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Materials and Technical

Basic Concepts of Rubber

What is “Rubber?”

“Rubber” refers to elastomeric compounds that consist of various monomer units forming polymers that are heat cured (vulcanized). Polymers are long molecular chains and are derived from the Greek “poly” (many) and “meros” (parts). The base monomer or monomers is used to classify the type of rubber, for example: Nitrile, Silicone or Neoprene.

What is a Rubber Compound?

Rubber is composed of many different ingredients that include the base elastomer, vulcanization agents, fillers and plasticizers. For example, the addition of fillers can reinforce or modify properties, or additional plasticizer can increase elongation and lower durometer.

Why Does Rubber Act “Rubbery?”

A polymer is considered a very viscous liquid or an elastic solid (i.e. rubber). The polymeric chains in rubber tend to be very long and flexible by nature and can rotate about their axis, which results in an entangled mass of contorted chains.

When a deformation of the rubber occurs, these tangled chains uncoil and recoil when the force is released. Therefore, elastic rebound or rubbery behavior is possible due to contortions of long, flexible polymeric chains, which allows rubber to be so resilient.

How is Rubber Made?

The elastomer is the basic component of all rubber recipes and is selected in order to obtain specific physical properties in the final product. Processing aids and softeners, such as oils and plasticizers, modify rubber to aid in mixing or molding operations. Sulfur is one of the most widely used vulcanizing agents to promote crosslinking which is used in conjunction with accelerators and accelerator activators to reduce cure times and enhance physical properties. Carbon black is one of the most common fillers because it reinforces the molecular structure. Antidegradants, such as antioxidants and antiozonants, retard the deterioration of rubber products. Lubricants, colors or any other miscellaneous ingredients may also be added.

What is Vulcanization?

The long, flexible polymeric chains of rubber, when heated, react with vulcanizing agents to form three-dimensional structures. These vulcanizing agents (usually sulfur or peroxide) are necessary to facilitate chemical crosslinking of polymeric chains. Once the rubber has been vulcanized or “cured”, physical properties are enhanced and the compound is more resistant to deterioration.

What is Compression Set?

Elastic recovery is a measure of the elastomer’s ability to return to its original shape once a compressive force has been removed. Failure of the seal to return to its original shape after compression is the condition termed “compression set” and all seals exhibit some degree of compression set. Determination of the amount of compression set is governed by ASTM designation D395 test procedure.

What is the difference between a Thermoset and Thermoplastic?

One classification method of polymeric materials is according to physical properties at elevated temperatures. Thermoset polymers become permanently “set” in the presence of heat and do not soften in the presence of subsequent heating. Conversely, a thermoplastic material will soften when heated (and eventually liquefy) and harden when cooled. This process is reversible and repeatable, as opposed to thermosetting polymers where the process is irreversible. Also, thermoset polymers possess superior mechanical, thermal, and chemical properties as well as better dimensional stability than thermoplastics. This is why thermoset (rubber) parts are generally preferred for sealing applications.

This section contains descriptions of the elastomers used in seal applications. These elastomers form the base of a wide variety of compounds, designated for specific applications. Every compound has specific characteristics and many compounds have common attributes. Therefore, it is important to consider all aspects of the compound prior to use. Also, as compound availability is customer-driven, lead times may vary.

Carboxylated Nitrile (X-NBR)

ASTM D1418 Designation: X-NBR

Elongation at break (up to): 250%

Description: Carboxylated Nitrile Butadiene Rubber (X-NBR) in this execution there are beside the sulfur bridges also carboxyl groups R-COO- on the double bond of the butadiene part. These groups will make ionic cross links with zinc (Zn2+) to give improved physical properties as compared to a non-carboxylated nitrile rubber. These ionic crosslinks are formed along with sulfur links. The carboxyl groups which are needed for these extra links are distributed randomly and are present at levels of 10% or less.

Applications: Oil resistant applications in automotive, marine and aircraft fuel systems where high strength and abrasion resistance is required.

Advantages: Good resistance to oils and non-polar solvents. Improved tear strength and abrasion resistance cf. NBR.

Disadvantages: Poor ozone and weathering resistance. Poor steam resistance.

Ethylene Propylene (EPDM)

Trade Names:

  • Nordel . . . Dow Chemical
  • Keltan® . . . Lanxess Corporation
  • Royalene . . . Chemtura Corporation

ASTM D1418 Designation: EPDM

ASTM D2000/SAE J200 Type, Class: AA, BA, CA, DA

Standard Color: Black

Description: A copolymer of ethylene and propylene (EPR), combined with a third comonomeradiene(EPDM), Ethylene Propylene has gained wide seal industry acceptance for its excellent ozone and chemical resistance characteristics.

Key Use(s): Outdoor weather resistant uses. Automotive brake systems. Automobile cooling systems. Water applications. Low torque drive belts.

Temperature Range
Standard Compound: -40° to +275°F / -40° to 135°C
Special Compound: -76° to +302°F / -60° to 150°C

Hardness (Shore A): 40 to 95

Features: When compounded using peroxide curing agents high temperature service can reach +350°F.  Good resistance to acids and solvents (i.e. MEK and Acetone).

Limitations: Have no resistance to hydrocarbon fluids.

Fluorocarbon (Viton®)

Trade Names:

  • Viton® . . . DuPont Performance Elastomers
  • Fluorel . . . 3M Company
  • Technoflon . . . SolveySolexis, USA

ASTM D1418 Designation: FKM

ASTM D2000/SAE Type, Class: HK

Standard Color: Black

Description: Combining high temperature resistance with outstanding chemical resistance, Fluorocarbon-based compounds approach the ideal for a universal O-ring material.

Key Use(s): Seals for aircraft engines. Seals for automotive fuel handling systems. High temperature/low compression set applications. Wide chemical exposure situations. Hard vacuum service.

Temperature Range
Standard Compound: -13° to +446°F / -25° to 230°C
Special Compound: -40° to +446°F / -40­° to 230°C

Hardness (Shore A): 55 to 90.

Features:High fluorine grades offer higher resistance to swell in high octane and oxygenated fuel blends. This give superior performance in Ethanol/Methanol blended gasoline. Base resistant grades offer improved resistance to amine based oil protectants found in new transmission oils.  Also, improved resistant to steam for higher temperature services. Low temperature bases can improve performance to -40°F.

New Polymers being offered have improved chemical resistance and low temperature performance.

Viton® Extreme ETP offers similar chemical compatibility as Kalrez with temperature performance with a TR(10) of -40°F and brittleness to -76°.

Limitations: Fluorocarbons (Viton®s) are not recommended for exposure to ketones, amines, low molecular weight esters and ethers, nitro hydrocarbons, hot hydrofluoric or chlorosulfonic acids, or Skydrol® fluids. They are also not recommended for situations requiring good low temperature flexibility.

Hydrogenated Nitrile (H-NBR)

Trade Names:

  • Zetpol . . . Zeon Co., Ltd.
  • Therban . . . Lanxess

ASTM D1418 Designation: HNBR

ASTM D2000/SAE J200 Type, Class: DH

Standard Color: Black

Description: HNBR is the product of the hydrogenation of Nitrile, resulting in varying degrees of saturation of the polymeric chain, with accompanying varying degrees of enhanced physical strength and chemical resistance properties.

Key Use(s): ALL oil resistant applications, including exposure to such oil additives as detergents, anti-oxidants and anti-wear agents. Exposure to oil soured with metal sludge. Seals for oil well applications. Seals for automotive fuel handling systems. Seals for general industrial usage.

Temperature Range
Standard Compound: -30° to +300°F / -34.44° to 148.89°C (Dry Heat Only)
Special Compound: -76° to +347°F / -60° to 175°C

Hardness (Shore A): 50 to 90

Features: Compounds can be formulated for service temperature ranging from -85°F to +350°F. Like Nitrile, increasing acrylonitrile content improves oil resistance at a cost of reduced low temperature performance. HNBR offers improved physical properties and better ozone resistance to Buna-N compounds. HNBR should be used when oil seals are also exposed to weather or environmental ozone.

Limitations: Like Nitrile, HNBR is not recommended for exposure to ethers, esters, ketones, or chlorinated hydrocarbons.

Natural Rubber (NR)

Trade Names:

  • None

ASTM D1418 Designation: NR

ASTM D2000/SAE J200 Type, Class: AA

Standard Color: Black

Description: Natural Rubber is the vulcanized product of the juice of the Hevea tree (latex).

Key Use(s): Mainly used for dampeners due to its ability to absorb vibration. Used for stoppers for cost and chemical compatibility. Most natural rubber still used for tires.

Temperature Range: Standard Compound: -58° to +158°F / -50° to 70°C (Dry Heat Only)

Hardness (Shore A): 40 to 90.

Features: Natural Rubber features high tensile strength; high resilience, high abrasion and high tear resistance properties, with a good friction surface and excellent adhesion to metals. Natural Rubber features good resistance to organic acids and alcohols, with moderate resistance to aldehydes.

Limitations: Not widely used in sealing industry due to poor compression set performance at elevated temperature and lack to resistance to petroleum based fluids. Medical industry has moved away from the use of Natural Rubber because of its allergic reactions after repeated contact.

Neoprene® (CR)

Trade Names:

  • Neoprene . . . DuPont Performance Elastomers
  • Baypren . . . Lanxess

ASTM D1418 Designation: CR

ASTM D2000/SAE J200 Type, Class: BC, BE

Standard Color: Black

Description: One of the earliest of the synthetic materials to be developed as an oil-resistant substitute for Natural Rubber, Neoprene is a homopolymer of chloroprene (chlorobutadiene).

Key Use(s): Numerous component uses in the transportation field. Recommended for exposure to weathering. Preferred sealing material for refrigeration industry. FDA approved for food & beverage industry use.

Temperature Range:
Standard Compound: -40° to +250°F / -40° to 121.11°C (Dry Heat Only)
Special Compound: -67° to +250°F / -55° to 121.11°C

Hardness (Shore A): 40 to 90.

Features: Neoprene can be used in innumerable sealing applications due to its broad base of such desirable working properties as: good resistance to petroleum oils; good resistance to ozone, sunlight and oxygen aging; relatively low compression set; good resilience; outstanding physical toughness; and reasonable production cost.

Due to its excellent resistance to Freon® and ammonia, Neoprene is also widely accepted as a preferred material for refrigeration seals.

Limitations: Neoprene is generally attacked by strong oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons. Because Nitrile is economically competitive with Neoprene, and generally has superior performance characteristics in most situations, it has largely replaced Neoprene in the O-rings of today.

Nitrile (NBR/Buna-N)

Trade Names:

  • Nipol . . . Zeon
  • Krynac . . . Bayer Polymer
  • Nysyn . . . DSM Elastomers
  • Chemigum . . . Eliokem

ASTM D1418 Designation: NBR

ASTM D2000/SAE J200 Type, Class: BF, BG, BK, CH

Standard Color: Black

Description: Presently the seal industry’s most widely used and economical elastomer, Nitrile combines excellent resistance to petroleum-based oils and fuels, silicone greases, hydraulic fluids, water and alcohols, with a good balance of such desirable working properties as low compression set, high tensile strength, and high abrasion resistance.

Key Use(s): Oil resistant applications of all types. Low temperature military uses. Off-road equipment. Automotive, marine, aircraft fuel systems. Can be compounded for FDA applications.

Temperature Range
Standard Compound: -40° to +257°F / -40° to 125°C
Special Compound: -76° to +275°F / -60° to 125°C

Hardness (Shore A): 40 to 90.

Features: Comprised of the copolymer butadiene and acrylonitrile, in varying proportions. Compounds can be formulated for service temperature ranging from -85°F to +275°F. Use of Carboxylated Nitrile can have superior abrasion resistance, while still having improved oil resistance.

Limitations: Nitrile compounds are attached by small amounts of Ozone. Phthalate type plasticizers are commonly used in compounding Nitrile Rubber. These plasticizers can migrate out and cause problems with certain plastics. Also, new regulations on certain phthalates have limited their use.

Perfluoroelastomer (Kalrez®)

Trade Names:

  • Chemraz . . . Green, Tweed & Co.
  • Kalrez® . . . DuPont Performance Elastomers
  • Tecnoflon PFR . . . Solvay Solexis

ASTM D1418 Designation: FFKM

ASTM D2000/SAE J200 Type, Class: N/A

Standard Color: Black

Description: FFKM parts are made from a Perfluoroelastomer (Kalrez®) possessing exceptional resistance to degradation by aggressive fluids and/or gases.

Key Use(s): Seals for use in the chemical and petroleum industries as well as for the manufacturing of semiconductors and analytical and process instruments. It is also used for high temperature applications and for paint and coating operations.

Temperature Range: Standard Compound: -13° to +600°F / -25° to 315.56°C

Hardness (Shore A): 65 to 90

Features: FFKM combines the toughness of an elastomeric material with the chemical inertness of Teflon™. It resists attack by nearly all chemical reagents and provides long-term service where corrosive additives can cause other elastomers to swell or degrade. In addition, FFKM parts are less likely to cold flow than Teflon seals.

Limitations: Withstanding degradation by virtually ALL chemicals, FFKM can swell significantly when exposed to some fluorinated solvents, fully halogenated freons and uranium hexafluoride. In addition, FFKM parts should not be exposed to molten or gaseous alkali metals.

As the thermal coefficient of expansion for FFKM is stated by the manufacturer to be “about 50% greater than for fluoroelastomers”, gland volume may have to be increased to allow for this expansion in elevated temperature situations.

Because of its high cost, FFKM is generally used when no other elastomer is appropriate.

Polyacrylate (ACM)

Trade Names:

  • HyTemp ACM . . . Zeon
  • Acralen A . . . Bayer Polymer

ASTM D1418 Designation: ACM

ASTM D2000/SAE J200 Type, Class: DH; DF

Standard Color: Black

Description: Polyacrylates are copolymers (ethyl acrylates) possessing outstanding resistance to petroleum fuels and oils.

Key Use(s): Sealing automatic transmissions & power steering systems. Sealing petroleum oils up to 350°F.

Temperature Range
Standard Compound: -25° to +300°F / -31.67° to 148.89°C
Special Compound: -40° to +347°F / -40° to 175°C

Hardness (Shore A): 40 to 90.

Features: With excellent resistance to hot oil, automatic transmission and Type A power steering fluids, the greatest use for Polyacrylate is found in automobile manufacture, where O-rings of this material are employed to seal components of automatic transmission and power steering systems.

Highly resistant to sunlight and ozone degradation, Polyacrylate also features an enhanced ability to resist flex cracking.

Limitations: While resistance to hot air aging is superior to Nitrile, Polyacrylate strength, compression set, water resistance properties and low temperature capabilities are inferior to many other polymers. Polyacrylates are also not generally recommended for exposure to alcohol, glycols, alkalis, brake fluids, or to chlorinated or aromatic hydrocarbons.

Polyurethane, Cast (PU)

Trade Names:

  • Vibrathane . . . Uniroyal
  • Cyanaprene . . . American Cyanamid
  • Polathane . . . Polaroid

ASTM D1418 Designation: N/A

ASTM D2000/SAE J200 Type, Class: N/A

Standard Color: Amber

Description: Cast Polyurethane is outstanding over other O-ring elastomers in abrasion resistance and tensile strength. Additionally, Cast Polyurethane surpasses the performance of Millable Polyurethane in its higher tensile strength, greater elongation, wider temperature range, and lower compression set characteristics.

Key Use(s): Seals for high hydraulic pressures. Situations where highly stressed parts are subject to wear. Used for wheels, rolls, slurry parts, bumpers, couplers, and shock absorbers. Wiper seals for axially moving piston rods.

Temperature Range: Standard Compound: -30° to +175°F / -34.44° to 79.44°C

Hardness (Shore A): 70 and 90.

Features: With tensile strength of up to 6,000 psi, elongation of 350 to 650%, compression sets of 10 to 25%, and exceedingly high abrasion resistance, the physical properties of Cast Polyurethane are among the best of all O-ring elastomers.

Although they swell slightly upon exposure, Cast Polyurethane compounds feature excellent resistance to mineral-based oils and petroleum products, aliphatic solvents, alcohols and ether. They are also compatible with hydraulic fluids, weak acids and bases, and mixtures containing less than 80% aromatic constituents.

Limitations: Cast Polyurethanes are not recommended for exposure to concentrated acids and bases, ketones, esters, very strong oxidizing agents, pure aromatic compounds and brake fluids. With the exception of special compounds, they are also not recommended for exposure to hot water or steam.

Polyurethane, Millable

Trade Names:

  • Millathane® . . . TSE Industries Inc.

ASTM D1418 Designation: AU, EU

ASTM D2000/SAE J200 Type, Class: BG

Standard Color: Black

Description: Millable Polyurethane is outstanding over most other O-ring elastomers in abrasion resistance and tensile strength.

Key Use(s): Seals for high hydraulic pressures. Situations where highly stressed parts are subject to wear.

Temperature Range: Standard Compound: -30° to +175° F / -34.44°C to 79.44°C

Hardness (Shore A): 40 to 90

Features: Millable Polyurethane offers superior seal performance in hydraulic situations, where high pressures, shock loads, or abrasive contamination is anticipated.

Millable Polyurethane possesses chemical compatibility similar to that of Nitrile, offering good resistance to petroleum-based oils, hydrocarbon fuels and hydraulic fluids, the oxidizing effects of ozone, and the aging effects of sunlight. It also has good tear resistance.

Good low temperature flexibility is also a feature of many Millable Polyurethane elastomer compositions.

Limitations: Unless specially compounded, at elevated temperatures Millable Polyurethane begins to soften, losing its physical strength and chemical resistance advantages over other polymers.

Tending to rapidly deteriorate when exposed to concentrated acids, ketones, esters, chlorinated and nitro hydrocarbons, Millable Polyurethanes are also prone to hot water and steam degradation.

Silicone (MQ, PVMQ, VMQ)

Trade Names:

  • Elastosil . . . Wacker
  • Silastic . . . Dow Corning
  • Silplus . . . Momentive Performance Materials

ASTM D1418 Designation: MQ, PMQ, VMQ, PVMQ

ASTM D2000/SAE J200 Type, Class: FC, FE, GE

Standard Color: Red

Description: A group of elastomers, made from silicon, oxygen, and hydrogen, Silicones are renowned for their retention of flexibility and low compression set characteristics, within one of the widest working temperature ranges for elastomers.

Key Use(s): Static seals in extreme temperature situations. Seals for medical devices, compatible with FDA regulations.

Temperature Range:
Standard Compound: -85° to +400°F / -65° to 204.44°C
Special Compound: -148° to +400°F / -100° to 204.44°C

Hardness (Shore A): 20 to 80

Features: Phenyl (PVMQ) based silicones can perform to -155°F. New polymers can take short term to 600°F.

Limitations: Generally, low abrasion and tear resistance, and high friction characteristics preclude silicones from effectively sealing some dynamic applications. Silicones are also highly permeable to gases and are generally not recommended for exposure to ketones (MEK,acetone) or concentrated acids.

Teflon (PTFE)

Trade Names:

  • Teflon . . . DuPont Performance Elastomers
  • TFM . . . Dyneon

ASTM D1418 Designation: FEP

ASTM D2000/SAE J200 Type, Class: N/A

Standard Color: White

Description: Teflon is a tough, chemically inert polymer possessing an incredible working temperature range.

Key Use(s): Seals for wide chemical exposure situations, with special emphasis on temperature extremes.

For static and SLOW INTERMITTENT dynamic situations.

Temperature Range: Standard Compound: -300° to +450°F / -184.44° to 232.22°C

Hardness (Shore A): 98.

Features: Teflon is inert to virtually all industrial chemicals, even at elevated temperatures. Seals fabricated from this material feature outstanding weather resistance, high resistance to ozone, and high resistance to the degrading effects of exposure to such solvents as acetone, MEK, and xylene. Possessing average elastomer characteristics of 2,500 to 3,500 psi tensile strength, and 300% elongation, they are tough, impact resistant, low friction, non-twisting performers over an extremely wide temperature range.

Limitations: Teflon is hampered by very poor elastic memory at room, or low temperatures. This presents problems in O-ring installation, requiring extra care to be taken in control over O-ring I.D. stretch. Heating Teflon in boiling water, or in a controlled oven, to 200°F is said to enable an O-ring stretch of 10 to 20% to be achieved, thereby assisting installation, and helping to assure a tight fit.

Because of its poor tear resistance, during O-ring installation particular care should be taken to avoid nicking or scratching Teflon, as imperfections will cause O-ring leakage.

Finally, the tendency of virgin Teflon to cold flow over time, when used in gasket type applications, may require special material compounding (with fillers) to control such “creep” in critical sealing situations.

TFE/P™ (Aflas®)

Trade Names:

  • Aflas® . . . Asahi Glass Co., Ltd.

ASTM D1418 Designation: FEPM

ASTM D2000/SAE J200 Type, Class: HK

Standard Color: Black

Description: A copolymer of tetrafluoroethylene/ propylene, TFE/P can offer a combination of high temperature and chemical resistance.

Key Use(s): Seals for oil field, aerospace, chemical and general industrial environments.

Temperature Range: Standard Compound: -14° to +446°F / -25.56° to 230°C

Hardness (Shore A): 60 to 90.

Features: Resistance to a wide range of chemicals, high temperature and electrical capabilities give broad application diversity. TFE/P have resistance to acids and bases, steam/hot water, corrosion inhibitors, oils and lubricants, and industrial solvents. TFE/P also offer improved low temperature properties over most fluoroelastomers.

Limitations: Tests have shown that other FKM elastomers are recommended for automotive fuels since they have less volume swell than TFE/P. Also, TFE/P has shown to have less than desirable results when exposed to toluene, ethers, ketones, and acetic acid.