GB2034907A - Magnetic toner for electrostatic photography - Google Patents

Abstract

Magnetic toner particles are provided which are easily charged to a suitable extent and provide images of good density with reduced grey background, even under conditions of high humidity. They comprise a magnetic material, a binder resin and a fluorine-containing material, e.g. a fluorine-containing surface active agent, preferably having a molecular weight of less than 10,000. The fluorine-containing material is present at the surface of the magnetic material.

Description

SPECIFICATION Magnetic toner for electrostatic photography This invention relates to magnetic toner for electrostatic image development.

In electrophotography an electrostatic image is usually formed on an image-bearing support i.e. a photosensitive element and as developers of the so-called two-component system which comprises toner and carrier, and developers so-called single-component system which comprises a magnetic toner containing a magnetic material but free of any independent carrier are used. In the two-component system, toner and carrier are respectively charged to opposite polarities by being stirred and rubbed together, and the charged tonger is brought into contact with the surface of a support bearing thereon an electrostatic image to attach to said surface by static attraction and thereby to develop the electrostatic image.In the case of developer of the single-component system comprising a magnetic toner alone, on the other hand, development of electrostatic image is carried out by a technique known as the magnetic brush process, wherein the magnetic toner is allowed by the action of a magnet to stick brushwise on the surface of a sleeve and then the magnetic toner is brought into contact with the surface of a support bearing an electrostatic image and thus to form an electrostatic image.

The developer of the two-component system, however, have such inherent disadvantages that, because only toner is consumed as the result of repeated development, the concentration of the toner in the developer reduces and, accordingly, in order to carry out development satisfactorily in the long run, the mixing proportion of the toner to the carrier in the developer must be maintained around a certain level by replenishing the developer with the toner.

Developers of the single-component system, on the other hand, do not have such disadvantages because no carrier is contained therein. This developer, however, has such drawbacks that because of relatively low electric capacity and poor acceptability the charge retained in the toner is liable to be lost by leak, in particular, under an atmosphere of high humidity, whereby the maximum image density of the developed toner image tends to decrease.

Moreover, the development using a magnetic toner often has such a drawback that a gray background image is liable to occur when electric resistance of the magnetic toner is relatively high. The reason for this phenomenon may be ascribable to the fact that in the development using magnetic toner, rubbing force produced against the surface of an electrostatic image-bearing support is small and accordingly even when bias voltage is applied to said support with the view of enhancing the rubbing force, the effect obtained is very small.In other words in the development using a developer of the twocomponent system, because rubbing force of a carrier against the surface of eectrostatic image-bearing support is large, the toner particles attached to the area to which said toner particles should not have attached could be removed by the carrier particles, whereas in the case of the magnetic toner, because the rubbing force of magnetic toner brush formed on a sleeve of a magnetic brush in a development unit against the surface of electrostatic image-bearing support is small, removing force of the toner particles attached to the undesirable portion of the support is very small. This is also inferable from the fact that the possibility for a gray background to occur is proportional to the number of rotation of the sleeve.

Electrostatic image is usually formed on an photosensitive element while a residual potential on the surface thereof still exists. In the development using the developer of the two-component system on the one hand this residual potential can substantially be diminished by applying to the electrostatic imagebearing support counter bias voltage through the carrier particles which consist mainly of a conductor such as iron powder. In the development using the magnetic toner, on the other hand, this residual potential cannot easily be diminished by the application of an counter bias voltage because no carrier is present in the developer and the toner particles are composed mainly of insulators (having a resistance of at least 1 013 cm), with the result that the toner particles tend to remain on the surface of electrostatic image-bearing support.

Further, fluidity of the insulating magnetic toner is generally poor and on this account, for example, cleaning characteristics of the surface of electrostatic image-bearing support is poor, and thus such an undesirable phenomenon that toner particles remaining on a master sheet cannot sufficiently be removed after transfer of the developed toner image to a transfer sheet, whereby the thus remaining toner particles cause staining on the surface of a photoconductor or formation of gray background on the transfer sheet.

An object of the present invention is to provide a magnetic toner for electrostatic image development, which magnetic toner is easily charged, having high charge quantity and excellent in charge acceptance and is capable of forming a toner image having sufficient maximum image density even under an atmosphere of high humidity.

Another object of present invention is to provide a magnetic toner for electrostatic image development, which magnetic toner is capable of forming a toner image with reduced gray background.

A still further object of the present invention is to provide a magnetic toner for electrostatic image development, which magnetic toner has excellent fluidity and cleaning characteristics.

In order to accomplish the objects a low molecular weight fluorine-containing compound is incorporated into the aforesaid magnetic toner particles, in particular, by previously allowing said compound to stick on the fine particles of the magnetic material. That is, the magnetic toner of the present invention is prepared in the usual way by the use of fine particles of a magnetic material, wherein the fine particles of the magnetic material are coated with the aforesaid low molecular weight fluorine-containing compound by immersing said fine particles in a solution which contains said fluorine-containing compound or spraying said fine particles with said solution.As a solvent for the fluorine-containing solvent, for example, water, methanol, aceton, iso-propylalcohol, iso-propylether, toluen, perchloroethylene, benzene, heptane or carbontetrachloride car be mentioned and these solvents may be used either singly or in combination.

In the present invention, a variety of fluorine-containing compounds can be used, and particularly fluorine-containing surface active agents are advantageously employed. Such fluorine-containing compounds are considered to have a sufficient affinity to the fine particles of the magnetic material as well as for binder resins.

In addition to such fluorine-containing surface active agents as referred to above, various fluorine containing compounds of different types may also used in the present invention, but low molecular weight compounds having a molecular weight of less than 10000 are preferable. The compounds having excessively high in molecular weight become less soluble in almost all organic solvents and thus there is a practical difficulty and the adhesion attained between such compound and the magnetic material during the aforesaid processing step is not very strong since functional groups of such high polymers are bound and blockaded during polymerization.Accordingly, in the step of using the developer prepared by the use of such high molecular weight compound, there is involved such drawback that the coatings of said compound formed on the magnetic material tend to peel off and cause changes in performance of said developer. Preferably, these low molecular weight fluorine containing compounds in the present invention are used in a quantity of 0.01 to 1.0 percent by weight based on the fine particle of magnetic material.

Thus according to the present invention magnetic toner is incorporated with a low molecular weight fluorine-containing compound in the aforementioned manner and when the present magnetic toner is used as is clear from the examples mentioned later, a toner image having sufficiently high maximum image density and reduced fog can be obtained even under the atmosphere of high humidity and cleaning characteristics of the surface of electrostatic image-bearing support is improved because of high fluidity of the present magnetic toner and accordingly an favorable transfer images can be obtained on a transfer sheet and these excellent characteristics of the present magnetic toner can be maintained for a long period of time when it is used repeatedly.

Although the reasons for such excellent effects of the present invention are -not very clear, it is considered that by incorporation of a low molecular weight fluorine-containing compound into the magnetic toner particles, not only the fine particles of magnetic material are enhanced in their ability of being charged but also, owing to a water repelling action which is inherent to the fluorine-containing compound, the resulting toner image can have sufficiently high maximum density.Furthermore, because of high acceptance the present magnetic toner can attach strongly to the surface of the image bearing support wherein electric charge is present imagewise and; even if the toner attaches to portion where no electric charge is present, the attached toner can be removed by a small force owing to high sliding property enhanced by the fluorine-containing compound contained on the surface of the magnetic toner and, moreover, no adverse effect is brought about on retention force of a magnetic toner brush formed on a magnet brush.For these reasons, the magnetic toner particles attached to an unintended portion can easily be removed by the toner brush with the result that possible formation of gray background can be restrained, and further that high fluidity imparted by the sliding property of the surface of magnetic toner is thought to assure the removal of the attached toner at the time of cleaning the electrostatic image-bearing support surface.

Furthermore, that these preferable actions may be attributed to the low molecular weight fluorine containing compound on the fine magnetic particles is also understood from the fact that a marked effect is exhibited when said fluorine-containing compound is coated on said particles rather than said fluorine-containing compound is homogeneously distributed in a binder resin.

Fluorine-containing surface active agents which are preferably employed in the present invention include, for example, such compounds as CqFgCOOH, C7F15S03Na,,C8F17OCH39 C4F9NC0, CFS03C2H5, C7F15CONH2, C4F9S03NH4, ClOF2lSNa,

In addition thereto, such compounds represented by the following general formulas (I) -- (V), respectively can also preferably be used in the present invention:

Wherein R1 and R4 independently represent an alkyl, aralkyl, aryl or heterocyclic group or a group derived therefrom, and at least one of R, and R4 represents any of said group in which the whole or part of hydrogen atoms have been substituted by fluorine atoms; R2, R3, R6a and R7 independently represent a lower alkyl group:R5 represents a divalent atomic group; R8 represents a group having at least three carbon atoms and fluorine atoms bonded to at least three carbon atoms; A represents a divalent atoms or atomic group represented by -O-, -NR-, -S-, -SO2NR-, -CONR-, -SO3-or-COO (in which R represents an alkyl or aryl group of 1 to 6 carbon atoms);R9 represents a divalent hydrocarbon group; R1O, R11 and R12 are individually an alkyl, aryl or aralkyl group of 1 to 6 carbon atoms, and R10 and R12, or R10, R11 and R12 may form, together with N-atom, a 5- or 6-membered ring; (X) and (Y)- independently represent an anion provided that X and Y may be the same; and n and m independently represent 0 or 1: Typical examples of the compounds represented by the aforesaid general formulas (i) to (V) are shown below with reference to respective types.

-CH2(CF2CF2)3H ] Cl

(V-2) [ C12F230 (CH2CH2O)3CH2CH2N+(CH3)3 ] (v - 3) [ C8F17SO2NHCH2CH2CH2- N+(CH3)3 ] (v - 4)

(v - 7) [ C9F17O (CH2)3N+(CH3)3 ] I

(v - 9) [ C7F15CONH (CH2)3N+(CH3)3 ] Cl

[ C7Fl5Coo(CH2)3N (CH3)3 1 CH3S04

These fluoroine-containing amineimide compounds represented by the aforesaid general formulas can be prepared by known method, and these compounds can be readily converted according to the usual way to acylhydrazinum salts by treatment with hydrochloric acid, hydrobromic acid, hydroiodic acid, peroxyacid, etc.

Furthermore the following fl@orine-containing surface active @@@@ts can also @e used whe@in @ represents an average polymerization degree.

C9F17O(C2H4O)nCH3(n is 1 to 20)

(n is O to 20. and R is an alkyl or substituted alkyl group of 1 to 6 carbon atoms)

C9F17O (C2H40)nCH2cH2N (CH3)3 I ( n is 1 to 20 )

(R is an alkyl or substituted alkyl group of 1 to 6 carbon atoms?

The fluorine-containing surface active agents preferably used in the present invention may have less than 40 fluorine atoms, since in this case solubility of a fluorine-containing surface active agent in organic solvents decreases with the number of fluorine atoms, with the result that usable solvents are limited in number and the use of highly toxic organic solvents may be necessary.

In addition to the fluorine-containing surface active agents mentional hereinbefore, the following fluorine type oiigomer compounds (having a molecular weight of less than 10000) can also be used in the present invention. Usable oligomer compounds include vinyl fluoride, binylidene fluoride, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, perfluoropropylnen, 1,1 dihydroperfluorooctyl methacrylate and polymers thereof, ethylene-tetrafluoroethylene copolymers, vinlidene fluoride-trifluorochloroethylene copolymers, styrene-perfluorobutyi methacrylate copolymers, vinyl fluoride-vinylidene fluoride copolymers, vinylidene fluoride-tetrafluoroethylene copolymers, vinylidene fluoride-hexafluoropropene copolymers, polyfluoroalkyl silane, terpolymer of tetrafluoroethylen-vinylidene fluoride-nonfluoride monomer fluorine-modified epoxy resins, fluorine modified alkyd resins, fluorine-modified poiyester, fluorine-modified melamin resins, fluorine-modified acrylic resins, fluorine-modified dioipolyestes, fluorochemical-modified urethane resins consisting of fluorofatty acid condensates, fluorocarbon-modified acrylate and methacrylate polymers and copolymers thereof, perfluoroalkyl-a-trifluoromethacrylate polymers, fluorocarbon polyethers, polymers derived from fluoroketone, polymers and condensation polymers of vinyl fluoride monomers having functional groups, etc. In the present invention, two or more lower molecular weight fluorine-containing compounds may also be used in combination.

Such fluorine type oligomer compounds may be prepared, for example, by polymerization of monomers with an excess of a polymerization initiator (e.g. benzoyl peroxide, azobisisobutyronitrile, etc.), or by polymerization of monomers with large amounts of a chain transfer agent (e.g carbon tetrachloride, carbon tetrabromide, etc.) and, if necessary, removing a high molecular compound having a molecular weight of more than 10000 by fractionation according to the fractional precipitation process.

Magnetic materials used in the present invention include nickel, cobalt, iron or oxides thereof or ferrite. Magnetite is preferable from a practical point of view. Binder resins used in the present invention may be any one which can conventionally used in the art, including styrene resins, cellulose resins, polyurethane resins, polyether resins, styrene-butadiene resins and mixtures thereof.

The magnetic toner particle according to the present invention may be incorporated with various additives such as coloring agents, charge controlling agents and these additives include carbon black, oil black, nigrosine dyes, Aniline Blue, Chalcoil Blue, Chrome Yellow, Ultramarine Blue, Methylene Blue chloride, Phthalocyanine Blue, Rose Bengale and other pigments or dyes.

The developers comprising the magnetic toners of the present invention can advantageously be applied to development of electrostatic images formed on a photosensitive element such as zinc oxide photosensitive element, selenium photosensitive element and organic photosensitive element.

The present invention is illustrated below with reference to examples. "Part or parts" in the .following examples are expressed in terms of all weight.

EXAMPLE 1 In a 0.5% methanol solution of a cation type fluorine-containing surface active agent "Florad FC-1 34" (a product of 3M Co.) of the chemical formula [C8F,7SO2NHCH2CH2pN+'{CH3)3]1- was immersed 56 parts of particulate magnetite 'BL-500' (a product of Titan Kogyo K.K.).The thus treated magnetite was incorporated with 40 parts af a styrene-acrylic resin (a copolymer of styrene: butyl methacrylate:methyl methacrylate = 5:3:2), 4 parts of an ethylene vinyl acetate resin "Ultrathene 630" (a product of Toyo Soda K.K.), 2 parts of carbon black "MA--100" (a product .of Mitsubishi Kasei K.K.), 3 parts of a polypropylene "Viscol 660P" (a product of Sanyo Kasei K.K.) and 1 part of a dye "Oilblack BS"(product of Orient Kagaku K.K.), and the resulting mixture was mixed and dispersed for 10 hours by means of a ball mill. The mixture thus treated was kneaded with a biaxiai extruder and cooled to solidify to obtain a mass.After having been rough ground, the mass was pulverized further with a jet mill to obtain finely divided particles, followed by heat treatment with a spray drier at an inlet temperature of 3500C. and an outlet temperature of 500C. and further by classification with a zigzag classifier (manufactured by Alpine Co.). There was thus obtained a magnetic toner of the present invention having an average particle diameter of 1 6 tjm. This magnetic toner was incorporated with 0.2% by weight of a hydrophobic silica "R-972" (a product of Nippon Aerosil K.K.) to prepare a sample according to the present invention.

Separately, a magnetic toner having an average particle diameter of 1 5 ,um was prepared in the same manner as above, except that the particulate magnetite was not treated with the fluorine containing surface active agent a comparative sample.

The sample and comparative sample were independently subjected to copying test of 1000 runs using an electrocopying machine "U-Bix 2000R" (manufactured by Konishiroku Photo Industry Co., Ltd.) equipped with a developing vessel for magnetic toner and a zinc oxide photosensitive element under the stomospheres at a normal humidity of 50% RH and at a high humidity of 80% RH. In this test, the temperature of a heat roller fixer was set at 1 800C.

As the result of the above test, clearly transferred and fixed images were obtained to both samples except that the sample of the present invention was far better than the comparative sample with respect to cleaning characteristics and gray background. In the case where the comparative sample was used, a decreasing ratio in maximum image density of the transferred image obtained under the atmosphere of high humidity to that obtained at normal humidity was as large as 32%, whereas the decreasing ratio in the case of the present sample was only 9% and thus the obtained image was found to have a sufficient image density.

Evaluation of the cleaning characteristics was made according to a method wherein an electrostatic image formed on a photosensitive element was developed and the developed image was cleaned out by means of a magnet brush while the image remained untransferred, then a white paper used as an original was subjected to white paper copying operation so that a residual image which had not been removed by cleaning may be transferred to transfer sheet and thereby to judge an extent of formation of the residual image on the transfer sheet.

Separately, using the sample of the present invention, running copying test for 20000 times was conducted using the same copying machine whereupon no practical difficulty was observed though at the final stage of copying the diameter of toner shifted to the side of larger diameter and some roughness was observed in transferred images, and formation of favorable transferred images was possible even when the test was further continued.

EXAMPLE 2 In a 0.1% acetone solution of an anionic fluorine-containing surface active agent "Ftergent" (a product of Neos Co.) of the bhemical formula CgFa70C6H4COONa was immersed 50 parts of a particulate magnetite "EPT-1 000" (a product of Toda Kogyo K.K.). The thus treated particulate magnetite was incorporated with 40 parts of the same styrene-acrylic resin as in Example 1, 1 0 parts of styrene-butadiene resin (styrene:butadiene = 95:5), 1 part of a carbon black "MA-1 00", 4 parts of polypropylene "Viscol 660P" and 2 parts of a dye "Nigrosine Base EX" (a product of Orient Kagaku K.K.), followed by mixing and dispersing for 10 hours with a ball mill.The resulting mixture was melt kneaded for 1 hour with a double roll, followed by the same procedures as in Example 1, to prepare a magnetic toner of the present invention.

Using this magnetic toner, the same copying test as in Example 1 was conducted at normal humidity, whereupon a sharp transferred image having high image density was obtained, and cleaning characteristics was found favorable and practically no gray background was observed. Further, the same copying test as above was conducted but using a moistened transfer sheet, whereupon it was confirmed that the present magnetic toner is high in moisture resistance, though the maximum image density of the transferled image obtained in this test was found to be lower by 13% than that obtained in Example 1.

Separately, a comparative sample of magnetic toner was prepared in the same manner as above except that the particulate mflgnetite was not treated with the fluorine-containing surface active agent, and the comparative sample thus prepared was subjected to the same test. As the result, it was found that in the transferred image obtained, as compared with that obtained by the use of the present magnetic toner, fog formation was observed to a considerable extent and also the cleaning characteristics evaluated was inferior. Further, as the result of the same moisture resistance test it was found that the decreasing ratio in maximum image density was more than 35%.

EXAMPLE 3 To 55 parts of magnetite treated with the same fluorine-containing surface active agent as used in Example 1 for the preparation of sample were added 40 parts of a styrene resin "Piccolastic D-1 25" (a product of Esso Petrochemical Co.), 5 part of the same styrene-butadiene resin as in Example 2. 2 parts of a carbon black "MA-8" (a product of Mitsubishi Kasei K.K.) and 1 part of the same dye as in Example 1, followed by mixing and dispersing for 10 hours with a ball mill. The resulting mixture was melt kneaded for 1 hour with a hot double roll and cooled down to room temperature to solidify. The resulting mass was roughly ground, followed by pulverizing with a jet mill.The finely divided particles thus obtained were subjected to heat treatment with a spray drier in the same manner as in Example 1 and then classified with a zig zag classifier to prepare the present magnetic toner having an average particle diameter of 13 pm.

Using this magnetic toner, copying test was conducted with an electrocopying machine "U-Bix 1500" (manufactured by Konishiroku Photo Industry Co., Ltd.) equipped with a magnetic toner developing vessel, whereupon a sharp transferred image which was favorably heat fixed and had high image density with practically free of a gray background. Further, after completion of copying for 1000 times, it was observed that stain in a master sheet was small and cleaning characteristics evaluated was found favorable. When a moistened transfer sheet was used, the decrease in maximum image density observed in the resulting transferred image was very slight.

On the other hand, a comparative sample of magnetic toner was prepared in the same manner except that the particulate magnetite was not treated with the fluorine-containing surface active agent.

The comparative sample was subjected to the same test whereupon the comparative sample was found to be inferior to the present sample with respect to gray background and cleaning characteristics, and the decrease in maximum image density observed was remarkably large.

EXAMPLE 4 60 parts of the magnetite used in Example 2 was treated with the fluorine-containing surface active agent in the same manner as in Example 1, and thereto were added 40 parts of the same styreneacryl resin and 8 parts of styrene-butadiene resin as in Example 2, 3 parts of a carbon black "Peerless 155" (a product of Columbia Carbon Co.), 4 parts of polypropylene and 0.5 part of a dye "Oilblack BS".

The same procedures as in Example 1 were then repeated to prepare the present magnetic toner having an average particle diameter of 17 ym.

Using this magnetic toner, the same copying test as in Example 1 was conducted, whereupon a sharp transferred image with less gray background and high image density was obtained. it was also observed that when the present magnetic toner is applied to hot roll fixing process, the temperature range within which the transferred image was fixed with no offset phenomenon was broadened and occurrence of stain on the master sheet after running operation could be greatly reduced.

Further, the same test using a moistened transfer sheet as in Example 2 was conducted, whereupon the decreasing ratio of the maximum image density was found to be 1 8%.

On the other hand, a comparative sample of magnetic toner was prepared in the same manner, except that the particulate magnetite was not treated with the fluorine-containing surface active agent, and the comparative sample was subjected to the same tests as above, whereupon it was found that the sample was inferior to the present sample with respect to fog formation and cleaning characteristics.

and the decreasing ratio of the maximum image density of the transferred image on a moistened transfer sheet was as high as 40%.

EXAMPLE 5 An electrostatic iatent image formed on an organic photosensitive element (manufactured by Kalle Co.) provided in an electrocopying machine "U-Bix 2000R" (manufactured by Konishiroku Photo Industry Co., Ltd.) was developed using the sample of the present magnetic toner and the comparative sample, respectively, described in Example 1, and the toner image obtained was transferred and fixed to form a transferred image. Wherein the development had to be carried out by rotating a development vessel in the same rotation direction as in a drum equipped with the organic photosensitive element since said photosensitive element, differently from a zinc oxide photosensitive element, has a smooth surface.

The transferred images thus obtained were compared with each other, whereupon it was found that favorable developability was attained when the sampel of the present magnetic toner was used, for example, a sharp transferred image having high image density as 1.2 with less gray background was obtained by development at ordinary temperature and normal humidity, and even by development under the atmosphere of high temperature and humidity, a transferred image having a maximum image density of about 0.7 and with less density decrease was obtained and, at the same time, a transfer rate decrease was small. It is reported that cleaning characteristics in the organic photosensitive element is generally poor. When the sample of the present magnetic toner was used, however, the cleaning was able to be carried out readily and sufficiently according to the fur brush technique.

On the other hand, in the transferred image developed by the use of the comparative sample, the image density thereof was markedly low, for example, the maximum image density of the transferred image obtained by development at ordinary temperature and normal humidity was 0.7, and that obtained in the case of development at high temperature and humidity was 0,5, and the transferred image was found poor in image quality. Furthermore, cleaning characteristics evaluated was poor and the cleaning was incomplete after several times of copying.

Claims (6)

1. Magnetic toner for electrostatic image development which comprises a magnetic material, a binder resin and a fluorine-containing compound. which is being made present on the surface of said magnetic material.

2. Magnetic toner for electrostatic image development according to claim 1 wherein said fluorinecontaining compound is being made present on the surface of said magnetic material by immersing said magnetic material in a solution containing said fluorine-containing material or by spraying said magnetic material with said fluorine-containing solution.

3. Magnetic toner for electrostatic image development according to claim 2 wherein said fluorinecontaining compound is a fluorine-containing surface active agent.

4. Magnetic toner for electrostatic image development according to claim 2 or 3 wherein said fluorine-containing compound has a molecular weight of less than 1 0000.

5. Magnetic toner for electrostatic image development according to claim 4 wherein said fluorine containing compound being present with respect to said magnetic material in a quantity ranging from 0.01 to 1.0 percent by weight.

6. A magnetic toner according to Claim 1 and substantially as hereinbefore described with reference to any of the Examples.