WO2003037630A1

WO2003037630A1 - Printing elements and methods of construction

Info

Publication number: WO2003037630A1
Application number: PCT/AU2002/001458
Authority: WO
Inventors: Graeme Kirk
Original assignee: Kirk Engineering Services Australia Pty Ltd
Priority date: 2001-10-29
Filing date: 2002-10-28
Publication date: 2003-05-08
Also published as: AUPR857301A0

Abstract

The invention provides a printing element having an ink receiving layer formed of a polymeric composition wherein the polymeric composition comprises a polyurethane matrix and a nanoparticle filler. A gravure printing cylinder (1) has a number of ink receiving cells (2) engraved onto its surface. A doctor blade (4) is provided to wipe excess ink from the surface of the cylinder. An impression cylinder (5) is provided opposite the printing cylinder and the material (6) to be printed on is fed through the nip of the printing cylinder (1) and the impression cylinder (5). The printing cylinder (1) has the ink receiving cells (2) formed with a nanocomposite polymeric material.

Description

PRINTING ELEMENTS AND METHODS OF CONSTRUCTION

Technical Field

The present invention relates to printing elements, such as printing cylinders and surfaces, and to their methods of construction. It relates particularly to gravure printing cylinders and surfaces.

Background Art

In a typical gravure printing process, a metal printing cylinder has a plurality of cells engraved into its surface. These cells represent the image to be printed, and receive ink from an ink reservoir through which the cylinder rotates. When the surface contacts a material that is to receive the print, the ink is transferred from the cells to the material to produce the required image (the image corresponding to the pattern of the ink cells and their dimensions). A doctor blade is used to remove excess ink from the cylinder's surface before the surface contacts the material.

Gravure printing is able to provide a high-resolution print, and can also provide continuous tone printing by varying the depth and/or opening size of the cells and so the amount of ink that each cell will transfer.

Although gravure printing is the method of choice for long run high quality prints, the process of producing a gravure printing cylinder is complex.

The process consists of the plating of a cylindrical metal substrate of steel or aluminium with a layer of copper. The coppered cylinder is machined to a desired dimension and tolerance, and is polished. The polished cylinder is then engraved and plated with a protective layer of chrome.

This process is slow and expensive, and includes health and environmental risks, as it requires the use of toxic heavy metals and cyanide. Furthermore, the cylinders produced are heavy and unwieldy.

It is an aim of the present invention to provide printing cylinders and other surfaces, such as plates, that do not have these problems.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application. Throughout the description and claims of this specification, use of the word "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps.

Disclosure of the Invention

Viewed from a first aspect, the present invention provides a printing element, such as a cylinder or plate, preferably for gravure printing, wherein the printing element has print surface that includes a polymeric nanocomposite material.

Thus, the print surface may comprise a polymeric matrix within which is provided nanoparticle filler material. Preferably, the polymeric matrix is a polyurethane.

The printing element of the present invention has an ink receiving layer formed of a polymeric composition wherein the polymeric composition comprises a polyurethane matrix and a nanoparticle filler.

The printing element may be, for instance, a printing cylinder or plate.

Preferably, the printing element is a cylinder. The printing element may be for use in forming an image on a substrate such as a gravure or relief printing element or it may be used to transfer ink in a controlled amount to another surface as an anilox printing element.

Preferably, the ink receiving layer is engravable. In a preferred form, the ink receiving layer is engraved for gravure or relief printing. The engraving provides the image on the surface of the element which is to be applied to the substrate. The ink receiving layer may be engraved by any suitable method such as electrochemical or laser engraving processes.

Nanoparticles can generally be thought of as particles having dimensions in the nanometer range, ie. from 1 to 1000 nm. They can also have a small size distribution. Such particles may be made from various different materials and in various different forms, as discussed further below.

Printing elements, such as gravure printing cylinders, made in accordance with the present invention are able to provide a number of advantages over the prior art.

The use of a polymeric layer for the print surface of the cylinder, rather than a copper layer, is less costly, does not involve toxic plating chemicals and metals, and can produce a more lightweight cylinder.

The use of the nanoparticles, meanwhile, is able to provide the polymeric layer with suitable characteristics to allow for good quality printing.

Thus, the nanoparticles can provide the polymer with sufficient hardness that ink cells engraved into the print surface can have well-defined edges without any "roll over" that might otherwise occur without the use of the particles. "Roll over" is where the edges of the ink cells are deformed during the engraving process, resulting in smudging and a lack of definition in the printed image.

Furthermore, due to their size, the nanoparticles allow for a high definition engraving of the polymer and will not occlude the small-dimensioned print cells used in gravure printing, and consequently will not produce visible defects in the printed image.

Another important aspect is that the nanoparticles can be evenly dispersed in the polymer, and can provide the polymer with a good uniformity of properties over its entire engravable surface.

The nanoparticles may also provide the polymeric material with other preferable characteristics, and can affect for example the mechanical, thermal, chemical and dimensional stability of the polymer. They can also increase coating hardness, scratch resistance and abrasion or wear resistance, e.g. by a factor of two to four.

They may for example provide the polymeric material with resistance to the solvents used in the printing inks, and may provide the material with lubricating properties. Good lubrication can be very important in gravure printing processes with respect to reducing wear and the prevention of e.g. "scumming" due to vibrations produced by a doctor blade against a printing cylinder's surface. The nanoparticles may also provide the polymeric material with electro- conductive properties. This would then allow an electric field to be applied between the cylinder and an impression roller on the other side of the material that is to be printed. The electric field may then assist in improving ink transfer from the cylinder inkwells to the print material.

The nanoparticles may for example be based on aluminium oxide, zinc oxide, and alkyl quarternary ammonium salts and Bentonite. Others, such as copper oxide, antimony tin oxide, cerium oxide, indium tin oxide, and titanium dioxide may also be used to obtain further characteristics. The nanoparticles may consist of a wide variety of materials including for example metal oxides, such as aluminium oxide, antimony tin oxide, cerium oxide, copper oxide, indium tin oxide, iron oxide, titanium oxide, yttrium oxide, zinc oxide, and the like. They may also be based on carbon material such as graphite, and silicates including alumino-silicates. Further, the nanoparticles may be in the form of clays and ceramics, such as montmorillonite clay (or kaolinites and illites), and also in other forms, such as graphite nanofibres and nano-tubes, e.g. carbon tubes made of coiled fibres, and bucky balls (buckminsterfullerenes), i.e. carbon balls. The nanoparticles may also comprise mixtures of any of the aforesaid materials. Various such nanoparticles are manufactured by for example Nanophase

Technologies of Illinois, USA, (e.g. under the trade name NanoTek), and Physitrόn, Inc, of Alabama, USA.

Preferably, the nanoparticles are selected from the group consisting of aluminium oxide, zinc oxide, silicates, and clays. Preferably, the nanoparticles have a dimension, e.g. diameter, grain size or thickness, which is less than about 200 nm, preferably less than about 150 nm and more preferably less than about 100 nm. Typically the nanoparticles are at least 10 nm preferably at least 40 nm and the range of between about 40 nm and about 100 nm is particularly preferred. Most preferably, the nanoparticles have a dimension that is in the range of about 50 to about 1 QOnm.

In a particularly preferred form, the nanoparticles may consist of aluminium oxide of about 56 nm particle size, zinc oxide of about 71 nm particle size and that is preferably greater than about 75 nm, and clay, e.g. alkyl quarternary ammonium salt and Bentonite clay, of about 75 nm particle size. When carbon nano-tubes are used, they may have for example diameters in the range of about 50 nm to about 100 nm, and may have lengths of for example between about 1 and about 10 microns.

Where the nano-particles comprise clay nano-particles, they may take on an intercalated arrangement. It is preferred, however, for the polymer-clay composite to take on an exfoliated structure, as this maximises the structural interaction with the polymer.

Preferably, the nano-particles are treated so as to ensure that they do not chemically and/or physically attach to one another, and/or to increase their wetting with the polymer matrix in which they are to be bound. This helps to promote a uniform dispersion of the nano-particles throughout the polymer. The treatment may comprise for example using surface treatments, e.g. as are known in the nanoparticle art. This may include the use of functional group attachments to the surface of the particles. The functional groups employed may for example include epoxide, amine, acrylate, vinyl, ether and/or ester groups.

The amount of nanoparticles used in proportion to the polymeric matrix may take any suitable value. Preferably, the polymeric matrix includes from about 2% to about 20% of nanoparticles. More preferably, the polymer includes from about 2 to 5% of nanoparticles.

The polymeric matrix comprises a polyurethane (e.g. a blend of polyols and MDI prepolymers and monopolymers).

The polymer may for example comprise single or two pack polyurethane.

Polyurethanes have been surprisingly found to provide a particularly suitable polymeric matrix for use with nanoparticles in the present invention. Polyurethanes may be produced by the reaction of a polyfunctional isocyanate, most often with hydroxyl compounds although other compounds containing functional groups that are also capable of reacting with isocyanates such as amines or thiols may also be used. Hydroxy-containing components cover a wide range of molecular weights and types, including polyester and polyether polyols. The polyfunctional isocyanates can be aromatic, aliphatic, cycloaliphatic, or polycyclic in structure and can be used directly as produced or modified. Preferably, the polyurethane comprises a blend of polyols, more preferably polyether polyols and an isocyanate. The isocyanate may be any suitable isocyanate or polyisocyanate prepolymer and can be based on, for example, toluenediisocyanate (TDI), diphenylmethane -4,4'-diisocyanate (MDI) and naphthalene 1 ,5-diisocyanate. Preferably the isocyanate or polyisocyanate prepolymer is diphenylmethane 4,4'-diisocyanate (MDI) or a derivative thereof.

The nanoparticles may be blended into e.g. a polyurethane mixture using for example a high shear mixer.

In a preferred embodiment, the surface of the polymeric layer is provided with a protective coating in order to resist wear. Wear may for example occur due to the printing process itself, when a printing cylinder contacts the material to be printed or an offset roller, or can occur due to the application of a doctor blade to the cylinder to remove excess ink.

The coating may take the form of a lubricating coating and/or a coating that increases the surface hardness of the layer.

The protective coating may be polymeric or ceramic, and may take the form of polytetrafluoroethylene, preferably micronised polytetrafluoroethylene.

In one possible form, the nanocomposite material may be plated, e.g. with chrome, for protection. To facilitate this, the polymeric material may include nanoparticles therein for making the polymeric material conductive.

The body of the printing element may be made from the nanocomposite material, or the printing element may be made from a substrate on which the nanocomposite material is mounted. Such a substrate may be of any suitable material, such as steel, aluminium, copper or carbon fibre. Where the nanocomposite polymer layer is provided over a substrate, the nanocomposite layer may provide a hard outer surface for preventing "roll over" and the like, and the ink ceils may extend through the nanocomposite layer into or onto the or another substrate below.

Viewed from another aspect of the present invention provides a coating composition for use in coating a print element comprising a polyurethane within which is provided a nanoparticle filter.

Various methods may be used to produce a printing cylinder, plate or other printing surface in accordance with the above teachings. In one preferred form, the body of the printing element cylinder may comprise the engravable polymeric material, and, viewed from a further embodiment, the present invention provides a method of making a printing element in which the element is formed from a nanocomposite material. The printing element may for example be made in a suitable mold and then machined. In one preferred form, a printing cylinder may be formed by a centrifugal coating method, and may then be machined to required dimensions and shapes.

In the centrifugal coating method, a sleeve is provided about a central cylindrical element and is sealed at each end. The assembly is then rotated as the polymer is fed into the gap between the sleeve and the cylindrical element. A restricted outlet is provided from the gap, and the polymeric material is pumped into the gap to increase the pressure of the fluid until fluid flows out of the outlet. The centrifugal forces produced in the polymeric material force the air in the material outwards, and out through the outlet. Once a steady flow of material is obtained out of the assembly, the outlet is sealed to allow the polymeric material to set. The sleeve and cylindrical element may be of e.g. steel, and the seals may be rubber and may be held mechanically in place against the pressure of the polymeric material. Viewed from a further aspect, therefore, the present invention provides a method of making a printing cylinder, e.g. a gravure printing cylinder, in which a nanocomposite polymer is formed into a tube by a centrifugal coating method.

In another preferred form, the printing element may comprise a core or substrate, e.g. a cylindrical core or substrate, onto which the engravable polymeric material is coated. The core or substrate may comprise any suitable material, and may for example include fibreglass, ceramic, plastics, aluminium, steel or other metals, which may be suitably shaped, machined or moulded.

Thus, viewed from a further aspect, the present invention provides a method of making a printing cylinder, e.g. a gravure printing cylinder, including the step of applying a nanocomposite polymer onto a cylindrical core or substrate.

The application of the polymer can be carried out in any suitable manner, and may be achieved by extrusion or rolling of the polymer. In a preferred embodiment, the polymer is sprayed onto the surface of the substrate or core, which is preferably rotating. In this case, the polymer is preferably of a suitable viscosity to avoid slumping of the polymer as the substrate rotates and before the polymer is sufficiently cured

When applied by spraying, the carrier media for the single pack polymers mentioned above can be solvent and/or water based.

The polymer may be sprayed onto the core using more than one pass, e.g. two or three passes, in order to avoid slump.

Preferably, the polymer is a two pack polymer. The two components of such a polymer may be pre-mixed and sprayed onto the substrate from the same outlets. Alternatively, the two components of the polymer are sprayed from independent outlets. Preferably, the two components are mixed together at the outlet of an application nozzle. This ensures that only the necessary amounts of the two components are mixed together, and can prevent the waste of unused premixed material. It also allows for fast setting formulae to be used. The polymeric matrix is a polyurethane preferably formed from a blend of polyols and an isocyanate. Preferably, the nanoparticle filler is mixed with the blend of polyols prior to mixing the blend of polyols and the isocyanate.

The mixing process may be based on a continuous output of a base raw material for the polymer, with e.g. a catalyst being added at a frequency and output regulated via e.g. an electronic interface linked to a computer control such as a PC. This can help to facilitate the constant density of material required when depositing it on the substrate.

Again, more than one pass may be used during the spraying process.

Examples of preferred two pack polymers are two pack solventless polyurethane and epoxy resin.

In order to prevent slumping when applying the polymeric coating to the core, the polymeric materials may have a high viscosity, e.g. 1 ,200 centipoises. This can however be reduced by elevating the material temperatures e.g. between about 12°C and about 32°C. This may require an adjustment of the reaction times of the raw materials to ensure that the polymer will not cure before it is applied to the cylinder.

Differential heating may also be used, so that one component of the two pack polymer is at a higher temperature than a second component or catalyst (which may also have a higher temperature than the standard temperature, so as to prevent e.g. too high a temperature differential between the two components). The temperature differential between the two components assists with maintaining an acceptable level of viscosity.

During application of the polymer, the cylinder may be kept rotating by a suitable driver unit which may rotate the cylinder using an electronically controlled motor to ensure a constant speed of rotation regardless of the diameter of the cylinder base. A robotic arm may be used in the spray operation.

Preferably, the printing cylinder bases are pre-heated prior to application of the polymeric coating. This may be achieved using an infra-red oven to heat the cylinder base quickly. The same oven may also be used to speed up curing of the polymeric coating after application.

During the application of the polymeric layer, its thickness may be monitored using a non-contact measuring device, such as a laser-based measuring device.

The invention further provides a method of forming a printing element comprising providing a structural support; forming a mixture of a polyol, a diisocyanate and a nanoparticle filler to provide a polyurethane composition; and applying a coating layer of the polyurethane composition to the support to form an ink receiving layer; and optionally engraving the print receiving layer for relief or gravure printing.

In a further preferred form, the printing cylinder may comprise a used printing cylinder, such as a currently used copper and chrome plated gravure printing cylinder, with the polymeric nanocomposite layer provided thereabouts. This then allows for the re-use of such cylinders and can result in a considerable saving in raw materials and other resources.

Thus, viewed from a further aspect, the present invention provides a method of making a printing cylinder, e.g. a gravure printing cylinder, in which a used printing cylinder is coated in a layer of nanocomposite polymeric material to produce an engravable layer.

When applying the nanocomposite polymeric layer to a substrate, the substrate may need to be pre-treated with a wetting and/or bonding agent in order to prevent delamination or separation of the polymer layer from the core or substrate during use or otherwise. The agent may for example be a heat stable phosphate. Preferably, the wetting agent is a polymer, such as a liquid polymer. It may for example comprise a polyolefinic primer or adhesive, e.g. of the type known as UNISTOLE produced by Mitsui Chemicals.

Once coated, the cylinders should be cured. The curing process may take any suitable form, and, in one embodiment, the cylinder is cured using an atomising gun. It may also or alternatively be placed within a curing oven, and curing may be carried out in stages, so that for example, the curing oven may have two chambers - a pre-heat chamber and a curing chamber at a higher temperature. A UV lamp may also be used in the curing process e.g. of epoxy based raw materials.

When re-using a cylinder, various steps may be needed before application of a new polymer printing surface. Thus, the cylinder may be machined to a new dimension, and the surface may be roughened to promote binding of the polymer. This new method of making cylinders suitable for re-use can eliminate the need for re-coppering and chroming to prepare a printing cylinder for re-use.

Roughening may be achieved by blasting the cylinder with a suitably abrasive material.

As said, the polymer may require a protective coating to be applied to its surface, and this may for example be achieved by spraying a coating of e.g. PTFE onto the surface, or by plating the surface with chrome.

The ink receiving layer may be provided with ink cells in any suitable manner, e.g. by etching or engraving.

When engraving the printing cylinders, any suitable method may be used, such as an electromechanical or laser engraving process. These engraving methods may be part of an electronic engraving system. In one embodiment, engraving is carried out using a diamond stylus cutting tool. In another embodiment, direct laser non-contact engraving is used for the creation of ink wells. The laser may be a C0₂, YAG or Diode type laser. Where diamond cutting is used, it has been found that a tool having a head geometry with an included angle of 110°, 120° and 130° can provide very good results. Such a tool can be obtained from Chardon Tool in Chardon, Ohio, USA.

Preferably, an air knife or vacuum system is used during engraving to remove chips of the polymer that are produced by the engraving process. These chips might otherwise cling to the surface of the polymeric layer and cause printing defects. Such a system may be used whether the engraving equipment is electromechanical or laser.

When wiping a cylinder during printing, a standard doctor blade of steel may be used. Alternatively, the doctor blade may also be made of a polymeric material, e.g. polyester. It could include a blade portion of polymeric nanoparticle material, the nanoparticle material providing suitable characteristics, such as lubricity, to the blade.

Although the use of a nanocomposite polymer printing surface in gravure printing has been emphasised, such a surface also has applications in other printing regimes, such as letterpress, general intaglio printing, flexographic and offset printing.

Besides use with printing cylinders and plates, the present invention may also extends to other printing elements, such as anilox cylinders for controlling the transfer of ink to flexographic printing plates, and further extends to the production of anilox rollers as used in embossing or in the application of adhesive, over-lacquer or the like to a web of material.

This then may allow consistent image quality for the various printing processes. The invention may also be applied to non-printing situations, in which an engraved surface is required.

Thus, viewed from a further aspect, the present invention provides an engraved surface, wherein the engraved surface includes a polymeric nanocomposite material. The present invention also provides a printing element including an engraved surface that includes a polymeric nanocomposite material.

This may be possible where for example high definition prints are not required, and/or where other steps are taken to avoid the problems of "roll over" of the engraved cell edges. For example, it has been found that using a diamond-engraving tool having the above-mentioned form allows for high definition engraving of various polymeric surfaces. The polyurethane components of the invention are particularly suited to engraving by this method. Also, the polymeric layer could be provided with a chrome or other plating, in which case the polymeric material may be made suitably electro-coηductive, e.g. through the use of a metal filler, e.g. a nanoparticle filler.

The present invention therefore extends to a printing element, such as a cylinder or plate, preferably for gravure printing, having an engraved surface of a polymeric material. The polymer is preferably selected such that the hardness and/or resilience of the polymer is suitable for engraving at the required resolution, and the polymeric material may include therein filler material to provide a suitable hardness to the polymer surface. Preferably, the hardness of the polymer is greater than about 80 Shore D, and may be 81 to 82 Shore D.

The nanoparticle filled polyurethane compositions used in the present invention also has been found to provide properties which can not be readily achieved with other polymers.

The present invention further extends to the re-use of a printing cylinder (or plate), such as current copper and chrome plated cylinders, including the step of coating the cylinder (or plate) in a polymeric layer and engraving this layer. The polymeric layer is a polyurethane matrix comprising a nanoparticle filler.

The printing cylinders provided by the present invention can advantageously be used in a cylinder and drive shaft assembly in which the drive shaft includes a pair of drive shafts ends that clamp the cylinder therebetween to provide a quick and simple cylinder assembly using for example a hydraulic coupling means, such as a hydraulic coupling sleeve as manufactured under the trademark KOSTYRKA by Ing. Peter Kostyrka Ltd of Stuttgart, Germany. Such drive shaft assemblies are disclosed in the co- pending International Patent Application deriving priority from Australian Provisional Patent Application PR8574 filed on the same day as the present application and entitled "Printing Cylinders and Methods of Construction", the contents of which are incorporated herein by reference. The present invention also extends to printing methods and printing systems using printing elements, e.g. cylinders or printing surfaces, in accordance with any of the above-mentioned cylinder or surface constructions, and to products printed using any such methods and systems. Brief Description of Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. It is to be understood that the particularity of the drawings does not supersede the generality of the preceding description of the invention.

In the drawings:

Figure 1 is a schematic drawing of a typical printing arrangement for gravure printing;

Figure 2 is a longitudinal cross-section through a printing cylinder in accordance with a first embodiment of the present invention;

Figure 3 is a longitudinal cross-section through a printing cylinder in accordance with a second embodiment of the present invention; Figure 4 is flow diagram showing the process of producing a cylinder in accordance with Fig. 2; and

Figure 5 is a flow diagram showing the process for producing a cylinder in accordance with Fig. 3.

Modes for Carrying out the invention

Referring to Fig. 1, a gravure printing assembly will generally comprise a print cylinder 1 that has a number of ink cells 2 engraved into its surface. These ink cells 2 represent an image to be printed, and may vary in depth and/or opening size to transfer more or less ink from each cell and provide continuous tone printing.

The print cylinder 1 is driven to rotate through an ink fountain 3 to take up ink into the ink cells 2, and a doctor blade 4 is provided to wipe excess ink from the surface of the cylinder 1. An impression cylinder 5 is provided opposite the printing cylinder 1 , and the material 6 to be printed on is fed though the nip of the printing cylinder 1 and impression cylinder 5. The impression cylinder 5 is generally a rubber covered metal cylinder, which is friction driven. It is used to urge the material 6 against the printing cylinder 1 to ensure proper transfer of ink from the ink cells 2 to the material 6, as well as to move the material 6 through the press.

In accordance with the present invention, the printing cylinder 1 has the ink cells 2 formed within a nanocomposite polymeric material. The polymer comprises polyurethane and nanoparticles which may comprise for example metal oxides, silicates, clays and/or ceramics, as well as carbon tubes or balls. It should be noted that various other polymers and nanoparticles could also be used as appropriate.

The use of a nanocomposite polymeric material allows the printing cylinder to be made more efficiently and with less time and expense than the prior art copper and chrome plated cylinders, and can be less harmful to health and the environment as the various toxic metals and chemicals used in plating are not needed.

Furthermore, the use of the nanoparticles provides the polymeric materials with sufficient hardness that the ink cells 2 can be engraved in a well- defined manner without "roll over" of their edges, whilst ensuring that even high- resolution ink cells are not occluded by protrusion of particulate matter from the cell edges.

The nanoparticles may comprise for example aluminium oxide, zinc oxide and/or Bentonite clay with e.g. alkyl quarternary ammonium salts.

The different types of nanoparticle may be applied individually or in combination, and may be chosen to vary one or more characteristics of the polymer, such as hardness, lubricity, ink resistance, electro-conductivity, and dimensional stability. Fig. 2 shows schematically a first embodiment of the printing cylinder 1 of

Fig. 1. In this embodiment, the cylinder 1 comprises a cylindrical shell 7 made from the nanocomposite polymeric material, which may be produced by centrifugal layering of the polymer. Each end of the shell 7 includes a stepped portion 8 for receiving a plastics end portion 9. The end portions 9 are chamfered to receive correspondingly shaped drive shaft ends, and include keyways 10 for receiving key portions of the drive shaft ends.

Fig. 3 shows schematically a second embodiment of the printing cylinder 1 of Fig. 1, in which the cylinder comprises a core 11 on which a nanocomposite polymeric layer 12 is coated. The core 11 may comprise any suitable material, and may be steel or another metal, but is preferably of lightweight material and construction, and may comprise for example plastics, fϊbreglass or aluminium moulded or cast into the appropriate shape and to include chamfered ends and keyways 10 for drive shaft ends.

The core 11 may be treated with a binder, such as a polyolefinic primer, before application of the nanocomposite polymer layer 12 so as to prevent its delamination.

The nanocomposite polymer layer 12 may be applied in any suitable manner, e.g. by extrusion or rolling of the polymer material or by spraying the material onto the core 11 as it is rotated.

The polymer layer 12 is then cured in any appropriate manner e.g. by the use of a heating oven and/or atomising guns and/or UV light.

In one modification to this second type of cylinder, the core 11 may comprise a used printing cylinder, such as the prior art copper and chrome plated cylinders or a used cylinder in accordance with the present invention (either of the first or second embodiment type). In this case, the core 11 may first be stripped of the previously engraved surface down to e.g. a steel, aluminium or plastics substrate before the application of the nanocomposite polymeric layer. This may be achieved by mechanical removal.

In order to protect the surface of the printing cylinder 1 from wear, the nanocomposite layer of both the first and second embodiments may be provided with a protective coating of e.g. PTFE that may be sprayed on to the polymeric layer 12 after it is cured and after engraving. With both types of cylinder, it is preferred to use the drive shaft systems disclosed in the co-pending International Patent application deriving priority from Australia Provisional Patent Application PR8574 filed on the same day as the present application and entitled "Printing Cylinders and Methods of Construction". This can then provide a particularly lightweight assembly which may be more easily transported and stored than the prior art cylinders.

Fig. 4 is a flowchart of the process for producing a printing cylinder in accordance with Fig. 2.

At step S1 , the nanocomposite polymeric shell 7 is first produced, e.g. by centrifugal layering. In centrifugal casting, the material is feed into the gap between a rotating sleeve and cylinder assembly under pressure so that the air in the material is forced outwards and is then bled off through a restricted outlet. Once a constant flow through the outlet is established, the outlet is closed and the fluid cast in the assembly. The shell 7 is then machined to the required dimensions at step S2, as are the plastics ends 8 made using identical raw materials cast as a solid rod at step S3.

The shell 7 and ends 8 are then assembled together at step S4, and the assembly polished to the required diameter of the cylinder at step S5. Equipment such as a Polish Master ™ machine, as are known in the art for plating cylinders, may be used to obtain the required polishing.

Once polished, the assembly is engraved at step S6. Engraving may be carried out using known electronic engraving techniques used for engraving copper plated cylinders. It has been found that a diamond stylus having a geometry with an included angle of 110°, 120°, or 130° can provide good sharp results.

Once engraved, the cylinder 1 is proofed at step S7 though the use of a proofing press.

The assembly is then ready for use in printing and at step S8 may be attached to a drive shaft using the hydraulic drive shaft mentioned above.

Fig. 5 is a flowchart of the process for making a printing cylinder in accordance with Fig. 3.

At step S1 , a suitable core element 11 , such as an aluminium or fibreglass cylinder is machined to a required diameter. Next, at step S2, the nanocomposite polymeric layer 12 is overlaid onto the core 11. This may be achieved by spraying a two pack polymeric material onto the core 11.

Once overlaid, the polymeric layer 12 is polished to the required cylinder diameter at step S3, and then the cylinder is engraved and proofed at steps S4 and S5 in the same manner as in the Fig. 4 process.

The cylinder may then be mounted on hydraulic shafts at step S6 in the same manner as in the Fig. 4 process, or it may be the case that the core 11 already has integral shafts as indicated in step S7. Where the core 11 comprises a pre-used printing cylinder, then step S1 would include stripping the previously engraved layer from the cylinder as. part of the machining of the core to the required diameter.

Prior to step S2, the core 11 may also be wetted to prevent delamination of the polymeric layer 12 by the use of polyolefinic primers.

In both the process of Figs. 4 and 5, a protective coating may be applied to the surface of the polymeric material, before or after engraving by for example spraying PTFE onto the surface of the cylinder. Alternatively, the surface could be chrome plated, and, to facilitate this a metallic nanoparticle composition could be used, so as to provide the cylinder with suitable electro- conductivity.

Besides producing printing cylinders, the use of the nanocomposite material may be applied to a gravure printing plate. Plates are generally used in sheetfed type presses, e.g. for producing postage stamps, bank notes or the like.

Also, the use of the nanocomposite polymers need not be limited to printing cylinders, but may be applied to anilox cylinders that e.g. are used to control the application of ink to a flexographic plate or the like. Such a cylinder requires an array of cells across its surface for holding a controlled amount of dye that it then transfers.

The invention may also be applied to anilox cylinders that are used to provide non-inking functions, such as in embossing or the application of adhesive, or an over-lacquer or some other substance to a web.

When using the diamond cutting stylus mentioned above, i.e. having a geometry of 110, 120 or 130, good ink cell definition may still be provided even without using nanoparticle fillers in the polymeric coating.

It is to be understood that various alterations, additions and/or modifications may be made to the parts previously described without departing from the ambit of the present invention, and that, in the light of the teachings of the present invention, the composition of the engravable surface and the construction of the surface and any support therefor may take a number of different forms.

Example 1 Example 1

A high impact polyurethane polymer of the composition detailed below was used to provide a layer of polymer on a gravure cylinder according to the present invention.

Polyether Polyol Functionality 3, Molecular weight 300-500 15-30 parts

Polyether Polyol Functionality 3, Molecular weight 800-1000 15-30 parts

Polyether Glycol Functionality 3, Molecular weight 100-200 3-7 parts Polyether Polyol Functionality 2, Molecular weight 400-500 1-3 parts

Polyether Polyol Functionality 4.5, Molecular weight 500-700 2-5 parts

Moisture scavenger 2-6 parts

Catalyst - Metal Acid salt 0.5-2 parts

Pigment - Titanium Oxide 10-15 parts Inorganic Filler 25-50 parts

Isocyanate component functionality 2.0 - 2.7 based on diphenyl methane diiscocyanate.

The above composition corresponds to the Aptane™ DP009-12/B910, 80-85°D elastomer system as supplied by Ariel Industries Pty Ltd of Victoria, Australia. The inorganic filler will comprise nanoparticles, e.g. aluminium oxide, zinc oxide and/or nano-clay.

Specific examples of appropriate nanoparticles available from Nanophase Technologies of Illinois, USA, (under the trade name Nanotex) include:

• Aluminium oxide 99.5% (Al₂0₃) with particles of 56 nm, bulk density 0.10 g/cc with a spherical morphology;

• Zinc Oxide 99.5% (ZnO) with particles of 71 nm, bulk density 0.15 g/cc with an elongated morphology; • Alkyl Quaternary Ammonium Salt with Bentonite with particles of

100 nm, bulk density 1.9 - 2.1 g/cc with an exfoliated morphology;

• The optimum percentage of addition of nanofiller has been identified as being between 2% and 5%.

The polymer may be applied to a cylindrical substrate, such as a steel or aluminium substrate, by a casting and/or spraying operation. Example 2

A polyurethane polymer of the composition detailed below was used to provide a layer of polymer on a gravure cylinder according to the present invention.

The use of a polyurethane that is a blend of:

• Polyether Polyol, Molecular weight 500-500 Functionality 3, Hydroxyl number 350-420 (20-40%) • Polyether Polyol, Molecular weight 800-1200

Functionality 3, Hydroxyl number 140-220 (20-40%)

• Polyether Polyol, Molecular weight 80-110 Functionality 3, Hydroxyl number 1600-2000 (10-20%)

• Polyether Polyol, Molecular weight 350-500 Functionality 3, Hydroxyl number 200-300 (2-10%)

• Polyether Polyol, Molecular weight 500-800 Functionality 3, Hydroxyl number 400-500 (5-15%)

• Moisture scavenger (2-10%)

• Clay-based Nano filler (2-5%) • Cyclo Organic Ester (4-10%)

• Polyurethane Gelatin Catalyst

Based on metal acid salts (0.5-2%)

Isocyanate component functionality of 2.1 based on diphenyl methane disocyanate. The isocyanate value is between 28% and 32%.

The clay based nanofiller used is an alkyl quaternary ammonium salt with Bentonite with particles of 1000 nm, bulk density 1.9 - 2.1 g/cc with an exfoliated morphology.

The polymer is applied to a cylindrical substrate, such as a steel or aluminium substrate, by a spray unit in combination with an impingement gun which mixes the polyols and the isocyanate in the spray nozzle. The clay- based nanofiller had been incorporated into the polyols component using a high shear rate mixer to ensure accurate and even dispersion within the blend. The spray unit applies the coating to the substrate, which is rotating, in a series of passages until the desired thickness is obtained.

Claims

1. A printing element having an ink receiving layer formed of a polymeric composition wherein the polymeric composition comprises a polyurethane matrix and a nanoparticle filler.

2. A printing element according to claim 1 wherein the polyurethane is formed from a polyether polyol composition and diisocyanate.

3. A printing element according to claim 2 wherein the diisocyanate is selected from diphenymethane-4,4'-diisocyanate and derivatives thereof.

4. A printing element according to claim 2 wherein the polyether polyol composition has a molecular weight in the range of from 100 to 20,000 and includes at least one polyether polyol of molecular weight in the range of from 1 to 500 and at least one polyether polyol of molecular weight over 500.

5. A printing element according to any one of claims 1 wherein the nanoparticle filler is selected from one or more of the group consisting of zinc oxide, aluminium oxide and clay.

6. A printing element according to claim 5 wherein the nanoparticle filler is a clay selected from quarternary ammonium salts of bentonite.

7. A printing element according to claim 5 wherein the nanoparticle filler has a dimension of less than about 200 nm.

8. A printing element according to claim 5 wherein the nanoparticles of the nanoparticle filler have a dimension in the range of from 1 to 200 nm.

9. A printing element according to any one of claims 1 to 8 wherein the nanoparticle filler comprises from about 2 to 20% by weight of the polymeric composition.

10. A printing element according to any one of claims 1 to 9 wherein the ink receiving layer is provided with a protective coating on the printing surface.

11. A printing element according to claim 10 wherein the protective coating is chrome.

12. A printing element according to claim 10 wherein the protective coating is polytetrafluoroethylene.

13. A printing element according to claim 1 further comprising a support for the ink receiving layer.

14. A printing element according to claim 1 in the form of a cylinder comprising a cylindrical metal support and said ink receiving layer on the outer surface thereof.

15. A printing element according to claim 1 in which the ink receiving layer has been engraved for gravure or relief printing.

16. A method of forming a printing element comprising providing a structural support; forming a mixture of a polyol, a diisocyanate and a nanoparticle filler to provide a polyurethane composition; and applying a coating layer of the polyurethane composition to the support to form an ink receiving layer; and optionally engraving the print receiving layer for relief or gravure printing.