Eutectic hardener from food-based chemicals to obtain fully bio-based and durable thermosets

Jonathan Tellers a, Philippe Willems b, Bôke Tjeerdsma b, Nathanael Guigo a and Nicolas Sbirrazzuoli *a
aInstitut de Chimie de Nice, Université Côte d'Azur, CNRS, UMR 7272, 06108 Nice, France. E-mail:;
bORINEO – original renewables, Acaciastraat 14, B-3071 Erps-Kwerps, Belgium. E-mail:

Received 24th January 2020 , Accepted 3rd March 2020

First published on 6th March 2020

This paper presents a facile strategy to activate citric acid (CA) for room temperature curing. The high melting point of CA is reduced by preparing a eutectic mixture of CA and ethyl lactate (EL). By using CA and EL in the molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1, the melting point of CA can be reduced from 200 °C to −50 °C. This eutectic mixture is then used to cure epoxidized linseed oil (ELO) at room temperature and higher temperatures. Room temperature curing was demonstrated by studying the evolution of the storage and loss modulus as a function of time, as well as by monitoring the changes in the glass transition temperature (Tg) with time. By isothermal curing experiments, the activation energy was determined to be 73 kJ mol−1. A comparison between the performance of the samples prepared at room temperature and that of samples prepared at higher temperatures was made, and it was found that full curing cannot be achieved at lower temperatures. By using only bio-based molecules, a fully bio-based thermoset with applicability in everyday life was obtained.


In the new age of increasing awareness of consumers and policy makers,1 synthetic polymer materials are under heavy environmental scrutiny.2 Synthetic plastics present issues, as they are produced from finite fossil feedstocks and pose a substantial threat to the environment.2–5 A worldwide global task force has been set in place by the United Nations between the local and regional government to define sustainable development goals (SDG) for 2030. Goal number 9 of this roadmap aims at “increasing resource-use efficiency and greater adoption of clean and environmentally sound technologies and industrial processes”.6,7 The current global market is still dominated by conventional plastics from fossil feedstocks,2 with many companies trying to promote bio-based materials from niche to consumer ready and high performing products.8 Factors holding bioplastics back are their cost, overall performance, and oftentimes lack of design. Epoxidized plant oils (unsaturated triglycerides) are a great source to obtain green thermosetting materials.9 These oils contain a large number of unsaturated carbon bonds that can be epoxidized,10 affording a molecule with multiple reactive moieties, which makes them suitable to obtain thermosetting materials. Despite not being considered industrially relevant 10 years ago,11 these oils are now commercially available on a large scale. What is missing is a convenient bio-based hardener, which can help in obtaining thermosets at room temperature.

Among hardeners, amines play a major role due to their high reactivity and suitability to afford thermosets with excellent properties. However, oftentimes their high toxicity and tendency to oxidize presents a major drawback.12–14 The use of carboxylic acids is less suitable, and researchers tend to focus on anhydrides and other commercial hardeners.15,16 These can be used to obtain thermosets with high glass transition temperatures (Tg ∼ 110 °C),17 but they are oftentimes only partially bio-based and difficult to handle as anhydrides can cause respiratory tract diseases.18 The major drawbacks hampering the use of carboxylic acids in a similar fashion are their high melting point and slower reactivity. Thus, curing is normally carried out at high temperatures involving convoluted methods.19–22 Clearly, these acids have to be activated in some way to reduce their melting point.

A common strategy to lower the melting point of a molecule is by preparing a binary eutectic mixture. An eutectic mixture is a formulation of two or more components that do not react with each other to form a new compound, but instead inhibit each other's crystallization, resulting in a lower melting point (Tm) of the eutectic mixture compared to the individual constituents alone.23 This strategy has been explored for electronic device production,24,25 pharmaceutical applications26,27 and nanotechnology,28 and as novel solvent systems.29–32 With regard to obtaining polymers and thermosets, the use of eutectic systems as solvents or monomers has recently gained significant interest,32–35 but no attention has been paid to enable high melting point natural acids for curing of thermosets.

Here, we present a strategy to liquefy citric acid (CA), a multifunctional and natural acid, by preparing a eutectic mixture of CA and ethyl lactate (EL), avoiding the excessive use of harmful solvents and water. This mixture is then used to cross-link epoxidized linseed oil (ELO) at room temperature and higher temperatures for analyzing the curing kinetics and properties of the final material.

Results and discussion

Design considerations – liquefied citric acid

Citric acid is attractive as a potential curing agent for epoxy resins for a plethora of reasons. CA has three carboxylic acid groups that provide high connectivity, it is a naturally occurring acid,36 is of a low cost, nontoxic, and sustainable as it is produced by microbial fermentation.37,38 The high melting point of CA compared to commercially used anhydrides hinders its use in a similar fashion. Thus, it is necessary to liquefy the acid to enable curing at room temperature and relatively moderate temperatures (<50 °C).39 This may be achieved by depressing the freezing point of the acid through the addition of compound B, which forms a eutectic mixture. The pure compound has lower entropy and a higher chemical potential than the impure mixture, which can be exploited to reduce the melting temperature (Tm) below room temperature.40,41 In order to obtain fully bio-based materials, compound B should also be a bio-based chemical. After screening many different molecules, ethyl lactate (EL) was selected as a promising candidate. It was found that when mixtures of CA and EL (e.g. 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar mixture) were heated above 130 °C, below the melting point of CA at 153 °C,42 a homogenous liquid was formed. The heating period depends strongly on the total mass of the components. When this mixture was cooled down to room temperature, it became viscous and remained stable for prolonged time periods (6 months and more). An overview of the chemicals employed and their appearance, as well as that of the mixture are shown in Fig. 1. To verify that no reaction occurred between the EL ester and the carboxylic acid groups during the liquefaction process, an NMR spectrum of the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar equivalent of the CA[thin space (1/6-em)]:[thin space (1/6-em)]EL mixture that was maintained at 150 °C for 30 min was recorded, which showed signals of a perfect mixture of the two materials without any new signals arising (see Fig. S1).
image file: d0gc00311e-f1.tif
Fig. 1 Overview of employed compounds. (A) Citric acid. (B) Ethyl lactate. (C) Mixture of citric acid and ethyl lactate obtained by heating them in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio.

The formation of hydrogen bonds between EL and CA was revealed by IR measurements (spectra in Fig. S2). Between 3600 cm−1 and 3000 cm−1, the O–H stretching peaks of EL100 (100 mol% ethyl lactate) and CA100 (100 mol% citric acid) are relatively narrow, which can be associated with relatively weak intermolecular interactions. In contrast, for the eutectic mixture much broader peaks (∼3200 cm−1) are observed as a consequence of stronger intermolecular interactions. In addition, a slight redshift is observed for the carbonyl band in the mixed systems (∼1700 cm−1), which is also in agreement with the modification of the hydrogen bond network.

The Tm of the eutectic mixtures was investigated as a function of its composition by subjecting liquefied eutectic mixtures of CA/EL to DSC measurements. The obtained phase diagram is shown in Fig. 2 (DSC traces are given in Fig. S3). For a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar mixture (0.5 mol% CA and 0.5 mol% EL) the melting point was determined to be ∼−52 °C, which shows a difference of 200 °C in comparison to that of molecular CA. The eutectic point is found at a high molar percentage of EL (∼85%), a composition which is less attractive since only CA will react with ELO to form a highly cross-linked material.

image file: d0gc00311e-f2.tif
Fig. 2 Binary phase diagram of CA/EL mixtures. The x-axis shows the part of CA in mol%, while the remaining part is made up of EL completely.

Influence of the liquefactor ratio on the properties of the final material

Next, curing with this eutectic mixture was investigated. As an epoxy matrix linseed oil was chosen. Linseed oil is commercially available and due to its high content of unsaturated double bonds10 it is an excellent candidate for epoxidation for large scale commercial applications. Epoxidized linseed oil (ELO) is liquid at room temperature, which allows its easy application, and can be formed into a desired shape. Homogeneous curing can be achieved when it is mixed with a liquid hardener such as the liquefied eutectic CA/EL mixtures. The content of epoxy groups and average molecular weight of one ELO molecule, required to match the appropriate amount of the carboxylic acid hardener, can be determined by NMR. Details of the calculation are given in the ESI, and a Mw = 944.86 g mol−1 and an epoxy group content of n = 5.545 (per molecule) were determined (NMR spectrum in Fig. S4). The best carboxylic acid to epoxy group ratio was determined by, image file: d0gc00311e-t1.tif for a fixed REutectic of 1, samples with RELO ranging from 0.6 to 1.0 were mixed and immediately subjected to curing during DSC and heating from 25 °C to 200 °C at 10 K min−1. In accordance with previous investigations on the curing of ELO,20 we found that an excess of epoxy groups results in the highest reaction enthalpy. This is due to the fact that hydroxy groups formed during the epoxy ring opening and the hydroxy group of CA can participate in the cross-linking reaction.43,44 A ratio of RELO = 0.8 was found to be ideal.

To study the influence of the liquefactor EL, three CA–EL ratios of image file: d0gc00311e-t2.tif were tested. In all cases, crystallization is sufficiently inhibited to push the Tm of these mixtures well below room temperature. The viscosities of these three eutectic mixtures are drastically different, with more EL leading to a lower viscosity (see Fig. S5). To ensure full curing, a high temperature curing protocol (HTP) was used to obtain rectangular slabs suitable for further analysis. Preheated ELO and hardener were mixed and poured into a rectangular mold, followed by curing at 60 °C for 1 h. This step facilitates the removal of air. Then, a post curing step at 160 °C was carried out for 2 h, ensuring full consumption of oxirane groups. Using this method, rectangular slabs with REUTECTIC of 0.5, 1, and 2 were prepared (details of the example mixture and calculations are given in the experimental section in the ESI). Photos of the slabs are shown in Fig. S6, and from a visual examination it can be concluded that the slab with REUTECTIC = 0.5 is not homogenous and shows streaks in the material. This is due to the higher viscosity of the hardener, which makes it difficult to achieve optimal mixing. The samples were subjected to DMA measurements, and the results are displayed in Fig. 3. The glass transition, calculated from the maximum of Tan Delta, is significantly affected by an increase in the amount of EL in the system, decreasing from 39.7 °C (R = 0.5) to 35.0 °C (R = 1) to 30.4 °C (R = 2). This comes as no surprise, since EL does not actively participate in the reaction and can act as a plasticizer in the system.45 Generally speaking, R = 1.0 gave a good compromise between ease of handling due to a reasonable viscosity and properties of the final material. By changing the EL ratio, one could control the room temperature flexibility depending on the desired application.

image file: d0gc00311e-f3.tif
Fig. 3 DMTA data (−50 °C to 150 °C, 1 Hz, 0.1% strain) of thermosets cured with image file: d0gc00311e-t4.tif using a fixed COOH/oxirane group-ratio of 0.8 (the symbols are meant to differentiate the curves better and do not represent the data points).

The tensile test data of these three samples (Table 1) are consistent with the previous results. With a decrease in the amount of EL in the mixture, Young's modulus (E) and thus the stiffness of the materials increases. We attribute the drastic increase from 33 MPa (REutectic = 1) to 630 MPa (REutectic = 0.5) to the difference in Tg between the samples. The tensile tests were carried out at room temperature (∼20 °C), a temperature range in which the sample properties of our materials change drastically. A slight difference in the Tg can have a profound influence on the material stiffness for a given measurement temperature. Like the material stiffness, the tensile strength at the yield point increases with a decrease in the amount of EL, while the strain at break reduces from 77% to 51% to 21% due to the increased brittleness of the materials with a low EL content. This is a known effect of plasticizers,46 further corroborating that EL acts as a plasticizer in this system. The reduced strain at break in conjunction with an increased tensile strength of the samples with R = 1 compared to R = 2 results in identical toughness values for both (0.64 ± 0.11 J m−3 and 0.65 ± 0.08 J m−3 respectively). The huge increase in the tensile strength of the sample with R = 0.5 resulting in a more than two-fold increase in the toughness to 1.43 ± 0.47 J m−3, is favourably comparable to recently fabricated ELO based thermosets.20,47REUTECTIC also affects gel fraction and swelling ratio of the final material. Since EL is incorporated into the matrix and does not cross-link the material, the cross-link density (inversely proportional to Mw between cross links) decreases with increasing EL content. This results in an increase in the swelling ratio and a decrease in the gel fraction with the increase of REutectic (see Table 1). For the intended purpose of room temperature curing the hardener mixture with REutectic = 0.5 is very difficult to use due to its high viscosity at around 20–25 °C. As a compromise, the hardener mixture with REutectic = 1 was used to further investigate room temperature curability.

Table 1 Tensile data and the sol/gel test of thermosets cured with image file: d0gc00311e-t5.tif using a fixed COOH/oxirane group-ratio of 0.8
R Eutectic Used protocol Tensile data Swelling ratio (%) Gel fraction (%)
E (MPa) σ Y (MPa) ε B (%) Toughness (J m−3)
With E = Young's modulus; σY = tensile strength at yield point; εB = strain at break; swelling ratio and gel fraction were determined in THF, details in the ESI;† HTP = high temperature protocol; RTP = room temperature protocol.
2 HTP 30 ± 12 4 ± 1 77 ± 4 0.65 ± 0.08 128 ± 13 86 ± 2
1 HTP 33 ± 4 6 ± 1 51 ± 4 0.64 ± 0.11 88 ± 6 89 ± 1
0.5 HTP 630 ± 38 18 ± 1 21 ± 6 1.43 ± 0.47 71 ± 5 92 ± 2
1 RTP 6 ± 1 1.1 ± 0.3 23 ± 4 0.081 ± 0.038 46 ± 25 68 ± 4

Room temperature curing capabilities of the eutectic CA/EL hardener

For the room temperature protocol (RTP), a mixture of ELO and an appropriate amount of hardener were poured into rectangular molds and left to cure at room temperature. For a sample with REUTECTIC = 1 the RT curing was followed by DSC, rheology, and IR spectroscopy, showing that indeed the sample is cured at RT. For rheology, a sample mixed at room temperature was placed on a parallel plate rheometer and cured at 24 °C (same temperature as the curing room) for many days, storage (G′) and loss (G′′) modulus and viscosity were monitored by applying a uniaxial stress of 0.5%. For DSC, multiple samples (DSC pans) were prepared from an initial mixture, placed in the same environment (24 °C), and left to cure. Then, periodically one of the samples was subjected to DSC scanning (details in the ESI) to follow the Tg of the materials as it cures. A combined graph of rheology data and Tg determined via DSC as a function of time is presented in Fig. 4. As is evident from this graph, curing indeed occurs at room temperature (24 °C). The time to reach the gel point (tα,gel), the point at which the material forms a network, is signified by the crossover of G′ and G′′, which is reached after ∼17 h. The Tg, as observed by DSC (traces in Fig. S7) increases until it reaches a value of about 2 °C, which is reached after around one week at this temperature.
image file: d0gc00311e-f4.tif
Fig. 4 Storage (G′) and loss (G′′) modulus monitored via parallel plate rheology (strain = 0.5%, ω = 1 Hz, T = 24 °C) and Tg determined over multiple days via DSC. The crossover point (G′ > G′′) is marked with a + at ∼17 h after the start of the reaction. The experimental Tg Data was fitted with a simple exponential growth function (Tg = Tg0 + A[thin space (1/6-em)]exp(R0 × t)). The cross on the fit represents the maximum inclination of the logarithmic curve (determined from the differential).

The curing at room temperature was also monitored by IR spectroscopy by taking periodic measurements of a sample curing at 24 °C (the spectra are plotted in Fig. S8). Specifically, the OH band at ∼3500 cm−1 is increasing due to the formation of new OH groups upon opening of the oxirane ring, and the oxirane band at 851 cm−1 slowly disappears with time as the reaction progresses. After 8 days of curing at room temperature, the sample was post cured in an oven for 2 h at 160 °C and its IR spectrum was measured again, which further showed an increase in the OH band absorption and the complete disappearance of the oxirane band. From rheology we know that the reaction is almost finished at the point of reaching a plateau of G′. However, since at higher conversion the reaction becomes diffusion controlled,48,49 there is not enough material transport to fully cure the material at room temperature, which is evidenced by the IR spectra of the post-cured sample.

This is also reflected in the mechanical performance when RTP cured materials are compared to HTP cured ones. In Fig. 5, the DMTA results for a HTP cured sample, a RTP cured sample, and a second run of the same RTP cured sample are displayed. As a result of partial curing, the RTP cured sample has a ∼30 °C lower Tg than the HTP cured sample. Additionally, at around 90 °C, the storage modulus increases (blue line with circles) due to further curing of the material during the measurement. A second scan of the same sample reveals that the Tg is nearly identical to that of the HTP cured sample. The rubber plateau modulus remains a bit lower, which can be attributed to the fact that during the long curing time part of the oxirane rings were consumed by the reaction with moisture in air, which would ultimately reduce the cross-link density and result in a lower storage modulus.50 Tensile tests revealed that partial curing also resulted in lessened material properties (Table 1). The tensile strength, strain at break, and toughness are comparable to those of ELO which is fully cured at high temperatures with a C32 dicarboxylic acid.20

image file: d0gc00311e-f5.tif
Fig. 5 DMTA data (−100 °C to 180 °C, 1 Hz, 0.1% strain) of RTP and HTP cured thermosets cured with REutectic = 1 using a fixed COOH/oxirane group-ratio of 0.8 (the symbols are meant to differentiate the curves better and do not represent the sole data points). RTP#2 represents a second run of the sample RTP#1.

Finally, the curing activation energy (EA) was determined via rheology. The gel point, as an iso-conversional phenomenon,51 can be used to determine the EA for the curing of ELO with the liquefied acids.52 To determine the EA, curing was carried out at temperatures between 24 °C–60 °C (isothermal curing) and monitored using rheology. The gel point can be identified as the cross-over point of G′ and G′′. The obtained curing curves are displayed in Fig. 6A. As is evident from this data, the time to reach the gel point is significantly influenced by the temperature in this case, taking as long as 1047 min at 24 °C, and as less as 46.7 min at 60 °C. The relationship between the gel point tα,gel (same degree of conversion α between samples at the gel point) and the applied isothermal temperature Ti is given by the Arrhenius equation, considering that gelation occurs at a constant degree of cure:49,53

image file: d0gc00311e-t3.tif(1)
where C is a constant and R is the universal gas constant. Fitting of a plot of ln(tα,gel) against the inverse temperature allows one to determine the EA from the slope of that fit. The data and fit are shown in Fig. 6B. Thereby, EA was determined to be around 73 kJ mol−1, which is similar to the value determined for other ELO thermosets cured with non-bio based dicarboxylic acids and anhydrides.20,54,55

image file: d0gc00311e-f6.tif
Fig. 6 Rheology data obtained from isothermal curing experiments. (A) Storage (G′) and loss (G′′) modulus as a function of time for different isothermal curing temperatures (60 °C, 50 °C, 45 °C, 40 °C, 35 °C, 30 °C, and 24 °C). (B) Plot of the logarithm of the time it took to reach the gel point (tα,gel) vs. the inverse temperature of curing.


The experimental details are given in the ESI.


We set out to prepare a fully bio-based thermoset with excellent properties using melting point depression by preparing a eutectic mixture of natural components. In particular, a eutectic mixture between ethyl lactate (EL) and citric acid (CA) was prepared. These chemicals present the advantage of being natural, abundant, inexpensive, non-toxic, and even food contact approved. Such a mixture is innovative in that it provides great advantages over traditional hardeners, especially when interior applications are foreseen (tabletop, children's room, flooring, kitchen, coating, etc.). Using this method, useable eutectic mixtures with difference of 150–200 °C in the melting point (compared to molecular CA) were obtained. These mixtures were then investigated as hardeners to obtain bio-based thermosets, using epoxidized linseed oil (ELO) as the epoxy matrix. By varying the ratio of EL to CA the tensile strength toughness can be increased, which comes at a cost of strain at break of the material and difficulty in preparation due to a higher viscosity when less amount of EL is used. Using this method, comparatively strong bio-based thermosets are obtained in a convenient manner. It was demonstrated that using a eutectic mixture allows mixing and even curing at room temperature with a high melting point natural acid. This latter point is of considerable importance to save energy (heat), which is often needed for cross-linking epoxies with diacids. This method presents a hitherto unexplored strategy for bio-based thermosets and considering the present surging interest in eutectic mixtures, a further compelling use case.

Conflicts of interest

The authors declare no conflict of interest.


This work was benefited from financial support from the French Government, managed by the National Research Agency (ANR) under the UCAJEDI Future Investments Project with reference number ANR-15-IDEX-01 and “La Maison de la Chimie”. Fruitful collaboration with Mettler Toledo is also acknowledged.


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Electronic supplementary information (ESI) available: NMR and IR spectra, and raw DSC data. See DOI: 10.1039/d0gc00311e

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