Double-sided printable PSA thermal labels

Abstract

The present invention relates to a double-sided printable PSA heat-sensitive label and a method for manufacturing such a label, which is suitable for machine-readable labeling of food containers, wherein the label is configured to include a base paper made of chemical pulp, which separates a front side and a back side, the front side includes a temperature-sensitive coating that does not contain bisphenol A and is suitable for direct thermal printing, and the back side includes a pressure-sensitive adhesive layer that is suitable for flexographic printing with UV-curable ink, and wherein the machine readability of the label is improved by affecting the static sensitivity of the temperature-sensitive coating of the label through the thickness of the pressure-sensitive adhesive layer.

Description

Technical Field

[0001] The present invention relates to a double-sided printable and pressure-sensitive adhesive heat-sensitive label suitable for labeling food containers and a method for manufacturing the label. Background Art

[0002] 2,2-Bis(p-hydroxyphenyl)propane, also known as bisphenol A, hereinafter also referred to as BPA, is a widely used developer in many thermal paper applications as well as thermal labels. However, tightening legislative initiatives such as EC 2016 / 2235 are imposing restrictions on the use of BPA. Therefore, there is a need to develop new thermal papers and thermal labels that do not contain BPA. While another type of bisphenol or phenolic or even urea-based developer can be used as a replacement for BPA in thermal papers, the lack of BPA has brought unexpected challenges to thermal labels. In particular, it has been observed that the lack of BPA in temperature-sensitive coatings causes specific problems for the duplex printability of PSA thermal labels suitable for food container labeling. This problem becomes apparent when the label is already printed from both sides so that the front side (face side) also contains direct thermal print. Replacing BPA with another developer may result in excessive print on the front side, which can reduce the machine readability of the label front side. Machine readability refers to the automatic computer-aided detection of product-specific data, which is an established practice in the food packaging industry.

[0003] refer to Figures 1a to 1c , these figures illustrate the problem. Double-sided printable PSA thermal labels LAB1 are generally used as information carriers in consumer products that are to come into contact with food. Before the thermal label is adhered to the surface of a product that may be transparent, one or both sides of the thermal label can be printed. When the product surface is transparent, the benefit of providing printing on the reverse side is that the print can be seen through the product surface. Therefore, each side of the label can be subjected to different printing techniques. The reverse side BLR1 of the label LAB1 generally contains general information, represented by the mark MRK1. The general information is generally static and can be, for example, a recipe, advertisement or trademark formed by symbols, characters and / or text. Therefore, the purpose of the mark MRK1 on the reverse side of the thermal label LAB1 is to provide visually readable information to people. If the container is sealed by a transparent film cover, the mark MRK1 can be viewed after the container is opened. The typical printing technology used to provide the mark MRK1 is UV flexographic printing, hereinafter also referred to as flexographic printing using UV light-curable printing ink, which does not require solvent drying. UV flexographic printing refers to a process in which a UV-curable ink is applied to the reverse side of the label and subsequently cured by radiation with UV wavelengths. UV-curable printing inks typically contain 1.5 to 2 times more pigment than solvent-based or water-based inks.

[0004] The front side FLR1 of the label LAB1 is typically arranged to contain product-specific data MRK2, which may vary between each labeled product. The product-specific data MRK2 may be, for example, logistic information, price and / or weight of the product containing the label. The product-specific data MRK2 is typically compressed into a machine-readable format. An example of a machine-readable format is a barcode, such as a linear and / or two-dimensional barcode format, such as an EAN barcode or a QR code. Printing on the front side is typically done separately from the back side. Printing on the front side may be part of a high-speed labeling operation, so a fast and reliable printing method is required, such as direct thermal printing. In the case where the container is sealed by a transparent film lid, the product-specific data MRK2 is typically read by a machine before the container is opened. Therefore, the purpose of the mark MRK1 on the front side is different from the purpose of the product-specific data MRK2 on the back of the thermal label, the front side being a machine-readable data carrier.

[0005] refer to Figure 1c . A particular problem associated with the replacement of BPA with another developer is the appearance of a flexographic print that has been printed with a UV-curable ink on the back side BLR1 of a thermal label. In a BPA-free thermal label, despite the presence of a barrier layer and a pre-coat layer, which can be used to prevent adhesive migration and to insulate the temperature-sensitive coating on the front side FLR1, respectively, the mirror image mark MRK3 of the flexographic print on the back side BLR1 of the thermal label may become visible on the front side FLR1 of the thermal label. As soon as the label has been subjected to direct thermal printing, the mirror image mark MRK3 becomes prominent. This at least partial appearance of a mirror image of the flexographic print on the front side of the thermal label is problematic for the machine readability of the label, since the mirror image mark MRK3 may overlap with the product-specific data MRK2, which can lead to a degradation of the reflection differences between the light and dark elements of the product-specific data. In the case of barcodes, the reflection differences are usually called print contrast signals or PCS. If the reflection differences are not high enough, the product-specific data MRK2 may not be reliably machine-readable or may not be readable at all. Summary of the invention

[0006] The scope of protection sought by various examples of the present invention is defined by the independent claims. Examples and features described in this specification that do not fall within the scope of the independent claims, if any, are to be construed as examples useful for understanding the present invention.

[0007] As disclosed herein, thermal paper refers to a special type of printable paper that contains a temperature-sensitive coating that is configured to change color when exposed to a specific amount of heat. Thermal paper typically begins to change color at temperatures ranging from 60°C to 100°C. The color change is due to the presence of a leuco dye, which is colorless at room temperature but undergoes a structural change when protonated in the presence of sufficient heat and a proton donor, resulting in a colored structure. The proton donor is often referred to as a developer, a developing agent, or simply a developer.

[0008] Thermal labels can be made from thermal paper. Figure 2 , which shows by way of example a cross-sectional view of a double-sided printable label LAB1, the label LAB1 comprising a base paper PAP1 separating a front side FLR1 from a back side BLR1, the front side FLR1 comprising a layer of temperature-sensitive coating LEU1 suitable for direct thermal printing, the back side BLR1 comprising a thickness H a A layer of pressure-sensitive adhesive ADH1, which has been adhered to the product surface S OBJ superior.

[0009] Thus, as disclosed herein, a thermal label refers to a printable multilayer label comprising a layer of pressure-sensitive adhesive on the reverse side suitable for adhering the thermally printable label to a product surface, and a layer of temperature-sensitive coating suitable for direct thermal printing on the front side. The front side of the thermal label is usually white before printing. The CIE whiteness of the front side can be determined by a standardized method (ISO 11475), which is based on reflectivity data obtained over the entire visible light spectrum. Direct thermal printing refers to a process having the following characteristics, wherein the thermal printing head is configured to heat a predetermined area on the front side of the label so that the temperature-sensitive coating in the predetermined area is protonated and a colored structure is produced. The colored structure is usually black, but other colors are also possible. The color that appears depends on the composition of the temperature-sensitive coating. The pressure-sensitive adhesive is denoted as PSA below. The thermal label comprising the PSA layer can be attached to the product surface by applying slight pressure. The double-sided printable thermal label in this context also comprises a reverse side suitable for flexographic printing with UV-curable printing ink.

[0010] The invention disclosed herein solves the above-mentioned problems associated with flexographic printing behavior by providing a BPA-free double-sided printable pressure-sensitive adhesive heat-sensitive label, which includes a base paper made of chemical pulp, which separates the front side and the back side, the front side is suitable for direct thermal printing, and the back side is suitable for flexographic printing with UV light-curable printing ink, and contains a layer of pressure-sensitive adhesive. BPA, which is commonly used in temperature-sensitive coatings, has been replaced by another color developer, preferably by another type of bisphenol color developer. The pressure-sensitive adhesive, preferably a water-based acrylic adhesive, has been selected to be suitable for labeling food containers. The combination of the BPA-replacement color developer and the pressure-sensitive adhesive layer provides a method for adjusting the double-sided printability of the heat-sensitive label, so that the back side of the label has better flexographic printing retention.

[0011] In the past, the thermal sensitivity of thermal paper was usually adjusted by calendering and pre-coating the base paper before applying the temperature-sensitive coating, or by sensitizers in the temperature-sensitive coating. However, for thermal labels comprising a BPA-free temperature-sensitive coating, empirical studies have shown that pressure-sensitive adhesives suitable for food container labeling can be used to provide double-sided printable thermal labels, in which the phenomenon of the reverse printing appearing on the front temperature-sensitive coating can be avoided. The results obtained with BPA-free thermal papers and labels surprisingly show that a PSA layer applied to the reverse side of the label can be used to prevent the flexographic print made of UV-curable printing ink from interacting with the temperature-sensitive coating on the front side of the label. The interaction with the temperature-sensitive coating can be further reduced by selecting the composition and / or thickness of the pressure-sensitive adhesive layer. According to standard ISO 5-4:2009 (E), the double-sided printability of such thermal labels can be determined by measuring the static thermal sensitivity of the label. According to standard ISO 5-4:2009 (E), the suitability of a base paper comprising a temperature-sensitive coating for a double-sided printable thermal label can be further determined by measuring the static thermal sensitivity of the base paper comprising a temperature-sensitive coating from both sides. Static thermal sensitivity indicates the relationship between temperature and the color change that occurs when a leuco dye reacts with a substituted developer. Static thermal sensitivity can be expressed as an optical density at a defined temperature, which refers to the portion of incident light reflected by the front side of the label. The optical density of a base paper comprising a layer of temperature-sensitive coating at a defined temperature can be determined from each side, i.e. the front side and / or the back side. It is particularly noteworthy that the results of experimental studies have shown that the suitability of thermal paper for double-sided printable labels may be related to the arithmetic mean optical density of the thermal paper. A suitable base paper comprising a layer of temperature-sensitive coating has an optical density of less than 0.1 after exposure to a temperature of 80°C or 95°C for 2 seconds when measured from the back and front side of the base paper as the arithmetic mean of three measurements using a 45° / 0° annular illumination optical element. Advantageously, the base paper including a temperature sensitive coating has different optical densities on the back and front sides, so that the portion of incident light reflected by the back side of the base paper label is smaller than the portion of incident light reflected by the front side of the base paper label including the temperature sensitive coating.

[0012] The pressure-sensitive adhesive layer on the reverse side provides a method of controlling the label properties so that the reverse printing can be avoided on the temperature-sensitive coating on the front side. The experimental results show that the PSA layer can be arranged to provide a lower static thermal sensitivity on the reverse side of the label. It has been observed that the lower static thermal sensitivity on the reverse side of the label is associated with a reduced tendency of the mirror image of the flexographic print made of UV-curable printing ink to appear on the front side of the thermal label. This has an impact on the machine readability of the label. Based on the experimental results, when the label is exposed to a temperature of 80°C for 2 seconds according to standard ISO 5-4:2009, the PSA thermal label disclosed herein has an optical density of less than 0.1, and the PSA thermal label is double-sided printable, so that the flexographic print made of UV-curable printing ink on the reverse side does not interfere with the machine readability of the front side of the label. When measured at higher temperatures, the lower static thermal sensitivity is associated with improved machine readability. Advantageously, the optical density is also less than 0.7 when determined from the front side of the label with 45° / 0° annular illumination optics, after exposure for 2 seconds at a temperature of 95°C, as the arithmetic mean of three measurements, according to standard ISO 5-4:2009(E). The machine-readable double-sided printable thermal label is capable of providing a minimum print contrast signal equal to or higher than 80% when determined by direct thermal printing of a trapezoidal barcode, according to standard ISO / IEC 15416:2016(E).

[0013] Food packaging is a specific industrial sector that uses a large number of thermal labels on transparent film surfaces and packaging. Transparent films are useful for providing users with a view of the contents of the package. Thermal labels can be used to provide various information and quality assurance of the product. The proximity to food limits the group of pressure-sensitive adhesives that can be used on the label. PSAs suitable for food packaging need to be approved for such use. The pressure-sensitive adhesive layer can be selected from a water-based acrylic adhesive or a hot melt adhesive suitable for food container labeling. Preferably, the PSA layer is a water-based acrylic adhesive, which includes an acrylic polymer and a tackifying resin, such as a rosin ester resin. It has been observed that water-based acrylic adhesives suitable for food container labeling can further improve the duplex printability of thermal labels, especially when used in combination with a temperature-sensitive coating containing a bisphenol developer. Water-based acrylic adhesives can be used to reduce the heat sensitivity of the reverse side. Based on the surface free energy results obtained in experimental studies, the surface energy level of water-based acrylic adhesives is low. Therefore, water-based acrylic adhesives can have an improved barrier effect on UV-curable printing inks that generally contain a higher pigment content. When the pressure-sensitive adhesive layer is a water-based acrylic adhesive, the layer can therefore have a better retention capacity of UV-curable printing ink. In other words, a PSA layer of a water-based acrylic adhesive can be provided to reduce the migration of UV-curable printing ink to the front of the label. When the PSA layer is a water-based acrylic adhesive, the static thermosensitivity of the front of the label can therefore be adjusted. The water-based acrylic adhesive further exhibits the thermal insulation of the reverse side of the heat-sensitive label, thereby effectively resisting the propagation of the flexographic printing made of UV-curable printing ink on the PSA layer. Advantageously, the PSA layer is a permanent adhesive, and when measured according to FINAT test method No. 9 (9th edition, 2009), it has a loop tack value equal to or higher than 10 Newtons. Advantageously, the thickness of the PSA layer is in the range of 10 to 20 microns. This thickness range is most conducive to controlling the thermal properties of the label.

[0014] The properties of the label can be further adjusted by a base paper that separates the front and back sides of the label. Base paper made of chemical pulp has a very high cellulose fiber content and therefore has excellent physical properties, such as stiffness and rigidity. Advantageously, the label includes a base paper made of bleached chemical pulp, which generally contains lignin in an amount of less than 1% by weight. Bleaching of chemical pulp significantly reduces the amount of hemicellulose, lignin, wood extractives and inorganics in the material. Therefore, labels with enhanced surface whiteness and brightness can be produced, because several layers on the front side of the thermal label are generally transparent to a certain extent before direct thermal printing. When measured according to ISO 11475, the front side of the label advantageously has an ISO whiteness equal to or higher than 105%, preferably equal to or higher than 120%, and most preferably equal to or higher than 130% before direct thermal printing. By providing a label with a high surface whiteness level, the reflection difference between dark and bright elements of thermal printing can be increased. This improves the printing contrast signal of the label, thereby improving the machine readability of product-specific data.

[0015] The double-sided printable label may also include additional layers, such as a transparent coating as the outermost layer on the front side on top of the temperature-sensitive coating. The coating typically comprises or consists of water-soluble starch, carboxymethyl cellulose, partially or fully hydrolyzed polyvinyl alcohol or its derivatives and serves to protect the temperature-sensitive coating.

[0016] Accordingly, there is provided a double-sided printable label suitable for machine-readable labeling of food containers, the label comprising:

[0017] - Base paper made of chemical pulp, which separates the following two sides:

[0018] ○ Suitable for direct thermal printing on the front and

[0019] ○Obverse side,

[0020] - the reverse side comprises a pressure-sensitive adhesive layer suitable for flexographic printing with UV-curable inks, and,

[0021] - the front side comprises a temperature-sensitive coating free of 2,2-bis(p-hydroxyphenyl)propane, and,

[0022] Therein, the thickness of the pressure-sensitive adhesive layer has been chosen so that the portion of the incident light reflected by the front side of the label, expressed as optical density, is less than 0.1 when determined as the arithmetic mean of three measurements with 45° / 0° annular lighting optics after exposure to a temperature of 80°C for 2 seconds in accordance with standard ISO 5-4:2009 (E).

[0023] Accordingly, there is provided a method for producing a double-sided printable label suitable for machine-readable labeling of a food container, the method comprising:

[0024] - providing a base paper made of chemical pulp, the base paper comprising a temperature-sensitive coating free of 2,2-bis(p-hydroxyphenyl)propane, and a primer layer between the base paper layer and the temperature-sensitive coating on a first side of the base paper layer, thereby obtaining a front side suitable for direct thermal printing, and

[0025] - coating the other side of the base paper with a pressure-sensitive adhesive suitable for UV flexographic printing, thereby obtaining a reverse side comprising a pressure-sensitive adhesive layer,

[0026] In this method, the thickness of the pressure-sensitive adhesive layer is chosen so that the portion of the incident light reflected by the front side of the label, when expressed as optical density, is less than 0.1 when determined according to standard ISO 5-4:2009 (E) with a 45° / 0° annular illumination optics after exposure for 2 seconds at a temperature of 80°C, using the arithmetic mean of three measurements.

[0027] Hereinafter, working examples that are helpful for understanding the present invention will be described in more detail. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1a A flexographic print is illustrated which has been printed with a UV curable ink on the reverse side of a thermal label.

[0029] Figure 1b A direct thermal print is illustrated which has been printed on the front side of a thermal label.

[0030] Figure 1c The front side of a BPA-free thermal label is illustrated, where a flexographic print that has been printed with a UV-curable ink on the reverse side of the thermal label becomes visible over the temperature sensitive coating on the front side and overlaps with the direct thermal print.

[0031] Figure 2 A cross-sectional view of a double-sided printable label is illustrated, the label comprising a base paper separating a front side from a back side, the front side comprising a layer of temperature sensitive coating suitable for direct thermal printing, and the back side comprising a pressure sensitive adhesive layer which has been adhered to a surface of an object.

[0032] Figure 3 An example is illustrated of a cross-sectional view of a double-sided printable label, the label comprising a base paper separating a front side from a back side, the front side comprising a layer of temperature-sensitive coating suitable for direct thermal printing, the back side comprising a pressure-sensitive adhesive layer, the label further comprising a barrier layer between the base paper layer and the pressure-sensitive adhesive layer, a pre-coating layer between the base paper and the temperature-sensitive coating, and a coating as the outermost layer of the front side on top of the temperature-sensitive coating.

[0033] Figure 4 Results of an experimental study investigating the static thermal sensitivity of double-sided printable labels are shown.

[0034] Figure 1a , 1b , 1c, 2 and 3 are schematic and not to any particular scale. The symbols SX, SY and SZ in the figure refer to orthogonal directions that are perpendicular to each other. DETAILED DESCRIPTION

[0035] Double-sided printable adhesive labels

[0036] A label generally comprises a face layer and an adhesive layer for adhering the face layer to an article. Thus, a label is a multi-layer product having a front side and a reverse side comprising an adhesive layer, wherein the reverse side is intended to be adhered to the surface of a product. Labels can be used to display information. Double-sided printable labels comprise a first side and a second side, wherein the two sides of the label can be printed separately. Double-sided printable adhesive labels also comprise a layer of adhesive material. Adhesive refers to a composition having a tendency to adhere to the surface of an object by chemical bonding or dispersion bonding. The adhesiveness of the adhesive depends on the type of adhesive and the surface material used. The strength of the adhesive may increase after the label is adhered to the surface of the product.

[0037] The properties of the labels can be determined by using standardized test methods, such as those described in the FINAT Technical Manual containing test methods for self-adhesive materials (9th edition, 2014). Unless otherwise stated, the values ​​characterizing the properties of the labels refer to the values ​​obtained by the FINAT method.

[0038] The thickness of the PSA layer can be determined according to ISO Standard 534.

[0039] Method for manufacturing double-sided printable label

[0040] refer to Figure 3 . The double-sided printable label LAB1 can be made of thermal paper. The thermal paper can be made of a base paper PAP1 coated with a layer of temperature-sensitive coating LEU1. Optionally, further layers can be provided, such as a pre-coating PRE1, a coating TOP1 on top of the temperature-sensitive coating LEU1 and / or a barrier layer REV1. The double-sided printable thermal label LAB1 further comprises at least one layer of pressure-sensitive adhesive ADH1. The layer of pressure-sensitive adhesive ADH1 can be applied to the reverse side BLR1 of the label LAB1. The pressure-sensitive adhesive ADH1 can be applied as a coating, for example as a contour coating, a spray or curtain coating, a film transfer coating or a blade / rod coating. The basis weight of the double-sided printable thermal label LAB1 is typically between 60 and 90 g / m 2 In the range of 62 to 80 g / m 2In the range of 63 to 75 g / m 2 - within the range.

[0041] The double-sided printable thermal label LAB1 can be manufactured from facestock, for example, by die-cutting the facestock. A method for manufacturing a double-sided printable label LAB1 may include:

[0042] - providing a base paper PAP1 having a first side and a second side,

[0043] - applying a pre-coating layer PRE1 on a first side of the base paper PAP1,

[0044] - applying a layer of temperature-sensitive coating LEU1 on a first side of the base paper PAP1, thereby forming a thermal paper, and

[0045] - A layer of pressure sensitive adhesive ADH1 suitable for flexographic printing with UV curable ink is applied on the second side of the base paper PAP1 to form a double-sided printable label LAB1. If necessary, other layers can be applied on the first or second side of the base paper PAP1.

[0046] Thermal printing advantages of high-quality papers. The quality of paper suitable for thermal printing can be defined by paper brightness, opacity, whiteness, smoothness / roughness and / or surface strength. Therefore, the base paper PAP1 is usually made of chemical pulp, preferably bleached chemical pulp. The base paper PAP1 usually has a thickness of 30 microns or more, preferably 40 microns or more, most preferably 50 microns or more, for example in the range of 30 to 60 microns, preferably in the range of 40 to 59 microns, and most preferably in the range of 55 to 58 microns. Chemical pulp has a very high cellulose fiber content and therefore has excellent physical properties, such as stiffness and rigidity. Bleaching of chemical pulp further significantly reduces the amount of hemicellulose, lignin, wood extractives and inorganic substances in the material. The bleached chemical pulp usually contains less than 1% by weight of lignin, preferably less than 0.5% by weight of the base paper PAP1, and most preferably less than 0.2% by weight of the base paper PAP1. Thus, base paper PAP1 made of bleached chemical pulp enables the production of labels with enhanced surface whiteness and brightness. When measured according to ISO 11475, the front side of the label before direct thermal printing advantageously has an ISO whiteness equal to or higher than 105%, preferably equal to or higher than 120%, most preferably equal to or higher than 130%.

[0047] The thermal label LAB1 includes a temperature sensitive coating LEU1. The temperature sensitive coating LEU1 contains chemical reagents, such as a color former, a developer and a sensitizer. The temperature sensitive coating LEU1 has been configured to melt when exposed to a sufficient amount of heat, thereby initiating a chemical reaction. The chemical reaction is typically a reduction or oxidation reaction, which occurs between the color former and the developer in a molten state and produces a color change. The temperature sensitive coating LEU1 is typically applied as an aqueous suspension to a base paper PAP1 or a pre-coat PRE1 and then dried into a solid layer. The temperature sensitive coating LEU1 typically has a thickness of 2 microns or more, for example in the range of 2 to 6 microns, preferably in the range of 2 to 5 microns, and most preferably in the range of 3 to 4 microns.

[0048] The temperature-sensitive coating LEU1 comprises a color former, which may be a leuco dye. Examples of leuco dyes are, for example, triarylmethanephthalide dyes, fluorane dyes and crystal violet lactone. Spironolactone compounds are usually used as leuco dyes. Advantageously, leuco dyes suitable for direct thermal printing usually have a five-membered spironolactone ring at one end of the molecule and a tertiary amino group at the other end of the molecule, which facilitates the ring opening of spironolactone.

[0049] The temperature-sensitive coating LEU1 contains a developer, which is a weakly acidic compound capable of transferring a proton to a color former, thereby initiating a chemical reaction. The developer is preferably a compound that is stable at hot printing temperatures and does not have strong acidity, since strong acidity can promote background imaging. A traditional example of a developer is 2,2-bis(p-hydroxyphenyl)propane, also known as 4,4'-isopropyldiphenol, also known as bisphenol A. Developers that can replace bisphenol A are, for example, other bisphenol developers, such as bisphenol S, bisphenol F, bisphenol C and their derivatives, in which the chemical structure has two hydroxyl groups connected to the benzene ring, like BPA. Other types of developers, such as bisphenol-free phenolic developers, are also possible. Typically, the chemical structure of a bisphenol-free developer has only one hydroxyl group connected to the benzene ring. The developer can alternatively be a phenol-free developer, such as a zinc salt, a substituted salicylic acid, or a compound that is a sulfone and / or urea derivative. An example of a sulfone-based developer is 3,3'-diallyl-4,4'-dihydroxy-diphenylsulfone. An example of a developer that is a urea derivative is N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl)urea.

[0050] The temperature-sensitive coating LEU1 generally comprises one or more sensitizers. A sensitizer can be used to reduce the melting point temperature of the temperature-sensitive coating LEU1. The sensitizer can thereby promote the initiation of a chemical reaction. The sensitizer can also be used as a temperature-dependent solvent for a color former and a developer, configured to initiate a chemical reaction when exposed to a sufficient amount of heat. The sensitizer can be, for example, a fatty acid amide, a wax or a carboxylic acid ester. Preferably, the sensitizer can be an aromatic ether, an aromatic ester or a biphenyl derivative, which is less likely to form residue accumulation on a thermal printing head during direct thermal printing.

[0051] The temperature sensitive coating LEU1 may also contain inorganic minerals or fillers, such as precipitated calcium carbonate, calcined kaolin, silica or calcined clay. These compounds do not melt in the same way as color formers, developers and / or sensitizers, and therefore can be used as a binding material to prevent the chemical substances involved in the chemical reaction from migrating in the molten state. Inorganic minerals or fillers can also be used to improve the whiteness of the front FLR1.

[0052] The heat-sensitive label LAB1 may include a precoat layer PRE1 between the base paper PAP1 and the temperature-sensitive coating LEU1. The precoat layer PRE1 is applied to the base paper PAP1 before the temperature-sensitive coating LEU1. The precoat layer PRE1 is typically a layer that can be applied as a liquid to provide surface smoothness and uniformity. The precoat layer PRE1 can further promote the anchoring of the temperature-sensitive coating LEU1 to the base paper PAP1. The precoat layer PRE1 may contain, for example, starch and / or polyvinyl alcohol and / or latex, such as styrene-butadiene latex (SB) or styrene acrylic (SA) latex. The precoat layer PRE1 can be used as an additional insulating layer on the front side FLR1 of the label LAB1. Therefore, the precoat layer PRE1 can provide thermal insulation between the temperature-sensitive coating LEU1 and the pressure-sensitive adhesive layer ADH1. The precoat layer PRE1 is typically applied at 2-15 g / m 2 The range of application is preferably 5-12g / m 2 The pre-coating layer PRE1 may have a thickness of 5 micrometers or more, for example in the range of 5 to 15 micrometers, preferably in the range of 8 to 13 micrometers, most preferably in the range of 9 to 11 micrometers.

[0053] The heat-sensitive label LAB1 may also include a coating TOP1 on top of the temperature-sensitive coating LEU1 as the outermost layer of the front side FLR1. The coating TOP1 is usually colorless and transparent. The coating TOP1 may include or consist of the components listed below: for example, water-soluble starch, carboxymethyl cellulose, partially or completely hydrolyzed polyvinyl alcohol or its derivatives. The outermost layer can be used to protect the temperature-sensitive coating LEU1 from mechanical wear, chemicals and exposure to ambient conditions. The coating TOP1 is usually 2-4 g / m 2The coating TOP1 typically has a thickness of 1 micrometer or more, for example in the range of 1 to 4 micrometers, preferably in the range of 1 to 3 micrometers, most preferably in the range of 2 to 3 micrometers.

[0054] The heat-sensitive label LAB1 may further include a barrier layer REV1 between the base paper layer PAP1 and the pressure-sensitive adhesive layer ADH1. The barrier layer REV1 may be applied as a coating on the reverse side REV1 and is used to prevent the pressure-sensitive adhesive from migrating through the base paper layer PAP1. The barrier layer REV1 may contain or consist of the components listed below: water-soluble starch, carboxymethyl cellulose, partially or completely hydrolyzed polyvinyl alcohol or its derivatives. The barrier layer REV1 typically has a thickness of 1 micron or more, for example in the range of 1 to 4 microns, preferably in the range of 1 to 3 microns, and most preferably in the range of 2 to 3 microns.

[0055] Reverse printing

[0056] The heat sensitive label LAB1 comprises a pressure sensitive adhesive layer ADH1 on the reverse side BLR1. The reverse side BLR1 of the double sided printable label LAB1 needs to be suitable for flexographic printing with UV light curable printing inks, which is widely used for flexible packaging printing in the food industry, such as for printing of plastic packaging, corrugated board and adhesive label laminates.

[0057] Flexographic printing is a mechanical relief printing method characterized by a soft and flexible printing plate. UV-curable printing inks are not based on the evaporation of solvents, but on a polymerization reaction, called curing, by UV radiation and oxygen. UV-curable printing inks typically contain a binder, functional monomers, pigments and additives such as photoinitiators. Photoinitiators are compounds that contain reactive groups and react with high-energy radiation to initiate a polymerization reaction, which is also called curing. The challenge faced by many photoinitiators used in UV-curable printing inks is that photoinitiators tend to be highly mobile components. However, compared with conventional printing inks, UV-curable printing inks have the following advantages: very fast curing speed, very high formability, good chemical resistance and scratch resistance. For UV-curable printing inks, the thickness of the printed ink layer is usually in the range of 0.8 to 2.5 microns.

[0058] Pressure sensitive adhesives suitable for food contact applications appear to provide a means of reverse printing with a wider range of UV-curable printing inks. Pressure sensitive adhesives have many advantages. PSAs can be applied to thermal paper by a coater. Example methods of applying PSAs are slot, gravure, reverse roller and curtain coating. The coating method can be selected according to the characteristics of the PSA. Preferably, the PSA is of a permanent type so that the label does not separate from the product surface prematurely. Permanent adhesives have high tack values. The bond strength of a permanent pressure-sensitive adhesive to a surface develops as a function of time, known as the adhesive setting time. For example, within a few minutes or half an hour after the adhesive contacts the packaging surface, a sufficient strength bond between the label and the product can be obtained almost immediately upon pressing. The adhesive can adhere to the surface at a minimum temperature of -40°C or higher, such as at a minimum temperature of -20°C or higher. The adhesive can adhere to the surface at a maximum temperature of 100°C or lower, such as at a maximum temperature of 80°C or lower. The adhesive can adhere to the surface, for example, in the range of -20°C to 100°C, or in the range of -40°C to 60°C. The temperature range in which an adhesive can adhere to a surface is called the adhesive's service temperature.

[0059] Pressure-sensitive adhesive ADH1 can be selected from water-based acrylic adhesives or hot melt adhesives suitable for food container labeling. Preferably, the pressure-sensitive adhesive layer is a water-based acrylic adhesive, most preferably a water-based acrylic adhesive, which includes an acrylic polymer and a tackifying resin, such as a rosin ester resin. It has been noted that the water-based acrylic PSA adhesive suitable for food container labeling is particularly suitable for reducing the heat sensitivity of the reverse side of the double-sided printable label. The adhesive layer can include an elastomeric component, such as acrylic acid, ethylene-vinyl acetate or styrene block copolymer. The final bonding strength can be formed in, for example, less than 4 hours of attachment. As mentioned above, the bonding strength can be expressed by a viscosity value. The viscosity value of the adhesive can be determined according to the FINAT test method (2014, the 9th edition), which is used for ring adhesion measurement, referred to as FTM9. The viscosity value of the adhesive can be determined by the combination of the adhesive layer attached to the label. The permanent adhesive suitable for label LAB1 can have a viscosity value equal to or higher than 10 Newtons, preferably equal to or higher than 13 Newtons, and most preferably equal to or higher than 16 Newtons. The tack values ​​of permanent pressure sensitive adhesives may range from 10 to 25 Newtons when measured using FTM9.

[0060] The suitability of the pressure sensitive adhesive ADH1 for flexographic printing with UV curable inks can be further qualified by the surface free energy of the pressure sensitive adhesive. The surface free energy of the PSA can be measured by drop shape analysis. The Kruss drop shape analyzer is an example of a device suitable for this drop shape analysis.

[0061] Print contrast signal and machine readability of double-sided printable PSA labels

[0062] refer to Figure 1b The front side FLR1 of label LAB1 is used for direct thermal printing. Figure 1b The thermal print, referred to as product specific data MRK2, is usually provided in a machine-readable format, such as a one-dimensional or two-dimensional barcode format. A linear barcode refers to a one-dimensional specific pattern consisting of lines and spaces of various widths. A trapezoidal barcode is an example of a linear barcode, which is printed in a direction parallel to the direction of transport of the thermal label during direct thermal printing. Correspondingly, a grid barcode refers to a linear barcode that is read in a direction perpendicular to the direction of transport of the thermal label during direct thermal printing. Trapezoidal barcodes are more difficult to print clearly. Typically, an LED light source is used for illumination, and the barcode is scanned (i.e., machine-read) at a visible light wavelength in the range of 630 to 650 nm.

[0063] The verification of the machine readability of the barcode can be evaluated by determining the contrast, resolution and / or defects of the direct thermal printing. Advantageously, the quality of the print displayed in a machine readable format can be evaluated by a print contrast signal denoted PCS. The machine readability of the label can be determined by direct thermal printing of a trapezoidal barcode and evaluating the print contrast signal of the resulting trapezoidal barcode according to standard ISO / IEC15416:2016(E).

[0064] Essentially, print contrast refers to the relative reflectivity difference between the light and dark elements of the resulting trapezoidal barcode, expressed as a percentage, where the light elements represent the non-printed area (i.e., background) and the dark elements represent the thermally printed area, resulting in color variation. Therefore, the print contrast signal (PCS) can be expressed according to the following equation 1:

[0065] PCS=(R1–Rd) / R1×100%

[0066] Where R1 is the reflectivity of the non-printed area and Rd is the reflectivity of the thermally printed area.

[0067] In practice, labels suitable for machine-readable labeling are those capable of providing on the front side a minimum print contrast signal equal to or higher than 80% at visible wavelengths in the range 630 to 650 nm, when determined by direct thermal printing of a trapezoidal barcode according to standard ISO / IEC15416:2016(E). If the reflectance difference is not high enough, the machine readability of the label is insufficient.

[0068] Static thermal sensitive double-sided printable PSA labels

[0069] Static thermosensitivity, also called static sensitivity, defines the temperature at which the color former (e.g. leuco dye) and the developer begin to melt. The static thermosensitivity of a thermal label is a measure of the temperature at which the temperature-sensitive coating begins to change color. The darkening phenomenon of the temperature-sensitive coating is called "direct thermal printing" or "inline thermal printing". Therefore, the static thermosensitivity of a thermal label indicates the relationship between temperature and the color change on the front side of the label. Static thermosensitivity refers to the portion of the incident light reflected by the label, which can be determined from the front side of the label. The static thermosensitivity of thermal paper can be further determined independently of the back side of the thermal paper. Static thermosensitivity can be expressed as the optical density at a defined temperature, which is typically in the range of 60°C to 150°C, for example at a temperature of 80°C and / or 95°C. The optical density of the surface is preferably determined as the arithmetic mean of three measurements made with a 45° / 0° annular illumination optics after 2 seconds of exposure. A densitometer X-Rite eXact can be used To determine the optical density of base papers and labels containing temperature sensitive coatings. According to the manufacturer's instructions, X-Rite eXact Conforms to industry standard ISO 5-4:2009(E) for measuring reflectance density characteristics.

[0070] Low static thermal sensitivity is associated with good contrast for direct thermal printing. In other words, when a thermal label comprises a low optical density value, the thermal label is an indication that the temperature-sensitive coating does not react during the cooling phase of the thermal print head used for direct thermal printing. Interestingly, experimental results obtained for BPA-free thermal labels comprising a PSA layer approved for food packaging applications surprisingly show that the lower static thermal sensitivity of the label is associated with a reduced tendency for a mirror image of a flexographic print using a UV-curable ink to appear on the front side of the label. Therefore, when the PSA has been approved for food packaging applications, the static thermal sensitivity can be used as an indicator of the duplex printability of a PSA label comprising a layer of BPA-free temperature-sensitive coating. In addition, the results show that when a layer of such a PSA is applied to the reverse side of a BPA-free label, the thickness of the PSA layer thus formed can be used to adjust the barrier properties of the reverse side of the label to UV-curable printing inks. For a base paper comprising a layer of temperature-sensitive coating, when expressed in optical density values, preferably, the portion of incident light reflected by the reverse side is smaller than the portion of incident light reflected by the front side. Experimental studies with BPA-free labels further showed that an optical density of less than 0.1, when determined from the front side of the label, can be used as a predictor of sufficient PSA layer thickness for duplex printability. Experimental studies with BPA-free thermal papers further showed that an optical density of less than 0.1, when determined from both the front and back sides of the label, can be used as a predictor of suitability for duplex printable labels.

[0071] It has been observed that the correlation between the low static heat sensitivity of double-sided printable PSA labels and the reduced tendency of mirror image formation of flexographic prints made using UV light-curable printing inks is most significant when the thickness of the PSA layer on the reverse side is in the range of 10 to 20 microns. In addition, water-based acrylic adhesives appear to be more effective than hot melt adhesives. When hot melt adhesives are used, the thickness of the pressure-sensitive adhesive layer is preferably in the range of 15 to 20 microns. When water-based acrylic adhesives are used, the thickness of the pressure-sensitive adhesive layer can be 15 microns or less, preferably in the range of 10 to 15 microns. The thickness of the pressure-sensitive adhesive layer can be determined from a sample according to ISO 534, for example by using an instrument designed for this purpose, such as an L&W Micrometer.

[0072] Example 1 - Effect of PSA on Static Thermal Sensitivity

[0073] In an experimental study, the effect of PSA on the static thermal sensitivity of thermal labels was investigated. The study consisted of three samples, S1, S2 and S3, each containing the same BPA-free thermal paper with a basis weight of 67 g / m 2 , with a thickness of 74 microns. Sample S1 is thermal paper only, without a PSA layer, and serves as a negative control sample for the effect of PSA. Sample S2 is a thermal label, in which the same thermal paper has been coated with a permanent hot melt PSA. Sample S3 is a thermal label, in which the same thermal paper has been coated with a permanent water-based acrylic PSA with a tack value of 18N, and this PSA has been approved for food container labeling. Water-based acrylic PSA is a dispersion adhesive based on acrylate polymers and tackifying resins such as rosin ester resins. The thickness of the PSA layer of sample S2 is 19 microns, and the thickness of the PSA layer of sample S3 is 15 microns. According to standard ISO 5-4:2009(E), an X-Rite Exact with 45° / 0° annular illumination optics was used. The static thermal sensitivity of the samples as optical density was measured five times (OD_1 to OD_5) from the front side of each sample using a densitometer, following the manufacturer's instructions. The optical density values ​​were recorded from samples S1, S2 and S3 after UV flexographic printing ink was applied to the reverse side of the samples using a Flexiproof 100 machine simulating flexographic printing and the UV flexographic printing ink was cured, using a 200 W / cm fluorescent lamp with a width of 100 mm, the distance of the light source from the reverse surface facing the light source was 10 mm, and the samples were transported under the lamp at a speed of 50 m / min. Therefore, all samples S1, S2 and S3 were subjected to the same treatment, so the differences in the experimental study were the presence of the PSA layer and its composition.

[0074] The results are listed in Table 1 (below).

[0075] Table 1. Optical density measurements of the front side of samples S1, S2, and S3. OD_1 to OD_5 refer to the measured values, OD_AVE refers to the average of five measurements in each sample, and STDEV refers to the standard deviation between the five measurements.

[0076] S1 S2 S3 OD_1 0,803 0,696 0,081 OD_2 0,925 0,623 0,082 OD_3 0,817 0,625 0,082 OD_4 0,818 0,538 0,088 OD_5 0,891 0,603 0,087 OD_AVE 0,851 0,617 0,084 STDEV 0,054 0,057 0,003

[0077] As demonstrated in this study, the static thermal sensitivity was less than 0.1 when measured as optical density from sample S3 containing a permanent water-based acrylic PSA. In addition, the optical density of sample S3 containing a permanent water-based acrylic PSA was significantly less than the optical density of thermal paper alone. In addition, the optical density of sample S2 containing a permanent hot melt PSA was less than the optical density of thermal paper alone. In addition, the optical density of sample S3 containing a permanent water-based acrylic PSA was significantly less than the optical density of sample S2. The results indicate that pressure-sensitive adhesive layers, particularly layers of water-based acrylic adhesives, can be used to adjust the thermal sensitivity of thermal labels. Based on the results of the experimental study, water-based acrylic adhesives appear to improve the barrier properties of thermal labels to UV flexographic printing inks and enhance the thermal insulation properties of the reverse side of thermal labels when represented by the optical density of the label.

[0078] The machine readability of the three samples was determined by direct thermal printing of a trapezoidal barcode on the reverse side of the label after flexographic printing with UV-curable ink. The print contrast signal was determined by the trapezoidal barcode produced on the front side of the label according to standard ISO / IEC15416:2016(E). In the tested label S3, the front side was able to provide a minimum print contrast signal equal to or higher than 80% at a wavelength of visible light in the range of 630 to 650 nm, according to standard ISO / IEC15416:2016(E), when determined by direct thermal printing of the trapezoidal barcode.

[0079] Example 2 - Static thermal sensitivity of BPA-free thermal labels at 80°C

[0080] In the second experimental study, the static thermal properties of BPA-free thermal papers were investigated by exposing two groups of thermal papers to temperatures ranging from 70°C to 135°C. The first group included two samples, Ex_1 and Ex_2, each of which contained a bisphenol developer in the temperature-sensitive coating. Sample Ex_1 contained a bisphenol S derivative as a developer in the temperature-sensitive coating. Sample Ex_2 contained bisphenol S as a developer in the temperature-sensitive coating. The grammage of sample Ex_2 was 68 g / m 2The second group of samples includes two samples C_1 and C_2, wherein, in the temperature sensitive coating, sample C_1 contains a bisphenol-free color developer (3,3'-diallyl-4,4'-dihydroxy-diphenyl sulfone), and sample C_2 contains a phenol-free color developer (N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl) urea).

[0081] Static thermal sensitivity was measured as the optical density of each sample as the arithmetic mean of three measurements on the back and front sides. The X-Rite Exact with 45° / 0° annular illumination optics was used according to standard ISO 5-4:2009(E) following the manufacturer’s instructions. Densitometer. The optical density values ​​were recorded after exposure for 2 seconds at temperatures of 70°C, 80°C, 95°C, 105°C, 130°C and 135°C. The selected temperature range can be used to determine the change in optical density with temperature, which is a measure of the portion of incident light reflected by the front and back sides of the thermal paper. The results of the experimental study are listed in Table 2 (below) and are presented in Figure 4 This is explained in .

[0082] Table 2. Optical density (OD) measurements of two groups of samples in the temperature range of 70 to 135°C. The optical density (OD) value of each sample is the arithmetic mean calculated from 6 measurements, 3 from the front side and 3 from the back side.

[0083]

[0084] refer to Figure 4and Table 2 and Table 1 (above). The results show that the optical density (OD) serves as an indicator of the temperature-dependent properties of a base paper comprising a temperature-sensitive coating, when measured as the arithmetic mean of three measurements from each side of the base paper comprising a temperature-sensitive coating, according to standard ISO 5-4:2009 (E), after exposure to a temperature of 80°C for 2 seconds with 45° / 0° annular illumination optics. Furthermore, the optical density (OD) can be used as an indicator of the temperature-dependent behavior of a double-sided printable label. When the average optical density at a temperature of 80°C is equal to or less than 0.1, the base paper comprising a temperature-sensitive coating exhibits a significantly reduced static thermal sensitivity over the entire measured temperature range. The difference in the average optical density between the first and second group of samples develops particularly clearly at a temperature of 95°C, where the average optical density of the first group of samples is less than 0.7, while in the second group of samples the average optical density is significantly higher. When combined with the results of Example 1, it is expected that bisphenol developers, especially bisphenol S and / or its derivatives, and permanent water-based acrylic PSA with a tack value of 18N approved for food container labeling are an advantageous combination in double-sided printable labels to obtain low static thermal sensitivity and improve the barrier properties of thermal labels to UV flexographic printing inks on the reverse side of the labels. It was also observed that on the reverse side of these base papers containing a layer of temperature-sensitive coating, the lower static thermal sensitivity was associated with a reduced tendency for the mirror image of the flexographic print made with UV light-curable printing ink to appear on the front side of the thermal paper.

[0085] As a general conclusion of the above studies, the pressure-sensitive adhesive layer on the reverse side thus represents a means of controlling the properties of heat-sensitive labels, making it possible to avoid the reverse side printing from appearing on the front side temperature-sensitive coating. Furthermore, the choice of a pressure-sensitive adhesive and a base paper comprising a layer of temperature-sensitive coating provides a means of producing labels in which the mirror image of a flexographic print made with a UV-curable printing ink appears on the front side of the heat-sensitive label with a reduced tendency.

[0086] When the average optical density of the base paper containing a layer of temperature-sensitive coating at a temperature of 80°C in the second group of samples C_1 and C_2 containing a bisphenol-free developer or a phenol-free developer is measured to be higher than 0.1, it can be expected that the interaction with the temperature-sensitive coating can be reduced by increasing the thickness of the pressure-sensitive adhesive layer. The results show that the interaction with the temperature-sensitive coating can be reduced by selecting the composition and / or thickness of the pressure-sensitive adhesive layer.

[0087] Example 3 - Surface Free Energy of BPA-Free Thermal Label

[0088] In a third experimental study, thermal labels comprising a hot melt PSA layer were compared with thermal labels comprising a water-based acrylic PSA in terms of the surface free energy of the PSA layer. This study consisted of four samples, where samples S2 and S3 were the same as in Example 1 (above). Sample S4 was another example of a thermal label comprising a water-based acrylic PSA, and sample S5 was another example of a thermal label comprising a hot melt PSA.

[0089] The surface free energy of each sample was measured using a Kruss drop shape analyzer. The PSA layer of each thermal label was measured at room temperature using the sessile drop method, where a drop of diiodomethane standard test solution was deposited on the surface of the PSA layer, and after the sessile drop was formed on the surface, the static contact angle of the drop was determined. In this study, a waiting time of 4 seconds was used to form the sessile drop. The surface free energy, including polar and dispersive interaction fractions, was calculated from the determined static contact angles according to the Wu model (harmonic mean) and the Owens-Wendt-Rabel-Kaelben (OWRK) model (geometric mean). To improve the accuracy of the drop shape analysis, the measurement was repeated 10 times. Therefore, a total of 10 drops were deposited at different locations of the PSA layer, from which the surface free energy was determined as the arithmetic mean.

[0090] Table 3 (below) shows the measured surface free energy values ​​for the PSA layers in Samples S2, S3, S4, and S5 in millinewtons per meter (mN / m).

[0091] Table 3. Surface free energy values ​​(mN / m) of samples S2, S3, S4 and S5 measured by the sessile drop method after 4 seconds.

[0092]

[0093] The experimental studies showed that there was a significant difference in the behavior of thermal labels comprising a hot melt PSA layer (samples S2 and S5) and thermal labels comprising a water-based acrylic PSA (samples S3 and S4). The surface free energy of the thermal label comprising a water-based acrylic PSA layer was significantly reduced. It was further observed that the water-based acrylic PSA seemed to repel the standard test solution more than the hot melt PSA. Interestingly, sample S3, which contained the same water-based acrylic PSA, was used in both Examples 1 and 3. Therefore, the results together show that thermal labels comprising a water-based acrylic PSA layer may have an improved barrier effect on UV-curable printing inks, which typically contain a higher amount of pigment. The water-based acrylic adhesive layer may also have a better retention capacity for UV-curable printing inks, which can be demonstrated by measuring the optical density of the label.

Claims

1. A double-sided printable label (LAB1) suitable for machine-readable labeling of food containers, the label (LAB1) comprising: - Base paper made of chemical pulp (PAP1) which separates the following two sides: front side suitable for direct thermal printing (FLR1), and Reverse (BLR1), - the reverse side (BLR1) comprises a layer of pressure sensitive adhesive (ADH1), wherein the pressure sensitive adhesive is a water-based acrylic adhesive and the layer is suitable for flexographic printing with UV-curable inks, and, - the front side (FLR1) comprises a temperature-sensitive coating (LEU1) which does not contain 2,2-bis(p-hydroxyphenyl)propane and which comprises a developer selected from bisphenol S, bisphenol F or bisphenol C, and, The thickness (H a ) is such that, expressed in optical density, the portion of incident light reflected by the front side (FLR1) of the label (LAB1) is less than 0.1 when determined as the arithmetic mean of three measurements with 45° / 0° annular illumination optics after exposure to a temperature of 80°C for 2 seconds, and wherein, after UV flexographic printing of the back side (BLR1), the front side (FLR1) is capable of providing a minimum print contrast signal equal to or higher than 80% at a wavelength of visible light in the range of 630 nm to 650 nm, as determined by direct thermal printing of a trapezoidal bar code according to standard ISO / IEC 15416:2016 E.

2. The double-sided printable label (LAB1) according to claim 1, wherein: The thickness (H a ) is in the range of 10 μm to 20 μm.

3. A double-sided printable label (LAB1) according to claim 1 or 2, wherein: The temperature-sensitive coating (LEU1) comprises a color former and a sensitizer.

4. A double-sided printable label (LAB1) according to claim 1 or 2, wherein: The developer is bisphenol S.

5. The double-sided printable label (LAB1) according to claim 1 or 2, wherein: When expressed as optical density, the portion of incident light reflected by the front side (FLR1) of the label (LAB1) is less than 0.7 when determined as the arithmetic mean of three measurements with 45° / 0° annular illumination optics after exposure to a temperature of 95°C for 2 seconds.

6. A double-sided printable label (LAB1) according to claim 1 or 2, wherein: The base paper (PAP1) comprising the temperature-sensitive coating (LEU1) is selected so that, when expressed as an optical density value, after exposure to a temperature of 80°C or 95°C for 2 seconds, as determined by the arithmetic mean of three measurements using a 45° / 0° annular illumination optical element, the portion of the incident light reflected by the back side (BLR1) of the base paper (PAP1) is smaller than the portion of the incident light reflected by the front side (FLR1) of the base paper (PAP1) comprising the temperature-sensitive coating (LEU1).

7. The double-sided printable label (LAB1) according to claim 1 or 2, wherein: The pressure sensitive adhesive layer (ADH1) is a permanent adhesive having a loop tack value equal to or higher than 10 Newtons when measured according to FINAT test method No.

9.

8. A double-sided printable label (LAB1) as claimed in claim 1 or 2, further comprising a barrier layer (REV1) located between the base paper (PAP1) and the pressure-sensitive adhesive layer (ADH1), which comprises water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

9. The double-sided printable label (LAB1) according to claim 1 or 2, further comprising a barrier layer (REV1) located between the base paper (PAP1) and the pressure-sensitive adhesive layer (ADH1), which is composed of water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

10. A double-sided printable label (LAB1) as described in claim 1 or 2, further comprising a coating (TOP1) as the outermost layer of the front side (FLR1) on top of the temperature-sensitive coating (LEU1), wherein the coating (TOP1) comprises water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

11. A double-sided printable label (LAB1) as described in claim 1 or 2, which also includes a coating (TOP1) as the outermost layer of the front side (FLR1) on top of the temperature-sensitive coating (LEU1), wherein the coating (TOP1) consists of water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

12. The double-sided printable label (LAB1) according to claim 1 or 2, further comprising a pre-coating layer (PRE1) between the base paper (PAP1) and the temperature sensitive coating layer (LEU1).

13. A double-sided printable label (LAB1) according to claim 1 or 2, wherein: The lignin content of the chemical pulp of the base paper (PAP1) is less than 0.5% by weight.

14. The double-sided printable label (LAB1) according to claim 1 or 2, wherein: The front side (FLR1 ) has a CIE whiteness equal to or higher than 105% before direct thermal printing, when determined according to standard ISO 11475.

15. The double-sided printable label (LAB1) according to claim 1 or 2, wherein: The front side (FLR1 ) has a CIE whiteness equal to or higher than 120% before direct thermal printing, when determined according to standard ISO 11475.

16. The double-sided printable label (LAB1) according to claim 1 or 2, wherein: The front side (FLR1 ) has a CIE whiteness equal to or higher than 130% before direct thermal printing, when determined according to standard ISO 11475.

17. A method for producing a double-sided printable label (LAB1) suitable for machine-readable labeling of food containers, the method comprising: - providing a base paper (PAP1) made of chemical pulp, said base paper comprising a temperature-sensitive coating (LEU1) free of 2,2-bis(p-hydroxyphenyl)propane and comprising a developer selected from bisphenol S, bisphenol F or bisphenol C, and a pre-coating (PRE1) between said base paper (PAP1) and the temperature-sensitive coating (LEU1) on a first side of the base paper (PAP1), so as to obtain a front side (FLR1) suitable for direct thermal printing, - coating the other side of the base paper (PAP1) with a pressure-sensitive adhesive suitable for UV flexographic printing, thereby obtaining a reverse side (BLR1) comprising a layer of pressure-sensitive adhesive (ADH1), said pressure-sensitive adhesive being a water-based acrylic adhesive, In the method, the thickness (H a ) such that the portion of incident light reflected by the front side (FLR1) of the label (LAB1) is less than 0.1 when expressed in optical density, when determined as the arithmetic mean of three measurements with 45° / 0° annular illumination optics after exposure for 2 seconds at a temperature of 80°C, and wherein the front side (FLR1) is capable of providing, after UV flexographic printing of the back side (BLR1), of a minimum print contrast signal equal to or higher than 80% at a wavelength of visible light in the range 630 nm to 650 nm, when determined by direct thermal printing of a trapezoidal bar code according to standard ISO / IEC 15416:2016E.

18. The method of claim 17, wherein: The thickness (H a ) is in the range of 10 μm to 20 μm.

19. The method according to claim 17 or 18, wherein: The temperature-sensitive coating (LEU1) comprises a color former and a sensitizer.

20. The method of claim 17 or 18, wherein: The developer is bisphenol S.

21. The method of claim 17 or 18, wherein: When expressed as optical density, the portion of incident light reflected by the front side (FLR1) of the label (LAB1) is less than 0.7 when determined as the arithmetic mean of three measurements with 45° / 0° annular illumination optics after exposure to a temperature of 95°C for 2 seconds.

22. The method of claim 17 or 18, wherein: The base paper (PAP1) comprising the temperature-sensitive coating (LEU1) is selected so that, when expressed as an optical density value, after exposure to a temperature of 80°C or 95°C for 2 seconds, as determined by the arithmetic mean of three measurements using a 45° / 0° annular illumination optical element, the portion of the incident light reflected by the back side (BLR1) of the base paper (PAP1) is smaller than the portion of the incident light reflected by the front side (FLR1) of the base paper (PAP1) comprising the temperature-sensitive coating (LEU1).

23. The method of claim 17 or 18, wherein: The pressure sensitive adhesive layer (ADH1) is a permanent adhesive having a loop tack value equal to or higher than 10 Newtons when measured according to FINAT test method No.

9.

24. The method according to claim 17 or 18, further comprising a barrier layer (REV1) located between the base paper (PAP1) and the pressure-sensitive adhesive layer (ADH1), which comprises water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

25. The method according to claim 17 or 18, further comprising a barrier layer (REV1) located between the base paper (PAP1) and the pressure-sensitive adhesive layer (ADH1), which is composed of water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

26. The method of claim 17 or 18, further comprising a coating (TOP1) as the outermost layer of the front side (FLR1) on top of the temperature-sensitive coating (LEU1), the coating (TOP1) comprising water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

27. The method of claim 17 or 18, further comprising a coating (TOP1) as the outermost layer of the front side (FLR1) on top of the temperature-sensitive coating (LEU1), the coating (TOP1) consisting of water-soluble starch, carboxymethyl cellulose, or partially or completely hydrolyzed polyvinyl alcohol.

28. The method of claim 17 or 18, wherein: The lignin content of the chemical pulp of the base paper (PAP1) is less than 0.5% by weight.

29. The method of claim 17 or 18, wherein: The front side (FLR1 ) has a CIE whiteness equal to or higher than 105% before direct thermal printing, when determined according to standard ISO 11475.

30. The method of claim 17 or 18, wherein: The front side (FLR1 ) has a CIE whiteness equal to or higher than 120% before direct thermal printing, when determined according to standard ISO 11475.

31. The method of claim 17 or 18, wherein: The front side (FLR1 ) has a CIE whiteness equal to or higher than 130% before direct thermal printing, when determined according to standard ISO 11475.

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