Preparation And Propoties Of Polyurea-based Double-walled Microcapsules/Epoxy Resin Self-healing Marine Protective Coatings

Marine protective coatings are highly susceptible to temperature changes,alternation of wetting and drying,and corrosive media erosion during service,resulting in decline of mechanical properties and corrosion resistance of coatings or even failure.In recent years,microcapsules-based self-healing coatings have become a research hotspot in the field of coatings,aiming at achieving the targeted repair of micro-cracks.However,most of the existing external microcapsule systems are single-wall microcapsule systems,which have low repair efficiency for the substrate materials and some problems,such as the flow of repair agent depends on specific temperature,the deactivation of catalyst,and the improvement of comprehensive performance after repair is not obvious.Moreover,microcapsule cross-linked polymer is still vulnerable to damage,and the stability of the wall structure remains to be further optimized.Therefore,the synthesis of polyurea-based double-walled microcapsules was carried out to increase the contact probability of the repair agent and the curing agent,which greatly improve the repair efficiency of the substrate.The wall materials were modified and enhanced by graphene oxide(GO)to meet the requirements of durability,solvent and corrosion resistance during preparation and service of microcapsules and modified coatings.the influence of synthesis and preparation parameters on the structure and properties of polyurea double-walled microcapsules was systematically studied,and the effects of emulsifier content,proportion between core and wall material,stirring speed and other factors on the structure and properties of polyurea double-walled microcapsules and graphene oxide(GO)modified microcapsules was emphatically explored.Optical microscope(OM)was used to observe the microscopic morphology and distribution of different droplets.Scanning electron microscope(SEM)was applied to observe the microscopic morphology of microcapsules under the optimal synthesis condition and Nano Measurer software was used to analyze the particle size distribution of microcapsules.The chemical composition of the microcapsules was analyzed by Fourier transform infrared spectroscopy(FTIR),and the thermostability of the syntheticcapsules and the amount of the encapsulated ratio of core materials was tested by thermogravimetric analyzer.Meanwhile,the mechanical properties such as tensile strength,elastic modulus and breaking strength of the composite coatings were characterized by tensile test.The gray-scale correlation coefficient maps obtained by digital speckle technique(DSCM)were used to track the crack propagation during the stretching process.Finally,electrochemical impedance spectroscopy(EIS)was used to evaluate the corrosion resistance mechanism,corrosion resistance and repair efficiency of the modified coating after seawater immersion.The results of microcapsule structure and performances show that when the mass ratio of the prepolymer emulsion,and the hexamethylenediamine emulsion and the single-walled microcapsule emulsion is 3.3:1.5:5.8,and the stirring speed was 1800 rpm,the synthetic polyurea double-wall microcapsules had good microscopic morphology.Meanwhile,the optimal parameters for the synthesis of graphene oxide-modified microcapsules are: the mass fraction of graphene oxide dispersion is 2.5wt%,the mass ratio of the modified prepolymer emulsion,and the hexamethylenediamine emulsion and the single-walled microcapsule emulsion when synthesizing the graphene oxide-modified double-walled microcapsules is 3.3:1.0:5.7.Infrared test showed that both of the synthesized microcapsules contained urea bond,and the XRD test indicates that GO has been successfully embedded in the microcapsule wall material.Thermogravimetric analysis showed that the microcapsules had higher thermal stability,the encapsulation ratio of obtained microcapsules was around 52.34%,and the coverage ratio of GO-modified microcapsules to the inner core material and outer core material was 26.75% and10.88% respectively.The results of coating self-healing performance study show that,in the tensile test,the mechanical properties of the epoxy resin coating with 3.0wt% microcapsules are excellent,and the repair efficiency can reach to 63.76%,and the repair efficiency of the coating containing 3.0wt% microcapsules increased to 80.43%.Therefore,the ability of microcapsule/epoxy resin coating to repair crack and inhibit crack propagation is mainly related to the fluctuation of St value of modified epoxy coating and generation-reduction-increased self-repair process.The repair efficiency of the epoxyresin coating containing 5.0wt% microcapsules was 84.46% after 72 h seawater immersion.While the coating efficiency of 8.0wt% GO modified microcapsules was374.15% after 572 h soaking.The continuous infiltration of the electrolyte solution can lead to an increase in the coating capacitance and a decrease in the transfer charge resistance.The "sprouting,repairing and expanding" repair process of microcracks in self-repairing coating under the action of medium invasion and coating matrix expansion was verified.