DE69620336T2 - Process for producing a lithographic printing plate with water as a developer - Google Patents

Description

1. Technical field of the invention.

The present invention relates to a method for producing a lithographic printing plate. The present invention particularly relates to a method in which the lithographic printing plate can be developed with tap water or an aqueous liquid.

2. General state of the art.

Lithographic printing is the process of printing from specially manufactured surfaces, certain areas of which will attract lithographic ink and other areas will repel the ink when wetted with water. The ink-attracting areas form the printing image areas, and the ink-repelling areas form the background areas.

In the field of photolithography, a photographic material is made to attract oily or greasy colors in the photo-exposed areas (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.

In the production of conventional lithographic printing plates, also referred to as surface lithoplates or planographic printing plates, a support which has an affinity for water or has acquired such an affinity through chemical processing is coated with a thin layer containing a radiation-sensitive composition. Suitable layers containing a radiation-sensitive composition are light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a variety of synthetic photopolymers. In particular, diazo-sensitized layer combinations are widely used.

During image-wise exposure of the light-sensitive layer, the exposed image areas become insoluble and the non-exposed areas remain soluble. The printing plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin contained in the non-exposed areas.

Commercial diazo plates typically use an anodized and roughened aluminum support with a hydrophilic surface. However, there are also commercially available plates with a flexible support such as a paper support coated with a hydrophilic layer. For example, the Lithocraft 10008 PHOTOPLATETM is a diazo plate that contains a hydrophilic layer on a paper support and a light-sensitive diazo layer coated over it. Depending on the plate supplier's instructions, a plate can be made by image-wise exposing the lithographic plate precursor or imaging element, mounting the exposed imaging element on the press, and wiping the plate surface with Lithocraft® 10008 Developer/Desensitizer. The plate supplier’s instructions also include a process that does not use a developer/desensitizer. However, such a process usually results in poor lithographic performance, so in practice a developer/desensitizer is almost always required.

The radiation-sensitive imaging elements described above, used to make a printing plate, have the particular disadvantage that they must be protected from light. This disadvantage is particularly evident in on-press development (development on the printing press), since the image-wise exposed imaging element is usually clamped in normal daylight, thus limiting the handling time for clamping the imaging element. In addition, aluminum-type diazo printing plates are not at all suitable for on-press development.

On the other hand, processes for producing printing plates are known in which image-forming elements are used that are more sensitive to heat than to light. For example, FR-A 1 561 957 describes an image-forming Material is described which contains hydrophobic thermoplastic polymer particles in a hydrophilic binder. JP-A 57102394 also describes a similar material which contains hydrophobic polymer particles in poly(vinyl alcohol) as a hydrophilic binder and is described as suitable for processing with water.

Research Disclosure No. 33303, January 1992, describes a heat-sensitive imaging element comprising a cross-linked hydrophilic layer comprising thermoplastic polymer particles and an infrared-absorbing pigment such as carbon black on a support. When exposed imagewise to an infrared laser, the thermoplastic polymer particles coagulate imagewise, rendering the surface of the imaging element ink-accepting at those areas without further development. A disadvantage of this process is the high susceptibility of the resulting printing plate to damage, since the non-printing areas can become ink-accepting when light pressure is applied to those areas. In addition, the lithographic performance of such a printing plate may be poor under critical conditions and such a printing plate will therefore have a limited lithographic printing latitude.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The object of the present invention is to provide a method in which a printing plate with excellent printing properties is produced in a practical and ecological manner.

Further objects of the present invention will become apparent from the following description.

The present invention provides a method for producing a lithographic printing plate characterized by the following steps.

(1) imagewise exposure to light from an imaging element which is (i) supported on a hydrophilic surface of a flexible substrate coated with a crosslinked hydrophilic layer support comprises an image-forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80ºC and (ii) a compound capable of converting light into heat and contained in the image-forming layer or a layer adjacent thereto,

(2) and developing an image-wise exposed imaging element thus obtained by mounting it on a printing drum of a printing press and supplying an aqueous fountain solution and/or printing ink to the image-forming layer while the printing drum is rotating.

The present invention also provides a method for producing multiple copies from an original, characterized by the following steps.

(1) imagewise exposure to light of an imaging element comprising (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80°C and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto,

(2) mounting, without prior development, an image-wise exposed imaging element thus obtained on a printing drum of a printing press,

(3) rotating the printing drum, whereby an aqueous fountain solution and/or ink are supplied to the imaging layer of the imaging element, and

(4) the transfer of ink from the image-forming element to a receiving element, usually a sheet of paper.

The present invention further provides a method for producing a lithographic printing plate characterized by the following steps.

(1) mounting an imaging element comprising (i) on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80ºC and (ii) a compound capable of converting light into heat and contained in the image-forming layer or an adjacent layer, onto a printing drum of a printing press,

(2) imagewise laser or LED exposure of the imaging element, and

(3) developing an image-wise exposed imaging element thus obtained by supplying an aqueous fountain solution and/or printing ink to the imaging layer while the printing drum is rotating.

The present invention also provides a method for producing multiple copies from an original, characterized by the following steps.

(1) mounting an imaging element comprising (i) on a hydrophilic surface of a flexible support coated with a cross-linked hydrophilic layer an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80°C and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto, on a printing drum of a printing press,

(2) imagewise laser or LED exposure of the imaging element,

(3) developing an image-wise exposed imaging element thus obtained by supplying an aqueous fountain solution and/or printing ink to the imaging layer while the printing drum is rotating, and

(4) the transfer of ink from the imaging element to a receiving element.

The present invention also describes a printing device comprising the following parts.

(i) at least one printing station comprising the following parts.

- a printing drum

- a means for mounting an imaging element on the printing drum,

- at least one laser or LED device,

- a means for directing and focusing the laser or LED radiation onto the surface of the imaging element,

- a means which controls the mutual movement of the conducting means and the carrier element so that the surface of the image-forming element is scanned by the laser or LED radiation,

(ii) a means for conveying dampening water to the printing drum,

(iii) a means for conveying printing ink to the printing drum,

(iv) means for conveying a receiving element to the printing station, and

(v) a means for transferring ink from the printing drum to the receiving member.

In yet another aspect, the present invention relates to a method for producing a lithographic printing plate characterized by the following steps.

(1) imagewise exposure to light from an imaging element containing (i) on a hydrophilic surface of a flexible support coated with a crosslinked hydrophilic layer an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80ºC and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto,

(2) and developing an image-wise exposed imaging element thus obtained by rinsing with tap water or an aqueous liquid.

4. Short description of the characters

The present invention is illustrated by the following figures, but is not limited thereto.

Fig. 1 is an isometric view of the cylindrical embodiment of a usable according to the invention and with a Diagonal writing matrix cooperating image generating device,

Fig. 2 is a schematic representation of the embodiment shown in Fig. 1, illustrating its working mechanism in more detail,

Fig. 3 is a front view of a writing matrix used for image formation according to the invention, in which the image-forming elements are arranged in a diagonal matrix,

Fig. 4 is an isometric view of the cylindrical embodiment of an image generating device usable according to the invention and cooperating with a linear writing matrix,

Fig. 5 is a cutaway perspective of a separate laser and a beam guidance system,

Fig. 6 is an enlarged partial sectional perspective view of a lens element that focuses a laser beam from an optical fiber onto the surface of an imaging element,

Fig. 7 is an enlarged partial sectional perspective view of a lens element with a built-in laser, and

Fig. 8 is a circuit diagram of a laser driver circuit usable according to the invention.

5. Detailed Description of the Present Invention An imaging element useful in accordance with the invention comprises on a hydrophilic surface of a lithographic support an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80°C. The hydrophilic binder used in the present invention is preferably not or only slightly crosslinked. The imaging element further comprises a compound capable of converting light into heat. Although this compound is preferably incorporated in the imaging layer, it can also be contained in a layer adjacent to the imaging layer.

The lithographic support comprises a flexible support, such as a paper support or a plastic film, coated with a cross-linked hydrophilic layer. A particularly suitable cross-linked hydrophilic layer can be obtained from a hydrophilic binder cross-linked with a cross-linking agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolyzed tetraalkyl orthosilicate. The latter binder is particularly preferred.

Hydrophilic (co)polymers such as homopolymers and copolymers of vinyl alcohol, acrylamide, methylolacrylamide, methylolmethacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride-vinyl methyl ether copolymers are suitable as hydrophilic binders. The hydrophilicity of the (co)polymer or (co)polymer mixture used is preferably higher than or equal to the hydrophilicity of at least 60% by weight, preferably at least 80% by weight, hydrolyzed polyvinyl acetate.

The amount of crosslinking agent, in particular tetraalkyl orthosilicate, is preferably at least 0.2 parts by weight per part by weight of hydrophilic binder, preferably between 0.5 and 5 parts by weight, particularly preferably between 1.0 part by weight and 3 parts by weight per part by weight of hydrophilic binder.

A crosslinked hydrophilic layer in a lithographic support used according to this embodiment preferably also contains substances which improve the mechanical strength and porosity of the layer. Colloidal silica can be used for this purpose. The colloidal silica can be used in the form of any commercially available water dispersion of colloidal silica with, for example, an average particle size of up to 40 nm, e.g. 20 nm. In addition, inert particles with a larger grain size than the colloidal silica can be added, e.g. silica prepared as described in J. Colloid and Interface Sci., Volume 26, 1968, pages 62 to 69, by Stöber, or alumina particles or particles with an average diameter of at least 100 nm, which are particles of titanium dioxide or other heavy metal oxides. By embedding these particles, the surface of the cross-linked hydrophilic layer acquires a uniform rough texture with microscopic peaks and valleys that serve as storage points for water in background areas.

The thickness of a cross-linked hydrophilic layer in a lithographic support used according to this embodiment may vary between 0.2 µm and 25 µm and is preferably between 1 µm and 10 µm.

Particular examples of suitable crosslinked hydrophilic layers usable according to the invention are described in EP-A 601 240, GB-P 1 419 512, FR-P 2 300 354, US-P 3 971 660, US-P 4 284 705 and EP-A 514 490.

As a flexible support of a lithographic support according to this embodiment, a plastic film is particularly preferred, for example a subbed polyethylene terephthalate film, a cellulose acetate film, a polystyrene film, a polycarbonate film, etc. The plastic film support can be opaque or translucent.

A polyester film carrier coated with an adhesion-improving layer is particularly preferred. Adhesion-improving layers particularly suitable for use according to the invention contain a hydrophilic binder and colloidal silica, as described in EP-A 619 524, EP-A 620 502 and EP-A 619 525. The amount of silica in the adhesion-improving layer is preferably between 200 mg/m² and 750 mg/m². Furthermore, the ratio of silica to hydrophilic binder is preferably above 1 and the specific surface area of the colloidal silica is preferably at least 300 m²/g, particularly preferably 500 m²/g.

In the present invention, an image-forming layer is coated onto a hydrophilic surface. One or more intermediate layers are optionally inserted between the lithographic support and the image-forming layer. An image-forming layer according to the invention contains thermoplastic polymer particles dispersed in a hydrophilic binder.

Suitable hydrophilic binders for use in an image-forming layer according to the invention are, for example, synthetic homo- or copolymers such as a polyvinyl alcohol, a poly(meth)acrylic acid, a poly(meth)acrylamide, a polyhydroxyethyl (meth)acrylate, a polyvinyl methyl ether or natural binders such as gelatin, a polysaccharide such as dextran, pullulan, cellulose, gum arabic and alginic acid.

The glass transition point of the hydrophobic thermoplastic polymer particles used according to the invention is preferably at least 90°C, particularly preferably at least 100°C.

The hydrophobic thermoplastic polymer particles used in the invention preferably have a coagulation temperature above 50°C and more preferably above 70°C. Coagulation can occur as a result of softening or melting of the thermoplastic polymer particles under the action of heat. Although the coagulation temperature of the thermoplastic hydrophobic polymer particles is not subject to a specific upper limit, it should be sufficiently below the decomposition temperature of the polymer particles. The coagulation temperature is preferably at least 10°C below the temperature at which decomposition of the polymer particles occurs. If the polymer particles are exposed to a temperature above the coagulation temperature, they coagulate and form a hydrophobic agglomerate in the hydrophilic layer, whereby the hydrophilic layer becomes insoluble in tap water or an aqueous liquid at these locations.

Specific examples of hydrophobic polymer particles that can be used according to the invention are, for example, polystyrene, polyvinyl chloride, polymethyl methacrylate, polyvinylidene chloride, polyacrylonitrile, polyvinyl carbazole, etc. or copolymers and/or mixtures thereof. Polymethyl methacrylate is particularly preferred.

The weight average molecular weight of the polymers can vary between 5,000 and 1,000,000 g/mol.

The particle size of the hydrophobic particles can vary between 0.01 mm and 50 µm, particularly preferably between 0.05 mm and 10 µm and most preferably between 0.05 µm and 2 µm.

The polymer particles are contained as a dispersion in the aqueous coating liquid of the image-forming layer and can be prepared by the processes described in US-P 3,476,937. In another process particularly suitable for the preparation of an aqueous dispersion of the thermoplastic polymer particles.

- the hydrophobic thermoplastic polymer is dissolved in an organic water-immiscible solvent,

- the solution thus obtained is dispersed in water or an aqueous medium, and

- the organic solvent is evaporated.

The amount of hydrophobic thermoplastic polymer particles contained in the image-forming layer is preferably between 20 wt.% and 65 wt.%, more preferably between 25 wt.% and 55 wt.%, and most preferably between 30 wt.% and 45 wt.%.

As suitable light-to-heat converting compounds, preferably infrared absorbing components are used, although the absorption wavelength is not of great importance as long as the absorption of the compound used falls within the wavelength range of the light source used for imagewise exposure. Particularly useful compounds are, for example, dyes and in particular infrared dyes, carbon black, metal carbides, metal borides, metal nitrides, metal carbonitrides, oxides with a bronze structure and oxides with a structure related to the bronze family but without the A component, e.g. WO2,9. Conductive polymer dispersions can also be used, such as conductive polymer dispersions based on polypyrrole or polyaniline. The lithographic performance achieved and in particular the print durability achieved depends on the heat sensitivity of the imaging element. In this respect, it has been shown that very good and favorable results can be achieved with soot.

Although a light-to-heat converting compound according to the invention is particularly preferably embedded in the image-forming layer, at least part of the light-to-heat converting compound can also be incorporated into an adjacent layer. Such a layer can be, for example, the crosslinked hydrophilic layer of a lithographic support according to the second embodiment of lithographic supports explained above.

According to a method according to the invention for obtaining a printing plate, the image-forming element is exposed imagewise and then mounted on a printing drum of a printing press. In a preferred embodiment, the printing press is then started and, while the printing drum with the image-forming element mounted on it is rotating, first the dampening rollers supplying dampening water and then the ink application rollers are lowered onto the image-forming element. In general, the first clear and usable prints are obtained after about 10 revolutions of the printing drum.

According to an alternative method, the inking rollers and dampening rollers can be lowered simultaneously or the inking rollers can be lowered first.

The fountain solutions that can be used according to the invention are aqueous liquids that generally have an acidic pH value and contain an alcohol such as isopropanol. The fountain solutions that can be used according to the invention are not subject to any particular restrictions and commercially available fountain solutions can be used.

It may be advantageous to wipe the image-forming layer of an image-wise exposed imaging element with, for example, a water-soaked cotton ball or sponge before mounting the imaging element in the printing press or at least before starting the printing press. This will remove certain non-image areas but will not initiate the actual development of the imaging element. However, this procedure has the advantage of avoiding the risk of significant contamination from the dampening system of the printing press and the printing ink used.

According to a further process, the image-forming element is first clamped onto the printing drum of the printing press and then image-formed directly on the press. exposed. After exposure, the imaging element can be developed as described above.

According to yet another method according to the invention, the imaging element can be image-wise exposed and then developed by rinsing with tap water.

Preferably during or after development of the image-wise exposed imaging element, the element is processed with a gum such as gum arabic, dextrans, dextrins, water-soluble cellulose derivatives, pullulan, etc. The image-wise exposure according to the invention is preferably an image-wise scanning exposure using a laser or an LED diode laser. According to the invention, it is very particularly preferred to use a laser emitting in the infrared (IR) and/or near infrared range, i.e. in the wavelength range between 700 and 1500 nm. Laser diodes emitting in the near infrared range are particularly preferred for use in the present invention.

A preferred imaging device for image-wise scanning exposure according to the invention preferably comprises a laser whose power is applied directly to the surface of the imaging element through lenses or other beam directing elements or whose power is transferred from a remote laser to the surface of an unexposed imaging element through a fiber optic cable. A controller and associated positioning hardware ensure precise orientation of the radiation to the surface of the imaging element, ensure radiation scanning across the surface, and turn the laser on at locations adjacent to selected points or areas of the imaging element. The controller is responsive to input image signals corresponding to the original document and/or image to be copied onto the imaging element to produce a precise negative or positive image of that original. The image signals are stored in a computer in the form of a bit match table file. Such files can be generated by a raster image processor (RIP) or other suitable means. For example, a RIP can input data in page description language which defines all elements to be transferred to the image-forming element, or as a combination of page description language and one or more image files. The bit correspondence tables are used to define the color tone as well as the screen frequencies and screen angles in amplitude-modulated screening. However, the present invention is particularly well suited for use in combination with frequency modulation screening, as described, for example, in EP-A 571010, EP-A 620677 and EP-A 620674.

The imaging device can be autonomous, acting only as a plate automaton, or can be built directly into a lithographic printing press equipped with a dampening system. In the latter case, printing can begin immediately after imagewise exposure and development, thus achieving a significant reduction in press preparation time. The imaging device is a flat sheet recorder or a flat drum recorder, in which the unexposed lithographic printing plate is attached to the cylindrical inner or outer surface of the drum. Of course, the structure of the outer drum is more suitable for in-situ use on a lithographic printing press, the printing drum itself being the drum component of the recorder or plotter.

In a preferred drum configuration, the required relative movement between the laser beam and the imaging element is obtained by rotating the drum (and thus also the imaging element mounted thereon) about the drum axis and by moving the laser beam parallel to the axis of rotation, thereby scanning the circumference of the imaging element and gradually obtaining an image "growing" in the axial direction. The movement of the laser beam can also be set parallel to the drum axis, whereby the scanning angle increases after each scanning pass over the imaging element, thereby obtaining a circumferentially "growing" image on the imaging element. In both cases, after completion of the scanning, complete laser scanning and development produces a true image on the surface of the imaging element. In the flatbed configuration, the beam scans along one of the axes of the imaging element and is indexed after each pass along the other axis. Of course, the required relative motion between the laser beam and the imaging element can be achieved by motion of the imaging element rather than by (or in addition to) laser beam motion.

Regardless of the scanning method of the laser beam, it is usually preferable to use a large number of lasers (for speed reasons), the power of which is used to drive a single writing matrix. The writing matrix is then indexed after each pass over or along the image-forming element, a distance which is determined by the number of laser beams originating from the matrix and the desired image resolution (i.e. the number of pixels per unit length).

A more detailed description of a preferred embodiment of an image forming device usable in accordance with the invention follows below.

a. External drum recording

Referring to Figure 1 of the various figures, there is shown an external drum embodiment of an imaging system useful in accordance with the present invention. The assembly includes a drum 50 around which an imaging member 55 is wound. Drum 50 includes a hollow segment 60 in which the outer edges of imaging member 55 are secured by conventional clamping members (not shown). The size of the hollow segment can vary widely depending on the environment in which drum 50 is used.

Drum 50 is preferably installed straight forward in the structure of a conventional lithographic printing press equipped with an element supplying dampening water to the image-forming element and serves as a printing drum the press. In a typical printing press design, the supply of ink and fountain solution to the imaging element 55 is accomplished by means of an ink feed line or a series of fountain rollers, the end rollers of which are in rolling contact with drum 50. The latter drum is also in rolling contact with a blanket cylinder which transfers ink to the receiving element, typically a sheet of paper. The printing press may be equipped with more than one such printing assembly in a linear matrix. Alternatively, a plurality of printing assemblies may be arranged around a large central printing cylinder in rolling contact with all of the blanket cylinders.

Drum 50 is mounted within a support frame and is driven by a standard electric motor or other conventional device (see schematic diagram in Fig. 2). The angular position of drum 50 is monitored by a hollow shaft encoder (see Fig. 4). A

Writing matrix 65 writes to the rotating imaging element 55. The axial movement of writing matrix 65 is controlled by the rotation of a step motor 72 which drives spindle 67 and thereby shifts the axial position of writing matrix 65. After each complete passage of writing matrix 65 over the entire surface of imaging element 55, step motor 72 is energized for the time that writing matrix 65 is above cavity 60. The rotation of step motor 72 shifts writing matrix 65 to the appropriate axial location to begin the next imaging passage.

The axial index distance between successive image forming passages depends on the number of image forming parts in the writing matrix 65 and their configuration in the matrix, as well as on the desired resolution. Fig. 2 shows a series of laser sources L1, L2, L3, ... Ln driven by suitable laser driver circuits, designated as a whole by reference number 75, the power of which is supplied to a fiber optic cable Gallium arsenide lasers are preferred as lasers, although other laser models are also possible.

The cables carrying the laser power are collected in a bundle 77 and exit as separate cables connected to the writing matrix 65. To avoid loss of laser power, it may be desirable to arrange the bundle in such a configuration that it does not have to be bent above the critical angle of refraction of the fiber (thus maintaining total internal reflection).

Fig. 2 also shows a control unit 80, which then controls the laser drivers 75 when the associated lasers come to lie opposite the points to be written on the image-generating element 55. Control unit 80 also ensures the drive of the step motor 72 and the drum drive motor 82. Control unit 80 is supplied with data from two sources. A sensor 85 constantly monitors the angular position of drum 50 in relation to the writing matrix 65 and supplies the corresponding position signals to control unit 80. In addition, data signals from an image data source (e.g. a computer) are also fed into control unit 80. The image data correspond to points on image-generating element 55 that are to be written with image points. To do this, controller 80 correlates the actual relative positions of the writing matrix 65 and image forming element 55 (as indicated by sensor 85) with the image data, thereby turning on the respective laser drivers at the appropriate times during the scanning of the image forming element 55. The control circuitry required to carry out this scheme is well known to those skilled in the art of scanners and plotters.

At their end, the laser power cables run into lens assemblies 96 mounted within the writing matrix 65, which preferably direct the laser beam precisely onto the surface of the imaging element 55. The following is a description of the construction of a suitable lens assembly, these assemblies being generally designated by reference number 96 in the present invention. An illustration of a A suitable arrangement is shown in Fig. 3, in which the lens assemblies 96 are arranged offset over the surface of the body 65.

The staggered arrangement of the lens systems makes it easier to arrange a larger number of lens arrangements in a single head than would be problematic with a linear arrangement. Since the image formation time is directly related to the number of lens elements, a staggered arrangement makes it possible to speed up the overall image formation. Controller 80 is either supplied with image data already arranged in a vertical column, each belonging to a different lens arrangement, or can gradually extract the contents of a memory buffer, in which a complete bit correspondence table of the image to be transmitted is stored, from the buffer, column by column. In both cases, controller 80 recognizes the various relative positions of the lens arrangements with respect to image-forming element 55 and only controls the appropriate laser when the associated lens arrangement is located over a point to be imaged.

An illustration of an alternative matrix structure is shown in Fig. 4, which also shows sensor 85 mounted on drum 50. In this case, the writing matrix, designated by reference numeral 150, comprises a long linear body which is fed with light guides from bundle 77. The inside of writing matrix 150, or a certain portion thereof, contains threads engaging spindle 67, rotation of the spindle advancing writing matrix 150 along imaging element 55 as discussed above. The individual lens assemblies 96 are each equidistantly spaced a distance B from one another. Distance B corresponds to the difference between the axial length of plate 55 and the distance between the first and last lens assemblies. Distance B corresponds to the total axial distance traveled by writing matrix 150 during a complete scanning pass. With each passage of writing matrix 150 over cavity 60, step motor 72 is switched on and moves matrix 150 by its rotation over an axial distance corresponding to the desired spacing between image forming passages (ie the print density). This distance is smaller than the distance indicated in the embodiment described above (write matrix 65) by a factor of n, where n is the number of lens arrays incorporated in write matrix 65.

b. Performance (guide and lens arrangement)

An illustration of suitable means for delivering laser power to the surface of an imaging element is shown in Figures 5 through 7. Referring to Figure 5, this shows a separate laser assembly which uses a fiber optic cable to deliver the laser pulses to the imaging element. In this assembly, a laser source 250 is powered by an electrical cable 252. Laser 250 is located in the rear segment of a housing 255. Two or more focusing lenses 260a and 260b are located in the front segment of the housing. The lenses focus the radiation from laser 250 onto the end face of a fiber optic cable 265 which is preferably (although not necessarily) secured within the housing 255 by means of a removable retaining capsule 267. Cable 265 conducts the power from laser 250 to an output arrangement 270 illustrated in detail in Fig. 6.

Fig. 6 shows how light guide cable 265 is passed through a (preferably removable) retaining capsule 274 in assembly 270. Retaining capsule 274 fits on a generally tubular body 276 which contains a series of threads 278. In the front part of body 276 are two or more focusing lenses 280a and 280b. Cable 265 is passed through a bushing 280 through a portion of body 276. Body 276 forms a hollow channel between inner lens 280b and bushing end 280. The end face of cable 265 is thereby located a predetermined distance A from inner lens 280b. Distance A and the focal lengths of lenses 280a and 280b are dimensioned so that the radiation delivered by cable 265 at normal beam distance to the imaging element 55 is precisely focused onto the surface of the imaging element. This distance can be adjusted to vary the size of an image element.

Body 276 may be secured to writing matrix 65 by any suitable technique. In the illustrated embodiments, a nut 282 engages threads 278 and compresses an outer flange 284 of body 276 against the outer surface of writing matrix 65. The flange may optionally be provided with a translucent window 290 to protect the lenses from possible damage.

Alternatively, the lens assembly can be mounted on a pivot point within the writing matrix which ensures rotation of the lens assembly in the axial direction (i.e., in Fig. 6, through the plane of the paper). This pivot point allows the axial position to be precisely adjusted. At a rotation angle of 4º or less, the circumferential error caused by the rotation can be compensated for electronically by shifting the image data before it is fed to the controller 80.

Fig. 7 shows an alternative embodiment in which the laser source irradiates the surface of the imaging element directly without transmission through a fiber optic cable. From this FIGURE it can be seen that laser source 250 is located within the rear segment of an open housing 300. In the front part of the housing 300 there are two or more focusing lenses 302a and 302b which precisely focus the radiation from laser 250 onto the surface of the imaging element 55. The housing can optionally be provided with a light-transmitting window 305 arranged in the same plane as the open end and a heat sink 307.

It should be noted that although optical fibers are used in the above discussion of the imaging embodiments and the associated FIGURES, in any event the use of fibers can be eliminated by using the embodiment shown in Figure 7.

c. Driver circuit ·

A suitable driver circuit for driving a diode-type laser (e.g., a gallium arsenide laser) is illustrated schematically in Fig. 8. The operation of the driver circuit is controlled by controller 80, which applies a fixed pulse width signal (preferably between 5 and 20 µsec in duration) to a high speed, high voltage MOSFET driver 325. The output terminal of driver 325 is connected to the gate of a MOSFET 327. Since driver 325 is capable of providing a high output voltage to quickly charge the capacitance of the MOSFET gate, the turn-on and turn-off times of MOSFET 327 are very short (preferably less than 0.5 µsec) despite the capacitive loading. The source terminal of MOSFET 327 is connected to a ground potential.

Once the MOSFET 327 is set in the conductive state, a laser diode 330 is controlled with current. An adjustable current limiting resistor 332 that controls the diode power is inserted between the MOSFET 327 and the laser diode 330. This control can be used, for example, to correct different diode powers and to generate identical output strengths in all lasers belonging to the system, or to vary the laser power in order to control the image size.

Opposite the terminals of laser diode 330 there is a capacitor 334 which prevents damage from current overshoots, which can occur, for example, due to conductor inductance in combination with low interelectrode capacitance of the laser diodes.

The present invention will now be illustrated by the following examples, but is not limited thereto. All parts are by weight unless otherwise stated.

EXAMPLE 1 Preparation of imaging element I (material)

To prepare an imaging element I according to the invention, the following coating composition 1 is prepared, coated in an amount of 30 g/m² (wet layer amount) onto a lithographic support as specified in the description and dried at 35°C.

Preparation of the casting composition 1

7 g of a 15% carbon black dispersion, 1 ml of an aqueous wetting agent solution and 73 g of water are added successively to 6.75 g of a 20% dispersion of polymethyl methacrylate (Tg 105°C) (average particle diameter of 90 nm) stabilized with Hostapal B (1%, based on the polymer) in demineralized water, and finally 12 g of a 5% aqueous solution of a 98% hydrolyzed polyvinyl acetate with a weight average molecular weight of 200,000 g/mol are stirred in.

Manufacturing of imaging element II (material)

An imaging element II according to the invention is prepared in identical manner to imaging element I, with the exception that the polymethyl methacrylate in the coating solution is replaced by a copolymer of butyl acrylate and methyl methacrylate (16/84 wt. %) (Tg 84.2°C).

Preparation of imaging element III (material)

An imaging element III (comparative material) is prepared in an identical manner to imaging element I except that the polymethyl methacrylate in the coating solution is replaced by a copolymer of butyl acrylate and methyl methacrylate (28/72 wt%) (Tg 61.5°C).

Production of printing plates and printing of proofs

The imaging elements described above are scanned with an Nd:YLF infrared laser emitting at 1050 nm (scanning speed 4 m/s, beam width 15 µm and laser power on the surface of the imaging element 670 mw). The image-wise exposed imaging elements thus obtained are mounted on an ABDICK 360TM offset press equipped with a VARWM KOMPAC II dampening system. The ink used is Van Son RB2329TM and the fountain solution is G671cTM (3% aqueous solution) sold by Agfa-Gevaert NV. After mounting an imaging element on the printing press, the press is started and the printing drum with the imaging element mounted on it is allowed to rotate. The dampening rollers of the printing press are then lowered onto the surface of the imaging element and supply fountain solution to the imaging element. After 10 revolutions of the printing drum, the ink supply rollers are also lowered onto the imaging element. After 10 more revolutions, imaging element I produces clear prints with no ink uptake in the non-image areas. Using the same procedure, imaging element II produces prints with very slight foaming in the non-image areas. Using the same procedure, imaging element III produces prints with complete mottling. Even if the printing drum is rotated 50 times at each stage of development, the lithographic plate still shows complete mottling. It is therefore clearly evident from this example that imaging elements T and II (imaging elements of the present invention) containing hydrophobic thermoplastic polymer particles having a Tg of at least 80°C give excellent to acceptable lithographic printing plates after exposure and development, while imaging element III (comparative imaging element) containing hydrophobic thermoplastic polymer particles having a Tg of less than 80°C gives excellent to acceptable lithographic printing plates after exposure and development. Development printing plates with complete spotting can be obtained.

Claims (7)

1. A process for making a lithographic printing plate characterized by the following steps. (1) imagewise exposure to light from an imaging element containing (i) on a hydrophilic surface of a flexible support coated with a crosslinked hydrophilic layer an imaging layer comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder having a glass transition temperature Tg of at least 80ºC and (ii) a compound capable of converting light into heat contained in the imaging layer or a layer adjacent thereto, (2) and developing an image-wise exposed imaging element thus obtained by rinsing with tap water or an aqueous liquid. 2. A method according to claim 1, characterized in that the compound capable of converting light into heat is selected from the group consisting of an infrared absorbing dye, carbon black, a metal boride, a metal carbide, a metal nitride, a metal carbonitride and a conductive polymer particle. 3. A method according to any one of the above claims, characterized in that the imagewise exposure is carried out with a laser, an LED diode or a plurality of lasers. 4. A method according to any one of the above claims, characterized in that the compound capable of converting light into heat is contained in the image-forming layer. 5. A process according to any one of the above claims, characterized in that the thermoplastic polymer particles have a coagulation temperature of at least 50°C. 6. A method according to any one of the above claims, characterized in that the hydrophilic binder in the image-forming layer is selected from the group consisting of polyvinyl alcohol, a poly(meth)acrylic acid, a poly(meth)acrylamide, a polyhydroxyethyl (meth)acrylate, a polyvinyl methyl ether and a polysaccharide. 7. A method according to any one of the above claims, characterized in that the hydrophobic thermoplastic polymer particles are selected from the group consisting of polystyrene, polyvinyl chloride, polymethyl methacrylate, polyvinylidene chloride, polyacrylonitrile, polyvinylcarbazole or their copolymers and/or mixtures.