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More stringent evaporative emission loss and vehicle life requirements imposed by CARB LEV II and EPA Tier II regulations are necessitating replacement of highly permeable Air Intake and Oil System gaskets with low permeation elastomers. Low permeation, however, is not the only requirement for these gaskets. This paper presents data on fuel permeation, compressive stress relaxation and the low temperature sealing properties of the various elastomer candidates for these seals in order to help the sealing system engineer optimize the overall performance of these gaskets.

Today's fuel systems place many demands on the seals containing liquid and vapor hydrocarbons. California Air Resource's LEV II and EPA's Tier 2 demands require fuel systems which are essentially hermetically sealed with a robust, long term (12-15 year), life. Two properties which are key to long-term seal life are the material's ability to retain it's sealing force, and the ability to resist fuel permeation. To evaluate these two fuel seal properties, testing was conducted on a number of rubber compounds including HNBR, an HNBR-fluoroplastic alloy, FVMQ (fluorosilicone), an FKM-FVMQ blend, and FKM. To evaluate permeation through a seal, Thwing Albert cups were fitted with stainless steel lids and sealing gaskets prepared from the various test materials. Fuel losses through the gaskets were determined at elevated temperatures. Long term, >1000 hour, stress relaxation testing was conducted in “hot” 60°C fuel and “sour” fuel on these compounds.

The fuel permeation rate of elastomers is an important factor to consider in the design of automotive fuel system components. Evaporative hydrocarbon emission losses are now tightly regulated. The new 2 gram in 24 hour limit in a 20-40°C diurnal SHED test must be met not only when the car is built, but also at any time up to 10 years or 100,000 miles of service, or for trucks 11 years or 120,000 miles. The SHED test protocol calls for draining the fuel system on a car that has been in service and refilling with a non-oxygenated test fuel. However, the car may have been using an oxygenated fuel prior to the test. There has been speculation that the more permeable oxygenated fuel will influence the permeation rates through the elastomeric components for some time after the original oxygenated fuel has been drained from the vehicle.

The low temperature properties of fluorohydrocarbon elastomers (FKM) have been well documented using traditional rubber laboratory tests such as brittle point, temperature retraction, glass transition, and stiffness by Clash-berg and Gehman methods. Although these tests are considered good indicators of low temperature behavior, these techniques have not proven to be entirely accurate in predicting the low temperature sealing capability of fluoroelastomer molded products such as O-rings. The purpose of this paper is to present an overview of the low temperature properties of standard fluoroelastomers and the newer improved low temperature types, by both traditional test methods and a recently developed static O-ring testing device. Through use of this static testing apparatus, the influence of several variables are investigated, which include polymer type, compound design, seal interference, temperature, and fuel plasticization of the FKM compounds.

In recent years, fluoroelastomers have been used in automotive fuel systems with increasing regularity. Automotive fuels have become increasingly aggressive on fuel handling polymers. Gasolines blended with alcohols, alcohol by-products, and additives place a broader challenge on fuel handling polymers. Fluoroelastomers, especially high fluorine types, are viewed by many engineers as a method of stabilizing part performance in the face of today's ever-changing fuels. This paper will compare a traditional fluoroelastomer used in automotive fuel systems to new development fluoroelastomers that are designed to help both the end user and the rubber part manufacturer. The testing done will focus on benefits available from the new FKM polymers, such as low swell in fuel/alcohol blends, improved low temperature flexibility, and improved compression set resistance.