ERIKS nv - O-ring Technical Handbook

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Sealing ElementsTechnical Handbook O-rings5. Designing with RubberAttack mechanisms: Chemical Compatibility• The process of chemical degradation or chemical incompatibilityis very complex. In general, degradation of the polymerbackbone and cross-link may occur by means of:• nucleophilic attack - nucleophiles are ions or molecules thatcan donate electrons. This is the main cross-linking mechanism.In certain chemical media, nucleophilic attack canresult in increased cross-linking and embrittlement.• dehydrofluorination - in fluorocarbon elastomers the attack ofaliphatic amines can result in the formation of unsaturatedbonds in the polymer backbone.• polar attack - swelling caused by electrostatic interactionsbetween the dipole and polymer chainNucleophilic Attack(Unsaturated)C = CPolar AttackH 2OOxidationO -Degradation may also occur due to interactions of the chemicalenvironment and elastomer filler systems. This type of degradationmay be caused by oxidation of fillers, or by chemical attackof certain fillers or process aids.Chemical Attack MechanismsIn many applications special considerations should be made forcontamination or vacuum performance. Contamination is criticalin semiconductor fabrication and medical applications. Thismay take the form of particle generation, extractable ions or otherresidual gas contamination.Test methods:ISO 1817 (Liquids)ASTM D471, D1460, D3137 (Liquids)Volume Swell:The most common measure of chemical compatibility is volumeswell. The following formula is used in reporting volume swellmeasurements. This takes into account dimensional changes inall three dimensions, and is more precise than specific dimensionalchange readings for most sealingapplications.Volume Swell:(Weight in Air-Wt. in Water) final- (Wt. in Air - Wt. in Water) initialVS (%) =x100(Weight in Air - Weight in Water) initialNote:The "Weight in Water" measurement is performed by suspendinga sample in a container of water and recording it's weight. Thistakes into consideration that the density of a solid is equal to it'sweight in air divided by the difference of it's weight in air and it'sweight in water.29All the information in this documentation has been compiled with the greatest of care.Despite this we can bear no responsibility whatsoever for any errors present in the documentation. The recommendations are intended as guidelines.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberThermal EffectsAll rubber is subjected to deterioration at high temperature.Volume change and compression set are both influenced byheat. Hardness is influenced in a complex way. The first effectof high temperature is to soften the compound. This is a physicalchange, and will reverse when the temperature drops. Inhigh pressure applications the O-ring may begin to flow throughthe clearance gap as the temperature rises, due to this softeningeffect. With increasing time at high temperature, chemicalchanges occur. These generally cause an increase in hardness,along with volume and compression set changes. Changes intensile strength and elongation are also involved. Being chemicalin nature, these changes are not reversible.The changes induced by low temperature are primarily physicaland reversible. An elastomer will almost completely regain itsoriginal properties when warmed.Thermal ExpansionCoefficient of linear thermal expansion is the ratio of the changein length per °F or °C to the original length at 0°F or 0°C.Coefficient of volumetric expansion for solids is approximately3 times the linear coefficient. As a rough approximation, elastomershave a coefficient of thermal expansion 10-times that ofsteel. With Fluoroelastomers and Perfluoroelastomers the coefficientof thermal expansion is even greater. This can be a criticalfactor at high temperature if the gland is nearly filled with theO-ring or at low temperature if the squeeze is marginal. Leakingcan be the result of seal failure at low temperature if the squeezeis small.There are certain reactions which in some circumstances causean O-ring to exert relatively high forces against the sides of thegroove. If the seal is completely confined and the gland is 100%filled, the dominating force is the force of thermal expansion ofthe rubber. The groove must always be sufficiently wide to allowfor the maximum expansion of the O-ring. There have beeninstances where a seal has ruptured a steel gland due to expansionwhen heated. Therefore it has to be considered that in nocase a gland fill in excess of 95% is allowed.This should be taken into consideration when designing O-ringgrooves for applications in excess of 300°F (150°C). Please contactyour ERIKS representative for assistance in groove design.Thermal ExpansionMaterial Thermal Stability x10 -5 / °CFKM 200°C / 392°F 16NBR 120°C / 250°F 23VMQ 230°C / 450°F 59-79FFKM 300°C / 570°F 23EPDM 150°C / 300°F 16Stainless - 1.04Aluminium - 1.3TEFLON 230°C / 450°F 5-8KEL-F 280°C / 540°F 4-7Polyimide 275°C / 530°F 530All the information in this documentation has been compiled with the greatest of care.Despite this we can bear no responsibility whatsoever for any errors present in the documentation. The recommendations are intended as guidelines.www.eriks.info
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ERIKS nv - O-ring Technical Handbook

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