Epoxy Comparisons and Technical Data Sheets
- Thread starter rharshberger
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Just to elucidate:
Apparently some people are under the impression that commercial epoxies contain fillers to attain a particular ratio of epoxy:curative. Some epoxies such as JBWeld do contain fillers---aluminum, aluminum oxide, steel, silicon dioxide (silica), etc. The fillers may improve thermal expansion, water absorption, shrinkage, etc. but the fillers make such epoxies opaque. Most clear epoxies (and most curatives) that we use for rocketry do not contain nonreactive fillers or solvents. They are "100% solids" meaning that all the different components react to form the final product.
There are literally hundreds of commercial epoxy resins (likewise curatives) with varying properties. The most common thick epoxy is DGEBA, the diglycidyl ether of bisphenol A. Trade names include DER 331, EPON 828, Araldite 6010, Epi-Rez 510, many others.
Thin epoxies used for laminating start with DGEBA or similar, then have "reactive diluents" added. These are low-viscosity, low molecular weight liquids, but unlike most solvents these diluents have epoxide groups that react the same as the epoxide groups on DGEBA. That means they are incorporated into the cured product. These include butyl glycidyl ether, octylene oxide, and others. By mixing different epoxy resins and different reactive diluents, and by using different curatives, almost any epoxy: curative ratio from 20:1 to 1:1 or even lower can be obtained.
Why not use the reactive diluent alone, since it reacts the same as does DGEBA? Because those diluents react quite rapidly, exotherm far too much, and usually result in considerable shrinkage of the final product. About 5-10% reactive diluent is considered optimum and gives a sharp reduction in viscosity.
Quiz on Friday.
Best -- Terry
PS: Lee and Neville have a number of books on epoxies; largely technical, but some really good information.
Apparently some people are under the impression that commercial epoxies contain fillers to attain a particular ratio of epoxy:curative. Some epoxies such as JBWeld do contain fillers---aluminum, aluminum oxide, steel, silicon dioxide (silica), etc. The fillers may improve thermal expansion, water absorption, shrinkage, etc. but the fillers make such epoxies opaque. Most clear epoxies (and most curatives) that we use for rocketry do not contain nonreactive fillers or solvents. They are "100% solids" meaning that all the different components react to form the final product.
There are literally hundreds of commercial epoxy resins (likewise curatives) with varying properties. The most common thick epoxy is DGEBA, the diglycidyl ether of bisphenol A. Trade names include DER 331, EPON 828, Araldite 6010, Epi-Rez 510, many others.
Thin epoxies used for laminating start with DGEBA or similar, then have "reactive diluents" added. These are low-viscosity, low molecular weight liquids, but unlike most solvents these diluents have epoxide groups that react the same as the epoxide groups on DGEBA. That means they are incorporated into the cured product. These include butyl glycidyl ether, octylene oxide, and others. By mixing different epoxy resins and different reactive diluents, and by using different curatives, almost any epoxy: curative ratio from 20:1 to 1:1 or even lower can be obtained.
Why not use the reactive diluent alone, since it reacts the same as does DGEBA? Because those diluents react quite rapidly, exotherm far too much, and usually result in considerable shrinkage of the final product. About 5-10% reactive diluent is considered optimum and gives a sharp reduction in viscosity.
Quiz on Friday.
Best -- Terry
PS: Lee and Neville have a number of books on epoxies; largely technical, but some really good information.
DES
Well-Known Member
I'll throw my two bits in on this subject - The best measure of brittleness in a composite or plastic material such as epoxy is going to be the tensile elongation at break. A brittle material will have minimal elongation, whereas a non-brittle material will deform (stretch) in a plastic manner quite a bit before it breaks. For comparison using West System data:
The West Systems 105 resin has a modulus of elasticity of 450,000 psi, and an elongation at break of only 4%. A stiff material, with little give.
The West Systems G Flex has a modulus of elasticity of 150,000 psi, but an elongation at break of 32%. Three times more flexible, with a lot of stretch, 8 times as much.
If you cast threads of the two epoxies about 6 inches long, you can bend the G Flex in a full circle if you are careful. But the 205 will break after very little deflection.
The Shore hardness isn't really a measure of brittleness, it is a measure of how much the plastic deforms under a point load. I suppose there is some correlation with brittleness, but if you have published data on tensile elongation, that is a more direct measure.
I like to use the 105 epoxy for structural bonding of fin roots, and the G Flex gel for the fillets. The idea being I get the high shear strength at the root, but a non-brittle, high elongation fillet that doesn't crack if the fin bends.
The West Systems 105 resin has a modulus of elasticity of 450,000 psi, and an elongation at break of only 4%. A stiff material, with little give.
The West Systems G Flex has a modulus of elasticity of 150,000 psi, but an elongation at break of 32%. Three times more flexible, with a lot of stretch, 8 times as much.
If you cast threads of the two epoxies about 6 inches long, you can bend the G Flex in a full circle if you are careful. But the 205 will break after very little deflection.
The Shore hardness isn't really a measure of brittleness, it is a measure of how much the plastic deforms under a point load. I suppose there is some correlation with brittleness, but if you have published data on tensile elongation, that is a more direct measure.
I like to use the 105 epoxy for structural bonding of fin roots, and the G Flex gel for the fillets. The idea being I get the high shear strength at the root, but a non-brittle, high elongation fillet that doesn't crack if the fin bends.
NotABoatBuilder
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Here’s a link to the uscomposites 635 elongation data. I loved the table!
https://www.scribd.com/document/439850785/635-thin-Epoxy-all-hardeners-pdf
https://www.scribd.com/document/439850785/635-thin-Epoxy-all-hardeners-pdf
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This looks like a good place to leave this info for posterity. I emailed U.S. Composites asking about ratios for mixing by weight, as I think I can be more accurate with a gram scale than with a meniscus when doing small quantities. Response I got is below:
rharshberger
Well-Known Member
Thank you, however experience has shown that mixing at ratios US Composites lists by volume also works by weight. I use US Composites 635 and 150 epoxies a lot, and for small amounts (too small for the pumps) I use a scale and mix at stated 2:1, 3:1, and 4:1 ratios with no issues. I have mixed quantities as low as 8 grams total. The volume ratios are so close to the weight ratios it seems to make no difference.
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With a good 0.01g scale and a couple of good syringes I can mix 2.5 grams (fast hardener, 4:1), probably less, and it cures just fine. I like to use epoxy for some parts of some models---couplers in particular---and have gotten used to working with smaller quantities. Half a century in laboratories helped a bit. I *think* I could mix a gram...
Definitely have to mix thoroughly or get possible soft spots. Scrape well.
And thanks for the weight ratios, NotABoatBuilder! I prefer to work to as close to ideal as possible.
I *think* I could mix a gram...Definitely have to mix thoroughly or get possible soft spots. Scrape well.
And thanks for the weight ratios, NotABoatBuilder! I prefer to work to as close to ideal as possible.
For most... However, I found out the hard way (err, sticky way) not true for TAP vinylester. What a mess.
