Composition, Structures And Properties Of Low Infrared Emissivity Coatings (LIREC)

With the improvement of infrared detection technology, the importance of the development and application of infrared stealth materials increases. Due to its outstanding advantages of infrared stealth coating, as infrared stealth coating, it is one of the most important species. But how to design and prepare infrared stealth coatings which has low infrared emissivity and the excellent performance of engineering applications becomes the key factor.In this paper, the ideal composition and structure of low infrared emissivity coating (LIREC) was designed by theoretical calculation and analysis, the physical model of radiation absorbing of containing flake particles of coating was designed, its infrared emissivity approximate calculation formula was derived, and the related experiments on the conclusions was designed and verified. High infrared transparency of EPDM rubber and low infrared transparency of polyurethane were choosed for two typical representatives, which were modified as resin-based material, and LIREC was prepared. The composition and structure of LIREC on infrared emissivity performance, mechanical properties, corrosion resistance and anti-aging properties of LIREC was studied, providing the basic guidance for LIREC. The main contents include:(1) The composition and structure design of LIREC:Through theoretical calculation and analysis, the effects of the pigment material, shape of pigment particles, the distribution of pigment particles in the coating and infrared transparency and thickness of the surface coating resins on infrared emissivity of coating were researched, and thus the ideal composition and structure of LIREC were designed. The physical model of radiation absorbing of coating containing flaky particles was designed and the infrared emissivity approximate formula was derived.(2) Study of the composition and structure on the infrared emissivity of coating:The related experiments were designed to validate the conclusions of the composition and structure design of LIREC. The results showed that, the infrared emissivity of coatings can significantly decrease whenâ‘ the flake metallic powder was elected as pigment, andâ‘¡the high-infrared transparent resin was elected as adhesive, andâ‘¢the polarity increases after grafting resin adhesive with polar groups, enhances its compatibility with the metallic paint and reduces the gap in the coating, andâ‘£the metallic flake pigment was spread out in order in the coating. And the coating with low infrared emissivity (about 0.1) was prepared.Through the preparation of the different attachment rates of Cu flake pigments, infrared coating resin layer thickness can be adjusted, thereby regulating the infrared emissivity of coating. The high attachment rate of the metal pigments can significantly reduce the infrared emissivity of coating. The impact of the infrared transparency of the resin on infrared emissivity of coating reduces significantly when the high attachment rate of Cu flake pigment was elected. Even if the less infrared transparency resin was used, LIREC can be obtained. Therefore, the choice scope of resin adhesive is broadened, and lay a solid foundation for the development of LIREC with good properties.After experimental verification, the infrared emissivity aformula can be used to calculate approximately the infrared emissivity of coating containing flaky particles and can reflect the main factors of the infrared emissivity of the coating.(3) Study of the composition and structure on the mechanical properties of the LIREC:The impacts of polar groups of EPDM-g-MAH, pigment content and resin structure on the mechanical properties of LIREC were analyzed. The results showed that, the mechanical properties of LIREC can be improved by designing the composition and structure of LIREC. 1) Grafting low polarity adhesive with polar groups. The XPS analysis shows that the polar groups of EPDM-g-MAH can form coordination bonds with the metal, significantly improve the mechanical properties of LIREC. 2) Adjusting pigment content. The adhesion of LIREC increased with the increased pigment content, and the hardness first increased and then decreased. The flexibility and resistance to impact strength were significantly reduced for rigid chain of PU-based coatings, while the flexibility and resistance to impact strength of the coating were less affected for the flexible chain resin EPDM-g-MAH. 3) Adjusting resin structure. The flexibility and impact strength resistance of LIREC prepared with EPDM-g-MAH were better than that prepared with polyurethane, while the hardness was somewhat less. The flexibility and impact strength resistance of LIREC prepared with epoxy modified polyurethane were greatly improved, while the adhesion and hardness remain basically unchanged.(4) Study of the composition and structure on the corrosion resistance of LIREC:The corrosion mechanism and process of LIREC was studied in aqueous salt solutions. The results showed that the corrosion failure of LIREC was mainly due to the corrosion of Cu powder paint generating CuCl, causing a substantial increase in infrared emissivity. The resin adhesive can provide protection to the pigments and slow down the corrosion process. Pre-corrosion, anti-seepage properties of the resin play an important role, the better in resin with good resistance to water seepage, the slower the infiltration rate of corrosion solution; at the latter part of corrosion the resin adhesion to metal pigments plays a more important role, the strong adhesion is able to slow down the interface of the sparkling progress. EPDM and polyurethane which were modified as resin adhesive, and LIREC were prepared. The composition and structure of LIREC on corrosion resistance was studied. The results showed that, the corrosion resistance of LIREC can be improved by designing the composition and structure of LIREC. 1) Grafting low polarity adhesive with polar groups. EPDM modified by the MAH increased the polarity, and significantly increased the compatibility and adhesion with metallic pigment, reducing the porosity of the coating and thus enhancing the corrosion resistance of LIREC. 2) Adjusting the composition and structure of resin. Resin adhesive modified by introducing the component with stronger corrosion resistance, and full consideration should be given on the surface tension between the various components. The polyurethane modification to introduce the epoxy which has a stronger adhesion to metal paint can improve corrosion resistance of LIREC. Due to the much difference in surface tension between fluorine composition and epoxy resin, the mixed capacity is poor, so the corrosion resistance of LIREC got worse when the both two were used. 3) Increasing the compactability of coating, and reducing the pores in the coating. The study of Epoxy-modified polyurethane (EPU) showed that proper curing temperature, composition ratio and the type of epoxy resin can improve the degree of crosslinking and reduce the structural pore, and appropriate defoamer usage and curing temperature can reduce the pores caused by the bubbles and solvent evaporation, thereby increasing the corrosion resistance of LIREC. 4) The corrosion resistance of LIREC which prepared with resin adhesive with stronger corrosion resistance is not determinately better. And full consideration should be given on the pigment content needed, the anti-seepage properties of resin adhesive and the adhesion between resin adhesive and metal pigment. 5) Smearing high infrared transparency finish on LIREC. The LIREC was prepared with high infrared transparency EPDM-g-MAH as the finish and EPU-based material, the infrared emissivity of the coating increases 0.05, but the corrosion resistance has substantially increased.(5) Study of the composition and structure on the heat aging and light aging properties of LIREC:The heat aging property of different LIREC was studied. The results showed that the reason for the increased infrared emissivity in thermal aging is oxidation of Cu powder pigment. The resin adhesive can provide protection to the pigments and slow down the oxidation process, and protect performance of resin adhesive with the good compatibility with pigment and the strong adhesion was better. EPDM-g-MAH/20% Cu and EPU/50% Cu coatings had good heat aging properties, and the coatings can be used for a long time below 433 K.The light aging property of different LIREC was studied. The results showed that, with the extension of time of artificial accelerated aging, the loss of light and color and relatively mild powder occurred on low infrared emissivity coating, and the infrared emissivity of coating was also gradually increased. The poor compatibility of binder components could result in two-phase separation, increasing the absorption of ultraviolet light and accelerating the aging; The decrease of the content of components vulnerable to UV-degradation can increase the resistance to artificial accelerated aging of the coating. The order of artificial accelerated aging-resistant properties of low infrared emissivity coatings was: FPU/50% Cu > PU/50% Cu > EPDM-g-MAH/20% Cu > EPU/50% Cu > EPDM-g-MAH@EPU/50% Cu, and was positively correlated with the resistance of artificial accelerated aging of the used adhesives.