TECHNICAL FIELD

This invention relates generally to the production of embossed thermoplastic film or sheet material and, more particularly, to the structure and to the method of production of embossed polymeric transfer foils, sheets and the like. It is particularly applicable to the production of thin, metallised and embossed thermoplastic transfer foils having unique or unusual optical characteristics well-suited for use as security devices in bank-notes, travellers cheques and other security tokens. Such uses and tokens are described in Australian Patent No 488,652. However, the products and methods of this invention are not necessarily confined to such uses and the invention may be simply employed to produce embossed film. BACKGROUND ART

It is known that thermoplastic sheet materials may be impressed with surface textures or patterns by passing them through the nip of heated metal embossing rolls, or between heated platens, into the surface of which the desired . texture or pattern has been etched.

Whichever technique is employed, however, it is most difficult to emboss thin thermoplastic film (less than about 10 microns thick). Not only are such materials difficult to handle through hot rolls or platens without severe

continuously by extruding a film of molten thermoplastic material into the nip of a pair of rollers, at least one of which is embossed, the film bearing the desired pattern being drawn off as the rollers turn. But, again, this method cannot produce thin films. The present invention overcomes the above mentioned difficulties.

In our prior Australian Patent No 488,652 a novel design of security token was disclosed which comprised a laminated plastic sheet incorporating very thin security devices designed to make forgery difficult and expensive. Examples of such devices disclosed in that patent were diffraction gratings and Moire patterns, it being envisaged that these thin, film-like devices would be individually inserted into the security token by automatic machines designed for production of laminated bank-notes. This prior patent also disclosed a method of production of gratings in which the desired diffraction pattern is embossed into a thin metal layer deposited on a crosslinkable polymer. This was then coated with another layer of crosslinkable polymer to seal in the metal layer and both layers were crosslinked to make them more difficult to separate. Diffraction gratings produced in this way were then coated on each side

with a heat-sealable polymer which provided adhesion of the grating to the substrate of the security token.

A serious problem associated with the use of metallised diffraction gratings as security devices is the possibility that they might be removed from the token and separated in such a way as to allow access to the metallised and embossed surface. This would then allow replication of the surface by known procedures. Moreover, this danger is exacerbated if conventional transfer foil techniques are used to handle the thin devices during manufacture as these techniques require, the use of release agents to release the devices from the backing sheet to which they are temporarily attached. Some of the release agent adheres to the transferred device and naturally inhibits secure bonding of a protective layer or coating to the device. In view of the very fine physical features of such devices and their extreme delicacy when unprotected, it is generally impractical to attempt to remove traces of the release agent prior to final coating or lamination. DISCLOSURE OF THE INVENTION

Accordingly, the present invention seeks, generally, to provide a technique for embossing thin film and, more particularly, for producing a transfer foil, by which technique embossed patterns of high line density can be imparted to thin films suitable for use in the manufacture of security devices for incorporation in bank-notes or other

security feocens, or for use in decorative or novelty ____ mcvtδrials.

The general technique of the present invention is based upon the surprising discovery that polymeric sheet materials may themselves be used as the embossing medium for thermoplastic materials without significant degradation of the embossed pattern during use, provided the material of the embossing medium has a softening point which is relatively high with respect to that of the embossed material♦

According to its more general aspect, therefore, the present invention comprises a technique for embossing a thin film of thermoplastic material characterised in that the film is supported on a carrier sheet of flexible material and brought into contact under pressure and heat with the surface of a flexible strip of polymeric material having a pattern, design or texture embossed therein, whereby the film is impressed with said pattern, design or texture, the flexible material of said carrier sheet and said strip having softening temperatures substantially above that of the film and being of substantially greater thickness than the film. The thin product film is thus embossed by pressing it between the embossing strip and the carrier sheet under the action of heat and pressure and, after embossing, it may be removed from the carrier sheet. Typically, the thin film will be less than 10 microns thick while the carrier sheet

and the embossing strip will be greater than 20 (and usually greater than 50) microns thick.

It is preferable, although not essential, for the film and the strip to be brought into contact by passing them together through the nip of a pair of rollers, at least one of which is heated. And in such an arrangement it is also preferable for the strip to have the form of a continuous flexible belt.

It will be appreciated that the use of a continuous belt in conjunction with heated pressure rollers allows the rapid production of embossed thermoplastic film and that, since the embossing belt can be produced by the conventional step-and-repeat platen-embossing method, the embossing pattern may be continuous, uninterrupted or of a regular nature. Furthermore, it has been found that when a thin coating or layer of low softening point thermoplastic material on a carrier is embossed by this method, it allows highly intricate fine-line patterns such as diffraction gratings to be transferred to the product sheet. In its more particular aspect, the present invention comprises a method of forming a transfer foil suitable for use in security tokens or the like, characterised in that a thin film of thermoplastic material is formed and embossed on a carrier sheet according to the more general aspect of this invention, and further characterised in that the embossed surface of the film is at least partially coated to

render the embossed design more readily visible and in that the surface of the coated and embossed film is sealed with a layer of transparent thermoplastic material which fills the indentations thereof. If a metal coating is used to render the design more visible, it may be subsequently oxidised to give a part-reflective, part-transmission grating. Alternatively, the embossed layer may be coated with a very thin layer of a material - for example calcium fluoride - having a sufficiently different refractive index to the embossed layer to form a transmission grating.

The metallised or coated surface of the embossed layer is coated with the second layer of thermoplastic material so as to fill the exposed indentations of the embossed impression and prevent access to the embossed pattern. This layer also provides the means whereby the embossed and metallised layer can be made to adhere to a suitable recipient substrate, this being done by the application of heat and pressure to the carrier sheet. Preferably, both - or at least one - of said layers of thermoplastic material adjacent to the embossed and coated surface are transparent.

By adjustment of heat, pressure and the speed of operation, it is readily possible to effect the transfer of the completed grating from the carrier sheet to a product substrate (bank-note or security token) by softening the first layer just sufficiently to allow it to separate from

the carrier sheet and by softening the second layer sufficiently to ensure that it adheres to the substrate. In this way, the newly exposed surface of the first layer of thermoplastic material is well-suited to a further laminating or coating operation because of complete absence of release agents thereon. Furthermore, it will be appreciated that, even before a protective coating has been applied to the transferred device, it will be extremely difficult to gain access to the metallised layer in a way that would allow its reproduction in a manner aforedescribed. Being itself very thin and being firmly bonded to the extremely thin and delicate embossed surface, either thermoplastic layer would be very difficult, if not impossible, to remove by mechanical means without damaging the embossed layer. Since both layers are of the same or very similar chemical compositions, any attempt to remove one layer by the use of a solvent is likely to affect the other layer and cause disruption of the embossed surface. Thus, as intended, the method of the present invention is well-suited to the production of security devices for use in bank-notes or like security tokens.

The invention also embraces novel transfer foils of the type described.

Having broadly portrayed the nature of the present invention a particular embodiment will now be described by way of example and illustration only. In the following

description reference will be made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a series of diagrams illustrating the four principal steps involved in the manufacture and use of a transfer foil formed in accordance with the present invention.

Figure 2 is a diagrammatic representation of the final transfer step, showing the transfer foil and the recipient substrate in longitudinal section.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The particular embodiment chosen involves the production of a transfer foil containing a unique diffraction grating of high line density for use on security tokens or the like. In such applications, it is essential to make it as difficult as possible for copyists to gain access to the diffraction grating by separating the layers of the foil. It will be appreciated also that a key security element is the unique design of the diffraction grating itself which cannot be reproduced without access to major items of equipment such as electron beam etchers and complex computer programs driving such equipment. However, the production of the unique diffraction grating and the preparation of the master does not form part of the present invention. Conventional diffraction gratings, produced on circular turning machines or by linear ruling engines, could be used and will have value for decorative purposes.

Referring now to Figure 1, it will be seen that the Figure comprises three diagrams (A to C) illustrating three steps in the process for the production of a transfer foil for a security token in the form of a diffraction grating. A number of intermediate steps are not illustrated but will be mentioned.

Figure 1A is illustrative of the broadest aspect of the invention i.e. the method by which a coated thermoplastic strip 20 can be embossed in a continuous process by the use of the embossing belt 10 it being understood that the coating of thermoplastic strip 20 has a softening point significantly below that of the belt 10. For example, with a nylon embossing belt, it is convenient to use an acrylic copolymer coated onto a poly(ethylene terephthalate) strip 20, the coating being of between 2 and 4 micron and the strip being between 20 and 30 micron in thickness. The strip is carried from a pay-off roll 22 to a take-up roll 24 through the nip of a pair of rollers 26 and 28, of which the former is cooled and the latter is heated, which are pressed together during operation. The upper roller 28 carries the belt 10 which is also supported by idler roll 30. In operation, the pressure rolls 26 and 28 rotate to draw strip 20 from the pay-off reel 22 through the 'nip and press it into contact with the embossing belt 10. In this way, an impression of the series of diffraction gratings on belt 10 is transferred onto the upper surface of the thermoplastic strip 20.

In the next process step (shown in Figure IB), the coated and embossed thermoplastic strip 20 is housed within a vacuum chamber 32 and transferred from a pay-off reel 34 to take-up reel 36 so as to continuously expose an intermediate section to aluminium vapours generated by the evaporation of aluminium 38 from crucibles or boats 40. (Strip 20 is wound so that the embossed side is lowermost). This technique of metallising plastic film is well-known, as are the methods of preparing the surface of the film for adhesion of the metal layer. In any event, it is important that deposition of the metal should be uniform over the surface of strip 20.

After metallisation, the embossed and metallised side of the strip 20 is coated with another thin (2-5 micron) layer of thermoplastic material of similar composition and solubility to the first (non-embossed) coating. This layer serves to fill the embossed indentations and prevent access to the metallised surface by would-be copyists. Referring to Figure 2, the first (embossed) coating is identified by reference numeral 42, the metal layer by 44 and the covering thermoplastic layer by 46. As indicated previously, layer 46 not only serves to fill the embossed pattern but also acts as a heat-activated adhesive by which the device can be stuck onto a bank-note substrate. Finally, referring to Figure 1C the embossed, coated and metallised thermoplastic strip of material 20 is used as

a transfer foil to impart selected sections of the metallised coating (i.e. the security devices) onto a product substrate 50 by hot stamping, the hot stamping plattens being shown diagramatically at 52 and 54 in Figure 1C.

An important consideration in connection with the hot stamping is the need to ensure even, reproducable and predictable transfer of the diffraction grating from the transfer foil 20 to the product substrate 50. In accordance with a subsidiary feature of the present invention, this is achieved by proper selection of materials and coatings which comprise the transfer foil. (Figure 2 shows the transfer foil and the product substrate in cross-section. ) Preferably, the strip 20 is a sheet of polyester or cellulose ester, coating 42 is formed from an acrylic copolymer and coating 46 is formed from an acrylic copolymer of similar properties to coating 42.

Thus, as diagramatically illustrated in Figure 2, transfer of security devices from the strip 20 to the product substrate 50 is readily effected by a hot stamping process in which heated platens or rollers squeeze strip 20 and substrate 50 together under pressure at selected spots, where the pressure is applied, transfer is effected and where it is not applied the diffraction grating 44 and its enclosing layers 42 and 46 remain adhered to the polyester or cellulose ester base - i.e. the backing strip 20 of the transfer foil.

INDUSTRIAL APPLICABILITY

It will be seen from the foregoing description of the present invention that a method has been provided for the production of embossed thermoplastic film and of a hot-stamp transfer foil containing high resolution embossed and metallised patterns. Furthermore, in accordance with the secondary objective of the present invention, a transfer foil technique has been described which is well suited for use in connection with the production of security tokens or the like security devices. Nevertheless, those skilled in the art will appreciate that many variations and modifications can be made to the techniques and equipment described without departing from the spirit or the scope of the present invention.