Dispersion

Abstract

Heat-sensitive layers for heat-sensitive recording media formed from dispersions that include a fatty acid amide as heat-sensitive particles that change from blocking to transmitting light due to heating present problems with scratch resistance, calender roll staining, and the like.　This dispersion is for forming a heat-sensitive layer that changes from blocking to transmitting light due to deliberate heating. The dispersion includes heat-sensitive particles that change due to deliberate heating, inorganic particles, and a binder. The heat-sensitive particles include at least one type of wax particles selected from among behenic acid, caranuba wax, and rice bran wax.

Description

dispersion

[0001] The present invention relates to a dispersion for forming a thermosensitive layer of a thermosensitive recording medium.

[0002] The heat-sensitive layer of a general-purpose thermal recording medium develops color through a thermal color-developing reaction between a colorless or pale-colored leuco dye and a color developer consisting of a phenol or organic acid. By intentionally applying heat to a specific region of the heat-sensitive layer of such a thermal recording medium, a colored image is formed. This allows for a compact recording device, facilitates maintenance, and reduces noise. Thermal recording mediums are widely used as recording paper for label printers and other issuing machines, automatic ticket vending machines, ATMs, order slip output devices in restaurants, and data output devices in scientific research equipment.

[0003] However, while phenolic color developers such as 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) and bis(4-hydroxyphenyl)sulfone (bisphenol S), which are used in the heat-sensitive layer of common heat-sensitive recording media, have good color-developing properties, they may be endocrine disruptors, and as a result, their use was banned in Europe in January 2020, raising concerns about the safety of the components that make up heat-sensitive recording media. Due to safety concerns about phenolic color developers, progress has been made in replacing them with color developers that do not have a phenol skeleton (hereinafter referred to as non-phenolic), but the safety of non-phenolic color developers has not been sufficiently verified, and issues with conventional heat-sensitive recording media remain, such as their inability to be used in food contact applications, including leuco dyes.

[0004] To address this issue, a thermosensitive recording medium has been proposed in which a thermosensitive layer is laminated onto a colored base layer (see Patent Document 1). In this thermosensitive recording medium, the colored base layer is, for example, high-quality paper coated with black paint or a resin film dyed black. The thermosensitive layer changes from light-shielding to light-transmitting when intentionally heated. In the light-shielding state, the thermosensitive layer blocks visible light, making the color of the base layer invisible. On the other hand, in the light-transmitting state, visible light passes through the thermosensitive layer, making the color of the base layer visible. This thermosensitive layer contains hollow polymer particles, inorganic particles, and a binder. The mechanism by which this thermosensitive layer changes from light-shielding to light-transmitting is thought to be as follows: Before heating, the light-shielding properties of the thermosensitive layer are maintained due to the light-scattering effect of the hollow polymer particles. On the other hand, when the hollow polymer particles are deformed by heating, their light-scattering effect decreases, and the thermosensitive layer in the heated areas becomes light-transmitting.

[0005] Recently, as microplastics have become an environmental issue, there is a risk that the use of the hollow polymer particles mentioned above in thermal recording media may be restricted. Patent documents 2, 3, and 4 propose using a heat-sensitive wax instead of hollow polymer particles. The heat-sensitive wax changes from light-blocking to light-transmitting upon intentional heating, and in the above examples, a fatty acid amide is used. That is, a thermal recording media has been proposed in which a heat-sensitive layer containing a fatty acid amide, inorganic particles, and a binder is laminated on a colored base layer.

[0006] International Publication No. WO 2010 / 48394 International Publication No. WO 2019 / 141751 European Patent No. EP 2993055 European Patent No. EP 3957489

[0007] The present inventors investigated the performance of a thermosensitive layer containing a fatty acid amide. Using stearic acid amide as the fatty acid amide, silica as the inorganic particles, and polyvinyl alcohol as the binder, a thermosensitive layer containing these was prepared and laminated on a black underlayer. Indeed, the thermosensitive layer exhibited high light-blocking properties and high whiteness before heating (Comparative Examples 2 to 5). However, the thermosensitive layer exhibited poor abrasion resistance. For example, the unheated area changed when rubbed with a finger, revealing the base color of the underlayer.

[0008] In addition, thermosensitive recording media equipped with the above-mentioned thermosensitive layer also present productivity issues. To smooth the surface of a thermosensitive recording medium, a smoothing treatment using a calender roll is generally required. However, a thermosensitive layer using a fatty acid amide is fragile, and when the calender roll is pressed against the thermosensitive recording medium, components of the thermosensitive layer may adhere (transfer) to the calender roll. When the calender roll becomes soiled in this way, the smoothness of the surface of the thermosensitive recording medium deteriorates, resulting in a decrease in printability. To avoid this, the calender roll must be cleaned, which also reduces productivity.

[0009] The inventors of the present invention have conducted extensive research to solve at least one of the above problems and have arrived at the present invention. That is, a first aspect of the present invention is defined as follows: A dispersion for forming a heat-sensitive layer, the heat-sensitive layer being changed from light-blocking to light-transmitting upon intentional heating, the dispersion containing heat-sensitive particles that change upon intentional heating, inorganic particles, and a binder, the heat-sensitive particles including at least one wax particle selected from behenic acid, carnauba wax, and rice wax.

[0010] When at least one wax particle selected from behenic acid, carnauba wax, and rice wax was used as the heat-sensitive particles, the resulting heat-sensitive layer exhibited superior abrasion resistance compared to conventional heat-sensitive layers using fatty acid amides. Furthermore, transfer to the calender roll was eliminated. This effect was maintained even when fatty acid amides were used in combination as heat-sensitive particles.

[0011] Wax-based heat-sensitive particles, such as ester waxes such as behenic acid, carnauba wax, and rice wax, and fatty acid amide waxes such as stearic acid amide, are referred to in this specification as Type A heat-sensitive particles. By using such Type A heat-sensitive particles, the wax component ensures the lubricity (printability) and releasability of the heat-sensitive layer.

[0012] The particles of behenic acid, carnauba wax, and rice wax are light-shielding and do not transmit visible light in their default state (the state before intentional heating; hereinafter, sometimes abbreviated as the pre-heating state). In this way, in the pre-heating state, they cooperate with the inorganic particles to make the heat-sensitive layer light-shielding, preventing the background color of the colored base layer from being visible from the outside. On the other hand, when heat (intentional heating) is applied from a general-purpose thermal head, the behenic acid, carnauba wax, and rice wax change and become light-transmitting.

[0013] The amount of at least one selected from behenic acid, carnauba wax, and rice wax in the thermosensitive layer can be determined appropriately in combination with the amounts of inorganic particles and other particles, but should be such that the color of the colored base layer is not visible when the thermosensitive layer is coated with the layer. A larger amount of inorganic particles can maintain the light-shielding properties of the thermosensitive layer before heating, even when the amount of thermosensitive particles is reduced. On the other hand, because inorganic particles do not become translucent even when intentionally heated, incorporating an excessive amount of inorganic particles can prevent the thermosensitive layer from becoming translucent. In other words, the amount of inorganic particles is adjusted to ensure the transparency of the thermosensitive layer after heating. According to the inventors' research, the amount of inorganic particles is preferably 30% by mass or less of all components in the dispersion other than the dispersion medium (hereinafter sometimes simply referred to as "total components").

[0014] Although there is no particular lower limit for the amount of inorganic particles, in order to ensure sufficient light-shielding properties in the heat-sensitive layer before heating, the amount of inorganic particles can be 20% by mass or more relative to the total amount of ingredients. Such inorganic particles are preferably white. Examples of white inorganic particles that can be used include titanium oxide, silica, alumina, aluminum hydroxide, calcium carbonate, bentonite, and kaolin, and can have any particle size. The use of colored inorganic particles can impart color to the heat-sensitive layer.

[0015] The binder binds the heat-sensitive particles, inorganic particles, and other particles together, and water-soluble polymers such as polyvinyl alcohol and cellulose derivatives can be used as the binder resin.

[0016] This does not preclude the use of amide-based waxes, as used in conventional examples, as heat-sensitive particles. The amount of at least one of behenic acid, carnauba wax, and rice wax blended can be appropriately selected depending on the application of the heat-sensitive recording medium. From the standpoint of ensuring abrasion resistance, it can be 10% by mass or more of the total blended components. More preferably, it is 20% by mass, and even more preferably, it is 30% by mass or more. The upper limit of the blended amount of the wax is not particularly limited, but it can be 80% by mass of the total blended components. Provided that the wax is contained in an amount of 10% by mass or more of the total blended components, the upper limit of the blended amount of Type A wax-based heat-sensitive particles is also not particularly limited, but it can be 80% by mass of the total blended components. More preferably, it can be 70% by mass of the total blended components.

[0017] Other heat-sensitive particles (Type B) can be blended into the dispersion. Examples of such Type B heat-sensitive particles include 1,2-diphenoxyethane particles and / or polylactic acid particles. The inclusion of these particles improves the whiteness of the heat-sensitive layer before heating. The 1,2-diphenoxyethane particles and polylactic acid particles also change from light-blocking to light-transmitting upon intentional heating with a thermal head. The blending amount of 1,2-diphenoxyethane particles and polylactic acid particles can be 20% to 60% by mass of the total blended components. Blending these particles at 20% by mass or more improves the whiteness of the heat-sensitive layer. On the other hand, blending them at more than 50% by mass hardly results in any improvement in whiteness.

[0018] The above-mentioned Type B heat-sensitive particles have a higher light-shielding property than Type A heat-sensitive particles before heating, so by increasing the amount of these particles, the whiteness of the heat-sensitive layer can be increased. At the same time, Type B heat-sensitive particles are harder than Type A heat-sensitive particles, so they can impart good abrasion resistance to the heat-sensitive layer. Such Type B heat-sensitive particles also include hollow polymer particles.

[0019] Depending on the application of the heat-sensitive layer, zinc stearate and other auxiliary agents can be blended into the dispersion to ensure printability (slipperiness, prevention of thermal head contamination, etc.). Water can be used as the dispersion medium for such dispersions. Of course, alcohol or other organic materials can also be used in combination as the dispersion medium. The dispersion auxiliary can be one or a mixture of two or more of anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, polymeric surfactants, cationic polymer compounds, and anionic polymer compounds. Adjusting the surface tension can improve the uniformity of the coating surface and whiteness. The following thickeners can be blended as auxiliary agents for such dispersions. Adjusting the viscosity can improve the storage stability of the dispersion and the coating suitability during application. Examples of thickeners include cellulose derivatives such as methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose, and hydroxypropyl cellulose, starch derivatives such as cationized starch and etherified starch, plant gums such as gum arabic, guar gum, and xanthan gum, animal polymers such as casein, chitosan, and chitin, and polyalkoxide polymers such as polyethylene glycol. The amount of thickener to be added is selected appropriately depending on the application of the dispersion, and can be, for example, 0.01 to 2.0% by mass based on the total mass of the coating material (dispersion) for the heat-sensitive layer.

[0020] The colored base layer on which the thermosensitive layer is laminated may have a surface color (on which the thermosensitive layer is laminated) that is different from the color of the thermosensitive layer before heating. Since the surface color of a commonly used thermosensitive recording medium is white before heating, it is preferable that the surface color of the colored base layer be black. This is to create a strong contrast with the white color of the thermosensitive layer before heating. The material of the colored base layer is not particularly limited, but it can be made of paper if it is to be used for the same purpose as general-purpose thermal paper. When the thermosensitive layer is used for another purpose, the material of the colored base layer can be selected as desired depending on the purpose.

[0021] The thickness of the thermosensitive layer can be selected as desired, subject to the following conditions: the color of the colored base layer must be invisible before heating, and the heated area must become translucent under the intended heating conditions to be applied to the thermosensitive layer. Specifically, the thickness of the thermosensitive layer can be 2 μm to 12 μm, preferably 3 μm to 10 μm, and more preferably 3 μm to 7 μm. The thermosensitive layer can also have a multilayer structure. For example, a first layer in contact with the colored base layer contains type A thermosensitive particles (e.g., fatty acid amide) other than at least one wax particle selected from behenic acid, carnauba wax, and rice wax, and a second layer laminated on this first layer contains at least one wax particle selected from behenic acid, carnauba wax, and rice wax.

[0022] FIG. 1 shows the barcode used for the evaluation of printing performance.

[0023] The behenic acid (Cas No.: 112-85-6, also referred to as behenic acid) used in the dispersion of the present invention is not particularly limited in terms of purity, and examples thereof include NAA-222S (behenic acid 85% by weight, melting point 78 to 82°C) manufactured by NOF Corporation, Lunac BA (behenic acid 85% by weight, arachidic acid 10% by weight, melting point 78 to 82°C) manufactured by Kao Corporation, and Behenic Acid 90% (behenic acid 90% by weight, melting point 78 to 82°C) manufactured by VVF (India) Limited.

[0024] The carnauba wax (Cas No.: 8015-86-9) used in the dispersion of the present invention is a natural product obtained by refining carnauba wax harvested from palm trees of the Arecaceae family. It is composed of 80 to 85% by mass of wax esters, 3 to 4% by mass of free fatty acids, 10 to 12% by mass of free alcohols, and 1 to 3% by mass of hydrocarbons. Carnauba wax is also suitable because it is widely known for its food safety, as evidenced by its use as a food additive. Commercially available carnauba waxes can be used, including, for example, Carnauba Wax No. 1, No. 2, and No. 3 (melting point 82 to 88°C) manufactured by Toyochem Co., Ltd. and Carnauba Wax No. 1, No. 2, and No. 3 (melting point 82 to 88°C) manufactured by Kato Yoko Co., Ltd.

[0025] The rice wax (CAS No. 8016-60-2) used in the present invention is a vegetable wax extracted from rice bran oil. Rice wax contains fatty acids and higher alcohols as its main components. The fatty acids mainly include palmitic acid, behenic acid, and lignoceric acid, and the higher alcohols mainly include 1-hexacosanol and triacontanol. Commercially available rice waxes can be used, such as the TOWAX-3 series (melting point 76-86°C) manufactured by Toa Kasei Co., Ltd. and XECO-0002B (melting point 82-88°C) manufactured by Nippon Seiro Co., Ltd.

[0026] The aforementioned behenic acid, carnauba wax, and rice wax may be used alone or in combination. The blending amount of these waxes is preferably 10% to 80% by mass relative to the total mass of the medium in the dispersion, i.e., the heat-sensitive particles, inorganic particles, binder, and other auxiliaries (hereinafter referred to as the blending components of the dispersion). If the blending amount is less than 10% by mass, the slipperiness may be insufficient, resulting in noise and poor printing when printing with a receipt printer, and insufficient release properties may lead to transfer to the calendar roll. Even if the blending amount exceeds 80% by mass, abrasion resistance and prevention of transfer to the calendar roll can be achieved. However, considering that approximately 10% by mass of the binder is required as an essential component, the blending amount of particles effective for light-blocking, such as inorganic particles, is limited, which may result in insufficient light-blocking properties of the heat-sensitive layer before heating. Ester-based waxes other than behenic acid, carnauba wax, and rice wax, which have melting points of 70 to 90°C, are considered to have similar functions. In particular, behenyl behenate (Cas No. 17671-27-1, melting point approximately 75°C) can be used alone or in combination, as it provides the same light-blocking properties, slipperiness, and release properties as carnauba wax and rice wax. The particle size of the heat-sensitive particles (Type A) is not particularly limited, but from the standpoint of ensuring dispersibility in the dispersion and the smoothness of the surface of the heat-sensitive layer, it is preferable that the average particle size be 0.5 to 10 μm. The method for measuring the average particle size is as described in the Examples.

[0027] Numerous pigments commonly used in thermosensitive recording media can be used as inorganic particles. For example, the pigment may be selected from the group consisting of calcined kaolin, natural kaolin, kaolinite, magnesium silicate hydrate, silicon dioxide, bentonite, calcium carbonate, calcium silicate hydrate, calcium aluminate sulfate, aluminum hydroxide, aluminum oxide, and boehmite. The amount of these inorganic particles is preferably 30% by mass or less of the total components of the dispersion. While there is no particular lower limit for the amount of inorganic particles, to ensure sufficient light-shielding properties in the thermosensitive layer before heating, the amount of inorganic particles can be set to 20% by mass or more of the total components of the dispersion. While there are no particular limitations on the particle size of the inorganic particles, an average particle size of 0.5 to 10 μm is preferred to ensure dispersibility in the dispersion and surface smoothness of the thermosensitive layer. The average particle size can be measured in accordance with the method described in the Examples.

[0028] The binder binds the heat-sensitive particles, inorganic particles, and other particles. Examples of such binder resins include water-soluble polymers such as polyvinyl alcohol and cellulose derivatives. Examples of polyvinyl alcohols that can be used include fully (partially) saponified polyvinyl alcohol, carboxy-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, and silicon-modified polyvinyl alcohol. Examples of cellulose derivatives include hydroxyethyl cellulose, methyl cellulose, and carboxymethyl cellulose. The amount of binder used can be selected appropriately depending on the application of the heat-sensitive recording medium, but can be 5 to 25% by mass based on the total medium of the dispersion.

[0029] The Type B heat-sensitive particles contained in the dispersion of the present invention can include 1,2-diphenoxyethane particles and / or polylactic acid particles. These particles have a higher whiteness than Type A heat-sensitive particles before heating, and therefore can improve the light-shielding properties (whiteness) of the heat-sensitive layer before heating. The 1,2-diphenoxyethane (Cas No. 104-66-5) used in the present invention is not particularly limited in terms of purity, and examples include KS-235 (melting point 97°C) manufactured by Sanko Co., Ltd.

[0030] The polylactic acid used in the present invention may be a polylactic acid polymerized using lactic acid or lactide obtained by thermal decomposition of lactic acid, or a copolymer of lactic acid and another hydroxycarboxylic acid.

[0031] The other hydroxycarboxylic acid in the copolymer of lactic acid and other hydroxycarboxylic acid is not particularly limited, but examples thereof include glycolic acid, 2-hydroxybutyric acid, 2-hydroxyvaleric acid, 2-hydroxycaproic acid, 2-hydroxyheptanoic acid, 2-hydroxyoctanoic acid, 2-hydroxy-2-methylpropionic acid, 2-hydroxy-2-methylbutyric acid, 2-hydroxy-2-ethylbutyric acid, 2-hydroxy-2-methylvaleric acid, 2-hydroxy-2-ethylvaleric acid, 2-hydroxy-2-propylvaleric acid, 2-hydroxy-2-butylvaleric acid, 2-hydroxy-2-butylvaleric acid, and 2-hydroxybutanoic acid. Examples of the hydroxycarboxylic acids include 2-hydroxy-2-methylcaproic acid, 2-hydroxy-2-ethylcaproic acid, 2-hydroxy-2-propylcaproic acid, 2-hydroxy-2-butylcaproic acid, 2-hydroxy-2-pentylcaproic acid, 2-hydroxy-2-methylheptanoic acid, 2-hydroxy-2-ethylheptanoic acid, 2-hydroxy-2-propylheptanoic acid, 2-hydroxy-2-butylheptanoic acid, 2-hydroxy-2-methyloctanoic acid, 3-hydroxypropionic acid, 4-hydroxybutyric acid, 5-hydroxyvaleric acid, 6-hydroxycaproic acid, and 7-hydroxyheptanoic acid. These may be used alone or in combination of two or more.

[0032] In the copolymer of lactic acid and another hydroxycarboxylic acid, the molar ratio of the hydroxycarboxylic acid monomer to the lactic acid monomer is not particularly limited, but is, for example, 0.2 or less, or 0.1 or less, or 0.05 or less. When polylactic acid is used as the polylactic acid resin, the D-lactic acid content is preferably 1 to 30 mol%, and more preferably 5 to 20 mol%. Lactic acid and hydroxycarboxylic acid may take the D-, L-, or D / L-configuration forms, and any of these forms is not limited.

[0033] The weight-average molecular weight of the polylactic acid resin is preferably 5,000 to 1,000,000, and more preferably 10,000 to 300,000. The weight-average molecular weight (Mw) of the polylactic acid resin can be determined, for example, by gel permeation chromatography (GPC) in accordance with the method described in the Examples section below, by comparing with a standard substance having a known molecular weight.

[0034] The amount of Type B thermosensitive particles can be selected arbitrarily depending on the application of the thermosensitive recording medium, but can be 60% by mass or less of the total components of the dispersion. If the amount of Type B thermosensitive particles exceeds 60% by mass, the amount of Type A thermosensitive particles is limited, making it difficult to ensure the lubricity (printability) and releasability of the thermosensitive layer. There is no particular lower limit for the amount of Type B thermosensitive particles. By incorporating inorganic particles, the amount of Type B thermosensitive particles can be reduced to zero. The particle size of the thermosensitive particles (Type B) is not particularly limited, but from the standpoint of ensuring dispersibility in the dispersion and the surface smoothness of the thermosensitive layer, it is preferable that the average particle size be 0.5 to 10 μm. The average particle size is measured in accordance with the description in the Examples.

[0035] Examples of the present invention will be described below. These examples relate to dispersions for obtaining a thermosensitive layer of a thermosensitive recording medium. The thermosensitive recording medium has a structure in which a thermosensitive layer is laminated on the surface of a colored underlayer, and the thermosensitive layer is obtained by applying the dispersion of the example to the surface of the colored underlayer and drying it. The colored underlayer on which the dispersions of the examples and comparative examples described below are laminated has a basis weight of 55 g / m. ２We used single-color origami paper (surface color: black) made by Toyo Co., Ltd.

[0036] In the following description, unless otherwise specified, "parts" and "%" refer to "parts by mass" and "% by mass", respectively. The average particle size is the value for 50% of the cumulative distribution obtained by measurement using a laser diffraction particle size distribution analyzer (Microtrac MT3300II manufactured by Microtrac-Bell Co., Ltd.). Details are explained in "Shikizai" 67[7], 463-473 (1994).

[0037] The preparation of the dispersion for the heat-sensitive layer is described below. (1) Preparation of Type A Heat-Sensitive Particles (1-1) Preparation of Behenic Acid Dispersion (Liquid A-1) 20 parts of fine powder of behenic acid (trade name: NAA-222S, manufactured by NOF Corporation, behenic acid content 85 wt %, melting point 74 to 78°C), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 88%, polymerization degree 500), and 60 parts of water were mixed and pulverized using a sand mill to an average particle size of 4.4 μm to obtain a behenic acid dispersion (Liquid A-1).

[0038] (1-2) Preparation of behenic acid dispersion (Liquid A-2) 20 parts of fine powder of behenic acid (trade name: NAA-222S, manufactured by NOF Corporation, behenic acid content 85% by weight, melting point 74 to 78°C), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 88%, polymerization degree 500), and 60 parts of water were mixed and pulverized using a sand mill () until the average particle size became 2.2 μm, to obtain a behenic acid dispersion (Liquid A-2).

[0039] (1-3) Preparation of Carnauba Wax Dispersion (Liquid A-3) 20 parts of fine powder of carnauba wax (trade name: Carnauba Wax No. 2, manufactured by Toyochem Co., Ltd., melting point 80 to 86°C), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 88%, polymerization degree 500), and 60 parts of water were mixed and pulverized using a sand mill to an average particle size of 2.5 µm to obtain a carnauba wax dispersion (Liquid A-3).

[0040] (1-4) Preparation of rice wax dispersion (Liquid A-4) 20 parts of fine powder of rice wax (trade name: XECO-0002B, manufactured by Nippon Seiro Co., Ltd., melting point 77-86°C), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 88%, polymerization degree 500), and 60 parts of water were mixed and pulverized using a sand mill to an average particle size of 2.5 μm to obtain a rice wax dispersion (Liquid A-4). The heat-sensitive particles and binders used in the examples and comparative examples are as follows:

[0041] (1-5) Preparation of Stearic Acid Amide Dispersion (Liquid A-5) Hydrin U-425CE (solid content concentration 35%, average particle size 3.5μ, melting point of stearic acid amide 100°C) manufactured by Chukyo Yushi Co., Ltd. was used.

[0042] (2) Preparation of Type B Heat-Sensitive Particles (2-1) Preparation of 1,2-Diphenoxyethane Dispersion (Liquid B-1) 20 parts of 1,2-diphenoxyethane (trade name: KS-235, manufactured by Sanko Co., Ltd., melting point 97°C), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 88%, polymerization degree 500), and 60 parts of water were mixed, and the mixture was pulverized using a sand mill to an average particle size of 0.8 μm to obtain a 1,2-diphenoxyethane dispersion (Liquid B-1).

[0043] (2-2) Preparation of Polylactic Acid Dispersion (Liquid B-2) 20 parts of fine powder of polylactic acid (trade name: LX930, D-lactic acid:L-lactic acid=8:92, manufactured by Total Corbion, Tg 60°C, melting point 130°C), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 88%, polymerization degree 500), and 60 parts of water were mixed and pulverized using a sand mill to an average particle size of 1.8 μm, to obtain a polylactic acid dispersion (Liquid B-2).

[0044] (3) Preparation of Inorganic Particle Dispersion (3-1) Preparation of Silica Dispersion (C) 20 parts of silica (trade name: Mizukasil P-527, manufactured by Mizusawa Industrial Chemical Co., Ltd.), 20 parts of a 10% aqueous solution of polyvinyl alcohol (saponification degree 98%, polymerization degree 500), and 60 parts of water were mixed, and the mixture was dispersed using a sand mill until the average particle diameter became 5.0 μm, to obtain a silica dispersion (liquid C).

[0045] (4) Preparation of binder (4-1) Polyvinyl alcohol A (D-1 solution) Gohsenol NL-05 (saponification degree 98-99.0 mol%, polymerization degree 500) manufactured by Mitsubishi Chemical Corporation was dissolved in water and used (solids concentration 7%). (4-2) Polyvinyl alcohol B (D-2 solution) Gohsenol N-300 (saponification degree 98-99.0 mol%, polymerization degree 30,000) manufactured by Mitsubishi Chemical Corporation was dissolved in water and used (solids concentration 7%). (4-3) Hydroxymethylpropylcellulose (D-3 solution) Metrose 60SH-05 manufactured by Shin-Etsu Chemical Co., Ltd. was dissolved in water and used (solids concentration 7%). (4-4) Methylcellulose (D-4 solution) Benecel A15 manufactured by Ashland Japan Co., Ltd. was dissolved in water and used (solids concentration 7%).

[0046] (Example 1) (4) Preparation of coating material for thermosensitive layer A composition consisting of 318.2 parts of A-1 liquid, 90.9 parts of C liquid, and 142.9 parts of D-1 liquid was mixed and stirred to obtain a coating material for a thermosensitive recording layer.

[0047] (5-2) Preparation of a thermal recording medium The above-mentioned colored underlayer has a basis weight of 55 g / m ２ The dispersion was applied to one side of a single-color origami paper (black) in an amount of 5.0 g / m2 after drying. ２ The coating was dried at 60°C for 3 minutes to form a thermosensitive recording layer, and the surface was smoothed with a calendar roll. 2 ) was calculated from the difference between the mass of the underlayer before the dispersion was applied and the mass of the thermosensitive recording medium on which the thermosensitive layer was laminated by drying.

[0048] Example 2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, D-1 solution was replaced with D-3 solution.

[0049] Example 3 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, A-2 solution was used instead of A-1 solution.

[0050] Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 272.7 parts of A-1 liquid, 136.4 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0051] Example 5 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in preparing the dispersion in Example 1, 227.3 parts of A-1 solution, 113.6 parts of C solution, 357.1 parts of D-1 solution, and 40.0 parts of water were used.

[0052] Example 6 A thermosensitive recording medium was obtained in the same manner as in Example 5, except that in the preparation of the dispersion in Example 5, A-2 solution was used instead of A-1 solution.

[0053] Example 7 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 181.8 parts of A-1 liquid, 28.6 parts of A-5 liquid, 113.6 parts of C liquid, 357.1 parts of D-1 liquid, and 80.0 parts of water were used.

[0054] Example 8 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, A-3 solution was used instead of A-1 solution.

[0055] Example 9 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, A-1 solution was replaced with A-4 solution.

[0056] Example 10 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 181.8 parts of A-1 liquid, 85.7 parts of A-5 liquid, 90.9 parts of C liquid, 142.9 parts of D-1 liquid, and 130.0 parts of water were used.

[0057] Example 11 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 181.8 parts of A-1 liquid, 90.9 parts of B-1 liquid, 136.4 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0058] Example 12 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 181.8 parts of A-1 liquid, 136.4 parts of B-1 liquid, 90.9 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0059] Example 13 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, A-2 solution was used instead of A-1 solution.

[0060] Example 14 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, D-1 solution was replaced with D-2 solution.

[0061] Example 15 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, D-1 solution was replaced with D-3 solution.

[0062] Example 16 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, D-1 solution was replaced with D-4 solution.

[0063] Example 17 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 136.4 parts of A-1 liquid, 181.8 parts of B-1 liquid, 90.9 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0064] Example 18 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 90.9 parts of A-1 liquid, 227.3 parts of B-1 liquid, 90.9 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0065] Example 19 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 45.5 parts of A-1 liquid, 272.7 parts of B-1 liquid, 90.9 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0066] Example 20 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, A-3 solution was used instead of A-1 solution.

[0067] Example 21 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, A-1 solution was replaced with A-4 solution.

[0068] Example 22 A thermosensitive recording medium was obtained in the same manner as in Example 12, except that in the preparation of the dispersion in Example 12, B-2 liquid was used instead of B-1 liquid.

[0069] Example 23 A thermosensitive recording medium was obtained in the same manner as in Example 22, except that in the preparation of the dispersion in Example 22, A-2 solution was used instead of A-1 solution.

[0070] Example 24 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 181.8 parts of A-1 liquid, 90.9 parts of B-1 liquid, 136.4 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0071] Example 25 A thermosensitive recording medium was obtained in the same manner as in Example 22, except that in the preparation of the dispersion in Example 22, A-3 solution was used instead of A-1 solution.

[0072] Example 26 A thermosensitive recording medium was obtained in the same manner as in Example 24, except that in the preparation of the dispersion in Example 24, A-1 solution was replaced with A-3 solution.

[0073] Example 27 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 136.4 parts of A-1 liquid, 57.1 parts of A-5 liquid, 90.9 parts of B-1 liquid, 90.9 parts of C liquid, 142.9 parts of D-1 liquid, and 90.0 parts of water were used.

[0074] Example 28 A thermosensitive recording medium was obtained in the same manner as in Example 27, except that in the preparation of the dispersion in Example 27, B-2 liquid was used instead of B-1 liquid.

[0075] Example 29 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 136.4 parts of A-1 liquid, 136.4 parts of B-2 liquid, 90.9 parts of C liquid, 285.7 parts of D-1 liquid, and 80.0 parts of water were used.

[0076] Comparative Example 1 Blue4est manufactured by Koehler Paper was used as a thermosensitive recording medium.

[0077] Comparative Example 2 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 200.0 parts of A-5 liquid, 90.9 parts of C liquid, 142.9 parts of D-1 liquid, and 220.0 parts of water were used.

[0078] Comparative Example 3 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 171.4 parts of A-5 liquid, 127.3 parts of C liquid, 171.4 parts of D-1 liquid, and 160.0 parts of water were used.

[0079] Comparative Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 142.9 parts of A-5 liquid, 113.6 parts of C liquid, 357.1 parts of D-1 liquid, and 80.0 parts of water were used.

[0080] Comparative Example 5 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 114.3 parts of A-5 liquid, 218.2 parts of C liquid, 171.4 parts of D-1 liquid, and 70.0 parts of water were used.

[0081] Comparative Example 6 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 114.3 parts of A-5 liquid, 136.4 parts of B-2 liquid, 90.9 parts of C liquid, 142.9 parts of D-1 liquid, and 90.0 parts of water were used.

[0082] Comparative Example 7 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 318.2 parts of B-1 liquid, 90.9 parts of C liquid, and 142.9 parts of D-1 liquid were used.

[0083] Comparative Example 8 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 227.3 parts of A-1 liquid, 159.1 parts of C liquid, 214.3 parts of D-1 liquid, and 40.0 parts of water were used.

[0084] Comparative Example 9 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 409.1 parts of A-1 liquid and 142.9 parts of D-1 liquid were used.

[0085] Comparative Example 10 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 318.2 parts of A-1 liquid and 428.6 parts of D-1 liquid were used.

[0086] Comparative Example 11 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 136.4 parts of A-1 liquid, 227.3 parts of B-2 liquid, 285.7 parts of D-1 liquid, and 80.0 parts of water were used.

[0087] Comparative Example 12 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 227.3 parts of A-1 liquid, 136.4 parts of B-2 liquid, 285.7 parts of D-1 liquid, and 80.0 parts of water were used.

[0088] Comparative Example 13 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 386.4 parts of A-1 liquid, 45.5 parts of C liquid, 71.4 parts of D-1 liquid, and 50.0 parts of water were used.

[0089] Comparative Example 14 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 22.7 parts of A-1 liquid, 227.3 parts of C liquid, and 642.9 parts of D-1 liquid were used.

[0090] Comparative Example 15 A thermosensitive recording medium was obtained in the same manner as in Example 1, except that in the preparation of the dispersion in Example 1, 22.7 parts of A-1 liquid, 136.4 parts of C liquid, and 928.6 parts of D-1 liquid were used.

[0091] The results of the examples and comparative examples are shown in Table 1.

[0092] A comparison of Examples 1 to 29 and Comparative Examples 1 to 7 in Table 1 shows that the use of at least one wax particle selected from behenic acid, carnauba wax, and rice wax as the heat-sensitive particles results in favorable coating film performance (no calendar staining and abrasion resistance of the heat-sensitive layer). A comparison of Comparative Example 13 with Examples 1, 8, and 9 shows that it is preferable to have at least one wax particle selected from behenic acid, carnauba wax, and rice wax account for 80% by mass or less of the total dispersion components. Furthermore, a comparison of Comparative Examples 14 and 15 with Example 19 shows that it is preferable to have these wax particles account for 10% by mass or more of the total dispersion components. Furthermore, Examples 7 and 10 show that favorable coating film performance can be maintained even when a conventionally used fatty acid amide is used in addition to the above waxes. The results of Comparative Example 8 show that printing performance deteriorates when inorganic particles are incorporated in an amount exceeding 30% by weight of the total dispersion components. The results of Comparative Examples 9 to 12 show that even when inorganic particles are not incorporated, the whiteness (hiding ability) decreases, preventing printing contrast and resulting in poor printing performance. The preferred amount of inorganic particles is thought to be 10% to 30% by weight of the total dispersion components. The results of the Examples suggest that a content of 20% to 30% by weight is preferable to maintain sufficient whiteness in the heat-sensitive layer. The results of Examples 22 to 26 and 29 show that the whiteness of the heat-sensitive layer can be improved by using biodegradable polylactic acid as Type B heat-sensitive particles.

[0093] The various properties were measured or evaluated by the following methods. (1) Calendering Stain Using a calendering device SC-28 manufactured by Yuri Roll Machine Co., Ltd., each thermosensitive recording medium was calendered at a nip pressure of 5 kg / cm to smooth the surface of the thermosensitive layer. At that time, transfer of the thermosensitive layer material to the calender roll was visually confirmed.

[0094] (2) Whiteness (Hiding Ability) Each thermosensitive recording medium was calendered at a nip pressure of 5 kg / cm using a calendering device SC-28 manufactured by Yuri Roll Machine Co., Ltd. to smooth the surface of the thermosensitive layer. The reflection density of the resulting thermosensitive layer was measured using a spectrodensitometer (X-Rite 504, manufactured by X-Rite). The reflection density of the colored underlayer, monochrome Origami (black) manufactured by Toyo Corporation, was 1.40. The lower the reflection density value, the higher the hiding ability of the thermosensitive recording layer, and the higher the value, the lower the hiding ability. In order to print barcodes or QR codes (registered trademark) and have them recognized by a reading device, the difference in black and white contrast is important, and higher whiteness is required.

[0095] (3) Abrasion Resistance The thermosensitive layers obtained from the dispersions of the Examples and Comparative Examples were subjected to a calendering process at a nip pressure of 5 kg / cm using a calendering device SC-28 manufactured by Yuri Roll Machine Co., Ltd., to smooth the surface of each thermosensitive layer. The surface of the obtained thermosensitive layer was rubbed five times with the pad of a finger, and the reflection density was measured to confirm the change in whiteness. If the whiteness decreases due to physical contact such as rubbing, the contrast decreases, which reduces the visibility of recorded characters and makes it difficult to recognize QR Codes (registered trademarks) and barcodes. Therefore, the change in whiteness after the abrasion resistance test is preferably less than 0.10. If the change in whiteness after the abrasion resistance test is 0.10 or more, the visibility of characters and the recognition rate of code printing decrease, which is problematic in practical use.

[0096] (4) Print Density Using a receipt printer (product name: TM-T88V, manufactured by Seiko Epson Corporation), recording was performed on each thermosensitive recording medium at a print density of 100%, a multi-tone print density of 100%, a print speed level of 10, and a power capacity level of 3 (high), and the reflection density of the resulting recorded portion was measured with a spectrodensitometer (X-Rite 504, manufactured by X-Rite Corporation) to determine the print density. A higher print density is preferable, and a print density of 1.1 or more is preferred so that the image is clearly visible to the human eye. A print density of less than 0.9 results in low sensitivity and many defects such as whiteouts, which is problematic in practical use.

[0097] (5) Printability (QR Code (registered trademark) and barcode recognition) Using a receipt printer (product name: TM-T88V, manufactured by Seiko Epson Corporation), a QR Code (registered trademark) and a barcode were recorded on each thermal recording medium at a print density of 100%, a multi-tone print density of 100%, a print speed level of 10, and a power capacity level of 3 (high). It was confirmed whether the QR Code (registered trademark) and the barcode could be recognized using the standard QR Code (registered trademark) recognition app on the iPhone (registered trademark) 13 mini (iOS version 16.4.1) and JAN Reader (a barcode reader app created by Itoh Tomoaki). The QR Code (registered trademark) measuring 1.4 cm x 1.4 cm and the barcode measuring 3.8 cm x 2.6 cm shown in Figure 1 were used for evaluation. In the table, ○ indicates recognition possible, and × indicates recognition impossible.

[0098] The present invention is not limited to the above-described embodiments of the invention, and various modifications within the scope of the claims and within the scope that can be easily conceived by a person skilled in the art are also included in the present invention.

Claims

1. A dispersion forming a heat-sensitive layer, the heat-sensitive layer changing from light-shielding to light-transmitting by intentional heating, the dispersion comprising heat-sensitive particles that change when intentionally heated, inorganic particles, and a binder, the heat-sensitive particles including at least one type of wax particles selected from behenic acid, carnauba wax, and rice wax, the at least one type of wax particles comprising 10 to 80% by mass of the total components of the dispersion.

2. The dispersion according to claim 1, further comprising fatty acid amide wax particles as said heat-sensitive particles.

3. The dispersion according to claim 1, further comprising 1,2-diphenoxyethane particles and / or polylactic acid particles as the heat-sensitive particles.

4. The dispersion according to claim 1, further comprising hollow polymer particles as said heat-sensitive particles.

5. The dispersion according to claim 1, wherein the amount of the inorganic particles is 30 mass % or less of the total components of the dispersion.

6. A method for producing a thermosensitive recording medium, comprising the steps of: applying the dispersion according to claim 1 to a colored underlayer; and removing the solvent from said dispersion to form a thermosensitive layer on said colored underlayer.

7. A heat-sensitive layer which changes from light-shielding to light-transmitting by intentional heating, comprising heat-sensitive particles which change by the intentional heating, inorganic particles, and a binder, the heat-sensitive particles including at least one type of wax particles selected from behenic acid, carnauba wax, and rice wax, the at least one type of wax particles being 10 to 80% by mass of the total components of the heat-sensitive layer.

8. The heat-sensitive layer according to claim 7, further comprising fatty acid amide wax particles as said heat-sensitive particles.

9. The heat-sensitive layer according to claim 7, further comprising 1,2-diphenoxyethane particles and / or polylactic acid particles as the heat-sensitive particles.

10. The heat-sensitive layer according to claim 7, wherein the heat-sensitive particles further include hollow polymer particles.

11. The heat-sensitive layer according to claim 1, wherein the amount of the inorganic particles is 30% by mass or less of the total components of the heat-sensitive layer.

12. A thermosensitive recording medium comprising the thermosensitive layer according to claim 7 and a colored underlayer.

13. The thermal recording medium according to claim 12, wherein said colored underlayer comprises paper.

Images