Preparation and properties of double-layer phenolic/polyurethane coated isophorone diisocyanate self-healing microcapsules

The synthesis mechanism of PF/PU-IPDI microcapsulesThe 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 1The schematic diagram of double-layer PF/PU-IPDI microcapsules.Morphology of microcapsulesThe 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 2Digital images of (a) PU-IPDI powder, (b) PF/PU-IPDI powder; optical micrographs of (c) PU-IPDI, (d) PF/PU-IPDI.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 3Surface morphology of (a) PU-IPDI microcapsules and (b) PF/PU-IPDI microcapsules.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 4The 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.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 5Size distribution analysis of PF/PU-IPDI microcapsules.Determination microcapsules componentsThe 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 6The infrared spectra of shell materials, core material and PF/PU-IPDI microcapsules.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 7The SEM image of cracked PF/PU-IPDI microcapsules.Mechanical property of microcapsulesThe 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 8SEM 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.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 9Depth-sensing indentation tests of microcapsules.Table 1 Depth-sensing indentation test results of microcapsules.Thermal property and core fraction of microcapsulesThe 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 10The TG analysis of (a) PU-IPDI microcapsules, (b) PF/PU-IPDI microcapsules.Thermal aging study of microcapsulesThe 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 11Isothermal TG curves of IPDI and different microcapsules.Stability study of microcapsulesThe 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 12Plot of the core content variations of PU-IPDI and PF/PU-IPDI microcapsules as a function of immersion time in water.Effects of microcapsules on coating propertiesIn 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.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 microcapsulesEpoxy 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 13Corrosion of two epoxy coatings (a neat epoxy coating; b 10 wt.% PF/PU-IPDI microcapsules).Figure 14The mechanism diagram of self-healing reaction.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 15The 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).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 16Curing agent released from ruptured microcapsules.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 17The healing efficiency diagram of epoxy coatings with different microcapsule content.