GB2277382A - Photoresist composition - Google Patents

Abstract

The photoresist composition comprises a photoresist and one or more dyestuffs which on exposure initially absorb substantially all incident radiation to which the photoresist is sensitive but as exposure progresses the dyes undergo a colour change such that the composition transmits a progressively increasing proportion of the radiation and as a result of that transmission the solubility characteristics of the photoresist are changed. Preferably, the photoresist generates an acid on irradiation and the dye(s) are indicator dye(s) which change colour. The compositions are preferably used to make printing plates.

Description

"Method and Composition" This invention relates to the production of images by reflex exposure, and is concerned more especially with the production of images on supported layers which, after suitable development, may be used inter alia as lithographic printing plates, as photographic film, as overhead projector sheets, as silks for silk screen printing or in the production of printed circuit boards.

This invention also relates to compositions for use as photosensitive layers.

It has previously been proposed to produce images by reflex exposure using a silver halide emulsion as the supported layer. Such layer compositions, however, are relatively expensive. They are also environmentally undesirable due to waste disposal problems.

The present invention provides a method of forming an image on a photo-sensitive layer by reflex exposure of an original using visible (400 to 750 nm) and/or ultraviolet incident radiation (5 to 400 nm) in which method the layer comprises a composition which includes one or more dyestuffs and is photo-sensitised to a predetermined wavelength region of the visible and/or ultra-violet spectrum, the composition also being such that, on exposure to the visible and/or ultra-violet radiation within the region to which the composition is sensitive, the layer initially transmits relatively little of the incident radiation in that region, but as the exposure progresses the composition undergoes a colour change under the influence of the directly incident radiation and the radiation reflected back from the original, the effect of the colour change being that the composition transmits a progressively increasing proportion of the radiation in the sensitised region, the effect of the increased transmission being to modify the solubility characteristics of the affected areas of the photosensitive layer, to contrast with those of the unaffected areas, and the exposure being terminated when the desired degree of contrast has been produced.

Surprisingly, the method of the invention can produce images exhibiting good contrast between the image and non-image areas, without the use of relatively expensive silver halide emulsions.

In principle in the method of the invention radiation in the sensitised wavelength region transmitted into the composition may be absorbed or may pass through the composition. After the dye has undergone the colour change there is greatly increased transmission of radiation in the sensitised region into the composition thereby leading to increased absorption by the sensitiser and a change in solubility characteristics. Radiation in the sensitised wavelength region which is transmitted through the composition will be reflected by the nonimage areas as is usual in reflex exposure and may then also be absorbed by the sensitiser.

Previously dyes and pigments have been employed in photosensitive layers as a means of checking the quality of the image formed on exposure. The present invention provides the use of a colour change of a colouring agent, in general a dyestuff, in a given environment, resulting from exposure to light in facilitating image formation in a photosensitive layer containing the colouring agent in the given environment.

If a colouring agent other than a dye is used then it should not stop the transmission of radiation of at least some wavelengths through the composition. For example, if a pigment is used it should be transparent or at least translucent.

The colour change of the dye from a colour which blocks transmission of radiation in the sensitised wavelength region to one which allows transmission of radiation in the sensitised region means that non-image areas are exposed to much higher levels of radiation in the sensitised wavelength region than image areas and therefore a good contrast may be achieved.

As the colour of the dye changes the dye allows increased transmission of radiation in the sensitised wavelength region and this leads to fast exposure.

After the colour change has occurred the dye may act as a filter and depending on its colour it may block the transmission of some or most incident radiation of wavelengths outside the sensitised wavelength region.

It is preferred that the dye is chosen to be of an appropriate colour after the colour change to act as a filter. For example, if the photosensitive layer is sensitised in the blue region then the dye would.prefer ably change to a blue colour thereby allowing the transmission and reflexion of blue light but absorbing radiation not in the blue region.

The method of the present invention allows an image to be created on a plate from an original without the need to use a camera, intermediate film negatives or positives or expensive hardware. The invention therefore also provides the use of image-wise reflex exposure of a photo-sensitive layer comprising a dyestuff which changes colour on exposure to make a lithographic plate, thereby avoiding the need for a camera, or intermediate film positives or negatives.

The present invention provides a composition for use as a photosensitive layer comprising one or more dyestuffs, which composition is photosensitised to a predetermined wavelength region of the visible (400 to 750 nm) and/or ultra violet (5 to 400 nm) spectrum, the composition also being such that, on exposure to visible and/or ultraviolet radiation within the region to which the composition is sensitive, the composition initially transmits relatively little of the incident radiation which is in the pre-determined wavelength region, but as exposure progresses the composition changes to a different colour (or to a colourless form) such that the composition transmits a progressively increasing proportion of the radiation in the pre-determined wavelength region and as a result of that increased transmission the solubility characteristics of the composition are changed.

Such a composition is of course suitable for use in a photo-sensitive layer which is exposed by reflex exposure as described in the above method. The composition may also be used in a photo-sensitive layer which is to be exposed by transmission exposure, for example, through a film positive or negative. A photo-sensitive element comprising a supported layer of a composition according to the invention may be used to make a lithographic printing plate or a film. Such layers and elements are also provided by the invention.

Preferably, the layer composition is so formulated that the effect of the colour change is to shift the transmission spectrum of the composition towards shorter wavelengths. For example, an initially red composition may be so formulated as to change to colourless or blue on irradiation, or an initially mauve composition (comprising a red dye and a blue dye) may be so formulated as to change to green.

The preferred wavelength range for the incident light will depend on the wavelength range to which the composition is sensitised and on the range in which it transmits only a relatively small proportion of the incident radiation. In general, it will be preferable for the sensitised range to be in the blue region of the spectrum (400 to 500 nm) and accordingly, while white light may be used, it will then also be preferable for the incident radiation to be in the blue (400 to 500 nm) and/or near ultra-violet (200 to 400 nm) region of the spectrum.

In principle, the layer composition may be positive-working or negative-working. In the case of a positive-working composition, the effect of the increased transmission through the layer and consequent reflection back from the original will be to increase the solubility of the affected layer areas in the developer (normally an aqueous alkaline solution), whilst in the case of a negative-working layer composition the solubility of the affected areas in the developer will be reduced. The unaffected areas of the coating will then be soluble as compared with the affected areas and may be removed by an appropriate developer. Advantageously the composition is chosen so that the developer may be water or an aqueous developer, for example, a dilute aqueous alkaline solution. This obviates the need for organic developers which often cause waste disposal problems.

In general, the layer composition may comprise a resin, a dye or suitable combination of dyes, and a photosensitive substance (such substance hereinafter referred to as a "Sensitiser").

The layer composition may contain any suitable resin base, for example, a novolak, colophony or shellac resin, or a polymer, more especially a copolymer of a vinylphenol, for example, as described in British Patent Specification No. 1 375 461.

For ease of application the layer composition may be dissolved in a solvent. Examples of solvents which may be used with the layer composition include mono- and dialkyl ethers of ethylene glycol (for example, those known by the trade names "Cellosolve" and Methyl "Oxitol"); dioxane, dimethylformamide, ethylene glycol esters, cyclohexanone, and methyl ethyl ketone; or a suitable mixture of solvents may be used.

The sensitiser may comprise a quinone diazide, more especially an o-quinone diazide or an o-naphthoquinone diazide, for example, an ester or amide of an onaphthoquinone diazide sulphonic or carboxylic acid or a halide of an o-naphthoquinone diazide-4-sulphonic acid.

As further examples of sensitisers there may be mentioned p-quinone diazides such as, for instance, the ss-naphthyl amide of a p-benzoquinone diazide sulphonic acid; pimino-quinone diazides; organic solvent-soluble condensation products of diazonium salts with formaldehyde; co-condensation products of aromatic diazonium salts with formaldehyde; aromatic diazonium salts; diazo sulphonates of aromatic and heterocyclic amines; and polymeric materials containing diazo, azido or other photo-sensitive groups.

The dye or dye combination is selected in conjunction with the sensitiser so that, on irradiation of the layer of the composition, there is initially significant inhibition of transmission in the said predetermined wavelength region and that, as the exposure continues, the desired colour change results.

Advantageously, the sensitiser is one which produces acid on irradiation, for example, a diazide or diazonium salt, and the dye or dye combination functions as an indicator which undergoes the desired colour change in the presence of the acid produced. Preferably, the dye or dye combination changes colour in the pH range of from 2.5 to 6.5.

Examples of dyes which may be used include triaryl methane dyes, azo dyes, and anthraquinone dyes, more especially basic dyestuffs from those groups. For instance, the composition may comprise one or more of the following: Crystal Violet (42 555), Crystal Violet Carbinol (42 555:1), Methyl Violet 2B (42 535), Rhodamine B (45 170), Quinaldine Red (B.D.H. Indicator dye), Acilan Brilliant Blue 5B (42 740), Victoria Blue B (44 045), Bromophenol Blue (B.D.H. indicator dye). The numbers in parentheses are the C.I. constitution number used in the five-volume work "Colour Index", 3rd edition (1971, London) for identifying the dyestuffs.

In certain cases it is advantageous for the layer composition to contain one or more acids, which may be inorganic (for example, sulphuric acid) but are preferably organic. An organic acid incorporated in the composition may be aromatic or aliphatic, and may contain one or more hydroxyl groups in addition to one or more carboxyl groups. Examples of organic acids which may be used include maleic acid, malonic acid, benzoic acid, phthalic acid, salicylic acid, fumaric acid, oxalic acid, tartaric acid and citric acid.

Advantageously, the layer composition is supported on a plastics film, for example, a polyester film. To permit the reflex exposure the support must of course be transparent or at least translucent. To obtain good reflex exposure the surface next to the coating should not be a matt surface but should be smooth and in most cases reflective.

In the methods of the present invention a supported photo-sensitive layer is generally and preferably placed with the composition directly adjacent to the original.

It is possible, however, to produce an image by the methods described when the support is placed directly adjacent to the original. That has the advantage that a right reading image is formed. In the former case a laterally reverse imaged is formed, and some applications, for example, use as an offset lithographic printing plate, require a right reading image. The gap between the original and the photo-sensitive layer caused by the thickness of the support may allow scattering of the light reflected from the original to take place thereby leading to a less clear image.

The image is also likely to be reduced in contrast with the non-image areas as compared with the case where the photo-sensitive layer is between the original and the support. In that case reflected light may be trapped between the original and the support, reflecting off each in turn until it is finally absorbed by the photosensitive layer. If the support is below the photosensitive layer then there is no layer above it to trap the light and part of the advantage of the reflex exposure process is lost.

It is therefore generally preferred that the methods are carried out with the composition directly adjacent to the original which removes the possibility of scattering and gives the best reflex exposure. If necessary a laterally reversed original may be used as described below to give a right reading image.

The thickness of the layer support may be in the range of from 0.02 to 0.3 mm, preferably from 0.1 to 0.12 mm.

Where the supported layer is to be used after exposure and appropriate developing as, for example, an overhead projector sheet then the colour of the image may be important. The dyestuff and sensitiser should in that case be chosen to give the desired colour after exposure.

In the case of a negative working image formed by reflex exposure it is the exposed areas which remain after developing and the dye should therefore be chosen to be the desired colour after it has undergone the colour change. In the case of a positive image formed by reflex exposure it is the relatively unexposed parts of the layer which remain after development and the dye should therefore be chosen to have the desired colour in its unexposed state.

Where the supported layer is to be used, after exposure and appropriate development of the resulting image, as a lithographic printing plate, the support is advantageously provided with a hydrophilic coating before the application of the layer composition. The hydrophilic pre-coating may comprise one or more inorganic materials such as, for example, titanium dioxide, silica and zinc oxide and, if desired, there may be provided an interlayer as described in British Patent Specification No. 1 205 651.

In the case of a positive-working layer composition, the relatively non-exposed areas constitute an insoluble oleophilic positive image, while the areas that are relatively highly exposed as a result of initial reflection from the original and consequent increased transmission and hence greater reflection undergo chemical change resulting in modified solubility characteristics, normally increased solubility in dilute alkali. Those areas can then be dissolved to uncover the underlying hydrophilic pre-coating. The result is a positive oleophilic image on a hydrophilic background.

In the case of a negative-working layer composition, the relatively greater exposure in the non-image areas renders the composition in those areas insoluble and hydrophobic. The relatively non-exposed areas remain soluble and can be removed by suitable development to expose the underlying hydrophilic pre-coating.

The formation of oleophilic areas is an advantage of the present invention as there is no requirement for further treatment before the plate may be used in lithographic printing. Silver compounds do not exhibit naturally oleophilic properties and when they are used in the production of lithographic printing plates further treatments are required to obtain oleophilic areas.

The quantity of layer composition applied to the support may be in the range of from 0.3 to 3.0 g/m2. For a positive-working composition, the quantity is preferably at least 2.0 g/m2, whilst for a negativeworking composition the quantity will typically be about 1 g/m2.

The duration of the exposure will depend on the thickness of the layer and of the layer support, and on the wavelength and intensity of the incident radiation.

It will be appreciated that to achieve a good image it is necessary to arrive at an exposure time which is sufficient to bring about the requisite chemical changes in the areas affected by reflection from the original as well as by direct irradiation, yet causes little or no change in the areas affected predominantly by direct irradiation alone. An exposure of about 30 seconds will be suitable in many cases.

The supported layer can serve, after suitable development, as a lithographic printing plate, as a film, as an overhead projector sheet, as a silk for silk screen printing or in the production of printed circuit boards.

When, as is preferred, the supported layer is placed with the composition directly adjacent to the original the image produced is laterally reversed with respect to the original. If the plate is required for offset lithographic printing then a right reading image is required and so the original must be reversed before exposure. One way in which that may readily be carried out is by the use of computing techniques which allow manipulation of images and text. The image once reversed may then be printed out as an original by, for example, use of a laser printer or ink- or bubble-jet printer.

The present invention therefore provides the use of computing techniques to produce a laterally reversed image from an original for use in a process using reflex exposure.

The composition may include other substances conventionally incorporated in light-sensitive layers, for example, plasticisers and surface-active agents.

The following Examples illustrate layer compositions which may be used in the method of the invention.

Example l Parts Kelrez (novolak condensation product of o-cresol and formaldehyde) 13 Methyl Oxitol 32 Salicylic acid 10 DET BF 4 (diazonium chloride sensitiser) 2 Methyl Violet 0.15 Rhodamine B (45 170) 0.05 Example 2 Crayvallac 280 26 Cellosolve 60 Benzoic Acid 10 O-Naphthoquinone diazide sensitiser 4 Crystal Violet 0.3 Rhodamine B (45 - 170) 0.1 Example 3 Cresol-based novolak resin 26 Methyl cellosolve 60 Malonic Acid 10 Sensitiser as in Example 2 4 Quinaldine Red (B.D.H. indicator dye) 0.4 Acilan Brilliant Blue 5B (42 - 740) 0.4 Example 4 Low molecular weight copolymer of styrene 26 and maleic anhydride Methyl cellosolve 50 Ethyl Acetate 10 Salicylic Acid t 10 Sensitiser as in Example 1 2 Methyl Violet 2B (42-535) 0.4 Example 5 Liacin 0 26 D.M.F. 60 Maleic Acid 10 Sensitiser as in Example 1 4 Quinaldine Red (B.D.H. indicator dye) 0.5 Victoria Blue B (44-045) 0.1 Example 6 Duraz Harz 175 24 Lytron 820 (Monsanto) 2 (Styrene/maleic anhydride copolymer) 2 Conc. Sulphuric Acid 2 Methyl Oxitol 40 Butyl Acetate 28 Sensitiser as in Example 1 4 Crystal Violet Carbinol (42-555:1) 0.2 Methyl Violet 2 B (42-535) 0.2 Example 7 SMA 2625 (Arco Chemical Co.) 16 Bakelite Harz 205 10 Maleic Acid 10 Sensitiser as in Example 1 4 Bromophenol Blue (B.D.H. indicator dye) 0.3 Crystal Violet (42-555) 0.1 A method according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing.

Referring to the drawing, the reference numeral 1 indicates an original (black outline on white background) placed on a rubber mat 2. Immediately above the original and in contact therewith is a photosensitive layer composition 3 coated on a support 4. The layer 3 may be a positive-working composition formulated, for instance, as in Example 1 above and the support 4 may comprise, for instance, a polyester such as a translucent, white polyethylene terephthalate having a hydrophilic precoating.

A glass sheet 5 keeps the supported layer composition flat and the components 1, 2, 3-4 and 5 are pressed together by suction, for example, in a vacuum frame.

The exposure will be described with reference to the layer composition of Example 1, but it will be appreciated that similar considerations apply to other compositions formulated for use according to the invention.

The dyes of Example 1 are such that the unexposed layer composition appears mauve, and the red component of the mauve initially inhibits the transmission of the blue light to which the composition is sensitised. On irradiation, a small proportion of the blue incident light passes through the coating and is reflected back up towards the coating from the non-image (white) areas of the original. Part of this reflected light may then pass through the coating and be reflected back by the support layer, this light will then reflect backwards and forwards between the original and the support until it is absorbed by the diazonium salt (the sensitiser).

The irradiated diazonium salt produces acid in the presence of which the dyes undergo chemical change and thereby a change of colour. The coating turns to a bluegreen colour and as this colour change occurs the coating transmits an increasing proportion of the blue incident light and the conversion of the composition to an alkalisoluble form proceeds relatively very rapidly as compared with the areas above the image. In those areas there is very little or no reflection from the original (black) and so very little colour change and therefore low transmission of the blue incident light.

After approximately 30 seconds, the exposure is ended, and subsequent development of the exposed layer removes the affected areas and yields a hydrophobic image on a hydrophilic background constituted by the exposed pre-coating on the support 4. As the layer composition 3 is in direct contact with the original, and is therefore between the original and the support, a laterally reversed image is formed. The resulting printing plate can therefore be used in direct lithographic printing.

If the plate is required for offset lithographic printing then a laterally reversed original would be needed and this could be achieved as discussed above.

Claims (28)

CLAIMS:

1. A method of forming an image on a photosensitive layer by reflex exposure of an original using visible and/or ultra-violet incident radiation in which method the layer comprises a composition which includes one or more dyestuffs and is photo-sensitised to a predetermined wavelength region of the visible and/or ultra-violet spectrum, the composition also being such that, on exposure to the visible and/or ultra-violet radiation within the region to which the composition is sensitive the layer initially transmits relatively little of the incident radiation in that region but as the exposure progresses the composition undergoes a colour change under the influence of the directly incident radiation and the radiation reflected back from the original, the effect of the colour change being that the composition transmits a progressively increasing proportion of the radiation in the predetermined wavelength region, the effect of the increased transmission being to modify the solubility characteristics of the affected areas of the photo-sensitive layer, to contrast with those of the unaffected areas, and the exposure being terminated when the desired degree of contrast has been produced.

2. A method as claimed in claim 1 wherein the incident radiation is in the blue region and/or the ultra-violet region of the spectrum.

3. A method as claimed in claim 1 or claim 2, wherein the photosensitive layer is supported on a plastics film.

4. A method as claimed in any one of claims 1 to 3, wherein there is a hydrophilic coating between a support and the photosensitive layer.

5. A method as claimed in any one of claims 1 to 4, wherein the layer composition is present on a support in a quantity within the range of 0.3 to 3.0 g/m2.

6. A method as claimed in any one of claims 1 to 5, wherein the original is a laterally reversed image.

7. A method as claimed in any one of claims 1 to 6, wherein the exposure time is about 30 seconds.

8. A lithographic printing plate made from a supported photosensitive layer on which an image has been formed by a method as claimed in any one of claims 1 to 7.

9. A film made from a supported photosensitive layer on which an image has been formed by a method as claimed in any one of claims 1 to 7.

10. A composition for use as a photosensitive layer comprising one or more dyestuffs which composition is photosensitised to a predetermined wavelength region of the visible and/or ultra violet spectrum, the composition also being such that, on exposure to visible and/or ultraviolet radiation within the region to which the composition is sensitive, the composition initially transmits relatively little of the incident radiation which is in the pre-determined wavelength region, but as exposure progresses the composition undergoes a colour change such that the composition transmits a progressively increasing proportion of the radiation in the pre-determined wavelength region and as a result of that increased transmission the solubility characteristics of the composition are changed.

11. A lithographic printing plate made from a supported photosensitive layer of a composition as claimed in claim 10.

12. A film made from a supported photosensitive layer of a composition as claimed in claim 10.

13. A photosensitive element comprising a supported photosensitive layer of composition as claimed in claim 10.

14. The invention as claimed in any one of claims 1 to 13, wherein the effect of the colour change is to shift the transmission spectrum of the composition towards shorter wavelengths.

15. The invention as claimed in any one of claims 1 to 14, wherein the predetermined wavelength region is in the blue region of the spectrum.

16. The invention as claimed in any one of claims 1 to 15, wherein the composition is such that after exposure the image may be developed with water or an aqueous developer.

17. The invention as claimed in any one of claims 1 to 16, wherein the composition comprises a resin base and a photosensitive substance as a sensitiser.

18. The invention as claimed in claim 17, wherein the resin base is a novolak, colophony or shellac resin or a copolymer of a vinyl phenol.

19. The invention as claimed in claim 17 or 18, wherein the photo-sensitive compound is a quinone diazide, an organic solvent-soluble condensation product of a diazonium salt with formaldehyde, a co-condensation product of an aromatic diazonium salt with formaldehyde, an aromatic diazonium salt, a diazo sulphonate of an aromatic or heterocyclic amine or a polymeric material containing diazo, azido or other photo-sensitive groups.

20. The invention as claimed in claim 19, wherein the photosensitive compound is an o-quinone diazide or an o-naphthoquinone diazide.

21. The invention as claimed in any one of claims 17 to 20, wherein the photosensitive substance produces acid on irradiation and at least one of the one or more dyestuffs acts as an indicator which undergoes the desired colour change in the presence of the acid produced.

22. The invention as claimed in claim 21, wherein the dyestuff which acts as an indicator changes colour in the pH range of 2.5 to 6.5.

23. The use of a colour change of a dyestuff, in a given environment, resulting from exposure to light in facilitating image formation in a photosensitive layer containing the dyestuff in the given environment.

24. The use of imagewise reflex exposure of a photo-sensitive layer comprising a dyestuff which changes colour on exposure to make a lithographic plate thereby avoiding the need for a camera, or intermediate film positives or negatives.

25. A method substantially as described with reference to the sole Figure herein.

26. A composition substantially as described in any one of Examples 1 to 7 herein.

27. The use of computing techniques to produce a laterally reversed image from an original for use in a process using reflex exposure.

28. Any new feature described herein or any new combination of herein described features.