Enamels Graphitization

Among nonmetallic materials, glass, chemical stoneware, enameled steel, acid-proof brick, carbon, graphite, and wood are resistant to iodine and its solutions under suitable conditions, but carbon and graphite may be subject to attack. Polytetrafluoroethylene withstands Hquid iodine and its vapor up to 200°C although it discolors. Cloth fabrics made of Saran, a vinyHdene chloride polymer, have lasted for several years when used in the filtration of iodine recovered from oil-weU brines (64). 

FIG. 5-12 Variation of absorptivity with temperature of radiation source. (1) Slate composition roofing. (2) Linoleum, red brown. (3) Asbestos slate. (4) Soft rubber, gray. (5) Concrete. (6) Porcelain. (7) Vitreous enamel, white. (8) Red brick. (9) Cork. (10) White dutch tile. (11) White chamotte. (12) MgO, evaporated. (13) Anodized aluminum. (14) Aluminum paint. (15) Polished aluminum. (16) Graphite. The two dashed lines bound the limits of data on gray paving brick, asbestos paper, wood, various cloths, plaster of parts, lithopone, and paper. To convert degrees Ranldne to kelvins, multiply by (5.556)(10 ). 

For cast iron enamelling the so-called grey iron is preferred. Its composition varies somewhat depending upon type and thickness of casting, but falls within the following limits 3-25-3-60% total C, 2-80-3-20% graphitic C, 2-25-3-OOVo Si, 0-45-0-65% Mn, 0-60-0-95% P and 0-05-010Vo S. 

Because of the corrosiveness of the reaction media, ceramic materials, glass, graphite, rigid-type PVC, and enameled or coated steel are recommended for pipes, cocks and valves, pumps, and cooling systems. 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

Plugs for graphite bridge initiators are made by molding the plastic about a twisted pair of enameled wires, and thea grinding the surface clean, bis ieaves a plastic surface with two metal islands separated by twice the thickness of the enamel. A droplet of colloidal suspension of graphite in water is applied over the point of closest approach of these islands to dry. Since both resistance... 

Coolers 5 and 7 are block graphite apparatuses with the heat exchange surface of 5.4 m2 cooler 9 is a steel enameled apparatus of the vessel in a vessel type with the heat exchange surface of 4 m2. 

No special strict requirements are put forward with respect to cast iron composition because in this case adhesion of enamel is of a mechanical nature. The carbon content is usually 3.4 —3.7%, of which 2.5 —3.0% is in graphite form. The base material should be mostly pearlitic, since free cementite in the form of larger crystals is responsible for bubble formation by oxidation. The quality of enamels is adversely affected by sulphur. 

Uses of polyamide-imides include pumps, valves, refrigeration plant accessories, and electronic components. The polymers have low coeflScient of friction, e.g., 0.2 (to steel), which is further reduced to as little as 0.02-0.08 by blending with graphite and Teflon. In solution form in N-methyl-2-pyrrolidone, Torlon has been used as a wire enamel, as a decorative finish for kitchen equipment, and as an adhesive and laminating resin in spacecraft... 

If there is a need to change the resistivity of a paint or varnish, suitable additives must be used. For example, the addition of 8% graphite to the chloro-vinyl enamel KhV-124 reduces the surface resistivity from 1 10 to 4.6 10 f2. The modified coating is electrically conductive, and adhesive interaction due to electrical forces may change as the time of particle contact with the surface is extended. 

Carbon black is also the base of most black typographical inks. This is a large market since an average of 16 kg of ink, containing 11 -13% by weight of carbon black, is required for every ton of news-print. Other applications are found in paints, enamels, lacquers, molded carbon and graphite, and many others. 

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