separation module. ITM module is the separation devices and the ITM wafers form the building blocks of the separation device that allows the selective permeation of oxygen at commercially relevant flux and purity. 

A variety of tests are available for determining film quality. Many are based on ASTM standards. For example, tests for permeation of oxygen, water vapor, or other substances may be carried out, and will be discussed in Chapter 14. Tests for impact strength, tensile strength, tear strength, and other mechanical properties are important for many applications. Tests particularly related to quality control in the film production process include those that relate in some fashion to the appearance of the film. 

R. A. Pasternak, M. V. Christensen, and J. Heller, Diffusion and Permeation of Oxygen, Nitrogen, Carbon Dioxide, and Nitrogen Dioxide through Polytetra-fluoroethylene, Macromolecules, 3, 366 (1970). 

In order for a polymer molecule to be attacked by oxygen, it must come in contact with it. This means that oxygen must be able to permeate into the material. Otherwise, all the oxidation will occur only at the surface. It was shown that oxidation occurs more readily in amorphous regimis of the polymers where permeation of oxygen is not hindered by the chains being packed tightly together in the crystallites. That is only tme, of course, at temperatures below I m of the polymer. 

A combination of a polymer layer with a board is used to package liquids such as milk, where the plastic layer provides the barrier to contain the milk within the package and the board the basic strength. Where it is necessary to store the beverage for long periods, such as fruit juices, additional barrier properties are required to prevent permeation of oxygen into the food product. To achieve this additional protection, an aluminum layer is incorporated within a plastic/board composite. 

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