GB2052774A - Toner for electrostatic photography - Google Patents

Abstract

A toner composition for electrostatic photography which comprises (A) toner powder comprising a base resin and a colouring agent and having an average particle size of not more than 30 microns and (B) silica powder coated with the base resin.

Description

SPECIFICATION Toner for electrostatic photography The present invention relates to a toner forelectrnstatic photography.

The formation of an image on the surface of a photoelectric conductor and the development thereof by an electrostatic means have been widely accepted from many years ago. Procedures for the development of such electrostatic images are roughly classified into the following two methods: (1) a liquid development method using a developing agent comprising an insulating organic medium and one or more kinds of colouring agents finely dispersed therein; and (2) a dry development method using a developing agent in fine powder (called a "toner") comprising a natural or synthetic resin and a colouring agent (e.g. carbon black or a dye) dispersed therein. Among the latter, there are known, e.g., the cascade method, the hair brush method, the magnetic brush method, the impression method and the powder cloud method.This invention pertains to a toner which is employable in the dry developmen method.

In the dry development method, a two-component development procedure such as the cascade method or the magnetic brush method is widely adopted as a practically valuable development method.

For such a two-component procedure, a mixture of an insulating toner having an average particle size ol 30 microns or less and a granular substance usually called a "carrier" is employed. In the cascade method, an insulating bead-like granular substance is used as the carrier. The toner is charged with electricity by friction with the carrier, adheres to the surface of the carrier and is thus transported to the development position. In the magnetic brush method, the carrier to be used is a magnetic granular substance (mainly iron particles) having an average particle size of about 70 to 100 microns. The carrier is attracted magnetically to a sleeve to make the shape of a brush. The toner is charged with electricity by friction with the carrier as in the cascade method, adheres to the surface of the carrier and is thus transported to the development position.In development, the electroconductive carrier acts also as a developing electrode being extremely close to the surface of the photoelectric conductor.

The two-component development procedure has some problems with regard to both the carrier and the toner. As to the carrier, it participates in the electrification and the transport of the toner and functions as a development electrode (in case of the magnetic brush method), but is not directly concerned with the development itself. Thus, it is not consumed at every time of copying, so that its deterioration progresses with an increase in the number of times of copying. The toner undergoes mechanical wear and tear due to the shearing force and the impact force during operations in the developing apparatus, and deterioration occurs after several thousand times of development operations.

For the prevention of such deterioration of the carrier, coating with various kinds of substances has been proposed to afford a certain effect of delaying the deterioration rate, but the effect is restricted due to the necessity of maintaining the quality of the developed picture.

In the case of the toner, there have also been proposed various methods for the prevention of deterioration, thus for example the use of high molecular weight resins which resist mechanical wear and tear, the incorporation of wear-reducing agents (e.g. molybdenum disulfide) into the toner composition and the addition of silica powder to the toner powder to increase powder fluidity produce a wear-reducing effect, and the incorporation of silica powder into the toner composition imparts thereto a polishing effect on the toner components which have adhered to the carrier and the photoelectric conductor.However, these proposals cause other problems such as e.g. an elevation of the fixing temperature, pollution of the carrier and the photoelectric conductor, confusion of the picture caused at the remixing of the recovered toner, wear and tear of the developing machine and a reduction in the miscibility between the carrier and the toner.

According to the present invention, we have sought to solve such problems as are mentioned above, and to provide a toner for electrostatic photography having superior characteristics in comparison with conventional toners, particularly a toner which is stable in powder and electric characteristics, constantly affords pictures with a high degree of resolution, prevents the formation of toner films on the surfaces of the carrier particles and of the photoelectric conductor, suppresses the confusion of pictures at the remixing of the recovered toner, reduces wear and tear of the developing machine and improves the workability at the mixing of the carrier with the toner.

This object can be attained by the use of a toner composition which comprises a toner in a fine powder form having an average particle size of 30 microns or less, and a silica powder whose surface is coated with the base resin in the toner.

According to the invention, there is provided a toner composition for electrostatic photography which comprises (A) toner powder comprising a base resin and a colouring agent and having an average particle size of not more than 30 microns and (B) silica powder coated with the base resin at the surface.

The toner powder to be used as the component (A) may be any conventional toner powder, for example, a toner powder comprising as the base resin polystyrene, a copolymer of styrene with an acrylate and/or a methacrylate, an epoxy resin, a polyacrylate, a polyamide resin, a polyester resin, a polyvinyl chloride, a copolymer of vinyl chloride with vinyl acetate or the like and, as additives, a colouring agent (e.g. carbon black or various organic dyes) and an electrification-regulating agent (an antistatic agent) (e.g. nigrosine base, nigrosine base salt, Neutral Red or Nile Blue). In the case of the average particle size being larger than 30 microns, the resolving powder of the copied picture is reduced, and fogging due to rough powder is caused so that a clear picture is not obtainable.

To achieve electrostatic photography with an excellent resolving powder and a high reflection concentration at the solid portion as well as a small toner layer thickness even in a heat roll fixing method suitable for high speed copying, the use of a toner powder comprising (a) a styrene resin having a weight average molecular weight of not less than 3,000 (b) furnace type carbon black of pH 6.5 to 9.5, (c) lower molecular weight polyolefin having a weight average molecular weight of not more than 10,000 and (d) an electrification-regulating agent in a weight proportion of 100 : 5-1 5 : 3-10: 3-5 is particularly recommended. Such toner powder preferably has a particle size of 5 to 20 microns and an average particle size of 9 to 10 microns.

The styrene resin (a) may be the one conventionally employed as a toner resin and having a weight average molecular weight of 3,000 or more, preferably of 3,000 to 100,000. Specific examples are polystyrene or a copolymer of styrene or its derivative (e.g. p-chlorostyrene or vinyltoluene) with any other vinyl monomer (e.g. ethylene, vinyl chloride, butadiene, methyl acrylate, butyl methacrylate, acrylonitrile, acrylamide, vinyl methyl ether, vinyl methyl ketone or N-vinylpyrrole). These styrene resins may be used alone or in various combinations.Commercially available products such as styrene resin are exemplified by "Highmer SBM 600, SBM 73" (trade mark), (manufactured by Sanyo Chemical Industries Ltd.), "Priolite S-6B, V1 L, VTACL, ACL, HML" (trade mark) (manufactured by Goodyear Tire Corp.), and "Picolastic D-1 50, D-1 25" (trade mark) (manufactured by Esso Research s Engineering Co.). When a styrene resin having a weight average molecular weight below 3,000 is used, the softening point is too low so that the storage stability is reduced and a "blocking phenomenon" is caused.

As the carbon black (b), there may be employed a neutral or weakly basic furnace type carbon black of pH 6.5 to 9.5, preferably pH 7 to 8.5. Usually, this carbon black has a particle size of 10 to 25 m,u, preferably of 1 5 to 20 my. The tinting strength index showing the colouring ability of carbon black is desired to be 100 or more, preferably 1 5 or more. As commercially available products of such carbon black, there may be mentioned, for example, "Monarch 700, 800, 880, 900" and "Black Pearls 700, 800, 880, 900" (trade mark) (manufactured by Cabot) and "RAVEN 1 170, 1250" (trade mark), (manufactured by Columbian Carbon Japan Ltd.). These products may be used alone or in various combinations.The carbon black is normally used in an amount of 5 to 1 5 parts, preferably 5 to 10 parts per 100 parts of the component (a). When the amount is below 5 parts, the reflection concentration of toner particles is lowered. When the amount is greater than 1 5 parts, the layer thickness of the toner image becomes insufficient, and the reflection concentration is lowered. The acidity (basicity) of the carbon black has a large influence upon the density of the toner image. When the pH is smaller than 6.5, fine gaps are readily formed on the toner image layer, and the toner layer thickness becomes large.

When the pH is larger than 9.5, the layer thickness of the toner image becomes too small, and a sufficient reflection concentration at the solid portion cannot be obtained.

The low molecular weight polyolefin (c) may be low molecular weight polyethylene or polypropylene conventionally used as an additive for toner powder in the heat roll fixing method and having a weight average molecular weight of 10,000 or less, preferably of 2,000 to 8,000. As commercially available products of such low molecular weight polyolefin, there may be exemplified "AC Polyethylene 6A, 617" (trade mark), (manufactured by Allied Chemical Corp.) and "Biscole 550P" (trade mark), (manufactured by Sanyo Chemical Industries Ltd.). These products may be used alone or in various combinations. The amount of the low molecular weight polyolefin to be used may be from 3 to 10 parts, preferably from 3 to 5 parts, per 100 parts of the component (a) from the viewpoint of the endowment of the releasing property and improvement of fluidity.When a low molecular weight polyolefin having a weight average molecular weight larger than 10,000 is used, the softening point is high so that the offset phenomenon cannot be prevented.

As the electrification-regulating agent (d), there may be employed conventional agents such as nigrosine dyes and metal-containing dyes. Examples of commercially available products of such electrification-regulating agent are "Brontron N-01, N-02, N-03, S--31" (trade mark) (manufactured by Orient Chemical). These products may be used alone or in various combinations. The amount of the electrification-regulating agent to be used may be from 0.5 to 5 parts, preferably from 0.5 to 2 parts, per 100 parts of the component (a). When the amount is smaller than 0.5 part, the amount of charged electricity is too small so that powdery smoke is generated at development, and fogging occurs on the copied paper. When the amount is larger than 5 parts, the amount of charged electricity is too large, and the reflection concentration at the solid portion is lowered.

The toner powder may be prepared by mixing the components (a) to (d) in the said proportion by a conventional procedure. For example, the components (a) to (d) are mixed preliminary, and the mixture obtained is subjected to melt-kneading with the aid of a heat roll or an extruder, rough crushing by a hammer mill, fine crushing by a jet mill and classifying by a zigzag classifier. The toner powder thus obtained is preferred to have a particle size of 5 to 20 microns and an average particle size of 9 to 10 microns. When the lower limit of the particle size is smaller than 5 microns, the durability of the toner powder is reduced, and contamination is increased. Besides, the powder fluidity is lowered causing deterioration of the developing property and preventing a smooth supply of toner powder.When the upper limit of the particle size is larger than 20 microns, the density of the toner image becomes small causing gaps to form in the layer and enlarging the layer thickness. Such enlargement of the layer thickness leads to the thermal conduction not being effected smoothly in the case of high speed copying, which results in the offset phenonmenon (cold off-set) due to insufficiency of fixation. This phenomenon can be prevented by increasing the amount of the releasing agent, for instance, to 1 5 to 20% weight relative to the toner resin, but in such a case, the powder fluidity is lowered to exert bad influences upon the developing property and the supply of toner powder. Nevertheless, the toner powder thus obtained shows adequate fluidity with an angle of repose of about 430 to 450.

The surface-coated silica powder to be used as the component (B) is obtainable by coating the surfaces of particles of silica with the base resin in the toner powder. Such base resin usually has a softening point of 50 to 1 300C, preferably of 70 to 1200 C.

As the starting uncoated silica, there may be used any conventional silica such as the so-called "dry method silica" obtained by the hydrolysis of silicon tetraoxide in an oxyhydrogen flame or the socalled "hydrophobic silica" obtained by etherifying at least 75% of the hydroxyl groups present on the surfaces of the particles of the dry method silica with hydrophobic organic groups.

The coating may be carried out, for example, by dispersing the starting silica in a solution of the base resin in a suitable solvent, and subjecting the resulting dispersion to drying, followed by granulating and sieving. As to the mixing porportion of the starting silica and the base resin, there is no particular limitation. Usually, from 1 to 10 parts (by weight), preferably 2 to 5 parts, of the base resin are used relative to 100 parts of the starting silica. In the case of the proportion of the base resin being smaller than the lower limit, the deterioration of picture quality may be caused when the recovered toner is used. In the case of the proportion being larger than the upper limit, the toner film formation on the surfaces of the carrier and the photoelectric conductor is not sufficiently prevented.

The particle size of the surface-coated silica is generally 5 microns or less, preferably 1 micron or less. With a particle size larger than 5 microns, a sufficient effect by the use of silica is not expected. For example, the number of particles of the silica powder held by one particle of the toner powder is decreased so that the characteristics are lowered and the prevention of the toner film formation on the surfaces of the carrier and the photoelectric conductor is reduced.

The toner composition of the invention comprises, as essential components, the toner powder and the surface-coated silica powder. The mixing proportion of these essential components may be optionally decided. Usually, the amount of the surface-coated silica powder may be in the range of 0.1 to 1% (by weight), preferably 0.2 to 0.8%, on the basis of the total weight of the toner composition.

When the amount is smaller than 0.1%, the effect for prevention of the toner film formation on the carrier or the photoelectric conductor surface is lowered. When the amount is larger than 1%, the disorder of the picture such as fogging, decrease of resolving power and lowering of concentration is readily caused in the case of continuous resupply of undeveloped toner as recovered to the developing macHine, and, at the same time, wear and tear of the developing machine are accelerated.

The preparation of the toner composition may be carried out, for example, by mixing the essential components in a conventional mixing machine (e.g. an Henschel mixer) at room temperature for a certain period of time (for instance, for 5 to 10 minutes in the case of the Henschel mixer).

The toner composition of the invention possesses excellent powder fluidity and electric charge stability as in a conventional toner composition comprising toner powder and uncoated-silica powder. In addition, toner film formation on the carrier or the photoelectric conductor can be efficiently prevented and the progress of deterioration is extremely delayed even in the case of a large number of repetitions of copying. In the conventionl toner composition containing uncoated-silica powder, scattering of the toner composition is caused at mixing with the carrier to contaminate the inside of the copying machine, and wear and tear of the developing machine becomes marked so as greatly to shorten the life of the machine.In the toner composition of the invention, its mixing with the carrier can be accomplished with ease, and wear and tear of the developing machine are reduced. A noteworthy advantage of the toner composition of the invention is that, even in the case of continuous resupply of the undeveloped toner as recovered to the developing machine, disorder of picture and lowering of picture concentration are not produced.

The present invention will be hereinafter explained further in detail by the following Examples in which Comparative Examples are also shown. In these Examples and Comparative Examples, part(s) and are by weight, unless otherwise indicated.

Example 1 Preparation of surface-coated silica powder Dry method silica powder ("Aerosil 200" (trade mark) manufactured by Degussa) (100 parts) is added to a resin solution obtained by dissolving a polystyrene resin ("Picolastic D-1 25" (trade mark) manufactured by Esso Research s Engineering Co.) (5 parts) in acetone (100 parts) and toluene (2900 parts), and the resulting mixture is stirred for about 30 minutes with the aid of a high speed stirrer to yield a dispersion, which is subjected to spray drying, granulation and sieving to obtain a surface-coated silica powder having a particle size of 5 microns or less.

Preparation of toner composition: To toner powder (100 parts) having an average particle size of 12 microns and comprising the above-mentioned "Picolastic D-1 25" (trade mark) as the base resin and carbon black, a metalcontaining dye and low molecular weight polyethylene as additives, the above-obtained surface-coated silica powder (0.5 parts) is added, and the mixture is kneeded uniformly with the aid of a Henschel mixer to yield a toner composition.

Copying test:- The toner composition (30 parts) is added to a surface-oxidized iron powder ("EFV--III" (trade mark) manufactured by Nihon Teppun K.K.) (1000 parts) as the carrier, and the resulting mixture is kneaded uniformly to make a developing agent, which is used in the copying test.

A copying machine having an amorphous selenium drum as the photoelectric conductor is remodelled so that the recovered toner composition after drum cleaning is automatically circulated into the toner-supplying part, and the thus remodelled apparatus is used for copying.

Copying is repeated continuously to yield 30000 sheets of copied paper. The picture obtained with the use of the toner composition shows a lower background concentration and a better degree of resolution and is superior in concentration of image-formed portions and in clearness of border portions, as compared with the pictures obtained by the use of the said toner powder itself (Comparative Example 1 ) and by the use of a mixture of the toner powder (100 parts) with hydrophobic silica powder ("Aerosil R 972" (trade mark) manufactured by Degussa) (0.5 part) (Comparative Example 2).The detailed results of the comparison are shown in Table 1, as follows:- TABLE 1

Toner Toner Present Comparati ve Comparative composition invention Example 1 Example 2 Thermal stability (45sC) & x () Moisture stability (29 'C, A 80%) Miscibility of toner x with carrier Initial Background picture concentration Resolving degree Concentration of image-formed portion Clearness of border portion Copied Background 0 x (fogging A (fogging picture concentration being caused being caused after after 10000 after 12000 30000 times of times of times of copying copying -- Resolving x (reduced I(reduced degree after 12000 after 20000 times of times of Concentration O x (reduced A (reduced of image-formed after 10000 after 12000 portion times of times of copying) copying) Clearness of x (reduced A (reduced h (reduced ; border portion after 12000 after 20000 times of times of copying) copying) !ring of toner Little Little Much Wear and tear of rotating I None None Observed on part of developing whole surface machine Example 2 Preparation of surface-coated silica powder:: To a resin solution obtained by dissolving the "Picolastic D-1 25" (trade mark) used in Example 1 (2 parts in acetone (100 parts) and toluene (2900 parts), hydrophobic silica powder ("Aerosil R 972") (trade mark) (100 parts) and vinyltris (P-methoxyethoxy)silane ("KBC-1003" manufactured by Shinestu Chemical Industry Co., Ltd.) (0.01 part) are added, and the resulting mixture is stirred for about 30 minutes with the aid of a high-speed stirrer to obtain a dispersion, which is subjected to spray drying, granulation and sieving to yield surface-coated silica powder having a particles size of 5 microns or less.

Preparation of toner composition: To the same toner powder having an average particle size of 12 microns as used in Example 1 (100 parts), the above-obtained surface-coated silica powder (0.8 part) is added, and the mixture is kneaded uniformly with the aid of a Henschel mixer to obtain a toner composition.

Copying test:- In the same manner as in Example 1, a developing agent is prepared, and continuous copying is carried out to yield 3000 sheets of cdpied paper. As will be understood from the test results shown in Table 2 below, the use of the toner composition affords an extremely advantageous effect in comparison with Comparative Examples 1 and 2.

TABLE 2

Toner Present Comparative Comparative composition invention Example 1 Example 2 . .

Thermal stability (45"C) x x Moisture stability A (20 C, 80%) Miscibility of toner x with carrier Initial Background picture concentration () Resolving degree Concentration of image-formed portion clearness of border portion Copied Background O x (fogging A (fogging picture concentration being caused being caused after after 10000 after 12000 30000 times of times of times of copying Resolving x (reduced h (reduced degree after 12000 after 20000 times of times of copying) cbpying) Concentration O x (reduced A (reduced of image-formed after 10000 after 12000 portion times of times of copying) copying) Clearness of x (reduced A (reduced border portion t) after 10000 after 20000 times of times of copying) copying) Scattering of toner Little Little Much inside copying apparatus Wear and tear of rotating None None Observed on part of developing machine whole surface Example 3 Preparation of surface-coated silica powder To a resin solution obtained by dissolving a styrene-acryl copolymer resin ("Highmer SBM 600") (trade mark) manufactured by Sanyo Chemical Industries Ltd.; Mw, 6000) (5 parts) in acetone (100 parts) and toluene (2900 parts), dry method silica powder (100 parts) is added, and the mixture is stirred for about 30 minutes with the aid of a high speed stirrer to obtain a dispersiin, which is subjected to spray drying, granulation and sieving to yield a surface-coated silica powder having a particle size of 5 microns or less.

Preparation of toner composition: "Highmer SBM "600" (trade mark) (100 parts), low molecular weight polypropylene ("Biscole 550P" (trade mark) manufactured by Sanyo Chemical Industries Ltd,; Mw, 4000) (5 parts), carbon black ("RAVEN 1250" (trade mark) manufactured by Columbian Carbon Japan Ltd.; pH, 7.0) (8 parts) and an electrification-regulating agent ("Bontron S-31" (trade mark) manufactured by Orient Chemical) (2 parts) are mixed preliminarily with the aid of a mixer and then melt-kneaded by a heat roll. The resulting mixture is subjected to a rough crushing, fine crushing and sieving to yield a toner powder having a particle size of 5 to 20 microns with an average particle size of 9 to 10 microns.The angle of repose of the thus obtained toner powder is 430 (determined with the aid of "Powder Tester" manufactured by Hosokawa Tekko).

To the toner powder (100 parts) as above prepared, the previousiy obtained surface-coated silica powder (0.5 part) is added, and the mixture is kneaded uniformly with the aid of a Henschel mixer to yield a toner composition.

The toner composition as obtained above may be used in the same manner as in Example 1.

Examples 4 to 9 In the same manner as in Example 1 but using a toner powder prepared from the components shown in Table 3, there is obtained a toner composition: TABLE 3

Example 4 5 6 7 8 9 Component. PicolasticD-150*1) 100 100 50 100 - - (part(s)) Picolastic D-125 *2) - - 50 - - - Highmer SBM 600 ~ ~ ~ ~ 100 100 Biscole 550P 5 3 - - 5 5 AC Polyethylene 6A *3) - - 5 - - - AC Polyethylene 617 *4) - - - 5 - - Monarch 880 8 8 8 8 ~ ~ ~ RAVEN 1250 2 - - 8 8 8 Bontron S-41 2 2 2 2 1 3 Angle of repose 44 430 440 440 43 45 Note: *l) Polystyrene resin manufactured by Esso Research & Engineering Co., Mw, about 5000; *2) Polystyrene resin manufactured by Esso Research & Engineering Co., Mw, about 3000; *3) Low molecular weight polyethylene manufactured by Allied Chemical Corp.; *4) Low molecular weight polyethylene manufactured by Allied Chemical Corp.; *s) Carbon black manufactured by Cabot, pH, 8.0.

Examples 10 and 11 In the same manner as in Example 1 but using a toner powder prepared from the components as shown in table 4, there is obtained a toner composition as follows: TABLE 4

Example 10 11 Component Priolite VTL *t) 100 100 (part(s)) AC Polyethylene 6A *2) 5 Biscole 500* - 5 Monarch 880*3) 8 8 Bontron N-O2 *4) 1.5 Bontron N-01 *5) - 2 Angle of repose 43O 44- Note: *l) Vinyltoiuene-butadiene copolymer resin manufactured by Goodyear, Mw, 78000; *2) Low molecular weight polyethylene manufactured by Allied Chemical Corp,; *3) Carbon black manufactured by Cabot, pH, 8.0; *4) Electrification-regulating agent manufactured by Orient Kagaku; *5) Electrlficatlon-regulating agent manufactured by Orient Kagaku.

Claims (9)

1. A toner composition for electrostatic photography which comprises (A) toner powder comprising a base resin and a colouring agent and having an average particle size of not more than 30 microns and (B) silica powder coated with the base resin in the toner powder at the surface.

2. A toner composition according to claim 1, wherein the toner powder (A) comprises (a) a styrene resin having a weight average molecular weight of not less than 3,000, (b) furnace type carbon black of pH 6.5 to 9.5, (c) low molecular weight polyolefin having a weight average molecular weight of not more than 10,000 and (d) an electrification-regulating agent in a weight proportion of 100:5-15:3-10:0.5-5.

3. A toner composition according to claim 1 or 2, wherein the content of the surface-coated silica powder (B) is 0.1 to 1% by weight based on the total weight of the toner composition.

4. A toner composition according to any one of claims 1 to 3, wherein the surface-coated silica powder (B) comprises silica powder and the base resin is coated on the surface of the silica powder in a weight proportion of 100:1-10.

5. A toner composition according to any one of claims 1 to 4, wherein the surface-coated silica powder has a particle size of not more than 5 microns.

6. A toner composition according to any one of claims 1 to 5, which is usable for the heat roll fixation method.

7. A toner composition according to any one of claims 2 to 6, wherein the low molecular weight polyolefin is a low molecular weight polyethylene or a low molecular weight polypropylene.

8. A toner composition according to any one of claims 2 to 7, wherein the electrificationregulating agent is a nigrosine dye or a metal containing dye.

9. A toner composition according to claim 1 substantially as herein described with reference to any of the specific examples.