4. Recycling Techniques
There has been considerable concern for the environment due to the continued use of finite resources and the need to address waste management, which has led to improved recycling of materials. Landfilling has in many cases been a comparatively economical method of disposing of waste based on polymer composites, since polymers are generally more difficult to recycle and the recycling process is also expensive. Typically, the production of carbon composites produces up to 40% scrap material, which can end up in landfills or waste incineration [73
]. To protect the environment, therefore, in addition to the use of economic means such as taxes, rules have been introduced to encourage recycling [74
]. Waste management had gained enormously in importance in the European Union and to reduce it, waste prevention should start at the production stage by reusing a product, the recycled material, the incineration of waste and the reduction of landfill. Compared to fast melting thermoplastics, thermoset composites have a cross-linked structure with the inability to be formed. Several thermoset polymers, such as polyurethane, can be easily converted into their starting monomer. Conversely, commercially available thermosetting resins, such as polyester and epoxy resin, are difficult to depolymerize into their starting monomers [75
]. At present, it can be assumed that full recovery of fibers, known as a direct structural recycling approach, will benefit the composites sector. The recycled fibers from this approach have an additional market value due to the low consumption of natural resources, energy and labor, together with a near virgin fiber quality [76
]. As far as the breakdown of recycling technologies of research and industry is concerned, solvolysis (24%), pyrolysis (31%) and mechanical grinding (18%) are characterized by the highest acceptance. 20% make up so-called “other” technologies [75
]. Numerous methods have been investigated and established. These include mechanical, thermal and chemical-based recycling approaches, as the choice of methods depends on the type of material to be recycled and the application in which it is reused [11
]. Furthermore, it is difficult to determine a standard recycling method among the various methods [77
]. Different recycling processes have been reported and promoted for thermoset composites, as it is depicted in Figure 4
. Basically, three classification processes have been reported so far: mechanical, chemical and thermal recycling. Mechanical recycling consists of mechanical shredding processes to reduce the waste into recyclates. Thermal recycling involves thermal processes to break down the waste material for material and energy and the chemical recycling involves dissolving the matrix from the fibers in a reactive medium.
Technology Readiness Level (TRL) is a framework used in many variations across industries to provide a measure of technology maturity from ideation (basic principles) to commercialization [78
]. Incineration and landfilling are considered to be at TRL 9, which means it is present as a currently operating system. Pyrolysis for carbon fibers and mechanical milling for glass fiber applications achieved average values of 8.3 and 8.2 and a median of 8, which places them at TRL 8. As far as conventional pyrolysis of recycled carbon fibers is concerned, the process is commercially available on an industrial scale (e.g., ELG Carbon Fibre Ltd., Bilston, UK and CFK Valley Stade Recycling GmbH & Co. KG, Wischhafen, Germany) [79
]. Pyrolysis for glass fibers and mechanical grinding for carbon fibers had a mean of 6.25 and 6.3 respectively with a median of 7. Fluidized bed pyrolysis and solvolysis had a mean value of 4.2 and 2.24 (median of 4). Microwave heating had a mean value of 3.2 (median of 3) [75
]. In the following, the mechanical, chemical and thermal recycling methods are briefly described, and the status of current research is summarized.
4.1. Mechanical Recycling
In general, mechanical recycling represents as a technique for shredding composite waste into smaller pieces also denoted as recyclates. In this process, mechanical recycling techniques start with cutting and shredding the scrap or discarded composites into smaller pieces. Subsequently, the different fragments are classified. The smaller size increases the separability of the fibers and the resin matrix (usually a thermosetting resin) from the composite structures. Slow speed cutting or shredding mills are normally used to reduce the size of the material to 50–100 mm. If the composite waste is homogeneous and does not contain metal components, high-speed milling is used to reduce the size between 50 μm and 10 mm [80
]. The recyclates are thereby divided into coarse recyclates with a higher fiber content and fine recyclates with a higher resin content using cyclones and screens. Effective reuse of recyclates is based on particle size as described in literature [80
]. This process is referred to as the pre-recycling process for various processes according to the current state-of-the-art [82
]. Nowadays, research is mainly focused on GRP [11
]. This might be since discontinuous recyclates and their reuse are applied in low-value applications such as filler or reinforcing materials [85
]. An important aspect here is the price. CFs are more expensive compared to GFs. Disruption of their physical integrity through mechanical recycling can lead to economic and fiber losses. There have been serious drawbacks since the early development of the process, although studies by some authors such as Mou et al. [86
] showed an improvement in the flexural strength of concrete after the addition of recycled GF filler. However, Pickering [80
] pointed out that the use of GF recyclates as fillers is not economical due to the availability of new fillers such as calcium carbonate or silica as a bilious alternative. To overcome the limitations, recent studies show acceptable improvements in the process. For example, Meira Castro et al. [87
] used computational intelligence for optimization and showed that compressive and flexural strength of polymer composites can be improved. Compared to thermal and chemical recycling, this optimized process was cost-effective. Kočevar and Kržan [88
] separated 70% of the GF with a normal hammer mill to further increase the yield without residues. The remaining 30% of the waste was then used as filler for thermoplastics. A study by Li and Englund [89
] describes how waste from the aerospace industry was crushed using a hammer mill and subsequent shredding. Recyclates were pressed into flat pallets and subjected to mechanical tests, which showed a decrease in mechanical properties of at least 50–60% compared to the original composite. As the particle size of the CF recyclate decreases, the mechanical properties increase as shown. Another promising high TRL technology is Co-Processing or Cement Kiln technology [41
] which involves mixing the shredded composite parts with other kind of waste to feed into cement kilns. The organic resin is burnt for energy production and the inorganic components become feedstock for cement. CFRPs can be treated the same way, but recycling is preferable [23
]. One important aspect of a more sustainable suggestion to mechanical recycling of both GFRP [90
] and CFRP [91
] is that higher recycling rates are more energy-saving, however, reaching a plateau at a certain recycling rate. Furthermore, it was shown that the composite matrix is used as a substitute fuel, enabling savings to be made in the use of other (fossil) fuels [41
4.2. Thermal Recycling
The main objective of the thermal recycling process is to separate the fibers from the matrix. This can be achieved by either (a) pyrolysis processes, (b) fluidized bed pyrolysis processes and (c) microwave pyrolysis processes. Microwave pyrolysis has very limited availability, even at pilot scale [79
]. In this process, heat is used to break up the composite in a thermal recycling process. Due to the higher operating temperature of 450–700 °C, the insignificant volatile materials may be burnt off, leaving the valuable fibers behind. Usually, the process temperature depends on the type of resin used in the scrap plastic. It should be noted that an unsuitable temperature can either leave char on the fiber surface (undercooked) or lead to a reduction in the diameter of the recovered fibers (overcooked) as described in the literature [11
]. Thermal recovery can be divided into three types [80
]. The principle for decomposition by heat remains the same, but the results are different for each process. Polymeric compounds have certain calorific values and therefore electricity can be generated by converting the waste compound into heat [11
]. However, the incineration process produces ash as a by-product, which is a major disadvantage. This ash can only be landfilled as so-called inert waste being detrimental to the progress of a circular economy. Another disadvantage is that when the heat is converted into electricity, an efficiency of only 35% can be achieved. Yet burning coal in the furnace is a much better option than burning CFRP. In more recent studies, research is focusing on the full recovery of fibers through thermal recovery processes such as fluidized bed process (FBP, easily scalable process) and pyrolysis [3
]. Controlled resin decomposition at optimal temperature instead of complete combustion of CFRP can lead to CF recovery with negligible surface damage. Matielli Rodrigues et al. [92
] studied resins thermally decomposed at 450 °C for 2 h. CF was recycled without significant surface damage. The decomposed epoxy resins are derived from the diglycidyl ether of bisphenol-A (DGEBA). This is difficult to recycle due to its cross-linked structure after the resin has cured. Hence, recycling with little surface damage is a better option than complete landfill. Under optimal thermal conditions, the decomposition of the resin in GF is not as efficient as in CF, but post-treatments of recycled glass fibers (rGF) can help to recover their properties [38
]. Unlike CF, thermal recycling of GF under high temperature operating conditions (300–600 °C) reduces the strength of the resulting GF by up to 80% and is difficult to recycle further due to the low reinforcement potential [94
]. Yang et al. [94
] investigated two chemical treatments for this purpose: chemical etching and post-silanization to treat decomposed GF (80% reduced tensile strength) at 500 °C for 30 min. Chemical post-treatment retained 30–70% of the lost mechanical properties in rGF. Thomason et al. [38
] restored 75% of the strength loss. The GF have been immersed in 3 M NaOH solution at 90 °C for 10 min and then neutralized (short-term hot sodium hydroxide NaOH treatment). Pender and Yang [96
] used catalysts such as CuO, CeO2
to accelerate the resin decomposition with them. The processing time was reduced by 20 min, 40% less energy. CuO had the highest resin removal efficiency at 375 °C, while CuO and CeO2
increased the strength retention capacity of GF by 20%.
4.3. Chemical Recycling
Chemical recycling is defined as the process in which polymers are chemically converted to monomers or partially depolymerized to oligomers through a chemical reaction. The polymer matrix present in the waste composite is broken down by dissolving it in any chemical solution in a chemical recycling process including, e.g., acids, bases and solvents. Chemical recycling is mainly used for CFRPs [11
]. Depending on the composition of the polymer substrate, suitable chemicals and solvents are selected [85
], whereas the solid composites are mechanically grinded before chemical recycling to obtain a larger surface area. Once the polymer matrix has been dissolved, the recycled fibers are washed to remove minor surface residues [76
]. The recovered fibers have retained long fibers with maximum mechanical properties. The process has a higher resin degradation. Concerning recent chemical recycling processes, resin degradation is obtained either with solvents by solvolysis or with water by hydrolysis. In the case of solvolysis, solvents are used under suitable conditions, especially reaction time and concentration, to depolymerize the polymeric part of a composite material. Water is used to break down the resin during hydrolysis [76
]. The use of harmful and concentrated chemicals can lead to a significant environmental impact [85
], therefore the chemicals must be replaced by water and alcohol under supercritical conditions. Furthermore, the disadvantage of improper fiber alignment in discontinuous rCF (longer than 5 mm) can be suppressed by the concept of a centrifugal alignment rig [98
] or by calendering by rolling [99
]. As far as solvolysis is concerned, research has been done at lab scale with the focus on carbon fiber composites, but a commercial application does not yet exist.
As far as solvolysis is concerned, this method comprises the decomposition behavior of CF reinforced epoxy resin composites in a molten KOH. CFRPs decomposed under atmospheric pressure at temperatures from 285 to 330 °C [100
Concerning supercritical solvolysis, a supercritical solvent is used (for water with temperatures >374 °C and pressure >221 bar). This method is mainly applied to CFRPs to recover CFs having a good quality without considering the products of the polymer degradation [11
]. The advantages and disadvantages of chemical recycling methods are reflected in Table 2
6. Summary & Conclusions
This work attempted to provide clear perspectives on the state-of-the-art of the recycling technology in the composite industry and guidelines for economically and environmentally sustainable End-of-Life (EoL) solutions and development of the composite material recycling.
History. The historic timeline of the composite industry was presented in six periods of its development. The trend during the last two decades has been an attempt to save weight, reduce the cost of composite materials, and to develop the composites recycling technologies. Composite materials recycling is now one of the fastest growing niches of the composite research. Among other notable niches are health monitoring and ageing/corrosion studies, that have reached a new wave of interest due to novel concepts and technologies.
State-of-the-art. The three main EoL solutions of treating composite waste are landfill disposal, incineration and recycling. Traditional disposal routes (landfill and incineration) are becoming increasingly restricted and banned due to creating a negative impact on the environment and ecosystem, and composites industry companies and their customers are looking for more sustainable solutions. However, landfill is still the most common route. This, however, is to change in the near future. The industries where composites are most employed are aerospace, automotive, marine and wind energy. The increased use of CFRPs and GFRPs in the industry coupled with landfill disposal restrictions and bans is resulting in a need to develop a sustainable composite recycling technology. A barrier to the increased use of GFRPs and CFRPs is the lack of effective and sufficient recycling solutions and facilities. Sociotechnical pressure will only grow in the current decade of 2020s as more countries are to restrict landfill options, and due to a fast-growing number of composites EoL waste.
Drivers. The main driver in the transportation industries (automotive, rail and aerospace) is the need for lightweight materials to improve efficiency and an environmental consideration. FRPs can replace existing materials to lower the environmental footprint and enable key applications contributing to a more sustainable society. The COVID-19 pandemic is accelerating decommissioning of aircrafts, making the recycling topic even more so important—retirement of 11,000 aircrafts is expected in the decade of 2020s alone. Furthermore, the content of composite materials used in aircrafts has been growing immensely in the last years. In the automotive industry, electric cars need lightweight materials for reaching longer ranges between recharging. Application of CFRP in car parts reduces the weight by ~30% of a standard car. Furthermore, there are directives that require at least 85–95% of a car to be recyclable. Lastly, Boris Johnson’s Green Industrial Revolution to completely ban sales of new gasoline and diesel cars in the UK by the end of 2020s decade will affect the composite industry. In the wind energy and offshore sectors, there are more than a third of a million utility scale wind turbines installed around the world, most of which are designed for service life of 20–25 years. Turbines from the first major wave of wind power in 1990s are reaching the EoL in the decade of 2020s. In the naval sectors, the use of composites is also advancing at a fast pace.
Market. The market drivers that determine the best disposal solutions of FRP waste are the market forces of demand and supply, the increasing cost of landfills, the increase in awareness on circular economy thinking, the markets for recycled products, government policies, and legislations on recycled FRP composites, breaching of new markets. It is anticipated that the worldwide market for end product composites will reach $114.7 billion by 2024. The nature of composites generated by the industry is known to be about 1/3 thermoplastics and 2/3 thermosets. GFRP still remains the dominant material in the composites market, over 90%. However, CFRP growth rate is the largest. Market comparison by reinforcement fiber type, i.e., glass, carbon, basalt, aramid, and their market share (~70% GF, ~12% CF, ~11% BF, ~7% AF), cost range and their mechanical properties were also described in this work.
Sustainable recycling technology. A key aspect for the development of sustainable recycling technology is to identify the optimal recycling methods for different types of composites. These include mechanical, thermal and chemical-based recycling approaches, and the choice of methods depends on the type of material to be recycled and the application in which it is reused. Mechanical recycling consists of mechanical shredding processes to reduce the waste into recyclates. Thermal recycling involves thermal processes to break down the waste material for material and energy and the chemical recycling involves dissolving the matrix from the fibers in a reactive medium.
Carbon fiber-reinforced composites (CFRP). CFs are more expensive compared to GFs. Disruption of their physical integrity through mechanical recycling can lead to economic and fiber losses. Possible savings with recycled CFs are already significant as of today. Virgin carbon products are typically between 30–40 €/kg, while recycled products are 10–20 €/kg. Furthermore, a rCF has less than 10% of the global warming potential of a virgin CF. As of today, chemical recycling is mainly used for CFRPs—the highest tensile strength values are obtained from fibers produced by chemical recycling and the lowest values by mechanical recycling.
Glass fiber-reinforced composites (GFRP). In GFRP waste recycling, it is crucial to develop low-cost approaches for recycling the GF, otherwise it will not be beneficial and affordable since commercial GF are not very expensive reinforcing fibers. Developing a method for obtaining high quality rGF at a competitive price with virgin GF is an important challenge. The critical technical challenge in the development of an effective GFRP recycling technology is the 80–90% drop in strength and the shortening of rGFs with every recycling cycle. The development of an economically viable process for regenerating mechanical properties of thermally rGF would have major technological, societal, economic, and environmental impacts. As of today, the highest tensile strength retainment is obtained in rGF produced by High Voltage Fragmentation and the lowest values by the pyrolysis and the Fluidized-bed processes. Economically viable recycling to get quality rGF needs yet to be demonstrated.
Basalt and aramid fiber-reinforced composites (BFRP, AFRP). The composite recycling industry has concentrated mostly on GFs and CFs, as of now. However, a sustainable and effective recycling technology of BF and AF-reinforced composites is yet required to be developed in the near future. It is expected that optimal recycling routes for BF should be somewhat similar to GF, whereas AF—more similar to CF.
Self-reinforced thermoplastic composites. Self-reinforced thermoplastic composites are promising due to their ultimate recyclability. Studies with polyethylene deserve special attention, given that this is the general-use polymer with higher production worldwide, and that it can be applied in actual high-performance composites being reinforced by, i.e., ultra-high-molecular-weight polyethylene (UHMWPE).
Mechanical recycling. Mechanical recycling is a cost-effective process and more suited for GFRP, while thermal and chemical recycling are more suited for CFRP. Nevertheless, it negatively affects the mechanical performance of GF strongly. One important aspect of a more sustainable suggestion to mechanical recycling of both GFRP and CFRP is that higher recycling rates are more energy-saving, however, reaching a plateau at a certain recycling rate. Mechanical recycling is suggested to obtain a filler powder from both thermoset and thermoplastic CFRP and GFRP.
Cement kiln. Another promising high TRL technology is co-processing or a cement kiln technology. It involves the use of the FRP waste as an alternative fuel in the cement industry. 100% of the composite waste is “recovered” in the form of energy and raw materials. Co-processing of GFRP waste seems to have no negative effect on the quality of the produced cement. Cement kiln method is more suited for GFRP, not CFRP. Co-processing in cement kiln is suggested for thermoset GFRP.
Thermal recycling. Currently the most common method in the industry for the recycling of FRPs is by pyrolysis. Thermal recycling enables the recovery of rCF largely maintaining their reinforcement capability, whereas rGF are quite damaged (at least 50–80% strength lost). Thermal recycling is suggested to obtain rCF from mostly thermoplastic CFRP. However, it is not sufficiently economically viable to recycle thermoset CFRPs using pyrolysis. In some sources, it is stated that it is economically viable to use pyrolysis for thermoset GFRPs. However, GFs lose 80% or more of their strength when exposed to temperatures typically found in GFRP thermal recycling processes, which makes them unsuitable for reuse as a composite reinforcement. This fact opens a great opportunity for developing a sustainable rGF regeneration technology, which has already been attempted, i.e., by Strathclyde University.
Chemical recycling. Chemical recycling enables the recovery of rCF largely maintaining their reinforcement capability, whereas rGF are quite damaged. Solvolysis is the most suitable method to recycle CF because it consumes less energy, and it contributes to the development of high-quality recycled CF. Unfortunately, the Recycling by Super Critical Fluid Solvolysis (SCFS) is not the most environmentally friendly recycling process compared to mechanical recycling or pyrolysis. The method can lead to a potential reuse of the matrix. Chemical recycling is suggested to obtain rCF from thermoset CFRP. Chemical recycling to obtain rCF from thermoplastic CFRP.
Energy demand. Energy demand involved in composite recycling methods is the following: Chemical recycling (21–91 MJ/kg); Pyrolysis (24–30 MJ/kg); Microwave Pyrolysis (5–10 MJ/kg); Mechanical recycling (0.1–4.8 MJ/kg).
Technology Readiness Level (TRL). Landfilling and incineration are considered to be at TRL 9. Among the recycling techniques, mechanical grinding for GF applications is considered as the most advanced, and pyrolysis most advanced for CF applications. Pyrolysis for CF and mechanical milling for GF applications achieved average values of 8.3 and 8.2 and a median of 8. Pyrolysis for GF and mechanical grinding for CF have a mean of 6.25 and 6.3, respectively, with a median of 7. Fluidized bed pyrolysis and solvolysis have a mean value of 4.2 and 2.24 (median of 4). Microwave heating have a mean value of 3.2 (median of 3). Industrial recycling applications—High TRL technologies—which are already implemented in the recycling facilities were presented in this manuscript.
Future trends and recycled composite products. The future holds novel concepts of reusing and repurposing of EoL composites and development of new recycling and recycled fiber regeneration technologies. Advancements in predictive modelling are also highly anticipated.
Regarding reuse and repurposing, large sections of wind turbine blades can be reused for architectural or other structural purposes; another application might be furniture from end-of-life wind turbine blades as has been demonstrated. Some other sustainable aspects include reuse of waste rubber, i.e., tires can be used as a source of valuable raw materials in different polymeric matrices for the manufacture of low-cost products and a cleaner environment. Rubbers have a role in sustainable development of the composite and automotive industries. Additionally, novel and promising matrix materials include vitrimers due to their increased recyclability. These materials may expand the horizons of thermoset materials.
As far as modelling is concerned, the industry needs mathematical models that depict the influence of (1) variance of processing parameters on the quality of recyclates for all recycling techniques, and (2) composite recycling processing parameters on the cost and environmental impact prediction. Cost and environmental impact are highly important to ensure sustainable development since recycling techniques must be not only technically feasible but also cost effective and environmentally friendly. The development of mathematical models for these recycling techniques is crucial for researchers to better understand and optimize them.
The future development of the composite industry is “green” and optimistic yet highly dependent on the rapidly developing recycling technology and implementation of Circular Economy (CE) and sustainable thinking.