epoxy joint

[Keith Black]

Brian, I look forward to Gray's answer, but I can share my experience with West Systems epoxy.

West Systems epoxy is not a 1:1 mixture like many of the hobby epoxies. As I understand it the 1:1 epoxies have extra fillers added to the hardeners to provide the 1:1 ratio to allow for easier to mixing. West Systems uses five parts resin to one part hardener. When I first started using the West epoxy I tried measuring the volume either in a small mixing cup or sometimes counting drops (when using very small amounts). This yielded very unreliable results, sometimes it hardened, sometimes I just made rubber. Then I broke down and bought a triple beam balance that measures to the 10th of a gram so I can mix by weight. Using this method I'm able to mix very small quantities and it always hardens properly.

My "guess" is that the hobby epoxies add fillers such that the volume of the hardener is equal to the volume of the resin.

BTW, for those using the epoxies from the hobby shop, I highly recommend trying the West Systems epoxy. This stuff is MUCH cheaper by volume and seems to be a much higher quality epoxy. You purchase one resin and different hardeners for different drying times. Also, they have a larger variety of fillers for various purposes, I personally like the Colloidal Silica (406) which will thicken up the epoxy to keep it from running, but unlike micro balloons it is extremely strong. (This is not my own discovery, Lance Van Nostrand sold me on the West Systems epoxy, thanks Lance!)

Keith Black 
  ----- Original Message ----- 
  From: brianyemail-nsrca at yahoo.com 
  To: discussion at nsrca.org 
  Sent: Tuesday, January 18, 2005 8:58 AM
  Subject: Re: Re: epoxy joint


  Gray, since it is epoxy school time. I was curious what is the best way to measure parts a and b for the epoxy mix. My method is to squeeze out a couple of lines of equal length on top of my mixing pad. I usually only have a tablespoon leftover of one or the other parts when I run out.

  Ive tried my gram scale as well, but its not sensitive enough to measure the small amounts Im mixing. So then I wonder is it best to mix by weight or volume or does it matter. 

  Ive always been curious how sensitive the ratio is to get the optimal properties.

  Thanks for the information.

  Brian

  Gray E Fowler <gfowler at raytheon.com> wrote:

    "Gray
    What are the negatives to thinning with alcohol. Suppose you add fiber filler the epoxy gets thicker.Suppose you thin to a just a little to work well.Have you hurt the joints strength.I do it and it never has failed. This way I use much less.
    Lotta talk here about epoxy and none about thinning.

    Jim Ivey" 


    Jim 

    So many negatives......where do I start. 
    Epoxy does not "dry" it cures. The epoxy resin reacts with a hardener which for our applications is usually a primary amine. The hardener must be added in a stoichiometric amount. Any excess hardener means it does not react in as all the epoxy reaction sites are used up. Too little hardener and then you have unreacted epoxy. Both of these situations cause a loss of properties. The most dramatic will be a reduction in the glass transistion temperature  (Tg) which is the temperature when the epoxy softens and no longer transfers stress because it is too soft. Considering that the room temp epoxies that we use have a max Tg of about 128F, a bad mix ratio could have a Tg of 100F which is a temperature that many people fly at in the summer. 
    Adding solvents means you just cured the epoxy with trapped unreacted stuff, which will do the same thing as a bad mix ratio. Alcohol can really be bad, if it has water in it.  You have not had any failures because once again, it is still strong enough for our applications. Trapped solvent will reduce the Tg and rubberize the epoxy, which will change over time as the solvent slowly escapes, but the epoxy will never reach full properties. 
    Using a "thinned" epoxy as a coating such as on a wing is different because in the very thin coat, the solvent can evaporate before the epoxy cures.  



    Gray Fowler
    Principal Chemical Engineer
    Composites Engineering
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