Epoxy & Heat

[Richard Smith]

1. If one uses a heat gun to soften and remove stray epoxy (say . . near a fillet) and in the process heats and softens the neighboring fillet epoxy, is that neighboring epoxy still OK after it cools - functionally sound?

2. Related (kinda): If one heats and softens a small section of cured epoxy and then adds more epoxy while the epoxy is still hot, is the bond a chemical link?

Jeezze, I wish the search function would work!

[JimConlin]

1 If what you're cleaning off is thin gobs and dribbles, then a larger mass of epoxy nearby is probably not getting as hot, especially below its surface. Most epoxies become somewhat stronger with 'post-curing' when baked at a moderately elevated temperature for a few hours, so it might just help. I wouldn't worry about it.

2 I don't think so.

[ 08-22-2005, 12:51 PM: Message edited by: JimConlin ]

[Denman James]

I'll preface this whole thing with the fact that I am not a chemist, but an engineer....

I've always been a little suspicious of the strength of epoxy that has been baked by a heat gun (which I use regularly for cleaning tools up). High heat does nasty things to polymers (and protein chains), and I can't think that it would be good for bond strength. However, wood is a great insulator. I have used it to scrape up drips and the like on occasion, and think that over a joint line, the wood should provide enough insulation that the epoxy in the joint won't get hot. The thin drips and streaks should come quickly to a hot enough temperature to scrape off easily without doing damage deeper down.

As for the bonding - I don't think that the process is the same for the curing of the mixed epoxy and the heating of the old epoxy. We did experiment a little with using heat to thin epoxy to get bubbles out while topcoating with mixed results.

Originally posted by Richard Smith:
1. If one uses a heat gun to soften and remove stray epoxy (say . . near a fillet) and in the process heats and softens the neighboring fillet epoxy, is that neighboring epoxy still OK after it cools - functionally sound?

2. Related (kinda): If one heats and softens a small section of cured epoxy and then adds more epoxy while the epoxy is still hot, is the bond a chemical link?

Jeezze, I wish the search function would work!

[Denman James]

I'll preface this whole thing with the fact that I am not a chemist, but an engineer....

I've always been a little suspicious of the strength of epoxy that has been baked by a heat gun (which I use regularly for cleaning tools up). High heat does nasty things to polymers (and protein chains), and I can't think that it would be good for bond strength. However, wood is a great insulator. I have used it to scrape up drips and the like on occasion, and think that over a joint line, the wood should provide enough insulation that the epoxy in the joint won't get hot. The thin drips and streaks should come quickly to a hot enough temperature to scrape off easily without doing damage deeper down.

As for the bonding - I don't think that the process is the same for the curing of the mixed epoxy and the heating of the old epoxy. We did experiment a little with using heat to thin epoxy to get bubbles out while topcoating with mixed results.

Originally posted by Richard Smith:
1. If one uses a heat gun to soften and remove stray epoxy (say . . near a fillet) and in the process heats and softens the neighboring fillet epoxy, is that neighboring epoxy still OK after it cools - functionally sound?

2. Related (kinda): If one heats and softens a small section of cured epoxy and then adds more epoxy while the epoxy is still hot, is the bond a chemical link?

Jeezze, I wish the search function would work!

[Keith Wilson]

I had a long conversation with the System 3 guy at the WB show one year about precisely this subject. He said several useful things:

- Epoxy softens at a fairly low temperature; 150F is more than sufficient to make it rubbery and easy to scrape off.

- Heating to this temperature (or even considerably higher) won't damage epoxy; once it cools, it's better than ever. In fact, the heating will probably cause it to react more completely ("postcure") and actually get stronger.

- The above is probably irrelevant anyway, because the epoxy inside a joint, or even in a fillet. won't get more than slightly warm unless one is an absolute maniac with a heat gun.

So I'd say go ahead, use the heat gun. I've cleaned up epoxy drips this way for ten years at least, and have never had a joint fail. Cleaning up drips with heat is one of the great advantages of epoxy.

About the second question: new epoxy sticks just fine to cured epoxy at ordinary temperatures. You may have to clean off amine blush, and sand if it has cured longer than a day or two, but he bond at room temperaure is quite stong enough; why bother heating?

[ 08-22-2005, 01:41 PM: Message edited by: Keith Wilson ]

[Todd Bradshaw]

Many high-tech composite constructions (some carbon masts, airplane parts, etc.) and baked when manufactured to strengthen them. I guess it also is kind of a "been there, done that" sort of thing in that doing so tends to raise the temperature at which the resins will soften in the future, making the piece better able to resist heat in use.

You do need to be a bit careful though with thin-ish fiberglass-covered wooden structures. A thin piece of heated wood (like a strip canoe hull's wooden core, for example) may expand enough to stretch the fiberglass covering more than it normally wants to stretch. This can result in lots of tiny fractures inside the glass laminations. Structurally, it's still probably fine, but if you have a beautiful varnished finish over clear glass, you might start seeing a whitish weave pattern and on a painted surface you may start to get some weave telegraphing that wasn't there before. I've seen it happen to boats stored up in the rafters of garages in hot weather or some with dark paint that were stored out in the summer sun.

[ 08-22-2005, 02:43 PM: Message edited by: Todd Bradshaw ]

[JimConlin]

A related point- When epoxy is post-cured whether by heating to a precise temperature or by incidental circumstances, it 'moves' a bit. Consequently, if it's been finished, there can be print-through of glass weave or such in the finish. So, when i have the presence of mind and time before finishing, i'll try to do a poor-man's post-cure by covering the victim with a dark tarp and putting it in the sun for an afternoon. Can't hurt.

[Richard Smith]

Thanks. Good discussion.

I'm familiar with print-through. This mast crutch is about 9 years old. This happened this summer – the first summer we have had ‘round these parts in a while w. temps frequently over 90F. When I first saw this I thought about it for a while - thinking it may have been caused by gas out. Then I realized it was a result of the epoxy softening - its getting dang hot under the cover!

http://i18.photobucket.com/albums/b126/Rnismith/PrintThrough.jpg
(Good digital cameras are amazing!)

Todd Bradshaw: "I've seen it happen to boats stored up in the rafters of garages in hot weather or some with dark paint that were stored out in the summer sun."
> Good point, an epoxy negative that should be publicized. God forbid it will ever get to the point that it will be cause for structural concerns, but as "climate change" continues this kind of thing may become an epoxy boat owners cosmetic concern - for those of us in normally cooler climates: "Just so you know, don't let it get too hot."

So, for re-heating, the consensus seems to be that it has the same effect as on lasagna: It is even better after it is reheated.

Keith Wilson: ". . . the bond at room temperature is quite strong enough; why bother heating?"
> I was thinking that sometimes there are situations when it is a PITA to get at the old epoxy spot to sand it - or one wants to form a "super-duper" bond to old (just in a small area). So far, it does not seem that "we" are too sure about if applying new epoxy to hot old for a chemical link will work.

Thanks for the banter. Speaking of lasagna . . .

[ 08-22-2005, 04:57 PM: Message edited by: Richard Smith ]