Recyclability of Composites in Commercial Aviation: Industrial Specificities, Readiness and Challenges

Abstract

The integration of composite materials into commercial aviation has transformed the industry by providing superior performance benefits, including enhanced fuel efficiency, reduced emissions, and improved structural integrity. With a significant shift towards aircraft featuring high contents of composite materials, the focus has also turned to the challenges associated with the end-of-life management of these materials. Unlike metals, composites are notoriously difficult to recycle due to the strong bonding between fibres and resin, creating significant environmental and economic challenges. The methodology employed—consisting of an extensive literature review that prioritises a holistic approach—aims to provide an overview of the status of composite recyclability in aviation. With this, the report investigates the durability of composites under operational conditions, the associated environmental factors, and their impact on the recycling potential. The dismantling processes for decommissioned aircraft are analysed to identify strategies that preserve material integrity for effective recycling. Established recycling methods are critically evaluated alongside innovative approaches. The study highlights the limitations of current techniques in terms of costs, energy consumption, and material degradation while exploring emerging technologies that aim to overcome these barriers. It is concluded that currently available techniques do not possess the industrial maturity required to handle the amount of composite materials being employed in aviation. Moreover, there is a clear discontinuity between the developments in the usage of composites and their end-of-life recycling, which can cause serious environmental and economic challenges in future years. By combining information regarding composite usage and aircraft retirements, assessing the environmental and economic implications of composite recycling as well as available techniques, and proposing pathways for improvement, this research underscores the importance of adopting sustainable practices in aviation. The findings aim to contribute to the development of a circular economy within the aerospace sector, ensuring the long-term viability and environmental responsibility of future composite-intensive aircraft designs. This is performed by calling for a multi-stakeholder strategy to drive recycling readiness and facilitate the evolution towards a circular economy in aviation, leading to more sustainable design, production, and dismantlement of aircraft in the future.

1. Introduction

In recent decades, the aviation industry has undergone a profound transformation, particularly in the materials used in aircraft construction. Historically, metals such as aluminium have dominated the manufacturing of aircraft structures due to their strength, lightness, and durability [1]. However, the need for greater fuel efficiency, lower emissions, and improved performance has driven the industry towards composite materials [2]. These materials, typically made of a polymer matrix reinforced with high-strength components like carbon or glass fibres, offer a superior strength-to-weight ratio and increased resistance to corrosion and fatigue compared to traditional metals.

While composites have brought significant advantages in terms of fuel efficiency and operational cost savings, they also present unique environmental challenges, particularly regarding end-of-life recycling. Unlike metals, which can be easily melted and reused, composite materials are difficult to recycle due to their complex chemical and structural composition. This is currently a critical issue in aviation: in about twenty years, a growing global fleet of aircraft which are built with a high portion of composites—the Boeing 787 and the Airbus A350 feature more than 50% composite materials in their structural composition (

Table 1. Composite usage in commercial aircraft [1,3].

Aircraft Model	Composites Content (% Weight)	Main Composite Structures
Airbus A380	25	GLARE in front fairing, upper fuselage shells, crown and side panels and the upper section of the forward and aft upper fuselage; carbon and glass fibre reinforced plastics in wings, fuselage sections, tail surfaces, and doors; composite honeycomb panels in the belly fairing
Airbus A350	53	Carbon composite wing, fuselage, skin, frame, keel beam and rear fuselage; complete horizontal and vertical tail plane
Boeing 787	50	All composite fuselage and wing box, engine fan blades and casing

) [3]—will be approaching retirement, meaning that sustainable disposal practices will become increasingly necessary.

Within composite materials, Carbon Fibre Reinforced Polymers (CFRP) have become particularly relevant. To quantify the increase in the usage of CFRP globally and across several industries, Figure 1 presents the evolution of global CFRP demand in recent years, as well as the estimations for the foreseeable future. Reference [4], from which this trend was derived, was published in 2024; the reduction in CFRP demand observed in 2023—highlighted in the plot—is analysed: a decrease of nearly 15% was observed relatively to the previous year, marking the first year since 1995 where a decrease in global CFRP demand was observed. It is, however, proposed, based on extensive industrial records and reports, that this was caused by an accumulation of inventory during the pandemic, which led to a delayed impact on demand in 2023, as the world progressively returned to normality. In fact, this demand is expected to continue growing in the upcoming years. It is also important to note that, by 2023, the aerospace industry alone represented nearly 22% of the CFRP demand (in weight), and generated a business volume of nearly 50%. In fact, the growing high-value applications found for CFRP in the aerospace industry are a powerful economic driver and are some of the main reasons for the presented growth predictions.

Focusing on the aircraft industry, it is important to present the metrics that support the urgent need to develop more recyclable materials and implement more effective recycling processes. With the advent of large aircraft, as has been observed for the past five decades, the number of circulating aircraft has increased significantly and, concomitantly, so has the number of vehicle retirements. In particular, in recent years, passenger flights have become much more accessible, leading to a global and unprecedented fleet growth. A long-term consequence of this growth is the overwhelming number of aircraft that will require retirement procedures. Figure 2 presents the evolution of the number of yearly retirements between 1980 and 2017. In general, the rate grew during these decades, with economic and business effects having an influence in recent years. However, it is expected that the rate will continue to grow. Given the current fleet expansion, over 20,000 aircraft are expected to be retired within the next 20 years [5].

Moreover, it is also relevant to address the expected retirement age of the most common aircraft, which is summarised in Figure 3. It is observable that, on average, a passenger aircraft is retired after 25 years of service. This provides us with a quantification of the time available for the development of new recycling techniques required for the decommissioning of the currently produced aircraft, which tend to feature significantly growing contents of CFRP materials. Only by promoting such developments will it be possible to mitigate the environmental impact of composite materials and continue to push the sustainability efforts in the aerospace industry.

The environmental impact of composite materials in aviation stems from several factors. First, composites are not biodegradable, meaning they persist in the environment if not properly managed. Secondly, current recycling methods for composites are either underdeveloped or inefficient, often resulting in down-cycling, where the recycled material has inferior properties compared to the original one. Furthermore, the energy-intensive processes required for the separation and recovery of fibres, particularly carbon fibres, from the polymer matrix add to the environmental footprint of these materials. In an era where the aviation industry faces increasing scrutiny over its carbon emissions and waste production, the inability to efficiently recycle composites poses a significant challenge [6].

The reasons for recycling composites are both environmental and economic. Environmentally, the recycling of composites can reduce the waste sent to landfills, minimise the depletion of raw materials (such as carbon fibres), and decrease the overall carbon footprint of the aviation sector. Economically, recovering valuable fibres from retired aircraft can lead to cost savings in manufacturing new components, as recycled fibres can be reused in secondary applications, such as automotive or consumer goods. Moreover, regulatory pressures, such as those related to the European Union Circular Economy Action Plan (EUCEAP) and the growing societal demand for sustainable practices further emphasise the need for advanced recycling solutions in the aviation industry.

This work aims to explore the durability and recyclability of composite materials used in commercial aviation, focusing on the challenges associated with the dismantling and recycling of these materials. It will examine the factors affecting the long-term durability of composites, such as environmental exposure and mechanical fatigue, and how these factors complicate end-of-life processes. Additionally, the research will address current recycling technologies, such as mechanical, thermal, and chemical recycling methods, and their limitations in recovering high-quality materials, particularly fibres. Emerging technologies and innovations in the field will also be discussed, with an emphasis on their potential to transform the sustainability of the aviation sector [7,8].

The primary objective of this study is, therefore, to evaluate the environmental impact of composite materials used in commercial aviation, particularly in relation to the challenges of waste generation and recycling. A key focus will be on identifying the technical obstacles involved in dismantling aircraft made with composite structures and understanding the specific difficulties these materials present during recycling. Furthermore, the study aims to explore current and emerging recycling technologies, assessing their effectiveness in recovering usable materials while minimising environmental harm. This research will also seek to highlight how improved recycling processes can contribute to greater sustainability and resource efficiency within the aviation industry, leading to a greener and more sustainable sector. Ultimately, based on the conducted study, the authors will present the outline of a possible approach to mitigate the challenges identified.

2. Methodology

This report presents the culmination of a structured literature review, aiming to investigate the recyclability of composite materials in aviation, addressing topics such as the durability of composites under operational conditions, the associated environmental factors, and their impact on the recycling potential. This study focuses on the identification of the most relevant publications addressing the durability of composite materials, end-of-life strategies, and innovative recycling technologies, contributing to the acknowledgement of the present situation regarding the viability of recycling composite materials in the aeronautical sector. Several topics are revealed to be of increasing concern in this industry, particularly the projections regarding the volume of composite-intensive aircraft that will be dismantled in the upcoming years, the definition of industrially viable processes, and the modest developments in innovative recycling techniques. With this, to produce the present review, the authors idealised three main research questions:

• What operational, environmental and economic factors need to be considered when recycling composite materials?
• What is the state-of-the-art regarding composite recycling techniques?
• What are the specificities of these topics in the aviation sector?

The literature search was conducted across major scientific databases, including Scopus, Web of Science, and ScienceDirect, and other renowned literary sources available in general search tools such as Google Scholar. An initial filtration was performed based on appropriate keywords pertinent to the scope of the review, the recency of the publications, and the relevance of the article title and abstract. Then, the selected articles were analysed in terms of their full content to assess their suitability for the purpose of the review. After applying all these criteria, the final articles were selected for a detailed analysis, forming the core of the discussion in this paper, offering insights into current practices, limitations, and future directions for sustainable composite material management in aviation.

Finally, the overall analysis methodology is intended to produce an all-encompassing document, providing a complete overview of the recyclability of composites in the aviation sector.

3. Composite Usage in Commercial Aircraft

The relevance of the present article is based on the significant rise that composite materials have seen in terms of their usage in aircraft construction in the past few decades. Therefore, it is relevant, as an introductory step, to analyse the reasons that motivate such usage, as well as some aircraft that can be seen as textbook examples of innovation as far as composite materials are concerned.

To better understand the reasons that supported the introduction (and keep supporting the development) of composite materials in the aeronautical industry, it is important to first understand the utilised criteria when selecting a material for an aircraft. The material choice for aircraft is, as in every industry, strongly (and mostly) connected to structural aspects of the vehicle.

The components of classical aircraft vehicles are often subjected, during their operation, to important loads that must be sustained in order to ensure the correct functioning of the system as a whole. For example, the wings of an airplane are responsible for sustaining the lift needed for the plane to fly, comprising several structural components that provide them with the properties required for this task: the ribs give shape to the wing section and serve as support for the skin, being responsible for transferring the loads from the skin and stringers to the spars; ribs can also serve as attachment points for the control surfaces, flaps, landing gear and engines [9], withstanding torsion moments (the moments resulting from aerodynamic forces) and shear stresses (due to the vertical and horizontal resultant of forces) [10]. Ribs and other elements act with the ultimate goal of increasing the structural integrity of the wing, with their design being mostly governed by static strength and elastic modulus [11]. In the specific case of ribs, these mostly operate under compression, so the drivers for their design must also consider compression resistance properties. However, once aircraft operation is a highly cyclic activity and often exposes materials to harsh environments, properties related to cyclic loading (fatigue) and to corrosion are also to be considered when projecting these components. As previously mentioned, ribs mainly suffer from torsion due to the aerodynamic moment, but they are also subjected to effects from bending due to the lift distribution, which is transmitted from the spars; they must also withstand the pressure exerted by the fuel stored in the wings, which is the case for most commercial aircraft. For all these reasons, these are highly stressed components, with the specific stiffness and strength being determining properties in the material choice associated with these elements [12], especially if a high strength-to-weight ratio is demanded, as for large commercial aircraft.

Along with the previously reviewed specific example of wing ribs, which provides a very pertinent overview of the structural phenomena and requirements that influence material choices in aircraft, another important aspect to consider is the requirements of the certifying authorities regarding the material behaviour. In particular, buckling and fatigue phenomena are points of concern, making high yield strength and high stiffness fundamental requirements for these components.

Taking this information into account, the material used in aircraft structural components must feature the following in general:

• High stiffness;
• High yield and compressive strengths;
• Low density, not to invalidate the design due to excessive weight;
• Resistance to fatigue and corrosion, to withstand cyclic loadings and demanding environmental conditions;
• Resistance to crack initiation and propagation, to allow the safe delay of maintenance interventions;
• Ductility, to reduce the residual stresses resulting from assembly (if applicable) [13].

For these reasons, aluminium alloys—such as the 2000 (copper alloyed) and 7000 (zinc alloyed) series—are the most used materials for this type of application [14], being known mostly for their high specific strength and low density. For example, the Al2024-T3 and Al7075-T651 are two alloys that appear quite often in commercial aircraft. This is also motivated by factors other than the ones analysed previously: the economic and industrial viability—such as manufacturability—of the materials also needs to be considered when selecting alloys. Therefore, alloys that are economically viable—allowing for a reduction in the aircraft cost—as well as easy to process and well-known to the industry, due to their long history of usage in these applications, are more likely to make it as the final options for aircraft materials.

This was, initially, an obstacle to the introduction of composite materials in the production of civil aircraft: firstly, the production technologies that allowed to obtain the intended properties were very costly, making these materials economically obsolete; secondly, when compared with more common materials—such as metallic ones, which had a very well-known behaviour under various circumstances—composite materials were a somewhat grey area in a lot of aspects regarding material behaviour, making them a possible safety liability or demanding expensive testing efforts; finally, the lack of a well-known history of usage raised concerns—both for the manufacturers and the certification authorities—about their general practical implementation, making more conventional material choices (such as aluminium alloys) preferable.

When composite materials became commercially viable, after relevant developments regarding their production and material behaviour, the industry was quick to assume that they would rapidly become dominant in terms of aircraft materials. Although this growth has not been as pronounced as initially thought, the history of recent aircraft developments still shows a clear upward trend in terms of the usage of composite materials. In fact, the combination of suitable mechanical properties and a characteristic low density makes composite materials a very fitting choice for the aeronautic industry, as they allow for obtaining the necessary safety characteristics, while keeping the structural weight below thresholds that would severely compromise aircraft efficiency. Furthermore, this class of materials is highly flexible in terms of properties and configurations, allowing them to be tailored to each specific application, based on the required properties or shape.

In fact, composite materials—particularly CFRP—are increasingly integral to the aircraft industry due to their exceptional properties, including high strength-to-weight ratio and corrosion resistance. Beyond weight savings, which enhance fuel efficiency and reduce emissions, these materials also offer reduced maintenance as they have higher corrosion resistance, are durable and less prone to cracks and offer more design flexibility [15]. CFRP also offer enhanced passenger comfort as they exhibit excellent damping properties, reducing vibrations and noise, and provide better insulation [16]. Among the most advanced commercially available composite materials are multi-directionally reinforced laminates of carbon fibres within a polymer matrix, where plies of a CFRP are stacked at different angles, obeying a previously designed and optimised stacking sequence.

With several goals being established by various entities regarding efficiency, safety, and sustainability in aviation, the research regarding composite materials has been highly encouraged by European and American institutions alike [17,18]. This has lead to the production, by the civilian aircraft industry, of passenger aircraft with very high composite contents, of which the Airbus A350 XWB airliner and the Boeing 787 Dreamliner are examples, both featuring over 80% (in volume) of composite materials in their composition (note that comparing the material percentages in volume is also an adequate way to analyse composite materials: due to their characteristic low density, comparing mass percentage would inevitably put those materials at an apparent disadvantage relatively to metallic materials, for example). For that reason, such aircraft can be seen as true engineering achievements, and will be taken here as a reference in terms of composite employment in the civil aerospace industry [19].

3.1. Airbus A350 XWB

The Airbus A350 XWB designation refers to an aircraft family of long-range, mid-size, wide-body, twin-engine jet airliners that can seat 250 to 350 passengers in typical seat layout configurations. To provide a reference in terms of the recency of its design, the Airbus A350 XWB had its first commercial flight in January 2015 [19].

Its structure features several advanced materials, ranging from composites in the fuselage, wings, and horizontal and vertical tails to aluminium-lithium alloys in floor beams, frames, ribs, and landing gear bays and to titanium alloys in the main landing gear supports, engine pylons, and certain attachments. On top of its fuselage featuring composite panels, it is relevant to mention that these are mounted on composite fuselage frames as well.

The Airbus A350 XWB design employs composite wings with blended tip winglets, thus diverging significantly from more traditional winglet designs. Additionally, the wings have an upward curvature over the final span-wise section, which adds to the complexity of the wing configuration. This configuration—which requires various camber profiles along the span as well as two different curvatures along the wing—can only be achieved due to the usage of composite materials for the wings, which can be adequately tailored to the wanted configuration. In fact, this type of wing is never observed in aircraft with metallic wings, because it cannot be efficiently achieved with such materials [19].

3.2. Boeing 787 Dreamliner

Similarly, the Boeing 787 Dreamliner is a family of long-range, mid-size, wide-body, twin-engine jet airliners that can seat 242 to 335 passengers in typical configurations. This allows us to conclude that the aircraft models discussed here belong to the same class. However, this aircraft established itself as the first major commercial airliner to use composites as the main materials in its frame, currently being Boeing’s most fuel-efficient airliner, associated with a remarkably low structural weight for its class [19]. The Boeing 787 Dreamliner had its first commercial flight in October 2011, making it a model designed prior to the previously seen Airbus A350 XWB.

In terms of composition, the Boeing 787 features an astonishing 80% volume content of composite materials, meaning that each Boeing 787 aircraft contains around 32,000 kg of CFRP composites (which translates to approximately 23,000 kg of carbon fibre). These are employed in components such as the fuselage, wings, tail, doors, and interior. A particularity of the production process of the Boeing 787 is that its fuselage sections are laid up on rotating mandrels, with Automated Fibre Placement (AFP) and Automated Tape Laying (ATL) being used to deposit layers of carbon-fibre epoxy resin prepreg to previously contoured surfaces. During this process, the reinforcement fibres are oriented in specific directions to deliver maximum strength along desired load paths. Then, the fuselage sections are cured in autoclaves, with the resulting monocoque shell having internal longitudinal stiffeners already embedded. This process allows one to produce a structure that requires significantly fewer fasteners than conventional metallic built-up airframes, where components were often added on top of each other, or required some type of fastening element to hold them together. Similar composite manufacturing techniques are applied to the wings, which, once again, show the tailoring that composite materials enable in terms of the production of aircraft components.

In a similar fashion to what was seen for the Airbus A350 XWB, the Boeing 787 features wings with some differentiating characteristics: its composite wings have raked wing-tips, in which the tip of the wing has a higher degree of sweep (the inclination of the wing towards the front or the back of the aircraft) than the rest of the apparatus—which improves fuel efficiency and climb performance while also reducing the required take-off length. Once again, the possibility of applying various camber shapes along the wingspan as well as a double-curvature configuration is enabled only by composite wings and cannot be adequately achieved in wings composed of metallic materials [19].

4. Durability and Environmental Impact in Composites

Now that the main reasons behind the rising usage of composite materials in aircraft have been discussed, it is important to understand their behaviour during operation, which is strongly connected to durability. Durability is considered one of the most important and complex quantifications regarding materials, and it is associated with highly demanding and complex environments, being particularly relevant in the automotive and aeronautical sectors. It is defined as the capacity of a certain material to withstand stress, lasting long periods of working time without sustaining a significant degradation of its mechanical integrity or sustaining any damage that could compromise the minimum capacity requirements for its application [20]. As previously mentioned, composite materials are gaining significant popularity in aeronautical applications, particularly due to their overall optimal mechanical properties under competitive strength-to-weight ratio demands. They are becoming more utilised throughout aircraft components with various finalities [21].

However, considering the challenging world of commercial aviation, particularly the high number of predicted flights for most types of aircraft, there is a need to optimise the maintenance procedures and durability analysis for all the materials involved to obtain a more profitable operation, while respecting high-security standards. As expected, the major threats associated with material longevity, especially in the aeronautical sector, are fatigue, corrosion, possible impacts, thermal variations, and UV radiation, which could potentially contribute to a lower lifetime and premature need for replacement. Being a regular option for modern aircraft components (Figure 4), the high demand for CFRPs is partially due to the fact that their great properties provide high resistance to the majority of the previously mentioned factors that influence the durability of the structure. However, their usage comes with some disadvantages that are worth the attention and analysis.

In terms of fatigue, composite materials are known to have higher resistance than the majority of metals and exhibit competitive behaviour for large numbers of cycles; however, they are prone to delamination—a phenomenon that consists of the separation of overlaying composite layers [22]. This is more likely to develop in areas that are exposed to cyclical loading or in higher-stress locations (namely rivets or joints) or due to errors associated with the alignment between the different composite layers, leading to separation with subsequent propagation, which will eventually compromise the mechanical properties of the composite and influence the structural integrity to failure [22,23]. In those cases, the principal methodology to overcome this issue is related to an active inspection procedure—ultrasound and X-ray, for example, or possibly the implementation of new adhesive systems and fatigue-resistant resins as well as an optimisation process of composite layup design.

Furthermore, factors like exposure to UV radiation could also accelerate the degradation of composite materials and lower their durability: long-period incidence of ultraviolet radiation can cause several phenomena in composite materials, especially in the polymer matrix, due to its major vulnerability, and can compromise its mechanical properties, since the matrix binds and transfers all types of loads to the reinforcing fibres [24,25]. The exposure during large periods and absorption of this type of radiation may contribute to the development of photochemical reactions throughout the composite, initiation of bonding breakage, decrease in performance, and development of microcracking or delamination. Those threats can be amplified by external environmental conditions in conjunction with UV radiation, namely the presence of water, elevated temperatures, and thermal variations [26], to which commercial aircraft are inevitably exposed. To enhance durability and material resistance for these phenomena, some mitigation strategies are utilised, which range from the application of UV radiation-resistant coatings in the external composite surfaces to the development of composites featuring advanced epoxy resins with more qualified thermal stability [24].

Additionally, another possible threat to composite material durability is associated with galvanic corrosion. Even though these materials are characterised by a high resistance to corrosion effects, they are usually assembled with metallic fasteners in aeronautical components, and those connections may lead to some undesirable phenomena. In those cases, if there is evidence of condensation of moisture or electrolyte over the contact area between the two materials, there may be ion conduction, and a galvanic couple will result. The composite will operate as a cathode, facilitating the dissolved oxygen reduction, while the metallic part will be established as an anode and will suffer oxidation [27]. This does not necessarily result in composite degradation, but it accelerates the oxidation of the assembly, which will hasten the necessity of replacement. This issue can be overcome by utilising certain types of coatings to reduce the direct contact between materials and delay the alloy corrosion. Other coatings can also be applied on the composite surface to eliminate possible reactions [28].

Given all the major threats to the durability decrease of polymers, it is important to correlate those phenomena with applications in the aeronautical sector, especially in commercial aviation and the corresponding expected flight conditions. In those scenarios, it is predicted that a high number of long-period flights will be performed to satisfy the demands of airlines while minimising maintenance costs, but withstanding wide-ranging variations of environmental conditions due to the diversity of itineraries.

A typical commercial aircraft is expected to fly at an altitude of around 11.5 km (37,500 feet) to maximise its efficiency with lower aerodynamic drag and avoid possible turbulence expected at lower altitudes. However, those cruising altitude levels are associated with low-temperature ranges at around −50 °C, which will influence the resistance of the materials, especially considering an expected high thermal variation between take-off, landing, and cruising flight attitudes.

Overall, factors like humidity and UV radiation will be regularly present in flight conditions, with a necessity for regular inspection and maintenance to detect and mitigate possible residual stresses, delamination, or even crack development. From a structural point of view, aircraft composite materials are expected to be subjected to a high number of cyclic loadings in several departure and landing procedures, leading to various concerns associated with fatigue and mechanical resistance that need to be properly considered with safety protocols.

5. Dismantling of Commercial Aircraft

Following the overview regarding the composite materials used in commercial aircraft and a series of processes that they experience throughout their operational life, it is important to understand, before proceeding to considerations regarding the recycling phase, how these materials can be dismantled from the decommissioned vehicles without compromising their suitability for recycling. Once this is the focus of the present study, all considerations will pertain to the specific case of commercial aircraft. This is relevant because dismantling procedures and disassembly techniques vary according to the type and usage of each aircraft, as do the conditions in which the materials reach the end-of-life.

In what concerns commercial aircraft, these usually have an operational life of 20 to 30 years, before they are deemed unsuitable to continue operating. Before proceeding to the dismantlement of the vehicle, analyses and deliberation must be performed, in order to decide the most advantageous course of action: it is common for civil aircraft to be converted into cargo aircraft or to continue their operation in other industries (military applications, humanitarian aid, firefighting), once these might have lower utilisation rates and less stringent safety requirements, allowing the vehicle to operate, even if under different conditions, for an expanded period [5].

Alternatively to the repurposing, aircraft can be simply recycled, with their parts being removed and treated in a way that enables the usage of those materials in new applications. This contemplates the usage of the extracted parts both inside the aviation market and in other industries. Evidently, the specificities of the disassembly process of an aircraft depend on the destination of the recycled parts, and different disassembly techniques should be chosen according to the intended outcome of the recycling process. These techniques include processes such as system disassembly, coarse and precise cutting, application of electromagnetic Eddy currents, sink-floating separation and crushing. The disassembling sequence can also be optimally defined depending on the type of output that needs to be achieved. Tools used for the aircraft disassembly mainly include plasma torches, different types of angle grinders, high-pressure water guns, chain saws and hydraulic clamps [29].

In the following subsections, an overview of each modality regarding the repurposing of aircraft parts is presented in order to introduce the main considerations about procedures, techniques, and processed outputs.

5.1. Disassembly and Repurposing Inside the Aviation Industry

Even in the event that an aircraft is taken out of operation, that does not mean that all of its parts have lost their operational suitability. In fact, some components of decommissioned aircraft can be returned to the aviation material market for normal circulation, after being appropriately dismantled, thoroughly examined, repaired and subjected to a strict quality testing before being reinstalled in a new aircraft [5], once compliance with the requirements of the relevant authorities must be continuously ensured. These are often referred to as “second-hand aviation materials”, and are very attractive from an economic point of view, which has recently caused this market niche to grow significantly [29].

Components such as the engine, the oil pump, the battery pack, the manipulator, the shock absorber, the cockpit wind-shield, and the cabin row seats are among the most recyclable ones, with very few structural components—such as fuselage or wing skin panels, wing spars and ribs—being included in the modality of repurposing. This is to be expected: structural components are what dictates the expected life of an aircraft (depending on crack propagation and other degradation effects), so, when an aircraft is decommissioned, it is very likely that it is due to the fact that the structural components can no longer perform the desired functions with the required level of confidence, thus not being suitable for repurposing inside the aeronautical industry. Structural components are also often more difficult and costly to replace, which limits the usage of second-hand structural parts.

It is also relevant to mention that the dismantling process of the aircraft can be essentially divided into two phases [5]:

• A first phase, including processes up to the removal of parts for repurposing in other aircraft. This phase is still part of the aviation industrial domain and is thus subject to the related regulations. This means that, during this phase, the retired aircraft is still under a valid certification. This phase pertains to the considerations reviewed in this subsection.
• A second phase, which encompasses final dismantling and recycling. The decommissioned aircraft has therefore lost its certification, and aviation regulations are no longer applicable. This phase pertains to the considerations of the next subsection.

5.2. Dismantling and Recycling of Composite Materials

After the valuable and reusable components of the aircraft have been disassembled, prompting the loss of certification validity, it is directed for repurposing inside the aviation industry, and the vehicle (now classified as “waste”) enters the dismantling phase. This is the phase where, through adequate handling, potentially recyclable parts are dismantled. Most of the time, these will undergo recycling processes with the ultimate goal of enabling their employment in non-aerospace industries [5]. The specific set of procedures employed to dismantle the aircraft depends not only on the specific model and materials but also on the recycling intentions: the ultimate output that is desired (carbon fibre filling for civil construction applications, for example) dictates the processes to be used later for recycling, which, in turn, might require a certain type of input (carbon fibres must be well conserved and easily separable from matrix residues, for example). However, in the case of composite materials, these are frequently removed manually [30], after the previously described phases of removal of hazardous materials and liquids and disassembly of reusable components. A summary of the disassembly and dismantling phases for civil aircraft is presented in Figure 5.

Moreover, as mentioned in previous sections, the civil aircraft industry is currently seeking to produce increasingly lightweight aircraft, in order to increase their efficiency and reduce the environmental impacts of the industry, leading to a very significant increase in the usage of composite materials for structural applications, which enables a relevant weight reduction.

However, one of the disadvantages of the rather fast growth in the usage of these materials is that the end-of-life consequences of composites—i.e., what needs to be done after they are withdrawn from operation—were not adequately considered: these materials are very difficult to recycle. In fact, previously used treatment methods were banned internationally due to being highly damaging to the environment [29]. On the other hand, some recycling techniques provide process outputs that are not competitive from an economic point of view, rendering the recycling investment unviable. Furthermore, in the case of aircraft which feature high composite material contents—such as the previously analysed Airbus A350 XWB and Boeing 787—, the decommissioning will demand significant disposal and recycling of composite materials, so viable treatment methodologies are required. It is therefore imperative to establish clean, economically viable recycling procedures that enable the attainment of usable and useful materials, while ensuring sustainability, which was the very motivation for the usage of composite materials in the aircraft industry. The state-of-the-art of such techniques is reviewed in the following sections.

6. Composite Recycling Processes

This section is subdivided into two parts: firstly, a general overview of the various types of composite materials is presented as far as matrix and reinforcement are concerned (Figure 6); secondly, the main existing recycling technologies are reviewed and summarised in

Table 2. Composite recycling techniques and features [3].

Type of Matrix	Applicable Techniques	Technology Features	Technology Status
Thermoplastic	Remelting and remoulding	No separation of matrix from the fibre	Mainly studied for manufacturing or scrap processing
		Regrinding followed by compression or injection moulding/extrusion
		Output as pellets or flakes for moulding
		Fibre breakage leads to property degradation
	Chemical recycling	Dissolution of the matrix	Lacks study
		Fibre breakage leads to property degradation
	Thermal processing	Combustion or incineration for energy recovery (option for old scrap); leads to material loss	Lacks study
Thermoset	Mechanical recycling	Comminution 1–grinding–milling	Found in commercial operations: ERCOM (Germany) Phoenix Fibreglass (Canada)
		Products: fibres and fillers Degradation of fibre properties
	Thermal recycling	Combustion/incineration with energy recovery	Promising technology
		Fluidised-bed thermal process for fibre recovery	Hindered by the reduced market for recycled fibres
		Pyrolysis for fibre and matrix recovery
	Chemical recycling	Chemical dissolution of the matrix	Only laboratory studies
		Solvolysis (supercritical organic solvent) or hydrolysis (supercritical water)	Promising
		Product: high-quality fibres, potential recovery of resin
		Inflexibility of solvent and potential pollution
Metal	Remelting casting	Die-cast scrap: direct remelting casting	MMC is much more expensive than the alloys or reinforcements
		Foundry scrap: direct remelting with cleaning (dry Ar)	Aiming at reuse of MMC
		Contaminated scrap: remelting–fluxing–degassing–cleaning
		Highly contaminated scrap: metal recovery only—remelting and refining to separate reinforcement from Al alloy

¹ Reduction in solid material particle size by fracture via grinding, milling, or similar processes.

.

Thermoplastic matrix composites offer a relatively simpler recycling process compared to thermoset composites due to their reversible bonding nature. Table 2 highlights three primary recycling methods: remelting and remoulding, chemical recycling, and thermal processing. Each method has its advantages and limitations, which are detailed below.

Remelting and remoulding are the most straightforward recycling techniques for thermoplastic matrix composites. This method does not separate the matrix from the fibres, but rather involves regrinding the composite material into smaller particles, such as pellets or flakes. These particles are then used in compression or injection moulding, allowing the recycled material to be reshaped into new products. However, the process results in significant property degradation, particularly in fibre-reinforced composites, due to fibre breakage during grinding and remoulding. Additionally, the matrix undergoes thermal stress, which may compromise the mechanical properties. Currently, this method is more often studied for processing scrap rather than end-of-life composites, as it is better suited for clean, uncontaminated scraps.

Chemical recycling involves the dissolution of the thermoplastic matrix, which enables the recovery of embedded fibres. The process typically employs solvents to dissolve the polymer matrix, leaving the reinforcement fibres intact. Similarly to remelting, fibre breakage and property degradation can occur. However, the technique allows for a cleaner separation of fibres, which can improve the quality of recovered reinforcements when compared to mechanical methods. Despite its potential, chemical recycling has not been extensively studied and remains in the experimental development phase. Its application in industrial processes still requires a higher technology readiness level.

Thermal processing techniques, such as combustion or incineration, focus on energy recovery by burning the polymer matrix. This method is particularly useful for composites where fibre recovery is not the main objective. The combustion process generates energy at the cost of significant fibre damage or their complete loss, making it unsuitable for fibre reuse. As a result, thermal processing is often considered as an energy recovery option for old or contaminated scraps. Despite its potential for energy generation, this method has not been widely studied or published, reflecting its limited practical application.

Recycling of metal matrix composites (MMCs) is particularly challenging due to the complexity of separating the metallic matrix from the reinforcement materials. As Table 2 outlines, remelting casting is the main method for recycling MMCs, with various approaches depending on the level of scrap contamination.

Remelting casting is the primary recycling method for metal matrix composites. The process involves melting the composite material, often under controlled conditions, to recover the metallic components. The degree of complexity increases based on the type of scrap. For clean die-cast scraps, direct remelting casting is applied without significant pre-treatment, making the process relatively efficient. However, foundry scrap requires additional cleaning, such as dry argon cleaning, to remove surface contaminants and ensure better-quality recycling [32].

In the case of contaminated scrap, more intensive processes are required, including fluxing and degassing. Fluxing helps to remove impurities, while degassing reduces porosity in the molten metal, improving the quality of the recycled material. For highly contaminated scrap, metal recovery processes are necessary, where the material is refined to separate the reinforcement from the metallic matrix. This step is particularly relevant for aluminium alloy-based MMCs, where reinforcement separation is essential to achieve high-purity aluminium.

While remelting casting is effective for recovering metallic matrices, this process is often expensive due to the energy and additional treatments often required. Furthermore, separating reinforcements, such as ceramics, adds another layer of complexity. The high cost associated with MMC recycling makes it less economically viable compared to recycling traditional alloys or reinforcement materials. However, ongoing efforts are focused on reusing MMCs in applications where slightly degraded properties are acceptable, which can help reduce the need for costly processing.

In conclusion, recycling thermoplastic and metal matrix composites presents unique challenges and opportunities. For thermoplastics, methods like remelting and chemical recycling focus on fibre and matrix reuse but suffer from property degradation. Meanwhile, metal matrix composites rely on remelting processes, which are often energy intensive but enable recovery of valuable metallic matrices and, in some cases, reinforcement separation. Continued research and technological advancements are needed to improve the efficiency and cost-effectiveness of these recycling techniques.

Given the fact that the majority of composites used in commercial aviation are thermoset matrix composites—despite the recent increase in the usage of thermoplastic composites—this class will be explored here in-depth, with more technical aspects of the recycling processes related to this family of composites being reviewed. Figure 7 depicts, in a rather simplified form, the general procedure of recycling, which starts with an end-of-life (EOL) product and culminates in recyclates, possibly with the desired market value.

6.1. Mechanical Recycling

Mechanical recycling is the simplest and most commonly used method for recycling composites. It involves shredding or grinding the composite material into smaller particles. These particles are typically used as fillers in lower-grade products, such as concrete, or as a reinforcement in non-structural plastics.

The composite waste is first sorted and then crushed into small fragments using specialised grinders and shafts, as can be seen in Figure 8. The resulting material is a mixture of fibres and resin particles, which can be used as reinforcement in new composites or as filler in less demanding applications. It is relevant to highlight two aspects pertaining to this process typology:

• material loss: The length of fibres is significantly reduced during the grinding process, which negatively impacts the mechanical properties of the material. Carbon fibre lengths, originally tens of millimetres long, can be reduced to just a few millimetres (often under 10 mm).
• limitations for structural applications: Due to the reduction in fibre length, mechanically recycled composites have 50–70% lower mechanical strength compared to virgin composites. This makes them unsuitable for high-stress applications, like aerospace or automotive structural parts, but they are commonly used in the construction industry in applications such as concrete reinforcement.

The stages of the recycling processes are depicted in Figure 9, where from a virgin set of carbon fibres, recycling procedures such as reclamation and re-manufacturing provide a material that can be used for less structurally demanding applications [3,33].

Figure 8. Illustration of a multiple-shaft grinding machine: (a) front and top views, (b) interior view, (c) CFRPs before shredding, and (d) CFRPs after shredding. Reprinted from [34,35] with permission from Elsevier.

Figure 8. Illustration of a multiple-shaft grinding machine: (a) front and top views, (b) interior view, (c) CFRPs before shredding, and (d) CFRPs after shredding. Reprinted from [34,35] with permission from Elsevier.

Figure 9. Stages of the recycling process [36]. Reprinted under Creative Commons CC-BY license.

Figure 9. Stages of the recycling process [36]. Reprinted under Creative Commons CC-BY license.

Additionally, it must be noted that one of the main problems with carbon fibres (CFs) being mechanically recycled is the adhesion between the reclaimed fibres and the new polymeric matrix: there is poor interfacial adhesion (Figure 10).

6.2. Thermal Recycling

Thermal recycling is used to recover fibres by breaking down the polymer matrix through high-temperature processes. The two main methods are pyrolysis and fluidised bed processing [39].

6.2.1. Pyrolysis

The composite is heated in an oxygen-free environment at 400–700 °C, which decomposes the resin and leaves behind the reinforcing fibres. Carbon fibres can be effectively recovered, but they may experience some degradation in properties, namely a loss of 10–30% in tensile strength, depending on the process: different variants can be used depending on whether matrix removal or the preservation of tensile/compressive strength is to be prioritised [40,41]. In some cases, it is reported that the fibres can be separated from the resin without affecting the fibre surface, namely in the case of aerospace prepreg waste [42]. A scheme of a pyrolysis process is shown in Figure 11.

6.2.2. Fluidised Bed

It involves placing the composite waste in a bed of sand heated to 450–550 °C, with a stream of air breaking down the resin. This method produces cleaner fibres than pyrolysis, but there is still some fibre damage due to the mechanical friction [43]. A schematic representation is depicted in Figure 12.

6.2.3. Efficiency and Costs

Both processes are effective at recovering carbon and glass fibres, but they are energy intensive. For example, pyrolysis consumes 15–20 GJ per ton of composite material, making it 3–4 times more energy-demanding than common aluminium recycling processes. Additionally, resins cannot be recovered in these processes, which reduces overall recycling efficiency and output.

Figure 11. Pyrolysis process [44,45]. Reprinted under Creative Commons CC-BY license.

Figure 11. Pyrolysis process [44,45]. Reprinted under Creative Commons CC-BY license.

Figure 12. Fluidised bed process [46,47]. Reprinted under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International.

Figure 12. Fluidised bed process [46,47]. Reprinted under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International.

6.3. Chemical Recycling (Solvolysis)

Chemical recycling, or solvolysis, is a more advanced process typology that uses solvents to break down the resin at moderate temperatures, typically between 90 (Figure 13) and 400 °C (Figure 14). This process can recover both the fibres and, potentially, the resin, although with reduced properties.

Solvolysis uses solvents like supercritical water (a supercritical fluid is a substance at a temperature and pressure above its critical point, where it behaves both like a gas and a liquid), alcohols or acetic acid solutions to dissolve the resin, leaving the fibres intact. The fibres recovered through solvolysis often retain 80–95% of their original tensile strength. Solvolysis is one of the most promising methods for producing high-quality recycled fibres. However, it is expensive and difficult to apply at a larger scale, with processing costs often being 10–20% higher than mechanical recycling. Additionally, the chemical waste produced can present environmental challenges [48,49].

7. Problems and Difficulties in Recycling Composites

Now that the main processes available for the recycling of composite materials have been reviewed, it is possible to outline some limitations that ultimately impact the viability of the recycling of these materials. In each of the following sections, additional subsections are found, summarising each of the approached topics.

7.1. Separation Between Resins and Fibres

The main technical challenge in composite recycling lies in the limited separability of resins and fibres, particularly in thermoset composites (which account for more than 90% of composites used in aerospace [50]), where the matrix cannot be swiftly melted or returned to a more fluid state, such as in thermoplastics. In the following subsections, additional details pertaining to the factors that make the fibre-resin separation such a complicated process are presented.

7.1.1. Thermosets vs. Thermoplastics

Unlike thermoplastics, which can be remelted and reshaped, thermosets cannot be reprocessed through heating. In fact, these materials undergo a curing process, where the resin chemically bonds with the fibres, creating a structure that cannot be melted or remoulded [3]. This makes their recycling particularly challenging because breaking the bonds between the resin and fibres typically leads to fibre degradation.

7.1.2. Energy and Cost

The processes that can effectively separate fibres, such as pyrolysis or solvolysis, are highly energy intensive and costly. For example, a thermoplastic composite recycling process may cost around EUR 300–400 per ton of output, while thermoset composites can cost up to EUR 1500 per ton to recycle [3].

7.2. Fibre Degradation

Fibre degradation is a common issue in composite recycling, particularly in thermal processes like pyrolysis and fluidised bed methods, as reviewed previously. During pyrolysis, carbon fibres can lose up to 25% of their tensile strength due to oxidation and high temperatures, which are an important part of the recycling process [1,41]. Glass fibres, which are more common in older planes such as the Airbus A380, are even more susceptible to degradation, often losing up to 50% of their original strength [1]. This degradation means that the recycled fibres often have to be used in lower-value applications where mechanical performance is less critical, such as in non-load-bearing automotive parts or consumer goods. In fact, while carbon fibres can be effectively recovered, they may experience property degradation—especially tensile strength—with losses ranging from 10% to 30% depending on the specific process parameters [40,51]. Maintaining temperatures below certain thresholds and optimising the process for the specific application can significantly improve the outcome, with recent studies showing up to 90% retention of tensile strength [41]. This highlights the importance of the fact that the reclaiming process can be optimised for specific applications (both in terms of available inputs and desired outputs), sometimes conserving most of the fibre properties and thus preventing the recycled fibres from being relegated to low-value applications [41].

7.3. Economic Viability

Recycling composites, especially through thermal or chemical methods, is far more energy intensive and expensive than recycling traditional materials like aluminium or steel. For example, the energy required to recycle carbon fibre composites is estimated to be 10 times higher than that for aluminium recycling, largely due to the complex separation process and high temperatures required. This energy usage raises both environmental and economic concerns.

The cost of recycling composites ranges from EUR 500 to EUR 2000 per ton of output, depending on the method used. This makes it less economically attractive compared to simply producing new fibres, which is why large-scale adoption of recycling practices remains limited [52].

Furthermore, composite materials are highly heterogeneous, often consisting of different types of fibres, resins, and additives. This variation makes the recycling process more difficult. A composite used in aeronautic or aerospace applications may contain epoxy resins, carbon fibres, glass fibres, and various coatings or fire-retardant additives. Each of these materials requires different processing conditions, making it difficult to develop a universal recycling method. The heterogeneity of composites also means that they must be carefully sorted before recycling, which increases logistical costs [39]. Overall, these processes pose a financial challenge, and alternative paths need to be identified both in terms of techniques and usage of the obtained recyclates.

8. Quality of Recycled Composites

The main factors when considering recycled materials are structural quality, since it heavily impacts the performance of a component with respect to loading capacity, security tests, and maintenance frequency, as well as aesthetic quality in industries where it is considered critical.

8.1. Structural Quality

The mechanical properties of recycled composites are often inferior to those of virgin materials due to fibre shortening and degradation during recycling processes. For example, in recycled carbon fibres, the tensile strength can drop by as much as 20–40%, depending on the recycling process. This limits their use in high-performance applications, such as aerospace or wind turbines, but makes them suitable for non-load-bearing components in automotive or consumer goods. Nevertheless, with the improvements in recycling processes, it is expected that this tendency will change, and in the future, more advancements will enable their usage in structural components.

8.2. Aesthetic Quality

Recycled composites can also suffer from surface imperfections, such as roughness or discolouration. Thus, their appearance is heavily impacted, being particularly evident in mechanically recycled materials, where the short fibres and fragmented matrix can create a rough, inconsistent surface. These composites are often painted or coated to mask these imperfections, but this adds additional steps and costs to their application. As a result, limitations become clear regarding the utilisation of recycled composites in applications where appearance is critical, such as in consumer electronics or high-end automotive interiors, unless they undergo significant post-processing [39].

8.3. Inferences

Despite the challenges in recycling composites, current methods like mechanical, thermal, and chemical recycling have enabled the recovery of fibres and resins to varying degrees of success. Recycled composites are progressively finding applications in sectors like the automotive and construction industries, though the quality of the recycled material remains a significant limiting factor. Developing more cost-effective and energy-efficient recycling processes will be critical to enhancing the viability and broader adoption of composite recycling in the future.

9. Innovation and Emerging Recycling Methodologies

The surging demand for composite materials across diverse sectors requires the development of sophisticated recycling methods. Traditional recycling processes often fall short in terms of efficiency, affordability, and environmental friendliness. To overcome these obstacles, innovative recycling approaches are being explored to enhance the recyclability of composite materials. These emerging technologies prioritise maximising material recovery, minimising waste, and fostering sustainable practices, ultimately driving the creation of a circular economy for composites.

9.1. Emerging Recycling Methods

As the global demand for composite materials escalates, the need to innovate recycling methods and arrange better solutions for material disposal has rapidly become a priority and a topic of great concern. Those concerns are partially related to the lack of options and limitations associated with applying conventional recycling methods to high-complexity composite structures and adapting to the material diversity of composites, leading to issues like the waste of material and inefficiency of the utilised methods, compromising economic and environmental drivers. With this, the importance of studying new methods or adapting conventional options arises to enhance the recyclability of composite materials. After extensive analysis and comparison with the methods previously mentioned, methodologies like microwave-assisted pyrolysis, co-pyrolysis with catalysts, enzymatic depolymerisation, supercritical ${\mathrm{CO}}_2$ recycling, fluidised bed processing, and advanced catalytic solvolysis were chosen for further interpretation and characterisation. Other than increasing the available methods for composite material recycling, these new procedures also promote a wide range of solutions for a circular economy, paving the way for the transformation of end-of-life materials into new components in several industries. These processes are reviewed in the following.

Microwave-assisted pyrolysis. This method utilises microwave energy to promote the development of thermal degradation, contributing to the depolymerisation of complex composite material structures, with special efficiency for polymers like polystyrene. This methodology is based on an iteration of the conventional pyrolysis method, in which the major difference is associated with the fact that the heating process is achieved internally and not externally, resulting in a more uniform and higher temperature variation and lower processing time and energy consumption. Additionally, a reduced pressure-controlled environment could also contribute to the process efficiency, namely the production of valuable products and minimisation of the formation of char (an unwanted solid residual resulting from processing carbonaceous materials) or impurities [53].

Usually, the final products after the microwave-assisted pyrolysis procedures are gas and oils, which are characterised by high quality and low viscosity and can be utilised in several applications, like fuel or raw material for new components, after some type of refinement. Hence, it is possible to promote an efficient recycling process with the possibility of reutilising these types of materials for other industries. Nevertheless, some challenges can be encountered, such as the variation of recyclability depending on the type of composite material and the fact that this is still considered a financially highly demanding method [54].

Co-pyrolysis with catalysts. This process consists of the thermal degradation of both biomass and waste plastics, being a methodology in which the working temperatures are considerably high—around 400 to 600 °C—in an oxygen-free controlled environment, thus contributing to the process of breaking polymers into simpler hydrocarbons. This methodology is considered an innovative and emerging process due to the application of innovative catalysts, which changes the final bio-oil product into a high-quality product with several energy and chemical applications. With that, the chemical reactions involved will be more precise, favouring the formation of desired products while minimising waste and unwanted by-products, leading to a more efficient chemical process. Additionally, it also transforms non-recyclable plastics and biomass material into new products for further applications, promoting a circular economy contribution [55].

The role of catalysts in this procedure is to accelerate and facilitate chemical reactions, promoting the development of lighter hydrocarbons and, consequently, better final quality products with higher yields, as well as a decrease in undesirable by-products. Regarding the microwave thermal variation, the functionality is equivalent to the principle of the previous methodology analysed, in which the thermal decomposition will also enhance the quality of the products [55].

This emerging process promotes waste management, reuse of composite material for other industries and a reduction in ${\mathrm{CO}}_2$ emissions, being considered a more environmentally friendly procedure.

Enzymatic depolymerisation. This methodology represents another innovative process for composite recyclability, that comprises the utilisation of enzymes to depolymerise polymers into monomers under mild conditions (neutral pH or low temperatures), being particularly suitable for natural fibre composites and biodegradable polymers. The operation employs enzymes that catalyse hydrolysis with the intent to target ester bonds of the material, promoting the transformation of polymers into monomers of high quality, which can be reinserted into new products for several industries. In these cases, throughout the process, one of the main advantages is the preservation of both polymer matrix and natural fibres, which is ideal for recycling systems. Since this recycling process works under environmentally friendly conditions, it does not produce any noxious by-products, and simultaneously promotes the use of natural and bio-based composite fibres, being regarded as one of the most promising emerging techniques. Nevertheless, some challenges were encountered, namely the incompatibility for a certain range of composites and the vulnerability of enzymes and their functionality, compromising the industrial-scale implementation [56].

Supercritical ${\mathbf{CO}}_{\mathbf{2}}$ recycling. This procedure utilises carbon dioxide (${\mathrm{CO}}_2$) in a special state as an agent to penetrate the composite material, promotes the separation of the fibres and matrix and further transformation, representing an innovative recycling method. The ${\mathrm{CO}}_2$ state is known as a supercritical state—achieved under certain conditions, like temperatures above 31 °C and pressure above 74 bar, acting both as a gas and a liquid-, considered an efficient solvent to break down composites and produce high-quality material [57]. During the process, ${\mathrm{CO}}_2$ enters the composite material, dissolving contaminants and causing polymer chain separation, separating the polymer matrix and fibres. While the fibres remain intact throughout the process, the matrix will be dissolved, and the ${\mathrm{CO}}_2$ utilised can be depressurised and recycled for further processes. In this procedure, the energy consumption and level of emissions are considerably lower than other technologies, and there is a reduction in the need for utilisation of toxic solvents, making the process more environmentally friendly. Furthermore, the carbon dioxide reutilisation enables a continuous circular economy and a recycling system that develops high-quality end material with no harm to the environment [57]. However, in this type of process, the required equipment and operational costs are particularly high in comparison to other alternatives, due to the necessity for the ${\mathrm{CO}}_2$ preparation in a temperature and pressure-controlled environment. Additionally, the ${\mathrm{CO}}_2$ applicability is reserved for a limited range of composites, in terms of recyclability effectiveness [57].

9.2. Applications of Recycled Composites

In addition to all the information outlined earlier, composite material recycling methods have undergone several developments in the past few years due to the increase in demand and exploitation in various industries, and are thus becoming progressively more efficient. However, the complexity involved in recycling composites and maintaining their integrity without compromising their mechanical properties is still considered a challenge. With the progress in the maintenance of mechanical properties and composite reapplication within similar levels of structural demands, their reutilisation is reaching more industries, namely the same application as the original composite. Industries like automotive, civil construction, or wind energy are considered the major application range of this type of material [58].

Regarding the automotive sector, considering the latest models and updates associated with the interiors and the level of detail, the utilisation of recycled composite materials has become more popular. The major application is essentially for non-structural components, namely carbon and glass fibres for internal panels and supports, with some structural applications, such as reinforcement structures that do not require maximum mechanical integrity [59]. Figure 15 overviews the usual composite recycling procedure leading to the automotive industry.

The developments of civil engineering and construction focused on the utilisation of composite materials as concrete reinforcement, promoting the increase in traction resistance and a lower necessity of metallic components, enhancing several properties like corrosion resistance and increasing their overall strength [60]. This reutilisation process is a key factor, especially for constructions subject to harsh environmental conditions, namely bridges (Figure 16).

In terms of the wind energy industry, the composite material content of eolic turbines is rising, making the possibility of composite reutilisation a crucial economic factor. Even though the application of this type of material in wind turbine blades (Figure 17) is structurally demanding, the availability of recycled composites with competitive mechanical properties is rising, and the positive environmental impact associated is aligned with the growing sustainability requirements that promote this type of construction [62].

10. Paradigm Shifting and Recommended Actions

During the development of the present study, the authors were able to identify some critical points in terms of the recyclability of composite materials (particularly CFRP) in the aircraft industry. In future efforts to change the current paradigm, it is important to ensure a strong cooperation between entities such as the Aircraft Fleet Recycling Association (AFRA) and central organisations such as the International Civil Aviation Organization (ICAO) and the International Air Transport Association (IATA). Moreover, it is paramount that the industry is actively included in this process, and that open, collaborative and regulated efforts are developed. Only a joint push can take the aviation sector to new levels when it comes to material recycling and economic circularity. With this in mind, the authors present, in Figure 18, an overview of a possible approach to this process, carefully designed taking into account the extensive literature review underlying the present work.

11. Conclusions

The aeronautical sector demand for composite materials has increased exponentially, driven by the progressively higher percentage of composite materials in newer commercial aircraft and the development of more qualified and manipulated materials to optimise their application. However, the higher number of composite-made aircraft will eventually make it challenging to dismantle these vehicles at the end of their lifetime: the recyclability of composites in high volumes will present several difficulties, given the maturity of these types of processes at the current time.

While several techniques have been developed—or are currently in development—even addressing the optimisation for specific applications and the creation of tailored recyclate products—current data regarding composite usage in aviation and expected aircraft retirements suggest that the advancements made so far are modest when compared to the volume of composite materials being employed in this sector. Therefore, a new paradigm is necessary in terms of the usage of composite materials, the development of new recycling techniques, and the employment of recyclates; discontinuities between these phases need to be tackled, namely by developing new materials while considering their end-of-life processes and industrial reintegration. Moreover, a better interface between the industry and certifying authorities must be promoted, enabling the simultaneous fulfilment of sustainability and economic drivers.

With this, future work should be focused on a more intensive optimisation of all the different types of processing technologies to enable rapid and efficient procedures. It is also necessary to compensate for the usual incompatibility and selectivity of the various methodologies for certain composites. Furthermore, to match sustainability standards, the carbon footprint needs to be considered throughout the development process to define efficient methods for a more circular economy and a healthier industrial ecosystem.