GB1576008A

GB1576008A - Radiation sensitive plates

Info

Publication number: GB1576008A
Application number: GB624476A
Authority: GB
Original assignee: Vickers Ltd
Current assignee: Vickers Ltd
Priority date: 1976-02-17
Filing date: 1976-02-17
Publication date: 1980-10-01
Also published as: DE2706901A1

Description

(54) IMPROVEMENTS IN OR RELATING TO RADIATION SENSITIVE PLATES (71) We, VICKERS LIMITED, a British Company, of Vickers House, Millbank Tower, Millbank, London SW1 P4RA, 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 radiation sensitive plates and is concerned with such plates which are for use in the production of lithographic printing plates capable of printing a range of tones.

As is well known, lithographic printing plates are frequently prepared from radiation sensitive plates comprising a substrate coated with a layer of radiation-sensitive material by image-wise exposing the material to actinic radiation so that the radiation-struck areas become more or less soluble in suitable developers than the nonradiation-struck areas. Thus, in the case of the so-called positive plates the radiation-struck areas of the layer become more easily removable from the substrate than the non-radiation struck areas and in the case of the so-called negative plates, the non-radiation-struck areas of the layer remain more easily removable from the substrate than the radiation-struck areas.

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 varia-.

tions create the optical illusions 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-im posed, as occurs when reproducing multicoloured originals, moire patterns may occur. Also, a special half-tone screen has to be employed at one stage in the reproduction method, and this is not only difficult to make and hence expensive but also requires considerable skill and expenditure of time by the user.

It is also known to print half-tone using plates with fine grained surfaces without the necessity of using a screen. In this case printing is done "from the grain of the plate". 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 linear relationship between the tone values of the original transparency and the corresponding tone values of the plate or printed image.

It is an object of this invention to provide a lithographic plate which gives a better range of printed tones and an improved linear relationship, without the necessity of using the extraneous half-tones screen and which is thus devoid of the tonal distortions, patterning and moire patterns caused by the use of such a screen.

According to one aspect of the present invention there is provided a radiation-sensitive plate suitable for the production of a lithographic printing plate, which radiation-sensitive plate comprises a substrate, a layer of radiation-sensitive material on said substrate, the layer having a surface topography consisting of peaks and depressions arranged in a random manner and of varying relative heights and depths, and discrete areas of opaque screening material (as hereinafter defined) located solely in the depressions.

According to another aspect of the pre- sent invention, there is provided a method of manufacturing a radiation-sensitive plate which comprises providing a substrate with a layer of radiation-sensitive material having a surface which has a topography consisting of peaks and depressions arranged in a random manner and of varying relative heights and depths, applying opaque screening material (as hereinafter defined) on to the surface of said layer, and removing screening material from the peaks to produce a random pattern of discrete areas of screening material located solely in the depressions.

The required topography of the surface of the radiation-sensitive layer may be pro duced either by coating a grained substrate with a sufficiently thin layer of radiationsensitive material such that the grain of the substrate is partially reproduced in the surface of the radiation-sensitive layer or by.mechanically roughening a smooth surfaced radiation-sensitive layer. In the latter case, a smooth surfaced radiationsensitive layer may be produced on a grained substrate by coating the substrate with a layer of radiation-sensitive material which is of a thickness such that it completely fills the grain of the substrate. The radiation-sensitive material may be a quinone diazide based material or other positive working material.

A preferred grained substrate is electrochemically or mechanically grained aluminium, which may additionally be anodised.

By "opaque screening material" there is meant material which shields to a greater or lesser extent the sensitive material underlying the same from the affects of the radiation during subsequent exposure.

Thus, the areas of screening material may completely prevent the transmission of radiation to the underlying sensitive material or they may merely reduce the amount of radiation transmitted to the underlying sensitive material as compared to the amount of radiation reaching the sensitive material in those parts not covered by the areas of screening material. The screening material is preferably a particulate colourant such as a pigment suspended in an oleo vehicle, such as an inking-in ink.

Typically the areas of screening material on the surface of the radiation-sensitive layer, have a maximum dimension varying from,5 microns to 40 microns. The discrete areas may be produced by applying th.e pigment suspension onto the radiationsensitive layer so that pigment enters the depressions - and then removing the excess suspqnsion. . -.

In. use, the radiation-sensitive plate is image-wise. exposed to actinic radiation though a continuous tone transparency.

The image-wise exposed plate is then treated to remove the screening material and the more soluble areas of the imagewise exposed material. The tonal range may be altered by giving the plate a flash exposure after removal of the screening material but before development.

The following non-limiting Examples illustrate the invention.

Example I An electrograined and anodised aluminium foil was coated with a mixture of an orthoquinone diazide sensitiser and novolak resin to produce a presensitised, positiveworking radiation-sensitive plate. The radiation sensitive layer was sufficiently thin that the peaks and depressions of the grained foil were partially repeated in the surface of the layer. A scarlet pigment suspended in an oleo vehicle was applied to the surface of the layer and rubbed in.

The excess was wiped off to leave irregularly spaced and sized areas of pigment in the depressions of the surface of the layer.

Measurement of these areas showed a variation in maximum dimension from 5 microns up to 40 microns, with the majority having a maximum dimension in the range 10 to 20 microns. The proportion of the plate surface underlying the areas of pigment was found to be approximately 25%.

The plate was then exposed to light from a mercury halide lamp for 1 minute beneath a continuous tone positive transparency of a pictorial subject. The screening material areas were then removed with benzene and the plate was finally processed in the usual manner with aqueous alkaline developer containing trisodium phosphate at pH 125 and the non-image areas were desensitised with an aqueous solution of gum arabic.

Examination showed that the lightest tone to be reproduced as a solid corresponded to the areas on the positive transparency with a density of 14.

For comparison, a similar but unscreened plate was similarly exposed and processed and examination showed the lightest tone to be reproduced as a solid corresponded to a density of 10 on the positive transparency.

Example 2 A mechanically grained aluminium foil was coated in the same manner as Example 1. A pigment suspension, similar to that of Example 1 except that the pigment was brown, was applied in the same way.

Measurement of the areas of screening material showed that the majority of them had a maximum dimension of 20 microns.

The plate was then exposed beneath a continuous tone positive transparency - to.

light from a mercury-halide lamp for one minute. The screening material areas were removed with white spirit and the plate was then developed and desensitised as in Example 1.

Comparison of the resultant plate with a similarly processed, but unscreened plate showed that the lightest tones to be repro ducked as solids corresponded respectively to the areas of the positive having densities of 1 2 and 0 9.

Example 3 A non-anodised, electrograined, aluminium foil was coated with a radiation-sensitive layer in the same manner as Example 1 and then treated with a pigment suspension similar to that used in Example 1 except that the pigment was Vulcan 3 carbon black. The suspension was rubbed in and the excess removed to leave grey, areas of screening material in the depressions on the surface of the radiation-sensitive layer.

After exposure as in Example 2, the plate was treated with a mixture of equal parts of cyclohexane and 2,6-dimethyI-4heptanone which removed the screening material and developed the plate.

The areas of the aluminium revealed on development were finally desensitised with an aqueous solution of gum arabic.

Examination revealed that the lightest tone to be reproduced as a solid corresponded to the area of the positive having a density of 21.

Example 4 A substantially smooth, chromium surfaced, multimetallic printing plate was coated with the positive-w & rking radiationsensitive material of Example 1. When thoroughly dry, the surface of the layer was irregularly roughened by mechanical means and provided with an in situ irregular pattern of areas of pigment in the manner described in Example 1. Best results were obtained when the pigment suspension was applied to the radiation-sensitive layer using a hard surfaced roller.

The plate was exposed and processed as in Example lj and the areas of chromium revealed on development were made hydrophilic by applying a mixture of phosphoric acid and gum arabic. When the plate was used for printing on a lithographic press, a good reproduction of the original positive was obtained.

Example 5 An electrograined and anodised aluminium foil was coated with radiation-sensitive material and screening material and exposed in the same way as in Example t.

After removal of the areas of screening material with benzene, the plate was flash exposed for 3 secs. to the same mercury halide lamp and then developed with a metasilicate developer.

Examination showed that the lightest tone to be reproduced as a solid corresponded to the areas on the positive transparency with a density of 215.

Example 6 Example 1 was repeated using, as the continuous tone original, a 015 density increment step wedge. Development was effected by hand using alkaline trisodium phosphate developer. The density of the original was compared with the density of the inked image on the plate and the results are shown graphically in Figure 1 of the accompanying drawings. Curve A represents the case where no screening material was used and curve B represents the case where screening material was present in accordance with the present invention.

It can be seen that the range of tones between clear and solid when operating in accordance with the invention is 6 to 7 whereas a range of only 3 to 4 is obtained when the screening material is omitted.

Example 7 Example 1 was again repeated using a 015 density increment step wedge as the continuous tone original. In this case, however, the development was effected in a tank using a metasilicate developer. The density of the original was compared with the density of the inked image on the plate and the results are shown graphically in Figure 2 of the accompanying drawings.

Curve A' represents the case where no screening material was present and shows that the range of tones between clear and solid was only 5 to 6 whereas curve B' represents the case where screening material was present in accordance with the present invention and shows that the range of tones between clear and solid was 8 to 9.

The experiment using screening material was repeated but in this case a flash exposure was given to the plate after the screening material had been removed and before development. The densities of the original and the inked image on the plate were compared and are shown in curve C of Figure 2. The range of tones between clear and solid was 11 to 12.

WHAT WE CLAIM IS:- 1. A radiation-sensitive plate which comprises a substrate,

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The plate was then exposed beneath a continuous tone positive transparency - to. light from a mercury-halide lamp for one minute. The screening material areas were removed with white spirit and the plate was then developed and desensitised as in Example 1. Comparison of the resultant plate with a similarly processed, but unscreened plate showed that the lightest tones to be repro ducked as solids corresponded respectively to the areas of the positive having densities of 1 2 and 0 9. Example 3 A non-anodised, electrograined, aluminium foil was coated with a radiation-sensitive layer in the same manner as Example 1 and then treated with a pigment suspension similar to that used in Example 1 except that the pigment was Vulcan 3 carbon black. The suspension was rubbed in and the excess removed to leave grey, areas of screening material in the depressions on the surface of the radiation-sensitive layer. After exposure as in Example 2, the plate was treated with a mixture of equal parts of cyclohexane and 2,6-dimethyI-4heptanone which removed the screening material and developed the plate. The areas of the aluminium revealed on development were finally desensitised with an aqueous solution of gum arabic. Examination revealed that the lightest tone to be reproduced as a solid corresponded to the area of the positive having a density of 21. Example 4 A substantially smooth, chromium surfaced, multimetallic printing plate was coated with the positive-w & rking radiationsensitive material of Example 1. When thoroughly dry, the surface of the layer was irregularly roughened by mechanical means and provided with an in situ irregular pattern of areas of pigment in the manner described in Example 1. Best results were obtained when the pigment suspension was applied to the radiation-sensitive layer using a hard surfaced roller. The plate was exposed and processed as in Example lj and the areas of chromium revealed on development were made hydrophilic by applying a mixture of phosphoric acid and gum arabic. When the plate was used for printing on a lithographic press, a good reproduction of the original positive was obtained. Example 5 An electrograined and anodised aluminium foil was coated with radiation-sensitive material and screening material and exposed in the same way as in Example t. After removal of the areas of screening material with benzene, the plate was flash exposed for 3 secs. to the same mercury halide lamp and then developed with a metasilicate developer. Examination showed that the lightest tone to be reproduced as a solid corresponded to the areas on the positive transparency with a density of 215. Example 6 Example 1 was repeated using, as the continuous tone original, a 015 density increment step wedge. Development was effected by hand using alkaline trisodium phosphate developer. The density of the original was compared with the density of the inked image on the plate and the results are shown graphically in Figure 1 of the accompanying drawings. Curve A represents the case where no screening material was used and curve B represents the case where screening material was present in accordance with the present invention. It can be seen that the range of tones between clear and solid when operating in accordance with the invention is 6 to 7 whereas a range of only 3 to 4 is obtained when the screening material is omitted. Example 7 Example 1 was again repeated using a 015 density increment step wedge as the continuous tone original. In this case, however, the development was effected in a tank using a metasilicate developer. The density of the original was compared with the density of the inked image on the plate and the results are shown graphically in Figure 2 of the accompanying drawings. Curve A' represents the case where no screening material was present and shows that the range of tones between clear and solid was only 5 to 6 whereas curve B' represents the case where screening material was present in accordance with the present invention and shows that the range of tones between clear and solid was 8 to 9. The experiment using screening material was repeated but in this case a flash exposure was given to the plate after the screening material had been removed and before development. The densities of the original and the inked image on the plate were compared and are shown in curve C of Figure 2. The range of tones between clear and solid was 11 to 12. WHAT WE CLAIM IS:-

1. A radiation-sensitive plate which comprises a substrate,

a layer of radiation-sensitive material on said substrate, the layer having a surface topography consisting of peaks and depressions arranged in a random manner and of varying relative heights and depths, and discrete areas of opaque screening material (as hereinbefore defined) located solely in the depressions.

2. A radiation-sensitive plate as claimed in Claim 1 wherein the areas have a maximum dimension of from 5 to 40 microns.

3. A radiation-sensitive plate as claimed in Claim 1 or 2 wherein the substrate is a grained substrate.

4. A radiation-sensitive plate as claimed in Claim 3 wherein the substrate is an electrochemically grained aluminium substrate.

5. A radiation-sensitive plate as claimed in any one of Claims 1 to 4 wherein the radiation-sensitive material comprises a quinone diazide.

6. A radiation-sensitive plate as claimed in Claim 1 substantially as hereinbefore described in any one of the Examples.

7. A method of manufacturing a radiation-sensitive plate which comprises providing a substrate with a layer of radiationsensitive material having a surface which has a topography consisting of peaks and depressions arranged in a random manner and of varying relative heights and depths, applying opaque screening material as hereinbefore defined) onto the surface of said layer, and removing screening material from the peaks to produce a random pattern of discrete areas of screening material located solely in the depressions.

8. A method according to claim 7 wherein the substrate is a grained substrate and wherein said layer is produced by applying to the grained substrate a coating of the radiation-sensitive material having a thickness which is such that the grain of the substrate is partially reproduced in the surface of the coating.

9. A method according to Claim 7 wherein said layer is produced by forming a coating of the radiation-sensitive material on the substrate and thereafter roughening the surface of the coating.

10. A method according to Claim 7, 8 or 9 wherein said areas are provided by applying to the surface of the layer a suspension of screening material in an oleo vehicle and thereafter removing the excess suspension from the surface of the layer to leave screening material solely in the depressions.

11. A method as claimed in any one of Claims 7 to 10 wherein the screening material is a pigment.

12. A method of manufacturing a radiation-sensitive plate substantially as hereinbefore described in any one of the Examples.

13. A radiation-sensitive plate whenever produced by the process claimed in any one of Claims 7 to 12.

14. A method of producing a lithographic printing plate which comprises image-wise exposing a radiation-sensitive plate as claimed in any one of Claims 1 to 6 or 13 to a continuous tone original, and thereafter removing the areas of screening material and developing the layer.

15. A method according to Claim 14 wherein the areas of screening material are removed prior to developing the layer.

16. A method according to Claim 15 and comprising the additional step of subjecting the plate to a flash exposure after the screening material has been removed and prior to development.

17. A method of producing a lithographic printing plate substantially as hereinbefore described in any one of the Examples.

18. A lithographic printing plate whenever produced by the method claimed in any one of Claims 14 to 17.