Degradation Mechanism And Modification Of Polyurethane Coating And Rapid Evaluation On Its Failure

Organic coatings often suffer failure during their service on aircrafts and ships due to many environment factors,such as UV irradiation,salt spray,ozone,wet-heat shock and so on.Different degradation mechanisms are caused by different ageing environments.Therefore,in order to improve the protection performance,it is necessary to study degradation mechanisms of organic coatings under different environments,which could help to make a modification research.In addition,the coating protection performance is monitored and fast evaluated,and the preventive maintenance window of service coating is given to prevent corrosion and structure damage of metal substrate,which has important application value for improving the service reliability of equipments.In this study,polyurethane（PU）series coaitngs on AA7075 aluminum alloy were studied.The degradation mechanism of fluorinated PU coating under salt spray test（SST）and ultraviolet A（UVA）irradiation were investigated.The modification of PU coating by adding different kinds of nano-fillers was also studied to improve its adhesion strength,corrosion resistance and durability.Furthemore,an electrochemical sensor suitable for non-destructive monitoring of coating failure was designed,and it could be used for on-site evaluation of coating performance by characteristic parameters based on EIS.The main research contents and results are summarized as follow:1.The degradation process of typical fluorinated polyurethane（FPU）coating under SST and UVA was studied.Microstructure changes of FPU coating under SST and UVA were analysed by EIS evolution combined with micro-morphologies observation by scanning elelctron microscopy（SEM）and atomic force microscopy（AFM）and thermal analysis by thermogravimetric analysis（TGA）.Laster Confocal Raman（CRM）and Infrared Spectroscopy（ATR-FTIR）were used to characterize change of molecular structure of coating from surface to interior.Some results could be obtained:the coating impedance decreased more rapidly under SST than UVA irradiation in the initial stage.Nevertheless,the UVA rather than the SST increased the glass transition temperature（T_g）significantly and enhanced the brittleness of FPU coating.In addition,the micro-morphologies observation indicated that many blisters and wrinkles could be found on the surface of FPU coating during UVA but only micropores were observed during SST.All of these micro-flaws could provide fast channels for aggressive ions to migrate into the interior of coating,and decrease the protection performance of coating to Al matrix.In addition,the molecular structures change from surface to interior of FPU coating during ageing studied by ATR-FTIR and CRM showed that post-curing process was caused at the early stage of UVA irradiation,which could increase degree of cross-linking and impedance of coating.But the breakdown of coating molecule chain was caused by UVA irradiation along with ageing time,resulting in the rapid decline of coating impedance.During the SST ageing process,SST had little influence on the polymer structures and coating impedance dropped slowly.Furthermore,the dynamics of coating degradation showed that the UVA irradiation mainly resulted in chemical deterioration through the photo-oxidation with the influenced depth of 0²0μm beneath FPU coating,which mainly increased the concentration of carboxyl group and reduced the content of C-N group.However,SST only promoted hydrolysis of the superficial layer 0¹0μm from surface to interior.2.Effects of nano-CeO₂,carbon nanotubes（CNTs）and composite nanoparticles CeO₂@CNTs and CeO₂@PDA@CNTs on corrosion resistance of PU coating were investigated.It could be obtained that the amounts of nano-CeO₂ was less than 0.5 wt%,the corrosion resistance could not be improved efficiently due to insufficient addition and coating resistance dropped rapidly with increase of testing time for the modified coating.While the coating resistance got markedly changed and maintained high value for a long time with additive 1.0 wt%nano-CeO₂.For CNTs as nano filler,the higher CNTs loading（2.0 wt%）could deteriorate the corrosion barrier of PU coating due to agglomeration of CNTs in PU matrix.However,CNTs wrapped by polydopamine（PDA）could improve the dispersibility of CNTs in PU matrix,which not only enhanced compatibility of CNTs with PU,but slowed transmission of aggression ions in the coating by physical barrier.So the overall protection performance of PU composite coating was improved obviously.Moreover,the composite nanoparticles CeO₂@CNTs and CeO₂@PDA@CNTs were synthesized by nano-CeO₂ and CNTs.It was found that the modified CNTs wrapped by PDA could increase loading of nano-CeO₂.And CeO₂@PDA@CNTs could achieve synergistic effect of nano-CeO₂ and CNTs to protect coating after cycling ageing test,which could not only reduce the water absorption of the coating itself,but also enhance the adhesion strength of the coating and improve the abilities against UV aging.This is due to the adsorption of CeO₂ nanoparticles at the interface of PU/Alwhich could restrict the local corrosion of Al matrix caused by aggressive ions infiltration,thereby effectively improving the durability of PU coating.3.A coating ageing non-destructive monitor and impedance sensor were designed based on EIS and microelectronics,which could be used for early diagnosis of coating performance in the field.Compared with the traditional three–electrode system,the EIS spectra of different kinds of PU coatings were measured under SST by sensor.The results showed that the coating impedance sensor could be used to sensitively and accurately monitor coating ageing process.Several critical indexes related to EIS such as breakpoint frequency（f_b）,phase angle(θ_10Hz,θ_15KHz),specific capacitance(C_10Hz,C_15KhHz)and impedance modulus(Z_0.1Hz),were proposed to evaluate the severity of coating degradation.The results indicated that,the impedance sensor could accurately monitor the degradation process of coating,and once Z_0.1Hz<10⁶Ωcm² f_b>100 Hz orθ_10Hz<20°,the coating may be regarded as completely degraded and fail to protect metal substrate.So,these parameters could be used as a threshold for coating failure to guide preventive renewal or maintenance of coating on aircrafts and ships.