GB2167876A - Positive working presensitised plate - Google Patents

Abstract

A positive working presensitised plate which is obtained by colouring a grained and then anodised aluminium or aluminum alloy plate with an ultraviolet light absorbing dye, and then coating the coloured plate with a positive working light-sensitive composition and which gives faithful tone reproduction, reproduces fine dots and hardly produces any film's edge image.

Description

SPECIFICATION Positive working presensitised plate This invention relates to a positive working presensitised plate from which a planographic printing plate is to be prepared.

Positive working presensitised plates (hereinafter referred to as "positive PS plates") which comprise a grained and anodised aluminium support having coated thereon a positive working light-sensitive composition are utilised in the preparation of planographic printing plates. For this purpose, they are exposed to light through a positive transparency and developed by a developer to dissolve and remove the light-sensitive composition in exposed areas, so that the hydrophilic surface of the support appears and a positive image is produced. The resulting planographic printing plate, however, generally shows dot gain when it is used in printing.To avoid this dot gain, excessive exposure has been used when preparing this printing plate to make the dots sharper than the original, or an aluminium support is used having the centreline surface roughness (Ha) of 0.6 to 1.2 microns and a surface reflectance of 50% or more as described in Japanese Patent Publication No. 59-26479. Recently, when preparing the original film, it has become possible to use a scanner to decrease the dot area in the film, and it is no longer necessary to make the dots sharper. However, with a lithfilm which is low in solid concentration, dot sharpening would result in fine dot disappearance. If the exposure value is reduced to minimise fine dot disappearance, an undesired image is formed on edge portions by light scatter and this edge image must be eliminated after development.For this reason, it has been necessary in such a situation to produce a plate which is high in dot sharpening effect.

However, this means that dot reproduction will not be very faithful and a plate having more faithful dot reproduction is now desired.

Japanese Patent Public Disclosure No. 57-118238 discloses the incorporation of a compound of the formula (I)

wherein R1, R2 and R3 each represent hydrogen atom or alkyl, alkoxy, aryl, substituted aryl, acyl, halogen, nitro or hydroxyl group, and groups R3 may be the same or different, into a lightsensitive layer used to prepare a presensitised plate having a high sensitivity and faithful tone reproduction. However, the resulting presensitised plate does not provide particularly faithful tone reproduction and has restricted development latitude.

It is also well known to produce a negative working presensitised plate (hereinafter referred to as "negative PS plate") comprising a hydrophilic support having coated thereon a negative working light-sensitive composition. This is exposed to light through a negative transparency to cure the light-sensitive composition or to increase adhesive strength between the support and the light-sensitive layer, and then developed by a developer to remove the light-sensitive composition in unexposed areas, so that the hydrophilic surface of the support appears and an oilsensitive positive image of the cured negative working light-sensitive composition is produced.

With such a negative PS plate, it is advantageous to prolong the exposure time because the exposed area of the light-sensitive layer is cured to a greater extent by a longer exposure time and consequently the bond strength between the support and the light-sensitive layer is further increased. As a result, a change in development conditions (e.g. time for development) will have little effect on the image strength, and therefore a stable image, that is, a printing plate, having high printing durability is obtained. On the other hand, a longer exposure time is disadvantageous in that halation or irradiation expands the image, and resolution is reduced, as a result of which printed matter obtained from such a printing plate possesses poor tone reproduction.It is known that these disadvantages are marked when a support of good light scattering material such as a grained aluminium plate or paper is used and such a tendency is increased when a grained and anodised aluminium plate is used.

Negative PS plates considered to be free from these disadvantages have been proposed. For example, US Patent No. 3,891,516 discloses a negative PS plate using an aluminium support having a surface of dark steel, i.e. grey, appearance which is obtained by graining an aluminium plate with a wet graining agent such as a pumice slurry, and then immediately anodising it in a sulphuric acid solution. However, this aluminium support has low light reflectance in the ultraviolet to visible part of the electromagnetic spectrum, and therefore the printed image obtained using the support is difficult to see clearly, and contrast between the oil-sensitive image areas and the exposed surface areas of the support which are produced by development is low, which makes plate examination employing elimination or correct techniques more difficult.

US Patent No. 3,458,311 discloses a negative PS plate comprising a hydrophilic support of a grained aluminium plate having thereon, as an undercoat, a dibasic acid dye on which a photopolymerisable composition is coated. This negative PS plate, comprising the negative working lightsensitive composition containing a light-sensitive diazo resin, possesses very low sensitivity and cannot be employed for practical use.

US Patent No. 3,280,734 discloses a negative PS plate comprising an aluminium support which as been anodised and then immediately coloured with an aqueous solution of Mordant Blue 69 or Mordant Yellow 59, on which a diazo resin is coated in the manner described in US patent No. 2,714,066. Furthermore, US Patent No. 4,277,555 discloses a negative PS plate comprising an aluminium support which has been anodised, treated with an aqueous solution of sodium silicate and then coloured with a basic dye, on which support a diazo resin is coated.

The purpose of colouring the support surface in these negative PS plates is to make visible the oil-sensitive image obtained by development but it does not improve the tone reproduction described earlier. This is further evidenced by the fact that the light-sensitive layer of these PS plates comprises only a diazo resin which is coated in an amount of from 10 to 30 mg/m2.

Such a thin light-sensitive layer will not cause any problem such as halation or irradiation and consequently will not cause degradation of tone reproduction. It has, however, been found that with these negative PS plates comprising a hydrophilic aluminium support coloured with a specific mordant dye or basic dye as described in the aforementioned patents, with a negative working light-sensitive composition comprising a diazo resin being coated on the support, colour staining is often produced in non-image areas or on printing plates obtained therefrom which have been stored for a long period, which causes background contamination on printed matter using such plates.

Many attempts such as described above, have been made to improve tone reproduction of negative PS plates, but were not successful.

In the field of positive PS plates, no efforts have been made to improve tone reproduction except for those described in Japanese Patent Public Disclosure No. 57-118238 and US Patent 3,891,516.

According to the present invention, there is provided a positive working presensitised plate which is obtained by colouring the surface of a grained and then anodised aluminium or aluminium alloy plate with an ultraviolet light absorbing dye and then coating the coloured surface with a positive working light-sensitive composition.

The positive working presensitised plate of this invention gives faithful tone reproduction and fine dot reproduction and also reduces colour staining (i.e. contamination in non-image areas of dye which has been added to a light-sensitive layer, adsorbed on an anodised coating and not been completely removed in the exposure and development steps), gives the high image-contrast without producing an image of the films edge and facilitates the use of elimination techniques.

Aluminium plates which can be used in this invention include pure aluminium and aluminium alloy plates. Various aluminium alloys can be used such as those containing silicon, copper, manganese, magnesium, chromium, zinc, lead, bismuth, and nickel. Generally, these alloys further contain iron and titanium and a negligible amount of other impurities.

Prior to graining, an aluminium plate can optionally be treated to remove rolling oil and to provide a clean surface. It is common practice to use, for this purpose, organic solvents such as trichloroethylene or surface active agents to remove rolling oil and alkali etching agents such as sodium or potassium hydroxide to provide a clean surface.

Mechanical, chemical or electrochemical graining methods may be used. Mechanical methods include ball graining, blast graining and brush graining wherein a plate is brushed with an aqueous dispersion such as pumice slurry using, for example, a nylon brush. Chemical methods include that disclosed in Japanese Patent Public Disclosure No. 55-31187 in which a plate is immersed in an aqueous saturated solution or an aluminium salt of a mineral acid. Electrochemical methods includes an electrolytic process using alternating current in an acidic electrolyte, for example hydrochloric acid, nitric acid or a mixture there. Of these graining methods, the preferred one is the graining method described in Japanese Patent Public Disclosure No.

55-137993 wherein a mechanical graining method and an electrochemical graining method are combined. An aluminium plate grained by this method can adsorb thereon a large quantity of dye and the bond strength between the plate and oil-sensitive image formed thereon is high.

Preferably, an aluminium plate is grained by any of the above-mentioned methods so as to obtain a centreline surface roughness (Ha) of 0.3 to 1.0 micron.

The aluminium plate thus grained is optionally washed with water and etched chemically. The etching agent used is an aqueous solution of a base or acid which dissolves aluminium. It is necessary to select an etching agent that does not produce on the etched surface a coating which is different from aluminium, which coating may be derived from aluminium or by a component of the etching solution. Examples of preferred etching agents include such basic substances as sodium hydroxide, potassium hydroxide, trisodium phosphate, disodium phos phate, tripotassium phosphate ans dipotassium phosphate; an acid etching agent such as sulphuric acid, persulphuric acid, phosphoric acid, hydrochloric acid or an acid salt thereof may also be used.Such metal salts are those of zinc, chromium, cobalt, nickel and copper which are lower in the activity series than aluminium are not suitable because they form an undesired coating on the etched surface. The concentration of the etching solution and temperature at which a plate is etched are preferably selected so that aluminium or its alloy dissolves at a rate of 0.3 to 40 g/m2.min., although a rate lower or higher than this range may also be employed.

Etching can be conducted by immersing an aluminium plate in the etching solution or coating the plate with the etching solution. Preferably, the amount of etching which occurs is in the range of 0.5 to 10 g/m2.

It is preferred to use an aqueous solution of a base because of its high etching speed. During etching, a temporary deposit is produced on the plate which is therefore usually cleaned by means of an acid such as nitric acid, sulphuric acid, phosphoric acid, chromic acid, hydrofluoric acid or hydrofluoboric acid.

The aluminium plate thus etched is optionally washed with water and then anodised in a conventional manner. More specifically, the aluminium plate is immersed in an aqueous or nonaqueous solution of for example sulphuric acid, phosphoric acid, chromic acid, oxalic acid, sulphamic acid, benzenesulphonic acid or mixtures thereof and a charge of direct to alternating current is passed therethrough to form an anodised coating on the aluminium plate surface.

Anodisation can usually be carried out under conditions of an electrolyte concentration of 1 to 80 percent by weight, a temperature of 5 to 70 C, a current density of 0.5 to 60 A/dm2, a voltage of 1 to 100 V and a duration of 30 seconds to 50 minutes, anodising conditions varying specifically depending upon the kind of electrolyte used.

One preferred anodisation method is that described in British Patent No. 1,412,768 wherein a plate is anodised at a high current density in a sulphuric acid solution; another preferred anodisation method is that described in US Patent No. 3,511,661 wherein a plate is anodised in a phosphoric acid bath.

The aluminium plate thus grained and anodised is optionally treated so as to render it hydrophilic. For example, the aluminium plate is treated by means of an aqueous solution of an alkali metal silicate such as sodium silicate as described in US Patents Nos. 2,714,066 and 3,181,461, potassium fluorozirconate as described in Japanese Patent Publication No. 36,22063 and polyvinyl phosphonic acid as described in US Patent No. 4,153,461.

The aluminium plate thus obtained is coated with an ultraviolet light absorbing dye. It is preferred that the absorption wavelength of the ultraviolet light absorbing dye is nearly the same as that of a light-sensitive compound to be used. Preferred dyes are those having an absorption wavelength of 340 to 450 nm which is the absorption wavelength of o-quinonediazides conventionally used as light sensitive compounds. Particularly preferred dyes are those having a maximum absorption wavelength of 340 to 450 nm.

These dyes are dissolved in water, organic solvents or mixed solvents of water and organic solvents and coating of the plate takes place in conventional manner, for example by dipping, roll coating or bar coating. These dyes are coated in such amount that, when o-quinonediazides are used as light sensitive compounds, the reflective optical density (Ds) at the maximum absorption wavelength of the dye within the absorption bands of o-quinonediazides becomes 0.08 to 0.4 higher than when dyes are not coated. The amount of UV light absorbing dye is preferably 30 mg/m2 or less. When the increase in Ds is lower than 0.08, tone reproduction cannot be improved, while when it is higher than 0.4, the sensitivity of the presensitised plate is greatly reduced. The reflective optical density (Ds) is determined by an integrating sphere type reflective spectrophotometer.When the amount of dyes coated is more than 30 mg/m2, the printing durability of the resulting printing plate becomes poor.

Various ultraviolet light absorbing dyes can be used in the practice of this invention. Among these, it is preferred to use monobasic acid dyes which have only one sulphonic or carboxyl group per molecule because such dyes are highly effective in reducing colour staining. Examples of such monobasic acid dyes are as follows: C.l. Acid Yellow 11

C.l. Acid Yellow 40

C.l. Acid Yellow 99 Chromium Complex

C.l. Acid Yellow 25

C.l. Acid Yellow 36

C.l. Acid Yellow 76

Compound (II)

These ultraviolet light absorbing dyes may be used alone or in combination.There may optionally be used in combination with these dyes gum arabic, carboxymethyl cellulose, carboxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, alginic acid, polyacrylic acid and its derivatives, copolymers of vinyl methyl ether and maleic anhydride, copolymers of vinyl acetate and maleic anhydride, polyvinylsulphonic acid and its salts, water soluble metal salts (e.g. zinc acetate), or fine particles of silica or titanium dioxide. There may also be included in a coating liquid comprising these dyes a pH adjustor such as an amine, amino acid, amine salt or amino acid salt.Examples of amines include ammonia, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, tripropylamine, monoethanolamine, diethanolamine, triethanolamine, allylamine, diallylamine, triallylamine, tributylamine, tertbutylamine, ethylenediamine, piperidine, pyridine, hydrazine, cyclohexylamine and aniline.

Examples of amino acids include glycine, alanine, valine, leucine, isoleucine, dihydroxyethylglycine, serine, threonine, aspartic acid, glutamic acid, lysine, arginine, phenylalanine, tyrosine, histidine, tryptophan, proline and hydroxyproline. Examples of acids with which amines or amino acids form salts include hydrochloric acid, nitric acid, sulphuric acid and acetic acid. Particularly preferred is hydrochloric acid. The pH of the coating liquid of the ultraviolet light absorbing dyes is selected so as to increase the absorbance of the dyes. The preferred pH range is 2.5 to 8.0.

When the pH is lower than 2.5, the dyes are strongly adsorbed on the anodised coating and cannot be removed, even after development, thereby leaving coloured non-image areas. When the pH is higher than 8.0, it is likely to lead not only to the decomposition of o-quinonediazides but also to the reduction of activity of a compound which forms a photolytic product which interacts with dye contained in the light-sensitive layer (explained later) to change colour tone.

It is advantageous to add such pH adjustors because they can increase the absorption of the dyes and reduce colour staining.

Finally, there is provided on the aluminium support thus treated a layer of a positive working light-sensitive composition. The positive working light-sensitive composition used in this invention preferably comprises an o-quinonediazide compound and a phenolic resin.

The o-quinonediazide compound used in this invention contains at least one o-quinonediazide group and increases its alkali solubility when irradiated with active light. There are many kinds of o-quinonediazide compounds having a variety of chemical structures. Examples of these compounds are set out in detail in J. Kosar, "Light-sensitive systems'', John Wiley & Sons, Inc. pp.

339-352. Particularly preferred are o-quinonediazide sulphonic acid esters and sulphonic acid amides obtained by reaction between the o-quinonediazide compounds and various aromatic polyhydroxy compounds or amines. Among these preferred o-quinonediazide compounds are esters formed by reaction between benzoquinone-1,2-diazidosulphonyl chloride and polyhydroxyphenyl and between naphthoquinone-1,2-diazidosulphonyl chloride and pyrogallol-acetone resin as described in Japanese Patent Publication No. 43-28403. Further examples of suitable o-quinonediazide compounds include esters between benzoquinone-1,2-diazidosulphonyl chloride or na phthoquinone-1,2-diazidosulphonyl chloride and phenolformaldehyde resin as described in US Patents Nos. 3,046,120 and 3,188,210.Other examples of o-quinonediazide compounds which may be used in this invention are described in many patents, e.g. Japanese Patent Public Disclosures Nos. 47-5303, 48-63802, 48-63803, 48-96575, 49-38701 and 48-13354, Japanese Patent Publications Nos. 41-11222, 45-9610 and 49-17481, US Patents Nos.

2,797,213, 3,454,400, 3,544,323, 3,573,917, 3,674,495 and 3,785,825, British Patents Nos.

1,227,602, 1,251,345, 1,267,005, 1,329,888 and 1,330,932 and West German Patent No.

854,890.

Phenolic resins which can be used in this invention include novolak resins and polyvinyl compounds having a phenolic hydroxyl group. Novolak resins include those obtained by polycondensation of phenols and formaldehyde in the presence of an acidic catalyst, and those modified with xylene or mesitylene. Typical examples of these novolak resins include phenol-formaldehyde resins, cresol-formaldehyde resins, p-tert-butylphenol-formaldehyde resins and phenol-modified xylene resins.

Examples of polyvinyl compounds having a phenolic hydroxyl group include hompolymers and compolymers of hydroxystyrene and homopolymers and copolymers of halogenated hydroxystyrene.

O-quinonediazide compounds are preferably used in the light-sensitive composition in an amount of from 10 to 50 percent by weight and preferably from 20 to 40 percent by weight.

Phenolic resins are preferably present in the light-sensitive composition in an amount of from 45 to 79 percent by weight and preferably from 50 to 70 percent by weight.

Preferably, the positive working light-sensitive composition used in this invention contains for the purpose of discrimination of image such dyes as Crystal Violet, Methyl Violet, Malachite Green, Fuchsin, Para Fuchsin, Victoria Blue BH (produced by HODOGAYA CHEMICAL), Victoria Pure Blue BOH (produced by HODOGAYA CHEMICAL), Oil Blue No. 603 (produced by ORIENT CHEMICAL), Oil Pink No. 312 (produced by ORIENT CHEMICAL), Oil Red 5B (produced by ORIENT CHEMICAL) and Oil Green No. 502 (produced by ORIENT CHEMICAL). These dyes can be used alone or in combination and in an amount of from 0.3 to 15 percent by weight based on the total amount of the light-sensitive composition.

There may further by contained in the light-sensitive composition a compound which can produce a photolytic compound which interacts with the aforementioned dyes to change colour tone, such as o-naphthoquinonediazido-4-sulphonylhalogenides described in Japanese Patent Public Disclosure No. 50-36209, trihalomethyl-2-pyrones and trihalomethyl triazines described in Japanese Patent Public Disclosure No. 53-36223, varous o-naphthoquinonediazide compounds described in Japanese Patent Public Disclosure No. 55-6244, an 2-trihalomethyl-5-aryi-1,3,4- oxadiazoles described in Japanese Patent Public Disclosure No. 55-77742. These compounds can be used alone or in combination and in an amount of 0.3 to 15 percent by weight of the light sensitive composition.

The light-sensitive composition used in this invention may contain acid anhydrides such as described in Japanese Patent Public Disclosure No. 52-80022 to improve the sensitivity thereof.

The light-sensitive composition may further contain various additives, for example fillers, surface active agents for improving coating properties, for example cellulose alkyl ethers and ethyleneoxides (e.g. FC-430 and FC-431 produced by 3M) and plasticisers for improving the physical properties of coatings, for example dibutyl phthalate, butyl glycolate, tricresyl phosphate and dioctyl adipate. The addition of fillers improves the physical properties of the coating and gives the light-sensitive layer a mat surface which improves vacuum contact between the presensitised plate and the original films and consequently prevents an image from becoming indistinct. Such fillers include talcum powder, glass powder, clay starch, wheat flour, corn powder and Teflon (trademark) powder.

The light-sensitive composition is dissolved or dispersed in a solvent and coated on the previously prepared plate. Examples of such solvents include ethylene dichloride, cyclohexanone, methyl ethyl ketone, methyl cellosolve acetate, propyleneglycolmonomethylether acetate, n-butyl propionate, 3,3-dimethylbutyl acetate, 2-ethoxytetrahydropyrone, ethyleneglycol mono-t-butylether, toluene, ethyl acetate, 2-heptanone and 2,4-pentanedione. These solvents can be used alone or in combination. It is necessary to select a solvent that does not redissolve the dye coated on the aluminium surface and can form a uniform halation layer. For this purpose, it is desired to select a solvent system mainly comprising esters.

The solids content of the coating liquid is preferably from 2 to 50 percent by weight. The coating liquid is typically coated in an amount of 0.5 to 3.0 g/m2, expressed as solids content.

The lower the amount coated, the greater the light-sensitivity but the lower the physical properties such as mechanical strength, chemical properties, development latitude and oil-sensitivity.

The developing agent used for the positive working light-sensitive composition of this invention may be an aqueous solution of an inorganic basic compound, for example sodium silicate, potassium silicate, sodium hydroxide, potassium hydroxide, lithium hydroxide, trisodium phosphate, disodium phosphate, triammonium phosphate, diammonium phosphate, sodium metasilicate, sodium bicarbonate or ammonium hydroxide, or an organic basic compound, for example monoethanolamine or diethanolamine. The concentration of the basic compound is preferably from 0.1 to 10 percent by weight and more preferably 0.5 to 5 percent by weight. The basic solution used may optionally contain surface active agents or organic solvents such as alcohols.

The following examples, wherein "percent" indicates percent by weight, unless otherwise indicated, illustrate this invention.

EXAMPLES I AND 2 AND COMPARATIVE EXAMPLE A An aluminium plate (1050) of 0.3 mm thickness was degreased with trichloroethylene, grained with a nylon brush and 400 mesh pumice-water suspension and washed with water. The plate was immersed in 25% sodium hydroxide solution in water at 45 C and etched for 9 seconds, washed with water, immersed in 20% nitric acid for 20 seconds and washed with water. The amount of aluminium removed by etching was about 8 g/m2. The plate was anodised in 7% sulphuric acid at a current density of 15 A/dm2 using direct current to produce an anodised coating of 3 g/m2, washed with water and dried. The plate was coated with a dye solution as described in Table 1 and dried.The plate was then coated with the following positive working light-sensitive composition liquid and dried to prepare a positive PS plate of Example 1 or 2 or Comparative Example A according to what, if any, ultraviolet dye coating liquid was used. The amount of the light-sensitive layer applied was 2.5 g/m7 after drying. The light-sensitive composition liquid had the following composition: Ester formed between naphthoquinone-1 ,2-diazido-5-sulphonylchloride and pyrogallol-acetone resin (Note 1) 0.90 g Cresol-formaldehyde resin 2.00 g t-butylphenol-formaldehyde resin (Note 2) 0.05 g Naphthoquinone- 1 ,2-diazido-4- sulphonylchloride 0.03 g Oil Blue No. 603 (produced by ORIENT CHEMICAL) 0.05 g Methyl ethyl ketone 8 9 Methyl cellosolve acetate 15 g Note 1...as described in Example 1 of US Patent No. 3,635,709 Note 2...as described in US Patent No. 4,123,279.

These positive PS plates were exposed to the light from a 3 kW metal halide lamp at a distance of 70 cm and developed with 1 to 8 diluted developer DP-4 (tradename of a developer produced by FUJI PHOTO FILM CO.LTD.) at 250C for 40 seconds using an automatic developing machine (800U produced by FUJI PHOTO FILM CO.LTD.). An appropriate exposure time was taken as the time required for step 4 of a step tablet having 0.15 density difference (produced by FUJI FILM CO.LTD.) to become clear and the time for the image of the film's edge not to appear (when step 5 of the step tablet became clear). The three plates showed the same step tablet sensitivity and had the same exposure time. Tone reproduction was evaluated by K value (line width in microns) of the strip film FOGRA-PMSK.Colour straining tests were also conducted after processing of 4 m2 of PS plate per litre of the developer. The results of the tests are also shown in Table 1.

Table 1

Formulation of UV Minimum line width reproduced Colourstaining absorbing dye coating (K value) (micron) liquid PS plate Water 50 g pH Amount of dye Optical density* Step 4 Step 5 coated cleared cleared Methanol 450 g (390 nm) After 4 m2/l (Film edge image (Film edge image treatment appeared) did not appear) Example 1 Acid yellow 6.83 10 mg/m2 0.47 12.5 15.5 0.025 25 (0.25 g) (gain of 0.16 (Slight thinning (Slight thinning over Compara- of 12 micron of 15 micron tive Example A) line) line) Example 2 Acid yellow 5.2 10 mg/m2 0.53 12 15 0.01 25 (0.25 g) (Triethanolamine- (gain of 0.22 HCl 6 mg/m2) over Compara (Triethanolamine- tive Example A) HCl (0.15 g) Comparative - - - 0.31 15 20 0.05 Example A * Autorecording Spectrophotometer Model 340 equipped with the integrating sphere R-10A (Produced by HITACHI).

Table 1 shows that the PS plates of Examples 1 and 2, in which the ultraviolet light absorbing dye, Acid Yellow 25 was provided in an undercoat, exhibit more faithful tone reproduction and reproduce fine dots (fine line: K value) better under the same exposure value, as compared with that of Comparative Example A. A line of 15 micron width could faithfully be reproduced on the PS plates of Examples 1 and 2 even when step 5 of the step tablet cleared and an image of film's edge did not appear. On the other hand, lines 15 micron in width could be reproduced in a lower exposure value under which step 4 cleared and an image of film's edge appeared.

Colour staining observed after processing of 4 m2/1 was lower in Examples 1 and 2 than in Comparative Example A, and therefore fine dots and an image of film's edge could easily be examined in Examples 1 and 2 and elimination of undesired images was facilitated.

The PS plate of Example 2 wherein triethanolamine was used to adjust the pH of the coating liquid (pH 5.2) shown higher optical density after the dye coating and showed slightly better tone reproduction and further colour staining, as compared with that of Example 1 wherein the pH of the coating liquid (pH 6.83) was not adjusted.

Printing was carried out using the three plates, to obtain excellent printing durability (100,000 copies were obtained) without staining.

EXAMPLES 3 AND 4 AND COMPARATIVE EXAMPLES B, C AND D The procedure of Example 1 was repeated except that the amount of Acid Yellow 25 was changed. The results obtained are as shown in Table 2.

Table 2

Methanol Formulation of UV absorbing Amount of Increase in Minimum ilne Reduction in Printing dye coating liquid Acid Yellow optical width repro- sensitivity durability Water 50 (g) coated density duced when (the number Methanol 450 (g) (mg/m) (390 nm) the Step of copies) (Amount of Acid Yellow (g)) 4 cleared (micron) Comparative - - 0 15 Control 100,000 Example A Comparative 0.0075 3 0.05 15 none 100,000 Example B Example 3 0.15 6 0.10 13.5 none 100,000 Example 2 0.25 10 0.16 12.5 none 100,000 Example 4 0.5 20 0.32 11 0.3 in grade 98,000 Comparative 0.7 28 0.45 10 1 in grade 95,000 Example C Comparative 0.9 36 0.58 9.5 1.5 in grade 85,000 Example D Tone reproduction was not improved in Comparative Example B where optical density was low like in Comparative Example A wherein UV absorbing dye was not applied as undercoat.

Examples 3, 2 and 4 each gave excellent results. The greater the increase in optical density, the better the tone reproduction. Example 4 showed a slight decrease in sensitivity (0.3 in grade) and printing durability (98,000 copies), which is no problem from the practical point of view.

Comparative Example C, wherein the increase in optical density was 0.45 and the dye was coated in an amount of 28 mg/m2, showed a decrease in sensitivity (1 in grade) and an acceptable lower limit of printing durability (95,000 copies). Comparative Example D wherein the dye was coated in an amount of 36 mg/m2 clearly showed a decrease in printing durability (85,000 copies).

EXAMPLE 5 AND COMPARATIVE EXAMPLES E AND F The procedure of Example 2 was repeated except that the pH of the absorbing dye-containing coating liquids was adjusted by IN-HCI to 3, 2 and 1, respectively. The results obtained are as shown in Table 3.

Table 3

Formulation of UV absorbing Colour staining dye coating liquid Minimum line width Increase in reproduced when Water 50 g optical density the Step 4 Methanol 150 g (390 nm) cleared (micron) From the From the Acid Yellow 0.25 g pH light-sensitive undercoated dye Triethanolamine layer (blue) (yellow) HCl 0.15 g Comparative not coated - 0 15 0.05 none Example A Example 2 coated 5.2 0.19 12 0.01 none Example 5 coated 3 0.20 12 0.01 none Comparative coated 2 0.22 11.8 0.01 partial yellow Example E staining Comparative coated 1 0.10 13.5 0.005 overall yellow Example F staining Table 3 shows that colour staining occurred when Acid Yellow 25 was used at pH of 2. Optical density was maximum at a pH of 2 and reduced at a pH of 1. When triethanolamine.HCl was added, the optical density remained high even at the pH of 5.2.

Table 4 shows the optical density of the coating obtained from the coating liquids of Examples 1 and 2 to which various amount of 1N-HCI had been added to adjust the pH. Table 4

HCl was added to the formulation HCl was added to the formulation of Example 1 of Example 2 pH of coating liquid Increase in Yellow density Increase in Yellow density optical density optical density (390 nm) (390 nm) 6.83 0.16 none - 5 0.165 none 0.19 none 4 0.168 none 0.195 none 3 0.20 none 0.20 none 2 0.264 partial 0.22 partial 1 0.11 overall 0.10 overall Table 4 shows that the increase in optical density in Example 1 when above pH 4 was lower than that in Example 2 wherein triethanolamine.HCI was used and the optical density did not depend on the pH change, but the optical density increased at this pH by from 3 to 2.The optical density of the coating obtained from the formulation containing triethanolamine.HCI was more stable against pH change than that obtained from the formulation not containing triethano lamine.HCI. In the formulation of Example 1, yellow staining appeared below pH 2.5, from the undercoated dye rather than colour staining from the light-sensitive layer so that it is practically impossible to use the formulation of Example 1 at below pH 2.5.

It is therefore desirable to add such a pH adjustor as triethanolamine.HCI to the UV light absorbing dye coating liquid so that high optical density is obtained, colour staining is reduced and the PS plate is stably obtained without being affected by pH change.

EXAMPLE 6, 7 AND 8 The procedure of Example 2 was repeated except that Acid Yellow 25 was replaced by the following yellow dyes. The same results as in Example 2 were obtained.

Example 6 Acid Yellow 29 Example 7 Acid Yellow 76 Example 8 Compound (II) COMPARATIVE EXAMPLE G The procedure of Example 1 was repeated except that the Acid Yellow 25 solution was replaced by Tartrazine solution in water. The increase in optical density of 400 nm was 0.16.

Tone reproduction was improved like in Example 1 but colour staining increased.

Tartrazine (C.I. Acid Yellow 23)

From the foregoing it will be seen that the positive working presensitised plate of this invention gives faithful tone reproduction, reproduces fine dots and hardly produces any image of film's edge. When a monobasic acid dye is used as an ultraviolet light absorbing dye, colour staining is reduced, high image contrast is obtained and eliminated of undesired image is facilitated.

Claims (20)

Claims:

1. A positive working presensitised plate which is obtained by colouring the surface of a grained and then anodised aluminum or aluminium alloy plate with an ultraviolet light absorbing dye and then coating the coloured surface with a positive working light-sensitive composition.

2. A positive working presensitised plate as claimed in claim 1, which has been grained to a centreline surface roughness (Ha) of 0.3 to 1.0 micron before colouring.

3. A positive working presensitised plate as claimed in claim 1 or 2, wherein aluminium has been etched to an extent of from 0.5 to 10 g/m2.

4. A positive working presensitised plate as claimed in any preceding claim, which has been treated to render it hydrophilic before colouring it.

5. A positive working presensitised plate as claimed in claim 4, which has been rendered hydrophilic by treatment with an alkali metal silicate, potassium fluorozirconate or polyvinyl phosphonic acid.

6. A positive working presensitised plate as claimed in any preceding claim, wherein the positive working light-sensitive composition comprises o-quinonediazide.

7. A positive working presensitised plate as claimed in claim 6, wherein the positive working light-sensitive composition comprises o-quinonediazide sulphonic acid ester or amide.

8. A positive working presensitised plate as claimed in claim 6, wherein the positive working light sensitive composition comprises an ester between benzoquinone-1,2-diazidesulphonyl chloride or naphthoquinone-1,2-diazidesulphonyl chloride and phenol-formaldehyde resin.

9. A positive working presensitised plate as claimed in any one of claims 6 to 8, wherein the positive working light sensitive composition comprises o-quinonediazide compound(s) in an amount of from 10 to 50% by weight.

10. A positive working presensitised plate as claimed in any one of claims 6 to 9, wherein the positive working light sensitive composition comprises a phenolic resin.

11. A positive working presensitised plate as claimed in claim 10, wherein the phenolic resin is a novolak resin or a polyvinyl compound having a phenolic hydroxyl group.

12. A positive working presensitised plate as claimed in claim 10 or 11, wherein the positive working light sensitive composition comprises the phenolic resin in an amount of from 45 to 79% by weight.

13. A positive working presensitised plate as claimed in any one of of claims 6 to 12, wherein the ultraviolet light absorbing dye is coated on the plate so that the reflective optical density (Ds) of the coloured plate becomes 0.08 to 0.4 higher at a maximum absorption wavelength in the range of 340 to 450 nm than when no dye is coated.

14. A positive working presensitised plate as claimed in claim 13, wherein the ultraviolet light absorbing dye is coated in an amount of 30 mg/m2 or less.

15. A positive working presensitised plate as claimed in claim 13 or 14, wherein the ultraviolet light absorbing dye is a monobasic acid dye.

16. A positive working presensitised plate as claimed in claim 15, wherein the monobasic acid dye is selected from C.l. Acid Yellow 11, C.l. Acid Yellow 29, C.l. Acid Yellow 40, C.l.

Acid Yellow 99 chromium complex, C.l. Acid Yellow 25, C.l. Acid Yellow 36, C.I. Acid Yellow 76 and a compound of the following formula

17. A positive working presensitised plate as claimed in any one of claims 13 to 16, wherein the ultraviolet light absorbing dye has been applied in a coating liquid which contains a pH adjustor.

18. A positive working presentised plate as claimed in claim 17, wherein the pH adjustor is selected from amines, amino acids, amine salts and amino acid salts.

19. A positive working presensitised plate as claimed in claim 17 or 18, wherein the pH of the coating liquid of the ultraviolet light absorbing dye is in the range of from 2.5 to 8.0.

20. A positive working presensitised plate substantially as described in any one of the foregoing Examples 1 to 8.