Microsoft Word - 001.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 66, 2018 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Songying Zhao, Yougang Sun, Ye Zhou Copyright © 2018, AIDIC Servizi S.r.l. ISBN 978-88-95608-63-1; ISSN 2283-9216 Study on the Coloration and Acid Resistance of Cobalt Blue Pigments on Ceramic Glazes Laiyi Yao Pingdingshan University, Pingdingshan 467000, China yaolaiyi96@sina.com The solid phase method was used to synthesize spinel CoAl2O4 and willemite (Co, Zn) 2SiO4 cobalt blue materials, and their color rendering and acid resistance were compared. The SiO2 and MgO were introduced to modify the Co2O3-Al2O3 system and the finely ground CoAl2O4 was taken as crystal nucleus to investigate its effect on the color rendering of the pigment and acid resistance. The on-glaze cobalt blue pigment with bright color rendering, high color saturation and good acid resistance was developed. The solid phase method was used to synthesize willemite cobalt blue material and then the lead-free flux with strong acid resistance was selected and mixed with the pigment in a certain proportion. The cobalt blue pigment was obtained by baking at a certain temperature. Through the single factor test and orthogonal test, the flux formula was optimized and the secret cobalt blue pigment with bright color rendering and good acid resistance was obtained. The results show that the spinel structure is superior to willemite structure in terms of acid resistance and the introduction of the finely ground CoAl2O4 crystal nucleus can improve the color rendering effect and acid resistance of the spinel cobalt blue material. XRD analysis shows that the main crystal phase of the optimal formula pigment is CoAl2O4, but there are SiO2 residues. 1. Introduction In the ceramic decoration, cobalt blue has always been the main color of ceramic decoration, and it is widely used in daily use, construction and sanitary ceramics. It can also be used as on-glaze, in-glaze and under- glaze pigment, color glaze and body pigment. The majority of the traditional cobalt blue materials are the willemite-structured CoO-ZnOSiO2 system and the spinel-structured CoO-Al2O3 system (Jose et al., 2016). Researchers in China began to study the problem of poor acid-resistance of cobalt blue materials from the 1980s (Zhang et al., 2018), but so far no cobalt-blue material with good acid resistance has been found on the market. The cobalt blue material has excellent color rendering effect whether it is used as on-glaze pigment, in-glaze color pigment or under-glaze pigment. Its color is light, bright and stable (Zhang et al., 2018). Most of the traditional cobalt blue materials are of the willemite-structured CoO-ZnO-SiO system. The on-glaze blue pigment on the glaze formulated with the lead flux has a very bright color rendering effect and high color saturation. Most of these pigments are used in domestic on-glaze porcelain lithography and silk screen decals. However, its acid resistance is poor and the amount of lead release is much higher than international standards (Bachar et al., 2018). Therefore, the improvement of the acid resistance and non-toxicity of the cobalt blue pigment has become a problem that needs to be solved urgently in the daily ceramic industry. This paper aims at the problem of poor acid resistance of the existing willemite ceramic pigments and improves the chemical stability of the pigment by adding crystalline phase with strong acid resistance formed by forming ions and coloring ions into the pollution-free ceramic flux. The research first studied the effect of different components of the flux on pigment luster, color rendering and acid resistance through single-factor experiments. Then, the orthogonal test was used to optimize the flux formulation and the on-glaze cobalt blue pigment formula with good effect was obtained. XRD analysis was performed on the optimized formulation samples (Jiang et al., 2018). At present, the acid resistance of cobalt blue pigments at home and abroad is not ideal. In this study, according to the solid-phase reaction mechanism and by adding crystal nucleus, the pigment with strong acid resistance is developed through the system experiment and improved preparation process of pigments. Also, DOI: 10.3303/CET1866031 Please cite this article as: Yao L., 2018, Study on the coloration and acid resistance of cobalt blue pigments on ceramic glazes, Chemical Engineering Transactions, 66, 181-186 DOI:10.3303/CET1866031 181 the lead-free flux developed by this research group was utilized to develop a lead-free and acid-resistant on- glaze cobalt blue pigments with good color rendering effect (Fischer et al., 2017). 2. Experimental method 2.1 Raw Material of Test Specimen Experimental materials: Co2O3 is the analytically pure; ZnO, MgO and boric acid are all chemically pure, Al2O3 and SiO2 are industrially pure, and the experimental flux is self-made lead-free flux by the research group; the equipment mainly includes muffle furnace, planetary mill, desktop drying oven and electronic analytical balance. All experimental formulations in this study are added with boric acid as the mineralizer with a total mass of 4% of the formulation. 2.2 Comparison between Co2O3-ZnO-SiO2 System and Co2O3-Al2O3 System The color rendering and acid resistance of the willemite-structured Co2O3-ZnO-SiO2 system and the spinel- structured Co2O3-Al2O3 system were compared by experiments. The color rendering and acid resistance of pigments were investigated through the change of the introduction amount of different raw materials and the conclusion was drawn that the Co2O3-ZnO-SiO2 system is superior to Co2O3-Al2O whether it was the color of the pigment or the color after baking (Li et al., 2015; but for acid resistance, color fading was found in both systems, but the overall performance of Co2O3-Al2O3 system is better than that of Co2O3-ZnO-SiO2 system. This may be due to the fact that on the one hand, the color resistance of spinel-structured is better than the willemite-structured; on the other hand it may be related to the crystal integrity of the pigment particles. Therefore, the Co2O3-Al2O3 spinel system was selected for subsequent experiments, and its acid resistance and color rendering ability were improved by modification. The number for the optimal formulation of color rendering and acid resistance of Co2O3-ZnO-SiO2 system and Co2O3-Al2O3 system was CZS and CA. 2.3 Modification of Co2O3-Al2O3 System The formulation of Co2O3-Al2O3 system with relatively good acid resistance was determined and then the SiO2 and MgO were introduced to improve its color rendering ability and acid resistance. The amount of alumina oxide and silicon dioxide remained unvarying, and the effect of the content of Co2O3 and MgO on the color rendering ability and acid resistance was investigated. The specific experimental arrangement is shown in Table 1. Table 1: Experimental arrangement for Co2O3-MgO-Al2O3-SiO2 system Index CO2O3 MgO AL2O3 SiO2 1-1 2 18 33 47 1-2 4 16 33 47 1-3 6 14 33 47 1-4 8 12 33 47 1-5 10 10 33 47 1-6 12 8 33 47 1-7 14 6 33 47 1-8 16 4 33 47 1-9 18 2 33 47 2.4 Introduction of CoAl2O4 Crystal Nucleus The pigment sample with the best effect of the Co2O3-Al2O3 system CA was taken and the wet grinding of the sample was conducted with in a planetary mill with a zirconia ball of diameter 5 mm until it completely passed through the 400 mesh. After drying, it was added as a nucleus to formula 1 to 5in the Co2O3-MgO- Al2O3-SiO2 system, which had the optimal effect(Ashkenazi et al., 2017). The volume of addition ranged from 2% to 13%, 1% every time. A total of 12 formulations were numbered 2-1 to 2-12.  1 2, ,..., nX a a a (1) 2.5 Experimental Process (1) Pigment preparation process The pigment was compounded, mixed, roasted, ground to passing through the 250 mesh screen. The baking system of pigments was: heating rate 6°C/min, synthesis temperature 1250°C, heat preservation 120min, 182 natural cooling, atmosphere being oxidizing atmosphere (Polić et al., 2015).  1 2, ,...,j nC C C C (2) (2) Baking process The ratio of fixed pigment and flux is 1:3; the baking temperature is 800°C; the heating rate is 10°C/min; and heat preservation time is 10 minutes. The specific process flow is shown in Figure 1. ij T T (3) Weighing Mixing Oil adjustment Screen printing Dried Applique Colorful grill Figure 1: Color baking process flow chart 2.6 Characterization Test According to the QB/T2455.2-99 standard, the sample was soaked in 4% acetic acid at 22±2°C for 24 hours. The corrosion of the flower surface was observed by visual observation and the acid resistance of the pigment was described. The German Brock D8Advance X-ray powder diffractometer was used to identify the crystal phase composition. CuKα radiation, target voltage 40kV, electric current 40mA, λ=0.15418nm, graphite monochromator filter, scanning range 2θ=10° to 70°, scan rate 0.02 (°)/s. The chromatic value of the sample was measured using the WSO-3A colorimeter from Beijing Kangguang Instrument Co., Ltd. (Betancur- Granados et al., 2017); the spectral reflectance of the sample was measured with a Lamda850 UV-Vis spectrophotometer of PerkinElmer Co., Ltd., whose wavelength range was 400- 780nm. 3. Results and discussions 3.1 Modification of Co2O3-Al2O3 System Table 2: Results of Co2O3-MgO-Al2O3-SiO2 system Index Appearance shape Color color Colorful roasting Acid resistance 1-1 Dispersion difference, Agglomeration Light blue Light blue No obvious fading 1-2 Dispersion difference, Agglomeration Light blue Light blue No obvious fading 1-3 Better dispersion, Slightly agglomerate Light blue Light blue No obvious fading 1-4 Better dispersion, Slightly agglomerate Light blue Light blue No obvious fading 1-5 Good dispersion, fluffy blue blue No obvious fading 1-6 Good dispersion, fluffy Bright blue blue No obvious fading 1-7 Good dispersion, fluffy Bright blue blue No obvious fading 1-8 Good dispersion, fluffy Bright blue blue No obvious fading 1-9 Good dispersion, fluffy Dark blue blue No obvious fading The modified experimental results for the Co2O3-Al2O3 system are shown in Table 2. It can be seen from Table 2 that the acid resistance and color rendering of the pigment obtained with the introduction of MgO and SiO2 based on the Co2O3-Al2O3 system are improved significantly. According to the acid resistance test standard, that is, soaking in 4% acetic acid at 22±2°C for 24 hours. There is no obvious fading, which meets the acid resistance requirements of ceramic on-glaze pigments, but the color saturation after baking is not 183 satisfactory (Veronesi et al., 2017). Therefore, considering the color rendering, acid resistance and cost, the formula CA with finely ground baking pigments was introduced as crystal nucleus based on formula 1-5 in subsequent experiments to improve the crystal integrity with a view to further improve the color rendering effect and acid resistance. 3.2 Experimental results of after Introducing CoAl2O4 Crystal Nucleus Table 3 shows the experimental results after the introduction of CoAl2O4 crystal nucleus. It can be seen from Table 3 that with the increase in the amount of crystal nucleus, the color rendering of the synthesized pigment is from light to dark. The dispersion is good and the particles are loose and easy to crush (Grazenaite et al., 2016). The color rendering of formula 2-5 is the optimal. The porcelain plate after baking is soaked in 4% acetic acid at 22±2°C for 24 hours and the results show that there is no significant fading in the pigment and there is slight fading in others. max min 2 i i C C (4) Table 3: Addition of CoAl2O4 as nucleating agent Index Appearance shape Color color Colorful roasting Acid resistance 2-1 Good dispersion, fluffy Light blue Light blue Slightly faded 2-2 Good dispersion, fluffy Light blue Light blue Slightly faded 2-3 Good dispersion, fluffy Light blue Light blue Slightly faded 2-4 Good dispersion, fluffy Light blue Light blue Slightly faded 2-5 Good dispersion, fluffy Bright blue blue Slightly faded 2-6 Good dispersion, fluffy Bright blue blue Slightly faded 2-7 Good dispersion, fluffy Bright blue blue No obvious fading 2-8 Good dispersion, fluffy Bright blue blue No obvious fading 2-9 Good dispersion, fluffy Dark blue blue No obvious fading 2-10 Good dispersion, fluffy Dark blue Dark blue Slightly faded 2-11 Good dispersion, fluffy Dark blue Dark blue Slightly faded 2-12 Good dispersion, fluffy Dark blue Dark blue Slightly faded 3.3 Color Rendering Analysis Table 4 shows the chroma test results of the pigment sample CA, CZS, and 2-5. Table 4: Results of colorimetric analysis Index L a b CA 47.68 5.81 -23.97 CZS 46.48 7.09 -30.56 2-5 46.94 6.34 -40.63 From Table 4, It can be seen that the difference between the L value and the a value of each pigment sample is not significant, but the difference of -b value (blue) is significant. The -b value of 2-5 is the highest, which means that the color is the bluest. Figure 2 is the spectral reflectance curve of the optimal formula 2-5 sample. It can be seen from Figure 2 that sample 2-5 has significant reflection of blue light at 430-480 nm and also there is a small reflection peak for cyan light at 480-500 nm; there is a significant absorption peak at 550-650 nm, indicating that it mainly absorbs yellow light, together with a small amount of yellow-green and orange light. The displayed color is mainly blue, with a slight purple and cyan tone. The reflection peak of the sample at the blue light wavelength of 450 nm is narrow, which indicates that the color saturation of the sample is relatively high. m m x X m   (5) 184 3.4 XRD Analysis The XRD analysis is performed on CZS, CA and 2-5 pigment samples and the results are shown in Figure 2 and Figure 3. Figure 2: XRD patterns of sample CZS with using (Co, Zn)2SiO4 Figure 3: XRD patterns of sample CA with using the CoAl2O4 The crystal phase of CZS pigment is willemite (Co,Zn)2SiO4 and the crystal phase of CA pigment is spinel CoAl2O4. The crystal phase of pigment 2-5 with the optimal color reddening and acid resistance is CoAl2O4 and SiO2. Due to the introduction of the CoAl2O4 crystal nucleus, the pigment particles grew continuously with CoAl2O4 as the crystal nucleus within the same temperature preservation time, resulting in the formation of CoAl2O4 spinel. This indicates that the solid solubility of Co2+ is relatively high and is less susceptible to corrosion by H+. In addition, the SiO2 in pigment 2-5 does not participate in the reaction to form a new phase. On the one hand, the acid resistance of SiO2 itself is stronger, which improves the acid resistance of the pigment. On the other hand, SiO2 may promote the formation and growth of spinel phase during the solid phase reaction process and the mechanism needs further study.  max min min 100 i i i C C Threshold C    (6) 4. Conclusions In this study, the synthesis technology of cobalt blue pigment by the solid phase method is studied by adding crystal nucleus and the on-glaze cobalt blue pigment with bright color rendering, high color saturation and good acid resistance was developed. Its b value is -40.63, and there is a significant reflection of blue light at 430-480 nm. Studies have shown that the acid resistance of the CoAl2O4 spinel structure is better than that of 2Thea(') 20 30 40 50 60 70 80 In te n s it y 10 4 0 0.5 1 1.5 2 2.5 3 2Theta 0 50 100 150 200 250 300 350 400 In te n s it y -7 -6 -5 -4 -3 -2 -1 0 185 the traditional willemite structure. On the basis of Co2O3-MgO-Al2O3-SiO2 system, 6% finely ground CoAl2O4 nucleation agent is introduced to improve the color rendering effect and acid resistance of the spinel cobalt blue pigment. The results of XRD analysis show that the crystalline phase of the optimal formula pigment is CoAl2O4, but there are SiO2 residues. 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