Synthesis and degradable properties of cycloaliphatic epoxy resin from renewable biomass-based furfural

Abstract

Biomass-based cycloaliphatic epoxy resin (Epoxide-A) with two epoxycyclohexyls linked with an acetal group was synthesized by utilizing furfural as a major raw material through a facile two-step preparation: synthesis of a diene precursor and subsequent epoxidation. The chemical structures of Epoxide-A and its precursor were confirmed by FTIR and ¹H NMR spectroscopy. Compared with the commercial cycloaliphatic epoxy resin ERL-4221, the cured Epoxide-A exhibits a similar glass transition temperature and thermal decomposition temperature but significantly higher mechanical modulus, shearing strength and lower coefficient of thermal expansion. More importantly, it is found that the cured Epoxide-A can readily degrade in an acidic aqueous solution due to the labile acetal linkages distributed within the cross-linked network, and the degradation apparently accelerates with the increase of the solution acidity. This peculiar degradation property provides a feasible after-treatment application for the products fabricated with epoxy resin, e.g., in the recovery of precious metal and carbon fiber from electronic waste and carbon fiber-reinforcing composite materials.

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Introduction

Cycloaliphatic epoxy resins are high performance thermosetting materials that have a variety of applications in electronic encapsulations, preparation of composite materials, coatings and adhesives, etc. After curing, the densely cross-linked network and the rigid cycloaliphatic structure confer on epoxy resins long-term service stability because of excellent chemical-heat resistance and high mechanical strength. Nevertheless, the insoluble and infusible nature of conventional epoxy resins also leads to recycling issues.^1–8 For example, when the epoxy resins are applied to microelectronic encapsulation, the three-dimensionally cross-linked network makes the subsequent disassembly operation very difficult in the event that the repair and recovery of the discarded or obsolete electronic products are required, which gives rise to increasing pressure on environment and economic costs.^9,10 A similar situation happens to the recovery of carbon fiber from carbon fiber-reinforced composites which are being widely used in many fields, including sports and gymnastic equipments as well as automobile, shipbuilding, aeronautics and astronautics industries.

Development of degradable epoxy resins in a controlled manner may provide a feasible solution to the above problems. In recent years, degradable epoxy resins that contain various chemical or thermal labile groups in the molecule have been reported. Tesoro et al. used aromatic diamines with disulfide linkages to cure epoxy resins. The results showed that the disulfide bonds in the cross-linked network could be cleaved and reformed under redox conditions.^11,12 Buchwalter et al. developed a series of cycloaliphatic epoxy resins comprising acid-labile linkages which led to an easy dissolution of the epoxy network.¹³ Shirai et al. synthesized a series of photo-cross-linkable epoxies containing thermally cleavable sulfonate ester groups. These cross-linked polymers could become soluble in organic solvents or water after heating at 120–200 °C.^14–16 Besides chemical degradation, epoxy resins that can controllably degrade in the desirable temperature range from 200 to 300 °C were also reported. For example, Ober and Wong investigated thermally cleavable epoxy resins with secondary/tertiary ester and carbamate/carbonate groups, respectively.^17–21 Our group has previously reported that the incorporation of weak secondary and tertiary carbon–ether linkages in the cycloaliphatic epoxy network displayed a thermal decomposition around 220 °C.²² Recently, several cycloaliphatic epoxy resins bearing thermally degradable sulfite and phosphate groups whose cured products underwent a thermal cleavage ranging from 185 to 280 °C were designed and synthesized.^23–25

However, a review of the literature reveals that almost all the afore-mentioned degradable epoxy resins are derived from non-renewable fossil resources. The utilization of biomass resources to produce epoxy resins has significant advantage taking account of the environmental conservancy and sustainable development of ecology and economy.^26–29

Furfural is the acid-degraded product of pentosans, whereas pentosans are abundantly available from agricultural residues like plant hulls and corn cobs. In this work, furfural was chosen as the major starting raw material to synthesize a novel bio-based acetal-linked cycloaliphatic epoxy resin (Epoxide-A). After curing, the acid-sensitive acetal groups are homogeneously incorporated within the three-dimensionally cross-linked network. The degradations of the cured Epoxide-A in acidic aqueous solutions with different acidity were examined and the degradation mechanism was studied by monitoring the degraded products through FTIR and GC/MS spectroscopy. In addition, the thermal and mechanical properties of cured Epoxide-A were evaluated by comparison with commercial cycloaliphatic epoxy resin ERL-4221 through the measurements of thermogravimetric analysis, thermal mechanical analysis, dynamic mechanical analysis and shearing strength.

Experimental

Materials

Furfural, cyclohex-3-enyl-1-methanol and OXONE (a monopersulfate compound: 2KHSO₅·KHSO₄·K₂SO₄) were purchased from Shanghai Regent Company and used without further purification. Commercial epoxy resin ERL-4221 was obtained from Xinjin Chemicals Company; its chemical structure is shown in Scheme 1. Hexahydro-4-methylphthalic anhydride (HMPA) and 2-ethyl-4-methyl-imidazole (EMI) were used as a curing agent and curing accelerator, respectively. Other chemical reagents including p-toluenesulfonic acid (p-TAS) were of analytical reagent grade and used as received.

Scheme 1 Synthetic route to acetal-linked cycloaliphatic epoxy resin (Epoxide-A).

Instruments

Fourier-transform infrared spectra (FTIR) were recorded on a Nicolet 5700 spectrometer. Liquid samples were measured by casting film on KBr salt tablets, whereas solid samples were measured using pallets prepared by compressing the dispersed mixture of sample and KBr powder.

¹H NMR and ¹³C NMR spectra were measured on an INOVA-400 NMR spectrometer (Varian) in CDCl₃ using tetramethylsilane (TMS) as an internal standard.

Electrospray mass spectrum (ESI-MS) was obtained on an Agilent 6310 quadruple LC/MS for electrospray ionization in positive and negative modes. The mass-to-charge (m/z) ratios of the ions were determined with a quadrupole mass spectrometer scanned from 10 to 3000 m/z.

Gas chromatography-mass spectrometry (GC-MS) measurements were determined on an Agilent 7000B triple quadrupole GC/MS. Samples were separated on a 5% phenyl methyl silox capillary column (HP-5MS). The column temperature was initially held at 80 °C for 1 min, then programmed to 220 °C at a rate of 10 °C min⁻¹, from 220 to 310 °C at a rate of 20 °C min⁻¹ and with a final hold time of 6 min. High purity helium was used as the carrier gas. Mass spectra were scanned at a rate of 1.5 scans per s. Electron impact ionization energy was 70 eV.

Dynamic mechanical analysis (DMA) measurements were conducted on an apparatus (TA DMA Q800) at a heating rate of 2 °C min⁻¹ and at a frequency of 1 Hz under nitrogen atmosphere in the linear viscoelastic range. The tests were carried out in the single cantilever mode using the specimens with dimension of 2 × 6 × 40 mm³.

Thermal mechanical analysis (TMA) measurements were recorded on an apparatus (TA TMA Q400) at a heating rate of 5 °C min⁻¹ under a nitrogen atmosphere with a constant stress of 0.5 N with a probe head of expansion. The specimens used for the tests were cylinders with dimensions of 6 × 50 mm².

Thermogravimetric analyses (TGA) were performed with a NETZSCH TG 209 thermal analyzer from 25 to 800 °C at a heating rate of 10 °C min⁻¹ and a nitrogen gas flow rate of 60 mL min⁻¹.

Shearing strength tests were performed on INSTRON-5567A according to the standard of ISO 4587-1979. Epoxy resin was cured between the joint of two steel plates with overlapping area (S) of 25–12.5 mm. The sample was measured with a gradually increased shearing force at a speed of 5 mm min⁻¹ until the bonded plane was broken. The maximum load (P, N) was recorded, and the shearing strength (r, MPa) was calculated from the equation: r = P/S. For each sample, five specimens were measured, and the values were averaged.

Degradation experiments of the cured Epoxide-A with dimension of 3 × 3 × 5 mm³ were conducted in 0.5 M methanesulfonic acid/THF/H₂O, 0.5 M p-toluene sulfonic acid/THF/H₂O, 2.0 M oxalic acid/THF/H₂O and 2.0 M acetic acid/THF/H₂O. In each case, the volume ratio of THF to H₂O is 4 : 1. At refluxing temperature, the residual weight percentages of samples at different time were recorded. For comparison, the degradation of the cured ERL-4221 in 0.5 M methanesulfonic acid/THF/H₂O was also conducted under the same condition.

Synthesis of 2-furylmethyl bis(cyclohex-3-enylmethyl)acetal (Olefin-A)

Cyclohex-3-enyl-1-methanol (33.6 g, 300 mmol), furfural (8.28 mL, 100 mmol) and p-toluenesulfonic acid (1.9 g, 10 mmol), anhydrous n-hexane (150 mL) and 5A molecule sieve (20 g) were charged into a 500 mL three-necked flask equipped with a mechanical stirrer and a thermometer. The mixture was reacted at 0 °C for 1 h. After filtration, the filtrate was treated with 15% of NaOH aqueous solution (2 × 50 mL) and deionized water (2 × 50 mL). The organic phase was dried over anhydrous MgSO₄ and concentrated under reduced pressure. The crude product was purified with column chromatography using hexane as eluent to afford 15.3 g Olefin-A as a colorless liquid having a boiling point of 160 °C at 5 mmHg. Yield: 51%. FTIR (cm⁻¹): 3141 (furan C=C), 3022 (cycloaliph. C=C), 2914 and 2838 (C–H), 1651 (cycloaliph. C=C), 1503 (furan C=C), 1151, 1096 and 1052 (C–O). ¹H NMR (CDCl₃/TMS, ppm): 7.41, 6.43 and 6.31 (3H, furan), 5.66 (4H, =CH–), 5.54 (1H, –HC–O), 3.35–3.48, (4H, –CH₂–O), 1.22–2.18 (14H, cycloaliph. –CH₂–, –CH–).

Synthesis of 2-furylmethyl bis(3,4-epoxycyclohexylmethyl)acetal (Epoxide-A)

Olefin-A (9.1 g, 30 mmol), dichloromethane (90 mL), acetone (90 mL) and 18-crown-6 ether (0.9 g, 3 mmol) were added in a 1000 mL four-necked flask equipped with a mechanical stirrer and two dropping funnels. The resulting mixture was vigorously stirred, and the temperature was decreased to 0 °C. Then OXONE (a monopersulfate compound: 2KHSO₅·KHSO₄·K₂SO₄, 45.0 g, 75 mmol) and ethylenediaminetetraacetic acid (0.06 g, 0.2 mmol) in 240 mL deionized water were added to the mixture. Meanwhile, a 0.75 M KOH aqueous solution was also added dropwise to keep the reaction mixture at pH 7.4–7.9. The system was stirred at 0 °C for 5 h. Then the organic phase was isolated, washed with deionized water, and dried over anhydrous MgSO₄. After filtration and concentration, the product was purified with column chromatography using hexane as eluent to obtain 8.2 g Epoxide-A as a colorless viscous liquid. Yield: 82%. FTIR (cm⁻¹): 3141 (furan C=C), 2986, 2924 and 2838 (C–H), 1503 (furan C=C), 1151, 1114, 1096, 1050 (C–O), 820 and 792 (cycloaliph. epoxide). ¹H NMR (CDCl₃/TMS, ppm): 7.39, 6.37 and 6.35 (3H, furan), 5.47 (1H, –H–O), 3.20–3.35, (4H, –CH₂–O), 3.10–3.20 (4H, C–H on epoxide ring), 1.00–2.19 (m, 14H, cycloaliph.–CH₂–, –CH–).

Curing of epoxy resins

Epoxide-A was mixed with HMPA at the molar stoichiometric ratio of 1 : 0.8 at room temperature. Into this mixture 0.5 wt% of EMI was added as a curing accelerator. The mixture was cured at 90 °C for 8 h, 150 °C for 2 h, and 170 °C for 1 h. As a comparison, the commercial epoxy resin ERL-4221 was also cured under the same condition as Epoxide-A.

Results and discussion

Synthesis and characterization of cycloaliphatic epoxy resin (Epoxide-A)

As shown in Scheme 1, the diene precursor Olefin-A was prepared by the nucleophilic addition between biomass furfural and commercial cyclohex-3-enyl-1-methanol, while the epoxidation product Epoxide-A containing acetal linkage was obtained through the reaction of Olefin-A with OXONE. The inorganic OXONE instead of organic peracid was used as an oxidant because the byproduct generated is convenient to be removed by simple washing with deionized water. In addition, ethylenediaminetetraacetic was added to catalyze the oxidation reaction. 18-Crown-6 ether and 5A-type molecule sieve were phase transfer catalyst and water removal agent, respectively.

Epoxide-A and its precursor Olefin-A were characterized by FTIR ¹H NMR, ¹³C NMR and MS spectroscopy. In the FTIR spectrum of furfural (Fig. 1a), the bands at 2850, 2814 and 2717 cm⁻¹ are attributed to the vibration of C–H bond of aldehyde group, whereas the strong absorptions at around 1670–1700 cm⁻¹ are assigned to C=O bond of aldehyde group. The bands at 3134 and 1503 cm⁻¹ are corresponded to stretching vibration of C–H and C=C bonds in furan ring, respectively.^30,31 After acetalization reaction with cyclohex-3-enyl-1-methanol, the above absorptions of aldehyde group disappear, and instead, there emerges the characteristic absorption of acetal bond –C–O–C–O–C– at 1000–1150 cm⁻¹. Moreover, the bands due to cycloaliphatic –C=C– bonds are observed at 3022 and 1651 cm⁻¹ (Fig. 1b). After epoxidation reaction, for Epoxide-A, the vibration of –C=C– double bond in the cyclohexenyl ring (Olefin-A) is absent, and the characteristic absorption of cycloaliphatic epoxy group at 792 and 820 cm⁻¹ appear (Fig. 1c).

Fig. 1 FTIR spectra of furfural (a), Olefin-A (b) and Epoxide-A (c).

¹H NMR spectra of Olefin-A and Epoxide-A are shown in Fig. 2. The signals of protons in furan ring are found at 7.41, 6.43 and 6.31 ppm. For Olefin-A, the peaks of the protons of –HC=CH– in cyclohexenyl rings locate at 5.66 ppm, while the signals of –CH–O– and –CH₂–O– corresponding to acetal group linked with furan and cyclohexenyl rings, respectively, appear at 5.54 ppm and 3.35 ppm. The other protons of cycloaliphatic CH or CH₂ groups are at 1.22–2.18 ppm. For Epoxide-A, the signal of –HC=CH– at 5.66 ppm disappears, whereas a new signal at 3.17 ppm belonging to the protons of epoxy rings emerges, demonstrating the complete epoxidation. In addition, all the signals in the ¹³C NMR spectra of Olefin-A and Epoxide-A (Fig. S1, ESI†) can be well assigned to the carbons of the corresponding compounds. The ESI-MS spectrum of Epoxide-A (Fig. S2, ESI†) shows that the MS data are consistent with the theoretical value of its molecular weight (334.398 g mol⁻¹).

Fig. 2 ¹H NMR spectra of Olefin-A (a) and Epoxide-A (b).

Thermal and mechanical properties of the cured epoxides

To evaluate the thermal stability of the cured Epoxide-A, the TGA measurement in nitrogen atmosphere was carried out and compared to that of the commercial epoxy ERL-4221. The TGA plots are presented in Fig. 3. The initial decomposition temperature of the cured Epoxide-A (302 °C) is slightly lower than that of ERL-4221 (339 °C), but their temperatures at maximum degradation rate are similar. Moreover, it is found that the cured Epoxide-A exhibits the apparently higher residual weight at 800 °C (7.6 wt%) than ERL-4221 (almost 0 wt%) due to the aromatization effect between the chain fragments derived from the furan rings.^32,33

Fig. 3 TGA thermograms of the cured epoxy resins under nitrogen.

Fig. 4 illustrates the DMA thermograms of the cured Epoxide-A and ERL-4221 as a function of temperature. The data of glass transition temperature (T_g) values derived from tan δ as well as the storage modulus below and above T_g are summarized in Table 1. According to the theory of rubber elasticity, the cross-linking densities (ρ) of a cured epoxy networks were calculated from the storage modulus in the rubbery region in term of equation: ρ = E′/3RT,³⁴ where E′ is the storage modulus at T_g + 30 °C, R is the gas constant and T is the absolute temperature.

Fig. 4 DMA thermograms of the cured epoxy resins.

Table 1 Data of DMA, TMA and shearing properties of cured epoxy resins

Sample	Tg-DMA (°C)	Storage modulus		ρ (10^−3 mol cm^−3)	CTE (ppm K^−1)	r^c (MPa)
		Glassy region^a (GPa)	Rubber region^b (MPa)
a Storage modulus at 50 °C.b Storage modulus at Tg-DMA +30 °C.c Shearing strength at 25 °C.
Epoxide-A	186	2.51	16.5	1.35	64.2	4.70
ERL-4221	191	2.30	11.7	0.95	67.1	3.29

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As shown in Table 1, the cross-linking density value of the cured Epoxide-A is 42% higher than that of the cured ERL-4221, indicating that, compared to ERL-4221, the cured Epoxide-A has the more compact network structure, which brings about higher storage modulus in the glass region. However, although the cured Epoxide-A has the higher cross-linking density, its T_g value (186 °C) is slightly lower than that of ERL-4221 (191 °C). The reason may be attributed to that the two epoxy cyclohexyl groups in Epoxide-A are linked via flexible ether bonds, whereas the linkage in ERL-4221 is a polar ester bond. As a result, it is seen that, relative to the cross-linking density of the network, the major factor that affects T_g is the polarity of polymer segment.

In addition, the denser cross-linking structure of the cured Epoxide-A leads to it the lower coefficients of thermal expansion (CTE) (64.2 ppm K⁻¹) than the cured ERL-4221 (67.1 ppm K⁻¹), as measured by thermal mechanical analysis (TMA) in the temperature range from 25 to 80 °C (Table 1). Low coefficients of thermal expansion is a favorable property for polymer materials taking account of the CTE matching requirement between polymer and various substrates, e.g., in LED or electronic encapsulation and the fabrication of polymer composite materials.

The adhesion property of the synthesized Epoxide-A was evaluated by measuring its shearing strength (γ) at room temperature according to the standard of ISO 4587-1979. For comparison, the shearing strength of the commercial epoxy resin ERL-4221 was also measured under the same condition. The results show that the cured Epoxide-A had a shearing strength of 4.70 MPa, whereas that of the cured ERL-4221 was only 3.29 MPa (Table 1). Relative to ERL-4221, the significantly improved adhesion property may be caused by its higher cross-linking density and storage modulus.

Acid degradation of the cured epoxy resins

The degradation behavior of the cured Epoxide-A was investigated under the various acidic conditions, including 0.5 M methanesulfonic acid/THF/H₂O (MSA) solution, 0.5 M p-toluene sulfonic acid/THF/H₂O (p-TSA) solution, 2.0 M oxalic acid/THF/H₂O (OXA) solution, and 2.0 M acetic acid/THF/H₂O (ACA) solution, following the order of decreased acidity. The residual weights of the samples were plotted against the degradation time, and the results are presented in Fig. 5. The degradation experiment of the cured ERL-4221 in MSA solution was also conducted for the purpose of comparison. It is seen that, the cured Epoxide-A exhibits a rapid weight loss in MSA and TSA solutions. After acid hydrolysis for 60 min and 120 min in MSA solution, the sample losses about 33% and 82% weight, respectively and completely degrades at 180 min. In addition, the degradation rate of the cured Epoxide-A is greatly affected by the acidity of the solution. For example, the degradation rate in aqueous acetic acid is the slowest among the four acids used in this work. Even so, the cured Epoxide-A can also completely degrades after acid hydrolysis for 720 min.

Fig. 5 Residual weight percentage as a function of degradation time for ERL-4221 in 0.5 M MSA solution (○) and Epoxide-A in 0.5 M MSA solution (■), 0.5 M p-TSA solution (●), 2.0 M OXA solution (▲), and 2.0 M ACA solution (▼).

On the other hand, the degradation process of the cured Epoxide-A was examined by visually observing the sample in 0.5 M methanesulfonic acid/THF/H₂O solution with 0 min, 30 min, 60 min and 90 min. The original size of the sheet sample is 12 mm in length, 8.0 mm in width and 1.0 mm in thickness (Fig. 6A). It was seen that the degradation firstly occurred at the edge and some tiny fragments had dropped from the sample at 30 min (Fig. 6B). The degradation was very rapid in the time interval of 30 to 60 min (Fig. 6C), and the sample completely disappeared after acid-treatment for 90 min (Fig. 6D). This result indicates that the embedded metals and/or carbon fibers can be effectively recovered if the product is fabricated with Epoxide-A.

Fig. 6 Illustration of the degradation process of the cured Epoxide-A treated with 0.5 M methanesulfonic acid/THF/H₂O solution for 0 min (A), 30 min (B), 60 min (C) and 90 min (D).

It is noteworthy that the cured ERL-4221 does not display degradation even after acid-treatment in the 0.5 M MSA solution up to 720 min, implying that the ester bonds in ERL-4221 cannot break under the acidic hydrolysis condition. The comparison of the degradation behavior between the two samples indicates that the acidic pyrolysis of the cured Epoxide-A should be attributed to the existence of large amounts of acetal linkages in the network.

The above deduction is demonstrated through monitoring the degradation products by FTIR spectroscopy. After hydrolysis in MSA solution for 90 min, the residual fragments were collected for FTIR measurement. For comparison, the FTIR spectrum of the original sample was also recorded. As shown in Fig. 7, after degradation treatment, although the absorption of acetal group is overlapped with other C–O groups, the characteristic bands of acetal linkage C–O–C–O–C at 1086, 1045 and 1006 cm⁻¹ apparently decrease. In addition, the absorption of conjugated C=C bond at 1503 cm⁻¹ in furan ring cannot be detected, indicating the furan moieties have been cleaved by the degradation reaction.

Fig. 7 FTIR spectra for the cured Epoxide-A sample before degradation treatment (a) and after degradation treatment in MSA solution at refluxing temperature for 90 min (b).

Furthermore, after the degradation treatment of the cured Epoxide-A sample, the remaining solution was examined by means of gas chromatography-mass spectrometry technique (GC/MS) (Fig. 8). The gas chromatography displays a series of peaks attributing to the various fragments cleaved from the network. The MS analysis shows that the component at the elution time of 4.5 min has the molecular ion peak at m/z = 96, which is consistent with the molecular mass of raw material furfural (96.081 g mol⁻¹). The FTIR and GC/MS results demonstrate that the acidic hydrolysis of Epoxide-A network is indeed due to the cleavage of acetal linkage.

Fig. 8 GC-MS spectra of the hydrolyzed products of cured Epoxide-A treated in MSA solution.

Conclusions

In this work, a novel cycloaliphatic epoxy resin (Epoxide-A) containing acetal linkage was successfully synthesized through the nucleophilic addition between bio-based raw material furfural and commercial cyclohex-3-enyl-1-methanol, and the epoxidation reaction. The chemical structure was confirmed by FTIR and ¹H NMR spectra. After curing, the resultant product possesses high glass transition temperature of 186 °C and the initial decomposition temperature over 300 °C. Compared with the commercially cycloaliphatic epoxy resin ERL-4221, the higher cross-linking density of the cured Epoxide-A brings about the lower coefficient of thermal expansion (64.2 ppm K⁻¹) and higher storage modulus (2.51 GPa). Moreover, the shearing strength of the cured Epoxide-A (4.70 MPa) is apparently higher than that of ERL-4221 (3.29 MPa), indicative of it excellent adhesion performance. On the other hand, the large amounts of acetal linkages in the cross-linked network lead to that the cured Epoxide-A can readily degrade in acidic aqueous solutions, and the degradation rate can be controlled by the variation of the solution acidity. The degradation mechanism of the cured Epoxide-A is attributed to the cleavage of acetal linkage as evidenced by the FTIR and GC/MS spectra. The degradation property is a great merit considering the potential applications in the treatments of electronic waste and epoxy-based composite materials if fabricated with Epoxide-A and recovery of precious metal and carbon fiber from them.

Acknowledgements

We thank the National Science Foundation of China (No. 51273031 and 51473026) and the Program for New Century Excellent Talents in University of China (No. NCET-06-0280) for financial support of this research.

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Footnote

† Electronic supplementary information (ESI) available: ¹³C NMR and ESI-MS data. See DOI: 10.1039/c5ra18658g

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