Heat Treatment for Epoxy?

[Bagmool]

I was corresponding with a person about building a wood/plywood plane. At one point, I realized that he had been reading about polymerization and believed that cured epoxy will turn to something like rubber if exposed to about 45-55°C (113-133°F), a temperature that might well be encountered in many parts of the world. This has something to do with epoxy's glass transition. This person's solution was to heat-treat the aeroplane, or at least the epoxy-containing parts of it, that is, bake them at 70°C (158°F) for 12-16 hours. I must say I was surprised. I had never heard about this need for heat treatment. Of course, I am just starting to learn about building aeroplanes. Is it really necessary?

[cub builder]

There is no need for heat treating the epoxies used in aircraft construction. However, what he did say does have some truth to it. Performing a post cure heat treatment will give epoxy resins better tolerance to heat. However, you are talking about an epoxy bonded wooden aircraft with what I am guessing will be glass skins. There is no need for any kind of heat treatment. You'll do more damage to the wood than you will good for the epoxy used for bonding the wood. And the glass skins are typically non-structural.

FWIW, I have a wood and glass plane that I completed some 18 years ago that now has well over 1000 hrs flight time. It flies in summer and winter, including some fly-ins in the heat in Arizona. So far, it hasn't debonded, delaminated, or otherwise melted during the occasions it has been out on the ramp in the Phoenix and Tucson areas.

Now, if you were building an all glass plane with a glass structure, sometimes a post lay up cure may be called out for certain structures. Generally speaking, the kit or plans supplier will cover that in the plans if it's needed. But usually, such parts that may require a heat cure are premolded with prepreg resins, so were already heat cured.

-Cub Builder

[George Sychrovsky]

No

[Bob H]

The softening temperature of a structural resin system is the Tg or glass transition temp. Most room temp curing epoxies have a Tg around 150F. Commercial aircraft use prepreg systems cured at 350F which results in a Tg around 330F. That's why commercial planes can be parked in hot environments without deformation.
The Tg of a room temp epoxy may be slightly increased with a postcure but the increase is perhaps 5-10F max. In doing so, the elongation of the resin is decreased which tends to make it more brittle and less damage tolerant to impact. So the bottom line is not to postcure as there are no major benefits.

High temp resins like bismalyimides or polyimides are routinely postcured to provide higher Tg but are not used by homebuilders.

[martymayes]

Just don't paint the topside of your plane black!

[Aaron Novak]

Heat treating room temp epoxys can help creep resistance, however it would probably not be useful for the posted application. Unfortunately creep is just part of "epoxy" resins, and is one of their biggest drawbacks.

[Bob H]

Creep would be an issue if resin temp got within 25F of Tg for prolonged, stressed conditions. In order to suffer creep deformation, you need temp close to Tg and a stressed state over 35% of ultimate strain. For RT cured epoxy and a plane just sitting on ground, keeping exposure temps below 120F should assure no deformation. This is why most composite homebuilts are painted with a white base coat to reflect sunlight and maintain acceptable temps.