It’s very likely that you have never heard the word “thermoset” before, but you probably use a bunch of them. Epoxy glue, acrylic resins, polymer clay, and silicone are all thermosets.
Were you to pick up and handle a thermoset, there wouldn’t be any clear way to distinguish it from a thermoplastic. That’s because their basic makeup is the same. They are polymers, long chains of repeating atomic elements. They can have a lot of similar properties in density, texture, and colour to thermoplastics like nylon or polyester or Worbla. There is an important difference, though, which has a couple of big implications.
While thermoplastics are basically jumbled messes of long, entangled molecules frozen together, thermosets are a cohesive matrix. An object made of a thermoset can be one huge, single molecule. The repeating atomic structure bonds form a three dimensional web.
One consequence of this is that thermosets don’t really melt. They can soften and become flexible, but since they’re one big molecule, they can’t become liquid, or even particularly squishy. Reshaping a thermoset would break that matrix, and usually ruin that item permanently.
On the flip side, thermosets can handle a fair bit of heat before they suffer any ill-effects. Until they get hot enough to cause new chemical reactions (i.e. burning), the material will keep its shape. Because of that cohesive structure, thermosets tend to also be stronger than thermoplastics.
Since you can’t melt a thermoset to form it, most come in a liquid or putty state, and then require some sort of reaction to turn them into a solid, also called curing them. There are three common forms this takes: two-part mixes, catalytic reactions, and thermal activation.
The two-part mixes are probably the ones you’ve seen the most. Epoxy glue, Green Stuff, and pretty much anything you pick up from Smooth-On are just some examples. There are two chemicals which are kept separate. When mixed together, usually in equal quantities, they undergo a series of chemical reactions which form that matrix.
Catalytic reactions are similar, but instead of mixing components in equal measure, there is a major component and a catalyst. The major component makes up most of the volume of the material and has all of the chemicals components needed within it to form the final solid, but it needs a kick to get those pieces to start joining up. The catalyst provides that kick and often requires only a few drops to do it. Acrylic resin and Bondo are common examples of this.
The term thermoset probably comes from the third group, those that cure with the addition of heat. These have all the chemistry needed to form that complex, interconnected molecular web inside them, but the chemical reactions that would cause that web to form need some extra energy to get going. The most common thermally activated thermosets that you’ll work with are polymer clays like Sculpey and Fimo.
There are also a lot of additives you can throw into the mix for varying effects. You can add fibers to create a composite material with a high tensile strength, like the fiberglass in Bondo. You can add metallic powders to get a lustrous faux-metal finish (also called cold casting). Pigments can be added to make the finished product clear, translucent, or opaque and any colour of the rainbow, and there are a range of chemicals that can alter the curing time and final properties of the material.
Along with their awesome versatility, thermosets do come with some own hazards and limitations. Once cured, they often cannot be reshaped at all, and many don’t take well to tooling. Some are very fussy about impurities like water and temperature variation when curing. Also, many of the chemicals are toxic in their liquid form and will ruin clothes and work surfaces, besides.
Thermosets offer a lot of flexibility to cosplayers. They let us make extremely detailed castings without fancy equipment. They allow for working times anywhere from five minutes to several years, and colour and texture variants from perfectly clear syrup to thick, black, sculptable clays. Just read the warning label.