An impact-resistant powder coating and a method for its production

Abstract

The application discloses an anti-impact powder coating and a preparation method thereof, and relates to the field of coatings.The application comprises the following steps: uniformly mixing epoxy resin, a phenolic epoxy curing agent, a methyl imidazole catalyst, part of a thermal expansion material, glass fiber, a leveling agent, benzoin and pure polytetrafluoro wax at room temperature to obtain premix; melt blending the premix, and low-temperature extrusion to obtain sheet material; crushing the sheet material into base powder with a particle size of 30-50 microns; uniformly mixing the base powder with the remaining thermal expansion material at room temperature and low speed to obtain a target powder coating.The double buffering network constructed by the application realizes multi-stage dissipation of impact energy; the specially-made super high epoxy equivalent weight resin and the special curing agent form a tough and elastic matrix network, so that the coating can not only withstand a strong impact of 80 cm in height, but also has extreme flexibility in bending through a 5 mm diameter shaft.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to an anti-impact powder coating and its preparation method. Background Technology

[0002] The chassis and lower body of a car are constantly exposed to complex road conditions and are highly susceptible to high-speed impacts from road debris such as gravel. This continuous physical impact directly damages the protective coating on the surface, exposing the base steel and causing corrosion, seriously threatening the vehicle's structural safety and service life. To address this challenge, the industry currently widely uses liquid anti-stone chip coatings based on polyvinyl chloride (PVC), applied to the chassis area using a high-pressure spraying process.

[0003] However, this traditional technical solution has several inherent drawbacks. First, during production and application, the coating releases a large amount of volatile organic compounds (VOCs), posing a significant risk to the environment and the health of operators, thus lacking environmental friendliness. Second, to achieve the necessary protective thickness, the coating is often too thick, which adds extra weight to the vehicle body, contradicting the trend of lightweighting in automobiles. Furthermore, the durability of this type of coating is limited; under long-term thermal-oxidative aging, salt spray corrosion, and mechanical stress, it is prone to gradually becoming brittle, cracking, and even peeling off from the substrate. Its typical service life is only about five years, making it difficult to provide long-term stable protection. From an application perspective, high-pressure spraying is inherently prone to problems such as sagging and uneven thickness, affecting the consistency of the coating's appearance and performance. It also has low application efficiency, and repairs after localized damage are quite cumbersome.

[0004] In summary, existing PVC stone chip protection coatings have limitations in terms of environmental friendliness, lightweight design, long-lasting protection, and ease of application. Meanwhile, the market demands higher overall performance from coatings, such as maintaining integrity under more severe impact conditions, possessing superior flexibility to resist deformation, and exhibiting excellent resistance to corrosion and chemical media.

[0005] Therefore, developing a new type of high-performance anti-stone chip coating that can comprehensively surpass existing technologies and has advantages such as environmental protection, lightweight, long-lasting effect, and easy construction has become a topic of significant industrial value and urgent technical importance. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned shortcomings and provide an anti-impact powder coating and its preparation method, thereby obtaining a new type of high-performance anti-stone impact coating with high impact resistance, high flexibility and environmental friendliness.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for preparing an impact-resistant powder coating includes the following steps:

[0009] S100. Epoxy resin, phenolic epoxy curing agent, methyl imidazole catalyst, some thermal expansion material, glass fiber, leveling agent, benzoin and pure polytetrafluoroethylene wax are mixed evenly at room temperature to obtain a premix.

[0010] S200: The above premixed material is melt-blended and extruded at low temperature to obtain sheet material;

[0011] S300: Crush the above-mentioned sheet material into base powder, controlling the particle size to 30~50 micrometers;

[0012] S400. Mix the above base powder with the remaining thermal expansion material at room temperature and low speed to obtain the target powder coating.

[0013] Unlike the conventional approach of blending all fillers at once, this invention introduces the thermally expandable material into the system in two stages. First, it is embedded into the resin matrix through melt extrusion. Second, it is physically mixed at low temperature to adhere to the surface of the base material particles. Ultimately, a dual-buffer network is constructed within the coating. The internal thermally expandable material acts as uniformly dispersed stress buffer points, while the surface thermally expandable material becomes an independent response unit more sensitive to external impacts. Both work synergistically when the coating is subjected to a sudden, violent impact, efficiently dissipating impact energy through multi-stage, multi-layered expansion and deformation.

[0014] To ensure the aforementioned buffering mechanism is fully utilized, this method employs a specially formulated ultra-high epoxy equivalent (4000~6000 g / eq) epoxy resin and a phenolic epoxy curing agent with a specific hydroxyl equivalent (180~250 g / eq). The former, with its extremely long molecular chain structure, provides ample deformation capacity and elastic space; the latter provides moderate and flexible crosslinking, preventing network embrittlement. The cured system formed by the combination of the two not only possesses excellent toughness and strength, but more importantly, it provides an indispensable elastic environment for the free expansion of thermally expandable microspheres and the realization of their buffering function.

[0015] Further, by weight, the composition includes: 800-1000 parts epoxy resin; 70-85 parts phenolic epoxy curing agent; 5-10 parts methylimidazolium catalyst; 15-20 parts thermal expansion material; 180-220 parts glass fiber; 8-12 parts leveling agent; 2-4 parts benzoin; and 4-6 parts pure polytetrafluoroethylene wax.

[0016] Of which, 5 to 10 parts are thermal expansion material and 10 to 15 parts are remaining thermal expansion material;

[0017] The epoxy resin has an epoxy equivalent of 4000~6000 g / eq, the phenolic epoxy curing agent has a hydroxyl equivalent of 180~250 g / eq, the thermally expanding material is unexpanded thermoplastic microsphere dry powder, the initial expansion temperature is 121~178℃, the volume expansion ratio is 2~5 times, and the particle size is 1~20 micrometers.

[0018] Epoxy resins have an epoxy equivalent as high as 4000~6000 g / eq, with extremely long molecular chains and low crosslinking density, designed to provide the coating with excellent flexibility and a large deformation space, enabling it to effectively absorb energy through chain segment movement when subjected to impact. The matching phenolic epoxy curing agent has a hydroxyl equivalent controlled at 180~250 g / eq. Its role is to provide moderate rather than excessive crosslinking points during the curing process, thereby imparting the necessary cohesive strength to the coating while preventing the network structure from becoming brittle due to excessive crosslinking, ensuring that the resin matrix possesses both rigidity and flexibility.

[0019] In this flexible matrix, a large amount of added glass fiber acts as a rigid reinforcing skeleton, significantly improving the coating's tensile strength, hardness, and scratch resistance, complementing the flexible resin. The thermal expansion material is made of thermoplastic microsphere powder, introduced in two parts through different process stages. This allows the microspheres to have a dual function in the coating: one part is uniformly dispersed within the matrix during melt extrusion, while the other part physically adheres to the surface of the powder particles later. When the coating cures or experiences localized high temperatures due to impact, these microspheres expand thermally, instantly providing additional volume buffering and stress dissipation for the coating, greatly enhancing its resistance to stone impacts.

[0020] In addition, methylimidazolium catalysts ensure efficient curing reaction; the combination of leveling agents, benzoin and pure polytetrafluoroethylene wax ensures the smoothness of the coating surface, eliminates bubbles and imparts excellent wear resistance and release properties.

[0021] The pure polytetrafluoroethylene wax can be made from commercially available low molecular weight polytetrafluoroethylene micro powder, such as PTFE micro powder with model numbers BM-054 and FM-005.

[0022] Furthermore, the method for preparing the epoxy resin includes the following steps:

[0023] Step 1: Etherify bisphenol A, epichlorohydrin and benzyltrimethylammonium chloride at 63-67°C for 3-5 hours; heat the system to 78-82°C and distill under vacuum for 1-2 hours to obtain the prepolymer;

[0024] Step 2: Adjust and stabilize the prepolymer temperature within the range of 68~72℃, and then start the cyclic reaction, with the total number of cycles controlled at 25 to 30; each cycle includes a ring-closing reaction stage and a chain extension reaction stage;

[0025] Step 3: After completing all chain extension cycles, add an alkaline solution dropwise and react at 67~72℃ for 100~140 minutes; wash the obtained resin solution clean, dehydrate it, and add an antioxidant to obtain the target epoxy resin.

[0026] Further, in step 1, the molar ratio of bisphenol A to epichlorohydrin is 1:8~12, and the amount of benzyltrimethylammonium chloride added is 1.5~2.5% of the mass of bisphenol A; in step 3, the alkaline solution is an aqueous solution of NaOH, and the total amount of alkaline solution added is 0.5~0.7:1 compared with the molar ratio of bisphenol A in step 1.

[0027] Further, in step 2, the closed-loop reaction stage includes the following: adding 28-32% w / w sodium hydroxide aqueous solution dropwise to the prepolymer at a rate of 1.5-2.5 mL / min, and after the addition is complete, continuing the reaction at 68-72°C for 50-70 minutes;

[0028] For a single closed-loop operation, the total amount of alkali solution used, calculated as NaOH, is in a molar ratio of 0.28 to 0.32:1 to bisphenol A in step 1.

[0029] The chain extension reaction stage includes the following: adding bisphenol A powder and benzyltrimethylammonium chloride to the system that has completed ring closure in one step, and reacting at 68~72°C for 80~100 minutes;

[0030] In a single chain extension, the molar ratio of the amount of bisphenol A added to that in step 1 is 0.14~0.16:1; the amount of benzyltrimethylammonium chloride added is 1.5~2.5% of the amount of bisphenol A added.

[0031] Furthermore, in step 2, after every 3 to 5 cycles, inter-cycle processing is performed; inter-cycle processing includes the following: pausing the reaction, washing the reaction system with deionized water at 55 to 65°C to remove the sodium chloride generated in the reaction.

[0032] This invention describes a method for preparing epoxy resin that involves multiple repetitive closed-loop chain extension cycles. During these cycles, precise control of the amount of alkali solution and bisphenol A added in each cycle ensures linear growth of the molecular chain unit by unit. Combined with a water washing and desalting operation after every 3-5 cycles, the byproduct sodium chloride is effectively removed, preventing catalyst poisoning, localized overheating, and uncontrolled molecular weight distribution caused by salt accumulation. This allows for the stable and reproducible construction of epoxy resins with an epoxy equivalent of 4000-6000 g / eq, extremely long molecular chains, and a regular structure.

[0033] In step 1, a ring-opening etherification reaction occurs. The catalyst, benzyltrimethylammonium chloride, provides protons to activate the epoxy ring. The phenoxy anion of bisphenol A (generated by trace amounts of water or acid catalysis) attacks the epoxy ring of epichlorohydrin, resulting in a ring-opening reaction and the formation of a chlorohydrin ether structure. This process repeats, and due to the excess of ECH, oligomers with chlorohydrins at both ends (-CH(OH)-CH2Cl) are mainly formed.

[0034] In step 2, the ring-closing reaction occurs: the added NaOH reacts with the terminal hydroxyl group of the chlorohydrin, deprotonating it to form an oxonium. This oxonium then attacks the adjacent carbon atom (the carbon atom bonded to Cl) through intramolecular nucleophilic substitution (S-N2), removing Cl. - A new epoxy ring is formed. Chain extension reaction: The newly generated highly reactive epoxy end group immediately undergoes ring-opening addition with the phenolic hydroxyl group of the added bisphenol A under the action of a catalyst, attaching another bisphenol A unit to the chain end, regenerating the chlorohydrin structure, and preparing for the next round of ring closure.

[0035] In step 3, after completing all chain extension cycles, the system still has a chlorohydrin structure at the end, requiring a final, thorough ring-closing reaction. Excess NaOH is added to ensure that all terminal chlorohydrin structures are completely converted to epoxy groups at high temperature and for a long time, overcoming the high viscosity mass transfer resistance caused by the high molecular weight. The reaction byproduct NaCl and excess alkali are thoroughly removed by washing with water, and the co-solvent toluene and water are removed by vacuum distillation, yielding a pure, stable, ultra-high epoxy equivalent solid epoxy resin.

[0036] The epoxy resin prepared in this invention, due to its extremely high epoxy equivalent, directly determines its unique network structure after curing. This structure possesses extremely low crosslinking density and ultra-long flexible molecular chain segments. This microstructural feature constitutes the fundamental material basis for the final coating's breakthrough in macroscopic performance. The vast molecular chain mobility endows the coating with extremely high deformation capacity, enabling it to effectively absorb and dissipate impact energy through the extension and slippage of molecular chains, rather than brittle fracture. This allows the coating to achieve extreme flexibility through a φ5mm shaft bending test. Simultaneously, the long-chain structure itself contributes to good cohesive strength and toughness. This is the chemical basis for achieving the core properties of high impact resistance and high flexibility in the entire anti-impact powder coating.

[0037] Furthermore, the preparation method of the phenolic epoxy curing agent includes the following steps:

[0038] Step 10: Under nitrogen protection, add formaldehyde solution to phenol and oxalic acid at a rate of 2-6 mL / min, raise the temperature to 95-100℃ at a rate of 0.5-1.5℃ / min, and maintain the temperature for 2-4 hours.

[0039] Step 20: After the reaction is complete, heat to 120~140℃ and dehydrate under reduced pressure, then cool to 70~80℃ and neutralize with alkali to pH=6~7;

[0040] Step 30: Wash with hot water above 80℃ until the aqueous phase is clear, and dehydrate under high vacuum at 120~140℃ for 1~2 hours to obtain the target phenolic epoxy curing agent.

[0041] Furthermore, in step 10, the molar ratio of phenol to formaldehyde is 1:0.82~0.88; the molar ratio of oxalic acid to phenol is 0.005~0.01:1.

[0042] Step 10 controls the formaldehyde dropping rate and heating rate, effectively mitigating the risk of explosive polymerization that might be caused by the intense exothermic reaction between phenol and formaldehyde in the initial stage, ensuring a smooth start and uniform reaction. By controlling the stoichiometry of the reactants, the degree of molecular chain growth is limited from the source. This is key to directly obtaining linear phenolic resins with a hydroxyl equivalent in the narrow target range of 180~250 g / eq and a relatively narrow molecular weight distribution.

[0043] Step 20, involving dehydration under reduced pressure at 120-140°C, efficiently removes the water generated during the reaction and unreacted free formaldehyde. This not only promotes the condensation reaction in the forward direction but also significantly improves the purity and storage stability of the final product. Secondly, step 30 involves repeated washing with hot water above 80°C to thoroughly remove water-soluble impurities such as residual catalyst salts from the resin. The subsequent deep dehydration under high vacuum conditions at 120-140°C ensures an extremely low moisture content in the finished curing agent.

[0044] Further, in step S100, the mixture is mixed at 15~28℃ for 5~15 minutes; in step S200, the extrusion temperature is 75~85℃; in step S400, the mixture is mixed at 20~35℃ for 10~20 minutes.

[0045] In step S200, melt extrusion is performed at a low temperature of 75-85℃. This ensures thorough melting and blending of the materials while preventing resin curing, and significantly suppresses early activation of the thermally expanding microspheres. First, this temperature window is still far below the significant expansion temperature of the thermally expanding material, providing a safety barrier that allows the material to be uniformly dispersed in its intact form and embedded in the molten resin matrix, achieving embedded composite. Second, the low temperature effectively inhibits premature curing reactions between the epoxy resin and the curing agent, preventing gelation of the material within the extruder and ensuring production continuity and stability. Third, the relatively low processing temperature helps maintain a suitable melt viscosity, which not only reduces extrusion energy consumption but also facilitates the uniform dispersion of glass fibers without excessive shear damage, thereby optimizing the coating's reinforcing effect.

[0046] An impact-resistant powder coating obtained by the preparation method described above.

[0047] The coating of this invention, while maintaining the inherent advantages of powder coatings such as zero VOC and convenient application, achieves comprehensive performance that is difficult for traditional anti-stone chip coatings to match. That is, while significantly reducing the coating thickness, it simultaneously achieves extremely high impact resistance, excellent flexibility, long-lasting corrosion resistance, and outstanding resistance to chemical media.

[0048] Compared with the prior art, the beneficial effects of the present invention are:

[0049] 1. The coating of this invention completely eliminates solvents, achieving zero VOC emissions and fundamentally solving the pollution and health hazards in production and construction. While achieving or even exceeding traditional protection levels, the coating thickness is significantly reduced, directly contributing to the lightweighting of the entire vehicle; the coating exhibits an ultra-long service life, with a salt spray resistance exceeding 1008 hours and the ability to withstand more severe chemical media erosion, which is expected to significantly extend the protection cycle; the use of electrostatic spraying technology avoids sagging defects, resulting in high material utilization and easier automation and local repair.

[0050] 2. The dual buffer network constructed by this invention realizes multi-stage dissipation of impact energy; and the ultra-high epoxy equivalent resin and special curing agent specially made to match this structure form a tough and elastic matrix network, which makes the core protective performance of the coating a qualitative leap. It can not only withstand the strong impact of 80 cm height, but also has the ultimate flexibility to bend through the φ5 mm axis. Detailed Implementation

[0051] The present invention will now be further described.

[0052] Example 1

[0053] The preparation method of epoxy resin includes the following steps:

[0054] Step 1, Prepolymer Preparation: Nitrogen gas is introduced into a dry reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen inlet pipe to replace the air. 1 mol of bisphenol A (BPA), 10 mol of epichlorohydrin (ECH), and 2% (by mass of BPA) of benzyltrimethylammonium chloride are added.

[0055] The temperature was slowly increased to 65°C, and the reaction was stirred at this temperature for 4 hours. Fourier transform infrared spectroscopy (FTIR) was used to monitor the reaction at 910 cm⁻¹. -1 The peak intensity at the epoxide (characteristic peak) decreased significantly, indicating that the etherification reaction was basically completed.

[0056] Removal of excess ECH: The system was heated to 80°C and distilled under reduced pressure at -0.095 MPa for 1.5 hours to recover unreacted ECH. A viscous chlorohydrin-terminated prepolymer was obtained.

[0057] Step 2, Controlled Chain Growth Cycle: Adjust and stabilize the temperature of the prepolymer at 70°C. Begin the following cycle, with a target of 28 cycles.

[0058] Closed-loop reaction:

[0059] Prepare a 30% (w / w) sodium hydroxide aqueous solution. Add the above alkaline solution (containing 0.3 mol of NaOH) dropwise to the reaction system at a constant rate of 2.0 mL / min using a precision metering pump.

[0060] After the addition was complete, the reaction was continued at 70°C for 60 minutes. Sampling was performed using FTIR monitoring until the sample reached 760 cm⁻¹. -1 The characteristic peaks near the C-Cl bond almost disappear, at 910 cm⁻¹. -1 The presence of a distinct epoxy peak indicates complete ring closure.

[0061] Chain extension reaction:

[0062] To the completed closed-loop system, add 0.15 mol of bisphenol A powder and benzyltrimethylammonium chloride in a single step. The amount of benzyltrimethylammonium chloride added is 2% of the amount of bisphenol A added. React at 70°C for 90 minutes.

[0063] Inter-cycle treatment: After every 4 cycles, pause the reaction, lower the system temperature to 40°C, add 200 mL of 60°C deionized water, stir for 15 minutes, and let stand to separate the layers. Separate the lower brine phase to remove the sodium chloride generated in the reaction.

[0064] Step 3: Final loop closure, termination, and purification:

[0065] Final loop closure: After completing all chain extension cycles, perform the final loop closure operation. Add 30% NaOH solution dropwise; the total amount of alkali added is 0.6 mol (based on NaOH). React at 70°C for 120 minutes.

[0066] Reaction termination and washing: Cool the system to 50°C, add 400 mL of deionized water and 200 mL of toluene (a co-solvent to facilitate separation). Stir for 30 minutes, transfer to a separatory funnel, allow to stand until complete separation, discard the lower aqueous phase, and check the pH until neutral (conductivity <50 μS / cm).

[0067] Repeat the water washing operation 4 times until the aqueous phase is discharged and tested with 0.1M silver nitrate solution and no white precipitate is found.

[0068] Dehydration and discharge: The washed resin solution is transferred to a dehydration kettle and distilled at 85℃ and -0.098 MPa vacuum to remove toluene and trace amounts of moisture.

[0069] After the solvent has completely evaporated and the material has become viscous and transparent, add 0.5 grams of antioxidant (BHT) and stir for 10 minutes.

[0070] While still hot, pour the molten resin into a preheated aluminum pan and cool it to room temperature to obtain an amber-colored transparent solid flake, which is the target epoxy resin.

[0071] Example 2

[0072] The preparation method of epoxy resin includes the following steps:

[0073] Step 1, Prepolymer Preparation: Nitrogen gas is introduced into a dry reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen inlet pipe to replace the air. 1 mol of bisphenol A (BPA), 8 mol of epichlorohydrin (ECH), and 1.5% (by weight of BPA) of benzyltrimethylammonium chloride are added.

[0074] The temperature was slowly increased to 63°C, and the reaction was stirred at this temperature for 3 hours. Fourier transform infrared spectroscopy (FTIR) was used to monitor the reaction at 910 cm⁻¹. -1 The peak intensity at the epoxide (characteristic peak) decreased significantly, indicating that the etherification reaction was basically completed.

[0075] Removal of excess ECH: The system was heated to 78°C and distilled under reduced pressure at -0.090 MPa for 1 hour to recover unreacted ECH and obtain chlorohydrin-terminated prepolymer.

[0076] Step 2, Controlled Chain Growth Cycle: Adjust and stabilize the temperature of the prepolymer at 68°C. Begin the following cycle, with a target of 25 cycles.

[0077] Closed-loop reaction:

[0078] Prepare a 30% (w / w) sodium hydroxide aqueous solution. Add the above alkaline solution (containing 0.28 mol of NaOH) dropwise to the reaction system at a constant rate of 1.5 mL / min using a precision metering pump.

[0079] After the addition was complete, the reaction was continued at 68°C for 50 minutes. Sampling was performed using FTIR monitoring until the sample reached 760 cm⁻¹. -1 The characteristic peaks near the C-Cl bond almost disappear, at 910 cm⁻¹. -1 The presence of a distinct epoxy peak indicates complete ring closure.

[0080] Chain extension reaction:

[0081] To the completed closed-loop system, 0.14 mol of bisphenol A powder and benzyltrimethylammonium chloride were added in a single batch. The amount of benzyltrimethylammonium chloride added was 2.5% of the amount of bisphenol A added. The reaction was carried out at 68°C for 80 minutes.

[0082] Inter-cycle treatment: After every 3 cycles, pause the reaction, lower the system temperature to 40°C, add 200 mL of 55°C deionized water, stir for 15 minutes, and let stand to separate the layers. Separate the lower brine phase to remove the sodium chloride generated in the reaction.

[0083] Step 3: Final loop closure, termination, and purification:

[0084] Final loop closure: After completing all chain extension cycles, perform the final loop closure operation. Add 30% NaOH solution dropwise; the total amount of alkali added is 0.7 mol (based on NaOH). React at 68°C for 100 minutes.

[0085] Reaction termination and washing: Same as in Example 1.

[0086] Dehydration and discharge: Same as in Example 1.

[0087] While still hot, pour the molten resin into a preheated aluminum pan and cool it to room temperature to obtain an amber-colored transparent solid flake, which is the target epoxy resin.

[0088] Example 3

[0089] The preparation method of epoxy resin includes the following steps:

[0090] Step 1, Prepolymer Preparation: Nitrogen gas is introduced into a dry reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen inlet pipe to replace the air. 1 mol of bisphenol A (BPA), 12 mol of epichlorohydrin (ECH), and 2.5% (by weight of BPA) of benzyltrimethylammonium chloride are added.

[0091] The temperature was slowly increased to 67°C, and the reaction was stirred at this temperature for 5 hours. Fourier transform infrared spectroscopy (FTIR) was used to monitor the reaction at 910 cm⁻¹. -1 The peak intensity at the epoxide (characteristic peak) decreased significantly, indicating that the etherification reaction was basically completed.

[0092] Removal of excess ECH: The system was heated to 82°C and distilled under reduced pressure at -0.098 MPa for 2 hours to recover unreacted ECH. A viscous chlorohydrin-terminated prepolymer was obtained.

[0093] Step 2, Controlled Chain Growth Cycle: Adjust and stabilize the temperature of the prepolymer at 72°C. Begin the following cycle, with a target of 30 cycles.

[0094] Closed-loop reaction:

[0095] Prepare a 30% (w / w) sodium hydroxide aqueous solution. Add the above alkaline solution (containing 0.32 mol of NaOH) dropwise to the reaction system at a constant rate of 2.5 mL / min using a precision metering pump.

[0096] After the addition was complete, the reaction was continued at 72°C for 70 minutes. Sampling was performed using FTIR monitoring until the sample reached 760 cm⁻¹. -1 The characteristic peaks near the C-Cl bond almost disappear, at 910 cm⁻¹. -1 The presence of a distinct epoxy peak indicates complete ring closure.

[0097] Chain extension reaction:

[0098] To the completed ring-closed system, 0.16 mol of bisphenol A powder and benzyltrimethylammonium chloride were added in a single batch. The amount of benzyltrimethylammonium chloride added was 2.5% of the amount of bisphenol A added. The reaction was carried out at 72°C for 100 minutes.

[0099] Inter-cycle treatment: After every 5 cycles, pause the reaction, lower the system temperature to 45°C, add 200 mL of 60°C deionized water, stir for 15 minutes, and let stand to separate the layers. Separate the lower brine phase to remove the sodium chloride generated in the reaction.

[0100] Step 3: Final loop closure, termination, and purification:

[0101] Final loop closure: After completing all chain extension cycles, perform the final loop closure operation. Add 30% NaOH solution dropwise; the total amount of alkali added is 0.7 mol (based on NaOH). React at 72°C for 140 minutes.

[0102] Reaction termination and washing: Same as in Example 1.

[0103] Repeat the water washing operation 5 times until the aqueous phase is discharged and tested with 0.1M silver nitrate solution and no white precipitate is found.

[0104] Dehydration and discharge: Same as in Example 1.

[0105] While still hot, pour the molten resin into a preheated aluminum pan and cool it to room temperature to obtain an amber-colored transparent solid flake, which is the target epoxy resin.

[0106] Comparative Example 1

[0107] The preparation method of epoxy resin includes the following steps:

[0108] Nitrogen gas was purged into a dry reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen inlet pipe to replace the air. 1 mol of bisphenol A (BPA), 2.2 mol of epichlorohydrin (ECH) (10% excess according to conventional theory), and 2% (by mass) of benzyltrimethylammonium chloride catalyst (BPA) were added. The temperature was slowly raised to 90°C, and the reaction was stirred for 1 hour to dissolve the bisphenol A and initiate the reaction. Subsequently, a 30% (w / w) aqueous solution of sodium hydroxide was slowly added dropwise over 2 hours via a dropping funnel, for a total addition of 2.2 mol of NaOH. After the addition was complete, the reaction was maintained at 90°C for another 3 hours. After the reaction was complete, the temperature was lowered to 60°C, and the mixture was washed several times with deionized water until the aqueous phase was neutral. The aqueous layer was separated. The organic layer was dehydrated under reduced pressure at 120°C and -0.095 MPa for 2 hours to obtain the epoxy resin product.

[0109] The performance parameters of the epoxy resins prepared by the methods of Examples 1-3 and Comparative Example 1 are shown in Table 1.

[0110] Table 1. Performance parameters of epoxy resins prepared by the methods of Examples 1-3 and Comparative Example 1

[0111]

[0112] As shown in Table 1, the comparative examples demonstrate that the traditional one-step method cannot synthesize resins with an epoxy equivalent of >4000 g / eq.

[0113] Examples 1-3, by adjusting the number of cycles (25 to 30 times), systematically achieved directional control of epoxy equivalent (4250-5870 g / eq) and molecular weight (11800-17600), demonstrating the controllability and repeatability of the epoxy resin preparation process of this invention. The low PDI value indicates that, even at such high molecular weights, the method of this invention can still maintain good molecular weight uniformity. This is due to the quantitative feeding in each cycle and the periodic intermittent desalination, which avoids the accumulation of side reactions—something that cannot be achieved by traditional processes.

[0114] The ultra-high epoxy equivalent (>4000) and moderate Tg of the final product ensure, from a chemical structure perspective, that when used as a coating matrix, it can form a curing network with low crosslinking density and flexible chain segments, providing the most fundamental material guarantee for the coating's extreme flexibility and impact resistance.

[0115] Example 4

[0116] The preparation method of the phenolic epoxy curing agent includes the following steps:

[0117] Step 10: Under nitrogen protection, add 37% w / w formaldehyde solution to phenol and oxalic acid at a rate of 4 mL / min, heat to 98°C at a rate of 1°C / min, and maintain the temperature for 3 hours; the molar ratio of phenol to formaldehyde is 1:0.85; the molar ratio of oxalic acid to phenol is 0.007:1.

[0118] Step 20: After the reaction is complete, heat to 130℃ and dehydrate under reduced pressure, then cool to 75℃ and neutralize with alkali to pH=6.5;

[0119] Step 30: Wash with hot water above 80℃ until the aqueous phase is clear, and dehydrate at 130℃ and -0.095MPa high vacuum for 1.5 hours to obtain the target phenolic epoxy curing agent.

[0120] Example 5

[0121] The preparation method of the phenolic epoxy curing agent includes the following steps:

[0122] Step 10: Under nitrogen protection, add 37% w / w formaldehyde solution to phenol and oxalic acid at a rate of 6 mL / min, heat to 100℃ at a rate of 1.5℃ / min, and maintain the temperature for 2 hours; the molar ratio of phenol to formaldehyde is 1:0.82; the molar ratio of oxalic acid to phenol is 0.005:1.

[0123] Step 20: After the reaction is complete, heat to 120℃ and dehydrate under reduced pressure, then cool to 70℃ and neutralize with alkali to pH=6.

[0124] Step 30: Wash with hot water above 80℃ until the aqueous phase is clear, and dehydrate at 120℃ and -0.095MPa high vacuum for 1 hour to obtain the target phenolic epoxy curing agent.

[0125] Example 6

[0126] The preparation method of the phenolic epoxy curing agent includes the following steps:

[0127] Step 10: Under nitrogen protection, add 37% w / w formaldehyde solution to phenol and oxalic acid at a rate of 2 mL / min, heat to 95℃ at a rate of 0.5℃ / min, and maintain the temperature for 4 hours; the molar ratio of phenol to formaldehyde is 1:0.88; the molar ratio of oxalic acid to phenol is 0.01:1.

[0128] Step 20: After the reaction is complete, heat to 140℃ and dehydrate under reduced pressure, then cool to 80℃ and neutralize with alkali to pH=7.

[0129] Step 30: Wash with hot water above 80℃ until the aqueous phase is clear, and dehydrate at 140℃ and -0.095MPa high vacuum for 2 hours to obtain the target phenolic epoxy curing agent.

[0130] Comparative Example 2

[0131] The preparation method of phenolic epoxy curing agent includes the following steps:

[0132] In a reactor equipped with a stirrer and a reflux condenser, 1 mol of phenol was added and heated to 50°C to melt it. While stirring, 1.5 mol of 37% formaldehyde solution (phenol / aldehyde molar ratio = 1:1.5) was added dropwise until the pH of the system reached 9-10. The temperature was then raised to 80-85°C and the reaction was maintained for 2 hours. After the reaction was complete, the mixture was neutralized with dilute hydrochloric acid, and then dehydrated under reduced pressure at 120°C to obtain a reddish-brown viscous resin.

[0133] The performance of the phenolic epoxy curing agents prepared by the method in Examples 4-6 and Comparative Example 2 is shown in Table 2.

[0134] Table 2. Performance of the phenolic epoxy curing agents prepared by the methods in Examples 4-6 and Comparative Example 2.

[0135]

[0136] As shown in Table 2, Comparative Example 2 yielded a high-functionality, potentially branched resin with a hydroxyl equivalent of ~110 g / eq, while the preparation method of the present invention yielded a low-functionality, linear phenolic resin with a hydroxyl equivalent of 180~250 g / eq.

[0137] In Examples 4-6, as the aldehyde ratio increases, the chances of molecular chain growth increase, leading to an increase in Mn, a higher hydroxyl equivalent, and a higher softening point. The low-temperature, slow-speed, long-time reaction in Example 6, compared to the high-temperature, rapid, short-time reaction in Example 5, resulted in a more complete reaction, thus producing lower levels of free phenol and a slightly higher molecular weight.

[0138] Example 7

[0139] 900 parts epoxy resin (prepared by the method of Example 1); 78 parts phenolic epoxy curing agent (prepared by the method of Example 4); 8 parts 2-ethyl-4-methylimidazolium; 18 parts unexpanded thermoplastic microsphere dry powder (model Expansionl920DU40); 200 parts glass fiber; 10 parts leveling agent (model BYK-361 N); 3 parts benzoin (diphenylethanol ketone); 5 parts pure polytetrafluoroethylene wax (model BM-054);

[0140] A method for preparing an impact-resistant powder coating includes the following steps:

[0141] S100. Epoxy resin, phenolic epoxy curing agent, methyl imidazole catalyst, 9 parts of unexpanded thermoplastic microsphere dry powder, glass fiber, leveling agent, benzoin and pure polytetrafluoroethylene wax are mixed at 20°C for 10 minutes to obtain a premix.

[0142] S200: The above premixed material is melt-blended and extruded at 80°C to obtain sheet material;

[0143] S300. Crush the above-mentioned sheet material into base powder, controlling the particle size to 40 micrometers;

[0144] S400. The above base powder and 9 parts of unexpanded thermoplastic microsphere dry powder are mixed in a V-type mixer at 28°C and 15 rpm for 15 minutes to obtain the target powder coating.

[0145] Example 8

[0146] 800 parts epoxy resin (prepared by the method of Example 1); 70 parts phenolic epoxy curing agent (prepared by the method of Example 4); 5 parts 2-ethyl-4-methylimidazolium; 15 parts unexpanded thermoplastic microsphere dry powder (model Expansionl920DU40); 180 parts glass fiber; 8 parts leveling agent (model BYK-361 N); 2 parts benzoin (diphenylethanol ketone); 4 parts pure polytetrafluoroethylene wax (model BM-054);

[0147] A method for preparing an impact-resistant powder coating includes the following steps:

[0148] S100. Epoxy resin, phenolic epoxy curing agent, methyl imidazole catalyst, 7 parts of unexpanded thermoplastic microsphere dry powder, glass fiber, leveling agent, benzoin and pure polytetrafluoroethylene wax are mixed at 15°C for 5 minutes to obtain a premix.

[0149] S200. The above premixed material is melt-blended and extruded at 75°C to obtain sheet material.

[0150] S300. Crush the above-mentioned sheet material into base powder, controlling the particle size to 30 micrometers;

[0151] S400. The above base powder and 8 parts of unexpanded thermoplastic microsphere dry powder are mixed in a V-type mixer at 20°C and 10 rpm for 10 minutes to obtain the target powder coating.

[0152] Example 9

[0153] 1000 parts epoxy resin (prepared by the method of Example 1); 85 parts phenolic epoxy curing agent (prepared by the method of Example 4); 10 parts 2-ethyl-4-methylimidazolium; 20 parts unexpanded thermoplastic microsphere dry powder (model Expansionl920DU40); 220 parts glass fiber; 12 parts leveling agent (model BYK-361 N); 4 parts benzoin (diphenylethanol ketone); 6 parts pure polytetrafluoroethylene wax (model BM-054);

[0154] A method for preparing an impact-resistant powder coating includes the following steps:

[0155] S100. Epoxy resin, phenolic epoxy curing agent, methyl imidazole catalyst, 10 parts of unexpanded thermoplastic microsphere dry powder, glass fiber, leveling agent, benzoin and pure polytetrafluoroethylene wax are mixed at 28°C for 15 minutes to obtain a premix.

[0156] S200: The above premixed material is melt-blended and extruded at 85°C to obtain sheet material;

[0157] S300: Crush the above-mentioned sheet material into base powder, controlling the particle size to 50 micrometers;

[0158] S400. The above base powder and 10 parts of unexpanded thermoplastic microsphere dry powder are mixed in a V-type mixer at 35°C and 20 rpm for 20 minutes to obtain the target powder coating.

[0159] Comparative Example 3

[0160] A method for preparing an impact-resistant powder coating includes the following steps:

[0161] S100. Add all the above materials into the high-speed mixer at once. Mix for 10 minutes at room temperature (20°C) until the materials are uniform in appearance, thus obtaining the premix.

[0162] S200: The premixed material is fed into a twin-screw extruder. The temperature of each section of the extruder is strictly controlled to ensure that the material is melted, mixed, and extruded at 80°C. The extrudate is cooled by the tableting rollers to obtain uniform flake material.

[0163] S300: The cooled flaky material is fed into a mechanical crusher for coarse crushing, and then into an air classifier mill for fine grinding. The speed of the classifier is adjusted to control the powder particle size at 40 micrometers.

[0164] S400. The base powder obtained in step S300 is put into a V-type mixer and mixed at 15 rpm for 15 minutes at 28°C to further homogenize the powder and obtain the final comparative powder coating product.

[0165] Other data and parameters are the same as in Example 7.

[0166] Comparative Example 4

[0167] The epoxy resin used in Example 7 (prepared by the method in Example 1) was replaced with a general-purpose epoxy resin (model E-12, epoxy equivalent ~500 g / eq). Other data parameters and preparation methods were the same as in Example 7.

[0168] Comparative Example 5

[0169] The phenolic epoxy curing agent used in Example 7 (prepared by the method in Example 1) was replaced with a general-purpose phenolic amine curing agent (model Hanamine1692, hydroxyl equivalent 95~100g / eq). Other data parameters and preparation methods were the same as in Example 7.

[0170] The anti-impact powder coatings prepared by the methods of Examples 7-9 and Comparative Example 3 were applied to a substrate (such as a steel plate) by electrostatic spraying and then cured by heating to form the final coating. The coating performance parameters are shown in Tables 3 and 4.

[0171] Existing technical specifications for anti-stone chip coatings:

[0172] Impact resistance: The coating should not crack or peel after being tested with a standard impact tester (e.g., impact from a height of 40cm-50cm);

[0173] Flexibility: The coating should show no cracks after a φ50mm shaft bending test;

[0174] Tensile strength / elongation: Tensile strength is typically ≥1.5 MPa, and elongation is ≥100%, ensuring that the coating can deform without cracking when the chassis steel plate vibrates;

[0175] Salt spray resistance: After being sprayed in a 5% NaCl solution at 35℃ for more than 720 hours (30 days), the coating should show no blistering, rust, or peeling.

[0176] Temperature resistance: It passed the extreme temperature cycle test from -40℃ (cold resistance) to +180℃ (heat resistance) without cracking or charring;

[0177] Chemical resistance: No abnormalities should occur after immersion in 1% sulfuric acid or 10% sodium hydroxide solution for 24 hours.

[0178] Table 3 Performance parameters of the final coatings of the impact-resistant powder coatings prepared by the methods in Examples 7-9

[0179]

[0180] As shown in Table 3, Examples 7-9 comprehensively surpass the existing anti-stone chip coating technical indicators in terms of impact resistance, flexibility, tensile strength, elongation, salt spray resistance, temperature change resistance, chemical media resistance, and appearance coating.

[0181] The impact resistance is far superior to existing indicators. The internal-external two-stage mixing process of this invention constructs a dual buffer network. The internally mixed microspheres are uniformly dispersed and absorb energy, while the externally mixed microspheres act as independent response units, realizing multi-stage energy dissipation, thus giving the coating superior impact resistance.

[0182] This invention uses an epoxy resin with ultra-high epoxy equivalent, forming a cured network with extremely low crosslinking density and extremely long molecular chains, providing unparalleled deformation capabilities compared to traditional resins. The coating of this invention exhibits superior flexibility; in a φ5mm shaft bending test, the coating showed no cracks.

[0183] In this invention, glass fiber provides rigidity reinforcement, while the ultra-flexible resin matrix provides deformation space, resulting in a coating with high strength and ultra-high elongation. The internal-external mixing process avoids damage to the matrix caused by microsphere pre-foaming; benzoin effectively degassing eliminates channels; and a leveling agent ensures a uniform coating. These three components synergistically form a dense barrier, preventing the penetration of corrosive media. This makes the coating of this invention exhibit salt spray resistance far exceeding existing standards.

[0184] The flexible resin network buffers thermal stress, and the special phenolic curing agent provides stable heat-resistant crosslinking points. The dense and defect-free coating avoids stress concentration, which together ensures the integrity of the coating over a wide temperature range and gives it excellent resistance to temperature changes.

[0185] Polytetrafluoroethylene micropowder migrates to the surface to form an inert protective layer; at the same time, the excellent coating density (as above) greatly delays the penetration and erosion of chemical media, making the coating of the present invention have a much higher tolerance to chemical media concentration (10%) than existing indicators (1%).

[0186] Table 4 Performance parameters of the final coatings of the anti-impact powder coatings prepared by the methods of Example 7 and Comparative Examples 3-5

[0187]

[0188] Note: In this invention, Examples 7-9 and Comparative Examples 3-5 were tested for chemical resistance using 10% sulfuric acid and 10% sodium hydroxide solutions, respectively.

[0189] As shown in Table 4, the performance indicators of Comparative Examples 3 to 5 are far lower than those of Example 7.

[0190] The performance of Comparative Example 3 declined across the board because it eliminated the core process of internal-external two-stage mixing, introducing all thermally expanding microspheres in the first step. During the extrusion process at 80°C, all microspheres were forced to undergo prolonged thermal shearing, inducing pre-activation or shell damage in some sensitive microspheres. This resulted in disordered and inefficient expansion behavior of the microspheres in the subsequent curing stage, making it impossible to form a carefully designed gradient buffer structure. This demonstrates that the mixing process of this invention is not a simple adjustment of conventional steps, but rather a reconstruction of the energy dissipation mechanism of the coating from the microstructure through precise control of the introduction method of functional fillers and thermal history.

[0191] The precipitous drop in flexibility and elongation in Comparative Example 4 exposes the limitations of traditional material systems. The high functionality of ordinary resins results in a rigid network with high cross-linking density after curing, severely restricting the movement of molecular chain segments. While possessing high tensile strength, it severely lacks flexibility, and stress cannot be effectively dispersed through chain segment slippage when subjected to impact or bending, instead rapidly concentrating and leading to brittle cracking. Its impact resistance plummets to industry benchmarks and fails rigorous bending tests, demonstrating that the ultra-low cross-linking density flexible network created by the ultra-high epoxy equivalent resin in this invention is an indispensable chemical basis for achieving the high elasticity of the coating.

[0192] Comparative Example 5, using a commercially available high-functionality phenolic amine curing agent, reacted with epoxy resin to form a highly rigid network with dense cross-linking points. While this network may provide good initial hardness and acid resistance, the extremely high cross-linking density leads to enormous internal stress. Under impact or bending, it cannot absorb energy through deformation, resulting in poor impact resistance and flexibility. Under alkaline immersion, the brittle network is more prone to microcracks due to swelling stress. This indicates that the specific hydroxyl equivalent of the phenolic epoxy curing agent prepared in this invention is not about the hydroxyl equivalent value itself, but rather about achieving synergy with the ultra-high flexibility epoxy resin in terms of reactivity, functional group matching, and final network toughness through a specific synthetic pathway. This synergy contributes to a balanced coating exhibiting both rigidity and flexibility, high toughness, and excellent durability.

Claims

1. A method for preparing an impact-resistant powder coating, characterized in that, Includes the following steps: S100. Epoxy resin, phenolic epoxy curing agent, methyl imidazole catalyst, some thermal expansion material, glass fiber, leveling agent, benzoin and pure polytetrafluoroethylene wax are mixed evenly at room temperature to obtain a premix. S200: The above premixed material is melt-blended and extruded at low temperature to obtain sheet material; S300: Crush the above-mentioned sheet material into base powder, controlling the particle size to 30~50 micrometers; S400. Mix the above base powder with the remaining thermal expansion material at room temperature and low speed to obtain the target powder coating. The epoxy resin has an epoxy equivalent of 4000~6000 g / eq, and the phenolic epoxy curing agent has a hydroxyl equivalent of 180~250 g / eq. The method for preparing the epoxy resin includes the following steps: Step 1: Etherify bisphenol A, epichlorohydrin and benzyltrimethylammonium chloride at 63-67°C for 3-5 hours; heat the system to 78-82°C and distill under vacuum for 1-2 hours to obtain the prepolymer; Step 2: Adjust and stabilize the prepolymer temperature within the range of 68~72℃, and then start the cyclic reaction, with the total number of cycles controlled at 25 to 30; each cycle includes a ring-closing reaction stage and a chain extension reaction stage; Step 3: After completing all chain extension cycles, add an alkaline solution dropwise and react at 67~72℃ for 100~140 minutes; wash the obtained resin solution clean, dehydrate it, and add an antioxidant to obtain the target epoxy resin.

2. The preparation method according to claim 1, characterized in that, By weight, epoxy resin 800-1000 parts; phenolic epoxy curing agent 70-85 parts; methylimidazolium catalyst 5-10 parts; thermal expansion material 15-20 parts; glass fiber 180-220 parts. Leveling agent 8-12 parts; Benzoin 2-4 parts; 4-6 parts of pure polytetrafluoroethylene wax; Of these, 7 to 10 parts are thermal expansion material and 8 to 10 parts are remaining thermal expansion material.

3. The preparation method according to claim 1, characterized in that, In step 1, the molar ratio of bisphenol A to epichlorohydrin is 1:8~12, and the amount of benzyltrimethylammonium chloride added is 1.5~2.5% of the mass of bisphenol A; in step 3, the alkaline solution is an aqueous solution of NaOH, and the total amount of alkaline solution added is 0.5~0.7:1 compared with the molar ratio of bisphenol A in step 1.

4. The preparation method according to claim 1, characterized in that, In step 2, The closed-loop reaction stage includes the following: adding 28-32% w / w sodium hydroxide aqueous solution dropwise to the prepolymer at a rate of 1.5-2.5 mL / min, and continuing the reaction at 68-72°C for 50-70 minutes after the addition is complete; For a single closed-loop operation, the total amount of alkali solution used, calculated as NaOH, is in a molar ratio of 0.28 to 0.32:1 to bisphenol A in step 1. The chain extension reaction stage includes the following: Bisphenol A powder and benzyltrimethylammonium chloride were added to the closed-loop system in one step, and the reaction was carried out at 68-72°C for 80-100 minutes. In a single chain extension, the molar ratio of the amount of bisphenol A added to that in step 1 is 0.14~0.16:1; the amount of benzyltrimethylammonium chloride added is 1.5~2.5% of the amount of bisphenol A added.

5. The preparation method according to claim 1, characterized in that, In step 2, after every 3 to 5 cycles, an inter-cycle treatment is performed; the inter-cycle treatment includes the following: pausing the reaction, washing the reaction system with deionized water at 55 to 65°C to remove the sodium chloride generated in the reaction.

6. The preparation method according to claim 1, characterized in that, The preparation method of the phenolic epoxy curing agent includes the following steps: Step 10: Under nitrogen protection, add formaldehyde solution to phenol and oxalic acid at a rate of 2-6 mL / min, raise the temperature to 95-100℃ at a rate of 0.5-1.5℃ / min, and maintain the temperature for 2-4 hours. Step 20: After the reaction is complete, heat to 120~140℃ and dehydrate under reduced pressure, then cool to 70~80℃ and neutralize with alkali to pH=6~7; Step 30: Wash with hot water above 80℃ until the aqueous phase is clear, and dehydrate under high vacuum at 120~140℃ for 1~2 hours to obtain the target phenolic epoxy curing agent.

7. The preparation method according to claim 6, characterized in that, In step 10, the molar ratio of phenol to formaldehyde is 1:0.82~0.88; the molar ratio of oxalic acid to phenol is 0.005~0.01:

1.

8. The preparation method according to claim 1, characterized in that, In step S100, the mixture is carried out at 15~28℃ for 5~15 minutes; in step S200, the extrusion temperature is 75~85℃; in step S400, the mixture is carried out at 20~35℃ for 10~20 minutes.

9. An anti-impact powder coating obtained by the preparation method according to any one of claims 1 to 8.

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