Mix resin by volume and/or by weight?

Hello, my name is Wil. I have a question about volume versus weight when mixing various resins. I understand the volume method and recognize that volume is routinely used with many resins, especially the epoxies, but am wondering about mixing by weight. It seems to me that "back in the day - yes I'm old" we used to mix polyester by weight when fabricating artificial limbs. That is, we may add 500 grams or ounces of resin and then add 1-2% of MEK(P) granular powder to the resin by weight. Can one of the chemical folks enlighten me about this? I've struggled to see anything about this in a forum. Thanks for your replies.

 

Epoxy can be mixed by weight. The ratio may be different then when mixing by volume because the densities of the two components are not exactly the same. Resin formulators/distributors will typically provide the ratio to use when mixing by weight. For example from Gougeon Brothers for West System epoxy:
To measure 105 Resin and 205 Fast Hardener or 206 Slow Hardener by weight or volume, combine five parts resin with one part hardener. To measure 105 Resin and 207 Special Clear Hardener or 209 Extra Slow Hardener by volume, combine three parts epoxy resin with one part hardener (by weight, 3.5 parts resin-1 part hardener). Dispensing & Mixing https://www.westsystem.com/instruction-2/epoxy-basics/dispensing-mixing/

 

I use mostly 2:1 epoxy (Resin Research, Greenroom, MAS) which are 2 parts resin to 1 part hardener by volume.
By weight it is different-100 parts resins to 43-44-45 parts hardener depending on the brand.
100 parts resin to 50 parts hardener by weight would be a mistake, adding 10% or more hardener than needed.
I prefer weight as I can use recycled containers and guesstimate resin needed by weighing the cut cloth
Less experience with poly but remember reading percents off of charts or doing the drops thing with Bondo resin.

 

I have used a lot of epoxy. I have had only about 3 small mixing failures in 300 gallons of use. Two of the failures were the same. Two ounces of hardener and one ounce of resin when I went away from rules for a 'quick batch'. Lots of scraping. The other was forgot to stir a decent size batch and laminated about 9 sqft with gum, basically.

I pour resin first always (except my errors). Why? Because it is easier to return a bit to the jug. Others pour hardener first. Touche'. But same all the time helps avoid did I add the other component, yet? Or, did I add the right volume of part A?

As a general rule, you will use more when pouring by volume. It is an inherent issue. Let me explain. So, let's say you are using a 2:1 epoxy that mixes 100:43 by weight. Three parts, by volume, makes a whole. So, it is easy to devolve to the measuring cup. Your cup may have 50ml graduations, in which case, you mix 100ml resin and 50ml hardener as a minimum amount. But on a scale, it is easy to mix a smaller batch. Or even 100g resin and 43g hardener, or 50/22,etc.

one gallon = 3785ml
Many epoxies are 9#/gallon (or so)
Or 4082 grams

Also, I tend to err on the hardener side when operating by volume. This is also an inherent issue. Around 240 gallons in, I was about half gallon short on hardener. This is an error rate of 6/10ths of a percent. Pretty close, but this was all high volume work.

So, as a general rule, small batches are far easier to measure by weight. Also, fairing compounds made in the shop are far more consistent when scale measured. This helps with things like pinholes in finishes, for example.

I have really lately only been using the scale, but for large volumes; it would be a bit of a pain. Also, even a scale can cause errors if zeroing problems or you walk away for a second.

So, for me, large volumes, by volume.
Small volumes, fairing mixes, by weight, for consistency, less bucket measuring loss.

 

The epoxy doesn't care which way you do it. But if you play around with a lot of different resin additives, and use multiple resin systems, it is often easier to keep track of the additive ratios by weight. I tend to mix resins by volume, then weigh the result. Then I add other volume based additives like liquid pigments and balloons, which are impossible to weigh. Then I check the original resin weight and compute the other additives.

 

Wow! Wonderful replies. Thank you. My main purpose for trying to find a "recipe" for mixing PE by weight had to do with experimenting with old resins to see what is going on with them. Are they good, or do they need catalyzed and then trashed. For example, I have an old gallon of a well known unwaxed PE that is clearly more than a year old. I got sidetracked with my project and am only now able to get back on it. I'm using the old stuff for non-structural use, waterproofing plywood under live-well lids, etc. My first two batches of resin cured fine, but are not the least bit tacky. ??? So, I poured a couple of test batches. I used 25 grams of resin and TWO drops of liquid MEK(P) and for the second batch, 25 grams of resin with 5 grams of MEK(P). The first light batch took about 15 hours to cure, but it did and, again, it was not tacky. It also cured at a much lighter color. The other batch cured in short order, maybe 20 minutes, and it too was non tacky. So I'm puzzled as to why this resin did not stay tacky. Was I sold a batch with wax in it, even though the container clearly says no wax added? When I use unwaxed resin, I expect it to be without wax and cure somewhat tacky so I can continue laying on layers without sanding, etc. I'll soon be out of this resin, so it is not a big deal, but mixing my test batches is a big deal because I am dealing with wide temperature ranges and humidity that I have little control over, so I want to know how quickly at my current temperature the resin is going to set. Of course I realize that a small test batch will set quicker than a thin soaked layer, but I can control that somewhat with heat when necessary. So, for an old guy like myself, working a bit slower than the young bucks, I need to know that I have enough time to work the resin while the pot life is still green. To close this episode, I guess I would recommend that the manufacturers provide both volume and weight ratios for their products so that folks with scales can experiment with a little more accuracy on small batches. For sure, I can make more accurate measurements with a digital scale than I can with eye-ball volumes for small batches. Does this make any sense at all? Perhaps, it's out there and I just haven't seen it yet. I realize that a dash or this and a dash of that is okay for many things in life, likely including resin mixes, which I've done more than once, but when material (money) and time wasted is involved, it seems to me that paying attention to some of the finer details just might pay off. Again, thanks for the replies. It is greatly appreciated.

 

Never mix by eye or use the glug glug method. Also mixing 25 grams of resin accurately is almost impossible, 100 grams is minimum for accuracy.

Polyesters have enough forgiveness in the chemistry that it doesn't make much of a difference in whether you mix by weight or volume, although in the lab it's done by weight. The equipment/spray guns used to spray and mix in a shop measure by volume.

Either method is fine for most situations.

 

For Bondo PE resin, 29oz/851ml is catalyzed with .27oz/11ml liquid= 1.29% [1.25% is a common chart value for PE resin] so 25g should be catalyzed with .325g or 8-9 drops figuring 28.3g/1 oz is 10 drops.
So your test batches were significantly over and under. I would try 1x and 1.5x of recommended for an additional test.

My old can of Bondo does not indicate whether it is waxed, nor does its MSDS sheet, and is not that sticky as cured either. I have used prepared waxed resins and have experienced the stickiness that happens if the wax does not surface as intended. The Bondo can says to sand 'if the surface dries hard or is no longer sticky...'

 

Drops are not accurate, the catalyst brand and temperature can vary the size of the droplets. We get calls every week from people using the drop method and not getting consistent results. Plus, many of the droplet charts are off right from the start. No resin manufacturers say to mix by counting drops.

You can get an accurate gram scale on Amazon for about $12, it needs to go two digits past the decimal point.

I forgot to answer the sticky resin part.

Most currently available resins are blends with a DCPD base resin. This goes for Ortho, ISO and many VEs.

DCPD resin isn't as air inhibited as the others. So when it's blended with them the surface tends to become tack free much sooner.

 

Just saying, this is quite a discussion being generated simply for the sake of a gallon of old cheap resin!
If the OP doesn’t trust it, chuck it! Give it to some kid to patch his surfboard!
Surely a quick simple experiment will give him all the information he wants.
Don’t forget to agitate the resin before pouring, and use fresh catalyst for best results.