Q: Is there a difference between polyester, acrylic and epoxy resins?

A: Acrylic resin (but not acrylic emulsion, which is the basis of acrylic paint) is a thermoplastic, which means it is one of a group of plastics which can be heated and manipulated repeatedly, whereas polyester resin and epoxy are thermosetting plastics, which use heat or a catalyst to solidfy into a solid mass that won't melt down. Acrylic is mixed from acrylic polymer, a dry powder, a methyl methacrylate monomer, a thin liquid, and usually an organic peroxide hardener of some sort. If pieces of any size are contemplated, an autoclave or hydraulic press is necessary for reducing air-bubbles and counteracting the internal stresses created by the strongly exothermic reaction. The dust is toxic, as is the monomer and its vapors. And the organic peroxides are especially poisonous, some of them being explosive and others causing instant blindness if they get in ones eyes.

Polyester resin, a syrupy clear liquid, is mixed with a small but variable amount of a strong catalyst, which causes the curing mass to heat up (and to craze if you've added too much.) It is versatile stuff, being useful for coating, casting, and building up composites, usually in conjunction with fiberglass cloth. It is not as hard or as clear as acrylic, having a somewhat yellowish tinge to it. And it- and especially the catalyst- is also highly toxic, and is persistantly evil-smelling as well.

Epoxy resin works similarly, doesn't smell as bad, but it -and the hardener that makes it set- is a sensitizer, meaning that you can get a nasty allergic reaction after repeated exposure. Some hardeners are not as bad as others in this respect. Epoxy won't set water-clear like acrylic, and doesn't resist sunlight (UV) degradation as well, but works better with high-tech cloths, like Kevlar and graphite.

Almost any dry pigment (with a few exceptions- test first on a small scale) can be used to color these resins, as well as various inert fillers which also add color; there are also special polyester dyes available. It can be made opaque or transparent- acrylic is used for casting "plexiglas" sheets, among other clear things. Be very careful when using any of these materials: these are generally considered industrial rather than art supplies, and you are expected to know how to protect yourself from their harmful effects. If you don't have the proper facilities for dealing with them, consider using safer alternative systems.

Andrew Werby

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