Dan, high enough peroxide concentration in the fuel will deteriorate polysulfide type sealant.
See:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA087267&Location=U2&doc=GetTRDoc.pdf
In the strongest (10 meq.) peroxide concentration, there was serious degradation.
From a practical standpoint, I have to think that if gasoline + air equals destruction of pro-seal, we'd see a lot more goo at the top of old gas tanks. As the study said, concerning MIL-S-8802 sealants: "These tank sealants are intended to provide very long service and are difficult to repair."
Ted,
Great find. What the paper is confirming is a mechanism for the breaking of previously crosslinked polymer linkages, and to Dan's point, proof that such a mechanism exists. Ted I know the paper specifically discusses JP5, but I'll make the case that the same potential exists in avgas. I've copied two relevant passages out of the paper to form the hypothesis.
This degradation is attributed to a reversion effect caused by polymer chain and/or crosslink scission. Polysulfide polymers are susceptible to attack by acid catalyzed hydrolysis at the backbone formal linkage. Another polysulfide degradation route is by direct oxidation at the formal linkage but this usually occurs at temperatures around 300 T. The polysulfides are generally considered to have good oxidation resistance at temperatures below 200'F. They are also cured by inorganic and organic oxidizing agents. From the available data it is uncertain whether catalyzed oxidation or acid catalyzed hydrolysis at the formal linkage is the primary degradation process. The latter mechanism is favored.
MIL-S-8802 elastomeric polysulfide sealants, particularly manganese dioxide cured type, are prone to severe degradation in 10 meq. peroxide fuel (1000-1350 hrs.) where the fuel acid number exceeds normal limits. It is uncertain whether a catalyzed direct oxidation or an acidic hydrolysis mechanism is responsible. The latter mode is believed more likely to occur. Elevated fuel acid numbers are believed due to peroxide decomposition by-products possibly present in the original prepared JP-5/peroxide concentrate and additionally accumulated in the stagnant immersion media during the test phase. Data on the effects of lower peroxide concentrations and acid numbers is needed to further assess damage potential with MIL-S-8802 sealants.
With evidence of the effects of high PV given above, the charge is made that avgas
could exhibit the same tendency should it have elevated PV and acid value (AV) numbers or given the right mechanism for PV / AV formation. If such conditions exist, polysulfide sealants, particularly those that are cured with manganese dioxide (which is probably the most commonly used sealant in RV construction), are prone to the effects as seen in the pictures given several posts ago. So the question is, does avgas have a tendency to have high PV and subsequently high AV?
One
potential route for avgas to have high PV would be the inadvertent blending of small quantities of jet fuel with avgas. Perhaps in the transferring process at the refinery, or perhaps in the transportation of both fuels, common pipes and or hoses would contain enough residual jet fuel to contaminate avgas, giving it the materials it needs to start forming PV and AV. My thinking is that PV contamination by this route isn't likely, but its a possibility.
The second route would be the formation of PV's in the avgas storage tank itself. This possibility is discussed below.
Storage Stability
Avgas instability involves multi-step reactions, some of which are oxidation reactions. Hydroperoxides and peroxides are the initial reaction products. These products remain dissolved in the fuel but may attack and shorten the life of some fuel system elastomers. Additional reactions result in the formation of soluble gums and insoluble particulates. These products may clog fuel filters and deposit on the walls of aircraft fuel systems, restricting flow in small-diameter passageways. Instability of avgas during storage is generally not a problem because of the way the fuel is manufactured (see page 66), and most fuel is used within a few months of its manufacture. Storage stability can be an issue at locations where fuel is stored for occasional or emergency use. Avgas that has been properly manufactured, stored, and handled should remain stable for at least one
year. Avgas subjected to longer storage or to improper storage or handling should be tested to be sure it meets all applicable specification requirements before use. (pg 50, Cheveron Aviation Fuels Technical Review FTR-3, 2006)
I take away from Chevrons explanation that, it
shouldn't be a problem because fuel is used relatively quickly, but , the potential is there should the fuel be abused or mishandled. What Cheveron does verify is that avgas
is susceptible to oxidation reactions that can cause PV formation. This leads into the third mechanism of PV and AV formation and its already been touched on several posts back...that is that a long term, slow fuel leak will cause the damage we've seen in the picture.
If Chevrons explanation is true, then fuel leaking from a tank, over a period of time would have high PV...not in the tank, but on the outer surface where the fuel is actively exposed to fresh oxygen. Obviously, oxidation reactions need...oxygen. That exposure occurs on the surface of the fuel wetted tanks. The fuel dribbling down on the outside of a leaking tank is thin, thin enough and spread out to have a very high surface area perfect for oxygen exposure. Over time, this oxidation reaction forms peroxides and given enough time, acids, which we now know have the ability to degrade polysulfide based sealants. Peroxides tend to be heavier than the base component from which the are formed, which means that as avgas is constantly dripping and wetting the surface, and as that avgas is evaporated away, it leaves behind peroxides that will stay in intimate contact with, you guessed it, polysulfide tank sealants. The reason you don't see (or shouldn't see) softening of the sealant inside the tank is that the residual oxygen levels inside a fuel tank are much lower than in free air. This coupled with fact that as you're out flying about, avgas is actively washing away any peroxides that might have formed and the engine consumes them, hence they never have a chance to build up inside the tank like they would on the outer surface of a fuel leaking tank.
Of course this is one possible explainable of the ONE case that we just happen to have photographic proof of, and its only a theory on how this kind of damage could be occurring.
Dan, you've mentioned mogas, and I was going to do a bit more research on that part of the PV equation before I commented, but I do believe that mogas is heavily "doped" with various goodies that could easily oxidize and form PV's and AV's. Its clear that the mechanism (PV / AV degradation) is a concern. Mogas, and the potential for it to have ethanol in it, (ethanol tends to be weakly acidic) significantly increases the chance to have PV and AV in enough concentration to cause problems. In an actively flying RV, this might not be a problem, but what about those planes that don't fly often that have mogas in the tanks? Interesting thought, and your point about bad fuel is correct. Its not bad fuel causing the problem, its the potential for
any particular fuel to form peroxides and acids that is concerning.
