Gaskets elastomers

As detailed in Table 26.1, various gasket elastomers are available which have chemical and temperature resistance coupled with good sealing properties. The temperatures shown are maximum therefore possible simultaneous chemical action must be taken into account. 

Polyacrylate elastomers find limited use in hydrauhc systems and gasket apphcations because of their superior heat resistance compared to the nitrile mbbers (219,220). Ethylene—acrylate copolymers were introduced in 1975. The apphcations include transmission seals, vibration dampers, dust boots, and steering and suspension seals. Further details and performance comparisons with other elastomers are given in reference 221 (see also Elastomers, SYNTHETIC-ACRYLIC ELASTOTffiRS). 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Gaskets in both dry gas and Hquid chlorine systems are made of mbberi2ed compressed asbestos. Eor wet chlorine gas, mbber or synthetic elastomers are acceptable. PTEE is resistant to both wet and dry chlorine gas and to Hquid chlorine up to 200°C. Tantalum, HasteUoy C, PTEE, PVDE, Monel, and nickel are recommended for membranes, mpture disks, and beUows. 

Extmsion techniques are used to make tubes, rods, gaskets, preforms, etc. Standard mbber equipment may be used to extmde fluorosihcone elastomers. The green strength of fluorosihcones is less than that of typical fluorocarbon elastomers, and this should be considered when designing the feed system. 

Equipment for storing and handling methanol may be made of carbon or stainless steel. Methanol is aggressive toward copper, zinc, magnesium, tin, lead, and alurninum, which should therefore be avoided. Gasket materials must also be chosen carefiiUy, because some elastomers swell and deteriorate when exposed to methanol. Similarly, the use of plastics for storage is not recommended. 

Enichem and DuPont ate two suppHers of acryHc elastomers. It is estimated there ate - 9000 t/yr of these elastomers used in the United States. The principal uses of acryHc elastomers ate in automotive appHcations requiring both oil and heat resistance. These include transmission, valve stem, crankshaft, pinion, and odpan gaskets and seals. In addition hose, tubing, toUs, and belts ate made from acryHc polymers. 

Thiokol elastomers possess fairly low tensile and tear properties. However, they have exceUent resistance to both aHphatic and aromatic solvents at room temperature and slightly elevated temperatures. The Thiokol division of Morton International Corporation is the suppHer of polysulftde elastomers in the United States. It is estimated that 1360—1600 t are used aimually in the United States. The primary use of polysulftde is in seals, gaskets, roUs, and diaphragms where solvent resistance and low permeabiHty are useful. 

Vluorosilicone. By fluorinating the siHcone polymer molecule it is possible to improve the solvent, fuel, and oil resistance of this already heat-resistant class of elastomers. The resulting polymers are especially useful in select automotive seals and gaskets as weU as military and downhole oilfield parts. 

Valve, trap, and baffle can be combined in such a manner that elastomers can be used in the valve and contamination is controlled from the valve-actuator mechanisms and from the gasket of the valve-plate seal (Fig. 15) (42). 

Antioxidants resistant to extraction by lubricants and gasoline are preferred for the stabili2ation of elastomers used in automotive appfications such as gaskets and tubing. Aromatic amine antioxidants, such as A/-phenyl-Ar-(p-toluenesulfonyl)-A-phenylenediamine [100-93-6] (37), with low solubifity in hydrocarbons, are extracted slowly from elastomers and are used for these appfications. 

Gaskets uti1i2ed in ethyleneamine service generally are made of Grafod flexible graphite or polytetrafluoroethylene (TEE). There is no single elastomer that is acceptable for the entire product line, although TEE may be considered as an alternative to elastomers across the product line. However, because TEE is not a tme elastomer, it may not always prove suitable as a replacement. 

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. 

Storage. Carbon steel and stainless steel should be used for all equipment in ethylene oxide service. Ethylene oxide attacks most organic materials (including plastics, coatings, and elastomers) however, certain fluoroplastics ate resistant and can be used in gaskets and O-rings. See Reference 9 for a hst of materials that are compatible with ethylene oxide. 

Like the 1-RTV systems, the two-part room temperature vulcanization systems (2-RTV) cure to produce flexible elastomers that resist humidity and other harsh environments. Interestingly, they display primerless adhesion property to many substrates, and are used in silicone adhesives, sealants, seals, and gaskets, to name a few. 

Figure 12.30 Potential uses of polyphosphazenes (a) A thin film of a poly(aminophosphazene) sueh materials are of interest for biomedical applications, (b) Fibres of poly[bis(trifluoroethoxy)phosphazene] these fibres are water-repellant, resistant to hydrolysis or strong sunlight, and do not burn, (c) Cotton cloth treated with a poly(fluoroalkoxyphosphazene) showing the water repellaney eonferred by the phosphazene. (d) Polyphosphazene elastomers are now being manufaetured for use in fuel lines, gaskets, O-rings, shock absorbers, and carburettor eomponents they are impervious to oils and fuels, do not bum, and remain flexible at very low temperatures. Photographs by eourtesy of H. R. Allcock (Pennsylvania State University) and the Firestone Tire and Rubber Company.

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. 

By far the largest group of small to medium elastomer components comprises seals and gaskets. Relaxation phenomena, which would result in loss of sealing ability, can become important. 

Because of its excellent range of properties and reliability, poly(fluoroalkoxyphosphazene) elastomers are used as seals, gaskets, and shock mounts in demanding military, aerospace, petroleum and industrial applications. In addition, applications under development for this elastomer include fuel hoses for artlc use, coated fabrics for protective clothing, sealants, coatings and medical devices. 

The Material of the Example. Poly(ether ester) (PEE) materials are thermoplastic elastomers. Fibers made from this class of multiblock copolymers are commercially available as Sympatex . Axle sleeves for automotive applications or gaskets are traded as Arnitel or Hytrel . Polyether blocks form the soft phase (matrix). The polyester forms the hard domains which provide physical cross-linking of the chains. This nanostructure is the reason for the rubbery nature of the material. 

In the first type (ISO-KF), the connections are made with conic profile flanges joined together by clamps, with an elastomer gasket (see Fig. 1.20). 

We have seen that several kinds of gaskets are used with the various types of flanges. For low and high vacuum at room temperature, elastomers O-rings (rubber, ethylenpropylen, silicon etc.) are used. Small pressures of flanges are needed. The maximum bakeout... 

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