On epoxies, curatives, diluents, and fillers

prfesser

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You can take the teacher out of the classroom but you can't take the 'teach' out of the teacher... :) What follows is somewhat simplified to minimize potentially confusing chemistry. Most of the information is from Lee and Neville's Epoxy Resin, very old but still valid. Technology for hobby epoxies is pretty much "mature" and has been for a while.

This is being posted under Techniques because that's where most questions about "which epoxy" show up. You do NOT need to know all this in order to use epoxies! Just use the manufacturer's recommended ratio of epoxy to curative, and mix really, really well. For best results you want a molecule-to-molecule mixing and that takes a while. For critical applications in a large/extreme project, weigh (significantly more accurate than volume), mix well, then pour without scraping into a second container and mix with a clean stick. That avoids the small amount of un-mixed epoxy or curative on the walls of the first container.

Epoxies are pretty forgiving, and for routine use the cure ratio isn't extremely critical, but get as close to the manufacturer's recommendations as is practical. It's good practice.

Epoxy resins

Epoxy systems--resin + curative--are thermosetting, meaning that the hardened epoxy can't be melted and potentially reused. Heating cured epoxy to a high enough temperature may liquefy it but that only happens because the stuff is decomposing.

The most common "basic" epoxy resin is the diglycidyl ether of bisphenol A (DGEBA). Examples are DOW's DER331, Shell's EPON828, Ciba's 'Araldite' 6020. The average epoxy equivalent weight of DGEBA is around 200, depending on purity; most commercial DGEBA has small amounts of higher or lower equivalent-weight epoxies as impurities. (These don't interfere with the reaction since the impurities are epoxies and react too.)

DGEBA is clear and nearly colorless and is about as thick as cold honey. It has a functionality of two, meaning that it has two reactive (epoxide) groups on each molecule. Stored at room temperature it has a virtually-indefinite shelf life. Some clear embedding epoxy resins are just DGEBA. Most hobby epoxies contain DGEBA plus additives for specific properties.

Curatives (hardeners)

Curatives react with epoxy resins to give what is essentially one (well, just a few) mega-molecules. The chemical bonds that hold the atoms together are much stronger than the forces between un-bonded molecules. DGEBA molecules can link to curative molecules end-to-end as well as side-to-side (cross-linking) forming a strong 3-dimensional network.

Curatives in hobby epoxy are usually amines (fishy-smelling) or amides (less fishy-smelling), or some combination of them. Some five-minute epoxies appear to use a sulfur-containing curative (from the smell), though I don't know exactly what it is.

Why do different epoxy systems have different ratios of epoxy to curative? Generally it's because of the equivalent weight of the curative or curative mixture. For example, a 1:1 ratio is obtained, NOT by additives or fillers, but by selecting a curative or devising a curative mixture that has the same effective equivalent weight as the epoxy, so that one epoxy molecule reacts with one curative molecule. For a 2:1 ratio the curative must have half the equivalent weight of the epoxy, and so on.

The functionality of the curative is important. Since the functionality of DGEBA is two, for effective cross-linking a curative functionality somewhat greater than two is used. The greater the functionality of the curative, the more cross-linking occurs. The 3-D matrix that forms has more chemical bonds and (theoretically) greater strength BUT...more crosslinking also means greater shrinkage when curing (bonding pulls the bonded atoms closer together than when un-bonded), and the product is more brittle. So there's a tradeoff of overall strength vs. brittle-ness and shrinkage. (Side note: phenolics made from phenol and formaldehyde are very highly crosslinked, which is why they are brittle and shrink during cure; usually they're cured under pressure to combat shrinkage.)

You may find, as I have, that one manufacturer's curative works with another manufacturer's epoxy resin. That points out the similarities between different manufacturers' resins and curatives, though cure ratios may vary somewhat. I've successfully used FibreGlast curative with US Composites resin---same cure ratio as FibreGlast resin.

(Reactive) Diluents

Laminating epoxy is much thinner than DGEBA so that it will easily wet fiberglass/carbon cloth. A solvent such as xylene can be added to thin the mixture, but xylene doesn't react, it's just a solvent. It needs to be removed before the epoxy cures or the product will have inferior properties.

Manufacturers use reactive diluents such as phenyl glycidyl ether or butyl glycidyl ether to thin DGEBA and other epoxies. A reactive diluent has one or more epoxide groups so it will react with the curative, just as with DGEBA. But the diluent has much smaller molecules and so is a very thin liquid. Only small amounts of most such diluents--around 10 parts per hundred parts of resin (10 phr)--are needed to thin the epoxy for laminating and other uses. Why not use more reactive diluent for even thinner epoxy? Too much increases exotherm behavior (smaller molecules = more effective collisions between molecules = faster reaction) and also increases shrinkage. Also the diluents have lower boiling points and evaporate more easily, so diluent can evaporate from an overly-warm batch.

At low levels typically used, reactive diluents do not significantly affect flexural strength or hardness, though they may decrease the heat distortion temperature. And diluents are skin irritants or worse (gloves!!).

Fillers

Fillers are nonreactive solids. They can improve certain properties for specific applications. For example, powdered aluminum increases thermal conductivity and heat resistance and improves machinability; fumed silica increases viscosity and can produce thixotropic properties; talc can reduce cost significantly. Most fillers will reduce shear strength to some degree, though Lee and Neville point out that aluminum oxide (Al2O3, aluminA not aluminUM nor aluminIUM) properly ground and suspended in the resin can increase shear strength by about a third at 25C, and by a factor of over 3 at 105C. Fillers must be dry, dry, dry for best results.

Epoxies containing solid fillers will be OPAQUE, not transparent. If the mixed epoxy is more-or-less transparent or even translucent, it probably doesn't contain significant amounts of solid fillers.

If you're adding your own fillers it's good practice to add a little at a time until you know how the mix responds. You may be surprised to find out how much filler is needed to change properties significantly. Some epoxies can hold a LOT of filler; I've added more talc than epoxy resin, to get a sufficiently thick mix.

Last note: Lee and Neville have a later book about three times as thick as the Epoxy Resins I have. Undoubtedly has a lot more than I can present here. Even if you don't understand the chemistry described, the tables of data will provide insight into properties of different epoxies, curatives, etc.

Quiz on Wednesday.:)

Best -- Terry
 

jderimig

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How do different spoxy brands advertise different strengths? Is there trade-offs made in the diluents or curatives or is the strength largely created in the marketing materials? For example, I am highly skeptical of the brand RocketPoxy.
 

prfesser

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How do different spoxy brands advertise different strengths? Is there trade-offs made in the diluents or curatives or is the strength largely created in the marketing materials? For example, I am highly skeptical of the brand RocketPoxy.
I'm not an epoxy expert, I just know what I've read, but my understanding is that choosing the strongest possible epoxy for rocket construction isn't terribly necessary except for extreme projects. Most epoxies are far stronger than the plywood, cardboard, or phenolic they're bonding. The substrate will break, or the bond will fail at the bondline (insufficient preparation) before the epoxy itself will break.

Making up numbers here: consider Brand A epoxy with tensile strength of 8000 psi and Brand B with 7000 psi. If the plywood being bonded has tensile strength of 3000 psi, it doesn't much matter which epoxy is used.

Diluents and curatives do affect the strength of the cured product but in most cases the effect isn't huge from what I've read. Say a 10-20% increase or reduction in strength.

Now if you're making a two-stage M-to-N all-fiberglass or carbon fiber rocket, attempting to hit Mach 3, then some serious consideration of the type of epoxy might be in order. Even so, most failures occur at the bondline. (A couple centuries ago my surface chem prof said that something like 90% of adhesive failure--not just epoxies---occurs from poor prep, with removal of moisture being pretty important.)

Best -- Terry
 

Kelly

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It seems to me that when using the small (4oz) bottles of hobby epoxy - JB Weld, Bob Smith, whatever - that if I mix 50:50 by weight, I usually end up with a significant amount of one bottle left over (I forget whether it's resin or hardener). Do you suppose that these are actually intended to be mixed 50:50 by volume? I would assume so, since the two bottles seem to be of equal volume. Does it matter if I mix by weight, which is generally more precise?
 

rharshberger

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It seems to me that when using the small (4oz) bottles of hobby epoxy - JB Weld, Bob Smith, whatever - that if I mix 50:50 by weight, I usually end up with a significant amount of one bottle left over (I forget whether it's resin or hardener). Do you suppose that these are actually intended to be mixed 50:50 by volume? I would assume so, since the two bottles seem to be of equal volume. Does it matter if I mix by weight, which is generally more precise?
They are, JB Weld is mixed by "sight"....as for Bob Smith it is by volume, most epoxies are (possibly for ease of use more than anything). Some epoxies will give ratios by weight, cant remember who right now but I have seen them, iirc US Composites will if requested tell you the ratio by weight if you would rather go that route, however their epoxy is so forgiving in many ways that it still works correctly using the volume ratio as though it were a weight ratio (what effect is has on properties I do not know, but it does work).
 

prfesser

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It seems to me that when using the small (4oz) bottles of hobby epoxy - JB Weld, Bob Smith, whatever - that if I mix 50:50 by weight, I usually end up with a significant amount of one bottle left over (I forget whether it's resin or hardener). Do you suppose that these are actually intended to be mixed 50:50 by volume? I would assume so, since the two bottles seem to be of equal volume. Does it matter if I mix by weight, which is generally more precise?
Bob Smith and the like are indeed intended to be mixed by volume. Usually just run two beads onto a piece of cardboard so they're the same dimensions, and mix. To mix by weight the densities (g/cm^3 or g/mL) of epoxy and curative must be known, or the manufacturer-provided weight-ratios are used.

Epoxy and its curative may have densities that are nearly the same, in which case the ratio by volume is about the same as by weight. However, I had some PC7 paste epoxy where the two parts had very different densities, probably because each had different fillers to give a paste. Mixing by volume and by weight were considerably different.

For JBW, BS and the like, mixing by volume is pretty forgiving and is adequate for run-of-the-mill rocketry.

Example of using density to find weight ratio: Consider an epoxy that is to use a 2:1 epoxy:curative ratio by volume. The SDS for the stuff says the epoxy has a density of 0.95 g/mL, and the curative, 1.15 g/mL. For the epoxy, two volumes or 2 mL x 0.95 g/mL = 1.90 g. For the curative, one volume or 1 mL x 1.15 g/mL is 1.15 g. So the ratio by weight is 1.90:1.15. Divide both numerator and denominator by 1.15 for an easier ratio of 1.65:1 epoxy:curative.

Best -- Terry
PS: if there's a Captain Pedantic among us, yes, the ratio is really by mass not weight. But unless you're mixing your epoxy in outer space it doesn't much matter in practice. 😁 Though I'd love to see some rich model rocketeer go into orbit and launch an Alpha on a B6-4, just for fun...
 

jqavins

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I've only read the first paragraph, and I already want to thank you!
------------------
Update: now I've read the whole thread, and I still want to thank you. :)

Several times you referred to "equivalent weight". Would you explain that please?

PS: if there's a Captain Pedantic among us, yes, the ratio is really by mass not weight. But unless you're mixing your epoxy in outer space it doesn't much matter in practice. 😁
That would make vacuum bagging interesting. Easy if you're inside a vehicle with a vent to outside, difficult if you're altogether outside.
Though I'd love to see some rich model rocketeer go into orbit and launch an Alpha on a B6-4, just for fun...
But it would need a vectored thrust stabilization system.
 
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prfesser

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Several times you referred to "equivalent weight". Would you explain that please?
<Prfesser takes a deep breath and girds his loins for a chemistry explanation>
For those not familiar with "mole" in chemistry, one mole is the mass of an element/ion/molecule that contains 6.02e+23 atoms/ions/molecules. That particular number is convenient because (look at a periodic table) one mole of an element/ion/compound has a mass exactly equal to its atomic or molecular weight ("molar mass") in grams. One mole of sodium, Na, weighs 22.9898 grams, one mole of chlorine, Cl, weight 35.453 grams. And both have the same number of atoms. (From here on I'll just say 'element" instead of element/ion/compound). The term "molar mass" is the presently accepted term for the mass of a mole of anything.

Since one Na reacts with one Cl to form table salt, NaCl, it follows that one mole of Na (22.9898 g, 6.02e+23 atoms) reacts with one mole of Cl (35.453 g 6.02e+23 atoms) to form one mole of NaCl (22.9898+35.453 = 58.443 g, 6.02e+23 "molecules"**) with nothing left over.

Now if you want to make one mole of, say, aluminum chloride, AlCl3, you need one mole of aluminum and THREE moles of chlorine (26.98 g Al, 3x35.453 g Cl). The ratio of the two depends on the number of each atom in the compound. So you need to know the formula of the compound, or the combining ratio, or the number of reacting groups on a molecule...

Equivalent weights simplify this because of the way equivalent weight (or just 'equivalent') is defined. One equivalent is the mass of an element that would react with one mole of electrons or one mole of H+, hydrogen ions. With that definition it turns out that one equivalent of any substance reacts with one equivalent of any other substance. We don't need to know formula, combining ratio, whatever.

So...if the equivalent weight of our epoxy is 220 g per equivalent, or just 220 g, and the equivalent weight of the curative is 140 g, that means that 220 grams of epoxy reacts with 140 grams of curative, with nothing left over. Simplifies calculations quite a bit.

Suppose you want to use two curatives with our 220 g/eq epoxy. Curative A has eq wt= 140 g/eq, curative B is 250 g/eq. The ratio needed would be two eq of epoxy, 440 g, to 140 g of A and 250 of B. Total, 440 g epoxy to 390 g of curatives.

The equivalent weight of an epoxy resin or a curative can be altered by making the rest of the molecule heavier or lighter. And an apparent eq wt can be obtained by judicious mixing of different epoxies or curatives. For example, if an epoxy chemist wanted a 1:1 cure ratio for the epoxy and curatives above, he could mix 54.5 grams of A and 145.5 grams of B and use that mix (average eq wt 440) with the epoxy resin. Or do similar convolutions with epoxy resins of different eq. wts. The permutations are literally unlimited.

I should start doing this professionally. Oh...wait...I did...and I quit, to do rocketry. ;)

Best -- Terry
**To be pedantic again, there's no such thing as a molecule of NaCl, because it's made of ions, not molecules. We say they're "formula units" of NaCl instead of molecules. But the basic idea remains.
 

jqavins

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Thanks. I really only needed the one paragraph.
Equivalent weights simplify this because of the way equivalent weight (or just 'equivalent') is defined. One equivalent is the mass of an element that would react with one mole of electrons or one mole of H+, hydrogen ions. With that definition it turns out that one equivalent of any substance reacts with one equivalent of any other substance. We don't need to know formula, combining ratio, whatever.

As to:
To be pedantic again, there's no such thing as a molecule of NaCl, because it's made of ions, not molecules. We say they're "formula units" of NaCl instead of molecules. But the basic idea remains.
Well, there is the gas phase to consider. (Maybe the liquid pahse too? Liquids are weird.)
 
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