epoxy joint

[brianyemail-nsrca at yahoo.com]

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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