US3507649A - Sensitized photoconductive zinc oxide - Google Patents

Description

United States Patent 3,507,649 SENSITIZED PHOTOCONDUCTIVE ZINC OXIDE Lee C. Hensley, 131 Mary St., Binghamton, N.Y. 13903 No Drawing. Filed Jan. 31, 1967, Ser. No. 612,781 Int. Cl. G03g 5/00, 7/00 U.S. Cl. 96--1.7 10 Claims ABSTRACT OF THE DISCLOSURE Dyestuff materials advantageously adapted for use as optical sensitizing agents with photoconductor materials employed in the electrostatic printing operations.

The present invention relates in general to optically sensitized photoconductive layers comprising zinc oxide and in particular to the provision of novel sensitizing dyestuffs for such purposes.

Electrostatic printing processes for the production of visible records or reproduction are well known in the art being extensively described in the literature both patent and otherwise. In general, such processes encompass as the salient features of operation the conversion of a light-image or electrical signal into an electrostatic charge pattern on an electrically insulating layer. Image-forming development can thereafter be effected according to any one of several procedures whereby to render the latent charge pattern visible. Electrophotographic processes based upon the utilization of photoconductive layers include of course the xerographic methods in which an electrically conductive support is first subjected to a uniform electrostatic charge in the dark this being accomplished for example by means of a high voltage corona discharge whereby an electrostatic charge is created on the element surface. Latent image formation can thereafter be effected by focussing a light-image on the charged surface, the light energy serving to selectively dissipate the electric charge in proportion to the intensity of the incident light radiation, i.e., an imagewise dissipation of the electric charge in accordance with the impressed lightimage. The residual charged areas of the photoconductive layers, i.e., those protected by the image areas of the original and thus unaffected by the exposure radiation provide what is tantamount to a latent electrostatic image pattern which can be readily rendered visible by application thereto of a suitable colorant, e.g., toner powder, having optical density sufficient to permit visible discernment of the image areas and which readily adheres to the residual charged areas. By the foregoing electrostatic development operation there is obtained a permanent visible image which provides an exact replica of the original. It is appreciated of course that a number of ramifications to the aforedescribed basic process by way of improvement have been promulgated in the art; invariably however, such methods depends for feasible practice upon the principal of light-induced charge-decay whereby to provide the surface of the image-recording member with a residual charge pattern capable of conversion to a readily visually comprehensible image. In general, it is found to be more effective practice to transfer the developed image which for example, may be defined by a pigmented resinous composition constituting a toner powder to a receiving sheet in contradistinction to methods wherein the photoconductive plate itself provides diice rect means for producing the desired photographic copy absent any transfer operation involving a receiving or master sheet. Photoconductive layers for use in electrophotographic reproduction processes of the foregoing type are conventionally prepared with such photoconductor materials as selenium, cadmium sulfide, zinc oxide, etc. For a plurality of reasons, zinc oxide has proved to be particularly beneficial for the vast majority of operations associated with electrography. The zinc oxide photoconductor material is conventionally employed in photoconductive layers in the manner described, i.e., a grounded support usually paper, is initially rendered sensitive to light by subjecting same to a blanket negative electrostatic charge on the zinc oxide layer in the substantial absence of any ultraviolet or visible radiation. As previously mentioned this step can readily be effected by means of ion transfer from a corona discharge. Following exposure, the resulting latent image areas, i.e., nonlight-struck portions of the photoconductive layer are developed, for example, with a pigmented resin powder having a charge opposite to the negative charge of the unexposed areas of the photoconductive layer. In this fashion, the pigmented powder firmly attaches itself via electrical attraction to such negatively charged areas. The strength of adhesion of the resin powder to the image bearing layer can be enhanced by a suitable fixing operation as for example by simply heating the resinous material to temperatures sufficient to fuse or melt same whereby such resin becomes permanently affixed to the surface of the image layer. It is to be understood of course that the temperatures employed in this operation should be selected so as to avoid any possibility of charring the paper support. In any event, suitable methods for effecting the development of a latent electrostatic image pattern are described in the prior art with recourse to a particular one depending primarily upon the requirements of the processor.

As previously mentioned, particularly beneficial results are obtained with the use of photoconductive layers containing as the photoconductor substance, zinc oxide. Despite the advantageous features inherent in the use of this material, certain problems are nevertheless encountered as regards attempts to impart optimum spectral response thereto. Since the effective photographic speed of the reproduction process vitally depends upon the actinic response of the photoconductor material, the overriding importance of this factor is readily evident. Thus, practically without exception, the commercial grade zinc oxide photoconductor materials provided specifically for use in connection with the formulation of photoconductive layers exhibit maximum or peak spectral response to but a rather limited region of the spectrum, primarily, the far blue and the ultraviolet. In contradistinction, the vast majority of the light sources customarily employed for electrophotographic exposures display maximum output in the visible spectral region, e.g., an ordinary tungsten light. Such restricted spectral response quite obviously imposes stringent and burdensome limitations upon the process in those instances wherein zinc 0X- ide is employed as the photoconductor material since the corresponding requirement is presented that a light source having the proper radiant emission be employed.

In view of the premier commercial importance of zinc oxide in the electrophotographic industry, considerable industrial activity has centered around the research and development of various means by which to extend the spectral response of zinc oxide photoconductors whereby to impart thereto peak sensitivity in those spectral regions forming the locus of the emissions characteriz ing those light sources which would ordinarily be employed. At this point it should be mentioned that one suggested remedy to the foregoing problem involves the use of photoconductive materials having a spectral response in the visible spectrum. Photoconductor substances which have heretofore been suggested for such purposes include, for example, the colored oxides, sulfides, selenides, tellurides, and iodides of such materials as cadmium, mercury, antimony, bismuth, thallium, molybdenuln, aluminum, lead, zinc, etc. Although providing some measure of improvement, procedures based upon the use of the latter photoconductor materials have nevertheless proved somewhat unsuitable for certain applications. Thus, for the most part, the industrial effort thus far expended has been concerned with the development of materials capable of absorbing radiant energy and of transferring the energy so absorbed to the photoconductor. Thus, it has been suggested to incorporate sensitizing dyes with the zinc oxide photoconductor for the purposes of imparting the requisite spectral sensitivity to the reproduction system. Representative dyestuff materials heretofore promulgated in this regard include, for example, rose bengal, eosin, malachite green, crystal violet, methylene blue, methylene grey, fluorescein and the like. Although the use of such dyestuffs has contributed greatly to resolving the problems associated with zinc oxide spectral sensitivity, other problems of a rather significant nature have nevertheless arisen as an incident thereto. Perhaps the primary objection to the sensitizing dyestuffs thus far suggested relates to their pronounced tendency to impart to the sensitizing formulation a spurious off-white tint or coloration thus vitiating to a significant extent attempts to achieve satisfactory contrast, gamma and the like. More specifically, such dyestuffs lead to the formation of tints which may be blue, green, yellow, orange or red as well as various shades and hues thereof. Moreover, as will be appreciated, the undesired coloration of the zinc oxide layer is objectionable from an aesthetic standpoint the latter being a relatively important consideration bearing directly upon the possibilities of commercial acceptance. In some instances, the recording element itself may be contemplated for further exposure sequence, e.g., the production of either black and white or color prints therefrom. The deleterious effects directly attributable to any spurious tint or off-white shade present in the image bearing layer will, practically without exception, be manifested in the form of inferior photographic quality in the resulting print. Such adverse effects are particularly evident with regard to color reproduction since the presence of spurious coloration in the recording element gives rise to faulty absorption densities, i.e., any fugitive color density will effectively modulate the exposure radiation and thus to this extent, effect undesired shifts in the color composition, color balance, etc., of the color print.

A further serious objection to a considerable number of the sensitizing dyestuffs thus far proposed concerns rather their instability under varying conditions of pH. This imposes rather severe limitations on the processors latitude of operations tending to circumscribe severely the range of selection of many of the remaining ingredients to be employed in the coating formulation. For example, the pH hypersensitivity of many known sensitizing dyestuffs restricts the processor, for example, in regard to the nature of the resin materials which may be efficaciously employed. For example, if the resin material selected is not correlated with the pH sensitivity characteristics of the sensitizing dye, undesired colorations develop in the coated layer and invariably become exceedingly more pronounced on standing and .4 thus are readily visually perceptible. Apparently, discoloration of the coated element results from the inter-layer diffusion of acidic or alkaline materials, as the case may be, such conditions being conducive to the creation of spurious, off-white tints. As explained hereinbefore, fugitive coloration of the electrophotographic element is not only aesthetically displeasing but, and perhaps more importantly, renders such element substantially unsuitable for further photocopying operations. Thus, as a concomitant to the use of sensitizing dystuffs possessed of the pH sensitivity property, it becomes incumbent upon the formulator to adjust or otherwise modify the coating composition by way of compensating for undesired shifts in coloration which would otherwise occur. In many instances, resort to the use of masking dyestuffs, i.e., dyestuffs having spectral absorption substantially complementary to that of the fugitive color tint, is made mandatory. As will be appreciated, remedial techniques of this nature can prove burdensome to the formulator, requiring relatively precise and predetermined adjustments in coating formulae. The costs involved in implementing such techniques may Well be prohibitive.

Other problems of equal significance which have been noted to attend the use of many of the sensitizing dyestuffs thus far provided relate to their sub-optimum compatibility with one or more of the ingredients comprising the electrostatic layer coating composition. In this regard, it is imperative to quality reproduction that the photoconductive coating composition containing the zinc oxide photoconductor be provided, prior to actual coating, in the form of a homogeneous and uniform dispersion of the involved ingredients. Any departure in this conection from optimum uniformity of dispersion render the final coating substantially incapable of uniform spectral response, i.e., the density equivalent of a given exposure product will in all likelihood vary throughout the coated layer. Thus, the possibility that the recording system will necessarily reflect the point-to-point density variations in the original to be reporduced in substantially emasculated.

Thus, in accordance with the discovery forming the basis of the present invention it has been determined, surprisingly, that a relatively limited class of dyestuff materials provide eminently suitable sensitizing agents for electrophotographic layer compositions, enabling the attainment of high quality reproduction absent the undesirable features characterizing sensitizing dyestuffs heretofore provided for such purposes.

Thus, a primary object of the present invention relates to the provision of novel dyestuff materials advantageously adapted for use with electrographic layer compositions and wherein the foregoing and related disadvantages are eliminated or at least mitigated to a substantial extent.

A further object of the present invention relates to the provision of optically sensitized electrophotographic layer compositions having excellent sensitometric properties, e.g., speed, contrast, gamma, ets., said compositions being capable of yielding high qaulity reproduction.

Another object of the present invention relates to the provision of optically sensitized electrophotographic layers having excellent actinic response and stability characteristics, said layers being substantially devoid of any tendency to develop spurious coloration.

Other objects and advantages of the present invention will become more apparent hereinafter as the description proceeds.

The attainment of the foregoing and related object is made possible in accordance with the present invention which in its broader aspects includes the provision of sensitizing dyestuffs having excellent sensitizing properties for zinc oxide photoconductive layers adapted for electrophotography, said dyestuffs corresponding to the following in one of the positions indicated, i.e., R, R and R The structural formula: term carboxyalkyl as used herein, is intended to connote a I --Y N Ra 2" R RI 'I CH=OH CHC o f l; 5 f

\ oon= on=o--on=on I i-R2 O: n-1 /qwherein R, R and R independently represent alkyl, e.gl, carboxy group in free acid form connected to the dyestufi' methyl, ethyl, propyl, butyl, isobutyl, etc.; aralkyl, e.g., molecule by an alkylene bridge; thus, such term includes benzyl, fi-phenethyl; hydroxyalkyl, e.g., hydroxethyl; carboxymethyl, carboxyethyl, carboxy-n-propyl, etc. The alkoxyalkyl, e.g., B-ethoxyethyl; carbalkoxyalkyl, e.g., improvements provided by the present invention have been carbomethoxymethyl, carboethoxymethyl, carboethoxyascertained to obtain to an optimum extent when such ethyl; acyloxyalkyl, e.g., [3acetoxyethyl, and the like, with group comprises either carboxymethyl or carboxyethyl. the provision that at least one of R, R and R represents As particular examples of dyestufi materials falling carboxyalkyl, e.g., carboxymethyl, carboxyethyl, etc.; R within the ambit of the above structural formula and and R represent hydrogen, alkyl, e.g., methyl, ethyl, etc.; found to provide exceptional advantage as sensitizing aryl, e.g., phenyl; aralkyl, e.g., benzyl, phenethyl, etc.; agents with photoconductive layers contemplated for use hydroxyalkyl, e.g., B-hydroxyethyl, etc.; m, n, p and q in electrophotography, there may be mentioned the foleach represents a positive integer of from 1 to 3 inclusive; lowing: X represents an acid anion, e.g., chloride, bromide, thiocyanate alkyl sulfate, such as, methylsulfate, ethylsulfate, gl s gigii fjggigigagfi i gi fi etc.; arylsulfate, etc.; arylsulfates, such as benzenesulfo- I nates, p-toluene sulfonate, etc.; Y and Y" each represents 3136F131z'benzoxazohdene)ethyhdene]4'oxo lodldethe non-methallic atoms necessary to complete a 5 or 6- Thlazohmum, 3 iarboxxethyl 2 (3'carboxyeth yl membered heterocycle, and Y represents a member ChloroZ'benzothlazolyhdenemethyl)5'[%'(516'd1' selected from the group consisting of oxygen, sulfur, $222 13- yl- -benzoxazolylidene)ethyl1dene14-0xo l Selemum and mtmgen' Thiazolinium, 2-(3-carboxyethylbenzoselenazolylidene- Typical representatives of the heterocyclic nuclei inmeth yl) 5-[2-(3-carboethoxymethyl-4-methylth1azolyl1- cluded Within the ambit of the definitlons for Y, Y and Y dene)ethylidene] 4 OXO iodide given abovelmclgde for thllazole f f zfig Thiazolinium, 3-carboxymethyl-2-(3-carboxymethyl-5- awe? :3- dialkyl indolenine, pyridine and the like. The aforemen- $222111:'ethylbenzoxazolyhdene)ethyhdeneM'om tioned heterocyclic nucleic may further contain one or Thrazolmium, 2-(3-carboxymethyI-Z-benzothiazolylidenemore groups which comprise conventlonal substrtuents with respect to dyestuffs of this general type. Such sub- 40 gig g g s 5 (3 methylbenzothlazolyhdene)4 l g iffi tgl gxi 2 222 5 3 3 gf ig fi' g figiffigfi Thiazolinium, 2(3-carboXymethyl-2-benzothiazolylidenee g chloro"iodo f alkoxye g methoxsl ethox; etc mgtgyl)-3-ethyl-5-(3-methylbenzothrazolylidene)4-oxo J 7 '7 7 '7. 7 a 3 i e EZSJSZJZZZZZYFLZZiZaififliiii'ilitifi fiii; specifically their relationship to dyestutf molecules of the igigzicggilglenemethyl)5'(3'methylthlazohnyhdene) type enumerated. The salient requirement with respect to such substituents is that they be essentially innocuous or izg i i fi s gig iifiifi i 332 2 non-reactive and exhibit no tendency to deleteriously afp H dege) 4 OXO iodidp y e y 1 feet the sensitizing properties of the dyestufl? molecule. As y specific examples of heterocyclic nuclei falling within the One of the particularly surprising discoveries of the definition of Y and Y" given above there may be menpresent invention concerns the fact that dyestuffs of the tioned the following: thiazole, 4-methylthiazole, S-methyltype described above not only exhibit an exceptionally thiazole, 4 phenylthiazole, 4,5 dimethylthiazole, benzohigh order of sensitizing efficiency, i.e., impart a high thiazole, 5,6 dimethylbenzothiazole, 4 chlorobenzoorder of spectral response to zinc oxide photoconductorthiazole, 4 -methylbenzothiazole, 5 -bromobenzothiazole, containing photoconductive layers, but in addition, are 5,6 diphenylbenzothiazole, 6 bromobenzothiazole, 5- highly stable over varying conditions of pH thus remethoxybenzothiazole, 6 iodobenzothiazole, 4 ethoxymoving any limitation as regards the nature of the resin benzothiazole, 5,6-dimethoxybenzothiazole, 5 hydroxybinder material employed. In addition, their compatibility benzothiazole, 4 methoxy-thianaphtheno 7, 6', 4, 5- With the various other ingredients conventionally emthiazole, 4 methyloxazole, 4 phenyloxazole, 4,5 diployed in photoconductive coating compositions presents methyloxazole, benzoxazole, 5 chlorobenzoxazole, 5- significant advantage. Such dyestuffs are further atypical Phenylbenzoxalole, dimethylbenzoXalole, Y' in that photoconductive elements containing same may be be l a 6 ChlofobeIlZOXaZOle, 5 hydroxybenzoxa' stored for extended periods of time either before or after Z016, 4 methylselenalole, 4 PhenylselenaZole, 13611205616 electrophotographic processing in the virtual absence of nazole, 5 chlorobenzoselenazole, 5 methoxybenzoselediscoloration Whether due to pH or other conditions nazole, 5 hydroxybenzoselenazole, a-naphthoselenazole,

fl naphthoselenazole, thiazoline, 4 methylthiazoline, gig ggi sg fifig l ig gf zi g g g i 33; -hl 'l',6-

qumohne 5 methylqumohne 8 C Oroqumo me 5 to about 75 milligrams per pound of 21110 oxide with ethoxyquinoline, 8 hydroxyquinoline, isoquinoline, 3,4- dihydroisoquinoline, 3,3 dimethylindolenine, pyridine, a range of from about 18 to about 24 mlnlgrams belng 4 methylpyridine, 3 5 dimothylpyridino, 4 ohloropwp particularly preferred. Such proportions are not critical di 3 hydroxypyridine, 4 pheny1pyridine, and the 1i per se merely encompassing those values found to assure It is of critical importance that the trinuclear complex the Obtelltion of optimum results- The requirements of merocyanine dye derivatives of the present invention a particular process may well dictate the propriety of contain at least one N-bonded carboxyalkyl group, i.e., departures therefrom. It will also be understood that such dyestuffs may be employed singly or in admixture comprising 2 or more depending primarily upon the requirements of the processor e.g., the peak sensitivity values desired in the photoconductive layer.

The photoconductive layer compositions of the present invention may be prepared according to conventional procedures described in the prior art, i.e. utilizing conventional solvents, coating aids, driers, etc., such ingredients being of an optional nature. In any event, the essential components of the photoconductive composition comprise the sensitizing dye, the photoconductor material, i.e., zinc oxide, the latter being dispersed in an insulating binder material having relatively high dielectric strength and good electrical insulating properties. As particular examples of film-forming insulating binders found to be suitable for use herein there may be mentioned the following: styrene-butadiene copolymers; silicone resins; soya-alkyd resins; poly(viny1 chloride); poly(vinylidene chloride); vinylidene chloride, acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate, vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral), polyacrylic and methacrylic esters, such as poly(methyl methacrylate), poly(n-butylmethacrylate), poly(isobutyl methacrylate), etc.; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly(ethylenealkaryloxyalkylene terephthalate); phenol-formaldehyde resins; ketone resins; polyamide; polycarbonates; etc. Methods of making resins of this type have been described in the prior art, for example, styrene alkyd resins can be prepared according to the method described in US. Patents 2,361,019 and 2,258,423. Suitable resins of the type contemplated for use in the photoconductive layers of this invention are sold under such trade names as Vitel PE-lX, Cymac, Piccopale 100, and Saran F-220. Other types of binders which can be used in the photoconductive layers of the invention include such materials as paraffin, mineral waxes, etc. Other polymeric materials found to be especially advantageous include, for example, a polyester material available commercially from the Celanese Corporation of America under the commercial trade name designation Epitex 1311, which is prepared by reacting epichlorohydrin with bisphenol A employing the former in slight molar excess and thereafter reacting the polyether obtained with a mixture of a dimerized fatty acid and soya fatty acid. The resultant product comprises a linear, acetone-soluble, nonheat curable polymer material containing epoxy groups. Methods for the preparation of such polymers are described, for example, in U.S.P. 2,970,983. A further material found to be admirably suited for use herein comprises a product available commercially from the Pennsylvania Industrial Chemical Company under the trade name designation Piccolastic which is identified as being a low molecular weight (on the order of approximately 400), low melting point (approximately 75 C.) polystyrene resin. In accordance with the present invention, it has been ascertained to be especially advantageous to employ the resinous polymeric materials in admixtures comprising 2 or more and thus to capitalize on the superior properties which may typify specific ones. The term resinous binder as used herein is thus to be accorded a significance consonant therewith, i.e., extending to either the singular or conjuctive use of such resin materials.

Suitable solvents for effecting homogeneous dispersion of the ingredients comprising the layer composition include, for example, toluene, Xylene, benzene, acetone, 2-butanone, chlorinated hydrocarbon, e.g., methylene chloride, ethylene chloride, etc. ethers, e.g., tetrahydrofuran or mixtures of such solvent materials.

Alternatively, the ingredients comprising the photoconductive coating composition may be provided in the form of an aqueous system in contradistinction to an organic solvent system. Improved sensitization results with either method. Again, recourse to either a solvent or aqueous system will be dictated in large part by the requirements of the processor.

Application of the photoconductive coating composition to the support material can be effected according to standardized methods, well known in the prior art. Thus, for example, coating methods such as doctor-blade, swirling, dip-coating and the like may be employed. The thickness to which the photoconductive layer composition is deposited may vary over a relatively wide range; in general, however, wet coating thicknesses Within the range from about .001" to about 0.01" are found to be eminently suitable for accomplishing the purposes of the present invention. Particularly beneficial results are found to obtain with the use of wet-coating thicknesses falling within the range from about .002" to about .006". The support material employed may be any of the conventional materials promulgated in the art for the fabrication of electrostatic recording elements the principal requirement being that such materials exhibit adequate electrical conductivity. Such materials include, for example, paper (at a relative humidity above about 20%); aluminum-paper laminates; metal foils, such as, aluminum foil, zinc foil, etc., metal plates, such as, aluminum, copper, zinc, brass, and galvanized plates; regenerated cellulose and cellulose derivatives; certain organic polymeric plastic materials, e.g., polyester and especially polyesters provided with a thin electroconductive layer, such as cuprous iodide coated thereon. Suitable supporting materials include in addition the humidity-independent conducting layers of semi-conductors dispersed in polymeric binders.

Other ingredients which may be incorporated into the coating composition for purposes of expediting the coating operation as well as to render the ultimate coating more suitable for use in the image recording process include, for example, plasticizers; e.g., polymeric hydrocarbons having a fair degree of aromaticity and low iodine value; drying agents, e.g., cobalt naphthenate, manganese naphthenate and the like.

The zinc oxide photoconductor materials contemplated for use herein are available commercially. Desirably, the zinc oxide should be provided in the form of relatively small particles having a mean diameter of less than about 0.5 micron. Particularly preferred for use herein is the zinc oxide product produced according to the French Process such as French Process, Florence Green Seal, pigment grade zinc oxide commercially available from the New Jersey Zinc Sales Company Inc. of New York. Other zinc oxide materials preferred for use herein include, for example, the product commercially known as St. Joe PC321 zinc oxide. Optimum realization of the advantages provided by the present invention is obtained by employing the insulating binder in amounts sufficient to insulate each of the zinc oxide particles from the remaining ingredients of the coating composition. Such proportions can be readily determined by rather route laboratory investigation.

The recording elements described herein can be advantageously employed with any of the well known electrophotographic processes based upon the use of photoconductive layers, e.g., the Xerographic process the latter being carried out by initially subjecting the electrophotographic element to a blanket electrostatic charge, e.g., by the use of a corona discharge. In view of the insulating character of the photoconductive layer, attributable to the presence of the insulating resin binder material, the uniform charge extent over the surface of the photoconductive layer is retained, such layer also having the property of negligible conductivity in the dark or, as more commonly stated, high dark resistivity. Exposure of the photoconductive layer to light erves to effect an imagewise dissipation of the electrostatic charge from the surface of the layer thus leading to the formation of a latent electrostatic charge pattern. The exposure may be effected through a negative by conventional exposure methods as for example, by contact printing techniques or alternatively by lens projection of an image. The extent of pointto-point charge dissipation depends correspondingly upon point-to-point intensity of the exposure illuminant. The residual charge pattern is thereafter rendered visible or otherwise developed by treatmnt with a suitable colorant, pigment, etc. comprising electrostatic particles having a charge opposite to that of the residual charge constituting the electrostatic latent image pattern, said developing agent being capable of ready visual comprehension. The developer agent may comprise, for example, a liquid developer in which the developing particles are suspended in an electrically insulating liquid carrier. Developing methods of this type are of course well known being described, for example, in U.S.P. 2,296,691 and in Australian Patent 212,315. Other developing methods depending upon, for example, heat fusion of resin particles, image transfer are likewise well known in the art and may be utilized to advantage in the practice of the present invention.

The following examples are given for purposes of illustration only and are not to be considered as necessarily limiting the present invention.

EXAMPLE I An electrophotoconductive coating composition is prepared in the following manner:

To a solution consisting of: Toluene250 ml.

Cobalt naphthenate0.2l gm. Manganese naphthenateO.21 gm. Xylene 6 ml.

is added, With stirring, 151 gm. of Epitex 1311 (a polyester obtained by reacting epichlorohydrin with bisphenol A to form a polyether and thereafter reacting the latter with a mixture of dimerized fatty acid and soya fatty acid as described in U.S.P. 2,970,983). Upon completion of the Epitex addition, 454 gm. of zinc oxide photoconductor (St. Joe P0321) is added to the solution while stiirring. With stirring being continued, a resin solution containing 35 gm. of piccolastic A75 (polystyrene resin having a molecular weight of approximately 400 and a melting point of approximately 75 C.) is added. The medium is thereafter stirred and milled until smooth. At this point there is added 20 mg. of the sensitizing dyestuff,

Thiazolinium, 3 carboxymethyl 2 (3 carboxyethyl-S- methoxy 2 benzoselenazolylidene methyl)5 [2- (3 benzyl- 2- benzoxazolylidene)ethylidene]4 oxo, iodide which has the following structural formula:

p, CCHCH=C 0 J3 o-on=o +1- -OCH3 N \N N CH2 i omooon cum coor-r dissolved in 20 ml. of methanol. The medium is thereafter stored for a period of approximately 30 minutes and then coated on Riegel 45 lb. conductive paper to a dried coating thickness of 20 lb. per 3000 sq. ft.

A coated paper is thereafter evaluated electrophotographically by exposure in a Bruning Copytron 2000 the latter comprising commercially available eletcrophotographic copying apparatus based upon dry toner development. The prints obtained are characterized by excellent density, contrast, etc. and are totally devoid of spurious coloration thus providing high contrast copy.

Moreover such prints exhibited no tendency to discolor upon standing for extending periods of time under varying conditions of heat, humidity, etc. In addition, the coated paper could be stored prior to electrophotographic process, exhibiting excellent stability upon aging for protracted time periods despite relatively severe conditions of heat, humidity, etc.

Upon examination of the various prints specimen it was readily evident that uniform dispersion of the sensitizing dyestuffs throughout the coating composition had been achieved in view of the prevailing uniformity of density, contrast, etc. The same observations were noted in connection with coated paper samples which had been subjected to extensive aging prior to electrophotographic processing.

The dyestutf employed in the above example is prepared in the following manner:

A solution comprising 0.8 part of methyl-p-toluene sulfonate and 0.42 part of the following compound:

(compound A) is heated at 131 C. for 20 minutes and allowed to cool. Thereupon, 0.3 part of the following compound:

N CH2 l COOH is added with 5 parts of methanol and 30 drops of triethylamine. The solution is then heated on a steam bath and glacial acetic acid is added, whereupon the initial stages of dye formation is detected. Thereupon, toluene is added as well as potassium iodide in acetone. The solution is then transferred to a centrifuge tube, centrifuged, decanted and the residue boiled out with toluene, recentrifuged (hot) and again decanted. The residue is boiled out with methanol, cooled and filtered to yield 0.2 part of dyestutf which, upon analysis, was determined to be:

Thiazolinium, 3 -carboxymethyl-2- 3-carboxyethyl-5- methoxy-2-benzoselenazolylidene-methyl) 5 [2- 3- benzyl-Z-benzoxazolylidene)ethylidene] 4-oxo, iodide having the structural formula depicted, exhibiting a peak spectral absorption in methanol at 572 Ill 1..

Compound A above is prepared by reacting 3-carboxymethyl rhodanine, in the presence of methanol and triethylamine with the following compound:

1 1 EXAMPLE 11 The procedure described in Example I is repeated except that the dyestuif employed comprises the following:

Thiazolinium, 3-carboxyethyl-2- 3-carboxyethyl-5-chloro- 2-benzothiazolylidene-methyl)5 [2 (5,6-dimethyl-3- ethyl2-benzoxazolylidene)ethylidene] 4-oxo, bromide having the following structural formula:

The above dyestuff which exhibits a peak spectral absorption in methanol at 586 mg is prepared in a manner identical with that described in Example I.

EXAMPLE III Example I is repeated except that the dyestuff employed comprises the following:

Thiazolinium, 2-(3-carboxyethylbenzoselenazolylidenemethyl 5- [2- 3-carboethoxymethyl-4-methylthiazolylidene)ethylidene] 4-oxo, iodide The dyestuif employed in Example III is prepared in the following manner:

A solution comprising 0.8 part of methyl-p-toluene sulfonate and 0.37 part of the following compound:

is heated at a temperature of 131 C. for 25 minutes and allowed to cool. Thereupon 0.38 part of the followin compound is added:

I COOH along with 4 parts of methanol and 30 drops of triethylamine. The solution is stirred, and warmed on a steam bath for 3 minutes whereupon 30 drops of acetic acid and 30 parts of toluene are added. The solution is allowed to stand for a brief time whereupon dye precipitation is effected by the addition of ether. The dye precipitates in the form of an oily substance which is thereafter decanted, triturated with ether, boiled out with toluene, decanted and boiled out twice with methanol. There is obtained 0.2 part of a dyestuff which upon analysis was determined to be:

Thiazolinium, 2-(3-carboxyethylbenzoselenazolylidenemethyl 5- [2- 3-carboethoxymethyl-4-methylthiazolylidene)ethylidene] 4-oxo, iodide having the structural formula previously illustrated and exhibiting a peak spectral absorption in methanol at 612 m In each of Examples II and III the prints obtained exhibited the superior properties described in Example I. Significantly, it is found by way of comparison that sensitizing dyestuffs of the type described in the prior art when subjected to the identical processing, yield prints of markedly reduced density and contrast. In fact, in order to achieve photographic speed, contrast, comparable to those typifying the use of the sensitizing dyestuffs of the present invention it was necessary to increase by a considerable margin the quantity of sensitizing dyestuff employed. Thus, one of the salient advantages of the present invention is at once apparent, namely synergistic sensitizing results can be obtained despite the use of the sensitiving dyestuif in reduced amounts. This effects a correlative mitigation in problems associated with spurious color tints arising from the use of the sensitizing dyestuffs in exaggerated quantities, i.e., on the order of those required for effective use with prior art sensitizers.

Similar results are obtained when the procedures exemplified are repeated but employing as the sensitizing dyestuff, the following:

Thiazolinium, 3-carboxymethyl-2 (S-charboxymethyl-S- chloro-2-benzothiazolylidene-methyl)5 [2,5,6-dimethyl-3-ethylbenzoxazolyidine)ethylidene] 4-oxo bromide.

Thiazolinium, 2-(3-carboxymethyl-2-benzothiazolylidenemethyl)-3-ethyl 5 (3-methylbenzothiazolylidene)4- oxo iodide.

Thiazolinium, 2-(3 carboxymethyl 2 benzothiazolylidenemethyl)-3-ethyl 5 (2-[ethyl-2-benzothiazolylidene] ethylidene 4-oxo iodide.

Thiazolinium, 3-carboxyethyl-2-(4,5 diphenyl-3-ethyl-2- oxoazolylidenemethyl)5 (3 methylthiazolinylidene) 4-oxo iodide.

Thiazolinium, I i-carboxymethyl 2 (3-ethyl-5-methyl- 4-phenyl 2 thiazolylidenemethyl)5-(3 methylthiazoliny1idene)4-oxo iodide.

The sensitizing dyestuffs of the present invention may be employed to advantage either singly or in admixture. Moreover, they may be used in combination with one or more of the dyestufl sensitizing materials heretofore described in the art. In any event, it is recommended practice to maintain the proportions of the instant dyestufi products within the concentration range hereinbefore specified in order to assure the obtention of optimum results. It is envisaged that in some instances sensitizer dyestuff concentration may be considered desirable which are substantially in excess of the delineated range. For example, it may be that the speed requirements of the process may dictate such a departure. In such instances it is advisable to include one or more additional dyestuffs in the photoconductive coating composition which perform primarily a masking function, i.e., to the substantial exclusion of a sensitizing function in order to assure the suppression of any possible fugitive tint which might otherwise develop. It will be further understood that such masking dyestuffs may be included as optional ingredients despite the use of the sensitizing dyestuffs of the present invention within the preferred concentration range; however, for the vast majority of commercial applications their use would hardly be required.

The present invention has been disclosed with respect to certain preferred embodiments thereof, and there will be obvious to persons skilled in the art modifications, equivalents or variations thereof which are intended to be included within the spirit and scope of this invention.

wherein R, R and R independently represent a member selected from the group consisting of alkyl, aralkyl, hydroxyalkyl, alkoxyalkyl, carbalkoxyalkyl, acyloxyalkyl with the provision that at least one of R, R and R represent ca'boxyalkyl; R and R represent a member selected from the group consisting of hydrogen, alkyl, aryl, aralkyl, hydroxyalkyl, m, n, p and q each represents a positive integer of from 1 to 3 inclusive, Y and Y" each represents the atoms necessary to complete a 5 or 6-membered heterocycle, Y represents a member selected from the group consisting of oxygen, sulfur, selenium and nitrogen, and X represents an acid anion.

2. A composition according to claim 1 wherein said sensitizing dyestufi comprises thiazolinium, B-carboxymethyl-Z-(3-carboxyethyl-5-methoxy 2-benzoselenazoly1- idene-methyl)5-[2-(3-benzyl 2 benzoxazolidene)ethylidene]-oxo iodide.

3. A composition according to claim 1 wherein said sensitizing dyestuff comprises thiazolinium, 3-carboxyethyl-2-(3-carboxyethyl 5 chloro 2-benzothiazo1ylidene-rnethyl(5-[2-(5,6-dimethyl 3-ethyl-2-benzoxazolylidene)ethylidene]4-oxo bromide.

4. A composition according to claim 1 wherein said sensitizing dyestuif comprises thiazolinium, 2-(3-carboxyethylbenzoselenazolylidene-methyl)5 [2 (Ii-carboethoxymethyl-4-methylthiazo1y1idene)ethylidene]4 oxo iodide.

5. A composition according to claim 1 wherein said sensitizing dyestulf comprises thiazolinium, S-carboxymethyl-2-(3-carboxymethyl 5 chloro-Z-benzothiazolyl- 14 idene-methyl)5-[2 ,5,'6-dimethy1 3 ethylbenzoxazolylidene)ethylidene]4-oxo bromide.

6. A composition according to claim 1 wherein said sensitizing dyestuff comprises thiazolinium, 2-(3-carboxymethyl-2 benzothiazolylidenemethyl) 3 thyl-S-(3- methylbenzothiazolylidene) 4-oxo iodide.

7. A composition according to claim 1 wherein said sensitizing dyestufl? comprises thiazolinium, 2-(3-carboxymethyl-2-benzothiazolylidenemethyl)3 ethyl 5-(2-[3- ethyl-2-benzothiazolylidene]ethylidene)4-oxo iodide.

8. A composition according to claim 1 wherein said sensitizing dyestuff comprises thiazolinium, 3-carboxyethyl-2-(4,5-diphenyl 3 ethyl-2-oxazolylidenemethyl) 5-(3-methylthiazolinylidene)4-oxo iodide.

9. A composition according to claim 1 wherein said sensitizing dyestuif comprises thiazolinium, 3-carboxymethyl-2-(3-ethyl 5 methyl-4-phenyl-2-thiazolylidenemethyl) 5-(3-methylthiazolinylidene) 4-oXo iodide.

10. An electrophotographic recording element comprising a backing member overcoated with the composition of claim 1.

References Cited UNITED STATES PATENTS 3,047,384 7/1962 Jones et al. 961.7 3,288,610 11/1966 Gotze 961.7 X

FOREIGN PATENTS 292,828 11/ 1953 Switzerland. 541,245 9/ 1955 Belgium.

GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner US. Cl. X.R.

" UNITED STATES PATENT OFFICE 6) CERTIFICATE OF CORRECTION Patent No, Dated 2]., Invenmfl Lee C. Hensley It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the title, Column 1, line 3, for "Lee C. Hensley, 131 Mary St., Binghamton, N.Y. 13903", insert,

--Lee C. Hensley, Binghamton, N.Y., assignor to GAF Corporation, New York, N.Y., a corporation of Delaware-- SIGNED AN SEALER o'cT 271970 SEAL Attest:

Edward M. Fletcher, 11-. v F R-1AM E. 'summm,

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Attcsting Officer sion of t nts