JP2005120187A - Black pigment particle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a black pigment particle which, even if discarded, little makes the environmental load and also little affects the human body. <P>SOLUTION: The black pigment particle has a non-magnetic core particle consisting of a light element with an atomic number of ≤22, an isoelectric point control layer that covers the core particle, and an oxide layer at least containing iron which covers the intermediate layer. The isoelectric point control layer is the one in which its isoelectric point is lower than each isoelectric point of the core particle and the oxide layer. It is preferable that the core particle comprises a complex hydroxide containing Mg and Al, that the charge control layer comprises an oxide of Si, and that the oxide layer comprises magnetite. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to black pigment particles. More specifically, the present invention relates to black pigment particles having a specific element composition and suitable mainly for paints, inks, toners, rubbers and plastics. The black pigment particles of the present invention are particularly suitable for non-magnetic toners that substitute for carbon black and for coloring engineering plastics that require high-temperature kneading.

  Black pigments used for paints, inks, toners, rubbers / plastics, etc. are required to be excellent in properties such as blackness, hue, coloring power, hiding power and the like and inexpensive. Currently, carbon black, iron oxide pigments such as magnetite, and other complex oxide pigments are used as black pigments depending on the application.

  Recently, in any of the aforementioned fields, demands other than high performance and high quality are increasing. Carbon black is refrained from use in view of environmental problems and effects on the human body. On the other hand, iron oxide pigments obtained by a wet oxidation reaction typified by magnetite are required to be low or non-magnetized because they have relatively high magnetization, although there are few problems like carbon black. It is unsuitable for use.

  As a technique relating to a black pigment for improving these problems, composite oxide pigments containing various inorganic components have recently attracted attention (see Patent Documents 1 and 2). For example, Patent Document 2 discloses a black pigment made of a complex oxide of cobalt, manganese, and iron.

Japanese Patent Publication No. 9-25126 Japanese Patent Laid-Open No. 10-231441

  However, since the composite metal oxide pigment is a material in which the main metal component is positioned as an environmental load substance such as cobalt or manganese, the problem of environmental load has not been solved. This fact is clear from the fact that these main metals themselves or their compounds are designated as specific chemical substances in accordance with the establishment of the domestic laws of the Environmentally Harmful Substances Release and Transfer Registration System (PRTR). In addition to such complex oxide pigments, even if a suitable core material particle is simply coated with a black material to obtain a pigment particle, such a particle causes the coating to peel off from the core material. Is likely to occur. In short, it is difficult to obtain an excellent black pigment while eliminating the cyclic loading substance as much as possible, even if it takes one blackness which is a severe characteristic of low magnetization or non-magnetic black pigment, and satisfactory materials are The reality is that it has not been found yet.

  Accordingly, an object of the present invention is to provide black pigment particles that are suitable mainly as black pigments for paints, inks, toners, rubbers and plastics, have low ring load, excellent blackness, and low magnetization. There is.

The present invention comprises a nonmagnetic core particle comprising a light element having an atomic number of 22 or less, an isoelectric point control layer covering the core particle, and an oxide layer containing at least iron and covering the intermediate layer. Black pigment particles having,
The isoelectric point control layer achieves the object by providing black pigment particles whose isoelectric points are lower than the isoelectric points of the core particles and the oxide layer.

  The present invention also provides an electrophotographic toner using the black pigment particles.

  Furthermore, the present invention provides an image forming method by an electrophotographic method using the above-described electrophotographic toner.

  Since the black pigment particles of the present invention do not contain a transition metal element or heavy metal element other than iron, even if discarded, the environmental load is small and the influence on the human body is very small. Moreover, it can be manufactured at low cost and is economically advantageous. Furthermore, the iron oxide layer is firmly fixed to the particles, and the peeling is unlikely to occur.

  Hereinafter, the present invention will be described based on preferred embodiments thereof. The black pigment particles of the present invention include non-magnetic core particles, an isoelectric point control layer that covers the core particles, and an oxide layer that contains at least iron and covers the intermediate layer (hereinafter referred to as iron oxide for convenience). At least a three-layer structure.

  The core particle is not particularly limited as long as it is a non-magnetic particle composed of a light element having an atomic number of 22 or less. The reason why a light element is used as the core particle in the present invention is as follows. Black pigment particles containing transition metal elements and heavy metal elements tend to have a high specific gravity due to their large molecular weight. Therefore, if a sufficient amount of black pigment particles are used for coloring, there is a disadvantage that the weight of the entire black pigment particles to be used is inevitably increased. On the other hand, by using a light element as the core particle, it is possible to reduce the weight of the pigment particle necessary for coloring the same level as the black pigment particle containing a heavy metal element or the like. That is, the black pigment particles of the present invention have a light specific gravity than conventional black pigment particles.

  Examples of the light elements used for the core particles include Li, Be, C, O, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti and the like when listed in order of atomic number. Examples of the constituent material of the core particles composed of these light elements include hydroxides, oxides, carbonates, sulfates, and phosphates of these light elements.

Examples of the hydroxide include composite hydroxides containing Mg and Al (so-called hydrotalcite compounds), composite hydroxides containing Ca and Al, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, Ca and Si. And a composite hydroxide containing Mg, Al, and Si. Examples of the oxide include magnesium oxide, aluminum oxide, calcium oxide, silicon dioxide, titanium dioxide, beryllium oxide, scandium oxide, Na ₂ O · 11Al ₂ O ₃ , K ₂ O · 11Al ₂ O ₃ , Li ₂ O · 11Al. ₂ O ₃ , a mixture of these oxides, a composite oxide containing Mg and Al, a composite oxide containing Ca and Al, a composite oxide containing Mg, Al and Si, a composite oxide containing Ca, Al and Si, Examples include composite oxides containing Mg, Al and Ti, composite oxides containing Ca, Al and Ti. Examples of the carbonate include magnesium carbonate and calcium carbonate. Examples of the sulfate include aluminum sulfate, calcium sulfate, titanyl oxide and the like. Examples of the phosphate include calcium phosphate (so-called apatite compounds). Of these, composite hydroxides containing Ca and Al, composite hydroxides containing Mg and Al (so-called hydrotalcites) in terms of ease of shape control, light specific gravity, availability, and cost ) Is preferably used.

  The shape of the core particles affects the shape of the black pigment particles finally obtained. For example, when the core particles have a plate shape, the resulting black pigment particles also have a plate shape. Similarly, when the core particles are spherical, the resulting black pigment particles are also spherical. When the core particles are spherical, the average particle size is preferably about 30 to 250 nm, particularly about 50 to 150 nm, from the viewpoint of good dispersibility of the particles after coating the iron coating layer. On the other hand, when the core particles are plate-like, the average plate diameter is preferably 100 to 1000 nm, particularly about 350 to 800 nm, and the plate thickness is preferably about 5 to 150 nm, particularly about 10 to 130 nm. The shape of the core particles depends on the pH of the reaction solution at the time of preparation. When the pH is relatively low, plate-like particles are obtained, and when the pH is relatively high, spherical particles are obtained. The particle diameter, plate diameter, and plate thickness of the particles were calculated from the ferret diameter after taking a 40,000 times photograph with a scanning electron microscope.

  The isoelectric point control layer covering the core particles is directly formed on the surface of the core particles. The isoelectric point control layer is used for successfully and stably forming the iron oxide layer. More specifically, when the iron oxide layer is directly formed on the surface of the core particle, if the isoelectric point of the core particle is close to the isoelectric point of the iron oxide layer, the core particle and the iron oxide layer Due to the electrical repulsion, it is extremely difficult to successfully form an iron oxide layer. Moreover, even if the iron oxide layer can be formed, it becomes unstable, and the peeling easily occurs. On the other hand, by forming an isoelectric point control layer on the surface of the core particle, the isoelectric point on the surface of the particle on which the iron oxide layer is formed (that is, the particle on which the isoelectric point control layer is formed on the core particle) However, it becomes lower than the isoelectric point of the core particle itself. As a result, the electric repulsion described above is less likely to occur, and the iron oxide layer is successfully formed. Further, the formed iron oxide layer becomes stable, and its peel resistance is improved.

From the above viewpoint, the isoelectric point of the isoelectric point control layer needs to be lower than the isoelectric points of the core particle and the iron oxide layer. In particular, the isoelectric point of the isoelectric point control layer is preferably at least 1 to 3 lower, particularly 3 to 7 or more lower than the lower isoelectric point of the core particles and the iron oxide layer. The isoelectric point is measured as follows. That is, the measurement solution is obtained by adjusting the sample to 0.01N KNO ₃ aqueous solution so that the sample concentration becomes 5% by weight. Using an ultrasonic zeta potential measurement device DT-1200 (manufactured by Dispersion Technology), the measurement solution is titrated to determine the zeta potential. For titration, 1N HNO ₃ or 1N KOH is used. The pH when the zeta potential is 0 mV is taken as the isoelectric point.

  If the relationship between the core particles and the iron oxide layer and the isoelectric point is as described above, the constituent material of the isoelectric point control layer is not particularly limited. However, as described above regarding the core particles, the black pigment particles of the present invention can be made to have a lighter specific gravity by constituting the isoelectric point control layer from light elements having an atomic number of 22 or less. From this point of view, the isoelectric point control layer is nonmagnetic and is preferably made of an oxide or hydroxide of Si, P, B, Al, or Ti.

Examples of the oxide include Si oxide (specifically SiO _{2 and the} like), Ti oxide (specifically TiO _{2 and the} like), Al oxide (specifically γ-AlOOH), B Oxides (specifically, B ₂ O _{3 and the} like) and P oxides (specifically, P ₄ O _{10 and the} like). Examples of the hydroxide include Al hydroxide (specifically α-Al (OH) ₃ ) and the like. Among these, it is preferable to use an oxide of Si, Al or Ti or an hydroxide of Al from the viewpoint of availability and cost.

  The isoelectric point control layer preferably has a thickness of about 0.1 to 50 nm, particularly about 1 to 30 nm, from the viewpoint of effectively performing isoelectric point control that can successfully form an iron oxide layer. .

The iron oxide layer covering the isoelectric point control layer is directly formed on the surface of the isoelectric point control layer. The iron oxide layer is a layer that gives blackness to the black pigment particles of the present invention. From this viewpoint, the iron oxide layer is preferably made of black iron-containing oxides such as magnetite, ilmenite (FeTiO ₃ ), and hercinite (FeAl ₂ O ₄ ). It is particularly preferable to use magnetite. When magnetite is used, the FeO content is preferably 12 to 18% by weight, more preferably 15 to 18% by weight from the viewpoint of further increasing the blackness.

  The black pigment particles of the present invention are preferably spherical or plate-like. By making it spherical, there is an advantage that the dispersibility of the black pigment particles of the present invention is improved. On the other hand, the coloring power can be increased by using a plate shape.

  From the viewpoint of increasing the blackness of the black pigment particles of the present invention, the iron oxide layer preferably has a thickness of 30 to 300 nm, particularly about 50 to 150 nm. Regarding the particle size, when the black pigment particles are spherical, the average particle size is preferably about 100 to 400 nm. On the other hand, in the case of a plate shape, the average plate thickness is preferably about 100 to 400 nm, and the average plate diameter is preferably about 200 to 1200 nm. If the size of the black pigment particles is about this level, the blackness can be effectively increased. In the case of plate-like particles, the dispersibility can be improved by the plate diameter and the plate thickness being within the above ranges.

  From the viewpoint of further increasing the blackness of the black pigment particles, when the black pigment particles are spherical, the average particle size is particularly preferably about 190 to 385 nm. On the other hand, in the case of a plate shape, it is particularly preferable that the average plate thickness is about 190 to 385 nm and the average plate diameter is about 300 to 800 nm. The reason is as follows. In general, in order to block visible light, if there is a substance with a thickness of ½ wavelength or more of the light, the light is canceled by the fixed-end reflection that occurs at the boundary between the inside and outside of the substance. Can be increased. Considering that the wavelength of visible light is from 380 nm to 770 nm, it is preferable that the average particle diameter (in the case of a spherical shape) or the average plate thickness (in the case of a plate shape) of the black pigment particles of the present invention is from 190 nm to 385 nm. At the same time, it is preferable to form a black iron oxide layer as the outermost layer of black pigment particles. In view of the average particle diameter of the core particles and the thickness of the isoelectric point control layer in the black pigment particles of the present invention, the particularly preferable thickness of the iron oxide layer is as described above. In the case of a plate, if the average plate diameter is within the above-mentioned particularly preferable range, dispersibility and coloring power are further improved when used as a pigment.

  As described above, the black pigment particles of the present invention have a light specific gravity mainly composed of light elements. Specifically, the true specific gravity is preferably 2.5 to 4.5, particularly preferably 3.5 to 4.5, and this value is compared with the specific gravity (about 5.2) of magnetite generally used as a black pigment. Small. Furthermore, compared with the true specific gravity of the copper-manganese-iron black pigment and the cobalt-manganese-iron black pigment described in the section of the prior art being about 5 to 7, the black pigment particles of the present invention It is clear that the specific gravity is small. The true specific gravity was measured in a room temperature 20 ° C. environment using a multi-volume density meter 1305 manufactured by Shimadzu Corporation.

The black pigment particles of the present invention preferably have a saturation magnetization at 79.6 kA / m of 10 to 50 Am ² / kg, particularly 10 to 45 Am ² / kg. If the saturation magnetization is within this range, sufficient blackness can be obtained, and even in an environment where a magnetic field is applied, it can be used without being affected by the magnetic field. In the black pigment particles of the present invention, the core particles are non-magnetic, and the isoelectric point control layer is preferably non-magnetic, so that the saturation magnetization can be easily in the above range.

The black pigment particles of the present invention preferably have a BET specific surface area of 10 to 100 m ² / g, particularly 15 to 60 m ² / g. If the BET specific surface area is in this range, the average particle diameter of the black pigment particles is not excessive and sufficient coloring power is expressed. Conversely, the average particle size is not too small, and the dependence on the environment can be reduced. In order to set the BET specific surface area within the above range, for example, the particle size of the black pigment particles may be adjusted as appropriate, or the thickness of the isoelectric point control layer and the thickness of the iron oxide layer may be adjusted.

  The black pigment particles of the present invention have a high blackness because they have the above-described configuration. Specifically, in the measurement of the blackness and hue of powder according to JIS K5101-1991, the L value by a color difference meter is preferably 25 or less, more preferably 22 or less, and the blackness is high. The a value is preferably 0.5 or less, and the b value is preferably 0.5 or less.

As an example of the black pigment particles of the present invention, the core particles are made of a composite hydroxide containing Mg and Al, the isoelectric point control layer is made of Si oxide, and the iron oxide layer is made of magnetite. It is done. In this case, the amount of Mg in the black pigment particles is 5 to 17% by weight, in particular 5 to 10% by weight, and the amount of Al is 2 to 8% by weight, in particular 2.5 to 5% by weight, This is preferable because sufficient blackness can be obtained. Moreover, it is preferable also from the point which can fully coat | cover a core particle with an iron oxide layer. If it cannot be coated, sufficient blackness may not be obtained. If the iron oxide layer is coated more than necessary, the composition of the black pigment particles will be close to Fe ₃ O ₄ and the saturation magnetization of the black pigment particles will be high, making it suitable for use under the influence of a magnetic field. It will disappear. The weight ratio of Mg to Al is preferably Mg / Al = 1.25 to 4.5, particularly 2 to 3 from the viewpoint of shape control of the core particles. The amount of Si is preferably 0.5 to 3.5% by weight, particularly 1 to 2% by weight, because the iron oxide layer can be successfully formed and the dependence on the environment can be reduced. The amount of Fe is preferably 30 to 60% by weight, particularly 35 to 55% by weight from the viewpoint that sufficient blackness can be obtained and saturation magnetization can be kept low.

  The black pigment particles can be produced, for example, by the following method. First, core particles made of a composite hydroxide containing Mg and Al are prepared. 1 mol of sodium carbonate is added to 1 liter of hot water at 50 ° C., and the temperature is raised to 95 ° C. To this is added 120 ml of a 12.5 mol / l aqueous sodium hydroxide solution. Further, a 4.23% by weight aluminum sulfate aqueous solution and a 1.25 mol / l magnesium sulfate aqueous solution are added. In this case, each aqueous solution is added so that 0.5 mol of aluminum sulfate and 1.25 mol of magnesium sulfate are added. Next, the pH of the reaction solution is adjusted to 10 using sodium hydroxide. The reaction solution is aged for 10 hours with slow stirring to produce core particles composed of a composite hydroxide containing Mg and Al. The core particles produced by this reaction are plate-like. In order to produce spherical core particles, the amount of sodium hydroxide aqueous solution added may be increased from 120 ml to 240 ml.

  Once the core particles are formed, 13.4% by weight aqueous sodium silicate solution is then added to the reaction solution. In this case, an aqueous solution is added so that 0.2 mol of sodium silicate is added. Next, the pH of the reaction solution is adjusted to 8 using dilute sulfuric acid to form an isoelectric point control layer made of Si oxide on the surface of the core particles.

Next, a 70 g / l iron (II) sulfate aqueous solution is added to the reaction solution. This aqueous solution also contains sodium hydroxide. In this case, the aqueous solution is added so that 0.25 mol of Fe ²⁺ is added. The concentration of sodium hydroxide is such that the pH of the reaction solution is maintained at 6.5 to 8.5. Air is blown into the reaction solution, and oxidation is continued until unreacted Fe ²⁺ is 1 g / l or less. The pH of the reaction solution changes due to the oxidation reaction. In that case, sodium hydroxide is additionally added so that the pH of the reaction solution is maintained at 6.5 to 8.5. The addition of this iron (II) sulfate aqueous solution and the Fe ²⁺ oxidation reaction are repeated a desired number of times. Thereby, an iron oxide layer having a desired thickness is formed on the surface of the isoelectric point control layer.

  In this way, the black pigment particles of the present invention are obtained. Next, the reaction solution is neutralized to a pH of about 6. The reaction solution is filtered to separate the solid and liquid, and the black pigment particles separated by filtration are dried and further pulverized to a desired particle size.

  The obtained black pigment particles can sufficiently exhibit the performance as a black pigment only by being subjected to a drying process for removing moisture. However, since the manufacturing method described above is a wet method, the blackness is somewhat affected by moisture. In order to further increase the blackness by eliminating the influence of moisture as much as possible and imparting high crystallinity, particles having an iron oxide layer formed on the isoelectric point control layer are placed in a non-oxidizing atmosphere. Heat treatment is effective.

  Examples of the non-oxidizing atmosphere include an atmosphere of hydrogen gas, carbon dioxide gas, nitrogen gas, inert gas, and the like. Of these atmospheres, it is preferable to use an atmosphere of hydrogen gas or nitrogen gas from the viewpoint of cost and availability. When hydrogen gas is used, it is preferable to perform heat treatment at a relatively low temperature of about 300 to 500 ° C. because hydrogen gas has strong reducing properties. Within this temperature range, it is easy to increase the blackness of the particles while preventing aggregation between the particles. On the other hand, when nitrogen gas is used, it is preferable to perform heat treatment at a relatively high temperature of about 800 to 1050 ° C. for the same reason as when hydrogen gas is used because nitrogen gas is not very reducible.

  As described above, according to the present invention, black pigment particles having a light specific gravity, high blackness, and low saturation magnetization can be obtained. Since the black pigment particles do not contain heavy metals, the environmental load is small and the influence on the human body is very small. The black pigment particles are economically advantageous because they can be produced at low cost.

  The black pigment particles of the present invention are suitably used as electrophotographic toner material powder, paint black pigment powder and the like. When used as an electrophotographic toner material powder, various images having excellent image characteristics such as image density, resolution, and gradation can be formed by an electrophotographic image forming method.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the scope of the present invention is not limited to such examples.

[Example 1]
(1) Production of non-magnetic core particles 1 mol of sodium carbonate was added to 1 liter of hot water at 50 ° C, and the temperature was raised to 95 ° C. To this was added 120 ml of a 12.5 mol / l sodium hydroxide aqueous solution. Further, a 4.23 wt% aluminum sulfate aqueous solution and a 1.25 mol / l magnesium sulfate aqueous solution were added. In this case, each aqueous solution was added so that magnesium and aluminum were added in the number of moles shown in Table 1, respectively. Next, the pH of the reaction solution was adjusted to 10 using sodium hydroxide. The reaction solution was aged with slow stirring for 10 hours to produce plate-like core particles composed of a composite hydroxide containing Mg and Al. The average plate diameter and plate thickness of the core particles are as shown in Table 1 (measurement method will be described later).

(2) Coating of isoelectric point control layer A 13.4 wt% sodium silicate aqueous solution was added to the reaction solution in which the core particles were formed. In this case, the aqueous solution was added so that the number of moles of silicon shown in Table 1 was added. Next, the pH of the reaction solution is adjusted to 8 using dilute sulfuric acid, and an isoelectric point control layer made of a material considered to be an oxide of Si, specifically SiO ₂ gel, is formed on the surface of the core particles. did. The thickness of the isoelectric point control layer is as shown in Table 1 (the measurement method is as described later).

(3) Coating of iron oxide layer A 70 g / l iron (II) sulfate aqueous solution was added to the reaction solution. This aqueous solution also contained sodium hydroxide. In this case, the aqueous solution was added so that Fe ²⁺ was added in the number of moles shown in Table 1. The concentration of sodium hydroxide was such that the pH of the reaction solution was maintained at 6.5 to 8.5. Air was blown into the reaction solution, and oxidation was continued until unreacted Fe ²⁺ was 1 g / l or less. The pH of the reaction solution changed due to the oxidation reaction. In that case, sodium hydroxide was additionally added so that the pH of the reaction solution was maintained at 6.5 to 8.5. The addition of this iron (II) sulfate aqueous solution and the Fe ²⁺ oxidation reaction were repeated several times. Thus, black pigment particles of the present invention in which an iron oxide layer having a thickness shown in Table 1 (the measurement method is as described later) was formed on the surface of the isoelectric point control layer were obtained. The iron oxide layer was composed mainly of magnetite.

[Example 2]
The numbers of moles of magnesium, aluminum, silicon, and iron (II) charged are shown in Table 1. In the production of the core particles, the amount of 12.5 mol / l sodium hydroxide aqueous solution added first was 240 ml. Other than these, black pigment particles were obtained in the same manner as in Example 1. The obtained core particles and black pigment particles were spherical.

Example 3
The black pigment particles obtained in Example 1 were heat-treated in a hydrogen gas atmosphere at 350 ° C.

Example 4
The black pigment particles obtained in Example 1 were heat-treated in a nitrogen gas atmosphere at 950 ° C.

Example 5
In Example 1, when the iron oxide layer was formed, iron (II) and Ti were added so as to have the charged mole numbers shown in Table 1. Other than this, black pigment particles were obtained in the same manner as in Example 1. Titanyl oxide was used as the Ti source. The iron oxide layer in the obtained black pigment particles mainly had an ilmenite structure. The particles were plate-like.

Example 6
In Example 1, when the iron oxide layer was formed, iron (II) and Al were added so as to have the number of moles charged as shown in Table 1. Other than this, black pigment particles were obtained in the same manner as in Example 1. Aluminum sulfate was used as the Al source. The iron oxide layer in the obtained black pigment particles mainly had a hercinite structure. The particles were plate-like.

Example 7
Black pigment particles were obtained in the same manner as in Example 1 except that plate-like Al ₂ O ₃ fine particles (manufactured by Kawai Lime Industry Co., Ltd.) were used as the core particles. At the time of use, 1 mol of Al ₂ O ₃ fine particles were dispersed in water to make a 2.5 liter dispersion, and the dispersion was dispersed for 1 minute using a homogenizer (IKRA ULTRA-TURRAX T50).

Example 8
Spherical CaO fine particles (manufactured by Hosokawa Micron Corporation) were used as the core particles. At the time of use, 1 mol of CaO fine particles were dispersed in water to make a 2.5 liter dispersion, and the dispersion was dispersed for 1 minute using a homogenizer (IKRA ULTRA-TURRAX T50). The number of moles of iron (II) charged during the formation of the iron oxide layer was set to the amount shown in Table 1. Other than this, black pigment particles were obtained in the same manner as in Example 1.

Example 9
Spherical TiO ₂ fine particles (manufactured by Hosokawa Micron Corporation) were used as the core particles. At the time of use, 1 mol of TiO ₂ fine particles were dispersed in water to make a 2.5 liter dispersion, and the dispersion was dispersed for 1 minute using a homogenizer (IKRA ULTRA-TURRAX T50). The number of moles of iron (II) charged during the formation of the iron oxide layer was set to the amount shown in Table 1. Other than this, black pigment particles were obtained in the same manner as in Example 1.

Example 10
In Example 1, an isoelectric point control layer was formed by adding titanyl sulfate in an amount shown in Table 1 in terms of Ti instead of sodium silicate to the reaction solution in which the core particles were generated. At this time, the pH of the reaction solution was maintained at 8. Titanyl sulfate was added in the form of an aqueous solution adjusted to pH 1 by adding sulfuric acid. Other than this, black pigment particles were obtained in the same manner as in Example 1. The isoelectric point control layer was composed of TiO ₂ .

Example 11
In Example 1, instead of sodium silicate, sodium tetraborate (Na ₂ B ₄ O ₇ .10H ₂ O) was added in the amount shown in Table 1 in terms of B to the reaction solution in which the core particles were formed. The isoelectric point control layer was formed by neutralizing to pH 8. Other than this, black pigment particles were obtained in the same manner as in Example 1. The isoelectric point control layer was mainly composed of B ₂ O ₃ .

Example 12
In Example 1, instead of sodium silicate, sodium phosphate (Na ₃ PO ₄ ) was added in the amount shown in Table 1 in terms of P to the reaction solution in which the core particles were formed, and then neutralized to pH 8 and so on. An electric point control layer was formed. Other than this, black pigment particles were obtained in the same manner as in Example 1. The isoelectric point control layer was mainly composed of P ₄ O ₁₀ .

Example 13
In Example 1, instead of sodium silicate, sodium aluminate (NaAlO ₂ ) was added in the amount shown in Table 1 in terms of Al to the reaction solution in which the core particles were formed, and then neutralized to pH 8 to obtain an isoelectric point. A control layer was formed. Other than this, black pigment particles were obtained in the same manner as in Example 1. The isoelectric point control layer was mainly composed of γ-AlOOH.

[Comparative Example 1]
Particles were obtained in the same manner as in Example 1 except that the operation of forming an iron oxide layer directly on the surface of the core particle was performed without forming the isoelectric point control layer.

[Comparative Example 2]
In Example 1, instead of sodium silicate used in forming the isoelectric point control layer, magnesium sulfate was used and added simultaneously with sodium hydroxide so that pH 10 was maintained. In this case, magnesium was added in the number of moles shown in Table 1. This formed a magnesium hydroxide layer on the surface of the core particles. Except for this, particles were obtained in the same manner as in Example 1.

[Performance evaluation]
The following measurements and evaluations were performed on the particles obtained in Examples and Comparative Examples. The results are shown in Table 1.

(1) Content of additive element The sample was dissolved and measured by ICP.

(2) FeO content The sample was dissolved in sulfuric acid and measured by redox titration using a potassium permanganate standard solution.

(3) Plate diameter (particle diameter) and plate thickness of core particles and black pigment particles The ferret diameter was measured by scanning electron micrograph (magnification 40,000 times), and the average plate diameter (average particle diameter in the case of spherical shape). ) And average plate thickness.

(4) Thickness of isoelectric point control layer Calculated by subtracting the ferret diameter of the core particles from the ferret diameter of the particles after formation of the isoelectric point control layer.

(5) Thickness of iron oxide layer The thickness was calculated by subtracting the ferret diameter of the particles after formation of the isoelectric point control layer from the ferret diameter of the black pigment particles.

(6) Specific gravity Using a multi-volume density meter 1305 manufactured by Shimadzu Corporation, the specific gravity was measured in an environment at room temperature of 20 ° C.

(7) Saturation magnetization Using a vibrating sample magnetometer VSM-P7 manufactured by Toei Industry Co., Ltd., measurement was performed with an external magnetic field of 79.6 kA / m.

(8) Black L value 1.4 cc of castor oil is added to 2.0 g of a sample and kneaded with a Hoover type Mahler. 7.5 g of lacquer was added to 2.0 g of this kneaded sample, and further kneaded, and then this was applied onto mirror-coated paper using a 4 mil applicator. After drying, the color difference was measured with a color difference meter (Tokyo Denshoku Color Analyzer TC-1800 type).

(9) BET specific surface area Measured with a 2200 type BET meter manufactured by Shimadzu Micromeritics.

(10) Evaluation of peel resistance of iron oxide layer 2 g of the particles obtained in each of the examples and comparative examples were put in a test tube, and 5 ml of pure water was added thereto. The test tube was immersed in a 100 W ultrasonic bath, and ultrasonic waves were applied for 1 minute. The test tube was pulled up from the ultrasonic bath, and the liquid in the test tube was separated into solid and liquid. The solid content was observed with an SEM, and the presence or absence of peeling of the iron oxide layer was examined and evaluated according to the following criteria.
◎ ・ ・ ・ No change compared to before applying ultrasonic waves.
○: About 10% of the surface of the plate-like hydroxide is visible compared to before the ultrasonic wave is applied.
X: The surface of the plate-like hydroxide can be seen about 50% compared to before the ultrasonic wave is applied, or the plate-like hydroxide is visible as it is before the ultrasonic wave is applied, and the iron oxide is another particle Can be seen as formed.

(11) Isoelectric point The actual isoelectric point was measured as described above.
For core particle examples 1 to 6 and 10 to 13 and comparative examples 1 and 2, the slurry after the generation of the core particles is sampled, the solid content is washed to remove excess salt, and then the core particles are filtered off. did. The measurement was performed after the core particles were dried at 50 ° C. For Examples 7 to 9, raw material particles were measured.
Isoelectric point control layer The slurry after forming the isoelectric point control layer on the surface of the core particles was sampled, the solid content was washed to remove excess salt, and then the particles were filtered off. The particles were measured after drying at 50 ° C.
For iron oxide layers Examples 1, 2, and 5-13 and Comparative Examples 1 and 2, the slurry after the formation of black pigment particles was sampled, the solid content was washed to remove excess salt, etc., and then the black pigment The particles were filtered off. The measurement was performed after drying the black pigment particles at 50 ° C. About Example 3 and 4, it measured after heat-processing.

As is clear from the results shown in Table 1, it can be seen that the particles (product of the present invention) obtained in each example have a small specific gravity and saturation magnetization despite having high blackness. Also, no peeling of the iron oxide layer was observed. In Comparative Examples 1 and 2, the iron oxide layer was not successfully formed on the surface of the core particle. The reason for this is considered to be that the surface of the core particle and the surface of the iron oxide layer were electrically repelled due to being negatively charged.

Claims (17)

Black pigment particles having nonmagnetic core particles composed of light elements having an atomic number of 22 or less, an isoelectric point control layer covering the core particles, and an oxide layer containing at least iron covering the intermediate layer Because
The isoelectric point control layer is a black pigment particle having an isoelectric point lower than that of each of the core particle and the oxide layer.   The black pigment particle according to claim 1, wherein the core particle is made of the light element hydroxide, oxide, carbonate, or sulfate.   The black pigment particles according to claim 1, wherein the isoelectric point control layer is made of an oxide or hydroxide of Si, P, B, Al, or Ti.   The black pigment particles according to claim 1, wherein the oxide layer is made of magnetite.   5. The isoelectric point of the isoelectric point control layer is at least 1 to 3 lower than the lower isoelectric point of the core particles and the oxide layer. 5. Black pigment particles.   The black pigment particles according to any one of claims 1 to 5, which are plate-shaped, have an average plate diameter of 200 to 1200 nm, and an average plate thickness of 100 to 400 nm.   The black pigment particles according to any one of claims 1 to 5, which are spherical and have an average particle diameter of 100 to 400 nm.   The black pigment particles according to claim 1, wherein the oxide layer has a thickness of 30 to 300 nm.   The black pigment particles according to any one of claims 1 to 8, having a true specific gravity of 3 to 4.5. The black pigment particles according to any one of claims 1 to 9, wherein the saturation magnetization at 79.6 kA / m is 10 to 50 Am ² / kg.   Black pigment particle | grains in any one of Claims 1-10 whose black L value is 25 or less. The black pigment particle according to any one of claims 1 to 11, which has a BET specific surface area of 10 to 100 m ² / g.   The black pigment particles according to any one of claims 1 to 12, which are obtained by heat-treating particles having the oxide layer formed on the isoelectric point control layer in a non-oxidizing atmosphere.   2. The black pigment particle according to claim 1, wherein the core particle is made of a composite hydroxide containing Mg and Al, the isoelectric point control layer is made of Si oxide, and the oxide layer is made of magnetite.   The amount of Mg is 5 to 17 wt%, the amount of Al is 2 to 8 wt%, the amount of Si is 0.5 to 3.5 wt%, and the amount of Fe is 30 to 60 wt%. Black pigment particles.   An electrophotographic toner using the black pigment particles according to claim 1. An image forming method using an electrophotographic method using the electrophotographic toner according to claim 16.