Heavy-duty fusion-bonded epoxy powder coating and method of manufacture

Abstract

The application discloses a heavy-duty fusion-bonded epoxy powder coating and a preparation method thereof, and belongs to the technical field of anticorrosive coatings. The coating comprises, by weight, 100 parts of an epoxy resin composition, 30-50 parts of a time-sequential temperature control curing system, 3-8 parts of a core-shell structure toughening agent, 30-60 parts of a functional filler composition and 1-3 parts of an additive composition. The time-sequential temperature control curing system comprises a main curing agent and a microencapsulated latent accelerator, which releases the accelerator when a preset trigger temperature is reached, realizes time-sequential control of leveling first and curing later, and effectively solves the contradiction between curing efficiency and coating compactness in thick coating construction. The epoxy resin composition is rigid-flexible compounded, and the core-shell toughening agent and the functional filler synergize, so that the impact resistance, flexibility and corrosion resistance of the coating are significantly improved, the coating is suitable for various processes such as electrostatic spraying and fluidized bed dipping, and the coating has good universality and construction stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of anticorrosive coatings, in particular to a heavy-duty fusion-bonded epoxy powder coating and a preparation method thereof. BACKGROUND

[0002] As a high-performance protective material that does not contain solvent and can obtain thick coating through one-time coating, the heavy-duty fusion-bonded epoxy powder coating plays a crucial role in the protection of infrastructure in key fields of the national economy such as oil and gas pipelines, marine engineering facilities, municipal water supply networks, and chemical equipment, due to its excellent chemical corrosion resistance, excellent mechanical strength, strong adhesion to the substrate, and environmental friendliness.

[0003] The patent for invention with publication number CN102559009B discloses a low-temperature bacteriostatic heavy-duty fusion-bonded epoxy powder coating. By carefully designing the ratio of curing agent to epoxy resin and introducing functional bacteriostatic components, the two technical requirements of achieving rapid curing at a lower construction temperature and inhibiting microbial corrosion are successfully met. The design idea of this scheme has positive significance for reducing construction energy consumption and broadening the application of the coating in specific biological corrosion environments.

[0004] The patent for invention with publication number CN113930133B addresses the common problems of poor powder flowability and easy caking in the roll-coating process of large-diameter steel pipes. By introducing specific functional fillers and leveling aids, the physical form and melting rheological behavior of the powder are optimized, significantly reducing the porosity of the coating during the roll-coating process, and providing an effective technical path for the automated and high-quality coating of specific large workpieces.

[0005] With the deepening of the understanding of the reliability of the whole life cycle of the coating system, and the increasing complexity and integration of application scenarios, the inherent technical limitations and coupling constraints between performance of the above-mentioned solutions focusing on solving single or local technical bottlenecks have gradually emerged.

[0006] The development path of the prior art largely follows a "performance substitution" mode, that is, in order to strengthen the performance of one dimension, the comprehensive performance of other dimensions is often sacrificed as a price, thereby forming a performance ceiling that is difficult to break through.

[0007] For example, low-temperature fast-curing systems represented by CN102559009B, the core of which is to use a high-reactivity curing system. This high activity not only gives the coating a low-temperature curing ability, but also causes the viscosity to increase too quickly during the melting and leveling stage. The molecular chain segments have not fully stretched and migrated, but have quickly crosslinked into a network structure. Under thick coating conditions (usually referring to a coating thickness of more than 400 microns), this rapid gelation process can severely hinder the escape of bubbles inside the coating and the complete fusion between powder particles, resulting in the formation of a high porosity inside the cured coating. This not only weakens the density of the coating, but also provides a potential channel for the penetration of corrosive media, fundamentally affecting the effectiveness of its long-term corrosion protection barrier.

[0008] At the same time, the introduction of functional additives such as antibacterial agents, as non-reactive or low-reactive entities, can interfere with the integrity of the cross-linked network of the epoxy resin matrix to some extent, forming stress concentration points and possibly leading to a decrease in the toughness and impact resistance of the coating. The process-specific formulation represented by CN113930133B is designed to match the unique shearing and flow environment of the roll-coating process, and its rheological properties are precisely controlled within a relatively narrow window. This high degree of process specificity makes it difficult to directly apply to other mainstream coating processes such as electrostatic spraying or fluidized bed dipping, as the latter two have completely different requirements for the electrification, fluidization performance, and leveling and anti-flowing ability of the powder after melting. This lack of process versatility limits the application range of this type of coating, increasing the complexity and cost of manufacturers needing to stock multiple types of coatings for different production lines.

[0009] The deep-seated contradiction in the prior art is how to decouple the reactivity, melt rheology, process adaptability, and physical and chemical properties of the coating at the molecular structure design and macro-formulation building levels, and break the technical dilemma of having to sacrifice the coating density, toughness, and application versatility in order to pursue a single performance indicator such as curing speed or specific process adaptability. This mutual balancing relationship between properties has become a core obstacle to the development of heavy-duty fusion epoxy powder coatings towards higher performance and wider application fields. SUMMARY

[0010] The purpose of the present application is to provide a heavy-duty fusion epoxy powder coating and a preparation method, aiming to overcome the technical problems of mutual balancing between coating density, toughness, and process versatility caused by the pursuit of a single performance indicator in the prior art.

[0011] To solve the above technical problems, the technical solution adopted by the present application is:

[0012] The heavy-duty fusion-bonded epoxy powder coating is composed of the following components in parts by weight: 100 parts of an epoxy resin composition; 30 to 50 parts of a time-temperature-control curing system; 3 to 8 parts of a core-shell structure toughening agent; 30 to 60 parts of a functional filler composition; and 1 to 3 parts of an auxiliary agent composition.

[0013] The epoxy resin composition is compounded by a first epoxy resin and a second epoxy resin as the main film-forming material of the coating.

[0014] Further, the first epoxy resin is a solid bisphenol A type epoxy resin with an epoxy equivalent weight (EEW) of 850 g / eq to 950 g / eq, which functions to build the basic skeleton of the coating, provide core mechanical strength, and provide chemical resistance in the coating formulation.

[0015] The second epoxy resin is an epoxy-terminated butyl nitrile rubber prepolymer (ETBN) with an epoxy equivalent weight of 400 g / eq to 500 g / eq, and the flexible butadiene-acrylonitrile segment contained in the molecular chain forms a micro-phase-separated rubber phase domain after curing, which is used to absorb and dissipate impact energy, significantly improving the toughness and anti-cracking performance of the coating. The weight ratio of the first epoxy resin to the second epoxy resin is 70:30. This fundamentally ensures that the cured coating has excellent impact resistance and flexibility while maintaining high hardness and strong adhesion.

[0016] The time-temperature-control curing system comprises a main curing agent and a microencapsulated latent accelerator. The main curing agent is a bisphenol A type phenolic resin with a hydroxyl equivalent weight of 150 g / eq to 170 g / eq and a functionality greater than 2.

[0017] The reaction of the main curing agent with the epoxy resin starts at a relatively high temperature and has a relatively flat reaction rate in the absence of an accelerator, providing the necessary flow time for the coating in the melting stage. The microencapsulated latent accelerator is composed of a core material and a shell material, wherein the core material is a high-activity imidazole accelerator, specifically 2-phenyl-4-methylimidazole; and the shell material is a crystalline polyester polymer material that is insoluble in the molten epoxy resin system and has a precise and steep melting point, which is set to 140℃±2℃.

[0018] In the process of coating heating and melting, the coating temperature rises from room temperature to 240℃, and the system viscosity is lower than 1000 mPa·s in the low viscosity leveling period of 100-130℃ for 60-90 seconds. When the internal temperature reaches 140℃±2℃, the microcapsules break and trigger rapid curing. At this time, the temperature is lower than the melting point of the shell material, so the microcapsules remain structurally intact, the high-activity accelerator in the core material is physically isolated and cannot contact the epoxy resin and the main curing agent, and the curing reaction basically does not occur or proceeds at a very low rate. This stage is defined as the "forced low viscosity leveling window period", and the duration is sufficient to allow the molten coating to flow and wet the substrate, and the air trapped in the thick coating due to powder accumulation can completely escape, thereby creating a prerequisite for forming a dense and pore-free coating. When the coating temperature continues to rise or is kept at a set temperature until the heat transfer causes the microcapsule temperature to reach the melting point of the shell material (140℃), the shell material rapidly melts and releases the high-activity 2-phenyl-4-methyl imidazole core material. Once released, the accelerator rapidly and uniformly disperses in the molten system and greatly catalyzes the etherification reaction of phenolic hydroxyl and epoxy groups, causing the curing reaction rate to instantaneously increase, the system viscosity to rapidly increase, and the crosslinking and curing process to be completed in a short time, forming a highly crosslinked three-dimensional network structure.

[0019] The amount of the main curing agent is 35 parts, and the amount of the microencapsulated latent accelerator is 1.5 parts, wherein the effective content of the core material in the microcapsule is 50%. The present application divides the traditional single and continuous "melting-curing" process of fusion epoxy powder coating into two independent stages, "melting-leveling-degassing" and "triggering-rapid crosslinking", which fundamentally solves the inherent contradiction between fast curing and low porosity.

[0020] The core-shell structure toughener is a core-shell copolymer particle with polybutadiene-styrene rubber as the core and polymethyl methacrylate as the shell, and the average particle size is 100 nm. When the core-shell structure toughener is blended with the epoxy resin system, uniform dispersion is achieved through the good compatibility of the outer shell with the epoxy matrix, and during the curing process, the rubber core acts as a stress concentration point to induce a large number of crazes and shear bands in the matrix, thereby absorbing fracture energy and achieving significant toughening effect. Its formation of macro and micro scale synergistic toughening mechanism with the aforementioned second epoxy resin (ETBN) makes the coating not prone to cracking or peeling when subjected to severe mechanical impact or thermal shock.

[0021] The functional filler composition is composed of barium sulfate, calcined kaolin and whisker wollastonite, and the ratio and particle size distribution are precisely controlled to achieve comprehensive optimization of the rheological properties, mechanical properties and barrier properties of the coating.

[0022] Specifically, the barium sulfate is prepared by precipitation method, and has a D50 median particle size of 2 μm. The barium sulfate mainly plays a role of increasing coating density, reducing cost and improving wear resistance in the formula. The calcined kaolin is treated by hydroxyl removal at a high temperature of 1200 DEG C, has low oil absorption and excellent chemical inertness, and has a sheet structure capable of forming a physical barrier layer in the coating to effectively delay the penetration path of corrosion medium. The whisker wollastonite is a needle-like structure filler, and has a length-diameter ratio of 15:1. The whisker wollastonite forms a three-dimensional network lap in the coating to play a role of reinforcing steel in reinforced concrete, and significantly improves the bending strength and modulus of the coating. The weight ratio of the barium sulfate, the calcined kaolin and the whisker wollastonite is 40:10:5. The composite use of the multi-morphology and multi-scale fillers not only fills the coating framework, but also realizes complementation and enhancement in function.

[0023] The auxiliary composition comprises a leveling agent, a degassing agent, an antistatic agent and fumed silica. The leveling agent is a polyacrylate high molecular polymer, which plays a role in reducing the surface tension of the molten coating, promoting the leveling of the coating film and eliminating surface defects such as orange peel. The degassing agent is benzoin, which sublimates at the melting temperature to provide a channel for the escape of internal gas of the coating, cooperates with the aforementioned “forced low viscosity leveling window period” mechanism, and further ensures the density and pore-free of the coating.

[0024] The antistatic agent is a quaternary ammonium salt compound, which ensures good and stable charging performance of the powder particles in electrostatic spraying construction by regulating the resistivity of the powder particles, and improves the powder application rate and coating uniformity. The fumed silica is a nanoscale hydrophobic treated silica, which is used as an anti-caking agent and fluidization aid in the powder coating to improve the storage stability of the powder and the fluidization effect in the fluidized bed construction. The synergistic effect of the above four auxiliary agents endows the powder coating of the application with excellent construction performance, so that it can be applied to various mainstream coating processes such as electrostatic spraying and fluidized bed dipping without difference, and exhibits wide process universality.

[0025] As a preferred embodiment of the application, the heavy-duty fusion epoxy powder coating has the following specific formula in terms of weight fraction:

[0026] The first epoxy resin (solid bisphenol A type, EEW 900 g / eq): 70 parts;

[0027] The second epoxy resin (ETBN, EEW 450 g / eq): 30 parts;

[0028] The main curing agent (bisphenol A type phenolic resin, hydroxyl equivalent weight 160 g / eq): 35 parts;

[0029] Microencapsulated latent promoter (core material: 2-phenyl-4-methylimidazole, shell material: polycaprolactone, number average molecular weight 8000-10000, melting point 140℃±2℃, core material content 50%): 1.5 parts;

[0030] Core-shell structure toughening agent (MBS type, particle size 100 nm): 5 parts;

[0031] Barium sulfate (D50=2 μm): 40 parts;

[0032] Calcined kaolin (flaky, D50=5 μm): 10 parts;

[0033] Whisker wollastonite (aspect ratio 15:1): 5 parts;

[0034] Leveling agent (polyacrylate type): 1.0 part;

[0035] Degassing agent (benzoin): 0.5 part;

[0036] Antistatic agent (quaternary ammonium salt type): 0.2 part;

[0037] Fumed silica (hydrophobic treatment): 0.3 part.

[0038] In addition, the application also discloses a preparation method of the heavy-duty fusion bonded epoxy powder coating.

[0039] Step 1, material pretreatment and premixing: the first epoxy resin, the second epoxy resin, the main curing agent, the core-shell structure toughening agent, the functional filler composition and the auxiliary agent composition except the fumed silica are measured according to the formula amount and are put into a high-speed mixer to be premixed in a dry method under the condition that the rotating speed is 1000 revolutions / minute, and the mixing time is 3 minutes, so that the macrodispersion of all powdery and granular solid materials is ensured to be uniform.

[0040] Step 2, melt extrusion blending: the uniformly premixed materials are continuously and stably fed into a same-direction double screw extruder through a loss weight feeder to be melt extruded. The screw length-diameter ratio of the extruder is 40:1, and the screw rotating speed is set to be 300 revolutions / minute. The barrel of the extruder is segmented and arranged to have the temperature, specifically, the temperature of the feeding area is set to be 80℃, the temperature of the melting area is set to be 100℃, the temperature of the homogenizing area is set to be 105℃, and the temperature of the head die is set to be 100℃. The temperature curve is designed to ensure that all resin components can be completely melt and uniformly dispersed with other components at the molecular level, and meanwhile, the highest temperature (105℃) of the whole process is strictly controlled to be far below the melting point (140℃) of the shell material of the microencapsulated latent promoter and the temperature at which the main curing agent significantly reacts, so that any degree of pre-solidification or crosslinking reaction in the extrusion process is avoided.

[0041] Step 3, tabletting and coarse grinding: the molten material extruded from the head of the extruder is immediately forced to cool and flatten by a double roller tabletting machine to form brittle sheets with a thickness of 1-2 mm. Then, the cooled sheets are fed into a toothed disc coarse grinder to be ground into granular semi-finished products with a size of less than 2 mm.

[0042] Step 4, post-addition and blending of key components: the granular semi-finished products obtained in step 3 are added into a low-shear conical screw ribbon mixer together with the microencapsulated latent accelerator and fumed silica in the amount as specified in the formula. Secondary mixing is performed at a speed of 30 rpm for 10 minutes. This step is another key innovation in the preparation process of the present application. By adding the heat-sensitive and shear-sensitive microencapsulated latent accelerator after the high-temperature and high-shear process of melt extrusion, the problem of premature curing of the system caused by premature rupture of the microcapsules due to mechanical force or local overheating is completely avoided, ensuring the integrity of the function of the latent accelerator. At the same time, the addition of fumed silica in this step allows it to be uniformly coated on the surface of the granular semi-finished products, playing its anti-caking and flow-aiding role.

[0043] Step 5, fine grinding and classification: the uniformly blended material in step 4 is fed into an air classifier mill (ACM) with a turbine classifier for fine grinding. The rotational speed of the classifier wheel is adjusted to control the particle size distribution of the outflow, so that the D50 median particle size of the final powder coating product is controlled between 35-45 microns, and the D90 is less than 90 microns. The grinding process uses a circulating air cooling system to ensure that the temperature in the grinding chamber is always below 40°C.

[0044] Step 6, screening and packaging: the ground and classified powder product is screened through a 200-mesh vibrating screen to remove any coarse particles or agglomerates that may be present, and the final product powder coating is obtained and immediately vacuum-sealed and packaged in a controlled temperature and humidity environment.

[0045] In the post-addition and blending step of step 1, the fumed silica in the auxiliary composition is also added into the mixing device together with the granular semi-finished products and the microencapsulated latent accelerator for secondary mixing; the mixing device is a low-shear conical screw ribbon mixer.

[0046] The fine grinding and classification of step 5 is performed using an air classifier mill with a turbine classifier, and the D50 median particle size of the final powder coating product is controlled between 35-45 microns, and the D90 is less than 90 microns; and a circulating air cooling system is used during the grinding process to ensure that the temperature in the grinding chamber is below 40°C.

[0047] Compared with the prior art, the present application has the following beneficial effects:

[0048] The present application realizes a fundamental breakthrough in coating density. Through a time-temperature control curing system, a low viscosity leveling window of up to 60-90 seconds is forcibly opened before the coating is cured after melting, so that the air inside the thick coating (thickness > 500 μm) can completely escape, and the porosity of the cured coating is tested to be lower than 0.5%, which is much better than the existing fast curing system which is generally higher than 2%, thereby building a more dense and defect-free physical barrier, greatly improving the long-term corrosion resistance.

[0049] Without sacrificing the final curing speed, the present application, through the synergistic effect of rigid-flexible epoxy resin composition and core-shell toughening agent, makes the coating maintain high adhesion (0 grade by crosshatch method) and high hardness (pencil hardness ≥ 3H), while its impact strength reaches 15 J, and the flexibility passes the 1° / mm shaft bending test, and the comprehensive mechanical properties surpass the existing technology.

[0050] The present application, through the systematic integration of antistatic agents, fluidizing aids and other functional additives, and combined with fine control of the powder particle size distribution, makes the coating of the present application show stable construction performance and excellent coating effect in both electrostatic spraying process which requires extremely high charging performance and fluidized bed immersion coating process which requires strict fluidization performance, solving the problem of strong specificity and poor universality of the existing technology.

[0051] The preparation process of the present application, which adds the microencapsulated latent accelerator after melting and extrusion, protects the structural integrity and functional effectiveness of the core functional components from the source, ensures the high stability and repeatability of product performance, and solves the process control problem commonly existing in the production of high activity latent systems in the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0052] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be considered as limiting the scope. For those skilled in the art, other related drawings can also be obtained without creative labor.

[0053] Figure 1 The overall flow chart of the preparation method of the present application is shown in the following figure. DETAILED DESCRIPTION

[0054] In the following, only some exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the embodiments of the present application. Therefore, the drawings and the description are considered to be exemplary in nature rather than limiting.

[0055] The application will be described in detail below with reference to the accompanying drawings Figure 1 The embodiments of the application will be described in detail.

[0056] The embodiment provides a heavy-duty fusion-bonded epoxy powder coating and a preparation method. By constructing a clear time sequence temperature control curing system, a melting leveling stage and a rapid cross-linking curing stage of the coating are decoupled in time and temperature dimensions, so that a technical bottleneck that a traditional fusion-bonded epoxy powder coating is difficult to balance high curing efficiency and low coating porosity in thick coating construction is fundamentally broken.

[0057] In one specific embodiment, the heavy-duty fusion-bonded epoxy powder coating is composed of 100 parts by weight of an epoxy resin composition, 30 to 50 parts by weight of a time sequence temperature control curing system, 3 to 8 parts by weight of a core-shell structure toughening agent, 30 to 60 parts by weight of a functional filler composition, and 1 to 3 parts by weight of an auxiliary agent composition. The functions, specifications and synergistic mechanisms of the components will be described in detail below.

[0058] The epoxy resin composition, as the basis for constituting the three-dimensional network skeleton of the entire coating, is composed of a first epoxy resin and a second epoxy resin in a precise ratio, and the total amount is 100 parts by weight.

[0059] Specifically, the first epoxy resin is a solid bisphenol A type epoxy resin, and the epoxy equivalent weight (EEW) is strictly controlled in the range of 850 g / eq to 950 g / eq. The selection of the epoxy equivalent weight range is the result of a large number of experimental optimizations. A lower epoxy equivalent weight will result in too low melting viscosity, and easy sagging in thick coating construction, while a higher epoxy equivalent weight will result in insufficient cross-linking density after curing, affecting the mechanical strength and chemical resistance of the coating.

[0060] In the present scheme, the first epoxy resin mainly undertakes the responsibility of constructing the rigid skeleton of the coating, and provides the basic hardness, adhesion and chemical resistance to corrosion medium for the coating.

[0061] The second epoxy resin is an epoxy terminated butadiene nitrile (ETBN) prepolymer with an epoxy equivalent weight of 400-500 g / eq. The ETBN contains a large number of flexible butadiene-acrylonitrile segments that can freely rotate in the molecular backbone. During the curing and crosslinking process with the bisphenol-A epoxy resin, micro-phase separation occurs due to thermodynamic incompatibility, forming rubber phase domains in the size range of nanometers to sub-microns, which are uniformly dispersed in the continuous phase of the rigid epoxy resin. These dispersed rubber phase domains act as pre-located micro energy absorbers, which can effectively induce the formation of crazes and shear bands in the matrix when the coating is subjected to external impact or internal stress due to rapid temperature change, thereby dissipating a large amount of energy and preventing the initiation and propagation of cracks.

[0062] The weight ratio of the first epoxy resin to the second epoxy resin is preferably 70:30. This ratio ensures that the significant toughening effect is achieved while not excessively sacrificing the hardness and heat resistance of the coating, achieving a high balance between rigidity and toughness.

[0063] The time-temperature controlled curing system is composed of a main curing agent and a specially designed microencapsulated latent accelerator. The main curing agent is a bisphenol-A type phenolic resin with a hydroxyl equivalent weight of 150-170 g / eq and an average functionality greater than 2. The phenolic resin curing agent is characterized in that, in the absence of catalysts or accelerators, the etherification reaction of the phenolic hydroxyl group and the epoxy group has a high activation energy, and the reaction initiation temperature is usually above 160°C, and the reaction process is relatively flat. This feature provides the basis for the construction of the "low viscosity leveling window" of the present application. When the powder coating melts on the surface of the substrate (for example at 120-130°C), the main curing agent and the epoxy resin are already miscible, but the chemical reaction rate is very low, and the system viscosity can be maintained at a relatively low level for a long time.

[0064] Further, in order to achieve precise "on-off" control of the curing reaction, a microencapsulated latent accelerator is introduced into the system. The accelerator is composed of a core material and a shell material that encapsulates the core material. The core material is a highly active imidazole-based accelerator, specifically 2-phenyl-4-methylimidazole. Such an accelerator has a strong catalytic effect on the epoxy-phenolic hydroxyl reaction, and once released, it can make the reaction rate increase exponentially. The shell material is the key to its latency, and it is selected from a specific crystalline polyester high molecular material. The polyester material is specially designed to have two key physical properties: first, it is almost completely insoluble in the molten epoxy resin system below 130°C, and can form a stable and dense physical barrier; second, it has a very precise and steep melting point, which is set at 140°C ± 2°C.

[0065] In the initial stage of the coating heating and melting, for example, the workpiece preheating temperature is 240℃, the powder coating melts rapidly after contacting the surface of the workpiece, and the internal temperature of the coating rapidly rises from room temperature. When the coating temperature reaches and maintains in the interval of 100℃ to 130℃; the epoxy resin and the main curing agent have both melted into liquid state, while the shell material of the microencapsulated latent accelerator keeps its structure intact due to the temperature not reaching its melting point, and physically isolates the high-activity core material inside from the reaction system.

[0066] In this stage, due to the lack of catalyst of the accelerator, the curing reaction is basically in a stagnant state, and the viscosity of the whole molten coating system is mainly determined by the melting viscosity of the resin itself, showing a low viscosity level. This stage is defined as the “forced low viscosity flow window period”. The existence of this window period provides sufficient time (usually up to 60-90 seconds) for the molten coating to flow fully to eliminate surface defects such as orange peel and completely wet the substrate surface to ensure the best adhesion. The forced low viscosity flow window period refers to the time period from the melting of the coating to the triggering of the microcapsule, during which the system viscosity remains below 1000 mPa·s, and the surface roughness Ra < 0.5μm, providing sufficient conditions for the escape of gas inside the thick coating.

[0067] More importantly, for thick coating applications, a large amount of air is inevitably entrapped when the powder particles are stacked. During this low viscosity window period, these entrapped gases have enough time and channels to escape from the extremely low viscosity melt, thereby laying the foundation for the final formation of a dense and pore-free coating.

[0068] With the continuous heat transfer from the substrate to the inside of the coating, when the temperature inside the coating exceeds the threshold of 140℃, the shell material of the microcapsule rapidly and completely melts, and its structure collapses instantaneously, thereby releasing the high-activity 2-phenyl-4-methyl imidazole core material inside to the molten epoxy resin and main curing agent system. The released accelerator molecules rapidly and uniformly diffuse and begin to efficiently catalyze the etherification reaction of phenolic hydroxyl and epoxy groups. Once the reaction is triggered, the curing and crosslinking rate rapidly increases, the system viscosity rapidly rises in a very short time, and quickly enters the gelation and complete curing stage, forming a three-dimensional network structure with high crosslinking density. Through this ingenious “flowing first and then curing” time separation design, the invention fundamentally solves the inherent defects of the traditional fast curing system, i.e. the air cannot escape and a large number of pinholes are generated inside the coating due to the too short gelation time and the too rapid viscosity rise.

[0069] In a preferred embodiment of the invention, the amount of the main curing agent is 35 parts, and the amount of the microencapsulated latent accelerator is 1.5 parts, wherein the effective content of the core material in the microcapsule is 50%.

[0070] The core-shell structure toughening agent is a core-shell copolymer particle prepared by emulsion polymerization process, the inner core is a polybutadiene-styrene rubber with low glass transition temperature, and the outer shell is polymethyl methacrylate (PMMA) which has good compatibility with the epoxy resin matrix. The average particle size of the toughening agent is about 100 nanometers.

[0071] During the melt blending process, due to the good compatibility of the PMMA shell with the epoxy resin, these nanoparticles can achieve uniform and stable dispersion. After the coating is cured and subjected to external force, these dispersed particles act as stress concentration points, and their soft rubber core can cavitate and induce a large number of micro-shear bands and crazes in the surrounding epoxy resin matrix. The above processes are efficient energy absorption mechanisms that can significantly improve the fracture toughness of the material. The core-shell structure toughening agent forms a macro and micro scale synergistic toughening system with the second epoxy resin (ETBN) described above, the sub-micron rubber phase formed by ETBN mainly absorbs large scale impact energy, while the nano-scale core-shell particles are responsible for inhibiting the propagation of micro-cracks on a more microscopic scale. The synergistic effect of the two makes the coating exhibit extremely high anti-cracking and anti-peeling performance when subjected to severe mechanical impact or thermal shock caused by a large change in environmental temperature.

[0072] The functional filler composition is compounded by barium sulfate, calcined kaolin and whisker wollastonite with precise particle size control and morphology selection. The design purpose of this composite filler system is to achieve comprehensive optimization of the rheological properties, mechanical properties and barrier properties of the coating.

[0073] Specifically, barium sulfate is prepared by chemical precipitation method to ensure its regular particle morphology and narrow particle size distribution, with a median particle size D50 of about 2 microns. In the formula, it mainly increases the density of the coating, improves the wear resistance and scratch resistance of the coating, and effectively shields part of the ultraviolet rays, while as a high cost-effective filler it reduces the cost. Calcined kaolin is the product of raw kaolin treated at a high temperature of 1200°C to remove hydroxyl groups, which eliminates the active hydroxyl groups on its surface, making it have extremely low oil absorption and excellent chemical inertness. The unique two-dimensional sheet structure tends to orient parallel to the substrate surface during the coating curing process, forming a stacked structure similar to a "brick wall", which greatly prolongs the path of corrosion media such as water, oxygen and chloride ions penetrating into the substrate, thereby significantly improving the physical barrier properties and long-term corrosion resistance of the coating.

[0074] Whisker wollastonite is a needle-shaped inorganic filler with a high aspect ratio (e.g. 15:1), which is randomly distributed in the coating and overlaps with each other to form a three-dimensional reinforcing network, which acts like the steel bars in reinforced concrete, effectively transferring and dispersing stress, thereby significantly improving the flexural strength, elastic modulus and thermal shock resistance of the coating.

[0075] In an optimized formulation, the weight ratio of barium sulfate, calcined kaolin and whisker wollastonite is 40:10:5, which makes the fillers of different forms and different sizes functionally complementary and synergistically enhanced.

[0076] The auxiliary composition, although in small amount, plays a crucial role in adjusting the final performance and application of the coating. The composition includes a leveling agent, a degassing agent, an antistatic agent, and fumed silica. The leveling agent is a polyacrylate high molecular polymer supported on a silica carrier, which can migrate to the surface of the coating in a molten state, effectively reducing the surface tension of the molten coating, promoting the leveling and spreading of the coating film, and eliminating surface defects such as orange peel and shrinkage caused by surface tension gradient.

[0077] The degassing agent is benzoin (diphenyl glycol), which sublimates at typical melt curing temperatures, and the sublimation gas provides an additional channel for the escape of internal coating gases (such as entrapped air, trace moisture in the material, etc.), which synergistically works with the aforementioned "forced low viscosity leveling window period" mechanism to further ensure the density and pore-free of thick coating.

[0078] The antistatic agent is a quaternary ammonium salt compound, which can moderately reduce the volume resistivity of powder particles, allowing them to efficiently and stably load static charges during electrostatic spraying, thereby improving the adsorption efficiency of powder on the workpiece (i.e., powder application rate) and improving the uniformity of coating on complex-shaped workpieces (such as Faraday cage areas).

[0079] The fumed silica, which is a nanoscale silica with a hydrophobic surface treatment, plays two main roles in the powder coating: first, during storage, its nanoparticles uniformly coat the surface of the powder coating particles, acting as a spacer to effectively prevent powder from caking due to changes in temperature and humidity or storage pressure; second, in fluidized bed dipping, it serves as an efficient fluidization aid to improve the aerodynamic performance of the powder, allowing it to form a uniform and stable gas-solid suspension in the fluidized bed, facilitating workpiece dipping.

[0080] As a preferred embodiment of the present application, the heavy-duty fusion epoxy powder coating has the following specific formulation by weight:

[0081] The first epoxy resin is a solid-state bisphenol A type epoxy resin with an epoxy equivalent weight (EEW) of 900 g / eq, and its amount is 70 parts;

[0082] The second epoxy resin is an epoxy-terminated nitrile rubber prepolymer (ETBN) with an epoxy equivalent weight (EEW) of 450 g / eq, and its amount is 30 parts;

[0083] The main curing agent is selected from a bisphenol A type phenolic resin with a hydroxyl equivalent weight of 160 g / eq, and the amount used is 35 parts;

[0084] The microencapsulated latent accelerator has a core material of 2-phenyl-4-methyl imidazole and a shell material of crystalline polyester, and the shell material has a melting point of 140 DEG C, the effective content of the core material in the microcapsule is 50%, and the total amount used is 1.5 parts;

[0085] The core-shell structure toughening agent is selected from an MBS type core-shell copolymer with an average particle size of 100 nm, and the amount used is 5 parts;

[0086] The functional filler composition includes 40 parts of precipitated barium sulfate with a D50 median particle size of 2 microns, 10 parts of flaky calcined kaolin with a D50 median particle size of 5 microns, and 5 parts of whisker wollastonite with an aspect ratio of 15:1;

[0087] The auxiliary agent composition includes 1.0 parts of a polyacrylate leveling agent, 0.5 parts of a degassing agent benzoin, 0.2 parts of a quaternary ammonium salt antistatic agent, and 0.3 parts of hydrophobic treated fumed silica.

[0088] The application also provides a preparation method of the heavy-duty fusion bonded epoxy powder coating, and specifically includes the following steps:

[0089] Step one, material pretreatment and premixing.

[0090] The first epoxy resin, the second epoxy resin, the main curing agent, the core-shell structure toughening agent, all the functional fillers (barium sulfate, calcined kaolin, and whisker wollastonite), and the leveling agent, the degassing agent, and the antistatic agent in the auxiliary agent composition are accurately metered according to the formula amount of the preferred embodiment, and are put into a jacketed high-speed mixer. Under the condition that the rotation speed of the mixer is set to 1000 rpm, dry high-speed premixing is carried out for 3 minutes. The purpose of this step is to disperse and uniformly distribute all the powdery and granular solid materials macroscopically, so as to provide uniform feeding for the subsequent melting and extrusion step.

[0091] Step two, melting and extrusion blending.

[0092] The pre-mixed materials are continuously and steadily fed into a co-rotating twin-screw extruder through a high-precision loss-in-weight feeder at a constant rate for melt-extrusion blending. The screw length-diameter ratio of the extruder is selected to be 40:1, and the screw rotation speed is set to be 300 rpm. In order to achieve sufficient melt-mixing while avoiding any pre-curing reaction, the barrel of the extruder is controlled by segmented temperature, and the typical temperature setting profile is as follows: the temperature of the first zone (feeding zone) is set to be 80°C to facilitate the feeding and venting of the materials; the temperature of the second and third zones (melt zone) is set to be 100°C to ensure the complete melting of the resin components; the temperature of the fourth and fifth zones (homogenization zone) is set to be 105°C to achieve the uniform dispersion of all components at molecular level through the kneading and shearing action of the screw elements; and the temperature of the sixth zone (die head) is reduced to 100°C to control the viscosity of the extruded melt. The key of this temperature profile design is that the highest material temperature (about 105°C) during the whole melt-blending process is strictly controlled to be far below the melting point of the shell material of the microencapsulated latent accelerator (140°C) and the temperature at which the main curing agent significantly reacts, thus fundamentally eliminating any degree of pre-curing or crosslinking reaction during the extrusion process and ensuring that the reactivity of the system is not lost.

[0093] Step three, tabletting and coarse crushing.

[0094] The hot melt material strip extruded from the die head of the extruder is immediately introduced into a double-roller tablet press. The cooling roller of the tablet press is circulated with cooling water to rapidly cool and calender the melt into brittle flakes with a thickness of 1-2 mm. Subsequently, the flakes cooled to room temperature are fed into a toothed-disc coarse crusher to be crushed into granular semi-finished products with a size of no more than 2 mm.

[0095] Step four, post-adding and blending of key components.

[0096] The granular semi-finished products obtained in step three are transferred into a low-shear conical ribbon mixer together with the microencapsulated latent accelerator and fumed silica measured according to the formulation. Secondary mixing is carried out under mild conditions with a rotation speed set to be 30 rpm for 10 minutes. By adding the microencapsulated latent accelerator, which is very sensitive to heat and shear force, after the high-temperature and high-shear melt-extrusion process, the problem of pre-curing of the system caused by premature rupture of the microcapsules and early release of the accelerator due to mechanical stress or local overheating during the extrusion process is completely avoided. This "post-adding" process is the technical guarantee for ensuring that the function of the time-sequential temperature-controlled curing system described in the present application is fully realized. At the same time, the fumed silica is added in this step to be uniformly coated on the surface of the granular semi-finished products under low shear, so as to most effectively play its anti-caking and flow-aiding functions, while avoiding excessive dispersion in the extruder due to high shear, which affects its efficiency.

[0097] Step five, fine grinding and classification.

[0098] The homogenously blended material from step four is fed into an air classifying mill (ACM) with a turbo dynamic classifier for the final fine grinding. By precisely adjusting the rotational speed of the built-in classification wheel, the on-line control of the particle size distribution of the discharged powder is achieved, so that the D50 median particle size of the final powder coating product is controlled between 35-45 microns, and the content of coarse particles with particle size greater than 90 microns (D90) is controlled at a very low level. The entire grinding process is equipped with a high-efficiency circulating air cooling system to ensure that the temperature of the material in the grinding chamber is always below 40°C, to prevent the powder from softening, adhering and caking.

[0099] Step six, screening and packaging.

[0100] The powder product obtained after grinding and classification is subjected to the final screening through a high-frequency vibrating screen with a 200-mesh aperture to ensure the removal of any agglomerates or foreign impurities that may have been generated during the conveying or collecting process. The final product after screening is immediately vacuum-sealed packaged in an environment with a temperature controlled below 25°C and a relative humidity below 50% to ensure its quality stability during storage and transportation. The present application divides the traditional single curing process into two distinct stages through precise timing temperature control:

[0101] First stage: 100-130°C low-temperature flow leveling, viscosity <1000 mPa·s, duration 60-90 seconds;

[0102] Second stage: ≥140°C trigger rapid curing, gel time <30 seconds;

[0103] This timing control is achieved through the precise phase transition temperature of the microcapsule shell material (140°C ± 2°C).

[0104] In order to facilitate further understanding of the present application by those skilled in the art, the present application will be further described below in conjunction with specific implementation cases.

[0105] Example 1:

[0106] A heavy-duty fusion epoxy powder coating is prepared according to the formula and preparation method of the preferred embodiment described above.

[0107] Specific formula (parts by weight):

[0108] First epoxy resin (EEW 900 g / eq) 70 parts;

[0109] Second epoxy resin (ETBN, EEW 450 g / eq) 30 parts;

[0110] Main curing agent (phenol-formaldehyde resin, hydroxyl equivalent 160 g / eq) 35 parts;

[0111] Microencapsulated latent accelerator (melting point 140°C) 1.5 parts;

[0112] Core-shell structure toughening agent 5 parts;

[0113] Barium sulfate 40 parts;

[0114] Calcined kaolin 10 parts;

[0115] Whisker wollastonite 5 parts;

[0116] Leveling agent 1.0 part;

[0117] Degassing agent 0.5 part;

[0118] Antistatic agent 0.2 part;

[0119] Fumed silica 0.3 part.

[0120] The preparation process strictly follows the above steps one to step six, especially in step four, the microencapsulated latent accelerator is added. The post-addition process is a key technical measure to ensure the integrity of the microcapsule structure:

[0121] Avoiding high shear force (300 rpm) of the extrusion process to damage the microcapsule shell material; preventing local overheating (maximum 105°C) from causing premature rupture of the microcapsule; ensuring the functional effectiveness of the time-temperature control curing system.

[0122] In this embodiment, the preparation method of the microencapsulated latent accelerator includes: dissolving 2-phenyl-4-methylimidazole in deionized water as the water phase, dissolving polycaprolactone in dichloromethane as the oil phase, in the presence of the emulsifier sodium dodecyl sulfate, high-speed emulsification at 10000 rpm for 10 minutes, then stirring at 40°C for 6 hours to volatilize the organic solvent, and after filtration, washing and drying, the microencapsulated latent accelerator with a core material content of 50% is obtained.

[0123] Comparative Example 1:

[0124] As a comparison, a heavy-duty fusion epoxy powder coating using a traditional fast curing system is prepared. Its formula is basically the same as that of Example 1, the main difference being the curing system.

[0125] The time-temperature control curing system (35 parts of main curing agent and 1.5 parts of microencapsulated latent accelerator) in Example 1 is replaced by a traditional fast curing system, which is composed of 35 parts of the same phenol-formaldehyde resin main curing agent and 0.5 parts of non-latent, high-activity conventional accelerator 2-methylimidazole (2-MI). The rest of the components and the amount remain consistent with Example 1.

[0126] In the preparation process, since 2-methylimidazole is a powder with good heat resistance, it is added to a high-speed mixer together with all other components in step one for premixing, and then directly melt-extruded without the post-adding process of step four.

[0127] The powder coatings prepared from Example 1 and Comparative Example 1 were coated and tested for performance using the same application process.

[0128] Application conditions: Q235 steel plate, sandblasted to Sa2.5 level, surface roughness 50-70 μm. The steel plate was preheated to 240°C, and applied by electrostatic spraying, with the dry film thickness controlled at 600±50 μm. The coating was cured in an oven at 230°C for 10 minutes. The cured sample was placed in standard laboratory conditions (23°C, 50% RH) for 7 days before performance testing. The test results are shown in Table 1.

[0129] The coating performance comparison results of Example 1 and Comparative Example 1 are shown in Table 1.

[0130] Table 1:

[0131]

[0132] From the data comparison in Table 1, it can be clearly seen that the technical solution of the present application exhibits significant comprehensive performance advantages. In terms of curing characteristics, the gel time of Example 1 (110 seconds) is significantly longer than that of Comparative Example 1 (45 seconds), which intuitively proves that the time-temperature control curing system of the present application successfully opens up an effective low-viscosity flow window period after melting and before gelation. The direct effect of this window period is reflected in the coating quality: the coating of Example 1 is smooth and flat at a thickness of 600 μm, without any pinholes, and passes the stringent spark test, proving that it is extremely dense inside. In contrast, Comparative Example 1 has a too fast curing speed, resulting in rapid increase in melt viscosity, which prevents the internal gas from escaping, forming a large number of pinholes, poor surface quality, and failing to pass the spark test.

[0133] In terms of mechanical properties, although the hardness of the two is comparable, the impact strength of Example 1 (15.0 J) is more than twice that of Comparative Example 1 (6.5 J), and its flexibility test result (passing 1.0° / PD) is much better than that of Comparative Example 1, showing excellent strength and toughness. This is due to the synergistic toughening mechanism of the hard and soft epoxy resin composition and the core-shell toughening agent in the present application, while Comparative Example 1, although containing the same toughening components, has a too fast curing rate, which may lead to insufficient phase separation structure and excessive curing internal stress, thereby weakening the toughening effect.

[0134] In terms of the anticorrosion performance of the core, low porosity and high toughness directly translate into super-strong corrosion resistance. In both key long-term anticorrosion performance tests, namely cathodic disbondment resistance and salt spray resistance, Example 1 outperforms Comparative Example 1. The extremely low coating porosity builds an excellent physical barrier, effectively blocking the invasion of corrosive media; while the excellent adhesion and toughness ensure that the coating is firmly combined with the substrate under harsh environments, and is not prone to peeling or blistering due to physical or chemical erosion.

[0135] The present application successfully provides a heavy-duty fusion epoxy powder coating and a preparation method thereof through its unique time sequence temperature control curing system design and precise synergistic effect between components. Not only does it fundamentally solve the contradiction between compactness and construction efficiency in thick coating construction, but it also achieves good results in mechanical properties and long-term corrosion resistance.

[0136] It should be noted that in the present embodiment, the viscosity is measured at 130°C, a shear rate of 10 s -1 under the condition of a Brookfield DV2T rotary rheometer.

[0137] Although preferred embodiments of the present application have been described, those skilled in the art, once aware of the basic inventive concept, can make additional changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0138] The above description is merely preferred embodiments of the present application and is not intended to limit the present application. It should be noted that any modifications, equivalent replacements and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A heavy-duty anti-corrosion fusion-bonded epoxy powder coating, characterized in that, By weight, it consists of the following components: 100 parts of epoxy resin composition; 30 to 50 parts of a time-controlled temperature curing system; 3 to 8 parts of core-shell toughening agent; 30 to 60 parts of functional filler composition; And 1 to 3 parts of the auxiliary composition; The time-controlled temperature curing system includes a main curing agent and a microencapsulated latent accelerator. The microencapsulated latent accelerator is composed of a core material and a shell material. The core material is an accelerator that promotes the reaction between epoxy resin and the main curing agent. The shell material is a polymer material that can maintain structural integrity to cover and isolate the core material below a preset trigger temperature, and can melt or rupture to release the core material when the trigger temperature is reached or exceeded. The core material of the microencapsulation latency promoter is an imidazole promoter, and the trigger temperature of the shell material of the microencapsulation latency promoter is set to 140℃±2℃. The epoxy resin composition is formed by compounding a first epoxy resin and a second epoxy resin; wherein, the first epoxy resin is a solid bisphenol A type epoxy resin, and the second epoxy resin is an epoxy-terminated nitrile rubber prepolymer. The core-shell toughening agent is a core-shell copolymer particle with polybutadiene-styrene rubber as the core and polymethyl methacrylate as the shell. The functional filler composition is composed of barium sulfate, calcined kaolin, and whisker wollastonite; the weight ratio of barium sulfate, calcined kaolin, and whisker wollastonite is 40:10:

5. The main curing agent is a bisphenol A type phenolic resin with a hydroxyl equivalent of 150 g / equivalent to 170 g / equivalent and a functionality greater than 2.

2. The heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 1, characterized in that: The imidazole accelerator is 2-phenyl-4-methylimidazole; the shell material of the microencapsulation latency accelerator is polycaprolactone with a number average molecular weight of 8,000-10,000, and the shell material is insoluble in molten epoxy resin system.

3. The heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 1, characterized in that, The first epoxy resin has an epoxy equivalent of 850 g / equivalent to 950 g / equivalent; the second epoxy resin has an epoxy equivalent of 400 g / equivalent to 500 g / equivalent.

4. The heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 3, characterized in that: The weight ratio of the first epoxy resin to the second epoxy resin is 70:

30.

5. The heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 1, characterized in that: The weight ratio of rubber core to plastic shell in the core-shell copolymer particles is 70:30, the glass transition temperature of the rubber core is -60℃ to -40℃, and the average particle size of the core-shell copolymer particles is 100 nanometers.

6. The heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 1, characterized in that: The barium sulfate is a particle with a median particle size of 2 micrometers in D50, prepared by precipitation; the calcined kaolin is a sheet-like filler that has undergone high-temperature dehydroxylation treatment; and the whisker wollastonite is a needle-like filler with an aspect ratio of 15:

1.

7. The heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 1, characterized in that: The additive composition comprises polyacrylate leveling agents, benzoin as a degassing agent, quaternary ammonium salt antistatic agents, and fumed silica as an anti-caking agent and fluidization aid.

8. A method for preparing a heavy-duty anti-corrosion fusion-bonded epoxy powder coating, used to prepare the heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to any one of claims 1-7, characterized in that, Includes the following steps: Step 1, Premixing: The epoxy resin composition, the main curing agent, the core-shell toughening agent, the functional filler composition, and some additives are put into a mixing device for premixing to obtain a premixed material; Step 2, melt extrusion: The premixed material is fed into an extruder for melt extrusion blending to obtain a melt blend, wherein the maximum material temperature during the extrusion process is controlled below the trigger temperature of the shell material of the microencapsulation latency promoter; Step 3, Cooling and coarse crushing: The molten blend is cooled and pressed into tablets, and the resulting tablets are coarsely crushed to obtain granular semi-finished products; Step 4, Post-blending: The granular semi-finished product obtained in Step 3, together with the microencapsulation latent accelerator and fumed silica measured according to the formula, are added to a low-shear conical ribbon mixer and mixed a second time at a speed of 30 rpm to obtain a uniformly blended material. Step 5, fine grinding and grading: The uniformly blended materials are finely ground and graded to obtain the finished powder coating.

9. The method for preparing a heavy-duty anti-corrosion fusion-bonded epoxy powder coating according to claim 8, characterized in that: In the melt extrusion process of step 2, the extruder barrel adopts segmented temperature control, with the feeding zone temperature set at 80℃, the melting zone temperature set at 100℃, the homogenization zone temperature set at 105℃, and the die head temperature set at 100℃; the mixing equipment in step 4 is a low-shear conical ribbon mixer; the fine grinding and classification in step 5 is carried out using an airflow mill with a turbine classifier, and the grinding temperature is controlled below 40℃. The median particle size of the final powder coating is between 35-45 micrometers in D50, and the D90 is less than 90 micrometers.

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