The basic material of elastomeric compounds is caoutchouc and produced either as natural rubber on plantations or manu- factured by the chemical industry. Today, a total of 32 synthetic caoutchoucs are known, the most important ones being listed in Table 5.1. For more details see Section 5.2. Elastomeric compounds consist of 50 to 60% caoutchouc and often are described simply as “rubber”. The remainder is made up of fillers, vulcanising agents, accelerators, ageing retardants and other additives permitting modification of the properties of the raw material to meet the particular requirements of a specific particular application. Elastomers used as seals, and particularly those used in O-rings, guarantee leak-free function if the following design requirements are observed:
Correct elastomer selection. To obtain the necessary chemi- cal and thermal compatibility it is necessary to select the correct elastomer Section 5.2 provides further information in a general form. For detailed information see our Medium Compatibility Table (Order no. 5703 E) which is available free of charge and contains information on elastomer compatibility with more than a thousand fluids and gases.
Compounds designed for optimum performance with constant properties. A recipe designed for optimum performance never will be changed, not even in its smallest detail. Minimum de- viation from a recipe could cause serious degrading of prop- erties of an elastomer so that it is unsuitable for a technical application, e.g. change in hardness, tensile strength, elonga- tion, wear resistance and not least compression set .
To avoid fluctuations in material quality, the Parker Seal Group have created CBI (Controlled Batch Identification). CBI assures that each material mix receives a serial number (batch number) which is unique. Each batch is tested in the laboratory and approved for production only when test results prove positive. Test results (i.e. values for hardness, specific gravity, tensile strength and elongation at break) are recorded. The batch number is found on every box of O-rings. In this way, a Parker O-ring can be traced back to its origin even after many years.
Regulation of temperature during vulcanization. Connections or “cross-links” between the polymer chains are formed dur- ing the vulcanisation process (fig. 5.1). These connections change the caoutchouc from a plastic-like material to an elas- tic material. After vulcanisation an elastomer has all the prop- erties which make for a good sealing material, i.e. resilience (compare with compression set, see Section 6.5).
Acryl-Nitrile-Butadiene Rubber (NBR)
Trade names: Perbunan® Bayer AG Chemigum® Goodyear Europrene N® Enichem Nipol N® Nippon Zeon
Nitrile rubber (NBR) is the general term for acryl-nitrile butadiene mixed polymer. The acryl-nitrile content varies in technical prod- ucts (18 to 50%) and influences the properties of the elastomer. The higher the acryl-nitrile content the better the resistance to oil and fuel. At the same time, the elasticity and the compression set is adversely affected (fig. 5.2). Often a compromise is drawn and a medium Acryl-Nitrile content is selected.
NBR has good mechanical properties when compared with other elastomers and a high wear resistance. NBR is not resistant to weathering and ozone. Heat resistance: up to +100°C, shorter life +120°C (at increased temperatures the speed of ageing accelerates, ageing in oil oc- curs at slower rate than in hot air). Cold flexibility: according to recipe between -20 °C and -55 °C Chemical resistance to:
aliphatic hydrocarbons (propane, butane, petroleum oil,
mineral oil and grease, Diesel fuel, fuel oils)
Butadiene Rubber (BR) Trade names: Buna CB® Bayer AG Europrene (Neocis)® Enichem
Polybutadiene rubber (BR) is mostly used in combination with other rubbers to improve cold flexibility and wear resistance. BR is primarily used in the tire industry, for drive belts and conveyor belts and does not suit use as a sealing compound.
Butyl Rubber (IIR) Trade names: Polysar Butyl® Bayer AG Exxon Butyl® Exxon Chem. Co. Butyl rubber (isobutylene, isoprene rubber, IIR) is produced by many companies in different types and differs widely in isoprene content. Isoprene is used for vulcanization. Butyl has a low per- meability rate and good electrical properties. Heat resistance: up to appr. 130°C Cold flexibility: down to appr. -40°C Chemical resistance to:
Not compatible with:
Chlorobutyl Rubber (CIIR) Trade name: Exxon Butyl® Exxon Chem.
Chlorobutyl rubber (CIIR) is produced by chlorinating butyl rub- ber. Its chloride content attains appr. 1.1% to 1.3%. Apart from the properties of butyl rubber (IIR), chlorobutyl rubber (CIIR) shows improved compression set properties and can be com- pounded with other rubbers.
Chloroprene Rubber (CR) Trade names:
Neoprene®
Du Pont
Baypren®
Bayer AG
Chloroprene was the first synthetic rubber and shows in general good ozone, ageing and chemical resistance, and good me- chanical properties over a wide temperature range. Heat resistance: up to appr. 100°C (120°C) Cold flexibility:
down to appr. -40°C
Chemical resistance to:
Chlorosulfonyl Polyethylene Rubber (CSM) Trade name: Hypalon® Du Pont
The ethylene monomer contains additional chlorine and sulphur groups. Chlorine gives the vulcanisate resistance to flame and mineral oil and also improves the cold flexibility.
Heat resistance: up to 120°C Cold flexibility: down to appr. -30°C
Limited compatibility:
Epichlorhydrin Rubber (CO, ECO) Trade name: Hydrin® Nippon Zeon
Epichlorhydrin is available in two types: the homopolymer (CO) and the copolymer (ECO). CO and ECO have both a good resist- ance to mineral oils, fuels and ozone. The high temperature re- sistance is good. Compression set and the tendency to corrosion of the sealing face increase at +150 °C. ECO has a good cold flexibility. CO has a high resistance to gas permeability. Heat resistance: up to appr. 135°C Cold flexibility: down to appr. -40°C Chemical resistance to:
Not compatible with:
Ethylene Acrylate Rubber (AEM) Trade name: Vamac® Du Pont Ethylene acrylate rubber is a mixed polymer of ethylene and me- thyl acrylate with the addition of a small amount of carboxylated curing monomer. Ethylene acrylate rubber is not to be confused with ethyl acrylate rubber (ACM). Heat resistance: up to 150°C (shorter life up to 175°C) Cold flexibility: according to application between -30°C and -40°C Ethylene acrylate rubber has a high resistance to ozone and oxygen attack. The compatibility with mineral oil is not as good as with ACM compounds. Ethylene acrylate swells in ASTM oil N°1 by appr. 5% to 10%, and in ASTM oil N°3 by appr. 45% to 55%. Ethylene acrylate rubber is not compatible with ketones, fuels and brake fluids. Ethylene Propylene Rubber (EPM, EPDM) (earlier abbreviations: APK, APTK, EPR)
Trade names:
EPM is a rubber manufactured as a copolymer of ethylene and propylene. Ethylene-propylene-diene rubber (EPDM) is pro- duced using a third monomer and is particularly useful to seal phosphate-ester hydraulic fluids and in brake systems which use fluid with glycol base. Heat resistance: up to 150°C (max. 180°C in water and water steam) Cold flexibility: down to appr. -50°C
Chemical resistance to:
Fluorocarbon Rubber (FKM) Trade names: Viton®
Dow Du Pont Elastomers Fluorel® Dyneon
Tecnoflon® Ausimont Dai-el® Daikon
Fluorocarbon rubber is the most significant compound develop- ment in sealing materials to come out of the 1950´s and is noted for its wide range of applications. FKM has an excellent resist- ance to high temperatures, ozone, oxygen, mineral oil, synthetic hydraulic fluids, fuels, aromatics and many organic solvents and chemicals. The low temperature resistance is not favourable and lies for static application at appr -25°C (in certain applications freedom from leakage has been achieved down to -40°C). Under dynamic conditions the lowest temperature expected is between -15°C and -20°C. The gas permeability is very low and similar to butyl rubber. Special FPM compounds have a higher resistance to acids, fuels, water and steam. Heat resistance: up to 200°C and higher temperatures with shorter lifetime
Cold flexibility: down to -25°C (partially -40°C)
Chemical resistance to:
Not compatible with:
Fluorosilicone Rubber (FVMQ) Trade name: Silastic® Dow Corning FVMQ contains trifluoropropyl groups next to the methyl groups. The mechanical and physical properties are very similar to VMQ. However, FVMQ offers an improved fuel and mineral oil resist- ance but poor hot air resistance when compared with VMQ.
Heat resistance: up to 175°C (200°C max.)
Cold flexibility: down to appr. -55°C
Chemical resistance to: same as for MVQ, additionally compat- ible with
Hydrogenated Nitrile Butadiene Rubber (HNBR) Trade names:
HNBR is a synthetic rubber that results from the hydrogenation of nitrile rubber (NBR). In this process the chemical double bonds in the NBR primary polymer chain undergo a hydrogenation proc- ess and therefore the term “hydrogenated nitrile” (HNBR).
The allowable temperature range extends to 150°C with short periods at higher temperature possible. By following design guidelines, effective sealing can be achieved at -40°C for static applications. For dynamic applications operating temperatures are limited to above -20°C. HNBR compounds possess superior mechanical characteristics, particularly high strength. For seal- ing applications this is an advantage as it prevents extrusion and wear.
Perfluorinated Rubber (FFKM) Trade names:
Kalrez has the chemical properties of PTFE (Teflon®) and the elastic properties of FKM-rubber. The processing of Perfluor Rubber is exceptionally difficult. Perfluor Rubber is only used in seldom cases because the raw material price is many times more expensive than Fluorocarbon (FKM). Normally alternative elastomers can be selected, FFKM only being taken in excep- tional cases.
Heat resistance: up to circa 310°C
Cold resistance: up to circa -15°C
Chemical resistance:
Polyacrylate Rubber (ACM) Trade names: Nipol AR® Nippon Zeon Hytemp® Nippon Zeon Europrene® Emichen
ACM or simply acrylate rubber consists of a base and a curing monomer. The basic monomer (rubber base) contains differing acrylate esters which influence the physical properties of the compound. Ethyl acrylate rubber has a good resistance to heat and mineral oil; on the other hand butyl acrylate has a better cold flexibility. Acrylate rubber has a good resistance to mineral oil, oxygen and ozone also at high temperatures. The water compat- ibility and cold flexibility of ACM are better than that of NBR.
Heat resistance: up to appr. 150°C (shortened lifetime up to appr. 175°C)
Cold flexibility: down to appr. -20°C
Chemical resistance to:
Not compatible with:
Polyurethane-Rubber (AU, EU)
Trade name:
Urepan®
Bayer AG
Considering the recipe of the particular polyole, one must differ- entiate between polyester urethane (AU) and polyester urethane (EU). EU shows better resistance to hydraulics. Polyurethane elastomers demonstrate in comparison with any other elastom- ers excellent wear resistance, high tensile strength and high elasticity. The permeability is good and comparable with IIR.
Heat resistance: up to appr. 90°C
Cold flexibility: down to appr. -40°C
Chemical resistance to:
Not compatible with:
Silicone Rubber (LSR, Q, MQ, VMQ)
Trade names: Silopren® Bayer AG
Silastic® Dow Corning STI
Elastosil® Wacker
Elastosil, Wacker The term silicone rubber covers a large group of materials in which methyl-vinyl-silicone (VMQ) is often central. Also Liquid Silicone Rubbers (LSR), which could be various coloured and are produced as two-component-compounds, belongs to this category.
Silicone elastomers as a group have a relatively poor tensile strength, tear resistance and wear resistance. However, they have many special properties: Silicones in general have good heat resistance up to +230°C and good cold flexibility down to - 60°C, weathering resistance, good insulating and physiologically neutral properties.
Heat resistance: up to appr. 210°C
(special compounds up to 230°C)
Cold flexibility: down to appr. -60/-55°C
(special compounds down to -100°C)
Chemical resistance to:
Not compatible with:
Styrol Butadiene Rubber (SBR) Trade names:
SBR probably is better known under its old names Buna S or GRS (government rubber styrene) and was first produced under government control between 1930 and 1950 replacing natural rubber. The basic monomers butadiene and styrol amount to appr. 23.5%. About one third of the world output of SBR is used in the tire production. SBR seals are mostly used in seals for non-mineral oil based brake fluid.
Heat resistance: up to appr. 100°
Cold flexibility: down to appr. -50°C
Chemical compatibility with:
Not compatible with:
The base elastomer and the hardness of the finished product are the main factors which enable a compound to resist heat, chemi- cal and physical influences.
The type of polymer in the compound is given by the prefix letter:
The hardness range of a compound is indicated by the suffice numbers, e.g. “70” means that the hardness is 70 ± 5 Shore A (measured on a flat piece). The recipe serial number is given between the suffix and the prefix. This recipe guarantees constant quality and never is changed. Each compounded batch is subject to extensive test- ing. Selection of base polymer. Temperature and compatibility with media are the most important parameters which must be considered when selecting a rubber base.
Only when these factors are known (including lubricants and cleaning fluids), a reliable recommendation can be given concerning selection of an elastomer. Normally, a compromise has to be made between high quality and cheaper products. The application temperatures given in Table 5.2 refer to long- term exposure to non-aggressive media. At higher temperatures new cross-link sites are formed between the polymer chains anlead to a loss of flexibility. The stiffness in the polymer chains can be observed as compression set in overloaded compounds and prevents an O-ring cross-section from returning to its original shape after deformation. After deformation a compound looses its original shape. A compound looses its elastic memory due to this mechanism and leakage can occur. Excess of the normal maximum temperature of a rubber com- pound results in reduced lifetime. Practically all elastomers suffer a physical or chemical change when contacting a medium. The degree of change depends on the chemistry of the medium and on the local temperature. An aggressivactive with increasing temperature.
Physical changes are caused by two mechanisms which can work concurrently:
The degree of volume change depends on the type of medium, structure of the rubber compound, temperature, geometrical shape (material thickness), and on the stressed condition of the rubber part. When deformed and exposed to a medium, rubber swells significantly less than in free state (up to 50%).
The limit of the permissible volume change varies with the ap- plication. For static seals a volume change by 25% to 30% can be tolerated. Swelling leads to deterioration of the mechanical properties, and in particular to those properties which improve extrusion resistance. In dynamic applications swelling leads to increased friction and a high wear rate. Therefore, a maximum swell by 10% should not be exceeded. Shrinkage should be avoided because deforma- tion will be reduced and the risk of leakage will increase.
The extraction of plasticiser from a material sometimes can be compensated for by absorption of the contact medium. This situ- ation, although compensated for by absorption, can lead to unex- pected shrinkage and leakage when an elastomer dries out. A chemical reaction between medium and elastomer can bring about structural changes in the form of further cross-linking or degrading. The smallest chemical changes in an elastomer can lead to significant changes in physical properties, e.g. embrittle- ment. The suitability of an elastomer for a specific application can be established only when the properties of both the medium and the elastomer are known under typical working conditions. If a particular material suits a medium, it is referred to as being “compatible” with that medium. Table 5.3 compares the various elastomeric materials ac- cording to their compatibility with media frequently found in practice. Several hundred further media can be found in our Medium Compatible Table 5703 E which well be sent to you on request without charge.
The greatest factor which influences the function of a seal is the assembly of an unsuitable elastomer. In spite of the most strin- gent customer quality control system it is still possible to confuse seals of the same colours and sizes. The elastomer base can only be established after extensive laboratory tests for compat- ibility or by spectroscopic analysis. For inside inspection of goods it often suffices to determine the specific gravity of a compound and to compare specific data of results with test values. The comparison of these values does not give any absolute conformation of compound identity.
The Col-O-Ring® system Where similar O-rings of different compounds are kept in the same store, surface colour coding is regarded as normal. This kind of marking is expensive and not durable so that selection of a replacement O-ring still is risky. Replacement of O-rings during maintenance is uncomplicated by the Col-O-Ring® system. The colour coding is permanent and positively excludes the confu- sion of similar sizes. Therefore Parker has developed coloured compounds having physical properties similar to the original black ones. Col-O-Ring® compounds are not a compromise; they combine a highly reliable positive identification system with properties expected from the highest quality Parker materials. The colour-elastomer combination has been selected to recom- mendations by the RMA (Rubber Manufacturers Association of the USA).
The American system of rubber classification, ASTM D 2000, is very complicated and not designed to existing European practic- es. The abbreviations used by that system are difficult to compre- hend by the user. The coding has been selected at random and is not easy to interpret. The user must code his requirements and the manufacturer must de-code them again. An ASTM descrip- tion of a single rubber compound can reach several lines length. The system is widely used in the USA but has not been estab- lished in Europe outside the automobile industry. In this industry, specifications tend to be raised internally by companies and bear little relation to any standard system.
Seals for gas supply and appliances
The following Parker compounds are approved by the DVGW (Deutscher Verband für Gas und Wasser e.V. – German Associa- tion for Gas and Water) for the given applications:
Seals for oxygen valves
According to tests carried out by the Deutsches Bundesamt für the following compounds can be used up to the specified tem-Materialprüfung (German Federal Authority for Material Tests) peratures and pressures:
Seals for preparation, storage and distribution of drinking water