A water-resistant mixed powder resin composition and powder paint and applications

Abstract

The application belongs to the technical field of powder coatings, and particularly relates to a water-vapor-resistant mixed powder resin composition, a powder coating and application, which comprises polyester resin and epoxy resin in a mass ratio of 1:0.8-1.2, the acid value of the polyester resin is 62-71 mgKOH / g, and the melt viscosity of the polyester resin at 200 DEG C is 3000-5000 mPa.s; the raw materials of the polyester resin comprise neopentyl glycol, 1,2-propylene glycol, trimethylolpropane, ethylene glycol, monobutyl tin oxide, terephthalic acid, isophthalic acid, trimellitic anhydride, antioxidant and curing accelerator, wherein the mass ratio of the 1,2-propylene glycol to ethylene glycol is 4:1-1:4, and the isophthalic acid accounts for 8-14% of the total mass of the raw materials of the polyester resin. The powder coating provided by the application has excellent long-term protection performance in a high-temperature water vapor and boiling water environment, and can be widely used in surface protection coatings of metal substrates in harsh humid and hot environments such as kitchen household appliances, bathroom equipment and medical devices.

Description

Technical Field

[0001] This invention belongs to the technical field of powder coatings, specifically relating to a water-resistant mixed powder resin composition and powder coating and its application, especially suitable for protective coatings on the surface of metal substrates such as kitchen appliances, bathroom equipment, and medical devices that are exposed to high-temperature water vapor or boiling water environments for a long time. Background Technology

[0002] Powder coatings, as a solvent-free and environmentally friendly coating technology, have been widely used in the automotive, home appliance, and building materials industries due to their excellent adhesion, abrasion resistance, and low VOC emissions. Among them, epoxy-polyester hybrid powder coatings are particularly widely used in China due to their good interior decorative properties and cost advantages. With the continuous expansion of powder coating application scenarios, especially in kitchen appliances (such as ovens, dishwashers, electric kettles, and rice cookers) and bathroom equipment, increasingly stringent requirements are being placed on the coating's long-term resistance to high-temperature steam or boiling water immersion.

[0003] However, existing powder coatings are prone to blistering, loss of gloss, decreased adhesion, and even peeling when exposed to high-temperature steam or boiling water for extended periods. The root cause of this failure lies in the hydrolytic breakage of ester bonds in the polyester resin molecular chain under the influence of high-temperature water molecules, leading to the destruction of the coating's cross-linked network structure. Simultaneously, residual hydrophilic groups such as hydroxyl groups in the polyester resin adsorb water molecules, which further hydrolyze adjacent ester groups at high temperatures, causing macromolecular chain breakage and reduced coating gloss. Moisture then penetrates between the substrate and the coating film, resulting in blistering, loss of gloss, discoloration, poor adhesion, and coating peeling. Therefore, regulating the hydrophobic-hydrophilic balance of the polyester resin at the molecular structure level and improving the hydrolysis resistance of ester bonds are key to solving the insufficient water vapor resistance of powder coatings.

[0004] Currently, the industry's solutions for improving water resistance or steam resistance mainly include the following aspects: One approach is to increase the resin crosslinking density to form a denser water-blocking network. For example, selecting a carboxyl polyester resin with a higher acid value can increase the crosslinking density. However, excessive crosslinking can lead to coating embrittlement and a sharp drop in impact resistance, making it difficult to balance water resistance and mechanical properties.

[0005] Secondly, monomers containing aromatic ring structures are introduced to enhance the steric hindrance and hydrophobicity of ester bonds. Studies have shown that increasing the content of isophthalic acid (IPA) is beneficial to improving the coating's resistance to boiling water. However, the excessive stacking of rigid benzene ring units can severely inhibit the freedom of molecular chain movement, causing the coating to lose its toughness and become brittle.

[0006] Third, optimize the types and proportions of polyol monomers to control the balance between rigidity and flexibility of the molecular chain. For example, monomers such as 2-ethyl-2-butyl-1,3-propanediol (BEPD) can be used to modify polyester resins. However, the introduction of different polyols often comes at the expense of one thing—improving hydrophobicity may sacrifice the leveling or flexibility of the coating.

[0007] In addition, existing technologies also improve coating density by adding water-resistant additives. For example, patent document CN106675346A discloses a high water-resistant mixed powder coating and its preparation method, which improves the coating's water resistance to 100°C for more than 30 hours by adding water-resistant additives containing cyano and ester functional groups and adhesion promoters containing hydroxyl functional groups. However, excessive addition of water-resistant additives can lead to poor surface leveling and deterioration of the coating's appearance. Patent document CN104861829A discloses a water-resistant epoxy polyester powder coating, its preparation method, and its application, which improves the anti-corrosion performance of epoxy polyester powder coatings by using composite carboxyl polyester resins. However, its water resistance limit is only 95°C / 24h, which is not suitable for long-term immersion in boiling water at 100°C. Patent document CN120025703A - A water-resistant aluminum powder coating and its preparation method, which improves the water resistance of the powder coating by mixing glycidyl methacrylate and carboxylic acid resin in a carboxyl-terminated polyester resin, but its raw material cost increases significantly, making it difficult to industrialize and popularize.

[0008] In summary, although existing technologies have made some progress in improving the water resistance and steam resistance of powder coatings, significant technical bottlenecks and application limitations remain. Whether by increasing crosslinking density, introducing rigid aromatic ring structures, optimizing polyol components, or adding functional additives, these strategies often fail to simultaneously improve water resistance while neglecting the coating's mechanical toughness, leveling appearance, and cost-effectiveness, resulting in a trade-off. Specifically, excessive crosslinking leads to coating embrittlement, excessive introduction of rigid groups weakens impact resistance, and the addition of additives can easily degrade the surface condition or increase raw material costs, severely restricting the large-scale application of epoxy polyester powder coatings in harsh high-temperature and high-humidity environments. More critically, existing improvement schemes mostly focus on macroscopic formulation adjustments or physical densification, lacking in-depth analysis and precise intervention of the hydrolysis mechanism of polyester resin molecular chains. The adsorption and penetration of water vapor in coatings, along with the hydrolysis of ester bonds, constitute a dynamic coupled process. A systematic approach is generally lacking that synergistically regulates the hydrolysis rate of ester bonds from the perspectives of the regular packing density of polyester resin segments and the hydrophobic microenvironment of side groups. Simply increasing crosslinking density or hydrophobicity is insufficient to fundamentally block the erosion pathways of water molecules on the network structure. Therefore, a new technical solution is needed to address these technical problems. Summary of the Invention

[0009] The purpose of this invention is to provide a water-resistant mixed powder resin composition, powder coating, and its application, in order to solve the problems mentioned in the background art, such as the lack of in-depth analysis and precise intervention of the hydrolysis mechanism of polyester resin molecular chains in improving the water boiling resistance and water vapor resistance of powder coatings, and the difficulty in fundamentally blocking the erosion path of water molecules on the network structure by simply increasing the crosslinking density or hydrophobicity, which seriously restricts the large-scale application of epoxy polyester powder coatings in harsh environments with high temperature and high humidity.

[0010] To achieve the above objectives, the present invention provides the following four technical solutions: Firstly, this invention provides a method for preparing polyester resin, the specific steps of which are as follows: S1. Add the prescribed amounts of neopentyl glycol, 1,2-propanediol, trimethylolpropane, ethylene glycol, terephthalic acid, and isophthalic acid to the reaction vessel, purge with nitrogen, heat to 100-120℃ to melt and stir evenly. S2. Add the amount of monobutyltin oxide in the formula, and heat to 160-220℃ at a heating rate of 5-10℃ / h to carry out an esterification reaction under normal pressure. At this time, the esterification reaction temperature is 180-190℃. Distill off the generated water, and control the column top temperature of the fractionation column to ≤102℃ until the acid value drops to 20-23 mgKOH / g. S3. Raise the reaction temperature to 230-250℃ and continue the esterification reaction at normal pressure. At this time, the high-temperature esterification temperature is 240-245℃ and the duration is 10-12 hours until the acid value drops to 10-18 mgKOH / g. S4. Cool to 220-230℃, add the prescribed amount of antioxidant under nitrogen protection, and perform polycondensation reaction at a vacuum of -0.095 to -0.1 MPa for 2-4 hours until the acid value drops to 7-10 mgKOH / g and the melt viscosity at 200℃ reaches 2000-3000 mPa·s. The antioxidant is one or more of triphenyl phosphite, antioxidant 1076, and antioxidant 168. S5. Purge with nitrogen to break the vacuum, cool to 200-210℃, add the formulated amount of trimellitic anhydride, and carry out the end-capping reaction for 2-3 hours under nitrogen protection and at a temperature of 205-210℃ until the acid value reaches 62-71 mgKOH / g and the melt viscosity at 200℃ reaches 3000-5000 mPa·s. S6. Cool to 180-200℃, add the curing accelerator of the formula amount and stir to react for 0.5-1 hour. Then discharge, cool and crush to obtain the polyester resin. The curing accelerator is one of triphenylethylphosphine bromide and butyltriphenylphosphine chloride.

[0011] The second aspect: This invention provides a water-resistant mixed powder resin composition, comprising an epoxy resin and a polyester resin prepared in the first aspect, wherein the mass ratio of the polyester resin to the epoxy resin is 1:0.8 to 1.2, the epoxy equivalent of the epoxy resin is 700 to 900 g / eq, the acid value of the polyester resin is 62 to 71 mgKOH / g, and its melt viscosity at 200°C is 3000 to 5000 mPa·s; the raw materials of the polyester resin include neopentyl glycol, 1,2-propanediol, trimethylolpropane, ethylene glycol, monobutyltin oxide, terephthalic acid, isophthalic acid, trimellitic anhydride, antioxidant, and curing accelerator, wherein the mass ratio of the 1,2-propanediol to the ethylene glycol is 4:1 to 1:4, and the isophthalic acid accounts for 8 to 14% of the total mass of the polyester resin raw materials.

[0012] Furthermore, the polyester resin comprises the following raw material components in parts by weight: 1-5 parts neopentyl glycol, 10-15 parts 1,2-propanediol, 1.0-2.0 parts trimethylolpropane, 6-15 parts ethylene glycol, 0.05-0.15 parts monobutyltin oxide, 51-60 parts terephthalic acid, 5-14 parts isophthalic acid, 5-10 parts trimellitic anhydride, 0.3-0.6 parts antioxidant, and 0.2-0.4 parts curing accelerator.

[0013] Furthermore, the epoxy resin is one or more of bisphenol A type epoxy resin and bisphenol F type epoxy resin.

[0014] Thirdly, the present invention provides a powder coating comprising the water-resistant mixed powder resin composition, filler, coating additives, and pigments described in the second aspect.

[0015] Fourthly: This invention provides an application of powder coating in the preparation of water-resistant coatings.

[0016] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention constructs an original triple molecular anti-hydrolysis protection system by synergistically defining four key characteristics: epoxy system parameters, polyester acid value and viscosity, diol compounding ratio, and IPA critical content. It solves the problem of coating failure in humid and hot environments from the root of hydrolysis reaction. Without sacrificing the coating's flexibility, appearance, production cost, and industrial feasibility, it achieves a leapfrog improvement in water vapor resistance, overcoming the long-standing technical prejudice in the industry that "water resistance and toughness cannot be achieved at the same time."

[0017] 2. This invention limits the final acid value of polyester to 62-71 mgKOH / g (optimal 68 mgKOH / g) and cures it with epoxy to form a three-dimensional cross-linked network with appropriate density. This not only relies on the appropriate cross-linking density to block water molecule penetration, but also avoids the problems of excessive hydrophilic carboxyl groups and stress concentration caused by excessive cross-linking due to high acid value. Compared with conventional commercially available polyester (acid value 62 mgKOH / g), the optimal acid value formula increases the gloss retention rate from 78.2% to 90.4% after boiling in water for 2 hours. At the same time, it maintains non-brittleness under both positive and negative impacts at 50 cm, solving the shortcoming of the sharp drop in the recoil performance of traditional high acid value resins.

[0018] 3. In the polyester resin raw material of the present invention, 1,2-propanediol and ethylene glycol are compounded in a mass ratio of 4:1 to 1:4 (preferably 2:1). The synergistic mechanism is as follows: the monomethyl side chain of 1,2-propanediol introduces spatial asymmetry into the molecular chain segment, disrupting the ordered hydrogen bond network of water molecules around the ester bond and reducing the effective collision frequency of local water molecules with the ester carbonyl group; ethylene glycol, as a symmetrical short-chain diol, has a compact molecular structure that helps to increase the regular packing density of the polyester chain segment, reduce the intermolecular gaps, and thus block the water vapor permeation channel. The two are complementary: the former provides a hydrophobic microenvironment at the chain segment level, and the latter provides physical barrier at the packing level, while a single polyol system cannot achieve both functions simultaneously. Comparative experimental data show that the light retention rate of the pure EG system without 1,2-propanediol after boiling in water for 2 hours is only 80.1%, while that of the pure 1,2-propanediol system without EG is 82.3%. However, the light retention rate of the two systems can reach 93.6% after boiling in water for 2 hours when they are mixed in the optimal ratio. Moreover, the light retention rate remains stable at 50 cm under both positive and negative impacts across the entire ratio range, achieving a balance between excellent water vapor resistance and flexibility.

[0019] 4. In this invention, isophthalic acid (IPA) accounts for 8-14% of the total mass of the polyester resin raw material. Its core function is reflected in three aspects: First, the meta-benzene ring of IPA reduces the electrophilicity of the carbonyl carbon in the ester bond through the conjugation effect, thereby essentially inhibiting the hydrolysis rate of the ester bond. Second, the spatial configuration of the meta-ester bond introduces irregular bends in the molecular chain compared to the para-ester bond (PTA), effectively shielding the ordered nucleophilic attack of water molecules on the ester bond. Third, the hydrophobicity of the benzene ring forms a local anti-water penetration network in the coating. When the IPA addition reaches 12%, the coating achieves a gloss retention rate of 96.7% after immersion in boiling water for 2 hours, while maintaining an excellent impact resistance of 50 cm, achieving the optimal balance of comprehensive performance. When the IPA addition increases to 14%, although the gloss retention rate can further increase, the impact resistance of the coating drops sharply to below 10 cm due to the excessive accumulation of rigid benzene rings. Therefore, 12% is the critical optimal value that balances water vapor resistance and mechanical toughness, thus achieving the simultaneous attainment of high water vapor resistance and excellent mechanical toughness, breaking through the long-standing technical bottleneck of "water resistance inevitably leads to brittleness" in the industry.

[0020] 5. This invention utilizes a water-resistant mixed powder resin composition that is less prone to clumping, poor melting, and curing defects, exhibits high batch-to-batch product performance consistency, and has a low production scrap rate. This powder coating can be stably used in scenarios involving long-term contact with boiling water and high-temperature steam, such as kitchen appliances (dishwashers, electric kettles, oven liners), bathroom metal components, medical device shells, and outdoor humid and hot metal parts. It overcomes the technical shortcomings of traditional mixed powders that cannot adapt to high humidity and high temperature conditions, expands the application boundaries of environmentally friendly solvent-free powder coatings, and has significant market value. Attached Figure Description

[0021] Figure 1 The graph shows the change in gloss retention rate of the coating after boiling in water for 2 hours under different polyester resin acid values ​​(62, 65, 68, 71 mgKOH / g). Figure 2 The graph shows the change in gloss retention rate of the coating after boiling in water for 2 hours under different mass ratios of 1,2-propanediol to ethylene glycol (8:2, 6:4, 4:2, 2:8). Figure 3 The graph shows the change in gloss retention rate of the coating after boiling in water for 2 hours under different IPA addition amounts (8%, 10%, 12%, 14%) according to the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described in detail below through specific embodiments. Unless otherwise stated, the raw materials used in the following embodiments are all commercially available industrial-grade products. Those skilled in the art should understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the present invention.

[0023] Example 1: Preparation of polyester resin A1 This embodiment provides a polyester resin A1, whose raw material composition is as follows: 3 parts neopentyl glycol (NPG), 14 parts 1,2-propanediol (1,2-PG), 1.5 parts trimethylolpropane (TMP), 7 parts ethylene glycol (EG), 0.10 parts monobutyltin oxide, 55 parts terephthalic acid (PTA), 12 parts isophthalic acid (IPA), 7 parts trimellitic anhydride (TMA), 0.4 parts antioxidant (triphenyl phosphite), and 0.3 parts curing accelerator (triphenylethyl phosphine bromide).

[0024] The preparation method of the above-mentioned polyester resin A1 includes the following steps: Step 1: Add the prescribed amounts of NPG, 1,2-PG, TMP, EG, PTA, and IPA to the reactor, purge with nitrogen for protection, and heat to 110°C to melt the materials and stir evenly.

[0025] Step 2: Add the prescribed amount of monobutyltin oxide, and heat to 185°C at a rate of 8°C / h to carry out an esterification reaction under normal pressure. Continuously distill off the generated water, controlling the temperature at the top of the fractionation column to not exceed 102°C, until the acid value of the reaction system drops to 21 mgKOH / g.

[0026] Step 3: Raise the reaction temperature to 242℃ and continue the esterification reaction at normal pressure for 11 hours until the acid value drops to 14 mgKOH / g.

[0027] Step 4: Cool down to 225℃, add the prescribed amount of antioxidant (triphenyl phosphite) under nitrogen protection, start the vacuum system, control the vacuum degree to -0.098 MPa, and carry out the polycondensation reaction for 3 hours until the acid value drops to 8 mgKOH / g and the melt viscosity at 200℃ reaches 2500 mPa·s.

[0028] Step 5: Purge with nitrogen to break the vacuum, cool to 205℃, add the prescribed amount of TMA, and carry out the end-capping reaction for 2.5 hours under nitrogen protection until the acid value reaches 68 mgKOH / g and the melt viscosity at 200℃ reaches 3621 mPa·s.

[0029] Step 6: Cool to 190℃, add the prescribed amount of curing accelerator (triphenylethyl phosphine bromide), and stir for 0.8 hours until uniformly dispersed. Then discharge, cool, and crush to obtain polyester resin A1.

[0030] Tests showed that polyester resin A1 had an acid value of 68 mgKOH / g, a melt viscosity of 3621 mPa·s at 200℃, and a glass transition temperature (Tg) of 55℃.

[0031] Example 2: Preparation of polyester resin A2 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that the final acid value is 62 mgKOH / g by adjusting the amount of trimellitic anhydride. The preparation method is the same as in Example 1.

[0032] Tests showed that polyester resin A2 had an acid value of 62 mgKOH / g, a melt viscosity of 3210 mPa·s at 200℃, and a Tg of 57℃.

[0033] Example 3: Preparation of polyester resin A3 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that the final acid value is 65 mgKOH / g by adjusting the amount of trimellitic anhydride. The preparation method is the same as in Example 1.

[0034] Tests showed that polyester resin A3 had an acid value of 65 mgKOH / g, a melt viscosity of 4205 mPa·s at 200℃, and a Tg of 56℃.

[0035] Example 4: Preparation of polyester resin A4 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that the final acid value is 71 mgKOH / g by adjusting the amount of trimellitic anhydride. The preparation method is the same as in Example 1.

[0036] Tests showed that polyester resin A4 had an acid value of 71 mgKOH / g, a melt viscosity of 3574 mPa·s at 200℃, and a Tg of 54℃.

[0037] Example 5: Preparation of polyester resin A5 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that: the final acid value of the polyester resin is fixed at 68 mgKOH / g, the total mass of the liquid polyol (1,2-PG+EG) remains unchanged, and the mass ratio of 1,2-PG to EG is adjusted to 8:2. The preparation method is the same as in Example 1.

[0038] Tests showed that polyester resin A5 had an acid value of 68 mgKOH / g, a melt viscosity of 3400 mPa·s at 200℃, and a Tg of 60℃.

[0039] Example 6: Preparation of Polyester Resin A6 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that: the final acid value of the polyester resin is fixed at 68 mgKOH / g, the total mass of the liquid polyol (1,2-PG+EG) remains unchanged, and the mass ratio of 1,2-PG to EG is adjusted to 6:4. The preparation method is the same as in Example 1.

[0040] Tests showed that the acid value of polyester resin A6 was 67 mgKOH / g, the melt viscosity at 200℃ was 3550 mPa·s, and the Tg was 57℃.

[0041] Example 7: Preparation of Polyester Resin A7 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that: the final acid value of the polyester resin is fixed at 68 mgKOH / g, the total mass of the liquid polyol (1,2-PG+EG) remains unchanged, and the mass ratio of 1,2-PG to EG is adjusted to 2:8. The preparation method is the same as in Example 1.

[0042] Tests showed that polyester resin A7 had an acid value of 68 mgKOH / g, a melt viscosity of 3800 mPa·s at 200℃, and a Tg of 53℃.

[0043] Example 8: Preparation of Polyester Resin A8 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that: the final acid value of the polyester resin is fixed at 68 mgKOH / g, the mass ratio of 1,2-PG to EG is 4:2, and the amount of IPA added is adjusted to 8% of the total mass of the polyester resin raw materials (i.e., the amount of IPA is reduced to 8 parts, and the amount of PTA is increased to 59 parts accordingly). The preparation method is the same as in Example 1.

[0044] Tests showed that polyester resin A8 had an acid value of 68 mgKOH / g, a melt viscosity of 3500 mPa·s at 200℃, and a Tg of 57.3℃.

[0045] Example 9: Preparation of Polyester Resin A9 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that: the final acid value of the polyester resin is fixed at 68 mgKOH / g, the mass ratio of 1,2-PG to EG is 4:2, and the amount of IPA added is adjusted to 10% of the total mass of the polyester resin raw materials (i.e., the amount of IPA is 10 parts and the amount of PTA is 57 parts). The preparation method is the same as in Example 1.

[0046] Tests showed that polyester resin A9 had an acid value of 68 mgKOH / g, a melt viscosity of 3550 mPa·s at 200℃, and a Tg of 54.1℃.

[0047] Example 10: Preparation of polyester resin A10 The composition of the polyester resin raw materials in this embodiment is basically the same as that in Example 1, except that: the final acid value of the polyester resin is fixed at 68 mgKOH / g, the mass ratio of 1,2-PG to EG is 4:2, and the amount of IPA added is adjusted to 14% of the total mass of the polyester resin raw materials (i.e., the amount of IPA is 14 parts and the amount of PTA is 53 parts). The preparation method is the same as in Example 1.

[0048] Tests showed that polyester resin A10 had an acid value of 68 mgKOH / g, a melt viscosity of 3700 mPa·s at 200℃, and a Tg of 45.8℃.

[0049] Example 11: Preparation of Powder Coating The polyester resin A1 obtained in Example 1 was mixed with epoxy resin (bisphenol A type epoxy resin with an epoxy equivalent of 820 g / eq) at a mass ratio of 1:1, and the following components were added: 300 parts of titanium dioxide, 280 parts of barium sulfate, 10 parts of leveling agent (polyacrylate), 3 parts of benzoin, and 8 parts of brightening agent.

[0050] Weigh the above components according to the proportions and put them into a high-speed mixer for premixing for 3 to 5 minutes. Then, melt-extrude the mixture through a twin-screw extruder at 95 to 105°C. After the extrudate is cooled by pressing, it is coarsely crushed and finely pulverized, and then passed through a 180-mesh sieve to obtain the powder coating.

[0051] Example 12: Preparation of Powder Coating The polyester resin A2 prepared in Example 2 was used, and the remaining operations were the same as in Example 11.

[0052] Example 13: Preparation of Powder Coating The polyester resin A3 obtained in Example 3 was used, and the remaining operations were the same as in Example 11.

[0053] Example 14: Preparation of Powder Coating The polyester resin A4 obtained in Example 4 was used, and the remaining operations were the same as in Example 11.

[0054] Example 15: Preparation of Powder Coating The polyester resin A5 prepared in Example 5 was used, and the remaining operations were the same as in Example 11.

[0055] Example 16: Preparation of Powder Coating The polyester resin A6 obtained in Example 6 was used, and the remaining operations were the same as in Example 11.

[0056] Example 17: Preparation of Powder Coating The polyester resin A7 prepared in Example 7 was used, and the remaining operations were the same as in Example 11.

[0057] Example 18: Preparation of Powder Coating The polyester resin A8 obtained in Example 8 was used, and the remaining operations were the same as in Example 11.

[0058] Example 19: Preparation of Powder Coating The polyester resin A9 obtained in Example 9 was used, and the remaining operations were the same as in Example 11.

[0059] Example 20: Preparation of Powder Coating The polyester resin A10 prepared in Example 10 was used, and the remaining operations were the same as in Example 11.

[0060] Example 21: Coating Preparation and Performance Testing The powder coatings prepared in Examples 11 to 20 were applied to cold-rolled steel sheet substrates that had undergone surface phosphate treatment by electrostatic spraying. The coating thickness was controlled to be 60 to 80 μm, and the coatings were cured at 180°C for 10 minutes to obtain water-resistant coatings.

[0061] The performance of each coating was tested using the following methods: (1) Acid value: Tested according to GB / T 6743-2008; (2) Melt viscosity: Tested according to GB / T 9751.1-2008, at a test temperature of 200℃; (3) Glass transition temperature (Tg): Tested according to GB / T 19466.2-2004, with a heating rate of 10 K / min; (4) Impact resistance: Tested according to GB / T 1732-1993, the coating is subjected to forward and reverse impact using a paint film impactor, and the maximum height of the coating without cracking is recorded; (5) Gloss: Tested according to GB / T 9754-2007, using a 60° gloss meter; (6) Resistance to boiling water / resistance to water vapor: Tested according to GB / T 1733-1993. The coated sample is immersed in a boiling water bath for 2 hours and then taken out to measure the gloss retention rate (gloss after test / initial gloss × 100%).

[0062] The performance test results of the coatings obtained in Examples 11-20 are shown in the table below: ; The following conclusions can be drawn from the results in the table above: As can be seen from the comparison of Examples 11-14 (acid values ​​62-71 mgKOH / g), and in conjunction with Figure 1 It can be seen that when the acid value increases from 62 mgKOH / g to 68 mgKOH / g, the gloss retention rate of the coating after boiling in water gradually increases from 87.3% to 90.4%, and the positive and negative impact resistance remains at an excellent level of 50 cm. This indicates that moderately increasing the acid value can improve the crosslinking density between the resin and the epoxy curing agent, forming a denser three-dimensional network that effectively prevents water molecules from penetrating into the coating. However, when the acid value is further increased to 71 mgKOH / g, the gloss retention rate decreases to 88.7%, and the negative impact resistance drops sharply to 30 cm. The reason is that an excessively high acid value leads to an excessively high crosslinking density, resulting in excessive rigid constraints and local stress concentration inside the coating. This makes it easier for water molecules to be retained at micro-defects and accelerates hydrolysis. At the same time, excessive carboxyl residues increase the hydrophilicity of the system, weakening the overall water resistance. Therefore, an acid value of 68 mgKOH / g is the optimal balance point.

[0063] As can be seen from the comparison of Examples 11, 15-17 (1,2-PG to EG mass ratio 8:2 to 2:8), and in conjunction with Figure 2It can be seen that as the EG ratio increases, the coating Tg continuously decreases from 60℃ to 53℃. This is attributed to the fact that the shorter carbon chain backbone of the EG molecule (only two methylene groups) gives the polymer chain segments higher conformational freedom and reduces the steric hindrance of chain segment rotation. The gloss retention rate after boiling in water shows a trend of first increasing and then decreasing, reaching a peak of 93.6% at 1,2-PG:EG = 4:2. From the perspective of molecular structure, the side methyl groups (-CH3) of 1,2-PG provide a hydrophobic barrier and steric hindrance effect, which can effectively resist the nucleophilic attack of water molecules on the ester bond. As a symmetrical short-chain diol, the compact molecular structure of EG is conducive to improving the regularity and packing density of polyester segments. However, excessive EG will enhance the hydrophilicity of the system due to the increased hydroxyl density, promoting the penetration of water molecules. When the ratio of the two is 4:2, the hydrophobic protection of the side methyl groups of 1,2-PG and the chain segment regularity and densification brought by EG achieve the optimal balance. At the same time, the ortho-hydroxy groups of EG help to form an intermolecular hydrogen bond network, further stabilizing the microstructure of the coating. Furthermore, the impact resistance of all embodiments remained at 50 cm, indicating that the adjustment of the polyol ratio mainly affects chain dynamics and polarity, rather than crosslinking density.

[0064] As can be seen from the comparison of Examples 11, 18-20 (IPA addition amount 8-14%), and in conjunction with Figure 3 It can be seen that as the amount of IPA added increases, the gloss retention rate of the coating increases monotonically from 94.5% to 98.2%, which fully demonstrates the significant contribution of the aromatic ring structure of IPA to the water vapor resistance: (1) The meta-benzene ring of IPA reduces the electrophilicity of the carbonyl carbon of the ester bond through the conjugation effect, thereby reducing the hydrolysis rate of the ester bond; (2) The steric hindrance of the meta-ester bond is greater than that of the ortho and para positions, effectively shielding the nucleophilic attack of water molecules on the ester bond; (3) The hydrophobic network of the benzene ring forms a dense water-resistant layer in the coating. However, when the IPA content exceeds 12% and reaches 14%, the impact resistance of the coating is completely lost (both forward and reverse impact <10 cm); but when the IPA content reaches 14%, the impact resistance of the coating is completely lost (both forward and reverse impact <10 cm). This indicates that the IPA addition of 12% is the critical point for achieving the optimal balance between water vapor resistance and mechanical toughness. However, when the IPA content exceeds 12% and reaches 14%, the coating's impact resistance is completely lost (both forward and reverse impact <10 cm). This is because the excessive accumulation of rigid benzene ring units severely inhibits the freedom of molecular chain movement, causing the coating to lose its toughness and become brittle. Therefore, an IPA addition of 12% is the critical point that achieves the optimal balance between water vapor resistance and mechanical toughness, at which point the coating possesses both an excellent gloss retention of 96.7% and an impact toughness of 50 cm.

[0065] Comparative Example 1: This comparative example uses a commercially available conventional 50:50 mixed polyester resin (acid value of approximately 62 mgKOH / g, melt viscosity at 200℃ of approximately 3000 mPa·s) mixed with epoxy resin (bisphenol A type, epoxy equivalent of approximately 820 g / eq) at a mass ratio of 1:1. The powder coating was prepared according to the same powder coating formulation and preparation method as in Example 11, and the coating was prepared and its performance was tested according to the same method as in Example 21.

[0066] Test results: The gloss retention rate was 78.2% after boiling in water for 2 hours, with a forward recoil of 50 cm and a reverse recoil of 35 cm. Compared with the embodiments of the present invention, this comparative example shows a significant difference in water vapor resistance.

[0067] Comparative Example 2: This comparative example uses the same polyester resin raw material composition as Example 1, except that isophthalic acid is not added (IPA dosage is 0, and PTA dosage is increased to 67 parts accordingly). The polyester resin is prepared according to the same preparation method as in Example 1, and the powder coating is prepared according to the same powder coating formulation and preparation method as in Example 11. The coating is prepared and its performance is tested according to the same method as in Example 21.

[0068] Test results: The gloss retention rate was 72.5% after boiling in water for 2 hours, with a forward and reverse recoil length of 50 cm. This indicates that the water vapor resistance of the coating without IPA is significantly reduced.

[0069] Comparative Example 3: This comparative example uses the same polyester resin raw material composition as Example 1, except that the mass ratio of 1,2-propanediol to ethylene glycol is 10:0 (i.e., no ethylene glycol is added, and the amount of 1,2-PG is 21 parts). The polyester resin is prepared according to the same preparation method as in Example 1, and the powder coating is prepared according to the same powder coating formulation and preparation method as in Example 11. The coating is prepared and its performance is tested according to the same method as in Example 21.

[0070] Test results: The light retention rate was 82.3% after boiling in water for 2 hours, with a positive and negative pressure of 50 cm. This indicates that the single 1,2-PG system, due to the lack of orderly chain stacking caused by ethylene glycol, has lower water vapor resistance than the compound system.

[0071] Comparative Example 4: This comparative example uses the same polyester resin raw material composition as Example 1, except that the mass ratio of 1,2-propanediol to ethylene glycol is 0:10 (i.e., no 1,2-PG is added, and the amount of EG is 21 parts). The polyester resin is prepared according to the same preparation method as in Example 1, and the powder coating is prepared according to the same powder coating formulation and preparation method as in Example 11. The coating is prepared and its performance is tested according to the same method as in Example 21.

[0072] Test results: The light retention rate was 80.1% after boiling in water for 2 hours, with a positive and negative pressure of 50 cm. This indicates that the single EG system, lacking the hydrophobic protective effect of the 1,2-PG side methyl group, has lower water vapor resistance than the compound system.

[0073] The industrial applicability verification is as follows: The powder coating obtained in Example 11 was further applied to the preparation of coatings for the following scenarios: (1) Kitchen appliances: The powder coating is electrostatically sprayed onto the metal substrate of the electric kettle shell and cured at 180°C for 10 minutes. After the coating is continuously immersed in boiling water at 100°C for 168 hours, the appearance is free of blistering, loss of gloss and peeling. (2) Bathroom equipment: The powder coating was electrostatically sprayed onto the metal parts of the shower head and cured at 180°C for 10 minutes. After the coating was continuously exposed to 85°C hot water steam for 240 hours, there was no change in appearance and the adhesion was good. (3) Medical device shell: The powder coating is electrostatically sprayed onto the surface of the metal substrate of the medical cart and cured at 180°C for 10 minutes. After the coating is sterilized by high pressure steam at 121°C for 30 minutes and 50 cycles, the coating has no blistering, cracking and peeling.

[0074] The above application verification results show that the powder coating provided by the present invention has excellent long-term protective performance in high-temperature water vapor and boiling water environments, and can be widely used as a surface protective coating for metal substrates in harsh humid and hot environments such as kitchen appliances, bathroom equipment, and medical devices.

Claims

1. A water-resistant mixed powder resin composition, comprising polyester resin and epoxy resin in a mass ratio of 1:0.8 to 1.2, characterized in that, The epoxy resin has an epoxy equivalent of 700–900 g / eq; the polyester resin has an acid value of 62–71 mgKOH / g and a melt viscosity of 3000–5000 mPa·s at 200°C; the raw materials of the polyester resin include neopentyl glycol, 1,2-propanediol, trimethylolpropane, ethylene glycol, monobutyltin oxide, terephthalic acid, isophthalic acid, trimellitic anhydride, antioxidants, and curing accelerators, wherein the mass ratio of 1,2-propanediol to ethylene glycol is 4:1 to 1:4, and the isophthalic acid accounts for 8–14% of the total mass of the polyester resin raw materials.

2. The water-resistant mixed powder resin composition according to claim 1, characterized in that, The polyester resin comprises the following raw material components in parts by weight: 1-5 parts neopentyl glycol, 10-15 parts 1,2-propanediol, 1.0-2.0 parts trimethylolpropane, 6-15 parts ethylene glycol, 0.05-0.15 parts monobutyltin oxide, 51-60 parts terephthalic acid, 5-14 parts isophthalic acid, 5-10 parts trimellitic anhydride, 0.3-0.6 parts antioxidant, and 0.2-0.4 parts curing accelerator.

3. The water-resistant mixed powder resin composition according to claim 1, characterized in that, The epoxy resin is one or more of bisphenol A type epoxy resin and bisphenol F type epoxy resin.

4. The water-resistant mixed powder resin composition according to claim 2, characterized in that, The specific steps of the polyester resin preparation method are as follows: S1. Add the prescribed amounts of neopentyl glycol, 1,2-propanediol, trimethylolpropane, ethylene glycol, terephthalic acid, and isophthalic acid to the reaction vessel, purge with nitrogen, heat to 100-120℃ to melt and stir evenly. S2. Add the amount of monobutyltin oxide in the formula, and heat to 160-220℃ at a heating rate of 5-10℃ / h to carry out the esterification reaction under normal pressure. Distill off the generated water, and control the top temperature of the fractionation column to ≤102℃ until the acid value drops to 20-23 mgKOH / g. S3. Raise the reaction temperature to 230-250℃ and continue the esterification reaction at normal pressure until the acid value drops to 10-18 mgKOH / g; S4. Cool down to 220-230℃, add the prescribed amount of antioxidant under nitrogen protection, and perform polycondensation reaction for 2-4 hours under vacuum of -0.095 to -0.1MPa until the acid value drops to 7-10 mgKOH / g and the melt viscosity at 200℃ reaches 2000-3000mPa·s. S5. Purge with nitrogen to break the vacuum, cool to 200-210℃, add the formulated amount of trimellitic anhydride, and seal the reaction under nitrogen protection for 2-3 hours until the acid value reaches 62-71 mgKOH / g and the melt viscosity at 200℃ reaches 3000-5000 mPa·s. S6. Cool to 180-200℃, add the curing accelerator of the formula amount and stir to react for 0.5-1 hour. Then discharge, cool and crush to obtain the polyester resin.

5. The water-resistant mixed powder resin composition according to claim 4, characterized in that, In S2, the esterification reaction temperature is 180–190℃.

6. The water-resistant mixed powder resin composition according to claim 4, characterized in that, In S3, the high-temperature esterification temperature is 240–245°C, and the duration is 10–12 hours.

7. The water-resistant mixed powder resin composition according to claim 4, characterized in that, In S5, the end-capping reaction temperature is 205–210℃.

8. The water-resistant mixed powder resin composition according to claim 4, characterized in that, In S4, the antioxidant is one or more of triphenyl phosphite, antioxidant 1076, and antioxidant 168; in S6, the curing accelerator is one of triphenylethylphosphine bromide and butyltriphenylphosphine chloride.

9. A powder coating, characterized in that, Includes the water-resistant mixed powder resin composition as described in any one of claims 1 to 8, fillers, coating additives, and pigments.

10. The application of the powder coating as described in claim 9 in the preparation of a water-resistant coating.

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