US2690395A - Composition for lithographic plate coatings - Google Patents

Description

Patented Sept. 28, 1954 COMPOSITION FOR LITHOGRAPHIO PLATE COATIN GS Vernon L. Gregory, Piedmont, Calif., assignor to Gregory & Falk, San Francisco, Calif., a co,-

partnership No Drawing. Application November 10, 1950, Serial N0. 195,131

4 Claims. (01. 95-7) This invention relates to the offset printing process, and is particularly directed to an improved coating for lithographic plates utilized in such a process.

In modern lithography, usually a zinc or aluminum plate is provided with a dried surface coating, and thereafter, the coated plate is exposed to light through a photographic film or the like whereby the exposed surface of the coating forms a relatively hard film, while the unexposed surface remains soft and water soluble. A suitable developing ink is then spread evenly over the plate and dried. After washing oif the water soluble portion of the coating, the remaining exposed or hardened film, which is receptive to greasy material, may be covered with a lithographic ink, and in the offset process, the inked design on the plate is transferred to a rubber blanket which is then brought into contact with the paper. With the surface coatings heretofore utilized, up to approximately 100,000 reproductions could be made from a plate under ideal conditions, but as an average, only 20,000 of such reproductions could be reasonably anticipated before the copies :became blurred, indistinct or otherwise objectionable. These previously used coatings usually included a colloid solution of albumen, dex'trin, casein or one of the soluble gums rendered light sensitive by the addition of a material such as ammonium bichromate or the sodium or potassium salt of bichromate. When a plate provided with such a coating is exposed to light, the exposed colloid and bichromate forms the hardened film which is receptive to lithographic ink; but as copies are reproduced by the plate in the aforementioned manner, the hardened film progressively loses its ink receptivity due in part to the physical wearing away of the film, and believed also to be due to chemical changes occurring therein. In connection with this latter point, it has also been found that if a plate is stored for any considerable length of time, the ink receptivity of the film is lessened, thereby resulting in poor reproductions.

It is therefore an object of the present invention to provide a coating for lithographic plates which will permit a maximum number of sharp, distinct reproductions to be printed thereby.

Another object of the invention is to provide a coating for lithographic plates which will retain its ink receptivity over a longer period of time and use than the coatings heretofore utilized in the art.

It is a further object of this invention to provide a coating possessing the above characteristics which will retain its ink receptivity even after the exposed surface thereof is worn away by contact with the rubber blanket, paper, ink, or other foreign material.

The invention possesses other objects and features of advantage which, together with the foregoing, will be specifically set forth in the following detailed description of the invention. It will be understood that the invention is not to be limited to the specific compositions or procedure herein described, as variations thereof may be employed which come within the scope of the appended claims.

In broad terms, the present invention relates to an image forming coating for lithographic plates, and includes a fatty substance emulsified with a colloid that is rendered light sensitive by the presence of ammonium bichroinate. Such a coating is utilized in the manner well known in the art as previously described, and it has been found that the addition of the fatty substance to the coating permits the latter to retain its ink receptivity over longer periods of time and under more conditions of use than any of the coatings previously used. This important factor of increased ink receptivity results from the natural afiinity of ink with fatty substances, and it is also believed that the metal plate itself is chemically changed by the fatty composition so that even after substantially all of the coating is worn away, the plate will still be receptive to the lithographic ink.

Although numerous fatty substances could be utilized in the composition, it is far more desirable to provide a natural fatty emulsion rather than requiring the addition of an emulsifier to other types of fat mixtures. For purpose of explanation, I have found the following substances to give excellent results in forming natural fatty emulsions when mixed with water: dried milk, dried whole egg, or natural soya bean powder. If desired, the emulsion may comprise liquid whole milk or synthetic mixtures of a fat and an emulsifier and water.

In actual practice, the plate coating composition is usually prepared in dried powder form, that is, the colloid base, the light sensitive material, and the fatty substance are thoroughly physically mixed together so as to form a substantially uniform mixture. In this dried condition, the composition may be easily shipped and stored until ready for use. When it is desired to utilize the same for a plate coating, a sulficient quantity of the mixture is added to the necessary amount of water in the familiar manner so as to produce the fatty emulsion composition heretofore described. The lithographic plate is then coated with the latter composition preparatory to light exposure.

To enable those skilled in the art to practice this invention, I have hereinbelow set forth examples of compositions which may successfully be utilized in forming lithographic plate coatings of the type desired.

Example N0. 1

Mix 800 ounces of a colloid such as casein, dextrin, albumen, or one of the soluble gums, with 320 ounces of a light sensitive material such as the potassium or sodium salts of bichromate, but preferably ammonium bichromate, and 230 ounces of dry whole milk powder, dried whole egg, or natural soya bean powder. If desired, approximately 12 ounces of thymol may be added to the above, the thymol acting as a preservative and odorant for the mixture. Sufficient water may then be added to the mixture to make 680 quarts of solution.

Example No. 2

If it is desired to use liquid whole milk, the amounts of colloid, light sensitive material and thymol will be the same as in the above example to which will be added 106 quarts of whole milk. Here too, sufficient water will be added to make up 680 quarts of solution.

It will be understood that the figures given in the foregoing examples are merely by way of illustration, as it is possible to deviate therefrom without departing from the scope of this invention. From these exampl%, it will be seen that of the total dry weight of the mixture, approximately 60% represents the colloid, 25% represents the bichromate or other light sensi tive material, and 15% represents the dried fatty substance. However, with regard to this latter percentage, it will be noted that a material such as dried whole milk has approximately a 30% fat content, and therefore, the actual fat content of the dried mixture is only in the neighborhood of 5% of the total weight. Although this fat content may be increased if the user takes extra precautions in developing the lithographic plate, I have found that by keeping the fat content in the above proportion, there will be little danger of excessive fat causing a soft exposed image on the plate.

The foregoing percentages are believed to give optimum results with a minimum of safeguards required to be employed by the user thereof. However, mixtures may and have been more or less successfully utilized in which the percentages of the ingredients to the total dry weight have fallen within the following ranges:

Per cent Colloid 40 to 80 Bichromate 5 to 50 Fat 0.5 to 20 It will thus be seen that a degree of flexibility is offered without radically departing or deviating from the present invention.

I claim:

1. An image forming coating for lithographic plates comprising a colloid selected from the group consisting of albumen, casein, and a soluble gum; a light sensitive material selected from the group consisting of ammonium bichromate, sodium bichromate, and potassium bichromate; and a substance which will form an emulsion in water selected from the group consisting of dried milk, dried whole egg, and natural soya bean powder.

2. An image forming coating for lithographic plates comprising a colloid selected from the group consisting of albumen, casein, and a soluble gum; a light sensitive material selected from the group consisting of ammonium bichromate, sodium bichromate, and potassium bichromate; and liquid whole milk.

3. Am image forming coating composition for lithographic plates including a natural saturated fatty substance selected from the group consisting of dried milk, dried whole egg and natural soya bean powder, the fat content of said fatty substance constituting from 0.5% to 20% of the total dry weight of the composition, a colloid, and a bichromate salt.

4. A dry composition for use as a lithographic plate coating comprising a colloid selected from the group consisting of albumen, dextron, casein and a soluble gum; a light sensitive bichromate salt; and a natural saturated fatty substance selected from the group consisting of milk, whole egg and natural soya bean and capable of forming an emulsion in water; the said colloid constituting approximately 60% by weight of the composition, and the fat content of the said fatty substance constituting approximately 5% by weight of the composition.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 66,102 Morvan June 25, 1867 134,470 Edwards Dec. 31, 1872 209,036 Freeman June 8, 1878 556,380 Stevens Mar. 17, 1896 2,574,258 Frost Nov. 6, 1951 2,598,189 Mullen May 27, 1952 2,624,673 Frost Jan. 6, 1953 FOREIGN PATENTS Number Country Date 267 Great Britain July 28, 1863 12,867 Great Britain Feb. 8, 1906 517,686 Great Britain Feb. 6, 1940

Claims (1)

1. AN IMAGE FORMING COATING FOR LITHOGRAPHIC PLATES COMPRISING A COLLOID SELECTED FROM THE GROUP CONSISTING OF ALBUMEN, CASEIN, AND A SOLUBLE GUM; A LIGHT SENSITIVE MATERIAL SELECTED FROM THE GROUP CONSISTING OF AMMONIUM BICHROMATE, SODIUM BICHROMATE, AND POTASSIUM BICHROMATE; AND A SUBSTANCE WHICH WILL FORM AN EMULSION IN WATER SELECTED FROM THE GROUP CONSISTING OF DRIED MILK, DRIED WHOLE EGG, AND NATURAL SOYA BEAN POWDER.