Somewhat OT - epoxy cure cycle question

Howzit all,

I’ve got a question for the epoxy gurus. There is a comment in System Three’s Epoxy Book on page 3, paragraph 1 regarding the relationship between curing and temperature:

“The curing rate will vary by about half or double with each 18 degrees Fahrenheit (10 deg. Celsius) change in temperature.”

I was just wondering if this 18-degree rule of thumb applies to all epoxies, or just to System Three’s formulations.

I ask because we’re having some curing issues with an epoxy encapsulation job at work, and it’s coinciding with some cooler temps (it dropped into the 30’s - in Florida - yikes!).

I’m ruling out variables one by one.



I think Greg Loehr said the same thing about Resin Researches temperature sensitivity, so it may be a universal thing.

the cure times of MAS epoxies were radically different for me

depending on whether I kept the room heated or not.

Most resin suppliers/manufacturers will supply a graph of geltime vs temperature. Certainly I use the graphs a lot when doing infusion as the viscosity / temperature / geltime / resinfront travel all gets kind of exciting, especially when somebody turns on the heating halfway through a job.

it dropped into the 30’s - in Florida - DUH!

Read the label… most resin’s (poly and epoxy) minimum temperatures are at about 65 degrees.

I’ve got another cure cycle question: how long can you wait before post-curing epoxy?

I’ve looked at the cure schedules I could find online, and read Greg Loehr’s comments on post-curing (esp the problems that can result if you just pop a room temperature piece straight into a hot oven). All of the schedules seem to assume going straight from initial to post-curing, though it would be a lot easier to do a few steps of laminating/sanding/cutlaps/etc. and then do one postcure step at the end.

I’m sure there are diminishing returns to the amount of high-temperature strength you get from post-curing the longer you wait to do it, and it will depend a lot on what passes for “room temperature” in the workroom.

With polyester it sounds like it’s still worth it to postcure a board a week or more after pouring the resin.

Know any rules of thumb for the epoxies you work with, or does anyone have a guess for RR2000?

Yeah, I can see how that part sounds. Actually, the room that the parts are in probably didn’t see temps much lower than 65. My main question was whether the time/temperature relationship mentioned in the Epoxy Book is universal. It was an interestingly specific detail that’s rattled around in my head for a long time. The product data sheet for this material only offers two references for cure time: 7 days at 77 degrees or 2 hours at 150. Obviously, there’s a curve in between those two points, but my math graphing skills are way too rusty to figure it out.

In this case, I think there’s something else going on, because even at 150 degrees, things are taking far longer than 2 hours to cure.

Thanks for the responses. Any other thoughts are welcome.


Well, to partly answer my own question, I just did the math (minus a fancy graph) and found this stuff doesn’t quite follow the 18-degree rule.

Guess I should know better. Universal rules are usually simpler, and more golden.



7 days at 77…

That sounds like some slow curing epoxy. If you are within range and it still isn’t curing as specified, there are other possibilities like double checking the mix ratio, shelf life of product, etc.

I’ve heard of factory errors and bad batches but typical snafus involve things like shelf life expiration or operator error.

If you have any left, check the expiration or packaging dates if they’re on the label. If it hasn’t expired, mix another small batch and double check your ratios, mixing technique and all that. Put it at 150 and see how long it takes to cure. If it goes off OK, consider an incorrect mixing ratio on your project. If it doesn’t, consider contacting manufacturer about a possible bad batch.

In my experience (which is based on a lot of reading and practical work) it doesn’t matter too much when you do a post cure. In theory with ambient cure resins you don’t need to post cure although all epoxies improve their mechanical properties with a post cure. I’d completed my longboard and “glossed” it before reading the small print on the epoxy.

“…once ambient cure is achieved a post cure is necessary of 5 hours at 80C or 8 hours at 60C”

I did some tests over a number of days and found that as long as a post cure was done at some stage then properties improved. Certainly leaving it (the resin glass sample) for 2 weeks and then postcuring yielded the same results as postcuring it after 1 day.

Hope that is of some help…


First you have to compare apples to apples. By that I mean cure rates are also a function of the mass and area that mass is spread over. For example, you cannot compare the cure rate of a 100 g epoxy mix in a cup to the cure rate of that same 100 g spread into a thin film. Anybody who has left a large epoxy batch in the cup can attest to this as the exotherm in sufficient mass can be compared to a self-fulfilling apocalyptic prophecy!

So if you are comparing the same mass to area ratio, then the general rule of thumb is doubling of reaction speed for every 10ºC for most room temperature curing epoxy, but that is a very general rule. In fact the relationship of cure speed with temperature is non-linear (it really approximates an negative exponential… look up the Arrhenius equation if you’re sufficiently geeky). That is why almost all epoxies, except very specialized formulas, will not fully cure below a certain temperature. Here the general rule of thumb is that temps. must be warmer than 50-60ºF depending on the specific epoxy.

This also has a bearing on post cures. At any temperature, as the reaction continues the rate slows down. In simplest terms, the two reactive molecules have a harder and harder time finding each other because there are less and less of them and they are being trapped in place by the transition from liquid to solid. Post cure heats helps to “re-liquidfy” things and the remaining reactive molecules can now find each other easier. Ideally, post cure heat should be applied in the Bstage before the reaction has gone too far because after a certain length of time the cross link density is too high to allow much movement. Nevertheless, a post cure at a temperature above the resin’s Tg (glass transition temperature is something around 130-150ºF for most room temp cure formulas) will do some good. The problem with surfboards is that high of temp can start to affect foam stability and cause out gassing.

Hope this helps.


Rikds is correct about the timing of the post-cure. And most epoxies are very temperature sensitive

with regard to their initial cure, you can totally suspend the reaction by placing the resin in a freezer.

We don’t do our lam process without proper resin and ambient temperature.

He shapes and is a chemist too! Mike you are the man…give me a shout when you get a chance.

Hey Rob, thanks for the compliment, but I’m just a shaper who hangs out with smart people

and keeps his ears open. Great explanation of cure/post cure in your first post, btw. For a resin

chemist to put it in nice simple terms that anyone can understand helps a lot.

And we’ll talk sometime, I’m just buried in foam and customer em’s and phone calls, etc.