Permeation constant

Some plastics are poor at offering resistance to the passage of fluids through them whereas others are excellent. Their relative performance may be quantified in terms of a permeation constant, k, given by... 

The main fluids of interest with plastics are oxygen and water vapour (for packaging applications) and CO2 (for carbonated drinks applications). Fig. 1.13 and Fig. 1.14 illustrate the type of behaviour exhibited by a range of plastics. In some cases it is necessary to use multiple layers of plastics because no single plastic offers the combination of price, permeation resistance, printability, etc. required for the application. When multi-layers are used, an overall permeation constant for the composite wall may be obtained from... 

A commercially available cellulose acetate film which we would now describe as homogeneous or isotropic, gave the results shown in Row 2 of Table I. The volumetric permeation rate of water per unit membrane area, called the water permeation flux Jl mVm day, and the water permeation constant. A, m m day atm were both very low, but a salt rejection of 94 percent was obtained. We define ... 

Unbeknownst to Sourirajan, Breton and Reid working at the University of Florida under Office of Saline Water sponsorship, also found that cellulose acetate is semipermeable to sea water electrolytes (3, 4). Comparative results of Breton and Reid are shown in Row 1 of Table I. It can be seen that the water permeation constant is considerably higher than that of Sourirajan. 

This difference is largely accounted for by the difference in thickness of the homogeneous membranes involved, such that the product of water permeation constant and membrane thickness is about the same for both membranes. The constant arises from the diffusion model of permeation in which ... 

Investigator Morphology % Type of Pressures, Water per- Water permeation Constant Solute... 

Comparative results with the Schleicher and Schuell mentorane are shown in the third row of Table 1(11). The flux and the water permeation constant increased by a factor of 40 over the previoiis results. Furthermore AX, the product of water permeation constant and total membrane thickness increased by a factor of 130. The most obvious explanation for these results is that the effective menijrane thickness was much less than the total membrane thickness. 

In contrast to the cases discussed above, the permeation constant Xp has no strong influence on the critical values. For dimensionless values Xp < 10-6 the critical values do not change at all with Xp. For large values variations within a factor of two are possible. The permeation constant is known to be very small. In our dimensionless units we expect it be of the order of < 10 9 for LMW thermotropic liquid crystals and neglect its influence on the critical values for this reason. In Sect. 2.2 we have emphasized that the y- and z-components of the velocity field are suppressed via Xp. These qualitative arguments are clearly confirmed by our numerical results. In all our calculations v jX is the dominating component of v and the ratio V >y/v i jA-... 

Both D and K, therefore O, are temperatnre dependent and have associated activation energies such that permeation is mnch higher at elevated temperatnres for all materials than at low temperatnres. For many materials, these permeation constants are too high for a given application and a permeation barrier mnst be considered. 

Siegel studied the permeability of the oral mucosa. He found that in a homologous series, an increase in lipid solubility resulted in an increased permeability of the oral mucosa. His permeation constants correlate exactly with Kow values J7 8 ... 

Film Forming and Selective Oxygen Permeation. Selective oxygen permeation was evaluated mainly for the polymers having short ollgoslloxane side chains. Oxygen permeation constants (Pq ) 1... 

If the oxygen permeation constant and the rate of oxygen consumption for the material remain relatively constant as a function of total absorbed dose, a steady state in heterogeneous oxidation will be approached. If the rate of oxygen consumption by reaction with free radicals generated by the radiation exceeds the rate of supply of oxygen from the edges of the polymer sample, distinct... 

Keys et al. (2000) explored five approaches to modeling the pharmacokinetics of di- -butyl phthalate and mono- -butyl phthalate. In a flow-limited version of the model, transfers between blood and tissues are simulated as functions of blood flow, tissue concentrations of di- -butyl phthalate or mono-n-butyl phthalate, and tissue blood partition coefficients, assuming instantaneous partitioning of the compounds between tissue and blood (Ramsey and Anderson 1984). In an enterohepatic circulation version of the model, the transfer of mono-n-butyl phthalate from the liver to the small intestine is represented with a first order rate constant (diffusion-limited) and a time delay constant for the subsequent reabsorption of mono- -butyl phthalate from the small intestine. In a diffusion-limited version of the model, the tissue transfers include a first order rate term (referred to as the permeation constant) that relates the intracellular-to-extracellular concentration gradient to the rates of transfer. This model requires estimates of extracellular tissue volume (ECV) and intracellular volume (ICV) ECV is assumed to be equal to tissue blood volume and ICV is assumed to be equal to the difference between tissue blood volume and... 

It is not possible to generalise about performance and level of permeabilities, as quantified in terms of the permeation constant (k), because some plastics have high permeabilities (poor in offering resistance to the passage of fluids or chemicals through them), and some have very low permeabilities [2]. 

Extracted Species Viscosity p of Oil Phase (Cp at 100-FT Permeation Constant ) (min ) pD ... 

Table 1. Moisture Solubility, Diffusivity and Permeation Constants...

Epoxidation also results in a substantial decrease in air permeability. Figure 14 records the permeation constants of various grades of ENR over a range of temperatures in comparison with those of NR and butyl rubber. The air permeability of ENR-50 is similar to that of butyl rubber and thus it can be used in applications requiring low air permeability. 

Environmental (e.g., surface and crop leaf residues) and work-place exposures (e.g., mixer-loaders, applicators, etc.) may result in the transfer of pyrethroids to skin where they may be absorbed. Dermal PBPK/PD models require compartment/ submodels and additional parameters (e.g., Kp, permeation constants, exposed surface area, evaporation rates, washoff fractions, etc.) to model the fate of pyrethroids deposited on the skin. 

The recovery of low to high percentages of oral dosages in the stomach, small and large intestine suggests that additional compartment and parameters (Caco-2-cell permeation constants, enterocytic metabolism, and enterohepatic circulation) may be needed to adequately describe gut absorption. 

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