GB2099371A - Finisher and lithographic printing plates - Google Patents

Abstract

A lithographic plate that has been exposed and developed is, before being heated to form a fused image, treated with an aqueous solution of a monoalkyl or dialkyl diphenylethersulphonic acid, wherein the or each alkyl group is a linear alkyl group of 14 to 18 carbon atoms.

Description

SPECIFICATION Finisher for lithographic printing plates This invention relates to lithographic printing plates, more particularly to a process for protecting developed plates, during fusion of the image, from contamination of the revealed hydrophilic substrate surfaces, without causing blinding of the images, by treatment of the developed plate with a special finishing agent prior to said fusion.

The difficulty and the unpredictability of producing lithographic plates which will lead to high quality printing are described in detail in the article entitled "Science on the Surface", British Printer, March, 1980.

The problems are also disclosed in U.K. Patent No. 1,513,368, which advocates treating a developed lithographic plate with a water soluble salt to provide a water-soluble layer which prevents contaminants from contacting the revealed substrate during fusing of the ink receptive printing surfaces.

The specific water soluble salts disclosed in the U.K. patent include a sodium dodecyl phenoxy benzene disulphonate, a sodium salt of alkylated naphthalene sulfonic acid, a disodium salt of methylene dinaphthalene sulfonic acid, a sodium dodecyl benzene sulphonate, a sodium salt of sulphonated alkyldiphenyl oxide or lithium nitrate. These materials will be hereinafter referred to as finishers or fusing gums in accordance with accepted nomenclature.

The difficulty with finishers hereinbefore proposed is that instructions for their use were not only time consuming but unless the instructions were strictly followed the finishers did not function properly.

Furthermore, certain known finishers resulted in the blinding of the image areas.

U.K. Patent No. 1,513,368 requires the following processing steps for heating a substrate coated with a radiation sensitive layer: 1. image-wise exposing the layer to radiation so that the layer includes radiation-struck areas and non-radiation struck areas of differing solubility; 2. developing the image-wise exposed layer to selectively remove the more soluble of the areas and reveal the substrate underlying these areas; 3. treating the member to provide a water soluble salt on the revealed substrate to prevent contaminants from contacting the revealed substrate; 4. heating the less soluble of the area remaining on the substrate after development, and 5. removing the water soluble layer.

In accordance with the present invention it has now been found that an improved lithographic plate processing method can be achieved by utilizing a special finisher which is a compound of the formula 1,

wherein the R1 and R2 are the same or different and each is hydrogen or a linear alkyl group of 14 to 18 carbon atoms with the proviso that R1 and R2 cannot both be hydrogen; and M is an alkali metal cation; and mixtures thereof.

In a preferred embodiment R1 is a linear alkyl group of 16 carbon atoms, R2 is hydrogen or R1 M is sodium, and mixtures thereof.

The number of carbons in the alkyl groups (R) is important as is the use of a linear alkyl group. Thus, for example, with lower alkyl chains such as linear Clo or branched C12 the treated plate showed a markedly inferior ink reception.

In general, the lithographic plates useful in the practice of the present invention comprises a metallic sheet or plate at least one surface of which is coated with a radiation-sensitive layer containing a radiationsensitive component whose solubility in developers differs before and after exposure to said radiation.

Conventional support members may be employed in the practice of this invention. The preferred supports may be any of the standard metal or metal surfaced plates which are commonly used in lithographic printing. Aluminum and zinc are two of the most preferred plates. It will be understood that the plates may be specially treated or be provided with intermediate coating or barrier layers well known to the art. In the case of aluminum, for example, it is often advantageous to chemically or electrolytically anodize its surface.

The metal surface can also be silicated.

The type of radiation-sensitive materials known as photopolymer materials become less soluble after exposure to radiation and therefore a negative transparency is used in the exposure. In this case it is the non-radiation struck areas which are removed by development and the radiation struck areas which remain on the substrate to form the image. Other materials which behave similarly are well known, e.g. the diazo resins, chromium sensitized colloids, diazonium or azide sensitized resins or polymers bearing such groups.

Members having a radiation-sensitive layer based on such materials are known as negative-working.

Radiation-sensitive materials such as those based on orthoquinone diazides become more soluble after exposure to radiation and therefore a positive transparency is used in the exposure. In this case it is the rndiation-struck areas which are removed by development and the non-radiation struck areas which remain on the substrate to form the image. Members having such radiation-sensitive alyers are known as positive-working.

The radiation-sensitive component in the form of a dispersion or solution may be applied to the support member by dipping, spraying, roller coating, brushing, or other conventional techniques. If desired, said protective coating may be supplied over said radiation-sensitive component to protect its photosensitivity.

Conventional procedures may be employed for the purpose.

In general, the printing plates of the invention are of the subtractive type and can be developed with water or with aqueous solutions in which the ingredients other than water are harmless and/or easily biodegradable or are present in extremely small quantities. The developed image accepts ink readily. The background is hydrophilic and oleophobic.

Generally, the developers should have a pH below 9 for speedy development. The higher the pH above 9 the slower the development. If the pH value is too low the substrate may be attacked.

After development is carried out the plate is rinsed, squeezed and, if desired, dried. If necessary the plate can be corrected following the initial rinsing step and then rinsed again prior to being squeezed and dried. All of these steps are conventional and do not pertain to the critical and/or essential features of the present invention.

After development of the exposed plate the finishing treatment according to this invention is performed.

Thus, the above finisher, of Formula I, is applied to the plate, which may be wet or dry, as an aqueous solution of about 5 to about 20% wt. solids content and excess solution is removed to provide a thin layer of finisher on the developed plate. If a solution of lower solids content is used, the plate must be pre-dried before this step.

The thus treated plate is then heated at about 18000 to about 24000 for about 5 to 10 minutes to fuse the ink receptive areas of the plate.

After fusing, the plate can be proofed directly in a sheetfed machine, such as a Roland-Favorit TM B, and after a few revolutions the copies are clean. In most cases, however, the plate is rinsed, after being cooled, to remove the finisher, squeezed and treated with a normal gum such as Dextrine, gum arabic, poly(vinyl alcohol), or the like. The plate can now be put in the printing machine.

The invention will be more fully understood by reference to the following illustrative embodiments.

Example 1 20% w. aqueous solutions of compounds of Formula I were applied to freshly developed printing plates (18 x 60 cm) whose image areas comprised a solid area and several halftone areas (screen area 60 line per cm and 20 and 80% dot area).

The thus treated plates were then heated, in a fusing oven, at 240or for 5 minutes and air-cooled. These plates were then used, without further treatment, such as washing, on a Roland-Favorit TM printing machine.

The efficiency of the finishing agent in protecting the developed plate during fusion is measured by the number of copies which must be run before an acceptable copy if produced. The results of such a determination using the above solutions is set forth below in Table I.

TABLE I Compound of Formula I, M=Na Number of Copies R C12 branched 26 Clolinear 18 0ie linear (acc. to invention) 11 The above data show that the greater the chain length of linear alkyl groups, the better was the ink reception and that compounds with branched chain alkyl groups showed a considerably worse ink reception.

Example 2 A further test of ink acceptance was carried out comparing the finisher of the present invention with two commercially available finishers RCTM 99 (Kalle) and Thermos~ (Howson-Algraphy).

The compound of Formula I, according to the invention, was used as described in Example 1, while the other products were used in accordance with the manufacturers' instructions and involved rinsing or washing the plate after fusion and prior to printing. The results using a positive lithographic plate (GPTM II, Polychrome Corp.), are set forth in columns 4 and 5 of Table II below, where "dry" and "wet" refer to printing of the fused plate in the absence and presence of an aqueous fountain solution.

Thus, it can be seen that whether in the "wet" or "dry" modes there is an approximate one-third reduction in the number of copies which must be run to obtain a clean copy, when the developed plate is prepared according to the invention as compared to plates finished in accordance with the prior art.

TABLE II Run Finisher Solids % Number of Copies Until First Clean Copy GP lI(dry) GP Il(wet) Finisher according to invention 20 20 17 2 RC 99 20 28 27 3 Thermotec 20 28 30 The above results show that the present invention led to results which are superior to competitive products. Furthermore, the composition according to this invention is 100% biodegradable, and avoids the disadvantages of the known fusing gums such as blinding of the image areas and the necessity of complying with time-consuming instructions in order to obtain the desired results.

Claims (9)

1. A process in which a lithographic plate that has been exposed and developed is heated to form a fused image and in which, before the heating, the developed plate is treated with an aqueous solution of at least one water soluble compound having the formula

wherein R1 and R2 are the same or different and each is hydrogen or a linear alkyl group of 14 to 18 carbon atoms with the proviso that R1 and R2 cannot both be hydrogen; and M is an alkali metal cation.

2. A process according to claim 1 in which R is a linear C16 alkyl group and R2 is R1 or hydrogen.

3. A process according to claim 1 or claim 2 in which M is sodium.

4. A process according to any preceding claim in which the aqueous solution has a solids content of from 5 to 20% by weight.

5. A process according to any preceding claim in which the heating is conducted at a temperature of 180 to 2400C for a time sufficient to effect fusion.

6. A process according to any preceding claim in which the fused plate is used directly in a web-offset machine.

7. A process according to any of claims 1 to 5 in which the fused lithographic plate is cooled, rinsed with water, squeezed and then preserved with a normal gum selected from gum arabic, poly(vinyl alcohol) and dextrine and the plate is then used in a printing machine.

8. A process according to any preceding claim in which the lithographic plate comprises, before exposure and development, a metal support surface carrying an overlying layer of a positive or negative acting composition that is reactive to readiation.

9. An exposed and developed lithographic plate that has been treated with a solution of a compound as defined in any of claims 1 to 3.