Low-temperature curing environment-friendly high-gloss powder coating and preparation method thereof

Abstract

The present application relates to the technical field of powder coating, in particular to a low-temperature curing environment-friendly high-gloss powder coating and a preparation method thereof.The prepared coating overcomes the problems of poor curing performance and poor environmental protection.The coating comprises carboxyl-terminated hyperbranched polyester, epoxy resin, crystalline dibasic acid, microcapsule accelerator and modified inorganic filler.The present application uses dimethyl carbonate green solvent and a reduced-pressure recovery process to prepare microcapsules, thereby eliminating the residues of formaldehyde and VOCs from the source;the instantaneous low-viscosity effect generated by the synergistic effect of hyperbranched structure and crystallization aid breaks through the low-temperature leveling bottleneck;the in-situ activation of superfine fillers by titanate coupling agent eliminates the interface defects and enhances the impact resistance.The coating has excellent storage stability and 130 DEG C rapid curing capacity, and the coating film has high gloss and high freshness, and meets the strict environmental protection standards.

Description

Technical Field

[0001] This invention relates to the field of powder coating technology, specifically to a low-temperature curing environmentally friendly high-gloss powder coating and its preparation method. Background Technology

[0002] Powder coatings typically have a curing temperature above 180℃. However, in the home building materials and high-end decoration sectors, the application of heat-sensitive substrates such as medium-density fiberboard (MDF), engineering plastics, and composite materials is becoming increasingly widespread. Due to the physical properties of the substrate, the moisture and volatile substances contained within the board will rapidly vaporize and be released at temperatures exceeding 140℃, leading to cracking, deformation, or the formation of pinholes and bubbles on the coating surface. Simultaneously, excessively high temperatures can also cause the plastic substrate to soften and deform. Therefore, if powder coatings could achieve complete curing within a temperature range of 120–130℃ or even lower, it would not only expand their application in the field of heat-sensitive materials and replace highly polluting solvent-based piano lacquer, but also significantly reduce energy consumption.

[0003] Currently, while low-temperature curing powder coatings for heat-sensitive substrates exist, these products face significant technical bottlenecks in achieving a high-gloss, mirror-like finish. Most existing low-temperature products tend to produce textured, wrinkled, or matte effects because highly active catalysts are often required to ensure the resin crosslinking reaction proceeds at low temperatures. However, this high activity leads to a rapid increase in the viscosity of the melt system, severely compressing the leveling time window and preventing the melt from spreading and smoothing out as effectively as with high-temperature curing.

[0004] Although a few existing technologies attempt to prepare low-temperature high-gloss powders by introducing acrylic resins or highly reactive epoxy systems, these methods suffer from severe orange peel and haze issues in practical applications. Furthermore, the large amounts of accelerators added to achieve low-temperature activity often sacrifice the powder's storage stability, making the product highly susceptible to physical agglomeration or chemical pregelation during summer transportation or storage, resulting in spray gun clogging and particulate defects on the coating surface.

[0005] Existing environmentally friendly low-temperature high-gloss powder coatings lack systematic and synergistic research addressing the triangular contradiction between reactivity, leveling performance, and storage stability. Furthermore, they often neglect the dispersion stability of inorganic fillers in low-temperature melts, resulting in coatings with high brittleness, poor impact resistance, and difficulty in eliminating haze caused by microscopic interface defects. Therefore, providing an environmentally friendly powder coating that combines low-temperature rapid curing, excellent storage stability, and a highly reflective mirror finish holds immense market potential for replacing traditional solvent-based coatings and promoting the greening of coating processes on heat-sensitive substrates.

[0006] To address this, a low-temperature curing environmentally friendly high-gloss powder coating and its preparation method are proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a low-temperature curing, environmentally friendly, high-gloss powder coating and its preparation method. The coating comprises carboxyl-terminated hyperbranched polyester, epoxy resin, crystalline diacid, microencapsulation accelerator, and modified inorganic filler. Microcapsules are prepared using dimethyl carbonate as a green solvent and a reduced-pressure recovery process, eliminating formaldehyde and VOC residues at the source. The instantaneous low viscosity effect generated by the synergistic effect of the hyperbranched structure and crystallization aid overcomes the bottleneck of low-temperature leveling. In-situ activation of the ultrafine filler using a titanate coupling agent eliminates interface defects and enhances impact resistance. This coating exhibits excellent storage stability and rapid curing capability at 130℃, producing a high-gloss and high-brightness film while meeting stringent environmental standards.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] This invention provides a method for preparing a low-temperature curing environmentally friendly high-gloss powder coating. The preparation method is as follows: activated modified inorganic filler, carboxyl-terminated hyperbranched polyester resin, epoxy resin, microencapsulation accelerator, crystalline diacid and additives are premixed, melt-extruded by a twin-screw extruder, cooled, cooled, pulverized and graded to obtain a low-temperature curing environmentally friendly high-gloss powder coating.

[0010] The activated modified inorganic filler is obtained by heating the inorganic filler to remove surface adsorbed water, and then spraying a titanate coupling agent into it under stirring for surface organic coating treatment;

[0011] The carboxyl-terminated hyperbranched polyester resin is prepared by polycondensation reaction of trimethylolpropane and chain extender monomers, followed by grafting and end-capping reaction of trimellitic anhydride.

[0012] The microcapsule promoter is obtained by emulsifying an oil phase mixture containing 2-methylimidazole and an aqueous phase solution containing polyvinyl alcohol, reacting and polymerizing them in a reactor, and then recovering the solvent, washing and drying the mixture after the reaction is complete. An oil phase mixture at 0-5℃ was added to an aqueous phase solution stirred at 8000 rpm at a rate of 5 mL / min, and emulsified for 3-5 min to form an O / W emulsion. To inhibit the diffusion of 2-methylimidazole into the aqueous phase and improve the coating rate, the aqueous phase solution was pre-saturated with dimethyl carbonate, and the pH was adjusted to 9-10 to inhibit imidazole protonation. The mixture was transferred to a reactor, the rotation speed was adjusted to 400 rpm, and the temperature was raised to 45℃ in a water bath. 35 parts of diethylenetriamine aqueous solution were added dropwise to initiate interfacial polymerization, and the temperature was raised to 60℃ and held for 3 h to cure. The concentration of the diethylenetriamine aqueous solution was 14.3%. After the reaction, the system was subjected to vacuum distillation at -0.08 MPa and 50℃ to recover the dimethyl carbonate solvent. The remaining product was filtered, washed with deionized water until neutral, and dried in a vacuum oven at 40℃ for 24 h to obtain a microcapsule promoter with a particle size of 5-8 μm.

[0013] The preferred method for preparing the activated modified inorganic filler is as follows: Rutile titanium dioxide and ultrafine barium sulfate, in a mass ratio of 3:1, are added to a high-speed mixer with a heating jacket. Heating is initiated, and when the material temperature reaches 100-110℃, stirring at 1500 rpm is started to remove adsorbed water from the filler surface. Maintaining the temperature and speed, isopropyltris(dioctylpyrophosphonooxy) titanate coupling agent is diluted with anhydrous ethanol at a 1:1 ratio and sprayed into the mixer through an atomizing nozzle. High-speed stirring is maintained for 10-15 minutes, allowing the alkoxy groups of the coupling agent to undergo a coupling reaction with the hydroxyl groups on the surface of the inorganic powder, forming a monomolecular organic coating layer, thus obtaining the activated modified inorganic filler. Cooling water is then turned on to lower the temperature to below 40℃ for later use. The high-temperature treatment is for dehumidification. The titanate coupling agent not only improves dispersion but also generates an internal lubrication effect during later melting due to its long-chain structure, significantly improving leveling.

[0014] The preferred method for preparing carboxyl-terminated hyperbranched polyester resin is as follows: 0.1 mol of trimethylolpropane (TMP) as the core molecule and 1.2 mol of 2,2-dimethylolpropionic acid (DMPA) as the chain extender monomer are added to a polymerization reactor equipped with a mechanical stirrer, a nitrogen inlet pipe, and a water separator; high-purity nitrogen is introduced at a rate of 0.5 L / min to replace the air for 5 min; 0.5 parts of p-toluenesulfonic acid catalyst are added; the temperature program is started, and the temperature is increased to 140°C at a rate of 5°C / min. After the material melts, stirring is started at 200 rpm; the temperature is continued to rise to 175-180°C for normal compression polymerization. During the reaction, the generated water is removed through the water separator, and the acid value change is monitored. The reaction is continued for 3-4 h until the acid value is lower than 5 mg KOH / g, obtaining the hydroxyl-terminated hyperbranched intermediate; the temperature is lowered to 160°C, and then... Tripterygian anhydride (0.6 mol) was used as a capping agent for grafting and capping. To prevent gelation, it was added in three batches with the same amount in each batch. The addition rate of trimellitic anhydride was 10 g / min, with a 20-min interval between each batch. The temperature was raised to 180-190℃, the stirring speed was increased to 350 rpm, and the reaction was maintained at this temperature for 2-3 hours. During this period, the acid value was sampled and tested every 30 minutes. When the acid value stabilized at 75-80 mg KOH / g, the reaction was stopped. A vacuum pump was turned on, and a vacuum of -0.08 MPa to -0.09 MPa was applied for 30 minutes to remove small molecules. After the vacuum was released, the material was discharged, cooled, pressed into sheets, and pulverized to obtain a carboxyl-terminated hyperbranched polyester resin with a number average molecular weight of 2500-3000 and a softening point of 105-110℃. Tripterygian anhydride was used to cap some hydroxyl groups and introduce carboxyl groups to ensure that the resin has acid functionality for reaction with epoxy resin.

[0015] This invention also provides a low-temperature curing environmentally friendly high-gloss powder coating. The raw materials for preparing the low-temperature curing environmentally friendly high-gloss powder coating include carboxyl-terminated hyperbranched polyester resin, epoxy resin, microencapsulation accelerator, and crystalline dicarboxylic acid. The low-temperature curing environmentally friendly high-gloss powder coating is prepared by any one of the above preparation methods. The particle size D50 of the low-temperature curing environmentally friendly high-gloss powder coating is 35±2μm.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] 1. By utilizing the low entanglement characteristics of hyperbranched molecules and the high-temperature solvent effect after the crystallization aid melts, it exhibits a low-viscosity rheological state within the low-temperature curing window of 120-130℃, overcoming the defect of insufficient leveling time caused by the rapid reaction of traditional low-temperature powders. This ensures that the coating can fully wet the substrate before rapid curing, achieving a high fullness mirror appearance of PCI9 or higher.

[0018] 2. By utilizing the highly cross-linked characteristics of the polyurea shell, the microcapsules are endowed with excellent shear toughness, preventing mechanical breakage and active leakage during the extrusion process; at the same time, the thermal response temperature of the shell is precisely controlled, so that it remains chemically inert when stored at 35℃ and rapidly breaks down and releases catalytic activity at 130℃, achieving a dual guarantee of storage stability and low-temperature curing efficiency, and no free formaldehyde was detected in the finished product.

[0019] 3. By eliminating the hydrophilic hydroxyl groups on the filler surface through the long-chain hydrophobic groups of the coupling agent, microscopic visual scattering and haze caused by micron-level agglomeration are prevented, significantly improving the clarity of the coating. At the same time, the chemical bonding effect is used to strengthen the interfacial bonding force, transforming the inorganic filler into a dissipative body of impact energy, so that the coating can maintain high hardness while achieving an excellent impact resistance of 50 kg·cm.

[0020] 4. By introducing surface-modifying components, a dense cross-linked network is formed on the coating surface using the highly active epoxy groups on their side chains. The surface densification effect effectively balances the surface tension gradient during the coating curing process, eliminating the short-wave orange peel effect. Combined with the high packing density of ultrafine barium sulfate, it synergistically imparts a deep reflective texture and extremely high imaging clarity to the coating, achieving the decorative standard of replacing liquid baking paint.

[0021] 5. By introducing vacuum distillation during the microcapsule preparation stage to achieve the recycling of dimethyl carbonate solvent, VOC emissions are reduced at the source. Combined with strict low-temperature control during extrusion and pulverization, pre-reaction of heat-sensitive materials is prevented. Powder coatings prepared using this process not only reduce curing energy consumption by more than 20% compared to traditional processes, but also meet stringent environmental emission limits, aligning with the industrial orientation of green and low-carbon manufacturing. Attached Figure Description

[0022] Figure 1The figures show the test results of gloss and leveling properties in Examples 1-5 and Comparative Examples 1-4 of the present invention. Detailed Implementation

[0023] 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. It should be noted that all parts in the present invention are parts by weight. The material information involved in the present invention is as follows: the CAS of trimethylolpropane is 77-99-6, the CAS of 2,2-dimethylolpropionic acid is 4767-03-7, the CAS of trimellitic anhydride is 552-30-7, the CAS of dodecanoic acid is 693-23-2, the CAS of 2-methylimidazole is 693-98-1, the CAS of diethylenetriamine is 111-40-0, polyvinyl alcohol is PVA-1788, the CAS of titanate coupling agent is 65345-34-8, and the CAS of rutile titanium dioxide is 13463-67-7. R-type (chlorination method), ultrafine barium sulfate CAS is 7727-43-7, D50<1.0μm, polyacrylate leveling agent WL-3090, mass percentage is 30-40%, the leveling agent is white powder, bulk density is 0.5-0.7g / cm³, volatile matter ≤1.5%.

[0024] Please see Figure 1 This invention provides a low-temperature curing environmentally friendly high-gloss powder coating and its preparation method, the technical solution of which is as follows:

[0025] Example 1

[0026] Cool the mixture to 0-5℃ in an ice-water bath. Dissolve 20 parts of 2-methylimidazole in 60 parts of dimethyl carbonate. While stirring, rapidly add 15 parts of isophorone diisocyanate, an oil-soluble shell material monomer, to obtain an oil phase mixture. Prepare the oil phase mixture fresh. Separately, dissolve 3 parts of polyvinyl alcohol in 200 parts of deionized water and add 0.5 parts of sodium dodecylbenzenesulfonate as an emulsifier to obtain an aqueous phase solution. Immediately after mixing the oil phase mixture, add it to the aqueous phase solution stirred at 8000 rpm at a rate of 5 mL / min, emulsifying for 3-5 minutes. In an O / W emulsion, the mixture was transferred to a reactor, the rotation speed was adjusted to 400 rpm, the water bath temperature was raised to 45°C, and 35 parts of diethylenetriamine aqueous solution were added dropwise to initiate interfacial polymerization. The temperature was then raised to 60°C and kept for 3 hours to cure. The concentration of the diethylenetriamine aqueous solution was 14.3%. After the reaction, the system was subjected to vacuum distillation at -0.08 MPa and 50°C to recover the dimethyl carbonate solvent, with a recovery rate of ≥95%. The remaining product was filtered, washed with deionized water until neutral, and dried in a vacuum oven at 40°C for 24 hours to obtain the microcapsule promoter.

[0027] After mixing 300 parts of rutile titanium dioxide and 150 parts of ultrafine barium sulfate and removing surface moisture, 10 parts of diluted titanate coupling agent solution were sprayed into the inorganic filler under stirring conditions of 110℃ and 1500rpm. The mixture was stirred for 15 minutes until the coating reaction was complete, and the activated modified inorganic filler was obtained. The filler was then cooled to 40℃ before use.

[0028] 13.4 parts of trimethylolpropane and 160.8 parts of 2,2-dimethylolpropionic acid were added to a polymerization reactor. Under nitrogen protection, 0.5 parts of p-toluenesulfonic acid were added. After the materials melted, stirring was started at 200 rpm. The temperature was continued to rise to 180℃ for normal compression polymerization. During the reaction, the generated water was removed. The reaction was carried out for 3.5 h until the acid value was 4.8 mg KOH / g, obtaining a hydroxyl-terminated hyperbranched intermediate. The temperature was lowered to 160℃, and trimellitic anhydride was added in three portions, 1 / 3 of the mass each time, for a total of 115.2 parts. The temperature was slowly raised to 185℃ at a rate of 2℃ / min, and the reaction was maintained at 350 rpm for 2.5 h until the acid value stabilized at 78 mg KOH / g. The reaction was stopped and vacuum was applied. After feeding, the product was tableted and pulverized to obtain a carboxyl-terminated hyperbranched polyester resin.

[0029] 455 parts of activated modified inorganic filler, 300 parts of carboxyl-terminated hyperbranched polyester resin, 300 parts of bisphenol A type epoxy resin (E-12 type), 5 parts of microcapsule promoter, 10 parts of crystallization aid dodecanoic acid, 8 parts of polyacrylate leveling agent, and 3 parts of benzoin were added to a high-speed mixer and premixed at 1000 rpm for 3 minutes. The premixed material was then fed into a twin-screw extruder. Since the microcapsule shell was changed to polyurea, the temperature zones were set as follows: feeding zone 85℃, melting zone 100℃, homogenization zone 105℃, and die 105℃; the screw speed was 350 rpm. The extruded sheet was cooled, pulverized by an ACM mill, and the cold air inlet temperature was ≤10℃ to obtain the finished product.

[0030] To investigate the nonlinear effects of the coupling of multiple factors such as component dosage, synthesis process, and preparation process on properties such as low-temperature curing and high gloss, five examples were designed below. Example 1 is the previously determined baseline scheme, while Examples 2-5 have been optimized and designed based on the core parameters, as shown in Table 1.

[0031] Table 1. Summary of key parameter variables for low-temperature curing environmentally friendly high-gloss powder coatings in Examples 1-5

[0032] Group Example 1 Example 2 Example 3 Example 4 Example 5 Dosage of carboxyl-terminated hyperbranched polyester (parts) 300 280 320 290 310 Dosage of bisphenol A type epoxy resin (parts) 300 320 280 310 290 Final acid value (mgKOH / g) based on the amount of carboxyl-terminated hyperbranched polyester used. 78 70 85 74 82 Dosage of microcapsule promoter (parts) 5 3.5 8 4.2 6.5 Dodecanoic acid dosage (parts) 10 15 8 12 9 Dosage of titanate coupling agent (parts) 5 8 3 6 4 Polymerization insulation temperature of microcapsule shell material (°C) 60 50 65 55 58 Temperature in the homogenization zone of the extruder (°C) 105 95 100 98 102

[0033] Comparative Examples 1-4: Based on Example 1, some parameters were adjusted, while other preparation methods remained the same as in Example 1.

[0034] Comparative Example 1: The microcapsule promoter was replaced with 20 parts of unencapsulated pure 2-methylimidazole.

[0035] Comparative Example 2: The carboxyl-terminated hyperbranched polyester resin was replaced with ordinary carboxyl-terminated polyester resin with an acid value of 30-35 mg KOH / g and a linear structure. Dodecanoic acid was removed and replaced with 150 parts of ordinary polyester resin, while maintaining the molar ratio of carboxyl groups to epoxy in the system at 1.1:1.

[0036] Comparative Example 3 did not undergo surface organic activation treatment of the inorganic filler, but directly used untreated rutile titanium dioxide and ordinary precipitated barium sulfate, and did not add titanate coupling agent.

[0037] Comparative Example 4 used melamine-formaldehyde resin as the microcapsule shell material, specifically through the synthesis of water-soluble melamine-formaldehyde prepolymer, preparation of core material emulsion, and in-situ polycondensation coating and curing reaction:

[0038] Add 30 parts of melamine, 60 parts of 37% formaldehyde aqueous solution, and 150 parts of deionized water to a three-necked flask equipped with a reflux condenser and mechanical stirrer; adjust the pH of the system to 8.5 using triethanolamine, as a weakly alkaline environment is conducive to the hydroxymethylation reaction; heat in a water bath to 70°C and stir at 300 rpm for 45-50 minutes; continue heating until the solution changes from turbid to transparent and a slight white mist appears when the sample is dropped into water, indicating that the critical solubility has been reached, and stop heating to obtain a water-soluble melamine-formaldehyde prepolymer solution, which is then allowed to cool naturally for later use.

[0039] 20 parts of 2-methylimidazole were dissolved in 60 parts of ethyl acetate and ultrasonically vibrated until completely transparent. 5 parts of styrene-maleic anhydride copolymer were dissolved in 300 parts of deionized water as a dispersant. The solution was heated to 50℃ to promote dissolution, and the pH was adjusted to 4.0-5.0. The oil phase was slowly poured into the aqueous phase dispersed in an 8000rpm high-shear disperser and emulsified for 10 minutes to obtain a stable oil-in-water emulsion with an average droplet size controlled at 5-8μm, thus obtaining the core material emulsion.

[0040] The stirring speed was reduced to 400 rpm, and the prepared water-soluble melamine-formaldehyde prepolymer solution was slowly added dropwise to the core material emulsion over a period of 30 min. After the addition was complete, the pH of the system was slowly adjusted to 4.0-4.5 using a 10% citric acid solution. The acidic environment initiated the polycondensation and crosslinking of the prepolymer. The temperature was slowly increased to 65°C at a rate of 1°C / min. Melamine-formaldehyde resin usually requires a higher curing temperature, but too high a temperature can cause the solvent to boil. Therefore, the temperature was controlled at 65°C, and the reaction was maintained for 3.5 h. After the reaction was completed, the mixture was filtered, washed three times with deionized water, and dried in a vacuum oven at 45°C for 24 h to obtain the melamine-formaldehyde type microencapsulated imidazole accelerator, i.e., the microencapsulation accelerator.

[0041] Test Example 1

[0042] To address the core appearance indicators of low-temperature curing environmentally friendly high-gloss powder coatings, the 60° gloss and coating leveling properties of the low-temperature curing environmentally friendly high-gloss powder coatings in the examples and comparative examples were tested. For the 60° gloss, in accordance with GB / T9754-2007, the powder coating was electrostatically sprayed onto a standard tinplate, with the film thickness controlled at 60-80 μm. After curing in a 130° oven for 10 min and cooling, a precision gloss meter was used to measure five points at different locations on the coating surface at an incident angle of 60°, and the average value was taken.

[0043] The leveling property of the coating was assessed using the PCI powder coating leveling visual standard. Under standard lighting, the cured sample was visually compared with the PCI standard sample (grades 1-10). Rating explanation: Grade 1 is the worst – severe orange peel / structure; Grade 10 is the best – perfectly smooth. The test results are summarized in Table 2 and... Figure 1 middle.

[0044] Table 2. Results of gloss and leveling tests in Examples 1-5 and Comparative Examples 1-4

[0045] Group 60° Gloss (GU) Coating leveling properties (PCI rating) Brief Review of Results Example 1 96.8 9 Excellent mirror finish, no visible orange peel. Example 2 97.5 9.5 Excellent leveling properties, close to those of piano lacquer. Example 3 93.2 8 The rapid curing process slightly limits leveling, but it is still excellent. Example 4 92.7 8 High filler content affects surface micro-smoothness Example 5 95.4 9 Overall performance balance Comparative Example 1 73.3 3 The surface is full of particles and has no luster. Comparative Example 2 82.1 4 Severe orange peel, like the surface of a grapefruit peel. Comparative Example 3 76.9 5 The surface has a hazy, dull appearance. Comparative Example 4 88.6 7 There are small pinholes or slight orange peel texture in some areas.

[0046] As shown in Table 2, the examples achieved good gloss and leveling properties through adjustments to components, catalyst morphology, resin structure, filler modification, and process. However, the cured appearance of the powder coating obtained in the comparative example differed significantly from that of the examples.

[0047] As shown in Comparative Example 1, 2-methylimidazole without microencapsulation exhibits extremely high reactivity. Its melting point is 142℃, but it initiates a ring-opening reaction as early as 80-90℃ upon contact with epoxy groups. When the mixing temperature in the twin-screw extruder reaches 105℃, the free imidazole undergoes a violent pre-curing reaction with the epoxy resin, generating partially cross-linked gel particles. These gel particles cannot remelt and level during subsequent spraying and curing at 130℃, remaining as foreign matter within the coating. This results in a surface covered with particles, severely hindering melt flow, causing a sharp drop in gloss and poor leveling properties. The results of Comparative Example 2 show that the ordinary carboxyl-terminated polyester resin has a linear molecular structure and does not contain the crystallizing agent dodecanoic acid. Under the low-temperature curing condition of 130°C, its melt viscosity is much higher than that of the hyperbranched polyester system in the examples, resulting in sluggish melt flow. Before the surface tension-driven leveling is completed, the crosslinking reaction has already occurred and frozen the surface morphology, resulting in severe orange peel defects. The gloss and vividness are greatly impaired. This profoundly reveals the strong synergistic leveling mechanism between the hyperbranched topology and the crystalline components in this invention. This mechanism plays an important role in instantly and significantly reducing the viscosity of the system and improving leveling. Comparative Example 3 shows that rutile titanium dioxide and barium sulfate contain a large number of hydroxyl groups on their surfaces, exhibiting hydrophilic and oleophobic properties, while the matrix is ​​an organic phase. Without surface modification using titanate coupling agents, the inorganic filler has extremely poor compatibility with the resin, making it difficult to be completely wetted and prone to agglomeration during melt extrusion. These agglomerates form microscopic protrusions and defects on the coating surface, causing diffuse reflection rather than specular reflection, directly leading to decreased gloss and severe haze. Simultaneously, the lack of internal lubrication from the long chains of the coupling agent increases melt flow resistance, further deteriorating leveling properties. Comparative Example 4 shows that although microcapsules were also added, the melamine-formaldehyde resin shell is a thermosetting rigid material with high brittleness and insufficient toughness. Under the high shear force of the twin-screw extruder, some microcapsules mechanically broke, causing premature release of the internal imidazole catalyst, which then came into contact with the resin, resulting in a slight increase in system viscosity and localized gelation. Ultimately, the coating film exhibited slightly lower gloss and decreased leveling properties.

[0048] Test Example 2

[0049] To accurately evaluate the reactivity kinetics of powder coatings and the degree of complete curing at low temperatures, the low-temperature gelation time and DSC curing degree of the examples and comparative examples were tested.

[0050] The low-temperature gelation time is determined according to GB / T 16995-1997. The standard hot plate temperature is set to 130℃ to simulate a low-temperature curing environment. A 0.5g powder sample is weighed and placed in the hot plate groove. The timer is immediately started, and the powder is stirred at a certain frequency with a stirring rod until it no longer forms strings and becomes a rubbery elastomer. The time is recorded. This indicator reflects the reactivity of the system. Too short a time is detrimental to leveling (resulting in orange peel), while too long a time leads to incomplete curing or low efficiency.

[0051] DSC curing degree is determined according to ISO 11357-5; the total reaction enthalpy ΔH of the uncured powder is measured. total After the powder coating was applied to the substrate and cured at 130℃ for 10 min, a sample of the coating was taken for a second DSC scan to determine its residual enthalpy of reaction (ΔH). res ); Calculation formula: Degree of cure = (1 - ΔH) res / H total (×100%). This indicator determines whether the chemical reaction has been completed. Low-temperature coatings require a curing degree of at least 90% at 130℃ to ensure mechanical properties and weather resistance. The test results are shown in Table 3.

[0052] Table 3. Reaction kinetic test results of Examples 1-5 and Comparative Examples 1-4

[0053] Group Gelation time at 130℃ (s) Curing degree(%) Example 1 240.2 98.5 Moderate reactivity, sufficient leveling window, and very complete curing. Example 2 265.8 96.1 Slightly slower curing allows for optimal leveling and ensures the required degree of curing is achieved. Example 3 185.4 99.3 Extremely fast reaction, suitable for high-efficiency production lines, and extremely high degree of curing. Example 4 232.7 95.6 The packing material slightly affects the reaction contact, but it's not a major issue. Example 5 218.9 97.4 A comprehensive balance, taking into account both efficiency and quality Comparative Example 1 45.3 / The reaction occurred during extrusion, and the test data showed pseudo-gelation; the coating is extremely brittle. Comparative Example 2 280.1 88.7 Excessive viscosity hinders the diffusion of functional groups, leading to incomplete curing at low temperatures. Comparative Example 3 245.5 92.9 The filler agglomerates and encapsulates the active sites, resulting in a slight decrease in the degree of curing. Comparative Example 4 160.6 97.2 If the gelation time is too short, latent failure will occur, affecting leveling.

[0054] As shown in Table 3, the overall curing performance of the examples is good, facilitating the subsequent use of coatings. In the comparative examples, adjustments to the components and process resulted in powder coatings with significantly different curing kinetics compared to the examples.

[0055] Comparative Example 1 shows that 2-methylimidazole, which was not protected by microcapsules and was directly exposed to the system, had an abnormally short gelation time of only 45.3 s. This is because during the extrusion process at 105 °C, the free imidazole catalyzed a large number of uncontrollable pre-reactions of the epoxy groups, consuming most of the active functional groups. When the gelation test was conducted at 130 °C, the powder was actually in a semi-cured state and lost its fluidity as soon as it melted. This explains why its leveling properties were extremely poor and the coating surface was covered with dead powder particles in the aforementioned tests. Although the DSC scan may show a low residual enthalpy at this time, what was formed was a brittle network with many defects, rather than a normal cured network. Comparative Example 2 shows that, without the use of hyperbranched resin and crystallizing agent, the system exhibits extremely high melt viscosity at 130°C. Ordinary linear resins not only have low concentrations of active functional groups and low acid values, but also lack crystalline diacids as reaction carriers. This results in rigid polymer chain segments exhibiting hindered movement at low temperatures, making effective collisions of active end groups difficult. As the reaction proceeds, the system prematurely enters the glass transition state, causing some active groups to be frozen within the rigid network and unable to participate in the reaction. Therefore, even with extended baking time, the final degree of curing is unlikely to exceed 90%, directly leading to unsatisfactory solvent resistance and mechanical strength of the coating. Comparative Example 3 shows that inorganic fillers not treated with titanate coupling agents agglomerate in the resin matrix. These micron-sized inorganic agglomerates constitute a physical barrier layer at the microscopic level, increasing steric hindrance to the movement of resin molecular chain segments and hindering the diffusion path of the curing agent into the resin interior. Simultaneously, differences in oil absorption rates on the filler surface lead to uneven local curing agent concentrations, resulting in a decrease in overall curing degree compared to Example 1. Although film formation is possible, the consistency of crosslinking density is poor. The results of Comparative Example 4 show that the gelation time of melamine-formaldehyde resin as the wall material is significantly shorter than that of the polyurea microcapsule system in Example 1. This is because the melamine-formaldehyde shell is brittle and partially mechanically breaks under the strong shearing action of twin-screw extrusion, leading to leakage of the imidazole catalyst. The leaked imidazole shortens the reaction latency of the powder, resulting in an excessively fast reaction rate in the early stage of heating. This premature reaction initiation not only compresses the time window for melt leveling but also increases the risk of slow chemical crosslinking during storage, demonstrating the superiority of the polyurea shell in shear resistance and thermal latency control.

[0056] Test Example 3

[0057] To investigate the physical anti-caking ability and chemical reaction latency stability of the powder under heated conditions, storage stability was assessed according to the following standards: Referencing GB / T21776-2008 "Assessment of Storage Stability of Powder Coatings" (equivalent to ISO8130-8). 50g of powder sample was placed in a wide-mouth bottle, uncovered, and placed in a constant temperature incubator at 35±2℃ for 240h (10 days). After removal and cooling to room temperature, the powder was poured out and its state was observed. Rating standards: Grade 10 (completely loose, no caking) to Grade 0 (hard caking, impossible to break). The gelation time of the powder after heat aging at 130℃ was tested, and the gelation time retention rate (or change rate) was calculated to determine whether early chemical cross-linking had occurred. The final change rate was calculated using the following formula: Change rate = (gelation time after aging - initial gelation time) / initial gelation time × 100%. The final test results for the examples and comparative examples are shown in Table 4.

[0058] Table 4. Results of physical anti-caking ability and chemical reaction latency stability of Examples 1-5, Comparative Examples 1 and 4

[0059] Group Appearance rating (0-10) at 35℃ for 240 hours Physical state description Gelatinization time change rate (%) Example 1 10 The powder is completely loose and flows as before. -1.2 Example 2 10 The powder is loose and free of visible clumps. -2.3 Example 3 9 Extremely lightweight Microsoft group, disperses with a light tap. -3.7 Example 4 10 The powder is completely loose -0.8 Example 5 10 The powder is loose and has good flowability. -1.9 Comparative Example 1 0 Hard, lumpy objects that cannot be crushed. N / A (gelled) Comparative Example 4 6 There are obvious clumps; they need to be squeezed out with force. -15.6

[0060] As shown in Table 4, the results of Examples 1-5 indicate that the polyurea shell prepared by interfacial polymerization exhibits excellent performance. The soft segment polyether / long-chain amine in its molecular chain endows the shell with excellent toughness and shear resistance. During twin-screw extrusion, the polyurea microcapsules can deform with the melt without breaking, perfectly preserving the internal imidazole active components. Under storage conditions of 35°C, the dense polyurea shell completely blocks the contact between the core material and the matrix resin, ensuring that the system is in a chemical dormant state. Therefore, the gelation time change rate is extremely low, ensuring the long-term loose and stable powder.

[0061] In the comparative example, by adjusting the encapsulation method of the component catalyst and the process, the storage stability of the powder coating obtained was significantly different from that of the example:

[0062] Comparative Example 1 shows that 2-methylimidazole is a highly active nucleophilic attack agent with a low melting point and a high sublimation potential. Without the physical barrier of the microcapsule shell, free imidazole molecules can continuously diffuse to the active sites of epoxy resin and carboxyl-terminated polyester in a thermal environment of 35°C. This diffusion triggers a slow but cumulative chain growth reaction that solidifies within 240 hours, resulting in irreversible chemical bonding between powder particles. Macroscopically, this manifests as the powder being completely sintered into a hard block, unable to recover its powder properties, and thus completely losing its usability. Comparative Example 4 shows that although microencapsulation technology was used, the melamine-formaldehyde resin shell is a relatively rigid thermosetting network with a lack of elasticity. During the high-shear mixing process in a twin-screw extruder, the brittle melamine-formaldehyde shell developed microscopic cracks or fractures, leading to partial leakage of the core imidazole catalyst. Although the concentration of these leaked catalysts was lower than that in Comparative Example 1, they were sufficient to trigger early cross-linking reactions in localized areas during long-term storage at 35°C, resulting in a significantly shortened gelation time, indicating that the system activity had shifted. At the same time, the leaked imidazole caused the particle surface to become sticky, resulting in obvious physical agglomeration of the powder and reducing the commercial grade of the product.

[0063] Test Example 4

[0064] To examine whether the decorative properties and mechanical durability of powder coatings are balanced, the following performance tests were conducted on the powders of the examples and comparative examples after curing. The curing method is as follows: A flat and clean standard tinplate with dimensions of 120mm × 50mm × 0.28mm was selected as the substrate and coated using electrostatic spraying, controlling the dry film thickness to 80μm; the sprayed sample was placed in a 130℃ forced-air oven for constant temperature curing for 10 minutes, then removed and air-cooled to room temperature before testing.

[0065] Impact resistance is tested according to GB / T 1732-2020. A 1kg hammer is dropped freely from different heights onto the coated sample using a heavy hammer impact tester. The coating is observed for cracks, wrinkles, or peeling, and the maximum height (in cm) from which the coating remains undamaged is recorded. These indicators reflect the coating's flexibility, adhesion, and resistance to instantaneous external forces. Low-temperature curing systems often become brittle due to uneven cross-linking density. High standards require a minimum impact resistance of 50kg·cm, meaning a height of 50cm is considered insufficient.

[0066] For vividness testing, referencing ASTM D5767, the cured coating surface was scanned using an orange peel analyzer to read the DOI value (0-100). Gloss only reflects the intensity of reflected light, while DOI reflects the clarity and sharpness of the image. The higher the DOI value, the closer the coating is to a mirror surface, with no microscopic ripples; a low DOI value results in a hazy appearance. High-end high-gloss powder coatings typically require a DOI >85. The final test results are shown in Table 5.

[0067] Table 5. Results of decorative and mechanical durability tests for Examples 1-5 and Comparative Examples 2-4

[0068] Group Impact resistance (1kg weight, cm) Visibility (DOI, 0-100) Brief Review of Results Example 1 50 92.4 The impact is perfect, and the image is clear and sharp. Example 2 50 94.1 Leveling limit, strongest mirror effect Example 3 45 85.3 The curing was too fast, which slightly reduced the DOI, but it was still acceptable. Example 4 45 82.7 Filler interference with imaging sharpness Example 5 50 89.8 Balanced performance Comparative Example 2 35 65.2 It is very brittle and has severe microscopic orange peel texture. Comparative Example 3 25 60.9 Interface peeling, surface with hazy shadows Comparative Example 4 40 78.5 Slightly reduced toughness, microscopic noise present.

[0069] As shown in Table 5, the final decorative and mechanical properties of the powder coatings obtained in the comparative example, through adjustments to the components and process, are significantly different from those in the examples. The specific mechanism analysis is as follows:

[0070] Comparative Example 2 shows that, despite using a common linear carboxyl-terminated polyester resin and removing dodecanoic acid, the melt viscosity of the system at 130°C was not optimized at the level of physical modification. High viscosity hindered the micro-spreading during the melt leveling stage, resulting in obvious short-wave ripples on the coating surface, leading to a sharpness of only 65.2, far lower than 92.4 in Example 1. At the same time, high viscosity reduced the resin's wetting ability on the substrate, making it easy for tiny voids to remain at the interface, causing stress concentration points. When subjected to a 1kg hammer impact, the impact energy could not be effectively dissipated through the dense interface layer, causing the coating to crack brittlely at a height of 35cm. The results of Comparative Example 2 conversely prove that the components in the technical solution of this invention, such as the special resin and crystallization aid, are not simply additive, but form an indispensable organic whole. As shown in Comparative Example 3, rutile titanium dioxide and barium sulfate, without treatment with titanate coupling agents, have high surface energy and are hydrophilic, resulting in natural interfacial repulsion with the oleophilic epoxy / polyester matrix. This leads to the inorganic fillers exhibiting micron-sized agglomerates in the coating. These agglomerates, distributed like gravel within and on the surface of the coating, cause diffuse reflection and scattering of light, resulting in extremely low DOI values ​​and a cloudy appearance. More seriously, the weakly bonded inorganic-organic interface is a weak link in the mechanical structure. Under impact, the interface rapidly peels off, causing a precipitous drop in impact resistance, manifested as chipping at the stress point or concentric cracking. Comparative Example 4 shows that although microencapsulation technology was used, the brittleness of the melamine-formaldehyde shell may cause fine fragments to be generated during extrusion and curing. In addition, due to the local pre-crosslinking caused by the premature release of some imidazole, these microgel particles are mixed in the coating film. Although they are difficult to detect with the naked eye, they will manifest as a decrease in imaging sharpness under optical instruments. In terms of mechanical properties, the melamine-formaldehyde shell is a rigid filler, while the polyurea shell in Example 1 is tough and can play a toughening role similar to the core-shell structure. Lacking the toughening effect of the polyurea shell, the impact resistance of Comparative Example 4 is acceptable, but it cannot reach the full score of 50cm in Example 1.

[0071] Test Example 5

[0072] The environmental performance of the low-temperature curing environmentally friendly high-gloss powder coatings prepared in Example 1 and Comparative Example 4 was tested according to the Chinese national mandatory standard GB 24409-2020. The test results are shown in Table 6.

[0073] Table 6 Environmental performance test results of Example 1 and Comparative Example 4

[0074] Testing items Example 1 Comparative Example 4 Standard Limits (GB24409) VOC content (g / L) 2 5 ≤20 Free formaldehyde (mg / kg) Not detected (ND) 125 ≤100 (some stringent standards) Lead (Pb) content (mg / kg) ND ND ≤1000

[0075] As shown in Table 6, the environmental performance test results indicate that Example 1 utilizes a green solvent system based on dimethyl carbonate, combined with a reduced-pressure recovery process and formaldehyde-free polyurea microcapsule technology. This results in a final product with extremely low VOC content and zero detection of free formaldehyde. In contrast, although Comparative Example 4 is also a powder coating, it uses melamine-formaldehyde resin as the microcapsule wall material, posing a risk of trace formaldehyde release during curing and long-term use, making it difficult to meet the stringent environmental requirements for interior decoration or high-end electronic products. Furthermore, the 130℃ low-temperature curing characteristic of Example 1 significantly reduces production energy consumption and carbon emissions compared to the traditional 180℃ curing process, aligning with the environmentally friendly concept of low-carbon manufacturing.

[0076] In summary, the carboxyl-terminated hyperbranched polyester synthesized using trimethylolpropane as the core in this invention, with its near-spherical low viscosity and numerous end-group active sites, combined with the high-temperature solvent effect of crystalline dodecanoic acid, constructs an extremely low viscosity leveling window in the early stage of low-temperature melting, eliminating orange peel and achieving a highly reflective paint appearance. The titanate coupling agent performs in-situ surface organic treatment of inorganic fillers, building a chemical bridge between the fillers and resin, eliminating interface defects, and synergizing with the toughness of the polyurea microcapsule shell to effectively transfer and dissipate external impact energy between the organic matrix and the inorganic framework. At the same time, the precise latency-burst mechanism of the thermosensitive polyurea microcapsules ensures both the early leveling time and complete curing at low temperatures, ultimately achieving a balance between decorative and functional properties in low-temperature curing powder coatings. Furthermore, although solvents are used to prepare the raw materials, there are no residues in the finished product, and the resulting low-temperature curing environmentally friendly high-gloss powder coating has excellent environmental performance.

[0077] 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 method for preparing a low-temperature curing, environmentally friendly, high-gloss powder coating, characterized in that, The preparation method is as follows: activated modified inorganic filler, carboxyl-terminated hyperbranched polyester resin, epoxy resin, microencapsulation accelerator, crystalline diacid and additives are premixed, melt-extruded by a twin-screw extruder, cooled, cooled, pulverized and graded to obtain the low-temperature curing environmentally friendly high-gloss powder coating; the crystalline diacid is dodecanoic acid. The activated modified inorganic filler is obtained by heating the inorganic filler to remove surface adsorbed water, and then spraying a titanate coupling agent under stirring to perform surface organic coating treatment. The carboxyl-terminated hyperbranched polyester resin is prepared by polycondensation reaction of trimethylolpropane and chain extender monomers, followed by grafting and end-capping reaction of trimellitic anhydride. The oil phase mixture is obtained by dissolving 2-methylimidazole in dimethyl carbonate, adding oil-soluble shell material monomer isophorone diisocyanate, and ultrasonically dispersing it evenly; the microcapsule promoter is obtained by emulsifying the oil phase mixture containing the 2-methylimidazole with an aqueous solution containing polyvinyl alcohol, reacting and interfacially polymerizing in a reactor, and recovering the dimethyl carbonate, washing and drying after the reaction is completed.

2. The method for preparing a low-temperature curing environmentally friendly high-gloss powder coating according to claim 1, characterized in that, The preparation method of the activated modified inorganic filler is as follows: rutile titanium dioxide and ultrafine barium sulfate are mixed and heated and stirred. Isopropyl tris(dioctyl pyrophosphate) titanate coupling agent and anhydrous ethanol are added and mixed. The mixture is then sprayed into a mixer through an atomizing nozzle and cooled to obtain the final product.

3. The method for preparing a low-temperature curing environmentally friendly high-gloss powder coating according to claim 1, characterized in that, The preparation method of the end-carboxyl hyperbranched polyester resin is as follows: the chain extender monomer is 2,2-dimethylolpropionic acid; the trimethylolpropane and the 2,2-dimethylolpropionic acid are added to a polymerization reactor, and the catalyst p-toluenesulfonic acid is added under nitrogen stirring. After programmed temperature rise, the polycondensation reaction is carried out; the trimellitic anhydride is added in portions to carry out the grafting and end-capping reaction; when the acid value reaches 70-85 mgKOH / g, vacuum is applied, the material is fed, compressed into tablets, and pulverized to obtain the final product.

4. The method for preparing a low-temperature curing environmentally friendly high-gloss powder coating according to claim 1, characterized in that, The aqueous solution is obtained by dissolving polyvinyl alcohol in deionized water.

5. The method for preparing a low-temperature curing environmentally friendly high-gloss powder coating according to claim 1, characterized in that, The preparation method of the microcapsule promoter is as follows: the oil phase mixture is added to the stirred aqueous phase solution to emulsify and form an oil-in-water emulsion; the oil-in-water emulsion is added to the reaction vessel, and after heating, diethylenetriamine aqueous solution is added dropwise; after heating, the mixture is kept at a certain temperature to solidify and carry out the interfacial polymerization reaction; after the reaction is completed, the remaining dimethyl carbonate is recovered by vacuum distillation; the obtained product is washed with water until neutral and then vacuum dried to obtain the microcapsule promoter.

6. A low-temperature curing, environmentally friendly, high-gloss powder coating, characterized in that, The raw materials for preparing the low-temperature curing environmentally friendly high-gloss powder coating include activated modified inorganic fillers, carboxyl-terminated hyperbranched polyester resin, epoxy resin, microencapsulation promoter, crystalline diacid, and additives; the low-temperature curing environmentally friendly high-gloss powder coating is prepared by the preparation method described in any one of claims 1-5.

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