Preparation And Properties Of Bio-Based Waterborne Epoxy Curing Agent And Its Anticorrosive Coatings

Waterborne epoxy anticorrosive coatings are widely used in petrochemical,transportation,aerospace,civil construction and other industries due to their advantages of excellent adhesion,good chemical resistance,high mechanical properties and low VOCs.Waterborne epoxy curing agent is the key component for waterborne epoxy resin to enter into substantive applications.Its structures and properties directly determine the physicochemical properties of waterborne epoxy coatings.Therefore,the research of waterborne epoxy curing agent is of great significance to the practical application of waterborne epoxy coatings.However,the application of waterborne epoxy curing agent in the field of anti-corrosion mainly has the following shortcomings.First,the most widely used and most important bisphenol A epoxy resins and polyamines in the preparation of waterborne epoxy curing agents are mostly derived from non-renewable petrochemical resources,which is not conducive to the sustainable development of waterborne epoxy coatings.Second,the preparation of waterborne epoxy curing agent usually requires the introduction of hydrophilic groups or the addition of surfactants to maintain the stability of the aqueous dispersions of epoxy curing agent.These polar groups would reduce the resistance of waterborne epoxy coatings to the penetration of corrosive media.In addition,the physical introduction of corrosion inhibitors into the waterborne epoxy anticorrosive coatings is prone to exudation and failure,which is not conducive to the continuous protection of coatings on the metal substrate.Based on the above problems,renewable bio-based epoxy resins,bio-based amine and organic corrosion inhibitors were selected to synthesize green waterborne epoxy curing agents,and polydimethylsiloxane grafted benzotriazole（PDMS-BTA）was used to provide dual protection of hydrophobic surface and corrosion inhibition.A hydrophobic self-cleaning waterborne epoxy anticorrosive composite coating was developed.（1）Castor oil,sorbitol and cardanol,three bio-based epoxy resins,1,8-terpene diamine and polyethylene glycol diglycidyl ether non-ionic hydrophilic segment were selected as the main raw materials,and 4-aminoethyl pyridine corrosion inhibitor was chosen to modify cardanol-based waterborne epoxy curing agent.An environmental-friendly bio-based waterborne epoxy curing agent was prepared by epoxy-polyamine addition reaction.The effects of raw material ratio,reaction temperature and reaction time on the conversion of epoxy were systematically discussed through single-factor parallel experiments,and the optimal reaction conditions were determined:the reaction temperature was 65°C,the reaction time was 5 h,the material molar ratio of amine to epoxy was 2.2:1.The effect of the hydrophilic segment PEGDEG content in the curing agent,the three bio-based epoxy resins and 4-aminoethylpyridine modified cardanol based waterborne epoxy curing agent on the mechanical properties and anticorrosive properties of the cured coatings were mainly studied by potentiodynamic polarization curves（TAFEL）and electrochemical impedance spectroscopy（EIS）.It was found that by using cardanol-based epoxy resin and adding 18.6 wt.%hydrophilic segment PEGDGE,the curing agent had good water dispersion stability,and the room-temperature cured coating could also have good flexibility and corrosion resistance,which could be comparable to the commercial curing agent.Moreover,the mechanical properties,water resistance and anti-corrosion properties of the cured coatings after being modified by 4-aminoethylpyridine are further improved,even better than the same type of waterborne epoxy curing agent available in the market.（2）Based on the above optimization of the process and raw material,benzotriazole grafted polydimethylsiloxane（PDMS-BTA）with a content of 5 wt.%was introduced into the cardanol-based waterborne epoxy coatings to provide coatings with low surface energy and corrosion protection.A new strategy for waterborne epoxy anticorrosion coatings was developed.The surface of the coatings was enriched with a large number of PDMS segments.Also,the surface of coatings became hydrophobic and adamant,smooth and flat.Dust and grit can slide off the surface with contamination droplets without leaving a trace,and it demonstrated excellent chemical stability and self-cleaning performance.In addition,the WE-PB_5%composite coating still showed good long-term corrosion resistance after being immersed in 3.5 wt.%Na Cl solution for 45 days.The OCP value of the composite coating moved positively to+0.00146 V,and the low-frequency impedance value(|Z|_f=0.01Hz)of the coating was two orders of magnitude higher than that of the plain waterborne epoxy coating.Meanwhile,the composite coating endowed the steel substrate with low corrosion current density(4.53×10^-9 A·cm^-2),extremely low corrosion rate(3.76×10^-4 mpy)and high protection efficiency（99.972%）.In addition,on the basis of the previous research,a waterborne epoxy anticorrosive paint with a pigment volume concentration（PVC）of 32%was developed by adding an appropriate amount of pigments and additives.The coating showed excellent mechanical properties and anti-corrosion properties.