Permeability-thickness relation

The fabric substrate used for the filter media will need to provide certain characteristics, dependent on the application. The typical properties required include abrasion resistance, stretch resistance, dimensional stability, and resistance to flex fatigue. All these performance criteria can be assessed by using suitable test methods. The fabric will also be tested for correct warp and weft sett, air permeability, thickness, density, and fabric tensile properties. All the factors will affect the performance of the filter as they are directly related. If woven fabrics are considered, the porosity will be directly related to the sett and therefore air permeability. The combinations of the fibre types, fabric constructions, and coating substrate will provide the filter manufacturer with a range of properties for use in a range of appUcations. 

Permeability P) is a measure of the amount of gas that passes through a film of thickness I and axea A within a finite amount of time f. It is the result of two distinct processes (i) initially penetrant molecules must dissolve into the surface of the film through a process governed by solubility, and (ii) these molecules must then diffuse across the film to the other surface. In order for permeation to occur, these two mechanisms—one thermodynamic (solubility) and the other kinetic (diffusion)— must both occur [14], Permeability is related to the solubility constant S and diffusion coefficient D by ... 

A third coefficient of some importance is the permeability, which is defined as the amount of a diffusing molecule that passes through a polymer film of unit thickness per second per unit area and a unit difference of pressure. Diffusivity, solubility, and permeability are related by the following ... 

Factors related to the geotextile filter are PSD, filtration opening size, permeability, thickness, compressibility, tensile strength, stiffness, and durability. 

Essentially equivalent information can be obtained during the formation of the filter cake, without the need for a second filtration. During filtration, particles are deposited as a layer of increasing thickness, so that the resistance to filtration increases. The resistance, R(m-1), is inversely related to permeability and is defined in terms of the volume flow rate ... 

Squier CA, Hall BK (1985a) The permeability of skin and oral mucosa to water and horseradish peroxidase as related to the thickness of the permeability barrier. J Invest Dermatol 84 176-179... 

In contrast, diffusion of MeOH measured via permeabilify measurements (assuming a partition coefficient of 1) was lower (1.3 x 10 and 6.4 x 10 cm s for Nafion 117 and BPSH 40, respectively) and showed no concentration dependence. The differences observed between the two techniques are related to the length scale over which diffusion is monitored and the partition coefficient, or solubility, of MeOH in the membranes as a function of concentration. For the permeability measurements, this length is equal to the thickness of the membrane (178 and 132 pm for Nafion 117 and BPSH 40, respectively), whereas the NMR method observes diffusion over a lengfh of approximately 4-8 pm. 

Variation in penetration at different anatomic sites Drug penetration is inversely related to thickness of the stratum corneum. Thus, permeability (and often toxicity) is greater in areas of thinner skin, such as the face or scrotum. 

A related system is that of the lipid-bilayer corked capsule membranes which are formed from ultrathin (about 1 pm thick), spongy, 2.0- to 2.5-mm-diameter, more-or-less spherical nylon bags in which multiple bilayers are immobilized (Fig. 43) [343-345]. They were considered to combine the advantages of mechanical and chemical stabilities of polymeric membranes with the controllable permeabilities of surfactant vesicles. Polymerization of the bilayers, in situ,... 

Thermal conductivity of a fabric is related to its air permeability, or movement of air between the interstices of the yam and fabric. For fabrics of a given thickness, the one that has greater air permeability allows greater heat dissipation by convection. Thus thermal insulation falls as air velocity rises. 

The lattice model of mass transfer gives self-consistent expressions for sorption isotherms and permeability coefficients for microheterogeneous membranes of variable thickness at an arbitrary degree of filling. The parameters of the lattice model can be related to the molecular structure of a matrix and to the parameters of the interaction between diffusant and matrix [191]. The lattice model can serve as a basis to construct phenomenological models, which are capable of describing the features of a molecular system diffusant-membrane matrix . 

The P s are now permeability coefficients and are related to the mobilities of the ions as in the original Nernst theory. The subscripts in and out refer to the concentrations of the ions inside and outside the membrane and the P s describe diffusion coefficients, mobilities, and the membrane thickness, but, in the Hodgkin-Huxley theory, were used as adjustable parameters. 

Thermophysical Properties. Several investigators have focused their work on evaluation of the thermophysical properties of clothing assemblies and either related the results to mannequins or wear trials or discounted the need for such trials and elaborate models. Total thermal resistance of a clothed body to heat transfer from the body to surrounding air was considered to be the sum of three properties thermal resistance of the textile, thermal resistance to heat transfer at the textile surface, and thermal resistance of the air interlayer. Relationships between thermal resistance of clothing assemblies, air permeability, wind speed, and assembly thickness were also explored (5J). A method for calculating the effects of wind speed on thermal resistance of clothing claims to be as reliable as tests that use mannequins (58). 

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