US 3671232 A
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United States Patent ()1 3,671,232 Patented June 20, 1972 ice 3,671,232 ELECTROPHOTOGRAPHIC MEMBER USEFUL AS A LITHOGRAPHIC MASTER Charles A. Kumins, Barrington Hills, lll., assignor to Addressograph-Multigraph Corporation, Mount Prospect, Ill. No Drawing. Filed Nov. 23, 1970, Ser. No. 92,191 Int. Cl. G03g 5/00, 7/00 US. Cl. 961.8 5 Claims ABSTRACT OF THE DISCLOSURE A method of making a lithographic master using a photoconductive member which contains infrared absorbing carbon black in the photoconductive coating in the amount ranging from 0.25% to 1% by weight of the photoconductive pigment. The method calls for fusing the ink receptive thermoplastic resin powder by infrared radiation. The presence of carbon black in the face coat achieves a unique balance in the amount of heat necessary to fuse the fine lines which form a part of the image, without fusing the spurious toner particles deposited in the background which can later physically be removed.
BACKGROUND OF THE INVENTION This invention relates to photographic members of the type in which a zinc oxide photoconductive pigment is dispersed in a resin binder and more particularly to the use of such electrophotographic members in the environment of lithographic duplication.
In the electrophotographic art, it is well known to pre pare a copy on a photoconductive layer by the electrostatic process whereby the copy is imaged with an oleophilic resin powder which is fixed to the photoconductive layer. Fixing is usually accomplished by the use of heat which causes the resin powder to coalesce and adhere to the photoconductive layer.
The copy sheet with the fused oleophilic image portions is then converted to a lithographic master by treatment with a conversion solution such as disclosed in US. Patent 3,323,451, which issued on June 6, 1967. The conversion step treats the resin binder portion so that it becomes Water receptive resulting in a planographic surface, that is differentially ink and water receptive. The ink receptive portions are the fused oleophilic resin images.
Equipment is provided whereby the entire process of making a master from a graphic original is rapidly and automatically accomplished such as disclosed in US. Patent 3,426,678, granted Feb. 11, 1969.
One of the important steps in the master making process is the fusing of the thermoplastic resin powder image onto the photoconductive surface so that it is strongly bonded thereto and will withstand the rigors of the planographic process thereby producing a long running master.
While it is significant that the master should be processed in a manner which is likely to produce a long run, it is desirable that all of the processing steps be rapidly and routinely accomplished employing conventional techniques of charging, exposing, developing, fusing, conversion and lithographic duplication. If any one of these steps require special considerations, such as variations in fusing, or the use of special developing materials or unusual conversion solutions, then the master imaging process becomes less than routine.
Further, the master imaging process and techniques should lend itself to a wide variety of graphic originals containing a wide range of subject matter thereon, most of which lends itself to reproduction by the lithographic techniques.
One of the problems in the master imaging system involves making lithographic reproductions of graphic originals containing low contrast subject matter so as to produce a light grey image on the photoconductive member, or else one that is composed of fine lines. The fusing step in the known master imaging machines usually employs infrared radiation so as to selectively fuse the black image areas having the greatest emissivity leaving unfused the spurious electroscopic thermoplastic oleophilic materials which are deposited in the non-image areas. The unfused particles at the end of the process are usually washed off and do not appear on the lithographic copies.
The selective fusing technique which is emissivity dependent, gives rise to additional problems in that the very light grey image areas, that is those which have a low emissivity, as well as those having fine line image portions, only partially fuse so that the life of the image on the lithographic master is seriously curtailed. Attempts to correct for this deficiency by increasing the amount of radiation results in fusing the spurious particles as well but also generates too much heat.
SUMMARY OF THE INVENTION In order to overcome the aforedescribed deficiencies of master imaging techniques known heretofore, it has been found that infrared absorptive materials may be incorporated into the photoconductive coating proper so that a unique balance is achieved whereby the coating is raised to a temperature above ambient so that all or most of the infrared radiation absorbed in these light grey image areas or low emissivity areas is used efficiently to fuse all image portions so that they are firmly fixed .to the photoconductive layer. And still, the surface is sufficiently reflective of the infrared radiation so spurious toner in the nonimage areas does not fuse.
It is a general object of the instant invention to provide an improved electrophotographic member in which the electroscopic thermoplastic oleophilic resin powder images of a wide range of image density are permanently heat fused by infrared radiation thereon to the exclusion of the spurious powder in the non-image areas. I
It is a further object of this invention to provide a electrophotographic member comprising a photoconductive pigment dispersed in a resin binder to which is added infrared absorptive materials so as to balance the amount of infrared radiation required to fuse the electroscopic thermoplastic oleophilic re'sin powder images of low and high emissivity to the exclusion of the spurious toner in the non-image areas.
It is a specific object of this invention to provide a lithographic master which may be prepared by electrophotographic imaging techniques which are rapidly fused thereon by means of infrared radiation to provide a long running lithographic master.
In carrying out the objects of the invention, the infrared absorptive characteristic of the photoconductive layer is controlled and balanced by the addition of material to the layer which is highly absorptive of the radiation. The material which has been found to give the best results in terms of producing a photoconductive member which meets at the requirements necessary for electrophotographic imaging and at the same time will respond to infrared radiation in the manner desired to rapidly fix the thermoplastic electroscopic powder is the use of carbon black.
The electrophotographic members which are generally employed in imaging systems of the type used for making lithographic masters are of the zinc oxide-resin binder type. The technology involved in the preparation of such electrophotographic members is well known. The photoconductive zinc oxide is dispersed in a wide variety of resin binders such as silicone, acrylic ester resins, styrenebutadiene copolymers, polyvinylchloride, polyvinylacetate, polyurethane resin systems, and thermoplast-aminoplast type resin binder systems. The technology involving various resin binders is Well developed and will not be dealt with in any further detail here.
Into the conventional coating formulations containing the zinc oxide pigment in a resin binder is dispersed carbon black in an amount ranging from 0.1% to 1.0% by weight based on the Weight of zinc oxide pigment in the formula. It will be appreciated that a wide variety of carbon blacks are commercially available and generally any carbon black may be employed which is compatible with the photoconductive properties of the resin binder system so as to maintain the surface resistivity in the range of from 10 to 10 ohm-centimeters.
The particle size range of the carbon black may vary from microns to 80 microns in particle size. Generally, it has been found that carbon blacks which are colloidal dispersions such as available from Columbia Carbon Black Corporation work eminently well. Other carbon blacks identified as channel blacks, such as is available from the Cabot Carbon Black Company may be successfully employed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples of electrophotographic members having incorporated therein carbon black pigments are given for the purposes of illustrating preferred embodiments of the invention. It will be understood, however, that this invention is not limited to these illustrative embodiments of photoconductive layers useful to practicing this invention.
Example I Material: Amount Toluene pounds 50 Methylethylketone do.. 120 Resin binder solution (60% solids) do 12.5
Silicone resin SR-82. General Electric Company. Zinc oxide photoconductive pigment New Jersey Zinc Company do 50 Carbon black-Columbia Carbon Company Colloidal Dispersion-Costyrene Black 921 grams 115 *Phloxine B-sensitizing dye do 136 Methanol mls 4000 The resin binder, toluene and methylethylketone solvents, are combined in a suitable vessel equipped with mixing equipment for the purpose of bringing these components into a uniform mixture. The zinc oxide pigment is gradually dispersed into the resin binder solvent mixture, the addition being slow so that a uniform dispersion results.
The carbon black is combined separately with the dye and methanol so that the dye is completely dissolved in the methanol into which is then dispersed the carbon black. This mixture is then added to the main mixture and agitation continued until it (the dye and carbon black) is uniformly dispersed. The photoconductive coating formulation is then applied to a conductive base support at a rate which gives a final dry coating weight of 15 pounds per 3000 square feet. The coated Web is passed through a forced hot air drying chamber maintained at 300 F., the residence time at this temperature being about 30 seconds.
Example II This example follows the [formulation of Example I with the exception that the amount of carbon black pigment added was 225 grams and was of the channel black variety manufactured by Cabot Carbon Company, substituted for the carbon black in Example I.
Example III This example follows the formulation of Example I with the exception that 56 grams of carbon black pigment of the channel black variety manufactured by Cabot Carbon Company was substituted for the carbon black in Example I.
Example IV Material: Amount Toluene pounds 50 Methylethyl'ketone do Resin binder styrene-butadiene copolymer,
manufactured by Goodyear Rubber Com- This example follows the formulation of Example IV with the exception that the amount of carbon black is 111 grams substituted for the 454 grams in Example IV.
Each of the foregoing photoelectrostatic members Were readily processed through the various steps necessary to create a lithographic master utilizing infrared radiation in order to fuse the thermoplastic powder image onto the surface.
The technique which is employed to create a powder image on the photoelectrostatic member is the conventional electrostatic process. The photoconductive member prepared in each of the examples was charged by applying a blanket electrostatic charge to the surface, and then exposed to a pattern of light and shadow creating a differential electrostatic pattern thereon. The pattern was then developed by the application of an electroscopic powder by any one of the conventional techniques such as cascade development, magnetic brush development or powder cloud.
In order to utilize the image member as a lithographic master it is then necessary to fuse the particle images to the surface and then to treat the surface so that it is rendered Water receptive in the non-image areas. The latter step is generally known as a conversion step and utilizes materials such as disclosed in US. Patent 3,323,451.
The advancement made in the instant invention resides in the incorporation into the photoconductive layer of the carbon black pigment so as to render the layer overall infrared absorptive. In the processing step of making an infrared master, exposing the member to infrared radiation causes the thermoplastic powder selectively to fuse and to adhere to the zinc oxide surface. This technique has the advantage of increasing the efficiency of processing the photoconductive layer. There is achieved a balance between that which is fused and that which remains unfused at a given radiation exposure so that the image portions are fused to the exclusion of the spurious particles inadvertently attracted to the background or non image areas. The use of infrared radiation as a heat source is desirable because it is more efficient than an oven-type heating arrangement.
As the photoconductive member of this invention is brought under the influence of the infrared radiation, the overall surface experiences a temperature rise due to the absorption into the coating of the infrared radiation where it is converted to heat energy. The image portions also absorb radiation but are heated to a higher temperature level than the non-image areas or the rest of the surface. It should be pointed out that while the carbon black pigment has been incorporated into the photoconductive coating it still gives the appearance of a white layer due to the predominance of the white zinc oxide photoconductive pigment and upon visual observation there is no evidence of any carbon black present in the coating. Accordingly, the background area or non-image area is highly reflective of infrared radiation in contrast to the highly black pigmented thermoplastic resin particles which are deposited in the image portions.
The temperature profile of the photoconductive member of this invention while under the influence of the infrared radiation may be described as having a first temperature level in the background area and a second higher temperature level in the image portions, said first temperature level being substantiall greater than ambient temperature. The first temperature level, that is, the temperature in the photoconductive layer is at a level which is insufiicient to fuse the powder but is at a sufficient level above ambient or the immediate environment to substantially decrease the rate of heat transfer from the image areas into the adjacent non-image areas and that portion of the photoconductive layer that is immediately underneath the powder. The rate of heat flow from the powder image is substantially reduced because the temperature differential between the image portions and the rest of the photoconductive member is substantially reduced thereby reducing the driving force that causes the rate of heat loss from the image. Therefore, the time required to cause the powder in the image areas to coalesce and fuse into the photoconductive coating is substantially reduced, while the amount of exposure is insufficient to fuse the spurious toner.
As a typical infrared processing arrangement, the photoconductive layer bearing the unfused powder image is passed beneath two infrared lamps such as manufactured by General Electric Company, identified as 6.13. T-3 lamp. The lamps contain a tungsten filament in a quartz envelope and attain a filament temperature in the range of 2300 to 2800 degrees K. The lamps are located anywhere from one to two inches from the photoconductive surface which is caused to move past the lamps at a rate of 5-20 feet per minute. In order to properly fuse a prior art photoconductive member, that is, one which did not have any carbon black or infrared absorptive pigment in the coating, the temperature in the vicinity of the surface of the photoconductive layer measured in the range of 500-600 F. in order to properly fuse the image area so as to achieve a suitably long running lithographic master. At these temperatures, the ability of the infrared radiation to selectively fuse the image portions to the exclusion of the non-image portions was seriously encumbered. These masters emerged from the fusing step with some spurious toner fused in the background area. This background would, of course, reproduce in a lithographic process.
Additionally, such high temperatures in the fusing area have a detrimental eifect on the mechanical parts adjacent to the fusing area causing early failure in the bearings and other materials of construction.
Attempts to lower the temperature in the range of 400 to 500 F. in the immediate vicinity of the surface of the photoconductive layer by reducing the power to the infra red radiators proved to be insufficient in terms of fusing those portions of the image which were of low emissivity such as the fine lines and punctuation marks and generally those areas of the original which were of very low contrast.
When processing photoconductive members having carbon black included in the photoconductive layer, the amount of radiation was adjusted so that the temperature in the vicinity of the surface of the masters was in the range of 425 to 500 F. Such lower temperature values gave the desired results, where before they proved to be too low in order to fuse the master. It was found that the photoconductive members of the instant invention at these temperature levels measured in the vicinity of the infrared radiators work more efficiently to selectively fuse the image portions without unduly heating up the entire photoconductive member or the adjacent mechanical structure of the particular processing machine.
One significant measurement of the success of the infrared technique of fusing the lithographic master is the length of run that is achieved on a lithographic press. The prior art materials, unless they were fused at the very high environmental temperatures described above, namely 500-600 F., the performance level was 50% of the rated 250-300 copies for the particular masters. When the same masters were prepared with the carbon black in the photoconductive layer and processed through an infrared fusing device where the temperature of the environment immediately adjacent the surface of the photoconductive layer one-half to one inch distant was in the range of 425-500 F., the length of run in the range of 250-300 copies was routinely obtained. Temperature measurement was made by a suitable thermocouple sensor.
it will be recognized that the reference to the temperature of the environment immediately adjacent the surface of the photoconductive layer provides the most practical location for obtaining a reference temperature value. With the reference temperature value in the range of 425 -500 F., the prior art materials gave short runs and neglected to fuse the fine or light image portions whereas the materials of the instant invention selectively fused the fine line image portions as well as effectively fusing the prominent image portions without fusing the spurious powder. This results in high quality lithographic masters which are capable of duplicating runs in the range of 250-300 copies.
What is claimed is:
1. The method of making a lithographic master comprising the steps of:
(a) applying a photoconductive coating comprising zinc oxide dispersed in a resin binder layer to a conductive substrate, said layer having incorporated therein carbon black pigment,
(b) charging the photoconductive layer rendering it responsive to electromagnetic radiation,
(c) exposing the charged layer to a pattern of light and shadow to produce a latent image and non-image thereon,
(d) applying electroscopic thermoplastic powder overall to the photoconductive surface to selectively adhere to the latent image portions to produce a visible image thereon,
(e) irradiating the powder image bearing surface with infrared radiation causing a temperature rise in the non-image areas to a first level, and a temperature rise in the image areas to a second level, said second level of temperature being sufficient to cause the thermoplastic particles to coalesce and said first temperature level operating to minimize heat loss from said image portions without appreciably fusing any thermoplastic particles which may be adhered in the non-image areas,
(f) treating the surface to render the non-image areas hydrophilic.
8 2. The method as defined in claim 1 wherein the amount References Cited of infrared absorbing carbon black is in the range of ITED TATE NTS 0.25% to 1.0% by weight based on the weight of zinc UN S S FATE oxide photoconductive pigment. 3573040 3/1971 Bach, 96.13 X 3,510,299 5/1970 Hernck et a1. 96-1 X 3. The method as defined in claim 2 wherein the particle 5 size of the carbon black is in the range of 5 to 80 microns.
4. The method as defined in claim 1 wherein the sur- GEORGE LESMES Pnmary Exammer face resistivity of the photoconductive layer is in the range I. R. MILLER, Assistant Examiner of 10 to 10 ohm-centimeters.
5. The method as defined in claim 1 wherein the rate of 10 movement of the photoconductive layer past the infrared 101 457, 462; radiation source is in the range of 5 to 20 feet per minute.