What can I use to slow down the curing of 2-part epoxy?

Question

I've been repairing a Chinese porcelain tea set for a friend using the faux-kintsugi technique involving gold mica powder and 2-part epoxy, based on Elmy's answer here.

I'm using UHU Plus Schnellfest 2-k-epoxidkleber, but the curing (or handling) time of 5 minutes ("5 min verarbeitbar - nach 20 min fest") is too fast to mend multiple pieces, and I can only make my batches so small (I use a single drop from both tubes).

Is there anything I can add (or do) to slow down the handling time of the epoxy?

Preferably this would be a common household ingredient, but all answers are welcome.

I realize I can add less of the hardening agent, but imagine this will just as easily compromise the durability.

Answer 1

Yes, keeping epoxy cold retards the curing time, but if the normal curing time is only 5 minutes, you won't win much.

Using too little hardener will ruin your resin and won't prolong the curing time at all. Imagine 2-part epoxy as one liquid of chains (this is the resin) and one liquid of locks (this is the hardener). When separate, they both are liquid, but once you mix them, whenever one lock encounters two chains in this mixture, it connects them and thereby hardens the epoxy. You always want enough locks in your mixture to connect all the chains, otherwise you leave uncured resin in there. If there are too few locks, they will work as quickly as always, but they simply won't be enough to cure all of the resin.

The speed at which the curing happens is an intrinsic property of the chemicals in the epoxy. Your best bet might be to use a different brand / type of epoxy. There are many different resin products with working times like 5 minutes, 30 minutes, 1 hour, 4 hours, 12-36 hours.

For your application I would suggest a working time of 5-30 minutes, because you'll have to stabilize the shards until the epoxy is cured enough to hold them. I honestly doubt that you'll be able to glue more than 2 or maybe 3 (if you're lucky) shards at the same time. My personal experience with epoxy is that the longer you need to handle it, the more of a hassle it is. You need to wear gloves while working (because epoxy can cause allergic reactions up to several hours after skin contact) which limits tactile feedback. It's so easy and so frustrating to accidentally smudge some wet epoxy all over the place. If I ever need to glue pieces with epoxy, I always try to prepare everything so I have to touch the pieces as little as possible (preferably not at all).

Maybe your current epoxy will work better for you if you use a different mixing technique. Put equal amounts of resin and hardener separated on a sturdy plastic sheet. Then take a stirring stick and transfer a small amount of resin into your "working area" of the plastic sheet. Use a second stirring stick to transfer a roughly equal amount of hardener into the "working area" and mix both with the second stirring stick. You can reuse the first stirring stick for each batch, but you'll need a second one for the hardener (or reuse one with completely cured resin on it), otherwise you risk partly curing the resin reserve on your sheet.

Answer 2

This is a bit of a frame challenge, but honestly the solution is to buy slow epoxy in the first place.

The difficulty is that consumer products seem to have divided into fast, unusable after a few minutes, and very slow, runny enough to flow for several hours (e.g. Araldite precision, which is what I tend to use).

If you can find one nominally rated to an hour, that's ideal. I've used these in industrial versions. Note that time ratings aren't accurate, and one manufacturer's 10 minutes is another's 5..

You're right that temperature can have an effect. Heating can cause quicker curing so an overnight product is strong in a few hours at about 50°C. With cooling, of the glue or the workpiece, you can only go so far, for two reasons. Firstly the unreacted components get too stiff to mix straight out of a freezer, and second, condensation on the glue or workpiece becomes a problem, which spoils the bond. One thing to note with heating is that the peak runniness is higher. This can make for a good bond as the epoxy wets the parts better, but can mean more flow than you want. By keeping your glue pot cool and your workpiece warm you can get best ratio of pot life to curing time.

Answer 3

Epoxy comes in different speeds

Of course, "5-minute epoxy" is what people go for, because it benefits from advertising. I actually use the West System 105 family, which is closer to 500 minute or 1000 minute depending on which hardener you use lol.

Remember, epoxy is often used for fiberglass layup - surfboards, Corvettes, boats, airplanes. When you're doing a big layup like that, you need time.

So you can get 2-minute, 10-minute, 20-minute, 60-minute, 180-minute or whatever speed of epoxy suits you, and a better hardware store will sell a spectrum.

5-minute epoxy is a compromise between working time and "the person having to hold the piece with their hands while it sets up". It's also not made to use with fillers such as adhesive or fairing filler (there wouldn't be time to mix it). So with longer cure epoxy, you simply need to find a way to hold it, brace it, clamp it, etc.

Make large enough batches to be sure of your ratio

Trying to do a batch of a single drop is inviting a mixing mistake. Base and hardener must be mixed at the correct ratio and not any other.

Too much base and long-chain molecules will not be nearly as long and it won't be as strong. Too much hardener and it will be contaminated with leftover hardener which will weaken it and make the repair poisonous to humans. Hardener is toxic - it's an immune system irritant. Skin blocks it reasonably well but ingestion is a bad idea.

Changing ratio will not change cure time. It will just make a mess. Unless the instructions tell you otherwise - some epoxies work a different way.

There's nothing wrong with throwing away epoxy - I use West System's metering pumps, so I get it in multiples of 23 cc and it costs me $0.50 USD. I have no problem with this. Epoxy is cheap.

But also, cured epoxy is non-toxic, unlike the B-part. You don't want to, 3-4 years later, find a mostly unused pack of epoxy which has leaked or dried out. Then you're forced to throw it in the trash with the B-part not reacted, and thus adding toxins to landfills.

Answer 4

The question asked

The question asks how to slow down the epoxy setting, and the reason is because the process you're using doesn't leave enough working time to glue multiple pieces, wasting a lot of materials. You've already identified cooling as a way to slow it down (probably the easiest way, although that won't buy you much time).

To answer the question you asked, the setting time is driven by the chemistry, so the ideal solution for gaining more working time is to use an epoxy system designed for a slower setting time. Once you select an epoxy system, I'm not aware of a good way to change its speed through chemistry (at least not with anything you would have at home). You especially don't want to reduce the hardener. That won't slow it down, it will just leave unreacted resin in the hardened mix.

Using a slower-setting epoxy or cooling the one you have would be the best solution for slowing the setting speed. However, slowing the setting speed isn't the best solution for your problem. The problem is really the process you're using.

Better solution to the problem

It sounds like two issues are driving the desire for more working time: blending and using the mica mixture to glue the parts, and lack of a good way to mix smaller batches, resulting in wastage if you don't try to glue multiple pieces to use it up.

The mica

Mixing the mica into the epoxy used to glue the pieces is a bad idea. It's not just using up some of the working time and complicating the glue-up, it's also affecting the alignment of the pieces.

Pieces of broken porcelain typical fit back together with zero gaps. Anything you put between the pieces will prevent a perfect reassembly. Each glue joint will throw the remaining pieces farther out of alignment. Adding the mica to the epoxy introduces solid material between the porcelain pieces that will prevent them from fitting back together perfectly.

Possible alternate assembly adhesive

For comparison, low viscosity superglue (cyanoacrylate) is probably the best adhesive for porcelain repair from the standpoint of pieces fitting back together. It works with a microscopic layer of glue, so pieces fit as perfectly as they're going to get. Unfortunately, superglue is very brittle, and even the waterproof ones won't handle water immersion if the tea set was to be used after repair. They also don't handle heat well.

If the tea set will only be displayed, you could use superglue to reassemble the pieces, then reinforce that with epoxy over the seams so it still looks like kintsugi. That's also the fastest way to repair it since there is virtually no curing time during assembly, especially if you use an accelerator. Just an alternative approach to consider.

Plain epoxy for assembly

But back to doing the repair entirely with epoxy, using just the epoxy without the mica to reassemble the pieces will let you compress the seams more for a better fit. Slowing the setting time will work against you. You will achieve the best fit by assembling several pieces to verify the alignment, then gluing just the piece most critical to aligning the other pieces. Use the least amount of epoxy that will hold the pieces together without excess, and squeeze the pieces together to spread the epoxy into the thinnest layer possible. If you can, use tape on the seam to stabilize the joint and keep a little compression on the seam. Let that do its initial cure, then repeat the process with the next piece. Slowing down the setting time will stretch the time needed for the repair.

The objective should be the best possible repair on the tea set. That will take as much epoxy as it takes, just don't unnecessarily waste it. Don't jeopardize the repair and the tea set to save a little epoxy. If the pieces interlock enough to stay together, you can try gluing more than one piece at a time, which will make better use of the epoxy. Without the mica at this stage, you're only wasting epoxy, not the mica, too. A faster setting time will reduce the chance of the parts moving during the cure.

Kintsugi

Once the pieces have been reassembled, the surface isn't likely to be perfect. There will be missing chips of porcelain, surface gaps where the epoxy didn't fill the whole seam, etc. Use the kintsugi for that. Mix an undersized batch of epoxy plus mica, and use a syringe or similar tool to paint it precisely along the seams. If you run out, add a little more. For this stage, you can make a batch big enough for the whole object (or as much of it as you can paint in the working time), and cooling the mixture to stretch your time will be beneficial. Use a disposable syringe since it won't be worth trying to clean it.

You might even want to use a fast-setting epoxy for the reassembly, and a slower-setting epoxy for the kintsugi.

Answer 5

I will post my results as an answer here, both to show them, and to inform others who are meaning to experiment with this faux-kintsugi method.

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Considerations of the existing answers

Elmy's method turned out to be the best option for my application and situation. I didn't want to buy a new slower-drying resin, as suggested by ChrisH, since I had glued most of the porcelain (and the next time I might want to try something closer to real kintsugi). I do suggest buying slower drying epoxy if you're still in a planning phase!

I tried fixer1234's solution as well, glueing parts with superglue, and applying the resin/mica mix afterwards. Glueing went a lot better, and at times indeed seemingly seamless (or 'seemless'), but applying the resin/mica mix afterwards turned out to be difficult, as I didn't want to ruin any of my brushes, and a (semi-)hard material either carries too little mix, or can't be controlled precisely enough for clean, thin lines.

It's good to know the ratio is so important for the material's bond and durability, as pointed out by Harper, but this was a purely cosmetic job, as I believe the epoxy wouldn't be food safe anyway. Throwing away large parts of otherwise perfectly fine epoxy resin was not a nice idea, either.

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Results

After the epoxy resin/mica mix had dried, the excess was remarkably easy to remove. I used a simple blade cutter to go over the lines and scratch the excess off. The porcelain (even on the recto side, which has a decorative print on it) didn't get damaged at all, and I was able to go from this:

To this:

The picture doesn't do it justice: both the line and colour are quite beautifully subtle.

On the more complicated breaks, the lines tend to be less subtle, but achieve a nice result nonetheless:

However, it certainly yields worse results when trying to glue several pieces together, or when parts are missing:

(I coloured the bare, rough surface of a chipped away part of the porcelain with the mix, but don't like the effect at all.)