CA1152797A - Method of manufacturing a lithographic printing plate by exposing a naphthoquinone diazide ester composition through a continuous tone master - Google Patents

Abstract

"Improvements in or relating to lithographic printing"

Abstract of the Disclosure In a method of manufacturing a lithographic printing plate a radiation sensitive plate comprising a grained substrate coated with a positive working radiation sensitive layer is exposed to actinic light through a continuous tone master, with no half tone screen between the plate and the light source. The layer comprises a naphthoquinone diazide ester and an alkali soluble resin in specified ratio, and the exposed plate is developed using a developer comprising an aqueous alkaline solution and a water miscible organic solvent, again in specified ratio. Plates produced by the method have an improved tonal range; the density range can be further extended by including in situ random spaced areas of opaque screening material on the surface of the layer such that the layer is struck by light which has passed through the screening material, optionally with a flash exposure before development.

Description

~152797 This invention rel~tes to a metl~od oE manu~acturing li~hographic printing plates and concerns methods of manufact-uring such plates for use in the printing of a range o~ tones As is well known, lithographic printing plates are frequently produced from radiation sensitive plates comprising a substrate coated with a layer of radiation sensitive material which on exposure to actinic radiation becomes more or less soluble in suitable developers than the unexposed material.
Thus, in the case of the so called positive-working plates the exposed portions of the layer become more easily removable from the substrate than the non-exposed portions and in the czse of the so called negative-working plates, the unexposed portions of the la~er remain more easily removable from the substrate than the exposed portions.
To produce prints from continuous tone originals using such radiation sensitive plates, it is the usual practice to create a photographic positive or negative master in which the tone comprises regularly spaced dots of various sizes, and to expose the radiation sensitive plate whilst it is in contact with said master, so that the image areas of the plate comprise regularly spaced dots, known as half-tone dots, which vary in size in direct relationship to the tones being matched. The dots are normally so small, however, that the presence of the individual dots is not readily distinguishable 25 to the naked eye, but their size variations create the optical - illusion of variation in tonal value.
However, this system suffers from the inherent and limiting diaadvantage that the resultant re~ularly disposed dot ''~

~1 ~52797 image sometimes clashes with the detail and form of the subject matter and results in "patterning". Moreover, when two or more similarly disposed images are super-imposed, as occurs when reproducing multi-coloured originals, moiré patterns may occur. Also, a special half-tone screen has to be employed at one stage in the reproduction method and not only is this difficult to make, and hence expensive, but also it requires considerable skill and expenditure of time by the user.
It is also known to print a limited range of tones using printing plates with fine grained surfaces without the necessity of using a screen. These are produced from radiation sensitive plates based on grained substrates by exposing the plate to a continuous tone master. In this case printing is done "from the grain of the plate". That this can be done is evidènt from the passage on pages 122-3 of "How to Make and Run Better Zinc Surface Plates", Lithographic Technical Foundation, Inc. 1953 where the use of the LTF Sensitivity Guide or the Kodak (registered trade mark) Number 2 Step Wedge is discussed. It is stated that ordinarily step numbers 1 to 5 or 1 to 6 will be solid black and if step 5 is the highest numbered solid black step, steps 6, 7, 8 and maybe more will show as grey tones which get weaker and weaker the higher the number. The Guide or Wedge described obviously consists of continuous tone areas ranging from white to black and thus can be regarded as a positive continuous tone master. This process is free from the disadvantages mentioned above but has the problem that it is difficult to obtain a wide range of printed tones at the same time as ensuring a reasonably realistic linear relationship between the tone values of the original master and the corresponding tone values of the printing plate or printed image. This situation has been taken advantage of by Ruderman in U.S. Patent Specification No. 3,282,208 but so far as is known the process described did not attain any commercial ' .
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;2797 success, except by the use of positive masters specially prepared with a limited tone range.
In our British patent 1,576,008, there is disclosed a manner of providing a wide range of printed tones with a suitable linear relationship by providing - an in situ screen on the radiation sensitive layer in the form of random spaced areas of opaque screening material.
According to the present invention there is - lO provided a method of manufacturing a lithographic printing plate which comprises:
(i) providing a radiation sensitive plate comprising a grained metal substrate coated with a positive working radiation sensitive layer comprising a naphthoquinone diazide ester and an alkali soluble resin, the ratio by weight of ester to resin being in the range l:l to 1:6.5;
(ii) exposing the plate to actinic light through a continuous tone master, with no half-tone screen between the plate and the light source, so that the layer is struck by the light;

115;2'797 and (iii) developing the exposed layer using a developer comprising from 99.5 to 75 p~rts by volume of an aqueous alkaline solution and from 0.5 to 25 parts by'volume of a water miscible organic solvent.
By means of the present invention it is possible to produce a lithographic printing plate which gives an improved range of printed tones and an improved linear relationship without the use of an extraneous half-tone screen or the use of specially prepared, limited tone positives.
Preferably, the substrate is grained by an electrograining process and has a surface roughness factor (as measured by gas adsorption) of from 2.5 to 12 m2 per m2.
The substrate may also be anodised to increase the printing wear life of the printing plate.
The alkali-soluble resin is preferably a novolak resin, and the ester is preferably the 1,2-naphthoquinone diazide-(2)-5-sulphonic acid ester of trihydroxybenzophenone.
It is preferred for the diazide ester and resin to be present in a ratio by weight of from 1:3 to 1:5. The coating weight of the radiation sensitive layer is preferably from 0.5 to 3 g/m .
The radiation sensitive layer of the plate may include additional components if desired. For example it may include a component which causes a colour change on exposure to light and thereby influences the visual contrast pr a component to increase the adhesion of the layer e.g.
of the type described in British Specification No.1,243,963.

~ ~i;2797 The plate may also comprise random spaced areas ofopaque screening material on the surface of the layer such that the layer is struck by light which has passed through the screening material. The material serves as an in situ half-tone screen (or "autoscreen") and permits the production of half tone lithographic printing plates from a continuous tone original without recourse to an extraneous half tone screen. This particular technique is disclosed and claimed in our British patent 1,576,008. In this case it is necessary for the surface of the layer to have a topography consisting of peaks and depressions arranged in a random manner and of varying relative heights and depths, and for the screening material to be provided so as to produce a random pattern of discrete areas of screening material loc~ted solely in the depressions.
In the case where no in situ half tone screen is provided it is not necessary for the surface of the layer to have such a topography since the required effect is produced by virtue of the graining in the substrate (see Adv. Print. Sci. Technol. Vol 5 (Inks, plates and print quality) Ed. Banks (1969) pp 229-239 by I. Pobboravsky and M. Pearson "Study of Screenless Lithography"). Where an in situ screen is provided it has been found that the density range can be extended by means of an overall flash exposure prior to the development step.
Examples of suitable alkaline solutions are sodium metasilicate solution, sodium hydroxide solution and trisodium phosphate solution. Examples of suitable organic solvent liquids are Carbitol (registered trade mark; also known as diethylene glycol monoethyl ether or ethyl digol), 2-methoxy ethanol, 2-ethoxyethanol, ethylene glycol, polyethylene glycol, diethylene glycol, iso-propyl alcohol, isobutyl alcohol, dimethyl formamide and ethanol.
Using conventional techniques on radiation sensitive plates of the type used in accordance with the present invention, a density range of 0.9 can )( ~5Z797 ordinarily be reproduced. However in practice due to the nature of the ink used in printing, this range becomes 0.7 or less when printed, which is not satisfactory for the reproduction of pictorial subjects. By using the method of the present invention, however, the density range can be extended by some 80%, for example to a range of 1.6 on the printing plate and a range of 1.2 on the print, which gives an adequate and satisfactory result.
If desired the printing plate may be treated in accordance with the teachings in our British Patent 1,584,350, to facilitate the acceptance of ink by the lighter tone areas.
The following non-limiting Examples, not all of which fall within the scope of the claims, illustrate the invention.
Example 1 An aluminium sheet was cleaned and electrograined in a bath of dilute hydrochloric acid to provide randomly located recesses with a surface roughness factor of 5.5 m per m2 (gas adsorption). This roughened surface was then anodised in 15% w/v sulphuric acid and, after washing and '.~

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l -~ 1~i2797 drying, was coated with a mixture o~ 1,2-naphthoquinone diazide-(2)-5- sulphonic acid ester o~ trihydroxybenzo-A phenone and novolak resin (Alnovol 429K) to provide apositive working radiation sensitive plate. The proportions by weight of diazide and novolak in the dry coating were li5 , respectively and the coating weight was 2.5 g/m2.
The plate was located under a 0.15 density increment ,.

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' ~' ' , ; : -~ ;2~97 continuous tone step wedge and exposed for 3 minutes to the light emitted by an Addalux mercury-halide lamp positioned at a distance of 1 metre.
Development was carried out at 70F for 3 minutes using a developer comprising 98% by volume of a 4% w/v aqueous solution of sodium metasilicate pentahydrate and

2% by volume of diethylene glycol monoethyl ether.
Processing was completed by washing with water and desensitising with an aqueous solution of gum arabic.
A density range of 1.65 was reproduced as grey steps on the plate.
Example 2 Example l was repeated twice using different developers.
Firstly, a developer comprising 98% by volume of a 0.3%
w/v aqueous solution of sodium hydroxide and 2~ by volume diethylene glycol monoethyl ether was used~ Secondly a developer comprising 93% by volume of a 5% w/v aqueous solution of trisodium phosphate and 7% by volume of diethy-lene glycol monoethyl ether was used. In both cases, density ranges of 1.65 were again obtained.
Example 3 A non-anodised electrograined aluminium sheet of surface roughness factor 9m2 per m2 was coated with a positive working radiation sensitive material similar to that of Example l but including the diazide and the resin in a ratio by weight of 1:6.5 respectively. The coating weight was 2.5g/m2. The resultant radiation sensitive plate was exposed in the manner of Example l.
Development was carried out using a developer comprising 82% by volume of a 6% w~v aqueous solution of _g_ - ,.

~ ~2 797 sodium metasilicate pentahydrate and 18% by volume of polyethylene glycol (molecular weight 300). After 2 minutes development the plate was washed with distilled water and dried. A thin layer of oleo ink was applied and allowed to dry. Subsequent cleaning of the non-image areas was effected using a conventional desensitising etch solution containing ammonium alginate, and the plate was protected by a thin layer of polyacrylamide before being placed on the printing press. The density range obtained was again 1.65.
Example 4 Example 3 was repeated using the following solution, as developer. A similar result was obtained sodium metasilicate pentahydrate 3.0%w/v 15 sodium hydroxide 0.7%w/v sodium benzoate 2.2%w/v Cibachrome* Water Red Dye trace (to colour) Polyethylene glycol 300 10% by volume Distilled water to 100%
Example 5 Example 1 was repeated twice except that the ratio of quinone diazide ester:novolak resin in the radiation sensitive layer was 1:3 and 1:4 respectively. Also, the exposure time was increased. Similar results were obtained.
Example 6 Example 1 was repeated except that the substrate of the radiation sensitive plate was a fine grained aluminium sheet (surface roughness factor 9m2 per m2) which had been anodised in phosphoric acid and that the coating weight was 0.6g/m . A density range of 1.65 was obtained.

* trade mark `~ 1 0-,2797 X~ample 4 ~as repeated except that the substrate of the radiation sensitive plate was an aluminium sheet that had been brush grained (surface roughness factor 3m2 per m2) but not anodised. A density range of 1.5 was obtained.
Example 8 Nine radiation sensitive plates were prepared and exposed as in ~xample l. ~hey were then developed for three minutes at 75~ using different developers. In each case the developer included a 4% w/v aqueous solution of sodium metasilicate pentahydrate as aqueous alkaline solution. In all cases except that of Plate l a water soluble organic solvent was additionally present. The results obtained and the constitution of the various developers used are shown in the following ~able-, Plate No. % by vol~ of Organic solvent and yO by Density aqueous vol thereof Range alkaline solution _ . .... . , 1 100% None 1.05 2 965O polyethylene glycol 4C~o 1. 8 96% diethylene glycol 1.65 monoethyl ether: 4%
4 96% 2-ethoxyethanol: 4% l. 5 96% diethylene glycol: 4% 1. 5 6 90% i so-propyl alcohol: lO~o 1. 65 7 96% isobutyl al¢hol : 4% 1. 5 8 96% dimethyl formamide: 4% l. 8 95% ethanol: 5% 1. 65 ~xample 9 Example 1 was repeated using different developers.

Stock "A" cjO~ llti on coml~ri ~ed ~OYo by volume of ~ 4y~/v aqueous solution of sodium metasilicate pentahydrate and 2~k by volume of polyethylene glycol (M.wt 300).
Stock "B" solution comprisel a mixture of an ~o w/v aqueous solution of sodium metasilicate pentahydrate, a w/v aqueous solution'~o~ disodium h~drogen phosphate, and 0.2 % w/v aqueous solution of tetrasodium ethylene diamine tetra acetic acid. Various combinations of these stock solutions were used as developer. A developer made up b~ taking 2 parts by volume of "A" and mixing it with l part by volume of "B"

produced a density range on the plate of 1.3. By using developers comprising different ratios of "A" and"B", the contrast and re~dering of the plate could be adausted as required. For (not within the scope of th,e, invention~
example, by using lO0~ of stock "B" as developer~,a contrastlng printing plate with a density range of 0.9 was obtained, which when overexposed to eliminate the lighter tones of the positive was suitable for the skeleton black printer of a duo-tone reproduction;by using lO~/o of the "A" solution as developer a long tonal range printing plate (density l.9~) was obtained frorn the same positive suitable for use as the fine-tone grey printer of the duo-tone reproduction.
~ y varying the proportions of solutions "A" and "B" as well as by adjusting the exposure time and altering the processing temperature and duration, an extremel~ wide range of reproductions of a continuous tone transparency could be obtained.
Example lO

A novolak resin was esterified to the extent of the replacement of 35% of its hydroxyl groups using 1,2-naphthoquinone ~i;2797 diazide(2)-~-sulphc)nic acid. 7g of the ester and 13 g of novolak were then admixed together and coated on to an aluminium sheet grained and anodised as an,Example 1.
The resultant radiation sensitive plate was then exposed as in Example 1 and developed using a developer comprising 89% by volume of a,4% w/v aqueous solution of sodium metasilicate pentahydrate and 11% by volume of diethylene glycol. The resultant printing plate had a density range of 1.65.
Example 11 Example 1 was repeated with the additional step of treating the developed plate according to our G.B.
Patent No.1,513,368 including heating the plate for 10 minutes at 230C.
The plate was found to produce more than 60,000 satisfactory copies under the same conditions as in Example 1 where the plate obtained gave 30,000 copies before any deterioration could be detected.
Exam~le _ Example 1 was repeated except that before exposure the radiation sensitive layer was covered with a thin layer of orange-coloured oleo varnish the excess of which was burnished away with a soft cloth so as to form an I in situ screen by leaving random areas of orange screening i material located in depressions in the surface of the radiation sensitive layer. In this case the exposure was carried out for 5 minutes and the orange screening material was removed with white spirit before development. A
plate having a density range of 1.95 was obtained.
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i;Z797 Example 13 Example 12 was repeated except that the plate was given a flash exposure of 4 seconds after the screening material had been removed and before development. A plate having a density range of 2.25 was obtained.
Example 14 Example 3 was repeated except that before exposure the radiation sensitive layer of the plate was covered with a brown coloured oleo ink. The excess ink was then burnished away with a soft cloth so as to leave areas of brown screening material located in depressions in the surface of the layer i.e. an in situ screen. In this case the exposure was carried out for 4~ minutes and the brown screening material was removed with petroleum spirit before development. A plate having a density range of 1.95 was obtained.
Example 15 Example 14 was repeated except that the plate was flash exposed for 4 seconds after the screening material had been used and before development. A plate having a density range of 2.1 was obtained.
Example 16.
Five electrograined and anodised aluminium foils were each coated with a mixture of a 1,2-naphthoquinone diazide-(2)-S-sulphonic acid ester of trihydroxy benzophenone and a novolak resin (Alnovol 429K) to form five presensitised positive-worklng radiation sensitive plates designated A, B, C, D and E. The ratio by weight of ester to resin in .

;2'~97 each dry coating was 1:5 and the roughened surface oI the foil of each plate was reproduced in the free surface of the radiation sensitive coating.
Plate A was exposed for 1~ minutes to light from a mercury halide lamp under a 0,15 density increment ; continuous tone step wedge and developed in a 6% w/v aqueous solution of sodium metasilicate pentahydrate (not within the scope of the claimed invention).
Plate B was provided with in situ screening material on its surface by rubbing into the surface a scarlet pigment suspended in an oleo vehicle. The excess suspension was then wiped off to leave irregularly spaced and sized opaque areas of pigment solely in the depressions of the surface. The plate was exposed in the manner of Plate A but for 4 minutes, cleaned of screening material with benzene, and then developed as Plate A (not within the scope of the claimed invention).
Plate C was provided with screening material, exposed and cleaned in the manner of Plate B but was developed with a solution comprising 98 parts by volume of a ~% w/v aqueous solution of sodium metasilicate penta-hydrate and 2 parts by volume of diethylene glycol mono-ethyl ether.
Plate D was treated in the same manner as Plate B but was given an overall flash exposure of 4 seconds before being developed ~not within the scope of the claimed ;~ invention).
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~1~i2797 Plate E was treated in the same manner as Plate C but was given an overall flash exposure of 4 seconds before development. All five plates were washed with water, desensitised with gum arabic and inked-up. The density of the step-wedge was compared with the density of the inked images on thé plates and the results are shown graphically in the single Figure of the accompanying drawing, which is a graph showing the variation of the density of the inked image as a function of the step number of the step wedge.
It can be seen that by carrying out the method of the present invention in combination with providing an in situ screen and a flash exposure, the density range reproduced as grey steps on the plate can be extended to as much as the 2.25 of Plate E.
Examples 17 to 23 The procedure of Example 16 was repeated by using the following alternative developers instead of the developer comprising sodium metasilicate pentahydrate and diethylene glycol monoethyl ether which was used on plates C and E.
Example 17: 96 parts by volume of a 6% w/v aqueous solution of sodium metasilicate pentahydrate and 4 parts by volume ethylene glycol.

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

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

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

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

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

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

Results substantiallysimilar to those of Example 16 were obtained.
Example 24 The nine lithographic printing plates produced in Example 8 were placed on a Heidelberger Kord single colour offset pressequipped with normal coated paper (white), good quality quick setting black ink and a fountain ,~
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solution consisting oI a 1 ~ aqueous solution ol acidi.Iied gum ~rabic. Fiity revolutions of the press were necessary - before the ink acceptance of the plate was adequate and then 25,000 satisfactory copies were produced without deterioration. On examination of the copies Plate 1 was found to have reproduced a density range of 0.8 whereas plate 2 was found to have reproduced a density range of 1.5.
The other seven plates gave corresponding results.
Example 25.
The lithographic printing plate produced in Example 10 was placed on a Solna offset press equipped with the same ink and paper as in Example 24. The fountain A solution consisted of a 1% aqueous solution of "Hydamp"~
supplied by the Howson-Algraphy Group of Vickers Ltd.
Some 50 revolutions of the press were needed before the ink acceptance of the plate was satisfactory and then 25,000 copies were produced without deterioration.
Examination of the copies showed the plate to have -reproduced a density range of 1.4.
Ex`ample 26 The lithographic printing plates produced in Example 16 were placed on a Heidelberger Kord press under the same conditions as Example 24. Similar results were obtained and examination of the copies showed that the density range reproduced was extended from 0.5 (plate A) to 2.0 (plate E).

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Claims (7)

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

2. A method according to claim 1, wherein the aqueous alkaline solution is selected from the group consisting of aqueous sodium metasilicate solutions, aqueous sodium hydroxide solutions and aqueous trisodium phosphate solutions.

3. A method according to claim 1 or 2 wherein the organic solvent is selected from the group consisting of 2-methoxy ethanol, 2-ethoxy ethanol, ethylene glycol, diethylene glycol monoethyl ether, polyethylene glycol, diethylene glycol, iso-propyl alcohol, isobutyl alcohol, dimethyl formamide and ethanol.

4. A method according to claim 1 wherein the substrate has a surface roughness factor of from 2.5 to 12 m2 per m2 (gas adsorption) and the coating weight of the layer is from 0.5 to 3 g/m

5. A method according to claim 1 wherein the alkali soluble resin is a novolak resin and the ester is the 1,2-naphthoquinone diazide-(2)-5-sulphonic acid ester of trihydroxybenzophenone.

6. A method according to claim 1 wherein the plate comprises in situ random spaced areas of opaque screening material on the surface of the layer such that the layer is struck by light which has passed through the screening material.

7. A method according to claim 6 which comprises the additional steps of removing the screening material after exposing the plate through the continuous tone master and subjecting the layer to a flash exposure prior to developing the layer.