Non-magnetic potassium ferrite pigments, process for preparing them and paint or resin containing them

Abstract: A non-magnetic pigment comprising plate-like potassium ferrite particles having an average diameter of 0.03 - 30.0 µm.

Description: The present invention relates to a process for producing non-magnetic plate-like potassium ferrite pigment particles. In recent years, in the manufacture of paints and the colouring of resins, a demand has emerged for pigment particles which exhibit improved dispersibility in vehicles or resins, improved workability and improved heat stability. This has arisen due to the desire to increase efficiency for the purpose of energy saving, and to improve the paint or resin properties. To improve dispersibility and workability, the pigment particles must be non-magnetic and plate-like, must be independently distributed, and must be of a suitable size. A particularly desirable particle diameter is from 0.03 to 30.0 µm. During the mixing of pigment particles with resins and injection-molding of the products, the temperature may rise to as high as about 300°C. It is therefore required that the mixture is stable at these temperatures without change of colour or degradation of properties. At the same time, with the recent improvement in cultural and living standards, it is no longer sufficient to improve the functional qualities of products; it is also necessary to give them an aesthetic, interesting external appearance. With regard to colour, for example, there is a strong demand for variegated hues and tones. The most typical example of a pigment with universal application is iron oxide powder. Hematite particles are widely used as a red-brown pigment, maghemite particles as a light brown pigment, and magnetite particles as a black pigment. Hematite particles, which are every well-known, are non-magnetic plate-like particles which are independently distributed and of a suitable size. The particles usually have a diameter of from 0.1 to 100 µm. Maghemite particles are magnetic, and they therefore reagglomerate easily. The particles are usually granular or acicular. When maghemite particles are heated at a temperature above 300 to 400°C, they begins to change into hematite, and the light brown colour of the maghemite particles, therefore, begins to show signs of red brown. Like maghemite particles, magnetite particles are magnetic and therefore tend to reagglomerate easily. They are also usually granular or acicular. When the magnetite particles are heated at a temperature above 200 to 300°C, the magnetite particles begin to change into maghemite, and the black colour starts to give way to light brown. At present, therefore, there is a strong demand for pigment powders with excellent dispersibility, workability and heat stability, and which also offer a variety of colours. The present invention provides a process for producing non-magnetic plate-like potassium ferrite pigment particles which have an average diameter of from 0.03 to 30.0 µm, the process comprising heat-treating, at a temperature of from 830 to 1200°C, a mixture of iron oxide particles, ferric oxide hydroxide particles or a mixture thereof, the said particles having an average particle diameter of from 0.003 to 30.0 µm, with potassium carbonate, from 0.5 to 300% by weight of at least one fusing agent which is an alkali metal chloride, sulfate, borate or silicate, and optionally boric acid as a further fusing agent, the mixing proportion of potassium carbonate with iron oxide particles and/or ferric oxide hydroxide particles being such that the proportion of K to Fe is not greater than 28 mol %. In one embodiment of the present invention the potassium ferrite particles contain Mn in the form of a solid solution and are dark brown in colour. In a further embodiment the potassium ferrite particles contain Mn and Al in the form of a solid solution and are dark brown in colour. The potassium ferrite particles may also contain Al in the form of a solid solution and are then a bright light brown colour. The particles which contain Mn may be prepared by a process as described above but wherein (i) Mn-containing iron oxide particles are used as the iron oxide particles, (ii) Mn-containing ferric oxide hydroxide particles are used as the ferric oxide hydroxide particles, or (iii) Mn compounds are added to the mixture, such that the quantity of Mn with respect to the total amount of Fe and Mn is from 0.1 to 20.0 atomic % and the proportion of K to Fe + Mn is not greater than 28 mol %. The particles which contain Mn and Al in the form of a solid solution may be prepared by a process as described above, but wherein (i) Mn and Al-containing iron oxide particles are used as the iron oxide particles, (ii) Mn and Al-containing ferric oxide hydroxide particles are used as the ferric oxide hydroxide particles, or (iii) Mn compounds and Al compounds are added to the mixture, such that the quantity of Mn with respect to the total amount of Fe, Mn and Al is from 0.1 to 20.0 atomic %, the quantity of Al with respect to the total amount of Fe, Mn and Al is not more than 60.0 atomic %, and the proportion of K to Fe + Mn + Al is not greater than 28 mol %. The particles which contain Al in the form of a solid solution may be prepared by a process as described above, but wherein (i) Al-containing iron oxide particles are used as the iron oxide particles, (ii) Al-containing ferric oxide hydroxide particles are used as the ferric oxide hydroxide particles are used as the ferric oxide hydroxide particles, or (iii) Al compounds are added to the mixture, such that the quantity of Al with respect to the total amount of Fe and Al is from 0.1 to 80.0 atomic % and the proportion of K to Fe + Al is not greater than 28 mol %. BRIEF EXPLANATION OF THE DRAWINGS Figs. 1 and 3 are electron micrographs showing the particle structure of the potassium ferrite particles obtained in Example 1 and Example 2 respectively. Fig. 2 is an X-ray diffraction pattern of the potassium ferrite particles obtained in Example 1. Fig. 4 is an electron micrograph (x 10,000) showing the particle structure of the potassium ferrite particles obtained in Example 11. Fig. 5 is an X-ray diffraction pattern of the potassium ferrite particles obtained in Example 11. Fig. 6 is an electron micrograph (x 50,000) showing the particle structure of the potassium ferrite particles obtained in Example 17. Fig. 7 is an X-ray diffraction pattern of the potassium ferrite particles obtained in Example 17. Pigments produced by the process of the present invention may be classified according to their colour as follows: (1) A non-magnetic liver brown pigment comprising plate-like potassium ferrite particles having an average particle diameter of from 0.03 to 30.0 µm. (2) A non-magnetic dark brown pigment comprising plate-like potassium ferrite particles having an average particle diameter of from 0.03 to 30 µm, containing Mn in the form of solid solution. (3) A non-magnetic dark brown pigment comprising plate-like potassium ferrite particles having an average particle diameter of from 0.03 to 30.0 µm, containing Mn and Al in the form of solid solution. (4) A non-magnetic bright light brown/orange-red pigment comprising plate-like potassium ferrite particles having an average particle diameter of from 0.03 to 30.0 µm, containing Al in the form of solid solution. The potassium ferrite particles produced by the process of the present invention are independent, are non-magnetic and heat-stable, and have a light brown, dark brown or bright light brown/orange-red colour. The potassium ferrite is composed of either potassium β'' ferrite (K₂O·nFe₂O₃, wherein n = 6.0 - 4.5), potassium β ferrite (KFe₁₁O₁₇), or a mixture of these two compounds. The potassium ferrite particles do not undergo any colour change when heated to a temperature of about 550°C, and have excellent heat-stability. In the Journal of Crystal Growth, Vol. 71 (1985), p. 253-255, for example, large monocrystalline particles of potassium ferrite of from 1 to 2 mm in size are disclosed; in Electrochemistry, Vol. 43 (1975), p. 273 - 277, a sintered body of potassium ferrite as a material for the study of ion conductivity is mentioned; and in JP-A-1 - 100027 (1989), a catalyst for production of styrene is described. There is, however, no description of the size, shape or colour of potassium ferrite particles for use as pigments, nor of the dispersibility and heat-stability thereof. The meaning of non-magnetic in the present invention is that the particles are effectively without any magnetization, more specifically that they have a magnetization of not greater than 2.0 emu/g. The meaning of plate-like in the present invention is that the particles are substantially thin, preferably that they have an aspect ratio (ratio of plate diameter: thickness) of from 3:1 to 20:1. In said light brown pigment (1), the potassium ferrite particles have a lightness L* of 30 - 45, a hue ϑ = tan⁻¹b*/a* of 33 - 55° , and a chroma C*ab = √ a*² + b*² of 15 - 27 [wherein L*, a* and b* are values represented by the (L*, a*, b*) uniform sensory colour space of the CIE, 1976]. In said dark brown pigment (2), the potassium ferrite particles containing Mn in the form of solid solution have a lightness L* of 20 - 38, a hue ϑ = tan⁻¹b*/a* of 20 - 50° , and a chroma C*ab = √ a*² + b*² of 1 - 30 [wherein L*, a* and b* are values represented by the (L*, a*, b*) uniform sensory colour space of the CIE, 1976]. In said dark brown pigment (3), the potassium ferrite particles containing Mn and Al in the form of solid solution have a lightness L* of 20 - 40, a hue ϑ = tan⁻¹b*/a* of 20 - 50° , and a chroma C*ab = √ a*² + b*² of 1 - 30 [wherein L*, a* and b* are values represented by the (L*, a*, b*) uniform sensory colour space of the CIE, 1976]. In said bright light brown/orange-red pigment (4), the potassium ferrite particles containing Al in the form of solid solution have a lightness L* of 30 - 48, a hue ϑ = tan⁻¹b*/a* of 30 - 50° , and a chroma C*ab= √ a*² + b*² of 15 - 50 [wherein L*, a* and b* are values represented by the (L*, a*, b*) uniform sensory colour space of the CIE, 1976]. If the particles are of approximately the same shape and, in particular, the same size, the orange colour increases and tends to become lighter as the amount of Al in the form of solid solution increases, so that a bright light brown/orange-red colour is obtained. As an iron starting material in the process of the present invention, iron oxide particles such as hematite, magnetite and maghemite, and/or ferric oxide hydroxide particles such as goethite, acagenite or lepidochrosite, both of average particle size of from 0.003 to 30.0 µm may be used. If the average particle size exceed 30.0 µm, the potassium ferrite particles having the particle size specified in the present invention are not obtained. There is a correlation between the size of iron oxide particles or ferric oxide hydroxide particles as the starting material and the size of potassium ferrite particles produced, the size of potassium ferrite particles tending to increase in proportion to the size of the starting material. In the present invention, the mixing proportion of iron oxide particles and/or ferric oxide hydroxide particles with potassium carbonate is such that the proportion of K to Fe, Fe + Mn, Fe + Mn + Al or Fe + Al is less than 28 mol % and is preferably from 18 to 28 mol %. If it is less than 18 mol %, hematite particles come to be mixed with potassium ferrite particles. If it is greater than 28 mol %, very unstable KFeO₂ is produced instead of potassium ferrite. Examples of the Mn compound used in the present invention are manganese dioxide and manganese tetraoxide. Examples of the Al compound used in the present invention are AlOOH, Al(OH)₃, γ-Al₂O₃ and η-Al₂O₃. In the case of said dark brown pigment (2), the Mn content of the iron oxide particles or of the ferric oxide hydroxide particles, or the amount of Mn compounds added to the mixture is such that the quantity of Mn in the heat-treated product is from 0.1 to 20.0 atomic % with respect to the total amount of Fe and Mn. If it is less than 0.1 atomic %, a non-magnetic dark brown plate-like pigment powder is not obtained, and if it is greater than 20.0 atomic %, compounds other than potassium ferrite are produced. In the case of said dark brown pigment (3), the Mn content of the iron oxide particles or of the ferric oxide hydroxide particles, or the amount of Mn compounds added to the mixture, is such that the quantity of Mn in the heat-treated product is from 0.1 to 20.0 atomic % with respect to the total amount of Fe and Mn. The Al content of the iron oxide particles or of the ferric oxide hydroxide particles, or the amount of Al compounds added to the mixture, is such that the quantity of Al in the heat-treated product is not more than 60.0 atomic % with respect to the total amount of Fe, Mn and Al. If it is greater than 60.0 atomic %, the chroma of the potassium ferrite particles obtained increases and a dark brown pigment is difficult to obtain. In the case of said bright light brown/orange-red pigment (4), the Al content of the iron oxide particles or of the ferric oxide hydroxide particles, or the amount of Al compounds added to the mixture, is such that the quantity of Al in the heat-treated product is from 0.1 to 80.0 atomic % with respect to the total amount of Fe and Al. If it is less than 0.1 atomic %, potassium ferrite particles containing Al in the form of solid solution are obtained, but they do not perform adequately as a bright light brown pigment. If it is greater than 80.0 atomic %, potassium ferrite particles containing Al in the form of solid solution are obtained, but they have a lower tinting strength and hiding power, which is undesirable for a pigment. The fusing agent used in the process of the present invention is one or more of an alkali metal chloride, sulfate, borate or silicate, and boric acid may also be used, if necessary. With alkali metal sulfates and silicates, as compared to alkali metal chlorides, the potassium ferrite tends to be finer, and this is particularly useful when it is desired to obtain finer particles. With alkali metal borates, as compared to alkali metal chlorides, the growth of potassium ferrite particles tends to be promoted. By combining these various fusing agents, therefore, the size of the potassium ferrite particles produced can be freely controlled depending on the objective. If boric acid is used alone in the present invention, potassium ferrite is not produced. If, however, it is used in conjunction with the alkali metal chlorides, sulfates, borates and silicates, the growth of potassium ferrite particles tends to be promoted. Suitable alkali metal chlorides for this purpose are KCl and NaCl. Suitable alkali metal sulfates for this purpose are K₂SO₄ and Na₂SO₄. Suitable alkali metal borates for this purpose are potassium borate and sodium borate. Suitable alkali metal silicates for this purpose are potassium silicate and sodium silicate. The amount of fusing agents added to the system is from 0.5 to 300 % by weight with respect to: (i) the total amount of iron oxide particles or ferric oxide hydroxide particles and potassium carbonate; (ii) the total amount of iron oxide particles or ferric oxide hydroxide particles and Mn compounds, or Mn compounds and Al compounds, or (iii) the total amount of iron oxide particles or ferric oxide hydroxide particles, Al compounds and potassium carbonate. If it is less than 0.5 % by weight, inter-particle sintering takes place with the result that an independent distribution of potassium ferrite particles is not obtained. If it is greater than 300 % by weight, an independent distribution of potassium ferrite particles is obtained; however there is no reason to add any more fusing agent than is necessary. The heat-treating temperature used in the process of the present invention is from 830 to 1200°C. If it less than 830°C, potassium ferrite is not produced. If it is greater than 1200°C, the fusing agents volatilize, and inter-particle sintering takes place. The quantity of Mn in the form of solid solution is from 0.1 to 20.0 atomic % with respect to the total amount of Fe and Mn. If it is less than 0.1 atomic %, the chroma increases and a dark brown pigment is difficult to obtain. If it is more than 20.0 atomic %, compounds other than potassium ferrite come to be mixed with the product. The quantity of Al in the form of solid solution with Mn in the form of the solid solution in the present invention is not more than 60.0 atomic % with respect to the total amount of Fe, Mn and Al. If it is more than 60.0 atomic %, the chroma of the potassium ferrite obtained increases, and a dark brown pigment is difficult to obtain. The quantity of Al in the form of solid solution in the present invention is from 0.1 to 80.0 atomic % with respect to the total amount of Fe and Al. If it less than 0.1 atomic %, the bright light-brown pigment of the present invention is not obtained. If it is more than 80.0 atomic %, the potassium ferrite particles containing Al in the form of solid solution have a lower tinting strength and hiding power, which is undesirable for a pigment. Said non-magnetic pigment (1) has a lightness L* of 30 - 45, preferably 31 - 42 ; a hue ϑ of 33 - 55°, preferably 33 - 50°; a chroma C*ab of 15 - 27, preferably 17 - 25; and a magnetization of not more than 2.0 emu/g, preferably not more than 1.0 emu/g; and has a light brown colour. The potassium ferrite particles of the pigment (1) have an average particle diameter of from 0.03 to 30.0 µm, preferably from 0.1 to 20.0 µm; and an aspect ratio of from 3:1 to 20:1, preferably from 5:1 to 20:1. Said non-magnetic pigment (2) has a lightness L* of 20 - 38, preferably 20 - 35 ; a hue ϑ of 20 - 50°, preferably 25 - 45° ; a chroma C*ab of 1 - 30, preferably 1 - 20 ; and a magnetization of not more than 2.0emu/g, preferably not more than 1.0emu/g; and has a dark brown colour. The potassium ferrite particles of the pigment (2) have an average particle diameter of from 0.03 to 30.0 µm, preferably from 0.1 to 20.0 µm; and an aspect ratio of from 3:1 to 20:1, preferably from 5:1 to 20:1. The quantity of Mn in the form of solid solution with respect to the total amount of Fe and Mn is from 0.1 to 20.0 atomic %, preferably from 5.0 to 15.0 atomic %. Said non-magnetic pigment (3) has a lightness L* of 20 - 40, preferably 20 - 35 ; a hue ϑ of 20 - 50°, preferably 25 - 45° ; a chroma C*ab of 1 - 30, preferably 1 - 20; and a magnetization of not more than 2.0emu/g, preferably not more than 1.0 emu/g; and has a dark brown colour. The potassium ferrite particles of the pigment (3) have an average particle diameter of from 0.03 to 30.0 µm, preferably from 0.1 to 20.0 µm; and an aspect ratio of from 3:1 to 20:1, preferably from 5:1 to 20:1. The quantity of Mn in the form of solid solution with respect to the total amount of Fe, Mn and Al is from 0.1 to 20 atomic %, preferably from 5.0 to 15.0 atomic %; and the quantity of Al in the form of solid solution with respect to the total amount of Fe, Mn and Al is not more than 60 atomic %, preferably from 5.0 to 15.0 atomic %. Said non-magnetic pigment (4) has a lightness L* of 30 - 48, preferably 35 - 48 ; a hue ϑ of 30 - 50°, preferably 35 - 50°; a chroma C*ab of 15 - 50, preferably 25 - 50; and a magnetization of not more than 2.0 emu/g, preferably not more than 1.0 emu/g; and has a bright light brown/orange-red colour. The potassium ferrite particles of the pigment (4) have an average particle diameter of from 0.03 to 30.0 µm, preferably from 0.1 to 20.0 µm; and an aspect ratio of from 3:1 to 20:1, preferably from 5:1 to 20:1. The quantity of Al in the form of solid solution with respect to the total amount of Fe and Al is from 0.1 to 80 atomic %, preferably from 5.0 to 60.0 atomic %. As the non-magnetic pigment powder comprises non-magnetic plate-like particles which are independently distributed and of a suitable size, in particular of an average particle size of from 0.03 to 30.0 µm, it has excellent dispersibility, workability and heat stability. It is, therefore, highly useful as a colouring pigment powder for paints, printing inks and resins. Furthermore as the potassium ferrite particles are plate-like, hard and heavy, they may have application as an anti-vibration filler; and as the electron donating ability of K⁺ is greater than that of Na⁺, they may also have application as a catalyst in the synthesis of styrene monomer. The present invention is now further described in the following Examples. Examples : The average particle diameters given in the following Examples and Comparative Examples, are average values measured from electron micrographs. The L* (lightness), and values of a* and b*, were measured using colour measurement test samples and a multi-source colour spectrophotometer MSC-IS-2D (Suga Test Instruments K.K.) in the Hunter Lab Space, and represented in terms of the uniform sensory colour space of the Commission Internationale de l'Eclairage, CIE (1976). Test samples were obtained by kneading 0.5 g of potassium ferrite particles together with 1.0 cc of castor oil in a Hoover type muller to make a paste, adding 4.5 g of clear lacquer and kneading the mixture to make a paint, and coating the paint onto cast-coated paper using a 6-mil applicator. Example 1 : A mixture of 14.5 g of spherical magnetite particles having an average particle diameter of 0.02 µm, 1.73 g of K₂CO₃ and 1.6 g of KCl (the quantity of KCl with respect to total weight of magnetite and K₂CO₃ was 10% by weight) was heat-treated at 875°C in air for 1.0 hour. The obtained product was washed with water, and then dried, thereby obtaining light brown particles. As shown by the electron micrograph in Fig. 1 (x 10,000), the light brown particles were plate-like particles of an average particle diameter of 1.5 µm, and were independently distributed. The obtained particles had a lightness L* of 32.87, a hue ϑ = tan⁻¹b*/a* of 37.96° , and a chroma C*ab = √ a*² + b*² of 19.62 . Further, as seen from an X-ray diffraction pattern in Fig. 2, peaks corresponding to potassium β ferrite and potassium β'' ferrite were found. In Fig. 2, peak A corresponds to potassium β ferrite, peaks B correspond to potassium β'' ferrite, and peaks C are due to both potassium β ferrite and potassium β'' ferrite. By the application of an external magnetic field of 10 KOe, the magnetization was approx. 0.83 emu/g, which is roughly about the same as for hematite. The obtained light brown particles were further heated at 550°C for 2.0 hours to examine their heat-stability. The resulting light brown particles had a lightness L* of 32.87, a hue ϑ = tan ⁻¹b*/a* of 37.96° , and a chroma C*ab = √ a*² + b*² of 19.62 . This confirmed that there was substantially no colour change, and that the powder had excellent heat stability. Examples 2 - 10 : Potassium ferrite particles were produced in the same way as in Example 1, except that the kinds and amounts of iron oxide particles and ferric oxide hydroxide particles, the amount of K₂CO₃ used, the kinds and amounts of fusing agents, and the heat-treating temperature and time were varied. Examination of electron micrographs showed that the potassium ferrite particles obtained in Examples 2 - 10 were independently distributed. Fig. 3 shows the electron micrograph of the potassium ferrite particles obtained in Example 2 (x 40,000). Comparative Example 1 : A red-brown particle powder was obtained in the same way as in Example 1, except that B₂O₃ was used as a fusing agent. The X-ray diffraction pattern showed that this red brown powder was hematite, and that potassium ferrite was not produced. Example 11 : A mixture of 14.5 g of spherical magnetite particles having an average particle diameter of 0.02 µm, 2.03 g of K₂CO₃, 2.61 g of Mn₂O₃ [corresponding to 15.0 atomic % calculated as Mn/(Fe+Mn)], and 1.9 g of KCl (the quantity of KCl with respect to total weight of magnetite, K₂CO₃ and Mn₂O₃ was 10.0 % by weight), was heat-treated at 875°C in air for 1.0 hour. The obtained product was washed with water, and then dried, thereby obtaining dark brown particles. As shown by the electron micrograph in Fig. 4 (x 10,000), the dark brown particles were plate-like particles of an average particle diameter of 1.5 µm, and were independently distributed. As a result of fluorescent X-ray spectroscopy, the quantity of Mn in the form of solid solution was found to be 15 atomic %. The obtained particles had a lightness L* of 24.0, a hue ϑ = tan⁻¹b*/a* of 37.0 , and a chroma C*ab = √ a*² + b*² of 3.2 . Furthermore, as seen from X-ray diffraction pattern in Fig. 5, peaks corresponding to potassium β'' ferrite was revealed. By the application of an external magnetic field of 10 KOe, the magnetization was approx. 0.89 emu/g, which is roughly about the same as for hematite. The obtained dark brown particles were further heated at 550°C for 2.0 hours to examine their heat-stability. The resulting dark brown particles had a lightness L* of 24.5, a hue ϑ = tan⁻¹b*/a* of 37.2 , and a chroma C*ab = √ a*² + b*² of 3.3 . This confirmed that there was substantially no colour change, and that the particles had excellent heat-stability. Examples 12 - 16 : Potassium ferrite particles were produced in the same way as in Example 11, except that the kinds and amounts of iron oxide particles and ferric oxide hydroxide particles, the Mn content, Al content and amounts used, the kinds and mixing amounts of Mn compounds, the kinds and mixing amounts of Al compounds, the kinds, proportions and amounts of fusing agents, and the heat-treating temperature and time, were varied. Tables 3 to 5 show the principal manufacturing conditions and properties of the products obtained. Examination of electron micrographs showed that the potassium ferrite particles obtained in Examples 12 to 16 were independently distributed. Comparative Example 2 : Dark brown particles were obtained in the same way as in Example 11, except that B₂O₃ was used as a fusing agent. The X-ray diffraction pattern of the obtained dark brown particles showed no peak due to potassium ferrite, and therefore that potassium ferrite was not produced. Example 17 : A mixture of 6.0 g of spherical magnetite particles having an average particle diameter of 0.02 µm, 2.03 g of K₂CO₃, 11.2 g of Al(OH)₃ [corresponding to 65 atomic % calculated as Al/(Fe+Al)] and 1.9 g of KCl (the quantity of KCl with respect to total weight of magnetite, Al(OH)₃ and K₂CO₃ is 10.0 % by weight), was heat-treated at 875°C in air for 1.0 hour. The obtained product was washed with water, and then dried, thereby obtaining bright light brown particles. As shown by the electron micrograph in Fig. 6 (x 50,000), the bright light brown particles were plate-like particles of an average particle diameter of 0.2 µm, and were independently distributed. As a result of fluorescent X-ray spectroscopy, the quantity of Al in the form of solid solution was found to be 65 atomic %. The obtained particles had a lightness L* of 44.1, a hue ϑ = tan⁻¹b*/a* of 45.3° , and a chroma C*ab = √ a*² + b*² of 48.0 . Furthermore, the X-ray diffraction pattern in Fig. 7 revealed peaks corresponding to the structure of potassium β'' ferrite. By the application of an external magnetic field of 10 KOe, the magnetization thereof was approx. 0.33 emu/g, which is roughly the same as for hematite. The obtained bright light brown particles were further heated at 550°C for 2.0 hours to examine their heat stability. The resulting bright light brown particles had a lightness L* of 44.2, a hue ϑ = tan⁻¹b*/a* of 45.3° , and a chroma C*ab = √ a*² +b*² of 48.3 . This confirmed that there was substantially no colour change, and that the powder had excellent heat-stability. Examples 18 - 22 : Potassium ferrite particles were produced in the same way as in Example 17, except that the kinds of iron oxide particles and ferric oxide hydroxide particles, the Al content and amount used, the kinds and mixing amounts of Al compounds, the kinds and amounts of fusing agents, and the heat-treating temperature and time, were varied. Tables 6 and 7 show the principal manufacturing conditions and properties of the products obtained. Examination of electron micrographs showed that the potassium ferrite particles obtained in Examples 18 to 22 were independently distributed. Comparative Example 3 : Red-brown particles were obtained in the same way as in Example 17, except that B₂O₃ was used as a fusing agent. The X-ray diffraction pattern of the obtained red-brown particles showed no peak due to potassium ferrite, and therefore that potassium ferrite was not produced. Examples Fusing agent Heat-treatment Kinds Weight ratio Amount (% by weight) Temperature (°C) Time (hrs) 11 KCl - 10 875 1.0 12 K₂SO₄+ KCl 1:1 10 875 1.0 13 KCl - 10 875 0.5 14 KCl+K₂SiO₃ 10:1 10 875 1.0 15 KCl+NaCl 10:1 10 875 1.0 16 KCl+B₂O₃ 10:1 30 890 1.0

Claims: A process for producing non-magnetic plate-like potassium ferrite pigment particles which have an average diameter of from 0.03 to 30.0 µm, the process comprising heat-treating, at a temperature of from 830 to 1200°C, a mixture of iron oxide particles, ferric oxide hydroxide particles or a mixture thereof, the said particles having an average particle diameter of from 0.003 to 30.0 µm, with potassium carbonate, from 0.5 to 300% by weight of at least one fusing agent which is an alkali metal chloride, sulfate, borate or silicate, and optionally boric acid as a further fusing agent, the mixing proportion of potassium carbonate with iron oxide particles and/or ferric oxide hydroxide particles being such that the proportion of K to Fe is not greater than 28 mol %. A process according to claim 1 wherein the particles produced have an aspect ratio of from 3:1 to 20:1. A process according to claim 1 or 2 wherein the particles produced have a magnetization of not more than 1.0 Am²/kg (emu/g). A process according to any one of claims 1 to 3, wherein the particles produced are a light brown colour and have a lightness L* of 30 - 45, a hue ϑ of 33 - 50° and a chroma C*ab of 15 - 27. A process according to any one of claims 1 to 3, wherein Mn-containing iron oxide particles and Mn-containing ferric oxide hydroxide particles are used as the iron oxide particles and ferric oxide hydroxide particles, or Mn compounds are added to said mixture, such that the quantity of Mn with respect to the total amount of Fe and Mn is from 0.1 to 20.0 atomic %, and the proportion of K to Fe + Mn is not greater than 28 mol %, thereby obtaining non-magnetic pigment particles which contain Mn in the form of a solid solution. A process according to claim 5, wherein the particles produced contain from 0.1 to 20.0 atomic % Mn with respect to the total amount of Fe and Mn. A process according to claim 5 or 6, wherein the particles produced are a dark brown colour and have a lightness L* of 20 - 38, a hue ϑ of 20 - 50°, and a chroma C*ab of 1 - 30. A process according to any one of claims 1 to 3, wherein Mn and Al-containing iron oxide particles and Mn and Al-containing ferric oxide hydroxide particles are used as the iron oxide particles and ferric oxide hydroxide particles, or Mn and Al compounds are added to said mixture, such that the quantity of Mn with respect to the total amount of Fe, Mn and Al is from 0.1 to 20.0 atomic %, the quantity of Al with respect to the total amount of Fe, Mn and Al is not more than 60.0 atomic %, and the proportion of K to Fe + Mn + Al is not greater than 28 mol %, thereby obtaining non-magnetic pigment particles which contain Mn and Al in the form of a solid solution. A process according to claim 8, wherein the particles produced contain from 0.1 to 20 atomic % Mn and not more than 60 atomic % Al, each with respect to the total amount of Fe, Mn and Al. A process according to claim 8 or 9, wherein the particles produced are a dark brown colour and have a lightness L* of 20 - 40, a hue ϑ of 20 - 50°, and a chroma C*ab of 1 - 30. A process according to any one of claims 1 to 3, wherein Al-containing iron oxide particles and Al-containing ferric oxide hydroxide particles are used as the iron oxide particles and ferric oxide hydroxide particles, or Al compounds are added to said mixture, such that the quantity of Al with respect to the total amount of Fe and Al is from 0.1 to 80.0 atomic % and the proportion of K to Fe + Al is not greater than 28 mol %, thereby obtaining non-magnetic pigment particles which contain Al in the form of a solid solution. A process according to claim 11, wherein the particles produced contain from 0.1 to 80.0 atomic % Al with respect to the total amount of Fe and Al. A process according to claim 11 or 12, wherein the particles produced are a bright light brown colour and have a lightness L* of 30 - 48, a hue ϑ of 30 - 50°, and a chroma C*ab of 15 - 50. A process according to any one of claims 1 to 13 which further comprises incorporating the particles in a paint, printing ink or resin.

Inventors: KATAMOTO TSUTOMU, TAMARI KOUSAKU, KATAMOTO, TSUTOMU, TAMARI, KOUSAKU
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1989
Main classification: C09C1
IPC codes: C09D7, C01G49, C09C1