Polyester resin for epoxy curing high-gloss high-leveling powder coating and preparation method thereof

Abstract

The invention belongs to the technical field of polyester resin, and particularly relates to polyester resin for an epoxy curing high-gloss high-leveling powder coating and a preparation method of the polyester resin. The polyester resin is prepared from the following components: neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1, 3-propylene glycol, trimethylolpropane, terephthalic acid, isophthalic acid, adipic acid, dimethylolbutyric acid and tetrahydrophthalic anhydride. The preparation method comprises the following steps: heating the polyhydric alcohol, adding the polybasic acid and the catalyst, and carrying out heat preservation reaction until the esterification water yield reaches 95% or above; then adding dimethylolbutyric acid, isophthalic acid and adipic acid for continuous reaction; tetrahydrophthalic anhydride is added for heat preservation; and finally, vacuumizing, reacting and discharging. The obtained polyester resin has a high glass transition temperature, and the prepared powder coating has high leveling, high gloss, full surface fullness, excellent definition, good mechanical properties and good anti-interference performance.

Description

Technical Field

[0001] This invention belongs to the technical field of polyester resins for powder coatings, specifically relating to an epoxy-cured high-gloss, high-leveling polyester resin for powder coatings and its preparation method. Background Technology

[0002] Thermosetting powder coatings are an environmentally friendly type of coating, mainly composed of resins, curing agents, pigments, fillers, and additives. Among them, pure polyester powder coatings and hybrid (epoxy / polyester) powder coatings are currently the most prevalent varieties in the domestic thermosetting powder coating market. With technological advancements and consumption upgrades, the market's demands for the comprehensive performance of coatings are increasing. For hybrid powder coatings, not only are excellent mechanical properties (such as adhesion and impact resistance) required, but also good surface finishes, such as high gloss and high leveling effects, to meet the application needs of high-end appearances.

[0003] Against the backdrop of increasingly stringent environmental policies, the environmental safety of powder coatings and their raw materials has received high attention. For example, trimellitic anhydride (TMA), a commonly used raw material for polyester resin synthesis, is strictly restricted in its use because it is listed as a respiratory sensitizer under the EU REACH regulation, and long-term exposure may damage lung function. This has prompted the industry to develop and apply safer and more compliant alternative raw materials, and the environmental friendliness of related products has become one of the key factors in market competitiveness.

[0004] In the synthesis technology of polyester resins for hybrid powder coatings, conventional processes typically involve esterification of polyols and polyacids under the action of a catalyst, followed by acid hydrolysis, vacuum polycondensation, and finally the addition of antioxidants and curing accelerators to obtain the finished resin. However, existing conventional technologies often struggle to achieve both high gloss and excellent leveling properties of the coating solely through the structural design of the resin itself, without relying on additional special surface additives (such as leveling agents and wetting accelerators). This presents a technical challenge. Existing technologies improve the water resistance of the coating film by adjusting the polymerization process, but the resulting coatings achieve a 60° gloss of approximately 94%, which does not reach the high gloss level (typically referring to 95% or higher).

[0005] In summary, no publicly reported solution exists in the current technology for achieving both high gloss (e.g., ≥95% gloss at 60°) and high leveling effect in epoxy-cured hybrid powder coatings through molecular structure design and synthesis technology innovation of polyester resins without relying on the addition of special surface additives. Therefore, developing a novel polyester resin whose own structure can endow coatings with excellent decorative properties is of great significance for meeting the demands of the high-end market, improving the environmental friendliness of products, and enhancing technological competitiveness. Summary of the Invention

[0006] The technical problem to be solved by this invention is to overcome the shortcomings of the aforementioned background technology and provide a TMA-free polyester resin for epoxy-cured high-gloss, high-leveling powder coatings. This polyester resin is free of trimellitic anhydride (TMA) and, in addition to having a high glass transition temperature, appropriate molecular weight and molecular weight distribution, suitable softening point, and suitable storage stability, also possesses a low melt viscosity, ensuring excellent wetting and dispersibility of pigments and fillers. Powder coatings prepared with this polyester resin exhibit high leveling, high gloss, full surface coverage, excellent clarity, good mechanical properties, and good anti-interference properties.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a polyester resin for epoxy-cured high-gloss and high-leveling powder coatings, which is prepared by reacting raw material components comprising the following mass ratio: neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol, trimethylolpropane, terephthalic acid, isophthalic acid, adipic acid, tetrahydrophthalic anhydride, and dimethylolbutyric acid, wherein the mass ratio of the above components is 1:(0.10-0.125):0.054:(0.015-0.035):(1.7-1.8):(0.21-0.25):(0.04-0.06):(0.07-0.13):(0.035-0.08).

[0008] Further improvements to polyester resins for epoxy-cured high-gloss, high-leveling powder coatings: Preferably, the reaction also includes an esterification catalyst, an antioxidant, and a curing accelerator, the sum of which is based on 0.1–2% of the total mass of the polyester resin.

[0009] Preferably, the esterification catalyst is a combination of zinc acetate and stannous oxalate in a mass ratio of 1:9.

[0010] Preferably, the antioxidant is a composition of triphenyl phosphite, hindered phenolic primary antioxidant, and phosphite secondary antioxidant.

[0011] Preferably, the antioxidant is a mixture of triphenyl phosphite, hindered phenolic primary antioxidant 1010, and phosphite auxiliary antioxidant 168 in a mass ratio of 1:1:2.

[0012] Preferably, the curing accelerator is a tertiary amine.

[0013] Preferably, the curing accelerator is one or a mixture of octadecyl dimethyl tertiary amine and hexadecyl dimethyl tertiary amine.

[0014] A second objective of this invention is to provide a method for preparing the polyester resin for epoxy-cured high-gloss, high-leveling powder coatings as described in any one of the above claims, comprising the following steps: (1) Mix polyols including neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol and trimethylolpropane, heat to 110-130°C in a reactor under normal pressure, add polybasic acid terephthalic acid and esterification catalyst, and heat the material to 170-180°C while stirring. (2) After the esterified water is distilled out, control the heating rate until the material temperature reaches 195℃±2℃, then adjust the heating rate until the material temperature reaches 235℃±2℃, keep the reaction at the temperature for 1-1.5 hours until the esterified water output reaches more than 95% of the theoretical output and the distillation head temperature is below 70℃, then end the heat preservation. (3) Cool the material to 200-205℃, add dimethylolbutyric acid, isophthalic acid and adipic acid, and keep it at 235℃±2℃ for 60-90 minutes. (4) Cool the material to 220-225℃, add tetrahydrophthalic anhydride, and keep it at 235℃±2℃ for 55-65 minutes. (5) Cool the material to 215-220℃, gradually evacuate the system until the absolute pressure is -0.090MPa to -0.097MPa, and continue the reaction for 90-120 minutes; (6) Cool the material to 190°C, add antioxidant and curing accelerator, stir and react for 45-55 minutes and then discharge the material.

[0015] As a further improvement to the above-mentioned method for preparing polyester resin for epoxy-cured high-gloss, high-leveling powder coatings: Preferably, in step (1), the heating rate is 2-3℃ / minute.

[0016] Preferably, in step (2), the heating rate from 170-180℃ to 195℃±2℃ is 2-3℃ every 30 minutes; the heating rate from 195℃±2℃ to 235℃±2℃ is 5-6℃ every 30 minutes.

[0017] The advantages of this invention compared to the prior art are as follows: (1) This invention provides a polyester resin for epoxy-cured high-gloss, high-leveling powder coatings, prepared by reacting the following components: neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol, trimethylolpropane, terephthalic acid, isophthalic acid, adipic acid, tetrahydrophthalic anhydride, and dimethylolbutyric acid. The polyester resin formulation prepared in this invention uses dimethylolbutyric acid and tetrahydrophthalic anhydride. Compared with conventional aromatic acids (terephthalic acid, isophthalic acid, etc.) and conventional fatty acids (1,6-adipic acid, succinic acid, etc.) monomers, the functional characteristics of these monomers are: 1) Dimethylolbutyric acid (DMPA) is a chain extender with dimethylol and carboxyl groups. The quaternary carbon atom connects to the dimethylol group to form a diamond-like covalent structure, which has high stability. The active groups in its molecular structure can improve the hardness, adhesion and chemical resistance of the coating film. DMPA is introduced into the polyester resin structure as a functional monomer to adjust polarity or reactivity. At the same time, DMPA is a neopentyl carboxylic acid with two active methylol groups, which is used to adjust the molecular chain structure in the prepolymer synthesis stage.

[0018] 2) Tetrahydrophthalic anhydride is an indispensable key raw material in the production of polyester resins for powder coatings. It is widely used in the automotive, home appliance, building materials, and industrial equipment industries by improving the resin's air-drying properties, weather resistance, adhesion, and heat resistance. With the continued growth of downstream application markets and increasingly stringent environmental requirements, the importance of tetrahydrophthalic anhydride and its derivatives in the powder coating industry will continue to increase.

[0019] Compared with existing technologies, this application introduces the functional monomer 2-ethyl-2-butyl-1,3-propanediol to reduce the viscosity of polyester resin and improve its wetting and dispersibility for pigments and fillers. It also employs dimethylolbutyric acid to control the molecular structure and introduces tetrahydrophthalic anhydride for effective end-capping. Furthermore, it controls the reaction process and melt viscosity. The resulting powder coating exhibits high leveling, high gloss, fullness, excellent clarity, good mechanical properties, and good anti-interference properties.

[0020] (2) In the formulation of this invention, zinc acetate combined with stannous oxalate is used as a catalyst. Zinc acetate has high efficiency and selectivity in catalyzing esterification reactions. The zinc acetate catalyst can reduce the activation energy of the reaction and increase the reaction rate. This invention uses an environmentally friendly catalyst, and the prepared polyester resin does not contain organotin and is not physiologically toxic. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0022] Examples 1-7

[0023] Examples 1-7 provide a method for preparing polyester resin for epoxy-cured high-gloss, high-leveling powder coatings. First, the corresponding mass parts of the raw materials (in grams) are weighed according to the component ratios in Table 1 below. The specific preparation method includes the following steps: (1) In a reactor equipped with a heating device, a stirrer and a distillation column, add the specified amount of polyol (diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol, neopentyl glycol and trimethylolpropane), heat the reactor to 125°C at atmospheric pressure, add polybasic acid (terephthalic acid) and esterification catalyst (zinc acetate, stannous oxalate), and while stirring, raise the material temperature to 175±2°C at a rate of 2.5°C±0.5°C per minute; (2) After the esterified water is distilled out, control the heating rate to 2.5±0.5℃ every 30 minutes, and raise the material temperature to 195℃±2℃; then adjust the heating rate to 5.5±0.5℃ every 30 minutes, and raise the material temperature to 235±2℃. Keep the reaction at this temperature for 1-1.5 hours until the amount of esterified water produced reaches more than 95% of the theoretical amount of water and the temperature of the distillation head is below 70℃, and then stop the heat preservation. (3) Cool the material to 203±2℃, add dimethylolbutyric acid, isophthalic acid and adipic acid, and keep it at 235±2℃ for 70 minutes. (4) Cool the material to 223±2℃, add tetrahydrophthalic anhydride, and keep it at 235±2℃ for 60 minutes. (5) Cool the material to 217±2℃, gradually evacuate the system until the absolute pressure is -0.095±0.002 MPa, and continue the reaction for 100 minutes; (6) Cool the material to 190°C, add antioxidant and curing accelerator, stir and react for 50 minutes, then discharge, cool and crush to obtain polyester resin for epoxy curing high gloss and high leveling powder coating, which are respectively referred to as polyester resin 1-7.

[0024] Comparative Example

[0025] This comparative example provides a method for preparing polyester resin for epoxy-cured high-gloss, high-leveling powder coatings. First, the corresponding mass parts of the raw materials (in g) are weighed according to the component proportions in Table 1 below. The specific preparation method includes the following steps: (1) In a reactor equipped with a heating device, a stirrer and a distillation column, add the specified amount of polyol (diethylene glycol, neopentyl glycol), heat the reactor to 120°C at atmospheric pressure, add polybasic acid (terephthalic acid) and esterification catalyst (zinc acetate, stannous oxalate), and while stirring, raise the material temperature to 175°C at a rate of 2.5±0.5°C per minute; (2) After the esterified water is distilled out, control the heating rate to 2.5℃±0.5℃ every 30 minutes, and raise the material temperature to 195℃±2℃; then adjust the heating rate to 5.5℃±0.5℃ every 30 minutes, and raise the material temperature to 235℃±2℃. Keep the reaction at this temperature for 1-1.5 hours until the amount of esterified water produced reaches more than 95% of the theoretical amount of water and the distillation head temperature is below 70℃, and then stop the heat preservation. (3) When the material temperature drops to 202±2℃, add isophthalic acid and adipic acid, and keep the reaction at 235℃±2℃ for 60-90 minutes. (4) When the material is cooled to 220-215℃, trimellitic anhydride is added, and the reaction is carried out at 210℃±2℃ for 60 minutes. (5) After the reaction is complete, the material is gradually evacuated to an absolute pressure of -0.090 to -0.097 MPa. The reaction continues for 90-120 minutes before the material is discharged, cooled and crushed to obtain ordinary polyester resin.

[0026] Table 1. Material proportions and performance indicators of polyester resins for Examples 1-7 and Comparative Examples

[0027] The performance tests of the polyester resins prepared in Examples 1-7 and the comparative tests are shown in Table 1 above. The test standards for the test items are as follows: 1. The acid value of polyester resin shall be tested in accordance with the standard GB / T 6743-2008.

[0028] 2. The viscosity of the polyester resin was tested in accordance with the standard GB / T 9751.1-2008.

[0029] 3. The glass transition temperature of polyester resin shall be determined in accordance with the standard GB / T 19466.2-2004.

[0030] 4. The softening point of polyester resin shall be determined in accordance with the standard GB / T 27808-2011.

[0031] As shown in Table 1, using polyols (neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol, trimethylolpropane), polybasic acids (terephthalic acid), and esterification catalysts (zinc acetate, stannous oxalate) as raw materials, along with appropriate preparation processes, polyester resins with suitable glass transition temperatures and melt viscosities can be obtained.

[0032] A comparison of Examples 1, 2, and 3 reveals that introducing different amounts of trimethylolpropane into the polyol has a certain impact on the viscosity and glass transition temperature of the resin. A comparison of Examples 4 and 5 reveals that introducing different amounts of dimethylolbutyric acid has a certain impact on the viscosity and glass transition temperature of the resin. A comparison of Examples 6 and 7 reveals that introducing different amounts of tetrahydrophthalic anhydride has a certain impact on the viscosity and glass transition temperature of the resin.

[0033] Example 8

[0034] This embodiment provides a method for preparing epoxy-cured high-gloss, high-leveling powder coatings, using the polyester resins obtained in Examples 1-7 and the comparative example above as raw materials. The specific steps are as follows: (1) Weigh the polyester resins prepared in Examples 1-7 and the comparative example respectively, and weigh the corresponding mass parts of epoxy resin E-12, barium sulfate, titanium dioxide, leveling agent, brightener 701 and benzoin. The composition ratio is shown in Table 2 below. Add them to a high-speed mixer and mix at a speed of 500-1000 r / min for 10-15 minutes to make each component initially dispersed evenly, and obtain the premixed material. (2) A twin-screw extruder is used, with the screw temperature set at 100-120℃, to melt and extrude the premixed material. Through the shearing and conveying action of the screw, the components are further promoted to disperse and mix evenly, forming a uniform melt. The extruded melt is rapidly cooled by a cooling device and then pressed into thin sheets for subsequent crushing. The cooled sheets are crushed and passed through a 200-mesh sieve to obtain a mixed powder coating, which is then electrostatically sprayed onto the sheet. (3) After the sprayed sample is placed in an oven at 190°C for 15 minutes, it is taken out and cooled, and the coating performance is tested.

[0035] Table 2. Coating samples and film properties prepared with different polyester resins in Example 8.

[0036] 1. Raw material description: Epoxy resin E-12: Epoxy equivalent is 833 g / mol; Leveling agent: Powder coating leveling agent (such as polyacrylate leveling agent GLP588 produced by Ningbo Nanhai Chemical Co., Ltd.); Brightener 701: Wetting accelerator for powder coatings (such as BLC701, a powder coating additive based on acrylate copolymers produced by Ningbo Nanhai Chemical Co., Ltd.).

[0037] 2. Testing Standards

[0038] The thickness of the coating was tested in accordance with the standard GB / T 13452.2-2008.

[0039] The flow of the coating on an inclined plate was tested according to the standard GB / T 21782.11-2010.

[0040] The gloss of the coating was tested according to the standard GB / T 1743-1979.

[0041] The solvent resistance test of the coating was conducted in accordance with the standard GB 23989-2009.

[0042] The adhesion of the coating was tested in accordance with the standard GB / T 9286-1998.

[0043] The impact performance of the coating was tested in accordance with the standard GB / T 1732-1993.

[0044] Level 1 represents the surface decoration performance achievable by ordinary mixed powder coatings; Level 2 represents a significant improvement over Level 1; and Level 3 corresponds to the best overall evaluation in terms of leveling, fullness, and clarity.

[0045] From the comparison of Examples 1 and 2-7 of this invention and the comparative examples, it can be found that the introduction of 2-butyl-2-ethyl-1,3-propanediol monomer into the synthetic polyester resin reduces the viscosity of the polyester resin, thereby improving the wetting and dispersibility of the polyester resin for pigments and fillers. The introduction of dimethylolbutyric acid monomer into the synthetic polyester resin can significantly control the melt viscosity of the polyester resin, effectively improving the leveling performance of the paint film. The introduction of tetrahydrophthalic anhydride monomer into the synthetic polyester resin results in a melt viscosity that has a decisive influence on the surface decorative properties of the paint film, such as gloss, leveling performance, fullness, and clarity. Furthermore, when dimethylolbutyric acid and tetrahydrophthalic anhydride monomers are used in combination, controlling the vacuum temperature and vacuum level during the polyester resin synthesis process yields a polyester resin that, when used to prepare a mixed powder coating, exhibits excellent surface decorative properties while meeting the requirements for mechanical and storage properties.

[0046] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments. It should be noted that many modifications and improvements can be made by those skilled in the art, and all modifications or improvements not exceeding the scope of the claims should be considered within the protection scope of the present invention.

Claims

1. A polyester resin for epoxy-cured high-gloss, high-leveling powder coatings, characterized in that, The following components are reacted to produce the following product: neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol, trimethylolpropane, terephthalic acid, isophthalic acid, adipic acid, tetrahydrophthalic anhydride, and dimethylolbutyric acid. Based on the mass of neopentyl glycol as 1, the mass ratio of neopentyl glycol to the other components is 1:(0.10-0.125):0.054:(0.015-0.035):(1.7-1.8):(0.21-0.25):(0.04-0.06):(0.07-0.13):(0.035-0.08).

2. The polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to claim 1, characterized in that, The reaction also includes an esterification catalyst, an antioxidant, and a curing accelerator, the sum of which is based on 0.1–2% of the total mass of the polyester resin.

3. The polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to claim 2, characterized in that, The esterification catalyst is a combination of zinc acetate and stannous oxalate in a mass ratio of 1:

9.

4. The polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to claim 2, characterized in that, The antioxidant is a combination of triphenyl phosphite, hindered phenolic primary antioxidant, and phosphite secondary antioxidant.

5. The polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to claim 4, characterized in that, The antioxidant is a mixture of triphenyl phosphite, hindered phenolic primary antioxidant 1010, and phosphite auxiliary antioxidant 168 in a mass ratio of 1:1:

2.

6. The polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to claim 2, characterized in that, The curing accelerator is a tertiary amine.

7. The polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to claim 6, characterized in that, The curing accelerator is one or a combination of two of octadecyl dimethyl tertiary amine and hexadecyl dimethyl tertiary amine.

8. A method for preparing a polyester resin for epoxy-cured high-gloss, high-leveling powder coatings according to any one of claims 1-7, characterized in that, Includes the following steps: (1) Mix polyols including neopentyl glycol, diethylene glycol, 2-butyl-2-ethyl-1,3-propanediol and trimethylolpropane, heat to 110-130°C in a reactor under normal pressure, add polybasic acid terephthalic acid and esterification catalyst, and heat the material to 170-180°C while stirring. (2) After the esterified water is distilled out, control the heating rate until the material temperature reaches 195℃±2℃, then adjust the heating rate until the material temperature reaches 235℃±2℃, keep the reaction at the temperature for 1-1.5 hours until the esterified water output reaches more than 95% of the theoretical output and the distillation head temperature is below 70℃, then end the heat preservation. (3) Cool the material to 200-205℃, add dimethylolbutyric acid, isophthalic acid and adipic acid, and keep it at 235℃±2℃ for 60-90 minutes. (4) Cool the material to 220-225℃, add tetrahydrophthalic anhydride, and keep it at 235℃±2℃ for 55-65 minutes. (5) Cool the material to 215-220℃, gradually evacuate the system until the absolute pressure is -0.090MPa to -0.097MPa, and continue the reaction for 90-120 minutes; (6) Cool the material to 190°C, add antioxidant and curing accelerator, stir and react for 45-55 minutes and then discharge the material.

9. The method for preparing the polyester resin for epoxy-cured high-gloss, high-leveling powder coating according to claim 8, characterized in that, In step (1), the heating rate is 2-3℃ / minute.

10. The method for preparing the polyester resin for epoxy-cured high-gloss, high-leveling powder coating according to claim 8, characterized in that, In step (2), the heating rate from 170-180℃ to 195℃±2℃ is 2-3℃ every 30 minutes; the heating rate from 195℃±2℃ to 235℃±2℃ is 5-6℃ every 30 minutes.

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