US3406063A - Xerographic material containing an inorganic photoconductor and nonpolymeric crystalline organic substances and methods of using of such material - Google Patents

Description

United States ABSTRACT OF THE DISCLOSURE An electrophotographic material is disclosed which includes a support and a photoconductive insulating layer coated thereon. The photoconductive insulating coating consists essentially of an inorganic photoconductor and nonpolymeric, crystalline organic substances. The organic substances may be acids, anhydrides, amine derivatives, phenol derivatives, esters, and aniline derivatives. Methods of using the electrophotographic material are also disclosed.

This invention relates to a xerographic printing material and method of use.

It is already known to produce electrophotographic printing elements using a photoconductive zinc oxide or other particulate photoconductor which is held to the surface of the support member by a film forming insulating polymeric binding material such as a resin.

It is known that the charge holding capacity of the element depends on the presence of the resin binder which in addition to being an insulator also prevents access of moisture to the photoconductor particles by virtue of forming a continuous film around the discrete particles embedded therein.

The normal binders which are used in the bonding of the photoconductor to the surface are insulating filmforming resins or gums, these binders being polymers and as such requiring careful curing to ensure the correct charge holding characteristics under varying climatic con ditions, and this curing must be carefully attended to in particular when the sheet is processed several times such as in overprinting colour work using liquid developers, as the repeated action of the solvents contained in the liquid developers may impair the electrical properties of the surface by softening the resin binder if such is not sufiiciently cured to develop complete solvent resistance.

A further disadvantage of the prior art resin binder coatings is the unsatisfactory continuous tone response of such coatings caused at least in part by the presence of the insulating resin binder, as the electrical properties of such binder are vastly different from those of the particulate photoconductor and any localised concentration of such binder in the coating or any uneven dispersion of the photoconductor particles in the resin matrix or any uneven wetting of the photoconductor particles by the binder may give rise to artifacts which form discontinuities in the necessary uniform photosensitivity of such coating.

It will be thus seen that the insulating resin binder coatings known heretofore have certain disadvantages which it is the object of this invention to overcome.

atent C 3,406,063 Patented Oct. 15, 1968 The electrophotographic process in accordance with this instant invention comprises the steps of providing an electrophotographic printing element consisting of a relatively conducting backing such as paper or metal sheet having deposited at least on one side thereof a photoconductive layer containing a particulate photoconductor and a nonpolymeric crystalline organic substance, forming a pattern of electrostatic charges on the surface of such layer, developing such pattern with electrostatically attractable toner material and fixing such developed pattern to the said surface.

The said pattern of electrostatic charges can be formed on the said surface of the photoconductive layer by the electrophotographic process wherein a uniform electrostatic charge is first formed on such surface which is then exposed to a pattern of electromagnetic radiation whereby the surface becomes discharged in accordance with the intensity of the incident radiation and thus a latent electrostatic image is formed on such surface, or alternatively the pattern of electrostatic charges can be formed on the said surface by selective charge deposition as known in electrostatic printing and more particularly in signal recording.

In carrying out research in connection with the improvement of photoconductive layers we have now surprisingly found that a photoconductive layer can be prepared from a particulate photoconductor such as zinc oxide or the like and an organic nonpolymeric substance which is of crystalline structure and thus is not a filmformer and does not form a binder-like matrix containing the photoconductor particles embedded therein and which substance thereby permits sedimentation of the photoconductor particles in closely packed form when applied to a surface whilst not inhibiting the charge holding and photoresponse characteristics of the photoconductor but forming with the photoconductor a coating of sufiicient mechanical strength for processing purposes as required in the electrophotographic or electrostatic printing process.

These substances do not require curing and therefore the production of coatings containing them is a much simpler matter than heretofore. It is also found that such substances impart a more uniform charge holding capacity to such coatings than was possible with the resins used heretofore, as they can be so selected that their volume resistivity is of a similar order to that of the particulate photoconductor being used.

We have found generally that as distinct from the prior art resin binders which had to be insulators having a volume resistivity in the order of 10 -10 ohm cm., nonpolymeric organic crystalline substances with volume resistivities greater than about 10 ohm cm., can be used in conjunction with particulate photoconductors. Such materials should in addition be of nonhygroscopic nature in order that the charge holding capacity of the so-formed photoconductor layer should not be unduly altered by changes in atmospheric moisture content. However such nonpolymeric organic crystalline substances may be water soluble to the extent of forming a 10% by weight solution in water at a temperature of the order of 20 C., provided such solubility does not indicate the material to be of hygroscopic nature. In addition the nonpolymeric substances should not react with the photoconductor at ambient temperatures, nor should they react with the solvents used at ambient temperatures. Thus the nonpolymeric organic crystalline substances useful in accordance Organic acids and anhydrides- Phthalic anhydride Z-furoic acid Phthalic acid Salicylic acid Salicylic anhydride Maleie anhydride Maleic acid Benzoic acid Itaconic acid Adipic acid Sulphosalicylic acid Alizarin Sebacic acid Iso-phthalic acid Laurie acid Myristic acid Tcrephthalic acid Amine derivativesp-Amino-azobenzine n-Tert-butylacrylamide Acetophenetidin Phenol derivatives Phenolphthalein Phenyl salicylate p-Phenyl phenol o-Phenylphenol Esters Beta-naphthyl-methyl ether l-napthyl salicylate Dimethyl terephthalate Aniline derivatives Diphenyl guanidine p-Benzyl aniline An obvious advantage of using materials according to this invention is that relatively thin layers such as of about 0.0015 gram per square centimeter or less of particulate photoconductors in closely packed form and thus of uniform photo-response can be held to a base as has not been heretofore possible, without the presence of a resin binder which requires curing and introduces discontinuities.

The degree of attachment of the layer to the base employing certain organic nonpolymeric crystalline substances as will be seen in the examples following this disclosure, is surprisingly at least as good as that of the prior art resin binder coatings.

However, when employing certain organic nonpolymeric crystalline substances particularly at relatively high pigment concentrations, it may be found that for some purposes the attachment of the layer to the base may not be sufiieient to allow subsequent extensive handling after development of an image, such layer is nevertheless capable of affording sufficient fixing to allow normal charging and exposure to an image pattern and subsequent development, and if the final image is to be fixed in the more permanent manner then a fixing medium may be used either in the liquid developer itself or subsequently applied, which could consist of such materials as hydrogenated rosin, esters of hydrogenated rosin, the long oil alkyds, the acrylic resins, calcium resinate and similar substances, which substances can conveniently be applied to the surface by dissolving them in a carrier liquid which will evaporate to leave the substance as a protective medium for the image. Alternatively, the surface containing the developed image may be sprayed with a protective substance.

In the electrophotographic layers in accordance with this invention the proportion of the particulate photoconductor to the organic nonpolymeric crystalline substance can be in the range of 1-8 parts by weight of the photoconductor to 1 part of the crystalline substance, depending on the density of the photoconductor and on the properties of the organic nonpolymeric substance. Mixtures of organic nonpolymeric crystalline substances can be also used.

The photoconductive coating material in accordance with this invention is prepared by first dissolving the non- 2,4,6-trichloroaniline 4 polymeric substance in a solvent therefor and subsequently dispersing the particulate photoconductor in the required quantity in such solution by ball-milling or other methods. Alternatively, the nonpolymeric substance can be mixed with the particulate photoconductor, then the solvent for the nonpolymeric substance can be added and such mixture can then be homogenised. The thus prepared coating material can be then applied by any known method to a base material and for such purpose the coating material can be diluted by the addition of more solvent or diluent.

Sensitising dyes as known in the prior art can be also added to the coating material to enhance or to affect the spectral response of the coating.

A further advantage will be thus seen to lie in the method of preparing the coating materials in accordance with this invention in that it is only necessary to break up the pigment aggregates and to ensure even particle distribution without prolonged milling operations as are necessary with the prior art resin binders, because in the layers provided by this invention there is no adsorbed layer of binder material around the discrete pigment particles since the organic nonpolymeric crystalline substances are not film-formers and thus lack wetting properties in the sense that this term is used in connection with conventional resinous materials.

It will be apparent that the photoconductive coatings to be used with the liquid developing process can contain only such nonpolymeric substances which are substantially insoluble in the carrier liquid contained in the liquid developer.

To enable the invention to be fully appreciated some examples will noW be given and it is to be noted that these examples refer to layers in which particulate photoconductors are held to a base such as paper by means of closely packed sedimentation in presence of the nonpolyrneric crystalline substances forming part of this invention, while in other examples a developer is included which has the effect of further fixing the image to the base firstly by including a substance in the liquid developer which will fix the developed image to the base and sec ondly by first using a developer without fixing properties and then efiecting a subsequent treatment to fix the image to the base. It will be realised that this invention is not restricted by the materials and combinations thereof cited in the following examples as one skilled in the art can utilise the teachings of this invention to select and to combine other organic nonpolymeric crystalline substances for the production of electrophotographic and electrostatic coatings.

EXAMPLE 1 A coating composition was prepared as follows:

Grams Phthalic anhydride (nonpolymeric substance) 50 was dissolved in Ethyl alcohol 200 Acetone 200 and to this solution was added Photoconductive zinc oxide 200 The mixture was ball-milled for 4 hours and then applied as a coating to paper. The coated paper was air dried and dark adapted for 24 hours after which it was used to produce an electrophotogra phic print thereon in the following manner:

The coated paper was placed on a metallic base plate in absence of activating radiation and subjected to a corona discharge from a series of points held at a potential of 7.5 kv. negative in relation to the positive base plate whereby the coated surface facing the said points accepted a negative surface charge of volts. A light pattern was then projected on to the charged surface and the resulting latent electrostatic image -was then rendered visible by developing, applying the so-called dry-method on one sample and the so-called liquid method on another sample.

In the dry developing method the latent image was developed with a commercially available black toner powder applied from a magnetic brush developing device. The developed image showed high density and nonirnage areas were clean. The image was heat fused.

In the liquid developing method the latent image was developed by immersing the sheet into a bath containing a liquid developer consisting of a liquid carrier n-heptane having dispersed therein 0.2% by weight a toner paste which paste was prepared by milling the pigment phthalocyanine blue with bodied linseed oil in equal proportion by weight.

EXAMPLE 2 In the coating composition of Example 1 the quantity of the zinc oxide photoconductor was reduced to 125 grams. The adhesion-cohesion characteristics of the layer were found to have been improved considerably and to be comparable with those of prior art resin bound layers, while the coated surface accepted a surface charge of 200 volts.

The coated paper was charged, exposed anddeveloped as described in Example 1.

EXAMPLE 3 Grams Benzoic acid 50 :was dissolved in Ethyl alcohol 350 and to this solution was added Photoconductive zinc oxide 150 The mixture was ball-milled and applied as in Example 1, and was found to be usable as a hotoconductive layer when used in conjunction with the dry toning technique of Example 1.

The above mixture was ball-milled and applied as a coating to paper as in Example 1.

The thus coated paper was charged, exposed and developed as in Example 1 except that the liquid developer contained 5% by weight of hydrogenated rosin dissolved in the carrier n-heptane to apply simultaneously with developing an overall protective layer to the surface.

EXAMPLE 5 The salicylic acid of Example 4 was replaced by the nonpolymeric substance sulphosalicylic acid.

EXAMPLE 6 The benzoic acid of Example 3 was replaced by 2- furoic acid as the nonpolymeric substance.

The coated paper was suitable for use with each of the dry and liquid developing processes described in Example 1.

EXAMPLE 7 The 2-furoic acid of Example 6 was replaced by an equal weight of the nonpolymeric substance maleic anhydride. The surface containing the developed image was dipped in a 7% solution of the resin ester gum in petroleum ether for protective purposes and dried.

6 EXAMPLE 8 The zinc oxide of Example 1 was replaced by the photoconductor mercury iodide.

EXAMPLE 9 The zinc oxide of Example 2 was replaced by the photoconductor tetragonal lead monoxide.

EXAMPLE 10 The zinc oxide of Example 4 was replaced by the photocon-ductor cadmium selenide.

EXAlMPLE 11 The zinc oxide of Example 6 was replaced by the photoconductor cadmium selenide.

The above mixture was ball-milled and applied as a coating to paper as in Example 1.

The coated paper was charged and exposed as in Example 1 but it was suitable only for the dry developing process as the myristic acid was not resistant to the solvent n-heptane contained in the liquid developer. The surface containing the developed and heat fused image was sprayed with an acrylic type protective medium.

EXAMPLE 13 The tetragonal lead monoxide of Example 12 was replaced by the photoconductor mercury iodide.

EXAMPLE 14 The 'myristic acid of Example 12 was replaced by the nonpolymeric substance lauric acid.

EXAMPLE 15 A paper coated with the composition of Example 4 was charged and exposed to X-rays. The object radiographed comprised an aluminum alloy coating with maximum wall thickness of one centimeter. The tube was operated at kvp.

A continuous tone radiograph was obtained by developing the exposed sheet in the liquid developer of Example 1.

EXAMPLE 16 A paper coated with the dye sensitised composition of Example 36 was charged and exposed to X-rays using a salt-screen placed against the charged surface. The saltscreen was one manufactured by DuPont and branded HiSpeed. The object radiographed and the tube voltage were as in Example 17.

A continuous tone radiograph was obtained by developing the exposed sheet in the liquid developer of Example 1.

EXAMPLE 17 Grams Phenolphthalein 50 was dissolved in Ethyl alcohol 225 Acetone 225 and to the solution was added Photoconductive zinc oxide 200 The mixture was ball-milled and coated as in Example 1, and was found to be suitable for use with each of the dry and liquid toning techniques described in Example 1.

7 EXAMPLE 1s The photoconductive zinc oxide of Example 17 was reduced in quantity to 150 grams. The so-formed coating showed improved adhesion-cohesion characteristics when compared with that of Example 17, while the images developed on this coating were also free of background nonirnagewise toner deposition.

EXAMPLE 19 Grams Ortho-phenyl phenol 70 was dissolved in Ethyl alcohol 420 and to the solution was added Photoconductive zinc oxide 175 The mixture was milled and the coating used as in Example 1.

prepared and EXAMPLE 20 Grams Para-phenyl phenol 80 was dissolved in Ethyl alcohol 400 and to the solution was added Photoconcluctive zinc oxide 160 The mixture was milled and the coatingprepared and used as in Example 1.

EXAMPLE 21 The zinc oxide of Example 17 was replaced by 200 grams of the photoconductor mercury iodide.

EXAMPLE 22 The zinc oxide of Example 17 was replaced by 250 grams of the photoconductor tetragonal lead monoxide.

EXAMPLE 23 The phenolphthalein of Example 18 was replaced with an equal weight of phenyl salicylate.

EXAMPLE 24 Grams Para-amino benzine 35 was dissolved in Ethyl alcohol 210 and to the solution was added Photoconductive zinc oxide 87 The mixture was milled and the coating prepared and used as in Example 1.

EXAMPLE 25 The para-amino benzine of Example 24 was replaced with an equal weight of n-tert-butyl acrylamide.

EXAMPLE 26 The para-aminobenzine of Example 24 was replaced with an equal weight of acetophenetidin.

EXAMPLE 27 Grams Beta-naphthyl-methyl ether 70 was dissolved in Ethyl alcohol 350 Ethyl ether 70 and to the solution was added Zinc oxide 175 The coating was prepared and used as in Example 1.

EXAMPLE 28 The beta-naphthyl-methyl ether of Example 27 was replaced with an equal weight of dimethyl terephthalate.

EXAMPLE 29 The beta-naphthyl-methyl ether of Example 27 was replaced with an equal weight of l-naphthyl salicylate.

The diphenyl guanidine of Example 30 was replaced with an equal weight of para-benzyl aniline.

EXAMPLE 3 2 The diphenyl guanidine of Example 30 was replaced with an equal weight of 2,4,6-trichloroaniline.

EXAMPLE 33 Grams Phenolphthalein 25 Phthalic anhydride 25 were dissolved in Ethyl alcohol 225 Acetone 225 and to the solution was added Photoconcluctive zinc oxide 200 The coating was prepared and used as in Example 1. Adhesion-cohesion characteristics of this coating were comparable with those of prior art resin bound photoconductor layers.

EXAMPLE 34 Grams Phenolphthalein 20 Phthalic anhydride 40 were dissolved in Ethyl alcohol 200 Acetone 200 and to the solution was added Zinc oxide 180 The coating was prepared and used as in Example 1. Adhesion-cohesion characteristics of this coating were also comparable with those of prior art resin bound photoconductor layers.

EXAMPLE 35 Grams Phthalic anhydride 50 was dissolved in Ethyl alcohol 225 Acetone 125 and to the solution was added Photoconductive zinc oxide 116 Anatase titanium dioxide 9 The coating was prepared and used as in Example 1.

This coating was also found to possess excellent adhesion-cohesion characteristics, and to hold a surface charge of 275 volts.

EXAMPLE 36 Grams Phthalic anhydride was dissolved in Ethyl alcohol 360 Acetone 200 and to the solution was added Photoconcluctive zinc oxide 200 which had been coated with a dye solution containing Rose bengal 0.05 Brilliant green 0.12 Dibromofluorescein 0.04

Distilled water and dried prior to mixing with the phthalic anhydride solution.

The coating was prepared and used as in Example 1,

and was found to be of a photographic speed of response 8 times faster than the nondye-sensitized coating of the same general formulation as described in Example 2.

EXAMPLE 37 The phthalic anhydride of Example 36 was replaced with 67 grams of phenolphtha'lein.

EXAMPLE 3 8 The phthalic anhydride of Example 36 was replaced with an equal weight of salicylic acid.

EXAMPLES 39-41 The dye solution of Examples 36-38 was replaced with Grams Erythrosin B 0.08

Ethyl alcohol 150 EXAMPLES 42-44 The dye sensitised zinc oxide of Examples 36-38 was replaced with an equal weight of high speed panchromatic zinc oxide of the type disclosed in United States Patent No. 2,727,807. A further enhancement in the speed of response was obtained.

EXAMPLE 45 The phenolphthalein of Example 17 was reduced to 25 grams. Adhesion-cohesion characteristics of the so-formed photoconductor layer were found to be adequate for electrophotographic processing.

EXAMPLE 46 The phthalic anhydride of Example 2 was increased in quantity to 125 grams and the weight of solvent was doubled.

The so-forrned coating also possessed excellent adhesion-cohesion characteristics while the increase in organic nonpolymeric crystalline substance content did not cause nonimage charge retention and toner deposition in background areas, as would normally occur with a prior art resin bound photoconductor layer containing equal weight of photoconductor and resin binder.

What we claim is:

1. An electrophotograp-hic process comprising the steps of exposing an electrostatically charged photoconductive insulating layer on a sheet selected from the group consisting of paper and metal to a light pattern to form an electrostatic image on said layer and developing the image on said layer by applying electroscopic particulate material, said photoconductive insulating layer consisting essentially of particles of an inorganic photoconductor and an organic nonpolymeric substance of crystalline structure which is not a film-former and does not form a binder like matrix containing the photoconductive particles embedded therein and which substance thereby permits sedimentation of the photoconductive particles in closely packed form without inhibiting the charge holding and photoresponse characteristics of the photoconductor and forming with the photoconductor a coating of sufiicient mechanical strength to withstand described process, said organic substance being selected from the group comprising organic acids, organic anhydrides, amine derivatives, phenol derivatives, esters, and aniline derivatives, and being nonreactive with the photoconductor at ambient temperatures.

2. An electrophotographic process according to claim 1 wherein the inorganic particulate photoconductor is present in the proportions from 1 to 8 parts by weight to 1 part by weight of the organic nonpolymeric substance of crystalline structure.

3. An electrophotographic process according to preceding claims 1 and 2 wherein at least one sensitising dye is incorporated within the photoconductive insulating layer.

4. An electrophotographic process according to claim 1 wherein the volume resistivity of the organic nonpolymeric substance of crystalline structure is at least 10 ohm cm.

5. Electrophotographic printing element comprising a support selected from the group consisting of paper and metal and coated thereon a photoconductive insulating layer consisting essentially of particles of an inorganic photoconductor and an organic nonpolymeric substance of crystalline structure which is not a film-former and does not form a binder like matrix containing the photoconductive particles embedded therein and which substance thereby permits sedimentation of the photoconductive particles in closely packed form without inhibiting the charge holding and photoresponse characteristics of the photoconductor and forming with the photoconductor a coating of sufficient mechanical strength to withstand described process, said organic substance being selected from the group comprising organic acids, organic anhydrides, amine derivatives, phenol derivatives, esters, and aniline derivatives, and being nonreactive with the photoconductor at ambient temperatures.

6. Electrophotographic printing element according to claim 13 wherein the inorganic particulate photoconductor is present in the proportions from 1 to 8 parts by weight to 1 part by Weight of the organic nonpolymeric substance of crystalline structure.

7. Electrophotographic printing element according to preceding claims 13 and 14 wherein at least one sensitising dye is incorporated within the photoconductive insulating layer.

8. Electrophotographic printing element according to claim 13 wherein the volume resistivity of the organic nonpolymeric substance of crystalline structure is at least 10 ohm cm.

References Cited NORMAN G. TORCHIN, Primary Examiner.

JACK C. COOPER, Assistant Examiner.