Commentary by Mike Beckmann-Bonstone Industries
There are “good” and “bad” formulations for all 3 chemistries, and many exceptions to the comments I’m going to make. For instance–not ALL epoxies can be used outdoors. And not ALL polyesters have poor water resistance. So remember that the following comments are for “generic” representatives of each type of chemistry)
Polyesters and acrylics cure the same way, in a manner that inherently causes shrinkage of the polymer as it cures. You can imagine that as it shrinks, it tends to pull away from the substrate, and thus the bond is stressed and weakened. Add additional stress by putting it outside (temperature extremes, freeze/thaw, moisture, sunlight, etc.), and you can have some real problems. If you’re working indoors only, and gluing something relatively porous, like marble for instance, you can get a pretty good mechanical “bite”, and the shrinkage is therefore not too much of a concern. With non-porous substrates, like granite– you WILL have a problem because of shrinkage.
Now, some differences between acrylic and polyester. Most polyesters used in the adhesive market are made using raw materials that are somewhat moisture-sensitive (primarily to keep the cost low). As a result, they are not generally recommended for moist environments. Acrylics are made using very different (and more expensive) raw materials, that don’t have this same moisture sensitivity tendency–they are therefore somewhat better than polyester for moist environments (but don’t forget that shrinkage factor we talked about earlier!) Now, uv-stability: polyesters are not–they will discolor and get even more brittle. But acrylics ARE uv-resistant–no change in color or gloss.
Now, on to epoxies–COMPLETELY different curing mechanism that results in practically no shrinkage, so you get a much stronger bond from the get-go. Also, as correctly mentioned in the previous posts, epoxy actually chemically bonds with most substrates. Compare this to the polyester and acrylic, where the bond is almost totally physical in nature. So the chemical bond of the epoxy, coupled with the low shrinkage, means you get a vastly stronger bond. Epoxies also GENERALLY cure slower than polyester or acrylic, but that’s one of the compromises that you may have to accept. (There are very fast-curing epoxies, nearly as fast as polyester–we have several–but they tend to cost more than “typical” epoxies). Now, regarding the uv-stability of epoxy–this is one area that is frequently misunderstood, and you need to be VERY careful when you hear claims of “uv-stable epoxy”. With very few (extremely expensive!) exceptions, ALL epoxies will discolor and chalk when exposed to sunlight. So how can some companies claim “uv-resistant epoxy”? Simple–this discoloration and chalking is a surface phenomenon only–it in no way affects the strength of the epoxy or the bond. So in that sense, if all you care about is that the assembly will not fall apart–then the epoxy is uv-stable for you! (If, however, you ARE concerned about the appearance of the joint–well then, we have a solution for that too, called Last Patch–but that’s another chemistry and another story….).
Bottom line–we recommend polyester for interior use only, in dry conditions; if you want an “upgrade” for uv-stability or slightly enhanced strength, try an acrylic; for anything outdoors, or for moist interior applications, and to get the best piece-of-mind for the strongest, most long-lasting assembly–we recommend epoxy.