US3701673A - Simultaneous conversion and fusion of electrostatically producer printing masters - Google Patents

Abstract

TO PROVIDE A PRINTING MASTER, BY THE ELECTROSTATIC PROCESS, A MASTER SHEET IS CHARGED AND SELECTIVELY DISCHARGED AND THEN TONED IN THE CONVENTIONAL MANNER TO PRODUCE AN IMAGE. THE TONED MASTER IS THEN BROUGHT INTO CONTACT WITH A HEATED CONVERSION SOLUTION TO SIMULTANEOUSLY FIX THE IMAGE AND CONVERT THE MASTER NON-IMAGE AREAS FROM A HYDROPHILIC CONDITION.

Description

United States Patent Office 3,701 ,673 Patented Oct. 31, 1972 SIIVIULTANEOUS CONVERSION AND FUSION OF ELECTROSTATICALLY PRODUCER PRINTING MASTERS Martin W. Hellar, Novelty, and Michael A. Kiener, Cleveland, Ohio, assignors to Addressograph-Multigraph Corporation, Cleveland, Ohio No Drawing. Filed Jan. 20, 1971, Ser. No. 108,214 Int. Cl. G03g 13/06 US. Cl. 117-175 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the production of planographic printing masters. Photoconductors are used to arrange toner material on a sheet in the image of an original subject. Then, it is usually necessary to treat such a master to make the background hydrophilic. This invention is useful in all such systems, but finds particular application in an electrostatically produced master of the type having a surface coating of a photoconductor normally oleophilic, which must then be chemically reacted to establish a hydrophilic condition. Hence, the preferred embodiment of the invention will be described in its application to an electrostatic master.

One method of producing a lithographic printing master is by the electrostatic process. In this process, a master sheet coated with a photoconductive material is charged, then exposed to dissipate the charge in the non-image areas, to provide the desired latent image thereon. A toner is then applied to the surface, which toner will adhere to the charged surface where the latent image is formed. Such toner, when held on the surface only by electrostatic charge, is easily removed. According to conventional prior art practice, therefore, this toner is then fixed at elevated temperatures, i.e. the toner is heated to such a degree that it firmly adheres to the paper which thus sets the toner on the surface. This fixing has conventionally been done with high wattage lamps, utilized to generate radiant energy, or by ovens.

Once the toner has been fixed, the entire surface of the master is then treated with a conversion solution. The conversion solution chemically converts the non-image areas, i.e. the background photoconductive material, to a water insoluble hydrophilic material. The fixed toner, in the form of the image, is and remains oleophilic. Thus, the master will receive ink in the image area and reject it in the background. All of this in well known in the art.

One of the disadvantages to the above described prior art process, is the large number of separate steps which must be performed. Each of these separate steps require a separate processing station and associated equipment in the devices designed for producing the masters.

Another drawback to the above entitled process, is the need for high wattage heat elements. Not only do these elements require a large power input, they also present a potential fire hazard because of the heat generated.

Another deficiency or drawback in the above described process, is the somewhat limited number of conversion soluitons which can be utilized at ambient temperatures, since many potentially useful conversion solutions react at rates so slow as to be commercially or conveniently unacceptable. This has necessitated utilizing relatively unstable solutions to achieve the necessary rate of conver- S1011.

SUMMARY OF THE INVENTION According to the present invention, a master has the latent image formed thereon in the conventional manner. and toner applied in a conventional manner. A conversion solution is maintained at, or above, the temperature. required to fix the toner. The master is contacted by the solution for a sufficient period of time to simultaneously fix the toner and convert the background.

DESCRIPTION OF THE PREFERRED EMBODIMENT The process of the present invention, as described in the summary above, contemplates the simultaneous background conversion, and fixing of an image of conventionally produced electrostatic master to convert the master to a suitable condition for lithographic printing.

In the practice of the present invention, any master. sheet having photoconductive material which can be converted to a water insoluble hydrophilic material can be utilized. Examples of such potential materials include metal oxides and sulphides. Of these, zinc oxide is by far the most commonly used material and hence is preferred for this invention.

The coated master is first given a uniform surface charge, and then selectively discharged by exposure to a pattern of light and shadow. The light struck area will discharge, leaving a charged area latent image thereon in the desired shape to be printed. Toner is then applied which will adhere to the charged latent image. There are various formulations of toners which can be utilized. Basically, all

of thees toners are natural or synthetic resins, or mixtures of resins, and have carbon black and/or dyes. The temperature required to fix these toners varies, and is determined by their composition. The preferred toner is a mixture of natural and synthetic resins with suitable dyes, and carbon black. This toner must be heated to approximately C., or above, to obtain the required degree of fixing. When the toner is heated to or above this temperature, it will firmly adhere to the sheet, forming a relatively permanent image configuration thereon. This is sometimes referred to as fusing the toner, although it is doubtful that complete or even any appreciable melting is required to obtain satisfactory adherence to the sheet. This toner is oleophilic and will accept grease ink for printing.

The above steps are all conventional prior art practice.

When the toner has been applied, the master is then brought into contact with a conversion solution heated to a temperature above the temperature required to fix the toner. Usually, with most conventional toners, the solution must be heated to at least 80 C. While there are many solutions or compounds which are suitable for converting the zinc oxide to a hydrophilic-oleophobic form, such as oxalates, citrates, and tartrates, the preferred solution of 10% mono-ammonium phosphate, with 5% glycerin, and 0.03% sorbic acid added thereto. The phosphate is the converting agent, the glycerin is a humectant and wetting agent, and the sorbic acid a mold inhibitor. This solution is quite stable at temperatures of 80 C. and above, and has a virtually indefinite shelf life. Other phosphates, such as monosodium phosphate, are also well adapted for conversion purposes.

The imaged and toned master is maintained in contact with the liquid solution at the elevated temperature a sufficient period of time to fix the image and convert the background to a water insoluble hydrophilic material. The fixing process, with most toners, is a time-temperature dependent relationship; the, higher the temperature, the less time required. to complete the fixing process. The upperlimit for the temperature is controlled only by the characteristics of the conversion solution and the material of the components of the master; i.e. the temperature cannot be too high to degrade or impair the conversion reac tion or to degrade any of the components of the master or solution. With aqueous solutions, the upper limit'is therefore,-about 100 C., the boiling point of water. However, it is normally desirable to operate at the minimum temperature which will give sufficiently rapid conversion, and effective fixing since this will minimize power requirements. In the preferred embodiment, temperatures of 81 to 84 C. are satisfactory, and adequate fixing is completed within about 0.3 second. Conversion is also completewithin this same time, andthe master is then ready for printing.

Thus, according to this invention, the fixing of the toner and converting of the background is accomplished in a single step, rather than in two steps, therebyeliminating one processing step. Also, the energy requirements to maintain the solution in the 81 to 84 C. range, are substantially less than the energy requirement to operate the heat lamps or ovens in the prior art devices. Further, the elimination of the heat lamp or ovens eliminates potential fire hazards.

Another benefit of the present process, is that phosphate conversion solutions are commercially practicable. These solutions are desirable since they are stable and have virtually indefinite shelf life. However, when phosphate solutions are used at ambient temperatures, i.e. about 20 C., the rate of the conversion reaction is so slow that they are not convenient in a machine copier. Hence, other converting agents, such as ferrocyanides have been used since they react rapidly enough at ambient temperatures. However, these agents are comparatively unstable, with a resulting short shelf life. Thus, by operating at temperatures of 80 C., and above, the rate of the conversion reaction of the phosphates with the zinc oxide is sufiiciently rapid to be commercial, thereby permitting their use and, hence, avoiding the undesirable characteristics of the ferrocyanide solutions.

What is claimed is:

1. In the process of forming a printing master wherein an electrostatic image is formed by selectively charging the surface of a master sheet having a photoconductive material thereon to form a latent image, and applying a toner to develop the image and wherein the toner must be fixed and the non-imaged areas converted to water insoluble hydrophilic material;

the improvement which comprises, providing a conversion liquid which will convert the non-imaged material at a temperature above that required to fix the toner, and contacting the surface having the unfixed toner thereon with said liquid at a temperature at least as high'as the temperature required to fix said toner and lower than that which would degrade any of the components of the master and liquid for a suflicient time to fix the toner and convert the non-imaged areas, whereby to simultaneously convert the background and fix the image. 2. The invention as defined in claim 1 wherein the liquid is an aqueous solution.

References Cited UNITED STATES PATENTS 3,001,872 9/1961 Hurz l0l463 3,107,169 10/1963 Bornarth 101-466 3,278,323 10/1966 Kalman et a1. 1l7-17.5 3,567,484 3/1971 White et a1 117-21 WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R.