Composite carrier particles for electrophotography and process for producing the same

Abstract: Composite carrier particles for electrophotography comprise (i) 80 to 99% by weight of ferromagnetic fine particles and (ii) a phenol resin, and have a number-average particle diameter of 10 to 1,000 µm, a bulk density of not more than 2.0 g/cm³ and a curved surface configuration.

Description: The present invention relates to composite carrier particles for electrophotography and to a process for their preparation. In electrophotography, the developing method which is prevalently used is one in which an electrostatic latent image is formed by various means using a photoconductive material such as selenium, OPC (Organic photoconductor) or α-Si and a toner electrically charged to the opposite polarity to the latent image is caused to adhere to the latent image by electrostatic force using, for instance, magnetic brush development, thereby developing the latent image. In the developing process, carrier particles which are usually referred to simply as carrier are used. An appropriate quantity of positive or negative electricity is applied to the toner through frictional charging, and the charged toner is transferred to the developing zone near the surface of the photoconductive layer where the latent image is formed, through the medium of a magnet-incorporated development sleeve, by making use of the magnetic force. Recently, with the increasing speeding up, continuation and higher performance of copying machines, there is a strong request for an improvement of the properties of the carrier to be used in such copying machines. The carrier is required to have a low bulk density, a large saturation magnetization and a high electric resistance. When the bulk density of the carrier particles is high, a large driving force is required for stirring in the developing apparatus, resulting in early mechanical wear, production of spent toner, deterioration of charging characteristics of the carrier itself and damage to the photoconductive layer. It is, therefore, keenly required that the carrier particles have a low bulk density. Low saturation magnetization weakens the magnetic adhesive force of the carrier to the development sleeve, thereby causing release of the carrier particles from the development sleeve and their adhesion to the surface of the photoconductive layer. Thus, a large saturation magnetization of the carrier particles has also been a strong requirement. It is required that the magnetic carrier has as high an electric resistance as possible because of the necessity to control the frictional chargeability of the toner for forming a clear image. Hitherto, iron-powder carrier, ferrite carrier and binder-type carrier (resin particles having fine magnetic particles dispersed therein) have been developed and practically used as the magnetic carrier. Magnetic carrier particles having a low bulk density, large saturation magnetization and high electric resistance are most keenly required at present, but there are as yet no magnetic carrier particles which can amply satisfy these requirements. Regarding the iron carrier particles, there are available flaky particles, sponge-like particles or spherical particles, but since the true specific gravity of these particles is 7 to 8, their bulk density is as high as 3 to 4 g/cm³ and their electric resistance is as low as 10² to 10³ Ω·cm, a large driving force is necessary for stirring in the developing apparatus. This leads to early mechanical wear of the apparatus, resulting in the production of spent toner, deterioration of the charging characteristics of the carrier itself and damage to the photoconductive layer. As a means for increasing the electric resistance, the subject particles are treated with an organic solvent containing a resin, thereby coating the surface of the iron-particles with the resin. However, because of the low throughput rate, the coating of the surface of the iron particles tends to become insufficient and non-uniform, and the effect of increasing the electric resistance is unsatisfactory. Therefore the same treatment must be repeated several times. This causes complex and troublesome operations. Thus this method is disadvantageous industrially and economically. Furthermore, the oxide coating film on the surface of the iron particles is liable to peel off and is also unstable as oxidation may take place and advance in certain environmental conditions. Thus, peeling and cracking of the resin coating tends to occur and the coated surface of the iron particles may be partly bared, thereby causing a disturbance of the charging characteristics. Ferrite carrier particles are spherical in shape, with their true specific gravity being about 4.5 to 5.5 and their bulk density being about 2 to 3 g/cm³. The ferrite carrier particles, therefore, can overcome the problem of weight which is the defect of the iron-powder carrier, but the ferrite carrier is still unable to adapt itself satisfactorily to high speed copying machines where the development sleeve or the magnet therein rotates at high speed, or high speed laser beam printers for general purpose computers. Binder-type carrier has a small bulk density (less than 2 g/cm³), but as described in JP-B-59-24416 (1984), it is produced by mixing and melting magnetic fine particles and a matrix resin, and then cooling and pulverizing the molten mixture. The produced particles, therefore, have a low magnetization, and accordingly they have the problem that their magnetic adhesive force to the development sleeve is weak, which tends to cause release of carrier particles from the development sleeve and adhesion to the photoconductive layer. These carrier particles also have irregular shape and poor fluidity, so that they are hard to stir and tend to cause non-uniformity in development. Thus this binder-type carrier is unsatisfactory for application to high-speed development where especially good fluidity of the developer is required. It has been attempted to obtain a binder-type carrier having a curved particle-surface, especially a spherical binder-type carrier. It is possible, as described in JP-A-59-31967 (1984), to obtain spherical particles by mixing a thermoplastic resin and ferromagnetic fine particles, pulverizing the resultant mixture and further subjecting it to hot-air treatment. But in this case, it is hardly possible to make the ferromagnetic fine particles content not less than 80% by weight, and there are cases where it is impossible to secure the magnetism necessary to prevent scattering of the carrier particles during high speed development, although the design of the developing apparatus is partly responsible therefor. In case of dispersing spinel ferrite particles such as magnetite particles for a pigment having a submicron diameter into a thermoplastic resin by kneading, usually when the content of such spinel ferrite particles exceeds 80% by weight, a tendency is noted that the hot-melt mixture increases in viscosity and lowers in fluidity. As a result it is difficult to perform the kneading. Even if the kneading can be performed, it is hardly possible to make the pulverized particles spherical by a hot-air treatment because of the high viscosity of the melt. In the production of a binder-type carrier, a thermoplastic resin is usually used as matrix resin, but in this case the produced magnetic particles have weak strength and may be split into finer particles, which may become a cause of fogging of the developed image. In JP-A-58-136052 (1983) the use of a thermosetting resin in place of the thermoplastic resin for improving the strength of magnetic particles carrier is proposed. But in this case it is also hardly possible to make the content of the magnetic particles not less than 80% by weight. In said application as a process for producing a binder-type carrier using a thermosetting resin, the thermosetting resin and magnetic fine particles are mixed, the resultant mixture is melted and then heat-cured by adding a curing agent, and the resulting cured product is pulverized and classified. According to this method, however, it is impossible to obtain spherical particles by a hot-air treatment since the resin is thermoset, and the unnecessary classified-out particles cannot be recycled, unlike in the case when a thermoplastic resin is used. Thus industrial application of this method is difficult in terms of cost. As another process for a producing binder-type carrier using a thermosetting resin, said application also discloses a method in which a thermosetting resin is dissolved in a solvent such as toluene, then magnetic fine particles are dispersed therein, and the resultant dispersion is sprayed for granulation and then dried to evaporate away the solvent. The resulting granulated particles are further heat-cured and classified to form the desired carrier particles. According to this method, it is easy to form spherical particles, but since the process involves evaporation of a large amount of solvent, voids are apt to form in the granulated particles, thereby impairing their strength. Also, apparatus for recovering a large amount of solvent is necessary, and the classified-out particles with undesired sizes cannot be recycled as in the case of said pulverization method. This method, therefore, is unsuited for practical application. As described above, a variety of carrier particles and processes for producing the carrier particles have been proposed, and some of them have been put to practical use. However, for use in digital copying machines having the latest digital techniques applied to electrophotography, laser beam printers, plain paper facsimiles and other high-technique office machines, carrier particles having a higher performance are required, that is particles which can enable even higher speed operations, higher image quality, higher fineness, and formation of clear colour images. Such particles are required to have a low bulk density, a curved surface configuration and a high content of ferromagnetic fine particles. The present invention provides composite carrier particles for electrophotography comprising: (i) from 80% to 99% by weight of ferromagnetic fine particles and (ii) a phenol resin, and having a number-average particle diameter of from 10 to 1,000 µm, a bulk density of not more than 2.0 g/cm³ and a curved surface configuration. A melamine resin may be coated on the surfaces of the particles. The present invention also provides a process for producing the above composite carrier particles, which comprises reacting a phenol and an aldehyde in the presence of ferromagnetic fine particles, a suspension stabilizer and a basic catalyst in an aqueous medium. The particles coated with a melamine resin may be produced by reacting a melamine and an aldehyde in the presence of the above composite particles in an aqueous medium thereby coating the surfaces of the composite particles with a melamine resin. The above particles have a curved surface configuration, a low bulk density, a high saturation magnetization and a high electric resistance. Figs. 1 and 2 are scanning electron microphotographs (× 300) showing the structure of the composite particles obtained in Examples 1 and 3, respectively. Fig. 3 is a scanning electrophotograph (× 3000) showing the structure of the surface of a composite particle before coating with a melamine resin obtained in Example 1. Fig. 4 is a scanning electron microphotograph (× 3,000) showing the structure of the surface of a composite particle coated with a melamine resin obtained in Example 9. The composite carrier particles of the present invention have a number-average particle diameter of 10 to 1,000 µm. When the number-average particle diameter is less than 10 µm, it becomes difficult to prevent adhesion of the carrier to a photoconductive layer, whilst when the number-average particle diameter exceeds 1,000 µm, it becomes difficult to obtain a clear image. The preferred range of the number-average particle diameter is from 30 to 200 µm, more preferably from 30 to 100 µm, for obtaining a high image quality. The composite carrier particles of the present invention also have a bulk density of not more than 2.0 g/cm³. In the present invention, there is no specific limitation to the lower limit of the bulk density of the particles, but practically the lower limit of the bulk density is around 1.0 g/cm³. The composite particles with such a low bulk density are deemed to be able to serve as a carrier capable of providing a high image quality. The curved surface configuration is also characteristic of the composite carrier particles of the present invention. The composite particles with the "curved surface configuration" include spherical particles, oval particles, flat disc-like particles, and warped particles with complex curvatures. Any one of these composite particles has a small contact area between the particles because of the curved surface configuration, and exhibit excellent fluidity. Spherical composite particles are especially preferred since the spherical particles have excellent fluidity, minimized distortion of the particle shape and high particle strength. In the composite carrier particles of the present invention, the content of the ferromagnetic fine particles is more than 80% by weight to not more than 99% by weight, preferably 80 to 97% by weight. When the content of the ferromagnetic fine particles is not more than 80% by weight, the saturation magnetization lowers, and when said content exceeds 99% by weight, the adhesion between the ferromagnetic fine particles by the phenol resin tends to weaken. In view of strength of the composite particles, the content of the ferromagnetic fine particles is preferably not higher than 97% by weight. The reason why the content of the ferromagnetic fine particles can be so high in the present invention is not clear, but it is supposed that the ferromagnetic fine particles are bonded fast to each other with a small amount of the phenol resin because the gelation proceeds simultaneously with the primary reaction. The composite carrier particles of the present invention preferably have a saturation magnetization of about 40 to 150 emu/g. When the saturation magnetization is less than 40 emu/g, adhesion of the carrier particles to the photoconductive layer tends to occur. It is difficult to obtain composite particles having a saturation magnetization of more than 150 emu/g because no ferromagnetic particles which can be practically used for the said purpose in the form of fine particles are known. The saturation magnetization of the ferrite carrier, which is known in the art, is about 70 emu/g at the highest (refer to Basis and Application of Electrophotographic Techniques, p. 481, 1988, Corona Pub,, Co.), but in the case of the composite carrier particles of the present invention, it is possible to obtain easily a saturation magnetization of higher than 70 emu/g by increasing the content of fine ferrite. As the ferromagnetic fine particles, there can be used fine iron oxide particles of magnetite and maghemite, spinel ferrite containing one or more metals other than iron (such as Mn, Ni, Zn, Mg or Cu), magnetoplumbite type ferrite such as barium ferrite, and iron or alloys having an oxide layer on the surface. The shape of the ferromagnetic fine particles may be granular, spherical or acicular. Ferromagnetic fine particles such as iron particles may be used in applications where especially high magnetization is required, but considering the chemical stability, it is preferred to use fine iron oxide particles of magnetite and maghemite, spinel ferrite or magneto-plumbite type ferrite such as barium ferrite. It is possible to obtain composite particles having a desired saturation magnetization by properly selecting the kind and content of the ferromagnetic fine particles. For example, when it is desired to obtain a magnetization of 40 to 70 emu/g, it is suggested to use magnetoplumbite type ferrite such as barium ferrite or spinel ferrite, and when it is desired to obtain a high magnetization of 70 to 100 emu/g, it is advised to use magnetite or spinel ferrite containing Zn. To obtain a magnetization of higher than 100 emu/g, fine particles of iron or an alloy having an oxide layer on the surface thereof may be used. The composite carrier particles of the present invention have a satisfactory strength as the ferromagnetic fine particles are bonded to each other with a cured phenol resin matrix. The coating weight of the melamine resin on the surface of the composite particles is preferably not less than 0.05% by weight based on the core composite particles. When the said coating weight is less than 0.05% by weight, the formed coating film may have unsatisfactory strength and be non-uniform, and as a result, it is difficult to obtain the effect of increasing the electric resistance purposed in the present invention. The preferred range of the coating weight is 0.1 to 10% by weight based on the core composite particles. A process for producing the composite carrier particles of the present invention essentially comprises reacting phenols and aldehydes in an aqueous medium in the presence of a basic catalyst by allowing the ferromagnetic fine particles and a suspension stabilizer to coexist in the aqueous medium. As the phenols used in the process of the present invention, phenol; alkylphenols such as m-cresol, p-tert-butylphenol, o-propylphenol, resorcinol, bisphenol A; and compounds having phenolic hydroxide groups such as halogenated phenols in which benzene nuclea or alkyl groups are partly or wholly substituted with chlorine or bromine atoms, may be exemplified. Among them, phenol is the most preferred. As the aldehydes used in the process of the present invention, formaldehyde in the form of formalin or para-formaldehyde and furfural may be exemplified. Formaldehyde is especially preferred. The molar ratio of aldehyde to phenol is 1:1 to 2:1, preferably 1.1:1 to 1.6:1. When the molar ratio is less than 1:1, it is hard to produce the composite particles, and even if the composite particles could be produced, the formed composite particles tend to have poor strength because it is difficult to proceed with the curing of the produced resin. On the other hand, when the molar ratio is higher than 2:1, the amount of aldehyde remaining unreacted in the aqueous medium after the reaction tends to increase. As basic catalysts used in the process of the present invention, those which are usually used in the production of resol resins can be used. Examples of such basic catalysts are ammonia water, hexamethylenetetramine and alkylamines such as dimethylamine, diethyltriamine and polyethyleneimine. The molar ratio of the basic catalyst to phenol is preferably from 0.02:1 to 0.3:1. The amount of the ferromagnetic fine particles used in the process of the present invention is preferably 0.5 to 200 times (by weight) the amount of phenol. In view of the saturation magnetization of the produced composite particles and the particle strength, it is more preferable that the amount of the ferromagnetic fine particles is 4 to 100 times (by weight) the amount of phenol. The ferromagnetic fine particles preferably have a diameter of 0.01 to 10 µm. The more preferred particle diameter is 0.05 to 5 µm in view of the dispersion of the fine particles in the aqueous medium and strength of the produced composite particles. As suspension stabilizer used in the process of the present invention, there can be used hydrophilic organic compounds such as carboxymethyl cellulose and polyvinyl alcohol; fluorine compounds such as calcium fluoride; and substantially water-insoluble inorganic salts such as calcium sulfate. Calcium fluoride is preferred from the viewpoint of dispersion of the ferromagnetic fine particles into the inside of the phenol resin matrix. The amount of such suspension stabilizer used in the process of the present invention is preferably 0.2 to 10% by weight, more preferably 0.5 to 3.5% by weight based on the phenols. When the amount of the suspension stabilizer added is less than 0.2% by weight based on the phenol, irregular particles tend to be produced. On the other hand, when the amount of the suspension stabilizer exceeds 10% by weight based on the phenol, the amount of the suspension stabilizer such as calcium fluoride remaining on the surface of the composite particles produced tends to increase. In the case of adding a substantially water-insoluble inorganic salt, it is possible either to directly add the substantially water-insoluble inorganic salt or to add two or more different kinds of water-soluble inorganic salts so that a substantially water-insoluble inorganic salt is produced by a reaction. For instance, instead of using calcium fluoride, it is possible to add at least one compound selected from, for example, sodium fluoride, potassium fluoride and ammonium fluoride as one of the water-soluble inorganic salts, while further adding at least one compound selected from calcium chloride, sulfate and nitrate as another water-soluble inorganic salt so that calcium fluoride is produced. The reaction in the process of the present invention is carried out in an aqueous medium. In this reaction, the amount of water supplied is so selected that the solids concentration is preferably 30 to 95% by weight, more preferably 60 to 90% by weight. For carrying out the reaction, the mixture is gradually heated at a rate of 0.5 to 1.5°C/min, preferably 0.8 to 1.2°C/min, under stirring, and the reaction is performed at a temperature of 70 to 90°C, preferably 83 to 87°C, for 60 to 150 minutes, preferably 80 to 110 minutes. In the process of the present invention, this reaction is accompanied by a gelation reaction to form a gelled phenol resin matrix. After the reaction and gelation have been completed, the reaction product is cooled to a temperature below 40°C, thereby forming a water dispersion of spherical particles comprising the ferromagnetic fine particles dispersed uniformly in the gelled phenol resin matrix. This water dispersion is separated into solid and water by a conventional method such as filtration or centrifugation, and the solid matter is washed and dried, thus obtaining composite particles having a curved surface configuration in which the ferromagnetic fine particles are dispersed uniformly in the phenol resin matrix. The coating with the melamine resin in the present invention is performed by reacting a melamine and an aldehyde in the presence of the composite particles under stirring in a neutral or weakly basic aqueous medium, and gelling the reaction mixture. The melamine and aldehyde are made into ultra-fine particles insoluble in water as the reaction proceeds, and a state of suspension is generated. It is, therefore, expedient to allow a suspension stabilizer to coexist in the reaction system. As the suspension stabilizer, there can be used hydrophilic organic compounds and water-insoluble inorganic compounds as in the case of the formation of the phenol resin described above. The gelation may be conducted in the presence of an acidic catalyst, if necessary. The gelled product is cured by a heat-treatment, preferably at a temperature of 130 to 150°C. The ultra-fine particles of melamine resin are coated uniformly and densely on the surface of the composite particles, thereby enabling effective improvement of the electric resistance of the composite particles. The coating of the ultra-fine particles of melamine resin enlarges the specific surface area of composite particles, thereby obtaining a high electric resistance. As the melamine, there can be used melamine and its formaldehyde addition products such as dimethylolmelamine, trimethylolmelamine and hexamethylolmelamine. A melamine-formaldehyde precondensate is also usable. Among them, melamine is the most preferred. In the process of the present invention, the melamine is preferably used in an amount of 0.5 to 10% by weight, more preferably 2 to 7% by weight based on the core composite particles. When the amount of the melamine used is less than 0.5% by weight based on the core composite particles, the desired coating cannot be obtained, and when it exceeds 10% by weight based on the core composite particles, ultra-fine particles of melamine resin are formed independently and the separation thereof from the thus obtained composite particles is difficult. As the aldehyde, formaldehyde or acetaldehyde is preferred, but it is also possible to use formaldehyde in the form of formalin or paraformaldehyde, and compounds such as furfural, which are decomposed to produce formaldehyde. The amount of the aldehyde used in the process of the present invention is 1:1 to 10:1, preferably 2:1 to 6:1, by molar ratio to melamine. When the molar ratio of aldehyde to melamine is less than 1.0:1, it is hard to produce a melamine resin, and when it exceeds 10:1, the amount of the aldehyde remaining unreacted in the aqueous medium after the reaction increases. As the acidic catalyst used, if necessary, in the process of the present invention, formic acid, phosphoric acid, oxalic acid, ammonium chloride and p-toluenesulfonic acid may be exemplified. The amount (molar ratio) of the acidic catalyst used to the melamines is preferably not more than 10:1. As the suspension stabilizer used, if necessary, in the process of the present invention, there can be used the same stabilizer as the one used in the composite particle forming reaction. The suspension stabilizer is preferably used in an amount of not more than 15% by weight, more preferably not more than 10% by weight, based on the melamine. When the amount of the suspension stabilizer is more than 15% by weight based on the melamines, the amount of suspension stabilizer such as calcium fluoride remaining on the particle surfaces tends to increase. The reaction in the process of the present invention is carried out in an aqueous medium. The amount of water supplied in this reaction is not particularly specified, but the amount of water supplied is so selected that the particle concentration is preferably 30 to 60% by weight. An example of the coating reaction with melamine resin in the process of the present invention is described below. Aqueous solutions of two or more compounds capable of forming the substantially water-insoluble inorganic salts, the melamine, the aldehyde and the above-described composite particles are added at normal temperature to an aqueous medium under vigorous stirring to prepare a mixed solution. After adjusting the pH of the mixed solution to 7 to 9.5, the resultant solution is heated at a rate of 0.5 to 1.5°C/min, preferably 0.8 to 1.2°C/min under stirring, till reaching 70 to 90°C, preferably 80 to 85°C, and reacted at this temperature for 10 to 30 minutes, preferably 15 to 20 minutes. The reaction mixture is cooled to a temperature below 30°C, and after adding an acidic catalyst, the reaction mixture is then heated gradually at a rate of 0.5 to 1.5°C/min, preferably 0.8 to 1.2°C under stirring, and further reacted at a temperature of 75 to 95°C, preferably 80 to 90°C for 60 to 150 minutes, preferably 80 to 110 minutes. As this reaction advances, there takes place concurrently a gelation reaction by which the surfaces of the composite particles are coated with a melamine resin. After completion of the reaction and coating, the reaction product is cooled to a temperature below 30°C, whereupon there is obtained a water dispersion of the composite particles having their surfaces coated with the ultra-fine particles of melamine resin. This dispersion is then separated into solid and liquid according to a conventional method such as filtration or centrifugation, and the obtained solid product is dried and heat treated at a temperature of, for example, 130 to 150°C, to cure the ultra-fine particulate melamine resin. Consequently, there are obtained composite particles having their surfaces coated uniformly with cured melamine resin in the form of ultra-fine particles. The composite particles to be coated with the melamine resin in the present invention may be ones which have been dried in vacuo, ones which have been dried under normal pressure, and ones which have just been filtered and are still in a wet state. The composite carrier particles comprising the ferromagnetic fine particles and the phenol resin according to the present invention have a low bulk density, for example, not more than 2.0 g/cm³, preferably not more than 1.95 g/cm³, have a curved surface configuration and a high electric resistance, for example, a volumetric electric resistance of not less than 1 × 10⁵ Ω·cm preferably not less than 1 × 10⁶ Ω·cm, and also have a high saturation magnetization, for example, not less than 40 emu/g owing to the high content of the ferromagnetic fine particles. Thus these composite particles are suited for use as a magnetic carrier for electrophotography. It is possible with the above-described process of the present invention to easily produce the composite particles composed of the ferromagnetic fine particles and the phenol resin. The composite carrier particles coated with the melamine resin of the present invention also have a low bulk density, for example, not more than 2.0 g/cm³, preferably not more than 1.85 g/cm³, more preferably not more than 1.70 g/cm³, a high saturation magnetization, for example, not less than 40 emu/g owing to the high content of ferromagnetic fine particles and a high electric resistance, for example, a volumetric electric resistance of not less than 1 × 10¹⁰ Ω·cm, preferably not less than 1 × 10¹¹ Ω·cm due to coating with the melamine resin. Thus these composite particles can be also used advantageously as a magnetic carrier for electrophotography. It is remarkable that the composite carrier particles having their surfaces coated with the melamine resin according to the present invention have an additional advantage of enhanced durability as the melamine resin is a thermosetting resin with high strength. The process of the present invention is capable of easily producing the composite carrier particles composed of the ferromagnetic fine particles and the phenol resin, and it is possible to sufficiently increase the electric resistance by the coating treatment with the melamine resin. Thus the process of the present invention is advantageous industrially and economically. EXAMPLES The present invention will be hereinbelow described more particularly in Examples and Comparative Examples. Each number-average particle diameter shown is the mean value of the diameters of 200 particles measured from a light micrograph. The bulk density was measured according to the method of JIS K-5101. The saturation magnetization was measured using a vibrating sample type magnetometer VSM-3S-15 (manufactured by Toei Industries Co., Ltd.). The electric resistance was measured by a High Resistance Meter 4329A (mfd. by Yokogawa Hewlett-Packard, Ltd.). The shapes of the composite particles were determined from observation through a scanning electron microscope S-800 (manufactured by Hitachi Co., Ltd.). Production of composite carrier particles EXAMPLE 1 50 g of phenol, 65 g of 37% formalin, 400 g of spherical magnetite particles having an average particle diameter of 0.24 µm, 7.8 g of 28% ammonia water, 1 g of calcium fluoride and 50 g of water were supplied into and stirred in a 1-litre three-necked flask. The mixture was heated to 85°C over a period of 40 minutes and reacted at this temperature for 180 minutes to produce composite particles composed of magnetite particles and gelled phenol resin. Then the resultant contents in the flask were cooled to 30°C and added with 0.5 litre of water. After removing the supernatant, the spherical particles in the lower layer were washed with water and air dried. They were then further dried at 50 to 60°C under reduced pressure (below 5 mmHg) to obtain spherical composite particles (hereinafter referred to as composite particles A). A scanning electron micrograph (× 300 magnification) of the composite particles A is shown in Fig. 1. EXAMPLE 2 By carrying out the same reaction and after-treatments as in Example 1 except for using 4.5 g of hexamethylenetetramine instead of 7.8 g of 28% ammonia water as basic catalyst, there were obtained spherical composite particles (hereinafter referred to as composite particles B). EXAMPLES 3 - 8 and Comparative Examples 1 and 2 By carrying out the same reaction and after-treatments as in Example 1 except that the kinds and amount of ferromagnetic fine particles and the amount of suspension stabilizer were changed as shown in Table 1, there were obtained the corresponding composite particles (hereinafter the composite particles obtained in Examples 3, 4, 5, 6, 7 and 8 and Comparative Examples 1 and 2 are referred to as composite particles C, D, E, F, G, H, I and J, respectively). A scanning electron micrograph (× 300 magnification) of the composite particles C obtained in Example 3 is shown in Fig. 2. REFERENTIAL EXAMPLE 1 Magnetic developers were prepared by mixing 100 parts by weight of each of the composite particles A - J (as carrier) obtained in Examples 1 - 8 and Comparative Examples 1 and 2, and 3 parts by weight of a commercially available toner. Each of the prepared developers was subjected to a copying-test in which, using each developer, 20,000 copies were taken on A4 size paper by an electrophotographic copying machine using α-Si as a photoconductive material. Thereafter, the state of the surface of the photoconductive layer and the state of the developer in the copying machine were examined. In the case of the developers containing composite particles A - H of the present invention as carrier, there no adhesion of composite particles on the surface of the photoconductive layer nor any break of the composite particles were observed. On the other hand, in the case of the developer containing comparative composite particles I, the particles were broken into finer sizes, and in the case of the developer containing comparative composite particles J, there was seen adhesion of the particles on the surface of the photoconductive layer. Production of composite carrier particles coated with melamine resin EXAMPLE 9 5.4 g of melamine, 10.5 g of 37% formalin, 160 g of composite particles A obtained in Example 1, 0.35 g of calcium fluoride and 200 g of water were supplied into a 500 ml three-necked flask. Under stirring, the solution was adjusted to a pH of 8.5 with sodium hydroxide, and the resultant mixture was heated to 85°C over a period of 40 minutes and reacted at this temperature for 15 minutes. Then the contents in the flask were cooled to 30°C, and after adding 30 g of 5% ammonium chloride, the resultant contents were heated to 85°C over a period of 60 minutes and reacted at this temperature for 90 minutes. The reacted product in the flask was again cooled to 30°C, transferred into a 1 litre beaker, washed with water several times and then air dried. The product was further dried at 100 - 150°C under reduced pressure (below 5 mmHg). The amount of melamine resin in the obtained melamine resin-coated composite particles, when calculated from measurement of magnetization, was 1.9% by weight based on the composite particles. The structure of the surface of the composite particle before coating with a melamine resin, that is,. the composite particle obtained in Example 1, is shown in Fig. 3 (scanning electron micrograph of 3,000 magnification). The melamine resin coat of the composite particles obtained in Example 9, as seen from a scanning electron micrograph (× 3,000 magnification) shown in Fig. 4, was sufficient and uniform, and it was also noted that the coating melamine resin was in the form of ultra-fine particles. EXAMPLE 10 Melamine resin coating was performed in the same manner as Example 9 except for using PVA instead of calcium fluoride as the suspension stabilizer. The main producing conditions in this process are shown in Table 3. The amount of melamine resin in the obtained melamine resin-coated composite particles, as calculated from measurement of magnetization, was 2.0% by weight based on the composite particles. The melamine resin coat of the composite particles obtained in Example 10, as observed by a scanning electron microscope, was sufficient and uniform, and the coat was composed of melamine resin in the form of ultra-fine particles. EXAMPLE 11 100 g of composite particles C obtained in Example 3, 3 g of melamine monomer, 8 g of 37% formalin and 100 ml of water were supplied into and mechanically stirred in a four-necked flask equipped with a condenser. The mixture was heated to 75°C and stirred for 2 hours while maintaining this temperature. Then the contents were cooled to room temperature, filtered, washed with water and dried and cured at 150°C under reduced pressure (below 5 mmHg) for 6 hours. The amount of melamine resin in the thus obtained melamine resin-coated composite particles, when calculated from the measurement of saturation magnetization, was 2.1% by weight based on the composite particles. Observation by a scanning electron microscope showed that the melamine resin coat of the composite particles obtained in Example 11 was sufficient and uniform, and also that the coat was composed of ultrafine particulate melamine resin. EXAMPLES 12 - 15 Melamine resin coating of composite particles was performed in the same manner as Example 11 except for changes of the kind and amount of composite particles, amount of melamine monomer, amount of aldehydes and amount of water. The main producing conditions in this process and various properties of the obtained melamine resin-coated composite particles are shown in Table 3. REFERENTIAL EXAMPLE 2 By using the melamine resin-coated composite particles obtained in Examples 9 - 15 as magnetic carrier, magnetic developers were prepared by mixing 100 parts by weight of the respective composite particles with 3 parts by weight of a commercial toner. Then, using each of the thus prepared developers, there was conducted a copying test in which 20,000 copies of A4 size paper were taken by an electrophotographic copying machine using α-Si as a photoconductive material. In the copying tests using the developers containing the magnetic carriers obtained in Examples 9 - 15, there were obtained clear copied images.

Claims: Composite carrier particles for electrophotography comprising: (i) from 80 to 99% by weight of ferromagnetic fine particles and (ii) a phenol resin, and having a number-average particle diameter of from 10 to 1,000 µm, a bulk density of not more than 2.0 g/cm³ and a curved surface configuration. Composite carrier particles according to claim 1, wherein the ferromagnetic fine particles are iron oxide particles of magnetite or maghemite, spinel ferrite or magneto-plumbite type ferrite. Composite carrier particles according to claim 1 or 2, which have a saturation magnetization of 40 to 150 Am²/kg (40 to 150 emu/g). Composite carrier particles according to any one of the preceding claims, which have a melamine resin coat on the surface thereof. Composite carrier particles according to claim 4, wherein the weight of the melamine resin coating is not less than 0.05% by weight based on the core composite particles. A process for producing the composite carrier particles of claim 1, which comprises reacting a phenol and an aldehyde in the presence of ferromagnetic fine particles, a suspension stabilizer and a basic catalyst in an aqueous medium. A process according to claim 6, wherein the molar ratio of aldehyde:phenol is from 1.1:1 to 1.6:1. A process according to claim 6 or 7, wherein the amount of the ferromagnetic fine particles is 4 to 100 times the weight of the amount of phenol. A process for producing the composite carrier particles of claim 4, which comprises reacting a melamine and an aldehyde in the presence of the composite particles of claim 1 in an aqueous medium thereby coating the surfaces of the composite particles with a melamine resin. A process according to claim 9, wherein the melamine is used in an amount of from 2 to 7% by weight based on the weight of the core composite particles.

Inventors: ASAMI KEIICHI, ECHIGO YOSHIAKI, FUJIOKA KAZUO, HAKATA TOSHIYUKI, KISHIMOTO SOUICHIRO, KURITA EIICHI, SAKAIDA TSUTOMU, TAKARAGI SHIGERU, TODA TETSURO, ASAMI, KEIICHI, ECHIGO, YOSHIAKI, FUJIOKA, KAZUO, HAKATA, TOSHIYUKI, KISHIMOTO, SOUICHIRO, KURITA, EIICHI, SAKAIDA, TSUTOMU, TAKARAGI, SHIGERU, TODA, TETSURO
Applicants: TODA KOGYO CORP, UNITIKA LTD, TODA KOGYO CORP., UNITIKA LTD.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9