US3419411A - Method for the transfer of developed electrostatic images using a lattice forming substance - Google Patents

Description

United States Patent METHOD FOR THE TRANSFER OF DEVELOPED ELECTROSTATIC IMAGES USING A LATTICE FORMING SUBSTANCE Robert J. Wright, Traumere, South Australia, Australia, assignor to Research Laboratories of Australia Pty. Limited, North Adelaide, South Australia, a corporation of Australia No Drawing. Continuation-impart of aplication Ser. No. 307,009, Sept. 6, 1963. This application Apr. 20, 1967, Ser. No. 632,184

1 Claim. (Cl. 11737) ABSTRACT OF THE DISCLOSURE A superior method of developing electrostatic images is achieved by incorporating, in a wet developer, a latticeforming substance which presents branched molecules affording an interlaced structure in the developer which locks the pigment particles in place and prevents the spreading thereof. Examples of the lattice-forming substance are parafiin wax, ethyl cellulose and rubber modified polystyrene.

This application is a continuation-impart of application Ser. No. 307,009, filed Sept. 6, 1963, now abandoned.

This invention relates to electrophotography, and in particular relates to the transfer of electrophotographic image deposits to unsensitized surfaces, such as for instance paper surfaces.

The invention also relates to the production of color prints by electrophotographic means utilizing a transfer process which produces monochromatic prints on separate sheets which can be transferred in register to any desired backing and in any sequence to produce a composite color print or prints.

Electrophotographic techniques usually involve the operations of charging, exposing and developing a photosensitive surface layer which may comprise a photoinsulator such as vitreous or alpha monoclinic selenium, or comprise a photoconductive pigment in an insulating resin or other matrix, such systems being represented by the well known zinc oxide containing layers. It is the usual practice to regard such zinc oxide containing layers as expendable, the image being retained on the sensitized material, whereas selenium layers are normally regarded as reusable, and consequently the image developed on such selenium surfaces is transferred to a non-sensitive suface such as paper.

At present the view is generally held that to achieve satisfactory transfer of the image it is necessary to have such image deposit of large particle size or alternatively, if the particle size is small, to work with smooth surfaces on both of the image and transfer sheets. The majority of such transfer processes are based on dry toning techniques wherein the size of the toner particles is preferably about 20 micron minimum for line work, with the finer toners associated with continuous tone work requiring a transfer sheet coated with polyethylene or the like which can be maintained in uniform contact with a selenium surface during the transfer operation.

Particulate photoconductor printing elements have not been investigated to the same extent with regard to transfer of images because of their relatively low cost, but it is known to transfer image deposits from zinc oxide containing printing elements with some degree of success.

I have found that the essential feature which has to be kept under control is the mobility of the particulate deposited image so that particles shall be free to move away from the printing element at the time of transfer. This is relatively difiicult to achieve with dry toning techniques,

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particularly for continuous tone applications, and also when volatile liquids are used in toner dispersion, as such carrier liquids usually evaporate too quickly to allow a satisfactory transfer operation. However, relatively viscous and slow evaporating carrier liquids of suitable electrical properties such as a volume resistivity greater than 10 ohm cm. and a dielectric contrast less than about 3, allow the desired mobility to be attained. Unfortunately such liquids, due to their viscous nature can also cause some spread of the image deposit during transfer with resultant loss of definition.

We have now found that high intensity high resolution transfer images can be obtained using in combination, a pigment material, slow evaporating carrier liquids supporting the pigment material, and a lattice-forming material which is capable of supporting the image deposit with respect to side spread during the transfer step. Such images may be transferred satisfactorily to untreated paper by absorption transfer, by electrostatic transfer or more usually by combination of these two. The transfer operation can be carried out without mechanical aids it desired although it is preferred to use mechanical means to obtain the necessary contact between the two surfaces. The slow evaporation liquids which have been found to operate satisfactorily are these which have an evaporation rate at least six times slower than the standard ether.

The lattice-forming materials are those which are capable of forming laced fibers within the image deposit and must be electrical insulators soluble in or dispersible in this form in the slow evaporation liquids.

Specifically, a lattice-forming substance in accordance with the present invention are polymeric materials presenting a branched as distinguished from either a linear or closed chain molecule, and is a substance which when in apparent solution in a liquid has a molecular structure in which one dimension is at least one order greater than its dimensions in two other dimensions at right angles to each other and which preferably has at least one bond from which a further branch can and does grow at a different direction to the major axis of the primary molecular chain. The lattice-forming substance will therefore afford an interlaced structure in the developer, much in the nature of the fibers of a paper slurry, which is to say that the branches of the molecular chain extend in many different directions, and, when caused to co-deposit with the pigment during electrostatic development of an image, form a lacy network which physically locks the pigment particles in place and prevents spreading thereof. Polymers presenting a closed chain molecule or a molecule linear in one direction only are not within the purview of the present invention, since these are not capable of forming a lacy fiber, open mesh network as contemplated hereby.

Yet more specifically, it is an object of the present invention to enhance the production of electrostatic images by preventing spreading thereof during transfer by incorporating in a wet electrostatic developer a latticeforming substance as above defined and selected from the group consisting of paraflin wax, ethyl cellulose, and rubber modified polystyrene. Such substances and their equivalents thus prevent image spread or bleeding and result in a much sharper image in comparison to electrostatic images heretofore developed using non-branched fixing agents such as non-branched resins or polymers.

For single color work the process steps employed in my invention are as follows:

An electrostatic pattern is produced on a sensitizable sheet by any known means. This pattern may be an electrophotographic or electroradiographic latent image produced on a selenium layer or on a zinc oxide or other particulate photoconductor containing layer, or it may be an electrostatic pattern produced on an insulating layer by electrical or mechanical modulation. The so-formed pattern is developed in a body of liquid dispersed toner or by the use of a mechanical applicator for such a liquid dispersed toner, such toner suspension containing at least a proportion of relatively slow drying liquid so that the desired particle mobility can be maintained during the subsequent transfer operation, and such toner also containing an agent or agents capable of supporting mechanically the image deposit during the transfer operation. The developed image may be washed in clean carrier liquid if desired to remove unwanted background material and the excess liquid removed by draining. The image bearing sheet is then placed on the base plate of the charging machine and the transfer sheet is brought into contact with the wet image bearing surface, preferably by rolled line contact. The relatively absorbent transfer sheet absorbs carrier liquid and this also allows absorption transfer to occur. The transfer efiiciency may be anhanced by charging the contacted materials during transfer, which allows a more uniform contact to be obtained between the two sheets, and the bulk charge temporarily acquired on the wet transfer paper also increases transfer efficiency. The two sheets may be separated before or after drying.

The same invention may be applied to transfer type of color printing.

It is known that color prints produced by successive overprinting on electrophotographic papers or plates usually suifer from the disadvantage that whites are of relatively poor quality due to the matte finish of most electrophotographic layers, and if for any reason a dye sensitized sheet is used, color balance of the finished print is impaired and a white print needs to be added in addition to the colors to restore whites lost by dye sensitization. In addition it is known that electrophotographic layers containing zinc oxide as a particulate photoconductor, while capable in many instances of accepting repeated charging and toning operations, do not necessarily acquire the same total charge on each successive operation, consequently color value and sensitivities of toners are usually adjusted to compensate for these differences, it thus becoming necessary to apply such color toners in a specified sequence. It has become customary to apply such toners in the sequence blue, red, black and yellow, which causes further restrictions in that transparent pigments are necessary for all colors except blue, this restriction being a serious limitation with regard to yellow pigments.

Electrophotographic color printing methods hereinbefore described are of particular interest in relation to lithography, and offer a relatively quick and simple method for the proofing of transparencies which are subsequently to be used to manufacture lithographic printing plates. However, such prior art processes have the disadvantage that the proof is not produced on the paper or other stock to be used subsequently in lithography, and thus color values are not necessarily an accurate representation of those of the lithographic print.

The present invention affords a method and means by which these several deficiencies may be overcome, in that color prints may be produced on any desired stock, and can thus reproduce in brightness and color value a result similar to lithography, and the colors can be applied in any preferred sequence as erratic charge holding characteristics of multiple exposed electrophotographic layers does not occur. This invention employs liquid dispersed toners due to their intrinsic high resolution and controlled color value, but does not suffer from the disadvantage of the prior art processes in that electrical control of toner overprinting characteristics is not required. In addition this invention teaches a method for the production of continuous tone color prints which is not possible using lithographic means.

The process steps involved in the utilization of this invention for the proofing of screened or continuous tone color separated transparencies is similar to that described earlier herein, the same basic principle being applicable for each color, the charging step and the subsequent exposure through the transparency pertaining to the particular color to be printed being followed by development of the sheet in a liquid dispersed toner of the desired color, such toner containing pigment, a binder for the pigment, and a support agent capable of maintaining the developed image without loss of resolution during the transfer operation, these materials being dispersed in a liquid as stated earlier herein. The developed image is then transferred. Where desired the image on the photosheet may be of sufficient intensity to allow more than one transfer to be made.

Subsequent colors are overlain on the transfer sheet by repetition of this process, a separate photosensitive sheet preferably being used for each color to avoid contamination, the successive images however being transferred in register to the same transfer sheet. Thus the final color print is obtained on any desired surface, and the colors are laid down in any desired order.

The following examples will serve further to illustrate the principles of this invention.

Example 1 An electrophotographic printing element was prepared as follows:

Grams Photoconductive zinc oxide 900 Rhodene M8/50 resin 600 Toluol 250 4% manganese naphthenate 2.5 4% cobalt naphthenate 2.5

The components are milled together, diluted if required, and deposited on a relatively conductive support by any known coating method.

A toner compatible with the above photoconductive base was formed as follows:

Grams Microlith black CT (pigment) Microlith blue 4GT (pigment) 50 Rubber modified polystyrene (lattice) 250 Solvesso 100 (slow evaporating liquid) 550 The above components were mixed and milled by ball milling for 24 hours, and the resultant paste was dispersible in a variety of solvents of which Solvesso 100 and Solvent K93 were found to be suitable for this application, these being slow evaporating liquids. The rubber modified polystyrene, besides acting as a dispersing agent for the pigment, also acts as a fixing agent for the deposited image and, due to its molecular construction forms a lattice which supports the image deposit so that lateral spread of the deposit does not occur during development, washing or transfer operations. The concentration of the toner was not critical, satisfactory transfers being obtained with concentrations varying between 0.2 and 3.0 grams paste/100 ml. dispersing liquid.

Example 2 This example illustrates the use of an oleoresinous varnish in the initial toner formulation, with addition of a compatible wax as the lattice agent. The varnish aids the grinding of the pigment and assists fixing of the image and is herein referred to as a grinding aid.

Varnish: Grams Polymerized linseed oil Calcium resinate (6% limed polypale rosin) 40 Toner:

Carbon black SRF (pigment) 17.5 Carbon black ISAF (pigment) 17.5 Brillfast geranium 3543 (pigment) 5 Varnish (grinding aid) 85 Paraffin wax (lattice) 4Q This example also shows the use of an oleoresinous varnish in conjunction with a wax, however in this instance the wax supporting agent was added to the diluted dispersed toner and not milled with the initial formulation.

Varnish: Grams Hydrogenated rosin 80 Polymerized linseed oil 320 Toner:

Microlith black CT (pigment) 80 Microlith blue 4GT (pigment) 40 Varnish (grinding aid) 325 These components were milled in a bar mill at a pressure of 250 lbs. and the resultant paste dispersed in Shellsol T in the proportions 0.2-3 grams toner/100 ml. dispersing liquid. A solution of paratfin wax (lattice forming material) in Solvesso 100 (slow evaporating liquid) was prepared and added to the dispersed tone so that the weight of wax added was half that of the toner concentrate in suspension. The relative proportions of toner and wax were not critical, the above-mentioned proportions being adequate for the production of high resolution transfer lmages.

Example 4 Color toners compatible with the above photoconductor coating and with suitable properties for precision transfer are as set out below.

Varnish:

Hydrogenated rosin 8O Polymerized linseed oil 320 The components are heated together at 500 F. for onehalf hour.

In each of the four toner formulations given, the toners were homogenized by heat blending, followed by milling. Milling was performed on a bar mill, operating pressure 100 p.s.i. Each concentrate was separately dispersed in a carrier liquid of relatively slow evaporation rate, such as Shellsol T, Solvesso 100, or Solvent K93, in the proportions 0.1-3 grams concentrate/100 grams dispersing liquid. The various monochrome prints prepared for transfer were each processed to completion before the subsequent one was started, that is the first print was prepared and transferred before the second was prepared. These toners, when dispersed in a liquid of KB value above about 40, such as for instance a mixture of equal parts Shellsol T and Solvesso 100, are capable of continuous tone reproduction, whereas when dispersed in the low KB value liquid Shellsol T each toner is only of sufficient fineness for the rendition of half toner screens of 180 lines per inch or coarser.

Example 5 The hydrogenated rosin varnish employed in each of the toners of Example 4 is replaced by equal parts of the following:

Grams Polymerized linseed oil 160 Calcium resinate (6% polypale rosin) 40 Toner formulated using this varnish was dispersible in perchlorethylene (slow evaporating liquid) in addition to the dispersing liquids of Example 4.

Example 6 In this example the various toners are of somewhat higher gloss when deposited, and toner deposits are compatible with quick drying acrylic glazing solutions which may be employed where desired to increase the total refiectance of the finished transfer print.

Black Toner. Grams Microlith black CT (pigment) Microlith blue 4GT (pigment) 50 Lucite 46 (50% N.V. in toluol) (lattice) 300 Ethyl cellulose (7.5% N.V. in Solvesso 100) (lattice) 50 Blue Toner:

Microlith blue 4GT (pigment) 30 Lucite 46 (50 N.V. in toluol) (lattice) Ethyl cellulose (7.5% N.V. in Solvesso 100) (lattice) 15 Red Toner:

Fastel Pink B supra (pigment) 30 Lucite 46 (50% N.V. in toluol) (lattice) 100 Ethyl cellulose (7.5% in Solvesso 100) (lattice) 15 Yellow Toner:

Permanent Yellow GG (pigment) 30 Lucite 46 (50% N.V. in toluol (lattice) 240 Ethyl cellulose (7.5% in Solvesso 100) (lattice) 20 These toners are prepared by mixing and ball milling, and the resultant concentrates were dispersible in Solvesso 100, Solvent K93 and perchlorethylene (slow evaporating liquid). Dispersions in each instance were suitable for continuous toner reproductions.

The following are descriptions of materials mentioned by trade names in the various examples.

Microlith black CT, a resinated black pigment, manufactured by Ciba Ltd.

Microlith blue 4GT, a resinated phthalocyanine blue pigment, manufactured by Ciba Ltd.

Fastel Pink B supra, an organic like pigment, manufactured by I.C.I.

Permanent Yellow GG, a transparent yellow pigment, manufactured by Hoechst.

Brillfast Geranium 3543, a red like pigment manufactured by I. W. & T. A. Smith Ltd.

Rhodene M8/50, an isophthalic alkyd resin, manufactured by Polymer Corporation.

Lucite 46, a polybutylmethacrylate, manufactured by Du Pont.

Solvesso 100, an aromatic hydrocarbon solvent, KB value 93, flash point 118 F., evaporation rate (toluo-l=1) 75, manufactured by Esso Standard Oil Company.

Solvent K93, a substantially aromatic hydrocarbon 7 solvent, flash point 113 F., manufactured by Vacuum Oil Co.

Shellsol T, an aliphatic hydrocarbon solvent, flash point 130 F., KB value 26, manufactured by the Shell Company.

I claim:

1. The method of transferring developed electrostatic images which comprises suspending pigment particles in a slow evaporating electrically insulating liquid having a volume resistivity greater than 10 ohm cm. and a dielectric constant less than about 3 and an evaporation rate at least six times slower than ether, including a lattice forming subsance in the slow evaporating liquid capable of forming laced fibers within the image deposit, said lattice forming substance being dispersible in the said liquid and being an electrical insulator, applying the developer liquid so formed to a surface containing an electrostatic latent image selective to the said developer particles whereby the pigment is deposited imagewise and the lattice forming substance in solution in the carrier liquid is deposited on the said surface to prevent spread of the wet developer when transfer pressure is applied thereto by forming an open lacy network in the developer deposit to mechanically prevent lateral movement of the developer particles, pressing a second surface onto the said developer while said developer is still wet, and applying an electrostatic charges to such contacted surface to transfer developer to said second surface from the lacy network, said lattice-forming substance being a material selected from the group consisting of parafiin wax, ethyl cellulose and rubber modified polystyrene.

References Cited UNITED STATES PATENTS 2,297,691 10/ 1942 Carlson 95-5 8 2,676,100 4/1954 Heubner 118-637 X 2,690,394 9/1954 Carlson 117-37 X 2,788,288 4/1957 Rheinfrank et al. 252-62.1 X 2,891,011 6/1959 Insalco 25262.1 2,879,397 3/1959 Lehmann 96-1 X 2,899,335 8/1959 Straughman 117-37 2,917,460 12/1959 Solar 252-621 2,919,191 12/1959 Walkup 96-1 2,962,375 11/1960 SchalTert 96-l 2,986,466 5/1961 Kaprelain 1l7-l7.5 X 3,013,890 12/1961 BiXby 25262.1 X 3,049,077 8/1962 Darnm 10 1-211 3,068,115 12/1962 Gundlach 117-37 3,076,722 2/1963 Greig 117-31 3,100,426 8/1963 Kaprelian -1.7 3,102,026 8/1963 Metcalfe et al. 117-37 X 3,102,045 8/1963 Metcalfe et al. 117-37 3,108,893 10/1963 Oliphant 117-10 3,120,446 2/1964 Hunter 117-37 X 3,241,957 3/1966 Fauser et al. 96-1 3,244,633 4/1966 Yellin et a1 25262.1 3,281,241 10/1966 Mihajlov 96-1 3,337,340 8/1967 Matkan 96-1 FOREIGN PATENTS 235,264 9/1961 Australia.

658,699 10 /1951 Great Britain.

WILLIAM D. MARTIN, Primary Examiner.

E. I. CABIC, Assistant Examiner.

, US. Cl. X.R. 25262.1; 96-1.4