Abstract:
The invention is directed toward improvements in security substrates, such as paper, used for making security documents, such as bank notes, having anti-counterfeit able features and in particular to security substrates incorporating an elongate security element and methods of making the substrate. The invention comprises a security substrate for making security documents and the like comprising a fibrous base substrate and an elongate security element at least partially embedded therein. At one surface of the security substrate one portion of the security element is exposed to provide a continuous track along the length of the security element. A plurality of other portions along at least one edge of the security element are partially covered by overlapping regions of the fibrous substrate. The invention further comprises a method of manufacturing such a security substrate.

Description:
CROSS-REFERENCE TO PENDING APPLICATIONS 
     This application is the United States National Phase of PCT Patent Application No. GB2009/000837 filed on 27 Mar. 2009, which claims priority to British Patent Application No. 0805916.4 filed 1 Apr. 2008, both of which are incorporated herein by reference. 
     The following documents are incorporated herein by reference:
         GB 1552853, “Improvements In Or Relating To Authentication Devices” in the name of Peter Denis Lee;   EP 0059056, “Sheet Materials, Methods For Making Sheet Materials and Security Documents” in the name of Portals Limited;   WO 2000/39391, “Improvements in Making Paper” in the name of De La Rue International Limited;   WO 2003/095188, “Improvements in Fibrous Sheets” in the name of De La Rue International Limited;   WO 2004/001130, “Fibrous Substrates” in the name of De La Rue International Limited;   WO 2005/106601, “Improved Optically Variable Devices” in the name of De La Rue International Limited;   WO 2006/095161, “Security Device Based On Customized Microprism Film” in the name of De La Rue International Limited       

     The invention relates to improvements in methods of making sheets for security documents having edge or corner reinforcing watermarks and a surface applied feature. 
     Folded or bent corners (corner folds or dog-ears) on banknotes present a significant problem for many banks, as they can cause problems in cash handling machines and can result in an artificially short note life. Many machines will reject such notes from circulation. One major European central bank has indicated that 80% of the rejections from their machines are due to such corner folds. Notes with folded corners can also be problematic in ATMs and cash dispensers and other note handling equipment. This is becoming a more significant problem as the use of such machines is becoming more and more widespread. 
     Efforts have been made to resolve this problem by providing note handling equipment with apparatus for flattening banknotes to enable a dog-eared or curled document to be fed without jamming. Such a system is described in U.S. Pat. No. 5,265,856. 
     Another problem, which occurs with banknotes in particular, results from the tendency of users to roll and fold notes for storage or keeping in wallets and purses. This gives rise to damage at the middle of the edges of the notes and similar problems arise in ATMs and other note handling equipment as occurs with dog-ears and corners. 
     Furthermore there is also a significant problem with edge tears in banknotes and the propagation of such edge tears. 
     The use of corner and edge reinforcing watermarks has been described in WO-A-03/046282, EP-A-1466755, and EP-A-1466756. The structures described in the prior art have been very successful in solving the aforementioned problems. 
     In addition to dealing with the aforementioned problems, bank notes and other security documents require security features that can be easily recognised and verified by the public without the need for additional verification devices. Security devices of the type described herein are intended for use on documents of value including, but not limited to, banknotes, fiscal cheques, travellers cheques, fiscal stamps, postal stamps, certificates of authenticity, brand protection articles, bonds, certificates, vouchers and the like. 
     It is widely accepted that, when producing security documents, a range of security features is required. This is both to overcome the efforts of counterfeiters and to enable inspection of the documents at a number of different levels. It is usually considered that the inspection of security documents falls into one of three categories; public, teller assist and covert. 
     Inspection of documents at a public level requires the feature to be overt, or not to require additional aids to verify. Examples of public security features include windowed and embedded threads, cylinder mold watermarks, holographic foils and stripes, intaglio print, colorshifting or optically variable inks, thermochromic features, embossed or printed latent features. All these features will be familiar to those skilled in the art and they are also widely discussed in many publications, including many published patent specifications. It is important to note that several of the above features are not apparent until viewed or handled in the correct manner, e.g. latent images. Such features, though not overt, can still be authenticated using the human senses and handling alone and do not require the use of a secondary device or piece of equipment. 
     The use of optically variable security features in the form of surface applied stripes and foils are very common on banknotes and other secure documents. The preferred method for transferring stripes or patches is known as the transfer method in which an optically variable device is formed on a carrier substrate and transferred to the security document in a subsequent working step. The optically variable device can be applied to the security document using an adhesive layer. The adhesive layer is applied either to the optically variable device or the surface of the secure document to which the optically variable device is to be applied. After transfer, the carrier substrate may be removed, leaving the optically variable device as the exposed layer. 
     Alternatively the carrier layer can remain as part of the structure acting as an outer protective layer. 
     Optically variable devices may take a variety of forms including holograms, diffraction gratings, micro-optical structures (such as those comprising microlenses and microprisms), angular dependent colored reflection generated from materials such as liquid crystal materials, thin film interference structures, multilayer polymeric structures and photonic crystal structures. 
     The application of a surface applied material to a sheet having corner or edge reinforcing watermarks has proved problematic due to the poor adhesion of the stripe or patch to the regions of variable profile resulting from the plurality of reinforcing bars forming the watermarks. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved method of making paper sheets for security documents which enables a surface applied security feature such as a stripe or patch to be used in conjunction with a corner or edge reinforcing watermark. 
     The invention therefore provides a method of making a sheet from a fibrous substrate, the sheet having corners and edges joined at the corners, comprising the step of forming the substrate by depositing fibers on a support surface, fanning at least one reinforcing watermark at a predetermined position in the substrate, which at least one reinforcing watermark increases the stiffness of the sheet in the watermarked region, the reinforcing watermark being a positive watermark formed by varying the grammage of fibers of the substrate to form at least two adjacent bars of increased grammage relative to the mean grammage of fibers in adjacent areas of the substrate, and further forming at least one raised region, formed by increasing the grammage of fibers, which at least one raised region joins the adjacent bars within the at least one reinforcing watermark to form a smooth planar surface, and applying a substantially planar security feature to a portion of at least one surface of the substrate across the smooth planar surface. 
     The substrate is preferably first formed as a web which is subsequently cut into a plurality of sheets or the substrate forms a single sheet. 
     The reinforcing watermarks are preferably formed in at least one corner of the sheet so as to increase the stiffness of the sheet in said corners and/or along at least one edge of the sheet so as to increase the stiffness of the sheet along said at least one edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example only, with reference to the accompanying drawings in which: — 
         FIG. 1  is a plan view of a section of a continuous web of a fibrous substrate for making the sheets according to method of the present invention with edge reinforcing watermarks; 
         FIG. 2   a  is a plan view of the section of a web of substrate of  FIG. 1  with a surface applied stripe; 
         FIG. 2   b  is a plan view of a sheet of substrate cut from the web of  FIG. 2   a;    
         FIG. 3   a  is cross sectional end elevation on the line III-III of  FIG. 2A ; 
         FIG. 3   b  is cross sectional end elevation similar to that of  FIG. 3   a  of a prior art sheet; 
         FIGS. 4 to 9  are alternative patterns for edge reinforcing watermarks for use in the present invention; 
         FIG. 10  is a plan view of a section of a web of substrate for making the sheets according to the method of the present invention with corner reinforcing watermarks; 
         FIG. 11  is a plan view of a section of an alternative web of substrate for making the sheets of the present invention with edge and corner reinforcing watermarks; and 
         FIG. 12  is a plan view of a section of another alternative web of substrate for making the sheets of the present invention with edge reinforcing watermarks. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  there is illustrated a section of a sheet of a fibrous substrate, such as paper, in the form of a continuous web  10 , which can be made by hand or using a known papermaking machine, such as a cylinder mold or Fourdrinier machine. The web  10  is subsequently cut along the marked cutting lines  12  to form individual smaller sheets  11 , of which three sheets  11   a ,  11   b ,  11   c , are shown in  FIG. 1 . The individual smaller sheets  11  will form security documents such as banknotes, passports identification cards and the like. A range of fiber types can be used in the making of such substrates, including synthetic or natural fibers or a mixture of both. The actual preparation of the fibers is unrestricted by the invention and will depend on what effect it is wished to produce in the finished substrate. Security paper used for security documents, such as banknotes, passports, identification cards and so on, needs to be hard wearing, resilient and self-supporting and so an appropriate fiber mix must be selected. 
     It should be noted that the method of the present invention may also be performed by making single sheets directly rather than making a web or a large sheet which is cut into smaller sheets. 
     A typical watermark is created by well-known techniques of varying the grammage of fibers, so that in some areas there is a higher grammage of fibers than the mean grammage in the base substrate layer, and in others there is a lower grammage. When viewed in transmitted light the areas of lower grammage are lighter and the areas of higher grammage are darker than the base substrate, and the contrast between the light and dark areas can be very clearly seen. It has been found that watermarks that locally increase the grammage of the paper along the edges of the document significantly reduces its propensity to tear initiation and propagation by increasing the stiffness in this area. One reason for this increase is because of the increase in the stiffness of the paper. It is well known, according to classical beam theory, that the stiffness of an object is proportional to the cube of its thickness, as described in “Pulp and Paper Technology and Treatment of Paper”, 1978, page 74 by J d&#39;A Clark, Freeman Publications Inc, San Francisco. Small increases in thickness do thus result in a significantly largely benefit in terms of stiffness. A typical stiffness measurement would be the L&amp;W test as specified in ISO 2493. 
     It has been found that the stiffness of the paper increased where the watermark was a positive watermark having the effect of adding bulk to selected areas (as compared to the thickness of the base paper layer), as opposed to a negative watermark where the main portion was thinner than that of the base paper layer. 
     In one embodiment of the present invention, edge reinforcing watermarks  13  are formed in the web  10  such that when the web  10  is divided into smaller sheets  11 , they are provided along at least one, and more preferably two opposing, edges  14  of the smaller sheets  11 . In another embodiment of the invention, the edge reinforcing watermarks  13  are provided along all edges  14  of the sheet  11 . The edge reinforcing watermarks  13  are preferably positive watermarks, which means that the grammage of fibers in the watermarked area is the same or greater than the mean fiber grammage of the substrate in the non-watermarked areas. The preferred form of the edge reinforcing watermarks  13  on the smaller sheets  11  is a bar  13   a  or a plurality of adjacent bars  13   a , and the watermarks  13  preferably have elements perpendicular to the direction of the tear propagation, i.e. parallel to the edges of the sheet. 
     The improved resistance to tear initiation and propagation arises from the increase in the grammage of the substrate in the watermark bars  13   a  compared to the non-watermarked area, and it has been observed that the wider the bar  13   a  the greater is the observed improvement. However, it is difficult to produce positive watermark bars  13   a  of a uniform thickness along the whole length of a sheet  11  and having a width greater than 5 mm, due to washout of the fibers during the papermaking process. Therefor; a plurality of adjacent watermark bars  13   a  are preferably formed at the relevant positions in the web  10 , each bar  13   a  having a preferred width in the range of 1 to 5 mm, and more preferably in the range of 2 to 4 mm. 
     If the paper web  10  is produced in a continuous manner on a papermaking machine, then in order to ensure that the edge reinforcing watermarks  13  comprise at least one bar  13   a  along each of the long edges of the small sheet  11 , the watermark formed in the large continuously formed web  10  comprises at least two, and more preferably three or more reinforcing bars  13   a  formed on and/or adjacent the horizontal cut-line  12 . Preferably the cut-line  12  runs through the center of the middle bar  13   a . This configuration provides a tolerance for the actual position of the cut-line  12 , as the cut-line  12  can be anywhere within the middle bar  13   a  or even in the spaces either side of the middle bar  13   a  to still ensure there is at least two bars  13   a  along the respective edge of each sheet  11 . 
     It has been observed that, if a stripe  15  in the form of a polymeric film or coating is applied over the bars  13   a  of the reinforcing watermark  13 , poor adhesion is observed between the substrate and the stripe  15  due to the variable profile of the substrate in the region of the reinforcing watermark  13 . The reason for this poor adhesion is illustrated schematically in  FIG. 3   b  which shows a cross-section of the web  10  with a stripe  15  applied over an edge reinforcing watermark  13  comprising three-bars  13   a . It can be seen that the stripe  15  only contacts the web  10  in the raised regions of the bars  13   a  resulting in areas  16  of poor adhesion between the bars  13   a  where the stripe  15  bridges the gaps between them and has very little contact with the paper surface. 
     The present invention solves this problem by creating a region  17  of increased grammage of substrate which is raised and joins up the bars  13   a  in the localized region where the surface stripe  15  is to be applied. This is illustrated in  FIG. 3   a . The presence of this raised region  17  provides a smooth surface which increases the contact area between the stripe  15  and the substrate and significantly improves the adherence of the stripe  15  to the substrate. 
     The bars  13   a  of the reinforcing watermark  13  and the raised region  17  are preferably created by depositing fibers onto a support surface of the papermaking machine which has portions which are sunken relative to adjacent areas of the surface. In cylinder-mold papermaking, paper is formed on a partially submerged wire-cloth covered mold cylinder, which rotates in a vat containing a dilute suspension of paper fibers. As the mold cylinder rotates, water is drawn through the wire cloth depositing fibers onto the cylinder surface. In the sunken regions the fibers deposit with a greater thickness to form a positive watermark. The sunken regions are generally created by embossing the wire-cloth. 
     It is preferred, but not essential, that there is a gradual transition between the raised regions  17  and the adjacent regions of base substrate grammage. Experiments have shown that such a gradual transition can be achieved by having the slope between the sunken regions of the support surface and the adjacent regions at an angle which is preferably in the range of 20° to 70°, more preferably in the range of 30° to 55° and most preferably approximately 45° to the normal of the substrate forming support surface. 
     A wide range of designs of edge reinforcing watermarks  13  may be used in the present invention, which comprise (in some form) a plurality of stripes of increased grammage. Some suitable designs are illustrated in  FIGS. 4 to 9 . 
     The present invention is equally applicable to the manufacture of sheets having one or more corner reinforcing watermarks  18 . The corner reinforcing watermarks may be located in one, two, three or all four corners and the latter is illustrated in  FIG. 10 . Suitable corner reinforcing watermarks  18  are as described in WO-A-03/046282. A particularly effective pattern for a corner reinforcing watermark  18  is one that results in bars  18   a  of higher grammage approaching the edges  14  of the sheet  11  at an angle between 35° and 55° to the edges  14 ,  19  and more preferably at 45° The preferred widths of the bars  18   a  are in the range of 1 to 2 mm wide and most preferably 1.5 mm wide. 
     Sheets  11  may also have both corner and center edge reinforcing watermarks  13 ,  18 , for example, as shown in  FIG. 11 . 
     In addition, the edge and/or corner reinforcing watermarks  13 , 18  can be combined with a further watermark  19 , such as a portrait watermark, to enhance the security and aesthetic effect of the sheet  11 . As shown in  FIG. 11  at least one of the corner reinforcing watermarks  18  has a shaped border (which is not necessarily a contiguous border), at least a section of which is complementary to at least a section of a border of the further watermark  19 . 
     The aforementioned shaped section of border allows the close juxtaposition of the two watermarks  18 , 19  to provide an aesthetic combination and space saving. This results in an improvement in security by using close registration and cooperation of the two watermark features which makes it hard to counterfeit. The gap between the two watermarks is preferably no less than 3 mm and more preferably in the range of 3 to 5 mm. 
     The individual reinforcing watermarks  13 , 18  may be discrete or they may be joined together with watermark areas so that the watermark appears as a continuous frame around the whole sheet  11 . Alternatively just some of the reinforcing watermarks  13 , 18  may be joined together, to provide an aesthetic pattern. 
     In a further embodiment of the present invention the smooth raised region  17  onto which the stripe  15  is applied can be extended fully across the width of the sheet  11 , i.e. from long edge  14  to long edge  14 , as illustrated in  FIG. 12 . 
     While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled.