Composite coating for metal anticorrosion and preparation method thereof

Abstract

The application provides a composite coating for metal corrosion prevention and a preparation method thereof. The composite coating for metal corrosion prevention comprises the following raw materials in parts by mass: 45 parts of epoxy resin, 4-8 parts of flaky filler, 8-15 parts of latent curing agent, 12-20 parts of hydroxyl fluorocarbon resin, 0.5-1.2 parts of dispersing agent, 0.1-0.3 parts of polyacrylate, 1-2 parts of adhesion promoter, 3-8 parts of pigment, 35-45 parts of cosolvent; the latent curing agent comprises latent curing agent A and latent curing agent B, wherein the mass ratio of the latent curing agent A to the latent curing agent B is 3.5-4.5:1; the latent curing agent A is a dissociative latent curing agent prepared by non-symmetrical addition and blocking reaction of isophorone diisocyanate, amino silane and imidazole compound; and the latent curing agent B is a 1-cyanoethyl-substituted C1-C4 alkyl imidazole compound. The composite coating for metal corrosion prevention is applied in metal substrate corrosion prevention.

Description

Technical Field

[0001] This application relates to the field of anti-corrosion coatings and coating technology, specifically to a composite coating for metal corrosion protection and its preparation method. Background Technology

[0002] Metal corrosion has long been a core problem in industrial systems, resulting in huge costs for equipment damage, maintenance downtime, and material replacement. As the most commonly used protective method, coating corrosion protection dominates among many corrosion protection technologies due to its advantages such as controllable cost and wide applicability. Among existing coating systems, epoxy resin has become the most mature and widely used material system in the field of heavy metal corrosion protection due to its excellent adhesion, mechanical strength, and chemical resistance.

[0003] Despite the wide application of epoxy resin coatings, they have revealed obvious contradictions and limitations in engineering projects.

[0004] The contradiction between room-temperature latency and high-temperature cross-linking network construction: High-performance anti-corrosion coatings are mostly two-component systems, requiring on-site mixing in proportions, which is cumbersome and results in a short pot life after mixing, easily leading to waste. To simplify construction, single-component coatings have become a research direction, but traditional single-component coatings cannot simultaneously meet the requirements of storage stability and high cross-linking density after curing. Although commonly used imidazole curing agents can lower the curing temperature, they are prone to early cross-linking or explosive polymerization due to the lack of shielding at room temperature. On the other hand, traditional latent curing agents such as dicyandiamide require baking at temperatures above 180°C, which places certain requirements on the metal substrate and energy consumption. Furthermore, existing closed or latent designs often adopt conventional stoichiometric thinking based on fully closed or completely balanced stoichiometry, which results in a simple cross-linking network structure of the cured anti-corrosion coating, making it difficult to provide stable and reliable anti-peeling performance under complex chemical media and stress environments.

[0005] Phase separation during the blending of thermodynamically incompatible resins: Pure epoxy resins are prone to chalking under outdoor ultraviolet radiation, leading to a decrease in weather resistance; long-term exposure to complex chemical media also degrades the density of the paint film, allowing corrosive media such as moisture, oxygen, and chloride ions to penetrate to the surface of the metal substrate, reducing the protective effect; although the introduction of hydroxyl fluorocarbon resins can improve weather resistance and chemical inertness, conventional physical blending is prone to phase separation during the curing stage due to the large difference in surface tension between the two, which destroys the original mechanical strength, causing the barrier effect of the sheet filler to fail and the weather resistance to decrease; and existing coupling agents or chemical crosslinking methods often lack the kinetic competition control between multiphase resins, making it impossible to establish effective chemical bonds at the interface between the two phases.

[0006] Therefore, there is an urgent need to develop a new type of metal anti-corrosion composite coating that can be stored stably as a single component at room temperature for a long time to simplify construction and avoid material waste, while being able to cure rapidly at medium and low temperatures to meet on-site construction efficiency, and breaking the thermodynamic incompatibility barrier between epoxy and fluorocarbon; and should also have excellent weather resistance and chemical stability to provide long-term reliable protection for metal substrates. Summary of the Invention

[0007] This application provides a composite coating for metal corrosion protection and its preparation method, aiming to solve the problems in the prior art, such as the difficulty in balancing room temperature storage stability and curing efficiency of single-component epoxy anti-corrosion coatings, the weak peel resistance due to the single crosslinking network, and the easy occurrence of thermodynamic phase separation when epoxy resin and fluorocarbon resin are physically blended.

[0008] In a first aspect, this application provides a composite coating for metal corrosion protection, the composite coating comprising the following raw materials in parts by weight: 45 parts epoxy resin, 4-8 parts flake filler, 8-15 parts latent curing agent, 12-20 parts hydroxyl fluorocarbon resin, 0.5-1.2 parts dispersant, 0.1-0.3 parts polyacrylate, 1-2 parts adhesion promoter, 3-8 parts pigment, and 35-45 parts co-solvent; the latent curing agent comprises latent curing agent A and latent curing agent B, wherein the mass ratio of latent curing agent A to latent curing agent B is 3.5-4.5:1; latent curing agent A is a dissociative latent curing agent prepared by isophorone diisocyanate, aminosilane and imidazole compound through asymmetric addition and blocking reaction; latent curing agent B is a 1-cyanoethyl-substituted C1-C4 alkyl imidazole compound.

[0009] Furthermore, the mass ratio of latent curing agent A to latent curing agent B is 3.8:1.

[0010] This application proposes a composite coating for metal corrosion protection based on the synergy of dual latent curing agents. By controlling the unsealing conditions, polarity distribution and multiple reaction sites of the latent curing agents, the composite coating for metal corrosion protection can be successfully combined with room temperature latency and medium-low temperature curing, and an interpenetrating polymer network structure can be constructed in situ. Specifically, during the room temperature storage stage, the latent curing agent A, through the reaction of the primary isocyanate group of isophorone diisocyanate with the secondary amine group of imidazole compound to form an acyl imidazole urea structure, constructs a steric shield for the tertiary amine nitrogen atom on the imidazole ring; simultaneously, the latent curing agent B utilizes the electron-withdrawing effect of the 1-position cyanoethyl to reduce the electron density and nucleophilic basicity of the imidazole ring; the synergistic effect of the two structures enables the composite coating for metal corrosion protection to maintain chemical inertness at room temperature, effectively preventing premature nucleophilic attack of the lone pair electrons of the tertiary amine nitrogen atom on the epoxy group of epoxy resin and the resulting homopolymerization reaction, thereby improving the room temperature storage stability of the composite coating for metal corrosion protection; During the leveling stage before curing, as the co-solvent evaporates in a gradient, the hydroxyl fluorocarbon resin, due to the large number of low surface energy fluorocarbon bonds in its molecule, spontaneously migrates to the surface and accumulates at the interface between the coating and air formed by the composite coating for metal corrosion protection, driven by the thermodynamic surface tension gradient. Meanwhile, the latent curing agent B, due to its small molecular weight and the presence of highly polar cyano and imidazole rings, exhibits a strong affinity for epoxy resins and mainly settles and accumulates in the middle and lower layers of the coating. The latent curing agent A, in its molecular structure, contains both siloxane groups with affinity for metal substrates and acyl urea backbone segments with good compatibility with epoxy and hydroxyl fluorocarbon resins, and can be relatively uniformly distributed in the longitudinal space of the coating from the bottom layer to the surface layer. During the curing stage, the acyl imidazourea structure in the latent curing agent A molecule undergoes thermal reversal cleavage and desealing, releasing free isocyanate groups and catalytically active free imidazolidinyl compounds in situ. The released free imidazolidinyl compounds, together with the activated latent curing agent B, expose the active sites on the imidazolidinyl ring. They utilize their lone pair electrons to nucleophilically attack the epoxy groups of the epoxy resin and cause them to open the ring, thereby initiating the ring-opening homopolymerization reaction of the epoxy resin matrix. This catalytic mechanism promotes spontaneous crosslinking between epoxy resin molecules, constructing a homopolymer network. During the thermal reversal fracture unsealing process, the free isocyanate groups released by the latent curing agent A can undergo an urethane addition reaction with the hydroxyl groups of the hydroxyl fluorocarbon resin to generate polyurethane segments. This in-situ chemical bonding crosslinks the epoxy resin homopolymer network and the hydroxyl fluorocarbon resin network in the thermosetting process, constructing an interpenetrating polymer network structure. This polymer network structure effectively suppresses the phase separation defects caused by the thermodynamic incompatibility between epoxy resin and hydroxyl fluorocarbon resin, giving full play to the excellent weather resistance and low surface energy protection advantages of fluorocarbon resin. By limiting the mass ratio of latent curing agent A to latent curing agent B, the rigidity and toughness of the coating formed by the composite coating for metal corrosion protection can be balanced. Within this specific ratio range, latent curing agent B catalyzes the ring-opening of epoxy resin and constructs a homopolymer network during the curing stage, giving the coating excellent rigidity and hardness. At the same time, after latent curing agent A is thermally desealed and participates in crosslinking, the aliphatic flexible alkyl segments derived from aminosilane and polyurea and polyurethane segments introduced into its structure can effectively disperse, buffer and absorb the curing shrinkage internal stress generated by the homopolymerization of epoxy resin, reducing coating defects caused by stress concentration. The silanoxy groups modified on the latent curing agent A can absorb trace amounts of water vapor and undergo hydrolysis and condensation reactions during the curing reaction stage. On the one hand, they form a local inorganic silicon-oxygen network inside the coating, and on the other hand, they undergo covalent condensation bonding with the hydroxyl groups on the surface of the pretreated metal substrate, thereby improving the adhesion and weather resistance of the coating. Furthermore, under the synergistic effect of the dispersant and polyacrylate leveling agent, the flake filler can construct a tortuous path within the coating that hinders the penetration of corrosive media; the pigment plays a role in physical filling and mechanical reinforcement; the adhesion promoter can improve the adhesion of the coating to the substrate; and the co-solvent enables the construction of a gradient-layered protective barrier in this composite coating for metal corrosion protection.

[0011] In some embodiments, the latent curing agent A is prepared by the following method: M1: Isophorone diisocyanate and aminosilane are dispersed in an aprotic polar solvent, and the isophorone diisocyanate and aminosilane undergo a nucleophilic addition reaction to obtain a dispersion of isocyanate intermediate containing silane groups. M2: The isocyanate intermediate dispersion containing silane groups is mixed with imidazole compounds to allow the isocyanate intermediate containing silane groups to undergo a nucleophilic addition reaction with the imidazole compounds, thereby obtaining latent curing agent A; The molar ratio of isophorone diisocyanate, aminosilane and imidazole compound is 1:(0.98~1):(0.98~1); The aminosilane includes at least one of 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; The imidazole compounds include at least one of 2-methylimidazole and 2-ethylimidazole; The aprotic polar solvent includes at least one of propylene glycol monomethyl ether acetate and butyl acetate.

[0012] Furthermore, the latent curing agent A is prepared by the following method: M1: Disperse isophorone diisocyanate and aminosilane in an aprotic polar solvent and react at 20-30℃ for 2-3 hours to obtain a dispersion of isocyanate intermediate containing silane groups. M2: The isocyanate intermediate dispersion containing silane groups is mixed with an imidazole compound and reacted at 45-60°C for 3-4 hours to obtain a latent curing agent A dispersion. The mass fraction of the latent curing agent A in the obtained dispersion is 45%~55%.

[0013] The above embodiments utilize the asymmetric reactivity difference between the primary and secondary isocyanate groups in the isophorone diisocyanate molecule structure; in the M1 stage, the primary amino group of aminosilane preferentially undergoes selective addition with the secondary isocyanate group of isophorone diisocyanate to construct the urea bond skeleton in situ, while retaining the primary isocyanate group on the other side. In the M2 stage, imidazole compounds are used to perform a thermally reversible blocking reaction on the residual primary isocyanate groups. The resulting acylimidazourea structure has chemical latency at room temperature, which constructs a chemical shield for the catalytic active center of imidazole. This effectively prevents the premature attack of the lone pair electrons on the imidazole ring on the epoxy group and the homopolymerization reaction initiated during room temperature storage, thus endowing the composite coating used for metal corrosion protection with excellent room temperature storage stability. At the same time, this asymmetric acylimidazourea structure has a low thermal dissociation activation energy, which enables effective thermal reversible pyrolysis during the curing stage.

[0014] In some embodiments, the sheet-like filler includes at least one of mica powder, sericite, and talc powder.

[0015] Through the above embodiments, the sheet-like filler constructs a physical barrier diffusion path with a certain tortuosity inside the coating during the coating formation and curing process, thereby extending the penetration path of the corrosive medium.

[0016] In some embodiments, the dispersant includes at least one of polyvinylpyrrolidone, polyhydroxystearic acid, and polyvinyl butyral.

[0017] Through the above embodiments, the dispersant can be physically adsorbed on the surface of the flake filler and pigment particles and form a steric barrier, thereby reducing flocculation, agglomeration and irreversible hard precipitation.

[0018] In some embodiments, the adhesion promoter includes at least one of γ-glycidoxypropyltrimethoxysilane and γ-methacryloyloxypropyltrimethoxysilane.

[0019] Through the above embodiments, the epoxy or acryloyloxy groups in the adhesion promoter can generate polar affinity, spatial entanglement or covalent crosslinking with the epoxy resin matrix in the composite coating used for metal corrosion protection, while the trimethoxysilyl groups can absorb trace amounts of water vapor and undergo hydrolysis during the film formation process, and undergo covalent condensation reaction with the active hydroxyl groups on the surface of the pretreated metal substrate.

[0020] In some embodiments, the pigment includes at least one of titanium dioxide, zinc phosphate, barium sulfate, and carbon black.

[0021] Through the above implementation methods, inert pigments such as titanium dioxide and carbon black can scatter and absorb ultraviolet light to delay the photodegradation of epoxy resin matrix; barium sulfate plays a role in physical filling and spatial reinforcement; and zinc phosphate can slowly release phosphate ions under the dissociation equilibrium of trace amounts of permeable water and undergo interfacial complexation precipitation reaction with metal anode to generate phosphate passivation film in situ.

[0022] In some embodiments, the latent curing agent B includes at least one of 1-cyanoethyl-2-ethyl-4-methylimidazole and 1-cyanoethyl-2-methylimidazole; The co-solvent is composed of ethyl acetate, xylene, and butyl acetate in a mass ratio of (1~3):(2~4):4.

[0023] Through the above implementation methods, during the leveling stage before curing, the co-solvent system constructed from ethyl acetate, xylene, and butyl acetate in a specific ratio exhibits a controlled gradient evaporation rate. This mechanism effectively reduces defects caused by excessively rapid flash evaporation of the surface solvent. At the same time, the gradient evaporation prolongs the hydrodynamic window of the composite coating used for metal corrosion protection, providing sufficient time for the latent curing agent B to thermodynamically settle to the underlying epoxy matrix. In conjunction with the surface migration mechanism of hydroxyl fluorocarbon resin, a functional protective barrier with spontaneous layering is constructed.

[0024] Secondly, a method for preparing a composite coating for metal corrosion protection according to the first aspect includes the following steps: S1: The epoxy resin, hydroxyl fluorocarbon resin, dispersant, cosolvent (accounting for 50%~80% of the total mass of cosolvent), pigment, flake filler, and adhesion promoter are mixed evenly to obtain the base material; S2: Add the polyacrylate, latent curing agent A, latent curing agent B and the remaining co-solvent to the base material and disperse them evenly to obtain a composite coating for metal corrosion protection.

[0025] Through the above implementation method, step S1 establishes a stable dispersion system base material while avoiding the adsorption of subsequent additives by the specific surface area of ​​the pigment; step S2 adds latent curing agent A and latent curing agent B to reduce the latent curing agent from premature desealing due to mechanical heat generation in step S1.

[0026] Thirdly, a method for corrosion protection of a metal substrate, characterized by comprising the following steps: P1: Provide the composite coating for metal corrosion protection as described in the first aspect; P2: The surface of the metal substrate is pretreated by degreasing, derusting and sandblasting to make the rust removal grade of the metal substrate reach Sa2.5 and the surface roughness Rz is 40~75μm; P3: The composite coating for metal corrosion protection is applied to the surface of the pretreated metal substrate, and after leveling and curing, an anti-corrosion coating is formed on the surface of the metal substrate.

[0027] Through the above implementation method, step P1 provides a suitable composite coating for metal corrosion protection; the pretreatment in step P2 removes impurities from the metal substrate and constructs a moderately rough surface, increasing the effective specific surface area of ​​the interface; and removes the weak boundary layer of oxides, allowing the exposed high-energy crystal plane dangling bonds to undergo rapid surface hydroxylation; step P3 realizes the construction of the final anti-corrosion coating.

[0028] Compared with the prior art, the beneficial effects of this application are at least as follows: During room temperature storage, latent curing agent A constructs a chemical shield and steric hindrance protection for the active center of imidazole compounds through the acyl imidazole urea structure; together with latent curing agent B, which exhibits low nucleophilic basicity due to the electron-withdrawing effect of 1-cyanoethyl, they jointly inhibit the nucleophilic attack of the nitrogen atom of the imidazole ring tertiary amine on the epoxy group, thereby ensuring the excellent room temperature storage stability of the single-component system. During the leveling stage, with the hydrodynamic window provided by the evaporation of the cosolvent gradient, the low surface energy hydroxyl fluorocarbon resin is driven by the tension gradient to migrate spontaneously to the outer layer, the highly polar small molecule latent curing agent B spontaneously settles to the middle and lower layers, while the latent curing agent A with biphasic affinity is distributed relatively uniformly in the longitudinal direction, thereby inducing the construction of a functional gradient anti-corrosion barrier in situ before curing. During the curing stage, the acyl imidazourea structure in the latent curing agent A undergoes thermal reversal unsealing, releasing active free isocyanate groups and free imidazolium compounds in situ. These, together with the thermally activated latent curing agent B, catalyze the ring-opening homopolymerization of epoxy resin to construct a homopolymer network. Simultaneously, the isocyanate groups released from latent curing agent A undergo an in-situ urethane addition reaction with hydroxyl fluorocarbon resin, crosslinking the multiphase resin network into an interpenetrating polymer network structure through chemical bonding. This effectively suppresses phase separation defects and fully releases the low surface energy protection advantage of hydroxyl fluorocarbon resin. Furthermore, by limiting the mass ratio of latent curing agent A to latent curing agent B within a specific range, the aliphatic flexible alkyl segments derived from aminosilane and the flexible nodes of polyurea and polyurethane introduced by the thermal desealing of latent curing agent A buffer and absorb the shrinkage internal stress generated by the homopolymerization and curing of epoxy resin. Combined with the chemical anchoring effect of the silanoxy groups on the side chain of latent curing agent A absorbing trace amounts of water vapor and undergoing hydrolysis, and undergoing interfacial covalent condensation with the hydroxyl groups on the surface of the metal substrate, the adhesion of the anti-corrosion coating is improved while achieving a balance between rigidity and toughness. Attached Figure Description

[0029] Figure 1 The infrared spectrum of isophorone diisocyanate prepared in Example 1 of this application.

[0030] Figure 2 The infrared spectrum of the isocyanate intermediate containing silane groups in Example 1 of this application; Figure 3 The image shows a comparison of the infrared spectra of the latent curing agent A in Preparation Example 1 of this application; wherein the isocyanate intermediate containing silane groups and the latent curing agent A are the solid products obtained after solvent removal from the corresponding dispersions obtained in steps M1 and M2 of Preparation Example 1. Detailed Implementation

[0031] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0032] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with an implementation or example that are included in at least one implementation or example of this application.

[0033] In this specification, the illustrative expressions of the terms used do not necessarily refer to the same implementation or example.

[0034] Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more implementations or examples.

[0035] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated.

[0036] Therefore, features specified as "first" or "second" may explicitly or implicitly include at least one of those features. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0037] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0038] The following describes embodiments of this application.

[0039] The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0040] Where no specific technology or conditions are specified in the embodiments, the technology or conditions described in the literature in this field or the product manual shall apply.

[0041] If the manufacturers of the reagents or instruments used are not specified, they are all conventional products that can be obtained commercially.

[0042] Epoxy resin, bisphenol A type, epoxy value 0.53 mol / 100g; Mica powder with an average flake diameter of 20 μm; The hydroxyl fluorocarbon resin liquid has a solid content of 64.7% and uses butyl acetate as the solvent; its dynamic viscosity at 25°C is 1570 mPa·s; as a resin solution, its acid value is 3.4 mgKOH / g and its hydroxyl value is 63.5 mgKOH / g; as resin solids, its fluorine content is 24%. Polyacrylate, weight average molecular weight 15000; Barium sulfate, average particle size 0.5 μm; Polyvinylpyrrolidone, CAS No. 9003-39-8, weight average molecular weight 40,000.

[0043] Preparation Example 1 Preparation of latent curing agent A dispersion: M1: Under nitrogen protection, 22.2 parts of isophorone diisocyanate were dispersed in 35 parts of butyl acetate. A mixed dilution of 22.1 parts of 3-aminopropyltriethoxysilane and 17.5 parts of butyl acetate was added dropwise at 20℃ and 250r / min. The addition was completed in 60min. The reaction temperature was controlled at 20~25℃ and the mixture was stirred for 2h to obtain a dispersion of isocyanate intermediate containing silane groups. M2: The isocyanate intermediate dispersion containing silane groups is mixed with 8.2 parts of 2-methylimidazole and reacted at 55°C and 250 r / min for 3 h to obtain a latent curing agent A dispersion. See Figure 1 , Figure 2 , Figure 3 Isophorone diisocyanate, silane-containing isocyanate intermediate, and latent curing agent A at 2932 cm⁻¹ -1 Clear aliphatic CH stretching vibration peaks were preserved in the vicinity, at 1458 cm⁻¹. -1 The strong absorption peak of the alicyclic skeleton angular vibration is retained nearby, which indicates that steps M1 and M2 did not destroy the inherent amorphous cyclohexane core skeleton structure of isophorone diisocyanate. Compared to isophorone diisocyanate, the silane-containing isocyanate intermediate exhibits several newly formed organic characteristic absorption peaks: at 3345 cm⁻¹ -1 The broad, gentle absorption peak at 1650–1500 cm⁻¹ is attributed to the NH stretching vibration of newly formed urea bonds under strong hydrogen-bonded association; while the absorption peak at 1650–1500 cm⁻¹ is attributed to the NH stretching vibration of newly formed urea bonds under strong hydrogen-bonded association. -1 Within this region, the newly formed urea bond skeleton promotes physical coupling and splitting of spectral lines, at 1636 cm⁻¹. -1 and 1563cm -1 The structure evolves into a W-shaped twin valley, belonging to the amide I band (C=O stretching vibration) and amide II band (NH bending and CN stretching vibration) unique to the urea bond skeleton, respectively; at 1200~1000 cm⁻¹-1 Within this region, a broad, gently overlapping absorption band appears, attributed to the asymmetric stretching vibration of the Si-OC siloxane backbone in the 3-aminopropyltriethoxysilane structure; simultaneously, due to the equimolar consumption of the secondary isocyanate group, an absorption band appears at 2264 cm⁻¹. -1 The intensity of the characteristic strong absorption peak of the -NCO group at the position decreased, and the bottom of the peak smoothly hovered near the 18% transmittance line, confirming that the primary amino group of 3-aminopropyltriethoxysilane in step M1 had preferentially achieved selective addition with the secondary isocyanate group of isophorone diisocyanate, constructing a urea bond skeleton in situ, and retaining the primary isocyanate group on the other side. Furthermore, compared to isocyanate intermediates containing silane groups, the infrared spectrum of the final latent curing agent A exhibits further structural evolution: at 2264 cm⁻¹ -1 The characteristic absorption peak of tert-NCO at 1700-1500 cm⁻¹ is almost completely exhausted, leaving only a weak baseline fluctuation; meanwhile, the characteristic absorption peak of tert-NCO at 1700-1500 cm⁻¹ is also present. -1 Within the region, the original unimodal coupling evolved into a characteristic multi-overlapping valley, particularly at 1630 cm. -1 Nearby, a strong amide I band (C=O stretching vibration) absorption peak, deeply penetrating to approximately 25% transmittance, is observed, with a peak at 1563 cm⁻¹. -1 and 1541cm -1 The presence of secondary twin valley envelopes corresponds to the overlap of the conjugate C=N stretching vibration signals of the amide II band of the urea bond backbone and the 2-methylimidazolium ring; furthermore, at 1400–1300 cm⁻¹... -1 Fingerprint area and 745cm -1 The peaks exhibited characteristic absorption and symmetrical single-valley rebound, corresponding to the imidazole ring skeletal vibration and the out-of-plane bending vibration of unsaturated CH. These qualitative abrupt changes in the spectral micro-fingerprint characteristics confirmed that the final product contained a complete closed imidazole urea bond and a stable 2-methylimidazolium cyclic skeletal structure. Combined with the process conditions of 55°C and 3h reaction in step M2, the above qualitative and quantitative spectral transformations indicated that the primary isocyanate group retained in the silane-containing isocyanate intermediate was basically consumed, and a highly efficient nucleophilic addition and fully closed interaction occurred with the secondary amine group on the 2-methylimidazolium ring, successfully obtaining the desired latent curing agent A.

[0044] Comparative Preparation Example 1 Preparation of dispersions of isocyanate intermediates containing silane groups: M1-1: Same as M1 in Preparation Example 1.

[0045] Comparative Preparation Example 2 Preparation of oxime-based blocked curing agent dispersions: M1-2: Same as M1 in Preparation Example 1; M2-2: The isocyanate intermediate dispersion containing silane groups is mixed with 8.7 parts of butanone oxime and reacted at 55°C and 250 r / min for 3 h to obtain an oxime-based blocked curing agent.

[0046] Comparative preparation example 3 Preparation of a fully enclosed isocyanate dispersion of diimidazole: Under nitrogen protection, 22.2 parts of isophorone diisocyanate were dispersed in 35 parts of butyl acetate. A mixed dilution of 16.4 parts of 2-methylimidazole and 17.5 parts of butyl acetate was added dropwise at 20℃ and 250 r / min. The addition was completed in 60 min, and the reaction temperature was controlled at 20~25℃. The mixture was stirred for 2 h, and then heated to 55℃ and reacted for another 3 h to obtain a fully enclosed isocyanate of bisimidazole.

[0047] Example 1 Preparation of a composite coating for metal corrosion protection: Raw materials by weight: 45 parts epoxy resin, 5 parts mica powder, 9.5 parts latent curing agent A (introduced in the form of 19 parts latent curing agent A dispersion from Preparation Example 1, containing 9.5 parts butyl acetate), 2.5 parts 1-cyanoethyl-2-ethyl-4-methylimidazole, 16 parts hydroxy fluorocarbon resin (introduced in the form of 24.7 parts hydroxy fluorocarbon resin liquid, containing 8.7 parts butyl acetate), 0.8 parts polyvinylpyrrolidone, 0.2 parts polyacrylate, 1.5 parts γ-glycidoxypropyltrimethoxysilane, 5 parts barium sulfate, 45 parts co-solvent (including 10 parts ethyl acetate, 15 parts xylene, and 20 parts butyl acetate; wherein 9.5 parts butyl acetate are introduced from the latent curing agent A dispersion, 8.7 parts butyl acetate are introduced from the hydroxy fluorocarbon resin liquid, and 1.8 parts are added butyl acetate); Preparation process: S1: Epoxy resin, hydroxyl fluorocarbon resin liquid, polyvinylpyrrolidone, 6 parts ethyl acetate, 9 parts xylene, barium sulfate, mica powder, and γ-glycidyl etheroxypropyltrimethoxysilane are stirred at 40℃ and 2000 r / min for 45 min to obtain the base material. S2: Add a dispersion of polyacrylate, 4 parts ethyl acetate, 6 parts xylene, and 1.8 parts butyl acetate to the base material, stir for 15 min at 25°C and 600 r / min, then add latent curing agent A dispersion and 1-cyanoethyl-2-ethyl-4-methylimidazole, stir for 12 min at 25°C and 200 r / min, and then filter at 25°C, normal pressure, and a 200-mesh filter to obtain a composite coating for metal corrosion protection.

[0048] Example 2 The method is largely the same as in Example 1, except that the co-solvent is 45 parts of butyl acetate (of which 9.5 parts are introduced from the latent curing agent A dispersion, 8.7 parts are introduced from the hydroxyl fluorocarbon resin liquid, and 26.8 parts are added butyl acetate).

[0049] Comparative Example 1 The preparation was largely the same as in Example 1, except that 9.5 parts of latent curing agent A and its introduced form (19 parts of latent curing agent A dispersion) were replaced with 17.52 parts of the isocyanate intermediate dispersion containing silane groups in Comparative Preparation Example 1 (which contained 9.5 parts of butyl acetate).

[0050] Comparative Example 2 The preparation was largely the same as in Example 1, except that 9.5 parts of latent curing agent A and its introduced form (19 parts of latent curing agent A dispersion) were replaced with 19.09 parts of the oxime-based blocked curing agent dispersion (containing 9.5 parts of butyl acetate) from Comparative Preparation Example 2.

[0051] Comparative Example 3 Similar to Example 1, except that 9.5 parts of latent curing agent A and its introduced form (19 parts of latent curing agent A dispersion) were replaced with 16.48 parts of the bisimidazole fully enclosed isocyanate dispersion (containing 9.5 parts of butyl acetate) from Comparative Preparation Example 3.

[0052] Comparative Example 4 The method is largely the same as in Example 1, except that 2.5 parts of 1-cyanoethyl-2-ethyl-4-methylimidazole are replaced with 2 parts of 1-cyanoethyl-2-ethyl-4-methylimidazole.

[0053] Comparative Example 5 The method is largely the same as in Example 1, except that 2.5 parts of 1-cyanoethyl-2-ethyl-4-methylimidazole are replaced with 2.8 parts of 1-cyanoethyl-2-ethyl-4-methylimidazole.

[0054] Comparative Example 6 It is largely the same as Example 1, except that 1-cyanoethyl-2-ethyl-4-methylimidazole is not added.

[0055] Test section Stability test: Referring to GB / T 6753.3-1986, the composite coatings of each embodiment and comparative example were steadily injected into a 0.5L standard coating sealed container (filling volume approximately 80% of the container volume) under a controlled environment of 25% relative humidity. Before sealing the container, nitrogen gas was introduced into the top space of the container, and then the container was tightly sealed with a lid with a built-in PTFE anti-corrosion liner to physically prevent interference from environmental moisture-induced non-hydrolytic condensation of silanes or isocyanate side reactions. Before sealing the container, the temperature was measured in a 25°C constant temperature water bath. Determine the initial viscosity ν0 of the composite coating; place the sealed sample container in a constant temperature oven set at 50℃ and let it stand continuously for 14 days; then remove the sample container and let it stand at 25℃ for 24 hours; after opening the container, first observe the macroscopic state and record whether irreversible skinning, phase separation, hard precipitation or gelation occurs; if the macroscopic state is good, determine the endpoint dynamic viscosity νt of the coating; use the formula: Δν=[(νt-ν0) / ν0]×100% to determine the viscosity growth rate.

[0056] Painting steps: P1: Select a standard Q235 carbon steel plate with dimensions of 150mm×70mm×2mm as the metal substrate. Wipe it with xylene and acetone in sequence to thoroughly remove surface oil stains. After drying, use 50-mesh brown corundum for dry sandblasting to thoroughly remove rust and reveal a uniform metallic color until the surface roughness of the metal substrate reaches Sa2.5 level and the surface roughness Rz reaches 40~75μm. After the dry sandblasting is completed, immediately use oil-free and water-free dry compressed air to blow away the residual dust and abrasive on the surface, and complete the subsequent steps within 4 hours. P2: Load the composite coating prepared in the examples or comparative examples (if premature gelation occurs and film cannot be formed, this coating step is not performed) into a gravity air spray gun; control the spraying air pressure to 0.3MPa, the nozzle diameter to 1.5mm, and the spray gun to be perpendicular to the substrate surface and maintain a working distance of 20cm; use a multi-pass spraying process, and flash dry at room temperature for 5min after each unidirectional spraying, control the cumulative wet film thickness, and ensure that the final dry film thickness after complete curing is maintained at 80±5μm; P3: The coated steel plate is placed horizontally in a dust-free and ventilated environment at 25℃ and 60% relative humidity, and allowed to stand for 15 minutes to level, thus obtaining a sample. The sample is then smoothly transferred into a constant temperature drying oven, and heated from room temperature to 60℃ at a rate of 2℃ / min, and held for 15 minutes. The temperature is then increased to 90℃ at a rate of 2℃ / min and held for 20 minutes. Finally, the temperature is increased to 120℃ at a rate of 2℃ / min and baked for 55 minutes to cure. After curing, the sample is allowed to cool naturally to room temperature in the oven and placed in a desiccator for 24 hours to form an anti-corrosion coating on the surface of the metal substrate, thus obtaining a test sample, which is then subjected to various performance tests.

[0057] Corrosion resistance test: In accordance with GB / T 1771-2007 standard, the edges and back of the test sample were subjected to strict anti-corrosion sealing treatment before testing. A scratch was prepared on the test surface using a standard single-edged scratching tool, cutting through the coating to the metal substrate. A neutral NaCl aqueous solution with a concentration of 50 g / L and a pH value maintained between 6.5 and 7.2 was continuously sprayed in a salt spray environment chamber at 35°C. After reaching the set cycle (1000 h), the residual salt deposits on the sample surface were immediately washed away gently with a clean, flowing weak water stream at 35°C, and then quickly dried with oil-free and water-free compressed air. Before evaluation, the loose coating that had lost its adhesion to the substrate on both sides of the scratch was carefully removed using a stiff brush or standard tape peeling method. Then, the maximum single-sided corrosion spread width (MCCW) at the scratch was accurately measured using vernier calipers.

[0058] Adhesion test: Dry adhesion test: Refer to GB / T 9286-2021 and strictly select a cross-cutting tool with a blade spacing of 2mm; cut 25 cross-cutting grids through the anti-corrosion coating surface to the metal substrate. After cleaning up the debris, apply and press the standard test tape firmly into the cross-cutting area; use an eraser or silicone finger cot to vigorously rub the back of the tape until the coating color is observed to be uniform through the tape, ensuring complete interface adhesion between the tape and the coating; after 5 minutes, pinch the suspended end of the tape and then peel it off smoothly at an angle of approximately 60° within 0.5~1s; under good lighting conditions, use a magnifying glass with a light source to observe the area of ​​anti-corrosion coating peeling off in the grid area, and determine the adhesion level according to the 0~5 standard; Wet adhesion test: Immerse half of the fully cured test sample in deionized water at 25°C for 48 hours. Remove the sample immediately with filter paper to dry the surface moisture. Within 8 minutes of removing the sample from the water, quickly perform the same cross-cut and tape peeling operation as the dry adhesion test on the immersed area. Determine the adhesion level according to the 0-5 standard.

[0059] Methyl ethyl ketone (MEK) rub test: Referring to GB / T 23989-2009, a solvent wiping tester equipped with a standard 1000g±10g load wiping head was used. The wiping head was strictly wrapped with four layers of degreased pure cotton gauze and thoroughly soaked with analytical grade methyl ethyl ketone (MEK) solvent. The MEK-soaked wiping head was placed vertically on the surface of the anti-corrosion coating and wiped linearly at a constant frequency of 1 stroke / second with a stroke of 100mm (each push forward and pull back once counts as one reciprocating wiping). During the test, 2ml of methyl ethyl ketone was added to the gauze area with a dropper every 50 wipings to ensure that the wiping interface always maintained sufficient solvent saturation. Wiping continued until the coating surface showed obvious loss of gloss, softening and stickiness, or local coating was completely dissolved and peeled off to expose the metal substrate. The test was stopped immediately and the current total number of reciprocating wipings (MRC) was recorded. If the coating surface still showed no obvious damage or exposure of the substrate after 200 wipings, the test was terminated and recorded as ">200 times".

[0060] The test results are shown in Table 1: Table 1

[0061] Note: "—" indicates that the test could not be performed because the sample had gelled or failed to form a film.

[0062] As shown in Table 1, the overall performance of each embodiment is better than that of each comparative example, indicating that the composite coating for metal corrosion protection proposed in this application based on the synergy of dual latent curing agents successfully balances room temperature latency and medium-low temperature curing of the composite coating for metal corrosion protection by controlling the unsealing conditions, polarity distribution and multiple reaction sites of the latent curing agents, and constructs an interpenetrating polymer network structure in situ. The reason may be that in Comparative Example 1, the acyl imidazolidinyl urea structure was not generated to build steric shielding for the nitrogen atoms of the tertiary amine, and the chemical inertness of the room temperature latency stage was lost. It could not effectively prevent the free groups from premature nucleophilic attack on the epoxy groups of the epoxy resin, which triggered premature homopolymerization reaction, resulting in direct gelation of the composite coating and loss of room temperature storage stability. In Comparative Example 2, the latent curing agent A, which does not have an acyl imidazole urea structure, was not used. After thermal reversal fracture and desealing during the curing stage, it could not release the catalytically active free imidazole compounds in situ. Therefore, it could not expose the active sites on the imidazole ring together with the latent curing agent B, and could not use its lone pair electrons to nucleophilically attack the epoxy groups of the epoxy resin and open the ring. This resulted in insufficient spontaneous crosslinking between epoxy resin molecules and insufficient construction of the ring-opening homopolymer network, which affected the rigidity, hardness and resistance to repeated wiping with methyl ethyl ketone (MEK). In Comparative Example 3, the latent curing agent was not modified with silanoxy groups in its molecular structure. During the curing reaction stage, it could not absorb trace amounts of water vapor to undergo hydrolysis and condensation reactions, and failed to covalently condense and bond with the hydroxyl groups on the surface of the pretreated metal substrate. At the same time, it lacked aliphatic flexible alkyl segments derived from aminosilane, and could not effectively disperse, buffer, and absorb the curing shrinkage internal stress generated by the homopolymerization of epoxy resin. This resulted in coating defects caused by stress concentration, a sharp drop in adhesion, and an expansion of the anti-corrosion spread width. In Comparative Example 6, without the addition of latent curing agent B, the highly polar cyano groups and imidazole rings were missing and settled and enriched in the middle and lower layers of the coating. This resulted in a lack of sufficient active sites to catalyze the ring-opening of the epoxy resin and the construction of a homopolymer network during the curing stage, which significantly reduced the physical corrosion resistance and mechanical properties of the coating.

[0063] In Comparative Examples 4 and 5, the failure to limit the latent curing agent A and latent curing agent B to a specific mass ratio range resulted in an inability to balance the rigidity and toughness of the coating formed by the composite coating used for metal corrosion protection. In Comparative Example 4, the latent curing agent B was relatively insufficient, leading to a weaker ability to catalyze the ring-opening of epoxy resin and the construction of a homopolymer network. In Comparative Example 5, the latent curing agent B was relatively excessive. Although it rapidly carried out nucleophilic attack using its lone pair electrons, the flexible polyurea and polyurethane segments provided by the unsealing of latent curing agent A were relatively insufficient, resulting in a decreased ability to buffer the internal stress of curing shrinkage, stress concentration, and coating defects, which in turn led to a decrease in dry adhesion and an aggravation of corrosion and erosion.

[0064] Example 1 exhibits better overall performance compared to Example 2. This is because the co-solvent ratio in Example 1 is more reasonable. During the leveling stage before curing, the co-solvent undergoes better gradient evaporation, resulting in a stronger driving force of the thermodynamic surface tension gradient. This causes the hydroxyl fluorocarbon resin containing low surface energy fluorocarbon bonds to migrate more thoroughly and spontaneously to the surface and accumulate at the interface, thus enabling the gradient layered protective barrier of the composite coating used for metal corrosion protection to be better constructed.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A composite coating for metal corrosion protection, characterized in that, The composite coating comprises the following raw materials in parts by weight: 45 parts epoxy resin, 4-8 parts flake filler, 8-15 parts latent curing agent, 12-20 parts hydroxyl fluorocarbon resin, 0.5-1.2 parts dispersant, 0.1-0.3 parts polyacrylate, 1-2 parts adhesion promoter, 3-8 parts pigment, and 35-45 parts co-solvent; the latent curing agent comprises latent curing agent A and latent curing agent B, wherein the mass ratio of latent curing agent A to latent curing agent B is 3.5-4.5:1; latent curing agent A is a dissociative latent curing agent prepared by isophorone diisocyanate, aminosilane, and imidazole compounds through asymmetric addition and blocking reaction; latent curing agent B is a 1-cyanoethyl-substituted C1-C4 alkyl imidazole compound.

2. The composite coating for metal corrosion protection according to claim 1, characterized in that, The latent curing agent A is prepared by the following method: M1: Isophorone diisocyanate and aminosilane are dispersed in an aprotic polar solvent, and the isophorone diisocyanate and aminosilane undergo a nucleophilic addition reaction to obtain a dispersion of isocyanate intermediate containing silane groups. M2: The isocyanate intermediate dispersion containing silane groups is mixed with imidazole compounds to allow the isocyanate intermediate containing silane groups to undergo a nucleophilic addition reaction with the imidazole compounds, thereby obtaining latent curing agent A; The molar ratio of isophorone diisocyanate, aminosilane and imidazole compound is 1:(0.98~1):(0.98~1); The aminosilane includes at least one of 3-aminopropyltriethoxysilane and 3-aminopropyltrimethoxysilane; The imidazole compounds include at least one of 2-methylimidazole and 2-ethylimidazole; The aprotic polar solvent includes at least one of propylene glycol monomethyl ether acetate and butyl acetate.

3. The composite coating for metal corrosion protection according to claim 2, characterized in that, The latent curing agent A is prepared by the following method: M1: Disperse isophorone diisocyanate and aminosilane in an aprotic polar solvent and react at 20-30℃ for 2-3 hours to obtain a dispersion of isocyanate intermediate containing silane groups. M2: The isocyanate intermediate dispersion containing silane groups is mixed with an imidazole compound and reacted at 45~60℃ for 3~4h to obtain a latent curing agent A dispersion. The mass fraction of the latent curing agent A in the obtained dispersion is 45%~55%.

4. The composite coating for metal corrosion protection according to claim 1, characterized in that, The sheet-like filler includes at least one of mica powder, sericite, and talc powder.

5. The composite coating for metal corrosion protection according to claim 1, characterized in that, The dispersant includes at least one of polyvinylpyrrolidone, polyhydroxystearic acid, and polyvinyl butyral.

6. The composite coating for metal corrosion protection according to claim 1, characterized in that, The adhesion promoter includes at least one of γ-glycidoxypropyltrimethoxysilane and γ-methacryloxypropyltrimethoxysilane.

7. The composite coating for metal corrosion protection according to claim 1, characterized in that, The pigment includes at least one of titanium dioxide, zinc phosphate, barium sulfate, and carbon black.

8. The composite coating for metal corrosion protection according to claim 1, characterized in that, The latent curing agent B includes at least one of 1-cyanoethyl-2-ethyl-4-methylimidazole and 1-cyanoethyl-2-methylimidazole; The co-solvent is composed of ethyl acetate, xylene, and butyl acetate in a mass ratio of (1~3):(2~4):

4.

9. A method for preparing a composite coating for metal corrosion protection according to any one of claims 1 to 8, characterized in that, Includes the following steps: S1: The epoxy resin, hydroxyl fluorocarbon resin, dispersant, cosolvent (accounting for 50%~80% of the total mass of cosolvent), pigment, flake filler, and adhesion promoter are mixed evenly to obtain the base material; S2: Add the polyacrylate, latent curing agent A, latent curing agent B and the remaining co-solvent to the base material and disperse them evenly to obtain a composite coating for metal corrosion protection.

10. A method for corrosion protection of a metal substrate, characterized in that, Includes the following steps: P1: Provide a composite coating for metal corrosion protection as described in any one of claims 1 to 8; P2: The surface of the metal substrate is pretreated by degreasing, derusting and sandblasting to make the rust removal grade of the metal substrate reach Sa2.5 and the surface roughness Rz is 40~75μm; P3: The composite coating for metal corrosion protection is applied to the surface of the pretreated metal substrate, and after leveling and curing, an anti-corrosion coating is formed on the surface of the metal substrate.

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