A polyester resin for powder coating having high hardness and excellent adhesion, and a method for preparing the same

Abstract

The present application relates to a kind of high hardness, the polyester resin for powder coating with excellent adhesion, which uses the raw material polymerization obtained including o-cresol formaldehyde epoxy resin by-product, xylene, isobutyric anhydride, adipic acid, terephthalic acid, tetraethylene glycol, phenyl succinic anhydride.In the preparation of polyester resin, the functionality is higher, and has the structure of o-cresol formaldehyde epoxy resin, as 70 / 30 polyester resin product for indoor powder coating is used, the final coating not only has high hardness, but also has excellent adhesion, other conventional performance such as coating appearance, impact resistance, gloss, etc. can also meet the requirements, at the same time, the high value-added comprehensive utilization of o-cresol formaldehyde epoxy resin by-product is realized, and has excellent economic and environmental value.

Description

Technical Field

[0001] This invention relates to the field of powder coating technology, specifically to a high-hardness, high-adhesion polyester resin for powder coatings, its preparation method, and its application in 70 / 30 powder coatings. Background Technology

[0002] Powder coatings have advantages such as no VOC generation, energy saving, and high efficiency, and have seen rapid development in recent years, being widely used in industries such as metal products, furniture, construction, automobiles, and electrical appliances. Indoor mixed powder coatings are mainly divided into 50 / 50, 60 / 40, and 70 / 30 types due to differences in application. Among them, the 70 / 30 type powder coating has a larger amount of polyester resin in its formulation. Since the price of E-12 epoxy resin is generally 2000-3000 yuan / ton higher than that of polyester resin, the 70 / 30 type powder coating has a certain cost advantage compared to the 50 / 50 and 60 / 40 types. However, because epoxy resin has better adhesion than polyester resin, the adhesion of conventional 70 / 30 powder coatings is worse than that of 60 / 40 and 50 / 50. How to improve the adhesion of 70 / 30 powder coatings is one of the research directions in this field.

[0003] o-Cresol-formaldehyde epoxy resin, as a primary resin raw material for electronic packaging, boasts advantages such as excellent adhesion, strong bonding properties, and good curing performance. It is widely used as a molding compound for electronic components such as semiconductor devices and integrated circuits to protect these components from environmental corrosion and maintain product performance and lifespan. Therefore, the requirements for o-cresol-formaldehyde epoxy resin are extremely high, namely, high purity and very low levels of residual chloride and sodium ions and hydrolyzable chlorine. Due to these stringent requirements for purity and chloride ion control, multiple purification processes are necessary in actual production. Generally, solvent extraction and multiple water washing steps are required to obtain high-purity o-cresol-formaldehyde epoxy resin. However, these multiple post-processing purification steps generate a large number of byproducts. These byproducts, after evaporation and concentration, are currently mainly used as low-end fillers in adhesives, and no truly high-value-added applications have yet been developed. Summary of the Invention

[0004] To address the aforementioned problems, this invention combines the excellent adhesion of o-crestyrene epoxy resin. The polyester resin of this invention is mainly obtained by polymerizing o-crestyrene epoxy resin byproducts, xylene, isobutyric anhydride, adipic acid, terephthalic acid, tetraethylene glycol, and phenylsuccinic anhydride. During the preparation process, the o-crestyrene epoxy resin byproducts are first purified, and then modified with isobutyric anhydride to adjust their functionality, ensuring the prepared polyester resin maintains suitable toughness and meets the requirements for impact resistance. Simultaneously, adipic acid is used for preliminary polymerization, followed by polymerization reactions of terephthalic acid and tetraethylene glycol, and a special end-capping agent, phenylsuccinic anhydride, is used for end-capping. The resulting polyester resin has high functionality and possesses the structure of o-crestyrene epoxy resin. The final coating film not only has high hardness but also excellent adhesion. Other conventional properties such as coating appearance, impact resistance, and gloss also meet the requirements. Furthermore, this invention achieves high-value-added comprehensive utilization of o-crestyrene epoxy resin byproducts, demonstrating excellent economic and environmental value.

[0005] A high-hardness, high-adhesion polyester resin for powder coatings comprises the following raw materials in parts by weight:

[0006] 10-12 parts of o-cresol formaldehyde epoxy resin byproducts;

[0007] 22-26 parts xylene;

[0008] Isobutyric anhydride 0.8-1.1 parts;

[0009] Adipic acid 6-9 parts;

[0010] 8-12 parts of terephthalic acid;

[0011] Tetraethylene glycol 12-15 parts;

[0012] 5-8 parts of phenylsuccinic anhydride;

[0013] The components also include catalysts and antioxidants;

[0014] The molecular structure of the o-cresol formaldehyde epoxy resin byproduct is as follows:

[0015] ,

[0016] n is 4-7.

[0017] For example, the o-cresol formaldehyde epoxy resin by-product has an epoxy equivalent of 350-500 g / mol, a hydroxyl equivalent of 150-250 g / mol, a chloride content of 1-3%, and a solid content of 75-85%, and was purchased from Jining Tangyi Chemical Co., Ltd.

[0018] The catalyst is tetrabutyl titanate, with a dosage of 0.4-0.7% of the total raw material mass; the antioxidant is antioxidant 1076, namely β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate octadecyl ester, with a dosage of 0.25-0.4% of the total raw material mass.

[0019] A method for preparing a polyester resin for powder coatings with high hardness and excellent adhesion includes the following steps:

[0020] A. Add the formulated amount of o-cresol epoxy resin byproduct to reactor 1, start stirring and heat to 80-90℃, then start the vacuum system to remove the remaining moisture from the byproduct under reduced pressure.

[0021] B. After no obvious water has been evaporated, stop the vacuum, add the amount of xylene solvent in the formula, heat to the boiling point of xylene to continue the water-removing reaction, and when the water content in the distilled xylene solvent is less than 0.5%, cool down to room temperature to allow the insoluble matter to precipitate and settle, and filter to obtain a transparent filtrate and filter residue. The filter residue is sent for wastewater treatment.

[0022] C. Pump the obtained filtrate into reactor 2, add the prescribed amount of catalyst and isobutyric anhydride, heat to 137-140℃ to carry out esterification end-capping reaction, so as to adjust and reduce the functionality of o-cresol epoxy resin by-products, and use xylene as a dehydrating agent to carry out water separation esterification reaction to obtain modified polymer.

[0023] D. Sampling and testing: When the acid value of the modified polymer is lower than 3 mg KOH / g, add the amount of adipic acid in the formula to carry out the preliminary chain extension polymerization reaction, while keeping the temperature at 137-140℃.

[0024] E. Take samples to test the acid value. When the acid value of the polymer reaches 10-18 mg KOH / g, add the prescribed amount of tetraethylene glycol and terephthalic acid, and gradually raise the temperature to 220-225℃. During the heating process, remove the xylene solvent and keep the reaction at the temperature.

[0025] F. Detect the acid value of the polymer in the system. When the acid value of the polymer reaches 25-34 mgKOH / g, add the prescribed amount of antioxidant and start the vacuum system to carry out the vacuum polycondensation reaction.

[0026] G. When the acid value of the polymer reaches 10-16 mgKOH / g, stop the vacuum system, add the prescribed amount of end-capping agent phenylsuccinic anhydride, heat to 230-235℃ for reaction, stop the reaction when the acid value of the polymer reaches 28-37 mgKOH / g, then discharge at high temperature, cool the polyester resin, crush and granulate to obtain the polyester resin.

[0027] In step A, the vacuum degree is controlled between -0.095MPa and -0.097MPa; in step E, the temperature is gradually increased to 220-225℃ at a heating rate of 8-11℃ / h; in step F, the vacuum degree is controlled between -0.096MPa and -0.098MPa; and in step G, the polyester resin is cooled with a steel belt.

[0028] For example, the method for preparing the polyester resin for powder coating with high hardness and excellent adhesion includes the following steps:

[0029] A. Add the formulated amount of o-cresol epoxy resin byproduct to reactor 1, start stirring and heat to 80-90℃, then start the vacuum system to remove the remaining moisture from the byproduct under reduced pressure, and control the vacuum degree at -0.095MPa to -0.097MPa.

[0030] B. After no obvious water has been evaporated, stop the vacuum, add the amount of xylene solvent in the formula, and heat to the boiling point of xylene to continue the water-removing reaction. When the water content in the distilled xylene solvent is less than 0.5%, it means that the water has been basically distilled off. At this time, cool down to room temperature to allow insoluble substances such as inorganic salts to precipitate and settle, and filter to obtain a transparent filtrate and some filter residue (inorganic salts). The filter residue is sent for wastewater treatment.

[0031] C. Pump the obtained filtrate into reactor 2, add the prescribed amount of catalyst and isobutyric anhydride, heat to 137-140℃ to carry out esterification end-capping reaction, so as to adjust and reduce the functionality of o-cresol epoxy resin by-products, and use xylene as a dehydrating agent to carry out water separation esterification reaction to obtain modified polymer.

[0032] D. Sampling and testing: When the acid value of the modified polymer is lower than 3 mg KOH / g, it indicates that the modification of isobutyric anhydride is basically complete. At this time, add the amount of adipic acid in the formula to carry out the preliminary chain extension polymerization reaction, and keep the temperature at 137-140℃.

[0033] E. Take samples to test the acid value. When the acid value of the polymer reaches 10-18 mgKOH / g, it indicates that the preliminary chain extension polymerization of adipic acid is complete. At this time, add the prescribed amount of tetraethylene glycol and terephthalic acid, and gradually raise the temperature to 220-225℃ at a heating rate of 8-11℃ / h. During the heating process, remove the xylene solvent and maintain the temperature for reaction. F. Test the acid value of the polymer in the system. When the acid value of the polymer reaches 25-34 mgKOH / g, add the prescribed amount of antioxidant, start the vacuum system to carry out vacuum polycondensation reaction, and control the vacuum degree at -0.096MPa to -0.098MPa.

[0034] G. When the acid value of the polymer reaches 10-16 mgKOH / g, stop the vacuum system, add the prescribed amount of end-capping agent phenylsuccinic anhydride, heat to 230-235℃ for reaction, stop the reaction when the acid value of the polymer reaches 28-37 mgKOH / g, then discharge at high temperature, cool the polyester resin with a steel belt, crush and granulate to obtain polyester resin.

[0035] The resulting product is a light yellow transparent granule with an acid value of 28-37 mgKOH / g and a softening point of 114-125℃.

[0036] This invention also relates to the application of the aforementioned high-hardness, high-adhesion polyester resin for powder coatings, or the high-hardness, high-adhesion polyester resin for powder coatings obtained by the aforementioned preparation method, in indoor 70 / 30 powder coatings. It is prepared using polyester resin, E-12 epoxy resin, titanium dioxide, barium sulfate, leveling agent, brightening agent, benzoin, etc. The coating preparation method is as follows: the materials are mixed according to the powder coating formula, extruded using a twin-screw extruder, pressed into sheets, crushed, and then the sheets are pulverized and sieved to produce powder coating. The powder coating is sprayed onto a surface-treated tinplate substrate using an electrostatic spray gun and cured to obtain the coating layer.

[0037] Beneficial effects:

[0038] This invention employs a cleverly designed polyester resin formulation, primarily utilizing byproducts of o-cresaldehyde epoxy resin, xylene, isobutyric anhydride, adipic acid, terephthalic acid, tetraethylene glycol, and phenyl succinic anhydride for polymerization. In this reaction system, isobutyric anhydride is mainly used to adjust the functionality of the o-cresaldehyde epoxy resin byproducts, while also possessing good hydrophobicity and the ability to lower the softening point of the polyester resin, thus improving leveling performance. Adipic acid is mainly used to adjust the segmental flexibility of the o-cresaldehyde epoxy resin byproducts, ensuring the overall impact performance of the polyester resin segments meets requirements. Tetraethylene glycol is mainly used to adjust the segmental flexibility of the polyester resin, lower the softening point and melt viscosity, achieving the desired impact, appearance, leveling, and gloss. The high activity of the terminal carboxyl group in phenyl succinic anhydride, along with the presence of the phenyl group, enhances the water resistance of the segments.

[0039] In the reaction process, the o-crestyrene epoxy resin byproducts are first purified, and then modified with isobutyric anhydride to adjust their functionality so that the prepared polyester resin maintains suitable toughness and meets the requirements for impact resistance. Simultaneously, adipic acid is used for preliminary polymerization, followed by polymerization reactions with terephthalic acid and tetraethylene glycol. A special end-capping agent, phenylsuccinic anhydride, is used for end-capping treatment. The resulting polyester resin has high functionality and the structure of o-crestyrene epoxy resin. When used as a 70 / 30 polyester resin product for indoor powder coatings, the final coating film not only has high hardness but also excellent adhesion. Other conventional properties such as coating appearance, impact resistance, and gloss also meet the requirements. This process achieves high-value-added comprehensive utilization of the o-crestyrene epoxy resin byproducts, demonstrating excellent economic and environmental value. Detailed Implementation

[0040] The present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. It should also be pointed out that, unless otherwise indicated, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terms "comprising" or "including" and similar words used in this invention refer to elements or objects preceding the word that encompass the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0041] All raw materials used in this invention are commercially available. For example, the molecular structure of the o-cresol formaldehyde epoxy resin byproduct is as follows:

[0042] ,

[0043] n is 4-7.

[0044] The o-cresol formaldehyde epoxy resin byproduct has an epoxy equivalent of 350-500 g / mol, a hydroxyl equivalent of 150-250 g / mol, a chloride content of 1-3%, and a solid content of 75-85%. It was purchased from Jining Tangyi Chemical Co., Ltd.

[0045] Preparation Example 1

[0046] A polyester resin for powder coatings with high hardness and excellent adhesion comprises the following raw materials in parts by weight:

[0047] 10 parts of o-cresol formaldehyde epoxy resin byproduct;

[0048] 22 parts xylene;

[0049] Isobutyric anhydride 0.9 parts;

[0050] 7 parts adipic acid;

[0051] 9 parts terephthalic acid;

[0052] 12 parts of tetraethylene glycol;

[0053] 6 parts of phenylsuccinic anhydride;

[0054] The catalyst is tetrabutyl titanate, and its dosage is 0.5% of the total mass of raw materials; the antioxidant is antioxidant 1076, and its dosage is 0.4% of the total mass of raw materials.

[0055] The method for preparing the high-hardness, high-adhesion polyester resin for powder coatings includes the following steps:

[0056] A. Add the formulated amount of o-cresol epoxy resin byproduct to reactor 1, start stirring and heat to 85°C, then start the vacuum system to remove the remaining moisture from the byproduct under reduced pressure, with the vacuum degree controlled at -0.095MPa.

[0057] B. After no obvious water has been evaporated, stop the vacuum, add the amount of xylene solvent in the formula, and heat to the boiling point of xylene to continue the water-removing reaction. When the water content in the distilled xylene solvent is less than 0.5%, it means that the water has been basically distilled off. At this time, cool down to room temperature to allow insoluble substances such as inorganic salts to precipitate and settle, and filter to obtain a transparent filtrate and some filter residue (inorganic salts). The filter residue is sent for wastewater treatment.

[0058] C. The obtained filtrate is pumped into reactor 2, and the prescribed amount of catalyst and isobutyric anhydride are added. The temperature is raised to 137°C to carry out the esterification end-capping reaction in order to adjust and reduce the functionality of the o-cresol epoxy resin by-product. Xylene is used as a dehydrating agent to carry out the dehydration esterification reaction to obtain the modified polymer.

[0059] D. Sampling and testing: When the acid value of the modified polymer is lower than 3 mg KOH / g, it indicates that the modification of isobutyric anhydride is basically complete. At this time, add the amount of adipic acid in the formula to carry out the preliminary chain extension polymerization reaction, and keep the temperature at 137℃.

[0060] E. Take samples to test the acid value. When the acid value of the polymer reaches 10 mg KOH / g, it indicates that the preliminary chain extension polymerization of adipic acid is complete. At this time, add the amount of tetraethylene glycol and terephthalic acid in the formula, and gradually raise the temperature to 220°C at a heating rate of 10°C / h. During the heating process, remove the xylene solvent and keep the reaction at the temperature.

[0061] F. Detect the acid value of the polymer in the system. When the acid value of the polymer reaches 25 mg KOH / g, add the prescribed amount of antioxidant, start the vacuum system to carry out the vacuum polycondensation reaction, and control the vacuum degree at -0.096 MPa.

[0062] G. When the acid value of the polymer reaches 10 mg KOH / g, stop the vacuum system, add the prescribed amount of end-capping agent phenyl succinic anhydride, heat to 230℃ for reaction, stop the reaction when the acid value of the polymer reaches the expected value, then discharge at high temperature, cool the polyester resin with a steel belt, crush and granulate to obtain polyester resin.

[0063] The resulting product is a light yellow transparent granule with an acid value of 35 mg KOH / g and a softening point of 116℃.

[0064] Preparation Example 2

[0065] A polyester resin for powder coatings with high hardness and excellent adhesion comprises the following raw materials in parts by weight:

[0066] 12 parts of o-cresol formaldehyde epoxy resin byproduct;

[0067] 26 parts of xylene;

[0068] Isobutyric anhydride 1.1 parts;

[0069] Adipic acid 9 parts;

[0070] 12 parts terephthalic acid;

[0071] 15 parts of tetraethylene glycol;

[0072] 8 parts of phenylsuccinic anhydride;

[0073] The catalyst is tetrabutyl titanate, and its dosage is 0.7% of the total mass of raw materials; the antioxidant is antioxidant 1076, and its dosage is 0.4% of the total mass of raw materials.

[0074] The preparation method is the same as in Preparation Example 1.

[0075] The resulting product is a light yellow transparent granule with an acid value of 36 mg KOH / g and a softening point of 122℃.

[0076] Preparation Example 3

[0077] A polyester resin for powder coatings with high hardness and excellent adhesion comprises the following raw materials in parts by weight:

[0078] 11 parts of o-cresol formaldehyde epoxy resin byproducts;

[0079] 24 parts of xylene;

[0080] 1 part isobutyric anhydride;

[0081] 8 parts adipic acid;

[0082] 10 parts terephthalic acid;

[0083] 14 parts of tetraethylene glycol;

[0084] 7 parts of phenylsuccinic anhydride;

[0085] The catalyst is tetrabutyl titanate, and its dosage is 0.7% of the total mass of raw materials; the antioxidant is antioxidant 1076, and its dosage is 0.4% of the total mass of raw materials.

[0086] The preparation method is the same as in Preparation Example 1.

[0087] The resulting product is a light yellow transparent granule with an acid value of 33 mg KOH / g and a softening point of 117℃.

[0088] Preparation Example 4

[0089] A polyester resin for powder coatings with high hardness and excellent adhesion comprises the following raw materials in parts by weight:

[0090] 11 parts of o-cresol formaldehyde epoxy resin byproducts;

[0091] 23 parts xylene;

[0092] Isobutyric anhydride 0.9 parts;

[0093] 8 parts adipic acid;

[0094] 10 parts terephthalic acid;

[0095] 13 parts of tetraethylene glycol;

[0096] 6 parts of phenylsuccinic anhydride;

[0097] The catalyst is tetrabutyl titanate, and its dosage is 0.7% of the total mass of raw materials; the antioxidant is antioxidant 1076, and its dosage is 0.4% of the total mass of raw materials.

[0098] The preparation method is the same as in Preparation Example 1.

[0099] The resulting product is a light yellow transparent granule with an acid value of 32 mg KOH / g and a softening point of 119℃.

[0100] Comparative preparation example 1:

[0101] It uses commercially available 70 / 30 polyester resin, model ZJ7031, from Huangshan Zhengjie New Materials Co., Ltd.

[0102] Examples 1-4 and Comparative Example 1 (prepared using the polyester resins from Preparations 1-4 and Comparative Example 1, respectively):

[0103] The general-purpose 70 / 30 mixed powder coating formulation, by weight, is as follows:

[0104] 420 parts of the above-mentioned polyester resin;

[0105] 180 parts of E-12 epoxy resin;

[0106] 180 parts of titanium dioxide;

[0107] 180 parts of barium sulfate;

[0108] 10 parts leveling agent;

[0109] 8 parts brightening agent;

[0110] Four portions of benzoin.

[0111] Coating preparation:

[0112] According to the powder coating formula, all materials are mixed evenly, extruded, pressed, and crushed using a twin-screw extruder, and then the sheets are pulverized and sieved (150-180 mesh) to produce powder coating. The powder coating is then sprayed onto the surface-treated tinplate substrate using an electrostatic spray gun, with a film thickness of about 80μm, and cured at 180℃ for 15 min to obtain the coating layer.

[0113] Performance Comparison

[0114] The coating index testing is based on GB / T 21776-2008 "Guideline for Testing Standards of Powder Coatings and Their Coatings". The pencil hardness test is conducted according to GB / T6739-2006 "Determination of Hardness of Paint Film by Pencil Method for Paints and Varnishes". The adhesion test standard is based on GB / T 9286-1998 "Cross-cut Test of Paint Films for Paints and Varnishes".

[0115] The results of testing the coating performance of the polyester resins prepared according to the coating formulations provided by the present invention are shown in Table 1 below.

[0116] Table 1 Performance of Powder Coating Film

[0117]

[0118] As shown in the table above, the cured coating of the product of this invention has a smooth and even appearance with a high gloss level, generally above 93%, and good impact resistance. These conventional properties are not significantly different from those of ordinary commercially available products. However, due to the special structure of the product of this invention, which introduces the high functionality of the o-cresphenolic epoxy resin byproduct, the hardness of the coating is significantly improved, all above 2H, which is higher than that of ordinary commercially available products. At the same time, it also brings about a significant improvement in adhesion, with conventional adhesion reaching level 0, and the adhesion does not decrease significantly after boiling in water, effectively overcoming the shortcomings of 70 / 30 mixed powder coatings. In addition, this solution can realize the high-value-added comprehensive utilization of o-cresphenolic epoxy resin byproducts, which has significant environmental and economic value.

[0119] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A polyester resin for powder coating, characterized in that, Includes the following raw materials in parts by weight: 10-12 parts of o-cresol formaldehyde epoxy resin byproducts; 22-26 parts xylene; Isobutyric anhydride 0.8-1.1 parts; Adipic acid 6-9 parts; 8-12 parts of terephthalic acid; Tetraethylene glycol 12-15 parts; 5-8 parts of phenylsuccinic anhydride; The components also include catalysts and antioxidants; The molecular structure of the o-cresol formaldehyde epoxy resin byproduct is as follows: ， n is 4-7; The o-cresol formaldehyde epoxy resin byproduct has an epoxy equivalent of 350-500 g / mol, a hydroxyl equivalent of 150-250 g / mol, a chloride content of 1-3%, and a solid content of 75-85%. The method for preparing the polyester resin for powder coating includes the following steps: A. Add the formulated amount of o-cresol epoxy resin byproduct to reactor 1, start stirring and heat to 80-90℃, then start the vacuum system to remove the remaining moisture from the byproduct under reduced pressure. B. After no obvious water has been evaporated, stop the vacuum, add the amount of xylene solvent in the formula, heat to the boiling point of xylene to continue the water-removing reaction, and when the water content in the distilled xylene solvent is less than 0.5%, cool down to room temperature to allow the insoluble matter to precipitate and settle, and filter to obtain a transparent filtrate and filter residue. The filter residue is sent for wastewater treatment. C. Pump the obtained filtrate into reactor 2, add the prescribed amount of catalyst and isobutyric anhydride, heat to 137-140℃ to carry out esterification end-capping reaction, so as to adjust and reduce the functionality of o-cresol epoxy resin by-products, and use xylene as a dehydrating agent to carry out water separation esterification reaction to obtain modified polymer. D. Sampling and testing: When the acid value of the modified polymer is lower than 3 mg KOH / g, add the amount of adipic acid in the formula to carry out the preliminary chain extension polymerization reaction, while keeping the temperature at 137-140℃. E. Take samples to test the acid value. When the acid value of the polymer reaches 10-18 mg KOH / g, add the prescribed amount of tetraethylene glycol and terephthalic acid, and gradually raise the temperature to 220-225℃. During the heating process, remove the xylene solvent and keep the reaction at the temperature. F. Detect the acid value of the polymer in the system. When the acid value of the polymer reaches 25-34 mgKOH / g, add the prescribed amount of antioxidant and start the vacuum system to carry out the vacuum polycondensation reaction. G. When the acid value of the polymer reaches 10-16 mgKOH / g, stop the vacuum system, add the prescribed amount of end-capping agent phenylsuccinic anhydride, heat to 230-235℃ for reaction, stop the reaction when the acid value of the polymer reaches 28-37 mgKOH / g, then discharge at high temperature, cool the polyester resin, crush and granulate to obtain the polyester resin.

2. The polyester resin for powder coating as described in claim 1, characterized in that, The catalyst is tetrabutyl titanate, and the amount used is 0.4-0.7% of the total mass of the raw materials.

3. The polyester resin for powder coating as described in claim 1, characterized in that, The antioxidant is antioxidant 1076, and the dosage is 0.25-0.4% of the total mass of the raw materials.

4. A method for preparing a polyester resin for powder coatings as described in any one of claims 1-3, characterized in that, In step A, the vacuum level is controlled between -0.095 MPa and -0.097 MPa.

5. The method for preparing a polyester resin for powder coating as described in claim 4, characterized in that, In step E, the temperature is gradually increased to 220-225℃ at a heating rate of 8-11℃ / h.

6. The method for preparing a polyester resin for powder coating as described in claim 4, characterized in that, In step F, the vacuum level is controlled between -0.096 MPa and -0.098 MPa; In step G, the polyester resin is cooled using a steel belt.

7. The application of the polyester resin for powder coating as described in any one of claims 1-3 or the polyester resin for powder coating with high hardness and excellent adhesion obtained by the preparation method described in any one of claims 4-6 in indoor 70 / 30 powder coating.

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