Anti-pinhole high flow haa curing system polyester resin for powder coating and method of making

Abstract

This invention provides a polyester resin for high-leveling HAA (high-efficiency autoclave) curing powder coatings with anti-pinhole properties and its preparation method, belonging to the field of polyester resin preparation technology. This application constructs a polyester skeleton with aromatic hard segments and a certain degree of flexibility by combining a diacid component with polyol monomers. Under the premise of segmental control of acid value, 2-butyl-2-ethyl-1,3-propanediol and trimethylolpropane are introduced, so that the resin retains sufficient carboxyl end groups and has a segmental environment conducive to dynamic transesterification. Zinc acetylacetonate and hindered phenolic phosphite are added to form a Zn-P synergistic system, which inhibits excessive catalysis and oxidative discoloration during synthesis and extrusion, and provides moderate transesterification catalytic activity during the curing stage. Resin particles are obtained by melt spreading and rapid cooling to fix the structure and additive distribution, providing support for HAA powder coatings to achieve thick-coat anti-pinhole properties and color stability while maintaining good leveling.

Description

Technical Field

[0001] This invention belongs to the field of polyester resin preparation technology, and relates to polyester resin for anti-pinhole high-leveling HAA curing system powder coating and its preparation method. Background Technology

[0002] Currently, HAA (β-hydroxyalkylamide) cured polyester powder coatings are widely used in the fields of household appliance casings, building profiles, and industrial equipment due to their solvent-free nature, convenient construction, and good environmental performance. However, existing HAA polyester resins still have significant shortcomings in terms of thick coating leveling and control of appearance defects.

[0003] On the one hand, to mitigate defects such as pinholes and craters caused by water release during HAA curing, existing technologies often control gas release by lowering the resin acid value, increasing the molecular weight, or increasing the amount of degassing agents such as benzoin. While this improves surface smoothness under thin film conditions to some extent, it often leads to insufficient carboxyl end groups, decreased curing reactivity, and problems such as incomplete curing and decreased mechanical properties. On thicker films, vertical surfaces, or complex structures, water and small molecules still have difficulty escaping in time, easily resulting in appearance defects such as pinholes, craters, and orange peel. Alternatively, simply introducing flexible diacids or diols to reduce melt viscosity can easily lead to an overly rigid crosslinking network after curing, making it difficult to release internal stress in time. This makes the coating more prone to micropores, ripples, and flow marks on thicker films and constrained substrate surfaces.

[0004] On the other hand, in order to reduce the curing temperature and shorten the curing time, existing formulations have also introduced metal salt catalysts such as zinc salts and tin salts or acidic co-catalysts to improve the reaction rate of the HAA system. However, such catalytic systems are prone to aggravating the degradation of the polyester backbone and oxidative yellowing at high temperatures, limiting their usage and narrowing the process window. Some systems reduce the amount of catalyst to avoid yellowing, but it is difficult to ensure sufficient curing and stress relaxation under thick coating or low temperature conditions. The self-healing ability of defects during the curing process is limited, and it is still difficult to balance the anti-pinhole and leveling properties of thick coating with the color stability and weather resistance after curing. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a polyester resin for high-leveling HAA-cured powder coatings with anti-pinhole properties and its preparation method. This application utilizes a combination of terephthalic acid / isophthalic acid and aliphatic dicarboxylic acids, along with neopentyl glycol and 1,4-cyclohexanediol, to construct a polyester backbone primarily composed of aromatic hard segments and doped with flexible aliphatic segments. This results in a resin possessing heat resistance, weather resistance, and moderate melt viscosity. The introduction of 2-butyl-2-ethyl-1,3-propanediol and trimethylolpropane, along with large side chains and branched structures, regulates the free volume and β-hydroxy ester content. Under segmented acid value control, this approach not only retains sufficient carboxyl end groups but also provides a favorable microenvironment for subsequent dynamic transesterification. Simultaneously, zinc acetylacetonate and hindered phenolic phosphites are added during the melt stage to form a Zn-P synergistic system compatible with the polyester. This system inhibits excessive catalysis and oxidative discoloration during synthesis and extrusion, while providing appropriate catalytic activity for transesterification during the curing stage. By using melt spreading and rapid cooling, the above-mentioned structure and additives are fixed in the resin flakes, resulting in resin particles with controllable particle size and water content. This provides a foundation for subsequent HAA powder coatings to achieve thick coating anti-pinhole and color stability while maintaining good leveling properties.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a polyester resin for a high-leveling HAA-curing powder coating system with anti-pinhole properties, the method comprising:

[0008] S1: Terephthalic acid, isophthalic acid and aliphatic diacid are mixed to obtain diacid components. Polyol monomers are added to obtain a mixture. After stirring and reacting, the temperature is increased and reacted to obtain a linear prepolymer polyester reaction liquid. Further polycondensation reaction is carried out to obtain a linear prepolymer polyester melt.

[0009] S2: Cool the linear prepolymer polyester melt, add zinc acetylacetonate and hindered phenolic phosphite antioxidant to obtain reaction solution A, stir the reaction to obtain polyester melt;

[0010] S3: After cooling the polyester melt, spread it to form a molten resin layer, cool and solidify it to obtain sheet polyester resin, crush it, and dry it to obtain polyester resin for powder coating with anti-pinhole high leveling HAA curing system.

[0011] As a preferred technical solution of the present invention, in step S1, the molar ratio of terephthalic acid, isophthalic acid and aliphatic dicarboxylic acid in the dicarboxylic acid component is (60-75):(5-15):(10-20), for example, it can be (60.0, 61.5, 63.0, 64.5, 66.0, 67.5, 69.0, 70.5, 72.0, 73.5 or 75.0):(5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15):(10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20), but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0012] The aliphatic dicarboxylic acid is any one of adipic acid, succinic acid, or sebacic acid.

[0013] In some optional embodiments, the ratio of total hydroxyl equivalent to total carboxyl equivalent in the polyol monomer is (0.9-1):1, for example, it can be 0.90:1, 0.91:1, 0.92:1, 0.93:1, 0.94:1, 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1 or 1.00:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0014] In some optional embodiments, the molar ratio of neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol, and trimethylolpropane in the polyol monomer is (20-40):(20-40):(5-15):(8-15), for example, it can be (20, 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40):(20, 22, 2...). 4, 26, 28, 30, 32, 34, 36, 38 or 40: (5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15): (8.0, 8.7, 9.4, 10.1, 10.8, 11.5, 12.2, 12.9, 13.6, 14.3 or 15.0), but not limited to the listed values, other unlisted values ​​within this range also apply.

[0015] In some optional embodiments, the temperature of the mixture stirring reaction is 120-160°C, for example, 120°C, 124°C, 128°C, 132°C, 136°C, 140°C, 144°C, 148°C, 152°C, 156°C or 160°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0016] In some optional embodiments, the reaction time of the mixture is 0.5-1.5 h, for example, it can be 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h or 1.5 h, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0017] In some optional embodiments, after the stirring reaction, the temperature is further increased to 180-230°C, for example, to 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C, 215°C, 220°C, 225°C or 230°C, but not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0018] In some optional embodiments, the mixture is stirred and then heated and reacted for 2-6 hours, for example, 2.0 hours, 2.4 hours, 2.8 hours, 3.2 hours, 3.6 hours, 4.0 hours, 4.4 hours, 4.8 hours, 5.2 hours, 5.6 hours, or 6.0 hours, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0019] In some optional embodiments, the acid value of the linear prepolymer polyester reaction solution is 25-50 mgKOH / g, for example, it can be 25.0 mgKOH / g, 27.5 mgKOH / g, 30.0 mgKOH / g, 32.5 mgKOH / g, 35.0 mgKOH / g, 37.5 mgKOH / g, 40.0 mgKOH / g, 42.5 mgKOH / g, 45.0 mgKOH / g, 47.5 mgKOH / g or 50.0 mgKOH / g, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0020] In some optional embodiments, the temperature of the polycondensation reaction of the linear prepolymer polyester reaction solution is 200-230°C, for example, 200°C, 203°C, 206°C, 209°C, 212°C, 215°C, 218°C, 221°C, 224°C, 227°C or 230°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0021] In some optional embodiments, the pressure of the polycondensation reaction of the linear prepolymer polyester reaction solution is 1-5 kPa, for example, it can be 1.0 kPa, 1.4 kPa, 1.8 kPa, 2.2 kPa, 2.6 kPa, 3.0 kPa, 3.4 kPa, 3.8 kPa, 4.2 kPa, 4.6 kPa or 5.0 kPa, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0022] In some optional embodiments, the polycondensation reaction time of the linear prepolymer polyester reaction solution is 1-4 hours, for example, it can be 1.0 hours, 1.3 hours, 1.6 hours, 1.9 hours, 2.2 hours, 2.5 hours, 2.8 hours, 3.1 hours, 3.4 hours, 3.7 hours or 4.0 hours, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0023] In some optional embodiments, the acid value of the linear prepolymer polyester melt is 18-28 mgKOH / g, for example, it can be 18 mgKOH / g, 19 mgKOH / g, 20 mgKOH / g, 21 mgKOH / g, 22 mgKOH / g, 23 mgKOH / g, 24 mgKOH / g, 25 mgKOH / g, 26 mgKOH / g, 27 mgKOH / g or 28 mgKOH / g, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of the present invention, in step S2, the linear prepolymer polyester melt is cooled to 180-210°C, for example, to 180°C, 183°C, 186°C, 189°C, 192°C, 195°C, 198°C, 201°C, 204°C, 207°C or 210°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0025] In some optional embodiments, the amount of zinc acetylacetonate added is 0.1-0.20% of the mass of the linear prepolymer polyester melt, for example, it can be 0.100%, 0.110%, 0.120%, 0.130%, 0.140%, 0.150%, 0.160%, 0.170%, 0.180%, 0.190% or 0.200%, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0026] In some optional embodiments, the amount of the hindered phenolic phosphite antioxidant is 0.1-0.5% of the mass of the linear prepolymer polyester melt, for example, it can be 0.10%, 0.14%, 0.18%, 0.22%, 0.26%, 0.30%, 0.34%, 0.38%, 0.42%, 0.46% or 0.50%, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0027] In some optional embodiments, the hindered phenolic phosphite antioxidant is tris(2,4-di-tert-butylphenyl) phosphite or antioxidant PEP-36.

[0028] In some optional embodiments, the temperature of the reaction mixture A during stirring is 180-200°C, for example, it can be 180°C, 182°C, 184°C, 186°C, 188°C, 190°C, 192°C, 194°C, 196°C, 198°C or 200°C, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0029] In some optional embodiments, the reaction time of the reaction solution A is 0.5-1 h, for example, it can be 0.50 h, 0.55 h, 0.60 h, 0.65 h, 0.70 h, 0.75 h, 0.80 h, 0.85 h, 0.90 h, 0.95 h or 1.00 h, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0030] As a preferred technical solution of the present invention, in step S3, the polyester melt is cooled to 160-180°C, for example, to 160°C, 162°C, 164°C, 166°C, 168°C, 170°C, 172°C, 174°C, 176°C, 178°C or 180°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0031] In some alternative embodiments, the thickness of the molten resin layer is 2-10 mm, for example, it can be 2.0 mm, 2.8 mm, 3.6 mm, 4.4 mm, 5.2 mm, 6.0 mm, 6.8 mm, 7.6 mm, 8.4 mm, 9.2 mm or 10.0 mm, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0032] In some alternative embodiments, the temperature at which the molten resin layer is cooled and cured is 5-25°C, for example, 5°C, 7°C, 9°C, 11°C, 13°C, 15°C, 17°C, 19°C, 21°C, 23°C or 25°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0033] In some optional embodiments, the cooling and curing time of the molten resin layer is 5-10 min, for example, it can be 5.0 min, 5.5 min, 6.0 min, 6.5 min, 7.0 min, 7.5 min, 8.0 min, 8.5 min, 9.0 min, 9.5 min or 10.0 min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0034] Secondly, the present invention provides a polyester resin for anti-pinhole high-leveling HAA curing powder coating prepared by the preparation method described above.

[0035] This application achieves synergy at three levels: polyester main chain structure, end-group acid value control, and metal-phosphorus synergistic catalysis system. This allows the resulting resin to balance high carboxyl end-group content, adjustable chain segment mobility, and moderate dynamic transesterification capacity during subsequent curing with HAA. This, in turn, facilitates better leveling and thick-coating anti-pinhole performance within a wider acid value window.

[0036] First, this application constructs a polyester backbone primarily composed of aromatic diacids and doped with certain flexible segments by combining terephthalic acid, isophthalic acid, and aliphatic diacids (adipic acid, succinic acid, or sebacic acid). Terephthalic acid provides high chain rigidity and heat resistance, which is beneficial to coating hardness and scratch resistance; isophthalic acid helps to break excessive regularity, improve internal stress distribution, and enhance crack resistance during processing; aliphatic diacids introduce flexible methylene segments, which can reduce melt viscosity, widen the melt flow window, and provide a buffer space for local chain rearrangement during subsequent curing. In the polyol portion, neopentyl glycol and 1,4-cyclohexanediol together provide relatively stable diol units, ensuring the polyester's hydrolysis and weather resistance on the one hand, and maintaining a suitable glass transition temperature of the resin on the other, thus balancing the storage stability of the powder coating and its flowability during curing. The total hydroxyl equivalent and total carboxyl equivalent of the dicarboxylic acid in the polyol monomer are controlled at (0.9-1):1, so that the carboxyl groups are slightly in excess. This is beneficial to make the resin end groups mainly composed of carboxyl groups, which provides sufficient reaction sites for subsequent HAA curing.

[0037] Secondly, this application regulates the free volume and branching degree of polyester segments by introducing 2-butyl-2-ethyl-1,3-propane and trimethylolpropane into the polyol component. 2-Butyl-2-ethyl-1,3-propanediol contains relatively large butyl and ethyl side chains, forming certain gaps and an internal lubricating environment between polyester chains. This is beneficial for maintaining the mobility of local segments near the curing temperature, ensuring limited molecular rearrangement space even after the crosslinked network is formed. This structural feature provides a favorable microenvironment for the subsequent dynamic ester exchange process. Trimethylolpropane, as a trifunctional alcohol, can introduce a certain number of branching points, increasing the potential crosslinking density and adding exchangeable structural units such as β-hydroxy esters, which is beneficial for forming a network structure with limited topological rearrangement capabilities. By segmenting and controlling the esterification and polycondensation processes, first controlling the acid value to 25-50 mgKOH / g and then reducing the polycondensation to 18-28 mgKOH / g, a moderate level of carboxyl end groups can be retained while obtaining sufficient molecular weight and mechanical properties.

[0038] Furthermore, this application introduces zinc acetylacetonate and hindered phenolic phosphite antioxidants into the polyester melt stage, constructing a metal-phosphorus synergistic system compatible with the main resin structure. The addition of zinc acetylacetonate to the polyester melt, along with the acetylacetonate ligand's effect on Zn...2+ It has a certain chelating effect, which helps to inhibit excessive catalytic ester bond breaking or degradation at resin synthesis and extrusion processing temperatures, and is beneficial to maintaining the stability of resin color and molecular weight; within the subsequent HAA curing temperature range, with changes in coordination equilibrium and local environment, Zn 2+ It may partially transfer to sites near carboxyl and β-hydroxy ester groups, thus playing a catalytic role in transesterification and allowing the cross-linked network to maintain a certain degree of chemical rearrangement after formation. Hindered phenolic phosphite antioxidants (tris(2,4-di-tert-butylphenyl) phosphite, PEP-36 type) can, on the one hand, act as a Zn... 2+ The auxiliary ligands participate in regulating the coordination environment and active release temperature range, making the catalytic effect more concentrated in the curing stage; on the other hand, it also functions as a secondary antioxidant, by capturing peroxides and free radicals, slowing down the oxidation discoloration and chain degradation caused by metal catalysis during high-temperature curing, which is beneficial to maintaining the color stability and weather resistance of the system under the premise of introducing metal catalysis.

[0039] Finally, this application fixes the above-mentioned structure and additive distribution in the solid resin by introducing the catalytic system during the melt stage, spreading it into a resin layer at 160-180℃, and rapidly cooling and curing it at 5-25℃. The faster cooling process helps maintain the molecular weight distribution and chain segment conformation at the end of synthesis, reducing the risk of further polycondensation, uneven crosslinking, or localized degradation caused by prolonged high-temperature holding. Curing the resin into sheets before crushing and drying helps obtain resin particles with moderate particle size and controllable moisture content, providing stable raw materials for subsequent powder extrusion and electrostatic spraying. The resulting polyester resin has both aromatic and aliphatic units in its skeletal structure, free volume and moderate branching in its chain segment structure, and a Zn-P latent catalytic system that can play a role in the subsequent curing stage at the end group and small molecule levels. This provides an inherent basis for achieving good leveling and thick coating anti-pinhole performance, while also ensuring color stability during processing and weather resistance during application.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] This application utilizes a combination of terephthalic acid / isophthalic acid and aliphatic dicarboxylic acids, along with neopentyl glycol and 1,4-cyclohexanediol, to construct a polyester backbone dominated by aromatic hard segments and doped with flexible aliphatic segments. This results in a resin with heat resistance, weather resistance, and moderate melt viscosity. The introduction of 2-butyl-2-ethyl-1,3-propane and trimethylolpropane, along with large side chains and branched structures, regulates the free volume and β-hydroxy ester content. Under segmented acid value control at 18-28 mg KOH / g, this approach not only retains sufficient carboxyl end groups but also provides a favorable microenvironment for subsequent dynamic transesterification. Simultaneously, the addition of zinc acetylacetonate and hindered phenolic phosphites during the melt stage forms a Zn-P synergistic system compatible with the polyester. This system helps mitigate over-catalysis and oxidative discoloration during synthesis and extrusion, and provides appropriate catalytic activity for transesterification during the curing stage. By using melt spreading and rapid cooling, the above-mentioned structure and additives are fixed in the resin flakes, resulting in resin particles with controllable particle size and water content. This provides a foundation for subsequent HAA powder coatings to achieve thick coating anti-pinhole and color stability while maintaining good leveling properties. Attached Figure Description

[0042] Figure 1 This is a physical image of the polyester resin used in the anti-pinhole high-leveling HAA curing system powder coating prepared in Example 1 of this application. Detailed Implementation

[0043] The technical solutions of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. The embodiments described herein are specific implementations of the present invention, used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary, and should not be construed as limiting the implementation methods or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can employ other obvious technical solutions based on the content disclosed in the claims and specification of this application. These technical solutions include any obvious substitutions and modifications made to the embodiments described herein.

[0044] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone further purification or processing.

[0045] Example 1

[0046] This embodiment provides a polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings and its preparation method. The preparation method of the polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings specifically includes the following steps:

[0047] S1: Terephthalic acid, isophthalic acid, and aliphatic diacid adipic acid are mixed in a molar ratio of 70:12:18 to obtain a diacid component. A polyol monomer is added to obtain a mixture, wherein the ratio of the total hydroxyl equivalent to the total carboxyl equivalent of the diacid in the polyol monomer is 0.98:1. The polyol monomer is neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol, and trimethylolpropane in a molar ratio of 35:30:10:12. The mixture is stirred at 150°C for 1.2 h and then heated to 220°C for 5 h. When the acid value reaches 45 mg KOH / g, a linear prepolymer polyester reaction solution is obtained. The polycondensation reaction is continued at 210°C and 4 kPa for 3 h until the acid value reaches 25 mg KOH / g to obtain a linear prepolymer polyester melt.

[0048] S2: The temperature of the linear prepolymer polyester melt is reduced to 200℃, and zinc acetylacetone and hindered phenolic phosphite antioxidant tris(2,4-di-tert-butylphenyl) phosphite are added to obtain reaction solution A, wherein the amount of zinc acetylacetone added is 0.18% of the mass of the linear prepolymer polyester melt, and the amount of hindered phenolic phosphite antioxidant added is 0.4% of the mass of the linear prepolymer polyester melt. The reaction is stirred at 195℃ for 0.8h to obtain polyester melt;

[0049] S3: After cooling the polyester melt to 175°C, spread it to form a molten resin layer with a thickness of 8mm. Cool and cure it at 20°C for 8 minutes to obtain sheet polyester resin. Crush and dry to obtain polyester resin for powder coating with anti-pinhole high leveling HAA curing system. Figure 1 The image shows a physical picture of the polyester resin used in the anti-pinhole high-leveling HAA curing system powder coating, which appears as colorless to light yellow transparent irregular flake-shaped particles.

[0050] Example 2

[0051] This embodiment provides a polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings and its preparation method. The preparation method of the polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings specifically includes the following steps:

[0052] S1: Terephthalic acid, isophthalic acid, and aliphatic diacid succinic acid are mixed in a molar ratio of 60:5:10 to obtain a diacid component. A polyol monomer is added to obtain a mixture, wherein the ratio of the total hydroxyl equivalent to the total carboxyl equivalent of the diacid in the polyol monomer is 0.9:1. The polyol monomer is neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol, and trimethylolpropane in a molar ratio of 20:40:5:8. The mixture is stirred at 120°C for 0.5 h and then heated to 180°C for 2 h. When the acid value reaches 25 mg KOH / g, a linear prepolymer polyester reaction solution is obtained. The polycondensation reaction is continued at 230°C and 1 kPa for 1 h until the acid value reaches 18 mg KOH / g to obtain a linear prepolymer polyester melt.

[0053] S2: The temperature of the linear prepolymer polyester melt is reduced to 180℃, and zinc acetylacetone and hindered phenolic phosphite antioxidant PEP-36 are added to obtain reaction solution A, wherein the amount of zinc acetylacetone added is 0.1% of the mass of the linear prepolymer polyester melt, and the amount of hindered phenolic phosphite antioxidant added is 0.1% of the mass of the linear prepolymer polyester melt. The reaction is stirred at 180℃ for 0.5h to obtain polyester melt;

[0054] S3: After cooling the polyester melt to 160°C, spread it to form a molten resin layer with a thickness of 2mm. Cool and cure it at 5°C for 5 minutes to obtain sheet polyester resin. Crush and dry to obtain polyester resin for powder coating with anti-pinhole high leveling HAA curing system.

[0055] Example 3

[0056] This embodiment provides a polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings and its preparation method. The preparation method of the polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings specifically includes the following steps:

[0057] S1: Terephthalic acid, isophthalic acid, and sebacic acid (an aliphatic diacid) are mixed in a molar ratio of 65:8:12 to obtain a diacid component. A polyol monomer is added to obtain a mixture, wherein the ratio of the total hydroxyl equivalent to the total carboxyl equivalent of the diacid in the polyol monomer is 0.92:1. The polyol monomer is neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol, and trimethylolpropane in a molar ratio of 30:25:12:10. The mixture is stirred at 130°C for 0.8 h and then heated to 200°C for 4 h. When the acid value reaches 30 mg KOH / g, a linear prepolymer polyester reaction solution is obtained. The polycondensation reaction is continued at 220°C and 2 kPa for 2 h until the acid value reaches 22 mg KOH / g to obtain a linear prepolymer polyester melt.

[0058] S2: The temperature of the linear prepolymer polyester melt is reduced to 190℃, and zinc acetylacetone and hindered phenolic phosphite antioxidant tris(2,4-di-tert-butylphenyl) phosphite are added to obtain reaction solution A, wherein the amount of zinc acetylacetone added is 0.12% of the mass of the linear prepolymer polyester melt, and the amount of hindered phenolic phosphite antioxidant added is 0.2% of the mass of the linear prepolymer polyester melt. The reaction is stirred at 185℃ for 0.7h to obtain polyester melt;

[0059] S3: After cooling the polyester melt to 165°C, spread it to form a molten resin layer with a thickness of 5mm. Cool and cure it at 10°C for 7 minutes to obtain sheet polyester resin. Crush and dry to obtain polyester resin for powder coating with anti-pinhole high leveling HAA curing system.

[0060] Example 4

[0061] This embodiment provides a polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings and its preparation method. The preparation method of the polyester resin for anti-pinhole, high-leveling HAA curing system powder coatings specifically includes the following steps:

[0062] S1: Terephthalic acid, isophthalic acid, and aliphatic diacid adipic acid are mixed in a molar ratio of 75:15:20 to obtain a diacid component. A polyol monomer is added to obtain a mixture, wherein the ratio of the total hydroxyl equivalent to the total carboxyl equivalent of the diacid in the polyol monomer is 1:1. The polyol monomer is neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol, and trimethylolpropane in a molar ratio of 40:20:15:15. The mixture is stirred at 160°C for 1.5 h and then heated to 230°C for 6 h. When the acid value reaches 50 mg KOH / g, a linear prepolymer polyester reaction solution is obtained. The polycondensation reaction is continued at 200°C and 5 kPa for 4 h until the acid value reaches 28 mg KOH / g to obtain a linear prepolymer polyester melt.

[0063] S2: The temperature of the linear prepolymer polyester melt is reduced to 210℃, and zinc acetylacetone and hindered phenolic phosphite antioxidant PEP-36 are added to obtain reaction solution A, wherein the amount of zinc acetylacetone added is 0.20% of the mass of the linear prepolymer polyester melt, and the amount of hindered phenolic phosphite antioxidant added is 0.5% of the mass of the linear prepolymer polyester melt. The reaction is stirred at 200℃ for 1 hour to obtain polyester melt.

[0064] S3: After cooling the polyester melt to 180°C, spread it to form a molten resin layer with a thickness of 10 mm. Cool and cure it at 25°C for 10 min to obtain sheet polyester resin. Crush and dry to obtain polyester resin for powder coating with anti-pinhole high leveling HAA curing system.

[0065] Comparative Example 1

[0066] This comparative example provides a polyester resin for a high-level HAA curing system powder coating with anti-pinhole properties. The difference from Example 1 is that neopentyl glycol is used instead of 2-butyl-2-ethyl-1,3-propanediol in the polyol monomer of S1. Other operating steps and process parameters are exactly the same as in Example 1.

[0067] Comparative Example 2

[0068] This comparative example provides a polyester resin for a high-level HAA curing system powder coating with anti-pinhole properties. The difference from Example 1 is that the degree of polycondensation in S1 is reduced so that the acid value of the linear prepolymer polyester melt is 10 mg KOH / g. Other operating steps and process parameters are exactly the same as in Example 1.

[0069] Comparative Example 3

[0070] This comparative example provides a polyester resin for a high-level HAA curing system powder coating with anti-pinhole properties. The difference from Example 1 is that zinc acetylacetone is not added in S2, while the other operating steps and process parameters are exactly the same as in Example 1.

[0071] Comparative Example 4

[0072] This comparative example provides a polyester resin for a high-level HAA curing system powder coating with anti-pinhole properties. The difference from Example 1 is that zinc acetate is used instead of zinc acetylacetonate in S2. Other operating steps and process parameters are exactly the same as in Example 1.

[0073] Comparative Example 5

[0074] This comparative example provides a polyester resin for a high-level HAA curing system powder coating with anti-pinhole properties. The difference from Example 1 is that in S1, the polyol monomers are adjusted to neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol and trimethylolpropane mixed in a molar ratio of 35:30:10:5. Other operating steps and process parameters are exactly the same as in Example 1.

[0075] The performance of the polyester resins used in the anti-pinhole high-leveling HAA curing powder coatings of Examples 1-4 and Comparative Examples 1-5 was tested, and the specific process is as follows:

[0076] The viscosity of the sample was tested according to GB / T 9751.1-2008;

[0077] The anti-pinhole, high-leveling HAA curing system powder coatings of Examples 1-4 and Comparative Examples 1-5 were prepared with polyester resin to obtain the same set of HAA powder coatings. After spraying and curing, performance tests were conducted. The specific test process is as follows:

[0078] The number of pinholes on a 100μm film thickness sample is determined according to the testing specifications in GB / T 21776-2008.

[0079] Its color difference was tested according to GB / T 11186.3-1989;

[0080] Its specular gloss (60°) was tested according to GB / T 9754-2007.

[0081] Impact performance was tested according to GB / T 1732-2020 (50cm normal impact).

[0082] The test results are shown in Table 1.

[0083] Table 1. Performance test results of polyester resins for anti-pinhole high-leveling HAA curing system powder coatings in Examples 1-4 and Comparative Examples 1-5

[0084]

[0085] From the test results of Example 1 and Comparative Example 1 in Table 1, it can be seen that in S1, the use of neopentyl glycol to replace 2-butyl-2-ethyl-1,3-propanediol resulted in a lack of free volume and intrinsic lubrication from the large-volume side chains, leading to increased melt viscosity. Insufficient melt flowability and reduced chain segment relaxation ability made it difficult to fill the micro-voids formed by water release during the thick coating curing process, resulting in more pinholes. The catalytic system and acid value levels were basically consistent, and the thermo-oxidative stability was similar, with little color difference. Due to insufficient leveling, increased surface orange peel and micro-defects, the coating surface was rougher, and the mirror gloss decreased. Poor network flexibility and insufficient release of internal residual stress made it more prone to cracking or whitening under a 50cm normal impact, resulting in decreased overall impact resistance.

[0086] From the test results of Example 1 and Comparative Example 2 in Table 1, it can be seen that reducing the degree of polycondensation in S1 resulted in an acid value of 10 mgKOH / g for the linear prepolymer melt, increased molecular weight and chain regularity, and increased melt viscosity. The reduction in carboxyl end groups and exchangeable sites led to insufficient HAA reaction and dynamic transesterification during thick coating curing, making it difficult for water-releasing channels to heal in time, resulting in an increase in pinholes. Reduced water release slightly alleviated thermo-oxidative aging and slightly improved color difference. Higher viscosity and insufficient stress relaxation resulted in increased orange peel and ripples on the surface, and a decrease in mirror gloss. The network crosslinking was denser but less flexible, making it more prone to fine cracks or whitening under a 50cm normal impact, and reducing impact resistance.

[0087] From the test results of Example 1 and Comparative Example 3 in Table 1, it can be seen that S2, without the addition of zinc acetylacetone, has a similar resin structure and little change in melt viscosity. However, lacking zinc catalytic centers, the transesterification reaction rate in the high-temperature curing zone is significantly reduced, and the dynamic network adjustment capability is insufficient. During the thick coating curing process, the network rigidly locks in, and the micropores formed by water release are difficult to repair through chemical rearrangement, resulting in an increase in the number of pinholes. Without the additional oxidation caused by metal catalysis, the color difference is better than in Example 1. The surface has more pinholes and micro-pits, poor leveling, and decreased mirror gloss. The network lacks dynamic relaxation capability, and is prone to cracking or even local peeling under a 50cm normal impact, resulting in decreased impact resistance.

[0088] From the test results of Example 1 and Comparative Example 4 in Table 1, it can be seen that in S2, zinc acetate was used to replace zinc acetylacetonate, and free Zn 2+ The high activity and localized degradation reduce the effective molecular weight and slightly decrease the melt viscosity. The catalysis is excessively vigorous and unevenly distributed; while it retains some transesterification capacity during thick-coat curing, the number of pinholes increases. The highly active zinc salt exacerbates oxidation and discoloration during curing, increasing color difference. The combination of uneven localized reaction and yellowing deteriorates the surface appearance and reduces mirror gloss. The coexistence of partial chain segment degradation and over-crosslinking makes it more prone to cracking or localized peeling under a 50cm normal impact, resulting in decreased impact resistance.

[0089] From the test results of Example 1 and Comparative Example 5 in Table 1, it can be seen that adjusting the molar fraction of trimethylolpropane to 5 in S1 reduces the number of branching points and β-hydroxy ester sites, making the chain segments more linear, increasing entanglement and flow resistance, and raising the melt viscosity. The dynamic exchangeable structure density is insufficient, resulting in weakened stress relaxation and defect healing capabilities during thick coating curing, and an increase in pinholes. The catalytic system and acid value levels are close, and the thermo-oxidative stability is similar, with little change in color difference. Leveling is limited, orange peel and surface micro-defects increase, and the mirror gloss decreases. Uneven distribution of crosslinking points and insufficient chain segment migration ability make it prone to continuous fine cracks or whitening under a 50cm normal impact, reducing impact resistance.

[0090] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A process for the preparation of an anti-pinhole high-levelling HAA-curing system polyester resin for powder coatings, characterised in that, The preparation method includes: S1: Terephthalic acid, isophthalic acid and aliphatic diacids are mixed in a molar ratio of (60-75):(5-15):(10-20) to obtain a diacid component. Polyol monomers are added to obtain a mixture, wherein the ratio of the total hydroxyl equivalent to the total carboxyl equivalent of the diacids in the polyol monomers is (0.9-1):

1. The molar ratio of neopentyl glycol, 1,4-cyclohexanediol, 2-butyl-2-ethyl-1,3-propanediol and trimethylolpropane in the polyol monomers is (20-40):(20-40):(5-15):(8-15). After stirring and reacting, the temperature is increased and reacted to obtain a linear prepolymer polyester reaction solution with an acid value of 25-50 mgKOH / g. Polycondensation reaction is continued to obtain a linear prepolymer polyester melt with an acid value of 18-28 mgKOH / g. S2: Cool the linear prepolymer polyester melt, add zinc acetylacetonate and hindered phenolic phosphite antioxidant to obtain reaction solution A, wherein the amount of zinc acetylacetonate is 0.1-0.20% of the mass of the linear prepolymer polyester melt, the amount of hindered phenolic phosphite antioxidant is 0.1-0.5% of the mass of the linear prepolymer polyester melt, and the hindered phenolic phosphite antioxidant is tris(2,4-di-tert-butylphenyl) phosphite or antioxidant PEP-36, and stir the reaction to obtain polyester melt; S3: After cooling the polyester melt, spread it to form a molten resin layer, cool and solidify it to obtain sheet polyester resin, crush it, and dry it to obtain polyester resin for powder coating with anti-pinhole high leveling HAA curing system.

2. The method of making an anti-pinholing, high-flow, HAA-curable, powder coating polyester resin according to claim 1, characterized in that, In S1: The aliphatic dicarboxylic acid is any one of adipic acid, succinic acid, or sebacic acid.

3. The method of making an anti-pinholing, high-flow, HAA-curable, powder coating polyester resin according to claim 1, characterized in that, In S1: The stirring reaction temperature of the mixture is 120-160℃.

4. The method of making an anti-pinholing, high-flow, HAA-curable, powder coating polyester resin according to claim 1, characterized in that, In S1: After the stirring reaction, the temperature is further increased to 180-230℃.

5. The method of making an anti-pinhole, high flow HAA cure system, polyester resin for powder coating according to claim 1, characterized in that, In S1: The polycondensation reaction temperature of the linear prepolymer polyester reaction solution is 200-230℃; The polycondensation reaction pressure of the linear prepolymer polyester reaction solution is 1-5 kPa.

6. The method of making an anti-pinholing, high flow HAA cure system, polyester resin for powder coating according to claim 1, characterized in that, In S2: The linear prepolymer polyester melt is cooled to 180-210℃.

7. The method of making an anti-pinholing, high-flow, HAA-curable, powder coating polyester resin according to claim 1, characterized in that, In S3: The polyester melt is cooled to 160-180°C; The thickness of the molten resin layer is 2-10 mm; The cooling and curing temperature of the molten resin layer is 5-25℃.

8. A polyester resin for use in powder coatings with a high-level HAA curing system and anti-pinhole properties, prepared by the preparation method according to any one of claims 1-7.

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