epoxy joint

[Gray E Fowler]

"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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