Fukuji SUZUKI,, Yukio MURUI, Maoya ADACHI, Kazuaki HASHIMOTO and Yoshitomo TODA

429 J. Jpn. Soc. Colour Mater., 81 11 429 436 2008 Fukuji SUZUKI,, Yukio MURUI, Maoya ADACHI, Kazuaki HASHIMOTO and Yoshitomo TODA Interference phenomena result from covering the surface of plate-shaped particles with metal-oxide substances. We changed the orders and thickness of the layers, which are Fe 2O 3 and TiO 2, and contemplated the condition of each pattern. As a result we found that pearl pigments with more saturated colors can be produced by coating the surface of mica with a lower layer of metal-oxide, Fe 2O 3, which has a strong light absorption property and high refractive property, followed by a top layer of TiO 2, rather than the other round. In addition we observed the gap in the flip-flop effect between the two patterns. Key-words Pearlescents pigments, Metal-oxide composites, High luster pearl pigment, Laminates of mica/ Fe 2O 3/TiO 2 Pearlescent pigments employing the phenomenon of optical interference are generally made by depositing highly refractive metal oxides on the smooth surfaces of platelets. Various types of products having different luster, color saturation, and particle effects can be made by altering the type and size of the substrates and changing the reflection and absorption properties of the metal oxides, thus improving the color qualities exhibited by the coated films 1. Typical pearlescent pigments in widespread use are made by coating fine particles of TiO 2 on smooth mica platelets. These TiO 2- coated mica pearlescent pigments have the appearance of a white powder imparting a luster originating from light scattering however, depending on the reflected colors of the substrate, the luster may become extremely low, particularly when the reflected color from the substrate is bright. Nihon Koken Kogyo Co., Ltd. 6-1-2 Ichibancho, Tachikawa, Tokyo 190-0033, Japan Department of Life and Environmental Sciences, Faculty of Engineering, Chiba Institute of Technology 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan Corresponding Author, E-mail f-suzuki@nihonkoken.co.jp Presently, colored pearlescent pigments that exhibit excellent color properties, originating from the powder appearance and the coated films, are available by laminating interference materials with light absorbing materials 2,3. Color development arising from interference and absorption is achieved by laminating Fe 2O 3, an optically absorbing material, on TiO 2-coated mica pearlescent pigments or by depositing a metal oxide on the mica surface. These colored metallic pearlescent pigments exhibit certain specific colors because the laminated films possess absorption properties at visible wavelengths, and color saturation by interference increases to a certain level and is then weakened as the laminated film thickness increases, due to the absorption of specific wavelengths. Such a flip-flop effect is quite limited, however. In particular, when only a thick Fe 2O 3 film is coated, the flip-flop effect almost disappears because of the strong color absorption. With a thin laminated TiO 2 film, the color interference phenomenon starts in accordance with the order of color circle from yellow, and the color changes repeatedly starting with a first interference, then a secondary interference as the film thickness increases 1. This phenomenon is caused by the change of interfering wavelength at longer wavelengths. It is theoretically impossible to obtain high luster colors or pure colors from 1

430 Original Research Paper J. Jpn. Soc. Colour Mater., 81 11 2008 Synthetic process of metal oxides laminates. the interference effect by changing the film thickness because newly appearing interference colors at short wavelengths overlap with one another, thus causing color mixing between the interference colors. In order to obtain the strongest color development from coated films, we have examined the best combination of high luster powders having strong brightness and high color saturation by laminating films of TiO 2 and Fe 2O 3 particles on the surfaces of mica substrates. Synthetic mica made by Topy Industries with an average particle size of 20 µm was used as the substrate, and Fe NO 3 3 9H 2O made by Jujo Chemicals and TiCl 4 48 solution, made by Sumitomo Titanium were used to obtain Fe 2O 3 and TiO 2 by neutralization with the method described below. As shown in, two reaction steps were needed to laminate the metal oxides on the substrates. After dispersing the substrates in distilled water to make a 10 slurry, aqueous solutions of the metal salt and NaOH were titrated at constant speed, while keeping the temperature at 80 and the ph at a desired level until the end point, and samples were removed at several stages to observe the film lamination growth and the interference color development. After the reaction was completed, the samples were washed, neutralized, and dried while observing the electrical conductivity of unreacted materials and sodium salt by-product, and were then calcined at 700 for 90 minutes. The final composite material was synthesized by laminating a second layer using the same process as above. The ph for depositing the metal oxides was 1.6 for TiO 2 and 3.0 for Fe 2O 3. Numerous samples were obtained by changing the laminating order of the metal oxides and by varying the film thickness. We employed an X-ray diffractometer Miniflex, manufactured by Rigaku to detect the crystal phase of the metal oxides, and an scanning electron microscope S-2100B, manufactured by Hitachi for examining the metal oxides to determine whether they were adhered evenly on the surfaces of the substrates. An energy dispersive X-ray spectrometer EDS was employed to measure the chemical elements of the laminated films. In the measurements of color saturation and luster of the films, a color measurement device CM2500d, manufactured by konicaminolta and gonio-spectrophotometers MA68II manufactured by X-Rite and GCMS-4 manufactured by Murakami Shikizai were used. The color of the dried film was measured after drawing-down dispersions of the sample and transparent nitrocellulose lacquer at a ratio of 1 15 coated onto lustrous black paper at a constant thickness. The powder color was measured after coating a fixed amount of powder on the surfaces of pieces of black synthetic leather. shows a graph of lightness L*, equivalent to luster versus color saturation C* of commercially available TiO 2-coated interference pearlescent pigment, TiO 2/Fe 2O 3-coated interference pearlescent pigment, and Fe 2O 3-coated colored pearlescent pigment, measured as described below. Measurement points for FeOOH and Fe 2O 3 are shown for comparison. By laminating TiO 2 and Fe 2O 3 particles on mica substrates, the color of the TiO 2-coated interference pearlescent pigment 2

Preparation of Interference Colors and High Luster Pearlescent Pigments Provided by Thin Film Laminations with Different Refractive Index 431 Fig. 4 SEM photographs of Mica/Fe2O3 and Mica/Fe2O3/TiO2. Fig. 2 Value and chroma graph of Mica/TiO2/Fe2O3 and Mica/Fe2O3 and Mica/TiO2 and inorganic pigment. Fig. 5 Fig. 3 The X-ray diffraction patterns of Mica/Fe2O3 and Mica/Fe2O3/TiO2. shifts toward higher luster, and the color of the Fe2O3coated interference pearlescent pigment shifts toward higher color saturation compared with yellow or red Fe2O3-coated tinting pigments however, no pigments exhibited both high luster and high color saturation simultaneously. 3.2 Structural Analysis of the Laminates by X-ray Diffraction Fig. 3 shows the X-ray diffraction pattern of a powder sample of Fe2O3 deposited on a synthetic mica substrate and a sample prepared by depositing a second coating of TiO2 on the first sample. An Fe2O3 peak was found in the first reaction step, and in the second reaction step, an Fe2O3 or co-existing TiO2 rutile and Fe2O3 peaks were found, because these two different metal oxides were laminated one after another. The same peaks were also observed when the lamination order was changed. Cross-sectional STEM image and element mapping. 3.3 Surface Microscopy of Metal Oxide-deposited Particles The coating conditions used when depositing the metal oxide on the synthetic mica substrate strongly influence the formation of interference colors and luster. In particular, the surface smoothness and uniformity of the laminated film after calcination are very important. Fig. 4 shows SEM images of Fe2O3/TiO2 laminated samples. On the surface of the Fe2O3 layer in the first image, some coarse particles were observed but the surface condition was uniform. In TiO2-laminated layer in the second image, free particles were attached to the surface, but a uniform layer was obtained overall. To confirm whether a smooth surface was obtained on the metal oxide coatings, we examined cross-sections of the particles and analyzed the element distributions using EDS. The results are shown in Fig. 5. By this examination, we successfully confirmed that the extremely thin film derived by coating Fe2O3 on the mica substrate and then depositing a TiO2 film on the Fe2O3 layer of the first coating are expressing respectively apparent laminated structures. 3.4 Optical Properties Our main goal in this study is to obtain pearlescent 3

432 Original Research Paper J. Jpn. Soc. Colour Mater., 81 11 2008 Evaluation method of luster and color saturation. Value and chroma relationship when Fe 2O 3 layer thickness is increasing. Interference colors by depositing TiO 2 and Fe 2O 3 L*/C*. pigment powders having excellent gloss and brilliant color saturation. In addition, when expressed in terms of optical evaluation properties, the reflected colors from the coated film should be strong and their luster and color saturation should as high as possible. The reflection properties of pearlescent pigments strongly depend on the incident/viewing angles and the specular reflection angle. The luster and interference color are maximized at viewing angles close to the specular reflection angle. To evaluate this effect, we examined correlations of the L* value luster and C* value color saturation from the reflection properties observed at 45 incident light angle and 30 viewing angle, shown in, using the X-rite MA-68 II goniospectrophotometer. The area where both L* and C* are maximized is a target area for obtaining pigments with high luster and high color saturation. shows how the L* and C* values depend on the coating film thickness when TiO 2 particles and Fe 2O 3 particles were individually deposited on mica substrates. In the case of the TiO 2, the L* value rapidly increased at low film thicknesses, and when the thickness reached an interference color zone, it shifted to a high color saturation area, resulting in a slight decrease in the L* value. In the case of Fe 2O 3 on the other hand, both the L* value and C* value gradually increased as the film thickness increased, reaching higher L* and C* values than in the case of TiO 2. When the thickness further increased, a sudden reduction in the L* and C* values was observed. This apparent higher luster exhibited by Fe 2O 3 might be attributed to the higher surface reflectivity originating from the higher refractive index than that of TiO 2. When the film thickness of the Fe 2O 3 reached the point where the light absorption was higher, absorption became stronger than reflection, resulting in decreased L* and C* values. 3.4.1 Optical Properties of Mica/TiO 2/Fe 2O 3 Laminates shows the behavior of the L*and C* values by varying the thickness of an Fe 2O 3 film coated on a TiO 2 film with a thickness of approximately 150 nm yellow interference on a mica substrate. The optimum luster and color saturation were obtained when the Fe 2O 3 thickness was 10 20 nm when the thickness was higher than 50 nm, the luster and chroma decreased. The film thickness in Fig. 8 is the optical thickness refractive index thickness calculated from the interference color of the Fe 2O 3. The color of the Fe 2O 3 layer in the double-layer metal oxide structure shifted to higher chroma when its thickness was smaller than the single coating of either TiO 2 or Fe 2O 3 mentioned above. The color of the Fe 2O 3 powder coated on the TiO 2 layer to a thickness of 10 20 nm was brilliant yellow, and when the thickness was increased to more than 50 nm, the color changed to red or dark red. This could be explained as follows The brilliant yellow color may have been caused by the 4

Preparation of Interference Colors and High Luster Pearlescent Pigments Provided by Thin Film Laminations with Different Refractive Index 433 Value and chroma relationship of Mica/TiO 2/Fe 2O 3 and Mica/Fe 2O 3/TiO 2 as thickness is increasing. Spectral reflection curves of Mica/Fe 2O 3/TiO 2 and Mica/TiO 2/Fe 2O 3 at yellow color. formation of TiFe 2O 5 by calcining the TiO 2 and Fe 2O 3, coupled with its interference properties. Then, by further increasing the Fe 2O 3 thickness in upper layer, the color changes to red or dark red possibly because of the coexistence of Fe 2O 3 particles which are not involved in the above reaction. We conclude that the ideal film thickness of Fe 2O 3 is 10 20 nm. A thicker Fe 2O 3 coating will inevitably cause changes in the color and interference properties, as well as reduced luster and color saturation. 3.4.2 Lamination Order of Fe 2O 3 shows how the L* and C* values change depending on the TiO 2 film thickness when Fe 2O 3 was coated as an under layer A and as an upper layer B at a fixed Fe 2O 3 film thickness of 10 nm. The TiO 2 film on both layers A and B started to impart yellow to orange interference colors and exhibited reduced luster and color saturation as the film thickness increased, the same behavior as seen in Fig. 7. This resulted in the appearance of interference color cycles, and interference colors were observed in the yellow to orange powders as a result of the flip-flop effect. In the yellow interference color area in the second cycle, the layers A and B both showed higher luster and chroma. We confirmed that providing the Fe 2O 3 film as the under layer provided a larger locus of the L*and C* values. Specifically, the ideal method to obtain maximum L*and C* values was to provide the Fe 2O 3 film as the under layer 10 nm and the TiO 2 film as the upper layer. 3.4.3 Optical Properties of Mica/Fe 2O 3/TiO 2 Laminates shows spectral reflection curves of Fe 2O 3 10 nm and TiO 2 160 nm films observed at a Color locus of Mica/TiO 2/Fe 2O 3 and Mica/Fe 2O 3/TiO 2 at yellow color. 30 viewing angle. The yellow color was located mainly at wavelengths 560 590 nm, and the properties such as intensity of reflection of luster-relating angular wavelength and the volume of chroma-relating color saturating substances were high in the Mica/Fe 2O 3/TiO 2 laminate, showing that it possesses suitable properties for use in production-scale pearlescent pigments. shows the reflectance indicated on a*/c* coordinate axes at each viewing angle, observed using the GCM- 45 gonio-spectrophotometer at 45 incident angle. In this test, to evaluate the powder color appearance, the pigment powders were uniformly applied to the surfaces of pieces of black artificial leather. From the figure, the area of color change by varying the viewing angle was greater and the chroma was higher when the Fe 2O 3 film was formed as the under layer, compared to the case where the TiO 2 was formed as 5

434 Original Research Paper J. Jpn. Soc. Colour Mater., 81 11 2008 High luster pearl pigment-coated automobile panel. L*value relations with the change of viewing angles of the laminates of Mica/Fe 2O 3/TiO 2. the under layer. Particular attention should be paid to the fact that the color at an incident angle of 45 was completely different when viewed at different viewing angles. At the specular reflection angle, the exhibited color was a greenish yellow, but at extreme shade angle, the color changed to vivid red as a result of the strong flip-flop effect. This phenomenon was not observed, however, when the under layer was TiO 2. shows the L* value for an incident light angle of 45, measured at various viewing angles. Regardless of whether the Fe 2O 3 formed the under layer or the upper layer, the L* value increased rapidly around the specular angle, reaching a maximum value. Such a high luster indicates that the surface of the metal oxide film was uniformly flat. However, a slight gap was observed between the maximum-reflectance angles for the different coating positions of the two films. One possible explanation for the 15 angular gap at the maximum L* value position when the upper layer was Fe 2O 3 is as follows when the incident light passes through the upper Fe 2O 3 film and reaches the surface of the TiO 2 film, its angle was strongly refracted because of the higher refractive index of the Fe 2O 3. Fig. 10 and Fig. 11 show that laminating Fe 2O 3 in the under layer and TiO 2 in the upper layer at a thickness ratio of 10 nm 160 nm results in higher luster and higher saturation color, showing the possibility of producing pigments exhibiting these effects. We coated the laminated pigments described above on the curved surface of an automotive panel. shows the coating exhibited an apparent flip-flop effect from red to yellow. When laminating films of highly refractive Fe 2O 3 and TiO 2 particles, the laminating order of the films and their thickness were confirmed to be important factors, and the coating of light-absorbing Fe 2O 3 particles in an under layer was essential for obtaining highly saturated colors. In performing this study, two different properties were observed by changing the lamination order of the films, though both films had the same thickness. First, absorption color was observed at the shade angle position when an absorbing film was formed in the under layer when laminating a combination of Fe 2O 3 and TiO 2. Second, the maximum-reflectance angle of the reflected light slightly deviated from the specular angle when the absorbing layer was formed in the upper layer. When we consider the interference phenomena in the thin light-absorbing Fe 2O 3 film in the laminate, the light that passes through the thin film will be absorbed in the layer of the wavelength of the film. The light which was not completely absorbed returns after being reflected from under layer and interferes with the light reflected at the surface. We believe that this limits the emergence of interference colors because the wavelengths at which the light is absorbed no longer exist. Although we observed the typical increase in luster and the gradual weakening of the interference colors as the film thickness of the Fe 2O 3 increased, the complementary colors did not disappear, and we observed the color cycling which is characteristic of interference in these kinds of thin films. In this phenomenon, the light which passed through the absorption layer and returned after being reflected was not necessarily absorbed completely at the thickness levels we used in our experiments. This suggests that wavelengths at which transmission and reflection occurred, but not absorption in the film, were the origin of the interference of the complementary colors. The interference color obtained from the mica/fe 2O 3/TiO 2 laminates in this study should have been yellow, and the shade color transmitted interference color 6

Preparation of Interference Colors and High Luster Pearlescent Pigments Provided by Thin Film Laminations with Different Refractive Index 435 Optical simulation result of color locus of Mica/ TiO 2/Fe 2O 3 and Mica/Fe 2O 3/TiO 2. should have been violet to orange however, it was actually a clear red. This phenomenon is difficult to explain. The gap between the maximum-reflectance angles caused by the different lamination orders might be explained by a phenomenon that occurs when a film with higher refractive index exists at the upper layer, and the incident light angle becomes larger at the same time. However, this explanation does not allow us to say whether the light that reaches the lower layer shifts to the higher index position than the incident angle. In either case, it is necessary to formulate a theory from the results obtained by repeated trial and error measurements to explain the properties of a multilayered structure that provides an absorbing layer against a layer of TiO 2 particles with higher refractive index. We simulated the interference properties of thin films by inducing parameters of refractive index and absorption coefficients and thickness distribution of pigments substance and spatial orientation of pigments on rough surface, and we calculated the changes in the interference properties by altering the lamination order, as shown in 4,5. As noted above, we confirmed the effectiveness of coating Fe 2O 3 in the under layer for obtaining higher luster. This simulation result agrees with the experiment results shown in Figs. 9 11 well. However, the results of the simulation 4,5 still did not explain the gap observed in the interference color at the shade angle and the gap between the maximumreflectance angles. We attempted to produce laminated films by coating Fe 2O 3 and TiO 2, both having high refractive index, on synthetic mica substrates to obtain pigments that exhibit high color saturation and high luster. We investigated the film thickness of Fe 2O 3 and TiO 2, as well as their lamination order. By coating a 10 nm Fe 2O 3 film as an under layer, followed by a 160 nm film of TiO 2 particles as an upper layer, the correct proportion to produce a yellow interference color, we obtained a yellow pearlescent pigments with very high brightness and high color saturation. This mica/fe 2O 3/TiO 2 laminate imparted a yellow color, tinted slightly with green. Additionally, it provided high luster and high color saturation at the specular reflection angle, and exhibited a red color flipflop effect of red color at angles far from the specular reflection angle. 1 49 th Pigments Guide Handbook, issued by Japan Color Material Association, p.63, August 30, 2007. 2 Japan patent, No.62-34962. 3 Japan patent, No.2939314. 4 H. Shiomi, E. Misaki, M. Adachi High chroma pearlescent pigment designed by optical simulation, Fatipec2006. 5 E. Misaki Reproducing beautiful complexion with the makeup foundation designed by multiangle image-capture and optical simulation, IFSCC, 2006. 7

436 Original Research Paper J. Jpn. Soc. Colour Mater., 81 11 2008, Corresponding Author, E-mail f-suzuki@nihonkoken.co.jp 2 8