A water fade pigment and a method for preparing the same

Abstract

The present application relates to the technical field of pigment material, and particularly relates to a water-caused fading pigment block and application thereof. The pigment block is in a solid block structure, and is prepared from leuco dye, anhydrous calcium sulfate, organic weak acid, water-soluble binder, humectant, dispersant, preservative, pH regulator and deionized water, wherein the leuco dye is a fluoran dye with a lactone ring closed structure, and the anhydrous calcium sulfate is used as an inorganic forming skeleton material. By controlling the component ratio and particle size range, the forming property of the pigment slurry and the system stability are considered. The pigment block can be coated on the surface of a water-impermeable substrate by kneading or externally applying oxalic acid aqueous solution, and can be removed by water wiping or washing. The pigment block has stable structure, simple preparation process, is suitable for use in a block form, and meets the requirements of repeated writing or coating and water washing removal.

Description

Technical Field

[0001] This invention relates to the field of pigment materials technology, specifically to a water-induced fading pigment block and its preparation method. Background Technology

[0002] Currently, pigment products are widely used in painting, marking, teaching, decoration, and entertainment. Among them, block pigments are popular among teenagers and families due to their portability and intuitive use. Most existing pigment blocks are solid watercolors, wax pigments, or water-soluble dyes. Their coloring process typically relies on water, oil, or solvents to adhere the pigment to the surface of substrates such as paper, wood, and plastic. However, these pigments often exhibit strong adhesion and stability after color development, making them difficult to remove without damaging the substrate.

[0003] To address the need for erasable or washable pigments, existing technologies have proposed some reversible color-changing or washable pigment solutions. These solutions involve adding water-soluble dyes, thermochromic materials, or photochromic materials to cause the pigment to change or weaken its color under specific conditions. However, these solutions typically rely on specific environmental conditions, such as temperature changes, ultraviolet light irradiation, or treatment with special chemical reagents. The process is relatively complex and imposes certain limitations on the application scenarios and operating methods. Furthermore, some washable pigments may still leave traces after drying, making them difficult to completely remove.

[0004] Therefore, how to provide a pigment product with stable structure, reasonable formulation, applicable to various application scenarios, and easy to remove under water has become an urgent problem to be solved. Summary of the Invention

[0005] To overcome the limitations of existing pigment products, such as restricted application scenarios, difficulty in rapid and residue-free removal after color development, and poor reusability, this invention provides a water-based fading pigment and its preparation method. This pigment exists in block form, can stably adhere to and quickly set on various waterproof substrates, achieves controllable color development through oxalic acid triggering, and completely fades without residue within 30 seconds upon contact with water after application. It supports at least 25 repeated painting-fading cycles on the same substrate surface. All raw materials meet food-grade or children's toy safety standards. The preparation process is simple, suitable for large-scale production, and can be widely used in various scenarios such as children's craft creation, teaching demonstrations, and home decoration.

[0006] The water-based fading pigment of the present invention is prepared from the following raw materials by mass percentage: Black-15 dye, chemically named 2-(2-4-dimethylamino)-3-methyl-6-diethylaminofluorane, with a molecular structure in the leuco form of a closed lactone ring, 15%–25%; anhydrous calcium sulfate, 30%–40%; oxalic acid, 1%–3%; water-soluble binder, 8%–12%; humectant, 5%–8%; dispersant, 2%–4%; preservative, 0.5%–1.0%; pH adjuster, 0.3%–0.8%; and the balance being deionized water.

[0007] Preferably, the anhydrous calcium sulfate is an analytical grade powder with a particle size of 200-300 mesh; the oxalic acid is a food-grade crystalline powder with a particle size of 100-200 mesh. Anhydrous calcium sulfate powder within this particle size range exhibits good uniformity and dispersibility, preventing excessively large particles from causing a grainy feel or clogging the ink flow during coating, while also preventing excessively fine particles from causing the pigment blocks to become too dense and brittle or too hard. The analytical grade ensures low impurities, improving product stability and safety. The 100-200 mesh particle size allows for a moderate dissolution rate of oxalic acid in the slurry, facilitating uniform distribution during slow addition and preventing excessively high local concentrations that prematurely trigger dye development. The food-grade grade ensures safety in accordance with children's toy standards, while the high purity and strong reaction specificity of the crystalline powder ensure gentle and controllable color development triggering.

[0008] Preferably, the water-soluble adhesive is selected from one or both of gum arabic and hydroxypropyl methylcellulose. Gum arabic dries quickly (setting in 3-5 minutes) and has strong adhesion, making it suitable for rapid setting scenarios; hydroxypropyl methylcellulose has good flexibility and is crack-resistant, making it suitable for flexible substrates; the combination can optimize adhesion and setting performance according to the scenario, without sagging or cracking.

[0009] Preferably, the humectant is selected from one or both of glycerin and propylene glycol. Both are food-grade and have excellent moisturizing properties, which can keep the pigment block moderately soft, prevent it from drying and cracking or becoming too hard, while extending the storage stability after opening (≥6 months), improving the coating feel and ink flow smoothness, and preventing the pigment block from becoming brittle or absorbing moisture and clumping during use.

[0010] Preferably, the dispersant is selected from one or both of sodium dodecylbenzenesulfonate and polysorbate 80. Both have high dispersion efficiency, effectively preventing the agglomeration of anhydrous calcium sulfate and dye particles, resulting in a uniform slurry with stable viscosity, which facilitates a smooth surface and no color spots on the pigment blocks after molding; at the same time, they have good compatibility and do not affect the oxalic acid-triggered color development reaction, ensuring that the final product has a uniform and full color coating.

[0011] Preferably, the preservative is selected from one or both of potassium sorbate and sodium dehydroacetate. Both are food-grade broad-spectrum preservatives, and a small amount (0.5% to 1.0%) can effectively inhibit the growth of mold and bacteria, significantly extend the shelf life of pigment blocks (18 months unopened, ≥6 months after opening), prevent the slurry or finished product from deteriorating in a humid environment, and do not affect pH and color development performance.

[0012] Preferably, the pH adjuster is selected from one or both of citric acid and sodium bicarbonate, ultimately adjusting the pH value of the pigment paste to 6.5–7.5. The combination of citric acid (acidic) and sodium bicarbonate (alkaline) allows for precise fine-tuning of the pH to a near-neutral range, preventing the paste from being too acidic and prematurely triggering dye development or too alkaline and affecting stability; a pH of 6.5–7.5 is close to the physiological value of skin, improving product safety, while also facilitating the effectiveness of preservatives and maintaining the overall stability of the formulation.

[0013] The method for preparing the water-induced fading pigment of the present invention includes the following steps: S1. Raw material pretreatment: Prepare Black-15 dye solution, grind and sieve anhydrous calcium sulfate, prepare water-soluble binder solution, and prepare oxalic acid solution respectively; S2. Slurry preparation: Anhydrous calcium sulfate, dispersant and Black-15 dye solution are first mixed and dispersed in a high-speed disperser. Then, water-soluble binder solution, humectant, preservative and pH adjuster are added in sequence. Finally, oxalic acid solution cooled to room temperature is slowly added at a rate of 1 mL / min. Dispersion is continued until a uniform paste slurry with a viscosity of 5000-6000 mPa·s is formed. S3. Molding: The slurry is injected into the mold and degassed for 15 minutes under a vacuum of -0.09MPa, and then pressed for 30 seconds under a pressure of 5MPa to obtain the pigment block blank. S4. Drying: Place the blank in an environment with a temperature not exceeding 50°C and dry for 4 to 5 hours to obtain water-induced fading pigment blocks.

[0014] Furthermore, the Black-15 dye solution is prepared by mixing Black-15 dye with deionized water at a mass ratio of 1:1.5 and stirring in a constant temperature water bath at 35°C to 40°C until completely dissolved; the oxalic acid solution needs to be cooled to room temperature before adding the slurry to avoid prematurely triggering the dye color development.

[0015] The pigment blocks of this invention are in a light gray, taupe state under storage and normal conditions. During use, by rubbing, friction, or by pre-applying a small amount of oxalic acid solution to the substrate surface, the oxalic acid is brought into full contact with the Black-15 dye. Under a mild acidic environment, the dye lactone ring rapidly opens, forming a quinone structure of an extended conjugated system, thus producing a highly saturated black color. After being coated onto impermeable substrates such as glass, ceramics, metals, plastics, enamel, and acrylic, it sets rapidly within 3-5 minutes without sagging or cracking.

[0016] When fading, simply wipe with water. Water molecules quickly penetrate to the dye-substrate interface, disrupting the local acidic microenvironment. At the same time, anhydrous calcium sulfate assists in the capillary penetration of water and further reduces the H⁺ concentration by combining calcium ions with oxalate ions. This causes the dye molecules to lose protons, the lactone ring to close again, and the dye to return to a colorless leuco form. The dye then detaches from the substrate surface with the water flow, achieving complete fading within 30 seconds with no residue.

[0017] Compared with the prior art, the present invention has the following significant advantages: 1. Strong adaptability to multiple scenarios: It can be used stably on various waterproof surfaces such as glass, metal, and plastic. It does not drip on smooth substrates and sets in 3-5 minutes. It adheres to flexible substrates without cracking. 2. Quick, reversible, and reusable: Color development is controllable, and the color completely fades within 30 seconds of contact with water. It supports ≥25 re-creations on the same substrate, significantly reducing material waste. Attached Figure Description

[0018] Figure 1 Test by wiping on glass; Figure 2 : Scratch test on plastic; Figure 3 : A test to remove the metal surface; Figure 4 : Erasure test on whiteboard. Detailed Implementation

[0019] The technical solution of the present invention will be further described below with reference to embodiments thereof. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Equivalent substitutions or modifications made by those skilled in the art without departing from the inventive concept should fall within the scope of protection of the present invention.

[0020] Unless otherwise stated, the raw materials used in the following examples are all commercially available qualified products, and the relevant proportions are by mass percentage or by mass parts; the preparation process is completed under conventional laboratory or industrial conditions, and the parameters are all conventional process parameters that can be understood and implemented by those skilled in the art.

[0021] Example 1

[0022] This embodiment provides a basic water-based fading pigment composition for verifying the color development and water-wash fading effects of the pigment on a waterproof, glass-like substrate. In this embodiment, the raw material composition of the composition is as follows (based on a total mass of 1000g): Leuco dye Black-15: 18%; Anhydrous calcium sulfate (300 mesh): 35%; Gum arabic: 35%; Glycerin: 10%; Sodium dodecylbenzenesulfonate: 6%; Potassium sorbate: 3%; Citric acid: 0.8%; Oxalic acid: 0.5%; Deionized water: balance.

[0023] The composition is prepared according to the following steps: (1) Prepare a leuco dye dispersion; (2) Screen anhydrous calcium sulfate; (3) Mix and disperse anhydrous calcium sulfate, dispersant and leuco dye dispersion; (4) Add binder, humectant, preservative and pH adjustment component in sequence; (5) Slowly add oxalic acid solution and stir evenly to obtain the target slurry.

[0024] The above slurry is molded and dried to obtain pigment blocks. In use, the pigment blocks are rubbed on the surface of a glass substrate to develop a clear and continuous pattern; subsequently, wiping with water completely removes the pigment layer within a short time, leaving no residue on the glass surface, as shown in the attached figure. Figure 1 As shown.

[0025] Example 2

[0026] This embodiment, based on Embodiment 1, adjusts the film-forming binder system to verify the applicability of the pigment of the present invention on the surface of waterproof plastic substrates. In this embodiment, the raw material composition of the composition is as follows (based on a total mass of 1000g): Leuco dye Black-15: 20%; Anhydrous calcium sulfate (300 mesh): 32%; Hydroxypropyl methylcellulose: 32%; Propylene glycol: 11%; Polysorbate 80: 7%; Sodium dehydroacetate: 3.5%; Sodium bicarbonate: 0.5%; Oxalic acid: 0.3%; Deionized water: balance.

[0027] The preparation method is basically the same as in Example 1, except that hydroxypropyl methylcellulose is used instead of gum arabic as the main binder. After the obtained pigment composition is made into a solid product, writing or coating operations are performed on the surface of a plastic substrate, resulting in a continuous and uniform colored pattern; after wiping with water, the colored layer is effectively removed, and the plastic surface retains its original state, as shown in the attached image. Figure 2 As shown.

[0028] Example 3

[0029] This embodiment obtains a high-strength solid pigment block suitable for repeated use by increasing the inorganic filler content, and uses it to verify its color development and fading performance on metallic substrates. The raw material composition of the composition is as follows (based on a total mass of 1000g): Leuco dye Black-15: 22%; Anhydrous calcium sulfate (300 mesh): 40%; Gum arabic: 30%; Glycerin: 6%; Sodium dodecylbenzenesulfonate: 5%; Potassium sorbate: 2%; Citric acid: 0.8%; Oxalic acid: 0.6%; Deionized water: balance.

[0030] After the above composition was formulated into block pigment, it was applied to a metal substrate for writing, resulting in clear and stable lines. Subsequently, wiping with water completely removed the pigment residue, leaving no obvious residue or damage on the metal surface. (See attached image.) Figure 3 As shown.

[0031] Example 4

[0032] This embodiment, based on the aforementioned embodiments, further optimizes the mixing uniformity and system stability during the slurry preparation stage. The key focus is on staged control of stirring speed and time to achieve a more stable distribution of each functional component within the system, thereby improving the overall consistency and stability of the resulting pigment during repeated color development and washing processes. In this embodiment, the raw material composition of the water-induced fading pigment composition is as follows (based on a total mass of 1000g): Leuco dye Black-15: 21%; Anhydrous calcium sulfate (300 mesh): 36%; Blended binder of gum arabic and hydroxypropyl methylcellulose: 32%; Blended humectant of glycerin and propylene glycol: 8%; Nonionic dispersant: 6%; Food-grade preservative: 2.5%; Weakly acidic pH adjuster: 0.8%; Oxalic acid: 0.5%; Deionized water: Balance.

[0033] The method for preparing the composition includes the following steps: (1) Add anhydrous calcium sulfate and the dispersant into a mixing container and perform pre-dispersion treatment at a stirring speed of 800-1000 r / min for 10-20 minutes to fully wet and initially uniformly disperse the anhydrous calcium sulfate particles in the liquid phase. (2) While maintaining a stirring speed of 800-1000 r / min, add the leuco dye dispersion to the above system and continue stirring for 15-30 minutes to make the leuco dye uniformly distributed on the surface of the inorganic filler and in the continuous phase, forming a stable basic dispersion system. (3) Then reduce the stirring speed to 400-600 r / min, and add the compound binder solution and compound humectant in sequence at this stirring speed, and continue stirring for 20-30 minutes to gradually transform the system into a uniform high-viscosity slurry; (4) Add preservatives and pH adjustment components under the above stirring speed conditions, and continue stirring for 10 to 15 minutes to stabilize the chemical environment of the system; (5) Finally, the stirring speed is further reduced to 200-300 r / min. Under this condition, the oxalic acid solution is slowly added and the stirring time is extended to 15-25 minutes so that the oxalic acid is evenly distributed in the system without forming local high concentration areas, thereby obtaining the target slurry with stable structure.

[0034] By controlling the stirring speed and time in stages as described above, structural damage caused by excessive shearing during the high viscosity stage can be avoided, while preventing the accumulation of oxalic acid in local areas, thus helping to maintain the leuco stability of the slurry during storage.

[0035] After the obtained slurry is molded and dried, a solid pigment product is obtained. In use, the pigment product can form continuous color marks on a non-permeable substrate surface under external force; after water treatment, the color marks can be effectively removed, and the substrate surface returns to its original state. After repeated color development and washing operations, the pigment product still maintains stable color development and fading properties, as shown in the attached figure. Figure 4 As shown.

[0036] As can be seen from Examples 1 to 4, the present invention, through the rational design of the leuco dye system, the ratio of inorganic fillers and the acid triggering conditions, enables the obtained water-induced fading pigment to achieve stable color development and water washing fading effects under different formulation conditions and usage scenarios, which reflects the preferred implementation of the technical solution of the present invention.

[0037] Comparative Example Two types of black painting materials on the market were selected as comparative examples and compared with the present invention in terms of "color development-fading" performance. The results are shown in the table below: As can be seen from the comparison, the present invention is significantly superior to traditional black painting materials in terms of color controllability, multi-scene adaptability, reusability, and safety, and is especially suitable for the needs of scenarios such as children's crafts and teaching demonstrations that require "on-demand color development and quick removal".

[0038] Although the principles of the present invention have been described in detail above with reference to preferred embodiments, those skilled in the art should understand that the above embodiments are merely illustrative explanations of the implementation of the present invention and are not intended to limit the scope of the present invention. The details in the embodiments do not constitute a limitation on the scope of the present invention. Any obvious changes, such as equivalent transformations or simple substitutions, based on the technical solutions of the present invention without departing from the spirit and scope of the present invention fall within the protection scope of the present invention.

Claims

1. A water fade pigment, characterized in that, It is prepared from the following raw materials by weight percentage: Black-15 dye, chemically named 2-(2-4-dimethylamino)-3-methyl-6-diethylaminofluorane, has a molecular structure in a closed lactone ring leuco form, and its content ranges from 15% to 25%. Anhydrous calcium sulfate, 30%–40%; Oxalic acid, 1%–3%; Water-soluble adhesive, 8%–12%; Moisturizer, 5%–8%; Dispersant, 2%–4%; Preservatives, 0.5%–1.0%; pH adjuster, 0.3%–0.8%; The remainder is deionized water.

2. The water fade pigment according to claim 1, characterized in that, The anhydrous calcium sulfate is an analytical grade powder with a particle size of 200-300 mesh; the oxalic acid is a food-grade crystalline powder with a particle size of 100-200 mesh.

3. The water-caused fading pigment according to claim 1, characterized in that, The water-soluble binder is selected from one or both of gum arabic and hydroxypropyl methylcellulose.

4. The water-induced fading pigment according to claim 1, characterized in that, The moisturizer is selected from one or both of glycerin and propylene glycol.

5. The water-induced fading pigment according to claim 1, characterized in that, The dispersant is selected from one or both of sodium dodecylbenzenesulfonate and polysorbate 80.

6. The water-induced fading pigment according to claim 1, characterized in that, The preservative is selected from one or two of potassium sorbate and sodium dehydroacetate.

7. The water-induced fading pigment according to claim 1, characterized in that, The pH adjuster is selected from one or two of citric acid and sodium bicarbonate, and the pH value of the pigment is adjusted to 6.5-7.

5.

8. A method for preparing a water-induced fading pigment as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Raw material pretreatment: Prepare Black-15 dye solution, grind and sieve anhydrous calcium sulfate, prepare binder solution, and prepare oxalic acid solution; S2. Slurry preparation: Anhydrous calcium sulfate, dispersant and dye solution are mixed in a high-speed disperser, and then binder solution, humectant, preservative and pH adjuster are added in sequence. Finally, the oxalic acid solution is slowly added at a rate of 1 mL / min to prepare a uniform paste slurry with a viscosity of 5000-6000 mPa·s. S3. Molding: After pouring the slurry into the mold, vacuum degassing is performed for 15 minutes at a vacuum degree of -0.09MPa, followed by pressing at a pressure of 5MPa for 30 seconds to form a pigment block blank; S4. Drying: Dry the green body at a temperature not exceeding 50°C for 4 to 5 hours.

9. The method for preparing the water-induced fading pigment according to claim 8, characterized in that, The oxalic acid solution prepared in step S2 needs to be cooled to room temperature before adding the slurry; the preparation process of the Black-15 dye solution is as follows: Black-15 dye and deionized water are mixed at a mass ratio of 1:1.5 and stirred in a constant temperature water bath at 35°C to 40°C.

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