A pre-screening of the solvolysis recycling process for CFRPs based on the mechanical properties of the recovered fibers.

Over the past few decades, composite materials, and specifically carbon fiber reinforced plastics (CFRPs), are finding increasing use in the automotive, aerospace, and aeronautics industries. As a result, the production of CFRPs has been significantly increased, thus leading to a corresponding increase in waste production. In the near future, landfill and incineration disposal of waste will likely be prevented due to legislation, thereby bringing forward the need to develop efficient recycling processes for CFRPs. However, recycling of CFRPs is very challenging, mainly due to the difficulty in removing the thermosetting matrix. This paper reports a pre-screening of the solvolysis recycling process for CFRPs on the basis of the mechanical properties of the recovered fibers. To this end, solvolysis tests were conducted on unidirectional CFRP samples under supercritical and subcritical conditions using acetone and water. The solvolysis tests were conducted for various conditions of temperature, pressure, and reaction time. The efficiency of the recycling processes has been evaluated by means of single-fiber tension tests on the recovered fibers, which were conducted according to the ASTM C 1557-14 standard. In most cases, the decomposition efficiency of the epoxy resin in the CFRP, measured in terms of mass, ranged between 90 and 100%. Moreover, the mechanical tests showed that the recovered fibers retained more than 58% of their initial Young’s modulus and tensile strength.


Introduction
Carbon Fiber Reinforced Polymers (CFRPs) find extensive applications in industries like aerospace, aviation, automotive, and energy [1].CFRPs are favoured due to their remarkable specific strength, high modulus, and resistance to oxidation, often surpassing traditional, heavier metals.This material choice is associated with expected weight savings, leading to reduced fuel consumption and environmental benefits.However, CFRPs currently suffer from resource inefficiency, involving high energy demands during manufacturing and limited recycling options, resulting in a significant portion of composites ending up in landfills [2].The continuous rise in regulations, increasing landfill expenses and the growing desire for lightweight structures have intensified the need for finding methods for recycling composites and specifically thermoset CFRPs [3].Current recycling methods for CFRPs can be broadly classified into three main categories: mechanical, chemical and thermal methodologies, as shown in Figure 1.Mechanical methodology involves grinding the CFRP to convert them into particles that can be used again in the manufacturing section.Thermal methods rely on elevated temperatures required for the polymer matrix to undergo vaporization, leaving behind recoverable fibers.Chemical processes use solvents to chemically break down the polymers, preserving the fibers for future use [4].Mechanical and pyrolysis processes were the primary methods identified for treating composite waste.However, these methods have limitations when it comes to recovering carbon fibers with mechanical properties comparable to those of virgin carbon fibers [5].

Figure 1. Main methodologies of recycling.
In recent years, chemical degradation has emerged as the most promising method for recycling CFRPs, offering the potential to recover residual value from both fibers and chemicals [6].It is imperative to enhance chemical recycling technology to make it applicable across a broad spectrum of uses, in order to become an asset for resource recovery and environmental impact reduction.Promising solvents for chemical processing include water, acetone and alcohols.In particular, water and alcohols stand out due to their eco-friendliness, wide availability, cost-effectiveness, and low toxicity [7].For instance, water can be classified as an environmentally friendly reaction medium due to its ready availability, cost-effectiveness, and low potential for toxicity.As the primary solvent, water effectively breaks down the epoxy resin, releasing the virgin fibers under varying decomposition conditions [8].Also, supercritical acetone offers several advantages, notably its proximity to the epoxy resin's Hansen solubility parameter and its effective solvent capability for epoxy decomposition [9].Many researchers have investigated chemical recycling and especially solvolysis using water and acetone as solvents.Kim et al. [10] investigated solvolysis using water as a solvent (hydrolysis) in supercritical conditions and specifically at temperature 405 ± 2 °C, pressure 280 ± 10 bar and reaction times of 10, 30, 60, and 120 minutes.They observed that the elimination efficiency is over 99% when the reaction time was 120 minutes.Yuyan et al. [8] achieved a complete decomposition using water in subcritical conditions.Furthermore, the tensile measurements of individual fibers showed that the reclaimed fibers only experienced a 1.8% reduction in tensile strength compared to the virgin.Okajima and Sako [9] explored the removal of epoxy resin using acetone in superheated and supercritical conditions at 350°C, with decomposition efficiencies increasing with reaction time up to 60 minutes to a maximum of 95.6%.Sokoli et al. [11] investigated the degradation of hybrid fiber composites using near-critical water or supercritical acetone.Notably, under specific conditions, supercritical acetone demonstrated the ability to achieve nearly complete resin degradation.Conversely, carbon fibers were effectively retrieved with no significant loss in tensile strength when either water or acetone was used.
In the present work, CFRP samples were subjected to solvolysis experiments in supercritical and subcritical water and supercritical acetone.The experiments were carried out under varying conditions of temperature, pressure, and reaction duration.The loading rate (volume of solvent/reactor volume) was constant.The effectiveness of the chemical recycling procedures was assessed by measuring the efficiency of epoxy resin removal, by characterizing the morphology and by conducting single-fiber tension tests on the reclaimed fibers.

Materials
The CFRPs were supplied by KVE Composites Group ([0]8) and the unidirectional prepregs (Sigrapreg C U600-0/SD-E501-33%) for the CFRPs were manufactured by SGL Carbon.The carbon fibers (SIGRAFIL C T50-4.0/240-E100) had a filament diameter of 7.0μm and the epoxy resin used was E501.In order to conduct the solvolysis experiments, the specimens were cut into the dimensions of 30x50x4 mm (width x length x thickness) by using a milling cutter.

Solvolysis experiments
The experiments were conducted using a non-stirred batch reactor (Parr Instrument Company).The reactor's vessel was a high pressure/high temperature vessel of 500 mL total volume capacity.The sample was put into the vessel at a loading rate of 0.6 (volume of solvent/reactor volume).Initially, the system was slowly heated to reach the target temperature.Subsequently, after the predetermined duration of each experiment, the system was cooled down to room temperature through a cooling system (with the use of tap water).In the first series of experiments (#01-03), de-ionized water was employed under various conditions and in the last two experiments (#04-05) utilized acetone, as detailed in Table 1.
Table 1 Acetone 350 230 40 Following this elapsed period, the reactor was unsealed, and the fibers, along with the liquid fraction containing the dissolved organic products resulting from the degraded resin, were collected and retrieved.Stringent safety precautions, including the use of personal protective equipment (such as gloves and safety glasses) and adequate ventilation, were implemented throughout the experimental procedures.A fume hood cupboard was used for ventilation to actively protect the operator from inhalation of toxic vapors due to resin decomposition.Also, safety and emergency equipment, such as fire extinguishers and first aid kit, is available in the laboratory.

Characterizations 2.3.1 Decomposition resin efficiency.
The ratio of the decomposition efficiency of resin in the CFRP samples, measured in terms of mass.Represents the ratio of the resin mass separated from the fibers to the total mass of the resin before recycling, serving as a direct indicator of the recycling reaction's effectiveness.It can be calculated using the resin content of the sample, the mass of each sample before the solvolysis experiment, and the mass of solid residue after recycling [8,12].The rate of epoxy resin removal in the samples was assessed in each experiment through the application of the following formula: (1)

Morphological characterization.
The recycled fiber samples underwent thorough analysis using Scanning Electron Microscope (SEM) analysis.The SEM examination was performed using a Zeiss SUPRA 35VP model, employing a high vacuum mode with a 1.7 nm resolution at a 10 kV accelerating voltage.This analysis aimed to evaluate aspects such as fiber morphology, diameter, and the visual identification of any resin residues.

Tensile properties.
The efficiency of the recycling processes has been evaluated by means of single-fiber tension tests on the recovered fibers, which were conducted according to the ASTM C 1557-14 standard [13].Tensile loss and Young Modulus of the recycled fibres were measured specifically.The single carbon fiber filaments underwent a meticulous separation process and were temporarily attached to a thick paper mounting tab with a 25 mm gauge length.Once properly positioned and aligned, they were securely bonded using epoxy adhesive.Single Fiber Tensile tests were then conducted using a Miniature Materials Tester named Minimat 2000 fitted with a 200N cell, using a crosshead speed of 2 mm/min.For each sample, a minimum of 25 filaments were subjected to testing.

Solvolysis experiments
Following the solvolysis experiments, the recovered fibers were subjected to a treatment process.Specifically, they were immersed in an acetone bath in order to facilitate the removal of any residual resin.Subsequently, it is essential to remove the humidity after the bath treatment, so the fibers were carefully placed in an oven for 24 hours at 50°Celsius.After the solvolysis process, a visual inspection as demonstrated in Figure 2 of the samples reveals that a great amount of epoxy resin has been removed and the fibers appear to be in a clean condition and the length of the fibers seems to be continuous (with negligible changes in the initial length of the fibers).Further analyses were conducted as described below for the characterization of the fibers.

Decomposition resin efficiency
The decomposition efficiency of the epoxy resin in the CFRP, was measured in terms of mass.For this reason, the samples were weighed before and after solvolysis processes.The rate of the decomposition efficiency was measured with equation (1) as detailed in Table 2.In the first three experiments that were conducted using water as the solvent, the removal efficiency increased with rising temperature (and pressure) as well.Higher rates of resin elimination were achieved under supercritical operating conditions.In experiment #03, was observed the highest resin removal rate, reaching almost 100%.However, in experiment #04, the rate was lower compared to the others, likely attributed to the lower pressure conditions.Finally, during the solvolysis recycling process, the choice of temperature and pressure can influence the effectiveness of the resin removal.

Morphological characterization
The recovered fibers underwent examination through a Scanning Electron Microscope (SEM).Figure 3 displays the SEM analysis results for the recovered fibers.Furthermore, with the utilization of the SEM multiple diameter measurements were conducted, and then analysis revealed that the mean diameter in every experiment exhibited negligible change.In Figure 3, some measurements of the diameters are also demonstrated.Notably, the reclaimed fibers exhibited negligible traces of resin residue on their surfaces, and there were no detectable signs of physical damage such as cracks, or any changes in morphology with regard to diameter.Specifically, in experiment #04 there are visible resin residues, a fact that confirms the low rate of resin removal.

Mechanical properties
Tensile strength and Young modulus measurements were conducted on the reclaimed fibers to assess how the recycling process affected their mechanical properties.For these mechanical properties of the virgin fibers, a mean value was used, which was derived from data available in the literature and, more specifically from results obtained through single fiber tensile tests [9,11,14,15].When examining the performance of reclaimed fibers, as detailed in Figure 4, in experiment #02 was observed a tensile strength reduction of 42%.This result signifies that under supercritical conditions with water as a solvent for a duration of 15 minutes indicates a substantial degradation of the material's mechanical properties.Furthermore, in all the experiments there is a visible degradation of the tensile strength.The minimum degradation of 21% was appeared in experiment #03, where the reaction time of the supercritical water was only 5 minutes.A parallel trend is observed in the elastic modulus in Figure 5.In the measurements of the Young modulus, it was observed a degradation in the reclaimed fibers.The recovered fibers retained more than 62% of their initial elastic modulus.The minimum degradation was demonstrated in the experiment #03, as the minimum degradation of the tensile strength.In the experiment with the supercritical acetone a slight degradation of the mechanical properties occurs.

Conclusions
Solvolysis seems to be a promising methodology for recycling.The recovered fibers by solvolysis process offer a great potential of reinforcement in the future and could possibly replace the virgin fibers.This work has demonstrated using SEM results, that in most experiments, the recycled fibers were clean, smooth and continuous, without negligible changes in the diameters of the fibers.Furthermore, the decomposition efficiency in 4 out of 5 experiments was ranged between 90 and 100% using only water or acetone (as a solvent) without any catalyst.The maximum resin removal occurred using supercritical water.The mechanical properties have been evaluated with single-fiber tension tests on the recovered fibers.The mechanical tests showed that the recovered fibers reclaimed more than 58% of their initial tensile strength and 62% of their initial Young's modulus.However, it is necessary to carry out a statistical test as the standard deviation of the results in the mechanical properties is high and further research is needed.

Figure 2 .
Samples after the solvolysis experiments ((a) for the experiment #01 operating conditions, (b) for the experiment #02 operating conditions, (c) for the experiment #03 operating conditions, (d) for the experiment #04 operating conditions, (e) for the experiment #05 operating conditions).

Figure 4 .
Figure 4.The tensile strength of the virgin fibers compared to the reclaimed fibers and the standard deviation.

7 Figure 5 .
Figure 5.The Young modulus of the virgin fibers compared to the reclaimed fibers and the standard deviation.
[1] Pantelakis Sp and Tserpes K (Eds.)2020 Revolutionizing aircraft materials and processes (Springer Nature Switzerland) [2] Borjan D, Knez Ž and Knez M 2021 Recycling of Carbon Fiber-Reinforced Composites-Difficulties and Future Perspectives Materials 14 4191 [3] Kao C C, Ghita O R, Hallam K R, Heard P J and Evans K E 2012 Mechanical studies of single glass fibres recycled from hydrolysis process using sub-critical water Composites Part A: Applied Science and Manufacturing 43 398-406 [4] Pietroluongo M, Padovano E, Frache A and Badini C 2020 Mechanical recycling of an end-oflife automotive composite component Sustainable Materials and Technologies 23 e00143 [5] Nunes A O 2015 Composites renforcés à fibres de carbone : récupération des fibres par vapothermolyse, optimisation du procédé phdthesis (Ecole des Mines d'Albi-Carmaux) [6] Das M and Varughese S 2016 A Novel Sonochemical Approach for Enhanced Recovery of Carbon Fiber from CFRP Waste Using Mild Acid-Peroxide Mixture ACS Sustainable Chem.Eng. 4 2080-7

.
Solvolysis operating conditions for the experiments.

Table 2 .
Decomposition efficiency of epoxy resin in terms of mass.