FUEL RESISTANCE

The fluoroelastomers possess good mbber properties with the added advantages of being nonburning, hydrophobic, and solvent- and fuel-resistant. In addition to these, because of flexibiHty down to about —60° C, these polymers have been used in seals, gaskets, and hoses in army tanks, in aviation fuel lines and tanks, as well as in cold-climate oil pipeline appHcations. These polymers have also found appHcation in various types of shock mounts for vibration dampening (14,17). 

During the 1930s gradual improvements in the product and processing overcame some of the drawbacks of this material. Nonetheless, the apphcations were limited and Thiokol Corp. stmggled to remain solvent. The first year Thiokol reported a profit was in 1941, 13 years after its foundation. This was realized when the U.S. Air Force discovered that the aUphatic polysulftdes were usehil as a fuel-resistant sealant for aircraft tanks and hoses. Polysulftdes also began to be used as sealants for boat hulls and decks. 

Vulcanizates of ECH homopolymer and ECH—EO copolymer are resistant to ASTM oils, aUphatic solvents, and aromatic-containing fuels, showing low swell after exposure. The polymers do not harden after exposure to these fluids, although plasticizer may be extracted. Overall, these polymers offer a good balance of heat, ozone, and fuel resistance over a broad temperature range. 

Two propylene oxide elastomers have been commercialized, PO—AGE and ECH—PO—AGE. These polymers show excellent low temperature flexibihty and low gas permeabihty. After compounding, PO—AGE copolymer is highly resiUent, and shows excellent flex life and flexibiUty at extremely low temperatures (ca —65°C). It is slightly better than natural mbber in these characteristics. Resistance to oil, fuels, and solvents is moderate to poor. Wear resistance is also poor. Unlike natural mbber, PO—AGE is ozone resistant and resistant to aging at high temperatures. The properties of compounded ECH—PO—AGE he somewhere between those of ECH—EO copolymer and PO—AGE copolymer (22). As the ECH content of the terpolymer increases, fuel resistance increases while low temperature flexibihty decreases. Heat resistance is similar to ECH—EO fuel resistance is similar to polychloroprene. The uncured mbber is soluble in aromatic solvents and ketones. 

A good metallic fuel resists air oxidation and moisture, has a high heat output per gram, and is obtainable at moderate cost in fine particle sizes. Aluminum and magnesium are the most widely used materials. Titanium, zirconium, and tungsten are also used, especially in militaiy applications. 

Chemical resistance of protective coatings is one of the major concerns especially when stability to fuels, resistance to acid rains and protection of the coated substrate from corrosion are required. Chemical resistance has been... 

The chemical and fuel resistance of polysulfides makes them useful as sealants and coatings for secondary containment areas, where they prevent chemicals, solvents, fuels, etc, from seeping into the ground in the event of spillage or a storage tank leak. Polysulfide-based coatings and sealants are also used for bridges, air fields, and road construction. 

D 3569 Specification for Joint Sealant, Hot Applied, Elastomeric, Jet Fuel Resistant Type for Portland Cement Concrete Pavements... 

Sealing Compound, One Part Silicone, Aircraft Firewall Adhesive/Sealant, Fluorosilicone Aromatic Fuel Resistant, One Part Room Temperature Vulcanizing... 

Sealing Compound, Noncuring, Groove Injection, Temperature and Fuel Resistant... 

The 38% of chlorine content found in the homopolymer creates the fuel resistance and promotes flame retardancy. The ECH/EO copolymer has improved low-temperature properties, e.g., flexibility. The very polar chloromethyl group responsible of the oil resistance properties is also the cross-linking site in these elastomers. 

VitonA (DuPont) CF2=CH2 + CF3CF=CF2 Elastomers used for sealants, O-rings, fuel-resistant seals for... 

The potential market for polysulphides is for water-industry sealants, and fuel-resistant sealants and coatings for secondary containments. As a construction, civil engineering and insulating-glass sealant polysulphides have a poor to fair market share. Table 7.1 summarises the specific applications of polysulphide systems in the construction industry. 

Reference standards have been adopted for polysulphide sealants in countries such as Australia (Doc. 1572-1970), Canada (19 GP 13a-1970), Japan (JIS-5724-1969) and other countries. However, a good two-component sealant can meet British Standard BS 4254, US Federal Specification TT-S-00227 E and ASTM C-920. Fuel-resistant two-part polysulphide sealants comply with BS 5212-75 and US Federal Specification SS-S-200 D. Sealants used for water retaining and excluding meet the requirement of BS 6920. Polysulphide sealants used for fire protection comply with BS 4254-1983. Additionally these sealants are non-combustible to BS 476, part 4 1984. All these categories will be discussed under separate head-... 

Figure 7.3 Joints in concrete slabs applied with fuel-resistant polysulphide sealants to prevent contamination of groundwater...

The different formulations of NBR/PVC blends marketed by Zeon under the trade name NipoP show either a co-continuous or dispersed structure. The NBR/PVC formulations vary from 50 50 to 70 30 wt%. The blends are processed mostly by extrusion into products such as cable jackets and hose covers. Injection molding is also being carried out. The blends show good ozone and abrasion resistance. Grades with higher AN content in the NBR display better ozone and fuel resistance while lower AN content results in improved low temperature flexibility. 

Goodyear markets blends of nitrile rubber with PVC, chlorinated PE, TPU or PA, or then-combination, under the trade name Chemigum TPE. The blends show either a disperse or co-continuous morphology. Properties include excellent oil and fuel resistance, high wear resistance and strength, a brittleness temperature below -45°C and a service temperature up to 120°C. Applications include gaskets, seals, joints, geomembranes, seals and liners. 

A PC/PEST blend has replaced an aluminum alloy in rotary gas powered mowers. The blend provides impact and fuel resistance, yet has a good, durable surface finish. 

Fuel resistance, low temperature flexibility, fill and drain convenience for manufacture becomes barrier to prevent vapor transmission through polyethylene and contain fuel where pin-holes may exist or appear in polyethylene. 

The impetus for their research was to discover a new class of viscoelastic polymers which exhibited good thermal stability and broad use temperatures for use as fuel-resistant sealants. The synthetic methods used to prepare polymers like 9 are similar to those methods which were used in studies mentioned previously. For 2 (x = 0), the bis(silanol) was prepared and then condensed using tetramethylguanidine 2-ethylhexoate as a catalyst to prepare the polymer. For 9 (x = 1) the appropriate bis(silanol) was condensed with a bis(dimethylamino)silane, and for 9 (x = 2) the appropriate bis(silanol) was condensed with a bis(dimethylamino)disiloxane. Table 3 relates some of the various properties found from polymers prepared using these methods. 

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