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Design of healable epoxy composite based on β-hydroxyl esters crosslinked networks by using carboxylated cellulose nanocrystals as crosslinker
Composites Science and Technology 181 (2019) 107677
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Design of healable epoxy composite based on β-hydroxyl esters crosslinked networks by using carboxylated cellulose nanocrystals as crosslinker
T
Chuanhui Xu∗, Zhongjie Zheng, Wenchao Wu, Lihua Fu, Baofeng Lin Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Interface Healing
Thermosetting resin-based composites are applied widely, but they are insoluble and infusible once complete curing so that they may be wasteful when the materials have some damages. Here, we use a simple and efficient method to design an epoxy composite which can be healed and recycled via interesterification reaction at elevated temperature. Carboxylated cellulose nanocrystals (CCNs) with a large number of carboxyl groups on the surface were isolated from cotton by hydrochloric acid and nitric acid, and then were used as crosslinker to react with diglycidyl ether of bisphenol A (DGEBA). Fourier transform infrared (FTIR) spectroscopy result indicates that β-hydroxyl ester bonds were formed between carboxyl groups and epoxy groups, while the equilibrium swelling experiment result confirms the formation of β-hydroxyl ester bond-crosslinked networks. Interestingly, the char yields of composites (> 16%), much higher than that of CCNs (7.05%) and DGEBA (4.44%), were about three times higher than the calculated values, showing a significantly improved thermal stability. It was found that the composite with a carboxyl/epoxy = 0.2 has a better and more obvious healing affect at 180 °C than others.
1. Introduction Thermosetting resin-based composites are applied widely in aerospace, construction, transportation, automobile industry and military manufacture due to the formation of covalently crosslinked structure which endows them with high strength and high modulus [1,2]. However, it is unable to reshape and recycle thermoset materials because the covalently crosslinked structure makes them insoluble and infusible once complete curing [3,4]. To overcome this shortcoming, studies have largely focused on forming reversible chemical crosslink network in thermoset materials. For example, in 2011, Leibler and coworkers pioneered a class of thermoset materials whose covalently crosslinked structure can be altered from previous topology to others by heating [5–9]. The materials were formed from the reaction of diglycidyl ether of bisphenol A (DGEBA) and fatty dicarboxylic or tricarboxylic acids with zinc acetate (Zn(Ac)2) as catalyst, having a network of β-hydroxyl esters which can occur transesterification reaction in thermally stimulation. They are named vitrimer in which the number of chemical bonds remains constant during network rearrangements. Therefore, this kind of thermoset materials have some thermoplasticity properties such as reprocessing and recycling. Later, many researchers committed to the research development of
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this novel material. They found new vitrimer-like materials which had new reversible covalent bonding, such as transcarbamoylation [10], disulfide exchange [11–16], transalkylation [17], imine exchange [18], transamination [8,9], boronic ester exchange [19] and olefin metathesis [20]. In addition, various nanofillers were incorporated into vitrimer materials to get new functions or improve original performances. For examples, Tang et al. [21] incorporated carbon nanodot into epoxidized natural rubber (ENR) to form a vitrimer, as well as improved the mechanical properties of ENR. Xu et al. [22] also prepared an ENRbased composite which was crosslinked by citric acid-modified bentonite (CABt). The ENR/CABt composites exhibited vitrimer-like behavior that could be recycled and healed at elevated temperatures. Liu et al. [23] used modified silica nanoparticles as a crosslinker to react with carboxyl group-grafted styrene-butadiene rubber which endowed styrene-butadiene rubber (SBR) with recycling ability. Yang et al. [24] incorporated carbon nanotube into the system of DGEBA and adipic acid, and made the materials had photothermal properties. Since their discovery, vitrimer materials are going to be applied in many fields, e.g. liquid-crystalline materials [25], shape memory materials [26], recyclable materials [27], 3D printing materials [28] and self-healing materials [29]. However, with constantly proposing environmental and sustainable
Corresponding author. E-mail address: [email protected] (C. Xu).
https://doi.org/10.1016/j.compscitech.2019.06.004 Received 7 May 2019; Received in revised form 1 June 2019; Accepted 5 June 2019 Available online 06 June 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
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concerns in recent years, the use of fillers from natural and renewable resources in composites have drawn more and more attentions. Cellulose nanocrysals (CNs) which are different from other nanofillers like carbon nanodot, carbon nanotube, silica [30], zinc oxide [31–33] and ferric oxide [34], are one of the “green nanofillers” that come from renewable resources and are frequently reported to increase the mechanical properties of composites due to their low density, high aspect ratio and high modulus and strength [35]. Moreover, the research of modified CNs was carried out simultaneously. Cheng et al. [36] utilized nitric acid and hydrochloric acid to oxidate CNs by a facile and one-step method where the –CH2OH groups were changed into –COOH groups. Cao et al. [37] used 2,2,6,6- tetramethyl-piperidinyl-1-oxyl radical (TEMPO) to oxidate the surface hydroxy groups of CNs into carboxyl groups, and then used it to react with ENR to prepared a self-healing and recycling material. Lin et al. [38] prepared acetylation cellulose nanocrystals by using CNs and acetic anhydride, and added them into poly (lactic acid) to increase the mechanical properties. Chen et al. [39] synthesized waterborne biodegradable polyurethane, and then used it to react with TEMPO oxidized cellulose nanofibrils to prepare composite films and 3D printing ink. According to above researches, we found that the oxidative cellulose nanocrystals had mass carboxyl groups which were able to react with epoxy groups and generate β-hydroxyl ester bonds. Therefore, in the present work, we prepared carboxylated cellulose nanocrystals (CCNs) firstly and then used it as crosslinker to cure diglycidyl ether of bisphenol A (DGEBA) via esterification, using zinc acetate as catalyst. The formation of β-hydroxyl ester bonds crosslinked network could make composites recyclable and healable at elevated temperatures due to the transesterification reaction. Moreover, the light mass and degradable ability of the “green” CNs could add additional advantages for CCNsDGEBA composites in various potential applications. We hope our report could enrich the vitrimer family.
g. The details of titration method are provided in Supporting Information. 2.3. Preparation of CCNs-DGEBA composites The preparation procedure was shown in Fig. 2. 5 g DGEBA was mixed with different amount of CCNs under magnetic stirring at 60 °C. When the mixture was uniform, Zn(Ac)2 was added to the mixture, and its amount was determined by the amount of carboxyl in system (25% of carboxyl). As water was volatilized, the mixture was concentrated to be a stable thicker homogeneous emulsion. Then, it was poured in a polytetrafluoroethylene mould, and vacuum dried at 50 °C to remove the residual water. Subsequently, it was cured at 110 °C for 6 h. Finally, the composites with different ratios of carboxyl/epoxy were prepared (carboxyl/epoxy = 0.04, 0.05, 0.07, 0.1, 0.2). CCNs-DGEBA samples were coded of CD-x, and x was the ratios of carboxyl/epoxy. The weight percentages of CCNs in CD-0.04, CD-0.05, CD-0.07, CD-0.1 and CD-0.2 were 20.0, 23.5, 28.8, 38.4 and 55.5%, respectively. 2.4. Fourier transform infrared spectroscopy (FTIR) FTIR test was conducted on a Nexus-470 spectrometer (Thermo Fisher Scientific, USA) under attenuated total reflectance (ATR) mode. The experiments were performed in wavenumber range of 4000–400 cm−1 for 32 scans and resolution of 4 cm−1. 2.5. Atomic force microscopy (AFM) The morphology of CCNs was observed through AFM measurement in an Innova Bruker Multimode 8 (Germany), tapping mode. Diluent suspension of CCNs was dropped onto a freshly cleaved mica and dried before measurement.
2. Experimental section
2.6. X-ray diffraction (XRD)
2.1. Materials
XRD was conducted in a D/max-Ultima IV X-ray diffractometer by using Cu Kα (1.5418 Å) X-rays with a current of 30 mA and a voltage of 40 kV. At room temperature, the date were collected in angular range of 5–40° at the rate of 2°/min in steps of 0.02°. The crystallinity index (CI) was calculated according to equation (1):
DGEBA was purchased from aladdin (Shanghai, China). Cotton used to isolate carboxylated cellulose nanocrystals were received from Hubei Chemical Fiber Co., Ltd. (Xiangfan, China). Hydrochloric acid (HCl, 36%), nitric acid (HNO3, 69.2%) and zinc acetate (Zn(Ac)2, 98%) were purchased from Kelong Chemical Reagent Factory (Chengdu, China). All the reagents were analytical pure and used as received.
CI =
I002 − Iam × 100% I002
(1)
where I002 is the peak intensity of (002) lattice plane at 2θ = 22.8°, and Iam is the intensity at 2θ = 18.5° (amorphous phase), respectively.
2.2. Preparation of carboxylated cellulose nanocrystals (CCNs)
2.7. Thermogravimetric analysis (TGA)
CCNs were extracted from cotton according to Cheng's method [36]. The reaction formula of preparation CCNs is shown in Fig. 1. Briefly, 15 g cotton was hydrolyzed in three-necked bottle at 110 °C for 3 h using suitable HCl/HNO3 mixed acid with a volume ratio = 7:3. HCl was 4 mol/L in magnetic stirring. After 3 h, the resultant suspension was cooled to room temperature, and then was washed three times by successive centrifugations with deionized water. Subsequently, the suspension was dialyzed by deionized water until the pH of deionized water was constant. The content of CCNs in the suspension which will be used to prepare composites was concentrated to 5 wt%. Then the suspension was treated with sonication to form a stable aqueous suspension. The concentration of carboxyl groups on CCNs was determined by titration method [40], and the experimental result was 0.802 mmol/
TGA were conducted in a NETZSCH TG 209 F1 thermogravimetric analyzer. 3–10 mg samples were placed in ceramic crucible. The measurements were performed from room temperature to 800 °C at 20°/min in N2 atmosphere with a flow rate of 20 mL/min. 2.8. Equilibrium swelling experiment Equilibrium swelling experiment was used to confirm the formation of CCNs cross-linked DGEBA network, which was performed by soaking CD-x (m1) in toluene at 40 °C for 3 d. Then the swollen samples were dried at 80 °C in oven until a constant weight (m2). Gel fraction (GF) Fig. 1. Reaction formula of preparation of CCNs.
2
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Fig. 2. The procedure of preparation of CCNsDGEBA composites.
corresponded to the stretching vibration of C]O of carboxyl group. The existence of carboxyl groups in CCNs strongly suggested that the oxidation reaction succeeded in changing –CH2OH into > C]O on the surface of CNs. In the reaction, the hydrogen ion of HCl and HNO3 exhibited the acidic behavior to destroy the amorphous region of cellulose while the HNO3 was oxidant to change –CH2OH into > C]O. In order to confirm whether the oxidation reaction destroyed the inside structure of CCNs, X-ray diffraction test was done for the CNs and CCNs. As shown in Fig. 3d, CNs and CCNs exhibited five reflection peaks in the same position at 14.8°, 16.7°, 20.6°, 22.8° and 33.9°, corresponding to reflection planes of cellulose (11(−) 0), (110), (012), (200) and (040) [41], respectively. In addition, the calculated CI value of CNs was 88.87% while that of CCNs was slightly increased to 91.50%. Therefore, the similar diffraction patterns implied that the crystal structures of the CNs and CCNs were not changed during the hydrolysis and oxidization in the mixed acid. In brief, the surface of crystalline cellulose was carboxylated but the inside of crystalline cellulose was not greatly affected by the HNO3. The thermal stability of CNs and CCNs was studied under N2 atmosphere, and the TGA and DTG curves were shown in Fig. 3e. Before 200 °C, CNs and CCNs experienced the first degradation step which was attributed to the evaporation of absorbed water. Then it was obvious to show that the degradation temperature of CCNs is higher than CNs’ (T5% of CNs and CCNs were 222 °C and 290 °C respectively, and Tmax were 241 °C and 361 °C respectively from DTG). The improved thermal stability of CCNs was possibly due to the its carboxyl groups possessed greater electronegativity than the hydroxyl groups [42], which resulted in more and stronger hydrogen bonds. This strengthened the interactions of CCNs, which contributed to the thermal stability of CCNs. Besides, the sulfate ester groups on the surface of CNs introduced by sulphuric acid hydrolysis also decreased the thermal stability of CNs. The above results also suggested that the chemical reaction between CNs and HCl/HNO3 was successful.
was calculated according to equation (2): GF = m1/m2 × 100%
(2)
2.9. Scanning electron microscopy (SEM) The morphology of CCNs-DGEBA composites was characterized by SEM (Hitachi Ltd, Japan) which was operated at voltage of 10 kV. Before observation, the samples were cryo-fractured in liquid nitrogen and then coated with a thin layer of gold. 2.10. Healing experiment For healing experiments, the sample was scratched by a penknife, leaving several deep scratches on the surface of sample. Then it was heated to 160 °C and took pictures every 2 h. After 4 h, the sample was heated to 180 °C, then took pictures after another hours. 2.11. Tensile experiment and recycling experiment The tensile properties of the CCNs-DGEBA composites were measured on a universal testing instrument (Shimadzu AG-1, 10 kN, Japan), tensile speed was 50 mm/min, according to ISO 527 at room temperature. Young's modulus was obtained from the initial slope of the stressstrain curve. For recycling experiment, the original samples were tested for tensile tests, and then they were broken into small pieces and hot pressed under 15 MPa for 10 min at 180 °C. The recycling samples were conducted tensile test again. 3. Results and discussion 3.1. Characterization of CCNs To better confirm the successful fabrication of CCNs, we prepared a reference of CNs through a well-known sulphuric acid hydrolysis method. The details of the preparation of CNs are provided in Supporting Information, and the AFM image of CNs is given in Fig. S1 (Supporting Information). As shown in Fig. 3a, the CNs and CCNs suspensions stored at 10 °C for 3 d show similar appearances without any visible precipitates. However, what should be pointed out is that a few of precipitates appeared inevitably in both CNs and CCNs suspensions after storing 5 d (10 °C). This suggested that the excellent stability of CCNS suspension could be maintained in 5 days when storing at low temperatures. The morphology of CCNs was characterized by AFM as displayed in Fig. 3b. It was found that the CCNs possessed a rod-like structure whose average length and width were about 200 nm and 30 nm, respectively. This clearly indicates that the cotton had been hydrolyzed into nanostructure by the treatment of HCl/HNO3 mixed acid. We also provide the 3D morphology of CCNs in Fig. S2 (Supporting Information). The successful fabrication of CCNs was effectively confirmed by FTIR in Fig. 3c. In the FTIR spectra of CNs and CCNs, the absorptions at 3350, 2899 and 1054 cm−1 correspond to the stretching vibrations of O–H, C–H and C–O [36], respectively. For the CNs, there was a typically absorption with the O–H bending vibration of bound water in 1645 cm−1 [37]. After the reaction with HCl/HNO3, there was a new absorption peak appeared at 1736 cm−1 which
3.2. CCNs covalently crosslinked DGEBA The epoxy-acid reaction between CCNs and DGEBA and the resultant network crosslinked via β-hydroxyl ester bonds are illustrated in Fig. 4a. Obviously, CCNs having multiple carboxyl groups on the surface could act as multifunctional linkages, which participated into the formation of covalently crosslinked network. As shown in Fig. 4b, DGEBA is white powder, which cannot be mould pressed into block material without crosslinker to open its epoxy groups. However, incorporation of CCNs successfully turned DGEBA powder into white blocks with regular geometries as shown in Fig. 4c, implying that the CCNs were critical to the formation of CCNs-DGEBA crosslink network. Besides, the DGEBA can be dissolved in toluene immediately at room temperature (Fig. 5a), while the CCNs-DGEBA composites cannot be dissolved but keep its original shape and only swollen slightly in toluene even at 40 °C for 5 days, as shown in Fig. 5b (CD-0.05) and 5c (CD-0.1), which strongly confirmed that a crosslinked network was indeed formed in the CCNs-DGEBA composites. The occurrence of the chemical reaction between DGEBA and CCNs was effectively confirmed by FTIR. As shown in Fig. 5d, the absorptions of stretching vibrations for carboxyl groups at 1736 cm−1 appeared in the CCNs-DGEBA composites. At the same time, the absorption peak intensity of epoxy group of the DGEBA at 969 cm−1 reduced with the 3
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Fig. 3. (a) Photos of CNs and CCNs suspensions; (b) AFM images of CCNs; (c) FTIR spectra of CNs and CCNs; (d) XRD diffraction patterns of CNs and CCNs; (e) TGA and DTG curves of CNs and CCNs.
increased to about 80% in the CD-0.2. It is worth mentioning that the GF was increased quickly from 59% in the CD-0.1–80% in the CD-0.2. This suggested that the epoxy-acid reaction occur more adequately when the content of CCNs reached an adequate amount. Furthermore, the increasing GF indicated that the network in the CCNs-DGEBA composites was developed with more carboxyl groups participated in reaction [43]. However, it must be pointed out that the GF in CD-0.04 and CD-0.05 was only 50% and 52%, which implied that about half of the DGEBA was not effectively crosslinked by CCNs. This was mainly
increase of CCNs. When the ratio of carboxyl/epoxy reached 0.2, the absorption of epoxy group disappeared in the cured CCNs-DGEBA composite. This confirmed that the esterification reaction between DGEBA and CCNs took place during curing of the CCNs-DGEBA composites and the CCNs were fully integrated into the vitrimer network. Fig. 5e shows the gel fraction (GF) of the CCNs-DGEBA composites obtained from the equilibrium swelling experiment. The GF of CCNsDGEBA composites increased with increasing the ratio of carboxyl/ epoxy. For examples, the GF was about 50.00% in the CD-0.04, while it
Fig. 4. (a) Illustration of the CCNs cross-linked DGEBA network based on epoxy-acid reaction; (b) photo of DGEBA powder; (c) photo of CCNs-DGEBA composite (CD0.1). 4
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Fig. 5. Photo of swelling experiments: (a) DGEBA dissolved in toluene at room temperature, (b) CD-0.05 immersed in toluene for 5 days at 40 °C, (c) CD-0.1 immersed in toluene for 5 days at 40 °C; (d) FTIR spectra of CCNs-DGEBA composites; (e) gel fraction of the samples.
due to that the amount of CCNs was insufficient so that effective crosslinking of CCNs and DGEBA was not formed. As a result, the samples of CD-0.05 and CD-0.04 were easily to break up into pieces rather than a whole rectangle. 3.3. Dispersion of CCNs in the composites The dispersion state of CCNs in DGEBA matrix was characterized by SEM. As shown in Fig. 6a and b, it is evident that the CCNs are uniformly dispersed in the DGEBA matrix without significant aggregate when the CCNs content is not high. The chemical reaction between DGEBA and CCNs prevented the thermodynamically favorable reaggregation of CCNs during sample preparation process. Furthermore, the carboxyl groups on the surface of CCNs improved its compatibility with the DGEBA, which also reduced the aggregation behavior between CCNs. However, when the content of CCNs further increased, e. g, CD0.1 (Fig. 6c) and CD-0.2 (Fig. 6d), slight reaggregation happened inevitably. Nevertheless, majority of CCNs can be achieved fine dispersion in the DGEBA matrix, which provided sufficient contacts between the carboxyl groups of CCNs and the epoxy groups of DGEBA molecules to generate β-hydroxyl ester bonds. Fig. 7a and b shows the magnified images of typical small CCNs aggregates we caught from CD-0.1 and CD-0.2, respectively. It is clearly seen that the single CCNs particles and small CCNs aggregates are well embedded in the DGEBA matrix, showing excellent interfacial compatibility between them [44,45]. Obviously, the formation of β-hydroxyl ester bonds between CCNs and DGEBA chains made CCNs to be parts of the covalently crosslinked network. That is to say, the CCNs played an important role in linking the DGEBA chains to be thermoset network. Note that the CCNs is different from the carbon nanodot, bentonite, silica and carbon nanotube as mentioned in the Introduction, it can be biodegraded. Once the CCNs was degraded completely, the difficulty in decomposing the
Fig. 6. SEM images of cryo-fractured surface of CCNs-DGEBA composites: (a) CD-0.05; (b) CD-0.07; (c) CD-0.1; (d) CD-0.2.
thermoset network will be decreased [46,47]. Therefore, from this point of view, the degradable ability of “green” CNs could add additional advantages for CCNs-DGEBA composites in various potential applications. 3.4. Thermal stability of the CCNs-DGEBA composites The thermal stabilities of CCNs, DGEBA and CCNs-DGEBA 5
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Where CY′ is the actual value of char yield. The results as given in Table S1 (Supporting Information). The calculated LOI of DGEBA and CCNs were 19.28 and 20.32, respectively. According to Ferdosian et al. [50], when the LOI value is below 20.95%, the materials can be burn easily in air. However, the calculated LOI of all the composites were above 24%, implying that the composites had a good stability at elevated temperature in air, comparing with CCNs and DGEBA. As a result, the CCNs-DGEBA composites have a good temperature stability which enables them to be healed at elevated temperature via transesterification reaction, without thermal degradation. Fig. 7. SEM images of CCNs embedded in DGEBA: (a) CD-0.1; (b) CD-0.2.
3.5. Healing behavior of the CCNs-DGEBA composites
composites were studied under N2 atmosphere and the TGA curves are shown in Fig. 8a. The T5% of DGEBA and CCNs were 213 °C and 290 °C, respectively, while it was increased to about 315 °C for the CCNsDGEBA composites. Besides, the temperatures of maximum weight loss rate of the CCNs-DGEBA composites was obviously increased as marked in Fig. 8a, which suggested that the thermal stability of CCNs-DGEBA composites was improved. Herein, it is interesting to discuss the char yield of the CCNs-DGEBA composites. As shown in Fig. 8a, the char yields of composites were higher than that of the raw materials. For instance, the char yield of DGEBA and CCNs were 4.44% and 7.05%, respectively. However, it was increased to 16–17% for the CCNsDGEBA composites. These results indicated that the formation of βhydroxyl ester bonds crosslinked network endowed the composites with better thermal stability so that it can be used at higher temperatures. According the char yield values of DGEBA and CCNs and the weight ratio of DGEBA/CCNs in the composites, the theoretical values of char yield of composites can be calculated from equation (3):
CY = 7.05x + 4.44(1 − x )
In theory, the network rearrangement can be realized through transesterification reaction of β-hydroxyl ester bonds at elevated temperature [51,52]. Since that the crosslinked network in the CCNsDGEBA composites was formed by the β-hydroxyl ester bond linkages on the surface of CCNs, the transesterification reaction potentially made CCNs-DGEBA composites healable at high temperatures. Considering that the CCNs-DGEBA composites were hard thermosets without any flexibilities, we designed an experiment to investigate the healing behavior of the CD-0.2. The sample was scratched by a clean penknife at room temperature, leaving a X-shaped scratch as shown in Fig. 9a. Then the sample was placed at 160 °C for 2 h and photo taking (Fig. 9b). After heating at 160 °C for another 2 h, the sample was taken photo again to record the changes of scar (Fig. 9c). Next, the sample was heated to 180 °C for 4 h and the photo of scratch was recorded (Fig. 9d). It is clearly seen that the scar after heated at 160 °C for 2 h faded slightly (Fig. 9b), and it was also changed slightly even heated at 160 °C for 4 h (Fig. 9c), indicating that reacted reactivity of the transesterification was too low to activate the healing of scar. When the temperature was increased to 180 °C, the scar faded significantly after healing for 4 h (Fig. 9d), showing obvious healing effect. Because of the chemical reaction between CCNs and DGEBA, CCNs served as a crosslinking center that was capable of covalently crosslinking of DGEBA through β-hydroxyl ester linkage on the surface (Fig. 4a). Therefore, the CD-0.2 with sufficient β-hydroxyl ester linkages could exhibit vitrimer-like properties at high temperature through transesterification reaction of β-hydroxyl ester, thus the interfacial exchange in the damage position was initiated as illustrated in Fig. 10 [53], and then the composite exhibited a healing behavior in macroscopic. However, we had to admit that, comparing with CD-0.2, the healing effect of the composites with lower CCNs contents (carboxyl/epoxy < 0.1) was not so good. Fig. 11 shows the changes of scar on the surface of CD-0.05 healed at 180 °C for 4 h. It was disappointing to find that there is no obvious healing sign for the scar on the surface of CD-0.05 due to the lower level of β-hydroxyl ester bond linkages in CD-0.05 [54,55]. Furthermore, the hard benzene ring of DGEBA could limit its
(3)
Where the CY is the theoretical value of char yield and x is mass fraction of CCNs, and 7.05 and 4.44 are the char yields of CCNs and DGEBA obtained from Fig. 8a, respectively. The results of calculated values of char yields are shown in Fig. 8b. As expected, the actual values of composites were about three times higher than the calculated values, suggesting that the CCNs and DGEBA had a positive synergistic effect on the thermal stability. The CCNs-DGEBA crosslinked structure persisted against thermo-degradation and endowed composites with a good heat-resistant ability, which effectively inhibited the disintegration of composites [48,49]. According to Van Krevelen-Hoftyzer equation, we calculated limiting oxygen index (LOI), the minimum amount of oxygen needed of polymeric materials flammability, for the CCNs-DGEBA composites. It can be calculated from equation (4) [50]:
LOI = 17.5 + 0.4CY ′
(4)
Fig. 8. (a) TGA thermograms of CCNs, DGEBA and CCNs-DGEBA composites; (b) actual values and theoretical values of char yield. 6
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Fig. 9. Healing process of scares on the surface of CD-0.2.
backbone moving free at larger scale which added additional difficulties in effective touch of reactive groups during transesterification in the CD-0.04, CD-0.05 and CD-0.07. However, it should be pointed out that there was quite a number of DGEBA did not participate into the formation of CCNs-DGEBA network due to the insufficient CCNs in the CD-0.04, CD-0.05 and CD-0.07. The epoxy groups of residual DGEBA might reacted with the formed secondary OH in β-hydroxyl esters to form new non-exchangeable crosslinks during healing, which hindered the potential healing effect of the CD-0.04, CD-0.05 and CD-0.07. To address this, the FTIR spectra and the gel fraction of the CD-0.04, CD-0.05 and CD-0.07 after healing were characterized and the results are provided in Figs. S3 and S4 (Supporting Information), respectively. It was found that their FTIR spectra (Fig. S3) before and after healing were similar. Especially, the hydroxyl groups (around 3400 cm−1) and epoxy groups (969 cm−1) had almost the same intensity before and after healing, indicating that the formed secondary OH did not react with the epoxy of residual DGEBA. Besides, the gel fractions of CD-0.04, CD-0.05 and CD-0.07 were almost invariable after healing, which further indicated that the non-crosslinking parts of samples were not increased in during healing process. This might be attributed to that the reaction between hydroxyl groups and epoxy groups in a solid material is more difficult than that between carboxyl groups and epoxy groups. In addition, the hard benzene ring of DGEBA structural unit could add additional difficulties in effective touch of reactive groups during transesterification. Therefore, the lower level of β-hydroxyl ester bond linkages in CD-0.04, CD0.05 and CD-0.07 should be the main reason for their non-healing behaviors.
Fig. 11. Healing process of scar on the surface of CD-0.05.
DGEBA composites could be reprocessed through hot-pressing at 180 °C. Unfortunately, only CD-0.1 and CD-0.2 could be successfully recycled in our experiment. The CD-0.04, CD-0.05 and CD-0.07 were unable to be hot-pressed from fragment to a whole due to the insufficient β-hydroxyl ester cross linkages, even the temperature was increased to 200 °C. We provided the photo of CD-0.07 that was hotpressed at 180 °C and 200 °C in Figs. S5 and S6 (Supporting Information), respectively. It is clearly seen that the boundary of individual fragments still existed. This result is similar to the healing behavior as observed in healing experiment. When the carboxyl/epoxy ratio increased to 0.1, the CCNs-DGEBA composites could be recycled. Fig. 12a shows the typical stress-strain curves of CD-0.1 and CD-0.2 before and after recycling and the inset is the photo of CD-0.2 that was hot-pressed at 180 °C for 10 min. The mechanical properties such as Young's modulus, tensile strength and strain of CD-0.1 and CD-0.2 before and after recycling are summarized in Table S2 (Supporting
3.6. Recycle performance of the CCNs-DGEBA composites Thanks to the transesterification reaction of β-hydroxyl ester, CCNs-
Fig. 10. Schematic illustration of healing of CCNs-DGEBA composite via exchange reactions of β-hydroxyl esters. 7
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Fig. 12. Mechanical properties of CCNs-DGEBA composite before and after recycling: (a) typical stress-strain curves; (b) retention of Young's modulus; (c) retention of tensile strength; (d) retention of strain.
China (21875047) and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology Foundation (2018Z006).
Information). Although the tensile property of CD-0.1 and CD-0.2 decreased slightly after recycling, the retention of the Young's modulus, tensile strength and strain could be maintained above 90% as shown in Fig. 12b, c and d, respectively.
Appendix A. Supplementary data 4. Conclusions
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compscitech.2019.06.004.
In this paper, CCNs were successfully prepared through the hydrolysis and oxidization of cotton in the mixture of HCl and HNO3. Because of the multiple carboxyl groups on the surface, CCNs acted as multifunctional linkages to participate into the formation of DGEBA covalently crosslinked network. This endowed the CCNs with excellent compatibility with DGEBA matrix, which helped CCNs to achieved a fine dispersion in the composites. At the same time, the CCNs and DGEBA had a positive synergistic effect on the thermal stability of their composites. It was found that the char yields of composites were > 16%, much higher than that of CCNs (7.05%) and DGEBA (4.44%), which were about three times higher than the calculated values. Due to the exchangeable β-hydroxyl ester bonds formed by the reaction of CCNs and DGEBA, composites could be healed and recycled at 180 °C. However, this healing and recycling behaviors were only found in the composites CD-0.1 and CD-0.2. Since the CCNs are renewable and abundant in nature, we imagine the CCNs-DGEBA composites which can be healed and recycled may get new applications as green materials. Furthermore, once the CCNs was degraded completely, the difficulty in decomposing the thermoset network will be decreased. Therefore, from this point of view, the degradable ability of “green” CNs could add additional advantages for CCNs-DGEBA composites in various potential applications.
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Acknowledgements This work was supported by National Natural Science Foundation of 8
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Design of healable epoxy composite based on β-hydroxyl esters crosslinked networks by using carboxylated cellulose nanocrystals as crosslinker
T
Chuanhui Xu∗, Zhongjie Zheng, Wenchao Wu, Lihua Fu, Baofeng Lin Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Interface Healing
Thermosetting resin-based composites are applied widely, but they are insoluble and infusible once complete curing so that they may be wasteful when the materials have some damages. Here, we use a simple and efficient method to design an epoxy composite which can be healed and recycled via interesterification reaction at elevated temperature. Carboxylated cellulose nanocrystals (CCNs) with a large number of carboxyl groups on the surface were isolated from cotton by hydrochloric acid and nitric acid, and then were used as crosslinker to react with diglycidyl ether of bisphenol A (DGEBA). Fourier transform infrared (FTIR) spectroscopy result indicates that β-hydroxyl ester bonds were formed between carboxyl groups and epoxy groups, while the equilibrium swelling experiment result confirms the formation of β-hydroxyl ester bond-crosslinked networks. Interestingly, the char yields of composites (> 16%), much higher than that of CCNs (7.05%) and DGEBA (4.44%), were about three times higher than the calculated values, showing a significantly improved thermal stability. It was found that the composite with a carboxyl/epoxy = 0.2 has a better and more obvious healing affect at 180 °C than others.
1. Introduction Thermosetting resin-based composites are applied widely in aerospace, construction, transportation, automobile industry and military manufacture due to the formation of covalently crosslinked structure which endows them with high strength and high modulus [1,2]. However, it is unable to reshape and recycle thermoset materials because the covalently crosslinked structure makes them insoluble and infusible once complete curing [3,4]. To overcome this shortcoming, studies have largely focused on forming reversible chemical crosslink network in thermoset materials. For example, in 2011, Leibler and coworkers pioneered a class of thermoset materials whose covalently crosslinked structure can be altered from previous topology to others by heating [5–9]. The materials were formed from the reaction of diglycidyl ether of bisphenol A (DGEBA) and fatty dicarboxylic or tricarboxylic acids with zinc acetate (Zn(Ac)2) as catalyst, having a network of β-hydroxyl esters which can occur transesterification reaction in thermally stimulation. They are named vitrimer in which the number of chemical bonds remains constant during network rearrangements. Therefore, this kind of thermoset materials have some thermoplasticity properties such as reprocessing and recycling. Later, many researchers committed to the research development of
∗
this novel material. They found new vitrimer-like materials which had new reversible covalent bonding, such as transcarbamoylation [10], disulfide exchange [11–16], transalkylation [17], imine exchange [18], transamination [8,9], boronic ester exchange [19] and olefin metathesis [20]. In addition, various nanofillers were incorporated into vitrimer materials to get new functions or improve original performances. For examples, Tang et al. [21] incorporated carbon nanodot into epoxidized natural rubber (ENR) to form a vitrimer, as well as improved the mechanical properties of ENR. Xu et al. [22] also prepared an ENRbased composite which was crosslinked by citric acid-modified bentonite (CABt). The ENR/CABt composites exhibited vitrimer-like behavior that could be recycled and healed at elevated temperatures. Liu et al. [23] used modified silica nanoparticles as a crosslinker to react with carboxyl group-grafted styrene-butadiene rubber which endowed styrene-butadiene rubber (SBR) with recycling ability. Yang et al. [24] incorporated carbon nanotube into the system of DGEBA and adipic acid, and made the materials had photothermal properties. Since their discovery, vitrimer materials are going to be applied in many fields, e.g. liquid-crystalline materials [25], shape memory materials [26], recyclable materials [27], 3D printing materials [28] and self-healing materials [29]. However, with constantly proposing environmental and sustainable
Corresponding author. E-mail address: [email protected] (C. Xu).
https://doi.org/10.1016/j.compscitech.2019.06.004 Received 7 May 2019; Received in revised form 1 June 2019; Accepted 5 June 2019 Available online 06 June 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
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concerns in recent years, the use of fillers from natural and renewable resources in composites have drawn more and more attentions. Cellulose nanocrysals (CNs) which are different from other nanofillers like carbon nanodot, carbon nanotube, silica [30], zinc oxide [31–33] and ferric oxide [34], are one of the “green nanofillers” that come from renewable resources and are frequently reported to increase the mechanical properties of composites due to their low density, high aspect ratio and high modulus and strength [35]. Moreover, the research of modified CNs was carried out simultaneously. Cheng et al. [36] utilized nitric acid and hydrochloric acid to oxidate CNs by a facile and one-step method where the –CH2OH groups were changed into –COOH groups. Cao et al. [37] used 2,2,6,6- tetramethyl-piperidinyl-1-oxyl radical (TEMPO) to oxidate the surface hydroxy groups of CNs into carboxyl groups, and then used it to react with ENR to prepared a self-healing and recycling material. Lin et al. [38] prepared acetylation cellulose nanocrystals by using CNs and acetic anhydride, and added them into poly (lactic acid) to increase the mechanical properties. Chen et al. [39] synthesized waterborne biodegradable polyurethane, and then used it to react with TEMPO oxidized cellulose nanofibrils to prepare composite films and 3D printing ink. According to above researches, we found that the oxidative cellulose nanocrystals had mass carboxyl groups which were able to react with epoxy groups and generate β-hydroxyl ester bonds. Therefore, in the present work, we prepared carboxylated cellulose nanocrystals (CCNs) firstly and then used it as crosslinker to cure diglycidyl ether of bisphenol A (DGEBA) via esterification, using zinc acetate as catalyst. The formation of β-hydroxyl ester bonds crosslinked network could make composites recyclable and healable at elevated temperatures due to the transesterification reaction. Moreover, the light mass and degradable ability of the “green” CNs could add additional advantages for CCNsDGEBA composites in various potential applications. We hope our report could enrich the vitrimer family.
g. The details of titration method are provided in Supporting Information. 2.3. Preparation of CCNs-DGEBA composites The preparation procedure was shown in Fig. 2. 5 g DGEBA was mixed with different amount of CCNs under magnetic stirring at 60 °C. When the mixture was uniform, Zn(Ac)2 was added to the mixture, and its amount was determined by the amount of carboxyl in system (25% of carboxyl). As water was volatilized, the mixture was concentrated to be a stable thicker homogeneous emulsion. Then, it was poured in a polytetrafluoroethylene mould, and vacuum dried at 50 °C to remove the residual water. Subsequently, it was cured at 110 °C for 6 h. Finally, the composites with different ratios of carboxyl/epoxy were prepared (carboxyl/epoxy = 0.04, 0.05, 0.07, 0.1, 0.2). CCNs-DGEBA samples were coded of CD-x, and x was the ratios of carboxyl/epoxy. The weight percentages of CCNs in CD-0.04, CD-0.05, CD-0.07, CD-0.1 and CD-0.2 were 20.0, 23.5, 28.8, 38.4 and 55.5%, respectively. 2.4. Fourier transform infrared spectroscopy (FTIR) FTIR test was conducted on a Nexus-470 spectrometer (Thermo Fisher Scientific, USA) under attenuated total reflectance (ATR) mode. The experiments were performed in wavenumber range of 4000–400 cm−1 for 32 scans and resolution of 4 cm−1. 2.5. Atomic force microscopy (AFM) The morphology of CCNs was observed through AFM measurement in an Innova Bruker Multimode 8 (Germany), tapping mode. Diluent suspension of CCNs was dropped onto a freshly cleaved mica and dried before measurement.
2. Experimental section
2.6. X-ray diffraction (XRD)
2.1. Materials
XRD was conducted in a D/max-Ultima IV X-ray diffractometer by using Cu Kα (1.5418 Å) X-rays with a current of 30 mA and a voltage of 40 kV. At room temperature, the date were collected in angular range of 5–40° at the rate of 2°/min in steps of 0.02°. The crystallinity index (CI) was calculated according to equation (1):
DGEBA was purchased from aladdin (Shanghai, China). Cotton used to isolate carboxylated cellulose nanocrystals were received from Hubei Chemical Fiber Co., Ltd. (Xiangfan, China). Hydrochloric acid (HCl, 36%), nitric acid (HNO3, 69.2%) and zinc acetate (Zn(Ac)2, 98%) were purchased from Kelong Chemical Reagent Factory (Chengdu, China). All the reagents were analytical pure and used as received.
CI =
I002 − Iam × 100% I002
(1)
where I002 is the peak intensity of (002) lattice plane at 2θ = 22.8°, and Iam is the intensity at 2θ = 18.5° (amorphous phase), respectively.
2.2. Preparation of carboxylated cellulose nanocrystals (CCNs)
2.7. Thermogravimetric analysis (TGA)
CCNs were extracted from cotton according to Cheng's method [36]. The reaction formula of preparation CCNs is shown in Fig. 1. Briefly, 15 g cotton was hydrolyzed in three-necked bottle at 110 °C for 3 h using suitable HCl/HNO3 mixed acid with a volume ratio = 7:3. HCl was 4 mol/L in magnetic stirring. After 3 h, the resultant suspension was cooled to room temperature, and then was washed three times by successive centrifugations with deionized water. Subsequently, the suspension was dialyzed by deionized water until the pH of deionized water was constant. The content of CCNs in the suspension which will be used to prepare composites was concentrated to 5 wt%. Then the suspension was treated with sonication to form a stable aqueous suspension. The concentration of carboxyl groups on CCNs was determined by titration method [40], and the experimental result was 0.802 mmol/
TGA were conducted in a NETZSCH TG 209 F1 thermogravimetric analyzer. 3–10 mg samples were placed in ceramic crucible. The measurements were performed from room temperature to 800 °C at 20°/min in N2 atmosphere with a flow rate of 20 mL/min. 2.8. Equilibrium swelling experiment Equilibrium swelling experiment was used to confirm the formation of CCNs cross-linked DGEBA network, which was performed by soaking CD-x (m1) in toluene at 40 °C for 3 d. Then the swollen samples were dried at 80 °C in oven until a constant weight (m2). Gel fraction (GF) Fig. 1. Reaction formula of preparation of CCNs.
2
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Fig. 2. The procedure of preparation of CCNsDGEBA composites.
corresponded to the stretching vibration of C]O of carboxyl group. The existence of carboxyl groups in CCNs strongly suggested that the oxidation reaction succeeded in changing –CH2OH into > C]O on the surface of CNs. In the reaction, the hydrogen ion of HCl and HNO3 exhibited the acidic behavior to destroy the amorphous region of cellulose while the HNO3 was oxidant to change –CH2OH into > C]O. In order to confirm whether the oxidation reaction destroyed the inside structure of CCNs, X-ray diffraction test was done for the CNs and CCNs. As shown in Fig. 3d, CNs and CCNs exhibited five reflection peaks in the same position at 14.8°, 16.7°, 20.6°, 22.8° and 33.9°, corresponding to reflection planes of cellulose (11(−) 0), (110), (012), (200) and (040) [41], respectively. In addition, the calculated CI value of CNs was 88.87% while that of CCNs was slightly increased to 91.50%. Therefore, the similar diffraction patterns implied that the crystal structures of the CNs and CCNs were not changed during the hydrolysis and oxidization in the mixed acid. In brief, the surface of crystalline cellulose was carboxylated but the inside of crystalline cellulose was not greatly affected by the HNO3. The thermal stability of CNs and CCNs was studied under N2 atmosphere, and the TGA and DTG curves were shown in Fig. 3e. Before 200 °C, CNs and CCNs experienced the first degradation step which was attributed to the evaporation of absorbed water. Then it was obvious to show that the degradation temperature of CCNs is higher than CNs’ (T5% of CNs and CCNs were 222 °C and 290 °C respectively, and Tmax were 241 °C and 361 °C respectively from DTG). The improved thermal stability of CCNs was possibly due to the its carboxyl groups possessed greater electronegativity than the hydroxyl groups [42], which resulted in more and stronger hydrogen bonds. This strengthened the interactions of CCNs, which contributed to the thermal stability of CCNs. Besides, the sulfate ester groups on the surface of CNs introduced by sulphuric acid hydrolysis also decreased the thermal stability of CNs. The above results also suggested that the chemical reaction between CNs and HCl/HNO3 was successful.
was calculated according to equation (2): GF = m1/m2 × 100%
(2)
2.9. Scanning electron microscopy (SEM) The morphology of CCNs-DGEBA composites was characterized by SEM (Hitachi Ltd, Japan) which was operated at voltage of 10 kV. Before observation, the samples were cryo-fractured in liquid nitrogen and then coated with a thin layer of gold. 2.10. Healing experiment For healing experiments, the sample was scratched by a penknife, leaving several deep scratches on the surface of sample. Then it was heated to 160 °C and took pictures every 2 h. After 4 h, the sample was heated to 180 °C, then took pictures after another hours. 2.11. Tensile experiment and recycling experiment The tensile properties of the CCNs-DGEBA composites were measured on a universal testing instrument (Shimadzu AG-1, 10 kN, Japan), tensile speed was 50 mm/min, according to ISO 527 at room temperature. Young's modulus was obtained from the initial slope of the stressstrain curve. For recycling experiment, the original samples were tested for tensile tests, and then they were broken into small pieces and hot pressed under 15 MPa for 10 min at 180 °C. The recycling samples were conducted tensile test again. 3. Results and discussion 3.1. Characterization of CCNs To better confirm the successful fabrication of CCNs, we prepared a reference of CNs through a well-known sulphuric acid hydrolysis method. The details of the preparation of CNs are provided in Supporting Information, and the AFM image of CNs is given in Fig. S1 (Supporting Information). As shown in Fig. 3a, the CNs and CCNs suspensions stored at 10 °C for 3 d show similar appearances without any visible precipitates. However, what should be pointed out is that a few of precipitates appeared inevitably in both CNs and CCNs suspensions after storing 5 d (10 °C). This suggested that the excellent stability of CCNS suspension could be maintained in 5 days when storing at low temperatures. The morphology of CCNs was characterized by AFM as displayed in Fig. 3b. It was found that the CCNs possessed a rod-like structure whose average length and width were about 200 nm and 30 nm, respectively. This clearly indicates that the cotton had been hydrolyzed into nanostructure by the treatment of HCl/HNO3 mixed acid. We also provide the 3D morphology of CCNs in Fig. S2 (Supporting Information). The successful fabrication of CCNs was effectively confirmed by FTIR in Fig. 3c. In the FTIR spectra of CNs and CCNs, the absorptions at 3350, 2899 and 1054 cm−1 correspond to the stretching vibrations of O–H, C–H and C–O [36], respectively. For the CNs, there was a typically absorption with the O–H bending vibration of bound water in 1645 cm−1 [37]. After the reaction with HCl/HNO3, there was a new absorption peak appeared at 1736 cm−1 which
3.2. CCNs covalently crosslinked DGEBA The epoxy-acid reaction between CCNs and DGEBA and the resultant network crosslinked via β-hydroxyl ester bonds are illustrated in Fig. 4a. Obviously, CCNs having multiple carboxyl groups on the surface could act as multifunctional linkages, which participated into the formation of covalently crosslinked network. As shown in Fig. 4b, DGEBA is white powder, which cannot be mould pressed into block material without crosslinker to open its epoxy groups. However, incorporation of CCNs successfully turned DGEBA powder into white blocks with regular geometries as shown in Fig. 4c, implying that the CCNs were critical to the formation of CCNs-DGEBA crosslink network. Besides, the DGEBA can be dissolved in toluene immediately at room temperature (Fig. 5a), while the CCNs-DGEBA composites cannot be dissolved but keep its original shape and only swollen slightly in toluene even at 40 °C for 5 days, as shown in Fig. 5b (CD-0.05) and 5c (CD-0.1), which strongly confirmed that a crosslinked network was indeed formed in the CCNs-DGEBA composites. The occurrence of the chemical reaction between DGEBA and CCNs was effectively confirmed by FTIR. As shown in Fig. 5d, the absorptions of stretching vibrations for carboxyl groups at 1736 cm−1 appeared in the CCNs-DGEBA composites. At the same time, the absorption peak intensity of epoxy group of the DGEBA at 969 cm−1 reduced with the 3
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Fig. 3. (a) Photos of CNs and CCNs suspensions; (b) AFM images of CCNs; (c) FTIR spectra of CNs and CCNs; (d) XRD diffraction patterns of CNs and CCNs; (e) TGA and DTG curves of CNs and CCNs.
increased to about 80% in the CD-0.2. It is worth mentioning that the GF was increased quickly from 59% in the CD-0.1–80% in the CD-0.2. This suggested that the epoxy-acid reaction occur more adequately when the content of CCNs reached an adequate amount. Furthermore, the increasing GF indicated that the network in the CCNs-DGEBA composites was developed with more carboxyl groups participated in reaction [43]. However, it must be pointed out that the GF in CD-0.04 and CD-0.05 was only 50% and 52%, which implied that about half of the DGEBA was not effectively crosslinked by CCNs. This was mainly
increase of CCNs. When the ratio of carboxyl/epoxy reached 0.2, the absorption of epoxy group disappeared in the cured CCNs-DGEBA composite. This confirmed that the esterification reaction between DGEBA and CCNs took place during curing of the CCNs-DGEBA composites and the CCNs were fully integrated into the vitrimer network. Fig. 5e shows the gel fraction (GF) of the CCNs-DGEBA composites obtained from the equilibrium swelling experiment. The GF of CCNsDGEBA composites increased with increasing the ratio of carboxyl/ epoxy. For examples, the GF was about 50.00% in the CD-0.04, while it
Fig. 4. (a) Illustration of the CCNs cross-linked DGEBA network based on epoxy-acid reaction; (b) photo of DGEBA powder; (c) photo of CCNs-DGEBA composite (CD0.1). 4
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Fig. 5. Photo of swelling experiments: (a) DGEBA dissolved in toluene at room temperature, (b) CD-0.05 immersed in toluene for 5 days at 40 °C, (c) CD-0.1 immersed in toluene for 5 days at 40 °C; (d) FTIR spectra of CCNs-DGEBA composites; (e) gel fraction of the samples.
due to that the amount of CCNs was insufficient so that effective crosslinking of CCNs and DGEBA was not formed. As a result, the samples of CD-0.05 and CD-0.04 were easily to break up into pieces rather than a whole rectangle. 3.3. Dispersion of CCNs in the composites The dispersion state of CCNs in DGEBA matrix was characterized by SEM. As shown in Fig. 6a and b, it is evident that the CCNs are uniformly dispersed in the DGEBA matrix without significant aggregate when the CCNs content is not high. The chemical reaction between DGEBA and CCNs prevented the thermodynamically favorable reaggregation of CCNs during sample preparation process. Furthermore, the carboxyl groups on the surface of CCNs improved its compatibility with the DGEBA, which also reduced the aggregation behavior between CCNs. However, when the content of CCNs further increased, e. g, CD0.1 (Fig. 6c) and CD-0.2 (Fig. 6d), slight reaggregation happened inevitably. Nevertheless, majority of CCNs can be achieved fine dispersion in the DGEBA matrix, which provided sufficient contacts between the carboxyl groups of CCNs and the epoxy groups of DGEBA molecules to generate β-hydroxyl ester bonds. Fig. 7a and b shows the magnified images of typical small CCNs aggregates we caught from CD-0.1 and CD-0.2, respectively. It is clearly seen that the single CCNs particles and small CCNs aggregates are well embedded in the DGEBA matrix, showing excellent interfacial compatibility between them [44,45]. Obviously, the formation of β-hydroxyl ester bonds between CCNs and DGEBA chains made CCNs to be parts of the covalently crosslinked network. That is to say, the CCNs played an important role in linking the DGEBA chains to be thermoset network. Note that the CCNs is different from the carbon nanodot, bentonite, silica and carbon nanotube as mentioned in the Introduction, it can be biodegraded. Once the CCNs was degraded completely, the difficulty in decomposing the
Fig. 6. SEM images of cryo-fractured surface of CCNs-DGEBA composites: (a) CD-0.05; (b) CD-0.07; (c) CD-0.1; (d) CD-0.2.
thermoset network will be decreased [46,47]. Therefore, from this point of view, the degradable ability of “green” CNs could add additional advantages for CCNs-DGEBA composites in various potential applications. 3.4. Thermal stability of the CCNs-DGEBA composites The thermal stabilities of CCNs, DGEBA and CCNs-DGEBA 5
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Where CY′ is the actual value of char yield. The results as given in Table S1 (Supporting Information). The calculated LOI of DGEBA and CCNs were 19.28 and 20.32, respectively. According to Ferdosian et al. [50], when the LOI value is below 20.95%, the materials can be burn easily in air. However, the calculated LOI of all the composites were above 24%, implying that the composites had a good stability at elevated temperature in air, comparing with CCNs and DGEBA. As a result, the CCNs-DGEBA composites have a good temperature stability which enables them to be healed at elevated temperature via transesterification reaction, without thermal degradation. Fig. 7. SEM images of CCNs embedded in DGEBA: (a) CD-0.1; (b) CD-0.2.
3.5. Healing behavior of the CCNs-DGEBA composites
composites were studied under N2 atmosphere and the TGA curves are shown in Fig. 8a. The T5% of DGEBA and CCNs were 213 °C and 290 °C, respectively, while it was increased to about 315 °C for the CCNsDGEBA composites. Besides, the temperatures of maximum weight loss rate of the CCNs-DGEBA composites was obviously increased as marked in Fig. 8a, which suggested that the thermal stability of CCNs-DGEBA composites was improved. Herein, it is interesting to discuss the char yield of the CCNs-DGEBA composites. As shown in Fig. 8a, the char yields of composites were higher than that of the raw materials. For instance, the char yield of DGEBA and CCNs were 4.44% and 7.05%, respectively. However, it was increased to 16–17% for the CCNsDGEBA composites. These results indicated that the formation of βhydroxyl ester bonds crosslinked network endowed the composites with better thermal stability so that it can be used at higher temperatures. According the char yield values of DGEBA and CCNs and the weight ratio of DGEBA/CCNs in the composites, the theoretical values of char yield of composites can be calculated from equation (3):
CY = 7.05x + 4.44(1 − x )
In theory, the network rearrangement can be realized through transesterification reaction of β-hydroxyl ester bonds at elevated temperature [51,52]. Since that the crosslinked network in the CCNsDGEBA composites was formed by the β-hydroxyl ester bond linkages on the surface of CCNs, the transesterification reaction potentially made CCNs-DGEBA composites healable at high temperatures. Considering that the CCNs-DGEBA composites were hard thermosets without any flexibilities, we designed an experiment to investigate the healing behavior of the CD-0.2. The sample was scratched by a clean penknife at room temperature, leaving a X-shaped scratch as shown in Fig. 9a. Then the sample was placed at 160 °C for 2 h and photo taking (Fig. 9b). After heating at 160 °C for another 2 h, the sample was taken photo again to record the changes of scar (Fig. 9c). Next, the sample was heated to 180 °C for 4 h and the photo of scratch was recorded (Fig. 9d). It is clearly seen that the scar after heated at 160 °C for 2 h faded slightly (Fig. 9b), and it was also changed slightly even heated at 160 °C for 4 h (Fig. 9c), indicating that reacted reactivity of the transesterification was too low to activate the healing of scar. When the temperature was increased to 180 °C, the scar faded significantly after healing for 4 h (Fig. 9d), showing obvious healing effect. Because of the chemical reaction between CCNs and DGEBA, CCNs served as a crosslinking center that was capable of covalently crosslinking of DGEBA through β-hydroxyl ester linkage on the surface (Fig. 4a). Therefore, the CD-0.2 with sufficient β-hydroxyl ester linkages could exhibit vitrimer-like properties at high temperature through transesterification reaction of β-hydroxyl ester, thus the interfacial exchange in the damage position was initiated as illustrated in Fig. 10 [53], and then the composite exhibited a healing behavior in macroscopic. However, we had to admit that, comparing with CD-0.2, the healing effect of the composites with lower CCNs contents (carboxyl/epoxy < 0.1) was not so good. Fig. 11 shows the changes of scar on the surface of CD-0.05 healed at 180 °C for 4 h. It was disappointing to find that there is no obvious healing sign for the scar on the surface of CD-0.05 due to the lower level of β-hydroxyl ester bond linkages in CD-0.05 [54,55]. Furthermore, the hard benzene ring of DGEBA could limit its
(3)
Where the CY is the theoretical value of char yield and x is mass fraction of CCNs, and 7.05 and 4.44 are the char yields of CCNs and DGEBA obtained from Fig. 8a, respectively. The results of calculated values of char yields are shown in Fig. 8b. As expected, the actual values of composites were about three times higher than the calculated values, suggesting that the CCNs and DGEBA had a positive synergistic effect on the thermal stability. The CCNs-DGEBA crosslinked structure persisted against thermo-degradation and endowed composites with a good heat-resistant ability, which effectively inhibited the disintegration of composites [48,49]. According to Van Krevelen-Hoftyzer equation, we calculated limiting oxygen index (LOI), the minimum amount of oxygen needed of polymeric materials flammability, for the CCNs-DGEBA composites. It can be calculated from equation (4) [50]:
LOI = 17.5 + 0.4CY ′
(4)
Fig. 8. (a) TGA thermograms of CCNs, DGEBA and CCNs-DGEBA composites; (b) actual values and theoretical values of char yield. 6
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Fig. 9. Healing process of scares on the surface of CD-0.2.
backbone moving free at larger scale which added additional difficulties in effective touch of reactive groups during transesterification in the CD-0.04, CD-0.05 and CD-0.07. However, it should be pointed out that there was quite a number of DGEBA did not participate into the formation of CCNs-DGEBA network due to the insufficient CCNs in the CD-0.04, CD-0.05 and CD-0.07. The epoxy groups of residual DGEBA might reacted with the formed secondary OH in β-hydroxyl esters to form new non-exchangeable crosslinks during healing, which hindered the potential healing effect of the CD-0.04, CD-0.05 and CD-0.07. To address this, the FTIR spectra and the gel fraction of the CD-0.04, CD-0.05 and CD-0.07 after healing were characterized and the results are provided in Figs. S3 and S4 (Supporting Information), respectively. It was found that their FTIR spectra (Fig. S3) before and after healing were similar. Especially, the hydroxyl groups (around 3400 cm−1) and epoxy groups (969 cm−1) had almost the same intensity before and after healing, indicating that the formed secondary OH did not react with the epoxy of residual DGEBA. Besides, the gel fractions of CD-0.04, CD-0.05 and CD-0.07 were almost invariable after healing, which further indicated that the non-crosslinking parts of samples were not increased in during healing process. This might be attributed to that the reaction between hydroxyl groups and epoxy groups in a solid material is more difficult than that between carboxyl groups and epoxy groups. In addition, the hard benzene ring of DGEBA structural unit could add additional difficulties in effective touch of reactive groups during transesterification. Therefore, the lower level of β-hydroxyl ester bond linkages in CD-0.04, CD0.05 and CD-0.07 should be the main reason for their non-healing behaviors.
Fig. 11. Healing process of scar on the surface of CD-0.05.
DGEBA composites could be reprocessed through hot-pressing at 180 °C. Unfortunately, only CD-0.1 and CD-0.2 could be successfully recycled in our experiment. The CD-0.04, CD-0.05 and CD-0.07 were unable to be hot-pressed from fragment to a whole due to the insufficient β-hydroxyl ester cross linkages, even the temperature was increased to 200 °C. We provided the photo of CD-0.07 that was hotpressed at 180 °C and 200 °C in Figs. S5 and S6 (Supporting Information), respectively. It is clearly seen that the boundary of individual fragments still existed. This result is similar to the healing behavior as observed in healing experiment. When the carboxyl/epoxy ratio increased to 0.1, the CCNs-DGEBA composites could be recycled. Fig. 12a shows the typical stress-strain curves of CD-0.1 and CD-0.2 before and after recycling and the inset is the photo of CD-0.2 that was hot-pressed at 180 °C for 10 min. The mechanical properties such as Young's modulus, tensile strength and strain of CD-0.1 and CD-0.2 before and after recycling are summarized in Table S2 (Supporting
3.6. Recycle performance of the CCNs-DGEBA composites Thanks to the transesterification reaction of β-hydroxyl ester, CCNs-
Fig. 10. Schematic illustration of healing of CCNs-DGEBA composite via exchange reactions of β-hydroxyl esters. 7
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Fig. 12. Mechanical properties of CCNs-DGEBA composite before and after recycling: (a) typical stress-strain curves; (b) retention of Young's modulus; (c) retention of tensile strength; (d) retention of strain.
China (21875047) and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology Foundation (2018Z006).
Information). Although the tensile property of CD-0.1 and CD-0.2 decreased slightly after recycling, the retention of the Young's modulus, tensile strength and strain could be maintained above 90% as shown in Fig. 12b, c and d, respectively.
Appendix A. Supplementary data 4. Conclusions
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.compscitech.2019.06.004.
In this paper, CCNs were successfully prepared through the hydrolysis and oxidization of cotton in the mixture of HCl and HNO3. Because of the multiple carboxyl groups on the surface, CCNs acted as multifunctional linkages to participate into the formation of DGEBA covalently crosslinked network. This endowed the CCNs with excellent compatibility with DGEBA matrix, which helped CCNs to achieved a fine dispersion in the composites. At the same time, the CCNs and DGEBA had a positive synergistic effect on the thermal stability of their composites. It was found that the char yields of composites were > 16%, much higher than that of CCNs (7.05%) and DGEBA (4.44%), which were about three times higher than the calculated values. Due to the exchangeable β-hydroxyl ester bonds formed by the reaction of CCNs and DGEBA, composites could be healed and recycled at 180 °C. However, this healing and recycling behaviors were only found in the composites CD-0.1 and CD-0.2. Since the CCNs are renewable and abundant in nature, we imagine the CCNs-DGEBA composites which can be healed and recycled may get new applications as green materials. Furthermore, once the CCNs was degraded completely, the difficulty in decomposing the thermoset network will be decreased. Therefore, from this point of view, the degradable ability of “green” CNs could add additional advantages for CCNs-DGEBA composites in various potential applications.
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Acknowledgements This work was supported by National Natural Science Foundation of 8
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