While it may seem recent, the quiet war between thermoset composites and thermoplastics has been waged since the late 1980’s. Plastics engineers have debated the benefits and pointed out the weaknesses of each as fiercely as a couple computer geeks debating Microsoft versus Linux. The battles will continue, and the thermoplastics folks will say that composites are going the way of the dodo bird, but the truth is that both materials have their place in the world and thermoset composites are far from dead.
Detractors cite thermosets limited shelf life and poor recyclability post-cure. Once a catalyst is added to a monomer, the molecular change is permanent and irreversible. That means that once cured, the compound, when exposed to heat, will degrade rather than melt. This is because the composite material degrades below its melting temperature. It also means that fabricators must carefully control the polymerization process because once the material is cured, it can’t be melted and remolded.
Thermoplastics, on the other hand, are touted for being easy to process. They are able to melt, pour, solidify, and remelt easily. Why, then, aren’t thermoplastics ubiquitous throughout the industry? Why are thermoset composites still used and preferred by some engineers?
- Thermosets have been around for a long time. They have an established place in the market and are trusted.
- Thermosets have a lower raw material cost.
- Easier forming and processing.
- High thermal resistance (brake pistons, jet-engines, and high performance supercars just to name a few).
- Thermosets still represent 95% of aerospace prepregs.
Thermoset composite parts are typically made from epoxy or polyester resins and (most commonly) reinforced with glass fibers. Depending on the application, polyester resin systems can be cured to be softer and more flexible or harder and more brittle. Polyester composites are used across a wide variety of consumer and industrial products from bathroom showers to auto body panels, to boat hulls. In the electrical industry, thermoset composites have a dielectric advantage, helping drive usage for both arc and track resistance.
Epoxy resins are cured to be harder and tougher, and have high resistance to solvents and alkalis. Circuit boards would be good examples of a glass-reinforced epoxy application.
Reinforcing material in thermoset composites isn’t solely glass. Other fiber reinforcement material includes: carbon, graphite, boron, and aramid (Kevlar) fiber. Glass comprises the vast majority of thermoset composite reinforcement due to its incredible tensile strength, but depending on the application, other reinforcement material may be utilized.
Thermoplastics innovation is advancing today in much the same way thermosets have advanced throughout history, but thermoplastics aren’t unseating and killing thermosets. Thermosets are still being utilized in trusted applications like aerospace and auto while thermoplastics continues to make strides into these industries.
Rather than pitting one against the other, the future will likely prove that each technology, like any technology, being utilized where it’s strongest and being replaced where its competition proves stronger. Like Microsoft at the desktop and Linux at the edge of the network, so shall thermosets and thermoplastics one day find their peace.
