Sealing materials chemical resistance

Storage and Handling. The acid should never be allowed to stand in a line completely sealed between two closed valves or check valves. Excessive pressure caused by thermal expansion of the Hquid can cause leaks or pipe mptures. AH lubricants and packing materials in contact with chlorosulfuric acid must be chemically resistant to the acid. Elanged connections are recommended over screwed fittings and flange guards should be used. 

Figure 3-1A. Sealless Magnetic Drive Centrifugal Pump, no seals, no leakage, no coupling. Chemical resistance depends on materials of construction. See Table 3-2. (By permission, LaBour Pump Co.)...

In a fully synthetic FR fluid, the fire resistance is due to the chemical nature of the fluid in the others, it is afforded by the presence of water. The other main distinction between the two groups is that the fully synthetic fluids are generally better lubricants and are available for use at operating temperatures up to 150°C (272°F), but are less likely to be compatible with the conventional sealing materials and paints than are water-based products. 

Nitrile rubbers, including fiber-reinforced varieties, are used both as radial shaft-seal materials and as molded packing for reciprocating shafts. They have excellent resistance to a considerable range of chemicals, with the exception of strong acids and alkalis, and are at the same time compatible with petroleum-based lubricants. Their working temperature range is from —1°C to 107°C (30°F to 225°F) continuously and up to 150°C (302°F) intermittently. When used on hard shafts with a surface finish of, at most, 0.00038 mm root mean square (RMS), they have an excellent resistance to abrasion. 

Elastomers are used for their flexibility in seals, gaskets and hoses and to resist abrasion (through absorption of the kinetic energy of the impinging particles). The range of materials includes natural and synthetic rubbers and modem elastomers with chemical resistance. 

Table XV Chemical resistance of commonly used elastomer gaskets and sealing materials...

Chemical compatibility is another important factor to reckon with in material selection as in the case of butterfly valves which are frequently used in industrial fluid flow systems where the valves are operated to load and unload different kinds of fluids for shipment or storage. These applications include tank trailers for transportation of fluids from one location to another as well as fluid flow pipelines. A typical butterfly valve usually incorporates a resilient rubber seal component which the valve pivots to seal off the flow lines and when opened allows the fluid to be conveyed to the flow line. The chemicals transported in tanks using butterfly valves are often caustic or acidic. These chemicals corrode the pivot area of the valve making its function ineffective. The rubber gasket material prevents leakage while the valve chamber is protected by a chemically resistant rubber material. 

To calibrate the cobalt source, three systems are most often used ferrous sulfate, ferrous sulfate-cupric sulfate, and ceric sulfate. Dosimeters of these solutions are prepared by filling 5-ml. chemical-resistant glass ampoules with approximately 5 ml. of solution and flame-sealing the ampoules. The ampoules are then arranged in phantoms of Masonite or similar materials (Figure 13) to simulate the food items. These phantoms are placed in containers similar to those used for food products, and arranged in the conveyor carrier in which they are transported into the irradiation cell. Because of the upper dose limit of the ferrous sulfate and ferrous sulfate-cupric sulfate dosimeters (40,000 and 800,000 rads, respectively), these systems can be used only to establish the dose rate in the facility and not to monitor the total dose during food irradiation. The ceric dosimeter which... 

A large variety of elastomers and plastics are currently available for seals in valves. At present, there is no single material suitably resilient to all pressures, temperatures and chemicals. Therefore, each resilient seat application should be selected after considering the specific fluid and service conditions. Where certain materials may be excellent with respect to chemical resistance, they may not be suitable for the intended service temperatures, and vice versa. 

Polyolefins. Polyethylene (conventional and linear) and polypropylene have excellent chemical resistance and are readily molded and machined, although they are rather soft. Conventional polyethylene adheres well to metals, and polyethylene tubing can be readily sealed around metal rods and wires to make simple electrodes suitable for use at temperatures below 60°C. The material is resistant to mineral acids and bases (except concentrated sulfuric and perchloric acids) and most organic solvents except halogenated or aromatic hydrocarbons. 

In operation containers constructed of microwave-transparent materials, (e.g. quartz or fluoropolymers), are used to hold multiple samples inside the ultraCLAVE . The interior of the stainless steel vessel is protected by a titanium nitride or multi-layer PTFE plasma coating for complete acid and chemical resistance. Sample containers may be open or covered by a lid. After the samples are loaded (manually or robotically) the ultraCLAVE cover is lowered into place by an electric motor controlled from the system s PC. The vessel closure is engaged and secured in place to seal the ultraCLAVE for high pressure operation. 

Material component evaluation and selection are critical steps in the development of a MDI formulation [140,141], The materials must be chemically resistant and compatible with all components of the formulation. Gaskets must have appropriate mechanical properties and work effectively as a seal, preventing leakage of the formulation and moisture ingress. While the basic components themselves have... 

Saran (Dow polyvinylidene dichloride) is a tough, chemically resistant plastic available in a variety of forms that are useful in the laboratory. Saran pipe or tubing can easily be welded to itself or sealed to glass and is useful for handling corrosive solutions. Thin Saran film, available commercially as a packaging material, is useful for windows, support films, etc. Mylar (du Pont polyethylene terephthalate) film and other polyester films are also useful for these purposes. Mylar is chemically inert and has excellent electrical properties for electrical insulation and for use as a dielectric medium in capacitors. Much thinner than these are films that can be made in the laboratory by allowing a dilute ethylene dichloride solution of Formvar (polyvinyl acetal) to spread on a water surface and dry. 

Many methods have been proposed to address this issue (see Chapter 9). Beside thermal and chemical resistances of the sealing materials other issues need to be considered as well. One such important issue is the mismatch of the thermal expansion coefficients between the membrane element and the sealing material or joining material. While similar material design and engineering problems exist in ceramic, metal and ceramic-metal joining developmental work in this area is much needed to scale up gas separation units ot membrane reactors for production. The efforts are primarily p ormed by the industry and some national laboratories. 

The development of an effective cleaning process has to take into account the nature of the foulants, the thermal and chemical resistance of the membrane material, membrane housing and seals. Typically, membrane manufacturers make specific recommendations regarding the cleaning agents and cleaning procedures that need to be used for different membranes and applications. 

The second sheet lining is also a fluorocarbon—Halar , but made entirely of this basic material. Though quite expensive, this material has probably the best overall chemical resistance of any of the generally available sheet linings. It is adhered to the substrate with a rubber-type adhesive, and though Halar can accept surface temperatures of 300°F and above without damage, the use of the sheet must be limited to the 220°F range due to the thermal limit of the elastomer adhesive. Joints are butted, tooled to accept a weld strip, and then a narrow strip is heat-sealed over the joint. Full data on the material, its installation and chemical resistance is available from the manufacturer. 

Innovation in the fabric softener business not only focused on compositions, but also on packaging. As an alternative to plastic bottles, heat-sealed flexible polyethylene pouches were introduced to the market in the early 1980s [7], followed by several other containers free-standing flexible pouches with a solid base, different bag in a box rigid units, and refill cartons coated with chemically resistant polymers. These novelties led to a 40% reduction in plastic bottle consumption, the use of more biodegradable, renewable, easily recycled material, and a decrease in the packaging and distribution cost. New bottles made of 100% post consumer recycled plastic sandwiched between two layers of virgin resin also became available [7],... 

The most widespread polymer currently in use for high-speed dynamic sealing applications in automotives is polyetheretherketone. These thermoplastic aromatic polymers are used in the aerospace, electronics, and nuclear industries [9], These materials have excellent mechanical- and chemical-resistance properties, which permit polyethere-therketones to be used in many engineering applications—often in harsh environments [10], These polyetheretherketones are the product of the reaction of 1,4-dihydroxyben-zene and 4,4 -di II non then /ophenonc [8]. Figure 6.5 shows this reaction. 

Facepiece materials must be chemically resistant and impermeable to agents ( chemically hard ) while being flexible (especially at the seals), comfortable, non-toxic to the skin, easily workable, inexpensive and have a long shelf-life. Eyepieces and visors must also be transparent. It is difficult to combine all of these features in one material in particular, flexible and chemically hard materials are typically difficult to work with and uncomfortable next to the skin. On the other hand, easily workable and comfortable materials tend to absorb agent (nerve agents are good solvents), which will then off-gas , e.g. in a collective protection system. In extreme cases, the agent can penetrate the material itself. Chemical hardness is of primary importance and most facepieces are made of chlorobutyl rubber. 

Joints must then be sealed with an elastomeric sealer in order to prevent seepage through the seam or crack. To seal control joints and cracks, a chemically resistant joint sealer is applied in the seam joint. There are many different sealant materials and specifications for installation which should be considered at the design stage. 

A second generation of phenolic dispersions, patented by J. S. Fry (33). involved the post dispersion of phenolic resins in a mixture of water and water-miscible solvents. To conform with air pollution regulations, the solvent was held to 20 volume %, or less, of the volatiles. A heat-reactive phenolic resin dispersion (34) and a phenolic-epoxy codispersion have become commercially available based on the above technology. Supplied at 40-45% solids, these products, which have a small particle size (0.75-1.0 ym), are better film formers than the earlier dispersions. Used alone or in blends with other waterborne materials, corrosion-resistant baking coatings may be formulated for coil coating primers, dip primers, spray primer-surfacers, and chemically resistant one-coat systems. Products of this type are also tackifiers for acrylic latexes, and such systems have been employed as contact, heat seal, and laminating adhesives for diverse substrates. 

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