DE69805613T2 - METHOD FOR IMAGING LITHOGRAPHIC PRINTING PLATES BY HIGH INTENSITY LASER - Google Patents

Description

This invention relates to the method for producing a lithographic printing plate, which comprises exposing a support coated with a melanophilic layer, a photothermal conversion layer and a melanophobic layer to a focused laser beam.

The art of lithographic printing is based on the immiscibility of oil and water, with the oily material or ink being preferentially retained by the image area and the water or fountain solution being preferentially retained by the non-image area. When a suitably prepared surface is wetted with water and then a printing ink is applied, the background or non-image area retains the water and repels the ink, while the image area accepts the ink and repels the water. The printing ink on the image area is then transferred to the surface of a material such as paper, cloth and the like on which the image is to be reproduced. Generally, the printing ink is transferred to an intermediate material called a blanket, which then transfers the printing ink to the surface of the material on which the image is to be reproduced.

A very common type of lithographic printing plate has a light-sensitive coating applied to an aluminium-based support. The coating can react to light by making the exposed part soluble so that it is removed in the development process. Such a plate is called positive working. Conversely, if this exposed part of the coating is hardened, a plate is called negative working. In both cases, the remaining image area receives ink or is oleophilic and the non-image area or background receives water or is hydrophilic. The differentiation between image and non-image areas is made in the exposure process, whereby a film with a vacuum to ensure good contact. The plate is then exposed to a light source, part of which is UV radiation. If a positive plate is used, the area on the film corresponding to the image on the plate is opaque so that light does not attack the plate, while the area on the film corresponding to the non-image area is clear and allows light to pass through to the coating which is then dissolved and removed. In the case of a negative plate, the reverse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating beneath the clear area of the film is hardened by the action of light while the area not attacked by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and accepts ink, while the non-image area which had the coating removed by the action of a developer is desensitized and is therefore hydrophilic.

Photothermal direct-write lithography plates are known under the name Kodak Direct Image Thermal Printing Plate. However, they require wet processing in alkaline solutions. It would be desirable to obtain a photothermal direct-write lithography plate that does not require development.

The art has attempted to produce such plates by a variety of means. All of them have failed to produce a plate with high writing sensitivity, high image quality, low curl and high run length without development.

US-A-5,372,907 describes a direct write lithographic plate which is exposed to a laser beam and then heated to crosslink and thus prevent development of the exposed areas while making the unexposed areas more developable, the plate then being developed in conventional alkaline plate developer solution. The problem with this is that developer solutions and the apparatus containing them require maintenance, cleaning and periodic replenishment of the developer, all of which is expensive and cumbersome.

US-A-4,034,183 describes a directly writable lithographic plate without development, wherein a laser-absorbing, hydrophilic cover layer applied to a support is exposed to a laser beam to burn the absorption agent and thereby convert it from an ink-repelling state to an ink-receptive state. All examples and teachings require a high energy laser and the run lengths of the lithographic plates are limited.

US-A-3,832,948 describes both a printing plate with a hydrophilic layer that can be removed from a hydrophobic support by strong light and a printing plate with a hydrophobic layer that can be removed from a hydrophilic support. However, no examples are given.

US-A-3,964,389 describes a development-free printing plate which is produced by laser transferring a material from a carrier film (donor) to a lithographic surface. The problem with this process is that small dust particles trapped between the two layers can cause image degradation. It is also more expensive to produce two parts.

US-A-4,054,094 describes a process for making a lithographic plate using a laser beam to etch away a thin overcoat of polysilicic acid on a polyester substrate, thereby making the exposed areas receptive to ink. No details are given of run time or print quality, but it is expected that a non-crosslinked polymer such as polysilicic acid will rub off relatively quickly and give a short run of acceptable prints.

US-A-4,081,572 describes a process for producing a printing form on a support by coating the support with a hydrophilic polyamic acid and then imagewise converting the polyamic acid to melanophilic polyimide under heat from a flash lamp or laser. No details are given about run length, image quality or ink/water balance.

US-A-4,731,317 describes a process for producing a lithographic plate by applying a polymeric diazo resin to a roughened, anodized aluminum lithographic support, exposing the image areas with a YAG laser and then processing the plate with a graphic varnish. The varnishing step is cumbersome and expensive.

Japanese Kokai No. 55/105560 describes a method for producing a lithographic plate by removing a hydrophilic layer applied to a melanophilic support with a laser beam, the hydrophilic layer containing colloidal Contains silica, colloidal alumina, a carboxylic acid or a carboxylic acid salt. The only examples given use colloidal alumina alone or zinc acetate alone without cross-linking agents or additives. No details are given on the ink/water balance or run length limitations.

WO 92/09934 describes and broadly claims a photosensitive composition containing a photoacid generator and a polymer having acid-unstable tetrahydropyranyl groups. This would include a hydrophobic/hydrophilic switching lithographic plate composition. However, such a polymer switch is known to provide poor separation between ink and water in the printing process.

EP 0 562 952 A1 describes a printing plate with a polymeric azide coated on a lithographic support and removal of the polymeric azide by exposure to a laser beam. No printing press examples are given.

WO 94/18005 describes a printing plate with a laser-absorbing layer, which is applied to a support with a cross-linked hydrophilic layer, which is removed by exposure to the laser. All examples teach a polyvinyl alcohol layer cross-linked with hydrolyzed tetraethylorthosilicate.

US-A-5,460,918 describes a thermal transfer process for producing a lithographic plate from a donor with an oxazoline polymer to a receiver with a silicate surface. A two-part system such as this gives rise to problems with image quality due to dust and the high cost of producing two parts.

It would be desirable to obtain a lithographic plate with high writing sensitivity, high image quality, short free-running and long run lengths without development. None of the examples in the field can satisfy this satisfactorily.

The present invention is a method of making a lithographic plate comprising a support coated with an ink-accepting laser-absorbing layer which is subsequently coated with a cross-linked hydrophilic layer having metal oxide groups on the surface. Exposure of this plate to a high intensity laser beam followed by mounting on a press results in excellent prints without chemical treatment.

The lithographic printing plate includes

a) a carrier with

b) a melanophilic, photothermal conversion layer with

c) a melanophobic layer comprising a cross-linked polymeric matrix containing a colloid of an oxide or a hydroxide of beryllium, magnesium, aluminium, silicon, gadolinium, germanium, arsenic, indium, tin, antimony, tellurium, lead, bismuth or of a transition metal, wherein the cross-linked polymeric matrix is derived from a cross-linking agent which is an alkyl silicate, an alkyl titanate, an alkyl zirconate or an alkyl aluminate.

The printing plate is exposed to a laser beam with an intensity greater than 0.1 mW/um² (mW/u²) for a duration sufficient for complete exposure to 200 mJ/cm² or greater .

The support of this invention can be a polymer, metal or paper film or a laminate of any of the three. The thickness of the support can vary as long as it can withstand the stress of the printing press and is thin enough to wrap around the printing form. A preferred embodiment uses polyethylene terephthalate with a thickness of 100 to 200 micrometers (microns). Another embodiment uses aluminum with a thickness of 100 to 500 micrometers (microns). The support should be stretch resistant enough that color printing records result in a full color image. The support can be coated with one or more "primer" layers to improve the adhesion of the final assembly. The back of the support can be coated with antistatic agents and/or slip or matting layers to improve the handling and "feel" of the lithographic plate.

The term "melanophilic" is Greek and means ink-attracting. Since most conventional inks are linseed oil-based, melanophilic is usually equated with oleophilic.

The photothermal conversion layer absorbs laser radiation and converts it into heat. It converts photons into phonons. To do this, it must contain a non-luminescent absorber. Such an absorber can be a dye, a pigment, a metal, or a number of dichroic materials that absorb because of their refractive index and thickness. The absorber can be in the hydrophilic layer or thermally close to the hydrophilic layer. This requires assumes that a significant portion of the heat generated by the absorber acts to raise the temperature of the hydrophilic layer to a level at which the change to the melanophilic state occurs. Examples of dyes useful as absorbers for near infrared diode laser beams can be found in US-A-4,973,572. One useful pigment is carbon.

The binder used to hold the dye or pigment in the photothermal conversion layer can be selected from a wide list of film-forming polymers. Useful polymers can be found in the families of polycarbonates, polyesters, and polyacrylates. Chemically modified cellulose derivatives such as nitrocellulose, cellulose acetate propionate, and cellulose acetate are particularly useful. Exemplary polymers can be found in U.S. Patent Nos. 4,695,286, 4,470,797, 4,775,657, and 4,962,081.

Surfactants may be included in the photothermal conversion layer to facilitate coating uniformity. A particularly useful surfactant for solvent-coated polymer layers is DC510, a silicone oil sold by Dow Corning Company of Midland, Michigan.

The melanophobic or hydrophilic layer is intended to be effectively wetted by an aqueous fountain solution in the lithographic printing process and, when wet, to repel the ink. In addition, it is useful if the hydrophilic layer is somewhat porous so that wetting is more effective. The hydrophilic layer must be crosslinked if long print runs are to be achieved, since a non-crosslinked layer will rub off too quickly. Many crosslinked hydrophilic layers are available. Those derived from di-, tri- or tetraalkoxysilanes or titanates, zirconates and aluminates are used in this invention. Examples are colloids of silicon hydroxide, aluminum hydroxide, titanium hydroxide and zirconium hydroxide. These colloids are formed by processes fully described in US-A-2,244,325, 2,574,902 and 2,597,872. Stable dispersions of such colloids can be purchased inexpensively from companies such as DuPont Company of Wilmington, Delaware. The hydrophilic layer is most effective when it contains a minimal amount of hydrophobic groups such as methyl groups or alkyl radicals. The hydrophilic layer should preferably contain less than 5% by weight of hydrocarbon groups. A preferred embodiment of the invention uses aminopropyltriethoxysilane as the crosslinking and polymer-forming layer with the addition of colloidal silica to impart porosity to the layer. The thickness of the crosslinking and polymer forming layer may be from 0.05 to 1 µm thick, and more preferably from 0.1 to 0.3 µm thick. The amount of silica added to the layer may be from 100 to 5000% of the crosslinking agent, and most preferably from 500 to 1500% of the crosslinking agent. Surfactants, dyes, laser absorbing agents, colorants useful for visualizing the written image, and other additives may be added to the hydrophilic layer as long as their content is low enough not to significantly affect the ability of the layer to hold water and repel ink.

The laser used to expose the lithographic plate of this invention is preferably a diode laser because of the reliability and low maintenance of diode laser systems, but other lasers such as gas or solid state lasers may also be used.

The layers are applied to the support by any well-known coating technique such as spin coating, knife coating, gravure coating, dip coating or extrusion coating with hoppers. The process for using the resulting lithographic plate comprises the steps of: 1) exposing the plate to a focused laser beam in the areas where ink is desired in the printed image, and 2) mounting the plate on a printing press. No heating, developing or cleaning prior to printing is required. A vacuum dust collector may be useful during the laser exposure step to keep the focus lens clean. Such a collector is fully described in US-A-5,574,493. The power, intensity and exposure level of the laser is fully described in the co-pending application referred to.

In the process for making the lithographic printing plate described above, it has been discovered that by exposing these elements to a focused laser beam having an intensity greater than 0.1 milliwatts/µm² (mW/u²) for a duration sufficient to achieve a complete exposure of about 200 millijoules/cm² or greater, the efficiency of the process is improved and better prints are obtained with less laser exposure energy. Good prints are defined as those having a uniform optical density greater than 1.0. This improvement in efficiency is unexpected since in general, when exposing lithographic printing plates of a film negative, it was found that the same degree of exposure, namely the same amount of joules per square centimeter, is required without taking the exposure lamp into account.

The following examples illustrate the invention.

example 1

A mixture of 10 g of carbon (Cabot Black Pearls 700) in 400 g of methyl ethyl ketone and 400 g of methyl isobutyl ketone with 21 g of nitrocellulose was mixed with 1 mm diameter zirconia beads (the amount of beads filled half the container) in a tumbler mixer for 24 hours. The beads were filtered off and the suspension was applied to polyethylene terephthalate as a 32.29 ml/m² (3.0 cc/ft²) wet coating. After drying, the support was coated with a solution of 120 g of colloidal silica stabilized with ammonia (Nalco 2326) mixed with 280 g of water, 2 g of aminopropyltriethoxysilane and 0.1 g of Zonyl FSN surfactant, the mixture being applied as 16 ml (cc) per square meter of wet coating. The coating was dried at 118°C for 3 minutes. The coating was then exposed to a focused diode laser beam at 830 nm wavelength on an apparatus similar to that described in US-A-5,446,477. The exposure level was about 600 mJ/cm2 and the intensity of the beam was about 3 mW/µm2 (mW/u2). The laser beam was adjusted to produce a halftone dot image. After exposure, the plate was mounted on an ABDick press and several thousand good impressions were made.

Example 2

A solution of 4 g of nitrocellulose and 2 g of 2-{2-{2-chloro-3-{(1,3-dihydro-1,1,3-trimethyl-2H-benz{e}indol-2-ylidene)ethylidene}-1-cyclohexen-1-yl}ethenyl}-1,1,3-trimethyl- 1H-benz{e}indolium 4-methylbenzenesulfonate in 200 ml (ce) of a 70:30 mixture of methyl isobutyl ketone and ethanol was coated on a polyethylene terephthalate support at 32.69 ml/m² (cc/m²). After drying, the support was coated with a solution of 120 g of colloidal silica stabilized with ammonia (Nalco 2326) mixed with 280 g of water, 2 g of aminopropyltriethoxysilane and 0.1 g of Zonyl FSN surfactant, the mixture being applied as a 16 ml/m² wet coat. The coating was dried for 3 minutes at 118ºC. The coating was then a focused diode laser beam at 830 nm wavelength on an apparatus similar to that described in US-A-5,446,477. The exposure level was about 600 mJ/cm and the intensity of the beam was about 3 mW/µm² (mW/u²). The laser beam was adjusted to produce a halftone dot image. After exposure, the plate was mounted on an ABDick press and several thousand good impressions were produced.

Example 3

Example 2 was repeated, but the nitrocellulose was replaced by cellulose acetate propionate and the mixture was applied at 18.88 g/m².

Example 4

Example 3 was repeated, but the cellulose acetate propionate was replaced by polyvinyl acetate.

Example 5

Example 3 was repeated, but the cellulose acetate propionate was replaced by Novolak.

Example 6

Example 3 was repeated, but the cellulose acetate propionate was replaced by α-cyanoacrylate and the solvent was acetonitrile.

Example 7

Example 1 was repeated, but the hardener used was a mixture of dimethyldimethoxysilane and methyltrimethoxysilane, sold as Z-6070 by Dow Corning Company. Several hundred good prints were printed.

Example 8

Example 7 was repeated, but the hardener used was glycidoxypropyltrimethoxysilane. Several hundred good prints were printed.

Example 9

A mixture of 10 g of carbon (Cabot Black Pearls 700) in 400 g of methyl ethyl ketone and 400 g of methyl isobutyl ketone with 21 g of nitrocellulose was mixed with 1 mm diameter zirconium oxide beads (the amount of beads filled half the container) in a tumbler mixer for 24 hours. The beads were filtered off and the suspension was coated onto polyethylene terephthalate as a 32.29 ml/m² (3.0 cc/ft²) wet coat. After drying, the support was coated with a solution of 120 g of colloidal silica stabilized with ammonia (Nalco 2326) mixed with 280 g of water, 2 g of aminopropyltriethoxysilane and 0.1 g of Zonyl FSN surfactant; the mixture was coated as a 16 ml/m² wet coat. The coating was dried for 3 minutes at 118°C. The coating was then exposed to a focused diode laser beam at 830 nm wavelength on an apparatus similar to that described in US-A-5,446,477. The laser intensity was adjusted stepwise in 40 steps from full intensity down to 6/256 of the total power at each step. Exposure was performed at four different drum rotation speeds. The resulting set of step-wedge exposures provides a set of exposures of different intensity with different lengths of time. After exposure, the plate was mounted on an ABDick press and 1000 prints were made. Print number 500 was selected and the last step of full density (with the lowest power) at each rpm was determined. The laser intensity for each step is obtained by the laser power at the step divided by the area of the laser spot. The area of the laser spot was measured by a laser beam profiler and was 25 x 12 micrometers (milciron) at the 1/e² point. For each of the lowest full density levels, the exposure and intensity were calculated and are presented in Table 1. Table 1

The data in Table 1 show that a higher intensity laser beam is more effective and requires less total exposure energy to achieve a good print.

Control 1

A solution of 5% colloidal alumina (Dispal 18N4-20) in water was coated at 21.5 ml/m² (cc/m²) onto the same carbon coated support as used in Example 1 and dried at 118°C for 3 minutes. The coating was then exposed to a focused diode laser beam at 830 nm wavelength on an apparatus similar to that described in US-A-5,446,477. The exposure level was about 600 mJ/cm² and the intensity of the beam was about 3 mW/gm² (mW/u²). The laser beam was adjusted to produce a halftone dot image. After exposure, the plate was placed on an ABDick press and prints were made. After about 20 prints, the background began to ton. After 100 prints, the image was ugly and unusable. This shows that the crosslinking agent is important for good print performance.

Control 2

Example 1 was repeated in every respect except that the crosslinking agent, aminopropyltriethoxysilane, was omitted. After exposure, the plate was mounted on an ABDick press and prints were made. The background never became completely white, but a faint low contrast image was visible on a few prints. After about 20 prints, the background was so dark that the image was essentially invisible. This control shows that the crosslinking agent is important for good print performance.

Control 3

A mixture of 1.76% titanium dioxide, 3.4% poly(vinyl alcohol) (Scientific Products, 96% hydrolyzed), 1.69% hydrolyzed tetraethyl orthosilicate (prepared by stirring 22 g tetraethyl orthosilicate, 44 g water, 44 g ethanol and 30 mg concentrated hydrochloric acid for 10 minutes), 0.22% nonylphenoxypolyglycidol (surfactant) was coated at 21.5 ml/m² (cc/m²) onto a polyethylene terephthalate support and dried at 118ºC for 3 minutes. The coating was then held at 100ºC for 1 hour. The coating was then exposed to a focused diode laser beam at 830 nm wavelength on an apparatus similar to that described in US-A-5,446,477. The exposure level was about 600 mJ/cm2 and the intensity of the beam was about 3 mW/µm2 (mW/u2). The laser beam was adjusted to produce a halftone dot image. After exposure, the plate was mounted on an ABDick press and prints were made. The first three or four prints gave a light but visible image. By the tenth print, full ink density was achieved, but the background toned to the point where the image was no longer visible. This control shows that the process and element described in WO 94/18005 is significantly inferior to the present invention.

Control 4

A mixture of 1.5% aminopropyltriethoxysilane in water was coated on the carbon-containing layer of Example 1. After drying, the coating was exposed and applied to the press as in Example 1. The plate took up ink all over and good images were not printed. This shows that both the hardener and the colloidal oxide (such as silica) are necessary for good printing performance.

Claims (19)

1. A method of making a lithographic printing plate comprising exposing an element comprising (a) a carrier, b) a melanophilic photothermal conversion layer applied to the support and c) a melanophobic layer comprising a cross-linked polymeric matrix containing a colloid of an oxide or a hydroxide of beryllium, magnesium, aluminum, silicon, gadolinium, germanium, arsenic, indium, tin, antimony, tellurium, lead, bismuth or a transition metal, wherein the cross-linked polymeric matrix is derived from a cross-linking agent which is an alkyl silicate, an alkyl titanate, an alkyl zirconate or an alkyl aluminate, with a laser beam with an intensity greater than 0.1 mW/um² (mW/u²) for a duration sufficient to provide complete exposure to 200 mJ/cm² or greater. 2. The method of claim 1, wherein the exposure time is 0.1 to 1 second per square centimeter. 3. The method of claim 1, wherein the support is a polyester film. 4. The method of claim 1, wherein the photothermal conversion layer comprises carbon dispersed in a cellulosic binder. 5. The method of claim 1, wherein the thickness of the melanophobic layer is 0.05 to 1 micrometer (micron). 6. The method of claim 1, wherein the colloid is silicon hydroxide. 7. The process of claim 1, wherein the colloid is aluminum hydroxide. 8. The method of claim 1, wherein the colloid is titanium hydroxide. 9. The method of claim 1, wherein the colloid is zirconium hydroxide. 10. The method of claim 1, wherein the colloid is silica. 11. The method of claim 1, wherein the crosslinking agent is a di-, tri- or tetraalkoxysilane. 12. The process of claim 1, wherein the crosslinking agent is aminopropyltriethoxysilane. 13. The method of claim 1, wherein the crosslinking agent is a mixture of dimethyldimethoxysilane and methyltrimethoxysilane. 14. The method of claim 1, wherein the crosslinking agent is glycidoxypropyltrimethoxysilane. 15. The method of claim 1, wherein the crosslinking agent is tetraethyl orthosilicate. 16. The method of claim 1, wherein the crosslinking agent is tetrabutyl titanate. 17. The method of claim 1, wherein the crosslinking agent is zirconium butoxide. 18. The method of claim 1, wherein the melanophobic layer contains 100 to 5000% colloid, based on the weight of the crosslinked polymeric matrix. 19. The method of claim 1, wherein the total exposure required to expose the element to provide a good print with uniform optical density greater than 1 decreases as the intensity of the laser beam increases.