Designing an effective fuel service seal is not easy. Taken together, there are literally hundreds of hydrocarbons, trace metals, and additives (such as oxygenates, corrosion inhibitors, and detergents) in any given gallon of gasoline. Variances in crude oil processing and changes in the fuel during storage further complicate the picture.

Though the variables are numerous, seal engineers are primarily concerned with two factors. The first is aromatic content. Aromatic hydrocarbons (those containing ringed carbons, such as benzene, toluene, and xylene) are used along with other additives (such as alkylates) to boost octane ratings in unleaded fuels. Higher ratings generally translate to increased engine efficiency. Unfortunately, aromatic hydrocarbons also cause greater elastomer swell compared to aliphatic hydrocarbons (those with straight-chain carbons, such as paraffins, olefins, and acetylenes) or other fuel constituents. The higher the aromatic content, the greater the potential swell. Since greater swell is linked to increased degradation of physical properties in elastomeric parts, aromatic content is one major concern.

The other major concern is the level of oxygenated additives (oxygenates), particularly alcohols and ethers. As with aromatic hydrocarbons, oxygenated additives raise octane numbers. Gasoline blends containing alcohols and ethers also extend the fuel supply and cut down on pollutants. The additional oxygen atomsthey provide allow cleaner engine combustion, thus producing less carbon monoxide (CO). Use of reformulated fuels containing oxygenated additives has been ordered by the Environmental Protection Agency (EPA) for cities with poor air quality. But oxygenates can be problematic for the seal designer. The presence of oxygenated additives in certain concentrations can make gasoline much more aggressive toward elastomeric compounds. This heightened aggression dramatically increases the likelihood that seals will be degraded to the point of failure.

The composition of fuels can thus have a number of effects on elastomers. As already noted, substantial volume change (most commonly elastomer swell) is a primary concern. Volume change is typically accompanied by changes in physical properties, including hardness, tensile strength, modulus, and elongation. Resistance to tearing and to compression set are also impacted as a result of volume change. Increasing swell means hardness and these other physical properties will decrease.

The elastomer’s resistance to fuel permeation is another major consideration, particularly in sealing applications. Even if permeability isn’t a problem, the elastomer may face chemical attack from “sour” fuel. Often seen in fuel-injected automotive systems, soured fuel results when oxygen combines with hydrogen to form what are known as hydroperoxides (O2H groups). These hydroperoxides later break into free radicals which, because they have at least one unpaired electron, are “anxious” to chemically react. A prime target: the elastomer’s chemical backbone. Depending on the circumstances, free radicals can cause the elastomer to become too soft (due to the breaking of chemical bonds, known as reversion) or too brittle (due to unwanted crosslinking; see Figure 54). Either way, the elastomer is compromised.

Additionally, compounds used in fuel systems must be able to withstand temperature extremes. Unless properly anticipated, high temperatures can contribute to other effects, especially elastomer swell and compression set. Low temperatures can be troublesome in dynamic applications.

Because fuel service can have such wide-ranging effects on elastomers, the American Society for Testing and Materials (ASTM) developed test method D 471 as a way to gauge the effects of fuels and other liquids on elastomeric samples. Samples are exposed to a fluid (e.g. Reference Fuel A) for a specific period of time (e.g. 70 hours) at a set temperature (e.g. 23° C). After exposure, the sample’s properties (e.g. hardness, tensile strength, elongation, and volume) are measured and compared with the properties as recorded prior to testing. Decisions can then be made as to the suitability of a particular compound for use with a given fuel.

ASTM Reference Fuels A through K have been specifically selected to test compounds in contact with gasolines or diesel fuels. Which tests are called for depends on which fluid(s) the elastomer will encounter. For example, Reference Fuel A is a 100% isooctane fluid which mirrors the shrinking or low-swell effects of gasolines composed primarily of aliphatic hydrocarbons. If the compound in question will be used around gasolines with a very high aliphatic content, then a test using Reference Fuel A is a good idea. Reference Fuel B is a 70% isooctane-30% toluene mixture. The toluene content lends the mixture a level of aromaticity, enabling Reference Fuel B to approximate the swelling effects of commercial gasolines. The ASTM Reference Fuels are listed in Table 17.

Peroxide-curable, high fluorine content fluorocarbon rubber (FKM) is currently the most common choice for fuel service. High fluorine content fluorocarbons traditionally have poor low temperature resistance, but Type GFLT fluorocarbons have improved low temperature properties similar to Type GLT in combination with fluid resistance analogous to Type GF. In lieu of fluorocarbon, some nitrile (NBR) compounds may be suitable, provided they have a high acrylonitrile (ACN) content to bolster fuel resistance. Epichlorohydrin rubber (ECO) is also used for fuel service, but it does not perform as well as fluorocarbon or nitrile, especially in sour fuel hydroperoxides.

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