Microporous sheets

A mixture of powdered poly(vinyl chloride), cyclohexanone as solvent, silica, and water is extruded and rolled in a calender into a profiled separator material. The solvent is extracted by hot water, which is evaporated in an oven, and a semiflexible, microporous sheet of very high porosity ( 70 percent) is formed [19]. Further developments up to the 75 percent porosity have been reported [85,86], but these materials suffer increasingly from brittleness. The high porosity results in excellent values for acid displacement and electrical resistance. For profiles, the usual vertical or diagonal ribs on the positive side, and as an option low ribs on the negative side, are available [86],... 

Eventually, the articles are formed into microporous sheets or films. Such microporous sheets and films may be used as labels, diffusion membranes, and separators in electrochemical devices, e.g., batteries, capacitors, and fuel cells. Special examples for formulations are presented in the literature (35). 

Membrane A microporous sheet or layer of material that is used to separate solids from liquids and, in some cases, solutes from solvents. 

Fig. 3. Kinetics of liquid sorption in 10 replications using samples cut from the same microporous sheet. The sorbent particles were comminuted crosslinked polyacrylamide (approx. 80% by weight) enmeshed in PTFE microfibers...

In the production of microporous sheets used as separation membranes, voids are internally created to make material permeable. Polypropylene was highly... 

In the fall of 1966, researchers at North Star Research Institute began a search for compression-resistant microporous substrates.19 This effort resulted in the development of microporous sheets of polycarbonate (Lexan) and poly-sulfone (Udel).20 Figure 5.4 shows a graph comparing the flux levels and flux stability for three membranes made at that time (a) float-cast cellulose acetate on microporous polysulfone, (b) float-cast cellulose acetate on a mixed cellulose ester microfilter support and (c) a standard asymmetric cellulose acetate membrane. The improvement in membrane fluxes was readily apparent, when switching from cellulosic substrates to the microporous polysulfone substrate. 

Figure 3 A supported liquid membrane device. A solid microporous sheet (e.g., polypropylene) is soaked in the membrane phase until its pores are saturated with the membrane phase. Usually, the membrane phase is an organic solvent and the microporous sheet is a hydrophobic substance, so the membrane fluid is readily retained in the pores by capillary action. Now this supported liquid membrane can be placed between the feed and receiving phases, and the same mechanism of transport as presented in Figs. 1 and 2 applies.

Figure 2.23 The microporous potassium sodium europium silicate AV-9 has a structure similar to the natural sodium calcium silicate monteregianite. Layers containing both edge-sharing NaOg (dark grey) and EuOg (medium grey) octahedra are separated by microporous sheets of silicate tetrahedra. The pore space is filled by extra framework potassium cations and zeolitic water.

The addition of relatively large proportions of insoluble modifiers or fillers can greatly alter the structure and properties of the microporous sheet. Addition of talc and cellulose powder are typical examples. While desirable effects on appearance, feel, absorbency, etc., may be achieved— perhaps also with significant cost reduction—mechanical properties usually deteriorate. 

Separators Microporous sleeves Microporous sheet and glass wool mat Microporous sheet and glass wool mat... 

Fig. 24. Adsorption of lithium on the internal surfaces of micropores formed by single, bi, and trilayers of graphene sheets in hard carbon.

Fig. 32. Reversible capacity of microporous carbon prepared from phenolic resins heated between 940 to 1 I00°C plotted as a function of the X-ray ratio R. R is a parameter which is empirically correlated to the fraction of single-layer graphene sheets in the samples.

Rubber media appear as porous, flexible rubber sheets and microporous hard rubber sheets. Commercial rubber media have 1100-6400 holes/in. with pore diameters of 0.012-0.004 in. They are manufactured out of soft rubber, hard rubber, flexible hard rubber and soft neoprene. The medium is prepared on a master form, consisting of a heavy fabric belt, surfaced on one side with a layer of rubber filled with small round pits uniformly spaced. These pits are 0.020 in. deep, and the number per unit area and their surface diameter determine the porosity of the sheet. A thin layer of latex is fed to the moving belt by a spreader bar so that... 

Micropores are invisible to the naked human eye thus for outsiders it is always surprising that separators of typically 60 percent porosity (i.e., 60 percent void volume, 40 percent solid material) present the impression of a compact, hole-free, nontransparent sheet. 

The alternative is hexane, which because of the explosion hazard requires a more expensive type of extractor construction. After the extraction the product is dull gray. The continuos sheet is slit to the final width according to customer requirements, searched by fully automatic detectors for any pinholes, wound into rolls of about 1 m diameter (corresponding to a length of 900-1000 m), and packed for shipping. Such a continuous production process is excellently suited for supervision by modern quality assurance systems, such as statistical process control (SPC). Figures 7-9 give a schematic picture of the production process for microporous polyethylene separators. 

Thin zeolite sheets offer improved mass transfer for possible rapid cycle adsorption processes. One of the first studies of zeolite as fillers in paper-making was issued to NCR in 1955, although at that time the term zeolite was more often used to describe any ion exchanger whether or not it was actually a crystalline microporous zeolite [95]. A later patent described the incorporation of micropo-rous zeolite powders in paper sheets [96]. More recently a number of patents described zeolite-containing papers in adsorption processes [97-99]. 

A separator is a porous membrane placed between electrodes of opposite polarity, permeable to ionic flow but preventing electric contact of the electrodes. A variety of separators have been used in batteries over the years. Starting with cedar shingles and sausage casing, separators have been manufactured from cellulosic papers and cellophane to nonwoven fabrics, foams, ion exchange membranes, and microporous flat sheet membranes made from polymeric materials. As batteries have become more sophisticated, separator function has also become more demanding and complex. 

One particular version of the lithium-ion gel polymer cells, also known as plastic lithium-ion cell (PLION). was developed by Bellcore (now Telcordia Technologies).In this case. Gozdz et al. developed a microporous plasticized PVdF—HFP based polymer electrolyte which served both as separator and electrolyte. In PLION cells, the anode and cathode are laminated onto either side of the gellable membrane. Good adhesion between the electrodes and the membranes is possible because all three sheets contain significant amounts of a PVdF copolymer that can be melted and bonded during the lamination step. 

In the supported liquid membrane process, the liquid membrane phase impregnates a microporous solid support placed between the two bulk phases (Figure 15.1c). The liquid membrane is stabilized by capillary forces making unnecessary the addition of stabilizers to the membrane phase. Two types of support configurations are used hollow fiber or flat sheet membrane modules. These two types of liquid membrane configuration will be discussed in the following sections. 

Of the existing flat-sheet RO membranes, cellulose acetate membranes of the Loeb-Sourirajan type give the best results because their open microporous substrate minimizes internal concentration polarization. Conventional interfacial composite membranes, despite their high water permeabilities and good salt rejections, are not suitable for PRO because of severe internal concentration polarization. 

The main difference between carbon nanotubes and high surface area graphite is the curvature of the graphene sheets and the cavity inside the tube. In microporous solids with capillaries which have a width not exceeding a few molecular diameters, the potential fields from opposite walls will overlap so that the attractive force which acts upon adsorbate molecules will be increased in comparison with that on a flat carbon surface [16]. This phenomenon is the main motivation for the investigation of the interaction of hydrogen with carbon nanotubes (Figure 5.14). 

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