Greg,

Without empirical test data to support my position, just observational...yes, I believe, they do change over time.

I've been hanging on to a batch of epoxy and hardener in partially filled cans for something like 2 years now. Yesterday I pulled out the batch to mix some fairing compound and, in a modestly heated shop, the spread material was still not completely hard after two days on the new deck I'm building for a canoe.

I'm of the opinion that some of the aromatic properties of the the hardener, especially, have evaporated over time. That goes along with an oxidation process that has shown itself through the darkening of the liquid. The components also exhibited a unique, somewhat milky appearance when being mixed. That could have come from the chemistry being somewhat cool relative to the air temps., I don't know.

These materials have also been exposed to some pretty wild temperature shifts here in Utah and that may have further destabilized the chemical reactions.

I haven't spoken to my epoxy supplier yet about the suspected behavior but as soon as I do I'll get back to you guys on my findings. I use PTM&W Epoxy, a SoCal manufacturer, which is typically used in the aerospace industry for production tooling and parts.

Chris Ostlind