Method of manufacturing of an electrophotographic toner

Abstract: An electrophotographic toner is made up of toner particles composed of irregularly-shaped core particles made chiefly of binder resin, and surface-modifying fine particles which are first dispersed over and attached to the surface of the core particles, and then affixed or formed into a film thereon. The BET specific surface area, based on N 2 adsorption, of the toner particles is less than 0.64 times the BET specific surface area of the core particles and surface-modifying fine particles combined together. Further, this value is 1.07 times the BET specific surface area of hypothetical toner particles which are perfect spheres. Consequently, the toner is not prone to problems such as filming, toner scattering, and fogging which are caused by peeling, separation, etc. of the surface-modifying fine particles, nor to poor cleaning due to spherical toner particles. Further, since the toner is manufactured with a quantitative grasp of the state of modification of the surface of the core particles by the surface-modifying fine particles, it is a toner in a stable state.

Description: The present invention concerns a method of manufacturing an electrophotographic toner which has undergone surface modification processing, for use in one-component or two-component developing agents used to develop electric or magnetic latent images in image-forming devices, such as copy machines and printers, which adopt the electrophotographic method. In image-forming devices, such as copy machines and printers, which use the electrophotographic method, images are generally formed as follows. First, toner having a positive or negative charge is electrostatically affixed to an electrostatic latent image formed on a photoconductive member (photoreceptor), so as to form a toner image. Then, this toner image is transferred to and fixed on a transfer material such as transfer paper. Toners used for this kind of image formation generally have an average particle diameter of 5µm to 20µm, and generally include at least a colorant and a binder resin for fixing the colorant, etc. to the transfer material (transfer paper etc.). In the past, various toners have been used as developing agents for developing latent images formed on photoreceptors in electrophotographic image-forming devices. One conventional method of manufacturing toner is, for example, grinding. This is a manufacturing method in which materials such as colorant, charge control agent, and anti-offset agent (mold release agent) are melted and kneaded together with a thermoplastic resin. This mixture is then cooled and hardened, and then ground and separated to produce toner particles. Another method is suspension polymerization, in which materials such as charge control agent are mixed and dispersed with polymerizable monomers, polymerization initiator, colorant, etc. This mixture is then polymerized in water. Again, there are wet methods such as the suspension granulation method, in which a colorant and a charge control agent are added to a synthetic resin. This mixture is then melted, suspended in a nonsolvent medium, and granulated. However, with toner produced by these manufacturing methods, the charge control agent, anti-offset agent, etc. exists within the toner particles. Further, only a small amount of these additives exists on the surface of the toner particles. For this reason, the charging quantity of the toner shows a wide distribution, and accordingly there are problems with toner scattering and image fogging. There are also cases when sufficient anti-offset effect cannot be obtained. Further, the charging quantity of the toner is generally controlled by a friction charging member such as a carrier or a charging blade. If the charging quantity is more than the optimum quantity, image density is too low, but if it is less than the optimum quantity, fogging, toner scattering, etc. occur, leading to deterioration of image quality. In order to prevent these kinds of problems, a charge control agent is generally internally added to the toner. For example, charge control agents added to positive-charging toners include nigrosine-based dyes, pyridinium salt, ammonium salt, and lake compounds of these. However, although these charge control agents internally added to the toner are fine particles, they have a wide particle size distribution, and have no set shape. Accordingly, control of the state of their dispersal within the particles of binder resin is difficult. For example, if the particles of charge control agent dispersed within the binder resin particles are too large in diameter, the charge control agent is likely to separate out during successive copying, dirtying the charging member (carrier etc.). Again, if the particles of charge control agent dispersed within the binder resin particles are too small in diameter, their charge controlling effect is weakened. This has the drawback that the supplied toner has a slow charging response, giving rise to image fogging, toner scattering, etc. Further, the proportion of internally added charge control agent which is exposed on the surface of the toner particles differs according to the dispersal conditions at the time of production. Accordingly, another drawback is that the charging quantity of the toner is difficult to stabilize. In addition, it is even more difficult to control the dispersal of the charge control agent with toners formed by polymerization. As discussed above, it is difficult to take full advantage of the effects of charge control agents, anti-offset agents, etc. if they are merely internally added to the toner. An alternative method of controlling toner charging is a technique for applying mechanical impact force, using a particle surface modification device, to attach to the surface of the toner particles chargeable inorganic particles made of a material such as silica, alumina, or titanium oxide, which have been surface processed with a material such as silane coupler or silicon oil. However, in order to give the toner sufficient chargeability using these chargeable inorganic fine particles, they must be used in great quantity. Again, in order to fully attach the chargeable inorganic fine particles to the surface of the toner particles by means of mechanical impact force, attachment processing must be continued until surface unevenness of the toner particles is eliminated, even when non-spherical toner particles are used. As a result, toner particles which have undergone attachment processing become perfect spheres without points, which impairs blade cleaning and leads to poor cleaning. In recent years, the development of high-speed copy machines, environment-responsive copy machines, etc., has created a need for development of toner capable of low-energy fixing (low-temperature fixing). Accordingly, as a means of attaining low-temperature fixing, methods using toners including binder resins with low glass transition points or softening points have been investigated. One example of a technique for attaining low-temperature fixing is Japanese Examined Patent Publication No. 36586/1982 (Tokukosho 57-36586), which discloses a toner which uses as binder resin a crystalline polymer having a melting point of 50°C to 150°C and an activation energy of 35kcal/mol or less. Further, Japanese Unexamined Patent Publication No. 87032/1975 (Tokukaisho 50-87032) (corresponding to US Patent No. 3,967,962) discloses a toner which uses a polymer formed by chemical bonding of a crystalline polymer with a melting point of 45°C to 150°C and a non-crystalline polymer with a glass transition point of 0°C or lower. Again, Japanese Unexamined Patent Publication No. 3446/1984 (Tokukaisho 59-3446) (US Patent No. 4,528,857) discloses a toner which uses a block co-polymer, in which a crystalline block, with a melting point of 50°C to 70°C, is included in a non-crystalline block molecule with a glass transition point 10°C higher than the melting point of the crystalline block. However, use of these conventional low-temperature-fixing toners was difficult because of such problems as toner filming phenomenon caused by the soft portion of polymers, deterioration of toner chargeability, photoreceptor characteristics, etc. in, for example, successive copying, and blocking phenomenon. In other words, attaining low-temperature fixing by using toners with low glass transition points or softening points had serious problems such as deterioration of the toner's resistance to blocking, not to mention filming phenomenon and cffset phenomenon. For this reason, in the past, a method of adding an external additive to prevent deterioration of the toner's resistance to blocking has been adopted. However, if this external additive is not attached to the toner particles but can move freely, it moves from the toner to the carrier when the carrier and toner are mixed, thus changing the quantity of charging, etc. As a result, the toner's stability over time (toner life during successive copying) deteriorates, which leads to impairment of image quality. In recent years, the electrophotographic process has been adopted in various fields such as printers, facsimiles, color copy machines, and high-speed copy machines, and thus toners are needed which combine various characteristics (such as control of charge polarity) corresponding to these various fields and functions. In response to this need, numerous electrophotographic toners of a type called "surface-modified toner," which gives the electrophotographic toner various characteristics, are being investigated. Some examples of surface-modified toners are a toner to which are added fine particles having various functions, such as charge control agent; an electrophotographic toner in which durability, fixing characteristics, etc. are improved by using fine particles of hardened resin to cover the surface of core particles having a low softening point; and a toner which improves charging characteristics, fluidity characteristics, etc. by means of processing to make the toner particles spherical. In particular, many surface-modified toners have been proposed in which surface-modifying fine particles of, for example, charge control agent are dispersed over and attached to the surface of core particles of colorant, and then affixed or formed into a film thereon. For example, Japanese Examined Patent Publication No. 17576/1989 (Tokukohei 1-17576) discloses an electrophotographic toner in which particles of colored resin powder are covered with a layer of a fine powder of resin or polymeric material having a particle diameter of not more than 1/10 of that of the colored resin powder. This toner is formed by coverage processing until the particles of fine powder are embedded over part of the surface of each particle of colored resin, and then heating to fuse the particles of fine powder together, forming a covering on each particle of colored resin. Again, Japanese Unexamined Patent Publication No. 3171/1992 (Tokukaihei 4-3171) (corresponding to US Patent No. 5,206,109) discloses a manufacturing method in which surface-modifying fine particles are attached to the surface of core particles, uniformly affixed thereto by application of mechanical impact force, and then uniformly fixed or turned into a film thereon by heating in a hot air flow at 200°C to 600°C. Again, Japanese Examined Patent Publication No. 56502/1993 (Tokukohei 5-56502) proposes a surface-modified toner in which mechanical impact force is applied to attach fine powder having various functions, 2µm or less in average particle diameter, to the surface of particles of a binder resin powder made chiefly of binder resin. In this toner, attachment is performed by imbedding the particles of fine powder in the surface of each particle of binder resin powder, so that the thickness of the surface layer produced will be 2µm or less, while heating at a temperature of at least 48°C, but below the melting point of the binder resin. Japanese Unexamined Patent Publication No. 34971/1993 (Tokukaihei 5-34971) discloses the following method of manufacturing electrophotographic toner. First, in a processing room, a rotating member is rotated, mixing toner core particles (chiefly made of at least resin) with surface-processing fine particles in a high-speed air flow. By means of this mixing, the fine particles can be uniformly dispersed and attached over the surface of each toner core particle. Then, by intensifying the mixing conditions, the fine particles attached to the surface of the toner core particles are fixed and/or turned into a film thereon. However, electrophotographic toners produced by the grinding or wet methods discussed above, which are not surface-modified toners, have the following problems. Generally, electrophotographic toners have charging characteristics (including polarity) which vary according to the needs of the object for which and the environment in which they are to be used. In other words, different types of electrophotographic toner include different quantities of charge control agent, etc. Accordingly, when a single electrophotographic toner manufacturing device is to be used to manufacture different types of electrophotographic toner, any previously manufactured toner remaining in the manufacturing device will cause problems such as increase of the quantity of toner with reverse polarity in the subsequently manufactured toner, decrease of the toner's charging stability, etc. In order to avoid these problems, different production lines are usually provided for toners with different polarity, or thorough maintenance cleaning of the manufacturing device is performed. Again, even with electrophotographic toners of the same polarity, according to the required characteristics, different types of charge control agents are used. The composition of toners also varies. Accordingly, even when manufacturing electrophotographic toners of the same polarity, if the same manufacturing device is to be used, thorough maintenance cleaning of the manufacturing device must be carried out, as above, in order to avoid contamination from different charge control agents or toner materials. In this way, when manufacturing electrophotographic toners of this type, maintenance cleaning of the manufacturing device must be performed whenever the type of toner is changed. This accordingly has drawbacks such as increase in the cost of manufacturing the toner arising from the costs of cleaning and of materials discarded and wasted at the time of cleaning. In addition, the foregoing conventional methods of manufacturing surface-modified toners merely propose methods of affixing/forming a film of surface-modifying fine particles on the surface of core particles by mechanical or heat processing, or toners produced by such methods. These conventional manufacturing methods perform mechanical impact or heat processing to obtain a toner with long life, in which the surface-modifying fine particles on the surface of the core particles will withstand the stress of use without peeling or separation. Accordingly, the toner particles produced are made spherical, which reduces friction with the cleaning device, leading to problems such as poor cleaning. Further, the actual state of the toner obtained by surface modification is determined only by visual means such as observing the surface of particles of surface-modified toner through an SEM (Scanning Electron Microscope). In other words, the state of the toner is not grasped quantitatively, either during or after manufacturing. Accordingly, with the conventional manufacturing methods, it is difficult to determine whether the surface-modified toner which has been manufactured sufficiently realizes desired functions. As a result, there is a great possibility that a toner will be manufactured which is not uniform and which lacks stability. In addition, none of the conventional art gives any consideration to the weight-average molecular weight of the polymer particles (surface-modifying particles) to be affixed or made into a film on the surface of the core particles. Incidentally, there is a method of evaluating the state of surface-modified toner produced which uses the BET specific surface area, based on N 2 adsorption. The BET specific surface area of surface-modifying particles to be affixed to the surface of core particles is specified in Japanese Unexamined Patent Publication No. 335357/1992 (Tokukaihei 4-335357). However, the BET specific surface area of the surface-modified toner produced is not discussed. Further, this disclosure does not hit upon the idea of quantitatively grasping the state of surface modification. If the core particles for surface-modified toner are to be manufactured by polymerization, facilities for control of dangerous substances such as monomers and initiators, processing of waste water, etc. are necessary, which requires large investments in facilities and increases the expenses of repayment of these investments. Further, washing and drying processes take a long time, thus reducing productivity. In addition, since the fine powder cannot be reused, manufacturing costs are increased in comparison with grinding. In addition, since in this case the electrophotographic toner particles obtained are nearly spherical, their reduced friction results in reduced attaching force. Spherical toner particles also have a negative effect on the cleaning process. This cleaning process is the removal, using a cleaning brush, etc., of untransferred toner remaining on the photoreceptor after transfer of the toner image. When the untransferred toner is spherical, it has insufficient attaching force with respect to the cleaning brush, and its removal is made difficult. Further, in actual use of electrophotographic toner in, for example, a high-speed copy machine (copy speed of 60 sheets/minute or more), there are cases when high stress may be applied within the developer, etc. At this time, this stress may cause peeling or separation of the fine particles of charge control agent from the surface of the core particles, leading to so-called image fogging. Accordingly, in such cases, stronger affixing/film formation of the fine particles of charge control agent on the surface of the core particles is needed. However, this kind of stronger affixing/film formation cannot be realized with manufacturing methods in which all processing is carried out in a surface modification device such as a Henschel-type mixer. For this reason, electrophotographic toner which is to be used in a device which applies high stress thereto should preferably be manufactured using a high-energy-applying surface modification device capable of affixing/film formation by applying high shearing force, high impact force, or high energy. However, if a high-energy-applying device is used from the stage of manufacturing at which the core particles and the fine particles of charge control agent are combined, affixing/film formation of the fine particles proceeds before the fine particles have been uniformly dispersed. As a result, the charge control particles may become affixed to the core particles in a non-uniform state, or a film of non-uniform thickness may be formed. This may lead to manufacturing of electrophotographic toner which lacks charging stability. Conventional art has also been proposed in which functional fine particles such as charge control agent are dispersed over and attached directly to colored core particles, and then affixed and/or formed into a film thereon. However, no electrophotographic toner has been proposed in which polarity can be controlled even when using the same core particles. Further, no proposal has noted the advantages and effects which could be obtained, in manufacturing electrophotographic toner, by providing a step after production of the core particles, in which they are given a charge of the required polarity. The present invention was created in order to solve the foregoing problems of the conventional art. Its first object is to provide a method of manufacturing of a surface-modified toner capable of improving stability over time (toner life during successive copying) by preventing problems such as filming, toner scattering, and image fogging due to peeling, separation, etc. of surface-modifying fine particles made of, for example, fine polymer particles, and to prevent poor cleaning due to spherical toner particles. Further, a second object of the present invention is to provide a method of manufacturing of a toner capable of low-temperature fixing, and which has superior heat resistance, i.e., storage stability (anti-blocking) characteristics. Further, a third object of the present invention is to provide a method of manufacturing electrophotographic toner which does not require provision of separate production lines for each type of electrophotographic toner to be manufactured, and which, when different types of electrophotographic toner are to be manufactured on the same production line, does not require thorough maintenance cleaning whenever the type of toner is changed. In order to attain the first object mentioned above, a method of manufacturing an electrophotographic toner according to the present invention is made up of irregularly-shaped core particles chiefly composed of binder resin, and surface-modifying fine particles which are first dispersed over and attached to the surface of the core particles, and then affixed or made into a film thereon, so as to produce toner particles, in which: the BET specific surface area, based on N 2 adsorption, of the toner particles satisfies: 0.64S 0 > S > 1.07×[3/(ρD/2)]; and S 0 = S 1 X+S 2 (1-X), where: S is the BET specific surface area of the toner particles; S 0 is the BET specific surface area of the core particles and the surface-modifying fine particles combined together; S 1 is the BET specific surface area of the core particles alone; S 2 is the BET specific surface area of the surface-modifying fine particles alone; ρ is the specific gravity of the toner particles; D is the average particle diameter of the toner particles by volume; and X is a ratio of the amount of surface-modifying fine particles (parts by weight) to the amount of the sum of surface-modifying fine particles and core particles (both parts by weight). With the foregoing structure, the toner's BET specific surface area is less than 0.64S 0 ; in other words, the surface-modifying fine particles are sufficiently affixed to the surface of the core particles, and thus problems like filming and toner scattering will not occur. Further, the toner's BET specific surface area is more than 1.07 times that of hypothetical toner particles which are perfect spheres; in other words, the toner particles are not spherical, and thus poor cleaning can be prevented. As a result, a surface-modified toner can be obtained in which the surface-modifying fine particles dispersed over and attached to the surface of the core particles are affixed or made into a film thereon strongly enough so that they will not peel or separate therefrom, but without producing spherical toner particles, thus avoiding problems such as poor cleaning. In addition, since the foregoing electrophotographic toner is obtained by exposure to a hot air flow of 150°C to 400°C after the fine polymer particles have been dispersed over and attached to the surface of the core particles, the fine polymer particles and the core particles are sufficiently fused without making the irregularly-shaped core particles spherical. As a result, problems such as filming, toner scattering, and image fogging, which are caused by peeling, separation, etc. of the fine polymer particles due to, for example, mechanical stress in the developing vessel during successive copying, can be prevented. Thus stability over time (toner life during successive copying) can be improved. Further, poor cleaning due to spherical toner particles can also be prevented. In addition, since the glass transition point of the fine polymer particles is higher than that of the core particles, the glass transition point of the core particles being 40°C to 65°C, and that of the fine polymer particles being 58°C to 100°C, the foregoing electrophotographic toner enables low-temperature fixing (low-energy fixing), and has superior heat resistance, i.e., storage stability (anti-blocking) characteristics. The method of manufacturing electrophotographic toner according to the present invention includes the steps of: (a) producing core particles for electrophotographic toner; and (b) using dry processing to attach fine particles to the surface of the core particles, and then to affix or form the fine particles into a film thereon; in which electrophotographic toners with different properties may be prepared by producing core particles having a common composition and by means of a common process, but changing the type or composition of the fine particles. With the foregoing method, even when manufacturing different types of electrophotographic toners, a single production line for the core particles is sufficient, after which the fine particle affixing step (b) may be performed by means of simple dry processing. Accordingly, there is no need to provide separate electrophotographic toner production lines for electrophotographic toners with different properties. Accordingly, investment in facilities may be reduced. Further, since the fine particle affixing step (b) is simple dry processing, there is little contamination of the interior of the manufacturing device. Accordingly, even when manufacturing different types of electrophotographic toner on the same electrophotographic toner production line, it is not necessary to perform thorough maintenance cleaning in order to remove previously manufactured electrophotographic toner remaining in the manufacturing device. In addition, the quantity of electrophotographic toner discarded at the time of cleaning can be reduced to a minimum. Accordingly, manufacturing costs of the electrophotographic toner can also be reduced. Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings. Figure 1(a) is an explanatory drawing showing the form of a core particle and surface-modifying fine particles which make up an electrophotographic toner according to the present invention. Figure 1(b) is an explanatory drawing showing the form of a combined particle made of the core particle and surface-modifying fine particles shown in Figure 1(a). Figure 1(c) is an explanatory drawing showing change in the state of surface modification of the combined particle shown in Figure 1(b) in accordance with hot air temperature. Figure 2 is an explanatory drawing showing a heat processing device for manufacturing electrophotographic toner according to the present invention. Figure 3 is an explanatory drawing showing the structure of a particle of electrophotographic toner according to the present invention. The present invention will be explained below. As shown in Figure 2, a heat processing device for manufacturing a surface-modified electrophotographic toner (hereinafter referred to simply as "toner") according to the present invention includes a hot air producing device 11, a fixed quantity supplier 12, a cooling/recovery device 13, and a diffusion nozzle 14. The following will explain the manufacture of toner using this heat processing device. Figure 1(a) is an explanatory drawing showing the form of a core particle 1 and surface-modifying fine particles 2. The core particle 1 is composed chiefly of binder resin, is irregularly shaped, and is obtained by a method such as grinding. Incidentally, "irregular shape" means any shape other than a perfect sphere. First, the core particle 1 and the surface-modifying fine particles 2, which, as shown in Figure 1(a), initially exist separately, are combined by attaching the surface-modifying fine particles 2 to the surface of the core particle 1, forming a combined particle 3. The form of the combined particle 3 is shown in Figure 1(b). Then, a predetermined quantity of combined particles 3, in which the surface-modifying fine particles 2 are uniformly dispersed over the surface of the core particles 1, are supplied to the fixed quantity supplier 12 shown in Figure 2. Next, the combined particles 3 are sprayed, along with compressed air, from the fixed quantity supplier 12 through the diffusion nozzle 14 and into a hot air flow area A. The hot air flow area A is hot air produced by the hot air producing device 11, the temperature of which is adjusted to a predetermined level. In the hot air flow area A, heat energy is instantly applied to the combined particles 3. Then, in order to affix or form a film of the surface-modifying fine particles 2 on the surface of the core particles 1, the combined particles 3, to which the heat energy has been applied, are guided into the cooling/recovery device 13 and immediately cooled by cold air. This cold air may be external air of normal temperature (approximately 25°C), or cooled air of adjusted temperature. Toner particles of a predetermined state, which have undergone surface modification in a heat processing device of this kind, are recovered at a temperature lower than the glass transition point of the chief resin of the core particles, and turned into commercial products. At this time, the surface-modified toner is manufactured so that the BET specific surface area, based on N 2 adsorption, of the toner particles satisfies: 0.64S 0 > S > 1.07S calc S 0 = S 1 X+S 2 (1-X) S calc = (surface area of perfect sphere) / (density × volume of perfect sphere) = 4π(D/2) 2 /[ρ×(4π/3)×(D/2) 3 ] = 3/(ρD/2) Here, S is the BET specific surface area of the toner particles; S 0 is the BET specific surface area of the core particles and the surface-modifying fine particles combined together; S 1 is the BET specific surface area of the core particles alone; S 2 is the BET specific surface area of the surface-modifying fine particles alone; ρ is the specific gravity of the toner particles; D is the average particle diameter of the toner particles by volume; and X is a ratio of the amount of surface-modifying fine particles (parts by weight) to the amount of the sum of surface-modifying fine particles and core particles (both parts by weight). Incidentally, the average particle diameter by volume is particle diameter based on a mass standard. The BET specific surface area based on N 2 adsorption is the surface area per unit mass of a powder, which is calculated from the volume of nitrogen (N 2 ) adsorbed by the powder by using the BET adsorption isotherm. It is preferable if the BET specific surface area of the toner particles is as shown by: 0.60S 0 ≥ S ≥ 1.10S calc Further, it is even more preferable if the toner's BET specific surface area is as shown by: 0.38S 0 ≥ S ≥ 1.12S calc Appropriate control of the various operating parameters of the manufacturing process is sufficient to ensure that the toner satisfies the conditions of equations (1), (4), and (5). These parameters include, for example, device conditions such as the quantity of combined particles processed, the temperature of the hot air produced by the hot air producing device 11, the duration of exposure of the combined particles in the hot air flow area A, the angle of the diffusion nozzle 14, and the rate of flow ratio (proportion of speed of particles to speed of hot air flow), and the composition, combination ratio, particle diameter, shape (chiefly the core particles), glass transition point, and molecular weight of the core particles and surface-modifying fine particles. In equations (1), (4), and (5), the value on the left side shows the extent of surface modification based on the extent of fusing of the surface-modifying fine particles, the way heat is applied, etc., and the value on the right side shows the extent to which the toner particles are made spherical (including surface smoothness). Accordingly, with this manufacturing method, the extent to which the toner particles are made spherical can be quantitatively grasped by means of the BET specific surface area based on N 2 adsorption, allowing control of the state of surface modification in order to manufacture a uniform and stable toner. Further, in the foregoing heat processing device, when affixing or forming a film of the surface-modifying fine particles on the surface of the core particles, heat is applied to the surface of the combined particles instantly (no more than 1 second) using hot air more than 100°C but less than 450°C in temperature, or more preferably 150°C to 400°C. By this means, a temperature above the softening point of the surface-modifying fine particles and the core particles is applied to the surface-modifying fine particles and the surface of the core particles, but a heat quantity sufficient to soften the core particles does not reach their interior. For this reason, as shown in Figure 1(c) at c2 and c3, it is possible to create a state in which the surface-modifying fine particles are fused and affixed or formed into a film on the surface of the core particle, but the irregular shape of the core particle is maintained. Incidentally, in Figure 1(c), the portion to the left of each broken line shows the state of affixing, in which the surface-modifying fine particles are affixed over part of the core particle. The portion to the right of each broken line shows the state of film formation, in which the surface-modifying fine particles are formed into a film covering the entire surface of the core particle. However, in the heat processing mentioned above, if the temperature of the hot air is less than 100°C, heat energy sufficient to affix or form a film of the surface-modifying fine particles cannot be applied (see Figure 1(c) at c1). Again, if the temperature of the hot air is more than 450°C, the core particles become more spherical (see c4), and mutual fusing and aggregation of the toner particles during surface modification occurs (see c5), sometimes making it impossible to obtain toner with a predetermined particle diameter. If processing speed is slowed in order to avoid this, problems arise, such as reduction of production efficiency and increase of production costs. In manufacturing the toner, in order to obtain the initial state of attachment, combination, and dispersal, a device such as the Mechano-mill (Okada Precision Industries Co., Ltd. product), the Mechanofusion System (Hosokawa Micron Co., Ltd. product), the Hybridization System (Nara Machinery Manufacturing Co., Ltd. product), or the Cosmos System (Kawasaki Heavy Industries Co., Ltd. product) may be used. Again, as a heat processing device, a device able to produce a hot air flow, such as the Suffusing System (Japan Pneumatic Industries Co., Ltd. product), may be used. A suitable state of the toner which satisfies equation (1) is a state in which the surface-modifying fine particles are attached and affixed or formed into a film on the surface of the core particles in such a way that the following toner particles (see Figure 1(c) at c2 and c3) are produced. Namely, the toner particles produced have a BET specific surface area, based on N 2 adsorption, which is less than 0.64 times the BET specific surface area (S 0 ) of the combined core particles and surface-modifying fine particles (which is calculated from the BET specific surface area (S 1 ) of the core particles alone, the BET specific surface area (S 2 ) of the surface-modifying fine particles alone, and the ratio of composition between the two kinds of particles), but is more than 1.07 times the BET specific surface area (S calc ) of hypothetical toner particles which are perfect spheres (which is calculated from the average particle diameter by volume of the toner produced). Further, it is more preferable if the toner particles produced also satisfy equations (4) and (5). When image formation is performed using a toner obtained in this way, there is no occurrence of phenomena such as filming, which is caused by surface-modifying fine particles peeling or separating from the core particles and becoming attached to the photoreceptor, or toner scattering and image fogging, which are caused by free toner particles. Accordingly, stable images can be obtained. Further, in order to obtain a toner which will not cause poor cleaning at the time of use, the toner must be manufactured giving consideration to a balance between (i) the extent to which the core particles are made spherical in surface modification processing and (ii) the extent to which the surface-modifying fine particles are affixed or formed into a film. Consideration may be given to this balance by using the BET specific surface area discussed above to control the conditions of manufacturing the toner, which is obtained by affixing or forming a film of the surface-modifying fine particles on the core particles. However, with toner particles in a state (see Figure 1(c) at c1) in which the BET specific surface area is more than the value on the left side of equation (1), i.e., more than 0.64S 0 , the surface-modifying fine particles will be insufficiently affixed. Accordingly, with particles in this state, separation, peeling, etc. of the surface-modifying fine particles occurs, causing such problems as filming and toner scattering. Again, with toner particles in a state (see Figure 1(c) at c4 and c5) in which the BET specific surface area is less than the value on the right side of equation (1), i.e., less than 1.07S calc , poor cleaning arises due to the detrimental effects of spherical toner particles, and image fogging occurs due to mutual fusing and aggregation of the toner particles. The binder resin used for the core particles of the toner may be, for example, polystyrene, styrene-acrylic copolymer, styrene-acrylonitryl copolymer, styrene-maleic anhydride copolymer, styrene-acrylic-maleic anhydride copolymer, polyvinyl chloride, poly-olefin resin, epoxy resin, silicone resin, polyamide resin, polyurethane resin, urethane-modified polyester resin, or acrylic resin, or a mixture of any of these, or a block copolymer or graft copolymer combining any of these. For binder resin, all materials may be used which have a molecular weight distribution well-known for use in toner, such as one-peak or two-peak distribution. Further, one or more well-known function-imparting agent may be mixed and dispersed into the binder resin forming the core particles. These function-imparting agents include, but are not limited to, charge control agents like azo-based dye, carboxylic acid metal complexes, quaternary ammonium compounds, and nigrosine-based dye; colorants like carbon black, iron black, nigrosine, benzine yellow, and phthalocyanine blue; and anti-offset agents like polyethylene, polypropylene, and ethylene-propylene copolymers. Further, magnetic powder may also be included. The core particles should preferably have heat characteristics whereby their glass transition point (Tg 1 ) is from 40°C to 70°C. By this means, low-temperature fixing of the toner can be improved. In contrast, core particles having a glass transition point of less than 40°C will easily melt when undergoing heat processing at over 150°C, thus becoming spherical. Accordingly, poor cleaning will arise in actual use. Again, with core particles having a glass transition point of more than 70°C, the toner produced will not melt sufficiently when being fused and fixed onto the paper in regular heat fixing. Since adhesion to the paper is impaired in this way, the image is likely to peel or rub off on surfaces it touches, because strong fixing cannot be obtained. Further, since the surface of the core particles is covered with surface-modifying fine particles having an even higher glass transition point, such a toner is not suitable for actual use. A core particle diameter similar to that of typical powdered toners is suitable. An average particle diameter by volume of 5µm to 15µm is appropriate. As surface-modifying fine particles to be attached to and affixed or formed into a film on the core particles, charge control agent, fluidizing agent, and/or colorant may be used. Again, organic fine particles and/or magnetic or non-magnetic inorganic fine particles intended to impart functions, such as anti-offset agent, may also be used. Examples of such inorganic fine particles include titanium and silicon. In particular, when thermoplastic organic fine particles are used, the foregoing toner manufacturing method, which is characterized by heat processing, can be made even more effective. Concrete examples of thermoplastic organic fine particles (organic surface-modifying fine particles) which may be used include methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and homopolymers or copolymers made of monomers such as styrene, p-methyl styrene, sodium styrensulfonate, vinyl benzyl chloride, acrylic acid, dimethyl aminoethyl acrylate, methacrylic acid, and dimethyl aminoethyl methacrylate. Further, examples of polymerization initiators which may be used in polymerization to give the thermoplastic organic fine particles a positive or negative charging function include potassium persulfate, ammonium persulfate, and amidinopropane-base, or a monomer having a polar group such as an amino group, an amide group, a carboxylic acid group, or a sulfonic acid group. Further, examples of substances which may be used to give the thermoplastic organic fine particles an anti-offset effect include polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, ethylene-ethylacrylate copolymer, and an ionomer having a polyethylene structure. It is especially preferable if the thermoplastic organic fine particles have an average particle diameter by volume of no more than 1µm. This is because, when combining the core particles and surface-modifying fine particles, uniform dispersal of the surface-modifying fine particles over the surface of the core particles is preferable in order to obtain good surface modification. If the surface-modifying fine particles are too large, dispersal and attachment of the surface-modifying fine particles over the surface of the core particles becomes difficult. In other words, if dispersal, attachment, and combination processing is performed using surface-modifying fine particles having an average particle diameter by volume of more than 1µm, it may be impossible to attach them to the surface of the core particles using weak forces such as electrostatic force and van der Waals force, and they may exist separately from the core particles. Further, in this case, since the layer of surface-modifying fine particles is thicker, instantaneous heat processing at 150°C to 400°C for 1 second or less does not result in the application of sufficient heat energy to the combined particles. This may make it impossible to sufficiently fuse and affix the surface-modifying fine particles to the core particles. Raising the temperature of heat processing in consideration of the foregoing makes the core particles spherical, and thus is not preferable. Accordingly, by selecting surface-modifying fine particles with an average particle diameter by volume of 1µm or less, strong affixing or film formation, which is more resistant to stress, can be obtained. As a result, a good surface-modified toner can be obtained which is free of peeling or separation during use, and which does not cause poor cleaning. The thermoplastic organic fine particles should preferably have heat characteristics whereby their glass transition point (Tg 2 ) is higher than that of the core particles (Tg 1 ), and within a range from 60°C to 100°C. If the glass transition point is higher than 100°C, heat processing at 150°C to 400°C for 1 second or less will not result in the application of sufficient heat energy. Accordingly, sufficient fusing and attachment is not possible. Further, if more heat energy than this is applied, the core particles become spherical, which may lead to problems such as toner scattering and filming. Again, if the glass transition point of the thermoplastic organic fine particles is less than 60°C, the toner produced will have poor preservation (stability in storage), and will be prone to mutual fusing and aggregation of toner particles. Further, the surface-modifying fine particles themselves, being brittle, will have inferior durability, making the toner unsuitable for actual use. The thermoplastic organic fine particles should preferably have heat characteristics whereby their weight-average molecular weight (Mw) is from 50,000 to 210,000. If the weight-average molecular weight is more than 210,000, instantaneous heat processing at 150°C to 400°C for 1 second or less will not result in the application of sufficient heat energy. This may make it impossible to sufficiently fuse and affix the surface-modifying fine particles to the core particles. If the heat energy is increased in order to fuse and affix the surface-modifying fine particles, the core particles become spherical, leading to problems such as toner scattering and filming. Again, if the weight-average molecular weight of the thermoplastic organic fine particles is less than 50,000, the toner produced will have inferior preservation (stability in storage), and the toner particles may mutually fuse or aggregate. Further, the surface-modifying fine particles themselves, being brittle, will have inferior durability, and the strength of the image formed will be impaired. By selecting surface-modifying fine particles with a weight-average molecular weight within the range specified above, a strong state of affixing or film formation, which is more resistant to stress, can be obtained, and thus a superior toner can be obtained which is free of peeling or separation during use, and which does not cause poor cleaning. With toner in which surface-modifying fine particles are first dispersed over and attached to, and then affixed or formed into a film on, core particles, the strength of attachment of the surface-modifying fine particles varies according to the compatibility between the affixed or filmed surface-modifying fine particles and the surface of the core particles. For example, with a combination such as water and oil, even if a film is formed, the fusing force at the interface between the two kinds of particles is weak, and the film will peel at the application of the slightest stress. Accordingly, by selecting a combination with good affinity, a toner with stronger attachment can be manufactured, which is not prone to problems in actual use such as toner scattering, image fogging, and filming. In particular, in the manufacturing method according to the present embodiment, heat processing of short duration is used to affix or form a film of the surface-modifying fine particles on the core particles without making the core particles spherical. Accordingly, compatibility of the core particles and surface-modifying fine particles (i.e., the surface characteristics between the core particles and the surface-modifying fine particles) is a more important issue than in manufacturing methods which, for example, embed the surface-modifying fine particles in the surface of the core particles by means of mechanical impact force. One index of the compatibility of an organic high molecular material is its solubility parameter (SP) value. This SP value is the square root of a value obtained by dividing the molar vaporization energy of liquid organic high molecular material by its molar volume. SP values of from 6 to 17 are typical. High molecular materials having close SP values are generally considered to have good compatibility. For example, the following materials widely used as binder resins for toner have the following SP values: styrene- (meth) acrylic resins, 8.3 to 9.5; polyester resins, around 10.7. Again, the following materials used as organic surface-modifying fine particles have the following SP values: polymethyl methacrylate (PMMA), 8.9 to 9.5; polybutyl methacrylate (PBMA), 8.4 to 9.5. Incidentally, these ranges in SP value are due to differences in the resins' molecular weight, composition, etc., the quantity of polymerization initiator added, etc. Here, in combining the core particles and organic surface-modifying fine particles, the two materials combined can be said to have good compatibility if the absolute value of the difference in their SP values is 2.0 or less. In this case, since strong affixing or film formation of the surface-modifying fine particles is possible, a good state, in which they will not peel or separate, can be obtained. However, with combinations in which the absolute value of the foregoing difference is more than 2.0, the surface-modifying fine particles are likely to peel or separate due to the stress of stirring within the developer, etc., causing such problems as toner scattering and filming. After calculating the quantity of surface-modifying fine particles needed to cover the surface of a core particle from the diameter of the surface-modifying fine particles, the quantity of organic surface-modifying fine particles to be added is generally determined by the percentage of the surface of the core particles to be covered, or by the qualities of the layer of surface-modifying fine particles to be attached. In the manufacturing method according to the present embodiment, any quantity of surface-modifying fine particles able to be attached to the surface of the core particles during attachment/combination processing can be affixed or formed into a film thereon during the surface modification processing. Generally, the quantity added will be no more than 20 parts surface-modifying fine particles to 100 parts core particles by weight. However, in the manufacturing method according to the present embodiment, it is preferable if the quantity of surface-modifying fine particles added is from 0.1 part by weight to 15 parts by weight. If less than 0.1 part by weight is added, the quantity of surface-modifying fine particles on the surface of the core particles will be too small. In this case, problems will arise, such as lack of preservation because of insufficient coverage of the surface of the core particles, loss of the effects of surface modification because the core particles easily become spherical, etc. Again, if more than 15 parts surface-modifying fine particles by weight are added, the layer of surface-modifying fine particles on the surface of the core particles will be too thick. In this case, with the instantaneous heat processing of the manufacturing method according to the present embodiment, sufficient heat will not reach the surface of the core particles, and the fusing needed to affix or form a film of the surface-modifying fine particles will not be attained, which is likely to lead to problems such as filming, scattering, and image fogging due to peeling or separation. Raising the temperature of the heat processing in order to avoid this is not preferable, because the core particles become spherical, and mutual fusing of toner particles occurs. For this reason, by selecting the quantity of surface-modifying fine particles to be added from within the range specified above, desired functions (charge control, improvement of preservation, etc.) can be imparted, a strong state of affixing or film formation, which is more resistant to stress, can be obtained, and thus a superior toner can be obtained which is free of peeling or separation during use, and which does not cause poor cleaning. Incidentally, improving cleaning characteristics by using irregularly-shaped toner particles has the opposite effect from improving charging characteristics and fluidity by making toner particles spherical. However, the charging characteristics and fluidity needed in a powdered toner vary according to the copy machine or printer used. Accordingly, it is not always necessary to improve charging characteristics and fluidity by making toner particles spherical. Concrete examples of toners according to the present embodiment will be explained below as concrete examples 1 through 6. (CONCRETE EXAMPLE 1) The core particles used in concrete example 1 were prepared by mixing, by weight, 100 parts styrene-acrylic copolymer binder resin, 6 parts carbon black, and 3 parts low molecular weight polypropylene in a Henschel mixer, melting and kneading this mixture at 150°C using a two-shaft extruding kneader, and then, after cooling, the kneaded mixture was first coarsely ground using a feather mill, and then ground and separated in a jet mill. These core particles were irregularly-shaped particles having an average diameter by volume of 10.5µm, and a BET specific surface area (S 1 ) of 1.70m 2 /g. The organic surface-modifying fine particles used were made of polymethyl methacrylate (PMMA), and had an average diameter by volume of 0.15µm, and a BET specific surface area (S 2 ) of 37.8m 2 /g. Using the foregoing core particles and surface-modifying fine particles, toner was prepared according to the following method. First, setting the amount of surface-modifying fine particles added at 5 parts by weight (X = 5/(100+5) ≒ 0.048) to 100 parts by weight of core particles, the two kinds of particles were put in a Henschel-type mixer and stirred at 1500rpm (peripheral speed 10m/s) for 30 minutes. In this way, the surface-modifying fine particles were dispersed over and attached to the surface of the core particles by van der Waals force and electrostatic force, yielding combined particles in an ordered mixture. Using the hot air flow surface modification device Suffusing System (Japan Pneumatic Industries Co., Ltd. product) for hot air flow processing (heat processing as shown in Figure 2) to affix or form a film of the surface-modifying fine particles, toner was obtained by exposing the combined particles to the hot air flow for a short duration of 1 second or less. Here, for measurement of the BET specific surface area of the core particles (S 1 ), the BET specific surface area of the surface-modifying fine particles (S 2 ), and the BET specific surface area of the toner obtained (S), a value obtained by the one-point measurement method using the BET specific surface area measurement device Gemini 2360 (Shimadzu Manufacturing product) was adopted. For measuring the average particle diameter of the core particles by volume and the average particle diameter of the toner by volume (D), the Multisizer II (Coulter Electronics Ltd. product) was used, and for measuring the average particle diameter of the surface-modifying fine particles by volume, the Mastersizer (Malvern Instruments Ltd. product) was used. In concrete example 1, samples T1 through T6, shown in Table 1, were obtained by changing the temperature of the hot air at the time of hot air flow processing. In other words, hot air processing at each of six temperatures from 100°C to 450°C was performed on combined particles formed by adding 5 parts by weight of PMMA surface-modifying fine particles (with an average diameter by volume of 0.15µm and a BET specific surface area of 37.8m 2 /g) to the surface of 100 parts by weight of irregularly-shaped core particles (with an average diameter by volume of 10.5µm and a BET specific surface area of 1.70m 2 /g). Here, since S 1 = 1.70[m 2 /g], S 2 = 37.8[m 2 /g], and X = 0.048[m 2 /g], the BET specific surface area of the combined particles (S 0 ), calculated using equation (2) above, is 3.43[m 2 /g]. SAMPLE NO. HOT AIR TEMPERATURE [°C] AVERAGE PARTICLE DIAMETER BY VOLUME [µm] BET SPECIFIC SURFACE AREA S [m 2 /g] T1 100 10.9 2.210 T2 150 10.9 2.060 T3 200 10.8 1.300 T4 300 10.9 0.637 T5 400 11.0 0.542 T6 450 11.2 0.523 Next, Table 2 shows the results of evaluation of actual copying after copying 10,000 sheets using each of the samples T1 through T6 with 0.3 parts by weight of silica (Nippon Aerosil Co., Ltd. product R972) mixed in as fluidizing agent. Table 2 also shows the values relating to the equations (1), (4), and (5) for each sample. Evaluation of actual copying was performed by successive copying of 10,000 sheets using a Sharp Co. copy machine (SF-2027) and then evaluating image fogging, toner scattering, filming, and poor cleaning. In the Table, "O" indicates that the evaluation after copying was good, "Δ" indicates the limit of acceptability for use, and "×" indicates a poor evaluation. In regard to the evaluation of sample T1, image deterioration due to filming was so marked that copying was suspended after 6,000 sheets. Further, in Table 2, S/S 0 corresponds to the coefficient of S 0 (the left side of equations (1), (4), and (5)), and S/S calc corresponds to the coefficient of S calc (the right side of the same equations). The specific gravity of the toner particles (ρ) was 1.1×10 6 [g/m 3 ]. As shown in Table 2, with sample T1, which underwent hot air flow processing at 100°C, affixing of the surface-modifying fine particles was insufficient, and image fogging and toner scattering occurred. Filming also occurred after approximately 5,000 copies. Again, with sample T6, which underwent hot air flow processing at 450°C, poor cleaning occurred after approximately 8,500 copies, and was accompanied by image fogging and toner scattering. As a result, it can be seen that a hot air temperature of more than 100°C but less than 450°C is preferable. In this case, the BET specific surface area conditions are 0.64 > S/S 0 > 0.14 and 4.42 > S/S calc > 1.07, and since the maximum limit of the toner's BET specific surface area is based on S 0 , and its minimum limit on S calc , the conditions obtained are: 0.64S 0 > S > 1.07S calc . With samples T2 through T5, good images which were at or better than the limit of acceptability for use were obtained in evaluation after 10,000 copies. Thus it can be seen that temperature conditions of 150°C to 400°C are preferable. In this case, the BET specific surface area conditions are 0.60 ≥ S/S 0 ≥ 0.15 and 4.12 ≥ S/S calc ≥ 1.12, and, for the same reasons as above, the conditions obtained are: 0.60S 0 ≥ S ≥ 1.12S calc . Further, with samples T3 through T5, all evaluations were good, confirming that temperature conditions of 200°C to 400°C were even more preferable. In this case, the BET specific surface area conditions are 0.38 ≥ S/S 0 ≥ 0.15 and 2.57 ≥ S/S calc ≥ 1.12, and, for the same reasons as above, the conditions obtained are: 0.38S 0 ≥ S ≥ 1.12S calc . Incidentally, there are cases in which the measured BET specific surface area of toner which has undergone hot air flow processing is close to the calculated BET specific surface area of a hypothetical toner with particles which are perfect spheres. This is due to smoothing of the surface of the particles. Examination with an SEM has confirmed that, with particles having a specific surface of at least 1.1 times that of the hypothetical particles which are perfect spheres, the particles have not become spherical, and maintain a sufficiently irregular shape. (CONCRETE EXAMPLE 2) Next, samples T4 and T7 through T10, shown in Table 3, were prepared in the same manner as in concrete example 1, except that the temperature of hot air flow processing was held constant while the average particle diameter of the surface-modifying fine particles by volume was varied. In other words, irregularly-shaped core particles having an average particle diameter by volume of 10.5µm and a BET specific surface area (S 1 ) of 1.70m 2 /g were used. Then, five types of combined particles (samples T4 and T7 to T10) were prepared by adding to the surface of the core particles, by weight, 5 parts PMMA surface-modifying fine particles with average particle diameters by volume ranging from 0.1µm to 2.0µm. Each type of combined particle was then processed in a hot air flow of 300°C. In addition, Table 4 shows the results of evaluation of actual copying after copying 10,000 sheets using each of the samples T4 and T7 through T10 with, as in concrete example 1, 0.3 parts by weight of silica (Nippon Aerosil Co., Ltd. product R972) mixed in as fluidizing agent. Table 4 also shows the values relating to the equations (1), (4), and (5) for each sample. The method of making these evaluations was the same as that of concrete example 1. Further, the specific gravity (p) of the toner particles was also the same as in concrete example 1, i.e., 1.1×10 6 [g/m 3 ]. As shown in Table 4, with sample T10, which used PMMA surface-modifying fine particles 2.0µm in average particle diameter by volume, fogging of white areas of the image and toner scattering occurred to such an extent that this toner was unsuitable for use. This is probably caused by a great amount of fine powder toner due to a large number of surface-modifying fine particles existing separately from the core particles, without being attached thereto, and by inferior charging stability due to failure to form a uniform film. Thus it can be seen that PMMA surface-modifying fine particles less than 2.0µm in average particle diameter by volume are preferable. In this case, the BET specific surface area conditions are 0.71 > S/S 0 and 2.75 > S/S calc , and since the maximum limit of the toner's BET specific surface area is based on S 0 , the conditions obtained are: 0.71S 0 > S. Further, with samples T7, T4, T8, and T9, copying characteristics which were at or better than the limit of acceptability for use were obtained. Thus it was confirmed that PMMA surface-modifying fine particles of from 0.1µm to 1.0µm in average particle diameter by volume are preferable. In this case, the BET specific surface area conditions are 0.33 ≥ S/S 0 and 1.83 ≥ S/S calc , and, for the same reasons as above, the conditions obtained are: 0.33S 0 ≥ S. Further, with samples T7, T4, and T8, all evaluations were good, confirming that PMMA surface-modifying fine particles of from 0.1µm to 0.4µm in average particle diameter by volume were even more preferable. In this case, the BET specific surface area conditions are 0.27 ≥ S/S 0 and 1.53 ≥ S/S calc , and, for the same reasons as above, the conditions obtained are: 0.27S 0 ≥ S. (CONCRETE EXAMPLE 3) Next, samples T4 and T11 through T14, shown in Table 5, were prepared in the same manner as in concrete example 1, except that the temperature of hot air flow processing was held constant while the quantity of surface-modifying fine particles added was varied. In other words, irregular-shaped core particles having an average particle diameter by volume of 10.5µm and a BET specific surface area (S 1 ) of 1.70m 2 /g were used. Then, five types of combined particles (samples T4 and T11 to T14) were prepared by adding to the surface of the core particles PMMA surface-modifying fine particles with an average particle diameter by volume of 0.15µm and a BET specific surface area (S 2 ) of 37.8m 2 /g in quantities ranging from 0.1 part to 20 parts by weight. Each type of combined particle was then processed in a hot air flow of 300°C. In addition, Table 6 shows evaluation of actual copying after copying 10,000 sheets using each of the samples T4 and T11 through T14 with, as in concrete example 1, 0.3 parts by weight of silica (Nippon Aerosil Co., Ltd. product R972) mixed in as fluidizing agent. Table 6 also shows the values relating to the equations (1), (4), and (5) for each sample. The method of making these evaluations was the same as that of concrete example 1. Further, the specific gravity (p) of the toner particles was also the same as in concrete example 1, i.e., 1.1×10 6 [g/m 3 ]. As shown in Table 6, with sample T14, in which 20 parts by weight of PMMA surface-modifying fine particles were added, image fogging and toner scattering occurred, as did filming after copying approximately 8,000 sheets, to such an extent that this toner was unsuitable for use. Thus it can be seen that addition of less than 20 parts by weight of PMMA surface-modifying fine particles is preferable. In this case, the BET specific surface area conditions are 0.79 > S/S 0 and 13.4 > S/S calc , and since the maximum limit of the toner's BET specific surface area is based on S 0 , the conditions obtained are: 0.79S 0 > S. Further, with samples T11, T12, T4, and T13, copying characteristics which were at or better than the limit of acceptability for use were obtained. Thus it was confirmed that addition of from 0.1 part to 15 parts by weight of PMMA surface-modifying fine particles is preferable. In this case, the BET specific surface area conditions are 0.57 ≥ S/S 0 and 7.74 ≥ S/S calc ≥ 1.10. Accordingly, for the same reasons as above, the conditions obtained are: 0.57S 0 ≥ S ≥ 1.10S calc . Further, with samples T12 and T4, all evaluations were good. Accordingly, it was confirmed that addition of from 1 part to 5 parts by weight of PMMA surface-modifying fine particles is even more preferable. In this case, the BET specific surface area conditions are 0.28 ≥ S/S 0 ≥ 0.19 and 1.27 ≥ S/S calc ≥ 1.13, and, for the same reasons as above, the conditions obtained are: 0.28S 0 ≥ S ≥ 1.13 S calc . The foregoing concrete examples 1 through 3 confirmed that preferred BET specific surface area conditions are 0.64S 0 > S > 1.07S calc . Further, it was confirmed that conditions of 0.60S 0 ≥ S ≥ 1.10S calc are more preferable, and that conditions of 0.38S 0 ≥ S ≥ 1.12S calc are even more preferable. (CONCRETE EXAMPLE 4) Next, samples T15 through T19, shown in Table 7, were prepared in the same manner as in concrete example 1, except that the glass transition point (Tg 2 ) and weight-average molecular weight (Mw) of the surface-modifying fine particles were held constant while the glass transition point of the core particles (Tg 1 ) was varied. In other words, core particles with average particle diameter by volume adjusted to 10.5µm, and having glass transition points ranging from 35°C to 75°C were used. Then, five types of combined particles (samples T15 through T19) were prepared by adding to the surface of each type of core particle 5 parts by weight of PMMA surface-modifying fine particles with an average particle diameter by volume of 0.15µm, a glass transition point of 72°C, and a weight-average molecular weight of 120,000. Each type of combined particle was then processed in a hot air flow of 300°C. In addition, samples T17 and T20 through T23, shown in Table 7, were prepared in the same manner as in concrete example 1, except that the glass transition point of the core particles (Tg 1 ) and the weight-average molecular weight of the surface-modifying fine particles (Mw) was held constant while the glass transition point of the surface-modifying fine particles (Tg 2 ) was varied. In other words, core particles with average particle diameter by volume adjusted to 10.5µm, and having a glass transition point of 55°C were used. Then, five types of combined particles (samples T17, T20 to T23) were prepared by adding to the surface of the core particles 5 parts by weight of PMMA surface-modifying fine particles with an average particle diameter by volume of 0.15µm, glass transition points ranging from 55°C to 108°C, and a weight-average molecular weight of 120,000. Each type of combined particle was then processed in a hot air flow of 300°C. In addition, Table 7 shows evaluation of actual copying after copying 10,000 sheets, fixing, and preservation using each of the samples T15 through T23 with, as in concrete example 1, 0.3 parts by weight of silica (Nippon Aerosil Co., Ltd. product R972) mixed in as fluidizing agent. The method of evaluating actual copying was the same as that of concrete example 1. Fixing was evaluated by a performing a rubbing test (1kgw) with a sand eraser (Lion Co., Ltd. product ER-502K) in a device for testing fastness to rubbing, and then measuring the percentage of fixed toner remaining after rubbing. In this evaluation, if 80% or more of the toner remained after rubbing, the toner was considered satisfactory for actual use. Preservation was evaluated by filling a cartridge for the copy machine (SF-2027) with 320g of toner, letting stand at 45°C for 2 weeks, and then checking for blocking. Since, as shown in Table 7, samples T16 through T18 had good copying evaluation, fixing, and preservation, it was confirmed that core particles with a glass transition point of 40°C to 70°C are preferable. In contrast, sample T15, which had core particles with a glass transition point of 75°C, had inferior fixing. Again, sample T19, which had core particles with a glass transition point of 35°C, had poor copying evaluation in each area, and preservation was impaired, making it unsuitable for actual use. Further, samples T21, T17, and T22 had good copying evaluation, fixing, and preservation. Accordingly, it was confirmed that surface-modifying fine particles with a glass transition point of 60°C to 100°C are preferable. In contrast, with sample T20, which had surface-modifying fine particles with a glass transition point of 108°C, image fogging and filming occurred, and fixing was also impaired. Again, with sample T23, which had surface-modifying fine particles with a glass transition point of 55°C, image fogging and poor cleaning occurred, and preservation was impaired. For these reasons, samples T20 and T23 were unsuitable for actual use. (CONCRETE EXAMPLE 5) Next, samples T17 and T24 through T27, shown in Table 8, were prepared in the same manner as in concrete example 1, except that the glass transition points of the core particles (Tg 1 ) and the surface-modifying fine particles (Tg 2 ) were held constant while the weight-average molecular weight (Mw) of the surface-modifying fine particles was varied. In other words, core particles with average particle diameter by volume adjusted to 10.5µm, and having a glass transition point of 55°C were used. Then, five types of combined particles (samples T17, T24 to T27) were prepared by adding to the surface of the core particles 5 parts by weight of PMMA surface-modifying fine particles with an average particle diameter by volume of 0.15µm, a glass transition point of 72°C, and weight-average molecular weights ranging from 45,000 to 250,000. Each type of combined particle was then processed in a hot air flow of 300°C. In addition, Table 8 shows evaluation of actual copying after copying 10,000 sheets, fixing, and preservation using each of the samples T17 and T24 through T27 with, as in concrete example 1, 0.3 parts by weight of silica (Nippon Aerosil Co., Ltd. product R972) mixed in as fluidizing agent. The method of making these evaluations was the same as that of concrete example 1. Further, the methods of evaluating fixing and preservation were the same as in concrete example 4. Since, as shown in Table 8, samples T25, T17, and T26 had good copying evaluation, fixing, and preservation, it was confirmed that a weight-average molecular weight of the surface-modifying fine particles of 50,000 to 210,000 is preferable. In contrast, with sample T24, which had a weight-average molecular weight of 45,000, fixing and preservation were impaired. Again, sample T27, which had a weight-average molecular weight of 250,000, had poor copying evaluation in each area, and fixing was also impaired. Accordingly, samples T24 and T27 were unsuitable for actual use. (CONCRETE EXAMPLE 6) Next, samples T28 through T30, shown in Table 9, were prepared using core particles of styrene-acrylic copolymer or polyester resin, and surface-modifying fine particles of PMMA or styrene-PBMA copolymer. In other words, two types of core particles with average particle diameter by volume adjusted to 10.5µm were used. Then, three types of combined particles (samples T28 through T30) were prepared by adding 5 parts by weight of surface-modifying fine particles with an average particle diameter by volume of 0.4µm, but with different SP values, to the surface of each type of core particle. Each type of combined particle was then processed in a hot air flow of 300°C, producing toners with an average particle diameter by volume of approximately 11.5µm. Table 9 also shows the results of evaluation of actual copying after copying 10,000 sheets using each of the samples T28 through T30 with, as in concrete example 1, 0.3 parts by weight of silica (Nippon Aerosil Co., Ltd. product R972) mixed in as fluidizing agent. As Table 9 shows, with sample T30, in which the difference in the SP values of the core particles and surface-modifying fine particles was 2.2, image fogging occurred due to peeling and separation of the surface-modifying fine particles from the surface of the core particles, and filming also occurred after approximately 7,000 copies. For this reason, it was confirmed that a difference in SP values of less than 2.2 is preferable. Again, since samples T28 and T29 had good evaluations in each area, it was confirmed that a difference in SP values of 2.0 or less is more preferable. As has been discussed above, electrophotographic toner according to the present embodiment is made up of irregularly-shaped core particles chiefly composed of binder resin, and surface-modifying fine particles which are first dispersed over and attached to the surface of the core particles, and then affixed or made into a film thereon, so as to produce toner particles, in which the BET specific surface area, based on N 2 adsorption, of the toner particles satisfies: 0.64S 0 > S > 1.07×[3/(ρD/2)]; and S 0 = S 1 X+S 2 (1-X), where: S is the BET specific surface area of the toner particles; S 0 is the BET specific surface area of the core particles and the surface-modifying fine particles combined together; S 1 is the BET specific surface area of the core particles alone; S 2 is the BET specific surface area of the surface-modifying fine particles alone; ρ is the specific gravity of the toner particles; D is the average particle diameter of the toner particles by volume; and X is a ratio of the amount of surface-modifying fine particles (parts by weight) to the amount of the sum of surface-modifying fine particles and core particles (both parts by weight). With the foregoing structure, the toner's BET specific surface area is less than 0.64S 0 ; in other words, the surface-modifying fine particles are sufficiently affixed to the surface of the core particles, and thus problems like filming and toner scattering will not occur. Further, the toner's BET specific surface area is more than 1.07 times that of hypothetical toner particles which are perfect spheres; in other words, the toner particles are not spherical, and thus poor cleaning can be prevented. As a result, a surface-modified toner can be obtained in which the surface-modifying fine particles dispersed over and attached to the surface of the core particles are affixed or made into a film thereon strongly enough so that they will not peel or separate therefrom, but without producing spherical toner particles, thus avoiding problems such as poor cleaning. Further, it is more preferable if the electrophotographic toner has toner particles with a BET specific surface area of no more than 0.60 times the BET specific surface area of the core particles and surface-modifying fine particles when combined together, and no less than 1.10 times the BET specific surface area of hypothetical toner particles which are perfect spheres. In this case, a better toner can be obtained, in which poor cleaning and peeling or separation of the surface-modifying fine particles do not occur. Further, it is even more preferable if the electrophotographic toner has toner particles with a BET specific surface area of no more than 0.38 times the BET specific surface area of the core particles and surface-modifying fine particles when combined together, and no less than 1.12 times the BET specific surface area of hypothetical toner particles which are perfect spheres. In this case, an even better toner can be obtained, in which poor cleaning and peeling or separation of the surface-modifying fine particles do not occur. In addition, it is preferable if the electrophotographic toner is made up of surface-modifying fine particles having a glass transition point which is higher than that of the core particles, and if the glass transition point of the core particles is 40°C to 70°C, and that of the surface-modifying fine particles is 60°C to 100°C. In this case, surface-modifying fine particles are used which have a higher glass transition point than that of the core particles. Surface-modifying fine particles which are within a range which does not sacrifice fixing performance are combined with core particles which are capable of low-temperature fixing while maintaining strong fixing. Thus, low-temperature fixing of the core particles can be realized, and the preservation of the surface-modifying fine particles can be improved, enabling a toner with superior low-temperature fixing and preservation. Further, with the foregoing combination, a toner can be obtained which is free of peeling or separation of the surface-modifying fine particles. Again, it is preferable if surface-modifying fine particles with an average particle diameter by volume of no more than 1µm are used in the electrophotographic toner. In this case, by using surface-modifying fine particles no more than 1µm in average particle diameter by volume, a strong state of affixing or film formation which is resistant to stress can be obtained, thus enabling a superior toner which is not prone to peeling or separation, and which will not cause poor cleaning. Further, it is preferable if the surface-modifying fine particles used in the electrophotographic toner are organic surface-modifying fine particles having a weight-average molecular weight of from 50,000 to 210,000. In this case, by using surface-modifying fine particles with a weight-average molecular weight within the foregoing range, a strong state of affixing or film formation which is resistant to stress can be obtained, thus enabling a superior toner which is not prone to peeling or separation, and which will not cause poor cleaning. In addition, it is preferable if the surface-modifying fine particles used in the electrophotographic toner are organic surface-modifying fine particles, and if the absolute value of the difference in the solubility parameter values of the organic surface-modifying fine particles and the core particles is no more than 2.0. In this case, since the difference in solubility parameter values of the organic surface-modifying fine particles and the binder resin of the core particles is no more than 2.0, the two materials have good compatibility, resulting in a strong state of affixing or film formation, thus enabling a superior toner which is not prone to peeling or separation, and which will not cause poor cleaning. In addition, it is preferable if the surface-modifying fine particles used in the electrophotographic toner are organic surface-modifying fine particles, and if 0.1 part to 15 parts by weight of the organic surface-modifying fine particles are added for 100 parts by weight of the core particles. In this case, by adding the organic surface-modifying fine particles in a quantity within the foregoing range, desired performance, such as charge control and improvement of preservation, can be imparted, and a strong state of affixing or film formation which is resistant to stress can be obtained, thus enabling a superior toner which is not prone to peeling or separation, and which will not cause poor cleaning. The method of manufacturing electrophotographic toner according to the present embodiment includes the steps of dispersing and attaching surface-modifying fine particles on the surface of irregularly-shaped core particles chiefly composed of binder resin, so as to produce combined particles; and affixing or forming a film of the surface-modifying fine particles on the surface of the core particles, so as to produce toner particles; in which the toner particles are manufactured so that their BET specific surface area, based on N 2 adsorption, satisfies: 0.64S 0 > S > 1.07×[3/(ρD/2)]; and S 0 = S 1 X+S 2 (1-X), where: S is the BET specific surface area of the toner particles; S 0 is the BET specific surface area of the core particles and the surface-modifying fine particles combined together; S 1 is the BET specific surface area of the core particles alone; S 2 is the BET specific surface area of the surface-modifying fine particles alone; ρ is the specific gravity of the toner particles; D is the average particle diameter of the toner particles by volume; and X is a ratio of the amount of surface-modifying fine particles (parts by weight) to the amount of the sum of surface-modifying fine particles and core particles (both parts by weight). With the foregoing manufacturing method, since the state of surface modification can be quantitatively grasped by means of the BET specific surface area, the state of surface modification can be controlled to produce a toner which is in a uniform and stable state. Here, the state of surface modification can be controlled by changing the various parameters of the manufacturing process (which include device conditions such as temperature, duration of exposure, and quantity processed, and the composition, combination ratio, particle diameter, shape, glass transition point, and molecular weight of the core particles and surface-modifying fine particles). In the foregoing method of manufacturing electrophotographic toner, it is preferable, in the step for producing the toner, to expose the combined particles to a hot air flow area in such a way that the temperature applied to the surface-modifying fine particles and to the surface of the core particles is at or above the softening point of these respective particles, but the temperature applied to the interior of the core particles is insufficient to soften the core particles, and then to cool the toner particles produced thereby. In this case, the surface-modifying fine particles can be affixed or formed into a film on the surface of the core particles while maintaining the irregular shape of the core particles, thus enabling production of a toner which will not cause poor cleaning. In addition, in the foregoing method of manufacturing electrophotographic toner, it is preferable if the temperature of the hot air flow area is more than 100°C but less than 450°C, and if the duration of exposure of the combined particles in the hot air flow area is less than 1 second. In this case, since the temperature of the hot air flow area is within the foregoing range, the surface-modifying fine particles are sufficiently affixed to the core particles without blocking of the toner. Further, since the exposure time is less than 1 second, processing speed is not slowed.

Claims: A method of manufacturing an electrophotographic toner comprising the steps of: (a) attaching and dispersing surface-modifying fine particles on the surfaces of irregularly-shaped core particles chiefly comprising binder resin, so as to produce combined particles; and (b) affixing or forming a film of the surface-modifying fine particles on the surfaces of the core particles, so as to produce electrophotographic toner particles; characterized in that the core particles are selected to have a BET specific surface area S 1 and the surface-modifying fine particles are selected to have a BET specific surface area S 2 whereby the electrophotographic toner particles have a BET specific surface area S (the BET specific surface areas being based on N 2 absorption) which satisfies the conditions 0.64S 0 > S > 1.07×[3/(ρD/2)]; and S 0 = S 1 X+S 2 (1-X), where: S is the BET specific surface area of the toner particles; S 0 is a BET specific surface area of the core particles and the surface-modifying fine particles combined together; S 1 is a BET specific surface area of the core particles alone; S 2 is a BET specific surface area of the surface-modifying fine particles alone; ρ is a specific gravity of the electrophotographic toner particles; D is an average particle diameter by volume of the electrophotographic toner particles; and X is a ratio of the amount of surface-modifying fine particles (parts by weight) to the amount of the sum of surface-modifying fine particles and core particles (both parts by weight). The method of manufacturing electrophotographic toner set forth in claim 1, wherein: in said step (b), the combined particles are exposed to a hot air flow area, such that a temperature applied to the surface-modifying fine particles and to the surfaces of the core particles is at or above softening points of the respective particles, but a temperature applied to the interiors of the core particles is insufficient to soften the core particles, and the combined particles are then cooled. The method of manufacturing electrophotographic toner set forth in claim 2, wherein: the hot air flow area has a temperature of more than 100°C, but less than 450°C, and the combined particles are exposed to the hot air flow for no longer than 1 second.

Inventors: AKAZAWA YOSHIAKI, IMAFUKU TATUO, ISHIDA TOSHIHISA, MORINISHI YASUHARU, MURAKAMI TOSHIHIKO, NAGAHAMA HITOSHI, NAKAMURA TADASHI, OGAWA SATOSHI, OUCHI TAKAKI, AKAZAWA, YOSHIAKI, IMAFUKU, TATUO, ISHIDA, TOSHIHISA, MORINISHI, YASUHARU, MURAKAMI, TOSHIHIKO, NAGAHAMA, HITOSHI, NAKAMURA, TADASHI, OGAWA, SATOSHI, OUCHI, TAKAKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1998
Main classification: G03G9
IPC codes: G03G9