I hesitate to use the word “sustainable” for the practice of gluing highly refined industrial products together to make products that are – for the most part – unnecessary. The fact is that composites processing can be incredibly wasteful and a huge volume of unrecyclable mixed-plastic trash is produced from even very standard processing techniques. Go look in your dumpster if you work in a composites processing facility and see the problem first hand – we are all doing bad things and we are not aware of it to the extent that we need to be!
With that in mind, our economic system is based on building things and selling them to eachother. We get the materials mainly by digging them out of the ground or growing and harvesting them. Unfathomable quantities of long-dead plant matter are sucked and dug up to fuel the refining of other materials – all so that we can mix them up exactly as we wish to create things we need. And the truth is we do need them – we have created an ingenious system that is so highly leveraged on “cost of extraction only” energy, complex supply chains and manufacturing processes that we literally can’t live without many industrial products. We need to build just to survive!
While in isolation, composites appear to be an ecological nightmare and crime against the earth and our future – when compared with other materials we could choose they have redeeming qualities. Metals take even more energy on average to extract and refine for a given useful unit. Raw plastics are often recycleable but have limited physical properties. Bad as it is, there is definitely room for plastic/fiber composites!
Reinforcement fibers are typically based on petrochemical (carbon) or mineral (fiberglass) precursors which take a lot of energy to produce – but not as much as metals. Typical thermoset resins (epoxies, polyesters, phenolics) are are based on industrial chemistry much of which is developed from petrochemical precursors and all of which requires lots of energy to refine, process and transport. Overall, there are many applications where the weight benefits of composites can reduce energy consumption over the lifetime of the product. This is perhaps the best argument for composite materials in the vehicles of the future – that we should build with petrochemicals instead of burn them.
Airliners are a great example of this. A Boeing 787 (see article) has a structure that is 50% composite materials. This allows for a weight savings of 20% – or it allows for 20% more airplane for the same weight. Compared to a similar plane made without composite materials, the Boeing 787 will use around 20% less fuel. It will use less fuel every hour of flight over decades of service. Given that people are unlikely to stop flying completely – this use of composite materials makes lots of sense.
On the low-volume side, current high performance thermoset composite practices are truly unsustainable in terms of the amount of energy used and waste produced for a given end product. Race-cars and yachts will continue to create huge dumpsters full of mixed plastic waste and there is no way to meaningfully reduce that and maintain the quality of one-off prepreg or infused parts. This is something we should all know, and be aware of in our use of these materials. If you can develop a way to reduce scrap parts or reduce material usage, that will help and should be a priority!
Thermoset resins can’t readily be recycled, but thermoplastics can. You just heat ’em up and reshape! For large scale production in automobiles and other “mobility devices” thermoplastics make a lot of sense. Typically thermoplastics are formed in a set of molds under pressure and heat – like injection molding but with fiber reinforcement. Getting the fibers to run where you want them is a challenge and often thermoplastic prepregs are combined with injection “overmolding” to create long-fiber reinforced net-shape parts. There is little process material and the cycle time is fast. Plenty of energy is used in the process but much less than casting or machining a similar part out of metal – and the composite part is lighter for the same strength.
The future of production composites is trending in a thermoplastic direction. For smaller volume parts, 3D printing is meeting molding somewhere in the tens to hundreds of parts. Thermoplastic 3D printing filament is already being combined with simple long-fiber reinforcement (see Markforged and others) to build one-off parts. This will only get better and faster as the technology improves. Composites will be more and more automated at smaller part volumes. Soon the line between “3D printing” and “composites” will be blurred more thoroughly by automatic fiber placement and additively manufactured molds and cores.
All this is good news for the sustainability of composite materials. The trend toward using automation to increase the efficiency of composite layup is good. The trend toward wider use of thermoplastics instead of thermosets is good. While we have to build things to survive, we should build them from the most appropriate materials available. Sometimes the energy expended in building a vehicle that is lighter will be paid off many times over through reduced energy use over it’s operating lifetime. Composites aren’t the answer for most products, but for things that have to fly or drive or last a long time in harsh environments, they are often the best available option.