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Double-layered surface decoration of flaky aluminum pigments with zinc aluminum phosphate and phytic acid–aluminum complexes for high-performance waterborne coatings
Journal Pre-proof Double-layered surface decoration of flaky aluminum pigments with zinc aluminum phosphate and phytic acid–aluminum complexes for high-performance waterborne coatings
Wenqi Li, Ling Shi, Junying Zhang, Jue Cheng, Xiaodong Wang PII:
S0032-5910(19)31056-3
DOI:
https://doi.org/10.1016/j.powtec.2019.11.097
Reference:
PTEC 14979
To appear in:
Powder Technology
Received date:
26 November 2018
Revised date:
12 September 2019
Accepted date:
25 November 2019
Please cite this article as: W. Li, L. Shi, J. Zhang, et al., Double-layered surface decoration of flaky aluminum pigments with zinc aluminum phosphate and phytic acid–aluminum complexes for high-performance waterborne coatings, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2019.11.097
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© 2019 Published by Elsevier.
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Double-layered surface decoration of flaky aluminum pigments with zinc aluminum phosphate and phytic acid–aluminum
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complexes for high-performance waterborne coatings
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Wenqi Li a, Ling Shi a,*, Junying Zhang a, Jue Cheng a, Xiaodong Wang b,*
Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing
b
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University of Chemical Technology, Beijing 100029, China
State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology,
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Beijing 100029, China
*Corresponding authors: Tel & fax: +86 10 6441 0145.
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Email: [email protected] (L. Shi); [email protected] (X. Wang).
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Abstract: This paper reported a double-layered surface decoration for aluminum flakes by coating a zinc aluminum phosphate passive film, followed by fabricating a phytic acid–aluminum complex sealing film. The chemical compositions and structures of surface-decorated aluminum flakes were characterized by Fourier-transform infrared, X–ray photoelectron, and energy-dispersive X–ray spectroscopy and their
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morphology and double-layered structure were confirmed by scanning and transmission electron
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microscopy. The surface-decorated aluminum flakes not only show excellent dispersibility in water due to
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the enhanced hydrophilicity, but also exhibit good corrosion resistance and chemical stability against water.
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Most of all, the double-layered surface decoration leads to an improvement in optical performance of
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coating surface such as glossiness, flop index and distinctness of image compared to the single-layered one when the surface-decorated aluminum flakes are applied for waterborne coatings. The surface-decorated
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aluminum flakes developed by this work can well serve as a good flaky aluminum pigment for
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high-performance waterborne coatings.
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Keywords: Aluminum flakes; Surface decoration; Phytic acid; Chemical conservation film; Corrosion resistance; Waterborne coatings
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1. Introduction Sustainable development has been vital for the survival and development of human beings, because all of the world have to face a wide range of common global challenges like environmental pollution, greenhouse emissions, climate change and global warming. There is a great demand for eco-friendly chemical products and “green” materials in various industrial and civil fields [1]. On the other hand, much
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stricter legislation and growing environmental consideration have forced paint manufacturers to develop
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coating systems with reduced content of volatile organic compounds (VOCs) in recent years [2,3].
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Waterborne coatings, containing a lot of water with small quantities of other organic solvents, are well
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known to be environmentally friendly due to an extremely low content of VOCs and therefore have
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received a broad range of application for automotive vehicles, buildings, furniture, domestic electrical appliances, official electronic equipments, and various metal and plastic parts [4]. To improve the
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rheological performance, adhesive force, glossiness, impact toughness, ultraviolet (UV) protection,
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abrasion resistance, scratch resistance and delamination resistance, various functional inorganic fillers such
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as TiO2, ZnO, SiO2, Al2O3, Fe2O3, clay nanoparticles and aluminum flakes are necessarily incorporated into waterborne coatings [5]. In most cases, the surface modification for these inorganic fillers becomes a vital procedure for enhancing the application performance of waterborne coatings due to low interfacial adhesion, high surface tension and poor chemical stability of pristine fillers [6]. Aluminum flakes are a type of important functional fillers, which can serve as pigments with silver-white metallic or pearlescent color. With special metallic luster, prominent shielding effectiveness, excellent mechanical properties, high reflection and good UV resistance, flaky aluminum pigments have been broadly applied for automotive coatings, printing inks, wood coatings, automotive coatings, industrial paints, furniture lacquers, plastic coatings, anti-corrosion coatings, etc [7]. Nevertheless, flaky aluminum 3
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pigments are easily corroded by water through an oxidation reaction in a weak acidic or alkaline medium due to its active chemical properties when used in waterborne coatings and paints. This leads to a change in color from silver white to grey as well as a reduction in metallic luster. Furthermore, the corrosion reaction results in a security issue during the production and storage of waterborne coatings and paints [8]. Therefore, it is essential to conduct a surface modification for flaky aluminum pigments to inhibit the
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corrosion reaction [9]. The researchers from pigment and coating industries have worked on this issue for a
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long time. The encapsulation and corrosion inhabitation are considered as two of the most effective surface
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modification methods for flaky aluminum pigments [10]. Ma et al. [11] reported a sol-gel encapsulation
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method for waterborne aluminum pigments with SiO2 through anchorage of H2O2, and a dense and smooth
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coating layer for corrosion resistance was formed on the surface of aluminum pigments with this method. He et al. [12] studied the surface modification of waterborne aluminum pigments with SiO2 and polymer
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brushes, and they found that the dispersibility and corrosion resistance of the resultant aluminum pigments
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in aqueous media were enhanced significantly. Pi et al. [13] reported the encapsulation of aluminum
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pigments with an organic-inorganic hybrid film and found that the anticorrosive and adhesive performance of the pigments was greatly improved in paint films. Liu et al. [14] synthesized the composite particles based on aluminum flakes and poly(trimethylolpropane triacrylate) through in-situ polymerization, and a performance investigation indicated that the corrosion resistance and adhesive property of composite particles were greatly improved due to the formation of a compact polymeric layer on the surface of aluminum flakes. Although the encapsulation of aluminum flakes with organic or inorganic films can form an adequate barrier to insulate the flaky aluminum pigment from the corrosion media, there are still some critical defects like easy crack on the protective layer, poor dispersibility of pigments, reduced glossiness of coating films and short cycle time for corrosion protection [15]. The introduction of chromates, phosphates, 4
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molybdates, fatty acids, amino acids, acrylic acid, nitro- and amino-phenols, heterocyclic compounds, saccharides and some small molecular organic inhibitors also can generate highly efficient corrosion inhibition for flaky aluminum pigments [16]. Müller et al. [17] investigated the application of citric acid as a corrosion inhibitor for flaky aluminum pigments and found that citric acid could act as a chelating agent to inhibit the corrosion reaction of aluminum pigments. Amin et al. [18] reported the effect of poly(acrylic
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acid) as a corrosion inhibitor on the anticorrosive performance of aluminum in weakly alkaline and found
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that its inhibition efficiency was strongly dependant on its concentration and molecular weight. Liu et al.
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[19] reported an investigation on the corrosion resistance of aluminum pigments with poly(methyl
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methacrylate) (PMMA) and found that there was a significant enhancement in corrosion inhibition by
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combination of PMMA and 3-methacryloxypropyltrimethoxysilane. Although chromates are considered as one of the most effective inhibitors for corrosion protection of flaky aluminum pigments, their utilization
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inhibition [20].
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has been restricted due to an environmental concern for heavy-metal free alternatives to chromate
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Phosphate inhibitors have received tremendous attention in recent years due to their eco-friendly nature as well as an outstanding anticorrosive effect on various metal materials [21]. Bastidas et al. [22] reported a comparative study on the anticorrosion of steel reinforcement by three sodium phosphates as corrosion inhibitors and gave a correlation between the phosphate content and steel corrosion rate. Naderi et al. [23] investigated the effects of zinc aluminum molybdenum orthophosphate hydrate and zinc calcium strontium aluminum orthophosphate silicate hydrate as new phosphate-based anticorrosion pigments on corrosion protection of mild steel, and they observed corrosion inhibition synergism between the two pigments. Tamilselvi et al. [24] reported the enhanced corrosion resistance of mild steel by fabricating a zinc phosphate coating layer through the development of ZnO nanoparticles in the phosphating bath. Feng 5
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et al. [25] investigated the efficiency of aluminum tripolyphosphate (ATP) as a corrosion inhibitor for carbon steel in carbonated concrete pore solutions and mortars and found the pitting corrosion of the steel could be prevented effectively due to an enhancement in stability of passive film on the steel surface by ATP. However, the surface modification with phosphate inhibitors generally produces an unusual porous structure on the metal surface, and these pores may adsorb the corrosive media and contaminants in the
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atmosphere. This may deteriorate the anticorrosion effect of protective layer on the protected metal
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materials [26]. In this case, there were few reports about the use of phosphate inhibitors for the
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anticorrosive modification of micro-size metal particles like flaky aluminum pigments. Therefore, to
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improve the corrosion resistance of phosphate-modified metal materials, the second protective layer is necessary to seal these pores produced by phosphate inhibitors. A facile sealing method is to fabricate a
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sealing film, or called “chemical conversion film”, upon the modified metal surface through the chelation
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of phytic acid and the relevant metal ions [27,28]. Phytic acid (PA) is a water-soluble organic compound
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known as inositol hexakisphosphate, and it has attracted special attention in the surface treatment of metal
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for many years due to its strong chelating capability for various metal ions including Fe3+, Cu2+, Zn2+, Mg2+ and Al3+ as well as a nontoxic nature. Although PA could be employed as a corrosion inhibitor to form a protective layer for many metals against corrosion, the PA–metal complex layer only presented a limited anticorrosion effectiveness toward aluminum and its alloys as a chemical conversion film [29]. On the other hand, PA is able to well serve as a sealing layer to protect the phosphate-based passive films from further corrosion reaction. Wang et al. [30] reported a surface sealing modification for the anodized aluminum with PA via a solution-immersion process and found that the anodized aluminum with a phytic acid–aluminum (PA–Al) complex thin film exhibited much better corrosion resistance in comparison with the unmodified one. It is expected that the combination of phosphate-based protective layer and PA–Al 6
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complex sealing layer can generate much better corrosion resistance for flaky aluminum pigments. In this study, we reported a double-layered surface decoration for flaky aluminum pigments with zinc aluminum phosphate (ZPA) and PA–Al complexes. In this new method, a phosphate-based passive film was first fabricated onto aluminum flakes through a surface passivation reaction, and then a PA–Al complex sealing film was formed via a chelation reaction between PA and Al3+ ions. The chemical
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compositions and microstructures of surface-decorated aluminum flakes were characterized extensively,
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and the synthetic strategy and reaction mechanism were described. Moreover, the effect of surface
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decoration on the dispersibility, optical performance and anticorrosion behaviors of aluminum flakes were
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investigated intensively. The aim of this study is to develop an eco-friendly surface decoration method for
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the flaky aluminum pigments used for waterborne coatings so as to achieve excellent corrosion resistance, good storage stability, high mechanical properties and good glossiness for high-performance waterborne
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2.1. Materials
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2. Experimental
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coatings.
Aluminum flakes were commercially supplied by Eckart Co., Ltd., Germany, and they have a mean diameter of 10–12 μm and an aluminum content of 99 wt %. PA [C6H6(H2PO4)6] with a concentration of 50 wt % in H2O was commercially obtained from Aladdin Co., Ltd., USA. Zinc phosphate [Zn3(PO4)2], diammonium phosphate [(NH4)2HPO4] and isopropanol were purchased from Tianjin Fuchen Chemical Co., Ltd., China. All reagents were used as received without further purification.
2.2. Preparation methods A double-layered surface decoration for flaky aluminum pigments was performed by coating a ZPA [Zn3Al(PO4)3] film upon aluminum flakes through a passivation reaction of aluminum with Zn3(PO4)2 and 7
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(NH4)2HPO4, followed by fabricating an Alx(HiPhy) film onto the surface of ZPA passive film through the chelation of PA and Al3+. In a typical synthetic process, 1.0 g of aluminum flakes was dispersed in 40 ml of isopropanol with stirring for 5 min. On the other hand, 0.4 g of diammonium phosphate was dissolved in 6 ml of deionized water, and 0.04 g of zinc phosphate was added under controlled agitation for 5 minutes. Afterwards, the resulting solution was added dropwise into the aqueous suspension containing aluminum
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flakes under mild agitation, and then a passivation reaction was performed at 25 ºC for 1 h to obtain the
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aluminum flakes surface-coated with a ZPA passive film. Finally, the as-prepared aluminum flakes were
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washed with deionized water several times and then dispersed in 40 ml of isopropanol. In succession, an
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aqueous solution containing PA was prepared by dissolving 1.0 g of PA in 6 ml of deionized water and then added dropwise into the isopropanol suspension containing the as-prepared aluminum flakes to perform a
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chelation reaction between PA and Al3+ at 30 ºC for 3 h. The surface-decorated aluminum flakes with a
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double-layered structure were obtained ultimately, and then they were washed with deionized water and
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dried in oven at 60 ºC for further use.
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2.3. Characterization and measuring method Fourier-transform infrared (FTIR) spectroscopy was conducted on a Bruker Alpha-T FTIR spectrometer at a scanning number of 32 using KBr sampling discs. Energy dispersive X-ray (EDX) spectroscopy was performed to analyze the surface elemental distributions of flaky aluminum samples using a Bruker XFlash 6160 energy dispersive X-ray spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Scientific ESCALAB 250 X-ray photoelectron spectrometer with a focused monochromatized Al–Kα radiation. The obtained XPS spectra were fitted with a Casa XPS software, in which a Shirley background was assumed. X-ray diffraction (XRD) powder patterns were obtained from a Rigaku D/Max 2500 VBZ+/PC X-ray diffractometer using Cu–Kα radiation operated at 40 8
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kV and 50 mA with a scan rate of 1 ºC/min. Scanning electron microscopy (SEM) was conducted to observe the morphologies of flaky aluminum samples on a JEOL JSM-7800 scanning electron microscope. The microstructures of flaky aluminum samples were investigated by transmission electron microscopy (TEM) on an FEI Tecnai G2 20 transmission electron microscope operating at an acceleration voltage of 200 kV.
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The static water contact angles of flaky aluminum samples were measured by a Dataphysics
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OCA15EC contact angle meter using deionized water drops. The mean contact angle value was obtained
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from at least five individual measurements for each sample. To characterize the dispersibility of flaky
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aluminum samples, aluminum flakes were well dispersed in deionized water under vigorous agitation. The
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resulting suspension was dripped onto a glass slide and then observed by an Olympus BX51 optical microscope equipped with a Sony CCDIRIS digital camera. The thermal stabilities of flaky aluminum
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samples were characterized by thermogravimetric analysis (TGA) on a TA Instruments SDT2960
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thermogravimetric analyzer at a heating rate of 10°C/min in nitrogen.
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The electrochemical corrosion behavior of flaky aluminum samples was characterized on an electrochemical workstation (Chenhua CHI760E, China) equipped with a standard three-electrode configuration. This three-electrode configuration consists of a nickel foam sheet with an area of 1.52.0 cm2 used as a working electrode, a platinum sheet as a counter electrode and an Ag/AgCl electrode as a reference electrode. To prepare the working electrode, a flaky aluminum sample was mixed with polyvinylidene fluoride binder in N-methyl-pyrrolidinone to form a slurry, and then the resulting slurry was pasted on the nickel foam sheet in an area of 11 cm2. The nickel foam sheets coated with flaky aluminum samples were immersed in a NaCl aqueous solution (3.5 wt %) to serve as a working electrolyte. The measurements of electrochemical impedance spectrometry (EIS) were carried out in the frequency range of 9
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buretter at room temperature. The corrosion resistance was determined by the production rate and total
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production volume of H2 gas within the same duration.
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A series of polyacrylate-based waterborne coatings were prepared by using different flaky aluminum
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samples at the same formulation. The resulting coatings were sprayed onto the steel plates with a dimension
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of 150 100 1 mm3 and then dried at room temperature for 24 h. The glossiness of coating surface was measured by a BYK 4601 haze–gloss reflectometer at the illuminating angles of 20° and 60°. The flop
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index of coating surface was measured by an X–Rite MA68 multi-angle spectrophotometer. The
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distinctness of image of coating surface was measured by a BYK 4840 wave-scan dual detector. The
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cross-cut tests of coating surface were carried out according to the method described by the ISO–2409 standard, and a classification of adhesion was achieved on the basis of the surface appearance of cross-cut area. All of the measurements were carried out at room temperature, and each of the reported data represented a mean value of five tests.
3. Results and discussion 3.1. Reaction mechanism and structural characterization Fig. 1 shows the synthetic strategy and reaction mechanism for the double-layered surface decoration of aluminum flakes with ammonium phosphate salt and phytic acid salt. It is noted that such a surface decoration is involved in two reaction steps. In the first step, Zn3(PO4) 2 and (NH4)2HPO4 reacted with the 10
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Al2O3 surface thin layer of aluminum flakes to produce Zn3Al(PO4)3, which deposited on the surface of aluminum flakes to form a passive film or “chemical conversion film”. It is well known that phosphate ions can react with various metal ions to form various insoluble phosphorous compounds, which can act as a protective layer to inhibit the corrosion of metal materials [1]. However, Zn3(PO4)2 has a poor solubility in the aqueous solution, thus depressing the release of phosphate ions. Moreover, phosphate ions hardly
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performed an oxidation reaction with aluminum due to their poor reaction capability, thus resulting in the
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formation of a loose protective layer through deposition of phosphate salt [31]. This is evidently
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disadvantageous to the corrosion inhibition of zinc phosphate. In this case, the incorporation of
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(NH4)2HPO4 not only can improve the solubility of Zn3(PO4)2 but also can generate a synergistic effect to
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enhance the reactivity of aluminum and phosphate ions so as to form a more compact and stable protective layer [23]. Nevertheless, the reaction of aluminum and phosphate ions normally forms a passive film
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consisting of a barrier inner layer and a porous outer layer. Such a passive film was reported to present an
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ordered array of cells [30], and it may easily absorb the corrosive substances and contaminants in the air,
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thus reducing the anticorrosive effectiveness of phosphate-based protective layer. Therefore, a PA–Al complex thin film was fabricated to seal this porous layer through a chelation reaction between PA and Al3+ ions in the second step as displayed in Fig. 1. With a strong chelating capability, PA could attack the aluminum substrate to make more Al3+ ions released and then reacted with these Al 3+ ions to produce a series of PA–Al complexes based on the chemical composition of Alx(HiPhy). These complexes can form a compact sealing layer as another chemical conversion film upon the ZPA passive film to provide an additional anticorrosive protection for aluminum flakes [32]. Moreover, the fabrication of this sealing film also improves the hydrophily of aluminum flakes due to the water-soluble nature of PA, which can enhance the dispersibility of flaky aluminum pigments in waterborne coatings. 11
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The chemical compositions and structures of surface-modified aluminum flakes were first characterized by FTIR spectroscopy, and the resulting infrared spectra are given in Fig. 2. As observed from the infrared spectrum of pristine aluminum flakes, there is a weak characteristic band distinguished at 664 cm-1 for the Al–O stretching vibration, implying the existence of Al2O3. Moreover, two absorption peaks are also observed at 3426 and 1638 cm-1 for the O–H stretching and bending variations of water,
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which is due to the absorbed water on the surface of aluminum flakes. On the other hand, the infrared
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spectrum of ZPA-decorated aluminum flakes not only shows the same absorption bands with pristine
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aluminum flakes but also exhibits a series characteristic absorption bands at 956 cm-1 for the P–O stretching
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vibration of phosphate group and at 1056 and 569 cm-1 due to the stretching and deformation vibration of
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P=O bond. A characteristic peak can be observed at 897 cm-1 due to the Zn–O stretching vibration. These results indicate the formation of ZPA on the aluminum flakes [33]. Moreover, there is a set of absorption
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bands found at 1349–1526 cm-1, which may be ascribed to the C–H deformation vibration of the organic
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solvents absorbed by the passive film. As for the Alx(HiPhy)@ZPA-decorated aluminum flakes, a set of
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new absorption bands centered at 796 cm-1 can be observed due to the P–O–Al stretching vibration derived from the chelation of phytic molecules with Al3+ ions [29]. Considering of a low resolution for the FTIR spectra obtained from powder specimens, the chemical compositions of surface-decorated aluminum flakes were further confirmed by EDX characterization, and the resulting spectra are presented in Fig. 3. As displayed in Fig. 3a, the EDX spectrum of ZPA-decorated aluminum flakes clearly reveals five peaks corresponding to C, O, P, Zn and Al elements on the flaky aluminum surface. The Al, Zn, O, and P signals imply the existence of ZPA on the surface of aluminum flakes, while the presence of C signal is attributed to the carbon diaphragm used for EDX measurements. Furthermore, the mass and atomic percentages could also be estimated by EDX analysis and was given as an inset in Fig. 3a, which clearly determines the 12
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chemical composition of the ZPA-decorated aluminum flakes. It is found in Fig. 3b that the Alx(HiPhy)@ZPA-decorated aluminum flakes show the same elemental signals in their EDX spectrum, and however the intensity of the P signal is enhanced relatively. It is also noteworthy from the elemental data that the mass and atomic percentages of P element are much greater than those in the ZPA-decorated aluminum flakes, whereas those of Zn element become smaller. Such a change in chemical composition
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indicates an overlap of the Alx(HiPhy) film onto the ZPA one.
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The chemical structures of coating layers were further determined by the XPS spectra shown in Fig. 4.
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The XPS survey spectra clearly show a series of intensive peaks assigned to the O, C, P and Zn elements
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for both of ZPA-decorated and Alx(HiPhy)@ZPA-decorated aluminum flakes, whereas the intensive signals
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corresponding to O, C and Al elements are found in the survey spectra of pristine aluminum flakes (see Fig. 4a). However, only weak signals corresponding to aluminum can be detected for the surface-decorated
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aluminum flakes due to the overlap of two chemical conversion films. In this case, all of the XPS spectra
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were further fitted with Gaussian-Lorentzian peaks by means of the Casa XPS software to find out O, C
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and Al elements in different environments. Fig. 4b–4i show the high-resolution XPS spectra of pristine and surface-decorated aluminum flake samples. Through the curve fitting, two deconvoluted peaks can be found at binding energy of 74.3 and 71.8 eV for the Al–O and Al–Al bonds in the high-resolution Al 2p spectrum of pristine aluminum flakes (see Fig. 4b), and meanwhile a deconvoluted peak can be observed at binding energy of 531.9 eV for the O–Al bond in their high-resolution O 1s spectrum (see Fig. 4c). These results identify the Al and Al2O3 substances as the chemical composition of pristine aluminum flakes. Therefore, the existence of Al2O3 in pristine aluminum flakes can be confirmed by the combination of FTIR and XPS analyses. It is observed in Fig. 4d–4f that there are a series of deconvoluted peaks for Al–Al (69.5 eV), Al–O-–P (72.6 eV), P–O-–Al (131.7 eV), P=O (132.5 eV), O-–Zn (529.2 eV), O-–Al (530.0 eV), O-–P 13
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(531.1 eV), and O=P (531.9 eV) in the high-resolution spectra of ZPA-decorated aluminum flakes. These deconvoluted peaks in Al 2p, P 2p and O 1s zones provide a firm evidence for the formation of ZPA film on the surface of aluminum flakes. In the case of the Alx(HiPhy)@ZPA-decorated aluminum flakes, the peaks for Al–Al (69.7 eV), Al–O-–P (72.6 eV), Al–O-–Phy (73.2 eV), P–O- (131.8 eV), O–P–O- (132.4 eV), P=O (133.0 eV), O-–Zn (528.9 eV), O-–Al (529.6 eV), O-–P–O (530.4 eV), P–O-–Al (530.8 eV), P–O–C (531.4
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eV) and O=P (531.8 eV) can be clearly distinguished on the basis of their deconvoluted Al 2p, P 2p and O
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1s spectra (see Fig. 4g–4i), suggesting the presence of Alx(HiPhy) outer layer accordingly. These XPS
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results further confirm the successful fabrication of Alx(HiPhy)@ZPA double protective layers onto the
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aluminum flakes. The crystalline structures of flaky aluminum samples were investigated by XRD, and
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resulting diffraction patterns are displayed in Fig. 5. The XRD pattern of pristine aluminum flakes is found to show a set of intensive diffraction peaks at 2θ values of 38.51, 44.80, 65.23 and 78.32, which are
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assigned to the (111), (200), (220) and (311) reflections of crystal phase of the aluminum substrate,
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respectively [33]. Although two surface-decorated flaky aluminum samples exhibit the similar XRD
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patterns as pristine aluminum flakes, a series of new diffraction peaks can be observed in their patterns. These weak diffraction peaks are attributed to the complicated crystal phases of ZPA and Alx(HiPhy) films coated on the surface of aluminum flakes according to the standard JCPDS Nos. 26-0034, 21-1488, 33-1474 and 51-1754.
3.2. Morphology and microstructure The morphologies and microstructures of flaky aluminum samples were investigated by SEM and TEM, and the obtained SEM and TEM micrographs are shown in Fig. 6 and Fig. 7, respectively. It is observed in Fig. 6a that pristine aluminum flakes exhibit an irregular shape with a size around 10 μm. These aluminum flakes seem to have a very smooth surface according to the magnified SEM micrograph in 14
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Fig. 6b. It is seen in Fig. 6c and 6d that the ZPA-decorated aluminum flakes present a more flat surface with a semi-ordered array of bulges, indicating the formation of a ZPA passive film. The presence of bugles is ascribed to the porous and loose structure of the passive film as reported by the relevant references [34–36]. Owing to a weak reactivity of (NH4)2HPO4, the release of Al 3+ ions from aluminum flakes is insufficient to be involved in the passivation reaction. As a result, an inhomogeneous loose structure was
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caused inside the passive film. The TEM micrographs in Fig. 7a and 7b also clearly demonstrate that the
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microstructure of the passive film includes a compact barrier layer and a loose outer layer. Moreover, the
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thickness of passive film can be estimated approximately as 4 nm according to the magnified TEM
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micrographs in Fig. 7b. As observed in the SEM micrographs of Alx(HiPhy)@ZPA-decorated aluminum
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flakes in Fig. 6e and 6f, it is interestingly found that the bulges seem to become insignificant in the presence of Alx(HiPhy) sealing film on the flaky aluminum surface. There is a noticeable increase in total
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thickness of coating films observed in the TEM micrographs in Fig. 7c and 7d due to the formation of
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Alx(HiPhy) sealing film. It is understandable that PA has a stronger reactivity with metal ions by chelation
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and thus can easily react with the Al3+ ions existing in the loose part of ZPA passive film as well as those released from the aluminum substrate. Such a chelation reaction not only can form an Alx(HiPhy)-based chemical conversion film on the surface of modified aluminum flakes but also can seal the pores inside the ZPA film to enhance the compactness of protective layer. The total thickness of chemical conversion films can also be determined as about 9 nm as observed in Fig. 7d, which evidently favors the anticorrosive capability of protective layer.
3.3. Dispersibility and thermal stability The wettability of flaky aluminum samples was investigated by measurement of static water contact angle, and the obtained results are illustrated in Fig. 8. According to the resulting data and relevant optical 15
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images, pristine aluminum flakes are found to have a water contact angle of 93.45º, indicating a hydrophobic nature. However, after the surface decoration with ZPA, the water contact angle of aluminum flakes decreases to 68.24º. This implicates that the hydrophilicity of flaky aluminum surface is remarkably improved due to a decrease in surface energy as a result of the fabrication of ZPA passive film. It is noteworthy that the water contact angle continues to drop down to 31.09º after the fabrication with an
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Alx(HiPhy) sealing film. This result suggests that the hydrophilicity of aluminum flakes is further enhanced
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due to the incorporation of hydrophilic phosphate groups in PA molecules, which may favor the dispersion
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of flaky aluminum pigments in waterborne coatings. The dispersibility of flaky aluminum samples in water
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was investigated by optical microscopic observation, and the obtained microscopic images are displayed in
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Fig. 9. It is noted in Fig. 9a that there is a serious aggregation phenomenon observed from the aluminum flakes in water, indicating that pristine aluminum flakes have poor dispersibility in water due to their
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hydrophobic nature. However, the ZPA-decorated aluminum flakes in water exhibit a lot of dispersed
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particles with a small size. This is attributed to the hydrophilic feature of ammonium phosphate salt in the
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passive film. Furthermore, the Alx(HiPhy)@ZPA-decorated aluminum flakes are observed to exhibit much smaller dispersed units in water compared to the ZPA-decorated ones. Such good dispersibility is due to the fact that the abundant hydrophilic groups in PA molecules can further improve the hydrophilic performance of aluminum flakes. These results are in good agreement with the data of water contact angle. Furthermore, the compatibility of Alx(HiPhy)@ZPA-decorated aluminum flakes with coatings will be verified by a series of their applicable experiments in waterborne coatings. The thermal stability of flaky aluminum samples was analyzed by TGA under a nitrogen atmosphere, and the resulting TGA and DTG curves are presented in Fig. 10. It is observed that pristine aluminum flakes present a slight weight loss in the temperature range of 50–400 ºC, followed by a rapid weight 16
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increase in the temperature range of 730–800 ºC. The losses of adsorbed and structural water were involved in the early weight decrease [37], and the later weight loss may be due to an oxidative reaction between the Al substrate and Al2O3 thin film until 400 ºC [13]. However, the nitrogen gas can react with aluminum flakes to produce AlN with an elevation of temperature above 700 ºC, which leads to a dramatic increase in specimen mass [13]. The ZPA-decorated aluminum flakes are found to exhibit a more significant weight
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loss than the pristine one in the temperature range of 50–600 ºC due to a relatively poor thermal stability of
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ZPA passive film. As observed in Fig. 10, there are two peaks corresponding to maximum decomposition
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temperature in their DTG curve. The first one is attributed to the thermal decomposition of the loose outer
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layer of ZPA passive film, and the second one is due to the thermal degradation of its compact barrier layer.
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It is noticeable that the fabrication of Alx(HiPhy) sealing film onto the ZPA-decorated aluminum flakes evidently enhances the thermal stability of aluminum flakes and thus reduces their weight loss in the early
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heating stage until 350 ºC. Nevertheless, the Alx(HiPhy) sealing film has to undergo a thermal degradation
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at a much higher temperature over 350 ºC, which results in a faster weight loss due to the pyrolysis of
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Alx(HiPhy). In summary, the Alx(HiPhy)@ZPA-decorated aluminum flakes exhibit a better thermal stability than the ZPA-decorated ones in the operating temperature range for waterborne coatings.
3.4. Electrochemical behavior and chemical stability EIS measurements were performed to investigate the electrochemical behavior of aluminum flakes before and after surface decoration. Fig. 11 shows the Nyquist spectra of flaky aluminum samples obtained from EIS tests as well as the equivalent circuit derived from the simulation of EIS data with the Zsimpwin software. The Nyquist spectra of these three samples are all found to present a semicircular diagram following with a diffusional element over the whole frequency region. It is well known that the impedance behavior of a metal material reflects its anticorrosive capability, and the magnitude of impedance can 17
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determine the resistance of chemical conversion films to the transportation of electrons and charges [38]. According to the simulated results, the equivalent circuit of three flaky aluminum samples is found to include a solution resistance (Rs), an interfacial charge-transfer resistance (Rt), the Warburg impedance (Zw) related to ionic diffusion and a non-intuitive circuit element (QCPE) representing the constant phase element [25]. Although three flaky aluminum samples exhibit a similar impedance behavior, pristine aluminum
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flakes show the smallest diameter among three Nyquist spectra. The diameter of semicircular diagram is
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observed to present a significant increase as a result of the double-layered surface decoration of aluminum
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flakes with ZPA and Alx(HiPhy). This means that the interfacial charge-transfer resistance of aluminum
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flakes is improved due to the formation of chemical conversion films, which can serve as effective physical
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barriers to prevent corrosive ions in the electrolyte from penetrating the films to induce the electrochemical corrosion of aluminum flakes. The corrosion resistance of aluminum flakes is enhanced as a result of the
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surface decoration. It is clearly observed in Fig. 11 that the Alx(HiPhy)@ZPA-decorated aluminum flakes
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exhibit the largest diameter, which means there is the highest value in the real and imaginary impedance of
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their semicircular diagram. This implicates that the double-layered surface decoration can provide a greater interfacial charge-transfer resistance for aluminum flakes than the single-layered one and forms a stronger anticorrosive layer for them accordingly. On the other hand, the impedance of constant phase element (ZCPE) can be given by Eq(1) [39].
ZCPE
1 Q0 ( j)n
(1)
where n is the exponent of constant phase element, j is an imaginary value, ω is the frequency of phase angle, and Q0 has the numerical value of the admittance (1/ZCPE) at ω = 1 rad/s. The QCPE represents a pure capacitance at n = 1, but it will become a pure resistance at n = 0. The value of ZCPE is associated with film thickness, surface heterogeneity and surface roughness [25]. In this work, the value of n was determined as 18
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approximate 0.85 on the basis of the fitting result of EIS data, which suggests that the corrosion resistance of aluminum flakes is not only controlled by the interfacial charge-transfer resistance but also influenced by the value of ZCPE. It is no doubt that the Alx(HiPhy)@ZPA-decorated aluminum flakes have a more compact and thicker chemical conversion film than the ZPA-decorated ones and thus gain a higher value of ZCPE. Accordingly, this makes them a better anticorrosive effect on aluminum flakes [39].
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The chemical stability of flaky aluminum samples was evaluated by the evolution of hydrogen gas in
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water boiling tests. Fig. 12 shows the plots of volume evolution of hydrogen gas as a function of time for
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the aluminum flakes before and after surface decoration. It is observed in Fig. 12 that pristine aluminum
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flakes exhibit high reactivity with water, and they released a high volume of hydrogen gas within 20 min in
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the water-boiling test. However, almost no hydrogen gas was released from both the ZPA-decorated and Alx(HiPhy)@ZPA-decorated aluminum flakes during the whole process of water boiling tests. There is no
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doubt that the fabrication of chemical conversion films on the flaky aluminum surface effectively prevents
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the aluminum substrate from reacting with water to produce hydrogen gas. It is noteworthy from the
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close-up evolution plots in Fig. 12 that there is still a small amount of hydrogen gas produced by the ZPA-decorated aluminum flakes in the water boiling test due to the loose outer layer of ZPA passive film, through which the water molecules may penetrate to react with the aluminum substrate. After coated with an Alx(HiPhy) sealing film, the surface-decorated aluminum flakes no longer react with water, and therefore, there is no release of hydrogen gas observed in the close-up evolution plot. Fig. 13 shows the SEM micrographs of the aluminum flakes after water boiling tests. According to these micrographs, there is no trace of water corrosion found on the surface of the Alx(HiPhy)@ZPA-decorated aluminum flakes, whereas some traces are distinguished on the surface of ZPA-decorated aluminum flakes due to a corrosive reaction with water. On the other hand, pristine aluminum flakes were completely reacted with water during the 19
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water boiling test, and nothing was left for SEM observation. These results indicate that the Alx(HiPhy)@ZPA-decorated aluminum flakes have excellent corrosion resistance against water and can maintain a long-term chemical stability in waterborne coatings.
3.5. Application performance The application performance of flaky aluminum samples in waterborne coatings was investigated in
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terms of the glossiness, flop index, distinctness of image (DOI) and classification of adhesion obtained
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from the dried coating surface, and the obtained results are presented in Fig. 14. As seen in Fig. 14a, the
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coating surface achieved glossiness values of 95.5% and 101.2% at the illuminating angles of 20º and 60º,
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respectively, when pristine aluminum flakes were incorporated into the waterborne coating. The glossiness
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of coating surface is found to decrease when the ZPA-decorated aluminum flakes were used in the waterborne coating. This may be due to the rougher surface of ZPA-decorated aluminum flakes than the
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pristine ones, which leads to a decrease in specular reflectance [40]. However, the waterborne coating
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containing the Alx(HiPhy)@ZPA-decorated aluminum flakes shows a slight improvement in surface
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glossiness, which is ascribed to the formation of more smooth Alx(HiPhy) sealing film upon aluminum flakes. A similar trend can be observed in the flop index and DOI value of coating surface as seen in Fig. 14b and 14c. The addition of ZPA-decorated aluminum flakes results in a considerable decrease in both flop index and DOI value of coating surface compared to the pristine ones. However, the use of Alx(HiPhy)@ZPA-decorated aluminum flakes for waterborne coatings can improve these two parameters evidently. The compact surface structure of chemical conversion film favors the orientation and distribution of aluminum flakes in the waterborne coating, thus improving the optical performance of coating surface [41]. Moreover, the cross-cut test results shown in Fig. 14d demonstrate that the edges of the cuts are completely smooth, and none of the squares of the lattice is detached from these three coating samples, so 20
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all of them can be classified to 0 grade in adhesion according to the classification of the ISO–2409 standard. Fig. 15 shows the digital photograph of two waterborne coatings individually containing the Alx(HiPhy)@ZPA-decorated aluminum flakes and pristine aluminum flakes after standing for 7 days. It is clearly observed that the waterborne coating containing the Alx(HiPhy)@ZPA-decorated aluminum flakes exhibited a homogenous state after long-term standing, and however there was a serious phase separation
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occurring in the waterborne coating containing pristine aluminum flakes. It is understandable that the poor
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compatibility of pristine aluminum flakes with the waterborne coating results in the phase separation. On
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the other hand, the surface decoration of aluminum flakes may enhance their compatibility with the
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waterborne coating due to the improvement of hydrophilic performance. These results confirm that the
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Alx(HiPhy)@ZPA-decorated aluminum flakes developed by this work the have better compatibility with the waterborne coating than the pristine ones due to surface decoration, and they can well serve as a good flaky
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4. Conclusions
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aluminum pigment for high-performance waterborne coatings.
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A double-layered surface decoration was successfully carried out up on aluminum flakes by fabrication of a ZPA passive film, followed by coating an Alx(HiPhy) sealing film. The chemical compositions and structures of the resultant aluminum flakes were determined by FTIR, EDX, and XPS spectroscopy. The morphologies of surface-decorated aluminum flakes were confirmed by SEM, and the double-layered structure of Alx(HiPhy)@ZPA-decorated aluminum flakes was verified by TEM. The double-layered surface decoration made aluminum flakes good hydrophilic performance, thus improving their dispersibility in water. The Alx(HiPhy)@ZPA-decorated aluminum flakes exhibited good anticorrosive performance and a chemical stability against water on the basis of the EIS analysis and water boiling test results. Most of all, an investigation on application performance indicated that the optical performance of 21
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coating surface was enhanced and the adhesion performance was well maintained when the Alx(HiPhy)@ZPA-decorated aluminum flakes were used for a polyacrylate-based waterborne coating. The surface-decorated aluminum flakes developed by this work can well serve as a high-performance pigment for waterborne coatings.
Acknowledgements
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This work is financially supported by the National Key Technology R&D Program of China with a
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grant number of 2014BAE12B03.
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Declarations of interest: none
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Figure Captions Figure 1
Scheme of synthetic strategy and reaction mechanism for the double-layered surface decoration of flaky aluminum pigments.
Figure 2
FTIR spectra of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes. EDX spectra of (a) ZPA-decorated aluminum flakes and (b) Alx(HiPhy)@ZPA-decorated
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f
Figure 3
aluminum flakes.
(a) XPS survey spectra of (1) pristine aluminum flakes, (2) ZPA-decorated aluminum flakes and
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Figure 4
flakes,
(d,
e,
f)
ZPA-decorated
aluminum
flakes
and
(g,
h,
i)
Pr
aluminum
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(3) Alx(HiPhy)@ZPA-decorated aluminum flakes; high-resolution XPS spectra of (b, c) pristine
Alx(HiPhy)@ZPA-decorated aluminum flakes.
XRD patterns of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c)
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Figure 5
SEM micrographs of (a, b) pristine aluminum flakes, (c, d) ZPA-decorated aluminum flakes and
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Figure 6
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Alx(HiPhy)@ZPA-decorated aluminum flakes.
(e, f) Alx(HiPhy)@ZPA-decorated aluminum flakes. Figure 7
TEM
micrographs
of
(a,
b)
ZPA-decorated
aluminum
flakes
and
(c,
d)
Alx(HiPhy)@ZPA-decorated aluminum flakes. Figure 8
Static water contact angles of flaky aluminum samples.
Figure 9
Optical microscopic images of the aluminum flakes dispersed in water: (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes.
Figure 10 TGA and DTA curves of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and 27
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(c) Alx(HiPhy)@ZPA-decorated aluminum flakes. Figure 11 Nyquist spectra of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes, in which the equivalent circuit was presented as an inset. Figure 12 Plots of volume evolution of H2 gas as a function of time for (a) pristine aluminum flakes, (b)
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ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes.
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Figure 13 SEM micrographs of (a) ZPA-decorated aluminum flakes and (b) Alx(HiPhy)@ZPA-decorated
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aluminum flakes after water boiling tests.
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Figure 14 (a) Glossiness, (b) flop index, (c) DOI value, and (d) adhesion of coating surface for the dried
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waterborne coatings containing (1) pristine aluminum flakes, (2) ZPA-decorated aluminum flakes and (3) Alx(HiPhy)@ZPA-decorated aluminum flakes.
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Figure 15 Digital photograph of the waterborne coatings containing (a) the Alx(HiPhy)@ZPA-decorated
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aluminum flakes and (b) pristine aluminum flakes.
28
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Highlights
We developed a new double-layered surface decoration for aluminum flakes.
Aluminum flakes were coated with a ZPA passive film and PA–Al complex sealing film.
The surface-decorated aluminum flakes achieved a good chemical stability and corrosion resistance.
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f
The surface-decorated aluminum flakes can serve as a high-performance pigment for
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Pr
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waterborne coatings.
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30
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Wenqi Li, Ling Shi, Junying Zhang, Jue Cheng, Xiaodong Wang PII:
S0032-5910(19)31056-3
DOI:
https://doi.org/10.1016/j.powtec.2019.11.097
Reference:
PTEC 14979
To appear in:
Powder Technology
Received date:
26 November 2018
Revised date:
12 September 2019
Accepted date:
25 November 2019
Please cite this article as: W. Li, L. Shi, J. Zhang, et al., Double-layered surface decoration of flaky aluminum pigments with zinc aluminum phosphate and phytic acid–aluminum complexes for high-performance waterborne coatings, Powder Technology(2019), https://doi.org/10.1016/j.powtec.2019.11.097
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© 2019 Published by Elsevier.
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Double-layered surface decoration of flaky aluminum pigments with zinc aluminum phosphate and phytic acid–aluminum
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complexes for high-performance waterborne coatings
a
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Wenqi Li a, Ling Shi a,*, Junying Zhang a, Jue Cheng a, Xiaodong Wang b,*
Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing
b
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University of Chemical Technology, Beijing 100029, China
State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology,
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Beijing 100029, China
*Corresponding authors: Tel & fax: +86 10 6441 0145.
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Email: [email protected] (L. Shi); [email protected] (X. Wang).
1
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Abstract: This paper reported a double-layered surface decoration for aluminum flakes by coating a zinc aluminum phosphate passive film, followed by fabricating a phytic acid–aluminum complex sealing film. The chemical compositions and structures of surface-decorated aluminum flakes were characterized by Fourier-transform infrared, X–ray photoelectron, and energy-dispersive X–ray spectroscopy and their
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morphology and double-layered structure were confirmed by scanning and transmission electron
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microscopy. The surface-decorated aluminum flakes not only show excellent dispersibility in water due to
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the enhanced hydrophilicity, but also exhibit good corrosion resistance and chemical stability against water.
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Most of all, the double-layered surface decoration leads to an improvement in optical performance of
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coating surface such as glossiness, flop index and distinctness of image compared to the single-layered one when the surface-decorated aluminum flakes are applied for waterborne coatings. The surface-decorated
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aluminum flakes developed by this work can well serve as a good flaky aluminum pigment for
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high-performance waterborne coatings.
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Keywords: Aluminum flakes; Surface decoration; Phytic acid; Chemical conservation film; Corrosion resistance; Waterborne coatings
2
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1. Introduction Sustainable development has been vital for the survival and development of human beings, because all of the world have to face a wide range of common global challenges like environmental pollution, greenhouse emissions, climate change and global warming. There is a great demand for eco-friendly chemical products and “green” materials in various industrial and civil fields [1]. On the other hand, much
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stricter legislation and growing environmental consideration have forced paint manufacturers to develop
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coating systems with reduced content of volatile organic compounds (VOCs) in recent years [2,3].
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Waterborne coatings, containing a lot of water with small quantities of other organic solvents, are well
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known to be environmentally friendly due to an extremely low content of VOCs and therefore have
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received a broad range of application for automotive vehicles, buildings, furniture, domestic electrical appliances, official electronic equipments, and various metal and plastic parts [4]. To improve the
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rheological performance, adhesive force, glossiness, impact toughness, ultraviolet (UV) protection,
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abrasion resistance, scratch resistance and delamination resistance, various functional inorganic fillers such
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as TiO2, ZnO, SiO2, Al2O3, Fe2O3, clay nanoparticles and aluminum flakes are necessarily incorporated into waterborne coatings [5]. In most cases, the surface modification for these inorganic fillers becomes a vital procedure for enhancing the application performance of waterborne coatings due to low interfacial adhesion, high surface tension and poor chemical stability of pristine fillers [6]. Aluminum flakes are a type of important functional fillers, which can serve as pigments with silver-white metallic or pearlescent color. With special metallic luster, prominent shielding effectiveness, excellent mechanical properties, high reflection and good UV resistance, flaky aluminum pigments have been broadly applied for automotive coatings, printing inks, wood coatings, automotive coatings, industrial paints, furniture lacquers, plastic coatings, anti-corrosion coatings, etc [7]. Nevertheless, flaky aluminum 3
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pigments are easily corroded by water through an oxidation reaction in a weak acidic or alkaline medium due to its active chemical properties when used in waterborne coatings and paints. This leads to a change in color from silver white to grey as well as a reduction in metallic luster. Furthermore, the corrosion reaction results in a security issue during the production and storage of waterborne coatings and paints [8]. Therefore, it is essential to conduct a surface modification for flaky aluminum pigments to inhibit the
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corrosion reaction [9]. The researchers from pigment and coating industries have worked on this issue for a
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long time. The encapsulation and corrosion inhabitation are considered as two of the most effective surface
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modification methods for flaky aluminum pigments [10]. Ma et al. [11] reported a sol-gel encapsulation
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method for waterborne aluminum pigments with SiO2 through anchorage of H2O2, and a dense and smooth
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coating layer for corrosion resistance was formed on the surface of aluminum pigments with this method. He et al. [12] studied the surface modification of waterborne aluminum pigments with SiO2 and polymer
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brushes, and they found that the dispersibility and corrosion resistance of the resultant aluminum pigments
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in aqueous media were enhanced significantly. Pi et al. [13] reported the encapsulation of aluminum
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pigments with an organic-inorganic hybrid film and found that the anticorrosive and adhesive performance of the pigments was greatly improved in paint films. Liu et al. [14] synthesized the composite particles based on aluminum flakes and poly(trimethylolpropane triacrylate) through in-situ polymerization, and a performance investigation indicated that the corrosion resistance and adhesive property of composite particles were greatly improved due to the formation of a compact polymeric layer on the surface of aluminum flakes. Although the encapsulation of aluminum flakes with organic or inorganic films can form an adequate barrier to insulate the flaky aluminum pigment from the corrosion media, there are still some critical defects like easy crack on the protective layer, poor dispersibility of pigments, reduced glossiness of coating films and short cycle time for corrosion protection [15]. The introduction of chromates, phosphates, 4
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molybdates, fatty acids, amino acids, acrylic acid, nitro- and amino-phenols, heterocyclic compounds, saccharides and some small molecular organic inhibitors also can generate highly efficient corrosion inhibition for flaky aluminum pigments [16]. Müller et al. [17] investigated the application of citric acid as a corrosion inhibitor for flaky aluminum pigments and found that citric acid could act as a chelating agent to inhibit the corrosion reaction of aluminum pigments. Amin et al. [18] reported the effect of poly(acrylic
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acid) as a corrosion inhibitor on the anticorrosive performance of aluminum in weakly alkaline and found
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that its inhibition efficiency was strongly dependant on its concentration and molecular weight. Liu et al.
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[19] reported an investigation on the corrosion resistance of aluminum pigments with poly(methyl
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methacrylate) (PMMA) and found that there was a significant enhancement in corrosion inhibition by
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combination of PMMA and 3-methacryloxypropyltrimethoxysilane. Although chromates are considered as one of the most effective inhibitors for corrosion protection of flaky aluminum pigments, their utilization
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inhibition [20].
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has been restricted due to an environmental concern for heavy-metal free alternatives to chromate
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Phosphate inhibitors have received tremendous attention in recent years due to their eco-friendly nature as well as an outstanding anticorrosive effect on various metal materials [21]. Bastidas et al. [22] reported a comparative study on the anticorrosion of steel reinforcement by three sodium phosphates as corrosion inhibitors and gave a correlation between the phosphate content and steel corrosion rate. Naderi et al. [23] investigated the effects of zinc aluminum molybdenum orthophosphate hydrate and zinc calcium strontium aluminum orthophosphate silicate hydrate as new phosphate-based anticorrosion pigments on corrosion protection of mild steel, and they observed corrosion inhibition synergism between the two pigments. Tamilselvi et al. [24] reported the enhanced corrosion resistance of mild steel by fabricating a zinc phosphate coating layer through the development of ZnO nanoparticles in the phosphating bath. Feng 5
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et al. [25] investigated the efficiency of aluminum tripolyphosphate (ATP) as a corrosion inhibitor for carbon steel in carbonated concrete pore solutions and mortars and found the pitting corrosion of the steel could be prevented effectively due to an enhancement in stability of passive film on the steel surface by ATP. However, the surface modification with phosphate inhibitors generally produces an unusual porous structure on the metal surface, and these pores may adsorb the corrosive media and contaminants in the
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atmosphere. This may deteriorate the anticorrosion effect of protective layer on the protected metal
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materials [26]. In this case, there were few reports about the use of phosphate inhibitors for the
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anticorrosive modification of micro-size metal particles like flaky aluminum pigments. Therefore, to
e-
improve the corrosion resistance of phosphate-modified metal materials, the second protective layer is necessary to seal these pores produced by phosphate inhibitors. A facile sealing method is to fabricate a
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sealing film, or called “chemical conversion film”, upon the modified metal surface through the chelation
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of phytic acid and the relevant metal ions [27,28]. Phytic acid (PA) is a water-soluble organic compound
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known as inositol hexakisphosphate, and it has attracted special attention in the surface treatment of metal
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for many years due to its strong chelating capability for various metal ions including Fe3+, Cu2+, Zn2+, Mg2+ and Al3+ as well as a nontoxic nature. Although PA could be employed as a corrosion inhibitor to form a protective layer for many metals against corrosion, the PA–metal complex layer only presented a limited anticorrosion effectiveness toward aluminum and its alloys as a chemical conversion film [29]. On the other hand, PA is able to well serve as a sealing layer to protect the phosphate-based passive films from further corrosion reaction. Wang et al. [30] reported a surface sealing modification for the anodized aluminum with PA via a solution-immersion process and found that the anodized aluminum with a phytic acid–aluminum (PA–Al) complex thin film exhibited much better corrosion resistance in comparison with the unmodified one. It is expected that the combination of phosphate-based protective layer and PA–Al 6
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complex sealing layer can generate much better corrosion resistance for flaky aluminum pigments. In this study, we reported a double-layered surface decoration for flaky aluminum pigments with zinc aluminum phosphate (ZPA) and PA–Al complexes. In this new method, a phosphate-based passive film was first fabricated onto aluminum flakes through a surface passivation reaction, and then a PA–Al complex sealing film was formed via a chelation reaction between PA and Al3+ ions. The chemical
f
compositions and microstructures of surface-decorated aluminum flakes were characterized extensively,
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and the synthetic strategy and reaction mechanism were described. Moreover, the effect of surface
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decoration on the dispersibility, optical performance and anticorrosion behaviors of aluminum flakes were
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investigated intensively. The aim of this study is to develop an eco-friendly surface decoration method for
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the flaky aluminum pigments used for waterborne coatings so as to achieve excellent corrosion resistance, good storage stability, high mechanical properties and good glossiness for high-performance waterborne
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2.1. Materials
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2. Experimental
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coatings.
Aluminum flakes were commercially supplied by Eckart Co., Ltd., Germany, and they have a mean diameter of 10–12 μm and an aluminum content of 99 wt %. PA [C6H6(H2PO4)6] with a concentration of 50 wt % in H2O was commercially obtained from Aladdin Co., Ltd., USA. Zinc phosphate [Zn3(PO4)2], diammonium phosphate [(NH4)2HPO4] and isopropanol were purchased from Tianjin Fuchen Chemical Co., Ltd., China. All reagents were used as received without further purification.
2.2. Preparation methods A double-layered surface decoration for flaky aluminum pigments was performed by coating a ZPA [Zn3Al(PO4)3] film upon aluminum flakes through a passivation reaction of aluminum with Zn3(PO4)2 and 7
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(NH4)2HPO4, followed by fabricating an Alx(HiPhy) film onto the surface of ZPA passive film through the chelation of PA and Al3+. In a typical synthetic process, 1.0 g of aluminum flakes was dispersed in 40 ml of isopropanol with stirring for 5 min. On the other hand, 0.4 g of diammonium phosphate was dissolved in 6 ml of deionized water, and 0.04 g of zinc phosphate was added under controlled agitation for 5 minutes. Afterwards, the resulting solution was added dropwise into the aqueous suspension containing aluminum
f
flakes under mild agitation, and then a passivation reaction was performed at 25 ºC for 1 h to obtain the
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aluminum flakes surface-coated with a ZPA passive film. Finally, the as-prepared aluminum flakes were
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washed with deionized water several times and then dispersed in 40 ml of isopropanol. In succession, an
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aqueous solution containing PA was prepared by dissolving 1.0 g of PA in 6 ml of deionized water and then added dropwise into the isopropanol suspension containing the as-prepared aluminum flakes to perform a
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chelation reaction between PA and Al3+ at 30 ºC for 3 h. The surface-decorated aluminum flakes with a
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double-layered structure were obtained ultimately, and then they were washed with deionized water and
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dried in oven at 60 ºC for further use.
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2.3. Characterization and measuring method Fourier-transform infrared (FTIR) spectroscopy was conducted on a Bruker Alpha-T FTIR spectrometer at a scanning number of 32 using KBr sampling discs. Energy dispersive X-ray (EDX) spectroscopy was performed to analyze the surface elemental distributions of flaky aluminum samples using a Bruker XFlash 6160 energy dispersive X-ray spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Scientific ESCALAB 250 X-ray photoelectron spectrometer with a focused monochromatized Al–Kα radiation. The obtained XPS spectra were fitted with a Casa XPS software, in which a Shirley background was assumed. X-ray diffraction (XRD) powder patterns were obtained from a Rigaku D/Max 2500 VBZ+/PC X-ray diffractometer using Cu–Kα radiation operated at 40 8
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kV and 50 mA with a scan rate of 1 ºC/min. Scanning electron microscopy (SEM) was conducted to observe the morphologies of flaky aluminum samples on a JEOL JSM-7800 scanning electron microscope. The microstructures of flaky aluminum samples were investigated by transmission electron microscopy (TEM) on an FEI Tecnai G2 20 transmission electron microscope operating at an acceleration voltage of 200 kV.
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The static water contact angles of flaky aluminum samples were measured by a Dataphysics
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OCA15EC contact angle meter using deionized water drops. The mean contact angle value was obtained
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from at least five individual measurements for each sample. To characterize the dispersibility of flaky
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aluminum samples, aluminum flakes were well dispersed in deionized water under vigorous agitation. The
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resulting suspension was dripped onto a glass slide and then observed by an Olympus BX51 optical microscope equipped with a Sony CCDIRIS digital camera. The thermal stabilities of flaky aluminum
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samples were characterized by thermogravimetric analysis (TGA) on a TA Instruments SDT2960
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thermogravimetric analyzer at a heating rate of 10°C/min in nitrogen.
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The electrochemical corrosion behavior of flaky aluminum samples was characterized on an electrochemical workstation (Chenhua CHI760E, China) equipped with a standard three-electrode configuration. This three-electrode configuration consists of a nickel foam sheet with an area of 1.52.0 cm2 used as a working electrode, a platinum sheet as a counter electrode and an Ag/AgCl electrode as a reference electrode. To prepare the working electrode, a flaky aluminum sample was mixed with polyvinylidene fluoride binder in N-methyl-pyrrolidinone to form a slurry, and then the resulting slurry was pasted on the nickel foam sheet in an area of 11 cm2. The nickel foam sheets coated with flaky aluminum samples were immersed in a NaCl aqueous solution (3.5 wt %) to serve as a working electrolyte. The measurements of electrochemical impedance spectrometry (EIS) were carried out in the frequency range of 9
Journal Pre-proof 0.01 Hz – 20 kHz. All of the electrochemical tests were repeated at least three times for statistical purposes. Water boiling tests were conducted to detect the corrosion resistance of flaky aluminum samples under the boiling water condition according to the Chinese national standard of HG/T 2456.5–2016. In a typical testing process, 1.0 g of flaky aluminum sample was dispersed in 200 ml of boiling water under magnetic agitation. The volume of H2 gas generated by reaction of aluminum and water was measured with a gas
f
buretter at room temperature. The corrosion resistance was determined by the production rate and total
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production volume of H2 gas within the same duration.
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A series of polyacrylate-based waterborne coatings were prepared by using different flaky aluminum
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samples at the same formulation. The resulting coatings were sprayed onto the steel plates with a dimension
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of 150 100 1 mm3 and then dried at room temperature for 24 h. The glossiness of coating surface was measured by a BYK 4601 haze–gloss reflectometer at the illuminating angles of 20° and 60°. The flop
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index of coating surface was measured by an X–Rite MA68 multi-angle spectrophotometer. The
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distinctness of image of coating surface was measured by a BYK 4840 wave-scan dual detector. The
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cross-cut tests of coating surface were carried out according to the method described by the ISO–2409 standard, and a classification of adhesion was achieved on the basis of the surface appearance of cross-cut area. All of the measurements were carried out at room temperature, and each of the reported data represented a mean value of five tests.
3. Results and discussion 3.1. Reaction mechanism and structural characterization Fig. 1 shows the synthetic strategy and reaction mechanism for the double-layered surface decoration of aluminum flakes with ammonium phosphate salt and phytic acid salt. It is noted that such a surface decoration is involved in two reaction steps. In the first step, Zn3(PO4) 2 and (NH4)2HPO4 reacted with the 10
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Al2O3 surface thin layer of aluminum flakes to produce Zn3Al(PO4)3, which deposited on the surface of aluminum flakes to form a passive film or “chemical conversion film”. It is well known that phosphate ions can react with various metal ions to form various insoluble phosphorous compounds, which can act as a protective layer to inhibit the corrosion of metal materials [1]. However, Zn3(PO4)2 has a poor solubility in the aqueous solution, thus depressing the release of phosphate ions. Moreover, phosphate ions hardly
f
performed an oxidation reaction with aluminum due to their poor reaction capability, thus resulting in the
oo
formation of a loose protective layer through deposition of phosphate salt [31]. This is evidently
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disadvantageous to the corrosion inhibition of zinc phosphate. In this case, the incorporation of
e-
(NH4)2HPO4 not only can improve the solubility of Zn3(PO4)2 but also can generate a synergistic effect to
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enhance the reactivity of aluminum and phosphate ions so as to form a more compact and stable protective layer [23]. Nevertheless, the reaction of aluminum and phosphate ions normally forms a passive film
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consisting of a barrier inner layer and a porous outer layer. Such a passive film was reported to present an
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ordered array of cells [30], and it may easily absorb the corrosive substances and contaminants in the air,
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thus reducing the anticorrosive effectiveness of phosphate-based protective layer. Therefore, a PA–Al complex thin film was fabricated to seal this porous layer through a chelation reaction between PA and Al3+ ions in the second step as displayed in Fig. 1. With a strong chelating capability, PA could attack the aluminum substrate to make more Al3+ ions released and then reacted with these Al 3+ ions to produce a series of PA–Al complexes based on the chemical composition of Alx(HiPhy). These complexes can form a compact sealing layer as another chemical conversion film upon the ZPA passive film to provide an additional anticorrosive protection for aluminum flakes [32]. Moreover, the fabrication of this sealing film also improves the hydrophily of aluminum flakes due to the water-soluble nature of PA, which can enhance the dispersibility of flaky aluminum pigments in waterborne coatings. 11
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The chemical compositions and structures of surface-modified aluminum flakes were first characterized by FTIR spectroscopy, and the resulting infrared spectra are given in Fig. 2. As observed from the infrared spectrum of pristine aluminum flakes, there is a weak characteristic band distinguished at 664 cm-1 for the Al–O stretching vibration, implying the existence of Al2O3. Moreover, two absorption peaks are also observed at 3426 and 1638 cm-1 for the O–H stretching and bending variations of water,
f
which is due to the absorbed water on the surface of aluminum flakes. On the other hand, the infrared
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spectrum of ZPA-decorated aluminum flakes not only shows the same absorption bands with pristine
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aluminum flakes but also exhibits a series characteristic absorption bands at 956 cm-1 for the P–O stretching
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vibration of phosphate group and at 1056 and 569 cm-1 due to the stretching and deformation vibration of
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P=O bond. A characteristic peak can be observed at 897 cm-1 due to the Zn–O stretching vibration. These results indicate the formation of ZPA on the aluminum flakes [33]. Moreover, there is a set of absorption
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bands found at 1349–1526 cm-1, which may be ascribed to the C–H deformation vibration of the organic
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solvents absorbed by the passive film. As for the Alx(HiPhy)@ZPA-decorated aluminum flakes, a set of
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new absorption bands centered at 796 cm-1 can be observed due to the P–O–Al stretching vibration derived from the chelation of phytic molecules with Al3+ ions [29]. Considering of a low resolution for the FTIR spectra obtained from powder specimens, the chemical compositions of surface-decorated aluminum flakes were further confirmed by EDX characterization, and the resulting spectra are presented in Fig. 3. As displayed in Fig. 3a, the EDX spectrum of ZPA-decorated aluminum flakes clearly reveals five peaks corresponding to C, O, P, Zn and Al elements on the flaky aluminum surface. The Al, Zn, O, and P signals imply the existence of ZPA on the surface of aluminum flakes, while the presence of C signal is attributed to the carbon diaphragm used for EDX measurements. Furthermore, the mass and atomic percentages could also be estimated by EDX analysis and was given as an inset in Fig. 3a, which clearly determines the 12
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chemical composition of the ZPA-decorated aluminum flakes. It is found in Fig. 3b that the Alx(HiPhy)@ZPA-decorated aluminum flakes show the same elemental signals in their EDX spectrum, and however the intensity of the P signal is enhanced relatively. It is also noteworthy from the elemental data that the mass and atomic percentages of P element are much greater than those in the ZPA-decorated aluminum flakes, whereas those of Zn element become smaller. Such a change in chemical composition
f
indicates an overlap of the Alx(HiPhy) film onto the ZPA one.
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The chemical structures of coating layers were further determined by the XPS spectra shown in Fig. 4.
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The XPS survey spectra clearly show a series of intensive peaks assigned to the O, C, P and Zn elements
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for both of ZPA-decorated and Alx(HiPhy)@ZPA-decorated aluminum flakes, whereas the intensive signals
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corresponding to O, C and Al elements are found in the survey spectra of pristine aluminum flakes (see Fig. 4a). However, only weak signals corresponding to aluminum can be detected for the surface-decorated
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aluminum flakes due to the overlap of two chemical conversion films. In this case, all of the XPS spectra
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were further fitted with Gaussian-Lorentzian peaks by means of the Casa XPS software to find out O, C
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and Al elements in different environments. Fig. 4b–4i show the high-resolution XPS spectra of pristine and surface-decorated aluminum flake samples. Through the curve fitting, two deconvoluted peaks can be found at binding energy of 74.3 and 71.8 eV for the Al–O and Al–Al bonds in the high-resolution Al 2p spectrum of pristine aluminum flakes (see Fig. 4b), and meanwhile a deconvoluted peak can be observed at binding energy of 531.9 eV for the O–Al bond in their high-resolution O 1s spectrum (see Fig. 4c). These results identify the Al and Al2O3 substances as the chemical composition of pristine aluminum flakes. Therefore, the existence of Al2O3 in pristine aluminum flakes can be confirmed by the combination of FTIR and XPS analyses. It is observed in Fig. 4d–4f that there are a series of deconvoluted peaks for Al–Al (69.5 eV), Al–O-–P (72.6 eV), P–O-–Al (131.7 eV), P=O (132.5 eV), O-–Zn (529.2 eV), O-–Al (530.0 eV), O-–P 13
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(531.1 eV), and O=P (531.9 eV) in the high-resolution spectra of ZPA-decorated aluminum flakes. These deconvoluted peaks in Al 2p, P 2p and O 1s zones provide a firm evidence for the formation of ZPA film on the surface of aluminum flakes. In the case of the Alx(HiPhy)@ZPA-decorated aluminum flakes, the peaks for Al–Al (69.7 eV), Al–O-–P (72.6 eV), Al–O-–Phy (73.2 eV), P–O- (131.8 eV), O–P–O- (132.4 eV), P=O (133.0 eV), O-–Zn (528.9 eV), O-–Al (529.6 eV), O-–P–O (530.4 eV), P–O-–Al (530.8 eV), P–O–C (531.4
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eV) and O=P (531.8 eV) can be clearly distinguished on the basis of their deconvoluted Al 2p, P 2p and O
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1s spectra (see Fig. 4g–4i), suggesting the presence of Alx(HiPhy) outer layer accordingly. These XPS
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results further confirm the successful fabrication of Alx(HiPhy)@ZPA double protective layers onto the
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aluminum flakes. The crystalline structures of flaky aluminum samples were investigated by XRD, and
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resulting diffraction patterns are displayed in Fig. 5. The XRD pattern of pristine aluminum flakes is found to show a set of intensive diffraction peaks at 2θ values of 38.51, 44.80, 65.23 and 78.32, which are
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assigned to the (111), (200), (220) and (311) reflections of crystal phase of the aluminum substrate,
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respectively [33]. Although two surface-decorated flaky aluminum samples exhibit the similar XRD
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patterns as pristine aluminum flakes, a series of new diffraction peaks can be observed in their patterns. These weak diffraction peaks are attributed to the complicated crystal phases of ZPA and Alx(HiPhy) films coated on the surface of aluminum flakes according to the standard JCPDS Nos. 26-0034, 21-1488, 33-1474 and 51-1754.
3.2. Morphology and microstructure The morphologies and microstructures of flaky aluminum samples were investigated by SEM and TEM, and the obtained SEM and TEM micrographs are shown in Fig. 6 and Fig. 7, respectively. It is observed in Fig. 6a that pristine aluminum flakes exhibit an irregular shape with a size around 10 μm. These aluminum flakes seem to have a very smooth surface according to the magnified SEM micrograph in 14
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Fig. 6b. It is seen in Fig. 6c and 6d that the ZPA-decorated aluminum flakes present a more flat surface with a semi-ordered array of bulges, indicating the formation of a ZPA passive film. The presence of bugles is ascribed to the porous and loose structure of the passive film as reported by the relevant references [34–36]. Owing to a weak reactivity of (NH4)2HPO4, the release of Al 3+ ions from aluminum flakes is insufficient to be involved in the passivation reaction. As a result, an inhomogeneous loose structure was
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caused inside the passive film. The TEM micrographs in Fig. 7a and 7b also clearly demonstrate that the
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microstructure of the passive film includes a compact barrier layer and a loose outer layer. Moreover, the
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thickness of passive film can be estimated approximately as 4 nm according to the magnified TEM
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micrographs in Fig. 7b. As observed in the SEM micrographs of Alx(HiPhy)@ZPA-decorated aluminum
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flakes in Fig. 6e and 6f, it is interestingly found that the bulges seem to become insignificant in the presence of Alx(HiPhy) sealing film on the flaky aluminum surface. There is a noticeable increase in total
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thickness of coating films observed in the TEM micrographs in Fig. 7c and 7d due to the formation of
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Alx(HiPhy) sealing film. It is understandable that PA has a stronger reactivity with metal ions by chelation
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and thus can easily react with the Al3+ ions existing in the loose part of ZPA passive film as well as those released from the aluminum substrate. Such a chelation reaction not only can form an Alx(HiPhy)-based chemical conversion film on the surface of modified aluminum flakes but also can seal the pores inside the ZPA film to enhance the compactness of protective layer. The total thickness of chemical conversion films can also be determined as about 9 nm as observed in Fig. 7d, which evidently favors the anticorrosive capability of protective layer.
3.3. Dispersibility and thermal stability The wettability of flaky aluminum samples was investigated by measurement of static water contact angle, and the obtained results are illustrated in Fig. 8. According to the resulting data and relevant optical 15
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images, pristine aluminum flakes are found to have a water contact angle of 93.45º, indicating a hydrophobic nature. However, after the surface decoration with ZPA, the water contact angle of aluminum flakes decreases to 68.24º. This implicates that the hydrophilicity of flaky aluminum surface is remarkably improved due to a decrease in surface energy as a result of the fabrication of ZPA passive film. It is noteworthy that the water contact angle continues to drop down to 31.09º after the fabrication with an
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Alx(HiPhy) sealing film. This result suggests that the hydrophilicity of aluminum flakes is further enhanced
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due to the incorporation of hydrophilic phosphate groups in PA molecules, which may favor the dispersion
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of flaky aluminum pigments in waterborne coatings. The dispersibility of flaky aluminum samples in water
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was investigated by optical microscopic observation, and the obtained microscopic images are displayed in
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Fig. 9. It is noted in Fig. 9a that there is a serious aggregation phenomenon observed from the aluminum flakes in water, indicating that pristine aluminum flakes have poor dispersibility in water due to their
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hydrophobic nature. However, the ZPA-decorated aluminum flakes in water exhibit a lot of dispersed
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particles with a small size. This is attributed to the hydrophilic feature of ammonium phosphate salt in the
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passive film. Furthermore, the Alx(HiPhy)@ZPA-decorated aluminum flakes are observed to exhibit much smaller dispersed units in water compared to the ZPA-decorated ones. Such good dispersibility is due to the fact that the abundant hydrophilic groups in PA molecules can further improve the hydrophilic performance of aluminum flakes. These results are in good agreement with the data of water contact angle. Furthermore, the compatibility of Alx(HiPhy)@ZPA-decorated aluminum flakes with coatings will be verified by a series of their applicable experiments in waterborne coatings. The thermal stability of flaky aluminum samples was analyzed by TGA under a nitrogen atmosphere, and the resulting TGA and DTG curves are presented in Fig. 10. It is observed that pristine aluminum flakes present a slight weight loss in the temperature range of 50–400 ºC, followed by a rapid weight 16
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increase in the temperature range of 730–800 ºC. The losses of adsorbed and structural water were involved in the early weight decrease [37], and the later weight loss may be due to an oxidative reaction between the Al substrate and Al2O3 thin film until 400 ºC [13]. However, the nitrogen gas can react with aluminum flakes to produce AlN with an elevation of temperature above 700 ºC, which leads to a dramatic increase in specimen mass [13]. The ZPA-decorated aluminum flakes are found to exhibit a more significant weight
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loss than the pristine one in the temperature range of 50–600 ºC due to a relatively poor thermal stability of
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ZPA passive film. As observed in Fig. 10, there are two peaks corresponding to maximum decomposition
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temperature in their DTG curve. The first one is attributed to the thermal decomposition of the loose outer
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layer of ZPA passive film, and the second one is due to the thermal degradation of its compact barrier layer.
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It is noticeable that the fabrication of Alx(HiPhy) sealing film onto the ZPA-decorated aluminum flakes evidently enhances the thermal stability of aluminum flakes and thus reduces their weight loss in the early
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heating stage until 350 ºC. Nevertheless, the Alx(HiPhy) sealing film has to undergo a thermal degradation
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at a much higher temperature over 350 ºC, which results in a faster weight loss due to the pyrolysis of
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Alx(HiPhy). In summary, the Alx(HiPhy)@ZPA-decorated aluminum flakes exhibit a better thermal stability than the ZPA-decorated ones in the operating temperature range for waterborne coatings.
3.4. Electrochemical behavior and chemical stability EIS measurements were performed to investigate the electrochemical behavior of aluminum flakes before and after surface decoration. Fig. 11 shows the Nyquist spectra of flaky aluminum samples obtained from EIS tests as well as the equivalent circuit derived from the simulation of EIS data with the Zsimpwin software. The Nyquist spectra of these three samples are all found to present a semicircular diagram following with a diffusional element over the whole frequency region. It is well known that the impedance behavior of a metal material reflects its anticorrosive capability, and the magnitude of impedance can 17
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determine the resistance of chemical conversion films to the transportation of electrons and charges [38]. According to the simulated results, the equivalent circuit of three flaky aluminum samples is found to include a solution resistance (Rs), an interfacial charge-transfer resistance (Rt), the Warburg impedance (Zw) related to ionic diffusion and a non-intuitive circuit element (QCPE) representing the constant phase element [25]. Although three flaky aluminum samples exhibit a similar impedance behavior, pristine aluminum
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flakes show the smallest diameter among three Nyquist spectra. The diameter of semicircular diagram is
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observed to present a significant increase as a result of the double-layered surface decoration of aluminum
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flakes with ZPA and Alx(HiPhy). This means that the interfacial charge-transfer resistance of aluminum
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flakes is improved due to the formation of chemical conversion films, which can serve as effective physical
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barriers to prevent corrosive ions in the electrolyte from penetrating the films to induce the electrochemical corrosion of aluminum flakes. The corrosion resistance of aluminum flakes is enhanced as a result of the
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surface decoration. It is clearly observed in Fig. 11 that the Alx(HiPhy)@ZPA-decorated aluminum flakes
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exhibit the largest diameter, which means there is the highest value in the real and imaginary impedance of
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their semicircular diagram. This implicates that the double-layered surface decoration can provide a greater interfacial charge-transfer resistance for aluminum flakes than the single-layered one and forms a stronger anticorrosive layer for them accordingly. On the other hand, the impedance of constant phase element (ZCPE) can be given by Eq(1) [39].
ZCPE
1 Q0 ( j)n
(1)
where n is the exponent of constant phase element, j is an imaginary value, ω is the frequency of phase angle, and Q0 has the numerical value of the admittance (1/ZCPE) at ω = 1 rad/s. The QCPE represents a pure capacitance at n = 1, but it will become a pure resistance at n = 0. The value of ZCPE is associated with film thickness, surface heterogeneity and surface roughness [25]. In this work, the value of n was determined as 18
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approximate 0.85 on the basis of the fitting result of EIS data, which suggests that the corrosion resistance of aluminum flakes is not only controlled by the interfacial charge-transfer resistance but also influenced by the value of ZCPE. It is no doubt that the Alx(HiPhy)@ZPA-decorated aluminum flakes have a more compact and thicker chemical conversion film than the ZPA-decorated ones and thus gain a higher value of ZCPE. Accordingly, this makes them a better anticorrosive effect on aluminum flakes [39].
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The chemical stability of flaky aluminum samples was evaluated by the evolution of hydrogen gas in
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water boiling tests. Fig. 12 shows the plots of volume evolution of hydrogen gas as a function of time for
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the aluminum flakes before and after surface decoration. It is observed in Fig. 12 that pristine aluminum
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flakes exhibit high reactivity with water, and they released a high volume of hydrogen gas within 20 min in
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the water-boiling test. However, almost no hydrogen gas was released from both the ZPA-decorated and Alx(HiPhy)@ZPA-decorated aluminum flakes during the whole process of water boiling tests. There is no
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doubt that the fabrication of chemical conversion films on the flaky aluminum surface effectively prevents
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the aluminum substrate from reacting with water to produce hydrogen gas. It is noteworthy from the
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close-up evolution plots in Fig. 12 that there is still a small amount of hydrogen gas produced by the ZPA-decorated aluminum flakes in the water boiling test due to the loose outer layer of ZPA passive film, through which the water molecules may penetrate to react with the aluminum substrate. After coated with an Alx(HiPhy) sealing film, the surface-decorated aluminum flakes no longer react with water, and therefore, there is no release of hydrogen gas observed in the close-up evolution plot. Fig. 13 shows the SEM micrographs of the aluminum flakes after water boiling tests. According to these micrographs, there is no trace of water corrosion found on the surface of the Alx(HiPhy)@ZPA-decorated aluminum flakes, whereas some traces are distinguished on the surface of ZPA-decorated aluminum flakes due to a corrosive reaction with water. On the other hand, pristine aluminum flakes were completely reacted with water during the 19
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water boiling test, and nothing was left for SEM observation. These results indicate that the Alx(HiPhy)@ZPA-decorated aluminum flakes have excellent corrosion resistance against water and can maintain a long-term chemical stability in waterborne coatings.
3.5. Application performance The application performance of flaky aluminum samples in waterborne coatings was investigated in
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terms of the glossiness, flop index, distinctness of image (DOI) and classification of adhesion obtained
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from the dried coating surface, and the obtained results are presented in Fig. 14. As seen in Fig. 14a, the
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coating surface achieved glossiness values of 95.5% and 101.2% at the illuminating angles of 20º and 60º,
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respectively, when pristine aluminum flakes were incorporated into the waterborne coating. The glossiness
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of coating surface is found to decrease when the ZPA-decorated aluminum flakes were used in the waterborne coating. This may be due to the rougher surface of ZPA-decorated aluminum flakes than the
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pristine ones, which leads to a decrease in specular reflectance [40]. However, the waterborne coating
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containing the Alx(HiPhy)@ZPA-decorated aluminum flakes shows a slight improvement in surface
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glossiness, which is ascribed to the formation of more smooth Alx(HiPhy) sealing film upon aluminum flakes. A similar trend can be observed in the flop index and DOI value of coating surface as seen in Fig. 14b and 14c. The addition of ZPA-decorated aluminum flakes results in a considerable decrease in both flop index and DOI value of coating surface compared to the pristine ones. However, the use of Alx(HiPhy)@ZPA-decorated aluminum flakes for waterborne coatings can improve these two parameters evidently. The compact surface structure of chemical conversion film favors the orientation and distribution of aluminum flakes in the waterborne coating, thus improving the optical performance of coating surface [41]. Moreover, the cross-cut test results shown in Fig. 14d demonstrate that the edges of the cuts are completely smooth, and none of the squares of the lattice is detached from these three coating samples, so 20
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all of them can be classified to 0 grade in adhesion according to the classification of the ISO–2409 standard. Fig. 15 shows the digital photograph of two waterborne coatings individually containing the Alx(HiPhy)@ZPA-decorated aluminum flakes and pristine aluminum flakes after standing for 7 days. It is clearly observed that the waterborne coating containing the Alx(HiPhy)@ZPA-decorated aluminum flakes exhibited a homogenous state after long-term standing, and however there was a serious phase separation
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occurring in the waterborne coating containing pristine aluminum flakes. It is understandable that the poor
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compatibility of pristine aluminum flakes with the waterborne coating results in the phase separation. On
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the other hand, the surface decoration of aluminum flakes may enhance their compatibility with the
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waterborne coating due to the improvement of hydrophilic performance. These results confirm that the
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Alx(HiPhy)@ZPA-decorated aluminum flakes developed by this work the have better compatibility with the waterborne coating than the pristine ones due to surface decoration, and they can well serve as a good flaky
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4. Conclusions
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aluminum pigment for high-performance waterborne coatings.
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A double-layered surface decoration was successfully carried out up on aluminum flakes by fabrication of a ZPA passive film, followed by coating an Alx(HiPhy) sealing film. The chemical compositions and structures of the resultant aluminum flakes were determined by FTIR, EDX, and XPS spectroscopy. The morphologies of surface-decorated aluminum flakes were confirmed by SEM, and the double-layered structure of Alx(HiPhy)@ZPA-decorated aluminum flakes was verified by TEM. The double-layered surface decoration made aluminum flakes good hydrophilic performance, thus improving their dispersibility in water. The Alx(HiPhy)@ZPA-decorated aluminum flakes exhibited good anticorrosive performance and a chemical stability against water on the basis of the EIS analysis and water boiling test results. Most of all, an investigation on application performance indicated that the optical performance of 21
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coating surface was enhanced and the adhesion performance was well maintained when the Alx(HiPhy)@ZPA-decorated aluminum flakes were used for a polyacrylate-based waterborne coating. The surface-decorated aluminum flakes developed by this work can well serve as a high-performance pigment for waterborne coatings.
Acknowledgements
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This work is financially supported by the National Key Technology R&D Program of China with a
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grant number of 2014BAE12B03.
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Declarations of interest: none
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Figure Captions Figure 1
Scheme of synthetic strategy and reaction mechanism for the double-layered surface decoration of flaky aluminum pigments.
Figure 2
FTIR spectra of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes. EDX spectra of (a) ZPA-decorated aluminum flakes and (b) Alx(HiPhy)@ZPA-decorated
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Figure 3
aluminum flakes.
(a) XPS survey spectra of (1) pristine aluminum flakes, (2) ZPA-decorated aluminum flakes and
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Figure 4
flakes,
(d,
e,
f)
ZPA-decorated
aluminum
flakes
and
(g,
h,
i)
Pr
aluminum
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(3) Alx(HiPhy)@ZPA-decorated aluminum flakes; high-resolution XPS spectra of (b, c) pristine
Alx(HiPhy)@ZPA-decorated aluminum flakes.
XRD patterns of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c)
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Figure 5
SEM micrographs of (a, b) pristine aluminum flakes, (c, d) ZPA-decorated aluminum flakes and
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Figure 6
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Alx(HiPhy)@ZPA-decorated aluminum flakes.
(e, f) Alx(HiPhy)@ZPA-decorated aluminum flakes. Figure 7
TEM
micrographs
of
(a,
b)
ZPA-decorated
aluminum
flakes
and
(c,
d)
Alx(HiPhy)@ZPA-decorated aluminum flakes. Figure 8
Static water contact angles of flaky aluminum samples.
Figure 9
Optical microscopic images of the aluminum flakes dispersed in water: (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes.
Figure 10 TGA and DTA curves of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and 27
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(c) Alx(HiPhy)@ZPA-decorated aluminum flakes. Figure 11 Nyquist spectra of (a) pristine aluminum flakes, (b) ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes, in which the equivalent circuit was presented as an inset. Figure 12 Plots of volume evolution of H2 gas as a function of time for (a) pristine aluminum flakes, (b)
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ZPA-decorated aluminum flakes and (c) Alx(HiPhy)@ZPA-decorated aluminum flakes.
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Figure 13 SEM micrographs of (a) ZPA-decorated aluminum flakes and (b) Alx(HiPhy)@ZPA-decorated
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aluminum flakes after water boiling tests.
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Figure 14 (a) Glossiness, (b) flop index, (c) DOI value, and (d) adhesion of coating surface for the dried
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waterborne coatings containing (1) pristine aluminum flakes, (2) ZPA-decorated aluminum flakes and (3) Alx(HiPhy)@ZPA-decorated aluminum flakes.
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Figure 15 Digital photograph of the waterborne coatings containing (a) the Alx(HiPhy)@ZPA-decorated
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aluminum flakes and (b) pristine aluminum flakes.
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Highlights
We developed a new double-layered surface decoration for aluminum flakes.
Aluminum flakes were coated with a ZPA passive film and PA–Al complex sealing film.
The surface-decorated aluminum flakes achieved a good chemical stability and corrosion resistance.
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f
The surface-decorated aluminum flakes can serve as a high-performance pigment for
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Pr
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waterborne coatings.
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30
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15