Linni Zhao,
Lin Zhang and
Zhonggang Wang*
State Key Laboratory of Fine Chemicals, Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Linggong Rd. 2, Dalian 116024, China. E-mail: zgwang@dlut.edu.cn
First published on 26th October 2015
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.
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.
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.
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 CO 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).
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 –HCCH– 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. 4 illustrates the DMA thermograms of the cured Epoxide-A and ERL-4221 as a function of temperature. The data of glass transition temperature (Tg) values derived from tanδ as well as the storage modulus below and above Tg 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 Tg + 30 °C, R is the gas constant and T is the absolute temperature.
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 Tg 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 Tg 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.
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.
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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 CC bond at 1503 cm−1 in furan ring cannot be detected, indicating the furan moieties have been cleaved by the degradation reaction.
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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.
Footnote |
† Electronic supplementary information (ESI) available: 13C NMR and ESI-MS data. See DOI: 10.1039/c5ra18658g |
This journal is © The Royal Society of Chemistry 2015 |