METHOD FOR FORMING FIXED IMAGES

Abstract: A method of forming fixed images comprising forming an electrostatic latent image; developing the electrostatic latent image, whereby a toner is applied to the electrostatic latent image to form a visible image; and simultaneously transferring and fixing the formed visible image onto a preheated recording medium.

Description: - 1 - DESCRIPTION METHOD OF FORMING FIXED IMAGES TECHNICAL FIELD The present invention relates to a method of forming fixed images used for plain paper copying machines, laser printers, plain paper facsimiles, etc. More particularly, it relates to a method of forming images in which transfer and fixing are simultaneously carried out using a recording medium whose surface is heated in advance. BACKGROUND ART Conventionally, when images are formed with copying machines, laser beam printers, etc., various methods such as an electrophotographic process and an electrostatic recording process have been generally used. Figure 3 shows a schematic view of an apparatus used in a conventional method of forming fixed images using an electrophotographic process. In the conventional method, after an electrostatic latent image formed on a photoconductor by optical means is developed in a developing process, it is transferred to a recording medium such as a recording paper in a transfer process and then fixed into the final image generally with heat and pressure in a fixing process. As the photoconductor is repeatedly used, a cleaning device is provided for cleaning the residual toner after the transfer process with its rotation. In the conventional method of forming fixed images using an electrophotographic process, however, the processes from the formation of the electrostatic latent image up to its fixing onto the recording medium are time consuming, which makes the apparatus used therein not only complicated but also large. In addition, since the transfer efficiency of the toner is poor in the transfer process, it poses such problems as extra labor needed for the disposal of the toner collected by cleaning the residual toner, and the pollution due to the scattering of the toner in and out of the apparatus. Therefore, a method of simultaneously conducting transferring and fixing has been proposed (U.S. Patent l,_i_{8,872). In this method, since the transferring and the fixing are simultaneously carried out by pressing the toner image developed on the dielectric drum to the recording medium, the simplification of the apparatus can be surely achieved. However, since only pressure is applied at the fixing, the fixing ability is poor, and a little improvement is achieved in the transfer efficiency. The fixing of the toner should be generally conducted at a high temperature due to the high melting temperature of the toner, thereby requiring an apparatus with a high thermal efficency. The fixing process usually works independently, and is carried out at such a high temeperature of around 200°C . Accordingly, expensive heat-resistant materials such as heat-resistant resins, heat-resistant rubbers, etc. have to be provided in the periphery of the fixing device. In addition, when the fixing is carried out at a high temperature, it is subject to problems such as curling and jamming of the paper, etc. Therefore, taking into consideration of the radiation from the apparatus, a device highly capable of radiating heat is in demand. Further, in view of solving the problems, there has been proposed a method of forming fixed images, wherein the transfer and fixing process is simultaneously carried out by adhering the toner onto a transfer film, which rotates while keeping it partly in close contact with a photoconductor to form a toner image, and putting the recording paper and the transfer film between a pressure roller and a heat roller provided away from the photoconductor (Japanese Patent Laid-Open No. 19788V1990) . However, in this method, the processes become more complicated because of the additional step using the transfer film, making the apparatus even larger. In addition, since the exposing is carried out through the transfer film, the exposure lowers, whereby the lines of electric forces corresponding to the electrostatic latent image formed on the photoconductor reduce, and the absorption of the toner decreases, resulting in a poor resolution at the time of developing. Alternatively, an electrostatic recording process comprises forming an electrostatic latent image by applying a signal voltage via a recording needle on an electrostatic recording sheet such as a recording paper; developing the electrostatic latent image using a developer device such as a magnetic brush; and transferring and fixing the electrostatic latent image by such means of heat, pressure, etc. However, the toner is adhered to the recording medium in excessive portions at the time of developing, causing so-called "greasing," and foreign materials such as paper powders are adhered to the recording needle at the time of applying voltage directly to the recording paper via the recording needle, making it less likely to obtain a normal image. Accordingly, it still has been difficult to obtain a stable, high-quality image continuously. In view of solving these problems, as disclosed in Japanese Patent Laid-Open Nos. 8U696/1980 and 13701/1991, methods comprising using a magnetic film or an insulating film, applying the toner on the surface of the film uniformly, supplying a thermal pulse corresponding to the signal by using a thermal head from the reverse side of the film to melt the toner, thereby forming fixed images on a recording paper have been known. However, in this method, the toner cannot be uniformly applied, causing the resulting fixed images being uneven in stripes. In addition, a part of the applied toner layer may be agglomerated by heat, and the resulting agglomerated toner is highly likely to act as a foreign material in the repetition of re-applying processes of the toner. Further, the toner is adhered to the recording medium such as a recording paper in excessive portions at the time of developing, making it likely to cause "greasing." As mentioned above, in either the electrophotographic process or the electrostatic recording process, various problems have been pointed out. From these standpoints, the development of a novel method of forming fixed images and a matching toner thereto is in demand. DISCLOSURE OF INVENTION An object of the present invention is to provide a novel method of forming fixed images, wherein transferring and fixing are simultaneously carried out onto the preheated recording medium, so that the fixing takes place only with a pressure roller, and practically all of the toner is transferred and fixed to the recording medium in the transfer and fixing process, thus providing only a trace amount of the toner lost and a little toner disposed, which makes the toner disposal box and its cleaning process simple and the resulting apparatus extremely compact. Therefore, in view of solving the above-mentioned problems, the present inventors have investigated a toner shell material which is fragile to heat at a low temperature. As a result, they have found that a thermally dissociating encapsulated toner produced by interfacial polymerization melts at a temperature of not more than 120 β C , and they have further investigated the image formation method using this encapsulated toner and have thus developed the present invention. More particularly, in view of solving the above problems, the method of forming fixed images of the present invention comprises forming an electrostatic latent image; developing the electrostatic latent image, whereby a toner is applied to the electrostatic latent image to form a visible image; and simultaneously transferring and fixing the formed visible image onto a preheated recording medium. In addition, the preheating temperature of the recording medium is in the range of not less than 50°C and not more than 160°C . According to the present invention, the visible image formed in the developing process is simultaneously transferred and fixed to the recording medium whose surface is heated in advance. Therefore, the transfer and fixing process can be remarkably simplified, and the radiator can be made much smaller due to its low fixing temperature, thereby achieving the miniaturization of an image-forming apparatus. BRIEF DESCRIPTION OF DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: Figure 1 is a schematic view of an apparatus used in the method of forming fixed images as defined by the present invention using an electrophotographic process; Figure 2 is a schematic view of an apparatus used in the method of forming fixed images as defined by the present invention using an electrostatic recording process; Figure 3 is a schematic view of an apparatus used for conventional methods of forming fixed images; Figure D is a schematic view showing the charging process in the method as defined by the present invention using the electrophotographic process; Figure 5 is a schematic view showing the exposing process in the method as defined by the present invention using the electrophotographic process; Figure 6 is a schematic view showing the developing process in the method as defined by the present invention using the electrophotographic process; and Figure 7 is a schematic view showing the transfer and fixing process in the method as defined by the present invention using the electrophotographic process. The reference numerals in Figures 1 through 7 denote the following elements: Element 1 is a photoconductor, element 1a a photoconductive layer, element lb a conductive supporter, element 2 an exposure device, element 3 a developer device, element 3a a rotating sleeve, element H a heater, element 5a a pressure roller, element 5b is a transfer device, element 6 a recording medium (a recording paper), element 7 a charger, element 8 a cleaner device, element 8a a toner collecting box, element 9 a charge eraser, element 10 a toner, element 12 a fixing roller, element 14 a heat roller, element 21 a voltage-applying device, element 22 a charge-remover, element 23 a developer device, element 2U a heater, element 25 a pressure roller, element 26 a recording medium, element 27 an insulating endless film belt, element 28 a cleaning web, element 29 a toner and element 30 a holding roller. BEST MODE FOR CARRYING OUT THE INVENTION The toner used in the present invention is a thermally dissociating encapsulated toner. The encapsulated toner according to the present invention comprises a heat-fusible core containing at least a coloring agent and a shell formed thereon so as to cover the surface of the core material. In the present invention, the thermally dissociating encapsulated toner means a toner which comprises a shell whose structure is fragile to heat, and a core material which can be fixed at a low temperature by pressure. More particularly, the shell structure changes with heat, and at the point when pressure is applied, the core material is discharged to effect the fixing of the toner. Depending on the raw materials and production method, a large variety of encapsulated toners are conceivable, and as long as they are within the range of the required thermal properties, there is no limitations on what production process or materials are used. The toner in the present invention is a thermally dissociating encapsulated toner, and any toner whose fixing temperature is maintained in the range of HO to 120 β C to the recording medium such as a recording paper can be properly chosen. As to the method for producing the encapsulated toners, the following can be mentioned. (1) Spray-drying method After the core material is dispersed in a non-aqueous solution of polymer or polymer-emulsion, the dispersed liquid is spray-dried. (2) Phase separation method (coacervation method) In a solution of ionic polymer colloids and the core material, phase separation is conducted around the core material. In other words, a simple emulsion is first prepared, which in turn is converted to a complex emulsion, in which the core materials are micro-encapsulated. (3) Interfacial polymerization method A core material solution or dispersion is dispersed in a water in oil or oil in water type emulsion system, while at the same time shell material monomers (A) are collected around the surfaces, where in the next method, monomers (A) and monomers (B) react. (M) Other methods include an in-situ polymerization method, a submerged cure coating method, an air suspension coating method, an electrostatic coalescing method, a vacuum vapor deposition coating method, etc. The particularly preferred toner include those produced by the interfacial polymerization method and the spray-drying method. While the spray-drying method has the merits of an easy function separation for the core material and shell material and a large choice of shell materials, the interfacial polymerization method not only has the merit of an easy function separation for the core material and shell material but also is capable of producing a uniform toner in an aqueous state. Moreover, substances of low softening points can be used for the core material in the interfacial polymerization method, making it particularly suitable from the aspect of fixing ability. Accordingly, in the present invention, the thermally dissociating encapsulated toner produced by the interfacial polymerization method among others is particularly preferred For shell materials, styrene resins (Japanese Patent Laid-Open No.205162/1983), polyamide resins (Japanese Patent Laid-Open No.66948/1983) . epoxy resins (Japanese- Patent Laid-Open No.148066/1984), polyurethane resins (Japanese Patent Laid-Open No.179860/1982) , polyurea resins (Japanese Patent Laid-Open No.150262/1987) and many others have been proposed. And as substances fixible under heat and pressure contained in the core material, thermoplastic resins having glass transition points (Tg) of between 10°C and 50°C such as polyester resins, polyamide resins, polyester-polyamide resins and vinyl resins can be used. As compared to the thermal properties of the core material, the structure and the thermal properties of the shell material concern themselves remarkably with the fixing ability of the entire toner. Since a particular polyurethane resin among the above-mentioned resins for the shell materials is thermally dissociating, having excellent storage stability and fixing ability at a low temperature, it is an extremely favorable material for the method of forming fixed images of the present invention. As principal components of such a shell material, resins obtainable from the reaction between an isocyanate compound and/or isothiocyanate compound and compounds containing a phenolic hydroxy group and/or a thiol group are preferably used (EP0453857A). The thermally dissociating encapsulated toner suitably used in the present invention can be produced by any known methods such as interfacial polymerization, etc., and this encapsulated toner is composed of a heat-fusible core material containing at least a coloring agent and a shell formed thereon so as to cover the surface of the core material, wherein the main components of the shell are a resin prepared by reacting: (A) an isocyanate and/or isothiocyanate compound comprising: (1) 0 to 30 ol % of a monovalent isocyanate and/or isothiocyanate compounds, and (2) 100 to 70 mol % of at least a divalent isocyanate and/or isothiocyanate compounds with (B) an active hydrogen compound comprising: (3) 0 to 30 mol % of a compound having one active hydrogen atom reactive with isocyanate and/or isothiocyanate groups and (4) 100 to 70 mol % of a compound having at least two active hydrogen atoms reactive with the isocyanate and/or isothiocyanate groups at a molar ratio of the component (A) to the component (B) of between 1:1 and 1:20, and wherein at least 30% of all of the linkages formed from the isocyanate or isothiocyanate groups are thermally dissociating linkages. According to the present invention, the thermally dissociating linkage is preferably one formed by the reaction between a phenolic hydroxyl and/or thiol group and an isocyanate and/or isothiocyanate group. Examples of the monovalent isocyanate compounds to be used as the the component (1) in the present invention include ethyl isocyanate, octyl isocyanate, 2-chloroethyl isocyanate, chlorosulfonyl isocyanate, cyclohexyl isocyanate, n-dodecyl isocyanate, butyl isocyanate, n-hexyl isocyanate, lauryl isocyanate, phenyl isocyanate, m-chlorophenyl isocyanate, 4-chlorophenyl isocyanate, p-cyanophenyl isocyanate, 3,4-dichlorophenyl isocyanate, o-tolyl isocyanate, m-tolyl isocyanate, p-tolyl isocyanate, p-toluenesulfonyl isocyanate, 1-naphthyl isocyanate, o-nitrophenyl isocyanate, m-nitrophenyl isocyanate, p-nitrophenyl isocyanate, p-bromophenyl isocyanate, o-methoxyphenyl isocyanate, m-methoxyphenyl isocyanate, p-methoxyphenyl isocyanate, ethyl isocyanatoacetate, butyl isocyanatoacetate and trichloroacetyl isocyanate. Examples of the divalent or higher isocyanate compounds to be used as the component (2) in the present invention include aromatic isocyanate compounds such as 2,4-tolylene diisocyanate, 2,4-tolylene diisocyanate dimer, 2,6-tolylene diisocyanate, p-xylylene diisocyanate, m-xylylene diisocyanate, 4,4' -diphenylmethane diisocyanate, 1,5- naphthylene diisocyanate, 3,3'-dimethyldiphenyl-4,4 1 - diisocyanate, 3.3'-dimethyldiphenylmethane-4,4 1 -diisocyanate, m-phenylene diisocyanate, triphenylmethane triisocyanate and polymethylenephenyl isocyanate; aliphatic isocyanate compounds such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, lysine diisocyanate and dimer acid diisocyanates; alicyclic isocyanate compounds such as isophorone diisocyanate, 4,4' -methylenebis- (cyclohexyl isocyanate), methylcyclohexane-2,4 (or 2,6)- diisocyanate and 1 ,3-(isocyanatomethyl)cyclohexane; and other isocyanate compounds such as an adduct of 1 mol of trimethylolpropane with 3 mol of tolylene diisocyanate. Examples of the isothiocyanate compounds include phenyl isothiocyanate, xylylene-1 ,4-diisothiocyanate and ethylidene diisothiocyanate. Among these isocyanate and isothiocyanate compounds, compounds having an isocyanate group directly bonded to an aromatic ring are preferred, because they are effective in forming a urethane resin having a low thermal dissociation temperature. According to the present invention, the monovalent isocyanate and/or isothiocyanate compound (1) also serves as a molecular weight modifier for the shell-forming resin and • can be used in an amount of at most 30 mol % based on the isocyanate component and/or the isothiocyanate component. When the amount exceeds 30 mol % , the storage stability of the obtained encapsulated toner is undesirably poor. Examples of compounds having one active hydrogen atom reactive with isocyanate and/or isothiocyanate groups to be used as component (3) in the present invention include aliphatic alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol, tert-butyl alcohol, pentyl alcohol, hexyl alcohol, cyclohexyl alcohol, heptyl alcohol, octyl alcohol, nonyl alcohol, decyl alcohol, lauryl alcohol and stearyl alcohol; aromatic alcohols such as phenol, o-cresol, m-cresol, p-cresol, 4-butylphenol, 2-sec-butylphenol, 2-tert-butylphenol, 3-tert-butylphenol, 4-tert-butylphenol, nonylphenol, isononylphenol, 2-propenylphenol, 3-propenylphenol, 4-propenylphenol, 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 3-acetylphenol, 3-carbomethoxyphenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2-bromophenol, 3-bromophenol, 4-bromophenol, benzyl alcohol, 1-naphthol, 2-naphthol and 2-acetyl-1- naphthol; and amides such as ε -caprolactam. Particularly, a phenol derivative represented by the following formula (I) is preferably used: wherein R. , R 2 . R3. Ru and R 5 each independently represents a hydrogen atom, an alkyl group having 1 to 9 carbon atoms, an alkenyl, alkoxy, alkanoyl, carboalkoxy or aryl group or a halogen atom. Examples of the dihydric or higher alcohols among the compounds having at least two active hydrogen atoms reactive with isocyanate and/or isothiocyanate groups to be used as the component (4) in the present invention include catechol, resorcinol, hydroquinone, 4-methylcatechol, 4-tert- butylcatechol, 4-acetylcatechol, 3-methoxycatechol, 4-phenylcatechol , 4-methylresorcinol, 4-ethylresorcinol , 4-tert-butylresorcinol, 4-hexylresorcinol, 4-chlororesorcinol, 4-benzylresorcinol, 4-acetylresorcinol, 4-carbomethoxyresorcinol, 2-methylresorcinol, 5-methylresorcinol, tert-butylhydroquinone, 2,5-di-tert- butylhydroquinone, 2,5-di-tert-amylhydroquinone, tetramethylhydroquinone, tetrachlorohydroquinone, methy1ca boaminohydroquinone, methylureidohydroquinone, benzonorbornene-3.6-diol , bisphenol A, bisphenol S, 3,3'- dichlorobisphenol S, 2,2' -dihydroxybenzophenone, 2, -dihydroxybenzophenone, 4,4' -dihydroxybenzophenone, 2,2' -dihydroxydiphenyl, 4,4' -dihydroxydiphenyl, 2,2' -dihydroxydiphenyl ethane, 3.4-bis(p-hydroxyphenyl) hexane, 1 ,4-bis(2-(p-hydroxyphenyl)propyl)benzene, bis(4- hydroxyphenyl)methylamine, 1 ,3-dihydroxynaphthalene, 1,4- dihydroxynaphthalene, 1 ,5-dihydroxynaphthalene, 2,6- dihydroxynaphthalene, 1 ,5-dihydroxyanthraquinone, 2-hydroxybenzyl alcohol, 4-hydroxybenzyl alcohol, 2-hydroxy- 3,5-di-tert-butylbenzyl alcohol, 4-hydroxy-3,5-di-tert- butylbenzyl alcohol, 4-hydroxyphenethyl alcohol, 2-hydroxyethyl 4-hydroxybenzoate, 2-hydroxyethyl 4- hydroxyphenylacetate, resorcinol mono-2-hydroxyethyl ether, hydroxyhydroquinone, gallic acid and ethyl 3,4,5- trihydroxybenzoate. Among these dihydric or higher alcohols, catechol derivatives reprsented by the following formula (II) and resorcinol derivatives represented by the following formula (III) are preferably used: wherein Re. R?. Re and R g each independently represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl, alkoxy, alkanoyl, carboalkoxy or aryl group or a halogen atom. OH wherein Ri 0 . R_ ι . R ι2 and Ri 3 each independently represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl, alkoxy, alkanoyl, carboalkoxy or aryl group or a halogen atom. Further, examples of the compounds having at least one isocyanate- or isothiocyanate-reactive functional group other than the hydroxyl group and at least one phenolic hydroxyl group include o-hydroxybenzoic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, 5-bromo-2- hydroxybenzoic acid, 3-chloro-4-hydroxybenzoic acid, 4-chloro-2-hydroxybenzoic acid, 5-chloro-2-hydroxybenzoic acid, 3.5-dichloro-4-hydroxybenzoic acid, 3-methyl-2- hydroxybenzoic acid, 5-methoxy-2-hydroxybenzoic acid, 3,5-di-tert-butyl-4-hydroxybenzoic acid, 4-amino-2- hydroxybenzoic acid, 5-amino-2-hydroxybenzoic acid, 2,5- dinitrosaϊicylic acid, sulfosalicylic acid, 4-hydroxy-3- ethoxyphenylacetic acid, catechol-4-carboxylic acid, 2,4- dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2,6- dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, 3,5- dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, m-hydroxycinnamic acid, p-hydroxycinnamic acid, 2-amino-4- methylphenol, 2-amino-5-methylphenol, • 5-amino-2- methylphenol, 3-amino-2-naphthol, 8-amino-2-naphthol, 1-amino-2-naphthol-4-sulfonic acid, 2-amino-5~naphthol-4- sulfonic acid, 2-amino-4-nitrophenol, 4-amino-2-nitrophenol , 4-amino-2,6-dichlorophenol , o-aminophenol , m-aminophenol , p-aminophenol , 4-chloro-2-aminophenol , 1-amino-4- hydroxyanthraquinone, 5-chloro-2-hydroxyaniline, -cyano-3- hydroxycinnamic acid, -cyano-4-hydroxycinnamic acid, 1-hydroxynaphthoic acid, 2-hydroxynaphthoic acid, 3-hydroxynaphthoic acid and 4-hydroxyphthalic acid. Further, examples of the polythiol compounds having at least one thiol group in each molecule include ethanethiol, 1-propanethiol, 2-propanethiol, thiophenol, bis(2- mercaptoethyl) ether, 1 ,2-ethanedithiol, 1 ,4-butanedithiol , bis(2-mercaptoethyl) sulfide, ethylene glycol bis(2- mercaptoacetate) , ethylene glycol bis(3-mercaptopropionate) , 2,2-dimethylpropanediol bis(2-mercaptoacetate) , 2,2- dimethylpropanediol bis(3-mercaptopropionate) , trimethylolpropane tris(2-mercaptoacetate) , trimethylolpropane tris(3-mercaptopropionate) , trimethylolethane tris(2-mercaptoacetate) , trimethylolethane tris(3-mercaptopropionate) , pentaerythritol tetrakis(2-mercaptoacetate) , pentaerythritol tetrakis(3-mercaptopropionate) , dipentaerythritol hexakis(2-mercaptoacetate) , dipentaerythritol hexakis(3-mercaptopropionate) , 1,2- dimercaptobenzene, 4-methyl-1 ,2-dimercaptobenzene, 3,6-dichloro-1 ,2-dimercaptobenzene, 3,4,5,6-tetrachloro-1 ,2- dimercaptobenzene, xylylenedithiol and 1 ,3,5-tris(3- mercaptopropyl) isocyanurate. In the thermally dissociating shell-forming resin used in the present invention, at least 30%, perferably at least 50% of all of the linkages formed from isocyanate or isothiocyanate groups are thermally dissociating linkages When the content of the thermally dissociating linkages is less than 30%, the strength of the shell in the heat-and- pressure fixing cannot be sufficiently lowered, making it less likely to exhibit any advantageous fixing performance of the core material. In the thermally dissociating encapsulated toner of the present invention, other compounds having an isocyanate- reactive functional group other than phenolic hydroxyl and thiol groups, which may be used as a shell-forming material in such an amount as not to lower the ratio of the linkages formed by the reaction of isocyanate and/or isothiocyanate groups with phenolic hydroxyl and/or thiol groups to the all of the linkages formed from isocyanate and/or isothiocyanate groups is less than 30%, include, for example, the following active methylene compounds such as malonate and acetoacetate, oxime such as methyl ethyl ketone oxime, carboxylic acid, polyol, polyamine, aminocarboxylic acid and aminoalcohol. According to the present invention, the compound having one active hydrogen atom reactive with isocyanate and/or. isothiocyanate groups as the component (3) may be used in an amount of at most 30 mol % based on the active hydrogen component. When the amount exceeds 30 mol %, the storage stability of the resulting toner is undesirably poor. Further, the molar ratio of (A) the isocyanate compound and/or isothiocyanate compound comprising the components (1) and (2) to (B) the active hydrogen compounds comprising the components (3) and (4) preferably lies between 1:1 and 1:20 in order to obtain a resin free from unreacted isocyanate groups. In the production of the encapsulated toner according to the present invention, the shell is preferably formed by an interfacial polymerization or an in-situ polymerization. Alternatively, it may be formed by a dry method comprising stirring in an air stream at a high rate matrix particles used as a core material together with particles used as a shell-forming material having a number-average particle size of one-eighth or less of that of the matrix particles. The resins to be used as core materials of the encapsulated toner according to the present invention are thermoplastic resins having glass transition points (Tg) of 10 to 50 β C , and examples thereof include polyester resins, polyester-polyamide resins, polyamide resins and vinyl resins, among which vinyl resins are particularly preferable. When the glass transition point (Tg) is less than 10°C , the storage stability of the resulting encapsulated toner is undesirably poor, and when it exceeds 50°C , the fixing strength of the encapsulated toner is undesirably poor. Examples of the monomers constituting the vinyl resins include styrene and its derivatives such as styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, -methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-chlorostyrene and vinylnaphthalene; ethylenically unsaturated monoolefins such as ethylene, propylene, butylene and isobutylene; vinyl esters such as vinyl chloride, vinyl bromide, vinyl fluoride, vinyl acetate, vinyl propionate, vinyl formate and vinyl caproate; ethylenic monocarboxylic acids and esters thereof such as acrylic acid, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, amyl acrylate, cyclohexyl acrylate, n-octyl acrylate, isooctyl acrylate, decyl acrylate, lauryl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, methoxyethyl acrylate, 2-hydroxyethyl acrylate, glycidyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methyl -chloroacrylate, methacrylic acid, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, amyl methacrylate, cyclohexyl methacrylate, n-octyl methacrylate, isooctyl methacrylate, decyl methacrylate, lauryl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, methoxyethyl methacrylate, 2-hydroxyethyl methacrylate, glycidyl methacrylate, phenyl methacrylate, dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate; ethylenic monocarboxylic acid derivatives such as acrylonitrile, methacrylonitrile and acrylamide; ethylenic dicarboxylic acids and derivatives thereof such as dimethyl maleate; vinyl ketones such -as vinyl methyl ketone; vinyl ethers such as vinyl methyl ether; vinylidene halides such as vinylidene chloride; and N-vinyl compounds such as N-vinylpyrrole and N-vinylpyrrolidone. Among the above core material resin-constituting monomers according to the present invention, the core material- orming resin contains, in the main skeleton of the resin, styrene or its derivatives preferably in an amount of 50 to 90 parts by weight, and the ethylenic monocarboxylic acid or an ester thereof preferably in an amount of 10 to 50 parts by weight to control the thermal properties of the resin, such as the softening point. When the monomer composition constituting the core material-forming resin according to the present invention contains a crosslinking agent, which may be also used, if necessary, as a mixture of two or more of them, any known crosslinking agents may be properly used. When the amount of the crosslinking agent added is too large, the resulting toner is less likely to be heat-fused, thereby resulting in poor heat fixing ability and heat-and-pressure fixing ability. On the contrary, when the amount is too small, in heat-and-pressure fixing, a part of the toner cannot be completely fixed on a paper but rather adheres to the surface of a roller, which in turn is transferred to a subsequent paper which creates the so-called "offset" or "offset phenomenon". Accordingly, the amount of the crosslinking agent is preferably 0.001 to 15% by weight, more preferably 0.1 to 10% by weight, based on the monomers used. The core material of the thermally dissociating encapsulated toner according to the present invention may further contain, if necessary, one or more offset inhibitor of any known kind for the purpose of improving offset resistance at heat-and-pressure fixing. These offset inhibitors are contained in an amount of 1 to 20% by weight based on the resin contained in the core material. The core material of the thermally dissociating encapsulated toner according to the present invention contains a coloring agent, which may be any one of the dyes and pigments used in the conventional toners. The coloring agent is generally contained in an amount of 1 to 15 parts by weight per 100 parts by weight of the resin contained in the core material. In addition, in the shell-forming materials of the thermally dissociating encapsulated toner according to the present invention and in the core material, a metal- containing dye which has been used for toners, for example, a metal complex of an organic compound having a carboxyl or nitrogenous group, such as nigrosine, may be added in an effective amount as a charge control agent. Alternatively, such a charge control agent may be mixed with the toner. The thermally dissociating encapsulated toner according to the present invention may contain, if necessary, a fluidity improver and/or a cleanability improver. Further, for the purpose of controlling the developability of the encapsulated toner, an additive, for example, finely powdered polymethyl methacrylate, etc. may be added. Furthermore, for the purposes of toning or resistance control, a small amount of carbon black may be used. The thermally dissociating encapsulated toner of the present invention preferably has a softening point of 80 to 150°C. ' If the softening point is lower than 80°C , the offset resistance of the resulting encapsulated toner is undesirably poor, and when it exceeds 150°C , the fixing strength of the encapsulated toner is undesirably poor. Although the particle size of the encapsulated toner according to the present invention is not particularly limited, the average particle size thereof is generally 3 to 30 ju m. The preferred thickness of the shell of the encapsulated toner is from 0.01 to 1 μ m. When the thickness ■ is less than 0.01 μ. m , the blocking resistance of the resulting encapsulated toner is poor, and when it exceeds 1μm, the heat fusibility of the resulting encapsulated toner is undesirably poor. Examples of the thermally dissociating encapsulated toners which is preferably used in the present invention will be described below, but the present invention is not restricted to these alone. The method of forming fixed images of the present invention are detailed below, referring to the drawings. The method of forming fixed images of the present invention comprises preheating a recording medium, and thereafter simultaneously transferring and fixing the visible image onto the preheated recording medium. Therefore, the recording system used for the method of forming fixed images of the present invention is not limitative to a particular one, and as long as the transferring and fixing to the recording medium can be carried out, any system can be applied. For instance, the method of forming fixed images can be applicable to systems using an electrographic process and electrostatic recording process. In addition, the present invention can be applicable to systems using other processes such as a magnetographic process and an ion deposition process. Figure 1 is a schematic view of an apparatus used in the method σf forming fixed images as defined by the present invention showing one example using the electrographic process. Element 1 is a photoconductor such as of amorphous silicon or organic photoconductor, etc. in which a photoconductive layer is provided on a conductive supporter. For photoconductors, those practically used are photoconductors of selenium, silicon, organic groups, etc., and any of these can be used. However, as described below, since the photoconductor is exposed to a considerable amount of heat from the recording medium heated in advance, the silicon photoconductors and the organic photoconductors having good heat resistance are preferred. In addition, - 25 - since the transferring and fixing are carried out on the surface of the photoconductor, the photoconductors having good security and wear resistance are preferably used. Element 7 is a charger located opposite to the photoconductor 1. The charging means is not particularly restricted, and any of, for instance* a corona charger, a brush charger, a roller charger, etc. can be used. Element 2 is an exposure device located opposite to the photoconductor 1 for forming electrostatic latent images on the photoconductor surface. For an exposure device 2, light sources such as laser beams, LED or EL arrays, etc. are used in combination with an image-forming optical system. Alternatively, a device based on optical systems projecting a reflected light of a document generally provided in the copying machine can be used. Element 3 is a developer device located opposite to the photoconductor 1 for making visible the electrostatic latent image formed on the photoconductor with the toner. For a developer device, any of the commonly used two-component magnetic brush developer device, the one-component magnetic brush developer device, the one-component non-magnetic developer device, etc. can be used. Element 4 is a heater, and element 5a is a pressure roller, and the heater 4 is disposed just before the point where the photoconductor 1 contacts the pressure roller 5a, so that the preheated recording medium can be conveyed to the pressure roller. In addition, although the distance between the photoconductor and the heater is preferably kept as little as possible for the purpose of preventing the decrease in the surface temperature of the paper, the heater 4 is preferably set at a proper distance from the photoconductor so as not to cause thermal effects or thermal deformation on the photoconductor. The heater 4 is a device for preheating the surface of the recording medium such as a recording paper, wherein the surface comes in contact with the toner. As long as it is a device capable of heating the surface of the recording medium up to 160°C , any type of heat source can be used for the heater 4. Heating bodies of the heater 4 include, ' for example, a hot plate, a quartz heater, a flash heater, a heating belt, a heating element, etc., with preference given to the quartz heater and the heating element. The pressure roller 5a is a means for pressure-welding the preheated recording medium onto the surface of the photoconductor. In an ordinary fixing device, it is necessary to use heat-resistant silicone rubbers, etc. to carry out fixing at a high temperature. However, in the present invention, it is not required to use the pressure roller having a particularly high heat resistance, since the pressure roller in contact with the reverse side of the preheated recording medium is not directly heated, and the temperature transmitted to the pressure roller is remarkably low. Therefore, as long as the materials for the pressure roller are elastic bodies having a good heat resistance at not less than 150°C , there are no limitations, and any of the ordinary inexpensive elastic materials including, for instance, heat-resistant polyurethane resins, acrylic resins, nitrile resins and non-conjugated diene terpolymer resins such as EPDM can be used. In addition, since the nip pressure of the pressure roller is usually 0.1 to 4.0 kg/cm, preferably 0.2 to 2 kg/cm, the durability thereof becomes longer. Incidentally, in the present invention, a belt may be used as a similar means in the place of the pressure roller. After the transfer and fixing process, the cleaner device 8 such as a cleaning web for removing trace amounts of the toner remaining on the surface of the photoconductor is arranged opposite to the photoconductor 1. The photoconductor 1 and the pressure roller 5a rotate at a constant peripheral speed in the direction shown in Figure 1 by a specified driving means not illustrated in the figure. In addition, a recording paper 6 used as a recording medium is conveyed in the manner shown in Figure 1. The recording paper 6 is then pressed between the pressure roller 5a and the photoconductor 1 to transfer and fix the visible image, and discharged out of the system by a paper discharging means not illustrated in the figure. In this connection, the conveying speed of the recording medium and the heating temperature of the heater is so regulated that the preheating temperature for the recording medium is maintained within the predetermined temperature ranges. Next, one example of the method of forming fixed images suitably using an electrostatic recording process is schematically shown in Figure 2. Element 21 is a voltage-applying device, element 23 a developer, element 27 an insulating endless film belt, element 29 a toner, element 24 a heater and element 25 a pressure roller. The voltage-applying device 21 is arranged opposite to the surface of the insulating endless film belt 27. It serves as a means for applying pulse voltage to a plurality of pin electrodes while driving the insulating endless film belt 27 along with the pin electrodes to carry out discharging on the insulating endless film belt, so that charges expressed by a character or a symbol are supplied onto the insulating endless film belt. The toner 29 in the developer 23 is applied to the obtained electrostatic latent image. The insulating film belt 27 having a thickness of 5 μ m to 20 μ m and made of heat-resistant films such as polyester resins, polyimide resins, polyamide resins, and other non- heat resistant films such as polypropylene films, polyethylene films, cellophane, etc. or conductive layers provided with dielectric coating can be used. The insulating endless film belt 27 is stretched with holding rollers 30. As in the case of the electrophotographic process, the heater 24 is disposed just before the point where the insulating endless film belt 27 contacts the pressure roller 25, so that the preheated recording medium 26 can be conveyed to the pressure roller 25. The heater 24 is a device for preheating the surface of the recording medium such as a recording paper, wherein the surface comes in contact with the toner. As long as it is a device capable of heating the surface of the recording medium up to 160 β C , any type of heat source can be used for the heater 24. The same devices as in the case of electrophotographic process can be used. The pressure roller 25 is a means for pressure-welding the preheated recording medium onto the surface of the insulating endless film belt 27. It is not required to use the pressure roller having a particularly high heat resistance, since preheated heat is not directly transmitted to the pressure roller. Therefore, as long as the materials for the pressure roller are elastic bodies having a good heat resistance at not less than 150 β C , there are no limitations on its materials, and any of the ordinary inexpensive elastic materials which can be used for the electrophotographic process can be also used. In addition, as in the case of the electrophotographic process, since the nip pressure of the pressure roller is usually 0.1 to 4.0 kg/cm, preferably 0.2 to 2 kg/cm, the durability thereof becomes longer. Incidentally, in the present invention, a belt may be used as a similar means in the place of the pressure roller. The conveying speed of the recording medium and the heating temperature of the heater is so regulated, as in the case of the electrophotographic process, that the preheating temperature for the recording medium is maintained within the predetermined temperature range. A cleaning web 28 is arranged opposite to the insulating endless film belt 27 not only to remove trace amounts of the residual toner but also to cool the insulating endless film belt 27 after transferring and fixing to the recording paper. Further, a charge remover 22 such as a conductive brush and a conductive roll removes the residual charges, and thereby the insulating endless film belt 27 is recovered to its initial state for reuse. Since the toner used in the method of the present invention is not subject to charging in the transfer process, not only insulating encapsulated toners but also conductive encapsulated toners can be used. Depending on the raw materials and production methods, a large variety of encapsulated toners are conceivable, and as long as they are within the range of the required thermal properties, there are no limitations on what production process or materials are used. Specifically, those having thermal properties capable of melting the toner on the recording medium heated in advance at a temperature range of between 50 β C and 160 β C and of fixing the toner by pressure of a pressure roller can be properly chosen. In general, the fixing temperature of the toner to the recording medium is in the range of between 40 β C and 120°C . Next, the individual processes of the method of forming fixed images by the present invention having the above- mentioned construction will be described. First, as a method using the electrophotographic process, Figure 4 shows a charging process, Figure 5 an exposing process, Figure 6 a developing process and Figure 7 a transfer and fixing process. In the charging process, as shown in Figure 4, a specified charge is uniformly supplied, e.g. by the corona charger 7 to the photoconductor surface. A photoconductor sensitive to a positive charge is taken here for an example, and the surface of the conductive supporter 1b is coated with the photoconductive layer 1a to form the photoconductor 1. A high voltage is applied by the corona charger 7 to the photoconductive layer 1a, thereby positively charging the surface of the photoconductive layer 1a. In the exposing process, as shown in Figure 5, a light from the exposure device 2 is irradiated to the surface of the related photoconductor, so that a leakage of charges occurs only in the exposed parts to form an electrostatic latent image on the photoconductive layer 1a. In the developing process, as shown in Figure 6, the toner triboelectrically charged inside the developer device is transported by the rotating sleeve 3a, and developed onto the photoconductor surface in proportion to the charge on the photoconductor surface. The developing process is an assortment of normal development in which a reversely polarized toner adheres to the charges by the Coulomb's force and of reverse development in which the toner adheres to the charges lost due to exposure to the light. The development process in the present invention applies to either method, but the case of the normal development is illustrated in Figure 6. In the transfer and fixing process, as shown in Figure 7, the visible image formed by applying the toner to adhere to a latent image on the surface of the photoconductor is conveyed. At the same time, a recording medium 6 such as a recording paper preheated by a heater 4 is pressure-welded onto the surface of the photoconductor by pressing the reverse side of the recording medium by a pressure roller 5a so as to synchronize with the initial end of the visible image, and thereby the visible image is simultaneously transferred and fixed onto the recording medium 6. In other words, when the toner adhered to the latent image formed on the surface of the photoconductor is pressure- welded to the recording medium, the deformation of the shell structure of the encapsulated toner due to the heat held in the recording medium takes place at the same time with the discharging of the core material in the encapsulated toner due to pressure of the pressure roller. When the temperature applied to the surface of the recording medium by the heater 4 is too high, the recording paper tends to curl, and when it is too low, sufficient fixing of the toner cannot be achieved, making record preservation difficult. Therefore, the surface of the recording medium is usually heated to a temperature of between 50 β C and 160 β C , preferably between 50°C and 120 β C . In the present invention, since substantially all of the toner is transferred to the recording medium, a toner collecting device is not required. Incidentally, although trace amounts of the toner may remain on the surface of the photoconductor 1 after the transferring of the toner to the recording medium 6, this toner can be removed by pressure- welding the photoconductor with such devices as a cleaning web arranged opposite to the photoconductor, making it possible to repeatedly use the photoconductor. Further, when the transfer and fixing process is completed, the charges remaining on the photoconductor are neutralized by a charge eraser 9 such as a charge erasing lamp arranged opposite to the photoconductor 1 , so that the photoconductor 1 is reused for the charging process. Alternatively, in the case of the electrostatic recording process, at first, charges represented by characters or symbols are supplied on the insulating endless film belt 27 by a voltage-applying device 21 as shown in Figure 2 to form an electrostatic latent image thereon. The resulting latent image is formed into a visible image by adhering the toner 29 in the developer. The transfer and fixing process is carried out in the same manner as in the case of the electrophotographic process by passing a recording medium 26, heated in advance by a heater 24, between a pressure roller 25 and the insulating endless film belt 27. Specifically, as shown in Figure 2, the insulating endless film belt rotates in a direction shown in the figure at a constant peripheral speed by a specified driving means not illustrated in the figure to convey the visible image formed by applying the toner to the surface of the insulating endless film belt. At the same time, the reverse side of the recording medium 26 is pressed by a pressure roller 25 to pressure weld the preheated recording medium 26 onto the surface of the insulating endless film belt 27 so as to synchronize with the initial end of the visible image. Therefore, the transferring of the toner to the recording medium 26 takes place at the same time of the fixing of the visible images. When the temperature applied to the surface of the recording medium by the heater 24 is too high, the recording paper tends to curl, and when it is too low, sufficient fixing of the toner cannot be achieved, making record preservation difficult. Therefore, the recording medium is heated to a temperature range similiar to that described for the case of the electrophotographic process. When the transfer and fixing process is completed, the recording paper is discharged from the system by a discharging means. In addition, the present invention is not confined to the above-mentioned embodiments, and specifications of the kinds of individual apparatus, processes etc. can be revised based on the principles of the present invention. By using the method of forming fixed images of the present invention, the following effects can be obtained: (1) Since the fixing is carried out at a low temperature by using a toner having good fixing ability and by only preheating the recording medium, the fixing takes place by only using a pressure roller, thereby making the apparatus extremely compact. (2) Since the heating temperature is low and the heat from the surface of the recording medium does not directly contact the pressure roller, a high heat resistance is not required for the pressure roller. Accordingly, inexpensive elastic members can be used as the materials of the pressure roller, and the duration of the roller becomes long. (3) Since substantially all of the toner is transferred to the recording medium in the transfer, and fixing process, little loss of the toner takes place, thereby causing substantially no toner to be discharged from the apparatus. Accordingly, the toner collecting box and the cleaning process can be simplified, thus making it possible to achieve low cost and miniaturization in the overall apparatus. (4) Since the toner for the low-temperature fixing is used, the temperature of the heating body in the fixing device can be set low with only a small rise in the temperature in the system, thereby making it possible to miniaturize the forced radiation device. (5) Since the surface of the recording medium is heated in a preheating process, a cardboard paper is also applicable for the method of the present invention. (6) Since the fixing takes place without going through an electrostatic process, the conductive toners can also be used, and an electrostatic inducing-type development capable of applying low voltage can also be used. The present invention is hereinafter described in more detail by means of the following working examples, but the present invention is not limited by them. Production Example of Encapsulated Toner: To a mixture comprising 70.0 parts by weight of styrene, 30.0 parts by weight of 2-ethylhexyl acrylate and 1.0 part by weight of divinylbenzene, 10.0 parts by weight of carbon black "#44" (manufactured by Mitsubishi Chemical Industries, Ltd.), 4.0 parts by weight of 2,2'- azobisisobutyronitrile, 9.5 parts by weight of 4,4'- diphenylmethane diisocyanate "Millionate MT" (manufactured by Nippon Polyurethane Industry Co., Ltd.) are added. The obtained mixture is introduced into an attritor (manufactured by Mitsui Miike Kakoki) and dispersed at 10°c for 5 hours to give a polymerizable composition. This composition is added to 800 g of a 4% by weight aqueous colloidal solution of tricalcium phosphate which has been preliminarily prepared in a 2-liter separable glass flask, so as to give a concentration of 30% by weight. The obtained mixture is emulsified and dispersed with a TK homomixer (manufactured by Tokushu Kika Kogyo) at 5°C and a rotational speed of 10000 rpm for 2 minutes. A four-necked glass cap is set on the flask, and a reflux condenser, a thermometer, a dropping funnel fitted with a nitrogen inlet tube and a stainless steel stirring rod are set thereon. The resulting flask is placed on an electric mantle heater. A solution of 22.0 g of resorcinol, 3.6 g of diethyl malonate and 0.5 g of 1 ,4-diazabicyclo [2.2.2] octane in 40 g of ion-exchanged water is prepared, and the resulting mixture is dropped into the flask in a period of 30 minutes through the dropping funnel while stirring. Thereafter, the contents are heated to 80 β C and reacted for 10 hours in a - 37 - nitrogen atmosphere while stirring. After cooling the reaction mixture, it is dissolved into 10%-aqueous hydrochloric acid. The resulting mixture is filtered and the obtained solid is washed with water, dried under a reduced pressure of 20 m Hg at 45°C for 12 hours and classified with an air classifier to give the encapsulated toner with an average particle size of 9 μ m having a shell made of a resin having a thermally dissociating urethane linkage. The glass transition point assignable to the resin contained in the core material is 30.2°C , and its softening point is 130.0°c. Production Example of Reference Toner: To 100 parts by weight of a polyester resin (Bisphenol- type polyester resin; softening point: 135°C ; Tg: 65°C ) , 7 parts by weight of carbon black (manufactured by Mitsubishi Kasei Ltd., MA8), 3 parts by weight of a polypropylene wax (Sanyo Kasei Ltd., Biscol 660P), and 2 parts by weight of a charge control agent (Hodogaya Kagaku Ltd., Aizenspilon Black TRH) are mixed, and the resulting mixture is kneaded by a pressurized kneader. After cooling the obtained mixture, it is pulverized with a pulverizing mill and then classified with a classifier to obtain a toner having a particle distribution range of 5 to 25 μ m and an average particle size of 10 # m. To 1 kg of the toner, 5 g of colloidal silica (Nihon Aerozil Ltd.: R972) is externally added to obtain a surface-treated reference toner. Test Example 1 : 50 g of the toner obtained in Production Example of Encapsulated Toner is blended together with 1 kg of a commercially available ferrite carrier by using a V-type blender to obtain a developer 1. The obtained developer 1 is used to carry out copying by using a modified apparatus of a commercially available copying machine as schematically shown in Figure 1. Specifically, a quartz heater is used as a heater and arranged at a distance of 5 cm away from the point where the photoconductor contacts the pressure roller. In addition, by varying the heating temperature and the conveying velocity, the temperature on the surface of the recording paper is properly adjusted so as to preheat the paper surface to a temperature of between 60°C and 160°C • The pressure roller used in the transfer and fixing is made of silicone rubber having a roller diameter of 30 mm , and transfer and fixing are carried out at a nip pressure of 0.5 kg/cm and a peripheral speed of 160 mm/sec. As a result, the lowest fixing temperature of the paper surface is 90°C , and substantially no melting of the toner to the surface of the photoconductor is observed at a temperature of between 80°C and 160°C . On the other hand, the toner obtained by the Production Example of Reference Toner is blended with a commercially available ferrite carrier to prepare a developer 2. Copying is carried out in the same manner as above using the modified apparatus. As a result, the lowest fixing temperature of the paper surface is 140°C . The lowest fixing temperature for the toner is the - 39 - temperature of the paper surface at which the fixing rate of the toner exceeds 70%. This fixing rate of the toner is determined by placing a load of 500 g on a sand-containing rubber eraser having a bottom area of 15 mm x 7.5 mm which contacts the fixed toner image, placing the loaded eraser on a fixed toner image obtained in the fixing device, moving the loaded eraser on the image backward and forward five times, measuring the optical reflective density of the eraser-treated image with a reflective densitometer manufacturecd by Macbeth Co., and then calculating the fixing rate from this density value and a density value before the eraser treatment using the following equation. Image density after eraser treatment Fixing rate = x 100 Image density before eraser treatment Test Example 2: The developer 1 obtained in Test Example 1 is used to carry out copying by using a modified apparatus of a commercially available copying machine as schematically shown in Figure 2. Specifically, a quartz heater is used as a heater and arranged at a distance of 5 cm away from the point where the insulating endless film belt contacts the pressure roller. In addition, by varying the heating temperature and the conveying velocity, the temperature on the paper surface of the recording paper is properly adjusted so as to preheat the paper surface to a temperature of between 60°C and 130°C • The pressure roller used in the transfer and fixing is made of EPDM having a roller diameter of 30 mmø , and transfer and fixing are carried out at a nip pressure of 0.5 kg/cm and a peripheral speed of 160 mm/sec. As a result, the lowest fixing temperature of the paper surface is 95°C , and substantially no melting of the toner to the surface of the insulating endless film belt is observed at a temperature of between 80°C and 130°C • On the other hand, the toner obtained by the Production Example of Reference Toner is blended with a commercially available ferrite carrier to prepare a developer 2. Copying is carried out in the same manner as above using the modified apparatus. As a result, the fixing rate at the highest fixing temperature of 130°c is 60%. From these test examples, it is confirmed that by utilizing the method of forming fixed images according to the present invention using a thermally dissociating encapsulated toner, the lowest fixing temperature can be remarkably lowered, thereby resulting in no curling or jamming of the paper sheets feeded to the copying machine. The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: CLAIMS 1. A method of forming fixed images comprising forming an electrostatic latent image; developing said electrostatic latent image, whereby a toner is applied to the electrostatic latent image to form a visible image; and simultaneously transferring and fixing the formed visible image onto a preheated recording medium. 2. The method according to claim 1, wherein said recording medium is preheated to a temperature of not less than 50 β C and not more than 160°C • 3. The method according to claim 1, wherein said toner is a thermally dissociating encapsulated toner which comprises a heat-fusible core material containing at least a coloring agent and a shell formed thereon so as to cover the surface of the core material, wherein the main component of • the shell is a resin prepared by reacting: (A) an isocyanate and/or isothiocyanate compound comprising: (1) 0 to 30 mol % of monovalent isocyanate and/or isothiocyanate compounds, and (2) 100 to 70 mol % of at least divalent isocyanate and/or isothiocyanate compounds with (B) an active hydrogen compound comprising: (3) 0 to 30 mol % of a compound having one active hydrogen atom reactive with the isocyanate and/or isothiocyanate groups and (4) 100 to 70 mol % of a compound having at least two active hydrogen atoms reactive with the isocyanate and/or isothiocyanate groups at a molar ratio of the component (A) to the component (B) of between 1:1 and 1:20, and wherein at least 30% of all of the linkages formed from the isocyanate or isothiocyanate groups are thermally dissociating linkages. 4. The method according to claim 3, wherein said thermally dissociating linkage is a linkage derived from reacting phenolic hydroxyl and/or thiol groups with the isocyanate and/or isothiocyanate groups. 5. The method according to claim 3, wherein said heat-fusible core material comprises a thermoplastic resin as its main component, whose glass transition point is 10°C 6. The method according to claim 3, wherein the softening point of said thermally dissociating encapsulated toner is 80°C to 150°C.

Inventors: KAWABE KUNIYASU, SASAKI MITSUHIRO, YASUDA SHIN-ICHIRO, KAWABE, KUNIYASU, SASAKI, MITSUHIRO, YASUDA, SHIN-ICHIRO
Applicants: KAO CORP, KAO CORPORATION
Application year: 1992
Main classification: G03G15
IPC codes: G03G9, G03G15