WO2008072919A1

WO2008072919A1 - Coating method of toner for realizing high resolution

Info

Publication number: WO2008072919A1
Application number: PCT/KR2007/006522
Authority: WO
Inventors: Woo Cheul Jung; Ji Hoon Lee; Hui Je Lee; Yu Na Kim; Chang Soon Lee; Jung Woo Kim
Original assignee: Lg Chem, Ltd.
Priority date: 2006-12-15
Filing date: 2007-12-14
Publication date: 2008-06-19
Also published as: KR20080055535A

Abstract

There is provided a coating method of a toner capable of controlling a coating condition to effectively introduce an external additive into a surface of a polymerized toner that has excellent performances when compared to toners prepared by the melt-mixing method. The coating method includes: preparing toner core particles and an external additive; and coating surfaces of the toner core particles with the external additive at a temperature between a temperature (4O0C) greater than room temperature and a glassification temperature (Tg). The coating method may be useful to prepare toner particles that may maintain a high transfer property and a high-resolution image after its long-term preservation since the toner core particles are easily coated with external additives without applying an excessive shearing force.

Description

COATING METHOD OF TONER FOR REALIZING HIGH

RESOLUTION

Technical Field

[I] The present invention relates to a coating method of a toner for realizing high resolution, and more particularly, to a coating method of a toner capable of controlling a coating condition to effectively introduce an external additive into a surface of a polymerized toner that has excellent performances when compared to toners prepared by the melt-mixing method.

[2]

Background Art

[3] Recently, there have been increasing demands for a copying machine and a laser printer along with the wide distribution of Personal Computers (PCs) and the office automation. Both the copying machine and the laser printer are image forming apparatuses that display a desired image by transferring a toner on a printing paper, and thus essentially uses toner to form an image.

[4]

[5] Generally, toner is a developer material that is used for development of electronic photographs, and used for printers or copying machines to deveolp an image on an image receptor in a transfer operation. Printing or copying processes of using toner in the copying machines or the laser printers are described as follows.

[6]

[7] 1. First, a charging step of uniformly charging a surface of a drum is performed. An

Organic Photo Conductor (OPC) drum and the like are generally used as the drum. The charging is conducted by electrostatically charging the drum surface using a charging rayon brush and the like.

[8]

[9] 2. Then, an exposure step of forming an electrostatic latent image by exposing the drum surface is followed. A charged body such as an organic photo conductor (OPC) on the unifromely charged drum surface functions as an insulator when light is not incident on the drum surface, but functions as a conductor for conducting charges in the presence of light. Thus, when the drum surface is exposed to the light such as laser beams, only the light-exposed portion is discharged or neutralized.

[10]

[I I] 3. Apart from the exposure step, a step of attaching a toner to a surface of a developer roller is carried out. This is a preliminary step, followed by a step of developing a toner image on the charged drum.

[12]

[13] 4. Subsequently, performed is a development step of developing the latent image on the surface of the drum with the toner attracted to a surface of the previously prepared developer roller, thereby forming an image. As described above, when the drum surface is exposed to light, the exposed portion thereof is discharged or neutralized. This is why, when the toner is charged with the same polarity as that of the drum surface, the no-exposed portion of the drum surface will repel toner to prevent toner from being transferred onto the latent image. However, the toner may adhere to the latent image in a desired image shape since the exposed portion of the drum surface does not repel toner.

[14]

[15] 5. After the development step, a step of transferring the toner image from the drum surface to an image-receiving paper (i.e., a printing paper) is performed. In the transfer step, a surface of the image -receiving paper is charged with a polarity opposite to that of the toner to generate an attraction force between the toner and the image-receiving paper, and the drum and the image -receiving paper are placed adjacent to each other in order to facilitate the transferring operation.

[16]

[17] 6. Since the toner is not permanently bonded to the image -receiving paper even though it is transferred to the image-receiving paper, a fusion step of fusing the toner to the image-receiving paper is followed. The fusion step is generally carried out by pressing the toner with heat and pressure while allowing the image-receiving paper, on which the toner image is formed, to pass between a pair of rollers including a heat roller and a pressure roller, and forming a coating layer around the toner using a binder in the toner.

[18]

[19] 7. Finally, prior to the recharging of the drum, a step of cleaning residual toner from the surface of the drum is carried out to charge the drum again for the next process cycle.

[20]

[21] As seen from the printing or copying process, one important principle used in the printing or copying using a toner is to use charged toner. That is, toner is charged by a developer roller and a doctor blade to allow its surface to gain electric charges, and the charged toner is developed onto a photoconductive drum in a pattern to be printed according to the charge state in a surface of the photoconductive drum, and transferred from the photoconductive drum to an image-receiving paper. Accordingly, poor charging characteristics of the toner makes it difficult to make easy progress of the de- velopment or transfer operation, and therefore it is difficult to realize a desired image having a high resolution.

[22]

[23] Accordingly, it is important in the fields of producing toner to provide toner having excellent charging characteristics.

[24] The charging characteristics of toner are realized by a charge control agent

(abbreviated "CCA") in the toner. The charging characteristics of toner become more excellent as the toner is in easy friction with a doctor blade.

[25]

[26] The methods of producing toner are mainly divided into two categories. One of them is to melting ad mixing (kneading) or extruding toner materials and mechanically pulverizing the resulting mixture to produce a toner having a fine particle size, which is called a melt-mixing method, and the other method is to finely dispersing toner materials in a dispersion medium such as water to prepare a suspension and polymerizing the suspended colloidal particles to produce toner particles, which is called a polymerization method.

[27]

[28] The melt-mixing process has been widely known up to now, and toner core particles prepared by the melt- mixing process have very irregular shapes such as acute edges, as well as irregular size and morphology. When the toner core particles have such irregular shapes, different pressures from a doctor blade are applied to the toner core particles, which leads to the different frictional forces generated in the toner core particles. Therefore, the toner particles prepared by the melt-mixing method do not have good charging characteristics or flowability.

[29] The polymerization method is developed to solve the above problems regarding the melt-mixing process, and has advantages that it is possible to produce particles that are more regular and spherical than the toner core particles prepared by the melt-mixing method.

[30]

[31] Because the toner core particles prepared by the above-mentioned melt- mixing or polymerization process do not show their own performances completely, an additional process of coating surfaces of the toner core particles with an external additive is followed. The external additive is a coating agent that is added to maintain charging characteristics for a long time after the toner shows the charging characteristics through the friction with a doctor blade, to control flowability and the like so that the toner can have a suitable range of frictional force when it is in the fraction with a doctor blade, and to focus distribution of particle charge amount into a narrow range. The external additive is mainly composed of fine organic particles, inorganic particles, silica, metal complexes or mixtures thereof.

[32]

[33] In general, the coating with the external additive is carried out by introducing an external additive into surfaces of the toner core particles, but it is not easy to completely introduce the external additive into the surfaces of the toner core particles using the conventional methods known in the art.

[34]

[35] Such problems are more serious in the toner core particles prepared by the polymerization method. This is why it is difficult to coat surfaces of the toner core particles with an external additive since a toner surface with spherical shape, e.g., the toner core particles prepared by the polymerization method, is more stable than that of the toner having irregular shape. That is, when the toner has a spherical shape, the external additive does not strongly bind to the toner surface, and thus is not properly fixed in the toner surface.

[36]

[37] When the external additive is not properly fixed in the toner surface, a developer roller is stained with the external additive during the printing process, or the external additive is peeled off from an image and observed as white spot when seen with the naked eye. Also, since the external additive is detached from the surface of the toner in the long-term preservation of the toner, a printed image may be not identical to an initial image, and charging characteristics may be deteriorated. Therefore, some methods known in the art have been used to facilitate the binding of an external additive to a surface of toner and prevent the detachment of the external additive from the toner surface, the methods comprising a step of introducing an external additive into a toner surface more easily under the same shearing force by lowering a glassi- fication temperature (Tg) of a binder resin constituting toner in the process of producing toner, or a step of introducing an external additive into a toner surface in a more simple manner by applying a high shearing force of a coating machine to the toner surface.

[38]

[39] However, the former methods have a problem that the toner may be easily melted since the glassification temperature of the binder resin is very low. Also, the later method has disadvantages that a large amount of energy is consumed, and the cost for apparatuses is also too high with the increased coating scale.

[40]

[41] Accordingly, there is a demand for developing a coating technique of toner capable of exhibiting high transferring properties and maintaining an image with high resolution after the long-term preservation of toner by effectively coating toner with an external additive under the same shearing force using conventional apparatuses, without causing problems regarding the melting of toner core particles. [42]

Disclosure of Invention

Technical Problem

[43] An object of the present invention is to provide a coating method of toner capable of exhibiting high transferring properties and maintaining an image with high resolution after the long-term preservation of toner by effectively coating toner with an external additive under the same shearing force using conventional apparatuses, without causing problems regarding the melting of toner core particles.

[44]

Technical Solution

[45] According to an aspect of the present invention, there is provided a coating method of a toner including: preparing toner core particles and an external additive; and coating surfaces of the toner core particles with the external additive at a temperature between a temperature (4O⁰C) greater than room temperature and a glassification temperature (Tg).

[46]

[47] In this case, the toner core particles may be prepared according to a process for preparing toner core particles including: adding a dispersant and optionally an anionic surfactant to an aqueous solution; preparing a monomer complex; adding the monomer complex to the aqueous solution; and pulverizing the monomer complex into fine colloidal particles by applying a shearing force to the monomer complex in the aqueous solution and polymerizing the pulverized fine colloidal particles.

[48] In order to produce the toner core particles, the dispersant may be added in a content of 0.1 to 10 % by weight, based on the total weight of the aqueous solution.

[49]

[50] In this case the dispersant may be at least one selected from the group consisting of inorganic dispersants such as calcium phosphate salts, magnesium salts, hydrophilic silica, hydrophobic silica and colloidal silica; or non-ionic polymeric dispersants such as polyoxyethylene alkylether, polyoxyalkylene alkylphenolether, sorbitan fatty acid ester, polyoxyalkylene fatty acid ester, glycerine fatty acid ester, polyvinyl alcohol, alkyl cellulose and polyvinyl pyrrolidone; and ionic polymeric dispersants such as polyacrylamide, polyvinyl amine, polyvinyl amine N-oxide, polyvinyl ammonium salt, polydialkyldiallyl ammonium salt, polyacrylic acid, polystyrene sulfonic acid, poly- acrylate, polystyrene sulfate and polyaminoalkyl acrylate.

[51] The selectively added surfactant may be present in a content of 0 to 20 % by weight, based on the total weight of the aqueous solution.

[52] The surfactant may be at least one selected from the group consisting of fatty acid salts, alkyl sulfuric ester salts, alkylaryl sulfuric ester salts, dialkyl sulfosuccinate and alkyl phosphate.

[53]

[54] Also, the monomer complex may include 30 to 90 % by weight of an aromatic vinyl monomer, 5 to 65 % by weight of at least one selected from the group consisting of acrylate monomer, methacrylate monomer and dien monomer, 0 to 30 % by weight of an acidic or basic olefin monomer, 1 to 20 % by weight of a color pigment or carbon black, 0.001 to 10 % by weight of a polar grafting agent, 0.01 to 20 % by weight of a charge control agent, 0 to 5 % by weight of a crosslinking agent and 0.01 to 5 % by weight of a polymerization initiator, and the balance of a chain transfer agent and wax.

[55] Furthermore, the surfaces of the toner core particles may be desirably crosslinked with each other.

[56]

Brief Description of the Drawings

[57] FIG. 1 is a perspective view schematically illustrating a conventional Hensel coating machine used in the coating with an external additive.

[58] FIG. 2 is a perspective view schematically illustrating a Hensel coating machine provided with an apparatus for controlling a temperature of toner core particles in coating with an external additive.

[59] FIG. 3 is a scanning electron microscopic diagram illustrating surfaces of the toner core particles prepared by adding silica to the surface of the toner core particles under the conditions as described in Examples 1 and 2 and Comparative examples 1 and 2, respectively.

[60]

Best Mode for Carrying Out the Invention

[61] Hereinafter, the present invention will be described in detail.

[62] The present inventors have attempted to meet the above objects of the present invention, and found that it is necessary to increase a coating temperature so as to attain the same coating efficiency without excessively increasing a shearing force of a coating machine, and therefore the present invention was completed on the basis of the above facts.

[63]

[64] That is, the present invention is characterized in that toner core particles are coated with an external additive while maintaining a coating temperature to a temperature (preferably, 4O⁰C or above) that is lower than a glassification temperature (Tg) but higher than a room temperature and applying a shearing force to the toner core particles and the external additive in the coating with an external additive. The glassi- fication temperature may be easily measured using an analysis system such as DSC, DMA, TMA, etc.

[65]

[66] The present invention is designed based on the fact that, when the toner core particles are heated to a temperature where a binder resin constituting the toner core particles is not melted completely so as to give chain mobility to the binder resin, the external additive is easily introduced into the toner core particles to give excellent coating property without causing problems that the toner core particles are fused and condensed with other particles. When the coating temperature of the external additive is higher than the glassification temperature, physical properties of the toner core particles may be deteriorated, which leads later to the adverse effects on charging characteristics of the toner. On the contrary, when the coating temperature of the external additive is less than the glassification temperature (Tg), it is not easy to coat the toner core particles with the external additive since the chain mobility is not given to the binder resin constituting the toner core particles. Accordingly, the coating temperature is preferably set to a temperature range between a room temperature and a glassification temperature (Tg), preferably 4O⁰C or above.

[67]

[68] In this case, the coating method of toner core particles is preferably carried out using a HENSEL coating machine as shown in FIG. 1, and the Hensel coating machine 1 has a rotatable blade 2 installed inwardly in the bottom thereof, thereby giving a shearing force to the toner core particles and the external additive.

[69]

[70] However, the temperature of the toner core particles and the external additive should be necessarily maintained to the above-mentioned temperature range to give chain mobility to a binder resin, as described above. In this case, the temperature of the toner core particles and the external additive may be easily controlled by using various kinds of heat sources, particularly by supplying water of a constant temperature through a hose 3 from a constant temperature bath 4 as shown in FIG. 2, the hose 3 being installed in an outer wall of the coating machine. Also, the above-mentioned temperature-controlling method may be carried out in a manner where a coating machine is put into an oven, a constant-temperature bath, or the like and operated, and the coating temperature may be controlled in other manners.

[71]

[72] Also, all of external additives such as organic matters, inorganic matters, metal complexes or silica may be used in the present invention if the external additives are used in the field of the conventional production of toner as described above. As a result, the external additives that have been developed and proposed in the art may be used in the present invention, regardless of the kind of the external additives. However, examples of the external additive include 0.1 to 5 parts by weight of silica, 0.01 to 0.5 parts by weight of a TiO₂-based organic/inorganic complex, 0.01 to 0.5 parts by weight of a PMMA-based polymer, 0.01 to 0.5 parts by weight of an organic/inorganic complex including Mg and Zn, based on 100 parts by weight of the toner core particles.

[73]

[74] And if the temperature where the toner core particles are coated with the external additive is set to a temperature range required for the present invention, there is no particular limitation on the condition of the shearing force that is applied to coat the toner core particles with the external additive. That is, since the present invention is related to a technique of increasing a coating efficiency of an external additive under the same condition of shearing force, all the conventional conditions of shearing force may apply to the present invention. As one example of the conditions of shearing force, a shearing force may be applied to the toner core particles and the external additive at a rotational speed of 1000 to 7000 rpm.

[75]

[76] All of generally used toner core particles may be used herein, and it is particularly preferred to use toner core particles that meet the following conditions. However, it is carefully considered that the term 'toner core particle' used in the present invention means particles prior to addition of an external additive, and the term 'toner particles' means particles that include all of an external additive or other additives as well as the toner core particles.

[77]

[78] The process of producing toner core particles is carried out by adding a dispersant and optionally a surfactant to an aqueous solution to prepare an aqueous phase in a reactor, the dispersant functioning so that raw materials for toner core particles can be present as fine colloidal particles prior to a polymerization process; preparing a monomer complex in a respective operation; adding the monomer complex to the aqueous solution; pulverizing the monomer complex into fine colloidal particles by applying a suitable temperature (50-90⁰C) and shearing force to the monomer complex in the aqueous solution; and suspension-polymerizing the pulverized fine colloidal particles. Surfaces of the toner core particles prepared by the method according to the present invention are preferably crosslinked with each other. This crosslinking system is required for preventing the off-set phenomenon in a printing process and the degradation in storage stability, which are required for toners conventionally. [79]

[80] The dispersant includes inorganic dispersants such as calcium phosphate salt, magnesium salt, hydrophilic silica, hydrophobic silica and colloidal silica; or non-ionic polymeric dispersants such as poly oxy ethylene alkylether, poly oxy alky lene alkyl- phenolether, sorbitan fatty acid ester, polyoxyalkylene fatty acid ester, glycerine fatty acid ester, polyvinyl alcohol, alkyl cellulose and polyvinyl pyrrolidone; and ionic polymeric dispersant such as polyacrylamide, polyvinyl amine, polyvinyl amine N- oxide, polyvinyl ammonium salt, polydialkyldiallyl ammonium salt, polyacrylic acid, polystyrene sulfonic acid, polyacrylate, polystyrene sulfate and polyaminoalkyl acrylate, and they may be used alone or in combinations thereof. The dispersant is preferably added in a content of 0.1 to 10 % by weight, based on the total weight of the aqueous solution. When the content of the added dispersant is less than 0.1 % by weight, a dispersion effect is not achieved sufficiently. On the contrary, when the content of the added dispersant exceeds 10 % by weight, side reactions are increasingly likely to occur, and therefore it is difficult to control polymer size distribution (PSD) of toner particles, which makes it disadvantageous to form toner particles having a desired particle size.

[81]

[82] Also, the surfactant, which is added optionally, is preferably at least one selected from the group consisting of fatty acid salt, alkyl sulfuric ester salt, alkylaryl sulfuric ester salt, dialkyl sulfosuccinate and alkyl phosphate. The anionic surfactant is preferably present in a content of 0 to 20 % by weight, based on the total weight of the aqueous solution. When the content of the anionic surfactant exceeds 20 % by weight, side reactions are increasingly likely to occur, and therefore it is difficult to control polymer size distribution (PSD) of toner particles, which makes it disadvantageous to form toner particles having a desired particle size.

[83]

[84] Also, the monomer complex preferably includes 30 to 90 % by weight of an aromatic vinyl monomer; 5 to 65 % by weight of one or two or more selected from the group consisting of acrylate monomer, methacrylate monomer and dien monomer; 0 to 30 % by weight of an acidic or basic olefin monomer; 1 to 20 % by weight of a color pigment or carbon black; 0.001 to 10 % by weight of a polar grafting agent; 0.01 to 20 % by weight of a charge control agent; 0 to 5 % by weight of a crosslinking agent; 0.01 to 5 % by weight of a polymerization initiator; and the balance of a molecular weight modifier and wax. Also, the monomer complex prepared from the above-mentioned compositions is preferably added in a content of 1 to 60 parts by weight based on 100 parts by weight of the aqueous solution, and polymerized.

[85] The components and their contents that form the monomer complex are described in detail, as follows.

[86]

[87] Here, the aromatic vinyl monomer includes styrene, monochlorostyrene, methylstyrene, dimethylstyrene, and the like, and they may be used alone or in combinations thereof. The aromatic vinyl monomer is preferably used in a content of 30 to 90 % by weight, based on the total weight of the monomer mixture. A reason for limiting the content of the aromatic vinyl monomer to a content range of 30 to 90 % by weight is generally to adjust a glassification temperature (Tg) of a polymerized toner. In this case, when the content of the aromatic vinyl monomer is less than 30 % by weight, a glassification temperature (Tg) of toner is too low, and therefore the toner is attached to a fusing roller in a printing process, which leads to the hot-offset problem. On the contrary, when the content of the aromatic vinyl monomer exceeds 90 % by weight, a glassification temperature (Tg) of toner is too high, and therefore fusion ability of the toner may be deteriorated since the toner is insufficiently fused in the printing process.

[88]

[89] Also, the acrylate monomer includes methylacrylate, ethylacrylate, n-butylacrylate, isobutylacrylate, dodecyl acrylate, 2-ethylhexylacrylate, etc. Here, the methacrylate monomer includes methyl methacrylate, ethylmethacrylate, n-butyl methacrylate, isobutyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, etc., and the dien monomer includes butadien, isoprene, etc., and they may be used alone or in combinations thereof. The acrylate monomer is preferably used in a content of 5 to 65 % by weight, based on the total weight of the monomer mixture. Here, a reason for limiting the content of the acrylate monomer to a content range of 5 to 69 % by weight is to prevent problems that may be caused in the printing process by adjusting a glassification temperature of a toner to a suitable extent, as described above in the aromatic vinyl monomer.

[90]

[91] Meanwhile, the acidic or basic olefin monomer is added if necessary. Among them, the acid olefinic monomer includes α,β-ethylene compounds containing a carboxyl group, and the basic olefinic monomer includes methacrylic acid esters, methacryl amides, vinyl amines, and diaryl amines of aliphatic alcohol containing amine or quaternary ammonium group, and ammonium salts thereof. The acid olefinic monomer and the basic olefinic monomer may be used alone or in combinations thereof, and they are preferably present in a content of 0 to 30 % by weight, based on the total weight of the monomer mixture. The acid olefinic monomer and the basic olefinic monomer are added to improve charging characteristics of a toner surface. In this case, when the content of the olefinic monomer exceeds 30 % by weight, reaction stability in the polymerization of toner may be degraded, which leads to the agglomeration of toner particles.

[92]

[93] And, the coloring agent such as a color pigment or carbon black is added to give colors to the toner. Because the coloring agent does not highly affect glassification temperature of toner core particles, there is no particular limitation on the content of the coloring agent if the coloring agent may express desired colors, but the content of the coloring agent is generally used in a content of 0.1 to 20 % by weight. The pigment includes inorganic pigments such as metal powder type, metal oxide type, carbon type, sulfide type, chromate type and ferrocyanide type pigments; and organic pigments such as azo type, acid dye type, basic dye type, mordant dye type, phthalocyanine type, quinacridone type and dioxane type pigments, and they may be preferably used alone or in combinations thereof.

[94]

[95] The polar grafting agent is added to function as a crosslinking agent and easily form a shell in a core-shell formation at the same time. Here, the polar grafting agent is preferably present in a content of 0.001 to 10 % by weight. The polar grafting agent includes ethylene dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, 1,6 hexamethylene diacrylate, allyl methacrylate, 1,1,1-trimethylol propane triacrylate, triallylamine, etc., and they may be used alone or in combinations thereof. When the content of the polar grafting agent is less than 0.001 % by weight, a hard shell that is formed on a core is not formed. On the contrary, when the content of the polar grafting agent exceeds 10 % by weight, the core is highly gelated, which leads to the degraded fusion ability of toner.

[96]

[97] The charge control agent is added to give charging characteristics to surfaces of the toner core particles. The charge control agent includes a cationic charge control agent, for example nigrosine dye, highly aliphatic metal salt, alkoxy amine, chelate, quaternary ammonium salt, alkylamide, fluorinated surfactant, metal salt of naph- thalenic acid, etc.; an anionic charge control agent, for example chlorinated paraffin, polyester, sulfonylamine of copper phthalocyanine, styrene-acryl polymer containing sulfonic acid group, etc; or an inorganic anionic charge control agent, for example chromium-containing azo metal complex, salicylic acid metal complex, chromium- containing organic dye, etc., and they may be used alone or in combinations thereof. The charge control agent is preferably present in a content of 0.01 to 20 % by weight. When the content of the charge control agent is less than 0.01 % by weight, the toner does not have a sufficient charge density in a printing process, whereas, when the content of the charge control agent exceeds 20 % by weight, the toner has an excessive charge density, which leads to the degraded image quality in the printing process.

[98]

[99] The crosslinking agent is added to increase resistance to moisture during the storage of the toner by crosslinking surfaces of the toner core particles with each other, or prevent toner from being melted by the local frictional heat when toner particles are passed through doctor blades in a developing machine. The crosslinking agent used herein preferably includes divinylbenzene, aryl methacrylate, etc. The crosslinking agent is preferably present in a content of 0 to 5 % by weight. When the content of the crosslinking agent exceeds 5 % by weight, the toner is highly gelated, which leads to the degraded fusion ability of toner. Therefore, it is preferred to limit the content of the crosslinking agent to a content range of 5 % by weight or less. Since the crosslinking agent may be optionally added, the lowest limit of the crosslinking agent is set to 0 % by weight.

[100]

[101] An oil- soluble initiator and a water-soluble initiator may be used as the polymerization initiator. Representative examples of the polymerization initiator include an azo initiator such as bisisobutyronitrile, azobisdimethylvaleronitrile, etc.; organic peroxide such as benzoyl peroxide, lauroyl peroxide, etc.; a conventional water-soluble initiator such as potassium persulfate, ammonium persulfate, etc. The polymerization initiator is preferablyused in an amount of 0.01 to 5 % by weight, based on the total weight of the monomer mixture. In this case, when the amount of the polymerization initiator is less than 0.01 parts by weight, non-reacted compounds remain in the monomer mixture, whereas reaction stability of toner may be deteriorated due to the excessively rapid reaction rate when the amount of the polymerization initiator exceeds 5 % by weight.

[102]

[103] The molecular weight modifier is added to improve fusion ability of toner in a printing process by reducing molecular weight of toner core particles to adjust a glassi- fication temperature of the toner core particles, and it may not be added, if necessary. Because the molecular weight modifier is added when the toner has poor fusion ability, the content of the molecular weight modifier is determined according to the fusion ability of the toner. Therefore, there is no particular limitation on the content of the molecular weight modifier. However, the molecular weight modifier is generally added in a content of approximately 0 to 8 % by weight. When the molecular weight modifier is added in an excessive amount, a glassification temperature of the toner core particles is too low, which leads to the hot-offset problem.

[104]

[105] One or two or more selected from the group consisting of mercaptan compounds, for example t-dodecyl mercaptan and n-dodecyl mercaptan are preferably used as the molecular weight modifier.

[106] The wax is added to give gloss to printouts after the printing of toner and fix the toner at a low temperature by reducing a melting point of the toner. There is no particular limitation on the content of the wax, but the wax may be added in a suitable amount, if necessary. In general, the wax is present in a content of approximately 0.01 to 30 % by weight. The wax includes petroleum-refined wax such as paraffin wax, mi- crocrystalline wax and ceresin wax; natural wax such as carnuba wax; and synthetic wax such as polyethylene and polypropylene, and they may be used alone or in combination thereof.

[107]

[108] Toner core particles are manufactured from the monomer complex having the above- mentioned compositions according to the conventional suspension polymerization methods. In this case, the monomer complex may be prepared by mixing (kneading) all components simultaneously. In order to prevent polymerization of the components before the suspension polymerization, it is more preferred to mix (knead) all components except for the polymerization initiator and add the polymerization initiator right before the suspension polymerization.

[109]

[110] When the aqueous solution and the monomer complex are prepared, followed is an operation of pulverizing the monomer complex into fine particles by applying a shearing force to the monomer complex in the aqueous solution and polymerizing the pulverized fine particles while suspending the fine particles. In this case, the shearing force may be applied in a manner where an impeller is rotated at a suitable rotational speed in a reactor. A polymerization temperature may be varied according to the used polymerization initiators, but the polymerization temperature is generally maintained to a temperature of about 50 to 9O⁰C. Also, the rotational speed of the impeller, which is used to give a shearing force, is generally set to a range of 10 to 700 rpm. As a subsequent process, an operation of obtaining toner particles from the suspension needs to be followed.

[I l l]

[112] For the subsequent process, a dispersant is removed from the suspension including the toner particles having a core-shell structure as prepared in the previous operation, by using any of suitable methods, and the dispersant-free suspension is then repeatedly washed and filtered to separate toner particles, and the toner particles are dried at a room temperature in a vacuum oven to obtain final toner particles. The toner core particles are dried until their moisture content is less than 1%. Here, the drying time may be varied according to the crosslinking level or the materials to be crosslinked. [113] According to the above-mentioned method according to the present invention, it is possible to obtain desired toner core particles. Then, an operation of adding an external additive onto the toner core particles is followed, as mentioned previously.

[114]

[115] That is, the coating method of toner according the present invention is characterized in that the coating method includes: preparing toner core particles and an external additive and coating surfaces of the toner core particles with the external additive under certain temperature conditions by applying a shearing force to the toner core particles and the external additive.

[116] Hereinafter, exemplary embodiments of the present invention will be described in detail referring to the accompanying drawings. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

[117]

Mode for the Invention

[118] Example 1: External Addition of Silica onto Toner Surface Using Hensel Coating Machine at 40 ⁰C

[119] A monomer mixture comprising 70% styrene, 15% butyl acrylate, 4.5% carbon black, 10% wax and a 0.5% charge control agent was added to the previously prepared aqueous phase (including a dispersant), and the resulting mixture was suspension- polymerized at 6O⁰C for 10 hours while stirring at 100 rpm to prepare toner core particles. The toner core particles were washed thoroughly, and dried. Then, lOOg of the toner core particles and 2 g of silica as an external additive were added into a coating machine, and the coating machine was then heated at 4O⁰C for 20 minutes. Then, the toner core particles were coated with the external additive for 3 minutes at a rotary speed of 4000 rpm to prepare a toner. Tg of the polymerized toner was 62⁰C.

[120]

[121] Example 2: External Addition of Silica onto Toner Surface Using Hensel Coating Machine at 50 ⁰C

[122] Toner core particles were prepared in the same manner as in Example 1 according to the suspension polymerization method. The prepared toner core particles were washed thoroughly, and dried. Then, lOOg of the toner core particles and 2 g of silica as an external additive were added into a coating machine, and the coating machine was then heated at 5O⁰C for 20 minutes. Then, the toner core particles were coated with the external additive for 3 minutes at a rotary speed of 4000 rpm to prepare a toner. [123]

[124] Comparative example 1: External Addition of Silica onto Toner Surface Using Hensel Coating Machine at 65 ⁰C

[125] Toner core particles were prepared in the same manner as in Example 1 according to the suspension polymerization method. The prepared toner core particles were washed thoroughly, and dried. Then, lOOg of the toner core particles and 2 g of silica as an external additive were added into a coating machine, and the coating machine was then heated at 65⁰C for 20 minutes. Then, the toner core particles were coated with the external additive for 3 minutes at a rotary speed of 4000 rpm to prepare a toner. For this Comparative example 1, when the toner core particles were coated with the external additive, the heating temperature was set to a temperature that is higher than Tg (e.g., 62⁰C) of the binder resin constituting the toner core particles.

[126]

[127] Comparative example 2: External Addition of Silica onto Toner Surface Using Hensel Coating Machine at Room Temperature

[128] Toner core particles were prepared in the same manner as in Example 1 according to the suspension polymerization method. The prepared toner core particles were washed thoroughly, and dried. Then, lOOg of the toner core particles and 2 g of silica as an external additive were added into a coating machine, and the toner core particles were then coated with the external additive for 3 minutes at a rotary speed of 4000 rpm, without any of additional heating process, to prepare a toner.

[129]

[130] Test Results

[131] Test for Measurement of Toner Particle Shape

[132] Shapes of the toner particles prepared in Examples 1 and 2 and Comparative examples 1 and 2 were measured using a scanning electron microscope (SEM). The results are shown in FIG. 3.

[133] As shown in scanning electron microscopic diagrams of FIG. 3 (a), (b) and (c), it was revealed that, when the toners were coated under the conditions as described in Examples 1 and 2, and Comparative example 1, surfaces of the toners were highly compactly coated with silica, compared the toner (FIG. 3 (d)) of Comparative example 2. This is indicates that chain mobility of toner is increased with the increasing temperature of 4O⁰C, 5O⁰C and 65⁰C when the toner surface is coated under the conditions of the same rotary speed (rpm) and the same time, and therefore the surface of the toner is more compactly covered with silica.

[134]

[135] Measurement of Charge Amount and Image Density of Toner

[136] The toners prepared in Examples 1 and 2 and Comparative examples 1 and 2 were measured for charge amount in an HP4600 color laser printer using a suction method. The results are listed in the following Table 1.

[137] Also, image densities of the toners on the printed paper were measured after the printing. The results are also listed in the following Table 1 [138] Table 1 [Table 1] [Table ]

[139] [140] As listed in the Table 1, it was revealed that the toners that were coated at 4O⁰C and 5O⁰C as in Examples 1 and 2 have a relatively higher image density than the toners coated at 65⁰C and a room temperature. This indicates that the silica, which functions to facilitate the transfer of toner by alleviating an adhesive force of the toner core particles and increasing fluidity of the toner, effectively reduces an adhesive force between toners since a surface of the toner is highly compactly coated with silica at 4O⁰C and 5O⁰C. Here, the most conspicuous point is that, although the toner coated at 65⁰C (Comparative example 1) is highly compactly covered with silica when compared to the toner of Comparative example 2 as shown above in the scanning electron microscopic diagram, the image density and the charging characteristics (Q/m) (meaning a charge amount per mass of toner (μC/g)) of the toner coated at 65⁰C (Comparative example 1) are shown to be poor, compared to those of the toners coated at different temperatures (Examples 1 and X). This is why, although the toner is effectively coated with silica due to the increased chain mobility of the toner at 65⁰C, physical properties of the toner are changed as the coating temperature approaches the glass transition temperature of the toner. This is why the increased temperature functions as one of parameters that leads to other changes (e.g., self-assembly) in the toner particle surfaces. Owing to these reasons, it was observed that the toner coated at 65⁰C has a very hard surface after the coating, compared to the other toners. Accordingly, it was confirmed from this experiment that the upper limit of the ideal coating temperature should be below the glass transition temperature of the toner to exclude other factors that may affect the toner.

[141] [142] Measurement of Transfer Rate of Toner [143] In the printing test, one of very important factors is to measure a transfer rate of toner. The ideal toner should have a high transfer rate, and have no significant change in the transfer rate even after its long-term storage. When the same toners are used herein, the high transfer rate and the maintenance of the transfer rate after the long- term storage are affected according to the kind and coating level of the external additive. To determine efficiency of the external addition according to the above- mentioned method, changes in the transfer rate were compared after 500 and 3000 sheets were printed in a printer.

[144] Table 2 [Table 2] [Table ]

[145] [146] As listed in Table 2, it was revealed that the toners whose surfaces are compactly coated with silica (Examples 1 and 2) are excellent in the transfer rate and the maintenance of the transfer rate, compared to the toner whose surface is not compactly coated with silica (Comparative example X). However, it was seen that the transfer rate of the toner of Comparative example 1 is not excellent but even poor when compared to that of the toner of Comparative example 2, like the previous results of the image density and the charge amount. This indicates that the toner is denatured by the coating temperature, as described above. Accordingly, it was re-confirmed that the toner is advantageously coated with an external additive within a suitable temperature range to manufacture a toner with high transfer rate.

[147]

Industrial Applicability [148] As described above, the coating method according to the present invention may be useful to prepare toner particles that may maintain a high transfer property and a high- resolution image after its long-term preservation since the toner core particles are easily coated with external additives without applying an excessive shearing force.

Claims

[1] A coating method of a toner, comprising: preparing toner core particles and an external additive; and coating surfaces of the toner core particles with the external additive at a temperature between 4O⁰C and glassification temperature (Tg).

[2] The coating method of claim 1, wherein the toner core particles are prepared according to a process for preparing toner core particles, the process comprising: adding a dispersant and optionally an anionic surfactant to an aqueous solution; preparing a monomer complex; adding the monomer complex to the aqueous solution; and pulverizing the monomer complex into fine colloidal particles by applying a shearing force to the monomer complex in the aqueous solution and polymerizing the pulverized fine colloidal particles.

[3] The coating method of claim 2, wherein the dispersant is added in a content of

0.1 to 10 % by weight, based on the total weight of the aqueous solution.

[4] The coating method of claim 3, wherein the dispersant is at least one selected from the group consisting of inorganic dispersants such as calcium phosphate salts, magnesium salts, hydrophilic silica, hydrophobic silica and colloidal silica; or non-ionic polymeric dispersants such as polyoxyethylene alkylether, poly- oxyalkylene alkylphenolether, sorbitan fatty acid ester, polyoxyalkylene fatty acid ester, glycerine fatty acid ester, polyvinyl alcohol, alkyl cellulose and polyvinyl pyrrolidone; and ionic polymeric dispersants such as polyacrylamide, polyvinyl amine, polyvinyl amine N-oxide, polyvinyl ammonium salt, polydial- kyldiallyl ammonium salt, poly aery lie acid, polystyrene sulfonic acid, poly- acrylate, polystyrene sulfate, and polyaminoalkyl acrylate.

[5] The coating method of claim 2, wherein the surfactant is present in a content of 0 to 20 % by weight, based on the total weight of the aqueous solution.

[6] The coating method of claim 5, wherein the surfactant is at least one selected from the group consisting of fatty acid salts, alkyl sulfuric ester salts, alkylaryl sulfuric ester salts, dialkyl sulfosuccinate and alkyl phosphate.

[7] The coating method of claim 2, wherein the monomer complex comprises 30 to

90 % by weight of an aromatic vinyl monomer, 5 to 65 % by weight of one or two or more selected from the group consisting of acrylate monomer, methacrylate monomer and dien monomer, 0 to 30 % by weight of an acidic or basic olefin monomer, 1 to 20 % by weight of a color pigment or carbon black, 0.001 to 10 % by weight of a polar grafting agent, 0.01 to 20 % by weight of a charge control agent, 0 to 5 % by weight of a crosslinking agent and 0.01 to 5 % by weight of a polymerization initiator, and the balance of a molecular weight modifier and wax.

[8] The coating method of claim 1, wherein the surfaces of the toner core particles are crosslinked with each other.