Liquid Developer for Use in Electrostatic Photography

(a) Field of the Invention

The present invention relates to a liquid developer for use in electrostatic photography, which developer contains wax and a filler for preventing transfer-crush.

(b) Description of the Prior Art

The wet developing method which comprises developing electrostatic latent images formed on electrophotographic sensitive materials, electrostatic recording materials and the like with a liquid developer (which normally comprises dispersing toner particles consisting essentially of a colorant and a resin in a highly insulating carrier liquid) is profitable in that the etching effect is small and so copied images of high resolving power can be obtained. The transfer method which comprises forming images on the above mentioned materials and thereafter transferring said images onto another substrate is profitable in that common paper and plastic film can be used as the transfer sheet, and so, the copies are easy to handle. Accordingly, it may be said preferable to form copies making use of the combination of the wet developing method with the transfer method. However, this system involves the questions that in case a transfer sheet of low smoothness is used, the solid area in the transfer image deteriorates in uniformity as copying is repeated, while in case a transfer sheet of high smoothness is used, the transfer image deteriorates in sharpness as copying is repeated.

Therefore, various proposals have usually been made concerning the liquid developer in order to solve these questions. For instance, it is known that wax should be contained in the toner particles for the purpose of improving the uniformity of the solid image and a spherical substance such as glass beads and a spacer such as polymethacrylate particles should be mixed with and included among the toner particles as the filler for preventing transfer-crush for the purpose of improving the sharpness of the transfer sheet (for instance, Japanese Laid Open Patent Application No. 34328/1974 corresponding to U.S. Pat. No. 3915874 discloses the use of the 0.5-15.mu. spherical substance and Japanese Laid Open Patent Application Nos. 178252/1982, 200049/1982 and 298351/1983 disclose that the spacer whose particle diameter is 20-70.mu. is used as a rule). However, these developers have merits and demerits respectively. The developer comprising the wax-containing toner particles can not achieve the effect of improving sharpness, while the developer which includes the filler for preventing transfer-crush therein can not obtain the uniformity of the solid image. In the latter developer, furthermore, in case the spherical substance is used, if the amount of the spherical substance used is in excess, and in case the spacer is used, if its particle diameter is too large, not only the effect of improving sharpness can not be obtained but also the deterioration in image density is brought about, and the image uniformity is sometimes reduced according to circumstances.

The object of the present invention is to provide a liquid developer for use in electrostatic photography, which developer is capable of obtaining copies of high image quality, irrespective of the high or low smoothness of the transfer paper used.

The liquid developer for use in electrostatic photography according to the present invention comprises dispersing toner particles consisting essentially of a colorant and resin in a highly insulating carrier liquid, wherein wax is further contained in said toner particles and a filler for preventing transfer-crush is mixed with and included among said wax-containing toner particles, said filler comprising 63.5 number %, or more, of particles having a particle diameter in the range of 10-20.mu., 6.5 number %, or less, of particles having a particle diameter of over 20.mu. particle diameter, and 30 number %, or less, of particles having a particle diameter of not more than 10.mu.. The term "number %" is defined as meaning the percentage of the total number of particles of filler.

In this connection, it is to be noted that the particle size distribution of fine particles is expressed in terms of number % or vol.% or the combination thereof. The relation between number % and vol.% is denoted by the following formula: number %=0.19.times.(vol.%).sup.1.18.

The reason why particle diameters of the filler for preventing transfer-crush have been defined as mentioned above will be given below. When the so-called fine powder of not more than 10.mu. is present in large amount, plenty of the filler becomes present on the photosensitive material on developing, whereby the image density is deteriorated, the particles effective for preventing transfer-crush are insufficient and so the sharpness is degraded. When the so-called coarse powder over 20.mu. increases, contrarily, poor transfer is caused and consequently the image density is deteriorated and a poor solid image is caused. Accordingly, it may be said to be desirable that all the filler for preventing transfer-crush ideally should be in the range of 10-20.mu.. However, it is inevitable that particles whose particle diameter is over 20.mu. or not more than 10.mu. will be present because the method for manufacturing particles and the method for classifying particles are not complete. Therefore, we have carried out various investigations to clarify that in case the particle diameter of the filler for preventing transfer-crush is mainly in the range of 10-20.mu., and the particles over 20.mu. is 6.5 number % or less and the particles not more than 10.mu. is 30 number % or less, the aforesaid problems are not caused. The filler for preventing transfer-crush, if too much is used, deteriorates the image density because said filler is present on the photosensitive material, and, if too little is used, is unable to prevent transfer-crush to the full. Accordingly, it is preferable that the amount of the filler for preventing transfer-crush contained in the developer should be in the range of 0.01-2 wt.%. As the filler for preventing transfer-crush there can be enumerated inorganic fine particles such as glass beads, zinc oxide, titanium oxide, silica and the like; synthetic resin such as polymethacrylate (for instance, methyl polymethacrylate and ethyl polymethacrylate), unsaturated polyester, polyvinyl chloride, polystyrene, polycarbonate, epoxy resin and the like; particles (for instance, dry toner) comprising these resins and the colorants referred to afterwards (the amount of resin is about 60-99 wt.% and the amount of colorant is about 1-40 wt.%); and the like.

The ingredients constituting the toner particles according to the present invention are colorant, resin and wax.

As the colorant there can be enumerated carbon black (goods on the market include Printex G and V and Special Black 15, 4 and 4-B produced by Degusa Inc.; Mitsubishi #44, #30, MA-11 and MA-100 produced by Mitsubishi Carbon K.K.; Laven 30, 40, 1035, Conductex SC, Mogal L, Elftex 8, Legal 400, and the like produced by Cabot Co.), Phthalocyanine Blue, Phthalocyanine Green, Sky Blue, Rhodamine Lake, Malachite Green Lake, Methyl Violet Lake, Peacock Blue Lake, Naphthol Green B, Naphthol Green Y, Naphthol Yellow S, Lithol Fast Yellow 2G, Permanent Red 4R, Brilliant Fast Scarlet, Hansa Yellow, Lithol Red, Benzidine Yellow, Lake Red C, Lake Red D, Brilliant Carmine 6B, Permanent Red F5R, Pigment Scarlet 3B, Alkali Blue, Oil Blue, Oil Violet, Methyl Orange, Fast Red, Methyl Violet and the like.

As the resin there can be used those which have usually been used in the wet toner. For instance, there can be enumerated graft copolymers of the vinyl monomer represented by the formula: ##STR1## [wherein, R stands for H or CH.sub.3 and X stands for COOC.sub.n H.sub.2n+1 (n=6-20)]

with glycidyl methacrylate, glycidyl acrylate, acrylic acid, methacrylic acid, vinylpyridine and the like, and copolymers of said graft copolymers with the vinyl monomer represented by the formula: ##STR2## [wherein, R stands for H or CH.sub.3 and Y stands for COOC.sub.n H.sub.2n+1 (n=1-5)] or the formula: ##STR3## (wherein, R stands for H or CH.sub.3, and Z stands for N(CH.sub.3).sub.2, N(C.sub.2 H.sub.5).sub.2 and CH.sub.2 CH.sub.2 OH.)

It is desirable that the amount of the vinyl monomer represented by the general formula (1) contained in the resin should be in the range of 30-95 wt.%.

The above mentioned resin, as occasion demands, may be used concurrently with natural resin such as ester gum, hardened rosin and the like; natural resin-modified thermo-setting resin such as natural resin-modified maleic resin, natural resin-modified phenol resin, natural resin-modified polyester resin, natural resin-modified pentaerythritol resin, epoxy resin and the like; and so forth.

The wax on the market includes the following:

In order to make the toner contain the above mentioned wax therein, the following methods and combinations thereof can be employed. Such methods include (1) a method comprising kneading the wax, as-powdered, together with the colorant and the resin in the presence of a small amount of carrier liquid (normally, the petroleum type aliphatic hydrocarbon as referred to afterwards is used therefor); (2) a method comprising heating and dissolving the wax in a non-aqueous solvent (normally, toluene, a petroleum type aliphatic hydrocarbon or its halide is used therefor), thereafter quenching same for separating and dispersing the wax in fine particle form and kneading this dispersion together with the colorant and the resin, or after the aforesaid method (1) is effected, heating the carrier liquid for dissolving the wax once and then quenching for separating and dispersing the wax in fine particle form; (3) a method comprising kneading the aqueous dispersion of the colorant and the non-aqueous solvent dispersion under heating and reduced pressure conditions for distilling out the solvent and water to thereby coat the colorant with wax and then kneading this wax-coated colorant together with the resin and, if necessary, the wax in the presence of a small amount of carrier liquid; and (4) a method comprising adding and dissolving wax in the process for the preparation of resin (said process being effected by heating) and kneading the obtained wax-containing resin together with a colorant in the presence of a small amount of carrier liquid, and the like. As kneading machines there can be used a kneader, an atritor, a ball mill, a kedy mill, a vibrating mill and the like. These steps can produce a wax-containing concentrated toner. The preferable amount of wax contained is 20-60 wt.% of the total amount of toner particles containing wax. When the amount of wax is small, toner layers do not contact on transferring and accordingly the recessed portion of a paper inferior in smoothness is not filled up on fixing. When the amount of wax is in excess, the obtained image is blurred.

The proper amounts of colorant, resin and solvent (or carrier liquid) used in the above concentrated toner are about 5-40 wt.%, about 5-40 wt.% and about 300-1,000 wt.% respectively against the total amount of the wax-containing toner particles.

The toner may be added with natural resin such as ester gum, hardened rosin and the like; natural resin-modified thermo-setting resin such as natural resin-modified maleic resin, natural resin-modified phenol resin, natural resin-modified polyester resin, natural resin-modified pentaerythritol resin; epoxy resin and the like in addition to the above components.

As the carrier liquid there may be used petroleum type aliphatic hydrocarbons such as cyclohexane, n-hexane, n-heptane, n-nonane, n-octane, isooctane, isododecane, ligroin and their mixtures (as the petroleum type aliphatic hydrocarbons on the market there may be enumerated Isoper E, G, H, L and K produced by Esso Standard Oil Co., Ltd., and Shellzol 71 and Solvesso 150 and the like produced by Shell Oil Co.)

The liquid developer according to the present invention may be prepared by diluting the thus obtained wax-containing concentrated toner about 5-10 times with a similar solvent or carrier liquid and further adding the filler for preventing transfer-crush thereto.

According to the present invention, a high image quality of copy can be obtained irrespective of high and low smoothness of the transfer paper used, because the toner particles contain wax therein and the filler for preventing transfer-crush has the above mentioned particle size distribution.

Next, the preparation example of the filler for preventing transfer-crush and the examples using the filler for preventing transfer-crush will be given. Part and % referred to herein are all by weight.

Styrene monomer: 50 parts

Dispersant (Tween 80 produced by Kanto Kagaku K.K.): 1.5 parts

Polymerization initiator (A.C.H.N produced by Otsuka Kagaku K.K.): 0.4 part

Water: 500 parts

These components were mixed together and held at 75.degree. C. for 5 hours while stirring vigorously. Then, the thus obtained massive matters were removed from this mixture. The mixture was washed 5 times repeatedly and dried. Thereafter, same was treated using the zigzag classifier (100 MZR produced by Alpine Company) while changing the classifying conditions to thereby obtain samples of the filler for preventing transfer-crush.

By repeating classification 6-7 times there was obtained a sample wherein all the particles were included within the particle diameter scope of 8.0-10.0.mu.. This sample was named No. 4. Classification was repeated so that No. 5 may have the particle diameter distribution of 10-13.mu., No. 6 the particle diameter distribution of 13-16.mu., No. 7 the particle diameter distribution of 16-20.mu. and No. 8 the particle diameter distribution of 20-25.mu.. Thus, a total of 8 samples were obtained.

50% Isopar-H dispersion of glycidyl methacrylatelauryl methacrylate-acrylic acid-methyl methacrylate copolymer: 40 parts

carbon black (Neospectra Mark II produced by columbia Co.): 13 parts

Alkali Blue (produced by Orient Kagaku K.K.): 2 parts

10% Isopar-H dispersion of OA WAX (produced by BASF Co., softening point 93.degree.-96.degree. C.): 250 parts

These components were placed in a ball mill, dispersed for 72 hours, thereafter added with 360 parts of Isopar-H, and further dispersed for one additional hour. 140 parts of this concentrated toner was diluted with 750 parts of Isoper H. Thus, a developer was prepared. The aforesaid samples No. 1-3 were placed in this developer so as to be 0.05%, and subjected to image formation with INFOTEC 8032R (produced by Kalley Co.).

As the result, No. 2 and No. 3 produced superior images but No. 1 was inferior in sharpness (evaluated based on the resolving power) and low in image density as compared with No. 2. Likewise, No. 4-8 were evaluated. The obtained results were as shown in the folowing table.

It can be seen from this table that No. 5-7 are superior in image density and No. 5-8 are superior in sharpness. Accordingly, it can be seen important that the particle diameter of the filler for preventing transfer-crush should be limited within the scope of 10-20.mu. in order to enhance sharpness without lowering image density.

500 g of water and 50 g of carbon black (Mogal A produced by Cabot Co.) were fully stirred in a flusher. Thereafter, 150 g of wax (DYNF, produced by Union Carbide Co.) was added thereto, and kneaded at 150.degree. C. for 2 hours. 250 g of carbon tetrachloride was further added thereto and same was kneaded for 2 hours. Thereafter, this mixture was subjected to reduced pressure for removal of water and carbon tetrachloride therefrom, and pulverized in a sweat mill to thereby obtain a wax-coated pigment.

Further, 80 g of isooctane was heated to 100.degree. C. in a flask and a mixture of 60 g of stearyl methacrylate, 20 g of methyl methacrylate, 10 g of maleic acid, 30 g of wax (DYNF produced by Union Carbide Co.) and 2 g of azobisisobutyronitrile was dropped in the flask for 2 hours and stirred. Further, 280 g of isooctane and 0.1 g of pyridine were added thereto, and said mixture was heated at 90.degree. C. for 6 hours to thereby obtain a wax-containing resin.

Next, 40 parts of the above mentioned wax-coated pigment, 80 parts of the above mentioned wax-containing resin and 180 parts of isooctane were dispersed for 48 hours in a ball mill. Thereafter, said dispersion was added with 300 parts of isooctane and dispersed for 1 hour to thereby obtain a concentrated toner. 250 parts of said toner was taken out and diluted with 1000 parts of isooctane. Thus, a developer was obtained.

Samples No. 1-No. 3 were put in this developer as in Example 1 for the purpose of image formation. No. 2 formed a superior image, but No. 1 was somewhat inferior in sharpness and inferior in image density. The contents of these samples were changed, but the obtained results were the same as obtained in Example 1.

The exactly same procedure as Example 1 except that the wax used in Example 1 was replaced by 310 MP (softening point: 122.degree. C.) produced by Mitsui Sekiyu Kagaku K.K. In the case of using a paper inferior in smoothness (for instance, a gilbert bond paper and the like) like a paper for use in typing, it was observed somewhat low in image density and large image uniformity as compared with Example 1, but in the case of using other transfer sheets there were obtained images substantially equal to Example 1.