GB2267355A

GB2267355A - Two-color developer for electrophotography

Info

Publication number: GB2267355A
Application number: GB9314518A
Authority: GB
Inventors: Hidehiko Soyama; Masanori Ichimura
Original assignee: Fuji Xerox Co Ltd
Current assignee: Fujifilm Business Innovation Corp
Priority date: 1989-11-02
Filing date: 1993-07-13
Publication date: 1993-12-01
Anticipated expiration: 2013-07-13
Also published as: DE4032702A1; GB2238395B; GB2238395A; GB9021918D0; GB2267355B; GB9314518D0

Abstract

A two-colour developer for two-color electrophotographic process comprising repeating exposure and development on a charged photo receptor for each of a first developer and a second developer to form a two-color toner image on the photoreceptor and transferring the toner image to a transfer medium all at once for fixing is disclosed, a toner of said first developer providing a first toner image having a smaller volume average particle size than that of a toner of the second developer. The image formed by first development with the first developer undergoes no disturbance on second contact development with the second developer to provide a copy of satisfactory image quality. The first and second developers are preferably chargeable to opposite polarities by adding negative and positive charge control agents to the first and second toners.

Description

TWO-COLOR DEVELOPER FOR ELECTROPHOTOGRAPHY FIELD OF THE INVENTION This invention relates to a two-color developer for electrophotography. More particularly, it relates to a developer for two-color electrophotographic process in which a charged electrophotographic photoreceptor is successively exposed to light followed by development to form a two-color toner image thereon, which is then transferred all at once to a transfer medium and fixed thereon.

BACKGROUND OF THE INVENTION Copying machines using a color developer have recently been developed. For example, a color copying system for providing a two-color image is known, which comprises the steps of carrying out the first exposure and development on a photoreceptor to transfer the first toner image to a copying paper and fix thereon, retaining the copying paper in a copying machine, and then carrying out the second exposure and development on the photoreceptor to transfer the second toner image to the copying paper and fix thereon. This color copying system, however, had a disadvantage of requiring a long time for copying since the steps of exposure, development, and transfer are repeated for each developer.

It has hence been proposed to obtain a color copy by repeating exposure and development for each of two developers of different colors to form a two-color toner image on a photoreceptor and then transfermas the toner image to a transfer medium all at once as disclosed in JP-A-1-287581 (the term "JP-A" as used herein means an "unexamined published Japanese patent application However, since the above-described system adopt contact development such as magnetic brush development, the toner image formed on a photoreceptor by a first developer undergoes disturbance due to abrasion at the time of second development only to produce an image of deteriorated quality.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a two-color developer for electrophotography which eliminates the above-described disadvantage.

That is, an object of the present invention is to provide a two-color developer for electrophotography with which second development can be carried out without disturbing a first toner image.

The present invention relates to a two-color developer for two-color electrophotographic process comprising repeating exposure and development on a charged photoreceptor for each of a first developer and a second developer to form a two-color toner image on the photoreceptor and transferring the toner image to a transfer medium all at once for fixing, (1) said first developer providing a first toner image being chargeable to have a higher charge quantity in absolute value than that of the second developer, or (2) a toner of said first developer providing a first toner image having a smaller volume average particle size than that of a toner of the second developer.

DETAILED DESCRIPTION OF THE INVENTION According to the first embodiment of the present invention, since the first developer charged to a higher charge quantity in absolute value than the second developer adheres to a photoreceptor more strongly, the first toner image formed on the photoreceptor undergoes no disturbance on second contact development.

The difference in charge quantity between the first and second developers is preferably at least 3 c/g in absolute value, arA more preferably 5 pc/g to 25 rc/g. It is preferable to control the charge quantity of the first developer within a range of from 10 to 30 Ac/g in absolute value for obtaining a satisfactory image. If the charge quantity of the first developer is less than 10 pc/g in absolute value, that of the second developer should be less than 7 rc/g in absolute value for making a difference of 3 pc/g. This being the case, disadvantages would result such that the second image has an excessive density or suffers from fog on the non-image area.

On the other hand, if the charge quantity of the first developer is more than 30 pc/g in absolute value, a sufficient first image density cannot be obtained. If the difference of charge quantity between the first and second developers is smaller than 3 pc/g in absolute value even with the charge quantity of the former developer falling within the above recited range, the first image is apt to suffer from an increase in line thickness and the like, resulting in deterioration of image quality.

The first and second developers may have the same polarity, but it is desirable that they are charged to opposite polarities in order to prevent a toner of one color from entering into a developing machine of the other color and undergoing re-development.

Methods for controlling charge quantity of the developer are not restricted. For example, the charge quantity can be controlled appropriately by selecting the kind and amount of a charge control agent in toner particles or an externally supplied additive, the kind of a binder resin in toner particles, or the kind of a carrier.

In the present invention, the charge quantity of developers was measured by means of a charge quantity measuring apparatus manufactured by TB-200 manufactured by Toshiba Chemical Co., Ltd.

In the second embodiment of the present invention, the toner of the first developer has a volume average particle size (D1) smaller than the volume average particle size (D2) of a toner of the second developer to decrease the toner layer thickness of the first toner image thereby to prevent disturbance of the first toner image on second contact development such as magnetic brush development with the second developer.

The volume average particle sizes of the toners D1 and D2 are preferably related so as to satisfy the following formulae (I) and (II): 4.0 m ' D1 < D2 b 20 m (I) D2 - D1 # 1.0 m (II) In the formula (I), D1 is less than 4.0 m, charge control of the toner of the first developer is difficult, failing to obtain a stable image. Further, if both D1 and D2 are more than 20 m, the resulting image has deteriorated graininess. In the formula (II), if the difference between D2 and D1 is less than 1.0 m, image disturbance cannot be sufficiently prevented.

Dl and D2 are more preferably related so as to satisfy the above formula (II) and the following formula (I'): 7.0 m # D1 # D2 # 18 m (I') In the present invention, the volume average particle size of toners was measured by means of a Coulter counter, Model TA-II manufactured by Nikkaki K.K.

The developer which can be used in the present invention comprises a toner and a carrier. The toner can be prepared.by coating colored particles with externally supplied additives.

The colored particles mainly comprise a colorant and a binder resin. Any of general thermoplastic resins is employable as a binder resin. Examples of suitable binder resins include homo- and co-polymers of styrene or derivatives thereof, e.g., chiorostyrene; monoolefins, e.g., ethylene, propylene, butylene, and isobutylene; vinyl esters, e.g., vinyl acetate, vinyl propionate, vinyl benzoate, and vinyl butyrate; amethylene aliphatic monocarboxylic acid esters, e.g., methyl acrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and dodecyl methacrylate; vinyl ethers, e.g., vinyl methyl ether, vinyl ethyl ether, and vinyl butyl ether; and vinyl ketones, e.g., vinyl methyl ketone, vinyl hexyl ketone, and vinyl isopropenyl ketone.

Typical examples of these binder resins are polystyrene, a styrene-alkyl acrylate copolymer, a styrene-alkyl methacrylate copolymer, a styrene-acrylonitrile copolymer, a styrenebutadiene copolymer, a styrene-maleic anhydride copolymer, polyethylene, and polypropylene. Further included in usable binder resins are polyester, polyurethane, epoxy resins, silicone resins, polyamide, modified rosin, paraffin, and waxes.

Colorants which can be used in the present invention are not particularly limited. Typical examples of suitable colorants are carbon black, nigrosine dyes, Aniline Blue, calcoil Blue, Chrome Yellow, Ultramarine Blue, Du Pont Oil Red, Quinoline Yellow, Methylene Blue chloride, Phthalocyanine Blue, Malachite Green oxalate, lamp black, Rose Bengale, C.I. Pigment Red 48:1, C.I. Pigment Red 122, C.I. Pigment Red 57:1, C.I.

Pigment Yellow 97, C.I. Pigment Yellow 12, C.I. Pigment Blue 15:1, and C.I. Pigment Blue 15:3.

The colored particles may contain a general charge control agent. Typical examples of suitable charge control agents include dyes, e.g., chromium-containing dyes, and those having an ionic structure, e.g., cetyl pyridinium chloride and potassium tetraphenylborate.

A volume average particle size of the colored particles is not greater than about 30 zm, and preferably from 3 to 20 zm.

A silica fine powder may be added to the colored particles. The silica .fine powder includes not only silica fine powder itself but silicon disulfide particles disclosed in JP-B-54-16219 (the term "JP-B" as used herein means an "examined Japanese patent publication") in which silicon atoms with 1 to 3 organic groups directly bonded thereto via an Si-C bond are chemically bonded via an Si-O-Si bond, said silicon atoms being present on the surface thereof. The silica fine particles may be subjected to surface treatment for rendering the surface thereof hydrophobic.

A cleaning aid or a charge control agent may also be added to the colored particles. Specific but non-limiting examples of suitable cleaning aids include a polyvinylidene fluoride powder and a polymethyl methacrylate powder. Specific examples of suitable charge control agent include tin oxide.

Carrier particles which can be used in the present invention include particles of iron, nickel, cobalt, iron oxide, ferrite, glass beads, granular silicone, etc. each of which has an average particle size of 500 pm or less.

These particles may be coated with a fluorine resin, a silicone resin. etc.

The first and second toners used in the present invention are preferably charged to opposite polarities each other.

According to the first embodiment of the present invention, disturbances of the first image during second contact development, such as an increase in line thickness, can be prevented to provide a two-color image of satisfactory quality while assuring a sufficient image density.

According to the second embodiment of the present invention, since the thickness of the toner image formed by first development can be reduced, disturbances of the first image during second contact development can be avoided to provide a two-color image of satisfactory quality.

The present invention is now illustrated in greater detail with reference to Examples, but it should be understood that the present invention is not deemed to be limited thereto.

All the parts, percents, and parts are by weight unless otherwise specified.

EXAMPLE 1 PreParation of Blue DeveloPer for First Development: Styrene-n-butyl methacrylate copolymer 100 parts (70/30) Copper phthalocyanine 10 parts KAYACHARGE N-3 (negative charge control 1 part agent produced by Nippon Kayaku Co., Ltd.) The above components were melt-kneaded, finely ground, and classified to obtain blue-colored particles having an average particle size of 12 rm.

100 parts of the blue particles were mixed with 1 part of hydrophobic silica ("R 972" produced by Nippon Aerosil K.K.) in a Henschel mixer to obtain a negatively chargeable blue toner.

100 parts of a carrier prepared by coating a ferrite core having an average particle size of 100 zm with a methyl methacrylate-n-butyl methacrylate copolymer (80/20) and 4 parts of the blue toner were mixed in a twin-cylinder mixer to prepare a blue developer for first development.

Preparation of Black Developer for Second Develosment: Styrene-n-butyl methacrylate copolymer (70/30) 100 parts Carbon black 10 parts Cetyl pyridinium chloride 1 part The above components were melt-kneaded, finely ground, and classified to obtain black particles having a mean particle size of 12 zm.

100 parts of the black particles and 0.5 part of aluminum oxide C (produced by Nippon Aerosil K.K.) were mixed in a Henschel mixer to obtain a positively chargeable black toner.

100 parts of a carrier prepared by coating a ferrite core having an average particle size of 100 zm with a vinylidene fluoride-trifluoroethylene copolymer (75/25) and 5 parts of the black toner were mixed in a twin-cylinder mixer to prepare a black developer for second development.

EXAMPLE 2 100 parts of the blue particles obtained in Example 1 and 2 parts of silica R 972" were mixed in a Henschel mixer to obtain a negatively chargeable blue toner, which was then mixed with a carrier in the same manner as in Example 1 to prepare a blue developer for first development.

The blue developer for first development was used in combination with the same black developer for second development as prepared in Example 1.

EXAMPLE 3 100 parts of the blue particles obtained in Example 1, 1 part of silica "R 972", and 1 part of a tine oxide fine powder were mixed in a Henschel mixer to obtain a negatively chargeable blue toner. The blue toner was mixed with a carrier in the same manner as in Example 1 to prepare a blue developer for first development.

100 parts of the black particles obtained in Example 1 were mixed with 0.5 part of aluminum oxide C and 0.5 part of a tin oxide fine powder in a Henschel mixer to obtain a positively chargeable black toner. The black toner was mixed with a carrier in the same manner as in Example 1 to prepare a black developer for second development.

COMPARATIVE EXAMPLE 1 100 parts of the blue particles obtained in Example 1 were mixed with 1 part of silica "R 972" and 2 parts of a tin oxide fine powder in a Henschel mixer to obtain a negatively chargeable blue toner. The blue toner was mixed with a carrier in the same manner as in Example 1 to prepare a blue developer for first development.

The blue developer was used in combination with the same black developer for second development as obtained in Example 1.

COMPARATIVE EXAMPLE 2 Blue particles were prepared in the same manner as in Example 1, except for increasing the amount of KAYACHARGE N-3 to 2 parts.

100 parts of the resulting blue particles and 3 parts of silica "R 972" were mixed in a Henschel mixer to obtain a negatively chargeable blue toner. The blue toner was mixed with a carrier in the same manner as in Example 1 to prepare a blue developer for first development.

The blue developer was used in combination with the same black developer for second development as prepared in Example 1.

COMPARATIVE EXAMPLE 3 A black developer for second development was prepared in the same manner as Example 1, except for increasing the amount of cetyl pyridinium chloride to 1.7 parts.

The black developer was used in combination with the same blue developer for first development as obtained in Example 1.

Cotv Test: Charge quantity of each of the developers prepared in Examples 1 to 3 and Comparative Examples 1 to 3 was measured.

Then, each developer was loaded on a first developing machine and a second developing machine of a copying machine shown in Fig. 9 of JP-A-1-287581 under 250C and 60% R.H., and copy test was carried out by using a test chart composed of a line image part and a solid image part having a density of 1.0 as original. Copied samples were evaluated according to the following evaluation, and the results obtained are shown in Table 1 below.

Increase in Line Thickness: Thickness of a line of the image formed by first development was measured and compared with that of the original. An increase in line thickness was calculated from: [(Thickness of Line in Copied Image)/(Thickness of Line in Original) - 1] x 100 Image Density: A solid original having a density of 1.0 as measured with a Macbeth densitometer was used. Image density of the image formed by first development was measured with the same densitometer.

Image Quality: The term "Good" in image quality means that the increase in line thickness of first development is not more than 10% and the image density of first development is at least 0.9. When the increase in line thickness or the image density is out of the above "good" range, image quality is regarded as "Bad".

TABLE 1 1st Imaae Judgment Charae Ouantitv Increase on 2-Color Example First Second in Line Image Image No. Developer Developer Thickness Densitv Oualitv (c/g) (4c/g) (%) Example 1 -20 13 4 1.0 Good Example 2 -28 13 3 0.9 Good Example 3 -10 7 8 1.1 Good Comparative -6 13 30 1.3 Bad Example 1 Comparative -38 13 2 0.7 Bad Example 2 Comparative -20 18 15 1.7 Bad Example 3 The following considerations can be derived from the results in Table 1.

In Example 1, the increase in line thickness in the first image over the original is very slight, and the density of the first image is sufficient.

In Example 2, the first image has an acceptable density, and the increase in line thickness is very slight.

In Example 3, the increase in line thickness in the first image is within an allowable range, and the density of the first image is sufficient.

In Comparative Example 1, because the charge quantity of the first developer is as low as -6 pc/g, the line in the first image shows a considerable increase over the original.

In Comparative Example 2, because the charge quantity of the first developer is as high as -38 Rc/g, a sufficient image density cannot be obtained.

In Comparative Example 3, because the difference in charge quantity between the first and second developers is as small as 2 rc/g in absolute value, the line in the first image shows a considerable increase over the original.

; EXAMPLE 4 Preparation of Black Developer for First Development: Styrene-n-butyl methacrylate copolymer (70/30) 100 parts Carbon black 10 parts KAYACHARGE N-3 1 part The above components were melt-kneaded, finely ground, and classified to obtain black particles having an average particle size of 9 pm 100 parts of the black particles and 1.5 parts of hydrophobic silica "R 972" having an average particle size of 0.016 zm were mixed in a Henschel mixer to obtain a negatively chargeable black toner.

4 parts of the black toner were mixed with 100 parts of a carrier prepared by coating a ferrite core having an average particle size of 100 zm with a styrene-n-butyl methacrylate copolymer (80/20) in a twin-cylinder mixer to prepare a black developer for first development.

PreDaration of Red Developer for Second Development: Styrene-n-butyl methacrylate copolymer (70/30) 100 parts Rhodamine dye 10 parts Cetyl pyridinium chloride 1 part The above components were melt-kneaded, finely ground, and classified to obtain red particles having an average particle size of 12 zm.

100 parts of the rèd particles and 0.5 part of aluminum oxide C having an average particle size of 0.020 zm were mixed in a Henschel mixer to obtain a positively chargeable red toner.

5 parts of the red toner and 100 parts of a carrier prepared by coating a ferrite core having an average particle size of 100 Wm with a vinylidene fluoride-hexafluoropropylene copolymer (75/25) in a twin-cylinder mixer to prepare a red developer for second development.

COMPARATIVE EXAMPLE 4 A two-color developer was prepared in the same manner as in Example 4, except that the black toner of the first developer had a volume average particle size of 11.5 zm.

COMPARATIVE EXAMPLE 5 A two-color developer was prepared in the same manner as in Example 4, except that the black toner of the first developer had a volume average particle size of 3.0 m.

COMPARATIVE EXAMPLE 6 A two-color developer was prepared in the same manner as in Example 4, except that the black toner of the first developer had a volume average particle size of 25.0 iim.

Copy Test: Copy test was carried out using each of the developers prepared in Example 4 and Comparative Examples 4 to 6 in the same manner as in Example 1. Evaluation was made in the same manner as in Example 1 except that line reproducibility was evaluated visually and organoleptically by using a Japanese chart. The results obtained are shown in'Table 2 below. TABLE 2 Volume Volume 1st Image Average Average Judgement Particle Size Particle Size Increase Line on 2-Color in First in Second in Line Image Reproduci- Image Example No. Developer Developer Thickness Density bility Ouality ( m) ( m) (%) Example 4 9 12 5 1.1 Good Good Comparative Example 4 11.5 12 30 1.2 Bad Bad Comparative Example 5* 3 12 3 0.5 Good Bad Comparative Example 6 25 12 45 1.4 Bad Bad *: Copy test for Comparative Example 5 was made under 10 C and 10%RH.

In Example 4, a satisfactory black image (the image formed by first development) with no image disturbance was obtained, in which an increase in line thickness was within +5.0% over the original line thickness.

In Comparative Example 4, an increase in line thickness in the black image reached +30.0% over the original line thickness.

In Comparative Example 5, the copy test was performed in an atmosphere at 100C and 10% RH. As a result, a sufficient black density of the first image was not obtained. In this case, the charge quantity of the first developer was -55 wc/g.

In Comparative Example 6, the black image formed by first development had considerably poor line reproducibility, failing to obtain sufficient image quality.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

1. A two-color developer for two-color electrophotographic process comprising repeating exposure and development on a charged photoreceptor for each of a first developer and a second developer to form a two-color toner image on the photoreceptor and transferring the toner image to a transfer medium all at once for fixing, a toner of said first developer having a smaller volume average particle size than that of a toner of the second developer.

2. A two-color developer as claimed in claim 1, wherein the volume average particle size D1 of a toner of said first developer and the volume average particle size D2 of a toner of said second developer satisfying the following formulae (I) and (II):

4.0 um < D1 < D2 z 20 ,um (I) D2 - D1 r 1.0 um (II)

3. A two-color developer as claimed in claim 2, wherein said first and second developers each comprises a toner and a carrier.

4. A two-color developer as claimed in claim 3, wherein the first and second toners for first and second developers are chargeable to opposite polarities each other.

5. A twocolor developer as claimed in any preceding claim, wherein said first developer is chargeable to have a higher charge quantity in absolute value than that of the second developer.

6. A two-color developer as claimed in claim 4, wherein said first developer is chargeable to have a charge quantity of from 10 to 30 c/g in absolute value, which is higher than that of the second developer by at least 3 c/g in absolute value.