A polyester resin for metal powder coating and its preparation method and application

Abstract

The present invention provides a polyester resin for metal powder coating, a preparation method and application thereof, wherein the raw materials for preparing the polyester resin for metal powder coating include: polyol, halogenated bisphenol A, terephthalic acid, phthalic anhydride, an acidolysis agent and tert-butyl glycidyl carbonate. The polyester resin for metal powder coating of the present invention enhances the polarity of the polyester resin due to the introduction of halogenated bisphenol A with halogen side groups, and the enhancement of polarity is beneficial to improving the surface tension of the resin. During the high-temperature curing process of the metal powder coating, the metal pigment can reduce the influence of gravity sedimentation under the pulling force of the resin surface tension, stabilize the metal pigment on the coating surface, and make the coating show a better metal effect.

Description

Technical Field

[0001] The invention belongs to the technical field of powder coatings, and in particular relates to a polyester resin for metal powder coatings and a preparation method and application thereof. Background Art

[0002] As people pay more attention to environmental protection, as environmentally friendly, high-performance, high-efficiency and economical powder coatings, with the rapid development of technology in recent years, the varieties are increasing and the application fields are gradually expanding. In terms of decoration, in addition to the ordinary powder coating varieties with different gloss and flat surface, the varieties and usage of orange grain, wrinkle, sand grain, pattern, hammer grain and metal powder coatings are also increasing, and gradually penetrate into various fields of solvent-based coatings. Among the above varieties, metal powder coatings are recognized by the industry as a high-end product in the field of powder coatings. Its products can be further subdivided into metal powder coatings such as metal flash, metal plating, metal pattern, hammer grain and metal gloss texture. The characteristic of this powder coating is that it contains metal pigments and fillers. By changing the type and specification of metal pigments and fillers, the coating formula and the manufacturing process, powder coatings with different performance and coating appearance can be obtained.

[0003] At present, most high-performance metal powder coatings on the market are prepared by thermal bonding. Since the bonding method has high requirements for bonding equipment and processes for preparing metal powder coatings, more research has been carried out on bonding equipment and processes. Research reports on metal powder coatings are basically concentrated on bonding equipment and processes, and there are basically no reports on the research of supporting polyesters. Due to the lack of matching polyester resins on the market, technologically advanced powder manufacturers rely on strong formula research and development design capabilities, and use advanced bonding equipment and strict process control to prepare metal powder coatings, but it is still difficult to solve the problem of instability between product batches. For most small and medium-sized enterprises with weak financial strength and insufficient technical capabilities, it is difficult to carry out the research and development and production of related metal powders. In order to promote the development of metal powder coatings, it is urgent to link up upstream and downstream to strengthen research on manufacturing equipment, processes and raw materials. In terms of supporting resins, there is currently a lack of research on polyesters specifically for metal powder coatings. In order to meet the market's demand for the rapid increase in the use of metal powder coatings, it is necessary to provide a polyester resin suitable for metal powder coatings to reduce dependence on equipment and process control, thereby promoting the rapid development of metal powder coatings. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the first aspect of the present invention provides a polyester resin for metal powder coating, and the bonding metal powder coating prepared by using the polyester has a prominent metal effect and a beautiful appearance.

[0005] The second aspect of the present invention provides a method for preparing the polyester resin for metal powder coating.

[0006] A third aspect of the present invention provides a metal powder coating comprising the polyester resin for metal powder coating.

[0007] According to a first aspect of the present invention, a polyester resin for metal powder coating is provided. The raw materials for preparing the polyester resin include: polyol, halogenated bisphenol A, terephthalic acid, phthalic anhydride, an acidolysis agent and tert-butyl glycidyl carbonate.

[0008] In some embodiments of the present invention, the polyester resin for metal powder coating is prepared from raw materials including, by mass fraction, 20wt% to 35wt% of polyol, 3wt% to 20wt% of halogenated bisphenol A, 30wt% to 55wt% of terephthalic acid, 2wt% to 10wt% of phthalic anhydride, 5wt% to 15wt% of acidolysis agent and 1wt% to 5wt% of tert-butyl glycidyl carbonate.

[0009] In some preferred embodiments of the present invention, the halogenated bisphenol A has a structural formula of Formula I:

[0010]

[0011] wherein each R is independently selected from H and / or halogen, and at least one R is H and at least one R is halogen.

[0012] Preferably, the halogenated bisphenol A comprises dihalogenated bisphenol A having the structural formula Ia:

[0013]

[0014] Wherein, R1 is halogen;

[0015] Further preferably, the halogenated bisphenol A has a structural formula of Formula Ib:

[0016]

[0017] In some preferred embodiments of the present invention, the amount of the halogenated bisphenol A is 5 wt % to 20 wt % by mass, preferably 5 wt % to 15 wt %.

[0018] In some more preferred embodiments of the present invention, the polyol includes at least one of neopentyl glycol, ethylene glycol, 2-methyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 1,3-propanediol, 1,6-hexanediol, 1,4-cyclohexanediol or trimethylolpropane; preferably, it is a combination of neopentyl glycol and other polyols. Further preferably, the ratio of neopentyl glycol to other polyols is greater than 5:1; further preferably, the ratio of neopentyl glycol to other polyols is greater than 6:1.

[0019] In some more preferred embodiments of the present invention, the amount of the polyol is 25 wt% to 35 wt% by mass; preferably 28 wt% to 33 wt%.

[0020] In some more preferred embodiments of the present invention, the amount of terephthalic acid is 30 wt% to 50 wt% by mass; preferably 35 wt% to 50 wt%; and more preferably 35 wt% to 45 wt%.

[0021] In some more preferred embodiments of the present invention, the amount of the phthalic anhydride acid is 3 wt% to 10 wt% by mass; preferably 4 wt% to 10 wt%; and more preferably 5 wt% to 10 wt%.

[0022] In some more preferred embodiments of the present invention, the acidolysis agent includes at least one of isophthalic acid, succinic acid, adipic acid or 1,4-cyclohexanedicarboxylic acid; preferably, the amount of the acidolysis agent is 7wt% to 15wt% by mass; preferably 7wt% to 12wt%.

[0023] In some more preferred embodiments of the present invention, the amount of versatile glycidyl carbonate is 1.5 wt% to 5 wt% by mass; preferably 2 wt% to 5 wt%; and more preferably 3 wt% to 5 wt%.

[0024] In some more preferred embodiments of the present invention, the raw materials for preparing the polyester resin for metal powder coatings further include an esterification catalyst, a curing accelerator, and an antioxidant. In the raw materials for preparing the polyester resin for metal powder coatings of the present invention, polyols, halogenated bisphenol A, terephthalic acid, phthalic anhydride, acidolysis agent, and tert-butyl glycidyl carbonate are components of the skeleton structure of the polyester resin, wherein tert-butyl glycidyl carbonate is used as a capping agent; the esterification catalyst is used to adjust the esterification polycondensation reaction rate and reaction degree, and the curing accelerator and antioxidant are used to improve certain properties of the polyester resin, respectively.

[0025] In some more preferred embodiments of the present invention, the esterification catalyst includes at least one of monobutyltin oxide, dibutyltin oxide or stannous oxalate. Preferably, the amount of the esterification catalyst is 0.05% to 0.15% by mass of the total material.

[0026] In some more preferred embodiments of the present invention, the curing accelerator includes at least one of benzyltriethylammonium chloride, triphenylphosphine, phosphine bromide, triphenylphosphine or tert-butylamine; the amount of the curing accelerator is 0.01% to 0.1% by mass of the total material.

[0027] In some more preferred embodiments of the present invention, the antioxidant includes a primary antioxidant and a secondary antioxidant, the primary antioxidant includes antioxidant 1076 and / or antioxidant 1010; the secondary antioxidant includes antioxidant 168 and / or antioxidant 626. Preferably, the mass ratio of the primary antioxidant to the secondary antioxidant is 1:2 to 2:1, and more preferably 1:1.

[0028] In some more preferred embodiments of the present invention, the acid value of the polyester resin for metal powder coating is 30 mgKOH / g to 35 mgKOH / g, the melt viscosity at 200°C is 3500 mp.s to 9000 mp.s, the glass transition temperature Tg is 60°C to 70°C, and the reactivity at 180°C is 120s to 360s. Preferably, the melt viscosity at 200°C of the polyester resin for metal powder coating is 4000 mp.s to 8000 mp.s, the glass transition temperature Tg is 62°C to 70°C, and the reactivity at 180°C is 180s to 360s. Further preferably, the melt viscosity at 200°C of the polyester resin for metal powder coating is 5000 mp.s to 7000 mp.s, the glass transition temperature Tg is 62°C to 68°C, and the reactivity at 180°C is 240s to 330s.

[0029] According to a second aspect of the present invention, a method for preparing a polyester resin for metal powder coating is provided, comprising the following steps:

[0030] S1: In an inert atmosphere, heat and melt the polyol and halogenated bisphenol A, then add terephthalic acid to carry out esterification and polycondensation reaction until the material becomes clear and transparent;

[0031] S2: After cooling, add phthalic anhydride to react until the material is clear and transparent; continue cooling, add acidolysis agent to react until the material is clear and transparent, and control the acid value to be between 45 mgKOH / g and 52 mgKOH / g;

[0032] S3: Cooling down to carry out vacuum polycondensation reaction, and releasing the vacuum when the acid value of the material is 35 mgKOH / g to 42 mgKOH / g;

[0033] S4: Continue to cool down, add tert-butyl glycidyl carbonate to carry out end-capping reaction, and obtain the polyester resin for metal powder coating.

[0034] In some embodiments of the present invention, in S1, the temperature of the esterification polycondensation reaction is 240°C to 245°C.

[0035] In some preferred embodiments of the present invention, in S2, the temperature is lowered to 230°C to 236°C, phthalic anhydride is added to react until the material becomes clear and transparent; the temperature is further lowered to 228°C to 233°C, an acidolysis agent is added to react until the material becomes clear and transparent, and the acid value is controlled to be between 45 mgKOH / g and 52 mgKOH / g.

[0036] In some more preferred embodiments of the present invention, in S3, the temperature of the vacuum polycondensation reaction is 225°C to 230°C, and the vacuum degree is -0.085 MPa to -0.1 MPa; preferably -0.095 MPa.

[0037] The method for preparing the polyester resin for metal powder coating comprises the following steps:

[0038] S1: In an inert atmosphere, heat and melt the polyol, halogenated bisphenol A, and esterification catalyst, and then add terephthalic acid to carry out esterification polycondensation reaction until the material becomes clear and transparent;

[0039] S2: After cooling, add phthalic anhydride to react until the material is clear and transparent; continue cooling, add acidolysis agent to react until the material is clear and transparent, and control the acid value to be between 45 mgKOH / g and 52 mgKOH / g;

[0040] S3: Cooling down to carry out vacuum polycondensation reaction, and releasing the vacuum when the acid value of the material is 35 mgKOH / g to 42 mgKOH / g;

[0041] S4: Continue to cool down, add tert-butyl glycidyl carbonate to perform end-capping reaction, and when the acid value of the material is 30 mgKOH / g to 35 mgKOH / g, cool down, add curing agent and antioxidant, and stir to obtain the polyester resin for metal powder coating.

[0042] According to a third aspect of the present invention, a metal powder coating is provided, comprising the polyester resin for metal powder coating.

[0043] The beneficial effects of the present invention are:

[0044] 1. The polyester resin for metal powder coating of the present invention has enhanced polarity of polyester resin due to the introduction of halogenated bisphenol A with halogen side groups, and the enhanced polarity is conducive to improving the surface tension of the resin. During the high-temperature curing process of metal powder coating, the metal pigment can reduce the influence of gravity sedimentation under the pulling force of the resin surface tension, stabilize the metal pigment on the coating surface, and make the coating show better metal effect.

[0045] 2. In the present invention, tert-butyl glycidyl carbonate is introduced into the main chain of the polyester resin as a capping agent. The tert-carbon group suspended in the structure of tert-butyl glycidyl carbonate can increase the wettability of the metal pigment, so that the metal pigment is better coated in the resin, thereby reducing the risk of peeling after the metal pigment is bonded to the resin.

[0046] 3. The preparation method of the polyester resin for metal powder coating of the present invention allows all raw materials to be fully polymerized into the main chain, bringing further optimized performance to the resin, which is reflected in: (a) the reaction activity of phthalic anhydride is significantly higher than that of terephthalic acid. Its addition in the second step can not only avoid the competitive polymerization with terephthalic acid resulting in incomplete reaction of terephthalic acid, but also ensure the complete reaction of halogenated bisphenol A; (b) tert-butyl glycidyl carbonate is a monofunctional group. Its addition in the early stage of synthesis will affect the polymerization degree of the main chain. At the same time, the larger side groups in its structure will significantly reduce the Tg of the polyester. The present invention utilizes the high reactivity of the epoxy group in tert-butyl glycidyl carbonate, uses it as a capping agent and reacts with the carboxyl group at a relatively low temperature to introduce it into the main chain, which will not affect the polymerization degree of the main chain. At the same time, placing the tertiary carbon group at the end of the main chain has a limited effect on the Tg of the polyester. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0048] Figure 1 The figure shows the distribution of silver powder in the metal powder coating of the polyester resin in Example 1 of the present invention.

[0049] Figure 2 The figure shows the distribution of silver powder in the metal powder coating of the polyester resin in Comparative Example 1 of the present invention.

[0050] Figure 3 The figure shows the distribution of silver powder in the metal powder coating of the polyester resin in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0051] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0052] Embodiments 1 to 5

[0053] S1: According to the formula composition in Table 1, the polyol, halogenated bisphenol A and esterification catalyst are heated under the protection of inert gas until they are melted, and then terephthalic acid is added, and the temperature is gradually increased to 240° C. to 245° C. to carry out esterification polycondensation reaction until the material is clear and transparent;

[0054] S2: Cool down to below 236°C, add phthalic anhydride, and continue the reaction until the material is clear and transparent;

[0055] S3: Cool down to below 233°C, add acidolysis agent and continue the reaction until the material is clear and transparent, and control the acid value in the range of 45mgKOH / g to 52mgKOH / g;

[0056] S4: Cooling to below 228°C for vacuum polycondensation reaction, and releasing the vacuum when the acid value of the reaction material reaches 35 mgKOH / g to 42 mgKOH / g.

[0057] S5: Cooling down to below 210° C., adding tert-butyl glycidyl carbonate for end-capping reaction, and when the acid value of the material reaches 30 to 35 mgKOH / g, cooling down, adding a curing accelerator and an antioxidant, stirring and dispersing the mixture evenly, and then discharging the material to obtain a polyester resin for metal powder coating.

[0058] Comparative Example 1

[0059] This comparative example prepares a polyester resin, which is different from Example 5 in that tert-butyl glycidyl carbonate is adjusted to the first step of feeding, and the specific process is as follows:

[0060] S1: According to the formulation in Table 1, polyol, halogenated bisphenol A, tert-butyl glycidyl carbonate and esterification catalyst are heated under the protection of inert gas until they are melted, and then terephthalic acid is added, and the temperature is gradually raised to the insulation temperature to carry out esterification polycondensation reaction until the material is clear and transparent;

[0061] S2: Cool down to below 236°C, add phthalic anhydride, and continue the reaction until the material is clear and transparent;

[0062] S3: Cool down to below 233°C, add acidolysis agent and continue the reaction until the material is clear and transparent, and control the acid value in the range of 42mgKOH / g to 46mgKOH / g;

[0063] S4: Cooling down to carry out vacuum polycondensation reaction, and releasing the vacuum when the acid value of the reaction material reaches 30 mgKOH / g to 35 mgKOH / g.

[0064] S5: Cooling the mixture to below 210°C, adding a curing accelerator and an antioxidant, stirring and dispersing the mixture evenly, and then discharging the mixture to obtain a polyester resin.

[0065] Comparative Example 2

[0066] This comparative example prepared a polyester resin, and the specific process was as follows:

[0067] 1) According to the formulation in Table 1, the polyol and the esterification catalyst are heated under the protection of inert gas until they are melted, and then terephthalic acid is added, and the temperature is gradually increased to the insulation temperature to carry out esterification polycondensation reaction until the material is clear and transparent;

[0068] 2) Cooling to below 235°C, adding an acidolysis agent to continue the reaction until the material is clear and transparent, and controlling the acid value in the range of 42 mgKOH / g to 46 mgKOH / g;

[0069] 3) The temperature is lowered to carry out vacuum polycondensation reaction, and the vacuum is released when the acid value of the reaction material reaches 30 mgKOH / g to 35 mgKOH / g.

[0070] 4) Cooling, adding curing accelerator and antioxidant, stirring and dispersing evenly, and then discharging the material to obtain polyester resin.

[0071] Table 1 Polyester resin formula composition

[0072]

[0073]

[0074] Test example

[0075] The polyester resins obtained in Examples 1 to 5 and Comparative Examples 1 to 2 were tested for performance, wherein:

[0076] The acid value is tested according to GB / T6743-2008;

[0077] Melt viscosity: ICI cone and plate viscosity measured at 200°C according to ASTM D4287-88;

[0078] Glass transition temperature (Tg): tested in accordance with GB / T 19466.2;

[0079] Surface tension: Tested in accordance with GB / T22237-2008;

[0080] Reactivity refers to the time required for the resin and curing agent to react to gel, and is measured at 180°C according to GB / T 16995-1997.

[0081] The test results are shown in Table 2:

[0082] Table 2 Properties of polyester resin

[0083]

[0084] Furthermore, the polyester resins prepared according to the methods of Examples 1-5 and Comparative Examples 1-2 are respectively used to prepare metal powder coatings. The preparation of metal powder coatings can be divided into two stages according to the process flow: powder coating base powder preparation and bonding processing: 1) Preparation of powder coating base powder: weigh the components according to the base powder formula in Table 3, and obtain the required powder coating base powder after premixing, melt extrusion, cooling and tableting, crushing and pulverizing and sieving; 2) Bonding processing: put the powder base powder, silver powder and bonding aid into the bonding pot according to the metal powder coating formula in Table 3, stir and heat at high speed, and bond. After bonding, quickly put the materials into a cold mixing pot for low-speed stirring and cooling, and obtain metal powder coating after sieving. The bonded metal powder coating is electrostatically sprayed on the pre-treated cold-rolled steel plate, cured at 200°C for 10 minutes to obtain a coating sample, and its coating performance and metal effect are tested. The test results are shown in Table 4 below.

[0085] Table 3 Composition of metal powder coatings

[0086]

[0087] The effect of metal powder is mainly to meet the apparent metal effect, and the judgment method is basically a visual method, which is also the method commonly used in the industry. Leveling mainly observes the flatness of the coating to see if there are orange peel phenomena, etc. It is generally compared with the PCI standard board, and 1-9 indicates flatness from poor to good. The uniformity of metal powder mainly depends on whether the metal powder is evenly distributed on the surface of the sample, whether there are areas with too much metal powder and some areas with less, which is called flower blooming in the industry. As a metal powder, if the bonding effect is not good, the free metal pigment will be easily adsorbed around the sample due to electrostatic adsorption, making the four sides shiny, which is what the industry calls the frame effect. If the metal pigment settles during the curing process, it will directly affect the metal effect of the coating. Only when the metal pigment floats on the surface of the coating can a coating with a metal effect be obtained. The distribution of the metal pigment on the surface of the coating can be observed microscopically through a scanning electron microscope. The electron microscope scanning images of the coatings in Example 1, Comparative Example 1 and Comparative Example 2 correspond to the following respectively. Figure 1 , Figure 2 and Figure 3 shown.

[0088] Table 4 Properties of metal powder coatings

[0089] Appearance leveling Metal powder uniformity Silver powder floating situation Border effect Example 1 5-6 Uniform Floating none Example 2 7-8 Uniform Floating none Example 3 4-5 Uniform Floating none Example 4 6-7 Uniform Floating none Example 5 6-7 Uniform Floating none Comparative Example 1 6-7 Uniform Slight sinking none Comparative Example 2 6-7 Flowering Obvious sinking obvious

[0090] From the comparison results in Table 4, it can be seen that when the polyester prepared by the formula and process of the present invention is used for the preparation of metal powder coating, the metal pigment and the base powder are fully bonded together, the metal pigment and the base powder are basically not peeled off, and the coating does not have flowery and frame effects. The distribution of silver powder in the coating was observed by scanning electron microscopy ( Figure 1-Figure 3 ) It can be seen that the silver powder in the coating prepared by the polyester of the present invention floats and is evenly distributed on the surface of the coating, the silver powder in the coating of Comparative Example 1 prepared by the formula provided by the present invention and the conventional process only sinks slightly, and the silver powder in the coating of Comparative Example 2 prepared by the conventional formula sinks significantly. In addition, the polyester of the present invention can meet the preparation of metal powder coatings with different leveling grades.

[0091] The above is a detailed description of the embodiments of the present invention, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A polyester resin for metal powder coating, characterized in that: The preparation raw materials include: the polyester resin for metal powder coating, the preparation raw materials include, by mass fraction: 20wt%~35wt% polyol, 3wt%~20wt% halogenated bisphenol A, 30wt%~55wt% terephthalic acid, 2wt%~10wt% phthalic anhydride, 5wt%~15wt% acidolysis agent and 1wt%~5wt% tert-butyl glycidyl carbonate; The preparation method of the polyester resin for metal powder coating comprises the following steps: S1: In an inert atmosphere, heat and melt the polyol and halogenated bisphenol A, then add terephthalic acid to carry out esterification and polycondensation reaction until the material becomes clear and transparent; S2: After cooling, add phthalic anhydride to react until the material is clear and transparent; continue cooling, add acidolysis agent to react until the material is clear and transparent, and control the acid value to be 45mgKOH / g~52mgKOH / g; S3: Cooling down to carry out vacuum polycondensation reaction, and releasing the vacuum when the acid value of the material is 35mgKOH / g to 42mgKOH / g; S4: Continue to cool down, add tert-butyl glycidyl carbonate to carry out end-capping reaction, and obtain the polyester resin for metal powder coating.

2. The polyester resin for metal powder coating according to claim 1, characterized in that: The halogenated bisphenol A has a structural formula of Formula I: ； wherein each R is independently selected from H or halogen, and at least one R is H and at least one R is halogen.

3. The polyester resin for metal powder coating according to claim 1, characterized in that: The polyol includes at least one of neopentyl glycol, ethylene glycol, 2-methyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 1,3-propylene glycol, 1,6-hexanediol, 1,4-cyclohexanediol or trimethylolpropane.

4. The polyester resin for metal powder coating according to claim 3, characterized in that: The polyol comprises a combination of neopentyl glycol and other polyols, and the ratio of the neopentyl glycol to the other polyols is greater than 5:

1.

5. The polyester resin for metal powder coating according to claim 1, characterized in that: The raw materials for preparing the polyester resin for metal powder coating also include an esterification catalyst, a curing accelerator and an antioxidant.

6. The polyester resin for metal powder coating according to claim 1, characterized in that: The polyester resin for metal powder coating has an acid value of 30 mgKOH / g to 35 mgKOH / g, a melt viscosity of 3500 mp.s to 9000 mp.s at 200° C., a glass transition temperature Tg of 60° C. to 70° C., and a reactivity of 120s to 360s at 180° C.

7. A method for preparing a polyester resin for metal powder coating, characterized in that: The following steps are involved: S1: In an inert atmosphere, heat and melt the polyol and halogenated bisphenol A, then add terephthalic acid to carry out esterification and polycondensation reaction until the material becomes clear and transparent; S2: After cooling, add phthalic anhydride to react until the material is clear and transparent; continue cooling, add acidolysis agent to react until the material is clear and transparent, and control the acid value to be 45mgKOH / g~52mgKOH / g; S3: Cooling down to carry out vacuum polycondensation reaction, and releasing the vacuum when the acid value of the material is 35mgKOH / g to 42mgKOH / g; S4: Continue to cool, add tert-butyl glycidyl carbonate to carry out end-capping reaction, and obtain the polyester resin for metal powder coating as claimed in any one of claims 1 to 6.

8. The method for preparing a polyester resin for metal powder coating according to claim 7, characterized in that: In S2, the temperature is lowered to 230°C~236°C, and phthalic anhydride is added to react until the material becomes clear and transparent; the temperature is further lowered to 228°C~233°C, and an acidolysis agent is added to react until the material becomes clear and transparent, and the acid value is controlled to be 45mgKOH / g~52mgKOH / g.

9. The method for preparing a polyester resin for metal powder coating according to claim 7, characterized in that: The method for preparing the polyester resin for metal powder coating comprises the following steps: S1: In an inert atmosphere, heat and melt the polyol, halogenated bisphenol A, and esterification catalyst, and then add terephthalic acid to carry out esterification polycondensation reaction until the material becomes clear and transparent; S2: After cooling, add phthalic anhydride to react until the material is clear and transparent; continue cooling, add acidolysis agent to react until the material is clear and transparent, and control the acid value to be 45mgKOH / g~52mgKOH / g; S3: Cooling down to carry out vacuum polycondensation reaction, and releasing the vacuum when the acid value of the material is 35mgKOH / g to 42mgKOH / g; S4: Continue to cool down, add tert-butyl glycidyl carbonate to perform end-capping reaction, and when the acid value of the material is 30 mgKOH / g to 35 mgKOH / g, cool down, add a curing agent and an antioxidant, and stir to obtain the polyester resin for metal powder coating as claimed in any one of claims 1 to 6.

10. A metal powder coating, comprising the polyester resin for metal powder coating according to any one of claims 1 to 6.

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