Synthesis, Processing, and Performance of a Furan-Based Glycidyl Amine Epoxy Resin

A furan-based glycidyl amine epoxy resin was synthesized using furfuryl amine and epichlorohydrin and monitored during synthesis by GPC and FTIR. Two major products, furan diepoxy (FDE) and α-chlorohydrin FDE, formed concurrently, with α-chlorohydrin FDE becoming dominant at longer reaction times.

Both products were purified and cured with an amine to study network formation using DSC, FTIR, and NMR. Thermomechanical properties measured by DMA showed that α-chlorohydrin FDE exhibited significantly higher stiffness and glass transition temperature than purified FDE. These enhanced properties were attributed to additional cross-linking through furan ring-opening, establishing a clear structure–property relationship.

The original article

Synthesis, Processing, and Performance of a Furan-Based Glycidyl Amine Epoxy Resin

Amy Honnig Bassett, Emre Kinaci, and Giuseppe R. Palmese*

ACS Omega 2025, 10, 46, 55432–55445

https://doi.org/10.1021/acsomega.5c05822

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Epoxy resins are conventionally bisphenol A (BPA) based, with the diglycidyl ether of BPA (DGEBA) making up about 70% of the epoxy usage. (1,2) BPA is a petroleum-based resource and a known human endocrine disruptor. (3,4) With a growing focus on a more sustainable future, researchers have sought BPA alternatives. Literature has shown furan to be a promising candidate as a phenolic alternative because of its aromaticity and availability. (5,6)

Furan is a heterocyclic, five-membered ring extracted from polysaccharides found in biomass wastes. (5) Furan-based compounds can possess a wide variety of functionality, including carboxyl, hydroxyl, amino, and vinyl moieties, leading to ease of use in multiple chemistries. (6) Epoxy-amine chemistry using furan-based compounds has been explored in numerous studies. Hu et al. successfully synthesized the furan-based epoxy monomer, 2,5-bis[(2-oxiranylmethoxy)-methyl]-furan (BOF) from 2,5-bis(hydroxymethyl) furan (BHMF). BOF cured with conventional aliphatic and aromatic amines had an increased room temperature storage modulus (E’) compared to benzene-based analogs. The improved property was hypothesized to result from potential hydrogen bonding associated with the oxygen atom in the furan ring. (7) A fully furan-based epoxy-amine system was also synthesized and cured. BOF was cured with difuran diamine (DFDA), synthesized from furfuryl amine. BOF/DFDA had a further increased T_g and room temperature E’ compared to BOF cured with a conventional aliphatic amine. The reasoning was again attributed to the potential hydrogen bonding of the oxygen in the furan ring with hydroxyl groups resulting from the epoxy-amine ring opening reaction. Additionally, the fully furan-based system also had increased thermal stability with a char yield in nitrogen at 750 °C of 40 wt %. (8) Other investigators found similar results when using 2,5-furan dicarboxylic acid (FDCA) as the starting furan material. The epoxy monomer based on FDCA cured with 3, 3′-diamino diphenyl-sulfone (33DDS) and 4, 4′-diamino diphenyl-sulfone (44DDS) had excellent E’ and T_g compared to nonfuran-based systems. (9) Potential hydrogen bonding and increased chain packing were attributed to the improved properties. The researchers proposed similar ideas for why furan increased viscoelastic properties, but did not provide fundamental evidence. The above-mentioned furan-based molecules require relatively complicated synthesis and purification processes to obtain thermosetting materials and have limited commercial availability. However, some furan-based molecules, such as furfuryl amine, are commercially available and can be readily functionalized to create thermoset materials. This investigation seeks to fundamentally understand the influence of the furan ring on (1) epoxy monomer synthesis, (2) epoxy-amine thermoset processing, and (3) polymer performance characteristics. The outcomes of this work will provide evidence for the role furan plays in improving the viscoelastic properties and promote the use of furan in thermosetting systems.

Epoxidizing furfuryl amine creates glycidyl amines instead of the glycidyl ethers shown in literature with BPA-based systems. Glycidyl ethers are characterized by an oxygen atom connected to the oxirane group and are the most widely used epoxy resins. Due to their popularity, extensive research has been done to fundamentally understand the synthesis and curing procedures to obtain the best properties with these materials. (10,11) Glycidyl amines, however, are less common, leading to a more limited fundamental understanding of these systems for achieving the best properties. Glycidyl amines are characterized by a nitrogen atom connected to the oxirane group. Generally, epoxies are synthesized using epichlorohydrin and an acidic hydroxyl group for glycidyl ethers or an amine-containing compound for glycidyl amines. (11,12) Despite similar synthetic routes, glycidyl amines present a unique challenge because of the formation of a tertiary amine upon complete glycidation. Tertiary amines are known to catalyze an epoxy’s ring opening, leading to homopolymerization and potentially creating unwanted side products during synthesis or cure. (10,12)

Limited foundational work has been done on glycidyl amine synthesis or cure. In the 1990s, a series of papers was published analyzing the impurities formed during the synthesis of glycidyl amines, the kinetics of intramolecular cyclization, and shelf life analysis. (13−17) A review was also published focused on the kinetics and mechanisms of cure of glycidyl amines. (18) Since the publication of these papers, there has been limited research on glycidyl amines. Additionally, the published papers focused on aniline-based molecules. Incorporating furan into a glycidyl amine will introduce further complications because of the structural differences between furan and benzene rings. To the best of our knowledge, there is no published work on understanding the interaction of the furan ring during the synthesis and cure of glycidyl amine.

This work has three aims: (1) to investigate the effect of synthetic conditions on glycidyl amine monomer formation, (2) to determine the interaction of the furan ring with glycidyl amine products during polymerization, and (3) to evaluate how the synthetic and polymerization conditions impact the final polymer properties of a furan-based glycidyl amine. The outcomes of this work will provide evidence for the unique ability of the furan ring to interact with synthetic products that are specific to glycidyl amine chemistry and how this can be tailored to impact the final polymer properties.

Experimental Section

Resin Characterization

All FDE and FGE resins were characterized using gel permeation chromatography (GPC). A Waters AQUITY Advanced Polymer Chromatography unit with an AQUITY refractive index detector was used. Tetrahydrofuran (THF) Optima was used as the eluent with a flow rate of 0.6 mL min^–1. A series of 4.6 × 150 mm AQUITY APC columns (XT 450 2.5 μm, XT 125 2.5 μm, and XT 45 1.7 μm) was used and maintained at 40 °C. Polystyrene standards were used to calibrate the instrument (PSS ReadyCal Kit, range: 474–2,500,000 g mol^–1; maximum Đ: 1.15). Samples were prepared by mixing 10–12 mg of resin into 2 mL of THF. The number-average molecular weight (M_n) and areas under the curve were determined. The hydrolyzable chlorine content was measured for Flashed-FDE and 52 h.-FDE following ASTM D1726–11 using Test Method A. (19) Test method A is used for 1 wt % or less hydrolyzable chlorine content.

The epoxy equivalent weight (EEW) was measured for Flashed-FDE and 52 h.-FDE. The EEW was measured following ASTM D1652–19 using the manual titration method. (20) However, it was found that for monosubstituted furan epoxies, the furan ring interacted with the perchloric acid used in the titration. A new titration method was developed to obtain an accurate EEW for monosubstituted furan epoxies based on methacrylation and acid number titration. Complete details on the EEW method are found in the SI.

Samples were also characterized by ¹H- and ¹³C{¹H}-NMR (24 °C in CDCl₃) using a Bruker Avance Neo 400 MHz NMR Spectrometer. ¹H NMR spectra with peak assignments are found in Figures S2–S4. Full ¹³C{¹H}-NMR spectra are found in Figures S5–S7. The ¹H NMR chemical shifts, number of protons represented by the signal, peak multiplicity, and coupling constants are listed below for the two pure products of Flashed-FDE and 52 h.-FDE. Additionally, the ¹³C{¹H}-NMR chemical shifts are denoted below for Flashed-FDE and 52 h.-FDE.

Reaction Kinetics for S-FDE and 1:10-FDE

A Thermo Scientific Nicolet iS50 FTIR spectrometer was used in transmission mode in the near-IR range (4000–8000 cm^–1) to track the formation of the epoxy-amine adduct over time. The reaction mixtures of 1:2.1 and 1:10 furfuryl amine: epichlorohydrin were mixed at room temperature and put into the FTIR cell setup. The cell setup was a sealed 1.5 mm outer diameter glass tube. A spectrum was taken every 30 min for 52 h, and each spectrum was 32 scans with a 4 cm^–1 resolution. The peak height of 5987 cm^–1 was used as a reference. The conversion of epoxy (EP, 6080 cm^–1), secondary amine (SA, 6545 cm^–1), and primary amine (PA, 4945 cm^–1) were calculated using eqs 1–3, where I(t = 0) and I(t) are the designated peak intensities before and during cure: (21−23)

Results and Discussion

Investigating the Effect of Reaction Conditions on FDE Resin Synthesis

Figure 2 includes the GPC traces for S-FDE and Flashed-FDE. The S-FDE trace had three peaks, suggesting the presence of high-molecular-weight oligomers in addition to pure FDE. Comparing the GPC traces of S-FDE and Flashed-FDE shows the location of the pure FDE peak, as Flashed-FDE has one peak at 8.56 min with an M_n value of 209 Da based on polystyrene standards. The molecular weight of FDE is 210 Da. Flash chromatography successfully removed oligomers and unreacted epichlorohydrin.

ACS Omega 2025, 10, 46, 55432–55445: Figure 2. GPC trace for S-FDE, 1:10-FDE, and Flashed-FDE

The GPC peak areas were used to calculate the percentage of pure FDE formed in S-FDE. By this method, S-FDE contained about 44% pure FDE, with the remaining being high-molecular-weight oligomers.

Using excess epichlorohydrin well above the stoichiometric value in the epoxidation of bisphenols and other alcohols yields products with limited oligomerization wherein the amount of n = 0 monomers can be more than 85%. (12) In an attempt to eliminate both the oligomers formed and the need for flash chromatography to purify the epoxidation products of furfuryl amine, a higher molar ratio of furfuryl amine to epichlorohydrin (1:10 mol) was used. Figure 2 also shows the GPC trace for the epoxidation products of 1:10-FDE.

¹H NMR spectroscopy was used to characterize the products formed during synthesis. The 52-h product, though similar to Flashed-FDE, particularly with respect to the monosubstituted furan ring (Figures S2–S4), had noticeable differences in the epoxy hydrogen peaks and the hydrogens on the carbon bridge between the furan and the nitrogen. In both regions, there was peak broadening, suggesting the formation of a product containing additional chemical groups.

¹³C{¹H}-NMR spectroscopy was used instead to characterize the products formed during the synthesis because the spectra had more apparent differences. Figure 5 shows ¹³C NMR spectra (A) showing the furan ring carbon region, and (B) showing the carbons between the epoxy ring and furan. The full spectra are found in Figures S5–S7.

ACS Omega 2025, 10, 46, 55432–55445: Figure 5. 13C{1H}-NMR for increasing reaction time of 1:10-FDE compared to Flashed-FDE with spectra offset for clarity. (A) Furan ring carbons with all carbons in the FDE structure labeled for all regions. Inserts are zoomed-in portions of the spectra. (B) Epoxy and chlorinated product carbons.

Model Study of 52 h.-FDE Cured with Furfuryl Amine

¹H- and ¹³C{¹H}-NMR techniques are better suited than FTIR for elucidating the bonds forming after furan ring opening. However, polymers that can be dissolved in solvents are required to use these techniques. Therefore, 52 h.-FDE was cured with furfuryl amine, a monoamine, to control network formation and allow the samples to be soluble in NMR solvent. Figure 8 shows reaction information for 52 h.-FDE cured with furfuryl amine, including A) a general reaction scheme, B) FTIR data, C) ¹³C{¹H}-NMR data, and D) ¹H NMR data for unreacted and cured samples.

ACS Omega 2025, 10, 46, 55432–55445: Figure 8. 52 h.-FDE cured with furfuryl amine up to 160 °C. A) General reaction scheme. B) FTIR spectra before and after cure. C) 13C{1H}-NMR at room temperature and 160 °C after 2 h. D) 1H NMR at room temperature and 160 °C after 2 h.

Conclusions

In this study, the synthesis, polymerization, and physical properties of the epoxidation of furfuryl amine with epichlorohydrin to create FDE were investigated. First, the synthesis and isolation of FDE were studied by varying the reaction conditions, including epichlorohydrin concentration and epoxy adduct formation time. When furfuryl amine was epoxidized with excess epichlorohydrin, a chlorinated side product formed in addition to the desired FDE monomer. The chlorinated side product was α-chlorohydrin FDE, referred to as 52 h.-FDE in this work.

The next step was to evaluate the effect of structural variations on epoxy-amine polymerization by comparing the polymerization of Flashed-FDE and 52 h.-FDE cured with PACM. Curing 52 h.-FDE with PACM promoted the formation of additional cross-linking reactions. A reaction pathway was proposed for the formation of additional cross-links. The hydrolyzable chlorine on 52 h.-FDE accelerated the epoxy-amine polymerization but also promoted the ring opening of furan. Furan is commonly accepted as a nonreactive group, but under specific conditions, it will act as a functional group to participate in network formation.

Finally, the increased cross-links were confirmed using cross-link density measurements, and the effect of additional cross-links on physical properties was evaluated. The experimental v_e for 52 h.-FDE/PACM was greater than the experimental v_e for Flashed-FDE/PACM, as well as the theoretical v_e values. The increase v_e supported the findings from the second step. The additional cross-linking resulted in an E′ of 4.4 GPa and T_g of 129 °C for 52 h.-FDE/PACM. Epoxy resins containing both a furan ring and hydrolyzable chlorine have the potential to increase the final properties due to the additional cross-linking reactions the furan ring can participate in.