MANUFACTURING THE SAME

TECHNICAL FIELD

The present invention generally relates to a substrate for security papers, in particular for banknotes and like high-security documents, and a method of manufacturing the same.

BACKGROUND OF THE INVENTION

A variety of substrate types are used in the high-security printing field, in particular for the production of banknotes, including (i) paper substrates which are typically made of cotton fibres, (ii) plastic or polymer substrates which are made of special polymer material (such as biaxially oriented polypropylene, or "BOPP") covered on both sides by white opacifying layers, as well as (iii) so- called hybrid or composite substrates combining paper and polymer materials or layers in one substrate medium.

A particular feature of polymer substrates, as for instance exemplified by the Australian Commemorative $10 banknote of 1988, resides in the provision of a transparent window formed by a region of the substrate where the white opacifying layers on both sides have been omitted to reveal a clear portion of the polymer material. This window portion is typically exploited to provide additional security by forming or applying features in the window such as embossings, printed patterns, and foil elements like optically-variable devices (OVDs).

Similar window features can be created in paper substrates and hybrid substrates provided the relevant paper layer(s) is/are provided with corresponding openings. In the case of hybrid substrates, the additional polymer layer can be exploited in order to close the window. In the case of paper substrates, the window-forming opening (which is produced e.g. by cutting) must typically be closed by a specific layer of material, such as foil material which is laminated on top of the window or otherwise embedded within the paper during manufacture. This process can be carried out at the paper mill or, more advantageously, at the printing works in accordance with the methods taught by International Patent Publications Nos. WO 2008/104904 A1 ,

WO 2009/1 12989 A1 and WO 2010/001317 A1 .

A potential problem with the paper-based window features resides in the fact that the opening in the paper layer(s) is prone to soiling and generates unevenness and non-uniformities in the substrate thickness which may affect processing and transport of the substrate through subsequent processing equipment. A solution is proposed in International Publication No.

WO 2005/1 16335 A1 which consists in filling the opening with transparent filling material, but this solution requires additional processing steps, after application of the foil material intended to cover the region of the window. Furthermore, adequate bonding of the transparent filling material within the opening becomes a challenge with such a solution.

There is a further trend in the high-security printing industry which is to combine micro-optical structures, such as lens structures, with printed patterns which are provided underneath and in register with the micro-optical structures so as to create sophisticated dynamic and/or optically-variable effects. An example of such a combination between a lens structure and a printed pattern is for instance disclosed in International Publication No. WO 2007/020048 A2.

As discussed in this publication, the dynamic and/or optically-variable effect is improved and optimized by maximizing the distance between the micro-optical structure and the printed pattern, namely by exploiting the entire thickness of the substrate material.

There is therefore a need for an improved substrate and a method of manufacturing the same.

SUMMARY OF THE INVENTION A general aim of the invention is to improve the known substrates used for the production of security documents, in particular banknotes.

A further aim of the invention is to provide such a substrate that suitably incorporates a window-based micro-optical structure.

These aims are achieved thanks to the invention as defined in the claims. Further advantageous embodiments of the invention form the subject- matter of the dependent claims and are discussed below. BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear more clearly from reading the following detailed description of embodiments of the invention which are presented solely by way of non-restrictive examples and illustrated by the attached drawings in which:

Figure 1 is a schematic view of a security document in the form of a banknote, as viewed from a recto side thereof, comprising a substrate according to the invention and which generally illustrates first, second and third embodiments of the invention ;

Figure 2 is a schematic cross-section of the banknote of Figure 1 , taken along line A-A, according to a first embodiment of the invention ;

Figure 3 is a schematic cross-section of the banknote of Figure 1 , taken along line A-A, according to a second embodiment of the invention ;

Figure 4 is a schematic cross-section of the banknote of Figure 1 , taken along line A-A, according to a third embodiment of the invention ;

Figure 5 is a schematic cross-section of a banknote in accordance with a fourth embodiment of the invention ; and

Figure 6 is a schematic cross-section of a banknote in accordance with a fifth embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will be described in the particular context of an application of the substrate for the production of banknotes. It is to be understood however that the substrate of the invention can be used for the production of a variety of security documents, such as visas, passports or like identity or travel documents. It should be appreciated that the substrate of the invention can take any appropriate substrate shape suitable for treatment in printing and processing equipment as conventionally used for the production of security documents, namely be provided in the form of individual sheets or a continuous web. In the context of the present invention, the expression "substrate" therefore designates sheet or web material in any shape, including individual sheets or a continuous web as used for the production of security documents, as well as the substrate of individual documents produced from such sheets or web, such as individual banknotes.

Figure 1 hereof shows an individual security document in the form of a banknote comprising a substrate according to the invention. As is typical in the art, banknotes are produced in the form of individual sheets or successive portions of a continuous web, which sheets or portions carry a matrix arrangement of individual note positions, which are processed (i.e. cut) into individual documents at the end of the production process. It shall therefore be appreciated that the corresponding substrate in sheet or web form basically comprises multiple repetitions of the banknote substrate as depicted in Figure 1 .

Figure 1 will be used to illustrate and describe first, second and third embodiments of the invention, the substrate being designated by reference S, S ^* , and S ^** respectively for the sake of distinguishing the embodiments. As shown in Figure 1 , the substrate S (S ^* , S ^** ) exhibits a window portion W, i.e. a portion of the substrate material that is substantially transparent, at least in part. While Figure 1 illustrates only one window, multiple windows of any desired shape could be provided.

As this will become apparent from reading the below description, the window is closed by a polymer layer designated by reference numeral 20 in the context of the first embodiment and by reference numerals 20' and 20" in the context of the second and third embodiments, respectively. This polymer layer can be made of any appropriate substantially transparent polymer material, such as biaxially oriented polypropylene (BOPP). The polymer layer can furthermore consist of a single homogenous layer of polymer material or of a laminate of two or more layers of polymer material.

The polymer layer 20 (20', 20") may extend only over a portion of the area of the substrate S (S ^* , S ^** ), as schematically illustrated by the dashed lines in Figure 1 which identify a strip of polymer material extending along the width of the banknote substrate S (S ^* , S ^** ). The polymer layer could however take any other shape suitably covering at least the area of the window portion W, such as a patch shape. In the context of the invention, the polymer layer 20 (20', 20") could advantageously extend over the whole area of the substrate S (S ^* , S ^** ), which simplifies manufacturing steps.

As further shown in Figure 1 , a micro-optical structure is formed in the region of the window W, which micro-optical structure is designated by reference numeral 30, 40 or 50 in the context of the first, second or third embodiment of the invention, respectively. This micro-optical structure 30 (40, 50) is preferably a lens structure, i.e. a structure comprising a multiplicity of individual lens elements such as semi-cylindrical or hemispherical lens elements. For the sake of illustration, it will be assumed that the micro-optical structure 30 (40, 50) consists in this example of multiple semi-cylindrical lens elements which are oriented horizontally in Figure 1 . Other micro-optical structures are possible, such as Fresnel lens structures or even more complex structures. In the context of the invention, the expression "micro-optical structure" should be understood as referring to any non-planar surface in or on the surface of the substrate S (S ^* , S ^** ) and which provides or creates an optical change in appearance when looking through the non-planar surface in question. The micro-optical structure 30 (40, 50) may extend over only part or all of the region of the window W or even beyond the boundaries of the window W depending on design choices and the manner in which the micro-optical structure 30 (40, 50) is formed on the substrate S (S ^* , S ^** ).

Turning to Figure 2, which illustrates a first embodiment of the invention, there is shown a cross-section of the banknote of Figure 1 taken along line A-A, through the substrate S and window region W, where the polymer layer 20 and micro-optical structure 30 are visible. As schematically illustrated, the polymer layer 20 exhibits, in the window-forming portion W, a thickness T (preferably of the order of 100 μιη) which is greater than the thickness t of the polymer layer 20 outside of the region of the window W. As further illustrated in Figure 2, first and second paper layers 1 1 , 12 are provided on each side of the polymer layer 20, which paper layers 1 1 , 12 do not cover the region of the window W. In other words, the polymer layer 20 is sandwiched between the first and second paper layers 1 1 , 12 and exhibits, in the region of the window W, portions protruding in the direction of both sides I, II of the substrate S. As depicted in Figure 2, the thickness T of the polymer layer 20 in the region of the window W is substantially equal to the added thickness of the paper layers 1 1 , 12 and of the polymer layer 20 outside of the region of the window W. In this way, the substrate S exhibits a substantially uniform and constant thickness T, it being understood that the thickness of the micro-optical structure 30 is not taken into consideration.

For the sake of illustration, the micro-optical structure 30 in the region of the window W is pre-embossed directly onto the side of the polymer layer 20, namely on the side coinciding with the upper side I of the substrate S. In other words, according to this first embodiment, the micro-optical structure 30 is an integral part of the polymer layer 20. The micro-optical structure could however be formed or applied other than by direct embossing of the polymer layer, as this will become apparent from the description of the second and third embodiments.

Figure 3 shows a cross-section of the banknote of Figure 1 , along the same line A-A, according to a second embodiment of the invention. For the sake of distinction, the substrate is designated by reference S ^* in this other example. The second embodiment shown in Figure 3 differs from the first embodiment illustrated in Figure 2 in that the micro-optical structure, designated by reference numeral 40, is pre-embossed onto a side of a foil element 45 which is applied, preferably by hot-stamping, onto the side of the polymer layer, designated by reference numeral 20'.

Figure 4 shows a cross-section of the banknote of Figure 1 , along line A- A, according to a third embodiment of the invention. This other embodiment differs from the previous embodiments described above in that the polymer layer 20" is exposed on one side (namely the upper side I in this example) of the substrate, designated by reference S ^** , and exhibits a substantially flat surface along this side I. A paper layer 15 is provided on the other side of the polymer layer 20" where a protruding portion is formed in the region of the window W. Like the paper layer 12 of the embodiments of Figures 2 and 3, the paper layer 15 is provided on the lower side of the polymer layer 20" so as not cover the region of the window W. The thickness T of the polymer layer 20" in the region of the window W is again substantially equal to the added thickness of the paper layer 15 and of the polymer layer 20" outside of the region of the window W, thereby resulting in a substrate S ^** exhibiting a substantially uniform and constant thickness T.

The micro-optical structure, designated by reference numeral 50 in this third embodiment, is formed by embossing of a side of a layer of material 55 (for instance a printed layer) which is applied, preferably prior to embossing, onto the side of the polymer layer 20". It is however to be appreciated that the micro- optical structure 50 could be pre-embossed directly onto a side (upper or lower side) of the polymer layer 20" (like in the first embodiment) or be pre-embossed onto a side of a foil element which is applied, preferably by hot-stamping, onto the side of the polymer layer 20" (like in the second embodiment).

It may be convenient to further apply an opacifying layer on top of the polymer layer 20" of Figure 3 (except in the region of the window W) as in the case of the known polymer substrates.

It is worth noting that the first and second paper layers 1 1 , 12 of the embodiments of Figures 2 and 3 and the paper layer 15 of the embodiment of Figure 4 could each consist of a single paper layer or of a laminate of multiple layers including at least one paper layer.

As illustrated by the above-described embodiments, an advantage of the substrate of the invention resides in that the substrate exhibits a substantially uniform and constant thickness, especially in the region of the window, thereby avoiding soiling issues. In addition, the polymer layer 20 (20', 20") in the region of the window W forms a substantially uniform and flat region enhancing the optical effect of the micro-optical structure 30 (40, 50). The window-forming portion of the polymer layer 20 (20', 20") is in particular advantageous in that it provides adequate support for printing a pattern on the side of the substrate S (S ^* , S ^** ) opposite to the side where the micro-optical structure 30 (40, 50) is located, thereby producing an optimal optically-variable effect when observing the window W from the upper side I of the substrate S (S ^* , S ^** ).

Figures 5 and 6 illustrate further embodiments of the invention highlighting that the polymer layer, designated by reference numeral 20 ^* , resp. 20 ^** , can exhibit a smooth thickness transition to the to the region of the window W, namely a smooth transition between the thick region of the polymer layer 20 ^* , 20 ^** forming the window W and the thinner region of the polymer layer 20 ^* , 20 ^** , outside of the region of the window W. The paper layers 1 1 ^* , 12 ^* in Figure 5 and the paper layer 15 ^* in Figure 6 similarly exhibit a gradually decreasing thickness as one moves towards the region of the window W.

The micro-optical structures 60, 70 depicted in Figures 5, 6 are similar to the micro-optical structure 30 of Figure 2. These micro-optical structures 60, 70 could alternatively be formed as in the alternative examples of Figures 3 and 4.

As far as the manufacture of the substrate is concerned, the method necessitates :

a) the provision of a polymer layer comprising at least one window- forming portion which exhibits a thickness which is greater than a thickness of the polymer layer outside of the window-forming portion ;

b) the provision of one or more paper layers in such a manner that the polymer layer is made to adhere to a side of at least one of the paper layers and that the resulting substrate exhibits a substantially uniform and constant thickness ; and

c) providing a micro-optical structure on at least one side of the window-forming portion of the polymer layer.

Techniques known as such in the art can be applied to ensure proper bonding and adhesion of the polymer layer onto the paper layer(s).

Various modifications and/or improvements may be made to the above- described embodiments without departing from the scope of the invention as defined by the annexed claims. In particular, as already mentioned, the invention is not limited to any particular substrate shape, but is preferably produced in the form of individual sheets or of a continuous web.

LIST OF REFERENCE NUMERALS USED THEREIN (if any)

S substrate, e.g. banknote or banknote sheet (first embodiment)

S ^* substrate, e.g. banknote or banknote sheet (second embodimen

S ^** substrate, e.g. banknote or banknote sheet (third embodiment) Sa substrate, e.g. banknote or banknote sheet (fourth embodiment)

Sb substrate, e.g. banknote or banknote sheet (fifth embodiment)

W window / window-forming portion of polymer layer 20, 20' or 20"

I first (upper) side of substrate S, S ^* , S ^**

II second (lower) side of substrate S, S ^* , S ^**

I I first (upper) paper layer (first and second embodiments)

12 second (lower) paper layer (first and second embodiments)

1 1 ^* first (upper) paper layer (fourth embodiment)

12 ^* second (lower) paper layer (fourth embodiment)

15 (lower) paper layer (third embodiment)

15 ^* (lower) paper layer (fifth embodiment)

20 polymer layer (first embodiment)

20' polymer layer (second embodiment)

20" polymer layer (third embodiment)

20 ^* polymer layer (fourth embodiment)

20 ^** polymer layer (fifth embodiment)

30 micro-optical structure, e.g. lens structure, pre-embossed on window- forming portion W of polymer layer 20 (first embodiment)

40 micro-optical structure, e.g. lens structure (second embodiment)

45 foil element carrying pre-embossed micro-optical structure 40

50 micro-optical structure, e.g. lens structure (third embodiment)

55 layer (e.g. printed layer) applied onto window-forming portion W of polymer layer 20" and subsequently embossed to form micro-optical structure 50

60 micro-optical structure, e.g. lens structure (fourth embodiment)

70 micro-optical structure, e.g. lens structure (fifth embodiment)

T thickness of substrate S, S ^* , S ^** and of window-forming portion W of polymer layer 20, 20', 20"

t thickness of polymer layer 20, 20', 20" outside of the window-forming portion W