Introduction

Steel and other metal-based materials are susceptible to corrosive damages upon long-term environmental exposure, which seriously affects their service lives1. Among various anti-corrosion methods, the utilization of polymeric coatings is one of the most popular routines. However, conventional coatings are inevitable to suffer from environmental and mechanical damage during transportation and daily usage. Failure to promptly and effectively repair these surface damages can result in the penetration of corrosive mediums such as water and oxygen into the bulk metal region, leading to severe internal corrosion2,3. In 2001, White et al.4 prepared dicyclopentadiene (DCPD) microcapsules, which were incorporated into epoxy resin to autonomous repair microcracks. Since then, researchers have developed different self-healing microcapsule systems4,5,6,7,8,9,10,11,12 and successfully applied them in epoxy coatings. In principal, when the coating is damaged, these microcapsules would automatically break, releasing the healing agent onto the crack surface where it undergoes polymerization to form a protective film that repairs the coating. Benefiting from the intelligent response to coating damage and self-healing capabilities, the rational design and high-throughput synthesis of microcapsule systems have become the research frontier13,14,15,16.

According to the types of healing agents, self-healing microcapsules can be categorized into corrosion inhibition, silicone grease, epoxy, siloxane, drying oil, and isocyanate types5. Among them, the isocyanate-type microcapsules have gained significant attention due to their distinctive self-healing properties (initiating the repair process without requiring a catalyst), which could greatly simplify the self-healing system17. The isocyanate core material exhibits high reactivity and rapidly reacts with water in the surrounding environment to generate polyurea macromolecules, thus effectively healing microcracks18,19. Therefore, abundant literatures focusing on the isocyanate-type microcapsules have been reported in recent years20,21,22,23,24,25,26,27,28,29,30,31,32. Sottos et al.20 introduced microcapsules containing a single diisocyanate and applied them in self-healing coatings. When the coating was damaged, the released diisocyanate reacted with environmental water to form a polymeric film to repair cracks, achieving effective anti-corrosion effects. Alizadegan et al.33 prepared isocyanate microcapsules by interfacial polymerization using polyurethane as wall material and isoporone diisocyanate as core material. The prepared microcapsules exhibited a particle size of 50–200 μm and a wall thickness of 2–20 μm, demonstrating appropriate healing performance when using this type of microcapsules for scratch tests. Lu et al.34 prepared polyurea coated IPDI microcapsules by using dodecyl benzene sulfonic acid (SDBS) and polyvinyl vinylpyrrolidone (PVP) as a compound emulsifier to form an oil-in-water system. The core material content decreased from 65.7 to 44.7% after soaking in water for 10 days. Haghayegh et al.35 synthesized IPDI microcapsules through interfacial polymerization, which were incorporated into epoxy resin to prepare a self-healing organic coating. The corrosion resistance and self-healing ability were investigated, demonstrating that the microcapsules could effectively protect the damaged coating areas against corrosive media. Moreover, a remarkable self-healing efficiency of 95% was achieved in the epoxy coating when the mass fraction of microcapsules was 10 wt.%.

When isocyanate microcapsules are applied to coatings, they will inevitably encounter challenges such as high temperatures, mechanical force and humid environment17. However, it is challenging for single-layered isocyanate microcapsules to meet all these requirements for practical applications. Currently, modifying the properties of shell materials to improve the overall resistance of isocyanate microcapsules have emerged as a primary strategies to circumvent these issues. A rationally designed double-layer shell structure for microcapsules can integrate the advantages of multi-wall materials to endow microcapsules with superior mechanical properties and stability, enabling a better application perspective. Credico et al.24 prepared double-layered IPDI microcapsules with urea–formaldehyde resin (PUF) as the outer shell and polyurethane (PU) as the inner shell using the toluene diisocyanate prepolymer, 1, 4-butanediol, ammonium chloride and formaldehyde as raw materials. The prepared bivalved microcapsules exhibited excellent thermal stability and extended shelf life. Sun et al.17 synthesized double-layer polyurea encapsulated hexamethylene diisocyanate (HDI) microcapsules by interfacial polymerization. The microcapsules exhibited a slight weight loss of 1.6 wt.% after being heat-treated at 100 °C for 60 min, and a core material loss of ~ 3 wt.% in weakly polar solvents and 87 wt.% in water, respectively. When 10 wt.% microcapsules were applied to an epoxy coating for scratch corrosion tests, the scratch area showed minimal corrosion within 24 h. Hu et al.36 prepared IPDI microcapsules with a dual shell polyurethane (PU)/melamine formaldehyde (MF) using a two-step method, incorporating butanediol (BDO), polyethylene glycol (PEG) 400, PEG 1000, and PEG 2000 as chain extenders. The influence of different chain extenders on the mechanical properties of IPDI microcapsules was investigated. Song et al.37 synthesized IPDI microcapsules coated with double-shell polyurethane (PU)/urea–formaldehyde resin (PUF) by interfacial polymerization and in-situ polymerization methods, and studied the impacts of process parameters (e.g. agitation rate, pH, core-to-shell weight ratio and temperature) on the synthesis of microcapsules. The microcapsules were then incorporated into epoxy coatings to evaluate their excellent self-healing properties for corrosion protection. Chen et al.38 prepared microcapsules coated with a hexamethylene-diisocyanate (HDI) solution of aggregation-induced emission luminogens (AIEgens), employing a two-step of interfacial polymerization and in-situ polymerization method to form a dual shell comprising polyurethane acrylate (PUA)/urea–formaldehyde (PUF). Their structural morphology, chemical composition and thermal stability were investigated systematically. Furthermore, the coating containing these microcapsules exhibited distinctive crack self-healing and self-warning functions.

Despite significant advances have been achieved in double-layer isocyanate-based microcapsules, most researches could only improve partial properties of microcapsules. In other word, achieving simultaneous enhancement of all these critical properties remains a persistent challenge. Therefore, to fulfill the practical requirements of self-healing coatings, isocyanate-based microcapsules should exhibit excellent mechanical performance, thermal resistance and stability properties. Due to the presence of a benzene ring in its molecular structure, phenolic resin exhibits enhanced stability owing to its large π bond, thereby improving its strength and rigidity. Additionally, its chemical stability and high temperature resistance, rendering it a decent choice as wall materials. Herein, a novel method for the preparation of double-layer shell phenolic resin (PF)/polyurethane (PU)—isophorone diisocyanate (IPDI) microcapsules was developed, which exhibited superior mechanical, thermal and aging properties. Its anti-corrosion performance in epoxy coatings was further comprehensively investigated.

Experimental

Materials

Isophorone diisocyanate (IPDI), 1, 4-butanediol (BDO), gum arabic (GA), epoxy resin (E-51) were purchased from McLean. Toluene diisocyanate prepolymer (Bayer L-75) was provided by Beijing Kemit Technology Development Co., Ltd. Ethyl acetate (EA), dimethyl silicone oil (PDMS) and anhydrous ethanol were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Tetrachloromethane was purchased from Tianjin Shentai Chemical Reagent Co., Ltd. Diethylenetriamine (DETA) and hydrophobic nano silicon dioxide were supplied by Aladdin. Alcohol-soluble phenolic resin and NL curing agent were purchased from Meisheng Plasticizing Co., Ltd. All the reagents were used directly without further purification.

Synthesis of single-layer PU-IPDI microcapsules

Firstly, 50 g of deionized water and 3.33 g of GA were added to a 250 ml three-mouth flask, and an aqueous solution was formed after stirring with a mechanical agitator at 500 r/min for 3 h at room temperature. Then, 2.75 g of L-75, 11.33 g of IPDI and 2.83 g of EA were dissolved to form a uniform oil phase solution. The oil phase solution was dropped into the aqueous solution and emulsified at a stirring speed of 600 r/min for 30 min to form an oil-in-water (O/W) emulsion. Then 2.25 g of BDO was added to the emulsion dropwise. The reaction temperature was raised to 50 ℃ for 1.5 h to obtain a microcapsule suspension. The PU-IPDI microcapsules were obtained by filtration, washing and drying, respectively.

Synthesis of double layer PF/PU-IPDI microcapsules

Firstly, 2.4 g hydrophobic nano silica was dispersed in 80 g PDMS to form a continuous phase dispersion. Then 1 g alcohol-soluble phenolic resin and 0.15 g NL curing agent was mixed in 2 g anhydrous ethanol to form a solution. 0.75 g PU-IPDI microcapsules were added into the solution and dispersed homogeneously. The mixture was added to PDMS and reacted at room temperature for 12 h at 500 r/min to obtain double layer PF/PU-IPDI microcapsules. The PDMS continuous phase was removed by centrifugation. Then PF/PU-IPDI microcapsules were obtained after being washed, filtered and dried in sequence.

Synthesis of self-healing coating containing PF/PU-IPDI microcapsules

Q235 steel was selected as the metal substrate in the experiment, which was polished with sandpaper, washed with deionized water and anhydrous ethanol respectively and dried prior to measurements. Epoxy resin (E51) was chosen as the coating matrix, and the curing agent DETA was added at a weight ratio of 10 wt.%. Firstly, epoxy resin was blended with DETA and thoroughly stirred for 10 min, followed by the addition of 10 wt.% PF/PU-IPDI microcapsules. Subsequently, the resin mixture was uniformly coated onto the steel substrate using a rod coating device at room temperature. The prepared samples were then placed in oven at 50 °C and dried for 24 h. The resulting coating thickness was around 500–600 µm. The blank coating was prepared followed the same conditions without adding self-healing microcapsules.

Characterization of PF/PU-IPDI microcapsules

The particle size, surface morphology, structure and shell thickness of prepared microcapsules were analyzed by scanning electron microscopy (SEM, JSM-7900F) and optical microscope (OM, DM2700P). The chemical composition of microcapsules was analyzed by Fourier transform infrared spectroscopy (FTIR, BRUKER TENSORII). The mechanical properties were analyzed by depth-sensing indentation analysis (DSI, Hysitron TS 77 Select). The thermal properties and core material content of microcapsules were studied by thermogravimetric analyzer (TGA, SETSYS EVOLUTION TGA 16/18). The temperature was raised from 25 to 600 ℃ in an argon atmosphere at a heating rate of 10 ℃/min. The thermal aging experiment of microcapsules was conducted by thermogravimetric analyzer (TG, SETSYS EVOLUTION TGA 16/18). The microcapsules were kept at 200 ℃ in an argon atmosphere for 8 h to measure the weight loss. Water immersion experiment was carried out to study the water penetration resistance of microcapsules. The microcapsules were immersed in deionized water for a period and dried at 60 ℃. Then the core material content of microcapsules was analyzed by a thermogravimetric analyzer.

Characterization of coating properties

The adhesion force of epoxy coating was tested using a QFZ film adhesion tester in accordance with the standard GB/T 1720-79(89). The impact performance was measured using a QCJ film impact tester in accordance with the standard GB/T 1732-1993. The roughness property was tested using a QTX film roughness tester in accordance with the standard GB/T 1731-79.

Tensile strength properties of the filled coatings

According to GB/T 1040-2006 standard test practice, the tensile strength of epoxy coating samples containing PF/PU-IPDI microcapsules were researched using a universal testing machine (SUNS UTM5205). At room temperature, the tester performed tensile tests on the sample at a speed of 2 mm/min. For each formulation, at least 3 duplicate samples were tested for statistical accuracy. A scalpel blade was used to create a crack in the specimen perpendicular to the drawing direction, with a length of 2 cm and a depth of about 100 μm. Tensile strength tests were carried out before, immediately and 24 h after crack creation. The healing efficiency of microencapsulated coated samples was calculated by comparing the tensile properties of the samples before and after crack creation.

Results and discussion

The synthesis mechanism of PF/PU-IPDI microcapsules

The synthesis mechanism of double-layer PF/PU-IPDI microcapsules is illustrated in Fig. 1. Firstly, toluene diisocyanate prepolymer (L-75), IPDI were dissolved with ethyl acetate (EA) to form an oil phase solution. GA was dissolved in water as an emulsifier to form a water phase solution. Then these two solutions were mixed and stirred to generate an oil-in-water emulsion. When chain extender BDO was added to the emulsion, polymerization reaction occurred at the oil–water interface, resulting in the formation of a polyurethane film that encapsulated the inner IPDI. Subsequently, the prepared PU-IPDI microcapsules were added into the solution composed of phenolic resin, NL curing agent and anhydrous ethanol. This mixture was then added into the PDMS dispersed with hydrophobic nano silica and continuously stirred. Due to the self-emulsification effect of PDMS, combined with the isolation and anti-sedimentation effect of nano silica, a layer of phenolic resin was coated onto PU-IPDI microcapsules using an ethanol solution. Upon crosslinking of the phenolic resin and NL curing agent, macromolecules would be formed and a layer of phenolic resin would be deposited onto the surface of PU-IPDI microcapsules, resulting in the formation of double-layer PF/PU-IPDI microcapsules.

Figure 1
figure 1

The schematic diagram of double-layer PF/PU-IPDI microcapsules.

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Morphology of microcapsules

The morphology of single-layer PU-IPDI and double-layer PF/PU-IPDI microcapsules is compared in Fig. 2. Owning to the colorless and transparent feature of polyurethane shell and inner isocyanate core, the single-layer PU-IPDI microcapsules exhibited as white powder under natural light (Fig. 2a). Comparatively, the outermost layer of phenolic resin on PF/PU-IPDI microcapsules exhibited a distinct pink coloration under natural light (Fig. 2c). Under the optical microscope, the single-layer PU-IPDI microcapsules exhibited irregularly spherical morphology with random surface collapse. The average particle size of these single-layer microcapsules were about 10–100 microns, as shown in Fig. 2b. The double-layer PF/PU-IPDI microcapsules exhibited similar morphology with slight variations compared with PU-IPDI microcapsules, except for a slightly pink color due to the outside phenolic layer of the microcapsules, as shown in Fig. 2d.

Figure 2
figure 2

Digital images of (a) PU-IPDI powder, (b) PF/PU-IPDI powder; optical micrographs of (c) PU-IPDI, (d) PF/PU-IPDI.

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In order to further investigate the surface morphology of prepared microcapsules, scanning electron microscopy (SEM) was employed for both single-layer PU-IPDI and double-layer PF/PU-IPDI microcapsules, as demonstrated in Fig. 3. The surface of PU-IPDI microcapsules appeared relatively smooth and compact, exhibiting noticeable depression areas in the shell material. Comparatively, the surface of double-layer PF/PU-IPDI microcapsules exhibited similar smooth while denser morphology. In addition, due to the presence of coated phenolic resin layer, any pits on the surface of microcapsules were gradually smooth out over time.

Figure 3
figure 3

Surface morphology of (a) PU-IPDI microcapsules and (b) PF/PU-IPDI microcapsules.

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The shell structures of these two isocyanate-based microcapsules were investigated in more details by crushing and observing them via SEM, as shown in Fig. 4. It is shown that the polyurethane shell of PU-IPDI microcapsules exhibited a single-layer structure with a thickness of 3–5 μm, as highlighted in Fig. 4a and b. On the other hand, the shells of PF/PU-IPDI microcapsules consisted of a double-layer structure, with a thickness of 3–5 μm for the inner polyurethane shell and ~ 1 μm for the outer phenolic resin layer, as shown in Fig. 4c and d. It is noted that there existed a distinctive interface between these two shell layers.

Figure 4
figure 4

The SEM images of crushed (a) PU-IPDI microcapsules, (b) magnified image of PU shell, and SEM images of (c) PF/PU-IPDI microcapsules, (d) magnified image of PF/PU shell.

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Moreover, the particle size distribution of PF/PU-IPDI microcapsules was analyzed via Nano measurer software based on overall 225 microcapsules, as summarized in Fig. 5. The measurement demonstrated a typical Gaussian distribution with the microcapsule size distribution ranging from 30 to 130 μm, and an average particle size of 75.37 μm with a standard deviation of 16.33 μm. Most microcapsules were found within the range of 50–100 μm, as shown in the inset of Fig. 5.

Figure 5
figure 5

Size distribution analysis of PF/PU-IPDI microcapsules.

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Determination microcapsules components

The chemical composition of PF/PU-IPDI microcapsules was analyzed using Fourier transform infrared spectroscopy (FTIR), which was performed on core material, shell materials and microcapsules respectively, as summarized in Fig. 6. In the infrared spectrum of the core material, the absorption peak at 2265/cm corresponds to the stretching vibration absorption peak of –N=C=O, while the absorption peak at 2958/cm represents the stretching vibration peak of –CH, and the absorption peak at 1462/cm indicates the shear vibration peak of –CH in isophorone diisocyanate. In the infrared spectrum of shell materials, absorption peak at 3360/cm is the stretching vibration peak of –OH and –NH, the absorption peak at 1560/cm is the stretching vibration peak of benzene ring –C=C–, and the characteristic peak of –NH–CO– is at 1640/cm. Analysis of double layer PF/PU-IPDI microcapsules revealed a distinctive absorption peak at 2265/cm, confirming their inclusion of IPDI core materials. Furthermore, the presence of absorption peaks at both 1640/cmand 1560/cm proved that phenolic resin and polyurethane were present in their shell materials. By comparing the IR spectra of microcapsules, core and shell materials, the IR spectrum of microcapsules was the superposition or combination of core and shell materials, which validated that the isophorone diisocyanate was successfully encapsulated in the double-layer PF/PU shell materials.

Figure 6
figure 6

The infrared spectra of shell materials, core material and PF/PU-IPDI microcapsules.

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The SEM image in Fig. 7 showed the crushed double-layer PF/PU-iIPDI microcapsules, clearly revealing extensive cracking and leakage of liquid core materials, thereby providing further evidence for the presence of IPDI core material within the microcapsules.

Figure 7
figure 7

The SEM image of cracked PF/PU-IPDI microcapsules.

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Mechanical property of microcapsules

The mechanical properties of single-layer PU-IPDI and double-layer PF/PU-IPDI microcapsules were investigated and analyzed. These two types of dried microcapsules were placed on a tablet press (769YP-24B), via which a pressure of 0.1 MPa was applied. The morphology of the pressed microcapsules was observed by SEM (Fig. 8). The single-layer PU-IPDI microcapsules were severely damaged as most microcapsules were collapsed and crushed to pieces, and the initial spherical state was no longer existed as shown in Fig. 8a. Comparatively, the overall damage degree of double-layer PF/PU-IPDI microcapsules was relatively milder. Some of the microcapsules were collapsed, while others only exhibited slight cracks, allowing for the preservation of their overall structure to a certain extent. This deformation results demonstrated that the mechanical properties of double-layer PF/PU-IPDI microcapsules were greatly improved duo to the incorporation of phenolic resin outer layer. By highlighting the magnified images of the shell regions, the single-layer polyurethane showed a brittle fracture surface as shown in Fig. 8c. In contrast, the phenolic resin outer layer contained a large number of benzene rings with a conjugated structure that guarantees the uniform distribution of π electrons within these rings. Therefore, the benzene ring structure remained relatively stable and could enhance the strength and rigidity of phenolic resin, enabling it to withstand high temperature and pressure effectively. When the double-layer PF/PU-IPDI microcapsules were pressed, initial failure occurs in the inner polyurethane layer followed by the crack propagation into the outer layer. However, due to its high strength and stiffness, the phenolic resin out layer is capable of withstanding greater pressures and preventing further crack spreading, thus avoiding the microcapsules structural failure, as highlighted in Fig. 8d.

Figure 8
figure 8

SEM images of microcapsules after pressure, (a) PU-IPDI; (b) PF/PU-IPDI; (c) fracture surface of PU shell; (d) fracture surface of PF/PU shell.

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In order to accurately evaluate the mechanical properties of microcapsules, depth-sensing indentation analysis was conducted on PU-IPDI and PF/PU-IPDI microcapsules respectively, as shown in Fig. 9. The detailed testing data are shown in Table 1. Under a maximum force of 300 μN, the single-layer PU-IPDI microcapsule exhibited a maximum probe depth of 306.2 nm, a contact depth was 245.5 nm and a contact stiffness of 3.7 μN/nm. Similarly, the double-layer PF/PU-IPDI microcapsules demonstrated a maximum probe depth of 171.1 nm, a contact depth of 118.8 nm, and a contact stiffness of 4.3 μN/nm. The elastic modulus of PU-IPDI microcapsules was measured to be 2.48 GPa, with a corresponding hardness of 172.85 MPa. In contrast, the double-layer PF/PU-IPDI microcapsules exhibited significantly enhanced mechanical properties, with an elastic modulus and hardness of 5.44 GPa and 618.06 MPa respectively. These findings highlight the substantial improvements in strength, stiffness and hardness through the incorporation of a phenolic resin outer layer on the microcapsules' surface. Specifically, the presence of this phenolic resin outer layer resulted in a remarkable increase in both elastic modulus and hardness by 2.2 and 3.6 times, respectively, compared to single-layer microcapsules alone. As a result, the double-layer microcapsules exhibit enhanced resistance against damage during storage and operation, ensuring reliable self-healing capabilities.

Figure 9
figure 9

Depth-sensing indentation tests of microcapsules.

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Table 1 Depth-sensing indentation test results of microcapsules.
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Thermal property and core fraction of microcapsules

The thermal properties of inner IPDI core, PU shell materials, PF/PU shell materials, single-layer PU-IPDI microcapsules and double-layer PF/PU-IPDI microcapsules were analyzed via thermogravimetric analysis. The TG curves of IPDI core, PU shell and single-layer PU-IPDI microcapsules are presented Fig. 10a. Specifically, the vaporization temperature (defined as 5 wt.% mass loss) of IPDI is 130.4 ℃ and the thermal decomposition temperature (defined as 5 wt.% mass loss) of PU shell is 244.8 ℃, respectively39. Therefore, it can be concluded that the core content of PU-IPDI microcapsules accounts for approximately 58.37 wt.%. Moreover, the temperature at which a mass loss of 5 wt.% occurs for single-layer PU-IPDI microcapsules is measured to be higher than that of the core materials by 30.1 ℃, indicating an inherent thermal protection effect on IPDI within these microcapsules. Figure 10b shows the TG curves of IPDI, PF/PU shell and double-layer PF/PU-IPDI microcapsules. Concretely, the decomposition temperature of PF/PU shell is 250.4 ℃, and the core content of double-layer microcapsules is calculated as 55.76 wt.%. The temperature for a 5 wt.% mass loss of double-layer PF/PU-IPDI microcapsules is 182.1 ℃, which is significantly higher by 51.7 ℃ and 21.6 ℃ compared to that of the core materials and single-layer PU-IPDI microcapsules respectively. Compared with single-layer PU-IPDI microcapsules, the TG curve of double-layer PF/PU-IPDI microcapsules shows a right-shift behavior, in other word, the required temperature for achieving the same core mass loss was increased; thus verifying that the thermal resistance of double-layer PF/PU-IPDI microcapsules has been sustainably improved. This enhanced thermal stability can be ascribed to a high concentration of benzene rings in phenolic resin, which possess strong bond energy due to sp2 hybridization configuration, leading to excellent thermal resistance. Therefore, these prepared double-layer PF/PU-IPDI microcapsules exhibit superior thermal performance enabling effectively protection of IPDI core materials while enhancing self-healing properties.

Figure 10
figure 10

The TG analysis of (a) PU-IPDI microcapsules, (b) PF/PU-IPDI microcapsules.

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Thermal aging study of microcapsules

The isothermal aging measurements of IPDI, single-layer PU-IPDI and double-layer PF/PU-IPDI microcapsules were carried out to evaluate their thermal aging properties. As shown in Fig. 11, rapid evaporation of IPDI occurred within 0.2 h with a steep mass loss due to its low vaporization temperature. The mass loss curve of single-layer PU-IPDI microcapsules slowed down at 27 min and reached a plateau stage after 96 min, resulting a final mass loss of 19.28 wt.% after 8 h thermal aging process. Comparatively, the mass loss curve of double-layer PF/PU-IPDI microcapsules reached a plateau at 22 min and the final mass loss was only 10.98 wt.%. Therefore, the double-layer PF/PU-IPDI microcapsules exhibited reduced core content loss and improved resistance to thermal aging, thereby extending their validity period for long-lasting self-healing efficacy when incorporated into epoxy coatings.

Figure 11
figure 11

Isothermal TG curves of IPDI and different microcapsules.

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Stability study of microcapsules

The shelf-life time of isocyanate-based microcapsules when exposed to the open air mainly depends on the humidity of the environment. The single-layer PU-IPDI and double-layer PF/PU-IPDI microcapsules were immersed in water for 1, 3, 7, 12 and 24 h respectively to investigate their stability performance in humid environment. Their core content was further analyzed by TGA. As shown in Fig. 12, the initial IPDI core content in single-layer PU-IPDI microcapsules was 58.37 wt.%. After soaking in water for 1, 3, 7, 12 and 24 h, respectively, the IPDI core content decreased to 43.22, 33.62, 28.26, 17.63 and 12.4 wt.%, respectively. In contrast, the IPDI core content in double-layer PF/PU-IPDI microcapsules decreased from 55.76 to 52.79, 51.9, 50.12, 49.56 and 41.11 wt.%, respectively. These results demonstrate that the retention rate of IPDI core content within double-layer PF/PU-IPDI microcapsules was significantly higher than single-layer PU-IPDI microcapsules, indicating that the introduction of phenolic resin second outer layer has improved the densification of the microcapsule shells. This denser and less-penetrable shell could effectively impede water molecules/moisture from diffusing into microcapsules, thus avoiding the reaction between water and IPDI. Consequently, in a humid environment, the denser double-layer PF/PU-IPDI microcapsules exhibit higher stability, and the double-layer shell can protect the core material from degradation and maintain the self-healing effect.

Figure 12
figure 12

Plot of the core content variations of PU-IPDI and PF/PU-IPDI microcapsules as a function of immersion time in water.

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Effects of microcapsules on coating properties

In order to investigate the effects of microcapsule incorporation on adhesion, impact resistance and roughness of epoxy coating, a series of relevant tests were carried out on pure epoxy coating and coatings containing 10 wt.% PF/PU-IPDI microcapsules. The corresponding results are shown in Table 2.

Table 2 Performance test results of epoxy coating.
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Table 2 demonstrates that the pure epoxy coating on tinplate substrate exhibiting excellent adhesion performance. The addition of microcapsules has minimal impact on its adhesion performance, maintaining it at the same level. However, both the pure epoxy and microcapsules incorporated coatings exhibit poor impact performance when subjected to a freely dropped weight from a height of 30 mm, resulting in cracks. In addition, cracks could be observed in both the pure epoxy coating and microcapsules incorporated coatings duirng the toughness test with a shaft rod diameter of 15 mm. Therefore, incorporating of 10 wt.% microcapsules minimally affects the adhesion, impact resistance and roughness properties of the epoxy coating.

Self-healing performance of microcapsules

Epoxy resin is one of the most widely used coating materials due to its excellent chemical resistance, strong adhesion and corrosion resistance28. Therefore, corrosion experiments were conducted on epoxy coatings to assess the self-healing properties of double-layer PF/PU-IPDI microcapsules. Specifically, two sets of samples were prepared: one consisting of a pure epoxy coating, while the other incorporated 10 wt.% PF/PU-IPDI microcapsules. Both sets were subjected to scratching with a scalpel. Subsequently, the samples were immersed in a 10 wt.% NaCl solution. Visual documentation of the corrosion at the scratched areas was recorded through photographs taken at 1, 3, and 7-day intervals (Fig. 13). Due to the damage of the pure epoxy coating, the barrier function was lost at both of the scratches and edges of the coating layer, allowing the corrosive media such as water, oxygen, etc. to directly contact with the steel substrate, leading to corrosion and rust formation, as shown in Fig. 13a. With the increment of immersion time, the corrosion region progressively intensified. As a contrast, the scratch corrosion test of epoxy coating with 10 wt.% microcapsules was carried out as shown in Fig. 13b. Remarkably, it is found that no corrosion was observed at the scratches even after 7 days, and the steel substrate remained in original state. This excellent anti-corrosion performance could be attributed to that the scratches would break the double-layer PF/PU-IPDI microcapsules, simultaneously releasing the curing agents inside the microcapsules. When these curing agents entered the crack surface, they can react with environmental water to form a polymer film through a cross-linking reaction, thereby repairing the scratches, restoring the barrier isolation function of epoxy coating and preventing corrosion and rust formation on the steel substrate (Fig. 14).

Figure 13
figure 13

Corrosion of two epoxy coatings (a neat epoxy coating; b 10 wt.% PF/PU-IPDI microcapsules).

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Figure 14
figure 14

The mechanism diagram of self-healing reaction.

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The morphology changes of the scratches were further observed using SEM, as shown in Fig. 15. Figure 15a displays the pure epoxy coating with a scratch width of ~ 60 μm, which cannot be repaired due to the absence of self-healing microcapsules. Comparatively, when the epoxy coating incorporating 10 wt.% microcapsules was scratched, accompanied with the ruptured microcapsules. The curing agent of IPDI was flowed out and reacted with water in the environment to form polyurea for self-repairing the crack surface, as highlighted in Fig. 15b. The generated polyurea exhibited a compact structure and filled up the entire scratched area effectively, preventing the NaCl solution from touching the steel substrate and protecting it from corrosion. These observations demonstrated that the double-layer PF/PU-IPDI microcapsules possess superior self-healing properties (Fig. 15c).

Figure 15
figure 15

The SEM micrographs of scratched area (a neat epoxy resin; b 10 wt.% PF/PU-IPDI microcapsules; c. Ployurea polymer as filled into the scratched area).

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The magnified SEM image in Fig. 16 shows the rupture of double-layer PF/PU-IPDI microcapsules and subsequent release of the curing agent, confirming the self-healing capability through the formation of macromolecule polyurea via reaction with released IPDI.

Figure 16
figure 16

Curing agent released from ruptured microcapsules.

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In this study, the healing efficiency η of the coating samples is calculated using tensile strength of intact, scratched and healed strip film samples (Eq. (1)):

$$ \eta = \left( {\frac{{W_{H} - W_{S} }}{{W_{I} - W_{S} }}} \right) \times 100\% $$
(1)

where WI, WS and WH represent tensile strength for intact, scratched and healed coating samples, respectively.

The healing efficiency for epoxy coating containing 0 wt.%, 5 wt.%, 10 wt.% and 15 wt.% PF/PU-IPDI microcapsules are shown in Fig. 17. As the content of microcapsules increased, the tensile strength of epoxy samples gradually decreases, which is attributed to the addition of microcapsules reduces the continuity of epoxy molecules and increases interface defects. Pure epoxy resin have no self-healing ability, while samples containing microcapsules have a certain repairing effect. When the microcapsule content is 5 wt.%, 10 wt.%, and 15 wt.%, the self-healing efficiency is 36.1%, 57.9%, and 52.4%, respectively. The content of repair agent increases with the increase of microcapsule content, which can better repair the crack surface and increase the strength of the epoxy sample. However, when the microcapsule content is too high, it will greatly reduce the basic strength of the epoxy resin and the repair efficiency will also decrease. Thus, the optimal microcapsule content is 10 wt.%, and the corresponding healing efficiency is 57.9%.

Figure 17
figure 17

The healing efficiency diagram of epoxy coatings with different microcapsule content.

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Conclusion

In this paper, a two-step method was successfully employed to prepare distinctive double-layer PF/PU-IPDI microcapsules. Optical microanalysis and scanning electron microscope analysis confirmed the formation of the double-layer structure of PF/PU and the presence of IPDI core within the microcapsules. Compared to single-layer PU-IPDI microcapsules, double-layer PF/PU-IPDI microcapsules exhibited a smooth surface with an average size of 75 μm. Fourier transform infrared spectroscopy analysis verified the existence of PF/PU shells and encapsulated IPDI core. Subsequently, comprehensive investigations were conducted on the mechanical, thermal, aging and stability properties of single-layer PU-IPDI and double-layer PF/PU-IPDI microcapsules. The results showed that the elastic modulus and hardness of PF/PU-IPDI increased by 2.2 and 3.6 times to PU-IPDI respectively. The thermal and aging properties of double-layer microcapsules were superior to those of single-layer counterpart. When these two microcapsules were immersed in water for 24 h, the core content of PU-IPDI decreased from 58.37 to 12.4 wt.%, while that of PF/PU-IPDI only decreased from 55.76 to 41.11 wt.%. The scratch corrosion tests demonstrated that the steel substrate exhibited no signs of corrosion even after being soaked in 10 wt.% NaCl solution for 7 days with double-layer PF/PU-IPDI microcapsules, thus verifying its excellent self-healing properties. The tensile test showed that when the microcapsule content of epoxy coating was 10 wt.%, the self-healing efficiency curve reached a peak of 57.9%. In conclusion, the double-layer PF/PU-IPDI self-healing microcapsules demonstrate excellent comprehensive properties, providing robust support for their prospective industrial applications.