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Author Topic: Heating Epoxy  (Read 4264 times)

Offline mccoy40

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Heating Epoxy
« on: August 06, 2021, 01:54:54 PM »
Hi All,

If I heat epoxy to get it thinner to make it easier to apply does that shorten the work time with the epoxy.

Will 5 minute epoxy start to set at four minutes if it is heated up?

Should I heat the two components separately?
Joseph Meyer
Philadelphia, PA

Offline Avaiojet

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Re: Heating Epoxy
« Reply #1 on: August 06, 2021, 02:54:48 PM »
I personally wouldn't even consider using heat to cure Epoxy. The opposite actually. it would increase the curing time.

If I elect to to separate two pieces of wood which were glued with Epoxy, doesn't matter how long, I would use heat.

In fact, when I removed the hard wood engine rails from a model I converted to Electric, I simply heated them up until the Epoxy softened then they separated and fell apart from gravity.

I had teardrop wheel pants which had the typical shape.

I cut them in half and used heat on the Epoxy to reshape them as they softened.

Couple of steps and I had perfect wheel pants for the Texaco 13. I have a build on this.

When Epoxy cools after heating, it hardens as if it was never heated.

I see no reason to heat Epoxy to speed up the hardening or curing process.

When I use 5 minute Epoxy, I put a dab of paint in with it so I don't use needless time over mixing. This is the reason my fillets look light yellow. Besides the Micro-balloons I add the color.

Works for me.

CB

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Offline Mike Alimov

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Re: Heating Epoxy
« Reply #2 on: August 06, 2021, 05:32:41 PM »
If heated, your 5 minute epoxy might become 5 second epoxy.
Thin it with acetone or full strength alcohol instead.  And I wouldn't use 5 minute epoxy for such applications anyway (ie building, laminating, etc). I would use at least 30-45 min epoxy. The 5 min stuff is for field repairs.

Offline Dan McEntee

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Re: Heating Epoxy
« Reply #3 on: August 06, 2021, 05:55:05 PM »
Hi All,

If I heat epoxy to get it thinner to make it easier to apply does that shorten the work time with the epoxy.

Will 5 minute epoxy start to set at four minutes if it is heated up?

Should I heat the two components separately?

    5 minute epoxy starts to cure the very second that the two parts come into contact with each other and while you are mixing it up you are using up part of that 5 minutes, which in actuality is about 3 minutes and 30 seconds.  5 minute epoxy, especially the cheap kind, never really completely cures like you might be used to seeing 30 minute epoxy or longer curing mixes appear when cured. I never keep 5 minute epoxy on hand because it can cause so many problems if you push the use of it nearing it's max curing time. About the only thing it is really useful for is gluing back together those chap foam gliders you see at the department store toy departments or putting a loose sole of your shoe back on. I strongly encourage newcomers to not even buy it. For some, 15 minute epoxy is too fast.
   If you doubt any of the above, just do a simple test. Have a stop watch handy and some 5 minute epoxy ready to mix. Hit the stop watch and start mixing a small batch of the glue, and keep stirring it a bit and playing with it while watching the clock. You are not worried about or distracted by actually applying it t anything so you can just observe. When you start seeing the mix start to get thick and hard to stir, it will be in the 2 to 3 minute range and if you haven't applied to  to anything by thin time it's too late. Once it gets past 5 minutes, keep playing with it. You will notice that it is still pliable, but absolutely not usable. Keep playing with it until you get to the 10 or 15 minute mark and it's still not hard.. This is why you don't want to use it for engine or motor mounts, or for spars of any kind.
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Offline Brett Buck

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Re: Heating Epoxy
« Reply #4 on: August 06, 2021, 06:24:18 PM »
Hi All,

If I heat epoxy to get it thinner to make it easier to apply does that shorten the work time with the epoxy.

Will 5 minute epoxy start to set at four minutes if it is heated up?


     Yes, epoxy cure rates are strongly dependent on temperature. JB-Weld cures in about 24 hours at room temperature, hit it with a heat gun and it might be a few minutes. Epoxy mixed in a cup will frequently heat itself up and cure faster, even boil itself, the cure in a fraction of the normal time. On the other end of the scale, 5-minute may take overnight under 50 degrees, or never at near-freezing.

     I have applied 5-minute epoxy fillets in a 105 degree garage and you could mix it really fast, get about one good swipe with your finger, and feel it getting thicker even in that one swipe.

     I think it is generally a bad idea to heat 5-minute in particular, depending on the application. If it is for sealing a surface (not a structural joint) and then only after it is applied, maybe, or if it is unusually cold when you apply it.

   Brett

Offline Brett Buck

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Re: Heating Epoxy
« Reply #5 on: August 06, 2021, 06:28:32 PM »
The opposite actually. it would increase the curing time.

   No.

    Brett

Offline Avaiojet

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Re: Heating Epoxy
« Reply #6 on: August 06, 2021, 06:49:15 PM »
   No.

    Brett

Brett,

Are you sure?

You might want to read me again.

If "normal" curing time was 5 minutes, heat would prolong the curing time. If you kept the heat gun on it, it'll never cure, but who's going to do this.

 It would increase the curing time, longer, not decrease the curing time, shorter.

"Increase." Prolong is a better word.

His goal was to make it cure faster. "Shorten," the curing time.
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Offline Robert Whitley

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Re: Heating Epoxy
« Reply #7 on: August 06, 2021, 07:18:26 PM »
From my experience the thinning and cure time can be summed up by: It all depends!
If the temp in my shop is cool and the epoxy might be thickening with age I will warm it with a heat gun to make it flow easier.

If cure time and volume are considerations I will choose a more appropriate type of epoxy. (15 minute, 30 minute etc.)

The only time I thin it is if I’m using it as a finishing resin or something of the sort.

Like everything else in our hobby just experiment on some test samples or simple shop jigs and fixtures to see what works.

Offline Dave Hull

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Re: Heating Epoxy
« Reply #8 on: August 07, 2021, 02:42:14 AM »
The rate of the chemical reaction that occurs in two-part epoxy when it is curing is highly dependent on temperature. For those that care, chemical reactions in general are characterized by the Arrhenius equation, which, simplistically says the rate of chemical reaction doubles for each 10C rise in the process temperature.

Preheating your 30-minute epoxy to reasonable temps, say 120F instead of room temp can be useful. By the same token, leaving your epoxy job in the unheated garage overnight not only will slow the cure, it also means that the epoxy will never come to a "full" cure. That is a whole discussion by itself, but for model airplanes using wood construction, it is mostly irrelevant. I would make an exception here for actual composite structures or epoxy composite jobs that depend on monocoque strength.

I think people may be getting confused between two things going on at the same time. When you hit the components, or the uncured mixed epoxy with heat, you are reducing the viscosity, even while you are accelerating the cross-linking. Grossly overheating the partially cured material--such as really getting on it with a heat gun--will reduce the viscosity but I would bet that you are also permanently damaging the material strength, notionally breaking chemical links that have already been made, and won't get remade as it cools. So as one person said  "who would do that?"

And then there is the extrapolation of a rule that's not a rule:  if you heat a completed assembly that is put together with epoxy it comes apart....  That's just an observation that has nothing to do with the original question of curing rates of epoxy. What may be helpful to understand is that epoxy is a thermoset resin. Cheap epoxies do not have very good hot strength. And epoxies that have been chemically altered with "stuff added" can only get worse. So you hit them with a heat gun that can put out air at least 450F (model usage; home or industrial guns put out at least twice that) and you're above the temp where hobby epoxies are going to have much (any?) strength. So the parts separate easily. And since it is not a thermoplastic, smooshing the parts together and hitting them with the heat gun does not "glue them back together."

Modelers are pretty creative and often insistent, so we do what we think is good enough for our needs. Here are examples I have seen promoted, many of which I have tried, along with some of my comments:

1. Thinning epoxy that already has a lot of filler in it (any 1:1 mix epoxy product has filler) is a make-do solution. There are epoxies available that are very low viscosity. You will hear guys tout their favorites, and the two that I have used both worked fine.

2. Thinning epoxy with a solvent means that the solvent has to evaporate too, and must migrate out of the unsolidified material or be trapped inside. Anything trapped inside is just weakening the epoxy. A really poor choice, which people will advocate simply because they have some on hand, is isopropyl alcohol. By definition, it has water in it. Water that will get trapped. That will alter the cure. Combine it with cold temps and you have a good chance of making a weak, rubbery, partially cured mess.  Acetone is probably a good deal better since it should evaporate out of the mixed uncured epoxy more completely, but it is still a "make do with what you have" solution.

3. Continuous heating of the material still to be used so that you can "save it" and finish the job just makes everything harder to do right. It won't be as strong and it poses handling issues as both ends of the spectrum: enough heat to get it to flow without running away everywhere, or; not quite enough heat so that you are trying to spread a stringy glob.

4. Epoxy paint is not epoxy glue or resin. Paint may also be two-part and it may say "Epoxy" on the can, but if you try to hold something together with it, let me know before you fly so I can stand way back from the circle. Waaaay back.....
« Last Edit: August 07, 2021, 03:06:59 AM by Dave Hull »

Offline Massimo Rimoldi

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Re: Heating Epoxy
« Reply #9 on: August 07, 2021, 05:05:20 AM »
Brett,

Are you sure?

You might want to read me again.

If "normal" curing time was 5 minutes, heat would prolong the curing time. If you kept the heat gun on it, it'll never cure, but who's going to do this.

 It would increase the curing time, longer, not decrease the curing time, shorter.

"Increase." Prolong is a better word.

His goal was to make it cure faster. "Shorten," the curing time.

Hi.
You probably confuse the fact that applying heat (say more than 200 ° F) can cause most epoxies to soften with the heat-accelerated curing process.  It happens due to the poor heat resistance of these resins (often below 160 ° F), however try putting a sample of any epoxy in an oven at 110-120 ° F and check what happens after half the time curing at room temperature ...

Massimo
« Last Edit: August 07, 2021, 05:27:58 AM by Massimo Rimoldi »

Offline Massimo Rimoldi

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Re: Heating Epoxy
« Reply #10 on: August 07, 2021, 05:12:33 AM »

4. Epoxy paint is not epoxy glue or resin. Paint may also be two-part and it may say "Epoxy" on the can, but if you try to hold something together with it, let me know before you fly so I can stand way back from the circle. Waaaay back.....

Hi.
Epoxy Paint is an epoxy resin dissolved in suitable solvents and with the addition of fillers and / or pigments

Massimo

Offline Brett Buck

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Re: Heating Epoxy
« Reply #11 on: August 07, 2021, 12:40:56 PM »
Are you sure?

    Yes.


    Brett

Offline Brett Buck

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Re: Heating Epoxy
« Reply #12 on: August 07, 2021, 12:48:05 PM »
Hi.
Epoxy Paint is an epoxy resin dissolved in suitable solvents and with the addition of fillers and / or pigments

Massimo

  Correct, but Dave is right in the sense that those fillers and pigments have the effect of greatly reducing the adhesive strength of the paint. But 2-part epoxy paint certainly is epoxy resin with a hardener and works the same fundamental way.

   As a paint it covers much better than dope because it uses the extremely superior adhesive qualities of epoxy to permit much more pigment to be used before it won't stick any more.

    Note also  that at least in the USA, there has been 1-part "epoxy paint" that is a typical air-drying enamel with some percentage of epoxy resin in it, for no particularly good reason that I could find. I tried it in my childhood search for the holy grail of fuel-proof paint (there was no place to get airplane dope within bicycle distance), it was just as bad as anything else from the local hardware store for fuel resistance, worthless.

    Brett

Offline Massimo Rimoldi

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Re: Heating Epoxy
« Reply #13 on: August 07, 2021, 05:00:49 PM »
 
    Note also  that at least in the USA, there has been 1-part "epoxy paint" that is a typical air-drying enamel with some percentage of epoxy resin in it, for no particularly good reason that I could find. I tried it in my childhood search for the holy grail of fuel-proof paint (there was no place to get airplane dope within bicycle distance), it was just as bad as anything else from the local hardware store for fuel resistance, worthless.

    Brett
The product you are referring to is Epoxy Ester Resin, there is also a water-based version.  It does not contain epoxy resins with a molecular weight> 700 and the polymerization mechanism is similar to that of common alkyd resins.  The fundamental characteristics of this type of products are the high adhesion to the support metals and the excellent passivation effect which gives excellent anticorrosive properties.

Massimo

PS: If you are interested in a technology that can replace epoxy and polyurethane paints (for our purposes), a new technology will probably be available in a few years that you have been developing in the USA for about five years.  In Italy it is called "BLOCK CHAIN ​​ADDITION" (I have no idea of ​​the chemical nature of the product or of the original USA name) and it seems to have absolutely exceptional technical characteristics: the total absence of isocyanates, aromatics, formaldehyde etc.  as well as mechanical and chemical resistance characteristics superior to the current paints used ...

Offline FLOYD CARTER

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Re: Heating Epoxy
« Reply #14 on: August 07, 2021, 05:50:30 PM »
Sometimes it is necessary to warm up the two bottles if they are stored in a cool place, just to get them to flow.

I don't normally use heat for cure, but sometimes the job at hand cannot be clamped.  So I hold the parts in my hand.  I use a Monokote heat gun if a hand is available.  Otherwise I get in front of my shop space heater.  I use Bob Smith 15 minute epoxy for everything.
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Offline mccoy40

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Re: Heating Epoxy
« Reply #15 on: August 08, 2021, 09:06:51 AM »
Thank you all forr your answers.

I'll explain my situation better and it may help.

I have a profile which is relatively solid- except where the wing meets the doublers and the front of the fuse. I noticed a slight wiggle(less than a 1/16 of an inch)  in the fuse and the joint needs re-gluing. I was hoping to use heated epoxy to get better flow into the tiny seam between the fuse and the wing leading edge.

I can just use white glue or titebond that has been thinned a little so it is runny.

Thanks again for all the great advice.

 H^^
Joseph Meyer
Philadelphia, PA

Offline Dave Hull

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Re: Heating Epoxy
« Reply #16 on: August 08, 2021, 11:18:19 PM »
Mickey Four-Oh,

Epoxy is just the ticket for the problem you mention. But there are variations that you can use/adapt. Heating the mixed epoxy to get the viscosity down would be very helpful. If the gap is tiny, then opening it up to get better epoxy penetration might make the joint effective, rather than just an outer skin of epoxy trying to carry the load. If you have a syringe that you can use with epoxy, you might drill one or more holes into the area of failure and inject the warmed, low viscosity epoxy in thru the holes as deep as you can go. Use the pressure to force it into the breaks.

Note that with regard to all the prior discussion, you would ideally have the warmed epoxy into the joint in just a couple of minutes, and it would be undisturbed after that. So no issue with continually heating it and thereby continuing to reduce it's final cured strength.

On jobs like yours, Sometimes I push the epoxy in, and the tape over the joint to keep any from running out. Sometimes you can use gravity to help keep the material where you want it. Slight over-filling, taped joints, tipped on the nose to keep the epoxy in the failure zone. That kind of thing....

Good luck with your repairs,

Dave

Offline Brett Buck

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Re: Heating Epoxy
« Reply #17 on: August 08, 2021, 11:33:50 PM »
Thank you all forr your answers.

I'll explain my situation better and it may help.

I have a profile which is relatively solid- except where the wing meets the doublers and the front of the fuse. I noticed a slight wiggle(less than a 1/16 of an inch)  in the fuse and the joint needs re-gluing. I was hoping to use heated epoxy to get better flow into the tiny seam between the fuse and the wing leading edge.

    That would be fine, but don't use 5-minute, use 2-hour epoxy, and if you have any significant gap, use milled fiberglass powder as a filler. Beware of it running through the other side and down the wing, this stuff flows everywhere.

    Brett

Offline Serge_Krauss

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Re: Heating Epoxy
« Reply #18 on: August 10, 2021, 12:29:35 PM »
Thanks for the great information, Dave (and others).

Not quite on topic, but related to a couple comments...

I had always thought that the old Hobby Poxy paint was "fuel proof" and wanted to get some finger prints off an old (30-40 years), not so practical model that I'd never flown. When I lightly dampened a cloth with rubbing alcohol to wipe off the oil, the Hobby Poxy softened and came off on the cloth. I had some of the old paint left in cans, re-coated the area, and sanded it smooth. Now there's another reason not to start that model. Ha! Anyway, I just wonder how Hobby Poxy was used on flying models in the 1960's.

SK

Offline Paul Smith

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Re: Heating Epoxy
« Reply #19 on: August 10, 2021, 05:03:13 PM »
I always loosen-up epoxy in the two container with a heat gun first.

Then I thin it out more after mixing.

Never a problem, unless you heat it too much.  Finesse.
Paul Smith

Offline Dave Hull

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Re: Heating Epoxy
« Reply #20 on: August 11, 2021, 01:02:00 AM »
Again, a bit tangent to the main question, but to follow up on what Serge said---

I use quite a bit of KlassKote paint. It is great stuff. It holds up really well when exposed to all of the fuels I have used, including high-nitro racing fuel for 1/2As. However, it does not like isopropyl alcohol. It softens the surface finish, and once you take the glossy outer skin off.....

I'm not a chemist, so I have no technical reason for the difference in ability to withstand one and not the other--except to say that the two types of alky simply are not the same.

Dave

PS--And what is that mid-chord intake for on your 1/2A Mouser?

Offline Avaiojet

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Re: Heating Epoxy
« Reply #21 on: August 11, 2021, 03:21:02 PM »
[quote author=Avaiojet

When Epoxy cools after heating, it hardens as if it was never heated.

I see no reason to heat Epoxy to speed up the hardening or curing process.


CB


That is total BS.
At first, we should never optimize construction with ease of dismantling in mind. We should focus on how to do things correctly and as strong as possible.
It’s been tested and well proven, that especially with faster 10…15 minute epoxies, the strenght of hardwood-hardwood (or any joint actually) joint will about double when cured in elevated temperature (50…80°C, depending on product).
That’s mainly due to better impregnation but also because resin cures more efficiently.
So, whenever possible, heat it. L

Total BS?

Nice comment. I stand by what I said.

Copied on line:

"Your basic DIY epoxy can withstand low amounts of heat, but anything between 20°C to 90°C (68 -195 Fahrenheit) will begin to cause a change in the molecular structure and the epoxy will soften and cause distortion. This means the epoxy will become something like rubber and lose its hard glass-like texture."
« Last Edit: August 11, 2021, 03:46:32 PM by Avaiojet »
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Offline Dan McEntee

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Re: Heating Epoxy
« Reply #22 on: August 11, 2021, 04:43:11 PM »
Total BS?

Nice comment. I stand by what I said.

Copied on line:

"Your basic DIY epoxy can withstand low amounts of heat, but anything between 20°C to 90°C (68 -195 Fahrenheit) will begin to cause a change in the molecular structure and the epoxy will soften and cause distortion. This means the epoxy will become something like rubber and lose its hard glass-like texture."

      Which is why you should not use that cheap epoxy. It can get that hot sitting n the back of your car or van. It will get within that range sitting in the sun. But , we have to remember that you never fly your models , so they never get tested to their fullest, but might fall apart hanging in a hot garage.

  Type at you later,
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Offline Trostle

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Re: Heating Epoxy
« Reply #23 on: August 11, 2021, 05:39:18 PM »

Total BS?

Nice comment. I stand by what I said.


A totally inappropriate response to an experienced builder and modeler who has contributed countless excellent materials to this forum.

I think I will listen to one who is a world class F2B competition pilot, a multi-time competitor at the World CL Championships, who is a master craftsman and a master machinist who builds his own engines of his design before I pay any attention to one who does not bother even to fly his own airplanes.

Keith
« Last Edit: August 13, 2021, 04:05:22 PM by Trostle »

Offline Brett Buck

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Re: Heating Epoxy
« Reply #24 on: August 11, 2021, 08:19:59 PM »
OMG this is worse than the venturi, will we ever figure it out??

    It is perfectly well-understood, you might note that the one person disputing the point makes a fool of himself every single time he posts - and was thrown off at least two other message boards and appears to be circling the drain here.

     Brett

Offline Serge_Krauss

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Re: Heating Epoxy
« Reply #25 on: August 16, 2021, 10:12:05 PM »
[PS--And what is that mid-chord intake for on your 1/2A Mouser?
[/quote]

I "designed" this little plane before I knew as much as I do now. That is the filler/vent for the aluminum tank that I should never have crafted, the main reason I never flew it. Thanks for the thoughts on alcohol. I should try a drop or so of actual fuel to see what happens.

SK

Offline mccoy40

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Re: Heating Epoxy
« Reply #26 on: August 23, 2021, 11:05:17 AM »
Thanks for all your suggestions!

The latest update is that the model is going to get fully refurbished at this point. After close examination I notice some further stress cracks in the fuselage and I'm just going to strip the front of the plane and re-attach with a fillet and some fiberglass tape and epoxy.

The plane is covered in Monokote, fuselage as well as wing. I'll strip the Monokote off and get a really good bond on the joint then use an epoxy and micro balloons fillet. I may also use some fiberglass tape with epoxy around the wing joint after that. I'm not sure about that last bit. I guess it will matter how successful the fillet and prior gluing works.

Thanks again.

P.S.     I have two of these planes and the second one is exactly the same - different color patterns of monokote


 
Joseph Meyer
Philadelphia, PA


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