A toner composition and image forming method using the same

Abstract: A toner composition for the development of an electrostatic latent image comprises a toner and a hydrophobic silica composed of a fine powder of silica treated with a compound having a polymethylsilyl group in the molecule.

Description: The present invention relates to a toner composition to be used for an image forming apparatus utilizing an electrophotographic method, and more particularly to a toner composition which shows good charge stabilization fluidity and delivering ability to a developing apparatus, and to an image forming method utilizing the same. Image forming apparatus utilizing electrophotographic methods are well known. In image forming apparatus utilizing electrophotographic methods, images are generally formed onto a sheet of copy paper by the following processes. After uniformly charging a photoconductor which services as an image-holding body, the surface of the charged photoconductor is exposed to a subject. A latent image is formed by attenuating electrostatic charges during the exposure of light. Then the electrostatic latent images are visualized by developing with toner to form a toner image. The toner images are transferred onto a sheet of copy paper. The above-described developing processes are classified into two categories; one is the two component development method using a developing agent in which a carrier composed of a magnetic material and a toner are mixed together, and the other is the single component method utilizing a toner only. In the two component development method, an appropriate charge is imparted to the toner by mixing with the carrier. In the single component development method, on the other hand, a predetermined charge is given to the toner by the friction among the toner particles, or by contacting with friction-charging parts such as the development sleeve, brush-cutoff regulating sleeve or toner press-adherering blade. As a result of this charging phenomenon the toner particles adhere to the photoconductor by an electrostatic force acting between the charged toner particles and the electrostatic charge building up the aforementioned electrostatic latent images, thereby forming visible images. The two component development method is described with reference to Figure 1 of the accompanying drawings which shows an image forming apparatus which comprises a photoconductor 10, a developing apparatus 5 and a toner container 1. A stirring roller 7 to mix the developing agent D homogeneously, and a development sleeve 8 are disposed inside a developing apparatus 5. A development sleeve 8 is composed of a magnet or the like, in the vicinity of which a magnetic brush composed of a chain-like array of the carrier in the developing agent D is formed. Toner T adheres to the carrier by frictional charging. The electrostatic latent images formed on the photoconductor 10 through the charging and exposure processes are developed by the above-described toner T. The toner image corresponding to the electrostatic latent image is transferred onto copy material such as a sheet of copy paper, followed by fixing on the copy material by the fixing equipment such as a fixing roller (a heated roller or a press roller). When the toner concentration in the developing agent D is decreased during the operation, the following system operates within the entire system to keep the predetermined toner concentration in the developing apparatus 5. A magnetic sensor 9 to detect the toner concentration in the developing agent D is equipped in the developing apparatus 5, and the magnetic sensor 9 is linked with the driving part of the toner-supply mechanism 11 disposed to the lower part of the toner container 1. When the toner concentration in the developing agent D is decreased, the magnetic sensor 9 transfers a signal to the driving part of the toner-supply mechanism 11, thereby feeding down a predetermined amount of toner T from the toner container 1 to the developing apparatus 5 by rotating the toner-supply mechanism 11. Good fluidity of the toner T is required to form an excellent quality of image in the two component development method. If fluidity of the toner T in the developing apparatus 5 is insufficient, output of the magnetic sensor 9 will fluctuate and charging will be unstable, thereby causing image overlapping and scattering of the toner particles. Particularly, if a toner-supply mechanism 11 by which toner is supplied to the developing apparatus 5 from the remote position using spiral screw 2 as is shown in Figure 1 is applied, efficiency of toner feed to the developing apparatus 5 will be decreased. Addition of fine particles of silica has been suggested to improve the fluidity of the toner. Since fluidity of a toner composition including fine particles of silica is rather good, feeding efficiency of the toner composition from the toner container 1 to the developing apparatus 5 and charge stability of the toner composition can be improved. In the above-described toner composition, however, fluidity of the composition is still unsatisfactory when micro-particles of the toner having the grain diameter of 10 µm or less are used to realize a high quality of images, or when formation of the images is performed under the circumstances of high temperature and high humidity. Some problems arise because feeding performance from the toner container 1 and charging stability decreased A laser printer, a LED printer and the like have been developed in recent years, wherein the image scanning part and the image output part are separated from each other. In these apparatuses, a reversed development method, i.e., the toner is made to adhere to the portions where the charge at the portions of the image exposure has been attenuated, is adopted (on the contrary, toner is made to adhere to the portions where charges are still remaining in the normal development method). In such a reversed development method, the images are exposed onto the photoconductor by a laser output of the digital processed image signals. In these methods, gradation is expressed by the proportion of the area in which the latent images are reproduced. A delicate gradation of the images cannot be expressed or a fine resolution of the line images cannot be reproduced precisely if the particle size of the toner is not uniform. Moreover, when the latent digital images are subjected to reversed development, lack of the images or irregularity in the image concentration will occur if the distribution of the toner in the magnetic brush formed on the development sleeve is not uniform. Therefore, the fluidity of the developing agent, particularly the delivering property of the developing agent on the development sleeve, should be uniform. A bias potential imposed on the development sleeve acts as a driving force for the development by the toner in the reversed development method. Since fluctuations in the toner charges will result in the adhesion of the toner to the portions where the images are not formed, the amount of the charges on the toner should be controlled precisely. A photoconductor comprising an organic photoconductive layer has been frequently used for the image forming apparatus in recent years. In this case, a "filming phenomenon" (a phenomenon in which the toner adheres on the surface of the photoconductor forming a film) tends to occur with repeated development because of high affinity between the resins forming a photoconductive layer and the toner. Consequently, a toner composition improved in cleaning performance is required. US-A-4868084 discloses a developer for developing electrostatic latent images comprising a hydrophobic silica fine powder that is obtained by treating silica fine powder with a silane coupling agent and treating further the treated silica fine powder with a silicone oil. US-A-3819367 discloses a developer material including a minor proportion of sub-microscopic silicone dioxide additive particles having at least a portion of the silicon atoms on the outside surface of the silicon dioxide particles directly attached through a silicon to carbon linkage to one to three organic groups. US-A-3900588 discloses an imaging technique and composition for developing electrostatographic latent images. A developer composition is disclosed which includes an abrasive material. Particularly preferred materials are those which have been surface modified to impart hydrophobic characteristics thereto. PA of Japan, Vol. 11, No. 181 (P-585) (2628), June 11, 1987 discloses an electric charge controlling agent in which charge controlling agents are bound with each other with the reactant of a reactive silicon compound as a binder on the surface of a toner. EP-A-0237038 discloses a toner for developing an electrostatic latent image including a hydrophobic aluminium oxide or a hydrophobic titanium oxide, a silica and lubricant. According to one aspect of the present invention, there is provided a toner composition for developing an electrostatic latent image on an image-holding body, the composition comprising toner particles having a mean particle size of about 6 to 10µm, characterized in that the toner has a toner particle distribution wherein 70wt % or more of the toner is 0.6 to 1.2 times the mean particle size, the amount of toner particles having a diameter of 5µm or less is at most 10wt %, and the amount of the toner particles having a diameter of 16µm or more is at most 2wt %, in that the toner composition further comprises particles of hydrophobic silica surface-treated with a polymer having a methylsilyl group, and wherein the compressibility of the toner composition is in the range of 35 to 40%. The hydrophobic silica particles are preferably present in a range of 0.05 to 2 parts by weight per 100 parts by weight of the toner particles. In a preferred embodiment, the toner composition further comprises fine particles of hydrophilic alumina. The weight rates of the hydrophobic silica to the hydrophilic alumina in the toner composition is preferably in the range of 1:0.2 to 1:3. According to a second aspect of this invention, there is provided an image forming method comprising the steps of: imparting a charge to an image-holding body by a charging method; forming an electrostatic latent image on the image-holding body by exposing the image on the image-holding body imparted by a charge by a digital exposure method; converting the latent image on the image-holding body into a toner image by a development method utilising a toner composition; and transferring the toner image on the image-holding body to a copy material by a transfer means wherein said toner composition is a toner composition according to the first aspect of the invention. It is preferred that the image-holding body be an organic photoconductive material. Thus, the invention described herein makes possible the objectives of: (1) providing a toner composition having good fluidity even when a toner with fine particle size and relatively inferior fluidity is used, and images are being formed under conditions of high temperature and humidity; (2) providing a toner composition which can attain the objectives of stability during toner feeding stability at the output from the sensor, prevention of blocking and coagulation and improvement of the initial charging ability; (3) providing a toner composition with which fogging and scattering of the toner around the images are prevented in a reversed development method; (4) providing a toner composition with which digital electrostatic latent images are reproduced precisely, whereby an image excellent in reproducibility of fine lines and grading is achieved; (5) providing a toner composition with little fluctuation in charging; (6) providing a toner composition for reversed development with durability and without there being filming of the toner on the organic photoconductive layer; (7) providing an image forming method in which fogging and scattering of the toner particles around the images are avoided in a reversed development method; and (8) providing an image forming method in which a digital electrostatic latent image formed by laser light exposure is developed precisely, with reproducibility of the fine lines and formation of images having good grading being realized. For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompanying drawing, wherein: Figure 1 is a schematic view of a development mechanism using a magnetic brush. Conventional fine silica powder is prepared by treating the surface of silica powder with a compound with relatively smaller molecular weight and having an alkyl group in the molecule. The hydrophobicity of such silica fine powder is, however, not sufficient. For example, the fluidity of a toner composition comprising fine toner particles with mean particle size of 10 µm or less and above-described fine silica, powder is not sufficient. The hydrophobic silica used in this invention can be prepared by treating the fine silica powder with a polymer having relatively high molecular weight and having a methylsilyl group in the molecule in contrast with the conventional surface treating agent. The hydrophobicity of the hydrophobic silica thus prepared has been observed to be largely improved. A toner composition comprising the hydrophobic silica and toner has excellent fluidity. Therefore, the toner composition displays excellent fluidity even when a toner of very fine particle size is used or the composition is used under the circumstances of high temperature and high humidity. The toner used in this invention may be a conventional toner which is used for the development method in a dry state in the prior art, having additives such as colorants dispersed in a resinous binder. Various kinds of resinous binders can be used, electrification and adhering properties being taken into account. These resinous binders include the olefine series of polymers such as styrene copolymers, acrylic copolymers, styrene-acrylic copolymers, polyethylene, chlorinated polyethylene, polypropylene, ionomers; and various kinds of polymers including polyvinyl chloride, polyester polyethers , polyamide, polyurethane, epoxy resin, diallyl phthalate resin, silicone resin, ketone resin, polyvinyl butyral resin, phenol resin, phenol resin modified by a rosin, xylene resin, maleic acid resin modified by a rosin, rosin ester and petroleum resin. Selection of these resins as a binder depends on the fixing method and the characteristics that are considered to be necessary. Of these resins, styrene polymers, acrylic polymers and styrene-acrylic copolymers are preferably used because of their grinding property and easy control of the molecular weight of the polymers, and styrene-acrylic copolymers are used more preferably. Preferable mean molecular weights of the resins are in the range of from 30,000 to 200,000, and more preferably in the range of from 50,000 to 150,000. One such resin or a mixture of two or more such resins can be used. The charging property of the toner through friction can be improved by using rosin ester, phenol resin modified by a rosin, maleic acid resin modified by a rosin, epoxy resin, polyester, fibrous polymers and polyether resin among the above-described resins. Softening temperature of the resinous binders are preferably in the range of 50 to 200°C, and more preferably in the range of 70 to 170°C. When a toner having a pressure fixing property is used, it is preferable to use a polymer which is deformed easily by pressure as a resinous binder including olefin polymers such as polyethylene and polypropylene, and polyamides. Other examples of resinous binders include, for example, polyvinyl acetate, ethylene-polyvinyl acetate copolymer, hydrogenated polyethylene, hydrogenated rosin ester, and aliphatic or aromatic petroleum resins. Conventional toners, dyes and pigments are used as colorants to be dispersed in the resinous binders. The colorants include carbon black as black pigment, and the copper phthalocyanine series of cyan pigments, azo series of yellow pigments, azo series of magenta pigments and quinacridone series of magenta pigments as colored pigments. Specific examples of colorants include carbon black, Lamp Black, Chrome Yellow, Hanza Yellow, Benzidine Yellow, Beslene Yellow, Quinoline Yellow, Permanent Orange GTR, Pyrazone Orange, Pulcan Orange, Watchang Red, Permanent Red, Brilliant Carmine 3B, Briliant Carmine 6B, Dupont-oil Red, Pyrazolone Red, Rysole Red, Rhodamine B Lake, Lake Red C, Rose Bengal, Aniline Blue, Ultramarine Blue, Chalco-oil Blue, Methylene Blue Chloride, Phthalocyanine Blue, Phthalocyanine Green, Malachite Green Octhalate and the like, and oil soluble dies such as C. I. Solvent Yellow 60, C. I. Solvent Red 27 and C. I. Solvent Blue 35. These colorants can be used independently or as a mixture of two or more thereof. The amount of colorant to obtain a sufficient amount for the image density is preferably in the range of 0.1 to 50 parts by weight per 100 parts of the resinous binder, and more preferably in the range of 1 to 20 parts by weight. Magnetic materials composed of magnetic bodies can be incorporated in the above-described toners to impart magnetization to the toner. The magnetic materials can be materials which show magnetization by themselves or are capable of being magnetized. Examples of these materials include metals exhibiting ferromagnetic properties such as ferrite, magnetite, iron, cobalt, nickel and manganese, or alloys thereof or metallic compounds including the above. Examples further comprise Fe 3 O 4 , Fe 2 O 3 , iron-zinc oxide, iron-yttrium oxide, cadmium oxide, iron-copper oxide, neodymium oxide, iron-barium oxide, iron-magnesium oxide, iron-manganese oxide and lanthanum oxide. The mean particle size of these magnetic materials is preferably in the range of 0.1 µm to 1 µm, and the materials can be used independently or in a mixed from of twc or more such materials. The amount of magnetic material is preferably in the range of 5 to 70 parts by weight per 100 parts by weight of the resinous binder, and more preferably in the range of 20 to 50 parts by weight. The above-described toner can contain a charge regulating agent to regulate charges. Examples of charge regulating agents include oil soluble dyes such as Nigrocine Dye, Oil Black and Spyrone Black; metallic soaps such as manganese, ferrous or ferric, cobalt, nickel, lead, zinc, cerium and calcium salts of naphthenic acid, salicylic acid, octoic acid, long chain fatty acids and rosin acid; or metalcontaining azo dyes pyrimidine compounds, and metal chelates of alkylsalicylic acid. The amount of the charge regulating agent can be in the range of 0.1 to 10 parts by weight per 100 parts of the resinous binder, and more preferably the amount can be in the range of 0.1 to 5 parts by weight. The above-described toner further comprises offset inhibitors to inhibit adhesion of the toner to the roller. Examples of the offset inhibitors include waxes such as low molecular weight polypropylene, low molecular weight polyethylene and paraffin wax; and low molecular weight olefin oligomer having a carbon number of four or more, fatty acid amides and silicone oil. The preferred amount of the offset inhibitor is in the range of 0.5 to 15 parts by weight per 100 parts by weight of the resinous binder. A toner composition which can satisfy the objectives of the present invention is prepared fine toner powder, the mean particle size of which is in the range of around 6 to around 10 µm, is used. The toner composition in this invention can be prepared using the above-described materials following conventional manufacturing methods. A melt is produced and is kneaded by using a two-axis extruder or a roll mill and the like, and the kneaded material is cooled, ground and fractionated to give a toner composition having the above-described appropriate particle size distribution characteristics. Sometimes rounding and trimming treatments may be performed during the process for manufacturing the toner composition. The hydrophobic silica used in this invention is prepared by treating high purity fumed silica (SiO 2 :99.8 wt%) with an organosilicone which is a polymer having a methylsilyl group in the molecule. The hydrophobicity of the hydrophobic silica is so strong that, when a toner composition is prepared by mixing the silica particles with the toner of the above-described particle size distribution, the particles of the hydrophobic silica disperse into the toner as primary particles, thereby improving the fluidity, charging ability through friction and stability in the atmosphere. As a consequence, the magnetic brushes having appropriate charge can be formed repeatedly. The particle size of the hydrophobic silica is, that typical of the primary grains, in the range of 0.0001 to 0.1 µm, and particularly preferably in the range of 0.001 to 0.01 µm. Commercially available fine silica powder includes Cabosil-TS720 manufactured by Cabot Co. The preferred amount of hydrophobic silica added is in the range of 0.05 to 2.0 parts by weight per 100 parts by weight of the toner, and more preferably in the range of 0.5 to 1.0 parts by weight. The toner composition of the present invention is prepared by mixing the above-described toner and hydrophobic silica. A conventional surface treating agent other than hydrophobic silica can also be blended with the toner composition in this invention. Preferably hydrophilic alumina particles are used. When alumina particles are included in the toner composition, the weight ratio of the hydrophobic silica to that of alumina particles is generally in the range of 1:0.2 to 1:3, and a ratio of 1:0.5 to 1:1 is particularly preferable. The toner composition of the present invention can be used either for the single component development method or for the two component development method. When the composition is used for the single component development method, a developing agent is prepared by mixing the toner containing above-described magnetic material with the hydrophobic silica. When the composition is used for the two component development method, a developing agent is prepared by blending the mixture of the toner and hydrophobic silica with the magnetic carrier. Particles composed of conventional magnetic materials can be used for the magnetic carriers. The materials include iron oxide, reduced iron, copper, nickel, cobalt, ferrites and the like; and alloys of them with zinc and aluminum. Of these materials, ferrite particles are particularly preferable. These ferrites include zinc ferrites, nickel ferrites, copper ferrites, manganese ferrites, nickel-zinc ferrites, manganese-magnesium ferrites, copper-magnesium ferrites, manganese-zinc ferrites and manganese-copper-zinc ferrites. The magnetic carrier may be coated with resin or not coated. Coating resins for the magnetic carrier include styrene resins, acrylic resins, silicone resins, fluoride resins, ketone resins, polyester resins, epoxy resins, melamine resins and polycarbonate resins. These resins are used separately or in a mixed form of two or more thereof. The particle size of the magnetic carrier is in the range of 50 to 120 µm, the range of 90 to 110 µm being preferable. The weight of the magnetic carrier to the toner composition is, preferably from 99:1 to 90:10, and particularly 98:2 to 96:4 Development conditions in the image forming method of the present invention are as follows: The amount of charge on the toner as measured by a blow-off method is preferably in the range of ±15 to ±25 µc/g, and particularly preferably in the range of ±17 to ± 23 µc/g. The surface charge of the image-holding body is preferably adjusted in the range of ±600 to ±800 V, and particularly in the range of ±650 to ±700 V. Bias voltage is preferably in the range of ±400 to ±550 V, and particularly preferably in the range of ±450 to ±500 V. In the toner composition of the present invention, the quality of the image improves when an organic photoconductor with a resin layer on its surface is used as a image-holding body, thereby preventing the occurrence of a cleaning fault. A toner composition well suited for the purpose of the reversed development of a digital electrostatic latent image is described hereinafter. A toner composition of the present invention suited the purpose of the reversed development of the digital electrostatic latent image comprises a toner having the particle size in a predetermined range and a sharp particle size distribution profile, and a hydrophobic silica treated with a polymer having a methylsilyl group in the molecule. The toner used in the toner composition preferably meets the following conditions. (a) Mean particle size of the toner is at least in the range of 6 to 10 µm, and 70 wt% or more of the total particles are in the range of 0.6 to 1.2 times the mean particle size. (b) The relative amount of fine toner particles with a particle size of less than 5 µm is 10 wt% or less based on the total amount of the toner, and the amount of coarse particles with particle size of 16 µm or more is 20 wt% or less based on the total amount of the toner. Compressibility of the toner composition is preferably in the range of 35 to 40%. When the toner composition filling the above-described conditions is subjected to the image forming method, the composition shows advantages comprising: improving the decrease of fluidity by making the particle size of the toner composition small; stabilizing the delivery quantity between the drum and the sleeve; and keeping the amount of the toner and charge constant in the magnetic brush. As a consequence, the toner is transferred precisely to the charge attenuating portions of the image area recorded by the laser light, thereby making the contrast of the light and shade portions of the image distinct and the grading of the image excellent. A line image with constant resolution of line width is also realized with good reproducibility of the original. When the average particle size of the toner exceeds the above-described range, reproducibility of the images of the ultra-fine lines decreases, thereby reproducing images of poor resolution. When the mean particle size of the toner is smaller than the above-described range, coagulation of the toner in the toner container occurs due to marked decrease of the fluidity of the toner, or irregularity in the images arises because of the instability of the amount of the toner in the magnetic brush formed on the development sleeve. Even when the mean particle size of the toner falls in the above-described range, mal-charged particles are incorporated in the magnetic brush if 70 wt% or more of the total toner particles fall in the range of 0.6 to 1.2 times of the mean particle size, thereby entailing a decrease in the image density or the occurrence of overlapping images. It is also important that the relative amounts of fine particles with a particle size of 5 µm or less and of coarse particles with a particle size of 16 µm or more are in the above-described range. A toner composition having a good cleaning characteristic and excellent image forming ability is obtained by making the relative amounts based on the total amount of the toner be in the prescribed range. In the above-described toner composition, fluidity can be improved by adding the hydrophobic silica to the toner composition. Accordingly, even when images are formed by using this toner composition under circumstances of high temperature and high humidity, the amount of toner delivered to the developing apparatus is maintained at a constant level, preventing mal-charging of the toner during continuous copying and forming a magnetic brush having a constant amount of toner by supplying a toner having an appropriate amount of charge to the development sleeve. Any cleaning deficiency during the development process is also prevented effectively. The amount of hydrophobic silica is in general in the range of 0.05 to 2 parts by weight per 100 parts by weight of the above-described toner, and particularly the amount in the range of 0.1 to 0.5 parts by weight is preferable. The compressibility of the toner composition may be adjusted so as to be in the range of 35 to 40%, and particularly the range of 36 to 38%. A toner composition whose compressibility is in the above-described range shows excellent mutual sliding ability as a result of interposing the hydrophobic silica uniformly among the particles, whereby the toner assumes an appropriately compacted state. The toner composition thus obtained can be maintained in a desirable state for supplying toner from the toner container to the development sleeve, in stirring performance in the stirring portion of the development apparatus and in the dispersion ability of the toner in the magnetic brush. When the compressibility is below the above-described range, the supplied amount of the toner composition from the toner container to the developing apparatus will be unstable. Excess supply of the toner will occur, causing scattering of the toner composition and fogging. When the compressibility of the toner composition is greater than in the above-described range, fluidity of the toner composition will decrease. Consequently, supply of the toner composition to the developing apparatus will be insufficient, causing deterioration in the image density and blocking in the stirring portion of the development, which entails adhesion of the toner to the development sleeve and stirring paddle. Compressibility is defined in this invention by the following relationship of [(compressed apparent specific gravity - loose apparent specific gravity) /compressed apparent specific gravity x 100(%)], which represents the mutual mixing state of the powder. Loose apparent specific gravity and compressed apparent specific gravity were measured by using a powder tester manufactured by Hosokawa Micron Co. The following examples illustrate the present invention. In the following examples, hydrophobic silica TS-720 (trademark, treated with a polymer having a methylsilyl group, manufactured by Cabot Co.) has the following chemical formula: The surface treatment agent of TS-720 has the following chemical formula: Additionally, hydrophobic silica R-972 (manufactured by Nihon Aerosil Co.) has the following chemical formula: The surface treatment agent of R-972 has the following chemical formula: Example 1 Toner material Parts by weight styrene-acrylic copolymer 100 carbon black 8.5 low molecular weight polypropylene Biscol 550P (trade mark, Sanyo Kasei Co.) 1.8 charge regulating agent, Bontron S-34 (trademark, Orient Kagaku Co.) 10 The toner materials described above were melted and kneaded together and the mixture was cooled, ground and fractionated to give a toner of mean particle size of 8.5 µm, wherein particle size distribution of 75 wt% of the toner based on the total particles was in the range of 5.1 µm to 10.2 µm, the relative amount of the toner with the coarse powder of a particle size of 16 µm or more was 0 wt% and that fine powder of particle size of 5 µm or less was 3 wt%. One hundred parts by weight of the toner was mixed with 0.5 parts by weight of the hydrophobic silica TS-720 (Cabot Co.) and 0.5 parts by weight of hydrophilic alumina [aluminum oxide C (Nihon Aerosil Co.)] by using a Henshel mixer to give a toner composition. The toner composition obtained was blended with a ferrite carrier of mean particle size of 104 µm coated with an acrylic polymer in a weight ratio of 3:97, thereby preparing a developing agent. Testing was carried out under the development conditions described in Table 2 by using the above-described toner composition and a reconstructed electrophotographic copying machine DC-1605 (trade mark, Mita Kogyo Co.) equipped with an exposure apparatus using a semiconductor laser beam. The results are listed in Table 2. Example 2 Toner materials as used in Example 1 were melted and kneaded together and the mixture was cooled, ground and fractionated to give a toner of mean particle size of 7.5 µm, the particle size distribution of 80 wt% of the toner based on the total particles being in the range of 4.5 µm to 9.0 µm, the relative amount of the toner with coarse particle size of 16 µm or more was 0 wt% and that of fine powder of particle size of 5 µm or less was 7 wt%. A developing agent was prepared by the method described in Example 1, except that 100 parts by weight of the toner was mixed with 0.7 parts by weight of hydrophobic silica TS-720 (Cabot Co.) and 0.5 parts by weight of hydrophilic alumina particles [aluminum oxide C (Nihon Aerosil Co.)] using a Henshel mixer. Testing was carried out by using the developing agent prepared under the development conditions listed in Table 2. The results are described in Table 2. Example 3 Toner materials as used in Example 1 were melted and kneaded together and the mixture was cooled, ground and fractionated to give a toner of mean particle size of 9.5 µm, wherein particle size distribution of 74 wt% of the toner based on the total particles was in the range of 5.7 µm to 11.4 µm, the relative amount of the toner with the coarse particle size of 16 µm or more was 1 wt% and that of fine powder of the particle size of 5 µm or less was 2 wt%. One hundred parts by weight of the toner were mixed with 0.5 parts by weight of hydrophobic silica TS-720 (Cabot Co.) and 0.5 parts by weight of hydrophilic alumina particles [aluminum oxide C (Nihon Aerosil Co.)] using a Henshel mixer to give a toner composition. The toner composition obtained was blended with a ferrite carrier of mean weight particle size of 90 µm coated with an acrylic polymer in a weight ratio of 3:97, thereby preparing a developing agent. Testing was carried out using the developing agent thus obtained under the development conditions described in Table 2. The results are described in Table 2. Comparative Example 1 The toner materials used in Example 1 were melted and kneaded together and the mixture was cooled, ground and fractioned to give a toner of mean particle size of 10.5 µm, wherein particle size distribution of 60 wt% of the toner based on the total particles was in the range of 6.3 µm to 12.6 µm, the relative amount of the toner with coarse particles of 16 µm or more was 5 wt% and that of fine powder of particle size of 5 µm or less was 3 wt%. A developing agent was prepared by the method described in Example 1, except that the toner of this example was used. Testing was carried out by using the developing agent obtained under the development conditions described in Table 2. The results are described in Table 2. Comparative Example 2 A developing agent was prepared by the same method described in Example 1, except that 100 parts by weight of the toner obtained by the method described in Example 1 and 0.5 parts by weight of a hydrophobic silica R-972 (mean particle size 0.001 µm, Nihon Aerosil Co.) were used. Testing was carried out by using the developing agent obtained under the development conditions described in Table 2. The results are described in Table 2. Comparative Example 3 The toner materials as were used in Example 1 were melted and kneaded together and the mixture was cooled, ground and fractionated to give a toner of mean particle size of 7.2 µm, wherein particle size distribution of 72 wt% of the toner based on the total particles was in the range of 4.3 µm to 8.5 µm, the relative amount of the toner with coarse particles of 16 µm or more was 0 wt% and that of fine powder of particle size of 5 µm or less was 11 wt%. A developing agent was prepared by the same method used in Example 1, except that 100 parts by weight of the toner of this example were mixed with 1.0 part by weight of a hydrophobic silica TS-720 (Cabot Co. ) and 0.5 parts by weight of hydrophilic alumina particles [aluminum oxide C (Nihon Aerosil Co.)] using a Henshel mixer. Testing was carried out by using the developing agent obtained under the conditions described in Table 2. The results are described in Table 2. Comparative Example 4 The toner materials used in Example 1 were melted and kneaded together and the mixture was cooled, ground and fractionated to give a toner of mean particle size of 5.8 µm, wherein particle size distribution of 80 wt% of the toner based on the total particles was in the range of 3.5 µm to 7.0 µm, the relative amount of the toner with coarse particles of 16 µm or more was 0 wt% and that of fine powder of particle size of 5 µm or less was 20 wt%. A developing agent was prepared by the method used in Example 1, except that 100 parts by weight of the toner of this example were mixed with 1.0 part by weight of a hydrophobic silica TS-720 (Cabot Co.) and 0.5 parts by weight of hydrophilic alumina particles [aluminum oxide C (Nihon Aerosil Co.)] using a Henshel mixer. Testing was carried out by using the developing agent obtained under the conditions described in Table 2. The results are described in Table 2. The evaluation methods for image quality as described in Table 2 were as follows: (1) Image densities and fog densities are the measured values obtained by using a reflection photodensitometer. (2) Half tones were evaluated visually with reference to standard copies. (3) Resolution of the images were expressed by the numbers of the fine line images reproducible in 1 mm width. (4) Irreguralities in the images were evaluated visually by observing the differences in the image density and image resolution among the center, both sides and top and bottom ends of the copy sheet. (5) Cleaning faults were expressed by the numbers of the copy sheets wherein cleaning faults occurred. The results described in Table 2 indicate that a good quality of images is maintained over a long period of use when copies are produced by using the toner compositions obtained in Examples 1-3. On the other hand, copying operations using the toner compositions obtained in Comparative Examples 1 - 3 cause problems and deficiencies as the copying operations are repeated, namely; decrease in image density, deterioration in resolution, irregularity in image quality and cleaning faults.

Claims: A toner composition for developing an electrostatic latent image on an image-holding body, the composition comprising toner particles having a mean particle size of about 6 to 10µm, characterized in that the toner has a toner particle distribution wherein 70wt % or more of the toner is 0.6 to 1.2 times the mean particle size, the amount of toner particles having a diameter of 5µm or less is at most 10wt %, and the amount of the toner particles having a diameter of 16µm or more is at most 2wt %, in that the toner composition further comprises particles of hydrophobic silica surface-treated with a polymer having a methylsilyl group, and in that the compressibility of the toner composition is in the range of 35 to 40%. A toner composition according to claim 1, wherein the hydrophobic silica particles are present in an amount of 0.05 to 2 parts by weight per 100 parts by weight of said toner particles. A toner composition according to claim 1 or 2, wherein said toner composition further comprises fine particles of hydrophilic alumina. A toner composition according to claim 3, wherein the weight ratio of hydrophobic silica to hydrophilic alumina in said toner composition is in the range of 1:0.2 to 1:3. An image forming method comprising the steps of: imparting a charge to an image-holding body by a charging method; forming an electrostatic latent image on the image-holding body by exposing the image on the image-holding body imparted by a charge by a digital exposure method; converting the latent image on the image-holding body into a toner image by a development method utilising a toner composition; and transferring the toner image on the image-holding body to a copy material by a transfer means, wherein said toner composition is a toner composition as claimed in any preceding claim. An image forming method according to claim 5, wherein said image-holding body is an organic photoconductor. An image forming method according to claim 5 or 6, which is a two component development method wherein a developing agent is used comprising said toner composition and a magnetic material as carrier therefor, and a charge is imparted to the toner material by mixing it with the carrier.

Inventors: HORIO HIROFUMI, IIDA TOMOHIDE, ITAKURA TAKAYUKI, TESHIMA TAKASHI, HORIO, HIROFUMI, IIDA, TOMOHIDE, ITAKURA, TAKAYUKI, TESHIMA, TAKASHI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9