Preparation And Properties Of Polydopamine Functionalized Graphene Oxide/Waterborne Polyurethane Composites

Compared with anti-corrosion measures such as cathodic protection,corrosion inhibitor and metal substrate modification,organic coatings are considered to be the most cost-effective anti-corrosion strategy due to their effective resistance to corrosion and a wide range of applications.Polyurethane coatings are widely used in industrial protection,wood furniture,automotive coatings and other fields due to their excellent weather resistance,chemical resistance,high hardness,high gloss,and strong adhesion.Conventional solvent-based polyurethane coatings use a variety of organic solvents as a dispersion medium,which inevitably generates a large amount of volatile organic compounds(VOCs)during production and application,which may cause harm to humans and the environment.With the continuous improvement of people's living standards and the sound laws and regulations related to environmental protection,it is an irresistible trend to replace the solvent type with water-based coating systems.Waterborne polyurethane(WPU)uses water as a dispersion system,emits almost no VOCs,and has good weatherability and mechanical properties.However,the hydrophilic groups contained in WPU cause poor water resistance and corrosion resistance.The performance of the coating is enhanced by the addition of nanomaterials,and a small amount of nanomaterial can effectively improve the performance of the coating.As the thinnest known material,graphene has excellent electrical,optical and mechanical properties,high specific surface area and impermeability,and its excellent barrier properties make it a huge potential for coating anti-corrosion and waterproof applications.However,the strong van der Waals force of the graphene layer results in poor dispersibility in the polymer and easy agglomeration.The high conductivity and metallicity of the graphene itself make it have corrosion-promoting activity,which is not conducive to the corrosion protection of the coating.The graphene oxide phase contains a large amount of oxygen-containing functional groups on the surface of the graphene,and the graphene can be functionalized and modified by reacting with an organic substance.The intermolecular force of the modified graphene sheet is weakened,and the introduced organic functional group imparts more unique properties to the graphene sheet while improving the compatibility of the graphene with the polymer.In this paper,graphene oxide(GO)is prepared by modified Hummer's method,and functionalized polydopamine modified graphene oxide(PGO)is easily prepared by dopamine one-step method.Through Fourier transform infrared spectroscopy(FT-IR).UV-vis absorption spectroscopy(UV-vis),Raman spectroscopy(Raman),X-ray photoelectron spectroscopy(XPS),atomic force microscopy(AFM)confirmed that polydopamine was successfully grafted onto the surface of graphene oxide.Preparation of waterborne polyurethane/graphene oxide(WPU/GO)and waterborne polyurethane/polydopamine functionalized graphite oxide with different GO or PGO additions(0 wt%,0.1 wt%,0.5 wt%,1 wt%)by solution blending and physics sonication.The effect of GO and PGO on the stability and uniformity of WPU aqueous dispersion was investigated by studying the particle size and zeta potential.The dispersion of GO and PGO in the coating was observed by field emission scanning electron microscopy(SEM).Electrochemical impedance spectroscopy and salt spray experimental tests were used to study the corrosion protection properties of composite coatings.The effects of nano-materials on the surface properties of waterborne polyurethane coatings were investigated by testing the adhesion,contact angle,hardness and gloss of the composite coatings.The thermal stability and water resistance of the composite coatings were also characterized.The results showed that 0.1 wt%of GO slightly enhanced the anti-corrosion performance of waterborne polyurethane coating.With the increase of GO addition,the corrosion resistance of coatings droped sharply.Adding 0.5 wt%,1 wt%GO,the waterborne polyurethane coatings exhibited less corrosion resistance than blank coatings.The water resistance of WPU was also greatly reduced due to the addition of GO,which is presumed to be due to the poor dispersibility of GO in the polymer matrix and its own hydrophilicity.The WPU/GO coatings contact angle showed little change compared to the WPU coating,indicating that GO had no effect on the hydrophilicity of the coatings.The hardness of WPU coatings with 0.1 wt%GO and 0.5 wt%GO did not change compared with WPU,while the hardness of WPU coating with 1 wt%GO increased by one grade,GO was not obvious enough to improve the WPU coating hardness.The gloss of the WPU coating showed a stepwise significant decrease as the amount of GO added increased.In addition,the addition of GO reduced the thermal stability of the WPU.GO does not significantly enhance the overall performance of aqueous polyurethane coatings and even exhibits negative effects in many respects.In contrast,with the addition of PGO,the anti-corrosion performance of WPU/PGO coatings first increased and then weakened,and the maximum resistance of the WPU/PGO coating with 0.5 wt%PGO was 1.77×10~8?cm~2.The maximum impedance modulus is also shown in the long-term corrosion test.After 168 hours of salt spray test,it can still provide effective protection to the substrate,and it also proved that its corrosion resistance was much better than other samples.1 wt%of PGO drastically reduced the protective properties of WPU coatings,even worse than blank WPU coatings.It was due to the organic modification of polydopamine improved the compatibility of graphene and polyurethane.The polydopamine covering the surface of graphene synergistically provided anti-corrosion effect,but excessive PGO will agglomerate in WPU and cannot provide ideal protective effect.The water resistance of the WPU/PGO coating was significantly higher than WPU coating.The contact angle of WPU/PGO coatings were nearly 10°higher than that of WPU and WPU/GO coatings,indicating that the hydrophobicity of the coating were enhanced.The hardness of the WPU coating with 0.1 wt%PGO was almost the same as that of the WPU coating,while the hardness of the WPU coating with 0.5 wt%PGO and 1 wt%PGO increased by 1 H and 2 H,respectively,to 3 H and 4 H.Compared to the WPU coating,the WPU coating with 0.1 wt%PGO and 0.5 wt%PGO had only a slight decrease in gloss,while the WPU coating with 1%PGO had the lowest gloss of all samples.The addition of PGO enhances the thermal stability of the WPU.In summary,the WPU coating with 0.5wt%PGO exhibited the best overall performance.