It can depend on the type of composite and the application area. There are some bio-based composite systems out there and a lot of research into potential recycling routes but this is quite challenging. The vast majority of high performance systems use both petrochemical derived resin systems and carbon-fibres along with fairly energy intensive manufacturing processes. On the face of it this doesn’t look good but these kind of composites really shine in their performance-to-weight ratio. For applications where weight corresponds to fuel use the large weight saving will give a net benefit. There have been several studies on this such as the one linked below which found that the break-even point in terms of crade-to-grave CO2 emissions on an example aerospace part is 70,000 km for carbon-fibre composite compared to an aluminium equivalent.
Most composites currently use petrochemical-based resin systems but there is intense research to replace these with bio-based materials. For example PLA thermoplastics are based on cornstarch, PFA thermosets are derived from sugar cane waste and it is possible to replace some of epoxy systems with waste from cashew nut shells. In time composite resins will be aligned with fully renewable polymers like other plastic industries.
Similarly fibres will move to bio-based verisons – glass will be replaced by natural fibres or regenerated fibres (e.g. vicose-like systems) and carbon fibre will be generated from precursors such as lignin derived from timber waste.
However, even now, the benefits of composites for lightweighting and materials reduction are contributing to reducing the CO2 footprints of airliners and vehicles.
Plastics have a bad rep at the moment but composites can achieve mechanical properties that other materials simply cannot, and often require less maintenance over their life-span. They can also offer massive weight savings. There is a lot of research being carried out into bio-based materials for composites including bio-based resins such as PFA and natural fibres including flax and jute. In terms of re-cyclability, thermoplastics can be re-moulded by applying heat and certain fibre types can be retrieved and re-used.
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