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Selecting seal materials can be an intimidating task. There are many types of e

Selecting seal materials can be an intimidating task. There are many types of elastomers and each is available in many different compounds. There are nine popular elastomers used in seals. This selection guide surveys popular elastomers intended for service at pressures up to 1,500 psi. Detailed information on compounds of each elastomer may be found in Parco’s material selection guides. If you believe your application may require a special compound not listed, please contact a Parco customer service representative. Elastomer Selection Criteria 1. Temperature Capabilities Elastomer performance becomes less predictable when a seal operates near the limits of its service temperature range. Consider the effects of temperature extremes when selecting an O-ring material. At low temperatures: • Elastomers become harder and less flexible until, at the brittle point or glass transition, the seal may crack. • Elastomers lose their rubber-like properties as the temperature drops. The TR-10 (temperature of 10% retraction) reflects the ability of an elastomer 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. Find the Right Elastomer for Your Application Ethylene Propylene Nitrile HNBR Polyacrylate Neoprene Fluorocarbon Silicone Aflas 0 100 200 300 400 -50 50 150 450 350 250 -100 Fluorosilicone Fig.1: Service Temperatures of Popular Elastomers Temperature (°F) Compounding affects performance at both high and low temperatures. Not all compounds of a given elastomer have the same temperature range. The temperature limits in the chart span the range of the compounds of each elastomer. • Changes in elastomers due to low temperature are physical, not chemical, and are generally reversible. However, 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, elastomers often undergo irreversible chemical changes. The polymer backbone may break or adjacent polymer molecules may cross- link, causing seals to become more rigid, reducing their resistance to compression set. • The rate of many chemical reactions doubles with each increase of 10°C (18°F). The relationship between reaction rate and temperature of these 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 1,000 hours at the upper rated temperature. An increase in operating temperature of 18°F may 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 represents the fluid compatibility of the principal elastomers from left to right. Very high swell, rapid deterioration or complete breakdown of the seal can occur if the elastomer is not compatible with the fluid. Factors such as chemical concentration, system pressure, operating temperature, and seal design must be considered when specifying a seal. Parco recommends that you evaluate the selected seal in a functional test before using it in production. Acids, dilute Alcohols Alkalis, dilute Brake fluid, non-petroleum Fuel oil Hydraulic oil, phosphate-ester Hydrocarbons, aliphatic Hydrocarbons, aromatic Ketones Mineral oil Solvents, chlorinated Steam, to 300°F Water Hydrochloric acid Methanol, ethanol Sodium hydroxide Wagner 21B®, Dextron® Diesel oils 1-6 Skydrol 500®, Hyjet® Gasoline, kerosene Benzene, toluene Acetone, MEK — Trichloroethylene — — ASTM D1418 Designation NBR FKM EPDM VMR CR ACM FVMR HNBR FEPM Fig.2: Fluid Compatibility by Elastomer Common Fluids Nitrile Fluorocarbon EPDM Silicone Neoprene Polyacrylate Fluorosilicone HNBR Aflas® Examples Legend: Recommended Moderate-to-severe effect Not recommended Minor-to-moderate effect (useful in some static applications only) Because so many applications involve hydrocarbons, a selection method based on the heat and oil resistance of the elastomers will encompass most users. In the ASTM D2000 system, elastomers are ranked by heat resistance (Type) and by oil resistance (Class). Employing the ASTM D2000 Type and Class system, Figure 3 displays the resistance of various elastomers to heat and to IRM 903, a common reference oil. However, compounds of a given elastomer 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 criteria for dynamic seals. Tear-resistant elastomers have superior ability to resist nicking, cutting, and tearing. Good tear resistance may be important in elastomer selection when the seal is to be installed by automated assembly equipment. Elastomers such as hydrogenated nitrile (HNBR) and Aflas are inherently abrasion resistant. Carboxylated nitrile (XNBR) offers significantly better abrasion resistance than standard nitrile. 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 Fig.3: ASTM D2000 Heat and Oil Resistance Designations Swell in IRM 903 Reference Oil (%) Elastomers fall into natural groups according to their heat and oil resistance. Those above the dotted line are recommended for elevated temperatures. Those to the right of the dotted line are preferred for use with hydrocarbons. Oil Resistance (Class) Heat Resistance (Type) Fig.4: Abrasion and Tear Resistance of Medium-Hardness Elastomers Silicone and fluorosilicone elastomers are used for static applications only. The elastomers lying to the right of the oblique line are suitable for either dynamic or static sealing. Abrasion and tear resistance vary with compound hardness. Tear Resistance (lbs/inch of thickness) Abrasion Rate (mg/rev) Aflas HNBR Nitrile EPDM Fluorocarbon Silicone Neoprene Polyacrylate 0.0 0.2 0.1 0.3 0.4 0.5 5 10 20 30 15 25 0.6 Fluorosilicone Dynamic or static Static only Excellent Poor A B C D E F G H K - No Req 170 120 100 80 60 40 20 10 H G F E D C B A Temp (°C⁄°F) 250⁄482 225⁄437 200⁄392 175⁄347 150⁄302 125⁄257 100⁄212 70⁄158 Aflas Fluorocarbon Fluorosilicone Polyacrylate HNBR Nitrile Neoprene EPDM Silicone Excellent Poor cause improperly specified O-rings to extrude, resulting in seal damage and eventual failure. Standard O-ring groove and gap dimensions cited in the MIL-G-5514 and AS4873 generally provide adequate sealing for differential pressures to 1,500 psi for all elastomers. Substantial improvement in extrusion resistance is attainable by 1) using harder O-rings, 2) decreasing the diametral clearance, or 3) using contoured hard rubber or plastic back-up rings. O-rings with high modulus and hardness are better able to resist extrusion. The 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. 5. Price Assuming that several elastomers meet all other requirements for a given application, Figure 7 should aid in making an economical selection. The prices of seals of the same elastomer may vary widely due to differences in compounding and processing costs. Fig.6: O-ring Extrusion from Differential Pressure O-ring extrusion is rare at conditions lying to the left of a seal’s performance line. For example, a 70-durometer seal with 0.005 inch gap (D=0.010) will seal to 800 psi but may extrude at higher differential pressures. For higher operating pressures, consult Parco’s nitrile and fluorocarbon selection guides for high-pressure applications. Total Diametral Clearance, D (in.) Differential Fluid Pressure (psi) 1000 10000 4000 6000 8000 2000 100 200 400 600 800 0 0.020 0.030 0.040 0.010 70-Durometer O-ring 80-Durometer O-ring 90-Durometer O-ring or 70-Durometer O-ring with Back-up rings No Extrusion Extrusion 1/2D Nitrile Neoprene EPDM Silicone Polyacrylate HNBR Fluorocarbon Aflas Fluorosilicone 0 2 4 6 8 10 12 14 16 18 Fig. 7: Relative Prices of Popular Elastomers This chart shows the prices of Parco O-rings made of the most popular compound of each elastomer and is intended to provide a rough estimate of relative price. These prices are based on a comparison of 30 popular sizes of O-rings for each compound. Elastomer Relative Price (Nitrile = 1) Fig.5: Back-up Rings Parco back-up rings serve as anti-extrusion devices. O-ring Back-up Ring Popular Elastomers The elastomers shown in the selection diagram (Figure 8) are the most popular used for O-rings. Variations in mechanical properties and seal performance exist among the compounds of a given elastomer, so price and suitability can vary accordingly. Nitrile Nitrile is the standard to which all the other elastomers are compared. Nitrile compounds are copolymers of acrylonitrile and butadiene. Acrylonitrile provides resistance to petroleum-based fluids such as oils and fuels, while butadiene contributes low- temperature flexibility. Standard nitrile is also known as Buna N rubber. Because they are versatile and inexpensive, nitriles are the most popular industrial seal material. Nitrile compounds provide excellent service with gasoline, crude oil, uploads/Geographie/ elastomer-selection-guide 1 .pdf