MW-Based Selective Molecular Recycling of Wind Turbine Blades and Other Fibre Reinforced Polymer Composites

Fibre reinforced polymer composites (FRPC) are materials defined by the combination of a polymer matrix and a reinforcing agent, mainly fibres. Due to their high strength-to-weight ratio and long durability, FRPCs - in particular carbon fibre reinforced polymers (CFRP) and glass fibre reinforced polymers (GFRP) - are widely used in the wind energy, aircraft-and automobile sectors. Waste cumulating from production (scrap or defective parts) and end-of-life parts, represents a major loss of resources and energy. A transition towards a more circular model would enable the retention of the maximum value of resources during their useful life, minimise waste production and resource use, and loop materials back into production.

While FRPC offers many advantages over homogenous materials, the same inherent properties that make composite materials so attractive make them complicated and costly to recycle.?This?is one of the reasons why there is no global scale polymer matrix composite waste recycling system. FRPC waste recycling is done in only a few recycling plants. FRPC plastic waste is mostly landfilled even though polymer matrix composites manufacturing is an expensive and energy intensive process. Potential recycling solutions would not only reduce the landfilled waste amount but also decrease the need for new FRPC manufacturing.

Appropriate waste management and lessening the need for virgin resources, require to systematically allow the re-use of the materials for the same or similar purposes: allowing polymer matrices to revert to monomers and avoiding fibre damage during the process (direct structural recycling approach).?Clean and intact fibres need to be obtained with comparable modulus on tensile properties to virgin glas fibre?(vGF) and virgin carbon fibre (vCF). For cost-effectiveness, avoidance of pre-processing, high energy-efficiency and the ability to produce a spectrum of different fractions of quality oils are critical.

Currently existing approaches - mechanical, thermal and chemical-based recycling approaches - have difficulties to meet all of the criteria above for effective, efficient and scalable recycling solutions.?

Current Recycling Approaches

Mechanical recycling is a technique used to reduce the size of scrap composites into smaller pieces (recyclates).?This is probably one of the simplest and most straight-forward methods to recycle composite.??However,?mechanical recycling requires the?careful collection, sorting and cleaning of the composite waste, and also quality control of the resultant filler recyclate.??Mechanical recycling can?impair?the physical integrity of the materials leading to fibre property loss. The effective reusing of recyclates is based on particle size. Re-incorporation with low-value applications like fillers or reinforcements is economically challenging, as cheaper virgin fillers such as calcium carbonate or silica are available.?Based on the insufficiencies, the role of?mechanical recycling techniques is getting limited to pre-recycling for other techniques such as thermal and chemical recycling.?

In chemical recycling the polymeric matrix is disintegrated using solvents (solvolysis) or water (hydrolysis).?Traditional chemical recycling using strong acids or solvents at various conditions has severe environmental impact. Fibres recovered, using chemical recycling,?can retain?long fibres with good mechanical properties.?Even though the process seems to be capable of recycling high quality and clean fibres with a crack-free surface and low energy usage, the disposal of such strong solvents is challenging.?

In thermal recycling, heat is used to break down the composite. The insignificant volatile materials are burnt, leaving the valuable fibres behind. Usually, the process temperature depends on the type of resin utilised in the scrap composite. Improper temperature control and consistency can either leave char on the fibre surface (undercooked) or result in reducing the diameter of the recovered fibres (overcooked). While the basic principle for decomposing composites using heat remains the same, the results are different for each process.

Complete fibre recovery using thermal recycling processes like fluidised-bed process (FBP) can result in CF recovery with negligible surface damage.?Commercial-scale FBP is capable of recycling clean and high-quality fibres with a fraction of the energy consumption needed to manufacture virgin fiber. Also, the process has a low environmental impact and a decent commercial-scale profit margin. However, the fibres are fluffy and discontinuous, limiting its use.

Since polymeric compounds have certain calorific values, electricity can be produced by converting the waste composite into heat. However, a major drawback in the combustion (incineration) process is the ash by-product, which can only be landfilled as inert waste and represents the lowest level of recycling.?

Another possibility is to pyrolyse the polymer matrix, whereby the fibre can be recovered. Pyrolysis is effective for the recycling of both CF and GF. Pyrolysis is a process in which an organic material is heated in an inert environment and the material is decomposed into low molecular weight substances.?The decomposing matrix produces oil and gas, along with the fibres and fillers (solid products). Conventional pyrolysis suffers from certain limitations, with the possibility of carbonaceous residues on the fibre surface considered the most challenging of all, impacting the mechanical properties of the recovered fibres. The char formation could be reduced with methods such as chemical treatment and post-heating of the fibres. Carbon dioxide (CO2) and water vapours could be used to remove the carbonacous residues formation.?

Microwave Depolymerisation

An alternative to conventional pyrolysis is microwave depolymerisation. This is a relatively new method.?Direct heat transfer, and the utilisation of microwave irradiation?are highly effective reverting polymer matrices to monomers, oligomers, polymers and hydrocarbon products and avoiding fibre damage during the process.?Heating by microwave radiation has the advantage that the bulk of the material is accurately and consistently heated throughout (volumetric heating). Polymers usually have low thermal conductivity. Thus, by using microwaves, the materials are not heated by convection and relatively moderate temperatures can be used, which is an advantage as the fibre will not degrade thermally. Sophisticated microwave depolymerisation is capable of recycling both CFRP and GFRP with fibres retaining very high mechanical properties – with either a clear fibre surface or intact coating.

Advanced microwave depolymerisation systems with high microwave absorption rates, can achieve high continuous throughput, as the reactor heats up rapidly, the polymers de-polymerise in minutes.?Due to the direct irradiation, the size of the feedstock doesn’t matter. Pending on the reactor size, large fractions of materials can be processed.?Less sophisticated microwave systems require shredding upfront or movement of materials in the reactor due to low microwave absorption rates.

The ability to sequentially depolymerise the polymer matrix at optimal temperatures and residence times allow for the gasification and subsequent distillation of different grades of quality hydrocarbon product, monomers, oligomers or polymers. The ability to recover a variety of different molecules in addition to gas and fibres is critical for the profitability and scalability of FRPC recycling. The fast extraction of gases and short volatile residence times do not only enable the production of high quality products, it is minimising the carbonaceous residues which impact the fibre performance negatively.

The other advantage of this agility is, that the ability to sequentially depolymerise the polymer matrix at optimal temperatures and residence times allows for the de-polymerisation of all polymers, mixed polymers and composites. Given the complex composition of FRCPs and the differences between products and changes over time, this is a necessary capability for profitable and scalable solutions.

The small footprint and continuous process allows for containerised solutions. Unlike conventional pyrolysis plants, advantage of containerising allows for simple up- and down-scale of capacity at different locations for most profitable solution and avoiding transportation of scrap FRCP. This is particular important for wind turbine blades, aircraft- and automotive sector.?

Sophisticated agile, modular, mobile and selective microwave depolymerisation solutions such as MWS allow to take the next leap in FRCP recycling towards a circular economy and sustainable future.?