A salt spray resistant and wear-resistant epoxy resin-based fastener electrophoretic coating and its preparation method

Abstract

This invention relates to the field of epoxy resin coating preparation technology, specifically to an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating and its preparation method. This invention overcomes the problem in existing technologies where epoxy resin coatings for fasteners cannot simultaneously achieve both corrosion resistance and wear resistance. The invention first synthesizes phosphate-esterified amine and then coats bisphenol A and phenolic epoxy resins with organosilicon. Subsequently, it utilizes phosphate-esterified amine and polyether amine for flexible ring-opening. Next, a KH560-modified multidimensional slurry is grafted into the resin backbone, and a blocked isocyanate crosslinking agent is introduced. After neutralization, it is blended with a cationic polytetrafluoroethylene suspension and subjected to high-shear reverse emulsification to obtain an epoxy resin emulsion, which is then coated and cured. The coating of this invention, through the synergistic effect of phosphate chelation, internal nano-thixotropic support, and surface self-lubrication, is suitable for surface corrosion protection and lubrication of high-strength fasteners in automotive chassis, wind power, and high-end hardware crafts.

Description

Technical Field

[0001] This invention relates to the field of epoxy resin coating preparation technology, specifically to an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating and its preparation method. Background Technology

[0002] Currently, cathodic electrophoretic coatings are widely used in the surface treatment of fasteners due to their excellent penetration and corrosion resistance. However, as existing fastener materials face the dual challenges of harsh mechanical friction and marine climate corrosion during mechanical assembly and long-term outdoor service, the existing epoxy resin electrophoretic coatings for fasteners have significant limitations in practical industrial applications.

[0003] Traditional bisphenol A type epoxy resin has a high crosslinking density after curing, and the coating is inherently brittle. When high-torque assembly is performed using pneumatic tools on hardware fastener production lines, the huge mechanical shear force can easily cause micro-cracks or even peeling off the coating in large pieces. Once the coating is damaged, the anti-salt spray barrier will fail instantly. At the same time, the hard resin debris that peels off acts as abrasive particles between the threads, which will exacerbate the friction loss.

[0004] To address wear resistance and lubrication issues, conventional techniques often employ physical blending, directly adding hard inorganic fillers or hydrophobic lubricants, such as polytetrafluoroethylene (PTFE) micropowder, to the coating formulation. However, in electrophoretic coating systems, this physical modification has fatal flaws. High-density, low-surface-energy powders are prone to agglomeration, sedimentation, and demulsification in the bath, resulting in extremely poor bath stability. Furthermore, the agglomeration of the inorganic phase disrupts the density of the resin network, opening permeation channels for water, oxygen, and chloride ions, leading to a significant decline in corrosion resistance. In addition, uneven filler distribution causes rough surfaces on finished products, resulting in a loss of high-decorative appearance, and causes drastic fluctuations in the torque coefficient of fasteners. Instability in the torque coefficient directly leads to different axial clamping forces for the same torque during assembly line work, causing fastener stripping, loosening, or breakage, resulting in assembly seizure or preload instability. Simultaneously, conventional resins are prone to edge collapse during high-temperature baking and curing, creating corrosion blind spots at the sharp corners of the threads.

[0005] In summary, existing technologies can solve the corrosion resistance of fastener electrophoretic coatings in a short time through simple physical blending. However, excessive blending of inorganic particles can lead to an imbalance in the overall performance of the coating. Existing technologies still suffer from the problem that epoxy resin electrophoretic coatings cannot simultaneously achieve both corrosion resistance and wear resistance.

[0006] Therefore, an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating and its preparation method are proposed. Summary of the Invention

[0007] The purpose of this invention is to provide an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating and its preparation method. This invention first synthesizes phosphate-esterified amine and modifies bisphenol A and phenolic epoxy resins with organosilicon; then, it utilizes phosphate-esterified amine and polyether amine for flexible ring-opening; next, it grafts KH-560-modified multidimensional slurry into the resin backbone, introduces a blocked isocyanate crosslinking agent, neutralizes it, and blends it with a cationic polytetrafluoroethylene suspension. After high-shear reverse emulsification, an epoxy resin emulsion is obtained, which is then coated and cured. This coating, through the synergistic effect of phosphate chelation, internal nano-thixotropic support, and surface self-lubrication, is suitable for corrosion protection and lubrication of high-strength fasteners in automotive chassis, wind power, and high-end hardware crafts.

[0008] To achieve the above objectives, the present invention provides the following technical solution: The mass fractions of the modified mixed slurry, phosphorylated amine solution, and coated PTFE suspension of this invention are all calculated based on the dry weight of the effective solid components (i.e., deducting the mass of solvents such as anhydrous ethanol, deionized water, or propylene glycol methyl ether). In actual batching, the weights are calculated based on the actual solid content of the prepared slurry or suspension. Unless otherwise specified, the mass fractions of the remaining components of this invention are all calculated based on the effective mass of the pure substance.

[0009] This invention provides a method for preparing an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating, comprising the following preparation steps in parts by weight: Dilute the epoxy electrophoretic emulsion with deionized water to control the pH value to 5.8-6.2 and the conductivity to 1000-1500 μS / cm, obtaining a working bath solution with a solid content of 12%-16%. Continuously cycle and mature the solution at 28℃-30℃ with a turnover frequency of 6-8 times / h for 24-48 hours to ensure full expansion of macromolecular chains and the formation of uniform electrophoretic micelles between nanomaterials and PTFE in the bath solution. The carbon steel fasteners are then subjected to standard degreasing, washing, surface conditioning, and zinc phosphating treatment to generate a dense crystalline phosphating film (film weight 2-3 g / m²). Finally, the fasteners are washed with pure water with a conductivity of less than 10 μS / cm to obtain the treated fasteners. Use the treated fasteners as cathodes, with the anode being a 316L stainless steel plate, and the electrode ratio (anode area: cathode area) controlled at 1:2-1:4. The bath temperature is 28℃-32℃. A soft-start mode is used. Within 30 seconds before power-on, the DC voltage is uniformly increased from 0V to the working voltage of 120V-140V to prevent excessive starting current from damaging the coating. Then, a constant voltage deposition is maintained at 120V-140V for 110s-140s to obtain a target dry film thickness of 15μm-20μm. After the fasteners are removed from the bath, they must be washed with ultrafiltrate recovery water and two rounds of pure water within 1 minute to thoroughly remove the physically attached paint and prevent water droplets and secondary sagging after baking. The fasteners are cured in three stages: the first stage is pre-baked in an 80-90℃ oven for 8-12 minutes; the second stage is heated to 110-120℃ and held for 8-12 minutes; and the third stage is heated to 130-140℃ and held for 15-25 minutes to obtain the fastener electrophoretic coating.

[0010] Preferably, the preparation of the epoxy electrophoretic emulsion includes the following steps: 30-40 parts of bisphenol A type epoxy resin E-20 and 10-15 parts of phenolic epoxy resin F-44 are added to a reaction vessel and heated to 105-115℃ for melting and dehydration. After cooling to 80-90℃, 0.1%-0.3% of the total resin amount of the catalyst dibutyltin dilaurate is added. 2-5 parts of polymethylphenylsiloxane intermediate are added dropwise at a uniform rate and the reaction is maintained at this temperature for 1.5-2.5 hours to undergo de-alcoholization polycondensation to obtain the epoxy precursor. Subsequently, phosphate-esterified amine solvent is added dropwise to the epoxy precursor at a uniform rate. 20-25 parts of liquid were reacted at 80-90℃ for 0.8-1.5 h, followed by the addition of 5-8 parts of polyetheramine D-400 for a flexible ring-opening reaction for 1.5-2.5 h to obtain a modified precursor. The modified precursor was then cooled to 65-75℃, and 3-6 parts of modified mixed slurry were added. The mixture was kept warm and stirred for 1-2 h, followed by the addition of 20-25 parts of crosslinking agent and 0.3-0.8 parts of bismuth neodecanoate. The mixture was stirred evenly at 200-400 rpm for 0.5-1 h to obtain a hybrid epoxy resin solution. Finally, the hybrid epoxy resin solution was cooled to 40-50℃. At 0℃, slowly add 2-5 parts of glacial acetic acid to neutralize and form salts, stirring for 20-40 minutes. Switch the equipment to a high-shear emulsification state at 3000-4000 rpm. First, slowly add 2-5 parts of coated PTFE suspension and disperse evenly. Then, add deionized water dropwise at a uniform rate to initiate the phase reversal of the system. Adjust the total solids content to 30%-35% to obtain an epoxy electrophoretic emulsion. The epoxy electrophoretic emulsion is a gray-black or dark gray-brown translucent to opaque homogeneous emulsion with a viscosity of 80-200 mPa·s at 25℃. The particle size exhibits a bimodal distribution. The main peak is concentrated at 80-120 nm, which is the organic nano micelles formed by the crosslinking agent encapsulated by the modified epoxy resin backbone; the secondary peak is at 1.5-2.5 μm (this is the mixed phase of PTFE and inorganic nano particles co-encapsulated by micelles), and the MEQ value is 35-45 mmol / 100g. The emulsion has a slightly acidic odor, no stratification, and no obvious coarse particle precipitation. After centrifugation at 3000 rpm for 30 min, the bottom precipitation rate is less than 2%, indicating that the nano inorganic phase and PTFE have been firmly encapsulated by the resin micelles and have not been released.

[0011] Preferably, the preparation of the modified mixed slurry includes the following process: 8-12 parts of graphene oxide, 4-6 parts of nano-silica, and 4-6 parts of molybdenum disulfide nanosheets were dispersed in a mixed solvent of anhydrous ethanol and deionized water. The mixture was treated with ultrasound at 35-45 kHz for 1.5-2.5 h. Glacial acetic acid was added to adjust the pH of the system to 4.5-5.5. Then, 5-8 parts of silane coupling agent KH-560 were slowly added dropwise. The mixture was heated to 70-80℃, refluxed, and mechanically stirred at 300-500 rpm for 10-14 h. Under these acidic reflux conditions, the methoxy groups of the silane coupling agent were completely hydrolyzed to generate silanol groups, which then underwent dehydration condensation with the hydroxyl groups on the surfaces of the inorganic phases such as graphene oxide and silica. A stable siloxane coating layer is formed; simultaneously, some of the epoxy groups that open due to hydrolysis are converted into vicinal diol structures, which undergo in-situ crosslinking and copolymerization with the isocyanate or amino groups in the system during subsequent reactions. After the reaction is completed, the mixture is centrifuged, washed, vacuum dried, and redispersed with an appropriate amount of propylene glycol methyl ether to obtain a modified mixed slurry. The modified mixed slurry is a uniform viscous paste that is dark brown to grayish-black, with a solid content (effective mass fraction) of 15%-25% and a viscosity of 2000-4500 mPa·s at 25℃. It exhibits extremely significant thixotropy and a particle size distribution of 0.8-2.0 μm. The measured value is its equivalent hydrodynamic diameter after it has fully expanded in the solvent. Preferably, the preparation of the coated PTFE suspension includes the following steps: PTFE micropowder with an average particle size of 1-3 μm was added to deionized water, and octadecyltrimethylammonium chloride was added at a ratio of 5%-8% of PTFE mass. The mixture was then subjected to shear dispersion at 2500-3500 rpm and synergistic ultrasonic treatment at 35-45 kHz for 0.8-1.5 h in a water bath at 55-65℃ to obtain a coated PTFE suspension. The coated PTFE suspension was a milky white uniform suspension with no obvious particle feel, a solid content (effective mass fraction) of 15%-25%, a viscosity of 50-150 mPa·s at 25℃, a particle size distribution of 1.2-2.5 μm, and a Zeta potential of 35 mV to 55 mV.

[0012] Preferably, the preparation of the phosphorylated amine solution includes the following steps: Add 15 parts of tetrahydrofuran to the reactor, heat to 50-60℃, add 1.5 parts of polyphosphoric acid and stir to dissolve at 200 rpm, then add 5 parts of diethanolamine dropwise at a uniform rate under mechanical stirring, controlling the dropping rate so that the exothermic temperature of the system does not exceed 80℃. After the addition is complete, keep the reaction at 70℃ for 3 hours to obtain a phosphorylated amine solution. Before preparing the epoxy electrophoretic emulsion, remove tetrahydrofuran by vacuum distillation and dissolve it in propylene glycol methyl ether to obtain the phosphorylated amine solution; or directly participate in the resinification reaction and remove it before emulsification. The phosphorylated amine solution is a colorless to slightly yellow transparent viscous liquid with a solid content (effective mass fraction) of 25%-35%, a viscosity of 150-300 mPa·s at 25℃, and a pH of 7.5-8.5.

[0013] The present invention also provides an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating, comprising a working tank liquid and a fastener to be treated; the working tank liquid is an epoxy electrophoretic emulsion with a solid content of 12%-15%, comprising epoxy resin, phenolic epoxy resin, polymethylphenylsiloxane, phosphoric acid esterified amine solution, polyetheramine, modified mixed slurry, crosslinking agent and coated polytetrafluoroethylene suspension.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In existing technologies, high-density inorganic fillers and hydrophobic lubricants are prone to sedimentation and demulsification in electrophoresis tanks. This invention utilizes the chemical bridging effect of KH-560 to graft multidimensional nanomaterials onto epoxy resin macromolecular chains in situ, while simultaneously using cationic surfactants to impart positive charges to PTFE micropowders. Under the high shear force of reverse emulsification, the organic / inorganic phases are stably encapsulated within nanoscale micelles, achieving anti-centrifugal sedimentation performance for several months, extending the working tank liquid circulation and turnover cycle, and solving the pain points of continuous coating in industrial production lines.

[0015] 2. To address the problem of large-scale coating peeling during fastener assembly and disassembly, this invention introduces phosphoric acid esterified amine and polyether flexible segments into the epoxy resin matrix. During gradient curing, the phosphate active groups form strong multidentate chelate coordination bonds with the iron ions or crystalline phosphating film on the fastener surface, upgrading conventional physical adhesion to chemical cross-boundary bonding. At the same time, the polyether segments provide excellent deformation stress absorption capacity, enabling the coating to maintain good adhesion even under repeated impacts from a pneumatic wrench, with an extremely low mechanical shear peeling rate.

[0016] 3. This invention overcomes the limitations of traditional coatings in balancing hardness and lubrication, constructing a multi-dimensional tribological network that combines both hardness and softness. Positively charged PTFE self-assembles and migrates to the coating surface during co-electrodeposition and leveling, forming a continuous transfer lubricating film with low surface energy, stabilizing the assembly torque coefficient. Simultaneously, embedded molybdenum disulfide nanosheets and nano-silica construct an inorganic micro-bearing support framework, effectively preventing rapid wear of the pure polymer film under repeated heavy loading and unloading, achieving long-term stable torque consistency.

[0017] 4. Conventional electrophoretic coatings are prone to a sudden drop in viscosity during high-temperature baking, leading to coating collapse and exposure at the sharp corners of the threads. This invention introduces grafted modified graphene oxide and employs a three-stage stepped curing process after coating. In the second stage leveling temperature zone, the two-dimensional sheet material and organic network rapidly construct microscopic thixotropic stress, firmly locking the molten resin and improving edge coverage. Combined with the labyrinthine dense barrier layer formed by graphene oxide and nano-silicon, the penetration of water, oxygen, and chloride ions is delayed, resulting in significantly improved corrosion resistance. In addition, the low-temperature curing process also avoids further damage to the resin matrix. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of obtaining fastener electrophoretic coating in Embodiment 1 of the present invention.

[0019] Figure 2 The image shows a radar comparison of the torque coefficient stability of the fastener coatings prepared in Example 1 of the present invention and Comparative Examples 9 and 10 during five consecutive tightening-unloading cycles. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise specified, all chemical reagents and raw materials used in the embodiments and comparative examples of this invention are commercially available. Specifically, the bisphenol A epoxy resin E-20 has an epoxy equivalent of 450-500 g / eq and a softening point of 64-76°C; the phenolic epoxy resin F-44 has an epoxy equivalent of 170-180 g / eq. g / eq, average functionality is 3.6; KH-560 is γ-(2,3-epoxypropoxy)propyltrimethoxysilane; polyetheramine D-400 is an amino-terminated polyether with a molecular weight of 400 and an amine equivalent of 115 g / eq; polymethylphenylsiloxane is a methoxy intermediate with a molecular weight of 1000-1500 and a viscosity of 200 mPa·s at 25℃; octadecyltrimethylammonium chloride CAS number 112-03-8 is a cationic surfactant that imparts a positive charge to PTFE; the effective phosphorus oxide content of polyphosphoric acid is above 80%, and it reacts with amines as a strong phosphorylation reagent; PTFE micro powder is a low surface energy solid lubricant with an average particle size of 1-3 μm; graphene oxide has a two-dimensional sheet structure with a single layer thickness of 0.8 nm-1.2 nm, a single layer ratio of not less than 80%, and a transverse sheet diameter of... The particle size is 0.5μm-2.5μm, the oxygen content (O / C atomic ratio) is 0.3-0.5, and the specific surface area needs to be greater than 400m² / g; the nano-silica is an amorphous zero-dimensional spherical nanoparticle with a particle size of 10nm-30nm and a specific surface area of ​​150m² / g-300m² / g; the molybdenum disulfide has a two-dimensional layered structure with a layer thickness of 1.5nm-5nm and a transverse sheet diameter of 0.2μm-1.5μm; the crosslinking agent of this invention uses hexamethylene diisocyanate trimer as the core framework and 3,5-dimethylpyrazole as a blocking agent to achieve low-temperature curing. The crosslinking agent is formed by reacting hexamethylene diisocyanate trimer and 3,5-dimethylpyrazole in a 1:1 molar ratio of isocyanate groups to active hydrogen in methyl isobutyl ketone solvent at 70-75℃ until the infrared spectrum reaches 2270cm. -1 The sample was prepared by completely eliminating the -NCO characteristic absorption peak at the point of dissolution, with a desealing temperature of 110-115℃ and a solid content of 70%.

[0022] Please see Figures 1 to 2 This invention provides an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating and its preparation method. The technical solution is as follows: Example 1

[0023] 15 parts of tetrahydrofuran were added to the reactor, and the temperature was raised to 60°C. 1.5 parts of polyphosphoric acid were added and stirred at 200 rpm to dissolve. 5 parts of diethanolamine were added dropwise at a uniform rate under mechanical stirring, and the dropping rate was controlled to stabilize the exothermic temperature of the system at 75°C. After the addition was completed, the reaction was kept at 70°C for 3 hours to obtain a phosphoric acid esterified amine solution. Before preparing the epoxy electrophoretic emulsion, tetrahydrofuran was removed by vacuum distillation, and the solution was dissolved in propylene glycol methyl ether to obtain a phosphoric acid esterified amine solution with a solid content of 30%. Ten parts of graphene oxide, five parts of nano-silica, and five parts of molybdenum disulfide nanosheets were dispersed in a mixed solvent of anhydrous ethanol and deionized water. The mixture was treated with ultrasound at a frequency of 40 kHz for 2 h. Glacial acetic acid was added to adjust the pH of the system to 4.5. Then, six parts of silane coupling agent KH-560 were slowly added dropwise. The mixture was heated to 80 °C, refluxed, and mechanically stirred at 400 rpm for 12 h. After the reaction was completed, the mixture was centrifuged, washed, vacuum dried, and redispersed with an appropriate amount of propylene glycol methyl ether to obtain a modified mixed slurry. PTFE micro powder was added to deionized water, and octadecyltrimethylammonium chloride was added at a ratio of 6% of PTFE mass. The mixture was then subjected to shear dispersion at 3000 rpm and synergistic treatment with ultrasonic waves at 40 kHz for 1.5 h at 60℃ water bath to obtain a coated PTFE suspension. 35 parts of bisphenol A type epoxy resin E-20 and 12 parts of phenolic epoxy resin F-44 were added to a reactor and heated to 110℃ for melting and dehydration. After cooling to 80℃, 0.2% of the total resin amount of the catalyst dibutyltin dilaurate was added, and 3 parts of polymethylphenylsiloxane intermediate were added dropwise at a uniform rate. The reaction was maintained at this temperature for 2 hours to undergo de-alcoholization polycondensation to obtain an epoxy precursor. Subsequently, a phosphorylated amine solution was added dropwise at a uniform rate to the modified epoxy precursor, and the reaction was carried out at 80℃ for 1 hour. Then, 6 parts of polyetheramine D-400 were added to carry out a flexible ring-opening reaction for 2 hours to obtain a modified precursor. The modified precursor was then cooled to 70℃, and 4 parts of modified mixed... The slurry was kept at a constant temperature for 2 hours. Then, 22 parts of blocked HDI trimer crosslinking agent and 0.5 parts of bismuth neodecanoate were added and mixed evenly at 300 rpm for 1 hour to obtain a hybrid epoxy resin solution. Finally, the hybrid epoxy resin solution was cooled to 40°C, and glacial acetic acid was slowly added dropwise to neutralize and form a salt. The mixture was stirred for 30 minutes. The equipment was switched to a high-shear emulsification state at 3500 rpm. First, 3 parts of coated PTFE suspension were slowly added and dispersed evenly. Then, deionized water was added dropwise at a uniform rate to initiate the phase reversal of the system. The rate of adding deionized water was controlled to maintain the system temperature below 50°C. The total solids content was adjusted to 35% to obtain an epoxy electrophoretic emulsion.

[0024] Deionized water was added to the epoxy electrophoretic emulsion to dilute it, resulting in a working bath solution with a solid content of 15%. The solution was continuously circulated and matured for 36 hours at 28℃-30℃ with a turnover rate of 6-8 times / h to form uniform electrophoretic micelles. Carbon steel fasteners were then subjected to standard degreasing, washing, surface conditioning, and zinc phosphating to generate a dense crystalline phosphating film. Finally, they were washed with pure water with a conductivity of less than 10 μS / cm to obtain the treated fasteners. These treated fasteners were used as cathodes, with 316L stainless steel plates as anodes. The electrode ratio (anode area: cathode area) was controlled at 1:3, and the bath temperature was maintained at 30℃ using a soft-start mode. Within 30 seconds before power-on, the DC voltage is uniformly increased from 0V to the working voltage of 120V; then, constant voltage deposition is maintained at 120V for 120 seconds to obtain a target dry film thickness of 15μm-20μm; after the fasteners are removed from the tank, they must be washed with ultrafiltrate recovery water and two pure water washes within 1 minute to thoroughly clean the surface paint physically adhering to it, preventing water droplets and secondary sagging after baking; then, the fasteners are cured in three stages: the first stage is pre-baked in an 85℃ oven for 10 minutes, the second stage is heated to 115℃ and held for 10 minutes, and the third stage is heated to 135℃ and held for 15 minutes to obtain the fastener electrophoretic coating.

[0025] Examples 2-4 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 1.

[0026] Table 1. Parameter changes in Examples 1-4 Graphene oxide mass dosage / part 10 8 9 12 Nano silica mass dosage / part 5 4 4.5 6 Molybdenum disulfide mass dosage / parts 5 4 6 5 KH-560 dosage (per serving) 6 5 7 8 Reflux stirring speed / rpm 400 300 450 500 Reflux mixing time / h 12 10 11 14 Epoxy Resin E-20 Mass Dosage / Part 35 30 32 40 Phenolic epoxy resin F-44 mass dosage / part 12 10 14 15 Mass dosage of polymethylphenylsiloxane / part 3 2 4 5 Polyetheramine mass dosage / part 6 5 7 8 Mass of phosphate-esterified amine solution used / part 20 15 18 25 Modified mixed slurry mass dosage / part 4 3 4.5 6 Mass of coated PTFE suspension (parts) 3 2 4 4.5 Trimeric crosslinking agent (mass amount / parts) 22 20 24 25 Epoxy electrophoretic emulsion solid content / % 35 30 32 34 Liquid-solid content in working tank / % 15 12 14 16 First stage pre-baking temperature / ℃ 85 80 82 90 Second stage insulation and leveling temperature / ℃ 115 110 112 120 Third-stage thermal insulation cross-linking temperature / ℃ 135 130 132 140 Comparative Example 1 is the same as Example 1, except that the phosphorylated amine pre-synthesized from polyphosphoric acid and diethanolamine is not used, and the ring-opening reaction is carried out with an equal amount of diethanolamine, while the rest of the process remains unchanged.

[0027] Comparative Example 2 is the same as Example 1, except that the flexible segment component polyetheramine D-400 is not added in the ring-opening reaction stage, while the rest of the process remains unchanged.

[0028] Comparative Example 3 is the same as Example 1, except that the low surface energy pre-modification step of epoxy precursor is not used, while the rest of the process remains the same.

[0029] Comparative Example 4 is the same as Example 1, except that silane coupling agent KH-560 is not added in the preparation of the modified mixed slurry, while the rest of the process remains unchanged.

[0030] Comparative Example 5 is the same as Example 1, except that in the preparation of the coated PTFE suspension, the cationic surfactant coating treatment is not used, while the rest of the process remains unchanged.

[0031] Comparative Example 6 is the same as Example 1, except that the three-stage step curing is not used, and it is directly placed in a 135°C oven for 35 minutes. The rest of the process remains the same.

[0032] Comparative Example 7 is the same as Example 1, except that graphene oxide is not added to the modified mixed slurry, and 10 parts of graphene oxide are replaced with an equal mass of inactive talc powder, while the rest of the process remains unchanged.

[0033] Comparative Example 8 is the same as Example 1, except that phenolic epoxy resin F-44 is not added, and an equal amount of bisphenol A type epoxy resin E-20 is added, while the rest of the process remains unchanged.

[0034] Comparative Example 9 is the same as Example 1, except that no coated PTFE component is added to the epoxy resin coating system, while the rest of the process remains unchanged. Comparative Example 10 is the same as Example 1, except that molybdenum disulfide is not added to the modified mixed slurry, and graphene oxide is replaced in equal parts by mass, while the rest of the process remains unchanged.

[0035] Comparative Example 11 is the same as Example 1, except that no silica component is added to the modified mixed slurry, and an equal mass fraction is replaced with graphene oxide, while the rest of the process remains unchanged.

[0036] Experimental Example 1: Adhesion Performance Test The epoxy electrophoretic emulsions prepared in Examples 1-4 and Comparative Examples 1-5 were subjected to stability tests, and the adhesion performance of the prepared fastener electrophoretic coatings was tested. According to GB / T 6753.3-1986, 10 mL of the epoxy electrophoretic emulsion obtained after reverse emulsification was accurately measured and injected into a graduated precision centrifuge tube, and the total mass was recorded. The centrifuge tube was symmetrically placed in a laboratory high-speed centrifuge, the speed was adjusted to 3000 rpm, and continuous high-speed centrifugation was performed for 30 min. After centrifugation, the tube was removed, and the upper emulsion suspension was carefully poured off, retaining the dense precipitate on the bottom and wall of the tube (mainly free or aggregated inorganic fillers and solid PTFE). A small amount of deionized water was added to the tube for washing and drying to constant weight. The mass of the non-volatile precipitate was weighed, and its percentage of the total non-volatile solid mass of the original emulsion was calculated, which is the centrifugal sedimentation rate. A laboratory micro-electrophoresis tank with a bottom circulation pump and an overflow weir, with a volume of 10... L was used to simulate the bottom push flow and surface overflow state of the industrial bath solution. The prepared high-solids epoxy electrophoretic emulsion was slowly diluted with deionized water to adjust the effective solids content of the working bath solution to 15%. The initial state of the working bath solution was tested. The circulation pump valve was adjusted to control the circulation and turnover frequency of the bath solution at 6-8 times / hour. The bath solution temperature was kept constant at 28℃±1℃ throughout the test. The bath was kept open during the test. The solids content was measured weekly, and electrophoretic emulsion was added according to the consumption to maintain the solids content within a dynamic range of 13%-15%. The pH value and conductivity change curves of the bath solution were recorded daily using high-precision instruments. When continuous circulation of the bath solution resulted in frequent coating defects and abnormal electrodeposition, the cumulative number of operating days at this point was recorded. This was done according to GB / T... According to 9286-2021, on flat areas of fasteners (such as the hexagonal face of bolt heads or unthreaded shanks), use a standard six-flute crisscross cutter, keeping the cutter tip perpendicular to the film layer, to uniformly draw crisscrossing grid lines that penetrate the coating to the substrate. Draw 6 lines in each direction, with a spacing of 1 mm depending on the film thickness (15-20 μm), forming 25 micro squares. Brush several times along the diagonal direction with a soft brush, then firmly adhere standard-compliant transparent pressure-sensitive adhesive tape to the crisscrossed area. After stabilizing, apply the tape at an angle of approximately 60° within 1 second. The tape was peeled off, and the integrity of the paint film at the edge of the grid cut was observed by comparing it with the standard chart and rated as grade 0-5. Using a standard pneumatic wrench and a hard socket, an overtightening torque of 300 N·m was applied to the M10 bolt and driven into the standard high hardness test base. Then it was unscrewed in the opposite direction. The tightening-unscrewing was repeated 5 times. A panoramic photograph of the thread root and side friction pair was taken using an ultra-microscopic stereomicroscope. By comparing the images before and after friction, the percentage of area of ​​color difference change was calculated to obtain the mechanical shear peeling rate. The test results are shown in Table 2.

[0037] Table 2 Test results of the examples and comparative examples Example 1 1.2 204 Level 0 1.1 Example 2 1.5 195 Level 0 1.4 Example 3 1.8 186 Level 0 1.7 Example 4 1.9 183 Level 0 1.3 Comparative Example 1 2.3 171 Level 2 14.5 Comparative Example 2 2.1 162 Level 1 12.6 Comparative Example 3 5.4 117 Level 1 6.8 Comparative Example 4 15.6 42 Level 1 9.2 Comparative Example 5 34.7 9 Level 2 11.9 As shown in Table 2, the adhesion performance and emulsion stability of the fastener electrophoretic coating obtained in the comparative example, through adjustments to the components and process, are significantly different from those in the example. In the example, the introduction of phosphated amine into the epoxy skeleton can achieve polydentate chelation with the iron ions of the matrix metal through chemical reaction, forming a cross-interface molecular stitching effect at the interface of cross-linking and curing, thus bonding the coating and the crystalline phosphate film together. In addition, the in-situ grafting reaction of polyetheramine D-400 and inorganic multidimensional nanomaterials grafted with KH-560 occurred during the preparation process. The polar and flexible segments after grafting not only endow the film with excellent toughness, but the low surface energy groups and positive charges in its molecular structure also significantly improve the electrorepulsion and free energy of the micromicelles, which is beneficial to the subsequent high-stability processing and electrophoretic coating process.

[0038] In Comparative Example 1, the lack of interfacial chemical bridging by phosphated amine means the coating and carbon steel substrate rely solely on physical molecular chain entanglement and van der Waals forces during curing. This results in significant interfacial shear stress during the curing process, and the physical entanglement is insufficient to maintain structural integrity, leading to low interlayer adhesion and large-area delamination. In Comparative Example 2, the increased proportion of rigid epoxy resin backbone leads to a lack of flexible deformation segments within the crosslinking channels. This results in stress release characteristics that do not fully match the complex metal substrate. The harder coating generates more complex internal stress concentrations under high-torque impacts. Furthermore, subsequent mechanical tests show significant stress concentration at the interface due to the high deformation resistance of the high-crosslink density functional areas, weakening the overall shear peel strength. In Comparative Example 3, the absence of a low surface energy pre-coating step indicates a higher proportion of purely polar polymer components in the resin. This slightly alters the rheological rate and water dispersion viscosity of the resin micelles, affecting the molecular chain diffusion depth during reverse emulsification with the aqueous phase and the presence of siloxanes. Differences in the dosage of the agent affect the initial dispersion stability and particle size distribution. However, in the later stages, excessive surface tension leads to micelle micro-flocculation and a decrease in bath life. In Comparative Example 4, without the addition of silane coupling agent KH-560, inorganic fillers such as graphene oxide and nano-silica cannot undergo grafting reactions with the subsequent epoxy resin backbone. As a result, the nanoparticles exist in the emulsion system only through physical encapsulation. Due to differences in specific surface area and surface energy, the inorganic phase is prone to agglomeration and sedimentation in long-period mechanical shear flow fields. The separation caused a sharp increase in the centrifugal sedimentation rate and a significant decrease in the working tank liquid circulation cycle. In Comparative Example 5, the lack of cationic surfactant modification resulted in the extremely hydrophobic PTFE micropowder lacking the coating of positively charged head groups. Its colloidal rheological properties were completely contrary to those of the positively charged cathodic epoxy emulsion, making it unable to be effectively wetted by the aqueous phase and causing severe phase separation and unstable flow during distribution and merging. This not only led to rapid demulsification and sedimentation of the system, but also caused the large free PTFE particles remaining inside the coating to form microscopic defects, destroying the adhesion continuity of the coating.

[0039] Experiment Example 2: Corrosion and Wear Resistance Test The fastener electrophoretic coatings prepared in Examples 1-4 and Comparative Examples 4-11 were subjected to corrosion resistance and wear resistance tests. According to GB / T 1771-2007, the cured standard carbon steel bolts were hung in a salt spray test chamber, with the sample placed at an angle of 15°-2° between the main thread surface and the vertical direction. The temperature inside the spray chamber was strictly controlled at 35°C, and a sodium chloride solution with a concentration of 50 g / L and a pH value of 6.5-7.2 was used for continuous spraying every 24 hours. Inspect the fastener surface, focusing on the thread root, flank, and shank. Record the cumulative test hours when the first characteristic, indelible red rust is observed with the naked eye. Randomly select fasteners after electrophoretic curing and use a precision cutter to cut them transversely along the thread axis. Place the cut thread tip section in an epoxy resin mold for cold mounting and curing. After curing, perform rough grinding, fine grinding, and polishing until the cross-section shows a clear, scratch-free metal-coating interface. Place the mounted sample under a metallographic microscope at 200-500x magnification and use image measurement software to measure the paint film thickness T1 at the thread tip and T2 on the flat thread flank. The edge coverage rate is calculated using the formula: Calculate the single-point value using T1 / T2×100%; according to GB / T16823.3-2010, assemble the electrophoretic coated fastener, standard washer, and standard nut into a connection pair, install them on a microcomputer-controlled fastener tightening characteristic testing machine, and set the drive speed to a uniform speed of 5 rpm; start loading, and when the axial force (clamping force) reaches 70% of the fastener's guaranteed load, automatically collect the tightening torque T and the actual axial force F at this time; calculate the initial K value according to the formula K=T / Fd (d is the nominal diameter of the thread); subsequently, without changing the connection pair, repeat the tightening-unloading cycle 5 times on the same bolt at the same speed, collect 5 sets of K value data, calculate its sample standard deviation, and use it to evaluate the fatigue wear resistance of the self-lubricating film; according to GB / T From 1768 to 2006, a standard flat-laid steel plate synchronous electrophoresis sample method was adopted to replace the abrasion tester. A standard carbon steel plate of the same material as the bolt (100mm×100mm×1mm) was subjected to cathodic electrophoresis and three-stage curing in the same electrophoresis tank and under the same process as the fastener. The initial mass V0 of the sample was weighed with a balance of 0.01%. The sample was fixed on the abrasion machine. CS-10 rubber hard grinding wheels were selected, and a constant load of 500g was applied to each wheel. After the instrument was started and rotated for 500 revolutions, the sample was removed, the surface debris was cleaned with a brush, and the mass V1 of the sample after abrasion was weighed again. The abrasion amount was calculated by the formula V=V0-V1. Three samples were tested and the abrasion amount was output. The test results are shown in Table 3.

[0040] Table 3. Test results of the examples and comparative examples Example 1 1200 52.3 0.124±0.003 0.004 12.1±0.4 Example 2 1150 48.7 0.131±0.005 0.006 14.8±0.5 Example 3 1180 50.4 0.128±0.004 0.005 13.9±0.3 Example 4 1240 46.9 0.132±0.006 0.007 15.3±0.8 Comparative Example 4 720 45.1 0.135±0.007 0.014 26.2±1.1 Comparative Example 5 620 43.6 0.147±0.008 0.018 29.4±1.6 Comparative Example 6 1160 15.2 0.130±0.005 0.011 21.5±0.9 Comparative Example 7 680 22.4 0.134±0.006 0.012 24.7±1.2 Comparative Example 8 850 41.5 0.139±0.007 0.009 18.6±0.7 Comparative Example 9 1020 51.9 0.285±0.016 0.045 48.3±2.5 Comparative Example 10 1140 49.3 0.146±0.008 0.023 32.1±1.4 Comparative Example 11 930 40.8 0.141±0.007 0.019 35.2±1.9 As shown in Table 3, the corrosion resistance and wear resistance of the fastener electrophoretic coatings obtained in the comparative examples, obtained by adjusting the components and processes, are significantly different from those in the examples. In Comparative Example 4, due to the absence of the silane coupling agent KH-560, there is a lack of chemical bonds between the inorganic composite particles and the epoxy resin backbone, relying solely on physical encapsulation. During the curing process, interlayer segregation at the interface between the inorganic and organic phases easily occurs, leading to abrasive wear of the particles under stress. Consequently, coating wear is aggravated, and the uneven distribution of the inorganic phase reduces the continuity of corrosion protection, resulting in a significant decline in salt spray resistance. In Comparative Example 5, the lack of cationic surfactant coating results in the extreme hydrophobic PTFE micropowder lacking surface charge coating. During electrodeposition, the rheological properties of the PTFE incompatibility with the epoxy emulsion increased the separation resistance of large PTFE particles during subsequent baking, leading to severe segregation defects and stress concentration within the coating film. This disrupted the continuity and density of the resin network, resulting in a significant deterioration in the coating's corrosion resistance and resistance to mechanical shear fracture. In Comparative Example 6, a one-step high-temperature curing method was used. The sudden high temperature caused the resin viscosity to drop to its lowest point during melting, and the thixotropic latching network formed by the two-dimensional inorganic layers during the gradient heating stage was missing, resulting in severe melt collapse of the coating at the fastener edges and reduced edge coverage. Simultaneously, the excessively rapid reaction rate deprived the PTFE self-lubricating components of time to migrate to the surface, preventing them from forming the ideal self-lubricating components. The lubricated surface caused significant fluctuations in the torque coefficient during repeated loading and unloading. In Comparative Example 7, the absence of graphene oxide and the lack of a high aspect ratio two-dimensional layered thixotropic framework resulted in a significantly lower yield stress of the coating film during heating and melting, leading to edge collapse and impaired edge coverage. Simultaneously, the lack of a physical barrier provided by the layered graphene oxide structure allowed for rapid penetration of corrosive media into the filler gaps, drastically shortening the salt spray resistance time. In Comparative Example 8, the resin network degenerated from multifunctional crosslinking to a pure E-20 bifunctional linear extension, resulting in a decrease in overall crosslinking density and aromatic ring density. Its melt viscosity decreased slightly during heating and flowing, having a minor negative impact on barrier resistance, reducing corrosion resistance, and increasing wear. In Comparative Example 9, the removal of the coated PTFE component in the coating system resulted in severe dry friction between the metal friction pairs due to the loss of the continuous self-lubricating transfer film with low surface energy on the coating surface. This led to a significant increase in the initial torque coefficient, and the resin backbone was rapidly worn down under intense tool shearing forces, resulting in high wear and large torque fluctuations after repeated loading and unloading. In Comparative Example 10, the absence of molybdenum disulfide component preserved the surface flexibility lubrication of PTFE. However, under high clamping force extrusion friction conditions, the loss of the micro-bearing support function of molybdenum disulfide, an inorganic solid lubricant with high load-bearing capacity, caused the PTFE lubricating film to be rapidly worn through. This resulted in significant fluctuations in the standard deviation of the friction coefficient after multiple loading and unloading operations, and increased wear.In Comparative Example 11, the removal of nano-silica components resulted in the loss of physical intercalation and isolation media between graphene oxide and two-dimensional sheet structures such as molybdenum disulfide. This led to severe secondary stacking and agglomeration of the two-dimensional materials during bath operation and electrophoretic film formation. Localized micro-agglomeration not only created stress defects, reducing wear and scratch resistance, but also caused uneven flow, affecting edge leveling and shape retention during curing, thus reducing edge coverage and corrosion resistance time.

[0041] from Figure 2 The results show that, from the inside out, Examples 1, 10, and 9 are respectively. It can be clearly seen that the K-value data points of Example 1 form a regular and compact inner polygon in 5 loading and unloading cycles; its initial torque coefficient average is as low as 0.124, and the standard deviation after 5 repeated loading and unloading cycles is only 0.004. This indicates that by introducing phosphated amine and polyetheramine D-400 into the epoxy resin skeleton, combined with the co-deposition of multidimensional inorganic nanomaterials and cationic polytetrafluoroethylene, a continuous self-lubricating transfer film with low surface energy is successfully constructed on the surface. With the high load-bearing capacity of molybdenum disulfide micro-bearing, it can maintain excellent friction coefficient stability under huge mechanical shear forces.

[0042] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A salt spray resistant and wear-resistant epoxy resin-based fastener electrophoretic coating, characterized in that, The fastener electrophoretic coating includes a working bath solution; the working bath solution is an epoxy electrophoretic emulsion with a solid content of 12%-15%, comprising 30-40 parts epoxy resin, 10-15 parts phenolic epoxy resin, 2-5 parts polymethylphenylsiloxane, 20-25 parts phosphorylated amine solution, 5-8 parts polyetheramine, 3-6 parts modified mixed slurry, 20-25 parts crosslinking agent, and 2-4.5 parts coated polytetrafluoroethylene suspension; wherein the modified mixed slurry comprises graphene oxide, silicon dioxide, molybdenum disulfide, and KH-560; and the coated polytetrafluoroethylene suspension comprises polytetrafluoroethylene and octadecyltrimethylammonium chloride.

2. The salt spray resistant and wear-resistant epoxy resin-based fastener electrophoretic coating according to claim 1, characterized in that, The epoxy resin is epoxy resin E-20; the phenolic epoxy resin is phenolic epoxy resin F-44; the phosphoric acid esterified amine solution includes polyphosphoric acid and diethanolamine.

3. A method for preparing an anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating as described in claim 1, characterized in that, The preparation process includes the following steps: diluting the epoxy electrophoretic emulsion with deionized water and obtaining electrophoretic micelles through continuous cyclic curing; using the treated fastener as the cathode and 316L stainless steel plate as the anode, electrophoretically coating to obtain a 15μm-20μm coated fastener; after washing with water, curing is carried out in three stages: the first stage is pre-baking in an oven at 80-90℃; the second stage is heated to 110-120℃ and held; the third stage is heated to 130-140℃ and held to obtain the electrophoretic coating for the fastener.

4. The preparation method of the anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating according to claim 3, characterized in that, The preparation of the epoxy electrophoretic emulsion includes the following steps: Epoxy resin E-20 and phenolic epoxy resin F-44 were added to a reaction vessel, and dibutyltin dilaurate and polymethylphenylsiloxane were added and reacted under heat to obtain an epoxy precursor. The phosphorylated amine solution was then added dropwise for reaction, and polyetheramine D-400 was added for ring-opening to obtain the modified precursor; Then, the modified mixed slurry is added, and the reaction is carried out under heat preservation. A hybrid epoxy resin solution was obtained by adding a crosslinking agent and mixing thoroughly with bismuth neodecanoate. Then, glacial acetic acid is added for neutralization, followed by shearing emulsification and the addition of a coated PTFE suspension. Deionized water is added to adjust the total solids content to 30%-35% to obtain the epoxy electrophoretic emulsion.

5. The preparation method of the anti-salt spray and wear-resistant epoxy resin-based fastener electrophoretic coating according to claim 3, characterized in that, The preparation of the phosphorylated amine solution includes the following steps: Propylene glycol methyl ether was added to the reactor, polyphosphoric acid was added and stirred to dissolve, and diethanolamine was added dropwise under mechanical stirring. The dropping rate was controlled so that the exothermic temperature of the system was 70-80℃, and the reaction was maintained at this temperature to obtain the phosphorylated amine solution.

6. The method for preparing the salt spray resistant and wear-resistant epoxy resin-based fastener electrophoretic coating according to claim 1, characterized in that, The preparation of the modified mixed slurry includes the following steps: dispersing 8-12 parts of graphene oxide, 4-6 parts of nano-silica and 4-6 parts of molybdenum disulfide in a mixed solvent of anhydrous ethanol and deionized water and ultrasonically treating; adding glacial acetic acid to adjust the pH of the system; then adding KH-560 dropwise and stirring to react; and obtaining the modified mixed slurry by centrifugation, washing, vacuum drying and dispersion with propylene glycol methyl ether.

7. The method for preparing the salt spray resistant and wear-resistant epoxy resin-based fastener electrophoretic coating according to claim 1, characterized in that, The preparation of the coated polytetrafluoroethylene suspension includes the following steps: adding polytetrafluoroethylene micro powder to deionized water, adding octadecyltrimethylammonium chloride, and performing shear dispersion and ultrasonic treatment to obtain the coated polytetrafluoroethylene suspension.

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