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Thread: Polyester, Acrylic and Epoxy.

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    SRN Editor SteveP's Avatar
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    Polyester, Acrylic and Epoxy.

    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.
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    SRN Member Dave K's Avatar
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    This is great info. I need a quicker, non flowing epoxy for seams at sinks. Any suggestions? The poly's others and myself have been using have been failing at the sink. I have been trying to encapsulate the poly by top coating with UV cure these days but if the water can get around it the poly will still fail in a few years.

    I have not had any problems with the poly or CA failing at other seams - just wet area's like sinks.

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    SRN Alumni DBrock's Avatar
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    Top - I have talked with Bonstone about their Express II Flowing and Knife Grade Epoxy and intend to try them in a seam repair project I have at a sink rail. I was told these have a 10 min working time with a 30 to 60 min set time that can be accelerated with heat. Have you looked into anything like this?

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    SRN Member Dave K's Avatar
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    I have used the express flowing but not the knifegrade. The flowing is awesone for rodding during fabrication if you only want to wait about an hour instead of the other Touchstone edge flowing which I used to set-up to cure over night. I should give the knifegrade a try. Thanks for the reminder.

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    I have problem with acrylics turning yellow over time. All interior use. Why is that happening?

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    I have seen the same thing??
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    SRN Senior Member glenn k's Avatar
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    Someone explained to me that it was about using too much hardner. I think it may have been Paul from Bonstone.
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    Wicked SRN Membah!! DUSTY's Avatar
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    All of the injectable pre colored acrylics work well for a semi flowing glue that can be used to replace popped out knife grade from the original install.
    You can also use it as a base and bonding glue in the seam.. then finish with a knife grade ( like akemi platinum) acry-poxy ( acrylic and epoxy) if you want to custom tint.

    these semi flowing glues like integra, chemical concepts, Altima etc.... aren't knife grade and are not as liquid as flowing glues, plus they automatically mix - to give the proper amount of hardener each time.


    the acrylic yellowing Roger and MM spoke of is similar to an acrylic coating on a floor - it yellows over time.

    if you're talking about it yellowing when first applied or when it just cured - that is because too much hardener was used. I will sometimes cook the acrylic too hot on purpose in order to get that yellowed color on some stones that have been resined - like delicatus


    I have found that in my area - acrylics can be more difficult to work with in the winter because the ambien temperature makes it take longer to cure. the house may be 70 degrees but the back of the van was 14 and the metal can will make the glue that temperature. making it take forever to cure, so I will be tempted to use more hardener than to play the waiting game.

    Akemi Platinum Acry Epoxy is my go to. its both acrylic and epoxy so it kicks quickly and is wicked strong - also works very well out doors.
    pale color and tints well
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