Parameters determining migration

An important quantity which can be calculated at equilibrium conditions is the amount of substance migrated into the food or food simulant at equilibrium, mF,e. Provided that the migration potential in the polymer, i.e. the initial amount of migrant dissolved in the polymer, mP0, is known then from mass balance calculations the following equation can be derived  

The volumes of the polymer material of the packaging and food simulant are given by VP and VF, the contact surface area between the two phases is given by A and the layer thickness of the polymer (for single sided contact) is given by dP 

Where cF.o is the initial concentration of the migrating component in the simulant. In contrast to Eq. (10-2) the partition coefficient K plays an important role here, where the sorption of component into the polymer is proportional to KPP 

in the case of Xj Xj then the polymer additive i will be overrun by the simulant before it can migrate out of the polymer. Depending on the solubility of the simulant in the polymer (KPj value), the migration behaviour of i will then be influenced in an accelerated way. In the case, however, that Xi Xj then the migration of i from the plastic is not influenced by the migration of the simulant. 

From the above equations (10-1 to 10-4) the following conclusions can be drawn with respect to the selection of a food simulant  

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First, with no exception, in all classical diffusion models one or more adjustable parameters enter in the formula of D. To calculate the magnitude of this/these param-eter/s a number of diffusion experiments must be performed with the very penetrant polymer system which one intends to simulate theoretically. In practice such experiments most often require quite sophisticated equipment to obtain the experimental data, and often non-trivial theoretical schemes to evaluate them. The attempt to save experimental work by using the adjustable parameters determined for a certain penetrant polymer system in order to estimate/predict Ds in a related system is generally not recomendable. Hence, in a first step, in order to use one or other of the classical diffusion models, one is forced to replace migration experiments with diffusion ones. 

Electrophoresis of nonconducting colloidal particles has been reviewed in this chapter. One important parameter determining the electrophoretic velocity of a particle is the ratio of the double layer thickness to the particle dimension. This leads to Smoluchowski s equation and Huckel s prediction for the particle mobility at the two extrema of the ratio when deformation of the double layer is negligible. Distortion of the ion cloud arising from application of the external electric field becomes significant for high zeta potential. An opposite electric field is therefore induced in the deformed double layer so as to retard the particle s migration. 

As the overall aim of parameter determination is the (simulation-based) prediction of the process behavior the final decision about the suitability of the isotherm equation can only be made by comparing experimental and theoretical elution profiles (Section 6.6). Depending on the desired accuracy, this may involve iteration loops for the selection of isotherm equations or in some cases even the methods (Fig. 6.16). In this context it should be remembered that, according to the model equations (e.g. Eq. 6.47), the migration velocity and thus the position of the profiles is a function of the isotherm slope. Therefore, it is most important for the reliability of process simulation that the slopes of the measured and calculated elution profile fit to each other. If the deviations are unacceptable, another isotherm equation should be tested. 

Volatility and Migration Rate. A study of the half-lives (T ) of the antioxidants in the polymer (see Table 1) at the same temperature suggests a reason for the lack of correlation between the two sets of results. In an oxygen absorption test, volatilization cannot occur, and the result is a true measure of the intrinsic activity of the antioxidant molecule. In an air oven test, on the other hand, physical loss of the antioxidant by migration and volatilization from the surface must dominantly influence the test results. Billingham and his coworkers ( 2) have shown that these two physical parameters determine the rate of loss of antioxidants from polymers. Increase in molecular mass generally decreases molecular mobility as well as volatility, and which factor dominates depends on the thickness of the sample ( 2). 

The first important parameter determining the final crystalline morphology is the nucleation density. N, (see Part 3.4.3.1). An increase in the nucleation density (per volume unit of the crystallizable material) due to migration of nuclei from one phase towards the other, or due to a nucleating activity at the polymer/polymer interface, results in the formation of more numerous, but smaller sphemlites. 

Electric field. The movement of ions on the paper is caused by the potential difference applied across the paper. The important parameter determining the extent of migration (d) is the voltage gradient, i.e. the applied voltage (V) divided by the distance (L) between the electrodes ... 

Sodium dodecyl-sulfate polyacrylamide gel-electrophoresis. With SDS-PAGE, the charge and shape differences of proteins are eliminated so that the only parameter determining the migration is the size of the protein. 

Optimization procedure is frequently carried out by the trial and error method (56-60,67) owing to the lack of a theoretical model of the multiple development process. Recently Markowski and Soczewinski (73,74) formulated the physical model for AMD, which is useful for describing the migration of the solute zones and computer analysis of various parameters determining the final optimization of gradient. 

Thermo-diffusion calculations analyze the migration of hazardous material from compartment to compartment to release in containment. These calculations use physico-chemical parameters to predict the retention of hazardous materials by filtration, deposition on cold surfaces and other retention processes in the operation. Containment event trees aid in determining the amount, duration and types of hazardous material that leaves the containment. 

Determination of initial recovery well (or trench) locations is an important design parameter. Floating LNAPL product tends to move in the direction of overall ground-water flow, as determined by the water table gradient. As a well or trench is pumped, the fluids (water and/or oil) migrate toward the area of lower pressure to fill the void. A cone of depression develops that extends outward. The fluid surface exhibits a rapid slope near the well, diminishing to a very low gradient at a distance. 

Determine the transport parameters that control pollutant migration through the subsurface environment (i.e., diffusion/dispersion and diffusion coefficients)... 

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