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

[0002] The manufacture of 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 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 microstructure 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 microstructure 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 microstructure 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 the materials for the microstructure relief image on the printing cylinder and the materials used for

the radiation curable lacquer to be printed are carefully selected and controlled then the holographic printing process and apparatus may be run at higher web speeds with no significant loss of image quality. It has been found that high quality microstructures e.g. holograms may be obtained at higher web speeds from cylinders with microstructure relief images that have been manufactured from a radiation curable material that has been selected from a different class of radiation curable materials used for the printing lacquer.

[0OO 8] The present invention therefore provides for a process for the manufacture of a surface relief microstructure by ISPR, which process comprises contacting a cylinder comprising a surface relief microstructure derived from either a radical, cationic or combined radical/cationic radiation curable composition, with a radiation curable lacquer composition derived from either a radical, cationic or combined radical/cationic radiation curable composition, wherein the radiation curable composition of the lacquer is a different class of radiation curable composition than that used to manufacture the surface relief microstructure of the cylinder. In a preferred embodiment the radiation curable lacquer is deposited on a web based substrate prior to contact with the cylinder comprising the surface relief microstructure.

[00 O 9] The radiation curable composition used for the manufacture of the cylinder surface and for the radiation curable lacquer will be a composition that is one of three types. The first are free radical polymerized or cured resin systems which are unsaturated resins or monomers, pre-polymers, oligomers etc that contain vinyl and/or acrylate unsaturation for example and which polymerize and/or cross-link through use of a photo initiator activated by the radiation source employed e.g. UV. These are typically referred to as radical systems. The second is cationic polymerizable or cured resins in which ring opening (e.g. epoxy rings) is effected using photoinitiators or catalysts which generate ionic entities under the radiation source employed e.g. UV. The ring opening is followed by cationic polymerization and/or intermolecular cross- linking. Also used as cationically curable systems are resins that contain one or

more groups with vinyl unsaturation. These systems are typically referred to as cationic systems. The third type is a hybrid cured systems in which bothi free radical cured resins and cationic cured resins are combined. These systems are typically referred to as radical/cationic systems or combined systems.

[0010] Detailed descriptions of the chemistries and formulations that may be used in radical, cationic and hybrid systems are described for example in the following references: United States Letters Patent No. 3,794,576; Crivello et. al. in Journal of Radiation Curing, Vol. 4, page 2 (1977); Crivello et. al. in Journal of Radiation Curing, Vol. 5, page 2 (January 1978); United States Letters Patent No. 4,090,936; United States Letters Patent No. 4,069,055; United States Letters Patent No. 4,058,401. "UV Curing: Science and Technology" edited by S. P. Pappas, Technology Marketing Corporation, Stamford, Connecticut; "Radiation Curing of Coatings", J.V. Koleske, Astm Intl, April 2002; "Cationic Radiation Curing", Joseph V. Koleske, June 1991 ; "Photoinitiation for Polymerization: UV & EB at the Millenium", D. C. Neckers, John Wiley & Sons, June 1999; "Radiation Curing in Polymer Science and Technology: Fundamentals and Methods", J. P. Fouassier; "Radiation Curing in Polymer Science and Technology: Polymerisation Mechanisms", J. F. Rabek, Kluwer Academic Print, March 1993; "Radiation Curing in Polymer Science and

Technology: Polymerisation Mechanisms", J. F. Rabek, Kluwer Academ ϊc Pub, January 1994; "Radiation Curing in Polymer Science and Technology: Practical Aspects and Applications", J. F. Rabek, Kluwer Academic, March 1993; "Radiation Curing in Polymer Science & Technology: Photoinitiating Systems, Vol. 2", J. P. P. Fouassier, J. F. Rabek, March 1993.

[0011] Typical resins useful in UV curable coatings are styrented polyesters and acrylics, such as vinyl copolymers of various monomers and glycidyl methacrylate reacted with acrylic acid, isocyanate prepolymers reacted with an hydroxyalkyl acrylate, epoxy resins reacted with acrylic or methacrylic acid, and hydroxyalkyl acrylate reacted with an anhydride and subsequently reacted with an epoxy.

[0012] In one embodiment the radiation curable lacquer is a composition which comprises a radiation curable composition as described above diluted with a solvent. In a preferred embodiment the radiation curable lacquer comprises up to 60% by weight of solvent with the balance being radiation curable composition. The solvent may be any solvent that is compatible with the radiation curable composition and which may be removed after coating of the radiation curable lacquer on the substrate. In a preferred embodiment the solvent is a material which is not designed to react with the radiation curable components on radiation curing.. Lacquers comprising solvents have relatively low viscosities e.g. water viscosities at ambient temperature and on application to the web based substrate provide relatively thin smooth and almost mirror like lacquer surfaces. The thickness is typically 1 to 2.5 μm. This provides a perfect varnish surface for high quality and high efficient replication of microstructures especially very fine microstructures. In a preferred embodiment the solvent is evaporated from the coated lacquer prior to contact with the cylinder comprising surface relief microstructures. This evaporation may be achieved through use of for example a hot air drying tunnel.

[0013] In a further embodiment solvent free lacquers may be used, which are approaching 100% reactive material. In this embodiment it is preferred that coating on the web based substrate is achieved using curtain coating techniques known in the art which enable coatings of controlled and relatively uniform thickness of from 1 to 80 μm to be achieved with a relatively smooth surface.

[0014] The radiation used to effect curing of the radiation curable composition will typically be UV radiation, alternatively or in combination the radiation source could include electron beam, visible, infra-red or higher wavelength radiation, depending upon the material, its absorbance and the process used.

[0015] In a preferred embodiment the surface relief microstructure is manufactured on a hollow cylinder that is transparent to UV. 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.

[0016] The cylinder comprising the surface relief microstructure may be and preferably is manufactured by the general process as described in WO94/18069, the whole contents of which are hereby incorporated by reference. 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 radiation curable formulation such as amines, which are transfered 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 radiation curable material into the master microstructures or that a suitable airless pump system is used.

[0017] In manufacturing the cylinder comprising the surface relief microstructure 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 microstructure relief surface using the general process as described in WO94/18069.

[0018] The cylinder comprising a surface relief microstructure for ISPR, which comprises in the cured state a radiation curable medium comprising one or more silicone compounds that have been incorporated by co-polymerization or cross-linking into the cured medium, may be used as the microstructure relief surface 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 are all hereby incorporated by reference.

[0019] The key aspect of the present invention is that in the process of the present invention the chemistry of the radiation cured coating used to manufacture the surface relief microstructure is selected to be a different chemistry from that used as the formulation for the> radiation curable lacquer which is to be imaged with the printing cylinder. Thus, if the cylinder has been manufactured with a free-radical, acrylate based formulation it has been found that if the radiation curable lacquer is selected to be a cationic epoxy based formulation then the apparatus and printing process may be operated at higher web speeds compared to the situation where both formulations are for example free-radical acrylate based formulations. This increase in speed is achieved with no loss of quality in the reproduced image. Trie same relationship is found when the cylinder image is manufactured from a cationic system and the lacquer is a free-radical acrylate based system. In either situation the formulation may be replaced by a hybrid formulation in combination with a free- radical system or a cationic system.

[0020] While the methods disclosed herein are primarily directed toward replication of microstructure surface relief 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 term surface relief microstructure 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.