GB2148523A

GB2148523A - Magnetizable encapsulated toner

Info

Publication number: GB2148523A
Application number: GB08426500A
Authority: GB
Inventors: Fujio Kakimi
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1983-10-20
Filing date: 1984-10-19
Publication date: 1985-05-30
Also published as: JPS6088963A; GB8426500D0; GB2148523B

Abstract

A magnetizable encapsulated toner for use in electrostatography comprising a shell, magnetizable particles and a core material comprising a colorant and a binder composition, is characterized in that at least 80% by weight of said magnetizable particles are present in the core.

Description

SPECIFICATION Magnetizable encapsulated toner BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a magnetizable encapsulated toner employable for producing a visible image from a latent image formed in such a recording process as electrostatography.

Description of prior arts As processes for fixation of a toner image in a recording process such as electrostatography, there are known three fixation processes, namely, a heat fixing process (fusion process), a solvent fixing process and a pressure fixing process. The pressure fixing process involving the use of neither a heating procedure nor a solvent produces no such troubles as inherently attached to the heat fixing process and the solvent fixing process. Moreover, the pressure fixing process can be used in combination with a high speed automatic copying and duplicating process, and the access time is very short in the pressure fixing process. Accordingly, the pressure fixing process is thought to be an advantageous fixing process inherently having a variety of preferable features.The present invention provides an encapsulated toner suitably employable for the above-mentioned pressure fixing process.

The pressure fixing process however has a variety of inadvantageous features, in addition to the above-stated advantageous features. For instance, the pressure fixing process generally provides poorer fixablity than the heat fixing process does, whereby the toner image fixed onto a paper sheet is apt to rub off easily. Further, the pressure fixing process requires very high pressure for the fixation, and such a high pressure tends to break the fibers of the support medium such as paper sheet and also produces glossy surface on the support medium.

Furthermore, the pressing roller is required to have relatively greater size, because the roller necessarily imparts very high pressure to the toner image on the support medium. Thus, size reduction of a copying and duplicating machine cannot exceed a certain limit defined by the size of a pressing roller.

There has been previously proposed an encapsulated toner which comprises toner particles enclosed with microcapsules, so as to overcome the above-described disadvantageous features of the pressure fixing process. The encapsulated toner is prepared by enclosing a core material (containing a colorant such as carbon black and a binder such as a polymer or a high boiling point solvent) with a shell which is rupturable by the application of pressure in the developing stage. The so prepared encapsulated toner has various advantageous features; for instance, fixation of the encapsulated toner does not require very high pressure, but the fixability is high.

Accordingly, the encapsulated toner is viewed as suitable for the use in the pressure fixing process. However, the encapsulated toners proposed up to now appear unsatisfactory in practical use, because they fail to meet certain requirements for providing smooth copying and duplicating operation and satisfactory toner image fixability and quality.

More in detail, it is required for the toner to be used as a developing agent in the electrostatography to have excellent powder characteristics (powder flowability) to give high development quality, and to be free from staining the surface of a photosensitive material on which a latent image is formed. Further, a toner for the use as a developing agent in the pressure fixing process is required to be satisfactory in the fixability under pressure and not to undergo off-setting on the roller surface, that is, phenomenon that the toner adheres to the roller surface so as to stain it.

In summary, the toner employed in the pressure fixing process is preferably at a high level in all characteristics such as powder characteristics (powder flowablity), fixability onto a support medium (e.g., paper sheet), as well as the preservability of the fixed image, resistance to the offsetting, and electron chargeability or electroconductivity depending on the system adopted. The previously known encapsulated toners are unsatisfactory in certain characteristics.

Various reasons can be assumed on the face that the previously known encapsulated toners are not satisfactory in the practical use.

According to the study of the present inventor, one reason resides in that the core is poorly coated with the shell in the encapsulated toner. The encapsulated toner is generally prepared by forming a shell of polymer over small droplets of core material (hydrophobic material comprising a colorant such as carbon black and an adhesive binder) dispersed in an aqueous medium to produce microcapsules which are then separated and dried. It has been discovered by the inventor that the small droplet of core material tends to be not completely coated with the shell in the formation of microcapsule in the aqueous medium, producing a defective microcapsule in which a portion of the core material is exposed outwardly. It has been noted that the exposure of a portion of the core material is observed particularly on a solid component of the core material.Thus, the exposure of a portion of the core material is sometimes observed in the case that heavy particles such as magnetizable particles are included in the core material, namely, in a magnetizable encapsulated toner which is generally employed in a one component-developing system. Thus, it has been surprisingly discovered that most of magnetizable particles which are employed in an ordinary magnetizable encapsulated toner and are to be present in the core are present not in the core but in the shell, resulting in protrusion of a portion of the magnetizable particles over the outer surface of the shell. Therefore, the electric resistance of the toner surface is decreased, so that the magnetizable encapsulated toner is apt to show unsatisfactory developing and image-transfer characteristics.Moreover, the shell containing the magnetizable particles therein is poor in air tightness, so that a volatile binder contained in the core likely escapes through the shell as the time passes, resulting in decrease of adhesive power.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetizable encapsulated toner being satisfactory in the powder characteristics, developing characteristics, electric resistance, and preservability of a visible image fixed on a support medium.

It is another object of the invention to provide a magnetizable encapsulated toner which is improved particularly in air tightness of the shell, whereby improving the storage stability of the magnetizable encapsulated toner.

The present invention provides a magnetizable encapsulated toner comprising a shell, magnetizable particles and a core material comprising a colorant and a binder composition, which is characterized in that at least 80% by weight of said magnetizable particles are present in the core.

More in detail, according to the study of the present inventor, the transfer of the magnetizable particles into the shell and further the protrusion of the magnetizable particles over the outer surface of the shell are effectively obviated by the employment of a binder composition showing an interfacial tension of not higher than 5 dyne/cm in water. Thus obtained magnetizable encapsulated toner is characteristic in that most of the magnetizable particles are present in the core and only a negligible or a very small amount of the particles are present in the shell and on the outer surface of the shell. Accordingly, the powder characteristics and electric characteristics are remarkably improved as compared with the conventional magnetizable encapsulated toner.

Therefore, the magnetizable encapsulated toner of the present invention provides a sharp and well-fixed visible image. Moreover, the off-setting is more improved because the exposure of the adhesive binder over the outer surface of the shell is reduced. Furthermore, since most of the magnetizable particles are present in the core and the shell is almost free from the solid magnetizable particles, the air tightness of the shell is very high. For this reason, evaporation of the liquid component of the binder contained in the core (vanishment of the liquid component of the core material due to escape through the shell during storage of the encapsulated toner) is effectively prevented, whereby giving a magnetizable encapsulated toner showing high storage stability.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 illustrates a schematic view of the section of the toner particle of the present invention given by SEM (scanning electron microscope).

Figure 2 illustrates a shematic view of the section of the conventional toner particle by SEM.

DETAILED DESCRIPTION OF THE INVENTION As described hereinbefore, there is already known a magnetizable encapsulated toner which is prepared by enclosing a core material containing a colorant such as carbon black, a polymer, and a binder such as a high boiling-point solvent (oily medium) with a shell being rupturable by application of pressure thereto. However, as described hereinbefore, most of the magnetizable particles are present in the shell or on the outer surface of the shell in the conventional magnetizable encapsulated toner.

The magnetizable encapsulated toner of the present invention, that is, an encapsulated toner in which at least 80% by weight, preferably at least 95% by weight, of the magnetizable particles are present in the core, can be prepared, for instance, by forming a shell over a core material containing a binder composition which shows distinctly lower interfacial tension than the previously known binder does, in an aqueous medium, utilizing a known encapsulating method such as interfacial polymerization method or outer polymerization method, separating the prepared microcapsule from the medium, and drying it.

The binder composition preferably employed in the preparation of the magnetizable encapsulated toner of the invention should show an interfacial tension of not higher than 5 dyne/cm in water. There is no other specific limitation on the binder composition, as far as the composition has functions of the binder of an encapsulated toner, that is, a function of keeping colorant, magnetizable particles, etc. within a core and a function of rapidly and securely fixing the visible toner image onto a support medium such as paper sheet in the stage of transfer of the toner image formed on the latent image.

The binder composition preferably employable in the above-mentioned process comprisies a polymer, an oily medium and an interfacial tension-controlling agent.

The core material contains a binder (adhesive material) for keeping the colorant within the core and assisting the fixation of the colorant onto the surface of a support medium such as paper sheet.

Examples of the polymer employable as a binder component include the following polymers: polyolefin, olefin copolymer, styrene resin, styrene-butadiene copolymer, epoxy resin, polyester, natural and synthetic rubbers, poly(vinylpyrrolidone), polyamide, cumarone-indene copolymer, methyl vinyl ether-maleic anhydride copolymer, maleic acid-modified phenol resin, phenol-modified terpene resin, silicone resin, epoxy-modified phenol resin, amino resin, polyurethane elastomer, polyurea elastomer, homopolymer and copolymer of acrylic acid ester, homopolymer and copolymer of methacrylic acid ester, acrylic acid-long chain alkyl methacrylate copolymer oligomer, poly(vinyl acetate), and poly(vinyl chloride).

In the present invention, the polymer of the binder composition is preferably selected from the group consisting of homopolymers and copolymers of acrylic acid esters (acrylates), homopolymers and copolymers of methacrylic acid esters (methacrylates), and styrene-butadiene copolymers.

The oily medium employable as a binder component include a solvent having a boiling point of higher than 1 50 C, examples being as follows: (1) Phthalic acid esters dibutyl phthalate, dihexyl phthalate, diheptyl phthalate, dioctyl phthalate, dinonyl phthalate, dodecyl phthalate, butyl phthalyl butyl glycolate, dibutyl monofluorophthalate; (2) Aliphatic dicarboxylic acid esters dimethyl malonate, diethyl malonate, dimethyl oxalate, diethyl oxalate, diethyl maleate; (3) Phosphoric acid esters tricresyl phosphate, trixylenyl phosphate, tris(isopropylphenyl) phosphate, tributyl phosphate, trihexyl phosphate, trioctyl phosphate, trinonyl phosphate, tridecyl phosphate, trioleyl phosphate, tris(butoxyethyl) phosphate, tris(chloroethyl) phosphate, tris(dichloropropyl) phosphate;; (4) Citric acid esters O-acetyl triethyl citrate, O-acetyl tributyl citrate, O-acetyl trihexyl citrate, O-acetyl trioctyl citrate, O-acetyl trinonyl citrate, O-acetyl tridecyl citrate, triethyl citrate, tributyl citrate, trihexyl citrate, trioctyl citrate, trinonyl citrate, tridecyl citrate; (5) Benzoic acid esters butyl benzoate, hexyl benzoate, heptyl benzoate, octyl benzoate, nonyl benzoate, decyl benzoate, dodecyl benzoate, tridecyl benzoate, tetradecyl benzoate, hexadecyl benzoate, octadecyl benzoate, oleyl benzoate, pentyl o-methylbenzoate, decyl p-methylbenzoate, octyl o-chlorobenzoate, lauryl p-chlorobenzoate, propyl 2,4-dichlorobenzoate, octyl 2,4-dichlorobenzoate, stearyl 2,4-dichlorobenzoate, oleyl 2, 4-dichlorobenzoate, octyl p-methxybenzoate;; (6) Aliphatic acid esters hexadecyl myristate, dibutoxyethyl succinate, dioctyl adipate, dioctyl azelate, decamethylene 1,10-diol diacetate, triacetin, tributin, benzyl caprate, pentaerythritol tetracaproate, isosorbitol dicaprilate; (7) Alkylnaphthalenes methylnaphthalene, dimethylnaphthalene, trimethyinaphthalene, tetramethylnaphtharene, ethylnaphthalene, diethylnaphthalene, triethylnaphthalene, monoisopropyinaphthalene, diisopropylnaphthalene, tetraisopropylnaphthalene, monomethylethylnaphthalene, isooctylnaphthalene; (8) Dialkylphenyl ethers di-o-methylphenyl ether, di-m-methyldiphenyl ether, di-p-methylphenyl ether; (9) Amides of fatty acid and aromatic sulfonic acid N, N-dimethyllauroamide, N,N-diethylcaprylamide, N-butylbenzenesulfonamide; (10) Trimellitic acid esters trioctyl trimellitate;; (11) Diarylalkanes diarylmethanes such as dimethylphenylphenylmethane, diarylethanes such as 1-methylphenyl1 -phenylethane, 1 -dimethylphenyl-1 -phenylethane and 1 -ethylphenyl-1 -phenylethane.

For the purpose of the present invention, the high boiling-point solvent is preferably selected from phthalic acid esters, phosphoric acid esters, alkylnaphthalenes and diarylalkanes.

In the invention, the oily medium preferably comprise a combination of such high boilingpoint solvent as mentioned above and an organic liquid having a boiling point of not lower than 1 20 C which is substantially incapable of dissolving the above-mentioned polymer or causing the polymer to swell (hereinafter referred to simply as organic liquid).

The organic liquid may be a paraffinic hydrocarbon or a naphthenic hydrocarbon.

The aliphatic saturated hydrocarbon of paraffinic or naphthenic type is generally obtained as a mixture of various hydrocarbon distillates showing boiling points in a certain range. Examples of the organic liquid which can be incorporated in a binder include paraffinic hydrocarbon mixtures having boiling point ranges of 158-177"C (e.g., ISOPAR G, tradename of Exxon Chemicals), of 174-189"C (e.g., ISOPAR H), of 188-210"C (e.g., ISOPAR L), and of 207-258tC (e.g., ISOPAR M) and naphthenic hydrocarbon mixtures having boiling point ranges of 162-210"C (e.g., SHELLSOL-D40, tradename of Shell Chemicals), of 185-220"C (e.g., SHELLSOL-D60) and of 195-251"C (e.g., SHELLSOL-D70).

The above-mentioned organic liquid preferably has a boiling point range of 140-220"C.

There is no limitation on a ratio (ratio by weight) of the organic liquid to the high boiling-point solvent, but the ratio of the former liquid/latter solvent preferably ranges from 9/1 to 1/9.

In the case that the polymer and the high boiling-point solvent are employed in combination.

the ratio of the polymer to the solvent preferably ranges 0.1 to 1 00.

In order to decrease the interfacial tension of the binder composition as defined in the invention, the binder preferably contains an interfacial tension-controlling agent. An example of the interfacial tension-controlling agent is a rosin-modified phenol resin. The interfacial tensioncontrolling agent is generally incorporated into the binder composition in an amount of 0.5 to 50% by weight, preferably 0.5 to 10% by weight, based on the total amount of the binder composition.

As the colorant of the electrostatographic toner, a black powder such as carbon black or grafted carbon black is generally employed. Also employed are chromatic pigments such as blue, yellow, and red pigments. Such known colorants can be used in the magnetizable encapsulated toner of the invention.

As the magnetizable particles to be incorporated into the core of the magnetizable encapsulated toner of the invention, there can be mentioned known magnetizable particles employable in the known magnetizable encapsulated toner. Examples of the magnetizable particles include magnetizable particles of metals, particles of metal alloys, and metal compounds, in which the metal can be cobalt, iron or nickel. If colored magnetizable particles such as black magnetite particles are employed as the magnetizable particles, they can serve as the colorants as well as the magnetizable particles.

If desired, the magnetizable encapsulated toner of the present invention can various additives such as a fluorine-containing resin which is effective in prevention of the off-setting can be also included.

There is known a process for the preparation of a microcapsule by forming a polymer shell over an oily droplet containing magnetizable particles, a colorant and a binder dispersed in an aqueous medium. The magnetizable encapsulated toner of the present invention can be prepared by the known method.

For instance, an interfacial polymerization method can be utilized for the preparation of a magnetizable encapsulated toner of the invention. Other examples of the method for the preparation of microcapsules include an internal polymerization method, phase-separation method, outer polymerization method, fusion-dispersion-cooling method, and coacervation method. These methods are not construed to be restrictive, and other methods can be employed for the preparation of the magnetizable encapsulated toner of the invention.

There is no specific limitation on the polymer material employed to form the shell. Examples of the polymer material include polyurethane resin, polyamide resin, polyester resin polysulfonamide resin, polyurea resin, epoxy resin, polysulfanate resin and polycarbonate resin. The shell of the polymer material can be easily prepared by the known interfacial polymerization method.

Among the above-mentioned polymer materials, the polyurea resin and polyurethane resin are particularly preferred. These resins can be produced in the form of a shell of microcapsule by reacting a polyisocyanate such as diisocyanate, triisocyanate, tetracyanate or polyisocyanate prepolymer, and a polyamine diamine such as diamine, triamine or tetraamine, a prepolymer having two amino groups, piperazine, its derivative, or polyol in an aqueous medium by interfacial polymerization method.

As the materials of the outer shell, other various polymers can be employed. Examples of the polymers employable for the preparation of the shell include melamine resin, urea resin, homopolymers and copolymers of styrene and its derivatives (e.g., polystyrene, polyparaclorostyrene, styrene-butadiene copolymer, styrene-acrylic acid copolymer, styrene-acrylic ester copolymer, styrene-methacrylic acid copolymer, styrene-methacrylic ester copolymer, styrenemaleic anhydride copolymer, styrene-vinyl acetate copolymer, etc.), polyvinyltoluene resin, acrylic ester homopolymer, methacrylic acid homopolymer, xylene resin, methylvinyl ethermaleic anhydride resin, vinylbutyral resin, polyvinylalcohol resin, and polyvinylpyrrolidone.

The shell of the encapsulated toner is preferably composed substantially of a complex layer formed of a variety of polymer materials.

The complex layer preferably employed in the invention comprises two or more polymers selected from the group consisting of a polyurethane resin, a polyurea resin and a polyamide resin. For instance, the complex layer can comprise a combination of a polyurea resin and a polyamide resin, a combination of a polyurethane resin and a polyamide resin, or a combination of a polyurea resin, a polyurethane resin and a polyamide resin.

The complex layer comprising a polyurethane resin and a polyamide resin can be prepared, for instance, using polyisocyanate, acid chloride, polyamine and polyol, by the interfacial polymerization method which comprises adjusting pH of the emulsified reaction liquid, and heating the reaction liquid. The complex layer comprising a polyurea resin and a polyamide resin can be prepared, for instance, employing polyisocyanate, acid chloride and polyamine, by an interfacial polymerization method which comprises adjusting pH of the emulsified reaction liquid, and heating the reaction liquid.Details of the process for the preparation of the complex layer such as the complex layer comprising a polyurea resin and a polyamide resin, or the complex layer comprising a polyurethane resin and a polyamide resin are shown in GB 2,107,480A. The shell composed of a complex layer is preferably employed in a magnetizable encapsulated toner containing the magnetizable particles in the core.

The shell of a melamine resin or a urea resin can be conveniently prepared by outer polymerization method. A process for the preparation of microcapsule having a shell of a melamine resin utilizing the outer polymerization method is disclosed in Japanese Patent Provisional Publications No. 55(1980)-15660, No. 55(1980)-47139 and No.

56(1981)-51238, and U.S. Patents No. 4,100,103 and No. 4,233,178 The preparation of microcapsules which comprises forming a urea resin around a core material containing a colorant and a binder in an aqueous medium is disclosed in Japanese Patent Provisional Publication No. 55(1980)-119438 and U.S. Patent No. 4,221,710.

The outer polymerization method employable for the formation of the shell of the magnetizable encapsulated toner of the invention can be carried out by the steps: emulsifying an oily liquid containing the core materials in an aqueous medium containing an anionic protective colloid; adding a combination of melamine and formaldehyde or a melamine-formaldehyde precondensate to the resulting emulsion; and adjusting pH value and temperature of the resulting mixture to cause outer polymerization around the oily core material, to form a capsule wail.

The microcapsules prepared by forming a shell around a core material are separated from the aqueous reaction medium, for instance, by spray drying to obtain a dry encapsulated toner.

The dried encapsulated toner particles are preferably heated to further improve their powder characteristics. The temperature for heating the dried encapsulated toner particles preferably ranges from 50 to 300"C, and more preferably ranges from 80 to 150"C. The period required for performing the heating varies with the heating temperature, the nature of the binder, etc.

Generally, the period ranges from 10 minutes to 48 hours, and preferably ranges from 2 to 24 hours.

There is no limitation on the means employed for performing the heating procedure. Examples of the heating means include an electric furnace, a muffle furnace, a hot plate, an electric drying oven, a fluid bed drying apparatus, and an infrared drying apparatus.

The resinous shell of the magnetizable encapsulated toner of the invention can be provided with a charge control agent such as a metal-containing dye or nigrosine, a flow improving agent such as hydrophobic silica, or other additive. These additive can be added to the shell of the encapsulated toner in an optional stage such as in the course of formation of the shell or after separating and drying the encapsulated toner.

The present invention will be illustrated by the following examples which are by no means intended to introduce any restriction into the invention.

Example 1 A binder composition was prepared by mixing 5 g. of rosin-modified phenol resin (trade name: Tamanol-350, available from Arakawa Chemical Industry Co., Ltd., Japan) and 25 g. of 1-isopropyl-phenyl-2-phenylethane solution containing 40 wt.% of polyisobutyl methacrylate (trade name: Acrybase MM-2002-2, available from Fujikura Chemicals Co., Ltd. Japan). The interfacial tension (y) of the resulting binder composition in water (containing 5% of methylcellulose) was measured by Wilhelmy Plate Method. The measured interfacial tension was 5 dyne/cm or less.

25 g. of 1-isopropyl-phenyl-2-phenylethane solution containing 40 wt.% of polyisobutyl methacrylate, 50 g. of magnetite particles (trade name: EPT-1000, available from Toda Industry Co., Ltd., Japan), and 1 g. of carbon black were mixed in an automortar to prepare a dispersion (magnetic ink).

Independently, 2.2 g. of terephthalic chloride, 9.9 g. of adduct of toluylene diisocyanate and trimethylolpropane (trade name: Vernock D-750, available from Dainippon Ink and Chemicals Inc., Japan), and 5 g. of the above-mentioned rosin-modified phenol were dissolved in 30 g. of trichloroethylene. The resulting solution was mixed with the above-obtained dispersion.

The resulting mixture was added in 200 g. of water containing 5% methylcellulose (trade name: Metholose-65SH, available from Shinetsu Chemical Industry Co., Ltd., Japan) to prepare an emulsion. The emulsion was controlled to contain oily drops of a mean diameter of approx.

13 ym, and after 10 min. 50 g. of aqueous 2.5% diethylenetriamine solution was added to the resulting emulsion. Then, aqueous 5% sodium carbonate solution was added to the emulsion to adjust the pH to 11.0. The emulsion was placed in a thermostat kept at 55'C for approx. 3 hours to complete the encapsulation reaction.

Thus obtained aqueous microcapsule dispersion was subjected to centrifugal separation to precipitate the microcapsules, and the supernatant was replaced with distilled water. Then, the microcapsules were dispersed in the distilled water, and the resulting suspension was subjected to the same washing procedure involving centrifugal separation and replacement of the supernatant with distilled water as above. The washing procedure was repeated 20 times. The microcapsules were then washed with distilled water and heated in an oven at 100cm for 5 hrs.

to give a dry powdery magnetizable encapsulated toner.

Thus prepared powdery toner flowed very easily. Microscopic observation on the toner indicated that the toner particles were present independently and that no bulky agglomerated particles were formed. Further, the microscopic observation on the surface of the toner particles indicated that the magnetizable particles neither protruded nor were exposed over the surface of the toner. A shematic view of the section of the toner particle given by SEM (scanning electron microscope) was illustrated by Fig. 1 in the attached drawing. Thus, approx. 100% of the magnetite was present inside of the core.

The magnetizable encapsulated toner was evaluated on the fixability under pressure in the following manner.

An electrostatic latent image formed in the conventional electrostatography was developed with the toner to give a toner image, which was then transferred onto a paper sheet.

The paper sheet carrying the visible toner image was treated under a pressing roller at a pressure of 350 kg/cm2. There was obtained a toner image with high sharpness and well fixed onto the paper sheet. Further, off-setting of the toner was at a very low level. The toner particles forming the visible image on the paper sheet was not removed by rubbing with a finger.

Example 2 A binder composition was prepared by mixing 30 g. of 1-isopropyl-phenyl-2-phenylethane solution containing 20 wt.% of polyisobutyl methacrylate (trade name: Acrybase MM-2002-2).

10 wt.% of polyisobutyl methacrylate (trade name: Acrybase MM-2002-1) and 20 wt.% of rosin-modified phenol resin (trade name: Tamanol-350) with 25 g. of isoparaffin (trade name: Isopar-H, available from Exxon Chemical Co. Ltd., Japan). The interfacial tension (y) of the resulting binder composition in water (containing 5% of sodium polystyrenesulfonate) was measured by Wilhelmy Plate Method. The measured interfacial tension was 5 dyne/cm or less.

The above-obtained binder composition, 50 g. of magnetite particles (trade name: EPT-1000), and 1 g. of carbon black were mixed for 5 min. in an automortar to prepare a dispersion (magnetic ink). The resulting dispersion was poured into 100 g. of water containing 5% sodium polystyrenesulfonate (trade name: Versa-TL-500, M.W.: approx. 500,000, available from National Starch) in a mixer to prepare an emulsion.

Independently, 6 g. of melamine was placed in a beaker, and 10 g. of aqueous 35% formaline and 64 g. of water were added to the melamine. The resulting mixture was heated at 60"C for 30 min. to prepare a melamine-formaline precondensate. The aqueous melamineformaline precondensate solution and the above-obtained magnetic ink were mixed and then adjusted to pH 6.0 with 0.1 N hydrochloric acid. The resulting mixture was stirred at 60"C for 2 hrs., and finally was adjusted to pH 10 with 0.1 N aqueous sodium hydroxide to complete the encapsulation reaction.

The microcapsule dispersion obtained as above was washed with distilled water in the same manner as in Example 1 to give a magnetizable encapsulated toner as a dry powder.

Thus prepared powdery toner flowed very easily. Microscopic observation on the toner indicated that the toner particles were present independently and that no bulky agglomerated particles were formed. Further, the microscopic observation on the surface of the toner particles indicated that the magnetizable particles neither protruded nor were exposed over the surface of the toner. A shematic view of the section of the toner particle given by SEM was illustrated by Fig. 1. Thus, approx. 100% of the magnetite was present inside of the core.

The magnetizable encapsulated toner was evaluated on the fixability under pressure in the same manner as in Example 1.

There was obtained a toner image with high sharpness and well fixed onto the paper sheet.

Further, off-setting of the toner was at a very low level. The toner particles forming the visible image on the paper sheet was not removed by rubbing with a finger.

Comparison Example 1 A binder composition was prepared by mixing 30 g. of 1-isopropyl-phenyl-2-phenylethane solution containing 24 wt.% of polyisobutyl methacrylate (trade name: Acrybase MM-2002-2) and 16 wt.% of polyisobutyl methacrylate (trade name: Acrybase MM-2002-1) with 25 g. of isoparaffin (trade name: Isopar-H). The interfacial tension (y) of the resulting binder composition in water (containing 5% of sodium polystyrenesulfonate) was measured by Wilhelmy Plate Method. The measured interfacial tension was 1 5 dyne/cm.

A dry powdery magnetizable encapsulated toner was prepared in the same manner as in Example 2 using the above binder compostion which showed high interfacial tension and contained no rosin-modified phenol resin.

The microscopic observation on the surface of the toner particles indicated that the magnetizable particles protruded or were exposed over the surface of the toner. Further, it was observed that the magnetizable particles and carbon black were concentrated in the shell. A shematic view of the section of the toner particle given by SEM was illustrated by Fig. 2. Thus, more than approx. 95% of the magnetite was present inside of the shell or over the outer surface of the shell.

Evaluation of Magnetizable Encapsulated Toner The tightness (air tightness) of the shell of the magnetizable encapsulated toners prepared in Examples 1 and 2 and Comparison Example 1 was examined by measuring evaporation of a liquid component in the core through the shell. This measurement was performed by taking 1 g.

of the powdery magnetizable encapsulated toner prepared in each example just after the preparation into an oven kept at 100"C for 1 6 hrs., and weighing thus heated toner, The results are set forth in Table 1.

Table 1 Before Heating After Heating Example 1 100% 90% Example 2 100% 85% Com. Example 1 100% 70% The results shown in Table 1 teach that the shells of the magnetizable encapsulated toners prepared using a binder showing an interfactial tension of not higher than 5 dyne/cm (Examples 1 and 2) have higher air tightness than that the shell of the magnetizable encapsulated toner prepared using a binder showing high interfacial tension (Comparison Example 1).

Claims

1. A magnetizable encapsulated toner comprising a shell, magnetizable particles and a core material comprising a colorant and a binder composition, which is characterized in that at least 80% by weight of said magnetizable particles are present in the core.

2. The magnetizable encapsulated toner as claimed in claim 1, in which at least 95% by weight of said magnetizable particles are present in the core.

3. The magnetizable encapsulated toner as claimed in claim 1, in which said binder composition shows an interfacial tension of not higher than 5 dyne/cm.

4. The magnetizable encapsulated toner as claimed in claim 1, in which said binder composition comprises a polymer, an oily medium and an interfacial tension-controlling agent.

5. The magnetizable encapsulated toner as claimed in claim 3, in which said interfacial tension-controlling agent is a rosin-modified phenol resin.

6. The magnetizable encapsulated toner as claimed in claim 5, in which said rosin-modified phenol resin is contained in said binder composition in an amount ranging from 0.5 to 50% by weight.

7. The magnetizable encapsulated toner as claimed in claim 1, in which said shell comprises a melamine resin or a urea resin.

8. The magnetizable encapsulated toner as claimed in claim 1, in which said shell comprises a polyurea resin or a polyurethane resin.

9. The magnetizable encapsulated toner as claimed in claim 8, in which said shell comprises is composed of a complex layer comprising a polyurea resin or a polyurethane resin and a polyamide resin.