Ceramics-Silikáty 63 (3), 291-296 (2019) www.ceramics-silikaty.cz doi: 10.13168/cs.2019.0023 LOW TEMPERATURE PHOSPHORIC ACID TREATMENT OF ZINC OXIDE FOR NOBEL WHITE PIGMENTS #HIROAKI ONODA, DAIKI HIGASHIDE Department of Informatics and Environmental Sciences, Kyoto Prefectural University, 1-5, Shimogamo Nakaragi-cyo, Sakyo-ku, Kyoto 606-8522, Japan #E-mail: [email protected] Submitted March 15, 2019; accepted accepted April 27, 2019 Keywords: Zinc oxide, Photocatalytic activity, Powder processing Zinc oxide that has the photocatalytic activity is used as white pigment for cosmetics. A certain degree of sebum on the skin is decomposed by the ultraviolet radiation in sunlight. In this work, zinc oxide was shaken with phosphoric acid to synthesize a white pigment for cosmetics. Zinc oxide was set with 0.1 mol∙l-1 of phosphoric acid at P/Zn = 1/1 and 1/2, and then shaking in water at 30, 40, 50 °C for 1, 3, 6 hours. The chemical composition, powder properties, photocatalytic activity, color phase, and smoothness of the obtained powder were studied. The P/Zn ratio in preparation had influence on the reaction between phosphoric acid and zinc oxide. The photocatalytic activity of zinc oxide was inhibited by phosphoric acid treatment. The treated samples at 50 °C had enough high reflectance at the visible light region. INTRODUCTION difficult to determine because the pore sizes of the skin are affected by such factors as age, gender, and climate As a white pigment, zinc oxide is used for cosmetic [9]. Furthermore, overly large particles are inappropriate applications [1]. This oxide is well known to have owing to a cracking of their coating on the skin [10]. photocatalytic activity [2]. Therefore, a certain degree It is therefore important to control the particle sizes of of sebum on the skin is decomposed by the ultraviolet the pigment. In earlier studies [11, 12], we prepared zinc radiation in sunlight. To repress this effect, technical phosphate pigments with no catalytic activity. However, processes of several kinds have been investigated and zinc phosphates had particle sizes larger than 1 µm. used. For example, as one such technique, composite Because these particles were too large, the novel process particles with silicon dioxide have been used [3]. was required to produce the smaller particles as a white However, these particle materials are too hard for use pigment. The zinc phosphate without photocatalytic on a human face. Mild materials are required for use activity had too much large particles, on the other hand, as a white pigment on a human face. In addition, the zinc oxide in small particle size had a photocatalytic activity. In previous work, to obtain the zinc oxide microfine zinc oxide is adsorbed through the skin [4]. particles with zinc phosphate coating, zinc oxide was A white pigment that is not adsorbed must be used. shaken in phosphoric acid at 60, 70, and 80 °C [13]. The Phosphates have been used for ceramic materials, target particles have the core-shell structure, which core catalysts, adsorbent, fluorescent materials, dielectric part is zinc oxide and shell part is zinc phosphate without substances, biomaterials, for metal surface treatment, photocatalytic activity. However, the obtained zinc oxide as fertilizer, detergents, food additives, in fuel cells, with zinc phosphate coating included larger particles pigments, and in other applications [5, 6]. Phosphate than 10 µm. Because of high temperature in phosphoric materials are well known to have high affinity for living acid treatment, zinc phosphate was much formed on the organisms [7]. Therefore, as a white pigment, phosphates surface of zinc oxide. Our purpose in this work was to are expected to be useful as cosmetics. obtain the white pigment in sub-micrometer size without When used as a cosmetic pigment, the particle photocatalytic activity by the control of the reaction shape and size distribution of the phosphate particles between zinc oxide and phosphoric acid. are important. Homogeneous spherical particles are In this work, zinc oxide was shaken with phos- expected to spread well on the skin [8]. However, phoric acid at lower temperatures, 30, 40, and 50 °C. overly small particles are unsuitable because the pig- The chemical compositions, powder properties, photo- ments might enter the pores of the skin. Generally, catalytic activity, color phases, and smoothness of the sub-micrometer size pigments are used. The standard obtained samples and their thermal products were studied size of white pigment particles used in cosmetics is for application to cosmetics. Ceramics – Silikáty 63 (3) 291-296 (2019) 291 Onoda H., Higashide D. EXPERIMENTAL images of the sample powders were observed (JSM- 5510LV; JEOL, Akishima, Japan). The particle size Zinc oxide was set with 0.1 mol·l-1 of phosphoric distributions of these materials were measured using acid (50 ml) at molar ratios of P/Zn = 1/1 and 1/2 in a glass a centrifugal precipitation particle-size distribution tube, and then this glass tube was shaking in hot water at (SA-CP3L, Shimadzu Corp., Kyoto, Japan). 30, 40, and 50 °C for 1, 3, and 6 hours (rate of shaking; The cosmetic properties were estimated according 100 times/min) [13]. The solutions were decantated off, to the photocatalytic activity, the color phase, and the and the powder samples were washed with water, and smoothness. The photocatalytic activity of samples was then dried at 50 °C over 3 days. All chemicals were of estimated with the decomposition of methylene blue commercial purity from Wako Chemical Industries Ltd. by 365 nm radiation [14, 15]. The 0.01 g of sample (Osaka Japan) and used without further purification. was placed in 4 ml of methylene blue solution (1.0 × The crystalline phase compositions of these mate- × 10-5 mol·l-1), and then this solution was radiated. The rials were analyzed using X-ray diffraction (XRD). The decrease of the absorption at about 660 nm was estimated XRD patterns were recorded on an X-ray diffractometer for 120 min. (MiniFlex; Rigaku Corp., Akishima, Japan) using mono- The color of phosphate pigments was estimated chromated CuKα radiation. The IR spectra were recor- using ultraviolet–visible (UV–Vis) reflectance spectra ded on a HORIBA FT-IR 720 (Horiba Ltd., Kyoto, with a spectrometer (UV2100; Shimadzu Corp., Kyoto, Japan) using the KBr disk method. A part of the samples Japan) (reference compound; BaSO4). The whiteness was dissolved in a nitric acid solution. The ratios of was also estimated in L*a*b* color space with TES135 phosphorus and zinc in the precipitates were also cal- plus color analyser (TES Electrical Electronic Corp., culated based on the inductively coupled plasma optical Taipei, Taiwan) (average of 5 times). emission spectrometry (ICP-OES) results of these solu- As the smoothness of sample powders, the static tions using a Thermo Scientific iCAP7000 from Thermo friction resistances were estimated on artificial leather Fisher Scientific (Waltham, USA). with Portable Friction Meter 94i-II (SHINTO Scientific The particle shapes and sizes of the precipitates Co., Ltd., Tokyo, Japan). The sample powders were were estimated based on scanning electron microscopy spread on the leather, and then a sensor was run over the (SEM) images and particle size distributions. The SEM powders. ZnO e) 50 °C, P/Zn = 1/1 e) 50 °C, P/Zn = 1/1 d) 50 °C, P/Zn = 1/2 d) 50 °C, P/Zn = 1/2 e y c) 40 °C, P/Zn = 1/2 c) 40 °C, P/Zn = 1/2 Intensit ansmittanc Tr b) 30 °C, P/Zn = 1/2 b) 30 °C, P/Zn = 1/2 a) original ZnO a) original ZnO 10 20 30 40 50 60 2000 1600 1200 800 400 2θ (°) Wavenumber (cm-1) Figure 1. XRD patterns of samples prepared under various Figure 2. IR spectra of samples prepared under various con- conditions (6 h). ditions (6 h). 292 Ceramics – Silikáty 63 (3) 291-296 (2019) Low temperature phosphoric acid treatment of zinc oxide for Nobel white pigments RESULTS AND DISCUSSION Figure 2 shows IR spectra of samples treated in various conditions. Original zinc oxide had no absorption Chemical composition at 1110-950 cm-1, on the other hand, samples prepared Figure 1 presents XRD patterns of samples treated in this work had some peaks at this region, due to zinc in various conditions. By phosphoric acid treatment at phosphate. The formation of zinc phosphate in the phos- higher temperature, the peak intensity became weaker. phoric acid treatment of zinc oxide was confirmed from IR results. The peaks at 400-550 cm-1, due to zinc oxide, Samples treated at P/Zn = 1/1 indicated weaker peaks of became weaker by phosphoric acid treatment. zinc oxide than those at P/Zn = 1/2. Nobel weak unknown Table 1 shows P/Zn ratio in samples prepared under peaks appeared in the XRD patterns of samples treated at various conditions. The P/Zn ratio of starting zinc oxide P/Zn =1/1. These changes were corresponding with the and zinc phosphate, Zn3(PO4)2, is 0 and 2/3, respectively. reaction between zinc oxide and phosphoric acid. Samples treated at higher temperature indicated higher P/Zn ratio. Samples treated at P/Zn = 1/1 had high Table 1. P/Zn ratio of samples prepared under various condi- P/Zn ratio. The high P/Zn ratio of samples means that tions. samples contained large amount of zinc phosphate. Shaking higher temperature and high P/Zn prepara- Temp. Preparation Time (h) tion ratio promoted the reaction with phosphoric acid. (ºC) P/Zn 1 3 6 When sample was the mixture of zinc oxide and phos- 30 1/2 0.228 0.215 0.183 phate in the molar ratio of ZnO / Zn3(PO4)2 = 2/1, the 40 1/2 0.270 0.257 0.340 P/Zn ratio became 2/5.