Double-coated heat-sensitive microcapsule, preparation method, application and plastic heat-sensitive paint

Abstract

The present application relates to the technical field of heat-sensitive technology, in particular to a double-layer coated heat-sensitive microcapsule, a preparation method, application and a plastic heat-sensitive coating. The double-layer coated microcapsule technology is adopted to encapsulate latent black dye in a high-temperature-resistant inner wall material and load a color developing agent in an outer wall material, so that the heat-sensitive components are physically isolated during high-temperature processing and early color development is completely avoided. The introduced thermoplastic resin matrix is homogeneous with PVC, so that the coating has the ability of synchronous thermoplastic deformation with the substrate and cracking is eliminated. The thermoplastic resin matrix also provides the plasticity of the coating and the adhesion with the substrate. The layer can be thermoplastically formed together with the PVC sleeve and can be used to produce an intelligent heat-sensitive sleeve product with high-definition on-site marking.

Description

Technical Field

[0001] This invention relates to the field of thermosensitive technology, and more specifically, to double-layer coated thermosensitive microcapsules, their preparation methods, applications, and plastic thermosensitive coatings. Background Technology

[0002] PVC heat shrink tubing is widely used for marking and protecting electronic wire harnesses due to its excellent insulation, flame retardancy, and cost advantages. In field operations, it is often necessary to print circuit numbers, specifications, and other information on the tubing. Traditional solutions often use pre-printing or inkjet coding, which suffers from problems such as unalterable information, poor abrasion resistance, and low construction efficiency. Thermal printing technology offers advantages such as immediate use, no ink required, and portable equipment, making it an ideal choice for on-site marking.

[0003] However, existing heat-sensitive coating technology has the following core drawbacks when applied to PVC heat shrink tubing: (1) High-temperature processing leads to functional failure (pre-development): The heat shrinking temperature of PVC is much higher than the color development temperature of the heat-sensitive coating. In existing coatings, even if the dye and color developer are separated by microcapsules, the wall material is not heat-resistant enough and will break during the heat shrinking process, causing the coating to turn black before printing and completely lose its information recording function.

[0004] (2) Stretching causes coating cracking and peeling: The shrinkage rate of PVC heat shrink tubing often exceeds 100%. During heat shrinking, the coating cannot stretch synchronously with the substrate, resulting in peeling and damage to the marking information.

[0005] (3) Color uniformity and color density difference: If the existing microcapsule coating is forcibly applied to heat shrink tubing, high-temperature shrinkage will destroy the spatial distribution of microcapsules, resulting in some areas of capsule accumulation and some areas of sparseness. During printing, the sparse capsule areas are lightly colored (OD<0.8), the accumulated areas are too dark, and the edges are easily blurred, making it impossible to form a clear mark.

[0006] (4) Lack of plasticity: Most existing coatings are thermosetting or brittle after curing and do not have plasticity. They cannot soften and deform when heated and set after cooling like PVC, which limits their application in irregular parts or in situations requiring secondary shaping.

[0007] In summary, existing technologies cannot simultaneously meet the requirement of not premature color development at temperatures above 150℃, nor can they achieve the effect of withstanding greater than 100% tensile stress without cracking. Currently, there is no dedicated heat-sensitive coating solution designed specifically for the characteristics of PVC heat shrink tubing processes. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a double-layer coated thermosensitive microcapsule, a preparation method, an application, and a plastic thermosensitive coating.

[0009] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: This invention provides a double-layered thermosensitive microcapsule, the microcapsule having a double-shell-core structure; the core component of the microcapsule includes a dye and a low-melting-point sensitizer, the low-melting-point sensitizer having a melting point of 80℃-100℃; the inner shell material of the microcapsule is a high-melting-point thermoplastic resin, and the outer shell material of the microcapsule includes a color developer and a low-softening-point resin; The glass transition temperature or melting point of the high-melting-point thermoplastic resin is greater than the thermoplastic processing temperature of PVC; the softening point temperature of the low-softening-point resin is lower than the glass transition temperature or melting point of the high-melting-point thermoplastic resin, and higher than room temperature.

[0010] Based on the above technical solution, the present invention can be further improved as follows.

[0011] Furthermore, the high-melting-point thermoplastic resin has a glass transition temperature or melting point greater than 180°C, the low-softening-point resin has a softening-point temperature of 80-120°C, and the temperature difference between the two is greater than 70°C.

[0012] Furthermore, the high-melting-point thermoplastic resin is melamine-formaldehyde resin, polyurea resin, or polyamide resin; the low-softening-point resin is ethyl cellulose, polyacrylate, or rosin-modified phenolic resin; and the color developer is bisphenol S or 4-hydroxy-4'-isopropoxydiphenyl sulfone.

[0013] Furthermore, the thickness of the inner shell is 0.2-0.5 μm, and the thickness of the outer shell material is 0.3-0.5 μm.

[0014] Furthermore, the average particle size D of the thermosensitive microcapsules 50 The thickness is 1.5-3.0 μm, and D 90 / D 50 The value is less than 2.0.

[0015] The present invention also provides a method for preparing the above-described double-layer coated thermosensitive microcapsule, wherein the inner shell material is coated on the outside of the core by in-situ polymerization, and the outer shell material is coated by solvent evaporation-induced phase separation.

[0016] Furthermore, the following steps are included: S1. Core Preparation: The dye and the low-melting-point sensitizer are mixed and heated to a melt mixture to obtain an oil phase; after cooling, the oil phase is added to the aqueous phase and emulsified to obtain an average particle size D. 50 It is an O / W emulsion with a thickness of 0.5-1.5 μm; S2. Inner shell coating: The high-melting-point thermoplastic resin is slowly added dropwise to the emulsion to obtain a reaction system. The pH of the reaction system is adjusted to acidic, and then heated, kept at a constant temperature, and reacted. After the reaction is completed, the pH of the reaction system is adjusted to neutral, and after cooling, a microcapsule slurry with an inner shell coating is obtained. The microcapsule slurry with the inner shell coating is centrifuged, washed, and dispersed in water to obtain a microcapsule suspension. S3. Outer shell coating: The color developer, organic solvent and low softening point resin are mixed to obtain an outer shell film-forming solution; under stirring, the outer shell film-forming solution is slowly added dropwise to the microcapsule suspension to obtain a double-layer coated thermosensitive microcapsule slurry; the organic solvent in the double-layer coated thermosensitive microcapsule slurry is removed to obtain a suspension, and the suspension is dried to obtain the double-layer coated thermosensitive microcapsules.

[0017] Furthermore, in step S1, the aqueous phase contains an emulsifier with a mass percentage of 4.5%-5.5%.

[0018] Furthermore, in step S2, the amount of the high-melting-point thermoplastic resin added is 25%-35% of the mass of the oil phase.

[0019] Furthermore, in step S2, the heating temperature is 65°C and the reaction time is 3 hours; before the reaction, the pH value of the reaction system is 4.5, and after the reaction, the pH value of the reaction system is 7.0.

[0020] The present invention also provides the application of the above-described double-layered thermosensitive microcapsules in the preparation of plastic thermosensitive coatings.

[0021] The present invention also provides a plastic heat-sensitive coating, comprising the double-layer encapsulated heat-sensitive microcapsules as described above, and further comprising a thermoplastic resin matrix, a plasticizer, and additives; wherein the glass transition temperature of the thermoplastic resin matrix is ​​70-90°C, and the elongation at break is greater than or equal to 200%.

[0022] Furthermore, the thermoplastic resin matrix is ​​polyvinyl chloride paste resin, chlorinated polyvinyl chloride, or thermoplastic polyurethane.

[0023] Furthermore, the mass fractions of each component are as follows: 100 parts of the thermoplastic resin matrix, 20-30 parts of the double-layer coated thermosensitive microcapsules, 65-75 parts of the plasticizer, and 18-25 parts of the additives.

[0024] The present invention also provides a thermosensitive PVC heat shrink sleeve having a thermosensitive coating, the thermosensitive coating being prepared using the plastic thermosensitive coating described above.

[0025] The beneficial effects of this invention are as follows: (1) The double-layer coated thermosensitive microcapsule of the present invention adopts a double-layer microcapsule structure of "inner encapsulation of dye and outer loading of color developer". It uses a high heat-resistant resin with a temperature greater than 180℃ as the inner barrier to ensure zero dye leakage during the heat shrinking process at 160-180℃, fundamentally solving the problem of "blackening when heated" that has plagued the industry for many years. After heat shrinking, the coating remains completely transparent (transmittance > 85%), laying the foundation for subsequent high-definition printing. (2) The double-layer coated thermal microcapsule of the present invention achieves “point-to-point delivery” of the color developer by enriching the color developer in the outer wall material of the microcapsule. Combined with the precise control of the microcapsule particle size, it ensures that the microcapsules are evenly distributed in the coating after stretching. During printing, each capsule can react with the surrounding high-concentration color developer at the moment of rupture, achieving a high color density with an OD value of >1.4 and an ultra-high uniformity with a range of <0.06. The printing effect is comparable to that of traditional thermal paper, and completely solves the technical pain point of mottled color development after stretching. (3) The method for preparing double-layer coated thermosensitive microcapsules of the present invention uses two coating methods in synergy and the process conditions are mild. This ensures the thermodynamic property gradient design of each of the double shell materials and ensures the concentrated particle size distribution and complete shell structure of the microcapsules. Finally, double-layer coated thermosensitive microcapsules with high heat resistance and processing stability and excellent printing color development performance are prepared. (4) The plastic heat-sensitive coating of the present invention uses a resin matrix that is completely compatible with PVC and has high ductility (elongation at break ≥250%), so that the coating extends synchronously with the substrate at 100% heat shrinkage rate, the molecular chains move in concert, stress is dispersed, microcracks and peeling are completely eliminated, and the adhesion remains at the highest level of 5B after heat shrinkage. (5) The plastic heat-sensitive coating of the present invention retains the thermoplasticity of the PVC matrix, so that the printed sleeve can still be heated and shaped again, adapting to various complex wiring environments and greatly expanding the application flexibility. (6) The plastic heat-sensitive coating of the present invention has a coating and plasticizing process that is highly compatible with existing PVC pipe production lines, requiring no large-scale equipment modification, and has extremely high industrialization value and market application prospects. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the process for preparing the double-layer coated thermosensitive microcapsules of the present invention. Detailed Implementation

[0027] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0028] The present invention relates to a double-layered thermosensitive microcapsule with a double-shell-core structure. The core component of the microcapsule includes a dye and a low-melting-point sensitizer, the melting point of which is 80°C-100°C. The inner shell material of the microcapsule is a high-melting-point thermoplastic resin, the glass transition temperature or melting point of which is greater than the thermoplastic processing temperature of PVC. The outer shell material of the microcapsule includes a color developer and a low-softening-point resin, the softening point of which is lower than the glass transition temperature or melting point of the high-melting-point thermoplastic resin and higher than room temperature.

[0029] The double-layer encapsulated thermosensitive microcapsule of the present invention uses double-layer encapsulation technology to encapsulate the dye in a high-temperature resistant inner wall material and load the color developer in an outer wall material, thereby achieving physical isolation of the thermosensitive components during high-temperature processing and completely avoiding premature color development.

[0030] The core contains a latent black dye that works in conjunction with a low-melting-point sensitizer to ensure rapid melting response and efficient color development during printing. The inner shell uses a high-melting-point thermoplastic resin with a glass transition temperature or melting point higher than that of PVC thermoplastic processing. This allows the microcapsules to maintain structural integrity during heat shrinking at 150-180℃, effectively preventing dye leakage and fundamentally solving the technical problem of overall blackening and failure of the coating caused by high-temperature pre-development. The outer shell contains a color developer and a low-softening-point resin, with the softening point of the low-softening-point resin falling between the melting point of the inner shell and room temperature. This design allows the outer shell to soften and melt preferentially during printing, creating conditions for the inner shell to rupture and release the dye under the pressure of the print head. It also enriches the color developer on the surface of the microcapsules, ensuring that the dye can fully contact and react with the high-concentration color developer immediately upon release, thereby achieving high color density. At the same time, this double-layer structure, combined with particle size control, ensures that the microcapsules remain uniformly distributed even after the coating undergoes significant stretching, ultimately guaranteeing uniform color development without spots, high color density, and good stability after heat shrinking.

[0031] Preferably, the glass transition temperature or melting point of the high-melting-point thermoplastic resin is greater than 180°C, ensuring that the internal latent dye is completely sealed and leak-free during the thermoplastic process at 150-180°C.

[0032] Preferably, the low softening point resin has a softening point temperature of 80-120℃, allowing it to soften rapidly during subsequent thermal printing heating. This causes the inner wall material to rupture under pressure, releasing the dye to come into contact with the developer for color development. The outer wall material simultaneously functions as a "developer reservoir" and a "buffer layer," protecting the inner layer from mechanical damage during processing.

[0033] Preferably, the temperature difference between the glass transition temperature or melting point of the high-melting-point thermoplastic resin and the softening point of the low-softening-point resin is greater than 70°C. This temperature gradient gives the two resins a significant difference in thermodynamic properties, thereby further ensuring the performance of the double-layer-coated thermosensitive microcapsules.

[0034] Preferably, the high-melting-point thermoplastic resin is melamine-formaldehyde resin, polyurea resin or polyamide resin, the low-softening-point resin is ethyl cellulose, polyacrylate or rosin-modified phenolic resin, and the color developer is bisphenol S or 4-hydroxy-4'-isopropoxydiphenyl sulfone.

[0035] Preferably, the outer shell material of the microcapsules contains 20-30 parts by weight of high-melting-point thermoplastic resin and 8-10 parts by weight of color developer.

[0036] Preferably, in the core components of the microcapsules, the dye comprises 10-15 parts by mass and the low-melting-point sensitizer comprises 20-25 parts by mass.

[0037] Preferably, the dye is a fluorane dye, specifically 3-dibutylamino-6-methyl-7-aniline fluorane, 2-(2-4-dimethylamino)-3-methyl-6-diethylaminofluorane, or 2-phenylamino-3-methyl-6-diethylfluorane, or other fluorane dyes. The low-melting-point sensitizer can be 4-benzylbiphenyl. Besides fluoranes, spironolactone dyes combined with acidic color developers can also be used. For special applications, color development systems for other colors such as blue and red can be developed.

[0038] Preferably, the thickness of the inner shell is 0.2-0.5 μm.

[0039] Preferably, the thickness of the outer shell material is 0.3-0.5 μm.

[0040] Preferably, the average particle size D of the thermosensitive microcapsules 50 The thickness is 1.5-3.0 μm, and D 90 / D 50 The value is less than 2.0; based on the inner shell material and the outer shell material, by further precisely controlling the particle size distribution of the microcapsules, it can be ensured that the microcapsules are evenly distributed and rupture synchronously after undergoing severe stretching, thereby achieving the technical effect of uniform color development and high color density (OD≥1.3) after heat shrinking.

[0041] The preparation method of the double-layer coated thermosensitive microcapsule of the present invention involves using an in-situ polymerization method to encapsulate the inner shell material outside the core, and then using a solvent evaporation-induced phase separation method to encapsulate the outer shell material.

[0042] An in-situ polymerization method is used to encapsulate an inner shell material around the core. This allows for precise control of the polymerization process of the inner shell material, enabling the uniform and dense deposition of high-melting-point thermoplastic resin on the surface of oil droplets containing latent dyes and sensitizers. This forms a barrier layer with controllable thickness and excellent heat resistance, ensuring the structural integrity of the microcapsules during subsequent high-temperature processing. A solvent evaporation-induced phase separation method is then used to encapsulate the outer shell material. Utilizing the phase separation characteristics of the polymer-developer mixture during solvent evaporation, the low-softening-point resin containing the developer is uniformly coated on the surface of the inner microcapsules. This method features mild process conditions that do not damage the inner shell structure and achieve uniform dispersion and stable loading of the developer in the outer shell material. The synergistic effect of these two encapsulation methods ensures both the thermodynamic property gradient design of the two shell materials and the concentrated particle size distribution and intact shell structure of the microcapsules. Ultimately, this results in a double-layered thermosensitive microcapsule with both high heat resistance and processing stability, and excellent printing and color development performance.

[0043] Specifically, such as Figure 1 As shown, the above preparation method includes the following steps: S1. Core Preparation: A latent black dye is mixed with a low-melting-point sensitizer and heated until molten to obtain an oil phase. After cooling, the oil phase is added to an aqueous phase and emulsified to obtain an average particle size D. 50 It is an O / W emulsion with a thickness of 0.5-1.5 μm.

[0044] Preferably, the aqueous phase contains 4.5%-5.5% by mass of emulsifier.

[0045] Preferably, the emulsifier is gum arabic or an aqueous solution of styrene-malacid anhydride.

[0046] Preferably, the oil phase is added to the aqueous phase and emulsified for 10 minutes at 6000 rpm using a high-speed shear machine.

[0047] S2. Inner shell coating: A high-melting-point thermoplastic resin is slowly added dropwise to the emulsion to obtain a reaction system. The pH of the reaction system is adjusted to acidic, and then heated, kept at a constant temperature, and reacted. After the reaction is completed, the pH of the reaction system is adjusted to neutral, and after cooling, a microcapsule slurry with an inner shell coating is obtained. The microcapsule slurry with an inner shell coating is centrifuged, washed, and dispersed in water to obtain a microcapsule suspension.

[0048] Preferably, the amount of high-melting-point thermoplastic resin added is 25%-35% of the mass of the oil phase.

[0049] Preferably, the heating temperature is 65°C and the reaction time is 3 hours; before the reaction, the pH value of the reaction system is 4.5, and after the reaction, the pH value of the reaction system is 7.0.

[0050] Preferably, the pH of the reaction system is adjusted with citric acid solution before the reaction and with sodium hydroxide solution after the reaction; the specific concentrations of citric acid solution and sodium hydroxide solution can be determined according to the actual situation.

[0051] S3. Outer shell coating: A color developer, organic solvent, and low softening point resin are mixed to obtain an outer shell film-forming solution; under stirring, the outer shell film-forming solution is slowly added dropwise to the microcapsule suspension to obtain a double-layer coated thermosensitive microcapsule slurry; the organic solvent in the double-layer coated thermosensitive microcapsule slurry is removed to obtain a suspension, and the suspension is dried to obtain double-layer coated thermosensitive microcapsules.

[0052] Preferably, the organic solvent is ethyl acetate.

[0053] Preferably, the double-layer-coated thermosensitive microcapsule slurry is heated to 40°C to completely evaporate the organic solvent, thereby removing the organic solvent.

[0054] Preferably, the drying method is spray drying, and the resulting double-layer coated thermosensitive microcapsules are powdered microcapsules. 50 It is 2.2 μm.

[0055] The application of the above-mentioned double-layer coated thermosensitive microcapsules of the present invention in the preparation of plastic thermosensitive coatings.

[0056] The plastic thermosensitive coating of the present invention includes the above-mentioned double-layer coated thermosensitive microcapsules, and also includes a thermoplastic resin matrix, a plasticizer and an additive; the glass transition temperature of the thermoplastic resin matrix is ​​70-90°C and the elongation at break is greater than or equal to 200%.

[0057] The plastic heat-sensitive coating of the present invention incorporates a thermoplastic resin matrix that is homogeneous with PVC, with a glass transition temperature (Tg) of 70-90°C. This makes it rigid at room temperature and highly elastic when heated to the thermoplastic temperature of PVC (>150°C), allowing it to be stretched synchronously with the PVC sleeve without cracking. This gives the coating the ability to thermoplasticly deform synchronously with the substrate, fundamentally preventing cracking. At the same time, the thermoplastic resin matrix also provides the coating with plasticity and adhesion to the substrate.

[0058] The plastic heat-sensitive coating of the present invention selects a thermoplastic resin that is compatible with PVC and has high ductility as the coating matrix, thereby fundamentally solving the problem of tensile cracking.

[0059] Preferably, the thermoplastic resin matrix is ​​polyvinyl chloride (PVC) paste resin (blended resin type), chlorinated polyvinyl chloride (CPVC), or thermoplastic polyurethane (TPU) that is completely compatible with PVC.

[0060] Preferably, the mass fractions of each component are: 100 parts of thermoplastic resin matrix, 20-30 parts of double-layer coated thermosensitive microcapsules, 65-75 parts of plasticizer, and 18-25 parts of additives.

[0061] Preferably, the viscosity of the plastic heat-sensitive coating is 1900-2100 mPa·s.

[0062] Preferably, the plasticizer is a phthalate plasticizer or a trimellitate plasticizer.

[0063] Preferably, the additives include heat stabilizers, ester additives, and viscosity reducers; more preferably, the heat stabilizer is a calcium-zinc composite heat stabilizer, which can prevent the coating from degrading during the processing of heat-sensitive PVC heat shrink tubing; the ester additive is epoxidized soybean oil, which has the functions of auxiliary stabilization and auxiliary plasticization; and the viscosity reducer is D80 solvent oil.

[0064] The preparation process of the plastic thermosensitive coating of the present invention is as follows: the components are mixed evenly using a low-speed mixer with a rotation speed of <500 rpm, and the plastic thermosensitive coating is obtained after vacuum degassing.

[0065] The heat-sensitive PVC heat-shrinkable sleeve of the present invention has a heat-sensitive coating, which is prepared by using the plastic heat-sensitive coating as described above.

[0066] Specifically, in one embodiment of the present invention, the coating and heating plasticizing process of the thermosensitive coating is as follows: Coating: A continuous dip coating method is used, in which the unexpanded transparent PVC sleeve is passed through a tank containing coating material at a coating speed of 4.5-5.5 m / min.

[0067] Pre-gelling: Place the coated sleeve in a drying equipment at 75℃-85℃ and heat for several minutes to make the coating lose its fluidity.

[0068] Heating and plasticizing: The temperature is increased to fully plasticize the PVC paste resin, and the coating forms a continuous film. Cooling and winding yields a heat-sensitive PVC semi-finished sleeve with a dry film thickness of approximately 8 μm.

[0069] Preferably, the heating and plasticizing temperature is 110-140℃.

[0070] After the above process, the PVC sleeve has a heat-sensitive coating. After heat shrinkage processing at 150-180℃, the heat-sensitive coating has no cracks, and the reflected light density OD value of the color display area after heat printing is ≥1.3, and the OD value uniformity is ≤0.1.

[0071] Heating and plasticizing at low temperatures (<140℃) forms the heat-sensitive coating, while expanding the sleeve at high temperatures (>150℃) achieves thermal shrinkage. The temperature difference ensures that the microcapsules are not damaged during processing.

[0072] The effects of the present invention will be illustrated below through specific embodiments and comparative examples.

[0073] Example In this embodiment, the method of the present invention is used to prepare double-layer coated thermosensitive microcapsules and coatings, wherein the resin matrix used in the coating is a PVC paste resin matrix.

[0074] The preparation process of the double-layer coated thermosensitive microcapsules in this embodiment is as follows: (1) Preparation of a core containing a latent dye: Ten parts of 3-dibutylamino-6-methyl-7-aniline fluorane (ODB-2, black dye) and 20 parts of 4-benzylbiphenyl (sensitizer, melting point 85℃) were melted and mixed at 120℃ to form the oil phase. The oil phase was then added to an aqueous phase containing 5% gum arabic as an emulsifier, and emulsified at 6000 rpm for 10 minutes using a high-speed shear press to obtain an average particle size D. 50 It is an O / W emulsion with a thickness of approximately 1 μm.

[0075] (2) Inner shell covering: Melamine-formaldehyde prepolymer (50% solids content) was slowly added dropwise to the above emulsion, at a rate of 30% of the oil phase mass. The pH was adjusted to 4.5 with citric acid, and the temperature was raised to 65°C and maintained for 3 hours. Melamine-formaldehyde resin was deposited and cured on the surface of the oil droplets, forming the inner wall material. After the reaction was completed, the pH was adjusted to 7.0 with NaOH, and the mixture was cooled to obtain the microcapsule slurry with inner layer coating.

[0076] After centrifuging and washing the inner layer microcapsule slurry, it was redispersed in an appropriate amount of water to obtain a microcapsule suspension.

[0077] (3) Outer shell covering: Eight parts of a color developer (bisphenol S) were dissolved in 20 parts of ethyl acetate, and then mixed with 30 parts of ethyl cellulose (10% ethanol solution) and stirred until homogeneous to form the outer wall material film-forming solution. While stirring, the outer wall material film-forming solution was slowly added dropwise to the microcapsule suspension. Solvent evaporation-induced phase separation was used to deposit the ethyl cellulose-color developer complex on the surface of the inner microcapsules, forming the outer shell.

[0078] The mixture was heated to 40°C and stirred for 2 hours to completely evaporate and remove the organic solvent, resulting in a suspension of double-layered thermosensitive microcapsules. After spray drying, powdered microcapsules were obtained. The D of these powdered microcapsules... 50 It is 2.2 μm.

[0079] The preparation process of the plastic heat-sensitive coating in this embodiment is as follows: Mix the components according to the types and proportions in Table 1, use a low-speed mixer (speed <500 rpm) to mix evenly, and degas under vacuum to obtain a plastic heat-sensitive coating.

[0080] Table 1. Composition and proportions of plastic heat-sensitive coatings The plastic heat-sensitive coating obtained in this embodiment is applied to the PVC sleeve and then plasticized and shaped. The specific steps are as follows: (1) Coating: The continuous dip coating method is adopted. The unexpanded transparent PVC sleeve (outer diameter 3mm) is passed through the tank containing the coating material at a coating speed of 5m / min.

[0081] (2) Pre-gelling: The coated sleeve is placed in an 80°C drying tunnel and heated for 2 minutes to make the coating lose its fluidity.

[0082] (3) Plasticization: The temperature is raised to 120℃ to fully plasticize the PVC paste resin, and the coating forms a continuous film. Cool and roll up to obtain the heat-sensitive PVC semi-finished sleeve, with a dry film thickness of about 8μm.

[0083] Simulated thermoplastic processing was used to heat-shrink the aforementioned sleeve. Specifically, the semi-finished sleeve was placed on a copper rod with a diameter of 1.5 mm and heated in a 120°C hot air oven for 1 minute. The sleeve rapidly shrank and tightly wrapped around the copper rod (shrinkage rate 100%). It was then removed and cooled.

[0084] Performance tests were conducted on the thermoplastic-processed sleeve. (1) Visual inspection: The surface of the heat-shrinkable sleeve coating is smooth, and no microcracks are observed under a high-magnification microscope (200x), indicating that it has the advantage of not cracking after heat shrinking. The adhesion (100-cross test) reaches 5B (no peeling).

[0085] (2) Pre-color development test: The heat-shrinked sleeve was observed at room temperature. The coating was transparent and colorless (colorless state) with a transmittance (550nm) of 86%, indicating that the microcapsules did not rupture during the heat shrinking process at 160℃, there was no dye leakage, and no premature color development occurred.

[0086] (3) Thermal color development performance test: A portable thermal printer (printhead energy 0.3mJ / dot, heating pulse width 1.0ms) was used to print continuous lines and solid blocks on the surface of the heat-shrinked tubing.

[0087] Color density (OD value): The optical density OD value of the printed area, measured using a reflectance densitometer, reached 1.45 for the black solid block.

[0088] Color uniformity: OD values ​​were measured at 10 random points within a 10cm long printing area. The maximum value was 1.48, the minimum value was 1.42, and the range was <0.06, proving that the color uniformity was extremely high.

[0089] Line edges: Printed lines have clear and sharp edges, uniform width, and no smudging or satellite dots.

[0090] Comparative Example 1 This comparative example uses single-layer coated microcapsules, meaning the microcapsules do not have a heat-resistant inner shell. Specifically, this comparative example uses conventional single-layer microcapsules with an ODB-2 core and an outer shell made solely of ethyl cellulose containing the color developer bisphenol S. These single-layer microcapsules are dispersed in the same PVC paste resin matrix as in Example 1. The types and amounts of other components, the preparation of the coating, the application, and the heat-shrink tubing processing are completely identical.

[0091] Testing revealed that after heat shrinking at 160℃, the entire sleeve turned completely black (OD value > 1.2), making further printing impossible. This is because the ethyl cellulose wall material lacks sufficient heat resistance, softening and cracking during the heat shrinking process, causing the dye and developer to react prematurely.

[0092] Comparative Example 2 This comparative example uses a non-PVC brittle matrix resin instead of the PVC paste resin matrix in Example 1, while the other components, amounts, and preparation parameters are exactly the same as in Example 1.

[0093] Testing revealed that the acrylic coating developed numerous cracks during heat shrinking at 160℃, with some areas peeling off in a fish-scale pattern. During printing, the cracked areas failed to develop color, resulting in incomplete markings.

[0094] Comparative Example 3 This comparative example uses D. 50 The microcapsules are large-particle-size microcapsules of 8 μm, and the other components, dosages and preparation parameters are exactly the same as in Example 1.

[0095] Testing revealed a rough coating appearance. After heat shrinking, printing tests showed uneven color development, with some areas having an OD value of only 0.7 and others as high as 1.5. Microscopic observation revealed that the large-diameter capsules were stacked during shrinkage, resulting in uneven distribution and uneven heating, leading to color spots.

[0096] Comparative Example 4 In this comparative example, the color developer bisphenol S was directly mixed with PVC paste resin. Therefore, the microcapsules used in this comparative example have a single-layer structure, and the microcapsules only encapsulate the dye. The shell material is the same as the inner shell material in Example 1, but lacks the outer shell material used in Example 1. Other components, amounts, and preparation parameters are completely identical to those in Example 1.

[0097] Testing revealed slight premature color development in the heat-shrinked coating. During printing tests, the OD value was only 0.9, and the color faded slightly over time after development. Analysis suggests that the dye released from the ruptured capsules during printing could not quickly contact a sufficient amount of developer, resulting in incomplete color development.

[0098] The test results of the above examples and comparative examples show that Example 1, through the precise design of the double-layer microcapsule (high heat-resistant inner layer + outer layer loaded with color developer) and the selection of a matrix homogeneous with PVC, successfully solved the problems of pre-color development and cracking in heat shrink processing, and achieved excellent color development performance with high uniformity (OD difference <0.06) and high color density (OD>1.4) after heat shrinking, which fully meets the requirements of high-precision marking of electronic wire harnesses.

[0099] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A double-layer coated thermosensitive microcapsule, characterized in that, The microcapsule has a double-shell-core structure; the core component of the microcapsule includes a dye and a low-melting-point sensitizer, the melting point of which is 80℃-100℃; the inner shell material of the microcapsule is a high-melting-point thermoplastic resin, and the outer shell material of the microcapsule includes a color developer and a low-softening-point resin. The glass transition temperature or melting point of the high-melting-point thermoplastic resin is greater than the thermoplastic processing temperature of PVC; the softening point temperature of the low-softening-point resin is lower than the glass transition temperature or melting point of the high-melting-point thermoplastic resin, and higher than room temperature.

2. The double-layer coated thermosensitive microcapsule according to claim 1, characterized in that, The high-melting-point thermoplastic resin has a glass transition temperature or melting point greater than 180°C, the low-softening-point resin has a softening-point temperature of 80-120°C, and the temperature difference between the two is greater than 70°C.

3. The double-layer coated thermosensitive microcapsule according to claim 2, characterized in that, The high-melting-point thermoplastic resin is melamine-formaldehyde resin, polyurea resin, or polyamide resin; the low-softening-point resin is ethyl cellulose, polyacrylate, or rosin-modified phenolic resin; and the color developer is bisphenol S or 4-hydroxy-4'-isopropoxydiphenyl sulfone.

4. The double-layer coated thermosensitive microcapsule according to claim 1, characterized in that, The thickness of the inner shell is 0.2-0.5 μm, and the thickness of the outer shell material is 0.3-0.5 μm.

5. A double-layer coated thermosensitive microcapsule according to any one of claims 1-4, characterized in that, The average particle size D of the thermosensitive microcapsules 50 The thickness is 1.5-3.0 μm, and D 90 / D 50 The value is less than 2.

0.

6. A method for preparing a double-layer coated thermosensitive microcapsule as described in any one of claims 1-5, characterized in that, The inner shell material is encapsulated outside the core using in-situ polymerization, and the outer shell material is then encapsulated using solvent evaporation-induced phase separation.

7. The method for preparing a double-layer coated thermosensitive microcapsule according to claim 6, characterized in that, Includes the following steps: S1. Core Preparation: The dye and the low-melting-point sensitizer are mixed and heated to a melt mixture to obtain an oil phase; after cooling, the oil phase is added to the aqueous phase and emulsified to obtain an average particle size D. 50 It is an O / W emulsion with a thickness of 0.5-1.5 μm; S2. Inner shell coating: The high-melting-point thermoplastic resin is slowly added dropwise to the emulsion to obtain a reaction system. The pH of the reaction system is adjusted to acidic, and then heated, kept at a constant temperature, and reacted. After the reaction is completed, the pH of the reaction system is adjusted to neutral, and after cooling, a microcapsule slurry with an inner shell coating is obtained. The microcapsule slurry with the inner shell coating is centrifuged, washed, and dispersed in water to obtain a microcapsule suspension. S3. Outer shell coating: The color developer, organic solvent and low softening point resin are mixed to obtain an outer shell film-forming solution; under stirring, the outer shell film-forming solution is slowly added dropwise to the microcapsule suspension to obtain a double-layer coated thermosensitive microcapsule slurry; the organic solvent in the double-layer coated thermosensitive microcapsule slurry is removed to obtain a suspension, and the suspension is dried to obtain the double-layer coated thermosensitive microcapsules.

8. The method for preparing a double-layer coated thermosensitive microcapsule according to claim 7, characterized in that, In step S1, the aqueous phase contains an emulsifier with a mass percentage of 4.5%-5.5%.

9. The method for preparing a double-layer coated thermosensitive microcapsule according to claim 7, characterized in that, In step S2, the amount of the high-melting-point thermoplastic resin added is 25%-35% of the mass of the oil phase.

10. The method for preparing a double-layer coated thermosensitive microcapsule according to claim 7, characterized in that, In step S2, the heating temperature is 65℃ and the reaction time is 3 hours; before the reaction, the pH value of the reaction system is 4.5, and after the reaction, the pH value of the reaction system is 7.

0.

11. The application of the double-layer encapsulated thermosensitive microcapsules as described in any one of claims 1-5 in the preparation of plastic thermosensitive coatings.

12. A plastic heat-sensitive coating, characterized in that, The product includes the double-layer coated thermosensitive microcapsules as described in any one of claims 1-5, and further includes a thermoplastic resin matrix, a plasticizer, and additives; the thermoplastic resin matrix has a glass transition temperature of 70-90°C and an elongation at break greater than or equal to 200%.

13. The plastic heat-sensitive coating according to claim 12, characterized in that, The thermoplastic resin matrix is ​​polyvinyl chloride paste resin, chlorinated polyvinyl chloride, or thermoplastic polyurethane.

14. The plastic heat-sensitive coating according to claim 12, characterized in that, The mass fractions of each component are as follows: 100 parts of the thermoplastic resin matrix, 20-30 parts of the double-layer coated thermosensitive microcapsules, 65-75 parts of the plasticizer, and 18-25 parts of the additives.

15. A heat-sensitive PVC heat-shrinkable sleeve, characterized in that, The heat-sensitive PVC heat-shrinkable sleeve has a heat-sensitive coating, which is prepared using a plastic heat-sensitive coating as described in any one of claims 12-14.

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