[NSRCA-discussion] epoxy question-I am back

Mon Nov 26 07:19:12 AKST 2007

If there's one thing I hate, it's the paint job on my bombs loosing
their gloss from excess shrinkage of the epoxy.  

From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Dr. Mike
Harrison
Sent: Monday, November 26, 2007 11:00 AM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] epoxy question-I am back

Gray,

I am pleased to see your scientific answer.  It is very helpful and I
would love to have the epoxy you could make up.  I thought about putting
my existing mix full strenght but I know I could never get all the
excess out.  Fortunately, having a weakened epoxy has no real
detrimental effects on glassing a wing.  With the info you have given it
will lead me to delay primer for a while.  I would think the solvent
evaporation rate would be inversely exponential, meaning most of the
evaporation/shrinkage would occur within a couple of months.  Since it
is a model and not a bomb, there is room for lax tolerances.  

Thanks

Mike

	----- Original Message ----- 

	From: Gray E Fowler <mailto:gfowler at raytheon.com>  

	To: NSRCA Mailing List <mailto:nsrca-discussion at lists.nsrca.org>

	Sent: Monday, November 26, 2007 9:23 AM

	Subject: Re: [NSRCA-discussion] epoxy question-I am back

	1.  "Thinning the epoxy saves no weight by evaporation.  There
is virtually no evaporation and no weight loss-a big time erroneous
assumption on my part.  So if you mix 2 ounces of epoxy and add 2 ounces
of thinner and put it all on the airplane, you have added 4 ounces to
the airplane.  " 

	After seeing the real question I expected a weight increase of
at least 9% (how much water in the mix). If you put 1oz (by weight) of
solvent in 1 oz of epoxy, and apply it as a coating , thin, you will
eventually end up with 1 oz of epoxy. Mike, your solvent got trapped,
and now it will have less adhesion, less tensile, Compression and
flexure strength, and will now shrink slowly as the alcohol exits the
cured epoxy, over a year or so (see Wayne's answer). 

	None of the solvent's listed will react with any epoxy blend.
Exotherm is negligible when applied as a thin coating, but significant
in mass. The ability to have the solvent evaporate out of the epoxy
coating is directly related to the mixed viscosity of the neat epoxy
blend and the cure rate of the epoxy. Those of you who ran experiments
with 5 minute epoxy reported bad results because of high viscosity and
fast cure rate.The high viscosity (even Mike's blend is apparently too
high) will cause a skin effect where the solvent evaporates from the
surface first, creating a high viscosity skin, then the epoxy starts to
cure increasing viscosity even more, thus trapping the solvent. Using
fiberglass makes this even worse. MIKE- in one year from now your wings
will weigh less. 

	So it sounds like the the Pattern Dudes of the world need a
solution.  What AeroSlave can do is provide a super low viscosity epoxy
for sale. This will be 100% solids (which means NO solvents). Chances
are, as a mixed epoxy, the viscosity will be lower than even blends with
some solvent in them. This can be used to apply fiberglass to wings or
as a general laminating resin. 

	How much interest would there be for such a a product? It would
probably cost about $50/ quart, $25/ pint. It would absolutely require
an accurate scale (+/- 1 gram) to WEIGH the epoxy and hardener into the
appropriate mix ratio. AND I offer no warranty, simply because you guys
are the biggest bunch of experimenters in the world, and I cannot
control, nor anticipate how this could get screwed up. 

	1. Operator cannot operate a scale. 
	2. Operator has a crappy scale. 
	3. Operator added solvent anyway because thats how he has always
done it...since 1976 
	4. Operators basement is 100% realative humidity..condensation
on the wing. 
	5. Operator added "more" hardener to make it cure faster. 
	6. Operator......(enter F/U here). 

	As you can see there is one common theme to all of these
possible problems, Operator, i.e. customer. 

	So if you are STILL interested please respond and Lance and I
will take it under advisement. 

	Gray Fowler
	Senior Principal Chemical Engineer
	Radomes and Specialty Apertures
	Technical Staff Composites Engineering
	Raytheon 

"Dr. Mike Harrison" <drmikedds at sbcglobal.net> 
Sent by: nsrca-discussion-bounces at lists.nsrca.org 

11/23/2007 01:57 PM 

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Subject

[NSRCA-discussion] epoxy question

	I have read with interest all the comments and some of you have
educated me on this issue.  If I may, I will share what I have learned. 

	1.  Thinning the epoxy saves no weight by evaporation.  There is
virtually no evaporation and no weight loss-a big time erroneous
assumption on my part.  So if you mix 2 ounces of epoxy and add 2 ounces
of thinner and put it all on the airplane, you have added 4 ounces to
the airplane.   
	2.  The properties of the epoxy are changed-it becomes rubbery. 
	3.  Changing the properties is not an issue regarding finishing
the airplane(painting) 
	4.  Using thinned epoxy is fine for applying glass cloth. 
	5.  I have tried MEK(epoxy thinner), denatured alcohol,
91%alcohol, acetone.  My preference is denatured alcohol because it is
the safest and most economical, I think.  I thought MEK would be the
answer.  It is the worst of the bunch.   
	6.  The best way to glass surfaces is to thin the epoxy and
apply as sparingly as possible.  A way to do that is to apply and wipe
off excess with paper towels.   
	7.  It is almost exactly 2 ounces to glass a wing panel
complete, so 4 ounces for a whole wing.  About 2 ounces for a stab.
Properly done wings and stab glassed and painted is 2-2.5 times the
weight of monokote.   
	8.  A second thinned coat of epoxy on the glassed wing is .75
ounces each wing panel-1.5 oz total. 

	Pick your poison. 

	Later, 
	Mike 
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