From black to green

The aerospace industry needs to focus on better ways of recycling carbon fibre
The aerospace industry needs to focus on better ways of recycling carbon fibre

Cascade Engineering’s sales manager of engineering, Grenville Williamson looks at the use of carbon fibre, whether it can be recycled or produced more sustainably and whether there are usable alternatives coming to market.

In common with all industries, building a more environmentally-friendly approach to design and manufacture plays a key role in the strategic thinking of structural engineering companies and their clients.

The UK government’s Net Zero Strategy: Build Back Greener outlines the key improvements needed to meet the government’s target. Each sector identified within the strategy has then needed to consider how to make these significant and sustainable gains. At Cascade we are particularly interested in sectors that might leverage efficient structures to achieve these aims, as the design and analysis of these is the specialist skillset we offer.

Greater global awareness of climate change and the wider issues surrounding sustainability have meant focus on end-of-life options for all components, which has highlighted high volumes of carbon fibre-reinforced plastics (CFRP) waste as being a potential problem. Additionally, the demand for CFRP products has exposed capacity issues in the raw material supply base, driving up costs. As such, the potential to recycle existing products – and investigate alternative options – has sparked a lot of interest.

Many thermoset products are consigned to landfill or incinerated for energy recovery
Many thermoset products are consigned to landfill or incinerated for energy recovery

Many aerospace CFRP products coming to their end of life use a thermosetting epoxy resin system that has made recycling difficult. In the past, these products would have been consigned to landfill or incinerated for energy recovery. Neither of these are ideal, so other recycling options are required.

With temperatures of around 1,000°C used in the process, manufacturing virgin carbon fibre is energy intensive. This is reflected in the end cost and in the resulting carbon footprint. Recycling processes use less energy and therefore can result in a cheaper product, more comparable to glass fibre in price, but with the improved properties carbon can bring. This makes recycled carbon fibre attractive in the wider markets, but any degradation in properties inhibits reuse in highly loaded, critical aircraft components.

Where does this degradation come from? Established composite recycling technologies often start with mechanical shredding to prepare the product for the processing plant. This immediately compromises fibre length – a key contributor to the strength and lifespan of the material.

Typically, variations of either pyrolysis or solvolysis techniques are then used to break down the composite materials. Pyrolysis techniques use heat in a chemically inactive environment to break down the parent material, allowing its components to be separated. Solvolysis is a thermochemical process that depolymerises the parent material using a solvent assisted by heat. These can be aggressive processes and, with fibre length compromised from the start, often result in the degradation of the material properties. The recycled materials are usually only suitable for use in less critical structures than they were originally.

Enhancing the process

More recently we have seen advances in pressolysis, the overarching name given to processes that separate resin and fibres using steam under pressure. This can be used for a variety of applications and suits composite materials well.

One noteworthy example is B&M Longworth’s Deecom process. This varies the heat and pressure during processing to soften, disconnect and separate the resin matrix from the fibres. By carefully controlling the process, high yields of clean fibres exhibiting near original virgin material properties are produced.

This cleaning removes the sizing (the preparation that promotes fibre impregnation and bonding) from the fibres, allowing the use of improved sizing chemistry to improve performance. Add to this the ability to design the associated apparatus to process larger parts without initial shredding, and the opportunity for reclaiming long fibres with improved performance is a possibility.

Carbon fibre alternatives

If recycling carbon fibre is either too expensive, too time-consuming or results in a lower quality product, what are the alternatives? When we talk about sustainability within the context of composite materials, we are focusing on investigating novel and innovative ways of approaching a complex problem. The good news is that we are seeing more of these novel materials being developed and used in practical applications.

Natural fibres were there at the start of composite structures, but their synthetic alternatives gave superior structural properties, better corrosion resistance and material compatibility, improved repeatability, and greater design flexibility.

With environmental impacts, regulations and public opinion putting a renewed focus on sustainability, there’s been a return to investigating natural fibres. On top of the environmental benefits, composites made from natural fibres have the potential for lower production costs and reduced weight in specific circumstances.

Grenville Williamson, engineering sales manager at Cascade Engineering
Grenville Williamson, engineering sales manager at Cascade Engineering

Natural fibres come from plants that can be readily farmed, making them sustainable and biodegradable. Flax, hemp, bamboo, jute and kenaf are all being considered with varying levels of maturity. Within aerospace their use for flying parts has been limited, but the focus shows a good intent for the future. Examples of where natural fibres may struggle is with specific strength for high-performance applications and moisture absorption, compromising service life and through-life maintenance.

Sustainable alternatives to natural fibres include recycled composites and as described above, recent developments in this field are showing positive signs for scalable and cost-efficient solutions in the near future.

Sustainability in the resin matrix is available through biodegradable products, though recyclable systems can help move industry toward sustainability while these are being developed further. Both have pros and cons.

There are some biodegradable resin matrix systems on the market and more in development. Bio-based polymers derived from renewable sources such as corn, soy, or sugarcane can be used as a sustainable composite matrix. The bio-based content varies in the products, and they need to be suitably matched to use, but they have a lower carbon footprint and reduce dependency on fossil fuels.

These systems are designed to be environmentally friendly and break down naturally over time to reduce the long-term effect on the planet. There are challenges though. Compared to their oil-based counterparts, they have low specific strength and stiffness as well as limited temperature and moisture resistance. As such, initial applications will be on non-critical or disposable components such as interior panels, non-structural parts or temporary structures.

Recyclable resin matrix systems are becoming increasingly feasible, and offer a route to inexpensive, high-quality materials. Both economic and environmental benefits will improve as technology advances. Recycling is ranked poorly in the best ways to manage waste though, behind avoidance, reduction and reuse. Also, chemical recycling to acquire the highest-grade materials is still in the research phase. Current techniques tend to downgrade the materials such that the recycled product is of lower structural capability than the original. Again, this limits their use on critical or highly loaded structures.

Working for a material future

Of course, some industries may be ahead of the game here. For example, specialists in the marine industry have been using natural and recyclable materials for some time. This knowledge is best shared across industries – pooling data and understanding to allow engineers and manufacturers to see how materials used in one application could be adapted to another.

If all industry sectors are to meet the expectations of the government and wider society in improving efficiency and reducing waste, opportunities for cross-fertilisation of ideas need to be taken. For aerospace this needs to focus on better ways of recycling carbon fibre to retain its material properties to allow use in critical structures, whilst complying with the needs of the regulatory authorities. This will provide a demanding market with a good supply of reliable, high-performance materials.

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