More from the world of Canard Airplane Builders:
Just a "for what it's worth" - Gary Hunter (our resident epoxy guru) replied
several years ago to a question on fuel compatibilities. His reply is below.
-Joe Person
Tech Counselor 4418
Bothell, WA
All the suppliers claim their "structural resins" are suitable for fuel
containment. And, they are - IF - the builder knows how to go about it and
does everything correctly. I have always been adamant about "post curing"
epoxies, especially the fuel tank. You can't buy cheaper insurance.
Safe-T-Poxy I or now EZ-Poxy 87 is probably by far the best in this
category, with or without a post cure. Chemical resistance is one of the
prime attributes of epoxies beyond homebuilding airplanes. They are used
for building fiberglass chemical storage tanks and piping and for lining of
steel tanks to prevent corrosion. In that industry the curing agent "type"
governs the degree of chemical resistance. Aromatic amines are by far the
best known curing agent type for overall chemical resistance - particularly
in fuels, solvents and strong acids. EZ-Poxy 87 is the only aromatic amine
curing agent available to the homebuilt world.
Next in line are "aliphatic amines". The original RAES & RAEF were
"modified" aliphatic amines. However, according to my tests the RAES
without a post cure was absolutely no good for fuel and marginal with. But,
the RAEF without a post cure was marginal and did just fine with a post
cure. Thus, the original Vari-Eze plans mandated RAEF for the fuel tanks.
Next in line are "cycloaliphatic" amines. Aeropoxy, MGS, Proset, and
EZ-Poxy 83 & 84 are "blends of modified aliphatic and cycloaliphatic amine
adducts". Modified - because the straight stuff has bad cure behavior.
Some modifications work out better than others. A prime example is the
Aeropoxy's sensitivity to temperature and moisture. I call these "quirky
cure characteristics". However, even the "quirkiest" of these curing
agents will resist fuel - IF - they are properly applied and fully post
cured.
DO NOT simply brush the resin on the inside surface of the already cured
fuel tank and expect it to cure like a 2 or 4 ply lay-up. This is where the
screen plugging flakes of epoxy come from.
Your original layups for all the inside tank components should be "wet" so
to be certain there are no dry spots, pinholes or voids. This is no place
to be too concerned about weight. Even the BID tapes in the corners should
be nice and wet. Peel ply only were secondary bonds will be needed. For
those of you than like the smooth surface of peel plying - add an extra
final ply of a fine weave light weight fiberglass deck cloth. It will give
you a similar smooth surface without introducing or hiding voids. Warm
shop temperatures and low humidity will reduce the curing agents "quirk"
factor.
There are many ways to obtain a post cure on the tank. Before the tops are
put on, you can use heat lamps to cure the inside surfaces. You can
pre-post cure the inside surface of the top too. After the top is bonded
in place you can post cure these bonds by heating the exterior surface with
heat lamps. The heat will work its way to the bonds.
OR, after the top of the tank is bonded on, you can circulate warm air
through the tank for several hours. I did this with the outlet end of my
vacuum cleaner inserted in to the fuel cap opening. About 140F is
sufficient.
RULE OF THUMB - If you can hold your hand on the surface to the count of 10
- the temperature is 140F or below.
Hope this is helps.
Gary Hunter
EAA Technical Counselor
Vari-Eze N235GH
EAA is Experimental Aircraft Association.
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