POLYMER TYPES & CHARACTERISTICS

NATURAL RUBBER (NR) Natural Polyisoprene

Advantages: Outstanding resilience; high tensile strength; superior resistance to tear and abrasion; excellent rebound elasticity; good flexibility at low temperatures; excellent adhesion to fabric and metals.

Limitations: Poor resistance to heat, ozone, and sunlight; very little resistance to oil, gasoline, and hydrocarbon solvents.

Comment: Natural rubber is a low cost material with excellent physical properties. Generally speaking, it is ideal for applications that require good resistance to abrasion, gouging, and cut growth. It is tough and long wearing and can be compounded for service at temperatures as low as -65F.

SBR (SBR, GR-S, Buna-S) Styrene Butadiene

Advantages: Excellent impact strength; very good resilience, tensile strength, abrasion resistance, and flexibility at low temperatures.

Limitations: Poor resistance to ozone and sunlight; very little resistance to oil, gasoline, and hydrocarbon solvents.

Comment: SBR is much like natural rubber in most of its properties and is the lowest cost and highest volume elastomer available. Although the physical properties are slightly poorer than those of natural rubber, SBR is tougher and slightly more resistant to heat and flex cracking and can be readily substituted for natural rubber in many applications with significant cost savings.

BUTYL (IIR) Isobutylene-isoprene

Advantages: Outstanding impermeability to gases and vapor; very good resistance to heat, oxygen, ozone, and sunlight; high energy absorption (damping); excellent resistance to alkalis and oxygenated solvents; good hot tear strength; superior resistance to water and steam.

Limitations: High compression set; poor resistance to oil, gasoline, and hydrocarbon solvents; low rebound elasticity (snap); fair processability, poor resilience.

Comment: Butyl is chemically unlike natural rubber or other synthetic elastomers in that it is inherently resistant to ozone and corrosive chemicals, including some mineral acids, ketones, and phosphate-ester-type hydraulic fluids. On the negative side, its creep, cold flow, and compression set characteristics leave much to be desired. The processing properties of butyl are only fair. Because of its low level of unsaturation, buytl is difficult to break down during mixing, and it is very susceptible to contamination. Slight contamination causes improper cure and poor physical properties. Chlorinated and brominated versions are less susceptible to contamination and offer lower compression set and improved heat resistance.

EPDM (EPDM, EPM, EPT) Ethylene-propelene, ethylene-propylene-diene

Advantages: Excellent resistance to heat, ozone, and sunlight; very good flexibility at low temperatures; good resistance to alkalis, acids, and oxygenated solvents; superior resistance to water and steam; excellent color stability.

Limitations: Poor resistance to oil, gasoline, and hydrocarbon solvents, adhesion to fabrics and metals is poor.

Comment: Because of its unique combination of physical properties, EPDM can be used in an unusually broad range of products. Aside from applications requiring resistance to oil and hydrocarbon solvents, there is scarcely an application in which EPDM is totally unsuitable. Because of its excellent resistance to ozone, sunlight and severe weather conditions, EPDM is ideal for outdoor service. In general, EPDM, is similar to butyl but has slightly less susceptibility to contamination.

NEOPRENE™ (CR) Polychloroprene

Advantages: Good inherent flame resistance; moderate resistance to oil and gasoline; excellent adhesion to fabrics and metals; very good resistance to weather, ozone, and natural aging; good resistance to abrasion and flex cracking; very good resistance to alkalis and acids.

Limitations: Poor to fair resistance to aromatic and oxygenated solvents; limited flexibility at low temperatures.

Comment: Neoprene™ is an excellent all-purpose elastomer with a nearly ideal balance of properties and few practical limitations. General purpose neoprenes are classified in two groups: a sulfur-modified type and a mercaptan-modified type. Sulfur-modified neoprene has increased tear strength and resilience and mercaptan-modified neoprene is superior in resistance to heat and compression set.

NITRILE (NBR, Buna-N) Acrylonitrile-butadiene

Advantages: Very good resistance to oil and gasoline; superior resistance to petroleum-based hydraulic fluids; wide range of service temperatures (-65 to 300F); good resistance to hydrocarbon solvents; very good resistance to alkalis and acids.

Limitations: Inferior resistance to ozone, sunlight, and natural aging; poor resistance to oxygenated solvents.

Comment: Nitrile and neoprene™ are the highest volume oil resistant elastomers. Nitrile is superior to neoprene in resistance to oil, gasoline, and aromatic solvents. However, it does not perform as well as neoprene in applications requiring exposure to weather, ozone, and sunlight. Furthermore, it has no inherent flame resistance.

The properties of nitrile vary considerably with the ratio of acrylonitrile and butadiene. In general, as the acrylonitrile content of the elastomer increases, oil and sovent resistance and abrasion resistance improve. When the acrylonitrile content decreases, these properties deteriorate, and low temperature flexibility and resilience improve.

When nitrile is modified by polyvinyl chloride (PVC) resins, its resistance to weather, ozone, and sunlight improves considerably without significant sacrifice in oil resisting properties. PVC/nitrile is similar to neoprene, in general, however, it is much inferior to neoprene in adhesion to fabrics and metals.

Carboxylated nitrile is generally tougher and more resistant to tear and abrasion than conventional nitrile, but it is less resilient and flexible at low temperatures.

URETHANE (AU, EU) Polyurethane diisocyanate

Advantages: Outstanding resistance to abrasion and tear; very high tensile strength with good elongation; excellent resistance to weather ozone, and sunlight; good resistance to oil and gasoline; excellent adhesion to fabrics and metals.

Limitations: Poor resistance to alkalis, acids, and oxygenated solvents; inferior resistance to hot water.

Comment: Urethane is notable for its combination of hardness with elasticity, and outstanding abrasion resistance and tear strength. It may be either ether or ester based. The ester based polymer is superior in resistance to abrasion and heat and the ether based polymer has better flexibility at low temperatures. Urethanes are sold in liquid or dry form and a casting process is usually used for viscous liquid form. Conventional rubber equipment is used for processing urethane in millable gum form. The dry type has physical properties slightly inferior to those of the liquid type. Urethane is a relatively expensive elastomer whose use is usually limited to applications that require a combination of its outstanding properties, such as toughness, tear strength, and abrasion resistance.

SILICONE (VSQ) Polysiloxane

Advantages: Outstanding resistance to high heat; excellent flexibility at low temperatures; low compression set; very good electrical insulation; excellent resistance to weather, ozone, sunlight and oxidation; superior color stability.

Limitations: Poor resistance to abrasion, tear, and cut growth; low tensile strength; inferior resistance to oil, gasoline, and solvents; poor resistance to alkalis and acids.

Comment: The most outstanding feature of silicone is its ability to retain rubbery properties through extremes in temperature. It is the most heat resisting elastomer in the market today and the most flexible at low temperatures. Service temperatures range from -150 to 500F. Silicone generally does not have high tensile strength, but much of the strength it does have can be retained at very high temperatures. Because silicone is relatively expensive, it is not normally used unless extreme temperature resistance is essential.

HYPALON™ (CSM) Chlorosulfonated polyethylene

Advantages: Good flame retardance; good abrasion resistance; superior resistance to weather, ozone, sunlight, and oxidation; excellent resistance to alkalis and acids; very good color stability; moderate resistance to oil and gasoline.

Limitations: Poor to fair resistance to aromatic solvents; limited flexibility at low temperatures; fair resilience and compression set.

Comment: Hypalon™ is a close match to neoprene™ in most properties, but it is superior in resistance to acids, solvents, ozone, and oxidation and has decidedly better color stability.

ACRYLIC (ACM) Polyacrylate

Advantages: Outstanding resistance to heat and hot oil; excellent resistance to weather, ozone, sunlight and oxidation; very good resistance to gasoline and oil, especially those that contain sulfur.

Limitations: Poor resistance to alcohols, alkalis, solvents, and aromatic solvents; limited flexibility at low temperatures; inferior resistance to water and steam; slow vulcanization rate.

Comment: Heat resistance of acrylic is superior to that of any other elastomer except silicone and fluorocarbon. It performs well in both hot air and hot oil for extended periods without significant loss of physical properties. Acrylic has moderate resistance to cut growth and flex cracking. Use of acrylic is limited by its inferior flexibility at low temperatures, poor processing properties, and slow vulcanization rate. A post cure is necessary to obtain the best characteristics. Low temperature service of acrylic is possible to only about -20F. However, some types of acrylic that have improved low temperature flexibility at the expense of tensile strength and oil resistance are available.

FLUOROCARBON (FKM) Fluorinated hydrocarbon

Advantages: Outstanding resistance to high heat; excellent resistance to oil, gasoline, hydraulic fluids and hydrocarbon solvents; good flame retardance; very good impermeability to gases and vapor; very good resistance to weather, oxygen, ozone, and sunlight.

Limitations: Poor resistance to tear and cut growth; very little resistance to oxygenated solvents; fair adhesion to fabrics and metals.

Comment: Fluorocarbon is a very expensive elastomer with outstanding resistance to a wide variety of oils, fuels, acids, and solvents at elevated temperatures; heat resistance that is almost as good as that of silicone; a resistance to hot oil that exceeds that of acrylic; and weathering properties superior to those of neoprene™. Because of its very high price, fluorocarbon is used only in applications requiring excellent stability under extremely severe operating conditions. Subjecting fluorocarbon to a two stage cure cycle produces the best balance of properties.

EPICHLOROHYDRIN (ECO, CO) Epichlorohydrin-ethylene oxide or polyepichlohydrin

Advantages: Excellent resistance to oil and gasoline; superior impermeability to gases and vapor; very good resistance to weather, ozone, sunlight, and oxidation; wide range of service temperatures; good resistance to petroleum based fluids and solvents.

Limitations: Inferior resistance to oxygenated solvents; poor resistance to steam and acids.

Comment: Epichlorohydrin is a specialty polyether elastomer that appears to possess a combination of many desirable properties of nitrile and neoprene™. On the negative side, its adhesion to fabrics and metals is much inferior to neoprene's and its superior impermeability to gasses often causes processing problems and product defects because of air entrapment. Epichlorohydrin is available as a homopolymer (CO) and a copolymer (ECO) of epichlorohydrin containing about 50 percent ethylene oxide. The homopolymer is superior in ozone resistance and gas impermeability and the copolymer is better in resilience and flexibility at low temperatures.

Oil resistance of the homopolymer and the copolymer is about the same. Epichlorohydrin is now also available as a terpolymer of epichlorohydrin, ethylene oxide, and a third monomer. The new terpolymer has properties similar to those of the copolymer. However, unlike the copolymer, the terpolymer can easily be blended with SBR and nitrile, and sulfur cured; and its adhesion to fabrics and metals is reportedly the best among the three versions of epichlorohydrin elastomer.