3.1. Self-Healing Agent and Microencapsulation Mechanism
To enhance the repair effect, dual-component microcapsules were selected and investigated in this work. The healing agents were chosen based on their adhesive capability, low toxicity, and thermal stability. As an aliphatic diisocyanate, IPDI shown in
Figure 1a was selected as one part of repair agents with two active –NCO active groups. These inherent characteristic features such as high solids content, low viscosity and high reactivity of hydrophobic PAE, also made it potentially suitable for utilization in high-performance coatings and subsequent potential scale-up of production. The –NH groups located in the molecular structure (
Figure 1b) could be sterically hindered aliphatic secondary diamine, while they could react with –NCO of IPDI at room temperature, and the reaction (as shown in
Figure 1d) usually proceeded within several minutes. The resultant product polyurea presents many advantages such as flexibility, weather resistance, water resistance and wear resistance. As a drying oil obtained from the nut of the tung tree, TO was selected as both solvent of PAE and a secondary-phase healing agent. TO was a triglyceride mainly composed of alpha-eleostearic acid, and the chemical structure of which was presented in
Figure 1c.
Owing to its low price, simple fabrication, good seal tightness and endurance, fire resistance, acid and alkaline resistance, MF resin was selected as the shell material. The formation of MF polymer was carried out by two steps as described in
Figure 1e. The first stage was the process of methylolation to form MF prepolymer. The nucleophilic addition between melamine and formaldehyde would occur in weak alkaline medium, resulting in a mixture of different methylolated melamine, and the trimethylolmelamine is more stable by comparison. The second stage was the resinification, and polycondensation reaction was carried out in acidic medium to form bridges between the triazine rings. There may exist two mechanisms: dehydration condensation of two methylol groups to form the ether bridge, and condensation of methylol and amino groups to form the methylene bridges. Then further polymerization occurred to form an insoluble and non-melting polymer with a bulk structure finally.
Self-healing microcapsule with MF shell was prepared by in situ polymerization, and the schematic fabrication was shown in
Figure 2. The hydrophobic phase (i.e., IPDI, PAE, PAE and TO) was added into the aqueous phase containing SMA and then emulsified to form a stable oil-in-water (O/W) emulsion. As an emulsifier, the molecular chain of SMA contains phenyl hydrophobic groups and the carboxyl hydrophilic groups, so they could be easily adsorbed to the interface during emulsification process and form stable emulsion. At the same time, the carboxylic ion groups located on SMA could absorb negative charge, while the amino groups on MF prepolymer could carry with positive charge [
29]. Both kind groups could migrate to the oil-water interface with the electrostatic interaction after MF prepolymer was added, and then polymerization started to form the insoluble cross-linked shell at certain temperature and acidic environment. There also existed some nanoparticles on the shell surface of microcapsule, which might be resulted from self-polymerization of MF prepolymer when the electric attraction became weaker as the microencapsulating proceeded.
3.2. Microencapsulation of IPDI
The OM micrographs in
Figure 3 displayed the morphology evolution of the IPDI microcapsules. Firstly, The O/W emulsion with uniform particle size could be obtained facilely by high-speed shear emulsification as shown in
Figure 3a, where obvious oil-water droplets interface could be observed clearly. After MF prepolymer as shell-forming monomer was added dropwise into the emulsion, the temperature was raised to 60 °C and maintained for 20 min, then the surface of the emulsion droplets became wrinkled as shown in
Figure 3b,c, indicating that the MF prepolymer aggregated around the surface of emulsion droplets under charge attraction and began to polymerize to form the solid shell of microcapsules. After continuous polycondensation for 2 h under above mentioned reaction condition, IPDI microcapsules with MF-coated shell were finally obtained. The synthesized microcapsules were spherical shaped with average diameter of around 3.2 μm as
Figure 3d,e shown, which was in accordance with the size of emulsion as well, so it confirmed the excellent stability of emulsion. Besides, there were some small solid particles with size less than about 100 nm on the surface, this could attribute to the fact that self-polymerization of prepolymer in water then adhered to the surface of microcapsules or gathered together in other existing mode. Moreover, the microcracks performance of microcapsules during self-healing process are greatly affected by the particle size. A partial enlargement of the capsule surface was shown in
Figure 3f, the outer surface was found quite smooth and compact, thus contributing to encapsulate the core material without leakage.
The IPDI microcapsules were more likely to be ruptured when subjected to certain external force, the micrographs of a single broken microcapsule were shown in
Figure 4. This illustrated that the shell material was sensitive to external forces, so the self-healing microcapsules distributed in substrate coatings could be ruptured when the substrate subjected external stimulate, and then released IPDI to target areas via capillary effects. In addition, we could find that the shell thickness of microcapsule was around 70 nm uniformly, and the inner surface of shell appeared smooth and compact.
To determine the successful microencapsulation, FTIR spectra was measured to identify the chemical structure of the microcapsules qualitatively, and the FTIR spectra of IPDI, microencapsulated IPDI and MF shell was shown in
Figure 5. The absorption bands at 2260 cm
−1 corresponding to –NCO stretch characteristic could be observed obviously on the spectrum of IPDI. The peaks at 2980 and 1470 cm
−1 referred to the stretching and bending vibration of –CH, moreover, stretching and bending vibration peaks of –CH
2 appeared at 2930 and 1380 cm
−1. The MF shell material could be manifested with wide stretching vibration peaks of N–H at about 3353 cm
−1 and stretching of triazine ring at 820 cm
−1. The peak at 2930 cm
−1 belonged to the stretching vibration of aliphatic –CH
2, and the peaks of 1366–1556 cm
−1 related to the stretching vibration of C=N. In addition, the stretching vibration peaks of C–N at 1000–1350 cm
−1 also existed in spectrum of shell, the peak of 2260 cm
−1 corresponding to –NCO and 3353 cm
−1 belonging to N–H and triazine ring at 820 cm
−1 appeared in the spectrum of microcapsules. The results indicated that IPDI was probably encapsulated with MF shell.
The thermal stability of self-healing microcapsule is a key parameter for its practical application, therefore, TGA measurements of IPDI, microencapsulated IPDI and MF shell were performed as shown in
Figure 6. As can be seen, the vaporization temperature (defined as 5 wt % mass loss) of IPDI was 163 °C approximately, while it increased to 236 °C after being encapsulated, and thus this illustrated that MF shell provided a thermal resistance for IPDI core, which could protect IPDI from permeation firmly. The shell material lost about 5% of weight below 150 °C presumably from cleavage of ether bond and elimination of H
2O and formaldehyde. There still around 15% residual mass remained at 800 °C mainly due to residual carbon. Besides, the weight loss of microcapsules was 56.2 wt % approximately from beginning to 350 °C and then lost weight with a higher rate until 500 °C. Compared with the weight loss curves of IPDI and shell, IPDI evaporated completely and shell lost weight about 15% at 350 °C, so it was reasonable to conclude that the weight loss was attributed to evaporation of the IPDI, residual water and formaldehyde from MF shell. Therefore, the core fraction of microcapsules was estimated as 48.5 wt % approximately.
3.3. Microencapsulation of Hydrophobic PAE-TO
To facilitate microencapsulation, the hydrophobic amine PAE was selected as another component of self-healing system. As shown in
Figure 7a, the stable O/W emulsion could be obtained after emulsification, and the size of droplets ranged from 2 to 5 μm. The emulsion was transferred to three-necked flask and MF prepolymer was dropped subsequently, however, it was worth to note that the particle size of oil droplets suddenly decreased from several microns to submicron or nanometer scale dramatically as
Figure 7b shown. Furthermore, from the SEM image of
Figure 7c, we could also find the PAE microcapsules exhibit smooth surface, and the mean diameter of microcapsules was 500 nm approximately, which was relatively uniform and consistent with the droplets’ size presented in
Figure 7b. This phenomenon might be attributed to the fact that partial reaction between the amino groups of PAE and hydroxyl or of the MF prepolymer occurred, resulting in the obvious diameter decrease of PAE microcapsules. However, it is considered hard for the PAE capsule with submicron or nanometer scale diameter to be ruptured to release self-healing PAE, and also less active self-healing PAE remained inside the shells.
As presented in
Figure 8, the FTIR spectra of PAE, microencapsulated PAE and the MF shell were measured to further study the molecular structure of microcapsule. On the spectrum of PAE, besides the peaks of –CH and –CH
2 related to stretching and bending vibration, the stretching and out-plane rocking vibration peaks of –NH appeared at 3340 and 858 cm
−1 respectively, meanwhile, the stretching vibration peak of C–N appeared at 1028 cm
−1, all of these characteristic peaks mentioned above were associated to amine group. The peaks at 1735 and 1177 cm
−1 corresponded to the stretching and bending vibration of C=O in ester group respectively, and the peak at 1260 cm
−1 related to the stretching vibration of C–O. On the spectrum of microcapsules, besides the characteristic peaks of shell appeared at about 3440 and 820 cm
−1, all the peaks belonging to PAE appeared. These results indicated that PAE was probably microencapsulated with melamine formaldehyde resin shell.
To solve the problem that core material partially reacted with the MF prepolymer, another self-healing agent TO was added also as solvent of PAE to reduce the reaction activity. TO and PAE are mutually soluble well, and TO could also employed as another self-healing material to consummate the dual-component repair system. We encapsulated PAE, PAE/TO (the mass ratio was 1:1) and TO under same encapsulation process, respectively. As shown in
Figure 9, the morphology of microcapsules containing different core material were observed using FE-SEM. The diameter of microencapsulated PAE was measured to be 500 nm approximately (
Figure 9a), and the particle size of microencapsulated TO was measured about ten times larger than that of microencapsulated PAE (
Figure 9c), besides, the surface of microencapsulated TO became much rougher than that of microencapsulated PAE which could enhance the microcapsule/coating interface bonding. While for the mixed core of PAE and TO, the diameter of microcapsules reached about 2 μm (
Figure 9b). This result indicated that the addition of TO could alleviate this problem effectively, and the confirmation of structure and properties required to be demonstrated by further characterization or other measurements.
Figure 10 showed the FTIR spectra of the PAE, TO, the mixture of PAE and TO, microencapsulated PAE/TO and the MF shell. The mixture spectra showed characteristic peaks of PAE at 3340 cm
−1 (–NH stretching vibration), 858 cm
−1(–NH out-plane rocking vibration) and 1028 cm
−1 (C–N stretching vibration). The spectrum also presented the characteristic peaks of TO at 3012 cm
−1 (=CH stretching vibration), 2927 and 2855 cm
−1 (–CH stretching vibration), 1745 cm
−1 (C=O stretching vibration) and 725 cm
−1 (–(CH
2)
n− (
n ≥ 4) bending vibration). FTIR curves confirmed that there was no new chemical bond generated during the mixing process of PAE and TO. In the spectrum of microcapsule, besides the characteristic peaks related to PAE and TO, the characteristic peaks of MF could be manifested at: 1366–1556 cm
−1 corresponding to the stretching vibration of –C=N and 820 cm
−1 associated to stretching of triazine ring. These results indicated that the mixture of PAE and TO as core was probably encapsulated by MF resin shell.
Thermogravimetry analysis were carried out for thermal degradation characterization of the amine microcapsules.
Figure 11 showed the TGA curves of PAE, TO, the mixture of PAE and TO, microencapsulated PAE-TO and MF shell. Firstly, the PAE was thermally stable up to 252 °C and degradation occurred in two stages. The first stage at 252–285 °C lost weight about 85%, the second stage ranged from 416 to 500 °C and decomposed completely at 500 °C. TO started to decompose at about 330 °C as seen from its weight loss curve. On the curve of the mixture of PAE and TO there existed three stages, i.e., 233 °C corresponding to lose weight of about 35%, 365 °C corresponding to around 49.5% and 442 °C corresponding to 14%, and this further indicated that there were no chemical bond or new material formed during the mix process when the PAE/TO was 1:1. After being microencapsulated by MF resin, thermal decomposition of microcapsule started around 195 °C and continued up to 305 °C, this was mainly attributed to the cleavage of ether bond and elimination of formaldehyde from degradation of MF shell above 200 °C. The second stage of decomposition occurred between 305 and 502 °C, which was likely attributed to the evaporation or decomposition of mixture of PAE and TO encapsulated in the shell. The thermal degradations of the core and shell took place in overlapping temperature range (252–485 °C).
3.4. Performance of Self-Healing Coating Containing Dual-Component Microcapsule
To evaluate the self-healing performance, the control coating and self-healing coating contained 25 wt % dual-component microcapsules were placed in the air for 12 h at room temperature (~20 °C). TCCM was used to observe the repair renderings, and the three-dimensional surface micrographs could also be obtained through confocal mode, besides, the roughness change of samples from 3D topography could be compared as well. As was clearly shown in
Figure 12a
1,a
2, an obvious crack could be observed in the control coating sample. By contrast, it could be found that the scratched area of the substrate coated with self-healing coating was nearly fully free of crack as shown in
Figure 12b
1,b
2. Furthermore, the repair results of crack could be found clearly by comparing the roughness of coating surface (
Figure 12a
3,b
3). Repairing property of self-healing coating could mainly attribute to the incorporation of microencapsulated IPDI and microencapsulated PAE/TO. The released repair agents from ruptured microcapsules could heal the crack automatically by reacting with each other, and subsequently the newly-formed polyurea was able to fill microcrack. The result clearly demonstrated the great potential of self-healing coating containing dual-component microcapsule in external environments.
Here, dual-component self-healing coating containing various contents of microcapsules were prepared to investigate the self-repairing capability under air circumstance for 12 h at room temperature (~20 °C). As results shown in
Figure 13, the crack was still obvious when 5 wt % self-healing microcapsules were added. With the content of microcapsules increased from 5 to 20 wt %, the repair of crack further tended to perfect. However, a small amount of micron-scale protrusions structure was formed on surface according to the images in
Figure 12b
2, which was mainly caused by excessive microcapsule accumulation, thus the properties such as smoothness, compactness and adhesion would be seriously affected when the content exceeds 20 wt %. Therefore, in comprehensive view, the appropriate content of microcapsules should no more than 20 wt % for optimal performance.
Furthermore, coatings containing different kinds of microcapsules were carried out to compare the self-healing effect of IPDI microcapsules and dual-component microcapsules in the air for 12 h at room temperature (~20 °C). From
Figure 14a we found the crack was clearly visible in epoxy coating without any microcapsules. By contrast, the coating containing 15 wt % IPDI microcapsules was repaired partly as shown in
Figure 14b, and this may be attributed to the slow reaction between IPDI and water vapor in the air, and the complete self-repair could not carry out in such a short time. In the end, the coating with dual-component microcapsules achieved optimal repair result presented in
Figure 14c, which mainly resulted from that the fast reaction of isocyanate-amine nucleophilic addition and the newly-formed polyurea layer. The healing efficiency with different healing system and microcapsules content were compared and summarized in
Table 3, the self-healing efficiency of dual-component microcapsules with content of 20 wt % could reached 98%. As a result, these results demonstrated the high efficiency of dual-component self-healing microcapsules and their potential application in anti-corrosion field.