PEEL-PROOF SECURITY FILM WITH HIGH TRANSPARENCY AND WITHOUT BREAKING POINTS

Description

Field of the invention

[0001] The present invention relates to peel-resistant security films made of a poly(meth)acrylate, in particular a polymethyl methacrylate, which contains a maximum of 10 wt% or no impact modifiers. These films can be available in highly transparent, translucent, or even white versions, are easy to print on, and can be applied without predetermined breaking points. The peel-resistant security films can thus be used in particular in documents such as chip cards or passports, in security labels such as vignettes, for theft protection of products, or as price tags. These security labels are also characterized by good weather resistance and, in particular, particularly good UV stability.

[0002] In particular, the present invention relates to the use of these security labels which have a thickness of at most 2 mm, contain at least 80 wt% polymethyl methacrylate and at most 10 wt% impact modifier particles or contain no impact modifier particles. State of the art

[0003] Security labels that cannot be removed non-destructively are used in a variety of applications. These include securing documents such as chip cards or passports, security labels such as vignettes, anti-theft labels for products, or price tags. A typical state-of-the-art chip card, for example, consists of up to 12 individual parts that are assembled and programmed in up to 30 steps. In addition to the carrier layer and the layer containing the magnetic stripe, separate layers are applied for each function. One or more layers are required to provide sufficient weather protection, scratch protection, and UV protection. A further layer, a security layer that cannot be removed non-destructively, is applied to maintain counterfeit protection.Often the print is also found on a separate layer, since the other layers listed are difficult to print on or only produce a poor print image.

[0004] State-of-the-art labels, such as those used as vignettes for gluing to glass panes, typically contain an optionally printable carrier layer made of PET, PVC, PE, or BOPP. A second layer, required for weather stabilization, is laminated to this layer on one side using an intermediate pressure-sensitive adhesive. This second layer, according to the state of the art, is typically made of polycarbonate, PET, or PVC. Since such layers may have limited brittleness solely for improved processability, the labels must be additionally structured, e.g., by cutting, to prevent non-destructive removal.

[0005] Pure PVC films are also available, especially as white films. While these have a desired low tear resistance, it can be observed that the path to complete break after tearing is relatively long. This means that a film that is minimally torn, for example, when lifting a security label, can under certain circumstances be removed from the substrate very carefully. A faster complete break would be more desirable in this case.

[0006] US 6,280,835 B1 discloses a thermoplastic brittle acrylic resin film consisting of one or two layers and in which no vinyl chloride resin is used. The acrylic resin film is made from a resin composition comprising 100 parts by weight of thermoplastic acrylic resin having an elongation at break of 10 to 180% and 10 to 70 parts by weight of at least one type of filler selected from the group consisting of quartz filler, aluminum hydroxide, barium sulfate, and calcium carbonate.

[0007] WO 2010 / 117771 A1 discloses a label comprising (a) a film having a first surface and a second surface and comprising a first film layer; and (b) an adhesive layer having a first surface and a second surface comprising an adhesive, wherein the first surface of the adhesive layer is adhesively bonded to the second surface of the film; (c) wherein the first film layer comprises at least one non-oriented thermoplastic polymer, wherein the thermoplastic polymer has a glass transition temperature in the range of about 40°C to about 190°C, and (i) a tensile modulus (ASTM D882) in the machine direction after heating that is less than the tensile modulus in the machine direction in ambient air, or (ii) an areal shrinkage (ASTM D1204) of at least about 5.3% after immersion in water at a temperature of about 80°C for at least about 3 minutes, or a combination of c(i) and c(ii). Task

[0008] In view of the state of the art, it was the object of the present invention to provide a method for producing peel-proof security films which can be realized without structuring with high transparency or a very good print image.

[0009] In particular, with regard to the state of the art, the task here was to provide a film for security labels that has both low tear resistance and a short tear path until the film breaks completely.

[0010] Furthermore, it was a particular object of the present invention to provide security films that can be manufactured and processed without tearing.

[0011] A further object of the invention was that the security films have particularly good yellow values ​​and an overall very high weather and UV resistance even after prolonged use or after weathering.

[0012] Furthermore, it was an object of the present invention to provide security films that help simplify the manufacturing process of chip cards.

[0013] Further objects underlying this invention may arise from the description, the prior art and the examples without being explicitly mentioned here. Solution

[0014] The objects underlying the invention are achieved through the novel use of polymethacrylate films as security labels. These polymethacrylate films are characterized in that the film was produced using an extrusion process; has a thickness between 20 and 75 µm and an elongation at break between 2.0% and 15% according to ASTM D1004; consists of 90 to 100 wt% of a polymethacrylate, which consists of 90 to 100 wt% of MMA and 0 to 10 wt% of acrylates with an alkyl radical having 1 to 4 carbon atoms; and that the film contains a maximum of 10 wt% or no impact modifiers.

[0015] The elongation at break is determined according to ISO 525, but can also be determined according to ASTM D1004.

[0016] As a rule, commercially available PMMA films, such as those used as weatherproof films, have an elongation at break of between 50% and 100%. It will be surprising to those skilled in the art that films with a significantly lower elongation at break can be used as security labels without special perforation. The elongation at break can be adjusted within the range of the invention with little effort by the skilled person. Various influencing factors can be varied to influence the elongation at break in the desired direction. For example, the elongation at break decreases, firstly, by increasing the glass transition temperature, which can be influenced by the precise monomer composition. In particular, a low proportion or the absence of acrylates or methacrylates with longer alkyl radicals of more than one carbon atom increases the glass transition temperature.

[0017] A second influencing factor is the type and especially the amount of impact modifiers. In particular, an increasing concentration of impact modifiers also increases the elongation at break, so that rather small amounts or the complete omission of impact modifiers contribute to an elongation at break according to the invention.

[0018] In the case of colored, especially white, films, pigments represent the third influencing factor. In addition to transparent versions, the films are preferably colored white. For this purpose, titanium dioxide, for example, is mixed into the matrix material of the film in concentrations of up to 40% by weight. The higher the titanium dioxide concentration, the lower the elongation at break, thus providing the expert with an additional influencing factor for white films.

[0019] These polymethacrylate films are characterized by the fact that the film consists of 90 to 100 wt% polymethacrylate. The poly(meth)acrylate consists of 90 to 100 wt% MMA and 0 to 10 wt% acrylates with an alkyl radical containing 1 to 4 carbon atoms.

[0020] The film has a thickness of between 20 and 75 µm. In particular, the film is distinguished—particularly in contrast to the prior art—by the fact that it contains no impact modifiers or contains a maximum of 10% by weight, preferably a maximum of 2% by weight. Impact modifiers are understood according to the invention to be particles having a soft phase by means of which the impact strength of films or molding compounds is improved. These are generally rubber particles or poly(meth)acrylate emulsion polymers. These emulsion polymers are generally present as core-shell or core-shell-shell particles, with the core or the middle shell representing a soft phase, usually with a high proportion of acrylates.

[0021] The embodiment of a transparent film preferably consists of 90 to 100 wt% polymethacrylate. The embodiment of a white security film preferably consists of 55 to 95 wt% polymethacrylate and preferably contains between 5 and 40 wt%, particularly preferably between 20 and 35 wt% titanium dioxide.

[0022] Detailed design of a preferred plant for producing the polymethacrylate films used according to the invention

[0023] The polymethacrylate film used in the invention is produced by an extrusion process. For this purpose, a system comprising at least the following components is used: an extruder, a melt pump, an optional melt filtration, an optional static mixing element, a flat film die, a polishing stack or a chill roll and a winder.

[0024] The polymethacrylate films used in the invention also exhibit particularly high resistance to weathering, especially to UV radiation. To further enhance this resistance, UV absorbers and / or UV stabilizers can be added to the polymethacrylate.

[0025] The polymethacrylate films used according to the invention can also be produced particularly easily and cost-effectively. This applies particularly to the preferred process for their production described above. Even during extrusion of the polymethacrylate films, the brittleness is so low, despite the lack of impact modifiers, that the risk of web breakage is further minimized and the film can be produced at high extrusion speeds. The polymethacrylate film can thus be produced using existing extrusion equipment and by known methods, preferably according to the extrusion equipment configuration described above.

[0026] Furthermore, the polymethacrylate films used in the invention have the advantage that they can be easily cut or punched to desired sizes. Particularly suitable methods for cutting the polymethacrylate film are laser cutting or laser punching.

[0027] The extrusion of polymers into films is widely known and is described, for example, in Kunststoffextrusionstechnik II, Hanser Verlag, 1986, p. 125 ff. Further embodiments of the individual system components are also described there.

[0028] In the process according to the invention, a hot melt is extruded from the extruder nozzle onto a gap between two smoothing rolls or onto a chill roll. The optimal melt temperature depends, for example, on the composition of the mixture and can therefore vary widely. Preferred temperatures of the PMMA molding compound up to the nozzle inlet are in the range of 150 to 300 °C, more preferably in the range of 180 to 270 °C, and most preferably in the range of 200 to 260 °C. The temperature of the smoothing rolls is preferably less than or equal to 150 °C, preferably between 60 °C and 140 °C.

[0029] In one embodiment, the nozzle temperature is preferably selected to be higher than the temperature of the mixture before entering the nozzle. The nozzle temperature is preferably set 10°C, more preferably 20°C, and most preferably 30°C higher than the temperature of the mixture before entering the nozzle. Accordingly, preferred nozzle temperatures are in the range from 160°C to 330°C, more preferably 190°C to 300°C.

[0030] The calender used consists, for example, of two or three smoothing rollers. Smoothing rollers are widely known in the art, with polished rollers being used to achieve a high gloss. However, in the process according to the invention, other rollers can also be used as smoothing rollers. These can be, for example, matte rollers. A film is formed through the gap between the first two smoothing rollers, which becomes a foil through simultaneous cooling. Fig. 1 schematically shows an embodiment with three smoothing rollers.

[0031] The alternative chill-roll rollers are also known to those skilled in the art. Here, the melt film is deposited on a single, cooled roller and transported further by it. Due to their simpler installation and operation, chill-roll rollers are preferred over polishing rollers for the production of the polymethacrylate films of the invention.

[0032] A particularly good surface quality of the polymethacrylate films can be ensured by the nozzle and roller having chrome surfaces, and especially by these chrome surfaces having a roughness Ra (according to DIN 4768) of less than 0.10 µm, preferably less than 0.08 µm.

[0033] To ensure that the resulting film is largely free of contaminants, a filter is optionally placed before the melt enters the die. The mesh size of the filter generally depends on the raw materials used and can therefore vary widely. It generally ranges from 300 µm to 20 µm. Filters with multiple screens of different mesh sizes can also be placed before the die inlet. These filters are commercially available. To obtain high-quality films, it is also advantageous to use particularly pure raw materials.

[0034] Optionally, a static mixing element can also be installed upstream of the flat film die. This allows components such as pigments, stabilizers, or additives to be mixed into the polymer melt, or up to 5 wt% of a second polymer, for example, in the form of a melt from a second extruder, can be mixed into the PMMA.

[0035] The pressure with which the molten mixture is forced into the die can be controlled, for example, by the screw speed. The pressure is generally in the range of 40 to 150 bar, without this limiting the process according to the invention. Accordingly, the speed at which the films can be obtained according to the invention is generally greater than 5 m / min, in particular greater than 10 m / min.

[0036] To ensure uniform melt flow, an additional melt pump can be installed in front of the flat film nozzle. Matrix materials used according to the invention

[0037] To produce the polymethacrylate films used according to the invention, molding compounds are used whose thermoplastic main component consists of at least 90% by weight, and particularly preferably at least 95% by weight, of polymethyl methacrylate (hereinafter referred to as PMMA). These polymers are generally obtained by radical polymerization of mixtures containing methyl methacrylate. These mixtures generally contain at least 90% by weight, and particularly preferably at least 95% by weight, of methyl methacrylate, based on the weight of the monomers. Security labels consisting essentially of polymethyl methacrylate, in particular, demonstrate particularly high quality.

[0038] These mixtures may also contain other (meth)acrylates that are copolymerizable with methyl methacrylate. The term (meth)acrylates includes methacrylates and acrylates, as well as mixtures of both.

[0039] In addition to the (meth)acrylates described above, the compositions to be polymerized may also contain other unsaturated monomers that are copolymerizable with methyl methacrylate and the aforementioned (meth)acrylates. These include, among others, 1-alkenes, acrylonitrile, vinyl acetate, styrene, substituted styrenes, or vinyl ethers. All of the above-mentioned monomers are preferably used in high purity.

[0040] The weight-average molecular weight M w of the homopolymers and / or copolymers can vary widely, with the molecular weight usually being tailored to the intended application and the processing method of the molding compound. However, it generally ranges between 20,000 and 1,000,000 g / mol, preferably between 50,000 and 500,000 g / mol, and particularly preferably between 80,000 and 300,000 g / mol, without any restriction being imposed. The weight-average molecular weight is determined by gel permeation chromatography (GPC) against polystyrene standards.

[0041] To produce the polymethacrylate films used according to the invention, various poly(meth)acrylates can be used as matrix materials, differing, for example, in molecular weight or monomer composition. Particularly preferred molding compounds of this type are commercially available under the trade name PLEXIGLAS® from Evonik Industries GmbH.

[0042] The molding compounds may contain conventional additives. These include, among others, antistatic agents, antioxidants, light stabilizers, and organic phosphorus compounds, weathering inhibitors, and plasticizers. However, the amount of additives is limited to the intended use. The security films according to the invention preferably comprise no more than 10% by weight, and more preferably no more than 2% by weight, of additives. Security films that comprise essentially no additives surprisingly demonstrate exceptional performance. The security labels

[0043] In addition to the described use, the applicable security films themselves are also part of the present invention. These are characterized by the film having a thickness between 20 and 75 µm and an elongation at break between 2.0% and 15%, preferably between 4% and 14%, and by the security film having at least the following layers in the specified order before application: a) a polymethacrylate film, produced by means of an extrusion process; which consists of 90 to 100 wt% of a polymethacrylate, which consists of 90 to 100 wt% of MMA and 0 to 10 wt% of acrylates with an alkyl radical having 1 to 4 carbon atoms, which is additionally characterized in that the film contains a maximum of 10 wt% impact modifier, and that it has a print on at least one side, b) a pressure-sensitive adhesive, c) a release coating and d) a carrier layer.

[0044] Layer a) preferably has a thickness without printing between 20 and 100 µm, preferably between 30 and 75 µm. The printing, in turn, generally has a thickness between 1.5 and 3 µm, while layer b) has a thickness between 20 and 30 µm, layer c) a thickness between 0.5 and 1.2 µm, and layer d) a thickness between 20 and 70 µm.

[0045] The specified thicknesses refer to the average of the smallest dimension of the respective layer measured perpendicular to the application surface. The thickness itself generally results from the respective application technology of the individual layers. For example, printing can be done using flexographic printing, digital printing, or even screen printing, while the pressure-sensitive adhesive can be applied using a roller or a doctor blade. For layers that are separate prior to the production of the security film, such as layers a) and d), in particular, the thickness of the layer can be determined using an underarm micrometer or similar known devices.

[0046] The embodiment of a transparent film preferably consists of 90 to 100 wt% of the polymethacrylate.

[0047] The extensions of the security labels in the other two dimensions are generally freely selectable, and their width is limited by the calender or extrusion die used for production. This means that the formats are virtually unlimited.

[0048] The polymethacrylate film is preferably cut by punching, cutting, laser cutting, or laser punching. Laser cutting or punching is particularly preferred.

[0049] In a special embodiment, the security film does not contain any impact modifiers.

[0050] Optionally, but not necessarily, the polymethacrylate films produced according to the invention can be additionally provided with grooves or notches to further prevent non-destructive removal of the labels. However, this is not actually necessary, since the polymethacrylate films used according to the invention as peel-resistant security labels cannot be removed non-destructively once they have been bonded to a surface.

[0051] The security films are primarily used for the production of chip cards, documents, security labels, other labels, or price tags. A prime example of their use is toll stickers, which are attached to the inside of vehicle windows. Examples Example 1

[0052] A molding compound composed of 98 wt% of a polymethyl methacrylate and 2 wt% impact modifier, in the form of a core-shell particle, is extruded to a film thickness of 50 µm under the following conditions: Screw temperature in the extruder: 240 to 270°C Die temperature: 240 to 260°C Melt temperature at the die: 240 to 260°C Roller temperature: 50 to 120°C

[0053] The material has a breaking elongation of 3%: Example 2

[0054] As in Example 1, except that 5% by weight of impact modifier was used. An elongation at break of 4.5% was measured. Comparison example 3 Composition: 3

[0055] As in Example 1, except that 30 wt% impact modifier and 20 wt% titanium dioxide were used. An elongation at break of 8% was measured.

Claims

1. Use of polymethacrylate films as security labels, characterized in that the film was produced by means of an extrusion process; having a thickness of between 20 and 75 µm and an elongation at break of between 2.0% and 15% according to ASTM D1004; comprising 90 to 100 wt% of a polymethacrylate, which comprise 90 to 100 wt% of MMA and 0 to 10 wt% of acrylates with an alkyl residue having 1 to 4 carbon atoms; and containing a maximum of 10 wt% or no impact modifiers.

2. Use of polymethacrylate films according to claim 1, characterized in that the polymethacrylate films are produced by means of a process which is characterized in that it is an extrusion process and that the equipment used consists of at least the following components: an extruder, a melt pump, optional melt filtration facility, an optional static mixing element, a flat film die, a polishing stack or chill roll, and a winder, the flat film die possessing a die lip having actuating elements for adjusting the die lip width, and the actuating elements having a distance of 11 to 15 mm from one another, the body having an external geometry adapted to the shape of the polishing rolls, the distance from the melt exit edge to the polishing nip being 80 mm or less.

3. Use of polymethacrylate films according to claim 2, characterized in that the flat film die is oriented relative to the polishing stack by means of laser and that the parallel deviation of the die relative to the smoothing rolls, measured at the two ends of one outer side of the die, exhibits a maximum deviation of 3 mm.

4. Use of polymethacrylate films according to at least one of claims 1 to 3, characterized in that the difference between the thinnest and thickest points of the polymethacrylate film is at most 5 µm.

5. Use of polymethacrylate films according to at least one of claims 2 to 4, characterized in that the die and roll have chrome surfaces and in that these chrome surfaces have a roughness Ra of less than 0.08 µm.

6. An application-capable security film, characterized in that the security film has a thickness of between 20 and 75 µm and an elongation at break of between 2.0% and 15% according to ASTM D1004, and in that the security film has before application at least the following layers in the order stated: a) a polymethacrylate film produced by an extrusion process; comprising 90 to 100 wt% of a polymethacrylate, which comprises 90 to 100 wt% of MMA and 0 to 10 wt% of acrylates with an alkyl residue having 1 to 4 carbon atoms, which is additionally characterized in that the film comprises at most 10 wt% of an impact modifier, and that it has printing on at least one side, b) a pressure-sensitive adhesive, c) a release coating, and d) a support layer.

7. Application-capable security film according to claim 6, characterized in that layer a) has a thickness without printing of between 20 and 75 µm, the printing has a thickness of between 1.5 and 3 µm, layer b) has a thickness of between 20 and 30 µm, layer c) has a thickness of between 0.5 and 1.2 µm, and layer d) has a thickness of between 20 and 70 µm.

8. Application-capable security film according to claim 6 or 7, characterized in that the film is transparent and comprises 90 to 100 wt% of polymethacrylate.

9. Application-capable security film according to at least one of claims 6 to 8, characterized in that the film does not contain any impact modifiers.

10. Use of the security films according to claim 6 or 9 for the manufacture of chip cards, documents, security labels, other labels, or price tags.