Encapsulation Of Aluminum Pigments And Their Applications In Coatings

Aluminum flakes (aluminum pigments) have been used in solvent-borne metallic paints or inks for many years due to their metallic appearance and"flop-effect". Recently, growing importance of environmental aspects has led the paint and coatings industry to the development of coating systems with a reduced content of volatile organic compounds (VOC). Waterborne coatings are the preferred way to solve the problem. However, since the waterborne coatings are mostly prepared in an alkaline medium, the aluminum pigments react with water to evolve much hydrogen, resulting in severe deterioration of metallic luster and dangerous pressure built up in the storage vessels. Therefore, it is necessary to treat the aluminum pigments first before they are applied in waterborne coatings.In this paper, inorganic-nano-SiO₂ encapsulated aluminum pigments (SiO₂/Al) were prepared by using TEOS as precursor with the traditional sol-gel method. The mechanism of the process of encapsulation and the effect of various factors on the encapsulation were analyzed. The structure and performance of the SiO₂/Al was characterized by Fourier transformation infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was found that when the mass of aluminum pigments is 2.0 g, the volume of ammonia is 3.0～4.5 mL, the molar ratios of Al/TEOS and H2O/TEOS are 5.5 and 30 respectively, the temperature and time of the reaction are 40℃and 6h respectively, compact SiO₂ films on the surface of aluminum pigments can be formed in the traditional sol-gel process, thus greatly improving the alkali resistance of the pigments. The excellent alkali resistance of the coated pigments is also verified by dipping it in an alkaline medium with a pH value of 11 for 60 days, without any hydrogen being evolved. However, the gloss of SiO₂/Al is greatly weakened in the sol-gel process. One of the possible measures to solve this problem is to add organic monomers in the hydrolysis process of TEOS.Encapsulated aluminum pigments were prepared by modified sol-gel method using tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) as precursors. The reaction conditions were optimized and the aluminum pigments prepared under the optimum conditions were characterized by Fourier transformation infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Subsequently the stability of the aluminum pigments in acid and alkaline aqueous media was examined. It was found that ratio of VTES/TEOS, reaction temperature, contents of ammonia and water have great influence on the sol-gel film forming on the surface of aluminum pigments and thus on the stability of the encapsulated pigments in corrosion media. In addition, both TEOS and VTES hydrolyze and condense to form a dense netlike layer on the surface of aluminum pigments. XPS analysis shows that TEOS-VTES successfully bonded to Al surface. The 0.64 eV shift of Al 2p binding energy indicates a new Si-O-Al bond formed on Al surface. The corrosion protection efficiencies of the TEOS-VTES coated aluminum pigments (TV/Al) prepared under the optimum experimental conditions reach 99.8% and 99.9% in acid media of pH1 and alkaline media of pH11 respectively, suggesting that TEOS-VTES sol-gel coatings can protect aluminum pigments well in corrosion media. The gloss of TV/Al decreased by 3.2% compared with that of uncoated pigments, which indicates that the luster of the pigments is kept well after encapsulation.In order to improve the compatibility of aluminum pigments with resins and other organic compounds contained in coatings, aluminum pigments were first coated with sol-gel film by using tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) as the precursor, followed by free radical copolymerization of styrene (St) and divinylbenzene (DVB) with the vinyl group of the VTES. The effects of the amouts of St, DVB and initiator on the morphology were investigated. The as-prepared encapsulated aluminum pigment was characterized by FTIR, SEM and XPS. Subsequently the stability of the aluminum pigments in acid and alkaline aqueous media, the gloss of the pigments, and compatibility of aluminum pigments with resins and other organic compounds contained in coatings was examined. It was found that when the mass of aluminum pigments is 2.0 g, the volume of TEOS and VTES are both 2.0 mL, the mass of St is 4.0～6.0 g, the contents of DVB and initiator are 20% and 1.0% of St respectively, uniform and compact polymeric films can be formed on the surface of aluminum pigments. The corrosion protection efficiency of the TEOS-VTES-St-DVB-coated aluminum pigments (TVSD/Al) in acid medium of pH1 and alkaline medium of pH11 are both 100%, indicating that the inorganic-organic hybrid composite layer of TVSD on the surface of the aluminum pigments can protect them well from alkaline and acid corrosion. The gloss of TVSD/Al decreased by 8.5% compared with that of uncoated pigments, which indicates that the luster of the pigments is kept basically after encapsulation. Furthermore, the TVSD/Al can be dispersed better in the solvent coatings than uncoated aluminum pigments so that the surface of the dry coatings is smoother.Therefore, it is a good way for aluminum pigments to be encapsulated by TVSD to avoid the alkaline and acid corrosion, keep the luster and be easily dispersed in the solvent coatings. It is necessary to improve the dispersibility of aluminum pigments in water in order to apply them in waterborne coatings. Hydrophilic-group-containing monomer maleic acid anhydride (MAA) was induced to the surface of TVSD/Al to form TEOS-VTES-St-DVB-MAA encapsulated aluminum pigments (TVSDM/Al). The process of encapsulation was investigated. The as-prepared encapsulated aluminum pigments were characterized by FTIR, SEM and XPS. Subsequently the stability of the aluminum pigments in alkaline aqueous media and dispersibility of aluminum pigments in water were examined. It was found that the sol-gel method can be used to encapsulate aluminum pigments. However, specific conditions are necessary to achieve good stability in alkaline aqueous media. The most important factor is to form a protecting layer on the surface of the aluminum pigments. In the sol-gel process, the TEOS and VTES can condense on the aluminum pigments surface as a barrier layer and further copolymerize with St, DVB and MAA monomers to form a compact protecting layer. The stability test shows that the sol-gel derived inorganic-organic hybrid coatings can give the aluminum pigments significantly improved storage stability in alkaline aqueous media. Furthermore, TVSDM/Al can be dispersed more easily than uncoated aluminum pigments.Nowadays, the most extensively used resin in waterborne coatings is acrylic resin. For the sake of dispersibility and compatibility of encapsulated aluminum pigments in waterborne coatings, methyl methacrylate (MMA) and acrylic acid (AA) were induced to the surface of TV/Al to polymerize with the vinyl group of the VTES to form organic/inorganic hybrid layer of TEOS-VTES-MMA-AA coated aluminum pigments (TVMA/Al). The as-prepared encapsulated aluminum pigments were characterized by FTIR, SEM and XPS. Subsequently the stability of the aluminum pigments in alkaline aqueous media and dispersibility of aluminum pigments in water was examined. It was found that poly-MMA exists on the surface of the pigments. TEOS-VTES encapsulate aluminum pigments first and poly-MMA and poly-AA encapsulate them subsequently to form compact TEOS-VTES-MMA-AA encapsulated aluminum pigments (TVMA/Al). The stability tests show that TVMA/Al exhibite excellent stability in acid and alkaline media. Furthermore, TVMA/Al can be dispersed uniformly in water so that it is promising for TVMA/Al to be applied in waterborne coatings in the future.Finally, applications of the encapsulated aluminum pigments prepared in the experiment in solvent and waterborne coatings were studied. It was found that the overall performances of the encapsulated pigments are comparative with that of Japanese products and the luster of the encapsulated pigments is even more excellent than that of Japanese products when they are applied in solvent coatings. But the performances of hiding power and adhesion with resin of the encapsulated aluminum pigments are deficient to some extent. The overall performances of the encapsulated aluminum pigments TVMA/Al are equivalent with that of Japanese products and the luster of the encapsulated pigments is even more excellent than that of Japanese products when they were applied in waterborne coatings. Therefore, it is promising for TVMA/Al to be applied in waterborne coatings in the future.