Chemical recovery of carbon fibers from composites via plasma assisted solvolysis

In this work plasma assisted solvolysis using nitric acid is proposed for chemical recovery of carbon fibers from carbon fiber-reinforced epoxy resin composites (CFRCs). Complete decomposition of the epoxy matrix could be achieved, regardless the composites’ geometry. The efficiency of the process was examined in terms of a) process duration, b) resin decomposition rate and c) materials properties. SEM and EDX data showed that the recovered fiber surfaces are almost free of epoxy resin residuals and their tensile strength was comparable to that of typical virgin carbon fiber. The process decomposition efficiency is very high without requiring additional organic solvents or high temperature / pressure, stressing the potential of this method for viable recycling of CFRCs.


Introduction
The demand for carbon fiber reinforced composites (CFRCs) is projected to reach 190 kilotons by 2050, as they present interesting combination of properties, strength, durability, high strength-to-weight ratios and corrosion resistance.CFRCs are extensively used in aerospace, automotive industries, wind turbine blades, and structural components.However, once cured and shaped, recycling at the end of their life cycle is very difficult because of the cross-linked nature of the resin.[1][2][3][4].
Therefore, the widespread application of CFRCs has brought a serious concern about their large amounts of waste, including leftover pieces, off-grade products, and end-of-service-life components.The current disposal methods are their storage to landfills or their combustion, but those solutions lead to severe environmental pollution and waste of resources [1].Furthermore, the cost of aerospace-grade carbon fibers currently ranges from €164 to €246 per kilogram, while in other applications such as industrial and wind energy the price ranges from €20 to €41 per kilogram.Thus, with the current solutions this valuable material is destroyed.
Recovery methods are mainly divided into three categories, i.e. mechanical, thermal and chemical recycling.The mechanical way includes crushing, shredding or milling the composites.The crushed composites are used as fillers or reinforcements to improve the mechanical properties of other materials.With this method, the recovered fibers are significantly damaged and long fibers cannot be recovered.On the other hand, chemical and thermal methods aim to the full recovery of the fibers and the removal of the polymer matrix.The thermal way is also known as pyrolysis and during this process, the organic matrix of the composite is decomposed through a heating process in a chemical inactive atmosphere.Thermal recycling can recover clean fibers and at the same time utilizes the resin into fuel.All thermal processes are typically very energy intensive and costly, often requiring temperatures over 400°C, and emitting harmful gases (CO2, CO etc.).Chemical recycling, also known as solvolysis, is a process that utilizes chemical reactions and is reported to cause lesser damage to the fibers, thus is claimed to be the most promising technology.Solvolysis, is the degradation and dissolution of the composite's resin into monomers, oligomers, or other substances by using solvents with or without catalyst, in order to retrieve non-defective carbon fibers [5][6][7][8].Three main solvolysis methods have been proposed: solvolysis using super and sub-critical fluids, alcoholysis and various wet oxidation methods.The unique features of super and sub critical liquids (water, organic solvents) enhance resin decomposition, therefore high-quality fibers can be recovered.However, the high pressure (>MPa) and temperature (>300 o C) needed for the process makes it rather energy-intensive and difficult for scaling up.Regarding the alcoholysis method, it is based on the solvolysis with organic solvents under alkaline catalysis (PEG-NaOH, K3PO4-ethanol, etc).Due to the lipophilicity of the epoxy resins, their degradation efficiency in organic solvents (mainly alcohols) is higher under catalysis.Alcoholysis, has intermediate operating conditions (100-200 0 C and 1-5 MPa) but the main disadvantages are the use of solvents with high boiling point, which can adversely affect the decomposition, and the high decomposition time [5,[8][9][10].
The wet oxidation methods are characterized by the use of high concentration acidic solutions (nitric acid, H2O2, etc) and low operating temperature on atmospheric pressure.Among the different acids that can be used, nitric acid has been demonstrated as the most effective oxidant.Nitric acid solvolysis of composites presents the highest decomposition rate while high quality fibers are recovered (strength retention up to 98 %).Although nitric acid enhances the decomposition process, the reported decomposition times are still high (20 -100 h), while environmental issues arise due to the production of nitrogen oxides and liquid wastes [5,11].
The approach of this work is to use plasma assisted solvolysis in order to increase the degradation rate of fiber reinforced epoxy resin composites.In particular, the use of nitrogen plasma inside concentrated nitric acid solution is proposed, as long as one can benefit from both typical nitric solvolysis and plasma chemistry.The decomposition of epoxy resins in nitric acid depends on the crosslinked network between the resin and the hardener of the composite.Most of the epoxy reinforced composites are either amino-cured or anhydrite-cured.The amino-cured epoxy resin is decomposed due to cleavage of the C-N bonds while the anhydrite-cured due to cleavage of the C-O bonds, both through hydrolysis.In both cases cleavage of the C-C bonds is the secondary reaction.Eventually when enough bonds have been cleaved, parts of the network start to detach [11,12].In the absence of plasma the reactive species that contribute to the resin decomposition are limited to: NO 2+, Η3Ο + , NO 3-, OH.When plasma is ignited in a nitric acid solution a plethora of reactive species are produced (NO2 + , Η3Ο + , OH • , H + , H • , NO2 • , H2O2, O •+ , HO •-, H2, O2, NO3 -, OH -, NO2 - , etc) which can enhance the resin degradation.Moreover, as the gas diffuses in the liquid phase, the bubbles explode near/on the composite surface accelerating the resin breakdown [11,13].
It is worth noting that even if the polymer matrix is completely broken down, resin fragments may remain on the recovered fibers surface.Thus, a second step of mild oxidation has been proposed using acetone -hydrogen peroxide mixtures [11].Acetone can dissolve the organic impurities detached to the fibers, while hydrogen peroxide produces oxygen and hydroxyl radicals that can oxidize the organic derivatives.
The aim of this work is to examine the efficiency of plasma enhanced solvolysis of carbon fiber composites in terms of the process time and the recovered fibers properties.In particular, three different types of composites typically used for aerospace, turbine wind blades and hydrogen storage were tested.The physicochemical properties of the recovered fibers were examined using Scanning Electron Microscopy (SEM) -Energy Dispersive X-Ray analysis (EDX) while the mechanical properties were evaluated according to ASTM D3379.Finally, the necessity of post treatment oxidation using C3H6O -H2O2 mixture was also evaluated [11,14].

Flow chart of the process
Figure 3 illustrates the flow chart of the plasma enhanced solvolysis process proposed in this work.Initially, the composite is pre-treated by immersion in a low concentration HNO3 solution (1-3 M) for 2 days, then the plasma assisted solvolysis in a high concentration HNO3 solution (14 M) takes place.The carbon fibers are mechanically collected after the end of the treatment, and they undergo ultrasonic bathing in a C3H6O -H2O2 mixture.The liquid waste remains in the reactor and small amount of H2O2 is added in order to regenerate concentrated HNO3 solution and reuse it in the following process cycles.The flue gas is directed to a wet scrubber where NOx is partially converted to HNO3 for reducing NOx wastes.When the HNO3 concentration of the scrubbing liquid reaches a value between 1-3 M, the liquid is used as the pre-treatment solution.Overall, this closed loop is centered in the optimization of the plasma assisted solvolysis step and the reduction of NOx emissions.

Plasma reactor setup
The plasma reactor setup consists of a cylindrical stainless-steel electrode through which 1 slm nitrogen gas flows into the system, a borosilicate container in which the composite and the nitric acid are placed and a stainless-steel plate electrode which is in contact with the container (figure 4).The cylindrical electrode is biased by a 30 kHz AC generator, while the plate electrode is grounded.In addition, the energy supply system includes a voltage amplifier, an oscilloscope and two probes for the electrical characterization of the discharge.The input power of the system was equal to 500 W. In all cases, the proportion between the composite mass and the amount of HNO3 inside the reactor is 1 g composite: 10 ml HNO3, while the solution temperature is always below 100 0 C.

Characterization of the recovered fibers
The recovered fibers were characterised by means of SEM-EDX.The SEM images provided information about the morphology of the fibers' surface and diameter while the presence of resin residuals on the fibers was evaluated by EDX analysis.Moreover, in order to evaluate the mechanical properties of the recovered materials, single fiber mechanical tests were performed according to ASTM D3379.

Results and discussion
In order to estimate the efficiency of plasma enhanced solvolysis on the degradation of different materials, specimens of the three composite types were treated up to complete matrix dissolution and fibers detachment.Regardless of the raw material, the process time was less than 180 min which is much shorter than the periods reported for conventional HNO3 solvolysis, indicating that plasma strongly enhances the decomposition process.However, the time required for complete dissolution differs between the composite types.Figure 5 illustrates the process time for each sample divided by the composites initial volume.The degradation of amine cured samples (Wind turbine blade scrap and aerospace scrap) are favored, compared to the anhydrite -cured composite (hydrogen storage material).Moreover, the resin/fiber ratio and the fibers arrangement in the raw material is estimated to have a secondary role on the decomposition of the materials.Specifically, composites with lower resin weight percentage seem to degrade faster, while the dissolution of the rich in resin hydrogen storage material is unfavorable.Moreover, the fibers were characterized by means of SEM-EDX.Firstly, the necessity of the posttreatment oxidation with C3H6O -H2O2 mixture was examined.Figure 6 illustrates the morphology and the atomic carbon content of the wind turbine blade fibers after the plasma assisted solvolysis (figure 6 a) and after their post-treatment with C3H6O -H2O2 mixture (figure 6 b).In both cases, minimal surface damage and polymer residuals are observed.The diameter of the recycled fibers (rCFs) is calculated ~ 7 μm which is equal to the virgin fibers (vCFs) and the atomic carbon content is calculated over 90%.As long as, no significant differences are detected between the two samples, the post oxidation of the rCFs could be skipped.Figure 7 illustrates the SEM-EDX results of the rCFs of the aerospace scrap.The fibers surface presents some defects and resin residuals are also detected.The EDX analysis revealed sulfur traces which are attributed to the hardener of the component (dapsone -C12H12N2O2S) while the carbon content of the material is up to 93%.The diameter of the rCFs is calculated ~ 5.2 μm which is equal to the vCFs.Figure 8a illustrates the rCFs from the hydrogen storage material and the vCFs (figure 8b) which were used for the material fabrication.The surface of the rCFs is slightly damaged but no resin residuals can be detected.It is worth noting that the carbon content of the recovered material was calculated equal to 90%, 1% less than that of the virgin fibers (91%).The diameter of the fibers is also calculated to 7 μm which is similar to the calculated size of the vCFs.Thus, in all cases no significant surface damage was detected and the calculated diameters of rCFs and vCFs were very close.However, significant resin residuals were detected on the fibers of the aerospace scrap.Single fiber mechanical tests took place according to ASTM D3379 for the rCFs from the aerospace scrap and their properties were compared to the properties of vCFs with similar diameter.The results are presented in figure 9.The recovered fiber has 0.0134 strain at break which is 74% of typical vCFs.The tensile strength is 3.584 GPa which is about 70% of vCFs and the Young's modulus is 268.4GPa which is about 95% of vCFs.The mechanical properties of the rCFs were comparable to the properties of vCFs indicating minimum damage of the fibers during the recycling process.Τhe damage of the fibers is minimum, therefore the degradation of their properties possibly occurs due to the alteration of their surface chemistry, since nitro and amino groups are added on the carbon fiber surface during the process [14].

Conclusions
In this work plasma assisted solvolysis was proposed for carbon fibers recovery from composites.Three different types of composites, typically used for wind turbine blade, aerospace and hydrogen storage, were tested as raw materials, while the process efficiency was estimated in terms of the process time and materials properties.
In all cases the time needed for the matrix dissolution and complete detachment of the fibers was significantly shorter than the time reported for conventional HNO3 solvolysis.However, the type of the cross-linked network and the fibers/resin ratio of the composite affect the process time, while the resin type seems to be a non-factor.In particular, the degradation of the amine cured composites (Wind turbine blade scrap and aerospace scrap) is favoured, compared to the anhydrite cured material (hydrogen storage material).SEM -EDX analysis revealed minimal to slight damage of the recovered fibers surface while significant resin residuals were detected on the surface of the aerospace material.In all cases the calculated diameters of the rCFs were similar to the vCFs.
Finally, the recovered fibers showed acceptable degradation of their mechanical properties, which is comparable to the other chemical methods of recovery.

Figure 1 .
Figure 1.Recycling methods of carbon fiber reinforced composites.

Figure 3 .
Figure 3. Flow chart of the process.

Figure 5 .
Figure 5. Treatment time per composite volume for different composites.

Figure 6 .
Figure 6.SEM-EDX characterization of rCFs from the turbine blade scrap a) after plasma assisted solvolysis and b) after plasma assisted solvolysis and bath with C3H6O /H2O2.

Figure 7 .
Figure 7. SEM-EDX characterization of rCFs from the aerospace scrap after plasma assisted solvolysis and bath with Acetone/H2O2.

7th
International Conference of Engineering Against Failure Journal of Physics: Conference Series 2692 (2024) 012017

Figure 8 .
Figure 8. SEM-EDX characterization of a) rCFs from the hydrogen storage material after plasma assisted solvolysis and b) vCFs which were used for this material.

Figure 9 .
Figure 9. Mechanical properties of rCFs from the aerospace scrap compared to the properties of vCFs with the same diameter (5.2 μm), according to single fiber mechanical tests (ASTM D3379).