A heat-sensitive recording material and a method for producing the same

Abstract

The application discloses a kind of thermosensitive recording material and preparation method thereof, it is related to printing technical field, including the following steps: S1, preparation bottom coating liquid and coating on support, air dry and form bottom layer;S2, preparation thermosensitive color layer coating liquid and coating on the bottom layer, air dry and form thermosensitive color layer;The thermosensitive color layer includes urea propyl trimethoxysilane as thermosensitive color developing agent.The method of the application innovatively selects urea propyl trimethoxysilane as thermosensitive color developing agent, constructs a kind of brand-new thermosensitive color developing system, to realize the comprehensive effect of environment and health friendly, low color developing starting temperature, good color developing image weather resistance, excellent color developing performance and high image stability simultaneously.

Description

Technical Field

[0001] This invention relates to the field of printing technology, and more specifically to a thermal recording material and its preparation method. Background Technology

[0002] Thermal recording materials are widely used in fields such as tickets, labels, faxes, and receipts due to the simple structure, noiseless operation, and easy maintenance of printing equipment. Their color development principle mainly relies on the thermal printhead's instantaneous heating, which causes a chemical reaction between the colorless dye (such as fluorescein compounds) in the thermal color-developing layer and the color developer, thereby opening the dye ring and revealing its color.

[0003] Traditional thermochromic reagent systems have long been dominated by phenolic compounds, with bisphenol A (BPA) being the most commonly used reagent due to its high colorimetric sensitivity and low cost. However, with technological advancements and increasing environmental and safety requirements, the BPA system has revealed several inherent drawbacks: First, studies have shown that BPA has endocrine-disrupting effects, posing environmental and health risks, and its use is facing increasingly stringent restrictions globally. Second, BPA itself has a high melting point, resulting in a high colorimetric onset temperature for thermosensitive recording materials, and the colorimetric images formed with colorless dyes have poor weather resistance. Under high temperature and humidity conditions or after prolonged exposure to light, background yellowing (haze) and image fading are likely to occur, affecting the long-term preservation of information. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art. This invention provides a thermal recording material and its preparation method. This method innovatively selects ureapropyltrimethoxysilane as a thermal colorimetric agent to construct a novel thermal colorimetric system, thereby simultaneously achieving the comprehensive effects of being environmentally friendly and health-friendly, having a low color development onset temperature, good weather resistance of the colorimetric image, excellent color development performance, and high image stability.

[0005] This invention provides a method for preparing a thermal recording material, comprising the following steps: S1. Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; S2. Prepare a thermosensitive color-developing layer coating solution and coat it onto the substrate, then air dry to form a thermosensitive color-developing layer; the thermosensitive color-developing layer includes ureapropyltrimethoxysilane as a thermosensitive color-developing agent.

[0006] Specifically, the preparation of the undercoat liquid includes: By weight, take 18-22 parts of calcined kaolin, 25-35 parts of 10wt% polyvinyl alcohol aqueous solution, 0.2-0.4 parts of dispersant, 15-20 parts of auxiliary binder and 25-35 parts of distilled water and mix them evenly to obtain the bottom coating liquid.

[0007] Specifically, the dispersant is an acetylenic diol-based nonionic surfactant; and / or The auxiliary adhesive is a polyvinyl acetate emulsion.

[0008] Specifically, the preparation of the thermosensitive colorimetric coating solution includes: By weight, take 5-7 parts of ureapropyltrimethoxysilane, 2-4 parts of 3-dibutylamino-6-methyl-7-anilinefluorane, 8-10 parts of 10wt% polyvinyl alcohol aqueous solution, 25-30 parts of distilled water and 8-10 parts of calcined kaolin, mix them evenly, and after wet dispersion, add 40-50 parts of 2wt% polyvinyl alcohol aqueous solution and mix evenly to obtain the thermosensitive color developing layer coating solution.

[0009] Specifically, the wet dispersion is achieved by grinding the particles using a wet sand mill until the average particle size is ≤0.5μm.

[0010] Specifically, the preparation of the thermosensitive colorimetric coating solution further includes: After the wet dispersion and before the addition of 2wt% polyvinyl alcohol aqueous solution, 0.5% to 3% of epoxy functionalized nano silica, accounting for the total dry matter mass of the system, is added, and the mixture is stirred and reacted at 50℃ to 65℃ for 30 min to 60 min.

[0011] Specifically, before use, the ureapropyltrimethoxysilane is pre-hydrolyzed with an acidic aqueous solution with a pH of 4-5 at 40-50°C for 30-50 minutes.

[0012] Specifically, the heat-sensitive color-developing coating solution also contains 1 to 3 parts by weight of sulfonated alkali lignin as a synergistic color-developing agent.

[0013] Specifically, a protective layer is coated and air-dried on top of the heat-sensitive color-developing layer; Based on the weight parts used in preparing the thermal color developing coating solution, the coating solution of the protective layer comprises 70 to 90 parts of waterborne polyurethane emulsion and 3 to 15 parts of waterborne wax paste; the solid content of the waterborne polyurethane emulsion ranges from 30% to 40%, and the solid content of the waterborne wax paste ranges from 5% to 10%.

[0014] The present invention also provides a thermal recording material, which is prepared by the method described above.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention is the first to propose and verify ureapropyltrimethoxysilane as a highly efficient and feasible primary colorimetric agent, fundamentally avoiding the potential hazards of traditional phenolic colorimetric agents and providing a safer, healthier, and more environmentally friendly colorimetric solution. Ureapropyltrimethoxysilane not only provides sufficient protons at lower temperatures for efficient dye development and ensures high colorimetric concentration, but its unique organosilicon molecular structure (silicon-oxygen bonds, silicon-carbon bonds) also endows the entire thermosensitive colorimetric layer with excellent thermal and chemical stability. This effectively overcomes the defects of background yellowing and image instability, resulting in materials with extremely low background haze and extremely high image retention even under high temperature and high humidity environments.

[0016] The method of this invention innovatively selects ureapropyltrimethoxysilane as a thermosensitive colorimetric agent to construct a novel thermosensitive colorimetric system, thereby simultaneously achieving the comprehensive effects of being environmentally friendly and health-friendly, having a low colorimetric initiation temperature, good weather resistance of the colorimetric image, excellent colorimetric performance, and high image stability. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic flowchart of the preparation method of the thermal recording material in an embodiment of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] This invention provides a method for preparing a thermal recording material. Figure 1 A schematic flowchart of a method for preparing thermal recording materials according to an embodiment of the present invention is shown, including the following steps: S1. Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; This step aims to form a smooth, highly adhesive, and absorbent substrate on the support, providing an ideal foundation for the subsequent coating of the heat-sensitive color-developing layer and color development.

[0021] The excellent adhesion ensures that the bottom layer can adhere firmly to the support and will not fall off during subsequent processing or use; on the other hand, it provides an ideal adhesion surface for the thermosensitive color development layer, preventing the thermosensitive color development layer from peeling off during use.

[0022] Excellent liquid absorption ensures that the substrate can effectively absorb the molten liquid (color reaction melt) generated by the thermosensitive color layer at the moment of printing, which can limit its lateral diffusion, making the color dots smaller and the edges sharper, thereby obtaining a clear and fine printed image and effectively improving image resolution. Moreover, the excellent liquid absorption of the substrate can make the molten liquid leave the reaction interface quickly and enter the relatively low temperature area inside the substrate, allowing it to cool and solidify rapidly, "locking" the color state and helping to improve the color development speed.

[0023] In some specific embodiments, the support can be a quantitative component with a mass of 50 g / m³. 2 ~100g / m 2 High-quality paper within the range, such as glassine paper, coated paper, or matte coated paper; this type of paper has a high degree of surface smoothness, good strength, and moderate cost, making it suitable for most commercial applications such as bills and labels.

[0024] The support can be synthetic paper, such as synthetic paper with polypropylene (PP) or polyethylene terephthalate (PET) as the main components; such supports have excellent dimensional stability, water resistance and mechanical strength, and are suitable for applications requiring high durability or for use in humid environments (such as outdoor labels, special industrial records).

[0025] The support can also be a plastic film, such as polyester film (PET), biaxially oriented polypropylene film (BOPP), or polyethylene film (PE). When such a support is selected, its surface can be corona treated to improve the adhesion of the coating liquid, thereby preparing a special thermal recording material with high flexibility and high tear resistance.

[0026] In some specific embodiments, the preparation of the undercoat liquid includes: taking 18 to 22 parts by weight of calcined kaolin, 25 to 35 parts by weight of 10 wt% polyvinyl alcohol aqueous solution, 0.2 to 0.4 parts by weight of dispersant, 15 to 20 parts by weight of auxiliary binder and 25 to 35 parts by weight of distilled water and mixing them evenly to obtain the undercoat liquid.

[0027] Calcined kaolin can be 18 parts, 20 parts, 22 parts, or any value within this range; its function is as an inorganic whitening pigment to increase the contrast of the underlayer; the calcined kaolin used in the underlayer also forms the skeleton of the underlayer, providing a porous structure to give the underlayer the required liquid absorption (or "porosity") and surface roughness.

[0028] A 10 wt% aqueous solution of polyvinyl alcohol (PVA) can be 25 parts, 30 parts, 35 parts, or any value within this range; this component acts as the main binder and thickener, providing both the primary bonding strength of the substrate and initial adjustment of the viscosity of the coating solution.

[0029] The dispersant can be 0.2 parts, 0.3 parts, 0.4 parts, or any value within this range; its function is to effectively wet the calcined kaolin particles, reduce the surface tension of the system, prevent agglomeration, and ensure the formation of a stable and uniform suspension.

[0030] The auxiliary adhesive may be 15 parts, or 17.5 parts, or 20 parts (in solids), or any value within this range; as an auxiliary adhesive, it works synergistically with PVA to enhance the cohesion, flexibility and adhesion to the support of the substrate.

[0031] Distilled water can be 25 parts, or 30 parts, or 35 parts, or any value within this range; as a solvent, it is used to precisely adjust the solid content of the entire coating system to a range suitable for coating processes.

[0032] According to a specific weight ratio, all the above components are placed in a high-speed disperser. Under room temperature conditions, the mixture is first stirred at a low speed of 500 rpm to 800 rpm for 5 minutes to initially wet the powdered components (such as calcined kaolin) and prevent dust from flying. Then, the speed is increased to 1200 rpm to 1500 rpm and stirring is continued for 30 to 40 minutes. When the bottom coating liquid is visually observed to be a uniform milky white suspension, and its fineness is measured to be ≤25 μm using a scraper fineness meter, it can be considered to be uniformly mixed, and the bottom coating liquid is obtained.

[0033] Specifically, the dispersant is an acetylenic diol-based nonionic surfactant (preferably at least one of tetramethyldecynyl diol and ethoxylated tetramethyldecynyl diol). Its molecular structure contains acetylenic bonds and hydroxyl groups, which can significantly reduce the surface tension of the liquid and provide excellent wetting and penetration effects at the solid-liquid interface. It can quickly surround and wet calcined kaolin particles, effectively preventing their agglomeration. At the same time, its nonionic properties make it stable within the pH range of the bottom coating solution, and it is not easy to have adverse interactions with components such as polyvinyl alcohol and auxiliary binders in the system. Through the steric hindrance effect, it ensures that the calcined kaolin particles are stably dispersed in the system for a long time, thereby obtaining a uniform and stable bottom coating solution, laying the foundation for the formation of a smooth bottom layer.

[0034] Specifically, the auxiliary adhesive is a polyvinyl acetate emulsion, which, together with the main adhesive polyvinyl alcohol (PVA), forms a high-performance composite adhesive system. After the water evaporates, the polymer particles in this emulsion deform and fuse to form a continuous and flexible transparent film. This film interpenetrates and complements the PVA film: PVA provides high bonding strength and rigidity, while polyvinyl acetate contributes excellent flexibility and internal plasticizing effect. Together, they overcome the brittleness that may exist in a single PVA film, resulting in a substrate with not only extremely high cohesive strength and excellent folding resistance, but also exhibiting strong adhesion to the support and providing an ideal anchoring interface for the thermosensitive color development layer. Furthermore, the emulsion uses water as the dispersion medium, making it environmentally friendly and safe. The solid content in the polyvinyl acetate emulsion is 35wt%~45wt%, which can be 35wt%, 40wt%, 45wt%, or any value within this range. Preferably, the solid content in the polyvinyl acetate emulsion is 40wt%, which has excellent compatibility with other components. It can achieve rapid and uniform mixing with other components, avoiding local agglomeration, and also ensures that the coating liquid as a whole has suitable rheological properties, which is convenient for subsequent rod coating or blade coating operations, thereby obtaining a coating with uniform thickness.

[0035] In some specific embodiments, the preparation of the undercoat solution includes: mixing 20 parts by weight of calcined kaolin, 30 parts by weight of a 10 wt% polyvinyl alcohol aqueous solution, 0.3 parts by weight of an acetylenol-based nonionic surfactant, 18 parts by weight of a 40 wt% polyvinyl acetate emulsion, and 31.7 parts by weight of distilled water to obtain the undercoat solution. This is a preferred weight ratio, which achieves the best balance of undercoat performance through the precise synergy of the components. The ratio of 20 parts calcined kaolin to 18 parts 40 wt% polyvinyl acetate emulsion (auxiliary binder) ensures optimal mechanical strength while maintaining the porous and absorbent properties of the substrate. The specific combination and dosage of PVA solution and polyvinyl acetate emulsion jointly construct a strong and tough bonding network, enabling the substrate to simultaneously possess excellent cohesive force, adhesion to the support, and bonding force to the thermosensitive color development layer. The dosage of 0.3 parts acetylenic diol-based nonionic surfactant can most effectively ensure the dispersion stability of calcined kaolin in this system, preventing sedimentation and agglomeration. The dosage of 31.7 parts distilled water adjusts the solid content and viscosity of this system to the optimal process window for subsequent coating.

[0036] In some specific embodiments, the coating and drying of the substrate includes: First, preparations before coating; The support is placed on the unwinding rack in the form of a roll and kept under constant tension. Before coating, it may be passed through a set of static elimination bars to remove surface static charge and prevent dust from being attracted by static electricity or uneven spreading of the coating liquid during the coating process. Furthermore, the prepared base coat liquid is transferred to the material tank of the coating machine and continuously stirred at low speed to maintain its uniformity and prevent solid particles from settling.

[0037] Next, apply the coating; Either bar coating or blade coating can be used. For example, when using a Mayer bar, the support passes through the gap between the feed trough and the bar at a constant speed (e.g., 10 m / min to 50 m / min). The bar scrapes away excess coating liquid, leaving a wet film of precisely measured thickness on the support surface. By selecting Mayer bars with different diameters and wire gauges, the wet coating amount can be precisely controlled. After drying, the dry coating amount of the underlayer is controlled at 5 g / m². 2 ~12g / m 2 Within the range, preferably 8g / m 2 ~10g / m 2 This coating thickness range ensures that the substrate effectively covers the support surface, providing an ideal interface, without compromising thermal conductivity and cost due to excessive thickness.

[0038] Next, air dry; The wet coating is immediately placed in a hot air drying tunnel for drying and curing. The temperature is controlled at 60℃~80℃, the air velocity is 2 m / s~5 m / s, and the drying time is 30 seconds~60 seconds. During this process, the moisture is effectively evaporated, and the polyvinyl alcohol and polyvinyl acetate emulsion form a film, which firmly binds the solid particles such as calcined kaolin together and forms a smooth, porous, and highly adhesive bottom layer on the surface of the support.

[0039] Finally, winding and post-processing; After being completely dried and cooled, the material is rewound into a roll using a winding device. Throughout the coating and drying process, the support operates on the equipment under constant tension to ensure the uniformity of the coating and the flatness of the product. At this point, the support with the bottom layer is ready for the next step of coating the thermosensitive color layer.

[0040] S2. Prepare a thermosensitive color-developing layer coating solution and coat it onto the substrate, then air dry to form a thermosensitive color-developing layer; the thermosensitive color-developing layer includes ureapropyltrimethoxysilane as a thermosensitive color-developing agent. This step is the process of forming the core coating with color development function. The key is to build a stable and highly sensitive color development system with ureapropyltrimethoxysilane as the thermosensitive color developer.

[0041] Traditional chromogenic agents (such as bisphenol A) have a phenolic hydroxyl group (-OH) at their core. This structure has been shown to have endocrine-disrupting effects, mimicking estrogen and posing potential risks to health and the environment. Ureapropyltrimethoxysilane, on the other hand, has a urea group (-NH-CO-NH-) as its core functional group. This is a stable structure widely found in nature and many pharmaceutical and chemical products, and it does not possess the hormone-disrupting properties of phenolic compounds. By replacing the phenolic hydroxyl group with the urea group as a proton donor, harmful structural units are eliminated from the molecular design stage, achieving environmental and health-friendly results.

[0042] The NH bond in the urea group has a certain acidity; when heated, this bond is relatively weak and easily breaks, providing a proton (H). + Compared to phenolic hydroxyl groups, which require higher energy to dissociate protons, urea groups can initiate the proton transfer process at relatively lower temperatures, thus reducing the onset temperature of color development. Furthermore, the organic portions (propyl chain and urea group) of ureapropyltrimethoxysilane exhibit good compatibility with organic dye molecules. This good compatibility ensures that the developer and dye achieve close molecular-level contact within the coating. When the thermal printhead heats up, protons can be transferred to the dye molecules more quickly and directly, reducing energy loss during the transfer process. This results in not only a lower onset temperature but also a faster color development response and higher color concentration.

[0043] In the aqueous environment of the thermosensitive color developer coating solution, the trimethoxysilane group of ureapropyltrimethoxysilane undergoes hydrolysis, generating highly reactive silanol groups (-Si-OH). These silanol groups undergo a condensation reaction with the abundant hydroxyl groups (-OH) on the surface of inorganic fillers (such as calcined kaolin) in the coating, forming a strong, irreversible Si-O-Si covalent bond. This chemical bond firmly anchors the color developer molecules to the framework of the thermosensitive color developer coating like an "anchor," completely preventing the color developer molecules from migrating to the coating surface, volatilizing, or pre-reacting with the dye during long-term storage or under high-temperature conditions. This is the fundamental reason for solving the problem of background yellowing (haze). Moreover, the strong anchoring also means that the coating is more robust, and it is less likely to generate color developer or dye dust under the high-speed friction of the printhead, greatly reducing printhead contamination.

[0044] In ureapropyltrimethoxysilane, the bond energy of the silicon-oxygen bond (Si-O) is about 460 kJ / mol, and the bond energy of the silicon-carbon bond (Si-C) is about 347 kJ / mol. These are both higher than the bond energies of the carbon-oxygen bond (CO, bond energy about 326 kJ / mol) and the carbon-carbon bond (CC, bond energy about 332 kJ / mol). This high bond energy means that more energy is needed to break it, resulting in high thermal stability and good chemical stability. In particular, the silicon-oxygen bond (Si-O), through its condensation with the hydroxyl groups (-OH) on the surface of the inorganic filler, can form a through-hole organic-inorganic hybrid network inside the coating. This network acts like a robust "cage," protecting the dye and developer within it, effectively improving the thermal and chemical stability of the coating. Thus, even in high-temperature environments, this hybrid network remains stable, protecting the internal colorimetric products from damage, resulting in extremely high image retention. At the same time, this hybrid network can effectively block the erosion of the colorimetric image by chemicals such as oxygen and plasticizers, improving the image's chemical resistance and weather resistance.

[0045] In summary, one end (urea group) of ureapropyltrimethoxysilane is responsible for "efficient operation," rapidly triggering the colorimetric reaction at low temperatures to ensure image quality. The other end (silane) is responsible for "stabilizing the structure," providing long-term stability to the entire system through chemical anchoring and hybrid networks, ensuring persistent image retention. This design, integrating the "working unit" and "stabilizing unit" into the same molecule, breaks the traditional separation between "colorimetric function" and "physical stability" in thermosensitive materials, which often requires trade-offs. This achieves a comprehensive effect of environmental friendliness, high sensitivity, and ultra-high stability.

[0046] Therefore, using ureapropyltrimethoxysilane as a highly efficient and feasible primary colorimetric agent fundamentally avoids the potential hazards of traditional phenolic colorimetric agents, providing a safer, healthier, and more environmentally friendly colorimetric solution. Ureapropyltrimethoxysilane not only provides sufficient protons at lower temperatures for efficient dye development and ensures high colorimetric concentration, but its unique organosilicon molecular structure (silicon-oxygen bonds, silicon-carbon bonds) also endows the entire thermosensitive colorimetric layer with excellent thermal and chemical stability. This effectively overcomes the defects of background yellowing and image instability, resulting in materials with extremely low background haze and extremely high image retention under high temperature and high humidity environments. In other words, by innovatively selecting ureapropyltrimethoxysilane as a thermosensitive colorimetric agent, a novel thermosensitive colorimetric system has been constructed, simultaneously achieving a comprehensive effect of environmental and health friendliness, low colorimetric onset temperature, good weather resistance of the developed image, excellent colorimetric performance, and high image stability.

[0047] In some specific embodiments, the preparation of the thermosensitive colorimetric coating solution includes: taking 5 to 7 parts by weight of ureapropyltrimethoxysilane, 2 to 4 parts by weight of 3-dibutylamino-6-methyl-7-aniline fluorane, 8 to 10 parts by weight of 10 wt% polyvinyl alcohol aqueous solution, 25 to 30 parts by weight of distilled water and 8 to 10 parts by weight of calcined kaolin, mixing them evenly, dispersing them wet, and then adding 40 to 50 parts by weight of 2 wt% polyvinyl alcohol aqueous solution, mixing evenly to obtain the thermosensitive colorimetric coating solution.

[0048] Ureapropyltrimethoxysilane can be 5 parts, 6 parts, 7 parts, or any value within this range; its function is as the main colorimetric agent, and the urea group (-NH-CO-NH-) at the end of its molecule is a proton donor. Under the heating of the thermal printhead, the urea group is activated, providing protons (H... + Ureapropyltrimethoxysilane triggers the ring-opening color development reaction of colorless dyes. It also acts as a stabilizer; the trimethoxysilyl group at the front of its molecule undergoes hydrolytic condensation, forming a strong Si-O-Si covalent bond with the hydroxyl groups on the surface of the inorganic filler (calcined kaolin). This effect "anchors" the color developer molecules within the coating network, significantly improving the coating's thermal stability and aging resistance, and fundamentally reducing printhead contamination caused by color developer migration. In short, ureapropyltrimethoxysilane not only achieves efficient color development but also imparts inherent stability and anti-fouling properties to the coating, which are not possessed by traditional phenolic color developers.

[0049] 3-Dibutylamino-6-methyl-7-aniline fluorane can be used in 2, 3, or 4 parts, or any value within this range; it is one of the commonly used black thermosensitive dyes. In its closed-ring state, it is colorless. When it accepts a proton from the developer, its intramolecular ester ring opens, forming a large π-conjugated structure, thus instantly turning into a dark color (usually bluish-black). 3-Dibutylamino-6-methyl-7-aniline fluorane, when combined with ureapropyltrimethoxysilane, can achieve high-contrast, high-definition color images; the ratio of its amount to the developer determines the final image color concentration.

[0050] A 10wt% polyvinyl alcohol aqueous solution can be 8, 9, or 10 parts, or any value within this range. It plays a dispersing and stabilizing role. In the wet dispersion stage, this high-concentration, high-viscosity PVA solution provides a suitable slurry viscosity for grinding and, through the steric hindrance effect of the polymer chains, prevents solid particles from re-aggregating under high-speed grinding, ensuring effective dispersion. The 10wt% polyvinyl alcohol aqueous solution also acts as a primary binder, encapsulating and binding the developer (ureapropyltrimethoxysilane), inorganic filler (calcined kaolin), and dye particles (3-dibutylamino-6-methyl-7-anilinefluorane) to form the skeletal structure of the coating, providing basic mechanical strength. The 10wt% polyvinyl alcohol aqueous solution ensures that the developer, inorganic filler, and dye particles can be efficiently ground to the submicron level, a prerequisite for achieving high-resolution printing quality.

[0051] Calcined kaolin can be 8 parts, 9 parts, 10 parts, or any value within this range; it mainly acts as an inorganic filler, providing a huge specific surface area, serving as both the site of the colorimetric reaction and the bonding point for the hydrolysis and condensation of ureapropyltrimethoxysilane, and improving the smoothness and wear resistance of the coating. Furthermore, calcined kaolin imparts a clean white background to the thermosensitive colorimetric layer, making the colorimetric image more vivid.

[0052] Distilled water can be 25 parts, or 27.5 parts, or 30 parts, or any value within this range; it serves as an aqueous medium for the thermosensitive color development coating solution, used to dissolve PVA, wet solid particles and provide a reaction site (such as the hydrolysis of silanes), and ensure the formation of a stable, uniform slurry system suitable for wet dispersion.

[0053] The 2wt% polyvinyl alcohol (PVA) aqueous solution can be 40 parts, 45 parts, 50 parts, or any value within this range. Adding a large amount of low-viscosity, low-concentration PVA solution after wet dispersion allows for gentle and precise adjustment of the viscosity and solids content of the entire thermosensitive color-developing coating solution to the optimal range for coating processes (such as bar coating or blade coating). Furthermore, the 2wt% PVA solution acts as a secondary binder, filling and perfecting the framework constructed by the primary binder (10wt% PVA solution), forming a more continuous and smooth coating film, and improving the coating's gloss and uniformity. By adding two mass concentrations of PVA solution stepwise, the dispersion stability and film formation control are managed in stages, ensuring the shear force and stability required in the wet dispersion stage while achieving precise control of the final coating viscosity, thus facilitating the acquisition of a high-quality, defect-free thermosensitive color-developing layer.

[0054] In some specific embodiments, the preparation of the thermosensitive colorimetric coating solution includes: taking 6 parts by weight of ureapropyltrimethoxysilane, 3 parts by weight of 3-dibutylamino-6-methyl-7-aniline fluorane, 9 parts by weight of 10 wt% polyvinyl alcohol aqueous solution, 27 parts by weight of distilled water, and 9 parts by weight of calcined kaolin, mixing them evenly, dispersing them wet, and then adding 46 parts by weight of 2 wt% polyvinyl alcohol aqueous solution, mixing evenly to obtain the thermosensitive colorimetric coating solution. This is a preferred weight ratio, which achieves the best balance of thermosensitive colorimetric layer performance through the precise synergy of each component. The optimal molar ratio of developer to dye (2:1) is achieved by using 6 parts ureapropyltrimethoxysilane and 3 parts 3-dibutylamino-6-methyl-7-anilinefluorane. This ratio ensures sufficient protons for complete ring-opening and color development of the dye during thermal activation, achieving the highest color concentration while avoiding the risk of background haze caused by excessive developer. The viscosity and solids ratio of the primary dispersion system (9 parts 10wt% PVA + 27 parts water, etc.) provide the best grinding efficiency environment for wet dispersion, protecting equipment and media while achieving the most effective particle breakage. Subsequent addition... The addition of 46 parts of 2wt% PVA solution precisely controls the total PVA content and final viscosity of the coating solution to the optimal process window for coating. Under this ratio, the coating solution has good leveling properties, and the coating formed after drying is continuous, smooth, and has high strength, which not only ensures excellent color development performance but also provides good protection for the printhead. The addition of 9 parts of calcined kaolin provides sufficient whiteness, thermal conductivity, and reaction surface area, while forming a perfect match with the total amount of binder. This ensures that the coating has ideal mechanical properties and a porous structure, and will not become brittle due to excessive filler, nor will it affect whiteness and heat absorption efficiency due to insufficient filler.

[0055] In some specific embodiments, the wet dispersion is achieved by grinding with a wet sand mill to an average particle size ≤0.5μm. Achieving this average particle size (≤0.5μm) significantly increases the contact area between the dye and the developer, resulting in a faster and more complete thermochromic reaction. Moreover, the fine particles ensure an extremely smooth and dense thermosensitive color-developing layer, resulting in higher resolution, clearer edges, and a grain-free printed image, while significantly reducing wear on the thermal printhead. Furthermore, ureapropyltrimethoxysilane can be more evenly distributed and contact the surface of the finer filler, promoting the formation of a more complete "in-situ hybrid" network and further improving the durability of the coating.

[0056] The grinding media utilizes high-hardness, high-wear-resistance zirconia beads with a particle size range of 0.4mm to 0.8mm. This size range provides sufficient shear force while preventing equipment clogging, achieving efficient particle crushing and dispersion. The grinding media fills 60% to 80% of the effective volume of the wet sand mill's grinding chamber. The grinding speed is controlled between 1000 rpm and 2000 rpm, and the grinding temperature is consistently kept below 40℃ to prevent heat-sensitive components (such as dyes and color developers) from undergoing pre-reaction or degradation during dispersion.

[0057] In some specific embodiments, the preparation of the thermosensitive colorimetric coating solution further includes: after the wet dispersion and before adding 2 wt% of the polyvinyl alcohol aqueous solution, adding 0.5% to 3% of epoxy-functionalized nano-silica by mass of the total dry matter of the system, and stirring and reacting at 50°C to 65°C for 30 to 60 minutes. Epoxy-functionalized nano-silica is an existing material (e.g., glycidyl methacrylate grafted modified nano-silica), which not only serves as a reinforcing filler, but its surface epoxy groups can also undergo ring-opening reactions with the hydrolysis products (silanol groups) of ureapropyltrimethoxysilane and the hydroxyl groups of polyvinyl alcohol, further forming an organic-inorganic hybrid cross-linked network within the coating. This structure further enhances the mechanical strength, heat resistance, and barrier properties of the coating, which is not present in traditional physical mixing formulations, resulting in unexpected improvements in thermal stability and durability.

[0058] Preferably, the preparation of the thermosensitive color development coating solution further includes: after the wet dispersion and before the addition of 2 wt% polyvinyl alcohol aqueous solution, adding 2% of epoxy functionalized nano silica by mass of the total dry matter of the system, and stirring and reacting at 60°C for 45 min.

[0059] In some specific embodiments, the ureapropyltrimethoxysilane is pre-hydrolyzed with an acidic aqueous solution of pH 4-5 at 40-50°C for 30-50 minutes before use. Pre-hydrolyzing the ureapropyltrimethoxysilane under acidic conditions allows its methoxy group (-OCH3) to be fully converted into more reactive silanol groups (-Si-OH). These activated silanol groups can not only react with the aforementioned nanomaterials but also condense with the hydroxyl groups on the surface of inorganic fillers such as calcined kaolin, greatly improving the interfacial bonding between the organic and inorganic phases. This optimizes the coating structure at the molecular level, thereby synergistically improving color uniformity and image durability.

[0060] Preferably, the ureapropyltrimethoxysilane is pre-hydrolyzed with an acidic aqueous solution with a pH of 4.5 at 45°C for 40 minutes before use.

[0061] Specifically, the acidic aqueous solution is prepared by adding a portion of the distilled water from the formulation of the thermosensitive color-developing coating solution, along with glacial acetic acid (i.e., acetic acid) or citric acid. Glacial acetic acid, as a weak organic acid, provides a mild and easily controllable acidic environment, preventing excessive condensation of silanes due to excessively low local pH, which could lead to gel formation. It is also easily volatilized during subsequent coating and drying processes, leaving no non-volatile residues in the final product, thus ensuring the purity and thermal stability of the coating. Citric acid is a safe polybasic organic acid and an excellent alternative.

[0062] In some specific embodiments, the thermosensitive color-developing coating solution also contains 1-3 parts by weight of sulfonated alkali lignin as a synergistic color-developing agent. A synergistic color-developing effect exists between ureapropyltrimethoxysilane and sulfonated alkali lignin; sulfonated alkali lignin itself is a polymer color-developing agent, and the sulfonic acid groups and phenolic hydroxyl groups in its molecule can form a dynamic hydrogen bond network with the urea groups of ureapropyltrimethoxysilane. This can, on the one hand, reduce the activation energy of the reaction during color development and improve the color-developing sensitivity; on the other hand, it can more stably fix the color morphology after color development, thus unexpectedly achieving both high color concentration and excellent image anti-aging properties without sacrificing sensitivity.

[0063] Preferably, the thermosensitive color-developing coating solution also contains 2 parts by weight of sulfonated alkali lignin as a synergistic color-developing agent. Sulfonated alkali lignin is an existing product, also known as lignin sulfonate.

[0064] In some specific embodiments, the coating and drying of the thermal color-developing layer includes: First, preparations before coating; The support with the bottom layer is placed on the unwinding rack in the form of a roll and kept under constant tension. Before coating, it can be optionally passed through a set of static elimination bars to remove surface static charge and prevent dust from being attracted by static electricity or uneven spreading of the coating liquid during the coating process. Furthermore, the prepared thermosensitive color development coating liquid is transferred to the material tank of the coating machine and continuously stirred at low speed to maintain its uniformity and prevent solid particles from settling.

[0065] Next, apply the coating; A precision rod coating machine is used, preferably with Mayer rods with a wire diameter of 30μm to 50μm for coating.

[0066] By precisely controlling the travel speed of the coating machine and the specifications of the Mayer rod, the wet coating amount of the thermal developer coating liquid is controlled at 15 g / m². 2 ~25g / m 2 Within the specified range. After drying, the dry coating weight of the thermosensitive color developing layer was precisely controlled at 4.0 g / m². 2 ~6.0g / m 2The coating amount range is an optimized key parameter that ensures both sufficient reactant concentration in the color developing layer to achieve high color concentration and a thin coating to facilitate rapid heat conduction, thereby achieving high-sensitivity color development.

[0067] Next, air dry; The wet coating is immediately placed in a hot air drying tunnel for drying and curing. The temperature is controlled at 60℃~80℃, the air velocity is 2 m / s~5 m / s, and the drying time is 90 seconds~120 seconds. During this process, the moisture is effectively evaporated, polyvinyl alcohol forms a film, and ureapropyltrimethoxysilane, 3-dibutylamino-6-methyl-7-aniline fluorane, and calcined kaolin are firmly bonded together, forming a smooth heat-sensitive color-developing layer on the bottom surface.

[0068] Finally, winding and post-processing; After being completely dried, the material is cooled to room temperature (20℃~30℃) by cooling rollers and then wound up with constant tension by a winding device. The semi-finished product after winding should be temporarily stored in a clean environment with a temperature ≤25℃ and a relative humidity ≤60% in preparation for subsequent coating of the protective layer or direct slitting.

[0069] In some specific embodiments, a protective layer is further coated and air-dried on the thermosensitive color-developing layer; the coating liquid of the protective layer comprises 70 to 90 parts of waterborne polyurethane emulsion and 3 to 15 parts of waterborne wax paste, based on the weight parts of the coating liquid when preparing the thermosensitive color-developing layer; the solid content of the waterborne polyurethane emulsion ranges from 30% to 40%, and the solid content of the waterborne wax paste ranges from 5% to 10%.

[0070] The polyurethane in the waterborne polyurethane emulsion can form a strong and continuous film, providing basic wear resistance; the wax particles in the waterborne wax paste form a lubricating layer on the coating surface, reducing the dynamic coefficient of friction of the material to 0.15~0.25; the final protective layer can significantly improve the scratch resistance and plasticizer resistance of thermal recording materials, and further reduce contamination of the printhead.

[0071] Preferably, based on the weight parts when preparing the thermosensitive color developing coating solution, the coating solution of the protective layer comprises 80 parts of waterborne polyurethane emulsion and 10 parts of waterborne wax paste; the solid content of the waterborne polyurethane emulsion is 35%, and the solid content of the waterborne wax paste is 7%.

[0072] Specifically, the preparation process of the protective layer includes: mixing the waterborne polyurethane emulsion and waterborne wax paste evenly under low-speed stirring (300 rpm to 500 rpm), then allowing it to stand to defoam, to obtain the protective layer coating liquid; using a microgravure coating method, precisely coating the protective layer coating liquid onto the dried thermosensitive color developing layer surface, with the wet coating amount controlled at 8 g / m². 2 ~12g / m2 Then, it is dried in hot air at 50℃~65℃ for 20 to 40 seconds to form a dry coating weight of 2g / m². 2 ~4g / m 2 The protective layer.

[0073] The process employs microgravure coating and medium-low temperature (50℃~65℃) drying, which avoids the physical or thermal damage that high-speed, high-temperature coating may cause to the underlying heat-sensitive color development layer.

[0074] The present invention also provides a thermal recording material, which is prepared by the method described above, comprising a support, a substrate formed on the support, and a thermal colorimetric layer formed on the substrate, wherein the thermal colorimetric layer comprises ureapropyltrimethoxysilane as a thermal colorimetric agent.

[0075] The thermal recording material of this invention uses ureapropyltrimethoxysilane as a highly efficient and feasible primary colorimetric agent, fundamentally avoiding the potential hazards of traditional phenolic colorimetric agents and providing a safer, healthier, and more environmentally friendly colorimetric solution. Ureapropyltrimethoxysilane not only provides sufficient protons at lower temperatures for efficient dye development and ensures high colorimetric concentration, but its unique organosilicon molecular structure (silicon-oxygen bonds, silicon-carbon bonds) also endows the entire thermal colorimetric layer with excellent thermal and chemical stability. This effectively overcomes the defects of background yellowing and image instability, resulting in a material with extremely low background haze and extremely high image retention under high temperature and high humidity environments. In other words, the thermal recording material of this invention, through the innovative selection of ureapropyltrimethoxysilane as a thermal colorimetric agent, constructs a completely new thermal colorimetric system, thereby simultaneously achieving a comprehensive effect of environmental and health friendliness, low colorimetric onset temperature, good weather resistance of the developed image, excellent colorimetric performance, and high image stability.

[0076] Furthermore, ureapropyltrimethoxysilane also acts as a stabilizer. The trimethoxysilane group at the front end of its molecule can undergo hydrolytic condensation to form a strong Si-O-Si covalent bond with the hydroxyl groups on the surface of inorganic fillers or inorganic pigments. This effect "anchors" the developer molecules within the coating network, significantly improving the coating's thermal stability and aging resistance, and fundamentally reducing printhead contamination caused by developer migration. In other words, ureapropyltrimethoxysilane not only achieves efficient color development but also additionally endows the coating with inherent stability and anti-fouling properties, which are not found in traditional phenolic developers.

[0077] Example 1 (1) Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; The preparation of the base coat solution includes: taking 20 kg of calcined kaolin, 30 kg of 10 wt% polyvinyl alcohol aqueous solution, 0.3 kg of acetylenol-based nonionic surfactant, 18 kg of 40 wt% polyvinyl acetate emulsion, and 31.7 kg of distilled water, and mixing them evenly to obtain the base coat solution.

[0078] (2) Prepare the thermosensitive color development layer coating solution and coat it on the substrate, then air dry to form a thermosensitive color development layer; The preparation of the thermosensitive colorimetric coating solution includes: taking 6 kg of ureapropyltrimethoxysilane, 3 kg of 3-dibutylamino-6-methyl-7-aniline fluorane, 9 kg of 10 wt% polyvinyl alcohol aqueous solution, 27 kg of distilled water and 9 kg of calcined kaolin, mixing them evenly, and then adding 46 kg of 2 wt% polyvinyl alcohol aqueous solution after wet dispersion, and mixing evenly to obtain the thermosensitive colorimetric coating solution.

[0079] Example 2 (1) Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; The preparation of the base coat solution includes: taking 20 kg of calcined kaolin, 30 kg of 10 wt% polyvinyl alcohol aqueous solution, 0.3 kg of acetylenol-based nonionic surfactant, 18 kg of 40 wt% polyvinyl acetate emulsion, and 31.7 kg of distilled water, and mixing them evenly to obtain the base coat solution.

[0080] (2) Prepare the thermosensitive color development layer coating solution and coat it on the substrate, then air dry to form a thermosensitive color development layer; The preparation of the thermosensitive color development coating solution includes: taking 6 kg of ureapropyltrimethoxysilane, 3 kg of 3-dibutylamino-6-methyl-7-aniline fluorane, 9 kg of 10 wt% polyvinyl alcohol aqueous solution, 27 kg of distilled water and 9 kg of calcined kaolin, mixing them evenly, and then adding 46 kg of 2 wt% polyvinyl alcohol aqueous solution after wet dispersion, and mixing evenly to obtain the thermosensitive color development coating solution; The preparation of the thermosensitive color development coating solution also includes: after wet dispersion and before adding 2 wt% of polyvinyl alcohol aqueous solution, adding 2% of epoxy functionalized nano silica by mass of the total dry matter of the system, and stirring and reacting at 60°C for 45 min.

[0081] Example 3 (1) Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; The preparation of the base coat solution includes: taking 20 kg of calcined kaolin, 30 kg of 10 wt% polyvinyl alcohol aqueous solution, 0.3 kg of acetylenol-based nonionic surfactant, 18 kg of 40 wt% polyvinyl acetate emulsion, and 31.7 kg of distilled water, and mixing them evenly to obtain the base coat solution.

[0082] (2) Prepare the thermosensitive color development layer coating solution and coat it on the substrate, then air dry to form a thermosensitive color development layer; The preparation of the thermosensitive color development coating solution includes: taking 6 kg of ureapropyltrimethoxysilane, 3 kg of 3-dibutylamino-6-methyl-7-aniline fluorane, 9 kg of 10 wt% polyvinyl alcohol aqueous solution, 27 kg of distilled water and 9 kg of calcined kaolin, mixing them evenly, and then adding 46 kg of 2 wt% polyvinyl alcohol aqueous solution after wet dispersion, and mixing evenly to obtain the thermosensitive color development coating solution; The preparation of the thermosensitive color development coating solution also includes: after wet dispersion and before adding 2 wt% of polyvinyl alcohol aqueous solution, adding 2% of epoxy functionalized nano silica by mass of total dry matter in the system, and stirring and reacting at 60°C for 45 min. Before use, ureapropyltrimethoxysilane should be pre-hydrolyzed with an acidic aqueous solution with a pH of 4.5 at 45°C for 40 minutes.

[0083] Example 4 (1) Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; The preparation of the base coat solution includes: taking 20 kg of calcined kaolin, 30 kg of 10 wt% polyvinyl alcohol aqueous solution, 0.3 kg of acetylenol-based nonionic surfactant, 18 kg of 40 wt% polyvinyl acetate emulsion, and 31.7 kg of distilled water, and mixing them evenly to obtain the base coat solution.

[0084] (2) Prepare the thermosensitive color development layer coating solution and coat it on the substrate, then air dry to form a thermosensitive color development layer; The preparation of the thermosensitive color development coating solution includes: taking 6 kg of ureapropyltrimethoxysilane, 3 kg of 3-dibutylamino-6-methyl-7-aniline fluorane, 9 kg of 10 wt% polyvinyl alcohol aqueous solution, 27 kg of distilled water and 9 kg of calcined kaolin, mixing them evenly, and then adding 46 kg of 2 wt% polyvinyl alcohol aqueous solution after wet dispersion, and mixing evenly to obtain the thermosensitive color development coating solution; The preparation of the thermosensitive color development coating solution also includes: after wet dispersion and before adding 2 wt% of polyvinyl alcohol aqueous solution, adding 2% of epoxy functionalized nano silica by mass of total dry matter in the system, and stirring and reacting at 60°C for 45 min. Before use, ureapropyltrimethoxysilane should be pre-hydrolyzed with an acidic aqueous solution with a pH of 4.5 at 45°C for 40 minutes. The heat-sensitive color-developing coating solution also contains 2 kg of sulfonated alkali lignin as a synergistic color-developing agent.

[0085] Example 5 (1) Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; The preparation of the base coat solution includes: taking 20 kg of calcined kaolin, 30 kg of 10 wt% polyvinyl alcohol aqueous solution, 0.3 kg of acetylenol-based nonionic surfactant, 18 kg of 40 wt% polyvinyl acetate emulsion, and 31.7 kg of distilled water, and mixing them evenly to obtain the base coat solution.

[0086] (2) Prepare the thermosensitive color development layer coating solution and coat it on the substrate, then air dry to form a thermosensitive color development layer; The preparation of the thermosensitive color development coating solution includes: taking 6 kg of ureapropyltrimethoxysilane, 3 kg of 3-dibutylamino-6-methyl-7-aniline fluorane, 9 kg of 10 wt% polyvinyl alcohol aqueous solution, 27 kg of distilled water and 9 kg of calcined kaolin, mixing them evenly, and then adding 46 kg of 2 wt% polyvinyl alcohol aqueous solution after wet dispersion, and mixing evenly to obtain the thermosensitive color development coating solution; The preparation of the thermosensitive color development coating solution also includes: after wet dispersion and before adding 2 wt% of polyvinyl alcohol aqueous solution, adding 2% of epoxy functionalized nano silica by mass of total dry matter in the system, and stirring and reacting at 60°C for 45 min. Before use, ureapropyltrimethoxysilane should be pre-hydrolyzed with an acidic aqueous solution with a pH of 4.5 at 45°C for 40 minutes. The heat-sensitive color-developing coating solution also contains 2 kg of sulfonated alkali lignin as a synergistic color-developing agent.

[0087] (3) A protective layer is formed by air drying on top of the heat-sensitive color developing layer; The coating liquid for the protective layer contains 80 kg of waterborne polyurethane emulsion and 10 kg of waterborne wax paste; the solid content of the waterborne polyurethane emulsion is 35%, and the solid content of the waterborne wax paste is 7%.

[0088] Comparative Example 1 Commonly used thermal recording materials on the market that use "bisphenol A (BPA)" as a thermal color developer include shopping receipts.

[0089]

[0090] Note: In evaluation systems, especially in this type of tiered evaluation of technical performance, the usual ranking from best to worst is: Excellent > Superior. + >Excellent >Good + >Good>Medium + >Medium>Poor.

[0091] As shown in Table 1, all examples (1-5) were rated "excellent" in terms of environmental and health friendliness, which completely solved the health and environmental risks of the "poor" rating of Comparative Example 1 (BPA system) due to the use of bisphenol A (which has endocrine-disrupting effects). This shows that the core innovation of the present invention - the use of ureapropyltrimethoxysilane as a colorimetric agent - has achieved the goal of environmental and health friendliness.

[0092] As shown in Table 1, the color development onset temperatures (90℃~95℃) of all Examples (1~5) were significantly lower than those of Comparative Example 1 (105℃), demonstrating that the new system has lower energy consumption and faster response speed. Furthermore, from Example 1 to Example 4, the color development onset temperature gradually decreased, reflecting the synergistic effect of the addition of epoxy-functionalized nano-silica, pre-hydrolysis treatment, and sulfonated alkali lignin, which jointly promoted the improvement of color development sensitivity. In addition, the color development concentration (OD=1.18) of Example 1 was slightly lower than that of Comparative Example 1 (OD=1.20), but with the optimization of the formulation, starting from Example 2, the color development concentration reached or exceeded the level of the comparative example, with Examples 4 and 5 reaching the highest at 1.25. This indicates that the new system, through optimization, fully achieved the requirement of high color development concentration while maintaining a low color development onset temperature.

[0093] As shown in Table 1, the initial background haze (OD value 0.05~0.08) of all Examples (1~5) was lower than that of Comparative Example 1 (OD value 0.10), and decreased progressively with formulation optimization, indicating that the new system itself has better background whiteness and initial stability. Comparative Example 1 showed a sharp increase in background haze after aging, with an image retention rate of only 70%; while Example 1 (85%) showed significant improvement, and with the construction of the crosslinked network (Example 2), interface optimization (Example 3), synergistic color development (Example 4), and physical protection (Example 5), the image retention rate steadily increased to 97%, and the background haze after aging also decreased to an extremely low 0.07; this fully demonstrates the decisive role of the organic-inorganic hybrid network of the present invention in improving image weather resistance and stability.

[0094] As shown in Table 1, Comparative Example 1 used bisphenol A (BPA) as the color developer. BPA molecules are prone to migration and volatilization, and their chemical properties are unstable under hot and humid conditions, leading to rapid yellowing of the background (the haze level increased from 0.10 to 0.25). Furthermore, the developed image was easily decomposed, resulting in extremely low image retention (70%), and its weather resistance was rated as "poor". Example 1 used ureapropyltrimethoxysilane as the color developer. Its silane end, through hydrolysis and condensation, forms Si-O-Si covalent bonds with the filler, "chemically anchoring" the color developer molecules and effectively inhibiting migration. This significantly improved background stability and image retention (increasing to 85%), laying a good foundation, and its weather resistance was rated as "good". Example 2 added epoxy-functionalized nano-silica, which acts as a crosslinking center, reacting with the colorant and binder to construct a three-dimensional organic-inorganic hybrid network. This network, like a "steel skeleton," greatly enhances the thermal stability and density of the coating, further locking in the components. Therefore, the initial background haze is lower (0.12), the image retention rate is higher (90%), and the weather resistance achieves a significant leap, rated as "Excellent." Example 3, based on Example 2, pre-hydrolyzed the colorant, allowing for a more complete and thorough reaction of the silane, achieving optimal interfacial bonding with the nano-silica and filler. The hybrid network is more complete and robust, consolidating and strengthening the weather resistance at the "Excellent" level, and increasing the image retention rate to 93%. Example 4 uses sulfonated alkali lignin as a synergistic color developer. It forms a dynamic hydrogen bond network with the main color developer. This network not only lowers the activation energy for color development but also more stably "fixes" the color morphology after color development, thereby enhancing the anti-aging ability of the color product itself. This is an additional molecular-level image-locking function on top of chemical stability, resulting in superior weather resistance (95% image retention rate), and is rated as "Excellent". + Example 5, building upon all the advantages of Example 4, adds a special protective layer (waterborne polyurethane + wax). This protective layer physically isolates the underlying heat-sensitive color-developing layer from external moisture, oxygen, plasticizers, etc., providing an ultimate physical barrier. This is a double guarantee for weather resistance, enabling the material to perform optimally in extreme environments, with an image retention rate as high as 97%, hence the evaluation of "excellent".

[0095] As shown in Table 1, the scratch / abrasion resistance of Example 1 is comparable to that of Comparative Example 1 (medium). Starting from Example 2, the abrasion resistance is improved to "good" and continues to be optimized due to the enhanced cohesion of the coating by the cross-linked network. Finally, Example 5 achieves a qualitative change in performance due to the addition of a special protective layer, reaching "excellent".

[0096] As shown in Table 1, Comparative Example 1 exhibits a "Poor" rating for printhead contamination due to BPA migration and volatilization issues. Example 1 uses ureapropyltrimethoxysilane, whose silane ends are "chemically anchored" in the coating network through hydrolysis and condensation, fundamentally preventing the migration of developer molecules; the source of contamination is significantly cut off, and contamination is fundamentally improved, resulting in a "Medium" rating. Example 2 adds epoxy-functionalized nano-silica, forming a dense cross-linked network that not only fixes the developer but also more firmly binds all components such as dyes and fillers together, enhancing the overall integrity of the coating and making it less prone to dust generation under printhead friction, further reducing contamination, resulting in a "Good" rating. Example 3 pre-hydrolyzes ureapropyltrimethoxysilane, forming a more complete and robust interfacial bond, maximizing the "anchoring" and "cross-linking" effects; the coating becomes more robust and uniform, almost eliminating contamination caused by component migration or coating damage, improving the rating to "Excellent". Example 4 added sulfonated alkali lignin, whose dynamic hydrogen bond network with the main color developer can more stably "lock" the color morphology after color development, further enhancing the stability of the color product and possibly reducing the generation of certain byproducts. Thus, it achieves even better anti-fouling performance on top of "excellent," and is therefore rated "excellent." + Example 5 adds a protective layer that physically isolates the printhead from the underlying thermal color development layer. The printhead directly contacts the smooth, wear-resistant, and chemically inert protective layer, almost completely eliminating the possibility of contamination and achieving the theoretically best "excellent" level.

[0097] In summary, the data in Table 1 clearly demonstrates the technical path of this invention, which involves gradual optimization and synergistic enhancement: Example 1 successfully constructed a new system foundation that is environmentally friendly, highly sensitive, and highly stable, completely replacing BPA; Examples 2 and 3 significantly enhanced the physicochemical stability (weather resistance and anti-fouling properties) of the coating by introducing crosslinking centers and optimizing interfacial reactions; Example 4 further optimized the color development performance (reducing the starting temperature and increasing the concentration) without sacrificing stability by introducing synergistic color-developing components; Example 5 provided the material with top-level physical protection and environmental isolation by adding a functional protective layer, achieving the ultimate goal of comprehensive performance.

[0098] The foregoing has provided a detailed description of a thermal recording material and its preparation method provided by the embodiments of the present invention. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing a thermal recording material, characterized in that, Includes the following steps: S1. Prepare the base coat liquid and apply it to the support, then air dry to form the base coat; The preparation of the undercoating solution comprises: taking 18-22 parts by weight of calcined kaolin, 25-35 parts by weight of 10wt% polyvinyl alcohol aqueous solution, 0.2-0.4 parts by weight of dispersant, 15-20 parts by weight of auxiliary binder, and 25-35 parts by weight of distilled water, and mixing them evenly to obtain the undercoating solution; the dispersant is an acetylenic glycol-based nonionic surfactant; the auxiliary binder is a polyvinyl acetate emulsion; S2. Prepare a thermosensitive color-developing layer coating solution and coat it onto the substrate, then air dry to form a thermosensitive color-developing layer; the thermosensitive color-developing layer includes ureapropyltrimethoxysilane as a thermosensitive color-developing agent. The preparation of the thermosensitive color-developing coating solution includes: taking 5 to 7 parts by weight of ureapropyltrimethoxysilane, 2 to 4 parts by weight of 3-dibutylamino-6-methyl-7-aniline fluorane, 8 to 10 parts by weight of 10 wt% polyvinyl alcohol aqueous solution, 25 to 30 parts by weight of distilled water and 8 to 10 parts by weight of calcined kaolin, mixing them evenly, dispersing them by wet method, and then adding 40 to 50 parts by weight of 2 wt% polyvinyl alcohol aqueous solution, mixing them evenly to obtain the thermosensitive color-developing coating solution; The preparation of the thermosensitive color development coating solution further includes: after the wet dispersion and before the addition of 2wt% polyvinyl alcohol aqueous solution, adding 0.5% to 3% of epoxy functionalized nano silica, accounting for the total dry matter mass of the system, and stirring and reacting at 50℃ to 65℃ for 30 min to 60 min. Before use, the ureapropyltrimethoxysilane is pre-hydrolyzed with an acidic aqueous solution with a pH of 4-5 at 40-50°C for 30-50 minutes.

2. The method for preparing the thermal recording material as described in claim 1, characterized in that, The wet dispersion is achieved by grinding the particles using a wet sand mill until the average particle size is ≤0.5μm.

3. The method for preparing the thermal recording material as described in claim 1, characterized in that, The thermosensitive color-developing coating solution also contains 1-3 parts by weight of sulfonated alkali lignin as a synergistic color-developing agent.

4. The method for preparing the thermal recording material as described in claim 1, characterized in that, A protective layer is also coated and air-dried on the heat-sensitive color-developing layer. Based on the weight parts used in preparing the thermal color developing coating solution, the coating solution of the protective layer comprises 70 to 90 parts of waterborne polyurethane emulsion and 3 to 15 parts of waterborne wax paste; the solid content of the waterborne polyurethane emulsion ranges from 30% to 40%, and the solid content of the waterborne wax paste ranges from 5% to 10%.

5. A thermal recording material, characterized in that, The thermal recording material is prepared by the method for preparing thermal recording material according to any one of claims 1 to 4.

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