# Recycling flax fibre composites by using a chemical solvolysis process in near- and supercritical conditions: linking fibre integrity to micro-mechanical performance

Coralie Buffet, Jing Wang, Sylvie Durand, Anthony Magueresse, Cyril Aymonier, Alain Bourmaud

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Coralie Buffet, Jing Wang, Sylvie Durand, Anthony Magueresse, Cyril Aymonier, et al. Recycling flax fibre composites by using a chemical solvolysis process in near- and supercritical conditions: linking fibre integrity to micro-mechanical performance. Composites Part A: Applied Science and Manufacturing, 2026, 203, pp.109595. 10.1016/j.compositesa.2026.109595. hal-05484407

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# Recycling flax fibre composites by using a chemical solvolysis process in near- and supercritical conditions: linking fibre integrity to micro-mechanical performance

Coralie Buffet<sup>a, *,</sup>, Jing Wang<sup>b</sup>, Sylvie Durand<sup>c</sup>, Anthony Magueresse<sup>a, #</sup>, Cyril Aymonier<sup>b</sup>, Alain Bourmaud<sup>a, @</sup>

## ARTICLE INFO

### Keywords:

Composites

Solvolysis

AFM

Micro-mechanical properties

## ABSTRACT

The growing demand for bio-based composites has intensified the need for efficient recycling strategies that preserve fibre integrity and reduce environmental impact. In this study, unidirectional flax-reinforced composites with epoxy and poly-amide 11 (PA 11) matrices are manufactured and subsequently recycled using a solvolysis-recycling based process under near- and supercritical conditions in a semi-continuous flow reactor. A water/ ethanol mixture (50/50 in mol) is employed as solvent at 250°C and 300°C under 250 bar. The recovered flax fibres are characterised using scanning electron microscopy (SEM), nanoindentation (NI) and atomic force microscopy (AFM). Mechanical tests on the original composites demonstrate tensile properties consistent with the best values reported in the literature, supported by low porosity levels (~2%). After solvolysis, flax fibres recovered from PA 11 composites exhibit clean surfaces, preserved cell wall organisation, and nano-mechanical properties close to virgin fibres. In contrast, fibres from epoxy composites experience substantial degradation, especially at 300°C, with pronounced ultrastructural damage and a 65.8% reduction in indentation modulus. AFM confirms localised degradation patterns and lumen-driven damage pathways. These results highlight the superior recyclability of Flax – PA 11 systems and the intrinsic limitations of Flax – Epoxy composites for fibre-preserving solvolysis. Overall, the study provides composite manufacturers and recyclers with new insights into fibre-scale degradation mechanisms and supports the development of more circular, eco-efficient bio-composite architectures.

## 1. Introduction

Interest and demand for thermoset and thermoplastic composite materials has been growing since the last two decades. According to the JEC Press kit 2023, the global composites market volume has risen from 8.5 in 2015 to 12.1 megatons (Mt) in 2022. Forecasts indicate that this figure will continue to grow, reaching 15.1 Mt in 2027. This is mainly due to their high strength-to-weight ratio and their long-term resistance. Motivated by the accelerating climate change in recent years [1,2], there is a need to better understand the environmental impact of these materials and reduce drastically the reliance on fossil-based resources [3,4]. Bio-composites, made by combining plant fibre reinforcements and/or bio-based polymer matrices, such as PLA, PHA, PBS and PA 11,

have therefore been developed in many industrial sectors: transport as automotive parts, sport, wind energy and even structural applications [5–9]. Indeed, flax fibres represent a good alternative to synthetic ones for several reasons: high specific mechanical properties, low density and better environmental benefits [10,11].

The increasing consumption of these materials has also led to a surge of their end-of-life management. However, composite waste recycling is complex due to their multiphasic structure which makes incineration and especially landfill the most common waste management options [12,13]. European environmental regulations, including landfill ban in Germany since 2009 and increased taxes, are driving to develop alternative and efficient solutions such as mechanical, thermal and chemical recycling methods [12].

<footnote><sup>a</sup> Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, France</footnote>

<footnote><sup>b</sup> Univ. Bordeaux, CNRS, Bordeaux INP, ICMCB, UMR 5026, F-33600 Pessac, France</footnote>

<footnote><sup>c</sup> UR1268 Biopolymères Interactions Assemblages, INRAE, F-44316 Nantes, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: coralie.buffet@univ-ubs.fr (C. Buffet).</footnote>
Concerning mechanical recycling, the feasibility of the process depends on the type of polymer matrix used [14]. Compared to thermosetting resins, which are cross-linked, thermoplastics plant fibre composites (PFC) can be recycled multiple times through thermomechanical process [15,16]. However, this end-of-life scenario does not enable the recovery of long fibres, as it reduces the aspect ratio of the recycled fibres, thereby limiting their potential for reuse in structural applications [17].

Currently, very few studies have focused on the chemical recycling of PFC. Recently, the alkaline hydrolysis of plant fibre (bleached softwood kraft pulp) reinforced PLA composites was investigated, achieving a maximum recovery rate of 78% [18]. By contrast, the thermal and chemical treatment of synthetic fibres, such as carbon and glass, reinforced thermoset and thermoplastic polymer composites has been studied in greater detail. These treatments enable to recover longer fibres with relatively good mechanical properties depending on the recycling process. The aim here is to degrade the polymer matrix, either by heating it at high temperature (400 – 750°C) or by using chemical solvents, in order to extract the fibres. A number of techniques have been developed for recovering synthetic fibres: combustion, fluidised bed, pyrolysis and solvolysis [19,20]. Pyrolysis is the most advanced industrial process, but still have a high human and environmental impact. A better compromise is achieved with fluidised bed method and chemical recycling, but the solvents used for the latter are very toxic to the environment and human health. Consequently, an increasing number of studies are focusing on solvolysis in near- or supercritical conditions using more environmentally friendly solvents at high temperatures under pressure [21–23], which could be a future alternative to the above-mentioned recycling processes.

Actually, Sugeta et al. have treated unsaturated polyester resin by supercritical water at 380°C and have demonstrated the preserved surface integrity and cleanliness of the recovered glass fibres [24]. Kamimura et al. have also shown that, using supercritical methanol in the presence of catalytic amounts of 4-(dimethylamino) pyridine (DMAP), the recovered glass fibres exhibited sufficient mechanical and surface properties to be reused [25]. Henry et al. have investigated the recycling of epoxy resin reinforced with carbon fibres using water and a water/ethanol (50/50%-vol) mixture, at different temperatures (350, 375 and 400°C) and a pressure of 250 bar, through a semi-continuous flow reactor. The diameter, tensile modulus and strength values obtained for the recycled carbon fibres are similar to those of the original fibres. Furthermore, the use of water/ethanol as reactive media has enabled to obtain clean fibres with less than 2%-wt of matrix residues [26].

In the present work, we introduce a comparative investigation of two technologically relevant flax-fibre composite systems, one thermoset (epoxy) and one bio-based thermoplastic (PA 11), to uncover, very innovatively, how matrix chemistry governs fibre recoverability and ultrastructural integrity under advanced solvolysis recycling conditions. Both thermosetting epoxy resin and thermoplastic polyamide 11 were treated by a water/ethanol (50/50 in mol) mixture, at two different temperatures (250 and 300°C) and a pressure of 250 bar. After visualising the recovered fibres through scanning electron microscopy (SEM), the study focuses on their micro-mechanical measurements by nanoindentation (NI) and Atomic Force Microscopy (AFM). Coupling these two methods is indeed a powerful tool to deeply characterise the potential structural and mechanical degradation of flax fibres [27].

# 2. Materials and methods

## 2.1. Plant fibres

The reinforcement used in this study was kindly provided by EcoTechnilin (Yvetot, France). It's a unidirectional (UD) preform tape with a weight of 110 ± 10 g/m2, named Flax Tape. This is an assembly of long combed flax fibres from different batches. According to the supplier, these flax fibres undergo no chemical or physical treatment

after the mechanical scutching and hackling processes.

## 2.2. Polymer matrices

Two polymer matrices were used: a thermoplastic one and a thermosetting resin. The fully synthetic epoxy resin (EPOLAM 2020) was supplied by Axson Technologies. Additionally, polyamide 11 (PA 11) pellets produced from a renewable resource (castor oil), under the trade name Rilsan®, were provided by Arkema. PA 11 pellets were extruded and calendered to produce films with a thickness of 60 µm in order to perform film stacking. Prior to extrusion, the pellets were dried under vacuum at 80°C for 12 h. A Brabender® single screw extruder fitted with a cast film die and nip rolls set to 25°C was used. The screw speed was set to 60 rpm and the barrel temperature was kept constant at 210°C.

## 2.3. Composite manufacturing

Unidirectional PA 11 and Epoxy laminated composites with flax reinforcement were manufactured by film stacking using a LABTECH® Scientific 50 T hydraulic press with a 16 × 16 cm² heating plate. First, the flax fibres were dried for 12 h at 60°C under vacuum. PA 11 films were also dried for the same duration at 80°C under vacuum. Two thicknesses of composite material were produced for each matrix. In fact, 1 mm-thick of flax fibre composites were used for solvolysis tests, while 2 mm-thick composites were prepared for mechanical tests. The number of stacks was determined by the density method, with the aim of achieving a constant fibre volume fraction of 50%.

Regarding the 2 mm-thick thermoplastic composite, twenty-one layers of unidirectional flax tape and fourteen layers of PA 11 film were interleaved. To ensure symmetry, three consecutive flax plies were stacked at the topmost, bottommost and central layers. To achieve a 1 mm-thick composite, twelve layers of unidirectional flax tape and eight layers of PA 11 film were interleaved. Thus, two consecutive flax plies were stacked at the topmost, bottommost and central layers to ensure symmetry once again. Both of these assemblies were placed between two aluminium cover plates before being inserted into the moulding press. The press was preheated to 210°C and the plates remained in contact at this temperature for 2 min to melt the polymer. Then, a pressure of 25 bars was applied in three successive steps of 5 bars each, followed by a final step of full pressing at 25 bars for 3 min. The total hot cycle at 210°C therefore lasted 8 min. The hot stacks were then pressed between cold plates at 25°C for 5 min with a pressure of 25 bars.

On the other hand, for both thermoset composites, the liquid epoxy resin was heated at 50°C to be more fluid and easier to apply. Resin drops were then applied manually and evenly between each layer of flax tape. Composite plates with thicknesses of 1 and 2 mm were obtained by assembling lay-ups comprising of eight and fifteen plies of unidirectional flax tape and intermediate epoxy resin, weighing a total of 15 and 30 g, respectively. As performed for thermoplastic composites, these assemblies were placed between two aluminium cover plates before being inserted into the moulding press. The same hot cycle as detailed above was achieved at room temperature, without the first pre-heating step.

The flax fibre volume fraction and the porosity rate of each composite were evaluated trough SEM observations of their transverse sections. Analysis of three cross-sections, each approximately 180 mm long, was performed using Image J® software.

## 2.4. Tensile testing

To determine the tensile properties of the composites, 150 × 15 × 2 mm specimens were prepared from UD plates in the longitudinal direction. To prevent slipping and breaking at the grips during tensile loading, glass fibre tabs (±45°) were bonded to the specimen ends using an epoxy adhesive (Araldite 420, Samaro). Tensile tests were conducted in a controlled environment of 23 ± 2°C and 50 ± 5% RH using an Instron 5566A instrument. The samples were conditioned in this
environment for 48 h prior to testing. A tensile testing machine equipped
with a 10 kN load cell was used, in accordance with the ISO 527-5
standard. The crosshead speed was 1 mm/min in the longitudinal di-
rection of the specimen. An Instron dynamic extensometer with a
nominal length of 25 mm was used to measure the sample strain. The
tensile modulus, calculated between a strain of 0.02% and 0.1%, as well
as the stress and strain at break, were determined from five specimens
for each formulation.

## 2.5. SEM analysis

Prior to any investigation, all the samples were sputtered with a thin
layer of gold (Edwards Sputter Coater). They were then viewed under a
JEOL JSM-IT500HR (Tokyo, Japan). In order to obtain topographical or
compositional images, observations were conducted under high vacuum
with an acceleration voltage of 3 or 5 kV for secondary (SED) or back-
scattered (BED-C) electrons, respectively. Three different samples were
prepared for SEM analysis. For the transverse section of UD composites,
the samples were first embedded in an epoxy resin (EPOLAM 2020) and
polished. Moreover, SEM images of the flax fibres recovered after the
different solvolysis conditions were taken at x1000 magnification. SEM
observations of the cross-sections of recycled flax fibres were also taken
at x1000 and x2000 magnification. For this step, the flax fibres were
embedded in low-viscosity agar epoxy resin (Agar Scientific UK) and
cured at 60°C overnight. The resulting samples were then machined and
their surfaces trimmed using an ultramicrotome (Ultracut S, Leica
Microsystems SAS, France) equipped with diamond knives (Trim and
Ultra AFM, Diatome, Switzerland). Their preparation is detailed by
Goudenhooft et al. [27]. The same samples were used for NI and AFM
investigations.

## 2.6. Chemical recycling process

Fig. 1 describes the set-up developed to perform the solvolysis ex-
periments summarised in Table 1.

The continuous-flow reactor was fabricated from stainless steel with an internal volume of 150 cm³. The composite samples were introduced into the reactor, and solvent was continuously delivered using a high-pressure pump. The reactor temperature was regulated by a heating jacket, while the system pressure was maintained at the desired level thanks to a back-pressure regulator. Once the steady-state conditions were reached, the solvolysis was maintained for the targeted duration. A filter was positioned at the reactor outlet to retain the flax fibres within the reactor while permitting the solvent, containing the solvolysis solution, to pass through. The stream was subsequently cooled down via a heat exchanger prior to collection in a vessel.

A range of operating parameters was investigated, including tem-
perature, pressure and reaction time (Table 1). Among these, tempera-
ture is frequently reported as the most critical factor in the literatures.
Hydrolysis temperatures for polyamides reported in industrial patents
and the literature range from 210 to 340°C [28]. In contrast, epoxy

resins exhibit hardener-dependent degradation: anhydride-cured epoxy
fully decomposes at 275°C without a catalyst in 60 min, whereas amine-
cured epoxy requires 330°C for the same duration [29]. In the present
study, experiments were performed at temperatures between 200°C and
350°C, a constant pressure of 250 bar and a reaction time of 60 min. A
mixture of water/ethanol (50/50 mol.%) was used as a solvent.

Based on these observations (fibres separated), 3 types of samples
have been selected for further characterisation of the recycled fibres:
recycled flax fibres from Flax - PA 11 composite at 250°C, recycled flax
fibres from Flax - Epoxy composite at 250°C and 300°C.

## 2.7. Nanoindentation (NI) measurements

Nanoindentation measurements were carried out under controlled
conditions of 23 ± 2°C and 50 ± 5% relative humidity (RH) using a
commercial Nanoindenter XP system (MTS Nano Instruments). The de-
vice was equipped with a Berkovich-type diamond indenter charac-
terised by a three-sided pyramidal geometry, which enabled the local
mechanical characterisation of fibre cell walls. The indentation depth
was set to a maximum of 120 nm for all samples. The experimental
protocol consisted of a loading phase at a strain rate of 0.05 s⁻¹, fol-
lowed by a 20-second holding period at maximum load, and an
unloading phase conducted at a constant rate of 10 µN/s.

The resulting load-displacement data were analysed using the Oliver
and Pharr method, which determines the indentation modulus from the
slope of the unloading curve [30].

In this study, the indentation modulus and unloading hardness were
selected as the key mechanical parameters, in order to avoid the impact
of potential structural changes during the loading phase.

## 2.8. AFM peak force QNM measurements

A MultiMode 8 AFM instrument (Bruker Corporation, USA) was
operated in PeakForce Quantitative Nanomechanical Mapping (PF-
QNM) mode. A RTESPA-525 probe (Bruker, USA), with a spring constant
between 145 and 160 N/m and an adjusted tip radius ranging from 25 to
35 nm, was used. The probe was calibrated using the Sader method to
ensure optimal measurement accuracy [31]. During the experiments,
the tip radius was further refined using a Kevlar reference sample,
characterised by an indentation modulus between 18 and 20 GPa. Image
resolution was set to 384 × 384 pixels. All AFM measurements were
carried out at a scan speed of 5 µm/s with a maximum applied load of
200 nN.

## 2.9. Thermogravimetric analysis (TGA)

The thermal stability of flax fibres was investigated using a Mettler
Toledo TGA/DSC 1 apparatus. For each measurement, approximately
32 mg of flax tape was placed in the instrument. Thermogravimetric
analyses were performed by recording mass variations as a function of
temperature, from 25°C up to 250 or 300°C at a heating rate of 10°C.

<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic flowsheet of the solvolysis recycling process under pressure and temperature.</figcaption></figure>
<table><caption>**Table 1**
Solvolysis recycling experiments.</caption><thead><tr><th>Number</th><th>Composite</th><th>Temperature (°C)</th><th>Pressure (bar)</th><th>Solvent</th><th>Time (min)</th><th>Observation</th></tr></thead><tbody><tr><td>1</td><td>Flax - PA 11</td><td>200</td><td>250</td><td>Water/Ethanol*</td><td>60</td><td>No change</td></tr><tr><td>2</td><td>Flax - PA 11</td><td>250</td><td>250</td><td>Water/Ethanol*</td><td>60</td><td>Fibres well separated</td></tr><tr><td>3</td><td>Flax - Epoxy</td><td>250</td><td>250</td><td>Water/Ethanol*</td><td>60</td><td>Fibres partially separated</td></tr><tr><td>4</td><td>Flax - Epoxy</td><td>300</td><td>250</td><td>Water/Ethanol*</td><td>60</td><td>Fibres well separated but degraded</td></tr><tr><td>5</td><td>Flax - Epoxy</td><td>350</td><td>250</td><td>Water/Ethanol*</td><td>60</td><td>Fibres degraded</td></tr></tbody></table>

min⁻¹. The tests were conducted under air atmosphere with a flow rate of 80 mL/min. Additionally, the degradation kinetics of flax fibres were assessed by isothermal TGA experiments, consisting of 60 min isotherm steps at 250°C and 300°C, corresponding to the temperatures applied in the solvolysis recycling processes.

## 2.10. Statistical analysis

A unilateral Student's t-test with a 95% confidence interval was conducted to evaluate whether the observed differences between the groups were statistically significant. Nanoindentation modulus and hardness values were analysed separately for each sample, and then statistically compared to the others. Thus, comparisons were made between each solvolysis condition and a reference sample (flax fibres that have not undergone any processing or solvolysis treatment). A p-value less than 0.05 was considered to indicate a statistically significant difference in terms of mechanical properties.

# 3. Results and discussion

## 3.1. Mechanical and structural assessment of the UD composite materials

A preliminary series of mechanical and structural tests was carried out to assess the quality and performance of the composite materials. Table 2 summarises the porosity rates and fibre volume fractions obtained through SEM image analysis, along with the tensile properties of the produced composites.

The mechanical properties of the fabricated materials are fully satisfactory and consistent with those reported in the literature for similar matrices and unidirectional reinforcements. For instance, Bourmaud et al. reported a modulus of 30.0 ± 2.1 GPa and a tensile strength of 257.9 ± 12.3 MPa for Flax - PA 11 composite with a fibre volume fraction of 50.4%, which are slightly higher than the values presented in this article, but correspond to a slightly higher fibre volume fraction [32]. The same applies to the Flax - Epoxy composite, where the measured properties are comparable to, or even exceed, the highest values reported for similar fibre volume fractions [33,34].

These high mechanical properties can be attributed to the quality of the manufactured composites, notably the low porosity content. With porosity around 2%, these materials meet the quality standards typically required in most sectors of the composite industry. The behaviour of these materials under solvolysis will be examined in the following sections. It can be concluded that the materials studied here are fully

<table><caption>**Table 2**
Unidirectional tensile properties of 2 mm-thick composite samples before recycling.</caption><thead><tr><th>Composite</th><th>Porosity rate (%)</th><th>Fibre volume fraction (%)</th><th>Young's modulus (GPa)</th><th>Stress at break (MPa)</th><th>Strain at break (%)</th></tr></thead><tbody><tr><td>Flax - Epoxy</td><td>2.2 ± 0.3</td><td>49.2 ± 3.2</td><td>30.8 ± 2.0</td><td>261 ± 16</td><td>1.0 ± 0.6</td></tr><tr><td>Flax - PA 11</td><td>2.1 ± 0.7</td><td>47.5 ± 2.8</td><td>23.6 ± 3.9</td><td>212 ± 20</td><td>1.3 ± 0.2</td></tr></tbody></table>

representative of those used in industrial applications, such as sports and leisure or transportation sectors.

## 3.2. Longitudinal and cross-sectional SEM analysis of the recycled flax fibres

A SEM analysis was performed in the longitudinal and transverse directions of the recovered flax fibres to provide an initial indication of their morphological state and the potential presence of matrix residues. Fig. 2 shows SEM observations of the recovered flax fibres in longitudinal direction for the 3 selected samples.

As highlighted in Fig. 2.A, flax fibres from the PA 11 composite recycled by solvolysis at 250°C reveal clean surfaces and are quite well individualised. These fibres can be easily recovered after the solvolysis process. Kink-bands, which are characteristic structural defects of flax fibres, can be also observed in Fig. 2.A but they don't seem to be exacerbated by the process [35]. Their morphology is indeed similar to observations on raw fibres (Fig. 3.C).

Nevertheless, as highlighted in Fig. 2.A, a small amount of the PA 11 matrix remains on the flax fibres. However, the quantity of remaining matrix depends on the location within the composite material. Fig. 4 illustrates that areas closer to the centre of the composite exhibit significantly more matrix residue. The same behaviour has already been observed in the literature [36]. ESEM micrographs of virgin carbon fibres and remaining composite after solvolysis using ultrapure water at 280°C for a reaction time of 30 min show that a higher concentration of residual matrix is also observed in the central regions of the carbon fibre-based composite. This can be controlled by the solvent flow rate correlated to the diffusion time of the solvent inside the composite. The slower the flow rate, the higher the diffusion time, the better is the resin extraction at the centre of the composite.

Regarding the epoxy resin composites shown in Fig. 2.B and C, the flax fibres exhibit cross-sectional breaks (Fig. 2.B) and at 300°C, they are completely damaged and unstructured (Fig. 2.C). An important quantity of resin residue can also be observed. For both solvolysis conditions at 250°C, the diameters of the flax fibres appear to be equivalent to those of virgin flax fibres in Fig. 3.A and B [37,38]. Conversely, at 300°C, the diameters of the observed "fibres" are very small, evidencing a pronounced degradation of the cell walls. Indeed, five sections of fibres with significantly reduced diameters seem to be detected in the middle of the SEM image (Fig. 3.C).

Fig. 5 shows the SEM observations of the cross-sections of the flax fibres after chemical recycling. Images (A), (B) and (C) were taken at the same scale, i.e. a magnification of x1000 magnification. This is also the case for the three images (D), (E) and (F) with a magnification of x2000. The chemical contrast confirms the presence of epoxy matrix residues in the Flax - Epoxy samples (Fig. 5.B and C), surrounding the flax fibres and fibre bundles. This is not the case for the recovered flax fibres with a PA 11 matrix in Fig. 5.A, confirming longitudinal observations (Fig. 2.A). This can be explained by the depolymerisation reactions that occur during solvolysis. Under identical process conditions, the PA 11 matrix degrades more easily through chemical recycling. This is related to the chemistry of this thermoplastic matrix, where the amide function can be attacked by water and/or ethanol. As can be seen in Fig. 2.A and Fig. 5.A and D, solvolysis effectively depolymerises the matrix, resulting in clean

<footnote>* Water/Ethanol (50/50 mol.%) – Critical point $p_c$ = 106 bar and $T_c$ = 280°C [52].</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Longitudinal SEM observations of flax fibres recycled by solvolysis (A) PA 11 – 250°C (B) Epoxy – 250°C (C) Epoxy – 300°C. One can notice that the observation scale is similar between the 3 figures, highlighting the strong fibre degradation when solvolysis is performed at 300°C.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. (A) Cross-section and longitudinal SEM observations at (B) 1000x and (C) 2000x of raw flax fibres from the flax tape preform.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Longitudinal SEM observations of flax fibres from the PA 11 composite recycled by solvolysis at 250°C.</figcaption></figure>

and individualised recovered flax fibres. In contrast, epoxy resins exhibit a higher resistance to solvolysis. This is likely due to the highly cross-linked nature of the epoxy network which requires more severe conditions to achieve substantial degradation. Increasing the processing temperature to at least 350°C is generally necessary to further decompose the epoxy resin [39]. However, while further thermal degradation of the resin would allow for better removal, it would also cause significant deterioration of the flax fibres. Therefore, a compromise must be found between the effectiveness of solvolysis and the preservation of the fibres properties.

For composite applications, the ultrastructure of flax fibres, namely the organisation of cell-wall layers, the microfibril orientation in the S2 layer, and the morphology of the lumen and middle lamella, directly governs how fibres deform and fail under load. Variations or degradation in these structural features influence cell-wall damage, fibre

splitting, and local stiffness, controlling stress transfer and failure mechanisms in the composite. Understanding ultrastructure is therefore essential for interpreting both reinforcement efficiency and ageing-related mechanical changes. Furthermore, the cell-wall ultrastructure of these fibres appears to be intact for Flax – PA 11 sample. This is consistent with the observations made in Fig. 2.A. However, cracks are noticeable in the fibre cross-sections in Fig. 5.A and B. The same observation was made on virgin fibres (before the manufacturing and recycling steps) as can be seen in Fig. 3.A. This is therefore most likely due to the sample preparation for SEM analysis, which requires a high vacuum. The cross-sections of the elementary fibres of the Flax – Epoxy sample recycled at 250°C have also preserved their main ultrastructure. However, their lumens begin to enlarge and open. For each sample, SEM images of the cross-sections of 25 elementary flax fibres were analysed. The surface area of the fibres and the lumen were thus defined, and the equivalent fibre diameter (i.e. considering the fibre as a perfect circle) as well as the average percentage of lumen content within them was calculated (Table 3). The high standard deviation value for each parameter is explained by the intrinsic variability (morphology, structure and mechanical properties) of plant fibres such as flax [40]. The lumen sizes of the reference sample and the Flax – PA 11 sample are considered statistically similar, while those of the epoxy composites are higher, with a lumen content within the fibre of 18.7 ± 7.8% after the recycling process at 300°C. This phenomenon indicates the beginning of a degradation via the lumen, a mechanism that has previously been observed in archaeological and compost-aged flax fibres and flax composite samples [41–43]. Here, the lumen effectively acts as a channel for the solvolysis solvent, causing flax fibre degradation. The impact of the different solvolysis conditions on the recovered flax fibre section is also evident. Indeed, their average surface areas are smaller than those of the reference sample (Table 3). Regarding the equivalent diameter values obtained for the reference sample and after solvolysis at 250°C for the PA 11 composite, there is a slight but meaningful decrease. The values still remain within the range of those reported in the literature [11,38]. However, there is a clear decrease in cross-sectional area and thus in equivalent diameter for the flax fibres from the epoxy composites with an average value of 81.7 ± 21.0 µm², corresponding to an equivalent diameter of 10.1 ± 1.4 µm, at a process temperature of 250°C, reflecting
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** SEM observations of the flax fibre composite cross-sections recycled by solvolysis at (A) x1000 and (D) x2000 of PA 11 – 250°C; (B) x1000 and (E) x2000 of Epoxy – 250°C; at (C) x1000 and (F) x2000 of Epoxy – 300°C. Composition contrast is observed in images (A), (B) and (C). Topographical contrast is observed in images (B), (E) and (F).</figcaption></figure>

<caption>**Table 3**</caption>

Mean cross-sectional areas of fibre and lumen determined by image analysis of 25 elementary fibres. Values with the same subscript letters can be considered as not statistically different (p > 0.05).

<table><thead><tr><th>Sample</th><th>Fibre area (μm²)</th><th>Lumen area (μm²)</th><th>Ratio (%)</th><th>Equivalent diameter (μm)</th></tr></thead><tbody><tr><td>Reference</td><td>204.4 ± 70.6<sup>a</sup></td><td>1.2 ± 0.7<sup>a</sup></td><td>0.6 ± 0.4<sup>a</sup></td><td>15.9 ± 2.8<sup>a</sup></td></tr><tr><td>PA 11 – 250°C</td><td>148.3 ± 54.7<sup>b</sup></td><td>1.5 ± 1.2<sup>a</sup></td><td>1.0 ± 0.6<sup>b</sup></td><td>13.5 ± 2.6<sup>b</sup></td></tr><tr><td>Epoxy – 250°C</td><td>81.7 ± 21.0<sup>c</sup></td><td>3.0 ± 2.0<sup>b</sup></td><td>3.6 ± 2.3<sup>c</sup></td><td>10.1 ± 1.4<sup>c</sup></td></tr><tr><td>Epoxy – 300°C</td><td>9.9 ± 6.4<sup>d</sup></td><td>1.9 ± 1.5<sup>c</sup></td><td>18.7 ± 7.8<sup>d</sup></td><td>3.4 ± 1.1<sup>d</sup></td></tr></tbody></table>

degradation of these flax fibres. Regarding the solvolysis test under the most extreme conditions (300°C), the cell-wall ultrastructure is completely damaged as already noticed in longitudinal observations (Fig. 2.C). Only fibre particles remain, with an average equivalent diameter of 3.4 ± 1.1 µm, which are difficult to separate from the visible residual epoxy resin (Fig. 5.C and F).

SEM images were then supplemented by TGA analysis, to simulate the isothermal ageing of the flax cell walls during solvolysis process and to dissociate the effect of temperature from solvent degradation. Table 4 synthesises the TGA results, and Fig. 6 shows the mass lost after 60 min isothermal analysis at 250°C and 300°C.

The first mass-loss event corresponds to the release of physically bound water and accounts for approximately 6–7% of the total mass in all tests. The second mass-loss event, calculated between the end of water release and the onset of each isothermal step (including the heating ramp at 10°C.min⁻¹), reaches 1.9% at 250°C and increases to 9.2% at 300°C. This marked difference indicates that flax fibres undergo

<caption>**Table 4**</caption>

Mass lost during the TGA isothermal analysis for the rax flax at 250°C and 300°C.

<table><thead><tr><th>Sample</th><th>1st mass lost (%)</th><th>2nd mass lost (%)</th><th>3rd mass lost (%)</th></tr></thead><tbody><tr><td>Raw flax 250°C</td><td>6.7</td><td>1.9</td><td>7.4</td></tr><tr><td>Raw flax 300°C</td><td>6.8</td><td>9.2</td><td>40.7</td></tr></tbody></table>

more severe thermal degradation prior to the start of the solvolysis recycling process at 300°C, which is here simulated by the isothermal stage.

The third mass-loss event occurs during the isothermal step and amounts to 7.4% at 250°C and 40.7% at 300°C. This substantial mass loss indicates that a significant fraction of organic compounds, primarily non-cellulosic polysaccharides (NCPs), degrades during the solvolysis recycling process. At the lower temperature, cellulosic components are clearly less affected.

These results are consistent with SEM observations and with the pronounced decrease in the mechanical properties of flax fibre cell walls after solvolysis at higher temperature. At 300°C, given the large amount of degraded material, cellulose degradation can also be assumed. Indeed, in flax fibres, the NCP fraction represents at most 15–20% of the parietal components, indicating that the observed mass loss at this temperature cannot be attributed solely to NCP degradation.

### 3.3. Nano-mechanical properties of the recovered flax fibres through NI measurements

To study the nano-mechanical properties (NI and AFM), a sample of virgin flax from the same preform used for composite processing that had not undergone any manufacturing or recycling processes, was prepared and analysed to provide a reference point for comparison with the other samples. Nanoindentation has been proven to be an effective technique for highlighting different degradation phenomena [27]. Thus, these experiments were conducted to evaluate the indentation modulus *E* and hardness *H* across several fibres per sample of interest (at least five fibres and a mapping of 15 × 15 or 20 × 20 with a spacing of 2 µm between indents). The results are presented in the form of a box plot in Fig. 7.

A clear tendency emerges from the nanoindentation modulus results. Each sample is statistically different (as shown as the *t*-test results, which are represented by different superscript letters in Fig. 7). For each composite and each solvolysis condition, the nanoindentation modulus of flax fibres progressively decreases compared to the reference sample. However, the fibres from the recycled Flax – PA 11 composite still exhibit good mechanical properties, with an average nanoindentation modulus of 19.7 ± 2.3 GPa. This slight decrease may be due to the processing of composite material causing a drastic change in the
<figure><img src="image_9.png" /><figcaption>Fig. 6. Simulated thermal degradation of flax fibres during solvolysis processes with 60 min isotherm at (A) 250°C and (B) 300°C under air.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 7. Box-plots of (A) nanoindentation modulus and (B) hardness values for flax fibres recycled by solvolysis under different conditions, alongside a reference sample – Values with the same subscript letters can be considered as not statistically different (p > 0.05).</figcaption></figure>

indentation modulus of flax fibres within the composite at temperatures above 210°C [44]. This value is still consistent with those found in previous studies [45,46]. The fibres recovered from the Flax – Epoxy composites exhibit a more significant decrease in nanoindentation modulus, indicating more pronounced degradation at the cell wall scale. The average values of 15.3 ± 1.6 GPa and 7.8 ± 1.3 GPa for solvolysis temperatures of 250°C and 300°C, respectively. The 65.8% reduction in the nanoindentation modulus compared to the reference sample indicates that the 300°C solvolysis conditions are too severe for flax fibres, as already morphologically observed through SEM analysis (Fig. 5.C and F). A similar trend is observed in the hardness values, albeit to a lesser extent. The hardness values of the two epoxy samples are relatively close: 278 ± 30 MPa and 265 ± 41 MPa, at solvolysis temperatures of 250°C and 300°C, respectively. These samples are therefore statistically different from the reference sample, indicating degradation of their cell wall properties due to chemical recycling. The hardness values obtained by nanoindentation for these plant fibres are generally between 300 and 500 MPa [8]. This range is consistent for the reference sample and for the PA 11 composite sample. Their respective average values are indeed 363 ± 34 MPa and 409 ± 37 MPa. A slight increase is indeed observed for the latter sample, suggesting an ultrastructural modification in the corresponding flax fibres following solvolysis.

Based on the nanoindentation results at the cell wall scale, these

recovered fibres show potential as reinforcement for composite materials [8]. Conversely, the flax fibres recovered from Flax – Epoxy composites exhibit significant degradation of their cell wall properties, underlining the impact of the solvolysis process, especially at high temperatures. It has been effectively demonstrated that the local mechanical properties of flax fibres decrease above 170°C [47].

The indentation modulus values correspond to the response of the cellulosic components. Cellulose, which has a high degree of crystallinity, is present primarily in the thickest layer of flax fibres, the S2-layer. This layer contains 80–85% cellulose and therefore exerts the greatest influence on the mechanical properties of flax fibres [48,49]. The hardness parameter is more influenced by non-cellulosic components such as lignin and pectin [50]. Thus, to better understand the link between ultrastructural properties and local features, it is important to work at a higher resolution, and AFM is a highly promising technique for precisely analysing altered materials [27].

### 3.4. Nano-mechanical features of the recovered flax fibres through AFM investigations

The degradation phenomena of flax fibres at the cell wall scale, originating from different environments (e.g. ageing over time, compost and water), have been extensively studied using AFM, in mechanical
mode, in the literature [27,41,43]. This characterisation method represents a powerful approach for the detailed investigation of local mechanical and structural damage at the micro- or nanoscale. The aim of this part of the study is therefore to enhance the mechanical study using nanoindentation modulus obtained by AFM in PF-QNM mode, while also specifying the local defects caused by the chemical recycling on flax fibres. The same reference sample studied previously using nanoindentation was also analysed in this section. The transverse cross-sections of flax fibres of the three recycled samples were investigated by AFM; the mechanical mapping obtained are shown in Fig. 8.

First of all, the mappings are in line with SEM observations (Section 3.2.) in terms of fibre morphology and observed damages. Fig. 5.C shows the degradation through the lumen of the flax fibres in the epoxy composite treated at 250°C. Comparable degradation mechanism was already observed by Melelli et al. [41] after 125 days in compost on Flax-PLA composites. The elementary fibres within the composite displayed internal holes through the lumen similar to our observations. All layers of this analysed fibre appear to be degraded, demonstrating a uniform decrease in the indentation modulus across the entire cross-section. In Fig. 8.C and D, two phases with different resin moduli values can be distinguished: the coating agar resin and the residual epoxy resin.

As already observed through SEM and nanoindentation experiments, at 250°C, fibres in epoxy matrix appear to be more damaged than in PA 11. The main difference arises from the polymer matrix reactivity. Solvolysis is highly effective for the thermoplastic PA 11 matrix at temperatures as low as 250°C thanks to the amide function. In contrast, solvolysis treatment at 250°C does not lead to complete depolymerisation of the epoxy resin. As a result, the degradation kinetics of the epoxy resin become slower relative to those of the fibres. Since the fibres exhibit higher reactivity under these solvolysis conditions, this leads to

more pronounced ultrastructural and micromechanical degradation. These phenomena promote solvent penetration through the fibre lumen. Finally, the chemical mechanisms involved in the depolymerisation of the epoxy resin remain difficult to fully elucidate. We therefore hypothesise that polymer decomposition in the thermosetting resin occurs predominantly via thermolysis rather than solvolysis, in contrast to the behaviour of the thermoplastic matrix dissolution.

At 300°C, solvolysis induces pronounced microstructural damage in the flax fibres cell walls, strongly compromising their AFM indentation modulus (Fig. 8.D). This mapping provides further information on the degradation induced by solvolysis: the primary and the secondary layers are both damaged by chemical recycling. Regarding the recovered flax fibres derived from the Flax-PA 11 composite, the indentation modulus was measured on the undamaged part of the fibres (Fig. 8.B). While its average value is certainly higher than that obtained for the reference, it remains within the range of “modern” untreated flax fibres [27]. In the case of this mapping, the onset of degradation can be seen in the outer layer of the fibres. The modulus of this area is effectively reduced with an average value of 16.7 ± 3.8 GPa, showing a decrease of 32% compared to the undamaged zone. Finally, the shape and size, as well as the ultrastructural integrity of the recovered flax fibres remain comparable to those of the reference sample, except under the most drastic solvolysis conditions (Fig. 8.D).

Fig. 9 shows the evolution of indentation modulus obtained by both nanoindentation and AFM PF-QNM tests. The box plots were obtained by selecting between 1000 and 2000 points from the AFM measurements. Although the two characterisation techniques differ in indentation size and depth, as well as in the number of measurements per fibre (ranging from 40 to 120 for NI values), the overall trend observed in Fig. 8 is consistent with that obtained from nanoindentation tests

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Indentation modulus mapping and average values obtained by AFM of flax fibres from (A) reference sample C (B) PA 11 – 250°C (C) Epoxy – 250°C (D) Epoxy – 300°C.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Indentation modulus box-plots given for each characterisation technique and sample.</figcaption></figure>

(Fig. 4). Except for the reference sample and for Flax – Epoxy treated at 300°C, a larger disparity in AFM values is obtained, highlighting the heterogeneity of flax fibre cell walls with local structural defects, non-visible at NI scale, each measurement being an average value due to size of each indent (approx. 6 – 7 mm compared to 50 – 60 nm for each AFM indent).

The complexity of calibrating the tips used in AFM PF-QNM mode, combined with the fact that nanoindentation and AFM measurements were not performed on the same fibre, may account for slight variations in stiffness values, likely due to the natural heterogeneity of flax cell walls. The modulus values obtained by AFM are generally higher than those obtained by NI, which is consistent with previous work [51]. Finally, AFM investigations allow to validate the NI results obtained previously, and its higher resolution provides more information on the degradation mechanisms of recycled flax fibres by revealing cracks, damaged areas, and other local defects.

## 4. Conclusion

This work demonstrates the feasibility of recycling unidirectional flax fibre composites through solvolysis in near- and supercritical conditions. The composites manufactured exhibited high mechanical performance and low porosity, making them representative of industrial-grade materials. Recycling behaviour was strongly dependent on the polymer matrix. For PA 11-based composites, fibres were efficiently recovered with preserved ultrastructure and mechanical properties, confirming the suitability of thermoplastic matrices for chemical recycling. In contrast, epoxy-based composites exhibited partially degraded fibres, structural damage, and marked reduction in nano-mechanical properties, particularly at 300°C. Advanced characterisation using SEM, NI and AFM revealed lumen-driven degradation pathways and highlighted the sensitivity of flax fibres to harsh chemical conditions.

Overall, these results highlight the strong potential of integrating bio-based thermoplastic matrices with flax fibres to create high-performance, fully recyclable composite systems, addressing a key challenge in sustainable materials design. The study also demonstrates the critical influence of solvolysis parameters on fibre preservation, pointing to the need for optimised protocols or alternative chemistries to enhance the recyclability of thermoset-based composites. By directly linking solvolysis conditions to fibre-scale structural and mechanical properties, this work provides a novel framework for predicting composite performance after recycling. These insights offer tangible guidance for industrial applications in transport, sports, and leisure, where balancing mechanical performance, durability, and circularity is

increasingly essential, thus advancing the design of next-generation eco-efficient bio-composites. Future work will focus on refining solvolysis conditions, exploring automated fibre handling techniques, and integrating single-fibre mechanical testing to further enhance process control and validate fibre recovery strategies for industrial-scale composite applications.

## CRediT authorship contribution statement

**Coralie Buffet:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Conceptualization. **Jing Wang:** Writing – original draft, Investigation, Formal analysis. **Sylvie Durand:** Writing – review & editing, Investigation, Formal analysis. **Anthony Magueresse:** Writing – review & editing, Methodology, Investigation. **Cyril Aymonier:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Alain Bourmaud:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors gratefully thank the ANR (Agence Nationale de la Recherche) for funding the RECYCOMP project (ANR-22-PERE-0005) in the frame of France 2030 plan with the PEPR Recyclage, Recyclability and Ré-utilisation des matières. They are also grateful to Antoine Kervoelen for his precious support in the composite manufacturing process. Régis Risani and Dr Lucile Nuez are also thanked for their respective contributions.

## Data availability

Data will be made available on request.

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