High Performance Recycled CFRP Composites Based on Reused Carbon Fabrics Through Mild Solvolysis Route

An original wet recycling method is developed for large carbon-fibers reinforced-polymers composite panels, addressed 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 under atmospheric pressure conditions. 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 virgin fibers exhibiting a smooth surface. The mechanical properties of composites based on neat and recycled fibers are determined from interlaminar shear strength, compression, compression after impact and Iosipescu shear tests. The recycled composites show promising shear and strength values with a deterioration of performances limited to about 10 to 33% 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 having lost their sizing, partly compensated by a good interface between fresh and residual cured epoxy thanks to mechanical anchoring as well as chemical reactions.


Introduction
Despite the high cost of carbon fibers reinforced polymer matrix composite (CFRP) solutions, the global demand for these materials keep raising due to the increasing number of applications in cars, boats, trains, windmills and airplanes. The main advantages stem from the high strength and stiffness over density performance indicators. The superior properties come from the carbon fibers (CFs) while the matrix, such as epoxy, ensures efficient load transfer provided that good adhesion with the fibers and sufficient ductility are sufficient to avoid premature failure.
However, CFRPs present challenges of environmental nature related first to the high embodied energy associated to CFs production and of the polymer-based matrices.
Furthermore, large volumes of waste are created at the end of the operational life time.
Recycling is the obvious solution to decrease the energy impact over the life cycle. The existing end-of-life solutions have limitations: incineration offers poor energy efficiency and generates polluting emissions while mechanical recycling only recovers lower performance reinforcements. Furthermore, landfill, while preferable as compared to incineration, will be restricted in a near future by legislation pushed by directives adopted by European Union to promote recycling and the use of waste as a resource [1][2][3][4].
New attempts have been made to recover individual CFs from composites waste by developing different physico-chemical approaches. The furthest advances have focused on pyrolysis and solvolysis treatments [5][6][7]. However, these processes often require prior milling or grinding steps, intense energy consumptions and often the use of environmentally unfriendly solvents. The result is often: (i) a product with insufficient quality which can only be reintroduced in the market for non-critical applications due to fiber length shortening, very drastic conditions and intrinsic mechanical properties abatement of the CFs [7,8] and (ii) an organic fraction recovered from the resin but, due to its very complex mixture varying from one composite to another, only exploited for energy recovery [9]. Recently Wang et al.
proposed a strategy to recycle valuable oligomers from cured epoxy resin composites via a selective cleavage of tertiary carbon-nitrogen bond using acetic acid associated to AlCl3 under mild conditions [10]. The claim was that the oligomers of the cured epoxy can be preserved, recycled and reused in resin manufacturing. The resulted CFs are individualized and, when tested, exhibit almost their original tensile strength and elastic modulus.
The development of recycling processes preserving the fiber woven architecture of cured waste remains a bold challenge. Indeed, unsized CFs are much more difficult to manipulate in remanufacturing and require realignment operations [9,11]. Novel technologies adressing this possibility have recently emerged. For example, Yu et al. have proposed a lab scale technique for recycling CFs from special composites with almost 100% yield [8]. The matrix (fatty acid mixture and epoxy) is a covalent adaptative network which is capable of transesterification type bond exchange reactions, fully dissolving in ethylene glycols by soaking at 180°C. The resulting dissolved polymer, after solvent elimination, when repolymerized, leads to the same thermomechanical properties as the fresh polymer. The woven fibers were reused in new applications with similar mechanical pattern and dimensions [8].
In this same context of recovered woven fibers, sub-critical and supercritical fluids such as water or alcohols offer new opportunities to recycle multi-layered composites. The recycled woven fabric layers, with retention of the fiber architecture, could be directly reused after minimal further processing to fabricate fiber composites. The residual recycled resin could be incorporated into fresh resin and cured. 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 x 45 mm x 2 mm). The recovered CFs maintained the shape of the plain fabric, at least in light of the published pictures, at the center of the recovered woven [15]. The studied sample could be mechanically constrained in order prevent possible entanglement with the reactor stirrer [12]. Studies revealed that efficient temperature and pressure levels significantly depend on the type of epoxy resin [9]. Unfortunately, the reaction rate when using larger pieces of composites upon scale up could be limited by diffusion requiring a new optimization of the selected parameters. [9].
Limited effort has been directed toward acid digestion as these processes are rightly considered dangerous in terms of health and environmental impact. Nitric acid has been successfully used to recycle CFs [16]. Other teams have also succeeded in recovering CFs of high quality using sulphuric or acetic acid as pretreatment and a solution of hydrogen peroxide for the oxidative degradation of the epoxy resin [17,18]. However, in these studies, the recovered CFs are too small, as the composite had to be cut into small slices (about 10mm), and the woven fabric shape structure is not preserved. This paper presents the main finding of an investigation on the reuse of fully reclaimed intact woven CF fabrics originating from an aerospace-grade composite made with a highperformance epoxy resin used in primary structural applications. The recycled CF mat in original A3 size was processed with the same epoxy resin and the same conditions (resin transfer molding) as the virgin composite. The procedure used in this study to extract the woven fabric from composite parts relies on the same principle as an existing wet industrial process set to separate aluminum-polyethylene composite packaging materials [19]. The process uses formic acid as separation reagent. The retrieved process tested in this work is based on direct formic acid digestion. The tests were carried out at room temperature, under atmospheric pressure and in a static mode to eliminate the cured epoxy resin and to regenerate the starting structured woven fabric.
To our knowledge, no similar studies have been published yet. This investigation, although not yet fully optimized, is the first of its kind and includes relevant information for scaling up the technology to an industrial level. This recycling process preserves the virgin woven architecture and format, which can be directly used to manufacture reclaimed CFRPs.
Moreover, the interest of this process is that it can treat mixed and contaminated materials with glass fiber fabrics, painted surfaces, foam cores or metal inserts, typically present in all composite structures. This paper targets therefore two complementary objectives: 1. To assess the effect of the recycling process on the degradation of the reinforcement potential of the carbon fibers. Scanning electron microscope (SEM) and thermogravimetric analysis (TGA) were used to characterize the residues of epoxy on the surface of the recovered carbon fibers.

2.
Materials and experimental details

Materials
The epoxy resin used in this study is the HexFlow® RTM6 supplied by Hexcel Composites, which is qualified for aerospace applications. The carbon fiber (CF) fabrics used as reinforcement consist of HexForce ® G0926 (HTA 6k) with a 5 harness satin weave (375 g/m 2 ) manufactured by Hexcel Composites. The solvent for the acid digestion is the formic acid 98-100% from Sigma-Aldrich.

Fabrication of reference and recycled composite
A reference composite panel based on the RTM6 epoxy resin and neat carbon fabrics was manufactured using the vacuum-assisted resin transfer molding process (VARTM) from Isojet.
After complete impregnation at 100 ml/min, a dwell pressure of 6.

Scanning electron microscopy (SEM)
Specimens for SEM analysis were mounted on stubs and coated with 8 nm chromium layer (Cressington sputter 208HR) to produce a thin conductive layer, minimizing degradation and drift due to thermal expansion. SEM analyses were performed on polished surfaces in a Jeol FEG SEM 7600F operating at 15 keV with a working distance of 8 mm.

Thermogravimetric Analysis (TGA)
Thermo-gravimetric measurements were performed using a TGA/SDTA851e from Mettler Toledo to determine the amount of residual epoxy resin still present on the treated fiber after wet treatment. Tests were performed under air environment with a flow rate of 50 ml/min.
Samples of around 15 mg are heated at 10°C/min. The evolution of the sample weight was recorded from 25 to 900°C. Reported values were average over a minimum of 3 tests.

Fourier transform infrared spectroscopy (FTIR)
FTIR was carried out by a Nicolet TM iN TM 10 infrared microscope from Thermo Fisher Scientific in attenuated total reflectance mode (ATR). The FTIR absorption spectrum of recycled carbon fibers was recorded in the wavenumber range from 4000 to 400 cm -1 with a spectral resolution of 4cm -1 .

Interlaminar shear strength (ILSS)
The interlaminar shear strength tests were performed according to the EN 2563 standard.
Rectangular specimens of 30 x 10 mm were tested with a rigid 3 points bending fixture on a Zwick Z250 universal testing machine operating at room temperature under a constant crosshead displacement speed of 1 mm/min. The span between the support cylinders was equal to 20 mm.

Compression
Compression tests were performed according to the ASTM D-6641 standard.

Compression after impact (CAI)
Compression after impact specimens were cut to 100 x 150 mm and subjected to a transverse impact of 24 J using an Instron Dynatup 9250HV impactor according to the AITM1-0010 standard. The size of the internal damage zone was determined by ultrasonic C-scan inspection. The damaged specimens were then loaded in a Zwick Z250 universal testing machine equipped with a load cell of 250 kN and tested using an in-plane compression fixture (WTF) in order to determine the residual strength.

Iosipescu shear test
The Iosipescu shear test consists of a V-notched specimen loaded according to ASTM D 5379. One side of the fixture is displaced vertically while the other side remains fixed and opposing force couples prevent any in-plane bending of the specimen. If properly executed, the stress state between the notches is a pure shear state, uniform in the minimum cross-section.
The square area within the minimum cross-section covered by the strain gages is referred to as the test region in this study. The measurements were performed under dry conditions.

Fiber morphology
The morphology and surface quality of virgin and recycled carbon fibers are presented in Figure 2. The "as received" CFs ( Figure 2a) show smooth fiber surfaces while the recycled carbon fibers (Figure 2b) are covered by some residual epoxy resin layer, alternating with few small "particles" along the fiber's axis. These particles result from an incomplete digestion of the cured resin (the exact amount of residual epoxy resin is quantified below by TGA). Indeed, epoxy residues are still present on the recycled carbon fibers showing that perfectly clean recycled fibers cannot be obtained even after 48h in formic acid and at room temperature, although a fraction of the fibers are devoid of residual resin as illustrated in the insert of Figure   2b. Interestingly, the applied chemical treatment does apparently not affect the quality of the surface of the carbon fibers, in particular the roughness. Additionally, it seems that the digestion does not initiate everywhere at the same time nor at the same rate. Indeed, when a composite is immersed into formic acid, the preferential digestion or etching first takes place at the edges and at the interlaminar spots instead of the intralaminar level where the fiber density is much higher, making the acid digestion longer. A certain amount of residual epoxy is then left between the fibers (Figure 2b). A schematic drawing of the digestion process in a cross-section of a composite is represented in Figure 3.

Morphological evaluation
The morphological characterization of the composite panels is investigated by SEM.
Cross-section images of panels made of (a) virgin or (b) recycled carbon fibers with fresh epoxy resin are illustrated in Figure 5. Although CFs are properly impregnated by the RTM6 epoxy resin, without any porosity in both cases, the presence of furrows (arrow on Figure 5b) surrounding the recycled carbon fibers indicate a lack of adhesion between the recycled fibers and the epoxy matrix. The observed lack of adhesion can be attributed to the destruction of the fiber sizing by the digestion process..

Interlaminar shear strength (ILSS)
The ILSS values of the reference and recycled composite panels are determined to quantify the influence of the reclaimed carbon fibers on the resistance to delamination damage.
The average maximum load and ILSS values of the virgin and the recycled composite panels are presented in Table 1. The ILSS average value of the recycled composite is 12% lower compared to the reference composite. This result indicates that the ILSS property predominantly depends on the matrix with a minor contribution of the fiber-reinforcement. The presence of residual cured epoxy resin on the carbon fibers and the lack of sizing after formic acid treatment, as shown in Figure 2b, explains the moderate decrease of the ILSS value. Figure 6 exhibits the compression modulus and compression strength of the virgin and recycled composites. A decrease by 12.5% and 25% is found for the recycled specimen compared to the reference, respectively. The strength is, as expected, more impacted than elastic stiffness. This decrease may be attributed, as discussed in the previous section, to the poor adhesion between the residual resin and the fresh resin, and between the unsized fibers and the fresh resin.

Compression after impact (CAI)
Virgin and recycled composites were impacted at static incident energy of 24 J. The delamination areas evaluated by C-scan measurements are shown in Figure 8, with the quantitative values listed in Table 2. The shape of the delaminated area of both composites is elliptical with a larger area for the recycled case, corresponding to an increase of the delamination surface by 46% compared to the reference. A comparison between the virgin and recycled composites in terms of maximum force and compression strength after impact is also given in Table 2. The recycled composite shows a decrease of the residual compression strength after impact by 33%. Impact properties are mainly dominated by the fibers, with a weak adhesion between the fibers and the matrix playing thus a major role.

Wrap-up
A spider chart summarizing the effect of the formic acid fiber recycling on the main mechanical properties is presented in Figure 10. The use of recycled carbon fabrics leads to a 10 to 33% decrease of the mechanical properties characterized in this work. Even though, it has not been demonstrated yet, one can anticipate that further recycling steps will not degrade more the properties. This will be confirmed in a further investigation.  [21][22]. The resulting amide groups are susceptible to react with fresh epoxy resin. The aromatic stretching band of bisphenol A is present at 1600 and 1515 cm -1 . The peak at 1215 cm -1 is assigned to aromatic-hydroxyl stretching. The hydroxyl functions could also react with epoxide groups present in RTM6 fresh resin. In summary, in addition to the observed mechanical anchoring, several chemical reactions can take place between the residual RTM6 resin after formic acid attack and the freshly injected one, which both can contribute to the preservation of mechanical properties. which shows the material index of the recycled composite is still above all metallic alloys.
Probably, in view of the strict performance, reliability and qualification constraints demanded in aerostructures, the recycled composites would not be acceptable in aeronautics applications.
However, they could find relevant application in other sectors such as in the automotive or marine fields.

Conclusion
The potential of the recycling of CRFP composites by solvolysis process under mild conditions, with low thermal and energy inputs has been demonstrated by comparing the performance of a reference composite to a nominally identical composite made with recycled carbon fibers. The main findings of the work are the following: • SEM and TGA results show the presence of 10 wt% residual RTM6 epoxy resin along the carbon fibers indicating that the applied conditions do not lead to a full dissolution of the cured epoxy resin.
• The digestion preferentially takes place along the edges of the composite and in the interlaminar regions due to the impact of lower carbon fibers density, favouring the acid digestion.
• The mechanical properties (ILSS, compression, CAI, Iosipescu shear test) of the recycled composites show an abatement of about 10 to 33% depending on the property when compared to the reference.
• The loss of performance is attributed to a poor adhesion between the fresh epoxy resin and the unsized carbon fibers but remain limited owing to the good interface between fresh and residual cured epoxy owing to a mechanical interlocking/anchoring effect as well as several well-identified chemical reactions.
Further recycling steps are not expected to lead to additional abatement of mechanical performance since only the interface behavior is involved. If the absence of further decrease of properties is confirmed, two options are left for the use of the recycled carbon fiber fabrics.
The first option is to use them in structural applications with lower constraints on mechanical performance, keeping in mind that the lower properties still maintain these recycled composites above metallic alloys in terms of specific stiffness and strength. The second option is to introduce a re-sizing step to potentially restore their original performance.