US3788930A

US3788930A - Method of fixing images obtained by liquid development in electrophotography

Info

Publication number: US3788930A
Application number: US00193957A
Authority: US
Inventors: S Honjo; O Fukushima
Original assignee: Fuji Photo Film Co Ltd
Current assignee: Fujifilm Holdings Corp
Priority date: 1970-10-30
Filing date: 1971-10-29
Publication date: 1974-01-29
Anticipated expiration: 1991-01-29
Also published as: CA975840A; FR2110139A5; DE2154144A1; GB1361670A; BE774763A

Abstract

A METHOD OF PERMANENTLY FIXING A TONER IMAGE ON AN ELECTROPHOTOGRAPHIC SENSITIVE MATERIAL, SAID TONER IMAGE BEING FORMED BY DEVELOPING AN ELECTROSTATIC LATENT IMAGE PROVIDED ON THE PHOTOCONDUCTIVE COATING OF SAID MATERIAL WITH A LIQUID DEVELOPER COMPRISING A CHARGED TONER DISPERSED IN A VOLATILE INSULATING CARRIER LIQUID, COMPRISING LAMINATING A TRANSPARENT PLASTIC FILM BY MEANS OF AN ADHESIVE LAYER PROVIDED ON SAID FILM ON SAID TONER-IMAGE BEARING SURFACE BY HEAT AND/OR PRESSURE WHILE SAID MATERIAL IS WET WITH THE VOLATILE LIQUID USED IN THE TREATMENT

OF SAID MATERIAL TO ASSIST THE LAMINATION OF SAID FILM ONTO SAID TONER-IMAGE BEARING SURFACE.

Description

Jan. 29. 1974 SATORU HOMO ET AL 3,788,930

METHOD OF FIXING IMAGES OBTAINED BY LIQUID DEVELOPMENT IN ELECTROPHOTOGRAPHY Filed Oct. 29, 1971 FIG. I

United States Patent O US. Cl. 156-307 4 Claims ABSTRACT OF THE DISCLOSURE A method of permanently fixing a toner image on an electrophotographic sensitive material, said toner image being formed by developing an electrostatic latent image provided on the photoconductive coating of said material with a liquid developer comprising a charged toner dispersed in a volatile insulating carrier-liquid, comprising laminating a transparent plastic film by means of an adhesive layer provided on said film on said toner-image bearing surface by heat and/or pressure while said material is wet with the volatile liquid used in the treatment of said material to assist the lamination of said film onto said toner-image bearing surface.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal cross-sectional view of an electrophotographic sensitive material used in practicing the fixing method according to the present invention.

FIG. 2 is a pictorial view illustrating the fixing method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for realizing quick and perfect fixing of toner images by forming a transparent surface layer on the image-bearing surface which has been treated with a liquid developer in electrophotography.

Electrophotography is already widely utilized in document copying for business use. In the case of dry development, an image formed by depositing dry powder toner is softened and fixed by applying heat or a solvent, while in the case of liquid development, a toner image deposited from the carrier liquid of a developer has the self-fixing property, so the image is firmly bonded onto the photosensitive layer by a resin ingredient included in the image after the evaporation of the carrier liquid.

In general, a photosensitive layer composed of a mixture of an inorganic photoconductor such as zinc oxide and a resin binder has a high volume content of powder and, hence, the surface is rough lacking in glossy appearance. However, when a toner image is formed and fixed thereon, the image portion comes to assume luster. That is, the background portion remains mat while the image portion becomes glossy. Such appearance, however, is not only far inferior to the uniform appearance of a conventional glossy silver-halide print, but also has the disadvantages that the background portion is likely to be damaged during storage and that the toner image may deteriorate when subjected to a strong external force. Particularly when the developed image has tonal gradation With a wide area of high concentration with a maxi- 3,788,930 Patented Jan. 29, 1974 mum optical reflective density as high as about 2.0, it takes a long time until the image portion is perfectly fixed and assumes a satisfactorily high mechanical strength. Accordingly, such image is likely to get damaged.

Therefore, the primary object of the present invention is to provide an improved fixing method which is free of the defects inherent to the conventional methods and to obtain an electrophotographic print which has an improved appearance and durability well comparable with those of the silver-halide prints.

According to the method of the present invention, the electrostatic latent images are formed on an electrophotographic light sensitive material having a support which is preferably flexible, then they are developed with a liquid developer and thereafter a transparent plastic film having an adhesive layer is laminated on the photoconductive coating while the material is still wet with the remaining developer thereby to let said film firmly adhere to the sensitive layer.

The material may preferably be washed with a washing liquid after development and prior to fixing operation. Such washing step is eifective to remove the excessive developer liquid present on the photoconductive coating, and thus to reduce background in the final print.

When the material is washed, the lamination is carried out while the material is still wet with such washing liquid. Anyway, the present process is characterized by that the lamination fixing is carried out on the coating which is still wet with a volatile liquid.

Now, the invention will be described in detail with reference to the accompanying drawings.

Referring to 'FIG. 1, there is shown a sectional view of an electrophotographic light-sensitive material, generally designated by 1. This sensitive material 1 consists of a support 10, an electroconductive subcoating layer 11, a back coating 13 and a photoconductive coating 12. The photosensitive material suitably used in the fixing method of the present invention may be free of the intermediate layers 11 and 13. The most preferred material for composing the support 10 is paper because of its air permeability, appearance and hand. It is also pos sible to use non-woven fabrics, various types of cloth or so-called synthetic paper.

According to the present invention, an electrophotographic material such as mentioned above is subjected to exposure and development treatment in the manner known in the art, and then a transparent sheet is laminated on said material while the latter is still wet with the volatile liquid which has been applied on the material. It was found against our intuitive expectation that no air bubbles are formed between the laminated sheet and the electrophotographic coating even if the support has no air permeability. In some typical examples, the lamination was conducted on an electrophotographic material with a paper support provided at its back side with a solvent penetration preventive layer, and an aluminumdeposited polyethylene terephthalate film. The photoconductive coating was a porous ZnO layer and wetted with an isoparaffinic solvent. Generally, a ZnO coating absorbs such organic liquid of around 10% of the coating weight. This liquid, however, produced no bubbles during the laminating operation, and the operation could be performed in a satisfactory manner even on a polyethylene terephthalate film. In the case of a paper with about 4 total thickness and provided with a back, solvent barrier coating, the sheet absorbed an organic liquid in an amount equivalent to 7-10% weight of the sheet, still lamination could be achieved satisfactorily without being accompanied by blistering. Air permeability of this paper, when measured by a Gurley-Hill testing machine, had an apparent value of more than 600 seconds. This value probably seems to mean the air leakage rate along the specimen surface.

These advantageous results are considered attributable to the fact that the volatile liquid was retained within the support in the case of the liquid-absorptive support. In the case of the non-absorptive support such as polyethylene terephthalate film, the successful result is supposedly ascribed to the limited amount of liquid retained on the material. At any rate, it seems important that the temperature at which the lamination is carried out should be kept substantially lower than the boiling point of the liquid remaining on the material.

In order to obtain a high quality image of continuous tone on a paper support, the surface thereof should prefererably be smooth. For this reason, it is desirable to pro vide a clay layer below the sensitive layer 12 (see FIG. 1). It is particularly desirable to use art paper, machine coated paper, photographic baryta paper or the like as base and provide an electroconductive layer 11 on such coated layer. The electroconductive layer 11 may be composed of colloidal alumina, polyvinyl benzene sodium sulfonate, polyvinyl benzyl trimethyl ammonium chloride, sodium or potassium polyacrylate or the like. The back coating 13 should preferably be one which can prevent penetration of the developer or pretreating liqiud which may be applied prior to development. The back coating also works to minimize curling of the sensitive materials as a whole. A higher electroconductivity is also desired also for the back coating. All of the above-mentioned materials for the electroconductive layer can be utilized. Also, CMC, gelatin, starch, casein, polyvinylalcohol or the like may be blended, if desired.

The photosensitive layer 12 should preferably be of a porous structure which allows impregnation of an organic solvent to some extent. A typical example is a homogeneous mixture of inorganic photoconductive powder such as zinc oxide, zinc sulfide, titanium oxide, cadmium sulfide or the like and binder resin. Organic photoconductive layers having porous structure may also be used. A photosensitive layer of a type composed of photoconductive powder and binder resin may have poor solvent permeability if the content of the binder is high. It is therefore desirable that particulate component is contained in a high concentration the preferred range of such concentration being from about 30 to 80% of the coating in volume. In the case of zinc oxide, the preferred mixing rate is 3:1 to :1 by weight ratio. In case of using an organic photoconductive layer, it is possible to easily obtain a desired porous structure by suitably adjusting the coating and drying conditions. It is also possible to additionally disperse powder uniformly therein. For example, a mixture of a good solvent having a low boiling point and a poor solvent having a higher boiling point may be used as the coating solvent for obtaining a desired porous structure. If such coating mixture is applied to the support and dried, the good" solvent first evaporates, allowing the poor solvent alone to remain, so that the film-forming component dissolved in the mixture finally separates out at a later stage of drying. The resultantly formed film has desired porosity and light-scattering property.

The electrostatic latent images are then formed on such photoconductive layer. A typical method for forming such electrostatic latent images comprises uniform charging of the layer in a dark place and image exposure. Other known methods can of course be utilized. For instance, Karmans method, where charging and exposure are conducted in the reversed order, may be employed. Also, only the light image exposure may be effected, without con- 4 ducting electrostatic charging, whereby a latent image is formed by the photovoltaic effect. The latent images thusformed are converted to visible images with a liquid developer prepared by dispersing fine charged particles in an insulating carrier liquid. When the high quality images, especially having tonal gradation is required, the latent image bearing surface may be wetted with an insulating toner-free pre-bath liquid prior to the development. This treatment with the pre-bath liquid has the effect to prevent undesirable background which may otherwise be caused by trapping of the toners on the latent image bearing surface by other than the electrostatic factors. Further, such pre-bath liquid delays attenuation of the latent images to elongate their file, time besides, penetrates into the back side of the sensitive material to prevent instrusion of the developer, thus also reducing the boiling of the back surfaces of the material. This operation, therefore, is particularly useful for a material having a back side which is absorptive of the treating liquid used. Washing of the developed material is also effective to improve the quality of the print. Particularly useful washing method is to use a liquid which acts to deposit resinous ingredients contained in the carrier liquid of the developer used, since the image is imparted with mechanical strength. (See German Patent Publication 1,908,298.) The washing liquid used should preferably be one which meets the above-said conditions and also has a faster evaporation rate than the developer. For practical use, an isoparaffin solvent having a low boiling point, such as, for example Isoper E (trade name) manufactured by Humble Oil Co., is preferred. In case where infiammability should be evaded, chloro-fluorinated hydrocarbons maybe used either solely or in combination with other solvents.

The material thus developed and washed is fulfilled mainly with the pre-bath liquid in the bulk (while a thin layer of washing liquid stays on the surface). Such material, therefore, is preferably squeezed lightly with rollers to reduce the impregnated amount of liquid so as to promote drying of the material. In this case, however, a due care should be given since the images are often damaged during the squeezing operation.

One of the advantages brought about by the present invention is that it mitigates the squeezing condition considerably, because the subsequent laminating operation can be successfully performed on the wet material.

The photosensitive material, which was squeezed in the manner described above and which is still wet, is then supplied to a fixing station such as shown in FIG. 2. In the figure, reference numeral 1 denotes a developed sheet, 10 a support, 12 a sensitive layer and 14 the toner images, respectively. A laminate film 2 is delivered from a stock roll not shown. This laminate film is composed of a transparent base 20 and a heat-sensitive or pressure-sensitive adhesive layer 21. Also in the figure, numeral 33 designates a tension roll, 31 a heating and pressure roll, and 32 a back-up roll. The heating-pressure roll 31 is made of metal and the back-up roll 32 is preferably provided with elastic surface comprising, for example, Teflon (polytetrafluoroethylene) or silicon rubber.

In case the adhesive layer 21 is heat-sensitive, the roll 31 should be one which can be heated to a suitable temperature, while in case said layer is pressure-sensitive, said roll may be an ordinary pressure roll. It is desirable that the adhesive layer 21 contains a material which is soluble or swellable in the liquid retained in the sheet.

It was unexpectedly found that when the laminate film 2 is pressed against the surface of the photosensitive material 1, the remaining liquid on said sensitive material surface or the presence of the volatile liquid in the photosensitive layer having a porous structure of said sensitive material 1 does not impede a firm bonding. It was also confirmed that any inclusion of air bubbles, which was feared to be caused by air permeability of the support 10, does not actually take place.

These results of experiments showed that it is possible to incorporate the laminate fixing mechanism in the elec trophotographic treating apparatus to treat the photosensitive material while it is still wet immediately after development treatment, with no need of perfectly drying the photosensitive material.

The laminate film used in the present invention may be prepared by applying a suitable bonding agent to a plastic film having a thickness of several ten to several hundred ,u. and made of, for example, polyethylene, polypropylene, ethylenevinylacetate copolymer, polyvinyl chloride, polyvinylidene chloride, cellulose diacetate, cellulose triacetate, cellulose acetate, polyethylene terephthalate, polycarbonate or the like.

The pretreatment of the plastic base films which is conducted before coating of the adhesive layer includes, for instance, corona discharge, flame treatment, chemical oxidation, ultraviolet ray irradiation or suitable undercoating. The composition of the adhesive layer is determined in association with the base material, and, hence, no further description may be given here. However, in case of the heat sensitive adhesive layer, usually a composition mainly comprising a thermoplastic polymer having a suitable softening point (around 100 C. or less) is used. For further information refer to pages 447 to 451 of 'Handbook of Adhesives by Irving Skeist (published by Reinhold Publishing Co. in 1962).

As mentioned before, it is desirable that the adhesive layer contains a material which is soluble or swellable in the insulating liquid remaining in the photosensitive material. Such insulating liquids include usually kerosene chlorofluorinated hydrocarbon, isoparaffin or n-parafiin type hydrocarbon, dimethylpolysiloxane or the like. Among the resin materials which are soluble in such solvents and may be incorporated in the adhesive layer include: polyisobutylene, polyoctylmethacrylate, polylaurylmethacrylate, polybutylmethacrylate, polybutene, petroleum resin, coumarone-indene resin, aliphatic polyester, long-oil type alkyd resin, polystyrene, phenolformaldehyde resin, glycerine ester of hydrogenated rosin, s-tyrene/ isobutylene copolyrner, styrene/butadiene copolymer, styrene/higher alkyl methacrylate copolymer and the like. Some of these materials show adhesive property at normal temperature, but for practical use, those which are nonadhesive at normal temperature are found convenient for handling. It is essential that the adhesive layer show discoloration for a long time. In this respect, one of the most preferred resin materials is polyalkylmethacrylate in which the alkyl contains carbon atoms more than 4.

It was found that even if the adhesive layer contains no component soluble in the remaining liquid in the photosensitive sheet, firm bonding is obtained as long as the composition of the photosensitive layer is mainly comprised of thermoplastic ingredients which are softened at the temperature where the bonding is practiced. And if the soluble components are present in the adhesive layer, it is possible to obtain strong bonding even if the photosensitive layer is not softened.

Generally speaking, the presence of such ingredient that is soluble or swellable in the remaining liquid in or on the electrophotographic material favors to realize a firm bonding between the laminate film and the material. This is also true in the case where the electrophotographic coating is thermoplastic. Usually, the laminating operation is performed at a temperature from room temperature to about 120 C. Too low a temperature weakens the bonding, while too high a temperature may cause thermal contraction of the laminate film and, in some cases, may give rise to air bubbles. Therefore in consideration of the bonding force and drying speed, the most preferred temperature range is 50 to 60 C. Particularly good result is obtained in case the adhesive layer is mainly composed of a thermoplastic resin having a secondary transition point within the range of about 15 to about 35 0., because, in such case, no lateral shear is produced at normal temperature and also no displacement of the laminate film of the obtained copy takes place during handling at normal temperature.

In the method of the present invention a state that an electrophotographic material is still wet a liquid implies that the material maintains the liquid in the range of about 4 to 30 g. per square meter. In a photosensitive zinc oxide paper with a solvent barrier backing, when it is subjected to a strong roller squeezing, the content of liquid e.g. isoparaffin (s.g. =ca. 0.8) may be reduced to 6 to 8 g. per square meter.

Example 1 An art paper measuring 130 g./m. which is available on the market, was coated on its coated side with 1.0 g./m. (measured in dried state) of colloidal alumina and on its back side with 1.5 g./m. of the same alumina. A mixture solvent consisting of methanol and water (70:30) was used in this back coating.

The air permeability of the paper thus obtained was more than 600 seconds as measured by Gurley-Hill testing machine.

Then a homogeneous dispersion liquid containing 100 parts of photoconductive zinc oxide, 10 parts of styrenemodified alkyd resin and 4 parts of polyisocyanate compound was applied on the front side of the paper which was treated as described above, such that the dry weight of this coating will measure 25 g./m.

After thorough drying the photosensitive paper obtained in this manner was subjected to dark adaptation, then charged with a negative corona and exposed to a light image to form an electrostatic latent image. This sheet was prebathed with kerosene and then developed by using a developer which was prepared by mixing carbon black and soybean alkyd resin as charge controlling agent in an isoparafiinic solvent. A development electrode was used during development. Upon completion of the development, the sheet was washed with the same isoparaflinic type solvent (trade name Isoper -E manufactured by *Esso or Humble Oil Co.) to remove the extra developing liquid. Finally, the sheet was lightly squeezed with squeezing rollers and then immediately passed between a heated roller (heated to C.) and a back-up roller together with a laminate film to be described later. The passing velocity was 5 cm./sec. and the diameter of the heated roller was 4 cm. The film was extended out so that it contacts with said roller at an angle of about 90. The laminate film used here was one which was prepared by exposing polypropylene film to corona discharge and then immediately applied thereon with a toluene solution composed of 80 parts of polybutylmethacrylate and 20 parts of aliphatic polyester resin (trade name D-tite 307NH manufactured by Dainippon Ink Co. and sold by Japan Reichhold Chemical Co.).

The laminate film adhered firmly to the photosensitive layer without producing any blister or peeling-01f. On another developer sheet the same adhesive film was laminated by reducing the heated roller temperature to 40 C. and 20 C., respectively; the resulting print confirmed strong adhesion and uniform appearance.

The photosensitive paper before lamination held 22 g./m. of solvent. Polybutyl methacrylate in the adhesive layer is soluble to the isoparafiinic solvent and kerosene, however, D-tite 307NH is insoluble to them.

'Example 2 A mixture comprising 20 parts of copolymer of styrene, butyl methacrylate and acrylic acid copolymerized in the ratio of 50:45 :5 and parts of photoconductive zinc oxide was added with a small amount of sensitizing dyes (a mixture of Fluorescein, Rose Bengal and tetrabromophenol blue) to prepare a coating mixture which was coated on the coated side of g./m. weight baryta paper treated as follows. The baryta surface was subbed with 2g./m. of 'ECR 34 (polycationic 1 conductive resin made by Dow Chemical Co.) and the back surface was coated with a thin layer comprising 60% polyvinylbenzene potassium sulfonate and 40% polyvinyl alcohol to give a dried coating weight of 1.5 g./m. No solvent permeation from the back side of this paper took place.

An electrostatic latent image was formed on the photosensitive layer of this photosensitive material and was developed by using a carbon black type developer where resin-modified phenol formaldehyde resin was added as charge controlling agent in a kerosene carrier liquid. After development the sheet was rinsed with an isoparaffin type solvent (Isoper E produced by Esso or Humble Oil 00.). After rinsing, the sheet was held vertically for a short time to let most of the liquid flow down. Under this condition, the sheet retained about g./m. of the liquid.

Then a polyvinyl chloride film having an adhesive layer composed of a mixture of 70' parts of polyvinyl acetate and parts of coumaron-indene resin was laminated on said already developed wet photosensitive material. The obtained copy had solid film adhesion and no air bubble was observed.

Example 3 A laminate film having an adhesive layer composed of 40 parts of glycerin ester of hydrogenated resin and 60 parts of D-tite 307N'H (trade name) was applied on an already developed photosensitive material obtained in the same manner as in Example 1. The result was excellent.

Example 4 A colored image was reproduced on a photosensitive paper of Example 1 according to an over-printing method using developers of three different colors. The carrier liquid was a mixture of 75 parts of decalin and 25 parts of kerosene. A varnish prepared by dissolving resinmodified phenol resin in linseed oil under heat was used as charge controlling agent. Rinsing was carried out with a mixture solution of Isoper and Diflon S-2 (CCI F-CCl F, produced by Daikin Industries). A laminate film formed by providing an adhesive layer composed of D-tite 307NH alone on a polypropylene film was pressed against the wet photosensitive sheet. There was resultantly obtained a duplicate having firm and uniform adhesion of said laminate film.

1 Polyvinylbenztrimethylammonlum chloride.

What is claimed is:

1. A method of permanently fixing a toner image on an electrophotographic sensitive material, said toner image being formed by developing an electrostatic latent image provided on the photoconductive coating of said material with a liquid developer comprising a charged toner dispersed in a volatile insulating carrier liquid, comprising laminating a transparent plastic film by means of an adhesive layer provided on said film on said toner-image bearing surface by heat and/or by pressure while said material is wet with the volatile liquid used in the treatment of said material to assist the lamination of said film onto said toner-image bearing surface.

2. A method of permanently fixing a toner image on an electrophotographic sensitive material, said toner image being formed by developing an electrostatic latent image provided on the photoconductive coating of said material with a liquid developer comprising a charged toner dispersed in a volatile insulating carrier liquid, comprising laminating a transparent plastic film by means of an adhesive layer provided on said film on said toner-image bearing surface by heat and/or by pressure while said material is wet with the volatile liquid used in the treatment of said material, the adhesive layer of the film containing an ingredient which is soluble or strongly swollen by the volatile liquid remaining on/in said material.

3. A method of permanently fixing a toner image on an electrophotographic sensitive material, said toner image being formed by developing an electrostatic latent image provided on the photoconductive coating of said material with a liquid developer comprising a charged toner dispersed in a volatile insulating carrier liquid, comprising laminating a transparent plastic film by means of an adhesive layer provided on said film on said toner-image bearing surface by heat and/or by pressure while said material is wet with the volatile liquid used in the treatment of said material, the amount of volatile liquid held by the sheet material just before the laminating operation ranging between 4 g./m. to 30 g./m.

4. A method of permanently fixing a toner image corresponding to a continuous tone original, said toner image being on an electrophotographic sensitive material and formed by developing an electrostatic latent image provided on the photoconductive coating of said material with a liquid developer comprising a charged toner dispersed in a volatile insulating carrier liquid, comprising laminating a transparent plastic film by means of an adhesive layer provided on said film on said toner-image bearing surface by heat and/ or by pressure while said material is wet with the volatile liquid used in the treatment of said material.

References Cited UNITED STATES PATENTS 3,669,859 6/1972 Merrill 961 R X 3,549,447 12/1970 Bresnick 961 R UX 3,427,242 2/ 1969 Mihajlov 96l.3 X

RALPH S. KENDALL, Primary Examiner D. A. SIMMONS, Assistant Examiner US. Cl. X.R.