Temperature-sensitive base paper surface glue and preparation method thereof

Abstract

The invention relates to the technical field of thermo-sensitive paper, and provides a thermo-sensitive paper surface adhesive and a preparation method thereof. The preparation method comprises the following steps: adding calcium carbonate, kaolin, nano silicon dioxide and expanded microspheres into deionized water, and adding a dispersing agent for dispersion treatment to obtain composite slurry; the preparation method comprises the following steps: adding chitosan into organic acid to prepare an acid solution of chitosan, then adding the acid solution of chitosan, cationic starch, polyvinyl alcohol and water-based acrylic emulsion into deionized water, stirring and dissolving to obtain a mother solution of the composite water-based adhesive; adding the composite slurry into a composite water-based adhesive mother solution, and stirring to obtain a surface adhesive coating solution; and coating the surface of thermosensitive base paper with the surface glue coating liquid, and carrying out drying treatment to obtain the thermosensitive base paper surface glue. According to the preparation method of the thermosensitive base paper surface adhesive, the heat insulation property, the surface strength, the flatness and the coating adaptability of the base paper are synchronously improved on the premise of not damaging the color development performance of a thermosensitive layer.

Description

Technical Field

[0001] This application relates to the field of thermal paper technology, and in particular to a thermal paper surface adhesive and its preparation method. Background Technology

[0002] Thermal recording paper is widely used in receipts, labels, and logistics documents due to its ink-free operation, fast imaging speed, and simple device structure. A typical thermal recording paper usually consists of a base paper layer, a surface adhesive layer, and a thermal coating layer. The surface adhesive layer, located between the base paper and the thermal coating layer, primarily functions to improve the surface smoothness of the base paper, enhance its load-bearing capacity for the thermal coating, and regulate heat conduction behavior, thereby ensuring image clarity and stability.

[0003] In existing technologies, the surface adhesive for thermal paper often employs a coating system composed of inorganic fillers such as calcium carbonate and kaolin, combined with polyvinyl alcohol, starch, or synthetic emulsion adhesives. While this system can improve paper smoothness and coating suitability to some extent, it still has several shortcomings. First, traditional inorganic fillers have large particle sizes and limited dispersibility, making it difficult to form a dense and uniform microstructure in the coating. This results in high porosity in the surface adhesive layer, uncontrollable heat transfer, and problems such as uneven heating of the thermal layer, low color development efficiency, or grayish background. Second, to compensate for insufficient coating strength and density, existing technologies often increase the coating amount or the amount of polymer adhesive, which not only increases costs but also leads to excessively high coating rigidity, reduced folding resistance, and even affects the lifespan of the printhead.

[0004] Therefore, there is an urgent need to provide a thermal paper surface adhesive and its preparation method to solve the above-mentioned technical problems. Summary of the Invention

[0005] In view of the above-mentioned shortcomings in the prior art, the purpose of this application is to provide a thermal paper surface adhesive and its preparation method.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows: In a first aspect, embodiments of this application provide a method for preparing a surface adhesive for thermally sensitive paper, comprising the following steps: S100. Calcium carbonate, kaolin, nano silica and expanded microspheres are added to deionized water, and a dispersant is added for dispersion treatment to obtain a composite slurry. S200. Chitosan is added to an organic acid to prepare an acid solution of chitosan. Then, the acid solution of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion are added to deionized water and stirred to dissolve, so as to obtain a composite aqueous adhesive mother liquor. S300. Add the composite slurry to the composite water-based adhesive masterbatch and stir to obtain the surface adhesive coating liquid. S400. Apply the surface adhesive coating liquid to the surface of the thermal base paper and dry it to obtain the surface adhesive for the thermal base paper.

[0007] In one alternative embodiment, the mass ratio of calcium carbonate, kaolin, nano-silica, and expanded microspheres is 1:(0.09-0.5):(0.02-0.17):(0.04-0.33); and / or the mass ratio of chitosan, cationic starch, polyvinyl alcohol, and aqueous acrylic emulsion is (0.03-0.6):(0.13-1.6):(0.2-2):1.

[0008] In an optional implementation, in S200, a silane coupling agent, a lubricant, and a defoamer are also added to the composite waterborne adhesive mother liquor; the mass ratio of the silane coupling agent, lubricant, and defoamer is 1:(0.2-4):(0.07-1.33).

[0009] In an optional implementation, in S100, the method for preparing the expanded microspheres includes the following steps: S110. Add thermoplastic polymer, foaming agent, initiator and stabilizer to an aqueous system, stir to form droplets, raise the temperature during stirring to form a polymer shell on the outer layer of the droplets, and obtain microspheres after cooling and drying. S120. The microspheres are heated to obtain expanded microspheres.

[0010] In an alternative embodiment, the thermoplastic polymer includes at least one or a combination of polyacrylonitrile, acrylate copolymers and polyvinylidene chloride; and / or the foaming agent includes at least one or a combination of isobutane, isopentane and n-pentane.

[0011] In one alternative implementation, the heat treatment temperature is 80-150°C.

[0012] In an optional embodiment, in S100, the dispersion speed is 800-2500 rpm; and / or in S300, the stirring speed is 400-700 rpm; and / or in S400, the drying temperature is 60-100°C.

[0013] In an optional implementation, in S400, the drying process is a segmented drying process; the segmented drying process includes a first stage drying process, a second stage drying process, and a third stage drying process.

[0014] In an optional implementation, S200 and S300 also include: S210. Add nanocellulose to the composite water-based adhesive mother liquor; The amount of nanocellulose used is 0.2-2 wt.% of the composite water-based adhesive mother liquor.

[0015] Secondly, embodiments of this application provide a thermal paper surface adhesive, which is prepared using the preparation method described in any one of claims 1-9.

[0016] The beneficial effects of this application include at least the following: (1) The method for preparing the surface adhesive of the thermal base paper in this application improves the heat insulation, surface strength, flatness and coating adaptability of the base paper simultaneously without damaging the color development performance of the thermal layer. (2) The preparation method of this application first constructs a composite slurry system with stable structure and complementary functions, then introduces a multi-component synergistic composite water-based adhesive, and finally forms a dense and interface-friendly surface adhesive layer on the base paper surface by low-temperature drying, which effectively avoids the problems of single surface adhesive, insufficient heat insulation, easy powdering or incompatibility with heat-sensitive layer in the prior art. Detailed Implementation

[0017] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are only for explaining this application, but the implementation of this application is not limited thereto.

[0018] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this application pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; the amounts of experimental reagents used are, unless otherwise specified, the amounts used in conventional experimental operations; and the experimental methods used are, unless otherwise specified, conventional methods.

[0019] Existing thermal paper surface adhesive technologies are mainly based on calcium carbonate, kaolin, and polymer adhesives. While improving paper suitability, they still face two major technical problems: First, the coating structure is not dense enough due to the characteristics of the filler, resulting in uneven heat conduction; second, the incremental process adopted to compensate for strength actually leads to poor coating flexibility and increased cost.

[0020] In a first aspect, embodiments of this application provide a method for preparing a surface adhesive for thermally sensitive paper, comprising the following steps: S100. Calcium carbonate, kaolin, nano silica and expanded microspheres are added to deionized water, and a dispersant is added for dispersion treatment to obtain a composite slurry. S200. Chitosan is added to an organic acid to prepare an acid solution of chitosan. Then, the acid solution of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion are added to deionized water and stirred to dissolve, so as to obtain a composite aqueous adhesive mother liquor. S300. Add the composite slurry to the composite water-based adhesive masterbatch and stir to obtain the surface adhesive coating liquid. S400. Apply the surface adhesive coating liquid to the surface of the thermal base paper and dry it to obtain the surface adhesive for the thermal base paper.

[0021] Preferably, the preparation method of the thermal paper surface adhesive of this application simultaneously improves the heat insulation, surface strength, smoothness, and coating adaptability of the base paper without damaging the color development performance of the heat-sensitive layer. This preparation method first constructs a structurally stable and functionally complementary composite slurry system, then introduces a multi-component synergistic composite water-based adhesive, and finally forms a dense and interface-friendly surface adhesive layer on the base paper surface through low-temperature drying. This effectively avoids the problems of single surface adhesive, insufficient heat insulation, easy powdering, or incompatibility with the heat-sensitive layer in existing technologies.

[0022] Preferably, in step S100, calcium carbonate, kaolin, nano-silica and expanded microspheres are first dispersed in an aqueous phase to fully utilize the complementary particle size and synergistic structural effects of the inorganic fillers in an independent system. Calcium carbonate and kaolin form the basic filling framework, nano-silica forms a microscale network to enhance the structural density, and expanded microspheres introduce a closed gas phase structure to significantly reduce heat conduction. This pre-dispersion step avoids the problem of filler agglomeration caused by the increase in binder viscosity during subsequent mixing. The dispersion process is carried out at a speed of 800-2500 rpm, which provides sufficient shear force. The dispersion process is divided into two stages: pre-dispersion and enhanced dispersion. Calcium carbonate and kaolin are micron-sized inorganic fillers that are easy to initially wet and deagglomerate. However, nano-silica has a large specific surface area and high surface energy, making it prone to secondary agglomeration. Expanded microspheres are hollow, low-density, and have limited shear resistance. If high shear or ultrasonic treatment is used from the beginning, it is easy to cause microsphere rupture or local overheating of the system. Therefore, pre-dispersion is carried out at 800-1200 rpm for 10-20 minutes, which helps the dispersant to be fully adsorbed on the surface of various fillers, so that the micron-sized fillers can be initially uniformly distributed, while reducing the probability of direct contact between nano-fillers. On this basis, high shear dispersion at 1500-2500 rpm for 10-30 minutes can effectively break the soft agglomerates of nano-silica, so that it can build a continuous nano network between the calcium carbonate and kaolin skeleton, while ensuring that the expanded microspheres maintain their intact structure. Calcium carbonate is used in large quantities, mainly to fill the skeleton, smooth the paper surface, and control costs. Kaolin is mainly used to improve the density and surface smoothness of the coating due to its lamellar structure. However, excessive use can lead to embrittlement of the coating and a decrease in oil absorption. Nano-silica has a large specific surface area and is easy to aggregate. A small amount can form a nanoscale network structure between fillers, which can significantly enhance the cohesive strength, wear resistance, and heat reflectivity of the coating. However, excessive use can lead to a sharp increase in the viscosity of the system and a decrease in workability. Expanded microspheres are mainly used to build a stable closed gas phase structure in the coating to improve thermal insulation performance. However, their strength is low. If the proportion is too high, it will weaken the overall mechanical properties of the coating. Therefore, it needs to form a synergistic reinforcing relationship with inorganic fillers. Therefore, the mass ratio of calcium carbonate, kaolin, nano-silica, and expanded microspheres is more suitable at 1:(0.09-0.5):(0.02-0.17):(0.04-0.33).

[0023] Furthermore, in S100, the preparation method of the expanded microspheres includes the following steps: S110. Add thermoplastic polymer, foaming agent, initiator and stabilizer to an aqueous system, stir to form droplets, raise the temperature during stirring to form a polymer shell on the outer layer of the droplets, and obtain microspheres after cooling and drying. S120. The microspheres are heated to obtain expanded microspheres.

[0024] Preferably, in step S110, a thermoplastic polymer, a foaming agent, an initiator, and a stabilizer are simultaneously introduced into the aqueous system. Uniform droplets are formed through stirring, and then an in-situ polymerization reaction occurs on the droplet surface under heating conditions, gradually generating a dense and continuous polymer shell, thereby reliably encapsulating the low-boiling-point foaming agent inside the shell. This process is essentially a typical suspension polymerization or emulsion polymerization microencapsulation process. Its advantage lies in the ability to precisely control the particle size range and shell thickness of the microspheres through stirring intensity, stabilizer type, and reaction conditions, preventing premature escape of the foaming agent or the formation of irregular pores. The microspheres obtained after cooling and drying are unexpanded microspheres with stable structure and good flowability, facilitating subsequent storage, transportation, and metering. Subsequently, the microspheres are heated to 80-150℃ in S120, gradually softening the shell. The internal foaming agent, a low-boiling-point alkane, vaporizes and generates internal pressure during this stage, driving the microspheres to expand uniformly and solidify upon cooling, ultimately yielding expanded microspheres with a stable hollow structure. Thermoplastic polymers include at least one or a combination of polyacrylonitrile, acrylate copolymers, and polyvinylidene chloride, due to their suitable glass transition temperature and thermal softening characteristics, maintaining shell density at room temperature while achieving controllable expansion within a set temperature range. The foaming agent includes at least one or a combination of isobutane, isopentane, and n-pentane, enabling high expansion ratios at relatively low temperatures, avoiding thermal damage to the base paper and adhesive system. The expanded microspheres prepared by this two-step method not only have uniform particle size and intact shells, but also controllable expansion ratios and initial expansion temperatures, making them ideal for maintaining structural stability during subsequent surface adhesive preparation and low-temperature segmented drying, thus forming a uniform and durable closed-loop gas-phase thermal insulation structure in the coating.

[0025] Preferably, in step S200, the composite water-based adhesive mother liquor is prepared separately to achieve functional division and interfacial synergy in the adhesive system. Chitosan and cationic starch form strong hydrogen bonds or electrostatic interactions with paper fibers through their polyhydroxy and polyamino structures, providing excellent fiber bonding strength. Polyvinyl alcohol (PVA) ensures continuous film formation, while the water-based acrylic emulsion compensates for water resistance and abrasion resistance, achieving a balance between environmental friendliness and performance in the resulting adhesive system. The composite water-based adhesive mother liquor is obtained by adding the acidic solution of chitosan, cationic starch, polyvinyl alcohol, and water-based acrylic emulsion to deionized water, heating to 85-95°C, stirring for 30-60 minutes, and then cooling to below 40°C. Because waterborne acrylic emulsions can provide continuous film formation, water resistance, and abrasion resistance; chitosan mainly utilizes its polyamino and polyhydroxy structures to enhance hydrogen bonding with paper fibers, thereby improving interfacial adhesion, but excessive amounts can lead to system swelling and decreased fluidity; cationic starch is used to improve the adsorption and water retention of paper fibers, while also improving coating uniformity; PVA plays a role in improving film formation continuity and coating toughness, complementing the acrylic emulsion and avoiding the brittleness or insufficient moisture resistance problems caused by a single adhesive system; therefore, the mass ratio of chitosan, cationic starch, polyvinyl alcohol, and waterborne acrylic emulsion is (0.03-0.6):(0.13-1.6):(0.2-2):1, which can achieve better results.

[0026] Furthermore, in step S200, a silane coupling agent, a lubricant, and a defoamer are added to the composite water-based adhesive mother liquor. The introduction of the silane coupling agent mainly strengthens the interphase interface bonding: since the surface adhesive system also contains various inorganic phases such as calcium carbonate, kaolin, and nano-silica, as well as organic phases such as chitosan, PVA, and acrylic emulsion, the interface is complex and prone to decreased moisture resistance due to insufficient bonding force. Therefore, the silane coupling agent can chemically bond or entangle with the hydroxyl groups on the surface of the inorganic filler and the active groups of the organic polymer through its two active groups, thereby significantly improving the structural density, water resistance, and overall stability of the coating; the addition of the lubricant is to cope with the high filler content, The friction problem brought about by the high thermal insulation structure, such as the problem that nano-silica and expanded microspheres increase the coefficient of friction on the coating surface while improving thermal insulation performance, thus accelerating printhead wear. An appropriate amount of lubricant can form a micro-lubricating phase in the coating, effectively reducing friction and ensuring printing smoothness and wear resistance without impairing its thermal conductivity. Since the system contains natural polymers, surfactants and other foaming components, if the bubbles cannot be eliminated in time during high-speed stirring and coating, defects such as pinholes and shrinkage cavities will be formed, affecting the smoothness of the coating and the uniformity of thermal insulation. Therefore, by adding defoamers, foam generation can be inhibited and defoaming can be promoted throughout the processing, thereby improving the construction adaptability and the integrity of the final coating. The addition of silane coupling agent, lubricant and defoamer should be controlled within the range of 0.4-1.5 wt.% relative to the total mass of the system. This can significantly improve the overall performance without adversely affecting cost, environmental protection and coating suitability. The mass ratio of silane coupling agent, lubricant and defoamer should preferably be controlled within the range of 1:(0.2-4):(0.07-1.33).

[0027] Furthermore, the S200 and S300 also include: S210. Add nanocellulose to the mother liquor of the composite water-based adhesive.

[0028] Preferably, the surface of nanocellulose molecules is rich in hydroxyl groups, which can form a multi-point physical cross-linked network with chitosan, cationic starch, and polyvinyl alcohol through hydrogen bonding. Introducing nanocellulose at this stage facilitates its full swelling, defibrillation, and uniform dispersion, thereby constructing a stable framework when subsequently compounded with inorganic fillers. Preferably, nanocellulose crystals (CNC) and nanocellulose fibers (CNF) are added, as they exhibit good dispersion stability and a significantly high aspect ratio in aqueous systems. The amount of nanocellulose used is 0.2-2 wt.% of the composite aqueous adhesive stock solution. On the one hand, this significantly improves the cohesive strength, crack resistance, and abrasion resistance of the adhesive layer, suppressing powdering and dusting problems in high-filler systems. On the other hand, the three-dimensional network structure formed by nanocellulose effectively limits the water migration rate, improving leveling and film uniformity during coating and drying, while avoiding excessive addition that could lead to a sharp increase in system viscosity, decreased workability, or coating embrittlement.

[0029] Preferably, in step S300, the composite slurry is introduced into the adhesive mother liquor under stirring. This allows for the uniform composite of the inorganic functional phase and the organic binder phase without damaging the microsphere structure and the dispersion state of the nanofillers, ensuring that the coating liquid has stable rheological properties and good construction adaptability. The stirring speed is 400-700 rpm, the time is 20-40 min, and the pH is adjusted to 7.5-9.0, with a viscosity of 800-2500 mPa·s. This speed range ensures thorough mixing of the filler and the adhesive phase while avoiding damage to the microspheres due to high shear. A suitable pH helps the chitosan, PVA, and acrylic emulsions to maintain a relatively stable dispersion and film-forming state. A suitable viscosity range balances the leveling properties during the coating process with the structural stability of the coating film.

[0030] Preferably, in step S400, the coating and drying process is adopted to allow the adhesive system to gradually form a film and co-cur with the base paper fibers at a lower temperature. This avoids potential thermal damage to the heat-sensitive layer at high temperatures and facilitates the formation of a dense, smooth, heat-insulating surface adhesive layer that is also friendly to subsequent heat-sensitive coatings. Through the rational design of the above steps, this preparation method features clear process layers, functional separation, and synergy. In terms of performance, it achieves a comprehensive improvement in heat insulation, surface strength, smoothness, and environmental friendliness, demonstrating significant technical rationality and application advantages. Furthermore, the drying process is a segmented drying process. This is because the adhesive system on the surface of the thermal paper is a multiphase, multi-component coating system that is highly sensitive to heat and moisture migration. If conventional one-stage drying is used, although it can quickly dehydrate the coating, it is very easy to cause the moisture on the coating surface to evaporate instantly and form a skin layer on the surface, which hinders the continued escape of internal moisture. This leads to defects such as blistering, pinholes, microsphere floating or collapsing, and voids inside the adhesive layer. At the same time, it will also lead to insufficient film formation of the bio-based adhesive system and uneven internal structure of the coating, ultimately resulting in powdering, insufficient surface strength, and poor adaptability of the thermal coating. The segmented drying method allows for the slow evaporation of surface free water during the low-temperature pre-drying stage, ensuring the coating is fully spread and fixing the initial spatial distribution of microspheres and nanofillers. During the medium-temperature main drying stage, bound water is removed in a concentrated manner, promoting the rearrangement of adhesive segments such as chitosan and PVA to form a continuous and dense film. During the higher-temperature final drying stage, residual moisture is further removed, allowing the adhesive layer to fully cure and stabilizing the coating size and moisture content, while avoiding excessively high temperatures that could cause secondary expansion of microspheres or thermal aging of the base paper.

[0031] Furthermore, the segmented drying process includes a first-stage drying process, a second-stage drying process, and a third-stage drying process. The first-stage drying process is conducted at a temperature of 60-70℃, mainly to slowly evaporate the free water on the surface, allowing the coating to complete its initial spreading and fixing the spatial distribution of microspheres and fillers. This avoids internal moisture retention due to instantaneous surface skinning, thereby preventing blistering, pinholes, and powdering. The second-stage drying process is conducted at a temperature of 75-90℃, which is the core stage for film formation and performance development. A large amount of bound water is removed, and the PVA and chitosan segments rearrange to form a continuous and dense film. The microspheres are stably embedded in the colloidal phase, significantly improving the surface strength and overall density of the coating. The third-stage drying process is conducted at a temperature of 90-100℃ to remove residual moisture and stabilize the moisture content and size of the coating. At the same time, the temperature is strictly controlled below the threshold of secondary expansion of microspheres and thermal aging of the base paper, thereby ensuring that the coating structure and performance do not undergo any adverse changes.

[0032] Secondly, this application also provides a thermal paper surface adhesive, which is prepared using any of the preparation methods described above. Therefore, it possesses all the beneficial effects of the preparation methods for thermal paper surface adhesives, and will not be elaborated further here.

[0033] Example 1 This embodiment provides a method for preparing a surface adhesive for thermal paper, including the following steps: S100. Calcium carbonate, kaolin, nano silica and expanded microspheres are added to deionized water in a mass ratio of 1:0.09:0.02:0.04. The mixture is first pre-dispersed at 800 rpm for 10 min, and then high-shear dispersed at 1500 rpm for 10 min to obtain a composite slurry. S200. Chitosan is added to an organic acid to prepare an acid solution of chitosan. Then, the acid solution of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion are added to deionized water in a mass ratio of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion of 0.03:0.13:0.2:1. The mixture is heated to 85°C, stirred for 30 minutes, and then cooled to below 40°C to obtain a composite aqueous adhesive mother liquor. S300. Add the composite slurry to the composite water-based adhesive stock solution, stir at 400 rpm for 20 min, and adjust the pH to 7.5 and the viscosity to 800 mPa·s to obtain the surface adhesive coating solution. S400. Apply the surface adhesive coating liquid to the surface of the thermal base paper and perform a three-stage drying process. The temperature of the first stage drying process is 60℃, the temperature of the second stage drying process is 75℃, and the temperature of the third stage drying process is 90℃ to obtain the surface adhesive of the thermal base paper.

[0034] Example 2 This embodiment provides a method for preparing a surface adhesive for thermal paper, including the following steps: S100. Calcium carbonate, kaolin, nano silica and expanded microspheres are added to deionized water in a mass ratio of 1:0.5:0.17:0.33. The mixture is first pre-dispersed at 1200 rpm for 20 min, and then high-shear dispersed at 2500 rpm for 30 min to obtain a composite slurry. S200. Chitosan is added to an organic acid to prepare an acidic solution of chitosan. Then, the acidic solution of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion are added to deionized water in a mass ratio of 0.6:1.6:2:1. The mixture is heated to 95°C and stirred for 60 minutes. Then, it is cooled to below 40°C. A silane coupling agent, lubricant and defoamer are added in a mass ratio of 1:0.2:0.07 to obtain a composite aqueous adhesive mother liquor. S300. Add the composite slurry to the composite water-based adhesive stock solution, stir at 700 rpm for 40 min, adjust the pH to 9.0 and the viscosity to 2500 mPa·s to obtain the surface adhesive coating solution. S400. Apply the surface adhesive liquid to the surface of the thermal base paper and perform a three-stage drying process. The temperature of the first stage drying process is 70℃, the temperature of the second stage drying process is 90℃, and the temperature of the third stage drying process is 100℃ to obtain the surface adhesive of the thermal base paper.

[0035] Example 3 This embodiment provides a method for preparing a surface adhesive for thermal paper, including the following steps: S100. Calcium carbonate, kaolin, nano silica and expanded microspheres are added to deionized water in a mass ratio of 1:0.3:0.1:0.2. The mixture is first pre-dispersed at 1000 rpm for 15 min, and then high-shear dispersed at 2000 rpm for 20 min to obtain a composite slurry. S200. Chitosan is added to an organic acid to prepare an acid solution of chitosan. Then, the acid solution of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion are added to deionized water in a mass ratio of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion of 0.3:1:1:1. The mixture is heated to 90°C, stirred for 30 minutes, and then cooled to below 40°C. A silane coupling agent, lubricant and defoamer are added in a mass ratio of 1:4:1.33 to obtain a composite aqueous adhesive mother liquor. S300. Add the composite slurry to the composite water-based adhesive stock solution, stir at 600 rpm for 30 min, adjust the pH to 8.0 and the viscosity to 1500 mPa·s to obtain the surface adhesive coating solution. S400. Apply the surface adhesive coating liquid to the surface of the thermal base paper and perform a three-stage drying process. The temperature of the first stage drying process is 65℃, the temperature of the second stage drying process is 80℃, and the temperature of the third stage drying process is 90℃ to obtain the surface adhesive of the thermal base paper. Between S200 and S300, there is also: S210. Add nanocellulose to the composite water-based adhesive mother liquor; The amount of nanocellulose used is 0.2 wt.% of the composite water-based adhesive mother liquor.

[0036] Example 4 This embodiment provides a method for preparing a surface adhesive for thermal paper, including the following steps: S100. Calcium carbonate, kaolin, nano silica and expanded microspheres are added to deionized water in a mass ratio of 1:0.3:0.1:0.2. The mixture is first pre-dispersed at 1000 rpm for 15 min, and then high-shear dispersed at 2000 rpm for 20 min to obtain a composite slurry. S200. Chitosan is added to an organic acid to prepare an acid solution of chitosan. Then, the acid solution of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion are added to deionized water in a mass ratio of chitosan, cationic starch, polyvinyl alcohol and aqueous acrylic emulsion of 0.3:1:1:1. The mixture is heated to 90°C, stirred for 30 minutes, and then cooled to below 40°C. A silane coupling agent, lubricant and defoamer are added in a mass ratio of 1:4:1.33 to obtain a composite aqueous adhesive mother liquor. S300. Add the composite slurry to the composite water-based adhesive stock solution, stir at 600 rpm for 30 min, adjust the pH to 8.0 and the viscosity to 1500 mPa·s to obtain the surface adhesive coating solution. S400. Apply the surface adhesive coating liquid to the surface of the thermal base paper and perform a three-stage drying process. The temperature of the first stage drying process is 65℃, the temperature of the second stage drying process is 80℃, and the temperature of the third stage drying process is 90℃ to obtain the surface adhesive of the thermal base paper. Between S200 and S300, there is also: S210. Add nanocellulose to the composite water-based adhesive mother liquor; The amount of nanocellulose used is 2 wt.% of the composite water-based adhesive mother liquor. In S100, the preparation method of expanded microspheres includes the following steps: S110. Polyacrylonitrile, isobutane, initiator and stabilizer are added to an aqueous system and stirred to form droplets. During stirring, the temperature is increased to form a polymer shell on the outer layer of the droplets. After cooling and drying, microspheres are obtained. S120. The microspheres are heated to 80℃ to obtain expanded microspheres.

[0037] Comparative Example 1 This comparative example provides a method for preparing a thermal paper surface adhesive. The difference from Example 1 is that in S400, a three-stage drying process is not used; only a one-stage drying process at 110°C is used.

[0038] Comparative Example 2 This comparative example provides a method for preparing a surface adhesive for thermal paper, which differs from Example 1 in that expanded microspheres are not added in S100.

[0039] This application has undergone multiple experiments, and some of the test results are presented here for reference to further describe the invention in detail. The following is a detailed description in conjunction with specific embodiments.

[0040] Performance testing The thermal paper surface adhesive prepared using the methods described in Examples 1-4 and Comparative Examples 1-2 was further used to prepare thermal paper, and the following tests were performed on the thermal paper: Surface strength test: Thermal paper samples with surface adhesive of thermal paper in Examples 1-4 and Comparative Examples 1-2 were respectively subjected to IGT surface test; Abrasion resistance test: Samples of thermal paper with surface adhesive of thermal paper in Examples 1-4 and Comparative Examples 1-2 were taken and subjected to Taber abrasion resistance test. The grinding wheel was CS-10, the load was 500g / wheel, and the number of revolutions was 100. Printing sensitivity test: Using the same printer, thermal paper with thermal base paper surface adhesive of Examples 1-4 and Comparative Examples 1-2 was printed at a speed of 150mm / s. The energy density was adjusted and the lowest energy required to achieve recognizable color development was recorded. The test results are shown in Table 1.

[0041] Table 1 As can be seen from Table 1, Comparative Example 1 uses a one-stage drying process, which leads to rapid skin formation on the coating surface, making it difficult for internal moisture to escape and resulting in a significant decrease in surface strength. Examples 1-4 use a three-stage drying process to achieve gradual curing of the coating structure, significantly reducing the defect rate and improving overall performance. Examples 3 and 4 incorporate nanocellulose, which significantly enhances the continuity of the colloidal phase, thereby giving the thermal paper better performance. Although Comparative Example 2 has high surface strength, the lack of expanded microspheres results in a lack of a closed gas phase structure, which increases the energy demand for thermal printing and the load on the printhead.

[0042] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for preparing a heat-sensitive base paper surface size, characterized by The preparation method comprises the following steps: S100, adding calcium carbonate, kaolin, nano-silica and expanded microspheres into deionized water, adding a dispersing agent for dispersion treatment to obtain a composite slurry; S200, adding chitosan into an organic acid to prepare an acid solution of chitosan, then adding the acid solution of chitosan, cationic starch, polyvinyl alcohol and water-based acrylic emulsion into deionized water, stirring and dissolving to obtain a composite water-based adhesive mother liquor; S300, adding the composite slurry into the composite water-based adhesive mother liquor for stirring treatment to obtain a surface glue coating solution; S400, coating the surface glue coating solution on the surface of the thermal sensitive base paper and performing drying treatment to obtain a thermal sensitive base paper surface glue.

2. The preparation method according to claim 1, wherein the mass ratio of the calcium carbonate, the kaolin, the nano-silica and the expanded microspheres is 1: (0.09-0.5): (0.02-0.17): (0.04-0.33); and / or the mass ratio of the chitosan, the cationic starch, the polyvinyl alcohol and the water-based acrylic emulsion is (0.03-0.6): (0.13-1.6): (0.2-2):

1. In the S200, the composite water-based adhesive mother liquor further needs to add a silane coupling agent, a lubricant and a defoaming agent; The mass ratio of the silane coupling agent, the lubricant and the defoaming agent is 1: (0.2-4): (0.07-1.33).

3. The preparation method according to claim 1, characterized in that, The preparation method of the expanded microspheres in the S100 comprises the following steps: S110, adding a thermoplastic polymer, a foaming agent, an initiator and a stabilizer into an aqueous system, stirring to form droplets, heating during the stirring process, forming a polymer shell layer on the outer layer of the droplets, and performing cooling treatment and drying treatment to obtain microspheres; 4. The method of claim 1, wherein, S120, heating the microspheres to obtain expanded microspheres.

5. The preparation method according to claim 4, wherein the thermoplastic polymer comprises at least one of polyacrylonitrile, acrylate copolymer and polyvinylidene chloride, or a combination thereof; and / or the foaming agent comprises at least one of isobutane, isopentane and n-pentane, or a combination thereof.

6. The preparation method according to claim 1, wherein the heating temperature is 80-150℃.

7. The preparation method according to claim 1, wherein in the S100, the rotation speed of the dispersion treatment is 800-2500 rpm; and / or in the S300, the rotation speed of the stirring treatment is 400-700 rpm; and / or in the S400, the drying temperature is 60-100℃. In the S400, The drying treatment is a segmented drying treatment; The segmented drying treatment comprises first, second and third drying treatments. Between the S200 and the S300, further comprising: S210, adding nano-cellulose into the composite water-based adhesive mother liquor; The amount of the nano-cellulose is 0.2-2 wt.% of the composite water-based adhesive mother liquor. ​ ​ 8. The method of claim 1, wherein, ​ ​ ​ 9. The method of claim 1, wherein, ​ ​ ​ 10. A heat-sensitive base paper surface size, characterized in that, The heat-sensitive base paper surface glue is prepared by the preparation method in any one of claims 1-9.

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