Cellophane, separators

Occasionally the zinc electrode is wrapped in a polypropylene fleece filled with inorganic substances, such as potassium titanate, in order to reduce the solubility of zinc since the problem of dendrite growth is aggravated even by the metallization of the cellophane separator due to the aforesaid silver reduction and its promoting the generation of shorts. 

Lewis, H. L. Hammersley, V. L. Wharton, S. P. NSWC evaluation of cellophane separation in model rechargeable silver-zinc cell. Presented at the 38th Power Sources Conference, Cherry Hill, NJ, 1998. 

The semipermeable membrane proposed for the demineralization of sea water is based on H. L. Calendar s theory that osmosis takes place through the membrane as vapor, condensing at the opposite membrane surface. The actual membrane being used consists of two sheets of untreated cellophane separated by a water-repellent powder, such as a silicone-coated pumice powder. The vapor gap is maintained by an air pressure in excess of the pressure on the sea water and the cellophane sheets support the capillary surfaces, which will withstand pressures up to 1500 p.s.i. A number of successful experiments are reported with over 95% desalinization. The present effort is directed toward obtaining reproducible experimental results and better methods of fabricating the vapor gap. 

Osmosis is the spontaneous process by which the solvent molecules pass through a semi-permeable membrane from a solution of lower concentration of solute into a solution of higher concentration of solute. A semipermeable membrane (e.g., cellophane) separates... 

Fig. 21. Polarographic vessel with a Cellophane separated reference electrode.

The reagent is conveniently stored as a solution in isopropyl alcohol. The molten (or solid) alkoxide is weighed out after distillation into a glass-stoppered bottle or flask and is dissolved in sufficient dry isopropyl alcohol to give a one molar solution. This solution may be kept without appreciable deterioration provided the glass stopper is sealed with paraffin wax or cellophane tape. Crystals of aluminium isopropoxide separate on standing, but these may be redissolved by warming the mixture to 65-70°. 

Under atmospheric conditions, 3-aminophenol is the most stable of the three isomers. Both 2- and 4-aminophenol are unstable they darken on exposure to air and light and should be stored in brown glass containers, preferably in an atmosphere of nitrogen. The use of activated iron oxide in a separate cellophane bag inside the storage container (116), or the addition of staimous chloride (117), or sodium bisulfite (118) inhibits the discoloration of aminophenols. The salts, especially the hydrochlorides, are more resistant to oxidation and should be used where possible. 

Cellophane or its derivatives have been used as the basic separator for the silver—ziac cell siace the 1940s (65,66). Cellophane is hydrated by the caustic electrolyte and expands to approximately three times its dry thickness iaside the cell exerting a small internal pressure ia the cell. This pressure restrains the ziac anode active material within the plate itself and renders the ziac less available for dissolution duriag discharge. The cellophane, however, is also the principal limitation to cell life. Oxidation of the cellophane ia the cell environment degrades the separator and within a relatively short time short circuits may occur ia the cell. In addition, chemical combination of dissolved silver species ia the electrolyte may form a conductive path through the cellophane. 

Silver—Zinc Separators. The basic separator material is a regenerated cellulose (unplastici2ed cellophane) which acts as a semipermeable membrane aHowiag ionic conduction through the separator and preventing the migration of active materials from one electrode to the other. 

A stronger separator is one made of sausage casiag material (ESC), a regenerated cellulose similar to cellophane but including some fibrous material. ESC is usually extmded ia tubes and electrodes are iaserted iato each ead of the tube. The tube is folded to form the so-called U wrap. 

The positive plates are siatered silver on a silver grid and the negative plates are fabricated from a mixture of cadmium oxide powder, silver powder, and a binder pressed onto a silver grid. The main separator is four or five layers of cellophane with one or two layers of woven nylon on the positive plate. The electrolyte is aqeous KOH, 50 wt %. In the aerospace appHcations, the plastic cases were encapsulated in epoxy resins. Most usehil cell sizes have ranged from 3 to 15 A-h, but small (0.1 A-h) and large (300 A-h) sizes have been evaluated. Energy densities of sealed batteries are 26-31 W-h/kg. 

The liquid membrane (thickness 0.2 cm) was separated from the aqueous solutions by two vertical cellophane films.The electrode compartments were filled with 0.05 M sulfuric acid solutions and were separated by the solid anion-exchange membranes MA-40. Binary mixtures contained, as a mle, 0.04 M Cu(II) and 0.018 M Pt(IV) in 0.01 M HCl. 0.1 M HCl was used usually as the strip solution. 

Thus in this system, in addition to the usual requirements, the separator has the task of delaying penetration for as long as possible. A membrane would be regarded as perfect which lets hydroxyl ions pass, but not the larger zincate ions. This requirements is best met by regenerated cellulose ( cellophane ) [10,11], which in swollen condition shows such ion-selective properties but at the same time is also chemically very sensitive and allows only a limited number of cycles the protective effects of additional fleeces of polyamide or polypropylene have already been taken into account. 

Pulp mills. These separate the fibers of wood or other materials, such as rags, Enters, waste-paper, and straw, in order to create pulp. Mills may use chemical, semichemical, or mechanical processes, and may create coproducts such as turpentine and tall oil. Most pulp mills bleach the pulp they produce, and, when wastepaper is converted into secondary fiber, it is deinked. The output of some pulp mills is not used to make paper, but to produce cellulose acetate or to be dissolved and regenerated in the form of viscose fibers or cellophane. 

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. 

For similar solvent polymeric membranes (78 wt.% dicresyl butyl phosphate in polyvinyl chloride) self-diffusion coefficients of the order of 10-7 cm2s 1 have been reported.12 These diffusion coefficients, as well as measurements of rotational mobilities,14 indicate that the solvent polymeric membranes studied here are indeed liquid membranes. This liquid phase is so viscous, however, that convective flow is virtually absent. This contrasts with pure solvent membranes where an organic solvent is interposed between two aqueous solutions either by sandwiching it between two cellophane sheets or by fixing it in a hole of a Teflon sheet separating the aqueous solutions.15 The extremely high convective flow is one of the reasons why the term membrane for extraction systems... 

Ultrafiltrates obtained using cellophane or polysulphone membranes at 20°C and a transmembrane pressure of c. 100 kPa are satisfactory, but the concentrations of citrate and calcium are slightly low due to sieving effects which are accentuated by high pressures. Dialysis of a small volume of water against at least 50 times its volume of milk (to which a little chloroform or azide has been added as preservative) at 20°C for 48 h is the most satisfactory separation procedure and agrees closely with results obtained... 

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