Elastomer Selection Guide
The most mportan t attributes of elastomeric seal materials are 1) temperature capabilities, 2) fluid compatibilIty 3) abrasion and tear resistance, 4) differential pressure resistance, and 5 price. Tb is brocli u re contains brief discussions of these five a nfl bu tes fol!ou ed by descriptions of Pa rcoc principal seal materials. A selection diagram at the end of the brochure will assist von in choosing the appropriate elastomer for you r sealing application.
Selecting sea] materials can be an intimidating task: there are many types of clastorners and each type is available in many different compounds.Fortunately, there are ten popular elastomers used in seals, and several cornpounds of each elastomer account for the bulk of all applications. This selection guide surveys popular elastomers intended for service at pressures to 1500 psi. Detailed information on compounds of each elastorner may be found in the Parco selection guides listed on the last page of this brochure. If, after reading these application may require a special compound not listed, contact your Parco distributor for further assistance. guides, you feel your
Elastomer Selection Criteria
1 — Temperature Capabilities
Elastomer performance becomes less predictable when a seal operates near the limits of its service ternperature range. Consider the effects of temperature extremes when selecting an 0-ring material.
At low temperatures:
• Elastomers become harder and less flexible until, at the brittle point, the seal may crack if struck.
• Elastorners lose their rubber-like properties as the temperature drops. The TR-10 (temperature of 10% retraction) reflects the ability of an elastorner to retract, that is, behave like rubber, at low temperatures.
• Fluid may penetrate the seal and act as a plasticizer, effectively lowering the brittle point below the value observed in dry air. In such cases, the seal may operate effectively below its rated service temperature. This must be confirmed on a case-by-case basis.
• Changes in elastomers due to low temperatures are physical, not chemical, and are generally reversible. 1-lowever, if the geometry of the gland changes while the seal is cold, the seal may be too stiff to adapt to the new shape and may fail. Movement may damage the seal while it is cold and inflexible.
At high temperatures:
• As temperatures approach the upper service limit, elastorners often undergo irreversible chemical changes: the polymer l)ackbone may break or adjacent polymer molecules may cross-link, causing seals to become more rigid and reducing their resistance to compression set.
• The rate of many chemical reactions doubles with each increase of 10°C (18F). The relationship between reaction rate and temperature of these so-called first- order reactions can be used as a rough guide in predicting the service life of a material. Figure 1 assumes a service life of 1000 hours at the upper rated temperature. An increase in operating temperature of 18°F may be expected to cut seal life in half. The added cost of a seal with a wider service range may be an excellent investment.
2— Fluid compatibility
Figure 2 lists a number of common fluids and general compatibility ratings for the most popular elastomers. Very high swell, rapid deterioration or complete breakdown of the seal can OCCU if the elastomer is not compatible with the fluid. Factors such as chemical concentration, system pressure, operating temperature and seal design must be considered concurrently when specifying a seal. Parco recommends that you evaluate the selected seal in a functional test before using it in production.
Because so many applications involve hydrocarbons, a selection method based on the heat and oil resistance of the elastomers will encompass most uses. In the ASTM DZ000 system, elastomers are ranked by heat resistance (Type) and by oil resistance (ClassL Employing the ASTM D2000 1ipe and Class system, Figure 3 displays the resistance of various elastoniers to heat and to IRM 903, a common reference oil that has replaced ASTM Oil No. 3. However, compounds of a given elastonier can have different rankings for both Type and Class. The selection diagram on the last page also uses heat resistance and hydrocarbon compatibility as principal elastomer selection criteria.
3— Abrasion and tear resistance
Abrasion-resistant seals are able to resist scraping or buffing. Abrasion resistance is generally a selection criterion for only dynamic seals.
Tear-resistant elastomers have superior ability to resist nicking, cutting and tearing. Good tear resistance may be important in elastorner selection when the seal is to be installed by automated assembly equipment.
Elastomers such as hydrogenated nitrile (FINBR), Atlas and hutyl are inherently abrasion resistant. Carboxvlated nitrile (XNBR) offers significantly better abrasion resistance than standard nit rile. The abrasion and tear resistance of many elastomers can be enhanced by compounding with internal lubricants such as Teflon or molybdenum disulfide.
4— Differential pressure resistance
Pressure applied evenly to both sides of a seal normally has no effect on sealing performance. When a pressure difference is anticipated, elastomer selection must also consider differential pressure resistance. High differential pressures will cause improperly specified 0-rings to extrude, resulting in seal damage and eventual failure.
Standard 0-ring groove and gap dimensions cited in the Parco 0-Ring Design Handbook and in MIL-G-5514 and AS4873 generally provide adequate sealing for differential pressures to 1500 psi for all elastorners.
Substantial improvement in extrusion resistance is attainable by 1) using harder 0-rings, 2) decreasing the diatnetral clearance, or 3) using contoured hard rubber or plastic back-up rings. 0-rings with high modulus and hardness are better able to resist extrusion. 1 he higher the modulus of a material, the greater the force required to stretch it. Similarly, the harder the material, the greater its resistance to indentation.
Assuming that several elastorners meet all other requirements for a given application, Figure 7 should aid in making a economical selection. The prices of seals of the same elastomer may vary widely due to differences in compounding and processing costs.
resistance to weathering and petroleum-based lubricants, a wide temperature range and exceptional economy.
Neoprenes have good abrasion and tear resistance and are suitable for use in heating, ventilating and air conditioning (HVAC) systems, refrigeration units and numerous dynamic applications.
Individual neoprene compounds have service temperatures within the range from -65 to +2l2°l including certain compounds formulated for lower temperatures. Parco’s most popular neoprene is 3110-70, which is FDA conforming, pO IXACRYLATE, also known as polyacrylic rubber, combines excellent resistance to hydrocarbon fuels with near imperviousness to ozone, IJV light and other forms of weathering. Polyacrylates have an upper service temperature similar to fluorosilicones at a much lower cost. important applications include automatic transmission seals and power steering assembly seals used with Type A fluid.
Polyacrylate compounds have service tern peratures from -20 to +350°F. Parco’s most popular polyacrvlate compound is 2930-70. FLUOROS ILl CONE has an inorganic silicon- oxygen polymer backbone like silicone, while incorporating fluorine-rich polar groups that provide resistance to non-polar fluids such as hydrocarbon fuels. While siicones have ASTM D2000 or ‘E’ fuel resistance designations, fluorosilicones
are classified as ‘K the highest level of fuel resistance (Figure 3). The heat resistance of the fluorosilicones is slightly below that of the silicones.
Fluorosilicones share the outstanding ozone, sunlight and weathering resistance of the silicones. They find their widest use in aggressive military/aerospace and automotive environments involving exposure to fuels over wide temperature ranges. They are not recommended for dynamic sealing due to poor abrasion resistance.
Fluorosilicone compounds have service temperatures from -80 to ÷350°l Parco’s most popular fiuorosilicone compound is 1903-70. YDROGENATED NITRII.E (HNBR), like conventional nitrile, is made from acrionitrile and butadiene monomers. After polymerization, a carbon-carbon double bond from the butadilene molecule is still present in the backbone of the nitrile polymer. These regions of unsaturation make the base polymer susceptible to uncontrolled cross-
linking by heat, ozone, hydrogen sulfide, sour crude and other oxidizing agents. Degradation of ordinary nitriles includes increased hardness, loss in elongation and tensile strength, and surface cracking. The weak link in the nitrile polymer can be eliminated by saturating (reacting with hydrogen) the remaining carbon-carbon double bond. Hydrogenated nitriles significantly outperform conventional nitriles in resisting heat and sour crude oil. FINBB compounds have a service range of -40 to +325°E They are recommended when upgrading from nitriles or as an economical alternative to more expensive fluorocarbon elasto mers. Parco’s most pop - ular hydrogenated nitrile compound is 2269-70. FLAS is a trade name for tetrafluoroethylene propylene
copolyrner. Aflas compounds have almost universal resistance to both acids and bases, steam, acid gases, crude oil and many types of corrosion inhibitors. Serviceability extends to 4O0F br long-term exposure or 550°F for brief exposures. With combined resistance to corrosion inhibitors and heat, Alias seals are able to resist the extremes of heat and pressure present in aggressive dowthole oil well environments. Aflas seals have very low rates of gas permeation and are highly resistant to explosive decompression, making them excellent choices for downhole packing elements.
Atlas compounds have service temperatures from -10 to ÷400°E Parco’s most popular Allas compound is 7115-75.
B(JTYL possesses the low gas permeability needed for vacuum or low-pressure applications where seals must prevent intrusion of air and other Contaminants. Butyl (chiorobuivi) compounds typically exhibit gas permeation rates much lower than the more popular nitrile and fluorocarbon elastomers. Soft butyl seals (hardness, Shore A less than 70) are especially effective at low pressures as they conform readily to minute irregularities in the groove to produce a more positive seal. Butyl rubber has few unsaturated double bonds, giving it good aging and weathering properties. Butyl compounds have service temperatures from -65 to +212°F. Parco’s most popular butyl is compound 0823-70.