epoxy composites for marine anticorrosive self-healing coating

Materials and Design 189 (2020) 108547

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GO-modified double-walled polyurea microcapsules/epoxy composites for marine anticorrosive self-healing coating Yanxuan Ma a,⁎, Yingrui Zhang a, Jiatong Liu a, Yajie Ge a, Xiaoning Yan a, Yi Sun b, Jian Wu c, Peng Zhang a a b c

Department of Material Science and Engineering, School of Civil Engineering, Qingdao University of Technology, Qingdao 266033, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Division of Advanced Nano-Materials and Division of Nanobionic Research, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Double-walled Polyurea microcapsules modified by graphene oxide were successfully prepared via interfacial polymerization. • The self-healing coatings showed better mechanical property and marine corrosion resistance than ordinary coatings. • Self-healing mechanism of the coatings was verified by Digital Speckle Correlation Method and Electrochemical Measurement.

a r t i c l e

i n f o

Article history: Received 24 December 2019 Received in revised form 1 February 2020 Accepted 3 February 2020 Available online xxxx Keywords: Self-healing Double-walled microcapsules Polyurea Digital speckle Anticorrosion

a b s t r a c t In order to achieve microcapsules' higher encapsulating capacity and better self-healing efficiency, the graphene oxide (GO)-modified double-walled Polyurea microcapsules were synthesized with 1, 6-Diaminohexane as the inner core and isophorone diisocyanate (IPDI)-based prepolymer as the outer core. Measured were the morphology, structure, thermal property and mechanical property of the microcapsules by using the Optical and Scanning Electron Microscopy (OM, SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA) and Nanoindentation tests. And the mechanical testing machine, Digital Speckle Correlation Method (DSCM) and Electrochemical Impedance Spectroscopy (EIS) measurement were used to evaluate their mechanical and anticorrosive properties. The results showed that the GO-modified microcapsules obtained possessed a good spherical shape with a mean diameter of 0.5 μm. The microcapsules with excellent thermostability containing a urea bond, the GO had been successfully embedded into the wall materials and the contents of the inner core materials and outer core materials were 26.75 wt% and 10.88 wt% respectively. The average Young's modulus, hardness and load during the holding time of the microcapsules were 2.708GPa, 0.088GPa and 28.51mN respectively. The mechanical and anticorrosive self-healing efficiency of epoxy coating with 3.0 wt% of microcapsules could gain up to 80.43% and 62.50% respectively. © 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (Y. Ma).

Organic coatings are widely used in the corrosion protection of substrates such as metals and concrete. However, they are susceptible to

https://doi.org/10.1016/j.matdes.2020.108547 0264-1275/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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Y. Ma et al. / Materials and Design 189 (2020) 108547

temperature changes [1,2], collisions, corrosive mediums erosion [3,4], solvent dissolution [5] and swelling [[6]6] during their service life, resulting in the degradation of coating mechanical properties and corrosion resistance, or even failure [7]. The failure of the coating originates from the generation of micro-cracks which are difficult to be observed and repaired timely. Researchers have proposed the microcapsulesbased self-healing coating [8,9,11[10]], aiming to repair the microcracks by the self-healing function of the coating, and to guarantee the improvement of long-term durability and reliability of the coating [12,13]. Due to the simplicity and feasibility of preparation process, interfacial polymerization has become one of the most well-established preparation methods for self-healing microcapsules. Zhang et al. successfully applied interfacial polymerization to synthesize self–healing microcapsules containing pure polyamine. It is showed that the highest healing efficiency of 111 ± 12% can be achieved within 48 h without any external intervention [14]. Using interfacial polymerization, Tatiya et al. [15] successfully prepared novel polyurea microcapsules with linseed oil as healing agent to enhance the composite coatings. It is found that the polyurea coating loaded with 5.0 wt% microcapsule showed excellent and satisfactory anticorrosive performance under an accelerated corrosion process. However, the study did not calculate the corrosion inhibition efficiency of the composite coating, and the mechanical properties of the composite were not studied. Although microcapsules are extensively applied in intelligent and anticorrosive coatings or other self-healing materials [16,17], they are still fragile and vulnerable, compared with the matrix or highly cross-linked polymer components. Therefore, the robust shell of microcapsules, which plays an indispensable role in the improvement of repairing efficiency, is generally required to satisfy these tough demands, such as survivability in harsh fabrication procedure, solvents resistance, mechanical strength and efficient rupture for releasing the healing agent. Many methods have been implemented to enhance the robustness of microcapsules so that the service life of smart composites, such as encapsulation of liquid agents with silica shells [18,19] or glass tubes [20,21] and modified shell with grapheme could be extended [22,23]. Li et al. synthesized self-assembled GO microcapsules by pickering emulsion method, which can simplify the preparation process of microcapsules and enhance the anticorrosion performance due to the physical barrier of GO in the shell [24]. Similarly, Li et al. reported that the GO modified by isocyanate groups possesses a high degree of

concentration in the solution of N-methyl-2-pyrrolidone (NMP). The results showed that the tensile modulus of the two MDI-modified graphene/hyperbranched poly(ether imide) nanocomposites ((GEMDI)-co-HBPEI(GE-MDI)-g-HBPEI)) increases by 41.3% and 27.0% respectively [25]. As shown in Scheme 1, the purpose of the fabrication of double-walled polyurea microcapsules with rigid shells modified by GO is to reduce the negative impact of the shell material where GO embedded. In addition, the anticorrosion properties of composite coatings including GO-modified microcapsules can be significantly enhanced, which could be attributed to that the pathway of the corrosive medium is extended by the two-dimensional layered structure of the GO as it can form a “maze” structure and improve the physical barrier of the coating [26–29]. The diffusion of the harmful medium in marine environment, like Cl− and H2O, could be restricted by the complicated barrier of GO modified shell. Moreover, the nanometer effect of GO can strengthen the mechanical performance of the wall material of microcapsules and ensure the repair efficiency [30]. In this research, explored were the preparation process and the characterization of the GO-modified microcapsules, including morphology and shell thickness, chemical composition, core content, size distribution of microcapsules, weight ratio of core to shell. Furthermore, the healing efficiency of the microcapsules in epoxy coatings has been evaluated by the tensile test, and the evaluation of the long-term corrosion resistance performance was disclosed by EIS after immersion for 720 h.

2. Materials and methods 2.1. Materials GO with a purity of 0.1 wt% was supplied by Naono Jicang Technology Co., Ltd. (Nanjing, China). IPDI (isomers content of 99%) and amino-terminated polyoxypropylene (average Mn~2000,D2000) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). 1,6-Diaminohexane used as chain extender and Sodium dodecyl benzene sulfonate (SDBS) as emulsifiers were provided by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Cyclohexane as a solvent during the preparation was received by Shanghai Aibi Chemical Reagent Co., Ltd. (Shanghai, China). Bisphenol A epoxy resins Epon 828 (equivalent of epoxy: 185–192)

Scheme 1. Self-healing mechanism of coating.

Y. Ma et al. / Materials and Design 189 (2020) 108547

and Epikure 3164 (equivalent of curing agent: 256) were purchased from Danbao Resin Co., Ltd. (Hebei, China). 2.2. Preparation of GO-modified double-walled polyurea microcapsules The encapsulation synthetic system was elaborately optimized to produce high-yield and dense microcapsules after many experiments. Four key steps were listed in the preparation process (Scheme. 2): (1) preparation of GO modified prepolymer emulsion; (2) preparation of 1, 6-Diaminohexane emulsion; (3) generation of initial singlewalled microcapsules through instantly interfacial polymerization between the prepolymer emulsion droplet and 1, 6-Diaminohexane droplet; (4) synthesis of double-walled microcapsules and coating of the second core material, which requires stable compound emulsion and immediate encapsulation of the obtained single-walled microcapsules during the enwrapped process. 2.2.1. Preparation of 1, 6-Diaminohexane emulsion 1.35 g of SDBS and 66.00 g of cyclohexane were added to a threenecked flask and stirred at 1000 rpm for 60 min. 4.50 g of 1, 6Diaminohexane were dissolved and stirred at 1500 rpm for another 1 h to obtain a transparent 1, 6-Diaminohexane emulsion. 2.2.2. Preparation of GO dispersion and modified prepolymer emulsion GO dispersion was prepared by adding 0.25 g GO emulsion to the 9.75 g solvent cyclohexane and dispersed for 8 h by a magnetic stirrer. Then, the 3.00 g of obtained GO dispersion, 0.72 g of SDBS and 110.00 g of cyclohexane were added in a three-necked flask. After that, 9.00 g of D2000 and 3.00 g of IPDI were respectively dissolved to the three-necked flask under N2 purging, and the mixture was stirred by mechanical agitation for 1 h to prepare a uniformly dispersed Gomodified prepolymer emulsion. 2.2.3. Synthesis of GO-modified single-walled polyurea microcapsules GO-modified double-walled polyurea microcapsules were prepared via interfacial polymerization. The prepolymer emulsion was slowly added to the 1, 6-Diaminohexane emulsion, with a weight ratio of 3.3:1.5 by injector. Due to the extremely high reactivity of amine with prepolymer, the preploymer droplet can instantly react with 1,6Diaminohexane at the interface and form the shell rather than dissolve in the solvents. With the rotation speed up to 1000 rpm and being stirred for 60 min, the single-walled microcapsule emulsion was obtained. 2.2.4. Synthesis of GO-modified double-walled polyurea microcapsules Double-walled microcapsule emulsion was synthesized by adding the 1, 6-Diaminohexane (chain extender) to obtain the single-walled microcapsule emulsion. The emulsion weight ratio between inner

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core, chain extender and single-walled was around 3.3: 1.0: (5.5–6.0). Then, the stir rate was increased to 1800 rpm, and the temperature of the mixture was raised to 40 °C and being kept for 2 h. Finally, the microcapsule powder was collected after cooling, centrifugating and vacuum drying. 2.3. Characterization microcapsules

of

GO-modified

double-walled

polyurea

2.3.1. OM and SEM test of the microcapsules The state of the obtained microcapsules in the emulsion was observed by an optical microscope (OM, UMT203, Aopu Optoelectronic Technology Co., Ltd., Chongqing, China), and scanning electron microscope (SEM, ULTRA-55, ZEISS Co., Ltd., Jena, Germany) were used to characterize the surface morphology and shell thickness of the prepared microcapsules. The microcapsule powder was mounted on a conductive adhesive tape and sputter-coated with a thin layer of gold on its surface. The size distribution and mean diameter were calculated by measuring N100 microcapsules using the image software (Nano Measurer). 2.3.2. FTIR and XRD test of the microcapsules The GO-grafted and ordinary microcapsules were tested by an X-ray diffraction (M21X, Shanghai Zhi Yuan Electronic Technology Co., Ltd., Shanghai, China) operated at 40KV and 200 mA with a size step of 0.02°. In addition, the FTIR spectroscopy of the synthesized GOmodified single-walled and double-walled microcapsules was analyzed to identify their chemical structure. FTIR was performed in KBr pellet on the FTIR spectrophotometer (Nicolet8700, Thermo Fisher Scientific, Waltham, MA, USA) with the range of 4000–500 cm−1. 2.3.3. TGA test of the microcapsules The thermal behavior of the synthesized GO-modified doublewalled microcapsules was evaluated by TGA (STA409PC, Germany), and compared with that of single-walled polyurea microcapsules, shell materials, inner and outer core material. The test was conducted from room temperature to 700 °C under N2 atmosphere with a heating rate of 10 °C/min. The TGA was also used to determine the inner core and outer core content encapsulated by synthesized microcapsules. 2.3.4. Nanoindentation test of the microcapsules To measure the mechanical properties of the of prepared microcapsules and neat coating, the Nano Indenter (G200, KLA-Tencor Corporation, Milpitas, USA) was applied to achieve displacement-load curves. The surface approach velocity was set at 50 nm/s, and the time for indentation was 10s. The samples obtained were solidified for 1–2 h at room temperature and each of the mechanical properties was measured 3 times on the different surface regions at each indentation load.

Scheme 2. The preparation process of GO modified double-walled microcapsules.

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2.4. Preparation and characterization of self-healing coating 2.4.1. Fabrication of self-healing coating The microcapsule-filled self-healing coatings were prepared by dispersing the different content of microcapsules (0.0 wt%, 3.0 wt%, 5.0 wt% and 8.0 wt%) into epoxy coating matrix at 25 °C. The Epon 828 epoxy resin and Epikure 3164 were weighed at the weight ratio of 1.7:1. Alcohol was first added into epoxy resin at 5.0 wt% of the total weight. To obtain homogeneous composite coatings, microcapsules with different contents were added into the epoxy after stirring for 5 min. The modified coatings were applied to the standard mold for tensile test and 10 mm × 10 mm steel panel with 500 μm film thickness.

2.4.2. Mechanical test of self-healing coating Tensile strengths of the film samples containing various microcapsules (0.0, 3.0, 5.0 and 8.0 wt%) were evaluated by a testing machine (MZ-4000D, Jiangsu Mingzhu Test Machinery Co., Ltd., Jiangsu, China), following the ASTMD 882 standard test practice. The process of crack propagation evolution was traced by digital speckle correlation method (DSCM) [31,32]. The standard samples of epoxy were stretched with the constant speed of 3.0 mm/min under the 500 N force at 20 ± 2 °C. Three replicate samples were tested for the statistical accuracy for each composite. The tensile strength test was conducted before (intact), on (scratched) and 72 h (healed) after the crack formation. An artificial scratch with 4 cm in length and 200 ± 24 μm in depth, being parallel to the direction of tensile test, was created on some of standard films using a scalpel blade to study their response to mechanical damages. The healing efficiency of microcapsule-based coating samples was

calculated by comparing the tensile properties of samples in the three different conditions. 2.4.3. Corrosion test of self-healing coating The anticorrosive properties of the self-healing coatings containing microcapsules with different content were tested by EIS (p4000A, Ametek Group Advanced Measurement Technology, Princeton, NJ, USA) with a frequency ranging from 105 to 10−2 Hz, and seawater as the electrolyte. With a scalpel blade the “X” shape of scratch with a length of 1 cm was created on the coated sample. After immersing in the seawater for 24 h, EIS measurements were carried out for all the testing samples with a 10 mm × 10 mm defined area. Three-electrode arrangements were composed of an Ag/AgCl reference electrode, the working electrode of the exposed steel panel, and the platinum counter electrode. The measurements were performed over 720 h at 20 ± 2 °C using 10 mV open circuit potential, and the collected data were fitted through ZSimDemo software using the electrical equivalent circuits. Meanwhile, polarization experiments were performed from −150 mV SCE to 250 mV SCE with a scan rate of 1mVs−1. The parameters of corrosion potential (Ecorr) and corrosion current density (Icorr) were calculated from the intersection of anode and cathode by using Tafel plots. 3. Results and discussion 3.1. Preparation and characterization of the synthesized microcapsules 3.1.1. Influence of mass ratio of core/shell on the synthesized microcapsules The effect of reaction conditions plays an important role in the formation of detached and well spherical microcapsules [33], especially, the emulsion weigh ratio of core, shell and single-walled microcapsules in the final synthesis. In this study, the surface and shell appearances of

Fig. 1. OM and SEM images of GO modified double-walled microcapsules with different emulsion weight ratios of core, shell and single-walled microcapsules. (a) OM image under ratio of 3.3:1:6.0, (b) SEM image under ratio of 3.3:1.0:6.0, (c) SEM image of shell SEM under 3.3:1.0:6.0, (d) OM image under ratio of 3.3:1.0:5.7, (e) SEM image under ratio of 3.3:1.0: 5.7, (f) SEM image of shell SEM under 3.3:1.0:5.7, (g) OM image under ratio of 3.3:1.0:5.5, (h) SEM image under ratio of 3.3:1.0:5.5, (i) SEM image of shell SEM under 3.3:1.0:5.5.

Y. Ma et al. / Materials and Design 189 (2020) 108547

modified microcapsules under different conditions were observed by OM and SEM. Fig. 1 showed the morphology of the double-walled microcapsules with different/core/shell/single-walled microcapsules emulsion ratios. It can be found that all microcapsules are nearly uniform and possess dense and roughness shell without evident defects, which was caused by precipitation, aggregation and deposition of polyurea, and the morphology of the shell with a rough surface was also reported [8]. Additionally, there was a decreasing trend in the shell thickness with an increasing emulsion of single-walled microcapsules. When the emulsion ratio of core/shell/single-walled microcapsules was 3.3:1.0:5.5, there were small numbers of the synthetic microcapsules and unreacted materials caused by the excessive inner core material. When core/shell/single-walled microcapsules emulsion ratio was changed to 3.3:1.0:5.7, the dense and well-dispersed double-walled microcapsules were formed, indicating a favorable ratio for double-walled microcapsules. However, the coalesced and ruptured microcapsules were obtained when the ration was adjusted to 3.3:1.0:6.0, which may result from the aggregation of unsteady emulsion droplets for the contents of the outer shell materials are relatively little, making it unable to form a compact and robust shell, and the rupture phenomenon was also described by Ye et al. due to the higher core/ shell weight ratio [34]. Morphology showed in the amplified SEM images indicated that microcapsules were well-separated and the surfaces were rough and dense (Fig. 2. a-c), which was attributed to the interlayer structure of GO. As the rough surface of microcapsules could enhance the interface

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interactions between microcapsules and coating matrix, the microcapsules can be well protected until the crack rupture it [35] Besides, the dense structure might reduce the diffusion and permeability of harmful chemicals in seawater, which can extend the service life of the coating matrix. In addition, the cross-section of broken single-walled microcapsules in Fig. 2. (d-e) demonstrated that active core materials, which can be released to the cracks, were successfully encapsulated by the obtained microcapsules. Moreover, the broken outer shell and impact inner microcapsule in Fig. 2. (f) and the structure of inner/outer shell from the cross-section morphology of double-walled microcapsules in Fig. 2. (g) can be observed clearly, which confirmed the growth of the double shell of microcapsules with healing agents. Furthermore, it must be pointed out that the quantity of healing agent microcapsules encapsulated for self-healing coatings and the size of microcapsules are extremely vital for filling the crack area [7]. The size distribution of GO modified microcapsules was displayed in Fig. 2. (h), and the average diameter was approximately 0.5 μm, which greatly reduced the negative impact on coating properties. 3.1.2. Chemical structure of the synthesized microcapsules X-ray scattering analysis was used to verify whether GO was successfully grafted into the shell of obtained microcapsules through the different crystalline structures, as shown in Fig. 2. (f). Compared with unmodified microcapsules, there was an obvious diffraction peak with 0.82 nm interlayer spacing at 10.8° as assigned to the (001) plane of GO [27], indicating a highly ordered structure. Besides, the two narrow

Fig. 2. GO modified double-walled microcapsules under different magnifications: (a) 4KX, (b) 10KX, (c) 20KX; Cross section of the broken single-walled microcapsules (d) with inner core, (e) with broken shell and inner core; Cross section of the broken double-walled microcapsules (f) with broken shell and healing agents (g) with outer shell and inner shell; (h) size distribution and (i) XRD patterns of ordinary and modified double-walled microcapsules.

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peaks (012 and 104) presented at 24.1° and 32.6° relating to GO can further demonstrate that GO was successfully loaded to polyurea shell. FTIR was applied to investigate the chemical components in the GO modified microcapsules. The transformation of 1,6-Diaminohexane and IPDI to polyurea (wall material) in the single-walled and double-walled products were observed by using the infrared absorption peaks, attributing to the polyurea bond (C\\O) at 1639 cm−1, 1559 cm−1 (N\\H), and 3475 cm−1 (N\\H) [36]. Additionally, the typical absorption of isocyanate group (–NCO) was tracked in the peaks at 2280 cm−1 in GO modified double-walled microcapsules, stating that the outer core material was successfully encapsulated. Furthermore, the successful modification of GO to wall materials was confirmed through a strong and broad peak presented at 3430 cm−1, as assigned to the O\\H stretching of C-OH. Besides, the stronger oxygen-containing functional groups (C=O) at 1090 cm−1 and the weaken absorption in OH in the doublewalled microcapsules indicated that a large amount of GO was inserted in the double-walled microcapsules. (See Fig. 3.) 3.1.3. Thermo stability and core content of the synthesized microcapsules The thermal stability of the modified microcapsules was characterized via TGA. In Fig. 4, it can be noticed that the heat resistance of the modified polyurea shell was quite stable before 325 °C. The reason can be attributed to the fact that the heat resistance of shell was enhanced after the functionalization with GO, which holds intermolecular hydrogen bonding interactions in polyurea shell. Apparently, the inner core material showed a rapid weight loss around 123 °C, through which the inner core content can be roughly estimated, 39.03 wt% and 26.75 wt% respectively in single and doublewalled microcapsules. At the same time, comparing that of the singlewalled microcapsules, the outer core content was about 10.88 wt% in double-walled microcapsules. What should be noticed was that due to the protective effect of the double robust shell, the heat resistance of the double-walled microcapsules was superior, and the inner core material was completely decomposed up to 202 °C. Therefore, both microcapsules modified by GO were excellent in heat resistance, and the thermal stability of core material could be significantly improved after coated due to the perfect barrier effect when GO was heated.

Fig. 4. TGA curves of shell, outer core, inner core, and single-walled and double-walled microcapsules.

nonlinear curve is caused by plastic deformation under loading, and the unloading curve was in consistent with the residual plastic deformation. The maximum load of neat coating was 61.18mN, which indicated that the maximum load double-walled microcapsules can sustain 46.60% of that neat one. This property also illustrated that the prepared microcapsules can release curing agent before matrix damage [37]. Besides, the average load during the holding time of double-walled microcapsule was 28.51mN, which was significantly higher than that of the singlewalled at 20.67mN. It demonstrated that the outer shell increased the loading force of the microcapsule, thus successfully improving the mechanical properties of cracked area. Moreover, the average hardness of single-walled and double-walled microcapsule were relatively lower than the neat coating (0.236GPa). And the neat coating possessed a higher Young's modulus of 4.668Gpa, therefore the microcapsules will break first and release the repair agent when the coatings suffer shear deformation. (See Table 1.)

3.1.4. Mechanical properties of the synthesized microcapsules Fig. 5 showed the load–displacement curves of the neat coating, prepared single-walled and double-walled microcapsules respectively, and this micromechanical behavior was extremely similar to that of PUF microcapsules researched by Wang et al. [36]. During the loading and unloading process, elastic deformation is related to linear displacement,

3.2. Deformation behavior and mechanical property of the self-healing coating

Fig. 3. FTIR spectra of GO modified single-walled and double-walled microcapsules.

Fig. 5. Load-displacement curve of the epoxy resin，pure material and the microcapsules.

3.2.1. Evolution process and mechanism of the self-healing coating's deformation behavior The mechanical properties are one of the most important basic properties of the coating. In order to study the effect of the microcapsules on

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Table 1 The mechanical properties of the neat coating, single-walled and double-walled microcapsule. Samples

Average Young's modulus (GPa)

Average hardness (GPa)

Average load during the holding time (mN)

Neat coating Single-walled microcapsule Double-walled microcapsule

4.668 1.682 2.708

0.236 0.065 0.088

61.18 20.67 28.51

self-healing epoxy coatings, coating samples containing different contents of microcapsules (0.0 wt%, 3.0 wt%, 5.0 wt% and 8.0 wt%) were prepared and tested by DSCM during the tensile testing. DSCM method is usually combined with uniaxial compression and tensile testing to explore the deformation field evolution characteristics [38,39]. Besides, displacement measured by DSCM was based on a comparison of the gray correlation of the two speckle fields. As shown in Fig. 6, taking the P in the field as the center and S as subset, the spot light intensity information near the center point can be obtained in the S area, which is called a two-dimensional sample space. During the deformation process, the P point moves to the P ‘point, and the spots in S move to the corresponding positions in S', thereby forming a new sample space. The equation of coefficient C was followed as Eq. (1): PP ðG  H Þ C ¼ P P  P P  H 2 1=2 ½ð G2 

ð1Þ

Where G represents the gray scale of each pixel in the initial image and H is that figure for the deformed image. Generally, deformation usually occurs in the y direction of samples, so deviation St of the gray correlation coefficient of the strain field in the y direction is used as the feature statistic. The St is calculated as

follows Eqs. (2) and (3): vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u m n  X u 2 1 St ¼ t X it −X t m  n−1 i¼1

Xt ¼

n X 1 m X m  n i−1 it

ð2Þ

ð3Þ

where Xit is the value of the image gray correlation coefficient of the ith small region in the image at time t or the strain value in the y direction at the ith point, and Xt is the mean value of Xit. Fig. 7 showed the curve of (Y-direction) strain feature quantity and Y-direction displacement, and gray correlation coefficient maps of film samples containing various microcapsules were shown in Figs. 8-11. As a large number of digital images were recorded during the entire loading process, a part of the representative images were selected and then 4 representative images in each sample were selected before the sample was completely broken during the calculation. In addition, in order to simplify these samples, the coating samples with different contents of GO-modified microcapsules would be defined as XGOMC. MC stands for the microcapsules, and X stands for the microcapsule concentration. For example, 5GOMC means that the coating sample incorporated 5.0 wt% microcapsules.

Fig. 6. (a) the schematic diagram of DSCM; (b) a calculation method of position of a speckle.

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Fig. 7. Relationship between (y-direction) strain quantity and y-direction displacement with load.

According to the variation characteristics of St values of the epoxy sample with the load, the evolution process of the sample can be roughly divided into three stages: (1) at the beginning of the tensile test, the St values increased sharply with the load. (2) In stage 2 (OB), the St values and Y-direction displacements grew relative slowly with the increased load, and St value showed some obvious fluctuations. (3) In the stage 3(BD), there was a trend for St values to increase or decrease sharply with slight fluctuations when the load increased. The results showed that, at the beginning of the tensile test, the St value of the 0GOMC sample was ≤0.196, and the overall gray correlation coefficient was relative higher in the test area, indicating that the whole of the film sample was in a uniform deformation state. Meanwhile, St values of the epoxy coatings with 3.0 wt%, 5.0 wt% and 8.0 wt%

microcapsules were all N0.20, demonstrating that some tiny local deformation regions were appeared. It can be seen clearly that deformation zones where the gray correlation coefficients of the area around y = 125–140 mm，y = 135-140 mm，y = 132-140 mm were significantly lower than those in other areas in Figs. 9(a), 10(a) and 11 (a) respectively. During OB stage, there wasn't significant non-uniform deformation in the 3GOMC sample proved by the lower St value. Moreover, it can be found that areas of y = 125–140 mm in Fig. 9 (a, b), and y = 6075 mm in Fig. 9 (c) with lower gray correlation coefficient were narrowed significantly, indicating that the self-healing process has been achieved by the ruptured microcapsules. Meanwhile, higher and fluctuating St values presenting in 5GOMC and 8GOMC samples indicated further expansion of the deformation areas, which was in line with the increase of the overall displacement in the Y direction and the expansion of the area with low gray correlation coefficient. As the incorporation of 5.0 wt% and 8.0 wt% microcapsules led to stress concentration of the coating during the stretching process and accelerated the crack propagation, the self-healing process wasn't presented by the variation of correlation coefficient. At the BD stage, subjected to the same load, the St values of 0GOMC and 3GOMC samples were still rising with numerical jitter, reaching maximum of 0.39 and 0.43 respectively. This phenomenon was mainly because that the coating samples still undergo local non-uniform deformation stage, indicating its excellent mechanical properties. There were downward trends for the St values and upward tendency for overall Ydirection strain feature quantity of 5GOMC and 8GOMC samples due to the fact that the crack evolution process gradually changed from local deformation to overall deformation. In Figs. 10(d) and 11(d), the overall gray correlation coefficient was reduced obviously in the observation area, and also presented the significant concentration bands with a low gray correlation coefficient. Therefore, it is necessary to further explore the balance between the contents of microcapsules and the mechanical properties of the coating, and further improve the mechanical response of the self-healing coating.

Fig. 8. Gray correlation coefficient map of 0GOMC.

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Fig. 9. Gray correlation coefficient map of 3GOMC.

3.2.2. Self-healing efficiency of the self-healing coating's mechanical property The main purpose of implanting microcapsules to the coating samples is to release the healing agent to repair the cracks, so it is very critical to quantify the repair efficiency of the self-healing coatings. Epoxy films (0GOMC, 3GOMC, 5GOMC and 8GOMC) were prepared and conducted in tensile tests to study the self-healing performance in three

different conditions (intact, healed and scratched). Stress-strain curves of epoxy film samples containing different content of microcapsules were shown in Fig. 12. A summary of mechanical properties of these films were shown in Table 2. The tensile data reported in Tables 2 presented the mean value ± standard deviation of at least 3 sampling. The results showed that, with the increase of the concentration of microcapsules in the coating, the tensile strength and elastic modulus

Fig. 10. Gray correlation coefficient map of 5GOMC.

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Fig. 11. Gray correlation coefficient map of 8GOMC.

gradually decreased. The elastic modulus for 0GOMC was calculated as 127.22 MPa, while for those with 3.0 wt%, 5.0 wt% and 8.0 wt% microcapsules, the elastic modulus decreased to 111.17 MPa, 95.60 MPa and 90.97 MPa respectively. Similarly, Alizadegan et al. [40] reported that when the contents of microcapsules in the coating increased from 0.0

to 10.0 wt%, the elastic modulus of the coating decreased from 998.7 MPa to 762.6 MPa due to the weak interface between microcapsules and epoxy matrix. Besides, the weaker mechanical properties of microcapsules compared to the cross-linked polymer matrix can also have a negative impact on that of the self-healing coating. Compared

Fig. 12. Stress-strain curves of epoxy coating samples containing various contents of microcapsules (0.0 wt%, 3.0 wt%, 5.0 wt% and 8.0 wt%) in three modes (intact, scratched and healed): (a) 0GOMC, (b) 3GOMC, (c) 5GOMC and (d) 8GOMC.

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Table 2 Tensile strength measurement results for coating samples in three different modes. Sample coding

Test mode

Elongation at break (%)

Tensile strength (MPa)

Elastic modulus (MPa)

Work at break (MPa)

0GOMC (Polyurea)

Intact Scratched Healed Intact Scratched Healed Intact Scratched Healed Intact Scratched Healed

1.60 1.37 1.60 0.92 0.59 0.69 0.53 0.44 0.75 1.14 0.92 0.47

41.83 23.72 39.94 38.39 26.26 31.33 31.49 28.97 27.39 25.68 22.85 25.81

127.22 ± 0.49 97.76 ± 2.57 134.36 ± 4.16 111.17 ± 3.76 85.59 ± 4.55 104.38 ± 4.93 95.60 ± 3.99 104.77 ± 2.55 94.07 ± 5.08 90.97 ± 5.07 84.19 ± 5.10 86.34 ± 3.62

38.85 23.71 34.64 38.39 29.70 31.40 31.49 28.88 30.12 26.17 22.81 24.68

3GOMC (Polyurea)

5GOMC (Polyurea)

8GOMC (Polyurea)

± ± ± ± ± ± ± ± ± ± ± ±

0.38 0.20 0.46 0.18 0.19 0.34 0.00 0.06 0.21 0.12 0.09 0.06

with 0GOMC, the breaking strength of 3GOMC microcapsules increased to 38.39 MPa, which was due to the enhanced mechanical properties by GO. Many methods have been reported by the previous researches to identify the healing efficiency of modified coatings, such as the fracture toughness tests, water vapor transmission test (WVT) [41] and so on. The results showed that tensile strength method is more reliable and suitable for calculating the efficiency, and this method was also employed by Haghayegh [9] and Alizadegan [40]. In our work, the healing efficiency of microcapsules filled films was identified via work at break based on the references [42,43]. HE% ¼

  W 1 −W H  100 W 1 −W S

ð4Þ

where, WI, WH and WS represent the work at break for intact, healed and scratched coatings respectively. Generally, the repair efficiency is related to the content of microcapsules in the coating, the stability of the microcapsules, the concentration and effectiveness of the repairing, and the curing agent encapsulated in the microcapsules. The increase of the accessible microcapsules with higher healing agents can promote healing efficiency. Fig. 13 displayed work at break for epoxy films containing different content of microcapsules. It could be found that the healing efficiency, as defined by Eq. (4), decreased from 80.43% to 52.49% by increasing the microcapsule contents from 3.0 wt% to 5.0 wt%. However, the healing efficiency declined to 45.34% when the coating contained 8.0 wt% microcapsules, which was mainly because of the stress concentration caused by the voids or defects left by the microcapsules after rupture. The results revealed that high incorporation of microcapsules was not always more effective.

Fig. 13. Healing efficiency for coating samples containing various wt% microcapsules based on comparison of work at break.

± ± ± ± ± ± ± ± ± ± ± ±

2.75 0.57 0.58 2.16 1.04 0.61 2.83 3.36 0.38 0.07 0.43 0.77

± ± ± ± ± ± ± ± ± ± ± ±

2.8 3.4 3.9 3.1 2.9 3.4 2.8 3.3 3.8 2.9 4.3 3.9

In this study, the best mechanical properties (based on self-healing efficiency) were obtained for the composite coating with 3.0 wt% microcapsules. 3.3. Marine anticorrosive performance of the self-healing coating 3.3.1. Electrochemical property of the self-healing coating EIS is a useful and effective method to evaluate the anticorrosive performance of microcapsules-based coatings due to its non-destructive and appropriate features [44,45]. The influence of GO modified double-walled polyurea microcapsules with various contents (0.0, 3.0, 5.0 and 8.0 wt%) on the anticorrosive properties of epoxy coating was detected by the impedance values over different immersion time in seawater. Figs. 14 and 15 showed the Nyquist, Bode and fitting plots of the coated steel with artificial defects after immersion for 24, 72, 120 and 192 h, and 240, 336, 576 and 720 h respectively. Impedance modulus (Zf = 0.01 Hz) of each bode plot shown in Fig. 16(a) was used to evaluate the corrosion resistance [46]. Before immersing in the seawater for 192 h, the impedance modulus (Zf = 0.01 Hz) of 3.0 wt% microcapsules-based coating was much higher than those of other samples, and it reached 1.17 × 106 Ω·cm2 at the beginning 24 h, indicating that the diffusion degree of corrosive mediums has been effectively inhibited by the self-healing microcapsules. At the same time, it can also be found that there was an upward trend of impedance values for coating filled with 8.0 wt% microcapsules, increasing from 2.17 × 105 to 7.73 × 105 Ω·cm2 after immersion for 336 h, which suggested the releasing process of healing agent encapsulated by microcapsules. Moreover, after 720 h's immersion, the impedance modulus at 0.01 Hz of coating samples filled with 3.0 wt%, 5.0 wt% and 8.0 wt% contents of microcapsules were still higher than that of pure epoxy coating (0.0 wt%), which evidenced that the healing products from GO-modified microcapsules can effectively reduce the penetration rate and enhance the protective effect of epoxy coatings. The Nyquist plots for all coating samples were showed in Fig. 14 (a1, b1, c1 and d1) and Fig. 15 (a1, b1, c1 and d1) after immersing in seawater for different periods of time. At the immersion period (24-192 h), modified coatings (with 3.0 wt%, 5.0 wt% and 8.0 wt% microcapsules) showed larger semicircle, and the gradually rising capacitive arcs in (b1, c1 and d1) of Fig. 14 proved the lowering reduction in the coating resistance and the improved anti-corrosion performance during the exposure time. Similarly, this phenomenon also appeared for the coating filled with 5.0 wt% microcapsules after immersing for 576 h in Fig. 14 (c1) and 8.0 wt% microcapsules-based coating during the immersion period (240-720 h) in (d1) of Fig. 15, indicating the excellent anticorrosive property of self-healing coatings. 3.3.2. Self-healing mechanism of the coating's electrochemical property The corresponding equivalent electric circuit models, shown in Fig. 16(d) to interpret the EIS results of the immersed coatings, were usually consisted with the solution resistance (Rs), coating resistance (Rc), charge transfer resistance (Rct), and constant phase element

12

Y. Ma et al. / Materials and Design 189 (2020) 108547

Fig. 14. Nyquist and bode plots of coated steel containing various contents of microcapsules (0.0 wt%, 3.0 wt%, 5.0 wt% and 8.0 wt%) after different immersion times (24, 72, 120 and 192 h) in seawater.

(CPE). The CPE related to coating capacitance (Qc) and double layered capacitance (Qdl) was employed to investigate the changes of capacitance behavior during corrosion [47,48]. Theoretically, the impedance of CPE is defined by Eq. (5):

Z CPE ¼

ðjwÞ−a Y

ð5Þ

where, j is imaginary number; ω is angular frequency; Y and α are constants [49]. Generally speaking, the destruction process of coating could be roughly divided into three stages. In stage 1, seawater and harmful chemicals penetrated into the interface through the micropores without reaching the coating/substrate surface. When the penetration of the electrolyte at the interface reached the saturation, a corrosive microbattery was formed at the interface and two time-constants were

shown (stage 2). At the end of the immersion (stage 3), rust spots or macroscopic holes were generated on the surface of the organic coating. EIS spectra in Fig. 14 presented the one-time constant feature in bode diagrams (except (b2)), implying that there was no under-film corrosion for the coated steel plate. Irregular time constant graph in (b2) was shown mainly due to formation of resistivity compound around the cutting edge, in which the corrosion resistance of the coating was still excellent, even if there were two time-constants. Besides, these figures displayed the bode diagrams (Fig. 15(b2)) of coating samples filled with 0.0 wt%, 3.0 wt% and 5.0 wt% after 576 h immersion, illustrated the degradation of corrosion resistance. The fast failure of pure epoxy coating occurs because of the existed pores and poor density caused by penetration of corrosive mediums. Furthermore, the variations of charge transfer resistance (Rct) and double layered capacitance (Qdl) obtained from fitting data were presented in Fig. 16 (b) and (c). The continuous penetration of electrolyte can lead to an increase in coating capacitance and a decrease in transfer charge resistance. It can

Y. Ma et al. / Materials and Design 189 (2020) 108547

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Fig. 15. Nyquist and Bode plots of coated steel containing various contents of microcapsules (0.0 wt%, 3.0 wt%, 5.0 wt% and 8.0 wt%) after different immersion times (240, 336, 576 and 720 h) in seawater.

be observed that coating samples with 3.0 wt% and 8.0 wt% microcapsules showed superior protective performance, which was in accordance with impedance values in 0.01 Hz. Moreover, the results of EIS indicated that the coating filled with 3.0 wt% microcapsules exhibited excellent anticorrosive properties before immersing for 192 h in seawater, and the coating embedded with 8.0 wt% microcapsules showed a high-speed increased trend for impedance values during the immersion period of 240-720 h. Therefore, GO nanostructure can effectively hinder the aggressive species penetration around scratched defects and microcapsules can effetely enhance anticorrosion performance of the coating. 3.3.3. Self-healing efficiency of the coating's anticorrosive property Fig. 17 showed the polarization curve of neat coating and composite coatings containing 3.0 wt%, 5.0 wt% and 8.0 wt% microcapsules for 24 h immersion in seawater. In Table 3 presented were the main corrosion parameters of composite coatings based on the Tafel extrapolation as corrosion potential (Ecorr), corrosion current density (Icorr), anodic (βa) and cathodic(βc). Polarization resistance (Rct) was calculated from the

Stern-Geary Eq. (6): Icorr ¼

βa j βc j 2:303RP ðjβc j þ βa Þ

ð6Þ

The corrosion current density of the coating can be influenced by the different contents of microcapsules, and the inhibition efficiency (IE) can be calculated by Eq. (7): 0

IEP ð%Þ ¼

icorr −i corr icorr

ð7Þ

where icorr and I'corr are corrosion current densities without and with self-healing microcapsules, respectively. It can be found that the neat coating with 8.0 wt% microcapsules showed a more negative Ecorr (−0.664 V) after 24 h's immersion owing to the severe corrosion process caused by agglomeration of microcapsules occurring at the interface. The corrosion potential of neat

14

Y. Ma et al. / Materials and Design 189 (2020) 108547

Fig. 16. (a) Impedance modulus (f = 0.01 Hz), (b) coating resistance (Rc), and (c) the double layer capacitance Qdl of the coated steel; (d) equivalent circuit models during immersion time.

coating was higher than composite coating (8.0 wt%) after 24 h of immersion. By comparison, the Ecorr values of the modified coating filled with 3.0 wt% and 5.0 wt% shift to the positive direction, and achieved more positive potential values (−0.543 V and −0.648 V). The Icorr values of composite coatings significantly decreased compared to that of the neat one due to the improvement of barrier properties of coating. Furthermore, the minimum Icorr value (0.1359 E-6) for the coating with 3.0 wt% microcapsules, demonstrating its excellent protective capacity, was superior to other modified coatings. Results obtained from other composite coatings modified by GO@PIH hybrids and polymeric micro/nanocapsules exhibited similar improvement behavior of coating corrosion resistance [50,51]. Moreover, The IEP values illustrated that

Fig. 17. The polarization curve of composite coatings after 24 h immersion.

the inhibition efficiency was more significant when the corporation is 3.0 wt%, reaching 62.50%, and an increase in contents of microcapsules to 5.0 wt% and 8.0 wt% didn't increase the IEp [52]. Compared to the mechanical self-healing efficiency, it can be noted that the optimal content of microcapsules in the coating was also 3.0 wt% since the exceed microcapsules may lead to resistance reduction of coating. Overall, the results showed that the corrosion resistance of composite coating could be enhanced by adding microcapsules. 4. Conclusion The GO-modified double-walled polyurea microcapsules filled with 1, 6-Diaminohexane and prepolymer had been smoothly prepared by the interfacial polymerization and the self-healable epoxy composite coatings with GO-modified double-walled polyurea microcapsule had been successfully developed. Prepared microcapsules were dense and well-dispersed with the average size of approximately 0.5 μm and shell thickness of 0.08 μm when emulsion weight ratio of core/shell/ single-walled was 3.3:1:5.7. The nanoparticles of GO embedded in the shell had been identified. DSCM analysis confirmed that the corporation of high content microcapsules to the coating formulation could lead to the deduction of coating's mechanical properties, and the best selfhealing efficiency was achieved in the coating with 3.0 wt% microcapsules, which was attributed to the healing agent encapsulated by robust microcapsules. The corrosion resistance of 3.0 and 8.0 wt% microcapsules dispersed in epoxy coating was super higher than the neat one, revealing that microcapsules can effetely enhance anticorrosion performance of the coating. In summary, the coatings showed an efficient self-healing function, which was ascribed to the healing agent encapsulated from robust microcapsules. Simultaneously, the reason of such excellent barrier and anticorrosive properties was that GO nanostructure effectively hinder the aggressive species penetration around scratched defects. Therefore, the modified coatings containing 3.0 wt%

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Table 3 Electrochemical parameters obtained after immersion in seawater for 24 h. System

Ecorr(Volts)

βa (mv)

βc (mv)

Rp(Ωcm2) E6

Icorr (Amp/cm2) E-6

IEP(%)

0.0 3.0 5.0 8.0

−0.649 −0.543 −0.648 −0.664

199.69 412.6 123.1 173.08

−141.96 −252.37 −98.56 −126.89

99.41 500.32 134.88 168.83

0.3624 0.1359 0.1762 0.1883

– 62.50 51.38 48.04

wt% wt% wt% wt%

GO-modified double-walled microcapsules have been proven to be appropriate to be used in marine environment for enhancing the toughness and impermeability of coatings where exceptionally anticorrosive performance is required. CRediT authorship contribution statement Yanxuan Ma:Conceptualization, Methodology, Investigation, Writing - review & editing.Yingrui Zhang:Formal analysis, Investigation, Writing - original draft.Jiatong Liu:Formal analysis, Investigation.Yajie Ge:Data curation, Formal analysis.Xiaoning Yan:Data curation, Formal analysis.Yi Sun:Methodology, Validation.Jian Wu:Methodology, Validation.Peng Zhang:Writing - review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the National Natural Science Foundation of China Project (No. 51408330 and 51922052), Natural Science Foundation of Shandong Province (No. ZR2018JL018) the Science and Technology Plans of Ministry Housing and Urbans-Rural Development of the People's Republic China, and Opening Projects of Beijing Advanced Innovation Center for Future Urban Design, Beijing University of Civil Engineering and Architecture (No. UDC2017031912). References [1] M.W. Urban, Dynamic materials: the chemistry of self-healing, Nat. Chem. 4 (2012) 1755–4349. [2] S.H. Lee, S.-R. Shin, D.-S. Lee, Self-healing of cross-linked PU via dual-dynamic covalent bonds of a Schiff base from cystine and vanillin, Mater. Design 172 (2019) 107774. [3] C. Chen, S. Qiu, M. Cui, S. Qin, G. Yan, H. Zhao, L. Wang, Q. Xue, Achieving high performance corrosion and wear resistant epoxy coatings via incorporation of noncovalent functionalized graphene, Carbon 114 (2017) 356–366. [4] M. Huang, J. Yang, Salt spray and EIS studies on HDI microcapsule-based self-healing anticorrosive coatings, Prog. Org. Coat. 77 (2014) 168–175. [5] J. Cui, X. Li, Z. Pei, Y. Pei, A long-term stable and environmental friendly self-healing coating with polyaniline/sodium alginate microcapsule structure for corrosion protection of water-delivery pipelines, Chem. Eng. J. 358 (2019) 379–388. [6] S. Qiu, W. Li, W. Zheng, H. Zhao, L. Wang, Synergistic effect of polypyrroleintercalated graphene for enhanced corrosion protection of aqueous coating in 3.5% NaCl solution, Acs. Appl. Mater. Inter. 9 (2017) 34294–34304. [7] X.M. Tong, T. Zhang, M.-Z. Yang, Q. Zhang, Preparation and characterization of novel melamine modified poly (urea–formaldehyde) self-repairing microcapsules, Colloid. Surface. A. 371 (2010) 91–97. [8] S. Lang, Q. Zhou, Synthesis and characterization of poly (urea-formaldehyde) microcapsules containing linseed oil for self-healing coating development, Prog. Org. Coat. 105 (2017) 99–110. [9] M. Haghayegh, S. Mirabedini, H. Yeganeh, Preparation of microcapsules containing multi-functional reactive isocyanate-terminated-polyurethane-prepolymer as healing agent, part II: corrosion performance and mechanical properties of a selfhealing coating, RSC Adv. 6 (2016) 50874–50886. [10] B. Qian, Z. Zheng, M. Michailidis, N. Fleck, M. Bilton, Y. Song, G. Li, D.G. Shchukin, Mussel-inspired self-healing coatings based on polydopamine coated nanocontainers for corrosion protection, Acs. Appl. Mater. Inter. 10 (2019) 10283–10291. [11] W. Li, Z. Jiang, Z. Yang, H. Yu, Effective mechanical properties of self-healing cement matrices with microcapsules, Mater. Design. 95 (2016) 422–430.

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