Smokin' Epoxy!

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PCUK

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Bonding in some ply backings for stanchions yesterday using West Epoxy. Mixed up enough to do the job in a paint mixing cup. Warmed the resin and hardener with a fan heater before mixing as it was around 4 degrees and really too cold for epoxy work. Bonded in the first one and then noticed a strong chemical smell. The paint cup was melting and the mix was smoking! Dropped it into a bucket of water and mixed small portions for each stanchion at a time. Finished the job and they were all curing nicely. All solid today so that pre-warming works well. Wouldn't do it on something more critical but happy with this job?
 
Yeah, it does that. Doesn't have to involve external heat; if you have a big enough lump of it in one place (eg, your mixing cup) when it starts to kick then it will get smoking hot and melt the pot. Spreading it out in some kind of tray, so that the exothermic heat can escape instead of speeding up the reaction, will slow it down if you're on the borderline of having enough pot life.

Pete
 
When patching up Coppercoat in Greece we have had the same problem. The speed of chemical reactions doubles with every 10 degree C increase, so an ambient of 25C and maybe letting the sun shine directly on the container sets it curing in very few minutes.
 
The trouble with epoxy is that when it starts to go off in the pot, you have very little time left compared to polyester. When polyester does that, it gets a bit lumpy and stringy in the pot - you then have about 10 minutes or so so use it up. But with epoxy resins, you maybe have less than a minute.
I sometimes use a hair dryer on a low heat and apply that to the resin when its in place - ok for a horizontal surface, but not so good on vertical as the resin gets very runny before it starts to kick. pls excuse the non-technical descriptins !

Epoxy resins are also very particular about the ratio of the 2 parts, it has to be pretty much exact, 3:1 or 2:1 ,whatever it says on the tin and dont be tempted to add a little more of the hardener. Again, polyesters are a lot more flexible and adding a little bit more hardener helps in colder conditions.

As long as what you are doing is not structural, and if the epoxy 'goes off' a bit too soon, its not a major problem.
Good luck with the repairs
 
The main thing I was worried about was the low ambient temperature, that's why I thought mixing in a cup would help. Helped a bit too much though! I don't think I'll be glassing today at minus 4 though, even with polyester to cover the bonded-in ply. I think welding is the ideal job today! Incidentally, I cut out sections of inner skin and foam sandwich to allow me to bond the ply direct to the outer skin which seems to give a far stronger job then trying to put spacers between the skins.
 
bernie_ni said:
Epoxy resins are also very particular about the ratio of the 2 parts, it has to be pretty much exact, 3:1 or 2:1 ,whatever it says on the tin and dont be tempted to add a little more of the hardener. Again, polyesters are a lot more flexible
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+1

The difference is that solid epoxy is made from the two chemicals, combined in the stated proportions. Any mismatch leaves molecules of one or the other without a partner, so they hang around as impurities in the structure, weakening it. Obviously a certain amount of this is inevitable, we're not weighing out individual molecules, but you want to minimise it as far as possible. Also, changing the proportions will not affect the speed of the reaction, you need to pick different chemical mixes (slow / fast hardener) for that.

In the case of polyester, the finished structure is made only from the resin part. The hardener merely catalyses the reaction, so adding more or less of it (within reason) changes the speed but not the final result.

Pete
 
I was once repairing a sea water pipe with an epoxy repair kit. I had mixed tin A with tin B and was applying and the bandage to my nicely prepared, cleaned and dried pipe section which I had removed from the system. Unfortunately I had placed the tin on the deck next to me under the rest of the pipework. A couple of drips of sea water landed in the open tin and within a second or two the thing went ballistic. It boiled up like boiling sugar, the label had caught fire and the whole tin of mixed epoxy had cured as hard as rock with the welding rod I used to mix it embedded like Excalibur. It was so violent that it actually gave me a rather large fright and had me running for a fire extinguisher.
 
To prevent an epoxy mix going of too soon/fast you only need to increase the surface area, so small cups and cans are out as are large quantities in larger containers containing more product.
As others have noted adding more hardener dose nothing, if you want a faster cure time, just add dry heat.

By the way, standard resins that cure rapidly because extra catalyst was added will fail sooner, they are slightly more brittle.
At least that's the result I got when testing to destruction some time ago now.

Good luck and fair winds. :)
 
If possible, it is better to warm the substrate than the resin. I seem to believe that, although it breaks the rules, the Gougeon brothers book says epoxy will cure down to about 7 Celsius, which is consistent with my experience. Some I did colder than that never stopped being tacky even after years.
 
cryan said:
I was once repairing a sea water pipe with an epoxy repair kit. I had mixed tin A with tin B and was applying and the bandage to my nicely prepared, cleaned and dried pipe section which I had removed from the system. Unfortunately I had placed the tin on the deck next to me under the rest of the pipework. A couple of drips of sea water landed in the open tin and within a second or two the thing went ballistic. It boiled up like boiling sugar, the label had caught fire and the whole tin of mixed epoxy had cured as hard as rock with the welding rod I used to mix it embedded like Excalibur. It was so violent that it actually gave me a rather large fright and had me running for a fire extinguisher.
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I wonder if the drips of sea water into the epoxy was just coincidental with the epoxy going balistic and overheating. I believe epoxy can be made to cure under sea water so it should have no effect. good luck olewill
 
William_H said:
I wonder if the drips of sea water into the epoxy was just coincidental with the epoxy going balistic and overheating. I believe epoxy can be made to cure under sea water so it should have no effect. good luck olewill
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No idea? It did cure, just that it cured in about 0.3nano seconds hence the heat. I have used those kits( the name escapes me) many times but that was the only time I got that reaction.
 
IMHO...

Prewarming the epoxy is not about helping it go off. It is simply about having the liquid runny enough that you can measure it out accurately (I use big syringes). Once you start mixing, you will notice that it goes runny anyway. Usually, only the resin needs prewarming, the hardener tends to stay runny anyway. If you can feel any kind of heat whatsoever when pre warming, you've gone too far...just shake the can to check it's a bit more liquid, that's all. If you store your epoxy in the house overnight before you need to use it, you should be fine anyway.

The heat from your hand can make the epoxy start to go off in the pot. Hold it at the top, above the layer of the resin. Those shallow plastic trays you get Chinese takeaway in are good for mixing...in short you want maximum surface area and minimum depth.

Only worry about fan heaters when the work is complete. IME you can get a lot of heat on the area simply with some kind of polythene tent, gaffer tape if necessary, and a well placed fan heater (in fact it's very easy to overdo it). I have worked on yacht rudders in situ in the freezing cold this way, and work is slowly continuing on my Flying Fifteen out in my garden, over the winter in this way. Your biggest enemy with the cold/damp is amine blush forming at the end...easily removed with the correct solvent or sanding.

If you do have the epoxy start to go off in the pot, if you catch it early you can give it a quick zap in the microwave! This gives you about another minute maximum where it will go very runny indeed, before setting rock hard. Not sure what Gougeon brothers would say about this, and I probably wouldn't use it anywhere very structural, but if it was a top layer of glass or perhaps for sealing wood, it's probably OK.

Personally, I only ever buy slow hardener.

Hope this helps.
 
Iain C said:
... if you catch it early you can give it a quick zap in the microwave! This gives you about another minute maximum where it will go very runny indeed, before setting rock hard. Not sure what Gougeon brothers would say about this,....
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Somebody whom I fear more than the Gougeon Brothers would veto this I think....
 
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