epoxy joint

[Jim Ivey]

 Thanks Gray that was a very comprehensive answer. Now we know about thinning.

Jim Ivey
> 
> From: Gray E Fowler <gfowler at raytheon.com>
> Date: 2005/01/18 Tue AM 09:37:10 EST
> To: discussion at nsrca.org
> Subject: Re: Re: epoxy joint
> 
> "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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