WO2004087795A1

WO2004087795A1 - Authentication means

Info

Publication number: WO2004087795A1
Application number: PCT/EP2004/003549
Authority: WO
Inventors: John Raseburn
Original assignee: Ucb, S.A.
Priority date: 2003-04-02
Filing date: 2004-04-02
Publication date: 2004-10-14
Also published as: GB0307615D0

Abstract

There is described a biaxially oriented polypropylene (BOPP) film which has dispersed therein rigid rod like UV absorbers compatible with the polymer such as . (E,E)-1,4- diphenylbuta-1,3-diene (DPBD); (E,E)-1,6-diphenylhexa-1,3,5-triene (DPHT); and beta, beta-carote n e (carotene) at typical concentrations of about 0.01% w/w. These absorbers preferentially align along one of the two orientation axes of the film and allow anisotropy in the film to be covertly detected by the difference in the UV fluorescence spectra of the film in along each axis. This provides a covert means to authenticate the film, which is useful as a substrate for security applications.

Description

Authentication means

The present invention relates to security features used to authenticate an article such as a security document comprising a substrate for example an orientated polymer film.

It is desirable to introduce features in a film used as a substrate for security documents and articles which are not readily apparent to a potential counterfeiter and which even if identified cannot be readily be reproduced.

Many polymeric films are orientated or stretched whilst heating in two perpendicular directions with respect to the film web (the machine direction, MD and the transverse direction, TD) to improve their mechanical properties. Generally differences in film orientation, including any localised modifications, can not be viewed under normal lighting conditions by the naked eye.

It is an object of the invention to exploit the properties of an orientated film to provide a means to authenticate a film.

The applicant has found that additives which absorb radiation which are large linear molecules with a degree of conformational rigidity will when incorporated into a polymer film be preferentially oriented with the polymer chains when such a polymeric film is oriented. This effect leads to a change in the radiation absorption properties of the additives. For use in a security film as a covert authentication feature the additive preferably absorbs radiation in the non visible region (i.e. is colourless) such as IR or UV radiation, preferably UV radiation.

Therefore broadly in accordance with one aspect of the invention there is provided a biaxially oriented polymeric film suitable for use as a substrate to provide improved authentication, the film comprising from about 0.001 % to about 5% by weight of a colourless additive dispersed in at least one layer of the film; where the additive exhibits a detectable difference in the spectral response when illuminated with polarised IR or UV radiation for which the axis of polarisation is parallel with one of the principal orientation axes (MD or TD).

The additive may also be a modification and/or functionalisation of the polymer rather than an additive per se. Preferably the additive preferentially aligns along one orientation axis of the film. Preferably the polymer film is a thermoplastic or biopolymer film, more preferably a polyolefinic film (which may be a homo-, bi, ter or co-polymer and/or any mixture(s) thereof) and/or cellulosic film. More preferably the film comprises polypropylene, polyethylene, mixtures and/or (co)polymer(s) thereof. Most preferably the film is a polypropylene homopolymer. The film may be a multilayer structure formed by any suitable method (such as co-extrusion and/or lamination) with one or more core or surface layers being as formed as described herein.

In one embodiment of the invention the film comprises biaxially oriented polypropylene (BOPP). The BOPP films may be prepared with substantially balanced physical properties, for example as can be produced using substantially equal machine direction and transverse direction stretch ratios or can be unbalanced where the film is significantly more oriented in one direction (MD or TD). Sequential stretching can be used, in which heated rollers effect stretching of the film in the machine direction and a stenter oven is thereafter used to effect stretching in the transverse direction or simultaneous stretching, for example using the so-called double bubble process or a simultaneous draw stenter. In fact the heater rollers in the double-bubble process effect MD orientation by causing TD shrinkage rather than deliberately exerting an MD tension directly. However, the action of the heat is to generate an MD shrinkage and therefore to exert an additional MD tension. The machine direction and transverse direction stretch ratios are preferably in the range of from 4:1 to 10:1 , and more preferably from 6:1 to 8:1.

Without wishing to be bound by any mechanism, in an unbalanced film of the invention preferred additives are those which will be more preferentially oriented in one direction and so the ansiotropic spectra of the film will be more pronounced. To improve the anisotropic effect of the additive in a balanced film which would otherwise necessarily be more subtle preferably some degree of anisotropy may be re-introduced for example in either the MD or TD or locally (e.g. using a laser heat source) to reduce the orientation in one direction or region and thus partially unbalance the film to increase the anisotropic orientation of the additive.

The films used in accordance with the present invention can be of a variety of thicknesses according to the requirements of the packages which are to be produced. For example they can be from about 10 to about 120 microns thick, and preferably from about 14 to about 40 microns thick.

Preferably the additive is a material which absorbs and/or fluoresces in the UV (hereinafter referred to as a UV absorber or UV additive). Certain UV-additives have anisotropic UV- absorption and/or UV fluorescence. Substrates that incorporate such UV additives may thus indicate by their absorbance or fluorescence the preferred physical orientation of additive within the substrate under UV irradiation. The preferred orientation reflects the overall molecular orientation of the additive in the substrate and may arise from the processing history of the substrate. This anisotropic behaviour may be particularly apparent when the UV light is filtered thought an appropriate polarising device where the intensity of the absorption or fluorescence may be dependent on the relative orientations of the polarisation of the UV irradiation and a suitable reference direction in the substrate. In the case of a manufactured film, a convenient reference direction is often the machine direction (i.e. the direction by which the film is conveyed and culminating in its being wound onto a reel.)

The dichroic ratio measures the maximum absorption or fluorescence intensity obtained when transverse direction of the film sample is aligned with the principal direction of the polarisng filter divided by the maximum intensity of absorption or fluorescence when the machine direction of the film sample is aligned with the principal direction of the polarising filter. For a film which has no preferential orientation of polymer chain molecules in either the MD or TD, and hence no preferential orientation of the UV additive, the ratio of the two intensities is approximately unity. For a film with a preferential orientation of chain trajectories towards the TD the dichroic ratio is greater than unity. For a film with a preferential orientation of chain trajectories towards the MD, the dichroic ratio is less than unity. As used herein "greater than unity" or "less than unity" implies that a statistically significant divergence from unity remains when the inherent experimental uncertainty in the values delivered by a given measurement has been taken into account.

Suitable anisotropic UV additives may comprise long, 'rod' shaped molecules which are compatible with the film polymer so they may be distributed in the polymer melt. The action of stretching the film during orientation may cause the 'rods' to aligned preferentially in a particular direction. The 'rods' can then influence the passage of UV radiation through the film. When polarised UV light is passed through the film in a direction A (parallel to the direction of the long axis of the molecules) a specific spectra will be displayed In a UV spectrophotometer or the UV light will be transmitted. When polarised UV light is passed through the film in a direction B (with polarisation axis perpendicular to the direction of the long axis of the molecules in the plane of the web), a different spectrum is displayed or then the radiation will be partially absorbed.

The difference in the two spectra results in a unique 'fingerprint' spectrum for the film.

The fluorescence of pigments (e.g. located in a package over which the film is wrapped) can be controlled by direction the polarised UV light is applied i.e. in direction A full fluorescence observed, direction B partial fluorescence observed.

Thus this method of marking a BOPP film is useful for security films and in packaging and overwrap applications. The additive can be detected at very low concentration, therefore does not need to be added in large amounts and there is thus no detrimental effect to the film properties.

Detection of the anisotropic properties of the film additive would normally be covert using a laboratory based spectrophotometer (although a hand held detector could be used in the field). The additive would not normally be detected by the user. This is advantageous as the counterfeiter would be unaware of the additive and even if the presence of the additive was detected it would be very difficult to mimic the ansiotropic effect by using any coating or printing techniques which might be available to a counterfeiter.

A localised change in the orientation of biaxially orientated polypropylene film can be achieved by the application of a continuous stream of laser energy (as described in the applicant's patent application WO 02/053473, incorporated herein by reference). The present invention can uses a subtle change in film orientation as a possible to make the authentication feature even more covert, so as well as unbalanced films it is possible to use balanced films which if necessary have been slightly unbalanced only enough so the anisotropic absorption can be detected .

Generally film orientation, including any localised modifications, can not be viewed under normal lighting conditions. However when viewed between two sheets of Polaroid film, any changes in orientation can be observed. Specific, 'rod like', tagents could be added to the base film material to indicate its degree of orientation. For example, the localised modification of orientation by the action of a continuous laser, could be made detectable by adding suitable pigments that react to any orientation change. Preferred UV-additives for use in the present invention may be selected from: polyenes, such as (trans, trans)-diphenyl butadiene, (trans, trans, trans)-diphenyl hexatriene, beta- carotene, vitamin A and its derivatives, cinnamoyl compounds and terpene resins; polycyclic aromatic hydrocarbons and their derivatives, such as fluorene, chrysene, perylene, phenanthrene, anthracene, and naphthalene; polyphenyl molecules, such as ortho- or para-terphenyl; heterocyclic compounds such as coumarins, rhodamines, fluorescein, flavins such as riboflavin (vitamin B₂) and amino acids such as tryptophan, phenylalanine and tyrosine, and derivatives of these susbtances such as methoxytryptophan; UV-absorbant polymers such as polyesters and grafted polymers such as UV-fluorescent polymers, such as poly(9,9-dioctylfluorenyl-2,7-yleneethynylene) and similar analogues, dyes and indicators, such as methyl red, phenolphthalein, Reactive Red, Reactive Yellow.

More preferred rod like UV additives which can be successfully dispersed in polypropylene to make BOPP films and used to determine the orientation with in the film using UV spectroscopy, comprise:

(E,E)-1 ,4-diphenylbuta-1 ,3-diene (DPBD,

(E,E)-1 ,6-diphenylhexa-1 ,3,5-triene (DPHT, and/or beta.beta- carotene (carotene,

As used herein the term balanced means that the film is oriented substantially the same in both directions, and the term unbalanced means that the film is oriented differently in each direction (conventionally the MD and TD). In one alternative emboidment of the present invention two or more rigid rod like species may be used as one of more UV absorber, UV fluorescent material, a material capable of Intermolecular non-radiative energy transfer (NRET) and/or a phosphorescent rnaterial.

Without wishing to be bound by any mechanism the following observations are made about each type of material.

With two or more UV absorbing rod species present in the film or article to be authenticated, each of which may exhibit a greater or lesser, or the same, orientational sensitivity as its counterpart, a more complicated UV absorption spectrum may be possible which can provide an enhanced means for authentication.

Some rod-like molecules fluoresce, i.e. when illuminated with UV light, the film appears to "glow". Both absorption and fluorescence are anisotropic, i.e. directionally-dependent, for molecules which themselves are anisotropic. Polarised UV illumination enables one to distinguish between bubble and stenter film via detection of the associated fluorescence, in an analogous fashion to the absorption method. With two or more UV fluorescent rod species present in said film or article, and where said species fluoresce at different wavelengths and which are isolated from one another on a molecular scale, a blend of fluorescence colours can occur. The intensity or colour of this blended light can be linked to orientation via appropriate detection. Optionally a third or further UV absorbing species (for example polyester) can be added to substantially block certain wavelengths in the applied illumination. Fluorescence may offer sensitivity advantages with regards to detection, which facilitates the design of the hand-held device. However, fluorescence is most effective in mobile phases where the rigid rod molecules experience a weaker physical connection to their molecular environment relative to more rigid phases. In a more mobile phase, it is likely that sensitivity to the orientation of the surrounding polymer chains is lower.

Materials capable of intermolecular non-radiative energy transfer (NRET) may also be used in the present invention. When the fluorescence wavelengths of one species overlap with the absorption wavelengths of another, transfer of UV light energy is possible between them. The result is that illumination at a wavelength suitable for causing the first species to fluoresce may, instead, cause the second species to fluoresce. A condition for this to occur is that they are in close proximity to one another, this distance being of the order of 1 - 5 nm. This close proximity could be effected by including both species within a polymer which is incompatible with polypropylene. Because of the low concentrations involved, detrimental effects on optical clarity are likely to be negligible. Subsequently, if the two species exhibit different orientational sensitivity, the orientational dependence of absorption and fluorescence can be exploited so as to selectively couple or decouple the transfer process, so that the same UV illumination would give different colours of fluorescence for BOPP films made by the bubble and stenter processes.

Phosphorescence refers to the "after-glow" effect which occurs with certain substances after an applied UV illumination has been switched off. It appears that rigid phases are more effective in this regard and phosphorescent materials may also be used in the present invention.

The authentication method of the invention using incident electromagnetic radiation (preferbaly UV illumination) to reflects the molecular orientation of the polymeric film (preferably BOPP film) . Hence the present invention provides a means to distinguish BOPP film produced by the bubble process (known herein as bubble film) which is generally balanced (i.e. substantially the same orienting process conditions in MD and TD as the film is orientated simultaneously) from BOPP film produced by the stenter process which is generally unbalanced (i.e. MD and TD orienting process conditions are different, as the film is oriented sequentially).

It will be appreciated that as well as the films made by the bubble process the term bubble film encompasses any similar oriented film of similar properties made by different techniques (such as LISM) where the film is also oriented simultaneously.

The principle of the "Rigid Rods" verification system is to include ultra-violet (UV) absorbing molecules within base film, thus affording a unique analytical signature for either forensic or field-detection. The molecules are preferably "rod-like", i.e. of an elongated and rigid nature, where rigid suggests that they do not easily change shape. As a result of their "rod-like" nature, their principal axes align with the directions of the polymer molecular chains in the film. The latter are governed chiefly by the manufacturing process by which the made. Hence, in the case of bubble OPP, the "rods" are distributed evenly, such that they point in all directions within the film. In the case of stenter film, in which the polymer chains are directed predominantly in the transverse direction, the "rods" will also be aligned chiefly in the transverse direction. By use of a polarised UV light source, the distribution of rod directions can be evaluated. This enables identification of the origin of the film sample under scrutiny in terms of its associated manufacturing process (bubble or stenter). Because of both the inclusion of the UV rods within the body of the film, and also the unusual nature of the bubble process, this method offers a means of verifying a UCB film sample as originating from UCB in a way which cannot be easily counterfeited.

The distinction between bubble and stenter process underlies the principle behind the Rigid-Rod Verification system. To simulate the bubble process, film samples were produced in the laboratory by drawing a heated plaque in two perpendicular directions simultaneously. To simulate a stenter process, the heated plaque was drawn first in one direction and then in a direction perpendicular to the first (sequential drawing). Although putatively similar processes in the sense that they are both biaxial, it is well established that simultaneous and sequential drawing give very different distributions of polymer chain directions, in the plane of the film, relative to a given reference direction.

Higher UV absorption occurs when the transverse direction is parallel with the polariser.

Schematic depiction of the uniform distribution of molecular axes in the plane of the bubble film is shown in Figure 9. Schematic depiction of the preferential distribution of molecular axes in stenter film towards the transverse direction is shown in Figure 10.

Factors which may be used to select an optimal material for use in the present invention as a rigid rod are sensitivity to film processing history (orientation); stability with regard to extrusion and subsequent film storage; fluorescence efficiency and/or price and cost effectiveness. DPBD was selected as a preferred material to use as rigid rod mainly on cost grounds. It will be appreciated that there will be many other suitable materials could be used as the rigid rod in the present invention. Preferred materials feature an elongated molecular geometry, more preferably comprise aromatic moieties, most preferably are selected from phenanthrenes, fluorenes, fluoresceins, flavins and/or aromatic hard-resins.

Because the intensity of the detected UV absorbance or fluorescence is a function of the total number of "rigid rods" through which the UV light beam passes, thicker films would likely require a lower concentration for optimal detection. Hence, the concentration would be best tailored to the film thickness of the particular grade being manufactured. It is worth noting that the propensity to absorb or fluoresce UV light is not the same for all the candidate Rod species. It is then possible that a molecule which absorbs strongly could be used at a lower concentration than another.

The UV-active species used as additives in the present invention are preferably those which possess a marked anisotropy in its interaction with UV irradiation such that, when largely immobilised in a host substrate, irradiation of the substrate with UV light gives an absorption spectrum or a fluorescence intensity which is indicative of a preferred orientation of the guest molecule with respect to any of the principal directions attributable to the substrate, such as the machine direction, transverse direction or normal direction.

It will be appreciated that certain rigid rod like species may degrade when exposed to the high-temperatures involved in polymer extrusion when a film is formed. It will be understood that such species are selected to be thermally stable and sufficiently stable to the UV during exposure of the film or article to UV during use and over the normal lifetime of the film or article.

Sufficiently stable to UV denotes that the active species remains active at a sufficient concentration so that the authentication method of the invention can be readily performed.

Preferably the security feature of the present invention may be incorporated directly into a suitable article and/or document or may be attached thereto (e.g. in a permanent or tamper evident manner) and/or is otherwise associated therewith as part of a security and/or authentication means. As used herein the term article includes but is not limited to printed matter such as documents.

Suitable articles which may be authenticated as described herein may comprise an integral part of a larger article and/or product (e.g. a high value article whose authenticity it is desired to check) where preferably that article or part thereof comprises a region of oriented polymer film . Alternatively the article may comprise for example a label and/or tag which is designed to be or attached to another article and/or for example comprise the packaging associated with another article.

An article of the present invention (in which the article and/or product to which the security article is attached, of which it is an integral part and/or with which it is associated), may preferably be one which would otherwise be susceptible to counterfeiting due to the high value, prestige and/or other importance associated with the article and/or product and/or where authentication of a genuine article and/or product is desired.

Preferably an article of the present invention comprises a security document and/or goods, such as one or more of; security tag, label, packaging, brand, trademark, logo, currency (such as bank note), cheque, share certificate, bond, stamp, passport, official document, ticket, security pass and the like.

In another preferred embodiment a film of the invention may package, wrap, be associated with, attached to and/or comprise an article selected from for example any of the following non-exhaustive list: antique objects; audio and/or visual goods for example blank and/or pre-recorded media in any format (e.g. compact disks, audio tapes and/or video tapes); chemical products for example pesticides, cleaning products, washing powders and/or detergents; tobacco products for example cigarettes, cigars, and/or tobacco goods; clothing articles for example leather articles; soft and/or alcoholic beverages for example wines or spirits; entertainment goods for example toys and/or computer games; foodstuffs for example tea, coffee, meats, fish, caviar and/or delicatessen produce, electrical and electronics parts for example computers and/or spare parts therefor, electronic objects and/or computer software, high technology machines and/or equipment; jewellery for example watches; leisure items for example binoculars and/or telescopes; perfumes and/or cosmetics for example shampoos, soaps, perfumes, deodorants, body lotions, creams, toothbrushes, toothpastes, razors and/or razor blades; products related to or for the treatment, diagnosis, therapy and/or prophylaxis of humans and/or animals, for example dental, medical and/or surgical equipment, blood transfusion pouches, medical infusion pouches, packaging for donated organs, osmotics bags, personal health equipment (e.g. optical glasses and/or sunglasses) and/or pharmaceutical products (e.g. in any suitable form for application for example pills, tablets, syrups and/or lotions); military equipment for example guns, gun sights, ammunition, rockets, military clothing, foodstuffs, gas-masks, mines, grenades and/or ordnance; photographic industry goods for example cameras and/or pellicles; scientific instruments and spare parts therefor, for example microscopes, chromatographic apparatus, spectrometric and/or nuclear magnetic resonance apparatus; machinery and spare parts for the transport industry for example parts for automotive, aerospace and/or aeronautical industry goods, cars, lorries/trucks, motorcycles, space vehicles, rocket ships, vehicle's windscreen stickers, tax discs, trains, coaches and buses, aeroplanes, tubes, trams, helicopters, deep sea exploration equipment, submarines, ships, boats, liners and/or merchant vessels; travel goods for example luggage; security documents and/or goods such as one or more of: security tag, label, packaging, brand, trademark, logo, currency (such as bank note), cheque, share certificate, bond, stamp, passport, official document, ticket, security pass and the like; sports articles for example sport shoes, tennis rackets, squash rackets and/or equipment for fishing, golf, climbing, skiing, shooting and/or scuba or other deep-sea diving; any other articles which are safety critical and/or where the failure of which would be critical and where authentication of a genuine and/or correctly prepared article is essential: any article which has utility in one or more of the uses to which the aforementioned articles may be used, any other instructional, recordal and/or promotional material with the article such as instruction manuals, guarantees, warranties, guides, log-books, records and the like and/or any other article which is suitable for attachment to (e.g. as a security label and/or tag) and/or association with (e.g. comprising the packaging) to any of the aforementioned articles.

An article made using the authentication means as described herein may comprise any other compatible security and/or authentication means in any compatible combination comprises, optionally in corresponding patterns the article: such as any of Moire inducing pattern, optical lens, Fresnel lens, multiple micro-lens, lenticular lens, distorting lens, metameric ink, micro-printing and polarising filter.

A further aspect of the invention broadly comprises a method of manufacturing a comprising the step of: applying an article and/or security document as described herein to the product as an integral part of the product, by attaching or associating the article to the product and/or by associating the article with the product.

Another aspect of the invention broadly comprises a method of authenticating a product comprising the steps of:

(a) illuminating the film of the invention with UV radiation with polarisation axis aligned in one of two direction along the film; and

(b) detecting a difference in the spectra of the film in each direction to authenticate the film

A still further aspect of the invention broadly comprises the use of UV absorbers, UV fluorescent materials, materials capable of intermolecular non-radiative energy transfer (NRET) and/or phosphorescent materials in a film and/or a method of the invention for the purpose of reflecting a preferred orientation distribution of the polymer chain trajectories in the film via approriate detection methods, i.e. UV absorption, UV fluorescence, dichroic UV absorption or dichroic UV fluorescence measurements, and thus betraying the manufacturing origin of the film in so far as it affects the orientation history of the film.

Still other aspects of the invention broadly comprise: Use of a product, article, security document and/or authentication means as described herein to provide a means of authentication.

A product authenticated by an article, security document and/or authentication means as described herein.

It is appreciated that certain features of the invention, which are for clarity described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely various features of the invention, which are for brevity, described in the context of a single embodiment, may also be provided separately and/or in any suitable sub-combination.

The term "comprising" as used herein will be understood to mean that the list following is non-exhaustive and may or may not include any other additional suitable items, for example one or more further feature(s), component(s), ingredient(s) and/or substituent(s) as appropriate.

Further and/or alternative features and aspects of the present invention are described in the claims.

Various non-limiting embodiments and examples of the present invention will now be described, by way of illustration only, with reference to the accompanying drawings in which:

Figures 1 and 2 are plots of the force versus time used to draw oriented films of Examples 9 and 10 respectively.

Figures 3 and 4 show infra-red absorbance as a function of the angle between the beam polariser and direction 1 for Examples 9 and 10 respectively

Figure 5 shows the known UV absorption spectrum of DPBD dissolved in hexane

Figure 6 UV absorption spectrum for Example 10 with direction 2 parallel with the floor of the spectrometer compartment.

Figures 7 and 8 are plots of UV fluorescence as described in Examples 7 and 8 Figure 9 is a schematic depiction of the even distribution of molecular axes in the plane of a BOPP film made by the bubble process Figure 10 is a Schematic depiction of the preferential distribution of molecular axes towards the transverse direction in a BOPP film made by the stenter process Examples

The examples were prepared as follows with reference to Table 1. Each UV additive (as given in Table 1 , both available commercially from Aldrich Chemcials) was dispersed in a pure powdered polyproylene (PP) homopolymer to which 0.3% of an antioxidant was added. The PP used was that available commercially without additives from Solvay SA under the trademark Eltex® P. The antioxidant was that available commercially from Ciba Speciality Chemicals under the trademark Irganox B225. The dispersions were prepared from an initial master batch (as specified in Table 1) using an air mixer. Subsequent mixing of the master batch with further quantities of polypropylene powder diluted the master batch sequentially to achieve the specified final concentration of UV absorber. Concentrations in units of g/kg denote the number of g of UV molecule in the number of kg of the homopolymer.

The diluted PP mixtures were subsequently melt-blended on a Prism Twin-Screw extruder to form granules from which solid plaques of polymer were produced in a heated press at 230°C, and then cast cold water to give sheets of cast PP film of thickness 1 mm. These cast sheets were cut into small square sheets of dimensions 1 mm x 60 mm x 60 mm, and stretched on a laboratory biaxial stretching machine at a temperature of 160°C. At this temperature, the polypropylene was slightly below its melting temperature, so that stretching took place in the solid state, affording biaxially-oriented polypropylene sheets of thickness approx. 30 μm. Two distinct biaxial drawing regimes were used simultaneously or sequentially.

Additional film samples were made analogously to Example 1 and 2 using Eltex® P terpolymer (KS 359) with larger concentrations (1.5 %) of UV additive (DPHT). The purpose of the higher concentration of UV additive was to give a clear signature in FTIR experiments for easier assessment of orientation distributions (OD's) for the additive. Terpolymer was used as thicker film samples could be drawn, which would assist FTIR analysis of OD's. It was found that if the film samples were made with slow-cooling rather than quenching the DPHT separated out as a clearly visible crystalline phase in the form of tiny dendrites.

Several sets of samples were prepared on a conventional manner on a laboratory long- stretcher, as summarised in Table 2. Samples were 1 mm in thickness and ~ 60mm x 60mm. For each sample, a heating time of 1 Vz minutes was allowed after emplacement in the stretcher and starting the heater.

Table 2

A temperature of 157° C was found to be the lowest temperature at which Eltex® P homopolymer could be drawn. At 165°C, the differences in the characteristics of Examples drawn under simultaneous or sequential conditions were fewer to those Examples drawn at lower temperatures. Although the example films were made by either drawing simultaneously or sequentially, in both cases the heating blowers were switched off a few seconds before stretching was applied, to prevent the film from melting in the vicinity of the blower outlet. Hence, for sequential draws, the second draw occurred at a lower temperature than the first.

Fiducial markings

It is customary to mark on the sample plaque a grid of fiducial markings using an ink- carrying stamp. This gives a grid of 16 squares on the sample, each square being 1 cm x1 cm. If one considers the grid to be a matrix of cells with indices (a,b) with a running from top left to top right and b from top left to bottom left, it is often found that cell (2,3) shows the most marked effect of drawing conditions. In the simultaneously-drawn sample, cell (3,2) measured approximately 7.5 cm x 7.5 cm after drawing. In the sequentially-drawn sample, this cell measured 8 cm x 11 cm, which illustrates the distinction in the two drawing directions for the sequentially-drawn sample.

Draw curves

Figures 1 and 2 illustrate the contrast in drawing forces for simultaneous and sequential experiments (respectively Examples 9 and 10). In each case the draw temperature was 157°C and the data acquisition rate was 20 Hz. For the sequential case, the sample was first drawn at constant width in the x-direction ('direction 1 ') and then at constant with in the y-direction ('direction 2'). It can be seen that comparatively dramatic drawing forces develop in the y-direction during the second draw, suggesting that the finished sample will be largely direction-2-oriented.

Tensile analysis

Some mechanical properties of the films of Examples 5 and 6 (DPHT additive drawn at 160°C) were briefly checked out on an Instron apparatus (settings 50 mm/min; samples 25 cm wide; gauge length 100 mm). As can be seen in Table 3, for sequentially-drawn samples, direction 2 was much stiffer and less extensible than direction 1.

Spectroscopic analysis

Several of the samples were subject to Fourier Transform Infra-red (FTIR) analysis. Additionally, Examples 9 and 10 (DPBD additive drawn at 157°C) were analysed by UV spectrometry to illustrate the principles of the present invention.

FTIR dichroism

The distribution of chain directions in the plane of the film were assessed using infra-red spectra which were plotted as absorbances versus wavenumber, where the absorbance is defined by "a=log₁₀(lo/l)" where l₀ the intensity of the reference beam, and I the intensity of the beam after passing through the sample. Hence, an absorbance of 3 indicates that the intensity of the beam is reduced to 1/1000th of its unhindered intensity. Absorbances above 3 are usually too noisy to be trustworthy for quantitative analysis.

Without wishing to be bound by any mechanism, the FITR may be explained as follows. For investigating polypropylene specimens, two peaks were particularly useful. The peak at -997 cm^"1 was attributed to crystalline-only vibrations, the peak at ~972cm^"1 was attributed to both crystalline and amorphous vibrations, but its intrinsic extinction coefficient may be different to that of the peak at 997 cm^"1. Both of these vibrations were taken as being directed along the chain axis. Figures 3 and 4 show infra-red absorbance as a function of the angle between the beam polariser and direction 1 for respectively Examples 9 (simultaneously-drawn) and Example 10 (sequentially-drawn). The data demonstrate the difference between molecular OD's in the planes of simultaneously-drawn and sequentially-drawn film samples. For the simultaneously-drawn case, there was a fairly even distribution of chain directions, albeit with a slight dip at about 45° to either of the two main drawing directions. For the sequentially-drawn case, there was a pronounced distribution in the second drawing direction, with a smaller population along the first drawing direction. Other FTIR dichroic data suggest that, for sequentially-drawn specimens, the population of chains oriented along direction 1 was reduced at higher drawing temperatures.

UV Spectrometry

It can be seen by comparing the known UV absorption spectrum of DPBD dissolved in hexane (Figure 5) with that of the sequentially oriented film of Example 10 (Figure 6) that the film sample exhibits a more-pronounced absorption at high-frequencies (-200 nm).

Without wishing to be bound by any mechanism this suggests that there is a preferential orientation of the DPBD molecule with respect to the plane of the film. Alternatively, perhaps there is some sort of intermolecular effect which affects the absorption characteristics in the solid-state, and which does not occur in solution. Evidently, the left- hand side of the spectrum provides a means to covertly distinguish between film orientation axis and thus authenticate the film of the invention.

It is also possible that a polarised UV beam could be usefully used for orientation analysis of the UV additives. Samples with balanced biaxial orientation or unbalanced biaxial orientation can be made conveniently on the long-stretcher. Lower temperatures accentuate the difference in properties according to film direction in sequentially-drawn (unbalanced) samples. Approx. 0.01% of UV molecule in the polymer was found a particularly suitable concentration for UV analysis.

Example 7 DPBD (100g) was blended with 25 kg of isotactic PP homopolymer (BP Eltex P HV001 PF) and the blended material introduced at a 1 % addition level to the extrusion system for the core layer of a BOPP bubble line. Samples from the process were obtained in which the finished film had been subjected to either a heat-setting process (i.e. thermally-induced relaxation of the film in its width (transverse) direction during its conveyance across heated rollers), or no heat-setting process. UV absorption spectroscopy using a polarising filter revealed a UV dichroic ratio or 1.0 for non-heat-set film and 0.8 for heat-set film, thus affording a clear indication of distinct differences in the processing history of the two sample types.

Example 8.

Film samples obtained as described in Example 2 were examined using a proprietary UV fluorescence measurement device in which was incorporated a polarising filter for the purpose of polarising the exciting UV irradiation at a wavelength of 360 nm. The fluorescence intensity was measured at a wavelength of 460 nm. The polarising filter was rotated in 5 degree increments with respect to the transverse direction of the film samples.

The variation in fluorescence intensity according to the rotational position of the filter are given in the Figures 7 (heat set) and Figure 8 (non heat set): Figure 7 is a plot of UV fluorescence intensity at 460 nm as a function of polariser angle for non-oriented BOPP film containing diphenylbutadiene (DPBD). [Excitation at 360 nm.] The solid line is a sixth-order polynomial regression fit, applied as a guide to the eye. Figure 8 is a plot of UV fluorescence intensity at 460 nm as a function of polariser angle for oriented BOPP film containing diphenylbutadiene (DPBD). [Excitation at 360 nm.] Heat-setting (at 135°C) has imparted slight MD molecular orientation. The solid line is a sixth-order polynomial regression fit, applied as a guide to the eye.

Polarised UV absorption measurements were used to assess the orientational distribution of DPBD molecules in films exemplified herein. The distribution was expressed as the ratio of peak heights in the associated UV absorption spectra . The results were as follows:

Film type Ratio of absorptive peak maxima (3 specimens), MD/TD

Non-heat-set 1.01 , 0.97, 1.17; average 1.05 Heat-set 1.11 , 1.20, 1.18; average 1.16

These data show that the Rigid Rod system used in the examples can distinguish the orientational history of the two film samples.

Claims

1 A biaxially oriented polymeric film suitable for use as a substrate to provide improved authentication, the film comprising from about 0.001 % to about 5% by weight of a colourless additive dispersed in at least one layer of the film; where the additive exhibits a detectable difference in the spectral response when illuminated with polarised IR or UV radiation for which the axis of polarisation is parallel with one of the principal orientation axes (MD or TD).

2 A film as claimed in the preceding claim, where the film is a biaxially oriented polypropylene film and optionally has a balanced oreintation.

3 A film as claimed in either preceding claim, where the additive possesses a marked anisotropy in its interaction with UV irradiation such that, when largely immobilised in the host substrate, irradiation of the substrate with UV light gives an absorption spectrum or a fluorescence intensity which is indicative of a preferred orientation of the guest additive with respect to any of the principal directions attributable to the substrate, such as the machine direction, transverse direction or normal direction.

4 A film as claimed in any preceding claim, where the additive has an elongated molecular geometry and is selected from one or more (optionally two or more) UV absorbers, UV fluorescent materials, materials capable of intermolecular non-radiative energy transfer (NRET), phosphorescent materials and/or any mixtures thereof..

5 A film as claimed in any preceding claim, where the additive is selected from: phenanthrenes, fluorenes, fluoresceins, flavins and/or aromatic hard-resins.

6 A film as claimed in any preceding claim, where the additive is selected from: (E,E)-1 ,4-diphenylbuta-1 ,3-diene (DPBD), (E,E)-1 ,6-diphenylhexa-1 ,3,5-triene (DPHT); and/or beta, beta- carotene (carotene).

7 A method of authenticating a product comprising the steps of:

(a) illuminating a film as claimed in any preceding claim with electromagnetic radiation (optionally UV radiation) with polarisation axis aligned in one of two direction along the film; and

(b) detecting a difference in the spectra of the film in each direction to authenticate the film.

8. A method as claimed in claim 6, comprising the steps of a) irradiating the film with a source of polarised (optionally UV) radiation to give a (optionally UV) absorption spectrum or UV fluorescence from the film b1 ) detecting the reflected , transmitted and/or fluoresced radiation from the film b2) authenticating the film by comparing the i) intensity of the radiation from the film with a known profile of radiation dependent on the relative orientation of the of the machine direction of the substrate and/or the polariser used for polarising the source irradiation and/or ii) distinguishing the polarisation of the irradiation from the film received by the detection system.

9 The use of UV absorbers, UV fluorescent materials, materials capable of intermolecular non-radiative energy transfer (NRET) and/or phosphorescent materials in a film as claimed in any of claims 1 to 5 and/or a method as claimed in either claim 6 or 7 for the purpose of reflecting a preferred orientation distribution of the polymer chain trajectories in the film via appropriate detection methods to determine the orientation history of the film.