Curing epoxy

I'm wondering if there is an impact on the strength of epoxy cured at different (high) temps. My basement is around 55 degrees and I've been less than thrilled at epoxy curing at that temp. I've read that folks out west in sunny Arizona put rockets outside to speed curing. I decided to implement this recently. I put some fillets on a rocket, then put the rocket in a box and sealed the box. I cut a hole and aimed my heat gun at the hole. Heat gun on low is supposedly 575 degrees, but it is a cheapo model.

Anyhow, I stuck a meat thermometer through the box and found that the temps got up to 185 inside. Needless to say my fillets cured in half and hour (I used West Systems). Normally in my chilly basement this would have taken four to six hours.

While I'm pleased with the results, is there such a thing as curing too fast? Can you goose that exothermic reaction to the point that it makes the epoxy weak?

West Systems is a somewhat unique epoxy system. Unlike nearly all other systems, elevating temperature during the cure beyond normal room temperatures does not improve the mechanical properties.

Roughly for each 10 degrees C increase in curing temperature, the curing time is cut in half. That should be true across all epoxy systems.

Epoxy which is cured at too high a temperature becomes brittle or weak. Epoxy that is cured at too low a temperature fails to completely cure and is therefore weak. Manufacturers of quality epoxy systems provide specifications for curing temperature range. For aerospace epoxy systems, one or more curing heat cycles will be specified. It is not generally a good idea to cure and/or post-cure an epoxy system outside of the manufacturer's specifications. If one does, the results will be different than what the manufacturer intended and likely inferior.

A very crude rule of thumb, applicable to most epoxy systems except for West, is that the glass transition temperature Tg of the cured epoxy is going to be roughly 10 degrees C or so above the peak post cure temperature.

Cured epoxy is a semi-crystalline solid. When the temperature of the cured epoxy is below the glass transition temperature, the epoxy behaves as a moderately rigid solid. At temperatures above the glass transition temperature, the epoxy loses its microcrystalline structure and becomes rubbery. In essence, all epoxy systems have a maximum service temperature that should not be exceeded. This is the glass transition temperature, or Tg. West Systems has one of the lowest Tg values of any professional epoxy system. This is probably a good feature for wooden boat construction, for which it was designed.

Gerald

Thanks Gerald, you are an epoxy genius. I checked the container of West and it gives a cure range of 50-100 degrees for the hardener I used. I don't have a reliable way to determine the mean kinetic temp of the epoxy as my reading was simple air temp. So in essence, I started with epoxy at 55 degrees and exposed it to 180 degrees, but couldn't speak to the epoxy temperature ramping during cure. The fg tubing was warm to the touch but could be handled without burning my hands.

Fortunately these were sport rockets where I will not be approaching materials performance thresholds. Your advice is definitely something to keep in mind for high performance projects. Next time I will switch to a lower temp hair dryer. And maybe I'll start reading directions. Thanks again.

I find IR probes to be nice for checking temperature of epoxy and for similar operations. I'm neither suggesting nor advocating this one; I just grabbed it as an example: https://www.amazon.com/dp/B002YE3FS4/?tag=skimlinks_replacement-20

Gerald

I don't like to speed up the curing of epoxy. The molecules cross link during the curing process, and the time required to do that has been considered by the manufacturer. In essence, that's why the stronger epoxies typically have longer cure times.

On the other hand, if I'm using epoxy in an application for which the ultimate strength isn't needed then I will hit it with a heat gun to speed up the process, and not be concerned about the loss in strength.

I use a small space heater, no need to cure epoxy in a cold basement

Dave A said:

I use a small space heater, no need to cure epoxy in a cold basement

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I'm in my basement right now enjoying 2 space heaters, one down low(feet), one up high. Overnight, a 60 watt light bulb close to the curing epoxy works good.