Study On The Properties Of Epoxy Conductive Coating Prepared By Electrostatic Spraying

Conductive coatings used in the petrochemical industry need to have high corrosion resistance,great bonding strength and wear resistance.Currently,metal particles and graphene are the main conducting media doped in conductive coatings.For conductive anticorrosive coatings applied via painting,the conducting particles may extensively precipitate because a large amount of the particles are poorly dispersive.Thus,the conducting particles content in the coating declines from bottom to surface.In this case,the excess conducting particles at the bottom lead to a low content of film-forming matter,further reducing adhesion between the coating and the matrix.In addition,the coatings become more prone to abrasion,also easily to bubble and peel.The low content of conductive particles in the surface layer leads to a lower conductivity than the design value of the coating.Therefore,the entire coating has a short service life,poor anti-corrosion and conductive effects,and it is difficult to meet the requirements of the petrochemical industry for conductive coatings.Although carbon nanotubes and graphene with superior conductivity have the potential to substantially lower the content of conducting particles,the application of conducting coatings via painting can still cause non-uniform conducting particle distribution.Electrostatic spraying involves the application of electric field stress and the secondary atomisation of particles.This may cause a dispersion effect on conducting particles.Regarding carbon nanotubes with one-dimensional linear structures and great draw ratios,as well as graphene with a two-dimensional layered structure,their influence mechanisms on the coating conductivity are still unclear,as well as the dispersion effects,influencing factors and dispersion mechanism of electrostatic spraying.In this research,epoxy resin commonly used in the chemical industry is used as the film-forming material,multi-walled carbon nanotubes(MWCNTs)and graphene are used as conductive media,and conductive coatings are prepared by electrostatic spraying.The dispersion mechanism and influencing factors of conductive medium in the coating during electrostatic spraying were explored.Additionally,exfoliation was conducted to determine the bonding strength of the prepared coatings.The coatings porosity was tested through the blue-point detection method,and the friction and wear properties of the samples were analysed using an HT-1000 high-temperature tribometer.Moreover,the surface resistivity of the coatings was measured using an RTS-9 double-electric-logging four-probe tester.The corrosion resistance of the coatings was explored through an electrochemical process.After the surface microtopography of the coatings was observed under a Zeiss Merlin Compact scanning electron microscope,the distributions of MWCNTs and graphene with different additive amounts in the coating were characterised.Finally,the following conclusions are drawn:(1)Electric field stress generated by electrostatic spraying was beneficial for the conducting media to be distributed in a direction of the electric field;consequently,the MWCNTs were distributed along the thickness,and graphene formed a layered structure in the thickness direction of the corresponding coating.This can effectively improve coating conductivity.This way,not only is the additive amount of conducting particles in the coating reduced,but the conducting particle precipitation can be prevented from elevating the coating surface resistance during the curing of epoxy resin.Therefore,the additive amount required for conducting coatings that reach the corresponding percolation threshold may be effectively lowered.(2)The electrical resistivity of conducting graphene/epoxy resin coatings prepared by electrostatic powder spraying may decline with increase in the graphene additive amount.Regarding the coatings porosity,abrasion resistance and corrosion resistance,the optimal values of the graphene additive amount should be obtained.For example,when 0.5 wt.%of the graphene was added,the volume resistivity of the coating decreased to 3397 ?·m;when the additive amount reached 3 wt.%,a conductive web was preliminarily formed inside the coating,and the percolation threshold was satisfied;the optimal porosity and corrosion resistance corresponded to coatings with 2 wt.%graphene.Considering that graphene plays a good lubricating role,the addition of 6 wt.%graphene produced the best abrasion resistance among the relevant coatings.Coatings with graphene content of above 6 wt.%exhibited reduced dispersity.Consequently,agglomeration is more likely to occur,which may reduce the friction and wear properties and corrosion resistance of the coatings.(3)For MWCNT/epoxy resin coatings prepared via electrostatic powder spraying,one-dimensional linearly structured MWCNTs can be more easily crosslinked and further form a conductive network,compared with graphene.Under the same additive amount,the electrical resistivity of MWCNT coatings was less than that of the graphene coating.More particularly,the conducting coating could reach its designed percolation threshold when the MWCNTs amount was 1.5 wt.%.Nevertheless,compared to coatings with the same additive amount of graphene,MWCNT-based coatings featured inferior bonding strength,corrosion resistance and friction and wear properties.(4)The acid treatment of MWCNTs improves their dispersity but impairs their structures.Under the same MWCNT content,the acidified coating had a higher resistivity value than the unacidified coating.In addition,surface treatment with sodium dodecyl sulphate(SDS)could also effectively enhance the dispersity of MWCNTs in the coatings,reducing their resistivity.Moreover,the designed percolation threshold could be satisfied only when the MWCNT additive amount subjected to surface treatment with SDS was 1 wt.%.(5)Graphene/epoxy resin and MWCNT/epoxy resin coatings were prepared through electrostatic liquid spraying.The upward electrostatic forces can effectively counteract the free settling of graphene,significantly improving the graphene dispersity.Moreover,graphene particles first sprayed on the matrix may collide with those previously sprayed on coatings on the matrix surface,thinning the epoxy resin coating covering the graphene surface and reducing the contact resistance between conducting particles.Under the action of electrostatic force,graphene and MWCNTs may arrange in the thickness direction of the coating,which favours the electricity passage formation and coating resistivity reduction.This way,the desired percolation threshold can be satisfied on the premise of a lower additive amount of conducting particles.In detail,the epoxy coating reached the corresponding percolation threshold when 0.5 wt.%MWCNTs was added,and the resistivity was only 26.1 ?·m.Compared with MWCNT coating interfaces with one-dimensional linear structures,the layer-structured graphene effectively extends the diffusion channels of the corrosive media.For this reason,the graphene-based conducting coating outperforms MWCNT-based conducting coatings in terms of corrosion and abrasion resistance.(6)For steel substrates,their coatings treated with low-temperature plasma(e.g.air,nitrogen and oxygen)exhibit significantly enhanced bonding strength.As the treatment time progresses,the bonding strength tends to first rise and then decline,and,the optimal plasma processing effects are generated by oxygen.Moreover,for Q235 steel surfaces treated with low-temperature oxygen plasma,the matrix surface can be cleaned.Moreover,plasma oxidation also occurs on the steel surface,producing Fe2O3 and FeOOH.When the time consumed by the oxygen plasma processing is rather short,the principal oxidative product is proved to be FeOOH.As it heaps up,the roughness of the corresponding steel surface is dramatically raised.Also,hydroxide radicals,effectively improving the polarity of the steel surface and surface free energy,react with polar groups in the epoxy coating to produce hydrogen bonds.This may further improve the bonding strength between the epoxy resin and the metallic matrix.With the extension of the oxygen plasma processing time,FeOOH in an oxide layer is decomposed at high temperature into Fe2O3,reducing both the free energy and polarity on the steel surface.Moreover,the oxide layer generated is non-conducive to the adhesion of the coating and the matrix because the layer is porous;thus,the coating-matrix bonding strength is lowered.