Biomass derived epoxy systems: From reactivity to final properties

Journal Pre-proof Biomass derived epoxy systems: from reactivity to final properties Guillaume Falco, Nicolas Sbirrazzuoli, Alice Mija

PII:

S2352-4928(19)30555-0

DOI:

https://doi.org/10.1016/j.mtcomm.2019.100683

Reference:

MTCOMM 100683

To appear in:

Materials Today Communications

Received Date:

12 June 2019

Revised Date:

3 October 2019

Accepted Date:

4 October 2019

Please cite this article as: Falco G, Sbirrazzuoli N, Mija A, Biomass derived epoxy systems: from reactivity to final properties, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100683

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Biomass derived epoxy systems: from reactivity to final properties

Guillaume Falco, Nicolas Sbirrazzuoli, Alice Mija*

Université Côte d’Azur, Université Nice-Sophia Antipolis, Institut de Chimie de Nice, UMR CNRS 7272, 06108 Nice Cedex 02, France Abstract:

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The present work focuses on the conception of bio-based epoxy resins arising from an increasing interest class of curing agents: the dicarboxylic acid. The hardener used was first synthesized from maleic anhydride and dipropylene glycol and then copolymerized with epoxidized linseed oil (ELO) to design resins from bio-resources. The anionic copolymerization between the two monomers has been highlighted by DSC while evolutions of chemical structures have been investigated by the corroboration of FT-IR and 2D NMR spectroscopies. DMA and DSC studies indicate that the copolymerization reaction is influenced by epoxy/dicarboxylic acid ratio and the presence of an initiator. Interestingly, the utilization of imidazole derivative as initiator provides selective reactions, which significantly increase the crosslink density and the α-relaxation temperature (Tα) of obtained networks. The resulting materials exhibit ductile and flexible behaviour with low Tα although keeping excellent thermal resistance (T10% > 315 °C).

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1.Introduction

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Keywords: Bio-renewable resources • Copolymerization • Epoxidized vegetable oil • Mechanical properties • Reaction mechanisms

Polymers from renewable resources are designing nowadays our sustainable future1,2

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via a strong development of platform molecules from bio-resources.3 That is why bio-based compounds have reached an important industrial and academic support leading to already marketed solutions. Furthermore, the green approaches and chemical pathways allow

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industries to anticipate the exhaustion of resources stemming from crude oil. In the thermosets market, epoxy resins have a major industrial place4,5 as structural

matrix, composites, coatings or adhesives due to their high-performances.6 In the last years, the research and production are more and more oriented to the eco-friendly epoxy resins, based on vegetable biomass.2,7,8,9 Among the eco-responsible solutions, the vegetable oils are old resources used to produce plasticizers or painting formulations opening in the present to new strategic platform for the production of high performance materials.10,11 The chemical

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structure of vegetable oils is constituted of triglycerides formed from a glycerol core and various fatty acids.12 Therefore, the vegetable oil will combine the properties given by the glycerol core (3D branching) and that of fatty acids (unsaturations and aliphatic length). Between all the vegetable oils, the linseed oil was chosen due to its high unsaturations content (with a high content in linolenic acid), allowing a high degree of functionalization13 (around 5.5 oxirane rings per triglyceride), and to its eco-friendly epoxidation process.14 Moreover, the Epoxidized Linseed Oil (ELO) showed its capacity to provide high-performance resins with high glass transition temperature (Tg) and toughened mechanical properties.15,16,17 To provide high-performance materials, the choice of the curing agent is a crucial step during the resins fabrication. In this work, we focused on a dicarboxylic acid type hardener

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for the copolymerization of epoxy monomers.18,19 The interest in these epoxy reticulating

systems increased in recent years20,21,22,23 due to the low toxicity of the carboxylic acids. In the sustainable chemistry area based on the conception of materials by using nontoxic and bio-resourced compounds, the dicarboxylic acids from renewable resources are one of

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attractive alternatives to the toxic hardeners used in most industrial formulations24 as aromatic or aliphatic amines. Nevertheless, the diacids have some practical drawbacks: their high

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melting temperatures (generally > 100 °C) and high polarity that make more difficult the compatibility with the epoxidized vegetable oils (apolar by aliphatic segments of triglyceride).

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Thus, the mixing of these two types of monomers can conduct to heterogeneous final epoxy resins. Some alternatives have been reported in the literature to solve this aspect and to obtain homogeneous epoxy/carboxylic acid systems by vegetable oils modification22,23 (to generate -

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COOH functions) or by solvents utilization25, for example. To increase the direct miscibility and compatibility, another possibility is to use a dicarboxylic acid that is liquid at room temperature. In this line, a bio-based dicarboxylic acid

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(named AcDiC) was synthesized26,27 by the reaction of the dipropylene glycol28 with the maleic anhydride.29,30

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The aims of this work are i) the synthesis and the reactivity study of bio-based

thermosets by the combination of non-toxic bio-resources and ii) to propose an alternative to epoxidized vegetable oils/anhydride systems that generate networks with high thermomechanical resistance and high Tg.15,17 To generate highly crosslinked networks, the curing reactions were optimized in terms of monomer ratios, amount of initiator, and curing temperature programs. The reactivity of the curing systems was studied by differential scanning calorimetry (DSC). Then, the co-esterification reaction between the two monomers has been evidenced by FT-IR and 2D NMR spectroscopies. Finally, the performances of the 2

obtained networks have been investigated by Dynamic Mechanical Analysis (DMA), tensile tests and Thermogravimetric Analysis (TGA).

2. Materials and methods Experimental section 2.1. Materials Epoxidized Linseed Oil (ELO) (Lankroflex L) (Figure 1a) was generously supplied by Ackros Chemical (actually Valtris). ELO is a viscous liquid (1,2 Pa.s at room temperature). It has a molar mass of about 980 g.mol-1 and contains 5.5 epoxy groups per triglyceride (epoxide equivalent weight (EEW) = 176 g.mol-1). Dipropylene glycol (Mw = 134.17 g.mol-1, purity = 99 %, b.p. 230.5 °C), maleic anhydride (Mw = 98.06 g.mol-1, m.p. = 53 °C, purity >

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99 %) and 2-methylimidazole (2-MI) (Mw = 82.10 g.mol-1, purity = 99 %, m.p = 143 °C) were obtained from Sigma-Aldrich Chemical. 2.2. Synthesis of AcDiC hardener

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AcDiC hardener – the dipropylene glycol dimaleate – (Figure 1b) was synthesized by the introduction of dipropylene glycol and maleic anhydride in a round-bottom flask under a

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nitrogen atmosphere fitted by a condenser. The mix was heated at 105 °C (± 1 °C) during 3 hours. The obtained product is a clear yellow liquid that was stored at room temperature under

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N2 atmosphere to avoid the hydrolysis of formed ester bonds.

2.3. 2D NMR characterization of ELO and AcDiC

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NMR investigations was conducted to confirm the chemical structure of ELO and AcDiC. All spectra were displayed in the Figure S1 and Figure S2 (SI). NMR characterizations of ELO and AcDiC were done in DMSO-d6 by HSQC 2D NMR, which

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allows making carbon-hydrogen correlations in 1J. The 1H and 13C characteristic peaks of the two compounds are assigned below according the numbering position of Figure 1.

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The NMR assignations signals of ELO are in agreement with previous

investigations16,31,32,33: 5.2 ppm (Hk); 4.0 – 4.3 ppm (Hj); 3.1 and 2.8 ppm (Hg); 2.3 ppm (Hf); 1.8 ppm (Hh); 1.7 ppm (He); 1.5 ppm (Hd); 1.2 and 1.4 ppm (Hc); 1 ppm (Hb); 0.9 ppm (Ha) for 1H spectra and 173.3 – 173.0 ppm (Ci); 69.6 ppm (Ck); 62.6 ppm (Cj); 54.1 – 58.0 ppm (Cg); 21.5 – 34.4 ppm (Cc, Cd, Ce, Cf and Ch); 11.2 – 14.8 ppm (Ca and Cb) for 13C spectra. Characteristic peaks of synthetized AcDiC are associated with: 13 ppm (H8); 6.4 ppm (H6); 6.3 ppm (H5); 5.0 ppm (H2); 3.50 – 3.8 ppm (H1); 1.0 – 1.2 ppm (H3) for 1H shifts and

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167.3 – 167.6 ppm (C7); 165.6 – 165.9 ppm (C4) ; 132.1 – 132.8ppm (C6); 129.1 – 129.8 ppm (C5); 73.4 – 74.0 ppm (C1); 70.6 – 71.4 ppm (C2); 16.8 – 18.6 ppm (C3) for 13C shifts. From these NMR investigations, we estimate a conversion of chemical reactions leading to AcDiC of 98%. The details of this assessment are displayed in Figure S3, SI section

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2.

AcDiC ELO

b)

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a)

2.3. Preparation of ELO/AcDiC resins

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Figure 1: Chemical structures of a) ELO and b) AcDiC). The numbering atoms position monomers is used for the NMR studies

ELO/AcDiC mixtures were prepared using ratios (R) of 1 and 0.8 according to the

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literature.15 The ratio (R) was defined as R = carboxylic acid groups/epoxy groupsR was defined as the ratio of carboxylic acid groups and epoxy groups which are present in all mixes

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(R = carboxylic acid groups/epoxy groups). Each ratio mixture was realized with and without 2-methylimidazole (2-MI) in order to highlight the contribution of the initiator on the polymerization. For the mixtures with the initiator, 1% mass of 2-MI initiator was added. The

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mixtures were prepared by firstly heating the ELO to around 65 °C to decrease it’s viscosity and to allow an optimal mixing with the AcDiC. ELO and AcDiC were mixed, thereafter the

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initiator was introduced into the homogeneous mixture. Each formulation was stirred at 65 °C during 4 min, then placed into a silicone mold and cured in oven at 150 °C during 30 minutes. A post curing at 180 °C during 30 minutes was applied to each formulation. ELO/AcDiC copolymers were compared to 100 % ELO crosslinked homopolymer.

This homopolymer was prepared by heating the ELO at 180 °C during 24 hours in a silicone mold. Curing temperature programs of resins have been optimized by preliminary investigations of reactivities and thermal stabilities by DSC and TGA experiments. The curing 4

conditions were selected to have no residual heat flow after isothermal steps and no signal related to the beginning of the degradation. 2.4. Experimental techniques Differential scanning calorimetry (DSC) measurements were carried out on a MettlerToledo DSC-1 apparatus equipped with a FRS5 sensor (with 56 thermocouples Au-Au/Pd) and STARe© software for data analysis. Temperature and enthalpy calibrations were performed by using indium and zinc standards. Samples of about 10 mg were placed in 40 μL aluminum crucible. The DSC measurements of crosslinking reactions were conducted at 10 °C.min-1 heating rate. Polymerization reactions were done directly in pans by heating of the

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reaction mixture. For the evolution of the degree of conversion at room temperature with time (Figure 4, in days), freshly mixes were stored at ~ 25 °C and DSC heating scan were

performed daily to evaluate the conversion degree reached. For ELO/AcDiC/2-MI mix using a ratio of 0.8, an isotherm at 90°C was performed to correlate the conversion degree estimated

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by DSC measurements with the evolution of chemical bounds measured by Fourier Transformer Infrared spectroscopy (FT-IR) at different time (Figure 6c).

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FT-IR was used to analyze the structures of ELO and AcDiC, and to investigate the reactivity between these two monomers. For this latter, an ELO/AcDiC/2-MI mix using a ratio of 0.8 was heated to 90 °C for 180 min. Samples were taken at 10, 30, 60 and 180 min

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and immediately quenched in liquid nitrogen. Then, each of these samples was referenced as a function of the degree of conversion, based on isothermal DSC curves. The FT-IR spectra

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were recorded on a Perkin Elmer Spectrum BX II spectrophotometer. An attenuated total reflectance (ATR) mode was used with a diamond crystal. 128 scans were accumulated with a resolution of 4 cm-1 and an interval of 2 cm-1. FT-IR spectra were recorded at different 1

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conversion degrees of polymerization and normalized at 1727 cm-1. H NMR and 13C NMR measurements were recorded in DMSO-d6 using a Bruker

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AVANCE I instrument at 25 °C. NMR studies were obtained with a direct probe working at 500.23 MHz for 1H and 125.75 MHz for 13C. The residual solvent signal at 2.50 ppm has been used as standard reference. 2D liquid state NMR (HSQC-type) was used to characterize the structure of ELO and AcDiC monomers, to designate the covalent interconnections developed during the copolymerization and to highlight if side reactions occurred during the crosslinking. As with FT-IR, an ELO/AcDiC/2-MI mix using a R ratio of 0.8 was used to appreciate the reactivity between the monomers. The sample was prepared by vigorously stirring of reactants in 50 % of DMSO-d6 until the achievement of a homogeneous solution. 5

The sample was heated at 70 °C during 96 hours to reach to the maximum degree of conversion of the polymerization. The dynamic mechanical properties were determined on a Mettler-Toledo DMA-1 equipped with STAR© software for curve analysis. The DMA was operated in traction mode with sample dimensions of 4.5 mm x 1.5 mm x 15.0 mm (± 0.1mm). Damping factors and elastic modulus values were collected from -150 to 75 °C at 2 °C.min-1 heating rate and 1 Hz frequency. The glass transition temperature, named below Tα, and assimilated to the αrelaxation, was assigned at the maximum of damping factor (tan δ = E”/E’). Tensile measurements of resins containing 2-MI were performed on a Testwell 112 with a

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load cell of 10 kN by using the ISO 527-2 standard. Dog-bone-shaped samples of 75.0 mm (length), 5.0 mm (width) and 2.0 mm (thickness) (± 0.1 mm) were deformed at room

temperature (~ 25 °C) until failure with a crosshead speed of 5 mm.s-1. The average values

extracted of measurements (Young modulus, stress and strain at break) were calculated from 6 measurements for each tested material.

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Thermogravimetric measurements (TGA) were made on a TGA 851e from MettlerToledo to evaluate the thermal stability of crosslinked copolymers. The microbalance has a

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precision of ± 0.1 μg. Samples of about 15 mg were placed into 70 μL alumina pans and

3. Results and discussion

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heated from 50 to 700 °C at 10 °C.min-1 heating rate under 50 mL.min−1 air flow.

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3.1. The anionic ELO/AcDiC copolymerization

The mechanism of dicarboxylic acid - epoxy rings polyesterification is that of a strictly living alternant anionic copolymerization (Figure 12a). The main esterification

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reaction could be accompanied with certain conditions (> 180 °C)34 by some sides reactions18 such as etherifications or homopolymerizations (Figure 12b). Condensations or hydrolysis are

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also other possible side reactions, which occur in low quantities. To promote the main reaction, initiators such as tertiary amines or imidazole are

generally used in combination with dicarboxylic acids for the crosslinking of epoxy monomers. For example, 2-methylimidazole (2-MI) is the most promising initiator for epoxy/dicarboxylic acid systems compared to other basic initiators such as 1methylimidazole, triethylamine, 1.8-diazabicyclo [4.0] undeca-7-ene (DBU), or dimethylaminopyridine (DMAP).21 In the presence of the 2-MI, the copolymerization is specific, i.e. the esterification between epoxy rings and carboxylic acid functions is 6

privileged.18,35 This copolymerization is initiated by the formation of oxyanions.20,36,37,38 Two ways of formation of active species are possible (Figure 12c). In the first scenario, the 2-MI initiates the reaction by opening the epoxy rings39 of ELO generating zwitterions. These zwitterions are constituted by a quaternary amine and an active oxyanion. Then, the oxyanion interact with a carboxylic acid group to form the carboxylate anion (-COO-). In the second scenario, a proton exchange between 2-MI and the -COOH function of AcDiC directly generates the carboxylate anion. In both cases, the -COO- leads to the copolymerization propagation. If systems reaction containing the initiator are still accompanied by some sides reactions, as schematized in Figure 12b, theses reactions do not start until the completion of

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the main reaction.40 Furthermore, the type and the number of secondary reactions will also depend on the functional ratio (R). For example, homopolymerization and etherification will

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be more favourable in systems with epoxy group’s excess.

a) General reaction

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Figure 2: Epoxy/carboxylic acid copolymerization reactions

c) Mechanism in presence of 2-MI 1/ Formation of active species via 2-MI action: - First possibility: 2MI/ epoxy reaction:

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Esterification

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b) Side reactions

Homopolymerization

- Second possibility: 2-MI/ acid function interaction:

2/ Formation of active species via proton exchange:

Condensation

Hydrolysis

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Etherification

3.2. ELO/AcDiC crosslinking Differential scanning calorimetry (DSC) was used to follow the reactivity of ELO/AcDiC copolymerization, through the study of thermal events occurring during a 7

dynamic heating program. Figure 2 shows the chemical structures of ELO and AcDiC. The DSC curves and corresponding degrees of conversion are presented in Figure 3a and Figure 3b respectively. These curves show the evolution of the copolymerization as the function of the temperature at different ratios, i.e. R = 0.8 or R = 1, with and without the 2-MI initiator. It is observed that the systems show different reactivity and reaction rates, according to the formulations used. As seen in the Figure 3a, the dominant exothermic event, which occurs from 50 °C until 180 – 250 °C, is assigned to the ELO/AcDiC copolymerization. 0,6

a)

R = 0,8 R=1 R = 0,8 + 2-MI R = 1 + 2-MI

0,5

0,3

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0,4



Heat flow / W.g

-1

exo up

0,5

0,2

0,1

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0,0 0,0 50

100

150

200

250

50

Temperature / °C

b)

R = 0,8 R=1 R = 0,8 + 2-MI R = 1 + 2-MI

1,0

100

150

200

250

Temperature / °C

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Figure 3: a) Dynamic DSC curing at 10 °C.min-1 and b) corresponding degree of conversion (α) of ELO/AcDiC systems in epoxy excess (R = 0.8) and in stoichiometric ratio (R = 1), with and without 2MI

In order to compare these four systems, the thermodynamic data of the corresponding reactions such as the heat of reaction (Q), the temperature at maximum of the peak (Tpeak) and

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the reaction interval of exothermic events were reported in the Table 1. The first ascertainment is that the ratio or the presence of the 2-MI does not significantly influence the start temperature of chemical reactions. This table also shows that formulations without 2-MI

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have higher Q values at same R. The epoxy/dicarboxylic acid copolymerization being not selective without initiator18, this peak certainly contains side reactions. Moreover, on the

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Figure 3a the thermal events of the systems without 2-MI are not Gaussian symmetrical: the second half peak (for T > 150 °C) has a lower slope than the first half (for T < 130 °C). This non-Gaussian non-symmetrical shape could show that secondary reactions occur at high temperature, as it is usually the case34 The Figure 3 and the Table 1 also exhibit that the mixes with 2-MI have a higher reactivity according to the temperature which is characterized by a higher slope of the reaction’s peaks, a faster conversion of chemical reactions and a lower Tpeak. The polymerization taking place in two steps40 in the presence of the initiator, firstly, we could 8

assign the copolymerization by esterification as a major event occurring in the first temperature interval (from ~ 50 °C until to ~ 160 °C). Then, the side reactions could occur at high temperatures with the appearance of a shoulder between 160 – 220 °C.

Table 1: Reaction heat (Q), temperature at the maximum of the peak (Tpeak) and reaction interval of ELO/AcDiC systems

Tpeak (°C) 143 ± 2 139 ± 2 134 ± 2 132 ± 2

Reaction interval (°C) 43 – 251 46 – 218 48 – 215 44 – 191

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R = 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI

Q (J.g-1) 253 ± 12 233 ± 11 228 ± 11 212 ± 10

An important specificity of the ELO/AcDiC combination is its ability to polymerize at

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room temperature as mentioned by Carter et al.26 Thus, the conversion degree of the

copolymerization at ~ 25 °C was quantified for each mix with the help of DSC measurements.

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These results are displayed as a graph in Figure 4 where the degrees of conversion are represented in percentage as the function of days (see the experimental part for further

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explications of the employed method).

For all the systems under study, the degree of conversion increases significantly until two days (> 60 %). The copolymerization rate of mixtures without 2-MI is slightly higher

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during the first days with a faster conversion for the R=1 mix. However, all systems reach an outstanding degree of conversion of ~ 85 – 90 % after 7 – 8 days. Then, the copolymerization does not more significantly evolve and the remaining 10 –15 % of conversion are attributed to

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side reactions, which can be occur only at higher temperatures.41

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degree of conversion / %

100

80

60

R = 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI

40

20

0 0

2

4

6

8

10

Days

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Figure 4: Degree of conversion (%) at room temperature of different ELO/AcDiC systems

3.3. FT-IR structural evolution during the copolymerization

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To identify structural evolutions during the copolymerization between ELO and

AcDiC, FT-IR spectra of each monomer were recorded. These spectra are presented in Figure

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5 and the corresponding assignments of ELO and AcDiC are reportedsummarized in Table 2

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ELO

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Asborbance a.u Absorbance // a.u

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the Table S1.

AcDiC

4000

3500

3000

1500

Wavenumber / cm

1000

-1

Figure 5: Raw FT-IR spectra of ELO and AcDiC

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For ELO, the principal peaks39,40,41 are that of the esters of triglycerides with the C=O stretching of ester groups (O=C-O-CH2-) at 1735 cm-1, the C-O stretching bond of ester groups (O=C-O-CH2-) at 1150 cm-1 and the C-O stretching ether bond in α-position of the ester groups (O=C-O-CH2-) at 1095 and 1115 cm-1. Characteristic peaks of ELO epoxy groups are the C-H wagging at 792 and 820 cm-1 and the C-O-C ether stretching bonds at 1240 and 1260 cm-1. Two other characteristic bands of ELO are the asymmetric and symmetric C-H stretching of aliphatic chains at 2920 and 2850 cm-1 respectively. Concerning the AcDiC hardener, some important bonds confirm its structure. At 1703 cm-1, the first characteristic bond is the C=O stretching of ester groups formed by the reaction

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of the hydroxyl groups of dipropylene glycol with the anhydride function of MA. The

resulting MA opened cycle generates a -COOH function, which is clearly visible by a wide

characteristic band from 2400 to 3700 cm-1. The C=C stretching bond at 1629 cm-1 and the CO-C stretching ether bond at 1060/1090 cm-1 also confirms the structure of the hardener.

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Finally, the small C=O band at 1780 cm-1 show that AcDiC contains some residue of MA. Several FT-IR spectra of the ELO/AcDiC/2-MI R = 0.8 mix were recorded to

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accurately explore the structural evolution during the copolymerization. The spectra were recorded on samples after isothermal polymerization at 90 °C from t = 0 – 180 min. The

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evolution of the FT-IR bands of the formed structures during the polymerization is displayed in Figure 6a and Figure 6b. The Figure 6c shows the isothermal heat flow evolution recorded by DSC associated with the degree of conversion reached for each time. Note that this

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isothermal reaction has the same enthalpy as that obtained by dynamic DSC measurement (Figure 3, Table 1) and which correspond to the maximal degree of conversion. Furthermore, this isothermal temperature allows a slow advancement of the reactions, which is an important

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parameter to evaluate the chemical structure evolution along the polymerization. To quantitatively compare each evolution bond, FT-IR spectra were normalized to the most

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intense band peak at 1727 cm-1. This band corresponds to the C=O elongation groups of ELO and AcDiC monomers and should not be affected by the polymerization.

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1.2

a)

AcDiC

0 min 10 min 30 min 60 min 180 min

ELO

1.1

1727

1.0

1722

1.0

Absorbance / a.u

1060 0.9

984

0.8

820

0.8

887 0.7 1740

0.6

1730

1720

1710

0.4

0.2

0.0 1800

1600

1400

1200

1000

800

-1

Wavenumber / cm

180 min

0.15

-1

0.05

b) 3800

3600

3400

Wavenumber / cm

3200

 = 0,68

0.10 30 min

 = 0,45 0.05

0.4

c)

10 min

 = 0,16

0

-1

0.2 0.0

0.00

3000

0.6

40

80

120

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0.00 4000

0.8

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60 min

0.10

1.0

 = 0,97

Degree of conversion

0 min 10 min 30 min 60 min 180 min

Heat flow / W.g

Absorbance / a.u

0.15

160

200

Times / minutes

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Figure 6 : FT-IR spectra of the ELO/AcDiC/2-MI R=0.8 mix showing the structural evolution during the copolymerization a) from 1800 to 700 cm-1 and b) from 4000 to 3000 cm-1. c) isothermal DSC curing thermogram at 90 °C and the degree of conversion corresponding for each time

At t = 0, the obtained signal (black curve on Figure 6a and Figure 6b) correspond only to the contribution from ELO and AcDiC. By heating, the confirmation of the

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copolymerization is emphasized by the consumption of epoxy ring bonds at 820 cm-1 and carboxylic acid bonds at 887 cm-1 and 3000 – 3600 cm-1. These variations are accompanied by the increasing of C-O ether stretching band at 1060 cm-1 formed by the esterifications

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between the two monomers. The epoxy ring decreasing bonds at 820 cm-1 are also connected with the apparition of characteristic -OH bonds of hydroxyl groups at 3500 cm-1 that are

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formed by the opening of epoxy group. Note that the C-O bands of epoxy rings do not disappear because of other ether bonds are present in the copolymer as C-O bonds of AcDiC or C-O bonds generated by side reactions. In addition, the increasing of the band at 984 cm-1, which may correspond to these latter cited ether bonds, is also noticeable. A shift of 5 cm-1 (from 1727 to 1722 cm-1) of C=O stretching of ester group bond is also noticeable. This slight difference confirms the conversion of C=O from carboxylic acid groups into C=O of ester groups. Finally, the disappearance of bond at 1780 cm-1 shows that the residual MA of AcDiC synthesis was consumed during the polymerization. 12

3.4. 2D NMR structural evolution during the copolymerization To complete the reactivity’s study, a multi-dimensional NMR investigation was conducted. As for FT-IR, the investigation was realized on the ELO/AcDiC/2-MI R = 0.8 system. In order to understand the evolution of polymeric structures during crosslinking, the characterization of start monomers was firstly done on 1H and 13C 1D NMR in DMSO-d6 (spectra were not shown here). The characteristic peaks of ELO and AcDiC are assigned below according the numbering position of Figure 2: -

The NMR assignations signals of ELO are in agreement with previous investigations16,39,40,41 : 5.2 ppm (Hk); 4.0 – 4.3 ppm (Hj); 3.1 and 2.8 ppm (Hg); 2.3

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ppm (Hf); 1.8 ppm (Hh); 1.7 ppm (He); 1.5 ppm (Hd); 1.2 and 1.4 ppm (Hc); 1 ppm (Hb); 0.9 ppm (Ha) for 1H spectra and 173.3 – 173.0 ppm (Ci); 69.6 ppm (Ck); 62.6 ppm (Cj); 54.1 – 58.0 ppm (Cg) ; 21.5 – 34.4 ppm (Cc, Cd, Ce, Cf and Ch); 11.2 – 14.8 ppm (Ca and Cb) for 13C spectra. -

Characteristic peaks of synthetized AcDiC are associated with: 13 ppm (H8); 6.4 1

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ppm (H6); 6.3 ppm (H5); 5.0 ppm (H2); 3.50 – 3.8 ppm (H1); 1.0 – 1.2 ppm (H3) for H shifts and 167.3 – 167.6 ppm (C7); 165.6 – 165.9 ppm (C4) ; 132.1 – 132.8ppm

18.6 ppm (C3) for 13C shifts.

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(C6); 129.1 – 129.8 ppm (C5); 73.4 – 74.0 ppm (C1); 70.6 – 71.4 ppm (C2); 16.8 –

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The structures formed during the copolymerization of ELO and AcDiC were followed by HSQC 2D NMR. HSQC-type NMR allows making carbon-hydrogen correlations in 1J. Thus, this structural investigation method gives access to all C-H bonds of the polymer network and

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their direct environment. Figure 7a display the HSQC cartography of the mixture at t = 0. The main peaks resulting of the contribution of ELO (green) and AcDiC (blue) were assigned

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according to the 1D NMR displacements previously listed. This figure also shows the peak of 2-MI (6.7 ppm for 1H-NMR and 133 – 135 ppm for 13C-NMR) and the residual peak of MA (7.4 ppm for 1H-NMR and 138.0 ppm for 13C-NMR) which corresponding to the C-H

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displacements of C=C bonds.

The cartography representing the structures after polymerization is exhibited in Figure

7b. The most significant modification is the disappearance of Cg/Hg peaks at 2.8 – 3.1 ppm for 1

H NMR and 54.1 – 58.0 ppm for 13C NMR (see dash green circle). These carbon-hydrogen

couples correspond to the methyne constituting epoxies rings of ELO. The new displacements of these methyne groups, at 3.3 – 4.6 ppm for 1H NMR and 70.0 – 85.0 ppm for 13C NMR (red see red rectangle), confirm the copolymerization by the epoxy rings opening.41 This 13

evolution is also accompanied by a shift of methylene functions in α-position of epoxies rings (Cd/Hd and Ch/Hh) from 1.5 – 1.8 ppm to 1.3 – 1.5 for 1H NMR and 11.2 – 14.8 ppm to 25.0 – 33.0 ppm for 13C NMR. Finally, the consummation of 2-MI and the residual MA during the polymerization is emphasized with the disappearance of its characteristic peaks. The HSQC-type NMR also allows to observe if unwanted reactions such as transesterification of ELO triglycerides or AcDiC monomer occurred during the copolymerization. For that, the methyne Ck/Hk was used as reference and assigned to an integral value of 1 at t = 0 and t = 96 h. The experimental and theoretical integrals of reactive functions of ELO and AcDiC are reported in the Table 32. For ELO, the integrals of the Cj/Hj

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and Cf/Hf couples are very similar at t = 0 (3.5 and 4.7 respectively) and t = 96 h (3.6 and 4.7 respectively). Therefore, we can assume that no transesterification reaction occurred on the

triglyceride esters of ELO. The same method was used to highlight if side reactions occurred on the AcDiC monomer structure. Thus, integrals at t = 0 and t = 96 h of C2/H2, C5/H5 and C6/H6 displayed in Table 32 are very close and highlight the absence of reactions of

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AcDiC monomer during the polymerization.

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transesterification (on C2/H2) or reaction of C=C double bonds (on C5/H5 and C6/H6) on the

14

1

H

13

C

ELO

CH2

AcDiC

CH, CH3

1

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a) H

13

C CH2

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CH, CH3

b)

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Figure 7: HSQC 2D NMR of ELO/AcDiC/2-MI R = 0.8 mix at (a) t = 0 and (b) after 96 hours at 70 °C. The principal characteristic peaks are surrounded in green for ELO and in blue for AcDiC. The new peaks highlighting the copolymerization are surrounded in red.

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Table 2: C/H couple integral values of reactive (or possible reactive) ELO and AcDiC groups at t = 0 and t = 96 h deducted with 2D NMR cartographies of Figure 7. The integration was measured using the Ck/Hk couple as reference

C/H couple

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Cj/Hj Cf/Hf C2/H2 C5/H5 + C6/H6

Integral values t=0 3.5 4.7 9.4 19.2

t = 96 h 3.6 4.7 9 19.3

15

3.5. Mechanical analysis 3.5.1 Dynamic mechanical studies To understand the role of the different parameters influencing the thermomechanical properties of the network, several factors have been studied. Considering the addition of AcDiC hardener, the ratio or the 2-MI initiator, the moduli evolutions and the glass/sub-glass transitions of the thermosets will be presented. First, the Figure 8a display tan delta curves corresponding to α-relaxation of crosslinked thermosets while corresponding moduli evolutions from the glassy to the rubbery state of the resin are shown in Figure 8b. The α-relaxation, which corresponds to cooperative motions of the main chains, is assimilated to the glass transition and gives direct indications

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on the viscoelastic state at room temperature, on mechanical properties and domain of applications of materials.4 Depending on the thermosets formulation (i.e. the addition of

AcDiC, the R ratio or the presence of 2-MI), α-relaxations are different. For the thermosets prepared without 2-MI, the α-relaxation peaks are larger, asymmetric and comprising a

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shoulder at around 4 °C that indicates the heterogeneity insight the structure of the network.

More precisely, there is no homogeneity insight the cooperative movements between the main chains of the crosslinked network. This is due to possible dual networks or several micro-

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domains, composed of shorter chains within the scaffold. Furthermore, the shoulder on each α-relaxation of thermosets without 2-MI is positioned in the same temperature range as the

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maximum of the tan δ peak of the ELO homopolymer. The homopolymer being the result of only homopolymerizations and etherifications, the presence of these shoulders on αrelaxations of the two thermosets without 2-MI would be due to numerous of competing

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reactions during crosslinking. This result is consistent with the reactivity part of our study that mentions the competition between main and side reactions in systems without 2-MI18. On the

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other hand, the α-relaxations of ELO/AcDiC networks obtained with 2-MI are narrower and show a Gaussian-like symmetrical shape. Therefore, we can assume that the utilization of 2MI as initiator increases the chemical selectivity allowing the synthesis of homogeneous final

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networks.

16

1.0

0.8

Tan delta

a) b)

R= 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI Homopolymer

0.6

0.4

0.2

0.0 -50

-25

0

25

50

75

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Temperature / °C

b) a)

-p

0.1

0.01

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R = 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI Homopolymer

1E-3 -50

-25

0

lP

Elastic modulus / GPa

1

25

50

75

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Temperature / °C

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Figure 8: (a) tan delta curves and (b) elastic modulus vs. temperature from -50 °C to 75 °C of cured resins

The glass transition temperatures of the thermosets, named below Tα, where assigned at the maximum of α-relaxation peaks and are reported in the Table 4 3. All values are around

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the ambient temperature. The corresponding Tα values are similar22 or superior20,21 to other vegetable oil/dicarboxylic acid systems. However, differences between Tα values exist among final networks. The Tα value of ELO/AcDiC without 2-MI is ~ 27 °C while that with 2-MI is ~ 41 °C. In the presence of 2-MI, the higher Tα could be due to a better control of the reaction’s selectivity, especially in the first stages of the copolymerization, as already shown in the Figure 3 and the Table 1. an increase of side reactions, which occurs at high temperature (~ 180 °C)18,34, i.e. after that the main esterification reactions have been done. 17

Despite the variation of the functional ratio, the Tα values are very close. Generally, it is more common to have higher Tα in thermosets with an excess of epoxy groups due to the densification of the network resulting by the inter-chains connectivity through the side reactions. On the other hand, the difference in network densities is noticeable with the amplitude of tan δ peaks of networks, that is directly correlated with the network density.42,43 The lower peak amplitude of R = 0.8 compared to R = 1 (0.74 and 0.96 respectively) combined with a smaller peak area show that the material undergoes less relaxations highlighting a higher crosslink density for the ELO/AcDiC/2-MI R = 0.8 network. Concerning the two thermoset prepared without 2-MI, the α-relaxation peaks are almost

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overlapped due the poorer selectivity of chemical reactions. Table 3: Glass transition (Tα), Elastic modulus (E’) at Tα + 30 °C, crosslinking density (ν) and thermal stability at 10 % mass loss (T10 %) for each ELO/AcDiC resin ν (mmol.cm-3) 0.41 ± 0.01 0.56 ± 0.02 0.59 ± 0.02 0.89 ± 0.04 0.77 ± 0.03

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E’ at Tα + 30 °C (MPa) 3.1 ± 0.1 4.6 ± 0.2 4.9 ± 0.2 7.6 ± 0.4 6.6 ± 0.3

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Homopolymer R = 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI

Tα (°C) 6±1 27 ± 1 27 ± 1 41 ± 1 41 ± 1

T10 % (°C) 348 ± 1 329 ± 1 315 ± 1 329 ± 1 320 ± 1

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Networks densities differences can be highlighted on moduli values in the rubbery regime (Figure 8b). Indeed, the E’ value at temperatures above to the α-relaxation is a parameter also characterizing the final crosslinking density. To quantify these differences

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between ELO/AcDiC networks, the crosslink densities were calculated following the Flory’s theory.44 From the elastic modulus value in the rubbery state, the crosslink density (ν) can be

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determined using the formula:45,46

𝜈=

𝐸′ 3𝑅𝑇

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where ν is the network crosslinking density (mol.cm-3), T is the temperature (K), E' is the elastic modulus value at T (MPa) and R is the gas constant. The above equation was applied at T = Tα + 30 °C46. The modulus and crosslinking density (ν) values are summarized in Table 4 3. By comparing the homopolymer and the ELO/AcDiC copolymers without 2-MI, we can observe that the presence of AcDiC into the formulations increases the crosslinking density by 40 %. The reported values also show that the addition of 1 % of 2-MI into the formulation

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increase the crosslinking density by 31 % for R = 1 and by 59 % for R = 0.8 in the final copolymer. This confirms the higher Tα obtained in the presence of the initiator (Figure 8a). At low temperatures, tan δ curves show more complex chain relaxation variations (Figure 9). Due to their homogeneity, only the sub-glass transitions of ELO/AcDiC/2-MI copolymers have been investigated. The first observation is that the sub glass transitions are significantly weaker than α one. From -150 °C to about -100 °C, the γ transition is associated to the hydroxyl side groups rotations which are formed by epoxy rings opening.35 These -OH groups are an important factor affecting the distances between the macromolecular chains as shown in Figure 10. As the secondary reactions imply the participation of the hydroxyl groups, the γ transition intensity could be proportional to the quantity of -OH in the final

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network. Thus, the γ transition intensity can be linked to the number of side reactions. The γ

transition being less intense for R = 0.8, this one could confirm that more secondary reactions took place in the epoxy excess ratio systems during the copolymerization.

R = 0.8 + 2-MI R = 1 + 2-MI Homopolymer

re

0.04

0.03

-100

-75

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-125

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Tan Delta

0.05

0.02 -150

-p

0.06

-50

Temperature / °C

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Figure 9: Sub-glass transitions of ELO homopolymer and ELO/AcDiC/2-MI copolymers

19

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Figure 10: Schematic representation (in green: the triglyceride chains of ELO, in red: AcDiC) showing shorter interchains distances due to etherification reactions, which take place from hydroxyl groups resulting of the opening of epoxy rings. Finally, β transitions (Tβ between ~ -100 °C and -65 °C) are observed for the three materials. Their positions compared to the ambient temperature indicates that all the final copolymers are ductile.4 This β transition is associated to the motions of main chains

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segments17 and more precisely to the ether bridges formed from hydroxyl groups.47 Therefore, the β transitions may be related to the γ transitions, i.e. to the side reactions as etherifications.

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Studying the evolution of γ and β transitions for ELO/AcDiC/2-MI R = 0.8 network, we can observe that the β transition is higher than the γ transition. This evolution is inverted for the

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stoichiometric ratio. Furthermore, the β transition is more intense for R = 0.8. These evolutions confirm the higher etherifications reactions in the R = 0.8 polymer, which gives it a higher crosslink density (Table 4 3) by more interchain connections. In ELO homopolymer,

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the very small β shows that few etherifications took place in this system, which is in agreement with its high γ transition and its lower mechanical behaviour.

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3.5.2 Tensile-tests

The mechanical properties of copolymers synthesized with 2-MI were also studied in

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non-linear regime by tensile-test measurements. The Figure 11 displays the stress-strain curves of the both ELO/AcDiC/2-MI systems, while the Table 5 4 groups the principal values of these two resins. These results show excellent mechanical properties of the two resins in the non-linear regime and confirm, by high strain values at break, the ductile behavior of the ELO/AcDiC/2-MI combination. Interestingly, the stoichiometric ratio system shows higher stress and strain at break values than the 0.8 ratio. Indeed, the curves of this latter follow the same shape of the

20

stoichiometric ratio until at its breaks, which occurs at ɛbreak ~ 80% and σbreak ~ 11 MPa, while the R=1 curve continuously increases until ɛbreak ~ 120% and σbreak ~ 15 MPa. The higher crosslink density of the R=0.8, calculated in Table 4 3, may explain the lower stress and strain values at break as the chain mobility in this 3D polymer network is poorer. In parallel, the lower crosslink density of the stoichiometric ratio allows the reorganization/alignment of polymer chains under stress, which is characterized with the reincrease of the slope from ɛ ~ 45 %. This one provides the polymer a higher stress and strain

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at the break.

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R = 0,8 + 2-MI R = 1 + 2-MI

10 8

-p

Stress / MPa

12

6

2 0 20

40

60

80

lP

0

re

4

100

120

Strain / %

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Figure 11: Stress-strain curves of the both ELO/AcDiC/2-MI system

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Table 4: Young modulus (MPa), stress (MPa) and strain (%) at break of the both ELO/AcDiC/2-MI systems.

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R = 0,8 + 2-MI R = 1 + 2-MI

Young modulus (MPa) 285 ± 7 365 ± 8

Stress at break (MPa) 11 ± 0.6 15 ± 0.3

Strain at break (%) 80 ± 5 118 ± 9

3.6. Thermal stability of bio-based thermosets The thermal stability of ELO/AcDiC resins was evaluated by thermogravimetry under regular airflow. TGA curves of each thermoset are shown in Figure 12. The degradation process can be divided into several stages. The first degradation step occurs in the temperature range 200 – 375 °C. This stage could be associated with chain pyrolysis, by i) the break of 21

ester bonds (formed during the polymerization between epoxy rings and carboxylic acid functions) and of ii) ether bonds formed by the secondary reactions. The mass loss of homopolymer during this step (i.e. until 375 °C) is lower compared to that of copolymers. 100

R = 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI Homopolymer

60

40

20

0 100

200

300

400

500

600

700

Temperature / °C

-p

0.0000

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Mass / %

80

a)

re

-0.0005

-0.0010

-0.0020

100

200

300

lP

R = 0.8 R=1 R = 0.8 + 2-MI R = 1 + 2-MI Homopolymer

-0.0015

400

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Derivative mass / % min

-1

b)

500

600

700

Temperature / °C

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Figure 12: (a) Thermogravimetric Analyses (TGA) and (b) Differential Thermogravimetric (DTG) curves vs. temperature of ELO/AcDiC systems As reported in Table 4 3, the T10%’s homopolymer shows a higher value with 19 to 33

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°C. This result could be explained by its chemical structure, mainly content to ether bonds, which are more thermally stable than the ester ones. Furthermore, the thermal degradation of the copolymer prepared in stoichiometric ratio starts at lower temperatures compared with that of R = 0.8 copolymer. This confirms that the degradation starts with the rupture of ester bonds as this type of linkage is higher within the R = 1 copolymer (due to the presence of a more quantity of AcDiC).

22

At higher temperature, the homopolymer degradation is accelerating from 375 °C reaching the copolymers curves at 475 °C. Finally, the last thermo-oxidative step, i.e. 475 – 630 °C, is the same for all networks. The T10% values are in the same range as other systems based on epoxidized vegetable oils combined with dicarboxylic acids20 or alcohols41 as curing agents. They are also in the same range as epoxidized vegetable oils/anhydrides17 systems which have higher Tα (> 130 °C). The thermal stability of these materials is also advantageous because it is comparable to that of commercial epoxy systems based on DGEBA.24,38,48,49 It can be concluded that the synthesized copolymers have a high thermal resistance, with slightly higher T10% values for the R = 0.8 formulations. Compared to the DMA results

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where the thermosets with 2-MI have higher α-relaxation values and higher crosslink

densities, thermogravimetric studies show that the ratio is the key parameter influencing the thermal stability.

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4. Conclusions

The elaboration of epoxy resins based on epoxidized vegetable oils is a sustainable

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alternative to petro-based and toxic compounds for the production of eco-friendly thermosets.

lead to bio-based epoxy resins.

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In this study, ELO was copolymerized with a dicarboxylic acid liquid at room temperature, to

In order to evaluate the potential of proposed formulations to copolymerize, reactivity studies and reaction mechanisms were firstly done. The mechanistic study conducted by FT-

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IR allowed following the copolymerization through the ELO/AcDiC systems evolution of structure, while the 2D NMR analyses confirmed that no undesired reactions as triglycerides

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transesterification occurred during the copolymerization. The DSC results have shown that the reactivity depends on the epoxy/dicarboxylic acid ratios or the addition of an initiator of reaction. The utilization of 2-MI as initiator allows to get selective reactions (favouring

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esterification in the first step) whereas side reactions such as etherification are favoured for R = 0.8 ratios. These differences of reactivity have no significant effect on thermal stability of synthesized thermosets (T10% > 315 °C) but they have an important impact on their mechanical behaviour. Indeed, DMA curves exhibit that the addition of 2-MI as initiator allows obtaining homogeneous materials, with a higher crosslinking density (from 31 % for R = 1 to 59 % for R = 0.8) and higher Tα values of 14 °C for the both ratios.

23

The low glass transition temperature values corresponding to synthesized ELO/AcDiC network reveal an alternative to high ELO/anhydride Tg systems. This combination allows designing vegetable oils based epoxy resins with glass transition from room temperature (with ELO/aliphatic dicarboxylic acid systems) to more than 130 °C (with ELO/anhydride systems) while keeping high mechanical properties and excellent thermal stability. conflict of interest

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We declare that this work don’t have any conflict of interest.

5. Acknowledgements

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The authors gratefully acknowledge the Région PACA (France) for the financial support of the ECOMOBIL project. Thanks to Dr. Marc Gaysinski from ICN (Institut de Chimie de Nice) for fruitful discussions and help with NMR experiments.

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