A toner composition and a method for preparing the same

Abstract: A toner composition is disclosed which comprises a mixture of toner particles with finely powdered acrylic polymer, and finely powdered silica dispersed in the mixture, wherein the finely powdered acrylic polymer is present in an amount of 0.05 to 0.15 parts by weight per 100 parts by weight of the toner particles and the weight ratio of silica to acrylic polymer is within the range of 1 to 5:1. Also dis­ closed is a method for preparing the toner composition.

Description: The present invention relates to a method of preparation of a toner composition for use in developer units for the development of electrostatic latent images as claimed in claim 1 and to the toner composition obtainable by this method, as claimed in claim 7. In electrophotographic copying machines, electrostatic latent images, formed on photosensitive layers containing inorganic or organic photoconductive materials, are rendered visible by a dry development process using powdered toners comprising resinous binders with additives such as colorants dispersed therein. The electrostatic latent images, which are formed on the photosensitive layer by a sequence of charge and exposure, are developed with the powdered toner to form toner images corresponding to the electrostatic latent images. These toner images are then transferred to a support such as paper. Finally, the toner images on the paper are fixed thereon by use of fixing means such as a heating or pressurizing roller. After each toner image has been transferred to the paper, the surface of the photosensitive layer is cleaned by scraping with a cleaning blade to remove residual toner. In order to achieve a satisfactory image formation, the toner desirably satisfies various requirements such as stable charge retention, absence of fogging or aerial scattering and lack of adhesion of residual toner on the surface of the photosensitive layer in the cleaning process. Techniques have been proposed for improving charge stability and cleaning characteristics by use of a toner composition containing acrylic polymers in finely powdered form. For example, a toner composition proposed in Japanese Laid-Open Patent Publication No. 60-186851 consists of toner particles with added fine acrylic polymer powder. This toner composition, however, has unsatisfactory flowability, in particular, when applied to continuous or high speed duplication of a large number of frames under high humidity conditions. A decrease in flowability causes toner scattering or image fogging in some cases. Developer units employed in recent copying machines are constructed such that toner fed from a toner cartridge is accommodated in a container called a hopper. A feed roller, composed of a porous or elastic material such as sponge, is fitted in the bottom of the hopper. Rotation of this feed roller allows toner to drop into a developer mixing unit to replace the toner consumed. However, if the toner mentioned above is used in this type of process, because of poor flowability, the toner composition may not drop despite the rotation of the feed roller. Moreover, this drawback may become extremely pronounced under high humidity conditions. According to the present invention, there is provided a method of preparing a toner composition, comprising the steps : (a) adding acrylic polymer particles to toner particles comprising a resinous binder and mixing them together to form a first toner mixture in which the weight ratio of the toner to the acrylic polymer is within the range from 30:1 to 50:1, the said acrylic polymer particles having a mean particle size within the range from 0.3 µm to 1.0 µm; (b) adding further toner particles to the first toner mixture so as to form a second toner mixture in which the concentration of the acrylic polymer is within the range from 0.05 to 0.15 parts by weight per 100 parts by weight of the toner; and (c) mixing finely powdered silica into the second toner mixture to form the said toner composition, the mean particle size of the finely powdered silica being within the range from 0.01 µm to 0.04 µm and the weight ratio of the finely powdered silica to the acrylic polymer, in the toner composition, being within the range from 1:1 to 5:1. Preferably, the concentration of the acrylic polymer in the said second toner mixture is within the range from 0.08 to 0.13 parts by weight per 100 parts by weight of the toner. It is also preferred that the weight ratio of the finely powdered silica to the acrylic polymer, in the toner composition, is within the range from 2.5:1 to 3.5:1. It is further preferred that the toner particles have a mean size within the range from 1 µm to 30µm. The method claimed in the present invention serves to produce a toner composition of satisfactory flowability, cleanability and charge stability, which can continue to form clear, sharp copied images over a long period of time without undergoing unacceptable changes in various characteristics, even during long periods of continuous copying. Such a toner composition can have superior moisture resistance, which undergoes little change in various characteristics and permits smooth feeding by a feed roller from a toner hopper even under conditions of high humidity, and does not cling strongly to the surface of a photosensitive layer so as to hinder cleaning thereof. The invention provides a method in which, firstly, acrylic polymer powder is added to and mixed together with toner particles in a weight ratio of toner to acrylic polymer within the range from 30:1 to 50:1. Then, additional toner is added to this mixture to reduce the relative content of the acrylic polymer to a prescribed concentration. This method can serve to improve the uniformity of dispersion of the acrylic polymer powder throughout the mixture and achieve uniform adhesion of the acrylic powder to the surfaces of the toner particles. When acrylic polymer powder is first added to the toner particles, a proportion of the acrylic polymer powder adheres strongly to the toner particles. The remainder of the acrylic polymer powder remains in a state of weak adhesion to the toner particles. When additional toner particles are added to this mixture and mixed therewith, the additional toner particles attract the weakly adhering particles of the acrylic polymer, resulting in greater uniformity of the mixture, which can be maintained thereafter. In contrast, if all the toner is mixed with the acrylic polymer in just a single operation, such uniform mixing of the two types of particles is not achieved, so that when a toner composition prepared in this manner is used in copying machines, feeding efficiency of the toner composition from the hoppers may deteriorate markedly. Moreover, when a running copy is being processed, reversely-polarized toner can form, resulting in scattering of the toner composition. Prior art examples of toner compositions prepared by such single step mixing operations are described in EP-A-0207628 and JP-A-63 198070. In the present invention, following the mixing of the additional toner particles with the initially mixed toner and acrylic polymer powder, finely powdered silica is added so that the resulting weight ratio of silica to acrylic polymer in the mixture is in the range from 1:1 to 5:1. This enables the particles of the acrylic polymer to maintain their uniform state of mixing with the toner particles for a prolonged period of time. Moreover, excellent flowability is imparted to the toner composition, permitting maintenance of satisfactory feeding from the hopper and stable charging over an extended period of time. Furthermore, no appreciable deterioration of the characteristics of the toner composition occurs even under conditions of high humidity. It is important that the addition of the finely powdered silica takes place after the mixing of additional toner particles with the initially mixed toner and acrylic polymer powder, since simultaneous addition of the silica and the acrylic polymer to the toner particles can lead to agglomeration of the acrylic polymer powder and a resulting deterioration in the feeding characteristics of the toner composition. Also with simultaneous mixing, poorly charged particles may be formed. In contrast, when finely powdered silica is added to the already mixed toner and acrylic polymer, the toner particles which have particles of acrylic polymer uniformly adhering to their surfaces are further covered by finely powdered silica. The acrylic polymer powder is then more stably maintained in a uniform state of dispersion and the overall flowability of the toner composition is thereby increased. The acrylic polymer powder can be in the form of spherical resin particles obtained by emulsion polymerization, soap-free polymerization, dispersion polymerization, suspension polymerization or by mechanical crushing of polymer blocks. The acrylic polymer powder employed herein typically has a mean particle size in the range from 0.3 µm to 1.0 µm, and preferably 0.4 µm to 0.6 µm. Amounts of 0.05 to 0.15, and preferably 0.08 to 0.13, parts by weight of the acrylic polymer per 100 parts by weight of the toner particles are employed. Amounts less than 0.05 parts by weight are undesirable because the number of poorly charged particles increases, resulting in a decrease of image density and the occurrence of image fogging. Also the cleaning characteristics of the toner composition deteriorate, so that residual toner cannot easily be completely removed. Amounts greater than 0.15 parts by weight are also undesirable, because flowability of such a toner composition exhibits a pronounced drop. Consequently, the efficacy of feeding from the hopper, as well as flowability within the developer unit deteriorate, and therefore image fogging and scattering of toner are prone to occur. The acrylic polymer can be a homopolymer of acrylic or methacrylic monomers, or can be a copolymer of acrylic or methacrylic monomers and free-radical polymerizable monomers. Examples of the acrylic or methacrylic monomers include acrylic acid, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, stearyl acrylate, cyclohexyl acrylate, phenyl acrylate, 2-hydroxypropyl acrylate, diethylaminoethyl acrylate, acrylamide, acrylonitrile, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate phenyl methacrylate, 2-hydroxypropyl methacrylate, diethylaminoethyl methacrylate, and the like. Examples of the free-radical polymerizable monomers include styrene derivatives such as styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, p-methoxystyrene, and p-chlorostyrene; olephinically unsaturated carboxylic acids such as maleic acid, fumaric acid, crotonic acid, and itaconic acid, or alkyl esters of these carboxylic acids; olephinic monomers such as ethylene, propylene, and butadiene; and vinyl compounds such as vinyl acetate, vinyl chloride, vinylidene chloride, vinylpyrrolidone, and vinylnaphthalene. The silica particles that are employed herein typically have a mean particle size between 0.01 µm and 0.04 µm and preferably between 0.02 µm and 0.03 µm. Preferably, hydrophobic silica particles are used. The weight ratio of silica to acrylic polymer is between 1.0:1 and 5.0:1, and preferably between 2.5:1 and 3.5:1. Ratios less than 1:1 are undesirable, because the overall flowability of the toner composition and the maintenance of dispersion of the powdered acrylic polymer will decrease. On the other hand, ratios greater than 5.0:1 are also undesirable, because image tailing will occur with a decrease in the amount of charge, and moreover, because the control of toner density by means of a sensor will become unstable. The toner particles that are employed herein can be those which contain additives such as colourants and the like, dispersed in the resinous binder as described below. Examples of the resinous binders include styrene polymers and copolymers, acrylic polymers and copolymers, styrene-acrylic copolymers, polyolefins such as polyethylene, chlorinated polyethylene, polypropylene, and ionomer, vinyl chloride polymers and copolymers such as polyvinylchloride, polyester resins, polyamide resins, polyurethane resins, polyether resins, epoxy resins, diallyl phthalate resins, silicone resins, ketone resins, polyvinylbutyral resins, phenol resins, xylene resins, rosin-modified phenol resins, rosin-modified maleate resins, rosin esters, and cellulose resins. The resinous binder that is employed herein typically has a weight average molecular weight between 30,000 and 200,000 and preferably between 50,000 and 150,000, and a softening point between 50° and 200°C and preferably between 70° and 170°C. One or more kinds of the above-mentioned resinous binders can be used, depending upon the fixing process or any other characteristics required. Because of high grindability and easy control of molecular weight distribution, styrene polymers and copolymers, acrylic polymers and copolymers, and styrene-acrylic copolymers are preferred with the styrene-acrylic copolymers being most preferred. To improve the frictional charging characteristics of toner particles, polyester resins, polyether resins, epoxy resins, rosin-modified phenol resins, rosin-modified maleate resins, rosin esters, and cellulose resins can be used for the resinous binders. Also, when the toner particles constitute pressure-fixative toner, the resinous binders can be polyolefins, polyamide resins, or other polymers and copolymers, the composition of which can readily be modified. These resins, polymers and copolymers may be used in admixture with other polymers and copolymers such as polyvinyl acetate, ethylene-vinylacetate copolymers, hydrogenated polyethylene, and hydrogenated rosin esters, or aliphatic, alicyclic, or aromatic petroleum resins. Examples of colorants which may be dispersed in the resinous binder include carbon black, lampblack, chrome yellow, Hansa yellow, benzidine yellow, threne yellow G, quinoline yellow, permanent orange GTR, pyrazolone orange, Vulcan orange, Watchung Red, permanent red, Brilliant Carmine 3B, Brilliant Carmine 68, du Pont oil red, pyrazolone red, Lithol 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 oxalate, and various oil-soluble dyes such as C.I. Solvent Yellow 60, C.I. Solvent Red 27, and C.I. Solvent Blue 35. One or more kinds of these colorants can be used to obtain adequate density of toner images, for example, in an amount of 1 to 30 and preferably 2 to 20 parts by weight per 100 parts by weight of the resinous binder. When the toner particles constitute magnetic toner, a magnetic material can be used together with or in place of the colourant. The magnetic materials are those which have magnetic properties or can be magnetized, including ferromagnetic metals such as iron, cobalt, and nickel, alloys or compounds of these metals, and other metals such as manganese, e.g., ferrite, magnetite, and the like. Such magnetic material may have a mean particle size between 0.1 µm and 1 µm. One or more kinds of these magnetic materials can be used, typically in an amount of 5 to 70 and preferably 20 to 50 parts by weight per 100 parts by weight of the resinous binder. The toner particles may contain a charge-controlling agent in order to control their charges. Examples of the charge-controlling agents include oil-soluble dyes such as Nigrosine base, oil black, and Spiron black; metallic soaps which are salts of various carboxylic acids, such as naphthenic acid, salicylic acid, octylic acid, fatty acid, and resin acid, with metals such as manganese, iron, cobalt, nickel, lead, zinc, cerium, and calcium; metal-containing azo dyes; pyrimidine compounds; and alkylsalicylate metal chelate compounds. Typically, amounts of 0.1 to 5 parts by weight of the charge-controlling agent per 100 parts by weight of the resinous binder are employed. The toner particles may contain an offset inhibitor in order to prevent them from adhering to fixing rollers. Examples of the offset inhibitors include low molecular weight polypropylene, low molecular weight polyethylene, various kinds of wax such as paraffin wax, low molecular weight polyolefin prepared from olefin monomers containing 4 or more carbon atoms, fatty acid amides, silicone oil, and the like. The offset inhibitor is preferably contained in an amount of 0.5 to 15 parts by weight per 100 parts by weight of the resinous binder. The toner particles that are employed herein typically have a mean particle size between 1 µm and 30 µm and preferably between 5 µm and 25 µm. The toner composition according to this invention can be useful for either a single developer or binary developer. When used as a single developer, the toner particles contain the magnetic material and are mixed with the acrylic polymer powder and finely powdered silica to form the single developer. When used as a binary developer, a mixture consisting of toner particles, powdered acrylic polymer, and finely powdered silica is further blended with carriers to form the binary developer. The carriers that are employed herein may be uncoated carriers such as glass beads, oxidized or unoxidized iron powder, or ferrite; or may also be coated carriers in which a magnetic material such as iron, nickel, cobalt, or ferrite, is coated with resins, polymers or copolymers such as acrylic polymers or copolymers, fluorocarbon resins, polyester resins, silicone resins, epoxy resins, or melamine resins. These carriers typically have a mean particle size of 50 µm to 2,000 µm. In such a developer comprising a toner composition and carriers, the concentration of toner composition is typically within the range from 2 to 15 percent by weight. A toner composition prepared in the above manner has adequate durability and moisture resistance in practical use, and even under conditions of continuous or high-speed copying, when the toner composition must be frequently replenished from the hopper and sharp fluctuations of toner consumption occur, the characteristics of the toner exhibit little change, with charge stability, cleaning characteristics, and flowability being stably maintained, permitting the formation of high quality images. In a toner composition prepared in accordance the method of this invention, finely powdered silica is dispersed so as to cover the toner particles bearing uniformly adherent acrylic polymer powder on their surfaces. Therefore, the charge control characteristics of the acrylic polymer powder are effectively manifested and the overall flowability of the toner composition is stably maintained, so that the characteristics of the toner composition undergo little change, thereby attaining invariant stable developing and cleaning characteristics. The following Examples illustrate this invention: Examples and Comparative Examples To 100 parts by weight of a styrene-acrylic copolymer as a resinous binder were added 10 parts by weight of carbon black as a colourant, 1 part by weight of a negative polarity dye as a charge-controlling agent, and 1.5 parts by weight of low molecular weight polypropylene as an offset inhibitor. After fusion and kneading by a conventional method, this mixture was cooled and ground to obtain toner particles with a mean particle size of 15 µm. Then, by varied addition procedures and with varied final concentrations, the toner particles obtained in this manner were mixed with an acrylic polymer powder consisting of PMMA (Polymethylmethacrylate) particles (the mean particle size thereof being 0.4 µm) and with hydrophobic silica (the mean particle size thereof being 0.016 µm, R972 supplied by Nippon Aerosil Co.), thus preparing a series of toner compositions. Each of the toner compositions obtained in this manner was mixed with a ferrite carrier having a mean particle size of 100 µm to form a developer, and the charging characteristics of these various developers were evaluated. Furthermore, using each of these developers, 70,000 copies were continuously taken both at ordinary temperature and humidity (20°C, 60%) and at high temperature and humidity (35°C, 80%) with a electrophotographic copying machine (DC-3285, manufactured by Mita Kogyo Co., Ltd.) modified so as to effect toner density control by means of a magnetic sensor, and the resulting picture quality characteristics were investigated in each case. The results of these tests are shown in Tables 1 and 2, wherein samples 1, 12, and 13 correspond to the preferred examples of this invention and the other samples correspond to the comparative examples. In Tables 1 and 2, the initial weight ratio of toner particles to acrylic polymer indicates the weight ratio of toner particles to powdered acrylic polymer in the initial mixture, before the content of acrylic polymer was adjusted to the final concentration. Except for samples 5, 11, and 14, the initial toner mixture consisting of toner particles and powdered acrylic polymer was thereafter mixed with additional toner particles in order to achieve the final concentration of acrylic polymer. Except for sample 10, wherein the powdered acrylic polymer and finely powdered silica were added simultaneously, the finely powdered silica was added after the mixture of toner particles and powdered acrylic polymer had been prepared. The concentration of powdered acrylic polymer is expressed in terms of parts by weight per 100 parts by weight of the toner particles. In the preparation of sample 5, the powdered acrylic polymer and finely powdered silica were simultaneously added to the entire amount of toner particles to be contained in the final product. Sample 11 was prepared by first adding the finely powdered silica to the entire amount of toner particles and afterward adding the powdered acrylic polymer, while sample 14 was prepared by first adding the powdered acrylic polymer to the entire amount of toner particles and afterward adding the finely powdered silica. The criteria for the evaluation of flowability indicated in these tables were as follows: ○...... Both replenishment from the hopper and flowability of the developer within the developing unit during development were excellent, entailing no problems; Δ...... Replenishment from the hopper and flowability of the developer within the developing unit gradually deteriorated; and ×...... The toner ceased dropping from the hopper, and development became impossible. The charging characteristics indicated in the tables were evaluated by measurement of the distribution of charge carried by the toner compositions after mixed with the carrier, and represent the proportion of toner particles carrying a charge of opposite polarity. Scattering of the toner was assessed by observing the contamination of the zone below the developing unit and the soiling of the reverse faces of the copies, and was graded according to the following scale: ○...... No scattering; Δ...... Almost no scattering; and ×...... Soiled copies appeared frequently. Cleaning characteristics were assessed on the basis of the images on the copies, and was graded according to the following scale: ○...... No fogging; Δ...... Almost no fogging; and ×...... Frequent occurrence of fogging. As can be seen from Tables 1 and 2, toner composition with excellent characteristics can be obtained by first mixing toner particles and powdered acrylic polymers at a prescribed ratio, subsequently admixing additional toner particles in order to adjust the content of powdered acrylic polymers to the desired final concentration, and finally admixing a specified amount of finely powdered silica. A toner composition made in accordance with the method of this invention has improved charging and cleaning characteristics as well as improved durability, which permit excellent image formation over long periods of continuously repeated copying, and also has significantly improved moisture resistance.

Claims: A method of preparing a toner composition, comprising the steps of: (a) adding acrylic polymer particles to toner particles comprising a resinous binder and mixing them together to form a first toner mixture in which the weight ratio of the toner to the acrylic polymer is within the range from 30:1 to 50:1, the said acrylic polymer particles having a mean particle size within the range from 0.3 µm to 1.0 µm; (b) adding further toner particles to the first toner mixture so as to form a second toner mixture in which the concentration of the acrylic polymer is within the range from 0.05 to 0.15 parts by weight per 100 parts by weight of the toner; and (c) mixing finely powdered silica into the second toner mixture to form the said toner composition, the mean particle size of the finely powdered silica being within the range from 0.01 µm to 0.04 µm and the weight ratio of the finely powdered silica to the acrylic polymer, in the toner composition, being within the range from 1:1 to 5:1. A method according to claim 1, wherein the concentration of the acrylic polymer in the said second toner mixture is within the range from 0.08 to 0.13 parts by weight per 100 parts by weight of the toner. A method according to claim 1 or 2, wherein the weight ratio of the finely powdered silica to the acrylic polymer, in the toner composition, is within the range from 2.5:1 to 3.5:1. A method according to claim 1, 2 or 3, wherein the said resinous binder is selected from the group comprising styrene polymers and copolymers, acrylic polymers and copolymers and styrene-acrylic co-polymers. A method according to any preceding claim, wherein the acrylic polymer is poly(methyl methacrylate). A method according to any preceding claim, wherein the mean particle size of the said toner particles is within the range from 1 µm to 30 µm. A toner composition obtainable by the method of any one of claims 1 to 6.

Inventors: INOUE MASAHIDE, SHIMIZU YOSHITAKE, TSUYAMA KOICHI, INOUE, MASAHIDE, SHIMIZU, YOSHITAKE, TSUYAMA, KOICHI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: G03G9
IPC codes: G03G9