PRODUCTION OF BANKNOTES OR THE LIKE

The present invention relates to a multilayer film for use in the production of paper-like products such as bank notes, security documents, including travellers and bank checks, and to a process for their production. More particularly, the present invention relates to a laminated multilayer film structure having the characteristics of the high quality papers typically employed in the production of bank notes and security documents.

In the production of bank notes and security documents rag paper has been employed for over 300 years. As is well known, rag paper has several properties which are highly desirable in such applications, including dead foldability, tear resistance, printablilty and embossability.

These highly desired properties may be characterized as follows: deadfold is the ability of a substrate to be creased or folded and to retain the fold without opening. Tear resistance is the ability of a substrate to resist both initiated and uninitiated tears and punctures. Printability is the ability of the substrate to adsorb and bond inks used during the lithographic printing process. Embossability is the ability of the substrate to deform under the pressures of the intaglio printing process to form a raised image on the resulting bank note or security document, with the intaglio ink remaining on the raised, deformed region resulting in a high degree of tactility or feel to the bank note or security document. As may be appreciated, these properties combine to give bank notes and the like their familiar feel and functionality. With the advent of color copiers and computer graphic scanners, the counterfeiting of bank notes has markedly increased. While there are active programs underway by major currency paper producers to make their substrate more secure through the use of watermarks, metallized threads and optical variable devices (OVDs) , such as photochromies, holographies, and diffraction gratings, these efforts do not appear to hold much promise of thwarting counterfeiters.

Plastic substrates offer a major security feature if a clear "window" is incorporated into the bank note. This window would ensure that a scanner or color copier could not copy the note. Additionally, other security features can be incorporated into or onto the bank note, including reverse printing of the note to protect the security devices and the print.

In accordance with the present invention, there is provided a curl-resistant, cross-sectionally symmetrical, laminated multilayer film substrate for use in the production of banknotes, security papers comprising:

(a) a first layer having inner and outer sides, comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94, said first layer being oriented in at least a first direction to a degree which is at least three times greater than the degree of orientation present in a second direction substantially normal to the first direction; and

(b) a second layer having inner and outer sides, comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94, said second layer being oriented in at least a first direction to a degree which is at least three times greater than the degree of orientation present in a second direction substantially normal to the first direction;

(c) a laminating adhesive resin disposed between the inner sides of (a) and (b) , which comprises a component selected from the group consisting of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) ; said second layer being laminated to said first layer so that the first direction of orientation of said second layer is substantially aligned with the first direction of orientation of said first layer. In another embodiment, the present invention relates to a method for preparing a bank note which comprises: i) laminating (a) a first layer having inner and outer sides, comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94 g/cc, said first layer being oriented in the transverse direction (TD) to a degree which is at least tl ee times greater than the degree of orientation present in the machine direction (MD) to

(b) a second layer having inner and outer sides, comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94, said second layer being oriented in the transverse direction to a degree which is at least three times greater than the degree of orientation present in the machine direction, with a laminating adhesive resin which comprises a component selected from the group consisting of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) , said second layer being laminated in the film substrate so that said transverse direction of orientation of said second layer is substantially aligned with said transverse direction of orientation of said first layer, thereby forming an embossable substrate; and ii) embossing said substrate. The resultant films exhibit good dead-fold characteristics and other properties, including a symmetrical structure which provides curl free film even at elevated temperatures, e.g., 66°C (150°F) , making them highly suited for the production of bank notes and other security documents. In still another embodiment, the present invention relates to a security token such as a banknote or stock certificate prepared from an embossable, laminated multilayer film substrate, said substrate comprising:

(a) a first layer having inner and outer sides, comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94, said first layer being oriented in the transverse direction (TD) to a degree which is at least three times greater than the degree of orientation present in the machine direction (MD) ; and (b) a second layer having inner and outer sides, comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94, said second layer being oriented in the transverse direction to a degree which is at least three times greater than the degree of orientation present in the machine direction, said second layer being laminated to said first layer so that the transverse direction of orientation of said second layer is substantially aligned with the transverse

direction of orientation of said first layer.

In still another embodiment, the present invention relates to a method for preparing an embossed, curl-resistant laminated multilayer film substrate for use in the production of banknotes, security papers and the like which comprises: i) providing copolymer polypropylene skins on both sides of an oriented high density polyethylene layer having inner and outer sides and comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.94, said layer being oriented in the transverse direction (TD) to a degree which is at least three times greater than the degree of orientation present in the machine direction (MD) ; ii) coating said copolymer propylene skin provided on one side of said layer with an adhesion promoting primer thereby providing a first primed layer (a) having a primed surface; iii) repeating steps i) and ii) to provide a second primed layer (b) having a primed surface; iv) providing a discrete security device on said primed surface of one of said primed layers; v) providing an orientable laminating adhesive resin between said primed layers and laminating said primed layers together while orienting said laminating adhesive resin to provide a substantially transparent laminate; vi) gravure printing at least one side of said substantially transparent laminate with an opacifying ink in register with said security device thereby providing a clear window in register with said security device to render it visible from at least one side, preferably from either side, of said film substrate.

In forming the multilayer film substrates for use in the production of the bank notes and other security documents of the present invention, at least two layers of the substrate are to contain a major proportion of a high density polyethylene (HDPE) having a density of at least 0.94, preferably at least 0.945. These film layers may be composed _^ xclusively of a single HDPE resin, a mixture of HDPE resins, or of HDPE containing a minor

proportion of another polymeric material, such as low density polyethylene (LDPE) , linear low density polyethylene (LLDPE) , polypropylene, ethylene vinyl alcohol (EVOH) copolymer, ethylene propylene (EP) copolymer or ethylene propylene butene-1 (EPB) copolymer, although a single HDPE resin or a blend of HDPE resins is particularly preferred in the practice of the present invention. The use of processing additives, such as microcrystalline wax or the like may be employed with the preferred HDPE resins to yield improved processing characteristics in the extruder by reducing extruder torque. Films made with either a blend of HDPE resins or with microcrystalline wax have been found to reduce the splittiness of the film which manifests itself as the tendency of the film to break in the TD direction. When blends of HDPE polymers are employed, such blends can comprise two or more polymers all of which preferably have densities of 0.94 or greater. Blends of HDPE polymers advantageously comprise a major proportion of HDPE having a melt index of 0.6 to 1.2 and one or more polymers having a different melt index.

Terblends may also be desirable. Suitable terblends generally compriεe 50 to 98 weight percent, preferably 84 to 96 weight percent of HDPE having a density of 0.96 or higher and a melt index of greater than 0.5 to 2.0; 1 to 25 weight percent, preferably 3 to 8 weight percent of HDPE having a density of 0.94 or greater and a melt index of 0.1 to 0.5; and 1 to 25 weight percent, preferably 3 to 8 weight percent, of HDPE having a density of 0.96 or higher and a melt index of greater than 2 to about 8. Preferably, the second and third HDPE polymers which are minor components are present in about equal amounts.

As is particularly preferred, the film substrate of the present invention includes a) a first layer comprising at least

50 weight percent of a high density polyethylene having a density of at least 0.94, the first layer oriented in at least a first direction, e.g., transverse direction (TD) , to a degree which is at least three times greater than the degree of orientation present in a direction substantially normal to the

first direction, e.g. , machine direction (MD) , and b) a second layer also comprising at least 50 weight percent of a high density polyethylene having a density of at least 0.95, the second layer also oriented in at least a first direction, e.g. , TD, to a degree which is at least three times greater than the degree of orientation present in a direction, e.g., MD, substantially normal to the first direction, the second layer being laminated to the film substrate so that the first direction of orientation of the second layer is substantially parallel to (or aligned with) the first direction of orientation of the first layer. A method of producing HDPE films with imbalanced biaxial orientation is disclosed in U.S. Patent No. 4,870,122.

As may be appreciated, to achieve the object of improved tear resistance in a multilayer film of the type describe herein, it has been discovered that imbalanced biaxially oriented HDPE films, laminated so that their primary directions of orientation are aligned substantially parallel to each other, can exhibit increased tear resistance where an orientable laminating resin, e.g. , LLDPE, is disposed between the layers and subjected to orienting so that the primary direction of orientation in the oriented resin is substantially normal to the primary direction of orientation of the first and second HDPE film layers. In one embodiment, such orientation in the primary direction of orientation of the laminating adhesive resin is achieved during the laminating step itself, preferably in the machine direction (MD) .

The degree of orientation of the HDPE film layers is an important aspect of this invention inasmuch as the proper degree of orientation provides desirable physical properties. Although higher density HDPE resin having a density of 0.957 or greater can be made directly into thin films by cast extrusion, problems of curling, uniformity and flatness exist. Accordingly, thin HDPE films of 0.02 to 0.04 mm (0.8 to 1.5 mils) having the best balance of properties are obtained using imbalanced biaxially oriented films prepared from films having a cast gauge of 0.3 to 0.5 mm (12 to 20 mils) which are reduced to the desired gauge

by stretching, i.e., orientation.

The films are produced and oriented in a conventional manner. The film is heated to its orientation temperature and first subjected to MD orientation between two sets of nip rolls, the second rotating at a greater speed than the first in an amount equal to the desired draw ratio. Then the film is TD oriented by heating and subjecting it to transverse stretching in a tenter frame. Typically MD orientation is conducted at 60° to 120°C and TD orientation at 110° to 145°C. While it is preferred that the degree of orientation in a first film direction be at least three times greater than the degree of orientation present in a direction substantially normal to the first direction, it is more particularly preferred that each HDPE film layer be oriented to an extent of 1.1 to 2.0 times in the machine direction (MD) and 6 to 12 times in the transverse direction (TD) . It has been found that the HDPE film layers can be produced with excellent quality at caster speeds of up to 110 fpm corresponding to line speeds of 140 fpm at 1.25 times MD orientation. When employed, this degree of imbalanced orientation produces an interesting effect in the HDPE components of the structure. The effect is a visible rippled and striated appearance, with the ripples being parallel to the transverse orientation direction. Under low magnification, in each square centimeter of HDPE film there will be seen from 5 to 30 discontinuous undulating ripples and striations generally parallel to the direction of orientation. This effect gives the film a slight translucent appearance, which tends to slightly blur distant objects viewed through the film. This effect indicates that the layers have been oriented in an imbalanced manner. The high density polyethylenes contemplated for use in the practice of the present invention include those disclosed in U.S. Patent No. 4,870,122.

To achieve the desired surface characteristics required of the paper-like products of the present invention, one or more skin layers can be applied, in any known manner, to the multilayer HDPE substrate material, for example by coating or

SUBSTTrUTESHEET(RULE26)

coextrusion before orientation or by coating the HDPE after one or both of the orientation operations. The skin layer can be any of the conventional materials used for this purpose in conjunction with polyolefin films, particularly polyethylene films. For example, to achieve a press-ready surface, a polymeric resin could be blended with fillers, fibers, pigments or the like, as necessary. Additionally, voided films, such as those disclosed in U.S. Patent Nos. 4,377,616, 4,632,869, 4,758,462 and others, may be laminated to the multilayer HDPE substrate to impart the properties of those structures to the films of the present invention.

The HDPE-containing layers (a) and (b) as disclosed above can further comprise copolymer polypropylene skins, e.g., ethylene-propylene-butene-1 terpolymer, provided on at least one side thereof, preferably on both the inner and outer sides thereof. In a particular preferred embodiment, the skins themselves can also comprise a component which is similar to components in the adhesive resin used to laminate layers (a) and (b) . For example, in a particularly preferred embodiment, low density polyethylene (LDPE) can make up from 10 to 20 wt% of the skins, with the balance being a copolymer polypropylene.

A suitable adhesion-promoting primer which provides greater adhesion between the laminated surfaces, e.g. , polymers selected from the group consisting of polyethyleneimide, epoxy, polyurethane, and acrylic, can be provided between the copolymer polypropylene skins (on the inner sides of (a) and (b) ) and the laminating adhesive resin. Primer compositions are disclosed in U.S. Pat. Nos. 4,447,494 and 4,681,803.

It is also envisioned that the substrate can be embossed, dyed, printed, texturised or otherwise treated before or after lamination; this being done on the internal or external surfaces of the laminated layers, so as to provide, for example, visual and/or tactile identification of the nature of a banknote, its significance or value. The laminating techniques which can be employed to effect the present invention are known in the art and include: adhesive-bonding or cementing, e.g., with laminating adhesive

SUBSTITUTESHEET(RULE25)

resins, preferably with a transparent agent; solvent-bonding, where a mist of solvent is sprayed over the surfaces to be bonded together; thermal lamination by heat-bonding where thermoplastic sheets are subject to a hot rolling or pressing operation; cast-lamination where one layer is cast onto the second and the second forms a substrate; or, extrusion or draw- lamination as in calendering operations known in the art.

When discrete security devices are incorporated within the substrate, e.g., optically-variable devices (OVDs), they can be enclosed in pouches affixed to the substrate. On the other hand, the optically-variable devices themselves may be incorporated in one (or both) layers of the laminated substrate or between the layers, it not being necessary to incorporate a physically discrete device within a clearly defined pouch formed between the layers.

Any suitable security device can be employed in the present invention such as one selected from the group consisting of optically variable devices (OVDs) , magnetic devices, electronic devices, and rare earth element-containing devices, with OVDs particularly preferred.

As employed in the present specification, the term "optically-variable" is used to denote any device which can readily be made to change appearance in a reversible, predictable and reproducible manner. The appearance of such devices may be altered, for example, by the application of body- heat or manual pressure, the variation of the angle of viewing and, the lighting conditions under which viewing takes place. The type of devices envisioned by the present invention are: diffraction gratings, liquid crystals, moire patterns and similar patterns produced by cross-gratings with or without superimposed, refractive, lenticular and transparent grids, such as Fresnel lenses, spaced partially-reflective, and partially transparent, coatings yielding variable interference patterns or the like, bi-refringent or polarizing layers or zone-plates. Generally, optically-active devices of this nature are readily recognized by unskilled persons and are yet extremely difficult to reproduce by photographic and printing techniques.

Moreover the production of any one such device in a reproducible fashion and the incorporation of such a device in a plastic laminate as described by the present invention is likely to be beyond the resources of the great majority of would-be forgers. Where a flexible paper-like product such as a bank-note is sought, it is of course preferable that the optically-variable devices should, themselves, be sheet-like, flexible and thin; it is also preferable for such devices to be compatible with the plastic material employed for the laminae to facilitate bonding and mitigate against reactive changes occurring with time.

According to the present invention, one preferred form of optically variable device may be a reflecting diffraction grating consisting of a metallized thermoplastic film embossed with a diffraction pattern. To prevent access to the embossed pattern for the purpose of illicit replication, it is preferable according to the present invention to employ a layer of thermoplastic material on each side of the metallized film which has similar solubility characteristics to that of the metal layer so that separation by preferential etching will be rendered extremely difficult. Another preferred device is a moire pattern formed by photographically reproducing fine line or dot patterns on each side of a thin film. The spacings of the dots and lines can be readily made too fine to be reproduced by printing techniques and yet the moire pattern can be displayed upon a much larger scale. Unique diffraction and moire patterns will often be preferred for use in bank notes and techniques are available for producing those by computer and photo-reduction methods.

In the production of low denomination bank notes, a suitable level of security against counterfeiting may be obtained by merely providing a clear "window" through the bank note. As indicated above, such a window would ensure that a scanner or color copier could not copy the note. Additionally, other security features can be incorporated into or onto the bank note, including reverse printing of the note to protect the security devices and the print.

It has been found that films similar to those of the

present invention but which are "cross-oriented," can be susceptible to curling at temperatures above 66° (150 ^β F) . Such cross-oriented films are similar to those of the present invention except that the second layer is laminated to the film substrate so that the first (primary) direction of orientation of the second layer is substantially normal to the first (primary) direction of orientation of the first layer. It is believed that such curling can result from differences in shrinkage at high temperatures in the machine direction and transverse direction of each layer. Additional curling susceptibility can result where coatings or skins of the layers have different coefficients of contraction from the HDPE component of the layers. Such imbalanced contraction and its attendant curling can be avoided by counterbalancing the overall shrinkage properties of one layer by laminating thereto a second layer identical to the first layer which is provided as a mirror image of the first layer. In other words, one half of the layered film structure is a mirror image of the other, with the plane of symmetry being along the horizontal midline of the layered film structure cross- section. This provides a cross- sectionally symmetrical layered film structure. Examples of such films include those of ABA, ABBA, ABCCBA and ABCDCBA construction where each letter represents a film layer, skin, coating, or adhesive layer. Such a cross-sectionally symmetrical layered film structure is necessarily a "parallel- oriented" structure, i.e., the primary direction of orientation of the firεt layer is parallel to the primary direction of orientation of the second layer in order to meet the mirror image requirement. Such a construction provides a symmetrical structure wherein the opposing shrinkage forces counteract each other to a significant degree. However, such a two layer parallel construction can be susceptible to poor tear properties in one direction, e.g, TD where two TD oriented films are employed. It has now been found that using a linear low density polyethylene (LLDPE) laminating resin oriented in the primary direction of orientation which is substantially perpendicular

to the primary direction of orientation of the layers which it ties, evinces reduced susceptibility to tearing. The laminating resin can itself be subjected to orienting to some extent during lamination, preferably after the resin has at least partially or completely solidified, and preferably in the machine direction. Such orienting can be from 1.5 to 10 timeε, preferably from 4 to 6 timeε, corresponding to a total drawdown of the laminating adhesive resin of greater than 10%, preferably greater than 20%, say, 75%. It has been found that as molecular weight of the laminating adhesive resin employed is increased, e.g., higher molecular weight LLDPE, orientation effects resulting from the lamination procedure are improved, resulting in a multilayer film of improved tear resistance in both machine direction and transverse direction while exhibiting curl resiεtance aε well.

In one particularly preferred embodiment, the extrusion laminating adhesive resin is itself of ABA construction, i.e., the resin has a core of LLDPE (B layer) and outer or skin layers of LDPE (A layerε) . The ABA conεtruction for the laminating resin provides an advantage inaε uch the A layers provide a high bond strength between the laminating adhesive resin and the oriented HDPE layers, while the higher molecular weight core (B layer) provideε tear resistance in the transverεe direction aε a reεult of orienting in the machine direction during lamination. A laminating adhesive resin comprising LDPE only can result in a bond strength ranging from 300 to 800 grams/inch unprimed, and 600 to 1400 g/in with primed HDPE layers. Such bond strengthε provide a product which iε very difficult to delaminate and which therefore reduceε the possibilities of tampering with any security device dispoεed between the HDPE layerε. Uεe of the ABA laminating adheεive resin provideε εimilar bond εtrengths while increasing transverse direction tear resiεtance (g/mil) by a factor of at leaεt two.

The following Exampleε illuεtrate the invention in which all partε are by weight unleεε otherwise specified. Example 1

This example demonstrates the preparation of a multilayer

film εubstrate produced in accordance with the present invention which is suitable for preparing banknotes having good dead fold characteristics.

A multilayer layer oriented film substrate having a 1.15 mil final thicknesε waε prepared by coextruding HDPE with copolymer polypropylene εkinε on both εideε to form a first layer (a) . The HDPE resin employed was Oxychem M-6211, available from Occidental Chemical Corporation, Dallas, Texas, having a density of 0.96 and a melt index of 1.0. The copolymer polypropylene skinε compriεed 90% Chisso 7510, an ethylene- propylene-butene-1 terpolymer available from Chisεo Corporation of Japan, and 10% Mobil LKA-753, a low denεity polyethylene available from Mobil Chemical Co., Norwalk, Connecticut. The HDPE comprised 90% of the resulting film layer (a) while the εkinε compriεe 10% (5% on each εide) . A second film layer (b) identical to (a) was also formed. The first and second filmε (a) and (b) were then oriented 1.4 times in the MD at about 115°C and 6 to 12 times, e.g., 10 times, in the TD direction at 115-140°C in a tenter frame. The copolymer skins on the inner side of oriented layers (a) and (b) were then coated with a polyethyleneimine (PEI) primer to form clear 1.15 mil OHD films.

On film (a) can be deposited optical varying devices (OVDs) , at predetermined distances from one another so as to provide an OVD at the same location on each of the banknotes which are ultimately prepared from the film.

Films (a) and (b) were laminated by providing therebetween a laminating adheεive LDPE reεin. Chevron 1017, available from Chevron Chemical Co., Houεton, Texas. The laminating adhesive resin undergoes orienting in the machine direction during the laminating step itself, thereby imparting enhanced tear resistance in the transverse direction (TD) . Lamination is carried out by conventional techniques, resulting in a multilayer film substrate having a final thickness of about 3.0 mils, the laminating adheεive resin itself providing about 0.7 mils thickness.

The resulting subεtrate was εubstantially clear and its

physical properties are summarized in the Table below.

The substrate was then gravure printed on both sides using an opacifying ink. The gravure printing waε carried out in such a way that the opacifying ink or coating was applied in register with the OVD embedded within the substrate, i.e., the coating was applied εo aε to avoid coverage of the OVD, thereby providing a clear window in regiεter with εaid device, rendering the OVD visible from both sides of the printed substrate. The gravure printed subεtrate iε 3.75 milε in thickness and can be taken up on a roll after printing or sheeted and guillotined in register to provide presε-ready sheets of approximately 30 inches by 30 inches which are then packaged in units of 5000 sheets. The product exhibited good dead fold properties, tear resiεtance in both directionε, printfastness, and provided better resolution while requiring lesε ink during printing than paper bank note εubεtrates. Moreover the product was not susceptible to curling at high storage temperature (180°F for 24 hours) . The characteristics of the product thus permit its advantageous substitution for conventional paper subεtrates used in printing banknoteε.

The product is also difficultly delaminatable, rendering isolation of an OVD for counterfeiting purposeε extremely difficult, owing to the high bond εtrengthε between the layers of films (a) and (b) . Example 2

Example 1 was repeated except that the laminating adhesive resin employed was an LLDPE laminating grade adheεive reεin, Dowlex 3010, available from Dow Chemical Co. of Midland, Michigan. The characteriεticε for the resulting substrate are set out in the Table below. Use of the higher molecular weight LLDPE as the adheεive resin in Example 2 resulted in increased TD tear εtrength. Example 3

Example 1 iε repeated except that the laminating adheεive resin employed is of ABA construction, A being LDPE, making up 20% of the total laminating resin (10% per side) and B being LLDPE, making up 80% of the total laminating resin. The

characteristics for the resulting substrate are set out in the Table below. Use of the ABA laminating adheεive resin between layers (a) and (b) provides high bond strengths between the layers of films (a) and (b) , which is believed attributable to the presence of LDPE components in both the laminating adhesive resin and the copolymer skins which are laminated to one another. Moreover, the presence of more readily orientable higher molecular weight LLDPE in the core of the laminating adhesive resin provides excellent TD tear strength resulting from orienting in the machine direction of the ABA laminating adhesive resin.

TABLE

Ex. 1 Ex. 2 Ex. , 3 Tensile MD 6 6 6

(psi x IO ³ ) MD 16 20 20

Modulus MD 365 380 380

Stiffneεs MD 17 20 20

(grams) TD 21 24 24 Tear Strength MD 89 63 80

(g/mil) TD 4 5 20