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
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
Though I'd love to see some rich model rocketeer go into orbit and launch an Alpha on a B6-4, just for fun...
