Black pigment particles

Abstract: Black pigment particles comprising polycrystalline particles having a mixed composition of Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution are useful as pigment particles for a developing toner and for coating materials such as paints.

Description: The present invention relates to black pigment particles which are non-magnetic, safe and harmless, and which have excellent workability and heat resistance. The black pigment particles of the present invention are particularly useful for use or, for example, a developing toner, colouring materials for paint or colouring materials for resins. Generally black pigment particles are widely used as a developing toner by mixing and dispersing the pigment particles in a resin and then molding the resin, or as coating materials by mixing and dispersing the pigment particles in a vehicle. Recently black pigment particles which are non-magnetic, safe and harmless, and which also have excellent workability and heat resistance have been strongly demanded from the standpoints of enhancement of working efficiency in the age of energy saving, safety and sanitation, and improvement of properties. To improve the workability of the pigment particles, it is imperative that the pigment particles are non-magnetic and have good dispersibility with an appropriate range of size, and are offered in the form of powder which is easy to handle by possessing a high apparent density and an appropriate range of size. Heat resistance is also a requisite for the black pigment particles because these particles, when used for a developing toner, are required to have a stable hue even at high temperatures above 150°C, since the developing toner, for which demand is growing with the spread of copying devices in recent years, is heated to a high temperature over 150°C in its production process. Hitherto, magnetite particles and carbon black particles have been popularly used as black pigment particles. Black pigment particles which are non-magnetic, safe and harmless, and which also have excellent workability and heat resistance are now most acutely required. Known magnetite particles, because of their magnetism, tend to repeat mutual agglomeration which deteriorates their dispersibility, resulting in poor workability. Furthermore magnetite particles begin to transform into maghemite particles at a temperature above 150°C and are change in colour from black to liver brown. They therefore have unsatisfactory heat resistance. On the other hand, a carbon black, although having excellent heat resistance, is hard to handle and has poor workability because it is a powder comprising ultra-fine particles with a size of 0.01 to 0.1 µm while having a high apparent density of around 0.1 g/cm³. Carbon black is also problematic in the aspect of safety and sanitation such as the carcinogenicity thereof. The present inventors have found that polycrystalline particles having a mixed composition of Fe₂TiO₅ and Fe₂O₃·Fe₂TiO₃ solid solution, obtainable by subjecting magnetite particles having their surfaces coated with a titanium compound, a mixture of magnetite particles and a titanium compound, or reduced particles obtained by reducing hematite particles having their surfaces coated with a titanium compound, to heat treatment at a temperature of not less than 700°C is a non-oxidizing atmosphere, and pulverizing the thus treated particles, are non-magnetic, safe and harmless, and have excellent a workability and heat resistance. Accordingly the present invention provides black pigment particles comprising polycrystalline particles having a mixed composition of Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution, the Fe₂TiO₅ being present in an amount of 1 to 40 wt%, and the Ti being present in the polycrystalline particles in an amount of 15 to 50 atom% based on the total amount of Fe(II) and Fe(III). The present invention also provides a process for the preparation of black pigment particles as defined above, which process comprises subjecting: (a) magnetite particles having their surfaces coated with a titanium compound, (b) a mixture of magnetite particles and a titanium compound, or (c) particles obtainable by reducing hematite particles having their surfaces coated with a titanium compound, to heat treatment at a temperature of not less than 700°C in a non-oxidising atmosphere and pulverising the particles thus obtained. The present invention additionally provides a developing toner comprising a resin and particles as defined above having an average particle size of from 0.1 to 0.5µm. The present invention further provides a coating material, for example a paint, comprising a vehicle and particles as defined above. Each particle is composed of both Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution. In the accompanying drawings: Fig. 1 and Fig. 2 are an electron micrograph (x100,000) showing the structure of a black pigment particle of the present invention and an X-ray diffraction pattern of the particle, respectively. The lightness L* of the black pigment particles of the present invention is 18 to 25, preferably 18 to 23, and the chroma C*ab = a*² + b*² is 0.2 to 4.0, preferably 0.2 to 2.0 (wherein L*, a* and b* are the values indicated by the (L*, a*, b*) uniform sensory colour space of CIE 1976). The black pigment particles of the present invention have a magnetization of about 0.5 to 5.0 emu/g, preferably 0.5 to 2.0 emu/g, and are almost non-magnetic. The pigment particles of the present invention also have an apparent density of 0.5 to 1.2 g/cm³, preferably 0.6 to 1.1 g/cm³. The size of the polycrystalline particles of the present invention is 0.05 to 2.0 µm, preferably 0.1 to 1.0 µm. When the particles of the present invention are used as colouring pigment particles for developing toner, it is preferable that the particles have an average size of 0.1 to 0.5 µm, considering facilitation of mixing and dispersion of the particles in resins. The polycrystalline particles having a mixed composition of Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution of the present invention are black in colour, non-magnetic, safe and harmless, easy to handle owing to an appropriate particle size and a high apparent density, and also have excellent heat resistance. The particle size of toner generally used at present is about 10 µm. There is, however, a tendency to decrease the particle size in conformity to enhance the performance of toner. This is is causing a complaint that the toner particles obtained by using carbon black with a low apparent density are hard to classify in the toner production process because of the small specific gravity thereof. The black pigment particles comprising polycrystalline particles of the present invention are free of such a problem because the toner particles obtained by using the said black pigment particles have a sufficiently large specific gravity owing to a high apparent density, for example 4.0 to 5.5 g/cm³, and are therefore easy to classify. The black pigment particles according to the present invention have excellent heat resistance. The colour thereof is scarcely changed even when they are exposed to a high temperature such as about 300°C. The black pigment particles of the present invention can be obtained by subjecting magnetite ( FeO x .FE₂O₃, wherein 0.5≦x≦1) particles having their surfaces coated with a titanium compound, a mixture of the magnetite particles and a titanium compound, or reduced particles obtainable by reducing hematite particles having their surface coated with a titanium compound, to heat treatment at a temperature of not less than 700°C in a non-oxidizing atmosphere, and pulverizing the particles thus obtained. Use of magnetite particles coated with a titanium compound as a starting material is preferred because it is easy to obtain the particles with low magnetization and such particles are essentially non-magnetic. The magnetite particles or hematite particles may be of any shape, for example granular, spherical or acicular. The size of the particles is from about 0.03 to 1.5 µm. There is correlation between the size of the starting particles and that of the product particles. Normally when using starting particles with a small size, the product particles are also small, and when using starting particles of a large size, there are obtained product particles of a large size. As the titanium compound coated on or mixed with the starting particles, there can be used hydrous oxides, hydroxides and oxides of titanium. When a titanium compound is mixed with magnetite particles, it is preferred to use a water-soluble titanium compound. The amount of titanium compound used in the particle preparation is 15.0 to 50.0 atom%, preferably 20 to 40 atom%, calculated as Ti, based on Fe(II) and Fe(III) in the magnetite particles. When less than 15.0 atom%, calculated as Ti, based on Fe(II) and Fe(III), is used the magnetization of the obtained black pigment particle increases. When more than 50.0 atom%, calculated as Ti, based on Fe(II) and Fe(III), is used though the desired non-magnetic black pigment particle can be obtained, no extra effect can be obtained. N₂ gas can, for example, be used as a non-oxidizing atmosphere in the heat treatment. When the atmosphere is oxidizing, the objective black pigment particles cannot be obtained. The amount of Ti in the polycrystalline particles of the present invention is 15 to 50 atom%, preferably 20 to 40 atom%, based on the total amount of Fe(II) and Fe(III) therein. The temperature for the heat-treatment is not less than 700°C, and is preferably 720 to 800°C. When the heat-treatment temperature is below 700°C, there is insufficient solid-phase reaction between the iron oxide and the titanium compound, and the desired black pigment particles cannot be obtained. Pulverization of the obtained particles can be accomplished by using a commonly employed pulverizer such as a ball mill, attritor or vibration mill. In the above process, the starting particles may be coated with a known sintering inhibitor before being subjected to the heat-treatment. In this case, sintering of the particle and between the particles in the heat-treatment is prevented, thereby obtaining black pigment particles having good dispersibility. A compound composed of at least one element selected from Al, Ti, Si, Zr and P and which does not impair the normal properties of the black pigment particles produced can be used as the sintering inhibitor. The amount of the sintering inhibitor element used in the present invention is 0.1 to 15.0 atom%, preferably 0.1 to 5.0 atom%, based on the amount of Fe and Ti. To obtain a satisfactory sintering inhibitive effect, it is preferable to add the sintering inhibitor in an amount of not less than 0.1 atom%. When its amount exceeds 15.0 atom%, magnetite is mixed in the produced black pigment particles, and as a result it is difficult to obtain the desired non-magnetic black pigment particles. The black pigment particles of the present invention suffer a decrease in lightness L* of not more than 3.0 and an increase in chroma C* of not more than 1.0 after the heat-treatment. These results demonstrate that the pigment particles remain almost free of colour change from black and have excellent heat resistance. The black pigment particles of the present invention are easy to handle and have excellent workability owing to their high apparent density,which is 0.5 to 1.2 g/cm³. Moreover, the pigment particles of the present invention are free of re-agglomeration and have good dispersibility as their magnetization is as low as 0.5 to 5.0 emu/g. As described above, the black pigment particles of the present invention are non-magnetic, safe and harmless, and also have excellent workability and heat resistance. Consequently they can be advantageously used as, for example, a developing toner, colouring material for paint or colouring material for coating resins. Examples The present invention is now further described in the following Examples. The average sizes of the particles shown in the following Examples and Comparative Examples are the average of the values determined from electron micrographs. The magnetization of the particles shown below are the ones determined in a magnetic field of 10 KOe. The ratio between Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution was shown by the value of strength ratio of main peaks on an X-ray spectrum commonly used as an index showing the existing proportions (by volume). For the L* value (lightness), a* value and b* value, the colour of each test sample was measured for L*, a* and b* by Hunter's Lab space by using amulti-light-source spectrocolorimeter MSC-IS-2D (mfd. by Suga Testing Machines Mfd. Co., Ltd.) and they (L*, a* and b*) were shown by the values indicated according to (L*, a*, b*) uniform sensory colour space of CIF 1976. Each test sample was prepared by kneading together 0.5 g of black pigment particles and 1.0 cm³ of castor oil by a Hoover muller to form a paste-like substance, then adding thereto 4.5 g of a clear lacquer, kneading them to form a coating material, and applying this coating material on a cast coated paper sheet by a 150µm (6-mil) applicator. Example 1 100 g of granular magnetite particles having an average particle size of 0.2 µm and a magnetization of 85.0 emu/g were mixed and dispersed in an aqueous solution containing 0.26 mol of TiOSO₄ (Ti/Fe = 20.0 atom%). This suspension was neutralized by adding NaOH, a hydroxide of Ti was deposited on the particle surfaces by adjusting the pH to 8, and then the obtained particles were filtered and dried. Fluorescent X-ray analysis of the obtained magnetite particles showed that they had their surfaces coated with a hydroxide of Ti and the amount of Ti(IV) in the magnetite particle was 21.0 atom% based on the sum of Fe(II) and Fe(III). 50 g of the granular magnetite particles thus obtained were heat-treated at 750°C under an N₂ gas stream for 120 minutes and then pulverized to obtain fine black particles. The black particles thus obtained had an average particle size of 0.25 µm as shown in the electron micrograph (x100,000) of Fig. 1 and were composed of a mixed composition of Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution as shown in the X-ray diffraction pattern of Fig. 2. In the diffraction pattern, peak A denotes Fe₂TiO₅ and peak B denotes Fe₂O₃-FeTiO₃ solid solution. The ratio of Fe₂TiO₅ to Fe₂O₃-FeTiO₃ solid solution, as determined by measuring the strength ratio of their respective main peaks A₂ and main X-ray peaks B₂, was 0.09 : 1. The black particles also had a lightness (L*) of 24.5, a chroma (C*ab = a*² + b*² )of 1.0, an apparent density of 0.8 g/cm³ and a magnetization of 0.6 emu/g. 2.0 g of these black particles were further heat-treated at 300°C in air for 60 minutes. The resulting product had a lightness L* of 21.7 and a chroma C* ab = a*² + b*² of 1.9, both of which were little different from those before the heat-treatment. Examples 2, 4 and 5 Black pigment particles were obtained by following the same procedure as Example 1 except for changing the starting particles, the kind, amount and coating method of the Ti compound, the amount of urea, enforcement of the treatment with a sintering inhibitor, the kind and amount of the sintering inhibitor, and the temperature and time of the heat treatment. The main producing conditions in the above Examples and the properties of the obtained products are shown in Table 1 and Table 2, respectively. Example 3 100 g of the same granular magnetite particles as used in Example 1 were mixed and dispersed in an aqueous solution containing 0.45 mol of TiOSO₄ (Ti/Fe = 35.0 atom%). To the suspension was added 2.72 mol of urea, the mixture was kept at 95°C under stirring for 180 minutes to hydrolyze the TiOSO₄, thereby depositing a hydroxide of Ti on the particle surfaces, and then the thus obtained particles were filtered and dried. Fluorescent X-ray analysis of the obtained granular magnetite particles showed that they had their surfaces coated with a hydroxide of Ti and the amount of Ti(IV) in the particle was 37.0 atom% based on the sum of Fe (II) and Fe(III). 50 g of the thus obtained granular magnetite particles coated with a hydroxide of Ti on the surfaces thereof were immersed in an aqueous solution containing 0.009 mol of Al₂(SO₄)₃, thereby depositing a hydroxide of aluminium on the hydroxide of Ti on the particle surfaces. The thus obtained particles were heat treated at 750°C in an N₂ gas stream for 120 minutes and pulverized to obtain the black particles. The main producing conditions in the above process and the properties of the obtained product are shown in Table 1 and Table 2, respectively. Referential Example 1 The same granular magnetite particles (lightness L* = 16.9, chroma C* ab = 1.10) as used in Example 1 were subjected to a heat resistance test in the same way as Example 1. The magnetite particles after the heat treatment had a lightness (L*) of 35.3 and a chroma (C* ab = a*² + b*² of 72.0, from which it is noted that the particles suffered a great colour variation in comparison with the particles before the heat-treatment, indicating poor heat resistance of these particles.

Claims: Black pigment particles comprising polycrystalline particles having a mixed composition of Fe₂TiO₅ and Fe₂O₃-FeTiO₃ solid solution, the Fe₂TiO₅ being present in an amount of 1 to 40 wt%, and the Ti being present in the polycrystalline particles in an amount of 15 to 50 atom% based on the total amount of Fe(II) and Fe(III). Particles according to claim 1, which have an apparent density of from 0.6 to 1.1 g/cm³. Particles according to claim 1 or 2, which have a lightness of L* of from 18 to 23, a chroma C* of from 0.2 to 2.0 and a magnetization of from 0.5 to 2.0 Am²/kg (0.5 to 2.0 emu/g). Particles according to any one of the preceding claims, wherein said polycrystalline particles have an average particle size of from 0.1 to 1.0 µm. Particles according to any one of the preceding claims, wherein the amount of Ti in said polycrystalline particles is from 20 to 40 atom% based on the total amount of Fe(II) and Fe(III). Particles according to any one of the preceding claims which have their surfaces coated with an oxide of at least one element selected from Al, Ti, Si, Zr and P. A process for the preparation of black pigment particles as claimed in any one of the preceding claims, which process comprises subjecting: (a) magnetite particles having their surfaces coated with a titanium compound, (b) a mixture of magnetite prticles and a titanium compound, or (c) particles obtainable by reducing hematite particles having their surfaces coated with a titanium compound, to heat treatment at a temperature of not less than 700°C in a non-oxidising atmosphere and pulverising the particles thus obtained. A developing toner comprising a resin and particles as claimed in any one of claims 1 to 6 having an average particle size of from 0.1 to 0.5 µm. A coating material comprising a vehicle and particles as claimed in any one of claims 1 to 6. A material according to claim 9, which is a paint.

Inventors: KATAMOTO TSUTOMU, KATAMOTO, TSUTOMU
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1990
Main classification: C09C1
IPC codes: C09C1, G03G9