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Catalytic synthesis of hematite--mica pearlescent pigments using a low-temperature method.

Abstract A novel low-temperature method is described for the direct synthesis of [Fe.sub.2][O.sub.3]/mica pearlescent pigments with ferric chloride as a precursor and trace Fe(II) as a catalyst which does not require any calcination step. The as-synthesized pigments are characterized in detail by means of different techniques including X-ray diffraction, field emission scanning electron microscopy, energy dispersive spectroscopy, and colorimetry. The influence of various factors on the transformation from Fe[(OH).sub.3] coating layer to crystalline hematite coating layer is investigated, respectively. Furthermore, this paper studies several critical technological parameters pertaining to the influences on morphology and color properties of the pigments such as the coating temperature and the pH value.

Keywords Catalytic synthesis, Mica, Hematite, Pearlescent pigments, Color property


Pearlescent pigments, which can create fascinating pearl luster, brilliance, and iridescent color effects, (1) have attracted an increasing interest because of their decorative and functional applications in automobile coatings, (2), (3) cosmetics, plastics, security printing, optical filters, (4-7) greenhouses, (8) ceramies, (9) and so on. The pearlescent pigments can be grouped into two fields: (1) substrate-free pigments that consist of basically one optically homogeneous material; (2) substrate-based pigments that have a layered structure on a substrate. (8), (10) The substrate-free pearlescent pigments are in some cases brittle and often mechanically not stable, (4) which limits their commercial applications. Consequently, the substrate-based pearlescent pigments are popular in various end user applications because they are chemically and thermally stable, noncombustible, not self-igniting, and innocuous to humans. (11-13) Generally speaking, the substrate-based pearlescent pigments are synthesized by coating transparent mica flakes with high refractive index materials such as metal oxides. The two main coating oxides generally used are [Fe.sub.2][O.sub.3] and Ti[0.sub.2]. (14-17)

These metal oxide coatings can be synthesized by several techniques such as metal--organic chemical vapor deposition (MOCVD), homogeneous hydrolysis, and titration methods. (8,18) The most common technique is the titration method, which gives better control of the coating layer quality. Moreover, the adhesion of the metal oxide is better when this method is employed. (19,20) In the case of ferric oxide layers, an aqueous solution of Fe[Cl.sub.3] is added continuously to a mica suspension. A coating of Fe[(OH).sub.3] on the mica flakes is obtained at a specific pH, whereby the pH is controlled by adding NaOH. The resulting Fe[(OH).sub.3] coating layer is transformed into a crystalline hematite coating layer by drying and calcinations processes.

In this article, a low-temperature method has been employed to synthesize directly [Fe.sub.2][0.sub.3]/mica pearlescent pigments with trace Fe(II) as a catalyst. This procedure is different from the reported methods in the literature without any requirement of calcination step at high temperature (e.g., 850[degrees]C). (21) Moreover, the novel method could maintain a great many advantages, such as simple equipment, convenience, low cost, etc.


The mica used as the substrate for pearlescent pigment was supplied by Langfang Ouke Fine Chemical Co., Ltd. as a gift. Analytical grade reagents (ferric chloride hexahydrate Fe[Cl.sub.3]*6[H.sub.2]0, ferrous chloride tetrahydrate Fe[CI.sub.2]4[H.sub.2]0, sodium hydroxide NaOH, and hydrochloric acid [HCL]) were purchased from Tianjin Chemical Reagent Company. The ferric salt solution was filtered through a 0.22 [micro]m Millipore filter to remove any particulate contaminants before use. The solvent used for the reaction was distilled water.

The schematic illustration for preparing [Fe.sub.2][0.sub.3]/mica pearlescent pigments using a low-temperature method is shown in Fig. 1. The suspending liquid was prepared by adding 7.5 g of mica powder to distilled water at a solid:liquid weight ratio of 1:25, and the suspending liquid was heated to 75[degrees]C in the reactor by water bath. We then added a small amount of hydrochloric acid to adjust the pH to 3.5. After that. 30 mL of 0.5 mol L (1) Fe[C1.sub.3] solution was added by means of a peristaltic pump while stirring, and the pH value of this system was maintained near 3.5 by the controlled titration of NaOH solution. After completing the addition, the pH of the system was adjusted to the desired pH 7 with a dilute NaOH solution. Then trace Fe(II) solution [n.sub.Fe(II)]/[n.sub.Fe(III)]= 0.04) was added into the system and the pH of the system was adjusted to 7 once again. After this the mixture was heated to boiling and kept at the boiling point for a certain time under vigorous stirring. The above liquid was filtered and repeatedly washed with distilled water and then dried. Finally, the hematite coated mica pearlescent pigments were obtained.


The X-ray diffraction (XRD) patterns of the samples were recorded using a Germany Bruker D8-ADVANCE X-ray diffractometer with Cu [K.sub.x] radiation at 40 kV and 40 mA. The uniformity of the coatings was observed by a HITACHI S-4800 field emission scanning electron microscopy (FESEM) at an accelerating voltage of 15.0 kV. With the same instrument, elemental composition was checked by energy dispersive speetroscopy (EDS) analysis. The color of the pigments was evaluated using the CIE L*a*b* parameters. These parameters were measured for an illuminant D65, using an SC-80 automatic colorimeter and a white ceramic tile (chromaticity coordinates: x = 0.3163, y = 0.3324) as a standard reference. The color difference between the prepared pigment and the reference pigment (Langfang Ouke Fine Chemical Co.. Ltd.) was obtained using the following equation:

[DELTA][E.sub.CIE]* = [[[([DELTA]a*).sup.2 +[([DELTA]b*).sup.2] + [([DELTA]L*).sup.2]].sup.1/2]

Results and discussion

The catalysis of Fe(II) on the transformation of Fe[(OH).sub.3] coated on mica

The suspension of Fe[(OH).sub.3]/mica is heated to boiling and kept at the boiling point for 6 or 0.5 11 in the absence or presence of trace amounts of Fe(II) ions, respectively. The XRD patterns of the samples are shown in Fig. 2. It can be seen that Fe[(OH).sub.3] coated on mica has transformed into hematite in the presence of trace amounts of Fe(II) ions (Fig. 2c), while no hematite peaks are found when Fe[(OH).sub.3] coated on mica is heated to boiling and kept at the boiling point for 6 h in the absence of trace amounts of Fe(II) ions (Fig. 2b). With the exception of mica diffraction pattern, the diffraction peaks at 24.1[degrees], 33.1[degrees], 40.8[degrees], and 54.0[degrees] in Fig. 2c can be indexed to the standard pattern for the hematite phase, which are in good agreement with reported data (JCPDS card No. 33-0664). (22) The sharp hematite peaks demonstrate the formation of hematite phase with good crystallinity, despite the low temperature of growth. The result reveals that Fe(II) has obvious catalytic effect on the transformation from an Fe[(OH).sub.3] coating layer to a pure crystalline hematite coating layer.


However, there are by-products produced when excess Fe(II) ions are added into solution. Therefore, in order to obtain a pure crystalline hematite coating layer, the dosage of Fe(II) ions is determined. In our experiments, the suspensions of Fe[(OH).sub.3]/mica with different [n.sub.Fe(II)]/[n.sub.Fe(III)] values are heated to boiling and kept at the boiling point for 0.5 h. The XRD patterns of the samples are shown in Fig. 3. It can be seen that when [n.sub.Fe(II)]/[n.sub.Fe(III)] < 0.08, all samples show similar spectra. The large peaks correspond to the mica substrate, and the peaks at 24.1[degrees], 33.1[degrees], 40.8[degrees], and 54.0[degrees] are assigned to hematite. The other hematite characteristic peaks are overlapped by intense mica crystalline peaks. And when [n.sub.Fe(II)]/[n.sub.Fe(III)] value is increased to 0.08, a group of new diffraction peaks arise beside the peaks of mica and hematite, which reveals the presence of [Fe.sub.3][0.sub.4] impurity (JCPDS card No. 19-0629). Table 1 gives the CIE L*a*b* values of [Fe.sub.3][0.sub.4]mica pearlescent pigments as a function of the increasing [.sup.n]Fe(II)/[.sup.n]Fe(III) value (0.02, 0.03, 0.04, 0.06, and 0.08). As shown in Table 1, when [.sup.n]Fe(II)/[.sup.n]Fe(III) < 0.08, the pigment color is mainly red, and when [.sup.n]Fe(II)/[.sup.n]Fe(III)[greater than or equal to]0.08, the pigment color will move to brown, so the purity of color will become worse.

Table 1: The CIE L*a*b* values of [Fe.sub.2]
[O.sub.3]/mica pearlescent pigments and their
difference with the reference pigment

[.sup.n]Fe(II)/                   L *    a *    B *

0.02                            56.30  21.62  29.88

0.03                            55.31  21.23  29.89

0.04                            54.90  22.99  29.00

0.06                            48.79  21.03  28.94

0.08                            46.67  15.25  25.39

[.sup.n]Fe(II)/                    Color difference
[.sup.n]Fe(III)                ([DELTA]E*.sub.CIE])

0.02                                          11.32

0.03                                          11.66

0.04                                          13.10

0.06                                          17.22

0.08                                          18.03

L *, brightness; a *, red--green index; b *,
yellow--blue index

The catalytic phase transformation time needed for completing the phase transformation of Fe[(OH).sub.3] coated on mica at different [.sup.n]Fe(II)/[.sup.n]Fe(III) values was determined. The results are shown in Fig. 4, which shows that the time shortens with the increase of the [.sup.n]Fe(II)/[.sup.n]Fe(III) value when [.sup.n]Fe(II)/[.sup.n]Fe(III)[less than or equal to]0.04, and the time is almost unchanged when [.sup.n]Fe(II)/[.sup.n]Fe(III) > 0.04. Therefore, the [.sup.n]Fe(II)/[.sup.n]Fe(III) value usually equals 0.04 in our experiments.


The effect of the coating temperature

The performance of [Fe.sub.2][O.sub.3]/mica pearlescent pigments as a function of the coating temperature is examined over the range of 65-90[degrees]C. In this study, the suspensions of Fe[(OH).sub.3]/mica obtained at different coating temperatures are heated to boiling and kept at the boiling point for 0.5 h. As the coating temperature is 65[degrees]C, some portion of the mica surfaces could not he covered completely (Fig. 5a). When the coating temperature is increased to 75[degrees]C, all the mica particles are observed to be uniformly coated by the present low-temperature method (Fig. 5b). Figures 5c and 5d show the FESEM micrographs of the [Fe.sub.2][O.sub.3]-coated mica at different high magnifications, corresponding to the white rectangular region in Fig. 5b. It can be seen that the coating layer, which consists of numerous tiny spherical hematite nanoparticles with a diameter of--50 nm, is smooth and dense and shows no signs of peeling. With further increasing the coating temperature to 90[degrees]C (Figs. 5e, 51), the spherical hematite nanoparticles are irregularly arranged on to the mica surfaces and the irregular agglomerated clusters are formed at some positions.


The interference color of [Fe.sub.2][O.sub.3]/mica pearlescent pigments was measured and compared to the analogous reference pigment (Table 2). When the coating temperature is elevated, the brightness of [Fe.sub.2][O.sub.3]/mica pearlescent pigments is first increased and then decreased, while the value of color difference is reduced to the lowest value and then increased. The reason is that the hydrolysis is an endothermic reaction which is influenced intensely by reaction temperature, and the reaction rate of hydrolysis is accelerated with the increase in reaction temperature. (11) Hence, at low coating temperature, the hydrolysis reaction is inadequate and slow, so the mica is not easy to be completely precipitated. However, if the temperature is too high, the reaction rate of hydrolysis is much higher than that of the sedimentation. Plenty of primary particles are generated in a very short moment so that they have not enough time to adhere to the surface of mica, so the primary particles are free in suspension. This results in the scattering of the light and the decrease of brightness. On the other hand, the formation of large-sized [Fe.sub.2][O.sub.3] aggregates on mica may also contribute to the scattering of [Fe.sub.2][O.sub.3]-coated mica. (23) Moreover, the increasing temperature causes the flocculation of the primary particles which makes the membrane on the substrate surface become loose. (24) To obtain optimum pearlescent pigment performance, a continuous, dense, and uniform coating layer is necessary. Based on the experimental results noted on the previous page, the best coating temperature is 75[degrees]C.
Table 2: The color characteristics of the samples
obtained at different coating temperatures and
their difference with the reference pigment
([DELTA][E *.sub.CIE])

Temperature     L *    a *    b *        Color difference
([degrees]C)                        ([DELTA][E*.sub.CIE])

65            54.82  23.26  31.48                   13.85
75            54.90  22.99  29.00                    1310
90            50.94  29.83  36.23                   22.48

In order to verify the surface distribution of Fe on mica powder, Fig. 6 compares the EDS spectra of mica powder and [Fe.sub.2][O.sub.3]/mica pearlescent pigment obtained at coating temperature 75[degrees]C. From Fig. 6, it can be seen that the peaks corresponding to Fe are not present in the mica, and they appear only after the low-temperature coating process, indicating successful [Fe.sub.2][O.sub.3] deposition on the mica surface by the present low-temperature method. In this experiment, 7.5 g of mica powder and 30 mL of 0.5 mol [L.sup.-1] Fe[Cl.sub.3] solution are employed to prepare [Fe.sub.2][O.sub.3]/mica pearlescent pigment. If the mica is coated by [Fe.sup.3+] completely, the theoretic coating ratio of [Fe.sup.3+] in the pearlescent pigment should be


0.03 x 0.5 x 56/7.5 + 0.03 x 0.5 x 56 x 160/112 x 100% = 9.66%

where the numerator is the weight of Fe added in the experiment, and the denominator is the total weight of

[Fe.sub.2][O.sub.3]/mica pearlescent pigment. Then the coating efficiency is obtained using the following equation

7.84%/9.66% x 100% = 81.16%

where 7.84% is the weight ratio of Fe by EDS, and 9.66% is the calculated ratio above. (25)

The effect of the initial pH value

The initial pH value is another key parameter for the [Fe.sub.2][O.sub.3]/mica pearlescent pigments' characterization. Here, we perform a group of experiments to observe the effect on the morphology and the brightness of pearlescent pigments with different initial pH values. The FESEM images of [Fe.sub.2][O.sub.3]/mica pearlescent pigments synthesized at different initial pH values are displayed in Fig. 7. From Fig. 7, it can be seen that the surface topographies of the [Fe.sub.2][O.sub.3] coating layer are significantly altered by the changes of the initial pH values. At low pH (2.0), the mica surface only shows patchy [Fe.sub.2][O.sub.3] deposition due to the low hydrolysis rate (Fig. 7a). While increasing the initial pH to 3.5, the surface of the mica is well covered with a continuous, dense, and uniform [Fe.sub.2][O.sub.3] coating layer (Figs. 7b, 7c). However, with the initial pH further increasing, the surface of the [Fe.sub.2][O.sub.3] coating layer becomes looser and rougher, and island-like structured [Fe.sub.2][O.sub.3] aggregates appear in some regions, which can be attributed to an accelerated rate of hydrolysis (Figs. 7c1, 7e, 7f).


As well as possessing different microstructural properties, [Fe.sub.2][O.sub.3]/mica pearlescent pigments synthesized at different initial pH values also have different color properties (Table 3). It can be seen that when adjusting the initial pH value from 2.0, 3.5, 4.0, to 5.0 successively, the brightness of pearlescent pigments varies from 52.04, 54.90, 53.74, to 51.76, respectively, and the value of color difference is found to be at minimum pH 3.5. Therefore, in our experiment, a pH of 3.5 in the range of 2.0-5.0 is considered to be the optimal value, which is in agreement with the results obtained by FESEM images (Fig. 7).
Table 3: The color characteristics of the samples obtained at
different initial pH values and their difference with the
reference pigment (DELTA[E *sub.CIE])

Initial pH       L *          a *        b *      Color difference
value                                         ([DELTA][E*.sub.CIE]

2.0            52.04        23.78      32.14                 16.42
3.5            54.90        22.99      29.00                 13.10
4.0            53.74        23.77      29.66                 14.56
5.0            51.76        27.16      30.92                 18.49


In summary, we have demonstrated a simple and novel approach to synthesize [Fe.sub.2][O.sub.3]/mica pearlescent pigments directly without ally requirement of a calcination step at high temperature. The surface morphologies and color properties of the pigments could be controlled by regulating the coating temperature or the pH value. Once the optimum parameters are selected, the obtained crystalline hematite coating layer has the advantages of uniformity, continuity, and densification. The results of EDS suggest that the coating ratio and coating efficiency of Fe are about 7.84% and 81.16%, respectively.


The authors gratefully acknowledge financial support from the Natural Science Foundation of China (20671028), and thank Ouke Company for providing mica powder.


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J. Cout. Technol. Res., 9 (6) 695-702, 2012

[c] ACA and OCCA 2012

L. Han

College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050016. China

Y. Chen

College of Mathematics and Information Science, Henan Normal University, Xinxiang 453007. Henan, China

M. You. Y. Wei *

College of Chemistry and Material Science, Hebei Normal University. Shijiazhuang 050016. China e-mail:

DOI 10.1007/s11998-012-9419-5
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Author:Han, Lihong; Chen, Yanchang; You, Modan; Wei, Yu
Publication:JCT Research
Date:Nov 1, 2012
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