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What types of resins are typically used in elements of aircraft, such as structural beams or sheets?

Are some of these commonly used resins more easy to recycle than others? I've that Pyrolysis and Solvolysis are used to recycle the carbon fibers, but it is unclear to me which resins make the carbon fiber composites able to be recycled.

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  • $\begingroup$ Are you looking for brands or chemistries? $\endgroup$
    – Pilothead
    May 23, 2018 at 20:39
  • $\begingroup$ I don't know the specifics of what Boeing or Airbus use, but the vast majority of carbon composites produced use epoxies, which are not easily recycled. However according to wiki they can be essentially pyrolised off in an oxygen-free environment, so the fibres can be recovered at least, though they'd likely have to be downcycled. $\endgroup$
    – Talisker
    May 23, 2018 at 20:47
  • $\begingroup$ Either is fine. I was thinking more along the lines of materials or families of materials. For example, would I want to look in the realm of thermoplastics in general? Or is there a specific epoxy that is widely used for the carbon fiber composites? $\endgroup$
    – H. Soukup
    May 23, 2018 at 20:48

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Currently major structural elements make the most use of bismaleimide and epoxy thermosets. PEEK thermoplastics are gaining share. There's a lot of these and other resins; one of the best-known suppliers with good detail on their website is Cytek.

Thermoplastics can be melted away and are partially recycled for low performance uses. Thermosets are non-recyclable. This, along with lower cost processes, is among the drivers for thermoplastics growth.

Now that composites are the new norm, some expect the next materials advancement metric after composite % to be thermoplastics percentage. The point isn't so much resin recovery as the ease and thus likelihood of recycling.

The recovered fiber is chopped up further and used in bulk in low performance applications. Most consumer goods and aftermarket car parts advertised as carbon only use a layer of woven CF on the outside, with the thickness built up by fiberglass or chopped CF mats with resin matrix.

Short (microns) chopped fiber is also used to fill [thermo]plastics for conventional bulk plastic manufacturing processes. That can be found in cell phone chassis and plastic structural parts in consumer products.

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This will give you some detail on thermoset resins, which solidify through chemical reaction. In summary:

  • Epoxy is the primary resin used in aerospace, at least for structures, as it is strong and wear resistant.

  • Phenolic is the primary non structural resin used in aerospace, as it is fire resistant. This is important in aircraft interiors.

  • Polyester is cheaper than epoxy but weaker, so it shows up in less structural or weight critical applications. Boats might use this.

  • Vinyl ester is somewhere in between epoxy and polyester, so not as good as either. I have never used this.

  • Bismaleimide (BMI) is used for high heat applications where maintaining strength is important. This is a slightly less expensive member of the Polyimide family, used for similar purposes.

  • Cynate ester apparently has military applications, with which I am not familiar.

  • PEEK as suggested in comment, is the only thermoplastic in this list, which is formed by melting and solidification as by injection molding. It can be recovered for reuse with reduced mechanical properties.

Thermoset resins are generally not recyclable. When composites are intended for recycling the fibers are the object, not the resin. The resin has to be burnt or dissolved away, creating an emissions problem that needs to be dealt with.

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  • $\begingroup$ You might want to add polyimide resins and PEEK for high temperature applications. And even the fibers are not fully recyclable; they mostly end up as short fibers for undirected reinforcement, not as woven material or unidirectional tapes. $\endgroup$
    – Peter Kämpf
    May 24, 2018 at 9:06

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