HK1151089A - Method of authenticating a polymer film - Google Patents

Description

Method for authenticating polymer films

The present invention relates to a method of authenticating a polymer film.

Polymer films are increasingly used as substrates in fields where security, authentication, identification and forgery prevention are important. In such fields, polymer-based products include, for example, banknotes, important documents (e.g., ID materials such as, for example, passports and land rights, stock and educational certificates), films for packaging high-value products for anti-counterfeiting purposes, and security cards.

Polymer-based security materials have advantages in terms of safety, functionality, durability, economy, cleanliness, processability, and environmental considerations. Perhaps the most notable of these advantages is the security advantage. For example, paper-based banknotes can be relatively easy to replicate, while countries using polymer-based banknotes are less subject to counterfeiting than paper-based banknotes. Polymer-based banknotes are also durable and not easily torn.

Polymeric film-based security materials can be modified to incorporate a variety of physical and hidden security features. Since the introduction of the first polymer banknotes about 25 years ago, security features have included Optically Variable Devices (OVDs), opacifying features, security threads printed with security features, embossing, transparent windows, and diffraction gratings. In addition to the complex security features, there is a more immediate advantage that if a counterfeiter attempts to simply reproduce a security material (e.g. a banknote) using such a machine, the high temperatures used in the reproduction machine often cause melting or deformation of the polymer-based material.

A variety of polymers may be used as the security matrix. Polypropylene films are one of these substrates. The three main processes for preparing polypropylene films are the tenter method, the casting method and the foaming method.

In the casting and tentering processes, polymer chips are typically placed in an extruder and heated so that the extrudate is extruded from a slot die onto a chill roll to form a film (in the case of the casting process) or a thick polymer ribbon (in the case of the tentering process). In the tenter frame process, the thick polymer tape is then heated again and then stretched in the longitudinal (length direction) direction (referred to as "machine direction") and in the transverse (width direction ") direction (referred to as" transverse direction ") to form a film.

In the foaming process, the polymer is extruded through an annular die, not through a slot die, to form a thicker extrudate in the shape of a hollow cylinder or "drainpipe" through which air is blown. The annular mould is on top of an apparatus which typically corresponds to several stories high (e.g. 40 to 50 metres). The extrudate is moved downward and continuously heated so that it expands to form bubbles. The bubble is then cut into two half-bubbles, each of which can be used individually as a "monoweb" film, or the two half-bubbles can be pinched and laminated together to form a twice thick film (or the bubble can be folded to form a twice thick film). In the die, there are typically three concentric rings, so that the hollow cylinder is a three-layer extrudate. For example, there may be a polypropylene core layer having a terpolymer skin layer on one side and another terpolymer skin layer on the other side. In this case, the single web consists of three layers of polypropylene in the middle, and the double web consists of five layers, since the layer in the middle is the same skin (terpolymer) of each half-bubble. A variety of other possible arrangements and compositions are possible depending on, for example, the number of rings, the type of skin layer, the type of core layer, etc.

Thus, the foaming process produces a film (e.g., 10 to 100 microns thick) by forming bubbles, while the tentering process produces a film by stretching the material. The foaming process produces a uniformly stretched film that is different from and advantageous for certain purposes over tentered films. Innovia Films ltd, Wigton, UK produce biaxially oriented polypropylene (BOPP) Films by a foaming process. In addition to polypropylene, other polymers (e.g., LLDPE, polypropylene/butene copolymers) can be formed into films using a foaming process.

There is a need to introduce features in films used as substrates for security documents, identity documents or value documents and articles, which are not readily apparent to potential unauthorized users or counterfeiters and which, even if identified, cannot be easily reproduced. The introduction of such security features may also be applicable to other tokens or items requiring authentication verification, such as access documents and tickets.

WO 2007/072426 discloses a method of making a polarization retardation film and the use of such a film in security applications.

US 2006/0187452 discloses a method for determining the level of birefringence of an optical material such as a polymer film.

US 5,737,298 discloses a technique for verifying the authenticity of a particular kind of pirated optical discs using a polarimeter.

JP 2005254643 discloses a multilayer transparent film having blue-shift properties, wherein two films of different refractive index are successively laminated.

US 2005/0109984 discloses a method for identifying a polymer tested as an identifiable polymer using a thermochromic compound.

WO 2005/086099 discloses a currency authenticity detection system using a number of optoelectronic sensors having both transmission and reflection properties.

In a first aspect, the invention provides a method of authenticating a polymer film comprising measuring the thickness of a layer therein by white light interferometry.

Preferably, the measurement is performed on the core layer of the film.

The transparent film material is reflective on both the front and back surfaces; the light reflected from these two surfaces will vary in optical path length depending on the distance determined by the thickness of the transparent film and the angle of incidence of the light. Thus, the reflected light waves will undergo a phase change related to the optical path length difference. Optical path length difference equal to [ (2n +1)^*A beam of λ/2 (where n is 0, 1, 2, 3 and λ is wavelength) will cause the echoes to be completely out of phase and will thus cause destructive interference, cancelling out any reflected light. An optical path length difference equal to n λ will cause the return light waves to be perfectly in phase with each other and in a state called constructive interference, whereby the return light is twice its previous intensity. An optical path length between these two cases will cause an intermediate degree of intensification or cancellation of the return light. Interferometry is a series of techniques that take advantage of the above phenomena to measure the distance and thickness of materials.

Monochromatic interferometry uses a single wavelength source to measure a single interference response. This technique has been successful in applications such as surface profilometry or in the measurement of thin optical films where the thickness of the substrate is well known. However, the use of monochromatic interferometry as a security verification method is limited by the small amount of data returned (single interference pattern) and the possibility that the resulting pattern is produced by disparate thicknesses (the above equations illustrate that optical path lengths of λ 2, 3 λ/2, 5 λ/2, 7 λ/2.

In contrast to monochromatic interferometry, the present invention uses white light interferometry. Applicants have found that it is most suitable for measuring multilayer polymer films in security applications. White light interferometry measures the interference pattern produced by a material over the entire wavelength range defined by the span and resolution of the spectrophotometer used. A film with an interference path length of any given size will interfere in the spectral range of the analysis; however, the degree of interference will be determined by how in or out of phase the reflected wave of a particular wavelength. Thus, the spectrum obtained in a white light interferometer will include a large number of different sized fringes, with the largest fringes being used for those n values that are most easily resolved. The thickness of the layer can be reliably determined via fourier transformation of the data to obtain the frequency of the fringes. Another advantage of white light interferometry is that the data collected is sufficient to allow the resolution of multiple interference layers and the individual thicknesses of those layers to be measured with a single measurement.

In a second aspect, the present invention provides a method of authenticating a polymer film comprising measuring the birefringence of a core layer therein.

Birefringence (birefringence) or double refraction (double refraction) is a property of a material that is caused by the difference in refractive index of two different polarizing materials, s-and p-materials. The resulting effect manifests itself as a rotation of the polarization angle of the light transmitted through the material; this effect is induced via interfacial interactions and propagates through the birefringent material; the observed birefringence is the product of the initial interfacial interaction (i.e., the angle of incidence) and the subsequent path length (subsequent path length) through the material. Birefringence can be observed in a variety of ways as exemplified below.

The present invention enables the film to be protected as it is. The particular inherent properties of the film are observed in the present invention and do not require the addition of any additional security or identification features. This identification can be used for authentication for security purposes and also to determine the origin of the film.

The films referred to herein are generally sheet-like materials and may be provided as individual sheets or as a web of material which may be subsequently processed (e.g. by die cutting) to provide a sheet or article form (article form) material. When reference is made to "film" in this specification, it is intended to include sheet-like, article-like, or web-like films unless explicitly stated otherwise.

The film may include a polyolefin film, such as polyethylene, polypropylene, blends thereof, and/or other known polyolefins. The polymer film can be prepared by any method known in the art including, but not limited to, cast sheet, cast film, or blown film. The film or sheet may be a single layer or a multilayer structure. Preferably, the film or sheet is a multilayer structure having at least one core layer therein. In the case of a single-layer structure, the single layer is a core layer. In case multiple core layers are provided, the measurement of the core layer thickness or the measurement of the core layer birefringence may be performed by white light interferometry on only one and/or on more than one core layer. The film may optionally be coated with, for example, an opacifying agent. The invention is particularly applicable to films comprising a hollow or non-hollow polypropylene film having a polypropylene core layer and skin layers substantially thinner than the core layer and formed from, for example, a copolymer of ethylene and propylene or a terpolymer of propylene, ethylene and butene. The film may comprise a biaxially oriented polypropylene (BOPP) film, which may be made into a balanced film using substantially the same machine and transverse direction stretch ratios, or can be an unbalanced film, wherein the film is significantly more oriented in one direction (MD or TD). Continuous stretching can be used, where heated rollers effect longitudinal stretching of the film and then tenters are used to effect transverse stretching. Or may be stretched simultaneously using, for example, a so-called foaming method, or may be stretched using a simultaneous stretch tenter (draw stenter).

The films used in the present invention can be of various thicknesses as desired for the application. For example, they can be from about 5 μm to about 240 μm thick, preferably from about 10 μm to about 120 μm thick, more preferably from about 12 μm to about 100 μm thick, and most preferably from about 14 μm to about 80 μm thick. The multilayer films of the present invention may be laminated together to form thicker substrates for use as, for example, security cards and the like. The thickness of such a laminate structure may be larger than the preferred maximum thickness of the non-laminated film, for example 250 μm or even more.

The film may comprise one or more additive materials. The additives may include: a dye; pigments, colorants; metallic and/or pseudometallic glitter coatings (e.g., aluminum); lubricants, antioxidants, surfactants, hardeners, gloss improvers, degradation aids, UV attenuating materials (e.g., UV light stabilizers); a sealant; tackifiers, anti-blocking agents, additives to improve ink adhesion and/or printability, crosslinkers (e.g., melamine formaldehyde resins), adhesive layers (e.g., pressure sensitive adhesives), adhesive release layers (e.g., for use as a substrate in a release sheet process for making labels). Additives also include those that reduce the coefficient of friction (COF), such as terpolymers.

Other additives include conventional inert particulate additives, preferably having an average particle size of from about 0.2 μm to about 4.5 μm, more preferably from about 0.7 μm to about 3.0. mu.m. The reduction in particle size improves the gloss of the film. The amount of the preferably spherical additive added to the or each layer is desirably greater than about 0.05% by weight, preferably from about 0.1% to about 0.5% by weight, for example about 0.15% by weight. Suitable inert particulate additives may include inorganic or organic additives, or mixtures of two or more such additives.

Suitable inorganic particulate additives include inorganic fillers such as talc, particularly metal oxides or non-metal oxides such as alumina and silica. Solid or hollow, glass or ceramic microspheres or microspheres may also be used. Suitable organic additives include particles of acrylic and/or methacrylic resins comprising polymers or copolymers of acrylic and/or methacrylic acid, preferably in spherical form. Such resins can be crosslinked by, for example, adding a crosslinking agent such as a methylated melamine formaldehyde resin thereto. The promotion of crosslinking may be aided by the introduction of appropriate functional groups, such as hydroxyl, carboxyl and amide groups, into the acrylic and/or methacrylic polymer.

For example, when performing white light interferometry, clarifying agents can be particularly preferred additives for reducing the haze and thereby increasing the signal strength of the film. The low haze films may enable the use of detuned white light interferometer (detuned white light interferometer) to generate measurable signals with smaller light sources, narrower wavelength ranges (e.g., such as that emitted by white light LEDs), shorter integration times, and/or lower optics and sensor requirements.

The present invention therefore particularly concerns the use of detuned light emitting devices and/or white light LED sources in interferometry of films comprising one or more clarifying agents.

Suitable clarifying agents may include diester salts of phosphoric acid, such as sodium 2, 2' -methylenebis (4, 6-di-tert-butylphenyl) phosphate; salts of monocarboxylic or polycarboxylic acids, such as sodium benzoate and aluminum tert-butylbenzoate; sorbitol derivatives, e.g. dibenzylidene sorbitol or C thereof₁-C₈Alkyl-substituted derivatives, such as methyl-, ethyl-or dimethyl-dibenzylidene sorbitol; inorganic additives such as silica, kaolin, or talc; or a mixture of two or more thereof. Other suitable clarifying agents or combinations thereof are known to those skilled in the art or are referred to, for example, in the plastics additives handbook, fifth edition; zweifel, h, ed; hanser Publ: munich, 2001.

Some or all of the desired additives listed above may be added together as a composition to be applied to the sheet of the present invention, and/or to form a new layer to which it may be applied (i.e., to form one of the inner layers of the final multi-layer sheet), and/or may form an outer or surface layer of the sheet. Alternatively, some or all of the foregoing additives may be added separately and/or directly into the body of the sheet (e.g., as part of the initial polymeric composition by any suitable method such as mixing, stirring and/or injection), optionally during and/or prior to sheet molding, and thus may or may not form a layer or coating on its own.

Such additives may be added to the polymer resin prior to the preparation of the film, or may be applied as a coating or other layer to the prepared film. If the additive is added to the resin, the mixing of the additive with the resin is accomplished by mixing the additive into the molten polymer using conventional techniques such as milling, mixing in a Banbury type mixer, or mixing in an extruder barrel. The mixing time can be reduced by mixing the additive with unheated polymer particles to achieve a substantially uniform distribution of the agent in the polymer mass, thereby reducing the time required for vigorous mixing at the melt temperature. The most preferred method is to mix the additive with the resin in a twin screw extruder to form a concentrate, which is then mixed with the resin of the film structure just prior to extrusion.

Formation of the films of the present invention (optionally oriented and optionally thermoset as described herein) comprising one or more additional layers and/or coatings can be conveniently accomplished by any lamination or coating technique well known to those skilled in the art.

For example, a layer or coating can be applied to another base layer by a coextrusion technique in which the polymeric components of each layer are coextruded into intimate contact while each layer is still molten. Preferably, the coextrusion is carried out from a multi-pass annular die such that the molten polymer components making up the individual layers of the multilayer film merge at their boundaries within the die to form a unitary composite structure, which is then extruded from a common die orifice in the form of a tubular extrudate.

One or more of the additives described herein may also be applied to the films of the present invention from a solution or dispersion of the additive in a suitable solvent or dispersant using conventional coating techniques. Aqueous latexes (e.g., prepared by polymerizing polymer precursors of the polymer additives) in an aqueous emulsion in the presence of a suitable emulsifier are preferred media from which the polymer additives or coatings may be applied.

Coatings and/or layers may be applied to one or both sides of the sheet. The or each coating and/or layer may be applied to any or all of the other coatings and/or layers sequentially, simultaneously and/or subsequently. If the gas barrier coating of the present invention is applied to only one side of the sheet, which is preferred, other coatings and/or layers may be applied to the same side of the sheet and/or to the opposite (other) side of the sheet.

Additionally or alternatively, more layers can be provided in the film by coextrusion from a multi-ring die to produce, for example, two, three, four, or more layers in the coextrusion exiting the die.

The coating composition can be applied to the treated side of the sheet (e.g., polymeric film) in any suitable manner, such as gravure printing, roll coating, bar coating, dipping, spray coating, and/or using a coating bar. Solvents, diluents and adjuvants may also be used in these processes, as desired. Excess liquid (e.g., aqueous solution) can be removed by any suitable means, such as press rolls, doctor blades, and/or air knives. The coating composition is generally applied in an amount such that, upon drying, a smooth, uniformly distributed layer having a thickness of from about 0.02 μm to about 10 μm, preferably from about 1 μm to about 5 μm, is deposited. Generally, the thickness of the applied coating is such that it is sufficient to provide the substrate with the desired properties. Once applied to the sheet, the coating may then be dried by hot air, radiant heat, or any other suitable method to impart the desired properties to the sheet of the present invention.

Combinations of more than one of the above methods of applying the additives and/or components thereof to the film may also be used. For example, one or more additives may be added to the resin prior to preparation of the film, and one or more other additives may be applied to the surface of the film.

In the multilayer film of the present invention having at least a substrate layer and a skin layer, the skin layer is preferably ink-printable. The skin layer has a thickness of about 0.05 μm to about 2 μm, preferably about 0.1 μm to about 1.5 μm, more preferably about 0.2 μm to about 1.25 μm, and most preferably about 0.3 μm to about 0.9 μm.

The film may have at least one region having reduced opacity thereon as compared to the surrounding film. The opacity of the film may be provided, at least in part, by the presence of void (or cavity) areas in the film. Such void regions may be created, for example, by providing at least one voiding agent (voiding agent) in the film. The manufacture of voided films is well known, and any suitable voiding agent may be used herein. The voiding agent is typically a particulate material and may be selected from organic, inorganic or polymeric materials. US 4,377,616 of Mobil Oil Corporation describes a number of these voiding agents. The voiding agent may be substantially spherical particles in nature, or may have a large aspect ratio. For example, the voiding agents described in WO-A-03/033574 may be used.

The opacity of the film may be supplemented with other materials that are not voiding agents, but are opacifying agents. In this connection, mention may be made, for example, of TiO₂The inorganic filler of (3).

The invention is particularly useful where the film is one prepared by a foaming process. The foaming process produces films with balanced orientation, well-defined and uniform thickness, and other properties (strong tensile strength, low elongation, high gloss and clarity, good puncture and flex crack resistance, oil and grease resistance, good water tightness), which define an "identification mark" indicating that the film has been prepared by the foaming process.

Previous attempts to protect membranes have included the addition of one or more markers to the membrane at low concentrations in an attempt to identify the membrane by detecting a stimulus response. However, the addition of a marker adds cost and complexity and can interfere with other additives. The present invention makes the evaluation and analysis of information inherent and already written into the structure of the film. Preferably, the identification of the present invention comprises identification of the core layer rather than the surface layer, whereby the security is enhanced to a certain extent by identifying the layer encapsulated in the film, i.e. it would be very burdensome and difficult to intervene or manipulate such a layer.

To distinguish films (e.g., BOPP films from other films), the total thickness of the films can be measured, as well as the thickness of individual layers, such as laminate layers. This enables the determination of specific characteristics depending on the specific method, e.g. the specific foaming method. Additionally or alternatively, the film's unique birefringence signature can be evaluated and used to determine whether the film was made by a particular method and thus whether it was, for example, a genuine note or a counterfeit note. Birefringence depends on the anisotropy of the material and films made by the foaming process have different anisotropy and therefore different birefringence properties from films made by other processes. Furthermore, the precise conditions used in the foaming process will affect the birefringence signature.

Thus, the present invention recognizes that the inherent properties of films made by a particular process, such as the foaming process, are unique and function as an identification mark without the need to add security or identification features.

The identification methods of the present application and the apparatus for such methods are generally applicable to a range of polymeric film materials and can be tailored to specific substrates and thicknesses by criteria known in the art. For qualification by measuring thickness using white light interferometry, the wavelength range of the spectrometer is determined by the row spacing of the grating spectrometer, the length of the detector array, and the focal length of the optical spectrometer. The thickness range that can be measured is related to the size of the spectral range used. It is preferred in the present invention to measure a thickness of 0.5 μm to 100 μm, which corresponds to a spectral range of about 500nm to 1000 nm. Depending on the type of equipment used, a balance can be struck between the need for a large spectral range and other factors, particularly in small equipment. Factors that ideally need to be balanced in a small device include spectral range, spectral resolution, and slit width; these factors determine the range of possible thicknesses, the ability to resolve thin layers, and the time it takes to obtain a response. In small devices, large spectral ranges can be cancelled out by using narrower slits in order to obtain satisfactory resolution, sufficient sensitivity and acceptable measurement time. According to the invention, a device can be provided which is particularly useful for identifying a specifically defined thickness.

The detection time of the spectrometer is determined by the minimum time required to saturate the detector array in the spectrometer. Thus, the brighter the light, the faster the saturation. However, the need for rapid detection times must be balanced by the need to avoid thermal damage to the identified membranes. The use of a pulsed light source such as a short-pulse xenon white flash lamp can alleviate this problem.

Accordingly, one aspect of the invention relates to the use of a pulsed light source in measurements.

Other aspects of the invention provide a detection cell designed for carrying out the method of the invention. Such a detection unit is suitable for identifying the properties of the material-specific identification mark based on the security polymer. The detection unit can be used for checking the reliability of the material; such as a banknote validator for use at a bank, ATM or cashier.

Other aspects of the invention provide the use of interferometry and/or birefringence measurement in determining the authenticity of a security article based on a polymer film.

Interferometry relies on optical interference between rays that are reflected at different surfaces within the film. Interference provides a measure of the optical path length between the surfaces and hence the thickness of the film (or layer). The uniformity of the boundary between layers also affects the measurement. Reflection occurs where the layers have different refractive indices.

The interferometry method used in the present invention is white light interferometry because monochromatic interferometry is not sufficiently discriminative.

A narrow beam of light is typically directed at the material to be measured and a detector detects light reflected from the boundary between layers. A series of peaks are obtained in the interferogram, which show the position of the corresponding layer. The multilayer film and web can thus be inspected quickly without the need to touch the material or destructively analyze the material.

Orientation in the polymer affects not only properties (mechanical, optical, blocking, and others) but also birefringence due to anisotropy in refractive index. Birefringence is the separation of light into two beams caused by such anisotropy and is apparently a function of the method of film production. For example, BOPP films prepared by the foaming process have special anisotropic properties due to uniform stretching.

Preferably, the membrane is a folded bubble membrane, i.e. comprising two half-bubbles laminated together. The preferred laminate layer is a terpolymer.

Certain aspects of the invention will now be described by way of non-limiting detailed examples with reference to fig. 1 to 3, which show elements of the apparatus of the invention for different methods of observing birefringence.

Referring to FIG. 1, a first method of observing birefringence is through the use of crossed polarizers. The linear polarizer allows either type of s-or p-polarized light to pass through it, so that when the second piece of linear polarizer is present and twisted by 90 ° with respect to the first piece, the remaining light resulting from a single polarization type is filtered; this technique is known as using orthogonal polarizers. Birefringent materials effectively rotate the polarization axis and thus when placed between two crossed polarisers will affect how much light is allowed to pass through them. While rotating the birefringent material between crossed polarisers causes the intensity of light to vary with the angle of birefringence. The thin polymer film operates on a first order of birefringence and will tend to rotate light rays between 0 ° and 90 °; a perfectly birefringent material would be different from a non-enhanced transmission between the polarizers to eliminate the effect of the first polarizer by passing rotating light through the second polarizer. This behavior forms the basis of a method for measuring film birefringence; the sample is typically placed between two motorized orthogonal polarizing filters, which are then rotated at 360 ° while maintaining the same rotational configuration as each other, and light from a light source is passed through the filter/sample/filter and its intensity is measured using a photodiode. The measured intensity will follow two 180 ° circles, the maximum and minimum of which are related to the birefringence of the film.

Referring to FIG. 2, a second method for birefringence measurement is to use two annular linear polarization filters comprising sectors of material, the filters having their own polarization angles that are related to the angular position of the sectors on the annular mirror. If two of these lenses are distinguished by their s-and p-orientation, the combination of the two will act as orthogonal polarizers for each sector. A single light source can be used to illuminate a sample placed between two such polarizers and the transmitted light from each sector can be injected into an optical fiber that in turn has a transmitted intensity measured using a photodiode. Thus the birefringence behaviour of the film can be measured in a single measurement without rotating the polariser-the resolution of this measurement will depend on the angular size of each sector-for example a sector up to 20 ° will provide 18 measurements and will be sufficient for finding the maximum and minimum transmission.

Referring to FIG. 3, a third method for birefringence measurement is the use of a quartz wedge. In this case, the birefringent material is placed between the polarizing filter and a calibrated quartz wedge while the light is directed towards an inspection system that measures the position of the fringes on the wedge.

In order to distinguish a given authentic film from other films, two approaches have been combined, which allow the user to exclude other types of films, i.e., given counterfeit films:

1. white light interferometry: white light interferometry, which measures the interference pattern produced by a substrate layer across the visible spectrum, provides the user with the thickness of the layers within the substrate, and knowledge of these layers precludes thinner films, films that have been laminated together (post-processing lamination yields thicker laminate layers), and films that have the correct thickness but have the wrong layer-to-layer combination. This excludes cast films (due to thickness) and severely limits the BOPP film options that counterfeiters can use.

2. Birefringence: BOPP films prepared by the tenter method are oriented more in the transverse direction than in the machine direction and therefore have more birefringence than BOPP films prepared by the double foaming method. The ability to precisely control birefringence using the double foaming process and thus provide a unique signature that can exclude more films.

The combination of the two tests can also exclude non-PP films depending on thickness, orientation or coextrusion limits.

Design scenario where the level of safety change is set for the method (level 1 is lowest safety; level 6 is highest safety) to determine if the material contains a film of about x microns thickness made of folded bubbles, each half bubble being about x/2 microns thick and containing BOPP as core layer and terpolymer as skin layers (the two inner skin layers then create a laminate layer).

Grade 1: genuine products and fake products are distinguished based on the thickness difference.

Grade 2: birefringent films and non-birefringent films (i.e., tentered or not tentered) are distinguished.

Grade 3: differentiating film structure (i.e., the film is made by laminating two halves x/2 microns thick or not)

Grade 4: a non-birefringent non-bubble film, made of two halves of x/2 μm laminate, is distinguished from a bubble film.

Grade 5: distinguishing between a blister film that has been subjected to folded bubble lamination and a blister film that has been treated separately after lamination-there is a difference in thickness and birefringence).

Grade 6: a distinction is made between x-micron folded blister films made by one particular foaming process and x-micron folded blister films made by a different foaming process.

Testing on any scale indicated success-scale 1 excluded 99% of commercially available membranes; the test of grade 2 removed the most common film similar to BOPP prepared by foaming. Level 4 is considered a successfully acceptable level; however, the detector is currently operating at level 5 and is therefore considered to be safe except for the impractical determination of attempts (i.e. to establish a double foaming method-which, however, may not fool the detector).

The above ranges do not indicate that films cannot be counterfeited-there is an unmet and still unknown way of achieving films of similar structure. However, counterfeiting a film will require a great deal of expertise on the part of the counterfeiter.

In fact, a counterfeit film is more likely to be purchased than made by the counterfeiter. There are several sources that can be classified into three main types:

1. cast film or blown film-cast films are prepared by extruding the polymer from a die onto a chill roll. Blown films are prepared by extruding a polymer from an endless film and expanding the bubbles in a semi-molten state. Cast and blown films are generally non-or slightly oriented and therefore have poor dimensional stability (i.e. they can be easily stretched), poor optics and thickness control.

2. Mono-oriented film-a mono-oriented film is prepared by extrusion from a die and stretching in the machine direction. Mono-oriented films are highly oriented, they have poor optical and poor lateral dimensional stability.

3. Biaxially oriented film-biaxially oriented Films are commercially available from Innovia Films Limited and a number of other manufacturers. Commercial grades of BOPP from many manufacturers are typically made by a tentering process in which the PP is extruded through a die onto a chill roll, stretched in the machine direction of a heated roll and stretched in the cross direction of a tenter frame. These films are themselves anisotropic and unlike BOPP made by the double foaming process, which is uniformly stretch oriented in all directions. The greatest possible threat of counterfeiting is that of laminating films together to imitate thicker films, which is why the next example was chosen to demonstrate that the method of the present invention is able to distinguish between these cases.

Examples

5-layer (real) film

Five-layer films were prepared using the so-called double foaming process. The molten polymer from the three extruders (one large core extruder and two smaller skin extruders) was extruded from an annular film to produce polypropylene tubes of 1mm to 2mm thickness. The tube is then reheated and stretched in both the transverse and longitudinal directions simultaneously by pulling the film in the longitudinal direction and blowing it into air bubbles to stretch the bond in the transverse direction. The resulting film has been stretched 8 × 8 to produce a film having a thickness of 20 μm to 50 μm. The bubble can be cut and pulled in two halves (to make a three layer film) or can be folded and laminated together to make a five layer film with a thickness of 50 μm to 100 μm. In this example, 5 layers of film were prepared, and then the film was annealed via reheating and rolled up into a roll. Five layers consist of two outer skin layers (< 0.5 μm), two main core regions (20 μm-55 μm) and a separate laminate layer (< 1 μm) holding the two core layers together. The thickness and birefringence of each layer were obtained from the total thickness of the sample. An interference pattern of the resulting film structure (set to "true") is shown in fig. 4.

5-layer (counterfeit) film

A hypothetical counterfeit film is made from a given authentic film by laminating the two webs of the "authentic" film together. An interference pattern of the resulting film structure (set to "counterfeit") is shown in fig. 5.

Claims (23)

1. A method of authenticating a polymer film comprising measuring the thickness of a layer therein by white light interferometry.

2. The method of claim 1, wherein the measurement is performed on a core layer of the film.

3. A method of identifying a polymer film comprising measuring the birefringence of a core layer therein.

4. A method of authenticating a polymer film comprising measuring the thickness of a layer therein by white light interferometry and/or measuring the birefringence of a core layer therein.

5. The method of claim 4, wherein the thickness measurement is performed on a core layer of the film.

6. A method as claimed in any preceding claim, wherein the measurement is performed on more than one layer.

7. A method as claimed in any preceding claim, wherein the measurement is performed using a pulsed light source.

8. A method as claimed in any preceding claim, wherein the measurement is performed using a detuned light source.

9. A method as claimed in any preceding claim, wherein the measurement is performed using an LED light source.

10. The method of any preceding claim, wherein the film comprises at least one clarifying agent.

11. A process according to any preceding claim wherein the film is prepared by a foaming process.

12. A method according to any preceding claim, wherein the film comprises a layer of biaxially oriented polypropylene (BOPP).

13. A method according to any preceding claim, wherein the film is a monoweb of three or more polymeric layers.

14. A method according to any preceding claim, wherein the film is a double web of five or more polymeric layers.

15. A method according to any preceding claim, wherein the method distinguishes between a film made by a foaming process and a film made by a different process.

16. A method of authenticating an object comprising a polymer film, wherein the authentication is performed by authenticating the polymer film of any preceding claim.

17. The method of claim 16, wherein the object is:

paper currency;

government documents;

non-government documents;

an identity document;

a passport;

a security thread;

optically Variable Devices (OVDs); or

Packaging or accessories for articles.

18. A method of identifying an authentic film or object based on the film or object meeting a prescribed predetermined thickness and/or birefringence criteria, using the method of any preceding claim.

19. A method of identifying an unauthentic or counterfeit film or object based on the film or object not meeting a prescribed predetermined thickness and/or birefringence criteria, using the method of any preceding claim.

20. A method according to claim 18 or 19, which is a forward compatible method suitable for identifying future films or objects to be manufactured.

21. A method according to claim 18 or 19, which is a backward compatible method for identifying already existing films or objects.

22. A detection unit adapted to perform the method of any preceding claim.

23. Use of white light interferometry and/or birefringence measurement to determine the authenticity of a security article comprising a polymer film substrate.