Epoxy Comparisons and Technical Data Sheets

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Jan 18, 2009
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According to Wikipedia (not the best source I am sure) a Barcol 60B is equivalent to a Shore 80D, IMO hardness is also going to affect how brittle the epoxy is, a hard epoxy is going to break before bending much. According to other sources I have seen there are no reliable ways to directly compare the two hardness standards, though there is a ASTM for doing so in metal materials, but both Shore D and Barcol are used primarily for relatively soft materials like rubber and rigid plastics, though iirc the Barcol test was developed originally to spot suspect/counterfeit rivets in aircraft manufacture.
Interesting . . . Thanks !

Dave F.


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May 7, 2017
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Murray, KY
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.


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May 7, 2016
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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.