GB2122762A

GB2122762A - Flexographic printing plates methods of their production and laminates for use in their production

Info

Publication number: GB2122762A
Application number: GB08216064A
Authority: GB
Inventors: David Russell Selwyn Fargus; Erik Elsberg
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co
Priority date: 1982-06-02
Filing date: 1982-06-02
Publication date: 1984-01-18
Also published as: DE3319676A1; AU557195B2; JPS58222892A; FR2527991A1; JPH0374380B2; GB2122762B; AU1449383A; CA1219488A; FR2527991B1

Abstract

The present invention relates to flexographic printing plates, methods of their production and laminates for use in their production. A flexographic printing plate comprises a layer of a grindable elastomer (1) having a thickness up to 1000 microns firmly bonded to a support layer (3). A layer of photopolymerised elastomeric material (5) is firmly bonded onto the face of the support layer (3) away from the grindable elastomer (1). The photopolymerised material (5) is produced by exposure of a photopolymerisable liquid to actinic radiation. The support layer (3) may be omitted, and anchor layer(s) (2, 4) may be included. The plate may be ground to uniform thickness. <IMAGE>

Description

SPECIFICATION Flexographic printing plates, methods of their production and laminates for use in their production The present invention relates to flexographic printing plates, methods of their production and laminates for use in their production.

There exist two common methods for the preparation of photopolymer flexographic printing plates. In the first a manufactured plate comprising a solid layer of photopolymerisable elastomer on a backing is image-wise exposed to crosslink desired portions of the solid layer.

Uncrosslinked portions of the layer are removed by washing with a solvent which dissolves the uncrosslinked material but not the crosslinked material. This leaves a plate having areas in relief.

The plate is treated in conventional manner to provide a smooth surface on the areas in relief.

Such manufactured plates are sold having a thin layer of non-tacky material on the face of the solid layer away from the backing. This in turn may be covered by a protective sheet to facilitate storage. The backing may comprise more than one layer, and in many cases comprises a relatively thin support layer in contact with the solid layer and a softer flexible layer on the face of the support layer remote from the solid layer.

Manufactured plates of this type are described, for instance, in British Patent Specification Nos.

1 489 193 and 1 525 965. Such manufactured plates are prepared for use merely by removing the protective layer.

As such plates are manufactured in a factory, their production can be closely controlled. The plates are therefore made to high tolerances, with each layer being of uniform thickness. Thus it can be ensured that the exposed and developed plates have uniform thickness over all of its areas in relief. However, manufactured plates are expensive as their cost includes the cost of the many layers of materials and the cost of production. Moreover, the material comprising the solid layer of photopolymerisable matter tends to be expensive and it must contain various components to prevent it from crosslinking during storage.

In the second method of producing photopolymerflexographic printing plates, a liquid photopolymerisable material is used to produce the exposed plate in situ. In a typical process, a negative is placed on a glass platen and covered with a thin layer of material. A quantity of a liquid photopolymerisable elastomeric material is spread evenly over the thin layer and is covered by a backing. Generally, the backing and the liquid material are applied simultaneously. The laminate thus formed is image-wise exposed to crosslink and therefore solidify desired areas of the liquid material and to bond it to the backing.

Uncrosslinked liquid material is washed off with a suitable solvent and the laminate is treated in conventional manner to produce the final flexographic printing plate.

It is possible to carry out the above process because the liquid material is formulated to have a high viscosity. It therefore remains in a discrete layer for a time long enough to allow image-wise exposure and crosslinking.

Unless plant conditions and machine maintenance are carefully controlled, the flexographic printing plates produced by this second method are not always of good quality.

The thickness of the layer of photopolymerised material is not uniform which leads to uneven inking of the plate and therefore badly printed products.

The variable thickness may be caused by one or a combination of the following factors. The platen on which the negative is placed may be uneven, either because of bad maintenance or because it is unevenly heated by the lights used to expose the plate. This causes it to warp. The equipment used to apply the backing and layer of liquid material to the thin layer may not be sophisticated enough to provide a layer of uniform thickness. Even if it is, if it is badly maintained, it may lose its ability to produce a uniform layer.

The viscosity of the liquid material may vary, either because of variations in atmospheric conditions or because of the uneven heating of the platen.

It has therefore proved difficult to produce high quality flexographic printing plates by this method.

According to the present invention there is provided a flexographic printing plate comprising, a layer of a grindable elastomer, having a thickness of up to 1000 microns, firmly bonded to layer of photopolymerised elastomeric material, which has been produced by exposure of a photopolymerisable liquid to actinic radiation.

Optionally a support layer, having a thickness typically from 75 to 150 microns, is interposed between and firmly bonded to the layers of grindable elastomer and photopolymerised elastomeric material.

The grindable elastomer may be any of the commonly available rubbers, such as polybutadiene, butadiene-acrylonitrile, butadienestyrene, isoprene-styrene, silicone, or polysulphide rubber. Preferably, the elastomer is a natural rubber, a polychloroprene rubber, or a polyurethane rubber. The elastomer may contain conventional fillers in order to enable it to be ground more easily. The elastomer should have a Shore A hardness of at least 30 but not more than 80. Preferably its hardness is between 40 and 60 Shore A.

The grindable elastomer may contain a reinforcing and stabilising matrix or web, such as textile or non-woven fabric, and this will be especially desirable when the support layer is absent.

If the liquid material is not rapidly polymerisable, the elastomer should be transparent to enable the liquid material to be exposed from both sides. However, if the liquid material polymerises rapidly on exposure, the elastomer may be opaque.

Preferably, the elastomer layer has a thickness from 100 to 500 microns, and most preferably is about 400 microns thick.

When present, the support layer may be any one of those commonly used in the art. It may comprise for instance a sheet of aluminium or other metal foil, a sheet of a plastics material or a layer of a crosslinked surface coating. The support layer may be reinforced, for instance by glass or textile fibres.

Preferably, the support layer comprises a film of polyester plastics material having a thickness of about 100 microns.

The support layer should provide a sufficientiy hard surface without impairing the flexibility of the finished plate. The thickness of the support layer should be determined with these criteria in mind, and for certain materials may fall outside the typical values indicated above.

Preferably, the liquid material from which the layer of photopolymerised elastomeric material is produced has a viscosity of about 500 to 500,000, more preferably 5,000 to 50,000, cP at 230C.

A suitable material for use in producing this layer is a methacrylate capped polyurethane polyether. This may have an average molecular weight of about 5000 to 50,000, typically about 25,000.

Other suitable liquids which are photopolymerisable to form solid elastomers and which have appropriate viscosities are known in the art.

Generally, the liquid material will contain a photoinitiator, such as benzoin or ethers formed from benzoin and methanol or isopropanol, or a photosensitiser such as benzophenone. The liquid may also contain chain transfer agents and molecular weight controlling agents. In the instance of the methacrylate capped polymer mentioned above, a proportion of acrylate or methacrylate monomer may be included.

The liquid material may include other conventional additives, such as thermal polymerisation inhibitors, (for instance pmethoxyphenol, hydroquinone or salts of Nnitrosocyclohexyl-hydroxylamine), antioxidants, and plasticisers.

It should be appreciated that the liquid material, once crosslinked by image-wise exposure to actinic radiation will be a solid having relief surfaces and having uncrosslinked material surrounding these surfaces. The uncrossiinked material will be removed in conventional manner to leave the relief surfaces above the rest of the plate. References to the thickness of the elastomeric material layer are to be construed as references to the thickness of the relief areas, unless otherwise indicated.

In some cases it may be necessary to provide an anchor layer between the grindable elastomer and the photopolymerised material. Where a support layer is present, an anchor layer may be provided between it and either or both of the other layers surrounding it. Such anchor layers are provided to promote bonding between adjacent layers.

The anchor layer may comprise a coating on the support layer and/or on the grindable elastomer. Alternatively, it may comprise a layer of adhesive applied to the support layer and/or the grindable elastomer. Clearly the nature of or necessity for an anchor layer will depend on the natures of the layers to be bonded. However, a person skilled in the art will find no difficulty in selecting suitable adhesive or coating for any particular application.

Preferably where a support layer is present, the anchor layer comprises a coating on both sides of the support layer. Where the support layer is a polyester, the anchor layer may be a coating of a mixture of polyvinylchloride and polyvinylacetate.

The flexographic printing plate of the present invention may be made by adhering a layer of grindable elastomer to a plate made in conventional manner using a liquid photopolymerisable material. However, it is preferred that the plate is made in a conventional apparatus using a laminate comprising the grindable elastomer and the support layer as the backing.

Therefore according to a second aspect of the present invention, there is provided a laminate for use in producing a flexographic printing plate, comprising a layer of a grindable elastomer having a thickness up to 1000 microns firmly bonded to a support layer having a thickness typically from 75 to 150 microns.

The flexographic printing plate will have been subjected to conventional processing after exposure to provide a flat surface on the relief areas of the plate. However, such a plate will still suffer from the disadvantage of having nonuniform thickness in the layer of elastomeric material. This disadvantage can be overcome by grinding off parts of the layer of grindable elastomer, the ground plate having a uniform thickness over the relief areas.

Therefore according to another aspect of the present invention, a process for producing a flexographic printing plate having uniform thickness comprises grinding a flexographic printing plate according to the first aspect of the invention to uniform thickness by removal of portions of the grindable elastomer.

The present invention also includes processes for preparing the intact flexographic printing plate, either by use of conventional processing using a laminate as previously described or by adhering a layer of grindable elastomer to a conventionally produced plate. The processes may also include the grinding step as set out above.

The conventional processing may include contact or off-contact exposure. Both types of processing are commonly used in the printing industry for the production of relief printing plates.

Off-contact exposure is often used in newspaper printing.

The grinding may be carried out on conventional grinding machines, and preferably should give a tolerance of not exceedingl40 microns.

By use of the flexographic printing plate of the present invention, it is possible to use the liquid method to produce a printing plate of uniform thickness, which will enable the production of high quality printed products.

The invention will now be described, by way of example only, with reference to the accompanying drawing which shows a diagrammatic cross-sectional side view of a flexographic printing plate according to the invention.

Referring now to the drawing, the flexographic printing plate comprises a layer 1 of a filled natural rubber having a Shore A hardness of 50.

This is firmly bonded by a layer of rubber adhesive 2 to a layer 3 comprising a polyester sheet which has been surface coated 4 to increase its adhesion to a layer 5. The surface-coated polyester sheet 3 is sold under the trade name Bexford LP 40 (Bexford Ltd. is a subsidiary of ICI).

As can be seen from the Figure, layer 5 has areas 6 in relief surrounded by areas 7 of lesser thickness. The layer 5 comprises a photopolymerised elastomeric material produced by photopolymerising a methacrylate capped polyurethane polyether. in the present case a commercially available liquid material sold by W.

R. Grace Ltd. of Park Royal, London, under the designation Flexopolymer Type 40 was used. The areas 6 in relief were those which were photopolymerised during image-wise exposure, whereas the other areas 7 are those from which uncrosslinked material has been washed away.

The portion of the layer 5 nearest to the support layer 3 was photopolymerised by back exposure to actinic radiation so that the areas 7 have some thickness.

Layer 1 is 400 microns thick, layer 3 is 100 microns thick and layer 5 is about 6 mm thick, although its thickness is not uniform.

The flexographic printing plate was produced using a conventional machine, such as a Model ALF machine manufactured by Asahi Chemical Industry Ct. Ltd., in the following manner.

A negative was placed on the glass platen of the machine and covered with a thin layer of a non-tacky, tear resistant transparent polypropylene film. A quantity of Flexopolymer Type 40 was applied to the polypropylene film and at the same time a sheet of Bexford LP 40 coated polyester film was applied to the top of the Flexopolymer Type 40. The laminate was then exposed on both sides to actinic radiation which caused at least some of the Flexopolymer Type 40 to crosslink and therefore solidify. It also caused the solidified material to bond to the polyester film.

The exposed plate was then treated in conventional fashion to remove uncrosslinked material and to prepare the surface of the relief areas for printing. After conventional processing, the plate was laminated to a sheet of filled natural rubber using a conventional rubber adhesive.

In another experiment, the same procedure was carried out except that the filled natural rubber was laminated to the polyester support before it was used in the ALF model machine.

Flexographic printing plates as shown in the Figure and made by either of the methods described above were ground to uniform thickness in a Vandercook rubber grinder. These ground plates were used in a flexographic printing process, and produced printed products with even ink distribution and of good quality.. In contrast, if the flexographic printing plates were used without grinding, the ink was spread unevenly over the relief surfaces, resulting in uneven printing and therefore products of lesser quality.

It can thus be seen that the present invention enables the use of liquid based flexographic printing plates which can be used to produce high quality printed products. Hitherto this has only been generally possible using expensive solid based manufactured plates.

Claims

1. A flexographic printing plate comprising a layer of a grindable elastomer, having a thickness of up to 1000 microns, firmly bonded to a layer of photopolymerised elastomeric material which has been produced by exposure of a photopolymerisable liquid to actinic radiation.

2. A flexographic printing plate according to claim 1, wherein the photopolymerisable liquid is a methacrylate capped polyurethane polyether having a molecular weight of about 25,000.

3. A flexographic printing plate according to claim 1 or 2, wherein the grindable elastomer is a natural, polychloroprene or polyurethane rubber.

4. A flexographic printing plate according to any one of claims 1 to 3, wherein the grindable elastomer contains a filler.

5. A flexographic printing plate according to any one of claims 1 to 4, wherein the grindable elastomer has a hardness from 30 to 80 Shore A.

6. A flexographic printing plate according to claim 5, wherein the grindable elastomer has a hardness from 40 to 60 Shore A.

7. A flexographic printing plate according to any one of claims 1 to 6, wherein the layer of grindable elastomer has a thickness from 100 to 500 microns.

8. A flexographic printing plate according to any one of claims 1 to 7, and including an anchor layer between the grindable elastomer and the photopolymerised material to promote adhesion between these layers.

9. A flexographic printing plate according to any one of claims 1 to 7, and including a support layer between the grindable elastomer and the photopolymerised material.

10. A flexographic printing plate according to claim 9, wherein the support layer has a thickness from 75 to 1 50 microns.

11. A flexographic printing plate according to claim 10, wherein the support layer has a thickness of about 100 microns.

12. A flexographic printing plate according to any one of claims 9 to 11, wherein the support layer comprises a metal foil, a sheet of plastics material or a layer of crosslinked surface coating.

13. A flexographic printing plate according to claim 12, wherein the support layer is reinforced by glass or textile fibre.

14. A flexographic printing plate according to any one of claims 9 to 12, wherein the support layer comprises a sheet of polyester material.

1 5. A flexographic printing plate according to any one of claims 9 to 14, wherein the support layer is provided on one or both faces with an anchor layer to promote adhesion between it and its respective adjacent layer(s).

1 6. A flexographic printing plate, substantially as hereinbefore described with reference to the accompanying drawing.

1 7. A laminate for use in producing a flexographic printing plate according to any one of claims 9 to 16, comprising a layer of grindable elastomer, having a thickness up to 1000 microns, firmly bonded to a support layer.

1 8. A laminate for use in producing a flexographic printing plate, substantially as hereinbefore described.

19. A process for the production of a flexographic printing plate according to claim 1, comprising applying a layer of a liquid photopolymerisable material to a suitable substrate, image-wise exposing the layer to actinic radiation to photopolymerise the liquid material in image areas, and treating the exposed assembly to provide flat relief areas in the photopolymerised layer.

20. A process according to claim 19, wherein the substrate is a layer of grindable elastomer.

21. A process according to claim 20, and including the step of providing an anchor layer on the face of the layer of grindable elastomer to which the photopolymerisable liquid is to be applied.

22. A process according to claim 19, wherein the substrate is a laminate according to claim 1 7 or 18.

23. A process according to claim 19, wherein the substrate is a support layer, and including the step of bonding a layer of grindable elastomer to the face of the support layer away from the photopolymerised material.

24. A process according to any one of claims 1 9 to 23, wherein a contact exposure process is used to expose the liquid to actinic radiation.

25. A process according to any one of claims 19 to 23, wherein an off-contact exposure process is used to expose the liquid to actinic radiation.

26. A process according to any one of claims 19 to 25, and including the step of grinding the plate to remove material from the layer of grindable elastomer to produce a plate having uniform thickness over its relief areas.

27. A process for the production of a fiexographic printing plate, substantially as hereinbefore described with reference to the accompanying drawing.