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Synthesis and Self-Healing Behavior of Thermoreversible Epoxy Resins Modified With Nitrile Butadiene Rubber.

INTRODUCTION

Due to the excellent properties such as good adhesion, high modulus, low shrinkage, and good corrosion resistance, epoxy resins have been used widely in fields of transportation, mechanics, construction, and so on [1-5]. However, cracks may present in the inner of the epoxy resin structure when under external impact and pressure, which can influence the comprehensive performance of epoxy resins and shorten service life seriously. Moreover, severe safety accidents may be caused sometimes [6-8]. Cracks can be remended if epoxy resins are endowed with self-healing property. Consequently, these problems can be resolved, and application fields of epoxy resins may be widened further.

In recent years, the self-healing polymer composite has attracted increasing attention, as it is able to solve the aforesaid problem [9-12]. There are basically two types of self-healing polymers, i.e., extrinsic and intrinsic materials [13-16]. Thermal reversibility Diels-Alder (DA) reaction is one of the most significant used methods for preparing intrinsic self-healing epoxy resins. For instance, Peterson et al. prepared a healable epoxy resin and the healing behavior was achieved with direct injecting a bismaleimide (BMI) solution in the crack surface. As a result, the repair is finished via the covalent DA bonding across the crack as well as swelling-induced physical interlocking of the crack faces [17-19]. Zhang et al. prepared two kinds of self-healing epoxy resins based on DA (EP-DA) reaction between furan group and BMI group [20-22], Li et al. prepared a kind of modified novolac epoxy resins with furan pendant groups and then crosslinked by bifunctional maleimide via DA chemistry and it can be used as smart material for the practical application of electronic packaging and structural materials [23]. However, the prepared materials exhibit intense brittleness. In general, toughening agents are added in order to overcome this problem. Many efforts have been made to improve the resins' toughness, including the use of rigid particles, preformed rubber particles, liquid crystalline epoxy resins, thermoplastics, and liquid rubbers. Liquid rubber modification is one of the most frequently used methods for toughening epoxy resins [24-26]. Pearson et al. studied the influence of size and quantity of rubber particles on the comprehensive mechanical properties of epoxy resin [27].

Many studies have been carried out for the liquid rubber toughing the epoxy resin. However, as far as we know, it has not been investigated of how the liquid rubber influence on the self-healing behavior of epoxy resin, while it is quite important for epoxy resins with self-healing capability. In this work, a new kind of self-healing EP-DA bonds and nitrile butadiene rubber (NBR) has been synthesized. The self-healing performance and mechanical properties of as-prepared epoxy resin are investigated by qualitative observation and quantitative measuring. More importantly, the effect of NBR on the self-healing behavior of the resultant epoxy resin is examined particularly, and results reveal that the thermoplastic NBR can accelerate the whole healing process. Consequently, the self-healing efficiency of epoxy resin is enhanced outstandingly. Meanwhile, the as-prepared self-healing epoxy resin also exhibits excellent reprocessing performance, and which makes it possible of recycling waste epoxy resins. As a result, the application fields of epoxy resins can be widened while the service life can be extended.

EXPERIMENTAL

Materials

Epichlorohydrin (ECH, 98%), tetraethylenepentamine (TEPA, 98%), and furfuryl alcohol (FAL, 99%) were purchased from Aladdin. Tetrabutylammonium hydrogen sulfate (TBAHS, 98%) and 1,1'(methylenedi-1,4-phenylene) BMI (98%) were provided by Energy Chemical. NBR ([M.sub.n] = 5,027 g/mol) was supplied by PetroChina Lanzhou Petrochemical Company.

Synthesis of FGE

ECH (92.50 g, 1 mol), TBAHS (4.0 g), and FAL (98.18 g, 1 mol) were charged into a 250-mL four-necked-round flask. The reaction was proceeded at 25[degrees]C for 4 h under mechanical stirring and nitrogen atmosphere, and then 160 mL of sodium hydroxide aqueous solution (50 wt%) was added and the reaction was carried out for 2 h. Finally, the reactant was extracted with diethyl ether, and then washed with water for three times. The organic layer was collected and the solvent was removed by vacuum drying. Consequently, a reddish brown liquid in a yield of 77.4% was resulted, and which is called as furfuryl glycidyl ether epoxy resin (FGE).

EP-DA and EP-DA-NBR Synthesis

FGE (20.00 g, 0.13 mol) and TEPA (2.75 g) were mixed well in a beaker and the reaction was proceeded at 25[degrees]C for 24 h and then BMI (17.92 g, 0.05 mol) was added. The mixture was poured into the mold and the reaction was proceeded at 67[degrees]C for 36 h. So the self-healing EP-DA reaction was synthesized, while the whole synthetic process can be expressed in Fig. 1. NBR-modified self-healing epoxy resin (EP-DA-NBR) was synthesized using the similar procedure, while NBR was mixed into FGE in advance. In our research, EP-DA-NBR containing 5% (namely, 5 phr, similarly hereinafter), 10%, and 15% of NBR relative to FGE weight, respectively, was synthesized, and which was labeled as EP-DA-NBR,- while a: denotes NBR contents.

Thermal Reversibility and Reprocessing Behavior Measurements

The thermal reversibility was investigated by sol-gel transition process. The experiment was carried out as follows: EP-DA-NBR with 20 wt% of N,N-dimethyl-formamide (DMF) was put into a transparent glass bottle. The phase change behavior upon 122[degrees]C and 60[degrees]C of heat treatment was observed.

The reprocessing behavior was examined by observing the recombining capability of EP-DA-NBR pieces under heat treatment, while the experiment was carried out as the following procedure: The EP-DA-NBR sample was impacted and EP-DA-NBR pieces were obtained. Then, these pieces are put into the mold and heat-treated upon 122[degrees]C/2 h and then 67[degrees]C/36 h, respectively. After heat treatment, the resultant sample is removed from the mold.

Self-Healing Property Evaluation

The self-healing property was evaluated by two means, i.e., observing the evolution of crack and measuring the change of mechanical property.

1. Observing the evolution of cracks in EP-DA-NBR plates at 122[degrees]C with a digital camera. The visible cracks were generated in advance by impacting a 40-mm-diameter/3-mm-thickness round sample with a 200 g of iron ball from a height of 300 mm before falling. The self-healing process was observed until crack disappeared. Photographs were taken at different time intervals during heat treatment process.

2. After heat-treated at 122[degrees]C for 45 min, samples were then treated at 67[degrees]C for 36 h. Then the flexural property of samples was measured via three-point bending test on a universal testing machine (AG-10 KN, Shimadzu) with a loading rate of 1 mm/min. The self-healing efficiency was examined quantitatively and which was calculated by the recovery rate of bending strength.

Characterization

The chemical structure of FGE, EP-DA, and EP-DA-NBR was examined with Fourier Transform Infrared spectra (FTIR) on an FTS3000 spectrometer (DIGILAB), while spectra were recorded in the range of 4,000-400 [cm.sup.-1]. The morphology of EP-DA-NBR fracture was observed using scanning electron microscopy (SEM, JSM6701F, Japan).

RESULTS AND DISCUSSION

Chemical Structure of FGE, EP-DA, and EP-DA-NBR

FGE is synthesized from the condensation between ECH and FAL using a two-step approach. The self-healing epoxy resin containing DA bonds (EP-DA) is prepared by the reaction between FGE and BMI while it is cured by TEPA.

The chemical structure of as-synthesized FGE, EP-DA, and EPDA-NBR is investigated by FTIR, while the investigated spectra are shown in Fig. 2. In FTIR spectrum of FGE (as shown in Fig. 2a), absorption of epoxy ring presents as reflected by the stretching vibration peaks at 1253 and 857 [cm.sup.-1] for C--O--C group, while the breathing vibration of epoxy ring appears at 918 [cm.sup.-1]. Meanwhile, a furan ring is indicated by peaks at 3,145 and 3,120 [cm.sup.-1] (namely, stretching vibration of C--H), 1,504 [cm.sup.-1] (i.e., stretching vibration of C=C), and 1,025 [cm.sup.-1] (that is breathing vibration of furan ring). These results indicate the successful synthesis of FGE.

It is worth nothing that an additional peak at 1775 [cm.sup.-1] appears in the spectrum of EP-DA (see as Fig. 2b), which is the characteristic absorption of DA adduct. It indicates that the DA reaction between FGE and BMI takes place successfully. Therefore, it can be confirmed that epoxy resin containing DA bonds has been synthesized successfully. The characteristic absorption of DA adducts at 1,775 [cm.sup.-1] appears in the spectrum of EP-DA-NBR (see Fig. 2c) as well, and it shows that the addition of NBR in EP-DA does not affect the DA reaction. Meanwhile, the stretching vibration absorption of C=N groups in NBR molecules presents at 2,240 [cm.sup.-1], and the hydroxyl group presents at 3,452 [cm.sup.-1]. All these characteristic absorptions manifest that EP-DA-NBR has also been prepared successfully.

Effect of NBR on Mechanical Property of EP-DA-NBR

Since NBR is a thermoplastic resin, the introduction of NBR in epoxy resin must have much influence on the microstructure and mechanical of EP-DA. First, the impact property of EP-DA-NBR is investigated, and the digital images of EP-DA-NBR with various contents of NBR after once impact with a 200 g of iron ball are shown in Fig. 3a. It is obvious that many cracks are present in EP-DA (i.e., sample containing 0 phr of NBR), indicating that EP-DA-NBR without introducing NBR has lower impact toughness and it is damaged easily upon impacting. By contrast, the situation is quite different when NBR is introduced into EP-DA. It can be found that the crack number decreases overwhelmingly for EP-DA-NBR containing NBR as compared with that of EP-DA, manifesting that the impact toughness of EP-DA has been improved greatly upon introducing of NBR. Moreover, Fig. 3 also shows that the number along with the width and depth of crack decreases gradually with the increase in NBR content in EP-DA-NBR, and it indicates that the damage degree of EP-DA-NBR reduces and the impact toughness enhances gradually.

In order to further investigate the effect of NBR on the mechanical property of EP-DA-NBR, the cross-sectional morphologies of bend fracture for EP-DA-NBR with various contents of NBR are examined with SEM, as shown in Fig. 3b. It can be found that many long and wide crazes are present at the fracture surface of EP-DA-NBR without containing NBR. Furthermore, the direction of crazes is relatively uniform, manifesting that the brittle fracture takes place for EP-DA (sample containing 0 phr of NBR) upon bending. On the contrary, the fracture mode changes from brittle rupture to ductile rupture gradually when NBR is introduced into EP-DA. Figure 3b shows clearly that a great deal of NBR aggregates (i.e., rubber particles) disperse uniformly in the epoxy resin matrix. It manifests that EP-DA-NBR containing NBR exhibits microphase-separated structure, and it is quite important for the mechanical enhancing of EP-DA. The number of the rubber particles increases gradually with the introduced NBR amount increasing. On the other hand, Fig. 3 shows that short and small crazes present at the fracture surface of EP-DA-NBR containing NBR. Moreover, the direction of crazes is chaotic. Meanwhile, the stress-whitened zone appears at the fracture surface of EP-DA-NBR containing NBR. All these results show that EP-DA-NBR-containing NBR presents ductile rupture behavior upon bending or stressing, indicating the toughness of EP-DA has been improved markedly upon the introduction of NBR. This is because Poisson's ratio between the NBR and the epoxy matrix is different and the bonding force between epoxy matrix and NBR is weak. Consequently, holes will generate in the interface between epoxy resins phase and NBR phase when EP-DA-NBR samples suffered from the external force. The resultant holes can effectively prevent crack formation and further reduce material deformation by binding [27-29]. On the other hand, these holes can also force EP-DA-NBR to generate high-elasticity deformation and this high elastic deformation can then absorb a great deal of damage energy. As a result, the toughness of epoxy resin is improved effectively at macro level.

In addition, Fig. 3b also shows that the most amount of stress-whitened zone presents at the fracture surface of EP-DA-NBR containing 10 phr of NBR, and it indicates that it is not at all the more of the amount of incorporated NBR, the better of the toughness of EPDA-NBR materials. Namely, the optimal toughness is obtained when 10 phr of NBR is introduced into EP-DA-NBR. The reason that leads to the result is that bridge phenomenon is generated when EP-DANBR sample contains too many rubber particles (such as NBR) upon suffering impact damage, and the bridge phenomenon is not conducive to the formation of holes between interfaces of epoxy resin and NBR [29]. Consequently, the toughness decreases contrarily when the incorporated NBR content in EP-DA-NBR exceeds 10 phr.

The effect of NBR on the mechanical property of EP-DA-NBR is further examined quantitatively, and the flexural property of EP-DA-NBR is measured by three-point bending test. The representative load-flexural displacement curves of EP-DA-NBR with various contents of NBR are depicted in Fig. 4. In general, the results reveal that the flexural displacement of EP-DA-NBR increases while the flexural load and strength decrease gradually with the increase in NBR additions. However, the flexural displacement of EP-DA-NBR begins to reduce when the content of incorporated NBR in EP-DA-NBR is more than 10 phr. The flexural property shows that NBR has preferable toughing effect on EP-DA when the incorporated NBR amount in EP-DA-NBR is 10 phr.

Therefore, results from digital camera, SEM along, and three-point bending test indicate that the toughness of EP-DA can be improved markedly by introducing NBR, while the optimal toughness is obtained when 10 phr of NBR is introduced into EP-DA-NBR.

Thermal Reversibility of EP-DA-NBR

EP-DA-NBR has thermal reversibility that can be reflected directly by sol-gel transition process. Figure 5 shows the observed phase transformation behavior of EP-DA-NBR upon treatment at different temperatures. EP-DA-NBR takes on brown and can be swelled by 20 wt % of DMF; however, it cannot flow. Upon heat treated at 122[degrees]C for 10 min, EP-DA-NBR sample is converted into a low viscous liquid, as shown in Fig. 5al. This manifests that EP-DA-NBR chains are broken into FGE and BMI because of r-DA reaction. As a result, the molecular weight of the sample decreases and then the viscosity reduces. Therefore, EP-DA-NBR takes on viscous liquid state and acquires some flowability. Interestingly, when the viscous liquid is cooled down to 67[degrees]C and kept at this temperature, the viscosity of the liquid will increase gradually and finally a gel forms again after 1 h, as shown in Fig. 5b 1. This can be explained by the reconnection of FGE and BMI once more. Namely, DA reaction takes places once again and DA bonds regenerate in EP-DA-NBR. Moreover, the phase transformation from sol to gel and then from gel to sol can be repeated many times, and Fig. 5 provides three cycles of phase transformation upon different heat treatment. These indicate that thermally reversible DA bonds are introduced into EP-DA-NBR successfully. Consequently, EP-DA-NBR exhibits excellent thermal reversibility.

Self-Healing Behavior and Repair Mechanism of EP-DA-NBR

Since the as-prepared EP-DA-NBR exhibits excellent thermal reversibility, it can be deduced that EP-DA-NBR must have self-healing property. The self-healing behavior of EP-DA-NBR upon heat treatment is first investigated by observing the evolution of cracks in EP-DA-NBR plates with a digital camera. The crack evolution process is observed when samples with cracks are heat-treated at 122[degrees]C, as shown in Fig. 6.

It is obvious in Fig. 6 that the number and area of cracks in EP-DA-NBR decrease gradually with the extension of heat-treatment for each sample. The cracks in EP-DA (namely, EP-DA containing 0 phr of NBR) disappear completely when heat treatment time reaches 8 min, indicating that EP-DA has excellent self-healing performance and the crack has been repaired apparently for 8 min of heat treatment. It is noteworthy that the introduction of NBR in EP-DA has much influence on the self-healing capability of EP-DA. It can be found from Fig. 6 that NBR can accelerate the self-healing rate effectively, and less time is needed for the disappearance of cracks with the increase in NBR content in EP-DA-NBR. Consequently, cracks in EP-DA-NBR15 have disappeared completely when the heat treatment time only reaches 6 min, indicating cracks in EP-DA-NBR15 can be repaired in a very short time.

The self-healing behavior is further examined quantitatively via investigating the change of flexural strength before and after the self-healing process while the self-healing procedure is conducted with heat treatment for 122[degrees]C/45 min and then 67[degrees]C/36 h. Figure 7 shows the flexural strength of both as-prepared and healed EP-DA-NBR with various contents of NBR.

Results show that the flexural strength for both as-prepared and healed EP-DA-NBR decreases gradually with the increase in incorporated NBR contents, respectively, as shown in Fig. 7. Moreover, as far as EP-DA-NBR containing the equal content of NBR is concerned, the flexural strength for the healed EP-DA-NBR decreases as compared with that of the as-prepared samples. However, the self-healing efficiency (recovery rate of flexural strength) is quite different for EP-DA-NBR containing various contents of NBR. The self-healing efficiency for EP-DA-NBR with 0, 5, 10, and 15 phr of NBR can reach 70.0%, 81.5%, 84.9%, and 88.6% one by one. It shows that the self-healing efficiency enhances gradually with the increase in NBR amount. Thereupon, it can be concluded that the incorporating NBR in EP-DA can improve the healing capability of EP-DA outstandingly.

In a word, results from visual inspecting of the crack evolution upon heat treatment reveal that the repair rate of EP-DA can be accelerated by introducing NBR, while results from three-point bending test indicate that the healing efficiency of EP-DA can be increased by incorporating NBR in EP-DA. The reason for the increase of the healing capability for EP-DA-NBR can be analyzed and the healing process can be schematically shown in Fig. 8. Both thermoreversible DA bonds and thermoplastic NBR are introduced into the epoxy resin, just as shown in Fig. 8a. Consequently, the as-prepared EP-DA-NBR exhibits both thermal reversibility and some thermoplasticity. Once EP-DA-NBR sample is impacted and cracks may engender in EP-DA-NBR. Figure 8b schematically shows a crack in EP-DA-NBR, and a large number of cracked polymer chains containing DA bonds are present at the crack interface. Upon heat treatment at 122[degrees]C, DA bonds break and r-DA reaction takes place. Consequently, polymer in EP-DA-NBR will be broken into short chains and molecules including FGE and BMI. The resultant short chains and molecules can move and diffuse across the crack under thermal action. Meanwhile, the thermoplastic NBR is prone to migrate upon heat treatment. The movement and diffusion of chains, including polymer, FGE, and BMI, can be further accelerated by NBR under the heat treatment at 122[degrees]C. As a result, the crack will be filled by all kinds of polymer chains and molecules after 45 min of treatment, just as shown in Fig. 8c. When the sample is further treated at 67[degrees]C, polymer chains and molecules continue to move. More importantly, ends of cracked FGE chains and BMI molecules may meet. Thus, DA reaction takes place and DA bonds regenerate once more. Consequently, long polymer chains form once again. As a result, the crack will be remended and the strength of EP-DA-NBR will be restored accordingly. Therefore, the complete EP-DA-NBR sample will be recovered, as shown in Fig. 8d. However, since the thermal movement range is restricted by the matrix structure and meanwhile side reaction exists, the strength of EP-DA-NBR cannot be restored completely.

Grounded on facts and explanation above, the healing mechanism of EP-DA-NBR can be deduced as follows: EP-DA-NBR exhibits excellent self-healing performance, and cracks in EP-DA-NBR are repaired by combined actions of thermoreversible DA reaction and thermal movement of molecular chains. Furthermore, the thermoreversible DA bonds contribute much to the recovery of mechanical property while the incorporated thermoplastic NBR accelerates the whole healing process.

Multiple Self-Healing Behavior of EP-DA-NBR

Multiple self-healing property is one of the most significant properties of self-healing materials. The first healed sample is obtained after treating the EP-DA-NBR sample with cracks at 122[degrees]C for 45 min and then 67[degrees]C for 36 h. And then, the resultant sample is impacted again so that new cracks are generated. Then, the similar heat treatment, i.e., 122[degrees]C/45 min and then 67[degrees]C/36 h, is conducted and the second healed sample is prepared. The third healed sample is prepared similarly. Figure 9 shows the flexural strength of EP-DA-NBR with various contents of NBR after each healed cycles. Meanwhile, the healing efficiency of each sample is calculated and included in Fig. 9 as well.

It can be seen from Fig. 9 that the healing efficiency decreases gradually with the increase of damage-repair cycles for EP-DA-NBR containing the equal content of NBR. Even in such cases, however, the healing efficiency still achieves 50% and above when the same sample is impacted and repaired for three cycles, indicating the asprepared EP-DA-NBR presents very good multiple self-healing capability. The decrease in healing efficiency for multiple damage-repair cycles results from the self-polymerization of BMI and incomplete reconnection of DA bonds [30], which can be probed as follows: too long time of heat treatment will bring about side reaction, and not all cracked FGE and BMI can take part in DA reaction again. As a result, part DA adducts have not regenerated in the next cycle of heat treatment.

On the other hand, it is noted that all healing efficiencies of EP-DA containing NBR are higher than those of EP-DA without containing NBR for the same damage-repair cycles, which further suggest that NBR can improve the healing effect of EP-DA markedly once more.

In addition, it can also be found from Fig. 9 that there is a distinct difference between the first healing efficiency of EP-DA-NBR and various contents of NBR, namely, 81.5%, 84.9%, and 88.6% for EPDA-NBR incorporated 5, 10, and 15 phr of NBR in turn. However, the healing efficiency tends to be equal after three damage-repair cycles, i.e., 56.2%, 56.3%, and 56.8% for EP-DA-NBR incorporated 5, 10, and 15 phr of NBR, respectively. It is because NBR has good thermal fluidity. NBR can promote the thermal movement of molecules and improve the self-healing effect of EP-DA-NBR; however, excessive amount of NBS will lead part NBR to migrate and move to the surface of the sample and then miss from the sample under multiple heat treatments. Consequently, the missed NBR will fail to promote the healing process of cracks. Therefore, the healing efficiencies of the third healed samples are similar.

Reprocessing Performance of EP-DA-NBR

EP-DA-NBR has excellent thermal reversibility and outstanding self-healing capability. Consequently, it is speculated that EP-DANBR has reprocessing performance and it ought to be reprocessed via treating the waste samples under 122[degrees]C and then 67[degrees]C for some time, respectively. Therefore, the reprocessing property of EP-DA-NBR is examined, and the reprocessing process is shown in Fig. 10. First, EP-DA-NBR is broken into pieces. Then, the pieces are put into a mold. The mold together with EP-DA-NBR pieces is treated under hot pressing upon for 122[degrees]C/2 h and 67[degrees]C/36 h in turn. The resultant sample is removed from the mold after the hot press molding finished. It can be found that EP-DA-NBR pieces have recombined together as a whole and a complete EP-DA-NBR sample is obtained again. Moreover, the experiment further indicates that the breakage-reprocessing of EP-DA-NBR can be repeated for many times.

The result indicates that EP-DA-NBR has excellent reprocessing performance, and the reason can be explained as follows: the distance among EP-DA-NBR pieces reduces gradually upon hot pressing owing to both pressure and movement of polymer chains. Meanwhile, r-DA reaction happens when treated at 122[degrees]C. Consequently, polymer chains in EP-DA-NBR pieces will break into short chains and molecules including FGE and BMI. The resultant short chains and molecules are apt to move and diffuse across gaps among EPDA-NBR pieces under thermal action. Moreover, the process can be accelerated when the thermoplastic NBR is included. As a result, gaps among EP-DA-NBR pieces will be filled by short chains and molecules. DA reaction takes place and the broken FGE and BMI molecules recombine by DA bonds once again when further treated at 67[degrees]C. Consequently, long chains regenerate and network structure forms once more. Thus, EP-DA-NBR pieces unite as a whole and a complete EP-DA-NBR sample regenerates again. Therefore, EP-DANBR exhibits excellent reprocessing performance, and this property makes it possible of realizing multiple recycling of waste epoxy resin, and it is quite important for reusing of industrial waste.

CONCLUSIONS

The epoxy resin containing DA bonds (EP-DA) is synthesized first via reaction between FGE and BMI. And then, NBR is introduced into EP-DA and a new kind epoxy resin (EP-DA-NBR) with both high strength and toughness is synthesized. Both EP-DA and EP-DA-NBR exhibit excellent self-healing performance based on dual actions of thermal reversibility of DA reaction and thermal movement of NBR molecular chains. Macroscopic observation and three-point bending test reveal that cracks in EP-DA-NBR can be remended upon heat treatment for 122[degrees]C/45 min and 67[degrees]C/36 h, and the healing speed is accelerated with the increase in NBR. The healing efficiency of EP-DA-NBR for once impacted and healed sample is as much as 80%, while the healing efficiency can still reach 56% when the same sample is damaged and repaired for three cycles. Therefore, the properties of epoxy resins can be improved by introducing both thermally reversible DA bonds and NBR, and application fields of epoxy resins may be widened further. In addition, the self-healing epoxy resin has outstanding reprocessing performance, which makes it possible of recycling waste epoxy resin.

ACKNOWLEDGMENTS

This research is supported by National Natural Science Foundation of China (Grant No. 51463010).

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Libang Feng (iD), (1) Hanwen Zhao, (1) Xia He, (1) Yanhua Zhao, (2) Liuxiaohui Gou, (1) Yanping Wang (1)

(1) School of Materials Science and Engineering, Lanzhou Jiaotong University, Lanzhou, 730070, China

(2) School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou, 730070, China

Additional Supporting Information may be found in the online version of this article.

Correspondence to: L. Feng; e-mail: lepond@hotmail.com Contract grant sponsor: National Natural Science Foundation of China.

DOI 10.1002/pen.25158

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Synthesis of EP-DA.

Caption: FIG. 2. FTIR spectra of FGE (a), EP-DA (b), and EP-DA-NBR (c). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Digital images (a) upon impacting and SEM fracture micrographs (b) after bending of EP-DA-NBR with various contents of NBR. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Representative load-flexural displacement curves of EP-DA-NBR with various contents of NBR. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Phase transformation behaviour of EP-DA treated with different temperatures/times for three cycles. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Visual inspecting of cracks evolution in EP-DA-NBR with various contents of NBR. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Schematic self-healing process of EP-DA-NBR. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Reprocessing process of EP-DA-NBR. [Color figure can be viewed at wileyonlinelibrary.com]
FIG. 7. Flexural strength of as-prepared and healed EP-DA-NBR with
various contents of NBR. [Color figure can be viewed at
wileyonlinelibrary.com]

     As-prepared   Healed
0      36.70       25.69
5      26.92       21.95
10     21.85       18.56
15     17.64       15.63

Note: Table made from bar graph.

FIG. 9. Flexura] strength of EP-DA-NBR with various
contents of NBR for different self-healing cycles. [Color figure
can be viewed at wileyonlinelibrary.com]

      as-prepared   1st Healed   2nd Healed   3rd Healed

0                     70.0%        63.5%        52.7%
5                     81.5%        71.4%        56.2%
10                    84.9%        70.3%        56.3%
15                    88.6%        71.5%        56.8%

Note: Table made from bar graph.
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Article Details
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Author:Feng, Libang; Zhao, Hanwen; He, Xia; Zhao, Yanhua; Gou, Liuxiaohui; Wang, Yanping
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Aug 1, 2019
Words:5255
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