Sustainability and the composites sector

3. Decarbonisation

Decarbonising industry is part of the UK Government’s legal commitment to achieve net zero greenhouse gas (GHG) emissions by 2050. For the composites supply chain, this means reducing the impact of the sourcing, production, use and disposal of composites. This needs to happen without limiting the ability of composites to deliver current needs, especially where they can be enablers for green technologies.

1. Low carbon feedstocks

The composites supply chain needs to move away from a linear economy that relies on non-sustainable feedstock and the landfilling of material. Instead, a circular economy needs to be developed where sustainably sourced materials are fed into a closed loop with minimal subsequent losses during manufacture and at end of life.

Nearly all polymer and fibre feedstocks used in the composites industry are derived from non-renewable sources. This introduces new carbon into the environment. Some companies are exploring alternative sources of carbon, including:

• bio-feedstocks such as ethanol derived from sugar cane, and certain grasses and bio-waste from agriculture and forestry
• waste materials, such as polymers and plastics (circular economy)
• the conversion of CO₂ itself to useful materials and products.

The chemical industry is also evolving to decarbonise the processes used to convert base materials to formulated products. This is being done by reducing the energy demand of chemical processes and by improving their efficiency.

Low carbon thermoplastics

Thermoplastics are one of the two main types of polymers used in composites, with polypropylene and polyamides particularly prominent. As companies reassess the way these plastics are produced, new technologies are emerging that could reduce carbon emissions during manufacture, albeit at a sizeable short-term cost.

Low carbon thermosets

The supply chain for thermosets is more complex than thermoplastics as they are multicomponent systems. They are created by combining mixtures of various agents, catalysts, oligomers and monomers. The two most common types are epoxy resins and unsaturated polyesters.

Progress is being made towards reducing the environmental footprint of thermosets. Examples of this include:

• Efforts to reduce the amount of carbon used to produce unsaturated polyester resins are underway.
• Low carbon routes to styrene monomer are being investigated so that bio-waste-derived feedstocks could be used.
• New bio-routes to glycols and anhydrides have been identified by a number of companies around the world. Some are in use commercially.

A bio-based epichlorohydrin derived from vegetable glycerol offers hope that epoxy resins could be produced with a significantly reduced carbon footprint. There are also developments in the supply chain that could help curb the carbon demands of the bisphenol A.

Bio-derived resins

Bio-derived resins can offer new chemical approaches to deliver composite performance at acceptable cost. Most bio-derived resins are derived from biomass feedstocks. These include plant oils, lignocellulosics and plant-based polysaccharides, and sugars from waste biomass.

Three promising resins and materials for industry are:

• polyfufuryl alcohol (PFA), the most widely adopted bio-derived matrix material to date in the composites industry. It is an excellent example of a bio-based polymer which has desirable properties that exceed its petrochemical equivalent: for example, high glass transition temperature and its fire, smoke and toxicity (FST) performance.
• polylactic acid (PLA), a thermoplastic with similar mechanical properties to polypropylene and has begun to be used as a replacement for other polymers in the packaging industry
• polybenzoxazines, thermosets with a very high glass transition temperature (up to 250 °C) which have been explored as potential replacements for phenolic resins in the aerospace industry.

Bio-derived, low carbon and natural fibres

Carbon fibre manufacture is predominantly petrochemical-based, using acrylonitrile monomer (ACN). ACN is polymerised and spun into fibre. A multi-stage pyrolysis process then removes any functionalisation leaving usable carbon fibre. This is a costly and energy-intensive process.

There are developments to reduce the carbon footprint of ACN. For example, bio-derived routes are now becoming available commercially (see the Solvay and Trillium case study). This aligns well with broader industry interests related to textile manufacture, where the sustainable sourcing of materials is a key consumer issue. Options for bio-derived drop-in replacements for polyacrylonitrile (PAN) in carbon fibre production are also being developed.

Read more about low carbon feedstocks in section 7.1 of our Chemistry-enabled sustainable composites technical report.

2. Manufacturing efficiency

Composite manufacturing is energy intensive and contributes to the carbon embodied within a composite part. It also generates thousands of tonnes of consumable waste, like vacuum bags that are single use and not designed with sustainability in mind.

The common approaches used to manufacture most composites are:

• resin and fibre combination through processes such as manual lay-up, infusion, and resin transfer moulding
• consolidation, typically with the form under vacuum to remove entrapped gas and compressed with applied pressure (for example, through autoclave, or within a press)
• curing (if using a thermoset).

There are examples of how industry is incorporating more sustainable practices into the manufacture of composites, including:

• fast-curing resins or low-temperature cures that reduce the energy required to run ovens
• UV curing which has proven itself in some areas, including aerospace and energy (for example, for wind turbine blade repair or cure)
• moving to a vacuum-bag-only approach, with just an oven, to significantly reduce energy consumption.

Read more about sustainability in composite manufacture in section 7.2 of our Chemistry-enabled sustainable composites technical report.

3. Increasing composite lifetime

Chemistry enables the durability of fibres and polymer matrices used in composites. It also plays an important role in the use of coatings and additives that protect them. This increases their resistance to environmental and in-service degradation, as well as fire. Chemistry can help by:

• sourcing more sustainable chemicals for additives from bio-derived or recycled sources (many performance-enhancing mechanisms rely on chemicals sourced from fossil fuels)
• collaborating more closely with material scientists and composite engineers to develop functional materials that enable detection of damage when it occurs.

Read more about increasing composite lifetime in section 7.3 of our Chemistry-enabled sustainable composites technical report.

4. Reuse and repair

Reuse represents an important strategy for reducing the environmental impact of FRPs. From a lifecycle perspective, reuse is generally preferential to recycling (see the waste hierachy). Reusing FRPs could be particularly valuable given their high inherent value and challenge to recycle.

Strategies for reusing FRPs:

• enhancing methodologies surrounding polymer identification (analytical chemistry)
• designing for the disassembly of large structural parts, for example, by developing reversible adhesives
• developing chemical strategies for structural health monitoring.

Reuse is still in its infancy. For example, the identification and requalification of composites remain significant challenges that would greatly benefit from more research and businesses working to facilitate it. However, chemical knowledge, techniques and research could play an important role in making reuse more commonplace.

FRPs can be damaged in a variety of ways, including impact, manufacturing defects, and environmental exposure. If damage is detected, repairing parts is a sustainable way of extending the lifetime of composites.

Strategies for repairing FRPs:

• composite patches that can be attached to a damaged structure
• liquid resin injection into pre-drilled holes within the damaged area, then curing it
• self-healing material that can be applied to design features, including section changes, ply drops, stringer run-outs, holes and joints. More research is needed to translate self-healing technologies into real-world applications.

Chemistry that underpins the development of easily repairable FRPs will accelerate the development of solutions in this field.

Read more about reuse and repair in section 7.4 of our Chemistry-enabled sustainable composites technical report.

5. Chemical recycling technologies

As composites are made of two or more materials, recycling them is more complex than conventional materials like plastic packaging and metals. Recycling techniques need to be able to recover multiple distinct materials to avoid many composite parts ending up in landfill.

As the composite market grows so too does the problem of composite recycling. For example, in the wind industry alone, it is estimated that, by 2050, 190,000 tonnes of carbon fibre-reinforced polymer (CFRP) waste will have been generated. So far, much of the research into recycling composite material has only focused on the recovery of carbon fibre, and less on lower-value fibres. However, some methods have been identified that can recover other fibres and resin at the end of life.

Chemical recycling processes, such as solvolysis and thermolysis, enable fibres with minimal performance drop to be recovered, and the resin collected for reuse. In order to make these processes viable value needs to be derived both from the recovered fibre and the recovered polymer fraction.

Promising examples of solvolysis are highlighted in Section 7.5.2 of our Chemistry-enabled sustainable composites technical report.

However, a common problem preventing recycling is that the chemical makeup of materials is not always known.  Composites often have a long lifetime and the materials used in their manufacture may not have been accurately logged as it passed along the supply chain. For composite recycling to become the norm:

• increased collaboration and information are needed along the supply chain to ensure that composites manufactured today can be easily identified
• sorting methodologies and analysis techniques need to be developed to identify and isolate the products of chemical recycling
• a better understanding of which chemical recycling techniques to apply to which composite types could highlight the standard approach for each composite class, adding great value to industry.

Several obstacles must still be overcome before there can be a new supply chain for recycled material but there are recycling technologies that show promise. To increase the uptake of composite recycling it must also become economically viable, which could be through increased penalties for non-recycling disposal strategies (i.e. legislation) or by ensuring the products recovered are of significant value.

Read more about chemical recycling technologies in section 7.5 of our Chemistry-enabled sustainable composites technical report.

6. Inherently recyclable composite materials

Chemical recycling is not always possible. For next-generation composites there is an opportunity to design them for recycling into new materials.

Vitrimer development is one approach that the composites industry could adopt for developing inherently recyclable materials. They are crosslinked networks containing dynamic covalent bonds, known as covalent adaptive networks (CANs). These polymer networks have been shown to be processable or recyclable through exchange reaction of the dynamic covalent bonds.

Several viable routes to triggered degradation thermosets have also been identified and could offer routes to more easily degraded polymer matrix structures at acceptable cost. Crucially, these chemistries will need to be designed with industrially viable chemical recycling technology in mind and the associated separation processes.

Read more about inherently recyclable composites in section 7.6 of our Chemistry-enabled sustainable composites technical report.