WO2025214710A1

WO2025214710A1 - Use of uv curable hot melt pressure sensitive adhesives in security tape and security label applications

Info

Publication number: WO2025214710A1
Application number: PCT/EP2025/057007
Authority: WO
Inventors: Stefan Mundt; Ricard PUJOLAR DURAN; Sonny Yuxia LIU
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2024-04-12
Filing date: 2025-03-14
Publication date: 2025-10-16
Anticipated expiration: 2026-10-12

Abstract

The present invention provides a security tape or security label comprising a substrate and a UV curable hot melt pressure sensitive adhesive (HMPSA), wherein the HMPSA comprises: 1) a UV-curable acrylic polymer that is a reaction product of a reaction composition comprising one or more acrylic monomers and one or more photoinitiators, and 2) optionally one or more additional ingredients, wherein said acrylic polymer is crosslinkable through said photoinitiator groups upon exposure to ultraviolet light.

Description

USE OF UV CURABLE HOT MELT PRESSURE SENSITIVE ADHESIVES IN SECURITY TAPE AND SECURITY LABEL APPLICATIONS

FIELD OF THE INVENTION

[0001] The invention relates to the use of UV-curable hot melt pressure sensitive adhesives in security tape and security label applications, and a security tape or security label comprising a substrate and the UV-curable hot melt pressure sensitive adhesives.

BACKGROUND OF THE INVENTION

[0002] Adhesive tapes and labels are attractive products from a consumer compliance perspective in that they are easily manipulated and stored, and they find utility in a wide range of bonding and masking applications, including bonding electrical, electronic, aerospace, and audio/video components.

[0003] Pressure sensitive adhesives are common components of adhesive tapes. They remain permanently tacky for a long period of time and need only minimum pressure to stick to a surface. Normally, they can be removed without leaving any adhesive residues, and without destroying the joined parts. However, there are specific applications in which it is necessary to leave behind adhesive residues: this is in the field of security, specifically tapes and labels that help prove when tampering of a product has occurred. The tamper-evident and ultra-destructible security tapes/labels are the ones providing the specific function of safeguarding product and transport packaging throughout the entire logistics chain with easily recognizable proof of first opening for maximum protection against tampering, theft, and product switching.

[0004] Tamper-evident and ultra destructible security tapes/labels are commonly used in industries that require a high level of fraud protection and quality assurance, as examples: Automotive, consumer goods, electronics, industrial, medical and pharmaceutical, military and government, packaging and transportation. There are known different methods to achieve the tamper evident effect, but the main ones are:

[0005] Hidden message/image: as soon as the tamper seal tape/label is opened or manipulated, somehow is easily delaminated before is completely removed, revealing multiple times a message (like “Void”, “Opened”, “Tamper evident”) or an image which in addition is irreversible. This first-opening effect remains visible even if an attempt is made to reseal the tamper evident tape/label. Both the overall label design as well as the opening effect (void effect) can be fully customized according to the packaging design and corporate identity. Figs 1 and 2.

[0006] Ultra-destructible tape/label material: once applied, the destructible tape/label is extremely difficult to peel off and upon attempted removal the labelling material fractures easily causing irreversible damage, making any tampering very clear. The material used as carrier or face material to produce the tape/label, easily breaks, tears, or destroys if its detaching is attempted. The peeled-up sections left behind are the evidence of manipulation preventing the tape/label from being removed and placed again in the same product/packing or other ones.

[0007] Pressure sensitive tapes and labels for tamper evident and ultra destructible security applications require the use of an adhesive providing particular properties and characteristics to fulfil the application requirements and achieve the required protection level against counterfeiting. Key properties include chemical resistance, high temperature resistance and low temperature resistance. This is because the most common method used by counterfeiters to violate the tamper evident tapes/labels is the use of low/high temperatures to reduce its adhesion on the substrate where are applied. For example, freon gas sprays (to freeze), or a hair dryer (to heat) are normally used on the tamper evident tape/label, because in both cases for most HMPSAs the extreme temperatures reached reduce the adhesion performance allowing the tape/label to be relatively easily cleanly removed without the delamination or destruction of the carrier/face material used. Then, when the temperature reaches again room temperature, the adhesive recovers its adhesion, and the tape/label can be repositioned not leaving evidence of manipulation.

[0008] It is possible for some HMPSAs to cover reasonably well the resistance to the low temperatures, or for other HMPSAs to cover reasonably well the resistance to high temperatures, but a HMPSA able to cover both extreme temperatures at same time is extremely difficult to find.

[0009] Due to the limitations of HMPSAs from the prior art, it has been necessary in the past to implement additional security methods in order to guarantee a complete protection against counterfeiting. For example, it has been required to use two different adhesives, or to use thermally sensitive (thermochromic) inks that change color usually when submitted to high temperatures. These are inadequate solutions because they add more production complexity and cost to the tamper evident and ultra-destructible security tape/label applications. SUMMARY OF THE INVENTION

[0010] The invention starts from the identification of the limitations presented by current available pressure sensitive tapes and labels used in tamper evident and ultra destructible security applications. The inventors have surprisingly found that a known class of hot melt pressure sensitive adhesives (HMPSAs) exhibit advantageous properties when applied to the security tape and label products.

[0011] HMPSAs initially developed and promoted for different applications, had not been identified nor promoted to be used successfully for those security applications overcoming the limitations of the adhesives used in this particular field.

[0012] According to a first aspect of the present invention, there is provided a security tape or security label comprising a substrate and a UV curable hot melt pressure sensitive adhesive (HMPSA) in applications, wherein the HMPSA comprises:

1) a UV-curable acrylic polymer that is a reaction product of a reaction composition comprising one or more acrylic monomers and one or more photoinitiators, and

2) optionally one or more additional ingredients, wherein said acrylic polymer is crosslinkable through said photoinitiator groups upon exposure to ultra violet light. A security tape or security label comprising a substrate and the UV cured hot melt pressure sensitive adhesive (HMPSA) also forms another aspect of the present invention.

[0013] According to another aspect of the present invention, there is provided the use of a UV curable hot melt pressure sensitive adhesive (HMPSA) in security tape and security label applications, wherein the HMPSA comprises:

2) optionally one or more additional ingredients, wherein said acrylic polymer is crosslinkable through said photoinitiator groups upon exposure to ultra violet light.

[0014] It has surprisingly been found that the UV-cured acrylic-based polymers defined herein provide good low temperature resistance and high temperature resistance at the same time, so find unexpected utility in tamper-evident and ultra-destructible tape/label applications. [0015] This new use of the UV-curable HMPSAs allows improved counterfeiting protection for security tapes and labels. Further, reduced production complexity is involved: only one single adhesive is required, without the need for additional security methods. This also results in a lower cost for the total security tape and label construction

[0016] According to another aspect of the present invention, there is provided a method of making a security tape or label as defined herein, the method comprising:

1) making a UV-curable acrylic polymer that is a reaction product of a reaction composition comprising one or more acrylic monomers and one or more photoinitiators,

2) removing any solvents and any volatiles from the UV-curable acrylic polymer,

3) mixing any optional ingredients into the UV-curable polymer to form a UV curable hot melt pressure sensitive adhesive (HMPSA), and

4) coating the UV curable hot melt pressure sensitive adhesive (HMPSA) on a tape or label substrate whereby forming the security tape or security label.

[0017] In order to cure the polymer, after step 4), the UV curable hot melt pressure sensitive adhesive (HMPSA) is cured using a UV-source.

[0018] The UV-cured acrylic polymer may exhibit a Glass Transition Temperature (Tg) < -10°C; preferably <-25°C. The Tg can be evaluated through the Dynamic mechanical analysis (DMA) method (DMA tan 5 peak as the Tg.

[0019] The UV-cured adhesive may exhibit a peel adhesion on steel of 1 to 30 N/inch; preferably 3 to 15 N/inch. A Peel Adhesion Test can be used to evaluate peel adhesion, for example according to Fl NAT TEST METHOD (FTM) 1 , orAFERA TEST METHOD 5001.

[0020] The UV-cured adhesive may exhibit a Shear Adhesion Failure Temperature (SAFT) at 1 kg/625mm² of greater than >50°C, preferably 60 - 100°C. The SAFT can be evaluated according to PSTC-17.

DESCRIPTION OF THE FIGURES

[0021] The invention is illustrated by the following non-limiting examples:

[0022] Figures 1 and 2: examples of hidden message/image security tapes/labels

[0023] Figures 3 and 4: examples of ultra-destructible tape/label materials [0024] Figure 5: results of performance test variation at Room Temperature.

[0025] Figure 6: results of performance test variation at very low temperatures.

[0026] Figure 7: results of performance test variation at high temperature.

DETAILED DESCRIPTION

[0027] As described above, the present invention relates to the use of a UV curable hot melt pressure sensitive adhesive (HMPSA) in security tape and security label applications, and a security tape or security label perse. The security tape or security label comprises a substrate and the HMPSA. The HMPSA comprises:

[0028] Thus, a reaction composition is formed from at least one acrylic monomer and at least one photoinitiator. The composition undergoes polymerisation to form the UV-curable acrylic polymer. At an appropriate point, the polymer is UV-cured to form a cured HMPSA. This HMPSA has utility in the field of security tapes and labels. For example, the uncured HMPSA can be applied to the tape or label, and the UV curing can take place to form the tape or label coated with the cured HMPSA. The tape or label is then ready to use.

[0029] As will be understood by the skilled person, the HMPSA may be obtainable by a free radical crosslinking mechanism, where the or one of the photoinitiators is a co-polymerizable photoinitiator which is copolymerizable with the acrylic monomer(s). Alternatively, the HMPSA is obtainable by a cationic crosslinking reaction of a cationic reactive functional group from one of the acrylic monomers.

Free Radical Polymerisation

[0030] As disclosed in, for example US 7,745,505 B2 (the entire disclosure of which is herein incorporated by reference), the HMPSA obtained by a free radical mechanism uses acrylic polymers comprising photoinitiators bound to the polymer chain. The co-polymerizable photoinitiators typically contain both a C=C double bond and a UV reactive chromophore of aromatic ketone. The photoinitiator typically also comprises a spacer containing both ethylene oxide and urethane, urea, carbonate or siloxane functional groups. The ethylene oxide is directly bonded to the chromophore (e.g., benzophenone) moiety, while the urethane, urea, carbonate or siloxane is closely linked to the polymerizable moiety, e.g., acrylic or styrenic C=C double bond.

[0031] In one embodiment of the invention, the acrylic polymers used are those that have a glass transition temperature between -10° C and -30° C. Thus, the monomers used are selected so as to achieve that glass transition range.

[0032] The acrylic polymer may be prepared by polymerizing a monomer mixture comprising at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH2=C(RA)(COORB), where RA is H or CH2, and RB is an optionally substituted, linear or branched C1-1000 alkyl chain. The monomer mixture suitably comprises this acrylic or methacrylic acid derivative and the copolymerizable photoinitiator. Alternatively, the monomer mixture comprises this acrylic or methacrylic acid derivative, the copolymerizable photoinitiator and at least one (meth)acrylic acid monomer. Suitably, the monomer mixture comprises (as monomers; ignoring reagents) one acrylic or methacrylic acid derivative and one photoinitiator, and optionally one (meth)acrylic acid monomer.

[0033] The or each of the acrylic or methacrylic acid derivatives of the formula CH₂=C(R^A)(COOR^B), may have as R^B is an unsubstituted C1-1000 alkyl chain. More preferably, the acrylic or methacrylic acid derivative is n-butyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, isodecyl acrylate, octyldecyl acrylate, caprolactone acrylate, or methyl acrylate, or a combination thereof. Most preferably, the acrylic or methacrylic acid derivative is 2-ethylhexyl acrylate. These monomers, especially the branched isomers, are able to undergo UV-initiated radical crosslinking with the photoinitiators or UV-reactive functional monomers.

[0034] Alternatively or in addition, the alkyl chain contains at least one functional group selected from the group consisting of a hydroxyl group and a carboxyl group. Preferably, R_B is a branched C1-1000 alkyl chain. Examples of acrylic and/or methacrylic acid derivatives useful as component (b) include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, and their corresponding methacrylates. It is possible to use vinyl compounds, instead of acrylates or methacrylates, such as vinyl acetate, styrene, a-methyl styrene, ethyl vinyl ether, acrylonitrile, and vinyl chloride. Vinyl monomers from the following exemplified groups may be used optionally as component (b): vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, vinylpyridine, vinyl compounds containing aromatic rings and heterocycles in the a position. [0035] The or each of the (meth)acrylic acid monomers is acrylic acid or p-carboxyethyl acrylate (P-CEA), or a combination thereof, and preferably is acrylic acid.

[0036] The or each acrylic monomer may also comprise polyethylene glycol (EO)n acrylic monomers, polyethylene/butylene mono-acrylic monomers, and/or siloxane containing acrylic monomers. Suitably, the or each acrylic monomer comprises 2-(2-ethoxyethoxy) ethyl acrylate, methoxy polyethylene glycol mono(meth)acrylate, or a combination thereof.

[0037] When present, the acrylic or methacrylic acid derivative of the formula CH₂=C(R^A)(COOR^B) (i) is present in an amount ranging from 50% to 99%, by weight of the reaction composition, preferably from 70% to 99% by weight of the reaction composition, more preferably from 80 to 99% by weight of the reaction composition, most preferably from 93 to 98% by weight of the reaction composition.

[0038] When present, the at least one (meth)acrylic acid monomer ii) is present in an amount ranging from 0.5% to 10% by weight of the composition, preferably from 1% to 8% by weight of the composition, more preferably from 2% to 6% by weight of the composition.

[0039] Examples of suitable acrylic or vinyl monomers containing carboxylic acid or anhydride functional groups useful as component (c) include acrylic acid, methacrylic acid, itaconic acid, maleic anhydride, itaconic anhydride, 4-(methacryloyloxyethyl)trimellitic anhydride and the like.

[0040] Examples of suitable acrylic or vinyl compounds useful as component (d), capable in particular of a hydrogen-abstraction reaction with UV photoinitiators or other UV reactive functional group, include amine, amide, and benzylic compounds of the following formula

[0041] The co-polymerizable photoinitiator may comprise a C=C double bond, a UV reactive chromophore of aromatic ketone, and optionally a spacer containing ether, sulfide, urethane, urea, carbonate or siloxane functional groups or a combination thereof. Suitably, the co- polymerizable photoinitiator has the structural formula wherein:

Z is S, O, CH₂, or NH,

R1-8 are independently H, Cl, Br, I, F, C1-24 alkoxy, C1-24 alkyl, or C1-24 aryl; and wherein at least one of RI_₈ must comprise at least one — W— X— Y group, wherein:

W is a covalent bond, or linear or branched C1-24 alkylene, C1-24 arylene, OC1-24, NHC1-24, or SC1-24,

X is a covalent bond, or a chemical divalent link of carbonate, urethane, urea, or tetramethyldisiloxane, and

Y is -Rg-C(Rio)=CH2, where R9 is OOC, or a linear or branched C1-24 alkylene, C1-24 arylene, OC1-24, NHC1-24, or SC1-24, wherein R_w is H or CH3.

[0042] Particularly preferred embodiments are photoinitiators having the following structural formula: where n=1-12, preferably 1-5, more preferably 1 , and

[0043] Preferably, the at least one photoinitiator is present in an amount ranging from 0.01% to 5%, by weight of the reaction composition, preferably from 0.1% to 0.5% by weight of the reaction composition, more preferably from 0.1 % to 0.4% by weight of the reaction composition.

[0044] For the process, the monomers in the reaction composition are converted by polymerization into the acrylic polymers. For polymerization the monomers are chosen such that the resulting copolymers can be used as HMPSAs, especially such that the resulting copolymers possess pressure sensitive adhesive properties in accordance with the “Handbook of Pressure Sensitive Adhesive Technology” by Donatas Satas (van Nostrand, N.Y. 1989). For these applications the static glass transition temperature of the resulting copolymers will advantageously be below about 25° C.

Cationic Polymerisation

[0045] As disclosed in, for example US 8,796,350 (the entire disclosure of which is herein incorporated by reference), the HMPSA obtained by a cationic mechanism is obtainable by a cationic crosslinking reaction of a cationic reactive functional group from one of the acrylic monomers with a UV light generated acid from a cationic photoinitiator.

[0046] The acrylic monomers i), ii) and iii) may be the same as those used in the free radical process. Also, an acrylic monomer with a cationic reactive functional group is used in the cationic process (hereinafter termed the “CRF acrylic monomer”). The CRF acrylic monomer may have the structural formula wherein

R¹ is O, S, C=O, or linear, branched, or cyclic alkylene, or oxyalkylene, arylene,

R² is linear, branched, and cyclic alkyl or alkoxy, aryl, H, halogen, C=O, or part of

R¹ as fused cycloaliphatic ring through a covalent bond connection,

R³ is (CH₂)n, n = 0-3,

X is acrylate or methacrylate, or comprises a -W-Y group, where

W is O, S, amide, carbonate, urethane, urea, siloxane or a combination thereof, and

Y is -R⁴-C(R⁵)=CH2, where R⁴ is a linear or branched C2-10 alkylene, C2-10 oxyalkylene, C=O, or arylene or derivative thereof, and R⁵ is H or CH3. [0047] The amount of the CRF acrylic monomer is from about 0.001 to about 0.015 equivalent per 100 g of the acrylic polymer.

[0048] The most frequently used cationic photoinitiators are either organic iodonium or sulfonium salts. The mechanism of a cationic photoinitiator, when irradiated, is that it forms an excited state which then breaks down to release a radical cation. This radical cation reacts with the solvent, or other hydrogen atom donors, and generates a protonic acid, which is the active species that initiates the crosslinking reaction.

[0049] Any of the many compounds known to initiate polymerization by a cationic mechanism may be used for the crosslinking reaction in this invention. These include, for example, diaryliodonium salts, triarylsulfonium salts, dialkylphenylsulfonium salts, dialkyl(hydroxydialkylphenyl)sulfonium salts and ferrocenium salts. The anions in theses salts generally possess low nucleophilic character and include SbF6-, PF6- , AsF6- , BF4- , B(C6 F5 )4 - or Ga(C6F5)4 - . Specific examples include Cyracure UVI-6976 (Dow Chemical). Particularly useful cationic initiators are soluble and red-shifted sulfonium salt photoinitiators, which have increased solubility in UV-crosslinkable compositions, promote efficient thick film UV curing, and exhibit increased thermal stability in UV crosslinkable compositions before cure, exhibit increased curing rates, and have a reduced dark cure time. They may have the structural formula below: where R is C₃H₇, C12H25, and

W is S, SO, SO₂ or CO.

The cationic photoinitiator is typically blended with the HMPSA before UV irradiation. Additional ingredient(s)

[0050] The HMPSA may optionally comprise various other additives, such as plasticizers, tackifiers, and fillers, all of which are conventionally used in the preparation of HMPSAs. As tackifying resins to be added, it is possible to use any known tackifying resins described in the literature. Representatively, mention may be made of pinene resins, indene resins, and rosins, their disproportionated, hydrogenated, polymerized, and esterified derivatives and salts, the aliphatic and aromatic hydrocarbon resins, terpene resins, terpene-phenolic resins, C5 resins, C9 resins, and other hydrocarbon resins. Any desired combinations of these or other resins may be used in order to adjust the properties of the resultant adhesive in accordance with the desired final properties.

[0051] In general it is possible to use any resin which is compatible with the corresponding acrylic polymers; reference may be made in particular to all aliphatic, aromatic, alkylaromatic hydrocarbon resins, hydrocarbon resins based on straight monomers, hydrogenated hydrocarbon resins, functional hydrocarbon resins, and natural resins. Explicit reference may be made to the depiction of the state of the art in the “Handbook of Pressure Sensitive Adhesive Technology” by Donatas Satas (van Nostrand, 1989).

[0052] In a further advantageous development one or more plasticizers, such as low molecular weight acrylic polymers, phthalates, whale oil plasticizers, or plasticizer resins, are added to the acrylic HMPSAs.

[0053] The HMPSAs may further be blended with antioxidants, light stabilizers, and compounding agents.

[0054] They may further be mixed with one or more fillers such as fibers, carbon black, zinc oxide, titanium dioxide, solid or hollow glass (micro)beads, microbeads of other materials, silica, silicates, and chalk. The addition of blocking-free isocyanates is also possible.

UV curing

[0055] The HMPSAs described herein can be crosslinked in air or nitrogen by irradiation with UV light in the range from 200 to 500 nm, preferably 250 to 320 nm. Irradiation may be done immediately while the adhesive compositions are still in melt, or after they cool to room temperature. [0056] The irradiation is done for a period of time sufficient to transform the low cohesive composition into an elastomeric adhesive of higher plasticity. The exact length of exposure will be dependent upon the nature and intensity of the radiation, the amount of photoinitiator, the polymer composition, adhesive formulation, the thickness of the adhesive film, environmental factors, and the distance between the radiation source and the adhesive film. The dosage or the length of exposure is controlled by the belt speed. It may be appropriate to adapt the lamp output to the belt speed or to shade off the belt partly, in order to reduce its thermal load.

[0057] The actual radiation used can be actinic light from any source, provided it furnishes an effective amount of UV radiation, since the adhesive compositions of the invention generally exhibit their maximum sensitivity to wavelengths in the ultraviolet range. Suitable sources of radiation are carbon arcs, mercury-vapor arcs, fluorescent lamps with special ultraviolet light emitting phosphors, electronic flash lamps and the like, lasers of specific wavelengths, LED bulbs or combinations of those. Preferred lamps are the electrodeless microwave powered lamps from Fusion Systems, or commercially customary high or medium pressure mercury lamps with an output of, for example, from 80 to 240 W/cm. When LED bulbs are used, the curing wavelength is typically around 365nm

[0058] In addition, the acrylic HMPSAs described in accordance with the invention may be crosslinked with electron beams. This type of crosslinking can also take place in addition to the UV crosslinking. Electron beam curing takes place by means of ionizing radiation.

Security Tapes and Labels

[0059] As described herein, the present inventors have surprisingly found that UV-curable HMPSAs can be used on tapes or labels in security applications. According to the present invention, there is provided a tape or label that comprises a substrate with the UV curable HMPSA as defined herein applied thereon. The method for making this may comprise:

1) making the reaction product of the reaction composition, i.e. by reacting the or each of the acrylic monomers and the or each of the photoinitiators,

2) removing the solvents and any volatiles,

3) mixing any optional ingredients to form the UV curable hot melt pressure sensitive adhesive (HMPSA), and

4) coating the UV curable HMPSA on the tape or label substrate.

[0060] At this point, the HMPSA has not been cured. Therefore, after step 4), the method may further comprise using a UV-source to form the tape or label with the cured HMPSA applied thereon. Thus, the present invention also provides a tape or label for use in security applications, wherein the tape or label comprises the substrate coated with the UV cured HMPSA as described herein.

Substrate

[0061] The tape/label substrate can be any material as long as it shows tamper evidence whenever efforts are made to remove it from where it is applied. It is well known in the field to use laminated films that are easy to delaminate in layers (with lower force than that required to remove the complete substrate). Such laminated films are typically double-sided adhesive films, with a PET carrier film in the middle.

[0062] Also, paper or films commonly named “ultradestructible” can be used - these are very easy to tear/break in small pieces (again, with lower force than that required to remove the complete substrate). Such laminated films are typically based on PET, BOPP, cellulose acetate, PVC, etc.

Polymerisation Process

[0063] As known by those skilled in the art, the preparation of acrylic polymers can be carried out by solution, emulsion, or bulk polymerization procedures using well-known polymerization techniques, such as free radical techniques. The copolymers can then be formed into hot melt adhesives by removal of the solvent, coagulation of the latex, or melt-processing of the neat polymers.

[0064] The polymerization may be conducted in the presence of one or more organic solvents and/or in the presence of water. Suitable organic solvents or mixtures of solvents are alkanes, such as hexane, heptane, octane, isooctane, and cyclohexane; aromatic hydrocarbons, such as benzene, toluene, and xylene; esters, such as ethyl, propyl, butyl and heptyl acetate; halogenated hydrocarbons, such as chlorobenzene; alkanols, such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; ethers, such as diethyl ether and dibutyl ether; or mixtures thereof.

[0065] In one advantageous embodiment of the process, the polymerization reactions proceed in an ethyl acetate solvent, thermally initiated by, for example, azobisisobutyronitrile (Al BN). [0066] To be used as HMPSAs, the acrylic polymers must be free of the solvent. For this purpose, the copolymers prepared as described above are concentrated to a solvent content of less than 2% by weight, preferably less than 0.5% by weight. Preferably, the process takes place using Thin Layer Evaporators (a continuous process) or stirred vacuum vessels (a batch process). The process may also take place in a concentration extruder, such as vent extruder, ring extruder, single-screw extruder, or twin-screw extruder, which are known to the skilled worker.

[0067] Any additional ingredients are either blended to the neat acrylic polymers in melt or added into the solutions of the copolymers at the end of the polymerization reactions. Upon the removal of the solvent, the mixtures are concentrated to give HMPSAs.

[0068] The acrylic polymers prepared will generally have a weight averaged average molecular weight (Mw) of from 10,000 to 2,000,000 g/mol, more preferably between 50,000 and 1 ,000,000 g/mol and most preferably between 100,000 and 700,000 g/mol. The Mw is determined by gel permeation chromatography (GPC) or matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS).

[0069] Any suitable process may be used to product security tape or label as disclosed herein. For example, direct coating or transfer coating.

[0070] In direct coating, the molten, uncured UV-curable HMPSA is coated directly on the tape/label substrate, and then cured with a UV source, for example a UV lamp.

[0071] One exemplary process comprises: a) coating the molten, uncured UV-curable HMPSA directly on to the tape/label substrate, b) curing with a UV source to form the UV-cured HMPSA c) siliconizing the side of the tape/label that is uncoated, and d) self-winding the coated tape/label to produce a roll protecting the HMPSA side to be exposed to undesired adhesion and/or contamination.

[0072] Another exemplary process comprises: a) coating the molten, uncured UV-curable HMPSA directly on to the tape/label substrate, b) curing with a UV source to form the UV-cured HMPSA c) laminating the coated side of the tape/label with a siliconized release liner that covers the UV HM PSA, and d) self-winding the coated tape/label to produce a roll protecting the HMPSA side to be exposed to undesired adhesion and/or contamination.

[0073] In transfer coating, the molten, uncured UV-curable HMPSA is coated is coated on a siliconized release liner, and then cured with a UV source, for example a UV lamp. The coated release liner is then laminated with the tape/label substrate and wound to produce a roll. As the coated layer of adhesive has no adhesion on the siliconized surface of the release liner, it “transfers” to the tape/label substrate with which is in contact.

[0074] Thus, the invention may be described according to the following statements:

[0075] Statement 1 : Use of a UV curable hot melt pressure sensitive adhesive (HMPSA) in security tape and security label applications, wherein the HMPSA comprises:

2) optionally one or more additional ingredients, wherein said acrylic polymer is crosslinkable through said photoinitiator groups upon exposure to ultraviolet light.

[0076] Statement 2. Use according to Statement 1 , wherein at least one photoinitiator is a copolymerizable photoinitiator, and the HMPSA is obtainable by a UV free radical crosslinking reaction through the copolymerizable photoinitiator which is copolymerized with the or each of the acrylic monomers.

[0077] Statement 3. Use according to Statement 2, wherein the co-polymerizable photoinitiator comprises a C=C double bond, a UV reactive chromophore of aromatic ketone, and optionally a spacer containing ether, sulfide, urethane, urea, carbonate or siloxane functional groups or a combination thereof.

[0078] Statement 4. Use according to Statement 2 or 3, wherein the co-polymerizable photoinitiator has the structural formula wherein: Z is S, O, CH₂, or NH,

[0079] Statement 5. Use according to Statement 1 , wherein at least one of the acrylic monomers has a cationic reactive functional group, and at least one photoinitiator is a cationic photoinitiator and the HMPSA is obtainable by a cationic crosslinking reaction of the cationic reactive functional group with a UV-generated acid from the or each of the cationic photoinitiators.

[0080] Statement 6. Use according to Statement 5, wherein the acrylic monomer with the cationic reactive functional group has the structural formula wherein

R¹ is O, S, C=O, or linear, branched, or cyclic alkylene, or oxyalkylene, arylene, R² is linear, branched, and cyclic alkyl or alkoxy, aryl, H, halogen, C=O, or part of R¹ as fused cycloaliphatic ring through a covalent bond connection,

R³ is (CH₂)n, n = 0-3,

X is acrylate or methacrylate, or comprises a -W-Y group, where W is O, S, amide, carbonate, urethane, urea, siloxane or a combination thereof, and

Y is -R⁴-C(R⁵)=CH2, where R⁴ is a linear or branched C2-10 alkylene, C₂- oxyalkylene, C=O, or arylene or derivative thereof, and R⁵ is H or CH3.

[0081] Statement 7. Use according to Statement 5 or 6, wherein the cationic photoinitiator is a sulfonium salt having the structure

where R is C₃H₇, CI₂H₂₅, and W is S, SO, SO₂ or CO.

[0082] Statement 8. Use according to any preceding Statement, wherein the or each acrylic monomer is selected from the group consisting of: i) at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH₂=C(RA)(COORB), where RA is H or CH₂, and RB is an optionally substituted, linear or branched C1-1000 alkyl chain, and/or ii) at least one (meth)acrylic acid monomer, and/or iii) an acrylic monomer that forms, alone or in combination with one or more different monomers, the UV-curable acrylic polymer having a glass transition temperature between - 10° C and -30° C.

[0083] Statement 9 Use according to Statement 8, wherein the or each acrylic monomer comprises or consists of i) at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH₂=C(R^A)(COOR^B), wherein R^B is an unsubstituted C1-1000 alkyl chain.

[0084] Statement 10. Use according to Statement 9, wherein the alkyl chain contains at least one functional group selected from the group consisting of a hydroxyl group and a carboxyl group.

[0085] Statement 11. Use according to Statement 9 or 10, wherein R_B is a branched C1-1000 alkyl chain.

[0086] Statement 12. Use according to any one of Statements 8 to 11, wherein monomer i) comprises or consists of n-butyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, isodecyl acrylate, octyldecyl acrylate, caprolactone acrylate, or methyl acrylate, or a combination thereof.

[0087] Statement 13. Use according to Statement 12, wherein monomer i) comprises or consists of 2-ethylhexyl acrylate.

[0088] Statement 14. Use according to any one of Statements 8 to 13, wherein monomer ii) comprises or consists of acrylic acid or p-carboxyethyl acrylate (P-CEA), or a combination thereof, preferably consists of acrylic acid.

[0089] Statement 15. Use according to any preceding Statement, wherein the or each acrylic monomer comprises polyethylene glycol (EO)n acrylic monomers, polyethylene/butylene mono-acrylic monomers, and/or siloxane containing acrylic monomers.

[0090] Statement 16. Use according to any preceding Statement, wherein the or each acrylic monomer comprises 2-(2-ethoxyethoxy) ethyl acrylate, methoxy polyethylene glycol mono(meth)acrylate, or a combination thereof.

[0091] Statement 17. Use according to any one of Statement 8 to 16, wherein the or each acrylic monomer comprises or consists of i) at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH₂=C(R^A)(COOR^B) as defined in Statement 8, in an amount ranging from 50% to 99%, by weight of the composition, preferably from 70% to 99% by weight of the reaction composition, more preferably from 80 to 99% by weight of the reaction composition, most preferably from 93 to 98% by weight of the reaction composition.

[0092] Statement 18. Use according to any one of Statements 8 to 17, wherein the acrylic monomer consists of two of i), ii) or iii), preferably consists of i) and ii).

[0093] Statement 19. Use according to any preceding Statement, wherein the at least one (meth)acrylic acid monomer ii) is present in an amount ranging from 0.5% to 10% by weight of the composition, preferably from 1% to 8% by weight of the composition, more preferably from 2% to 6% by weight of the composition.

[0094] Statement 20. Use according to any preceding Statement, wherein the at least one photoinitiator is present in an amount ranging from 0.01% to 5%, by weight of the reaction composition, preferably from 0.1% to 0.5% by weight of the reaction composition, more preferably from 0.1% to 0.4% by weight of the reaction composition. [0095] Statement 21. Use according to statement 4 and any statement dependent on statement 4, wherein the copolymerizable photoinitiator is where n = 1.

[0096] Statement 22. Use according to statement 4 and any statement dependent on statement 4, wherein the copolymerizable photoinitiator is where n = 1.

[0097] Statement 23. Use according to statement 4 and any statement dependent on statement 4, wherein the copolymerizable photoinitiator is

[0098] Statement 24. Use according to statement 4 and any statement dependent on statement 4, wherein the copolymerizable photoinitiator is

[0099] Statement 25. Use according to any preceding Statement, where the Tg of the UV- curable acrylic polymer is from -60°C to 20°C, preferably from -60°C to -30°C. [0100] Statement 26. Use according to any preceding Statement, wherein the weight average molecular weight (Mw) of the UV-curable acrylic polymer is from 50,000 to 2,000,000 g/mol.

[0101] Statement 27. Use according to any preceding Statement, wherein the viscosity of the UV-curable acrylic polymer ranges from 45,000 to 90,000 mPas at 130°C, preferably from 50,000 to 70,000 mPas at 130°C.

[0102] Statement 28. A tape or label for use in security applications, wherein the tape or label comprises a substrate with the UV curable HMPSA as defined in any one of the preceding Statements applied thereon.

[0103] Statement 29. A method of making a tape or label as defined in Statement 28, the method comprising:

2) removing any solvents and any volatiles from the UV-curable acrylic polymer,

[0104] Statement 30. A method according to Statement 29, wherein, after step 4), the UV curable hot melt pressure sensitive adhesive (HMPSA) is cured using a UV-source.

[0105] Statement 31. A tape or label for use in security applications, wherein the tape or label comprises the substrate coated with the UV cured HMPSA prepared according to Statement 30. Examples

[0106] Tests were carried out on Free Adhesive Film (FAF) samples. FAF describes a layer of adhesive and UV cured between 2 siliconized release liners with different resistance to separation. Thus, one of the two release liners is very easy to remove without damaging the film layer, allowing the adhesive layer remaining on the other release liner to be coated by transfer on any desired substrate.

[0107] Free Adhesive Film samples of an exemplary UV-curable acrylic hotmelt pressure sensitive adhesive (PSA) were coated with a coat weight of 20 gsm and were cured with a UV- C dose of 30 mJ/cm.

[0108] The exemplary free radical crosslinkable HMPSA is based on a co-polymer containing by weight:

• 96% of 2- EH A

• 3.5% of Acrylic acid

• 0.5% of an acrylic monomer having a benzophenone functional group as the photoinitiator

[0109] The exemplary cationic crosslinkable HMPSA is based on a co-polymer containing by weight:

• 98% of 2- EH A

• 1 .5% of acrylic monomer having an epoxy functional group

• 0.5% of a cationic photoinitiator

[0110] One Free Adhesive Film was transferred to the right side of a market typical "VOID" film that can generate the "VOID" image when removing the coated film once applied on a surface. The coated films (after the transfer process) were kept for at least 24 hours at room temperature (25°C).

Test 1 : Performance test variation at Room Temperature (25°C):

[0111] A test plate was prepared by applying one stripe (about 10 cm length and 25mm width) of the void film to a PE test plate (using Finat roller). The applied stripe was immediately removed by hand (in length direction) and visually checked to see if some kind of "void" image was created.

[0112] The results are documented in Figure 5. Test 2: Performance test variation at very low temperatures:

[0113] A stripe was applied as described before in Test 1. The stripe was immediately frozen by spraying for 3 seconds with freon cold spray (-45°C), followed by immediate removal of the stripe. A visual check was done to see if some kind of "void" image was created.

[0114] The results are documented in Figure 6.

Performance test variation at high temperature:

[0115] A stripe was applied as described before in Test 1. The stripe was immediately heated by a hair drier for 10 seconds, followed by immediate removal of the stripe. A visual check was done to see some kind of "void" image is created.

[0116] The results are documented in Figure 7.

[0117] As can be seen from Figures 5 to 7, successful use of the HMPSA occurred at room temperature, as well as low and high temperatures. More specifically, it can be seen that every test resulted in residue of HMPSA being left behind after the stripe was removed, thus providing evidence of tampering.

Claims

1. A security tape or security label comprising a substrate and a UV curable hot melt pressure sensitive adhesive (HMPSA), wherein the HMPSA comprises:

2. A security tape or security label according to claim 1 , wherein at least one of the photoinitiators is a copolymerizable photoinitiator, and the HMPSA is obtainable by a UV free radical crosslinking reaction through the copolymerizable photoinitiator which is copolymerized with the or each of the acrylic monomers.

3. A security tape or security label according to claim 2, wherein the copolymerizable photoinitiator comprises a C-C double bond, a UV reactive chromophore of aromatic ketone, and optionally a spacer containing ether, sulfide, urethane, urea, carbonate or siloxane functional groups or a combination thereof.

4. A security tape or security label according to claim 2 or 3, wherein the copolymerizable photoinitiator has the structural formula wherein:

Z is S, O, CH₂, or NH,

RI-8 are independently H, Cl, Br, I, F, C1-24 alkoxy, C1-24 alkyl, or C1-24 aryl; and wherein at least one of RI_₈ must comprise at least one — W— X— Y group, wherein:

X is a covalent bond, or a chemical divalent link of carbonate, urethane, urea, or tetramethyldisiloxane, and Y is -Rg-C(Rio)=CH2, where Rg is OOC, or a linear or branched C1-24 alkylene, C1-24 arylene, OC1-24, NHC1-24, or SC1-24, wherein R_w is H or CH3.

5. A security tape or security label according to claim 1 , wherein at least one of the acrylic monomers has a cationic reactive functional group, and at least one photoinitiator is a cationic photoinitiator and the HMPSA is obtainable by a cationic crosslinking reaction of the cationic reactive functional group with a UV-generated acid from the or each of the cationic photoinitiators.

6. A security tape or security label according to claim 5, wherein the acrylic monomer with the cationic reactive functional group has the structural formula wherein

R¹ is O, S, C=O, or linear, branched, or cyclic alkylene, or oxyalkylene, arylene, R² is linear, branched, and cyclic alkyl or alkoxy, aryl, H, halogen, C=O, or part of

R¹ as fused cycloaliphatic ring through a covalent bond connection, R³ is (CH₂)n, n = 0-3,

Y is -R⁴-C(R⁵)=CH2, where R⁴ is a linear or branched C2-10 alkylene, C2- oxyalkylene, C=O, or arylene or derivative thereof, and R⁵ is H or CH3.

7. A security tape or security label according to claim 5 or 6, wherein the cationic photoinitiator is a sulfonium salt having the structure where R is C3H7, C12H25, and W is S, SO, SO₂ or CO.

8. A security tape or security label according to any one preceding claim, wherein the or each acrylic monomer is selected from the group consisting of: i) at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH₂=C(RA)(COORB), where RA is H or CH₂, and RB is an optionally substituted, linear or branched C1-1000 alkyl chain, and/or ii) at least one (meth)acrylic acid monomer, and/or iii) an acrylic monomer that forms, alone or in combination with one or more different monomers, the UV-curable acrylic polymer having a glass transition temperature between -10° C and -30° C.

9. A security tape or security label according to claim 8, wherein the or each acrylic monomer comprises or consists of i) at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH₂=C(R^A)(COOR^B), wherein R^B is an unsubstituted C1-1000 alkyl chain.

10. A security tape or security label according to claim 9, wherein the alkyl chain contains at least one functional group selected from the group consisting of a hydroxyl group and a carboxyl group.

11. A security tape or security label according to claim 9 or 10, wherein R_B is a branched C1-1000 alkyl chain.

12. A security tape or security label according to any one of claims 8 to 11 , wherein monomer i) comprises or consists of n-butyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, isodecyl acrylate, octyldecyl acrylate, caprolactone acrylate, or methyl acrylate, or a combination thereof.

13. A security tape or security label according to claim 12, wherein monomer i) comprises or consists of 2-ethylhexyl acrylate.

14. A security tape or security label according to any one of claims 8 to 13, wherein monomer ii) comprises or consists of acrylic acid or p-carboxyethyl acrylate (P-CEA), or a combination thereof, preferably consists of acrylic acid.

15. A security tape or security label according to any one preceding claim, wherein the or each acrylic monomer comprises polyethylene glycol (EO)n acrylic monomers, polyethylene/butylene mono-acrylic monomers, and/or siloxane containing acrylic monomers.

16. A security tape or security label according to any one preceding claim, wherein the or each acrylic monomer comprises 2-(2-ethoxyethoxy) ethyl acrylate, methoxy polyethylene glycol mono(meth)acrylate, or a combination thereof.

17. A security tape or security label according to any one of claims 8 to 16, wherein the or each acrylic monomer comprises or consists of i) at least one monomer selected from an acrylic or methacrylic acid derivative of the formula CH2=C(R^A)(COOR^B) as defined in claim 8, in an amount ranging from 50% to 99%, by weight of the composition, preferably from 70% to 99% by weight of the reaction composition, more preferably from 80 to 99% by weight of the reaction composition, most preferably from 93 to 98% by weight of the reaction composition.

18. A security tape or security label according to any one of claims 8 to 17, wherein the acrylic monomer consists of two of i), ii) or iii), preferably consists of i) and ii).

19. A security tape or security label according to any one preceding claim, wherein the at least one (meth)acrylic acid monomer ii) is present in an amount ranging from 0.5% to 10% by weight of the composition, preferably from 1% to 8% by weight of the composition, more preferably from 2% to 6% by weight of the composition.

20. A security tape or security label according to any one preceding claim, wherein the at least one photoinitiator is present in an amount ranging from 0.01% to 5%, by weight of the reaction composition, preferably from 0.1% to 0.5% by weight of the reaction composition, more preferably from 0.1% to 0.4% by weight of the reaction composition.

21. A security tape or security label according to claim 4, wherein the copolymerizable photoinitiator is where n = 1.

22. A security tape or security label according to claim 4, wherein the copolymerizable photoinitiator is where n = 1.

23. A security tape or security label according to claim 4, wherein the copolymerizable photoinitiator is

24. A security tape or security label according to claim 4, wherein the copolymerizable photoinitiator is

25. A security tape or security label according to any one preceding claim, where the Tg of the UV-curable acrylic polymer is from -60°C to 20°C, preferably from -60°C to -30°C.

26. A security tape or security label according to any one preceding claim, wherein the weight average molecular weight (Mw) of the UV-curable acrylic polymer is from 50,000 to 2,000,000 g/mol.

27. A security tape or security label according to any one preceding claim, wherein the viscosity of the UV-curable acrylic polymer ranges from 45,000 to 90,000 mPas at 130°C, preferably from 50,000 to 70,000 mPas at 130°C.

28. A security tape or security label, wherein the security tape or security label comprises a substrate with the UV curable HMPSA as defined in any one of the preceding claims applied thereon.

29. A security tape or security label comprising a substrate and a UV cured hot melt pressure sensitive adhesive (HMPSA), wherein the HMPSA comprises:

1) a UV-cured acrylic polymer that is a reaction product of a reaction composition comprising one or more acrylic monomers and one or more photoinitiators, and

2) optionally one or more additional ingredients, wherein said acrylic polymer is crosslinked through said photoinitiator groups upon exposure to ultra violet light.

30. A method of making a security tape or security label, the method comprising:

2) removing any solvents and any volatiles from the UV-curable acrylic polymer,

31. A method according to claim 30, wherein, after step 4), the UV curable hot melt pressure sensitive adhesive (HMPSA) is cured using a UV-source.