Preparation Of Wet-Cured Isocyanate Microcapsules And Study On Anticorrosive Properties Of Self-Healing Coatings

The corrosion of metal materials is a problem in our national economy.One of the most cost-effective anti-corrosion measures is to impart an organic coating to its surface,of which self-healing coatings can respond to changes in external conditions in time to repair physical damage and extend the corrosion protection of the coating,making it a popular research topic in the corrosion protection.Wet-curable isocyanates are one of the most promising restorative agents for the development of one-component,catalyst-free self-healing microcapsules,however,the commonly used wet-curable cores are less reactive isocyanate monomers and research is yet to be done on more reactive monomers or tougher prepolymer cores.This paper proposed two design options for microcapsules from a theoretical analysis of the molecular structure,one is to prepare microcapsules with a more reactive liquefied diphenylmethane diisocyanate(MDI)as the core material,the other is to prepare microcapsules with a mixture of isophorone diisocyanate(IPDI)and isocyanate prepolymer as the core material,which ensures the content of isocyanate groups while increasing the toughness of the core material.The synthesis process was optimised and microcapsules with two different core materials were prepared.Subsequently,the structure and properties of the microcapsules were analysed and dispersed in an epoxy resin coating to prepare a metal anti-corrosion coating,and the self-healing anti-corrosion properties of the coating were investigated.The details are as follows:(1)Microcapsules of encapsulated MDI were prepared by simple interfacial polymerisation using liquefied MDI as both core and wall material,and the structure of the core MDI was confirmed by Fourier infrared spectroscopy(FTIR)tests.The effects of different emulsifiers,emulsification rate,emulsification time,chain extender and oil to water ratio on the morphology,mean particle size and stability of the synthetic microcapsules were investigated using optical microscopy(OM),scanning electron microscopy(SEM)and laser particle size analyser.The results showed that the best MDI microcapsules were prepared when the emulsifier was Lignosulfonate(Na LS)Pickering emulsion,the emulsifying time was 8 min,the emulsifying speed was 2500 r/min,the chain extender was ethylenediamine(EDA),and the emulsion oil-water ratio was 1:4.The microcapsules presented a uniform spherical shape with tight outer surface and no damage.The average particle size is 9.5μm,the wall thickness is about 0.9μm,and the core material content is about 56.0%.(2)By molecular design,a mixture of IPDI and isocyanate prepolymer(CPU-IPDI)with17%isocyanate group content was synthesized by reaction of excess IPDI and castor oil.Microcapsules encapsulating CPU-IPDI are prepared by interfacial polymerization using CPU-IPDI as the core material and MDI as the wall material.FTIR confirmed the existence of-NCO group in microcapsules,and TGA verified the existence of IPDI and prepolymer in core material.During the experiments,it was concluded by various analytical means that the prepared microcapsules had homogeneous morphology,smooth and undamaged outer surface and good thermal stability with an average particle size of 26.3μm,a wall thickness of 1.44±0.9μm and a core content of about 51.3%,when the amount of emulsifier Na LS Pickering emulsion was1 wt%and the reaction time of microcapsules forming was 3 h.(3)The self-healing metal coatings were prepared by incorporating MDI microcapsules and CPU-IPDI microcapsules into epoxy resin coatings respectively.Then the self-healing properties and corrosion resistance were analysed by OM,SEM,salt water erosion tests and electrochemical impedance tests.And the adhesion and hardness of the coating was investigated by means of a drawing circle method adhesion tester and Vickers hardness tester.The results show that both self-repair coatings have good self-healing properties,and the coating containing10 wt%CPU-IPDI microcapsules showed no significant corrosion after 20 days of salt water attack,exhibiting superior corrosion protection than the MDI microcapsule coating at the same content,with an electrochemical|Z|_f=0.01 Hz value of approximately 1.9×10~6Ωcm~2,three orders of magnitude higher than the blank coating and exhibited good adhesion and hardness.