epoxy curing??

[Boathook]

I gave up using dispensing nozzles and purchased a "cheap", well under 10 quid, electronic scales with a zero button. Can use any container (nearly!) for the resin and hardener and measure out to the nearest gram. Use the scales in a clear poly bag to save from spilt resin.

 

[Lakesailor]

I use lolly sticks (mixing spatulas) as measuiring devices.
I just mark it in 3 divisions. Then put in 2 parts resin and top up to top mark with 1 part hardner (that's the proportions for the system I use.)
Seems to work well.

 

[Robin2]

West System resin normally takes a long time to cure (as mentioned elsewhere). However it generates a lot of heat itself during the chemical reaction and if that heat cannot dissipate the curing epoxy can get very hot. I normally mix small quantities in throw-away plastic drinking beakers and the waste (i.e I mixed too much) frequently heats up sufficiently to melt the beaker. I have seen a larger waste quantity start smoking.

While it takes a long time to set, it may go off beyond the point where it is workable in 20 minutes or less if the weather is warm - must be quite a problem in the USA (hence the slow hardener, I presume). Good idea to pour the mixed epoxy into a wide shallow container to prolong its usable time - i.e. to allow it to stay cool.

I suspect that the quantity in the small holes went off quicker due to self heating.

 

[William_H]

High Tony no you should not worry about damage to the integrity of your hull. By the time you make a polyester fibreglass boat stiff enough to take the rigging loads the hull itself is way overly strong.
As for the patch. Provided you have a larger diameter on the outside as well as the inside compared to middle then if the epoxy stays stiff it can't fall out. Any real overheat damge is accompanied by much smoke. I suggest in a short time you will forget about the repair and be busy worrying about everything else. olewill

 

[jamesdestin]

Thanks to all for comments

Its looking Ok at the moment. I'm just off to scrabble in the skip for a spoonfull of discarded anti-foul.

Tony

 

[Avocet]

Re: Thanks to all for comments

I've been doing quite a big project with epoxy this year and last (and probably next year the way I'm going)! There are a number of things I've learned talking to various people during the course of this:

1. DON'T heat it greatly to make it go off quicker as this weakens it. Epoxy is supposed to take a long time to set, as has already been discussed in this thread. The reason for this is that the long chain molecules from which the resin is composed can move about during the cure process and "cross-link". The longer they have to do this, the stronger the cured product becomes. I've been using SP Systems resin and they have graphs of cured strength - the stuff made with the "slow" hardener is a bit stronger than the "fast" hardener for that reason. Whether or not that's a problem in your case is debatable because it would be stronger than the parent fibreglass (for the same thickness) anyway and that bit of boat might have been too strong for the job it was doing in any case - clearly, the boat hasn't fallen apart with nothing at all there when the seacock was in it anyway!

2. If you still have any the left-over resin in the mixing pot, try bending a piece of it and see if it bends or snaps. It should be very brittle. If you can plastically deform it without it snapping, it probably hasn't fully cured.

3. I've tried both hardeners and up here in the Northern UK, I've never really needed to use the slow hardener - it just doesn't get hot enough!

4. The "waxy" surface deposit doesn't always appear (although I'm mixing mine with microballoons and that might have an effect). Even then, it doesn't seem to clog the paper once it is well-cured but this can take several days in the spring and autumn - even in summer it can take a day.

5. It needs mixing incredibly well. The hardener is much less willing to react with the resin than it is with atmospheric moisture - hence the need to have it at least touch-dry before the evening when dew forms. For the same reason, it also needs mixing like mad - a couple of minutes stirring every nook and cranny of the mixing pot is time well spent.

5. Gentle warming whilst still liquid dramatically affects it's viscosity so when you put the hot air gun on it, you probably went "backwards" for a bit. It was still curing but also thinning out. People often warm it a bit to make it flow better when they're trying to get it to "wick" into the parent substrate. Sometimes, when patching old fibreglass, it is worth mixing some neat resin and warming it up (body temperature or so) and then painting it on to the exposed ends of the glass fibres so that it "wicks" up them and forms the best possible bond. As soon as this starts to go "tacky", put the main mix on top.

6. Those pump dispensers are sensitive to speed and smoothness of "press". I've given up on them and use the £10 digital scales as has been suggested elsewhere.

7. It will keep curing down to about 5 degrees C but incredibly slowly. I'm told that for some motorsport and aerospace applications they use pre-impreganted cloth with the catalysed resin already in it. This is stored in big freezers until ready for use and will last a couple of years like that. The curing process starts up again as soon as the temperature gets much above freezing but to get into the 1-2 days bracket it does need to be about 15 degrees or more. The disadvantage of excessive cure times is that for all the time the hardener is exposed to atmospheric moisture, it prefers to react with this than with the resin so you might end up with an under-cured final mix.

 

[graham]

Re: Thanks to all for comments

I think Tonies patches are fine. It wasnt heated excessively.The instructions say 60°max, It certainly wasnt that hot.

The reason we needed it to gel fairly quickly was the black thunderclouds gathering upwind of us /forums/images/graemlins/frown.gif. I sail with Tony ,I certainly will not be concerned about the epoxy patches,other things I may worry about ,but not the patches. /forums/images/graemlins/grin.gif

 

[Bajansailor]

Re: Thanks to all for comments

[ QUOTE ]
I'm told that for some motorsport and aerospace applications they use pre-impregnated cloth with the catalysed resin already in it. This is stored in big freezers until ready for use and will last a couple of years like that.

[/ QUOTE ]

Not just motorsport and aerospace industries - this is also the way in which Green Marine build the hulls for racing yachts and the RNLI offshore lifeboats.
The pre-preg glass is kept in the fridge until required, and then laid out in the mould in similar fashion to laying down 'ordinary' fibreglass.
It does not have to be rolled out too assiduously, as it is then vacuum bagged, which ensures uniform distribution of the resin through the lay-up.
The lay-up in the mould is then 'cooked' in a huge oven to promote the curing process. Not terribly hot - I think maybe around 40C (it was a long time ago now since I spent a day at Green Marine to see how they do it).