WO1995007496A1

WO1995007496A1 - A light sensitive printing plate

Info

Publication number: WO1995007496A1
Application number: PCT/GB1994/000857
Authority: WO
Inventors: Robert Michael Organ; Christopher David Mccullough; Peter Andrew Reath Bennett
Original assignee: Horsell Graphic Industries Ltd.
Priority date: 1993-09-09
Filing date: 1994-04-22
Publication date: 1995-03-16
Also published as: AU6573294A

Abstract

A lithographic printing plate comprising a substrate, a plurality of particles provided in an array on a surface of the substrate, and a layer of a light sensitive material on the surface of the substrate, the particles being arranged between the surface of the substrate and the layer of light sensitive material so that formations are provided on the surface of the layer of light sensitive material as a result of the particles under the layer. The particles can be deposited on the surface by a dry deposition technique, for example using a plasma. The particles facilitate removal of air from the space between the plate and a film positioned over the plate for exposure.

Description

A LIGHT SENSITIVE PRINTING PLATE

This invention provides a light sensitive printing plate, and a method of making a light sensitive printing plate. The plate of the invention might have an image formed on it, for use in printing processes such as lithography.

Image and non-image areas can be created on light sensitive printing plates by processes which include a step of exposing a layer of image material on the surface of the plate to radiation. By these processes, differences can be introduced in the solubility of the image material, so that the image material can be removed selectively from the plate, to leave the plate with a pattern on it corresponding to the image.

Image and non-image areas of the plate can be defined with respect to one another by applying a mask, for example in the form of a film, to the plate, in which the mask is transparent to radiation only in selected regions. For accurate definition of the image and non-image areas of the plate, it is important that the mask and the plate be in close surface to surface contact, and it is conventional to draw the mask and the plate towards one another by means of a vacuum.

GB-A-2025646 discloses a printing plate which comprises a substrate, a layer of photosensitive material on the substrate, and particles of a powder applied to the layer of photosensitive material. The particles stand proud of the surface of the layer of photosensitive material, so that there are channels between the particles through which fluid between the surface of the plate and a mask applied to the surface can escape. The provision of surface formations in this way thereby facilitates maintenance of close surface to surface contact, between a mask and a plate when pulled towards one another, for example by means of a vacuum. The present invention provides a technique for providing surface formations on a light sensitive printing plate in which particles are provided on the surface of the substrate, under a layer of light sensitive material. The particles may be provided by means of a deposition technique.

Accordingly, in one aspect, the invention provides a litho¬ graphic printing plate which comprises :

(a) a substrate,

(b) a plurality of particles provided in an array on a surface of the substrate, and

(c) a layer of a light sensitive material on the said surface of the substrate, the said particles being arranged between the surface of the substrate and the layer of light sensitive material so that formations are provided on the surface of the layer of light sensitive material as a result of the particles under the layer.

The plate of the present invention has the advantage that by selection of a light sensitive material with appropriate properties, for example viscosity when applied to the substrate, channels are provided on the surface of the plate between the surface and a mask, as a result of the formations on the surface of the plate arising from the presence of the particles. Fluid such as air, located between the surface of the plate and a mask applied to the surface, can escape. The provision of surface formations in this way thereby facilitates maintenance of close surface to surface contact, between a mask and the plate when pulled towards one another, for example by means of a vacuum. A further significant advantage of the provision of formations in this way is that the surface of the plate is provided by a continuous layer of light sensitive material, so that the likelihood of particles providing the formations interfering with access of radiation to the light sensitive material (as when particles are provided on the surface of light sensitive material of a printing plate) is reduced.

In another aspect, the invention provides a method of making a lithographic printing plate, which comprises:

(a) providing a plurality of particles on the surface of a substrate,

(b) providing a layer of a light sensitive material on the said surface of the substrate, the said particles being arranged between the surface of the substrate and the layer of light sensitive material.

The plurality of particles will generally be deposited on the surface of the substrate. The particles may be deposited by means of a wet deposition technique, for example involving distribution of a slurry or a solution, and evaporation of a solvent.

Preferably, the particles on the surface of the substrate are bonded to the surface as a result of deposition on the surface, especially so that the bond between the particles and the substrate surface is not broken during processing of the plate, for example to develop light sensitive material.

Preferably, the particles are deposited by means of a dry deposition technique, for example thermal spraying and sputtering. .An example of a thermal spraying technique is flame spraying, which has the advantage that particles are deposited on the substrate with a relatively low kinetic energy so that, even when heated to elevated temperature, the particles do not tend to flatten significantly as they impact with the surface of the substrate. A preferred technique for deposition of the particles involves plasma spraying.

When the deposition process employs a plasma spraying technique, it will generally be preferred for the plasma to be created from an inert gas, for example of hydrogen, nitrogen or argon, or mixtures of these or other gases. The gas is heated in an electric arc to elevated temperature, for example of at least 10" °C, generally at least 2 x 10⁴ °C.

The particles can conveniently be provided on the surface of the substrate during another step by which the substrate is prepared for use in a printing plate. For example, a process is disclosed in WO-A-94/05507 by which a coated substrate for use in lithography can be prepared, involving creating a surface layer on the surface of substrate base material using a deposition technique, which preferably is a dry deposition technique such as one using a gas plasma. An image layer is created on the deposited surface layer. The surface layer can be made using particles with two or more size distributions, so as to produce a substrate with a surface layer with discrete regions of higher roughness. This technique can be used to provide the particles in the plate of the present invention. Subject matter disclosed in WO-A-94/05507 is incorporated in the specification of the present application by this reference to the document.

The particles applied to the surface of the substrate might be applied to the substrate after it has been subjected to another processing step, such as one in which the surface of the substrate is provided with a surface layer of another material. For example, a substrate might be provided with a surface layer of a material such as aluminium oxide such as by a technique disclosed in WO-A-94/05507 referred to above or another technique, and then particles might be applied to the surface in accordance with this invention. The steps might be carried out in the reverse order. When the particles are deposited on the plate after it has been provided with a surface layer, the layer can be provided by a technique which is related to that by which the particles are provided or by a different technique. For example, the layer and the particles might both be provided by means of a technique such as disclosed in WO-A-94/05507 referred to above. Alternatively, the layer or the particles may be provided by another technique, for example electrochemically or by wet deposition technique.

The particles and a surface layer on the plate can be provided in a single step for example using two materials (which differ in respect of, for example chemical or physical properties, especially particle size), or in separate steps. When the particles and the layer are provided in separate steps, suitable methods include the deposition of the particles on the layer, and creation of the layer on the particles.

The particles might be deposited in conjunction with another step in the processing of a printing plate, for example in cleaning, roughening or other surface treatment steps during preparation of a substrate. The particles can be deposited onto a substrate which has been prepared for coating with light sensitive material using electrochemical processing techniques. The particles might also be deposited during a processing step such as when a gas plasma is used to clean or to roughen a plate.

Preferably, the height of the particles above the said surface of the substrate is at least about 0.5 μm, more preferably at least about 1 μm, for example about 2 to 3 μm.

Preferably, the height of the particles above the said surface of the substrate is less than about 30 μm, more preferably^'less than about 20 μm, especially less than about 10 μm. Preferably, the transverse size of the particles (which will be a diameter in the case of particles with a circular cross- section) which are used for deposition on the surface of the substrate is greater than about 1 μm, more preferably greater than about 2 μm, especially greater than about 10 μm. Preferably, that transverse size is less than about 35 μm, more preferably less than about 20 μm, especially less than about 15 μm.

A preferred size of particles for deposition onto the surface is in the range of about 7 to 20 μm, especially when Al₂0₃ is used.

It will be preferred for many applications for the density of particles on the surface of the plate to be at least about 50 cm^"2, preferably at least about 10³ cm^"2, for example at least about 5 x 10⁴ cm^"2. Preferably, the density of particles is less than about 5 x 10⁶ cm^'2, and might suitably be less than about 10⁶ cm^"2 for many applications.

The particles will generally be formed from a material which does not react adversely with the light sensitive material of the plate.

Preferably, the particles are formed from a material which is capable of exhibiting ceramic-type properties. Desirable properties can include hardness, chemical resistance, and resistance to abrasion. Such properties can arise from rapid solidification of the deposited material on contact with the base material of the substrate. The use of a material for the particles which has ceramic-type properties has the advantage of enhancing the ability of the substrate to withstand harsh physical conditions during use. Examples of materials capable of forming surface layers on a substrate, with ceramic-type properties include certain silicas, Al₂0₃, Cr₂0₃, Ti0₂, Zr0₂, WC and blends of these materials, such as blends of Al₂0₃ and Si0₂. The material of the substrate may comprise a metal, which might be a substantially pure elemental metal or an alloy. Suitable metals include, for example, iron based materials such as certain steels, copper and copper based alloys, nickel and cobalt alloys, and aluminium, magnesium and titanium and alloys based on these metals. Non-metallic materials might be used, such as ceramic materials, polymeric materials (such as certain polyesters) and paper based materials.

The method of the invention will include appropriate steps to prepare the material of the substrate for deposition of the particles. These might include, for example, cleaning, etching, texturing, anodising, grinding or polishing of the surface.

Especially preferred materials for the substrate include aluminium and aluminium based alloys, certain steels and certain polyesters.

The substrate produced by the method will generally be in the form of a sheet. The sheet might be in discrete pieces, or in the form of a continuous web, perhaps provided on a roll. The substrate will generally be produced continuously on a sheet of moving substrate base material, by moving a sheet through production equipment. Preferably, the sheet has a width measured in a direction perpendicular to the machine direction of at least about 0.2 m, more preferably at least about 0.3 m, especially about 0.5 m.

The invention will now be described by way of example with reference to the accompanying drawing, which is a schematic cross-sectional view through a plate according to the invention.

The drawing shows a plate 2 which comprises a substrate formed from an aluminium sheet 4. An array of particles 6 of aluminium oxide are provided on the surface of the sheet . A layer 8 of a light sensitive material is provided on the surface of the sheet, over the particles of aluminium oxide. The layer has a viscosity when applied to the substrate such that formations are formed in the layer, as a result of the particles provided under the layer on the surface of the substrate.

In use, a film 10 is applied to the plate 2, and the plate with the film on it, is exposed to UV light. The film is transparent -to the light only in selected areas, so that the plate is exposed to the light only in those areas . When the film 10 is applied to the plate, channels 12 are defined between the film and the surface of the plate, along which air can escape from the space between the film and the plate when in a vacuum light frame. In this way, close contact between the film and the plate can be achieved.

EXAMPLES

Description of general techniques

Spraying method

Sheets of the material to be sprayed were secured around a 200 mm diameter roller which acted as a heat sink. Spraying was performed by translational movement of a plasma spraying torch along the axis of the roller, as the roller rotated at 300 rpm. The spraying system comprised a Plasma-Technik M1100C control unit, a Plasma-Technik F400MB torch, and a modified Plasma-Technik Twin 10 powder feed unit with NL55 spreaders. The air cooling facility was operated. The modifications entailed introducing a pipe into the unit to allow a further flow of 9 l.min^"1 of argon above the powder, this was in addition to the standard carrier gas flow of 9 l.min^"1 of argon associated with the unmodified unit. The following spray conditions were used to create the substrate:

Primary plasma gas Argon

Secondary plasma gas Hydrogen

Primary gas flow 30 l.min^"1

Secondary gas flow 6 l.min^"1

Current 500 A

Nozzle diameter Tapered 7 to 10 mm

Nozzle-sheet distance 40 mm

Powder injector position 90°

Powder injector nozzle 4 mm

Torch traverse speed 30 mm.s^"1

Powders

All powders sprayed were selected from the range of alumina powders sold by Abralap Ltd under the trade mark ABRALOX. The powders were dehydrated prior to their introduction into the feed unit.

Printing plate manufacture

Printing plates were made by application of a coating used by Horsell Graphic Industries Limited in their plates sold under the trade mark CAPRICORN. The coating was applied using a laboratory bar coating technique to produce a dry coating weight of 2.1 g.m^"2.

Test met od

The effectiveness of the applied particles was measured using a drawdown test. The drawdown test comprises placing a specially designed film onto the printing plate in a vacuum frame. The film measures 281 x 248 mm, and includes a circular disk at about its centre measuring 26 mm wide and 1.1 mm. The film is clear except for a thin ring, radius 32 mm, imaged concentric with the disk. The film is placed on the printing plate in a vacuum frame. Air is drawn out between the film and the plate. The time taken for the air to be drawn out at the radius of the imaged ring gives a measure of the effectiveness of the particles as distance holder, maintaining a gap between the plate and the film.

Example 1 Use of multi-modal powder distributions

Sheets of aluminium alloy of designation AA1050 were plasma sprayed simultaneously and in single pass operations with alumina powders comprising mixtures of powders with distributions centred on 3 μm and 15 μm. The 3 μm powder was chosen because previous experiments had shown that the powder produced printing plates of desirable properties; the 15 μm powder was selected to provide the distance holder effect . A relative disk speed (as defined by Plasma-Technik) in the powder feed unit of 60% was used. The substrates produced were coated with the coating used by Horsell Graphic Industries Limited in their CAPRICORN plates, and drawdown tests were performed.

A summary of the drawdown times is presented in Table 1. It was found that the drawdown time could be lowered by increasing the weight fraction of 15 μm powder used.

TABLE 1

3 μm powder 15 μm powder Drawdown time (wt %) (wt %) (s)

100 None 23

99 1 22

90 10 18

Example 2 Overspraying onto plasma produced substrate Sheets of AA1050 aluminium alloy were plasma sprayed with 3 μm alumina powder in single pass operations, using a powder feed unit relative disk speed of 60%. The sheets were oversprayed with single distributions of alumina powders centred on 12 μm and 15 μm. The overspraying was carried out using a powder feed unit disk speed of 6% to produce the appropriate number of distance holder particles on the substrate surface to create the distance holder effect . The substrates created were coated with the coating used by Horsell Graphic Industries Limited in their CAPRICORN plates, and drawdown tests were performed.

A summary of the drawdown times is presented in Table 2. It was found that the drawdown time decreased with increase in the size of particle used in the overspraying operation.

TABLE 2

Powder size Drawdown time

(μm) (s)

None 23

12 16

15 15

Example 3 Spraying onto electrochemically produced substrate

Sheets of electrochemically produced substrate were plasma sprayed in a number single pass operations with alumina powders of distributions centred in the range of 3 μm and 15 μm. A relative disk speed in the powder hopper of 6% was used to produce the appropriate number of large particles on the surface to create the distance holder effect. The oversprayed substrates were coated used by Horsell Graphic Industries Limited in their CAPRICORN plates, and drawdown tests were performed.

A summary of the drawing times is presented in Table 3. It was found that overspraying larger particles onto traditionally produced substrate creates a substantial reduction in drawdown time compared with a plate made produced electrochemically such as one sold by Horsell Graphic Industries under the CAPRICORN trade mark, which incorporates silica particles in the coating material to provide the distance holder effect. The drawdown time appears to decrease with increase in the particle size.

TABLE 3

Distance holder type Powder size Drawdown time

(μm) (s)

None None 57

Silica in coating None 40

Alumina particles 3 28

Alumina particles 12 24

Alumina particles 15 15

The products of Examples 1, 2 and 3 were the subjects of electron micrographs, and were compared with the a coated plate made electrochemically with silica in the coating, as sold by Horsell Graphic Industries Limited under the trade mark CAPRICORN. The surface configurations of the plates were similar.

Additional example

A sheet of aluminium was wrapped around a roller and sprayed in non-vacuum conditions with a mixture of aluminium oxide powders, particle sizes 5 μm and 9 μm in equal amounts by weight, in a gas plasma consisting of a mixture of argon and hydrogen.

A layer of a light sensitive material, supplied as a component of a printing plate by Horsell Graphic Industries under the trade mark LIBRA GOLD, was coated on the substrate to a thickness of 1.5 g.m^"2. The plate was evaluated by exposing a test film onto the plate. The film had bonded to the surface which contacts the plate a circular disk, radius 12.5 mm and thickness 1 mm. A series of concentric circles were arranged around the disc, the outer most circle labelled "1" having a radius 32 mm and the inner most circle labelled "10" having a radius 14 mm. The film was_, placed on the plate, and air between the film and the plate was withdrawn. The degree to which air is withdrawn is apparent by the number of circles which appear on the plate after exposure to light, since the images of the circles will only be clearly visible on the plate in regions of the plate which are in direct contact with the film.

In accordance with this test, the plate made using the technique described above and exposed through the disk-bearing film, produced clear images of rings 1 to 6. This performance is satisfactory, and compares favourably with plates made without any surface formations and with plates in which surface formations are provided by mixture of particles with light sensitive coating material.

Claims

CLAIMS :

1 . A lithographic printing plate which comprises :

(a) a substrate ,

2. A printing plate as claimed in claim 1, in which the height of the particles above the said surface of the substrate is at least about 0.5 μm.

3. A printing plate as claimed in claim 1 or claim 2, in which the height of the particles above the said surface of the substrate is less than about 30 μm.

4. A printing plate as claimed in any one of claims 1 to 3, in which the transverse dimension of particles is at least about 1 μm.

5. A printing plate as claimed in any one of claims 1 to 4, in which the transverse dimension of the particles is less than about 35 μm.

6. A printing plate as claimed in any one of claims 1 to 5, in which the particles comprise a material which is capable of exhibiting ceramic properties.

7. A printing plate as claimed in claim 6, in which the ceramic material comprises aluminium oxide.

8. A printing plate as claimed in claim 6, in which the ceramic material comprises silica.

9. A method of making a lithographic printing plate, which comprises:

(a) providing a plurality of particles to the surface of a substrate,

10. A method as claimed in claim 9, in which the plurality of particles are provided on the surface of the substrate by a dry deposition technique.

11. A method as claimed in claim 10, in which the plurality of particles are provided on the surface of the substrate by thermal spraying.

12. A method as claimed in claim 10, in which the plurality of particles are provided on the surface of the substrate by plasma spraying

13. A method as claimed in claim 9, in which the plurality of particles are provided on the surface of the substrate by a wet deposition technique.