[NSRCA-discussion] epoxy question-I am back

Mon Nov 26 08:37:31 AKST 2007

Mixed viscosity is about 250 centpoise at 77F. Glass transisiton is about 
128F. But can be increased to 165F......IF it is cured or post cured at 
160F. A Tg of 109F is not possible in an epoxy without the application of 
about 210F heat. Any manufacturer that tells you otherwise is either 
ignorant or a liar.
A 128F Tg DOES NOTmean degredation of the epoxy itself, it means where 
structural strength is lost and this is reversable.

Gummy is the result of water or bad mix ratio. If you apply during high 
humidity the top surface can be sticky, This is just the very top of the 
cured epoxy and does not affect the structural integrity, yet is a pain 
for us. There are chemicals that I can formulate in the blend to make the 
cure less sensitive to this problem. 

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

"Mike Hester" <kerlock at comcast.net> 
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11/26/2007 10:50 AM
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Re: [NSRCA-discussion] epoxy question-I am back

I'm interested...what kind of viscosity and what will be the transitional 
temp? Most of the resins I have run across for this purpose have a 
transitional temp that is really too low. If you take a temp reading of 
your plane's surface in the summer time, some of the darker colors can 
approack 190 degrees f. On a wood fuse this can mean bubbling, 
delamination etc. 

What Id like to see is a transitional temp of at least 190+ and a 
viscosity of around 200cps. One that actually cures HARD and not gummy 
without reducer.

I'd buy it by the gallon =)
-Mike

----- Original Message ----- 
From: Gray E Fowler 
To: NSRCA Mailing List 
Sent: Monday, November 26, 2007 10:23 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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