Improvement Of The Mechanical Properties And Dirt Resistance Of PVDF Coatings

Poly(vinylidene fluoride)(PVDF) coatings are one of the best of high-performance coatings. It has the advantages of extremely high weatherability, corrosion resistance, thermal stability, chemical resistance and so on, and it often used as steel topcoats for outdoor application. However, PVDF coatings exhibit relatively poor mechanical strength and low dirt resistance under serevely-polluted environment, i.e., in China, which absolutely do not favour its wide applications. In this thesis, one or more kinds of additives including inorganic nanoparticles, organic hydrophobic additives, hydrophilic additives, and photocatalytic nano-TiO2particles are introduced into original PVDF coatings to adjust the mechanical properties, wettability and dirt resistance of PVDF coatings. Influence of inorganic nanoparticles on the mechanical properties of PVDF was studied. The methods to prepare superhydrophobic and highly(supra)-hydrophilic PVDF coatings were estabilished. The relationship between the surface characters and the dirt resistance of PVDF coatings were correlated. The detailed research contents and results are as follows:Inorganic nanopartices including nano-silica, nano-titania, nano-alumina and nano-barite are added into original PVDF coatings. Influence of the types and dosage of inorganic nanoparticles on the mechanical properties, such as, hardness, abrasion resistance, impact resistance and scratch resistance, of PVDF coatings, were studied. It was shown that the mechanical strength of PVDF coatings can be improved by adding a small amount of inorganic nanoparticles. However, the change trend of mechanical properties are dependent on the type of nanoparticles as well as on the mechanical term. Better mechanical improvement was obtained when the PVDF coatings contained2wt%(based on the mass of dry film) nano-alumina or hydrophobic nano-silica (nano-silica-CHs). UV accelerated weathering experiments indicated that the PVDF nanocomposite coatings had improved weathering resistance.In order to increase the hydrophobicity of PVDF coatings, one or more kinds of additives including hydrophilic or hydrophobic inorganic nanoparticles, fluorinated polyssiloxane, and poly(tetrafluoroethylene) are introduced into original PVDF coatings. Influences of the type and dosage of additives, resin/pigment ratio, and preparation method on the wettability and gloss of PVDF coatings were investigated. Superhydrophobic PVDF coatings with a water contact angle (WCA) of157.6°and a sliding angle of7.3°were prepared by addition of nano-silica-CH3particles at resin/R-210/nano-silica-CH3=32.5/12.5/14.75(wt/wt). The methyl groups at the surface of nano-silica-CH3particles can be easily decomposed, being attributed to the poor durability of the superhydrophobic PVDF coatings. The fluorinated polysiloxane, PMSF, can effectively improve the durability of superhydrophobic PVDF coatings, that is, the WCA remained above140°after350h of UV accelerated weathering experiments.The dirt resistance of PVDF coatings with various surface characters were determined by both the national standard tests (GB/T9757-2001) and outdoor exposure experiments, meanwhile, it was compared with those of commercial steel topcoats (polyurethane coating, FEVE coating and polysiloxane coating). It indicated that the dirt resistance of PVDF coatings depends on the surface wettibility and the type of pollutants rather than on the surface roughness and composition. In outdoor exposure experiments, superhydrophilic PVDF paints exhibited excellent self-cleaning performance and thus the best dirt-resistance. Superhydrophobic PVDF coatings exhibited self-cleaning performance initially, but, became dirty finally due to piling of PM2.5pollutants. Nevertheless, the superhydrophobic nanoparticle coatings can be its long-term superhydrophobicity in outdoor exposure experiments. This fact teaches us that long-term durbable superhydrophobic PVDF coatings may be achieved through accelerated chalking of surface and therefore removing the chalked surface under nature force.Hydrophilic additives including silane coupling agent (SCA), fluorinated SCA, and photocatalytic nano-TiO2particles, are introduced into original PVDF coatings to prepare highly-hydrophilic PVDF coatings. Both UV accelerated weathering experiment and outdoor exposure experiments are empolyed to investigate the change of WCAs of different PVDF coatings in service. Influence of the type and dosage of additives, the amount of nano-TiO2and the ratio of resin-to-pigment on the the change trends of the WCAs of PVDF coatings were examined. It was shown that fluorinated SCA (HA-1) is an effective additive to get hydrophilic PVDF coatings. Higher HA-1content and higher pigment/resin ratio are beneficial to the hydrophilicity of PVDF coatings. The lowest WCA of32°can be obtained at the pigment/resin ratio of60:40and5wt%HA-1. The hydrophilic additive favors the gloss of PVDF coatings without affecting the surface morphology. Superhydrophilic PVDF coatings can be attained by incorporation of photocatalytic nano-TiO2particles and subsequent UV-irradiation. However, nano-TiO2particles deteriorates the weatherability of original PVDF coatings. The outdoor exposure experiment showed that the hydrophilic PVDF coatings with HA-1additive demonstrated the better dirt resistance.