JP4829491B2 - Magnetite particles - Google Patents

Description

  The present invention forms a uniform and high-density silica layer on the surface of magnetite particles to provide oxidation resistance, and at the same time, by not greatly impairing its magnetic properties, particularly saturation magnetization, laser beam printers, copies, etc. The present invention provides magnetite particles suitable for a toner for developing an electrostatic latent image.

  Electrostatic copying type image forming apparatuses are widely used as copies and printers. These devices are roughly divided into color and monochrome. Among them, the monochrome toner is classified into a one-component toner and a two-component toner according to a difference in a toner transport method to the paper surface. Further, the one-component toner is classified into a magnetic one-component toner and a non-magnetic one-component toner depending on whether the toner contains a magnetic material. On the other hand, the two-component toner is of two types: a toner and a carrier as a carrier. In general, in this method, a non-magnetic toner is used as the toner, and the carrier as the carrier is classified into iron powder, ferrite carrier, magnetite carrier, and binder type carrier.

  Among these toners, the magnetic one-component toner needs to contain a black pigment as a coloring material and a magnetic powder for adding magnetism, but generally, iron oxide particles that are black and have magnetism, that is, magnetite, are widely used. It has been. However, magnetite has essential drawbacks when used in toners.

Magnetite is composed of divalent iron and trivalent iron as represented by the chemical formula of FeO.Fe ₂ O ₃ . For this reason, magnetite is easily oxidized, and is gradually oxidized by oxygen in the air and converted into maghemite even at room temperature. Alternatively, when heated in air, it rapidly oxidizes with heat generation and changes to maghemite or hematite. Magnetite is black, while maghemite is generally brown and hematite is red.

  Furthermore, maghemite and hematite have different magnetic properties from magnetite. Maghemite is a ferromagnetic material, but its theoretical saturation magnetization is smaller than that of magnetite, and hematite is a completely non-magnetic material. That is, when magnetite is oxidized and modified to maghemite or hematite, it becomes an unsuitable material for use in a magnetic one-component toner in terms of color and saturation magnetization. Industrially, there is a problem that the degree of progress of oxidation cannot be made constant from the production of magnetite to the completion of toner production.

  First, if the storage period and storage conditions from magnetite production to toner production differ, the oxidation level cannot be made constant. In toner production, it is difficult to control the degree of oxidation in a pulverization method applied with a magnetic one-component toner by heating and melting the resin and kneading it with magnetite. Thus, if the degree of progress of oxidation from magnetite production to toner production is different, a toner having a color and saturation magnetization different from the original design is produced. Therefore, how to suppress the oxidation of magnetite is an important issue.

  In order to suppress the oxidation of magnetite, it is effective to block contact of oxygen in the air. However, it cannot be denied that it is disadvantageous in terms of cost and workability to use a packaging material that does not transmit oxygen or to keep the inside of the apparatus in a nitrogen atmosphere in toner production.

In order to solve the above-described drawbacks, it has been proposed to form an inorganic, organic or composite layer of these on the surface of magnetite. For example, JP-A-8-48524 (Patent Document 1) discloses a magnetite particle having a particle surface coated with a thin film of iron-zinc oxide and a thin film of iron-silicon oxide, The main effect is to improve the fluidity and magnetic properties in a well-balanced manner, but it is obvious that the coating layer also has an oxidation inhibiting effect. However, according to the embodiment, coating the surface of the magnetite particles with nonmagnetic silicon oxide decreases the saturation magnetization per 1 wt% of SiO ₂ by about 1 Am ² / kg. Is not a suitable material.

  When the saturation magnetization of the magnetite is reduced, a larger amount of magnetite must be contained in order to obtain a toner having the saturation magnetization required for the apparatus. For this reason, the degree of freedom in designing the toner becomes narrow, which is not preferable. For example, although the toner is finally fixed on the paper surface by being melted by heating, the magnetic one-component toner using magnetite usually contains about 40 to 50% by weight of magnetite. Then, when the toner is heated and melted, sufficient fluidity cannot be obtained, and the fixability is deteriorated.

Therefore, even if a non-magnetic coating layer is formed on the magnetite surface to suppress the oxidation of the magnetite, the magnetic properties, particularly the saturation magnetization, does not change or is ideally smaller than expected. It is. However, it is common sense that it is theoretically difficult to obtain such a coating layer, and the present invention has not yet been made.
JP-A-8-48524

  The present invention has been made to solve the above-mentioned problems of the prior art, and provides a magnetite suitable as a toner material having a high-density silica coating layer on the particle surface for suppressing changes in color tone and magnetic properties due to oxidation. The purpose is to do.

Means for Solving the Problem and Embodiment of the Invention

That is, the present invention is a magnetite particles coated with the silicon compound on the particle surface, X (wt%) of the silicon compound coverage in the magnetite particles as SiO _2, the saturation magnetization when the measurement magnetic field of 398 kA / m It is related with the magnetite particle | grains characterized by satisfying the following (1) Formula, when it is set to Y (Am < ² > / kg) and it calculates by regression correlation also including the value of a base | substrate .
[Equation 1]
Y = K × X + C (1)
(0.5 ≦ X ≦ 5.0, −0.60 ≦ K ≦ −0.36 , 85 ≦ C ≦ 88)

Since the magnetite particles of the present invention satisfy the equation (1), the conventional magnetite-coated magnetite particles have a saturation magnetization larger than that predicted from the silicon compound coating amount as SiO ₂ . The saturation magnetization predicted from the silicon compound coating amount in the conventional magnetite-coated magnetite particles here is the saturation magnetization Y ′ calculated from the following equation (2), where Y> Y ′. is there.
[Equation 2]
Y ′ = Y ₀ × (100−X) / 100
Y '; predicted saturation magnetization ^(Am 2 / k g)
Y _0; base saturation magnetization of magnetite ^(Am 2 / k g)
X: SiO ₂ coating amount (% by weight)

K in the above equation (1) represents the saturation magnetization (Am ² / kg) when the measured magnetic field is 398 kA / m, which decreases per 1% by weight of the SiO ₂ coating amount, and the decrease (the absolute value of K) is 0. .34 to 0.62 Am ² / kg. On the other hand, in the conventional silicon compound-coated type magnetite particles, it is calculated as 0.84 to 0.88 Am ² / kg per 1% by weight of SiO ₂ coating from the equation (2). Accordingly, the amount of decrease in saturation magnetization per 1% by weight of SiO ₂ coating is smaller in the magnetite particles of the present invention. That is, it can be seen that if the magnetite particles serving as the base are the same, the present invention has a high saturation magnetization even with the same silicon compound coverage as SiO ₂ , and is superior. The formula (1) is obtained by regression correlation from the data of 5 to 6 samples obtained in the present invention. As shown in the examples, the saturation magnetization of the magnetite particles has a SiO ₂ coating amount. Up to about 1% by weight, it is the same as or slightly larger than the substrate, and there is a feature that no decrease is observed. C represents the saturation magnetization of the magnetite particles serving as the base when X = 0.

  The silicon compound referred to in the present invention is preferably silica. Silica refers to crystalline as well as amorphous silicon dioxide. In the present invention, the silica partly contains silicon hydroxide. The magnetite particles of the present invention have the advantages as described above because of the production method for coating magnetite particles with high-density silica, which will be described later.

  The magnetite particles of the present invention may be spherical, hexahedral, octahedral, dodecahedron, etc., but are preferably spherical in terms of fluidity when used in toner.

Further, magnetite particles of the present invention, a silicon compound containing 0.5 to 1.5 wt% as SiO ₂ inside the particles, average primary particle size 0.10～0.25Myuemu, specific surface area 6～15M ² / G is preferable.

If SiO ₂ is less than 0.5% by weight, it is difficult to obtain spherical magnetite, which is not preferable in terms of fluidity. On the other hand, if it exceeds 1.5% by weight, the magnetic susceptibility is lowered due to the presence of the free silicon compound, and the particle size distribution and the shape distribution are widened. When the average primary particle diameter is less than 0.10 μm and the specific surface area exceeds 15 m ² / g, the magnetite particles do not exhibit a black color due to a physical optical cause, and the average primary particle diameter exceeds 0.25 μm. Is less than 6 m ² / g, it is not preferable because coloring power is insufficient.

The magnetite particles of the present invention have an Fe ²⁺ content of 17 to 22% by weight and an Fe ²⁺ content retention rate of 40% or more after heat treatment in air at 200 ° C. for 1 hour. If the Fe ²⁺ content is less than 17% by weight, the blackness is inferior, and if it exceeds 22% by weight, it becomes difficult to suppress oxidation, which is not preferable. The reason ^why the Fe ²⁺ content retention after heat treatment at 200 ° C. for 1 hour in air is 40% or more is that the magnetite particles of the present invention have excellent oxidation resistance in combination with the large saturation magnetization. Means.

  The magnetite particles of the present invention are obtained by coating magnetite particles serving as a substrate with high-density silica, and are typically produced by the following method.

  First, a ferrous hydroxide precipitate is generated by neutralizing a ferrous salt aqueous solution and an alkali hydroxide, and the oxidation is advanced while adjusting the pH, temperature and air blowing amount. Particles are obtained. The size and shape of the obtained magnetite particles can be arbitrarily adjusted according to the ratio of ferrous salt aqueous solution and alkali hydroxide, pH, oxidation rate, iron concentration, and type and amount of the added component. The shape to be obtained is polyhedron, octahedron, hexahedron, spherical shape, needle shape, scale shape, etc., but it is preferably spherical in terms of fluidity as described above. In terms of size, those having an average primary particle diameter of 0.10 to 0.25 μm are preferred in practice.

  Examples of ferrous salt aqueous solutions include ferrous sulfate, ferrous chloride, ferrous nitrate, etc., but generally, iron sulfate by-produced during the production of sulfuric acid method titanium oxide, along with the surface cleaning of steel sheets Use by-product iron sulfate. Examples of the alkali hydroxide include alkali metals such as sodium hydroxide, potassium hydroxide, and calcium hydroxide, hydroxides of alkaline earth metals, ammonium hydroxide, ammonia gas, and the like.

A ferrous hydroxide precipitate is produced by neutralizing a ferrous salt aqueous solution and an alkali hydroxide. To obtain spherical magnetite particles, the amount of alkali hydroxide relative to the ferrous salt is It is preferable that it is 0.90-1.0 equivalent with respect to Fe < ²⁺ > in iron salt aqueous solution. If it is less than 0.90 equivalent, the production rate of spherical particles is low, and goethite is by-produced. Moreover, when it exceeds 1.0 equivalent, it will become an area | region where a cube, an octahedron, and a polyhedron are easy to produce | generate, and it is difficult to obtain spherical magnetite particles.

  The reaction temperature when oxidizing ferrous hydroxide in the present invention is 60 to 100 ° C. Below 60 ° C., the saturation magnetization becomes small. Even if it exceeds 100 degreeC, a magnetite particle is obtained, but it is not industrial.

  As the oxidation method, there are a method of passing an oxygen-containing gas such as air or oxygen gas through the liquid, a method of adding an oxidizing agent such as hydrogen peroxide, etc., but a method of passing an oxygen-containing gas is common. .

  Next, a method for coating the base magnetite with high density silica will be described.

  The suspension of the base magnetite is concentrated by an appropriate method, and the pigment concentration is adjusted to 50 g / L to 200 g / L. If it is less than 50 g / L, it is not preferable because a processing apparatus with a larger capacity is required to obtain the same amount of product, and if it is more than 200 g / L, the viscosity of the suspension is increased and uniform stirring is achieved. It is not preferable because it is difficult to become.

  This suspension is dispersed, the temperature of the suspension is raised to 80 to 90 ° C., and the pH is adjusted to 9.5 to 11, and then an alkali silicate such as sodium silicate or potassium silicate as a silica source. Is added and the mixture is kept stirring in the above pH range. Next, the pH is adjusted stepwise to 8-9 to coat the surface of the magnetite particles of the substrate with uniform high density silica.

  If the temperature is less than 80 ° C., it is difficult to coat the high-density silica layer, and even if the temperature exceeds 90 ° C., the magnetite of the present invention can be obtained, but it is economical in terms of energy cost for maintaining at a high temperature. Not right. Next, an alkali hydroxide aqueous solution such as sodium hydroxide or potassium hydroxide can be used as the pH adjuster of the alkali silicate, but when an alkaline earth metal such as magnesium hydroxide or calcium hydroxide is used, the alkali silicate The alkali metal ions are ion-exchanged with alkaline earth metal ions, and silica is immediately precipitated due to solubility, which is not preferable. Further, even when the pH at the time of addition of alkali silicate is less than 9.5, the solubility of silica is remarkably reduced, and when it exceeds 11, the magnetite particles of the present invention can be obtained, but the amount of alkali hydroxide used increases. Industrially unfavorable.

  The suspension after addition of the alkali silicate is stirred and held until the liquid-phase alkali silicate and the solid-phase silica are in an equilibrium state. Whether or not an equilibrium state has been reached can be detected by measuring the pH of the suspension. Since alkali ions are released when the alkali silicate in the solution is precipitated as solid phase silica, the pH of the suspension increases with time if the equilibrium is not reached sufficiently. Empirically, it takes 30 minutes to 2 hours for the pH to reach its peak and stop changing.

  The pH is then adjusted stepwise to 8-9 with an inorganic acid. In this case, an aqueous solution of sulfuric acid or hydrochloric acid can be used as the inorganic acid, but it is not preferable to use a compound having an oxidizing action such as nitric acid because it oxidizes magnetite. Inorganic acid is added to lower the pH by 0.2 to 0.5 and stirred for about 30 minutes until the liquid-phase alkali silicate and the solid-phase silica are in an equilibrium state, that is, until there is no increase in pH. Keep stirring. Thereafter, the same operation is repeated to adjust the pH stepwise from 8 to 9, whereby a uniform high-density silica layer is formed on the surface of the magnetite particles. In the case where the pH adjustment is performed once or gel-like silica as described in Comparative Examples described later is formed on the particle surface, the silica layer becomes non-uniform or not dense. Therefore, it is not preferable. On the other hand, if the pH is less than 8, the solubility of silica in the suspension becomes substantially constant, which is meaningless. If it exceeds 9, the alkali silicate remains in the suspension, which is not preferable.

  After that, for example, when it is desired to set the pigment pH near neutral, the pH may be further adjusted to 6-7. The suspension is washed, filtered and dried to obtain magnetite particles coated with the high-density silica of the present invention. When it is desired to adjust the properties as a powder, operations such as crushing, crushing, and compacting may be added in the subsequent steps. As described above, it is possible to obtain magnetite particles having excellent oxidation resistance and having a large magnetic property, particularly a large saturation magnetization, despite having a silica nonmagnetic inorganic coating layer. Although it is not clear why the decrease in saturation magnetization is suppressed by coating with high-density silica, the decrease width of the gel-like silica coating in the comparative example described later is almost as calculated, so magnetite It is estimated that the coating state of the silica on the surface may have an effect.

  Below, the measuring method of each characteristic is demonstrated.

(Average primary particle size)
Using a transmission electron microscope (“JEM-200CX” (trade name) manufactured by JEOL), the shape of the sample particles was observed at a magnification of 30,000 times, and the particle diameter of the equivalent circle diameter of 300 particles was measured. The average particle size was calculated as the average primary particle size.

(Magnetic properties)
Using a vibrating sample magnetometer (“VSM-3” (trade name) manufactured by Toei Kogyo Co., Ltd.), saturation magnetization and residual magnetization were measured with an external magnetic field of 398 kA / m.

(SiO ₂ and Al ₂ O ₃ content)
The test powder was dissolved, and ICP (“ICAP-5” manufactured by Nippon Jarrel-Ash) was used.
75 "(trade name)). The coating amount after the coating treatment is obtained by analyzing the SiO ₂ and Al ₂ O ₃ contents of the base magnetite that has not been coated beforehand, and subtracting the content before the coating treatment from the content analysis value after the coating treatment. Asked.
(Heat resistance test)
3 g of the test sample was spread on an aluminum pan having a diameter of 60 mm and a height of 10 mm, put into an oven maintained at 200 ° C., and taken out after 1 hour. The samples before and after the heat treatment were dissolved in a dilute sulfuric acid solution, and the Fe ²⁺ content was determined by oxidation-reduction titration using potassium permanganate. Further, the Fe ²⁺ content after the heat treatment was divided by the Fe ²⁺ content after the heat treatment and multiplied by 100 to obtain the Fe ²⁺ content retention rate.

Hereinafter, examples showing the effects of the present invention will be described. However, the following examples are provided for illustrative purposes only, and the scope of the invention is not limited by these examples.
Example 1
A magnetite powder containing 0.90% by weight of a silicon compound as SiO ₂ , an average primary particle size of 0.16 μm, and a saturation magnetization of 86.9 Am ² / kg was used for the substrate. As for the heat resistance of the magnetite particles, as a result of the heat resistance test, the Fe ²⁺ content retention rate was 22%. 6 kg of this magnetite was made into a suspension having a pigment concentration of 100 g / L and a volume of 60 L, and dispersed for 30 minutes with a TK homomixer manufactured by Tokushu Kika. The suspension was heated to 85 ° C., and adjusted to pH 10 by adding a 50 g / L aqueous sodium hydroxide solution. Next, 0.21 L of 400 g / L No. 3 water glass as SiO ₂ was added and kept stirring for 30 minutes, and then the pH of the suspension was adjusted to 9.75 by adding 1.8 mol / L dilute sulfuric acid solution. After stirring for 30 minutes, a 1.8 mol / L dilute sulfuric acid solution was added to adjust the pH of the suspension to 9.50. This operation was repeated until the pH of the suspension was 8.50. After 30 minutes of stirring and holding at pH 8.50 was completed, a 1.8 mol / L dilute sulfuric acid solution was added again to adjust the pH of the suspension to 6.0, and stirring was continued for 30 minutes. This suspension was washed, filtered, and dried to obtain magnetite powder. The obtained magnetite powder has a high-density silica coating layer of 0.86% by weight, a saturation magnetization of 87.0 Am ² / kg, a Fe ²⁺ content retention rate by a heat resistance test of 60%, and is excellent in heat resistance. Moreover, it was a magnetite powder with no decrease in saturation magnetization.

(Examples 2 to 5)
Magnetite powder was obtained in the same manner as in Example 1 except that the amount of No. 3 water glass added was changed.

(Examples 6 to 10)
A magnetite powder containing 0.90% by weight of silicon compound as SiO _{2 in the} base, an average primary particle size of 0.14 μm, a specific surface area of 11.4 m ² / g, and a saturation magnetization of 85.9 Am ² / kg is used as No. 3 Magnetite powder was obtained in the same manner as in Example 1 except that the amount of water glass added was changed.

(Examples 11 to 15)
A magnetite powder containing 0.92% by weight of a silicon compound as SiO _{2 in the} base, an average primary particle size of 0.12 μm, a specific surface area of 12.7 m ² / g, and a saturation magnetization of 86.1 Am ² / kg is used as No. 3 Magnetite powder was obtained in the same manner as in Example 1 except that the amount of water glass added was changed.

(Examples 16 to 20)
A magnetite powder containing 1.16% by weight of a silicon compound as SiO _{2 in the} base, an average primary particle diameter of 0.25 μm, a specific surface area of 7.9 m ² / g, and a saturation magnetization of 85.4 Am ² / kg is used as No. 3 Magnetite powder was obtained in the same manner as in Example 1 except that the amount of water glass added was changed.

(Comparative Example 1)
Magnetite powder was obtained in the same manner as in Example 1 except that No. 3 water glass was not added.

(Comparative Examples 2 to 5)
A gel is prepared by simultaneously dropping No. 3 water glass and 1.8 mol / L dilute sulfuric acid solution into the magnetite suspension of Example 1 at 40 ° C. so that the pH of the suspension is maintained at 5-7. The silica is coated.

(Comparative Examples 6-9)
Magnetite powder was obtained in the same manner as in Example 1 except that sodium aluminate was used instead of No. 3 water glass.

Table 1 shows the physical properties of the magnetite powders obtained in the examples and comparative examples.

As can be seen from K in Table 1 (the reduction width of σs per 1% by weight of the coating (Am ² / kg)), the examples were as small as −0.60 to −0.36 and the saturation magnetization σs. The Fe ²⁺ content retention rate by the heat resistance test is 40% or more and is excellent in heat resistance. On the other hand, in the comparative example, K is larger on the minus side than −0.82, and σs is lower than the calculated value.

[The invention's effect]
As described above, since the magnetite particles of the present invention have high-density silica on the particle surface, the amount of decrease in saturation magnetization per unit coating amount as SiO ₂ is small and the oxidation resistance is excellent. The toner is preferably used for a toner for developing an electrostatic latent image, particularly a magnetic one-component toner.

Claims (3)

A magnetite particle having a particle surface coated with a silicon compound, wherein the silicon compound coating amount in the magnetite particle is X ₂ (% by weight), and the saturation magnetization when the measured magnetic field is 398 kA / m is Y (Am ² / kg), and a magnetite particle satisfying the following formula (1) when calculated by regression correlation including the value of the substrate:
[Equation 1]
Y = K × X + C (1)
(0.5 ≦ X ≦ 5.0, −0.60 ≦ K ≦ −0.36 , 85 ≦ C ≦ 88) _2. The magnetite particle according to claim 1, wherein the particle contains 0.5 to 1.5% by weight of a silicon compound as SiO ₂ and has an average primary particle size of 0.10 to 0.25 μm. 3. The magnetite according to claim 1, wherein the Fe ²⁺ content is 17 to 22% by weight, and the Fe ²⁺ content retention after heat treatment at 200 ° C. for 1 hour in air is 40% or more. particle.