Epoxy curing extremely slowly

Well, what you're describing could amount for a degree or two, but would that really change the pot life from 22 minutes to 3 hours? I mean, I worked with small batches hundreds of times before, and never once did I ever see pot life past 30 minutes...

 

Most vendors supply volume mixing ratios, which can be different than weight mixing ratios, check with the formulator. Small amount mixing can be a problem without not only precise metering, but also mixing. With small volumes you can easily smear enough on the side of the mixing cup to decrease the ratio considerably. Most lower mix ratio epoxies (1:1, 2:1 and 3:1) can tolerate about a 5% error in ratio, before you have a cure issue, though the higher the mix ratio, the more important this becomes. On small batches, I use syringes to dispense the two parts and mix in a flat tray, never a cup. Don't try to cheat or add/subtract hardener or resin, you're just guessing, which is simply a quick way to waste more epoxy.

This wouldn't be the first time a formulator screwed the pooch on a batch of goo. It happens, though darn difficult to tell with just phone or email conversations. Your temperatures are fine, maybe a tad high for room temperature cure epoxies (most designed for 77F cure). Test normal size batches, at normal temperatures and see where you are, but given what you've done, I'd consider sending it back to the supplier and asking them to make it cure.

 

Thanks for replying, PAR! I'm not sure if you read my updates on the previous page, but I borrowed some more of the same epoxy (same manufacturer, same "model") from another guy who bought it a year ago, and tested his epoxy and my epoxy side-by-side. Both his and mine cure extremely slowly, and at the same rate, so I don't have any reason to suspect that the seller messed up my order.

The specification calls for 2:1 by volume, or 100:44 by weight. I always do it by weight, because I have super-accurate scales (checked them today, still accurate), down to 0.01 g. I mix with an accuracy of about 0.05%, which is way below the 5% borderline that you mentioned. I do it in cups, and yes, these are small amounts, but I do mix very thoroughly (3-4 minutes, all directions, scooping of the walls, etc.), so I don't really think mixing could amount for the difference between 22 minute theoretical pot time and the 3 hour actual time that I get.

Besides, I used to make batches as small as 3 grams in cups (with another epoxy from different manufacturer, but a similar ratio), and I never had any problems with curing, because I'm obsessed with accuracy and mixing properly, so 15 grams should be plenty to eliminate any errors... I mean, I can still test a larger batch, but I don't want to waste much for testing purposes, epoxy is damn expensive, and I'm sure the result won't be significantly different.

 

I would test a 100g batch, or volumetric 2 oz : 1 oz, or so.

Keep track of all the times. Keep it in the cup and check it for change/exotherm at 20 minutes then every minute until it kicks.

Contact the manufacturer if it doesn't meet specs.

I am guessing it got mislabeled as a slower hardener.

If it never hardens; you need to stop using it.

What sort of wirk are you doing for such small batching?

 

Well, I can do 100 g batch experiment, but that is wasting a lot of epoxy...

I'd agree that the hardener could be mislabeled, but as I said, another guy borrowed me a sample of his epoxy (same manufacturer, same brand, just bought a year ago), and it hardens just as slow, so either the manufacturer always mislabels the hardener (extremely unlikely), or there's another problem. By the way, there are no custom hardeners for this epoxy, just this one. This is not West Systems, you don't get to choose the hardener

The first batch I made 3 days ago finally hardened... It is about as hard as most plastics. I can still dent it with a steel yawl, but then again, I could do that with all epoxies I ever worked with.

Not sure what you meant by "wirk". If you intended to write "work", then, well, for example, gluing some small pieces, or filling a screw hole, patching some minor dent or gouge, making filler for some small patch I overlooked before... It's rare that I ever need to make batches above 30 grams - and then, only when fiberglassing, making fillets, or laminating stuff like spars... As I said, epoxy is expensive as hell around here, and I would hate to waste it.

 

The smallest batch I ever make is 3 oz unless it is going in a syringe.

The classic tests for epoxy are generally 100-150g tests. So if you are using a lower batch size; the time spec won't be met.

Epoxy exotherms in a mass faster, so a smaller mass WILL take longer.

I have never attempted to quantify it.

 

When it gets older it takes longer to set ,and the ambient temperature is more important than with polyester I have found, because you can't alter the catalyst according to air temperate . But 25 celcius with no draught should be perfect. The date of the batch number may give a clue. There are different types of epoxy, the sort I used was 4 to 1 and responded similarly to your description and worked well. It stopped acting as fly paper after about 4 to 6 hours. The extra fairing added significantly to the building time. It didn't really slump though and is seriously tough. Leaving it as a mass to heat[5mins recommended] longer and begin to cure before spreading would be nerve wracking but would speed up touch dry, I wouldn't..ha, there is far more workable window in mid set than polyester. Oh and the manufacturer would not guarantee it after sitting for 12 months, probably because of the slow curing.

 

If you are stuck with the stuff; use a water bath with hot water under your mix cup when you mix if it wasn't mentioned.

You basically find a container that your mix cup can sit in and let the cup warm and then mix above it as well. You could also leave the resin in for awhile and let it heat, too.

 

I guess that's what it is. Probably the 22 minute pot life in the specification was determined by making tests with batches large enough to make exotherms noticeable. Anyway, the epoxy does harden finally, it's just the initial cure that's so damn slow. Well, maybe it will be an advantage once the summer sets in, and I will have limited time to use up these batches before they harden in the pot Thank you for your help, fellow boatbuilder.