Thermal recording media

Abstract

Provided is a thermal recording medium and method of making the thermal recording medium, wherein the thermal recording medium comprises a substrate; a first layer carried by the substrate, wherein the first layer is a pre-coat layer comprising a dyestuff; and a second layer at least partially covering the first layer, wherein the second layer is a light scattering layer that is configured to become substantially transparent at selected print locations upon application of sufficient heat at the selected print locations; wherein the first layer comprises organic polymeric nanoparticles having an average particle size in the range of about (100) to about (250) nm, and a minimum film forming temperature of between about (40) °C to about (80) °C, and a dyestuff carrier, and / or the second layer comprises organic polymeric nanoparticles having an average particle size in the range of about (100) to about (250) nm, and a minimum film forming temperature of between about (40) °C to about (80) °C, and an agglomerating agent.

Description

P608908PC00Thermal Recording MediaField of the Invention

[0001] The present invention relates to direct thermal recording media and methods for making the same.Background

[0002] Manufacturers of direct thermal recording media / materials, such as thermal paper i.e., Point of Sale (POS) receipts, currently face several problems.

[0003] Thermal paper is commonly used in POS systems since no additional ink is involved in the process, which means that the printers can be small and more manageable than traditional printers that require either toner or ink cartridges to be regularly replaced. In principle thermal printing is a printing method where the print is generated at a speed from 10 mm / s to 100 mm / s by locally heating the surface of a thermosensitive coated substrate, which then locally irreversibly changes colour.

[0004] Thermoprinting has typically been based on a chemical reaction between an acidic developer and a leuco dyestuff. Thermal paper such as POS receipts have typically consisted of leuco dyes which undergo a chemical reaction upon the application of heat (from a thermal printer, for example) to develop a recorded image on the thermal paper. Typically leuco dyes, such as triaryl methane phthalides, are used to create different colours once exposed to an acid and heat, as well as developers which are acids that provide the colour when combined with leuco dyes and heat is applied. Typically, the developers are organic acids and are phenols such as bisphenol A (BPA) and bisphenol S (BPS).

[0005] However, the use of bisphenol A has been made illegal in thermoprintable papers in many countries since it is carcinogenic. Its use is continually being banned in more countries over time. Bisphenol S has been used in place of bisphenol A but its use in thermoprintable paper is also gradually being banned in a significant number of countries. Further alternatives to bisphenol A and S have been difficult to find because of price concerns and supply chain restrictions.

[0006] Therefore, there has been a need to develop alternative thermal recording media which do not require the use of leuco dyes such as bisphenol A. This has led to the development of direct thermal recording media that are designed to operate based on a thermally-induced change of state rather than a thermally-induced chemical reaction between a leuco dye and an acidic developer. Examples of these alternatives can be found in US 2019 / 0291493 A and US 2021 / 0086524 A, relying on the use of hollow sphere pigments (HSPs).

[0007] The principle of these alternative solutions is the application of an opacifying layer of HSPs on a dark substrate. Such thermal printing media typically includes a fiber substrate, a pre-coat layer either comprising carbon black or having a carbon black layer on top of it and a top-coat which comprises the HSPs. The top layer becomes transparent in the specific locations at which heat is applied such that the carbon black colouring beneath becomes visible. However, such products have considerable drawbacks sinceP608908PC00there are significant problems involved with using carbon black since it is generally not recyclable and particularly not in the large quantities required to generate sufficient colour contrast in the thermoprintable paper. Carbon black is also generally quite expensive. Prior art methods have also found it difficult to use other colourants and dyestuffs as a substitute for carbon black, without experiencing issues such as the dyestuff migrating into the substrate causing issues such as insufficient colour contrast.

[0008] Furthermore, the prior art solutions typically require the fiber-based substrate to be processed with a yankee cylinder to produce the substrate with a sufficient degree of smoothness for the prior art pre-coats to be applied thereon - which can be time consuming and energy intensive. The pre-coat layer in these prior art solutions is usually designed to be heat insulating and normally contains a low-density insulator such as calcined clay (bulky pigment particles produced by sintering conventional platy clay into bulky agglomerates) to prevent the applied heat during use from damaging the substrate. Alternatively, the pre-coat may contain hollow sphere particles which also act as a heat insulator. Hollow sphere pigments are used which consist of hollow particles with an acylate / methacylate ester polymer shell, whereby the particle interior is filled with water. On drying, water evaporates and particles with a definite internal void volume are obtained. However, the presence of such an insulating layer or insulating components can limit the speed at which the thermal media can be printed, as well as increasing costs due to the additional complexities involved in having an insulating layer present.

[0009] Further still, such alternative solutions fail to address one of the main principles of thermoprintable papers which is the generation of a maximum brightness contrast between printed and unprinted areas. These alternative solutions struggle to generate a sufficiently high brightness contrast between the printed and unprinted areas by having the carbon black layer below the opacifying layer. They also typically require a top-coat of high thickness in order to avoid the carbon black underlayer being visible to the user in the unprinted areas and to ensure that sufficient contrast can be generated. A top-coat of high thickness, however, results in higher production costs, as well as requiring more heat to sufficiently heat through the top-coat, which results in a higher energy consumption and longer heating times.

[0010] Therefore, there is a need to identify better alternative direct thermal recording media that are designed to operate based on a thermally-induced change of state rather than a thermally-induced chemical reaction between a leuco dye and an acidic developer in order to avoid the use of BPA and BPS.

[0011] In this respect, the inventors of the present invention have developed thermal recording media which are recyclable, have excellent properties, do not require the presence of an insulating pre-coat, that enable a top-coat of lower thickness and which is also excellent in terms of brightness contrast between the printed and unprinted areas, amongst other advantages discussed herein.Statements of Invention

[0012] In accordance with a first aspect of the invention, there is provided a thermal recording medium, comprising:a substrate;P608908PC00a first layer carried by the substrate, wherein the first layer is a pre-coat layer comprising a dyestuff; anda second layer at least partially covering the first layer, wherein the second layer is a light scattering layer that is configured to become substantially transparent at selected print locations upon application of sufficient heat at the selected print locations; whereinthe first layer comprises organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250 nm, optionally determined according to ISO 22412, and a minimum film forming temperature of between about 40 °C to about 80 °C, optionally determined according to ISO 2115, and a dyestuff carrier, and / orthe second layer comprises organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250 nm, optionally determined according to ISO 22412, and a minimum film forming temperature of between about 40 °C to about 80 °C, optionally determined according to ISO 2115, and an agglomerating agent.

[0013] In an embodiment of the invention, the organic polymeric nanoparticles in the precoat layer are at least partially coated by the dyestuff, preferably wherein the dyestuff is adsorbed onto the surface of the nanoparticles via the dyestuff carrier.

[0014] In an embodiment of the invention the pre-coat layer is configured such that upon the application of sufficient heat at the selected print locations, the organic polymeric nanoparticles at the selected print locations melt to liberate the dyestuff and make the pre-coat layer appear visibly darker at the selected print locations, preferably wherein sufficient heat is a temperature at or above the minimum film forming temperature of the organic polymeric nanoparticles.

[0015] In an embodiment of the invention, the organic polymeric nanoparticles in the topcoat are agglomerated by the agglomerating agent.

[0016] In an embodiment of the invention, the organic polymeric nanoparticles in the topcoat layer are sufficiently agglomerated such that the top-coat layer is substantially opaque, preferably wherein the second layer is configured such that upon the application of sufficient heat at the selected print locations, the organic polymeric nanoparticles at the selected print locations melt to reduce the degree of agglomeration to become substantially transparent, preferably wherein sufficient heat is a temperature at or above the minimum film forming temperature of the organic polymeric nanoparticles.

[0017] In an embodiment of the invention, the thermal recording medium comprises both the pre-coat layer and top-coat layer as defined in the first aspect of the invention; preferably wherein the dyestuff carrier and the agglomerating agent are both the same or different and / or preferably wherein the organic polymeric nanoparticles in the pre-coat and top-coat layer are the same or different.

[0018] In an embodiment of the invention, the dyestuff is an anionic dyestuff, preferably a black anionic dyestuff.

[0019] In an embodiment of the invention, the organic polymeric nanoparticles are a polymer based on monomers of one or more of styrene, butadiene, acrylate andP608908PC00vinylacetate, preferably wherein the polymer is an emulsion polymer, further preferably wherein the organic polymeric nanoparticles are a styrene-butadiene polymer.

[0020] In an embodiment of the invention, the dyestuff carrier and / or agglomerating agent is or comprises a polymer containing primary amino groups, preferably wherein the polymer containing primary amino groups is a polyalcohol amine, further preferably obtained by reacting a polyvinyl alcohol with cyanoguanidine in the presence of an acid.

[0021] In an embodiment of the invention, the acid is orthophosphoric acid and / or the polyvinyl alcohol is at least 99% hydrolyzed, preferably fully hydrolyzed.

[0022] In an embodiment of the invention, the pre-coat layer comprises the organic polymeric nanoparticles in an amount of between about 70 wt% to about 98 wt%, the dyestuff in an amount of between about 1 wt% to about 25 wt% and / or the dyestuff carrier in an amount of between about 1 wt% to about 25 wt%.

[0023] In an embodiment of the invention, the top-coat layer comprises the organic polymeric nanoparticles in an amount of between about 55 wt% to about 95 wt% and / or the agglomerating agent in an amount of between about 5 wt% to about 45 wt%.

[0024] In an embodiment of the invention, the pre-coat layer has a coat weight of between about 3 to about 8 g / m2, optionally between about 4 to about 7 g / m2, or optionally between about 4 to about 6 g / m2; and / or the top-coat layer has a coat weight of between about 3 to about 8 g / m2, preferably between about 4 to about 7 g / m2or optionally between about 4 to about 6 g / m2.

[0025] In an embodiment of the invention, the thermal recording medium does not comprise one or more, or all, of carbon black, hollow sphere pigments, calcined clay, bisphenol A and bisphenol C, preferably wherein the top-coat does not contain any dyestuff.

[0026] In a second aspect of the invention, there is provided a method of making a thermal recording medium, preferably the thermal recording medium according to any preceding aspect or embodiment, the method comprising:i) applying a first layer as defined in any preceding aspect or embodiment to a substrate; andii) applying a second layer as defined in any preceding aspect or embodiment on the first layer,optionally further comprising the step of preparing the dyestuff carrier and / or agglomerating agent by reacting a polyvinylalcohol with cyanoguanidine in the presence of water, wherein the reacting step further comprises the addition of an acid and wherein the dyestuff carrier and / or agglomerating agent is a polyalcoholamine compound.Detailed Description

[0027] The present invention generally relates to a thermal recording medium such as a Point of Sale (POS) receipt and / or a thermoprintable paper / receipt, and methods of making the thermal recording medium, as defined herein.P608908PC00

[0028] The use of organic polymeric nanoparticles in either, or both, the pre-coat and topcoat has been found to be particularly advantageous. This helps to overcome the aforementioned issues related to the use of BPA and BPS in thermal recording media that rely on a thermally-induced chemical reaction involving leuco dyes and an acidic developer, while also overcoming disadvantages relating to known direct thermal recording media that are designed to operate based on a thermally-induced change of state rather than a chemical reaction.

[0029] Surprisingly it has been identified that organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250nm and having a minimum film forming temperature (MFT) of between about 40 °C to about 80 °C, can be used together with a particular dyestuff carrier and a dyestuff to obtain an improved pre-coat layerthan the prior art, in a thermal recording medium that undergoes a thermally-induced change of state. It has also been identified that organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250nm and having a minimum film forming temperature (MFT) of between about 40 °C to about 80 °C can be used together with particular agglomerating agents to obtain an improved top-coat layer than the prior art, in a thermal recording medium that undergoes a thermally-induced change of state.

[0030] In this respect, the present invention avoids the necessity of using environmentally unfriendly carbon black in the thermal recording media by overcoming issues relating to the incorporation of dyestuffs into thermal recording media. The present invention also enables a thermal recording medium to be produced having reduced top-coat and precoat weights, as well as a reduced amount of dyestuff, which results in cost savings. The present invention also highly advantageously generates an excellent brightness contrast between the top-coat and pre-coat, which results in a more visible print to the human eye, especially without requiring higher top-coat / pre-coat weights. Since carbon black is not used, the thermal recording media of the present invention are also more environmentally friendly.

[0031] In the present invention, the general structure of the thermal recording medium is a substrate having a pre-coat directly thereon, and a top-coat applied directly onto the pre-coat - although in certain embodiments additional layers can be present in between the substrate / pre-coat and pre-coat / top-coat. The organic polymeric nanoparticles of the invention can be used in either or both of the pre-coat and top-coat. When present in both the pre-coat and top-coat, the organic polymeric nanoparticles can be the same or they can be different.

[0032] In the pre-coat, it has been found that a dyestuff, preferably an anionic dyestuff, can be adsorbed onto the surface of the organic polymeric nanoparticles. Preferably this is achieved by a dyestuff carrier, which is less susceptible to melting than the organic polymeric nanoparticles and which is highly effective at adsorbing the dyestuff onto the surface of the organic polymeric nanoparticles. In this embodiment, the pre-coat therefore contains the organic polymeric nanoparticles having a dyestuff adsorbed onto the surface thereof by the dyestuff carrier, preferably a vinylalcohol-vinylamine copolymer, and so the dyestuff is dispersed throughout the pre-coat. In this embodiment therefore the pre-coat contains (1) organic polymeric nanoparticles, a (2) dyestuff and (3) a dyestuff carrier. It has been found particularly advantageous to use a dyestuff carrier containing primary amino groups since, without being bound by theory, it is understood that they generate strong internal crosslinking which helps to trap the anionic dyestuff at the surface of theP608908PC00organic polymeric nanoparticles. The presence of the primary amino groups also helps to stabilize the pre-coat composition via the Pickering emulsion effect. Secondary or tertiary amino groups generally fail to provide such additional benefits. In the absence of the use of any dyestuff carrier, the dyestuff will generally not be adsorbed onto the surface of the nanoparticles and instead adsorbs with an aqueous fraction in the substrate coating and will problematically move into the substrate.

[0033] In accordance with this embodiment of the invention, such a pre-coat may be used with any top-coat that is capable of becoming substantially transparent or transparent at selected print locations upon application of sufficient heat at the selected print locations. Such examples are described in US 2019 / 0291493 A and US 2021 / 0086524 A. As explained herein, the top-coat is configured such that visible light incident on an exposed surface of the top-coat which propagates through the surface of the top-coat is scattered in many directions by one or more of reflection, refraction, and diffraction. As a result of the light scattering, any dyestuff or colorant present in the pre-coat layer (beneath the top-coat) is not substantially visible to a human observer and so the observer would only see the white or light-colored appearance created by the scattering action of the particles in the top-coat. Upon the application of sufficient heat and pressure, for a sufficient amount of time, (e.g., from a thermal print head in a conventional thermal printer such as a POS receipt printer) the top-coat becomes transparent or substantially transparent in the heated area. In US 2021 / 0086524 A, for example, this is achieved by second particles in the thermal recording medium melting, while first particles do not substantially melt, flatten, collapse, or otherwise deform. Due to the proximity of the second particles to the first particles and the porosity of the first light-scattering layer, the melted particles rapidly flow into and fill some or substantially all of the spaces between the first particles, as well as inside the empty interiors of such particles. Upon cooling (after passing the thermal print head), the melted particles form a solid matrix which is no longer light scattering. As a result, visible light rays strike the outer surface of the top-coat and propagate through the modified layer, reaching and impinging upon the pre-coat layer.

[0034] In the present invention, prior to the application of heat from a thermal printing head, the pre-coat layer is also light scattering. This is because the organic polymeric nanoparticles are very small in size, with a high surface area, which causes high levels of light scattering. In view of the dyestuff being significantly dispersed across the pre-coat layer on the surfaces of the organic polymeric nanoparticles, the pre-coat layer exhibits a colour that is a light version of the dyestuff present. For example, if a black anionic dyestuff is used, the pre-coat layer appears a light blue colour. However, upon said application of sufficient heat and pressure, for a sufficient amount of time, by a thermal printing head, not only does the top-coat undergo a change of state. The top-coat undergoes a change of state such that it transforms from light-scattering to being at least substantially transparent. At the same time, the heat applied by the thermal print head is sufficient to cause the organic polymeric nanoparticles in the pre-coat to melt i.e., coalesce, in view of the organic polymeric nanoparticles having a minimum film forming temperature (MFT) of between about 40 °C to about 80 °C. This causes a coalescence of the organic polymeric nanoparticles. The melting of the organic polymeric nanoparticles generates a homogenous film which becomes transparent. Since the organic polymeric nanoparticles become transparent or substantially transparent and the nanoparticles fuse into a molten mass, once cooled (i.e., after passing the thermal print head), the pre-coat becomes darker in colour at the heated locations. This is because the dyestuff becomes considerably more visible (since the dyestuff essentially is now more concentrated and theP608908PC00organic polymeric nanoparticles have become substantially transparent at the heated locations) i.e. a dark image in the pre-coat layer in the area of heat application is generated. It is considered that the dyestuff carrier adsorbed on the organic polymeric nanoparticles water surface is disrupted by exposure to heat and the dyestuff is reverted to its original molecular configuration, absorbing light over the entire range of the visible spectrum.

[0035] Therefore, upon the application of heat at a specific location of the thermal recording medium, the top-coat becomes at least substantially transparent and the precoat becomes visible at that specific location as a dark mark or area surrounded by the lighter colour of the top-coat that has not been subject to heat treatment. The pre-coat at the specific heated location also becomes darker and thus the observer observes a dark image at the heated location. Therefore, by applying heat at particular locations of the thermal recording medium, a desired image / text can be generated based on the dark precoat being made visible to the observer through a combination of the top-coat becoming substantially transparent and the pre-coat becoming darker at the heated location.

[0036] Compared to prior art thermoprintable paper, since the pre-coat of the present invention also undergoes a colour change, improved colour contrast is achieved. In this respect, the colour of the pre-coat will always be slightly visible through the top-coat since the top-coat is unlikely to be completely light scattering. Therefore, when the prior art thermal recording medium uses something like a carbon black layer, the brightness contrast of the thermally printed image will be lower than the present invention, since the black printed parts will have a lower brightness contrast to the surround areas visible to the observer, which are made darker by the slightly visible carbon black layer under the top-coat. In contrast, in the present invention the surrounding areas of the pre-coat (i.e., the parts of the pre-coat surrounding the area of the pre-coat layer that has been subject to heat) are a lighter colour than the heated parts (i.e., as explained herein, the dispersed dyestuff means that non-heated areas of the pre-coat layer are a lighter variant of the colour of the dyestuff e.g., if the dyestuff is black, then the pre-coat appears a light blue). Thus, in the present invention, if a black dyestuff is used it will appear a light blue colour in the non-heat-treated parts and black in the heat treated parts of the pre-coat. Thus, a better colour contrast between printed and non-printed areas will be visible to an observer.

[0037] This is also advantageous since it means that a thinner (i.e., lower weight) top-coat can be used in the present invention. Whereas known thermal recording media require a top-coat of high thickness to negate the darkness of the colourant below, such as carbon black, (i.e., so that the top-coat generally appears a lighter colour), since non-heat-treated parts of the pre-coat of the present invention are already a lighter colour, a thinner topcoat layer can be used. Naturally this is advantageous because it is cost-saving in that less material is required for the top-coat and also since it means that the thermal printing machine can heat the top-coat / pre-coat sufficiently in a quicker manner, which results in quicker printing and a lower energy consumption.

[0038] It has also been identified that the organic polymeric nanoparticles of the present invention can be used in the top-coat of the thermal recording medium in addition to, or instead of, in the pre-coat layer. The use of the organic polymeric nanoparticles in the top-coat achieves a highly effective top-coat layer that becomes substantially transparent at selected print locations upon application of sufficient heat at the selected print locations.P608908PC00

[0039] The same organic polymeric nanoparticles are used in the top-coat layer as in the pre-coat layer - although it would obviously not be necessary to use exactly the same nanoparticles in both layers, provided that the nanoparticles are as defined herein. In the top-coat layer, the organic polymeric nanoparticles are preferably used together with an agglomerating agent. The agglomerating agent used in the top-coat is the same as the dyestuff carrier used in the pre-coat - although it would obviously not be necessary to use exactly the same substance in both layers, provided that the substance falls under the definition of the agglomerating agent / dyestuff carrier herein. However, in the top-coat the agglomerating agent functions to agglomerate the organic polymeric nanoparticles rather than to help adsorb dyestuff to the surface of the organic polymeric nanoparticles. In this embodiment, the organic polymeric nanoparticles are agglomerated in the top-coat by the agglomerating agent. Upon the application of heat to the pre-coat, the organic polymeric nanoparticles melt and transform into a molten form such that they become transparent (or substantially transparent) so that visible light can reach the pre-coat layer - such that the colour of the pre-coat layer becomes visible to an observer. Since the pre-coat layer will be a darker colour than the top-coat and will now be visible, a dark image or mark will become visible to the observer. Thus, the top-coat according to the present invention could be used with any conventional pre-coat (e.g., even those containing some carbon black).

[0040] However, most preferably, both the pre-coat and top-coat according to the present invention (i.e., containing the organic polymeric nanoparticles) are combined. Thus, most preferably the pre-coat comprises the organic polymeric nanoparticles, dyestuff carrier and dyestuff and the top-coat comprises the organic polymeric nanoparticles and the agglomerating agent. The organic polymeric nanoparticles and agglomerating agent / dyestuff carrier used in the two layers can be the same or they can be different, as long as they fall within the definition of these components provided herein. The combined use of both these pre-coat and top-coat layers results in an optimal brightness distinction between the two layers. If the top-coat of the present invention was combined with conventional means, such as a carbon black layer, then a thicker top-coat would be required to counteract the darkness of the carbon black layer, as explained already herein in order to generate sufficient contrast.

[0041] It is important that in both layers the organic polymeric nanoparticles have an average particle size in the range of about 100 to about 250 nm, so that they are light scattering when formed into an agglomerate-like structure, but are no longer light scattering once in a melted form since such an average particle size is well below the wavelength range of visible light, which is between 400 nm to 800 nm and are therefore generally not light scattering once they have been transformed out of the agglomerate form. It is also important that the organic polymeric nanoparticles have a minimum film forming temperature of between about 40 °C to about 80 °C, so that they are able to melt upon the application of a sufficient amount of heat by a conventional thermal printing machine.

[0042] In certain embodiments, the thermal recording medium according to the present invention does not comprise one or more of the following: calcined clay, an insulating layer (particularly an insulating layer between the pre-coat layer and the substrate), hollow sphere pigments (HSPs) e.g., as described in US 2021 / 0086524 A, carbon black, leuco dyes, acid developers, bisphenol A and bisphenol C. Preferably, the thermal recording medium does not contain any further layers than a substrate, pre-coat and top-coat.P608908DE00Preferably, the thermal recording medium does not contain a thermally insulating layer in between the substrate and pre-coat or between the pre-coat and top-coat layers.

[0043] In certain embodiments, the pre-coat of the thermal recording medium according to the present invention does not comprise one or more of the following: calcined clay, an insulating layer (particularly an insulating layer between the pre-coat layer and the substrate), hollow sphere pigments (HSPs) e.g., as described in US 2021 / 0086524 A, carbon black, leuco dyes, acid developers, bisphenol A and bisphenol C.

[0044] In certain embodiments, the top-coat according to the present invention does not comprise one or more of the following: calcined clay, an insulating layer (particularly an insulating layer between the pre-coat layer and the substrate), hollow sphere pigments (HSPs) e.g., as described in US 2021 / 0086524 A, carbon black, leuco dyes, acid developers, bisphenol A and bisphenol C. The top-coat does not contain any dyestuff.Thermal recording medium

[0045] Thermal recording medium or media takes its usual meaning in the art. It may also be known as a "thermal printing recording medium" or a "thermal transfer recording medium" or a "heat-sensitive recording material / medium" or "direct thermal recording medium". Medium herein refers to medium in the singular, whereas media refers to medium in the plural form. The media are well-known in the art and the application of sufficient heat and pressure, for a sufficient amount of time, causes a recorded image to develop, for example due to a chemical reaction taking place or due to a physical change of state. Examples of such products are provided in, for example, US 2021 / 0086524 A.

[0046] Such media are standard in several industries such as POS receipts. Therefore, in a preferred embodiment the thermal recording medium is a thermal paper, most preferably a thermal paper receipt. Alternatively, the product may be defined as a thermoprintable paper.

[0047] The thermal recording medium according to the present invention generally comprises, consists of, or consists essentially of a substrate, supporting a first layer (precoat layer) either directly or indirectly, and a second layer (top-coat layer) at least partially covering the first layer. Preferably, the at least one surface of the substrate is partially, substantially or completely covered by the first layer (pre-coat layer), which in turn has its opposite face preferably partially, substantially or completely covered by the second layer (top-coat layer). In preferred embodiments, the first layer (pre-coat layer) is directly supported on and completely covers one face of the substrate, and the second layer (topcoat layer) is directly supported on and completely covers an opposite face of the first layer i.e., the thermal recording medium has a conventional layered structure, preferably with no other layers than the substrate, pre-coat and top-coat layers. The surfaces covered / adjacent to one another are generally the major surface of the substrate / pre-coat layer / top-coat layer. There is typically a hard divide between each of the layers, with minimal overlap or leaching between layers.SubstrateP608908PC00

[0048] The thermal printing recording medium of the present invention contains a substrate. Preferably the substrate is a fiber-based substrate e.g., a cellulose-fiber containing substrate. Any substrate suitable for the preparation of a thermal printing recording medium may be used. In particular, any substrate suitable for use as a substrate in products such as a Point of Sale (POS) receipt. The substrate may also be any substrate suitable for use in other paper products known to the person skilled in the art. In certain embodiments, the substrate can comprise or consist of Old Corrugated Cardboard / Containers (OCC).

[0049] The substrate is preferably thin, substantially planar, and flexible. The substrate has a thickness defined by its opposed major surfaces. The substrate may preferably be or comprise a cellulose material, such as a conventional paper. The paper may have a base weight in a range from about 30 to about 90 g / m2, but other suitable base weights may also be used. Preferably the substrate has a thickness of between about 30 pm to about 100 pm. The paper may also be treated with one or more agents, such as a surface sizing agent. Uncoated base papers, including unsized, conventionally sized, and lightly treated base papers, can be used. The substrate may be simple in construction, and devoid of glossy coatings, or of other substantial, functional coatings. The substrate may, for example, be substantially uniform in composition throughout its thickness, rather than a multilayered construction or material to which one or more separate, functional coatings have already been applied. In some cases, however, it may be desirable to treat, prepare, or otherwise work the substrate in preparation for coating onto it the other layers shown in the figure. The substrate and its major surfaces may also be light-diffusive and opaque in character.

[0050] In certain embodiments, the substrate has not undergone a Yankee cylinder treatment method step. In the present invention, treatment with a Yankee cylinder is not necessary. This is because the high levels of smoothness that are typically required for the substrate in typical leuco dye thermal recording media is not required for the pre-coat layer of the present invention. In particular, the presence of the dyestuff carrier in the pre-coat layer allows application of the pre-coat onto a relatively rough substrate surface. Not requiring treatment with a Yankee cylinder is particularly advantageous since Yankee cylinder treatment is a very expensive process step and slow down the speed at which the substrate can be prepared.Pre-coat layer (herein "first layer"]

[0051] The thermal recording medium of the present invention contains a pre-coat layer. The pre-coat layer is carried by the substrate. Preferably the pre-coat layer is applied directly onto the substrate i.e., with no other layer in between. However, in alternative embodiments, the pre-coat is applied indirectly onto the substate i.e., other layers are positioned in between the pre-coat and the substrate. Generally, the pre-coat layer covers all or substantially all of one surface of the substrate.

[0052] In the event that the organic polymeric nanoparticles are contained in the top-coat, in alternative embodiments the pre-coat layer can be any conventionally known pre-coat layer and can include the use of an insulating layer and carbon black, for example, as described in US 2021 / 0086524 A. However, preferably the pre-coat layer contains the organic polymeric nanoparticles and is as defined herein.P608908PC00

[0053] As explained earlier, in the past it has been found difficult to substitute carbon black for dyestuffs in thermal recording media, which would be desirable to enhance the recyclability of the recording medium and provide other advantages. If dyestuffs are incorporated into the pre-coat, they problematically mostly end up being dissolved in the coating serum and migrate on application into the substrate. It is therefore difficult to obtain a substrate with low enough brightness and such efforts requires large amounts of dyestuff. Also, recyclability of such a paper will no longer be acceptable with such high dyestuff loading. There therefore exists a need to both generate a matrix with high light absorption efficiency combined with the lowest possible usage of dyestuff, so as not to impair the recyclability of the improved thermal printing paper. To prevent the dyestuff from penetrating into the base stock it should be adsorbed on the surface of pigments like calcined clay. However, the intense light scattering provided by the calcined clay makes it nearly impossible to obtain a dark substrate. Even by application of large quantities of dyestuff the coating will mostly show a greyish appearance and is not suitable for application in the contrast pre-coat layer. To achieve a high enough contrast between the pre-coat and the top-coat both flocculation and migration of the dyestuff must be prevented in the pre-coat layer.

[0054] Surprisingly it has been found that synthetic emulsion particles can be encapsulated with specific cationic polymers without coagulation in the pre-coat. The modified particles adsorb anionic dye (such as black dye) in a thin layer, so that only partial light absorption occurs. The polymeric particles scatter light efficiently so that, depending on the amount of dye used, a bright surface results. The backscattered light is predominantly in a wavelength between 400 to 500 nm, so that the coatings appear blue or green. Therefore, if heat is applied to the later described light scattering layer on the surface of the thermal recording medium, the light scattering layer becomes transparent and the accumulated black dye which has previously been adsorbed on the emulsion particles will absorb most of the incident light. A significant brightness contrast between the virgin coating and the areas exposed to heat results. The pre-coat layer may therefore be referred to as a thermosensitive layer. It is therefore also preferable that the dyestuff carrier has a higher minimum film forming temperature than the organic polymeric nanoparticles so that the dyestuff carrier does not melt upon passing through a conventional thermal printing machine (alternatively can be defined as having a higher melting point). The use of a precoat that is thermosensitive means that no additional insulating layers are required in the thermal recording medium.

[0055] In particular, it has been surprisingly found that organic (polymeric) nanoparticles can be used to formulate the pre-coat with excellent light scattering properties which, when exposed to heat will transform from an opacifying layer into a transparent film. It has been found to be possible to use certain polymers to effectively adsorb a thin layer of dyestuff onto the surface of organic nanoparticles in the pre-coat, meaning that when the organic nanoparticles are exposed to sufficient heat (i.e., heat above the MFT of the precoat), the organic nanoparticles become transparent and the dark colour of the dyestuff becomes more visible through the transparent top-coat. Since the pre-coat generates contrast itself between the heated and non-heated parts, a higher overall colour contrast is observed by a person viewing the thermal recording medium of the invention. Any colour change that takes place in the pre-coat is generally a local, irreversible colour change.P608908PC00

[0056] In particular, it has been found advantageous that the pre-coat layer comprises organic polymeric nanoparticles, a dyestuff carrier such as a polymer containing primary amino groups and a dyestuff, wherein the organic nanoparticles have an average particle size in the range of about 100 to about 250nm and a minimum film forming temperature of between about 40 °C to about 80 °C.

[0057] In certain embodiments, the pre-coat layer has a coat weight of between about 3 to about 8 g / m2, optionally between about 4 to about 7 g / m2, or optionally between about 4 to about 6 g / m2. In certain embodiments, the pre-coat layer comprises the organic polymeric nanoparticles in an amount of about 70 to about 98 wt%, preferably about 80 to about 98 wt%. In certain embodiments, the pre-coat layer comprises the dyestuff carrier in an amount of about 1 to about 25 wt%, preferably about 1 to about 15 wt%. In certain embodiments, the pre-coat layer comprises the dyestuff in an amount of about 1 to about 25 wt%, preferably about 1 to about 10 wt%.

[0058] In certain embodiments, the pre-coat layer comprises a general structure of the organic polymeric nanoparticles having the dyestuff adsorbed onto the surface of the nanoparticles through the dyestuff carrier. Therefore, the dyestuff is dispersed throughout the pre-coat, preferably together with the organic polymeric nanoparticles and the dyestuff agent. Preferably, the dyestuff is substantially evenly distributed / dispersed throughout the pre-coat layer.The organic polymeric nanoparticles

[0059] Organic polymeric nanoparticles are known in the art and are used extensively in paints, binders, adhesives etc. Organic polymeric nanoparticles referred to herein takes its usual meaning in the art. Nanoparticles refer to particles of matter 1 to 100 nanometres (nm) in diameter. The nanoparticles could also be referred to as particulate polymers or scattering particles or latex nanoparticles.

[0060] The organic polymeric nanoparticles have an average particle size in the range of about 100 to about 250nm, preferably between about 120 nm to about 180 nm. The average particle size is measured via dynamic light scattering, optionally using a laser based optical system, preferably in accordance with ISO 22412 (such as ISO 22412:2008 or preferably in accordance with ISO 22412:2017). It is advantageous that the organic nanoparticles have this size so that they are comfortably smaller than the wavelength range of visible light. This means that they are not light scattering for visible light themselves since the wavelength range of visible light is between 400 nm and 800 nm, far above the average particle size of the nanoparticles. Thus, the particles may be light scattering when they are sufficiently packed but upon melting can become transparent. Smaller particles below 100 nm average particle size yield films with little light scattering and a high degree of transparency which makes them unsuitable for use in thermoprintable coatings. Larger particles have a smaller light scattering surface area. As a result, sensitivity of the thermal printing layer is adversely impacted, e.g. more energy is required to produce a transparent area exposing the dark precoat. In certain embodiments, the organic polymeric nanoparticles have a BET specific surface area of about 30 to about 40 m2 / g. In alternative embodiments, the average particle size referred to herein may refer to an "average particle diameter" as defined in accordance with the measurement methods referred to herein, or may refer to a d99 or dgo particle size distribution. In certainP608908PC00embodiments, averages referred to herein refer to either weight average or number averages, as will be understood by a person skilled in the art. For the avoidance of doubt, while the nanoparticles of the present invention are characterized as having a particular average particle size, in alternative embodiments the nanoparticles may also not necessarily have an average particle size within the stated ranges and may have a different, undefined average particle size i.e., they may be defined in a manner absent a specific average particle size range.

[0061] The organic polymeric nanoparticles have a minimum film forming temperature (MFT) of about 40 °C to about 80 °C, optionally between about 50 °C to about 70 °C. MFT takes its usual meaning in the art, namely the lowest temperature at which a latex, emulsion or adhesive will uniformly coalesce when laid on a substrate as a thin film. The minimum film forming temperature is determined in accordance with ASTM D 2345 and / or ISO 2115 (e.g., ISO 2115-1996). It is advantageous that the organic polymeric nanoparticles have this minimum film forming temperature so that they appropriately (and irreversibly) melt and coalesce upon passing through a thermal printing machine. The organic polymeric nanoparticles are configured to melt and / or coalesce upon the application of sufficient heat from a thermal printing machine.

[0062] The monomer units used to generate the organic polymeric nanoparticles are not particularly limited as long as organic polymeric nanoparticles are generated which meet the required average particle size and melt flow index requirements. Any monomer containing one or more polymerizable vinyl group could conceivably be employed. In certain embodiments, the organic polymeric nanoparticles comprise or are a polymer based on monomers of one or more of styrene, butadiene, ethylene, vinyl acetate, vinyl propionate, vinyl versatate, vinyl chloride, vinylidene chloride, vinyl formamide, acrylic acid, itaconic acid maleic acid, maleic anhydride, acrylamide, methacrylamide and Cl to C4 esters of acrylic and methacrylic acid, and the like. Preferably the organic nanoparticles are or comprise styrene butadiene copolymer nanoparticles. Styrene-butadiene copolymers are preferred, and conveniently are widely available. An example of commercially obtainable suitable organic polymeric nanoparticles is Trinseo HPC 57 Experimental Latex, which is a nanoparticle polymer emulsion comprising 45-55 wt% of a styrene-butadiene copolymer and 45-55 wt% water.

[0063] There are many well-known ways of preparing the organic polymeric nanoparticles and any of these known methods would be suitable, although they are mainly separated into bottom up and bottom down processes. The main bottom down method of producing such organic polymeric nanoparticles is to produce the nanoparticles from solid thermoplastic polymers with a softening point of about 40 °C to about 80 °C involving a cryogenic grinding process. However, cryogenic grinding can be very expensive due to considerable consumption of liquid nitrogen and the unavoidable generation of a considerable amount of particles in the micrometre range. Preferred are bottom-up processes, which are best described as producing large molecular structures from individual molecules by the following chemical reaction processes: radical polymerization, cationic anionic polymerization, polycondensation, polyaddition and controlled polymer precipitation.

[0064] Most organic polymeric nanoparticles are produced by radical emulsion polymerization of ethylenically unsaturated monomers as an emulsion. The monomers are emulsified with a blend of ethoxylated linear or branched alkylethoxylates with an alkyl chain length of 10-12 carbon atoms and a polyoxyethylene chain length of 6 to 10 ethyleneP608908PC00oxide units and an alkyl sulfonate like lauryl sulfonate. The vinyl monomers are emulsified under stirring to an aqueous monomer emulsion and then continuously fed to the pressurized reactor. The polymerisation will take place under a pressurized nitrogen blanket. As initiators, persulfates like ammonium or potassium persulfate are mostly used. However other initiators like organic peroxides or azo-initiators can be used, which are water soluble and show a precise polymerisation initiation temperature by releasing the azo group as gaseous nitrogen.

[0065] Emulsion polymerization can be carried out by adding all reaction partners, especially the monomer emulsion to a suitable reactor and adding initiator in a single shot or continuously to the reactor. However, general practice is now to feed the monomer emulsion and initiator solution on a continuous basis to the reactor. The polymerization reaction takes place in emulsifier micelles. Monomer emulsified in micro sized droplets migrates into the emulsifier micelles where spontaneous radical polymerization occurs at the very beginning of the process.

[0066] Each emulsifier micelle can be considered as a kind of individual nano reactor in which polymer chains grow by radical addition of monomer to the growing polymer chain. To improve mono modality of the growing nanoparticles a so-called seed dispersion can be used. A seed dispersion is an emulsion polymer with a particle size between 50 nm to 100 nm produced by pre-reacting a limited quantity of monomers. If the monomer mix and initiator have been fed into the reactor, the reaction will be allowed to continue at a temperature between 60°C to 90°C for several hours. This process step is meant to reduce the quantity of unreacted monomers in the emulsion to the lowest possible level. Another procedure to reduce residual monomers is a post addition of initiator. If these reaction steps are not sufficient to reduce residual monomers level to a low enough concentration, the emulsion must be subjected to a so-called thin film steam distillation step. Apart from environmental considerations residual monomer content should be as low as possible, mostly because of objectionable odour.

[0067] The organic polymeric nanoparticles are generally provided in an emulsion form i.e., a copolymer in water emulsion, and are generally present in the thermal recording medium in this emulsion form. Preferably the emulsion comprises about 45 to 55 wt% copolymer and about 45 to 55 wt% water (such that the total is 100 wt%). Preferably, the emulsion polymer has a kinematic viscosity of less than about 500 cS. The organic polymeric nanoparticles are generally provided as an emulsion having a dry solids level of between about 40 wt% to about 60 wt%, preferably about 50%. An emulsion polymer with the desired minimum film forming temperature can be obtained by controlling by the ratio of monomers yielding polymers with high MFT versus low MFT. Classic examples are the co-monomer styrene / butadiene ratio and the ethylacrylate / butylacrylate ratio which allows to vary MFT between about -5 °C up to about 80 °C. For example, the MFT of an emulsion consisting of a styrene / butadiene copolymer can increase to 60°C to 70°C with a styrene content between about 70 wt% to about 75 wt%.Dyestuff

[0068] The dyestuff to be used in the pre-coat layer of the present invention is not particularly limited but it is preferable that an anionic dyestuff is used. Further preferably, a black anionic dyestuff is used, although any colour dyestuff may be used.P608908PC00

[0069] The dyestuff is present in the pre-coat of the invention only i.e., it is not present in the top-coat or in the substrate. The dyestuff is preferably dispersed throughout the pre-coat layer. Particularly, the dyestuff is adsorbed onto the surface of the organic polymeric nanoparticles, preferably through the use of a dyestuff carrier. Preferably, the anionic dyestuff is mixed with the organic polymeric nanoparticles and dyestuff carrier throughout the pre-coat layer i.e., substantially evenly dispersed across the layer. This structure helps to prevent the dyestuff from leaching into the substrate.

[0070] Since conventional dyestuffs can be used in the pre-coat layer, rather than carbon black for example, the thermal recording medium can be completely recycled since conventional dyestuffs can easily be bleached in a recycling process. Non-bleachable components such as carbon black prevent the thermal recording medium from being recyclable. Furthermore, as explained herein the thermal recording medium of the present invention requires less dyestuff than conventional thermal recording media which results in cost savings and also makes the thermal recording medium more recyclable. Therefore, the pre-coat of the present invention generally contains only small amounts of the dyestuff.Dyestuff carrier

[0071] It has been found that certain polymers can function to adsorb on acid aqueous interfaces (i.e., at the interface of the organic polymeric nanoparticles) and are then capable of binding dyestuff to the interfaces containing lewis acid groups. The dyestuff adsorbed on the aqueous interface is no longer capable of migrating and is locked in the coating matrix. The effect of avoiding migration of the dyestuff into the substrate is therefore achieved and the dyestuff is fixed to the organic polymeric nanoparticles through the dyestuff carrier i.e., the polymer is configured to prevent migration of dyestuff out of the pre-coat layer.

[0072] Preferred polymers are those possessing strong nucleophilic groups, in particular those containing primary, secondary or tertiary amino groups, amidine groups, lactam groups and lactone groups. Examples of suitable polymers containing lactam groups include polyvinylpyrrolidone and polyviniylcaprolactam or polymers with grafted lactam groups. Polymers, whose functionality is based on the presence of amino groups are polyvinyl amine, polyvinyl amine-vinyl alcohol, copolymers, polyallylamine and chitosan.

[0073] A particularly effective dyestuff carrier has been identified as a fully or partially hydrolysed polyvinyl alcohol (PvOH) (such as with a viscosity of 5.5 to 24 mPas, 4% solution, measured using an Ubbelohde type viscometer) reacted with cyanoguanidine under acidic conditions. Carbocations formed by reaction of the secondary hydroxyl groups of the polyvinyl alcohol chain under acidic conditions react with the nitrile groups of the cyanoguanidine, the first reaction step being the synthesis of an amidine structure. If the reaction proceeds further, hydrolysis occurs and a primary amine group is inserted into the polyvinyl alcohol chain. As a result, a copolymeric vinyl alcohol-vinylamine polymer results. An acid may be used to catalyse the reaction. This product exhibits extraordinarily strong affinity to polar surfaces and is capable of binding acidic dyestuffs in over stochiometric quantities.

[0074] Without being bound by theory, it is believed that the dyestuff carrier acts as a heat switch by altering remission from blue to black upon heat exposure (when a black dyestuff is used). It is assumed that protonation of the amine group controls the interaction modeP608908PC00of the polymer with solid surfaces and hydrophilic polymers. If the amino groups are protonated, mainly ionic interactions will occur whereas the free primary amino group undergoes strong interaction with, for instance, hydroxyl groups in, for instance, fully hydrolysed polyvinyl alcohols. By introducing primary amino groups into a fully hydrolysed polyvinyl alcohol-chain, polymer solutions which show no signs of gelation can be obtained. Upon drying a supramolecular polymer network is formed which can bind considerable quantities of dyestuff to solid / liquid interfaces.

[0075] Preferably, the dyestuff carrier is a polyalcohol amine compound, preferably a vinylalcohol-vinylamine copolymer. Such a dyestuff carrier has been particularly effective because it greatly reduces penetration of soluble material into the substrate (e.g., dyestuff) and its combination with the organic polymeric nanoparticles enables a high smoothness coated sheet to be obtained even on rough, uncalendared substrates. Issues relating to stickies can also be avoided. These benefits result from the excellent interaction potential of the vinylalcohol-vinylamine copolymer with the substrate.

[0076] Preferably, the polyalcohol amine compound is obtained by reacting a polyvinyl alcohol with cyanoguanidine in the presence of water, wherein the reacting step further comprises the addition of an acid. Preferably, the reacting step comprises an initial step of mixing the polyvinyl alcohol with cyanoguanidine in the presence of water and a subsequent step of adding the acid. Preferably, the initial step comprises heating and stirring the polyvinyl alcohol and cyanoguanidine mixture to at least partially, preferably completely, dissolve the polyvinyl alcohol and cyanoguanidine mixture in water. Preferably, the initial step comprises heating the mixture to a temperature of between about 80 to about 100 °C. Preferably the acid is orthophosphoric acid, preferably orthophosphoric acid having a concentration of about 80 to about 90%. Preferably the polyvinyl alcohol is at least 99% hydrolyzed, preferably fully hydrolyzed.

[0077] It has been found that alcohols like polyvinyl alcohol can be converted to partial polyamines by reaction with cyanoguanidine in the acid environment. If polyvinyl alcohol is dissolved in the presence cyanoguanidine a complex is formed between the hydroxyl groups of the polyvinyl alcohol and the amidine groups present in the cyanoguanidine. Without being bound by theory, a complex between the hydroxyl groups of the polyvinyl alcohol and the amidine group of cyanoguanidine is formed, which can break up the partially crystalline polymer structure of polyvinyl alcohol and thereby promotes better solubility of the fully hydrolysed polyvinyl alcohol. With most of the cyanoguanidine complexed by polyvinyl alcohol, addition of mineral acids to the solution promotes an addition reaction of the nitrile group to the secondary carbon atoms of the polyvinyl alcohol chain. A direct carbon-nitrogen bond is established. Eventually after a prolonged reaction time a molecule of guanyl acetic acid is split off from the cyanoguanidine molecule attached to the polyvinyl alcohol polymer chain. Hydroxyl groups of the polyvinyl alcohol have been transformed into primary amine groups. Depending on the stochiometric ration between alcohol groups and cyanoguanidine a certain percentage of amine groups will have been inserted into the polymer chain. The resulting compound has been found to be particularly effective as the dyestuff carrier. Therefore, preferably the dyestuff carrier is a polyalcohol amine compound obtained from a reaction of cyanoguanidine and polyvinyl alcohol in the presence of an acid.

[0078] In this respect, these dyestuff carriers have been found to be particularly effective at fixing anionic dyestuffs to the organic polymeric nanoparticles. The primary amino group interact with lewis acid sites at the solid-liquid interface. Most dyestuffs containP608908PC00sulfonic acid and / or carboxyl groups which will bind to the primary amino group of the vinylalcohol-vinylamine polymer. This results in less dyestuff migrating away from the pre-coat layer which is better for recycling purposes and also enhances the brightness of the colour of the pre-coat layer with less dyestuff addition required.

[0079] Alternative dyestuff carriers can be prepared by converting hexamethyl methylol melamine (C10H24N6O) in an acid environment to polymeric cationic nanoparticles which will also be capable of fixing dyestuff to an aqueous interface. The reaction proceeds through condensation of the methylmethylol groups under release of methanol and formaldehyde to methylene groups. Such a polymer can be obtained by reacting polyethylene glycol (e.g., polyethylene glycol 600) with cyanoguanidine. The mixture is heated and stirred to melt the polyethylene glycol. An acid is then added, such as orthophosphoric acid, under stirring and heat until all the cyanoguanidine has dissolved and is then heated for a certain period of time until hexamethyl methylol melamine is added and stirred under heat to obtain a suitable polymer to act as the dyestuff carrier.

[0080] Further alternatives include the use of carboxymethyl cellulose (CMC). In water, CMC is added together with cyanoguanidine and stirred under heat. An acid such as organophosphoric acid may be added and then the mixture is reacted under heat for a certain period of time to obtain a suitable polymer to act as the dyestuff carrier.

[0081] The dyestuff carrier has a higher minimum film forming temperature than the organic polymeric nanoparticles, such that the dyestuff carrier does not melt when processed in a conventional thermal printing device. The dyestuff carrier is different to the organic polymeric nanoparticles.Second layer (herein "top-coat layer")

[0082] The thermal recording medium of the present invention contains a top-coat layer. The top-coat layer is carried by the pre-coat layer. Preferably the top-coat layer is applied directly onto the pre-coat layer i.e., with no other layer in between. However, in alternative embodiments, the top-coat is applied indirectly onto the pre-coat layer i.e., other layers are positioned in between the pre-coat and the substrate. Generally, the top-coat layer covers all or substantially all of one surface of the pre-coat layer.

[0083] In the event that the organic polymeric nanoparticles are contained in the pre-coat, in certain embodiments the top-coat layer can be any conventionally known top-coat layer and can include the use of hollow sphere pigments (HSPs), for example, as described in US 2021 / 0086524 A. However, preferably the top-coat layer contains the organic polymeric nanoparticles.

[0084] As long as the top-coat is configured to become substantially transparent at selected print locations upon application of sufficient heat at the selected print locations, it is suitable for use in the present thermal recording media. The light scattering layer typically comprises light scattering particles, which are preferably configured to melt upon application of sufficient heat from a conventional thermal printer. The application of heat causes the light scattering particles to melt, causing the light scattering layer to become transparent, or substantially transparent, such that the first (pre-coat) layer can become visible.P608908PC00

[0085] As used herein reference to becoming substantially transparent and transparent refers to the layer changing state such that more light can pass through so that objects behind become more visible, preferably so that the object behind can be distinctly seen and they take their usual meaning in the art. As is to be expected, the term transparent does not require the layer to necessarily be completely transparent. Furthermore, reference herein to the top-coat becoming transparent refers to a local irreversible change.

[0086] The top-coat layer comprises organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250nm and a minimum film forming temperature of between about 40 °C to about 80 °C, both measured in accordance with the methods described herein. The organic polymeric nanoparticles used in the top-coat are as defined herein in relation to the organic polymeric nanoparticles used in the precoat layer and so all of the features relating to the organic polymeric nanoparticles in the pre-coat are applicable here but are not specifically repeated for reasons of brevity.

[0087] Preferably, the top-coat layer also comprises an agglomerating agent. The agglomerating agent functions to agglomerate the organic polymeric nanoparticles in the top-coat. The agglomerating agent is defined in the same way as the dyestuff carrier in the pre-coat layer and therefore all the features relating to the dyestuff carrier are also applicable to the agglomerating agent but are not specifically repeated here for reasons of brevity. In the top-coat, the agglomerating agent / dyestuff carrier compound obviously no longer acts as a dyestuff binder since no dyestuff is present in this layer but instead acts as an agglomerating agent for the organic polymeric nanoparticles. Specifically for the agglomerating agent, high molecular weight hydrophilic polymers are preferred and are mixed with the nanoparticle emulsion. The hydrophilic polymer swells and binds large amounts of water. As this water originates from the nano-emulsion, the interparticle distance decreases. The reduced interparticle distance allows long range van der Waals attractive forces to become dominant and agglomerates with reduced water content e.g. higher solids level are formed. The agglomerates consist of isolated particle groups with higher solids level, but by application of shear the agglomerates can be easily redispersed. Examples of polymers suitable to function as the agglomerating agent are in particular those which will generate osmotic flocculation such as cellulose ethers like carboxymethyl cellulose, carrageenans, alginates, galactomannans, polyvinyl alcohol, etc.

[0088] Structurally the top-coat layer comprises the organic polymeric nanoparticles agglomerated by the agglomerating agent. The agglomerating agent makes the top-coat light scattering by transforming the organic polymeric nanoparticles into large agglomerates that are capable of scattering visible light. The agglomerates should be composed of a limited number of original particles and are weakly bound together, that means the agglomeration is reversible under mild shear conditions but is reconstituted, if the coating layer immobilizes, due to water loss after application on a substrate. Upon the application of heat from, for example, a conventional thermal printing machine, the organic polymeric nanoparticles melt and fuse into a molten mass together with the agglomerating agent in order to transform from light scattering to a transparent or substantially transparent layer. Therefore, it is preferable that the agglomerating agent has a higher minimum film forming temperature than the organic polymeric nanoparticles so that the agglomerating agent does not melt upon passing through a conventional thermal printing machine.

[0089] When present in both the top-coat and pre-coat layers, the organic polymeric nanoparticles and the agglomerating agent in the top-coat may be the same or may beP608908PC00different to the organic polymeric nanoparticles and dyestuff carrier respectively in the pre-coat layer. Obviously, for ease of processing it may be more convenient to use the same substances for both components throughout the thermal recording medium.

[0090] In certain embodiments, the top-coat layer has a coat weight of between about 2 to about 10 g / m2. In certain embodiments, the top-coat layer comprises the organic polymeric nanoparticles in an amount of about 55 to about 95 wt%, preferably about 70 to about 95 wt%. In certain embodiments, the top-coat layer comprises the agglomerating agent in an amount of about 5 to about 45 wt%, preferably about 5 to about 30 wt%.Methods of preparing the thermal recording medium

[0091] In certain embodiments, a suitable substrate as defined herein is prepared or commercially obtained. To prepare the pre-coat, organic polymeric nanoparticles as defined herein are prepared (as explained herein) or commercially obtained and preferably mixed with a suitable dyestuff carrier and a dyestuff, and subsequently applied to a surface of the substrate using conventionally known means and methods. The order of addition of the substances is not particularly important, but it may be preferable to mix the dyestuff and dyestuff carrier first or the dyestuff carrier and the organic polymeric nanoparticles first, and then subsequently add the third component. To prepare the top-coat, the organic polymeric nanoparticles as defined herein are prepared (as explained herein) or commercially obtained and preferably mixed with a suitable agglomerating agent and then applied directly onto the pre-coat layer. The layers are mixed and applied using conventional means for manufacturing thermal printing media. Conventional methods are used to prepare the thermal recording medium and so further specific details are not provided herein. For example, conventional means of preparing a thermoprintable paper are used.In certain embodiments, also generally disclosed / described herein is the following embodiments:• Use of a vinylalcohol, vinylamine polymer capable of generating a dyestuff polymer complex encapsulating pigment particles.• Usage of a vinylalcohol-vinylamine polymer in quantities of 5 to 20 pts dry relative to nano-emulsion.• Usage of equivalent amounts of anionic black sulfonated azo dyestuff relative to carrier polymer.• Usage of styrene-butadiene nano emulsion with a minimum form forming temperature of 60°C to 80°C and a particle size range from 100 nm to 250 nm.• Usage of styrene-acrylate nano emulsion with a particle range of 100 to 200 nm and a minimum film forming temperature of 60°C to 80°C.• Contrast formulation comprising 100 pts of a nano emulsion with an MFT of 60°C to 80°C, a vinylalcohol vinylamine polymer in quantities of 1.5 to 20 pts dry preferably between 5 to 15 pts dry and an anionic sulfonated black azo dyestuff (reactive black dye) in quantities of 1.5 to 20 pts commercial solution to obtain a contrast precoat formulation.• Application of 2 to 10 g dry precoat formulation on a suitable base sheet, employing a wide range of fibers, including brown OCC- (old corrugated container) fibers.P608908PC00• Application of a dyestuff containing precoat formulation in a coat weight range from 2g / m2to 10g / m2to obtain a pre-coated sheet providing a brightness of 20 to 40 brightness points.• A precoated wood free sheet which is coated with a formulation according to any formulation herein, yields a sheet brightness of 20 to 40, which on exposure to a heat drops to a sheet brightness of 2 to 10 brightness points. Such a sheet provides excellent contrast as precoat for a thermal recording medium.• A top-coat formulation comprising a nano emulsion with a minimum film forming temperature between 60°C to 80°C and 1.5 to 10 pts dry of a vinylalcohol, vinylamine with a degree of substitution between 0.1 to 0.2 of hydroxyl groups and a viscosity of 5 to 24, preferably 10 to 14 mPas 4% solution.• The formulation is applied at coat weights ranging from 2 g / m2to 10 g / m2on a precoated sheet, to obtain a thermal recording medium.• A topcoat formulation comprising 100 pts nano emulsion 1 to 1.5 pts polyvinyl alcohol with a viscosity range from 5 to 10 mPas and 2 to 10 pts of a styrenebutadiene emulsion with a minimum film forming temperature of -5°C to 15°C are formulated to yield a top coat with excellent covering properties and good thermoprintability.Examples

[0092] The following non-limiting examples are provided.Example 1 - Preparation of a medium viscosity vinyl alcohol - vinylamine copolymer

[0093] In a bottle neck flask equipped with drip funnel speed controllable stirrer and thermometer, 850 ml demineralized water is added at room temperature. 120g solid polyvinyl alcohol powder characterized by the following parameters:Viscosity 4% solution: 14 mPasDegree of hydrolysis: 98 + / - 1%pH 4% solution: 6.5 + / - 0.05is slowly added under vigorous stirring to prevent lump formation.

[0094] 30g of cyanoguanidine powder is added under stirring to the mix. The addition of cyanoguanidine prevents formation of PVOH lumps and a relatively homogeneous mixture is obtained. The mixture is heated to around 95°C to 100°C and held at this temperature for at least 30 min. After this time, a clear homogeneous solution was obtained. The reaction between PVOH and cyanoguanidine was started by drastically lowering the pH of the mixture by addition of mineral acid, preferably orthophosphoric acid.

[0095] 8 g of 85% orthophosphoric acid was added under strong stirring and the mixture held at this temperature for 120 min. During the reaction which proceeds via interaction of carbocations formed due to the split off of the secondary hydroxyl group from the polyvinyl alcohol molecule at low pH. During the reaction pH increases from 1.5 to 2.0 to about 5 to 5.5 after a reaction of about 120 to 180 min. If a pH - figure within this range is obtained most of the cyanoguanidine has reacted with the polyvinyl alcohol and a degree of substitution of 0.1 to 0.2 has been reached. Degree of substitution in this context meansP608908PC00that 10% to 20% of hydroxyl groups in the polyvinyl alcohol chain have been substituted with primary amino groups. Upon cooling and filtration, a clear viscous liquid was obtained which can be used in the discussed application without any further modification. The substitution of hydroxyl groups with amino groups eliminates the gelling behaviour normally associated with solutions of fully hydrolysed PVOH. However, due to the high viscosity PVOH used as raw material, the finished product must be diluted to a 15% dry solids level for a viscosity low enough for pumping. The modified vinyl alcohol, vinylamine polymer has a Brookfield viscosity 50 Rpm spindle 3 at 23° C of 1200 to 1600 mPas.

[0096] After cooling a clear viscous liquid is obtained. The solution does not show any gelling tendency during storage at room temperature.

[0097] The substitution of hydroxyl groups with amino groups leads to intramolecular interactions. As consequence a highly compact particle like polymer structure is obtained. This type of polymer does not uncoil if absorbed on a solid liquid interface but acts as voluminous steric stabilizer for particles. Formation of interparticle bridges leading to particle agglomeration is thus prevented. If used in combination with nanoparticles coating layers with extremely homogeneous particle distribution can thus this be obtained. If used in combination with the described nanoparticles coating layers with good light scattering properties and homogeneous coat weight distribution can be obtained.Example 2

[0098] 100 g of a 75% - solution of a commercial Hexamethyl-methylol-melamine resin Madurit MW 112 produced by Prefere resins are diluted with 275 g demineralized water under slight stirring to a 20% solution. 5 g of a 60% - acetic acid is added to adjust pH of the mixture to 4.5 to 5. The mixture is stirred at room temperature for about 12 hours. After that time due to self-condensation a slightly turbid cationic colloidal suspension was obtained. The resulting cationic suspension acts as an effective carrier for anionic dyestuffs and can absorb at least its equivalent weight of for instance black anionic dyestuff solution. The dyestuff loaded suspension can be applied in combination with carrier pigments like nanoparticle emulsion to a suitable base paper to generate a contrast coating layer.Example 3

[0099] 4 g of a commercial Montmorillonite Rheomont OP, Imerys minerals are dispersed under strong agitation in 96 g of demineralized water. The mixture is stirred for about 12 hours at room temperature until most of the mineral has defoliated. Defoliation in this context means, that due to swelling phenomenaes particle sheets with a diameter of 6 to 10 nm and a sheet length of 0.5 pm to 1.0 pm are obtained. The thin particle sheets are transparent for visible light and can act as carrier for cationic nanoparticles. 100g of a 75% Hexa-methylol-melamine solution is added to the Montmorillonite suspension together with 5 g of an anionic black dyestuff solution(Trupocor CSP, Kapp Chemie). 2 pts of a 60% solution of acetic acid is added to adjust pH to about 4.5 to 5, to promote condensation of the melamine resin. The mixture is stirred for 12 h at room temperature until the ensuing resin-dyestuff complex is fixed to the surface of the Montmorillonite particles. A black homogeneous suspension is obtained which can be used in the precoat formulation as a contrast enhancing material.P608908PC00Example 4

[0100] 100 pts of a 50% - dispersion of a styrene-butadiene emulsion Trinseo HC57 with an average, particle diameter of 0.14 pm and a minimum film forming temperature of 60° C are mixed under moderate stirring with 10 g of a 20% polyvinyl alcohol - polyvinyl amine solution based on a fully hydrolysed PVOH with the viscosity of 14 mPa.s according to Example 1. Upon addition of the PVOH / PVAm solution moderate thickening of the mixture with a viscosity of 150 to 250 mPas Brookfield, 50 Rpm at 47% solids occurs. 2g of a black dyestuff solution Trupocor CSP are added under mild stirring. Upon addition of the dyestuff the mixture assumes a deep green blue colour.

[0101] A 40 g / m2fully sized and slightly calendered wood free base sheet is used as a substrate. The coating mixture is applied with wound Myerrods on DIN A4 sheets with a laboratory coater. Depending on rod size a coat weight of 3g / m2to 8 g / m2was achieved. The coated sheets were dried at a temperature of 35°C to 40°C for 3 min in a drying oven. Intense blue coated sheets with high smoothness were obtained. Brightness of the coated sheets varied from 25 to 30.

[0102] If exposed to heat exceeding the minimum film forming temperature of the nano emulsion, brightness dropped to a level of 5 to 10 brightness points depending on coat weight. These brightness levels provide sufficient contrast between topcoat and precoat if printed with a commercial thermoprinter (as shown in Table 1).Example 5

[0103] The precoated sheets were topcoated with the following formulation:

[0104] 100 pts Trinseo HC57, 10 pts of a 20% solution of PVOH / PVAm according to Example 1 followed by addition of 2 pts dry Aluminium-restinate. The PVOH / PVAm -solution was added under slight stirring to the styrene - butadiene emulsion. The mixture was homogenized under moderate agitation for about 10 min. After that time, a coating mix with a viscosity of 200 mPas to 300 mPas Brookfield spindle 3, 50 Rpm 23° was obtained. The coating mix was applied with a laboratory coater to the precoated sheets at coat weight from 3 g / m2to 7 g / m2. Brightness was increased after top coating as shown in Table 1. An extremely homogeneous glory scratch resistance, top-coated product was obtained.Example 6

[0105] Nano polyvinyl acetate emulsion Vinnapas H65, Wacker Chemie Burghausen MFT 40°C pH 5, 65% solids was used instead of the styrene butadiene emulsion. The emulsion is stabilized during production by using polyvinyl alcohol as a polymeric stabilizer. Because of the low film forming temperature an increase of film forming temperature is required, to provide an opaque coating layer. This can be achieved by adding a small quantity of aluminium salts, which complex with the PVOH used as a stabilizer.P608908PC00

[0106] To lOOpts Vinnapas H65 were added under agitation 2 ml of a 2% aluminium sulphate solution followed by addition of 13.3 ml of 15% - solution of partially hydrolysed polyvinyl alcohol, with a viscosity of 14 mPas 4% solution.

[0107] Viscosity of the mix increased to a Brookfield viscosity 50 Rpm, spindle 3, 23° C of about 800 mPas. If this formulation was applied at a coat weight of 5 to 7g / m2to a precoated sheet as described in Example 4 a thermoprintable sheet of 40 brightness was obtained.Example 7

[0108] 100 pts of a styrene butadiene (organic nanoparticle) dispersion Trinseo HC57 minimum film forming temperature 60° C with pH 8 to 8.5 was mixed under gentle stirring with 2 pts of a 25% solution of a partially hydrolysed PvOH with a viscosity of 5 to 5.5 mPas, 4% solution, degree of hydrolysis 88% under gentle stirring. The Brookfield viscosity of the mix increases to 600 to 800 mPas Brookfield viscosity spindle 3.

[0109] 10 pts of a styrene-butadiene latex Trinseo 4020, with a pH 5 to 5.5 was added under gentle stirring. Viscosity of the blend increases slightly to 800 to 900 mPas Brookfield viscosity. The coating formulation was applied laboratory coater to a precoated basic sheet using a precoat formulation as described in Example 4. Coat weight is 5g / m2to 7 g / m2. A top coated sheet with a brightness of 40 to 45 brightness points was obtained, which is thermoprintable.Example 8

[0110] To 100 pts of a styrene acrylic co-polymer with a particle size between 100 to 250 nm and a minimum film forming temperature of 40° C to 80° C 2 pph of a graft polymer of 20 pts 2-Dimethylaminoetylmethlacrylate and 80 pts PVOH 5 - 5.5 mPas viscosity hydrolysis degree 88% are added as a 25% solution. After stirring a coating formulation with a viscosity of 800 mPas and a dry solids level of 48% is obtained. When applied on a precoated sheet at a coat weight between 2 to 8g / m2a thermoprintable sheet with a brightness depending on coat weight and substrate between 40 and 55 was obtained.Table 1: Effect of using pre-coats and top-coats prepared in accordance with the present inventionP608908PC00

[0111] Both pre-coat and top-coat are thermoprintable. That means if heat is applied a brightness drop is realized in the heat exposed areas. It was now found that the brightness decrease obtained only with the contrast giving precoat layer is a function of coat weight. Depending on the amount of black dye used in the formulation a coating brightness of 15 to 35 was realized. However, if the coating is exposed to heat and becomes transparent the achievable brightness drop is a function of the amount of black dye available for light scattering in the transparent areas, hence coat weight. It is now possible by using a higher precoat weight and low dyestuff concentration to obtain a sheet with relatively high brightness and still realise in the printed areas a brightness drop, which allows for good image legibility. As a consequence, top coat weight can be reduced to such an amount as to realize a low brightness increase of the top coat sheet from 35 to 44 with a coat weight of only 3 g / m2to 4 g / m2. The advantage of the pre-coat I top-coat weight distribution, e.g. high pre-coat weight low top-coat weight, would be enhanced sensitivity of the paper e.g. lower energy demand to achieve a high print density.

[0112] If it is desired to achieve an extremely low brightness precoat level with a high addition rate of black dyestuff topcoat weight must be increased substantially to 6 g / m2to 8 g / m2to obtain a top coated sheet with a brightness of 45 to 48. However, a highertopcoat weight will adversely impact print sensitivity of the thermo sheet. It appears that adequate thermoprintability can be achieved by combining a high precoat weight with low dyestuff addition, hence relatively high brightness of the precoated sheet with a relatively low topcoat weight of 3g / m2to 5g / m2. The commercial carbon black coated paper 'Blue Forest' (BLUE4EST) (described in the Comparative Example below) operates with a very low precoat brightness and must realize an adequate brightness drop of 39 to 40 with a necessary relatively high topcoat weight of 8g / m2to 10g / m2. As this coating is formulated with a hollow sphere pigment with higher costs per kilogramme material, compared to the nano emulsion-based solution as described in this application the new thermopaper concept allows for significant cost savings.Comparative Example

[0113] A commercially available thermal paper, BLUE4EST®, from Koehler was tested as a comparative example. This product is a blue thermal paper based on the use of a pre-coat having a carbon black layer and a top-coat containing hollow sphere pigments (HSPs). The properties of this comparative example were analysed and the results of this analysis are shown in Table 2 below.Table 2P608908PC00Example 9

[0114] A polyethylene glycol MW 600 was molten, and 80 g of the molten PEG added to a vessel equipped with stirrer. Under mild agitation 20g of cyanoguanidine was added together with 2g of an orthophosphoric acid. The mixture was heated under agitation to 95°C to 100°C and kept at this temperature for about 120 min. Upon cooling a clear liquid with a viscosity of 100 mPas to 200 mPas Brookfield 50 Rpm spindle 3 was obtained. The product could be directly used as an anionic dyestuff carrier. The solution is basically water free.Example 10

[0115] Instead of polyvinyl alcohol other water-soluble hydrocolloids could be reacted with cyanoguanidine, in order to introduce primary amino groups into the polymer molecule. Carboxymethylcellulose, Finnfix FF5 supplied by Nouryon Chemicals OY Finland was used as a hydrocolloid. 800ml of demineralized water was added to a vessel, equipped with a stirrer and 150g of Finnfix FF5 was added together with 50g cyanoguanidine. The mixture was stirred at room temperature until a clear solution was obtained. 10g of an 85% orthophosphoric acid was added and the mixture heated to 95°C to 100°C for 120 min. A yellowish solution was obtained with good carrier properties for anionic dyestuff.Dyestuff carrier

[0116] To produce a usable dyestuff carrier, the amount of cyanoguanidine relative to polyvinyl alcohol should not exceed a molar ratio 1 to 5. Higher amounts of cyanoguanidine yield insoluble condensation products. The preferred molar ratio cyanoguanidine to PVOH is 1 to 10. If lower quantities of cyanoguanidine are employed dyestuff carrier properties of the product are negatively impact. The viscosity of the polyvinyl alcohol ranges between 5 to 25 mPas Brookfield 4% solution. As catalyst for the condensation reaction between PVOH and cyanoguanidine any strong mineral acid could be used. However, it was found that orthophosphoric acid provides the best compromise between the desired formation of primary amino groups and then undesirable formation of higher molecular weight condensates. The preferred amount of catalyst for the condensation reaction is between 0.05 to 0.1 mol relative to 1 mol polyvinyl alcohol.P608908PC00

[0117] Another dyestuff carrier can be obtained by grafting a reactive amine like Dimethylethylaminomethacrylate onto the polyvinyl alcohol chain. It is also possible to first react polyvinyl alcohol with cyanoguanidine followed by a grafting reaction with Dimethylethylaminomethacrylate. The grafted PVOH produces a more pronounced viscosity increase in combination with the nano emulsion Trinseo HC57.

[0118] Example:

[0119] In a vessel equipped with stirrer, reflux condenser drip funnels and thermometer 85 g of powered PVOH 5-88, viscosity 4% solution 5 - 5.5mPas Brookfield, 50 Rpm, spindle 3 23°C are dispersed in 290. ml dem. water. The mixture is heated to a temperature of 95°C to 100°C under moderate agitation and held at this temperature for 30min, until a clear homogeneous solution is obtained. Upon cooling to 70°, 15g of Dimethylethylaminomethacrylate together with 20g of a 1%- Ammonium persulfate solution is added over a period of 30min. The reaction mixture is held at a temperature of 70° to 75°C during 120min and then cooled to room temperature. The product is an efficient carrier for anionic dyestuff.

[0120] Example:

[0121] A dyestuff carrier with excellent carrier properties and good viscosity increasing properties was obtained by introducing both primary and secondary amino groups into the PVOH molecule. To a multi-neck vessel equipped with stirrer, reflux condenser and thermometer are added 290ml of dem. water. Under strong agitation 85 g of PVOH 5 - 99 were added followed by 10 g of cyanoguanidine. The mixture was heated to 95°C to 100°C and after this temperature was reached 4ml of 85% orthophosphoric acid was added. The mixture was reacted for 90min at 95°C and then cooled to 70°C. 5g of Dimethylethylaminomethacrylate was added followed by 10 ml of 1%- solution of Ammonium persulfate. The mixture was reacted for about 120min at 70°C. After cooling to room temperature, a clear viscous liquid was obtained, which can be used as dyestuff carrier.

[0122] Some other non-limiting examples (1A to 14A):

[0123] Example 1A: In a bottle neck flask equipped with stirrer and reflux condenser fully hydrolyzed 120g polyvinyl alcohol with a viscosity of 24 mPas are dissolved in 850 ml demineralized water at 95° C under vigorous stirring. 30g cyanoguanidine is added until a clear solution is obtained. 8g orthophosphoric acid at 85% concentration is added and the mixture is reacted for 120 min at a temperature of 90 °C to 95 °C. After cooling a clear viscous liquid is obtained. The solution does not show any gelling tendency during storage at room temperature.

[0124] Example 2A: Instead of using a high viscosity polyvinyl alcohol as in Example 1A, lower viscosity polyvinyl alcohols can be used at higher solution concentrations instead. In this Example, a polyvinyl alcohol had a viscosity of 14 mPas was processed in the same way as in Example 1A to obtain a vinyl alcohol / vinyl amine polymer yielding a 20%-dry solids solution. Alternatively, a polyvinyl alcohol having a viscosity of 5 mPas was used yielding a 25%-dry solids solution with a Brookfield viscosity spindle 3 between 850 to 1250 mPas / 23 °C.

[0125] Example 3A: A 75%-solution of Hexamethyl methylol melamine was diluted to 20% solids and pH-adjusted with an acid to a pH of 4,5 - 5.0. The mixture is allowedP608908PC00to react at room temperature for about 12 hours, until a slightly turbid suspension is obtained. This generates very small-crosslinked macromolecules that could also be used as the agglomerating agent and / or dyestuff carrier such as those described in Examples 1A and 2A.

[0126] Example 4A: In a further embodiment 100 parts of a 4% pre-dispersed dispersion of Rheomont PO (montmorillonite from Imerys minerals) are mixed with 5 parts of blue Trupocor CSP dyestuff and 100 parts of a 75%-solution of water based Hexa-methylol-melamine. After homogenizing 2 parts of acetic acid are added. During the condensation reaction, the dyestuff is fixed and immobilized on the surface of the mineral. The reaction is stopped by neutralizing the system with caustic.

[0127] Example 5A: 100 pts of a 50% dispersion of styrene / butadiene emulsion with a minimum film forming temperature of 50° C is mixed with 2.0 pts of an anionic black dye solution. 10 pts of the modified VA / VAm as described in Example 1A are added while vigorously stirring. The mixture is applied to a wood free base stock at a coat weight of 2-10 g / m2preferably 2 to 8g / m2. Upon drying a paper with a brightness of 15 -25 is obtained.

[0128] Example 6A: 100 pts of a 50% dispersion of styrene / butadiene with a minimum film forming temperature of 50° C is mixed with 10 pts polymer solution described in Example 4A. Upon mixing 0.5 pph of an anionic black dye solution are added. The mixture is applied with a Myer rod onto a suitable base paper with a coat weight of 2 - 9 g / m2preferably 2 to 6 g / m2. Upon drying a paper with a brightness of 5-10 is obtained.

[0129] Example 7A: 100 parts of polyvinyl acetate Vinnapas H65 (TG + 40°C, pH 5, Vise. 15000 mPas, 65% Solids) were diluted to 40% solids and mixed with 2 parts of 14-88 polyvinyl alcohol to increase viscosity. This was followed by the addition of 2 ml of a 2% - Al2(SC>4)3 solution. The white opaque dispersion was coated on the black precoated substrate which results in a white opaque layer. The opaque layer is thermoprintable. The agglomerated product is applied to a precoated sheet according to Example 5A. A bright opaque coating with the brightness of 45 to 50 at a coat weight of 5 to 6g / m2is obtained.

[0130] Example 8A: 100 pts of a styrene butadiene (organic nanoparticle) dispersion, 50% solids, pH 7.5 to 8, minimum film forming temperature 40 °C to 80 °C. This dispersion is mixed with 2 pph of a 25% PVOH solution viscosity 5 to 5.5 hydrolysis range 88% 10 pph of a 50% styrene butadiene dispersion with a minimum film forming temperature of 0°C to 14° C and a pH of 5 to 5.5 are added under stirring. The interaction of the acid SB- dispersion with the alkaline dispersion generates soft agglomerates. If the mixture is applied to a precoated sheet at a weight of 2 to 10g / m2a coating with a brightness of 45 to 60 is obtained. The product is thermal printable.

[0131] Example 9A: To 100 pts of a styrene acrylic co-polymer with a particle size between 100 to 250 nm and a minimum film forming temperature of 40° C to 80° C 2 pph of a graft polymer of 20 pts 2- Dimethylaminoethylmethlacrylate and 80 pts PVOH 5 - 5.5 mPas viscosity hydrolysis degree 88% are added as a 25% solution. After stirring a coating formulation with a viscosity of 800 mPas and a dry solids level of 48% is obtained. If applied on a precoated sheet at a coat weight between 2 to 8g / m2a thermoprintable sheet with a brightness depending on coat weight and substrate between 40 and 55 is obtained.

[0132] Example 10A: To 100 pts of a styrene butadiene dispersion with a minimum film forming temperature of 40° C to 80° C are added 1 pph of VA / Vam modified polymerP608908PC00as described in Example 1A. A coating with a Brookfield viscosity of 500 to 800 mPas is obtained. The coating can be applied to a dark precoated substrate at coat weights from 2 to 10g / m2. A bright coating with thermoprintable properties is obtained.

[0133] Example 11A: 850ml of demineralized water was added to a vessel equipped with a stirrer. 120g of PvOH (grade 24-99) and 30g cyanoguanidine were added under vigorous stirring. The granular PvOH dissolves readily without formation of lumps by heating to a temperature of 90°C. After the PvOH is completely dissolved 8g of 85% solution of orthophosphoric acid was added. The mixture was allowed to react at a temperature of 85°C to 90°C for 90 minutes. After cooling a clear viscous liquid was obtained, which could be used as the dyestuff carrier / agglomerating agent without further modification.

[0134] Example 12A: 750g of demineralised water was added to a vessel equipped with a stirrer. 50g cyanoguanidine and 200g of fully hydrolysed polyvinylalcohol were added under vigorous stirring. Upon heating the polyvinylalcohol dissolves rapidly. When a temperature of 90° C was reached, 8g of an 85% solution of orthophosphoric acid was added. The further reaction was performed as described in Example 11A. Upon cooling a clear viscous liquid was obtained which could be used as the dyestuff carrier / agglomerating agent without further processing.

[0135] Example 13A: 750 g of polyethylene glycol 600 were added to a vessel equipped with stirrer. 150g of cyanoguanidine were added under stirring and the mixture was heated to melt the polyethylene glycol. 8g of 85% solution of orthophosphoric acid was added to the mixture under stirring and heated to 90°C until all of the cyanoguanidine was dissolved. The mixture was kept under stirring at this temperature for about 60 minutes. Under cooling to 70°C, 100g of a 75% solution of Hexamethoxymethylmelamine was added and the mixture stirred for another 60 minutes at this temperature. Upon cooling a clear viscous liquid was obtained that can be used directly as the dyestuff carrier / agglomerating agent.

[0136] Example 14A: 800ml of demineralized water was added to a vessel equipped with a stirrer and 150g of a low viscosity carboxymethylcellulose (CMC FF5) was added, together with 50g of cyanoguanidine, and stirred under heating to 90°C until a clear solution was obtained. 10g of an 85% solution of orthophosphoric acid was added. The mixture was reacted for 90 minutes at 90 °C and, after cooling to room temperature, a yellowish clear solution was obtained, which can be used as the dyestuff carrier / agglomerating agent without further processing.

Claims

P608908PC00Claims1. A thermal recording medium, comprising:a substrate;a first layer carried by the substrate, wherein the first layer is a pre-coat layer comprising a dyestuff; anda second layer at least partially covering the first layer, wherein the second layer is a light scattering layer that is configured to become substantially transparent at selected print locations upon application of sufficient heat at the selected print locations; whereinthe first layer further comprises organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250 nm, preferably determined according to ISO 22412, and a minimum film forming temperature of between about 40 °C to about 80 °C, preferably determined according to ISO 2115, and a dyestuff carrier, and / orthe second layer comprises organic polymeric nanoparticles having an average particle size in the range of about 100 to about 250 nm, preferably determined according to ISO 22412, and a minimum film forming temperature of between about 40 °C to about 80 °C, preferably determined according to ISO 2115, and an agglomerating agent.

2. The thermal recording medium according to claim 1, wherein the organic polymeric nanoparticles in the pre-coat layer are at least partially coated by the dyestuff, preferably wherein the dyestuff is adsorbed onto the surface of the nanoparticles via the dyestuff carrier.

3. The thermal recording medium according to any preceding claim, wherein the precoat layer is configured such that upon the application of sufficient heat at the selected print locations, the organic polymeric nanoparticles in the pre-coat layer melt at the selected print locations to liberate the dyestuff and make the pre-coat layer appear visibly darker at the selected print locations, preferably wherein sufficient heat is a temperature at or above the minimum film forming temperature of the organic polymeric nanoparticles in the pre-coat layer.

4. The thermal recording medium according to any preceding claim, wherein the organic polymeric nanoparticles in the top-coat are agglomerated by the agglomerating agent.

5. The thermal recording medium according to claim 4, wherein the organic polymeric nanoparticles in the top-coat layer are sufficiently agglomerated such that the topcoat layer is substantially opaque, preferably wherein the second layer is configured such that upon the application of sufficient heat at the selected print locations, the organic polymeric nanoparticles in the top-coat layer melt at the selected print locations to reduce the degree of agglomeration to become substantially transparent, preferably wherein sufficient heat is a temperature at or above the minimum film forming temperature of the organic polymeric nanoparticles in the top-coat layer.P608908PC006. The thermal recording medium according to any preceding claim, wherein the thermal recording medium comprises both the pre-coat layer and top-coat layer as defined in claim 1;preferably wherein the dyestuff carrier and the agglomerating agent are both the same or different and / or preferably wherein the organic polymeric nanoparticles in the pre-coat and top-coat layer are the same or different.

7. The thermal recording medium according to any preceding claim, wherein the dyestuff is an anionic dyestuff, preferably a black anionic dyestuff.

8. The thermal recording medium according to any preceding claim, wherein the organic polymeric nanoparticles are a polymer based on monomers of one or more of styrene, butadiene, acrylate and vinylacetate, preferably wherein the polymer is an emulsion polymer, further preferably wherein the organic polymeric nanoparticles are a styrene-butadiene polymer.

9. The thermal recording medium according to any preceding claim, wherein the dyestuff carrier and / or agglomerating agent is or comprises a polymer containing primary amino groups, preferably wherein the polymer containing primary amino groups is a polyalcohol amine, further preferably obtained by reacting a polyvinyl alcohol with cyanoguanidine in the presence of an acid.

10. The thermal recording medium according to claim 9, wherein the acid is orthophosphoric acid and / or the polyvinyl alcohol is at least 99% hydrolyzed, preferably fully hydrolyzed.

11. The thermal recording medium according to any preceding claim, wherein the precoat layer comprises the organic polymeric nanoparticles in an amount of between about 70 wt% to about 98 wt%, the dyestuff in an amount of between about 1 wt% to about 25 wt% and / or the dyestuff carrier in an amount of between about 1 wt% to about 25 wt%.

12. The thermal recording medium according to any preceding claim, wherein the topcoat layer comprises the organic polymeric nanoparticles in an amount of between about 55 wt% to about 95 wt% and / or the agglomerating agent in an amount of between about 5 wt% to about 45 wt%.

13. The thermal recording medium according to any preceding claim, wherein:the pre-coat layer has a coat weight of between about 3 to about 8 g / m2, optionally between about 4 to about 7 g / m2, or optionally between about 4 to about 6 g / m2; and / orthe top-coat layer has a coat weight of between about 3 to about 8 g / m2, preferably between about 4 to about 7 g / m2or optionally between about 4 to about 6 g / m2.

14. The thermal recording medium according to any preceding claim, wherein the thermal recording medium does not comprise one or more or all of carbon black, hollow sphere pigments, calcined clay, bisphenol A and bisphenol C, preferably wherein the top-coat does not contain any dyestuff.P608908DE0015. A method of making a thermal recording medium, preferably the thermal recording medium according to any preceding claim, the method comprising:i) applying a first layer as defined in any preceding claim to a substrate; and ii) applying a second layer as defined in any preceding claim on the first layer, optionally further comprising the step of preparing the dyestuff carrier and / or agglomerating agent by reacting a polyvinylalcohol with cyanoguanidine in the presence of water, wherein the reacting step further comprises the addition of an acid and wherein the dyestuff carrier and / or agglomerating agent is a polyalcoholamine compound.

Images