A new strategy of reusing abandoned carbon fiber reinforced plastic: Microstructures and properties of C/C composites based on recycled carbon fiber

Abstract

A new strategy of recycling and reusing abandoned carbon fiber reinforced plastics (CFRP) is proposed: CFRPs are first fully carbonized to CF reinforced carbon (C/C) preforms, and then are manufactured into high value-added C/C composites. The results showed that the carbon residue rate of epoxy-resin (EP) matrix was fully recovered as the decomposition route of EP matrix was changed by charring agent. The recycled CF (rCF) was not markedly oxidized or thermally damaged, and possessed comparable properties with those of the virgin CF (vCF) after pyrolysis. The pyrolytic char had no obvious negative effect on the densification efficiency of the rCF reinforced carbon (rCF/C) composites. Both of the rCF/C and vCF reinforced carbon (vCF/C) composite bodies were quite dense, and exhibited almost no difference in their microstructures. The rCF/C and vCF/C composites therefore had quite close interface bonding strength (12.6 MPa and 13.0 MPa, respectively), and bending strength (106.4 MPa and 111.5 MPa, respectively). Furthermore, the rCF/C composites possessed comparable ablative rate with that of the vCF/C composites. The rCF/C composites derived from abandoned CF/EP composites present a great potential to be used as substitutes for vCF/C composites owing to their indistinguishable properties.

Keywords

Recycling carbon fiber C/C composites carbon fiber reinforced plastics

Introduction

Due to the outstanding properties such as specific strength, specific modulus, designable, and corrosion resistance, the carbon fiber reinforced plastics (CFRP) have received much recognition in aerospace, automotive, and energy sector applications. It was reported that the annual global demand for carbon fiber (CF) was anticipated to be from approximately 16,000 to 72,000 tonnes, and expected to exceed 140,000 tonnes by 2020.^1–4 With a steady growth in the usage of CFRP across the globe, the amount of unqualified products in the production process and waste that was generated at the end of the service life have increased dramatically. To realize the sustainable development of CFRP industry, recycling and reusing abandoned CFRP has become a priority issue.

To reduce the waste of resources and the negative impact of abandoned CFRP on the environment, many recycling CFRP processes have been proposed (mainly including grinding,^5–8 thermal processes,^9–13 and solvolysis^14–18). Among all recovery methods, pyrolysis is considered to be the most promising method owing to it outstanding advantages of low energy demand, cost effectiveness, and relatively good mechanical properties of the resulting recycled CF (rCF).¹³ The aim of the pyrolysis process is to separate the CF and the plastic matrix, and to obtain rCF with good mechanical properties as well as clean surface, so as to restore the rCF to the virgin CF (vCF) properties to the greatest extent. However, an irreconcilable contradiction arises between the amount of residual pyrolytic carbon and the mechanical strength of the rCF: if inert atmosphere is used in the pyrolysis process, rCF will not be damaged by high temperature oxidation, and the degradation of rCF mechanical properties can be avoided, but a layer of pyrolytic char from plastic matrix will inevitably adhere to the surface of rCF, which may worsen the mechanical properties of the re-prepared CFRP with new resin. On the contrary, once the oxidation atmosphere is used in the pyrolysis process, clean rCF can be obtained but its mechanical strength must be sacrificed.⁸,⁹ At present, most studies tend to use oxidation atmosphere in pyrolysis and the reasons are as follows: on the one hand, to make rCF fully dispersed without reunion, so as to facilitate subsequent reuse. On the other hand, clean surface of fiber is a necessary condition to obtain high bond strength interface in re-fabricated CFRP, hence the plastic carbon should be removed as much as possible.^{9–13,19–22} Actually, although the waste CFRP has been recycled efficiently, the problems caused by pyrolysis are extended to the reuse stage of re-fabricating CFRP.

The CF recovered from CFRP by pyrolysis has been widely considered for re-preparing CFRP with new resin. Most often rCF is in disorderly, fluffy, and discontinuous after recovery treatment.²³ Therefore, rCF is first incorporated into random discontinuous fiber composites such as bulk molding compound (BMC) and sheet molding compound (SMC).^24–27 Unfortunately, the mechanical properties of re-prepared composites based on rCF are not comparable with those of vCF controlled, even it can be said that there is a big gap between them. For instance, Elghazzaoui et al.²⁵ used rCF as they were without further treatment to fabricate BMC. The resulting composites showed inhomogeneity in fiber distribution and thus in the thickness of composites, leading to a large number of matrix pore and interface defects in composites. The ultimate tensile strength and flexural modulus of the re-fabricated composites decreased by 5% and 15% than those of the vCF controlled group, respectively. To reuse rCF in high value, researchers turned to believe that the fiber reforming and reweaving is a good means since the reformed veil can make the rCF-based composites increase in both tensile stiffness and strength.^23,28–30 However, in general, the current remanufactured rCF-based composites are not recommended for application in structural materials because of their poor reliability and repeatability, which also may be the reason why rCF-based composites so far are not available in the market.²³

The recovery of abandoned CFRP by pyrolysis results in pyrolytic oil as well. Commercially, the pyrolytic oil is available for reuse, but some thorny issues exist in practical engineering application. The pyrolytic oil recovered from the degradation of plastic is mainly phenolic and acid derivatives, such as phenol, resorcinol, and phthalic.³¹ Theoretically, these monomers could be reused as pre-polymer to manufacture new plastics. However, this path was judged not viable because the pyrolytic oil contains a great deal of complex compounds, which cause a wide range of boiling point distributions.¹³ Consequently, difficulty arises in separating the pyrolytic oil based on their boiling temperature, and also leads to a sharp rise definitely in the recovery cost. Actually, the researchers considered that the energy recovery thus far may be the most appropriate solution that reuses the pyrolytic oil efficiently.⁸ However, the gross calorific value of the pyrolytic oil (32,000–37,000 kJ·kg⁻¹) is lower than that of conventional hydrocarbon liquid fuels owing to the high oxygen content.^31,32 The pyrolytic oil was blended to petrol and was attempted to be used in daily life, but with the closed-cup flashpoint of the mixed oils below the limits specified (legislation in US or UK), that is, users would have to assume extra safety responsibility.³¹,³² At present, the reuse of CFRP matrix is still in its infancy, and there is an urgent need for more innovative paths.

From overview above, great efforts were made on recycling and reusing both the plastic matrix and CF of abandoned CFRP. Although much work has been done, the development in the field of CFRP recycling is still far from the level of industrialization. Therefore, more novel strategies should be constantly tried for reducing waste of resources and reusing recyclates effectively, ultimately realizing the sustainable development of CFRP industry. In this work, a new strategy of recycling and reusing of CFRP is proposed, to be specific, the abandoned CFRPs are first fully carbonized to CF-reinforced carbon (C/C) preforms, and then are re-manufactured into high value-added C/C composites. This method completely distinguishes from the conventional one: to begin with, the plastics matrix in CFRP will not be removed, on the contrary, it will be retained to a maximum extent in the form of pyrolytic carbon with the assistance of charring agent under argon atmosphere. Furthermore, the as-received rCF with rich pyrolytic carbon will not be fabricated into new CFRP, but into high value-added C/C composites. The CF-reinforced epoxy resin (CF/EP) composites are used as the research object as they are widely used across the world. It was reported that the phosphazene compounds, such as hexaphenoxy-cyclotriphosphazene (HPCTP), can significantly improve the carbon residue rate of EP,³³,³⁴ and it is used as a charring agent for EP in this work. The overall framework of this work is as follows: the charring mechanism of EP under the action of HPCTP is first analyzed, and then the mechanical properties and microstructures of rCF are investigated in detail. Furthermore, the microstructures, mechanical properties, and ablation resistance of rCF-reinforced carbon (rCF/C) composites are characterized and tested systematically. The vCF-reinforced carbon (vCF/C) composites are fabricated in the same way and acted as reference materials for comparing the properties of rCF/C composites. This is the first time in the open literature that the abandoned CFRPs are recycled and reused in this way.

Materials and methods

Materials

The density of the abandoned CF/EP composites is 1.24 g·cm⁻³ with CF needled felts (PAN-based T300, Toray, Japan) of 43.51 wt% and EP of 56.49 wt%. The number-average molecular weight of furfural–acetone resin is ∼750 g·mol⁻¹ that was offered by Shijiazhuang Shiyi Furfural Resin Co., Ltd (China). The HPCTP (industrial grade) was purchased from Shanghai Jizhi Biochemical Technology Co., Ltd (China).

Methods

The process flowsheet of recycling CF/EP by carbonization and preparation of rCF/C composites are shown in Figure 1. First of all, the weighed CF/EP composites were cut into 100 mm × 50 mm blocks and pre-pyrolyzed with the following conditions: temperature, 25°C to 350°C; heating rate, 10°C·min⁻¹ and maintained at 350°C for 5 min; atmosphere, argon. After the pre-pyrolysis treatment, the dense CF/EP was transformed into porous materials and the weight of the residual EP matrix was around 80% of that before pre-pyrolysis. Secondly, the as-received porous CF/EP composites were impregnated into HPCTP molten liquid at 150°C for 10 min and cooled to room temperature. In this step, the HPCTP molten liquid was infiltrated into the porous CF/EP under the action of capillary. Thirdly, the porous CF/EP composites with rich HPCTP were pyrolyzed into rCF/C preforms: temperature, 25°C to 800°C; heating rate, 10°C·min⁻¹ and maintained at 800°C for 60 min; atmosphere, argon. Lastly, the as-obtained rCF/C preforms were densified into high-density rCF/C composites by impregnated-pyrolysis method (four cycles) using furfural–acetone resin: temperature, 25°C to 1000°C; heating rate, 2°C·min⁻¹ and maintained at 1000°C for 60min; atmosphere, argon.

Characterizations

Thermogravimetric analysis (TGA) was used to study the thermal decomposition process and residue rate of the EP, HPCTP, and EP/HPCTP. The CF structure was investigated by a DXR Raman spectroscopy. Archimedes’s method was used for measuring the apparent densities and porosity of composites. A Testometrix Micro 350 testing for the mechanical properties of the CF: CF gage length, 25 mm; tensile rate, 0.5 mm·min⁻¹; sample number, 40. A TESCAN MAIA3 scanning electron microscope was used for viewing the microstructures of CF and C/C composites. The interfacial shear strengths (τ) between the carbon matrix and the fiber were tested by a Brooke Hysitron Ti 980 atomic force microscope single-fiber push-out: sample thickness: ∼100 μm; flat-end indenter tip displacement rate: 30 nm·s⁻¹. More than eight push-out tests were carried out on selected CFs until fiber complete debonding. τ can be obtained by equation (1)³⁵

τ = \frac{P}{2 π r_{f} L}

(1)

where P is the load at push-out, r_f is the fiber radius, and L is the fiber length.

A three-point-bending test was carried out for investigating the mechanical properties of C/C composites through WDW-100 machine with samples size of 50 mm × 4 mm × 4 mm, span of 40 mm, and crosshead speed of 0.5 mm·min⁻¹. The mass and linear ablation rates of C/C composites were carried out by oxygen–acetylene with ablation temperature/time of 1400°C/120 s.

Results and discussions

Thermogravimetric analysis

The EP, HTCTP, and EP/HPCTP (mass ratio 8:2) were detected by TGA for exploring the basic thermal decomposition process of the EP, and the ability of HPCTP to increase the EP carbon residue rate, Figure 2 plots the results. It can be seen that the onset of decomposition of EP begins with an evident weight loss at around 325°C, and then a single decomposition step is followed in range of 325–510°C. An almost completion of decomposition is detected during further heating (over 510°C) and the EP constant weight is 9.7%, strongly suggesting that the EP has a low carbon residue rate. For the HPCTP curve, the initial thermal decomposition is about 344°C, followed by a severe weight loss within 344–420°C, and then a final residual weight rate only 2.1% is detected at 800°C, indicating that HPCTP possesses a very low carbon residue rate. The EP/HPCTP exhibits a similar curve pattern with that of the EP and HPCTP, but the weight of EP/HPCTP at 800°C is increased to 24.8%, which is much higher than that of EP (9.7%) and HPCTP (2.1%), implying that HPCTP can effectively form a carbonization system with EP, and increase the charring yield of EP by interaction with each other.

According to the TGA results above and available literature,³⁴,³⁵ the reasons why the carbon residue rate of EP is increased substantially by HPCTP are as follows (Figure 3): in the beginning, the HPCTP decomposes in the process of heating and forms a kind of phosphate active molecules ( ${PO}_{x}^{θ}$ ), which are easy to catch H⁺ in the complex decomposition process and form polyphosphoric acid with strong dehydration ability. Secondly, the high-viscosity polyphosphoric acid adheres to the EP surface firmly at high temperature and interacts with –O–, which is broken and removed in the form of H₂O by the action of the acid. Lastly, the aromatization of EP molecular chain at the removal –O– sites and the retention of more carbon element in the form of pyrolytic carbon are occurred. Therefore, the residue rate of EP/HPCTP is obviously higher than that of EP without HPCTP.

Micromorphologies of rCF/C preforms

The micromorphologies of the obtained rCF/C preforms are shown in Figure 4. The rCF bundles are completely covered by an uneven layer of continuous pyrolytic char and are therefore bonded together tightly (Figure 4(a)). However, comparing with that of rCF bundle surfaces, the content of pyrolytic carbon inside the bundles is relatively less (Figure 4(b)). It can be very clearly observed that the large pores existing in the rCF bundles and the rCF are surrounded by discontinuous pyrolytic char (Figure 4(c)). Figure 4(d) shows that the pyrolytic char layer in the rCF bundles is very thin, and this ascribes to that the EP matrix was pyrolyzed to generate great quantities of small volatile organic compounds, some of which were captured by polyphosphoric acid and deposited to form a thin layer eventually. The micromorphologies above suggest that the obtained rCF/C preforms are rich in EP pyrolytic carbon, which would be beneficial to the subsequent rapid preparation of high-density rCF/C composites. Moreover, the pyrolytic char layer should not negatively affect the efficiency of densification owing to no closed-cell structures being formed in the received rCF/C preforms.

Structures and mechanical properties of rCF

The crystal of the rCF was examined through Raman spectra. Due to the effect of the carbon atomic lattice defect and C–C stretching vibration of graphite lattice in-plane, the CF possesses D-peak and G-peak.³⁶ In Raman spectra, the intensity (I_D and I_G) of these two peaks can reflect the crystal perfection (R) of CF, i.e. R = I_D/I_G. Generally, the larger the R value, the lower the crystal integrity of CF.³⁶

The Raman spectra of rCF and vCF are shown in Figure 5. As can be clearly seen, the D-peak and G-peak of the rCF fully resemble those of the vCF, strongly suggesting that the rCF was not significantly damaged after pyrolysis treatment. According to the intensity values of these two peaks, the R of the vCF is 0.8861, and rCF is 0.8715 and can be easily harvested, which indicates that the rCF keeps its original crystal structures well.

The mechanical properties of rCF and vCF were further investigated through single filament tensile. The uniaxial tension force–elongation curves of the rCF and vCF are shown in Figure 6. As can be seen, both of the uniaxial tension force–elongation curves of the rCF and vCF present a linear growth and brittle fracture behavior after reaching a maximum value. Moreover, it also can be observed that the force–elongation curves of these two types of CF present a certain degree of divergence.

For the sake of comparing the mechanical properties of the vCF and rCF intuitively, the uniaxial tensile strength (σ) and modulus (E) of the vCF and rCF were both calculated according to the corresponding fiber force and diameter, respectively, and the distribution of their σ and E are presented in Figure 7. As can be seen, the σ of the rCF is in a near-normal distribution, which is very similar to that of the vCF. From the results of the σ fitting probability distribution of CF tensile strength, it can be found that the most probable σ of rCF and vCF are 3285 MPa and 3344 MPa, respectively, suggesting that although the rCF was subjected to pyrolysis treatment, significant damage did not occur. Similarly, it can be found that the most probable E of the rCF and vCF are, respectively, 231 GPa and 237 GPa, further proving that the rCF is not evidently damaged and possesses comparable properties with that of the vCF.

Morphologies of C/C composites

Figure 8 shows the changes in the apparent density and porosity values of the rCF/C and vCF/C composites after each impregnation–pyrolysis cycle. It can be seen that the density of the rCF/C preforms (0.76 g·cm⁻³) are evidently higher than that of the vCF (0.55 g·cm⁻³) owing to massive pyrolytic char preserved from the EP matrix. The densities of the rCF/C preforms increased to 1.18 g·cm⁻³, 1.31 g·cm⁻³, 1.42 g·cm⁻³, and 1.48 g·cm⁻³ after each cycle of densification, while those of the vCF/C preforms increased correspondingly to 0.95 g·cm⁻³, 1.25 g·cm⁻³, 1.39 g·cm⁻³, and 1.50 g·cm⁻³, strong implying that there is no significant distinction in densification efficiency between the rCF/C and vCF/C composites, that is, evident negative effect of pyrolytic char does not occur on the densification rates of rCF/C preforms. On the contrary, the EP pyrolytic char can even increase the densification efficiency at the initial stage of the rCF/C composites densification process.

The photographs of the rCF/C and abandoned CF/EP composites are shown in Figure 9. As can be seen, the as-obtained rCF/C composites (Figure 9(a)) basically keeps the size of the original CF/EP (Figure 9(b)), indicating that CF/EP was recycled in near full size. In addition, no obvious deformation is found in the needled structure of CF felt in the rCF/C composites (Figure 9(c)). This ascribes to that the 2.5D needled structure of CF felt hinders its severe expansion during pyrolysis, and the great quantity of pyrolytic char generated from EP is retained to form a carbon layer, which acts as a matrix to fix the needled structure of rCF felt.

The microstructures of the rCF/C and vCF/C composites are illustrated in Figure 10. As can be seen, both of the rCF/C (Figure 10(a)) and vCF/C (Figure 10(b)) composites are very dense and the apparent large size pore defects are not found either among or inside the CF bundles, and there are no evident differences between the rCF/C and vCF/C composites found from their overall micromorphologies. The pores between fibers in the rCF (Figure 10(c)) and vCF (Figure 10(d)) bundles are fully filled with matrix carbon, and the carbon matrix is tightly attached to the fiber surface. From the angle of CF section, it can be clearly observed that the CF is surrounded by carbon matrix and there is no apparent pores found in both of the rCF/C and vCF/C composites (Figure 10(e) and (f)), indicating that these two types of C/C composites have been fully densified and the pyrolytic carbon from EP matrix has no negative effect on the microstructures of rCF/C composites. As is known from Figure 4, great quantities of pyrolytic char transformed from EP were retained, but the pyrolytic carbon seems to have disappeared in the resulting rCF/C composites in Figure 10. The reasons for this phenomenon may be as follows: the pyrolytic char generated from EP is of glass carbon and the same goes for furfural–acetone resin, and hence these two types of carbon although derived from different resins, they are the same stuff. During the impregnation–pyrolysis, the EP char and furfural–acetone carbon in rCF/C composites may combine with each other to form a continuous whole. The microstructures of the rCF/C hence resemble that of vCF/C composites, as exhibited in Figure 10.

Mechanical properties of C/C composites

To investigate the effect of EP pyrolytic char on the interfacial strengths of rCF/C composites, the interfacial shear strengths (τ) between the carbon matrix and fiber were tested using single-fiber push-out method. The load–displacement curves of single-fiber push-out of rCF/C and vCF/C composites are shown in Figure 11(a) and (b). As observed, the load–displacement curves of rCF/C and vCF/C composites exhibit quite similar four segments: near-linear growth (I), platform (II), nonlinear growth (III), and near-linear dramatic decrease (IV). Wherein, the maximum debonding load (P) of the composites interface appears between I and II segments (at the intersection of tangents I and II segments). It can be seen that P ranges from 30 to 40 mN for both rCF/C and vCF/C composites, suggesting that the interface bonding strength (τ) values of rCF/C and vCF/C composites are pretty close. To compare the τ of rCF/C and vCF/C composites more intuitively, the τ values are calculated through equation (1) and the distributions of the obtained τ values are drawn based on the Weibull model,³⁷ as shown in Figure 11(c). It can be seen that both of the rCF/C and vCF/C composites τ values show a near-normal distribution, and most τ values of rCF/C and vCF/C composites overlap. Moreover, the most probable τ values of rCF/C and vCF/C composites are pretty approximate, 12.6 MPa and 13.0 MPa, respectively. This result reveals that although the pyrolytic char derived from EP has a certain degree of negative impact on the interface of rCF/C composites, the effect is very limited. It can be predicted that the macromechanical strength of rCF/C and vCF/C composites would be close. As is evidenced in Figure 11(d), the fiber is debonded from the carbon matrix after the push-out test (rCF/C composites), without cracking the fiber or matrix.

Figure 12 shows the curves of bending stress–displacement of rCF/C and vCF/C composites. As can be seen, both the curves of rCF/C and vCF/C composites present a quasi-linear increase in the initial segment and an elastic region is followed by an increasing near-linearly stress up to a maximum. In the failure section, the bending stress decreases sharply, strongly implying that these two types of C/C composites experience a brittle fracture behavior. The flexural strength of rCF/C composites is 106.4 MPa, which is very close to the bending stress of vCF/C composites, which is 111.5 MPa, implying that they have a similar microstructures.

Figure 13 shows the microstructures of fracture surfaces of rCF/C and vCF/C composites after three-point bending test. It can be observed that a great amount of fiber is pulled out on the fracture surfaces of rCF/C composites (Figure 13(a) and (b)), and the pulled-out fiber has relatively smooth surfaces, whereas the bending test result showed that rCF/C composites exhibited a brittle fracture behavior as seen in Figure 12, indicating that the fiber was rapidly pulled out from the carbon matrix, i.e. the interfacial strength between rCF and carbon matrix is relatively weak. Similarly, substantial pulled-out fiber is found on the fracture surfaces of vCF/C composites just like those of rCF/C composites, but it can be observed that a small quantity of irregular microparticles adhering to the surfaces of the vCF were distinguished from that of the rCF control group, suggesting that vCF/C composites have a slightly higher interface strength than that of rCF/C composites. The reasons may ascribe to that the pyrolytic char decomposed from the EP matrix is in disordered lamellar structures, and these structures can easily lead to tiny bubbles retained in rCF/C composites during the impregnation–pyrolysis process. Therefore, pore defects are easy to be generated in rCF/C composites, and eventually resulting in a weaker interface. This conclusion corresponds to the interfacial bonding strength test in Figure 11 as well.

Ablation resistance of C/C composites

rCF/C composites were first considered for ablative material, so their ablation resistance is investigated in detail. The mass and line ablation rates of rCF/C and vCF/C composites are shown in Table 1. It can be seen that the mass and line ablation rates of rCF/C composites are, respectively, 21.6 mg·s⁻¹ and 0.36 mm·s⁻¹, while those of vCF/C composites are 18.7 mg·s⁻¹ and 0.29 mm·s⁻¹, suggesting that the ablation resistance of rCF/C composites is slightly worse than that of vCF/C composites. This is principally because the fact that rCF/C composites have more pore defects (see Figure 11), which make the oxidation molecules more likely to invade the composite bodies, and lead to a more active oxidation ablation sites in rCF/C composites. However, their ablation values are at the same level, revealing that there is no essential difference between these two materials.

Table 1.

Mass and line ablation rates of the rCF/C and vCF/C composites.

Sample	Mass ablation rate (mg/s)	Linear ablation rate (mm/s)
rCF/C	21.6	0.36
vCF/C	18.7	0.29

rCF/C: recycled carbon fiber reinforced carbon; vCF/C: virgin carbon fiber reinforced carbon.

The microstructures of rCF/C and vCF/C composites after ablation are shown in Figure 14. The surface of C/C composites inevitably react with the oxidizing atmosphere in the air owing to the C/C composites beinge heated to 1400°C in a very limited amount of time, and the ablation often occurs first at the composite pore defects. Therefore, the matrix of rCF/C (Figure 14(a)) and vCF/C (Figure 14(b)) composites is consumed preferentially leaving a complete CF felt architecture. With the continuous ablation, the carbon matrix within the fiber bundles is also rapidly consumed, and the CF is gradually involved in the ablation reaction. In this stage, the fiber in rCF/C and vCF/C composites is gradually washed into needle-like shape. These microstructures imply that there is no essential difference between rCF/C and vCF/C composites in organizational structure, i.e. vCF/C composites can be replaced by rCF/C composites as ablation material completely.

Conclusions

In this work, a new strategy of recycling CF/EP composites by carbonization and being reused to fabricate C/C composites is proposed. Owing to the decomposition route of EP was changed by HPCTP, the carbon residue rate of EP was increased from 9.7% to 24.8%, and the density of the obtained rCF/C preforms are therefore increased from CF felt of 0.55 g·cm⁻³ to 0.76 g·cm⁻³. The rCF was not markedly oxidized or thermally damaged, and kept comparable properties to those of the vCF. The preparation process of C/C composites suggested that there was no significant distinction between the rCF/C and vCF/C composites in densification efficiency, and even the density of the rCF/C was slightly higher than that of vCF/C composites in the initial stage due to EP was fully converted to pyrolytic char. Both of the rCF/C and vCF/C composite bodies were quite dense, and the microstructures of rCF/C composites were quite similar to those of vCF/C composites. Because of the pyrolytic char had a very limited impact on the microstructures of the rCF/C composites, the τ of the rCF/C and vCF/C composites are pretty approximate, 12.6 MPa and 13.0 MPa, respectively. Their bending strengths were at a same level as well, 111.5 MPa for the vCF/C and 106.4 MPa for the rCF/C composites. Moreover, the ablation testing results showed that rCF/C composites had comparable ablative rate with that of the vCF/C composites. The rCF/C composites derived from abandoned CF/EP composites possess an enormous potential to be used as substitutes for the vCF/C composites owing to their indistinguishable properties. This work opens up a new path for recycling and reusing of abandoned CFRPs in high value.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Wenjian Guo

Shuxin Bai

References

Witik

Teuscher

Michaud

, et al. Carbon fibre reinforced composite waste: an environmental assessment of recycling, energy recovery and landfilling. Compos Part A: Appl Sci Manuf 2013; 49: 89–99.

Nahil

Williams

PT.

Recycling of carbon fibre reinforced polymeric waste for the production of activated carbon fibres. J Anal Appl Pyrol 2011; 91: 67–75.

Broekel

Scharr

The specialities of fibre-reinforced plastics in terms of product lifecycle management. J Mater Process Technol 2005; 162: 725–729.

Kraus

Kühnel

Composites Market Report 2014 Market developments, trends, challenges and opportunities. The Global CRP Market, 2014.

Palmer

Savage

Ghita

, et al. Sheet moulding compound (SMC) from carbon fibre recyclate. Compos Part A: Appl Sci Manuf 2010; 41: 1232–1237.

Kouparitsas

Kartalis

Varelidis

, et al. Recycling of the fibrous fraction of reinforced thermoset composites. Polym Compos 2002; 23: 682–689.

Palmer

Ghita

Savage

, et al. Successful closed-loop recycling of thermoset composites. Compos Part A: Appl Sci Manuf 2009; 40: 490–498.

Pickering

SJ.

Recycling technologies for thermoset composite materials-current status. Compos Part A: Appl Sci Manuf 2006; 37: 1206–1215.

Meyer

Schulte

CFRP-recycling following a pyrolysis route: process optimization and potentials. J Compos Mater 2009; 43: 1121–1132.

10.

Lester

Kingman

Wong

, et al. Microwave heating as a means for carbon fibre recovery from polymer composites: a technical feasibility study. Mater Res Bull 2004; 39: 1549–1556.

11.

Zheng

Shen

, et al. A novel approach to recycling of glass fibers from non-metal materials of waste printed circuit boards. J Hazard Mater 2009; 170: 978–982.

12.

López

Rodríguez

Alguacil

, et al. Recovery of carbon fibres by the thermolysis and gasification of waste prepreg. J Anal Appl Pyrol 2013; 104: 675–683.

13.

Pimenta

Pinho

ST.

Recycling carbon fibre reinforced polymers for structural applications: technology review and market outlook. Waste Manage 2011; 31: 378–392.

14.

Bai

Wang

Feng

Chemical recycling of carbon fibers reinforced epoxy resin composites in oxygen in supercritical water. Mater Design 2010; 31: 999–1002.

15.

Piñero-Hernanz

García-Serna

Dodds

, et al. Chemical recycling of carbon fibre composites using alcohols under subcritical and supercritical conditions. J Supercrit Fluid 2008; 46: 83–92.

16.

Dang

Kubouchi

Sembokuya

, et al. Chemical recycling of glass fiber reinforced epoxy resin cured with amine using nitric acid. Polymer 2005; 46: 1905–1912.

17.

Oliveux

GLSE

Bailleul

JL.

Chemical recycling of glass fibre reinforced composites using subcritical water. Compos Part A: Appl Sci Manuf 2012; 43: 1809–1818.

18.

Liu

Jiang

, et al. Chemical recycling of carbon fibre reinforced epoxy resin composites in subcritical water: synergistic effect of phenol and KOH on the decomposition efficiency. Polym Degrad Stabil 2012; 97: 214–220.

19.

Meng

Olivetti

Zhao

, et al. Comparing life cycle energy and global warming potential of carbon fiber composite recycling technologies and waste management options. ACS Sustain Chem Eng 2018; 6: 9854−9865.

20.

Meng

McKechnie

Pickering

SJ.

An assessment of financial viability of recycled carbon fibre in automotive applications. Compos Part A: Appl Sci Manuf 2018; 109: 207–220.

21.

Meng

McKechnie

Turner

, et al. Energy and environmental assessment and reuse of fluidised bed recycled carbon fibres. Compos Part A: Appl Sci Manuf 2017; 100: 206–214.

22.

Meng

McKechnie

Turner

, et al. Environmental aspects of use of recycled carbon fiber composites in automotive applications. Environ Sci Technol 2017; 51: 12727–12736.

23.

Oliveux

Dandy

Leeke

GA.

Current status of recycling of fibre reinforced polymers: review of technologies, reuse and resulting properties. Prog Mater Sci 2015; 72: 61–99.

24.

Wong

Pickering

Rudd

CD.

Recycled carbon fibre reinforced polymer composite for electromagnetic interference shielding. Compos Part A: Appl Sci Manuf 2010; 41: 693–702.

25.

Elghazzaoui

Contribution à l’étude de la dégradation des composites carbone/époxy par solvolyse dans l’eausubcritique et supercritique en vue de leur recyclage. PhD Thesis, Université de Nantes, France, 2012.

26.

Connor

ML.

Characterization of recycled carbon fibers and their formation of composites using injection molding. Master Thesis, North Carolina State University, USA, 2008.

27.

Szpieg

Development and characteristics of a fully recycled CF/PP composite. PhD Thesis, Luleå University of Technology, Sweden, 2011.

28.

Wong

Mohammed

Pickering

, et al. Effect of coupling agents on reinforcing potential of recycled carbon fibre for polypropylene composite. Compos Sci Technol 2012; 72: 835–844.

29.

Oliveux

Bailleul

Gillet

, et al. Recovery and reuse of discontinuous carbon fibres by solvolysis: realignment and properties of remanufactured materials. Compos Sci Technol 2017; 139: 99–108.

30.

Werken

Reese

Taha

, et al. Investigating the effects of fiber surface treatment and alignment on mechanical properties of recycled carbon fiber composites. Compos Part A: Appl Sci Manuf 2019; 119: 38–47.

31.

Cunliffe

Williams

PT.

Characterisation of products from the recycling of glass fibre reinforced polyester waste by pyrolysis. Fuel 2003; 82: 2223–2230.

32.

Torres

de Marco

Caballero

, et al. Recycling by pyrolysis of thermoset composites: characteristics of the liquid and gases fuels obtained. Fuel 2000; 79: 897–902.

33.

Yang

Wang

Huo

, et al. Synthesis of phosphorus/nitrogen-containing compound based onmaleimide and cyclotriphosphazene and its flame-retardant mechanism on epoxy resin. Polym Degrad Stabil 2016; 126: 9–16.

34.

Leng

, et al. Synthesis of a novel flame retardant based on cyclotriphosphazene and DOPO groups and its application in epoxy resins. Polym Degrad Stabil 2016; 123: 105–114.

35.

Michael

Derek

Determination of band shifts as a function of strain in carbon fibers using Raman microline focus spectrometry (MIFS). Carbon 1993; 31: 1057–1064.

36.

Guo

Bai

, et al. A new strategy for high-value reutilization of recycled carbon fibre: preparation and friction performance of C/C-SiC brake pads based upon recycled carbon fibre felt. Ceram Int 2019; 45: 16545–11653.

37.

Naito

Inoue

Fukuda

The effect of surface modification with carbon nanotubes upon the tensile strength and Weibull modulus of carbon fibers. J Mater Sci 2012; 47: 8044–8051.