How to Select Elastomer

When selecting elastomeric seals for specific applications, there are a number of important selection criteria including the anticipated service conditions, the design and inspection requirements for the particular application and material specification and traceability.

The design requirements of the particular sealing application are also critical including:

Material specification and traceability are also critical for proper seal selection.

When selecting materials for your particular sealing application, the guiding principle should be "value-in-use." When evaluating seal performance, seal life and maintenance costs must be included. A seal made from EPM may be appropriate for many general applications where heat and steam are encountered, but inappropriate at higher temperatures when contaminated steam and frequent maintenance are required. The relatively high price of a fluoroelastomer or perfluoroelastomer will be recouped many times over by a seal's long service life. Specifying the proper high performance seal can also prevent costly unscheduled downtime and dangerous leakage.

SAE J200 (ASTM D2000) is a classification system for specifying rubber products for automotive use which has been adopted by other industries as a general guideline for relative performance. The following chart positions elastomers by their resistance to heat aging and swelling in oil. Although this system is a valuable tool in characterizing and positioning elastomers, it does not address the chemical and thermal resistance in hostile environments encountered in chemical processes. Due to the chemical attack on the backbone and or crosslinks of the rubber part, one must rely on field experience, laboratory testing and guidance from material experts.

Elastomers are rated for their compatibility to a variety of media including fluids, weather, ozone, etc. Usually, elastomers must perform in more than one media so it is important to understand all the elements that the elastomer will be exposed. In chemical processing environments, it is important to understand the elastomers resistance to a variety of fluids.

Elastomers in a fluid environment may absorb the liquid and swell, react chemically with the fluid and change the polymer structure, or solubles may be extracted from the elastomer causing an actual decrease in volume. In some cases, it is not the polymer itself which is adsorbing or desorbing, but the other ingredients that the compounder has used to make the part. For this reason, it is important to contact the part manufacturer in critical applications.

Most chemical resistance guides (including this one), report the percentage of swelling to indicated part performance in a certain chemical, with below 5 to 10 percent swelling considered excellent. Although swelling is a key criterion for part compatibility and one that is generally accepted as a standard for compatibility, other properties should be measured as well. A knowledgeable supplier or part manufacturer can provide additional guidance.

The mechanical properties of an elastomer will generally change after prolonged exposure to high temperatures. Natural rubber, for example, will become gummy compared to neoprene which slowly hardens. The extent to which hardening or softening is undesirable will depend upon the service required. The rate at which properties of an elastomer change increases logarithmically with the temperature. Relative small changes in temperature may, therefore, cause large differences in the degree of deterioration.
Heat aging tests are usually based on 70-hour exposure in thermally controlled hot air. While this may correlate well with longer exposure periods, it may not correspond to higher temperature in other chemical media. In critical applications, engineers should consult with the part manufacturers to see if long-term exposure in a particular media have been evaluated and documented. In addition, tests should be performed under conditions which closely mirror the actual process conditions.

There are several mechanical properties that will help determine the serviceability of elastomer parts in particular applications, most of these properties can be manipulated by the part manufacturer through the compounding process. Compounders can alter the ingredients and/or cure system to impart the required characteristics.

There are many mechanical properties that influence an elastomers performance including compression set, tensile strength, elongation, hardness and abrasion resistance. In determining specification requirements, some of these properties (or other properties) may be significant.

Compression set - Rubbers deform under load and rarely return completely to their original form. The difference between the initial and final dimensions is known as compression set or permanent set. The main difficulty with the interpretation of compression set is that the testing time is so short in relation to the time in service. Gaskets for water mains, for example must retain their sealing properties for decades, whereas the tests are done for hours. Of equal or greater importance is how the elastomer behaves while it is under conditions of constant stress or strain for long periods.

Tensile strength - Tensile strength is the maximum tensile stress reached in stretching a test piece, usually a flat dumbbell shape, to its breaking point. Tensile tests can also be made after exposure to heat, chemicals, etc. Retention of tensile after exposure is much more important than the values after exposure.
Most rubbers that have a tensile strength below 7 MPa are usually rather poor in most mechanical properties and those above 21 MPa have good mechanical properties. However, those elastomers that fall in between those numbers usually have adequate mechanical properties. In addition, rubber components are rarely loaded in tension above 1 MPa, so elastomers with a higher tensile strength rarely reach the degree of their ultimate strength.

Elongation - Elongation or strain, is the extension between bench marks produced by a tensile force applied to the test piece and is expressed as a percentage of the original distance between the marks. Elongation at break or ultimate elongation, is the elongation at rupture.

Hardness - Hardness is measured by the depth of indentation on a ball and the results are converted to International Rubber Hardness Degrees (IRHD). This scale ranges from 0 (infintely soft) to 100 (infnitely hard). The Shore durometer is a typical hand instrument that approximates to IRHD degrees. Because of the limitations of the testing instruments, hardness is usually within -/+5.

Abrasion resistance - In many applications resistance to wear is one of the most difficult to analyze and measure. Wear is usually considered in terms of abrasion, which is defined as the loss of material that results from mechanical action on a rubber surface. Abrasion resistance is complicated and depends on many things, resilience, stiffness, thermal stability, resistance to cutting and tearing, etc. Because these variables cannot be reproduced accurately in the field, abrasion tests are not recommended for specifications.

Depending on the application, other properties may be required. If the elastomer is used in exteriors, then weathering should be considered. For wire and cable applications, electrical properties should be considered. Below are a few other properties that may be considered in specific applications.

ELECTRICAL - Elastomers are used extensively in electrical applications because they provide an excellent combination of flexibility and electrical properties. Electrical testing is a complex and specialized subject. An understanding of the basic principles is necessary when specifying elastomers in electrical applications.

WEATHERING - Deterioration in physical properties can occur when elastomers are exposed to weather. This can be cracking, peeling, chalking, color changes and other surface defects that ultimately may lead to failure in surface. The most important cause of deterioration is ozone. Sunlight, oxygen, moisture and temperature also effect elastomers. Most defects can be avoided through proper compounding. Synthetic elastomers are inherently more resistant than natural rubber.

PERMEABILITY - Permeability is a measure of the ease with which a liquid, vapor, or gas can pass through an elastomeric film or laminate. Permeability is an important factor in many applications of elastomers. Linings for reservoirs, flexible fuel tanks, gaskets and seals, diaphrams, etc are some applications where permeability must be kept within reasonable limits.

classes of elastomers. The elastomers listed are frequently used in the chemical processing industry for fluid handling. Information regarding these elastomers was supplied by industry sources, including the Fluid Sealing Association. Trademarked products are manufactured by DuPont Dow Elastomers. For more specific information about these polymers, including data sheets, refer to the section titled: Products of DuPont Dow Elastomers.

Elastomers are formulated by compounders to afford specific engineering properties for an application. Vulcanization converts the thermoplastic compound into the desired thermoset shape. Crosslinks among the polymer chains impart strength and elasticity to the sealing product. It is important for seal specifiers to consult with the seal manufacturer to determine the appropriateness of particular materials for an application.

Elastomers versus plastics -
Elastomer's are long-chain polymers connected by crosslinks that impart strength, resilience and elasticity, and these crosslinks are very stable to heat and high pressure.

Plastics are also long-chain polymers, but they are not connected by chemical crosslinks. Plastics acquire their strength when the chains orient with one another and become crystalline in regions. These regions may deform or melt under high pressure and temperature.

· Retention of sealing force during pressure and temperature cycling (a major problem with many plastics)

Ratings are at a room temperature. For ratings at other
temperatures, refer to the Viton or Kalrez?/a> guides.

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

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

Butyl is unlike other synthetic elastomers or natural rubber in that it is inherently resistant to ozone and corrosive chemicals. On the negative side, butyl behaves like a plastic, in that it creeps, cold flows and has poor compression set.

Viton

Please note: more than one type of Viton?may be rated as "recommended." If so, sealing performance in service must also include considerations of other factors such as resistance to compression set, mechanical strength at service temperatures, seal design, and seal cost.

Chemical Resistance Guide

The following chemical resistance evaluation of various elastomers has been assembled by The Los Angeles Rubber Group, Inc based on the published literature of various polymer suppliers, rubber manufacturers and sources including:

The criteria used for the ratings were primarily volume swell resistance, compression set resistance, and in addition, aging resistance. For the most part the ratings were arrived from specific data or general agreement of the above sources. When no data or agreement was found, the ratings were arrived at by theory and analogy. In some cases they are the considered opinion of experienced compounders. We cannot guarantee their accuracy nor assume responsibility for their use. Several factors must always be considered when using a rubber part in service. The most important as we see them are:

a. The temperature of service. Higher temperatures increase the effect of all chemicals on polymers. The increase varies with the polymer and the chemical. A compound quite suitable at room temperature might fail miserably at elevated temperatures.

b. Conditions of service. A compound that swells badly might still function well as a static seal yet fail in a dynamic application.

c. The grade of polymer. Many types of polymers are available in different grades that vary greatly in chemical resistance.

d. The compound itself. Compounds designed for other outstanding properties may be poorer in performance in a chemical than one designed especially for fluid resistance.

Each polymer is rated for use in individual chemicals at room temperature. Where multiple chemicals are in use, refer to the rating of the most aggressive fluid when evaluating polymer performance. Polymers are rated as:

TLARGI wishes to thank DuPont Dow Elastomers for their assistance in updating this chemical resistance guide.

	Material	Chemical Group	Generally Resistant to	Generally Attacked by
NR, IR	Natural rubber, Isoprene	Polyisoprene	Most moderate wet or dry chemicals, organic acids, alcohols, ketones, aldehydes	Ozone, strong acids, fats, oils, greases, most hydrocarbons
SBR, BR	Butadiene, Styrene Butadiene	Styrene, Butadiene Copolymer, Polybutadiene	Similar to natural rubber	Similar to natural rubber
IIR	Butyl	Isobutylene, Isoprene, polymer	Water and steam	Petroleum solvents, coal, tar, solvents, aromatic hydrocarbons
EPM, EPDM	Ethylene Propylene	Ethylene Propylene copolymer and terpolymer	Water, steam and brake fluids	Mineral oils and solvents, aromatic hydrocarbons
NBR	Nitrile	Butadiene, Acrylonitrile copolymer	Many hydrocarbons, fats, oils, greases, hydraulic fluids, chemicals	Ozone, ketones, esters, aldehydes, chlorinated and nitro hydrocarbons
HNBR	Hydrogenated nitrile	Butadiene, Acrylonitrile copolymer	Similar to NBR but with improved chemical resistance and higher service temperature	Ozone, ketones, esters, aldehydes, chlorinated and nitro hydrocarbons
CO₁ ECO	Epichlorohydrin	Epichlorohydrin polymer and copolymer	Similar to nitrile with ozone resistance	Ketones, esters, aldehydes, chlorinated and nitro hydrocarbons
CR	Neoprene	Chloroprene polymer	Moderate chemicals and acids, ozone, oils, fats, greases, many oils, and solvents	Strong oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons
CSM	Hypalon®	Chlorosulfonated polyethylene with improved acid and ozone resistance	Similar to Neoprene	Concentrated oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons
CM, CPE	Tyrin®	Chlorinated polyethylene	Similar to Neoprene with improved acid and ozone resistance	Concentrated oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons
AU, EU	Urethane	Urethane polymer	Ozone, hydrocarbons, moderate chemicals, fats, oils, greases	Concentrated acids, ketones, esters, chlorinated and nitro hydrocarbons
T	Polysulfide	Organic polysulfide polymer	Ozone, oils, solvents, thinners, ketones, esters, aromatic hydrocarbons	Mercaptons, chlorinated hydrocarbons, nitro hydrocarbons, ethers, amines, hetercocyclics
Si, VMQ	Silicone	Organic silicone polymer	Moderate or oxidizing chemicals, ozone, concentrated sodium hydroxide	Many solvents, oils, concentrated acids, dilute sodium hydroxide
FSI, FVMQ	Fluorosilicone	Fluorinated organic silicone polymer	Moderate or oxidizing chemicals, ozone, aromatic chlorinated solvents, bases	Brake fluids, hydrazine, ketones
TFE/P	Tetrafluoroethylene/ Propylene	Fluorinated copolymer	Steam, amines and amine corrosion inhibitors, caustics, high pH media, wet sour gas, oil	Aromatic hydrocarbons, chlorinated solvents, ethers, limited in low temperatures
ACM	Polyacrylate	Copolymer of acrylic ester and acrylic halide	Ozone, extreme pressure, lubricants, hot oils, petroleum solvents, animal and vegetable fats	Water, alcohols, glycols alkali, esters, aromatic hydrocarbons, halogenated hydrocarbons, phenol
FKM #1	Fluoroelastomer	Standard fluorocarbon dipolymer 66% fluorine	All aliphatic, aromatic and halogenated hydrocarbons, acids, animal and vegetable oils	Ketones, low molecular weight esters and alcohols and nitro-containing compounds
FKM #2	Fluoroelastomer	Standard or specialty type fluorocarbon. Typically, >66% fluorine	Same as FKM#2. Greater chemical resistance	Ketones, low molecular weight esters and nitro-containing compounds
FFKM	Perfluoroelastomer	Fully fluorinated fluorocarbon	Best fluid resistance of any elastomer	Fluorocarbon-containing refrigerants cause minor effects

For over 40 years, Viton® fluoroelastomers have helped end users by preventing seal failures, extending maintenance intervals, handling aggressive fluids and higher temperatures, increasing safety and meeting stringent environmental regulations.

As the need grew for more resistant sealing materials, new types of Viton were created; they include specialty fluoroelastomers as well as general types. The Chemical Resistance Guide for Viton® will assist you in choosing the best type of Viton for service in the media selected at 23ºC. If data exists for temperatures other than 23ºC, that will be provided also. Selecting the best type of Viton will improve mean time between failures, improve equipment safety and reduce your plant's total operating costs. Where either the temperature or media extend beyond the capability of Viton, please consult The Chemical Resistance Guide for Kalrez®.

Despite the introduction and adoption of the specialty types of Viton in the automotive industry, part suppliers to the chemical processing industry have continued to use standard dipolymer fluoroelastomer or Viton A. Although parts made from Viton A have served the chemical processing industry well for many years, improved seal performance may be possible with specialty types of Viton. For example, Viton A is rated D for service in methanol at 23ºC because of volume swell, but Viton F, GFLT and Extreme ETP are rated A. If end users specify an "off-the-shelf" fluoroelastomer in methanol service, chances are he will receive a standard dipolymer type that may or may not even be made of DuPont Dow Viton. In this case, he may be at risk of premature seal failure.

Unfortunately for end users, seal standards in the chemical processing industry are not uniform or "standardized" across the industry. Thus, there is a proliferation of seals supplied by many distributors from many seal manufacturers around the world. These manufacturers may use different polymers, blend them with non-fluoroelastomers, incorporate "off-spec" material, reprocess scrap (repro) or apply inadequate quality assurance procedures. No wonder that seal performance may vary so widely卐ven though they are promoted as "equivalents" to DuPont Dow Viton.

To protect your plant's equipment while complying with OSHA 1910.119 regulations, it's essential for engineers, seal specifiers and purchasers to fully understand and protect equipment from seal failure. To do so, select and specify elastomer seals with the same rigor as the metallic components.

To assist end users in selecting manufacturers of seals made of Viton, DuPont Dow has selected a group of licensees, each of which guarantees the pedigree of their product as DuPont Dow Viton. Licensees will provide documentation that the seal they are supplying is made with DuPont Dow Viton. The "mark" is listed below. When end users specify specific types of Viton manufactured by Viton licensees, it is a major step in helping to guarantee seal quality and improve seal performance.

There are three major families of Viton® fluoroelastomer: A, B, and F. Viton® A types are dipolymers of vinylidene fluoride (VF₂) and hexafluoropropylene (HFP) in a ratio to give a polymer fluorine content of 66%. Viton® B types are terpolymers of VF₂, HFP, and tetrafluoroethylene (TFE) in a ratio to give a polymer fluorine content of 68%. Viton® F types are terpolymers of VF₂, HFP, and TFE in a ratio to give a polymer fluorine content of 70%. Viton® A, B and F types are generally cured using bisphenol AF (curative) and a suitable accelerator system. Other speciality Viton® types are available which may contain additional or different main monomers to impart "speciality" end use properties (e.g. improved low temperature flexibility) and these are usually cured using a peroxide system. Examples of such polymers are Viton® GLT, GFLT and ETP. These speciality types contain, in addition to the main monomers, a cure site monomer to facilitate peroxide curing.

Viton® fluoroelastomers are identified as A, B, or F types, based upon their relative resistance to fluids and chemicals. Differences in the fluid resistance among these families are the result of the fluorine level of the polymer, determined by the type and the relative amounts of the specific monomers, which comprise the polymer.

Among the standard A, B, and F types of Viton® fluoroelastomer, fluid resistance generally improves with increasing levels of fluorine, as shown below (note the volume increase after aging in methanol at 23ºC[73ºF]). As the fluorine level increases, however, low-temperature flexibility decreases; a compromise must be accepted between the fluid resistance and low-temperature flexibility of the final vulcanizate.

Fluorine Content vs. Fluid Resistance and Low-Temperature Flexibility
	Standard Types			Specialty Types
	A	B	F	GLT	GFLT	ETP
Nominal Polymer Fluorine Content, wt%	66	68	70	64	67	67
Percent Volume Change in Fuel C, 168 hr. at 23ºC(73ºF)*	4	3	2	5	2	4
Percent Volume Change in Methanol, 168 hr. at 23ºC(73ºF)*	90	40	5	90	5	5
Percent Volume Change in Methylethyl Ketone, 168 hr. at 23ºC (73ºF)	>200	>200	>200	>200	>200	19
Percent Volume Change in 30% Potassium Pydroxide 168 hr. at 23ºC(73ºF)*	(Samples too swollen and degraded to test)					14
Low-Temperature Flexibility, TR-10, ºC*	-17	-13	-6	-30	-24	-12

^*Nominal values, based on results typical of those obtained from testing a standard, 30 phr MT (N990) carbon black-filled, 75 durometer vulcanizate.

The physical properties of vulcanizates based on Viton® fluoroelastomers are determined to a large extent by the type and amount of the filler(s) and curative(s) used in the formulation and by the temperature and duration of the curing cycle used in their manufacture.

In terms of resistance to compression set, low-temperature flexibility, and resistance to certain classes of fluids, some inherent differences exist among various types, or families, of Viton® polymers. They are the natural result of the differences in types and relative amounts of monomers that are used in the manufacture of the many various grades of Viton®.

The differences in physical property characteristics, which exist among various types of Viton® fluoroelastomer products, are outlined below. As an example, resistance to compression set is an important property for seals, and if this property was considered to be the most important feature for a particular part, then one of the A-types of Viton® would be the best choice for the job. However, if resistance to the widest possible range of fluids is a more important consideration than compression set, then F-type Viton® would well be a better choice for that particular end-use application. Further, if both fluid resistance and low-temperature flexibility are equally important requirements for maximizing end-use performance of a given seal, then products in the GFLT family of Viton® would represent the best overall choice of products.

Physical Property Differences Among Types of Viton®
Type of Viton Fluoroelastomer	Resistance to Compression Set	General Fluids/Chemical Resistance	Low-Temperature Flexibility*
A	1	3	2
B	2	2	2
F	2	1	3
GB, GBL	2	2	2
GF	3	1	3
GLT	2	3	1
GFLT	2	1	1
ETP	3	1	2

1 = Excellent-Best Performance capability of all types
2 = Very Good
3 = Good-Sufficient for all typical fluoroelastomer applications

^*Flexibility, as measured by Temperature of Retraction (TR-10), Gehman Torsional Modulus, Glass Transition (Tg), or Clash-Berg Temperature. Brittle-Point tests are a measure of impact resistance only and do not correlate at all with a vulcanizate's ability to maintain a seal at low-temperatures.

As in the case of physical properties, different types of Viton® have different levels of resistance to chemical media.

Below are the differences that exist among the types of Viton, in terms of their resistance to various classes of fluids and chemicals.

In as much as certain types of Viton exhibit performance that is superior to other types in one regard, but not quite as good in some other aspect, it is important to consider the in-service performance criteria of the parts to be manufactured, in terms of both the physical property requirements and their fluid or chemical resistance needs.

Differences in Fluid Resistance Among Types of Viton®
	Type of Viton Fluoroelastomer
	A	B	F	GB	GF	GLT	GFLT	ETP
	Cure System
	Bisphenol			Peroxide
Hydrocarbon Automotive, Aviation Fuels	1	1	1	1	1	1	1	1
Aliphatic Hydrocarbon Process Fluids, Chemicals	1	1	1	1	1	1	1	1
Oxygenated Automotive Fuels (containing MeOH, EtOH, MTBE, etc.)	NR	2	1	2	1	NR	1	1
Aromatic Hydrocarbon Process Fluids, Chemicals	2	2	1	1	1	2	1	1
Aqueous Fluids; Water, Steam, Mineral Acids (H₂SO₄, HNO₂, HCl, etc.)	3	2	2	1	1	1	1	1
Low Molecular Weight Keytones, Esters	NR	NR	NR	NR	NR	NR	NR	3
High Ph Solutions, Strong Caustics, Organic Base	NR	NR	NR	NR	NR	NR	NR	1

In addition to inherent differences among the various types of Viton® fluoroelastomer, compounding variables have major effects on the physical property characteristics of the final vulcanizates. One very important variable is the crosslinking (curing system) that is used to vulcanize the elastomer.

Diamine curatives were introduced in 1957 (DIAK-1) for crosslinking Viton® A. While these diamine curatives are relatively slow curing and do not provide the best possible resistance to compression set, they do offer unique advantages, such as excellent adhesion to metal inserts and high hot tensile strength.

Most fluoroelastomers are currently crosslinked with Bisphenol AF, a curative first introduced in 1970. In 1987, an improved bisphenol curative was introduced, which was made available in several different precompounds: Viton® A-201C and A-401C. The modified system provides faster cure rates, improved mold release, and slightly better resistance to compression set, compared to the original bisphenol cure system used in Viton® E-60C and E-430. Additional precompounds of Viton®, incorporating this modified curative, were introduced in 1993, including Viton® A-331C, A-361C, B-201C and B-601C.

In 1976, peroxide curing of fluoroelastomers was made possible for the first time. The peroxide cure system provides fast cure rates and excellent physical properties in polymers, such as GLT and GFLT, which cannot be readily cured with either diamine or bisphenol crosslinking systems. In the case of polymers, such as Viton® GF, GBL-200, and GBL-900, the peroxide cure has been shown to provide enhanced resistance to aggressive automotive lubricating oils and steam and acids. Vulcanizates of Viton® polymer cured with peroxide do not generally show any significant difference in resistance to other fluids and chemicals, compared to the same polymer cured with bisphenol.

A comparison of the various processing and physical property characteristics of compounds using the different cure systems is shown below.

A Comparison of Cure Systems Used in Crosslinking Viton®
Characteristic	Type of Cure System
Characteristic	Diamine	Bisphenol	*Peroxide/Coagent**
Processing Features
Processing Safety (Scorch)	P-F	E	E
Fast Cure Rate	P-F	E	E
Mold Release/Mold Fouling	P	G-E	F-G
Adhesion to Metal Inserts	E	G	G
Performance Features
Compression Set Resistance	P	E	G
Steam, Water, Acid Resistance	F	G	E

^*Luperco 101-XL (trademark of Penwalt Corporation) and Varox Powder (trademark of R. T. Vanderbilt Co., Inc.) are commonly used.