NO127472B - - Google Patents

Description

Planographic printing form.

The present invention relates to planographic printing forms, comprising a paper carrier provided with a planographic coating which essentially consists of a layer of a water-insoluble, 'hydrophilic organic binder in which a finely divided inorganic filler is dispersed, and where said coating has an image which is made up of an oleophilic azo dye which is the reaction product of a light-sensitive benzene-diazonium salt and an aqueous, buffered phloroglucin solution with a pH between 4 and 8.

By planographic or lithographic methods, e.g. offset printing, printing forms are used where an oleophilic (printing ink-receptive) image is present on a hydrophilic base material. When printing, the printing form is continuously moistened with water and printing ink. The water is selectively absorbed by the hydrophilic material and the printing ink by the oleophilic areas in the printing surface. If, however, there are irregularities in the lithographic surface, which irregularities cause defects in the water film that must be constantly present in the hydrophilic areas, the plate will be tinted, i.e. the hydrophilic the areas will absorb printing ink either in spots or over the entire surface. The printing plate on which the oleophilic image is formed can e.g. consist of

an anodically oxidized aluminum foil or a paper plate provided with a planographic coating.

In its simplest form, the planographic paper plate consists of a paper carrier and a planographic coating, which contains a substantial inorganic pigment and a water-insoluble hydrophilic organic binder. Such plates with a single coating are only suitable for producing a few hundred print copies. If it is desired to obtain multiple copies with a planographic paper plate, usually one or more water-repellent coatings and/or adhesive coatings are applied between the lithographic coating and the support.

Several compositions have been proposed for planographic coatings. As pigments in such coatings are used, for example: zinc oxide, titanium dioxide, barium sulphate, antimony trioxide, carbon black, calcium carbonate, magnesium carbonate, lithopone, aluminum oxide, calcium fluoride, magnesium oxide, calcium oxide, silicon dioxide, kaolin, bentonite, aluminum silicate, diatomaceous earth, satin white , talc, Fuller's earth, Georgian clay or calcined clay. As binders for such coatings, the literature mentions, among other things: gelatin, casein, zein, soy protein, albumin, animal glue, hydroxyethyl cellulose, sulfoethyl cellulose, carboxymethyl cellulose, cellulose sulfate, methyl cellulose, oxidized cellulose, carboxymethyl hydroxyethyl cellulose, methyl ethyl cellulose, starch, hydroxyethyl starch, glucosan- 2-monocarbamate, oxidized starch, starch acetate, enzyme-converted starch, acid-modified starch, chlorinated starch. amioca starch, starch glycolate, reaction products of hydroxyethyl starch with aminoformaldehyde resins, pectin derivatives, alginic acids, gum arabic, carrageenan, chitosan, dextrin, dextran, John's br6d gum, tragacanth gum, mesquite gum, karaya gum, guar gum, polyvinyl alcohol, polyacrylic acid, polyacrylamides, methylolpolyacrylamides, copolymers of acrylic acid, methacrylic acid and their amides with hydrophobic monomers such as styrene, vinyl acetate or ethyl acrylate, polyamides and copolymers of styrene with

carboxylic acids.

Several curing agents and/or precipitating agents can be used to make the binders insoluble. The oleophilic image on the planographic coating can either be formed mechanically (e.g. by typesetting) or photographically. On the so-called presensitized printing plates, diazo resin or an o-naphthoquinone diazide is generally used as a light-sensitive compound. By diazo resins is meant condensation products of an aromatic diazo compound such as p-diazodiphenylamine, with an aldehyde such as formaldehyde. Upon exposure, the diazo resins will give oleophilic decomposition products. The printing form is developed by washing away the undecomposed diazo compound with water so that printing plates sensitized with diazo resins are normally negative. The o-naphthoquinone diazides used in practice are mostly either water-insoluble esters which are not easily dissolved by organic solvents, or amides of o-naphthoquinone diazide sulfonic or carboxylic acid. On exposure, the diazides decompose and form a benzocyclopentadiene carboxylic acid which is insoluble in aqueous alkaline liquids. Printing plates sensitized with o-naphthoquinone diazides are either positively or negatively working depending on the composition of the light-sensitive coating and the selected development method. The development can be effected by selective dissolution of either the non-decomposed o-naphthoquinone diazide or its photo-decomposition product. With the help of the o-naphthoquinone diazides, positively working planographic plates of high quality can be produced. However, these plates are relatively expensive even when the carrier material is paper, because the light-sensitive compound must be applied using an organic solvent and consequently the planographic coating must meet special requirements to prevent the sensitizing liquid from penetrating too deeply into the carrier. The necessary development method also causes difficulties due to the strongly alkaline and consequently aggressive liquid that is needed for the purpose.

There is a great demand for a cheap, light-sensitive, positively working printing plate for simple production of forms calculated for approx. 5° to 200 impressions. So far, such printing plates have not appeared on the market.

German patent no. 857 888 describes a method in which an anodically oxidized aluminum plate, sensitized with a benzenediazonium salt such as the chlorozincate of 4-diazonium-N-methylaniline or of 4-diazonium-2-methyl-N-ethylaniline, is exposed and then developed according to a diazotyping method to convert the undecomposed diazonium salt into an oleophilic azo dye.

If the photosensitive coating also contains an azo coupling component, the azo dye is formed by exposure of the plate to ammonia vapour. However, the print form can also be developed with a solution containing an azo coupling component. The azo coupling component is preferably a polyhydric aromatic hydroxyl compound such as 2,4>2',4'-tetrahydroxydiphenyl. The aluminum printing form thus obtained will show a blurred image due to the flow of dyes during development, if the development is carried out with liquid. The selective receptivity to printing ink and the adhesion of the image to the plate is insufficient.

British patent no. 1 064 128 describes a method where a light-sensitive printing plate obtained by sensitizing a so-called photographic baryte paper is used, where the paper's baryte coating contains barium sulphate as a pigment and a protein as a binder, with a light-sensitive diazonium salt of high coupling activity . After exposure, development is effected with an aqueous alkaline phloreglucinole solution and after this the plate with the image is treated with an acidic solution of a hydrophilic organic binder to make the plate's background sufficiently hydrophilic. The plate's background surface is also far too porous. The alkaline phloroglucinol solution has a poor shelf life.

British patent documents No. 1 O38 279 °S No. 1 082 889 describe methods for the production of printing forms where a commercially available plate for direct image formation is sensitized with a diazonium salt with hSy coupling activity. From the plates thus obtained, a good print form can be provided by exposure and development with an aqueous buffered neutral fluoroglucinol solution. A disadvantage associated with the plates in the above-mentioned British patent documents for sensitizing is that the planograph in spoon coating, which contains barium sulfate as pigment and a water-insoluble protein as binder, is too porous, which results in the diazo compound locally needing deep in and as a consequence of this the coating's complete fading and coupling is difficult. During development, a part of the diazo compound which has penetrated too deeply will still form an oleophilic azo dye, but this azo dye is not present on the surface and will therefore not affect the surface's oleophilic properties. In this way, a number of hydrophilic points will become visible in the oleophilic area in the form of white spots in the impression image. On old records, particularly strong penetration has a very disruptive effect.

The purpose of the present invention is to fulfill

the need for an inexpensive, light-sensitive positive working printing plate, which can be converted into a printing form in a very simple way by sensitizing a paper printing plate with a light-sensitive benzenediazonium salt, by exposing the sensitized coating and subsequently developing the plate with an aqueous buffered phloroglucin solution, where the paper carrier material is provided with a specially selected planographic coating.

According to the present invention, there is thus provided a planographic printing form comprising a paper carrier provided with a planographic coating which essentially consists of a layer of a water-insoluble, hydrophilic organic binder in which a finely divided inorganic filler is dispersed, and where said coating has an image formed by an oleophilic azo dye which is the reaction product of a light-sensitive benzene-diazonium salt and an aqueous buffered phloroglucin solution with a pH of between 4 and 8, characterized in that the planographic coating contains polyvinyl alcohol with a degree of hydrolysis of over 98% as a binder.

It is surprising that with this planographic be-

simply put, a single-coated printing plate sensitized with a benzenediazonium salt can be obtained. The planographic coating which has sufficient adhesion to the paper without the use of an adhesive coating, can be sensitized without too deep penetration of the diazo compound and has a sufficient affinity for the azo dye to give the printing form produced from the plate a capacity of from 5° - 2oo passes.

Titanium dioxide, diatomaceous earth and Lustra clay can be used as pigments in the planographic coating, either alone or mixed with other pigments.

It is often advantageous to add water glass, a silicon dioxide sol, an aluminum oxide sol or a stannous anhydride sol to the pigment dispersion. The polyvinyl alcohol used as a binder for the coating has a degree of hydrolysis that is higher than 98%. The cross-linking agent used to make the polyvinyl alcohol water-insoluble can be selected from the agents known for this purpose. Good results can e.g. is achieved with glyoxal.

The planographic coating should be cured in such a way that

its water-absorbing capacity is small. To determine this capacity, the so-called Cobb test is used (see Tappi Standard T 441 os-63 or British Standard PBMA Test Method PTI5). It has been shown that the value for the water-absorbing capacity of the planographic coating, measured according to the Cobb test, should amount to less than 6. (For most plates for direct imaging, this value is considerably higher, approx. 10-20).

In the planographic coating chosen according to the present invention, the sensitizing liquid should not penetrate too deeply into the plate so that the diazo compound is almost exclusively present on the surface.

The sensitization is also uniform, i.e. patchy penetration of the sensitizing liquid will not occur.

The planographic coating used in the invention differs in both of the above-mentioned respects from the coatings which contain another binder such as oxidized starch, hydroxyethyl cellulose, carboxymethyl cellulose, hydroxyethyl carboxymethyl cellulose or alginic acid.

The planographic coating according to the invention preferably has a weight of 9~12 g/m<2.> The ratio between filler and binder is preferably between 4 and 6. The coating can be applied directly to a paper with a sufficiently smooth surface, such as super-calendered paper.

Sensitization of the planographic coating is preferably carried out with an aqueous solution of a light-sensitive benzenediazonium salt. A phloroglucinol solution with a pH value of between 4 and 8 is used as developer, as the diazonium salt must have a relatively high coupling activity. Diazo compounds with a coupling activity in aqueous solution higher than p-diazo o-chloro-N,N-diethylaniline are suitable. (The coupling activity can be measured according to British patent no. 957 836). In British patents no. 1 064 128, 1 064 129 and 347 430 and in Dutch patent applications no 6 508 566, 6 708 503 and 6 610 037, light-sensitive benzenediazonium salts are described which have a suitable high coupling activity and which can also be used with good results in sensitizing the planographic coating due to their other properties.

During the sensitisation, other liquid aids can be used to improve the properties of the light-sensitive coating. As stabilizers for the light-sensitive coating, the acids which are known for use for this purpose, such as phosphoric acid, boric acid and tartaric acid, are particularly suitable.

An unexpectedly appropriate stabilization is achieved with a mixture of boric acid and tartaric acid, the boric acid making up the largest part of the mixture.

Development of the print form is carried out with an aqueous buffer solution of fluoroglucinol with a pH value of between 4 °g 8. Buffering of the solution can be carried out with the help of known buffer salts used for diazotype developers. Good results can e.g. is achieved with tartrate, adipate, acetate, phosphate and citrate buffers.

The exposure is preferably carried out by exposing the light-sensitive printing plate, which is in contact with a transparent original, to actinic light. From originals obtained by typewriting on translucent paper, printing forms can be obtained that can be used from loo to 250 times. When using an original produced by typewriting on a translucent sheet whose substrate has good light permeability, e.g. a thin polyester or cellulose acetate film, and whose surface coating consists mainly of an inorganic transparent pigment such as amorphous or crystalline silicon dioxide and a hydrophobic organic film-forming binder such as polyvinyl acetate or cellulose acetate butyrate, however, a few thousand copies can be made using the provided printing form if the ratio between pigment and binder is greater than 0.8. This ratio is preferably between 1 and 3<

Example I

Supercalendered paper of 106 g/m is coated with a dispersion containing: 500 ml of a solution of 66 g of polyvinyl alcohol with a degree of hydrolysis of 99*8$ in lOO ml of water,

22o ml of an aqueous dispersion containing 73° g/l titanium dioxide which is doped with A^O^ and SiC^ and which has a particle size of o.2 µm,

1 ml glass of water,

8l ml of a 4°$ solution of glyoxal in water,

55 ml isopropanol and

as much water as is necessary to bring the volume up to looo ml.

The coating is dried and cured by heating for 10 - 20 seconds at 155 or 160°C. The resulting coating weighs 12 g/m<2>.

Of each of the following diazo compounds:

a) 4-Denz°ylam:i-no 2,5-di-n-butoxybenzene-diazonium chloride, zinc chloride double salt, b) 4-(4,-methoxyphenylthio) 5-methoxy 2-N-methyl N-ethoxycarbonylamino- benzene-diazonium chloride, zinc chloride.double salt, c) 4-dimethylamino 3-chloro 2-(4'-methylphenyl)thiobenzene-diazonium chloride, zinc chloride double salt, d) 4-bis (2'-acetoxyethyl)araino 3-(2', 4 '> 5'-trichlorophenoxy)-6-chlorobenzene-diazonium chloride, zinc chloride double salt,

dissolve 0.06 mol in a liquid containing 8.3 g boric acid and 1.7 g tartaric acid dissolved in 4°o ml isopropanol and 600 ml water. A plate prepared according to example 1 is sensitized with each of the sensitizing liquids so that each plate contains approximately 0.5 millimoles of diazo-

p

connection per m after drying.

The plates are marked a, b, c and d respectively.

The light-sensitive printing plates are exposed under a transparent paper original provided with a strongly U.V-absorbing typewritten text until all the diazo compound has been bleached out under the image-free areas of the original. The plate is then developed with a solution containing:

8 g phloroglycinol

4 g of thiourea

2 g of citric acid

5o g trisodium citrate (2 aq)

1 g potassium hydroquinone monosulphate

loo g of sodium chloride

6oml propyl glycol and

water up to looo ml.

The pH value of this solution is approx. 6.5.

Immediately after development, the plates are clamped onto an offset printing machine.

Plate a gives 150, plate b 2oo, plate c loo and plate d 250 good impressions.

Example II

The following dispersions are produced:

a) A dispersion containing:

600 ml of a solution containing loo g of polyvinyl alcohol with a degree of hydrolysis of 99-1°°$ in lOO ml of water, 250 ml of an aqueous dispersion containing 73° 'g/l of titanium dioxide which is doped with Al20^ and SiC>2 and has a couple-tick size of approx. o.2^um,

9o ml of glyoxal solution ($40),

50 ml isopropanol and

60 ml of an aqueous alkali-stabilized dispersion of silicon dioxide with a particle size of between 7 and

30 nm.

b) A dispersion as specified under a) in which 600 ml of a solution containing loo g of oxidized starch in looo ml of water however

is used instead of 600 ml of the polyvinyl alcohol solution.

c) A dispersion as stated under a) where the 600 ml polyvinyl alcohol solution is however replaced by: 600 ml of a solution

containing 3° g of the sodium salt of carboxymethylhydroxyethyl cellulose (containing 0.3 carboxymethyl and 0.7 hydroxyethyl groups per glucose unit) in 100 ml of water.

d) A dispersion as indicated under a) where the 600 ml polyvinyl alcohol solution is however replaced by 600 ml of a solution

containing loo g of sodium carboxymethylcellulose in looo ml of water.

e) A dispersion as indicated under a) where the 600 ml polyvinyl alcohol solution is however replaced by 600 ml of a solution

containing 66 g of hydroxyethyl cellulose in 100 ml of water.

f) A dispersion as indicated under a) where the 600 ml polyvinyl alcohol solution is however replaced by 600 ml of a solution

containing 5° g of sodium alginate in lOO ml of water.

Super-calendered paper of 106 g/m is treated with each of the six dispersions so that a coating of approx. lo g/m p is present on each sheet after drying and curing of the coatings. The plates are marked a, b, c, d, e and f respectively. The six plates thus obtained are sensitized with a solution containing: 30 g of 4-(4'-methoxyphenylthio) 5-methoxy 2-N-methyl-N-ethoxy-carbonyl-aminobenzene-diazonium chloride, zinc chloride double salt,

8 g boric acid

1.6 g tartaric acid

3oo ml of isopropanol

700 ml of water

and dried.

The plates are exposed and then developed with a solution containing:

12 g phloroglucinol

<1>5-3 S citric acid

87 g disodium phosphate (2 aq)

5° g of sodium chloride

5o ral n-propanol,

water up to looo ral.

Printing is done with the six printing forms obtained. Only the printing form of plate a gives a good result.

Example III

a) The following dispersion is prepared:

250 ml of an aqueous dispersion containing 730 g/l titanium dioxide which is not doped and which has a particle size of o.2 µm

600 ml of an aqueous solution containing loo g/l polyvinyl alcohol with a degree of hydrolysis of 99.8$,

9° ml glyoxal solution (4.0%),

5o ml of isopropanol,

water up to looo ml.

b) The following dispersion is prepared:

250 ml of an aqueous dispersion containing 650 g/l "Lus-tra" clay (particle size 0.5-2 yum)

600 ml of an aqueous solution containing lop g/l pply-vinyl alcohol with a degree of hydrolysis of .99*8%

90 ml glyoxal solution (40%)

5° ml of isopropanol

water up to looo ml.

c) 'The following' dispersion is prepared:<1>

250 ml of an aqueous dispersion containing 400 g/l diatomaceous earth (particle size 1-3 /um)

600 ml of an aqueous solution containing loo g/l polyvinyl alcohol with a degree of hydrolysis of 99*8%

90 ml glyoxal solution (4°$)

50 ml isopropanol

water up to looo ml.

Super-calendered paper provided with a planographic coating of 9 g/m p is treated with the dispersions a, b and c. The plates are sensitized as described in Example II. After drying, an oleophilic azo dye image is formed on the light-sensitive plates during exposure and development. The development is carried out with a solution containing:

12 g phloroglucinol

11 g of citric acid

87 g disodium phosphate (2 aq)

in loooo ml of water.

The printing forms produced in this way give approximately 200 impressions.

Claims (1)

Planographic printing form comprising a paper carrier provided with a planographic coating, which essentially consists of a water-insoluble, hydrophilic organic binder in which a finely divided inorganic filler is dispersed, and wherein said coating has an image formed by an oleophilic azo dye which is the reaction product of a light-sensitive benzene-diazonium salt and an aqueous buffered phloroglucin solution with a pH of between 4 °g 8, characterized in that the planographic coating contains polyvinyl alcohol with a degree of hydrolysis of over 98% as binder.