DESCRIPTION

1. Field of the invention.

The present invention relates to a heat mode recording material for making a lithographic plate for use in lithographic printing. The present invention further relates to a method for imaging said heat mode recording material e.g. by means of a laser.

2. Background of the invention.

Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (hydrophobic areas) whereas other areas will not accept ink (hydrophilic areas) . According to the so called conventional or wet printing plates, both water or an aqueous dampening liquid and ink are applied to the plate surface that contains hydrophilic and hydrophobic areas. The hydrophilic areas will be soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink.

When a laser heat mode recording material is to be used as a direct offset master for printing with greasy inks, it is necessary to have hydrophobic-hydrophilic mapping of the image and non-image areas. In the case of heat mode laser ablation it is also necessary to completely image wise remove a hydrophilic or hydrophobic topcoat to expose the underlying hydrophobic respectively hydrophilic surface of the laser sensitive recording material in order to obtain the necessary difference in ink-acceptance between the image and non-image areas.

For example DE-A-2448325 discloses a laser heat mode "direct negative" printing plate comprising e.g. a polyester film support provided with a hydrophilic surface layer. The disclosed heat mode recording material is imaged using an Argon laser thereby rendering the exposed areas hydrophobic. An offset printing plate is thus obtained which can be used on an printing press without further processing. The plate is called a "direct negative" plate because the areas of the recording material that have been exposed are

rendered ink accepting.

Other disclosures in DE-A-2448325 concern "direct negative" printing plates comprising e.g. hydrophilic aluminium support coated with a water soluble laser light (Argon-488nm) absorbing dye or with a coating based on a mixture of hydrophilic polymer and laser light absorbing dye (Argon - 488nm) . Further ex-amples about heat mode recording materials for preparing "direct negative" printing plates include e.g. US-A-40341183, DE-A-2607207, DD-A-213530, DD-A-217645 and DD-A-217914. These documents disclose heat mode recording materials that have on an anodized aluminium support a hydrophilic layer. The disclosed heat mode recording materials are image-wise exposed using a laser. Laser exposure renders the exposed areas insoluble and ink receptive, whereas the non exposed image portions remain hydrophilic and water soluble allowing to be removed by the dampening li-quid during printing exposing the hydrophilic support. Such plates can be used directly on the press without processing.

DD-A-155407 discloses a laser heat mode "direct negative" printing plate where a hydrophilic aluminum oxide layer is rendered oleophilic by direct laser heat mode imaging. These printing plates may also be used on the press without further processing.

From the above it can be seen that a number of proposals have been made for making a 'direct negative' offset printing plate by laser heat mode recording. They have such disadvantages as low recording speed and/or the obtained plates are of poor quality.

3. Summary of the invention

According to the present invention it is an object to provide an alternative heat mode recording material of high sensitivity for making a negative working lithographic printing plate of high quality.

For ecological reasons it is another object of the present invention that this lithographic printing plate of high quality preferably can be obtained by development under essential dry conditions or with plain water or by using the imaged non processed plate directly on the press, these processing methods are referred to as 'ecological processing'.

Still further objects of the present invention will become clear from the description hereinafter.

According to the present invention there is provided a heat mode recording material comprising on a support having a ink receptive

surface or being coated with an ink receptive layer a substance capable of converting light into heat and a hardened hydrophilic surface layer having a thickness not more than 3μm.

According to the present invention there is provided a method for making a lithographic printing plate comprising the steps of:

— image wise exposing a heat mode recording material comprising on a support having a ink receptive surface or being coated with an ink receptive layer a substance capable of converting light into heat and a hardened hydrophilic surface layer having a thickness not more than 3μm and

- subsequently removing said hardened hydrophilic surface layer at the exposed areas by rubbing a thus obtained image-wise exposed heat mode recording material under essentially dry conditions or by using water or an aqueous solution.

4. Detailed description of the invention.

It has been found that in accordance with the present invention negative working lithographic printing plates can be obtained having a high recording speed, yielding a high printing endurance, high sharpness, good contrast and an excellent resolution in an ecologically more acceptable way.

To obtain the effects of the present invention it is important that the thickness of the hydrophilic surface layer is not more than 3μm preferably not more than lμm. When the thickness of the hydrophilic surface layer becomes more than 3|lm a heat mode recording material will be obtained having a low recording speed and yielding printing plates of poor quality. The minimum thickness of the surface layer is not particularly critical but is preferably more than 0.05|im. A particularly interesting range for the thickness of the hydrophilic surface layer is between 0.05μm and 0.5μm.

In order to obtain good lithographic printing properties, especially to obtain a good printing endurance, it is also a requirement that the hydrophilic surface layer be hardened.

According to a preferred method of the present invention the heat mode recording material is image-wise exposed using a laser. Preferably used lasers are e.g. semiconductor lasers, YAG lasers e.g. Nd-YAG lasers, Argon lasers, ect.. The laser may have a power output between 40 mW and 10000 mW and preferably operates in the infrared part of the spectrum.

Subsequent to the image-wise exposure the hydrophilic surface layer is removed at the exposed imaged areas. Removing can be done by e.g. rubbing using e.g. a brush or a cotton pad. Such rubbing may be carried out during image-wise exposure as described in US-P-5.148.746. Rubbing of the heat mode recording material is preferably carried out in absence of a liquid in essentially dry conditions. However plain water or a water based developer may be used also. Removing the hydrophilic surface layer on the imaged areas can further be done by ultrasonic treatment of the imaged recording material immersed in plain water or a water based developer, by means of a pressure jet of plain water or a water based developer. Removal of the exposed areas may also be done by directly mounting the plate on a printing machine so that the dampening liquid dissolves and removes the hydrophilic surface layer at the exposed areas during startup of the printing machine.

The substance capable of converting light into heat may be present in the support and/or in a separate recording layer provided between the support and the hardened hydrophilic surface layer. The thickness of such a separate recording layer may be varied over a wide range. However when the thickness of such recording layer becomes more than 3μm image—wise exposure should then be carried out through the hardened hydrophilic surface layer. In this case it also preferred not to include an additional layer between such recording layer and the hardened hydrophilic surface layer or to keep the thickness of such additional layer to a minimum preferably not more than 0.5μm. Furthermore when using a recording layer having a thickness of more than 3μm such layer should then be ink receptive.

On the other hand when the thickness of the recording layer is less than 3μm exposure can take place through the support, when transparent, or through the hardened hydrophilic layer. Further in this case the recording layer will generaly not be required to be ink receptive as long as the underlying surface or layer is ink receptive and total removal of the recording layer together with the hydrophilic surface layer at the exposed areas is possible.

In any case, irrespective of the thickeness of the recording layer, exposure through the hardened hydrophilic surface layer is preferred since it has been found that the printing plates obtained in this way have a better resolution and sharpness.

According to a preferred embodiment of the present invention a recording layer is used having a thickness between O.Olμ and 30μm.

Examples of substances capable of converting light into heat that may be used in connection with the present invention are e.g. carbon black, infrared or near infrared absorbing dyes or pigments, metals such as Bi, Sn, Te etc. or a combination thereof. Suitable infrared dyes are disclosed in e.g. US-4833124, EP-321923,

US-4774583, US-4942141, US-4948776, US-4948777, US-4948778, US-4950639, US-4950640, US-4912083, US-4958552, US-5024990, US-5023229 etc.. Suitable infrared pigments are e.g. HEUCODOR metal oxide pigments available from Heubach Langelsheim. When a metal such as e.g. bismuth is used as a heat converting substance the recording layer is preferably a vacuum or vapour deposited metal layer.

According to a particular embodiment of the present invention the recording layer may be a vacuum deposited aluminium layer. The thickness of such an aluminium layer however should be less than 250 A and more preferably between lOOA and 225A.

However the substance capable of converting light into heat may alos be dissolved or dispersed in a binder. Polymeric binders for the recording layer are preferably hydrophobic such as e.g. cellulose esters e.g. cellulose acetate, cellulose nitrate, a copolymer of vinylidene chloride and acrylonitrile, poly (meth) acrylates, polyvinyl chloride, etc.. Preferably the polymeric binder in the recording layer is heat sensitive: e.g. a polymer containing nitrate ester groups (e.g. self oxidizing binder cellulose nitrate as disclosed in GB-1316398 and DE-A-2512038) ; e.g. a polymer containing carbonate groups (e.g. polyalkylene carbonate); e.g. a polymer containing covalently bound chlorine (e.g. polyvinylidene chloride) .

Suitable supports for a heat mode recording material used in connection with the present invention are preferably non-metallic supports having a hydrophobic (ink receptive) surface e.g. a polyester film support, paper coated with a polyolefin such as polyethylene, polycarbonate film, polystyrene film etc.. However metallic support such as e.g. aluminium can also used in connection with the present invention. In case the surface of the support is not or insufficiently hydrophobic it may be provided with an intermediate hydrophobic primer coating whereon a recording layer may be deposited. Alternatively the recording layer can be the hydrophobic layer at the same time.

According to the present invention the recording layer may comprise additional substances such as plasticizers, crosslinking agents and resins, particulate materials such as e.g. polyethylene

or fluorinated polymer dispersions, etc.. to impart toughness, print durability and oleophilic surface characteristics of the ablated image portions of the recording layer, wetting agents, matting agents, anti-oxidizing agents etc.. Preferably the recording layer will be hardened when it includes a binder.

Concerning the composition of an optional undercoat layer any polymeric binder is suitable provided the adherance between the recording layer and the support is thereby improved. It has also been found that when the undercoat layer contains a polymer with heat labile groups (e.g. covalently bound chloride) adjacent to the recording layer the speed of the recording material can be improved as disclosed in EP 92201633.2.

According to the most preferred embodiment of the present invention imaging of the heat mode recording material takes place through the hydrophilic surface layer. In this way excellent image quality (high sharpness, good contrast, excellent resolution) and increased sensitivity are obtained.

Different kinds of hardened hydrophilic surface layers are suitable in connection with the present invention. The hydrophilic coatings are preferably cast from aqueous compositions containing hydrophilic binders having free reactive groups including e.g. hydroxyl, carboxyl, hydroxyethyl, hydroxy-propyl, amino, aminoethyl, a inopropyl, carboxymethy1, etc.. along with suitable crosslinking or modifying agents including e.g. hydrophilic organotitanium reagents, ali.aminofo.rmyl acetate, dimethylol urea, melamines, aldehydes, hydrolyzed tetraalkyl orthosilicate, etc..

Suitable polymers for hydrophilic layers may be selected from the group consisting of gum arabic, casein, gelatin, starch derivatives, carboxymethyl cellulose and Na salt thereof, cellulose acetate, sodium alginate, vinyl acetate-maleic acid copolymers, styrene-maleic acid copolymers, polyacrylic acids and salts thereof, polymethacrylic acids and salts thereof, hydroxy-ethylene polymers, polyethylene glycols, hydroxypropylene polymers, polyvinyl alcohols, and hydrolyzed polyvinylacetate having a hydrolyzation degree of at least 60% by weight and more preferably at least 80% by weight.

Hydrophilic layers containing polyvinylalcohol or polyvinylacetate hydrolyzed to an extent of at least 60% by weight hardened with a tetraalkyl orthosilicate, e.g. tetraethyl orthosilicate or tetramethyl orthosilicate, as disclosed in e.g. US—P-3476937 are particularly preferred because their use in the present heat mode recording material results in excellent

lithographic printing properties .

A further suitable hardened hydrophilic surface layer is disclosed in EP 91201227.5. The hydrophilic layer disclosed in this EP-application comprises the hardening reaction product of a (co)polymer containing amine or -amide functions having at least one free hydrogen (e.g. a ino modified dextrane) and aldehyde.

According to the present invention the hardened hydrophilic surface layer may comprise additional substances such as e.g. plasticizers, pigments, dyes etc.. The hardened hydrophilic surface layer may also comprise particulate materials such as e.g. Tiθ2 or colloidal silica to increase the strength and/or hydrophilic character of the hydrophilic layer.

The present invention will now be illustrated with the following examples without however limiting it thereto. All parts are by weight unless otherwise specified.

EXAJtfPLE 1

To a polyethylene terephthalate support provided with a layer of a copolymer of vinylidene chloride (88 mol%) , methylacrylate (10 mol%) and itaconic acid (2mol%) in an amount of 170mg/m was vacuum deposited a bismuth layer as a recording layer such that the optical density thereof was 4.5 (thickness of about 0.30 μm) .

To this recording layer was then coated the following coating dispersion for the hydrophilic layer: to 82 grams of a dispersion containing 21.5 % of Ti02 (average particle size 0.3 to 0.5 μm) and 2.5 % of polyvinylalcohol in deionized water were subsequently added, while stirring, 639 grams of a 5 % polyvinyl alcohol solution in water, 77 grams of a hydrolyzed 22 % tetramethyl orthosilicate emulsion in water and 22 grams of a 10 % solution of a wetting agent. To this mixture was then added 180 grams of deionized water and the pH was adjusted to pH=4. The coating dispersion was coated on the recording layer to a dry thickness of 0.5 μ to obtain a hydrophilic surface layer. After drying at 30°C the thus obtained element was subjected to a temperature of 80°C for several hours.

The thus prepared heat mode recording material was image-wise exposed through the hydrophilic surface layer with a Nd-YLF laser (1053 nm) at a linear writing speed of 32.8 m/s, with a spot diameter of 18 μm (l/e2) and a power output at the surface of the heat recording material of 1600 mW. After imaging the hydrophilic surface layer was image—wise removed by rubbing with a dry cotton

pad to expose the hydrophobic image portions of the support . The optical density of the imaged image portions was decreased to about 0.05 compared to the non imaged areas having an optical density of 4.5. This difference in optical density between imaged and non imaged portions allows visual inspection of the recorded image on the plate.

The printing plate thus prepared was mounted on a printing press .ABDick 360 and was used to print with a commonly employed ink and dampening solution. The achieved image-wise hydrophobic-hydrophilic differentiation was satisfactory and print copies of good quality were obtained.

EXAMPLE 2

The imaged heat mode recording material as described in example 1 was image-wise exposed as described in example 1 and subjected to ultrasonic treatment in plain water during 2 minutes.

The printing plate thus prepared was mounted on a printing press ABDick 360 and was used to print with a commonly employed ink and dampening solution. The image-wise achieved hydrophobic-hydrophilic differential was satisfactory and print copies of good quality were obtained.

EX2*iMPLE 3

The imaged heat mode recording material as described in example 1 was image-wise exposed as described in example 1 and was then mounted without further processing on a printing press .ABDick 360. The recording material was used to print with a commonly employed ink and dampening solution. The image-wise achieved hydrophobic- hydrophilic differential was satisfactory and print copies of good quality were obtained.

EJCAMPLE 4

To a polyethylene terephthalate support was coated a dispersion for the recording layer having the following composition: 9.5 grams of low molecular weight nitrocellulose E330 (1), 9.5 grams of carbon black Special Schwarz 250, 0.2 grams of Solsperse 5000 (2), 0.8 grams of Solsperse 24000 (3), 80 grams of n-butanone. The recording layer was applied to a dry coating thickness of about 0.5 μ

corresponding to an optical density of about 2.

(1) E330 is a low molecular nitrocellulose, supplier Wolff Walsrode.

(2) Solsperse 5000 is a dispersing agent from ICI .

(3) Solsperse 24000 is a dispersing agent from ICI.

To this recording layer was then coated the coating dispersion from example 1 for the hydrophilic layer to a dry thickness of 0.5 μ to obtain a hydrophilic surface layer. After drying at 30°C the thus obtained element was subjected to a temperature of 80°C for several hours .

The thus prepared heat mode recording material was image-wise exposed through the hydrophilic layer with a Nd-YLF laser (1053nm) at a linear writing speed of 32.8 m/s, with a spot diameter of 18 μ (l/e2) and a power output at the surface of the heat mode recording material of 1600 mW. After imaging the hydrophilic surface layer was image—wise removed by rubbing with a dry cotton pad to expose the hydrophobic image portions of the remaining recording layer. The optical density of the imaged image portions was decreased to about 1 compared to the non imaged areas having an optical density of 2. This difference in optical density between imaged and non imaged portions allows visual inspection of the recorded image on the plate.

The printing plate thus prepared was mounted on a printing press ABDick 360 and was used to print with a commonly employed ink and dampening solution. The achieved image—wise hydrophobic-hydrophilic differentiation was satisfactory and print copies of good quality were obtained.

EXA1YIPLE 5

To a polyethylene terephthalate support was coated a dispersion for the recording layer according the composition of example 4. The recording layer was applied to a dry coating thickness of about 6 μ corresponding to an optical density of >5.

To this recording layer was then coated the coating dispersion from example 1 for the hydrophilic layer to a dry thickness of 0.5 μm to obtain a hydrophilic surface layer. After drying at 30°C the thus obtained element was subjected to a temperature of 80°C for several hours .

The thus prepared heat mode recording material was image-wise

exposed through the hydrophilic surface layer with a Nd-YLF laser (1053 nm) at a linear writing speed of 32.8 m/s, with a spot diameter of 18μ (l/e2) and a power output at the surface of the heat mode recording material of 1600 mW. After imaging the hydrophilic surface layer was image-wise removed by rubbing with a dry cotton pad to expose the hydrophobic image portions of the remaining recording layer. The image portions showed a decreased optical reflectance compared to the non imaged areas. The difference in optical reflectance between imaged and non imaged portions allows visual inspection of the recorded image on the plate.

The printing plate thus prepared was mounted on a printing press ABDick 360 and was used to print with a commonly employed ink and dampening solution. The achieved image-wise hydrophobic-hydrophilic differentiation was satisfactory and print copies of good quality were obtained.

EXAMPLE 6

To a polyethylene terephthalate support was coated a dispersion for the recording layer according the composition of example 4. The recording layer was applied to a dry coating thickness of 0.5 μm corresponding to an optical density of about 2.

To this recording layer was then coated following coating composition for the hydrophilic surface layer: prior to application was added to lOOg of a solution containing 2 % of carboxy- methylcellulose (CMC) 4.5 grams of a hydrolyzed 22 % tetramethyl orthosilicate emulsion in water and 2 grams of a 10 % solution of a wetting agent. The pH of the composition was adjusted to pH=4. The coating dispersion was coated on the recording layer to a thickness of 0.5 μm. After drying at 30°C the thus obtained element was subjected to a temperature of 130°c for 5 minutes. After polishing with a wet cotton pad a very though hydrophilic surface layer with a thickness of <0.5 μm was obtained on the hydrophobic nitrocellulose- carbon black based recording layer.

The thus prepared heat mode recording material was image-wise exposed through the hydrophilic toplayer with a Nd-YLF laser (1053 nm) at a linear writing speed of 32.8 m/s, with a spot diameter of 18 μm (l/e2) and a power output at the surface of the heat mode recording material of 1600 mW. After imaging the hydrophilic surface layer was image-wise removed by rubbing with a dry cotton pad to expose the hydrophobic image portions of the remaining recording

layer. The optical density of the imaged portions was about 1 compared to the on imaged areas showing an optical density of 2. This difference in optical density between imaged and non imaged portions allows visual inspection of the recorded image on the plate.

The printing plate thus prepared was mounted on a printing press .ABDick 360 and was used to print with a commonly imployed ink and dampening solution. The acchieved image-wise hydrophobic-hydrophilic differentiation was satisfactory and print copies of good quality were obtained.

EXAMPLE 7

The imaged heat mode recording materials prepared and exposed according to examples 4, 5 and 6 were subjected to ultrasonic treatment in plain water during 2 minutes.

The printing plates thus prepared were mounted on a printing press ABDick 360 and were used to print with a commonly used ink and dampening. The image-wise achieved hydrophobic-hydrophilic differential was satisfactory and print copies of good quality were obtained.

EXAMPLE 8

The imaged heat mode recording materials prepared and exposed according to examples 4, 5 and 6 were mounted without further processing on a printing press .ABDick 360.

The recording materials were used to print with a commonly employed ink and dampening solution. The image—wise achieved hydrophobic-hydrophilic differential was satisfactory and copies of good quality were obtained.