GB1591988A

GB1591988A - Lithographic printing

Info

Publication number: GB1591988A
Application number: GB4323376A
Authority: GB
Original assignee: Vickers Ltd
Current assignee: Vickers Ltd
Priority date: 1977-12-29
Filing date: 1977-12-29
Publication date: 1981-07-01

Description

(54) IMPROVEMENTS IN OR RELATING TO LITHOGRAPHIC PRINTING (71) We, VICKERS LIMITED, a British Company of Vickers House, Millbank Tower, Millbank, London SW1P 4RA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to lithographic printing plates and concerns lithographic printing plates for use in the printing of a range of tones.

As is well known, lithographic printing plates are frequently produced from radiation sensitive plates comprising a substrate coated with a layer of radiation sensitive material which on exposure to actinic radiation becomes more or less soluble in suitable developers than the unexposed material. Thus, in the case of the so called positive-working plates the exposed portions of the layer become more easily removable from the substrate than the non-exposed portions and in the case of the so called negative-working plates, the unexposed portions of the layer remain more easily removable from the substrate than the exposed portions.

To produce prints from continuous tone originals using such radiation sensitive plates, it is the usual practice to create a photographic positive or negative master in which the tone comprises regularly spaced dots of various sizes, and to expose the radiation sensitive plate whilst it is in contact with said master, so that the image areas of the plate comprise regularly spaced dots, known as half-tone dots, which vary in size in direct relationship to the tones being matched. The dots are normally so small, however, that the presence of the individual dots is not readily distinguishable to the naked eye, but their size variations create the optical illusion of variation in tonal value.

However, this system suffers from the inherent and limiting disadvantage that the resultant regularly disposed dot image sometimes clashes with the detail and form of the subject matter and results in "patterning". Moreover, when two or more similarly disposed images are super-imposed, as occurs when reproducing multi-coloured originals, moire patterns may occur. Also, a special half-tone screen has to be employed at one stage in the reproduction method and not only is this difficult to make, and hence expensive, but also it requires considerable skill and expenditure of time by the user.

It is also known to print a limited range of tones using printing plates with fine grained surfaces without the necessity of using a screen. These are produced from radiation sensitive plates based on grained substrates by exposing the plate to a continuous tone master. In this case printing is done "from the grain of the plate". That this can be done is evident from the passage on pages 122-3 of "How to Make and Run Better Zinc Surface Plates", Lithographic Technical Foundation, Inc. 1953 where the use of the LTF Sensitivity Guide or the Kodak (registered trade mark) Number 2 Step Wedge is discussed. It is stated that ordinarily step numbers one to five or one to six will be solid black and if step 5 is the highest numbered solid black step, steps, 6, 7, 8, and maybe more will show as grey tones which get weaker and weaker the higher the number. The Guide or Wedge described obviously consists of continuous tone areas ranging from white to black and thus can be regarded as a positive continuous tone master. This process is free from the disadvantages mentioned above but has the problem that it is difficult to obtain a wide range of printed tones at the same time as ensuring a reasonably realistic linear relationship between the tone values of the original master and the corresponding tone values of the printing plate or printed image. This situation has been taken advantage of by Ruderman in U.S. Patent Specification No. 3,282,208 but so far as is known the process described did not attain any commercial success, except by the use of positive masters specially prepared with a limited tone range.

In our co-pending Application No. 6244/76 (Serial No 1576 008), now in order for acceptance, there is disclosed a manner of providing a wide range of printed tones with a suitable linear relationship by providing an in situ screen on the radiation sensitive layer in the form of random spaced areas of opaque screening material.

According to the present invention there is provided a method of manufacturing a lithographic printing plate which comprises (i) providing a radiation sensitive plate comprising a lithographically grained metal substrate coated with a positive working radiation sensitive layer comprising a naph thoquinone diazide ester and an alkali soluble resin, the ratio by weight of ester to resin being in the range 1:1 to 1:6.5; (ii) exposing the plate to actinic light through a continuous tone master, with no half-tone screen between the plate and the light source so that the layer is struck by the light; and (iii) developing the exposed layer using a developer comprising from 99.5 to 75 parts by volume of an aqueous alkaline solution and from 0.5 to 25 parts by volume of a water miscible organic solvent.

By means of the present invention it is possible to produce a lithographic printing plate which gives an improved range of printed tones and an improved linear relationship without the use of an extraneous half-tone screen or the use of specially prepared, limited tone positives.

Preferably, the substrate is grained by an electrograining process and has a surface roughness factor (as measured by gas adsorption) of from 2.5 to 12 m2 per m2. The substrate may also be anodised to increase the printing wear life of the printing plate.

The alkali-soluble resin is preferably a novolak resin and it is preferred for the diazide ester and resin to be present in a ratio by weight of from 1:3 to 1:5. The radiation sensitive layer of the plate may include additional components if desired. For example it may include a component which causes a colour change on exposure to light and thereby influences the visual contrast, a component which serves as an in situ screen as described in our Application No.6244/76 (Serial No 1576 008), or a component to increase the adhesion of the layer e.g.

of the type described in British Specification No. 1,243,963. The coating weight of the radiation sensitive layer should preferably be from 0.5 to 3g/m2.

Examples of suitable alkaline solutions are sodium metasilicate solution and trisodium phosphate solution and examples of suitable organic solvent liquids are Carbitol (registered trade mark; also known as diethylene glycol monoethyl ether or ethyl digol), 2-methoxy ethanol, 2-ethoxyethanol, and ethylene glycol.

Using conventional techniques, a density range of 0.9 can be reproduced using radiation sensitive plates of the type used in accordance with the present invention. In practice due to the nature of the ink used in printing, this range becomes 0.7 or less when printed which is not satisfactory for the reproduction of pictorial subjects. Using the method of the present invention, however, the range can be extended by 80% i.e. there can be obtained a range of 1.6 on the printing plate and a range of 1.2 on the print, which improvement in tonal range gives an adequate and satisfactory result.

If desired the printing plate may be treated in accordance with the teachings in our co-pending Application No. 31136/76 (Serial No 1584 350), now in order for acceptance, to facilitate the acceptance of ink by the lighter tone areas.

The following non-limiting Examples illustrate the invention: Example 1 An aluminium sheet was cleaned and electrograined in a bath of dilute hydrochloric acid to provide randomly located recesses with a surface roughness factor of 5.5m2 per m2 (gas adsorption). This roughened surface was then anodised in 15% w/v sulphuric acid and, after washing and drying, was coated with a mixture of 1,2-napththoquinonediazide (2)-5sulphonic acid ester of trihydroxybenzophenone and novolak resin (Alnovol 429K) to provide a positive working radiation sensitive plate. The proportions by weight of diazide and novolak in the dry coating were 1:5, respectively and the coating weight was 2.5 g/m2.

The plate was located under a 0.15 density increment continuous tone step wedge and exposed for 3 minutes to the light emitted by an Addalux mercury-halide lamp positioned at a distance of 1 metre.

Development was carried out at 700F for 3 minutes using a developer comprising 98 % by volume of a 4% w/v aqueous solution of sodium metasilicate pentahydrate and 2% by volume of diethylene glycol monoethyl ether. Processing was completed by washing with water and desensitising with an aqueous solution of gum arabic. A density range of 1.65 was reproduced as grey steps on the plate.

Example 2 Example 1 was repeated twice using different developers. Firstly, a developer comprising 98% by volume of a 0.3% w/v aqueous solution of sodium hydroxide and 2% by volume diethylene glycol monoethyl ether was used. Secondly a developer comprising 93% by volume of a 5% w/v aqueous solution of trisodium phosphate and 7% by volume of diethylene glycol monoethyl ether was used. In both cases, density ranges of 1.65 were again obtained.

Example 3 A non-anodised electrograined aluminium sheet of surface roughness factor 9m2 per m2 was coated with a positive working radiation sensitive material similar to that of Example 1 but including the diazide and the resin in a ratio by weight of 1:6.5 respectively. The coating weight was 2.5g/m2. The resultant radiation sensitive plate was exposed in the manner of Example 1. Development was carried out using a developer comprising 82 % by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 18% by volume of polyethylene glycol (molecular weight 300). After 2 minutes development the plate was washed with distilled water and dried. A thin layer of oleo ink was applied and allowed to dry.

Subsequent cleaning of the non-image areas was effected using a conventional desensitising etch solution containing ammonium alginate, and the plate was protected by a thin layer of polyacrylamide before being placed on the printing press. The density range obtained was again 1.65.

Example 4 Example 3 was repeated using the following solution, as developer. A similar result was obtained sodium metasilicate pentahydrate 3.0%w/v sodium hydroxide 0.7 Sow/v sodium benzoate 2.2%w/v Cibachrome Water Red Dye trace (to colour) Polyethylene glycol 300 10% by volume Distilled water to 100% Example 5 Example 1 was repeated twice except that the ratio of quinone diazide ester:novolak resin in the radiation sensitive layer was 1:3 and 1:4 respectively. Also, the exposure time was increased. Similar results were obtained.

Example 6 Example 1 was repeated except that the substrate of the radiation sensitive plate was a fine grained aluminium sheet (surface roughness factor 9m2 per m2) which had been anodised in phosphoric acid and that the coating weight was 0.6g/m2. A density range of 1.65 was obtained.

Example 7 Example 4 was repeated except that the substrate of the radiation sensitive plate was an aluminium sheet that had been brush grained (surface roughness factor 3m2 per m2) but not anodised. A density range of 1.5 was obtained.

Example 8 Nine radiation sensitive plates were prepared and exposed as in Example 1. They were then developed for three minutes at 750F using different developers. In each case the developer included a 4% w/v aqueous solution of sodium metasilicate pentahydrate as aqueous alkaline solution. In all cases except that of Plate 1 a water soluble organic solvent was additionally present. The results obtained and the constitution of the various developers used are shown in the following Table.

Plate No. % by vol. of Organic solvent and % by Density aqueous vol thereof Range alkaline solution 1 100% None 1.05 2 96% polyethylene glycol:4% 1. 8 3 96% diethylene glycol 1.65 monoethyl ether: 4% 4 96% 2-ethoxyethanol: 4% 1. 5 5 96% diethylene glycol: 4% 1. 5 6 90% iso-propyl alcohol: 10% 1.65 7 96% isobutyl alcohol : 4% 1. 5 8 96% dimethyl formamide: 4% 1. 8 9 95% ethanol: 5% 1.65 Example 9 Example 1 was repeated using different developers.

Stock "A" solution comprised 80% by volume of a 4% w/v aqueous solution of sodium metasilicate pentahydrate and 20% by volume of polyethylene glycol (M.wt. 300).

Stock "B" solution comprised a mixture of an 8% w/v aqueous solution of sodium metasilicate pentahydrate, a 2% w/v aqueous solution of disodium hydrogen phosphate, and 0.2% w/v aqueous solution of tetrasodium ethylene diamine tetra acetic acid. Various combinations of these stock solutions were used as developer. A developer made up by taking 2 parts by volume of "A" and mixing it with 1 part by volume of "B" produced a density range on the plate of 1.3. By using developers comprising different ratios of "A" and "B", the contrast and rendering of the plate could be adjusted as required. For example, by using 100% of stock "B" as developer, a "contrasting" printing plate with a density range of 0.9 was obtained, which when overexposed to eliminate the lighter tones of the positive was suitable for the skeleton black printer of a duo-tone reproduction; by using 100% of the "A" solution as developer a long tonal range printing plate (density 1.95) was obtained from the same positive suitable for use as the fine-tone grey printer of the duo-tone reproduction.

By varying the proportions of solutions "A" and "B" as well as by adjusting the exposure time and altering the processing temperature and duration, an extremely wide range of reproductions of a continuous tone transparency could be obtained.

Example 10 A novolak resin was esterified to the extent of the replacement of 35% of its hydroxyl groups using 1,2-naphthoquinone diazide(2)-4-sulphonic acid. 7g of the ester and 13g of novolak were then admixed together and coated on to an aluminium sheet grained and anodised as in Example 1. The resultant radiation sensitive plate was then exposed as in Example 1 and developed using a developer comprising 89% by volume of a 4%w/v aqueous solution of sodium metasilicate pentahydrate and 11 % by volume of diethylene glycol. The resultant printing plate had a density range of 1.65.

Example 11 Example 1 was repeated with the additional step of treating the developed plate according to our GB patent No. 1 513 368 including heating the plate for 10 minutes at 230"C.

The plate was found to produce more than 60,000 satisfactory copies under the same conditions as in Example 1 where the plate obtained gave 30,000 copies before any deterioration could be detected.

Example 12 Example 1 was repeated except that before exposure the radiation sensitive layer was covered with a thin layer of orange-coloured oleo varnish the excess of which was burnished away with a soft cloth to leave random areas of orange screening material located in depressions in the surface of the radiation sensitive layer. Also the exposure was carried out for 5 minutes and the orange screening material was removed with white spirit before development. A plate having a density range of 1.95 was obtained.

Example 13 Example 12 was repeated except that the plate was given a flash exposure of 4 seconds after the screening material had been removed and before development. A plate having a density range of 2.25 was obtained.

Example 14 Example 3 was repeated except that before exposure the radiation sensitive layer of the plate was covered with a brown coloured oleo ink. The excess ink was then burnished away with a soft cloth so as to leave areas of brown screening material located in depressions in the surface of the layer. Also the exposure was carried out for 4+ minutes and the brown screening material was removed with petroleum spirit before development. A plate having a density range of 1.95 was obtained.

Example 15 Example 14 was repeated except that the plate was flash exposed for 4 seconds after the screening material had been used and before development. A plate having a density range of 2.1 was obtained.

Example 16 Five electrograined and anodised aluminium foils were each coated with a mixture of a 1,2-naphthoquinone diazide-(2)-5-sulphonic acid ester of trihydroxy benzophenone and a novolak resin (Alnovol 429K) to form five presensitised positive-working radiation sensitive plates designated A, B, C, D and E. The ratio by weight of ester to resin in each dry coating was 1:5 and the roughened surface of the foil of each plate was reproduced in the free surface of the radiation sensitive coating.

Plate A was exposed for 1 i minutes to light from a mercury halide lamp under a 0.15 density increment continuous tone step wedge and developed in a 6%w/v aqueous solution of sodium metasilicate pentahydrate.

Plate B was provided with screening material on its surface by rubbing into the surface a scarlet pigment suspended in an oleo vehicle. The excess suspension was then wiped off to leave irregularly spaced and sized opaque areas of pigment solely in the depressions of the surface. The plate was exposed in the manner of Plate A but for 4 minutes, cleaned of screening material with benzene, and then developed as Plate A.

Plate C was provided with screening material, exposed and cleaned in the manner of Plate B but was developed with a solution comprising 98 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 2 parts by volume of diethylene glycol monoethyl ether.

Plate D was treated in the same manner as Plate B but was given an overall flash exposure of 4 seconds before being developed.

Plate E was treated in the same manner as Plate C but was given an overall flash exposure of 4 seconds before development. All five plates were washed with water, desensitised with gum arabic and inked-up. The density of the step-wedge was compared with the density of the inked images on the plates and the results are shown graphically in the single Figure of the accompanying drawing, which is a graph showing the variation of the density of the inked image as a function of the step number of the step wedge.

It can be seen that by carrying out the method of the present invention in combination with providing an in situ screen and a flash exposure, the density range reproduced as grey steps on the plate can be extended as far as the 2.25 of Plate E.

Examples 17 to 23 The procedure of Example 16 was repeated but using the following alternative developers instead of the developer comprising sodium metasilicate pentahydrate and diethylene glycol monoethyl ether which was used on plates C and E.

Example 17: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume ethylene glycol.

Example 18: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume 2-ethoxy ethanol.

Example 19: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume diethylene glycol.

Example 20: 90 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 10 parts by volume isopropyl alcohol.

Example 21: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume isobutyl alcohol.

Example 22: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume dimethyl formamide.

Example 23: 95 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 5 parts by volume ethanol.

Results substantially similar to those of Example 16 were obtained.

Example 24 The nine lithographic printing plates produced in Example 8 were placed on a Heidelberger Kord single colour offset press equipped with normal coated paper (white), good quality quick setting black ink and a fountain solution consisting of a 1 % aqueous solution of acidified gum arabic. Fifty revolutions of the press were necessary before the ink acceptance of the plate was adequate and then 25,000 satisfactory copies were produced without deterioration. On examination of the copies Plate 1 was found to have reproduced a density range of 0.8 whereas plate 2 was found to have reproduced a density range of 1.5. The other seven plates gave corresponding results.

Example 25 The lithographic printing plate produced in Example 10 was placed on a Solna offset press equipped with the same ink and paper as in Example 24. The fountain solution consisted of a 1% aqueous solution of "Hydamp" supplied by the Howson-Algraphy Group of Vickers Ltd.

Some 50 revolutions of the press were needed before the ink acceptance of the plate was satisfactory and then 25,000 copies were produced without deterioration. Examination of the copies showed the plate to have reproduced a density range of 1.4.

Example 26 The lithographic printing plates produced in Example 16 were placed on a Heidelberger Kord press under the same conditions as Example 24. Similar results were obtained and examination of the copies showed that the density range reproduced was extended from 0.5 (plate A) to 2.0 (plate E).

WHAT WE CLAIM IS: 1. A method of manufacturing a lithographic printing plate which comprises (i) providing a radiation sensitive plate comprising a lithographically grained metal substrate coated with a positive working radiation sensitive layer comprising a naphthoquinone diazide ester and an alkali soluble resin, the ratio by weight of ester to resin being in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Plate B was provided with screening material on its surface by rubbing into the surface a scarlet pigment suspended in an oleo vehicle. The excess suspension was then wiped off to leave irregularly spaced and sized opaque areas of pigment solely in the depressions of the surface. The plate was exposed in the manner of Plate A but for 4 minutes, cleaned of screening material with benzene, and then developed as Plate A. Plate C was provided with screening material, exposed and cleaned in the manner of Plate B but was developed with a solution comprising 98 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 2 parts by volume of diethylene glycol monoethyl ether. Plate D was treated in the same manner as Plate B but was given an overall flash exposure of 4 seconds before being developed. Plate E was treated in the same manner as Plate C but was given an overall flash exposure of 4 seconds before development. All five plates were washed with water, desensitised with gum arabic and inked-up. The density of the step-wedge was compared with the density of the inked images on the plates and the results are shown graphically in the single Figure of the accompanying drawing, which is a graph showing the variation of the density of the inked image as a function of the step number of the step wedge. It can be seen that by carrying out the method of the present invention in combination with providing an in situ screen and a flash exposure, the density range reproduced as grey steps on the plate can be extended as far as the 2.25 of Plate E. Examples 17 to 23 The procedure of Example 16 was repeated but using the following alternative developers instead of the developer comprising sodium metasilicate pentahydrate and diethylene glycol monoethyl ether which was used on plates C and E. Example 17: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume ethylene glycol. Example 18: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume 2-ethoxy ethanol. Example 19: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume diethylene glycol. Example 20: 90 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 10 parts by volume isopropyl alcohol. Example 21: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume isobutyl alcohol. Example 22: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume dimethyl formamide. Example 23: 95 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 5 parts by volume ethanol. Results substantially similar to those of Example 16 were obtained. Example 24 The nine lithographic printing plates produced in Example 8 were placed on a Heidelberger Kord single colour offset press equipped with normal coated paper (white), good quality quick setting black ink and a fountain solution consisting of a 1 % aqueous solution of acidified gum arabic. Fifty revolutions of the press were necessary before the ink acceptance of the plate was adequate and then 25,000 satisfactory copies were produced without deterioration. On examination of the copies Plate 1 was found to have reproduced a density range of 0.8 whereas plate 2 was found to have reproduced a density range of 1.5. The other seven plates gave corresponding results. Example 25 The lithographic printing plate produced in Example 10 was placed on a Solna offset press equipped with the same ink and paper as in Example 24. The fountain solution consisted of a 1% aqueous solution of "Hydamp" supplied by the Howson-Algraphy Group of Vickers Ltd. Some 50 revolutions of the press were needed before the ink acceptance of the plate was satisfactory and then 25,000 copies were produced without deterioration. Examination of the copies showed the plate to have reproduced a density range of 1.4. Example 26 The lithographic printing plates produced in Example 16 were placed on a Heidelberger Kord press under the same conditions as Example 24. Similar results were obtained and examination of the copies showed that the density range reproduced was extended from 0.5 (plate A) to 2.0 (plate E). WHAT WE CLAIM IS:

1. A method of manufacturing a lithographic printing plate which comprises (i) providing a radiation sensitive plate comprising a lithographically grained metal substrate coated with a positive working radiation sensitive layer comprising a naphthoquinone diazide ester and an alkali soluble resin, the ratio by weight of ester to resin being in

the range 1:1 to 1:6.5; (ii) exposing the plate to actinic light through a continuous tone master, with no half-tone screen between the plate and the light source, so that the layer is struck by the light; and (iii) developing the exposed layer using a developer comprising from 99.5 to 75 parts by volume of an aqueous alkaline solution and from 0.5 to 25 parts by volume of a water miscible organic solvent.

2. A method according to Claim 1, wherein the aqueous alkaline solution is an aqueous sodium metasilicate solution.

3. A method according to Claim 1, wherein the aqueous alkaline solution is an aqueous sodium hydroxide solution or an aqueous trisodium phosphate solution.

4. A method according to Claim 1, 2 or 3, wherein the organic solvent is 2-methoxy ethanol, 2-ethoxy ethanol, or ethylene glycol.

5. A method according to Claim 1, 2 or 3, wherein the organic solvent is diethylene glycol monoethyl ether, polyethylene glycol, diethylene glycol, iso-propyl alcohol, isobutyl alcohol, dimethyl formamide, or ethanol.

6. A method according to any preceding claim wherein the substrate has a surface roughness factor of from 2.5 to 12 m2 per m (gas adsorption).

7. A method according to any preceding claim wherein the substrate is an anodised substrate.

8. A method according to any one of the preceding claims wherein the alkali soluble resin is a novolak resin.

9. A method according to any one of the preceding claims wherein the ester is the 1,2-naphthoquinone diazide -(2)-5-sulphonic acid ester of trihydroxybenzophenone.

10. A method according to any one of the preceding claims wherein the ratio by weight of ester to resin is from 1:3 to 1:5.

11. A method according to any one of the preceding claims wherein the coating weight of the layer is from 0.5 to 3g/m2.

12. A method according to any one of the preceding claims wherein the plate comprises in situ random spaced areas of opaque screening material on the surface of the layer such that the layer is struck by light which has passed through the screening material.

13. A method according to claim 12 and comprising the additional steps of removing the screening material after exposing the plate through the continuous tone master and subject- ing the layer to a flash exposure prior to developing the layer.

14. A method of manufacturing a lithographic printing plate in accordance with claim 1 substantially as hereinbefore described in Examples 1, 3, 4, 9 or 11.

15. A method of manufacturing a lithographic printing plate in accordance with Claim 1 substantially as hereinbefore described in any one of Examples 2, 5 to 8, 10 and 12 to 23.

16. A lithographic printing plate whenever manufactured by the method claimed in any preceding claim.

17. A method of lithographic printing which comprises obtaining a lithographic printing plate as claimed in claim 16, applying water and ink to the lithographic printing plate, and thereafter contacting the lithographic printing plate with a receptor sheet to transfer ink from the lithographic printing plate to the sheet.

18. A method according to claim 17 substantially as described in any one of Examples 24 to 26.

19. A printed sheet whenever produced by the method claimed in claim 17 or 18.