The present invention relates to a method of producing a liquid developer and a liquid developer produced by the method.

As a developer used for developing an electrostatic latent image formed on a latent image carrier, there are known two types. One type of such a developer is known as a dry toner which is formed of a material containing a coloring agent such as a pigment or the like and a binder resin, and such a dry toner is used in a dry condition thereof. The other type of such a developer is known as a liquid developer which is obtained by dispersing toner particles into a carrier liquid having electric insulation properties.

In the developing method using such a dry toner, since a solid state toner is used, there is an advantage in handleability thereof. On the other hand, however, there is fear of an adverse effect on human body and the like caused by toner powder. Further, this method also involves problems such as contamination caused by dispersal of toner and insufficient uniformity of toner particles when dispersed in the insulation liquid. Further, in such a dry toner, aggregation of toner particles is likely to occur during the preservation thereof, and thus it is difficult to make the size of each toner particle sufficiently small. This means that it is difficult to form a toner image having high resolution. Furthermore, there is also a problem in that when the size of the toner particle is made to be relatively small, the problems resulted from the powder form of the dry toner described above becomes more serious.

On the other hand, in the developing method using the liquid developer, since aggregation of toner particles in the liquid developer during the preservation thereof is effectively prevented, it is possible to use very fine toner particles and it is also possible to use a binder resin having a low softening point (a low softening temperature). As a result, the method using the liquid developer has the advantages such as good reproductivity of an image composed of thin lines, good tone reproductivity as well as good reproductivity of colors. Further, the method using the liquid developer is also superior as a method for forming an image at high speed.

Conventionally, such a liquid developer is produced by a grinding method in which toner particles are produced by grinding a resin (see JP-A No. 07-234551 , for example), a polymerization method in which monomer components are polymerized in a solution having electric insulation to produce resin fine particles which are not soluble in the electric insulation solution (see JP-A No. 07-234551 , for example), or a precipitation method in which a solution is obtained by dissolving a resin material and a pigment in a non-water-based solvent, and then a solvent which is insoluble to the resin material is added to the thus obtained solution with being stirred to thereby precipitate the resin material (see JP-A No. 2003-345071 , for example).

However, these conventional liquid developer producing methods involve such problems as described below.

Namely, in the grinding method, it is difficult to grind a resin material so that toner particles can have sufficiently small size (e.g. 5 µm or less). This means that it takes very long time or it requires very large energy to obtain toner particles having a sufficiently small size that can exhibit properly the effects resulted by the use of the liquid developer as described above, thus leading to extremely low productivity of a liquid developer. Further, in the grinding method, a particle size distribution of toner particles is likely to be large (that is, there is large variations in particle sizes), and the shapes of the toner particles are liable to be irregular and nonuniform. As a result, obtained toner particles are likely to have variations in their properties (e.g. charge properties). Further, it may be conceived that a resin material is to be subjected to dry grinding instead of wet grinding in a nonpolar solvent (insulation liquid). In this case, however, even if very fine particles are obtained by the dry grinding, these particles are likely to get aggregated with each other, thus it is difficult to make the size of each toner particle sufficiently small.

Further, in the polymerization method, it is difficult to set polymerization conditions appropriately. This means that it is difficult to produce a resin material having a desired molecular weight and form toner particles having a desired size. Further, it is also difficult to make the variations in the size of the toner particles sufficiently small. As a result, stability of quality of a toner and reliability thereof are likely to be low. Further, since the polymerization method requires a relatively long time for the formation of the toner particles, the productivity of the liquid developer is not so high. In addition, the polymerization method generally requires large production machines and facilities.

Furthermore, in the precipitation method, each of the materials (especially, a pigment) is likely to get aggregated when precipitating a resin material. Therefore, there is a problem in that obtained toner particles are likely to have variations in compositions and properties among the toner particles. In addition, since a pigment is likely to get aggregated in this precipitation method as described above, it is difficult to form an image having sufficient transparency (a clear image) using an obtained liquid developer.

Moreover, the liquid developers produced by the conventional methods involve a problem in that toner particles generally have poor fixing properties to a recording medium such as a paper. In addition, there is also a problem in that an offset of toner particles is likely to occur after fixation of toner particles to a recording medium such as a paper is carried out.

Accordingly, it is an object of the present invention to provide a liquid developer which includes toner particles having small particle size distribution, uniform shape, and excellent fixing properties to a recording medium, a liquid developer which includes toner particles having small particle size distribution and uniform shape and being capable of exhibiting a property of each of materials constituting the toner particles sufficiently, a liquid developer having excellent anti-offset properties (mold release properties) and further a method of producing a liquid developer capable of producing such a liquid developer efficiently. In particular, it is an object of the present invention to provide such a liquid developer as described above by a method harmless to the environment.

These objects are achieved by the present invention described below.

A first aspect of the present invention is directed to a method of producing a liquid developer which comprises an insulation liquid and toner particles dispersed in the insulation liquid. The method comprises the steps of: preparing a water-based dispersion liquid comprising a dispersoid composed of a material containing a resin material and a water-based dispersion medium constituted from a water-based liquid in which the dispersoid is dispersed; removing the water-based dispersion medium by spraying the water-based dispersion liquid in the form of droplets to obtain toner particles, each of the toner particles being formed by aggregation of a plurality of particles of the dispersoid contained in each droplet of the water-based dispersion liquid; and dispersing the toner particles into the insulation liquid directly.

According to the liquid developer producing method described above, it is possible to provide a liquid developer producing method which makes it possible to produce effectively (with good productivity) a liquid developer which includes toner particles having small particle size distribution, uniform shape, and excellent fixing properties to a recording medium. In particular, it is possible to provide a liquid developer producing method which makes it possible to produce a liquid developer which includes toner particles having small particle size distribution, uniform shape, and excellent fixing properties to a recording medium by a method harmless to the environment.

In the liquid developer producing method according to the present invention, it is preferred that an average particle size of the particles of the dispersoid contained in the water-based dispersion liquid is in the range of 0.01 to 1.0 µm.

This makes it possible to obtain toner particles having sufficiently high roundness and uniformity in properties and shape of the respective particles (toner particles). Further, this also makes it possible to further stabilize spray conditions of the water-based dispersion liquid.

In the liquid developer producing method according to the present invention, it is preferred that when an average particle size of the particles of the dispersoid contained in the water-based dispersion liquid is defined as Dm (µm) and an average particle size of the toner particles is defined as Dt (µm), a relation of 0.005 ≤ Dm/Dt ≤ 0.5 is satisfied.

This makes it possible to make variations in shape and size of the toner particles especially small.

In the liquid developer producing method according to the present invention, it is preferred that an average particle size of the droplets is defined as Dd (µm) and an average particle size of the particles of the dispersoid contained in the water-based dispersion liquid is defined as Dm (µm), a relation of Dm/Dd < 0.5 is satisfied.

This makes it possible to make variations in particle size of the toner particles smaller, while exhibiting a feature of the dispersion liquid sufficiently (e.g. good formation of droplets) when manufacturing the toner particles.

In the liquid developer producing method according to the present invention, it is preferred that an average particle size of the droplets is defined as Dd (µm) and an average particle size of the toner particles is defined as Dt (µm), a relation of 0.05 ≤ Dt/Dd ≤ 1.0 is satisfied.

This makes it possible to obtain very fine toner particles having a high degree of roundness and sharp particle size distribution relatively easily.

In the liquid developer producing method according to the present invention, it is preferred that the dispersoid includes various kinds of dispersoids respectively formed from different materials, and each droplet of the water-based dispersion liquid contains the various kinds of dispersoids.

According to the liquid developer producing method described above, it is possible to provide a liquid developer producing method which makes it possible to produce effectively (with good productivity) a liquid developer which includes toner particles having small particle size distribution and uniform shape and being capable of exhibiting a property of each of materials constituting the toner particles sufficiently. In particular, it is possible to provide a liquid developer producing method which makes it possible to produce a liquid developer which includes toner particles having small particle size distribution, uniform shape, and excellent fixing properties to a recording medium by a method harmless to the environment.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid contains the various kinds of dispersoids, and the droplets of the water-based dispersion liquid are formed by ejecting the water-based dispersion liquid.

This makes it possible to remove the water-based dispersion medium from the droplets to thereby form toner particles as aggregates of a plurality of particles of the dispersoid.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid is prepared by mixing a first dispersion liquid in which a first dispersoid is dispersed and a second dispersion liquid in which a second dispersoid comprising a material different from a material constituting the first dispersoid is dispersed.

Even if a constituent material of the toner particles includes components which are hard to be dispersed or dissolved with each other, by preparing different dispersion liquids (first dispersion liquid and second dispersion liquid) containing each of the components and mixing the dispersion liquids, it is possible to obtain a water-based dispersion liquid in which various kinds of dispersoids each formed from the component are dispersed more uniformly. As a result, in a finally obtained liquid developer, variations in properties of the toner particles can be made smaller.

In the liquid developer producing method according to the present invention, it is preferred that aggregates constituting the toner particles are formed by colliding first droplets of a first dispersion liquid in which a first dispersoid is dispersed and second droplets of a second dispersion liquid in which a second dispersoid comprising a material different from a material constituting the first dispersoid is dispersed so that the first and second droplets are joined together to obtain droplets of the water-based dispersion liquid, and then removing the water-based dispersion medium from the droplets of the water-based dispersion liquid.

This makes it possible to make compositions of formed droplets substantially uniform, even if specific gravity or the like of the first dispersion liquid is different from that of the second dispersion liquid, and the dispersion liquids are hard to be mixed with each other. As a result, in a finally obtained liquid developer, variations in properties of the toner particles can be made smaller.

In the liquid developer producing method according to the present invention, it is preferred that only one of the first dispersion liquid and the second dispersion liquid contains a coloring agent and only the other dispersion liquid contains the resin material.

This makes it possible to prevent occurrence of a color shift, a color blur or the like effectively, when the finally obtained toner particles are transferred onto a transfer member (recording medium) such as a paper.

In the liquid developer producing method according to the present invention, it is preferred that only one of the first dispersion liquid and the second dispersion liquid contains a coloring agent and only the other dispersion liquid contains a charge control agent.

When a coloring agent is used in combination with a charge control agent, there is a case that the function of the charge control agent is inhibited by contact with the coloring agent depending on the kinds of the coloring agents (especially, in the case where the coloring agent is a carbon black). However, by dispersing each of the coloring agent and the charge control agent into a different dispersion medium, the coloring agent and the charge control agent can be existed in a state that they are adequately away from each other in each obtained toner particle. Therefore, the finally obtained toner particles can have excellent charge properties while keeping superior color development.

In the liquid developer producing method according to the present invention, it is preferred that the toner particles contain water more than an amount of water absorption of the resin material.

This enables the fixing properties of the toner particles to a recording medium to become particularly excellent.

In the liquid developer producing method according to the present invention, it is preferred that a water content of the toner particles is in the range of 0.3 to 5.0 wt%.

According to this method, it is possible to make dispersibility of the toner particles excellent while making charge properties of the toner particles sufficiently excellent, thus enabling the toner particles to have particularly excellent fixing properties to a recording medium.

In the liquid developer producing method according to the present invention, it is preferred that the average particle size of the droplets is in the range of 1.0 to 100 µm.

In this case, the water-based dispersion medium can be removed more efficiently. Further, it is possible to form toner particles having appropriate particle diameter more reliably.

In the liquid developer producing method according to the present invention, it is preferred that the method further comprises the step of heating an aggregate dispersion liquid obtained by dispersing aggregates constituting the toner particles into the insulation liquid.

According to the liquid developer producing method described above, it is possible to produce easily a liquid developer having excellent anti-offset properties. In particular, it is possible to provide a liquid developer producing method which makes it possible to produce a liquid developer which includes toner particles having small particle size distribution, uniform shape, and having excellent anti-offset properties.

In the liquid developer producing method according to the present invention, it is preferred that when a heating temperature of the aggregate dispersion liquid is defined as T (°C) and a softening point of the resin material is defined as T_1/2 (°C), a relation of T_1/2 - 40 ≤ T ≤ T_1/2 + 30 is satisfied.

This makes it possible to replace water by the insulation liquid effectively, while maintaining the shape of the aggregates.

In the liquid developer producing method according to the present invention, it is preferred that the average particle size of the particles of the dispersoid contained in the water-based dispersion liquid is in the range of 10 to 1000 µm.

This makes it possible to form spaces having adequate size in each toner particle. Further, this makes it possible to prevent undesirable bonding (aggregation) of the particles of the dispersoid reliably, thereby enabling the size of finally obtained toner particles to be optimum size and roundness.

In the liquid developer producing method according to the present invention, it is preferred that the average particle size of the droplets is in the range of 0.5 to 100 µm.

This makes it possible to carry out the removal of the dispersion medium more efficiently. Further, this makes it possible to form toner particles each having adequate particle size more effectively.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid contains fine particles manufactured by an emulsion polymerization method as the dispersoid.

This makes it possible to form the particles each having an uniform particle diameter.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid is prepared using fine particles obtained by a grinding method.

This makes it possible to make size of the particles of the dispersoid constituting the water-based dispersion liquid sufficiently small, whereby size of the toner particles can be made sufficiently small.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid is prepared using a kneaded material containing the resin material and a coloring agent.

According to this method, variations in compositions and properties of the respective toner particles can be made particularly small.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid is prepared through a method which comprises the steps of: dissolving the kneaded material into a solvent which can dissolve at least a part of the kneaded material to obtain a solution; and dispersing the solution into the water-based liquid.

According to this method, it is possible to make variations in shape and size of the toner particles small, and thus variations in properties (such as charge properties) of the toner particles can be made small. Further, it is also possible to make the diameter of each toner particle smaller.

In the liquid developer producing method according to the present invention, it is preferred that the water-based dispersion liquid is obtained by removing the solvent after the solution is dispersed into the water-based liquid.

This makes it possible to prevent undesirable aggregation between the particles of the dispersoid and between the toner particles more effectively, and as a result thereof, uniformity in shape and size of the toner particles can be made especially excellent. Further, since a deairing treatment can be performed together with the removal of the solvent, it is possible to prevent formation of toner particles having irregular shapes effectively. Further, the water-based dispersion medium (water) can enter the inside of the particles of the dipsersoid effectively, thus it is possible to obtain the toner particles containing appropriate amount of water.

A second aspect of the present invention is directed to a liquid developer produced using the liquid developer producing method as described above.

According to the liquid developer described above, it is possible to provide a liquid developer which includes toner particles having small particle size distribution, uniform shape, and excellent fixing properties to a recording medium. Further, it is possible to provide a liquid developer which includes toner particles having small particle size distribution and uniform shape and being capable of exhibiting a property of each of materials constituting the toner particles sufficiently. Furthermore, it is possible to provide a liquid developer having excellent anti-offset properties (mold release properties).

A third aspect of the present invention is directed to a liquid developer. The liquid developer comprises an insulation liquid; and toner particles dispersed into the insulation liquid, wherein each of the toner particles has an outer surface and spaces formed therein, the spaces each having an opening opened at the outer surface or in the vicinity thereof, the opening communicating with the outer surface, and a portion having a diameter larger than that of the opening inside the toner particle, wherein the insulation liquid is retained in the spaces of the toner particles.

This makes it possible to provide a liquid developer having excellent anti-offset properties (mold release properties).

In the liquid developer according to the present invention, it is preferred that when an average diameter of the openings of the spaces is defined as X (nm) and an average greatest diameter of the spaces inside the toner particles is defined as Y (nm), a relation of 0.01 ≤ X/Y ≤ 10 is satisfied.

This makes it possible to easily extrude the insulation liquid outside the toner particles during the toner particles are fixed onto a recording medium such as a paper, while retaining the insulation liquid inside the toner particles more reliably before the fixing process.

In the liquid developer according to the present invention, it is preferred that the average diameter of the openings of the spaces is in the range of 1 to 500 nm.

This makes it possible to easily extrude the insulation liquid outside the toner particles during the toner particles are fixed onto a recording medium such as a paper, while retaining the insulation liquid inside the toner particles more reliably before the fixing process.

In the liquid developer according to the present invention, it is preferred that the average greatest diameter of the spaces inside the toner particles is in the range of 90 to 4950 nm.

This makes it possible to retain the insulation liquid inside the toner particles more reliably before the fixing process.

In the liquid developer according to the present invention, it is preferred that a porosity of the toner particles is in the range of 1 to 70%.

This makes it possible to easily extrude the insulation liquid outside the toner particles during the toner particles are fixed onto a recording medium such as a paper, while retaining the insulation liquid inside the toner particles more reliably before the fixing process.

In the liquid developer according to the present invention, it is preferred that the insulation liquid comprises silicone oil.

Since silicone oil has excellent insulation properties and exhibits excellent anti-offset properties, the silicone oil is preferably used as the insulation liquid.

In the liquid developer according to the present invention, it is preferred that an average particle size of the toner particles is in the range of 0.1 to 5 µm.

This makes it possible to make variations in properties of the toner particles such as charge properties and fixing properties, and therefore the reliability of the liquid developer as a whole can be made especially high, and the resolution of an image to be formed using the liquid developer (toner) can also be made especially high.

In the liquid developer according to the present invention, it is preferred that a standard deviation of the particle size among the toner particles is 1.0 µm or less.

This makes it possible to make variations in properties such as charge properties and fixing properties particularly small, thereby enabling to improve reliability of the liquid developer as a whole.

• Fig. 1 is a vertical cross-sectional view which schematically shows one example of the structure of a kneading machine and a cooling machine both used for producing a kneaded material used for preparing a water-based emulsion (water-based dispersion liquid).
• Fig. 2 is a vertical cross-sectional view which schematically shows a first embodiment of a liquid developer producing apparatus used in producing a liquid developer of the present invention.
• Fig. 3 is an enlarged sectional view of a head portion of the liquid developer producing apparatus shown in Fig. 2.
• Fig. 4 is an enlarged sectional view of a head portion of the liquid developer producing apparatus shown in Fig. 2.
• Fig. 5 is a vertical cross-sectional view which schematically shows a third embodiment of a liquid developer producing apparatus used in producing a liquid developer of the present invention.
• Fig. 6 is a cross-sectional view which schematically shows a toner particle contained in a liquid developer of the present invention.
• Fig. 7 is a vertical cross-sectional view which schematically shows a fourth embodiment of a liquid developer producing apparatus used in producing a liquid developer of the present invention.
• Fig. 8 is a cross-sectional view of one example of a contact type image forming apparatus to which the liquid developer of the present invention can be applied.
• Fig. 9 is a cross sectional view of one example of a non-contact type image forming apparatus to which the liquid developer of the present invention can be applied.
• Fig. 10 is a cross-sectional view which shows one example of a fixing apparatus to which the liquid developer of the present invention can be applied.
• Fig. 11 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 12 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 13 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 14 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 15 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 16 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 17 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 18 is an illustration which schematically shows another example of the structure in the vicinity of the head portion of the liquid developer producing apparatus of the present invention.
• Fig. 19 is one example of an electron micrograph of a toner particle contained in the liquid developer of the invention.

Hereinbelow, with reference to the accompanying drawings, a preferred embodiment of a method of producing a liquid developer according to the present invention and a liquid developer manufactured by the method will be described in details.

First, the first embodiment of the present invention will be described.

Fig. 1 is a vertical cross-sectional view which schematically shows one example of the structure of a kneading machine and a cooling machine both used for producing a kneaded material used for preparing a water-based emulsion (water-based dispersion liquid), Fig. 2 is a vertical cross-sectional view which schematically shows a first embodiment of a liquid developer producing apparatus used in producing a liquid developer of the present invention, and Fig. 3 is an enlarged sectional view of a head portion of the liquid developer producing apparatus shown in Fig. 2. In the following description, the left side in Fig. 1 denotes "base" or "base side" and the right side in Fig. 1 denotes "front" or "front side".

The liquid developer producing method of the present invention is characterized by comprising:

• a water-based dispersion liquid preparing step for preparing a water-based dispersion liquid comprising a dispersoid composed of a material containing a resin material and a water-based dispersion medium constituted from a water-based liquid in which the dispersoid is dispersed;
• a dispersion medium removal step for removing the dispersion medium by spraying the water-based dispersion liquid to obtain toner particles; and
• a dispersing step for dispersing the toner particles directly into an insulation liquid.

Although the water-based dispersion liquid used in the present invention can be prepared by any methods, the one prepared using a kneaded material containing a coloring agent and a resin material is used in this embodiment.

A kneaded material obtained in the kneading step described below contains a component which forms a toner particle of a liquid developer, and the kneaded material contains at least a binder resin (resin material) and a coloring agent.

First, a description will be made with regard to a constituent material used for preparing the kneaded material.

Generally, toner particles contained in a liquid developer are constituted from a material which contains a resin (binder resin) as its main component.

In the present invention, although a type of a resin (binder resin) constituting the kneaded material is not particularly limited, it is preferred to use a self-dispersible type resin which has dispersibility to a water-based liquid described later. By using the self-dispersible type resin, dispersibility of a dispersoid in the water-based dispersion liquid becomes particularly excellent and an appropriate amount of water (moisture) can be contained in the dispersoid. As a result of this, it is possible to obtain toner particles having appropriate water content (moisture content). In this regard, it is to be noted that in this specification, the term "self-dispersible" means properties having dispersibility to a dispersion medium without using a dispersant, and the term "self-dispersible type resin" means a resin material having such self-dispersibility.

No particular limitation is imposed on the self-dispersible type resin, and examples of such a self-dispersible type resin include a resin having a plurality of groups which are lyophilic (hydrophilic) to a water-based liquid described below.

Examples of groups (functional groups) having such lyophilic property (hydrophilic property) include -COO^-group, -SO₃ ^-group, -CO group, -OH group, -OSO₃ ^-group, -COO-group, -SO₃-, -OSO₃-group, -PO₃H₂, -PO₃ ^2-, -PO₄-group, and quatemary ammonium, and salts thereof. Since such a self-dispersible type resin has particularly excellent dispersibility to a water-based liquid, it is possible to prepare a water-based dispersion liquid (water-based emulsion and water-based suspension) described later without using any dispersant or by using an extremely small amount of dispersant. As a result, it is possible to prevent effectively occurrence of a problem resulted from the fact that a dispersant is contained in a liquid developer finally obtained. In more details, it is possible to effectively prevent a dispersant from giving an adverse effect to a charge property of toner particles. Further, it is also possible to prevent foam formation by a lowered antifoaming property resulted from the use of a dispersant for preparation of a dispersion liquid, thereby enabling to improve an ejection stability when the water-based dispersion liquid (water-based suspension) described later is ejected. Furthermore, since a dispersant or charge control agent are likely to be absorbed when resin particles are dispersed into a carrier solution which constitutes a liquid developer, it is possible to further stabilize dispersibility and charge properties.

The above-mentioned groups themselves have properties which are easily charged. Therefore, use of such groups is advantageous in improving charge properties of toner particles themselves.

Further, among the groups mentioned above, -COO-group and -SO₃-gourp are particularly preferred. A self-dispersible type resin having such a group has particularly superior dispersibility against the water-based liquid as well as appropriate water retention ability. Further, it can be manufactured relatively easily and be available at a relatively low cost. As a result, it is possible to further reduce production cost of the liquid developer.

It is preferred that the group mentioned above exists at a side chain of a polymer constituting a resin material. This makes it possible to make hydrophilic property against the water-based liquid more excellent, and thereby to make dispersibility of a dispersoid constituted from a self-dispersible type resin in a water-based dispersion liquid (water-based emulsion and water-based suspension) especially excellent. Furthermore, it is also possible to obtain a dispersion liquid having especially excellent dispersion state without using any organic solvents in the manufacturing process of a liquid developer, and thus to manufacture the liquid developer by a method which is harmless to the environment since it does not use any organic solvents.

The self-dispersible type resin described above can be manufactured by bonding the material having the functional group described above to a raw resin material (raw resin) or its monomer, dimer, oligomer, and the like.

For example, a self-dispersible type resin having -COO-group can be manufactured by graft copolymerization or block copolymerization of a low water-soluble or water-insoluble resin (raw resin) with unsaturated carboxylic acids, or random copolymerization of a monomer constituting a thermoplastic resin with unsaturated carboxylic acids.

Example of such unsaturated carboxylic acids include unsaturated monocarboxylic acids, unsaturated dicarboxylic acid or anhydrides thereof such as (meth) acrylic acid, maleic acid, fumaric acid, tetrahydrophthalic acid, itaconic acid, citraconic acid, crotonic acid, isocrotonic acid, nagic acid, maleic anhydride, citraconic anhydride; ester compounds such as monoester and diester of methyl, ethyl, and propyl of the unsaturated carboxylic acids; salts of unsaturated carboxylic acids such as alkali metal salts, alkaline earth metal salts, ammonium salts, and the like.

Further, the self-dispersible type resin having -SO₃ ^-group can be manufactured by, for example, graft copolymerization or block copolymerization of a thermoplastic resin (raw resin) with unsaturated sulfonic acids, random copolymerization of an unsaturated monomer constituting an addition polymerization type thermoplastic resin with a monomer containing unsaturated sulfonic acids, or polycondensation of a monomer constituting a polycondensation type thermoplastic resin with a monomer containing unsaturated sulfonic acids.

Examples of such unsaturated sulfonic acids include styrene sulfonic acids, sulfoalkyl (meth) acrylate, metal salts thereof, and ammonium salts, and the like. Further, examples of a monomer containing sulfonic acids include sulufo-isophthalic acid, sulufo-terephthalic acid, sulfo-phthalic acid, sulfo-succinic acid, sulfo-benzoic acid, sulfo-salicylic acid, and metal salts thereof, and ammonium salts, and the like.

Examples of a resin (raw resin) which can be used as a constituent material of the self-dispersible type resin include (meth)acrylic resin; polycarbonate resin; a homopolymer or a copolymer of styrene resin that includes styrene or a styrene substitution product, such as polystyrene, poly-&agr;-methylstyrene, chloropolystyrene, styrene-chlorostyrene copolymer, styrene-propylene copolymer, styrene-butadiene copolymer, styrene-vinyl chloride copolymer, styrene-vinyl acetate copolymer, styrene-maleic acid copolymer, styrene-acrylate ester copolymer, styrene-methacrylate ester copolymer, styrene-acrylate ester-methacrylate ester copolymer, styrene-&agr;-methyl chloroacrylate copolymer, styrene-acrylonitrile-acrylate ester copolymer, styrene-vinylmethylether copolymer, or the like; polyester resin; epoxy resin; urethane modified epoxy resin; silicone modified epoxy resin; vinyl chloride resin; rosin modified maleic acid resin; phenyl resin; polyethylene-based resin; polypropylene; ionomer resin; polyurethane resin; silicone resin; ketone resin; ethylene-ethyl acrylate copolymer; xylene resin; polyvinyl butyral resin; terpene resin; phenol resin; aliphatic or alicyclic hydrocarbon resin; or the like. These resin components can be used alone or in combination of two or more.

As described above, the self-dispersible type resin can be manufactured, for example, by polymerizing a precursor having a functional group described above (that is, corresponding monomer, dimmer, oligomer, and the like).

The number of the functional groups (hydrophilic groups) contained in the self-dispersible type resin is preferably in the range of 0.001 to 0.050 mol with respect to 100 g of the self-dispersible type resin, and more preferably in the range of 0.005 to 0.030 mol. This makes it possible to improve dispersibility of the dispersoid mainly formed of the self-dispersible type resin while maintaining effectively properties necessary as a toner particle.

The content of the self-dispersible type resin in the kneaded material (that is, the content of the self-dispersible type resin in the composition used for preparing the kneaded material) is not particularly limited to any specific value, but it is preferably in the range of 55 to 95 wt%, more preferably in the range of 60 to 90 wt%, and even more preferably in the range of 65 to 85 wt%. If the content of the self-dispersible type resin is less than the above lower limit value, there is a case that it is not possible to raise the dispersibility of the dispersoid in the water-based dispersion liquid (water-based emulsion and water-based suspension) sufficiently. On the other hand, if the content of the self-dispersible type resin exceeds the above upper limit value, the amount of the coloring agent is relatively decreased so that it becomes difficult to form a visible image having a sufficient contrast when a resultant liquid developer is actually used.

The kneaded material may contain other resin materials in addition to the self-dispersible type resin described above. As for such resin materials (that is, resin materials other than the self-dispersible type resin), resin materials such as those mentioned above as the raw material resins can be used.

The softening point of the resin (resin material) is not particularly limited to any specific value, but it is preferably in the range of 50 to 120°C, more preferably in the range of 60 to 115°C, still more preferably in the range of 65 to 115°C. In this specification, the term "softening point" means a temperature at which softening begins under the conditions that a temperature raising speed is 5°C/min and a diameter of a die hole is 1.0 mm in a high-floored flow tester. Further, in a case where the resin material contains two or more types of resins, the softening point of the resin material is determined by the weighted average of these resins.

A toner contains a coloring agent. As for a coloring agent, pigments, dyes or the like can be used. Examples of such pigments and dyes include Carbon Black, Spirit Black, Lamp Black (C.I. No. 77266), Magnetite, Titanium Black, Chrome Yellow, Cadmium Yellow, Mineral Fast Yellow, Navel Yellow, Naphthol Yellow S, Hansa Yellow G, Permanent Yellow NCG, Benzidine Yellow, Quinoline Yellow, Tartrazine Lake, Chrome Orange, Molybdenum Orange, Permanent Orange GTR, Pyrazolone Orange, Benzidine Orange G, Cadmium Red, Permanent Red 4R, Watching Red Calcium Salt, Eosine Lake, Brilliant Carmine 3B, Manganese Violet, Fast Violet B, Methyl Violet Lake, Prussian Blue, Cobalt Blue, Alkali Blue Lake, Victoria Blue Lake, Fast Sky Blue, Indanthrene Blue BC, Ultramarine Blue, Aniline Blue, Phthalocyanine Blue, Chalco Oil Blue, Chrome Green, Chromium Oxide, Pigment Green B, Malachite Green Lake, Phthalocyanine Green, Final Yellow Green G, Rhodamine 6G, Quinacridone, Rose Bengal (C.I. No. 45432), C.I. Direct Red 1, C.I. Direct Red 4, C. I. Acid Red 1, C.I. Basic Red 1, C.I. Mordant Red 30, C.I. Pigment Red 48:1, C.I. Pigment Red 57:1, C.I. Pigment Red 122, C.I. Pigment Red 184, C.I. Direct Blue 1, C.I. Direct Blue 2, C.I. Acid Blue 9, C.I. Acid Blue 15, C.I. Basic Blue 3, C.I. Basic Blue 5, C.I. Mordant Blue 7, C.I. Pigment Blue 15:1, C.I. Pigment Blue 15:3, C.I. Pigment Blue 5:1, C.I. Direct Green 6, C.I. Basic Green 4, C.I. Basic Green 6, C.I. Pigment Yellow 17, C.I. Pigment Yellow 93, C.I. Pigment Yellow 97, C.I. Pigment Yellow 12, C.I. Pigment Yellow 180, C.I. Pigment Yellow 162, and Nigrosine Dye (C.I. No. 50415B); metal oxides such as metal complex dyes, silica, aluminum oxide, magnetite, maghemite, various kinds of ferrites, cupric oxide, nickel oxide, zinc oxide, zirconium oxide, titanium oxide, magnesium oxide, and the like; and magnetic materials including magnetic metals such as Fe, Co, and Ni; and the like. These pigments and dyes can be used singly or in combination of two or more of them.

In preparing the kneaded material, additional components other than the above components may be used. Examples of such other components include a wax, a charge control agent, a magnetic powder, and the like.

Examples of such a wax include hydrocarbon wax such as ozokerite, ceresin, paraffin wax, micro wax, microcrystalline wax, petrolatum, Fischer-Tropsch wax, or the like; ester wax such as carnauba wax, rice wax, methyl laurate, methyl myristate, methyl palmitate, methyl stearate, butyl stearate, candelilla wax, cotton wax, Japan wax, beeswax, lanolin, montan wax, fatty ester, or the like; olefin wax such as polyethylene wax, polypropylene wax, oxidized polyethylene wax, oxidized polypropylene wax, or the like; amide wax such as 12-hydroxystearic acid amide, stearic acid amide, phthalic anhydride imide, or the like; ketone wax such as laurone, stearone, or the like; ether wax; and the like. These waxes can be used singly or in combination of two or more.

Examples of the charge control agent include a metallic salt of benzoic acid, a metallic salt of salicylic acid, a metallic salt of alkylsalicylic acid, a metallic salt of catechol, a metal-containing bisazo dye, a nigrosine dye, tetraphenyl borate derivatives, a quaternary ammonium salt, an alkylpyridinium salt, chlorinated polyester, nitrohumic acid, and the like.

Further, examples of the magnetic powder include a powder made of a magnetic material containing a metal oxide such as magnetite, maghemite, various kinds of ferrites, cupric oxide, nickel oxide, zinc oxide, zirconium oxide, titanium oxide, magnesium oxide, or the like, and/or magnetic metal such as Fe, Co or Ni.

Further, the constituent material of the kneaded material may further contain zinc stearate, zinc oxide, cerium oxide, silica, titanium oxide, iron oxide, aliphatic acid, or aliphatic metal salt, or the like in addition to the materials described above.

Furthermore, the constituent material of the kneaded material may further contain materials used as a solvent such as inorganic solvent, organic solvent and the like. This makes it possible to improve kneading efficiency so that the kneaded material in which each component thereof is mixed with each other more homogeneously can be obtained.

Hereinbelow, a description will be made with regard to one example of a method for obtaining a kneaded material K7 by kneading a material K5 which contains the above-mentioned components.

The kneaded material K7 can be manufactured using a kneading apparatus as shown in Fig. 1, for example.

The material K5 to be kneaded contains the components as described above. By containing a coloring agent, air such as air contained by the coloring agent is also contained in the material K5, which means that there is a possibility that air bubble could enter the inside of the toner particle. However, since the material K5 is subjected to the kneading process in this step, it is possible to eliminate air contained in the material K5 efficiently, and therefore it is possible to prevent air bubble from entering the inside of the toner particle effectively, that is, prevent air bubble from remaining inside the toner particle effectively. Therefore, it is preferred that the material K5 to be kneaded is prepared in advance by mixing the above-mentioned various components.

In this embodiment, a biaxial kneader-extruder is used as the kneading machine, a detail of which will be described below.

The kneading machine K1 includes a process section K2 which kneads the material K5 with conveying it, a head section K3 which extrudes a kneaded material K7 so that an extruded kneaded material can have a prescribed cross-sectional shape, and a feeder K4 which supplies the material K5 into the process section K2.

The process section K2 has a barrel K21, screws K22 and K23 inserted into the barrel 21, and a fixing member K24 for fixing the head section K3 to the front portion of the barrel K21.

In the process section K2, a shearing force is applied to the material K5 supplied from the feeder K4 by the rotation of the screws K22 and K23 so that a homogeneous kneaded material K7 is obtained.

In this embodiment, it is preferred that the total length of the process section K2 is in the range of 50 to 300 cm, and more preferably in the range of 100 to 250 cm. If the total length of the process section K2 is less than the above lower limit value, there is a case that it is difficult to mix and knead the components contained in the material K5 homogeneously. On the other hand, if the total length of the process section K2 exceeds the above upper limit value, there is a case that thermal modification of the material K5 is likely to occur depending on the temperature inside the process section K2, or the number of revolutions of the screws K22 and K23, or the like, thus leading to a possibility that it becomes difficult to control the physical properties of a finally obtained liquid developer (that is, resultant toner) sufficiently.

In this connection, when the temperature of the material (material temperature) during the kneading step is preferably in the range of 80 to 260°C, and more preferably in the range of 90 to 230°C though it varies depending on the composition of the material K5 and the like. In this regard, it is to be noted that the temperature of the material inside the process section K2 may be constant throughout the process section K2 or different depending on positions inside the process section K2. For example, the process section K2 may include a first region in which an internal temperature is set to be relatively low, and a second region which is provided at the base side of the first region and in which an internal temperature is set to be higher than the internal temperature of the first region.

Moreover, it is preferred that the residence time of the material K5 in the process section K2, that is the time required for the material K5 to pass through the process section K2, is 0.5 to 12 minutes, and more preferably 1 to 7 minutes. If the residence time of the material K5 in the process section K2 is less than the above lower limit value, there is a possibility that it is difficult to mix the components contained in the material K5 homogeneously. On the other hand, if the residence time of the material K5 in the process section K2 exceeds the above upper limit value, there is a possibility that production efficiency is lowered, and thermal modification of the material K5 is likely to occur depending on the temperature inside the process section K2 or the number of revolutions of the screws K22 and K23, or the like, thus resulting in a case that it is difficult to control the physical properties of a finally obtained liquid developer (that is, a resultant toner) satisfactorily.

Although the number of revolutions of the screws K22 and K23 varies depending on the compositions of the binder resin or the like, 50 to 600 rpm is preferable. If the number of revolutions of the screws K22 and K23 is less than the above lower limit value, there is a case that it is difficult to mix the components of the material K5 homogeneously. On the other hand, if the number of revolutions of the screws K22 and K23 exceeds the above upper limit value, there is a case that molecular chains of the resin are cut due to a shearing force, thus resulting in the deterioration of the characteristics of the resin.

In the kneading machine K1 used in this embodiment, the inside of the process section K2 is connected to a pump P through a duct K25. This makes it possible to deaerate the inside of the process section K2, thereby enabling to prevent the pressure inside the process section K2 from raising due to heated-up or heat generation of the material K5 (kneaded material K7). As a result, the kneading step can be carried out safely and effectively. Further, since the inside of the process section K2 is connected to the pump P through the duct K25, it is possible to prevent air bubble (in particular, relatively large air bubble) from being contained in the obtained kneaded material K7 effectively, so that it becomes possible to obtain a liquid developer (that is, a toner) having excellent properties.

The kneaded material K7 which has been kneaded in the process section K2 is extruded to the outside of the kneading machine K1 via the head section K3 by the rotation of the screws K22 and K23.

The head section K3 has an internal space K31 to which the kneaded material K7 is sent from the process section K2, and an extrusion port K32 through which the kneaded material K7 is extruded.

In this connection, it is preferred that the temperature (temperature at least in the vicinity of the extrusion port K32) of the kneaded material K7 in the internal space K31 is higher than a softening temperature of the resin materials contained in the material K5. When the temperature of the kneaded material K7 is such a temperature, it is possible to obtain a toner particle in which the components thereof are more homogeneously mixed, thereby enabling to make variations in its properties such as charge properties, fixing properties, and the like particularly small.

The concrete temperature of the kneaded material K7 inside the internal space K31 (that is, the temperature of the kneaded material K7 at least in the vicinity of the extrusion port K32) is not particularly limited to a specific temperature, but is preferably in the range of 80 to 150°C, and more preferably in the range of 90 to 140°C. In the case where the temperature of the kneaded material K7 in the internal space K31 is within the above range, the kneaded material K7 is not solidified inside the internal space K31 so that it can be extruded from the extrusion port K32 easily.

The internal space K31 having a structure as shown in Fig. 1 includes a cross sectional area reduced portion K33 in which a cross sectional area thereof is gradually reduced toward the extrusion port K32. Due to the cross sectional area reduced portion K33, the extrusion amount of the kneaded material K7 which is to be extruded from the extrusion port 32K becomes stable, and the cooling rate of the kneaded material K7 in a cooling process which will be described later also becomes stable. As a result of this, variations in properties of each toner particle can be made small, whereby enabling to obtain a liquid developer (that is, a liquid toner) having excellent properties as a whole.

The kneaded material K7 in a softened state extruded from the extrusion port K32 of the head section K3 is cooled by a cooler K6 and thereby it is solidified.

The cooler K6 has rolls K61, K62, K63 and K64, and belts K65 and K66.

The belt K65 is wound around the rolls K61 and K62, and similarly, the belt K66 is wound around the rolls K63 and K64.

The rolls K61, K62, K63 and K64 rotate in directions shown by the arrows e, f, g and h in the drawing about rotary shafts K611, K621, K631 and K641, respectively. With this arrangement, the kneaded material K7 extruded from the extrusion port K32 of the kneading machine K1 is introduced into the space between the belts K65 and K66. The kneaded material K7 is then cooled while being molded into a plate-like object with a nearly uniform thickness, and is ejected from an ejection part K67. The belts K65 and K66 are cooled by, for example, an air cooling or water cooling method. By using such a belt type cooler, it is possible to extend a contact time between the kneaded material extruded from the kneading machine and the cooling members (belts), thereby enabling the cooling efficiency for the kneaded material to be especially excellent.

Now, during the kneading process, since the material K5 is subjected to a shearing force, phase separation (in particular, macro-phase separation) and the like can be sufficiently prevented. However, since the kneaded material K7 which went through the kneading process is free from the shearing force, there is a possibility that phase separation (in particular, macro-phase separation) will occur again if such a kneaded material is being left standing for a long period of time. Accordingly, it is preferable to cool the thus obtained kneaded material K7 as quickly as possible. More specifically, it is preferred that the cooling rate of the kneaded material K7 (for example, the cooling rate when the kneaded material K7 is cooled down to about 60°C) is faster than -3 °C/s, and more preferably in the range of -5 to -100 °C/s. Moreover, the time between the completion of the kneading process (at which a shearing force has been eliminated) and the completion of the cooling process (time required to decrease the temperature of the kneaded material K7 to 60°C or lower, for example) is preferably 20 seconds or less, and more preferably 3 to 12 seconds.

In the above embodiment, a description has been made in terms of an example using a continuous biaxial kneader-extruder as the kneading machine, but the kneading machine used for kneading the material is not limited to this type. For kneading the material, it is possible to use various kinds of kneading machines, for example, a kneader, a batch type triaxial roll, a continuous biaxial roll, a wheel mixer, a blade mixer, or the like.

Further, although in the embodiment shown in the drawing the kneading machine is of the type that has two screws, the number of screws may be one or three or more. Further, the kneading machine may have a disc section (kneading disc section).

Furthermore, in the embodiment described above, one kneading machine is used for kneading the material, but kneading may be carried out by using two kneading machines. In this case, the heating temperature of the material and the rotational speed of the screws of one kneading machine may be different from those of the other kneading machine.

Moreover, in the above embodiment, the belt type cooler is used, but a roll type (cooling roll type) cooler may be used. Furthermore, cooling of the kneaded material extruded from the extrusion port K32 of the kneading machine is not limited to the way using the cooler described above, and it may be carried out by air cooling, for example.

The kneaded material K7 obtained through the cooling process described above is ground. By grinding the kneaded material K7, it is possible to obtain a water-based dispersion liquid (water-based emulsion and water-based suspension) (described later) in which a finer dispersoid is dispersed relatively easily. As a result, it becomes possible to make the size of the toner particles smaller in a liquid developer finally obtained, and such a liquid developer can be preferably used in forming a high resolution image.

The method of grinding is not particularly limited. For example, such grinding may be carried out by employing various kinds of grinding machines or crushing machines such as a ball mill, a vibration mill, a jet mill, a pin mill, or the like.

The grinding process may be carried out by dividing it into a plurality of stages (for example, two stages of coarse and fine grinding processes). Further, after the grinding process, other treatment such as classification treatment may be carried out as needed. Such classification treatment may be carried out using a sieve or an air flow type classifier or the like.

By subjecting the material K5 to the kneading process as described above, it is possible to eliminate air contained in the material K5 effectively. In other words, the kneaded material K7 obtained through such a kneading process hardly contains air (air bubble) in the inside thereof. By using such kneaded material K7, it is possible to prevent generation of toner particles of irregular shape (such as void particles, defect particles, fused particles, and the like) effectively in a water-based dispersion liquid spraying step which will be described later. As a result, in a liquid developer finally obtained, it is possible to prevent occurrence of a problem such as lowered transfer property and cleaning property which are caused by such toner particles having irregular shape.

In the present embodiment, a water-based dispersion liquid is prepared using the kneaded material described above. In particular, a water-based emulsion is firstly prepared using the kneaded material described above and a water-based suspension is then prepared using the water-based emulsion.

By using the kneaded material K7 in preparing the water-based dispersion liquid (water-based emulsion), the following effects can be obtained. Namely, even in a case where a constituent material of toner particles contains components which are difficult to be dispersed in a dispersion medium or difficult to be mutually soluble to each other, these components are mutually soluble to each other satisfactorily and finely dispersed in the obtained kneaded material by way of the kneading step described above. In particular, most of pigments (coloring agent) have poor dispersibility to a liquid used as a solvent described later. However, in this embodiment, because the kneading step has been carried out before the kneaded material is dispersed into a solvent, the outer periphery of each particle of a pigment is coated with a resin component effectively. Therefore, dispersibility of the pigment to the solvent is improved (particularly, the pigment can be finely dispersed in the solvent), color development of a finally obtained liquid developer becomes excellent. For these reasons, even in a case where a constituent material of toner particles contains a component having poor dispersibility to a dispersion medium of a water-based dispersion liquid (water base-emulsion and water-based suspension) (water-based dispersion medium) which will be described later (hereinafter, this component will be referred to as "poor dispersibility component") or a component having poor solubility to a solvent contained in a dispersion medium of a water-based emulsion (hereinafter, this component will be referred to as "poor solubility component"), it is possible to make dispersibility of a dispersoid in the water-based dispersion liquid (water base-emulsion or water-based suspension) more excellent. With these results, in a finally obtained liquid developer, variations in compositions and properties of respective toner particles can be made small, and therefore the liquid developer can have excellent properties as a whole.

Next, by using the kneaded material K7, a water-based emulsion comprised of a water-based dispersion medium constituted from a water-based liquid in which a dispersoid constituted from a toner material is dispersed is prepared (water-based emulsion preparing step).

Further, in the water-based emulsion used in the present invention, since a dispersoid is in a liquid sate (that is, a dispersoid has fluidity so that it can be deformed relatively easily), there is a tendency that each particle of the dispersoid is formed into a shape having a relatively high roundness (sphericity) due to its surface tension. Accordingly, in a suspension (water-based suspension) prepared using the water-based emulsion, there is also a tendency that each particle of the dispersoid is formed into a shape having a relatively high roundness (sphericity). As a result, it is possible to finally obtain toner particles having a relatively high roundness (sphericity). Further, in the emulsion containing a dispersoid in a liquid state (that is, a dispersoid having fluidity so that it can be deformed relatively easily), it is possible to raise uniformity in the size of the dispersoid relatively easily by stirring the emulsion.

The method for preparing the water-based emulsion is not particularly limited, but in the present embodiment, a water-based emulsion is prepared by obtaining a solution in which at least a part of the kneaded material K7 is dissolved, and then by dispersing such a solution into a water-based liquid. In this connection, it should be noted that in this specification the term "emulsion" means a dispersion liquid comprised of a liquid state dispersion medium and a liquid state dispersoid (dispersion particles) dispersed in the dispersion medium, and the term "suspension" means a suspension liquid (including suspension colloid) comprised of a liquid state dispersion medium and a solid state dispersoid (suspension particles) dispersed in the dispersion medium. Further, in a case where both a liquid state dispersoid and a solid state dispersoid exist in a dispersion liquid, the term "emulsion" means a dispersion liquid in which the total volume of the liquid state dispersoid is larger than the total volume of the solid state dispersoid, while the term "suspension" means a dispersion liquid in which the total volume of the solid state dispersoid is larger than the total volume of the liquid state dispersoid.

Hereinbelow, a description will be made with regard to the method for preparing the water-based emulsion.

In the present embodiment, a kneaded material solution (a solution of the kneaded material) in which at least a part of the kneaded material is dissolved is obtained.

The solution is prepared by mixing the kneaded material with a solvent in which at least a part of the kneaded material can be dissolved.

As for the solvent used for preparing the solution, various solvents can be used so long as at least a part of the kneaded material can be dissolved thereinto, but normally, solvents which have low mutual solubility to a water-based liquid described later (that is, a water-based liquid used for preparing the water-based emulsion) are used. For example, a liquid having a solubility of 10 g or less with respect to 100 g of a water-based liquid at a temperature of 25°C is used.

Examples of such solvents include inorganic solvents such as carbon disulfide, and carbon tetrachloride, and organic solvents such as ketone-based solvents (e.g., methyl ethyl ketone (MEK), methyl isopropyl ketone (MIPK), and 2-heptanone), alcohol-based solvents (e.g., pentanol, n-hexanol, 1-octanol, and 2-octanol), ether-based solvents (e.g., diethyl ether, and anisole), aliphatic hydrocarbon-based solvents (e.g., hexane, pentane, heptane, cyclohexane, octane, and isoprene), aromatic hydrocarbon-based solvents (e.g., toluene, xylene, benzene, ethyl benzene, and naphthalene), aromatic heterocyclic compound-based solvents (e.g., furan, and thiophene), halide-based solvents (e.g., chloroform), ester-based solvents (e.g., ethyl acetate, isopropyl acetate, isobutyl acetate, and ethyl acrylate), nitrile-based solvents (e.g., acrylonitrile), and nitro-based solvents (e.g., nitromethane and nitroethane). These materials can be used singly or in combination of two or more of them.

The amount of the solvent contained in the solution is not limited to any specific value, but is preferably in the range of 5 to 75 wt%, more preferably in the range of 10 to 70 wt%, and even more preferably in the range of 15 to 65 wt%. If the amount of the solvent contained in the solution is less than the above lower limit value, there is a possibility that it is difficult to dissolve the kneaded material sufficiently depending on the solubility of the kneaded material to the solvent. On the other hand, if the amount of the solvent exceeds the above upper limit value, a time required for removing the solvent in the subsequent step becomes long, the productivity of the liquid development is lowered. Further, if the amount of the solvent is too much, there is a possibility that the components which were sufficiently and homogeneously mixed to each other in the kneading step described above are phase-separated, and thereby making it difficult to make variations in the properties of the toner particles of a finally obtained liquid developer sufficiently small.

In this regard, it is to be noted that it is sufficient if at least a part of the components which constitute the kneaded material is dissolved (including a swelling state), and therefore components which were not dissolved may exist in the solution.

Next, a water-based emulsion is obtained by mixing the above mentioned solution with a water-based liquid. Normally, in the thus obtained water-based emulsion, a dispersoid which contains the solvent and the constituent material of the kneaded material are dispersed in the water-based dispersion medium formed from the water-based liquid.

In the present invention, the term "water-based liquid" means a liquid which contains at least water (H₂O), and it is preferred that the water-based liquid is mainly constituted from water. The water content of the water-based liquid is preferably 50 wt% or higher, more preferably 80 wt% or higher, and even more preferably 90 wt% or higher. In this regard, the water-based liquid may contain components other than water. It may contain, for example, a component having excellent mutual solubility with water (for example, a material which has solubility of 30 g or higher with respect to 100 g of water at a temperature of 25°C). Examples of such components include alcohol-based solvents such as methanol, ethanol, propanol, and the like, ether-based solvents such as 1,4-dioxane, tetrahydrofuran (THF), and the like, aromatic heterocyclic compound-based solvents such as pyridine, pyrazine, pyrrole, and the like, amide-based solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and the like, nitrile-based solvents such as acetonitrile and the like, and aldehyde-based solvents such as acetaldehyde, and the like.

Further, in preparing the water-based emulsion (water-based dispersion liquid), a dispersant or the like may be used for the purpose of improving the dispersibility of the dispersoid. Examples of such a dispersant include: inorganic dispersants such as tricalcium phosphate, and the like; nonionic organic dispersants such as polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycol, and the like; anionic organic dispersants such as tristearic acid metal salts (e.g., aluminum salts), distearic acid metal salts (e.g., aluminum salts and barium salts), stearic acid metal salts (e.g., calcium salts, lead salts, and zinc salts), linolenic acid metal salts (e.g., cobalt salts, manganese salts, lead salts, and zinc salts), octanoic acid metal salts (e.g., aluminum salts, calcium salts, and cobalt salts), oleic acid metal salts (e.g., calcium salts and cobalt salts), palmitic acid metal salts (e.g., zinc salts), dodecylbenzenesulfonic acid metal salts (e.g., sodium salts), naphthenic acid metal salts (e.g., calcium salts, cobalt salts, manganese salts, lead salts, and zinc salts), resin acid metal salts (e.g., calcium salts, cobalt salts, manganese salts, lead salts, and zinc salts), polyacrylic acid metal salts (e.g., sodium salts), polymethacrylic acid metal salts (e.g., sodium salts), polymaleic acid metal salts (e.g., sodium salts), metal salts of acrylic acid-maleic acid copolymers (e.g., sodium salts), polystyrenesulfonic acid metal salts (e.g., sodium salts); and cationic organic dispersants such as quaternary ammonium salts (e.g., dodecyltrimethylammonium chloride); and the like. By using the dispersant as described above in preparing the water-based emulsion, it is possible to improve the dispersibility of the dispersoid. Further, it is also possible to make variations in shape and size of the dispersoid in the water-based emulsion particularly small relatively easily, and also possible to make the shape of each particle of the dispersoid roughly spherical shape. With these results, it is possible to obtain a liquid developer which is comprised of toner particles each formed into a roughly spherical shape and having uniform shape and size. Further, it is possible to make storage stability of the water-based emulsion particularly excellent by using the above-mentioned dispersant in the preparation of the water-based emulsion.

It is preferred that the solution is mixed with the water-based liquid while at least either the solution or the water-based liquid is being stirred. This makes it possible to obtain an emulsion (a water-based emulsion) in which a dispersoid having small variations in size and shape is homogeneously dispersed easily and reliably.

Examples of methods for mixing the solution and the water-based liquid include a method in which the solution is added (for example, dropped) into the water-based liquid contained in a container, a method in which the water-based liquid is added (for example, dropped) into the solution contained in a container, and the like. In these methods, the water-based liquid or the solution which is contained in a container is preferably being stirred. This makes it possible to exhibit the above effect more conspicuously.

The amount of the dispersoid in the water-based emulsion is not particularly limited, but preferably in the range of 5 to 55 wt%, and more preferably in the range of 10 to 50 wt%. This makes it possible to prevent undesirable bonding or aggregation of particles of the dispersoid more reliably, thereby enabling to make productivity of the toner particles (liquid developer) particularly superior.

The average particle diameter (size) of the particles of the dispersoid in the water-based emulsion is not particularly limited, but preferably in the range of 0.01 to 1. 0 µm, and more preferably in the range of 0.05 to 0.5 µm. This makes it possible to prevent undesirable bonding or aggregation of particles of the dispersoid in the water-based emulsion more reliably, thereby enabling to make the size of the toner particles finally obtained optimum. In this regard, it is to be noted that the term "average particle diameter" means an average particle diameter based on a predetermined volume of particles.

Further, although the above description was made with regard to the case that the components of the kneaded material are contained in the dispersoid in the water-based emulsion, a part of the components of the kneaded material may be contained in the dispersion medium.

Furthermore, the water-based emulsion may contain additional components other than the above-mentioned components. Examples of such additional components include a charge control agent, magnetic powder and the like.

Example of the charge control agent include metal salts of benzoic acid, metal salts of salicylic acid, metal salts of alkyl salicylic acid, metal salts of catechol, metal-containing bisazo dyes, nigrosine dyes, tetraphenylborate derivatives, quaternary ammonium salts, alkyl pyridinium salts, chlorinated polyesters, nitrohumic acid, and the like.

Examples of the magnetic powders include powders of metal oxides such as magnetite, maghemite, various ferrites, cupric oxide, nickel oxide, zinc oxide, zirconium oxide, titanium oxide, magnesium oxide, and the like, and powders of magnetic materials containing magnetic metals such as Fe, Co, and Ni.

The water-based emulsion may further contain, for example, zinc stearate, zinc oxide, or cerium oxide, in addition to the above-mentioned materials.

The thus obtained water-based emulsion may be used as a spray liquid for producing toner particles as it is. However, in the present embodiment, a water-based suspension 3 comprised of a dispersion medium (water-based dispersion medium) 32 and a solid state dispersoid 31 dispersed in the dispersion medium 32 is obtained based on the water-based emulsion (in which the liquid state dispersoid is dispersed in the water-based dispersion medium), and the thus obtained water-based suspension is used as a spray liquid for producing toner particles. This makes it possible to prevent undesirable aggregation between particles of the dispersoid or between the toner particles more effectively, and as a result thereof, the uniformity in the shape and size of the toner particles can be made especially excellent. Further, a deaerating treatment can be carried in addition to the removal of the solvent, which means that it is possible to prevent formation of toner particles having irregular shape more effectively. Furthermore, since the water-based dispersion medium (water) can enter the inside of particles of the dispersoid effectively when removing the solvent, it is possible to obtain toner particles having appropriate water content (moisture content).

Hereinbelow, a detailed description will be made with regard to a method for preparing the water-based suspension 3.

The water-based suspension 3 can be prepared by removing the solvent which constitutes the disperoid from the water-based emulsion.

The removal of the solvent can be carried out, for example, by heating or warming the water-based emulsion or placing it in an atmosphere under reduced pressure. However, it is preferred that the water-based emulsion is heated under reduced pressure. This makes it possible to obtain a water-based suspension 3 containing a dispersoid 31 having particularly small variations in size and shape thereof relatively easily. Further, by removing the solvent as described above, it is possible to carry out a deaerating treatment in addition to the removal of the solvent. By the deaerating treatment, it is possible to reduce the amount of the dissolved air in the water-based suspension 3, and therefore when the dispersion medium 32 is removed from the droplets 5 of the water-based suspension 3 in the water-based dispersion medium removal section M3 of the liquid developer producing apparatus M1, it is possible to prevent generation of air bubble in the water-based suspension 3 in an effective manner. As a result, it is possible to prevent toner particles having irregular shapes (such as void particles and defect particles) from entering (or being mixed into) a finally obtained liquid developer effectively.

When the water-based emulsion is heated (or warmed), the heating temperature is preferably in the range of 30 to 110°C, and more preferably in the range of 40 to 100°C. If the heating temperature is set to a value within the above range, it is possible to remove the solvent immediately while preventing generation of a dispersoid 31 having irregular shapes effectively (that is, preventing rapid vaporization (boiling) of a solvent from the inside of the dispersoid of the water-based emulsion).

Further, when the water-based emulsion is placed in an atmosphere under reduced pressure, the pressure of the atmosphere in which the water-based emulsion is placed is preferably in the range of 0.1 to 50 kPa, and more preferably in the range of 0.5 to 5 kPa. If the pressure of the atmosphere in which the water-based emulsion is placed is within the above range, it is possible to remove the solvent immediately while preventing generation of a dispersoid 31 having irregular shapes effectively (that is, preventing rapid vaporization (boiling) of a solvent from the inside of the dispersoid of the water-based emulsion).

In this regard, it should be noted that it is sufficient that the removal of the solvent is carried out to the extent that at least the dispersoid is transformed into a solid state. It is not necessary to remove substantially all the solvent contained in the water-based emulsion.

The average particle diameter (size) of particles of the dispersoid 31 contained in the water-based suspension 3 is not limited to any specific value, but preferably in the range of 0.01 to 1.0 µm, and more preferably in the range of 0.05 to 0.5 µm. This makes it possible to prevent undesirable bonding (aggregation) of the particles of the dispersoid reliably, thereby enabling the size of finally obtained toner particles to be optimum size and roundness.

Next, the water-based suspension (water-based dispersion liquid) 3 is sprayed in the form of droplets 5. By spraying the water-based suspension 3, the dispersion medium (water-based dispersion medium) 32 is removed from the water-based suspension 3 (droplets 5), a plurality of particles of the dispersoid 31 contained in each of droplets 5 are aggregated to thereby form toner particles 8, and the thus formed toner particles 8 are directly dispersed into an insulation liquid 9 (water-based dispersion liquid spraying step). By this process, a liquid developer 10 in which the toner particles 8 are dispersed in the insulation liquid 9 is obtained. Further, since the dispersion medium in the dispersion liquid used as a spray liquid is constituted from a water-based liquid, it is possible to obtain a liquid developer by a method which is harmless to the environment.

The spray of the water-based suspension (water-based dispersion liquid) may be carried out by any methods, but preferably carried out by intermittently ejecting droplets of the water-based suspension. This makes it possible to carry out the removal of the water-based dispersion medium efficiently while preventing undesirable aggregation of the dispersoid effectively, whereby the productivity of the liquid developer is improved. Further, since the removal of the water-based dispersion medium is carried out by intermittently ejecting droplets of the water-based suspension, even in the case where a part of the solvent remains in preparing the water-based suspension, it is possible to remove the remaining solvent together with the water-based dispersion medium in an effective manner.

In particular, in the present embodiment, the removal of the water-based dispersion medium is carried out using a liquid developer production apparatus as shown in Figs. 2 and 3.

As shown in Fig. 2, the liquid developer production apparatus M1 includes head portions M2 for intermittently ejecting the water-based suspension (water-based dispersion liquid) 3 in the form of droplets 5 as described above, a water-based suspension supply portion (water-based dispersion liquid supply portion) M4 for supplying the water-based suspension 3 to the head portions M2, a dispersion medium removal portion M3 in which the dispersion medium is removed while the water-based suspension 3 (droplets 5) in the form of droplets (fine particles) ejected from the head portions M2 is being conveyed, thereby to obtain toner particles 8 and an insulation liquid storage portion M5 for storing the insulation liquid 9.

The water-based suspension supply portion M4 is not particularly limited as long as it has the function of supplying the water-based suspension 3 to the head portions M2. The water-based suspension supply portion M4 may be provided with a stirring means M41 for stirring the water-based suspension 3 as shown in Fig. 2. By providing such a stirring means M41, even in the case where the dispersoid 31 is hard to be dispersed in the dispersion medium (water-based dispersion medium) 32, it is possible to supply the water-based suspension 3 which is in a state that the dispersoid 31 is sufficiently homogeneously dispersed in the dispersion medium to the head portions M2.

Each of the head portions M2 has a function of ejecting the water-based suspension 3 in the form of fine droplets (fine particles) 5. Further, each of the head portions M2 has a dispersion liquid storage portion M21, a piezoelectric device (element) M22, and an ejection port (nozzle) M23.

In the dispersion liquid storage portion M21, the water-based suspension 3 is stored. The water-based suspension 3 stored in the dispersion liquid storage portion M21 is ejected from the ejection port M23 in the form of droplets 5 into the dispersion medium removal portion M3 when a pressure pulse (piezoelectric pulse) is applied by the piezoelectric device M22.

The feature of the present invention resides in the use of the dispersion liquid as an ejection liquid (spray liquid). By using the dispersion liquid as an ejection liquid, it is possible to obtain the following effects.

Specifically, by using the dispersion liquid as an ejection liquid, at the time when the ejection liquid (dispersion liquid) is ejected through the ejection port, the dispersion liquid is selectively cut somewhere in the dispersion medium having a low viscosity from a microscopic standpoint, so that the dispersion liquid is ejected in the form of droplets. Therefore, the ejected droplets of the dispersion liquid have small variations in size. As a result, the formed toner particles also have small variations in particle size.

Then, the droplets ejected through the ejection portion are quickly formed into a spherical shape due to the surface tension of the dispersion medium. Further, each of droplets formed from the dispersion liquid contains a plurality of particles of the dispersoid. Therefore, since such droplets have excellent shape stability, they are conveyed in the dispersion medium removal portion while maintaining a substantially spherical shape as a whole, to thereby form toner particles. As a result, the formed toner particles have a high degree of roundness and small variations in particle shape.

On the other hand, in the case where a solution or a molten liquid is used as an ejection liquid, such effects as described above cannot be obtained. Specifically, such an ejection liquid has a uniform viscosity when microscopically observed, and therefore the ejection portion tends to have difficulty in cutting the ejection liquid when ejecting (spraying) the ejection liquid. As a result, each droplet of the ejection liquid tends to have an elongated tail. Therefore, in the case where a solution or a molten liquid is used as an ejection liquid (spray liquid), the formed toner particles tend to have large variations in particle size and shape, and a low degree of roundness.

Further, by using the dispersion liquid as an ejection liquid, even if the particle size of toner particles to be formed is sufficiently small, it is possible to make the roundness of the toner particles sufficiently large and make the particle size distribution of the toner particles sharp easily. This makes it possible to obtain toner particles having uniform charge properties and form a thin layer having uniform thickness and high density on a development roller, in the case where the toner particles are used for printing. As a result, it is possible to prevent defects such as photographic fog and the like from occurring, and as a result, it is possible to form a sharper image.

The shape of the ejection portion M23 is not particularly limited, but preferably it is formed into a substantially circular shape. By forming the ejection portion M23 into such a shape, it is possible to raise sphericity of the ejected water-based suspension 3 and the toner particle 8 formed in the dispersion medium removal portion M3.

When the ejection portion M23 has such a substantially circular shape, the diameter thereof (that is, nozzle diameter) is preferably in the range of 0.5 to 100 µm, more preferably in the range of 0.8 to 50 µm, and even more preferably in the range of 0.8 to 15 µm. If the diameter of the ejection portion M23 is less than the above lower limit value, clogging is likely to occur and therefore there is a case that variations in the size of the droplets 5 to be ejected become larger. On the other hand, if the diameter of the ejection portion M23 exceeds the above upper limit value, there is a possibility that the water-based suspension 3 (droplets 5) to be ejected contains air bubbles inside thereof depending on the relative power balance between the negative pressure of the dispersion liquid storage portion M21 and the surface tension of the nozzle.

Further, it is preferred that the a portion in the vicinity of the ejection portion M23 of each head portion M2 (that is, an inner surface of the nozzle aperture of each ejection portion M23 and a surface of the head portions M2 in which the ejection portions M23 are provided (the lower surface in the drawing)) has a liquid repellency (water repellency). This makes it possible to prevent the water-based suspension 3 from adhering around the ejection portion effectively. As a result, it is possible to prevent a poor formation of droplets and occurrence of defective ejection of the water-based suspension 3. Further, since adhering of the water-based suspension 3 around the ejection portion is prevented effectively, the shape stability of the droplets to be ejected is improved (variations in shape and size of the respective droplets are made small), and thus variations in shape and size of toner particles to be finally obtained can also be made small.

Examples of a material having such a liquid repellency include fluoro-based resins such as polytetrafluoroetylene (PTFE) and silicone-based materials.

As shown in Fig. 3, each of the piezoelectric devices M22 is formed by laminating a lower electrode (a first electrode) M221, a piezoelectric element M222, and an upper electrode (a second electrode) M223 in this order from the bottom side. In other words, each of the piezoelectric devices M22 has a structure in which the piezoelectric element M222 is provided between the upper electrode M223 and the lower electrode M221.

The piezoelectric device M22 functions as a vibration source, and the diaphragm M24 is vibrated by the piezoelectric device (vibration source) M22 to instantaneously increase the internal pressure of the dispersion liquid storage portion M21.

In particular, in each of the head portions M2, the piezoelectric element M222 keeps its original shape in a state where a predetermined eject signal from a piezoelectric device driving circuit (not shown in the drawings) is not inputted, that is, in a state where a voltage is not applied across the lower electrode M221 and the upper electrode M223 of the piezoelectric device M22. At this time, since the diaphragm M24 also keeps its original shape, the volume of the dispersion liquid storage portion M21 is not changed. That is, the water-based suspension 3 is not ejected through the ejection portion M23.

On the other hand, the piezoelectric element M222 changes its shape when a predetermined eject signal from the piezoelectric device driving circuit is inputted, that is, when a predetermined voltage is applied across the lower electrode M221 and the upper electrode M223 of the piezoelectric device M22. As a result, the diaphragm M24 is significantly bent (toward the lower side in Fig. 3), so that the volume of the dispersion liquid storage portion M21 is reduced (changed). At this time, the pressure in the dispersion liquid storage portion M21 is instantaneously increased, so that the water-based suspension 3 is ejected in the form of droplets through the ejection portion M23.

When single ejection of the water-based suspension 3 is finished, namely one droplet is formed, the piezoelectric device driving circuit stops a voltage from being applied across the lower electrode M221 and the upper electrode M223. As a result, the piezoelectric device M22 is returned to its almost original shape so that the volume of the dispersion liquid storage portion M21 is increased. At this time, since pressure is exerted on the water-based suspension 3 in the direction from the water-based suspension supply portion M4 to the ejection portion M23 (that is, in the positive direction), it is possible to prevent air from entering the dispersion liquid storage portion M21 through the ejection portion M23. Then, the water-based suspension 3 in an amount equal to the ejected amount thereof is supplied to the dispersion liquid storage portion M21 from the water-based suspension supply portion M4.

By carrying out predetermined periodic application of a voltage in such a manner as described above, the water-based suspension 3 in the form of a droplet is repeatedly ejected due to vibration of the piezoelectric device M22.

As described above, by carrying out ejection (discharge) of the water-based suspension 3 by the use of a pressure pulse due to vibration of the piezoelectric element M222, it is possible to eject the water-based suspension 3 intermittently drop by drop with the shape of each droplet 5 being stable. As a result, it is possible to make variations in shape and size of respective toner particles extremely small, thereby enabling to produce toner particles having high sphericity (a shape close to a geometrically perfect spherical shape) relatively easily.

Further, by ejecting the dispersion liquid (water-based dispersion liquid) by the use of vibration of the piezoelectric element, it is possible to eject the dispersion liquid at predetermined intervals more reliably. This makes it possible to effectively prevent collision or aggregation between the ejected droplets 5 of the dispersion liquid, thus resulting in preventing formation of defective toner particles 8 effectively.

The initial velocity of the water-based suspension 3 (droplets 5) at the time when the water-based suspension 3 is ejected from the head portions M2 into the dispersion medium removal portion M3 is preferably in the range of, for example, 0.1 to 10 m/sec, more preferably in the range of 2 to 8 m/sec. If the initial velocity of the water-based suspension 3 is less than the above lower limit value, productivity of toner particles is lowered. On the other hand, the initial velocity of the water-based suspension 3 exceeds the above upper limit value, the finally obtained toner particles tend to have a lower degree of sphericity.

The viscosity of the water-based suspension (water-based dispersion liquid) 3 ejected from the head portions M2 is not limited to any specific value, but is preferably in the range of, for example, 0.5 to 200 (mPa·s), and more preferably in the range of 1 to 25 (mPa·s). If the viscosity of the water-based suspension 3 is less than the above lower limit value, it is difficult to control the size of each droplet of the water-based suspension to be ejected properly, thus resulting in a case where the finally obtained toner particles have large variations in size. On the other hand, if the viscosity of the water-based suspension 3 exceeds the above upper limit value, there is a tendency that each of the formed droplets has a larger diameter, the ejecting velocity of the water-based suspension 3 becomes low, and the amount of energy required to eject the water-based suspension 3 becomes large. In a case where the viscosity of the water-based suspension 3 is especially high, it is impossible to eject the water-based suspension 3 in the form of droplets.

The water-based suspension (water-based dispersion liquid) 3 to be ejected from the head portions M2 may be cooled in advance. By cooling the water-based suspension 3 in such a manner, it is possible to prevent undesirable evaporation (volatilization) of the dispersion medium 32 from the water-based suspension 3 at the vicinity of the ejection portions M23 effectively. As a result, it is possible to prevent changes in the ejected amount of the water-based suspension 3 which are caused by the fact that the diameter of each ejection portion is reduced with the elapse of time, thereby enabling to obtain toner particles having small variations in shape and size of respective particles.

Further, the average particle diameter (size) of the droplets 5 ejected from the head portions M2 also varies depending on the content of the dispersoid 31 in the water-based suspension (water-based dispersion liquid) 3, but is preferably in the range of 1.0 to 100 µm, more preferably in the range of 1.0 to 50 µm, and even more preferably in the range of 1.0 to 30 µm. By setting the average particle diameter of the droplets 5 of the water-based suspension 3 to a value within the above range, it is possible to obtain toner particles 8 each having an appropriate particle diameter.

In the meantime, in general, each droplet 5 ejected (sprayed) from the head portions M2 has enough large size as compared to that of each particle of the dispersoid 31 contained in the water-based suspension (water-based dispersion liquid) 3. That is, a plurality of particles of the dispersoid 31 are dispersed (contained) in each droplet 5. Therefore, even in the case where the particles of the dispersoid 31 have relatively large variations in particle diameter, the ratio of the dispersoid 31 contained in the ejected droplets 5 is substantially uniform. For this reason, by allowing the ejected amount of the droplets 5 to be substantially uniform, it is possible to obtain toner particles 8 having small variations in particle diameter, even in the case where the particles of the dispersoid 31 have relatively large variations in particle diameter. Such a tendency becomes more conspicuous, by satisfying the following relation. Namely, when the average particle diameter of the droplets 5 is defined as Dd (µm), and the average particle diameter of the particles of the dispersoid 31 contained in the water-based dispersion liquid 3 is defined as Dm (µm), it is preferred that the relation of Dm/Dd < 0.5 is satisfied and more preferred that the relation of Dm/Dd < 0.2 is satisfied.

Further, when the average particle diameter of the droplets 5 is defined as Dd (µm), and the average particle diameter of the produced toner particles 8 is defined as Dt (µm), it is preferred that the relation of 0.05 ≤ Dt/Dd ≤ 1.0 is satisfied and more preferred that the relation of 0.1 ≤ Dt/Dd ≤ 0.8 is satisfied. By allowing Dd and Dt to satisfy the above relation, it is possible to relatively easily to obtain very fine toner particles 8 having a high degree of roundness and sharp particle size distribution.

The frequency of the piezoelectric device M22 (the frequency of an piezoelectric pulse) is not limited to any specific value, but is preferably in the range of 1 kHz to 500 MHz, and more preferably in the range of 5 kHz to 200 MHz. If the frequency of the piezoelectric device M22 is less than the above lower limit value, productivity of toner particles is lowered. On the other hand, if the frequency of the piezoelectric device M22 exceeds the above upper limit value, there is a possibility that the ejection of the water-based suspension 3 cannot follow the frequency of the piezoelectric device M22 so that the sizes of the droplets of the water-based suspension 3 become different from each other. As a result, there is a possibility that toner particles 8 finally obtained have large variations in their size.

The liquid developer production apparatus M1 shown in Fig. 1 is provided with a plurality of head portions M2. From each of the head portions M2, a water-based suspension 3 in the form of droplets (droplets 5) is ejected to the dispersion medium removal portion M3.

The water-based suspension 3 may be ejected at substantially the same time from all the head portions M2, but it is preferred that the water-based suspension 3 is ejected in such a manner that the timing of ejection is different in at least two adjacent head portions M2. This makes it possible to prevent collision and undesirable aggregation effectively between the water-based suspension 3 in the form of droplets 5, namely between the droplets 5 ejected from the adjacent head portions M2, before the toner particles 8 are formed.

Further, as shown in Fig. 2, the liquid developer production apparatus M1 has a gas stream supply means M10, and the gas stream supply means M10 is adapted to inject gas at a substantially even pressure through a duct M101 from each of the gas injection openings M7 provided between the adjacent head portions M2. This makes it possible to convey the droplets 5 of the water-based suspension 3 intermittently ejected from the ejection portions M23 with the distance between the droplets 5 being maintained, thereby enabling to prevent collision and aggregation between the droplets effectively to obtain toner particles 8. As a result, it is also possible to obtain toner particles 8 having small variations in their size and shape.

Further, by injecting gas supplied from the gas stream supply means M10 through the gas injection openings M7, it is possible to form an air stream flowing in substantially one direction (that is, in a downward direction in Fig. 2) in the dispersion medium removal portion M3. Such a gas stream makes it possible to efficiently convey the toner particles 8 produced in the dispersion medium removal portion M3. As a result, collection efficiency of the toner particles 8 is improved, and thus productivity of a liquid developer is also improved.

Furthermore, by injecting gas through the gas injection openings M7, an air flow curtain is formed between the droplets 5 ejected from the adjacent head portions M2. Such an air curtain makes it possible to prevent collision and aggregation between the droplets more effectively.

The gas stream supply means M10 is equipped with a heat exchanger M11. By providing such a heat exchanger M11, it is possible to set the temperature of gas to be injected from the gas injection openings M7 to an appropriate value, thereby enabling to efficiently remove the dispersion medium 32 from the water-based suspension 3 in the form of droplets which have been ejected into the dispersion medium removal portion M3.

Further, by providing such a gas stream supply means M10, it is possible to control the dispersion medium removal rate for removing the dispersion medium 32 from the droplets of the water-based suspension 3 ejected from the ejection portions M23 easily by adjusting the amount of a gas stream to be supplied.

The temperature of gas to be injected from the gas injection openings M7 varies depending on the compositions of the dispersoid 31 and the dispersion medium 32 contained in the water-based suspension (water-based dispersion liquid) 3, but is preferably in the range of 0 to 70°C, and more preferably in the range of 15 to 60°C. By setting the temperature of gas to be injected from the gas injection openings M7 to a value within the above range, it is possible to remove the dispersion medium 32 effectively from the droplets 5 while maintaining shape uniformity and shape stability of the obtained toner particles 8 at a sufficiently high level.

The humidity of gas to be injected from the gas injection openings M7 is preferably 50 %RH or less, and more preferably 30 %RH or less. By setting the humidity of gas to be injected from the gas injection openings M7 to 50 %RH or less, it is possible to remove the dispersion medium 32 contained in the water-based suspension 3 efficiently in the dispersion medium removal portion M3, thereby further improving the productivity of the toner particles 8.

The dispersion medium removal portion M3 is constructed from a tubular housing M31. In order to maintain the inside of the dispersion medium removal portion M3 at a temperature within a predetermined range, a heat source or a cooling source may be provided inside or outside the housing M31, or the housing M31 may be formed as a jacket having a passage of a heat medium or a cooling medium.

In the liquid developer production apparatus shown in Fig. 2, the pressure inside the housing M31 is adapted to be adjusted by a pressure controlling means M12. By adjusting the pressure inside the housing M31, it is possible to produce the toner particles 8 more effectively, and as a result, productivity of a liquid developer is improved. Further, in the structure shown in the drawing, the pressure controlling means M12 is connected to the housing M31 through a connecting pipe M121. Further, an enlarged diameter portion M122 is formed in the vicinity of the end portion of the connecting pipe M121 at a side which is connected to the housing M31, and a filter M123 for preventing the toner particles 8 and the like from being sucked into the pressure controlling means M12 is provided in the end of the enlarged diameter portion M122.

The pressure inside the housing M31 is not limited to any specific value, but is preferably 150 kPa or less, more preferably in the range of 100 to 120 kPa, and even more preferably in the range of 100 to 110 kPa. By setting the pressure inside the housing M31 to a value within the above range, it is possible to prevent effectively the dispersion medium 32 from being removed rapidly from the droplets 5 (that is, boiling phenomenon of the droplets 5). As a result, it is possible to produce the toner particles 8 effectively while preventing formation of defective toner particles 8 reliably. In this connection, it is to be noted that the pressure inside the housing M31 may be substantially the same or different from each other at various positions thereof.

Further, a voltage apply means M8 for applying a voltage to the inner surface of the housing M31 is connected to the housing M31. By applying a voltage of the same polarity as the toner particles 8 (droplets 5) to the inner surface of the housing M31 by the use of the voltage apply means M8, it is possible to obtain such effects as described below.

Generally, the toner particles 8 are positively or negatively charged. Therefore, when there is any charged matter of polarity opposite to that of the toner particles 8, the phenomenon in which the toner particles 8 are electrostatically attracted and adhere to the charged matter occurs. On the other hand, when there is any charged matter of the same polarity as that of the toner particles 8, the charged matter repels each another, thereby effectively preventing the phenomenon in which the toner particles 8 adhere to the surface of the charged matter. For this reason, by applying a voltage of the same polarity as that of the toner particles 8 to the side of the inner surface of the housing M31, it is possible to prevent effectively the toner particles 8 from adhering to the inner surface of the housing M31. As a result, it is also possible to prevent effectively the formation of defective toner particles 8 as well as to improve the collection efficiency of the toner particles 8.

The housing M31 further includes an expanded-diameter portion M311 in the bottom portion thereof. In the expanded-diameter portion M311, the inner diameter thereof is expanded toward the lower side in Fig. 2. By providing such an expanded-diameter portion M311, it is possible to prevent the toner particles 8 from adhering to the inner surface of the liquid developer production apparatus M1 (in particular, the inner surface of the housing M31 or the insulation liquid storage section M5) more effectively. As a result, it is possible to increase production efficiency of the liquid developer 10 as well as to prevent defective toner particles from being mixed into the liquid developer 10 effectively, whereby enabling to improve reliability of the liquid developer 10.

Normally, the respective toner particles 8 formed in the dispersion medium removal portion M3 (housing M31) as described above are obtained as an aggregate of a plurality of particles of the dispersoid 31 contained in each droplet 5. This makes it possible to make variations in shape and size of respective toner particles small as well as to make variations in properties of respective toner particles small, even in the case where variations in shape and size of the particles of the dispersoid contained in the water-based dispersion liquid (water-based suspension) are relatively large. As a result, the liquid developer can have high reliability as a whole.

Further, as described in the above, the toner particles 8 are produced using the water-based dispersion liquid (water-based emulsion and water based suspension) which contains the dispersion medium constructed from the water-based liquid. Water constituting the water-based liquid has a relatively high boiling point and relatively low vapor pressure at around room temperature among various types of liquids. Therefore, the toner particles 8 formed in the dispersion medium removal portion M3 (housing M31) contain predetermined amount of water while maintaining sufficient shape stability. In addition, the inventors discovered that the toner particles containing predetermined amount of water as above have excellent fixing properties to recording mediums such as a paper and the like. This is because of the reasons described below.

Namely, since an insulation liquid (carrier) which constitutes a liquid developer needs to have insulation properties and low dielectric constant, it is normally constructed from molecule having no functional group of high polarity. On the other hand, recording mediums such as a paper and the like used for image forming with a liquid developer is normally constructed from a material having hydrophilic functional group (e.g., hydroxyl group) such as cellulose. Therefore, in a conventional liquid developer, if an insulation liquid remains in surfaces of toner particles, the insulation liquid impairs fixing properties of the toner particles (adhesion between the toner particles and the recording medium). On the other hand, in the liquid developer of the present invention, since the toner particles contain predetermined amount of water, the water contained in the toner particles exhibits a function of improving adhesion between the toner particles and the recording medium, whereby resulting in excellent fixing properties of the toner particles.

Further, in the present invention, since each toner particle is obtained as an aggregate of a plurality of particles of the dispersoid, an adequate amount of water can be reliably retained in spaces which are formed between the particles of the dispersoid constituting each toner particle. In this way, since in the liquid developer (unfixed toner particles) the water is reliably retained, leakage of the water outside the toner particles is prevented effectively. On the other hand, the toner particles can extrude the water effectively due to pressure applied during fixing process, so that adhesion of the toner particles (toner image) against a recording medium can be made excellent.

The toner particles 8 are not particularly limited to any specific one as long as the respective toner particles 8 contains predetermined amount of water therein, but it preferably contains water more than the amount of water absorption of a resin material which constitutes the toner particles 8. According to this, fixing properties of the toner particles 8 to a recording medium can be made particularly excellent. In this regard, the term "amount of water absorption" in the present invention means the largest amount of water which can be contained in the toner material (resin material) itself and it does not include the amount of water adsorption (amount of water adsorbed in the surface of the resin material by functional group).

Further, the water content of the toner particles 8 is not particularly limited to any specific value, but is preferably in the range of 0.3 to 5.0 wt%, more preferably in the range of 0.5 to 2.5 wt%, and even more preferably in the range of 0.5 to 2.0 wt%. When the water content of the toner particles 8 is set to a value within the above range, it is possible to make charge properties of the toner particles 8 sufficiently good as well as to make fixing properties of the toner particles 8 to a recording medium particularly excellent.

The toner particles 8 produced in the above manner are introduced into the insulation liquid storage portion M5 and mixed with the insulation liquid 9. In this way, a liquid developer 10 comprised of the insulation liquid 9 and the toner particles 8 dispersed in the insulation liquid 9 is obtained. As described in the above, in the present invention, the formed toner particles 8 are directly mixed with the insulation liquid 9 without being collected as fine particles. This makes it possible to prevent aggregation and the like of the toner particles sufficiently as well as to make productivity of the liquid developer excellent.

In the structure shown in the drawing, the insulation liquid storage portion M5 includes a stirring means M51 for stirring the insulation liquid 9. The stirring means M51 enables the toner particles 8 to be dispersed in the insulation liquid 9 homogeneously enough even in a case where a difference of specific gravity between the insulation liquid 9 and the toner particles 8 is relatively large (for example, absolute value of the difference is 0.3 g/cm³ or more). Further, it becomes possible to keep an excellent dispersion state of the toner particles 8 stably for a long period of time in the obtained liquid developer 10. Furthermore, it is possible to prevent effectively the toner particles 8 from being suspended in the vicinity of the surface of the insulation liquid 9 as well as to prevent the aggregation and the like of the toner particles 8 in an efficient manner.

Various liquids may be used as the insulation liquid 9 if the liquids have sufficiently high insulation properties. Specifically, a liquid having an electric resistance of 10⁹ &OHgr;cm or more at room temperature (20°C) is preferably used, more preferably a liquid having an electric resistance of 10¹¹ &OHgr;cm or more is used, and even more preferably a liquid having an electric resistance of 10¹³ &OHgr;cm or more is used.

Further, it is preferred that the insulation liquid 9 has a dielectric constant of 3.5 or less.

Examples of such insulation liquids 9 that satisfy the above conditions include octane, isooctane, decane, isodecane, decaline, nonane, dodecane, isodecane, cyclohexane, cyclooctane, cyclodecane, benzene, toluene, xylene, mesitylene, various types of silicone oils, vegetable oils (e.g., linseed oil, soybean oil), ISOPAR E, ISOPAR G, ISOPAR H, ISOPAR L ("ISOPAR" is a product name of Exxon Mobil Corporation), SHELLSOL 70, SHELLSOL 71 ("SHELLSOL" is a product name of Shell Oil), Amsco OMS, Amsco 460 solvent ("Amsco" is a product name of Spirit Co., Ltd.).

The liquid developer obtained as described above is constituted from toner particles having superior fixing properties to a recording medium. Further, in the liquid developer obtained as described above, variations in shape and size of the toner particles are small. Therefore, in such a liquid developer, toner particles are easy to migrate in the insulation liquid (that is, in the liquid developer), and thus it is advantageous in high speed development. Further, since the toner particles have small variations in their shape and size and thus they have superior dispersibility, so that settle down and floating of the toner particles in the liquid developer are prevented effectively. Therefore, such a liquid developer can keep superior storage stability for a long period of time.

The average particle size (diameter) of the toner particles 8 in the liquid developer 10 obtained as described above is preferably in the range of 0.1 to 5 µm, more preferably in the range of 0.4 to 4 µm, and even more preferably in the range of 0.5 to 3 µm. If the average particle size of the toner particles 8 is within the above range, it is possible to make resolution of a toner image formed from the liquid developer (toner) sufficiently high with small variations in properties of the toner particles 8 such as chargeable properties or fixing properties, and especially high reliability as a whole of a liquid developer 10.

Further, it is preferred that the standard deviation of particle size among the toner particles 8 which constitute the liquid developer 10 is 1.0 µm or less, more preferably in the range of 0.1 to 1.0 µm, and even more preferably in the range of 0.1 to 0.8 µm. When the standard deviation of particle size lies within the above range, variations in charge properties, fixing properties and the like of the toner particles 8 become especially small, thereby further improving the reliability of the liquid developer 10 as a whole.

Further, when the average particle diameter of the particles of the dispersoid 31 contained in the water-based suspension 3 is defined as Dm (µm), and the average particle diameter of the produced toner particles 8 is defined as Dt (µm), it is preferred that the relation of 0.005 ≤ Dm/Dt ≤ 0.5 is satisfied and more preferred that the relation of 0.01 ≤ Dm/Dt ≤ 0.2 is satisfied. By allowing Dm and Dt to satisfy the above relation, it is possible to obtain a liquid developer 10 containing the toner particles 8 having very small variations in particle shape and size.

Furthermore, it is also preferred that an average roundness R represented by the following formula (I) is 0.85 or higher, more preferably in the range of 0.90 to 0.99, and even more preferably 0.95 to 0.99. $$\mathrm{R}={\mathrm{L}}_0/{\mathrm{L}}_1$$

wherein L₁ (µm) represents the circumference of projected image of a toner particle 8 that is a subject of measurement, and L₀ (µm) represents the circumference of a perfect circle (a geometrically perfect circle) having the same area as that of the projected image of the toner particle 8 that is a subject of measurement.

When the average roundness R of the toner particles 8 is within the above range, the transfer efficiency and the mechanical strength of the toner particles 8 can be made excellent while the particle size of the toner particles 8 are made sufficiently small.

In this case, it is preferred that a standard deviation of the roundn