US3376134A

US3376134A - Photoconductive compositions comprising zinc oxide and methods for using such

Info

Publication number: US3376134A
Application number: US440315A
Authority: US
Inventors: Frederick A Stahly; Westlake L Goehring
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1965-03-16
Filing date: 1965-03-16
Publication date: 1968-04-02
Anticipated expiration: 1985-04-02

Description

United States Patent 3 376,134 PHGTOCGNDUCTIVE COMPOSITIQNS COMPRIS- ING ZHNC OXIDE AND METHODS FOR USING SUCH Frederick A. Stahly and Westlake L. Goehring, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Mar. 16, 1965, Ser. No. 440,315

9 Claims. (Cl. 96--1.8)

This invention relates to improved coating compositions for use as photoconductive layers in electrophotographic elements and to elements comprising such coatlllgS.

It is known that photoconductive zinc oxide can be employed in photoconductive insulating layers coated on paper, to make elements useful in electrophotography.

In a known electrophotography process employing zinc oxide in photoconductive layers, the photoconductive insulating coating is first given a blanket negative electrostatic charge, in the substantial absence of actinic radiation, by means of a corona discharge. The photoconductive layer is then exposed imagewise to actinic radiation. Exposed areas of the photoconductive coating will lose, Wholly or in part (depending upon amount of exposure), the negative electrostatic charge while unexposed portions of thephotoconductive layer retain the negative electrostatic charge. Photographic speed is proportional to the exposure required to dissipate the charge and is measured by methods similar to those used for measuring photographic speed of silver halide films. The resulting electrostatic image can be developed by conventional methods, for example using a pigmented resin powder which has a charge opposite to the negative charge of the unexposed areas of the photoconductive layer. The pigmented powder is attracted to the negatively charged areas and can then be affixed to the photoconductive layer by simply heating the resinous vehicle of the toner to fuse it to the photoconductive coating. Various other means of developing the latent photoconductive image to a visible image have been described in the prior art.

A disadvantage of zinc oxides normally used in such photoconductive layers is that the photosensitivity of the zinc oxide layer normally is at its greatest in the ultraviolet region of the spectrum, whereas an exposing source may have its maximum output in a visible region of the spectrum. Various sensitizing materials are known to increase the sensitivity of zinc oxide to the visible region of the spectrum and these may be incorporated in coating compositions of the present invention. By addition of sensitizer dyes, the photographic speed of photoconductive layers containing zinc oxide can be increased by a factor of 200 or more over that of the unsensitized composition. However, at speeds greater than about six times that of the unsensitized composition, the sensitizers begin to cause objectionable coloration.

Therefore, an object of the present invention is to provide improved photoconductive coating compositions com prising zinc oxide for making electrophotographic elements having improved photographic speed. It is another object of the present invention to provide photoconductive compositions that can employ lower concentrations of sensitizing dyes to achieve a given photographic speed. It is still another object to provide electrophotographic elements based on photoconductive zinc oxide and having improved photographic speeds and less coloration. Other objects will be apparent from the following detailed description and claims.

In accordance with the present invention, to a conventional photoconductive zinc oxide coating composition which contains a suitable insulating binder, and preferably containing at least in part a silicone resin, we add the product of dimerization of a C to C unsaturated fatty acid. The dimerization product, which is preferably formed from an unsaturated fatty acid having from 16 to 22 carbon atoms, is generally employed in a concentration of at least 3 percent based on the weight of the zinc oxide. As indicated hereinabove, the photoconductive composition may contain in addition to the said dimerization product a photosensitive dye such as a merocyanine or carbocyanine dye or any of the other photosensitive dyes that are suitable for sensitizing photoconductive zinc oxide.

The coating compositions of the present invention may be mixed in accordance with techniques generally known in the art. Incorporation of the fatty acid dimerization product in the coating requires no special processing techniques. For example, compositions of the present invention can be prepared by blending the ingredients with a liquid diluent in conventional mixing equipment. Hydrocarbon solvents and polar organic solvents such as alcohols, ketones, and esters are usually employed as liquid diluents. In general, it is preferred to add photoconductive zinc oxide, usually French process zinc oxide, to the liquid diluent first and to follow by addition of a silicone resin (when employed), the sensitizer dye, the fatty acid dimerization product and other additives, and last, add the other dielectric binder resin. However, it is to be pointed out that the order of addition of ingredients to make the composition is not a critical element of the present invention.

The resulting composition is then thoroughly mixed and coated onto a suitable support, usually paper and the diluent is evaporated. It is preferred to evaporate substantially all of the solvent in order to avoid interference with the dielectric properties of the coating. Coating techniques have been developed and are known in the art for application of photoconductive zinc oxide coating compositions to supports and these conventional coating methods are suitable for coating compositions of the present invention.

The resins employed as binders may be selected from a wide variety of resinous materials. While there are, of course, some preferred binder resins, in general, we may use any resin that can be dried at a safe temperature, that has good dielectric properties and that adheres well to the zinc oxide and the support. In order to improve electrical response at high humidity, it is preferred to employ a silicone resin which usually is used in combination with other resins, but which may also be used by itself. In preferred coatings in accordance with the present invention, silicone resins are employed in a concentration range of about 5 to 50 percent of the binder composition. Other preferred resins suitable for use in binder compositions include polystyrene and styrene copolymer resins, ABS resins, polyvinyl halide resins, polyvinyl acetate resins, acrylate resins, alkyd resins, and silicone resins. Resins suitable for use as binders in zinc oxide electrophotographic elements have been described in the prior art and need not be detailed here.

The dimerization product of an unsaturated fatty acid monomer, employed in the compositions of the present invention, is prepared by a process for polymerization of the monomer at the double bond. Such products are available commercially. The dhnerized acid isthe principal ingredient of the dimerization product, which usually also contains lesser amounts of unreacted fatty acid monomer and some higher polymerization products, i.e., trimers, tetramers, etc. For convenience, in this specification, we may refer to the dimerization product by the term dimer. The invention includes use of dimers of such unsaturated fatty acid monomers as oleic acid, palmitoleic acid, petroselenic acid, and erucic acid in zinc oxide photoconductive coating compositions. The preferred unsaturated acid monomers are those containing from 16 to 22 carbon atoms and especially preferred are the unsaturated fatty acids containing 18 carbon atoms. The acid monomer may contain one double bond or may contain, more than one double bond as in the case of linoleic acid or linolenic acid. The dimers are employed in a concentration of at least 3 percent based on the weight of the zinc oxide. Suitable concentrations range from about 3 to about 1 percent by weight of the zinc oxide. Still higher concentrations are operable, but beyond about 10 percent the additional dimer does not effect much additional improvement.

The unsaturated fatty acid dimers improve the speed of Zinc oxide containing photoconductive compositions about equally for whatever wave length of light is employed; hence, photoconductive compositions of zinc oxide and the dimer retain maximum sensitivity in the ultraviolet region. If it is desired to shift or extend maximum sensitivity to the visible region, it is still necessary to add sensitizers, usually one or more of the known sensitizing dyes. The presence of such sensitizers in most cases Will not interfere with the effect of the dimers and in fact, presence of the dimer will reduce the amount of dye necessary to obtain desired speed at the sensitized Wave lengths.

The preferred zinc oxide photoconductive materials are of relatively small particle size, generally less than 0.5 micron mean diameter. Such zinc oxide materials are readily available and can be purchased under a variety of trade names, such as XX78 and Photox 801 (New Jersey Zinc Company), etc. Sufficient binder should be employed to insulate each of the zinc oxide particles from the surrounding particles in the photoconductive insulating coatmg.

Exposure of a charged photoconductive insulating layer to actinic radiation causes a loss or reduction of electrical resistivity in exposed areas of the photoconductive coating and a consequent discharge of the static charge in such areas. The degree of discharge will depend on the intensity and time of exposure. The resulting latent electrostatic image can then be developed to a visible image by any of several methods known in the art. A useful method of developing the latent electrophotographic image is the magnetic brush process described in US. Patent No. 2,874,063 patented Feb. 17, 1959 to Harold G. Grieg.

Uses and advantages of certain preferred embodiments of the invention are illustrated by the following example.

EXAMPLE This example describes preparation of a number of photoconductive compositions with and without a preferred additive of the present invention and compares the photographic speeds obtained with coatings thereof.

Composition A To a Waring Blendor were added 314.7 g. of toluene, 256.0 g. of zinc oxide (modified with 1.0 10 eq. HCl per 56 g. of zinc oxide), 21.3 g. of a 60 percent solution of a silicone resin (Dow-Corning Z-6018) in toluene, and 170.6 g. of a 30 percent solution of a specially heat treated 50/50 n-butyl-isobutyl methacrylate copolymer (Du Pont Co. Lucite 2046) in toluene in that order and sheared for 20 minutes. Then 37.4 g. of methyl alcohol was added to the mixture which was gently stirred for 5 minutes.

Composition B To a Waring Blendor were added 314.3 g. of toluene, 256.0 g. of zinc oxide (modified with 1.0 eq. HCl per 56 g. of Zinc oxide), 20.8 g. of a 60 percent solution of a silicone resin (Dow-Corning Z-6018) in toluene, and 171.5 g. of a 30 percent solution of a specially heat treated 50/50 n-butyl-isobutyl methacrylate copolymer (Du Pont Co. Lucite 2046) in toluene in that order 4 and sheared for 20 minutes. Then 0.00690 g. of a sensitizer dye having the following composition:

Percent.

Crystal violet 28.6

Fluorescein 34.6 3-carboxymethyl-5,3-eth.yl-2-( 3-hydro) benzothiazolylidene rhodanine 36.7

dissolved in 37.4 g. of methyl alcohol was added to the mixture which was gently stirred for 5 minutes.

Composition C A mixture of 1905 g. of toluene, 95.2 g. of a 60 percent solution of the said silicone resin at 1143 g. of the said zinc oxide was processed through a laboratory model Sand Grinder. The mixture was then sheared for 15 minutes in a Polytron mixer after the addition of 34.3 g. of the product of dimerization of unsaturated C fatty acid (Emery 3020-R dimer acid, Cincinnati, Ohio). This composition containsabout 5 percent of the C acid, 73 percent of the. C dicarboxylic dimer acid and 22 percent of the C tricarboxylic trimer acid. The mixture was sheared for another 30 minutes following the addition of 761.8 g. of the 30 percent methacrylate in toluene solution described for Composition A and 1153 g. of additional toluene.

To 658.2 g. of this composition was added 27.8 g. of

methanol and the mixture was stirred for 15 minutes.

Composition D To 658.2 g. of Composition C without the added methanol was added 0.00513 g. of the sensitizer dye of Composition B dissolved in 27.8 g. of methanol and the mixture was gently stirred for 15 minutes.

Composition E To 658.2 g. of Composition C Without the added methanol, was added 0.0102 6 g. of the sensitizer dye of Composition B dissolved in 27.8 g. of methanol and the mixture was gently stirred for 15 minutes.

Compositions A through B were coated on St. Lawrence C2S paper at a dry Weight coverage of 3.0 g./ft. The

so-coated papers were .then charged under a corona discharge and exposed for one-half second in a sensitometer to tungsten illumination. The exposed coating was then developed by magnetic brush development using small iron carrier particles and black pigmented sulfur toner- TABLE Composition: Relative white light speed A 10 B 25 C 20 D 75 E It will be apparent that Composition A must be compared to Composition C and Composition B to Com-= positions D and E. Despite the higher concentration of sensitizer dye in Composition E, n0 objectionable stain was observed.

The foregoing example is intended to illustrate the preparation of certain preferred compositions of the present invention and advantages to be derived therefrom and a preferred use for the coating compositions. The example is not intended to limit the scope of the invention to the embodiments described. Other resins and sensitizer dyes having the above-indicated requisite properties may be employed. Other dimers derived from dimerization C to C fatty acids may be similarly employed. Variations and modifications will be apparent to persons skilled in the art and are included within the scope of this invention as defined in the appended claims.

What is claimed is:

1. A photoconductive coating composition comprising a high dielectric resinous binder insulator having uniforml dispersed therein photoconductive zinc oxide particles and at least 3 percent 'by weight of the zinc oxide of the product of dimerization of an unsaturated fatty acid monomer in which said fatty acid monomer has from 12 to 24 carbon atoms.

2. A photoconductive composition comprising a high dielectric resinous binder insulator having uniformly dispersed :therein photoconduotive zinc oxide, a sensitizing dye, and at least 3 percent by weght of the Zinc oxide of the product of dimerization of an unsaturated fatty acid monomer in which the fatty acid monomer has from 12 to 24 carbon atoms.

3. The photoconduc-tive composition of claim 1, wherein the high dielectric resinous binder insulator contains from 5 to 50 percent of the mixture of a silicone resin.

4. The 'photoconductive composition of claim 1, Wherein the high dielectric resinous binder insulator is a mixture of a methacrylate resin and from 5 :to 50 percent 'by weight of the mixture of a silicone resin.

5. The composition of claim 1, wherein the dimerization product is derived from an unsaturated fatty acid monomer having 18 carbon atoms.

6. The photoconductive composition comprising a mix- .ture of a .methaicrylate resin and from 5 to 50 percent by weight of the mixture of a silicone resin having uniformly dispersed there-in photoconductive zinc oxide, a sensitizing dye, and from 3 to 10 percent based on the Weight of the zinc oxide of a product of dimerization of an unsaturated fatty acid monomer in which the acid monomer has 18 carbon atoms.

7. The photoconductive composition of claim 6, wherein the methacrylate resin is an n-butyl-isobutyl methacrylate copolymer.

8. A photographic element for electrophotography comprising a .paper support and a relatively thin layer of a composition comprising a high dielectric resinous binder insulator having unifiormly dispersed therein photoconductive zinc oxide particles and at least 3 percent by weight of the zinc oxide of the product of dimerization of an unsaturated fatty acid :monomer in which the fatty acid has from 12 to 24 carbon atoms.

9. The process of claim 8, wherein the resinous binder is a mixture consisting of a meth acrylate copolymer and from 5 to percent by weight of the mixture of a silicone resin and wherein the dimerization product is a dimer of an unsaturated fatty acid monomer containing 18 carbon atoms.

References Cited UNITED STATES PATENTS 5/1964 Stahley 96-1 4/1966 Sh-ulm'an 961

Claims

1. A PHOTOCONDUCTIVE COATING COMPOSITION COMPRISING A HIGH DIELECTRIC RESINOUS BINDER INSULATOR HAVING UNIFORMLY DISPERSED THEREIN PHOTOCONDUCTIVE ZINC OXIDE PARTICLES AND AT LEAST 3 PERCENT BY WEIGHT OF THE ZINC OXIDE OF THE PRODUCT OF DIMERIZATION OF AN NSATRATED FATTY ACID MONOMER IN WHICH SAID FATTY ACID MONOMER HAS FROM 12 TO 24 CARBON ATOMS.