WO2006032494A1

WO2006032494A1 - Method and apparatus for high speed manufacture of microstructures

Info

Publication number: WO2006032494A1
Application number: PCT/EP2005/010249
Authority: WO
Inventors: Johannes Matthiesen
Original assignee: Securis Limited
Priority date: 2004-09-23
Filing date: 2005-09-20
Publication date: 2006-03-30
Also published as: GB0421165D0

Abstract

Surface relief holograms may be replicated rapidly and with accuracy on a web based printing unit using the technique of in-situ polymerization replication (ISPR). The printing cylinder surface has a printing layer with surface relief microstructure that is derived from a silicone elastomeric material preferably a two-pack silicone elastomer. The printing cylinder may be used at faster web speeds than conventional ISPR cylinders having microstructures manufactured with radiation cured media.

Description

METHOD AND APPARATUS FOR HIGH SPEED MANUFACTURE OF

MICROSTRUCTURES

[0001] The invention relates to an apparatus and method for the manufacture of surface relief microstructures e.g. holograms.

[0002] The manufacture of surface structures with dimensions between the nanometer and micrometer level e.g. surface relief holograms has been undertaken in the past via a number of different methods. One commonly used method is thermal embossing where a hard embossing cylinder is utilized typically with pressure and temperature to transfer the image from the cylinder to a suitable thermoformable plastic such as PVC. A further method utilizes solvent based casting where a plastic dissolved in a solvent is coated onto the master with surface relief hologram and allowed to dry by evaporation, and the resulting dry layer of plastic is peeled off the master surface relief.

[0003] A more recently developed method is that of in-situ polymerization replication (ISPR). With this technology a liquid polymer or resin, usually deposited on a substrate such as a polymer film or web, is cast or molded against the microstucture to be replicated e.g. holographic image or optically variable effect profile, in a continuous fashion. The molded profile is retained in the polymer or resin on or after removal from the microstucture mold by use of a curing stage. Examples of this approach are described in United States Patent Nos. 3,689,346, 4,758,296, 4,840,757, 4,933,120, 5,003,915, 5,085,514 and in DE-A-4, 132,476, WO88/09252 and WO94/18609. In most of these prior art techniques the microstructure to be molded is provided as a relief in a metal surface on a rotary cylinder. They all utilize radiation curable media as the liquid polymer or resin media for casting and therefore this technology is sometimes referred to as UV casting.

[0004] In WO94/18609 the curing of the radiation curable media is achieved by the use of a UV source that is located within the bore of a hollow quartz cylinder that is carrying the microstructure relief image to be molded. In one embodiment the relief image is formed in a polymer sleeve that has been placed or cast on the outer surface of the quartz cylinder. The polymer sleeve is substantially transparent to the UV radiation that is used to cure the cast radiation curable resin. In a further embodiment the microstructure relief image is cast on the cylinder using UV curable resin system. Typically the thickness of the cast layer containing the surface relief microstructure is in the range from 3μm up to 300 μm. Also, as the surface relief microstructure located on the quartz cylinder is a cured organic material it is relatively soft compared to the metal surfaces used in more conventional other ISPR processes.

[0005] ISPR processes are attractive processes as they are web based and lend themselves to continuous/semi-continuous operations for bulk manufacture of surface relief structures. One of the enduring problems with web based processes however is that there is always a limit to the speed at which the web can be processed and this limit usually manifests itself in the limit at which acceptable quality may be achieved or the limit of the casting technology used; this is usually the maximum curing speed for the radiation curing resin to be cast. UV curing technologies are known in the printing industry that can run at web speeds in excess of 100 meters per minute. However, it is difficult to achieve such high web speeds with the ISPR processes described in the prior art especially those as described in WO94/18609, as attempts to run at higher speeds result in a reduction on the quality of the surface relief microstructure images. Image quality is critically important in many applications as the resultant application relies upon good quality optical effects which are directly related to the quality of the transferred image, respectively microstructures of the refraction grating.

[0006] Thus there is a need for ISPR processes and equipment that may be operated at higher web speeds whilst maintaining image quality. [0007] It has now surprisingly been found that if a particular group of materials are used to manufacture the surface relief microstructure image the resulting cylinder carrying the surface relief microstructure image may be used in holographic printing process at significantly higher web speeds with no significant loss of image quality, especially when compared to cylinders having surface relief microstructures manufactured using UV curable resin formulations. The particular group of materials that enable this to be achieved are the so called silicone elastomers especially tow-part silicone elastomers.

[0008] The present invention therefore provides in a first aspect a process for the manufacture of a surface relief microstructure cylinder for ISPR, which process comprises contacting a surface relief microstructure master with a silicone elastomer in contact with a cylinder for ISPR and curing of the silicone elastomer. The cylinder may be a conventional cylinder for use in ISPR processes e.g. a solid metal cylinder or preferably it is a hollow cylinder, which is made of a material e.g. quartz, that transmits UV light.

[0009] In a second aspect the present invention provides a surface relief microstructure cylinder for ISPR, which comprises a cured silicone elastomer adhered to a cylinder and having a surface relief microstructure defined therein. The cylinder may be a conventional cylinder for use in ISPR processes e.g. a solid metal cylinder or preferably it is a hollow cylinder, which is made of a material e.g. quartz, that transmits UV light.

[0010] In a third aspect the present invention provides an ISPR process for the manufacture of a microstructure, which process comprises contacting a surface relief microstructure master with a curable medium such that a surface relief microstructure corresponding to that of the master is formed in the surface of the curable medium that is in contact with the master and curing of the curable medium, wherein the surface relief microstructure of the master comprises a silicone elastomer. Preferably the master comprises a surface relief microstructure surface for ISPR according to the second aspect of the present invention and/or as manufactured by the process of the first aspect of the present invention.

[0011] In a fourth aspect the present invention provides an apparatus for the manufacture of surface relief microstructures using ISPR, which apparatus comprises a surface relief microstructure cylinder according to the second aspect of the present invention or as manufactured using the process of the first aspect of the present invention, means for contacting a curable medium with the surface relief microstructure of the cylinder and means for curing the curable medium. In a preferred embodiment the means for contacting the curable medium is a web of material e.g. Mylar onto which the curable medium has been deposited before contact with the cylinder, preferably the curable medium is a radiation e.g. UV curable medium. In a further preferred embodiment the means for curing the curable medium is a source of UV radiation and in a particularly preferred embodiment the UV source is located within the bore of a hollow cylinder transparent to UV.

[0012] In all aspects of the present invention the silicone elastomer may be a one part or more preferably a two-part silicone elastomer resin system. The resin system may be curable by the action of moisture, heat or addition of catalysts such as amines or alcohols. Preferably, the silicone elastomer system is substantially clear. By substantially clear is meant a resin system which, in the pre-cured state, is a straw color or water white. It is preferred that the silicone elastomer system in the cured state exhibits low levels of UV absorbency especially at the normal maximum emission wavelengths for UV lamps typically used for UV curing systems and that may be used in the ISPR processes described in WO94/18609. By low levels of UV absorbency is meant that less than 50% of the total UV emitted from the UV lamp is absorbed by the silicone elastomer, preferably less than 40%, more preferably less than 30%, more preferably less than 20%, more preferably less than 10% and most preferably less than 5%. In a preferred embodiment the silicone elastomer is substantially free of any additives or components that exhibit significant absorbance in the UV spectrum. The silicone elastomer is free of photoinitiators that are typically used in UV curing and/or their residues or degradation products. In a preferred embodiment the silicone elastomer is free of any UV absorbing chromophores. In a preferred embodiment when the resin system is cured it is preferred that that the curing mechanism is such that it does not result in a significant exotherm. In a further preferred embodiment the resin exhibits minimal shrinkage on curing and cures to a scratch resistant and flexible elastomeric material. These silicone elastomer based curable resins are systems that cure by heat or by the action of moisture or curing agents such as amines or alcohols.

[0013] Examples of suitable silicone elastomers are resins which may contain aliphatic epoxy and/or aliphatic urethane components to the resin system. Preferred silicone elastomers are those that are based on a siloxane backbone such as poly(dialkylsiloxanes), preferably poly(dimethylsiloxanes) and preferably those derived from tetra(trimethylsiloxy)silane. It is preferred that the silicone elastomer is free of aromatic based resin materials. The silicone elastomers in the pre-cured state preferably have viscosities as determined by ASTM D1084 of between 2000 to 10000 mPa.s, preferably 3000 to 8000 mPa.s, more preferably 4000 to 7000 mPa.s and most preferably 4000 to 6500 mPa.s.

[0014] It is preferred that the silicone elastomer is derived from a two-part composition wherein the curing agent is present at a ratio of polysiloxane to curing agent of 1 :1 or 10:1 , preferably the ratio is 10:1. Preferably, the curing agent is a tetramethyltetravinylcyclotetrasiloxane curing agent. Examples of suitable silicone elastomers are those manufactured and supplied by Dow Corning under the trade name SYLGARD^®., especially SYLGARD^® 184 and similar materials.

[0015] It is preferred that in the cured state that the silicone elastomer has a Durometer hardness, Shore A, as determined by ASTM D1084 within the range of 25 to 65, preferably 30 to 65, more preferably 35 to 60, more preferably 40 to 60 and most preferably 45 to 55. It is also preferred that in the cured state the silicone elastomer has a tensile strength as determined by ASTM D1084 within the range of 3 to 15 MPa, preferably 4 to 10 MPa, more preferably 5 to 9 MPa, and most preferably 5.5 to 8.5 MPa.

[0016] In a preferred embodiment the surface relief microstructure is manufactured on a solid cylinder for conventional ISPR and on a hollow UV transparent cylinder for use in the process descried in WO94/18069. The cylinder may be manufactured from any material that is substantially transparent to UV radiation. The cylinder may be manufactured from a polymer material or quartz. When quartz is used the cylinder may be manufactured from synthetic/processed or natural quartz. Preferably the cylinder is manufactured from synthetic/processed quartz.

[0017] The cylinder comprising the surface relief microstructure either with solid or hollow cylinders may be manufactured for example but not exclusively by the general process as described in WO94/18069, the contents of which are hereby incorporated by reference, with the exception that the radiation curable resins described therein are replaced by the silicone elastomer compositions described above. The general cylinder casting process as described in WO 94/18069 using vacuum in order to speed up the casting process. It has been found that the use of vacuum may cause problems due to micro-bubbles arising by contaminants or low molecular weight components of the curable formulation such as amines, which are transferred into their gas phase due to the vacuum. For this reason it is preferred that the cylinder manufacturing process utilizes positive air pressure during manufacture to force curable material into the master microstructures or that a suitable airless pump system is used.

[0018] In manufacturing the cylinder comprising the surface relief microstructure surface in a first step an original surface relief microstructure e.g. hologram is prepared by well-known means. To produce the original hologram, an object is first recorded in a first hologram by standard off-axis recording techniques. A Benton hologram is then recorded from the first hologram onto a surface relief medium such as photoresist, thus producing the original surface relief hologram. The next step is the manufacture of the hologram master. From the original surface relief hologram, a hologram master may be made by any of various techniques. One well-known way technique is to electroform nickel onto the original surface relief hologram, thereby producing a reversed metal replica of the original. This master may then be used to manufacture the surface relief microstructure using the general process as described in WO94/18069.

[0019] The cylinder comprising a microstructure relief surface for ISPR, which comprises a silicone elastomer based curable medium in the cured state with surface relief microstructures defined thereon, may be used as the surface relief microstructure master for any ISPR process such as those described in United States Patent Nos. 3,689,346, 4,758,296, 4,840,757, 4,906,315

4,933,120, 5,003,915, 5,085,514 6,214,443 6,344,245 6,436,483 and in DE-A- 4,132,476, EP 0,338,378, WO88/09252, WO94/18609, WO99/38704, the contents of which for each and every reference are all hereby incorporated by reference.

[0020] While the methods disclosed herein are primarily directed toward replication of surface relief microstructure patterns, it is clear that the methods are also useful for replication of any kind of surface relief pattern. In the claims which follow, the word microstructure relief surface is used to mean holograms, diffractive patterns and any structure that may provide an optical effect. The term also encompasses structures at the nanometer to micrometer scale that are not designed to provide optical effects.

[0021] In the forgoing disclosure reference is made to a cylinder based system. It is to be understood that in all aspects of the present invention the cylinder may be replaced by a belt of material e.g. Mylar onto which the surface relief microstructure bearing silicone elastomer may be deposited.

Claims

1. A process for the manufacture of a surface relief microstructure cylinder for ISPR, which process comprises contacting a surface relief microstructure master with a silicone elastomer in contact with a cylinder for ISPR and curing of the silicone elastomer.

2. A process as claimed in claim 1 wherein the silicone elastomer medium is heat curable.

3. A process as claimed in claim 1 or claim 2 wherein the cylinder is a solid cylinder.

4. A process as claimed in claim 1 or claim 2 wherein the cylinder is a hollow cylinder.

5. A process as claimed in claim 4 wherein the hollow cylinder is quartz.

6. A surface relief microstructure cylinder for ISPR, which comprises a cured silicone elastomer adhered to a cylinder and having a surface relief microstructure defined therein.

7. A surface relief microstructure cylinder for ISPR as claimed in claim 6 wherein the silicone elastomer medium is heat curable.

8. A surface relief microstructure cylinder for ISPR as claimed in claim 6 or 7 wherein the cylinder is a solid cylinder.

9. A surface relief microstructure cylinder for ISPR as claimed in claim 6 or 7 wherein the cylinder is a hollow cylinder.

10. A surface relief microstructure cylinder for ISPR as claimed in claim 9 wherein the hollow cylinder is quartz.

11. An apparatus for the manufacture of surface relief microstructures using ISPR, which apparatus comprises a surface relief microstructure cylinder as claimed in claims 6, means for contacting a curable medium with the surface relief microstructure of the cylinder and means for curing the curable medium.

12. An apparatus as claimed in claim 11 wherein the means of contacting the curable medium is a web or a continous / endless loop upon which the curable medium has been coated.

13. An apparatus as claimed in claim 11 wherein the means for curing the curable medium is a source of UV radiation.

14. An apparatus as claimed in claim 11 wherein the curable medium is a radiation curable medium.

15. An apparatus as claimed in claims 11 to 14 wherein a UV source is located within the bore of a hollow cylinder transparent to UV.

16. An ISPR process for the manufacture of a surface relief microstructure, which process comprises contacting a surface relief microstructure master with a curable medium such that a surface relief microstructure corresponding to that of the master is formed in the surface of the curable medium in contact with the master and curing of the curable medium, wherein the surface relief microstructure of the master comprises a silicone elastomer.

17. A process as claimed in claim 16 wherein the curable medium is an acrylate based radiation curable medium.

18. A process as claimed in claim 16 wherein the curable medium is a cationic or radical radiation curable medium.

19. A process as claimed in any one of claims 16 to 18 wherein the curable medium is cured with a source of UV radiation.

20. A process as claimed in claim 16 wherein the microstructure is a surface relief hologram.

21. A process, cylinder or apparatus as claimed in any one of the preceding claims wherein the silicone elastomer is a two-part silicone elastomer.

22. A process, cylinder or apparatus as claimed in any one of the preceding claims wherein the silicone elastomer is a two-part silicone elastomer having a curing agent present in a ratio of siloxane to curing agent of 1:1 or 10:1.

23. A process, cylinder or apparatus as claimed in any one of the preceding claims wherein the silicone elastomer is substantially transparent to UV.