Preparation And Properties Of Multiwalled Carbon Nanotubes/Polyurethane Anticorrosive Conductive Coatings

The existing conductive coatings are oil-based coatings which can release organic solvents into the air during the process of preparation,construction and curing,it can pollute the environment and give rise to serious security risks.Moreover,the addition of a large number of conductive particles can make the antistatic property of the coatings satisfy the application requirements,but the dispersion of the conductive particles would be uneven during brushing especially multi-layer brushing,reducing the properties of adhesion,corrosion resistance and hydrophobicity of the coatings.To resolve the above problems,the eco-friendly waterborne polyurethane(WPU)was used as a film-forming material and the multiwalled carbon nanotube(MWCNT)was used as a conductive medium in this paper.Firstly,the MWCNTs/WPU anticorrosive conductive coatings were prepared by brushing to reveal the dispersion disciplinarian and the conductive mechanism of the MWCNTs with different contents in the WPU matrix.Then,coatings with the MWCNT contents in the gradient distribution were used for multi-layer brushing,during curing the free settling of the MWCNTs was utilized to translate the adverse factors of the agglomeration and sedimentation into the secondary diffusion of the MWCNTs,the secondary dispersion mechanism of the MWCNTs was discussed.Moreover,WPU anticorrosive conductive coatings with low MWCNTs contents were prepared by a self-designed electrostatic spraying device for liquid materials,the effect of the electrostatic spraying on the dispersibility of the MWCNTs was maninly studied.On the basis of the above studies,the polytetrafluoroethylene(PTFE)emulsion with low surface energy was added to improve the surface energy which can play a synergistic effect with the micro-rough surface structure constructed by the MWCNTs,then,the effects of the unmodified and coupling agent modified hydrophobic fillers on the hydrophobic performance of the WPU anticorrosive conductive coatings were respectively researched.It will provide a foundation for the optimization of the anticorrosive conductive coatings and a new technology for the anticorrosion in petrochemical industries.The main conclusions are shown as followsThe low MWCNTs contents in the MWCNTs/WPU anticorrosive conductive coatings which were prepared by brushing can make the MWCNTs particles stretch well in the WPU matrix,the extended MWCNTs were conducive to the formation of a conductive network,but the mesh of the whole coating was so large that partial regions of the coatings were conductive,leading to the uneven conductivity.When the MWCNTs contents were high,a small amount of the stretched MWCNTs were scattered in the top areas of the coatings and a large number of the agglomerated MWCNTs in the bottom areas of the coatings.Additionally,the conductivity of the coatings was increased with an increase in the MWCNTs contents.The coatings with the MWCNTs contents less than 0.3 wt%were dominated by capacitive conductivity.The coatings with the MWCNTs contents of(0.3?0.6)wt%were mainly tunnel conductivity.The coatings with the MWCNTs contents more than 0.6 wt%were tunnel conductivity which was principally in the top areas of the coatings and conductor conductivity which was primarily in the bottom areas of the coatings.When the MWCNTs contents of the coatings were over 0.6 wt%,the aggregated and deposited MWCNTs increased the defects in the bottom areas,so that the corrosion resistance,adhesion,surface hardness and wear resistance were decreased with the increase of the MWCNTs contents.As the MWCNTs content was 0.6 wt%,the MWCNTs dispersed uniformly and intertwined with each other to form the conductive path,the volume resistivity and adhesion of the WPU coating with 0.6 wt%MWCNTs can conform to the standard requirements and its corrosion current density was about one order of magnitude less than that of the pure Q235 steelThe underlayer coatings were prepared by brushing the WPU dispersion with 0.4 wt%MWCNTs,then,the WPU dispersions with(0.6?1.5)wt%MWCNTs were respectively casted on the uncured underlayer coatings to prepare the upperlayer coatings,thus,the MWCNTs/WPU anticorrosive conductive coatings were obtained by this multilayer brushing In this case,the MWCNTs in the upperlayer coatings settled during curing and then penetrated to the underlayer coatings,leading to the homogeneous distribution of the MWCNTs in the whole multilayer coatings.The higher MWCNTs contents of the upperlayer coatings made the conductivity of the conductive coatings better.However,there were more agglomerated MWCNTs in the conductive coatings as the MWCNTs contents in the upperlayer coatings were over 0.9 wt%,causing the conductive coatings to generate more defects and form interlayer fracture,so that the corrosion resistance was reduced.The dispersion of the MWCNTs in the multilayer coating with 0.4 wt%MWCNTs in the underlayer and 0.9 wt%MWCNTs in the upperlayer was the most uniform,so that the multilayer coating held the best electrical conductivity,its corrosion rate was reduced to 0.0029 mm·a-1,its volume resistivity was decreased by at least 1 orders of magnitude and its adhesion was increased by 10%,compared with those of the coating with 0.6 wt%MWCNTs which was prepared by single-layer brushing.Electrostatic spraying made the MWCNTs disperse more evenly and produce a spatial network structure in the WPU,leading to that the coating structure became more compact and the amount of the MWCNTs in the top areas of the coatings was equivalent to that in the inside areas,therefore,the problem that the high MWCNTs contents in the top areas and the low MWCNTs contents in the bottom areas of the coatings prepared by brushing was overcome.As the WPU was cured,the micro-rough surface structures of the coatings were observed due to that the MWCNTs particles were partially exposed over the coating surfaces.With an increase in the MWCNTs contents,the surface hardness and conductivity of the coatings was increased,however,its adhesion,corrosion resistance and wear resistance were first increased and then decreased.The volume resistivity can meet the industry requirements when the MWCNTs content was only 0.2 wt%,which was far lower than that of the conductive coatings prepared by brushing.The structure and comprehensive performances of the coating with 0.3 wt%MWCNTs were the best among all the obtained coatings,its volume resistivity was 6.8×1052·cm.Furthermore,its adhesion and impedance value were respectively increased by 10.18%and about one order of magnitude compared with the pure WPU coating,and its wear rate was decreased by 50.21%.After adding the PTFE with low surface energy,the WPU dispersions containing PTFE and MWCNTs were sprayed by electrostatic spraying.When the conductive coatings were produced,the low surface energy of the PTFE could play a synergistic effect with the micro-rough structure arising from the MWCNTs,making the obtained conductive coatings hold the superhydrophobicity.After adding the PTFE,the permeation threshold of the conductive coatings rose and the friction coefficients were decreased.In addition,there could be some interfaces between the PTFE resins and the WPU resins due to the poor compatibility,reducing the adhesion and corrosion resistance of the coatings,so that the coatings were easy to peel off during wear as the increase of the PTFE contents.Remarkably,the structure and composite properties of the coating with the WPU,PTFE,and MWCNTs at a mass ratio of 7:3:0.2 were the most outstanding among all the obtained coatings,its volume resistivity,WCA and adhesion were respectively 2.3×104 ?·cm,154° and 5.01 MPa.In addition,its friction coefficient and corrosion current density were respectively decreased by about 1 order and 3 orders of magnitude in contrast to those of the pure WPU coating.A kind of silane coupling agent which can be coupled with polar and non-polar groups was incorporated into the WPU coatings,then,the WPU,PTFE and MWCNTs were linked into a whole by the coupling agent,improving the uniformity of the coating and reducing the curing interfaces between the WPU and the PTFE,so that the corrosion resistance of the coating was enhanced.Additionally,the dehydration reaction that occurred between the silane coupling agent and the polar surface of Fe formed the-NH2 goups,leading to the decreased adhesion of the coating to the substrate,so that the problems of the coating without adding the coupling agent solving,including low adhesion,easy delamination and easily peeling off during wear,were solved.Adding an appropriate amount of the coupling agent can reduce the contents of the PTFE and the MWCNTs.As an increase in the contents of the hydrophobic fillers modified by the coupling agent,the conductivity and hydrophobicity of the coatings were improved and the adhesion,wear resistance and corrosion resistance were first increased and then declined.Noticeably,the structure and combination properties of the coating with the WPU,PTFE,MWCNTs and KH-550 at a mass ratio of 7:1.5:0.1:0.032 were the most excellent,the volume resistivity and WCA of the obtained coating were respectively 1.5×104 ?·cmand 155°.Besides,compared with the pure WPU coating,the adhesion was decreased by 7.9%,the friction coefficient and corrosion current density were reduced by about 2 and 3 orders of magnitude,respectively.