GB2075700A - Lithographic printing plates and light-sensitive compositions for use in them - Google Patents

Abstract

A light-sensitive composition as, or for, the image layer of a surface lithographic printing plate comprises a homogeneous mixture of (i) the esterification product, insoluble in dilute aqueous alkali, of an alkali-soluble novolac resin with a quinone diazide sulphonic acid in an amount sufficient to esterify no more than 15% of the free hydroxyl groups of the alkali-soluble novolac resin, and (ii) an alkali-soluble resin with a molecular weight that is low enough to give the compatibility necessary for stability of the homogeneous mixture but high enough for the resin to have a useful reinforcing effect, eg. vinyl chloride/vinyl acetate co-polymer, polyvinyl acetate polyvinyl acetal or an epoxy resin derived from epichlorohydrin and bisphenol A. The plates have good sensitivity, alkali-solubility differential, and press life. i

Description

SPECIFICATION Lithographic printing plates and light-sen'sitive compositions for use in them This invention relates to 'surface' lithographic printing plates of the kind in which a resinous coating on a hydrphilic base is sensitised with a quinone diazide compound which, on photo- decomposition, generates a carboxylic acid group that renders the coating susceptible to removal by a dilute aqueous alkali developer.

In practice, the photosensitive radial of quinone diazide sensitiser is almost always one of the two ortho naphthoquinones, and they are almost always used in the form of an ester of a sulphonic acid; a great variety of alcohol residues may be used.

The resinous base of the coating is a sparingly alkali-soluble resin, the amount of sensitiser (alkali-insoluble) used being sufficient to render the mixture insoluble (in relation to the specific developer to be used), while the lightdecomposition product makes the exposed coating more soluble than the base resin.

The most commonly used alkali-soluble resins are those derived from a phenol and formaldehyde known as Novolacs, and in some cases a more intimate relation has been obtained between the resin and the sensitiser by chemically bonding them together by esterfying the residual hydroxl groups of the Novolac with a quinone diazide sulphonic acid; but major problems have arisen because it has not proved possible to obtain a sufficient solubility differential between exposed and unexposed areas, as an increase in the degree of substitution designed to achieve this produces too great, and too inconsistent, a decrease in the overall solubility (presumably because the residual hydroxyl groups determine alkali solubility).

In photosensitive coatings of the same Novolac/quinone diazide mixture type used for other purposes, (for example as etch resists for the manufacture of bimetallic lithographic plates, 'printed' circuits and metal stencils) use has been made of reinforcing resins that are not themselves soluble in alkali but are so compatible with the Novolac that they do not form a self-supporting residue when the exposed Novolac/sensitiser mixture dissolves.

The use of such reinforcing resins has not hitherto been seriously considered for surface lithographic plates, because any residue remaining on the plate would be sufficiently oleophilic to produce 'scumming'.

Our investigations have shown that if reinforcing resins are carefully chosen (with regard to chemical type and molecular weight) the amount of resin remaining on the plate can in fact be reduced to a negligible level.

(For example, a partially-hydrolysed vinyl chloride/vinyl acetate copolymer containing 2% hydroxyl groups (Vinylite VROH from Bakelite Ltd.) can be used satisfactorily at a rate of about 14% in a coating based on a Novolac (Alnovol 429K from Hoechst A-G) sensitised with the bis (2-diazo-1-oxo napthaquinone diazide sulphonic acid) ester of 2,4 dihydroxybenzophenone).

More specifically, however, and quite surprisingly, we have found that reinforcing resins, almost without regard to chemical type, can be used with certain Novolac-quinone diazide sulphonate esters to give consistent coatings with adequately high sensitivity (photographic speed) and góod solubility differential.

In accordance with the invention, therefore, a light-sensitive composition for surface lithographic plates comprises a homogenous mixture of (i) the esterification product, insoluble in dilute aqueous alkali, of an alkali-soluble Novolac resin with a quinone diazide s#ulphonic acid in an amount sufficient to esterify no more than 15% (preferably not more than 10%) of the free hydroxyl groups of the alkali- soluble Novolac resin., and (ii) an alkali-insoluble resin with a molecular weight that is low enpugh to give the compatibility necessary for stability of the homogenous mixture but high enough for the resin to have a useful reinforcing effect.

The esterification product (i) can of course be prepared from suitable functional derivatives of the reagents indicated.

The composition may include other conventional ingredients, such as dyes or pigments for enhancing the visiblity of the developed image or pH indicators for giving an immediate visible image on exposure; and when the composition is prepared for coating it will also include a solvent.

The proportions of the principal ingredients, namely the sensitive resin (i) and the reinforcing resin (ii) will depend to some extent on the particular materials chosen, but in most cases the proportion of reinforcing resin should be in the range 5-25% by weight of total solids.

The chemical nature of the reinforcing resin (ii) is material only to the extent that it influences compatibility. This must be tested empirically, but useful guidance may be obtained by considering the solubility parameters of the two ingredients. When using benzoquinone diazide sulphonic acid Novolac esters, we have found vinyl chloride-vinyl acetate copolymers, polyvinyl acetate, polyvinyl acetal and epoxy resins derived from epichlorohydin and bisphenol A to be satisfactory, and would expect good results from many other polymers containing oxygen and/or other electronegative hetero atoms in a proportion of about 1 to each 2 to 2.5 carbon atoms. The required molecular weight depends somewhat on chemical nature but is relatively low, say around 5000 to 15000 for typical polymers.

The invention includes printing plates coated with the compositions described, whether unexposed, exposed, or exposed and developed.

Any suitable hydrophilic base may be used, but we prefer an anodised and silicated aluminium base. Alternatives include chromium, grained but unanodised aluminium, cellulose foil and coated papers. Coating weights and techniques may be conventional.

Apart from the benefits of good sensitivity and solubility differential already described, the incorporation of the reinforcing resins of the invention has the advantages of increased abrasion resistance and consequently improved run length, better water-resistance and consequently improved non-blinding character, and reduced cost (as the reinforcing resins can be much cheaper than the light-sensitive resins).

The following are examples of the invention: Example I An alkali-soluble phenol/formaldehyde novolac resin (AlnovolPN430 from Hoechst A-G) was esterified by a conventional technique with 2-diazo 1-oxo napthoquinone-5-sulphonic acid until 10% of the free hydroxyl groups of the novolac were reacted. 79 of the product and 0.7g of a low molecularwweight polyvinyl acetate (Mowilith 20 from Hoechst A-G) were dissolved in 100 ml of ethylene glycol monoethylene ether acetate ("cellosolve acetate") and whirl coated on a slurry brush grained, anodised and silicated aluminium plate.

After drying at 80 C for 12 minutes, the plate was exposed under a 21-step Stouffer Step Wedge for 90 seconds to a 2kW tungsten-iodine lamp in a Nuarc Flip-top printingdown frame, and developed in 7% sodium metasilicate solution by swabbing for 60 seconds.

The resulting plate showed that the area covered by density step 2 on the Stouffer Step Wedge was completely clear of coating, while step 7, although slightly discoloured by the developer, showed that the resin had not been even partially removed by the developer and on mounting the plate on an offset printing press, more tham 30,000 satisfactory prints were produced before step 7 of the wedge started to break down. The non-image areas meanwhile printed cleanly throughout.

A A similar plate coated with the novolac ester but not containing the polyvinyl acetate, exposed and processed in the same manner, showed the same clear step on the step wedge but step 9 was the first that was not attacked by the developer. On the printing press only step 9 and those above it were capable of printing more than 30,000 copies.

Example 2 7g of the same esterfied novolac as used in Example 1 and 1.1 g of a partially-hydrolysed vinyl chloride/vinyl acetate copolymer (Vinylite VROH from Bakelite Ltd.) with a molecular weight of about 8000 were dissolved in 100 ml of the same solvent. The solution was whirl coated on an electrograined and anodised aluminium plate and exposed and developed as in Example 1. A clear step 2 and solid step 7 were obtained and survived over 30,000 copies on the press without deterioration.

A similar plate coated with the novolac ester but not containing the vinyl chloride/vinyl acetate copolymer, exposed and processed in the same manner, showed the same clear step 2 on. the step wedge but step 9 was the first that was not attacked by the developer. On the printing press only step 9 and those above it were capable of printing more than 30,000 copies.

Example 3 This was identical with Example 2 except that the amount of the vinyl chloride/vinyl acetate copolymer was reduced to 1 Og and 0.6g of a vinyl chloride/vinyl acetate/maleic acid copolymer (Vinylite VMCH from Bakelite Limited and containing 4% maleic acid) was added.

A A similar plate was also prepared without either of the vinyl copolymer resins and the two plates were mounted side by side on an offset printing press abnd printed until the images had so deteriorated that satisfactory copies could not be produced. Whereas the plate containing the Vinylite resins printed 70,000 copies, only 50,000 could be produced from the plate without the Vinylite resins before the printed copies were unsatisfactory.

Example 4 The phenol formaldehyde resin, Alnovol PN430, was esterified with 2-diazo-1-oxo naphthoquinone-5-sulphonic acid until 15% of the free hydroxyl groups of the novolac were reacted.

7 gm of this resin and 0.7 gm VROH (as in Example 2) were dissolved in 100 ml of the same solvent as was used in the preceding examples.

This solution was whirl coated on to a slurry-grained, anodised and silicated aluminium plate and dried at 80 C for 12 minutes.

The plate was exposed under a 21 step Stouffer step wedge for 150 seconds to a 2 kW lamp and processed as in Example 1. The step wedge showed a clear step 2 and solid step 6 and on the printing press clean copies were produced.

When the amount of VROH was increased by 20% by weight in the coating solution and the plate coated and processed as before and placed on the printing press scumming occur red in the non-image areas.

This shows that in some cases the amount of alkali insoluble resin must be limited; suitable levels are easily determined by simple trial-and-error experiments.

Example 5 7 gm of the resin ester as in Example 1 was dissolved with 0.7 gm of the same solvent as was used in the preceding examples; the solution was a copolymer of epichlorhydrin and Bisphenol A having a molecular weight averaging 1000 (sold by Shell Chemicals Limited under the trade mark "Epikote" as Epikote 1001) into 100 ml of the same solvent as was used in the preceding examples; the solution was coated on to a slurry-grained, anodised and silicated aluminium plate.

When the plate was exposed and processed as in Example 1 and placed on the printing press clean copies were produced.

When a higher molecular-weight copolymer 1 (Epikote 1009) was used only 2% could be incorporated before the resulting plate scummed on the printing press.

This example shows the advantage of lower molecular weight in polymers of this class.

Claims (10)

1. A light-sensitive composition comprising a homogenous mixture of (i) the esterification product, insoluble in dilute aqueous alkali, of an alkali-soluble novolac resin with a quinone diazide sulphonic acid in an amount sufficient to esterify no more than 15% of the free hydroxyl groups of the alkali-soluble novolac resin, and (ii) an alkali-insoluble resin with a molecular weight that is low enough to give the compatibility necessary for stability of the homogeneous mixture but high enough for the resin to have a useful reinforcing effect.

2. A light-sensitive composition comprising a homogenous mixture of (i) the esterification product, insoluble in dilute aqueous alkali, of an alkali-soluble novolac resin with a quinone diazide sulphonic acid in an amount sufficient to esterify no more than 15% of the free hydroxyl groups of the alkali-soluble novolac resin, and (ii) 5-25% by weight of total solids of an alkali-insoluble resin with a molecular weight that is low enough to give the compatibility necessary for stability of the homogeneous mixture but high enough for the resin to have a useful reinforcing effect.

3. A composition as claimed in Claim 1 or Claim 2 including also a dye or pigment.

4. A composition as claimed in Claim 1 or Claim 2 including also a pH indicator.

5. A composition in accordance with Claim 1 and substantially as described in one of the numbered examples.

6. A coating composition in accordance with any one of the preceding claims including a solvent.

7. A surface lithographic printing plate comprising a hydrophilic base coated with a light-sensitive composition comprising a homogeneous mixture of (i) the esterification product, insoluble in dilute aqueous alkali, of an alkali-soluble novolac resin with a quinone diazide sulphonic acid in an amount sufficient to esterify no more than 15% of the free hydroxyl groups of the alkali-soluble novolac resin, and (ii) an alkali-soluble resin with a molecular weight that is low enough to give the compatibility necessary for stability of the homogeneous mixture but hight enough for the resin to have a useful reinforcing effect.

8. A printing plate as claimed in Claim 7 in which the coating composition is as claimed in any one of Claims 2 to 5.

9. A printing plate as claimed in Claim 7 or Claim 8 when exposed to form an image.

10. An exposed and developed surface lithographic printing plate comprising a hydro- philic base exposed in non-image areas and coated in image areas with an oleophilic, light-sensitive composition comprising a homogenous mixture of (i) the esterification product, insoluble in dilute aqueous alkali, of an alkali-soluble novolac resin with a quinone diazide sulphonic acid in an amount sufficient to esterify no more than 15% of the free hydroxyl groups of the alkali-soluble novolac resin, and (ii) an alkali-insoluble resin with a molecular weight that is low enough to give the compatibility necessary for stability of the homogeneous mixture but high enough for the resin to have a useful reinforcing effect.