WO2017112507A2

WO2017112507A2 - Tamper-evident label containing an irreversible photochromic substance

Info

Publication number: WO2017112507A2
Application number: PCT/US2016/066797
Authority: WO
Inventors: Maciej P. PIETRAS
Original assignee: 3M Innovative Properties Company
Priority date: 2015-12-21
Filing date: 2016-12-15
Publication date: 2017-06-29
Also published as: GB201522491D0; WO2017112507A3

Abstract

A tamper-evident adhesive label stock (10) comprising a first layer (1), a second layer (2) and a release liner (3), wherein the second layer (2) is positioned between the first layer (1) and the release liner (3) and contains an irreversible photochromic substance that undergoes a color change upon exposure to a UV-light source and wherein the first layer (1) is UV-light blocking such that the irreversible photochromic substance in the second layer (2) is not activated as long as it it covered by the first layer (1), and wherein the first and second layers are bonded to each other and wherein the label stock (10) further comprises either in the second layer or in an additional bottom layer (6) positioned between second layer (2) and release liner (3) an adhesive material (7) by which the label (10) can be adhered to a substrate (8) and wherein photochromic substance is activated by UV-light to undergo a color change that is visible to the naked eye and that is irreversible. Further provided is the label stock applied as label to a substrate, and a method of preparing the label stock.

Description

TAMPER-EVIDENT LABEL CONTAINING AN IRREVERSIBLE

PHOTOCHROMIC SUBSTANCE

Field

The present disclosure relates to a tamper-evident label stock and label comprising an irreversible photochromic substance and to a method of preparing the label.

Background

There has long been a need for visual evidence that a container, for example a container of ingestible products such as drugs, has been opened. There has also been a need to apply serial numbers or price tags to articles such as automobiles, passports, and items for sale so that they cannot be surreptitiously transferred to a different article.

These needs have been answered in large measure by tamper evident labels. Such labels bear a message (such as a repeating pattern of the word "void") that becomes visible if the label is opened. Several examples of such labels are described, for example, in US patent number 5,153,042. In essence, such labels are multi-layered articles and contain a release coating printed on a bottom layer of the label. The bottom layer may be colored or contains a different color than the top coating. The bottom layer is adhered to the container and provides a strong bond to the container surface. The release coating is covered by a varnish which strongly adheres to the bottom layer but weakly adheres to the areas covered by the release coating. Usually such article then contains another layer covering its surface. When breaking the label to open it the top layer(s) get(s) removed along the pattern printed by the release coating which is the weakest bond in the label thus exposing a warning message. Several variations of this type of tamper-evident labels are known.

However, these types of tamper evident labels may not easily allow to detect tampering in cases where the label had been opened but its top layer has been replaced by a new layer to conceal that the label had been opened. A prospective purchaser of an article protected by such a label might inspect the label with insufficient care to detect that it had been tampered with. Alternative tamper evident labels use so called "taggants". Taggants are materials that are not visible to the naked eye but become visible when using an appropriate reader.

Taggants include fluorescent materials, infrared-active materials or materials that become visible using an UV-lamp. An example of this type of tamper-evident labels is disclosed in WO2013/134654. A disadvantage of these type of labels is that a specific device is necessary to examine the label.

Therefore, there still exists the need for alternative tamper-evident labels. Desirably, such labels do not require external readers to identify indicia indicating tampering. Favorably, indicia indicating tampering are noticeable to the naked eye.

Summary

In one aspect there is provided a tamper-evident adhesive label stock (10) comprising a first layer (1), a second layer (2) and a release liner (3), wherein the second layer (2) is positioned between the first layer (1) and the release liner (3) and contains an irreversible photochromic substance that undergoes an irreversible color change upon exposure to a UV-light source and wherein the first layer (1) is UV-light blocking such that the irreversible photochromic substance in the second layer (2) is not activated while it is covered by the first layer (1), and wherein the first and second layers are bonded to each other and wherein the label stock (10) further comprises either in the second layer or in an additional bottom layer (6) positioned between second layer (2) and release liner (3) an adhesive material (7) by which the label (10) can be adhered to a substrate (8) and wherein the color change that is visible to the naked eye and wherein the UV-light source is daylight.

In another aspect there is provided a tamper-evident label (10') bonded to a substrate (8) by an adhesive material (7), wherein the label (10') comprises a first layer (1) and a second layer (2), wherein the second layer (2) is positioned between the first layer (1) and the substrate (8) and contains an irreversible photochromic substance that undergoes an irreversible color change upon exposure to a UV-light source and wherein the first layer (1) is UV-light blocking such that the irreversible photochromic substance in the second layer (2) is not activated as long as it is covered by the first layer (1), and wherein the first and second layers are bonded to each other and wherein the label (10') further comprises either in the second layer (2) or in an additional bottom layer (6) positioned between second layer (2) and substrate (8) the adhesive material (7) and wherein the color change is to the naked eye and the UV-light source is daylight.

In a further aspect there is provided a method of preparing a tamper-evident label stock according to any one of claims 1 to 9, comprising

(i) applying a first layer (1) to a second layer (2) wherein the second layer comprises an irreversible photochromic substance that undergoes an irreversible color change upon exposure to a UV-light source wherein the color change is visible to the naked eye and is irreversible and wherein the first layer is UV-light blocking such that the irreversible photochromic substance in the second layer (2) is not activated as long as it is covered by the first layer (1), wherein either the second layer (2) or an additional bottom layer (6) comprises and adhesive material (7) by which the labelstock can be adhered to a substrate (8) or a release liner (3) and wherein the UV-light source is daylight.

Description

The label of the present disclosure is a multi-layered adhesive tape. The multi-layered material without or with release liner is referred to herein as "label stock". The label stock adhered to a substrate other than a release liner is referred to as a "label".

The present disclosure provides a tamper evident adhesive label comprising a first layer, a second layer, and a release liner. The second layer is positioned between the first layer and the liner and comprises an irreversible photochromic substance. An "irreversible photochromic substance" as referred to herein changes color or becomes colored upon exposure to UV-light. The color change is irreversible, which means the color change does not get reversed when the exposure to UV light has ceased.

Generally, the first layer comprises at least one material that blocks UV light such that the first layer blocks sufficient UV light to minimize or inhibit a UV-light induced color change of the photochromic substance. The first layer may fully cover the entire second layer, or it may cover an area of the second layer containing the photochromic substance. In either case, a color change becomes visible when the first layer gets damaged or removed.

The first and second layers are connected to each other by a releasable adhesive bond. The connection may be direct connections or the layers may be connected to each other indirectly, by way of one or more intermediate layers positioned between the first and second layers.

The materials of the layers are chosen so that when a force is applied to remove the label the bond connecting the first and second layer ruptures first. Therefore, the adhesives used to provide a bond to the substrate or to intermediate layers are selected to create a stronger bond than the bond connecting the first and second layers.

The tamper evident label stock and label of the present disclosure will now be described in greater detail and with reference to specific embodiments wherein like elements are provided with the same or similar reference numbers. In one embodiment the present disclosure provides a tamper evident adhesive label stock (10) comprising a first layer (1), a second layer (2), a release liner (3) and various optional layers (4), (5) and (6). Optional layer (4) represents a top layer. Optional layer (5) represents an intermediate layer and optional layer (6) represents a bottom layer. Each optional layer (4), (5), and (6) may in fact represent a plurality of separate optional layers.

The second layer (2) is positioned between the first layer (1) and the release liner (3) and contains an irreversible photochromic substance. Therefore, the second layer (2) is also referred to herein as "photochromic layer". When the photochromic substance becomes exposed to UV light it gets activated and changes color or becomes colored. The color change as such can indicate that the label had been opened or damaged. Alternatively, the photochromic substance can be arranged in a pattern that displays information, for example a warning message. When the photochromic substance becomes activated the messages appears. For example, the layer underneath and on top of the second layer may be transparent and also the second layer may be transparent or opaque and upon activation the photochromic substance becomes colored. Likewise, the photochromic substance may be already colored in its non-activated stage, for example it may be green and changes into red upon activation. If the photochromic substance is arranged in a pattern to display a warning message, this message is invisible to the naked eye, if it is placed on a

background that is of the same green color, for example by using a colored bottom layer (6). When the photochromic substance gets activated it changes its color into red and the message stands out from the green background and becomes visible to the naked eye. The color change is irreversible. Therefore once the message is visible it indicates the label had been opened before even when the original top layer has been replaced by a new top layer to disguise the fact that the label had been broken, opened or damaged.

To prevent premature activation of the photochromic material the photochromic layer (2) is covered, at least in parts, by the first layer (1). The first layer absorbs or reflects UV light, such that the photochromic substance in the second layer (2) is not activated as long as it is covered by the first layer (1). Therefore, the first layer (1) is also referred as "blocking layer". The blocking layer (1) may form the outmost top layer of the label stock (10) or it may be covered by optional layer (4), which then forms the outmost top layer of the labelstock (lO).

The tamper evident label stock further comprises an adhesive material (7) by which it can be adhered to a substrate (8). The adhesive material (7) may be part of the second layer (2) or it may be present in the form an additional adhesive layer (6) which forms the bottom layer (6) and which is covered by the release liner (3) to prevent premature adhesion and allows the label stock (10) to be stored, for example wound up into a roll. First layer (1):

The first layer (1) is a UV light blocking layer. This means the layer either absorbs or reflects UV light sufficiently such that the photochromic substance in the underlying second layer (2) does not get activated as long as it is covered by the first layer (1). UV light as used herein means the ultraviolet light from day light having a wavelength of from 200 to 450 nm from daylight. The first layer may be made of a UV-light blocking material of sufficient thickness such that UV-light from daylight does not penetrate the first layer (1) to reach the second layer (2) to set off the colour change of the photochromic substance. Such material may be, for example a polymeric material, which may contain UV-light absorbers or UV-light reflectors in sufficient amounts to prevent the photochromic substance from being activated. UV-light absorbers can be used as known in the art. Such absorbers may be organic or inorganic and include, for example, benzophenones,

phenyltriazines,benzotriazoles, oxalanilides and hindered amine light stabilizers

("HALS"). HALS include, for example, 2,2,6,6,-tetramethylpiperidin and derivatives. In typical UV-reflectors include zinc oxide, iron oxide and titanium oxide particles. In particular titanium dioxide is available in very small particles sizes such that polymers or varnishes containing them may appear transparent.

Instead of a single UV-blocking layer, several UV-blocking layers may be used, for example a combination of first layer (1) and optional top layer(s) (4).

In one embodiment the first layer (1) is a polymeric film. In principle any polymeric film may be used that is sufficiently UV-light blocking sufficiently strong and durable to remain substantially unmarred while keeping protected containers closed, for example, while they are being handled in distribution. It also should be sufficiently flexible to allow application over discontinuities of the closures of ordinary containers. The first layer (1) may be a thermoplastic polymeric film. Particular examples of suitable polymers include acrylates, polyesters, poylcarbonates and polyvinyls. Acrylates include polymers containing repeating units derived from acrylates and methacrylates. Polyesters include polymeric reaction products of dicarboxylates and glycols. Specific polyesters include cellulose acetates, polyethylene terephthalates (PET), polybutylene terephthalates (PBT). Examples of polyvinyls include polymers containing vinyl chlorides, styrenes and naththalenes and combinations thereof. The polymers may be homopolymers or copolymers. A particularly preferred material is PET, more preferably biaxially oriented polyethylene terephthalate. In a preferred embodiment of the present disclosure the first layer (1) is transparent such that the second layer (2) is visible to the naked eye.

In some embodiments the first layer (1) is birefringent and typically comprises one or more birefringement polymers.

In one embodiment the first layer (1) is a metallic film. The metallic film may be bound to the second layer (2) by an adhesive intermediate layer (5). It may also vapour-deposited onto an intermediate layer (5) or onto the second layer (2).

The first layer (1) may typically have a thickness of from about 5 μπι to about 5 mm. In a typical embodiment the first layer (1) may have a thickness of from about 40 μπι to about 250 μιη. In some embodiments, the first layer (1) is the outmost top layer. The first layer (1) may be printable and/or may contain a pattern displaying information or it may be colored.

Optional top layer (4)

In one embodiment, the first layer (1) is not the outmost top layer. In this embodiment the first layer (1) may be covered by one or more top layers (4), which may then form the outmost layer. Preferably, the top layer(s) (4) may be bonded to the first layer (1) by a stronger bond than those connecting the first (1) and second layers (2) or that of optional intermediate layer (5) and second layer (2) to provide a breakline (9) between second layer (2) and the layer(s) covering it.

The top layer (4) may be printable. It may be printed to contain information. Printing may be done by known methods including offset printing, gravure printing, screen printing and thermoprinting. It may also be colored and may contain one or more colorants. The same materials may be used to make the top layer (4) as are described for producing the first layer (1).

Top layer (4) and first layer (1) may be bonded together by an adhesive bond using an adhesive or by lamination or coextrusion, by chemical reaction (i.e. curing). The top layer may also be a varnish or a coating and may be applied to the first layer (1) in liquid form, for example as a solution or as melt.

In some embodiments the top layer (4) is selected from metallic films, polyesters, polyamides, polyester, polyurethanes, polyacrylates, butadiene-styrenes, and polyolefins, Preferably, the top layer (4) is a polyolefinic film, for example a polyethylene-based or polypropylene film, meaning a polymer film containing more than 50% by weight of a polymer containing repeating units derived from ethylene or propylene, respectively. Second layer (2) and photochromic material (20)

The second layer (2) contains the photochromic material (20). Photochromic material (20) and UV-blocking material in the first layer (1) are chosen that the wave length of the UV light to which the photochromic colorant is responsive by colour-change is blocked. Thus the UV-blocking material may only block the wavelengths of the UV light spectrum to which the photochromic colorant is reactive to. The photochromic material according to the present disclosure are compounds that undergo an irreversible color change upon activation by UV light. Preferably, the change in color is visible to the naked eye. It includes a change from one color to another color and also a change from not-colored to becoming colored. Preferably, the UV light contained in natural light is sufficient to activate the photochromic substance. The activation is irreversible. This means the color- change does not reverse when the exposure to UV-light is discontinued or ceased completely.

Preferably, the color change occurs in less than 60 minutes, more preferably in less than 60 seconds and most preferably in 5 to 50 seconds upon exposure to a source of UV light. In a preferred embodiment the source of UV-light is natural light, i.e. day light.

Irreversible photochromic substances that are activated by UV light are known and include, but are not limited to, 1,2-dihydroquinolines, as described for example in US 4,812,171, incorporated herein by reference. The irreversible photochromic substances are used in effective amounts to achieve the color change in the desired time and/or upon exposure to the desired light source, such as for example natural light. Generally, the greater the concentration of photochromic substances used the faster occurs the color change. Typical amounts include from 0.1 to 10% by weight based on the total weight of the second layer (2). The second layer (2) typically may have a thickness of from about 5 μιη to about 300 μπι, preferably from about 10 μπι to about 60 μιη.

In one embodiment the photochromic substance is arranged to display information. In its simplest form just the occurrence of the color change is indicating the label has been opened. For this purpose it may be sufficient to provide the photochromic substance as a dispersion or solution in the second layer (2) or in shaped form, for example in dots, lines or areas. In more sophisticated embodiments the photochromic substance is distributed in or on the layer in a pattern that displays information, including texts, symbols, pictograms, bar codes or geometric patterns.

For example, the photochromic substance may be printed or coated onto a bottom layer (6), or it may be incorporated into the second layer (2), for example by mixing or dissolving it with the material that forms the second layer (2) and then applying the material onto a release liner (3) or bottom layer (6) to provide the second layer (2).

In one embodiment the second layer (2) comprises the adhesive material (7). In this embodiment the photochromic material (20) may be mixed or dissolved with the adhesive material to produce the second layer (2). The adhesive material may be the material by which the label stock (10) is adhered to a substrate or it may be a different adhesive material and the label stock (10) may contain an additional adhesive layer, for example in the form of a bottom layer (6) by which the label stock is adhered to a substrate (8). Any adhesive well known to those skilled in the art may be used in the context of the present disclosure. The adhesive material may contain one or more adhesives. Preferably the adhesives are pressure sensitive adhesives, for example adhesives that adhere to a substrate by applying finger tip pressure and without requiring heating or evaporation of solvent to provide an adhesive bond. The adhesive material may be based on polyacrylates, polyesters, polyolefines, polyamides, polyurethanes, silicone polymers, natural and synthetic rubber such as polybutadiene and copolymers, polyisoprenes and copolymers, as well as hydrogenated derivatives thereof with or without resin, fillers and cross linked agents. Polyacrylates include those which have been modified for use on a variety of surface having very different surface energies, such as steel/polyethylene.

Useful alkyl acrylates (i.e. acrylic acid alkyl ester monomers) for the preparation of polyacrylates include linear or branched monofunctional unsaturated acrylates or methacrylates of non-tertiary alkyl alcohols, the alkyl groups of which have from 4 to 18 and, in particular, from 4 to 12 carbons atoms. Examples of these lower alkyl group include but not limited to n-butyl acrylate, isobutyl, acrylate, hexyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, n-octyl acrylate, n-octyl methacrylate, 2-methylbutyl acrylate, isononyl acrylate, n-nonyl acrylate, isoamylacrylate, n-decyl acrylate, isodecyl acrylate, isodecyl methacrylate, isobornyl acrylate, 4-methyl-2-pentyl acrylate and dodecyl acrylate. Particularly preferred adhesives include copolymers of alkyl acrylates which have a straight chain of from 4 to 12 carbon atoms and a minor proportion of a highly polar copolymerizable monomer such as acrylic acid. Such adhesives are pressure-sensitive and are described for example in U.S. Pat. No. 2,973,286 incorporated herein by reference. Particularly preferred are acrylate based adhesive materials such as copolymers of isooctyl acrylate and acrylic acid, which typically are pressure-sensitive adhesives. Examples include those disclosed in US Patent No. 2,884, 126 incorporated herein by reference.

Optional intermediate layer (5)

The second layer (2) of the labelstock is positioned between the first layer (1) and the release liner (3) or the substrate (8) in is used as label. Between first and second layer on or more intermediate layers (5) may be placed. Layer (5) may be an adhesive layer to provide a bond between first and second layer. It may be used to weaken the bond between first and second layer to make sure the bond between first and second layer fails before the bond between second layer (and optional bottom layers) and the substrate (8) fails. For example the optional layer (5) may contain a release coating. The release coating may be any material that provides a weak bond to the first layer (1) to create a break line (9) between first (1) and second layer (2). Preferably, the release coating is as thin as possible while still serving that function, e.g., normally from 100 to 300 nm in thickness. Release coatings are known in the art. Preferred release coatings include polyvinyl alcohol, silicones, fluorinated chemicals, and waxes. Especially preferred are solutions of polyvinyl alcohol dissolved in isopropyl alcohol and water. Preferably, the release coating is transparent.

Also a combination of release coating and primer may be used. The primer may be used to smooth out any irregularities on the top surface of the second layer (2) which may result, for example, by printing a pattern of photochromic material on the second layer (2). The primer is preferably transparent. The thickness of the primer should be sufficient to afford a continuous, planar surface to which the first layer (1) can be applied and bonded. Its thickness preferably is about 4 to 6 times the thickness of the release coating. The primer may be, for example, a solvent-based adhesive.

Bottom layer (6)

One or more optional bottom layers (6) may be placed between second layer (2) and release liner (3) or substrate (8). The optional bottom layer typically is an adhesive layer comprising an adhesive material as described above in connection with the second layer (2). It may be the same or a different adhesive material. The materials are chosen such that they provide a strong bond between substrate (8) and second layer (2).

Release liner (3)

The adhesive bottom layer (either optional layer (6) or second layer (2) of the label stock is protected by a release liner (3). Release liners as known in the art may be used. The release liner (3) preferably is adhesive-repellent. It typically comprises a paper or plastic film which has been coated or modified with compounds of low surface energy relative to adhesive to which the release liner is applied. Typical examples of such compounds include organo silicone compounds, fluoropolymers, polyurethanes and polyolefins. The liner may also be a polymeric sheet produced from polyethylene, polypropylene, PVC, polyesters, fluoropolymers with or without the addition of adhesive-repellant compounds. Typically, the release liner (3) may have thickness of from about 10 μπι to about 1000 μπι, preferably from about 30 μπι to about 120 μπι. Substrate (8)

According to the present disclosure label stock can be prepared that can be applied on various containers and packaging materials by selecting the appropriate adhesives.

Containers include boxes, cans, bottles and vials. The containers include containers for pharmaceuticals, food and beverages, cosmetics and electronic goods. Packaging materials include paper and paper-like materials like cardboard, carton, glass,

thermoplastics, metals, wood and coatings. Preferred containers include containers for pharmaceuticals, in particular pills. .In one embodiment of the present disclosure there is provided an article containing on its surface a tamper evident adhesive label according to the present disclosure. Such articles may be selected from packages for solid

pharmaceutical products including pills, and containers bottles for liquids, including pharmaceutical liquids and food containers such as bottles and vials.

The label stock can be prepared by known methods for producing adhesive multi-layered products. For example, the layers may be prepared by coating, co-extrusion, lamination or a combination thereof. Regarding the use of the photochromic material care has to be taken to avoid premature activation by exposure to UV light or day light, which means the photochromic material may have to be applied in a dark room. In a preferred embodiment the label is transparent. In another preferred embodiment at least the layers covering the second layer (2) are transparent.

The present disclosure is now further illustrated by examples without intending to limit the disclosure to the examples.

Examples

Example 1

A transparent polyester sheet from Mitsubishi was used as UV blocking layer (1). The sheet was coated with an acrylic adhesive material (commercially available as A40 from 3M Company). Before the adhesive material is applied to the polyester sheet the adhesive material is mixed with 15% wt (based on 100% wt of the adhesive material) with a photochromic ink. A paper liner can then be applied to the adhesive material to provide a label stock.

The labelstock was prepared in a dark room with no exposure to daylight to prevent premature color change.

The release liner is removed and the label stock is applied to a substrate. When trying to remove the label from the substrate the polyester layer breaks off and a color change in the adhesive layer can be observed.

Example 2

A photochromic ink can be applied on a polyolefin sheet in a series of dots in a dark room. A transparent adhesive material can be coated over the polyolefin sheet on the side that contains the photochromic ink. The adhesive layer can be coated in various areas with a polyvinyl alcohol coating. A polyester sheet can be applied to the adhesive material that provide UV blocking of the underlying photochromic polyolefin sheet. Another adhesive material can be applied to the side that is opposite to the photochromic side of the polyolefin sheet and then can be subsequently covered by a release liner.

The release liner can be removed and the label stock can be applied with its adhesive side to a substrate. When the label is removed from the substrate, the label splits between first and second layer and the dots become visible.

Claims

1. A tamper-evident adhesive label stock (10) comprising a first layer (1), a second layer (2) and a release liner (3), wherein the second layer (2) is positioned between the first layer (1) and the release liner (3) and contains an irreversible photochromic substance that undergoes an irreversible color change upon exposure to a UV-light source and wherein the first layer (1) is UV-light blocking such that the irreversible photochromic substance in the second layer (2) is not activated while it is covered by the first layer (1), and wherein the first and second layers are bonded to each other and wherein the label stock (10) further comprises either in the second layer or in an additional bottom layer (6) positioned between second layer (2) and release liner (3) an adhesive material (7) by which the label (10) can be adhered to a substrate (8) and wherein the color change that is visible to the naked eye and wherein the UV-light source is daylight.

2. The tamper-evident label stock (10) according to claim 1 having an outermost top layer that is (a) printable, (b) comprises a pattern displaying information, or both (a) and (b), and wherein the outermost top layer is either the first layer (1) or an additional layer (4) covering the first layer (1).

3. The tamper-evident label stock (10) according to any one of the preceding claims, wherein the first layer (1) comprises a polyester.

4. The tamper evident label stock (10) according to any one of the preceding claims, further comprising one or more additional layer (5) between first (1) and second layer (2).

5. The tamper-evident label stock (10) according to any one of the preceding claims, wherein the second layer (2) contains the irreversible photochromic substance in a pattern that displays information.

6. The tamper evident label stock (10) according to any one of the preceding claims wherein the first layer (1) and any additional layers (5) between first (1) and second layers (2) are sufficiently transparent such that the second layer is visible to the naked eye.

7. The tamper evident label stock (10) according to any one of the preceding claims wherein the first layer (1) and any additional layers (5) between first (1) and second layers (2) and the second layer (2) are sufficiently transparent such that a layer underneath the second layer is visible to the naked eye.

8. The tamper-evident label stock (10) according to any one of the preceding claims, wherein the second layer (2) contains the irreversible photochromic substance in a pattern that displays information.

9. A tamper-evident label (10') bonded to a substrate (8) by an adhesive material (7), wherein the label (10') comprises a first layer (1) and a second layer (2), wherein the second layer (2) is positioned between the first layer (1) and the substrate (8) and contains an irreversible photochromic substance that undergoes an irreversible color change upon exposure to a UV-light source and wherein the first layer (1) is UV-light blocking such that the irreversible photochromic substance in the second layer (2) is not activated as long as it is covered by the first layer (1), and wherein the first and second layers are bonded to each other and wherein the label (10') further comprises either in the second layer (2) or in an additional bottom layer (6) positioned between second layer (2) and substrate (8) the adhesive material (7) and wherein the color change is to the naked eye and the UV-light source is daylight.

10. The label according to claim 9 having an outermost top layer that is (a) printable, (b) comprises a pattern displaying information, or both (a) and (b), and wherein the outermost top layer is either the first layer (1) or an additional layer (4) covering the first layer (1).

11. The label according to claim 9 or 10, wherein the first layer (1) comprises a polyester.

12. The label according to any one of claims 9 to 11 further comprising one or more additional layer (5) between first (1) and second layer (2).

13. The label according to any one of claims 9 to 12, wherein the second layer (2) contains the irreversible photochromic substance in a pattern that displays information.

14. The label (10') according to any one of claim 9 to 13 wherein the first layer (1) and any additional layers (5) between first (1) and second layers (2) are sufficiently transparent such that the second layer (2) is visible to the naked eye.

15. The label (10') according to any one of claims 9 to 14 wherein the first layer (1) and any additional layers (5) between first (1) and second layers (2) and the second layer (2) are sufficiently transparent such that the substrate (8) is visible to the naked eye.

16. The label (10') according to any one of claims 9 to 15, wherein the bond between second layer and substrate and including any bonds of intermediate layers positioned between second layer (2) and substrate (8) is stronger than the bond between fist layer (1) and second layer (2) including the bonds of any optional layers between first (1) and second (2) layer.

17. The label (10') according to any one of claims 9 to 16, wherein the substrate (8) is a container.

18. A method of preparing a tamper-evident label stock according to any one of claims 1 to 9, comprising