Preparation of titanium dioxide compound pigments based on kaolin substrates.Abstract Titanium dioxide compound pigments were prepared by hydrolysis of tetrabutyl titanate on the surface of kaolin substrates. The coating processes were investigated by various factors in single-factor tests, and the products were tested by oil absorption value and whiteness. The results show that the optimum compound pigments reach a whiteness of 94.0% and an oil absorption value of 35.5 mL/100 g. The products were further characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and zeta potential analyzer. The XRD analysis indicates that the microstructure of the coating on a kaolin substrate is mainly anatase. Transmission electron microscopy images clearly reveal that the layer of [TiO.sub.2] consists of randomly oriented small particles with no agglomeration outside the kaolin surface.
Keywords Titanium dioxide, Kaolin substrates, Compound pigments, Oil absorption value, Whiteness
Titanium dioxide has been studied extensively because of its wide application in white pigments for coating, paints, papers, and other fields. (1-8) However, the high cost and scarce resources of titanium white have limited its application to a large extent. Substrates coated with titanium dioxide have been initiated for the purpose of producing compound materials. Many theoretical calculations have been explored for pigments coating, such as a calculation designed to test three models including pigment sphere within microvoid spherical shell, micro-void spherical within pigment spherical shell, and separate pigment and microvoid spheres that might simulate the light-scattering properties of coatings, (9) a theoretical calculation of particle size distribution upon the luminance and color of a concentric sphere model, (10) and an analysis of light-scattering efficiency of core-shell white pigments. (11) The core-shell mathematical model has been applied in suitable sphere substrates, including silica and glass beads. For instance, whitener pigment particles obtained from spherical silica cores were surrounded by a thin shell of titanium dioxide. (12) Glass beads were coated with a uniform titanium dioxide layer by adding titanium chloride solution. (13) The compound materials consist of an inexpensive, readily manufacturable core material that is surrounded by a concentric layer or shell of titania. The substrate material can also be a flaky structure. For example, [Al.sub.2][O.sub.3] flakes were coated with titanium dioxide via hydrolysis of a titanium salt solution using the titration method. (14) Mica flakes were deposited on a thin layer of titanium dioxide to make nacreous mica pigments described in US4,038,099, (15) US3,650,790, (16) and US3,087,828. (17) Mica compound pigments have been reported in other patents, such as the calcination of mica particles coated with titanium dioxide at elevated temperatures and extended times, (18) and the preparation of titanium dioxide-coated mica pigments with an antidiscoloring property. (19) Mica flakes can also be used as substrates for titanium white. For instance, the deposition of rutile [TiO.sub.2] nanoparticles on lamellar sericite, (20) the growth of textured [TiO.sub.2] thin films on muscovite mica using pulsed laser deposition, (21) and the coating of [TiO.sub.2] on mica particles produced by the hydrolysis method. (22) The coating of flakes is quite different from that of spherical particles, since the spheres follow the concentric sphere model with theoretical calculations of the light-scattering efficiencies of core-shell white pigments, but the flaky structure is surrounded, as described, by a layer of titania in mica substrate with a film formation on the surface of flakes. Mica compound pigment has been widely investigated as a substitute for titanium white. However, only a few works have been reported on the growth of [TiO.sub.2] on kaolin flaky substrates. (23) kaolin, as another kind of mineral pigment, has the advantage of high-quality, cheap, and abundant storage so it can be used to prepare composite titanium white to reduce costs and expand the application of titanium dioxide.
In this paper, we review the preparation of titanium dioxide compound pigments based on a kaolin substrate. The process of coating [TiO.sub.2] on the surface of kaolin particles was studied by hydrolysis method. It is shown that the tetrabutyl titanate dosage, slurry concentration, pH value, hydrolysis temperature, aging time, calcination temperature, and calcination time are important factors that influence the effect of the coating. The coating products were tested by whiteness and oil absorption value, and further characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and zeta potential analyzer. The results show that titanium dioxide coating on the kaolin powders can greatly improve the property of kaolin. Kaolin-based compound pigment can supply a reference for the coating product as a substitute for titanium white.
Kaolin clay was collected from Suzhou in the Jiangsu province in mideastern China, and typical characteristics of kaolin samples are given in Table 1.
Table 1: Typical characteristics of kaolin Particle size ([mu]m) Sample Whiteness (%) Oil D90 D50 D10 pH value (in absorption water, 20%) value (mL/100 g) Kaolin 90.0 54 9.59 3.09 1.01 6.0
* Chemical composition of kaolin: [SiO.sub.2] 54.56%, [Fe.sub.2] [O.sub.3] 0.47%, [Al.sub.2][O.sub.3] 43.15%, [K.sub.2]O 1.24%.
Tetrabutyl titanate (Ti[([OC.sub.4][H.sub.9]).sub.4], 99.0% AR, Tianjin Chemical Reagent Company), absolute alcohol ([CH.sub.3][CH.sub.2]OH, 99.7% AR, Tianjin Chemical Reagent Company), and hydrochloric acid (HCl, 37.36% AR, Tianjin Chemical Reagent Company) were commercially available products and were used without further purification.
Preparation of [TiO.sub.2]-coated kaolin
Kaolin particles coated with titanium dioxide were prepared by the hydrolysis method. Tetrabutyl titanate was mixed with absolute alcohol and hydrochloric acid, and the mixture solution was added into the slurry of kaolin and deionized water in thermostatic water baths. After the reaction aged for some hours, the compound slurry was desiccated in a vacuum oven. The dry compound particles were calcined in a muffle furnace for several hours.
The whiteness of products was tested by a whiteness meter (DN-B whiteness instrument, P.R. China). The whiteness values were measured according to international standards (ISO2470, R457), and the oil absorption values were measured according to national standards (GB 5122.15-88, China). The zeta potentials of materials were determined by measuring their electrophoresis mobility with a zeta potential analyzer (Zeta, BI-90Plus, USA). The power structure of materials was characterized by X-ray diffractometry (X Pert PRO DY2198, Holland). The morphologies and particle sizes of the samples were observed by scanning electron microscope (SEM, JSM-35CF, Japan) and transmission electron microscope (TEM, CM12/STEM, Holland).
Results and discussion
Effect of various factors on [TiO.sub.2]-coated kaolin particles
The obtained [TiO.sub.2]-coated kaolin particles arc influenced by various factors, including tetrabutyl titanate dosage, slurry concentration, pH value, hydrolysis temperature, aging time, calcination temperature, and calcination time. To reach optimum results, the effect parameters are adjusted and listed as follows:
The effect of tetrabutyl titanate dosages on whiteness and oil absorption value is shown in Figs. 1a and lb. When the tetrabutyl titanate dosage is increased, the whiteness of compound pigments is improved from 92.0% to 94.0%. In the meantime, the oil absorption value is reduced to the lowest value with tetrabutyl titanate dosage 6 mL/20 g kaolin and then increased. In the first stage, [TiO.sub.2] particles grow evenly on the surface of flaky kaolin particles with the hydrolysis of tetrabutyl titanate. The produced coating films smooth out, in part, the surface roughness of the kaolin substrate and decrease the oil absorption value of compound pigments. If excessive tetrabutyl titanate is added into the system, however, [TiO.sub.2] particles are prone to aggregate rather than coat on the surface of kaolin. The produced porous structure of [TiO.sub.2] compound pigments result in an increase of oil absorption value and a decrease of whiteness. So, the tetrabutyl titanate dosage of 6 mL/20 g kaolin is chosen for the experiment.
[FIGURE 1 OMITTED]
Slurry concentration is also a parameter for the pigments' characterization. Similar tests are observed and the results are shown in Figs. 2a and 2b. When the slurry concentration is increased, the whiteness of compound pigments is improved from 92.2% to 93.0%, and the oil absorption value decreases from 41 mL/100 g to 40 mL/100 g with gradual hydrolysis of tetrabutyl titanate. However, when the slurry concentration is further increased, it is too high to stir evenly, and the hydrolysis product of tetrabutyl titanate cannot (or not totally) coat on the surface of slurry. The uneven thickness and irregular grain of the oxide layer on kaolin flakes led to the decrease of whiteness and increase of oil absorption value. The optimum result of the test is high whiteness and low oil absorption value, so the slurry concentration of 11.0% was chosen for the experiment.
[FIGURE 2 OMITTED]
The pH value is an important factor for [TiO.sub.2]-coated kaolin. The effect of pH value on whiteness and oil absorption value is shown in Figs. 3a and 3b. When the pH value is adjusted from 1.0 to 7.0, the whiteness of compound pigments is first increased and then reduced, while the value of oil absorption is reduced to the lowest value and then increased. When tetrabutyl titanate is hydrolyzed at a pH value of 7.0, it hydrolyzes tetrabutyl titanate so fast that [TiO.sub.2] particles are deposited on the surface of kaolin flakes by hetero-flocculation (flocculation of dissimilar particles that have already formed). The formed [TiO.sub.2] particles are not uniformly agglomerated together, which will affect the whiteness and oil absorption of [TiO.sub.2]-coated kaolin particles. While at a pH value of 1.0, it is inadequate and slow to hydrolyze tetrabutyl titanate on the surface of kaolin flakes by heteronucleation (growth of [TiO.sub.2] layers a surface of different chemistry). The kaolin flakes are partly coated with [TiO.sub.2] layers, which also affect the performance of products. When the pH is adjusted from 1.0 to 3.0, tetrabutyl titanate is hydrolyzed to support [TiO.sub.2] layer growth using heteronucleation until it reaches the optimum pH value of 3.0, so the whiteness of compound pigments is first increased from 1.0 to 3.0. However, when the pH is changed from 3.0 to 7.0, excessive tetrabutyl titanate is hydrolyzed to form dissimilar [TiO.sub.2] particles by heteroflocculation, so the whiteness of compound pigments is reduced from 3.0 to 7.0. [TiO.sub.2]-coated kaolin particles at different pH value were also observed on a TEM (Fig. 11). From the results of Figs. 3 and 11, we can see that the optimum pH value is 3.0.
[FIGURE 3 OMITTED]
[FIGURE 11 OMITTED]
The effect of hydrolysis temperature on whiteness and oil absorption value is observed in Figs. 4a and 4b. When the hydrolysis temperature is elevated, the whiteness of the compound pigments is increased, but the value of oil absorption is first reduced and then increased. At first, the hydrolysis temperature is low, at 25[degrees]C, which is not beneficial to an endothermic reaction of the hydrolysis process. However, when the temperature is elevated to 40[degrees]C, the hydrolysis reaction of tetrabutyl titanate is promoted by the outer temperature, and the elevated temperature quickens the formation of [TiO.sub.2] particles on the surface of kaolin. The improvement in whiteness and oil absorption value is obvious. When the temperature is further enhanced, the irregularities in particle shape and size could reduce the performance of compound pigments. To obtain optimum pearlescent pigment performance, thickness control and a fine structure of the metal oxide are necessary. So, the optimum hydrolysis temperature is 40[degrees]C.
[FIGURE 4 OMITTED]
Aging time is another parameter for the pigments' characterization. The effect of aging time on whiteness and oil absorption value is shown in Figs. 5a and 5b. As the aging time extended, small titanium dioxide particles were coated on the surface of flaky kaolin and then aggregated together. So, the whiteness of compound pigments is primarily improved and then reduced. From the result, we can see the optimum aging time is 16.0 h.
[FIGURE 5 OMITTED]
After hydrolysis for 16.0 h, the products are further calcined to improve the property of compound pigments. The effect of calcination temperature on whiteness and oil absorption value is shown in Figs. 6a and 6b. [TiO.sub.2]-coated kaolin has an inflexion with higher whiteness and low oil absorption value at the calcination temperature of 800[degrees]C. When the calcination temperature is increased from 650 to 800[degrees]C, the coating is converted from a hydrous titanium dioxide to the crystalline anatase form, which is supported by X-ray diffractometry analysis in Fig. 8b, consisting only of anatase crystalline form at a calcination temperature of 800[degrees]C. However, when the calcination temperature is further increased, especially at a elevated temperature higher than 800[degrees]C, the phase transformation from anatase to rutile crystalline form occurs. (14)
[FIGURE 6 OMITTED]
[FIGURE 8 OMITTED]
At the calcination temperature of 800[degrees]C, different calcination times are tried from 1 to 6 hours in the experiment. From the results shown in Figs. 7a and 7b, we can see an inflexion with higher whiteness and lower oil absorption value at the calcination time of 3 h. The coating of hydrous titanium dioxide is transformed into anatase crystalline form at 800[degrees]C, and the transformation lasts for 3 h. The products provide improved performance with an increase in whiteness and a decrease in oil absorption value. However, when the calcination time is further increased, part of anatase [TiO.sub.2] particles are self-aggregated during the extending calcination process, which leads to a slight increase in oil absorption value. So the optimum calcination time is 3 h.
[FIGURE 7 OMITTED]
By using single-factor tests such as those above, the process of creating [TiO.sub.2]-coated kaolin compound pigments is confirmed with the optimum parameters as follows: a tetrabutyl titanate dosage of 6 mL/20 g kaolin, a slurry concentration of 11.0%, a pH value of 3.0, hydrolysis temperature at 40[degrees]C, an aging time of 16.0 h, calcination temperature at 800[degrees]C, and a calcination time of 3 h. The optimum compound pigments reach a whiteness of 94.5% and an oil absorption value of 36.0 mL/100 g.
XRD test of kaolin and [TiO.sub.2]-coated kaolin particles
The chemical compositions of kaolin substrate and [TiO.sub.2]-coated kaolin particles were measured by X-ray diffractometry and the results are shown in Figs. 8a and 8b.
From the XRD images, the kaolin particles have diffraction peaks at 7.1841, 4.4482, 4.1405, 3.5871, 3.3485, 3.2549, 2.3277, and [TiO.sub.2]-coated kaolin composites have diffraction peaks at 3.5147, 2.3787, 2.3370, 1.8975, 1.6911, 1.6617, which could be indexed as tetragonal anatase titanium dioxide, according to the stand card.
The morphologies of kaolin and [TiO.sub.2]-coated kaolin particles
The morphology of kaolin particles was characterized by SEM. From the image in Fig. 9, we can see that the surface of a kaolin particle is a laminated structure. The kaolin and [TiO.sub.2]-coated kaolin particles were further tested by TEM. As shown in Fig. 10a, kaolin particles have a special flaky structure. The image of [TiO.sub.2]-coated kaolin is shown in Fig. 10b, and an enlarged image of the coating is presented in Fig. 10c. It can be seen that the [TiO.sub.2] metal oxide layer is composed of randomly oriented small particles--approximately 20 nm in diameter--and no significant amounts of metal oxides aggregated outside the kaolin surface are observed. [TiO.sub.2]-coated kaolin particles become rounded and smooth, which is important to slow down oil absorption values and improve the property of compound pigments.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
[TiO.sub.2]-coated kaolin prepared at different pH values was investigated by using TEM. The results in Fig. 11 show that [Ti.O.sub.2] particles are coated uniformly on the surface of kaolin substrates at a pH value of 3.0 (Fig. 11b). In contrast, the image of pH 1.0 (Fig. 11a) shows kaolin substrates with little [TiO.sub.2] particles, and the image of pH 7.0 (Fig. 11c) shows the sample with severe agglomeration. It can be explained that it is inadequate to hydrolyze tetrabutyl titanate at a pH value of 1.0 while, at a pH value of 7.0, tetrabutyl titanate is hydrolyzed so fast that [TiO.sub.2] particles are deposited nonuniformly and agglomerated together.
Micro-electrophoresis Zeta value
The effect of the pH value on [TiO.sub.2] coatings can also be investigated with micro-electrophoresis using a zeta potential analyzer. The zeta potentials of kaolin particles were measured at different pH values and the results are presented in Fig. 12. The isoelectric point of kaolin particles is 2.2 in water, while that of hydrated titania is 6.7. When pH value is adjusted between 2.2 and 6.7, negatively charged hydrated titania can electrostatically adsorb on the surface of kaolin particles with positive charges and form a uniform film of [TiO.sub.2] coating. In addition, hydrated [TiO.sub.2] coatings have great free energy due to their small particles. They are prone to absorb on the surface of kaolin substrate to reduce their free energy. So, [TiO.sub.2] coatings on kaolin can be considered the common results of electrostatic attraction and reduced free energy.
[FIGURE 12 OMITTED]
Compound pigments based on kaolin particles coated with metal [TiO.sub.2] oxides were investigated using single-factor tests. The compound pigments reach a whiteness of 94.5% and an oil absorption value of 36.0 mL/100 g, with the optimum parameters of tetrabutyl titanate dosage of 6 mL/20
g kaolin, slurry concentration of 11.0%, pH value of 3.0, hydrolysis temperature at 40[degrees]C, aging time of 16.0 h, calcination temperature at 800[degrees]C, and calcination time of 3 h. X-ray diffraction analysis shows that [TiO.sub.2] coating is anatase after calcination at 800[degrees]C. Kaolin-based coatings at different pH values were investigated using TEM and zeta potential analyzer. The results indicate that the coating layer is composed of randomly oriented [TiO.sub.2] particles with no agglomeration at a pH value of 3.0.
Acknowledgments The authors thank Professor Weidong Zhou (Analysis Center, Yangzhou University) very much for kindly supporting SEM and TEM measurements of the samples. This work was financially supported by research funds from the Chinese Education Department (2003406).
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[C] FSCT and OCCA 2009
Q. Yan, Y, Lei ([??]), J. Yuan
School of Resource and Environmental Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, P.R. China