US3467603A - Method of preparing electrophotographic photoconductive compositions - Google Patents

Description

United States Patent F 3,467,603 METHOD OF PREPARING ELECTRO- PHOTOGRAPHIC PHOTOCONDUCTIVE COMPOSITIONS James A. Brown, Midland, and Walter L. Garrett, Freeland, Mich., and Henry Wielicki, Philadelphia, Pa., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Deiaware No Drawing. Continuation-impart of application Ser. No. 396,453, Aug. 14, 1964. This application July 28, 1967, Ser. No. 656,690

Int. Cl. G03c 1/68; G03g 7/00 US. Cl. 252501 9 Claims ABSTRACT OF THE DISCLOSURE The present invention concerns a method of preparing a photoconductive composition for use in electrophotographic printing containing a photoconductive zinc oxide and an insulating film forming resinous binder which comprises milling the zinc oxide in the presence of a fluidizing quantity of a fluidizing agent and in the absence of a binder quantity of said resinous binder; admixing said binder in binder quantities with the zinc oxide somilled at substantially the termination of the milling period; and preferably miXing said zinc oxide and the resin binder by agitation to a substantially uniform mixture.

This application is a continuation-in-part of copending application Ser. No. 396,453 filed Aug. 14, 1964, now abandoned.

Exemplary of an electrophotographic printing process employed herein is one whereby high quality thereon, may be produced, which plates in turn, after being etched, may be used to reproduce said images, for example, as newspaper print or display advertisements. This process comprises, in general, first providing a static negative electric charge in a subdued light to a recording element to a level, for instance, of at least about 300 volts, said element comprising a prepared photoconductive composition or mixture coated on a black plate material of, for example, magnesium or zinc base metal. The photoconductive compositions may, for example, consist essentially of a photoconductive zinc oxide suspended in a crosslinkable resinous insulating binder, such as, for example, in SR-82 silicone resin made by the General Electric Company, and preferably a sensitizing dye, The element so charged is then exposed to an illuminated image desired to be reproduced, for instance, either by contact printing or by focusing said image through a lens on the coated plate, thereby to produce a latent electrostatic image thereon. The recording element, within a proper time after charging and exposing, depending on its socalled dark decay characteristic, that is, its loss of charge as a function of time while in the dark, is then developed and rendered visible. This latter developing operation may comprise, for example, a sequence of steps which includes: (a) contacting said charged-exposed plate with positively charged catalyst particles, for example, catalyst particles suspended in an inert carrier liquid, such as, for example, aluminum octoate suspended in n-heptane, (b) rinsing the so-contacted plate in an inert hydrocarbon liquid, for example, a mixture of iso-octane and heptane, (c) heating at an elevated temperature and time sufiicient to cure and cross-link the resin areas catalyzed and (d) removing the non-catalyzed, thus nonhardened non-cross-linked resin acreas, including the removal, for example, by means of an acidic rinse, of any surface conversion coating present on the base metal in said non-hardened areas. The element now exhibiting Patented Sept. 16, 1969 a visible image pattern, and exposed bare metal in the non-image areas, may then be subjected to an etching treatment, preferably to the action of the recently developed and well-known powderless etching process. In addition to the liquid developing techniques, as described immediately above, the charged and exposed plate may also be developed by means of a powder cloud, cascade, or magnetic brush technique, as commonly practiced by those skilled in the art. This electrophotographic printing process and similar processes are described in the RCA Review, Vol. 15, No. 4, December 1954, and US. Patents 3,231,374 and 3,052,540.

Heretofore, preparation of the photoconductive mixtures have been by milling zinc oxide, preferably a photoconductive zinc oxide made by the so-called French Process, by means, for example, of a blender or ball mill in the presence of all the insulating binder resin employed in the coating, for example SR-82 resin, preferably a sensitizing dye such as, for example, fiuorescein disodium salt, and a diluent such as xylene. The milling is primarily for the purpose of breaking up and reducing the size of the zinc oxide agglomerates and particles. Illustrative of this prior art method of milling the zinc oxide in all of the resin are US. Patents 3,110,591 and 3,052,504.

The term binder quantities of resin, or equivalent or similar terms, as used in the present invention, refers to quantities of insulating resin added for the purpose only as a binder for the zinc oxide particles and for the photoconductive composition in general (as opposed to a fluidizing purpose in milling zinc oxide). Ordinarily a ratio of about 1.0 and 3.0 parts by weight of zinc oxide per part of binder, and preferably about 2 parts zinc oxide to 1 part of binder will be employed in the final composition. A fluidizing amount of resin as used herein refers to a small quantity of resinous fluidizing agent defined herein necessary only to suspend or fiuidize the zinc oxide during milling thereof, whereupon, binder quantities may be added to prepare the final photoconductive composition after said milling of the zinc oxide has been accomplished.

Preparing the mix in the conventional manner, however, results in various and serious difiiculties. First, coating mixes were obtained which, when applied, were characterized by a glossy appearing coating. Such coatings are inferior and substantially ineffective as to its photoconductive properties, namely, its slow light decay characteristic and poor resolution. To illustrate, they exhibit a light decay characteristic, such that when the electrostatically charged coating is exposed, for examp e, to light of an intensity of .8 foot candles, a period of about 3 or more minutes is required to substantially dissipate said electrostatic charge therefrom. Such a period of time is normally too long, therefore, rendering these coatings commercially unacceptable. An acceptable light decay would be, for example, from about 0.5-1 second to about seconds. In addition, these compositions heretofore gave relatively poor resoution. This apparently is a result of the presence of excessively large zinc oxide agglomerates. Moreover, by mixing all the ingredients together at one time, the resulting viscosity during mixing may be so low as to prevent efiicient milling. Mixing in this manner also causes other disadvantages. For example, the composition mixes so-obtained are frequently of inconsistent quality varying to a large extent in their protoconductive properties.

The term light-decay as used herein refers to the property of a photoconductor or photoconductive composition to dissipate an electrostatic charge thereon when exposed to electromagnetic radiation as a function of time.

In preparing the photoconductive compositions as described hereinbefore, the mixing procedure of said composition has been found to be very important in order to obtain maximum photoconductivity and suitable viscosities of the final mix prior to application on the plate. It is apparent, from the foregoing, therefore, that an improved method of preparing electrophotographic printing plates and photoconductive compositions of the type described aforesaid is needed to produce a mix of consistent high photoconductive quality, particularly useful in electrophotographically producing photoengraved printing plates for printing newspapers, display ads, and the like.

The principal object of the present invention, therefore, is to provide a novel and improved method of preparing high quality photoconductive compositions for application on a backing material, from which photoengravings may be produced, which avoids the aforementioned difficulties and disadvantages.

A further object of the invention is to provide a novel method of preparing a high quality photoconductive composition having a fast light decay characteristic, a slow dark decay characteristic, and satisfactory resolution properties for use in electrophotographic printing.

These and other objects and advantages will become apparent from the detailed description of the invention which follows hereinafter.

Accordingly, the novel method of the present invention comprises, in general, milling, grinding, or otherwise reducing the agglomerate and/or particle size of the photoconductive zinc oxide component of the composition in the presence of a fluidizing amount of one or more fluidizing agents, each said agent containing one or more reactive hydroxyl groups, said milling and the like being accomplished in the absence, however, of binder quantities of the insulating resin. The zinc oxide is normally and preferably milled to, for example, submicron sized particles and/or small agglomerates as a minimum size and up to about, for example, 50 microns in size as a maximum. The zinc oxide particle size as milled, however, may vary from that stated while still obtaining acceptable photoconductive compositions, providing the particle size is uniform and the particles are not excessive in size, such as, for example, not over 100 microns. Fluidizing agents suitable for use in the present novel process, for example, may be one or more fluidizing liquids selected from the group consisting of an organic liquid glycol, for example, ethylene glycol, dipropylene glycol, including ether glycols such as, e.g., ethylene glycol ethyl ether, propylene glycol methyl ether; a substantially anhydrous monohydric alcohol, for example, ethyl alcohol, n-decyl alcohol; and including ether alcohols, for example, 4-methoxy-4-methyl pentanol-2 (Pent-o-Xol), 2-ethoxy ethanol-l and the like. Only when substantially all said milling has been accomplished is the insulating film forming resin (in binder quantities), and normally a sensitizing dye, added to the mix. The term milling as used herein refers to a milling or working action, as in a ball or rod mill, grinding, abrading or other means or action of working the zinc oxide particles and agglomerates to obtain the proper size, whereas, the term mixing or blending as used herein refers to the action of agitating or otherwise mechanically moving one or more materials of the composition to obtain a uniform mixture, dispersion, blend, or consistency.

Insulating film forming resinous binders which may be used in the photoconductive coating prepared in accordance with the present invention include silicone intermediate resins having one or more reactive hydroxyl groups and a molecular weight within the range of from about 500 to about 5000, an epoxy resin containing one or more reactive hydroxyls, e.g., an eopxy diglycidyl ether of bisphenol-A having an epoxide equivalent weight of from about 250 to 5500, an epoxidized polyolefin, alkyd resins (polyester type) and generally the resins described in US. Patent 3,110,591. Also, the fluidizing resins defined hereinafter may in general be employed as the insulating resin as well.

The sensitizingdye, when used, is employed to modify the spectral response to the photoconductive zinc oxide so as to render it particularly responsive to light within the range of from, for example, approximately 4000 to about 8000 Angstroms. Though use of a sensitizing dye is preferred in that it renders the zinc oxide more responsive to light or particularly responsive to light of a certain wave length, the present novel composition may be prepared and used satisfactorily without said dye being present.

Total milling time of the ingredients should be held between about 3 to about 72 hours, and preferably from 4 to 18 hours, since milling times shorter than about 3 hours give coatings that are not free of detrimentally large agglomerates of zinc oxide. Milling times longer than approximately 72 hours detrimentally reduces the light decay speed of the coating. In actual practice, the dye is ordinarily added to the mix during the last 30 minutes of milling or grinding, whereas, the resin in binder quantities is only added thereto at a time preferably not greater than about 5 minutes before termination of milling. The sensitizing dye, however, may if desired, be added at any time during preparation of the composition. By preparing the photoconductive composition in accordance with the invention, little, if any, encapsulation of the zinc oxide occurs. In any event, no detrimental encapsulation is encountered by resin.

In addtion to the organic liquid fluidizers prescribed for use hereinbefore, fluidizing quantities within the range of from about 0.0-1 percent by weight to about 3.0 percent by weight, preferably 0.01 to 0.2 percent of a fluidizing resin selected from the group consisting of (a) an intermediate silicone resin having one or more reactive hydroxyls, characterized by either a straight or cyclic chain, for example, the SR-82 silicone resin aforesaid or Dow Corning Z6018 silicon intermediate resin, having a molecular weight within the range of from 500 to 5000 and (b) an epoxy resin contaiing one or more reactive hydroxyls, for example, an epoxy diglycidyl ether of bisphenol-A, or an epoxidized polyolefin may also be used. These and other resins, such as, for example, alkyd resins (polyester type) and polyglycols, in such fluidizing quantities (that is, few drops), together with an inert diluent, such as, for example, xylene, or toluene, to reduce the mix viscosity, act very effectively as fluidizers and, as aforesaid, cause no detrimental amount of encapsulation of the zinc oxide when used in these fluidizing amounts.

When employing the resins specified above in fluidizing amounts for dispersing the zinc oxide particles for milling, it is normally necessary to incorporate into the mixture an inert diluent to obtain a proper mix viscosity for milling. A proper viscosity has been found to be, for example, one within the range of from about 2500 to about 11,000 centipoises, preferably from about 5000 to about 6000 centipoises, Suitable diluents for use in the present invention include, for example, xylene, toluene, Pent-o-Xol, and ethylene glycol ethyl ether, depending on the fluidizer being used.

Use of a diluent for proper viscosity during mixing may also be desirable when using one of the non-resin fluidizers, that is, when using one of the aforesaid organic liquid fluidizers specified above, which has an inherent high viscosity (that is, too high for proper milling). In this case, a less viscous organic liquid fluidizer may be admixed therewith, or a non-fluidizing inert diluent such as, for example, xylene, may be added. The less viscous organic liquid fluidizers, of course, inherently serve as their own diluent.

During milling and mixing, of course, one or more of the aforementioned fluidizers will be present, but upon applying the photoconductive composition on a backing and subsequently curing the coating applied, both the nonresinous fluidizers and diluents will be substantially completely dispelled, with only residues, if any, remaining in the coating. The resinous fluidizers, of course, remain in the composition as part of the resinous binder component.

In general, this zinc oxide to be used in preparing the photoconductive compositions described for use in the present invention is a photoconductive zinc oxide which is substantially electrically non-conductive in the dark and which exhibits a surface photoconductivity of at least a minimum level sufiicient for practical use in the photoconductive compositions prepared using the present invention. An example of such a zinc oxide is photoconductive zinc oxide made by the French Process which has a value of surface photoconductivity of at least about ohmcm. When subjected to light of a wavelength of at least about 3900 A. Another example is Florence Green Seal No. 8 zinc oxide made by the New Jersey Zinc Co.

In addition to the foregoing, various other sensitizing agents or dyes may be employed in the photoconductive composition prepared by the present invention to vary the spectral response of, for example, the French Process zinc oxides. Satisfactory sensitizing agents include, for example, fluorescein disodium salt (Color Index No. 45350), Rose Bengal (Color Index No. 45440), and Brom Phenol Blue used in an amount sufiicient to modify the spectral response of the zinc oxide component to within a desired light or radiation range. Such an amount will vary, of course, with the particular sensitizing dye employed and response desired and can normally be determined by one skilled in the art.

The following examples serve to further illustrate the present invention but are not intended to be construed as limiting it thereto.

EXAMPLE I The following ingredients were charged into a No. lBF Burundum fortified ceramic ball mill jar of 1.3 gallons Electrometer Results then tested in an electrometer to determine both the light and dark decay characteristics thereof. The electrometer test comprised charging the coupon to, for example, 300 or more volts in the dark in the electrometer, then exposing the plate, still in the device, to 0.8 foot candle light and measuring the voltage loss. In addition, representative 9" x 12" samples of the sheets themselves were negatively charged by means of a corona wire and imaged by exposure to an illuminated image under varying conditions of light. The image test copy (that is, the image to be reproduced) containing typed matter, a 65 line per inch half-tone area, a 65 line step scale ranging from 5 to 90 percent tone, a reverse area, and a 2" x 2 solid area. The plate so-ima-ged was then developed by contact with a catalyst material (dimethoxy aluminum octanoate) suspended in an inert carrier liquid, heated to about 475 F. to cure the composition, and then the uncured, non-hardened composition removed therefrom, whereupon, the properties of the coating in general, were determined by imaging tests. These tests included (a) an examination of the solid area of the image as to Whether they were completely filled in and uniform, (b) a close examina'ion of the shadow dots in the half tone areas to see whether the areas between said dots were open or filled in (plugged), (c) an examination for sharpness of the edge of the image pattern in general, (d) an examination as to resolution, and (e) a determination of the cleanliness of the background areas, that is, of the non-image areas.

The results of the electrometer and other tests are presented in the table below which also indicates the various conditions under which said tests were run. It can be seen, therefore, that the compositions set forth in the table and described in the specification provide recording elements satisfactory for use in electrophotographic printing processes.

TABLE 9 x 12 Imaged Plate Results Dark Decay Light Decay Coupon Initial Shadow Sharp- Reso- Back- Samples Voltage 30 sec. 60 sec. 10 sec. 20 sec. 30 sec. 40 sec. Sohds Dots ness lution 1 ground (1) 445 320 278 100 30 10 Good Open... Good..- Good Clean. (2) 435 327 287 175 50 25 d d (1 D0. (3) 458 335 292 145 25 D0. (4) 452 320 281 130 45 25 D0. (5) 457 330 285 115 40 25 D0. (6) 425 315 277 105 35 20 D0. (7) 430 295 255 115 40 25 D0.

1 250 lines per inch.

N oTE.Tl1e plates were exposed to 32 Luxometer Units using quartz lights on a Kem'o 18 camera at an F stop of 16. capacity containing 6,000 grams of /2 x /2" Burundum cylindrical grinding media:

(a) 864 grams of Florence Green Seal No. 8 zinc oxide of the New Jersey Zinc Co.

(b) 1200 ml. of Shell Chemical Companys Pent-o-Xol solvent (a 5 carbon ether alcohol, 4-methoxy-4-methyl pentanol-Z) So-charged, the mill jar was rotated for 5%. hours at 60 r.p.m., stopped, and 12 ml. of a 4.16 percent solution of a fluorescein disodium salt in anhydrous methyl alcohol added thereto. Rotation was then continued for another 30 minutes, whereupon, 762 ml. of SR82 resin was added to the mill and rotation continued approximately for another 5 minutes.

The milling and grinding having been accomplished, the mix was removed from the jar and placed in a container where it was slowly agitated for a minimum of about 16 hours to allow the viscosity to stabilize between 25 and cps. So stabilized, the mix was then screened through a 500 mesh screen (US. Standard Sieve). Thereafter, the mix was placed in a spray machine where it was applied by spray coating to a clean previously prepared magnesium photoengraving sheet stock to a coating thickness of preferably about 0.55 mls., plus or minus 0.05 mls.

A" x 4" coupon samples of these coated sheets were EXAMPLE II A photoconductive composition was prepared as follows using 0.175 cc. of SR82 silicone resin as a fluidizer and xylene as diluent vehicle.

864 grams of the zinc oxide used in Example I was admixed with 1,005 grams of the xylene diluent and the fluidizer, and placed in a ball mill for milling purposes and milled by rotating it for about 5 /2 hours similarly as in Example I. At the end of this period, 9.6 mls. of 2.08 percent fluorescein dye in methyl alcohol, as a sensitizing agent, were added and the milling continued for about 30 minutes longer, whereupon, 762 mls. of the said SR82 resin as a binder was added and the agitation continued for another 3 to 5 minutes.

Similar tests, as performed in Example I, on plates prepared from the composition showed a light decay characterized such that an initial voltage of 300 volts was dissipated to 10 volts upon being exposed to a 0.8 foot candle light for about 20 seconds. Examination further showed the image to have a satisfactory resolution.

The experiment was repeated successfully using toluene as a diluent. Essentially the same good test results were obtained.

EXAMPLE III A photoconductive composition was prepared similarly as in Example 1, except that ethylene glycol ethyl ether (Dowanol EE) was used as the fiuidizer. Acceptable light and dark decay characteristics, together with satisfactory resolution was obtained from plates prepared from the composition prepared by this means.

EXAMPLE IV Another photoconductive composition was prepared similarly as in Example 1, except that 1 gram of SR-82 was used to fluidize 2160 grams of ZnO, along with 1100 ml. of Pent-o-Xol-i-llOO ml. of xylene acting in this case as a diluent to obtain the proper viscosity during a milling time of 8 hours total. The mixture so-prepared produced a very acceptable photoconductive composition with respect to light and dark decay properties.

Similarly as in the foregoing examples, an epoxy resin, as specified hereinbefore, may also be used as a fluidizer as described, along with a suitable diluent, or, in combination with one or more of the other resinous and nonresinous fluidizers in milling zinc oxide for periods within the range specified.

After preparation and prior to application if, as by spraying, the photoconductive composition described 'hereinbefore is preferably first stabilized to a viscosity of from about 20 to about 200 centipoises by slowly agitating the mix for a period, for example, of about 12 hours.

Reference to termination of milling herein with respect to when a binding amount of the resin may be added to the zinc oxide during preparation of the photoconductive composition means that the binder resin may be added substantially at or near to the end of said milling period such as, for example, about minutes prior to termination of milling. Of course, the binder may also be added after the milling period and during mixing of the composition as descrbed hereinbefore.

The present invention may be modified or changed by those skilled in the art Without departing from the spirit or scope thereof, and it is understood that we limit ourselves only as defined in the appended claims.

We claim.

1. A method of preparing a photoconductive composition for use in electrophotographic printing containing a photoconductive zinc oxide and an insulating film forming resinous binder which comprises: milling said zinc oxide for a period of time to provide by working a zinc oxide having a maximum particle size of about 100 microns, the milling being accomplished in the presence of at least a fluidizing quantity of a fluidizing agent and in the absence of a binder quantity of said resinous binder, said fluidizing agent being at least one member of a group consisting of an organic liquid glycol, a substantially anhydrous monohydric alcohol, a cyclic or straight chained silicone resin having at least one reactive hydroxyl group, an epoxy resin having at least one reactive hydroxyl group, and an alkyd resin; admixing said binder in binder quantities with the zinc oxide so-milled at substantially the termination of said milling period, and mixing said zinc oxide and the resin binder by agitation to a substantially uniform mixture, any of said resins when used being in solution form.

2. The method of claim 1 wherein the photoconductive composition comprises a ratio of from about 1.0 to about 3.0 parts by weight of said photoconductive zinc oxide per part of said resin binder.

3. The method of claim 1 wherein the photoconductive zinc oxide is milled in a ball mill for a period of time within the range of from about 3 to about 72 hours at a milling viscosity within the range of from about 2500 to about 11,000 centipoises.

4. The method of claim 1 wherein the photoconductive zinc oxide is milled in a ball mill for a period of time within the range of from about 4 to about 18 hours.

5. The method of claim 1 wherein the photoconductive zinc oxide is milled in the presence of from about 0.01 to about 3.0 percent by weight of a fiuidizing resin, characterized by having at least one reactive hydroxyl, selected from the group consisting of a silicone resin having a straight or cyclic chain structure, an epoxy resin, an alkyd resin, and a polyglycol.

6. The method of claim 2 wherein the photoconductive composition comprises a ratio of from about 1.0 to about 3.0 parts by weight of said photoconductive zinc oxide per part of said resin binder, and a sensitizing dye in an amount sutficient to modify the spectral response of the zinc oxide to light in a preselected radiation range is added to the composition after the milling step.

7. The method of claim 1 wherein the photoconductive zinc oxide is admixed after said termination of milling with a binder amount of a silicone resin.

8. The method of claim 6 wherein the sensitizing dye is a fiuoroscein disodium salt.

9. The method of claim 1 wherein the uniform mixture of zinc oxide and binder is stabilized to viscosity suitable for application by spraying of from about 20 to about 200 centipoises.

References Cited UNITED STATES PATENTS 3,052,540 9/1962 Greig 25250l X 3,110,591 11/1963 Stewart 252-50l X RICHARD D. LOVERING, Primary Examiner US. Cl. X-R. 96--107, 1.8

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,467,603 September 16, 1969 James A. Brown et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 35, after "quality" insert photoengraved printing plates, having image patterns line 43, "black" should read back line 69, "acreas" should read areas Column 2, line 5, "techniques" should read technique line 25, "3,052,504" should read 3,052,540 line 32, "and" should read to line 64, "protoconductive" should read photoconductive Column 3, line 68, "eopxy" should read epoxy Column 4, line 37, "contaiing" should read containing Column 5, line 13, "When" should read when Column 6, line 52, "0.175" should read 0. 75 Column 7, line 21, "if, as" should read as line 34, "descrbed" should read described Signed and sealed this 16th day of June 1970.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents