Magnetic particles used for electrostatic latent image developer and process for producing the same

Abstract: Magnetic particles suitable for use as an electrostatic latent image developer which particles comprise composite particles comprising from more than 80% by weight to not more than 99% by weight of fine ferromagnetic particles and a cured phenol resin, and having a number-average particle diameter of from not less than 1µm to less than 10µm and an apparent density of not greater than 1.5 g/cm³.

Description: The present invention relates to composite particles used in an electrostatic latent image developer, comprising fine ferromagnetic particles and a phenol resin, as well as a process for producing the same. Heretobefore, as one of developing methods for electrostatic latent images, a developing method with a so-called one component-type magnetic toner using, as a developer, composite particles comprising fine ferromagnetic particles mixed and dispersed in a resin, without using a carrier has been generally known and put to practical use. For the magnetic toner, an electroconductive magnetic toner prepared by adding an electroconductive material such as carbon black to fine ferromagnetic particles and a resin has generally been known. In a developing method using the electroconductive magnetic toner, the magnetic toner is held on a non-magnetic sleeve by the magnetic force of a magnet roller and is electrostatically charged to a polarity opposite to that of a latent image by electrostatic induction when the toner is brought closer to the latent image. The magnetic toner charged to the opposite polarity is the deposited and developed on the surface of the latent image, overcoming the magnetic attraction. The image developed using the electroconductive magnetic toner described above has a problem in that electrostatic transfer on the other recording member is difficult. For overcoming such drawback, there has been proposed a method of conducting development using a magnetic toner of high volumic electric resistivity of not less than 10¹² ohm.cm instead of the conductive toner. However, although the developing method using a magnetic toner of high electric resistivity can improve the transferability, the developability is worsened. In view of the above, JP-A-56-142540 has proposed a method of improving both the transferability and developability by using a mixture of a magnetic toner of high electric resistivity and magnetic particles having an average particle diameter of less than that of the magnetic toner. As various properties of the magnetic particles used in the proposed method, it is strongly demanded that the particles have an average particle diameter of not less than 1 µm to less than 10 µm, and have a softness such that no injuries are caused to a roller upon fixing an image by means of the roller, that is, a softness of not greater than about 1.5 g/cm³ as expressed by an apparent density and a volumic electric resistivity of less than 10¹² ohm.cm, particularly not greater than 10⁹ ohm.cm. More preferably, it is required that the magnetic particles have excellent fluidity. The fact that particles with an average particle diameter of not less than 1 µm to less than 10 µm are required for the magnetic particles is apparent from the fact that the one component-type magnetic toners used at present have average particle diameters of from 5 to 20 µm and from the description in JP-A-56-142540 that "the conductive magnetic particles have a volume average particle diameter of from about 1/5 to 4/5 of that of the magnetic toner, preferably from about 3/10 to 2/3", because, as described therein, "it is important in the developer according to the present invention that the average particle diameter of the conductive magnetic particles 5b is less than that of the magnetic toner 5a of high electric resistivity. If the magnetic particles 5b are larger than the magnetic toner 5a, the periphery of the magnetic particles is covered with the smaller magnetic toner. Since the magnetic attraction to the magnet 6 is increased as the magnetic particles become larger, magnetic particles carrying the magnetic toner therearound are just removed from the electrostatic latent images to cause white spots, referred to as blanking on the image. On the other hand, it is not preferred that the conductive magnetic particles are too small. That is, if the particle size is too small, fine magnetic particles are attracted strongly to the periphery of the magnetic toner by means of van der Waals forces to form a similar structure to that in the conventional conductive magnetic toner made conductive at the periphery thereof, thereby worsening the electrostatic transferability". In addition, it is stated that "In the present invention, the electroconductivity of the conductive magnetic particles is defined such that the volumic electric resistivity is not greater than 10⁹ ohm.cm, whereas the high electric resistance of the magnetic toner is defined such that the volumic electric resistivity is not less than 10¹² ohm.cm. ...". The electric resistance of the magnetic particles is required to have an electric resistance of less than that of the magnetic toner with high electric resistivity of not less than 10¹² ohm.cm, that is, an electric resistivity of less than 10¹² ohm.cm, preferably not greater than 10⁹ ohm.cm. Referring further to the fluidity, it has been known that the fluidity of the developer controls the behavior of the developer in a developing machine and gives an undesired effect on the charging characteristics of the developer. As a result, if the fluidity is poor, for instance, unevenness tends to occur in the image and, in an extreme case, this causes a problem, for example the image may not be obtainable. Accordingly, improvement in the fluidity of the magnetic particles is also demanded. As magnetic particles, there are known coagulated particles prepared by (i) washing fine ferromagnetic particles sufficiently with water and, subsequently, rapidly drying them with stirring in a drier, or (ii) fluidizing the fine ferromagnetic particles in a fluidized layer and spraying an organic polymeric material, for example, onto the particles, as described in JP-A-56-159653. Magnetic particles having an average particle diameter of from not less than 1 µm to less than 10 µm, an apparent density of not greater than about 1.5 g/cm³ and a volumic electric resistivity of less than 10¹² ohm.cm are most strongly demanded at present. However, although coagulated particles with an apparent density not greater than about 1.5 g/cm³ can be obtained by any one of the known methods described above, since particle control is difficult and a great number of coagulated particles with an average particle diameter of not less than 10 µm are present together, the particle size distribution is wide and the particles cannot be used unless they are classified. In particular, in the case of using method (i), since the fine ferromagnetic particles are merely coagulated by drying, the coagulation is easily disintegrated, so that a large amount of fine particles having a size of not greater than 1 µm are mixed together, making the particle size distribution broader. In method (ii), since the organic polymeric material as the binder is present between each of the fine ferromagnetic particles, the coagulated particles formed are not easily disintegrated. On the other hand, the amout of the organic polymeric material in the coagulated particles is increased, which results in a drawback that the volumic electric resistivity is increased to not greater than 10¹² ohm.cm. Thus, the amount of the organic polymeric material contained in the magnetic particles has to be minimized while considering the bondability between each of the fine ferromagnetic particles and the electric resistance. Composite particles comprising fine ferromagnetic particles and an organic polymeric material such as a resin have been generally known. They are obtained, for example, by mixing the fine ferromagnetic particles and the resin in predetermined amounts in a molten resin and, subsequently, pulverizing the obtain mixture as described in JP-A-59-31967. However, the content of the fine ferromagnetic particles is generally less than 80% by weight and the content of the fine ferromagnetic particles cannot be increased. Accordingly, it is difficult to obtained composite particles with an electric resistivity of less than 10¹² ohm.cm by reducing the resin content. It is, therefore, strongly demanded to provide composite particles having an average particle diameter of from not less than 1 µm to less than 10 µm and an apparent density of not greater than 1.5 g/cm³, the content of the fine ferromagnetic particles being increased as much as possible. As a result of the present inventors earnest studies, it has been found that composite particles comprising fine ferromagnetic particles and a phenol resin prepared by reacting phenols and aldehydes in an aqueous medium in the presence of fine ferromagnetic particles and a basic catalyst to form fine composite particles comprising fine ferromagnetic particles and a cured phenol resin, in which the concentration of the fine ferromagnetic particles is controlled or the surface of the ferromagnetic particles is made hydrophobic, have a number-average particle diameter of from not less than 1 µm to less than 10 µm, an apparent density of not greater than 1.5 g/cm³ and the content of the fine ferromagnetic particles of from more than 80% by weight to not more than 99% by weight. These particles are useful as magnetic particles for an electrostatic latent image developer. The present invention has been attained based on such a finding. EP-A-384 697, citable under Article 54(3) EPC, describes composite carrier particles which, inter alia, have a number-average particle diameter of from 10 to 1,000 µm. In a first aspect of the present invention, there are provided composite particles suitable for use in an electrostatic latent image developer, which particles comprise (i) from more than 80% by weight to not more than 99% by weight of fine ferromagnetic particles and (ii) a cured phenol resin, and have a number-average particle diameter of from not less than 1 µm to less than 10 µm and an apparent density of not greater than 1.5 g/cm³. In a second aspect of the present invention, there are provided composite particles as defined above wherein said fine ferromagnetic particles have been subjected to a surface hydrophobic treatment. In a third aspect of the present invention, there is provided a process for producing composite particles as defined in the first aspect, comprising reacting a phenol and an aldehyde in an aqueous medium in the presence of fine ferromagnetic particles and a basic catalyst, the ferromagnetic particles being at a concentration of not greater than 65% by weight in the aqueous medium. In a fourth aspect of the present invention, there is provided a process for producing composite particles as defined in the second aspect, comprising reacting a phenol and an aldehyde in an aqueous medium in the present of fine ferromagnetic particles subjected to a surface hydrophobic treatment and a basic catalyst. In a fifth aspect of the present invention, there is provided an electrostatic latent image developer comprising composite particles as defined above. Figures 1 and 2 are scanning type electron microscope photographs (x 10,000) showing the particle structures of composite particles of Examples 3 and 8 of the present specification. The number-average particle diameter of the composite particles according to the present invention is from not less than 1 µm to less than 10 µm. If it is less than 1 µm or not less than 10 µm, it is not preferred for the particles to be used in an electrostatic development. Since the particle diameter of the composite particles has a close relation with the particle diameter of the magnetic toner used together, it is properly selected to be from not less than 1µm to less than 10 µm, preferably from 2 to 8 µm. The apparent density of the composite particles according to the present invention is not greater than 1.5 g/cm³. The low apparent density enables a higher image quality. Additionally, since the particles are of low apparent density and soft, they do not injure a roller, for example, upon fixing by the roller. The electric resistivity of the composite particles according to the present invention, as expressed by the volumic electric resistivity, is generally less than 10¹² ohm.cm, preferably not greater than 10⁹ ohm.cm. The composite particles according to the present invention have excellent fluidity. The fluidizing rate is, for example, not less than 0.45 g/s, preferably not less than 0.48 g/s. The content of the fine ferromagnetic particles in the composite particles according to the present invention is from more than 80% by weight to not more than 99% by weight, preferably from 82 to 95 % by weight. If it is not greater than 80% by weight, the content of the insulative resin component is increased, and composite particles with a volumic electric resistivity of less than 10¹² ohm.cm are not obtained. If it exceeds 99% by weight, the bonding force between each of the fine ferromagnetic particles is reduced and the fine ferromagnetic particles are separated individually which increases the amount of fine particles having a particle diameter of less than 1µm. Examples of the fine ferromagnetic particles usable in the present invention are fine iron oxide particles of magnetite and maghemite, spinel ferrite containing one or more metals other than iron (for example Mn, Ni, Zn, Mg or Cu), magnetoplumbite type ferrite such as barium ferrite, as well as iron or an alloy having an oxide layer on the surface thereof. In addition, oxides of metals other than iron (for example Mn, Ni, Zn, Mg or Cu) can also be incorporated together with the fine ferromagnetic particles. The shape of the fine ferromagnetic particles may be, for example, granular, spherical or acicular. Composite particles having the desired saturation magnetization can be obtained by properly selecting the kind and the content of the fine ferromagnetic particles. For instance, to obtain a saturation magnetization of from 40 to 70 emu/g, magnetoplumbite type ferrite such as barium ferrite or spinel ferrite may be used. To obtain a high saturation magnetization of from about 70 to 100 emu/g, magnetite or Zn-containing spinel ferrite may be used. To obtain a higher saturation magnetization of greater than 100 emu/g, fine particles of iron or an alloy having an oxide layer on the surface thereof may be used. The particle size of the fine ferromagnetic particles used in the present invention is preferably about from 0.01 to 0.3 µm. The content of the fine ferromagnetic particles is preferably from 0.5 to 200 times by weight based on the amount of the phenol. In consideration of the saturation magnetization and the strength of the particles of the resultant composite particles, the content of the fine ferromagnetic particle is preferably from 4 to 100 times by weight based on the amount of the phenol. The content of a cured phenol resin in the composite particles according to the present invention is particularly preferably from not less than 1% by weight to less than 20% by weight. In the present invention, when using fine ferromagnetic particles, the surfaces of which have been subjected to a hydrophobic treatment, composite particles with an average particle diameter from not less than 1 µm to less than 10 µm can be obtained without controlling the concentration of the fine ferromagnetic particles to the specified narrow range as described later. The fine ferromagnetic particles subjected to a hydrophobic treatment can be obtained by any method, such as a method of simply mixing fine ferromagnetic particles and a hydrophobic treating agent, or a method of mixing fine ferromagnetic particles and a hydrophobic treating agent in an aqueous medium thereby adsorbing the hydrophobic treating agent to the surface of the particles. As the hydrophobic treating agent, there can be used a coupling agent such as a titanate or silane, a silylating agent or a silicone oil, as well as various surfactants. As the titanate coupling agent, there can be mentioned, for example, isopropyltrisostearoyl titanate, isopropyltridecylbenzene sulfonyl titanate, isopropyltris(dioctylpyrophosphate) titanate, bis(dioctylpyrophosphate) oxyacetate titanate and bis(dioctylpyrophosphate) ethylene titanate. As the silane coupling agent, there can be mentioned, for example, vinyl trichlorosilane, vinyltriethoxysilane, vinyl tris(β-methoxyethoxy)-silane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-aminopropyltriethoxysilane and N-β(aminoethyl)-γ-aminopropyltrimethoxysilane. As the silylating agent, there can be mentioned, for example, hexamethyldisilazane, trialkylalkoxysilane and trimethylethoxysilane. As the silicone oil, there can be mentioned, for example, dimethyl silicone oil and methyl hydrogenated silicone oil. As the surfactant, any of the commercially available surfactants may be used. Those having functional groups capable of coupling with hydroxyl groups present on the surfaces of the fine ferromagnetic particles are preferred. In view of the ionic property, a cationic or anionic surfactant is preferable. The treatment with the hydrophobic treating agent is preferably applied such that the hydrophobic degree of the fine ferromagnetic particles is from 20 to 60% by weight, preferably from 25 to 55 % by weight. If the degree is less than 20% by weight, the hydrophobic effect for the fine ferromagnetic particles is not sufficient, and electroconductive magnetic particles having excellent fluidity, which is an object of the present invention, are not obtained. If it exceeds 60% by weight, it is difficult to obtain composite particles with an average particle diameter of from not less than 1 µm to less than 10 µm. at a high yield. The basic catalysts used for reacting phenol and aldehyde, are generally those usually used for the production of resol resins. There can be mentioned, for example, aqueous ammonia or alkylamines such as hexamethylene tetramine, diethylene triamine or polyethylene imine. The molar ratio of the basic catalyst to the phenol is not less than 0.4:1, preferably 0.4:1 to 2.0:1, when considering the particle diameter of the resultant composite particles. As the phenol used, there can be mentioned phenol, those compounds having phenolic hydroxy groups such as alkylphenols, for example, m-cresol, p-tert-butylphenol, o-propylphenol, resolsinol or bisphenol A, and halogenated phenols in which a benzene ring or alkyl group is partially or entirely substituted with chlorine or bromine atoms. Phenol is the most preferred. As the aldehydes used, there can be mentioned, for example, formaldehyde in the form of formalin or paraformaldehyde, and fulfural. Formaldehyde is particularly preferred. The molar ratio of the aldehyde to the phenol is generally from 1:1 to 4:1, preferably from 1.2:1 to 3:1, considering the particle diameter of the resultant composite particles. If the molar ratio of the aldehyde to the phenol is less than 1:1, the composite particles cannot be formed easily, or if they are formed, the strength of the resin tends to be reduced. On the other hand, if the molar ratio of the aldehyde to the phenol is more than 4:1, more unreacted aldehyde tends to remain in the aqueous medium after the reaction. The concentration of the fine ferromagnetic particles in the present invention is such that the average particle diameter of the resultant fine composite particles is from not less than 1 µm to less than 10 µm. The reaction in the present invention is carried out in an aqueous medium, in which the amount of water charged is adjusted such that the concentration of the fine ferromagnetic particles is 40 to 65% by weight, preferably 43 to 60 % by weight. If the concentration exceeds 60% by weight, it becomes difficult to obtain composite particles with a number-average particle diameter of from not less than 1 µm to less than 10 µm in a high yield. On the other hand, if the concentration of the fine ferromagnetic particles is less than 40% by weight, it is difficult to obtain the desired composite particles. In the case of using fine ferromagnetic particles subjected at the surface thereof to the hydrophobic treatment, the amount of water charged is preferably controlled such that the concentration of the fine ferromagnetic particles is from 35 to 95% by weight, more preferably from 50 to 90% by weight. If it is less than 35% by weight, it is difficult to obtain composite particles with a number-average particle diameter of from not less than 1 µm to less than 10 µm in a high yield. If it exceeds 95% by weight, it is difficult to obtain the desired composite particles. In the reaction of the present invention, the temperature is generally gradually elevated under stirring at a rate of 0.5 to 1.5°C/min, preferably, 0.8 to 1.2°C/min and the reaction is generally conducted at a temperature of from 70 to 90°C, preferably, 83 to 87°C for 60 to 150 min, preferably, 80 to 110 min. In the reaction, the curing also proceeds simultaneously with the reaction to form a matrix of the cured phenol resin. After the reaction, when the reaction product is cooled to less than 40°C, an aqueous dispersion of composite particles in which fine ferromagnetic particles are uniformly dispersed in the matrix of the cured phenol resin can be obtained. Subsequently, after filtering the aqueous dispersion and carrying out a solid-liquid separation in accordance with a customary method such as centrifugal separation, composite particles having fine ferromagnetic particles uniformly dispersed in the matrix of the phenol resin can be obtained by washing and drying. In the reaction of the present invention, a suspension stabilizer may also be present if necessary. As the suspension stabilizer, there can be mentioned hydrophilic organic compounds such as carboxylmethyl cellulose and polyvinyl alcohol, and water-insoluble inorganic salts such as fluoro compounds, for example, calcium fluoride and calcium sulfate. Considering the dispersion of the fine ferromagnetic particles to the inside of the phenol resin matrix, calcium fluoride is preferred. The suspension stabilizer, when used, is preferably used in an amount of from 0.2 to 10% by weight based on the phenol. If it is exceeds 10% by weight, the amount of the suspension stabilizer such as calcium fluoride remaining on the surface of the composite particle tends to be increase. The composite particles according to the present invention have a number-average particle diameter of from not less than 1 µm to less than 10 µm, an apparent density of not greater than 1.5 g/cm³ and an electric resistivity of less than 10¹² ohm.cm, preferably not greater than 10⁹ ohm.cm, due to the increase in the content of the fine ferromagnetic particles, and the improved bondability between each of the fine ferromagnetic particles. Accordingly, they are most suitable for use as the magnetic particles used in a electrostatic latent image developer demanded most strongly at present. Since the composite particles have excellent fluidity, they are more suitable as the magnetic particles used in a electrostatic latent image developer. In addition, since the average particle diameter of the resultant composite particles can be controlled easily, composite particles having a number-average particle diameter of from not less than 1 µm to less than 10 µm can be obtained at a high yield without classification, which is extremely useful from the industrial and economic points of view. The magnetic particles in accordance with the present invention can be applied not only in an electrostatic latent image developer but also in general application uses as magnetic composite particles, that is in various application fields such as in a toner, paint, colourant such as an ink, column filler, electromagnetic wave absorber or damping agent. EXAMPLES The present invention is non explained in more details in the following Examples. (1) The number-average particle diameter was a number-average value of diameter measured for 200 particles by optical microscope photography. (2) The apparent density was measured in accordance with the method of JIS K5 101. (3) The saturation magnetization was measured by using a "Vibration Specimen Type Magnetometer VSM-3S-15" (manufactured by Toei Industry Co.) at a external magnetic field of 10 KOe. (4) The volumic electric resistance was measured by a High Resistance Meter 4329A (manufactured by Yokogawa Hewlett-Packard, Ltd.). (5) 50 g of composite particles were filled in a glass flask (opening : 75 , height: 75 mm, inner diameter at a conical portion: 6 , length of the straight portion: 30 mm) and the particles falling time (s) when applying a predetermined vibration, was determined. A fluidizing rate was shown as a value obtained by dividing the weight of the composite particles by the particle falling time described above. (6) 0.2 g of fine ferromagnetic particles and 50 g of water were placed in a 500 cm³ volume Erlenmeyer flask, to which methanol was continuously added under stirring. The point at which the initially separated ferromagnetic particles began to settle and disperse in water-methanol mixed solution was then determined as a terminal point, the amount of methanol used up to that point was determined and the hydrophobic degree was shown by the value calculated in the following equation: Hydrophobic degree % = Amount of methanol Amount of water 50g + Amount of methanol × 100 Example 1 To a one liter three-necked flask were charged 30 g of phenol, 60 g of 37% formalin, 400 g of spherical magnetite with an average particle diameter of 0.24 µm, 12 g of an aqueous 25% ammonia and 280 g of water (corresponding to 51% by weight of concentration of the fine ferromagnetic particles) under stirring. After stirring for a while at room temperature, the temperature was elevated to 85°C for 40 min under stirring and a reaction was conducted at that temperature for 180 min to form composite particles comprising magnetite and a cured phenol resin. The contents in the flask were then cooled to 30°C and, after adding 0.5 liters of water, the supernatant was removed and, further, the composite particles in the lower layer were washed with water and then dried in air. Subsequently the obtained composite particles were dried under reduced pressure (less than 5 mmHg) at 50 to 60°C to obtain spherical composite particles (hereinafter referred to as composite particles A). The properties thereof are shown in Table 2. Example 2 Reaction and after-treatment were conducted in the same procedures as those in Example 1 except for using 8 g of hexamethylene tetramine instead of 12 g of the aqueous 25% ammonia as the basic catalyst to obtain spherical composite particles (hereinafter referred to as composite particles B). Table 1 shows the principal production conditions and Table 2 shows various properties of the resultant composite particles. Examples 3-7 Comparative Examples 1, 2 Reaction and after-treatments were conducted in the same procedures as those in Example 1 except for changing the kind, the amount and the concentration of the fine ferromagnetic particles as shown in Table 1 (the composite particles in the Example 3-7 and the Comparative Example 1 are referred to, respectively, as composite particles C - H). Table 1 shows the principal production conditions and Table 2 shows various properties of the resultant composite particles. Fig. 1 shows a scanning electron microscope photograph (x,10000) for composite particles C obtained in Example 3. In Comparative Example 2, no composite particles were obtained and a large amount of fine ferromagnetic particles was suspended in liquid. Example 8 After charging 400 g of spherical magnetite with an average particle diameter of 0.24 µm into a Henshel mixer and stirring sufficiently, 2 g of a titanate coupling agent (Plainact TTS : manufactured by Ajinomoto Co.) was added, the temperature was elevated up to about 100°C and they were mixed and stirred sufficiently for about 30 min. 50 g of phenol, 75 g of an aqueous 37% formalin, 400 g of magnetite subjected to a hydrophobic treatment, 18 g of an aqueous 28% ammonia and 150 g of water (corresponding to 57.7% by weight of the concentration of fine ferromagnetic particles) were charged under stirring into a one liter three-necked flask, the temperature was elevated to 85°C for 40 min and the reaction was conducted at a temperature for 180 min to obtain composite particles comprising spherical magnetite and a cured phenol resin. Then, the contents in the flask were cooled to 30°C and, after adding 0.5 liters of water, the supernatant was removed and, further, the composite particles in the lower layer were washed with water and then dried in air. Subsequently, the obtained composite particles were dried under reduced pressure (less than 5 mmHg) at 50-60°C to obtain spherical composite particles (hereinafter referred to as composite particles I). Fig. 2 shows a scanning electron microscope photograph (x 10000) of composite particles I and Table 4 shows various properties of them. Examples 9-14 Composite particles were obtained by the same procedures as those in Example 8 except for changing the kind of the fine ferromagnetic particles, the kind and the amount of the,hydrophobic treating agent used, the kind and the amount of the basic catalyst, the amount of phenol, the amount of aldehyde and the amount of water (the composite particles obtained in Examples 9-14 are hereinafter referred to as composite particles J - O respectively). Table 3 shpws the principal production conditions and Table 4 shows various properties of the composite particles. Application Example Developers were prepared by mixing 25 parts by weight of each of composite particles A - O obtained in Examples 1 - 14 and Comparative Example 1 described above with 100 parts by weight of a commercially available magnetic toner. Then, when copying was conducted to common paper in an electrophotographic copying machine using Se as photosensitive material using developers containing composite particles A - G and I - 0 respectively obtained in Examples 1 - 14, clear copy image with no blanking could be obtained in all cases. On the other hand, white spots, referred to as white blanking, were shown in the developer containing composite particles H of Comparative Example 1.

Claims: Composite particles suitable for use in an electrostatic latent image developer, which particles comprise (i) from more than 80% by weight to not more than 99% by weight of fine ferromagnetic particles and (ii) a cured phenol resin, and have a number-average particle diameter of from not less than 1 µm to less than 10 µm and an apparent density of not greater than 1.5 g/cm³. Composite particles according to claim 1 having a volumic electric resistivity of less than 10¹² ohm.cm. Composite particles according to claim 1 or 2, wherein said fine ferromagnetic particles have been subjected to a surface hydrophobic treatment. Composite particles according to claim 3, having a fluidizing rate not less than 0.45 g/s. Composite particles according to claim 3 or 4, wherein the hydrophobic degree of said surface treated fine ferromagnetic particles is from 20 to 60% by weight. A process for producing composite particles as defined in claim 1 or 2, comprising reacting a phenol and an aldehyde in an aqueous medium in the presence of fine ferromagnetic particles and a basic catalyst, the ferromagnetic particles being at a concentration of not greater than 65% by weight in the aqueous medium . A process according to claim 6, wherein the concentration of the fine ferromagnetic particles in the aqueous medium is from 40% by weight to 60% by weight. A process for producing composite particles as defined in claim 3, 4 or 5, comprising reacting a phenol and an aldehyde in an aqueous medium in the presence of fine ferromagnetic particles subjected to a surface hydrophobic treatment and a basic catalyst. A process according to claim 8, wherein the concentration of said fine ferromagnetic particles in the aqueous medium is from 35 to 95% by weight. A process according to any on of claims 6 to 9, wherein the molar ratio of said basic catalyst to said phenol is not less than 0.4:1. An electrostatic latent image developer comprising composite particles as claimed in any one of claims 1 to 5.

Inventors: FUJIOKA KAZUO, HAKATA TOSHIYUKI, TAKARAGI SHIGERU, TODA TETSURO, FUJIOKA, KAZUO, HAKATA, TOSHIYUKI, TAKARAGI, SHIGERU, TODA, TETSURO
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9