[NSRCA-discussion] epoxy question-I am back

Mon Nov 26 07:00:58 AKST 2007

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 Please respond to
              NSRCA Mailing List <nsrca-discussion at lists.nsrca.org> 

       To "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>  
              cc  
              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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