AU2005252433A1

AU2005252433A1 - Security device with temperature activatable polymeric layer

Info

Publication number: AU2005252433A1
Application number: AU2005252433A
Authority: AU
Inventors: Christopher John Eastell; John Philip Winchcombe
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2004-06-10
Filing date: 2005-06-09
Publication date: 2005-12-22
Also published as: DE602005003666D1; PL1753915T3; GB0412981D0; EP1753915B1; DE602005003666T2; ATE380268T1; GB2414962B; ES2297722T3; GB2414962A; EP1753915A1; WO2005121450A1; GB0511753D0

Abstract

Security device 10 has layers 12,15 of a copolymer which switches from opaque to transparent at a specified activation temperature. These layers comprise a hard segment and a crystallisable soft segment or at least one soft segment which is crosslinked and contains a crystallisable soft segment. The device may include demetallised indicia 14 in metal layer 13. The layers 12,15 are opaque at temperatures below the activation temperature (preferably 35-37 {C) and switch to become transparent when warmed above it, revealing the indicia 14. The polymers may be oligomers. Adhesive layers 16 bond the device as a thread 17 on to a substrate from which a secure document 11 is made. Alternatively, the device may overlay an optically variable feature (30, figure 13) which becomes visible through the polymeric film 12 above the activation temperature.

Description

WO2005/121450 PCT/GB2005/002283 SECURITY DEVICE WITH TEMPERATURE ACTIVABLE POLYMERIC LAYER The present invention relates to improvements in security devices that can be used in varying shapes and 5 sizes for authenticating or security applications. In particular, the invention refers to a durable security device comprising a layer that switches from opaque to transparent at a specified temperature. 10 It is widely known to use in banknotes, security elements in the form of strips or threads which are made from a transparent film provided with a continuous reflective metal layer, vacuum deposited aluminium on polyester film being the commonest example. Banknotes 15 made using such security devices have been in general circulation in many countries for many years. When such security elements are embedded in security paper and the paper is subsequently printed to provide the security document, e.g. a banknote, the security element cannot 20 readily be discerned in reflected light but is immediately apparent as a dark image when the document is viewed in transmitted light. Such security elements are extremely effective against counterfeiting by printing or photocopying, since the optically variable effect which 25 is provided cannot be accurately simulated, for example by printing a line on the paper. The composition, size and positioning of security elements currently in use vary depending on the desired 30 security of the document. Typical security elements are composed of a polymeric film, such as polyester, which WO2005/121450 PCT/GB2005/002283 2 may be metallised or coloured and may include microprinted lettering denoting a title or message. The lettering can be produced by printing onto the substrate or by de-metallising a metallic layer on the substrate. 5 One example of a microprinted security thread can be found in GB-A-1095286. The security element can be slit to produce either registered lettering with respect to the edge of the thread, or unregistered lettering designed so that the message always appears irrespective 10 of thread slitting. The widths of security elements typically used vary from 0.5 mm to 6.0 mm and may have thicknesses typically ranging from 12 micrometres up to 50 micrometres. 15 The positioning of the security element within the document may be strictly controlled to predetermined criteria and may be concurrent with additional security features, such as a watermark. Security elements may be fully embedded within security paper in such a way that 20 paper fibres cover both sides of the security elements, making it considerably less visible in reflective light, but clearly visible in transmitted light. In recent times, however, in order to enhance 25 security documents against modern counterfeiting techniques making use of sophisticated colour separation, printing and colour photocopy technology, it has become common to use a security element comprising a thin layer of aluminium on a plastic support which is exposed on one 30 side of the sheet at intervals along the length of the security element, the region of exposure being referred to as a "window". GB-A-1552853 and GB-A-1604463 disclose WO2005/121450 PCT/GB2005/002283 3 banknotes containing such windows. Paper for use in producing such banknotes can be made using the method disclosed in EP-A-0059056. The dimensions of the windows typically used are from 3 mm to 14 mm lengthways, with 5 "bridges", being the areas of the paper substrate covering the security element between the windows, ranging from 4 mm to 30 mm there between. Again, the positioning of the windows may be controlled to allow registration of the window with respect of the document 10 and other security features such as watermarks. This latter development has resulted in enhanced security, and windowed paper has been used for banknotes in many countries. A banknote of this type provides added 15 security against counterfeiters as, when viewed in transmitted light, the security element is seen as a dark line and when viewed in reflected light on the appropriate side, the bright shining aluminium portions which are exposed at the windows are readily visible. 20 However, there is a need for even greater security by the use of more sophisticated security devices in order to render the task of a would-be counterfeiter more difficult as the reflected light appearance of the exposed aluminium portions of a security device can be 25 simulated to a degree by modern materials and techniques, for example, by the use of hot foil stamping. Other security elements in use are coated with protective lacquers and these may contain, for example, 30 fluorescent inks which are only visible on illumination with ultra violet light of a specific wavelength, e.g. 366 nm.

WO2005/121450 PCT/GB2005/002283 4 It is also possible to make security elements that are machine-readable by incorporating into or onto the substrate, substances which can be identified by a 5 detector, such as magnetic material. Other known security elements can be a single layer or multiple layers bonded or laminated together, and in some instances each layer may be coated with metal such 10 as aluminium, stainless steel, tin and/or metal 6xide such as tin oxide. The coating may be protected with a lacquer which may also contain coloured or invisible luminescent pigments, for example, rare earth chelates, such as europium acetylacetonate and the like. 15 Another prior art solution to enhancing the security of documents is the use of security devices comprising layers that switch from opaque to transparent at a set temperature to reveal hidden features, for example 20 indicia or optically variable devices. EP-A-0608078 discloses a security thread containing a thermochromic coating which changes from a coloured state to a colourless state to reveal underlying indicia. WO-A 0116426 discloses a security element also containing a 25 thermochromic layer, which is combined with a second layer such as an optically variable layer or a machine readable layer. In one particular embodiment of WO-A 0116426 a thermochromic layer is applied over a metallised holographic device, such that on heating above 30 a certain temperature the thermochromic layer is activated so that it becomes transparent and reveals the underlying hologram. WO-A-0050249 describes the use of WO2005/121450 PCT/GB2005/002283 5 thermochromic liquid crystal materials that become transparent or opaque when warmed and their application in security elements. An alternative approach is suggested in FR2698390 which describes the use of 5 piezochromic materials in an authenticating element such that a reversible change occurs from an opaque state to a transparent state under the action of a pressure. Thermochromic materials can be divided into two 10 categories; leucodye thermochromics and liquid crystal thermochromics. The use of the leucodye thermochromic materials in security inks, coatings and devices is limited due to its very poor lightfastness, while the liquid crystal thermochromic and piezochromic materials 15 require expensive and complex processing not suitable for mass produced articles. There is a requirement for a film or coating which can switch on touch from opaque to transparent which is both durable and easy to process. 20 According to the present invention there is provided a security device for use with a security document comprising an activatable layer of polymeric material comprising a hard segment and crystallisable soft segment 25 or at least one soft segment which is crosslinked and contains a crystallisable soft segment, said segments being selected to provide a predetermined activation temperature whereby the layer is opaque at temperatures below the activation temperature and becomes transparent 30 at temperatures above the activation temperature.

WO2005/121450 PCT/GB2005/002283 6 The present invention provides a significant improvement over the prior art in that it is very durable and easy to process. 5 The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross sectional side elevation of a security device according to the present invention in the 10 form of an embedded security element, on the line I-I of Figures 2 and 3; Figures 2 and 3 are plan views of a security document incorporating the security element of Figure 1 under different viewing conditions; 15 Figure 4 is a cross sectional side elevation of an alternative embodiment of a security device in the form of a windowed security element, on the line IV-IV of Figures 5 and 6; Figures 5 and 6 are plan views of a security 20 document incorporating the security element of Figure 4 under different viewing conditions; Figures 7 and 8 are cross sectional side elevations of further alternative embodiments of the security device in the form of a windowed security element; 25 Figure 9 is a cross sectional side elevation of the security device of Figure 8 on a transfer material so that the device can be used in the form of a patch or stripe; Figure 10 is a cross sectional side elevation of 30 another alternative embodiment of a security device WO2005/121450 PCT/GB2005/002283 7 applied to a banknote, on the line X-X of Figures 11 and 12; Figures 11 and 12 are plan views of the security document of Figure 10 under different viewing conditions; 5 Figures 13 and 14 are plan views of a further alternative security device incorporating a hologram applied to a security document under different viewing conditions; Figures 15 and 16 are cross sectional side 10 elevations of two further alternative security device incorporating a holographic layer for use as a patch or stripe; Figure 17 is a cross sectional side elevation of a security substrate incorporating a further embodiment of 15 the security device, so that it is exposed on both sides; Figure 18 is a cross sectional side elevation of an embodiment of a security element suitable for use in the substrate of Figure 17; and Figures 19 to 22 are plan views of the substrate of 20 Figure 17 incorporating the security element of Figure 18 under different viewing conditions. The security device of the current invention utilises a polymeric film or coating that switches from 25 opaque to transparent at a specified temperature. The polymeric systems suitable for use in the current invention comprise at least one hard segment and at least one crystallisable soft segment, or alternatively at least one kind of soft segment which is cross-linked and 30 contains a crystallisable soft segment, without the presence of a hard segment. The term "segment" refers to WO2005/121450 PCT/GB2005/002283 8 a block or sequence of polymer forming part of the polymeric system. Such polymeric systems are used in shape memory polymers and have been described in detail in US-B-6720402. The term "segment" refers to a block or 5 sequence of polymer forming part of the polymeric system. The terms hard segment and soft segment are relative terms, relating to the softening point (melting point or glass transition temperature) of the segments. The hard segment(s) has a higher softening point than the soft 10 segment(s). The polymeric systems suitable for the current invention can be thermoplastic, thermoset, interpenetrating networks, semi-interpenetrating 15 networks, or mixed networks. Polymers can be a single polymer or a blend of polymers. Polymers can be linear, branched, thermoplastic elastomers with side chains or any kind of dendritic structural elements. 20 According to the present invention the security device utilises a thermoplastic polymeric film or coating. The thickness of the thermoplastic polymeric film or coating is in the range 0.5-100pm and more preferably in the range 1-30pm. The thermoplastic polymer 25 is constructed such that when in the form of a film or coating the hard segment is substantially transparent and the soft segment is substantially opaque due to its high degree of crystallinity). In this case the film or coating will appear opaque below the melting point of the 30 crystalline soft segment and will become substantially transparent when heating above the melting point of the soft segment. The hard and soft segments are preferably WO2005/121450 PCT/GB2005/002283 9 oligomers. The term oligomer refers to a linear chain molecule having a molecular weight up to 15000 Daltons. The polymers forming the segments are selected based on the desired softening temperatures. The activation 5 temperature, i.e. the melting point of the soft segment, can be controlled by changing the monomer composition and the kind of monomer. For the current invention the activation temperature is in the range 250C to 800C, preferably 300C to 50 0 C. In a preferred embodiment the 10 material can be formulated such that the switch from opaque to transparent occurs at body temperature, -35 37 0 C, and therefore can be activated by touch. Alternatively, the security device of the present 15 invention utilises a thermosetting polymeric film or coating. The thickness of the thermosetting polymeric film or coating is in the range 0.5-100 tm and more preferably in the range 1-30pm. A thermosetting polymeric system suitable for the current invention 20 comprises a covalently cross-linked network, comprising one or more soft segments, and a crystallisable switching segment (soft segment). The activation temperature, at which the thermosetting polymeric film or coating switches from opaque to transparent, is the melting 25 temperature of the crystalline switching element. For the current invention the activation temperature is in the range 250C to 800C, preferably 300C to 50 0 C. In a preferred embodiment the material can be formulated such that the switch from opaque to transparent occurs at body 30 temperature, ~35-37 0 C, and therefore can be activated by touch.

WO2005/121450 PCT/GB2005/002283 10 Examples of polymer segments that are suitable for this invention include, but are not limited to, polyamides, polyester amides, poly(amino acid)s, 5 synthetic poly(amino acids), polyanhydrides, polycarbonates, polycaprolactones, polyacrylates, polyalkylenes, polyacrylamides, polyalkylene glycols, polyalkylene oxides, polyalkylene terephthalates, polyortho esters, polyvinyl ethers, polyvinyl esters, 10 polyvinyl halides, polyvinylpyrrolidone, polyesters, polylactides, polyglycolides, polysiloxanes, polyurethanes, ethylene vinyl acetate, poly(meth)acrylic acid, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyvinylphenol, and copolymers and mixtures 15 thereof. Specific examples of thermoplastic polymer systems comprising a soft segment with a high degree of crystallinity and a substantially transparent (that is to 20 say predominantly amorphous) hard segment are detailed in a review article "Shape Memory Polymers" in Angew. Chem. Int. Ed. 2002, 41, 2034-2057. One particular example suitable for the current invention is a segmented block copolyester urethane containing physical cross-links 25 (hard segment) and a crystallisable soft segment comprising polycapralactone units. Examples of thermosetting polymer systems comprising a cross-linked network of one or more soft segments, and 30 a crystallisable switching segment (soft segment) can be found in the same "Shape Memory Polymer" review article. One particular example is an AB polymer network based on WO 2005/121450 PCT/GB2005/002283 11 oligo(z-caprolactone) diols as the component that forms a crystallisable switching element. In this example the oligo(e-caprolactone) diols are functionalised with end groups to form a soft segment that can undergo a 5 polymerisation reaction. The oligo(e caprolactone)dimethacrylate segments are then combined with n-butyl acrylate to form the polymer network. The molecular weight of the two segments control the crystallinity, the activation temperature and the 10 mechanical properties. The polymeric material of the current invention can be produced as a film or as a coating and therefore can be incorporated into a security substrate which is used 15 to manufacture a document of value, such as a banknote, using a number of known processes. The polymeric material is incorporated into a security device such as a thread, stripe or patch in the form of a substrate or as a coated layer. In one embodiment of the invention, such a device 20 is applied to the surface of a document, as in the case of a stripe or patch. In another embodiment it is partially embedded into the document so that it is visible only partly on the surface of the document in the form of a windowed security element. In a further 25 embodiment the device is incorporated into the document such that regions of the device are viewable from the both sides of the document. One method for incorporating a security device such 30 that it is viewable from both sides of the document is described in EP-A-1141480. Alternatively, the polymeric WO2005/121450 PCT/GB2005/002283 12 material is applied directly to the surface of the document of value as an ink or coating. The function of the polymeric layer of the current 5 invention is to switch from opaque to transparent at a set activation temperature. The switch from opaque to transparent can in itself act as a security feature but in preferred embodiments it is used in combination with a second security element/layer which has further 10 properties which can be checked by either public, teller and/or machine to authenticate the device. Examples of security elements/layers that can be used include, but are not limited to, holograms, colour shifting optical effect pigments, colour shifting liquid crystal films, 15 demetallised designs, printed indicia, luminescent pigments and magnetic ink and codings. The following are examples of preferred embodiments of the present invention. 20 Referring to Figures 1 to 3, there is shown a cross section of a security device 10 for incorporation into a security document 11 as an embedded thread 17. The security device 10 comprises a polymeric film 12 of the 25 type described above that switches from opaque to transparent at a set activation temperature. The polymeric film 12 has been metallised, using vapour deposited aluminium to provide a metal layer 13, and contains demetallised indicia 14 by a method, such as 30 that described in EP-A-0319157. Alternatively the vapour deposited metallised layer 13 can be replaced with a WO2005/121450 PCT/GB2005/002283 13 metallic conductive or non-conductive ink applied in a non-continuous manner to provide indicia. A polymeric coating 15, that switches from opaque to 5 transparent at the same activation temperature as the polymeric film, is then applied over the metal layer 13. The coating 15 may be applied, by a number of techniques using, for example, a roll coater or alternatively using a printing press by flexographic, offset lithographic or 10 gravure techniques. An adhesive layer 16 is then.applied to both sides of the security device 10 to promote bonding of the device 10 to the secure substrate, from which the secure document 11 is made, during manufacture. 15 In reflected and transmitted light the visibility of the thread 17 is considerably reduced due to the opaque white appearance of the polymeric film and coating (Figure 2). On warming the security device 10 above the activation temperature, preferably 35-370, the polymeric 20 film 12 and coating 15 both switch from opaque to transparent revealing the negative (demetallised) indicia 14 when viewed in transmitted light (Figure 3). An alternative construction is shown in Figure 4 in 25 which a metallised polyester film 20, with demetallised indicia 14, is coated on both sides with a polymeric coating 15 that switches from opaque to transparent at a set activation temperature. An adhesive 16 is then applied to both sides of the security device 10 to 30 promote bonding of the device 10 to the secure substrate, from which the secure document 11 is made, during WO2005/121450 PCT/GB2005/002283 14 manufacture. Figure 5 and 6 illustrate the incorporation of such a device 10 into a secure document 11 as a windowed thread 17. In reflected light the thread 17 can be seen in the windows 18 having the opaque white colour 5 of the polymeric coating 15 (Figure 5). In transmitted light the visibility of the thread 17 is considerably reduced due to the opaque white appearance of the polymeric coating 15. On warming the security device 10 above the activation temperature, preferably 35-37 0 C, the 10 polymeric coatings 15 switch from opaque to transparent revealing the negative indicia 14 when viewed in transmitted light (Figure 6). In reflected light, when warming above the activation temperature, the windows 18 switch from white opaque to metallic with light lettering 15 or indicia. A further construction, suitable for incorporation as a windowed thread 17, is shown in cross-section in Figure 7. The security device 10 comprises a polymeric 20 film 12 that switches from opaque to transparent at a set activation temperature. The polymeric film 12 has been metallised, using vapour deposited aluminium 13, and contains demetallised indicia 14 by the method such as that described in EP-A-0319157. As before, the vapour 25 deposited metallised layer can be replaced with a metallic conductive or non-conductive ink applied in a non-continuous manner to provide indicia. An adhesive 16 is then applied to both sides of the security device 10 to promote bonding of the device 10 to the secure 30 substrate, from which the secure document 11 is made, during manufacture.

WO2005/121450 PCT/GB2005/002283 15 In reflected light, if the thread 17 is metal side up, demetallised indicia 14, appearing opaque white on a metal background, can be viewed in the windows 18. The visibility of the thread 17 in the embedded regions of 5 the thread 17 is considerably reduced. If the thread 17 is positioned metal side down, then the thread 17 can be seen in the windows 18 in reflected light as a white opaque strip. On warming the security device 10 above the activation temperature, preferably 35-37 0 C, the polymeric 10 film 15 becomes transparent. In reflected light the thread 17 can be viewed in the windows 18 as metallised strips with clear lettering or indicia. In transmitted light the thread 17 appears as a continuous dark line with clear lettering or indicia. 15 Figure 8 shows a cross-sectional view of a further embodiment of the security device 10 of the current invention. The device 10 is intended for use as a security label and comprises a clear polymer substrate 20 20 such as polyester onto which is printed identifying indicia 22 using inks or dyes. A polymeric coating 15, that switches from opaque to transparent at a set activation temperature, preferably 35-370C, is then applied over printed indicia 22. At room temperature the 25 device 15 has a uniform white opaque appearance but on warming above the activation temperature the polymeric coating 15 switches from opaque to transparent and reveals the hidden printed indicia 22. An adhesive layer 16 is applied to one side of the device 10 and over this 30 is applied a glassine carrier layer 19. The glassine layer 19 allows the label to be easily removed for WO2005/121450 PCT/GB2005/002283 16 reapplication to a document, or other item requiring protection. Figure 9 shows the label device 10 applied to a 5 substrate. The glassine layer is first removed to expose the adhesive layer 16. The label device 10 is then applied to the substrate, the adhesive used may be a pressure sensitive or hot melt adhesive and may be permanent or temporary. The use of temporary adhesives 10 can be of use where a label needs to be removed:and reapplied to another article. However, it is more likely that the label must be applied in a permanent manner. To prevent removal and reapplication of a permanently applied label the label may also be provided with other 15 tamper evident features e.g. frangible substrate layers, kiss cuts, and the like. A further embodiment of the current invention is shown in Figure 10 in which a polymeric film 15 of the 20 type described above that switches from opaque to transparent at body heat (35-370C) is applied over a serial number 25 of a banknote 26. The cross-section of the banknote 26 in the region of the polymeric film 15 is shown in Figure 10. The polymeric film 15 can be prepared 25 as a patch and applied using a release layer 24 and a carrier film 23 as is well known in the production of foil transfer devices, as described in US-A-4728377. At room temperature (<35 0 C) the serial number 25 is obscured (Figure 11) but becomes visible when the device 10 is 30 warmed by touch and can be checked against a second serial number printed on the note (Figure 12). In an alternative method a polymeric coating 15 that switches WO2005/121450 PCT/GB2005/002283 17 from opaque to transparent at body heat (35-37 0 C) is printed over a serial number 25 of a banknote 26. As a further alternative it would be possible to 5 apply the patch or printed polymeric film 12 onto the banknote 26 prior to application of the serial number(s) 25. A serial number 25, or design, can then be applied under the patch or film 12 by use of a suitable laser marking apparatus. By tuning the power level of the 10 laser correctly it should be possible to mark the surface of the banknote 25 without affecting the patch or polymeric film 12 on top. This can be made easier by printing the banknote 26 with a suitable laser marking additive (see Datalase from Sherwood Technology on their 15 web site http://www.sherwoodtech.com/index.asp). Such materials allow the use of a lower power laser and thus make it easier to mark through the patch or film 12 without affecting it. The second not obscured number could also be applied by laser or by more normal 20 processes such as letterpress. This would be a preferred approach as it would allow the numbering of the documents as a final step after all other print processes. This avoids the problem of having to renumber documents due to spoil during the application of the opaque to transparent 25 polymer. Figure 13 shows a further example of the current invention in which a polymeric film 12 that switches from opaque to transparent at body heat (35-37oC) is applied to 30 a secure document 11 such that it partly covers an optically variable feature, such as a metallised hologram 30. On touch the covered part of the hologram 30 will be WO2005/121450 PCT/GB2005/002283 18 revealed (Figure 14) and can be compared to the exposed section. The polymeric film 15 can be prepared as a patch and applied using a release layer 24 and a carrier film 23 as described with reference to Figure 9. 5 Figure 15 shows an embodiment of a security device 10 which is suitable for application to a secure document 11 as a patch or a stripe, in which a polymeric coating 15 that switches from opaque to transparent at a set 10 activation temperature is combined with a holographic layer or hologram 30. The security device 10 comprises a polymeric coating 15, a thermoplastic lacquer embossed with a holographic structure 30, and a reflective aluminium layer 13 that ensures that the holographic 15 structure 30 can be observed in reflection. The multilayer structure is applied to a carrier strip 23, such that the carrier strip 23 and the polymeric coating 15 are separated by a release layer 24. In a modified embodiment the reflective aluminium layer 13 can be 20 replaced with a thin reflection enhancing layer for e.g. a thin transparent layer of zinc sulphide. The security device 10 can be applied to the document 11 using an adhesive layer 16. The adhesive 25 layer 16 is applied either to the metal layer 13 or the surface of the secure document 11 to which the device 10 is to be applied. After transfer the carrier strip 23 is removed leaving the polymeric coating 15 as the exposed layer. On warming the security device 10 above the 30 activation temperature, preferably 35-37'C, the polymeric coating 15 becomes transparent and reveals the hologram.

WO2005/121450 PCT/GB2005/002283 19 Figure 16 shows a further embodiment of the security device 10, suitable for application to a secure document 11 as a patch or a stripe, in which a polymeric coating 15 that switches from opaque to transparent at a set 5 activation temperature is combined with a hologram. The security device 10 comprises a polymeric layer 15,31, a thermoplastic lacquer embossed with a holographic structure 30, and a reflective aluminium layer 13 that ensures that the holographic structure 30 can be observed 10 in reflection. The polymeric layer 15,31 comprises a polymeric coating 15 that switches from opaque to transparent at 35-37 0 C applied in the form of indicia or any type of identifying information. The remaining areas of the polymeric layer 15,31 comprise an opaque ink 31 15 which has the same visual appearance as the polymeric coating 15 when it is in its opaque state at room temperature. The multilayer structure is applied to a carrier strip 23 in the same manner as described previously with reference to figure 15. In a modified 20 embodiment the reflective aluminium layer 13 can be replaced with a thin reflection enhancing layer for e.g. a thin transparent layer of zinc sulphide. At room temperature (<35 0 C) the security device 10 25 has a uniform opaque appearance across its whole surface, but on warming the security device 10 above the activation temperature, preferably 35-37 0 C, only the regions containing the polymeric coating 15 become transparent to reveal holographic areas which have the 30 outline form of the identifying indicia.

WO2005/121450 PCT/GB2005/002283 20 A security device 10 comprising a polymeric film 12 or coating 15 of the current invention can also be incorporated into a secure substrate 26 such that regions of the device 10 are viewable from the both sides of the 5 substrate 26, or document made therefrom. One method for incorporating a security device 10 such that it is viewable from both sides of the substrate 26 is described in EP-A-1141480. Here a security thread 17 is selectively exposed on one side of the security substrate 26 and 10 fully exposed on the second side to produce a window 18, as illustrated in Figure 17. This method allows for the insertion of considerably wider security threads 17 into substrates. 15 Figure 18 shows a cross-sectional view of a security device 10 that could be incorporated in the manner described in EP-A-1141480. The security device 10 comprises a polymeric film 12 that switches from opaque to transparent at 35-37 0 C. On one side the polymeric film 20 12 has been metallised, using vapour deposited aluminium 13, and subsequently demetallised to form an identifying image. An optional protective lacquer 34 can then be applied over the metallised layer 13. On the other side of the polymeric film 13 in Figure 18, is coated an 25 adhesive layer 16 to promote bonding of the device 10 to the secure document 11. The security thread 17 is adhered to the document 11 such that one side 32 is fully exposed on the front of the document (Figure 19) and the other side 33 is exposed in a window 18 on the back of the 30 document 11 (Figure 20).

WO2005/121450 PCT/GB2005/002283 21 At room temperature the polymeric film 15 is opaque and when the security device 10 is viewed from the front of the document 11 in reflection a reflective metallised layer is observed containing a number of light 5 demetallised images with an opaque white appearance (figure 19). In contrast when viewed from the back of the document 11 (Figure 20) in reflection the security device 10 has an opaque white appearance. On warming the security device 10 above the activation temperature, 10 preferably 35-37 0 C, the polymeric film 12 becomes transparent such that in reflection the metallised layer containing a number of light demetallised images can be seen from both sides of the document 11. In transmitted light the demetallised images appear bright against a 15 dark background (Figures 21 and 22). The above embodiments are illustrative only and there are numerous other possibilities. 20 25

Claims

1. A security device for use in a security 5 substrate comprising an activatable layer of a polymeric material having a hard segment and a crystallisable soft segment, said segments being selected to provide a predetermined activation temperature, whereby the layer is opaque at temperatures below the activation 10 temperature and becomes transparent at temperatures above the activation temperature.

2. A security device as claimed in claim 1 in which the polymeric material has an amorphous hard 15 segment.

3. A security device as claimed in claim 1 or claim 2 in which the polymeric material is a thermoplastic material in which the hard segment is 20 substantially transparent and the soft segment is substantially opaque due to its high degree of crystallinity, wherein the activation temperature is the melting temperature of the soft segment. 25

4. A security device for use in a security substrate comprising an activatable layer of a polymeric material having at least one soft segment which is cross linked and contains a crystallisable soft segment, said segments being selected to provide a predetermined 30 activation temperature, whereby the layer is opaque at temperatures below the activation temperature and becomes transparent at temperatures above the activation temperature. WO2005/121450 PCT/GB2005/002283 23

5. A security device as claimed in claim 4 in which the polymeric material is a thermosetting material comprising a covalently cross-linked network of one or more soft segments and a crystallisable soft switching 5 segment, wherein the activation temperature is the melting temperature of the crystalline switching element.

6. A security device as claimed in any one of the preceding claims in which the activation temperature lies 10 in the range of 250C to 80 0 C.

7. A security device as claimed in claim 6 in which the activation temperature lies in the range of 30'C to 50C. 15

8. A security device as claimed in claim 7 in which the activation temperature is body temperature.

9. A security device as claimed in any one of the 20 preceding claims in which the hard and soft segments are oligomers.

10. A security device as claimed in any one of the preceding claims further comprising a carrier layer to 25 which is applied the activatable layer.

11. A security device as claimed in claim 10 in which opaque indicia are applied to the carrier layer and the activatable layer is applied over the indicia. 30

12. A security device as claimed in claim 10 or claim 11 in which the carrier layer is also an activatable layer. WO2005/121450 PCT/GB2005/002283 24

13. A security device as claimed in claim 11 or claim 12 in which the indicia are provided by a layer of metal which has been partially demetallised to form the 5 indicia.

14. A security device as claimed in any one of claims 10 to 13 comprising a further activatable layer of the copolymer applied to a surface of the carrier layer 10 opposing the surface to which the indicia are applied.

15. A security device as claimed in any one of the claims 1 to 10 in which indicia are formed by an opaque ink and the activatable layer is applied in register 15 therewith, the opaque ink and activatable layer being of the same visual appearance such that at temperatures below the activation temperatures the security device has a uniform opaque appearance, whilst above the activation temperature opaque indicia are revealed. 20

16. A security device as claimed in any one of claims 1 to 14 in which indicia are applied to a surface of the activatable layer. 25

17. A security device as claimed in any one of the preceding claims further comprising an adhesive layer applied to one side of the device, to which adhesive layer is applied a release layer. 30

18. A security device as claimed in any one of the preceding claims further comprising an optically variable feature located beneath the activatable layer. WO2005/121450 PCT/GB2005/002283 25

19. A security device as claimed in claim 18 in which the optically variable feature is a holographic feature. 5

20. A security device as claimed in any one of the preceding claims wherein the polymeric layer is a film.

21. A security device as claimed in any one of claims 1 to 19 wherein the polymeric layer is a coating. 10

22. A security substrate comprising a base substrate and a security device as claimed in any one of the preceding claims. 15

23. A security substrate as claimed in claim 22 in which the security device is applied to at least one surface of the base substrate.

24. A security substrate as claimed in claim 22 in 20 which the security device is at least partially embedded within the security substrate and visible at least one aperture or window in one or both surfaces of the substrate.

25 25. A security substrate as claimed in any one of claims 22 to 24 comprising a plurality of security devices.

26. A security document made from a security 30 substrate as claimed in any one of claims 22 to 25. WO2005/121450 PCT/GB2005/002283 26

27. A security substrate substantially as hereinbefore described with reference to and as shown in the accompanying drawings.