Preparation, with graphene, of novel biomimetic self-healing microcapsules with high thermal stability and conductivity

Abstract

This paper reports a comparative study of microcapsules with enhanced thermal stability and electrical conductivity inspired by the bionic thermal insulation of birds’ feathers for self-healing aged asphalt. The work is based on an in situ polymerization with composite shell components of graphene and hexamethoxymethylmelamine resin. By using graphene, microcapsules with rough surfaces are achieved, improving the interface between microcapsules and asphalt. In addition, the microcapsules’ initial thermal decomposition temperature is appropriately high, so that the stability of the microcapsule in the asphalt highway system is protected. The proportion of graphene in the microcapsule shell can regulate the microcapsule’s heat resistance because graphene modifies the shell’s structural makeup. Additionally, the microcapsules’ electrical conductivity is relatively high. The self-healing capability of bitumen sharply increases, providing benefit to the effect of microcapsules on the properties of aged asphalt.

Graphical abstract

Keywords

graphene / microcapsule / bitumen / heat insulation / conductivity

1 Introduction

Carbon material additives can effectively improve the electrical conductivity and thermal conductivity of asphalt composites, enhancing the self-healing and snow melting capabilities of asphalt-based materials [1,2]. The aging of asphalt, with poor dispersion of carbon nanomaterials, is accelerated when electric heating is applied repeatedly [3]. Therefore, it is important to research improvement of the self-healing and snow melting ability of asphalt-based materials. The research in the literature on asphalt aging shows that it is mainly caused by the loss of light components during service. Compared with other methods for repairing aging asphalt, microcapsule self-healing technology is efficient and environmentally friendly [4–7]. It is possible for aged bitumen to recover its original viscoelasticity via a rejuvenator, i.e., an engineered cationic emulsion. Rejuvenator should be well dispersed in the molten asphalt by the way of microencapsulation [8]. Rejuvenator can be coated with epoxy resin to produce microcapsules, so that it is difficult for the rejuvenator to leak quickly in case of cracks, and porous sand can be used as an adsorbent carrier [9,10]. To improve the compactness of the microcapsule shell, methanol etherified melamine formaldehyde resin can be applied to manufacture microcapsules by an in situ polymerization method [4]. Compared with epoxy capsules, the polymer microcapsules with regular morphology and smooth surface could be more evenly dispersed in asphalt. It has been shown that the core/shell mass ratio can be reduced by improvement in the shell density and heat resistance [5]. The repairing efficiency of microcapsules has been reduced due to less core material content. The microcapsules of calcium alginate shell have been successfully prepared, with porous structure [11,12]. The compressive strength of the shell may be influenced by the dehydration of the alginate cross-linking structure when the temperature exceeds 100 °C [13]. It is necessary to increase the integrity of microcapsule shell materials and reduce the core/shell ratio to better protect the rejuvenator in microcapsules; doing so may cause the shell structure of microcapsules to become hard and not easy to crack, creating counter-influence between the contributing capabilities of microcapsules and their own stability. This paper reports on inorganic compound microcapsules that can help to solve such problems.

When used in composite materials, graphene offers excellent properties regarding electrical conductivity, chemical corrosion resistance, mechanical properties, and heat resistance. Graphene is a two-dimensional planar structure material, formed by sp2 hybridization of carbon atoms, with excellent mechanical, thermal and electrochemical properties [14]. It has important value for forming composite materials with outstanding performance. So far, several ways have been developed to generate graphene, such as mechanical stripping, oxidation-reduction, and epitaxial growth, but the most commonly used method is chemical vapor deposition [15]. Research on graphene-epoxy nanocomposites suggests that the addition of graphene sheets could enhance electrical conductivity [16]. It can be seen that the dispersion of graphene in the epoxy has an influence on the dielectric characterization, which has been found to be associated with the electrical percolation threshold. Chen et al. [17] have demonstrated that addition of graphene to an epoxy matrix improves anti-corrosion and wear resistance properties. The non-covalent π–π interactions between dispersant and graphene layers can be harnessed to control their agglomeration. Owing to the oxygen containing functional groups of graphene oxide, chemical modification can be applied to produce composites. The mechanical properties of composites in terms of elastic modulus, tensile strength and elongation at break, and thermal stability have been improved relative to those of epoxy alone [18]. Huangfu et al. [19] fabricated the anti-magnetic interference epoxy nanocomposites containing graphene via a template-casting method. The results indicated that heat resistance index and electrical conductivity were increased to 171.3 °C and 52.1 S/cm, respectively.

The properties of graphene make it possible to design a conductive and heat-resistant microcapsule material. The designed microcapsules have excellent thermal stability and perfect conductive path in the matrix material, which has both self-healing performance and ice melting function. Unlike nano inorganic particles, graphene, as a flexible laminated material, does not become separated from the resin base, which can cause holes in the surface [20]. Zhang et al. [21] have demonstrated that the hierarchical honeycomb microstructures with graphene and ceramic can be used to achieve high electrical conductivity and excellent thermal-insulation performance simultaneously. Voids formed between graphene and ceramics due to pores provide the key to temperature resistance. The microcapsules shell has a fluffy and lamellar structure, which can increase the thermal insulation effect in a similar way to behavior of bird feather structure. The method has been applied in the field of architecture [22].

In this paper, a new preparation method is designed, using graphene content to change the morphology of microcapsules, so that they have high conductivity and temperature resistance. Different preparation methods for addition of graphene produce, with variations such as stirring speed and time, graphene content and core-shell ratio result in different influences on crosslinking structure of resin. Pure hexamethoxymethylmelamine (HMMM) resin is also generated for reference and the crosslinking process is observed. Two different microcapsule preparation methods were designed in the article. Under the same raw material ratio, microcapsule structures with different formation processes can be obtained. Some testing methods, such as scanning electron microscopy (SEM) testing, infrared spectrum testing, thermogravimetric testing and X-ray diffraction (XRD) testing, are used to analyze the structural variations of materials.

2 Experimental section

2.1 Materials

Graphene sheets were purchased from Suzhou Carbonium Graphite Technology Co., Ltd. (Suzhou, China). The prepolymer of highly methylated melamine formaldehyde resin (HMMM ≥ 98%), in situ polymerized on the surface of rejuvenator oil droplets, was supplied by Foshan Shengyi Biotechnology Co., Ltd. (Foshan, China). Styrene maleic anhydride (SMA) copolymer, as the emulsifier, was provided by Ashland (Shanghai, China). The rejuvenator of bitumen as the core material was obtained from Anshan Senyuan Co., Ltd. (Anshan, China). Sodium hydroxide (NaOH ≥ 99%) and acetic acid were used to adjust the pH values, and were purchased from Tianjin Chemical Factory (Tianjin, China). All the chemicals were used as-received without further purification.

2.2 First technique for fabricating graphene microcapsules (TGM-1)

SMA was added to an alkaline solution with a pH value of 10 at 40 °C, and mixed at 300 r/min for 2 h. Graphene sheets were dispersed in the prepolymer solution and treated by ultrasound for 30 min. Rejuvenator of aged bitumen was added into the surfactant solution containing graphene and dispersed into droplets using a high-speed shearing machine for 30 min. The homogeneously mixed prepolymer mixture was dropwise added into the suspension aqueous solution of rejuvenator and mixed at 300 r/min. The graphene sheets were dispersed around oil drops in the oil-in-water (O/W) system and fixed in the resin layer through the polymerization process of prepolymer molecules. Owing to electrostatic adsorption, HMMM molecules were continuously condensed on the surface of droplets. Acetic acid solution was applied to adjust the pH value of the solution to pH 3–4. The temperature of the solution was raised from 40 to 80 °C, in steps of 10 °C, being kept at constant temperature for 15 to 20 min at each step, and held at the higher temperature for 2 h. After crosslinking, the solution temperature was reduced to room temperature. The microcapsules were centrifugally filtered, cleaned, and dried. Finally, the product was obtained in the form of powder.

2.3 Second technique for fabricating graphene microcapsules (TGM-2)

Unlike in the first method, graphene was in two parts: one part is on the surface of oil droplets and the other part is in the resin layer. SMA was hydrolyzed and swelled in the alkali solution for 2 h. The rejuvenator was dispersed in the solution at high rate in the form of droplets. Meanwhile the solution containing graphene was dripped into the emulsion, where flake graphene was then adsorbed on the surface of the oil drops with strong specific surface area. Stable O/W emulsion was obtained by high-speed shearing. Adding the blend of HMMM prepolymer solution could be considered as the next important step in the experiment. Sufficient time was left for polymerization, which was crucial to the formation of the whole reaction. The pH value of the system was adjusted to 3–4 by adding acetic acid. The temperature of solution was increased at the same heating rate of the method of TGM-1. After curing reaction, the suspension was centrifuged. After being washed for many times, the solid was obtained and dried by vacuum filtration.

2.4 Characterization

SEM (SEM HITACHI SU8020) was applied to observe the surface morphologies. The dried and scattered microcapsules were adhered to conductive tape on the sample table. The microcosmic structures of microcapsules were characterized by SEM employing an accelerating voltage of 20 kV. Fourier transform infrared spectroscopy measurements (FTIR ThermoFisher Scientific Nicolet iS50) were performed on microcapsules to obtain the chemical functional groups of shell materials. The range of the infrared spectrum was 7800–100 cm⁻¹, and the precision of wavenumber was 0.005 cm⁻¹. X-ray diffractometer (XRD X’Pert Panalytical) with Cu Kα radiation (λ = 0.154 nm) was applied for qualitative analysis of phase and to assess the relationship between grain size and micro stress. The samples were evaluated by the XRD method to investigate microstructural changes and the effect of graphene on the resin matrix. The preparation mechanism of the two kinds of graphene microcapsules was analyzed according to the above three different test methods. The heat resistance of the samples was tested by thermogravimetric analyzer (TGA STA449F3 Netzsch) at the heating rate of 10 °C/min under N₂ atmosphere with a flow of 40 mL/min. The initial decomposition temperature and maximum weight loss temperature was obtained to analyze the stability of shell materials. The dynamic shear rheometer (DSR DHR-1 TA Instruments) test was applied to evaluate the self-healing ability of matrix asphalt and the asphalt containing microcapsules.

3 Results and discussion

3.1 Morphological investigation

The performance of microcapsules can be changed due to their structure. Based on the deformation and cracking process of microcapsules under external forces, it was found that the stress on microcapsules is affected by morphologies. Microcapsules shells are squeezed by the surroundings when they are acting on bitumen. The stress uniformity of microcapsules is limited by its smooth and regular spherical structure. Once the deformation occurs, there is internal stress, which is like a “weakness” in the whole. The distribution and adhesion of microcapsules in asphalt will also be changed by its surface structure. In this paper, the effect of graphene on shell materials of microcapsules is analyzed to study the influence of different preparing techniques. The initial crystalline structure of microcapsules is destroyed by the addition of graphene, creating new shell structures. To observe the morphology of microcapsules prepared by different processes, five typical specimens of microcapsules were generated, as shown in Tab.1. The proportion of graphene in all microcapsules was the mass ratio of graphene to resin (graphene/HMMM ratio). TGM-0, defined as the sample number of microcapsules without graphene, the vacuity contrast group used to compare the effect of graphene addition on the performance of microcapsules. The state distribution of graphene in the shell is influenced by the size of microcapsules. Therefore, the effect of microcapsule size on their performance was observed, and was changed by adjusting the dispersion speed.

Fig.1 illustrates the morphology of microcapsules which are added the content of graphene by the method of TGM-1. It can be seen that the shapes of microcapsules are deformed and the surfaces are not smooth. Outcome for bituminous rejuvenator embedded in ordinary resin is shown in Fig.1(a). It can be seen that the microcapsules are uniformly dispersed and the shell is regular and compact. Fig.1(b) shows the results for microcapsules with the content of 5 wt.% graphene. Unlike in the plain microcapsules, the agglomeration state is present in the microcapsules with graphene in the shell. The crosslinking structure of the shell was destroyed by the addition of graphene. In the experiment, with the increase of graphene content, the resin was combined with graphene to form a new structure. The oil droplets were not completely wrapped to generate microcapsules and they leaked, as shown in Fig.2(a). It means that the in situ polymerization of microcapsules was prevented by excessive addition of graphene.

TGM-2 is another method for preparation of microcapsules containing graphene, with graphene embedded in the resin shell to improve stability. Meanwhile, it can be seen that the surface of the sphere is rough and regular without adhesion, owing to high-speed dispersion of graphene, as shown in Fig.1(c). Tab.1 indicates that the amount of graphene added in the two preparation methods is the same. However, the results present quite different structures of the whole. With the increase of graphene content, the crosslinking structure was also influenced by excess graphene. Fig.2(b) provides the surface morphology of a microcapsule shell with 7% graphene. It can be seen that small encircling cracks appear on the surface of microcapsules, and these cracks affect the compact structure of microcapsules. The surface of the oil droplets acted so as to build graphene into a multilayer structure. The local internal stress in the shell structure was caused by the increased graphene during the subsequent resin polymerization, resulting in the appearance of debris on the surface of shell.

In previous research, the results have shown that the size of microcapsules is affected by stirring speed and core/shell ratio, and especially by the stirring speed [5]. In the present preparation processes, the distribution of graphene is influenced by the size of microcapsules, and both change the stability microcapsules. The specimens TGM-1-2 and TGM-2-2, as shown in Fig.1(d) and Fig.1(e), were produced to compare the performance of microcapsules. The results from the current paper indicate that reducing the stirring speed can indeed increase the size of microcapsules. However, when the stirring time is controlled for more than 30 min, the stirring speed is no longer the main factor. In other words, the size of microcapsules depends primarily on the effect of stirring speed only within the 10–30 min dispersion time.

Fig.3 illustrates the macroscopic state of microcapsules containing 0 and 5% graphene, which were generated by two methods. Compared with the microcapsules sample TGM-1-1, it can be shown that the sample TGM-2-1 is dull-colored and has better dispersion due to the preparation processing. The graphene in TGM-2 is embedded in the possible interior, and so it has little effect on the morphology and structure of microcapsules under the condition of a certain proportion. The obviously different point, perhaps, is that the probability of fragmentation of microcapsules produced by the method of TGM-2 is much less than that of TGM-1. From Fig.2(b), it can be seen that the surface of microcapsules prepared by TGM-2 method appears to be similar to chapping.

3.2 Shell structure

Crosslinking and curing reactions are hindered by the addition of inorganic substances. Previous reports have studied nanoparticles used for the synthesis of microcapsules to form the inorganic shell [20]. The results of surface observation have shown that a porous structure is generated, owing to the falling off of nanoparticles, reducing the sealing property of microcapsules. It can be concluded that the structure of resin shell is affected by the morphology and characteristics of the inorganic materials. In the current work, FTIR and XRD were also used to test the effect of graphene on the structure of the microcapsule resin layer. Fig.4 illustrates that no new kind of chemical bond was formed in the two processes. In other words, both are entanglement and bonding of physical actions. The samples have similar characteristic absorption peaks, for which the stretching vibrations of C−H and N−H bonds are 2925 and 1578 cm⁻¹, respectively. Besides that, there is bending vibration of C−H bond at 699 cm⁻¹. No matter which method is used, resin layer structure of microcapsule is indeed influenced by the addition of graphene, as shown in Fig.5. Compared with the control group, the types of crystal structure of microcapsules modified with graphene were increased. The result is related to the composite structure of graphene and resin, and whether the graphene was inserted on the surface or inside of shell. Compared with no graphene, the continuity and integrity of the resin shell would be destroyed by the addition of graphene. The one similar point, perhaps, is that the wide main peak appears in each test result of Fig.5, which is caused by the presence of rejuvenator. Compared with the microcapsule shell prepared by the method of TGM-1, the impact of shell produced by the method of TGM-2 is larger. The spectrum of XRD did not shift to the right, which ruled out the separation of graphene layers with small molecular structure, resulting in honeycomb structure and thermal insulation [23].

Fig.6(a) shows the growth process of HMMM resin with high degree of methyl etherification. Resin molecules polymerized along the beaker wall to form polymers. The polymerization process of resin small molecule is a transition from needle like structure to multi-layer sheet. It can be seen that the prepolymer needs physical support when crosslinking to form macromolecules. Physical support, or framework, for resin polymerization is provided by the oil drops in the reaction process. In situ polymerization was carried out on the surface of oil droplets in the form of mixture that is composed of graphene dispersed by ultrasound and resin prepolymer. According to the zeta potential test, the surface of a microcapsule is electronegative, while the prepolymer is electropositive. Electrostatic force between them leads to the formation of dense structure. The two-dimensional lamellar structure of graphene is stacked on the surface of microcapsules under the traction of prepolymer molecular structure. Meanwhile, a small convex structure is produced due to the growth of small molecular structures on the surface of core materials. Some small microcapsules have been formed on the surface of the carrier by the polymerization of free small molecules and generate a dense layer to coat the large-scale microcapsule.

Unlike the microcapsules prepared by the method of TGM-1, microcapsules prepared by the method of TGM-2 were coated with graphene on the surface of oil drops. Nanoparticles were adsorbed on the surface of the core materials, according to the previous research. The prepolymer material was inserted through a gap between two particles in the case of in situ polymerization [24,25]. It can be concluded that graphene can enhance the corrosion resistance of resin through the laminated structure. The graphene layer was penetrated by resin molecules to form a sandwich structure which fixed graphene layer to prepare the inorganic/organic composite [26]. In the preparation of a sample, graphene was attached to the surface of the oil droplets, resulting in a graphene film structure at the interface between the oil phase and the water phase, and then the prepolymer molecules penetrated into the porous structure to create the composite shell. The rough outer surface of microcapsules was beneficial for the interaction between bitumen and microcapsules [27]. Compared with TGM-1 microcapsules, the microcapsules prepared by the method of TGM-2 not only ensure the roughness of the surface caused by graphene, but also make the microcapsule particles have better sealing performance and reduce the damage to the resin layer.

3.3 Thermal stability

According to the above research results, a new chemical bond was not formed between the added graphene and the resin matrix. The thermal stability of microcapsules was affected by the change of structure that was caused by the addition of graphene. It can be concluded that the heat resistance of organic compounds has been improved by the addition of inorganic materials [28]. Fig.7(a) illustrates that the initial decomposition temperature of pure resin is 99.6 °C. According to the above research, it can be seen that the resin matrix is a dense layer formed by continuous adsorption and stacking polymerization on the surface of core material droplets, while three-dimensional compact structure cannot be produced by the pure resin sheet. The loose system is caused by the change of degree of polymerization reaction, resulting in the poor heat resistance of the resin. In other words, it is precisely because of their structure that the microcapsules have high temperature resistance. The core material is protected by the dense structure from external interference.

Fig.7(b) illustrates the temperature condition that results in the shell rupture of microcapsules without graphene. It can be seen that the initial decomposition temperature is 143 °C; at that temperature it is not guaranteed that the microcapsules are intact in the molten asphalt. Although their survival rate under high temperature has been enhanced, compared with that for pure resin, the high temperature resistance of microcapsules needs to be improved given the application of microcapsules in asphalt and the technological conditions of paving asphalt concretes on pavement. It is also a goal of this study to improve the thermal stability of microcapsules without reducing the content of core material. Fig.7(c) indicates the behavior of thermal decomposition of TGM-1-1 microcapsules at high temperature. It can be seen that the thermal stability of microcapsules has been improved by adding 5% graphene and the initial decomposition temperature is 283 °C. However, compared with pure resin, the addition of graphene destroys the polymerization structure of the shell of microcapsules, which affects the integrity of the resin and reduces the heat resistance temperature, according to the above research. Therefore, the heat resistance of microcapsules will be improved by the addition of only a small amount of graphene which balances the structural changes. Fig.7(d) shows that the circumstances of high temperature resistance of microcapsules prepared by method TGM-2-1. It has been found that the initial decomposition temperature of microcapsules prepared by method TGM-2, 261 °C, is slightly lower than that of method TGM-1. It means that the decomposition of the resin molecules in the inner layer will be protected by the graphene in the outer layer, thus improving the heat resistance. Previous studies have shown that the core materials could be protected from leakage. According to the test results of XRD, the dense and uniform structure of the resin layer is destroyed by excess added graphene. TGM-1 method has more serious damage to the compact structure of the resin, and its initial cracking temperature is higher.

As mentioned above, the thermal stability of microcapsules is influenced by overall structure and varies with the size of microcapsules. Fig.8(a) shows thermal stability of microcapsules prepared by the method TGM-1 with an oil droplet shear dispersion rate of 500 r/min. It can be seen that the initial decomposition temperature of the microcapsules at 302 °C, with the decrease of the dispersion speed is larger than that of the sample of TGM-1-1, although the morphology of the microcapsules becomes more regular and the size becomes larger. With regard to the previous research theory, the results were expected to be different from the conclusion that the smaller the size of microcapsules without graphene, the higher the initial pyrolysis temperature. Owing to altering the stirring speed, the thickness of resin layer of sample TGM-1-2 is smaller is smaller than that of sample TGM-1-1. From previous studies, it might be expected that the initial decomposition temperature of sample TGM-1-2 is lower than that of sample TGM-1-1 according to previous studies. The above data analysis shows that their better heat resistance is caused by the microstructural regularity of microcapsules. The same results have been found in microcapsules TGM-2-2, as shown in Fig.8(b). Compared with microcapsules prepared by TGM-2-1 method, the initial decomposition temperature of TGM-2-2 is higher, which is 293 °C. The distribution of graphene on the surface of the microcapsules is influenced by the size. The state of the shell layer of the microcapsules is the main reason for the high temperature stability. In the absence of inorganic substances, the thermal stability of the microcapsules is affected by the thickness and compactness of the dense layer of the resin matrix. When the inorganic substances that play the role in heat insulation and temperature resistance exist, graphene and the resin layer cannot be fully combined to form a gap layer. With increase of microencapsulated particles, the content of graphene adsorbed on the surface is increased, and the thermal stability is enhanced. As we all know, birds’ fluff formed by the superposition of scales has a certain insulating effect, and the hollow structure is isolated from air flow, so that internal temperature is protected by air with small thermal conductivity [22]. The addition of graphene in the resin has a similar effect, as shown in Fig.9. The interior of the resin “fluffiness” was produced by the lamellar structure, resulting in the resin having the effect of heat insulation and temperature resistance, protecting the internal core material from high temperature.

3.4 Electrical conductivity

Based on previous research results, the addition of graphene improves the electrical conductivity of the resin structure. In other words, the conductive microcapsules can form a conductive path when acting on asphalt. An environment-friendly conductive, high-temperature- and corrosion-resistant system can be produced to facilitate subsequent research. In this paper, the resistivities of the microcapsules of TGM-1-2 and TGM-2-2 were measured by the powder resistivity tester. The resistivity of TGM-2-2 microcapsule was found to be large, and beyond the test range of the instrument, while the resistivity of TGM-1-2 microcapsule was 40.68 Ω·m. The adhesion of graphene on the surface of microcapsules could be effectively increased by the method of TGM-1.

3.5 Self-healing ability

The purpose of research of this kind of heat-resistant microcapsule with high core/shell ratio is to prolong the service life of asphalt pavement, and so reduce the maintenance cost and resource consumption of the pavement [29]. The self-healing effect on asphalt of microcapsules is also an important feature. The self-healing ability of aged asphalt containing microcapsules that were prepared by the method of TGM-1-2 and had excellent thermal stability was evaluated by DSR test. The self-healing index (HI) was introduced to characterize the repair ability of aged asphalt [30]. Fatigue cracking occurred in the samples under shear stress at a certain pressure (0.3 MPa) and temperature (25 °C). The whole process can be divided into the following three steps, as shown in Fig.10. (1) The microcracks of specimen were produced under the applied stress and the microcapsules were broken. In this process, the complex modulus decreased rapidly until it reached 40% of the original modulus. (2) In the second stage, the recovery period of one hour was applied to the sample. The complex modulus of asphalt would recover to a certain level during this time period. For aged asphalt, the recovery of initial properties was accelerated due to the penetration and diffusion of rejuvenator which flowed out due to the rupture of microcapsules. (3) The loading mode of the first stage to test would be repeated, resulting in the complex modulus of the test sample. The HI of asphalt containing microcapsules was calculated using Eq. (1):

(1)HI=\frac{G_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}}^{\ast }-G_{\mathrm{d}\mathrm{a}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}}^{\ast }}{G_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}^{\ast }-G_{\mathrm{d}\mathrm{a}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}}^{\ast }}\times \frac{t_1}{t_2},

where G_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}}^{\ast }, G_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}^{\ast }, and G_{\mathrm{d}\mathrm{a}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}}^{\ast } represent the modulus after healing, the initial modulus, and the damage modulus, respectively; t₁ is the time required for the first stage modulus to change from G_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}}^{\ast } to G_{\mathrm{d}\mathrm{a}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}}^{\ast }; t₂ is the time required for the third stage modulus to change from G_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}}^{\ast } to G_{\mathrm{d}\mathrm{a}\mathrm{m}\mathrm{a}\mathrm{g}\mathrm{e}}^{\ast }.

The time factor was taken into account in the calculation of HI in this paper. It could be seen that the decreasing rate of modulus of asphalt before and after intermission is related to loading time. The repair capability is affected by the complex modulus and the loading time. The results of HI are shown in Fig.11, which the HI of original aged asphalt and aged asphalt containing microcapsules was 23% and 33%, respectively. In other words, the repair capability of aged asphalt was enhanced by the addition of microcapsules.

4 Conclusions

Microcapsules were successfully prepared with high heat resistance inspired by birds’ feathers. The composite shell was generated by an in situ polymerization with HMMM resin and graphene. The thermal stability and conductivity of microcapsules were improved by the protective structure. The following conclusions can be drawn.

1) The morphological characteristics of microcapsules have changed rather than regular spheres, compared to Fig.1(a) and 1(b). The stress distribution of the shell was altered by the morphological changes of microcapsules, which was caused by the addition of graphene. The investigation shows that the dispersion speed had little effect on the size of microcapsules when the dispersion time was extended to 30 min in the preparation of microcapsules.

2) The heat insulation of the microcapsules was improved by the addition of graphene. The two methods of preparing microcapsules with graphene were physical entanglement and adsorption in resin base. The cross-linking of resin layer was hindered by the increased graphene, resulting in destruction of the dense shell of the microcapsule. A thermogravimetric experiment study on microcapsules with graphene was performed. The thermal stability of sample TGM-1-2 was better than that of the sample TGM-2-2. The high temperature resistance of the two types of microcapsules was determined by the distribution form of graphene in shell, and the size of the particle was not the decisive factor. Considering that the practical temperature of asphalt would not exceed 200 °C, the microcapsules prepared by the two methods were found to be suitable for application.

3) The addition of graphene ameliorates the conductivity of the microcapsule shell. Due to the adhesion of graphene on the surface of microcapsules prepared by TGM-1 method, the microcapsules have certain electrical conductivity. The resistivity of new graphene microcapsules could reach 40.68 Ω·m.

4) The self-healing capability of aged asphalt was improved by adding graphene microcapsules. Based on the above research, microcapsule particles prepared by TGM-1 method were selected as the material for repairing aged asphalt. The self-healing ability of asphalt was determined by DSR. The cracking of microcapsule shells was not affected by the addition of graphene, which laid a foundation for the follow-up study of the electrical conductivity and thermal conductivity of materials.

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