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Old 29-09-2009, 14:14   #1
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A Not Very Important Question

Firstly, a number of people to whom I have talked have noticed the same thing.....
Why is it, that when you mix up a bit of West System epoxy to do a little fibreglassing or whatever on a nice sunny day with a temperature of around 75 degrees F, the epoxy will cure hard within an hour.
Why is it, that on a cold day of around 60 degrees F, you go inside (or in my case down below when the wife's away) put on the heater and warm up the cabin to 75 degrees F, bring inside your unmixed resin to acclimatise for an hour or so, then mix up your epoxy and do your little job in a nice warm cabin, but it take hours and hours to cure?
In what way is artificial heat different to natural heat when it comes to curing epoxy?

Chris
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Old 29-09-2009, 15:10   #2
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Outside on a warmday everything is uniformly warm or warmer. Inside on a cold day everything is not uniformly heated by the heater and is, therfore, either warm or colder or cold. I suspect that you do not have uniform heating....no one does.

Try using a hairdryer or heat-shrink gun on the epoxy mix and watch it set up pronto!
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Old 29-09-2009, 15:43   #3
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Might have something to do with UV.
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Old 29-09-2009, 16:20   #4
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Might have something to do with UV.
It might, but my guess is evaporation. Notice how, when working outside, the smell of the epoxy is not very strong? When working in the cabin, the smell is much stronger. It's my guess that the air in the cabin becomes saturated with the epoxy catalyst, & the epoxy therefore cures much more slowly. You could try increasing the airflow- but I don't know how much that would be offset by the drop in temperature that would result.
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Old 29-09-2009, 16:36   #5
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I think that the thickness of the layer/coat of epoxy is key (a thin layer takes much longer to cure). Second thought is the humidity...

cheers,
Nick.
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Old 29-09-2009, 17:16   #6
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Originally Posted by Chrisc View Post
Why is it, that on a cold day of around 60 degrees F, you go inside (or in my case down below when the wife's away) put on the heater and warm up the cabin to 75 degrees F, bring inside your unmixed resin to acclimatise for an hour or so, then mix up your epoxy and do your little job in a nice warm cabin, but it take hours and hours to cure?

Chris
Chris you just explained it yourself. The air temp inside the boat is now at a comfy 75 degrees. The epoxy would also be at 75 or so you would think. The problem is that the surface you are applying the epoxy to is not at 75 degrees, its considerably colder and what ever heat the heater is putting to it via air transferr is being bled away super fast. Unless you keep that heater on for a day or so, the surface will always be lower.

Several years ago I got involved in a project to build an Electric car for racing. One of the things we did was remove the sun roof and rear window and replace them with foam in the sunroof opening and a light wood frame and lexan in the rear window. We did this in a high school auto shop, and they had the thermostat locked down. It was so cold in there that at times you could see your breath. I needed to do the epoxy work on site, and at first it was a nightmare. The car body never ever got warm even with heaters blowing warm air at it from a foot away. It was just too big. Plus the epoxy resin was just too cold.

We solved this by stealing the large coffee urn from the staff room, and keeping the resin can in there with it turned on to warm. Ditto with the hardener. We heated up the body in the area we needed to do the work by direct radiation using an infrared heater focused on the work area. This actually raised the body temp up to a nice workable level. We then did the epoxy work and when complete the infrared heater was focused on the inside of the car body and a tent was built around the work area, enclosing as small a volume as possible. In this we placed a regular hot air heater to keep the outside air warm. That solved the problem.

In your case I would suggest application of heat to the work area, as well as heating the interior. That way the resin will kick quite happily.


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