WO2020126753A1

WO2020126753A1 - Aqueous adhesive layer

Info

Publication number: WO2020126753A1
Application number: PCT/EP2019/084691
Authority: WO
Inventors: Hubertus Van Aert; Jessica MAURIELLO; Nick VALCKX
Original assignee: Agfa-Gevaert Nv
Priority date: 2018-12-18
Filing date: 2019-12-11
Publication date: 2020-06-25

Abstract

An aqueous adhesive layer comprising a first and a second binder, characterized in that the first binder includes a PVDC and the second binder includes an acrylate having a glass transition temperature between 25°C and 120°C.

Description

Aqueous adhesive layer

Technical field of the Invention

[001] The invention relates to aqueous adhesive layers, in particular aqueous adhesive layers for PVC and PET substrates.

Background art for the invention

[002] Adhesive layers, also referred to as primers, are applied on various substrates to improve the adhesion of coatings, foils, sheets, etc. towards that substrate.

[003] Security cards, such as ID cards, drivers licenses, credit cards, etc. typically consist of an overlay laminated on a core support. The core support may carry visible information, such as alphanumeric information, logos and a picture of the card holder, and optionally also digital information stored in a magnetic strip or in an electronic chip (so-called smart card). Also information, which become visible upon exposure to radiation, UV or IR, may be provided. After lamination of the overlay on the core support, information may also be provided on the overlay.

[004] A principal objective of security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original. Such modification usually involves de-laminating the overlay from the core support, altering the visible information on the card and relaminating, i.e. closing again, the card.

[005] The most widely used material in security cards is PVC (polyvinyl chloride), because of its low cost and its ease of printing and laminating. The biggest disadvantage of a PVC card body is the low durability, resulting in an effective lifetime of only 1 -3 years, much lower than the lifetime of the often expensive electronic chips. PC (polycarbonate) can be used for longer-life and more secure ID cards, but has a high production cost and a low resistance to torsion, scratching and chemicals. Other materials occasionally used for security card, like Teslin™ and ABS are only suitable for very low-end or single-use cards.

[006] PET-G (Polyethylene Terephthalate Glycol) is a kind of amorphous polyester which has not been toughened like crystalline polyester but rather processed to become more compatible with standard PVC card manufacturing. Durability of PET-G cards is comparable to that of PVC cards.

[007] There is growing interest for crystalline polyethylene terephthalate (PET-C) especially for the overlay in security cards. PET-C is a material that is very durable and resistant to mechanical influences (flexion, torsion, scraicnesj, chemical substances, moisture and temperature ranges. Untreated PET-C cannot be sealed or laminated to itself or to other materials, but requires additional adhesive coatings and layers to accomplish this.

[008] To avoid counterfeiting it is thus very important that the overlay has a very good adhesion to the core.

[009] As mentioned above, the overlay is typically laminated onto a core material, which is partly or even completely covered with ink. The adhesion of the overlay to such a core covered with ink may be different compared to the adhesion to a core having no ink on its surface.

[010] W02009/063058 (AGFA GEVAERT) discloses an overlay comprising a biaxially oriented polyester substrate and adhesion layer. The adhesion layer includes a polyester, a polyester-urethane or a copolymer of a chlorinated ethylene.

Although these overlays adhere well to a PVC, PC and PET-G core, they lose their adhesive properties when the core surface is printed completely or to a large extent with ink. Therefore a need exists to provide overlays based on crystalline polyethylene terephthalate (PET-C) that adhere well to both printed and unprinted PVC, PC and PET-G cores without requiring an extra ink receiving layer on the core body.

[011] EP-A 2567812 (AGFA GEVAERT) disclose an overlay comprising an adhesive layer of which the binder is a mixture of a first and a second polymer, wherein the first polymer is a copolymer comprising vinylacetate and vinylchloride and the second polymer is a copolymer of styrene, butadiene and methylmethacrylate or a copolymer of vinyl butyral, vinyl acetate and vinyl alcohol.

[012] Most prior art adhesive layers are solvent based layers, i.e. they are coated from solvent based compositions

[013] Aqueous adhesive layers, i.e. adhesive layers which are coated from an aqueous composition, are preferred for Health and Safety considerations. Also, drying efficiency of aqueous adhesive layers is typically higher compared to solvent borne adhesive layers. This results in more cost effective manufacturing of such aqueous adhesive layers.

[014] It is however often difficult with aqueous adhesive layers to combine good

adhesion properties with a sufficient water resistance. Summary of the invention

[015] It is an object of the present invention to provide an aqueous adhesive layer that has good adhesion properties and water resistance.

[016] The object of the invention is realized by the adhesive layer as defined in claim 1.

[017] It is another object of the invention to provide a security card that is difficult to counterfeit.

[018] That object is realized by the security card as defined in claim 12.

[019] Further objects of the invention will become apparent from the description

hereinafter.

Brief description of the drawings

[020] Figure 1 shows a schematic representation of an embodiment of a security

document according to the invention.

[021] Figure 2 shows a schematic representation of another embodiment of a security document according to the invention.

Detailed description of the invention

Definitions

[022] The terms“support” and“foil”, as used in disclosing the present invention, mean a self-supporting polymer-based sheet, which may be associated with one or more subbing layers. Supports and foils are usually manufactured through (co-)extrusion of polymer(s).

[023] The term“layer”, as used in disclosing the present invention, is considered not to be self-supporting and is manufactured by coating it on a support or a foil.

[024] The term“lamella”, as used in disclosing the present invention, includes one or more foils and one or more layers.

[025] “PET” Is an abbreviation for polyethylene terephthalate.

[026] “PET-G” is an abbreviation for polyethylene terephthalate glycol, the glycol

indicating glycol modifiers which are incorporated to minimize brittleness and premature aging that occur if unmodified amorphous polyethylene terephthalate (APET) would be used in the production of cards.

[027] “PET-C” is an abbreviation for crystalline PET, i.e. an oriented polyethylene

terephthalate. Such a polyethylene terephthalate support has excellent properties of dimensional stability. [028] The definitions of security features correspond with the normal definition as adhered to in the“Glossary of Security Documents - Security features and other related technical terms” as published by the Consilium of the Council of the European Union on August 25, 2008 (Version: v.10329,02.b.en) on its website: http://www.consilium.europa.eu/prado/EN/glossaryPopup.html.

Adhesive layer

[029] The adhesive layer according to the present invention is an aqueous adhesive layer comprising a first and a second binder, characterized in that the first binder is polyvinylidene (PVDC) and the second binder is an acrylate having a glass transition temperature (Tg) between 25°C and 100°C.

[030] An aqueous adhesive layer is an adhesive layer provided from an aqueous

composition.

[031] An aqueous composition includes water as main solvent. The aqueous

composition preferably contains at least 70 wt% of water, more preferably at least 80 wt%, most preferably at least 90 wt% of water relative to the total weight of all solvents of the composition.

[032] As the adhesive layer is prepared from an aqueous composition, the PVDC and acrylate are preferably provided to the composition as a latex. A latex as used herein is an aqueous dispersion of a polymer particle.

[033] The adhesive layer may further comprise other ingredients as long as it does not deteriorate its adhesion properties. Such ingredients include, for example surfactants to enhance the quality of the coating or colorants to provide an aesthetical or functional purpose. The adhesive layer may also comprise colloidal particles preferably being inorganic or organic colloidal particles. Preferred inorganic colloidal particles are colloidal silica particles. Suitable organic particles are crosslinked polystyrene or polymethylmethacrylate particles.

[034] The adhesive layer may also comprise UV blocking agents, thickeners, antistatic agents, biocides, light stabilizers, other binders, solvents, etc.

[035] The aqueous composition may comprise other solvents. A particularly preferred solvent is 2,5,7,10 T etraoxaundecane (TOU). It has been observed that the addition of TOU may improve the adhesion. The ratio of the amounts of water and TOU is preferably 85wt%/15wt%, more preferably 90wt%/1 Owt%, most preferably 95wt%/5wt% all relative to the total weight of the solvents.

[036] The adhesive layer is preferably provided on a support by coating. Any

conventional coating technique, such as dip coating, knife coating, extrusion coating, spin coating, slide hopper coating and curtain coating may be used for this purpose.

[037] However, adhesive layer may also be provided on a support using a spraying or a jetting device, such as an inkjet print head. Using an inkjet print head allows the composition to be provided according to a pattern or image. The composition may also be provided using a screen printing device.

[038] The coated adhesive layer is then preferably dried to remove water and other solvents from the layer. In the drying process, heat is supplied to evaporate the water from the coated layer. Also, when the drying temperature is above the film forming temperature of the latex particles, these will coalesce to form a

continuous film.

[039] The drying temperature may be optimized to obtain maximal adhesion properties.

Preferred drying temperatures are between 40 °C and 160 °C.

[040] In the manufacturing process of a support, the adhesive layer can be applied inline or offline. In an online process, there is the opportunity to carry out, after coating the adhesive layer onto the support, a stretching of the support including the adhesive layer. For example, when the support is a biaxially stretched PET film (BOPET film), the adhesive layer may be applied after a first stretching, preferably the longitudinal stretching, but before the second stretching, preferably the transversally stretching.

[041] The adhesive layer may also be applied inline after biaxially stretching the

support.

[042] The dry coating weight of the adhesive layer is preferably between 1 g/m² and 30 g/m². For offline coatings the dry coating weight is preferably between 5 and 30 g/m², for inline coatings the dry coating weight is preferably between 1 and 10 g/m².

PVDC

[043] Polyvinylidenechloride (PVDC) as used herein may be a homopolymer of

vinylidenechloride but is preferably a copolymer of vinylidenechloride with another monomer.

[044] PVDC may be susceptible to yellowing upon exposure to daylight and/or UV light.

Such yellowing has to be avoided in security cards.

[045] This yellowing may be linked to different aspects of PVDC, i.e. the crystallinity, the glass transition temperature and the copolymer composition. [046] Crystalline PVDC is preferably used because it is less susceptible to yellowing when compared to more amorphous PVDC. The crystallinity can be measured by Differential Scanning Calorimetry (DSC) or Fourier Transform Infrared

spectroscopy (FTIR).

[047] The crystallinity of PVDC may also be characterized by a so-called Crystallinity Index as described in WO2017/157963 paragraph [0024] Such a Crystallinity Index (C.l.) of the PVDC latex is preferably higher than 1 .1 , more preferably at least 1.2, most preferably at least 1 .3.

[048] A more crystalline PVDC typically has less chain imperfections, which may

causes chain branching, and therefore a lower amount of tertiary chlorine atoms. A lower amount of tertiary chlorine atoms may lead to less dehydrochlorination and therefore less double bond formation, which may be the reason for yellowing. The susceptibility towards yellowing is therefore in most cases linked to a so- called dehydrochlorination constant, for example measured according to

DIN53381 or ISO 182-3:1993.

[049] The influence of the presence of tertiary chlorine atoms on the stability of PVC is widely described, for example in“PVC Degradation and Stabilization”, George Wypych, 3rd Edition (ISBN9781895198850). The same principles apply for PVDC.

[050] The PVDC preferably has a high glass transition temperature (Tg) because a higher Tg may result in less mobility of the polymer and therefore a lower susceptibility towards yellowing. The PVDC preferably has a Tg of at least 12°C, more preferably from 13 to 28°C, most preferably from 17 to 20°C.

[051 ] The yellowing of PVDC based coatings may also be influenced by the PVDC copolymer composition. The copolymer composition may influence, for example, the water uptake and oxygen barrier properties of the coatings, both of which may influence the formation of double bonds and therefore the yellowing.

[052] As a conclusion, the yellowing of PVDC copolymers may be influenced by many parameters, such as crystallinity, glass transition temperature and copolymer composition. Moreover, these parameters may influence each other.

[053] For example the presence of a comonomer may decrease the crystallinity of the polymer dependent on the copolymerisation parameters of the comonomer with vinylidene chloride. A comonomer, which forms a more random or statistical distribution with the vinylidene chloride segments, give generally shorter segments of polyviny!idene chloride resulting in a lower crystallinity. A comonomer which gives a more blocky or sequential structure will resun in a higher crystallinity.

[054] The PVDC used in the present invention is preferably a copolymer comprising, besides a vinylidenechloride monomer, another monomer selected from the group consisting of an acrylate (for example methyl acrylate, methyl

methacrylate, etc.), acrylonitrile, vinyl acetate and a carboxylated monomer (such as acrylic acid, itaconic acid, maleic acid, etc.). The PVDC is more preferably a copolymer comprising a vinylidenechloride monomer and methylacrylate.

[055] Examples of such PVDC latexes that are commercially available are the Daran® and Serfene® latexes from Owensboro Specialty Polymers, PVDC Latex grades from Asahi Kasei, Diofan® latexes from Solvay Speciality Polymers and

Permax® latexes from Lubrizole Performance Coatings.

[056] As mentioned above, the adhesion properties of the aqueous adhesion layer is of major importance. In many cases however PVDC properties that have a positive influence on yellowing may have a negative influence on the adhesion properties. For example, the adhesion properties of more crystalline PVDC is typically worse than those of more amorphous PVDC, probably due to a higher chain mobility of more amorphous PVDC. The same apply for PVDC that have a high glass transition temperature (Tg).

[057] It is thus very difficult to combine good yellowing and adhesion properties of PVDC based coatings.

[058] It has now been found that coatings based on a combination of a PVDC latex having sufficient yellowing properties with specific acrylic copolymers are characterized by good yellowing and adhesion properties.

Acrylate

[059] The acrylate has a glass transition temperature from 25°C to 120°C, more

preferably from 30°C to 100°C, most preferably from 40°C to 70°C.

[060] The acrylate is preferably an ethyl acrylate - methyl methacrylate copolymer.

[061] By varying the ethyl acrylate/methyl acrylate ratio, the Tg of the acrylate

copolymer may be optimized to fall in the 25°C to 100°C region.

[062] A preferred acrylate is a ethyl acrylate - methyl methacrylate copolymer wherein the ratio of methyl acrylate to ethyl acrylate is at least 50wt%/50wt%, more preferably at least 60wt%/40wt%, most preferably 70wt%/30wt%.

[063] The acrylate is preferably prepared by emulsion polymerization using anionic and/or non-ionic surfactants. When using copolymerizable monomeric surfactants, also referred to as surfmers, an improved water resistance and wet adhesion may be obtained.

[064] It seems that the addition of the acrylate to the adhesive layer slows down the crystallization speed of the PVDC. This may explain why using a PVDC having a low amount of tertiary chlorine atoms, which would typically result in fast crystallization, in combination with the acrylate latex results in a sufficient adhesion.

[065] An advantage of an acrylate is its good UV stability, its low cost and its ease to apply.

[066] However, other water based polymers such as vinyl chloride copolymers,

polyurethane copolymers, polyurethane-acrylic hybrid polymers, polyesters, polyamides, polyolefin based polymers, or chlorinated polyolefine based polymers may also be used to slow down the crystallization speed of the PVDC in a coating.

Stabilizers

[067] The adhesive layer may further include so called stabilizers, which slow down the PVDC dehydrochlorination reaction described above. Preferred stabilizers are those disclosed in“PVC Degradation and Stabilization”, George Wypych, 3rd Edition (ISBN9781895198850).

Laser additives

[068] Laser marking, i.e. writing information with a laser, is often used to provide

security information on a security card. Preferably the information is laser marked inside the security card making it more difficult to counterfeit.

[069] When the support of the overlay is transparent for the laser radiation, laser

marking may be carried out in the adhesive layer.

[070] The adhesion layer may contain a laser additive, which renders the security document more sensitive to laser radiation. A preferred laser radiation used to laser mark is infrared radiation, more preferably near infrared laser radiation.

[071] It is however important that the laser additive does not impart unwanted

background colouration to the security document. This may realized by using only small amounts of the laser additive and/or selecting laser additives that has minimal absorption in the visible region of the spectrum.

[072] Suitable laser additives include antimony metal, antimony oxide, carbon black, mica (sheet silicate) coated with metal oxides and tin-antimony mixed oxides. In WO 2006/042714, the dark coloration of plastics is obtained by the use of additives based on various phosphorus-containing mixed oxides of iron, copper, tin and/or antimony.

[073] Suitable commercially available laser additives include mica coated with

antimony-doped tin oxide sold under the trade name of Lazerflair™ 820 and 825 by MERCK; copper hydroxide phosphate sold under the trade name of

Fabulase™ 322 by BUDENHEIM; aluminium heptamolybdate sold under the trade name of AOM™ by HC STARCK; and antimony-doped tin oxide pigments such as Engelhard Mark-it™ sold by BASF.

[074] In a preferred embodiment the laser additive is carbon black. This avoids the use of heavy metals in manufacturing these security documents. Heavy metals are less desirable from an ecology point of view and may also cause problems for persons having a contact allergy based on heavy metals.

[075] Suitable carbon blacks include Special Black 25, Special Black 55, Special Black 250 and FarbrussT^M FW2V all available from EVONIK; Monarch™ 1000 and Monarch™ 1300 available from SEPULCHRE; and Conductex™ 975 Ultra Powder available from COLUMBIAN CHEMICALS CO.

[076] The use of carbon black pigments as laser additives may lead to an undesired background colouring of the security document precursor. For example, a too high concentration of carbon black in the adhesive layer of the security document having a white core may result in grey security documents. For that reason it is preferred to use carbon black particles having a numeric average particle size smaller than 300 nm, preferably between 5 nm and 250 nm, more preferably between 10 nm and 100 nm and most preferably between 30 nm and 60 nm. The average particle size of carbon black particles can be determined with a

Brookhaven Instruments Particle Sizer BI90plus based upon the principle of dynamic light scattering.

[077] Infrared absorbing dyes having substantial no absorption in the visible region may also be used as laser additives. Such dyes, as disclosed in for example WO2014/057018 (Agfa Gevaert), are particular suitable for use with a NIR laser, for example with a 1064 nm laser.

Overlay

[078] In a preferred embodiment, the overlay (10) comprises an adhesive layer (100) as described above provided on a support (200). [079] The overlay according to this embodiment is then preferably laminated on a core (300) thereby forming the security card.

[080] The overlay may be laminated on one or both sides of the core, as shown in Figure 2.

[081] In another embodiment, the adhesive layer (100) containing the halogenated co- or homopolymer is coated directly on the core. The coating composition of the adhesive layer can be coated using any conventional coating technique, such as dip coating, knife coating, extrusion coating, spin coating, slide hopper coating and curtain coating on the support.

[082] Another layer or foil may then be applied on the coated adhesive layer.

Support

[083] The overlay preferably comprises a support (Figure 2, 100), more preferably a transparent polymeric support.

[084] Suitable transparent polymeric supports include cellulose acetate propionate or cellulose acetate butyrate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyamides, polycarbonates, polyimides, polyolefins, polyvinylchlorides, polyvinylacetals, polyethers and polysulphonamides.

[085] Examples of suitable polyester substrates are based on polyesters like

polyethylene terephthalate (PET), polyethylene naphthalate (PEN),

(co) polyesters based on cyclohexyldimethanol (CHDM) (PETG), (co)polyesters based on 2 , 5-f u rand icarboxyl ic acid (FDCA) (PEF), copolyesters based on isosorbide and polylactic acid (PLA).

[086] The support is preferably an oriented polyester support. Orienting a polyester support is achieved by stretching the support in a longitudinal direction, a transversal direction or both directions. The highest crystallinity of the polyester support is obtained by biaxially stretching.

[087] The polyester is preferably biaxially stretched with a stretching factor of at least 2.0, more preferably at least 3.0 and most preferably a stretching factor of about 3.5. The temperature used during stretching is preferably at least 100°C, more preferably at least 140°C and most preferably about 160°C.

[088] The oriented polyester support is preferably a polyethylene terephthalate or a polyethylene napthalate support.

[089] In the most preferred embodiment, the oriented polyester support is a biaxially stretched polyethylene terephthalate support. Such a polyethylene terephthalate support has excellent properties of dimensional stability and is very durable and resistant to scratches and chemical substances.

[090] The biaxially stretched polyethylene terephthalate substrate should be sufficiently thick to be self-supporting, but thin enough to be flexed, folded or creased without cracking. Preferably, the biaxially stretched polyethylene terephthalate substrate has a thickness of between about 7 pm and about 100 pm, more preferably between about 10 pm and about 90 pm, most preferably between about 25 pm and about 80 pm.

[091] The manufacturing of polyester foils and supports is well-known in the art of preparing suitable supports for silver halide photographic films. For example,

GB 81 1066 (ICI) teaches a process to produce biaxially oriented polyethylene terephthalate (BOPET) foils and supports.

[092] The support preferably comprises subbing layers to improve the adhesion

between the support and layers provided thereon.

Subbing Layers

[093] The support (100) preferably comprises subbing layers to improve the adhesion between the support and layers provided thereon.

[094] Useful subbing layers for this purpose are well known in the photographic art and include, for example, polymers of vinylidene chloride such as vinylidene chloride/acrylonitrile/acrylic acid terpolymers or vinylidene chloride/methyl acrylate/itaconic acid terpolymers.

[095] Suitable vinylidene chloride copolymers include: the copolymer of vinylidene chloride, N-tert.-butylacrylamide, n-buty! acrylate, and N-vinyl pyrrolidone

(e.g.70:23:3:4), the copolymer of vinylidene chloride, N-tert.-butyiacrylamide, n- butyl acrylate, and itaconic acid (e.g. 70:21 :5:2), the copolymer of vinylidene chloride, N-tert.-butylacrylamide, and itaconic acid (e.g. 88:10:2), the copolymer of vinylidene chloride, n-butylmaleimide, and itaconic acid (e.g. 90:8:2), the copolymer of vinyl chloride, vinylidene chloride, and methacrylic acid (e.g.

65:30:5), the copolymer of vinylidene chloride, vinyl chloride, and itaconic acid (e.g. 70:26:4), the copolymer of vinyl chloride, n-butyl acrylate, and itaconic acid (e.g. 66:30:4), the copolymer of vinylidene chloride, n-butyl acrylate, and itaconic acid (e.g. 80: 18:2), the copolymer of vinylidene chloride, methyl acrylate, and itaconic acid (e.g.90:8:2), the copolymer of vinyl chloride, vinylidene chloride, N- tert.-butylacrylamide, and itaconic acid (e.g. 50:30:18:2). Ail the ratios given between brackets in the above-mentioned copolymers are ratios by weight. [096] In a preferred embodiment, the support is provided with a subbing layer inciuamg a copolymer selected from the group consisting of a hydroxyl-functional, partially- hydrolyzed vinyl chloride/vinyl acetate copolymer and a polyester-urethane.

[097] In a particular preferred embodiment, the support is provided with a subbing layer including a binder based on a polyester-urethane copolymer.

[098] In a more preferred embodiment, the polyester-urethane copolymer is an ionomer type polyester urethane, preferably using polyester segments based on terephthalic acid and ethylene glycol and hexamethylene diisocyanate.

[099] Suitable polyester-urethane copolymers are Hydran APX101 H, Hydran AP40N and Hydran AP20, all from DIC Europe GmbH or mixtures of different polyester- urethanes or mixtures of polyester-urethanes with other polymers.

[0100] The application of subbing layers is well-known in the art of manufacturing

polyester supports for silver halide photographic films. For example, the preparation of such subbing layers is disclosed in US3649336 (AGFA) and GB 1441591 (AGFA).

[0101] In a preferred embodiment, the subbing layer has a dry thickness of no more than 2 pm or preferably no more than 200 mg/m2.

[0102] A preferred method of providing the subbing layers on the support is disclosed in EP-A 2374602 an EP-A 2567812 both from Agfa Gevaert.

[0103] A preferred method comprises the steps of a) stretching a polyester substrate in either a longitudinal or a transversal direction; b) coating and drying a subbing layer on the stretched polyester substrate ; c) stretching the coated polyester substrate in the longitudinal or transversal direction not selected in step a) in order to obtain a coated biaxially stretched polyester substrate having a subbing layer.

Outer layer

[0104] The overlay may also comprise an outer layer provided at a side of the support opposite to the side of the support upon which the adhesive layer is provided.

[0105] Such an outer layer is preferably an ink receiving layer or a receiver layer for Dye Diffusion Thermal Transfer (D2T2) printing.

[0106] The presence of such a layer enables the addition of information or other security information to the security document by for example inkjet printing or D2T2 printing. Laser markable Saver

[0107] To provide laser marking properties to the overlay, an additional laser markable layer may be provided. Such a laser markable layer is preferably provided on the adhesive layer.

[0108] Any laser markable composition may be used to form the laser markable layer.

[0109] According to one embodiment, the laser markable layer is capable of forming a black colour upon exposure to infrared radiation by carbonization of ingredients, typically the binder, of the laser markable layer.

[01 10] Such a laser markable layer, disclosed in for example EP-A 2335967, preferably comprises polymers selected from polycarbonate (PC), polybutylene

terephthalate (PBT), polyvinyl chloride (PVC), polystyrene (PS) and copolymers thereof, such as e.g. aromatic polyester-carbonate and acrylonitrile butadiene styrene (ABS). An optothermal converting agent, which absorbs infrared radiation and converts that radiation into heat, may be added to the laser markable layer to increase the marking density upon exposure to such infrared radiation.

[01 11] Other laser markable compositions that may be used are those disclosed in for example W02002/074548, comprising a binder and an oxyanion of a multivalent metal, such as ammonium octamolybdate (AOM), which may be laser marked using a CO2 laser; W02006/018640 and WO2012/114121 , both comprising a diacetylene compound and which may be laser marked using a UV laser;

WO2007/141522 comprising a marking component, for example AOM, and a metal salt, for example reduced indium oxide, that absorbs laser irradiation at 780 to 2500 nm and may be laser marked using a NIR laser.

[01 12] Preferred laser markable compositions include a leuco dye. Such laser markable compositions are disclosed in for example EP-A 2648920. A leuco dye is a substantially colourless compound, which may react with for example a colour developing agent to form a coloured dye. The reaction may be triggered by exposure to laser irradiation. Depending on the type of leuco dyes, or mixture of leuco dyes, any colour may be obtained.

[01 13] The colour laser markable layers may comprise an optothermal converting agent such as an infrared absorbing dye (IR dye) or an infrared absorbing pigment (IR pigment), both absorbing the IR radiation and converting it into heat.

[01 14] Preferred laser markable compositions comprises a leucodye, an optothermal converting agent and a colour developing agent or colour developing agent precursor. The composition may further comprise an acid scavenger and a UV absorber. [01 15] Aqueous laser markable compositions, compared to solvent based compositions, are preferred for health and safety reasons.

[01 16] Aqueous laser markable compositions are disclosed in for example for example W02006/052842, W02008/030428 and WO2014/124052.

[0117] Particular preferred aqueous laser markable compositions are disclosed in the applications PCT/EP2016/061069 (filed 18-05-2016) and PCT/EP2016/060533 (filed on 11-05-2016).

Core

[0118] The security document comprises a core.

[0119] The core can be transparent, translucent or opaque.

[0120] The core is preferably opaque. The advantage of an opaque core, preferably of a white colour, is that any information of the security document is more easily readable and that a colour image is more appealing by having a white

background.

[0121] Suitable polymers for the core of the security document include cellulose acetate propionate or cellulose acetate butyrate, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyamides, polycarbonates, polyimides, polyolefins, polyvinyl chlorides, polyvinylacetals, polyethers and polysulphonamides.

[0122] Preferred polymeric cores are based on polycarbonate (PC), polyvinylchloride (PVC), and polyethylene terephthalate (PET).

[0123] Other preferred cores are based on so-called synthetic papers such as Synaps™ or Teslin® synthetic papers, respectively from Agfa Gevaert and Teslin.

[0124] The core may also be based on paper, such as polyethylene or propylene coated paper.

[0125] The core may be a single component extrudate, but can also be co-extrudate.

[0126] Examples of suitable co-extrudates are PET/PETG and PET/PC.

[0127] Instead of a coloured or whitened support, an opacifying layer can be coated onto a transparent support. Such opacifying layer preferably contains a white pigment with a refractive index greater than 1.60, preferably greater than 2.00, and most preferably greater than 2.60. The white pigments may be employed singly or in combination. Suitable white pigments include C.l. Pigment White 1 , 3, 4, 5, 6, 7, 10, 1 1 , 12, 14, 17, 18, 19, 21 , 24, 25, 27, 28 and 32. Preferably titanium dioxide is used as pigment with a refractive index greater than 1.60. Titanium oxide occurs in the crystalline forms of anatase type, rutile type and brookite type. In the present invention the rutile type is preferred because it has a very high refractive index, exhibiting a high covering power.

[0128] Methods to obtain opaque polyethylene terephthalate and biaxially oriented films thereof of have been disclosed in, e.g. US 2008238086 (AGFA).

Security Document

[0129] Preferred security documents are security cards, which are widely used for

various applications such as identification purposes (ID cards) and financial transfers (credit cards).

[0130] The security document may include an electronic chip and optionally an antenna.

[0131] In a preferred embodiment the security document is a so-called radio frequency identification card or RFID-card.

[0132] The security document may contain various security features, such as anti-copy patterns, guilloches, endless text, miniprint, microprint, nanoprint, rainbow colouring, 1 D-barcode, 2D-barcode, coloured fibres, fluorescent fibres and planchettes, fluorescent pigments, kinegrams™, overprint, relief embossing, perforations, metallic pigments, magnetic material, Metamora colours, microchips, RFID chips, images made with OVI (Optically Variable Ink) such as iridescent and photochromic ink, images made with thermochromic ink, phosphorescent pigments and dyes, OVD’s, watermarks including duotone and multitone watermarks, ghost images and security threads

[0133] A combination with one of the above security features increases the difficulty for falsifying a security document.

EXAMPLES

Materials

[0134] All materials used in the following examples were readily available from standard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS (Belgium) unless otherwise specified. The water used was deionized water.

[0135] DIOFAN® P530 is a PVDC latex available from Solvay.

[0136] DARAN® SL159 is a PVDC latex based on vinylidene chloride and methyl

acrylate, available from Owensboro Specialty Polymers.

[0137] Tivida® FL2500 is a fluorosurfactant available from Merck.

[0138] TOU is 2,5,7,10 Tetraoxaundecane, a solvent available from Lambiotte. [0139] Chemfac PB133 is an alkyl ether phosphate surfactant from Chemax Inc. Preparation of subbed polyester PET-1

[0140] A coating composition SUB-1 was prepared by mixing the components according to Table 1 using a dissolver.

Table 1

[0141] After stretching a 1100 pm thick polyethylene terephthaiate substrate

longitudinally, the coating composition SUB-1 was coated onto the longitudinally stretched PET and dried.

[0142] The coated longitudinally stretched PET was then transversally stretched to produce a 63 pm thick transparent and glossy subbed biaxially stretched polyethylene terephthaiate substrate PET-1.

[0143] The dry thickness of the subbing layer coated from SUB-1 was 211 mg/m².

Preparation of the acrylate latex LX-01 to LX-03

[0144] The acrylate latexes LX-01 to LX-03 include a methyl methacrylate (MM A) - ethyl acrylate copolymer having a varying methyl methylacrylate - ethyl acrylate ratio.

Preparation of LX-01 = polv(MMA/ethyl acrylate 30/70)

[0145] 8.10 g Chemfac PB133, 1.35 g NaHCCb and 651.15g of water were added to a 1 liter double jacketed reactor. The reaction mixture was heated under nitrogen atmosphere to 75°C. Then, 1.5 % of the MMA/ethylacrylate monomer mixture (1.89 g ethyl acrylate and 0.81 g of MM A) was added and the reaction mixture was kept at 75°C for 15 minutes. Then, 50% of the initiator solution was added to the reaction mixture (29.7 g of a 2% sodium persulfate solution in water; ana me reaction mixture was heated within 30 minutes to 80°C.

[0146] The polymerization of this so-called“seed” fraction results in the formation of a opalescent latex.

[0147] Then, the remaining part of the monomer (124.1 1 g of ethylacrylate and 53.19 g of MM A) was added during 180 minutes to the reaction mixture simultaneously with the remaining part of initiator solution (29.7 gr of the 2% sodium persulfate solution in water).

[0148] After the addition of the monomers and the initiator, the reaction mixture was kept at 80°C for 60 minutes. Then, the residual monomer was removed by distillation under reduced pressure during 90 minutes where the pressure is gradually reduced to 400 mBar. After cooling to room temperature, the reaction mixture was filtered over a coarse S & S paper.

[0149] The resulting latex had a concentration of 21.45 wt% and a particle size of 46 nm (measured with a Malvern Zetasizer Nano S).

Preparation of LX-02 = poly(MMA/ethyl acrylate 50/50)

[0150] LX-02 was prepared using the same method as for LX-01 but with different

MM A/ethyl acrylate amounts.

[0151] To prepare the“seed” fraction, 1.35 g of ethyl acrylate and 1.35 g of MMA was added.

[0152] Then, 88.65 g of ethylacrylate and 88.65 g of MMA was added to complete the reaction.

[0153] The resulting latex LX-02 had a concentration of 23.06 wt% and a particle size of 44 nm (measured with a Malvern Zetasizer Nano S).

Preparation of LX-03 = ( MM A/ethyl acrylate 30/70)

[0154] LX-03 was prepared using the same method as for LX-01 but with different

MM A/ethyl acrylate amounts.

[0155] To prepare the“seed” fraction, 0.81 g of ethyl acrylate and 1.89 g of MMA was added.

[0156] Then, 53.19 g of ethylacrylate and 124.11 g of MMA was added to complete the reaction.

[0157] The resulting latex LX-03 had a concentration of 23.25 wt% and a particle size of 42 nm (measured with a Malvern Zetasizer Nano S). Methods

Adhesion

[0158] An overlay (OL) was prepared by hand coating an adhesive layer at 45°C onto the subbed polyester PET-1 at a wet coating thickness of 30 mih. The coating was subsequently dried at 130°C during 15 minutes.

[0159] The overlay was then laminated onto a PVC or PETG core (thickness 350 pm) using an Oasys laminator or a Lauffer laminator. The lamination times, pressures and temperatures were optimized for the different cores and overlays. Both plain or preprinted cores (using UV waterless offset printer and Supra UV WL Process Inks) were used.

[0160] After lamination, the security cards were then cut in a standard format for ID cards.

[0161] Dry adhesion was tested through a simple knife resistance test: a pen knife was used to slice the overlay and to attempt to peel away the overlay from the core material. If this was successfully removed with no, or very little resistance a quantitative analysis of 0 would be given to the dry adhesion of that product. Should the overlay not be able to be detached or damaged, a grade of 1 would be awarded to the dry adhesion. A scale ranging from 0 to 1 was applied for those cards with some resistance or minor detachment.

[0162] This test was repeated after 24 hours water submersion. This would often result in easier delaminable cards. Finally, a further adhesion test carried out was hot plate delamination where the card was placed on a heated surface for a few seconds before the overlay was attempted to be removed following the same technique and quantification as outlined for the dry adhesion tests.

[0163] If the adhesion was good, a PVC core the surface of which was pre-printed with different colours and loadings of ink, was used to measure the adhesion as described above for the bare PVC core.

Crystallinity Index PVDC latex

[0164] The Crystallinity Index (C.l.) of the PVDC latex was determined by the method disclosed in WO2017/157963 paragraph [0024]. The C.l. was determined after annealing the spincoated samples at 48 hours for 45°C.

[0165] The C.l. of DARAN SL159 and DIOFAN P530 used in the examples was

respectively 1.47 and 1.09. The g!ass transition temperature PVDC latex

[0166] The glass transition temperature of the PVDC latex was measured using

Differential Scanning Calorimetry (DSC) using the following heating/cooling cycle: The sample was kept at -80°C for 5 minutes. Then, the temperature was raised to 150°C at a rate of 20°C per minute. The sample was kept at 150°C for 20 minutes and then slowly cooled to -80°C at a rate of 0.5°C per minute followed by heating it again to 150°C at a rate of 20°C/minute. The Tg was measured in the second heating run.

[0167] The sample was kept under nitrogen during the measurement.

Yellowing

[0168] Yellowing was evaluated by comparing the minimum density before and after 24 hours exposure to UV radiation.

Example 1

[0169] The adhesive layers COMP-01 to COMP-03 and INV-01 to INV-03 were coated from an aqueous coating solution as described above. The dry weight of the ingredients are shown in Table 2

Table 2

[0170] The adhesion of the adhesive layers towards PETG and PCV was evaluated as described above. The results are shown in Table 3 The coating quality of some adhesive layers was too bad (due to non-compatibility of the binders) to evaluate the adhesion. These are indicated in Table 3 as Not Applicable (NA). Table 3

Claims

1. An aqueous adhesive layer comprising a first and a second binder, characterized in that the first binder includes a PVDC and the second binder includes an acrylate having a glass transition temperature between 25°C and 120°C.

2. An adhesive layer according to claim 1 , wherein the glass transition temperature of the PVDC is at least 15 ^"C.

3. An adhesive layer according to claim 1 or 2, wherein the PVDC is a vinylidene chloride - methyl acrylate copolymer.

4. An adhesive layer according to any of the preceding claims, wherein the PVDC has a Crystallinity Index measured according to the method disclosed in the description that is higher than 1.1.

5. An adhesive layer to any of the preceding claims, wherein the acrylate is a copolymer of methylmethacrylate.

6. An adhesive layer according to any of the preceding claims, wherein ratio of the

acrylate content to the PDC content is from 1/9 to 9/1.

7. The adhesive layer according to any of the preceding claims further comprising a laser additive.

8. The adhesive layer according to claim 7 wherein the laser additive is carbon black.

9. An overlay comprising the adhesive layer as defined in any of the claims 1 to 8

provided on one side of a support.

10. The overlay according to claim 9 wherein the support is a biaxially oriented

polyethylene terephthalate.

1 1. The overlay according to claim 9 or 10 further comprising an outer layer on the other side of the support.

12. A security document comprising an overlay as defined in any of the claim 9 to 1 1

provided on a core.

13. The security document according to claim 12 wherein the core is a PVC or a PET-G core.

14. The security document according to claim 12 or 13 wherein a surface of the core is pre-printed.