Polyester resin vs Epoxy Resin

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Postby Corwin C » Sun Feb 14, 2010 10:45 pm

angib wrote:Nothing wrong with polyester - loads of boats were sheathed in it and lasted decades. Undoubtedly epoxy is better, but I think polyester is good enough. It's a bit like the hardwood/softwood debate - is anyone unwilling to live in a house made from 'just' softwood?

Andrew


I wouldn't want to live in a house made of balsa (technically a hardwood.)
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Postby GreatPumpkin » Wed Feb 24, 2010 2:25 pm

Disclaimer: I have never worked with either type of resin...

For a recent project for my wife's birthday, a friend of hers wanted me to make some coasters with an clock design. I remember years ago when I was a kid, there was some kind of resin? kit that was sold to encapsulate bugs or whatever - you could make a paperweight or...

Anyway I started thinking about that and ran across this product in the craft/floral department. It's marketed as "Acrylic Water Kit". It's designed use is for making artificial floral arrangements that appear to be in water.

I used it for the coasters, routing out a circle about 3/16" deep into 4" square pieces of bamboo. I then glued a picture of a clock face into the bottom and mixed the "Acrylic Water". It comes in two bottles, each 4 ounces. One is labeled "epoxy" and the other is labeled "hardener". It says it has a cure time of 24-48 hours. I coated the coasters entirely, filling the rest of the routed void.

24 hours later, it was still very sticky/tacky, and the top of the thicker part was forming kind of a "skin". I was trying this in my unheated garage to hide them. I then moved them into the house and they cured solid over the next 24 hours.

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Longwinded way to get to my question - Is this fiberglass epoxy, or some other animal completely? My online research http://tinyurl.com/yfktkss would seem to point to epoxy vs polyester as this product uses a 1:1 mixing ratio.

Seems to be an extremely strong product - I had a hard time prying it loose from the metal angle edges I had it sitting on to cure.

It's only $7 for a total of 8 mixed ounces. While that may be more expensive (per ounce) than buying a well known brand of epoxy, it might be a viable alternative for someone needing just a small amount...
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Postby Corwin C » Wed Feb 24, 2010 10:22 pm

The difference between epoxy and polyester is the polymer chain which creates the final product. Essentially it's the chemical compound that is used to create the resulting material. It's kind of similar to the difference between nylon and silk. They are both strong, durable fabrics, but they also have obvious differences. I prefer epoxy, others polyester, to each their own. (BTW, don't ever try to bond anything with polyester...it will work poorly at best.) Acrylics are another class of polymer different in their chemical composition from both epoxy and polyester.

You can not tell by the mix ratio what type of material you are dealing with. The only way to know for sure is to READ THE LABEL. If it says epoxy on the label then you have epoxy, if it says polyamine, polyethylene terephthalate (PET), polycarbonate, polybutyrate (and many others) you are working with a polyester. Also, some epoxies have a combination of epoxy and forms of polyester in their matrix as well. I have worked with epoxy that was 1:1 mix by weight and another manufacturer was 1:1 by volume (yes it did make a difference). I have also seen epoxy with 2:1, 3:1, and 5:1 ratios. The result would be disaterous if you made the mistake of mixing in the wrong ratio. Again, READ THE LABEL.

Most of the "pour on" finishes are an epoxy (but not all) and they are generally very slow to set (to allow bubbles to make it to the surface and not spoil the finish). I dislike the very slow setting materials when working with fiberglass because they tend to "run away" from where you want them while you're not looking. (My attention span can be somewhat limited at times :roll:)

Also, I would match the epoxy (or polyester) for the task that you are performing. Trying to lay fiberglass (or any other composite material) with an unsuitable or even incompatible matrix is probably one of the most common mistakes that beginners make. If you are looking for a clear finish, the material will be formulated different than a UV resistant finish for example. They will look and perform quite differently.
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Postby frank_a » Tue Aug 31, 2010 1:05 pm

I restore old fiberglass boats, and have had great luck over the years with Bondo Polyester Resin. Generally I coat the resin saturated mat and/or cloth with two part epoxy for finish coating. I'm considering doing that for the teardrop I build someday.

If I was building a fiberglass skin for a teardrop (or most anything), I would start with mat, and finish with cloth. Mat gives you strength, cloth gives you la nicer finish.

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