Seals, gaskets, and o-rings for high pH applications, including organic acid technology ("OAT") coolant, require precise definition. Unfortunately, there is a lot of loose talk regarding base resistant elastomers, or BREs, in the world of fluoroelastomers. However, a hot, high pH fluid will distinguish a counterfeit base resistant elastomer within a couple thousand hours.
The materials engineer should be careful selecting a base resistant seal material for highly alkaline or caustic environments. There are sealing products loosely referred to as base resistant, which in fact are not base resistant. An old adage says that a woman cannot be "kind of" pregnant. She is either pregnant or she is not. So it goes with fluoroelastomers: they are either base resistant or they are not. The presence of any vinylidene fluoride ("VDF") in the elastomer backbone will render the elastomer susceptible to dehydrohalogenation. In other words, the elastomer is not base resistant if VDF is present.
It is common knowledge that a typical FKM class elastomer (fluoroelastomer), containing 40% to 80% vinylidene fluoride (also known as VF2 or VDF) will fail in a moderately basic environment. The polymer manufacturers have never suggested otherwise. However, in the early 1970’s DUPONT had just developed a fluoroelastomer, now called VITON® GF, with only 7% of VF2. Many suppliers and distributors, to this day, suggest that having only a little bit of VF2 induces base resistance. The low vinylidene fluoride or VF2 elastomers referred to as a BRE polymers often suggest the same. This is simply not true.
In the early 1970’s, Seals Eastern performed testing for the Johns-Manville Supertemp-Tite ® line of steam/hot water pipes. Multiple AS568-214 o-rings of a low VF2 content fluorocarbon were aged in a time sequence up to 12,000-hours of 150C continuous steam. The saturated steam / hot water was a controlled pH of 7.0-7.5 that would simulate a factory steam loop. Usually, a slightly basic pH is maintained to prevent corrosion in all such steam systems.
The o-rings aged in the steam loop represented a fluorocarbon elastomer containing only a small amount of VF2. The results illustrated the degrading effect on the elastomer backbone of any VF2 in a mildly basic environment. Remember, a pH scale is logarithmic so each pH point (1 thru 14) is 10X greater than the previous point (e.g. 7-8-9-10 is 1-10-100-1000x more aggressive in terms of nucleophilic reagents).
The major breakdown product of a VF2 containing elastomer is hydrogen fluoride and is the cause of the black corrosion noted in both our SAE Paper 2000-01-0923 (Fluoride Ion Concentration) and numerous field failures. Professor H.J. Harwood, in his peer-reviewed paper developed for the "Third Geothermal Seals Technical Session-1980", defined the breakdown process in great detail. His paper was subsequently published in ASTMs prestigious "Journal of Testing & Evaluation" and is available (JTVEA, Vol. 11, No. 4, July 1983, pp. 289-298. This work and data is twenty-five years old: nothing has changed since Harwoods 1980 work.
Dr.W.W. Schmiegel (Dupont Dow Elastomers), in his 2003 paper entitled "A Review of Recent Progress in the Design and Reactions of Base-Resistant Fluoroelastomers" presented at The International Rubber Congress, states "[i]t is an inescapable fact that amorphous fluoroelastomers of the VF2/HFP/TFE and VF2/HFP/PMVE can not be made resistant to gross dehydrofluorination and subsequent embrittlement due to overcrosslinking on contact with strong nucleophilic bases." This has been a market reality for more than twenty-five years. This is why Seals Eastern has spent so much time and money to develop its 7182TM brand AFLAS® technology. Further evidence is found in DuPonts recent introduction of their TBR ( VTX-8802 ) polymer in an effort to share in this growing market.
Historically, the major fluoroelastomer producers and fabricators not having a non-VF2 offering have resorted to “bad-mouthing” AFLAS by pointing out what was perceived to be the low-temperature shortcomings of Aflas. Specifically, the material possesses a glass-transition point (“Tg”) slightly above 0˚C. Tg is the temperature at which a polymer becomes "glassy". Seals Eastern has repeatedly found, that in many engine applications, a high Tg is simply not an issue regarding seal integrity. Nevertheless, we have the same situation today. Fabricators, who have nothing to offer for high-temperature, high pH environment applications, confuse potential end-users of Aflas ® or DuPont’s TBR with the low-temperature issue. The confusing premise is that a low VF2 content FKM polymer with a (-12°C) Tg will somehow perform better than an Aflas with its 7ºC Tg. The tragedy of this argument is that attention is diverted from the catastrophic consequences of a high temperature, high pH solution (i.e. hot engine coolant / oil) in contact with a VF2 containing polymer, no matter how small the VF2 content.
Silicone rubber offers no reprieve for hot OAT coolants. It is a widely know fact that silicone, a condensation polymer, will simply revert in the presence of a strong base under elevated temperatures such as you might find in engine "hot spots". One simply has to research the massive warranty issues (seal and gasket failures) experienced by several of the major engine manufacturers.
Fortunately, there is something called "the market place". Whether or not a VF2 containing elastomer works in the market for alkaline sealing applications, such as OAT coolants, will be discovered. Unfortunately, the truth is often only realized after considerable expense and the damage done.
Seals Eastern Inc.
Dan Hertz, Jr., President & Technical Director
April 29, 2004
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