Preparation of Acid-Resisting Ultramarine Blue by Novel Two-Step

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Preparation of Acid-Resisting Ultramarine Blue by Novel Two-Step Silica Coating Process Sifang Li,* Miao Liu, and Lan Sun Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ABSTRACT: Acid-resisting ultramarine blue pigment was prepared by a novel two-step silica coating process which is dense liquid coating followed by a sol
gel process. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and BET surface area analysis were used to characterize the composition of elements, surface structure, and morphology on the coating films. Uniform, smooth, and dense silica films on the irregular particles of ultramarine pigment were obtained via novel two-step coatings. Acid resistance of the silica-coated ultramarine blue was evaluated by immersing the pigment in 10 wt % hydrochloric acid. The silica-coated ultramarine blue prepared by the novel two-step process shows the 10th grade of acid resistance, which exhibits much higher acid resistance than that by dense liquid, sol
gel, and conventional two-step coatings. Furthermore, leaching experiments indicated that no sulfur was leached out for the sample prepared by novel two-step coatings, while hydrogen sulfide concentrations from 0.018 to 0.34 mg/L were detected in the leaching solutions for the samples prepared by the conventional two-step coating, dense liquid coating, and sol
gel process.

’ INTRODUCTION Ultramarine is a nontoxic inorganic pigment with a blue to purplish pink hue, and it consists of silicate, alumina, and sodium oxide plus sulfur. It is widely used as a colorant for plastics, rubbers, coatings, and cosmetics.1
6 However, in contact with acid, the pigment is rapidly decomposed and discolored.7 In practice, there are many circumstances under which the pigment may come into contact with acids such as acid rain, fruit juices, carbonated water, and acidic compositions during product processing. Therefore, producing acid-resisting ultramarine pigments for these applications has received much interest. The coloration of ultramarine that has a cage-cavity structure is attributed to its special crystal lattice and the location of the sodium polysulfide in the cavity, and the sensitiveness to acids is also due to this structure, so it is impossible to make acid-resisting ultramarine by changing its interior construction.8,9 To obtain acid-resisting ultramarine, attempts have been made through coating the surfaces of ultramarine particles with transparent SiO2 films to protect them from the corrosion of acids. The sol
gel process is a conventional method for silica coatings on finely dispersed particles.10 Under basic conditions, it is wellknown that the condensation of hydrolyzed units of alkoxides results in silica polymers with a higher degree of branching than under acidic conditions.11 The silica gelatin is obtained by the incomplete hydrolysis of tetraethyl orthosilicate (TEOS) and simultaneous condensation, which remains with many Si
OR terminal groups. Accordingly, the films of silica coated on the substrate in the sol
gel process have a porous and loose structure.12 The dense liquid process is another commonly used method for coating fine particles.9,13
16 It is proven that dense silica films on the surface of particles are formed under wellcontrolled conditions in dense liquid coating. However, the silica-coated particles are easily clumped, and the surfaces are not smooth. In a certain range of pH, the increase of pH value will r 2011 American Chemical Society

lead the formed particles of silica to become rougher and accelerate the settling speed of the silica gelatin, which makes the coated films of silica not uniform. Furthermore, many silica microspheres may grow up and deposit on the films. Recently, Liu et al.17 reported a two-step coating process for coating magnetic particles with silica, in which the magnetic particles were first coated by the sol
gel process and subsequently coated by the dense liquid method. In this paper, a novel two-step method is proposed for the preparation of acid-resisting ultramarine blue particles with highly dense, smooth, and uniform silica films. To be different from the conventional two-step process proposed by Liu et al.17 for coating magnetic particles, ultramarine blue particles are first coated with silica by the dense liquid process and then coated with silica by the sol
gel process. The silica-coated ultramarine blue prepared by this novel two-step method shows the highest acid resistance and the least odor compared with those by dense liquid, sol
gel, and conventional two-step coatings, respectively. To our knowledge, this is the first report on the silica coating of ultramarine blue in this way.

’ EXPERIMENTAL SECTION Materials. Ultramarine blue from Fujian Pucheng Max Pigment Company was dried in a vacuum oven at 120 °C for 24 h prior to the coating experiments. The average particle size was found by scanning electron microscopy (SEM) to be ca. 1.46 μm. Commercially available reagent grade sodium silicate, ammonium chloride, ammonium hydroxide solution (30%), and TEOS Received: February 19, 2011 Accepted: May 7, 2011 Revised: May 6, 2011 Published: May 07, 2011 7326

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Figure 1. Narrow-scan X-ray photoelectron spectra of silica-coated ultramarine blue.

were purchased from Sinophram Chemical Reagent Co., Ltd. Absolute ethanol was used as the solvent in the sol
gel process. Dense Liquid Coating. Five grams of ultramarine blue was suspended in 200 mL of water under sonication for 15 min to form a uniform slurry. Then the slurry was held at 90 °C and raised to pH 9.5 by adding 0.1 M NaOH. To the slurry being stirred, a 2.64 M aqueous sodium silicate solution (25 mL) and 5.28 M aqueous solution of NH4Cl (25 mL) were added simultaneously at the rate of 40 mL/h. The slurry was allowed to stand for 3 h, and then it was cooled to room temperature. The coated particles were collected by a B€uchner funnel, rinsed with deionized water three times and then with ethanol, dried at 135 °C, and stored in a desiccator. Sol
Gel Coating. Five grams of ultramarine blue was suspended in 120 mL of absolute ethanol under sonication for 15 min. Subsequently, 66 mL of TEOS solution (16 mL of TEOS in 50 mL of absolute ethanol) was added as the organic silica source. Then, 13 mL of ammonium hydroxide solution (30%) and 17 mL of deionized water were added in a dropwise manner into the suspension under vigorous stirring. The coating was conducted for 5 h. The coated particles were collected by the B€uchner funnel, rinsed with absolute ethanol, dried at 135 °C, and stored in a desiccator. Conventional Two-Step Coating. Five grams of ultramarine blue was suspended in 120 mL of absolute ethanol under sonication for 15 min. Subsequently, 58 mL of TEOS solution (8 mL of TEOS in 50 mL of absolute ethanol) was added as the organic silica source. Then, 6.5 mL of ammonium hydroxide solution (30%) and 8.5 mL of deionized water were added in a dropwise manner into the suspension under vigorous stirring. The coating was conducted for 5 h. The coated particles were collected by the B€uchner funnel, rinsed with absolute ethanol,

and dried at 135 °C. The dried particles were dispersed in 200 mL of water under sonication for 15 min to form a uniform slurry. Then the slurry was held at 90 °C, and raised to pH 9.5 by adding 0.1 M NaOH. To the slurry being stirred, a 1.32 M aqueous sodium silicate solution (25 mL) and 2.64 M aqueous solution of NH4Cl (25 mL) were added simultaneously at the rate of 40 mL/h. The slurry was allowed to stand for 3 h, and then it was cooled to room temperature. The coated particles were collected by the B€uchner funnel, rinsed with deionized water three times and then with ethanol, dried at 135 °C, and stored in a desiccator. Novel Two-Step Coating. Five grams of ultramarine blue was suspended in 200 mL of water under sonication for 15 min to form a uniform slurry. Then the slurry was held at 90 °C, and raised to pH 9.5 by adding 0.1 M NaOH. To the slurry being stirred, a 1.32 M aqueous sodium silicate solution (25 mL) and 2.64 M aqueous solution of NH4Cl (25 mL) were added simultaneously at the rate of 40 mL/h. The slurry was allowed to stand for 3 h, and then it was cooled to room temperature. The coated particles were collected by the B€uchner funnel, rinsed with deionized water three times and then with ethanol, and dried at 135 °C. The dried particles were dispersed in 120 mL of absolute ethanol under sonication for 15 min. Subsequently, 58 mL of TEOS solution (8 mL of TEOS in 50 mL of absolute ethanol) was added as the organic silica source. Then, 6.5 mL of ammonium hydroxide solution (30%) and 8.5 mL of deionized water were added in a dropwise manner into the suspension under vigorous stirring. The coating was conducted for 5 h. The coated particles were collected by the B€uchner funnel, rinsed with absolute ethanol, dried at 135 °C, and stored in a desiccator. Characterization. X-ray photoelectron spectroscopy (XPS) of the samples was carried out on a Quantum-2000 instrument 7327

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with an Al KR anode (hν = 1486.6 eV). The powder samples were pressed to tablets against a foil adhesive tape under 10 MPa for 10 s. Fourier transform infrared (FT-IR) spectroscopy of the samples was measured by the KBr pellet method on a Nicolet Table 1. Comparison of Four Kinds of Coated Ultramarine Blue Samples by Different Processes sample weight (g) before

after

coating

coating

(wt %)

dense liquid

5

5.65

11.5

32

sol
gel

5

8.78

43.1

159

conventional two step novel two step

5 5

8.25 7.92

39.4 36.9

140 128

coating method

silica content coating thickness (nm)

Nexus FT-IR spectrophotometer in the wavenumber range 4000
400 cm
1. The morphologies of the samples were characterized by a Hitachi S-4800 scanning electron microscope. Image processing software Nano Measurer 1.2.0 was used to determine the average particle size. Surface areas calculated by the BET method were determined from nitrogen adsorption
desorption isotherms at liquid nitrogen temperature by use of a Micromeritics TriStar 3000 instrument. Acid-Resistance Test. To evaluate the resistance to acids of the coated ultramarine blue samples, 0.5 g of the sample was mixed with 20 mL of 10 wt % (pH 1) hydrochloric acid solution. The mixture was agitated vigorously for 5 min and placed for 24 h. Then the solid sample was collected and rated for color change by contrasting with the color chart (GB 250-2008, ISO 105/ A02:1993 “Textiles—Tests for colour fastness—Part A02: Grey scale for assessing change in colour”). The levels of the acid

Figure 2. SEM images of the samples before and after coating: (a), (b) uncoated ultramarine; (c), (d) silica coated by conventional two-step process; (e), (f) silica coated by novel two-step process. 7328

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resistance are divided into 10 grades. The first grade refers to total color loss, and the tenth stands for no color change. The leaching solution was also collected and analyzed for hydrogen sulfide concentration by the methylene blue spectrophotometric method.18

’ RESULTS AND DISCUSSION Characterization of Silica Coating Films. The narrow-scan X-ray photoelectron spectra of uncoated ultramarine blue and four kinds of coated ultramarine blue samples prepared by the dense liquid, sol
gel, conventional two-step, and novel two-step processes are shown in Figure 1. The existence of a silicon band at 103.4 eV and an oxygen band at 532.8 eV on the spectra of coated samples is in accordance with the bands of silicon and oxygen in silica, which verifies silica coatings on ultramarine blue particles. Neither aluminum nor sulfur is observed on the surface of the samples coated by these four kinds of methods, while large amounts of aluminum and sulfur are shown on the surface of the uncoated sample. Table 1 represents silica contents and coating thicknesses of four kinds of coated ultramarine blue samples prepared by the dense liquid, sol
gel, conventional two-step, and novel two-step processes. An equivalent to 4 g of SiO2 was applied for each process. The silica content, the amount of silica coated on ultramarine blue, is defined as the weight fraction of silica in the coated particle. The average coating thickness was calculated using the approach described by Bergna et al.19 From Table 1, it can be seen that the sample prepared by the dense liquid process shows the least silica content and the thinnest silica layer, while the sample prepared by the sol
gel process exhibits the most silica content and the thickest silica layer. The silica content of the sample prepared by novel two-step coatings is lower than that by conventional two-step coatings. The surface morphologies of coated and uncoated ultramarine particles are shown in Figure 2. The uncoated ultramarine blue particles display rough surfaces with various sizes and shapes. The coated ultramarine blue particles show higher dispersion than that of uncoated ones. It can be seen that the silica surface of particles prepared by novel two-step coatings is much more uniform, smoother, and denser than that by conventional twostep coatings. This can be explained as follows: In the conventional two-step process, sol
gel coating is first applied. Partial hydrolysis of TEOS produces intermediate species such as RO
Si
(OH)3 or (RO)2
Si
(OH)2. These functional molecules can link together in a condensation reaction to form a siloxane [Si
O
Si] bond: RO
SiðOHÞ3 þ RO
SiðOHÞ3 f RO
SiðOHÞ2
O
SiðOHÞ2
OR þ H2 O

ð1Þ SiðORÞ4 þ RO
SiðOHÞ3 f RO
SiðOHÞ2
O
SiðORÞ3 þ ROH

ð2Þ where the alkyl group R = C2H5. Thus, polymerization is associated with the formation of a one-, two-, or three-dimensional network of siloxane [Si
O
Si] bonds accompanied by the production of H2O and ROH species. This type of reaction can continue to build larger and larger silicon-containing molecules. Therefore, the particles have the porous films of silica polymers with many Si
OR terminal groups. Because of the

Figure 3. FT-IR spectra of uncoated and coated ultramarine blue samples.

hydrophobic property of the Si
OR groups, the binding force between the surface and the precipitated silica by the following dense liquid coating is weak, leading to a uniform but less dense coating. On the contrary, dense liquid coating is first applied in the novel two-step process. Si(OH)4 condenses with OH groups of MOH surfaces, where M represents a metal that will form a silicate at the pH and temperature involved.19 The presence of silanol groups SiOH on the surface is good for the following sol
gel coating and a uniform and dense film is formed. In order to further characterize the surface morphologies of coated ultramarine particles, BET surface areas were measured to be 9.03 m2/g for the sample prepared by conventional two-step coatings and 6.86 m2/g for the sample prepared by novel twostep coatings. The much lower BET surface area for the sample prepared by novel two-step coatings indicates a denser coating with a lower roughness, which is in accordance with the observation from SEM images in Figure 2. The average particle sizes increase respectively from 1.46 μm to 1.51 μm (dense liquid), 1.76 μm (sol
gel), 1.78 μm (conventional two-step), and 1.72 μm (novel two-step), which indicate that the average coating thicknesses are 25 nm (dense liquid), 150 nm (sol
gel), 160 nm (conventional two-step), and 130 nm (novel two-step), respectively. These results are in accordance with the calculations represented in Table 1. Figure 3 shows the infrared spectra of uncoated ultramarine blue and four kinds of coated ultramarine blue samples prepared by the dense liquid, sol
gel, conventional two-step, and novel two-step processes. For uncoated ultramarine blue, the strongest vibration at 1010 cm
1 is assigned to the Si
O
Al asymmetric stretching of internal tetrahedra. The next strongest band at 450 cm
1 is assigned to the Si
O bending mode. The absorption band at about 700 cm
1 is due to the Si
O
Al symmetric stretching vibration. The band corresponding to the blue chromophore S3
is found at 580 cm
1. The wide band at 3440 cm
1 is assigned to the
OH stretching vibration. However, all the coated ultramarine pigments show two very strong absorption bands at 1108 cm
1 (Si
O
Si) and 1010 cm
1 (Si
O
Al). Significant differences between these bands for different kinds of coated ultramarine blue samples can be seen. Compared with the uncoated sample, the intensity of the band related to S3
significantly decreased for the coated samples and even disappeared for the sample prepared by the novel two-step process. Acid Resistance. Figure 4 indicates the acid resistance of these four kinds of coated ultramarine blue compared with uncoated ultramarine blue. The sample prepared by novel two-step coatings 7329

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coating films of the dense liquid method and the uniform coating films of the sol
gel process. The coating sequence is the dense liquid process followed by sol
gel coating, which is in contrast to the conventional two-step coatings. The ultramarine blue prepared by the novel two-step coatings shows 10th grade resistance to 10 wt % hydrochloric acid and no sulfur being leached out, which is much superior to that by the other methods above. Such coated ultramarine blue will have a much wider usage as a colorant than uncoated.

’ AUTHOR INFORMATION Corresponding Author

Figure 4. Acid resistance of samples prepared by different processes.

Table 2. Sulfur Leaching Test for Coated Ultramarine Blue coating method

H2S in 10 wt % HCl (mg/L)

odor

dense liquid

0.021

light

sol
gel

0.34

heavy

conventional two step

0.018

light

novel two step

nil

nil

exhibits the highest grade of acid resistance (the specification is 10) and no color change is observed. The uncoated sample is completely discolored and rated to the lowest grade of acid resistance (the specification is 1). The sample prepared by the sol
gel process shows relatively lower resistance to acid, although it has a very high silica content and a thick coating layer. This is because the silica coating film of the sample prepared by the sol
gel process is porous. This suggests that increasing the thickness of the silica films cannot neutralize the impact of porosity of sol
gel coating films. The samples prepared by dense liquid and conventional two-step coatings display rather higher resistance to acid, which are due to forming dense silica films. Ultramarine is a compound containing sulfur element. Immersing the pigment in hydrochloric acid causes generation of hydrogen sulfide. Therefore, if it is not well coated, the sample will still generate hydrogen sulfide slowly and show an odor when it is in contact with hydrochloric acid. Hydrogen sulfide concentrations of the leaching solutions for the samples having been mixed with 10 wt % hydrochloric acid for 24 h were determined (Table 2). It is evident that the amount of hydrogen sulfide leached out became undetectable for the sample prepared by novel two-step coatings. However, hydrogen sulfide concentrations from 0.018 to 0.34 mg/L were detected in the leaching solutions for the samples prepared by the conventional two-step coatings, dense liquid method, and sol
gel process. From appearances, there is no odor for the sample prepared by novel two-step coatings, while there is a heavy odor for that by the sol
gel process and light odors for that by dense liquid and conventional two-step coatings. Great improvement in the odor is achieved for novel two-step coatings compared with conventional two-step coatings.

’ CONCLUSIONS We have prepared acid-resisting ultramarine blue with uniform, smooth, and dense silica films by a novel two-step process. This method integrates the principal advantages of the dense

*Tel.: þ86 592 2186195. Fax: þ86 592 2186195. E-mail: sfli@ xmu.edu.cn.

’ ACKNOWLEDGMENT The authors thank Fujian Pucheng Max Pigment Co. for financial support of this study. ’ REFERENCES (1) Guilmin, T. Ultramarine Blue, an old pigment. In Coloring Technology for Plastics; Harris, R. M., Ed.; William Andrew: New York, 1999; pp 55
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