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Abstract

A new bio-based unsaturated polyester resin was blended with a bio-based reactive diluent and evaluated. Furan-based monomers were selected as the main monomers in both the resin and the bio-based reactive diluent to enhance thermomechanical properties and address solubility issues typically seen in bio-based resins and diluents. The resin was synthesized from 2,5-furan dicarboxylic acid, isosorbide, and glycerol, and the resulting polymer intermediate was then end-capped with methacrylic anhydride to introduce reactive sites for cross-linking reaction. The resin was then mixed with either different percentages of bio-based reactive diluent (2,5-bis(hydroxy-methyl) furan methacrylate) or with styrene to study the thermomechanical and rheological behavior of obtained resins. FT-IR, ¹³C-NMR, and ¹H-NMR were used to determine the chemical structure of the bio-based reactive diluent. The thermomechanical properties of resin containing bio-based reactive diluent or styrene are characterized and compared by DMA, TGA, and DSC. The synthesized resin had good solubility in the bio-based reactive diluent but not in the styrene. The different mixtures of resin and bio-based reactive diluent showed glass transition temperatures ranging from 166 °C to 174 °C, which was higher than the commercial fossil-based unsaturated polyester resin used as a reference in this study. With thermal and mechanical properties comparable to commercial petroleum-based thermosets, these bio-based resins are promising candidates for high-performance composites, coatings, and other thermoset-based applications.

Keywords

bio-based unsaturated polyester resins isosorbide FDCA bio-based reactive diluent furan-based unsaturated polyester resin and reactive diluent

Introduction

Thermoset unsaturated polyester resins are used in various applications, such as matrices in fiber-reinforced composites. They are valued for their easy processing and adjustable curing conditions, as well as their good thermomechanical properties, which include high strength and resistance to heat and chemicals.^1,2 Currently, most commercial unsaturated polyesters (UPE) are made from prepolymers composed of fossil molecules like propylene glycol, maleic anhydride, isophthalic acid, phthalic anhydride, and terephthalic acid. The prepolymer is mixed in a reactive diluent, mainly styrene, which reacts in the curing to form covalent crosslinks between the prepolymers. There are some promising examples of using bio-based molecules to synthesize replacements for these fossil-based UPEs, but many of them still rely on styrene for cross-linking.^3–7 However, styrene is volatile, fossil-based, and considered a hazardous air pollutant. Therefore, there is a need to develop fully bio-based and environmentally friendly UPE thermosets. Bio-based reactive diluents are an effective strategy in this regard, as they reduce volatile organic compound emissions by being incorporated into the polymer network rather than evaporating during processing. Their renewable origin supports reduced reliance on fossil resources and contributes to circular material flows. Recent research has focused on creating bio-based UPEs containing bio-based reactive diluents, demonstrating the possibility of achieving completely bio-based unsaturated polyester thermosets.^8–13 The synthesis of these resins often involves the use of aliphatic monomers in the prepolymer or the reactive diluent structures, resulting in a resin with limited thermomechanical properties once cured. Therefore, the substitution of traditional monomers with bio-based alternatives presents a significant challenge. Furan-based monomers, known for their aromatic structure and the possibility to obtain from many different biomass resources, show promise as replacements for petroleum-derived phenyl components in commercial UPEs. These monomers are considered valuable due to their ability to enhance glass transition temperature (T_g) and mechanical strength in comparison to aliphatic monomers, thanks to their rigid and aromatic structures.^14–17 Key furan derivatives such as (hydroxymethyl) furfural and furfural, are produced through the conversion of polysaccharides or pentose and hexose sugars. The United States Department of Energy has identified 2,5 furan dicarboxylic acid (FDCA) as a leading candidate among 12 molecules likely to drive the future green chemistry industry.¹⁸

The adoption of furan derivatives in UPEs can be approached according to two main strategies: first, as a reactant in the synthesis of the UPE prepolymer, and second, as a reactant to synthesize a reactive diluent. According to the first approach, various bio-based UPEs were synthesized using furan-based monomers through the polycondensation of a mixture of diols and diacids. Sousa et al. synthesized FDCA-based UPE through noncatalyzed bulk polycondensation, resulting in a thermally stable crosslinked resin with high T_g (up to 104 °C) and storage moduli. They used 2-hydroxyethyl methacrylate as the reactive diluent instead of styrene.¹⁹ Lomelí-Rodríguez et al. developed polyester resins for coil coatings by combining bio-based isosorbide (IS), FDCA, succinic acid (SA), and 1,5-pentanediol. Instead of focusing on detailed compositions and chemical characterization, the study aimed to evaluate the feasibility of bio-based coil coatings using common mechanical testing methods.²⁰ Suriano et al. developed a range of UPEs using different combinations of FDCA, itaconic acid, diols, and bio-based reactive diluents like butanediol dimethacrylate and dimethyl itaconate. After cross-linking, a T_g of 75 °C was achieved.²¹ Hofmann et al. also presented UPEs based on FDCA and other bio-based building blocks with T_g reaching up to 102 °C.¹⁰ Dai et al. created fully bio-based UPEs from FDCA, itaconic acid, SA, and 1,3-propanediol through direct polycondensation. The introduction of FDCA due to its rigid, aromatic structure significantly enhanced the modulus, flexural strength, and thermal properties of the cured resins, demonstrating the potential to replace fossil-based UPEs.²² In another approach, researchers utilized furan-derived monomers to create reactive diluents for UPE or vinyl ester resins. Sadler and colleagues described the use of two furanic reactive diluents, furoic acid glycidyl methacrylate, and furfuryl methacrylate, which were synthesized from furoic acid glycidyl and furfuryl alcohol, respectively. Blends of vinyl ester resins with furoic acid glycidyl methacrylate and another furanic diluent exhibited lower T_g (82–100 °C) and higher viscosities (4-234 cP) compared to styrene-based vinyl esters.²³ Mixing furfural methacrylate (35%) diluent with high viscosity glycidyl methacrylate isosorbide significantly decreased viscosity to 2234 cP at 25 °C, with the cured specimen exhibiting a T_g of 101 °C.²⁴ Furthermore, a toughening agent hyperbranched acrylate and a bio-based diluent tetrahydrofuryl acrylate were integrated into UV-curable coating formulations of acrylated epoxidized soybean oil. The addition of hyperbranched acrylates enhanced the adhesion, mechanical properties, hardness, and thermomechanical characteristics of the UV-curable coating, while tetrahydrofuryl acrylate enhanced its flexibility.²⁵

A new approach involves the use of furan-based components in both the prepolymer and diluent, which has not been previously reported. By synthesizing UPE with furan-based monomers in both the prepolymer and the reactive diluent, a resin with good thermomechanical properties, a high T_g, and improved solubility can be achieved. Xu et al. synthesized a bio-based UPE containing 1,4 butanediol, succinic anhydride, isosorbide, and maleic anhydride. The resin was diluted with isosorbide methacrylate as a bio-based reactive diluent. Isosorbide was chosen as the main monomer in both the reactive diluent and UPE to improve the rigidity of the final product, increase thermal and mechanical properties, and improve the solubility of bio-based UPE and reactive diluent.²⁶

In a recent study, we introduced a bio-based branched UPE made from FDCA, isosorbide, and glycerol as a bio-based resin alternative. The resin had a T_g of 173 °C, showing potential high-temperature applications..²⁷ Glass fiber reinforced composite made from this resin exhibited good mechanical properties.²⁸ However, the resin’s high viscosity and difficult processability for hand lay-up impregnation were noted as drawbacks. The high viscosity of the resin was mainly due to the branched structure of the resin resulting from glycerol (GLY) as a core molecule.²⁷ In such cases, viscosity can be adjusted with styrene or a bio-based reactive diluent.

This study aimed to synthesize a bio-based reactive diluent to blend with the prepolymer for viscosity reduction and enhanced processability. The study involved the first step of synthesizing branched low molecular weight polymer intermediates with FDCA to obtain a rigid skeletal and aromatic structure, with glycerol as a branching point and isosorbide for a unique bicyclic rigid structure to enhance thermomechanical properties. Methacrylic anhydride was then introduced as an end-cap to provide reactive cross-linking sites. 2,5-Bis(hydroxy-methyl) furan methacrylate was synthesized as a reactive diluent by the method mentioned by Chu et al.²⁹ The synthesized prepolymer was mixed in the second step with 40,50,60,65 % reactive diluent and properties were compared with the prepolymer mixed with 40,45,50% styrene. FT-IR, DMA, TGA, and DSC techniques were used to study the obtained UPEs.

Experimental

Materials

2,5-furan dicarboxylic acid (97%, Apollo Scientific), isosorbide (98%, TCI), glycerol (99%, Sigma-Aldrich) and tin (II)-2-ethyl hexanoate (95%, Sigma-Aldrich) catalyst, methacrylic anhydride (≥94%; Sigma-Aldrich), and hydroquinone (99% from Sigma-Aldrich) were utilized in the preparation of the prepolymer. The experiments were carried out under a nitrogen atmosphere (N₂, 99.5%). Xylene (≥98.5%; Sigma-Aldrich)/isopropyl alcohol (99.8%; Sigma-Aldrich) mixture (1:1), potassium hydroxide (≥85%; Sigma-Aldrich) solution in absolute ethanol (VWR Chemicals), 0.02 M, was used for acid number titration, with phenolphthalein (98.5% Acros, 1% solution in ethanol) serving as an indicator. Potassium hydrogen phthalate (99.95%; Sigma-Aldrich) in distilled water was used as a standard solution. The synthesized resin was mixed with or a reactive diluent made from 2,5-bis(hydroxy-methyl) furan (Apollo Scientific), 4-styrene (99%, Sigma-Aldrich) dimethylaminopyridine (DMAP, 99%, Apollo Scientific), and methacrylic anhydride. The resin was cross-linked using Luperox® A75 (75%, remainder water, Sigma-Aldrich) as the initiator for free radical initiation. CRYSTIC 115 NT³⁰ (Scott Bader) orthophthalic polyester resin containing 40 wt% styrene was used as a reference resin.

Synthesis of FDCA-based UPE

The resin was synthesized following the procedure outlined in our previous publication.²⁷ To summarize, the resin was made using bulk polymerization in two stages. Initially, glycerol (0.033 mol), FDCA (0.1 mol), and isosorbide (0.11 mol) were combined in a round-bottom flask with a mechanical stirrer, condenser, nitrogen inlet, and thermometer. Tin (II)-2-ethyl hexanoate (0.2% w) was then added, and the mixture was heated to 200 °C until the reactants were completely mixed (the complete homogenization of acid and diols and formation of a transparent mixture, about 15 hours after starting the reaction). The reaction progress was monitored for 48 hours until a constant acid value was achieved. In the second stage, the reaction solution was cooled to 110 °C, and hydroquinone (0.3 w % of total monomers mass) was added as an inhibitor. Methacrylic anhydride (0.11 mol) was then added slowly, and the temperature was maintained at 110 °C for 4 hours. Figure 1 shows the schematic synthesis of resin.

Figure 1.

Reaction scheme for the synthesis of resin.

Synthesis of 2,5-bis(hydroxy-methyl) furan methacrylate

2,5-Bis(hydroxy-methyl) furan methacrylate was synthesized following the procedure described by Chu et al.²⁹ (Figure 2). In a 100 mL three-necked flask with an addition funnel, condenser, thermometer, and magnetic stirring bar, 2,5-bis(hydroxymethyl) furan (10 g) and DMAP (0.954 g) were placed. The flask was heated to 80 °C using an oil bath to melt 2,5-bis(hydroxy-methyl) furan and DMAP. Methacrylic anhydride (25.26 g) was then added dropwise over 1 hour. The reaction continued at 80 °C for 24 hours under stirring. The product, a combination of 2,5-Bis(hydroxy-methyl) furan methacrylate and methacrylic acid, was collected and utilized as a bio-based reactive diluent.

Figure 2.

Reaction scheme for the preparation of reactive diluent.

Curing of the resin

The curing method included blending the UPE resin with 40,50,60,65 wt% reactive diluent or 40,45,50 wt-% styrene and then mixing it with 2.0 wt% of Luperox® P peroxide. Subsequently, the mixture was poured into a Teflon mold and placed in a 140 °C preheated oven for 60 min. The cured specimens were removed from the mold and cooled down to room temperature for further characterization.

Characterization techniques

Titration

Titration was used to monitor the progress of the condensation reaction by evaluating the percent conversion of carboxyl groups during the synthesis process. A specimen weighing 0.1-0.2 grams was collected, diluted with a 1:1 mixture of xylene and isopropyl alcohol, and titrated with a 0.02 M KOH solution in absolute ethanol. The titration used 1% phenolphthalein in ethanol as an indicator to measure the amount of potassium hydroxide consumed.

Rheological properties

The viscosity of the resins was analyzed using a Brookfield instrument (DV-E viscometer) with spindle 31 at rpm range from 6-100 at 25 °C, and the values were recorded in Pa.s.

Nuclear magnetic resonance

The samples were dissolved in CDCl₃ and analyzed using a Bruker AVANCE III Spectrometer with a cryogenic probe head, operating at 800 MHz. The ¹³C NMR data was obtained using a 30° excitation pulse and a 10-second relaxation delay. These experimental parameters were chosen to yield semi-quantitative ¹³C NMR spectra. The ¹H NMR spectrum was obtained using 30 degree excitation pulse and 2.5 s relaxation delay.

Fourier transform infrared spectroscopy

Fourier Transform Infrared Spectroscopy was carried out using a Nicolet 6700 FT-IR spectrometer to analyze the samples at different reaction stages and the cured resin. The spectra were recorded between 4,000 and 400 cm^-1 after 64 scans with a 4 cm^-1 band resolution.

Dynamic mechanical analysis

Dynamic Mechanical Analysis was performed on cured resin mixtures using a Q800 instrument from TA Instruments. Specimens (35 mm length, 2 mm thickness, and 5 mm width) were heated in a single cantilever bending clamp from -25 °C to 200 °C in an inert atmosphere at a heating rate of 2 °C per minute, with an amplitude and frequency set at 15 μm and 1 Hz, respectively.

Thermogravimetric analysis

Thermogravimetric Analysis was conducted using a TGA instrument (Q500 from TA Instrument) to explore the thermal stability of specimens. A small, cured resin specimen (8-15 mg) underwent heating from room temperature to 600 °C at a rate of 10 °C/min under a nitrogen purge stream.

Differential scanning calorimetry

Differential Scanning Calorimetry measurements were carried out using a TA Instrument DSC Q 1000 device on samples (5-15 mg) heated from 0 to 200 °C at a rate of 10 °C/min in aluminum pans. Both cured and uncured (mixed with 2% peroxide) resin samples underwent analysis to investigate the efficiency of the crosslinking reaction under a nitrogen atmosphere.

Results and discussion

Resin viscosity

The characteristics of thermoset resins during processing are influenced by their viscosity. In industrial settings, processing methods for composite manufacturing, such as pultrusion, vacuum infusion, spray and hand lay-up, and filament winding, require resins with low viscosity. Resins with high viscosity are difficult to process and may result in inadequate reinforcement impregnation. The viscosity of the resins was measured at 25 °C. The main drawback of the FDCA/IS/GLY resin was its high viscosity (semi-solid) at room temperature.²⁷ To address this issue, different percentages of the synthesized reactive diluent and styrene were added to the resin. The measured viscosities of the resin mixtures with different amounts of reactive diluent and styrene, and commercial UPE, were determined and tabulated in Table 1. The viscosity of the FDCA/IS/GLY/reactive diluent mixture decreases by increasing the reactive diluent percentage (14.5 Pa.s, 2.9 Pa.s, and 1.04 Pa.s for resins containing 40%, 50% and 60% reactive diluent respectively) and the resin containing 65% reactive diluent reached the viscosity of commercial UPE containing 40% styrene (the viscosity of commercial UPE and, FDCA/IS/GLY resin containing 65% reactive diluent is 0.4 Pa.s and 0.36 Pa.s respectively). The rapid decrease in viscosity indicates that FDCA/IS/GLY exhibits good miscibility with the reactive diluent; the reactive diluent is completely miscible in the FDCA/IS/GLY resin at varying percentages of bio-based reactive diluent. On the contrary, increasing the styrene content in the FDCA/IS/GLY resin caused an increase in the viscosity of the resins for the resin containing 45% and 50% styrene. This increase is due to the formation of two phases in the mixture of resin and styrene couple of minutes after mixing (Figures 3 and 4). The resin was miscible with styrene up to 40%, and above that, the resin was temporarily miscible (making two phases after a couple of minutes). This explanation is inferred from the observed viscosity trend and resin behavior; further studies, such as microscopy or dynamic rheology, would be required to fully characterize the phase behavior at higher styrene contents. An increase in the viscosity of resin due to the addition of reactive diluent in higher percentages has been observed in other studies as well..³¹ One reason for that is the polarity of isosorbide. Isosorbide is a key component to obtaining a resin with good thermomechanical properties; however, it is a highly polar molecule and could hinder the solubility of the resin in non-polar reactive diluents like styrene.^5,32 According to the findings, FDCA/IS/GLY mixed with 60% and 65% reactive diluent and 40% styrene in terms of viscosity can be tailored to meet the requirements of different thermoset manufacturing processes like spray and hand lay-up, pultrusion, vacuum infusion, and filament winding.

Table 1.

Viscosity of the resin mixtures and diluents.

Resin	Viscosity (Pa.s)	Miscibility
Commercial UPE (40% Styrene),	0.4	-
FDCA/IS/GLY Resin	-¹	-
FDCA/IS/GLY+40% reactive diluent	14.5	miscible
FDCA/IS/GLY+50% reactive diluent	2.9	miscible
FDCA/IS/GLY+60% reactive diluent	1.04	miscible
FDCA/IS/GLY+65% reactive diluent	0.36	miscible
FDCA/IS/GLY+40% Styrene	0.85	miscible
FDCA/IS/GLY+45% Styrene	6.1-6.9	Temporary miscible
FDCA/IS/GLY+50% Styrene	15-20	Temporary miscible

¹Not possible to determine due to the high viscosity.

Figure 3.

A mixture of FDCA/IS/GLY resin with left: 60% styrene, right: 60% reactive diluent.

Figure 4.

A mixture of FDCA/IS/GLY resin with 45% styrene.

Fourier-transform infrared spectroscopy

This technique was used to confirm the structure of the synthesized reactive diluent (Figure 5), the end-functionalization of the resin with methacrylic anhydride (Figure S1, S2, see supplementary materials), and the curing reaction of the resins (Figure S3, S4, see supplementary materials). The complete reaction of 2,5-bis(hydroxy-methyl) furan and methacrylic anhydride was confirmed by the absence of a hydroxy group peak which can be seen for 2,5-bis(hydroxymethyl) furan (3226-3312 cm⁻¹) and the formation of a new ester group (emerging peak of C=O at 1717 cm⁻¹ for 2,5-Bis(hydroxy-methyl) furan methacrylate and 1693 cm⁻¹ for methacrylic acid) as well as peaks at 1634 cm⁻¹ and 939 cm⁻¹ corresponding to the C=C stretching and =CH₂ bending respectively (Figure 5). The IR spectrum of FDCA/IS/GLY resin was similar to the previous study.²⁷ FDCA/IS/GLY peaks covered the reactive diluent peaks due to similarity in structure especially for functional groups like C=O and C=C but styrene peaks (The unsaturated C-H stretching peaks for benzene ring are at 3081 cm⁻¹, 3058 cm⁻¹, and 3026 cm⁻¹, ring bending peak at 694 cm⁻¹ and aromatic C-H band at 773 cm⁻¹) were obvious in the different mixtures of resin with styrene. The =CH₂ bond at 905 cm⁻¹ for resin/styrene and carbon-carbon double bond peaks at 1634 cm⁻¹ for both resin/reactive diluent and resin/styrene mixture disappeared as the produced resins were cured by a free radical reaction.

Figure 5.

FTIR results for (a) 2,5-bis(hydroxy-methyl) furan, (b) reactive diluent.

Nuclear magnetic resonance

In the NMR spectra (Figure S5, S6, see supplementary materials), it is possible to identify the two main components, methacrylic acid and 2,5-bis(hydroxy-methyl) furan methacrylate, in approximately 2:1 molar concentration, respectively.

Methacrylic acid, ¹H NMR (CDCl₃ =7,27, 298 K) d: 10.0 (bm, OH), 6.12 (pentuplet about 1 Hz, vinyl), 5.54 (pentuplet about 1 Hz), 1.86 (m, CH₃); ¹³C NMR d: 172.37 (COOH), 136.11 (s, C-2), 126.68 (t, vinyl CH2), 17.87 (q, CH₃).

2,5-Bis(hydroxy-methyl) furan methacrylate, ¹H NMR (CDCl₃ =7,27, 298 K) d: 6.32 (s, H-3,4), 6.05 and 5.50 (vinyl), 1.86 (m, CH₃); ¹³C NMR d: 166.90 (COO), 150.12 (C-2,5), 135.86 (s, Methacryl C-2), 126.04 (t, vinyl CH₂), 111.38 (d, ¹J_CH=175.8 Hz, C-3,4), 58.24 (t, ¹J_CH=149.5 Hz, CH₂O), 18.15 (q, ¹J_CH=128.3 Hz, CH₃).

Dynamic mechanical analysis

Dynamic mechanical analysis was carried out to explore the viscoelastic properties of the cured specimen. The commercial reference UPE exhibited a T_g of 106 °C and a storage modulus of 4105 MPa, acting as a benchmark for comparing different FDCA/IS/GLY resin with reactive diluent or styrene blends. The FDCA/IS/GLY resin displayed a T_g of 173 °C and a storage modulus of 3925 MPa. In the resin/reactive diluent mixture, as the reactive diluent content increased, the T_g and storage modulus also increased (T_g: 164 °C, 164 °C, 173 °C, 176 °C and storage modulus: 3674 MPa, 3809 MPa, 4275 MPa, 4292 MPa for 40%, 50%, 60% and 65% reactive diluent in the resin, respectively). Initially, adding the reactive diluent to the resin prepolymer (40% and 50%) resulted in a slight decrease in T_g and storage modulus. However, with higher reactive diluent content (60% and 65%), these values increased significantly, exceeding those of the neat resin. The T_g and modulus of crosslinking thermosets are primarily influenced by crosslinking density and chain segment rigidity. Increasing the weight fraction of reactive diluent in the resin enhances the rigidity of the chain segment, which might impede chain mobility in the crosslinking of the systems and increase the T_g and storage modulus of the thermosets. At the same time, the reactive diluent introduces ester linkages and flexible segments that partially counterbalance the effect of increased crosslink density, limiting a pronounced rise in stiffness. As a result, once a sufficiently crosslinked network is formed (≥60 wt% reactive diluent), further increases mainly improve network packing and homogeneity rather than significantly increasing effective crosslink density, leading to a relatively modest change in storage modulus.³³ A significant decrease in the storage modulus and T_g values in the resin/styrene mixture compared to resin/reactive diluent mixtures indicates the positive impact of a furan-based component presence in the reactive diluent structure. This could be attributed to nanopores or micropores weakening the material’s mechanical properties. Undissolved styrene, being more volatile than reactive diluent due to its lower molecular weight, may evaporate during the initial stages of crosslinking, creating pores throughout the material.³⁴ The lower miscibility of resin in styrene compared to reactive diluent also contributes to the diminished mechanical properties. Non-miscible styrene will function as a plasticizer and thus affect mechanical properties such as stiffness.^31,35 Increasing the styrene content in the resin/styrene mix resulted in lower storage modulus values (1892 MPa, 1410 MPa, and 1484 MPa) and Tg values (164 °C, 138 °C, and 124 °C) for resins with 40%, 45%, and 50% styrene, respectively. All resin mixtures exhibited higher T_g values than commercial UPE because of the presence of FDCA and IS in the resin and the star-shaped structure of the pure resin. Detailed results are presented in Table 2.

Table 2.

The storage modulus, loss modulus, and the glass transition temperatures of different resins.

Resin type	Tan δ peak (T_g °C)/(SD)	Storage modulus at 25°C (MPa)/(SD)
Commercial UPE (40% Styrene)	106 (0.53)	4105 (58)^*
FDCA/IS/GLY Resin	173 (0.65)	3925 (66)^^
FDCA/IS/GLY+40% reactive diluent	164 (0.63)	3674 (78)
FDCA/IS/GLY+50% reactive diluent	164 (0.4)	3809 (92)
FDCA/IS/GLY+60% reactive diluent	173 (0.72)	4275 (68)
FDCA/IS/GLY+65% reactive diluent	176 (0.65)	4292 (62)
FDCA/IS/GLY+40% Styrene	164 (0.53)	1892 (29)
FDCA/IS/GLY+45% Styrene	138 (0.48)	1410 (33)
FDCA/IS/GLY+50% Styrene	124 (0.46)	1484 (30)

*The difference between results from the DMA test in the previous paper²⁷ and here is due to styrene content. The commercial resin had a styrene content of 30% (reported as 30-40% in the datasheet), and the styrene content was adjusted to 40% here.

**The difference between results from the DMA test in the previous paper²⁷ and here is due to a different curing procedure.

Differential scanning calorimetry

The investigation of the curing behavior of resins involved detecting any residual exothermic heat in the DSC thermograms of cured specimens. The findings indicated that there were no remaining unreacted double bonds in the cured resins, confirming their complete curing (Table 3). FDCA/IS/GLY resin showed slightly lower exothermic heat evolved at 211 J/g, than the commercial UPE (recorded at 227 J/g). This is related to the presence of styrene as a reactive diluent in the resin. When the reactive diluent was added to the FDCA/IS/GLY resin, the exothermic heat increased significantly, with values recorded at 265 J/g, 288 J/g, 280 J/g, and 285 J/g for resin mixtures containing 40%, 50%, 60%, and 65% reactive diluent, respectively. This was attributed to the reaction of the double bonds in the reactive diluent with the unsaturated carbon-carbon bonds in the FDCA/IS/GLY backbone, forming a complex three-dimensional network. FDCA/IS/GLY resin containing styrene as a diluent demonstrated a lower heat of exotherm (recorded at 240 J/g, 230 J/g, and 234 J/g for the resin containing 40%, 45%, and 50% styrene, respectively) than FDCA/IS/GLY resin containing bio-based reactive diluent. A possible explanation is that the reactive diluent contains two double bonds, whereas styrene has only one. Since the curing process involving double bonds is exothermic, having more double bonds in the diluent leads to a higher heat of exotherm. The results also revealed that all resin/reactive diluent mixtures displayed two distinct exothermic peaks during the curing process, starting with a broad peak around 90 ˚C and continuing with a sharper peak around 150 ˚C. The first broad peak arises primarily from the curing of the reactive diluent, including both its self-reaction of active double bonds and its reaction with unsaturated sites on the FDCA/IS/GLY resin backbone. The second sharper peak corresponds mainly to the curing of the FDCA/IS/GLY resin backbone, completing the crosslinked network. Specifically, neat reactive diluent exhibited a sharp peak at 93 ˚C due to its active double bonds, while the bio-based resin/reactive diluent mixtures showed a broader peak at 91 ˚C, reflecting the decreased amount of reactive diluent in the mixtures (Figure 6). These findings align with the results of a DSC test on a bio-based UPE resin synthesized by Xu et al., containing 1,4 butanediol, IS, succinic anhydride, and maleic anhydride. In their study, the resin was diluted with isosorbide methacrylate as a bio-based reactive diluent.²⁶

Table 3.

DSC results for cured and uncured resins.

Resin type	Heat of exotherm for uncured resin (J g⁻¹) (SD)	Peak temperature for the uncured resin (°C)
Commercial UPE (40% Styrene)	227 (3.8)	121
FDCA/IS/GLY Resin	211 (6.2)	148
FDCA/IS/GLY+40% reactive diluent	265 (0.85)	147
FDCA/IS/GLY+50% reactive diluent	288 (2.2)	143
FDCA/IS/GLY+60% reactive diluent	280 (6.4)	142
FDCA/IS/GLY+65% reactive diluent	285 (5.1)	143
FDCA/IS/GLY+40% Styrene	240 (3.9)	143
FDCA/IS/GLY+45% Styrene	230 (1.8)	142
FDCA/IS/GLY+50% Styrene	234 (6.6)	148

Figure 6.

DSC curve for uncured resins.

Thermogravimetric analysis

The assessment of thermal stability in thermosets is crucial for determining their potential applications. Thermogravimetric Analysis (TGA) was used to analyze weight changes in different resin mixtures under varying temperatures (revised Table 4). Parameters such as the temperature at which 10% weight loss occurs, the maximum degradation temperature, and the residue at 600 °C are key indicators of thermal stability in cured resins. Comparatively, all synthesized resins exhibited superior thermal stability when compared to the reference UPE, with higher residue at 600 °C, 10% weight loss temperatures above 330 °C, and maximum degradation temperatures above 400 °C, which recorded 292 °C and 390 °C, respectively, for commercial UPE. This improvement is primarily attributed to the chemical structure of the synthesized resins, which include rigid furan units and branched architectures that increase crosslink density, restricting polymer chain mobility and enhancing thermal resistance. The incorporation of bio-based reactive diluents further contributes to network rigidity, resulting in higher decomposition temperatures and greater char formation. Specifically, the FDCA/IS/GLY resin containing reactive diluent exhibited greater thermal stability in terms of weight loss, maximum degradation temperature, and solid residue compared to the resin containing styrene. FDCA/IS/GLY containing 65% reactive diluent recorded the best result with 10% weight loss temperatures at 371 °C and maximum degradation temperatures above 445 °C. At lower temperatures (<300 °C), only minimal weight loss was observed, suggesting negligible degradation from unreacted monomers or residual volatiles, indicating efficient crosslinking during curing. These findings underscore the influence of the presence of a furan unit in both the resin and reactive diluent on thermal stability.

Table 4.

TGA results for cured resins.

Resin type	Degradation temperature at 10 wt% loss (°C) (SD)	Maximum degradation (°C) (SD)	Solid residue at 600 °C (%) (SD)
Commercial UPE (40% Styrene)	292 (3.4)	390 (7)	1,4 (0.11)
FDCA/IS/GLY Resin	344 (2.2)	403 (2)	7,6 (0.25)
FDCA/IS/GLY+40% reactive diluent	368 (2.8)	421 (5.4)	13,56 (0.21)
FDCA/IS/GLY+50% reactive diluent	370 (0.8)	425 (5)	13,63 (0.3)
FDCA/IS/GLY+60% reactive diluent	368 (1.7)	421 (7.3)	11,99 (0.53)
FDCA/IS/GLY+65% reactive diluent	371 (3.5)	445 (1.8)	14,69 (0.18)
FDCA/IS/GLY+40% Styrene	336 (0.95)	393 (3)	6,7 (0.1)
FDCA/IS/GLY+45% Styrene	320 (2.2)	400 (1.6)	8,1 (0.5)
FDCA/IS/GLY+50% Styrene	323 (5.7)	402 (6)	8,2 (0.25)

Conclusions

This study aimed to synthesize a bio-based reactive diluent to blend with the prepolymer to reduce viscosity and enhance processability. A range of bio-based unsaturated polyester resins was formulated using furan-based monomers as the rigid core constituent in both the resin and diluent. The bio-based resin (FDCA/IS/GLY) was created from 2,5-furan dicarboxylic acid, isosorbide, and glycerol, and then end-capped with methacrylic anhydride. By blending FDCA/IS/GLY resin with varying amounts of a bio-based reactive diluent (2,5-bis(hydroxy-methyl) furan methacrylate), a bio-based UPE was obtained. The thermomechanical properties of the fully bio-based resin mixed with the bio-based reactive diluent were compared to a partially bio-based resin made up of FDCA/IS/GLY resin and styrene as a diluent. Viscosity measurements demonstrated that the FDCA/IS/GLY resin had better compatibility with the bio-based reactive diluent compared to styrene. Thermomechanical analyses showed that the thermosets made with the bio-based reactive diluent exhibited superior properties, with a 10% weight loss occurring at temperatures up to 371 °C, glass transition temperatures ranging from 164 °C to 176 °C, and storage moduli up to 4292 MPa. The incorporation of furan-based monomers in both the resin and diluent resulted in improved thermomechanical properties compared to a traditional fossil-based unsaturated polyester resin, highlighting the potential of these bio-based thermosets as sustainable alternatives to petroleum-based UPEs. Based on the results, UPE formulations containing 60% and 65% of the bio-based reactive diluent met the requirements for viscosity and thermomechanical properties as a thermoset resin used in the industry. The synthesized resin and diluent show promise, and future research should focus on producing composites by reinforcing the resin and reactive diluent with fibers such as glass fibers or natural fibers.

Supplemental material

Supplemental Material - Synthesis of a bio-based unsaturated polyester resin from 2,5-furan dicarboxylic acid and isosorbide blended with a bio-based reactive diluent synthesized from 2,5-furandimethanol

Supplemental Material for Synthesis of a bio-based unsaturated polyester resin from 2,5-furan dicarboxylic acid and isosorbide blended with a bio-based reactive diluent synthesized from 2,5-furandimethanol by Samira Akbari, Tõnis Pehk, Ivo Heinmaa, Mikael Skrifvars, Sunil Kumar Ramamoorthy and Dan Åkesson in Polymers and Polymer Composites.

Footnotes

Acknowledgment

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The NMR tests were supported by the Estonian Research Council grant No. PRG1702.

ORCID iDs

Samira Akbari

Tõnis Pehk

Ivo Heinmaa

Mikael Skrifvars

Sunil Kumar Ramamoorthy

Dan Åkesson

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Supplemental material

Supplemental material for this article is available online.

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