Mixing Epoxy by Weight Instead of Volume

...which begs the question...

... How does the percentage of hardener in the mix affect the cured epoxy? Does more hardener make it more brittle? Does less hardener make it more pliable? Softer? What's the realistic tolerance for percentages?

Ellis

Re: Mixing Epoxy by Weight Instead of Volume

I always live in fear of not having put in enough hardner such that I end up with a gummy mess so I probably put in too much all the time anyway .

Ellis, asks a very good question.

Bill

Re: ...which begs the question...

Those are all good questions, Ellis.  I do not know the answers, and I’m hopeful that other members of the forum who have more experience and knowledge with epoxy and its chemistry will chime in. 

I believe the ratio of resin to hardener would be fairly important because the instructions on the System 3 container specified the weight ratio resin:hardener as 100:44.  That’s pretty specific and precise.  However, I have no sense of the potential problems that could occur with less than careful adherence to those values, and, no sense of the degree of variance that would be problematic.

Paul in Hunt Valley, MD

Re: ...which begs the question...

Aside from possibly affecting the properties of the glue joint, accurate mixing ensures that both components run out at the same time when the containers reach empty.

Re: ...which begs the question...

I think with WEST it is 3 percent.  Some folks mix up catalysts, vs hardeners.  Catalysts trigger the reaction but are not entrained in it.  In materials like foam insulation, and polyester resin as in "fiberglass",  installers or formulators often mess with the catalyst to meet their needs, not realizing that when all is done the catalyst is a contaminant in the mix.  This caused very expensive repairs, or disposal of generations of boats, including whole series of smaller naval boats, given that the plastic was permeable, and the catalyst was a salt, creating a situation not unlike what happens with freeze thaw action in permeable rocks.  Or causing the mess associated with early, and occasionally current, spray foam installations in houses.

With epoxies the two components complete each other, so ideally the mix is a perfect 1-1 mix of molecules (though the volumes are not necessarily on a 1-1 ratio, the better epoxies tend to be 2-1 or better still 5-1, wherein each molecule of resin finds it's match.  And not being a catalized bond, it is imperative that the mix be thorough as the hardener can't particularly go looking for it's mate.

Insufficient hardener may yield a softer mix, this has been observed.  Generally, structurally, soft is bad.  This may well be a false analogy, but the most common tensile re-enforcement is literally glass...  What happened is that WEST staked out the wood epoxy market, having had friendly access to the DuPont labs that at the time had the majority of the epoxy component market , and thereafter for some time.  The formula DuPont/WEST came up with for wood was hard, because that was what the testing showed, everyone who followed was pretty much left with little cost or structural room to innovate a harder product,  so they promoted soft as though it was the same thing as flexible, etc...  There are a wide range of usable product on the market today at different price points.

If one mixes an under or over batch, hardener wise, you have un-reacted toxic components in the mix.  Which is not something I would want to sand, though in general I don't sand epoxy if I can avoid it.

Re: ...which begs the question...

If one mixes an under or over batch, hardener wise, you have un-reacted toxic components in the mix.  Which is not something I would want to sand, though in general I don't sand epoxy if I can avoid it.

That's a factor. I guess you can't avoid some amount of unreacted component, so it makes sense to avoid sanding epoxy. That's usually hard to avoid, so safety gear and good ventilation are in order.

Re: In summary:

Follow the instructions on the containers you are mixing. Do not trust your memory from the last batch you mixed up. Do the math. We all have access to both postal gram scales (for mixing by weight) and to accurately graduated mixing cups (for mixing by volume). Mix resin and hardener thoroughly, before adding any additives. For small (e.g., pint size) batches, as a rule of thumb, I set a timer and mix West with a stick for two minutes, but I have no experience with slab river tables or trimarans or Homer bucket* size batches. For those, go find someone with lots of successes to ask for advise on times and mixing equipment.

* A.k.a., blue collar briefcase

Re: Mixing Epoxy by Weight Instead of Volume

Epoxy is by far outside my comfort level of knowledge, but...

1. Would weight of 100:44 be equivalent to a volume of 2:1?

2. For epoxy my measures are SWAG's. 1:1 blobs in a paper plate is easier to visualize than 2:1, which is why I favor 1:1 mixes.

Re: Mixing Epoxy by Weight Instead of Volume

  1. Would weight of 100:44 be equivalent to a volume of 2:1?

For the System 3 brand I was using this equivalence is specified on the label.  I do not know if that equivalence would apply to other epoxy brands. If the container directions do not specify a weight ratio for resin:hardener, I would contact the manufacturer’s technical support department for the information.

The difference between volume and weight ratios is an important point, especially since resin and hardener can have different densities. I’d also be cautious about assuming that adding extra hardener will simply make the epoxy cure “better.” With a two-part system, the recommended stoichiometric ratio is there for a reason, and changing it can affect cure, hardness, flexibility and final properties. Using the manufacturer’s specified weight ratio seems like the safest approach when working by weight.

I use both methods depending on what amount I'm mixing.
a- more than 1/4 cup, use 2:1 volume measurement.  Note that I use two different measuring cups for resin and hardener that get cleaned after every mix session.*
b- less than 1/4 cup, I use a gram scale to mix to 1 - .43 as specified by Total Boat.

*I find the "metered" pumps to be fairly accurate if I don't feel like playing with the scale or the measuring cups.
                                                  
I also don't skimp on the mixing, and, when advantageous, put the mix under vacuum for 2 minutes to get the entrained air out of the liquid.