# Oxidative self-polymerization of conjugated biobased free fatty acids produces thick crosslinked polyester films with low surface instabilities

Maxime Bès, Vincent Lefloch, Paul Kouadio, Mohamed Hanafi, Diakaridja

Nikiema, Eric Lacroux, Romain Valentin, Artem Kovalenko

► To cite this version:

Maxime Bès, Vincent Lefloch, Paul Kouadio, Mohamed Hanafi, Diakaridja Nikiema, et al. Oxidative self-polymerization of conjugated biobased free fatty acids produces thick crosslinked polyester films with low surface instabilities. Progress in Organic Coatings, 2026, 213, pp.109957. 10.1016/j.porgcoat.2026.109957. hal-05457556

HAL Id: hal-05457556

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Submitted on 14 Jan 2026

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Oxidative self-polymerization of conjugated
biobased free fatty acids produces thick cross-
linked polyester films with low surface
instabilities

Maxime Bèsª, Vincent Leflochª, Paul Kouadioª, Mohamed Hanafiª, Diakaridja Nikiemaᵇ, Eric Lacrouxᶜ, Romain Valentinᶜ, Artem Kovalenkoª,\*

ª - Laboratoire Sciences et Ingénierie de la Matière Molle, ESPCI Paris, PSL University, Sorbonne Université, CNRS, F-75005 Paris, France

ᵇ - Laboratoire de Constitution Et Réaction de La Matière, LCRM, Université Félix Houphouët Boigny Cocody, Equipe Synthèse Structurale, 22, BP 582, Abidjan 22, Côte d'Ivoire

ᶜ – Université de Toulouse, Toulouse INP, INRAE, LCA (Laboratoire de Chimie Agro-industrielle), Toulouse, France

\* corresponding author: artem.kovalenko@espci.fr
**Abstract**

The ability of polyunsaturated vegetable oils to form solid polymer films by oxygen-induced radical polymerization explains their major role as binders in paints and coatings. However, at large film thicknesses, the diffusion of the oxygen in the bulk is limited which induces gradients of composition and surface instabilities in the film. To avoid this effect, in industrial applications the oil is first prepolymerized, or blown, at high temperature. Here we show that millimeter-thick coatings may be produced by oxidative thermal curing of conjugated polyunsaturated free fatty acids without addition of any catalyst. These free fatty acids (FA) are produced by enzymatic hydrolysis of biobased triglycerides (TG) from *Ricinodendron heudelotii* oils rich in α-eleostearic acid (18:3). While both the crude oil (TG) and the free fatty acids (FA) are transformed into rubbery polyester networks insoluble in organic solvents, the FA samples show much less surface instabilities. Using infrared spectroscopy and differential scanning calorimetry, we reveal the pathway of the network formation in the TG and FA upon polymerization. By rationalizing these results in comparison with dynamic mechanical analysis of FFA polyesters, we suggest the possible network formation mechanism which may explain lower surface instabilities of the FA samples with respect to the TG ones.

**Keywords:** Biobased material, Unsaturated free fatty acid, Cross-linking; Mechanical properties, FTIR spectroscopy
# 1. Introduction

Drying oils, such as linseed, tung, urushi and poppyseed oils, have been used since very early times to fabricate biobased polyester binders[1–3] for artist paints and protective varnishes. These oils are composed of triglycerides of polyunsaturated fatty acids and self-polymerize in air via a free radical mechanism[4–6] initiated by the peroxides formed by addition of oxygen to the C=C double bonds. Because the oxidative radical polymerization is slow at room temperature, in painting applications the reaction is usually catalyzed and applied as a complementary slow curing step for prepolymers formed by more straightforward reactions of drying oils with comonomers, such as (trans)esterification with polyols[7,8] or polycondensation with anhydrides[9]. A major limitation of the oxidative curing is the decrease of the conversion with depth due to the low oxygen diffusion which creates gradients in mechanical properties[10] and induced surface instabilities when the film thickness attains about 100-200 µm [11,12]. While the relationship between the oil structure, curing conditions and the final polymer mechanical properties were determined for heritage[13,14] and synthetic[15] paints, its influence on the surface instabilities still remains puzzling.

One particular class of highly reactive drying oils are conjugated oils[7,16,17]. These oils are naturally present in some plants (tung[18,19], *Ricinodendron heudelotii* [9,20–23]) and may be also produced by catalytic or enzymatic treatment of non-conjugated oils[17]. The conjugation of the double bonds not only increases the kinetics of oxidative drying but also promotes the addition of the macroradicals to the conjugated double bonds, as shown[7,12,16,24,24–26] by size exclusion chromatography and mass-spectrometry. The products of the oxidative polymerization of conjugated linoleic acid ester (18:2)[7] or tung oil[16,24] rich in conjugated α-eleostearic acid (18:3) show the presence of much higher molecular weight oligomers with lower peroxide values compared to those formed by oxidation of non-conjugated linseed or urushi oils. The efficient self-polymerization of conjugated drying oils may be used for a green and fully biobased synthesis of polyester films and combined with non-oxidative curing approaches[12]. Further investigations are needed to propose curable oil formulations and polymerization protocols leading to coatings with tunable mechanical properties and surface morphology.

Recent studies bring in light a naturally available polyunsaturated oil extracted from seed kernels of *Ricinodendron heudelotii* [9,20–23], a tree which uses are emerging in central Africa and has a significant industrial potential. This oil is rich in conjugated α-eleostearic acid. It has been shown that manual dehulling allows extraction of an oil with a higher content of α-eleostearic acid compared to traditional extraction technique which uses boiling water [20]. The α-eleostearic acid (18:3 *cis*, *trans*, *trans*) in the obtained oil may be isomerized into β-eleostearic acid (18:3 *trans*, *trans*, *trans*) using artificial or natural sunlight[21]. It is possible to hydrolyze the triglycerides (TG) by chemical[9] or enzymatic[27] catalysis, to generate glycerol and free fatty acids (FA), introducing additional reactive sites that can participate in further chemical transformations. While several studies[27–29] reported the nutritional properties and stability studies of *Ricinodendron heudelotii* oil, only few works[9,30,31] considered its potential as a biobased monomer for polymer material science.

In this work, we investigate the covalent network formation by thermal oxidative drying of triglycerides (called “TG”) and free fatty acids (called “FA”) from *Ricinodendron heudelotii* oil without addition of comonomers or catalysts. We first characterize and compare the
chemical transformations in these two oils of different chemical nature using Fourier
Transformation Infrared analysis (FTIR). Next, we show the evolution of mechanical and
thermal properties of cured FA samples by Dynamic Mechanical Analysis (DMA) and
Differential Scanning Calorimetry (DSC). Finally, we discuss the possible network structures
and the differences between the polymers formed from TG and FA oils.

**2. Experimental**

2.1. *Materials*

The oil is extracted in Ivory Coast, through the local traditional method of dehulling[20]. Crude
*Ricinodendron heudelotii* oil major component is α-eleostearic acid triglycerides, an
unsaturated fatty acid with *cis*, *trans*, *trans* (CTT) configuration. These oil (called as “TG” for
brevity) was hydrolyzed to free the fatty acids and glycerol using lipase from *Candida rugosa*[27], and free fatty acids (FA) were separated by centrifugation. The fatty acid
composition of the TG and FA oils was identified by gas chromatography using a previously
described protocol [27] in accordance with European Standard DIN EN ISO 12966-3. Briefly,
the product was solubilized in tert-butylmethyl ether and methylated with trimethylsulfonium
hydroxide (in methanol). The resulting fatty acid methyl esters were analyzed by gas
chromatography (GC) in Varian 3900 system using a CP-Select 624 CB column (50 m × 0.25
mm, film thickness 0.25 µm) and helium as the carrier gas. The identification of fatty acid
methyl esters was based on the comparison of retention times obtained with those of a
commercial mixture of 37 fatty acid methyl ester standards (FAME Mix 37, Sigma Aldrich)
and those obtained by fatty acid profiling of tung oil and catalpa oil (*Catalpa bignonioides*).
The results are shown in Table 1 and confirm that in terms of fatty acid profile the FA samples
is very similar to the TG oil.

The glyceride composition of the oils was determined by High Performance Steric Exclusion Chromatography (ALLTECH Model 3300 ELSD detector) coupled to an Evaporative Light Scattering Detector (HPSEC-ELSD) (Ultimate 3000, DIONEX, Sunnyvale, USA) using tetrahydrofuran as eluent. Identification and quantification of the various partial esters (monoglycerides MG, diglycerides DG), triglycerides (TG) and free fatty acids (FA) were based on comparison of the retention times obtained with those of commercial standards: trilinolein, 1,3-dioleoyl-glycerol, 1-linoleoyl-rac glycerol and linolenic acid (Sigma-Aldrich, Saint-Louis, USA).
The results showed that FA samples contain 0.5% of monoglycerides and 8.1% of diglycerides. The *Ricinodendron heudelotii* oil (TG) and free fatty acids (FA) were stored at 4°C.

<table><caption>Table 1: Fatty acids profile of TG and FA samples before polymerization[27].</caption><thead><tr><th>Fatty acid profile (%)</th><th>Insaturations type</th><th>Triglycerides (TG)</th><th>Free Fatty acids (FA)</th></tr></thead><tbody><tr><td>α-eleostearic acid</td><td>C18:3 CTT*</td><td>60.1 ± 0.2</td><td>60.0 ± 0.1</td></tr><tr><td>β-eleostearic acid</td><td>C18:3 TTT*</td><td>1.1 ± 0.1</td><td>1.2 ± 0.1</td></tr><tr><td>Linoleic acid</td><td>C18:2 CC</td><td>22.8 ± 0.0</td><td>22.2 ± 0.2</td></tr><tr><td>Oleic acid</td><td>C18:1 C</td><td>7.1 ± 0.1</td><td>7.6 ± 0.2</td></tr><tr><td>Saturated fatty acids</td><td>-</td><td>8.9 ± 0.1</td><td>9.0 ± 0.2</td></tr></tbody></table>

'T' = trans alkene, 'C' = cis alkene, '*' = conjugated alkenes.

## 2.2. Polymer preparation

The oil was pre-heated at 60°C for 5 minutes and poured in rectangular Teflon molds (80 x 20 x 1 mm) to form a homogeneous film with a thickness of 0.5 mm. The mold was positioned horizontally in an oven at 120°C with a natural air convection. To avoid photo-isomerization, the samples were protected from the sunlight. Once polymerized, the samples were left to cool in the open air, then separated from the mold using a spatula. In the rest of the manuscript, the samples are denominated in function of their composition (TG or FA) followed by the time of heating. The cured samples in the form of strips measuring 80 mm by 20 mm were observed under optical microscopy, and a cross-sectional image was captured for each sample to determine its thickness using ImageJ (typically around 0.5 mm).

## 2.3. Fourier Transformation Infrared (FTIR)

FTIR-ATR spectra were measured on a Tensor 27 infrared spectrometer from Bruker, equipped with an ATR module. Atmospheric and ATR baseline corrections are performed using functions integrated into OPUS software. Spectra are obtained by averaging 64 scans of both the reference and sample, spanning from 4000 cm⁻¹ to 600 cm⁻¹ with a resolution of 2 cm⁻¹. The spectra were normalized using the absorption band at v = 1465 cm⁻¹, corresponding to the deformation δ<sub>C-H</sub>, as it was unchanged for all samples. Preliminary tests showed that peak
intensities were very similar from both sides of the sample. The bottom side of the sample was selected for future spectra acquisition.

The peak deconvolution was performed using Microsoft Excel software with Lorentzian functions. First, for each region of interest, the background was approximated by a linear line and removed. Next, characteristic narrow vibration bands were identified and their center positions were determined and set fixed in future deconvolution process. The remaining non-characteristic large bands were added and their parameters were set as variables. The optimization of the band intensities and half-widths was performed using Excel solver three times starting from significantly different initial values of parameters. Using the obtained band intensities, the standard deviation was calculated and used as an estimation of the error of the data treatment.

## 2.4. Differential Scanning Calorimetry (DSC)

The samples were analyzed by Differential Scanning Calorimetry (DSC) using TA Instruments® DSC25. A representative piece of sample with a mass of about 10 mg was placed into the aluminum pans and mechanically sealed. The experiments were carried out in ultra-pure nitrogen to avoid oxidation. For all samples, an isotherm at 100°C for 5 minutes were performed prior to the measurement to avoid thermal history effects. Subsequently, the samples were cooled to -90°C and then heated to 100°C at the same temperature change rate of 10°C/min.

## 2.5. Dynamic Mechanical Analysis (DMA)

Viscoelastic properties were obtained using Dynamic Mechanical Analysis (DMA) using a DMA850 from TA Instruments, equipped with a liquid nitrogen temperature controller. Sample strips with a width of 4 mm were cut from initial films using a die cutter and a hydraulic press. The measurements were performed in tensile mode in the linear regime at a maximum strain amplitude of 0.2% and oscillation frequency of 1 Hz. Because of strong surface instabilities of TG samples, it was not possible to test thick samples in the DMA. The temperature dependence of the loss factor tan(δ) of a thin sample TG-24h with a thickness of about 100 µm is given in the Supporting Information.

## 2.6. Tensile tests

The ultimate properties of the samples were tested in an Instron tensile testing machine, equipped with a 100 N load cell and a video extensometer. Dumbbell-shaped specimens were cut from the sample strips, with a central width of 4 mm and a length of 20 mm. The imposed strain rate on the samples was 0.01 s⁻¹. The Young's modulus E is calculated between 0% and 1% strain.

## 2.7. Gel fraction

For two selected samples, the gel fraction was determined by slow swelling and unswelling in ethyl acetate, which is a good solvent for the TG and FA. The samples were first weighed and placed in pure ethanol (bad solvent for the polymer) for a night. Ethyl acetate was slowly added by small portions and the solvent bath was regularly refreshed to obtain complete swelling while maintaining the sample integrity. The swollen sample was weighted and its dimensions measured with a caliper to estimate the swelling ratio. The sample was then slowly unswollen
by adding ethanol and dried under vacuum. The gel fraction was calculated as a ratio between the weight of the sample after and before swelling.

**3. Results**

3.1. *Formation of polymer networks*

<figure><img src="image_2.png" /><figcaption>Figure 1 – Photographs of the triglycerides (left) and free fatty acid (right) samples in the molds at different curing times.</figcaption></figure>

Upon heating at 120°C in air, the samples undergo a transformation from a fluid and transparent oil to a yellow polymer, as shown on Figure 1. The TG sample has less intense color than the FA one and exhibits a strong surface instability. The yellowing of the drying oils was attributed to the oxidation of natural contaminants present in tiny amounts in their composition[32]. The film morphology is still slowly evolving after the gel point attained after few hours of curing and the final film shape is obtained at about one day of curing. The similar surface instability has been observed polymerization of alkyd paints[33] and tung oil[12] films and may be explained by the polymerization of a surface film with different properties from the bulk. Following this explanation, the swelling of the surface film by the unreacted monomers leads to lateral expansion and wrinkling under compressive stress induced by the mold walls. In the case of FA samples, the polymerization leads to much lower instability that is present only on the sample edges. The possible explanation of this difference will be proposed in the section 3.5.
To confirm the formation of a covalent network for the FA samples, we performed the swelling and gel fraction analysis of the FA samples cured for 20 and 80 hours. To avoid cracks, the swelling tests were performed by gradual solvent displacement from ethanol to ethyl acetate, which are respectively a bad and good solvent for the initial free fatty acid. The results show that samples are insoluble and do swell in ethyl acetate. The linear swelling ratio and the gel fraction are equal to 1.53 and 69.4% for the sample cured for 20 hours and 1.38 and 85.8% for the sample cured for 80 hours. The high gel fraction which increases with the curing time shows that most of the oil molecules are linked to the network. The increase of swelling ratio indicates between 20 and 80 hours of curing indicates the presence of inter-chain reactions that increase the cross-linking degree of the networks. Compared to tung oil films cured via catalyzed UV or thermal radical polymerization, high gel fractions were obtained without use of any catalyst.

## 3.2. *Chemical transformations followed by FTIR-ATR*

We used FTIR-ATR to characterize chemical reactions happening in the oils during curing, in comparison with the literature data for drying oils [2,4,5]. The results are shown on Figure 2. For triglycerides (Figure 2a-c), we may note the appearance of a large v(O-H) band centered at 3430 cm⁻¹ and the increased intensity at ~1780 cm⁻¹ and 1695 cm⁻¹, caused by oxidation. The v(C=O)ₐₙ₆₇₄ band of TG at 1744 cm⁻¹ is broadened because of the formation of oxidation products such as free acids, ketones and aldehydes. The v(C=CH) band at 3010 cm⁻¹ attributed to *cis* unconjugated alkenes completely disappears after 30 hours of heating. In addition, we observe the evolution of the bands ω(C=C) which are characteristic of the triple conjugated *cis*-trans-*trans* (CTT) system of the α-eleostearic acid chain (the highest fraction in the TG sample). These bands situated at 991 and 965 cm⁻¹ evolve in a single band at 986 cm⁻¹ which is typical of a double conjugated *trans*-trans* (TT) system. We may conclude that polymerization leads to the disappearance of at least one double bond as expected for radical chain transfer reactions, in contrast to oxidative termination reactions which should lead to isomerization of conjugated bonds. Our findings are supported by the findings of Oyman et al. [16] who reported lower concentrations of oxygenated species in products of oxidation of conjugated polyunsaturated oils with respect to the non-conjugated ones.

Similar transformations of the alkene bands are observed in the case of free fatty acids, i.e. the disappearance of the v(C=CH) band at 3010 cm⁻¹ and of two ω(C=C)ₐₗₜₜ bands at 991 and 965 cm⁻¹, and the appearance of a ω(C=C)ₐₗₜₜ band at 986 cm⁻¹. (Figure 2d-f). The major difference between TG and FA spectra concerns the bands of acid and ester groups. The uncured free fatty acids show a large band at 3430 cm⁻¹ and an intense band at 1710 cm⁻¹ corresponding, respectively, to the stretching vibrations v(O-H) and v(C=O)ₐₗₐᵢₗ of the free carboxylic acid. The shoulder at 1744 cm⁻¹ reveals the presence of some residual ester groups in these oils. Upon curing, the intensity of the v(C=O)ₐₗₐᵢₗ band decreases while v(C=O)ₐₙ₆₇₄ increases and a new band at 1164 cm⁻¹ corresponding to the v(C-O)ₐₙ₆₇₄ appears. We may explain this by esterification of the free fatty acid via reactions with newly formed OH groups that are secondary products of the double bond oxidation.

On Figure 3 we plotted the intensity of the characteristic infrared bands for triglycerides (figure a) and free fatty acids (figure b) as a function of curing time. We may distinguish two regimes with fast and slow kinetics, corresponding respectively to the time before and after the gelation point which takes place at about 7 hours. The similar presence of two regimes was previously
observed upon drying of the linseed oil and may be explained by the reduction of chain mobility and reaction rate by network gelation[4].

<figure><img src="image_3.png" /><figcaption>Figure 2 – FTIR-ATR spectra of TG (a-c) and FA (d-f) samples at different curing times.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3 – Normalized intensity of characteristic FTIR-ATR bands for triglycerides (a) and free fatty acids (b) as a function of curing time. The wavenumbers of the bands are the following: v(C=CH) at 3010 cm⁻¹, v(O-H) at 3430 cm⁻¹, v(C=O)ₐster at 1744 cm⁻¹, v(C=O)ₐcid at 1710 cm⁻¹.</figcaption></figure>

### 3.3. Thermal properties

The polymerization affects drastically thermal properties of the sample. On the Figure 4a, we compare the DSC curves for bare TG and FA with polymers obtained after variable time of oxidative curing. In the uncured state, both TG and FA show multiple exothermal crystallization peaks upon cooling and endothermal melting peaks upon heating. The higher temperature of crystallization of the uncured FA with respect to uncured TG may be explained by easier packing ability of unsaturated chains and presence of hydrogen bonding between acid groups. These peaks completely disappear after about 1 day of curing (samples TG-24h and FA-19h). In comparison with the literature for oxidative curing of linseed oil [34], may suggest that crystallization is hindered by the change in chain conformation after the radical addition to the conjugated chains. For greater curing times, a broad glass transition is observed in the region from -50 to -10°C. The wide region of the glass transition may be explained by the heterogeneous polymer network structure due to the different radical pathways of the oxidative polymerization. The shift of the glass transition region to higher temperatures with increased curing time is due to the reduction of chain mobility by cross-linking points. The glass transition temperatures obtained by tangent method for TG and FA cured for a maximum tested time of 120 hours were equal to -12°C and -10°C, respectively. These temperatures are very similar between them and are close to the Tg of air-cured linseed oil [14,34].

<figure><img src="image_5.png" /><figcaption>Figure 4: DSC cooling curves of TG (a) and FA (b) before and after polymerization.</figcaption></figure>
### 3.4. Mechanical properties

The samples of free fatty acids cured for more than 20h were analyzed by dynamic mechanical analysis (DMA) and tensile testing. The Figure 5a and b show the DMA curves at oscillation frequency of 1 Hz for samples with different curing times. With increasing temperature, the samples transit from a glassy state with high elastic modulus $E'$ close to 1 GPa to a rubbery state with $E'$ of the order of 1 MPa. The mechanical glass transition temperature $T_α$ was determined as the temperature of the maximum of the loss factor $tan(δ)$. The value of $T_α$ of about 0°C were obtained for FA-20h and FA-25h. These values are similar to that obtained for a thin film sample TG-24h presented in the Supporting Information. The mechanical glass transition of the FA shifts to higher temperatures with increasing the curing time from 20 to 80 hours, which shows similar evolution to DSC. The value of $T_α$ is about 20°C higher than the $T_g$ obtained from DSC, which may be explained by the strain rate dependence of glass transition. We also observe the tendency of increasing the rubbery plateau modulus with curing time which indicates higher cross-linking density and lower uncured monomer fraction in the samples cured for longer times. The values of the tensile moduli in the rubbery state were obtained by tensile testing at low strain rate and are reported on the Figure 5c together with the values of $T_α$ obtained from DMA. We observe a good correlation and almost linear increase of $E$ and $T_α$ with curing time, which is similar to the general curing kinetics reported in the synthetic review in the reference[13] for slow aging oil and alkyd paints. In our case, the increase of elastic modulus is mostly due to the increase of the cross-linking density as curing proceeds.

<figure><img src="image_6.png" /><figcaption>Figure 5: Temperature dependence of the storage modulus $E'$ (a) and loss factor $tan δ$ (b) for fatty acid samples cured for different times obtained by dynamic mechanical analysis; (c) tensile modulus $E$ at 25°C (from tensile testing) and glass transition temperature $T_α$ (from DMA) of the same polymers as function of the curing time.</figcaption></figure>

### 3.5. Hypothetical network structure

As we showed in the previous sections, the triglycerides and free fatty acids obtained from *Ricinodendron heudelotii* oil conduct to cross-linked polymers with rather similar thermal and mechanical properties, besides the difference in the chemical nature of initial compounds and the chemical transformations revealed by infrared spectroscopy. Interestingly, the only major macroscopic difference between the cured TG and FA systems is the surface instability formed upon oxidative polymerization. In this section, we suggest the hypothetical structures of the TG and FA polymer networks and discuss the implications for their properties. The chemical structure of main compounds, major reactions and polymer network structures are represented on the Figure 6. For simplicity, we assume that the initial oils are composed of derivatives of the main free fatty acid component, which is α-eleostearic acid. We consider four major types
of bonds which are represented on the Figure 6a. The triple conjugated CTT bonds are present in both TG and FA oils and are reactive with respect to oxidative (i.e. with formation of new C-O-C or C-O-O-C bonds) and non-oxidative (i.e. with formation of new C-C bonds) radical addition. To highlight the similarity with the free radical monomers, we schematize the CTT system as “M”, and its oxidized version as “Mox”. We should note that a large variety of oxidized moieties may be formed as reported in the literature[1,4], however only peroxide and ether “bridges” will lead to polymerization. Taking in account the relatively high curing temperature (120°C) at which peroxides should be unstable, we adapted a general structure of the “Mox” group with one ether bond and two remaining conjugated TT bonds, shown on Figure 6a. This structure was proposed in the literature for conjugated linoleic acid[17] and is supported by disappearance of the triple conjugated CTT bonds and the appearance of a double conjugated TT bond on the FTIR spectra. Schematically, we represent the oxidative radical polymerization of CTT bonds as addition of the “Mox” species to the available monomer “M” and propagation to other monomers forming “M” chains, as represented on Figure 6b. Finally, the ester bonds between the alcohols “A” and free fatty acid residues “B” are dominant in the TG oils. These groups are present in a small amount in the FA oil due to the 0.5% wt. of monoglycerides (“MG”, with functionality $f_{MG} = 3$) and 8.1% wt. of diglycerides (“DG”, with functionality $f_{DG} = 3$) remaining after the enzymatic hydrolysis. Upon oxidation of FA oils, new ester bonds may be formed via esterification of free carboxylic acids with OH groups in oxidized “Mox” species (Figure 6b). The esterification is evidenced by the increase of $(C=O)_{ester}$ bands and the decrease of the $(C=O)_{acid}$ bands on FTIR spectra with increase of curing time of FA samples. It is favored by the evaporation of the resultant water from the sample at 120°C.

The described reactions lead to two different covalent network structures of TG and FA samples schematized, respectively, on the Figure 6c and d. In the network obtained from TG oil (Figure 6c), the covalent backbone is formed by a sequence of the TG moieties attached together through the reaction of the unsaturated parts “M” via oxidative radical mechanism. Because the formation of long “M” chains by radical oxidative polymerization of TG should be hindered by steric constraints. For simplicity of our explanation, we will first limit the discussion to the case of bimolecular reaction between “M” groups without formation of the “M” chains. In this case, the functionality of the TG molecule in polymer network will be $f_{TG} = 3 * f_{M} = 3$ (where $f_{M}$ in the functionality of “M” equal to 1), and the gel point of the network (i.e. the critical “M” group conversion) will be[35]:

$$p_{gel}^{TG} = 1/(1 - f_{TG}) = 0.5 \quad (1)$$

In contrast, the formation of a covalent network from FA oils may not be explained only by reaction between the “M” groups because the FA molecule contains one polyunsaturated block “M” and one carboxylic acid COOH (shown as “B”, functionality $f_{B} = 1$), leading to the overall functionality $f_{FA} = f_{M} + f_{B} = 2$, which is insufficient to form a network[35]. Taking in account the FTIR data, we may hypothesize that linear chains in the covalent backbone in FA samples may be formed via esterification of oxidized species “Mox” with the COOH groups and formation of “M” chains (Figure 6d). Additionally, free carboxylic acids connected to the network through the “M” or “Mox” group may form a second physical network through hydrogen bonding. While we did not find any thermal signature of the dissociation of this physical network on DSC up to 100°C, the FA samples cured for 25 and 40 hours show a softening upon heating in the temperature range from 40 to 60°C (Figure 5a) which may be caused by progressive dissociation of the hydrogen bonds. However, the FA samples are
insoluble in solvents and are thus covalently cross-linked. To explain this, we should emphasize the important role of the MG and DG impurities and “M” chains that can act as cross-linking points with functionality equal to 3. To estimate the effect of residual MG and DG, we may calculate the average molecular functionality of FA, MG and DG as following:

$$ \langle f \rangle_{FA} = n_{FA}f_{FA} + n_{MG}f_{MG} + n_{DG}f_{DG} \quad (2) $$

Where $n_{FA}$, $n_{MG}$ and $n_{DG}$ are molar fractions of FA, MG and DG, respectively. The obtained value $\langle f \rangle_{FA} = 2.06$ may be used to estimate the hypothetical gel point (i.e., the average conversion of “M” and “B” groups) in the FA sample:

$$ p_{gel}^{FA} = 1/(1 - \langle f \rangle_{FA}) \approx 0.94 \quad (3) $$

<figure><img src="image_7.png" /><figcaption>Figure 6 – Proposed mechanism of network formation from TG and FA oils: a) the chemical structure of reactive building blocks; b) main reactions leading to polymerization; c) schematics of the TG network; d) schematics of the FA network.</figcaption></figure>

Higher gel point of the FA network with respect to the TG one may be the key factor explaining the lower tendency to surface wrinkling of FA. Indeed, as shown by Basu et al.[11] for alkyd paint film curing, the wrinkles appear due the stresses induced by swelling of the gel film formed at the surface in contact with air by the unreacted monomer in bulk. If we assume the similar curing reaction kinetics between TG and FA (Figure 7), the surface of the TG samples will attain the gelation point faster while the surface of FA samples will be most composed of linear chains. Thus, the surface gel swelling will be most pronounced for the TG samples and will lead to surface wrinkling.

In our explanation, we ignored the fact that FA molecules with “M” or “Mox” groups may incorporate in “M” chains and act as cross-linking points with functionality equal to 3 (Figure 6d). To estimate the relative contribution of impurities (MG and DG) and cross-linking by “M” chains, we may analyze the cross-linking density from the DMA plateau modulus. The FA sample cured for 80 hours shows a stable plateau Young’s modulus $E \sim 3$ MPa which allows to estimate the molecular weight between cross-linking points from the James and Guth’s equation
$M_c = 3\rho RT/E$, where $\rho$, R and T are the material density, the universal gas constant and temperature, respectively. Roughly estimating $\rho$ as 900 kg/m$^3$, we obtain the molecular weight between cross-linking points $M_c \sim 2500$ g/mol, which corresponds to about one cross-linking point per nine molecular units of eleostearic acid. The molar fraction of MG and DG in the FA sample is about 6% and corresponds approximately to one molecule per fourteen molecular units of FA (the two FA units connected to the glycerol were subtracted). Thus, approximately half of cross-linking points is due to MG and DG, and half is due to the “M” chains.

<figure><img src="image_8.png" /><figcaption>Figure 7 – Scheme explaining the difference in the surface gelation and wrinkling of TG oils (a) and FA oils (b). The gradient of the color corresponds to the gradient in conversion rate due to oxygen diffusion inhibition.</figcaption></figure>

**4. Conclusion**

We showed that free fatty acids (FA) obtained from *Ricinodendron heudelotii* kernels may be used for synthesis of polyester films with less surface instabilities compared to the TG oils. Without any additives or comonomers, the oxidative radical polymerization at 120°C leads to a covalently cross-linked polyester network. Similarly to the TG oils, the oxidative polymerization leads to the disappearance of one double bond in the triple conjugated CTT system of the α-eleostearic acid chain. The covalent network of FA samples is formed due to esterification of the free acid groups with mono- and diglyceride impurities and with products of oxidation of the double bonds. The cured FA samples have rubbery behavior at room temperature with $T_g$ of about -10°C. Dynamic mechanical analysis and tensile testing of cured FA samples showed that mechanical glass transition temperature $T_α$ and rubbery tensile modulus *E* slowly increase with curing time after gel point. The lower tendency to surface wrinkling of FA compared to TG may be explained by the fact that FA samples should reach the gel point at higher overall conversion and, as consequence, gelation should happen more homogeneously in depth of the sample.

**Conflict of interests**

The authors declare no competing financial interest.
Acknowledgments

This work was funded by French Agence Nationale de la Recherche, grant MAGELAn <ANR-19-CE06-0024>.

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