View Full Version : Epoxy Strength

08-12-2001, 01:25 PM
Anybody out there got any idea whether heated (softened) epoxy cools and rehardens to its original strength after heating? It would be great to know as it;s the subject of many a long debate over here.

Thanks, J.B

08-12-2001, 01:54 PM
It depends how warm the epoxy gets. You can usually warm epoxy to its glass transition temperature (Tg)without harm. The material is unpredictable after Tg. Almost all manufacturers will give you the Tg of their epoxy if you ask them.

If you heat epoxy much past its Tg, permanant damage is possible. Epoxy is a thermoset plastic system. This means once cured the molecules are permanently locked (set)into position. Heating up to the Tg will Soften the epoxy but not damage it.

The opposite of a thermoset plastic is a thermoplastic. Most of the materials we think of as plastic fall into this catagory such as polytheylene, polyproplene and nylon. These materials can be melted and reformed many times with minimal degredation. Epoxy cannot be softened enough to reform it after it has initially cured.

I don't know what your application is but if you plan on introducing large temperature changes, pay attention to differences in thermal expansion rates. Damage can result to the bond if the difference and/or temperature change is too great. Sometimes this can be useful, for example, when using a heavy duty soldering iron to release a metal fastener that has been epoxied in place. This works well because the heat softens the epoxy (bond) and because the different expansion rate of metal and epoxy concentrates a high strain exactly at the bond line.

George Roberts
08-12-2001, 03:20 PM
There are other issues involved also.

Properties of most room temperature epoxies are improved by post curing. The exact amount of improvement and the temp for the postcure depends on the brand of epoxy and hardener used.

I have postcured my epoxy at 140 degrees with reasonable benefits and no problems. I also have postcured under black plastic on 110 degree days (I have no idea what the epoxy temp was).

I suspect any reasonable boat usage (away from engine parts) of room temperature epoxies will not damage the epoxy.

Mike Keers
08-12-2001, 06:40 PM
All I'll add is that the type and mix ratio affects the properties significantly.

I use FCI epoxy, generally the 1:1. The specs for their epoxies vary (they cite the "heat distortion temp", which I'm assuming is the 'Tg' BBIsle speaks of) from 121F for the 1:1 up to 130F for the 5:1 mix.

I asked specifically about the lower 1:1 temp for boatbuilding, and they recommended I simply don't paint a 1:1 coated hull black for example; that the epoxy simply begins to measurably deform under lab testing at that temp, it doesn't fail or let go or run off the boat. http://media5.hypernet.com/~dick/ubb/wink.gif

I would guess it would return to it's harder state if only heated to the lower limits. I live in the desert, and it's been 115 degrees here. I have a few wood and epoxy boats sitting around outside in the sun, and none has ever failed or come apart yet (or melted). The decks must certainly get up to 120.

08-12-2001, 11:24 PM
Black painted epoxy on an 80 degree day will reach about 140 degrees. Exposure to such temperatures is enough to cause blemishes when using some epoxy systems, though most would need higher temperatures.

There are literally hundreds of comercially available epoxy and hardener combinations usable for boatbuilding. Some of them would not give good service when hotter than 115 degrees. On the other hand, a few will give good service to 250 degrees.

Post cure is a little different than service temperature or heating to the point of damage. But you are correct in believing it has something to do with service temperature. As epoxy cures and becomes more rigid, molecules have a harder time finding reactive sights. Post curing adds energy needed to complete the molecular crosslinking. An easy way to think about it is that heat causes the molecules to vibrate or move so they can find their reaction sites. Correct post curing increases the service temperature.

Remember, the strength and stiffness of epoxy plus wood, or epoxy plus fiberglass (or any other material), is affected by temperature change. Anytime two materials are bonded with different expansion rates, changing temperature will change the combined strength and stiffness. Why? If the epoxy and other material stretch differently when exposed to thermal changes, some of the bond strength is used up holding the two together. Thus, this used up strength is not available to help with the normal loads, trailer, motor mount, hull strength and so forth.

How much degradation? For wooden boat applications, we are talking about 5% or so degradation of strength if the temperature is below the Tg.

Other things come into play if temperature is higher than the Tg. Epoxies become much more flexible after the Tg is reached though it is hard to predict how much. They will eventually no longer hold the wood in place. This is made worse by water intrusion which softens the epoxy somewhat more. Basically, after the Tg has been passed we know the epoxy is losing strength at an unpredictable rate. Back to the original question, it may or may not regain full strength after such exposure. To be safe choose an epoxy with adequate Tg for your service needs.

08-12-2001, 11:39 PM
Mix ratios may or may not determine the temperature capability of an epoxy. A lot depends on the chemistry of the hardner system, the amount of fillers and so forth. For example, it is absolutly possible to buy a 5:1 epoxy and hardener combination with poor temperature capability. Your best bet is to check with the manufacturer for the glass transition temperature (Tg).

Heat distortion temperature is slightly different than Tg but almost the same value. It is just as good as the Tg as far as this discussion is concerned.

Since you haven't reported any problems with your paint, you must have chosen the correct epoxy system for your desert exposure.

Bruce Hooke
08-13-2001, 10:14 AM
For what it's worth you can get the Tg data for Gougeon's epoxy by going to:


It varies based on which hardener you use. They also list the "Onset of Tg by DSC (F)" as well as the "Ultimate Tg by DSC (F)" (plus lots of other data).