US3679412A - Lithographic printing plates and methods for preparation thereof - Google Patents

Abstract

LITHOGRAPHIC PLATES ARE PREPARED BY (A) PROVIDING METALLIC SUPPORT WITH GELATINOUS IMAGE; (B) TREATING THE GELATINOUS IMAGE WITH AN AQUEOUS SOLUTION COMPRISING DICHROMATE, HALIDE, AND ACETIC ACID; AND (C) SUBJECTING THE TREATED IMAGE TO HEAT.

Description

United States Patent Office 3,679,412 Patented July 25, 1972 3 679 412 LITHOGRAPHIC PRINTING PLATES AND METH- ODS FOR PREPARATION THEREOF William E. Guthrie and Anthony J. Parrinello, both of 1669 Lake Ave., Rochester, N.Y. 14650 No Drawing. Continuation-impart of application Ser. No. 769,430, Oct. 21, 1968. This application May 4, 1970, Ser. No. 34,541

Int. Cl. G03c 7/02 US. CI. 9633 Claims ABSTRACT OF THE DISCLOSURE Lithographic plates are prepared by (a) providing metallic support with gelatinous image; (b) treating the gelatinous image with an aqueous solution comprising dichromate, halide, and acetic acid; and (c) subjecting the treated image to heat.

This application is a continuation-in-part of copending US. patent application Ser. No. 769,430 filed Oct. 21, 1968, now abandoned.

This invention relates to photographic elements and processes. In one of its aspects this invention relates to lithographic printing plates. In another of its aspects this invention relates to methods and materials for the preparation of lithographic plates. In a further aspect this invention relates to lithographic plates having oleophilic gelatinous image areas on hydrophilic metallic supports and methods and materials for the production thereof.

Lithographic methods and materials are assuming an ever increasing role in the printing industry. The increasing demand for and use of lithographic products has been accompanied by consumer sophistication at a level which is difficult for the artisan to fulfill. The desire to satisfy the consumer requests and resultant artisan demands for better quality at reasonable production cost have stimulated vast research efforts to find plates which have a long press life, plate materials which require short exposure times and minimal detailed processing for plate preparation, and plates having wide press latitude.

The research efforts have, for example, resulted in lithographic support materials which are capable of functioning effectively for a large number of press copies. Metallic supports, such as aluminum or zinc, have been successfully employed. In recent years aluminum has been particularly attractive as a lithographic support material due in part to its cost, availability, and light weight. Efforts to utilize aluminum supports, however, with damera speed light sensitive materials which require minimal skill in processing, for example, light sensitive silver halide gelatinous compositions, have been fraught with difiiculty. Extensive precautionary measures have had to be taken to avoid interaction of the aluminum support and the light sensitive materials. Even with these carefully manufactured plate materials excessive exposure times and highly skilled artisans were often required to convert the plate materials into useful printing plates.

The utilization of silver halide-gelatin compositions for long lasting plates has been a highly desirable goal due at least in part to the low exposure time and minimal processing required for such compositions. Although silver halide compositions are particularly susceptible to environmental problems, some success has been achieved using such compositions together with aluminum supports, e.g., by separating the silver halide material from the aluminum with a plastic interlayer or by using a transfer system whereby light insensitive gelatin is imagewise transferred to metallic supports. It has nonetheless been difficult to prepare plates having gelatinous images which will retain oleophilicity and which have wide utility with various combinations of lithographic inks, fountain solutions, etc.

The use of various methods to increase the oleophilicity (respectivity to greasy printing ink) has been helpful, but usually results in plates having unsatisfactory press life, such as results from using image conditions, or preparative steps which are not compatible with automatic processing machines.

According to the present invention lithographic plates which have wide press latitude and long press life can be prepared by forming a plate precursor comprising gelatinous images on a metallic support and treating the gelatinous images with a specific type of treating solution and exposing the treated images to heat.

Thus, the present invention provides methods for preparing lithographic plates having gelatinous images which retain oleophilicity throughout long press runs disposed on the metallic supports, which methods involve only steps which are accomplished in a relatively short period of time.

The invention further provides methods for producing lithographic plates having aluminum supports with gelatinous images thereon, which methods involve steps which are readily compatible with automated equipment.

This invention also provides methods for preparing lithographic plates having aluminum supports with gelatinous images thereon which methods employ substantially non-corrosive solutions.

Furthermore, the present invention provides novel lithographic printing plates which may be effectively utilized for long press runs under varying press conditions and with a wide variety of lithographic inks, i.e., greasy printing inks.

Other embodiments and advantages of the present invention will be apparent from the following description.

We have found that by forming plate precursors having gelatinous images on metallic lithographic supports, treating the images with an aqueous solution having a pH of from about 2 to about 6 and containing dichromate ions, halide ions, and acetic acid, and exposing the treated images to heat at a temperature of from about 250 to about 800 F. (about 122 to about 425 C.) for a period of from about 20 seconds up to about seconds, lithographic plates are formed which have wide press latitude and function effectively for long press runs.

For the formation of the plate precursor, a metallic support, desirably an aluminum support, is provided with gelatinous image by any suitable manner, for example, by gelatin image transfer to an aluminum support, by beginning with a light sensitive gelatinous layer on an aluminum support from which non-image areas are selectively removed by etch bleach, washoff, etc.

To convert the plate precursor into a lithographic plate according to the present invention at least the gelatinous images of the precursor are treated with an aqueous solution having a pH of from about 2 to about 6, advantageously about 3, and containing:

(1) Acetic acid and/or soluble residues thereof;

(2) Dichromate ions, which can be furnished to the solution by the addition of soluble dichromates, especially metal dichromates, advantageously alkalimetal dichromates such as sodium dichromate, potassium dichromate,

however, to treat the gelatinous images with solutions containing in a liter of total solution the dissolved residues from:

(1) About -50 ml. glacial acetic acid;

(2) About 20 to about 40 g. dichromate, e.g., ammonium or metal dichromate advantageously an amount which would furnish dichromate in an amount approximately equal to that furnished by g. potassium dichromate; and

(3) About 20 to about 150 g. metal or ammonium halide, desirably 20 to about 120 g. alkali metal or ammonium halide and advantageously an amount of alkali metal or ammonium halide that would furnish halide in an ion concentration approximately equal to that furnished by about 120 g. potassium bromide.

The time and temperature involved in treating the gelatinous images with the solution, of course, vary depending upon how the solution is applied, etc. The plate precursor may, for example, be simply passed through a bath at room temperature at a speed such that the image contacts the bath for at least about 20 seconds. Longer times, e.g., up to two minutes, can be employed but most often merely slow the overall operation. Shorter dwell times may be employed especially if the temperature of the bath is increased. It should be recognized, of course, that during the initial phases of the heating step the gelatinous images have imbibed therein or are in contact with the treating solution or reacted residues thereof. Other means for applying the solution to the gelatinous images, for example, by spraying, swabbing, etc. may be used.

The treated images are then subjected to heat approximately equal to that amount that would be furnished by placing the initially damp treated plate precursor in an oven for 20 to 90 seconds at a temperature of from about 250 F. to about 800 F., (about 122425 (2.), advantageously for about 30 to 60 seconds at about 280-600 F. (about 135-315 C.). The heat may be applied, for example, by placing the plate in a hot gaseous environment for the desired time or by impinging hot gas on the plate.

Optimization of process conditions within the indicated boundaries may vary depending upon many factors, including heating facility space, e.g., oven space, time alloted for plate manufacture, etc.

The gelatin images as mentioned previously may be provided by any suitable method which results in an imagewise disposition of gelatin on a hydrophilic support. A particularly advantageous method of forming an image is to utilize a first brushed then phosphoric acid anodized aluminum support on which a photographic silver halide emulsion is coated, exposed to a light image, and developed in a developer such as Kodak Developer D-19. Following development, the silver halide emulsion is immersed in an etch-bleach bath such as Kodak Etch Bleach Bath EB-4 containing cupric chloride, citric acid, urea and hydrogen peroxide. This results in bleaching silver grains and at the same time degrading or softening gelatin so that it is removed in the exposed areas. The resulting lithographic plate precursor is then treated in the desired 4 manner with the dichromate solution and heated as indicated above and used on the press. We have unexpectedly found that the treating solutions function very effectively even though no substantial developed silver resides in the gelatinous image to be treated.

The photographic silver halide emulsions which can be used according.to this advantageous embodiment of the invention include silver halide emulsions such as silver chloride, silver bromide, silver iodide, silver chlorobromide, silver chlorodide, silver bromiodide, silver chlorobromoiodide, etc. A particularly useful emulsion is a high contrast chlorobromide emulsion in which the silver halide contains at least 60 mole percent silver chloride.

Although it is preferred to use gelatin as the sole binding agent for the silver halide, it will be appreciated that other hydrophilic colloids can be used with gelatin to give a layer which can be hardened to a point where it has a melting point above about 180 F. and generally below about 300 F. Suitable hydrophilic colloids include colloidal albumin, cellulose derivatives, synthetic resins, particularly polyvinyl compounds and the like. Water insoluble polymerized vinyl compounds, e.g., polymers of alkyl acrylates or methacrylates, can also be included.

The coverage of the silver halide emulsion can be varied depending upon the use intended. A useful range is about to 800 milligrams per square foot of gelatin, preferably 100 to 400 milligrams per square foot and about 50 to 200 milligrams per square foot of silver as silver halide, preferably 50 to milligrams per square foot. The photographic emulsions described herein can be chemically sensitized such as with compounds of the sulfur groups, noble metal salts such as gold salts, reduction sensitized with reducing agents, combinations of these, etc. Furthermore, emulsion layers can be hardened with any suitable hardener such as aldehyde hardeners, aziridine hardeners, hardeners which are derivatives of dioxane, oxypolysaccharides, such as oxystarch, oxy plant gums, and the like.

The photographic silver halide emulsions can also contain additional additives particularly those known to be beneficial in photographic emulsions including, for example, stabilizers or antifoggants, particularly the water soluble inorganic acid salts of cadmium, cobalt, manganese, and zinc as disclosed in US. Pat. 2,829,404, substituted triazaindolines as disclosed in U.S-. Pats. 2,444,605 and 2,444,607, speed increasing materials, plasticizers, absorbing dyes, and the like. Sensitizers which give particularly good results in typical emulsions useful in our invention are the alkylene oxide polymers which can be employed alone or in combination with other materials, such as quaternary ammonium salts as disclosed in US. Pat. 2,886,437 or with mercury compounds and nitrogen-containing compounds, as disclosed in US. Pat. 2,751,299. The emulsions can be blue sensitized, orthochromatic, panchromatic, infrared sensitive, etc.

' The types of emulsions which may be employed according to the present invention, of course, include both negative and positive emulsions. Reversal emulsions or direct positive emulsions, such as hardened solarizing silver halide emuslions and hardened internal latent image silver emulsions forming the latent image mostly inside the silver halide grains, may in some instances be advantageously employed.

To increase sharpness, it may be desirable to include an antihalation pigment or dye in the emulsion. Typical dyes and pigments used in antihalation layers may be used provided they are inert to the emulsion and, when the etch-bleach system is utilized, do not affect the etch-bleach reaction. In a preferred embodiment, a carbon pigment is used. A useful amount of antihalation dye or pigment is 20 to 50 grams per sil ver mole. Including carbon pigment also may facilitate proofing of the resultant lithographic plate.

It will be understood that the emulsion can be coated using methods known in the art. For the etch-bleach sys- Zem of forming the plate image the emulsion must be sufiiciently hardened so that it will have a melting temperature of at least about 180 F., generally above about 200 F. and preferably above about 230 F.

It will be appreciated that any of the conventional silver halide developing agents can be used in the practice of this invention. Such developing agents can be incorporated into the element contiguous to silver halide, e.g., in the emulsion layer or in a contiguous layer. Typical developing agents include hydroquinone'and substituted hydroquinone, such as bromohydroquinone, chlorohydroquinone, toluhydroquinone, morpholinomethylhydroquinone, etc. It will also be appreciated that an auxiliary developing agent can be used in an amount of to 20% of the hydroquinone or substituted hydroquinone in order to improve the speed without affecting the developing reaction.

Typical auxiliary agents include 3-pyrazolidone, developing agents known in the art as well as Elon (N- methyl-p-aminophenol sulfate), and the like. Particularly useful auxiliary agents are 1-phenyl-3-pyrazolidone and 1-phenyl-4,4-dimethyl-3-pyrazolidone.

The developed silver halide emulsion advantageously is processed to a lithographic plate with an etch-bleach solution. Such solutions typically contain an oxidizing agent, such as hydrogen peroxide, an insoluble silver salt former, such as chloride ion, and a metal ion catalyst, such as cupric ion. A gelatin softener, such as citric acid and/or urea, may also be incorporated in the etch-bleach bath. The etch-bleach solution can be applied by spraying, dipping, immersing, swabbing, etc. in the areas where silver has been formed resulting in bleaching silver image and at the same time degrading or etching gelatin in these same areas. The etch-bleach application normally removes the gelatin in the exposed area. However, the emulsion may be washed to remove the etch-bleach solution and any remaining softened colloid. If desired, the emulsion can then be re-exposed to regular room light and then redeveloped to provide an image in those areas 'which were not etched. This provides a darkened image area for proofing purposes on the plate. Of course, the emulsion can be hardened after the etch bleach, if desired, to compensate for any surface softening due to the etch bleach. However, this is not necessary. The plate can be treated immediately following the etch-bleach operation, heated to provide durably oleophilic gelatinous images, and inked to provide a darkened image area and run on the lithographic press.

The etch-bleach solution may be one of those containing cupric chloride, citric acid, and hydrogen peroxide, such as Kodak Etch-Bleach 'Bath EB-3 or EB-4, as follows:

KODAK ETOH-BLEACH BATH EB-3 Water, at 125 F.-750 cc. Cupric chloride-l0 g.

Citric acidg.

Water to make 1 liter Hydrogen peroxide, 3%r1 liter KODAK ETOH-BDEACH BATH -EB-4 Water, at 125 F.600 cc. Cupric chloride-10 g.

Citric acidl50 g.

Urea150 g.

Water to make 1 liter Hydrogen peroxide, 3%1 liter Another suitable etch-bleach bath containing copper sulfate, citric acid, potassium bromide, and hydrogen peroxide is known as Kodak Etch-Bleach Bath EB-Z. However, an etch-bleach bath containing cupric chloride, citric acid, urea and hydrogen peroxide in which there is at least 20 grams per liter of cupric chloride is particularly suitable in producing a clean removal of the gelatin in the image areas in a period of time as short as 20 seconds. Such etch-bleach solutions are described in Albert Sieg application Ser. No. 650,616, filed July 3, 1967. Various other oxidizing compounds may be used in place of hydrogen peroxide, such as hydrogen peroxide precursors and the like. However, oxidizing agents which are used in place of hydrogen peroxide must be those which act selectively on the image area where the silver image is located rather than attacking the complete emulsion layer which would be the case, for example, if nitric acid was utilized.

Although aluminum is a particularly useful support in the practice of our invention, it will be appreciated that other supports can be used including metals such as copper, zinc, steel and the like which have been suitably hydrophilized or provided with hydrophilic surface. Advantageously, the aluminum is anodized at least on the side which is to receive the light sensitive emulsion, desirably in accordance with the disclosure of Rauner et al.,

. U.S. Ser. No. 567,031, filed July 1, 1966. The aluminum may, however, be grained or 'brushed and optionally thereafter anodized. The emulsion is desirably coated directly on the support especially when the support is anodized aluminum.

The following examples are intended to illustrate the invention and/or advantages thereof but not to limit it in any way.

EXAMPLE 1 EXAMPLE 2 The procedure of Example 1 is followed except that before thev etch-bleached material is inked the image is treated with an image conditioner as disclosed in US. Ser. No. 650,864, filed July 3, 1967. The plate tones badly even with high viscosity inks, such as Speed King Jet Halftone Black, Interchemical Corporation, Elizabeth, NJ.

EXAMPLE 3 The procedure according to Example 2 is followed except that before the plate is used on the press it is placed in an oven at about 400 F. for about 1 minute. The plate tends to tone very badly even with high viscosity inks such as Speed King Jet Halftone Black.

EXAMPLE 4 The procedure according to Example 1 is followed except that before the plate is put on the press, it is placed in a bath having the following ingredients:

Acetic acid (glacial)35 ml. Water to make 1 liter and then heated at a temperature of about 380 F. for about one minute. The resultant plate is readily inked. When run on a press, the image appears durably oleophilic and produces in excess of 25,000 copies of excellent quality. The type of ink employed with this plate does not appear to significantly afiect plate quality. Highquality prints are obtained using: high viscosity inks, such as Speed King Jet Halftone Black, Interchemical Corporation; medium viscosity inks, such as Print Gloss Offset 7 Black, Pope and Gray Company; and low viscosity inks, such as Web Offset Black, General Printing Ink Com- P EXAMPLE The procedure according to Example 4 is followed except that sodium bromide is substituted for potassium bromide and the treated plate is heated at 580 F. for 40 seconds. Similar results are achieved.

EXAMPLE 6 The procedure according to Example 5 is followed except that sodium dichromate is used in place of potassium dichromate. Similar results are achieved.

EXAMPLE 7 The procedure according to Example 4 is followed except that potassium chloride is used in place of potassium bromide and the plate is heated at 600 F. for a period of 35 seconds. Similar results are achieved.

EXAMPLE 8 The procedure according to Example 4 is followed except that ammonium dichromate is used in place of potassium dichromate. Similar results are achieved.

EXAMPLE 9 The procedure according to Example 4 is followed except that the plate is heated to 500 F. for about 30 seconds. Similar results are achieved.

EXAMPLE 10 The procedure according to Example 4' is followed except that the plate is put in an oven having a temperature of 325 'F. for about one minute. The plate works very well with low and medium viscosity inks but the plate tends to blind with high viscosity mks.

EXAMPLE 11 The procedure according to Example 4 is followed except that the plate is put in an oven having a temperature of about 420 F. for about one minute. Results sunilar to those of Example 4 are achieved.

EXAMPLE 12.

The procedure according to Example 4 is followed except that the plate is placed in an oven having a temperature of 250 F. The plate tends to bhnd with high viscosity inks but functions verywell with low viscosity inks,;such as Web Offset Black.

EXAMPLE 13 The procedure according to Example 4 is followed ex- ,cept that glycolic acid is substituted for acetic acid in the treating solution. The ink receptivity of the resultant plate is substantially less than the plate of Example 4. Similar diminished ink receptibity is noticed when other acids, such as diglycolic acid, hydrochloric acid, phosphoric acid, and citric acid, are substituted for acetic acid.

EXAMPLE 14 The procedure according to Example 4 is followed except that a grained or brushed aluminum support is utilized in place of the anodized aluminum support. Analogous results are achieved.

EXAMPLE 15 A gelatinous imagewise disposition is provided on a chemically grained aluminum support by a gelatin transfer system of the type disclosed in Clark et al., U.S. Pat. No. 2,763,553. The resultant plate is then treated in accordance with the procedure outlined in Example 4. Over 25,000 high-quality copies are obtained. No satisfactory copies are obtained when the same system is utilized with no heating step.

8 EXAMPLE 16 A photographic emulsion is prepared by combining the following ingredients:

of 10% Alkanol B solution g 10.0 (111) 4-phenyl catechol dispersion prepared as follows:

50 g. 4-phenyl catechol dissolved in 100 cc. di-

butylphthalate at 60 -70 cc. dispersed in 500 cc. 10% photographic gelatin and 50 cc. 7 /2% saponin solution and passed through the colloid mill five times g 10.0 15% saponin solution g 1.0 Water cc 20.0

The emulsion is coated at the rate of 6.5 grams per square foot on a phosphoric acid anodized aluminum support. This element is exposed to a line negative, activated for 30 seconds in an 8-percent solution of potassium phosphate, and vigorously washed for 30 seconds under a spray water jet. The photographic emulsion in the exposed and developed image area adheres to the surface of the aluminum while the emulsion in the non-exposed background area is completely washed away by the scrubbing action of the water jet. The processed element is then hardened by immersion for 30 seconds in an acidic solution of potassium bichromate and halide ion. This element is then placed on an offset lithographic printing press and printed copies made which are poor in quality because the gelatin image does not readily accept a variety of printing inks.

EXAMPLE 17 The element exposed and processed as in Example 16 is heated in an oven to 500 F. for a period of 30-60 seconds. This element is then placed on the offset lithographic printing press and many thousands of high quality copies are produced with a variety of printing inks.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A method for'preparing lithographic plates comprising:

(a) forming a gelatinous image on a metallic support;

(b) contacting the gelatinous image with an aqueous solution having a pH of from about 2 to about 6 and comprising dichromate ions, halide ions, and acetic acid; and

(c) heating the treated image in an environment maintained at a temperature of from about 250 F. to about 800 F. for a period of from about 20 to about seconds.

2. A method as in claim 1 and wherein the treated image is subjected to a temperature of from about 280 F. to about 600 F. for a period of from about 30 to about 90 seconds.

3. A method as in claim 1 and wherein the dichromate ions are furnished to the solution as ammonium or alkali metal dichromate, and the halide furnished as alkali metal halide.

4. A method as in claim 1 and wherein the aqueous solution has a pH of about 3 and consists essentially of water and the dissolved residues of ammonium or alkali metal dichromate, ammonium or alkali metal halide, and acetic acid. a

5. A method as in claim 1 and wherein the metallic support comprises aluminum, the aqueous solution has a pH of about 3 and consists essentially of water and the dissolved residues of about 20 to about 50 ml. acetic acid,

about 20 to about 40 g. ammonium or alkali metal dichromate, and about 20 to about 120 g. ammonium or alkali metal halide per liter of solution and wherein the treated images are subject to a temperature of from about 280 F. to about 600 F. for a period of from 30 to 90 seconds.

6. A lithographic plate prepared by:

(a) providing a metallic support with a gelatinous image;

(b) treating the gelatinous image with an aqueous solution comprising dichromate ions, halide ions, and acetic acid; and

(c) heating the treated image in an environment having a temperature of from about 250 F. to about 800 F. for a period of from about 20 to about 90 seconds.

7. The invention in accordance with claim 6 and wherein the metallic support is provided with a gelatinous image by:

(i) providing an element comprising an aluminum support having a light sensitive gelatinous silver halide emulsion thereon;

(ii) exposing the light sensitive emulsion to an image;

(iii) developing the exposed element; and

(iv) removing the areas containing developed silver by etch bleach.

8. The invention according to claim 6 and wherein the aqueous solution has a pH of from about 2 to about 6 and consists essentially of water and dissolved residues of soluble ammonium or metal dichromates, soluble ammonium or metal halides, and acetic acid.

9. The invention according to claim 6 and wherein the aqueous solution has a pH of about 2 to 6 and consists essentially of water and dissolved residues of alkali metal dichromates, alkali metal halides, and acetic acid, and the heating step involves subjecting the treated image to a gaseous environment having a temperature of from about 135 to about 315 C.

10. The invention according to claim 6 and wherein the aqueous solution has a pH of about 3 and consists essentially of water and the dissolved residues of about 30 g. potassium dichromate, about 120 g. potassium (bromide), about 35 ml. acetic acid per liter of solution.

11. A method for preparing a lithographic plate from a precursor having a metallic support with a gelatinous image thereon said method comprising:

(a) treating the gelatinous image with an aqueous solution having a pH of from about 2 to about 6 and comprising dichromate ions, halide ions, and acetic acid; and

(b) heating the treated image at a temperature of from 10 about 250 F. to about 500 F. for a period of no more than about seconds.

12. The method according to claim 11 and wherein the treating is accomplished by contacting the gelatinous image for a period of up to two minutes with an aqueous solution having a pH of from about 2 to about 6 and containing dichromate ions, halide ions, and acetic acid.

13. The method according to claim 11 and wherein the treating of step (a) accomplished by immersing the gelatinous image for a period of up to two minutes in an aqueous solution having a pH of from about 2 to about 6 and containing dichromate ions, halide ions, and acetic acid, and the heating of step (b) is at a temperature of from about 280 F. to about 500 F. for a period of about 90 seconds to about 30 seconds.

14. The method according to claim 11 and wherein the treating of step (a) is accomplished by immersing the gelatinous image in an aqueous solution having a pH of from about 2 to about 6 and consisting essentially of water and the dissolved residues from about 20 to about 50 ml. acetic acid, about 20 to about 40 g. ammonium or alkali metal dichromate, and about 20 to about g. ammonium or alkali metal halide per liter of solution, and the heating of step (b) is at a temperature of about 450 F. to about 280 F. for a period of from 30 to 90 seconds.

15. The method according to claim 11 and wherein the aqueous solution has a pH of about 3 and consists essentially of the dissolved residues of about 20 to about 40 g. ammonium or alkali metal dichromates, about 20 to about 120 g. ammonium or alkali metal bromide or chloride, and about 20 to '50 ml. acetic acid per liter of solution, and the heating step (b) is at a temperature of about to about 232 C. for a period of about 90 to about 30 seconds.

References Cited UNITED STATES PATENTS 2,882,153 4/1959 Cohn 9675 2,882,154 4/1959 Cohn 9675 2,407,290 6/1946 Pursell 9'55.6 2,462,570 7/1949 Terry 99-33 X 2,690,967 10/1954 Sharples 1l7-34 X 3,497,358 2/ 1970 Sieg et a1. 96111 2,310,223 2/ 1943 Eaton et a1. 96111 NORMAN G. TORCH'IN, Primary Examiner E. C. KIMLIN, Assistant Examiner US. Cl. X.R. 9648, 111