Interface Modification And Properties Of Metal/Polyurethane Infrared Low Emissivity Composite Coatings

With the rapid development of infrared detection devices and infrared-guided weapons, infraredlow emissivity coatings (IRLEC) have recently become a topic of considerable interest. Especially,the metal/polyurethane composite coatings composed of copper (Cu) filler and polyurethane (PU)have the potential for applications in IRLEC. However, the poor corrosion resistance and mechanicalproperties of Cu/PU coating are the major causes limiting its application. In the present work,experiments were carried out to assess the influence of interface modification on the corrosion andmechanical properties of low emissivity coating. Furthermore, the lifetime of the IRLEC wascalculated for engineering applications.Surface of Cu powder was chemically modified using gamma-aminopropyltriethoxy silane (KH550)in order to improve the interfacial interaction between Cu and PU polymer, and therefore, expectablecorrosion resistance of the Cu/PU coating with infrared low emissivity was acquired. An an obviousinteraction between Cu and PU was induced by the addition of KH550. Results have shown that theproper amount of KH550is benefit to the dispersion of Cu and induces the strong chemical interfacialinteraction, which often keeps the infrared low emissivity and increases the corrosion resistance of theCu/PU coating. Mechanical properties increase with increasing KH550content.Synergy effect of the surface modification of Cu by surfactant, cetyl trimethyl ammonium bromide(CTAB) and KH550was evaluated. An obvious interaction between Cu and PU was induced by theaddition of KH550, which implied that KH550can improve the chemical interfacial interaction whileCATB only improved the physical interaction between Cu and PU. The interfacial interaction betweenCu and PU was improved by the synergy effect of proper amount of KH550and CTAB, benefiting tothe dispersion of Cu and the low porosity of Cu/PU coating, which keeps the infrared low emissivityand increases the corrosion resistance of the Cu/PU coating. The mechanical properties of Cu/PUcoatings increase obviously after synergy effect of the surface modification.Surface modification of Cu with silver (Ag) using a ball-milling method evaluated was evaluated. Itwas found that Ag was homogeneously distributed in Cu and the encapsulation of oil layer on thesurface of Ag-Cu composite powders was formed after ball-milling, therefore, compatibility withorganic phase was improved, which often keeps the infrared low emissivity and enhances theanti-corrosion performance of the coating. After surface modification with Ag, the impact strength of(ball-milled Ag-Cu)/PU coatings keeps unchanged, and the impact strength of (ball-milledAg@Cu)/PU coatings is better. Modification of Cu/PU interface with aluminum (Al) to Al/PU interface was evaluated. Due toincreasing the electrical conductivity with increasing Al content, the infrared emissivity is deceasingobviously. The relationship between the Al content and mechanical properties presents a "U" type, andAl/PU composite coating has good adherence and impact strength at Al content of40wt.%. Corrosiontest results showed that the Al2O3from Al oxidation do not sacrifice the corrosion resistance of PUitself, and the low emissivity composite coatings exhibited favorable corrosion resistance.Modification of Cu/PU interface with bronze and epoxy to Cu-Sn/epoxy-polyurethane (EPU)interface was evaluated. Due to increasing the electrical conductivity with increasing bronze content,the infrared emissivity is decreasing obviously. The bronze/EPU composite coating had good adherenceand impact strength at bronze content below50wt.%, and then mechanical properties decreased in thebronze content range from50wt.%to60wt.%. The low emissivity bronze/EPU composite coatingsexhibited favorable corrosion resistance. By comparing bronze/EPU, Cu/PU,(ball-milled Ag-Cu)/PUand Al/PU coatings, the bronze/EPU coatings with40wt.%bronze have the best adherence, infraredlow emissivity and good corrosion resistance.The microstructural-electrochemical model is employed to predict the lifetime of infrared lowemissivity composite coatings in chloride environments. Electrochemical data collected in3.5wt.%NaCl solution is presented for the IRLEC, and these values are used as inputs for a mechanistic-basedcorrosion model which yields the salt spray life of the coating. To check the calculated results, themodel predictions were compared with the results of salt spray tests. The current work showed thatthe model was able to predict lifetime of IRLEC under salt spray, but tended to under-predict lifetimeat short times and over-predict at long times. Under-prediction may be associated with corrosionprotection of metallic pigment particles by polymer. Over-prediction by the model at longer exposuretimes may be associated with the fact that an influence of porosity is not included as a part of thissimple model.The mechanical properties of IRLEC decreased with increasing heating temperature and time, andthe maximum-tolerance temperature of the coating was573K. Moreover, the Arrhenius relationshipwas employed to calculate the mechanical lifetime of infrared low emissivity composite coatings, andthe mechanical lifetime at different Mach numbers was calculated when the impact strength wasdecreased to a particular level. The calculated results when compared with observation data validatethe effectiveness of the model predictions.The emissivity of IRLEC increased with increasing heating temperature and time, and that theemissivity of coating failure increased with decreasing Mach number. Moreover, the Arrheniusrelationship was employed to calculate the lifetime of infrared low emissivity composite coatings.When compared with observation data the calculated results validate the effectiveness of the model predictions.The emissivity of IRLEC increased with increasing heating temperature and time in damp heat.Moreover, the Arrhenius relationship was employed to calculate the lifetime of infrared lowemissivity composite coatings in damp heat. The calculated results were compared with observationdata and validated the effectiveness of the model predictions.