CRG: Chemical Resistance of Viton™ (2024)

For over 50 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.

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-S and Extreme™ ETP-S 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 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". 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…even though they are promoted as "equivalents" to Viton™.

To protect your process 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™, Chemours has selected a group of licensees, each of which ensures the pedigree of their product as Viton™. Licensees will provide documentation that the seal they are supplying is made with Viton™. The "mark" is below. When end users specify specific types of Viton™ manufactured by Viton™ licensees, it is a major step in seal quality and improved seal performance.

There are four major families of Viton™ fluoroelastomer: A, B, F and ETP. 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.

Viton™ Extreme™ ETP-600S is a terpolymer with ethylene/tetrafluoroethylene/PMVE. ETP-600S is peroxide cured. Other specialty Viton™ types are available which may contain additional or different main monomers to impart "specialty" 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-600S and GFLT-600S. These types contain, in addition to the main monomers, a cure site monomer to facilitate peroxide curing.

Specialty types of peroxide cured Viton™ made with Advanced Polymer Architecture (APA) were commercialized in 2002 that have improved processing and end-use performance characteristics. These types have an "S" designation.

Differences in the fluid resistance among various families of Viton™ 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.

*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.
Specialty types of Viton™ made with Advanced Polymer Architecture (APA) were commercialized in 2002 that have improved processing and end-use performance characteristics. These specialty types have an "S" designation.

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

*Bisphenol cured
**Peroxide cured
***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. Specialty types of Viton™ made with Advanced Polymer Architecture (APA) were commercialized in 2002 that have improved processing and end-use performance characteristics. These specialty types have an "S" designation.

* Bisphenol cured
** Peroxide cured

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 (DuPont™ 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 and B-651C.

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-S and GFLT-S, which cannot be readily cured with either diamine or bisphenol crosslinking systems. In the case of polymers, such as Viton™ GF-S, GBL-200S, and GBL-600S, 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.

Specialty types of peroxide cured Viton™ made with Advanced Polymer Architecture (APA) were commercialized in 2002 that have improved processing and end-use performance characteristics. These types have an "S" designation.

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

Rating: E = Excellent, G = Good, F = Fair, P = Poor

*Luperco™ 101-XL (trademark of Arkema Inc.) and Varox™ DBPH (trademark of R. T. Vanderbilt Co., Inc.) are commonly used.