A low-temperature saturated color-developing thermosensitive coating and its preparation method and application

Abstract

The present invention belongs to the technical field of thermosensitive materials and provides a thermosensitive coating with low-temperature saturated color development, as well as its preparation method and application. The thermosensitive coating includes a developer dispersion, a colorless dye microcapsule dispersion, and a thermosensitive sensitizer dispersion; the developer dispersion includes a primary developer and a secondary developer in a specific ratio, and both the developer dispersion and the thermosensitive sensitizer dispersion use a cellulose dispersant and an acetylene glycol surfactant as dispersants. The colorless dye microcapsules are encapsulated with a polyurethane shell. The colorless dye microcapsules are prepared using a polyurethane prepolymer, a plasticizer, and a polyol chain extender as the main raw materials. The raw materials for preparing the polyurethane prepolymer include polyols and isocyanate curing agents. The initial and saturated color development temperatures of the thermosensitive coating are both 70-80°C, and the base color is normal.

Description

Technical Field

[0001] The present invention relates to the technical field of thermosensitive materials, and more particularly to a thermosensitive coating with low-temperature saturated color development, a preparation method thereof, and applications thereof. Background Art

[0002] Thermal paper is a specialty paper coated with a heat-sensitive coating. It uses a color reaction between a colorless dye and a color developer, generated by heating, to produce the recorded image. It eliminates the need for ink and is widely used in retail, finance, apparel, logistics, healthcare, and other fields. For example, thermal paper can be used in thermal printers, fax machines, cash registers, credit card terminals, and other devices. Portable smart thermal printers, in particular, are widely used in our studies, work, and daily lives. However, existing thermal paper requires extremely high heat energy for printing. Prolonged high-temperature printing can cause severe wear and damage to the printhead. Furthermore, the color coating of thermal paper is squeezed by the high heat of the printhead, causing the material in the coating to melt and solidify upon cooling, accumulating on the printhead. This affects heat dissipation and can lead to persistent problems such as carbon deposits and paper sticking. This in turn significantly damages the thermal printhead, increasing the cost of thermal printing and limiting its application in more applications, especially high-end applications.

[0003] At present, the thermal color-forming coating in thermal paper is mainly made by mixing a colorless dye dispersion and a color developer dispersion. Since the colorless dye and the color developer can be in direct contact, if the reaction activity of the two types of materials is high, a color development reaction can occur at room temperature, which can easily lead to coloration of the thermal coating at room temperature and abnormal coating background color (optical density value > 0.25 at room temperature, affecting the printed image presentation effect). Therefore, the current thermal paper usually needs to adjust the type and ratio of the colorless dye and the color developer to reduce the reactivity of the two. While taking into account the normal background color (color development optical density value ≤ 0.25 below 70℃, GB / T 28210-2011), the initial color development is basically above 70℃, but it takes up to about 130℃ to reach saturated color development (color development optical density value ≧1.0, GB / T 28210-2011). Therefore, traditional thermal paper cannot achieve saturated color at a relatively low temperature of 70-80℃ while taking into account the normal base color. This makes the printing energy consumption of thermal paper high and also limits its application in more fields.

[0004] Therefore, there is an urgent need to develop a thermal paper that can achieve saturated color development at a relatively low temperature of 70-80°C while taking into account the normal background color. Summary of the Invention

[0005] The present invention aims to address at least one of the technical problems existing in the aforementioned prior art. To this end, the present invention provides a low-temperature, saturated color-developing thermal-sensitive coating, its preparation method, and its application. The thermal-sensitive coating provided by the present invention has a low initial color-developing temperature and a low saturated color-developing temperature (both between 70-80°C). Further, when the thermal-sensitive coating is applied to a substrate, the resulting thermal-sensitive material not only has a normal base color but also can achieve saturated color development at relatively low temperatures of 70-80°C. For example, when applied to a paper substrate, low-temperature, saturated color-developing thermal paper can be produced.

[0006] A first aspect of the present invention provides a heat-sensitive coating that develops saturated color at low temperature.

[0007] Specifically, a low-temperature saturated color-emitting thermosensitive coating comprises the following components:

[0008] Color developer dispersion,

[0009] Leuco dye microcapsule dispersion,

[0010] Thermal sensitizer dispersion;

[0011] The developer dispersion comprises a primary developer and a secondary developer; the primary developer is a phenol developer; the secondary developer is an aromatic carboxylic acid zinc salt developer and / or a sulfonylurea developer; the mass ratio of the primary developer to the secondary developer is (2-7):1;

[0012] The developer dispersion and the heat sensitizer dispersion include a dispersant, and the dispersant includes a cellulose dispersant and an acetylene glycol surfactant;

[0013] The colorless dye microcapsule comprises a polyurethane shell, in which a colorless dye is encapsulated; the raw materials for preparing the colorless dye microcapsule comprise a polyurethane prepolymer, a plasticizer, and a polyol chain extender; and the raw materials for preparing the polyurethane prepolymer comprise a polyhydroxy compound and an isocyanate curing agent.

[0014] Low temperature saturated color: refers to the fact that at a relatively low temperature of 70-80℃, the color optical density value of the thermal sensitive coating can reach ≧1.0, which is a saturated color.

[0015] The thermosensitive coating provided by the present invention: (1) uses a main and a secondary color developer to cooperate with each other, wherein the main color developer is a phenol color developer containing a phenol group, has high reactivity and strong color sensitivity; the secondary color developer is a zinc salt compound of an aromatic carboxylic acid color developer and / or a sulfonylurea color developer, which has good hydrogen bond acceptor and donor properties, is conducive to forming a stable colored complex, can widen the color temperature range and improve the saturated color optical density, ensure that the thermosensitive material can be used in multiple scenarios, and further enhance the storage stability of the thermosensitive material. By using the main and secondary color developers to react with the colorless dye together, an excellent synergistic effect is achieved, which improves the reactivity and color optical density value with the colorless dye. (2) In the colorless dye microcapsule, the colorless dye is wrapped in a polyurethane shell, and the isolation between the colorless dye and the external color developer is high. The colorless dye and the color developer cannot directly contact each other, preventing the free active molecules of the two types of materials from meeting, thereby avoiding the color reaction without heating and causing the coating to color. In addition, the polyurethane shell is a polyurethane compound terminated with isocyanate, and the isocyanate curing agent forms a hard segment to ensure the mechanical strength of the polyurethane shell; the polyhydroxy compound forms a soft segment, which helps to lower the softening point temperature of the polyurethane shell and improve thermal response. The present invention also adds a polyol chain extender, which can also form a soft segment. By adjusting the soft and hard segments of the polyurethane shell, the present invention further reduces the softening point of the polyester shell, increases the thermal sensitivity of the coating, and achieves the purpose of low-temperature saturated color development, and avoids the high background color (optical density value >0.25 at room temperature, affecting the rendering effect of printed images) and color loss of color-developed images caused by high temperature and high humidity. The raw materials for preparing the leuco dye microcapsules of the present invention also contain a plasticizer, which helps to increase the mobility of the polyurethane shell molecular chain. While increasing the plasticity of the polyurethane shell, it also reduces its softening point, improves the microcapsule post-processing performance and improves the thermal sensitivity of the leuco dye microcapsules. (3) The present invention improves the color sensitivity by adding a thermosensitive sensitizer. The thermosensitive sensitizer can reduce the softening point temperature of the colorless dye microcapsule, further reducing the initial thermal response temperature. At the same time, it can also improve the blackness, reaction speed and other aspects of the thermosensitive material, achieving the purpose of low-temperature saturated color development, and has excellent effects on improving the printing quality and speed of thermosensitive materials. (4) The present invention uses a combination of cellulose dispersants and acetylene glycol surfactants as dispersants to disperse the color developer and thermosensitive sensitizer, greatly increasing the degree to which the main and secondary color developer solid particles are wetted by water, causing the particles to carry the same charge and generate electrostatic repulsion and move away, preventing particle flocculation and improving grinding efficiency. In addition, the cellulose dispersant can allow the dispersion to maintain a certain viscosity, further ensuring the uniformity of the dispersion system, improving the suspension performance, and preventing precipitation for a long time; it also effectively improves the grinding dispersion efficiency and stability, and can meet the requirements of small particle size. The fine particles have a higher thermal sensitivity, further improving the color development ability of the coating.

[0016] Preferably, the low-temperature saturated color-developing thermosensitive coating comprises the following components in parts by weight:

[0017] 55-65 parts of developer dispersion,

[0018] 20-30 parts of colorless dye microcapsule dispersion,

[0019] 1-10 parts of thermal sensitizer dispersion.

[0020] Further preferably, the low-temperature saturated color-developing thermosensitive coating comprises the following components in parts by weight:

[0021] 60-65 parts of developer dispersion,

[0022] 25-30 parts of colorless dye microcapsule dispersion,

[0023] 5-10 parts of thermal sensitizer dispersion.

[0024] Preferably, the raw materials for preparing the leuco dye microcapsules include 5-15 parts of polyurethane prepolymer, 5-15 parts of plasticizer, and 5-15 parts of polyol chain extender, calculated by weight.

[0025] Further preferably, the raw materials for preparing the leuco dye microcapsules include, by weight, 10-12 parts of a polyurethane prepolymer, 5-10 parts of a plasticizer, and 5-10 parts of a polyol chain extender.

[0026] Preferably, the main color developer is at least one of 4,4'-isopropylidene diphenol, 4,4'-cyclohexylene diphenol, 2,4'-dihydroxydiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfone, 4-hydroxy-4'-isopropoxydiphenyl sulfone, 4-hydroxy-4'-allyloxydiphenyl sulfone, 4-hydroxy-4'-diphenyl sulfone, bis(p-hydroxyphenyl)methyl acetate, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, and 1,4-bis[a-methyl-a-(4'-hydroxyphenyl)ethyl]benzene.

[0027] Further preferably, the main color developer is at least one of 4-hydroxy-4'-isopropoxy diphenyl sulfone, 4-hydroxy-4'-allyloxy diphenyl sulfone, 4-hydroxy-4'-allyl diphenyl sulfone, bis(p-hydroxyphenyl)methyl acetate, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, and 1,4-bis[a-methyl-a-(4'-hydroxyphenyl)ethyl]benzene.

[0028] More preferably, the primary developer is 4-hydroxy-4'-isopropoxydiphenyl sulfone, which has a lower melting point and reduces the initial thermal response temperature.

[0029] Preferably, the aromatic carboxylic acid zinc salt compound type developer is a zinc salicylate type developer.

[0030] Further preferably, the zinc salicylate type developer is at least one of 4-(n-octyloxy) zinc salicylate, 4-[2-(p-methylphenoxy)ethoxy] zinc salicylate, 4-[3-(p-toluenesulfonyl)propoxy] zinc salicylate, and 5-[p-(2-p-methoxyphenoxyethoxy)isopropylphenyl] zinc salicylate.

[0031] Preferably, the sulfonylurea color developer is N-(p-toluenesulfonyl)-N'-(3-benzenesulfonyloxyphenyl)urea and / or 3-(3-methylureido)phenyl-4-methylbenzenesulfonate.

[0032] More preferably, the sulfonylurea color developer is N-(p-toluenesulfonyl)-N'-(3-benzenesulfonyloxyphenyl)urea.

[0033] Preferably, the heat-sensitive sensitizer is at least one of 2-naphthol benzyl ether, tetrabutylammonium bromide, dibutyl terephthalate, stearamide, stearic acid methylenebisamide, and stearic acid ethylenebisamide.

[0034] More preferably, the thermal sensitizer is 2-naphthol benzyl ether.

[0035] Preferably, the mass ratio of the cellulose dispersant to the acetylene glycol surfactant is (10-40):1.

[0036] More preferably, the mass ratio of the cellulose dispersant to the acetylene glycol surfactant is (20-30):1.

[0037] Preferably, the cellulose dispersant is at least one of carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl methyl cellulose.

[0038] More preferably, the cellulose dispersant is carboxymethyl cellulose.

[0039] Preferably, the molecular weight of the cellulose dispersant is 1000 to 3000. If the molecular weight of the cellulose dispersant is too high, the viscosity of the dispersion will increase, which is not conducive to the particle size control of the developer dispersion and the thermal sensitizer dispersion.

[0040] Preferably, the D of the leuco dye microcapsules 50 0.5-2μm.

[0041] Further preferably, the D of the leuco dye microcapsules 50 0.5-1μm.

[0042] Preferably, the mass ratio of the polyol to the isocyanate curing agent is 1:(1-3).

[0043] More preferably, the mass ratio of the polyol to the isocyanate curing agent is 1:(0.5-2).

[0044] Preferably, the polyol is polyether polyol and / or polyester polyol.

[0045] More preferably, the polyhydroxy compound is a polyether polyol.

[0046] Preferably, the polyether polyol is at least one of polyoxypropylene glycol, polytetramethylene glycol, and tetramethylene glycol-propylene oxide copolymer.

[0047] Preferably, the weight average molecular weight of the polyhydroxy compound is 800-2000.

[0048] More preferably, the weight average molecular weight of the polyhydroxy compound is one of 800, 1000, 1500 and 2000.

[0049] Preferably, the isocyanate curing agent is at least one of toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), diphenylmethane diisocyanate (MDI), dicyclohexylmethane diisocyanate (HMDI), hexamethylene diisocyanate (HDI), and lysine diisocyanate (LDI).

[0050] More preferably, the isocyanate curing agent is diphenylmethane diisocyanate (MDI).

[0051] Preferably, the plasticizer is at least one of phthalate plasticizers, phosphate plasticizers, and amide plasticizers. The above plasticizers help to further reduce the softening point temperature of the polyurethane shell and improve thermal response performance.

[0052] More preferably, the plasticizer is a phthalate plasticizer.

[0053] More preferably, the plasticizer is dibutyl phthalate.

[0054] Preferably, the polyol chain extender is at least one of ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, glycerol, and trimethylolpropane.

[0055] More preferably, the polyol chain extender is ethylene glycol.

[0056] Preferably, the raw materials for preparing the leuco dye microcapsule dispersion further include at least one of a leuco dye, an emulsifier, an organic solvent, a protective colloid, and a catalyst.

[0057] Preferably, the leuco dye is a black color-developing leuco dye.

[0058] Further preferably, the colorless dye is 3-pyrrolidinyl-6-methyl-7-anilinofluoran, 3-diethylamino-7-(m-trifluoromethylanilino)fluoran, 3-(N-isopentyl-N-ethylamino)-7-(o-chloroanilino)fluoran, 3-(N-ethyl-p-toluylamino)-6-methyl-7-anilinofluoran, 3-(N-ethyl-N-2-tetrahydrofurfurylamino)-6-methyl-7-anilinofluoran, 3-diethylamino-6-chloro-7-anilinofluoran, 3-di(n-butyl)amino-6-methyl-7-anilinofluoran, 3-di(n-pentyl)amino-6-methyl-7-anilinofluoran, 3- -(N-isopentyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-(N-n-hexyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-di(n-butyl)amino-(2-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-anilinofluoran, 3-diethylamino-6-methyl-7-(3-toluinyl)fluoran, 3-diethylamino-6-methyl-7-(2,6-dimethylanilino)fluoran, 3-diethylamino-6-methyl-7-(2,4-dimethylanilino)fluoran, 2,4-dimethyl-6-(4-dimethylaminoanilino)fluoran.

[0059] More preferably, the leuco dye is 3-di(n-butyl)amino-6-methyl-7-anilinofluoran.

[0060] Preferably, the emulsifier is at least one of an acetylenic glycol modified surfactant, a fatty alcohol ether phosphate, and polyoxyethylene laurate.

[0061] Preferably, the organic solvent is a low boiling point solvent.

[0062] More preferably, the organic solvent is at least one of ethyl acetate, acetone, petroleum ether, carbon tetrachloride, and chloroform.

[0063] Preferably, the protective colloid is at least one of polyvinyl alcohol, carboxymethyl cellulose, polyvinyl pyrrolidone, and cyclodextrin.

[0064] Preferably, the catalyst is dibutyltin dilaurate and / or triethylamine.

[0065] More preferably, the catalyst is dibutyltin dilaurate.

[0066] Preferably, the leuco dye microcapsules are prepared by interfacial polymerization.

[0067] Preferably, the method for preparing the leuco dye microcapsule dispersion comprises the following steps:

[0068] A polyol and an isocyanate curing agent are mixed to obtain a polyurethane prepolymer through an interfacial polymerization reaction, which is then mixed with a colorless dye, an organic solvent, and a plasticizer to obtain an oil phase. A protective colloid and an emulsifier are mixed to obtain an aqueous phase. The oil phase and the aqueous phase are mixed, and then water and a polyol chain extender are added, homogenized and emulsified, and a polymerization reaction is carried out to obtain a colorless dye microcapsule dispersion.

[0069] Preferably, the polymerization reaction temperature is 70-80° C., and / or the polymerization reaction time is 5-10 h.

[0070] More preferably, the polymerization reaction temperature is 75-80° C., and / or the polymerization reaction time is 7-9 h.

[0071] Preferably, the components of the heat-sensitive coating further include fillers, adhesives, cross-linking agents, wetting and leveling agents, and water.

[0072] Further preferably, the components of the heat-sensitive coating further include 2-5 parts of filler, 1-10 parts of adhesive, 0.5-3 parts of cross-linking agent, 3-6 parts of wetting and leveling agent and 70-90 parts of water.

[0073] More preferably, the components of the heat-sensitive coating further include 2-3 parts of filler, 5-10 parts of adhesive, 1-3 parts of cross-linking agent, 4-6 parts of wetting and leveling agent and 80-90 parts of water.

[0074] Preferably, the filler is at least one of kaolin, calcined kaolin, and silicon dioxide. The filler can increase the volume, reduce the cost, and enhance the mechanical properties of the coating.

[0075] Preferably, the adhesive is at least one of an aqueous solution of a synthetic polymer (such as an aqueous solution of polyvinyl alcohol (PVA), a water-dispersible adhesive, an emulsion-type water-based adhesive (such as polyvinyl acetate (PVAc), a polyacrylate adhesive, a polyurethane adhesive, and a synthetic rubber latex (such as styrene-butadiene, nitrile-butadiene, and chloroprene rubber latex).

[0076] Preferably, the cross-linking agent is at least one of glyoxal, aziridine, and polycarbodiimide.

[0077] Preferably, the wetting and leveling agent is at least one of polyether silicone, fluorocarbon surfactant, and acetylenic alcohol surfactant.

[0078] Preferably, the initial color development temperature of the heat-sensitive coating is 70-80°C, and / or the saturated color development temperature of the heat-sensitive coating is 70-80°C.

[0079] A second aspect of the present invention provides a method for preparing a low-temperature saturated color-developing thermosensitive coating.

[0080] A method for preparing a low-temperature saturated color-developing thermosensitive coating comprises the following steps:

[0081] The components are mixed to prepare the low-temperature saturated color heat-sensitive coating.

[0082] Preferably, the primary developer and the secondary developer are first mixed with the dispersant and ground to 0.2≤D 50 ≤0.8μm, heat-sterilize at 60-70℃ for 2-3h to obtain a primary developer dispersion and a secondary developer dispersion, respectively, and mix the primary developer dispersion and the secondary developer dispersion to obtain a developer dispersion.

[0083] Preferably, the thermosensitive sensitizer, dispersant and water are mixed and ground to 0.2≤D 50 ≤0.7μm, heat-sterilize at 60-70℃ for 2-3h to obtain a thermal sensitizer dispersion.

[0084] Particle size D of developer dispersion and thermosensitizer dispersion 50 It should not be too small or too large. If it is too large, it will affect the activity of the reaction with the dye, resulting in a decrease in the thermal sensitivity of the thermosensitive chromogenic layer. If it is too small, it will increase the production time and energy, resulting in high costs and uneconomical.

[0085] A third aspect of the present invention provides an application of a heat-sensitive coating that develops saturated color at low temperatures.

[0086] The invention discloses an application of a low-temperature saturated color-developing thermosensitive coating in the preparation of thermosensitive recording materials.

[0087] A fourth aspect of the present invention provides a thermosensitive material that develops saturated color at low temperature.

[0088] A low-temperature saturated color-producing thermosensitive material comprises a substrate and a thermosensitive color-producing layer in sequence. The thermosensitive color-producing layer is made of the low-temperature saturated color-producing thermosensitive coating.

[0089] Preferably, the heat-sensitive material is heat-sensitive paper, and the substrate is a paper base.

[0090] Preferably, the heat-sensitive material further comprises a primer layer and a protective layer, and the heat-sensitive material comprises a substrate, a primer layer, a heat-sensitive color-forming layer, and a protective layer stacked in sequence.

[0091] The base coat layer can improve the smoothness of the base paper and also serve as a heat insulating layer to more efficiently transfer heat to the heat-sensitive color-forming layer.

[0092] Preferably, the raw materials for preparing the primer layer include water-based latex, kaolin and polyvinyl alcohol. The use of layered kaolin and water-based latex can provide good heat insulation function.

[0093] Further preferably, in parts by weight, the raw materials for preparing the primer layer include 60-80 parts of aqueous latex, 10-20 parts of kaolin and 10-20 parts of polyvinyl alcohol.

[0094] Preferably, the water-based latex is a styrene acrylic emulsion.

[0095] Preferably, the protective layer is prepared from raw materials including polyvinyl alcohol, silicon dioxide, an organosilicon surfactant, a UV absorber, and a filler. The UV absorber can absorb some of the light that causes chemical reactions in the thermal paper coating, thereby mitigating deterioration of the printing paper and further protecting the printer's thermal components from damage. The combination of the UV absorber and filler can effectively reduce damage to the printing components during printing.

[0096] Further preferably, the raw materials for preparing the protective layer include, by weight, 40-50 parts of polyvinyl alcohol, 5-10 parts of silicon dioxide, 5-10 parts of organic silicon surfactant, 5-10 parts of ultraviolet absorber and 15-30 parts of filler.

[0097] Preferably, the filler is at least one of stearate emulsion (such as zinc stearate emulsion), carnauba wax emulsion, and paraffin emulsion.

[0098] A fifth aspect of the present invention provides a method for preparing thermal paper with low-temperature saturated color development.

[0099] A method for preparing a low-temperature saturated color-emitting thermosensitive material comprises the following steps:

[0100] The heat-sensitive coating is coated on the surface of a substrate and dried to obtain a heat-sensitive color-producing layer, thereby preparing the heat-sensitive material having low-temperature saturated color.

[0101] Preferably, the raw materials of the primer layer, the raw materials of the thermosensitive chromogenic layer, and the raw materials of the protective layer are first used to prepare the primer coating, the thermosensitive coating, and the protective layer coating respectively; then the primer coating, the thermosensitive coating, and the protective layer coating are sequentially coated on the surface of the substrate to prepare the thermosensitive material, which includes a stacked substrate, a primer layer, a thermosensitive chromogenic layer, and a protective layer.

[0102] Preferably, the coating amount of the primer layer is 3-6 g / m 2 .

[0103] Preferably, the coating amount of the thermosensitive color-forming layer is 4-5 g / m 2 .

[0104] Preferably, the coating amount of the protective layer is 1-2 g / m 2 .

[0105] Preferably, the solvents of the primer, heat-sensitive coating and protective layer coating are all water. Using water as a solvent is environmentally friendly and safe.

[0106] Preferably, the substrate is at least one of a paper base and / or a plastic film.

[0107] Preferably, the plastic film is a film made of at least one of polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyester film (PET), and polypropylene (PP).

[0108] Preferably, the coating method is at least one of air knife coating, rod blade coating, bar coating, short dwell coating, curtain coating, slot die coating, and die coating. The coating can be applied to a substrate such as paper using the conventional coating process to obtain a corresponding coating.

[0109] Compared with the prior art, the present invention has the following beneficial effects:

[0110] The main components of the heat-sensitive coating of the present invention include a developer dispersion, a colorless dye microcapsule dispersion, and a heat-sensitive sensitizer dispersion; the developer dispersion includes a primary developer and a secondary developer in a specific ratio, the primary developer is a phenol developer, the secondary developer is a zinc salt compound of an aromatic carboxylic acid developer and / or a sulfonylurea developer, and a combination of a cellulose dispersant and an acetylene glycol surfactant is selected as the dispersant; the colorless dye microcapsules include a polyurethane shell, the polyurethane shell encapsulating the colorless dye, and the colorless dye microcapsules are prepared using a polyurethane prepolymer, a plasticizer, and a polyol chain extender as main raw materials, and the polyurethane prepolymer is prepared using a polyhydroxy compound and an isocyanate curing agent. The thermal coating of the present invention has low initial color development temperature and saturated color development temperature (both at 70-80°C). The thermal coating is further used to prepare thermal materials, such as thermal paper, to give the substrate a thermal coating with high thermal sensitivity. Since the heat required for printing is low, printing energy consumption can be greatly reduced, and the material will basically not cool down and solidify and accumulate on the print head after melting, so there will be no stubborn problems such as carbon deposition and sticking to the paper, which reduces the adhesion of debris to the thermal print head or the wear of the thermal print head, further reduces the failure rate of thermal printing, and extends the storage time of the thermal paper. There is no crystallization, precipitation, or flocculation after at least three months. The colorless dye microcapsules in the coating have high isolation from the outside, which can avoid problems such as high background color of the thermal paper and discoloration of the color image caused by high temperature and high humidity, reduce the production cost and printing cost of the thermal material, and broaden the application field of the thermal paper. At the same time, the color development optical density is high, the background color is normal, the color development is high, the printed image is clear, and long-term continuous production can be carried out without problems such as drying and delamination. DETAILED DESCRIPTION

[0111] In order to make the technical solution of the present invention more clearly understood by those skilled in the art, the following examples are given for illustration. It should be noted that the following examples do not limit the scope of protection claimed by the present invention.

[0112] Unless otherwise specified, the raw materials, reagents, and apparatus used in the following examples can be obtained from conventional commercial sources or by conventionally known methods. "Parts" and "%" refer to "parts by mass" and "% by mass," respectively, unless otherwise specified. "Room temperature" refers to 25°C.

[0113] Example 1

[0114] A low-temperature saturated color-developing thermosensitive coating, comprising the following components in parts by weight:

[0115] 60 parts of developer dispersion (including 40 parts of primary developer dispersion and 20 parts of secondary developer dispersion, where the mass ratio of primary developer to secondary developer is 2:1),

[0116] 25 parts of colorless dye microcapsule dispersion,

[0117] 5 parts of thermal sensitizer dispersion,

[0118] 5 parts of 10% PVA 227 polyvinyl alcohol solution (adhesive),

[0119] 1 part glyoxal (cross-linking agent),

[0120] 4 parts of 10% concentration of Wet-270 (Germany Digo, wetting and leveling agent),

[0121] 80 parts deionized water.

[0122] A method for preparing a low-temperature saturated color-developing thermosensitive coating comprises the following steps:

[0123] (1) Preparation of polyurethane prepolymer:

[0124] 15 parts of polytetrahydrofuran diol (polyhydroxy compound) after dehydration are dissolved in 54 parts of ultra-dry tetrahydrofuran, stirred at room temperature to obtain a polytetrahydrofuran diol solution, and then the temperature is raised to 70°C; 30 parts of diphenylmethane diisocyanate (MDI, an isocyanate curing agent) are added to the above solution, and 1 part of dibutyltin dilaurate (catalyst) is added dropwise, and the temperature is maintained at 70°C for 6 hours to obtain an isocyanate-terminated polyurethane prepolymer.

[0125] (2) Preparation of leuco dye microcapsule dispersion:

[0126] 50 parts of 3-di(n-butyl)amino-6-methyl-7-anilinofluoran (leuco dye) were heated to 90°C and dissolved, and 10 parts of polyurethane prepolymer, 15 parts of ethyl acetate (organic solvent) and 5 parts of dibutyl phthalate (plasticizer) were mixed to obtain an oil phase; 10 parts of an aqueous solution of polyvinyl alcohol (Japan Kuraray PVA-203, mass concentration 10%, protective colloid) and 10 parts of acetylene glycol-modified surfactant (Tianjin Hepufei Le, Polyes-60, emulsifier) ​​were mixed to obtain a water phase; the oil phase was slowly added to the water phase, and emulsified and dispersed using a homogenizer by stirring at 10,000 rpm to obtain an emulsified dispersion; 20 parts of water and 5 parts of ethylene glycol (Shanghai Aladdin Biochemical Technology, polyol chain extender) were then added for homogenous emulsification, and the temperature was raised to 75°C for a 7-h polymerization reaction to obtain a polyurethane shell and a colorless dye microcapsule dispersion with an average particle size of 0.7 μm. The mixture was diluted with water to obtain a colorless dye microcapsule dispersion having a solid content of 25%.

[0127] (3) Preparation of primary developer dispersion:

[0128] 50 parts of carboxymethyl cellulose aqueous solution (Japan Daicel, CMC2200, viscosity of 2200mPa·s, mass concentration of 10%, dispersant) were added to a dispersion tank and stirred, and then 40 parts of 4-hydroxy-4'-isopropoxydiphenyl sulfone (D8, Lucky Chemical, main color developer) were added and stirred for 30 minutes. Then, 5 parts of acetylene glycol modified surfactant (Tianjin Hepferle, DS-260, dispersant) and 0.5 parts of Airex 902W (Germany Digo, antifoaming agent) were added and stirred for 30 minutes. The dispersion was then put into a vertical rod-type ball mill and ground for 10 hours to a particle size of D 50 The dispersion was then transferred to a heat preservation tank, heated to 65°C and maintained at 65°C for 2.5 hours for thermal characterization. After cooling naturally, it was diluted with deionized water to a concentration of 30%. Experiments have shown that the primary developer dispersion can be stored at room temperature for more than three months without crystallization, precipitation, or flocculation.

[0129] (4) Preparation of secondary developer dispersion:

[0130] 50 parts of a 10% carboxymethyl cellulose aqueous solution (Japan Daicel, viscosity 2200 mPa·s, mass concentration 10%, dispersant) were added to a dispersion tank for stirring, and then 32.5 parts of N-(p-toluenesulfonyl)-N'-(3-phenylsulfonyloxyphenyl) urea (PF200, Henan Weitixi Chemical Technology Co., Ltd., secondary color developer) were added. After stirring for 30 minutes, 1.2 parts of an acetylene glycol modified surfactant (Tianjin Hepfele, DS-260, dispersant) and 0.1 parts of Airex 902W (Germany Digo, antifoaming agent) were added. After stirring for 30 minutes, the dispersion was put into a vertical rod-type ball mill and ground for 10 hours to a particle size of D 50 0.4-0.7μm; transfer the dispersion to an insulated tank, raise the temperature to 65°C, maintain it at 65°C for 2.5 hours, and then naturally cool and dilute it to 30% concentration with deionized water. Experiments have shown that the secondary developer dispersion can be stored at room temperature for more than 3 months without crystallization, precipitation, or flocculation.

[0131] (5) Preparation of thermosensitive sensitizer dispersion:

[0132] 62 parts of a 10% carboxymethyl cellulose aqueous solution (Japan Daicel, viscosity 2200 mPa·s, mass concentration 10%, dispersant) were added to a dispersion tank for stirring, and then 32.5 parts of 2-naphthol benzyl ether (Shanghai Aladdin Biochemical Technology, heat sensitive sensitizer) were added. After stirring for 30 minutes, 1.2 parts of an acetylene glycol modified surfactant (Tianjin Hepferle, DS-260, dispersant) and 0.1 parts of an antifoaming agent Airex 902W (Germany Digo) were added. After stirring for 30 minutes, the dispersion was put into a vertical rod-type ball mill and ground for 10 hours to a particle size of D 50 0.4-0.7μm; transfer the dispersion to an insulated tank, raise the temperature to 65°C, maintain it at 65°C for 2.5 hours, and then naturally cool and dilute it to 30% concentration with deionized water. Experiments have shown that the heat-sensitive sensitizer dispersion can be stored at room temperature for more than 3 months without crystallization, precipitation, or flocculation.

[0133] (6) Preparation of heat-sensitive coating:

[0134] 40 parts of primary developer dispersion, 20 parts of secondary developer dispersion, 25 parts of colorless dye microcapsule dispersion, 5 parts of thermosensitive sensitizer dispersion, 5 parts of 10% mass concentration PVA 227 polyvinyl alcohol solution (adhesive), 1 part of glyoxal (cross-linking agent), 2 parts of kaolin (filler), 2 parts of polyether silicone wetting agent Wet-270 (Germany Digo, wetting and leveling agent) and 80 parts of deionized water were mixed and stirred to obtain a thermosensitive coating.

[0135] Example 2

[0136] A low-temperature saturated color-producing thermosensitive coating, which differs from Example 1 in that polytetramethylene glycol is replaced by 10 parts and diphenylmethane diisocyanate (MDI) is replaced by 30 parts.

[0137] Example 3

[0138] A low-temperature saturated color-producing thermosensitive coating is different from Example 1 in that 30 parts of polytetramethylene glycol are replaced and 30 parts of diphenylmethane diisocyanate (MDI) are replaced.

[0139] Example 4

[0140] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that the chain extender in the colorless dye microcapsule dispersion is replaced by 10 parts of ethylene glycol (Shanghai Aladdin Biochemical Technology).

[0141] Example 5

[0142] A low-temperature saturated color-developing thermosensitive coating is provided, which differs from Example 1 in that 5 parts of dibutyl phthalate (plasticizer) in the colorless dye microcapsule dispersion are replaced by 10 parts.

[0143] Example 6

[0144] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that 5 parts of the thermosensitive sensitizer dispersion in the thermosensitive coating are replaced by 10 parts.

[0145] Example 7

[0146] A low-temperature saturated color-developing thermosensitive coating is provided, which differs from Example 1 in that 40 parts of the primary developer dispersion in the thermosensitive coating are replaced by 52.5 parts, and 20 parts of the secondary developer dispersion are replaced by 7.5 parts.

[0147] Comparative Example 1

[0148] A low-temperature saturated color-developing heat-sensitive coating is provided, which differs from Example 1 in that polytetramethylene glycol is not added during the preparation of the polyurethane prepolymer.

[0149] Comparative Example 2

[0150] A low-temperature saturated color-developing heat-sensitive coating is provided, which differs from Example 1 in that diphenylmethane diisocyanate (MDI, an isocyanate curing agent) is not added during the preparation of the polyurethane prepolymer.

[0151] Comparative Example 3

[0152] A low-temperature saturated color-developing thermosensitive coating is different from Example 1 in that dibutyl phthalate (plasticizer) is not added during the preparation of the colorless dye microcapsule dispersion.

[0153] Comparative Example 4

[0154] A low-temperature saturated color-developing thermosensitive coating is different from Example 1 in that ethylene glycol (polyol chain extender) is not added during the preparation of the colorless dye microcapsule dispersion.

[0155] Comparative Example 5

[0156] A low-temperature saturated color-developing thermosensitive coating differs from Example 1 in that the carboxymethyl cellulose in both the developer dispersion and the thermosensitive sensitizer dispersion is replaced by an acetylene glycol-modified surfactant of equal mass.

[0157] Comparative Example 6

[0158] A low-temperature saturated color-developing thermosensitive coating is provided, which differs from Example 1 in that the acetylene glycol-modified surfactant is replaced with an equal mass of carboxymethyl cellulose in both the developer dispersion and the thermosensitive sensitizer dispersion.

[0159] Comparative Example 7

[0160] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that no thermosensitive sensitizer dispersion is added to the thermosensitive coating.

[0161] Comparative Example 8

[0162] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that no secondary developer dispersion is added to the thermosensitive coating.

[0163] Comparative Example 9

[0164] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that the primary developer dispersion in the thermosensitive coating is replaced with 26.9 parts, and the secondary developer dispersion is replaced with 33.1 parts, wherein the weight ratio of the primary developer to the secondary developer is 1:1.

[0165] Comparative Example 10

[0166] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that the primary developer dispersion in the thermosensitive coating is replaced with 52 parts, and the secondary developer dispersion is replaced with 8 parts, wherein the weight ratio of the primary developer to the secondary developer is 8:1.

[0167] Comparative Example 11

[0168] A low-temperature saturated color-developing thermosensitive coating, which differs from Example 1 in that the 10% mass concentration of carboxymethyl cellulose aqueous solution (Dacel 2200) in the developer dispersion and the thermosensitive sensitizer dispersion is replaced by a 10% mass concentration of PVA-203 polyvinyl alcohol aqueous solution (Gosei Kagaku, degree of polymerization 500) in the developer dispersion and the thermosensitive sensitizer dispersion.

[0169] Application Example 1

[0170] A thermal paper comprises a substrate (base paper layer), a primer layer, a thermal color-forming layer, and a protective layer stacked in sequence. The thermal color-forming layer is made of the thermal coating of Example 1.

[0171] A method for preparing thermal paper comprises the following steps:

[0172] (1) Preparation of primer coating solution:

[0173] 10 parts of styrene acrylic emulsion, 2.5 parts of kaolin, 2.5 parts of polyvinyl alcohol and 85 parts of deionized water were mixed and stirred to obtain a primer coating liquid with a solid content of 15%.

[0174] (2) Preparation of protective layer coating liquid:

[0175] 10 parts of polyvinyl alcohol, 2 parts of silicon dioxide, 2 parts of paraffin emulsion, 2 parts of carnauba wax emulsion, 2 parts of zinc stearate emulsion, 1 part of silicone surfactant, 1 part of ultraviolet absorber and 80 parts of deionized water are mixed and stirred to obtain a protective layer coating liquid with a solid content of 20%.

[0176] (3) Preparation of thermal paper:

[0177] The substrate surface was coated three times in sequence. Specifically, the primer coating liquid was evenly coated on the base paper layer by a scraper (the coating amount was controlled at 3g / m 2 Then the heat-sensitive coating of Example 1 is evenly coated on the primer layer by a scraper (the coating amount is controlled at 4g / m 2 ), drying to obtain a heat-sensitive color-forming layer; finally, the protective layer coating liquid is evenly coated on the heat-sensitive color-forming layer by an air knife (the coating amount is controlled at 2g / m 2 ), drying to obtain a protective layer and prepare thermal paper.

[0178] Application Example 2-7

[0179] The thermal papers provided in Application Examples 2-7 differ from those in Application Example 1 in that the thermal color-forming layers are made of the thermal coatings of Examples 2-7, respectively.

[0180] Comparative Application Examples 1-11

[0181] The thermal papers provided in comparative application examples 1-11 differ from those in application example 1 in that the thermal color-forming layers are made of the thermal coatings of comparative examples 1-11, respectively.

[0182] Product effect testing

[0183] 1. The volume average particle size of the colorless dye microcapsule dispersion, primary developer dispersion, secondary developer dispersion, and thermosensitive sensitizer dispersion of the examples and comparative examples was tested. The volume average particle size was measured using a Malvern Mastersizer 3000 laser diffraction particle size analyzer.

[0184] 2. Storage period test of each dispersion: The average particle size of the colorless dye microcapsule dispersion, primary developer dispersion, secondary developer dispersion, and thermosensitive sensitizer dispersion is tested every week. If the particle size remains unchanged compared to the initial particle size, it is considered normal.

[0185] 3. Thermal paper performance test method: Referring to the National Standard of the People's Republic of China GB / T28210-2011 "Thermal Paper", the thermal papers prepared in the above application examples and comparative application examples were tested for static color development performance and image preservation performance (heat resistance, light resistance, and moisture resistance).

[0186] Static color development performance: The color development optical density of thermal paper at temperatures of 68°C, 70°C, 72°C, 74°C, 76°C, 78°C and 80°C is tested respectively, where the color development optical density of 0.15 is the color development optical density of the blank area, and the color development optical density of 1.3 is the saturated color development optical density.

[0187] The above results are shown in Table 1.

[0188] Table 1 Performance test results of each dispersion and thermal paper

[0189]

[0190]

[0191]

[0192] Note: Flocculation means that the particle size is too large, agglomeration occurs, and it is abnormal.

[0193] From the results in the above table we can see that:

[0194] 1. Combining the static color development performance test results of the thermal paper prepared with the thermal coating of Examples 1-3 and Comparative Example 1, it can be seen that the thermal paper containing a soft segment (polyether polyol) in the polyurethane shell of Examples 1-3 can start to develop color at 70°C, and its initial color development temperature is at least 10°C lower than that of Comparative Example 1. As the proportion of polyether polyol increases, the color development optical density at the same temperature increases.

[0195] 2. Based on the static color performance test results of Example 1, Example 4, and Comparative Example 4, it can be seen that: (1) In Examples 1 and 4, the addition of a polyol chain extender (ethylene glycol) resulted in the inclusion of a soft segment in the polyurethane shell, and the initial color development temperature was 4°C lower than that of Comparative Example 4. Moreover, the higher the soft segment ratio, the higher the color development optical density at the same temperature. (2) In Comparative Example 4, the absence of a chain extender (ethylene glycol) resulted in the colorless dye being unable to be encapsulated, resulting in a larger particle size. Therefore, it can be seen from the above that a polyol chain extender plays an important role in the formation of the polyurethane shell (microcapsule).

[0196] 3. Combining the static color development performance test results of Example 1, Example 5 and Comparative Example 3, it can be seen that when a plasticizer (dibutyl phthalate) is added to the polyurethane shells of Example 1 and Example 5, the initial color development temperature is 4°C lower than that of Comparative Example 3, indicating that the plasticizer helps to lower the initial color development temperature.

[0197] 4. Combining the static color performance test results of Example 1, Example 6 and Comparative Example 7, it can be seen that the thermosensitive sensitizer is added to the thermosensitive coatings of Example 1 and Example 6, and the initial color development temperature is 4°C lower than that of Comparative Example 7. The higher the proportion of the thermosensitive sensitizer, the higher the color development optical density at the same temperature.

[0198] 5. From the above (1)-(4), it can be seen that the polyhydroxy compounds, polyol chain extenders, plasticizers, and thermosensitive sensitizers used in the present invention have a significant effect on lowering the softening point of the polyurethane shell, enhancing the thermal response of the system and achieving the purpose of low-temperature saturated color development.

[0199] 6. Combining the test results of Example 1 and Comparative Example 2, it can be seen that the hard segment (isocyanate curing agent) in Example 1 can improve the mechanical properties of the polyurethane shell, while Comparative Example 2 lacks a hard segment (isocyanate curing agent), resulting in the colorless dye being unable to be wrapped and directly reacting with the developer to produce color.

[0200] 7. Combining the test results of Example 1 and Comparative Example 8, it can be seen that Comparative Example 8, which does not contain a secondary developer, quickly reaches a maximum color development optical density (1.2) at 74°C, but cannot reach a higher color development (1.3). The saturated color development optical density is reduced by 0.1, which limits the use of thermal paper and increases printing costs. Example 1, which adds a secondary developer, reaches saturated color development only at 78°C, has a wider color development range (can be used to print patterns with color contrast), and can reach a color development optical density of 1.3.

[0201] 8. Combining the test results of Example 1 and Comparative Example 9, it can be seen that when the mass ratio of the primary color developer to the secondary color developer in Comparative Example 9 is 1:1, the initial color development temperature increases by 4°C. This is because the secondary color developer has a high melting point and its excessive amount will reduce the color development sensitivity of the system.

[0202] 9. Combining the test results of Example 1 and Comparative Example 10, it can be seen that when the mass ratio of the primary developer to the secondary developer in Comparative Example 10 is about 8:1, the color optical density will decrease by 0.1.

[0203] 10. From the above (8)-(9) and in combination with Example 1 and Example 7, it can be seen that the best effect is achieved when the mass ratio of the primary developer to the secondary developer is controlled at (2-7):1.

[0204] 11. Combining the test results of Example 1, Comparative Examples 5, and 6, it can be seen that carboxymethyl cellulose plays a key role in the grinding effect, preventing particle flocculation. In addition, the acetylene glycol-modified surfactant plays a role in improving grinding efficiency. The carboxymethyl cellulose and the acetylene glycol-modified surfactant synergistically improve grinding efficiency and prevent particle flocculation, ensuring that the required particle size is achieved within the specified time (the total grinding time for the developer dispersion and the thermosensitizer dispersion in each Example and Comparative Example is 10 hours).

[0205] 12. Based on the test results of Example 1 and Comparative Example 11, it can be seen that using the currently commonly used polyvinyl alcohol instead of carboxymethyl cellulose has low grinding efficiency and a short shelf life.

[0206] 13. From the test results of Comparative Examples 5, 6 and 11, it can be seen that the lack of dispersant or the change of dispersant type will cause the particle size of each dispersion to be larger, and the sensitivity to the color development of the system will also be weakened, resulting in a higher saturated color development temperature.

[0207] 14. The image preservation performance test results show that after testing the light resistance, heat resistance, and humidity resistance, the optical density values ​​of the blank portion and the color density values ​​of the image color portion of Examples 1-7 did not change significantly before and after the tests. This indicates that the colorless dye microcapsules in the thermal coating are highly isolated from the outside, thus avoiding problems such as high background color of the thermal paper and discoloration of the color image caused by high temperature and high humidity.

[0208] In summary, first, the colorless dye microcapsules in the thermal coating of the present invention use a polyurethane shell to wrap the colorless dye inside, so that the colorless dye does not come into direct contact with the developer. The colorless dye microcapsules have high isolation from the outside, avoiding problems such as high background color, background fog, and color loss of the developed image caused by high temperature and humidity. The printing temperature is reduced, thereby reducing debris adhesion or thermal print head wear. In addition, by regulating the ratio of the soft and hard segments of the polyurethane shell, the initial color development temperature and saturation color development temperature of the thermal coating are both between 70-80°C, achieving low-temperature saturated color development. Compared with traditional thermal paper, the temperature required for saturated color development is lower, thereby greatly reducing printing energy consumption, reducing thermal printing costs, and reducing the occurrence of stubborn problems such as carbon deposition and paper sticking. It further reduces the failure rate of thermal printing, extends the storage time of thermal paper, reduces the cost of thermal paper, and broadens the application field of thermal paper. In addition, a thermosensitive sensitizer and plasticizer are used in conjunction to lower the softening point of the polyurethane shell, enhancing thermal response and helping to achieve low-temperature saturated color development. Secondly, the present invention utilizes a combination of primary and secondary color developers to enhance the thermal response of the system. While achieving low-temperature saturated color development, the color development achieves relatively high optical density, reducing costs and achieving excellent results. Third, the present invention utilizes dispersants to prepare corresponding dispersions of the developer and thermosensitizer, which are then used to prepare the coating. Furthermore, a combination of an acetylene glycol-modified surfactant and a cellulose-based dispersant is selected as the specific dispersant. The acetylene glycol surfactant adsorbs on the surface of solid particles, reducing the liquid-liquid or solid-liquid interfacial tension and facilitating wetting of the surface of the agglomerated solid particles. The cellulose-based dispersant homogenizes the system, enhances suspension performance, and prevents precipitation. The acetylene glycol-modified surfactant and cellulose-based dispersant work together to refine the ground particles, further increasing the saturated color optical density and ensuring high-definition printed images. Furthermore, both the developer dispersions (including the primary and secondary developer dispersions) and the thermosensitizer dispersions exhibit no crystallization, precipitation, or flocculation after storage for more than three months. Fourth, the thermosensitive coating of the present invention can be further used to prepare thermal paper, which requires minimal production equipment and has low manufacturing costs. Furthermore, the paper is suitable for use in various commercially available thermal printers, and prints high-definition images and barcodes.

Claims

1. A heat-sensitive coating, characterized in that: Includes the following components: Color developer dispersion, Leuco dye microcapsule dispersion, Thermal sensitizer dispersion; The developer dispersion comprises a primary developer and a secondary developer; the primary developer is a phenol developer; the secondary developer is an aromatic carboxylic acid zinc salt compound developer and / or a sulfonylurea developer; the mass ratio of the primary developer to the secondary developer is (2-7):1; The developer dispersion and the thermosensitive sensitizer dispersion include a dispersant, wherein the dispersant includes a cellulose dispersant and an acetylene glycol surfactant; the mass ratio of the cellulose dispersant to the acetylene glycol surfactant is (10-40):1; The leuco dye microcapsules include a polyurethane shell, wherein the polyurethane shell encapsulates a leuco dye; the raw materials for preparing the leuco dye microcapsules include a polyurethane prepolymer, a plasticizer, and a polyol chain extender; the raw materials for preparing the polyurethane prepolymer include a polyhydroxy compound and an isocyanate curing agent; the weight average molecular weight of the polyhydroxy compound is 800-2000; The leuco dye microcapsule dispersion is prepared by a preparation method comprising the following steps: A polyol and an isocyanate curing agent are mixed to obtain a polyurethane prepolymer through an interfacial polymerization reaction, which is then mixed with a colorless dye, an organic solvent, and a plasticizer to obtain an oil phase. A protective colloid and an emulsifier are mixed to obtain an aqueous phase. The oil phase and the aqueous phase are mixed, and then water and a polyol chain extender are added, homogenized and emulsified, and a polymerization reaction is carried out to obtain a colorless dye microcapsule dispersion. The initial color development temperature of the heat-sensitive coating is 70-80°C, and / or the saturated color development temperature of the heat-sensitive coating is 70-80°C.

2. The heat-sensitive coating according to claim 1, characterized in that: According to parts by weight, it includes the following components: 55-65 parts of developer dispersion, 20-30 parts of colorless dye microcapsule dispersion, 1-10 parts of thermal sensitizer dispersion.

3. The heat-sensitive coating according to claim 1, characterized in that: The D 50 0.5-2μm.

4. The heat-sensitive coating according to claim 1, characterized in that: The mass ratio of the polyhydroxy compound to the isocyanate curing agent is 1:(1-3).

5. The heat-sensitive coating according to claim 1, characterized in that: In parts by weight, the raw materials for preparing the leuco dye microcapsules include 5-15 parts of polyurethane prepolymer, 5-15 parts of plasticizer, and 5-15 parts of polyol chain extender.

6. The method for preparing the heat-sensitive coating according to any one of claims 1 to 5, characterized in that: The steps include: The components are mixed to prepare the heat-sensitive coating.

7. Use of the thermosensitive coating according to any one of claims 1 to 5 in the preparation of thermosensitive recording materials.

8. A heat-sensitive material, characterized in that: The invention comprises a substrate and a thermosensitive chromogenic layer in sequence, wherein the thermosensitive chromogenic layer is made of the thermosensitive coating according to any one of claims 1 to 5.

9. The heat-sensitive material according to claim 8, characterized in that: The heat-sensitive material is heat-sensitive paper, and the substrate is a paper base.

10. The heat-sensitive material according to claim 8, characterized in that: The heat-sensitive material further includes a primer layer and a protective layer.

11. The heat-sensitive material according to claim 10, characterized in that: The heat-sensitive material comprises a substrate, a primer layer, a heat-sensitive color-forming layer, and a protective layer which are stacked in sequence.

12. The method for preparing a thermosensitive material according to claim 8 or 9, characterized in that: The steps include: The heat-sensitive coating is coated on the surface of a substrate and dried to obtain a heat-sensitive color-forming layer, thereby preparing the heat-sensitive material.

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