US3476559A - Electrostatic printing element comprising a dye sensitized zinc oxide coating on a high resistivity paper backing sheet and a process of using said element - Google Patents

Description

United States Patent Oflice 3,476,559 Patented Nov. 4, 1969 3,476,559 ELECTROSTATIC PRINTING ELEMENT COMPRIS- ING A DYE SENSITIZED ZINC OXIDE COATING ON A HIGH RESISTIVITY PAPER BACKING SHEET AND A PROCESS OF USING SAID ELEMENT Irving Panken, Brooklyn, N.Y., assignor, by mesne assignments, to lug. C. Olivetti & C. S.p.A., Ivrea, Italy, a corporation of Italy No Drawing. Continuation-f application Ser. No. 379,724, July 1, 1964. This application Jan. 29, 1968, Ser. No. 701,468

Int. Cl. G03g 5/08 U.S. Cl. 961.7 7 Claims ABSTRACT OF THE DISCLOSURE An electrostatic recording element is provided having a paper base sheet, zinc oxide as a photoconductor, a combination of sensitizing dyes for the photoconductor, and a binder for the dye and photoconductor for binding the two on the sheet. The recording element comprising the above combination of elements is panchromatically sensitive and possesses a resistivity, attributable to the base sheet only, from 10 ohm-cm. to 5 l0 ohm-cm. Moreover, this resistivity is displayed at low humidity conditions of 15% relative humidity at 70 F. while at high humidity conditions, e.g., 90% at 70 F., the recording element is still successfully utilized. Of the dyes used, Thioflavin is preferred in an amount of 35% of the other dyes used, such as Auramine O, Bromophenol Blue, and Rose Bengal.

This application is a continuation of application Ser. No. 379,724, filed July 1, 1964 and now abandoned.

This invention relates to electrostatic recording elements, a process of electrostatic recording, and, in particular, to an improved panchromatically sensitive paper and an improved photoconductive coating for such paper.

In operation a typical electrostatic printing process utilizes an element, such as a paper sheet, coated with a composition comprising a photoconductive material bonded by a resin or held in a matrix. Such a coating, in the dark, has a high resistivity. When the photoconductive coating is exposed to light, such as that reflected from a page to be copied, the photoconductive material, on those parts of the coating which are exposed to the light, becomes conductive compared to the material which is not exposed to light or only exposed to a relatively small amount of light. An electrostatic charge is imparted to the surface of the element by means known to the art, for example by corona-discharge, and this charge is dissipated where the photoconductive material on the surface of the sheet becomes conductive by exposure to light. Portions of the surface which have not been subjected to any significant amount of light remain in a charged condition, thereby forming an invisible pattern or-latent image of electrostatic charges on the element. This pattern, for example, corresponds to the dark areas, e.g., the printing on the page to be copied.

The electrostatic image pattern is made visible by applying to the element a toner powder which is selectively attracted and held by the electrostatical charges of the pattern. This toner powder is converted to a permanent printing by heating sufficiently to fuse the resin or other powder in the toner.

Apparatus for applying static charges to the photoconductive surface, exposing the surface to the desired light pattern, distributing, and fusing the toner on the surface are well known in the art.

The use of dye-sensitised photoconductive surfaces for obtaining panchromatically-responsive surfaces for use in electrostatic printing processes is old in the art. Greig describes the use of dyes in the preparation of panchromatic zinc oxide-organic binder coating composition in U.S. Patent 3,052,540. It has been generally accepted that such light-sensitive photoconductive compounds must be used on a high resistance conductive base, i.e., one having a resistivity of up to about 10 or 10 ohms-cm. under conditions of normal humidity and temperature, to form electrostatic recording elements. Ordinary paper had been considered too non-conductive to be useful as a carrier element. However, the resistivity of the paper decreases with increased moisture content but at higher humidity it increases. Either excessively high or low conductivity of the carrier sheet may make impossible the imparting of a suitable static charge and maintaining the latent image for a suitable period of time.

To avoid excessively high resistivity during periods of low humidity, hurnectants are added to such paper; in some cases, water-receptive resins are coated on the backside of the paper. Such treated papers, however, have inferior performance characteristics during periods of high humidity because of excessive conductivity resulting from increased moisture content of the paper.

Thus it is a problem in this art to provide a backing that will be suitable at any normal temperature and humidity conditions. Among other problems that are common to the art is the obtaining of sharp contrast between the area in which the toner is fused and the light background areas of the photoconductive surface, also the minimizing of time required for imparting a desired conductivity to the light areas of the photoconductive surface on its being exposed, and maximizing the time the electrostatic image will remain on the paper after initially being imparted thereto.

Another problem in the preparation of suitable photoconductive surfaces, e.g., for use in oflice copying, etc., is the difiiculty in providing a panchromatically photoconductive surface which is substantially white, i.e., free from tints imparted by different dyes utilized to obtain the desirable panchromatic response to light irradiation.

Therefore, it is an object of this invention to provide a copying sheet coated with a photoconductive material having a substantially white appearance and preferably a panchromatic response to light irradiation.

It is another object of this invention to provide a conductive sheet which requires only a relatively short period of exposure to light in order to discharge the surfaces exposed to such light.

Itis likewise an object of the invention to provide a composition of rapid photoconductive response.

It is another object of the invention to provide a photoconductive paper of sufficiently high resistance to facilitate the acquisition and retension, in the dark, of a relatively high surface charge on the photoconductive surface thereof when subjected to static-producing means such as corona.

It is still another object of the invention to provide a photoconductive sheet, with a base material useful for substantially trouble-free copying over a wide range of humidity conditions.

Applicants have met these objects by utilizing a paper backing having a relatively high resistivity as measured through the backing at 15 relative humidity and 70 F. Use of such a sheet of high resistivity, i.e. of 10 ohmcm. up to about 5X10 ohm-cm, has been found to give successful operation of electrostatic printing processes even under conditions of very high humidity, obtaining and maintaining high electrostatic charges on the photoconductive surface of the backing, and permitting the photoconductive surface to have suitable electrostatic dark-decay factor of about 0.1, that is the voltage across the photoconductive surface will decay from 200 volts to 100 volts in about 10 seconds. Such high-resistivity backings may be utilized successfully in electrostatic printing processes even after being stored for extended periods at relative humidities as high as 90% at 70 F.

The photoconductive surface of the sheet responds to light with excellent and surprising rapidity. This rapidity is thought to be due to the sensitizing dyes utilized by applicants. These sensitizing dyes also have the advantage of contributing relatively little reddish or yellowish tint to the paper.

As a photoconductive coating useful with this high resistance paper backing, there may be used a composition generally comprising from 10% to 40% of an organic polymer binder, 65 to 90% of photoconductive zinc oxide, and from to about 0.5% of photosensitizing dyes.

Especially advantageous photoconductive coating materials are those comprising photoconductive agents such as zinc oxide (especially that sold by New Jersey Zinc Company under the trade name Florence Green-Seal 8) and resin binders such as those known to the art as petroleum hydrocarbon type resin e.g., as sold by Pennsylvania Industrial Chemical Corporation under the trade name Piccopale 100, which is soluble in hydrocarbons, most esters and ketones, in some alcohols; copolymers of styrene and butadiene, e.g., that sold under the trade designation Pliolite S-D by Goodyear Tire and Rubber Company, acrylic resins, e.g., those sold under the trade designations B-82, B-44, and B72 by Rohm & Haas, terpolymers such as the indenstyrene-acrylonitrile terpolymer, sold under the name Piccoflex 105, by the Pennsylvania Industrial Chemical Corporation, and other resins known to the art to provide suitable nonconductive coatings which can carry and bind the photoconductive agents dispersed therein for coating on a backing.

Plasticizers are useful on such a coating for modifying the film-forming properties of these resins, advantageously the vinyl-type resin plasticizer sold under the trade designation Hercoflex 150 by Hercules Powder Company; but other plasticizers known in the art to be useful can serve this purpose, being chosen to suit the particular resin and to keep the low-conductivity of the coating.

Dyestuffs which may advantageously be added, to extend the range of chromatic response of the photoconductive coatings, include among others those listed in U.S. Patent 3,052,540. An advantageous fast-acting composition particularly adapted for use on the high-resistivity sheet of the present invention comprises Thioflavin, Auramine O, Bromophenol Blue, and Rose Bengal with proportions adjusted to give a balanced spectral absorption. Most advantageous dyestuff mixtures comprise at least 35% or more of Thioflavin. Some of these dyes may be omitted or replaced by equivalent dyes.

A toner, i.e. a thermally activatable adhesive powder (with or without color) adapted to contrast with the coating, is distributed over the coated sheet and is held electrostatically on the charged areas which have not been rendered conductive by light irradiation. Such toner advantageously comprises a low conductivity carbon black, e.g. that sold under the trade designation Raven by Columbian Carbon Company, and a resin binder essentially comprised of a heat activatable material such as thermoplastic indene-styrene-acrylonitrile terpolymer sold under the trade name Piccoflex 120 by Pennsylvania Industrial Chemical Corporation and a low molecular weight resin sold under the trade name Piccolastic P-lO O by the same company. These are mixed with a magnetic material such as magnetic iron oxide powder to orient it into a magnetic brush.

Generally, the toner composition comprises from 1% to 25% carbonblack, 5% to 40% iron oxide and 30% to 80% of resin binder.

The base sheet of high-resistivity may advantageously comprise a clay-coated paper, e.g., of sixty-pound Weight and thickness about 0.0032 inch, obtainable from Allied Paper Company of Kalamazoo, Michigan. The photosensitive coating is applied at a rate of from 10 to .40 lbs. per ream (24" x 36" sheets), i.e., about 3,000 square feet of paper. An especially advantageous coat is about 21 lbs. per ream with a coating of about a 0.065 mil thickness.

In this application, I have described a preferred embodiment of my invention and suggested various alternatives and modifications thereof; but it is to be understood that these are not intended to be exhaustive, and other changes and modifications can be made within the scope of the invention. The particular descriptions and suggested variations herein are selected for purposes of illustration in order that others skilled in the art will more readily and fully understand the invention and principles thereof and will be able to embody it in a variety of forms and to modify them, each as may be best suited to the conditions of a particular use.

Example 1 The following materials were compounded by grinding,

e.g. in a paint mill, as used in the coating art until the zinc oxide was dispersed to a fineness of 5-6 on the A dispersion of dyes comprising the following materials was then mixed into the composition:

Rose Bengal mg 10 Bromophenol Blue mg 2 Auramine O mg 10 Thioflavin P mg 25 Methyl alcohol cc The resulting composition was coated on 60-pound bond paper having a resistivity of 2X10 cm. as measured at 15% relative humidity and F, The coating was spread at a rate of about 0.007 lb. (dry weight) per square foot of paper and formed a coating of about 0.65 mils thick. This coating was dried. The coated paper formed an effective electrostatic recording sheet.

The paper was stored under dark conditions and used in an electrostatic printing machine, e.g. such as described in a copending application Ser. No. 295,652 of Ira Sage now known to the art, .and passed through a 6,000-vo1t, corona-discharge unit to cause a polarization of electrical charges on the coated paper and thus establish a negative charge on the coated surface of the element. The element was then exposed to a light image, focused on the charged surface, giving light and shadow corresponding to the image to be printed, e.g., a plurality of thin horizontal black shadow lines. This light exposure lasted about 0.075 second. After the exposure, the element was powdered with a toner comprising 15 parts carbon black and 55 parts of a fusible resin binder mixed with 20 parts magnetic iron oxide; this mixture is then mixed with an iron powder, which powder acts as a magnetic brush when in a magnetic field. This binder was previously compounded to form an apparently homogeneous composition and reduced to a powder having a particle-size of about 2 microns.

The sheet was inverted and shaken lightly to remove toner in uncharged areas; the toner, however, adhered to the areas or lines which had not been exposed to light. On heating this sheet, the retained toner was fused to form a permanent printing resembling in appearance lines formed of India ink.

Example 2 The procedures used in Example 1 were repeated using the following quantities of materials to form the photoconductive coating compositions:

Zinc oxide grams 210 Styrene-butadiene copolymer (Pliolite S-SD) grams 29.5 Plasticizer (Hercoflex 150) do 6.5 Toluene do 1400 Rose Bengal mg 18 Auramine O mg 21 Thioflavin P mg 40 Methyl alcohol cc 110 Bromophenol Blue mg 4 What is claimed is:

1. A panchromatically-sensitive recording element for electrostatic copying consisting essentially of .a paper backing sheet having a resistivity of from ohm-cm. to 5X10 ohm-cm. as measured at relative humidity at 70 F., and a coating on said sheet comprising photoconductive zinc oxide and a sensitizing dye mixture dispersed in an organic polymer-matrix, said sensitizing dye mixture comprising Rose Bengal, Bromophenol Blue together with at least one dye selected from the group consisting of Thioflavin and Auramine O.

2. A panchromatically-sensitive recording element as in claim 1 wherein said sensitizing dye mixture comprises Rose Bengal, Bromophenol Blue, and Thioflaven.

3. A panchromatically-sensitive recording element as in claim 1 wherein said sensitizing dye mixture comprises Rose Bengal, Bromophenol Blue and Auramine O.

4. A panchromatically-sensitive recording element as in claim 1 wherein said sensitizing dye mixture comprises at least of T hioflavin.

5. A panchromatically-sensitive recording element as in claim 2 wherein said sensitizing dye mixture comprises a major amount of Thioflavin.

6. A process of electrostatic printing comprising imparting an electrostatic charge to a recording element consisting of a paper backing sheet having a resistivity of from 10 ohm-cm. to 5x10 ohm-cm. as measured at 15% relative humidity and F. and a photoconductive coating on said sheet comprising a major portion of photoconductive zinc oxide and a sensitizing dye mixture dispersed in an organic polymer matrix, said sensitizing dye mixture comprising Rose Bengal, Bromophenol Blue together with at least one dye selected from the group consisting of Thioflavin and Auramine O, exposing said sheet to a light image, coating said sheet with a toner pow der, removing excess toner to leave a pattern on said element, and fusing said toner.

7. A process as in claim 6 wherein a major proportion of the dye mixture is Thioflavin.

References Cited UNITED STATES PATENTS 3,052,540 9/1962 Greig 96-1.7 3,113,003 12/1963 Teiser et a1. 961.5 3,116,147 12/1963 Uber et a1. 961.8 3,121,006 2/1964 Middleton et al. 96-15 OTHER REFERENCES Young et a1., R.C.A. Review, December 1954, pp. 469-478. Waltwin, The Lithographers Manual, vol. 2, Waltwin (1958) pp. 1333 to 13:33.

GEORGE F. LESMES, Primary Examiner I. C. COOPER, Assistant Examiner