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MATERIALS FOR FUEL SERVICE

Careful selection can help you avoid getting burned.

by Rick Hudson

Even after 30 years in the rubber and sealing industries, I’ll be the first to admit that some applications are just plain difficult. Designing an effective elastomeric component for fuel service is a perfect example. Taken together, there are literally hundreds of hydrocarbons, trace metals, and additives (such as antioxidants, corrosion inhibitors, and detergents) in any given gallon of gasoline. As if that weren’t enough, variances in crude oil processing and changes in the fuel during storage further complicate the picture. Though the variables are numerous, all seals, diaphragms, hoses, and custom-molded parts destined for fuel service must be resistant to degradation. Here’s a closer look at what makes fuel service so problematic, as well as some ways in which designers are arriving at solutions.

COMPOSITIONAL CONCERNS Fuel service designers are primarily concerned with two main compositional aspects. The first of these is aromatic content. Aromatic hydrocarbons (those containing ringed carbons, such as benzene, toluene, and xylene) are used in conjunction with other additives (such as alkylates) to boost octane ratings in unleaded fuels. Higher ratings typically translate to increased engine efficiency. Unfortunately, aromatic hydrocarbons also engender greater elastomer swell than aliphatic hydrocarbons (those containing straight-chain carbons, such as paraffins, olefins, and acetylenes) or other fuel constituents. The higher the aromatic content, the greater the swell to be expected. Because 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 atoms they 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 troublesome in terms of component design. 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 or other molded parts will be degraded to the point of failure.

EFFECTS ON ELASTOMERS 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 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). Either way, the elastomer is compromised.
Additionally, any compounds used in conjunction with fuel systems must be able to withstand temperature extremes. Unless properly anticipated, high temperatures can contribute to other effects, particularly elastomer swell and compression set. Low temperatures can also be challenging, especially in dynamic applications.

CHOOSING THE MATERIAL Peroxide-curable, high fluorine content fluorocarbon rubber (FKM) is currently the most common choice for fuel service applications. High fluorine content fluorocarbons traditionally have poor low temperature resistance, but Viton® GFLT has improved low temperature properties similar to Viton® GLT in combination with fluid resistance analogous to Viton® GF. In lieu of fluorocarbon, some nitrile (NBR) compounds may be suitable, provided they have a high acrylonitrile (ACN) content to bolster fuel resistance. In some instances, hydrogenated nitrile (HNBR) or fluorosilicone (FVMQ) are used. Epichlorohydrin rubber (ECO) is also used for fuel service, but it does not perform as well as either fluorocarbon or nitrile, especially in the presence of sour fuel hydroperoxides. If you need assistance on a fuel service application, please don’t hesitate to call us at 1-800-722-6766. We’ll be happy to help.

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