A composite coating material with fluorescent self-indication function for micro stress detection and a preparation method and application thereof

Abstract

The present application belongs to the technical field of intelligent coating, and relates to a composite coating material with fluorescent self-indication function for micro stress detection and a preparation method and application thereof. The present application realizes high sensitivity detection of micro cracks by adding fluorescently labeled microcapsules capable of breaking at the crack and releasing fluorescent dyes at the fixed point, and effectively avoids the adverse effects of microcapsule introduction on the transparency and flexibility of the coating through refractive index matching, particle size optimization and film thickness design, thereby being suitable for early warning of corrosion risk of various substrates, providing sufficient window period for equipment maintenance and coating repair, and significantly improving the safety and reliability of the protective coating.

Description

Technical Field

[0001] This invention belongs to the field of intelligent coating technology, and relates to a composite coating material with fluorescence self-indication function for micro-stress detection, its preparation method and application. Background Technology

[0002] In numerous applications such as precision instrument packaging, protection of high-end electronic equipment, and cultural relic preservation, transparent or semi-transparent polymer coatings (such as thermoplastic polyurethane (TPU), epoxy resin, and acrylic resin) are widely used for protecting substrates against oxygen, moisture, and corrosion. However, during use, these coatings are prone to developing microcracks that are difficult to detect with the naked eye due to aging, scratches, and stress fatigue. If these microcracks are not detected in time, they will gradually expand, eventually causing the coating's protective function to fail, exposing the substrate to risks such as corrosion and oxidation.

[0003] In the existing technology, although there have been attempts to use fluorescent microcapsules for material damage tracing, the technical path is mostly aimed at opaque substrates (such as cement and fabrics). If this technology is directly applied to transparent polymer coatings, it will face new challenges: (1) Transparent coatings have strict requirements for optical performance. The introduction of microcapsules must maintain extremely high visible light transmittance and avoid background fluorescence interference; (2) The interfacial compatibility between the coating and the microcapsules directly affects its flexibility and durability, which is fundamentally different from the mechanical anchoring effect pursued in opaque hard materials such as cement-based materials in terms of evaluation criteria; (3) For protective coatings, once microcracks are generated, the risk of substrate corrosion or oxidation will follow. More emphasis is placed on high sensitivity early warning of early micron-level cracks.

[0004] Therefore, developing a microcapsule technology that can be widely applied to transparent coating systems and takes into account both the mechanical / optical properties of the substrate and the function of high-sensitivity crack detection has a clear practical need and significance. Summary of the Invention

[0005] The purpose of this invention is to address the problems in existing technologies regarding the detection of microcracks in transparent polymer coatings, such as stringent optical performance requirements, mismatched interface compatibility evaluation standards, and insufficient sensitivity for detecting early-stage micron-level cracks. The invention proposes a composite coating material with fluorescent self-indicating function for micro-stress detection, along with its preparation method and applications.

[0006] One objective of this invention is achieved through the following technical solution: A composite coating material with fluorescent self-indicating function for micro-stress detection, comprising the following raw materials: Matrix resin; and fluorescently labeled microcapsules dispersed in the matrix resin; The fluorescently labeled microcapsule comprises a polymer capsule wall and an oil-soluble fluorescent dye encapsulated inside the polymer capsule wall; The polymer capsule wall is made of at least one of polyurea-formaldehyde, polyurea, or polyurethane. The average particle size of the fluorescently labeled microcapsules is 10~200μm.

[0007] Preferably, the difference between the refractive index of the polymer wall of the fluorescently labeled microcapsule and the refractive index of the matrix resin after curing is <0.03.

[0008] Preferably, the dry film thickness of the composite coating material is greater than the average particle size of the fluorescently labeled microcapsules. Preferably, the amount of fluorescently labeled microcapsules added is 1-20 wt% of the mass of the matrix resin.

[0009] More preferably, the amount of the fluorescently labeled microcapsules added is 1 to 5 wt% of the mass of the matrix resin.

[0010] Preferably, the matrix resin has a transmittance of >90% in the visible light band, and the matrix resin is selected from at least one of polyurethane resin, epoxy resin or acrylic resin.

[0011] Preferably, the dry film of the composite coating material has a transmittance of ≥80% in the visible light band.

[0012] Further preferably, the dry film of the composite coating material has a transmittance of ≥85% in the visible light band.

[0013] Preferably, the decrease in the transmittance of the dry film of the composite coating material in the visible light band is 0-10% compared to the transmittance of the matrix resin in the visible light band.

[0014] Preferably, the fluorescently labeled microcapsules have an average particle size of 50~150μm.

[0015] Preferably, the preparation method of the fluorescently labeled microcapsules includes: dispersing an oil-soluble fluorescent dye in an oil-phase monomer as an oil phase, adding an aqueous phase containing an emulsifier, forming an emulsion under high-speed shearing, adding an aqueous phase monomer, and carrying out an interfacial polymerization reaction at 50~70℃ for 1~6h.

[0016] Further preferably, the oil-soluble fluorescent dye is selected from at least one of Rhodamine 6G and oil-soluble fluorescein.

[0017] Further preferably, the oil phase monomer is a polyisocyanate; the polyisocyanate is selected from at least one of isophorone diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate or hexamethylene diisocyanate.

[0018] Further preferably, the aqueous monomer is an amine and / or a polyol.

[0019] More preferably, the amine is selected from at least one of ethylenediamine, hexamethylenediamine, diethylenetriamine, or polyethyleneimine; The polyol is selected from at least one of ethylene glycol, 1,4-butanediol, polyethylene glycol, or polyester polyol.

[0020] More preferably, when the aqueous monomer is an amine, the capsule wall of the fluorescently labeled microcapsule is made of polyurea.

[0021] More preferably, when the aqueous monomer is a polyol, the capsule wall of the fluorescently labeled microcapsule is made of polyurethane.

[0022] Further preferably, the emulsifier in the aqueous phase containing the emulsifier is an anionic emulsifier and / or a nonionic emulsifier, selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and polyvinyl alcohol.

[0023] Further preferably, the solvent in the aqueous phase containing the emulsifier is water.

[0024] Preferably, the preparation method of the fluorescently labeled microcapsules includes: dispersing an oil-soluble fluorescent dye in an aqueous phase containing an emulsifier to form a dispersion; reacting urea and formaldehyde at pH 8-9 and 50-70 °C to obtain a urea-formaldehyde resin prepolymer; adding the urea-formaldehyde resin prepolymer to the above dispersion, adjusting the pH to 1-4, and stirring at 40-70 °C to carry out an in-situ polymerization reaction, thereby obtaining fluorescently labeled microcapsules with polyurea-formaldehyde as the capsule wall material.

[0025] Further preferably, the oil-soluble fluorescent dye is selected from at least one of Rhodamine 6G and oil-soluble fluorescein.

[0026] Further preferably, the emulsifier in the aqueous phase containing the emulsifier is an anionic emulsifier and / or a nonionic emulsifier, selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and polyvinyl alcohol.

[0027] Further preferably, the solvent in the aqueous phase containing the emulsifier is water.

[0028] Further preferably, the mass ratio of urea to formaldehyde is 1:(0.1~1.5).

[0029] More preferably, the mass ratio of urea to formaldehyde is 1:1.

[0030] Preferably, the dry film thickness of the composite coating material is 50~300μm.

[0031] Preferably, the dry film of the composite coating material has a tensile strength ≥30MPa and an elongation at break ≥400%.

[0032] Preferably, the dry film of the composite coating material has a signal-to-background ratio ≥9 at the microcracks.

[0033] Preferably, the micro-stress detection limit of the polymer capsule wall of the fluorescently labeled microcapsule is <10 μm.

[0034] Further preferably, the micro-stress detection limit of the polyurea capsule wall is ≤6μm; The micro-stress detection limit of the polyurethane capsule wall is ≤8 μm; The micro-stress detection limit of the polyurea-formaldehyde capsule wall is ≤4 μm.

[0035] The second objective of this invention is achieved through the following technical solution: A method for preparing a composite coating material with fluorescent self-indicating function for micro-stress detection, the preparation method comprising the following steps: Fluorescently labeled microcapsules are uniformly dispersed in 100g of matrix resin at an addition amount of 1-20g and mixed evenly; the matrix resin is thermoplastic polyurethane resin. Alternatively, fluorescently labeled microcapsules are uniformly dispersed in 100g of matrix resin at an addition amount of 1-20g to obtain a premixed slurry, and then an equivalent amount of curing agent is added to the matrix resin and mixed evenly; the matrix resin is epoxy resin.

[0036] Preferably, the difference between the refractive index of the polymer wall of the fluorescently labeled microcapsule and the refractive index of the matrix resin after curing is <0.03.

[0037] Preferably, the composite coating material is applied to the surface of the substrate and cured to form a composite coating with fluorescent self-indicating function.

[0038] More preferably, the substrate is selected from at least one of metal, glass, and ceramic.

[0039] Preferably, the curing agent is selected from at least one of polyetheramine, isophorone diamine, and diethylenetriamine.

[0040] Preferably, the thickness of the composite coating is 50~300μm, and the thickness of the composite coating is greater than the average particle size of the fluorescently labeled microcapsules.

[0041] The third objective of this invention is achieved through the following technical solution: A method for monitoring the integrity of a protective coating on a substrate surface, the method comprising coating a composite coating material with fluorescence self-indication function for micro-stress detection onto the substrate surface to form a protective coating, wherein when the protective coating develops microcracks or is damaged, fluorescent marker microcapsules located at the cracks rupture and release fluorescent dye, generating a fluorescent signal under ultraviolet light irradiation.

[0042] Compared with the prior art, the present invention has the following beneficial effects: 1. The composite coating material with fluorescence self-indication function for micro-stress detection of the present invention incorporates fluorescently labeled microcapsules. The rupture of these microcapsules at cracks and the targeted release of fluorescent dye can capture micron-level damage in the early stages of coating service, achieving high-sensitivity detection of microcracks.

[0043] 2. In the preparation of the composite coating material with fluorescence self-indication function for micro-stress detection of the present invention, the adverse effects of microcapsule introduction on the transparency and flexibility of the coating are effectively avoided by means of refractive index matching, particle size optimization and film thickness design.

[0044] 3. The fluorescent self-indicating composite coating material for micro-stress detection of the present invention can be applied to the monitoring of a variety of substrates, realize early warning of substrate corrosion risk, provide sufficient window period for equipment maintenance and coating repair, and significantly improve the safety and reliability of protective coating. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the fluorescently labeled microcapsules in Example 1 of the present invention.

[0046] Figure 2 The images show fluorescence images of the microcracked region (left) and the crack-free region (right) of the composite coating in Example 1 of this invention. Detailed Implementation

[0047] The technical solution of the present invention will be further described and illustrated below through specific embodiments. It should be understood that the specific embodiments described herein are only for the purpose of helping to understand the present invention and are not intended to limit the present invention.

[0048] Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used in the art, and the methods used in the embodiments are all conventional methods in the art.

[0049] In this article, the raw materials include: Thermoplastic polyurethane resin (TPU), model Elastollan® 1185A, with light transmittance ≥90% and refractive index 1.49; Epoxy resin: E-51; Curing agents: polyetheramine D-230, diaminodiphenylmethane DDM; An equivalence curing agent to epoxy resin refers to one in which, theoretically, the amount of reactive groups provided by the curing agent is exactly equal to the amount of epoxy groups in the epoxy resin.

[0050] The performance tests in this article include: Transmittance: The composite coating material was coated onto a transparent substrate and tested using a UV-Vis spectrophotometer; Tensile strength and elongation at break: tested according to ASTM D638; Crack induction and fluorescence detection: A universal tensile testing machine was used to perform three-point bending fatigue tests on the specimens (amplitude 5%, frequency 2Hz, 1000 cycles) to simulate microcrack generation in TPU under flexible service conditions. The specimens with microcracks were then excited under a 365nm ultraviolet lamp, and fluorescence images were captured using a laser scanning confocal microscope. The signal-to-background ratio (SBR) was calculated using image analysis software. The crack width corresponding to an SBR ≥ 3 was defined as the detection limit (LOD). Salt spray test protection performance: Low carbon steel samples coated with composite coatings are placed in a neutral salt spray test chamber, and the time of fluorescence signal appearance and the time of substrate corrosion are observed periodically to evaluate the protective performance and early warning capability of the coating.

[0051] Example 1

[0052] The preparation method of the fluorescently labeled microcapsules and the composite coating with fluorescent self-indicating function for high-sensitivity micro-stress detection in this embodiment includes the following steps: (1) Dissolve 3g of urea in 8.1g of a 37% formaldehyde aqueous solution, adjust the pH to 8.4±0.2 with triethanolamine, and stir at 500 rpm for 1 hour at 70℃ to obtain a urea-formaldehyde resin prepolymer. Mix 0.1g of oil-soluble rhodamine 6G with 78g of deionized water, add 0.8g of emulsifier (sodium dodecyl sulfate), and emulsify at 1200 rpm for 40 minutes at 50℃ to form an aqueous dispersion. Add the above urea-formaldehyde resin prepolymer to the aqueous dispersion, adjust the pH of the system to 2.5±0.2 with ammonium chloride, and stir at 800 rpm for 1 hour at 50℃; then add 0.3g of curing agent (resorcinol), raise the temperature to 60℃, and continue stirring at 800 rpm for 1.5 hours. After the reaction is complete, cool the resulting suspension to room temperature, let it stand, and discard the supernatant and suspended impurities. The remaining precipitate was washed three times with distilled water, filtered through a Buchner funnel after each wash, until the pH of the filtrate was neutral. The filter cake was dried in a 50°C oven to constant weight, and finally sieved to obtain fluorescently labeled microcapsules with a particle size of 60 μm. The capsule walls were made of polyurea-formaldehyde and had a refractive index of ~1.47. Figure 1As shown, the obtained microcapsules are regular spherical in shape and uniformly dispersed.

[0053] (2) The fluorescently labeled microcapsules with an average particle size of 60 μm were uniformly dispersed in 100 g of thermoplastic polyurethane (TPU) resin at an addition amount of 5 g and stirred until uniformly mixed; coated on the surface of a low carbon steel substrate and cured to form a composite coating with a dry film thickness of 200 μm and fluorescent self-indicating function.

[0054] Performance testing was conducted. In this embodiment, the composite coating exhibits a tensile strength of 35 MPa and an elongation at break of 450%. The transmittance of this composite coating in the visible light band is 85%. After inducing microcracks using the three-point bending method, a clear fluorescence signal was observed at the crack location under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 15.8 and a detection limit (LOD) of 2 μm. Figure 2 As shown, obvious fluorescence aggregation is observed at the microcracks (left), while no fluorescence signal is observed in the crack-free areas (right), which directly demonstrates the high-sensitivity detection effect described in this embodiment. In the neutral salt spray test, the composite coating of this embodiment showed a fluorescence warning after 24 hours, and substrate corrosion was delayed until 48 hours; indicating that the composite coating of this embodiment can achieve early tracing of protective coating failure and effectively delay substrate corrosion to a certain extent.

[0055] Example 2

[0056] The preparation method of the fluorescently labeled microcapsules and the composite coating with fluorescent self-indicating function for high-sensitivity micro-stress detection in this embodiment includes the following steps: (1) 5 g of isophorone diisocyanate (IPDI) and 0.1 g of oil-soluble rhodamine 6G were dissolved in 20 g of toluene, and 1.0 g of emulsifier polyvinyl alcohol (PVA) was dissolved in 80 g of deionized water as the aqueous phase. Under high-speed shear conditions, the resulting oil phase was added dropwise to the resulting aqueous phase to form a stable oil-in-water emulsion. An aqueous solution containing 3 g of ethylenediamine was added dropwise to the resulting emulsion, and the mixture was stirred at 700 rpm at 55 °C for 3 hours to undergo interfacial polymerization and form polyurea capsule walls. After the reaction was completed, the resulting suspension was cooled to room temperature, filtered, and the precipitate was washed with distilled water and dried in an oven. Finally, it was sieved to obtain fluorescently labeled microcapsules with an average particle size of 60 μm.

[0057] (2) The fluorescently labeled microcapsules were uniformly dispersed at an addition amount of 3 g in 100 g of E-51 epoxy resin without curing agent to prepare a premixed slurry; an equivalent amount of polyetheramine D-230 curing agent was added to the E-51 epoxy resin, mixed evenly, and then coated onto the surface of a low-carbon steel substrate. After curing, a composite coating with fluorescent self-indicating function with a dry film thickness of 150 μm was formed. Performance testing was performed. The transmittance of the composite coating in this embodiment was 84% ​​in the visible light band. The refractive index of the polyurea capsule wall in this embodiment was ~1.52, and the refractive index of the epoxy resin after curing in this embodiment was ~1.54, with a refractive index difference of 0.02.

[0058] After inducing microcracks using the three-point bending method, clear fluorescence signals were observed at the cracks under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 11.5 and a detection limit (LOD) of 5 μm.

[0059] Example 3

[0060] The preparation method of the fluorescently labeled microcapsules and the composite coating with fluorescent self-indicating function for high-sensitivity micro-stress detection in this embodiment includes the following steps: (1) 10 g of poly(1,4-butanediol adipate) and 2.5 g of isophorone diisocyanate (IPDI) were reacted at 80 °C for 3 hours to obtain a terminal isocyanate prepolymer. 0.1 g of oil-soluble rhodamine 6G was dissolved in it as the oil phase. 1.5 g of sodium dodecylbenzenesulfonate emulsifier was dissolved in 85 g of deionized water as the aqueous phase. Under high-speed shear conditions, the obtained oil phase was added dropwise to the obtained aqueous phase to form a stable oil-in-water emulsion. An aqueous solution containing 1.0 g of 1,4-butanediol was added dropwise to the obtained emulsion, and the mixture was stirred at 700 rpm at 60 °C for 4 hours to undergo interfacial polymerization to form a polyurethane capsule wall with a refractive index of ~1.53. After the reaction was completed, the obtained suspension was cooled to room temperature, filtered, and the precipitate was washed with distilled water and dried in an oven. Finally, it was sieved to obtain fluorescently labeled microcapsules with an average particle size of 60 μm.

[0061] (2) The fluorescently labeled microcapsules were uniformly dispersed in 100 g of E-51 epoxy resin without curing agent at an addition amount of 3 g to obtain a premixed slurry; polyetheramine D-230 curing agent with an equivalent amount to E-51 epoxy resin was added, and after being mixed evenly, it was coated on the surface of low carbon steel substrate and cured to form a composite coating with fluorescent self-indication function with a dry film thickness of 150 μm.

[0062] Performance testing was conducted. In this embodiment, the composite coating exhibits a transmittance of 83% in the visible light band. After inducing microcracks using the three-point bending method, a clear fluorescence signal was observed at the crack under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 10.2 and a detection limit (LOD) of 6 μm.

[0063] Example 4

[0064] The preparation method of the composite coating with fluorescence self-indication function for high-sensitivity micro-stress detection in this embodiment includes the following steps: Fluorescently labeled microcapsules with an average particle size of 60 μm from Example 1 were uniformly dispersed at an addition rate of 3 g in 100 g of E-51 epoxy resin without curing agent to prepare a premixed slurry. An equivalent amount of polyetheramine D-230 curing agent was added to the obtained premixed slurry, and after being mixed evenly, it was coated on the surface of a low-carbon steel substrate and cured to form a composite coating with a dry film thickness of 150 μm.

[0065] Performance testing was conducted. In this embodiment, the composite coating exhibits a transmittance of 83% in the visible light band. After inducing microcracks using the three-point bending method, a clear fluorescence signal was observed at the crack under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 9.2 and a detection limit (LOD) of 6 μm.

[0066] Example 5

[0067] The preparation method of the fluorescently labeled microcapsules and the composite coating with fluorescent self-indicating function for high-sensitivity micro-stress detection in this embodiment includes the following steps: (1) The procedure was carried out according to step (1) of Example 1, except that the rotation speed in the high-speed emulsification was 800 rpm, and fluorescently labeled microcapsules with an average particle size of 140 μm were obtained; (2) Following step (2) of Example 1, a composite coating with a dry film thickness of 200 μm was obtained.

[0068] Performance testing was conducted. In this embodiment, the composite coating has a transmittance of 80% in the visible light band.

[0069] After inducing microcracks using the three-point bending method, clear fluorescence signals were observed at the crack locations under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 12.0 and a detection limit (LOD) of 4 μm. In this embodiment, the composite coating could still be effectively detected, but the transmittance was slightly lower.

[0070] Comparative Example 1 The preparation method of the microcapsules and composite coating in this comparative example includes the following steps: The procedure was carried out according to step (1) of Example 1, except that the oil-soluble Rhodamine 6G was replaced with an equal amount of water-soluble fluorescein sodium.

[0071] Because water-soluble sodium fluorescein has poor solubility and dispersibility in the oil-phase prepolymer and subsequent acidic reaction environment, it cannot be effectively encapsulated in the polyurea-formaldehyde capsule wall, resulting in a significant reduction in the fluorescence encapsulation rate of the obtained microcapsules, making them unsuitable for subsequent composite coating preparation.

[0072] Performance tests were conducted. In this comparative example, the tensile strength of the composite coating was 37.9 MPa, and the elongation at break was 415%. After inducing microcracks using the three-point bending method, no obvious fluorescence signal was observed at the cracks under 365 nm ultraviolet light irradiation, and the fluorescence signal-to-background ratio (SBR) was 1.5, which was insufficient to effectively detect the microcracks.

[0073] Comparative Example 2 The method for preparing the composite coating in this comparative example includes the following steps: Add 5g of oil-soluble Rhodamine 6G directly to 100g of thermoplastic polyurethane (TPU) resin and stir to mix evenly; apply to the surface of a low-carbon steel substrate and cure to form a composite coating with a dry film thickness of 200μm.

[0074] Performance tests were conducted. In this comparative example, the tensile strength of the composite coating was 41 MPa, and the elongation at break was 545%.

[0075] In this comparative example, the fluorescent dye was directly dispersed in the matrix without microcapsule encapsulation, resulting in uniform dye distribution in the coating and the inability to generate localized high-concentration fluorescence at the cracks. After inducing microcracks using the three-point bending method, no obvious fluorescence signal was observed at the cracks under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 0.8, making it impossible to effectively detect microcracks.

[0076] Comparative Example 3 The preparation method of the microcapsules and composite coating in this comparative example includes the following steps: (1) The procedure was carried out according to step (1) of Example 1, except that the rotation speed in the high-speed emulsification was 2000 rpm; fluorescently labeled microcapsules with an average particle size of 10 μm were obtained. (2) Follow the steps of Example 1 (2), except that the fluorescently labeled microcapsules obtained in this comparative example are used; a composite coating with a dry film thickness of 200 μm is obtained.

[0077] Performance tests were conducted. In this comparative example, the composite coating had a transmittance of 88% in the visible light band. After inducing microcracks using the three-point bending method, the fluorescence signal-to-background ratio (SBR) was 6.2 and the limit of detection (LOD) was 12 μm under 365 nm ultraviolet light irradiation, indicating low sensitivity.

[0078] Comparative Example 4 The preparation method of the microcapsules and composite coating in this comparative example includes the following steps: (1) The procedure was carried out according to step (1) of Example 1, except that the rotation speed was 500 rpm during high-speed emulsification; fluorescently labeled microcapsules with an average particle size of 200 μm were obtained. (2) Follow the steps of Example 1 (2), except that the fluorescently labeled microcapsules obtained in this comparative example are used; a composite coating with a dry film thickness of 200 μm is obtained.

[0079] Performance testing was conducted. In this comparative example, the transmittance of the composite coating decreased to 68% in the visible light band. After inducing microcracks using the three-point bending method, a clear fluorescence signal was observed at the crack location under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 14.1 and a detection limit (LOD) of 5 μm. Although the detection limit of the composite coating in this comparative example was still acceptable, its optical performance was severely degraded.

[0080] Comparative Example 5 The method for preparing the composite coating in this comparative example includes the following steps: Fluorescently labeled microcapsules with an average particle size of 60 μm from Example 1 were uniformly dispersed in 100 g of thermoplastic polyurethane (TPU) resin at an addition amount of 25 g and stirred until uniformly mixed; the mixture was then coated onto the surface of a low-carbon steel substrate and cured to form a composite coating with a dry film thickness of 200 μm.

[0081] Performance tests were conducted. The transmittance of this comparative composite coating in the visible light band was 78%. After inducing microcracks using the three-point bending method, a clear fluorescence signal was observed at the crack under 365 nm ultraviolet light irradiation, with a fluorescence signal-to-background ratio (SBR) of 10.5 and a detection limit (LOD) of 8 μm.

[0082] In this comparative example, the excessive amount of microcapsules added to the composite coating resulted in a significant decrease in the mechanical properties of the coating, with a tensile strength of 18 MPa and an elongation at break of 250%, which are far lower than the 545% and 41 MPa of the pure TPU coating.

[0083] Comparative Example 6 The method for preparing the composite coating in this comparative example includes the following steps: Fluorescently labeled microcapsules with an average particle size of 60 μm from Example 1 were uniformly dispersed in 100 g of thermoplastic polyurethane (TPU) resin at an addition amount of 0.5 g and stirred until uniformly mixed; the mixture was then coated onto the surface of a low-carbon steel substrate and cured to form a composite coating with a dry film thickness of 200 μm.

[0084] Performance testing was conducted. In this comparative example, the microcapsule content in the composite coating was too low, resulting in weak fluorescence signals at the cracks, making it impossible to effectively calculate the signal-to-background ratio and detection limit.

[0085] Comparative Example 7 The method for preparing the composite coating in this comparative example includes the following steps: Fluorescently labeled microcapsules (polyurea capsule walls) with an average particle size of 60 μm from Example 2 were uniformly dispersed at an addition amount of 3 g in 100 g of E-51 epoxy resin without curing agent to prepare a premixed slurry; an equivalent amount of diaminodiphenylmethane (DDM) curing agent was added, and after being mixed evenly, it was coated on the surface of a low-carbon steel substrate and cured to form a composite coating with a dry film thickness of 150 μm.

[0086] Performance testing was conducted. In this comparative example, the composite coating exhibited a hazy, cloudy appearance, with a transmittance of 75% in the visible light band. The refractive index of the polyurea capsule wall in this comparative example was ~1.52, while the refractive index of the cured epoxy resin in this comparative example was ~1.58, a difference of 0.06. This refractive index mismatch caused the composite coating to strongly scatter light (including UV excitation light and fluorescence released after microcapsule rupture), making it difficult for the fluorescence signal to be effectively transmitted to the surface. After inducing microcracks using the three-point bending method, under 365 nm UV irradiation, the fluorescence signal-to-background ratio (SBR) was 5.8, the detection limit (LOD) was 15 μm, and the sensitivity was low. This demonstrates that the refractive index mismatch between the fluorescently labeled microcapsule wall and the composite coating substrate not only reduces the transparency of the composite coating but also severely weakens the fluorescence warning function.

[0087] Comparative Example 8 The method for preparing the composite coating in this comparative example includes the following steps: The process is carried out according to step (2) of Example 1, except that a composite coating with a dry film thickness of 400 μm is formed by curing.

[0088] Performance tests were conducted. The composite coating exhibited a transmittance of 83% in the visible light band, a fluorescence warning time of 36 hours in the neutral salt spray test, and a corrosion time of 60 hours. In this comparative example, the excessive thickness of the composite coating reduced transmittance and also led to decreased flexibility, affecting its application.

[0089] Comparative Example 9 The method for preparing the composite coating in this comparative example includes the following steps: The process is carried out according to step (2) of Example 1, except that a composite coating with a dry film thickness of 30 μm is formed by curing.

[0090] Performance tests were conducted. The surface of the composite coating in this comparative example was uneven, the repeatability of the fluorescence signal was poor, the fluorescence warning time was 12 hours and the corrosion time was 24 hours in the neutral salt spray test, indicating insufficient protective performance.

[0091] Comparative Example 10 The preparation method of the microcapsules and composite coating in this comparative example includes the following steps: (1) Follow the steps of Example 1 (1), except that the anionic emulsifier sodium dodecyl sulfate (SDS) is replaced with an equal amount of cationic emulsifier cetyltrimethylammonium bromide (CTAB).

[0092] (2) Follow the steps (2) of Example 1, except that the microcapsules in this comparative example are used.

[0093] In this comparative example, the microcapsules aggregated and the integrity of the capsule wall was damaged.

[0094] Performance tests were conducted. The composite coating in this comparative example exhibited a transmittance of 72% in the visible light band and a fluorescence signal-to-background ratio (SBR) of 4.5.

[0095] Comparative Example 11 The preparation method of the microcapsules and composite coating in this comparative example includes the following steps: Fluorescently labeled microcapsules with an average particle size of 60 μm from Example 1 were uniformly dispersed in 100 g of thermoplastic polyurethane (TPU) resin at an addition amount of 5 g, and then 0.5 g of benzene ring-containing organosilicon defoamer was added and stirred to mix evenly. The mixture was then coated onto the surface of a low carbon steel substrate and cured to form a composite coating with a dry film thickness of 200 μm and fluorescent self-indicating function.

[0096] Performance testing was conducted. In this comparative example, the benzene-ring-containing silicone defoamer migrated to the crack area and reacted with the released Rhodamine 6G dye, causing dye fluorescence quenching. The composite coating in this comparative example had a transmittance of 84% in the visible light band; the fluorescence signal-to-background ratio (SBR) was 2.1, which was insufficient for effective detection.

[0097] In summary, this invention achieves highly sensitive detection of microcracks by incorporating fluorescently labeled microcapsules that can rupture at the crack and release fluorescent dye at a specific point. At the same time, through refractive index matching, particle size optimization, and film thickness design, the adverse effects of microcapsules on the transparency and flexibility of the coating are effectively avoided. Therefore, it is suitable for early warning of corrosion risks on various substrates, providing a sufficient window period for equipment maintenance and coating repair, and significantly improving the safety and reliability of the protective coating.

[0098] All aspects, embodiments, and features of this invention should be considered illustrative in all respects and not limiting of the invention; the scope of the invention is defined only by the claims. Other embodiments, modifications, and uses will become apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0099] In the preparation method of this invention, the order of the steps is not limited to the listed order. For those skilled in the art, variations in the order of the steps without creative effort are also within the scope of protection of this invention. Furthermore, two or more steps or actions can be performed simultaneously.

[0100] Finally, it should be noted that the specific embodiments described herein are merely illustrative examples of the invention and are not intended to limit the implementation of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them; it is neither necessary nor possible to exemplify all embodiments here. However, these obvious variations or modifications derived from the essential spirit of the invention still fall within the scope of protection of the invention, and interpreting them as any additional limitation would contradict the spirit of the invention.

Claims

1. A composite coating material with fluorescent self-indicating function for micro-stress detection, characterized in that, Its raw materials include: Matrix resin; and fluorescently labeled microcapsules dispersed in the matrix resin; The fluorescently labeled microcapsule comprises a polymer capsule wall and an oil-soluble fluorescent dye encapsulated inside the polymer capsule wall; The polymer capsule wall is made of at least one of polyurea-formaldehyde, polyurea, or polyurethane. The average particle size of the fluorescently labeled microcapsules is 10~200μm.

2. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 1, characterized in that, The difference between the refractive index of the polymer wall of the fluorescently labeled microcapsule and the refractive index of the matrix resin after curing is <0.

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3. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 1, characterized in that, The dry film thickness of the composite coating material is greater than the average particle size of the fluorescently labeled microcapsules.

4. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 1, characterized in that, The amount of fluorescently labeled microcapsules added is 1 to 20 wt% of the matrix resin.

5. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 1, characterized in that, The matrix resin has a transmittance of >90% in the visible light band, and the matrix resin is selected from at least one of polyurethane resin, epoxy resin or acrylic resin. The dry film of the composite coating material has a transmittance of ≥80% in the visible light band.

6. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 1, characterized in that, The preparation method of the fluorescently labeled microcapsules includes: dispersing an oil-soluble fluorescent dye in an oil-phase monomer as an oil phase, adding an aqueous phase containing an emulsifier, forming an emulsion under high-speed shearing, adding an aqueous phase monomer, and carrying out an interfacial polymerization reaction at 50~70℃ for 1~6h.

7. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 6, characterized in that, The oil-soluble fluorescent dye is selected from at least one of Rhodamine 6G and oil-soluble fluorescein; And / or, the oil phase monomer is a polyisocyanate; the polyisocyanate is selected from at least one of isophorone diisocyanate, toluene diisocyanate, diphenylmethane diisocyanate or hexamethylene diisocyanate; And / or, the aqueous monomer is an amine and / or a polyol, wherein the amine is selected from at least one of ethylenediamine, hexamethylenediamine, diethylenetriamine, or polyethyleneimine, and the polyol is selected from at least one of ethylene glycol, 1,4-butanediol, polyethylene glycol, or polyester polyol; when the aqueous monomer is an amine, the capsule wall of the fluorescently labeled microcapsule is made of polyurea; when the aqueous monomer is a polyol, the capsule wall of the fluorescently labeled microcapsule is made of polyurethane. And / or, the emulsifier in the aqueous phase containing the emulsifier is an anionic emulsifier and / or a nonionic emulsifier, selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and polyvinyl alcohol; the solvent in the aqueous phase containing the emulsifier is water.

8. The composite coating material with fluorescent self-indicating function for micro-stress detection according to claim 1, characterized in that, The preparation method of the fluorescently labeled microcapsules includes: dispersing an oil-soluble fluorescent dye in an aqueous phase containing an emulsifier to form a dispersion; reacting urea and formaldehyde at pH 8-9 and 50-70 ℃ to obtain a urea-formaldehyde resin prepolymer; adding the urea-formaldehyde resin prepolymer to the above dispersion, adjusting the pH to 1-4, and stirring at 40-70 ℃ to carry out an in-situ polymerization reaction to obtain fluorescently labeled microcapsules with polyurea-formaldehyde as the capsule wall material; The oil-soluble fluorescent dye is selected from at least one of Rhodamine 6G and oil-soluble fluorescein; The emulsifier in the aqueous phase containing the emulsifier is an anionic emulsifier and / or a nonionic emulsifier, selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and polyvinyl alcohol; the solvent in the aqueous phase containing the emulsifier is water.

9. A method for preparing a composite coating material with fluorescent self-indicating function for micro-stress detection as described in any one of claims 1 to 8, characterized in that, The preparation method includes the following steps: Fluorescently labeled microcapsules are uniformly dispersed in 100g of matrix resin at an addition amount of 1-20g and mixed evenly; the matrix resin is thermoplastic polyurethane resin. Alternatively, fluorescently labeled microcapsules are uniformly dispersed in 100g of matrix resin at an addition amount of 1-20g to obtain a premixed slurry, and then an equivalent amount of curing agent is added to the matrix resin and mixed evenly; the matrix resin is epoxy resin. The difference between the refractive index of the polymer wall of the fluorescently labeled microcapsule and the refractive index of the matrix resin after curing is <0.

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10. A method for monitoring the integrity of a protective coating on a substrate surface, characterized in that, The method includes coating a composite coating material with fluorescent self-indicating function for micro-stress detection as described in any one of claims 1 to 8 onto the surface of a substrate to form a protective coating. When the protective coating develops microcracks or is damaged, the fluorescent marker microcapsules located at the cracks rupture and release fluorescent dye, generating a fluorescent signal under ultraviolet light irradiation.

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