Influence of aluminum source on the color tone of cobalt blue pigment
Graphical abstract
Introduction
Pigments are insoluble powders used for coloring, and can be classified as organic and inorganic pigments. Inorganic pigments generally have a low Chroma, low tinting strength, and high hiding power, compared to organic pigments. Inorganic pigments exhibit good weather and chemical resistance, and are generally low cost. Cobalt blue is an inorganic pigment with an intense blue color, and consists mainly of cobalt aluminate (CoAl2O4) spinel. Cobalt blue is chemically and thermally stable, and is widely used to color materials like ceramics, plastics, and fibers [1], [2], [3], [4], [5], [6].
Cobalt blue has been synthesized by many methods, including the sol-gel method [2], hydrothermal method [3], and low-temperature combustion method [4], [5]. Solid-state reaction is the most common method used to industrially synthesize cobalt blue, because of its simple production flow and high cost-performance. Srisawad et al. [6] synthesized cobalt blue by solid-state reaction, using cobalt chloride hexahydrate and two types of gibbsite with different particle sizes. They reported that decreasing the starting particle size resulted in a lower crystallization temperature of CoAl2O4 formation. They also carried out syntheses using various cobalt salts and gibbsite [7], and reported the catalytic activities of the products. Melo et al. [8] synthesized cobalt blue using cobalt oxalate and alumina, with various calcination temperatures. They compared the color tones of the cobalt blue, when it was used as a glaze. While cobalt blue has been synthesized from various materials, there are no reports on the color tone of cobalt blue synthesized from different starting materials. Thus, the relationship between starting material and color tone has not yet been clarified. In the current study, we systematically investigated the influence of different starting materials on the color tone of cobalt blue, synthesized by solid-state reaction.
Because there are no harmful gas generation when sintering, aluminum hydroxide (Al(OH)3) and alumina (Al2O3) can be used as aluminum sources for synthesizing cobalt blue. Bolt et al. [9] investigated the rate of metal aluminate formation from metal oxide and alumina. They found that γ-Al2O3 reacted with cobalt oxide at a much higher rate than α-Al2O3. In addition, heating aluminum hydroxide (Al(OH)3) at low and high temperature reportedly yields spinel (γ-Al2O3) and corundum (α-Al2O3), respectively [10]. These reports suggest that vivid cobalt blue pigment should be synthesized from γ-Al2O3 which has a high reaction rate, or from Al(OH)3 which forms γ-Al2O3 during heating. Thus, we selected three sources, Al(OH)3, γ-Al2O3, and α-Al2O3, and experimentally investigated the influence of aluminum source on the color tone of the resulting cobalt blue.
Section snippets
Materials
Cobalt oxide (Co3O4, High Purity Chemicals, Osaka, Japan) was used as the cobalt source. Aluminum hydroxide (Al(OH)3, Wako), γ-alumina (γ-Al2O3, High Purity Chemicals), and α-alumina (α-Al2O3 (S), High Purity Chemicals, α-Al2O3 (L), Wako, Osaka, Japan) were used as the aluminum source. These regents were used as received without further purification.
Synthesis method
Cobalt blue was synthesized by the dry mixing of cobalt oxide and various aluminum sources, followed by calcination. Cobalt oxide and one of four
Results and discussion
Fig. 1(A) and (B) show SEM images and cumulative size distributions of the starting materials, respectively. Table 1 shows the mass median diameters (Dp50) and geometric standard deviations (σg) of the starting materials. Both the SEM images and mass median diameters indicate that Al(OH)3 and α-Al2O3 (L) have the smallest and largest particle sizes, respectively. The particle sizes of γ-Al2O3 and α-Al2O3 (S) are comparable. The SEM image of α-Al2O3 (S) shows agglomerated particles. For
Conclusion
We experimentally investigated the influence of aluminum source on the color tone of cobalt blue, synthesized by solid-state reaction. Four different aluminum sources were used. The color tone was characterized by the lightness L* and Chroma C* values. XRD measurements and particle size distribution analyses shows that the lightness L* value is determined by the mass median diameter of the cobalt blue, and is not affected by the type of starting material or crystallite size. The particle size
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