WO2002034517A1

WO2002034517A1 - Compositions comprising a pigment

Info

Publication number: WO2002034517A1
Application number: PCT/US2001/047059
Authority: WO
Inventors: Geoff Horne
Original assignee: Kodak Polychrome Graphics Company, Ltd.
Priority date: 2000-10-26
Filing date: 2001-10-22
Publication date: 2002-05-02
Also published as: WO2002034517B1; EP1333977A1

Abstract

The use of a pigment as an organic liquid resistance additive in a positive working composition provided as a solid coating on a substrate, for example a printing plate precursor, provides the coating with improved resistance to degradation by certain organic liquids including petroleum ethers, glycols and alkanols. Suitable compositions include those having the property that when provided as a coating on a substrate, regions which have been heated imagewise selectively dissolve in an aqueous developer, leaving behind regions which have not been heated. Infra-red laser radiation may be used to deliver the heat imagewise.

Description

COMPOSITIONS COMPRISING A PIGMENT

SPECIFICATION

FIELD OF THE INVENTION The present specification relates to methods of imaging articles having imagable coatings, for example to make lithographic printing plates, masks or electronic parts, such as printed circuits. The invention relates further to certain novel precursors and compositions per se.

BACKGROUND OF THE INVENTION A generally used type of lithographic printing plate precursor (by which is meant a coated printing plate prior to exposure and development) has a radiation sensitive coating applied to an aluminum substrate. A positive working precursor has a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. Only the remaining, image, area of the coating is ink-receptive.

The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing plate precursor with a vacuum to ensure good contact. The printing plate precursor is then exposed to a radiation source; conventionally this has been a UV radiation source. In the case where a positive printing plate precursor is used, the area of the film that corresponds to the image in the printing plate precursor is opaque so that no light will strike the printing plate precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed on development. In the manufacture of electronic parts such as printed circuits, after exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements, for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive working systems are widely used. In particular, in the main there have been used alkali developable positive working resists mainly composed of alkali-soluble novolac resins.

The types of electronic parts whose manufacture may use a resist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.

Imagable compositions may also be applied to plastics films in order to form masks. The required pattern is formed on the mask, which is then used as a screen in a later processing step, in forming a pattern on, for example, a printing plate or electronic part precursor.

Common to virtually all commercial applications of positive working systems employing UV radiation over several decades have been compositions comprising alkali soluble phenolic resins and naphthoquinone diazide (NQD) derivatives. The NQD derivatives have been simple NQD compounds used in admixture with resins, or NQD resin esters in which the photoactive NQD moiety has been chemically attached to the resin itself, for example by esterification of the resin with an NQD sulfonyl chloride.

The run length of many printing plates can be significantly increased by subjecting them to a heat treatment step ("baking step") after their development. However, subjecting developed plates to a baking step is not always desirable or practicable.

As demands on the performance of UV sensitive positive working coatings have increased so NQD technology has become limiting. In addition, digital and laser imaging technology is making new demands on coatings.

New positive working heat sensitive systems have been devised to meet the new demands. Such systems and methods are the subject of patent applications WO 97/39894, WO 99/01796, WO 99/01795, WO 99/08879, WO 99/21715, WO 99/21725 and WO 99/11458. Heat is delivered to the coatings described by conduction, using a heated body such as a stylus, or by charged particle radiation, or, preferably, by means of infra-red radiation, the coatings then containing suitable infra-red absorbers. Such systems are very effective but it would be desirable to improve their resistance to organic liquids. Positive working printing plate coatings often have poor resistance to chemicals used in a press room environment. For example the solvents used to clean certain inks from printing plates after initial printing may degrade the remaining coating, and make re-working and further use impossible. Certain inks and fount solutions may contain organic liquids which attack the coatings. These deficiencies are particularly marked with coatings containing novolac resins.

It is an object of embodiments of the invention to provide heat sensitive coatings with improved resistance to organic liquids, notably those used in printing processes and in PCB manufacture.

SUMMARY OF THE INVENTION In accordance with the present invention it has been found that the inclusion of pigments in heat sensitive coatings is effective in improving their resistance to organic liquids.

According to the present invention, there is provided an imagable positive working polymeric composition comprising a pigment as an organic liquid resistance additive. The composition has the property that when provided as a solid coating on a substrate, regions which have been imaged selectively dissolve in an aqueous developer, leaving behind regions which have not been imaged.

Compositions comprising the pigment exhibit improved resistance to certain organic liquids, for example (1) petroleum ethers, (2) alkane diols, for example hexane diol, other glycols, glycol ethers and (3) alkanols, including straight- chain alkanols, for example ethanol, branched alkanols, for example isopropanol, alkoxyalkanols, for example l-methoxypropan-2-ol, cycloalkanols, for example cyclohexanol, and beta-ketoalkanols, for example diacetone alcohols (ie 4-hydroxy-4- methyl-2-pentanone). A composition or coating described as resistant to organic liquids is preferably resistant to organic liquids of at least one of these classes, more preferably to organic liquids of at least two of them; and most preferably to organic liquids of all three of them (i.e., to petroleum ethers; glycols and glycol ethers; and alkanols). In preferred aspects of the invention heat is responsible for the imaging.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 depicts the effect of IR absorber on fount resistance.

DETAILED DESCRIPTION OF THE INVENTION

Suitable imagable polymeric compositions for use in the present invention include heat sensitive positive working compositions and positive working polymeric compositions which are imagable by the photolytic action of electromagnetic radiation, especially UV radiation. A preferred imagable polymeric composition for use in the present invention is a heat sensitive polymeric composition.

Suitably the pigment constitutes at least 0.25%, preferably at least 0.5%, more preferably at least 1%, most preferably at least 2%, of the total weight of the composition. Suitably the pigment constitutes up to 25%, preferably up to 20%, and most preferably up to 15%, of the total weight of the composition. There may be more than one pigment. References herein to the proportion of such compound(s) are to their total content.

In this specification weight percentages of components are expressed with reference to a solid composition. Suitable pigments include carbon black, lamp black, channel black, furnace black, graphite, iron blue, insoluble azo pigments, azo lake pigments, condensed azo pigments, chelate azo pigments, phthalocyanine-based pigments, anthraquinone-based pigments, perylene or perynone-based pigments, thioindigo- based pigments, quinacridone-based pigments, dioxazine-based pigments, isoindolinone-based pigments, quinophthalone-based pigments, vat dyeing lake pigments, azine pigments, nitroso pigments, nitro pigments, metal carbides, metal borides, metal nitrides, metal carbonitrides, and bronze structured oxides. Other suitable pigments include the pigments described in the Colour Index (CI) Handbook, "Latest Pigment Handbook" (edited by the lapan Pigment Technical Association, published 1977), "Latest Pigment Applied Technology" (CMC publications, published 1986) and "Printing Ink Technology" (CMC publications, published 1984). Especially useful pigments are carbon black, lamp black, channel black, furnace black, iron blue, metal carbides, metal borides, metal nitrides, metal carbonitrides and bronze structured oxides. The "pigments" required in the present invention are distinct from the

"dyes" proposed for use, for example, in the methods of US 4708925. Pigments are generally insoluble in the compositions and so comprise particles therein. Generally pigments are broad band absorbers, preferably able efficiently to absorb electromagnetic radiation and convert it to heat over a range of wavelengths exceeding 200 nm, preferably exceeding 400 nm. Generally they are not decomposed by the radiation. Generally they do not have any marked effect on the solubility of the unheated composition, in the developer. In contrast, dyes are generally soluble in the compositions. Generally dyes are narrow band absorbers, typically able efficiently to absorb electromagnetic radiation and convert it to heat only over a range of wavelengths not exceeding 100 nm. Consequently, dyes must be carefully selected having regard to the wavelength of the radiation which is to be used for imaging. Generally dyes are decomposed by the radiation. Frequently dyes have a marked effect on the solubility of the unheated composition in the developer, typically making it much less soluble. Preferably the polymeric composition comprises a polymer having hydroxyl groups. Preferably the polymer having hydroxyl groups is present in a greater amount by weight than said pigment, or of said pigments in total. Preferably the composition contains at least 40%, more preferably at least 50%, still more preferably at least 70%, and most preferably at least 80% of such a polymer having hydroxyl groups, by weight based on the total weight of the composition.

Particularly useful polymers having hydroxyl groups in this invention are condensation reaction products between appropriate phenols, for example phenol itself, C-alkyl substituted phenols (including cresols, xylenols, p-tert-butyl-phenol, p- phenylphenol and nonyl phenols), diphenols e.g. bisphenol-A (2,2-bis(4- hydroxyphenyl)propane), and appropriate aldehydes, for example formaldehyde, chloral, acetaldehyde and furfuraldehyde and/or ketones, for example acetone. Dependent on the preparation route for the condensation, a range of phenolic materials with varying structures and properties can be formed. Particularly useful in this invention are novolac resins, resole resins and novolac/resole resin mixtures. Most preferred are novolac resins. The type of catalyst and the molar ratio of the reactants used in the preparation of phenolic resins determines their molecular structure and therefore the physical properties of the resin. An aldehyde: phenol ratio between 0.5:1 and 1 :1, preferably 0.5:1 to 0.8:1 and an acid catalyst is used to prepare novolac resins.

Examples of suitable novolac resins have the following general structure

wherein the ratio of n:m is in the range of 1 :20 to 20:1 , and preferably 1:3 to 3: 1. In one preferred embodiment, n=m. In certain embodiments, n or m may be zero.

Other polymers suitable for inclusion in the composition, notably in admixture with a phenolic, preferably novolac, resin include: a polymer or copolymer of styrene, a polymer or copolymer of hydroxystyrene, notably of 4-hydroxystyrene or 3-methyl-4-hydroxystyrene, a polymer or copolymer of an alkoxystyrene, notably of 4-methoxystyrene, a polymer or copolymer of acrylic acid, a polymer or copolymer of methacrylic acid, a polymer or copolymer of acrylonitrile, a polymer or copolymer of acrylamide, a polymer or copolymer of vinyl alcohol, an acrylate polymer or copolymer, a polymer or copolymer of methacrylamide, a sulfonamido or imido polymer or copolymer, a polymer or copolymer of maleiimide or of alkylmaleiimide or of dialkylmaleiimide, a polymer or copolymer of maleic anhydride (including partially hydrolysed forms), a hydroxycellulose or a carboxycellulose.

Although the pigment used in the invention acts as a radiation absorbing compound, capable in preferred embodiments of converting radiation to heat, the composition may comprise one or more further radiation absorbing compounds. A large number of compounds, or combinations thereof, can be utilized as further radiation absorbing compounds in preferred embodiments of the present invention.

The further radiation absorbing compound is suitably an infra-red absorbing dye able to absorb the radiation selected for imaging and convert it to heat. Preferably the further radiation absorbing compound is one whose absorption spectrum is significant at the wavelength output of the laser which is (in preferred embodiments) to be used in the method of the present invention. Usefully it may be a dye of the squarylium, merocyanine, cyanine, phthalocyanine, indolizine, pyrylium or metal dithioline classes. Alternatively the pigment present as an organic liquid resistance additive may be the only radiation absorbing compound present.

A preferred coating for use in the present invention includes a modifying agent effective to alter the dissolution rate of the composition in a developer, in imaged regions and/or in non-imaged regions, in comparison with a corresponding coating not having the said modifying agent. Said modifying agent may be covalently bonded to said hydroxyl group-containing polymer of the composition. Alternatively it may be a compound which is not covalently bonded to said hydroxyl group-containing polymer of the composition.

The modifying agent may comprise a compound which is not covalently bonded to the polymer but which acts to inhibit the dissolution in an aqueous developer of the coating, such inhibition being reduced or entirely removed upon imaging. Such a compound is hereinafter referred to as a "reversible insolubiliser compound".

A large number of reversible insolubiliser compounds are known. Although it is possible for a reversible insolubiliser compound to be in a separate layer from the composition comprising the polymer, for example a barrier layer preventing the developer from contacting the composition, preferably it is incorporated by admixture in the composition. Suitably, in such embodiments, the reversible insolubiliser compound constitutes at least 1 %, preferably at least 2%, preferably up to 15%, more preferably up to 25% of the total weight of the composition. Thus a preferred weight range for the reversible insolubiliser compound may be expressed as 1-15% of the total weight of the composition. There may be more then one reversible insolubiliser compound. References herein to the proportion of such compound(s) are to their total content. Said reversible insolubilizer(s) may be selected from:

- functional groups as described in WO 99/01795. - separate reversible insolubilizer compounds, being diazide moieties

(in particular quinone diazide moieties) as described in WO 99/01796.

- separate reversible insolubilizer compounds, not being diazide moieties, and being as described in WO 97/39894, WO 99/08879, WO 99/11458, WO 99/21 15 and WO 99/21725. Examples described include nitrogen-containing compounds wherein at least one nitrogen atom is either quatemized or incorporated in a heterocyclic ring; or quatemized and incorporated in a heterocyclic ring. Examples of useful quartemized nitrogen containing compounds are cationic triaryl methane dyes such as Victoria Blue (CI Basic Blue 7), Crystal Violet (Gentian Violet, CI Basic Violet 3) and Ethyl Violet (CI Basic Violet 4). WO 97/39894 describes lithographic printing applications and WO 99/08879 describes electronic part applications of this technology. WO 99/21715 describes improvements to this technology brought about by use of a heat treatment carried out as part of the manufacture of articles bearing the composition. WO 99/21725 describes improvements to this technology brought about by the use of certain developer resistance aids, notably siloxane compounds. Certain compositions useful in the present invention have the property that when provided as a coating on a substrate the solubility of the coating in an aqueous developer is not substantially increased by ambient ultraviolet radiation.

Preferred compositions useful in the present invention are thermally imaged and suitably do not contain diazide moieties, especially quinonediazide moieties. However when the invention is applied using non-thermal technology, particularly UV imaging methods, the compositions may contain quinonediazide moieties.

With certain preferred compositions useful in the present invention heat imaging is believed to produce areas of the coating which have transient increased solubility in the developer. After an interval such areas may partially or wholly revert to their original, non-imaged level of solubility. Thus the mode of action of such preferred coatings does not require heat-induced lysis of the modifying means but, more likely, the break-up of a physico-chemical complex, which can reform. Consequently, in such embodiments the precursor is contacted with a developer within a time period of 20 hours or less of the exposure to imaging heat, preferably within about 120 minutes of exposure, and most preferably within 5 minutes of exposure.

An especially preferred composition for use in the present invention thus has an infra-red absorbing compound to convert infra-red radiation to heat and a said separate reversible insolubilizer compound as described in WO 97/39894 or WO 99/08879; or an infra-red absorbing compound which converts infra-red radiation to heat and which also functions as a reversible insolubilizer compound, for example a cyanine dye having both such characteristics.

Suitably the composition contains a developer resistance agent as defined in WO 99/21725, suitably a siloxane, preferably constituting l-10wt% of the composition. Preferred siloxanes are substituted by one or more optionally- substituted alkyl or phenyl groups, and most preferably are phenylalkylsiloxanes and dialkylsiloxanes. Preferred siloxanes have between 10 and 100 -Si(R')(R²)O- repeat units. The siloxanes may be copolymerised with ethylene oxide and/or propylene oxide. Preferred siloxanes are defined in WO 99/21725.

The compositions useful in the invention may contain other ingredients such as stabilising additives, inert colorants, and additional inert polymeric binders as are present in many positive working coatings.

In accordance with a second aspect of the invention, there is provided a positive working lithographic printing plate precursor, mask precursor or electronic part precursor, the precursor having a solid coating on a substrate, the coating comprising a composition as defined above.

The coating may be laid down from a liquid form of the composition, from which a solvent is removed by evaporation, to form the dried coating. Alternatively the coating may be applied to a plastics film, the film bearing the coating then being heat laminated onto the substrate. After the provision of the coating on the substrate, the resultant precursor may be subjected, as part of its manufacture, to a stabilizing heat treatment step. In a preferred embodiment, the heat treatment is carried out at a temperature of at least 40°C, preferably at least 50°C, most preferably at least 60°C. As regards the upper limit, the temperature is preferably not in excess of 90°C, more preferably not in excess of 85°C, most preferably not in excess of 75°C. In general, heat treatments in which the maximum temperature does not exceed the glass transition temperature (Tg) of the composition (as measured by differential scanning calorimetry (DSC) at a heating rate of 10°C/minute) are favored. Such heat treatments are suitably carried out on a stack of precursors or on a precursor coil, and so are efficient.

In a preferred embodiment, the heat treatment is carried out for at least 4 hours, and more preferably for at least 24 hours, and most preferably for at least 48 hours.

Preferably such a heat treatment takes place under conditions which inhibit the removal of water from the precursor, for example by wrapping the precursor (or preferably a stack or coil thereof) in a water impermeable material and/or by using humidity control. Further information on heat treatments useful in the present invention is disclosed in WO 99/21715.

The coating may contain polymeric particles in order to improve its mechanical properties. Suitably the polymeric particles constitute at least 0.25%, preferably at least 0.5%, more preferably at least 1%, yet more preferably at least 2%, most preferably at least 5%, and, especially, at least 7%, by weight of the coating.

Suitably the polymeric particles constitute up to 50%, preferably up to 40%, more preferably up to 30%, yet more preferably up to 25%, most preferably up to 20%), and, especially, up to 14%, by weight of the coating.

In this specification weight percentages are expressed with reference to the solid coating without the organic solvent.

Preferably the mean diameter of the polymeric particles is in the range 0.5-15 m, preferably 1-10 m, especially 3-7 m, as determined visually by an operator using scanning electron microscopy and a scale. Preferably the mean diameter of the polymeric particles, as thus measured, is larger than the mean thickness of the coating. Whilst not intending to be bound by any theory as to how the invention works, the presence of the particles may have a stress relieving effect and/or facilitate crack termination; and/or may protrude from the surface and are the parts contacted by objects, and thus may protect the rest of the coating from contact with objects.

An important factor is also believed to be the surface tension at the interfaces between the particles and the matrix material. Preferred particles for use in the present invention are those which are evenly dispersed in the coating, and which have relatively low critical surface tension

( _c). Critical surface tension ( _c) is discussed in Principles of Polymer Science, 3^rd edition, Ferdinand Rodriguez, ISBN 0891161767 at pages 367-370. Figures given herein are measured by the standard test described therein at 20°C. Preferably the particles are of a material which has a _c value of less than 50 mNm^"1, preferably less than 40, more preferably less than 35, and, especially, less than 25. Most preferred of all is a _c value of less than 20.

Preferably the polymeric particles are selected from optionally substituted polyolefin, polyamide and polyacrylic particles. More preferably they are selected from polyolefins and halogenated, especially fluorinated, polyolefins.

Polyethylene and polytetrafluoroethylene particles ( _c values typically about 31 and about 18.5 respectively) are especially preferred.

In accordance with a third aspect of the invention there is provided a method of manufacturing a precursor of the invention as defined herein, the method comprising applying a liquid to a substrate, the liquid comprising the composition of the invention defined herein dissolved in a solvent, and subsequently evaporating the solvent to form a solid coating.

A substrate may comprise a metal layer. Preferred metals include aluminum, zinc, copper and titanium. A substrate in embodiments of the invention intended as printing plate precursors may be arranged to be non-ink-accepting. Said substrate may have a hydrophilic surface for use in conventional lithographic printing using a fount solution or it may have an ink-repelling surface suitable for use in waterless printing. For printing applications the substrate may be aluminum which has undergone the usual graining, anodic and post-anodic treatments well known in the lithographic art for enabling a radiation sensitive composition to be coated thereon and for its surface to function as a printing background. Another substrate which may be used in the present invention in the context of lithography is a plastics material base or a treated paper base as used in the photographic industry. A particularly useful plastics material base is polyethylene terephthalate which has been subbed to render its surface hydrophilic. Also a so-called coated paper which has been corona discharge treated can be used.

Preferred printing plates have a substrate which has a hydrophilic surface and an oleophilic ink-accepting coating.

For electronic part applications the substrate may comprise a copper sheet, for example a copper/plastics laminate. After imaging and development an etching agent may be used to remove exposed metal regions, leaving, for example, a printed circuit.

For certain mask applications the substrate may be a plastics film.

Thus in preferred positive working embodiments a pattern may be obtained after pattemwise exposure and development of a precursor made by the method of the present invention. The developer solubility of the coating after it has been subjected to heat during pattemwise exposure is greater than the solubility of the corresponding unexposed coating. In preferred embodiments this solubility differential is increased by means of additional components and/or by resin modification, as described herein, and in patent applications which are referred to hereinabove. Preferably such measures reduce the solubility of the polymeric composition, prior to the pattemwise exposure. On subsequent pattemwise exposure the exposed areas of the coating are rendered more soluble in the developer, than the unexposed areas. Therefore on pattemwise exposure there is a change in the solubility differential of the unexposed coating and of the exposed coating. Thus in the exposed areas the coating is dissolved, to form the pattern.

The preferred coated precursor produced by use of the invention may in use be pattemwise heated indirectly by exposure to a short duration of high intensity radiation transmitted or reflected from the background areas of a graphic original located in contact with the recording material. The developer is dependent on the nature of the coating, but is preferably an aqueous developer. Common components of aqueous developers are surfactants, chelating agents such as salts of ethylenediamine tetraacetic acid, organic solvents such as benzyl alcohol and phenoxy ethanol, phosphates, and alkaline components such as inorganic metasilicates, hydroxides and bicarbonates, and mixtures of the foregoing.

Suitably a film-forming composition useful in the invention is inherently soluble in an alkaline developer. Suitably it may be rendered insoluble in an alkaline developer by means of one or more insolubilizer(s). Preferably, in use, provided as a coating, it is more soluble in an alkaline developer that it is in neutral liquids such as water. Certain useful coatings are substantially insoluble in neutral liquids, such as water. Preferably an aqueous developer is an alkaline developer containing one or more inorganic or organic metasilicates.

In the specification the term "developer soluble" refers to a coating that is soluble in a selected developer, to an extent useful in a practical development process. The term "developer insoluble" refers to a coating that is not soluble in the selected developer, to an extent useful in a practical development process.

In accordance with a fourth aspect of the invention there is provided a method for preparing a printing plate, mask or electronic part from a positive working precursor of the invention as defined herein, the method comprising the steps of (i) exposing the coating as described herein, imagewise; and (ii) removing the exposed regions of the coating using a developer liquid. Preferably exposure is effected by imagewise heating the coating. The heating of selected areas is preferably effected by the use of infrared electromagnetic radiation, the coating preferably containing a radiation absorbing compound as defined above, or a radiation absorbing compound being provided as a separate layer. Alternatively, charged particle radiation could be used to deliver heat. Alternatively, heat could be delivered directly, by a heated body applied to the coating or to the reverse face of the substrate. In this case no radiation absorbing compound is needed, provided, however, that a pigment is present as a solvent resistance additive.

In preferred methods the electromagnetic radiation employed for exposure is of wavelength at least 600nm, preferably at least 700nm, and more preferably at least 750nm. Most preferably it is at least 800nm. Suitably the radiation is of wavelength not more than 1350nm, preferably not more than 1300nm, more preferably not more than 1200nm, and most preferably not more than 1 150nm. The radiation may be delivered by a laser under digital control. Examples of lasers which can be used to expose coatings suitable for the method of the present invention include semiconductor diode lasers emitting at between 600nm and 1400nm, especially between 700nm and 1200nm. One example is the Nd YAG laser used in the Barco Crescent 42/T thermal image setter, which emits at 1064nm and another is the diode laser used in the Creo Trendsetter thermal image setter, which emits at 830nm, but any laser of sufficient imaging power and whose radiation is absorbed by the coating to produce heat, can be used.

In accordance with a fifth aspect of the invention there is provided an article bearing a pattern in a coating thereon, produced by the method of the fourth aspect. The article may be a mask or an electronic part but is preferably a printing plate, ready for printing. If wished such a printing plate may undergo a baking step after its chemical development for still further increased run length but this is not needed for most printing applications.

In accordance with a sixth aspect of the invention there is provided the use of a pigment as an organic liquid resistance additive in a heat imagable positive working printing plate precursor or electronic part precursor or mask precursor having a coating on a substrate, the coating comprising a polymeric material and being capable of absorbing incident radiation in the wavelength range 600-1400nm and converting it to heat, the composition having the property that when provided as a coating on a substrate, imagewise heated and subjected to an aqueous developer, regions which have been heated dissolve in the aqueous developer leaving behind regions which have not been heated; wherein the coating has improved chemical resistance in comparison with a corresponding coating not containing the pigment (that is, in which the weight proportion which would have been constituted by the pigment is instead constituted by further polymeric material of the coating). In accordance with a seventh aspect of the present invention there is provided a method of improving the chemical resistance of a heat imagable positive working coating comprising a polymeric material provided on a printing plate precursor or electronic part precursor or mask precursor, the method comprising adding a pigment as an organic liquid resistance additive in the composition which is to provide the coating, wherein the resultant coating has improved chemical resistance in comparison with a corresponding coating not containing the pigment (that is, in which the weight proportion which would have been constituted by the pigment is instead constituted by further polymeric material of the coating).

All references cited herein, including WO 97/39894, WO 99/01795, WO 99/01796, WO 99/21715, WO 99/21725, WO 99/08879 and WO 99/1 1458, are incorporated herein by reference in their entirety.

.The following examples more particularly serve to illustrate the present invention described hereinabove.

Example 1

Materials and equipment

KF 654B: an infra-red absorbing dye as supplied by Allied Signal, Middlesex

UK, believed to have the structure:

Br Θ

LB6564: a 1 :1 phenol/cresol novolak resin supplied by Bakelite, UK

LB744: a cresol novolak as supplied by Bakelite, UK

Crystal Violet: (Basic Violet 3). An insolubilizer compound as supplied by Ultra Colours and Chemicals of Cheadle Hume, Cheshire, UK, and having the structure:

Silikophen P50X: a phenyl methyl siloxane as supplied by Tego Chemie Services GmbH of Essen, Germany Dowanol PM: l-methoxy-propan-2-ol

Printex XE2: the pigment carbon black as supplied by Degussa Paris Blue: the pigment iron blue as supplied by Kremer

Creo Trendsetter 3244: a commercially available platesetter, operating at a wavelength of 830 nm, as supplied by Creo Products. Gretag Macbeth Densitometer: a commercially available densitometer as supplied by Color Data Systems Limited of the Wirral, UK Kodak Polychrome Graphics Mercury Mark V processor: a commercially available processor as supplied by Kodak Polychrome

Graphics, Leeds, UK The pigments used in this example were the Printex XE2 and Paris Blue materials, mentioned above. These materials also act as IR absorbers as well as organic liquid resistance additives, and as a consequence were compared against the dye KF 654B. The following formulations were prepared and coated onto an electrolytically grained, anodized and polyvinylphosporic acid sealed aluminium substrate.

Formulation (to coat)

* Printex XE2 is provided as a mill base, at 30% w/w ratio Printex XE2 1:4 LB6564 ** Paris Blue is provided as a mill base, at 40% w/w ratio Paris Blue 55: 45 LB6564

Plates were coated using a 6 thousandth of an inch (0.15 mm) wire bar and dried at 110°C for 90 sec. The resultant dry film weight was 2.00 g/m^*.

Plates A, B and C were then subsequently given a stabilizing heat treatment under the following conditions: Plate A 55°C 3 days

Plates B and C 70°C 3 days The reason for the higher temperature used for plates B and C was that earlier work had shown that coatings containing pigments require a slightly higher temperature than coatings containing KF 654B.

Exposure Using a Creo Trendsetter (trade mark) 3244 platesetter at 10 watt, plates were imaged with a 2% to 95% screen. Where the experimental density matched the theoretical density was taken to be the optimum exposure. The optimum imaging energies were taken to be: KF654: 200mJ/cm² Printex XE2 : 35 OmJ/cm²

Paris Blue: 250m J/cm"

Plates were developed using a Mercury Mk V processor at 750 mm min, containing developer having 14 wt% sodium metasilicate. Densities were measured with a Gretag densitometer.

Solvent Resistance Test

Plates made of according to Example 1 containing a 50% screen were immersed at 25°C in an aqueous mixture of isopropylalcohol (166g/litre) + 44g/litre of SUBSTIFIX. (SUBSTIFIX is a trade name of an alcohol replacement fount, available from Horstmann-Steinberg, of Germany). After one hour, plates were rinsed thoroughly in water, and evaluated for image attack.

A visual appraisal of image attack was given, the results of the image attack being shown in Fig. 1 , and gave the following results: Plate A: Severe image attack Plate B Minimal image attack

Plate C Minimal image attack

The addition of Printex XE2 or Paris Blue pigment gives a significant increase in alcohol/fount resistance

I S

Claims

1. An imagable positive working polymeric composition comprising a pigment as an organic liquid resistance additive.

2. The composition as claimed in claim 1, wherein the pigment is resistant to organic liquids of at least one class of compounds selected from the group consisting of petroleum ethers, alkanols, and glycols and glycol ethers.

3. The composition as claimed in claim 2, wherein the pigment is resistant to organic liquids of petroleum ethers, alkanols, and glycols and glycol ethers.

4. The composition as claimed in claim 1, wherein the pigment constitutes between 0.25% and 25% of the total weight of the composition.

5. The composition as claimed in claim 4, wherein the pigment constitutes between 2% and 15% of the total weight of the composition.

6. The composition as claimed in claim 1, wherein the pigment is at least one compound selected from the group consisting of carbon black, lamp black, channel black, furnace black, graphite and iron blue.

7. The composition as claimed in claim 1, wherein the imagable positive working composition is a heat-sensitive positive working composition.

8. The composition as claimed in claim 1, wherein the imagable positive working composition is a positive working composition imagable by the photolytic action of electromagnetic radiation.

9. The composition as claimed in claim 1, wherein the composition contains a polymer having hydroxyl groups.

10. The composition as claimed in claim 1, wherein the pigment also acts as a radiation absorbing compound, capable of converting radiation to heat.

11. The composition as claimed in claim 8, wherein the composition contains a further radiation absorbing compound in addition to the pigment.

12. The composition as claimed in claim 1, wherein the coating includes a modifying means effective to alter the dissolution rate of the composition in a developer, in non-imaged regions and/or in imaged regions, in comparison with a corresponding coating not have the said modifying means.

13. A positive working lithographic printing plate precursor or electronic part precursor, the precursor having a solid coating on a substrate, the coating comprising an imagable positive working polymeric composition comprising a pigment.

14. A method of manufacturing a precursor having a solid coating on a substrate, the coating comprising an imagable positive working polymeric composition comprising a pigment, the method comprising applying a liquid to a substrate, the liquid comprising the composition dissolved in a solvent, and subsequently evaporating the solvent to form the solid coating.

15. A method for preparing a printing plate, mask or electronic part from a positive working precursor having a solid coating on a substrate, the coating comprising an imagable positive working polymeric composition comprising a pigment, the method comprising the steps of:

(i) exposing the coating imagewise; and

(ii) removing the exposed regions of the coating using a developer liquid.

16. An article bearing a pattern in a coating thereon, produced by the method of: (i) imagewise exposing a coating comprising an imagable positive working polymeric composition comprising a pigment; and

(ii) removing the exposed regions of the coating using a developer liquid.

17. A method of improving the chemical resistance of a heat imagable positive working coating comprising a polymeric material provided on a printing plate precursor or electronic part precursor or mask precursor, the method comprising adding a pigment as an organic liquid resistance additive to the composition that provides the coating, wherein the resultant coating has improved chemical resistance in comparison with a corresponding coating not containing the pigment.