Curious epoxy question?

I had some epoxy left over from my last project, in the form of a half gallon of resin and a quart of hardner (MAS 2:1 mix) that I wanted to use what little was left for another project.  The pumps were a little sluggish and in need of a cleaning to ensure a good ratio mix.  Conventional wisdom says vinegar cleans the pumps and I did just that, to avoid having to purchase new ones for the little I had left and wanted to use.  However, I was a little startled when I opened the hardner container and strong fumes of ammonia wafted my way.  Not alarmed, just surprised.  I had no clue that epoxy hardener had any ammonia in its chemistry I'm only posting this to be sure my hardener isn't spoiled, despite that it's supposed have a very long shelf life unmixed.  Comments anyone?  Thanks.  Bob H

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RE: Curious epoxy question?

The diaper pail/old litterbox smell is normal for old hardener. I'll let a real chemist go into the details, but basically the nitrogen and hydrogen in the amines (which also cause blush and allergies) come loose and form ammonia (NH3).

Make sure that you get all the vinegar out of the pumps before putting them back into the jugs.



RE: Curious epoxy question?

Thanks Laszlo, is "old hardner" that smells like amonia still usable? Best,  Bob H

RE: Curious epoxy question?

Sure is, I just used some yesterday. Only thing, it seems to get slower as it ages. I noticed that my fresh-opened bottle cured a lot faster than the 3-year-old one, even though they were both medium. But the old stuf did cure hard after a couple of days.



RE: Curious epoxy question?

Thanks again Laszlo; I had the same effect, a longer than normal cure, but on the second day it was a done deal.  Almost threw the stuff out; glad I didn't now.  Best,  Bob H

RE: Curious epoxy question?

Hm.  Laszlo, I have always thought the YOU were the real chemist in this group. 

Anyway, until the job is filled, here is my guess about the chemistry involved in the two phenomena you describe--the ammonia smell and increased cure time associated with old hardener.

(Note that this 'helpful shop tip' may do absolutely NOTHING to help you build a kayak faster, cheaper, or better.) 

  1. Epoxy gets its strength when a matrix is formed, which happens because "amine" (NH3) groups on each hardener molecule form strong bonds with "epoxy" groups on each resin molecule.  (This part I am sure of; all the rest is a guess.)
  2. When hardener ages, some of the NH3 bonds are broken by water and CO2 in the air. NH4 (ammonia gas) is released (thus the smell), and amine blush molecules are formed.
  3. Now there are fewer NH3 groups to bond with the epoxy groups.  Thus, the curing reaction time increases.



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