EP0838731A1

EP0838731A1 - Developer

Info

Publication number: EP0838731A1
Application number: EP96913736A
Authority: EP
Inventors: Kimikazu Nagase; Tatsuro Tsuchimoto; Satoshi Yoshida
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1996-05-13
Filing date: 1996-05-13
Publication date: 1998-04-29
Also published as: WO1997043697A1; KR19990028919A

Abstract

A developer, in which coloring particles at least consisting of a resin and a colorant are dispersed in an insulating liquid, characterized by being 4 cst in the difference between the kinematic viscosity at 10°C and that at 50°C and 1 cst to 5 cst in the kinematic viscosity at 20°C.

A developer stable against the changes of temperature and process can be obtained.

Description

Technical Field

The present invention relates to a developer used for copiers and laser beam printers. Particularly, the present invention relates to a developer which is used for an image forming method containing the steps of supplying a developer onto an electrostatic latent image on an electrostatic latent image carrier by a developing roller, developing it to obtain a visible image, electrostatically transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, and re-transferring the transferred image on the intermediate transfer medium onto a recording medium.

Background Art

In the conventionally adopted saturation development method, development is effected sufficiently till the latent image charges on an electrostatic latent image carrier or electrostatic recording paper are neutralized by toner particles. This method has an advantage that if the electrification characteristics of the toner in the developer are controlled, a stable development density can be obtained and is little affected by the temperature change of the printer. However, the saturation development method has a problem that since it takes time for development to reach saturation, the printing speed of the printer cannot be raised.

To solve the problem, Japanese Patent Laid-Open (Kokai) No. 5-273865 proposes to balance the developing time and developing speed using a developing machine with a roller, for developing without reaching saturation, as a method for achieving a higher printing speed.

However, this method has such problems that the developing speed of the developer is affected by the temperature change of the printer, to affect the development density and that a higher printing speed changes the process, to affect the development density. So, a developer stable against the changes of temperature and process is demanded.

Disclosure of the Invention

The object of the present invention is to provide a developer with its developability little affected by the changes of temperature and process.

The object can be achieved by the present invention constituted as follows. The present invention is a developer, in which coloring particles at least consisting of a resin and a colorant are dispersed in an insulating liquid, characterized by being 4 cst or less in the difference between the kinematic viscosity of the developer at 10°C and that at 50°C, and being 1 cst to 5 cst in kinematic viscosity at 20°C.

Most Preferred Embodiments of the Invention

In the developer of the present invention, coloring particles mainly consisting of a pigment and a resin are dispersed in an insulating liquid by 0.1 to 10 wt%. A preferable amount is 0.5 to 5 wt%.

If the amount of coloring particles dispersed is less than 0.1 wt%, the printing density becomes too low disadvantageously, and if more than 10 wt%, the white ground is contaminated disadvantageously.

As for the kinematic viscosity of the developer, it is preferable that the difference between the value at 10°C and that at 50°C is 4 cst or less, and more preferable is 3 cst or less. It is preferable that the kinematic viscosity at 20°C is 1 cst to 5 cst, and more preferable is 1.5 cst to 4 cst.

If the kinematic viscosity at 20°C is less than 1 cst, there occurs a problem that when the developer of the non-image area on the electrostatic latent image carrier is attempted to be mechanically removed, for example, by a roller rotating in the reverse direction, the removal efficiency is low because of the low viscosity, causing the developer to remain in the non-image area, to contaminate the white ground. If more than 5 cst, it takes a long time for development, to lower the printing speed of the printer, since the mobility of the coloring particles in the developer declines.

If the difference between the kinematic viscosity at 10°C and that at 50°C is more than 4 cst, the developability depends on the mobility of the toner particles in the developer in the image forming method in which the developing time and the developing speed are balanced, for example, by supplying the developer through a developing roller for development. Since the mobility depends on the viscosity of the solvent, the dependence of the kinematic viscosity on temperature results in the dependence of the development density on temperature, and such a problem occurs that the printing density of the prints is affected by temperature.

To obtain the developer of the present invention, it is preferable to use a hydrocarbon based solvent with a volume resistivity of 10¹⁴ Ω·cm or more as the insulating liquid. It is preferable that the solvent has a dry point of 280°C or lower in distillation and an initial boiling point of 180°C or higher, and more preferable are a dry point of 270°C or lower and an initial boiling point of 200°C or higher.

If the initial boiling point in distillation is lower than 180°C, the solvent is volatilized on the electrostatic latent image carrier to change the developer concentration at the time of the transfer to the intermediate transfer medium, thus destabilizing the transfer, in the image forming method containing the steps of supplying a developer to the electrostatic latent image on the electrostatic latent image carrier by a developing roller, developing it to obtain a visible image, electrostatically transferring it in contact with the intermediate transfer medium with a silicone rubber layer on the surface, and re-transferring the transferred image on the intermediate transfer medium onto the recording medium.

If the dry point in distillation exceeds 280°C, the energy required for transfer from the intermediate transfer medium to paper and fixing becomes large when the solvent is volatilized, and this is unpreferable in view of equipment constitution.

If the volume resistivity is less than 10¹⁴ Ω·cm, the electrophoresis of the toner in the developer is not good, and the latent image cannot be sufficiently developed.

The use of any solvent other than a hydrocarbon based solvent is not preferable in view of safety, chemical stability, cost, etc.

Preferable hydrocarbon based solvents include Isopar L and M (produced by Exone Kagaku K.K.), JWS-8947 (produced by Esso Sekiyu K.K.), Isozole 400 (produced by Nippon Sekiyu Kagaku K.K.), NP-LC, NP-HCS and NP-SH (produced by Mitsui Texaco Chemical K.K.), NAS-4 (produced by Nippon Oils and Fats Co., Ltd.), etc. More preferable ones are Isopar M, JWS-8947, Isozole 400, NP-LC, NP-HCS, NP-SH and NAS-4. Further more preferable ones are JWS-8947 and NP-HCS.

It is preferable that the normal paraffin content of the hydrocarbon based solvent is 70 wt% or more, and more preferable is 80 wt% or more.

If the normal paraffin content in the hydrocarbon based solvent is less than 70 wt%, the discharge electrode is liable to be contaminated in the electrophotography to uniformly electrify the electrostatic latent image carrier by corona discharge. This is considered to have relation with the problem of ozone evolution by corona discharge.

As for the composition of normal paraffins, it is preferable that those with 12, 13 and 14 carbon atoms account for 80 wt% or more, and more preferable is 90 wt% or more.

If the normal paraffins with 12, 13 and 14 carbon atoms account for less than 80 wt%, the difference between the initial boiling point and the dry point is widened, and the balance between the stability of the transfer to the intermediate transfer medium and the transferability and fixability onto paper may be lost.

Furthermore, it is preferable that the ultraviolet light absorbance in a wavelength range of 260 to 350 nm, of the extract obtained by extracting the hydrocarbon based solvent with dimethyl sulfoxide is 0.1 or less.

If the ultraviolet light absorbance in a wavelength range of 260 to 350 nm, of the extract obtained by extracting the hydrocarbon based solvent with dimethyl sulfoxide exceeds 0.1, the solvent is colored in long-lime running probably because of the influence of corona discharge, etc., and especially when the solvent is used for a full-color printer, etc., such a problem that colors become turbid may occur. Furthermore, a problem occurs, that an aromatic component considered as an extracted component emits an offensive odor.

It is preferable that the average particle size of the coloring particles used in the present invention is 0.1 to 2.5 µm, and more preferable is 0.5 to 2 µm.

In the image forming method by using an intermediate transfer medium, since the electrostatic latent image carrier and the intermediate transfer medium contact each other when the toner is transferred from the electrostatic latent image carrier to the intermediate transfer medium, the latent image on the electrostatic latent image carrier is pressurized. So, if the average particle size is less than 0.1 µm, the amount of the solvent existing between particles increases, and the strength of the latent image cannot withstand the pressure applied at the time of transfer, and dot tailing in the printing direction occurs. If the average particle size exceeds 2.5 µm, the particle size of the toner becomes large compared to the pixels, and the image of thin lines, etc. on the prints is coarsened.

The coloring particles mainly contain a pigment or dye and a resin. Coloring particles with a pigment dispersed in a resin are preferable. It is preferable that the pigment content of the particles is 5 wt% to 30 wt%.

If the pigment content is less than 5 wt%, the tinting strength is insufficient, and if more than 30 wt%, the resin content becomes small, to cause insufficient fixing, lowering the gloss of the prints disadvantageously.

The resins which can be used here include alkyd resin, styrene resin, phenol resin, acrylic resin, styrene-acrylic resin, polyester resin, rosin modified phenol resin, rosin modified maleic acid resin, etc. The use of rosin modified maleic acid resin is preferable.

As the pigment or dye, general pigments and dyes can be used. Black ones include carbon black (marketed products include #30, #40, #50, MA-7, 11, 100 and 220 produced by Mitsubishi Kasei, Monarch 800, 900, Mogul L, Black Pearls 130, REGAL 330, 400 and 660R produced by Cabot, Raven 1255, 1020 and 1000 produced by Colombian Carbon, etc.), and also carbon black containing alkali blue.

Yellow ones include C.I. Pigment Yellow - 1, 3, 4, 5, 6, 12, 13, 14, 15, 16, 17, 18, 24, 55, 65, 73, 74, 81, 83, 87, 93, 94, 95, 97, 98, 100, 101, 104, 108, 109, 110, 113, 116, 117, 120, 123, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 156, 168, 169, 170, 171, 172 and 173. The use of C.I. Pigment Yellow - 12, 13, 14, 17, 81, 95, 109 and 154 is preferable.

Red ones include C.I. Pigment Red - 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 15, 17, 18, 22, 23, 31, 37, 38, 41, 42, 48 : 1, 48 : 2, 48 : 3, 48 : 4, 49 : 1, 49 : 2, 50 : 1, 52 : 1, 52 : 2, 53 : 1, 54, 57 : 1, 58 : 4, 60 : 1, 63 : 1, 63 : 2, 64 : 1, 65, 66, 67, 68, 81, 83, 88, 90, 90 : 1, 112, 114, 115, 122, 123, 133, 144, 146, 147, 149, 150, 151, 166, 168, 170, 171, 172, 174, 175, 176, 177, 178, 179, 185, 187, 188, 189, 190, 193, 194, 202, 208, 209, 214, 216, 220, 221, 224, 242, 243, 243 : 1, 245, 246 and 247. The use of C.I. Pigment Red - 31, 48 : 3, 57 : 1, 81 and 122 is preferable.

Blue ones include C.I. Pigment Blue - 1, 2, 9, 14, 15, 16, 17 : 1, 19, 21, 22, 24, 25, 56, 60, 61, 63 and 64, and the use of C.P. Pigment Blue - 15 is preferable.

In addition to the above, Phthalocyanine Green, Oil Violet, Methyl Orange, Methyl Violet, etc. are used.

The coloring particles mainly contain a pigment and a resin, and polyethylene glycol or polypropylene glycol, etc. can also be added to adjust the softening point.

Furthermore, the particles can also have a charge controlling agent, a wax, etc. for charge control and dispersibility improvement on their surfaces.

To produce the developer of the present invention, a resin and a colorant are molten and kneaded with heating using a two-screw extruder or kneader, etc. for coarse grinding, and the mixture is dispersed together with such additives as a charge controlling agent and wax in a hydrocarbon based insulating solvent using a ball mill, vibration mill or attritor, etc., to set the dispersion conditions for obtaining a predetermined particle size and particle size distribution.

The average particle size of the present invention is measured by using Shimadzu Centrifugal Precipitation Type Particle Size Distribution Measuring Instrument, SA-CP3.

The viscosities of the developer and the solvent are measured by using a Brookfield viscometer.

The normal paraffin content was measured by accurately measuring 1.0 g of a sample in a 10 ml measuring flask, adding chloroform (special grade chemical, produced by Kishida Kagaku) to make a 10 ml solution, measuring the solution using Gas Chromatograph HP5890SII (produced by Hewlett Packard) with DB-1 column produced by J&W, and determining the normal paraffin content from the peak area of the peak corresponding to the standard product.

The ultraviolet light absorbance in a wavelength range of 260 to 350 nm, of the extract obtained by extracting the hydrocarbon solvent with dimethyl sulfoxide is measured by taking 25 ml of the hydrocarbon solvent, extracting using 5 ml of dimethyl sulfoxide for absorption spectrum, and measuring the ultraviolet light absorbance in a wavelength range of 260 to 350 nm of the extract by an ultraviolet light absorption measuring instrument.

The image forming method of the present invention is described below.

The developer of the present invention is used to develop an electrostatic latent image formed on an electrostatic latent image carrier, to form an image. It is preferable that the developer is used for an image forming method which contains the steps of supplying a liquid developer onto an electrostatic latent image on an electrostatic latent image carrier by a developing roller, developing it to obtain a visible image, electrostatically transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, and re-transferring the transformed image on the intermediate transfer medium onto a recording medium.

For development stable against temperature and process, preferable is an image forming method which contains the steps of supplying a liquid developer with coloring particles consisting of a resin and a colorant dispersed in an insulating liquid onto an electrostatic latent image on an electrostatic latent image carrier by a developing roller, developing it to obtain a visible image, transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, and re-transferring the transferred image on the intermediate transfer medium onto a recording medium, and in which the insulating liquid is removed mechanically or electrically on the electrostatic latent image carrier, to keep the solid content of the liquid developer on the electrostatic latent image carrier at 10 to 30 wt% before electrostatic transfer onto the intermediate transfer medium, characterized by being 4 cst or less in the difference between the kinematic viscosity at 10°C of the liquid developer and that at 50°C.

If the difference between the kinematic viscosity at 10°C of the liquid developer and that at 50°C is more than 4 cst, the solid content of the liquid developer on the electrostatic latent image carrier before electrostatic transfer onto the intermediate transfer medium is not kept in a range of 10 to 30 wt%, and the image trails in the printing direction to degrade the image quality.

Furthermore, for development stable against temperature and process, preferable is an image forming method which contains the steps of supplying a liquid developer to an electrostatic latent image on an electrostatic latent image carrier by a developing roller, developing it to obtain a visible image, electrostatically transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, re-transferring the transferred image on the intermediate transfer medium onto a recording medium, and providing a liquid removing means for removing the solvent absorbed or acting as a solvent in the silicone rubber layer of the intermediate transfer medium, characterized by using a hydrocarbon based solvent with a dry point of 280°C or lower in distillation and an initial boiling point of 180°C or higher, as the insulating liquid.

If the dry point in distillation exceeds 280°C, it is difficult to remove the solvent absorbed or acting as a solvent in the silicone rubber layer of the intermediate transfer medium, and a larger apparatus is required. If the solvent is accumulated in the silicone rubber layer, the force required for separating paper from the intermediate transfer medium becomes large, and paper jam, etc. occur.

Furthermore, it is also preferable that the developer is used for an image forming method, in which a process containing the steps of supplying a liquid developer to an electrostatic latent image on an electrostatic latent image carrier by a developing roller, developing it to obtain a visible image and electrostatically transferring it in contact with an intermediate transfer medium is repeated to form a full color image on the intermediate transfer medium, and in which the full color image on the intermediate transfer medium is re-transferred onto a recording medium.

In the above description, "in contact with an intermediate transfer medium" means that the intermediate transfer medium and the electrostatic latent image carrier contact each other at two or more points without any development effected by the developer. When the development by the liquid developer occurs and when the visible image is transferred, it can be considered that the intermediate transfer medium and the electrostatic latent image carrier contact each other through the toner image.

The intermediate transfer medium of the present invention has a silicone rubber layer on a highly heat resistant rubber such as fluorine rubber through an adhesive layer, or a sponge rubber, rubber layer and silicone rubber layer laminated in this order on cloth, etc.

The printing medium in the present invention is not limited in material, and can be any material such as paper, plastic film, metal, cloth or sheet, etc. which allows ordinary printing.

Example 1

One thousand five hundred and forty grams of rosin modified maleic acid resin (FGM-310 produced by Arakawa Kagaku, with an acid value of 108 and a softening point of 120°C), 400 g of carbon black (MOGUL-L) and 60 g of polyethylene glycol (PEG6000 produced by Sanyo Chemical) were preliminarily mixed by a mixer, and molten and kneaded using a two-screw extruder (at a feed rate of 2 kg/h and at 100°C). The obtained kneaded mixture was cooled and coarsely ground to an about 50 µm coarse powder using a sample mill. Two hundred grams of the coarse powder, 110 g of an acrylic high polymer type charge controlling agent solution (14 wt% in solid content, produced according to the method described in Example XI of US Patent No. 3900412), 15 g of carnauba wax and 1250 g of petroleum hydrocarbon based solvent NP-HCS with the following properties (produced by Mitsui Texaco Chemical, with a volume resistivity of 10¹⁸ Ω·cm or more, initial boiling point of 220°C, dry point of 260°C, normal paraffin content of 98.7 wt%, and ultraviolet light absorbance peak value of 0.04 (wavelength 260 nm) in a wavelength range of 260 to 350 nm, of the extract obtained by extracting the hydrocarbon solvent with dimethyl sulfoxide) were mixed and ground by a vibration mill for 5 hours, to obtain a liquid toner with an average particle size of 1.5 µm. The raw solution was diluted by solvent NP-HCS, to have a solid content of 2 wt%, producing a developer.

The developer was 2.9 cst in the kinematic viscosity at 20°C and 2.05 cst in the difference between the kinematic viscosity at 50°C and that at 10°C.

At a room temperature of 20°C, a Se drum was used as a sensitive element, and development was effected using the obtained developer with the linear speed of the sensitive element set at 120 mm/sec. The toner image formed on the sensitive element was transferred onto an intermediate transfer medium stuck on a drum with silicone rubber as the outermost layer, and the toner image on the intermediate transfer medium was transferred onto art paper at a linear pressure of 20 kg with the pressure roller temperature kept at 200°C at a linear speed of 40 mm/sec. An image with a printing density of 1.31 free from white ground contamination could be obtained.

After transfer onto the paper, a 200°C metallic roller was kept in contact with the intermediate transfer medium, to remove the solvent absorbed or acting as a solvent in the silicone rubber layer.

With the room temperature raised to 30°C, printing was repeated, and when the temperature of the sensitive element reached 40°C, printing was effected similarly. An image with a printing density of 1.41 free from white ground contamination could be obtained. The prints were expanded by a microscope and examined and it was found that a good image free from dot tailing with 30 µm smooth thin lines reproduced without being coarsened could be obtained, and that the prints were not greatly affected by temperature.

In succession, 200 sheets were printed. The change of the printing density could be kept within 0.2, and good prints could be obtained with good reproducibility without any problem such as paper jam.

Example 2

Printing was carried out as described in Example 1, except that a squeeze roller rotating in the reverse direction and a charger were installed as means for removing the solvent from the developer of Example 1 on the sensitive element.

To measure the solid content of the developer on the electrostatic latent image carrier before electrostatic transfer onto the intermediate transfer medium, the printer was stopped during printing and the toner on the sensitive element was collected before transfer. The difference of the solid content before drying and that after drying was 21.3 wt%.

As a result, an image with a printing density of 1.28 free from white ground contamination could be obtained. The prints were expanded by a microscope and examined, and it was found that a good image free from dot tailing with 30 µm smooth thin lines not coarsened could be obtained, and that the prints were not greatly affected by temperature.

In succession, 200 sheets were printed. The change of printing density could be kept within 0.13, and good prints could be obtained with good reproducibility.

Example 3

A yellow toner with an average particle size of 1.1 µm was obtained as described in Example 1, except that 400 g of C.I. Pigment Yellow-13 (Seikafast Yellow 2600 produced by Dainichi Seika) was used instead of carbon black.

A magenta toner with an average particle size of 1.4 µm was obtained as described in Example 1, except that 400 g of C.I. Pigment Red-57 : 1 (Seikafast Carmine 6B 1476 produced by Dainichi Seika) was used instead of carbon black.

A cyan toner with an average particle size of 0.8 µm was obtained as described in Example 1, except that 400 g of C.I. Pigment Blue - 15 (Chromofine Blue 5187 produced by Dainichi Seika) was used instead of carbon black.

At room temperature 20°C, a Se drum was used as the sensitive element, and the four color developers obtained like this were used for development one after another using developing rollers with the linear speed of the sensitive element set at 120 mm/sec, and the yellow, magenta, cyan and black toners were transferred one by one onto the intermediate transfer medium stuck on a drum and with a silicone rubber as the outermost layer, to form a full color image on the intermediate transfer medium. The full color image was transferred onto art paper at a linear pressure of 20 kg at a pressure roll temperature of 200°C at a linear speed of 40 mm/sec. As a result, a good image with a yellow printing density of 1.3, magenta printing density of 1.3, cyan printing density of 1.5 and black printing density of 1.4 and free from white ground contamination could be obtained. Furthermore, 500 sheets were continuously printed, and as a result, it was found that the change of the printing density could be kept within 0.2, and that the color superimposition could be reproduced well.

Example 4

One thousand five hundred and forty grams of rosin modified maleic acid resin (FGM-310 produced by Arakawa Kagaku, with an acid value of 108 and a softening point of 120°C), 400 g of carbon black (MOGUL-L) and 60 g of polyethylene glycol (PEG 6000 produced by Sanyo Chemical) were preliminarily mixed by a mixer and molten and kneaded using a two-screw extruder (at a feed rate of 2 kg/h and 100°C). The obtained kneaded mixture was cooled and made into a coarse powder with a particle size of about 50 µm using a sample mill. Two hundred grams of the coarse powder, 110 g of an acrylic high polymer type charge controlling agent solution (with a solid content of 14 wt%, produced according to the method described in Example XI of US Patent No. 3900412), 15 g of carnauba wax and 1250 g of petroleum hydrocarbon based solvent JWS8947 with the following properties (produced by Esso Sekiyu: with a volume resistivity of 10¹⁶ Ω·cm or more, initial boiling point of 225°C, dry point of 242°C, normal paraffin content of 99.8 wt%, with the amount of normal paraffins with 12, 13 and 14 carbon atoms accounting for 99.9 wt% in all the normal paraffins, and of 0.03 (wavelength 260 nm) in the ultraviolet light absorbance peak value in a wavelength range of 260 to 350 nm, of the extract obtained by extracting the hydrocarbon based solvent with dimethyl sulfoxide) were mixed, and ground by a vibration mill for 5 hours, to obtain a liquid toner with an average particle size of 1.5 µm. The raw solution was diluted by solvent JWS8947 to achieve a solid content of 2 wt%, producing a developer.

The developer had a kinematic viscosity of 2.8 cst at 20°C and was 1.93 cst in the difference between the kinematic viscosity ad 50°C and that at 10°C.

The developer was used for printing as described in Example 1, and an image with a printing density of 1.35 free from white ground contamination could be obtained.

With the room temperature raised to 30°C, printing was repeated, and when the temperature of the sensitive element reached 40°C, printing was carried out similarly, to obtain an image with a printing density of 1.42 free from white ground contamination. The prints were expanded by a microscope and examined, and it was found that an image free from dot tailing and with 30 µm smooth thin lines not coarsened could be obtained, and that the prints were not greatly affected by temperature.

In succession, 200 sheets were printed. The change of the printing density could be kept within 0.16, and good prints could be obtained with good reproducibility.

Comparative Example 1

A liquid black toner with an average particle size of 1.2 µm and of 4.5 cst in the difference between the kinematic viscosity at 50°C and that at 10°C was produced as described in Example 1, except that Crystol 52 (produced by Esso Sekiyu) was used instead of NP-HCS.

The toner was used for printing as described in Example 1, except that the linear speed of the pressure roller was 10 mm/sec. At room temperature 20°C, an image with a printing density of 0.85 was obtained. Printing was repeated, and when the temperature of the sensitive element reached 40°C, printing was carried out similarly. It was found that an image with a printing density of 1.23 was obtained and that the dependence of the printing density on temperature was large.

Comparative Example 2

A liquid black toner with a kinematic viscosity of 5.4 cst at 20°C and with an average particle size of 1.2 µm was produced as described in Example 1, except that Lytol White (produced by Witco) was used instead of NP-HCS.

The toner was used for printing as described in Example 1. At room temperature 20°C, the printing density was 0.9, and to achieve the same printing density as achieved in Example 1, the linear speed of the sensitive element had to be lowered to 80 mm/sec, to lower the printing speed.

Comparative Example 3

A liquid black toner with a kinematic viscosity of 0.73 cst at 20°C and with an average particle size of 2.0 µm was produced as described in Example 1, except that Nisseki Isozol 200 (produced by Nippon Sekiyu Kagaku) was used instead of NP-HCS.

The toner was used for printing as described in Example 1. White ground contamination showing a printing density of 0.015 occurred, and good prints could not be obtained.

Industrial Applicability

The developer of the present invention is small in the fluctuation of developability caused by the changes of temperature and process, and stable prints can be obtained at a high speed.

Claims

A developer, in which coloring particles at least comprising a resin and a colorant are dispersed in an insulating liquid, characterized by being 4 cst or less in the difference between the kinematic viscosity at 10°C and that at 50°C and 1 cst to 5 cst in the kinematic viscosity at 20°C.
A developer, according to claim 1, wherein coloring particles at least comprising a resin and a colorant are dispersed in an insulating liquid, wherein the insulating liquid is a hydrocarbon based solvent with a dry point of 280°C or lower in distillation and an initial boiling point of 180°C or higher.
A developer, according to claim 2, wherein the insulating liquid is a hydrocarbon based solvent with a dry point of 270°C or lower in distillation and with an initial boiling point of 200°C or higher.
A developer, according to claim 1 or 2, wherein the insulating liquid is a hydrocarbon based solvent, and the normal paraffin content in the hydrocarbon based solvent is 70 wt% or more.
A developer, according to claim 4, wherein normal paraffins with 12, 13 and 14 carbon atoms account for 80 wt% or more in all the normal paraffins of the insulating liquid.
A developer, according to claim 1 or 2, wherein the insulating liquid is a hydrocarbon based solvent, and the ultraviolet light absorbance in a wavelength range of 260 to 350 nm, of the extract obtained by extracting the hydrocarbon based solvent with dimethyl sulfoxide is 0.1 or less.
A developer, according to claim 1 or 2, which is used for an image forming method containing the steps of supplying a developer onto an electrostatic latent image on an electrostatic latent image carrier, developing it to obtain a visible image, electrostatically transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, and re-transferring the transferred image on the intermediate transfer medium onto a printing medium.
An image forming method which contains the steps of supplying a liquid developer onto an electrostatic latent image on an electrostatic latent image carrier, developing it for obtaining a visible image, electrostatically transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, and re-transferring the transferred image on the intermediate transfer medium onto a printing medium, and in which the insulating liquid is mechanically or electrically removed from the liquid developer on the electrostatic latent image carrier, to keep the solid content of the liquid developer at 10 to 30 wt% on the electrostatic latent image carrier before electrostatic transfer onto the intermediate transfer medium, characterized by being 4 cst or less in the difference between the kinematic viscosity at 10°C and that at 50°C.
An image forming method which contains the steps of supplying a liquid developer onto an electrostatic latent image on an electrostatic latent image carrier, developing it for obtaining a visible image, electrostatically transferring it in contact with an intermediate transfer medium with a silicone rubber layer on the surface, re-transferring the transferred image on the intermediate transfer medium onto a printing medium, and providing a liquid removal means for removing the solvent absorbed or acting as a solvent in the silicone rubber layer of the intermediate transfer medium, characterized by using a hydrocarbon based solvent with a dry point of 280°C or lower in distillation and with an initial boiling point of 180°C or higher, as the insulating liquid.