Permeability, Permeation lamination

For parallel permeation pathways within a single layer of such a laminate, the total permeability may be calculated as the sum of the permeabilities [49], For example, when assuming that permeation may occur through both a lipid fraction and a protein fraction of the stratum corneum, the total permeability is calculated according to Eq. 34 ... 

Layered Structures. Whenever a barrier polymer lacks the necessary mechanical properties for an application or the barrier would be adequate with only a small amount of the more expensive barrier polymer, a multilayer structure via coextmsion or lamination is appropriate. Whenever the barrier polymer is difficult to melt process or a particular traditional substrate such as paper or cellophane [9005-81-6] is necessary, a coating either from latex or a solvent is appropriate. A layered structure uses the barrier polymer most efficiently since permeation must occur through the barrier polymer and not around the barrier polymer. No short cuts are allowed for a permeant. The barrier properties of these structures are described by the permeance P which is described in equation 16 where Pt and L are the permeabilities and thicknesses of the layers. 

The blistering process is thought to be osmotic. Water molecules, very slowly, permeate the resin, and any voids in the laminate pick up moisture. If the resin system is hydrolytically unstable, i.e. able to decompose by reaction with water, then salts can be formed in the water. The gel coat acts as a semi-permeable membrane and an osmotic cell is created where pressure builds up which can be sufficient to create blisters. 

Permeability is the permeation of a penetrant (liquid, vapor, gas, or volatile odor) through a polymeric membrane barrier which may be film, laminate, or coating. The rate of transmission at which a penetrant will pass through a polymeric membrane is governed by different factors. Some of these factors are dependent on the... 

A blister pack or strip is a container with a foil that is shaped so that it can contain separate dosages [26]. An aluminium lidding foil closes the form foil. Blisters are mostly used to package tablets or capsules. For blister packs a form foil of PVC or laminate of PVC/PVDC (polyvinylidene chloride) is used. The form foil is warmed in a blister machine and a mold station makes round or capsule-Uke pockets with compressed air. PVC is permeable to water vapour and therefore not suitable for tablets or capsules that are moisture sensitive. To package such products the PVC form foil should include an outside layer of PVDC. PVDC permeates less water vapour. Dependent on product sensitivity to moisture, different PVDC-thicknesses can be chosen. When complete resistance to water vapour is necessary a form foil that consists of a formable aluminium laminate (consisting of polyamide, soft aluminium and PVC) has to be used. 

Permeability. Many polymers are used in packaging and, in particular, for food. In this latter case the permeability to gases and vapors is of prime importance. The permeation or transmission of a gas or vapor is a function of the solubility of a gas or vapor in the polymer and the rate of diffusion through the matrix. The permeability coefficient, diffusion constant, and solubility coefficients can all be measured and are influenced by the chemical structure and morphology. In order to achieve the required permeability characteristics it is common to co-extrude a series of polymers to form a laminated structure. Such materials allow selective permeation of a specific species and enhance the life of the product (190,191) (see Transport Properties). 

Concerning the oxygen permeability, improvement factors of 30 and at least 300 were obtained depending on the coating support (due to the extremely low permeability of the multilayer laminate it is not possible here to give exact permeation values, see Fig 21-20). 

B.4 Permeability of Composite Laminates. Consider a laminar composite structure, in which the laminae, L are normal to the direction of permeation. These laminae can be either slabs, or hollow cylinders, or spherical shells (Fig. 4.23). Prove that the composite permeability, P, is given by the relationship... 

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