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Chemical structures of epoxy resin and curing agent: (a) epoxy tetraglycidyl-4,4’-diaminodiphenylmethane (TGDDM); (b) curing agent 4,4’-diaminodiphenylsulfone (DDS) and (c) cross-linked polymer network. 

Chemical structures of epoxy resin and curing agent: (a) epoxy tetraglycidyl-4,4’-diaminodiphenylmethane (TGDDM); (b) curing agent 4,4’-diaminodiphenylsulfone (DDS) and (c) cross-linked polymer network. 

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Over the past few years, there has been great deal of interest in recycling carbon-fibre-reinforced polymer composites. One method that has shown promising results involves the use of supercritical fluids to achieve separation between matrix and fibres by effectively degrading the resin into lower molecular weight compounds. In addition, the solven...

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... have very low toxicity. Supercritical fluids are suitable for recycling CFRPs because they are able to degrade the resin without extensive damage to the fibres [11]. Supercritical water and alcohols possess a combination of properties, such as low viscosity, high mass transfer coefficients and high diffusivity. In the supercritical or near-critical region, the dielectric constant is significantly reduced and hydro- gen bonding essentially disappears. Therefore, they behave similarly to common organic solvents and have high sol- vation power for organic compounds [12]. All of these properties are beneficial for resin degradation and removal. Piñero-Hernanz et al. [13,14] explored the use of acetone, methanol, ethanol and 1-propanol, as well as water. They were able to achieve 98.0 wt% resin elimination using supercritical 1-propanol without a catalyst at 350 ◦ C in a semi-continuous reactor system. Resin elimination of 95.4 wt% was achieved using supercritical water with 0.5 M KOH catalyst in a batch reactor at 400 ◦ C and 27.5 MPa (15.5 min reaction time). Bai et al . [15] utilized supercritical water to achieve similar results of high resin elimination and observed retention of the original fibre strength. In this study, recycling aerospace-grade epoxy carbon fibre composites using supercritical water was explored. The resin elimination efficiency (REE) was determined for the reclaimed fibre. In addition, the morphology of the reclaimed fibre was characterized and the tensile property measured. We also demonstrated the possibility of recycling multi-layered composites. Single-layer pre-impregnated (prepreg) epoxy carbon fibre composites were cured and used for the recycling study. The prepreg material employed in this study is one of the most widely used resin / carbon fibre combination for aerospace composite applications. The fibres were Hexcel IM7 eight-harness satin weaved (SGP 370-8H). The resin was Hexcel 8552, a toughened multi-component resin system whose main epoxy component is tetraglycidyl-4,4’- diaminodiphenylmethane (TGDDM); the curing agent was 4,4’-diaminodiphenylsulfone (DDS) (see Figure 1). The prepreg sample was cut to 6 cm × 6.5 cm and weighed and cured following the manufacturer’s recommended cure protocol. After curing, the sample was weighed again to determine the mass of resin lost during curing. Figure 1(c) is a representation of the infinitely cross-linked three- dimensional polymer network that is formed after complete curing. A high-temperature, high-pressure reactor system (Parr Instruments), equipped with mechanical stirring, was used for the recycling study. De-ionized (DI) water was used as the solvent, and potassium hydroxide (KOH) was used as the catalyst (0.05 M). The solution was delivered man- ually to the reaction vessel. The desired mass of solvent was calculated using the modified Benedict–Webb–Rubin (MBWR) equation of state (EOS), based on the volume of the reactor and target reaction pressure and ...

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... Before designing the experiments for this study, the most updated literature was compiled and may be found in Appendix A [13,[20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Apart from the current studies, there is a significant need for developing an effective solution to handle composite wastes. This study used a solvent treatment method-acid digestion-by employing sulfuric acid, nitric acid, acetone, and deionized (DI) water to dissolve the matrix under different decomposition conditions. ...
... Also, the thickness of material would be a contributing factor for deciding the time of treatment. Supercritical fluids and solvent-based processes, proposed by by the authors [25,34] can often handle a large variety of composites and materials without much difficulty. However, these processes are executed by maintaining high temperatures and pressure, which consumes a lot of energy. ...
Article
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Purpose The composites industry is constantly being formed by myriad forces—technologies, markets, people- all encouraging innovative ways to apply carbon, Kevlar®, and glass fiber composites to produce vital parts for a wide range of applications. The expanded demand for fiber-reinforced plastic (FRP) composites has prompted high manufacturing scrap and end-of-life waste volumes. Limited clearance on landfills and the high energy for virgin material production motivate the companies for practical composite recycling techniques. The work described in this study involves an array of experiments including acid treatment of outdated resin-impregnated composite fibers (prepreg) to study its effects and reclaiming the fibers for future sustainable manufacturing. Method The experiments were carried out at two different temperatures: 25 °C and also 60 °C. Sulfuric acid, nitric acid, acetone, and distilled water were used in the process, with varying treatment times of 60, 120, 240, 360, and 420 s. The recovered fibers were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Result The optimum treatment time, and temperature were different for all three types of fibers. Initially, the glass fiber yielded promising results at room temperature and with a minimal 120-s processing time. Carbon fiber treatment was successful at 60 °C with a 420-s treatment time. However, some surface damage was observed in the Kevlar® fiber at 60 °C. Conclusion The chemical recycling process, is the most sustainable, energy- and cost-efficient approach compared to all other available recycling processes. Also, it is possible to recover much cleaner fibers with the weave intact with an acid treatment and solvent-based recovery. Graphical Abstract
... However, the equipment is quite demanding since it must be capable of operating safely at the necessary conditions for supercritical fluid processing (high pressure and temperature). A catalyst is often necessary 12 , but not always 13,14 . In this last case, a semi-flow type reactor allows the loading of larger multi-layers (200 mm × 45 mm × 2 mm). ...
Article
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A novel environmentally friendly recycling method is developed for large carbon-fibers reinforced-polymers composite panels whose efficiency is demonstrated through a proof-of-concept fabrication of a new composite part based on recycled fibers. The recycling process relies on formic acid as separation reagent at room temperature and atmospheric pressure with efficient recycling potential of the separating agent. Electron microscopy and thermal analysis indicate that the recycled fibers are covered by a thin layer of about 10wt.% of residual resin, alternating with few small particles, as compared to the smooth virgin fibers. The recycled composites show promising shear strength and compression after impact strength, with up to 93% retention of performance depending on the property as compared to the reference. The recycled carbon fibers can thus be reused for structural applications requiring moderate to high performances. The loss of properties is attributed to a lower adhesion between fresh epoxy resin and recycled carbon fibers due to the absence of sizing, partly compensated by a good interface between fresh and residual cured epoxy thanks to mechanical anchoring as well as chemical reactions. The room temperature and atmospheric pressure operating conditions combined to the recyclability of the forming acid contribute to the sustainability of the entire approach.
... Before designing the experiments for this study, the most updated literature was compiled and may be found in Appendix A [13,[20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Apart from the current studies, there is a significant need for developing an effective solution to handle composite wastes. This study used a solvent treatment method-acid digestion-by employing sulfuric acid, nitric acid, acetone, and deionized (DI) water to dissolve the matrix under different decomposition conditions. ...
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Purpose Carbon fiber, Kevlar® fiber, and glass fiber are the most widely used polymer prepregs in manufacturing high-performance composites to produce vital parts for a wide range of applications. The production of carbon and Kevlar® fibers is an energy-intensive process, requiring 198–595 MJ to produce 1 kg of virgin carbon fiber. However, chemically recycling these expired prepregs takes only 38.4 MJ/kg, which could be significantly sustainable. The work described in this study involves an array of experiments involving acid treatment of outdated prepreg composite fibers to study its effects and reclaim the fibers for future applications. Method The experiments were carried out at two different temperatures: 25°C and also 60°C. Sulfuric acid, nitric acid, acetone, and distilled water were used in the process, with varying treatment times of 60, 120, 240, 360, and 420 seconds. The recovered fibers were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Result The optimum treatment time and temperature were different for all three types of fibers. Initially, the glass fiber yielded promising results at room temperature and with a minimal 120-s processing time. Carbon fiber treatment was successful at 60°C with a 420-s treatment time. However, some surface damage was observed in the Kevlar® fiber at 60°C. Conclusion The chemical recycling process is the most sustainable, energy- and cost-efficient approach compared to all other available recycling processes. Also, it is possible to recover much cleaner fibers with the weave intact with an acid treatment and solvent-based recovery.
... Generally, recycling using sub-and supercritical fluids is a quick and simple method and can be operated semi-continuously [74]. Furthermore, the decomposition of polymers proceeds rapidly and selectively, and the fibers compare favorably to the virgin fibers, with only a slight loss of tensile strength [75,76]. ...
Article
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Carbon fiber-reinforced composites present an exciting combination of properties and offer clear advantages that make them a perfect replacement for a spread of materials. Consequently, in recent years, their production has dramatically increased as well as the quantity of waste materials. As future legislations are likely to prevent the use of landfills and incineration to dispose of composite waste, alternative solutions such as recycling are considered as one of the urgent problems to be settled. This study presents the leading technologies for recycling carbon fiber-reinforced composites, focusing on chemical recycling using sub- and supercritical fluids. These new reaction media have been demonstrated to be more manageable and efficient in recovering clean fibers with good mechanical properties. The conventional technologies of carbon fibers recycling have also been reviewed and described with both advantages and drawbacks.
... Unfortunately, the Recycling by Super Critical Fluid Solvolysis (SCFS) is not the most environmentally friendly recycling process [126] compared to mechanical recycling or pyrolysis. However, the process can produce high quality rCF in as little as 15 to 120 min and can lead to potential reuse of the matrix (e.g., for matrix: to produce epoxy [127,128]). ...
Article
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Recently, significant events took place that added immensely to the sociotechnical pressure for developing sustainable composite recycling solutions, namely (1) a ban on composite landfilling in Germany in 2009, (2) the first major wave of composite wind turbines reaching their End-of-Life (EoL) and being decommissioned in 2019–2020, (3) the acceleration of aircraft decommissioning due to the COVID-19 pandemic, and (4) the increase of composites in mass production cars, thanks to the development of high volume technologies based on thermoplastic composites. Such sociotechnical pressure will only grow in the upcoming decade of 2020s as other countries are to follow Germany by limiting and banning landfill options, and by the ever-growing number of expired composites EoL waste. The recycling of fiber reinforced composite materials will therefore play an important role in the future, in particular for the wind energy, but also for aerospace, automotive, construction and marine sectors to reduce environmental impacts and to meet the demand. The scope of this manuscript is a clear and condensed yet full state-of-the-art overview of the available recycling technologies for fiber reinforced composites of both low and high Technology Readiness Levels (TRL). TRL is a framework that has been used in many variations across industries to provide a measurement of technology maturity from idea generation (basic principles) to commercialization. In other words, this work should be treated as a technology review providing guidelines for the sustainable development of the industry that will benefit the society. The authors propose that one of the key aspects for the development of sustainable recycling technology is to identify the optimal recycling methods for different types of fiber reinforced composites. Why is that the case can be answered with a simple price comparison of E-glass fibers (~2 $/kg) versus a typical carbon fiber on the market (~20 $/kg)—which of the two is more valuable to recover? However, the answer is more complicated than that—the glass fiber constitutes about 90% of the modern reinforcement market, and it is clear that different technologies are needed. Therefore, this work aims to provide clear guidelines for economically and environmentally sustainable End-of-Life (EoL) solutions and development of the fiber reinforced composite material recycling.
... The addition of alkali catalyst (NaOH) increases the resin removal efficiency [126,127]. In 2012, when repeating the same approach without any change in the process parameters, Knight et al. [128] achieved a resin decomposition rate of 95.9-99.2 wt% and recycled woven based CF using supercritical water (deionised water). ...
... The studies related to the second approach can also be implemented to recycle GFRP waste, as both forms of waste (CFRP and GFRP) involve similar polymer structures [48]. [128] 2012 KOH (alkali catalyst) 410 °C, 28 Mpa for 120 min and 0.5 M KOH catalyst Yuyan et al. [129] 2009 Sulfuric acid 260 °C, 1 M sulfuric acid, solvent feedstock ratio of 1:5 g/ mL for 105 min Kim et al. [130] 2019 ...
Article
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The growing use of carbon and glass fibres has increased awareness about their waste disposal methods. Tonnes of composite waste containing valuable carbon fibres and glass fibres have been cumulating every year from various applications. These composite wastes must be cost-effectively recycled without causing negative environmental impact. This review article presents an overview of the existing methods to recycle the cumulating composite wastes containing carbon fibre and glass fibre, with emphasis on fibre recovery and understanding their retained properties. Carbon and glass fibres are assessed via focused topics, each related to a specific treatment method: mechanical recycling; thermal recycling, including fluidised bed and pyrolysis; chemical recycling and solvolysis using critical conditions. Additionally, a brief analysis of their environmental and economic aspects are discussed, prioritising the methods based on sustainable values. Finally, research gaps are identified to highlight the factors of circular economy and its significant role in closing the life-cycle loop of these valuable fibres into re-manufactured composites.
... AFM and SEM were used for analysing the superficial morphology and found cleaned recycled carbon fibre with a great surface. Further, Knight et al. (2012) used supercritical water along with 0.05 M catalytic agent KOH (potassium hydroxide) in order to recover carbon fibre from the waste aerospace epoxy CFRP (Hexcel 8552/IM7). The tensile strength of the recovered fibre was tested and found same strength as of the virgin fibre (5.25 GPa). ...
Article
Materials have become an integral part of our lives owing to their wide usability, but at the same time, they are affecting the nature antithetically. Fibre Reinforced Plastic (FRP) is highly sought materials in an automobile, aerospace, structural, transportation and other industries due to its excellent mechanical performance, durability, and lightweight. Currently, miscellaneous industries manufacture engineered composite products using FRP materials, especially the products of hardbound thermoset resins, which are difficult to reprocess or reuse. Landfills and incineration are the most common technique for discarding the non-degradable FRP waste that has created an inimical impact on the environment and ecosystem. In order to reduce the ecological burden, we need to evaluate economical and feasible FRP recycling techniques. Extensive investigations conducted over the past decades have proven their efficacy in substituting the currently existing recycling processes. This review article enumerates the mechanical, thermal (fluidised bed and pyrolysis), chemical (low temperature and supercritical temperature) recycling methodologies, and their efficiency in degrading copious FRP materials like glass, carbon, natural etc. In addition, the economic and environmental aspects of fibre reinforced plastic materials based on life cycle assessment (LCA) has been discussed.
... From building and road construction materials like concrete (Khalid et al., 2017;Letelier et al., 2017;Liu et al., 2013) to composite polymers (Ogi et al., 2007;Wei et al., 2018;Ignatyev et al., 2014), recycling is economically viable. There is an increase in the study of recycling of composite 77polymers (Knight et al., 2012;Oliveux et al., 2015;Ignatyev et al., 2014), plastic waste (Jaivignesh and Sofi, 2017;Siddique et al., 2008;Rebeiz and Craft, 1995), composite wastes (Halliwell, 2006;Kratz et al., 2017;Meng et al., 2017a,b) and nanocomposites (Hasan et al., 2019). However, there are limitations and issues to recycling composites, particularly the fiber reinforced composites. ...
... Recycling of CFRP by supercritical fluids was developed in recent years (Akio et al. 2008;Chase et al. 2012;Haihong et al. 2016;Okajima, Masataka, and Sako 2011;Okajima et al. 2002;Raúl et al. 2008). However, the quantitative relation between degradation rate of epoxy resin and the process parameters had never been reported, and it was unknown to how to estimate process parameters accurately. ...
Article
The high-performance carbon fibres can be recycled from waste carbon fibre/epoxy resin composites by supercritical n-butanol. Recycling experiment designed by response surface method was used to investigate quantitative relation between degradation rate of epoxy resin and process parameters. Thus, the optimum process parameters could be obtained, and effects of process parameters and layers on degradation rate and mechanical performance of the recycled carbon fibres were analysed. The tensile strength of the recycled carbon fibre under the optimum process parameters was 94.53% of that of the original carbon fibre, tensile modulus was 93.57% of that of the original carbon fibre and interfacial shear strength was 90% of that of the original carbon fibre.
... Carbon fiber, due to its high price, seems to be the choice for recycling or reclaiming as much as possible. [7][8][9] Currently, recycled fibers are either used to make pellets or are used as fillers when combined with thermoplastics or thermosets. [10][11][12] A novel approach being studied is the process of wet-laid or hydroentanglement, which disperses fibers in water and consolidates them into preforms. ...
Article
The flexibility of processing PA6-based discontinuous carbon fiber panels using vacuum-assisted resin transfer molding was studied. The ease of incorporating various reinforcements namely baseline, tow in the center of preform, fabric in the center of preform and fabric on the outside as skin was investigated. Mechanical characterization was conducted on all the variations made. There was an average increase of about 3%, 20% and 47% in the tensile properties of tow in the center, fabric in the center and fabric on the outside as skin, respectively, as compared to the baseline. A similar increase in properties was noticed in its flexural and impact strength. The data showed a correlation between the mechanical properties and the total surface area of additional reinforcements used. As the surface area of the reinforcement increased, the mechanical properties increased as well. It also showed that reinforcements on the surface of the preform as a skin performed the best. DMA analysis showed the effect of reinforcement on the storage modulus and tan delta across temperatures ranging from 30°C to 150°C. SEM analysis showed that the fibers and the additional reinforcements were coated with PA6 which translated into consistent mechanical performance.