| Flaky aluminum pigment is one of the most used metallic effect pigments, and it is widely used in solvent-based coatings and inks due to its bright metallic luster and angle-dependent optical effect. As solvent-based coatings and inks are gradually being replaced by waterborne coatings and inks, the development of waterborne aluminum pigment is imperative. However, application of aluminum pigment in the waterborne is limited by two problems:Firstly, hydrogen from the reaction of aluminum pigments with water not only seriously weakened the metallic luster but also bring security risks in production and storage. Secondly, poor compatibility and dispersity of aluminum pigment in aqueous medium would lead to uneven coatings, low glossiness and aluminum shedding. Hence, surface modification of aluminum pigment for its application in aqueous medium is necessary.Currently, large numbers of researches focus on the inorganic and organic encapsulation of aluminum pigment. Dispersity of aluminum pigment in aqueous medium could be improved by grafting polymers, which first introduces the coupling agents containing double bonds, then conducts the situ polymerization of second monomer. But polymerization might be suppressed by this approach owing to the reactivity ratio.In this dissertation, the method of "grafting from" was introduced to immobilize azo initiator on the surface, which guarantee the following polymerization. The SiO2 coated Al (SiO2@Al) was synthesized by sol-gel method, then new waterborne aluminum pigment was prepared by grafting three different polymers from the surface of SiO2@Al using the method of "grafting from". Structure and morphology of SiO2@Al were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), gel permeation chromatography (GPC), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Performance of SiO2@Al was studied by corrosion resistance tests, BET adsorption, thermogravimetric analysis (TG), Zeta potential and particle size distribution. Main results are as follows:1. Synthesis and characterization of SiO2@AlSiO2 was coated on the surface of aluminum pigment in the ethanol using Tetraethoxysilane (TEOS) as precursor and triethylamine as catalyst. Results demonstrated that Si-O-Si bond was formed through the reaction between the hydrolysates of TEOS and Al-O-Si bond was formed by the reaction of hydrolysates of TEOS with hydroxyl groups on the surface of Al. The concentration of precursor, amount of water, reaction temperature and reaction time affected the coating performance, thereby affecting the corrosion resistance and dispersity. Optimum coating conditions were obtained by single factor and orthogonal experiments:the amount of aluminum pigment was 2.0 g, the molar ratio of silicon and aluminum was 5.2, the molar ratio of TEOS and water was 1:30, the concentration of catalyst was 0.13 mol/L, ethanol volume was 100 mL, reaction temperature was 45 ? and reaction time was 9 h. Corrosion inhibition efficiency of SiO2@Al in the sodium hydroxide solution with pH=12 was 97% under these conditions. BET specific surface area was 63.6 m2/g, void volume was 0.09 m3/g and the average pore diameter was 5.4 nm, suggesting that it had a dense surface. Although the surface of SiO2@Al was hydrophilic and easily infiltrated by water, it was still inclined to agglomerate in aqueous medium, resulting in decreasing glossiness. The agglomeration became more obvious as the increasing amount of coated SiO2.2. Synthesis and structural characterization of three kinds of polymer brushes-modified aluminum pigmentThree kinds of polymer brushes-modified aluminum pigment including weak polyelectrolyte brushes (PAA), strong polyelectrolyte brushes (PSS) and polymer brushes (PVP) were synthesized respectively on the basis of SiO2@Al. The molecular weight of PAA-modified aluminum pigment (PAA-SiO2@Al), PSS-modified aluminum pigment (PSS-SiO2@Al) and PVP-modified aluminum pigment (PVP-SiO2@Al) were 10768-22908,7921-20318 and 10957-31228, respectively. The grafting density of these brushes were 0.13-0.14 chains/nm2,0.09-0.11 chains/nm2, 0.16-0.17 chains/nm2. BET specific surface area tests indicated that after grafting polymer brushes, BET specific surface area and void volume decreased from 63.6 m2/g, 0.09 m3/g to 57.9 m2/g,0.06m3/g (PAA),43.7 m2/g,0.07m3/g (PSS),54.6 m2/g, 0.08m3/g (PVP), showing a denser surface.3. Corrosion resistance and dispersity of three kinds of polymer brushes-modified aluminum pigmentCorrosion inhibition efficiency of PAA-SiO2@Al, PSS-SiO2@Al and PVP-SiO2@Al in the sodium hydroxide solution with pH=12 were 100%,100% and 97.5%. Malvern particle size distribution suggested that d(0.5) of SiO2@Al declined from 24.6 ?m to 14 ?m,11.6 ?m and 14.7?m after grafting PAA, PSS and PVP, respectively. Dispersity of three kinds of polymer brushes-modified aluminum pigment in aqueous medium was improved obviously and agglomeration could not be found almost. It was also found that better dispersity could be obtained by larger molecular weight. The study also found that PVP was not sensitive to the pH and showed a low absolute value of Zeta potential (<13 mV). Dispersity of PVP-SiO2@Al was improved by steric hindrance. PSS was not sensitive to the pH as well, however, due to high charge amount of the surface, which increased with the increase of molecular weight, improvement of dispersity of aluminum pigment arised from the synergies between electrostatic repulsions and steric hindrance. In contrast, pH had significant effect on the Zeta potential of PAA. Microscopic examination also comfirmed that of dispersity of SiO2@Al could be dramatically improved, by polymer brushes.4. Properties of waterborne silver inkWaterborne silver inks were prepared using SiO2@Al, PAA-SiO2@Al, PSS-SiO2@Al and PVP-SiO2@Al as pigment in the waterborne inks. It was implied that poor dispersity of SiO2@Al was found in the waterborne silver inks. The coatings might not be smooth enough and glossiness of coatings ranged from 19.8 to 58.2 Gs as variable viscosity and temperature, polymer brushes-modified aluminum pigment could be well dispersed in the waterborne silver inks. The PAA-SiO2@Al coating showed a glossiness of 82 Gs, grade 0 of adhesion and hardness of 3H under optimum conditions, which had good smoothness and corrosion resistance. The PSS-SiO2@Al coating with a glossiness of 87.8 Gs, grade 1 of adhesion and hardness of 3H under optimum conditions represented good smoothness and corrosion resistance. The PVP-SiO2@Al coating with a glossiness of 95.4 Gs, grade 0 of adhesion and hardness of 3H under optimum conditions also showed good smoothness and corrosion resistance. Except for the hardness, these coatings had shown basically equal performance with imported products. |
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