A few more words of wisdom and experience from Gary Hunter that I thought
worth passing on. There's no substitute for experience, even better if it's
someone elses?!?
Graham
Is Nat's statement that if ... the glass wets out and goes transparent
> (I'll be using MGS epoxy) it's OK, and that if it stays white it isn't
wetting
> out and is no good ... a reliable test? It makes sense to me, but you're
the
> expert.
>
> Please help me out here .... Jim S.
As usual, Nat is pretty much correct.
Glass fibers have a "SIZING" applied to them when they are made. Most
resins do not inherently wet out or adhere to the glass fibers very well by
themselves. Primarily because of a disparity in their surface tensions.
The sizing is applied to the fibers as a very very dilute dispersion or
solution in water. The sizing solution does several things. The water
(introduced as a fog or spray) is used to cool the fiber from its molten
state. It is also the carrier solvent for the sizing. The solution is
also a lubricant for the downstream processing of the fiber up until it is
put on a spool and dried in an oven. Most importantly, the sizing
reduces the disparity in surface tensions between the glass fiber and the
resin. The sizing formulation is tailored for the specific kind of
"RESIN" that will be used to wet out the glass fibers. A polyester (boat
resin) requires a different kind of sizing than an EPOXY resin. There
are some sizings designed to be "dual" compatible. The only way know
what you have is make sure you buy the fiberglass specifically called out
by the designer of the aircraft.
Most of fiberglass you buy at the auto parts store, or marine supply house
is intended for polyester resins. However, with West Systems Epoxy being
sold through many auto and marine supply stores for auto body and boat
repair, most of the fiberglass is probably dual compatible. You can
probably use that fiberglass to make or repair your wheel pants or fairings
and such. But, under no circumstances should you consider that glass
source for structural components of your aircraft.
One way you can tell if the fabric sizing is compatible with he resin you
are using is a visual indication of "wet out". Just like Nat said. If it
doesn't wet out the glass fabric, it doesn't turn transparent or
translucent . It stays whitish.
Since these sizings are introduced to the glass fibers as a water solution,
they generally contain an extremely minute amount of surfactants to keep
them dispersed in water while they are being applied to the fibers. These
surfactants remain on the fiber after the water is dried. As such, if
water is re-introduced to the glass fabric, the sizing can be washed off or
moved about. You have probably noticed that if you had accidently dropped
a bead of sweat onto you glass fabric, it makes a permanent mark in your
laminate, even if you allowed it to dry before you did the lay up. This is
because the sizing was washed off the fiber in that particular area.
Similar results can occur if the entire roll of fiberglass were to become
wetted.
DO NOT USE glass that has become wet.
Exposure to high humidity generally doesn't hurt the fiberglass like direct
contact with water. However, it can run the risk of mildew forming on the
fabric. Generally, this will only occur on the first layers of the roll.
Further down in the roll, the glass fabric is probably OK.
SO, if you think the glass is suspect, throw away the first few layers, and
check the fabric below before using it.
Properly stored, the fiberglass we use should have an infinite shelf life.
I ALWAY store my fiberglass wrapped up in polyethylene plastic to protect
it from contamination of all sorts. A 3 or 5 mil polyethylene painters
tarp, rolled around the roll several times. Make sure the polyethylene
plastic is about 1 foot wider than the roll. Tuck the ends of the
polyethylene plastic into the ends of cardboard roll, and wedge it into
place with a mixing cup. This is very handy, and eliminates hassles with
masking tape and stuff.
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