GB1573414A

GB1573414A - Magnetic onecomponent developer

Info

Publication number: GB1573414A
Application number: GB5324976A
Authority: GB
Original assignee: Elfotec AG
Current assignee: Elfotec AG
Priority date: 1975-12-29
Filing date: 1976-12-21
Publication date: 1980-08-20
Also published as: DE2559018A1; CA1091970A; BE849894A; CH623942A5; FR2337360B1; DE2559018C2; NL7614467A; JPS5283232A; FR2337360A1

Abstract

The invention relates to the structure and composition of a magnetic single-component toner. Electrically conducting constituents of the toner are mainly located inside the toner particles. Even magnetic or magnetisable particles are mainly located inside the toner particles. The toner particles have a surface consisting mainly of an insulating binding means which generally has no conductive constituents. A method for producing such a single-component toner with variants is described. Use in electrophotography.

Description

(54) A MAGNETIC ONE-COMPONENT DEVELOPER (71) We, ELFOTEC AG., a body corporate organised under the laws of Switzerland, of Geissacherstrasse 6, CH-8126 Zumikon Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a magnetic one-component developer and to a process for its production.

So-called two-component developers frequently used in electrophotographic image production are a mixture of magnetic or magnetisable particles and toner particles, a magnetic brush being formed by means of the magnetic particles of the developer mixture. With a magnetic brush such as this, a latent charge image on a photoconductor may be subsequently developed in known manner by means of triboelectrically charged toner particles. Through the release of toner particles from the magentic brush, the mixture gradually becomes deprived of toner, jeopardising the maintenance of a desired image quality unless provision is made for topping up the toner. Unfortunately, this involves considerable difficulties so far as dosage is concerned.

By contrast, a so-called "one-component developer" is not a mixture of magnetic or magnetisable particles and separate, triboelectrically chargeable toner particles, instead each particle of a one-component developer internally contains both magnetic or magnetisable material and also pigment or dye embedded in an insulating binder.

The magnetic one-component developers normally used for electrophotographic image production are of relatively low resistivity, in other words their volume resistivity amounts for example to from 102 to 106 ohm.cm. This low resistivity is attributable to the incorporation of conductive carbon particles in, or their accumulation at, the surface of the developer particles. This low resistivity promotes charging of the developer induced by the charge image on a photoconductor (in contrast to the triboelectric charging of a two-component developer).

In cases where the developed charge image on the photoconductor is directly used as a copy, the above-mentioned low resistivity of the developer generally has no adverse effect upon the image quality obtained. However, in cases where an indirect so-called transfer process is used for transferring the charge image developed on the photo conductor to another image support, this low resistivity is troublesome because it causes both weaker edge definition and also inferior image resolution.

To enable this effect to be understood, the sequence of events taking place in the transfer process is briefly described in the following: After the development of a charge image on the photoconductor of a copier by means of a one-component developer of relatively low resistivity, the charge image is present on the photoconductor with, for example, a positive induced charge. By applying a negative potential to a transfer roller, the one-component developer is transferred to the receiving sheet, for example a sheet of paper, situated between the developer image and the roller for definitive recording of the image.Since the sheet of paper is not highly insulating and since the onecomponent developer is of relatively low resistivity, the one-component developer, where it is present on the surface of the paper, can undergo a reversal of charge, in other words this one-component developer now receives a negative charge on the sheet of paper and, accordingly, begins to migrate in the opposite direction, i.e. back to the photoconductor. This sequence of events is repeated for as long as the developer particles in question are situated within the range of influence of the transfer field. It is pointed out that the charge-reversal process described above takes place more quickly, the lower the resistivity of the onecomponent developer.

Since the migration of the developer back and forth does not take place strictly uniformly, even in a static system of electrodes, an unacceptable lack of definition and an equally unacceptable loss of resolution are obtained in the transferred image with the contact time of the transfer image, the wide field zone, the moving photoconductor and the low resistivity single component developer normally applied in practice.

In principle, the use of a magnetic single component developer in electrographic image production provides for the construction of a much simpler copying machine than is the case with conventional two-component developers. For example, it is possible to construct a copying machine in which one and the same magnetic developer applicator may be used both for developing the latent charge image and also for cleaning the re-usable photoconductor on completion of the transfer process.

However, it has been found that conventional magnetic one-component developers with the above-mentioned relatively low volume resistivity values give rise to considerable practical difficulties, especially in regard to the desirable or necessary image definition and resolving power.

It has now been found that the relatively low resistivity of conventional magnetic one-component developers is responsible for these difficulties.

It is pointed out that conventional highly resistive two-component developers only receive the required fluidity by the addition of the magnetic or magnetisable particles and show a tendency towards agglomeration in the absence of this addition.

In the case of one-component developers, it is only the relatively low resistivity types, with a volume resistivity of less than about 106 ohm.cm, which show adequate free-flow properties, properties which, by contrast, are missing from one-component developers of relatively high resistivity.

Accordingly, an object of the present invention is to provide a magnetic onecomponent developer with which the difficulties referred to above are obviated and which, therefore, is in particular considerably more resistive than conventional magnetic one-component developers and which, in addition, satisfies the other requirements imposed on an electrophotographic one-component developer.

According to the invention there is provided a magnetic one-component developer for electrophotographic image production, the developer comprising electrically insulating binder particles, the particles having an electrically conductive constituent comprising a magnetic or magnetisable material embedded therein in such a manner that the surface of each particle is substantially free of any conductive constituent and essentially comprises the electrically insulating binder, the volume resistivity of the developer being at least 1013 ohm.cm.

The electrically conductive constituents preferably also comprise pigment particles.

The invention also relates to a process for producing the one-component developer defined above wherein a crude developer produced from a mixture of magnetic or magentisable material, pigment and/or dye and an electrically insulating binder is aftertreated to improve its properties.

Several embodiments of the invention are described in detail in the following.

Achieving the desired properties for the magnetic one-component developer depends very largely inter alia upon the choice of suitable starting materials, upon the quantitative ratio between the individual constituents, upon the particular procedure adopted and upon the aftertreatment of the crude developer.

MATERIALS: 1. Magnetic or magnetisable material: iron oxide, magnetite, iron powder or other iron, nickel or chromium compounds.

2. Pigment and/or dye: carbon black and/or iron oxide black; nigrosine black, Sudan deep black and similar substances.

3. Binder: polystyrene, poly(meth)acrylates and copolymers thereof: softening point preferably in the range from 70 to 1200C.

The above-mentioned materials are preferably mixed in the following proportions: 1. Magnetic material: 40 - 60% 2. Pigment: 0 -12% 3. Dye: 0 - 2% 4. Binder (resin): 30 - 70%.

PROCEDURE: For achieving the required properties of the above-mentioned component developer, it is important for the electrically conductive pigment particles and the magnetic or magnetisable particles to be arranged in the interior of the developer particles rather than being retained at their surface because this would result in an undesirable degree of conductivity of the developer. The following processes have proved to be suitable for achieving this objective: 1. Spray drying the developer dispersion from an aqueous or non-aqueous dispersant.

2. Producing the developer by melting, cooling and grinding the originally solid developer mass.

3. After-treating the crude developer produced in step 1 or 2. This after-treatment includes obtaining a suitable particle size distribution, because the crude developer produced in step 1 or 2 has a particle size distribution ranging from less than 1 llm to about 50 to 60 llm. A particle size distribution as wide as this would be detrimental both to the quality of image obtained and also to the potential service life of the re-usable photoconductor which comes into contact therewith.

Accordingly, a first after-treatment step comprises sorting the particles according to size, for example by means of a grading machine known per se. The aim of this after-treatment step is to obtain a particle size distribution of from about 5 to 20 llm.

A second after-treatment step comprises improving the free-flow properties of the developer. The crude developer referred to has relatively poor free-flow properties which are unsuitable for the formation of a satisfactorily functioning magnetic brush.

This disadvantage may be obviated, for example, by the following after-treatment: (a) The surface of the developer particles may be mechanically treated or heat treated, i.e. for example by intensive stirring in a mixer or in a grading machine, for example of the Majac type manufactured by the Donaldson company of the United States.

(b) The surface of the developer particles may be coated with a flow-promoting agent such as, for example, carbon black and/or silicon oxide. In cases where the particles are surface-coated with carbon black, however, it is important to ensure that the concentration of carbon black does not exceed about 0.7% by weight because otherwise the volume resistivity of the completed developer would be unacceptably reduced.

It is believed that the aftertreatment of developer particles promotes the migration of the conductive constituent toward the interior thereof.

Some examples of one-component develops according to the invention are given in the following: EXAMPLE 1 200 g of 50% styrene(meth)acrylate copolymer, 98 g of magnetite, 2 g of carbon black and 100 g of toluene are homogenised for 1 hour in a ball mill, the dispersion is diluted to 12.5% by weight with a mixture of toluene and methylene chloride (1:3) and dried in a spray dryer (NIRO-Minor). The dry developer, consisting entirely of spherical particles, is treated for 5 minutes in a high speed mixer (Waring blender) in the presence of 0.3% by weight of carbon black and subsequently subjected to particle grading (3 - 45 microns, preferably 5 - 20 mic rons). The developer is free-flowing and has a volume resistivity of 4.1013 ohm.cm.A copy of extremely good quality is obtained after development of the charge image on a ZnO-photoconductor and corona transfer to plain paper.

EXAMPLE 2 In the production of developer in the same way as in Example 1, but with 5% by weight of carbon black and using a styrene methyl (meth) acrylate, adequate fluidity was obtained merely by treatment in the mixer, i.e. in the absence of flow-promoting agents. The volume resistivity amounted to 1013 ohm.cm. and the copy obtained was of good quality.

EXAMPLE 3 75 g of styrene(meth)acrylate suspended in toluene and methylene chloride (1:3), 72 g of magnetite and 3 g of carbon black were homogenised with stirring at 1900C, the developer mass was cooled, size-reduced to a particle size of approximately 1 mm and wet-ground in a ball mill in the presence of approximately 2% of a wetting agent (for example Levapon 4241, a product of Bayer AG), followed by decantation, filtering, washing and drying at 30 to 400C. After drying, the developer was further processed in the same way as described in Example 1.

A sharp copy with good blackening of large areas was obtained with this developer.

EXAMPLE 4 The procedure was as in Example 3 except that, after initial size-reduction to a particle size of 1 mm, the developer was finely ground, for example in an air jet mill.

WHAT WE CLAIM IS: 1. A magnetic one-component developer for electrophotographic image production, the developer comprising electrically insulating binder particles, the particles having an electrically conductive constituent comprising a magnetic or magnetisable material embedded therein in such a manner that the surface of each particle is substantially free of any conductive constituent and essentially comprises the electrically insulating binder, the volume resistiviity of the developer being at least 1013 ohm.cm.

2. A one-component developer as claimed in claim 1, wherein the particles undergo a mechanical after treatment to improve their flow properties.

3. A one-component developer as claimed in claim 1, wherein the particles undergo a thermal after treatment to improve their flow properties.

4. A one-component developer as

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. cooling and grinding the originally solid developer mass. 3. After-treating the crude developer produced in step 1 or 2. This after-treatment includes obtaining a suitable particle size distribution, because the crude developer produced in step 1 or 2 has a particle size distribution ranging from less than 1 llm to about 50 to 60 llm. A particle size distribution as wide as this would be detrimental both to the quality of image obtained and also to the potential service life of the re-usable photoconductor which comes into contact therewith. Accordingly, a first after-treatment step comprises sorting the particles according to size, for example by means of a grading machine known per se. The aim of this after-treatment step is to obtain a particle size distribution of from about 5 to 20 llm. A second after-treatment step comprises improving the free-flow properties of the developer. The crude developer referred to has relatively poor free-flow properties which are unsuitable for the formation of a satisfactorily functioning magnetic brush. This disadvantage may be obviated, for example, by the following after-treatment: (a) The surface of the developer particles may be mechanically treated or heat treated, i.e. for example by intensive stirring in a mixer or in a grading machine, for example of the Majac type manufactured by the Donaldson company of the United States. (b) The surface of the developer particles may be coated with a flow-promoting agent such as, for example, carbon black and/or silicon oxide. In cases where the particles are surface-coated with carbon black, however, it is important to ensure that the concentration of carbon black does not exceed about 0.7% by weight because otherwise the volume resistivity of the completed developer would be unacceptably reduced. It is believed that the aftertreatment of developer particles promotes the migration of the conductive constituent toward the interior thereof. Some examples of one-component develops according to the invention are given in the following: EXAMPLE 1 200 g of 50% styrene(meth)acrylate copolymer, 98 g of magnetite, 2 g of carbon black and 100 g of toluene are homogenised for 1 hour in a ball mill, the dispersion is diluted to 12.5% by weight with a mixture of toluene and methylene chloride (1:3) and dried in a spray dryer (NIRO-Minor). The dry developer, consisting entirely of spherical particles, is treated for 5 minutes in a high speed mixer (Waring blender) in the presence of 0.3% by weight of carbon black and subsequently subjected to particle grading (3 - 45 microns, preferably 5 - 20 mic rons). The developer is free-flowing and has a volume resistivity of 4.1013 ohm.cm.A copy of extremely good quality is obtained after development of the charge image on a ZnO-photoconductor and corona transfer to plain paper. EXAMPLE 2 In the production of developer in the same way as in Example 1, but with 5% by weight of carbon black and using a styrene methyl (meth) acrylate, adequate fluidity was obtained merely by treatment in the mixer, i.e. in the absence of flow-promoting agents. The volume resistivity amounted to 1013 ohm.cm. and the copy obtained was of good quality. EXAMPLE 3 75 g of styrene(meth)acrylate suspended in toluene and methylene chloride (1:3), 72 g of magnetite and 3 g of carbon black were homogenised with stirring at 1900C, the developer mass was cooled, size-reduced to a particle size of approximately 1 mm and wet-ground in a ball mill in the presence of approximately 2% of a wetting agent (for example Levapon 4241, a product of Bayer AG), followed by decantation, filtering, washing and drying at 30 to 400C. After drying, the developer was further processed in the same way as described in Example 1. A sharp copy with good blackening of large areas was obtained with this developer. EXAMPLE 4 The procedure was as in Example 3 except that, after initial size-reduction to a particle size of 1 mm, the developer was finely ground, for example in an air jet mill. WHAT WE CLAIM IS:

1. A magnetic one-component developer for electrophotographic image production, the developer comprising electrically insulating binder particles, the particles having an electrically conductive constituent comprising a magnetic or magnetisable material embedded therein in such a manner that the surface of each particle is substantially free of any conductive constituent and essentially comprises the electrically insulating binder, the volume resistiviity of the developer being at least 1013 ohm.cm.

4. A one-component developer as

claimed in any preceding claim, wherein the particles are coated with a flow-promoting agent to improve their flow properties.

5. A one-component developer as claimed in claim 4, wherein the flowpromoting agent comprises carbon black, the concentration of carbon black on the surface of the particles being at most 0.7% by weight.

6. A one-component developer as claimed in any of claims 4 or 5, wherein the flow promoting agent comprises silicon oxide.

7. A one-component developer as claimed in any preceding claim, wherein the particles are substantially spherical.

8. A process for producing a magnetic one-component developer as claimed in claim 1, wherein a particulate crude developer is produced from a mixture of magnetic or magentisable material, pigment and/or dye and an electrically insulating binder and is aftertreated the aftertreatment comprising sorting the particles of crude developer by size to obtain a predetermined size distribution and treating the surface of the particles to improve the free flow properties thereof thereby to produce particles whose surfaces are substantially free of any conductive constituent and essentially comprising the electrically insulating binder, the volume resistivity of the developer being at least 10l3 ohm.cm.

9. A process as claimed in claim 8, wherein the crude toner is produced by spray drying.

10. A process as claimed in claim 8, wherein the crude developer is produced by grinding a solid toner melt.

11. A process as claimed in claim 8, wherein substantially the following mixing proportions and procedures are applied: 200 g of 50 styrene(meth)acrylate copolymer, 98 g of magnetite, 2 g of carbon black and 100 g of toluene are homogenised for 1 hour in a ball mill, the dispersion is diluted to 12.5% by weight with a mixture of toluene and methylene chloride (1:3) and dried in a spray dryer, after which the dry developer, consisting of substantially spherical particles, is treated for 5 minutes in a high speed mixer in the presence of 0.3% by weight of carbon black and subsequently subjected to particle grading to obtain a particle size distribution of from 3 to 45 Fm.

12. A process as claimed in any of claims 8 to 10, wherein styrene methyl (meth)acrylate is used as binder.

13. A process as claimed in claim 9, wherein the components are mixed in the following proportions: 75 g of styrene(meth)acryl suspended in toluene and methylene chloride (1 : 3), 72 g of magnetite and 3 g of carbon black are homogenised with stirring at 1900C after which the toner mass is allowed to cool and is then size-reduced to a particle size of about 1 mm, followed by wet-grinding ina ball mill in the presence of about 2% of a wetting agent, decantation, filtration, washing and drying at 30 to 400C.

14. A process for producing a magnetic one-component developer as claimed in claim 1, substantially as herein described in any of the Examples.