Porous Fabrics

Highly porous fabric stmetures, eg, Gore-Tex, that can be used as membranes have been developed by exploiting the unique fibrillation capabiHty of dispersion-polymerized PTFE (113). 

Kwon and coworkers prepared a series of nano- to microstmctured biodegradable PCLA porous fabrics by electrospinning. The nanoscale-fiber porous fabrics were electrospun with PCLA (1 1 mole ratio, approximately 0.3-1.2 mm in diameter) using l,l,l,3,3,3-hexafluoro-2-propanol as a solvent. 

Ruorocarbon treatment generally improves the penetration resistance of porous fabrics, both woven and nonwoven, to liquids and particles. The fluorocarbon reduces die differences between the resistances of 100% cotton and various cotton/polyester blends. Durable press finishes reduce penetration resistance to liquids. Studies of soil releasing treatments are inconclusive. Laundering reduces the effectiveness of finishes diey must be renewed periodically. 

Mn02(s) and graphite, separated from the anode by a porous fabric. The battery is sealed in a steel can to reduce the risk of any of the concentrated KOH escaping. 

The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Fig. 9.1 The mesh is made of porous fabric or stainless steel. A stencil is produced on the screen either manually or photochemically defining the image to be printed. Thus, the design of the stencil allows to obtain a wide range of screen-printed electrodes in which the electrodic configuration, as well as the size and form of these electrodes can be controlled. 

Vacuum-assisted resin transfer molding (VARTM) is a process to form and shape FRP composites. Fibers are layered on a solid mold base and covered by a vacuum bag. By generating a vacuum, the vacuum bag is compacted owing to the action of atmospheric pressure, and, therefore, air bubbles are ehminated. Porous fabrics absorb excess resin and the entire material is cured to obtain better mechanical properties. This processing technology is suitable to a certain level of automation and ensures a good geometrical control and a smooth surface. 

The most common primary (nonrechargeable) battery is the alkaline battery ( Figure 20.20). The anode is powdered zinc metal immobilized in a gel in contact with a concentrated solution of KOH (hence, the name alkaline battery). The cathode is a mixture of Mn02 (s) and graphite, separated from the anode by a porous fabric. The battery is sealed in a steel can to reduce the risk of any of the concentrated KOH escaping. The cell reactions are complex but can be approximately represented as follows ... 

Since wind barriers are located at the cold side of the thermal insulation (in cold climates) they should combine a sufficient air and water tightness with a high vapour permeance. Materials which combine these properties and are often recommended as wind barriers are the spunbonded plastic films (also called house-wraps). These films are composed of rolled synthetic filaments (typically polypropylene or polyethylene fibres) that are welded together to form a continuous porous fabric. Measuring procedures and results of vapour transfer properties of spunbonded plastic films are reported in detail by Janssens and Hens [36]. The diffusion thicknesses of the films are a few centimetres and of the same order of magnitude as an air boundary layer. 

The difficulty here is that textiles are optimised as soft, flexible, porous fabrics with low fiber-volume content... 

It is successful that the patency rate of this vascular prosthesis was about 87 percent throughout the world. This new vascular prosthesis which is a porous molded tube, devised by combining the properties of non-porous molded tube developed by Carrel in 1912 with the porous fabric tube by Voorhees. 

Walter Crum in a paper read to the Glasgow Philosophical Society in 1843, mentions Hellot, etc., and Le Pileur d Apligny as teachii a physical theory, whilst Bergman and Berthollet held a chemical one. Crum proposed what is really a physical theory of adsorption, comparing the adhesion of the dye to a porous fabric with the adhesion of dissolved substances to charcoal. 

Flow of liquid thermoset polymer through porous fabric preform. Source Reprinted with permission from reference 1, copyright 2010 CRC Press,Taylor Francis Group.)... 

Let Ya be the mass supply of the ath component due to, for example, a chemical reaction, and let be the mass that is absorbed per unit area on the surface of the porous fabric that composes the solid phase. Then the mass conservation law of the ath component can be written as... 

The measurement of porosity can be obtained by the ratio of the fabric density and the fibre density (see Eq. [6.4]). In this direct method, both the bulk volume of a piece of porous fabric and the volume of the solid fibre materials are measured, and then the porosity can be calculated according to their definition. 

There are two main types of liquid transport in fabrics. One is the liquid absorption, which is driven by the capillary pressure in a porous fabric, in which liquid is taken up by a fabric through a negative capillary pressure gradient. The other type of liquid transport is called forced liquid flow, in which liquid is driven through the fabric by an external pressure gradient. 

The separator is an important component in a MFC, which physically separates the anode and cathode. A variety of separators have been explored for MFCs, including a salt bridge, cation exchange membrane (CEM), anion exchange membrane (AEM), bipolar membrane (BPM), microfiltration (MF) membrane, ultrafiltration (UF) membrane, and porous fabrics and porous materials. 

Separators are often overlooked in MFC research, both in its cost and in its contribution to internal resistance. Ion exchange membranes are about a few hundred US dollars per square meter. The high cost and mediocre performance limit their potential commercial application in MFCs. While producing the highest reported volumetric power density, the cost of porous fabrics can be <1 US m . Further optimization of thickness and control of biofthn development is needed. 

Figure 10.13. Effect of thickness of the porous fabric material of different porosities on the coefficient of surface heat transfer in case of forced convection [Source Reference 8].

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