When selecting the best elastomer for a Sealing application, an engineer or designer must be prepared to answer a series of questions about the physical, mechanical and chemical resistance properties required for the seal. Unless the material specifications have been pre-designated, gathering this information about the material’s end use and the functional requirement of the seal in the application will help to ensure the material sustains its physical properties through temperature, environmental factors and time.
The best place to start gathering information is by taking a close look at what the application will require from the compound. For replacement products, examine the properties and service conditions of the original material. This information can provide the foundation for what properties are desired, and what are not, in the new material. For new products, the properties of materials used in similar applications can provide valuable information.
The following checklist provides several characteristics that need to be taken into consideration when specifying a material.
- Tensile strength and elongation*
- Hardness range (dense), or compression deflection requirements (sponge)
- Compression set at room and operating temperatures
- UV, ozone, heat, storage requirements
- Color (if color matching is desired or necessary)
- Speed of recovery from deflection
- Permeability to gases
Chemical Resistance Properties:
- Resistance to acids or caustics
- Hydrocarbon solvents
- Oxygenated solvents
- Flame or temperature extremes
Listed on the next page are brief summaries of the properties of seven of the most versatile polymers, as well as physical and chemical resistance comparison charts. These lists are by no means all-inclusive. However, the majority of sealing solution needs can be met by using these popular materials. For a complete list of polymers, including specialty and high-tech materials, please contact a KASTRADE sales representative.
Silicone (Polysiloxane) features excellent resistance to ozone, sunlight and oxidation, and is very color stable. It maintains excellent flexibility at low temperatures, has outstanding resistance to high heat, low compression set and is a very good electrical insulator. However, it is not recommended for applications that require abrasion, tear and cut growth resistance, or high tensile strength. It is also not recommended for resistance to oil, gasoline, solvents, alkalis and acids.
EPDM (Ethylene-Propylene-Diene-Monomer) is found in a wide range of applications and is excellent for outdoor use. It is well known for its excellent ozone, weathering and aging resistance. It also has excellent water and steam resistance, maintains its flexibility at low temperatures, features excellent resistance to alkalis, acids and oxygenated solvents, and is very color stable. However, it is not recommended for resistance to oil, gasoline and hydrocarbon solvents.
Neoprene (Polychloroprene) is generally considered an excellent all-purpose elastomer with a solid balance of properties and few limitations. The polymer has moderate resistance to oils and gasoline. It features good flame resistance, weathers well and has very good resistance to brasion, flex cracking,alkalis and acids. However, neoprene provides poor resistance to aromatic and oxygenated solvents and has limited flexibility at low temperatures.
Nitrile (Acrylonitrile-Butadiene) has very goodoil, gasoline and abrasion resistance. Resistance to alkalis and acids increases as the nitrile content of the compound increases. Nitrile must be specially compounded for resistance to ozone, sunlight and natural aging. It has poor resistance to oxygenated solvents. Nitrile is superior to neoprene in oil and solvent resistance, but is not recommended for applications where it is exposed to severe weathering.
Fluoroelastomers (Viton) provide premium performance and long-term reliability in very harsh and corrosive environments. Their exceptional resistance to heat, aging and a broad range of fuels, solvents and chemicals makes them ideal for many demanding aerospace, automotive and industrial applications. Fluoroelastomers also offer excellent resistance to weather, ozone, oxygen and sunlight and are inherently more flame retardant than hydrocarbon rubbers. Fluoroelastomers have poor resistance to ketones, esters, ethers, amines and aqueous bases (i.e., ammonia and sodium hydroxide).
SBR (Styrene-Butadiene-Rubber) has excellent impact strength, good resilience, tensile strength, abrasion resistance and maintains its flexibility at lower temperatures. However, it is not the polymer of choice where resistance to oil, gasoline and hydrocarbon solvents is required. SBR materials are also not suitable for exposure to the outside elements, such as UV and ozone. Typical uses of SBR have been in tire treads, conveyor belt covers, mats and even shoe soles.
TPE (Thermoplastic Elastomers) are materials that have comparable properties and performance to their rubber counterparts but are processed like other thermoplastic materials. TPE is a collective name for several differentfamilies of elastomers that may contain differences in composition and molecular structure. Their performance properties are similar to conventional thermosets, such as natural rubber, SBR, EPDM or neoprene. The important features of TPEs are their flame and ozone resistance, excellent flex fatigue, and performance in alcohol. TPEs, however, are limited by their poor resistance to aliphatic and aromatic hydrocarbon solvents as well as oil and gasoline.