Find the Right Elastomer for Your Application - …

1 Selecting seal materials can be an intimidatingtask. There are many types of elastomers and eachis available in many different compounds. Thereare nine popular elastomers used in seals. Thisselection guide surveys popular elastomersintended for service at pressures up to 1,500 information on compounds of eachelastomer may be found in Parco s materialselection guides. If you believe your applicationmay require a special compound not listed, pleasecontact a Parco customer service representative. Elastomer Selection Criteria1. Temperature CapabilitiesElastomer performance becomes less predictablewhen a seal operates near the limits of its servicetemperature range. Consider the effects oftemperature extremes when selecting an low temperatures: elastomers become harder and less flexibleuntil, at the brittle point or glass transition, theseal may crack. elastomers lose their rubber-like properties asthe temperature drops.

2 The TR-10 (temperatureof 10% retraction) reflects the ability of anelastomer to retract, that is, behave like rubber,at low temperatures. Fluid may penetrate the seal and act as aplasticizer, effectively lowering the brittle pointbelow the value observed in dry air. In such cases,the seal may operate effectively below its ratedservice temperature. This must be confirmed on acase-by-case the Right Elastomer for your Application Ethylene Propylene Nitrile HNBR Polyacrylate Neoprene Fluorocarbon Silicone Aflas 0100200300400-5050150450350250-100 Fluorosilicone Fig. 1:Service Temperatures of Popular ElastomersTemperature ( F)Compounding affects performance at both high and lowtemperatures. Not all compounds of a given elastomerhave the same temperature range. The temperature limits in the chart span the range of the compounds ofeach Elastomer . Changes in elastomers due to low temperatureare physical, not chemical, and are generallyreversible.

3 However, if the geometry of the glandchanges while the seal is cold, the seal may be toostiff to adapt to the new shape and may may damage the seal while it is coldand high temperatures: As temperatures approach the upper servicelimit, elastomers often undergo irreversiblechemical changes. The polymer backbone maybreak or adjacent polymer molecules may cross-link, causing seals to become more rigid, reducingtheir resistance to compression set. The rate of many chemical reactions doubleswith each increase of 10 C (18 F). The relationshipbetween reaction rate and temperature of thesefirst-order reactions can be used as a rough guidein predicting the service life of a material. Figure 1assumes a service life of 1,000 hours at the upperrated temperature. An increase in operatingtemperature of 18 F may to cut seal life in added cost of a seal with a wider servicerange may be an excellent Fluid CompatibilityFigure 2 represents the fluid compatibility of theprincipal elastomers from left to Right .

4 Very highswell, rapid deterioration or complete breakdownof the seal can occur if the Elastomer is notcompatible with the fluid. Factors such as chemicalconcentration, system pressure, operatingtemperature, and seal design must be consideredwhen specifying a seal. Parco recommends thatyou evaluate the selected seal in a functional testbefore using it in , diluteAlcoholsAlkalis, diluteBrake fluid, non-petroleumFuel oilHydraulic oil, phosphate-esterHydrocarbons, aliphaticHydrocarbons, aromaticKetonesMineral oilSolvents, chlorinatedSteam, to 300 FWaterHydrochloric acidMethanol, ethanolSodium hydroxideWagner 21B , Dextron Diesel oils 1-6 Skydrol 500 , Hyjet Gasoline, keroseneBenzene, tolueneAcetone, MEK Tr ichloroethylene ASTM D1418 DesignationNBRFKM EPDMVMRCRACM FVMR HNBRFEPMFig. 2:Fluid Compatibility by ElastomerCommon FluidsNitrileFluorocarbonEPDMS iliconeNeoprenePolyacrylateFluorosilicon eHNBRA flas ExamplesLegend:RecommendedModerate-to-se vere effectNot recommendedMinor-to-moderate effect (useful in some static applications only)Because so many applications involvehydrocarbons, a selection method based on theheat and oil resistance of the elastomers willencompass most users.

5 In the ASTM D2000 system, elastomers are ranked by heat resistance (Type)and by oil resistance (Class). Employing the ASTMD2000 Type and Class system, Figure 3 displays theresistance of various elastomers to heat and toIRM 903, a common reference oil. However,compounds of a given Elastomer can havedifferent rankings for both Type and Class. Theselection diagram on the last page also uses heatresistance and hydrocarbon compatibility asprincipal Elastomer selection Abrasion and Tear ResistanceAbrasion-resistant seals are able to resist scrapingor buffing. Abrasion resistance is generally aselection criteria for dynamic seals. Tear-resistantelastomers have superior ability to resist nicking,cutting, and tearing. Good tear resistance may beimportant in Elastomer selection when the seal is tobe installed by automated assembly such as hydrogenated nitrile (HNBR) andAflas are inherently abrasion nitrile (XNBR) offers significantlybetter abrasion resistance than standard nitrile.

6 Theabrasion and tear resistance of many elastomerscan be enhanced by compounding with internallubricants such as Teflon or Differential Pressure ResistancePressure applied evenly to both sides of a sealnormally has no effect on sealing a pressure difference is anticipated, Elastomer selection must also consider differentialpressure resistance. High differential pressures willFig. 3:ASTM D2000 Heat and Oil Resistance DesignationsSwell in IRM 903 Reference Oil (%) elastomers fall into natural groups according to theirheat and oil resistance. Those above the dotted line arerecommended for elevated temperatures. Those to theright of the dotted line are preferred for use with Resistance (Class)Heat Resistance (Type)Fig. 4:Abrasion and Tear Resistance ofMedium-Hardness ElastomersSilicone and fluorosilicone elastomers are used for staticapplications only. The elastomers lying to the Right of theoblique line are suitable for either dynamic or static sealing.

7 Abrasion and tear resistance vary with Resistance (lbs/inch of thickness)Abrasion Rate (mg/rev) or staticStatic onlyExcellentPoorA BCDEFG HK-No Req 1701201008060402010 HGFEDCBATemp( C F)250 482225 437200 392175 347150 302125 257100 21270 158 AflasFluorocarbonFluorosiliconePolyacryl ateHNBRN itrileNeopreneEPDMS iliconeExcellentPoorcause improperly specified O-rings to extrude,resulting in seal damage and eventual O-ring groove and gap dimensions citedin the MIL-G-5514 and AS4873 generally provideadequate sealing for differential pressures to 1,500psi for all elastomers . Substantial improvement inextrusion resistance is attainable by 1) using harderO-rings, 2) decreasing the diametral clearance, or 3)using contoured hard rubber or plastic back-uprings. O-rings with high modulus and hardness arebetter able to resist extrusion. The higher themodulus of a material, the greater the forcerequired to stretch it.

8 Similarly, the harder thematerial, the greater its resistance to PriceAssuming that several elastomers meet all otherrequirements for a given Application , Figure 7should aid in making an economical selection. Theprices of seals of the same Elastomer may varywidely due to differences in compounding andprocessing 6:O-ring Extrusion from Differential PressureO-ring extrusion is rare at conditions lying to the left of aseal s performance line. For example, a 70-durometer sealwith inch gap (D= ) will seal to 800 psi but mayextrude at higher differential pressures. For higher operating pressures, consult Parco s nitrile and fluorocarbonselection guides for high-pressure Diametral Clearance, D (in.)Differential Fluid Pressure (psi) O-ring80-Durometer O-ring90-Durometer O-ring or70-Durometer O-ring with Back-up rings No ExtrusionExtrusion1/2 DNitrileNeopreneEPDMS iliconePolyacrylateHNBRF luorocarbonAflasFluorosilicone0246810121 41618 Fig.

9 7:Relative Prices of Popular ElastomersThis chart shows the prices of Parco O-rings made of themost popular compound of each Elastomer and is intended to provide a rough estimate of relative prices are based on a comparison of 30 popularsizes of O-rings for each Price (Nitrile = 1)Fig. 5:Back-up RingsParco back-up rings serve as anti-extrusion ElastomersThe elastomers shown in the selection diagram(Figure 8) are the most popular used for in mechanical properties and seal performance exist among the compounds of agiven Elastomer , so price and suitability can is the standard towhich all the otherelastomersare compared. Nitrilecompounds are copolymers ofacrylonitrile and butadiene. Acrylonitrile providesresistance to petroleum-based fluids such as oilsand fuels, while butadiene contributes low-temperature flexibility. Standard nitrile is alsoknown as Buna N rubber. Because they areversatile and inexpensive, nitriles are the mostpopular industrial seal compounds provide excellent service withgasoline, crude oil, power steering fluid, hexane,toluene, water, water-based hydraulic fluids, anddilute bases such as sodium hydroxide.

10 Becausenitriles contain unsaturated carbon-carbon bondsin the base polymer, they are not suitable forexposure to ozone, sunlight, and than 50% of sealing needs can be met nitrile compounds have servicetemperatures within the range from -65 to +250 F,including certain compounds formulated for lowertemperatures. Parco s most popular nitrilecompound is elastomerscommand a substantial shareof the seal withstand avery broad spectrum of chemicals over atemperature range second only to that of are commonly rated for continuousservice temperatures from -20 to +400 F, withintermittent exposures as high as 500 spite of their higher cost, fluorocarbons havereplaced nitriles in many applications because oftheir superior resistance to compression set, high-temperature, and a wide range of compounds have servicetemperatures from -20 to +400 F. Parco s mostpopular fluorocarbon compound is 9009-75.