Thermal sensitive paper and preparation method thereof

Abstract

The invention provides thermo-sensitive paper and a preparation method thereof. The preparation method comprises the following steps: making raw paper; carrying out surface sizing on the raw paper; wherein the surface sizing agent comprises surface sizing starch, a cross-linking agent A and a water repellent agent, and the cross-linking agent A comprises any one or a combination of more of polyacrylamide, polyamide polyamine epichlorohydrin, glyoxal and polymethylene polyphenyl isocyanate; carrying out pre-coating on the surface of the sized paper and forming a pre-coating layer; and coating the surface of the pre-coating layer to form a thermosensitive layer. According to the preparation method of the thermo-sensitive paper provided by the embodiment of the invention, besides ensuring the strength in a single coating, cross-linking systems with different properties are selected, and four-layer structures of the body paper, the body paper sizing, the thermo-sensitive pre-coating and the thermo-sensitive coating are subjected to interlayer combination through a proper adhesive and a cross-linking type to form an interlayer structure comprehensive reinforcing system; better printing performance is achieved, and the printing strength is remarkably enhanced.

Description

Technical Field

[0001] The present application relates to the technical field of papermaking, and in particular to a thermal paper and a preparation method thereof. Background Art

[0002] Thermal paper is also called thermal recording paper, thermal copy paper, and thermal fax paper. By applying heat energy to the surface of the paper, the developer and the colorless dye react chemically to produce the printed image. Thermal paper plays an important role in the fields of computer terminal output printing, fax communication records, securities printing, medical equipment, and various thermal testing instruments due to its advantages such as light printing equipment, simple printing operation, no noise, no odor, and high image resolution.

[0003] It is precisely because of the wide application of thermal paper that its marketing value has gradually attracted people's attention. In addition to printing on the coated surface (front) of thermal paper and displaying the required graphic information, the non-coated surface (back) of thermal paper cannot be processed by any thermal printing. For this reason, more and more customers use refined printing on the non-coated surface of thermal paper to achieve the purpose of anti-counterfeiting or publicity; special printing is performed on the coated surface of thermal paper to highlight important information, such as brand trademarks, product information, etc. This undoubtedly puts higher requirements on the printing adaptability of thermal paper. The conventional thermal paper production process does not make special designs for printing adaptability, so problems such as printing powder loss and false printing often occur, which has a certain negative impact on the product brand. Summary of the invention

[0004] A first aspect of an embodiment of the present application provides a method for preparing thermal paper, the method comprising:

[0005] Making base paper;

[0006] Surface sizing is performed on the base paper; wherein the surface sizing agent comprises surface sizing starch, a crosslinking agent A and a water repellent, wherein the crosslinking agent A comprises any one or more combinations of polyacrylamide, polyamide polyamine epichlorohydrin, glyoxal and polymethylene polyphenyl isocyanate;

[0007] Pre-coating and forming a pre-coating layer on the paper surface after sizing;

[0008] A heat-sensitive layer is formed by coating on the surface of the pre-coating layer.

[0009] In some optional embodiments, in parts by weight, the surface sizing starch is 80-100 parts, the cross-linking agent A is 2-10 parts, and the water repellent is 5-15 parts.

[0010] In some optional embodiments, the water repellent includes any one or more combinations of a polyamide polyurea water repellent, a melamine formaldehyde resin water repellent, and a water-based silane coupling agent.

[0011] In some optional embodiments, in the step of pre-coating and forming a pre-coating layer on the paper surface after sizing, the coating of the pre-coating layer includes: pigment A, adhesive and additives, the additives include defoamer and thickener; in parts by weight, the pigment A is 100 parts, the adhesive is 20-35 parts, the defoamer is 0-0.5 parts, and the thickener is 0.5-1.0 parts.

[0012] In some optional embodiments, the pigment A is selected from one or more combinations of calcined kaolin, hollow spheres, talc, bentonite, diatomaceous earth, alumina, nano-calcium carbonate, and hollow mesoporous silica; the adhesive is selected from one or more combinations of carboxyl-modified polyvinyl alcohol, polyhydroxypolybutadiene (PHPB), esterified starch, etherified starch, acrylic latex, styrene-acrylic emulsion, and styrene-butadiene latex; the thickener is selected from one or more combinations of carboxymethyl starch (CMS), bentonite, polyurethane polymer, silicone gel, attapulgite, molecular sieves, and acrylate copolymers; the defoamer is selected from one or more combinations of mineral oil, acetylene alcohol, fatty alcohol, polyether-modified silicone, polyether, silicone, and modified polymethylsiloxane.

[0013] In some optional embodiments, in the step of coating the pre-coating surface to form a heat-sensitive layer, the coating of the heat-sensitive layer includes: pigment B, a developer, a sensitizer, a colorless dye, an adhesive, a cross-linking agent B and a lubricant.

[0014] In some optional embodiments, in parts by weight, the pigment B is 100 parts, the developer is 20-50 parts, the sensitizer is 15-45 parts, the colorless dye is 15-50 parts, the adhesive is 10-40 parts, the crosslinking agent B is 5-10 parts, and the lubricant is 15-30 parts.

[0015] In some optional embodiments, the pigment B is selected from one or a combination of light calcium carbonate, ground calcium carbonate, calcium silicate, washed kaolin, montmorillonite, titanium dioxide, and silicon dioxide; the developer is selected from 2,2-bis(4-hydroxyphenylpropane), 4,4'-dihydroxydiphenyl sulfone, 4-[[4-(2-allyloxy)phenyl]sulfonyl]phenol, 2,4-diphenylsulfonylphenol, 4-hydroxy-4'isopropoxydiphenyl sulfone; phenol-free N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl)urea, 4,4"-bis(p-toluenesulfonylurea)-diphenylmethane; the sensitizer is selected from 2-naphthol benzyl ether, 1,2-diphenoxyethane, diphenyl sulfone,

[0016] The invention relates to a method for preparing the present invention, wherein the present invention comprises: a first step of preparing the present invention and a second step of preparing the present invention; a second step of preparing the present invention and a second step of preparing the present invention; wherein the present invention comprises a first step of preparing the present invention and a second step of preparing the present invention; and a first step of preparing the present invention and a second step of preparing the present invention. The present invention comprises a first step of preparing the present invention and a second step of preparing the present invention. The present invention comprises a first step of preparing the present invention and a second step of preparing the present invention. The present invention comprises a first step of preparing the present invention and a second step of preparing the present invention.

[0017] In some optional embodiments, the coating of the heat-sensitive layer also includes: a surfactant, a defoamer and a thickener; in parts by weight, the surfactant is 0-5 parts, the defoamer is 0-0.5 parts, and the thickener is 0-1.0 parts; the thickener is selected from one or more combinations of carboxymethyl starch (CMS), bentonite, polyurethane polymer, silicone gel, attapulgite, molecular sieves, and acrylate copolymers; the defoamer is selected from one or more combinations of mineral oil, acetylenic alcohols, fatty alcohols, polyether-modified silicones, polyethers, silicones, and modified polymethylsiloxanes.

[0018] In a second aspect, an embodiment of the present application provides a thermal paper, which is prepared by the preparation method described in the above embodiment.

[0019] The preparation method of thermal paper provided in the embodiment of the present application, in addition to ensuring the strength of a single coating itself, selects a cross-linking system with different properties, and combines the four-layer structure of base paper, base paper sizing, thermal pre-coating layer and thermal coating layer through a suitable adhesive and cross-linking type to form an interlayer structure comprehensive reinforcement system, thereby achieving better printing performance and significantly enhancing printing strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0021] Figure 1 It is a schematic flow chart of an embodiment of a method for preparing thermal paper of the present application;

[0022] Figure 2This is a schematic diagram of the structure after a surface sizing layer is formed on the base paper in the embodiment of the present application;

[0023] Figure 3 It is a schematic diagram of the structure after a pre-coating layer is formed on a thermal paper base paper in an embodiment of the present application;

[0024] Figure 4 It is a schematic diagram of the structure after a thermosensitive layer is formed on the pre-coating layer in an embodiment of the present application. DETAILED DESCRIPTION

[0025] The present application is further described in detail below in conjunction with the accompanying drawings and examples. It is particularly noted that the following examples are only used to illustrate the present application, but are not intended to limit the scope of the present application. Similarly, the following examples are only some embodiments of the present application rather than all embodiments, and all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present application.

[0026] The terms "first", "second", and "third" in the embodiments of the present application are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Thus, the features defined as "first", "second", and "third" can expressly or implicitly include at least one of the features. In the description of the present application, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined. In the embodiments of the present application, all directional indications (such as up, down, left, right, front, back...) are only used to explain the relative position relationship, movement, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication also changes accordingly. The terms "including" and "having" in the embodiments of the present application and any of their variations are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but optionally also includes steps or units that are not listed, or optionally also includes other steps or components inherent to these processes, methods, products, or devices.

[0027] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0028] The present invention develops a thermal paper with high printing adaptability. Different from the production process of conventional thermal paper, the present invention develops a special surface sizing process and thermal coating process, aiming to improve the surface strength of thermal paper and achieve better printing performance.

[0029] The coating process of thermal paper is divided into pre-coating layer and thermal layer. The pre-coating layer needs to provide thermal insulation performance, and porous mineral pigments such as calcined kaolin and diatomaceous earth will be selected. Due to its large specific surface area, the demand for adhesives in the formula is large, but in order to ensure thermal insulation performance, the proportion is limited; and in addition to conventional pigment adhesives, the thermal functional layer also has a high proportion of color-forming functional materials, so the proportion of adhesives is also limited. At present, most of the processes for improving printing applicability are aimed at strengthening a single coating. The technology in the patent of this invention is that in addition to ensuring the strength of a single coating itself, a cross-linking system with different properties is selected, and the four-layer structure of base paper, base paper sizing, thermal pre-coating layer and thermal coating is interlayered through a suitable adhesive and cross-linking type to form an interlayer structure comprehensive reinforcement system to achieve better printing performance.

[0030] See also Figure 1 , Figure 1 1 is a flow chart of an embodiment of a method for preparing thermal paper of the present application, and the preparation method includes but is not limited to the following steps.

[0031] Step S100, making base paper.

[0032] The base paper pulp may include bleached kraft softwood pulp, bleached kraft hardwood pulp, bleached chemical thermomechanical pulp and filler, etc. The detailed features of this part are within the scope of understanding of those skilled in the art and will not be elaborated here.

[0033] Step S200, applying glue to the surface of the base paper.

[0034] The surface sizing agent comprises surface sizing starch, crosslinking agent A and water repellent; in parts by weight, the surface sizing starch is 80-100 parts, the crosslinking agent A is 2-10 parts, and the water repellent is 5-15 parts.

[0035] Optionally, the surface sizing starch can be one of cassava starch and corn starch; the crosslinking agent A can include one or a combination of crosslinking agents that can react with hydroxyl groups, such as polyacrylamide (PAM), polyamide polyamine epichlorohydrin (PAE), glyoxal, polymethylene polyphenyl isocyanate (PAPI), etc.; the water repellent is selected from one or a combination of polyamide polyurea water repellent, melamine formaldehyde resin water repellent, and water-based silane coupling agent.

[0036] See also Figure 2 , Figure 2It is a schematic diagram of the structure after a surface sizing layer is formed on the base paper in an embodiment of the present application, wherein 101 in the figure indicates a base paper layer, and 102 indicates a surface sizing layer, wherein the surface sizing layer 102 may be formed on one side surface or both sides surface of the base paper layer 101.

[0037] Please continue reading Figure 1 The preparation method in this embodiment further includes step S300, performing pre-coating on the paper surface after sizing and forming a pre-coating layer.

[0038] After sizing, the base paper is dried in a drying cylinder, calendered, and rolled to obtain the heat-sensitive base paper, and then a pre-coating is applied on the front side of the base paper. Figure 3 , Figure 3 It is a schematic diagram of the structure after a pre-coating layer is formed on a thermal paper base paper in an embodiment of the present application, wherein the pre-coating layer 103 is formed on a surface sizing layer 102 .

[0039] Optionally, the coating of the pre-coating layer 103 in this embodiment includes: pigment A, adhesive and additives, the additives include defoamer and thickener; in parts by weight, pigment A is 100 parts, adhesive is 20-35 parts, defoamer is 0-0.5 parts, and thickener is 0.5-1.0 parts.

[0040] Among them, pigment A is selected from one or more combinations of calcined kaolin, hollow spheres, talc, bentonite, diatomaceous earth, alumina, nano calcium carbonate, and hollow mesoporous silica; the adhesive is selected from one or more combinations of carboxyl-modified polyvinyl alcohol, polyhydroxypolybutadiene (PHPB), esterified starch, etherified starch, acrylic latex, styrene-acrylic emulsion, and styrene-butadiene latex; the thickener is selected from one or more combinations of carboxymethyl starch (CMS), bentonite, polyurethane polymer, silicone gel, attapulgite, molecular sieve, and acrylate copolymer; the defoamer is selected from one or more combinations of mineral oil, acetylenic alcohol, fatty alcohol, polyether-modified silicone, polyether, silicone, and modified polymethylsiloxane.

[0041] Please continue reading Figure 1 The preparation method in this embodiment further includes step S400, coating the surface of the pre-coating layer to form a heat-sensitive layer.

[0042] See also Figure 4 , Figure 4 It is a schematic diagram of the structure after a thermosensitive layer is formed on the pre-coating layer in an embodiment of the present application, wherein the thermosensitive layer 104 is formed on the pre-coating layer 103 .

[0043] The coating of the thermosensitive layer 104 includes: pigment B, developer, sensitizer, colorless dye, adhesive, crosslinking agent B and lubricant. Optionally, in parts by weight, the pigment B is 100 parts, the developer is 20-50 parts, the sensitizer is 15-45 parts, the colorless dye is 15-50 parts, the adhesive is 10-40 parts, the crosslinking agent B is 5-10 parts, and the lubricant is 15-30 parts.

[0044] Optionally, pigment B is selected from one or a combination of light calcium carbonate, ground calcium carbonate, calcium silicate, washed kaolin, montmorillonite, titanium dioxide, and silicon dioxide; the developer is selected from one or a combination of 2,2-bis(4-hydroxyphenylpropane), 4,4'-dihydroxydiphenyl sulfone, 4-[[4-(2-allyloxy)phenyl]sulfonyl]phenol, 2,4-diphenylsulfonylphenol, 4-hydroxy-4'isopropoxydiphenyl sulfone; phenol-free N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl) urea, 4,4"-bis(p-toluenesulfonylurea)-diphenylmethane.

[0045] The sensitizer is selected from one or more combinations of 2-naphthol benzyl ether, 1,2-diphenoxyethane, diphenylsulfone, 1,2-bis(3-methylphenoxy)ethane, and stearic acid amide; the colorless dye is selected from one or more combinations of 2-phenylamino-3-methyl-6-dibutylaminofluoran, 2-phenylamino-3-methyl-6-diethylaminofluoran, crystal violet lactone, and 3,3-bis(4-dimethylaminophenyl)-6-dimethylaminophenyl peptide). The adhesive is selected from one or more combinations of carboxyl-modified polyvinyl alcohol, polyhydroxy polybutadiene (PHPB), esterified starch, etherified starch, acrylic latex, styrene-acrylic emulsion, and styrene-butadiene latex; the crosslinking agent B is selected from one or more combinations of ethylene glycol, melamine formaldehyde resin, isocyanate, acrylate, tert-butyl peroxide, acetyl peroxide, and water-based polyurethane; the lubricant is selected from one or more combinations of zinc stearate or calcium stearate.

[0046] Optionally, the coating of the thermal layer 104 in this embodiment may further include: a surfactant, a defoamer and a thickener; in parts by weight, the surfactant is 0-5 parts, the defoamer is 0-0.5 parts, and the thickener is 0-1.0 parts; wherein the thickener is selected from one or a combination of carboxymethyl starch (CMS), bentonite, polyurethane polymer, silicone gel, attapulgite, molecular sieve, and acrylate copolymer; the defoamer is selected from one or a combination of mineral oil, acetylenic alcohol, fatty alcohol, polyether-modified silicone, polyether, silicone, and modified polymethylsiloxane.

[0047] Several specific embodiments and comparative embodiments are introduced below for illustration.

[0048] Example 1

[0049] After the base paper is dried, the surface is sized, and the sizing agent includes 100 parts of surface starch, 5 parts of polyacrylamide, and 5 parts of polyamide polyurea water-repellent agent to obtain a heat-sensitive base paper;

[0050] The thermal paper is pre-coated with a heat-insulating layer. The pre-coating coating is prepared by mixing 40 parts of calcined kaolin, 60 parts of hollow spheres, 10 parts of carboxyl-modified polyvinyl alcohol, 15 parts of styrene-butadiene latex, 0.05 parts of polyether-modified silicone defoamer, and 0.5 parts of acrylic copolymer. The coating has a solid content of 25% and a viscosity of 320cps. The coating amount is controlled to be 7g / m by using a metering rod coating. 2 , and obtain thermal pre-coated paper.

[0051] After the thermal pre-coated paper is dried in an oven, the thermal layer is coated. By weight, the thermal coating is prepared by mixing 90 parts of light calcium carbonate, 10 parts of titanium dioxide, 45 parts of 4-[[4-(2-allyloxy)phenyl]sulfonyl]phenol, 30 parts of diphenylsulfone, 20 parts of carboxyl-modified polyvinyl alcohol, 40 parts of 2-phenylamino-3-methyl-6-dibutylaminofluoran, 2 parts of melamine formaldehyde resin, 0.05 parts of modified polymethylsiloxane defoamer, 10 parts of zinc stearate, and 1 part of polyurethane thickener. The coating has a solid content of 30% and a viscosity of 360cps. The coating amount is controlled to be 4g / m by curtain coating. 2 , and obtain thermal paper with high printing adaptability.

[0052] Example 2

[0053] After the base paper is dried, the surface is sized, and the sizing agent includes 80 parts of cassava starch, 7 parts of glyoxal, and 10 parts of water-based silane coupling agent to obtain a thermal base paper;

[0054] The thermal paper is pre-coated with a heat-insulating layer. The pre-coating coating is prepared by mixing 50 parts of diatomaceous earth, 50 parts of hollow medium silica, 5 parts of polyhydroxy polybutadiene, 25 parts of styrene-butadiene latex, 0.03 parts of mineral oil defoamer, and 0.6 parts of polyurethane copolymer. The coating has a solid content of 30% and a viscosity of 280cps. The coating amount is controlled to be 6g / m by using a metering rod coating. 2 , and obtain thermal pre-coated paper.

[0055] After the thermal pre-coated paper is dried in an oven, the thermal layer is coated. By weight, the thermal coating is prepared by mixing 60 parts of calcium silicate, 40 parts of silicon dioxide, 40 parts of 2,4-diphenylsulfonylphenol, 35 parts of 1,2-diphenoxyethane, 10 parts of etherified starch, 15 parts of acrylic latex, 30 parts of 2-phenylamino-3-methyl-6-diethylaminofluoran, 3 parts of isocyanate, 0.04 parts of silicone defoamer, and 18 parts of zinc stearate. The coating has a solid content of 35% and a viscosity of 320cps. The coating amount is controlled to be 4.5g / m by curtain coating.2 , and obtain thermal paper with high printing adaptability.

[0056] Example 3

[0057] After the base paper is dried, the surface is sized, and the sizing agent includes 70 parts of corn starch, 6 parts of polymethylene polyphenyl isocyanate, and 8 parts of polyamide polyurea water repellent to obtain a thermal base paper;

[0058] The thermal paper is pre-coated with a heat-insulating layer. The pre-coating coating is prepared by mixing 70 parts of bentonite, 30 parts of diatomaceous earth, 5 parts of carboxyl-modified polyvinyl alcohol, 20 parts of styrene-acrylic emulsion, 0.06 parts of polyether defoamer, and 0.2 parts of carboxymethyl starch. The coating has a solid content of 35% and a viscosity of 400cps. The coating amount is controlled to be 5g / m by using a metering rod coating. 2 , and obtain thermal pre-coated paper.

[0059] After the thermal pre-coated paper is dried in an oven, the thermal layer is coated. By weight, the thermal coating is 100 parts of PCC, 50 parts of 2,2-bis(4-hydroxyphenylpropane), 40 parts of 1,2-diphenoxyethane, 20 parts of polyvinyl alcohol, 40 parts of 3,3-bis(4-dimethylaminophenyl)-6-dimethylaminophenyl peptide), 3 parts of tert-butyl peroxide, 0.08 parts of modified polymethylsiloxane defoamer, and 18 parts of stearic acid. The coating has a solid content of 28% and a viscosity of 340cps. Curtain coating is used and the coating amount is 6g / m 2 , and obtain thermal paper with high printing adaptability.

[0060] Comparative Example 1:

[0061] After the base paper is dried, a sizing agent is applied to the surface, wherein the sizing agent comprises a mixture of 62 parts of cassava starch and 5 parts of an aqueous silane coupling agent to obtain a thermal base paper;

[0062] The thermal paper is pre-coated with a heat-insulating layer. The pre-coating coating is prepared by mixing 100 parts of diatomaceous earth, 20 parts of styrene-acrylic latex, 0.05 parts of polyether-modified silicone defoamer, and 0.5 parts of acrylate copolymer. The coating has a solid content of 25% and a viscosity of 220cps. The coating amount is controlled to be 7g / m by using a metering rod coating. 2 , and obtain thermal pre-coated paper.

[0063] The thermal pre-coated paper is oven dried and then coated with a thermal layer. The thermal coating is prepared by mixing 90 parts of montmorillonite, 10 parts of washed kaolin, 40 parts of 2,2-bis(4-hydroxyphenylpropane), 35 parts of 2-naphthol benzyl ether, 20 parts of polyvinyl alcohol, 30 parts of ODB, 0.03 parts of modified polymethylsiloxane defoamer, 15 parts of zinc stearate, and 0.7 parts of polyurethane thickener. The coating has a solid content of 35% and a viscosity of 560cps. Curtain coating is used and the coating amount is 5g / m2 , and obtain thermal paper.

[0064] Comparative Example 2 A certain factory currently uses thermal paper.

[0065] The strength comparison between the embodiments of the present application and the comparative embodiments is shown in the following table.

[0066] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Thermal surface RI dry strength 6 5 5 2 5 Non-coated surface RI dry strength 7 6 6 5 6

[0067] It can be seen that the printing strength of the thermal paper in this embodiment is significantly enhanced.

[0068] The preparation method of thermal paper provided in the embodiment of the present application, in addition to ensuring the strength of a single coating itself, selects a cross-linking system with different properties, and combines the four-layer structure of base paper, base paper sizing, thermal pre-coating layer and thermal coating layer through a suitable adhesive and cross-linking type to form an interlayer structure comprehensive reinforcement system, thereby achieving better printing performance and significantly enhancing printing strength.

[0069] The above descriptions are only some embodiments of the present application, and do not limit the protection scope of the present application. Any equivalent device or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly used in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A method for preparing thermal paper, characterized in that: The preparation method comprises: Making base paper; Surface sizing is performed on the base paper; wherein the surface sizing agent comprises surface sizing starch, a crosslinking agent A and a water repellent, wherein the crosslinking agent A comprises any one or more combinations of polyacrylamide, polyamide polyamine epichlorohydrin, glyoxal and polymethylene polyphenyl isocyanate; Pre-coating and forming a pre-coating layer on the paper surface after sizing; A heat-sensitive layer is formed by coating on the surface of the pre-coating layer.

2. The preparation method according to claim 1, characterized in that: In parts by weight, the surface sizing starch is 80-100 parts, the cross-linking agent A is 2-10 parts, and the water repellent is 5-15 parts.

3. The preparation method according to claim 2, characterized in that: The water repellent includes any one or more combinations of polyamide polyurea water repellent, melamine formaldehyde resin water repellent, and water-based silane coupling agent.

4. The preparation method according to claim 1, characterized in that: In the step of pre-coating and forming a pre-coating layer on the paper surface after sizing, the coating of the pre-coating layer includes: pigment A, adhesive and additives, and the additives include defoamer and thickener; in parts by weight, the pigment A is 100 parts, the adhesive is 20-35 parts, the defoamer is 0-0.5 parts, and the thickener is 0.5-1.0 parts.

5. The preparation method according to claim 4, characterized in that: The pigment A is selected from one or more combinations of calcined kaolin, hollow spheres, talc, bentonite, diatomaceous earth, alumina, nano calcium carbonate, and hollow mesoporous silica; the adhesive is selected from one or more combinations of carboxyl-modified polyvinyl alcohol, polyhydroxypolybutadiene (PHPB), esterified starch, etherified starch, acrylic latex, styrene-acrylic emulsion, and styrene-butadiene latex; the thickener is selected from one or more combinations of carboxymethyl starch (CMS), bentonite, polyurethane polymer, silicone gel, attapulgite, molecular sieve, and acrylate copolymer; the defoamer is selected from one or more combinations of mineral oil, acetylenic alcohol, fatty alcohol, polyether-modified silicon, polyether, silicone, and modified polymethylsiloxane.

6. The preparation method according to claim 1, characterized in that: In the step of coating the surface of the pre-coat layer to form a heat-sensitive layer, the coating of the heat-sensitive layer includes: pigment B, developer, sensitizer, colorless dye, adhesive, cross-linking agent B and lubricant.

7. The preparation method according to claim 6, characterized in that: In parts by weight, the pigment B is 100 parts, the developer is 20-50 parts, the sensitizer is 15-45 parts, the colorless dye is 15-50 parts, the adhesive is 10-40 parts, the crosslinking agent B is 5-10 parts, and the lubricant is 15-30 parts.

8. The preparation method according to claim 7, characterized in that: The pigment B is selected from one or a combination of light calcium carbonate, ground calcium carbonate, calcium silicate, washed kaolin, montmorillonite, titanium dioxide, and silicon dioxide; the developer is selected from one or a combination of 2,2-bis(4-hydroxyphenylpropane), 4,4'-dihydroxydiphenyl sulfone, 4-[[4-(2-allyloxy)phenyl]sulfonyl]phenol, 2,4-diphenylsulfonylphenol, 4-hydroxy-4'isopropoxydiphenyl sulfone; phenol-free N-(p-toluenesulfonyl)-N'-(3-p-toluenesulfonyloxyphenyl)urea, 4,4"-bis(p-toluenesulfonylurea)-diphenylmethane; the sensitizer is selected from one or a combination of 2-naphthol benzyl ether, 1,2-diphenoxyethane, diphenyl sulfone, 1,2-bis(3-methylphenoxy)ethane, hard The invention relates to a fatty acid amide, wherein the colorless dye is selected from one or more combinations of 2-phenylamino-3-methyl-6-dibutylaminofluoran, 2-phenylamino-3-methyl-6-diethylaminofluoran, crystal violet lactone, and 3,3-bis(4-dimethylaminophenyl)-6-dimethylaminophenyl peptide); the adhesive is selected from one or more combinations of carboxyl-modified polyvinyl alcohol, polyhydroxy polybutadiene (PHPB), esterified starch, etherified starch, acrylic latex, styrene-acrylic emulsion, and styrene-butadiene latex; the crosslinking agent B is selected from one or more combinations of ethylene glycol, melamine formaldehyde resin, isocyanate, acrylate, tert-butyl peroxide, acetyl peroxide, and water-based polyurethane; the lubricant is selected from one or more combinations of zinc stearate or calcium stearate.

9. The preparation method according to claim 7, characterized in that: The coating of the heat-sensitive layer also includes: a surfactant, a defoamer and a thickener; in parts by weight, the surfactant is 0-5 parts, the defoamer is 0-0.5 parts, and the thickener is 0-1.0 parts; the thickener is selected from one or a combination of carboxymethyl starch (CMS), bentonite, polyurethane polymer, silicone gel, attapulgite, molecular sieve, and acrylate copolymer; the defoamer is selected from one or a combination of mineral oil, acetylenic alcohol, fatty alcohol, polyether-modified silicon, polyether, silicone, and modified polymethylsiloxane.

10. A thermal paper, characterized in that: The thermal paper is prepared by the preparation method according to any one of claims 1 to 9.

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