A polyester resin for transparent powder coating with low curing temperature and high leveling property, and its preparation method and application

Abstract

This application discloses a polyester resin for transparent powder coatings with a low curing temperature and high leveling properties, as well as a preparation method and application. The polyester resin for transparent powder coatings comprises, by weight, 20-35% neopentyl glycol; 3.0-10% aliphatic polyols with cyclic structures; 2.5-5.0% aliphatic polyols with asymmetric side chain groups; 0.5-2.0% linear aliphatic polyols without side groups; 35-45% aromatic polyacids; 5-15% aliphatic polyacids with cyclic structures; 0-2.0% linear aliphatic polyacids; 5-8% end-capping agent; 0.05-0.15% esterification catalyst; 0.075-0.2% curing accelerator; and 0.3-0.5% antioxidant. The transparent powder coating prepared from the polyester resin for transparent powder coatings provided herein has a curing temperature below 200°C when sprayed on mirror panels, and can reach a minimum of 150°C. The cured coating film has high transparency, excellent leveling performance, excellent flexibility and can be post-processed.

Description

Technical Field

[0001] The present application relates to the technical field of polyester resins, and in particular to a polyester resin for transparent powder coatings with low curing temperature and high leveling properties, as well as a preparation method and application thereof. Background Art

[0002] Powder coatings, as a green and environmentally friendly industrial coatings category, are characterized by their absence of organic solvents, pollution-free operation, zero VOC emissions, high recycling rates, simple coating processes, and a high degree of automation. They have gained widespread market recognition and are rapidly growing, gradually replacing solvent-based coatings in the market share, becoming the preferred industrial coatings category in the context of the "dual carbon" initiative. In some applications, a topcoat is required over a primer, making clear powder coatings the preferred environmentally friendly topcoat.

[0003] Transparent powder coating systems mainly include: acrylic resin and polyacid curing system, hydroxyl polyester and blocked isocyanate curing system, polyester and TGIC curing system, etc. Transparent powder coatings prepared with acrylic resin and polyacid curing system are mainly used for surface finishing of high-end products such as automotive aluminum wheels, hardware metals, and sanitary ware. However, acrylic resin has poor dispersion performance and requires relatively high extrusion and mixing effects, that is, it has high requirements for equipment and processes. The powder of acrylic resin and polyacid curing system has a low glass transition temperature (about 35°C) and poor storage stability; at the same time, its low surface tension characteristics can easily lead to surface defects of the coating film, so it needs to be produced in a dust-free workshop and transported by refrigeration, which greatly limits its application and promotion.

[0004] Transparent powder coatings based on hydroxyl polyester and blocked isocyanate curing systems are made with a polyester resin as the base resin and a blocked isocyanate as the curing agent. Curing releases unblocked curing agents, requiring recovery equipment. Furthermore, they are poorly compatible with conventional polyester systems and exhibit high brittleness, which limits their large-scale application. Furthermore, polyurethane curing temperatures are mostly around 200°C, leaving limited curing agent options.

[0005] Transparent powder coatings of polyester and TGIC curing systems are generally prepared with polyester resin as the base resin and triglycidyl isocyanurate as the curing agent. When used as a topcoat, it has good compatibility with the underlying coating film and a mature production process. It is a variety that has developed rapidly in recent years. However, ordinary polyester is slightly worse than the acrylic system in transparency, and its leveling performance cannot reach the level of acrylic transparent powder coatings, resulting in poor flatness after topcoating, and slight pinhole defects on the topcoat, which significantly affects the topcoat effect of the base coating. Summary of the Invention

[0006] In response to the above problems, the present application provides a polyester resin for transparent powder coatings with high transparency, good leveling performance, and a curing temperature below 200°C and above 150°C.

[0007] The present application discloses a polyester resin for transparent powder coatings with low curing temperature and high leveling properties. The raw material components include, by weight percentage:

[0008] Neopentyl glycol: 20-35%;

[0009] Aliphatic polyols with cyclic structure: 3.0-10%;

[0010] Asymmetric aliphatic polyols with side chain groups: 2.5-5.0%;

[0011] Non-side group linear aliphatic polyol: 0.5-2.0%;

[0012] Aromatic polybasic acid: 35-45%;

[0013] Aliphatic polybasic acid with cyclic structure: 5-15%;

[0014] Straight-chain aliphatic polybasic acid: 0-2.0%;

[0015] Capping agent: 5-8%;

[0016] Esterification catalyst: 0.05-0.15%;

[0017] Curing accelerator: 0.075~0.2%;

[0018] Antioxidant: 0.3-0.5%.

[0019] This application introduces a cyclic aliphatic polyol into the polyester resin raw material components. The rigid structure of the cyclic aliphatic alcohol ensures that the synthesized polyester resin still has a glass transition temperature of approximately 60°C in the presence of a flexible polyester chain (a linear aliphatic polyol without side groups and a linear aliphatic polyacid). This high glass transition temperature imparts excellent processing properties such as crushing and grinding, as well as good storage stability, to the polyester resin when prepared into a powder coating.

[0020] At the same time, the introduction of asymmetric aliphatic polyols with side chain groups creates a non-crystalline nature for the polyester product, reducing viscosity and improving transparency. Combined with a linear aliphatic polyol without side groups and a linear aliphatic polyacid, the polyester's main chain and side chain structures possess strong resistance to bending under external forces, resulting in a stable molecular structure and excellent flexibility for the polyester resin.

[0021] The amount of aromatic polyacid is controlled, and aliphatic polyacids with cyclic structures and aliphatic polyacids with straight-chain structures are introduced. Compared with aromatic polyacids, aliphatic polyacids with cyclic structures and aliphatic polyacids with straight-chain structures have higher reactivity and can undergo esterification reaction at a relatively low temperature, thereby reducing high-temperature side reactions and yellowing of polyester resins during the synthesis process. The obtained polyester resin has a light chromaticity. Compared with polyester resins with a yellowish phase, polyester resins with a light chromaticity absorb less visible light, and the prepared transparent powder coating film has higher transparency.

[0022] The present application cleverly does not add a branching agent to the polyester raw material components, ensuring that the resin in the esterification and polycondensation stages always has a linear structure and controls the molecular weight from increasing too quickly.

[0023] Several optional methods are also provided below, but they are not intended to be additional limitations on the above-mentioned overall solution. They are merely further supplements or optimizations. Under the premise that there are no technical or logical contradictions, each optional method can be combined separately for the above-mentioned overall solution, or multiple optional methods can be combined.

[0024] Optionally, the aliphatic polyol having a cyclic structure is tricyclodecane dimethanol or a combination of tricyclodecane dimethanol, 1,4-cyclohexane diol, and hydrogenated bisphenol A.

[0025] Optionally, the mass percentage of tricyclodecane dimethanol in the raw material components is 3.0-5.0%. The special spatial asymmetric structure of tricyclodecane dimethanol makes the synthesized polyester less likely to form crystalline regions, has lower viscosity, and has higher transparency than the well-structured polyols and polyacids.

[0026] Optionally, the asymmetrical aliphatic polyol with side chain groups is selected from one or a combination of 2-methyl-2-propyl-1,3-propanediol, 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethyl-1,3-pentanediol.

[0027] Optionally, the side-group-free linear aliphatic polyol is selected from one or a combination of 1,6-hexanediol, 1,4-butanediol, and 1,5-pentanediol.

[0028] Optionally, the aliphatic polyacid having a cyclic structure is selected from one or a combination of 1,4-cyclohexanedicarboxylic acid, hexahydrophthalic anhydride, 1,2-cyclobutanedicarboxylic acid, and furandicarboxylic acid.

[0029] Optionally, the aromatic polyacid is terephthalic acid and isophthalic acid.

[0030] Optionally, the straight-chain aliphatic polyacid is selected from one or more combinations of 1,6-hexanedioic acid, 1,10-decanedioic acid, 1,8-octanedioic acid, 1,12-dodecanedioic acid, 1,9-nonanedioic acid, and 1,7-pimelic acid; preferably one or more combinations of 1,6-hexanedioic acid, 1,10-decanedioic acid, and 1,8-octanedioic acid.

[0031] Optionally, the end-capping agent is a combination of tert-butyl glycidyl carbonate EP-10 and hydrogenated trimellitic anhydride and / or trimellitic anhydride.

[0032] The end-capping agent is a combination of tert-butyl glycidyl ester EP-10 and hydrogenated trimellitic anhydride or trimellitic anhydride: first, it can ensure that the end-capping agent can be carried out at a lower temperature after polycondensation, and the color of the resin is lighter; second, trimellitic anhydride or hydrogenated trimellitic anhydride has high functional group characteristics. Even if one molecule of the polyester molecular segment reacts with tert-butyl glycidyl ester EP-10, three active carboxyl functional groups are still retained.

[0033] The esterification catalyst is one or a combination of stannous oxalate, n-butyl titanate, and tetraisopropyl titanate.

[0034] The antioxidant is a combination of a hindered phenol antioxidant and a phosphite antioxidant.

[0035] The hindered phenol antioxidant is one or a combination of tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]pentaerythritol ester, β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,3,5-(3,5-di-tert-butyl-4-hydroxyphenyl)-s-triazine-2,4,6(1H,3H,5H)trione or N,N'-bis-[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hexanediamine.

[0036] The phosphite antioxidant is one or a combination of tris(2,4-di-tert-butylphenyl) phosphite, bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite or bis(2,6-di-tert-butyl-4-methylphenyl) pentaerythritol diphosphite.

[0037] The curing accelerator is selected from one of triphenylethylphosphonium bromide, triphenylbutylphosphonium bromide, tetrabutylammonium bromide, tetrabutylammonium chloride, tetraethylammonium chloride, tetrapropylammonium bromide, methyltriethylammonium chloride, benzyltrimethylammonium chloride, and benzyltriethylammonium chloride, or a combination thereof.

[0038] The present application also provides a method for preparing a polyester resin for a transparent powder coating having a low curing temperature and high leveling property, comprising the following steps:

[0039] S1. Mix neopentyl glycol, aliphatic polyol with a cyclic structure, aliphatic polyol with asymmetric side chain groups, straight-chain aliphatic polyol without side groups, and an esterification catalyst, and heat to 110-130° C. at normal pressure to melt.

[0040] S2. Add aromatic polybasic acid, aliphatic polybasic acid with a cyclic structure, and aliphatic polybasic acid with a linear structure, and under the protection of protective gas, heat to 170-180°C within 0.5-2 hours, and distill off the esterification water; heat to 193-195°C within 2-6 hours; heat to 230-235°C within 3-5 hours, and keep warm for 1-1.5 hours; when the reaction degree reaches 95%, the heat preservation ends;

[0041] S3, continue the reaction under reduced pressure for 60 to 150 minutes;

[0042] S4, lowering the material temperature, adding the end-capping agent, and reacting for 90 to 120 minutes;

[0043] S5. The material is kept at 210-215° C., a curing accelerator and an antioxidant are added, and the mixture is stirred and reacted for 20-60 minutes to obtain the polyester resin for transparent powder coating described above.

[0044] The preparation method disclosed in the present application has simple procedures, is environmentally friendly and has high production efficiency.

[0045] Optionally, in step S4, when the material temperature drops to 220-225°C, trimellitic anhydride and / or hydrogenated trimellitic anhydride in the capping agent is added, the temperature is controlled at 213-217°C, and the reaction is carried out for 60-80 minutes; then tert-butyl carbonate EP-10 in the capping agent is added, the temperature is controlled at 210-215°C, and the reaction is carried out for 30-40 minutes.

[0046] The highly active carboxyl functional groups in the polyester molecular chain, combined with a controlled acid value of 38-42 mgKOH / g, combined with a curing accelerator, reduce the activation energy of the reaction system, enabling low-temperature curing. The high carboxylic acid concentration also ensures the density of the resulting powder coating film. This low-temperature curing capability is expected to be used in coatings on heat-sensitive substrates, further expanding the application prospects of transparent polyester powder coatings.

[0047] The present application cleverly does not add a branching agent to the polyester component, ensuring that the resin always has a linear structure during the esterification and polycondensation stages, controlling the rapid increase of the molecular weight, and combining a low-temperature polycondensation process (230-235°C, conventional polyester: 240-245°C) to reduce the viscosity of the synthesized polyester resin.

[0048] The active epoxy groups in versatate glycidyl carbonate EP-10 react with the carboxyl groups in the polyester resin to produce hydroxyl groups. The large side groups in the molecular formula of versatate glycidyl carbonate EP-10 enhance the fluidity of the polyester resin, ensuring a mirror-like leveling effect in the resulting powder coating. When the powder coating is used as a topcoat, this improves compatibility with the base film. The small amount of hydroxyl groups formed during the end-capping reaction enhances adhesion between the coating and the metal substrate, contributing to superior impact resistance when subjected to external forces or secondary processing.

[0049] The present application also provides a method for preparing a transparent powder coating having a low curing temperature and high leveling properties, comprising uniformly mixing the above-described polyester resin for transparent powder coating, a curing agent, a leveling agent, and a degassing agent, dispersing the mixture through an extruder, and grinding and screening the mixture to produce the transparent powder coating. The curing agent is preferably TGIC.

[0050] The coating film has excellent flexibility, good processing properties such as crushing and grinding, and good storage stability.

[0051] This application innovates the design of the polyester resin main chain structure and end-capping structure to prepare a polyester resin with properties such as light chroma, a higher glass transition temperature than acrylic resin, low viscosity, and high reactivity. The transparent powder coating prepared using the polyester resin disclosed in this application, which has a low curing temperature and high leveling properties, can be cured below 200°C and at a minimum of 150°C when sprayed on a mirror panel. The cured coating film has high transparency and excellent leveling properties. At the same time, the coating film has excellent flexibility and can be subjected to secondary processing such as bending and stamping. If sprayed on a primer with a pattern, the primer pattern can be made realistic. DETAILED DESCRIPTION

[0052] The contents of the present application are described in more detail below through examples. However, the following examples are only used to illustrate the contents of the present application rather than to limit them. Therefore, any changes within the meaning and scope equivalent to the claims of the present application should be deemed to be included in the scope of the claims.

[0053] The reagents, methods and equipment used in this application are conventional reagents, methods and equipment in this technical field. Unless otherwise specified, the reagents and materials used in this application are commercially available.

[0054] Table 1 shows the raw material formulations and performance indicators of the polyesters synthesized in Examples 1-3 and Comparative Examples 1-7. The acid value of the polyester resin was measured according to GB / T 6743-2008; the viscosity was measured according to GB / T 9751.1-2008; and the glass transition temperature was measured according to GB / T 19466.2-2004.

[0055]

[0056] Preparation of polyester resin:

[0057] Examples 1 to 3 and Comparative Examples 1 to 5 were synthesized using the following process:

[0058] Step S1: In a reaction kettle equipped with a heating device, a stirrer, and a distillation column, neopentyl glycol, a cyclic aliphatic polyol, an asymmetric aliphatic polyol with side chain groups, a linear aliphatic polyol without side groups, and an esterification catalyst are added according to the materials and amounts listed in Table 1, mixed evenly, and heated to 110-130° C. at normal pressure to melt;

[0059] Step S2: adding an aromatic polyacid, an aliphatic polyacid having a cyclic structure, and an aliphatic polyacid having a linear structure according to the materials and amounts recorded in Table 1, stirring under nitrogen protection and heating at a rate of 2-3°C per minute; when the material temperature rises to 170-180°C, esterification water begins to be generated and evaporated through the distillation column; at this time, the heating rate is controlled to increase the temperature by 2-3°C every 30 minutes; when the material temperature rises to 195°C±2°C, the heating rate is controlled to increase the temperature by 4-6°C every 30 minutes, and when the material temperature rises to 230-235°C, the reaction is kept warm for 1-1.5 hours; when the temperature at the top of the distillation column drops to less than 70°C and the esterification water output reaches 95% or more of the theoretical output, the insulation is terminated;

[0060] Step S3: gradually evacuating the mixture to -0.098 to -0.095 MPa at 230 to 235° C., and continuing the reaction for 60 to 150 minutes;

[0061] Step S4: lowering the material temperature to 220-225° C., adding trimellitic anhydride and / or hydrogenated trimellitic anhydride in the end-capping agent in the formula amount, controlling the temperature at 215° C.±2° C., and reacting for 60-80 minutes; then controlling the material temperature at 215° C.±2° C., adding tertiary glycidyl carbonate EP-10 in the end-capping agent in the formula amount, controlling the temperature at 210-215° C., and reacting for 30-40 minutes;

[0062] Step S5: The material is kept warm at 210-215° C., a curing accelerator and an antioxidant are added, and the mixture is stirred and reacted for 40 minutes. The polyester resins of Examples 1-3 and Comparative Examples 1-5 are obtained by discharging the material.

[0063] Comparative Example 6 Synthesis of polyester resin:

[0064] In the formulation system of Comparative Example 6, tert-butyl glycidyl carbonate EP-10 as the end-capping agent was not included. According to the above process, the step of adding tert-butyl glycidyl carbonate EP-10 and reacting in step S4 was omitted to synthesize the polyester resin.

[0065] Comparative Example 7 Synthesis of polyester resin:

[0066] The formulation system of Comparative Example 7 does not contain trimellitic anhydride or hydrogenated trimellitic anhydride end-capping agent. If the above process is used, the reaction will be incomplete, affecting the quality indicators of the synthesized polyester resin.

[0067] Comparative Example 7 was synthesized according to the following process:

[0068] Step 1: In a reaction kettle equipped with a heating device, a stirrer, and a distillation column, add neopentyl glycol, aliphatic polyol with a cyclic structure, aliphatic polyol with asymmetric side chain groups, aliphatic polyol without side groups, and an esterification catalyst according to the materials and amounts listed in Table 1, mix them evenly, and heat to 110-130°C at normal pressure to melt.

[0069] Step 2: Add aromatic polyacids, aliphatic polyacids with a cyclic structure, and aliphatic polyacids with a linear structure according to the materials and amounts recorded in Table 1, and under nitrogen protection, stir and heat at a rate of 2-3°C per minute; when the material temperature rises to 170-180°C, esterification water begins to be generated and evaporated through the distillation column; at this time, the heating rate is controlled to increase the temperature by 2-3°C every 30 minutes; when the material temperature rises to 195°C±2°C, the heating rate is controlled to increase the temperature by 4-6°C every 30 minutes, and when the material temperature rises to 235°C±2°C, the reaction is kept warm for 1-1.5 hours; when the temperature at the top of the distillation column drops to less than 70°C and the esterification water output reaches 95% or more of the theoretical output, the insulation is terminated;

[0070] Step 3: When the material temperature drops to 220-225° C., add the corresponding amounts of isophthalic acid and adipic acid in the end-capping agent according to Table 1, raise the temperature to 240° C., and react at 240° C. for 60-90 minutes;

[0071] Step 4: At 230-235°C, gradually evacuate to -0.098-0.095 MPa and continue the reaction for 60-150 minutes;

[0072] Step 5: The material temperature is controlled at 215°C ± 2°C, tertiary ester of glycidyl carbonate EP-10 in the formula amount of the end-capping agent is added, the temperature is controlled at 210-215°C, and the reaction is carried out for 30-40 minutes;

[0073] Step 6: The material is kept warm at 210-215° C., a curing accelerator and an antioxidant are added, the mixture is stirred and reacted for 40 minutes, and the polyester resin is obtained by discharging the material.

[0074] Preparation of clear powder coating:

[0075] Table 2 Transparent powder coating component ratio (unit: g)

[0076] name polyester resin curing agent Leveling agent Degassing agent quality 454 39.5 4 2.5

[0077] In Table 2, the curing agent is Huntsman PT810 triglycidyl isocyanurate; the leveling agent is Troy Powdermate 486CFL; and the degassing agent: benzoin (benzoin) is MIWON BENZOIN.

[0078] The three polyester resins synthesized in Examples 1 to 3 and the seven polyester resins synthesized in Comparative Examples 1 to 7 were added with a curing agent (TGIC), a leveling agent, a degassing agent, etc. according to the proportions listed in Table 2, and uniformly mixed, dispersed by an extruder, and powdered and sieved. After electrostatic spraying, the mixture was cured at 150°C / 15min-20min to form a film, and the coating performance was tested.

[0079] Performance test of transparent powder coating:

[0080] Transparency and appearance of the coating film: The prepared powder coating was sprayed on a mirror stainless steel plate. After curing, the transparency was evaluated by visually comparing the clarity of the mirror image after spraying the powder coating with that of the pure mirror image.

[0081] The coating film thickness, gloss, leveling grade, impact resistance, adhesion, bending resistance and other properties are tested by spraying onto ordinary steel plates.

[0082] The leveling grade of the coating film adopts the PCI grade, which specifically refers to visually comparing the coating film to be tested with the PCI leveling grade control plate (grades 0-10) to determine the corresponding leveling grade of the coating film to be tested. This is a common leveling grade determination method in the industry.

[0083] The thickness test of the coating film is carried out in accordance with the standard of GB / T 13452.2-2008;

[0084] The gloss test of the coating film is carried out in accordance with the standard of GB / T1743-1979;

[0085] The impact performance test of the coating is carried out in accordance with the standard of GB-T1732-1993;

[0086] The adhesion test of the coating film is carried out in accordance with the standard of GB / T 9286-1998;

[0087] The bending resistance test of the coating film is tested in accordance with the standard GB / T30791-2014;

[0088] The gel time test of powder coatings is carried out in accordance with the standard GB / T 16995-1997;

[0089] The test results are shown in Table 3.

[0090] Table 3 Performance test results of transparent powder coatings

[0091]

[0092] As can be seen from Table 3, the transparent powder coatings prepared in Examples 1 to 3 are cured at 150°C for 15 to 20 minutes, achieving low-temperature curing. In addition, the prepared coatings have a smooth, delicate surface, good transparency, and a leveling grade of 9 to 10, with a mirror effect. The bending performance can reach 0T, and the impact performance can pass both positive and negative 50kg.cm. The coatings have excellent flexibility and can be applied to workpieces requiring post-processing.

[0093] Compared with Comparative Example 1, Example 1 shows that the transparent powder coating has better processing performance, better transparency, and higher leveling grade, indicating that the aliphatic polyol with a cyclic structure has a greater influence on the glass transition temperature of the polyester resin; at the same time, it helps to reduce the viscosity of the system, and the transparent coating film achieves a higher leveling grade and transparency.

[0094] Comparing Example 1 with Comparative Example 2, the coating film has better transparency and a higher leveling grade. Combined with Comparative Example 1, this demonstrates that cyclic aliphatic polyols play an important role in the glass transition temperature, viscosity, and transparency of polyester resin systems. Furthermore, the presence of tricyclodecane dimethanol in a certain amount is essential to ensure low viscosity and high transparency in the system.

[0095] Comparing Example 2 with Comparative Example 3, the system of Comparative Example 3 has high viscosity, poor leveling (leveling grade ≤ level 6), poor impact (poor recoil), poor bending performance (1T), and poor transparency (gloss 128%), indicating that asymmetric aliphatic polyols with side chain groups play a positive role in obtaining a transparent powder coating with high leveling, post-processing ability, and high transparency.

[0096] Comparative Example 4 shows worse bending performance (2T) compared with Example 2 and Comparative Example 3, and its transparency is comparable to that of Example 2, indicating that the non-side-group linear aliphatic polyol plays a key role in the flexibility of the coating film in the system.

[0097] Comparing Comparative Example 5 with Example 1, while no cyclic aliphatic polyacid was added and the amount of aromatic polyacid was increased, the viscosity of the polyester resin increased significantly, and the chromaticity shifted to yellow. The film in Comparative Example 5 exhibited a significant decrease in transparency, a surface tinge of orange peel, and a significant decrease in transparency. Impact and bending properties also declined. This demonstrates that the amount of aromatic polyacid used in synthesizing a polyester resin for a transparent powder with high leveling, post-processing properties, and high transparency requires controlled use. While ensuring processability (a high glass transition temperature), a certain amount of aliphatic polyacid must be used.

[0098] Comparison of Example 3 with Comparative Example 6 reveals that the inclusion of tertiary glycidyl carbonate EP-10 in the end-capping agent plays a crucial role in controlling the viscosity and transparency of the polyester resin, thereby ensuring the transparency of the coating film. Comparison of Example 3 with Comparative Example 7 demonstrates that the powder coating cannot cure at low temperatures without the use of trimellitic anhydride or hydrogenated trimellitic anhydride in the end-capping agent.

[0099] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form, and any technical solutions obtained by equivalent replacement or equivalent transformation fall within the scope of protection of the present invention.

Claims

1. A polyester resin for transparent powder coatings having low curing temperature and high leveling properties, characterized in that: In terms of weight percentage, the components of the raw materials include: Neopentyl glycol: 20-35%; Aliphatic polyols with cyclic structure: 3.0-10%; Asymmetric aliphatic polyols with side chain groups: 2.5-5.0%; Non-side group linear aliphatic polyol: 0.5-2.0%; Aromatic polybasic acid: 35.0-45%; Aliphatic polybasic acid with cyclic structure: 5-15%; Straight-chain aliphatic polybasic acid: 0-2.0%; Capping agent: 5-8%; Esterification catalyst: 0.05-0.15%; Curing accelerator: 0.075~0.2%; Antioxidant: 0.3-0.5%; The aliphatic polyol with a cyclic structure is tricyclodecane dimethanol or a combination of tricyclodecane dimethanol and at least one of 1,4-cyclohexane diol and hydrogenated bisphenol A; the mass percentage of tricyclodecane dimethanol in the raw material components is 3.0-5.0%; the asymmetric aliphatic polyol with side chain groups is selected from one or a combination of 2-methyl-2-propyl-1,3-propanediol, 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethyl-1,3-pentanediol; and the end-capping agent is a combination of tert-butyl carbonate EP-10 and hydrogenated trimellitic anhydride and / or trimellitic anhydride.

2. The polyester resin for transparent powder coatings with low curing temperature and high leveling properties according to claim 1, characterized in that: The aliphatic polybasic acid with a cyclic structure is selected from one or a combination of 1,4-cyclohexanedicarboxylic acid, hexahydrophthalic anhydride, 1,2-cyclobutanedicarboxylic acid, and furan dicarboxylic acid.

3. The polyester resin for transparent powder coatings with low curing temperature and high leveling properties according to claim 1, characterized in that: The aromatic polyacids are terephthalic acid and isophthalic acid.

4. A method for preparing a polyester resin for transparent powder coatings with low curing temperature and high leveling properties, characterized in that: The following steps are involved: S1. Mix neopentyl glycol, aliphatic polyol with a cyclic structure, aliphatic polyol with asymmetric side chain groups, straight-chain aliphatic polyol without side groups, and an esterification catalyst, and heat to 110-130° C. at normal pressure to melt. S2. Add aromatic polybasic acid, aliphatic polybasic acid with a cyclic structure, and aliphatic polybasic acid with a linear structure, and under the protection of protective gas, heat to 170-180°C within 0.5-2 hours, and distill off the esterification water; heat to 193-195°C within 2-6 hours; heat to 230-235°C within 3-5 hours, and keep warm for 1-1.5 hours; when the reaction degree reaches 95%, the heat preservation ends; S3, continue the reaction under reduced pressure for 60 to 150 minutes; S4, lowering the material temperature, adding the end-capping agent, and reacting for 90 to 120 minutes; S5. Keep the material heated at 210-215° C., add a curing accelerator and an antioxidant, and stir the mixture for 20-60 minutes to obtain the polyester resin for transparent powder coating according to any one of claims 1-3.

5. The method for preparing a polyester resin for transparent powder coatings with low curing temperature and high leveling properties according to claim 4, characterized in that: In step S4, when the material temperature drops to 220-225° C., trimellitic anhydride and / or hydrogenated trimellitic anhydride in the end-capping agent is added, the temperature is controlled at 213-217° C., and the reaction is carried out for 60-80 minutes; then tertiary ester of glycidyl carbonate EP-10 in the end-capping agent is added, the temperature is controlled at 210-215° C., and the reaction is carried out for 30-40 minutes.

6. A method for preparing a transparent powder coating with low curing temperature and high leveling properties, characterized in that: include: The transparent powder coating prepared by the preparation method according to any one of claims 4 to 5 is prepared by uniformly mixing polyester resin, curing agent, leveling agent and degassing agent, dispersing through an extruder, grinding and screening, and preparing a transparent powder coating.

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