# DLP 3D printing of biobased, soft and degradable thiol-ene networks from cardanol-based polyesters

Emilie Rojtman, Rakine Mouhoubi, Estelle Metay, Sébastien Blanquer, Sylvain Caillol, Vincent Lapinte

► To cite this version:

Emilie Rojtman, Rakine Mouhoubi, Estelle Metay, Sébastien Blanquer, Sylvain Caillol, et al., DLP 3D printing of biobased, soft and degradable thiol-ene networks from cardanol-based polyesters. European Polymer Journal, 2025, 235, pp.114076. 10.1016/j.eurpolymj.2025.114076. hal-05126677

HAL Id: hal-05126677

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Submitted on 23 Jun 2025

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# DLP 3D printing of biobased, soft and degradable thiol-ene networks from cardanol-based polyesters

Emilie Rojtman<sup>a,b</sup>, Rakine Mouhoubi<sup>a,b</sup>, Estelle Metay<sup>b</sup>, Sébastien Blanquer<sup>a,b</sup>, Sylvain Caillol<sup>a,*<sup>b</sup></sup>, Vincent Lapinte<sup>a,*<sup>b</sup></sup>

## ABSTRACT

Cardanol is an attractive biobased building block derived from cashew nut shells, offering a sustainable alternative to fossil-based substances without competing with the food industry. Here, we introduce an approach for Digital Light Processing (DLP) 3D printing soft and degradable thiol-ene networks from cardanol-based polyesters. A cost-effective and eco-friendly synthesis of a cardanol-based diol was first developed using glycerol carbonate generated *in situ*, followed by two-step melt polycondensation with succinic acid to obtain a fully biobased polyester with reactive alkene groups. Cardanol-based resins were then DLP 3D printed *via* thiol-ene chemistry using a trifunctional thiol crosslinker. The printed networks exhibited soft mechanical behavior with high thermal stability, a low T<sub>g</sub> around -20 °C, and a Young modulus between 0.2 and 2 MPa. Their softness enabled the fabrication of complex structures, including highly deformable re-entrant auxetic architectures. Hydrolytic degradation studies performed in alkaline conditions showed complete degradation within two days, demonstrating the potential of cardanol-based networks for scalable and sustainable applications.

## 1. Introduction

Digital Light Processing (DLP) stands out among additive manufacturing technologies by enabling the high-resolution fabrication of complex 3D structures with high precision and speed [1,2]. Based on the photopolymerization of photo-sensitive resins, DLP offers advantages such as reduced material waste and scalability, making it a promising candidate for sustainable manufacturing processes [3]. However, the growth of the 3D printing market raises the need for more sustainable, degradable and recyclable polymer feedstocks due to concerns about plastic waste and fossil-based resources [4,5]. Sustainable polymers from renewable resources are therefore increasingly regarded as a viable alternative to fossil-based substances, addressing environmental, economic, and social challenges [6,7]. In particular, the development of biobased photo-sensitive resins for DLP 3D printing has gained considerable attention as it helps to reduce environmental impact [5,8]. Renewable feedstocks investigated for this purpose include plant oils [9], algae [10], cellulose [11], chitosan [12] and starch [13]. A promising strategy for further improving sustainability is the development of resins derived from industrial by-products [8]. Indeed, it is crucial to identify platform chemicals or building blocks that can be efficiently produced from abundant feedstocks while avoiding competition with the food industry and maintaining ecological balance.

In this context, a few industrial by-products such as lignin [14] and waste cooking oil [15] have been explored for use in DLP. However, many other promising by-products remain largely unexplored.

Among them, Cashew Nutshell Liquid (CNSL), a by-product of cashew nut processing, has gained significant attention as a renewable alternative to fossil-based resources. Easily extracted from cashew nut shells, CNSL contains phenolic compounds with multiple reactive sites, making it highly functionalizable [16]. Notably, cardanol is the most widely used compound derived from CNSL, as it constitutes the primary component of technical CNSL, with an annual production of approximately 800 kt [17]. Structurally, cardanol features a phenol-like core with a C15 alkyl chain at the meta position, which can exhibit up to three unsaturations. On average, cardanol contains approximatively 5-8 % saturated chain, 48-49 % monoene, 16-17 % diene and 29-30 % triene. Among the polymers that can be synthesized from cardanol derivatives, polyesters stand out for their potential biodegradability [18] as their ester bonds can be easily hydrolyzed. However, cardanol-based polyesters have been poorly studied. Only one paper has focused on the properties of fully cardanol-based polyesters [19]. In this work, Vallin et al. demonstrated the use of a commercially available cardanol-based diol to synthesize low-molecular weight polyesters to be used as plasticizer for poly(lactic acid) materials. Furthermore, no polymer networks derived from cardanol-based polyesters have been reported so far.

<footnote><sup>a</sup> ICGM, University of Montpellier, CNRS, ENSCM, Montpellier, France</footnote>

<footnote><sup>b</sup> Institut de Chimie et Biochimie Moléculaires et Supramoléculaires (ICBMS), CNRS, Université Lyon 1, Villeurbanne, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: sylvain.caillol@enscm.fr (S. Caillol), vincent.lapinte@umontpellier.fr (V. Lapinte).</footnote>
However, cardanol has been widely used to create crosslinked materials through thermal or UV processes, involving various polymerization reactions, leading to epoxies [20–27] polyurethanes, [28,29] acrylate/methacrylate-based thermosets [30–33] or thiol-ene networks [34,35]. These materials are typically thermally stable and rigid, exhibiting high glass transition temperatures ($T_g$), which limits their potential applications. However, two studies have shown that thiol-ene networks derived from cardanol-based precursors can also demonstrate soft behavior, paving the way for the development of soft cardanol-based devices [34,35].

More broadly, thiol-ene chemistry is emerging as a promising approach for the DLP 3D printing of soft biobased materials, offering rapid and uniform network formation under UV light at room

temperature. In addition, oxygen inhibition is significantly reduced in thiol-ene photo-polymerization [36]. Given these benefits, current research focuses on selecting reagents that allow the DLP 3D printing of thiol-ene networks that are both biobased and fully degradable [5,37–40]. Thiol-ene chemistry has been applied in the fabrication and DLP 3D printing of bio-based monomers such as levuglucosan, [41,42] levuglucosenone [37,38] terpenes, [39,43] or isosorbide [39,40], but not for cardanol-based polymer networks. To our knowledge, only one study, from Chen et al. [32] has reported the DLP 3D printing of cardanol-based materials, using a previously methacrylated cardanol-based precursor. However, the resulting structures were stiff, exhibiting a high $T_g$, and were printed into simple and non-degradable architectures.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** DLP 3D printing of cardanol-based materials. (A) Formulation of the resin using cardanol-based polyester PCSGu, the trifunctional thiol crosslinker TMTMP, the solvent diethyl carbonate, and the photo-initiator BAPO. (B) Schematic representation of the DLP 3D printing process, converting the cardanol-based resin into a crosslinked cardanol-based network.</figcaption></figure>
Here, we introduce an approach to DLP 3D print soft and degradable thiol-ene networks from cardanol-based polyesters. A cost-effective and eco-friendly synthesis of a cardanol-based diol was first developed using in situ generated glycerol carbonate, followed by the two-step melt polycondensation of this diol with succinic acid to produce a fully bio-based polyester with reactive alkene groups. By taking advantage of these readily reactive alkene groups, cardanol-based samples were DLP 3D printed via thiol-ene chemistry using a trifunctional thiol crosslinker, as shown in Fig. 1. They were then characterized for their thermal, mechanical, and thermo-mechanical properties. To further demonstrate the potential of cardanol-based networks in complex 3D-printed structures, re-entrant auxetic architectures were fabricated via DLP 3D printing, and their hydrolytic degradation was investigated. Overall, this study demonstrates the potential of cardanol for the synthesis of soft biobased thermosets through a simple and scalable chemistry, opening new opportunities for industrial applications.

## 2. Materials and methods

### 2.1. Materials

Cardanol (≥98 %) was provided by Orpia Group. Glycerol (≥99 %), potassium carbonate (99 %), deuterated chloroform (CDCl₃), tin(II) 2-ethylhexanoate (92.5–100.0 %), trimethylolpropane tris(3-mercaptopropionate) TMTMP (≥95 %) and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide BAPO (≥97 %) were purchased from Sigma Aldrich. Diethyl carbonate (99 %) was purchased from Thermo scientific. Magnesium sulfate was purchased from VWR chemicals. Hydrochloric acid and toluene were purchased from Honeywell. Ethyl acetate and sodium chloride were purchased from Fisher Scientific. Dichloromethane and methanol were purchased from Carlo Erba.

### 2.2. Synthesis of Cardanol diol from Glycerol (CG)

Cardanol (1.000 g, 3.31 mmol), glycerol (0.914 g, 9.93 mmol), diethyl carbonate (0.782 g, 6.62 mmol) and potassium carbonate (0.229 g, 1.65 mmol) were added into a 25 mL double-neck round bottom flask equipped with a magnetic stirrer and a condenser. The mixture was heated under argon atmosphere at 140 °C for 4 h. Afterward, the mixture was diluted in 25 mL of ethyl acetate, acidified with 1 M HCl and washed three times with distilled water by liquid/liquid extraction. The organic phase was dried over MgSO₄. After filtration, the filtrate was concentrated under vacuum. An orange viscous liquid was obtained with a yield of 84 % (1.064 g).

¹H NMR (400 MHz, CDCl₃) δ 7.23–7.09 (m, 1H, **b**), 6.79 (m, 1H, **a**), 6.72 (m, 2H, **c** + **d**), 5.82 (m, 1H, **l**), 5.49–5.26 (m, 3H, **i**), 5.11–4.93 (m, 1H, **m**), 4.12 (m, 1H, **p**), 4.00 (m, 2H, **o**), 3.89–3.71 (m, 2H, **q**), 2.87–2.68 (m, 3H, **j**), 2.54 (m, 2H, **e**), 2.03 (m, 3H, **h**), 1.60 (m, 2H, **f**), 1.33 (m, 13H, **g**), 0.91 (m, 2H, **n**). (Fig. S2).

### 2.3. Synthesis of cardanol-based polyester (PCGSu)

CG (80.00 g, 0.21 mol), succinic acid (25.11 g, 0.21 mol) and about 4 g/w/w% of toluene (~5 mL) were added in a 100 mL glass reactor equipped with a mechanic stirrer fitted with a mini propeller, a Dean-Stark, a thermometer and a gas inlet. The mixture was heated to 180 °C under nitrogen atmosphere during one hour. Then, 1 mol% of Sn (Oct)₂ (0.86 g) is added after dilution in 4 mL toluene and vacuum (0.5 to 0.1 bar) was applied to the system gradually over 7 h. Finally, the resulted polymer was diluted with about 40 mL of chloroform and precipitated in 500 mL of cold methanol to obtain a viscous orange polymer after drying under vacuum (83.66 g, yield: 80 %).

¹H NMR (400 MHz, CDCl₃) δ 7.16 (m, 1H, **b**), 6.83–6.75 (m, 1H, **a**), 6.71 (m, 2H, **c** + **d**), 5.91–5.71 (m, 1H, **l**), 5.49–5.26 (m, 4H, **i**), 5.01 (m, 1H, **m**), 4.54–3.50 (m, 5H, **o** + **p** + **q**), 2.80 (m, 2H, **j**), 2.73–2.59 (m, 4H, **r**), 2.55 (m, 2H, **e**), 2.03 (m, 3H, **h**), 1.59 (m, 2H, **f**), 1.47–1.08 (m,

13H, **g**), 0.90 (m, 2H, **n***). To prevent any confusion with the degree of polymerization **n**, **n*** has been chosen for proton assignment. (Fig. S4).

### 2.4. DLP 3D-printing of the cardanol-based resins

Three cardanol-based resins were formulated by mixing 5 g of PCGSu, the chosen mass of TMTMP (0.453, 0.679 or 1.359 g), 100 % w/ w of diethyl carbonate and 2 % w/w of BAPO in a 25 mL vial. The vial was then placed in an ultrasonic bath for 5 min at 40 °C to obtain a homogeneous resin. 3D models were designed using the software Rhinoceros 3D and exported as STL files. The 3D structures were then built from the cardanol-based resins by DLP stereolithography using a commercial Asiga Max X43 machine (KREOS supplier, France) with an integrated 385 nm LED and a pixel resolution of 43 µm. The ideal layer thickness was fixed at 100 µm. The optimum irradiation time was determined using Jacob's equation [44] by measuring the generated curing depths at 50 mW/cm². After construction, the structures were post-cured under 365 nm UV light at 20 mW/cm² for 20 min. They were then swelled in diethyl carbonate for 24 h and dried in a vacuum oven at 90 °C for 24 h.

### 2.5. Gel fraction and swelling ratio tests

To determine gel fractions and swelling ratios, dumbbell-like samples from all three cardanol-based networks were dried in a vacuum oven at 90 °C for 24 h before the initial weight was determined (mᵢ). The samples were then swollen in diethyl carbonate for 24 h and weighed in a swollen state (mₛ). They were then dried in a vacuum oven at 90 °C for 24 h before the final weight was determined (mᵢ). The gel fraction and swelling ratio for each cardanol-based network was calculated using Eqs. (1) and (2):

$$ \text{Gel fraction} = \frac{m_f}{m_i} \times 100 \qquad (1) $$

$$ \text{Swelling ratio} = \frac{m_s - m_i}{m_i} \times 100 \qquad (2) $$

### 2.6. Degradation tests

Cardanol-based materials were put in about 40 mL of 2 M NaOH aqueous solution in a sealed glass jar and left for several days to degrade completely. At the end of the degradation test, approximately 20 mL of ethyl acetate was added and a liquid–liquid extraction was performed to clean and isolate organic phase. The latter was then dried with MgSO₄ and filtered before being dried.

### 2.7. ¹H NMR spectroscopy

¹H NMR analyses were performed using a Bruker Avance 400 MHz spectrometer at 25 °C. NMR samples were prepared with CDCl₃ as solvent.

### 2.8. Gas chromatography (GC)

GC analyses were performed on a Shimadzu Gas Chromatograph GC-2025 equipped with a ZB-5-MS column (30.0 m × 0.25 mm × 0.25 µm) and with a FUSED SILICA capillary column (10.0 m × 0.1 mm × 0.1 µm film thickness) purchased from Sigma-Aldrich and an FID (flame ionisation detector). The carrier gas was N₂ at a flow rate of 1.27 mL min⁻¹ and H₂ gas was used as a carrier at 40 mL min⁻¹. The column temperature was initially 240 °C for 1 min, then gradually increased to 260 °C at 5 °C min⁻¹ and at 20 °C min⁻¹ up to 310 °C. Finally, the temperature was kept at 310 °C for 1.5 min. The injector temperature was set at 315 °C.
## 2.9. FTIR spectroscopy

FTIR spectra were recorded using a ThermoScientific Nicolet i50 spectrometer equipped with a Golden-Specac heating cell and ATR module. Data were collected with a resolution of 4 cm⁻¹ and averaged over 32 scans. The spectra were normalized to the C=O ester stretching vibration peak at 1739 cm⁻¹ when comparing the three cardanol-based networks together.

## 2.10. Rheology measurements

Shear rate sweep tests were performed using an Anton Paar Rheometer MCR 302 equipped with a cone plate geometry (diameter: 25 mm, angle: 2°, cone truncation: 105 µm). Measurements were carried out at 25 °C, and the shear rate was varied from 0.001 to 0.1 s⁻¹. The values were compared with a shear rate where all the samples had reached a plateau.

## 2.11. Size-exclusion chromatography (SEC)

SEC analyses were conducted using a triple-detection GPC from Agilent Technologies. The system was equipped with a guard column (Agilent Mesopore, 50 × 7.5 mm) and two columns (Agilent Mesopore and Resipore, 300 × 7.5 mm). The mobile phase was tetrahydrofuran (THF) at a flow rate of 1.0 mL min⁻¹. The detector used was a 390-LC PL0390-0601 refractive index detector. The entire system was thermostated at 35 °C. Polystyrene (PS) standards were used for calibration between 162 and 371 100 g/mol using Agilent GPC/SEC v1.2 software. A kinetic monitoring was performed during the polycondensation resulting in PCGSu. Around 10 mg of the reaction product was diluted in 2 mL of THF and then filtered with a PTFE filter of 0.20 µm before analysis.

## 2.12. Thermogravimetric analysis (TGA)

TGA analyses were performed on a TA Q50 Instruments. The washed and dried samples, weighing between 5 and 10 mg, were placed in an aluminum pan. The analyses were carried out with a gradient temperature from 20 to 580 °C was applied at a heating rate of 20 °C·min⁻¹, under nitrogen. Tmax was defined as the temperature at the maximum degradation rate obtained using the derivative curve DTGA.

## 2.13. Differential Scanning Calorimetry (DSC)

DSC was used to evaluate the thermal properties of the cardanol-based networks. DSC analyses were carried out using a Netzsch DSC 3500 Sirius. The washed and dried samples, weighing between 5 and 15 mg, were loaded into standard aluminum DSC pans. The samples were equilibrated at -100 °C and heated to 130 °C at a rate of 20 °C·min⁻¹. They were then cooled back to -100 °C and reheated to 130 °C at a rate of 20 °C·min⁻¹. Data were reported from the second heating scans. The glass transition temperature Tg, DSC was defined at the mid step change in the slope of the heat flow signal.

## 2.14. Uniaxial tensile testing

The mechanical properties of the cardanol-based networks were assessed using tensile tests with an Instron 3366L5885 mechanical tester equipped with a 100 N load cell. Tensile specimens measuring around 22 × 7 × 1.4 mm³ were printed, washed and dried before being tested at a displacement rate of 5 mm s⁻¹. The Young modulus (in MPa) was measured by taking the slope in the linear region of the stress-strain curve between 0 and 5 % strain. The strain at break was defined as the maximum strain that the specimen reaches before breaking. The stress at break was then defined as the stress value corresponding to the strain at break.

## 2.15. Dynamic Mechanical Analysis (DMA)

DMA was employed to study the thermo-mechanical properties of the cardanol-based networks. DMA tests were performed using a Mettler Toledo DMA instrument. Tensile specimens measuring around 22 × 7 × 1.4 mm³, were tested in tensile mode, with an active length of 10 mm. The samples were equilibrated at -50 °C. They were then subjected to a strain of 0.1 % at 1 Hz and heated from -50 to 120 °C at a rate of 2 °C/min. Tg, DMA and TX, DMA were respectively defined as the temperatures corresponding to the first and second peak maxima of the loss tangent (tan δ) curve. The crosslinking density (mol/m³) was calculated using Eq. (3):

$$ \nu_c = \frac{E_{Tg+50^\circ C}}{3R(T_{g, DMA} + 50^\circ C)} \quad (3) $$

where E'$_{Tg}$ +50 °C (MPa) is the storage modulus at the rubbery plateau and R is the universal gas constant (8.314 K⁻¹ mol⁻¹).

## 2.16. Gas chromatography coupled to mass spectrometry (GC-MS)

GC-MS analysis was performed with a QP2010 SE Shimadzu chromatographer (Canby, OR, USA) surmounted with a Zebron ZB-5 ms column (30 m × 0.25 mm × 0.25 µm). The carrier gas was He, the column flow was set at 1 mL·min⁻¹ and the injection mode was split (ratio 1:30). The injector temperature was set at 300 °C. The GC detector was composed of a quadrupole system and the MS detector was an electronic impact. The oven was set at 250 °C for 2 min, then the temperature was raised to 325 °C at 20 °C·min⁻¹. Finally, the temperature was maintained at 325 °C for 10 min. The sample was diluted in ethyl acetate to a concentration of approximately 1 mg for 2 mL of solvent.

# 3. Results and discussions

## 3.1. Synthesis of cardanol-based diol and polyesters

To enable polyester synthesis from cardanol, its functionalization into diols was first required. However, although cardanol-based diols are biobased, their synthesis often requires toxic reagents and several synthesis steps, making them difficult to scale up. Over the last decade, a specific cardanol-based diol, labeled CG, has been functionalized via Williamson etherification with epichlorohydrin [45] or α-chlorohydrin [46]. CG contains a propane-1,2-diol unit attached through an ether bond at the hydroxyl group of cardanol. While this method achieves high conversion and purity in just one or two steps, it still relies on toxic reagents. To avoid the use of CMR reagents, a patent demonstrated an alternative approach in which CG is obtained through the ring-opening of glycerol carbonate [47]. Glycerol carbonate is a highly promising starting material for the synthesis of various building blocks due to its high reactivity [48]. However, despite being commercially available, its cost remains too high for large-scale polymer manufacturing. Addressing this limitation, Truscello et al. [49] demonstrated that phenolic derivatives can be functionalized to obtain diols from glycerol by generating in situ glycerol carbonate in a one-pot reaction. Applying this approach to the functionalization of cardanol enabled the synthesis of CG through a more environmentally friendly method (Fig. 2A). This one-pot synthesis involves two stages: first, the in situ formation of glycerol carbonate, and second, the nucleophilic attack of cardanol on the cyclic carbonate. To prevent the transcarbonation of the resulting product, a large excess of glycerol was required. The reaction mechanism, inspired by the study of Galletti et al. [50] is proposed along with all potential by-products in Fig. S1. After optimization, it was determined that for 1 equivalent of cardanol, 3 equivalents of glycerol and 2 equivalents of diethyl carbonate yielded the highest conversion in the shortest reaction time. Furthermore, the addition of 0.1 equivalent of K₂CO₃ was found to be sufficient to deprotonate the phenolic compound and to promote the
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Synthesis of the cardanol-based polyester PCGSu from the cardanol-based diol CG. (A) Etherification of cardanol using glycerol as a starting reagent. (B) Solvent-free polycondensation of CG with succinic acid to synthesize PCGSu. (C) ¹H NMR spectra of cardanol, CG, and PCGSu in CDCl₃ as deuterated solvent. (D) Kinetic monitoring of the polycondensation of CG with succinic acid by GPC in THF, using a RI detector with PS calibration.</figcaption></figure>

in situ formation of glycerol carbonate within a relatively short time. Following previous studies on phenolic compounds [50], the temperature was set at 140 °C throughout the reaction to promote both glycerol carbonate formation and nucleophilic attack of cardanol on cyclic carbonate. During the first stage, the temperature was limited by the boiling point of diethyl carbonate, around 126 °C. Once most of the diethyl

carbonate had reacted to form glycerol carbonate, the temperature naturally increased to 140 °C, promoting the formation of CG. After 4 h, cardanol was fully converted into the desired product. The conversion of cardanol into CG was monitored by ¹H NMR spectroscopy, based on the chemical shift of the proton in the ortho position relative to the hydroxyl and C₁₅H₃₁-₂N groups, which shifted from 7.15 ppm for proton a to 7.19 ppm.
ppm for proton a' (Fig. 2C and Fig. S2). After purification and drying, CG was obtained in good yield (84 %), with no solvent residues and a relatively high purity of 96 % confirmed by GC analysis (Fig. S3). These results are critical for polycondensation, as achieving high molecular weights requires precise knowledge of monomer purity to ensure the correct stoichiometric conditions. Furthermore, in the case of CG, the presence of a secondary hydroxyl group in the monomer makes attaining high molecular weights more challenging, as it is less reactive than a primary hydroxyl group.

The synthesis of cardanol-based polyester was then carried out via two-step melt polycondensation (Fig. 2B). In the first step, the reaction is self-catalyzed and conducted under a nitrogen flux at 180 °C for one hour to promote monomer esterification. In the second step, the reaction is catalyzed by tin octanoate and carried out under vacuum at the same temperature for seven hours to facilitate molecular weight growth. During the first step, succinic acid condensed on the reactor walls, causing a stoichiometric imbalance in favor of CG. This occurs because succinic acid has time to accumulate before reacting, due to the low reactivity of the secondary hydroxyl group of CG. To minimize this effect, approximately 5 % w/w toluene, relative to the total monomer mass, was added to induce reflux and form an azeotrope with water. This helps wash the reactor walls while facilitating the efficient removal of water from the reaction medium. Once conversion exceeded 70 %, as shown by ¹H NMR with the disappearance of the α-hydroxy protons *d'* (3.89–3.71 ppm) in favor of the α-ester protons *d''* (4.54–4.14 ppm) (Fig. 2C), a slight vacuum was applied to remove most of the water formed during the first step. The catalyst was then added, and the vacuum was progressively reduced from 0.5 to less than 0.1 bar. At the end of the reaction, the fully biobased cardanol-based polyester PCGSu was reached, with a DPn of 9 corresponding to a Mn of 4400 g/mol, and a dispersity index of 1.5, as determined by SEC of PCGSu after precipitation (Fig. S5). The kinetic monitoring curves by SEC revealed that oligomers with higher molecular weights were effectively formed during the second step. Furthermore, the distinct peaks corresponding to various DPn values demonstrated the ability of the column to efficiently distinguish the shortest oligomers (Fig. 2D). At the end of the synthesis, a highly viscous liquid is obtained, with a faint yellow/orange color. The structure of PCGSu was confirmed by ¹H NMR (Figs. 2C and S4), while FTIR spectroscopy verified the polyester synthesis through the appearance of the C=O ester stretching vibration peak at 1739 cm⁻¹ (Fig. S6). In addition, a higher reaction temperature could not be applied, as the double bonds in the alkyl chain of CG could lead to undesired side reactions. Specifically, in the presence of a Lewis acid such as tin octanoate, Friedel-Crafts reactions may occur, leading to CG crosslinking [51–53]. However, under the applied reaction conditions, no loss of double bonds was observed, as confirmed by comparing the ¹H NMR spectra of raw cardanol, CG and PCGSu (Fig. 2C).

## 3.2. Formulation and DLP 3D printing of cardanol-based resins

PCGSu was used to prepare three photo-curable PCGSu-based resins for DLP 3D printing of degradable thiol-ene networks. PCGSu contains alkene groups along the aliphatic side chains that can undergo thiol-ene chemistry. The materials used for the resin formulation, shown in Fig. 1A, were carefully selected. To enable efficient photo-crosslinking, TMTMP was selected as a thiol crosslinker due to its trifunctionality, ensuring the formation of a well-structured network. In addition, TMTMP introduces ester bonds that, along with those in the polyester backbone, enhance hydrolytic degradability. Diethyl carbonate was chosen because it is a green solvent with a boiling point high enough to prevent evaporation during printing and can be easily removed during material drying [54]. Finally, BAPO was chosen as the photo-initiator due to its strong UV absorption [55]. To tailor material properties through the formulation of the three resins, the thiol content was varied to modulate the C=O to SH ratio, which influences the crosslinking density of the resulting material. To formulate these three resins, we

assumed that only one double bond in the aliphatic side group of PCGSu participated in the thiol-ene photo-crosslinking. Indeed, although cardanol can contain up to three double bonds, not all are sufficiently reactive in thiol-ene reactions due to steric hindrance. Considering the distribution of the different forms of cardanol (monoene, diene and triene) and assuming that only one double bond reacts per form, the average number of reactive double bonds is close to 1 [16]. Therefore, with a DPn of 9, PCGSu has approximately 9 reactive double bonds. As a result, its functionality was considered to be 9, representing the number of reactive double bonds available for crosslinking. The first resin was formulated so that nearly all double bonds reacted with the thiol crosslinker. The resulting network was labelled PCGSu-TMTMP_1:1. The second resin was formulated so that only half of the double bonds reacted with the thiol crosslinker. The resulting network was labelled PCGSu-TMTMP_2:1. Finally, the third resin was formulated so that one-third of the double bonds reacted with the thiol crosslinker. The resulting network was labelled PCGSu-TMTMP_3:1. (Fig. 3A). After optimizing the formulations, 100 % w/w diethyl carbonate relative to the total mass of reagents was added to significantly reduce viscosity and enable the printability of the resins, as viscosity is a crucial parameter for achieving high-quality printed objects. Indeed, the printable viscosity range for DLP 3D printing typically spans from 2 × 10² to 1 × 10⁴ mPa.s, ensuring good flowability and high molecular diffusion [9]. These factors enhance reactivity during the photocuring process and facilitate uniform layer formation. Initially, PCGSu exhibited a high viscosity of approximately 3 × 10⁴ mPa.s, whereas the formulated resins achieved a significantly lower viscosity of around 5 × 10¹ mPa.s, making them suitable for printing (Fig. S7). Before printing, a Jacob's working curve was established for each resin to determine the optimal printing parameters. This enables to prevent over-crosslinking and ensures optimal resolution for printed objects. A semi-logarithmic plot of curing depth versus light irradiation dosage was generated for each formulation, and the data points were fitted linearly to determine the appropriate exposure time for accurate layer curing (Fig. 3B). The optimal exposure times at 50 mW/cm² were determined to be 8 s for the resin resulting in the PCGSu-TMTMP_1:1 network, 9 s for the resin resulting in the PCGSu-TMTMP_2:1 network, and 10 s for the resin resulting in the PCGSu-TMTMP_3:1 network, each corresponding to a layer thickness of 100 µm. In particular, increasing the thiol content enhanced printing speed by increasing the frequency of interactions between thiol and alkene groups.

Based on the determined printing parameters, dumbbell-like samples were DLP 3D-printed from the three resins (Fig. 3C). The layers were oriented along the length of the dumbbell-like samples, so that they were aligned with the tensile direction. This configuration makes it possible to assess material strength without stressing the interfaces between layers. The samples exhibited gel fractions ranging from 60 % to 85 %, calculated using Eq. (1), indicating moderate crosslinking (Fig. S8). Swelling tests, calculated using Eq. (2), confirmed the expected trends in network crosslinking density (Fig. S9). PCGSu-TMTMP_1:1 exhibited the lowest swelling ratio, indicating the highest crosslinking density, as nearly all double bonds were incorporated in the network. In contrast, PCGSu-TMTMP_3:1 showed the highest swelling ratio, indicating the lowest crosslinking density. To confirm the presence of unreacted pendant alkene groups that did not participate in the photo-crosslinking for PCGSu-TMTMP_2:1 and PCGSu-TMTMP_3:1, the washed and dried dumbbell-like samples were analyzed using FTIR spectroscopy (Fig. 3D). In PCGSu-TMTMP_1:1, the characteristic C=H stretching vibration at 3025 cm⁻¹ almost completely disappeared, indicating that most of the double bonds were consumed during photo-crosslinking. In PCGSu-TMTMP_2:1 and PCGSu-TMTMP_3:1, as the amount of thiol crosslinker used decreased, the intensity of the C=H stretching vibration increased, confirming the presence of unreacted alkene groups in these networks.
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** DLP 3D printing of cardanol-based resins. (A) Schematic representation of the three different network structures. (B) Jacob's working curves for the three resins. (C) Photographs of DLP-printed cardanol-based dog-bone samples. (D) FTIR spectra of the three networks, with a zoom-in on the $\equiv$C-H stretching vibration region.</figcaption></figure>

### 3.3. Characterization of DLP 3D-printed cardanol-based materials

The thermal stability of PCGSu and PCGSu-TMTMP networks was assessed using TGA, as shown in Fig. 4A. First, by comparing the temperatures of maximum degradation rate ($T_{max}$), PCGSu exhibited a higher $T_{max}$ than the corresponding cardanol-based monomer CG, indicating that polymerization enhances thermal stability (Figs. S10 and S11). The $T_{max}$ value of PCGSu aligns with previously reported values for cardanol-based polyesters, around 400 °C, despite their structures incorporating the cardanol motif within the repeating unit [19]. Furthermore, PCGSu-TMTMP networks show a comparable $T_{max}$ to PCGSu (Fig. 4A and S11). A slight increase in thermal stability is observed as crosslinking density is reduced, from 405 °C for PCGSu-TMTMP_1:1 to 412 °C for PCGSu-TMTMP_3:1. The thermal properties of PCGSu and PCGSu-TMTMP networks were then assessed using DSC analyses, as shown in Fig. 4B. The values of $T_g$, $DSC$ for PCGSu-TMTMP

networks are given in Table 1. The $T_g$, $DSC$ value of PCGSu at -30 °C is consistent with that of another cardanol-based polyester of similar chain length reported in the literature [19]. This low $T_g$ value is attributed to the aliphatic nature and flexibility of the succinate-based segments. For PCGSu-TMTMP networks, $T_g$, $DSC$ values are relatively similar, with the least crosslinked network, PCGSu-TMTMP_3:1, exhibiting the lowest $T_g$, $DSC$ of -20 °C. Such low $T_g$ values are uncommon for cardanol-based networks. Only one study, from Lin et al., [35] has reported a cardanol-based thiol-ene network with a $T_g$ of -13 °C, while most cardanol-based networks, including epoxy, polyurethane, and acrylate systems, typically have high $T_g$ values above room temperature [22,29,32,56-60].

To evaluate mechanical properties of PCGSu-TMTMP networks under static loading conditions, tensile tests were conducted, as shown in Fig. 4C. The values of Young modulus, strain at break, and stress at break are listed in Table 1. For PCGSu-TMTMP networks, stiffness
<figure><img src="image_5.png" /><figcaption>Fig. 4. Thermal, mechanical, and thermo-mechanical properties of the DLP 3D-printed cardanol-based materials. (A) TGA curves of the three networks. (B) DSC thermograms of the three networks. (C) Stress-strain curves of the three networks. (D) Storage modulus and loss tangent curves of the three networks.</figcaption></figure>

<table><caption>**Table 1**

Summary of the thermal, mechanical, and thermo-mechanical properties of DLP 3D-printed cardanol-based materials.</caption><thead><tr><th></th><th>T<sub>max</sub><br>(°C)</th><th>T<sub>g, DSC</sub><br>(°C)</th><th>Young Modulus<br>(MPa)</th><th>Stress at break<br>(MPa)</th><th>Strain at break<br>(%)</th><th>T<sub>g, DMA</sub><br>(°C)</th><th>T<sub>x, DMA</sub><br>(°C)</th><th>E'<sub>Tg +50 °C</sub><br>(MPa)</th><th>ν<sub>c</sub> (mol/m<sup>3</sup>)</th></tr></thead><tbody><tr><td>PCGSu-TMTMP_1:1</td><td>405</td><td>-14</td><td>1.93 ± 0.08</td><td>0.48 ± 0.03</td><td>31.6 ± 2.7</td><td>-9</td><td>12</td><td>2.4</td><td>3.1 × 10<sup>2</sup></td></tr><tr><td>PCGSu-TMTMP_2:1</td><td>410</td><td>-14</td><td>0.78 ± 0.06</td><td>0.12 ± 0.01</td><td>20.1 ± 3.3</td><td>-14</td><td>14</td><td>1.0</td><td>1.3 × 10<sup>2</sup></td></tr><tr><td>PCGSu-TMTMP_3:1</td><td>412</td><td>-20</td><td>0.29 ± 0.06</td><td>0.07 ± 0.01</td><td>29.5 ± 6.6</td><td>-17</td><td>18</td><td>0.6</td><td>0.8 × 10<sup>2</sup></td></tr></tbody></table>

decreases as crosslinking density is reduced, with Young modulus values ranging from 1.9 MPa for PCGSu-TMTMP_1:1 to 0.3 MPa for PCGSu-TMTMP_3:1. As expected lower crosslinking density allows for greater chain mobility, which reduces network resistance to deformation under applied stress. However, no clear trend can be drawn from the strain at break values, which remain around 30 % strain. Finally, as observed for Young modulus in PCGSu-TMTMP networks, stress at break decreases with crosslinking density, as a network with lower crosslinking density requires less stress to reach a given strain. The mechanical properties of these networks exhibit a soft behavior, distinguishing them from typical

cardanol-based networks, which generally have Young modulus values higher than 10² MPa. Compared to the only study reporting DLP 3D printing of cardanol-based networks [32], where Young modulus ranges between 250 and 450 MPa and a strain at break not exceeding 6 %, the networks developed here show significantly lower Young modulus values (0.3–1.9 MPa) and higher strain at break (~30 %). This highlights that our approach enables the DLP 3D printing of more flexible cardanol-based networks. To further assess the thermo-mechanical properties of PCGSu-TMTMP networks under dynamic loading conditions, DMA was performed, as shown in Fig. 4D. The values of T<sub>g, DMA</sub>, T<sub>x, DMA</sub>, E'<sub>Tg +</sub>
50 °C, and $v_c$ are listed in Table 1. Regarding $T_g$, DMA, the values are within the same temperature range as those obtained by DSC when taken at the peak of the tan δ curve. However, DMA provides a more precise analysis than DSC, offering a clear distinction between transition peaks. Although the variation is small, $T_g$, DMA decreases as crosslinking density is reduced, highlighting the influence of the network structure on chain mobility. A second relaxation, not observed in DSC, was identified by DMA. While tan δ typically exhibits a single peak corresponding to the glass transition, here, a broad tan δ curve extends from $T_g$, DMA to $T_X$, DMA, a higher temperature where a second peak appears with an even greater tan δ value. In particular, $T_X$, DMA follows an inverse trend compared to $T_g$, DMA, increasing as crosslinking density decreases. To our knowledge, this behavior has not been previously reported in cardanol-based polymer networks. However, it is similar to what is observed in liquid crystal elastomers, which also exhibit a broad tan δ curve between their glass transition temperature and the nematic-to-isotropic transition temperature [61]. In our case, the cardanol motif is located in PCGSu side chain, similar to side-chain liquid crystal elastomers, [62] allowing for potential π-π stacking interactions within the network. Although this effect is less pronounced than in liquid crystal elastomers, this second relaxation at $T_X$, DMA could therefore be attributed to the organization of aromatic motifs, leading to an extended energy dissipation region. Furthermore, as crosslinking density decreases, meaning that the fraction of aromatic motifs within the network increases, $T_X$, DMA shifts to a higher temperature. This shift in $T_X$, DMA supports our hypothesis that these interactions could contribute to energy dissipation in the network, as more energy would be required to disrupt π-π stacking interactions when more cardanol motifs are present, provided they self-organize. The storage modulus $E'$$_{Tg}$ + 50 °C reflects the stiffness and elastic response of the networks under dynamic loading conditions. The stiffness trends observed in Young modulus are consistent with those in $E'$$_{Tg}$ + 50 °C, confirming that at temperatures close to ambient, higher crosslinking density results in increased stiffness. Finally, the crosslinking density $v_c$ was calculated from $E'$$_{Tg}$ + 50 °C using Eq. (3). Determining $v_c$ is essential as it supports the observed trend in crosslinking density and its role in influencing the mechanical and thermomechanical properties of the networks. The calculated $v_c$ values are consistent with previously reported data, with PCGSu-TMTMP_3:1 exhibiting the lowest $v_c$ value. Furthermore, $v_c$ values are on the order of $10^2$ mol/m$^3$, indicating that the networks are lightly crosslinked. These cardanol-based networks exhibit $v_c$ values typically reported in the literature, ranging from $10^2$ to $10^3$ mol/m [3.20,25,29,35,56,57,59,60] This low crosslinking density, combined with the chain mobility provided by the crosslinking of low-$T_g$ PCGSu, contributes to the soft behavior observed in the 3D-printed cardanol-based networks. In particular, the 3D-printed dumbbell-like samples from all three networks can be easily folded in half without breaking, confirming their low crosslinking density and soft mechanical behavior. They also exhibit high elasticity, instantly returning to their original shape after folding them repeatedly (Videos S1).

### 3.4. DLP 3D printing and degradation of a cardanol-based auxetic structure

Subsequently, we aimed to demonstrate the potential of these cardanol-based soft networks in complex 3D-printed structures using DLP. In particular, their observed softness and high elasticity make them suitable for auxetic structures. Auxetic structures stand out due to their negative Poisson's ratio, which allows them to expand laterally under tension and densify under compression [63]. This unique mechanical behavior provides high anisotropy, enhanced energy absorption, permeability, and fracture toughness. As a result, they are widely used in aerospace engineering, footwear, and biomedical implants for their superior mechanical performance [64]. To print these auxetic structures with DLP, we selected the resin formulation resulting in PCGSu-TMTMP_3:1, as this network exhibited the softest behavior. Two re-

entrant auxetic structures were then printed: one with small unit cells and a thickness of 2 mm, and another with larger unit cells and a thickness of 5.3 mm (Fig. 5A, B and S12A). The thinner re-entrant structure could be easily folded in half and even in four, demonstrating its elasticity (Fig. 5C). The thicker re-entrant structure exhibited a remarkable mechanical response, compressing to half its original size, whereas the non-auxetic material tends to exhibit brittle failure under compression. In particular, the thicker re-entrant structure was manually compressed multiple times and instantly returned to its original shape upon unloading, demonstrating its shape-recovery ability (Fig. 5D and Video S2). In addition, cardanol-based gyroid structures were DLP 3D-printed to further demonstrate the high resolution of the obtained structures (Fig. S12B and C). After 6 months of exposure to ambient air, no visible signs of degradation were observed on the printed structures.

The alkaline hydrolysis of the 3D-printed objects was subsequently investigated, as both the ester bonds in the polyester backbone and those in the thiol crosslinker are susceptible to cleavage (Fig. 6). The aim was to demonstrate the ability of the networks to degrade, not into polyester fragments, but directly into simple building blocks that could be reutilized. To achieve complete hydrolysis of the crosslinked network, the 3D-printed structures were immersed in a 2 M NaOH aqueous solution. The thinner re-entrant auxetic structure based on the PCGSu-TMTMP_3:1 network, with a thickness of 2 mm, underwent complete hydrolysis within 44 h. This is attributed to its porous architecture, which significantly increased the surface area exposed to the alkaline medium. For comparison, dumbbell-like samples based on PCGSu-TMTMP_1:1, PCGSu-TMTMP_2:1, and PCGSu-TMTMP_3:1 networks were subjected to the same conditions. Unlike the re-entrant auxetic structure, these samples did not achieve full degradation even after 75 h, despite having a lower thickness of 1.4 mm. Furthermore, no significant difference in hydrolysis time was observed among the different dumbbell-like samples, indicating that the crosslinking ratio had a negligible impact on degradation kinetics under these conditions. Following hydrolysis, the degradation products of the re-entrant auxetic structure were separated by acidification of the medium with hydrochloric acid, followed by liquid-liquid extraction using ethyl acetate. The aqueous phase could not be analyzed because it contained too much sodium chloride salt. The organic phase was first analyzed by GC-MS (Fig. S13). The solvent peak appeared at around 1.9 min retention time and CG peak appeared at 7.7 min, confirmed by mass analysis with *m/z* equal to 376.4. At 5.4 min, another peak could be visible and was attributed to cardanol. However, as shown by the NMR spectrum (Fig. S14), cardanol peaks were not visible. This identification was attributed to fragmentation of an amount of CG during MS analysis. A yield of 67 % of CG "recovered" was obtained. Indeed, the auxetic structure was based on the PCGSu-TMTMP_3:1 network, meaning that only one-third of the double bonds participated in the crosslinked network, leaving two-thirds of the CG units intact. The experimental yield was consistent with the theoretical calculated value of 62 % w/w. The small difference may be due to incomplete thiol-ene reaction, as some thiol crosslinkers did not fully react. The remaining cardanol-based molecules likely correspond to modified CG and are probably water-soluble. However, the high NaCl content in the aqueous phase produced after acidification prevented analysis using conventional techniques such as NMR, FTIR, and GC-MS. Recovering the CG monomer in the organic phase enables its reuse for new cardanol-based polyesters or other crosslinked materials, highlighting the potential for material circularity in this process. Overall, as shown in Table S1, among other studies on biobased thiol-ene networks, this work stands out as a promising strategy to obtain soft and degradable networks with low $T_g$ via thiol-ene chemistry from a renewable resource.

### 4. Conclusion

This work presents an approach to DLP 3D print soft and degradable thiol-ene networks from cardanol-based polyesters. The strategy first
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** DLP 3D printing of auxetic structures based on the resin resulting in PCGSu-TMTMP_3:1. (A) 3D models of the re-entrant auxetic structures. (B) Photographs of the thicker re-entrant auxetic structure. (C) Folding of the thinner re-entrant auxetic structure. (D) Compression of the thicker re-entrant auxetic structure.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Hydrolysis of the thinner 3D-printed re-entrant auxetic structure over time.</figcaption></figure>
introduces a cost-effective and eco-friendly synthesis method for the
cardanol-based diol monomer CG. Unlike other approaches that rely on
toxic reagents or expensive glycerol carbonate, cardanol was function-
alized using in situ generated glycerol carbonate in a one-pot reaction,
producing high-purity cardanol diols from glycerol. A fully biobased
cardanol-based polyester containing reactive alkene groups was subse-
quently synthesized *via* a two-step melt polycondensation of CG with
succinic acid, resulting in PCGSu with a DPn of 9 and a Mn of 4400 g/
mol. This study also demonstrates the use of thiol-ene chemistry for the
DLP 3D printing of soft cardanol-based materials. Several resins were
formulated using PCGSu and a trifunctional thiol crosslinker, with the
thiol content adjusted to modulate the C=C to SH ratio, which influences
the crosslinking density. The resulting cardanol-based networks
exhibited a soft mechanical behavior with high thermal stability, low Tg
values around -20 °C, and a Young modulus ranging from 0.2 to 2 MPa.
Taking advantage of the softness and elasticity of these networks,
complex 3D-printed architectures such as gyroids and re-entrant auxetic
structures were fabricated. The auxetic structures were easily
compressible to twice their original size without failure, highlighting
their high mechanical adaptability and deformability. Finally, the hy-
drolytic degradation of the auxetic structures was investigated, showing
complete degradation in less than two days and successful recovery of
the CG monomer in the organic phase *via* liquid-liquid extraction.
Overall, this work highlights the potential of cardanol-based polyesters
for the scalable production of sustainable and degradable materials *via*
DLP 3D printing, contributing to the development of biobased thermo-
sets and reinforcing their viability for future applications.

CRediT authorship contribution statement

**Emilie Rojtman:** Writing – original draft, Resources, Investigation, Conceptualization. **Rakine Mouhoubi:** Writing – original draft, Resources, Investigation, Conceptualization. **Estelle Metay:** Writing – review & editing, Resources, Investigation. **Sébastien Blanquer:** Writing – review & editing, Resources, Methodology. **Sylvain Caillol:** Writing – review & editing, Supervision, Project administration. **Vincent Lapinte:** Writing – review & editing, Writing – original draft, Supervision, 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.

Acknowledgments

The authors would like to thank Orpia Innovation (France) for
providing cardanol used in this study and for supporting previous
research that partially contributed to this work.

Appendix A. Supplementary data

Supporting Information contains reaction scheme of CG synthesis, ¹H NMR spectra (CG, CG recovered in the organic phase, PCGSu), GC chromatogram of CG, SEC chromatogram of PCGSu, FTIR spectra of CG and PCGSu, rheological measurements of PCGSu and cardanol-based resins, gel fractions and swelling ratios of DLP 3D-printed cardanol-based samples, TGA of CG and DTGA of CG and PCGSu, photographs of DLP 3D-printed complex structures, videos of folded DLP 3D-printed dumbbell-like sample and compressed DLP 3D-printed auxetic structure. The files are available free of charge. Supplementary data to this article can be found online at https://doi.org/10.1016/j.eurpolymj.2025.114076.

Data availability

Data will be made available on request.

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