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8 8.1 IntroductIon Seal and Gasket Design The importance of the chemist’s role in automotive design is illustrated well by the necessity for interaction between the mechanical and chemical fields in sealing and gasket design. The chemist’s knowledge of material and behavior under thermal and mechanical loads plays a key role in the design of these compounds. Maximum service use temperatures, compression set, flexural modulus, tear strength, dielectric properties, chemical resistance, ozone resistance, etc. are just some of the properties that must be considered by the material engineer or automotive chemist when designing a sealing system. Obviously, compression, tear, and service use temperatures have a higher weighted importance in seal and gasket applications. Seals and gaskets prevent fluid transfer between different regions of an operating system. Key in choosing a material is the environment in which the seal will have to function. A chemist will typically have to ask questions such as: What temperature extremes will the seal or gasket experience? What continuous use temperature will be required? How much relative movement will be between the two sealing surfaces? Will the seal or gasket be exposed to chemicals, ozone, weather factors, etc.? If exposure to chemicals occurs, what functional groups will interact with the seal? What are the compression set and the performance expectations? After answering these questions about the environment and performance requirements, the chemist can examine data from the various databases of material choices and make an informed decision. A successful seal design will ensure adequate compressive force while minimizing stresses acting upon the seal or gasket. 8.2 tear strengtH One of the major considerations that must be taken into account is the tear strength of the material. Shear forces are often applied by the two surfaces being sealed (Figure 8.1). In this situation, the automobile chemist must take into account the forces being applied and the temperature at which the shear is occurring. Figure 8.2 shows a diagram of tear versus temperature for some common elastomer compounds used in the automobile industry. The slope of the diagram should be considered if one is operating on a wider range of service temperatures. For instance, if an 111
112 The Role of the Chemist in Automotive Design Tear Strength in N/mm 2 25 20 15 10 Butyl rubber Compressed O-ring Seal in Shear FIgure 8.1 Diagram of compressed O-ring in shear. Natural rubber Nitrile rubber Polyethylene rubber 0 0 50 100 150 200 250 Temperature in °C application approaches 150°C in continuous use and tear is a factor, then it would be preferable to shy away from natural rubber, nitrile, or PE rubber as a choice for the seal. 8.3 tHermal servIceabIlIty range Tear strength of elastomers (post-heat aging) Fluoro rubber Silicone rubber FIgure 8.2 Tear strength of elastomers. (From Tomanek, A. Silicones and Industry, Wacker-Chemie GmBH, Munich, 1984. With permission.) The thermal serviceability or usage range is the temperature range in which a sealing material can be used and expected to function without degradation of properties. Table 8.1 shows the ranges of some compounds typically used in automotive sealing.
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THE ROLE OF THE CHEMIST IN AUTOMOTI
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CRC Press Taylor & Francis Group 60
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Contents Preface...................
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Contents ix 6.6 Thermal Properties
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Contents xi 11.4 Glass Microspheres
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The Author Herman Phlegm was born i
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2 The Role of the Chemist in Automo
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10 The Role of the Chemist in Autom
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3 3.1 IntroductIon Component Materi
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Component Materials in Automobiles
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Page 130 and 131: Seal and Gasket Design 113 table 8.
Page 132 and 133: Seal and Gasket Design 115 The main
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190 Index Carbon dioxide emissions,
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192 Index heat exchanger designatio
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194 Index Power train, 95 Power tra
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