Degradable and recyclable bio-based thermoset epoxy resins

Xianchao Chen ab, Sufang Chen *b, Zejun Xu a, Junheng Zhang a, Menghe Miao c and Daohong Zhang *a
aKey Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education & Hubei Key Laboratory of Catalysis and Materials Science, Hubei R&D Center of Hyperbranched Polymers Synthesis and Applications, South-Central University for Nationalities, Wuhan 430074, China. E-mail: zhangdh27@163.com
bKey Laboratory for Green Chemical Process of Ministry of Education, Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei 430205, PR China. E-mail: sufangchen@wit.edu.cn
cCSIRO Manufacturing, 75 Pigdons Road, Waurn Ponds, Victoria 3216, Australia

Received 9th April 2020 , Accepted 26th May 2020

First published on 27th May 2020


The design of a degradable high-performing thermoset without using organic solvents is critical for the understanding and sustainable development of homogeneous structures with simultaneous reinforcement and toughening functions. Here, we report a novel degradable and recyclable thermoset hyperbranched epoxy resin (EFTH-n) synthesized from bio-based 2,5-furandicarboxylic acid (FDCA). EFTH-n showed excellent performance on common diglycidyl ether of bisphenol-A (DGEBA) with simultaneous improvements in the impact strength, tensile strength, flexural strength, storage modulus and elongation by up to 181.84%, 60.22%, 24.08%, 32% and 58.0%, respectively. The homogeneous microstructure of EFTH-n/DGEBA composites was systematically analyzed using in situ Raman imaging, AFM, SEM, DMA, dynamic light scattering and the positron annihilation lifetime spectroscopy, which enabled us to attribute the improvements to the synergistic effect of the crosslinking density, free volume, intermolecular cavity, hyperbranched topological structure and good compatibility between the components, which was explained by an in situ reinforcing and toughening mechanism. We concluded that EFTH-n could significantly facilitate the degradation of the cured composites under mild conditions without using organic solvents together with a FDCA recycling yield of 56.8 wt%.


1. Introduction

Thermoset epoxy resins1 have an irreplaceable position in the polymer industry because of their high mechanical performance, and chemical and thermal resistance, and they are widely used in wind power and aerospace structures. However, their high viscosity, brittleness (low toughness) and difficulty of degradation are obstacles for continuing sustainable utilization and further developments. The main challenges2 for the use of epoxy resins and their composites include the design of high processability (low-viscosity) epoxy resins and cured epoxy resins with high strength and toughness that can be recycled and reused.

Hyperbranched epoxy resins (HERs) as important epoxy resins belong to one sub-class of dendrimers, and contain highly branched topological structures with a high content of functional groups.2 Because of their deformable topological structure, good solubility1 with other matrixes or solvents, high chemical stability3 and low viscosity,4 HERs can be used as toughening agents for thermoset materials,5 multi-functional initiators,6 and modifiers for epoxy resins.7 In recent years, scientists2 have made significant progress in synthetic methods of low viscosity HERs. The synthetic approaches of HERs2 include esterification, etherification, proton/group transfer polymerization,8 oxidation of double bonds,9 hydrosilylation reactions,10 thiol–ene (Michael addition and coupling) click reactions,11 thiol-isocyanate click reactions,12etc. The low viscosity of HERs13 can improve the rheological properties of diglycidyl ether of bisphenol-A (DGEBA) by separating the entangled molecular chains of DGEBA.

Among wide applications of HERs, one of the important uses in the industrial field is their simultaneous reinforcing and toughening function on DGEBA. Existing methods of simultaneous reinforcing and toughening of DGEBA include the use of nanomaterials,14 block polymers15 and hyperbranched epoxy resins.2 The homogeneous dispersion and size of nanoparticles, as well as good adhesion between DGEBA and nanoparticles are critical factors14 influencing the degree of improvement of strength and toughness. Inverse artificial nacre16 (epoxy-graphene layered nanocomposites) could improve the fracture toughness of epoxy resins by about 4.2 times, which was attributable to the fact that the layered structures alleviated high local stress and dissipated much of the energy. A mollusk shell laminated structure could significantly improve toughness due to a “tablet sliding mechanism”,17 which reproduces the nonlinear deformations over large volumes. Organic rigid-rod sulfonated polyamides18 could be dispersed uniformly in trifunctional epoxy resins, and produce a strong interfacial interaction due to the formation of covalent bonds between the matrix and polyamides. The impacted surface could deflect plastic deformation of voids and a large amount of cracks, resulting in high strength and toughness. Nanostructured block polymers,15 such as the “octopus”-like nanostructure, could increase the tensile strength and fracture toughness of epoxy resins by about 1.31 and 2.92 times, respectively. The reinforcing and toughening mechanism was explained by the integrating effect of strong interfacial adhesion between the matrix and nanostructure, voiding, cavitation and deformation. Polyrotaxanes19 could disperse homogeneously in epoxy resins without phase separation and simultaneously increase the adhesive strength and fracture toughness due to intermolecular hydrogen bonding.

Many types of HERs2 prepared by our group can disperse uniformly in DGEBA. With an increase in the HER content, the mechanical performance (the impact, tensile, and flexural strengths) of cured HER/DGEBA composites first increased and then decreased. The deformable topological structure in the core of HERs could dissipate energy and provide high toughness, and the crosslinkable groups in the shell of HERs could increase the crosslinking density so as to improve the strength of DGEBA. The protonema and cavities that appeared on the micrographs of the fractured surface, were attributed to an in situ reinforcing and toughening mechanism.2 On the other hand, the intramolecular cavities have a negative effect on the tensile and flexural strengths of the composites, as one would expect. HERs with an intermediate degree of branching13 and intermediate epoxy content20 have shown high strength and high toughness. The excellent compatibility between HERs and DGEBA, and their blends without microphase separation was also confirmed by SEM, DMA, and AFM techniques, which substantiated qualitatively the in situ reinforcing and toughening mechanism. However, the compatibility and homogeneous structure of the HERs/DGEBA blends are yet to be quantified using microscopic evidence, microphase morphology,21 phase distribution22 and interfacial morphology.23

HERs have shown superior benefits for modifying DGEBA. However, the cured DGEBA has proven to be difficult to degrade, reprocess and recycle due to their strong chemically crosslinked three-dimensional networks,24 resulting in solid wastes25,26 that have become a heavy burden on the environment. The main approaches for recycling epoxy resins25 include thermal degradation,27,28 pyrolysis,29 mechanical recycling30 and biodegradation,31 besides direct incineration and landfill. Both thermal degradation and pyrolysis29 require high temperature and consume a large quantity of organic solvent, which is energy-intensive and harmful to the environment. Cured materials could also be ground into fine powder and used as a cheap filler in new composites30 for mechanical recycling. Biodegradation31 takes much long time. Yet another available method is to introduce reversible dynamic covalent groups32 into the molecular structure of epoxy resins so that the resulting resin can be degraded under gentle decomposing conditions. Acetal, disulfide, imine and hexahydro-s-triazine are the main structures with reversible dynamic covalent bonds.33 Epoxy resins carrying acetal, imine and disulfide bonds and their composites can decompose at relatively low temperatures, but the mechanical strength of the resultant composites are usually not sufficient to meet application requirements due to the poor stability of their chemical structures. The exceptions are the epoxy resins33 containing hexahydro-s-triazine structures, which have not only high strength but also high glass transition temperature, and the cured product can be degraded completely in sulfuric acid solution in about 24 hours. The topological structure of epoxy resins greatly influences the degradation degree of cured resins.34 To improve their degradability, we changed from linear to hyperbranched structure and synthesized hyperbranched epoxy resins with a trimellitic acid structure,34 citric acid structure,35 hexahydro-s-triazine structure,1 and isocyanate and hexahydro-s-triazine structure.12 The ester bond or hexahydro-s-triazine in HER with a hyperbranched topological structure is easier to hydrolyze than the ether bond in DGEBA. The degradation of the ester bond or hexahydro-s-triazine results in the rapid disintegration of the three-dimensional network structure, which in turn leads to an increase in the degradation rate. Therefore, the synthesized HERs improved the tensile strength and toughness of DGEBA as well as the degradation degree of DGEBA cured by them by 3–8 times under more gentle degradation conditions. However, most of these degradable HERs were prepared using unsustainable fossil fuel-based materials. Bio-based resources may be used to prepare thermoset materials with properties comparable to those of fossil fuel-based systems.36 Many bioresources as raw materials, including plant oils,37 tannic acid,38 chitin,39 vanillin,40,41 and 2,5-furandicarboxylic acid (FDCA)42 have been investigated for preparing thermoset materials.43 The epoxy resin cured by vanillin-based epoxy vitrimer showed high thermal (Tg >120 °C), mechanical (tensile strength >60 MPa, Young's modulus >2500 MPa), and recycling properties.44 FDCA-based poly((ether)ester)s showed high thermal stability, promising mechanical properties and fast degradation rate especially in neutral and alkaline solutions.45 Bio-based FDCA is one of the most promising candidates for the manufacture of epoxy thermosets. FDCA has been highly rated as a biomass-based material for use as a substitute for petro-based diglycidyl terephthalate and widely reported as a precursor for developing many new monomers46 and FDCA-based epoxy resins.42,47 Identifying sustainable natural materials that can be synthesized into high-performance degradable HERs is still a great challenge for the recycling of epoxy resin and composite solid wastes.

Faced with these challenges, including highly efficient recycling, improving our understanding of the homogeneous reinforcing and toughening mechanism, and sustainable development of thermoset epoxy resins, we prepared degradable hyperbranched epoxy resins (EFTH-n) by an esterification and a thiol–ene click reaction based on tris(2-hydroxyethyl)isocyanurate (THEIC) and high-performance, inexhaustible bio-based FDCA. The cured EFTH-n/DGEBA composites showed excellent mechanical properties and degradability, including high tensile strength, high toughness, highly efficient degradation and recycling, as a result of the combined effects of hyperbranched topological structure of EFTH-n, dis-entangled DGEBA chains of EFTH-n, and good compatibility between EFTH-n and DGEBA. NMR, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) and dynamic light scattering (DLS) were used to characterize the chemical structure, molecular weight, molecular size and topological structure of EFTH-n. In situ Raman imaging technology was used to analyze the microscopic phase structure and interfacial compatibility of EFTH-n/DGEBA blends. The in situ reinforcing and toughening mechanism of the hyperbranched epoxy resins was investigated by integrating the surface micrographs obtained from atomic force microscopy (AFM) and scanning electron microscopy (SEM), the phase structure was determined by dynamic mechanical thermal analysis (DMA), and the free-volume was determined by positron annihilation lifetime spectroscopy (PALS). Finally, we studied the effect of some critical factors on the degradability of cured EFTH-n/DGEBA composites under more environmentally-friendly conditions of ethylene glycol/hydrogen peroxide and phosphoric acid solution without any organic solvents, and investigated the composition of the degraded products to understand the degradation and recycling mechanism.

2. Results and discussion

2.1 Characterization of EFTH-n

The bio-based degradable hyperbranched epoxy resin (EFTH-n, n = 3, 6, 9, and 12) was synthesized by a three-step method, shown in Scheme 1. The hydroxyl-terminated hyperbranched polymer (HFTH) was obtained by a reaction between 2,5-furandicarboxylic acid (FDCA) and tris(2-hydroxyethyl)isocyanurate (THEIC). An esterification reaction between the hydroxyl group and 3-mercaptopropionic acid (MPA) was carried out to produce TFTH-n at 120 °C by using p-toluenesulfonic acid as a catalyst. The epoxy groups were then grafted onto TFTH-n by a thiol–ene click reaction between the mercapto group and carbon–carbon double bond at room temperature. Experimental details can be found in the ESI. The chemical structures of HFTH and EFTH-n were confirmed from the FT-IR, 1H NMR and MALDI-TOF-MS spectra (Fig. S1 and S2).
image file: d0gc01250e-s1.tif
Scheme 1 Synthesis of HFTH and EFTH-n (n = 3, 6, 9, 12).

As shown in Fig. S2a, the wide absorption peak at around 3400 cm−1 is attributed to the vibration of the –OH group. The strong peak at 3100 cm−1 belongs to the –CH bond. The absorption peak at 1720 cm−1 corresponds to the vibration of the ester group. The appearance of the absorption peak at 911 cm−1 shows that the epoxy group has been generated. The 1H NMR spectra of EFTH-n in DMSO-d6 is shown in Fig. S2b. The presence of the epoxy group is confirmed by the signals at 3.41 ppm (Hm) and 2.71–2.48 ppm (Hn, Ho). The peak at δ 7.29 ppm is attributed to the chemical shift of –CH2– (Ha) on the furan ring. The chemical shifts at 4.41 ppm (Hb) and 4.11 ppm (Hc) are attributable to –CH2– attached to the ester group. Other proton peaks also appear at the corresponding chemical shifts. The number-average molecular weights of EFTH-3, EFTH-6, EFTH-9 and EFTH-12 are 4346 g mol−1, 4926 g mol−1, 5303 g mol−1 and 6118 g mol−1 (Fig. S2c), which is close to their theoretical molecular weights (4309 g mol−1, 4916 g mol−1, 5523 g mol−1, and 6129 g mol−1), respectively, indicating an increase in the molecular weights of EFTH-n with an increase in the number of their epoxy groups. The epoxy equivalent weights of EFTH-3, EFTH-6, EFTH-9 and EFTH-12 are 1677 g mol−1, 870 g mol−1, 679 g mol−1 and 628 g mol−1, respectively, in agreement with an increase in the n value from 3 to 12, and their viscosities are 3.2 Pa s, 2.8 Pa s, 2.3 Pa s and 2.0 Pa s, respectively (Fig. S3). These results suggest that EFTH-n have been successfully synthesized.

2.2 Mechanical performance

Fig. S4 shows that EFTH-n has a remarkable positive effect on the impact, tensile, and flexural strengths of the cured DGEBA. Each variation in the investigation was increased until the mechanical properties started to drop. One can say that similar trends indicate a certain correlation between the properties. With an increase in the EFTH-n content and n value, the mechanical performance (the impact, tensile, and flexural strengths) of cured EFTH-n/DGEBA composites first increased, reached a maximum value at 12 wt% EFTH-6/DGEBA, and decreased afterwards. As an example, the chemical structure of EFTH-6 is shown in Scheme S1. The impact strengths of cured EFTH-6/DGEBA with different EFTH-6 contents from 3 wt% to 15 wt% are 19.98 kJ m−2, 25.13 kJ m−2, 35.31 kJ m−2, 40.36 kJ m−2, and 23.33 kJ m−2, which show increases of 39.53%, 75.49%, 146.58%, 181.84%, and 62.92% over the cured pure DGEBA (14.32 kJ m−2), respectively. The tensile strengths are increased by 28.25%, 37.71%, 43.78%, 60.22%, and 43.36%, and the flexural strengths are improved by 13.01%, 17.85%, 20.73%, 24.08%, and 17.05%, over that of the pure DGEBA, respectively. The incorporation of 12 wt% EFTH-n and medium molecular weight of EFTH-n provided optimum toughness and strength.

The tensile stress–strain curves and impact force and displacement curves of EFTH-6/DGREBA composites at different mixing ratios and different EFTH-n/DGREBA composites (n = 3, 6, 9 and 12) at 12 wt% mixing ratio are shown in Fig. 1. With an increase in the EFTH-6 content, the elongation at break increases from 2.17% (cured DGEBA) to 3.00% (12 wt% EFTH-6/DGEBA) and then decreases to 2.45% (15 wt% EFTH-6/DGEBA), as shown in Fig. 1a. Increasing the epoxy group number n from 3 to 12 (Fig. 1b) increases the elongation at break from 2.41% (EFTH-3/DGEBA) to 3.00% (EFTH-6/DGEBA), and then decreases for EFTH-9/DGEBA and EFTH-12/DGEBA. The impact force–displacement curves of the cured EFTH-n/DGEBA shown in Fig. 1c and d show similar trends to that of the tensile stress–strain curves shown in Fig. 1a and b. With an increase in the EFTH-n content and n value both the impact force and displacement increase and then decrease. The cured 12 wt% EFTH-6/DGEBA composites shows a maximum impact force of 560 N (Fig. 1c) and a displacement of 4.66 mm (Fig. 1d), showing increases of 9.8% and 58.0% over the cured pure DGEBA (510 N and 2.95 mm, respectively). In addition, the toughness represented by the area under the force–displacement curve48,49 shown in Fig. 1c, is 1690 N mm for cured 12 wt% EFTH-6/DGEBA, which is about 2.22 times that of cured DGEBA (760 N mm). These results indicate that the cured 12 wt% EFTH-6/DGEBA composite has the highest impact strength, toughness and elongation.


image file: d0gc01250e-f1.tif
Fig. 1 The stress–strain and impact force–displacement curves of cured EFTH-n/DGEBA composites ((a) content of EFTH-n, (b) functionality (n value) of EFTH-n, (c) content of EFTH-n, and (d) functionality (n value) of EFTH-n).

The effects of the EFTH-n content and functionality on the thermomechanical properties of cured EFTH-n/DGEBA are shown in Fig. 2. The narrow α-relaxation peak indicates that there is no phase separation and the compatibility between EFTH-n and DGEBA is good. Increasing the EFTH-6 content slightly decreases the glass transition temperature (Tg) corresponding to the α-relaxation peak shown in Fig. 2a. And the α-relaxation peak of the cured 12 wt% EFTH-n/DGEBA composites increases with an increase in the functionality of EFTH-n from n = 3 to 12, as shown in Fig. 2b. The β-relaxation peak area of cured 12 wt% EFTH-6/DGEBA, as shown in Fig. 2c and d indicates high toughness.49 These results are in very good agreement with their mechanical properties and are shown in Fig. S4. In particular, the β-relaxation peak temperature (−22.21 °C) of the cured 12 wt% EFTH-6/DGEBA composite is much lower than that (−12.47 °C) of pure DGEBA, indicating an improvement on the low temperature resistance of the DGEBA.


image file: d0gc01250e-f2.tif
Fig. 2 DMA curves of cured DGEBA and EFTH-n/DGEBA in various relaxations ((a and b) α-relaxation and (c and d) β-relaxation of the cured systems).

The storage moduli (E′) of the cured EFTH-n/DGEBA composites are much higher than that of cured pure DGEBA, as shown in Fig. 3a and b, and the E′ of the cured 12 wt% EFTH-6/DGEBA composites is the highest at about 1865 MPa, about 32% greater than that of cured DGEBA (1408 MPa) at room temperature (20 °C). These results also substantiate the high tensile strength of the cured 12 wt% EFTH-6/DGEBA composite, as shown in Fig. S4. The crosslinking density of the cured EFTH-6/DGEBA composites reaches the peak at 12 wt% EFTH-6 content, as shown in Fig. 3c. Increasing the functionality (n value) of EFTH-n causes a continuous increase in the crosslinking density of the composites, as shown in Fig. 3d, which is also in agreement with their mechanical performance, as shown in Fig. S4.


image file: d0gc01250e-f3.tif
Fig. 3 Storage modulus curves (a and b) and crosslinking density (c and d) of cured EFTH-6/DGEBA.

The positron annihilation lifetime spectrum (PALS)50 is used to analyze the free volume properties of the hyperbranched polymers. Fig. 4 shows the PALS spectra of cured DGEBA and EFTH-n/DGEBA composites. The PALS data and free volume fraction are provided in Table 1. Compared with the fv of the cured DGEBA (3.202%), the free volume fractions of all the cured EFTH-n/DGEBA composites are lower. The fv of the composites decreases with an increase of the EFTH-6 content and the functionality (n value) of EFTH-n. The latter is attributed to the fact that the EFTH-n with ellipsoidal topological structure can penetrate the molecular chain of DGEBA, fill some voids and decrease the distance between the molecular chains. So the decrease of free volume plays a positive role in the simultaneous increase of the strength and toughness of the DGEBA, which can be explained by the mechanism of improving strength and ductility of the polymer.51


image file: d0gc01250e-f4.tif
Fig. 4 Positron annihilation lifetime spectrum of cured EFTH-n/DGEBA and DGEBA.
Table 1 PALS data and free volume fractions of cured DGEBA and EFTH-n/DGEBA composites
Sample τ 3 (ns) I 3 (%) R (nm) V (nm3) f v (%)
DGEBA 1.586 29.450 2.434 60.411 3.202
6 wt% EFTH-6/DGEBA 1.574 29.934 2.421 59.423 3.201
12 wt% EFTH-6/DGEBA 1.650 28.896 2.433 60.716 3.158
12 wt% EFTH-3/DGEBA 1.569 29.736 2.416 59.081 3.162
12 wt% EFTH-9/DGEBA 1.605 28.116 2.455 61.973 3.136


The improvement of the mechanical properties of DGEBA by EFTH-n may be attributed to the synergistic effect of crosslinking density,2 free volume fraction,51 intermolecular cavity,2 hyperbranched topological structure52 and good compatibility. The non-crosslinkable hyperbranched topological structure and free volume fraction have a positive effect on the impact strength and a negative effect on the tensile and flexural strengths and modulus while the crosslinking density has an opposite effect. In other words, intramolecular cavities and high crosslinking density have opposite effects on the mechanical properties of the composites. Additionally, the hyperbranched topological structure53 and intermolecular cavity can absorb the impact energy, deform and transfer load, resulting in high elongation, and high tensile and impact strengths. The incorporation of 12 wt% EFTH-n and medium molecular weight EFTH-n provided the right balance to achieve high toughness as well as high tensile and flexural strengths. Good compatibility can help the reciprocal penetration of molecular chains to reduce the distance among molecular chains so as to increase the tensile strength and modulus. With the increase in the EFTH-6 content, the crosslinking density first increased and then decreased (Fig. 3c). The free volume fraction reduces (Table 1) when both the hyperbranched topological structure and the intermolecular cavity content increase. With an increase in the functionality (n value) of EFTH-n, the crosslinking density increased slowly (Fig. 3d), and the free volume fraction decreased (Table 1), while the contents of the large-size hyperbranched topological structure and intermolecular cavity increased. The combined effect of crosslinking density, free volume, hyperbranched topological structure and good compatibility results in the simultaneous improvements of tensile, flexural and impact strength, and modulus and elongation of the cured EFTH-n/DGEBA composites, as explained by the in situ reinforcing and toughening mechanism.2

2.3 Degradation and recycling of cured EFTH-n

EFTH-6 was chosen as an example to obtain the basic degradation conditions for the complete decomposition of the cured 12 wt% EFTH-6/DGEBA composite. The effects of acid type, acid solution concentration, degradation time, and temperature, and the mass ratio of cured materials to solution and mass ratio of ethylene glycol (MEG) to H2O2 on the degradation degree of cured 12 wt% EFTH-6/DGEBA were studied, and the results are shown in Fig. S5 and S6. 0.4 g of the cured 12 wt% EFTH-6/DGEBA composite was degraded completely in 10.0 g of 2.0 mol L−1 H3PO4 formulated with equal amounts MEG and H2O2 at 90 °C after maintaining for 4 h. After this, we studied the effects of the EFTH-6 content and functionality (n value) in EFTH-n on the degree of degradation under the same conditions, as shown in Fig. 5. Increasing the EFTH-6 content can improve the degradation degree of the cured EFTH-6/DGEBA composites, as shown in Fig. 5a. The degree of degradation reached 99.8% when the EFTH-6 content was increased to 12 wt% or greater. This was because the ester bonds in EFTH-6 with a hyperbranched topological structure were more easily hydrolyzed than the ether bonds in DGEBA.34 As the number of ester bonds increased with an increase in the EFTH-6 content, it resulted in an increase in the degree of degradation.
image file: d0gc01250e-f5.tif
Fig. 5 The effects of the (a) EFTH-6 content and (b) functionality (n value) of EFTH-n on the degradation degree.

The gas chromatography mass spectrum of the degradation solution after extraction is shown in Fig. S7, and the degradation compositions and chemical structures obtained are given in Table S1. The proposed degradation mechanism of the composites is shown in Scheme S2. The main degradation product 3 is derived from the cleavage of the ether bond and part of the C–N bond in DETA-AN. The source of 7, similar to 3, includes the cleavage of the C–N bond and the hydrolysis of nitriles. Products 1 and 5 obtained by DGEBA degradation account for about 10.76% of the total, which may be attributed to the degradation of the cured 12 wt% EFTH-6/DGEBA. The products 2 and 4 obtained by EFTH-6 degradation account for about 4.59%, and their molecular structures indicate that the degradation is due to the breaking of the ester bonds. The presence of product 6 further proves that the degradation is due to the breaking of the ester and ether bonds. With the same content of EFTH-6 and DGEBA in cured EFTH-6/DGEBA, products 2 and 4 obtained by EFTH-6 degradation account for about 38.25%. Under the same conditions, the degradation rate of the cured EFTH-n/DGEBA composites is significantly improved after adding EFTH-n. Therefore, the degradation degree of the cured DGEBA is lower than that of cured EFTH-n/DGEBA composites, which is consistent with the experimental results shown in Fig. 5b. These results also reflect that the ester bonds in the EFTH-n with a hyperbranched topological structure are more easily hydrolyzed than the ether bonds in DGEBA with a linear structure.

The solution degraded from the cured 12 wt% EFTH-6/DGEBA composite was neutralized by 2.0 mol L−1 NaOH solution to approximately pH = 7. The degradation solution was separated by column chromatography to obtain a degradation product FDCA with a recycle yield of about 56.8 wt%. Its FT-IR, 1H NMR, 13C NMR and liquid chromatography-mass spectrometry (LC-MS) spectra are shown in Fig. 6. The absorption peaks at 3156 cm−1, 1666 cm−1 and 1571 cm−1 are attributed to the stretching vibrations of the –OH bond, the carbon–oxygen double bond (C[double bond, length as m-dash]O) of the carboxyl group and the carbon–carbon double bond (C[double bond, length as m-dash]C) of FDCA (Fig. 6a), respectively. Both peaks at δ 7.28 ppm (a) and δ 13.6 ppm (b) belong to the proton atoms of –CH in the five-membered ring and in the carboxyl group, as shown in Fig. 6b. Simultaneously, the peaks of the three kinds of carbon atoms appear at δ 159.3 ppm (a), δ 147.3 ppm (b) and δ 118.8 ppm (c) in the 13C NMR spectrum of Fig. 6c. All the peaks for recycled FDCA matched with those of carbon and hydrogen atoms of pure FDCA.54 In addition, the molecular weight of the recycled FDCA is 157.1 g mol−1 (Fig. 6d), which is close to its theoretical molecular weight (156.09 g mol−1) + H+. These results confirm that FDCA has been successfully recycled from the degradation solution of the cured EFTH-6/DGEBA composites.


image file: d0gc01250e-f6.tif
Fig. 6 Spectra of recycled FDCA ((a) FT-IR, (b) 1H NMR, (c) 13C NMR and (d) LC-MS).

2.4 Homogeneous structure and the reinforcing and toughening mechanism

As discussed earlier, EFTH-n can effectively toughen and reinforce DGEBA (Fig. S4), and the microstructure has been investigated using the DMA technique. We further explored the in situ reinforcing and toughening mechanism of hyperbranched epoxy resins by interrogating the homogeneous microstructure using Raman, AFM, SEM and PALS techniques.

The Raman spectra of both DGEBA and EFTH-6 are shown in Fig. 7. The characteristic vibration peak of epoxy groups appear at about 3000 cm−1, and both the peaks at 1107 cm−1 and 828 cm−1 belong to the antisymmetric and symmetrical stretching vibration of the C–O–C groups. The difference in the spectra of both DGEBA and EFTH-6 is that the peak at 1608 cm−1 belongs to aromatic C–C in DGEBA, and the peaks at 1737 cm−1 and 1511 cm−1 are attributed to C[double bond, length as m-dash]O and C[double bond, length as m-dash]C groups in EFTH-6, respectively. The peak at about 1511 cm−1 is distinct and is not disturbed by weak side peaks, which is selected as a reference for Raman mapping spectrum.


image file: d0gc01250e-f7.tif
Fig. 7 Raman spectra of pure DGEBA and EFTH-6.

Fig. 8a shows the Raman video image of the 12 wt% EFTH-6/DGEBA blend before curing, indicating a flat surface due to flowable liquid. After curing, the surface is filled with some rugged micro-regions (Fig. 8b) because of curing shrinkage and deformation. The mapping image shown in Fig. 8c for local Fig. 8a shows the detailed composition distribution of the EFTH-6 components in the blend. The light color region indicates a relatively low EFTH-6 content and relatively high concentration of DGEBA while the dark color region indicates a relatively high concentration of EFTH-6 and low content of DGEBA.55 Regions where both light and dark colors mix with each other indicate uniform distribution and good compatibility between EFTH-6 and DGEBA. After curing, the Raman mapping image in Fig. 8d (for local Fig. 8b) shows similar result as Fig. 8c, and the colors are much clearer, which is a result of the curing reaction, suggesting a homogeneous structure without microphase separation. The average diameter (DH) of DGEBA and EFTH-6 are 573.60 nm and 188.65 nm, as shown in Fig. 9a, respectively, but the average DH of the 12 wt% EFTH-6/DGEBA blend is 310.79 nm, which is attributable to the fact that the ellipsoidal topological structure of EFTH-n can disentangle the molecular chain of DGEBA, penetrate each other and reduce chain aggregation.13 So the average particle size of 12 wt% EFTH-6/DGEBA blend appears to be between those of EFTH-6 and DGEBA, as shown in Fig. 9a, indicating a homogeneous structure and good compatibility.


image file: d0gc01250e-f8.tif
Fig. 8 Raman video images and mapping regions of 12 wt% EFTH-6/DGEBA composites ((a–c) before curing and (c and d) after curing).

image file: d0gc01250e-f9.tif
Fig. 9 Particle size distributions (a) of DGEBA and 12 wt% EFTH-6/DGEBA blend, Raman spectra focused on lines MN (b) and ST (c), and peak area (d).

To further analyze the phase microstructure of the 12 wt% EFTH-6/DGEBA blend, we randomly selected an A–B line, as shown in Fig. 9b, and fixed the two most differentiated points M and N. The Raman spectra of ten equidistant points between M and N are shown in Fig. 9b. Similarly, the Raman spectra of ten equidistant points between S and T for the cured 12 wt% EFTH-6/DGEBA composite are shown in Fig. 9c. The peak areas at about 1511 cm−1 corresponding to the ten points shown in Fig. 9b and c are calculated and are shown in Fig. 9d. The peak areas are distributed mostly around 1800, indicating homogeneous dispersion of EFTH-6 into the matrix (DGEBA) and a homogeneous phase microstructure. Additionally, the compatibility number (Nc, experimental probe size/domain size of the phases)55 between EFTH-6 and DGEBA is also obtained by the Raman technique. Nc = 0, 1 and >1, imply incompatibility, semicompatibility and good compatibility of blends. Herein our case, the Raman scanning probe size and the domain size are 1.03 μm and 0.29 μm, respectively, which gives an Nc value of 3.55 for the 12 wt% EFTH-6/DGEBA blend, indicating good compatibility.

Fig. 10a and b compares the SEM micrographs of the impact-fractured surfaces of the cured DGEBA and the cured 12 wt% EFTH-6/DGEBA composites, displaying a remarkable change in the surface morphology from smooth (Fig. 10a) to rough (Fig. 10b) upon the addition of EFTH-6, indicating a transition from brittle to ductile. Many rugged “protonemas” appear on the surface of cured 12 wt% EFTH-6/DGEBA with a relatively uniform distribution without phase separation, which is consistent with its high impact strength. The intramolecular hyperbranched topological structure of EFTH-6 deforms due to the external crosslinked structure and the internal non-crosslinkable structure, which explain the ductile surface.


image file: d0gc01250e-f10.tif
Fig. 10 SEM micrographs of the impact fracture surface of cured ((a) DGEBA and (b) 12 wt% EFTH-6/DGEBA), and AFM images of cured 12 wt% EFTH-6/DGEBA composites ((c) height image, (d) phase image, and (e) three-dimensional image of phase).

Atomic force microscope (AFM)56 was also used to investigate the microstructure of the phase. Fig. 10c–e shows the height, the corresponding phase and a three-dimensional phase images of the cured 12 wt% EFTH-6/DGEBA composite. The lighter and darker regions shown in Fig. 10c represent higher and lower surface topographies. There is no distinct change in color. The phase and the three-dimensional phase images in Fig. 10d and e show uniform dispersion of many micropores. The relatively homogeneous phase images57 of different domains further illustrate the good compatibility between EFTH-6 and DGEBA, and the homogeneous microstructure.

3. Conclusions

Bio-based degradable hyperbranched epoxy resins (EFTH-n, n = 3, 6, 9, 12) were successfully prepared and used to improve the toughness, strength, modulus and elongation of DGEBA. With the increase in the content and functionality (n value) of EFTH-n, the mechanical performance of cured EFTH-n/DGEBA composites increased first and then plateaued, with the 12 wt% EFTH-6/DGEBA composite achieving the highest mechanical properties. The simultaneous in situ reinforcing and toughening mechanism and homogeneous microstructure were confirmed by Raman imaging, AFM, SEM, DMA, dynamic light scattering and positron annihilation lifetime spectroscopy techniques. The synergistic effect of the crosslinking density, free volume, intermolecular cavity, hyperbranched topological structure and compatibility resulted in the improvement. The EFTH-n could significantly promote the degradation of the cured composites under mild conditions without using any organic solvents, and the FDCA could be recycled at a yield of 56.8 wt%. The degradation mechanism was investigated by GC-mass spectrometry. This study provides an efficient and environmentally-friendly method for the synthesis and degradation of thermoset epoxy resins.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (51873233 and 51573210), the Hubei Provincial Natural Science Foundation (2018CFA023) and the Fundamental Research Funds for the Central Universities (CZP20006).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0gc01250e

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