PLATELET-LIKE COLORED PIGMENTS AND PROCESS FOR PRODUCING THE SAME

Abstract: Colored pigments on the basis of platelet-like substrates containing TiO2, one or more suboxides of titanium and an oxide or oxides of one or more different metals and/or non metals, whereby the concentration of the titanium oxides in the coating layer is maximum in the proximity of the substrate surface and gradually decreases towards the pigment surface and the concentration of an oxide or oxides of one or more different metals and/or non metals is maximum at the pigment surface. The different metals and/or non metals are preferably Li, Na, K, Mg, Ca, B, Al, Si, Zn and Fe in an amount of from 1 to 50 % by weight, relative to the amount of titanium in the pigment.

Description: Platelet-like Colored Pigments and Process for Producing the Same The present invention relates to a platelet-like colored pigment comprising titanium dioxide, one or more suboxides of titanium, and an oxide or oxides of one or more different metals and/or non metals. There is a continuously increasing demand for pigments with intense pearlescant color and/or metallic luster in the field of paints, coat:„rgs, inks, plastics, cosmetics and especially for exterior coatings. Therefore, the development in the field of pigments aims at lustrous and hiding pigments which can be new effect pigments or replace the metallic pigments such as aluminum flakes with their known disadvantages. It is known that coatings of Ti0 2 -layers on platelet-like substrates produce the so-called pearlescent effect. For this kind of pigments it is possible to control the interference color by changing the thickness of the Ti0 2 -layer. It is known from DE 3433657, US 4623396 and EP 0332071 to use suboxides, e. g. suboxides of titanium, for the coating of platelet-like substrates whereby for the reduction of the Ti0 2 ammonia gas is used. However, the reduction of solid particles by a reducing gas is not suitable for an industrial production, because the color of the product as a function of the degree of reduction is difficult to control. It is dependent on the size of the particles, the temperature, the gas flow and the time of treatment. Furthermore special facilities for dangerous or toxic gas are necessary. It is also known that such suboxides provide functional properties such as electric conductivity. P-A-1-158077 discloses a pigment consisting of a titanium oxide layer containing a dark color region on a mica core and a color tone adjusting layer as covering layer comprising at least one of silicon oxide, aluminum oxide and zinc oxide or a composite oxide thereof. The dark color region is composed of a dark color metal oxide, such as titanium oxide or iron oxide of lower order, of titanium nitride, titanium oxide or carbon black. The pigment is manufactured in a first step by heating and reducing titanium dioxide-coated mica flakes at a temperature of from 500 °C to 1000 °C in a reducing gas; or by blending titanium dioxide-coated mica flakes with titanium metal and heating the resulting blend in a vacuum at a temperature of from 500 to 1000 °C for a long time of more than 6 hours. The covering layer is deposited in a second step in an aqueous medium by precipitating of silicon dioxide hydrate on the first layer containing the titanium suboxides and drying or firing the pigment. This additional coating improves heat stability of the suboxide containig layer. However, the functional properties such as electric conductivity of the first layer is shielded by this second layer. The pigment has the disadvantage that it is manufactured in a complicated two-step process which raises the price. The reduction in vacuum requires expensive facilities and complicated opera¬ tion and high temperature and longer reaction time causes low production efficiency. Furthermore the used titanium metal is expensive. Accordingly, it is an object of this invention to provide platelet-like colored pigments having improved heat stability without losing the functional properties of colored layers which are producible in a simple one-step process. This object ' is solved according to the present invention by a platelet-like colored pigment on the basis of a platelet-like substrate which is covered by a coating layer containing Ti0 2 , one or more suboxides of titanium and an oxide or oxides of one or more different metals and/or non metals. The concen- tration of said titanium oxides in the coating layer is maximum in the proximity of the substrate surface and gradu¬ ally decreases towards the pigment surface and the concentra¬ tion of said oxide or oxides of one or more different metals and/or non metals is maximum at the pigment surface and gradually decreases towards the substrate surface whereby mixed phases of these oxides exist inside the coating layer. The different elements are preferably selected from alkali metals, alkaline earth metals, B, Al, Si, Zn or Fe. Further- more all other elements with sufficient reactivity can be used. The amount of the one or more different metals and/or non metals is from 1 to 50 % by weight, preferably from 2 to 30 % by weight, relative to the amount of titanium in the pigment. Furthermore this object is solved by a process for producing platelet-like colored pigments by mixing a platelet-like substrate coated with Ti0 2 with at least one solid reducing agent, preferably an alcaline earth metal B, Al, Si, Zn, Fe, LiH, CaH 2 , A1 4 C 3 , Mg 2 Si, MgSi 2 , Ca 2 Si or CaSi 2 and heating the mixture in a non-oxidative gas atmosphere at a temperature of more than 600 °C for more than 10 minutes. As starting materials platelet-like substrates like mica, kaoline or glass platelets which optionally may be coated with one or more metal oxide layers, or metal or metal oxide platelets coated with Ti0 2 are used. The particle sizes of the substrates are 1 to 500 μm in diameter and 0.1 to 1 μm in thickness. The materials are coated with Ti0 2 according to processes well-known in the art and described for example in US-A-3553001. The Ti0 2 -layer has a thickness in the range of from 10 to 1000 nm, preferably from 40 to 500 n . The reduction reaction takes place in a non-oxidative gas atmosphere such as N 2 , Ar, He, C0 2 , C--H y , H 2 , NH 3 , N 2 or Ar are preferred. In case of N 2 or NH 3 , TiN or TiON may be formed in addition to Ti0 2 - x . It forms mixed solid suboxides or oxide bronzes on the platelet¬ like substrates as soft solid powders at temperatures of more than 600 °C, preferably in the range from 700 to 1100 °C for more than 10 minutes, preferably for 15 minutes to 60 min¬ utes. Some examples will illustrate the reactions: 1000 °C 2 Al + 6 Ti0 2 /M > A1 2 0 3 x 3 Ti 2 0 3 /M Ar 1000 °C 2 Al + 3 Ti0 2 /M > ■ A1 2 0 3 x 3 TiO/ Ar 1000 °C Si + 6 Ti0 2 / » Si0 2 x 2 Ti 3 0 5 /M N 2 1000 °C Si + 4 Ti0 2 /M > ■ Si0 2 x 2 Ti 3 0 5 /M M = mica As reducing agent fine powdered elements like for example Al, Si, Mg, Ca and B or combinations thereof are particularly preferred. Furthermore alloys of metals or metal borides, carbides and suicides can be used. Well-known reducing agents like the alkali metals may be used in liquid or gas phase. Other reducing agents are hydrides like LiH or CaH 2 . In addition, combinations of these reducing agents with each other are possible. The Ti0 2 -pigment and the reducing agent are mixed in a ratio from 200:1 to 5:1 preferably from 100:1 to 10:1. The obtained pigments exhibit strong pearlescent color and/or metallic lustre. The color effect of the pigments can be controlled by chang¬ ing - the particle size of the platelet-like substrate (smaller particles lead to a soft silky luster, larger particles lead to a glittering luster) , - the thickness of the Ti-oxides layer (interference col¬ or) , - the kind of the solid reducing agent (low reduction potential yields grey to blueish black, high reductionpo- tential yields black to yellowish black) and - the amount of the solid reducing agent. The reduction reaction is accelerated in the presence of a halide, preferably a chloride as pointed out in Table 1. LiCl, NaCl, KC1, MgCl 2 , CaCl 2 , CuCl 2 , CrCl 3 , MnCl 2 , FeCl 2 , FeCl 3 , CoCl 2 , NiCl 2 or CeCl 3 are preferred. The reaction temperature can be reduced in the presence of chloride by 150 to 300 °C. In example I the calcining temperature has to be 1000 °C without the admixture of chloride in order to reach a reaction. It is reduced to 840 °C by admixing of CaCl 2 . The amounts of the halide can vary from 0,1 to 40 %, prefera¬ bly from 0,5 to 10 % relative to the platelet-like substrate coated with Ti0 2 . The products show a depth profile regarding the distribution of titanium and the one or more different metals and/or non metals inside the coating layer. Figure 1 shows a depth profile of Iriodin 120, a pearlescent Ti0 2 -pigment manufactured by E. MERCK. The concentration of titanium on the pigment surface is 22 atomic % contrary to the silicon concentration which is only about 5 atomic %. Figure 2 shows a depth profile of the same pigment after the reduction with metallic silicon according to example 1. The titanium concentration is decreased to 2 atomic % and the silicon concentration is increased to more than 28 atomic %. During the reduction reaction metallic silicon penetrates into the Ti0 2 -layer and is oxidized to Si0 2 . The silicon concentration decreases continously in the direction to the substrate and is about 9 % in a depth of 30 nm. Figure 3 shows the distribution of titanium and silicon in the pigment described in JP-A-1-158 077 beginning from the surface of the coating layer in direction to the substrate. The known pigment has been manufactured by a two-step process as described in the comparative examples 1 and 2. The two pigments show a very different structure. The known pigment has distinct layers. The top layer precipitated from an aqueous medium containing approximately 100 % of silicon dioxide is more than 3 nm thick. In contrary, the pigment according to the present invention does not possess any distinct layers. The concentration of the components in the coating layer is gradually changing with the depth of this layer. A pigment which is used in an organic solvent system such as paint or ink is often required to have a certain level of conductivity in order to avoid the danger of explosion by electrostatic spark. Table 2 shows the temperature resistance and the electroconductivity of the known pigment after the first process step (reduction of Ti0 2 -layer with Ti metal; PM 21-2) and after the second process step (precipitation of Si0 2 ; PM 21-3) and of the pigment according to the present invention (PM 21-1) . The pigment according to the present invention has a significantly better electroconductivity than the pigment produced by the two-step process. Additional coating with Si0 2 (PM 21-3) reduces the conductivity. Furthermore the pigments according to the present invention show a better application behaviour. That means better results regarding the humidity test in car paint application and regarding the photoactivity test in out-door application. Photoactivity test results are shown in Table 3. The pigment according to the invention has been compared with known pigments. If desired the pigment can be provided with high magneticity as shown in Table 4. The invention will now be described more specifically with reference to examples thereof, as well as comparative exam¬ ples, which are not intended to limit the scope of the inven¬ tion. Example 1 15 g of a Ti0 2 -coated mica powder, particle size distribution 5-25 μm, exhibiting white mass tone with white reflection color (Iriodin® 120; produced by E. Merck) and 1.5 g of Si powder, particle size < 150 m (from E. Merck), was mixed thoroughly in a 500 ml plastic bottle. The mixture was put into a quartz boat. The boat was placed inside a quartz tube (inner diameter 5 cm, length 100 cm) . Both ends of this tube were equipped with gas * pipes, one for introducing gas and the other for exhausting gas. Then blowing of N 2 gas into this tube was started at room temperature with the flow rate of 0.25 1/min. After 15 in., the tube was placed into an oven which was kept at 1000 °C, and maintained for 30 minutes. The tube was put out of the oven and let cool down. Until the tube was cooled down, the blowing of N 2 gas was maintained. The calcined powder was washed with water and dried at 120 °C over night, and then the powder was passed through a 40 μm sieve. To evaluate the color characteristics of the pigment produced according to the invention, 0.9 g of each pigment samples is worked in 53.6 g of an acrylic modified nitrocellulose lac¬ quer. After homogenizing with a propeller mixer at 1000 rpm for 10 min the dispersion is allowed to stand for one hour to remove air bubble. Then the pigmented lacquer is coated with an applicator (500 μm wet film thickness) on a block and white card. The visual evaluation of the color is expressed as masstone and interference color. The masstone color is the observation out of luster (45°/0°) and the interference color is the observation at reflection angle (22.5°/22.5°) . The CIE Lab color coordinates are measured with Hunter Lab Color Meter model D-25 tinder the conditions of 45°/0° with white background and 22.5°/22.5° with black background. The obtained powder exhibits blue black masstone with blue interference color (coloristics see also Table 4) . By ESCA measurement with combination of Ar sputtering tech¬ nique, depth distribution of Si atoms in the coated layer of this pigment was determined. As shown above in Fig. 2, Si atoms penetrate into Ti-oxide layer more than 30 nm and the distribution of Si atom shows gradation of concentration along the depth. X-ray diffraction of this powder indicates the presence of γ-Ti 3 0 5 and Si0 2 (quartz) . Example 2 100 g of a Ti0 2 -coated mica powder, particle size distribution 5-25 μ , exhibiting reddish white mass tone with green inter¬ ference reflection color (Iriodin® 231; produced by E. Merck) , 3 g of the same Si powder as in Example 1 and 1 g of CaCl 2 were mixed throughly in a 3 1 plastic bottle. This mixture was calcined at 800 °C for 30 minutes under N 2 gas flow of 3.0 1/min. with the same manner as in Example 1. The calcined powder was washed with water and dried at 120 °C overnight, and then the powder was passed through a 40 μ sieve. The obtained powder exhibits bluegreen black masstone with bluegreen interference color. This sample showed 7.5 kΩ ■ cm of powder conductivity which is enough to avoid electrostatic spark (see Table 2) . Comparative Example 1 This comparative example demonstrates the reduction with metallic titanium according to JP-A-1-158 077 as the first step of the two-step process. 100 g of the same Ti0 2 -coated mica as in Example 2 and 5 g of Ti powder (particle size 150 μ , from E. Merck) were mixed throughly in a 3 1 plastic bottle. This mixture was calcined at 1000 °C for 30 minutes under N 2 gas flow of 0.25 1/min. with the same manner as in Example 1. The calcined powder was washed with water and dried at 120 °C overnight, and then the powder was passed through a 40 μ sieve. The obtained pigment exhibits bluegreen black mass tone with blue green interference color. The sample showed a powder conductivity of 31.5 MΩ • cm. Consequently, the pigment according to the present invention has a significantly better electroconductivity than the comparison pigment. Comparative Example 2 As the second step of the two-step process described in JP-A-1-158 007 silicon oxide hydrate was deposited on the surface of the pigment obtained according to Comparative Example 1. 50 g of the pigment was dispersed into 500 ml of deionized water with stirring. During the reaction the stirring was maintained. The dispersion was heated up to 90 °C and the pH was adjusted by adding 5 wt% NaOH aq. solution. To this dispersion 75 ml of 10 wt% Na 2 Si0 3 aq. solution was added at the rate of 2 ml/min. with keeping the pH at 9 by simulta¬ neous addition of I N HC1 aq. solution. 62 ml of 1 N HC1 aq. solution was needed. The temperature and stirring were maintained for an addi¬ tional hour and then the mixture was cooled. The solid was filtered, washed with water and dried at 200 °C overnight. The obtained pigment exhibits bluegreen black mass tone with bluegreen interference color. ESCA analysis with combination of Ar sputtering technique indicates that a more than 3 nm thick silicon oxide layer covers the titanium oxide layer (see Fig. 3) . The sample shows a powder conductivity of > 125 MΩc . This value makes clear that the conductivity of the first layer is shielded by the silicon oxide layer. Examples 3-14 Mixtures of a Ti0 2 -coated mica pigment and a reducing agent and in some cases a chloride were prepared in the same manner as in Example 1. The ratio of the reactants is listed in Table 4. The mixtures were calcined under the conditions described in Table 4. The calcined powders were washed with water, dried at 120 °C overnight and then passed through a 40 μm sieve. The appearance colors and measured color values are listed in Table 4. • All of these Examples exhibit grey to dark color indicating the formation of suboxides by the reaction. Examples 15-32 These Examples demonstrate the influence of halides which accelerate the reduction reaction and which make it possible to lower the reduction reaction temperature. The samples were prepared in the same manner as in Example 1. The mixing ratios and the calcination conditions are listed in Table 4. The color appearance and the measured values are described in Table 4. The colors of the pigments in the Examples 16-19 are darker than the color of the pigment in Example 15. In this Example no halide was added. The darker color indicates the reduction reaction was accelerated. The colors of the pigments in the Examples 20-28 are dark indicating a reduction reaction took place, but without a halide the reduction reaction does not take place at such low temperatures. The Examples 29 to 32 show that with decreasing amount of the halide in the mixture the color of the pigment becomes lighter. Example 33 This Example demonstrates the production of a dark colored conductive pigment. 15 g of Ti0 2 -coated mica pigment (the same as in Example 1) and 3 g of an Al powder (particle size < 250 μ , from E. Merck) was mixed in the same manner as in Example 1 and calcined at 1000 °C for 30 minutes under N 2 gas flow of 0.25 1/min. The calcined material was washed with water, dried at 120 °C over night and passed through a 40 μ sieve. The obtained pigment exhibits yellowish black mass tone with brown reflection color. The X-ray diffraction of this pigment indicates the presence of Ti 2 0 3 , TiN (Osbornite syn) and Al 2 0 3 . The electroconductivity of this pigment was determined as 20 Ω • cm. Example 34 This Example demonstrates the production of a pigment with magnetic properties. 15 g of the same Ti0 2 -coated mica pigment as in Example 2, 1.35 g of the same Si powder as in Example 1 and 0.15 g of CaCl 2 were mixed in the same manner as in Example 1. The mixture was calcined at 1000 °C for 30 minutes under N 2 gas flow of 1.5 1/min. The calcined material was washed with water, dried at 120 °C overnight and passed through a 40 μ sieve. The obtained pigment exhibits black mass tone with brown reflection color and magnetic properties. Table 1 Example Ti0 2 coated Reduction No. mica pig. agent I Ir. 123 Si/CaCl 2 PM 11 comparison Ir. 123 Si II Ir. 123 A1/KC1 PM 13-2 comparison Ir. 123 Al III Ir. 123 B/KC1 PM 12-4 comparison Ir. 123 Ir. means Iriodin®, a trade name for interference pigments, manufactured by E. MERCK. Table 2 Sample Process Ti0 2 coated Reduction Mixing ratio Gas flow Calcine Electro- mica pigment agent pig./red. rate temp. conductivity* Example 2 Ir. 231 Si/CaCl 2 100/3/1 (PM 21-1) Comparative Example 1 Ir. 231 Ti 100/5 (PM 21-2) Comparative Example 2 PM 21-2 is coated with Si0 2 • nH 2 0 > 125 MΩ •. cm (PM 21-3) Na 2 Si0 3 + HC1 »Si0 2 • nH 2 0 *Specific restistance of pigment powder, resistance of pressed powder between two metal pistons in a tube with 10 kg/3 cm 2 was measured and calculated according to following formula: A R sP ~ R ( R s : specific resistance [Ω • cm]; R: measured resistance [Ω]; A: cross- d sectional area of a tube [cm 2 ]; d: thickness of powder layer [cm]) . Table 3 Sample Gas flow Calcine Color Humidity* 1 -) Photoactivity rate tempi. masstone/ 16 h/66 °C UV lamp, 24 h interference Ir. 103 Mg 100/3.8 Ar BOO °C dark grey/ A 0.25 1/min silver Ir. 103 Si/CaCl 2 100/2/0.5 N 2 900 °C grey/silver 3 1/min. Ir. 103 white/white Ir . 123 Si/B/KCl 100/4/1/0 .5 N 2 700 °C dark grey/ 3 1/rnin , silver Ir . 123 - white/white Test system.1-coate acrylic-melamine thermosetting paint. 4 wt% pigment content based on total paint. 2 ) 1 phr pigment incorporated in Pb compound containing PVC plate. rank A: less degradation B: fair C: much degradation Ir. means Iriodin®, a trade name for interference pigments, manufactured by E. MERCK. Table 4 Table 4 (continued) Table 4 (continued) Table 4 (continued) Table 4 (continued) Table 4 (continued) Electro¬ Magnetivity conductivity tivity 28 kΩcm Much Table 4 (continued) Table 4 (continued) Example. Color meter values Electro¬ Magnetivity No. 4570° white 22.5722.5° black conductivity background background tivity L a b L a b

Claims: Claims 1. Platelet-like colored pigments on the basis of platelet¬ like substrates coated with layers containing Ti0 2 , one or more suboxides of titanium and an oxide or oxides of one or more different metals and/or non metals, characterized in that the concentration of the titanium oxides in the coating layer is maximum in the proximity of the sub¬ strate surface and gradually decreases towards the pig- ment surface, that the concentration of an oxide or oxides of one or more different metals and/or non metals is maximum at the pigment surface and gradually decreases towards the substrate surface and that mixed phases of these oxides exist inside the coating layer. 2. - Pigments according to claim 1, characterized in that the different metals and/or non metals are preferably Li, Na, K, Mg, Ca, B, Al, Si, Zn and Fe. 3. Pigments according to claims 1 and 2, characterized in that the amount of said one or more different metals and/or non etals is from 1 to 50 % by weight, preferably from 2 to 30 % by weight, relative to the amount of titanium in the pigment. Process for preparing platelet-like colored pigments, characterized in that platelet-like substrates coated with Ti0 2 are mixed with at least one solid reducing agent, preferably an alkaline earth metal, B, Al, Si, Zn, Fe, LiH, CaH 2 , A1 4 C 3 , Mg 2 Si, gSi 2 , Ca 2 Si or CaSi 2 and that the mixtures are heated in a non oxidizing gas atmosphere at a temperature of more than 600 °C. 5. Process according to claim 4, characterized in that said substrate coated with Ti0 2 and said reducing agent are mixed in a ratio from 200:1 to 5:1, preferably from 100:1 to 10:1. 6. Process according to claims 4 and 5, characterized in that a halide is admixed to said platelet-like substrate coated with Ti0 2 in an amount of from 0.1 to 40 % by weight, preferably from 0.5 to 10 % by weight. 7. Process according to claim 6, characterized in that said halide is preferably LiCl, NaCl, KC1, MgCl 2 , CaCl 2 , CuCl 2 , CrCl 3 , MnCl 2 , FeCl 2 , FeCl 3 , CoCl 2 , NiCl 2 or CeCl 3 .

Inventors: AMBROSIUS KLAUS, FRANZ KLAUS-DIETER, NITTA KATSUHISA, WILHELM STEFAN, AMBROSIUS, KLAUS, FRANZ, KLAUS-DIETER, NITTA, KATSUHISA, WILHELM, STEFAN
Applicants: MERCK PATENT GMBH, MERCK PATENT GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG
Application year: 1993
Main classification: C09C1
IPC codes: C01G23, C09C1, C09C3