Curing Fiberglass

[Milan]

so I had some small fiberglass patching to do yesterday.
Wanted to get it to harden faster.

I looked it up on the net.
First thing i saw was it is an Exothermic reaction = gives off heat when curing

second thing I saw in same sentence, was to put it in the sun or add heat to make it cure faster.

Well according to basic chemistry, those 2 things are exact opposite of each other.

To increase the rate of an exothermic reaction, you would remove the heat in order to drive the equation to the " Products " side.

For endothermic reactions, heat would drive the reaction faster.

M

[pidjones]

Good fiberglass usually cures faster than I'd like, anyway. I'm not very quick.

[m in sc]

yeah, it takes like no time. slower the better IMHO so it sets up strong. ive never had to play with curing methods though.

[sav0r (CL MotoTech)]

For polyester resin you can mix in more hardener. For epoxies, you should just mix them at the right ratio. I use a 120 minute epoxy, it's nice an easy to work with. If you are wetting out big projects, a little slower set is good. I don't like to add any heat, room temp at like 70f is good.

There are many books on this stuff, but I've always fancied Burt Rutan's Moldless Composite Aircraft Construction book. It puts stuff out there in a very down to earth manner.

[Milan]

It is correct that the slower it cures, or hardens, the better.

M

[irk_miller]

With all due respect, slower curing fiberglass doesn't make it harder, but it does make it bond better.  Your strength comes from your substrate.  You don't want the epoxy harder.  It's not concrete.  As mentioned, cure time is dependent on temperature.  With polyester you can adapt cure time with hardener, but epoxy has to be mixed in the correct ratio.

The reason adding heat, like the from the sun, speeds up the cure is because the heat from the exothermic reaction has to escape to slow the cure down.  The less heat escapes, the faster it cures.  For every 18 degrees of temperature, cure time cuts in half.  This is why mixing epoxy in large quantities in tall containers is a really bad idea.  The cure gets exponentially quicker as it heats up and can ultimately produce enought heat to melt plastic. Spreading the epoxy over glass slows the cure down.  You can get faster curing epoxy meant for cooler temps to quicken cure.  The question I have it why you want it to cure quicker.  Is it because of a vertical repair?  If that is the case, then you want to use fused silica to thicken the epoxy so it can be spread vertically and not run.