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Elastomers

Introduction

Elastomers (rubbers) are special polymers that are very elastic. They are lightly cross-linked and amorphous with a glass transition temperature well below room temperature. They can be envisaged as one very large molecule of macroscopic size. The intermolecular forces between the polymer chains are rather weak. The crosslinks completely suppress irreversible flow but the chains are very flexible at temperatures above the glass transition, and a small force leads to a large deformation 1 Thus, elastomers have a low Young’s modulus and very high elongation at break when compared with other polymers. The term elastomer is often used interchangeably with the term rubber, although the latter is preferred when referring to vulcanized rubbers.
Elastomers can be classified into three broad groups: diene, non-diene, and thermoplastic elastomers. Diene elastomers are polymerized from monomers containing two sequential double bonds. Typical examples are polyisoprene, polybutadiene, and polychloroprene. Nondiene elastomers include, butyl rubber (polyisobutylene), polysiloxanes (silicone rubber), polyurethane (spandex), and fluoro-elastomers. Non-diene elastomers have no double bonds in the structure, and thus, crosslinking requires other methods than vulcanization such as addition of trifunctional monomers (condensation polymers), or addition of divinyl monomers (free radical polymerization), or copolymerization with small amounts of diene monomers like butadiene. Thermoplastic elastomers such as SIS and SBS block copolymers and certain urethanes are thermoplastic and contain rigid (hard) and soft (rubbery) repeat units. When cooled from the melt state to a temperature below the glass transition temperature, the hard blocks phase separate to form rigid domains that act as physical crosslinks for the elastomeric blocks.
Manufacturing elastomeric parts is achieved in one of three ways: injection molding, transfer molding, or compression molding. The choice of the molding process depends on various factors, including the shape and size of the parts, the required tolerance, as well as the quantity, type of elastomer, and raw material cost.
As with almost any material, selecting the right elastomeric product for the application requires consideration of many factors, including mechanical and physical service requirements, exposure to chemicals, operating temperature, service life, manufacturability of the parts, and raw material and manufacturing cost.

Elastomers That We Offer

Elastomers is essentially another word for rubber. Elastomers have a variety of properties such as hardness, tensile strength, and elongation.

As the name would indicate, this elastomer occurs naturally and comes from the latex of certain trees and plants. After the latex is processed, it becomes an elastomer with excellent mechanical properties. It has excellent tensile, elongation, tear resistance and resilience. It has good
abrasion resistance and excellent low temperature flexibility. Without special additives, it has poor resistance to ozone, oxygen, sunlight and heat. It has poor resistance to solvents and petroleum products. Useful temperature range is -67° F to +180° F (-55° C to +82° C).

Fluoroelastomers or Fluorocarbons (FKM), also known as Viton®, are highly fluorinated polymers that are suitable for continuous use at elevated temperatures. Various grades are available, including copolymer and terpolymers. The most common grades consist of tetrafluoroethylene,
hexafluoropropylene, and vinylidene fluoride. Fluoroelastomer compounds are widely used in the chemical, automotive, aerospace, and energy industry. They are used for hoses, diaphragms, accumulator bladders, gaskets, O-rings & seals, all operating in especially harsh environments.The typical service temperature of FKMs is between -20°C (-5°F) and +230°C (+445°F). They can, however, withstand temperatures up to 300°C for a short period of time. However, at high service temperatures FKMs are weak, so that the design must prevent any high loads.

This is a terpolymer of ethylene, propylene and a diene monomer. It has outstanding resistance to oxygen, ozone, and sunlight. Its resistance to polar materials such as phosphate esters, many ketones and alcohol. It has good electrical properties, low temperature flexibility, excellent heat, water and steam resistance. Its resistance to petroleum products is poor. Useful temperature range is -58° F to +300° F (-50° C to +150° C).
This is a copolymer of acrylonitrile and butadiene. It has excellent physical properties, however its claim to fame is based on its resistance to water, petroleum products and fuels. When compounded properly, it has good low temperature properties as well as good heat resistance. It does not have good ozone, oxygen or sunlight resistance without the addition of special additives. Useful temperature range is -40° F to +275° F (-40° C to +135° C).
This elastomer is made by the polymerization of Chloroprene. It has excellent physical properties. It is moderately resistant to petroleum products, sunlight, ozone and heat. It is flame resistant and will not support combustion. Useful temperature range is -40° F to +275° F (-40° C to +135° C).
This elastomer is made by the polymerization of Chloroprene. It has excellent physical properties. It is moderately resistant to petroleum products, sunlight, ozone and heat. It is flame resistant and will not support combustion. Useful temperature range is -40° F to +275° F (-40° C to +135° C).
This is a copolymer of styrene and butadiene. It has similar properties to natural rubber. Its resistance to solvents and petroleum products is about the same as natural rubber. Water resistance is better. Without special additives, it is vulnerable to ozone, oxygen and sunlight. Useful temperature range is -67° F to +180° F (-55° C to +82° C).
Silicone is made from sand and alkyl or aryl halides. It is predominately inorganic material. It has outstanding resistance to temperature extremes. It has excellent vibration damping, and reasonable physical properties such as tensile and elongation. Tear and abrasion resistance are generally poor. Useful temperature range is -148° F to +600° F (-100° C to +315° C).
This is a synthetic fluoropolymer of tetrafluoroethylene and is known for being one of the most versatile plastics available. It is made by the free-radical polymerisation of many tetrafluoroethene molecules, and is suitable for a wide range of applications in industries as diverse as aerospace, the food and drink industry, pharmaceuticals and telecoms. PTFE is commonly known as Teflon™ (owned by Chemours) and offers remarkable nonstick properties in cookware applications such as kitchen pans and baking trays. PTFE is produced by AFT Fluorotec in rods or tubes of any size, or filled with glass, carbon, stainless steel or many other materials to increase wear resistance and strength, whatever your project or build, we are sure to have a material that will work for you.
FFKM are one of the most expensive elastomers. Typical applications include core sleeves, gaskets, O-rings in chemical processing equipment for applications where the parts are exposed to harsh environments and high temperatures. Some grades are suitable for continuous use at 325°C (620°F), with chemical resistance being almost universal and unrivaled by any other elastomer. Applications – Chemical and hydrocarbon processing, semiconductor manufacturing, aerospace engines, FDA-compliant food, pharmaceuticals and beverages.
A unique elastomer. It can be produced from conventional nitrile rubber by hydrogenation of the unsaturated bonds in the butadiene units of the polymer. The properties of hydrogenated nitrile rubber (HNBR) depend upon the acrylonitrile content and the degree of hydrogenation of the butadiene copolymer. HNBRs have better oil and chemical resistance than nitrile rubbers & can also withstand much higher temperatures. Like NBR, this type of elastomer has excellent resistance to oils and fuels but it also has excellent resistance to many chemicals, heat (steam / hot water) & ozone. The mechanical properties like tensile & tear strength, elongation, & abrasion resistance are also excellent. Furthermore, HNBRs have good dynamic behavior at elevated temperatures. Disadvantages include higher cost & limited resistance to aromatic oils & polar organic solvents, poor electrical properties, & poor flame resistance. The typical working temperature range is -25°C to +160°C (-40°F to +320°F). Special grades that are sulfur or peroxide cured have improved dynamic applications but also have a lower maximum application temperature. As with nitrile, many properties can be influenced by varying the acrylonitrile content in the rubber. High-nitrile HNBR elastomers have better resistance to mineral oils, whereas peroxide/sulfur cured HNBRs have the best compression set and heat resistance.
AFLAS is a unique fluoroelastomer that has superior amine resistance & electrical insulation compared to conventional fluoroelastomers. It is highly suitable for critical applications where exceptional reliability is required. The molecular structure of AFLAS® gives it outstanding heat-resistance allowing it to be used continuously at 200°C. Its exceptional heat-resistance even enables it to withstand temperatures of 250°C. It has extremely low aromatizing properties compared to other rubber materials. It can be considered extremely effective for use in gaskets and packing on piping components for production lines, where lingering or transferred smells are a concern.

Heat Resistance

Elastomer performance becomes less predictable and reliable when an elastomer is used near the limits of its service temperature range. If, for example, the temperature drops, elastomers become harder and less flexible & when the temperature reaches the glass transition temperature, they loose their rubber-like properties entirely. At even lower temperatures, i.e. at the brittle point, they may crack. Changes in elastomer properties due to low temperature are usually physical, and fully reversible unless the elastomeric part is exposed to large tensions which can cause damage below the brittle or glass transition temperature. The opposite is true when an elastomer is exposed to high temperatures, that is to temperatures near or above the service temperature limit.
At these temperatures, elastomers often undergo irreversible chemical changes. For example, the polymer backbone may undergo chain scission or the polymer molecules may crosslink, causing the elastomeric part to become either (much) softer or more rigid, which, in turn, reduces their resistance to compression set. The maximal service temperature can greatly vary from elastomer to elastomer. The highest continuous service temperatures do have silicone and fluorocarbon elastomers which can exceed 400°F (230°C)2, followed by polyacrylic and hydrogenated nitrile elastomers with a maximal service temperatures between 320 and 350°F (160 – 180°C), whereas more ordinary elastomers such as Neoprene and Nitrile have a maximal operating temperature between 210 to 250°F (100 – 120°C).

Fluid Compatibility

Strong swelling and rapid deterioration or complete breakdown of an elastomeric part may occur if the elastomer is not compatible with the fluid it is exposed to. Factors such as chemical concentration, operating temperature and pressure affect the stability / compatibility with the chemicals. When in doubt, the elastomer should be evaluated in functional tests prior use.
Because many applications involve hydrocarbon oils, elastomeric parts such as seals are classified according to their heat & oil resistance. For example, in the ASTM D2000 system, elastomers are ranked by heat resistance (type) and by oil resistance (class). Fluorosilicone and fluorocarbon elastomers have excellent oil resistance at elevated temperatures (> 200°C). Other elastomers with good oil but only medium heat resistance include NBR, ACM & HNBR. In the case of ACM & HNBR, the operating temperature in hydrocarbon oils should not exceed 150°C and in the case of NBR 100°C. Silicone & Neoprene elastomers have only medium oil resitance. However, silicone elastomers can be operated at much higher temperatures than Neoprene. Poor oil resistance can be expected for EPDM, SBR, butyl (IIR, CIIR, BIIR) and natural rubber based elastomers (NR, IR).

Abrasion & Tear Resistance

Abrasion resistance is generally an important selection criteria for dynamic seal and tire applications of elastomers whereas good tear resistance may be important for other mechanical applications where the elastomers have to resist nicking, cutting and tearing. Elastomers such as hydrogenated nitrile (HNBR), polyester (AU) and polyether urethanes (EU), isoprene rubber (NR/IR), styrene butadiene rubber (SBR) and tetrafluoroethylene propylene copolymers have inherent abrasion resistance, whereas silicone (VMQ,), butyl (IIR), and perfluoro elastomers (FFKM) have poor abrasion resistance.
In many cases, the abrasion and tear resistance can be enhanced by compounding with internal lubricants such as Teflon® or molybdenum disulfide. Nitrile and and acrylic elastomers have fair abrasion resistance. However, carboxylated nitrile (XNBR) offers noticeably better abrasion resistance. Most elastomers with good abrasion resistance have also good tear resistance and elastomers with poor abrasion resistance have usually poor tear resistance. For example silicone and fluorosilicone are only suitable for static applications due to their poor tear and abrasion resistance.

Price

Cost is one of the most important selection criteria. Assuming that more than one elastomer meets all other requirements for a given application, price will usually dictate which elastomer will be chosen. The prices of elastomers may vary widely due to differences in raw material, compounding and processing costs. Inexpensive elastomers are styrene-butadiene (SBR) < natural rubber (NR) < isoprene (IR) < neoprene (CR) < nitrile (NBR) rubbers, whereas EPDM < urethane < silicone < polyacrylate (ACM) < butyl (IIR) < hydrogenated nitrile (HNBR) are somewhat more expensive but often still an economical choice. Expensive elastomers are fluorocarbons (FKM) (copolymers) < perfluorocarbons (FFKM) < fluorosilicones (FVMQ). These elastomers are usually only chosen if no other elastomer can meet the requirements.

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