Technical Field

[1 ] The present invention relates to methods of simulating wear and evaluating wear-resistance of a functionalised substrate for preparation of or use as a security document. In particular, the methods involve applying a wear-simulation procedure to a virgin specimen of the functionalised substrate, the wear-simulation procedure including both a mechanical stress test and a chemical stress test.

Background of Invention

[2] In circulation, bank notes are frequently exposed to harsh use conditions which limit the lifetime of the notes. Variables affecting the lifetime of bank notes include the frequency of transfer, the form of storage, environmental conditions such as climate, humidity and temperature, physical damage (such as stapling) and chemical interactions (exposure to soaps and other cleaning agents). As a result of dissimilar physical environments and divergent cultural uses of money, bank notes are exposed to very different stress conditions in different countries which may substantially affect the lifetime of bank notes.

[3] Bank notes and other security documents typically comprise a substrate coated with one or more coating layers, which is then overprinted with design features and provided with one or more security features (such as security threads, functional inks, embossed features, optically variable devices, etc) to deter counterfeiting. The increasing acceptance of polymeric substrates for bank notes has improved the durability of bank notes in circulation. However, even polymer bank notes are subject to degradation, which may be accelerated by harsh use conditions in different contexts. In particular, damage to the coating layers on polymeric substrates affects both aesthetic and security aspects of bank notes. Furthermore, various security features applied to bank notes may be particularly vulnerable to damage in circulation, potentially allowing counterfeiters to pass off poor quality reproductions as genuine used bank notes. [4] There is therefore a need for methods to simulate, in a laboratory environment, the wear of security documents under a variety of different conditions in circulation. Such tests should produce simulated wear, in a short time period suitable for a laboratory test, which is representative of the degradation mechanisms expected over long periods of time in circulation. Furthermore, methods of rigorously quantifying or classifying the effect of wear on bank note integrity are also needed. Such methods may be needed both in the context of developing new security documents suitable for anticipated use conditions, and for quality control during manufacture.

[5] To this end, bank notes and other security documents have previously been subjected to various laboratory tests. Typical tests are based on simulating the effect of a single stress variable on a virgin bank note (or precursor substrate thereof). For example, a standard "crumple test" is used to evaluate the adhesion of bank note coatings under high mechanical wear conditions. Various chemical tests simulate the effect of a chemical or environmental exposure stress on the integrity of bank notes.

[6] These single variable tests on their own do not accurately simulate the conditions experienced by bank notes during actual physical use by the general public. They are therefore inadequate tools for predicting the durability of bank notes in a variety of different use environments, or for developing new bank notes able to withstand such environments. The availability of results from multiple single variable tests (such as separate mechanical and chemical stress tests) does not adequately remedy this deficiency, as this does not simulate the complex interplay of mechanical, environmental and chemical stresses that affect bank note lifetime in circulation. Use simulation tests which simultaneously expose a bank note to multiple stresses may partially address this shortcoming, yet such tests have been poorly adapted to separately vary the contribution of each of the applied stresses in accordance with a wide variety of different use conditions to which a bank note may be exposed. Furthermore, subjecting bank notes to ambient temperature mechanical wear tests, even with added chemical stressors, does not satisfactorily simulate the long term failure modes of bank notes in many demanding conditions.

[7] There is therefore a need for an improved test which can simulate harsh use conditions where multiple mechanical, environmental and chemical stresses are applied to a bank note in varying degrees, and evaluate the effect of such conditions on the mechanical integrity of the bank note over long periods of use.

[8] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention

[9] The inventors have found that a wear-simulation procedure comprising the application of both mechanical and chemical stress tests is particularly useful for simulating field performance, including adhesion of coatings and/or security features, of security documents such as bank notes when exposed to a wide range of use conditions and extended time periods in circulation. The wear-simulation procedure employs at least one, and preferably both, of high temperature chemical stress test conditions (such as above 50 °C) and sequential application of the mechanical and chemical stress tests.

[10] High temperature chemical test conditions may advantageously accelerate the chemically or environmentally mediated degradation mechanisms responsible for bank note deterioration under field conditions, allowing an accurate simulation to be achieved in a lab test within a short time period. Sequentially applied mechanical and chemical stress tests allow the individual contributions of the mechanically and chemically mediated degradation mechanisms to be separately controlled, allowing for improved simulation of a wide variety of use conditions to which bank notes in circulation may be exposed.

[1 1 ] In accordance with a first aspect, the invention provides a method of evaluating wear-resistance of a functionalised substrate for preparation of or use as a security document, the method comprising: applying a wear-simulation procedure to a virgin specimen of the functionalised substrate to produce a worn specimen, the wear- simulation procedure comprising applying: a) a mechanical stress test; and b) a chemical stress test, wherein the chemical stress test is performed at a temperature of at least 50 °C, and performing a mechanical failure test to determine a resistance to failure of the worn specimen. Preferably the chemical stress test is applied at a temperature of at least 70 °C, more preferably of at least 80 °C, such as about 90 °C.

[12] In a preferred embodiment of the first aspect, the mechanical stress test and the chemical stress test are applied sequentially.

[13] In accordance with a second aspect, the invention provides a method of evaluating wear-resistance of a functionalised substrate for preparation of or use as a security document, the method comprising: applying a wear-simulation procedure to a virgin specimen of the functionalised substrate to produce a worn specimen, the wear- simulation procedure comprising sequentially applying: a) a mechanical stress test; and b) a chemical stress test, and performing a mechanical failure test to determine a resistance to failure of the worn specimen.

[14] In a preferred embodiment of the second aspect, the chemical stress test is applied at a temperature of at least 50 °C, preferably of at least 70 °C, more preferably of at least 80 °C, such as about 90 °C.

[15] In some embodiments of the first and second aspects, the mechanical failure test comprises an adhesion failure test such as an adhesive tape test. Such tests allow the effect of the simulated wear to be accurately and easily quantified, particularly when the wear-simulation procedure has not itself caused visually obvious or easily quantifiable damage. Moreover, adhesive tape tests are particularly well suited for quantifying the effect of wear on the adhesion of coating layers and/or applied security features to a security document substrate. Other adhesion failure tests, such as pick up adherence tests for coated offset papers, using high viscosity oils, may been utilised instead of an adhesive tape test.

[16] In some embodiments of the first and second aspects, determining a resistance to failure of the worn specimen comprises a binary assessment of whether the worn specimen has mechanically failed or remains intact after the mechanical failure test. As used herein, a binary assessment is a determination of whether the bank note passes or fails a test relative to a predetermined metric. For example, in an adhesion failure test, a specimen may be determined to have "failed" the test if any areas of coating has been removed from the substrate, and "passed" if all areas remain intact. A binary mechanical failure assessment may be particularly useful as a screening tool for routine quality control analyses or as an early stage screening test for new bank note designs and innovations, considering that subjectivity of the user is minimised.

[17] In some embodiments where determining a resistance to failure of the worn specimen comprises a binary assessment of whether the worn specimen has mechanically failed or remains intact, the method of evaluating wear-resistance further comprises, if the worn specimen remains intact after the mechanical failure test, repeatedly applying to the worn specimen the wear-simulation procedure and mechanical failure test until either the worn specimen has mechanically failed or a predetermined maximum number of wear-simulation procedure repeats has been applied. This advantageously allows bank notes to be assigned to three or more wear resistance grade classifications in a highly controlled and reproducible manner.

[18] The wear-simulation procedure of the invention may also be applied for other purposes beyond mechanical failure tests. For example, visual inspection or various analytical tests (including non-destructive techniques) may be performed on a worn specimen to obtain useful information on the degradation of a security document subjected to the wear-simulation procedure.

[19] In accordance with a third aspect, the invention provides a method of simulating wear of a functionalised substrate for preparation of or use as a security document, the method comprising: applying a wear-simulation procedure to a virgin specimen of the functionalised substrate to produce a worn specimen, the wear- simulation procedure comprising applying: a) a mechanical stress test; and b) a chemical stress test, wherein the chemical stress test is performed at a temperature of at least 50 °C. Preferably the chemical stress test is applied at a temperature of at least 70 °C, more preferably of at least 80 °C, such as about 90 °C.

[20] In a preferred embodiment of the third aspect, the mechanical stress test and the chemical stress test are applied sequentially.

[21 ] In accordance with a fourth aspect, the invention provides a method of evaluating wear-resistance of a functionalised substrate for preparation of or use as a security document, the method comprising: applying a wear-simulation procedure to a virgin specimen of the functionalised substrate to produce a worn specimen, the wear- simulation procedure comprising sequentially applying: a) a mechanical stress test; and b) a chemical stress test.

[22] In a preferred embodiment of the fourth aspect, the chemical stress test is applied at a temperature of at least 50 °C, preferably of at least 70 °C, more preferably of at least 80 °C, such as about 90 °C.

[23] In particularly preferred embodiments of the first to fourth aspects, the mechanical stress test is applied before the chemical stress test is applied. It is believed that at least some of the chemical or environmental degradation mechanisms that affect the integrity of security documents in circulation (particularly the coating layers thereon, or adhered security features) only become operative once mechanical damage has already been caused by physical stress conditions. Therefore the application of a chemical stress test (preferably a high temperature test) after a physical stress test is believed to provide more accurate simulation of wear conditions in circulation than has been available before.

[24] In embodiments of the first to fourth aspects, the functionalised substrate is a coated substrate, i.e. is comprises one or more coatings. The presently disclosed methods have been found to be particularly useful for simulating and evaluating the wear-resistance of coatings applied to security documents under a variety of field conditions. The functionalised substrate may additionally, or alternatively, comprise one or more security features, such as adhered security features. For example, it is envisaged that the presently disclosed methods could be used to evaluate the wear- resistance, including to a loss of adhesion, of a security feature proposed for use on a security document, under a variety of different use conditions.

[25] In embodiments of the first to fourth aspects, the mechanical stress test is a crumple test, such as a crumple test performed using an American National Bureau of Standards (NBS) crumpling device. Standardised crumple tests, such as the NBS crumple test, have previously been disclosed in the art for evaluating wear resistance of bank notes to physical stresses. The specimen may be crumpled according to a standardised procedure involving a set number of crumples, conducted at a set crumple pressure. In one embodiment of the first to fourth aspects, the specimen is crumpled 32 times under a fixed pressure of 100N.

[26] In embodiments of the first to fourth aspects, the chemical stress test is a sweat test comprising exposing the functionalised substrate to artificial perspiration. Such sweat tests may comprise immersing the specimen in artificial perspiration at a temperature of between 50 °C and 100 °C, preferably between 70 °C and 100 °C, more preferably about 90 °C. The specimen may be immersed in the artificial perspiration for about 30 minutes to about 20 hours, preferably about 6 hours.

[27] The artificial perspiration typically comprises an aqueous solution, and in its simplest embodiment may be water. However, it is preferred that the artificial perspiration comprises one or more solutes representative of the components of human perspiration. Degradation mechanisms associated with human handling of bank notes is thus more accurately simulated. The artificial perspiration may thus comprise an aqueous solution containing one or more of urea, lactic acid and at least one inorganic salt. Suitable inorganic salts include sodium chloride, potassium chloride, sodium sulphate and ammonium chloride. In some embodiments, the aqueous solution comprises urea, lactic acid and one or more salts selected from sodium chloride, potassium chloride, sodium sulphate and ammonium chloride. Preferably the aqueous solution comprises each of urea, lactic acid, sodium chloride, potassium chloride, sodium sulphate and ammonium chloride.

[28] In a preferred embodiment, the artificial sweat comprises an aqueous solution comprising:

a. urea in an amount of from about 0.01 wt% to about 0.1 wt%, preferably about 0.02 wt%;

b. lactic acid in an amount of from about 0.1 wt% to about 1 .0 wt%, preferably about 0.3 wt%;

c. sodium chloride in an amount of from about 0.1 wt% to about 1 .0 wt%, preferably about 0.45 wt%;

d. potassium chloride in an amount of from about 0.01 wt% to about 0.1 wt%, preferably about 0.03 wt%; e. sodium sulphate in an amount of from about 0.01 wt% to about 0.1 wt%, preferably about 0.03 wt%; and

f. ammonium chloride in an amount of from about 0.01 wt% to about 0.1 wt%, preferably about 0.04 wt%.

[29] The worn specimen may be dried after applying the chemical stress test, typically prior to performing a mechanical failure test or other inspections or analyses on the worn specimen. The worn specimen may be dried using an absorbant material, such as a paper towel, to absorb free water, and by exposure to ambient air for a period of at least one hour.

[30] In embodiments of the first to fourth aspects, the functionalised substrate comprises a polymer substrate. Coated polymer substrates are of increasing importance as security documents, offering advantages including durability and security. In some embodiments, the functionalised substrate is thus a bank note, or a substrate intended to be used in the manufacture of a bank note. In view of the inherent durability of polymeric bank notes, the methods of the invention are particularly advantageous relative to known tests, as they allow simulation of the degradation mechanisms over the long periods in circulation that such bank notes are expected to withstand.

[31 ] Where the terms "comprise", "comprises" and "comprising" are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[32] Further aspects of the invention appear below in the detailed description of the invention.

Definitions

Security Document

[33] As used herein, the term security document includes all types of documents of value and identification documents including, but not limited to the following: items of currency such as bank notes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licences, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

Substrate

[34] As used herein, the term substrate refers to the base material from which a security document is formed. The base material may be paper or other fibrous materials such as cellulous; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), biaxially-oriented polypropylene (BOPP); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

Functionalised substrate

[35] As used herein, the term functionalised substrate refers to a substrate to which one or more functional features have been applied, including coatings, printed features, design elements and security features. The functionalised substrate may be a coated substrate, to which one or more layers of coating have been applied over at least part of one or both surfaces of the substrate. The coating layers may include one or more opacifying layers, which may be applied on one or both sides of a polymeric substrate, and which may optionally include transparent windows and half windows. The coated substrate may also include coating layers such as primers, inks, embossed polymeric layers and other layers disclosed in the art of security document manufacture.

[36] The functionalised substrate may be a finished security document, such as a bank note (or part thereof). Alternatively, the functionalised substrate may be a precursor to the manufacture of a security document, or a material under investigation for potential application as a security document.

Opacifying Layers

[37] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT<LO where Lo is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat- activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Transparent Windows and Half Windows

[38] As used herein, the term window refers to a transparent or translucent area in the security document compared to the opaque region to which printing is applied. The window maybe fully transparent so as to allow the transmission of light substantially unaffected, or it may be partly transparent or translucent, partly allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[39] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting at least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate, a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

[40] A partly transparent or translucent area herein after referred to as a "half- window", may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that "half-window" is not fully transparent but allows sunlight to pass through without allowing objects to be viewed clearly through the half- window.

[41 ] Alternatively, it is possible for the substrates to be formed from a substantially opaque material, such as paper or fibrous material, without an insert of transparent plastics material inserted into a cut out or recessed into the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Security Device or Feature

[42] As used herein, the term security device or feature includes any one of a large number of security devices, elements or features intending to protect a security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate. A security document substrate may thus be functionalised with security features by any suitable techniques, including coating, embossing, printing, adhesion, hot stamping and the like. Security features may take a wide variety of forms such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent or phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic, or peizochromic inks; printed or embossed features including release structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gradients, holograms and diffractive optical elements (DOEs).

Virgin Specimen

[43] In the methods of the invention, the wear-simulation procedure is applied to a virgin specimen of the functionalised substrate. As used herein, a virgin specimen means a specimen of the functionalised substrate substantially as manufactured, which has not been subjected to stresses which degrade its functional properties. Where the functionalised substrate is a bank note, the virgin specimen may be a freshly manufactured complete bank note, or only a part thereof.

Brief Description of Drawings

[44] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[45] Figure 1 depicts a coated substrate with a virgin specimen cut out therefrom for subjecting to the wear-simulation procedure of the invention. [46] Figure 2 depicts the virgin specimen cut out from the coated substrate of Figure 1 , inserted into the rolling apparatus of an American NBS crumpling device.

[47] Figure 3 depicts schematically an apparatus for performing a sweat test on the specimen of coated substrate that has been crumpled in the crumpling device of Figure 2.

[48] Figure 4 depicts a worn substrate, crumpled in the crumpling device of Figure 2 and subsequently subjected to a sweat test in the apparatus of Figure 3, which has failed an adhesive tape test.

Detailed Description

[49] The present invention relates to methods of simulating wear and evaluating wear-resistance of a functionalised substrate for preparation of or use as a security document. The methods comprise applying a wear-simulation procedure to a virgin specimen of the functionalised substrate to produce a worn specimen, the wear- simulation procedure comprising applying both a mechanical stress test and a chemical stress test. The wear-simulation procedure includes one or both of the following features: a) the chemical stress test is performed at a temperature of at least 50 °C, and b) the mechanical stress test and the chemical stress test are applied sequentially. The methods of evaluating wear-resistance further comprise performing a mechanical failure test to determine a resistance to failure of the worn specimen.

[50] In an embodiment of the invention, and with reference to Figures 1 to 4, virgin specimen 10, having dimensions of 65 mm by 65 mm, is cut out of functionalised substrate 1 1 with a knife. As depicted in Figure 1 , functionalised substrate 1 1 is a coated substrate precursor for manufacture of a bank note, consisting of biaxially oriented polypropylene film uniformly coated with a primer layers and white opacifiying layers on both surfaces. However, substrate 1 1 may alternatively be a complete bank note having one or more of windows or half windows, printed design features and a variety of security features, including adhered security features. In this case, a number of virgin specimens may be cut from different regions of the bank note to ensure that the wear across the entire bank note is representatively studied. [51 ] Virgin specimen 10 is then subjected to a mechanical stress test in the form of a crumple test performed using an American National Bureau of Standards (NBS) crumpling device. Suitable such devices are manufactured by IGT Testing Systems Pty Ltd. Figure 2 depicts virgin specimen 10 inserted into rolling apparatus 13 of crumpling device 12. Specimen 10 is then rolled into a tube and inserted into crumpling chamber 14. A downward force is then applied to knob 15, until a force of 100N through the crumpled note is exceeded (causing counterweight 16 to be displaced upwards). Specimen 10 is then removed and uncrumpled. The rolling and crumpling process is then repeated, for a total of 32 crumples conducted sequentially in successively different rolled-up configurations, i.e. with four crumples in each possible configuration. It will be appreciated that a square specimen can be rolled up in 8 different configurations (each of four edges inserted through the rolling apparatus slot, with either surface uppermost).

[52] Although a standard 32 crumple, 100N test has been described, the skilled person will appreciate that the number of crumples or the maximum crumple pressure can be varied without departing from the scope of the invention. Moreover, such variations may be desirable in order that the test may more accurately simulate specific use conditions.

[53] Other mechanical stress tests may also be applied instead of a crumple test, provided that the test is adapted to apply a standardised and reproducible mechanical stress to the substrate specimen. Another suitable mechanical stress test is a standardised mechanical stretch test such as the Zarick test. Another suitable mechanical stress test is an abrasion test, for example using a TABER® Rotary Platform Abrasion Tester. In yet another suitable mechanical stress test, the substrate is perforated, for example by stapling. In the circulation of security documents such as bank notes, abrasion or perforation may expose underlying layers of a security document to chemical attack, leading to further deterioration of the document. The preferred mechanical stress test for a given application may thus depend on the type of physical stresses that the security document is expected to be exposed to in the field. In this context, it will be appreciated that a wide variety of standard or new techniques from applying a mechanical stress to a substrate may be employed. [54] As depicted in Figure 3, crumpled specimen 10 is then subjected to a chemical stress test using sweat test apparatus 17. Specimen 10 is immersed in 500 ml of artificial perspiration 18 contained in beaker 19, which is heated with hotplate 20. The temperature of artificial perspiration 18 is measured, and controlled at 90 ± 2 °C, with thermocouple 21 . Specimen 10 is held under the surface of artificial perspiration 18 by taping it to glass rod 22, which is held in place by retort stand 23. Specimen 10 is immersed in artificial perspiration 18 for a time of 6 hours (360 min).

[55] Artificial sweat 18 is an aqueous solution comprising 0.02 weight % urea, 0.3 weight % lactic acid, 0.45 weight % sodium chloride, 0.03 weight % potassium chloride, 0.03 weight % sodium sulphate and 0.04 weight % ammonium chloride. Water lost from beaker 19 due to evaporation during the test is replenished as needed with hot water.

[56] Although a six hour sweat test at 90 °C with artificial perspiration comprising each of urea, lactic acid and inorganic salts has been described, the skilled person will appreciate that various modifications of the chemical stress test may be made without departing from the scope of the invention. For example, the artificial perspiration may be pure water, or contain only some of the solutes described herein. Moreover, the temperature and time of the test may be varied. Again, such variables may be adjusted, independently of the mechanical stress test conditions, to more accurately simulate the field conditions of interest. The skilled person, with the benefit of this disclosure, would be able to select appropriate conditions for both the mechanical stress test and the chemical stress test without undue burden. For example, the wear-simulation procedure of the invention may be optimised by comparing the properties of virgin bank notes subjected to the procedure with those of bank notes withdrawn from circulation.

[57] The temperature of the chemical stress test is preferably at least 50 °C. Elevated temperature has been found to accelerate the degradation mechanisms that operate on coated substrates over long periods in circulation, in accordance with the Arrhenius equation. This allows a realistic simulation of environmental wear conditions, including conditions of high humidity or vigorous handling, to be obtained in a short lab test. Moreover, conducting the chemical stress test after first performing the mechanical stress test is also particularly preferred, as the physical damage caused by mechanical stresses is believed to allow penetration of chemical reagents into the layered substrate in the subsequent chemical stress test. For example, mechanical damage may expose an inherently weaker primer layer between the polymeric substrate and the opacifying layer to contact with chemical degradants. The degradation of coated substrates resulting from sequential application of a mechanical stress and a chemical stress test has been found to be representative of the degradation obtained in circulation in a variety of harsh use conditions.

[58] After the chemical stress test, worn specimen 10 is removed from artificial perspiration 18, rinsed in water and dried, initially with paper towel and then by exposure to ambient air for at least one hour.

[59] Worn specimen 10 is then subjected to a mechanical failure test to determine the resistance to failure. The mechanical failure test is in the form of an adhesive tape test, using Scotch 500 tape (manufactured by the 3M Company). A piece of Scotch tape, with dimensions of 30 mm x 65 mm, is adhered to the specimen, and then peeled off.

[60] The resistance to failure of worn specimen 10 is determined as a binary pass/fail assessment of whether the worn specimen has mechanically failed or remains intact after the mechanical failure test. If any portion of the substrate coating, or a security feature applied to the substrate, has been removed by the adhesive tape, as assessed through visual inspection, worn specimen 10 is determined to have a failed the mechanical failure test. This may be indicative of poor wear resistance under use conditions in circulation. However, if the substrate coating or security feature remains intact, as assessed through visual inspection, worn specimen 10 is determined to have a passed the mechanical failure test. This may be indicative of acceptable wear resistance under use conditions in circulation.

[61 ] Figure 4 depicts an example of worn specimen 10 after performing the adhesive tape test. The coating has been visibly damaged in region 24, indicating a failure of functionalised substrate 1 1 to withstand the simulated wear conditions applied. [62] For the case where worn specimen 10 remains intact after performing the adhesive tape test, and is thus assigned a pass, repeat cycles of sequential wear- simulation procedure (comprising both the sequential mechanical and chemical stress tests) and mechanical failure test may be applied to the worn specimen. The cycles may be repeated until either the worn specimen has mechanically failed or a predetermined maximum number of wear-simulation procedure repeats has been applied. For example, if a maximum of three wear-simulation procedures is performed, specimens tested may be assigned to one of four different wear resistance grade classifications, namely 1 ) failure after one wear-simulation procedure; 2) failure after two wear-simulation procedures; 3) failure after three wear-simulation procedures; and 4) remains intact after three wear-simulation procedures.

[63] Although an adhesive tape test has been described herein, a pick up adherence test using high viscosity oil, such as is applied for evaluating the adherence of coated offset papers, may be alternatively utilised as the adhesion failure test. A suitable device for such an analysis is a printability tester manufactured by IGT Testing Systems Pty Ltd.

[64] Apart from adhesion failure tests, the mechanical failure test may encompass other techniques useful for determining the resistance to failure of functionalised substrates subjected to wear. Suitable mechanical failure tests may thus include tests such as tensile tests, impact tests and the like. It will be appreciated that the preferred mechanical failure test for a given application of the invention may be selected with regard to the particular failure mode of concern for a security document. For example, if the failure mode of concern is loss of adhesion of a coating or adhered security feature, the preferred mechanical failure test may be an adhesion failure test. As another example, if the failure mode of concern is tearing, a tensile test may be most appropriate.

[65] Moreover, it may be preferred to conduct other analyses on the worn specimen which do not involve a potentially destructive mechanical failure test. Such analyses may include visual inspection of the worn specimen and any suitable analytical tests, including microscopy. [66] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.