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Metal corrosion causes huge economic losses. In China alone, the annual corrosion cost exceeds 2.1 trillion yuan, accounting for 3.34% of its GDP. Polymer coatings play an important role in metal corrosion protection as a physical barrier between the metal substrate and the external corrosive environment. However, anti-corrosion coatings will inevitably produce microscopic damage during use, and even cracks at the micro-nano scale will form microscopic channels for corrosive media to penetrate and contact the metal substrate. If this coating failure behavior is not discovered and repaired in time, it will cause continuous and severe corrosion to the metal. Therefore, the development of smart coatings with micro-damage self-repair and self-reporting capabilities is of great significance to extend the service life of anti-corrosion coatings in marine environments.
The main point of this paper
Characteristics and limitations of PVA:
PVA has good oil resistance, solvent resistance and fiber-forming properties, and is suitable as a shell material for coaxial electrospun nanofibers.
PVA lacks functional groups that chemically bond to metal substrates, resulting in low interfacial bonding strength between it and the metal substrate.
Application of phytic acid (PA):
Phytic acid (PA) contains 6 phosphate groups and 12 hydroxyl groups, which can form strong chelation and coordination effects with metals.
PA can enhance the adhesion and bonding strength of metal surfaces, form a dense monomolecular protective film, and has a corrosion inhibition effect.
Fluorescent dyes and TPE:
Traditional fluorescent dyes do not have a fluorescent "switch" function and cannot evaluate the healing state of microcracks.
TPE (aggregation-induced emission stellar luminescent source) emits weak ultraviolet light in dilute solution and emits enhanced visible blue light in the aggregated state, which can be used as a fluorescent "switch".
Synthesis of PVA-PA electrospinning solution:
PVA-PA electrospinning solution was synthesized to improve the adhesion strength of PVA to the metal substrate while maintaining the spinnability of PVA.
Preparation of sandwich microvascular network (SMN):
SMN was prepared by coaxial electrospinning technology with PVA-PA as the shell material and epoxy resin E51, TPE and polyamide resin as the core material.
The porous structure of SMN allows epoxy resin to be directly spin-coated on it to form a PVA-PA/SMN/EP composite coating.
Characterization and testing:
The morphology, composition and fluorescence indication effect of SMN were characterized using SEM, TEM, EDS, FTIR and fluorescence microscopy.
The dry and wet adhesion of the composite coating on the mild steel surface was tested using a pull-out tester.
The self-healing property and corrosion resistance of the composite coating were evaluated by stereomicroscopy, CLSM, EDS and EIS.
Technical advantages:
Coaxial electrospinning technology can prepare nanofibers with core-shell structure for encapsulating active substances.
Due to its unique structural and functional properties, this technology has wide application potential in catalysis, energy storage, medicine, filtration and sensing.
Polyvinyl alcohol grafted phytic acid (PVA-PA):
The synthesized PVA-PA electrospinning solution improves the adhesion strength between PVA and metal substrates.
PVA-PA as a shell material enhances the interfacial bonding strength with the metal substrate, thereby improving the application potential of anti-corrosion coatings.
Sandwich microvascular network (SMN):
SMN is prepared from PVA-PA solution as the shell material, epoxy resin 51 (E51), tetraphenylethylene (TPE) and polyamide resin as the core materials.
The high porosity of SMN allows epoxy resin to be directly spin-coated on it to form a composite coating (PVA-PA/SMN/EP).
Improved adhesion:
Due to the strong chelation and coordination between PA and mild steel, the pull-off adhesion of the PVA-PA/SMN/EP composite coating on mild steel is improved by 0.92 MPa.
Structural optimization:
By optimizing the relative viscosity, miscibility, conductivity and saturated vapor pressure of the core liquid and shell liquid, the microvascular structure of the nanofibers is improved and the fluidity of the internal active substances is maintained.
In summary, we developed an SMN based on coaxial electrospinning technology and applied it to monitor the microscopic damage location and healing status of anti-corrosion coatings. The results showed that PA grafted onto the PVA molecular chain can form P-O-Fe bonds through its own polar phosphate groups without affecting the fiber-forming properties of PVA, thereby improving the interfacial bonding strength between PVA electrospun nanofibers and mild steel substrates. Compared with the PVA/SMN/EP composite coating, the dry pull-out adhesion of the PVA-PA/SMN/EP composite coating on the mild steel substrate was improved by 0.92 MPa. In addition, by systematically optimizing the relative viscosity, injection speed ratio, miscibility, conductivity and saturated vapor pressure between the two jets of core liquid and shell liquid in coaxial electrospinning, the microvascular structure of coaxial electrospun nanofibers was improved and the fluidity of the internal active substances was maintained. When the PVA-PA/SMN/EP composite coating produces microcracks due to mechanical damage, the active substances encapsulated in the SMN flow out through capillary force, and the three-dimensional cross-linked network formed by the curing of E51 and polyamide resin enhances the spatial interaction between TPE molecules, making TPE emit bright blue fluorescence. This work provides a new way to prepare self-healing and self-reporting dual-functional coatings, and demonstrates the application prospects of coaxial electrospinning technology in smart anti-corrosion coatings.
