Calculations: mixing two epoxy curatives, and mixing epoxy by weight

[prfesser]

In another thread I suggested the use of US Composites laminating resin and curatives. Like most vendors they have three curatives, slow (2:1 epoxy/curative by volume), medium (3:1 ratio) and fast (4:1 ratio). An intermediate cure rate can be had by mixing the two curatives. The calculations may seem daunting but it's actually pretty simple.

Example: You want a medium speed cure for 120 mL of resin, but only have the slow and the fast on hand. Just calculate the amount of slow for half the resin: 60 mL needs half that, or 30 mL of slow. The other 60 mL needs one-fourth or 15 mL of fast. So measure 120 mL resin, 30 mL of slow, and 15 mL of fast.

[FWIW I was told by US Composites that they actually make their medium-curative by mixing the slow with the fast.]

For a somewhat faster cure, three-fourths of the resin (90 mL) needs one-fourth or 22.5 mL of fast; the other 30 mL of resin needs one-half or 15 mL of slow.

Mixing by weight can be much more accurate than by volume, especially for small amounts. A 10 mL syringe might be accurate to about 0.2 mL (roughly 0.2 g) at best, while a $10 scale can weigh to the nearest 0.01 g--- 20 times better. However, the mix ratios may differ.

US Composites' data sheets say that the ratios of resin to curative by weight are 100:46 (slow), 100:32 (medium), and 100:26 (fast). Makes calculations a little more complicated, but not overly so.

Say you want to cure 20 g of their epoxy resin. You have the fast and slow curatives, and you want the cure to be a little faster than "medium".

Three-fourths of the resin is 15 g, one-fourth is 5 g. The amount of fast needed is 15 x (26/100) or 3.9 g of fast. The amount of slow is 5 x (46/100) or 2.3 g of slow.

Put your container on the scale and press 'Zero' or 'Tare'. Add resin til the scale reads 20.00 g (you can get by with 20.0 g and ignore the hundreths, but better accuracy is almost always better). 'Tare' again. Add fast curative til it reads 3.90 g. 'Tare' again. Add slow curative til it reads 2.30 g. Mix thoroughly, scraping down frequently. Mix some more. And mix some more. For extreme projects, pour the contents into a second container WITHOUT scraping the sides, and mix again. Use it.

Best -- Terry
(You can take the teacher out of the classroom, but you can't take the 'teach' out of the teacher.)

 

[rharshberger]

I have always mixed the 3:1 635 by weight only without worrying about how it actually compares to volume, with no issues on curing. While its not the "right" way it works and no extra calulations needed. 15g of epoxy to 5 g of hardener, again its not the right way but it does work for the 3:1.

This is good info to have though, thank you Terry.

 

[XrayLizard]

Good post thank you.
Now I know why the hardener bottle says shake well.. not exactly easy to shake. I’ll try stirring it.

 

[Nathan]

That's why I like Aeropoxy. 1:1 mixing ratio. Just put two ketchup cups side by side and get them both to the same level.

 

[Lurch]

I once asked US Composites about the ratios by weight on the 635 with the medium hardenr. It turned out that weight is so close to volume that the difference was not worth worrying about. Errors in weighing/measuring would likely be more. Unfortunately, I don't have the email thread anymore for the exact numbers but the conclusion was weight or volume - doesn't really matter.

 

[jqavins]

OK, Teach, why transfer WITHOUT scraping the sides? It seems that would through the ratio(s) off.

• If the mixture were already uniform then you wouldn't need this step at all.
• Since the mixture is not yet uniform, you can't be sure that what's left behind, or thus what's transferred, has the same ratio(s) as the total that you so carefully measured.

Of course, any error so introduced would be well within both the margin of error in the measurements and the tolerance in the requirements. Still, what's the harm in scraping the sides?

 

[prfesser]

OK, Teach, why transfer WITHOUT scraping the sides? It seems that would through the ratio(s) off.

• If the mixture were already uniform then you wouldn't need this step at all.
• Since the mixture is not yet uniform, you can't be sure that what's left behind, or thus what's transferred, has the same ratio(s) as the total that you so carefully measured.

Of course, any error so introduced would be well within both the margin of error in the measurements and the tolerance in the requirements. Still, what's the harm in scraping the sides?

Scrape the sides while mixing, but if the end result is critical, the epoxy manufacturers/dealers say to pour into another container w/o scraping the sides. When I mix the sides (and the bottom) get scraped down frequently, but not everyone is so vigilant. The sides are where the resin or curative are not mixed as well as they are in the bulk of the mixture.

For our (rocketry) purposes it's rarely necessary, but if one wanted a REALLY strong epoxy mixture with no excess of either resin or curative, AND if one is working with relatively large (half-cup or more) amounts of epoxy, scrape down the sides while mixing, but transfer to another container for final use without scraping. It's not just my suggestion, this is what the boatmakers are told to do.

Best -- Terry

 

[jqavins]

Yeah, I understood the direction. The question was why that should be direction. I guess it's "because the manufacturer says so."

 

[prfesser]

Yeah, I understood the direction. The question was why that should be direction. I guess it's "because the manufacturer says so."

The amount and degree of mixing needed to get a molecule-for-molecule mixture is underestimated by most people. In analytical chem lab, we mixed a flask by inverting while shaking, let bubble rise to the top, repeat at least 50 times. 100 times is better. Students who ignored the advice often found that repetitive results from that solution didn't match all that well.

It's all but impossible to mix too much. One exception: mixing too long and the stuff starts hardening. Beyond that, more mixing is like more knowledge: almost always better than less.

Best -- Terry

 

[DES]

Yeah, I understood the direction. The question was why that should be direction. I guess it's "because the manufacturer says so."

The idea is that all of your batching and mixing are done in the first cup, hopefully with thorough scraping of sides and bottoms.

When you transfer to the second cup, any little bits of unmixed resin or hardener clinging to the sides are left behind. A quick remix, and then dispense onto the cloth. That way, when you pour onto your cloth from cup #2, or dip your brush into your cup, you won't pick up any traces of unmixed hardener or resin and streak them into the cloth where they will remain inactive. So it is more of a purity control than a mixing issue.

Personally, I don't bother with a second cup. Since I'm usually only batching 1-3 oz at a time, I just mix and scrap the bejeezus out of the mix. Also, since it is a smaller volume, dispense the hardener into the middle of the resin, don't let it drip on the side of the cup. I usually alternate layers of resin and harder in the cup as I'm dosing it, before I mix.

 

[FredA]

The second cup deal may help a bit on end purity, but it sounds more like a salesman upping his quota telling you to throw away some of every mix.

 

[DES]

Again, probably little value when you are mixing a couple of ounces in a Dixie cup with a popsicle stick. Probably more of an issue when you are mixing a couple of quarts or a gallon at a time in a bucket with a paddle mixer on a cordless drill, a lot harder to get the sides scraped clean.