[NSRCA-discussion] epoxy question-I am back

Mon Nov 26 08:28:25 AKST 2007

Mike/All

You are correct about the evaporation rate. Warmth always helps. That 9% 
water in your IPA may be locked in forever. Most of the commercial epoxy 
hardeners use a percentage of a chemical called polyamide. Having a 
percentage of polyamide in the epoxy hardener blend allows for mix ratio 
errors and still get a good cure. The polyamide is mix ratio forgiving. 
Polyamide is also so viscous that it will not pour at room 
temperature-hence opposite from what we are trying to do here with  our 
super thin fiberglass on wings. Another use for polyamide is for coatings 
because this chemical also allows for application on wet surfaces, that 
is, water has little detrimental effect on the polyamide. Polyamide is 
what cures the epoxy putty used to plug underwater pool leaks. So that 9% 
water in the IPA is most likely locked up in the polyamide, and will not 
come out without excessive heat.

The majority of the epoxy hardeners is a primary poly- amine. They are 
slightly soluble with water and once that happens polyamine will no longer 
cure epoxy correctly. Getting the IPA out of the cured epoxy will make the 
epoxy less rubbery and you will get back some of the physical properties, 
and as you mentioned, it will be strong enough to keep the fiberglass on 
the wing. Right now I think this is about removing the dead weight of the 
solvent.

In a thin coating as your wing skins are and having a 100F temperature for 
a week (your attic?) should go a long way to help evaporating the IPA 
trapped in your epoxy blend.  Too much temperature too fast can cause 
blistering, so be careful. Try a hair dryer but NOT a heat gun.

I know I just stated a bunch of stuff, but hopefully this makes sense.

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

"Dr. Mike Harrison" <drmikedds at sbcglobal.net> 
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11/26/2007 10:00 AM
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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 
To: NSRCA Mailing List 
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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