Developing Electrostatic Latent Images With A Mixture Of Positive And Negative Toners

Abstract

This invention relates to a developer and process for developing an electrostatic latent image, which process comprises applying to the image, in the absence of a carrier, a mixture consisting of effective amounts of at least one fusible, organic toner capable of acquiring a positive charge, and at least one infusible, inorganic toner of approximately the same effective particle size capable of acquiring a negative charge. The developed image is fused by heating and then treated with an alkaline liquid which removes the inorganic toner particles and renders hydrophilic the image free areas.

Parent Case Text

This application is a continuation of Ser. No. 598,152, filed Dec. 1, 1966, and now abandoned, which is a continuation-in-part of my copending application Ser. No. 123,099, filed July 11, 1961, and now abandoned.

Description

The present invention relates to an electrophotographic developer consisting of a mixture of toners, of about equal grain size and carrying small charges of opposite polarity, which is capable of uniformly developing even fairly large areas. In particular, the present invention relates to a process for preparing offset printing plates by electrophotographic means, using the developer.

It is known to electrophotographically prepare images by a process in which a photoconductor coating on a support is provided in the dark with an electrostatic charge. Then, the material is exposed, either by the contact process by illumination through a master or by image projection using an optical apparatus, an electrostatic image corresponding to the master being thus formed. This image is developed by brief contact with a developer consisting of a toner and a carrier, in the course of which the toner adheres to the image portions. The image is then fixed, for example by heating or by treatment with solvent vapor. In this way, copies are obtained electrophotographically which are resistant to abrasion.

In the mixture of carrier and toner hitherto used for the development of electrostatic latent images, particles were used for the carrier which were ten to a hundred times larger and of greater specific gravity then those used for the toner, e.g., grains with a diameter of 0.2 to 0.6 mm. Because of their size, gravity acting on the carrier particles is greater than the attraction of these particles to the charged portions of the photoconductor coating so that they do not adhere thereto.

Further, developer powders are known which consist of a mixture of two toners of opposite charge which are of about the same size but of different color. The developers of this type serve for developing electrostatic images in various colors. In these developers, both toners are fusible.

As toner, fine pigmented or non-pigmented thermoplastic or fusible electroscopic resin powder has been used.

When the developer is moved about, the toner acquires a charge, as a result of frictional electricity, which is the opposite of that on the carrier. Therefore, the fine resin powder is loosely held by the carrier material so that the two components do not become dissociated. When the electrostatic image is powdered over with the developer, the toner particles are drawn, in the case of opposite polarity, to the image that is being developed and are held there, while the carrier particles roll away. Thus, according to the polarity of the toner contained in the developer, or that of the electrostatic image, either the image portions or the image-free portions of the electrophotographic material are covered on development.

Latent electrostatic images also have been made visible by means of finely divided resin powder contacted with the electrostatic image by means of a current of air. With this method of development, a relatively large number of particles are deposited on the image-free portions and cause an undesirable "background."

The known developers all have the disadvantage that they do not uniformly develop image portions covering rather large areas, but are deposited only around the edges leaving the inside portions of such areas to all intents and purposes undeveloped.

The present invention provides a developer for charge images which uniformly adheres even to relatively large areas of the same charge and yields a visible image of good marginal definition, which is free of fog and undesirable edge effects. This is of particular importance in the case of offset printing plates prepared by electrophotographic means. In this case, it is an additional requirement that the background, i.e., the image-free areas, be clean and easily rendered hydrophilic.

The developer of the invention consists of a mixture of at least one fusible organic resinous toner capable of acquiring a positive charge and at least one infusible, inorganic toner capable of acquiring a negative charge.

Organic materials suitable for the resinous positive toner of the developer include natural and synthetic resins, e.g., colophony, copals, dammar resin, asphalts, colophony modified phenol resins, ketone resins, maleic resins, coumarone resin, polyacrylic acid resin and polystyrenes. Mixtures of these resins also can be used. Inorganic and/or organic pigments and/or dyestuffs are added to these resins to give them a definite positive polarity. These include, for example, carbon black, zinc oxide, titanium dioxide, barium sulfate, red lead and the following dyestuffs from Schultz' "Farbstofftabellen," Vol. I, 7th Edition (1931): Helio Red RMT (No. 200), Helio Fast Blue (No. 1188), Sudan dyestuffs, e.g., I (No. 33), II (No. 92), III (No. 532), CB (No. 127), R (No. 149), Cellitone dyestuffs (Color Index, Vol. I, 2nd Edition, 1956, pages 1655-1742), Spirit-Soluble Nigrosine (No. 985), Pigment Black B (No. 1361), Alizarin Blue Black B (No. 1195), Diamond Black F (No. 614), Fanal Violet LB (No. 803), and also mixtures of such dyestuffs and/or pigments. Frequently, very small quantities of these substances are sufficient, e.g., 0.5 percent of carbon black, to give the resins definite positive character. It is also often advantageous for small quantities, e.g., 0.1 to 10 per cent, preferably 1 to 5 per cent, of waxes and/or low melting point organic compounds and substitution products thereof, to be added to the positive toners so that the melting point and the adherence of the toner mixture is adjusted in the desired manner.

Waxes suitable for this purpose include natural waxes such as carnauba wax, beeswax, Japan wax, montan wax and ceresine, synthetic waxes such as those known under the denominations A Wax, OP Wax, SPO Wax, V Wax, O Wax, E Wax, Hard Wax H and Hard Wax W, various so-called "Ruhr" waxes, and in particular the products marketed under the name "Gersthofener" waxes with the codings S, L, O and OP, and waxlike substances such as hard paraffin, stearic acid, high-pressure hydrated waxes and stearic alcohol.

As low-melting point organic compounds, substituted and unsubstituted aromatic compounds with melting points between about 40 and 150.degree. C. are particularly of interest. Such compounds are naphthols, e.g., 1-naphthol and 2-naphthol, and also aromatic compounds such as acenaphthene, acylamino compounds such as acetanilide, halogen aromatic compounds such as p-dibromobenzene, amino compounds such as 2,4-diamino-toluene, o-phenylene diamine and phenols such as resorcinol and diphenylamine and derivatives thereof.

The positive toners are prepared as follows: The starting materials, preferably in small pieces, are ground together very finely, the mixture is heated to the melting point, the melt is stirred until a high degree of homogeneity is attained, and it is then cooled. Alternatively, the fusible starting materials may be liquefied by heating and the remaining components then introduced with stirring, after which the material is cooled. The toner mass obtained in this way is then finely ground and sieved. For the toner, fractions with an average grain size of about 1 to 50 .mu., preferably the fractions of about 1 to 10 .mu. or of about 10 to 20 .mu., are used. The fractions of smaller size are preferably used for the reproduction of fine screens, while the coarser toners are used for normal written text.

As the inorganic toner which acquires a negative charge, finely pulverized inorganic substances such as salts, e.g., potassium sulfate, calcium sulfate, ammonium chloride, sodium chloride, potassium bromide, copper sulfate, aluminum/potassium sulfate and sodium sulfate; oxides, e.g., ferric oxide, titanium dioxide, zinc oxide, aluminum oxide, and copper oxide; silicates, e.g., kieselguhr, silica gel, talcum, and glass powder; borates, e.g., sodium metaborate, and potassium borate; carbonates, e.g., calcium carbonate, magnesium carbonate, and potassium carbonate, may be used.

The inorganic substances also are finely ground and sieved where necessary; average grain sizes of about 1 to 50 .mu., preferably about 1 to 10 .mu. or 10 to 20 .mu., are used. Mixtures of different inorganic toners also may be used with advantage; mixtures of kieselguhr and borax are preferred.

For the preparation of the developer, one or more of the inorganic toners which acquire a negative charge and one or more of the organic resinous toners which acquire a positive charge are mixed together in the proportions of 1:1 to 20:1. About equal proportions of the two types of toner are preferable.

The developers are applied to latent electrostatic images which are to be developed in known manner and are then fixed, preferably by heating.

The method by which the latent electrostatic image is produced is immaterial, e.g., by the action of visible, ultraviolet, infra-red or X-rays on layers of appropriate sensitivity, or by direct electronic production of the image on an insulating layer, or by the transfer of an electrostatic image to an insulating layer. It is also possible for the permanent fixing of the toners on the photoconductor coating to be carried out by the use of solvents for the toners or for the photoconductor coating, particularly solvents in vapor form. Also, the electrostatic image being developed can be transferred, before fixing, to another material, particularly by means of a corona discharge, and then fixed to the transfer material.

The present invention also provides a process for preparing an offset printing plate which comprises developing an electrostatic latent image on a photoconductive layer with a mixture of at least one fusible organic resinous toner capable of acquiring a positive charge and at least one infusible, inorganic toner capable of acquiring a negative charge, wherein both toners, which are of approximately the same effective particle size, have an average particle size in the range of about 1 to 50 .mu. and wherein 1 to 20 parts by weight of inorganic toner are present per part by weight of organic toner, heating the developed powder image to a temperature somewhat below the softening point of the layer to fuse the organic toner, and treating the image with an alkaline liquid, whereby the loosely adhering negative inorganic toner is removed and the image-free areas are rendered hydrophilic.

The image-free areas of electrophotographic layers containing zinc oxide as the photoconductive substance may be rendered hydrophilic easily by means of aqueous alkaline solutions containing hexacyano iron acid salts.

If the photoconductor coating is applied to a supporting material which is suitable for electrophotographic purposes and satisfies the requirements of planographic printing, i.e., has a hydrophilic surface, the image-free portions, if they are aklali-soluble, can be dissolved away completely in a subsequent operation by treatment with an alkaline liquid, while the image portions carrying the hydrophobic toner are inked up with greasy ink. A printing plate prepared in this way is suitable for planographic printing. Organic alkaline solvents are used as decoating liquids; liquid aliphatic amines or amino alcohols admixed with mono- or polyhydric alcohols are preferred.

Normally, the process according to the present invention is performed as follows: A presensitized electrophotographic printing plate consisting of an aluminum foil having a thin coating of an organic photoconductor in an organic binder is charged in the usual manner and then exposed. The latent electrophotographic image formed is developed by drawing the foil through the developer mixture of the present invention or by moving the mixture repeatedly to and fro over the foil. The positive toner of the developer adheres during this process to the negatively charged portions of the image, and the negatively charged toner to the uncharged portions. If fixing is then effected by heating, the positive organic toner melts and becomes firmly anchored to the coating. If the fixing temperature is below the melting and softening point of the coating, the negative toner afterwards can be removed since it has not become anchored to the layer. The most favorable fusing temperature is best determined by tests. The temperature selected should be such that the organic component on the image areas melts uniformly and becomes anchored to the layer, whereas the inorganic component does not stick to the layer. As a rule, the most favorable temperature is in the range between 130 and 160.degree. C.

The toners have the advantage that, because of the small charges they carry, they are capable of developing large areas and do not adhere merely to the edges of the image. Moreover, letters become sharper. Since the inorganic component surrounds the organic component, the latter is prevented from running during the melting process. In addition thereto, the inorganic component has a cleaning effect which is particularly pronounced in the preparation of printing plates where the plate is wiped with an alkaline liquid. Since the inorganic toner adheres to the image-free areas, those organic toner particles which normally cause a contamination of the background areas deposit on the inorganic toner component. When the plate is wiped over with the alkaline liquid, they are removed together with the inorganic component, and thus can not be fused to the image-free areas, as would be the case otherwise.

The invention will be further illustrated by reference to the following specific examples:

EXAMPLE 1

Eight parts by weight of polyvinyl carbazole and 0.025 part by weight of Rhodamine B extra are dissolved in 100 parts by volume of toluene; this solution is applied in known manner to a paper and then dried. The electrocopying material thus prepared is negatively charged by a corona discharge and then exposed by the contact process, by illumination through a positive master. Development is then effected with a developer consisting of 10 parts by weight of kieselguhr and 10 parts by weight of a pigmented resin mixture. For the preparation of the pigmented toner, 30 parts by weight of a polystyrene of low melting point ("Polystyrol" LG), 30 parts by weight of a maleinate resin e.g., "Hobimal" P 59), and six parts by weight of Pigment Red B are melted together. After cooling, the melt is ground and sieved. The fraction of a particle size of about 10 .mu. is mixed with kieselguhr in equal proportions. When this developer is scattered on the latent electrostatic image, the red-pigmented, positively charged toner is attracted by the negative charge image so that a visible red image is formed and the colorless negative toner (kieselguhr) is repelled. The latter adheres to the uncharged parts of the electrocopying material. The toner image is fixed by heating to about 130.degree. C. Very sharp copies with large colored areas are obtained.

EXAMPLE 2

An aluminum plate upon which selenium has been vacuum-deposited is positively charged by a corona discharge of 6,000 volts and then illuminated through a negative master. The electrostatic image produced is powdered over with a developer consisting of 50 parts by weight of finely ground silica gel of an average particle size of 10 to 20 .mu. and 25 parts by weight of a toner made up of 20 parts by weight of a maleinate resin ("Beckacite K 105"), 10 parts by weight of polystyrene, four parts by weight of carbon black and two parts by weight of Pigment Black. These substances are melted together and, after cooling, the melt is ground and sieved and the fraction of a particle size of 10-20 .mu. is used. The positive pigmented toner is repelled by the positively charged portions and is deposited on the uncharged portions. A positive image becomes visible; the image that has been made visible can be transferred to any supporting material, e.g., paper, aluminum or plastic. Images rich in contrast with large black areas are obtained.

EXAMPLE 3

A foil made of bright, rolled aluminum that has been cleaned with trichloroethylene, is coated with a mixture of 30 parts by volume of ethylene glycol monomethylether, one part by weight of 2,5-bis-(4'-diethylaminophenyl-1')-1,3,4-oxadiazole and 0.8 part by weight of a styrene/maleic anhydride interpolymer with a specific gravity of 1.26-1.28 and a decomposition temperature ranging from 200 to 240.degree. C.; it is then dried. For the production of images on the electrocopying material thus obtained, the coating is negatively charged by a corona discharge and then illuminated through a positive master with a 125-watt high pressure mercury vapor lamp for one second. The electrostatic image of the master thereby produced is treated with a mixture consisting of 10 parts by weight of borax and three parts by weight of the positive toner described in Example 2. The two toners have a particle size of about 10 .mu.. When this developer is distributed on the electrostatic image, the black-pigmented positive toner is attracted to the negative charge image and the colorless negative toner is repelled. The latter therefore adheres to the uncharged parts of the electrocopying coating. The toner image is fixed by heating to about 160.degree. C. Images rich in contrast are obtained, in which extensive black areas are also evenly developed.

EXAMPLE 4

The procedure described in Example 3 is followed but, for development, a toner mixture is used which consists of 20 parts by weight of purified kieselguhr and 10 parts by weight of a ketone resin (AP resin) pigmented with carbon black and spirit-soluble nigrosine. The purification of the kieselguhr is effected in the following manner: 20 parts by weight of a commercial product are first boiled for one hour with 300 parts by volume of 15 per cent hydrochloric acid. After cooling, the material is filtered, washed with water, and dried. The product is then heated to glowing (about 600-800.degree. C.). The kieselguhr purified in this manner, in association with the pigmented toner, gives especially uniformly developed areas.

After the electrophotographic image has been fixed by heating to 140.degree. C., it can be converted into a printing plate if the image-free portions, and the kieselguhr adherent thereto, are wiped over with a solution consisting of 10 per cent mono-ethanolamine, 5 per cent of sodium silicate, 20 per cent of glycerine, 10 per cent of triglycol, and 55 per cent of methanol. After a brief rinsing with water, the plate can be inked up with greasy ink and used for printing in an offset machine.

EXAMPLE 5

A foil made of brushed aluminum is coated with a mixture of 100 parts by volume of glycol monomethyl ether, four parts by weight of 2,5-bis-(4'-diethylaminophenyl-(1') )-1,3,4-triazole, four parts by weight of a maleinate resin with an acid number of 110-130 (e.g., "Alresat" 400 C), and 0.001 part by weight of Crystal Violet; it is then dried. For the production of images on the electrocopying material thus obtained, the coating is negatively charged with a corona discharge, exposed beneath a positive master to a 100-watt incandescent lamp for two seconds, and the latent electrostatic image of the master thus produced is treated with a mixture consisting of 10 parts by weight of kieselguhr, 10 parts by weight of borax, and eight parts by weight of the positive toner described in Example 2. The powders have a particle size of about 10-20 .mu.. When this developer is distributed on the electrostatic image, the pigmented positive toner is attracted by the negative charge image and the mixture of inorganic toners is repelled. The latter therefore adhere to the uncharged margins of the black image and, after the image has been fixed by heating to 160.degree. C., they can be wiped away with a cotton pad. Images rich in contrast with good full tones are obtained.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

Claims

What is claimed is:

1. A process of developing an electrostatic latent image on a supported photoconductive layer which comprises

applying to the image a mixture consisting of effective amounts of at least one fusible, organic resinous toner capable of acquiring a positive charge and at least one infusible, inorganic toner of approximately the same effective particle size capable of acquiring a negative charge, the two toners having opposite charges when in admixture,

fusing the developed image by heating, and

treating the layer with an alkaline liquid, whereby the inorganic toner particles are removed and the image-free areas of the photoconductive layer are rendered hydrophilic.

2. A process according to claim 1 in which the layer having the fused image thereon is treated with an organic alkaline liquid, whereby the image-free areas of the layer are removed from the support.

3. A process according to claim 1 in which the toners have an average particle size in the range of about 1 to 50 .mu..

4. A process according to claim 1 in which about 1 to 20 parts by weight of inorganic toner are present per part by weight of organic toner.

5. A process according to claim 1 in which the organic toner comprises a colored mixture of polystyrene and a maleinate resin.

6. A process according to claim 1 in which the organic toner comprises a colored ketone resin.

7. A process according to claim 1 in which a small quantity, in the range of about 0.1 to 10 per cent by weight, of a compound selected from the group consisting of waxes, aromatic compounds melting in the range of about 40 to 150.degree. C., and substitution products thereof, is added to the organic toner.

8. A process according to claim 1 in which the toners have an average particle size in the range of 1 to 10 .mu..

9. A process according to claim 1 in which the toners have an average particle size in the range of 10 to 20 .mu..

10. A process according to claim 1 in which the inorganic toner is kieselguhr.

11. A process according to claim 1 in which the inorganic toner is silica gel.

12. A process according to claim 1 in which the inorganic toner is borax.

13. A process according to claim 1 in which the inorganic toner is a mixture of kieselguhr and borax.