Preparation Of Lignin Microcapsules Based Self-Healing Coatings And Their Performance Research

With the development of the economy,a large number of engineering and facilities for metal materials have emerged.Due to factors such as moisture and ultraviolet they are easily corroded and their performance is degraded,resulting in shortened service life and huge economic losses.The most cost-effective way is to apply coating on the surface of metal materials.The coating is potentially damaged by external forces,resulting in loss of protection of the material.Not only the self-healing coating forms a protective layer on the metal surface to isolate the corrosive medium in the environment,but also the healing agent in the coating can flow and repair the crack areas when the coating breaks,thus,the self-healing coating becomes a research hotspot.However,self-healing coating also has problems with poor aging resistance.The aromatic skeleton and polyhydroxyl structure endow biomass lignin with excellent dispersibility,reactivity,and UV-blocking property.Herein,the amphiphilic structural characteristics of the lignin can be used to prepare the microcapsules by embedding the healing agent in the oil-in-water emulsion,and then,the microcapsule-based self-healing coating is obtained by the reaction between the hydroxyl groups of the lignin and the isocyanate groups.The prepared self-healing coating can quickly repair cracks,effectively protect metal materials,and has anti-UV aging ability.This study has guiding significance for the application of lignin in the field of self-healing materials.First,spherical microcapsules（LS-IPDI）loaded with isophoronediisocyanate（IPDI）as core materials were prepared via interfacialpolymerization in an oil-in-water emulsion,using sodium lignosulfonate（LS）as emulsifier and shell material.The hydroxyl group for LS on the surface of microcapsules sequentially reacted with hexamethylene diisocyanate（HDI）and an amine component to form a self-healing polyurea coating.Small amount of polymeric methane diisocyanate（PMDI）was added to reinforce the shell of the capsules.FTIR tests showed that PMDI reacted with water to form a urea bond structure in the shell.The optimal synthesis process of LS-IPDI was determined:the LS content was 1 wt%,the water to oil ratio was 1.5:1,the emulsification time was 30s,and the rotational speed was 11,000 rpm/min.The prepared microcapsules under this condition had an IPDI loading of approximately72.6%,an average particle size of 55.2μm,and a wall thickness of 1.91μm.The microcapsules had a compact surface and good thermal stability.The prepared self-healing polyurea coating had a distinct self-healing effect.The corrosion rate of the prepared self-healing polyurea coating after immersion in 10 wt%NaCl solution for 48h was two orders of magnitude lower than that of the pure polyurea coating.Compared with the pure polyurea coating,the self-healing polyurea had better adhesion performance.After UV irradiation,the self-healing coating structure remained good,the color change leveled off,the surface roughness increased by 20%,the breaking strength and the elongation at break of 40%were maintained.Self-healing epoxy coating prepared by single-use LS-IPDI had self-healing effect,but the mechanical properties of the coating are not good.Therefore,modified lignin/silica nano composites（PAL/SiO₂）were used as emulsifier to embed IPDI to prepare Pickering emulsion,And a reaction product of PAL/SiO₂ and PMDI was used as shell materials to prepare microcapsules（PAL/SiO₂-IPDI）.Self-healing epoxy coating was prepared by direct mixing microcapsules with epoxy.The optimal synthesis process of PAL/SiO₂-IPDI was determined:the PAL/SiO₂ content was 0.5 wt%,the water to oil ratio was 1.5:1,the reaction temperature was 20^oC,and the rotational speed was 11,000 rpm/min.The prepared microcapsules under this condition had an IPDI loading of approximately 82.8%,an average particle size of 73.1μm,and a wall thickness of 2.36μm.The anti-corrosion tests in the high salt concentration solution showed that the epoxy coating of containing PAL/SiO₂-IPDI microcapsules could heal quickly after scratching,which significantly reduced the corrosion current and corrosion rate of the iron substrate.In addition,the nano indentation tests showed that the hardness value of the epoxy coating increased from 249.99 MPa to 302.98 MPa after adding PAL/SiO₂-IPDI microcapsules,and the elastic modulus was also improved.