# Proof of concept for manufacturing of composite tapes with household PolyEthylene Terephthalate (PET) as matrix

Elise Mathouillot, Monica Francesca Pucci, Pierre-Jacques Liotier

► To cite this version:

Elise Mathouillot, Monica Francesca Pucci, Pierre-Jacques Liotier. Proof of concept for manufacturing of composite tapes with household PolyEthylene Terephthalate (PET) as matrix. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2026, 728, pp.138464. 10.1016/j.colSURFA.2025.138464. hal-05301125

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https://imt-mines-ales.hal.science/hal-05301125v1

Submitted on 7 Oct 2025

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# Proof of concept for manufacturing of composite tapes with household PolyEthylene Terephthalate (PET) as matrix

Élise Mathouillot<sup>a,*</sup>, Monica Francesca Pucci<sup>b,†</sup>, Pierre-Jacques Liotier<sup>a,‡</sup>

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Recycling

Impregnation

Composite tapes

Drifted polymers

## ABSTRACT

New recycling method for PET is investigated by manufacturing thermoplastic tapes using recycled PolyEthylene Terephthalate (PET) as matrix. PET, which is widely used in the packaging industry, especially for bottles, remains a challenge due to the degradation of its properties during mechanical recycling. Faced with these challenges, a new recycling method for PET is investigated by manufacturing recycled and recyclable thermoplastic tapes using recycled PET as matrix. The process is based on a dissolution-precipitation method inspired by the phase inversion membrane technique. A solubility theory have been used to identify the best solvent/non-solvent combinations.

Experimental results show the efficiency of three solvents: Trifluoroacetic Acid (TFA), Hexafluoroisopropanol (HFIP) and Gamma-Valerolacone (GVL) combined with water and ethanol as non-solvent and evaporation to dissolve and precipitate PET in the fibrous reinforcement to create the thermoplastic tape. Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) analyses show that the chemical structure of PET has been preserved. SEM images

<footnote><sup>a</sup> Polymers Composites and Hybrids (PCH), IMT Mines Ales, Ales, France</footnote>

<footnote><sup>b</sup> LMGC, IMT Mines Ales, Univ Montpellier, CNRS, Ales, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: elise.mathouillot@mines-ales.fr (É. Mathouillot).</footnote>
reveal the influence of the choice of solvent/non-solvent combination on porosity and pore distribution in the material.

Proof-of-concept studies carried out with TFA and HFIP suggest that the HFIP/ethanol combination is the most suitable for minimizing degradation and maximizing the matrix content in the fibrous reinforcement. This work therefore lays the foundations for the development of this new recycling method as part of a move towards a circular economy and a reduction in the impact of composite materials, which are difficult to recycle.

# 1. Introduction

Polyethylene terephthalate (PET) is a thermoplastic polymer widely used in the packaging industry, particularly for bottles [1,2]. The global demand for PET reached 27 million tonnes in 2022, of which around 30% is recycled, making it one of the most recycled plastics [3]. Recyclability is a major asset in the current context of plastic waste management [4–6]. There are various methods of recycling plastics, including mechanical recycling, which involves grinding plastic waste and remelting it. Chemical recycling allows the plastic to be returned to its original monomer form [7], and there are other methods such as pyrolysis [8] and selective dissolution of materials [9,10], among others. PET is mainly recycled mechanically but mechanical recycling has significant limitations. Recent studies have shown that this process degrades the physical and chemical properties of PET, reducing the quality of the recycled material and limiting the potential applications of recycled PET [11–15]. On the other hand, composite materials offer interesting properties such as being lightweight and having high mechanical strength [16,17]. These properties make them ideal candidates for numerous industrial applications, especially in the transport industry and renewable energy sectors. Recent advances in this field, such as automated tape laying (ATL), have considerably improved manufacturing processes and enlarged the application possibilities for these materials [18,19]. However, despite their potential, composite materials present significant challenges in terms of recyclability. Their complex structure, made of different materials, makes them both difficult and costly to recycle [20,21]. This aspect is all the more important in the current context, where the circularity of materials has become an environmental and economic priority.

Faced with the growing constraints of PET recycling [22] and the complexity of recycling composite materials, an innovative approach has emerged to solve both problems in a single process. This solution consists in an innovative process for manufacturing thermoplastic tapes using recycled PET as a matrix, with the aim of creating materials that are both recycled and recyclable. The use of recycled PET in this process avoids dependence on virgin resources and minimize the environmental impact of composite materials, which are traditionally difficult to recycle at the end of their life. According to statistics, the global PET demand is set to reach 42 million metric tons by 2030 [3]. This growth shows the importance of developing innovative and efficient recycling solutions to meet growing demand while minimizing environmental impact.

The process is based on a PET dissolution-precipitation technique, commonly used in membrane manufacture [23–29]. This method is based on the system solvent/non-solvent/polymer, where a polymer solution is first created. Then the non-solvent induces precipitation when added to the polymer solution. A non-solvent is then used to precipitate the polymer into the fibrous reinforcement. The main focus of this study is to select an optimal solvent/non-solvent combination for the use of PET in the manufacture of thermoplastic tapes. This selection is essential to guarantee the efficiency of the process and the quality of the tapes produced. Optimizing this key parameter will establish a solid basis for the future development of this promising technology, enabling larger-scale production of recycled and recyclable composite materials. Fig. 1 shows schematically the concept of the new process and its circularity.

To achieve this, Hansen's solubility parameters will be used [30,31], which allow us to predict the interactions between molecules and

thus optimize solvent and non-solvent selection. The characterization of the samples produced is an essential step in this study. Several techniques will be used to assess the quality of the tapes produced. Differential scanning calorimetry (DSC) will be used to analyze the thermal properties of the recovered PET. Fourier transform infrared spectroscopy (FTIR) will be used to identify functional groups and assess potential chemical modifications of PET during the recovery process. Scanning electron microscopy (SEM) will provide essential information on material morphology and structure, including porosity within the tape and the fiber impregnation quality. These analyses will also be used to define a criteria to select the best solvent/non-solvent combination in terms of fiber impregnation, tape porosity and polymer degradation

Optimizing this pairing is crucial to ensure efficient PET dissolution and controlled precipitation in the fibrous reinforcement. Once the optimum solvent/non-solvent combination has been identified, manufacturing thermoplastic tapes using recycled PET as the matrix will become possible. The methodology involve dissolving the PET in the selected solvent, followed by impregnation of the reinforcing fibers with this solution. The non-solvent will then be introduced to precipitate the PET within the fibrous reinforcement, creating a thermoplastic tape. This approach is inspired by the manufacture of membranes by phase inversion [23], but adapted to the production of thermoplastic tapes. To validate our approach, a proof of concept was carried out. This crucial step aimed to demonstrate the feasibility of the process and estimate the initial properties of the tapes produced. SEM analyses were carried out on these preliminary samples to characterize the quality of fiber impregnation and the internal structure of the tapes.

# 2. Theory

## Hansen theory

The solubility of a polymer in solvents can be predicted by Hansen solubility theory [31]. It is based on three solubility components: $\delta_D$, $\delta_P$, $\delta_H$ which are the dispersive, polar and hydrogen-bonding component of the solubility parameter $\delta$ (Eq. (1)).

$$ \delta = \sqrt{\delta_D^2 + \delta_P^2 + \delta_H^2} \qquad (1) $$

Solvents with similar components are more likely to dissolve the polymer. $R_a$ (Eq. (2)) is defined as the distance between the polymer and the solvent in the Hansen space and $R_0$ is the radius of the sphere. Theoretically, if the Relative Energy Distance (RED) which define the distance between positions of solvent and polymer in the tridimensional coordinate system of Hansen [31]. (Eq. (3)) is under 1, the polymer will be dissolved by the solvent.

$$ R_a^2 = 4(\delta_{D2} - \delta_{D1})^2 + (\delta_{P2} - \delta_{P1})^2 + (\delta_{h2} - \delta_{h1})^2 \qquad (2) $$

$$ RED = R_a/R_0 \qquad (3) $$

with 1 for the polymer and 2 for the solvent.

## Membranes Theory

Wet phase inversion by precipitation (Fig. 2) is a widely used technique for membrane manufacture [24,26–28]. This process involves the controlled transformation of a homogeneous polymer solution from a liquid to a solid state. The process is divided into several steps:

* A polymer solution is prepared by dissolving the polymer in a suitable solvent
<figure><img src="image_4.png" /><figcaption>Fig. 1. Circularity of the new process of recycling PET.</figcaption></figure>

* The resulting solution is spread on a support to create a thin film

* The film is immersed in a coagulation bath containing a non-solvent, selected for the properties to impart to the membrane.

* The exchange between solvent and non-solvent causes precipitation of the polymer, forming the membrane structure.

Membrane formation is governed by two main phenomena [23]:

* Thermodynamic phase separation, occurring when the ternary system (polymer, solvent and non-solvent) becomes thermodynamically unstable.

* Mass transfer, the exchange between solvent and non-solvent leading to concentration gradients and influencing the kinetics of phase separation.

These two phenomena determine the final membrane characteristics such as porosity, pore size and asymmetric structure.

In addition, several factors influence membrane structure [23], such as polymer concentration in solution, choice of solvent and non-solvent, coagulation bath temperature and evaporation time prior to immersion. These factors impact the rate of precipitation and thus the final structure of the membrane, which can be spongy, dense or porous.

# 3. Materials and methods

## 3.1. Materials

Virgin PET pellets are a general purpose food grade suitable for water, soda, alcoholic beverages and oil. They were purchased from Indorama Ventures under the codename RAMAPET N1.

Some PET bottles were recovered after use and cut into small pieces. Moreover, bottles from beaches were donated by Effetmer and also cut into small pieces.

To solubilize the PET, different solvents were used. All were purchased from Sodipro and used without further purification.

As reinforcements for the tapes, carbon IM7 12k yarns from Hexcel reinforcements were used.

## 3.2. PET recovering

### PET Dissolution methodology

In order to dissolve PET, suitable solvents for this polymer had to be identified through the Hansen theory. Several tests were carried out

<table><caption>Table 1
Hansen parameters [30].</caption><thead><tr><th>Solvents</th><th>δ<sub>D</sub></th><th>δ<sub>P</sub></th><th>δ<sub>H</sub></th><th>RED</th></tr></thead><tbody><tr><td>Acetone</td><td>15.5</td><td>10.4</td><td>7</td><td>1.04</td></tr><tr><td>Dimethyl Sulfoxide (DMSO)</td><td>18.4</td><td>16.4</td><td>10.2</td><td>1.64</td></tr><tr><td>Tetrahydrofuran (THF)</td><td>16.8</td><td>5.7</td><td>8</td><td>0.49</td></tr><tr><td>N-Methyl-2-Pyrrolidone (NMP)</td><td>18</td><td>12.3</td><td>7.2</td><td>0.91</td></tr><tr><td>Trifluoroacetic Acid (TFA)</td><td>15.6</td><td>9.7</td><td>11.4</td><td>1.2</td></tr><tr><td>Hexafluoroisopropanol (HFIP)</td><td>17.2</td><td>4.3</td><td>9.6</td><td>0.64</td></tr><tr><td>Ethyl Cyanoacetate</td><td>16.7</td><td>7.9</td><td>8.3</td><td>0.58</td></tr><tr><td>Dibasic Esters (DBE)</td><td>16.2</td><td>6.5</td><td>8.4</td><td>0.67</td></tr><tr><td>Diethyl Succinate</td><td>16.2</td><td>6.8</td><td>8.7</td><td>0.7</td></tr><tr><td>Dichloromethane</td><td>17</td><td>7.3</td><td>7.1</td><td>0.4</td></tr><tr><td>Gamma-Valerolactone (GVL)</td><td>15.5</td><td>4.7</td><td>6.6</td><td>0.87</td></tr></tbody></table>

after pre-selection using Hansen's solubility parameters [30,31]. Table 1 lists the different solvents tested on PET along with their Hansen solubility parameters, from [30].

According to Hansen Theory, several solvents were proved to be functional. Trifluoroacetic Acid (TFA) and Hexafluoroisopropanol (HFIP) are effective at room temperature. Gamma-valerolactone (GVL) works at higher temperatures: 170 °C for complete dissolution [1]. Other solvents tested did not work. An amount of PET (pellets or cutted pieces) was immersed in the various solvents. Some solvents dissolved the pellets, rather quickly (TFA, HFIP). For some, dissolution only takes place at a certain temperature (GVL).

### PET Precipitation methodology

After dissolution, precipitation was performed. The polymer solution of PET/solvent (TFA and HFIP) was precipitated with a non-solvent. The choice of non-solvent is very important for the morphology of the PET to be recovered. The non-solvent has a direct influence on the formation and the morphology of pores in the precipitated polymer structure. Not only is the choice of non-solvent important, but also the quantity added and therefore the concentration of polymer in the mixture. To find out the suitable proportion between those three elements of the system, the principles of wet phase inversion membrane manufacture were applied [23]. Table 2 lists the different non solvent tested and their Hansen parameters. For GVL, precipitation is carried out without non-solvent, by evaporation in a laboratory fume hood for 24 h.
<figure><img src="image_5.png" /><figcaption>Fig. 2. Wet inversion phase principle.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Process of recycling PET by manufacturing thermoplastic tapes.</figcaption></figure>

<table><caption>**Table 2**
Hansen parameters [30].</caption><thead><tr><th>Non solvents</th><th>δ<sub>D</sub></th><th>δ<sub>P</sub></th><th>δ<sub>H</sub></th></tr></thead><tbody><tr><td>Water</td><td>15.5</td><td>16</td><td>42.3</td></tr><tr><td>Ethanol</td><td>15.8</td><td>8.8</td><td>19.4</td></tr></tbody></table>

## Fabrication of Tapes

Manufacturing thermoplastic tapes from recycled PET is based on the phase inversion method used in the production of polymer membranes [26], while adapting it to the present needs. The process begins by dissolving the recycled PET in the selected solvent, creating a homogeneous polymer solution. This solution is then used to impregnate fibers, ensuring uniform polymer distribution within the fibrous medium network. The next step is to introduce the non-solvent, which causes the precipitation of PET directly into the fibrous reinforcement (Fig. 3). This phase inversion phenomenon enables the formation of a thermoplastic tape. Controlling parameters such as polymer solution concentration, solution impregnation rate, precipitation rate and environmental conditions is essential to control the final morphology and properties of the composite such as pore density and the amount of polymer in the semi-product.

### 3.3. PET characterization

#### DSC

A Perkin-Elmer 8000 DSC was used to obtain various thermal data of virgin and recycled PET. Measurements were carried out between 40 °C and 280 °C twice with a cooling step between the two heatings. The heating rate has been fixed at the level of 10 °C/min and the cooling rate at 5 °C/min. An empty capsule was used as a reference and the sample weights varied from 9.8 to 11.5 mg. Melting temperature and crystallinity were determined thanks to Eq. (4) [32]. These data are essential for assessing whether the recycling process has altered the molecular structure of PET, for example through changes in the organization of amorphous and crystalline regions. A change in the degree of crystallinity or a shift in characteristic temperatures could indicate thermal or chemical degradation of the material. By comparing the DSC curves of recycled PET with those of virgin PET, it is possible to verify if the thermal and structural properties essential for guaranteeing the material's performance in future applications have been retained.

$$X_c = \frac{\Delta H_m}{\Delta H_m^0} \times 100 \qquad (4)$$
<figure><img src="image_7.png" /><figcaption>Fig. 4. FTIR Spectras PET and characteristic peaks.</figcaption></figure>

with $X_c$: the crystallinity (%) ; $\Delta H_m$: the measured enthalpy of fusion of a material (J/g) and $\Delta H_m^0$: the theoretical enthalpy of fusion of a 100% crystalline material (J/g) that is equal to 140 J/g

### FTIR - ATR

PET characterization by FTIR was performed on a Bruker Vertex 70 equipped with a ATR sampling accessory. FTIR spectra were recorded in the range of 4000–450 cm⁻¹ in KBr chips. These spectra have been used to examine the chemical structure of recycled PET samples and detect any molecular changes caused by the recycling process. These spectra can reveal signs of degradation, that can assess the potential preservation of the chemical structure of recycled PET. FTIR analyses of the recovered PET samples will be compared with the spectrum of vir- gin PET (Fig. 4), taking into account the following characteristic peaks, corresponding to specific vibrations and chemical groups [27,33]:

* 1724 cm⁻¹: Stretch of the C=O bond.

* 1410 cm⁻¹: Band associated with the benzene ring.

* 1262 cm⁻¹: Stretching of the C–C–O bond.

* 1100 cm⁻¹: Stretching of the O–C–C bond.

* 1018 cm⁻¹: Band associated with the benzene ring.

* 873 cm⁻¹: Band associated with the benzene ring.

* 727 cm⁻¹: Band associated with the benzene ring.

These references will enable to verify that the characteristic PET structures are unchanged during the recovery process.

### SEM images

SEM analyzes were performed using a Quanta FEG FEI 200 MEB. The use of SEM images to analyze recycled PET samples is essential to study the presence and nature of void in the precipitated polymer. By characterizing the size, distribution and density of pores depending on the non-solvent used. Enabling gathering data in order to select the best non-solvent to use to obtain a homogeneous, dense material.

## 4. Results

To dissolve PET, 2 solvents were effective at room temperature: TFA and HFIP. GVL requires higher temperatures. To maximize the precipitation process and minimize pore formation in the recovered material, a significative miscibility between solvent and non-solvent is required [25]. The effect of changing both solvent and non-solvent was investigated.

<figure><img src="image_8.png" /><figcaption>Fig. 5. DSC Spectras of PET recovered with TFA, TFA1 : first heating curve, TFA2 : second heating curve.</figcaption></figure>

### 4.1. Recovering with TFA

The dissolution of PET with TFA is simple, takes place at room temperature, and is very fast. However, TFA is highly toxic and ranked as eternal pollutant. With regard to its nature, it is important to check that TFA, an extremely corrosive solvent (pKa = 0.23), does not degrade PET. Two non-solvents were used with TFA. To achieve this, 2 nonsolvents were selected for TFA: water and ethanol.

#### DSC analysis

DSC analyses (Table 3) reveal that PET melting temperatures remain relatively stable, indicating that the recycling process does not lead to major changes in overall thermal structure of the material. This stability reflects good conservation of the chemical configuration of polymer chains and crystalline zones, which has an effect on the melting temperature of PET. The DSC curves (Fig. 5) show a similar trend for the first and second heating cycles. An enthalpy relaxation phenomenon is observed during the first heating cycle, which is common in PET samples [34].

#### FTIR spectra

Fig. 6 shows the infrared spectra of PET samples recovered using TFA and the two non-solvents. The PET characteristic peaks at 3433 cm⁻¹ and 2965 cm⁻¹ are present in both spectra. A broad band between 3600 cm⁻¹ and 3400 cm⁻¹ is observed in both spectra. This may indicate the presence of O–H groups, potentially due to partial hydrolysis of PET [35,36]. For water, the band is slightly wider, which may be explained by residual water from precipitation. If it is confirmed, it could lead to more pronounced risks of hydrolysis, which is not recommended. The characteristic ester bands at 1724, 1260 and 1100 cm⁻¹ are present in both samples. However, they are significantly less intense with ethanol than with water. This may shows that ethanol tends to have a greater effect on ester bonds than water. Absorption bands at 1018, 873 and 727 cm⁻¹ are still present and have the same appearance for both spectra, indicating that the aromatic structure of PET seems to remains intact.

#### SEM images

SEM images reveal differences in the density and voids of recycled polymers depending on the non-solvent used, i.e. water or ethanol. Samples precipitated with water (Fig. 7) show larger pores and a relatively homogeneous distribution across the structure, indicating higher porosity and lower density. This suggests that water, as a non-solvent, promotes uniform phase separation, but at the expense of final
<table><caption>**Table 3**

Thermal parameters of recovered PET.</caption><thead><tr><th>Samples</th><th>Melting temperature 1 (°C)</th><th>Melting temperature 2 (°C)</th><th>Crystallinity 1 (%)</th><th>Crystallinity 2 (%)</th></tr></thead><tbody><tr><td>PET Bottle</td><td>248.42 ± 0.4</td><td>241.9 ± 1</td><td>30.35 ± 1</td><td>22.7 ± 0.3</td></tr><tr><td>Recovered PET Bottle TFA/WATER</td><td>257.38 ± 2.5</td><td>249.15 ± 2.7</td><td>28.42 ± 1.3</td><td>22.35 ± 0.7</td></tr><tr><td>Recovered PET Bottle TFA/ETHANOL</td><td>250.91 ± 0.6</td><td>246.34 ± 0.1</td><td>27.65 ± 1</td><td>22.5 ± 0.1</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** FTIR Spectras of PET Bottle dissolved with TFA and precipitated with water and ethanol.</figcaption></figure>

material density. Conversely, samples precipitated with ethanol (Fig. 8) show smaller pores and a less uniform distribution, but overall a higher density, which is favorable for applications requiring a compact and robust matrix. These observations suggest that ethanol might be more suitable for tape manufacturing. These results thus highlight the strategic importance of non-solvent selection on density optimization and minimizing undesirable porosity, tailoring the material's final properties to the specific needs of its application.

## 4.2. Recovering with HFIP

Dissolving with HFIP is just as simple as with TFA. It can be done at room temperature and quickly, although not as relatively quickly as with TFA which is almost instantaneous. As with TFA, care must be taken when handling it, and protective measures must be observed. HFIP is less corrosive than TFA (pKa = 9.3), but still toxic and ranked as an eternal pollutant. It is important to check that its highly corrosive nature does not degrade the PET. In this part of the study, water is not used as a non-solvent. Despite its high miscibility with water (1000g/L at 25 °C), the tests carried out showed that there was no immediate precipitation of the polymer. In the literature, ethanol was also used rather than water for membrane manufacturing [24].

## DSC analysis

DSC analyses (Table 4) revealed no significant changes in melting temperature or polymer crystallinity after the recovery process, confirming the absence of significant degradation in the recovered PET. However, the results show a relatively high standard deviation for both crystallinity measurements. This variability could be explained by a heterogeneous distribution of crystalline zones in the material after recovery. The DSC curves (Fig. 9) for samples recovered with HFIP also show enthalpic relaxation during the first heating cycle, which is

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** SEM image of recycled PET dissolved with TFA and precipitated with water.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** SEM image of recycled PET dissolved with HFIP and precipitated with ethanol.</figcaption></figure>
<table><caption>**Table 4**

Thermal parameters of recovered PET.</caption><thead><tr><th>Samples</th><th>Melting temperature 1 (°C)</th><th>Melting temperature 2 (°C)</th><th>Crystallinity 1 (%)</th><th>Crystallinity 2 (%)</th></tr></thead><tbody><tr><td>PET Bottle</td><td>248.42 ± 0.4</td><td>241.9 ± 1</td><td>28.43 ± 1.3</td><td>22.7 ± 0.3</td></tr><tr><td>Recovered PET Bottle<br>HFIP/ETHANOL</td><td>249.5 ± 1.1</td><td>243.5 ± 0.8</td><td>30 ± 5.6</td><td>26 ± 6.7</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** DSC Spectras of PET recovered with HFIP, HFIP1 : first heating curve, HFIP2 : second heating curve.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** SEM image of recycled PET dissolved with HFIP and precipitated with ethanol.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** FTIR Spectras PET Bottle dissolved with HFIP and precipitated with ethanol.</figcaption></figure>

more pronounced than for TFA. This can also be explained by solvent evaporation.

### FTIR spectra

The infrared spectrum (Fig. 10) shows a slightly broad band between 3600 and 3400 cm⁻¹ which indicates a more pronounced presence of O-H groups than in the spectrum of virgin PET (Fig. 4) and may be characteristic of PET hydrolysis. The other bands characteristic of PET are present, indicating firstly that the ester bonds are preserved and that the aromatic skeleton seems also to be preserved.

### SEM images

SEM analysis of PET samples dissolved with HFIP shows a structure with very little porosity and a generally heterogeneous surface. The size of the pores suggests that HFIP appears as a more suitable solvent for this study than TFA. However, the heterogeneous distribution of pores on the surface of the material is not optimal for further PET recovery and the manufacture of thermoplastic tapes (see Fig. 11).

### 4.3. Recovering with GVL

Dissolution with the GVL is a two-stage process. First, a swelling stage is performed at 120 °C and lasts one hour at 120 °C. The PET is then dissolved by heating at 170 °C for 30 min. No non-solvent is used to recover the dissolved polymer, and precipitation proceeds by evaporation of the solvent.

### DSC analysis

Table 5 shows the parameters calculated using DSC graphs. The crystallinity during the first heat-up cannot be calculated. The curve for the first heating does not have a standard shape. An increase in crystallinity is observed with the second heating. This means that something has happened to the precipitated polymer in the crystalline structure. PET's structure is more crystalline. The DSC curves (Fig. 12) of the recovered sample with GVL look slightly different than those of other solvents. The melting temperature remains roughly the same, but the Tg cannot be calculated using these curves (see Table 5).

### FTIR spectra

Characteristic peaks of PET is observed in Fig. 13 suggests that the structure of PET is preserved. This indicates that no chemical changes
<table><caption>**Table 5**

Thermal parameters of recovered PET.</caption><thead><tr><th>Samples</th><th>Melting temperature 1 (°C)</th><th>Melting temperature 2 (°C)</th><th>Crystallinity 2 (%)</th></tr></thead><tbody><tr><td>PET Bottle</td><td>248.42 ± 0.4</td><td>241.9 ± 1</td><td>22.7 ± 0.3</td></tr><tr><td>Recovered PET<br>Bottle/GVL</td><td>242.63 ± 1.2</td><td>242.14 ± 1.2</td><td>28 ± 1.5</td></tr></tbody></table>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** DSC Spectras of PET recovered with GVL, GVL1 : first heating curve, GVL2 : second heating curve.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 13.** FTIR Spectras PET Bottle dissolved with GVL.</figcaption></figure>

could be the origin of the crystallinity modification observed in DSC. However, the region between 3800 and 3250 cm⁻¹ shows a larger band. This shows a degradation which can be explained by degradation or oxidation in the precipitated polymer [37,38].

### SEM images

SEM images of PET recovered with GVL show a fairly homogeneous, dense structure (Fig. 14). However, on some parts of the surface, pieces of non-dissolved PET can be observed as some parts of the surface (Fig. 15). Even when dissolved at 170 °C, dissolution was thus not complete after 30 min which appears as problematic for the targetted application.

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** SEM image of PET dissolved with GVL.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** SEM image of PET dissolved with GVL with undissolved fragments of PET.</figcaption></figure>

## 5. Proof of concept for tape manufacturing

Proofs of concept were carried out with TFA and HFIP. On the other hand, it was not possible to carry out a proof of concept with GVL because there was no spontaneous impregnation of the polymer solution in the fibers, possibly due to the temperature (170 °C) at which the solution had to be set to maintain a liquid/colloid form.

### 5.1. TFA/PET tape

#### SEM images

SEM observations of tapes impregnated with a polymer solution and TFA precipitated with water (Fig. 16) and ethanol (Fig. 17) show significant differences in structure depending on the nonsolvent used
<figure><img src="image_19.png" /><figcaption>**Fig. 16.** Carbon fibers impregnated with a solution of PET/TFA precipitated with water.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Carbon fibers impregnated with a solution of PET/TFA precipitated with ethanol.</figcaption></figure>

for precipitation. The sample precipitated with water has a more homogeneous structure overall but with larger porosities. This can be explained by a rapid precipitation mechanism due to the high affinity between TFA and water, which favors the appearance of voids as channels for TFA diffusion in water. In contrast, the sample precipitated with ethanol shows a denser but less homogeneous structure. This suggests that the precipitation is slower in ethanol, which limits porosities but results in uneven pore distribution in the semi-product. Cross-sectionals have been observed and SEM images highlight the disparity between the surface and the inner structure of the tow. For the sample precipitated in water (Fig. 18), the fibers appear as far apart, with a heterogeneous distribution in contrast to the surface distribution. In ethanol, the distribution is much more homogeneous, although the fibers remain rather distant (Fig. 19). This may be explained by a slower precipitation, which would enable a more uniform deposit in the carbon tow. However, repeatability and the effect of sample preparation for observation still have to be assessed.

<figure><img src="image_21.png" /><figcaption>**Fig. 18.** Cross-section of Fig. 16.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Fig. 19.** Cross-section of Fig. 17.</figcaption></figure>

## 5.2. HFIP/PET tape

### SEM images

Observation using SEM on the HFIP tape precipitated with ethanol reveals a structure with no apparent porosity, and a homogeneous polymer distribution on the carbon fibers. Fibers appear to be covered with matrix but there is none between the fibers themselves (see Fig. 20).

## 6. Discussion

The three functional solvents founded after the solubility tests were TFA, HFIP and GVL. The 4 solvent/non-solvent that have been studied of interest are TFA/water, TFA/ethanol, HFIP/ethanol and GVL with evaporative precipitation. The aim was to gather data in order to find the best solvent/non-solvent combination.

The DSC results show that there are no significant changes in the thermal characteristics of the PET samples recovered, suggesting that dissolution in the 3 solvents does not result in significant modifications to the structure of the polymer. However, infrared analysis showed
<figure><img src="image_23.png" /><figcaption>Fig. 20. Carbon fibers impregnated with a solution of PET/HFIP and precipitated with ethanol.</figcaption></figure>

degradation of the polymer, particularly with TFA, where a large band between 3600 and 3400 cm-1 was observed, potentially related to hydrolysis. In addition, this region also features a larger band for GVL and may also suggest hydrolysis of the precipitated polymer. Furthermore, when PET is precipitated with ethanol, a marked effect on PET ester liaisons (1719, 1260 and 1100 cm-1) is observed, indicating that the combination of these two compounds (TFA with ethanol) has a degradative effect on PET. For HFIP, this degradation is less significant. Dissolution with HFIP and GVL is therefore softer than with TFA. SEM analysis revealed the significant influence of solvent and non-solvent selection on porosity formation and sample surface homogeneity. These results suggest that the kinetics of pore formation, influenced by the precipitation rate, as well as the interactions between solvent and non-solvent, play a crucial role in the formation of the recovered PET microstructure. TFA/water samples tend to have a more homogeneous microstructure, but with larger pores. Ethanol, whether with TFA or HFIP, tends to have smaller pores but a more heterogeneous structure. SEM analysis also revealed that the PET recovered had not been completely dissolved by GVL, calling into question the dissolution conditions used for the samples.

As with PET alone, analyses of the tapes show similarities in terms of porosity and pore distribution. With ethanol, the analyses show smaller pore sizes and lower porosity than with water as a non-solvent. However, pore distribution remains more homogeneous with water as non-solvent.

The results obtained in this study are promising for the development of the PET recycling process explained above, but several avenues remain to be explored to deepen our understanding of degradation and pore formation mechanisms. Firstly, although FTIR analyses revealed signs of degradation, it is important to note that this study did not fully characterize the chemical degradation of PET. Additional analyses, such as rheological tests, could be considered to determine changes in chain lengths of the recovered PET, which would enable a more accurate assessment of the extent of degradation. Furthermore, DSC analyses for thermoporosimetry could provide information on the pore size distribution in PET samples [39,40]. This will be investigated in further works. In addition, the study of alternative solvents, which have not been tested here, could pave the way for new, more suitable solvent/non-solvent couples, offering an optimum compromise between degradability and porosity. Further cross-sectional analysis will be required in future studies to verify whether resin-fiber interactions influence the results obtained, and to analyze impregnation within the

fibrous reinforcement. Further studies on polymer concentration in solution and its influence on porosity formation would also be relevant to better understand the impact of PET concentration on the formation of porous structures. In addition, impregnation kinetics could be studied through capillary rise and surface tension measurements as a function of polymer concentration, in order to gain a better understanding of the dissolution mechanism and pore formation dynamics. It will also be interesting to characterize the precipitated polymer after a melting step to see whether porosities created during the process can be closed by melting.

## 7. Conclusion

This work has enabled us to select the best solvent/non-solvent combination for PET dissolution/precipitation by carrying out analyses to determine the thermal, chemical and microstructural degradation of recovered PET samples. The results showed that none of the selected combinations degraded the thermal properties of PET. However, TFA did lead to degradation of PET, notably by hydrolysis and GVL potentially has the same effect. TFA had an even greater effect on ester bonds [35] (1719, 1262 and 1100 cm-1) when precipitated in ethanol. For HFIP, degradation is not detected. In terms of PET microstructure, ethanol appears to be the non-solvent giving the least porosity compared with water. However, water tends to create a more homogeneous microstructure than ethanol. The proof of concept shows that it is possible to impregnate carbon fibers and achieve thermoplastic tape with TFA and HFIP. The PET/GVL solution does not allow impregnation on the fibers. It therefore appears that the HFIP/ethanol combination is best suited to the needs of the process.

The originality of this work lies in the manufacturing process which uses this study in its implementation. In fact, the selection of the solvent/non-solvent pair will enable us to continue the analysis of the process and enable the manufacture of recycled and recyclable composites from thermoplastic waste used as matrix. In further work, the recyclability of the tapes will be proved. The same dissolution-precipitation method will be used, with an additional step to recover the carbon fibers. Analyses of the influence of polymer concentration on porosity formation will be carried out. In a future study, wicking of solution in the carbon tows will be analyzed [41]. The focus will be on the impregnation of the polymer solution into the fibrous reinforcement, by carrying out wicking and determining the surface tension as a function of the polymer concentration in the solution. As the chosen solvent is harmful to aquatic ecosystems and is a CMR substance, further study of the project's viability, including an LCA, will be necessary to prove the sustainability of the recycling process. Despite this limitation, this work marks a step forward in the understanding of porosity formation and solvent-dependent PET degradation, but it also enables to make progress on the design of an innovative recycling process.

### CRediT authorship contribution statement

Élise Mathouillot: Writing – original draft, Methodology, Investigation, Data curation, Conceptualization. Monica Francesca Pucci: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Pierre-Jacques Liotier: Writing – review & editing, Supervision, Project administration, Methodology, 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.

### Data availability

Data will be made available on request.
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