epoxy still soft

Hi everyone, I think I may have messed up a batch of epoxy I put down 3 days ago (in 30 degree celcius heat) as it is still quite soft. It is not sticky and is quite firm but it is pliable and can be marked quite easily.

The other areas are fine, but this one isn't. Is there any hope? I have left it outside in the sun today and will see what happens, but think I may have to start scraping.

Any thoughts. Cheers

jason

 

What brand of epoxy?

 

Even super slow epoxy should have gotten reasonably firm at this point, in that temperature range. Yep, you figured it out and now are facing the undesirable task of scrapping it all down, then sanding back to solid substrate, before you can start the process, all over again. I'll bet you're going to be pretty anal about mixing procedures after you're through with this set back. This is the number one reason for failure, using epoxy, not following procedures. This could be anything from not removing the blush before a recoat, improper hardener/resin ratios, failing to wet out a joint with un-thickened goo first, etc. That runabout will get back to the water, just keep on plugging at it, you can do it.

 

yeah my thoughts exactly

thanks PAr and nero. It was trojan brand (genkem) that is sold in Australia. Everything else has set up really nicely, but this one... started scraping today...

 

Maybe a heat gun will soften the goo even more. Might make it easier to scrape off.

 

Yep, the problem likely springs from a pocket of resin in your mixing can that never got fully mixed with the hardener. Scraping it down and redoing it is the only option; the hardener can't diffuse into the resin once it's on the surface and gelling up.

 

try ******

 

Other replies above are good, valid. Especially the one about anal with respect to mixing ratios. My epoxy suppliers handbook says that too much hardener will keep the epoxy from reaching full hardness. Conversely too little hardener will make it brittle.

I have created my own disasters in the past. Now I portion resin/hardener by weight. I use a triple beam balance and weigh carefully. Never a problem since adopting that method. In most systems the hardener is lighter than the resin so a little arithmetic is needed to get the volumes just right.

Best of luck
Gene

 

The problem with epoxy is that every resin molecule has to find a hardener molecule. If there is too much resin or too much hardener some are left without a partner and become lonely and cranky and all sorts of problems arise. Therefore it is so important to get the mix ratio right so everybody is happy.

The catalyst in polyester is a bit like a wake up pill. It starts the reaction in the resin. The more catalyst the more reaction. Therefore there is more room for error. It just takes longer until every molecule has been woken up by the party.

 

Excellent analogy

 

Most epoxy hardeners are "reactive amine groups" (nitrogen atoms with a one or more hydrogen atoms attached) These amine groups react with the glycidol groups (resin - or diglycidol ether of bisphenyl A - a manufactured molecule)

The ratio of amine hydrogens to glycidol oxygens (accounting for the various molecular weights and densities involved) decides the final resin to hardener ratio.

Changing the manufactures recommended ratio will leave unreacted oxygen or hydrogen atoms (depending on which is in excess) and the result is a product that doesn't have a complete crosslink - a weaker goo. Sometimes, if the ratio is sufficiently wrong, one that will not react.

Epoxy hardeners aren't catalysts (like it is with polyester) Epoxy hardener mates with the resin, becoming part of the finished material, greatly contributing to it's structure. Catalysts promote a reaction, but aren't chemically pat of the finished product. Catalysts usually flash off (evaporate) or are changed into something else (a function of the reaction) which isn't part of the crosslink, but embalmed in the cured goo as an inert material (like a filler would be), just along for the ride.

 

don't use a heatgun

Using a heatgun will produce epoxy fumes that are more toxic then liquid epoxy and breathing this can really f**ck you up. If you need to use heat make sure to use a fan to blow the fumes away from you