banktoturn

strato911,

As you point out, a reaction that produces heat is exothermic, and one that requires added heat is endothermic. Completely independent of this is the fact that the speed of most chemical reactions increases as temperature increases. The reaction that cures epoxy is exothermic. It also is accelerated by higher temperatures ( I just checked this fact again, using a Google search ). There is no conflict here: the fact that chemical reactions are accelerated by higher temperatures is independent of whether they are endothermic or exothermic. The only difference is whether the reactions tend to increase or decrease the temperature.

I don't know specifically how the epoxy cure time varies with temperature, but the rule of thumb I learned in high school Chemistry is that most chemical reactions double in speed for every 10 deg. F increase in temperature. The fact that the viscosity of epoxy initially decreases when it is heated does not mean that the curing process has slowed down.

banktoturn

strato911

I too learned in chemistry class (that's a while ago) that most chemical reactions are accellerated by an increase in temperature. I can only assume epoxy is included in that catagory.

However, when I placed a 100 watt light bulb above a joint I was gluing (to heat it), after 12 hours, it still wasn't firm, so I removed the bulb, and let it sit for another 24 hours. Nowadays the only time I heat epoxy is if I want it to flow easier for better penetration into the wood.

CafeenMan

I can vouch for the heat part. In the middle of the summer here in Florida, 30 minute epoxy becomes 10 minute epoxy.

strato911

I can't explain why mine acted the way it did, but it wasn't a defective batch (the rest worked fine), and heat significantly increased curing time.

CafeenMan

One thing I've also noticed is that some brands are really sensitive to the climate. For example, I have some Pacer Z Poxy (30 minute) that appears to be sensitive to humidity. Every other batch cures really rubbery and to be sure I've taken time to dispense it accurately and mix it thoroughly. I finally tossed the stuff out. I'm not sure what the problem was but I'm guessing the moisture in the air had a strong effect on it.

I'm using Devcon now and not having any problems. The Z-poxy worked fine in less humid climate (Germany). Maybe it was just old, but I had it less than a year. I've had other brands that lasted much longer than that.

strato911

I don't think humidity was my problem - Canadian winters in Winnipeg have pretty dry air, especially my parent's house is heated entirely with electricity. As I mentioned, the batch didn't appear defective, the rest of the batch (before and after) worked fine. The only thing different was the heat (and light) from the light bulb.

DBCherry

My Feedback: (4)

Now we're onto something. It was the LIGHT that slowed your cure time. I almost always turn the shop lights off for part of the 24 hour cure time. I think the dark helps.