GB2070030A

GB2070030A - Toner for Electrophotography and Method of Preparing the Same

Info

Publication number: GB2070030A
Application number: GB8102721A
Authority: GB
Original assignee: Konica Minolta Inc
Current assignee: Konica Minolta Inc
Priority date: 1980-02-04
Filing date: 1981-01-29
Publication date: 1981-09-03
Also published as: DE3103591A1; GB2070030B

Abstract

A toner for developing electrostatic latent images is prepared by polymerizing a monomer (especially a vinyl monomer and especially by aqueous suspension polymerization) in the presence of a substance capable of endowing the monomer with thixotropic properties (e.g. hydrophilic silica, aluminium oxide, aluminium silicate or titanium oxide). Such a toner avoids the disadvantages of prior art methods involving crushing (toner coagulation cost) or polymerization (poor image quality or covering power).

Description

SPECIFICATION Toner for Electrophotography and Method of Preparing the Same The present invention relates to toner for development of electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing, etc. and a method for preparing such toner.

Methods to develop the electrostatic latent image are grossly classified into two categories, the so-called wet development process and dry development process. The wet development process uses various sorts of developer composed of various pigments or dyes finely dispersed in insulating solvent while the dry development process to which the cascade development method, fur brush development method, magnetic brush development method, impression development method, powder cloud method, etc.

belong uses developer of fine powdery form composed of so-called toner containing a compound of natural or synthetic resin with a coloring agent such as carbon black dispersed therein. Thus, the present invention relates specifically to toner used in the latter process or dry development process.

Toner used in the dry development process is prepared by mixing a compound of natural or synthetic resin with a coloring agent such as carbon black, fusing and kneading the mixture at high temperature, and crushing and pulverizing the kneaded mixture after cooling down to room temperature. However, toner that has heretofore been prepared as described above has wellrecognized defects as described below.

Namely, since fusing and pulverizing steps can not be dispensed with, it is necessary that the compound used can be fluidized at proper temperature to allow uniform dispersion of a pigment or the like therein and that the compound loaded with such pigment or the like can be crushed and pulverized into powder of a desirable grain size at a considerable rate by a pulverizer used. However, a compound that can readily be crushed and pulverized, if used, is liable to further crushing and pulverization in the electrophotographic copier system, causing failures, such as soiling of the copier system and grey background in copy images.Further, as toner is crushed and pulverized during the copy process, the pigment or dye embedded in the resin compound appears in the surface of toner grains, which may make toner grains slightly partially nonuniform in their frictional charging performance and further cause a problem of inferior wetproof performance depending on the type of pigment or dye used. On the other hand, use of a compound that is just readily fusible may cause toner caking as well as so-called "toner filming" on the surface of photoconductor.

Defects of the above toner which are concieved more significant are the shape and size distribution of its grains. Since toner grains prepared by crushing and pulverization are irregular and angular in shape, so they are liable to coagulate, which may presumably be an undesirable factor in the stability of toner in stock and dispensing performance of toner in use. The most significant defect is that it is very difficult to crush and pulverize the compound into a fine powder to desirable grain sizes uniform in grain size distribution.Toner used for developer of the dry electrophotographic process generally has a mean grain size of 10 y in order but if a compound that may be crushed and pulverized at an economical rate is selected and processed by the crushing and pulverizing method, considerably finer grains having a grain size smaller than 1 y are also produced and yet coarser grains larger than several tens microns are still left mixed in the final product though small in number. Generally, these few finer or coarser grains affect the performances of toner very adversely in the image quality, and particularly the resolution, sharpness and grey background.Further, comprising multiple process steps of fusion, kneading, crushing and pulverization, and classification, the crushing and pulverization method has an additional defect that the production cost is expensive.

Beside toner prepared by the crushing and pulverizing method as described above, toner for the dry development process prepared by the polymerization process was proposed by Japanese Patent Examined Publication Nos.

10,231/1961, 14,895/1976, 17,735/1978, 17,736/1978, 17,737/1978 and 51,830/1972.

These publications proposed methods of the socalled suspension polymerization process by which a mixture of monomer, or monomers, polymerization initiator, dispersion stabilizer and coloring agent is suspended in water and polymerized to prepare toner directly. What are common to all toner products prepared by these polymerization methods are the spherical shape of toner grains and embedding of the pigment in toner compound. The problems inherent to the crushing and pulverization methods, problem of toner coagulation due to unfavorable shaping of toner grains, problem of expensive production cost, etc. or the problems that are the defects of the crushing and pulverizing method have thereby been improved.However, these toner products were found still unsatisfactory in its performance regarding the image quality, such as resolution, image sharpness and grey background, and further there was a defect with them that having not been strongly shorn, they perform poorer in the covering power.

Accordingly, it is the primary object of the present invention to provide both new toner for development of electrostatic latent images that covers up various defects of not only toner prepared by the crushing and pulverizing process but one by the polymerization process of prior art, and a method of preparing such new toner.

It is another object of the present invention to provide both toner for development of electrostatic latent images that exhibits excellent performances in the image quality, such as resolution, image sharpness and grey background, and a method of preparing such toner.

It is another object of the present invention to provide both toner for development of electrostatic latent images that is improved in the covering power, and a method of preparing such toner.

These and other objects of the present invention will be achieved with toner for development of electrostatic latent images which comprises a polymer compound prepared by polymerizing a monomer under presence of a substance capable of endowing such monomer with a thixotropic property (the above toner and substance will hereafter be called "toner of the present invention" and "thixotropic substance", respectively).

Toner of the present invention is characterized by remarkable improvements not only in various defects toner prepared by the crushing and pulverizing process and one by the polymerization process of prior art have, for example, inferior performances in the image quality, such as resolution, image sharpness and grey background, but in the covering power. These improvements will never be expected from conventional toner.

In the present invention, the term "thixotropic property" refers to a characteristic state that continuous quasi structures are formed in a monomer mass due to tractive forces between particles dispersed therein and in case shearing forces are applied these quasi structures are destructed to increase the fluidity of monomer mass while when the monomer mass is left to stand these quasi structures are restored to increase the consistency of monomer.

A judgement whether a given monomer sample is endowed with the thixotropic property can be made by determing the relation between the shearing velocity and shearing force generated in that sample with use of a Couette type viscometer.

This thixotropic property was discussed and its measuring method was described, for example, by H. Green and R. N. Weltmann in their papers, "Industrial and Engineering Chemistry Analytical Edition", vol. 1 5, pp. (1943) and ibid, vol. 18, pp. 167 (1946), and by H. Green in his book "Industrial Rheology and Rheological Structures", John Wiley Ga Sons, New York (1949).

The thixotropic property can be measured using a Couette type viscometer as mentioned above by a method as follows.

A vessel having an external cylinder and an internal one is filled between these two cylinders with a sample whose thixotropic property is to be determined and the external Cylinder is driven at a certain angular speed [Q]. The sample liquid then moves at a shearing velocity [D] in the sence of revolution of external cylinder driving the internal cylinder by an angle of [0], when the shearing stress [S] generated in the sample decreases as the quasi structures are destructed with time till a steady state, at which the destruction and generation of quasi structures are presumably equilibrated, is reached.

As the angular velocity [Q] of external cylinder and the shearing velocity [D] given to the sample liquid increase, more quasi structures are destructed to further decrease the shearing stress generated in the sample and thereby reduce the angle of revolution [(i)] of the internal cylinder.

Therefore, if the shearing velocity given to the sample is first increased from a low level to a predetermined limit level and then decreased while the shearing stress generated in the sample being recorded, a fluidity characteristic curve composed of two paths or so-called histerysis loop is obtained. If a sample is endowed with a thixotropic property, the relation between the shearing velocity and shearing stress will be represented by a hysterysis loop as mentioned above.

For the thixotropic substance of the present invention, various substances may be used, among which hydrophilic silica, aluminium oxide, aluminium silicate or titanium oxide is preferable for use.

The thixotropic substance of the present invention is usually used in a quantity of 0.01 to 300 parts by weight per 100 parts by weight of the monomer component used. However, the above thixotropic substances or hydrophilic silica, aluminium oxide, aluminium silicate and titanium oxide can fully achieve the intended objects of the invention using 0.01 to 100 parts by weight per 100 parts by weight of the monomer component and, therefore, by contrast to other thixotropic substances that are used in larger quantities, they are unlikely to affect the product toner adversely in its charging performamce or make the polymerizing monomer more viscous and, therefore, difficult to handle during the preparing process.

In the present invention, if an anisotropically shaped hydrophilic silica, aluminium oxide, aluminium silicate or titanium oxide is used, addition of 0.01 to 3.0 parts by weight to 100 parts by weight of the monomer component is enough to fully achieve its intended objects. The thixotropic substance of the present invention preferably has a grain size of 0.005 to 1.0 and more preferably 0.01 to 0.1 ,u.

The monomer or monomers used in the present invention can be widely selected from all polymerizable monomers, among which vinyl monomers are preferable for use.

Examples of the vinyl monomers are styrene and its derivatives including o-methylstyrene, mmethylstyrene, p-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-n-butylstyrene, p-tert butylstyrene, p-n-hexylstyrene, p-n-octylstyrene, p-n-nonlystyrene, p-n-decylstyrene, p-ndodecylstyrene, p-methoxystyrene, pphenylstyrene, p-chlorostyrene and 3,4dichlorostyrene, among which styrene is most preferable for use.Other examples of vinyl monomers are ethylenic unsaturated monoolefins including ethylene, propylene, butylene and isobutylene, vinyl halides including vinyl chloride, vinylidene chloride, vinyl bromide and vinyl fluoride, vinyl esters including vinyl acetate, vinyl propionate, vinyl benzoate and vinyl butyrate, esters of a-methylene aliphatic nonocarboxylic acids including methyl acrylate, ethyl acrylate, nbutyl acrylate, isobutyl acrylate, propyl acrylate, noctyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methyl achloroacrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate, vinyl ethers including vinyl methyl ether, vinyl ethyl ether and vinyl isobutyl ether, vinyl ketones including vinyl methyl ketone, vinyl hexyl ketone and methyl isopropenyl ketone, and vinylnaphthalenes.

Toner of the present invention contains a polymer compound prepared by polymerizing a monomer or monomers under presence of a thixotropic substance. For the method of polymerization to prepare the above polymer compound, an arbitrary method of polymerization that is applied to the synthesis of the ordinary toner binder, for example, the suspension polymerization can widely be applied, though the suspension polymerization is most preferable as a method of polymerization used in the present invention. The polymer compound of the present invention can be loaded with additives to improve toner in its performances, such as coloring agent, charge control agent, fluidizing agent, parting agent (anti-offset agent) and magnetic material, during the process of its polymerization. Toner thus prepared may be used as it is or further pulverized to a desirable grain size for toner.In any case, toner is grained 1 to 50 and preferably 7 to 30 in grain size.

Further, toner of the present invention can be prepared by loading a polymer compound of the present invention that is formed with the above additives to improve toner in its performances, mixing and kneading the loaded compound, and crushing and pulverizing the kneaded compound to a desirable grain size after cooling.

If iarger than 50 14 in grain size, toner of the present invention is unsuitable for practical reproduction of images since it produces remarkably granular appearance. Thus, the preferable grain size of such toner is smaller than 30 y. On the other hand, toner of the present invention that is finer than 1 y in grain size is unsuitable for use, since it is liable to produce soiling of the surface of photoconductor or the socalled toner filming phenomenon, lowered sensitivity of photoconductor, inferior image quality, etc. Thus, the preferable grain size is larger than 7 y.

The method of preparing toner of the present invention will be understood more fully by the following detailed description. The suspension polymerization is executed by an ordinary method at 45OC to 1 000C under flowing nitrogen gas and under presence of a dispersing agent, for example, gelatin, starch, polyvinyl alcohol, barium sulfate, calcium sulfate, barium carbonate, magnesium carbonate, calcium phosphate, talc, clay, silicic acid, or powder of a metal oxide.

Toner of the present invention contains a polymer compound, which can be prepared by polymerizing one of the aforementioned monomers or by copolymerizing or individually polymerizing a plurality of these monomers.

Further, such monomer or monomers can be polymerized or copolymerized under presence of an additional polymer that is used in the conventional toner.

Further, the polymer compound of the present invention that is prepared may be loaded with the above additional polymer for mixing and kneading, and crushing and pulverization to prepare toner of the present invention.

For the coloring agent used in the present invention, a proper pigment or dye is arbitrarily used. Many toner coloring agents are wellknown.

Examples of such toner coloring agents are carbon black, nigrosine dye, aniline blue, calco oil blue, chrome yellow, ultramarine blue, Du Pont oil red, quinoline yellow, methylene blue chloride, phthalocyanine blue, malachite green oxalate, lamp black, rose bengale, and their mixtures. The pigment or dye used must present in toner in a quantity enough to strongly color such toner so clearly visible images may be formed on paper.

Thus, for example, for conventional exographic copies of printed documents, toner may be colored, for example, by a black pigment such as carbon black or a black dye such as amaplastblack dye. This pigment or dye is preferably used in a quantity of about 3 to 20 wt % on the basis of the total weight of toner.

Further, in the present invention, a magnetic material that is useful for toner to be used as a single component type developer may also be used.

The above magnetic material is a substance that is strongly magnetized in the direction of any field that presents, preferably colored black, well dispersible in resin, chemically stabilized, and further preferably readily available in a form of fine powder having a grain size of 1 1,t4 or smaller.

More specifically, the grain size of magnetic material is preferably 0.01 to 1 ju and more preferably 0.1 to 1 y. Among others, magnetite (ferrosoferric oxide) is most preferable for use.

Typical magnetic or magnetizable materials are metals including cobalt, iron and nickel, alloys between metals, such as aluminium, cobalt, steel, lead, magnesium, nickel, tin, zinc, antimony, beryllium, bismuth, cadmium, calcium, manganese, selenium, titanium, tungsten and vanadium, and mixtures of these alloys, metal compounds including metal oxides, such as iron oxide, copper oxide, nickel oxide, zinc oxide and magnesium oxide, fire resistant nitrides including vanadium nitride and chromium nitride, carbides including tungsten carbide, ferrites, and mixtures of the above. The quantity to add to toner is preferably about 50 to 300 parts by weight and more preferably 70 to 200 parts by weight per 100 parts by weight of the resin component of toner.

In preparing toner of the present invention, a polymerization initiator may be added when a monomer or monomers start polymerization.

For the above polymerization initiator, an ordinary oil soluble polymerization initiator may be used in a temperature range of ordinary use.

For example, benzoyl peroxide, lauroyl peroxide, 2,2'-azobisisobutyronitrile, 2,2 '-azobis-(2,4dimethylvaleronitrile), orthochlorobenzoyl peroxide, orthomethoxybenzoyl peroxide, etc. may be used. The polymerization is conducted under the atmospheric pressure or a higher pressure.

In the present invention, as a method of polymerization, the cross-linking polymerization may be used. Though an arbitrary means may be used for the cross-linking polymerization ordinarily, a cross-linking agent, such as divinylbenzene, ethylene glycol dimethacrylate, trimethylolpropane triacrylate or acryl methacrylate is added for this purpose.

In the present invention, a lower molecular weight olefin polymer may be added at any step of the process of preparing.toner of the present invention to endow the product toner with the offset-free property.

This lower molecular weight olefin polymer used in the present invention is an olefin polymer or olefin/olefin copolymer containing only an olefin monomer or monomers, or olefin/non-olefin copolymer containing a non-olefin monomer or monomers beside the olefin component, having a comparatively low molecular weight.

In the above, the term "comparatively low molecular weight" is used in its usual meaning, which is preferably between 1,000 and 45,000 and more preferably between 2,000 and 6,000.

Examples of olefin monomers are all olefins including ethylene, propylene, butene-1, pentene 1, hexene-1, octene-1, their isomers whose unsaturated bond is located differently, and their derivatives with an alkyl side chain or chains, such as 3-methyl- 1 -butene and 3-methyl-2-pentene.

Examples of non-olefin monomers to be used with olefin monomers to form olefin/non-olefin copolymers are vinyl ethers including vinyl methyl ether, vinyl n-butyl ether and vinyl phenyl ether, vinyl esters including vinyl acetate and vinyl butyrate, haloolefins including vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, vinyl chloride, vinylidene chloride and tetrachloroethylene, acrylate and methacrylate esters including methyl acrylate, ethyl acrylate, nbutyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, stearyl methacrylate and acrylic derivatives including acrylonitrile and N,N-dimethylacrylamide, diethyl fumarate, and p-pinene.

When provided as a copolymer, the lower molecular weight olefin polymer used in the present invention is either an olefin/olefin copolymer containing a plurality of olefin monomers as mentioned above or an olefin/nonolefin copolymer containing at ieast one olefin monomer as mentioned above and at least one non-olefin monomer as mentioned above.

Examples of the above olefin/olefin copolymer are ethylene/propylene, ethylene/butene, ethylene/pentene, ethylene/butene, propylene/pentene, ethylene/3-m ethyl- 1 -butene and ethylene/propylene/butene copolymers while examples of the above olefin/non-olefin copolymer are ethylene/vinyl acetate, ethylene/vinyl methyl ether, ethylene/vinyl chloride, ethylene/methyl methacrylate, ethylene/methyl methacrylate, ethylene/acrylic acid, propylene/vinyl acetate, propylene/vinyl ethyl ether, propylene/ethyl acrylate, propylene/methacrylic acid, butene/vinyl methyl ether, butene/methyl methacrylate, pentene/vinyl acetate, hexene/vinyl butyrate, ethylene/propylene/vinyl acetate and ethylene/vinyl acetate/vinyl methyl ether copolymers.

When an olefin/non-olefin copolymer is used for the lower molecular weight olefin polymer of the present invention, its olefin content is preferably as high as possible, for there is a general tendency that the smaller the olefin content of a copolymer used, the lower the parting performance of such copolymer and further the more inferior the performances of the product toner is fluidity, image quality, etc.

Therefore, it is desirable to increase the olefin content of the copolymer as high as possible.

Particularly, a copolymer whose olefin content is about 50 wt % or more is effectively used in the present invention.

The lower molecular weight olefin polymer used in the present invention is added in a quantity of 1 to 20 parts by weight and preferably 3 to 1 5 parts by weight per 100 parts by weight of the polymer compound of the present invention, for 1 part by weight or less, if added, is hardly effective while addition of 20 parts by weight or more is liable to cause gelation in the process of preparing the polymer compound.

The polymer compound prepared as described above may have an arbitrary molecular weight though its preferable range is between 10,000 and 1,000,000 and its more preferable range between 50,000 and 500,000.

The polymer compound of the present invention is useful when its softening point as measured by the ball and ring method is between 100 and 1 700C and also when the glass transition point is between 40 and 11 00C. If the softening point is lower than 100 C, there is a high possibility of excessive pulverization of toner and appearance of toner filming phenomena while at a softening point beyond 1 700C the compound becomes too hard to be crushed and pulverized and further it requires larger thermal energy for fixing to lower the thermal efficiency at the time of fixing.On the other hand, in case the glass transition point is lower than 400 C, the product toner becomes liable to coagulate due to the cold flow phenomenon since a temperature below 400C is specified for the keeping condition of toner, while at a glass point above 11 00C toner has such a defect that, notwithstanding the fixing roll, except when made of metal, exhibits limited heat resistance because of its material, for example, a fixing roll made of tetrafluoroethylene or teflon (supplier: Du Pont) becoming liable to wear and also decompose above 2500C, and therefore its maximum working temperature is rather limited, toner requires a higher fixing temperature, so particularly when fixed at a high rate it cannot be fixed fully.

The following examples are included merely to aid in the understanding of the invention, and variations may be made by one skilled in the art without departing from the spirit and scope of the invention. It is noted that in these examples the term "parts of weight" will be abbreviated "parts".

Example 1 70 part of styrene, 30 parts of n-butyl methacrylate, 5 part of Mitsubishi Carbon Black MA-600 (supplier: Mitsubishi Gas Chemical), 0.3 part of Azo Oil Black (R) (supplier: National Aniline), 2 parts of lauroyl peroxide and hydrophilic silica Aerosil-200 (supplier: Nihon Aerosil) were mixed by a sand stirrer for full dispersion. The mixture was then added in a 2 liter separable flask filled with 1.25 wt % aqueous polyvinyl alcohol.

The mixture was then heated up to 650C under agitation using a TK homogenizer (supplier: Tokushu Kogyo) driven at a rate of 4,000 r.p.m.

30 min after the temperature was elevated, the mean grain size of suspended grains was estimated to 10 to 1 5 15,fb. Thereafter, the TK homogenizer was switched over to an ordinary agitator, which was driven at a rate of 100 r.p.m.

to complete the polymerization reaction of 6 hr.

After completion of the polymerization, the mixture was cooled and the grained compound was filtered out, repeatedly dehydrated and washed, and then dried to produce toner of the present invention of negative polarity that has a mean grain size of 13 y.

5 parts of toner prepared as above and 95 parts of Iron Powder Carrier DSP (supplier: DOWA Teppun Kogyo) were mixed to prepare a two component developer. This developer was used with a commerically available PPC unit of model U-Bix V (supplier: Konishiroku Photo Industry) to conduct a copy test reproducing copy images. The copy images thus obtained were very clear and sharp showing excellent performance in reproduction of details and also high image darkness.

Example 2 The same procedure as in Example 1 was repeated to prepare toner except that 3 parts of Aluminium Oxide C (supplier: Nihon Aerosil) instead of hydrophilic silica Aerosil-200. Further, a two component developer was prepared by quite the same method as in Example 1. The same copy test as in Example 1 was then made, when copy images reproduced were very clear and sharp, showing excellent performance in reproduction of details and also high image darkness.

Comparative Example 1 Control toner of negative polarity was prepared by quite the same method as in Example 1 except that Aerosil-200 was omitted from the formulation of Example 1. A two component developer was prepared by the same method as in Example 1 using the above toner and quite the same copy test as in Example 1 was conducted.

The grey background was detected and the resolving power was lower than when toner of the present invention as prepared in Example 1 was used.

Example 3 The same procedure as in Example 1 was repeated to prepare toner of the present invention except that 3 parts of aluminium silicate Osmos-N (supplier: Shiraishi Kogyo) was added instead of Aerosil-200 in the formulation of Example 1. This toner was used to prepare a two component developer by quite the same method as in Example 1 and the same copy test as in Example 1 was conducted, when copy images reproduced were very clear and sharp, showing excellent performance in reproduction of details and high image darkness.

Example 4 85 parts of styrene, 1 5 parts of butyl acrylate, Misubishi Carbon Black MA-600, 1 part of charge control agent Oil Black BW (supplier: Orient Chemical), 2 parts of azobisbutyronitrile and 0.5 part of hydrophilic chain silica were mixed by a sand stirrer for full dispersion. The dispersed mixture was added to 1.0 wt % aqueous tricalcium phosphate suspension. The mixture was heated to 700C under agitation using a TK homogenizer driven at a rate of 4,000 r.p.m. 30 min after the temperature was elevated, the mean grain size of suspended grains was estimated to 10 to 1 5 y. Thereafter, the TK homogenizer was switched over to an ordinary agitator, which was driven at a rate of 100 r.p.m. to complete the polymerization reaction of 6 hr.After completion of the polymerization, tricalcium phosphate was removed by a treatment with hydrochloric acid and grained compound was then washed and dried to prepare toner of the present invention of negative polarity having a mean grain size of 13 ,u. This toner was used to prepare a two component developer by the same method as in Example 1 and the same copy test as in Example 1 was conducted, when clear and sharp images without grey background were reproduced.

Example 5 70 parts of styrene, 30 parts of n-butyl methacrylate, 5 parts of carbon black "Raven 1250" (supplier: Colombia Carbon), 0.2 parts of Oil Black BW, 3 parts of azobisisobutyronitrile, 5 parts of lower molecular weight polypropylene Viscol 550P (supplier: Sanyo Chemical Industries) and 3 parts of Titanium Oxide P-25 (supplier: Degussa) were mixed in a ball mill for full dispersion to prepare a polymerizable composition. This composition was added into a 2 liter separable flask filled with 1.5 wt % aqueous gelatin and the mixture was agitated 30 min by a TK Homogenizer driven at a rate of 4,000 r.p.m.

Thereafter, the TK homogenizer was switched over to an ordinary agitator, which was driven at a rate of 200 r.p.m. to complete the polymerization reaction of 8 hr to 80 C. After completion of polymerization, the grained compound was filtered out, washed with water and dried to prepare toner of the present invention of negative polarity having a mean grain size of 13 y. This toner of the present invention was mixed with carrier by the same method as in Example 1 to prepare a two component developer.

The above two component developer was used to conduct a copy test by the same method as in Example 1, when clear and sharp images of high darkness without any grey background were obtained. Thus, a combined use with a lower molecular weight polypropylene in the present example was found to nothing affect the performances of toner as mentioned above, excellent offset-free performance being exhibited.

Example 6 67 parts of ferrosoferric oxide powder "Mapico Black BL)500" (supplier: Titan Kogyo), 0.3 parts of charge control agent methylene blue chloride, 100 parts of styrene, 0.5 parts of benzoyl peroxide, 0.3 parts of cross-linking agent triethylene glycol methacrylate and 3.0 parts of Aerosil-200 were mixed for dispersion to prepare a polymerizable composition. Meanwhile, 1.0 part of tricalcium phosphate (supplier:Taihei Kagaku) or an inorganic salt hardly soluble in water was put into a 2 liter separable flask and distilled water was added thereto. The above polymerizable composition was then added to the flask and agitated 30 min by a TK Homogenizer driven at a rate of 4,000 r.p.m. Thereafter, the TK homogenizer was switched over to an ordinary agitator, which was driven at a rate of 100 r.p.m.

to complete the polymerization of 8 hr at 800C.

After completion of the polymerization, the grained compound was filtered out and dried to obtain toner for use as a single component developer. This toner was used for the developer as it was with a commercially available PPC unit of Model U-BixT(supplier: Konishiroku Photo Industry) to reproduce toner images, which were very clear and sharp showing excellent performance in reproduction of details and high image darkness.

Comparative Example 2 Control toner for use as a single component developer was prepared by the same method as in Example 6 except that Aerosil-200 was omitted from the formulation of Example 6. Copy images were reproduced by quite the same method as in Example 6. These copy images were not so sharp and dark as those reproduced in Example 6.

Example 7 90 parts of styrene, 10 parts of n-butyl acrylate, 5 parts of Mitsubishi Carbon Black 2300 (supplier: Mitsubishi Chemical Industries), 0.1 part of Oil Black BW, 2 parts of 2,2'-azobis-(2,4dimethylvaleronitrile), 0.5 parts of trimethylelpropane triacrylate, 5 parts of Viscol 550P and 3 parts of Aluminium Oxide C were mixed to prepare a polymerizable composition.

Meanwhile, a 2 liter of separable flask was filled with 1.25 wt % aqueous polyvinyl alcohol, to which the above polymerizable composition was added. From this point on, the same procedure as in Example 5 was conducted to prepare toner of the present invention of negative polarity having a mean grain size of 13 ,u for use in two component developer. This toner was used to prepare a two component developer by the same method as in Example 1. The developer thus prepared was used to conduct quite the same copy test as in Example 1, when clear and sharp copy images were reproduced, showing excellent performance in reproduction of details and high image darkness.

Claims

1. A toner for the development of an electrostatic latent image which comprises a polymer prepared by polymerizing a monomer in the presence of a substance cable of endowing the monomer with thixotropic properties.

2. A toner as claimed in Claim 1, wherein the monomer is a vinyl monomer.

3. A toner as claimed in Claim 1 or 2 wherein the substance capable of endowing the monomer with thixotropic properties is hydrophilic silica, aluminium oxide, aluminium silicate or titanium oxide.

4. A toner as claimed in any preceding Claim which further comprises a low molecular weight polyolefin having weight average molecular weight of 1,000 to 45,000.

5. A toner as claimed in any preceding Claim which further comprises a coloring agent.

6. A toner as claimed in any preceding Claim which consists of fine particles having average diameter of 1 to 50 microns.

7. A toner as claimed in Claim 1 and substantially as hereinbefore described with reference to any of Examples 1 to 7.

8. A method of preparing toner for the development of an electrostatic latent image which comprises polymerizing a monomer in the presence of a substance capable of endowing the monomer with thixotropic properties.

9. A method as claimed in Claim 8, wherein the monomer is a vinyl monomer.

10. A method as claimed in Claim 8 or 9, wherein the substance capable of endowing the monomer with thixotropic properties is hydrophilic silica, aluminium oxide, aluminium silicate or titanium oxide.

11. A method as claimed in any of Claims 8 to 10 wherein polymerization is effected in the presence of a low molecular polyolefin having a weight average molecular weight of 1,000 to 45,000.

12. A method as claimed in any of Claims 8 to 11, wherein polymerization is effected in the presence of a coloring agent.

13. A method as claimed in any of Claims 8 to 12, wherein the substance capable of endowing the monomer with thixotropic properties is present in an amount from 0.01 to 300 parts by weight, based on 100 parts by weight of monomer.

14. A method as claimed in Claim 13, wherein the amount is from 0.01 to 100 parts by weight.

1 5. A method as claimed in Claim 13 wherein the substance capable of endowing the monomer with thixotropic properties is anisotropicallyshaped colloidal silica, aluminium oxide, aluminium silicate or titanium oxide, and is present in an amount from 0.01 to 3.0 parts by weight, per 1000 parts by weight of monomer.

1 6. A method as claimed in any of Claims 8 to 1 5 wherein the monomer is polymerized by suspension polymerization.

17. A method as claimed in Claim 1 and substantially as hereinbefore described with reference to any of Examples 1 to 7.