O rings

With a batch process, such as hot isostatic compaction (HIP), heat exchange as used in a continuous reactor is not possible, and it is common practice to provide a furnace within the pressure vessel which is thermally insulated to ensure that the temperature of the vessel does not rise above 300°C. Most HIP operations involve gas pressures in the range 70—200 MPa (10—29,000 psi) and temperatures of 1250—2000°C, occasionally 2250°C (74). The pressure vessel may have a bore diameter from 27 to 1524 mm (75) and is nearly always provided with threaded closures sealed with O-rings made of elastomer provided the temperature is low enough. 

Fig. 23. O-ring backed up by miter ring to seal vessel (121).

Rubber Articles. The soHd polysulftde mbber products, Thiokol FA and Thiokol ST, are used ia the manufacture of printing roUers, printing blankets, gas meter diaphragms, O-rings, and specialty molded items. 

Economics and price of the final article often dictate a specific type of mbber that can be used. The expected usable life for the product is controlled by many factors including end customer awareness, competitive situation in the marketplace, safety, reUabiUty, and other factors. Rubber is almost always used as a functional part of another system. For example tires, hoses, belts, O-rings, and numerous mbber components are used in manufacturing automobiles and tmcks. The overall life of the vehicle as well as its performance level often control or direcdy relate to the service life or quaUty level of the mbber parts. 

Dow Corning, General Electric, and Shinetsu Chemical Company are the principal suppHers of these elastomers in the United States. It is estimated that the volume used in the United States is between 2700 and 3600 t/yr. The primary uses of these elastomers are in O-rings and shaft seals, as weU as wire and cable, and electrical coimectors. 

Bayer, USI, and DuPont are U.S. suppHers. There is an estimated 2300—4000 t/yr of EVA used in the United States. Uses include automotive electrical system components, eg, ignition cable, footwear, weatherstripping, and misceUaneous O-rings and seals for industrial appHcations. 

Compatibility and Corrosion. Gas turbine fuels must be compatible with the elastomeric materials and metals used in fuel systems. Elastomers are used for O-rings, seals, and hoses as well as pump parts and tank coatings. Polymers tend to swell and to improve their sealing abiUty when in contact with aromatics, but degree of swell is a function of both elastomer-type and aromatic molecular weight. Rubbers can also be attacked by peroxides that form in fuels that are not properly inhibited (see Elastomers, synthetic Rubber, natural). 

Potential areas of apphcation for carboranes include adhesives, gaskets, and O-rings (27). Research has been reported on improving the oxidation resistance and strength of carbon materials by coating with a carborane siloxane polymer (36). 

Compression Set Resistant. One property of fluorocarbon elastomers that makes them uniquely valuable to the sealing industry is their extreme resistance to compression set. Figure 4 plots compression set vs time for compounds prepared especially for compression set resistance (O-ring grades). 

Fig. 4. Compression-sets of fluorocarbon elastomers at 200°C, 3.5 mm O-rings A, Compound 1 (see Table 4) B, Compound 11 (see Table 4).

With a clear idea of use requirements and mbber response to specific additives, a formulation may be selected. Uses generally fall into one of three classes O-rings, molded goods, and extmded forms. 

Ring S. In O-ring appHcations, the primary consideration is resistance to compression set. A fluorocarbon elastomer gum is chosen for O-ring apphcations based on its gum viscosity, cross-link density, cure system, and chemical resistance so that the best combination of processibiUty and use performance is obtained. Sample formulations for such uses are given in Table 4. 

Extrusion. Extmsion techniques are used in the preparation of tubing, hose, O-ring cord, preforms and shaped gaskets. Typical extmsion conditions are 70 to 85°C for the barrel temperature and 95 to 110°C for the head temperature. The extmded forms are normally cured in a steam autoclave at 150 to 165°C. Some special grades of peroxide curable fluorocarbon elastomers can be hot air vulcanized. 

Commercially available fluorocarbon elastomers meet automotive specifications in the HK section of ASTM D2000 and SAE J-200. ASTM D1418 specifies designations of composition, eg, fluorocarbon elastomers are designated CEM, EKM, or EEKM. Commercially available fluorocarbon elastomers offer a balance of those properties needed to meet the major O-ring specifications, such as AMS 7276, AMS 7280A, AMS 7259, MIL 83248 Amendment 1 Type II, Class I and II. 

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