A low-light type low-temperature curing polyester resin for powder coating and a preparation method thereof

Abstract

The present application relates to a kind of low light type low temperature curing polyester resin for powder coating, mainly using terephthalic acid, butyl malonic acid, neopentyl glycol, dipropylene glycol, tartaric acid, 2-hydroxyethyl benzimidazole, 3,3',4,4'-benzophenone tetracarboxylic dianhydride etc. as raw material to obtain by polymerization. The polyester resin obtained finally is used for TGIC powder coating, which can be fully cured at low temperature, and the coating film after final curing is not only smooth, but also has high impact resistance, and the leveling grade is also high, the boiling water boiling resistance is excellent, and the coating film gloss is lower, the extinction effect under low temperature curing is realized, and other conventional properties meet the application requirements of TGIC powder coating.

Description

Technical Field

[0001] This invention belongs to the field of powder coating technology, specifically relating to a polyester resin for low-gloss, low-temperature curing powder coatings and its preparation method. Background Technology

[0002] Powder coatings are solid powdered synthetic resin coatings composed of solid resins, pigments, fillers, and additives. Unlike ordinary solvent-based and water-based coatings, their dispersion medium is air, not solvents or water. They are characterized by solvent-free production, 100% film formation, and low energy consumption, making them widely used in indoor and outdoor coatings. Outdoor weather-resistant powder coatings are mainly HAA and TGIC system products. Compared to HAA system products, TGIC system powder coatings have advantages such as better weather resistance and no pinholes in thick coatings, leading to their widespread application. TGIC, also known as triglycidyl isocyanate, is a curing agent with three active epoxy groups in its molecular structure, exhibiting the best overall performance. With the increasingly widespread application of decorative powder coatings, the demand for low-gloss powder coatings is also growing. A common method is to add external matting agents. However, the addition of matting agents not only consumes a large amount of TGIC curing agent, but also, due to the physical mixing method, their compatibility with polyester resins is poor, often leading to defects in film leveling. Therefore, the preparation of matte powder coatings requires a long grinding process to ensure a high yield, resulting in high energy consumption. Meanwhile, with increasing demands for energy conservation and emission reduction, and the expansion of the range of powder coating substrates, low-temperature curing powder coatings are an important direction for industry development. Therefore, how to design and synthesize TGIC powder coatings from the perspective of polyester resin molecular structure to achieve a gloss level of 15-30% while enabling low-temperature curing is of significant practical importance. Summary of the Invention

[0003] To address the aforementioned issues, this invention focuses on the design of the chain segment structure and the end-group structure for TGIC curing, and develops a low-gloss, low-temperature curing polyester resin for powder coatings. It is mainly obtained by polymerization using terephthalic acid, butylmalonic acid, neopentyl glycol, dipropylene glycol, tartaric acid, 2-hydroxyethylbenzimidazole, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride as raw materials. The resulting polyester resin has a relatively soft main chain structure, which lowers the softening point and increases its high-temperature fluidity, resulting in better leveling properties of the coating film under low-temperature curing. The appropriate amount of tartaric acid chain extender enhances the functionality of the polyester resin and improves the water resistance of the cured coating film. 2-hydroxyethylbenzimidazole and 3,3',4,4'-benzophenone tetracarboxylic dianhydride serve as special end-capping agents, giving the polyester resin end groups highly active imidazole and anhydride curing groups. This not only allows for full curing of the epoxy groups in the TGIC curing agent at low temperatures, but also the high initial curing activity of the imidazole groups allows them to begin curing with the epoxy groups in TGIC at 125℃, while the curing temperature of anhydrides generally needs to be above 140℃. This difference in curing temperature between the imidazole and anhydride groups leads to differences in the surface curing of the coating film. The final prepared coating film has a low gloss, generally between 15-30%, achieving a matte finish under low-temperature curing. Other conventional properties meet the application requirements of TGIC powder coatings.

[0004] This invention relates to a polyester resin for low-gloss, low-temperature curing powder coatings, comprising the following raw materials in molar proportions:

[0005]

[0006] The raw materials also include a catalyst and an antioxidant; the molar amount of the catalyst is 0.05-0.1% of the molar amount of terephthalic acid; the molar amount of the antioxidant is 0.03-0.05% of the molar amount of terephthalic acid.

[0007] Preferably, the catalyst is tetrabutyl titanate; the antioxidant is antioxidant 1076.

[0008] The preparation method of a low-gloss, low-temperature curing powder coating polyester resin as described above includes the following steps:

[0009] A. Add the prescribed amounts of neopentyl glycol, dipropylene glycol, and catalyst to the reaction vessel, stir, and heat to 125-130℃ to mix thoroughly.

[0010] B. Add the prescribed amounts of terephthalic acid and butylmalonic acid to the above mixture, and gradually raise the temperature to 210-220℃ under nitrogen protection, and then carry out the heat-preserving esterification polymerization reaction.

[0011] C. Take samples to test the acid value of the reacting polymer. When the acid value is 38-45 mg KOH / g, add the chain extender tartaric acid in the formula amount, and then gradually raise the temperature to 225-230℃ and continue the heat-preserving polymerization reaction.

[0012] D. When the acid value of the polymer reaches 30-35 mg KOH / g, add the prescribed amount of antioxidant, start the vacuum system, and carry out a vacuum polycondensation reaction at 225-230℃ to obtain a product with a higher degree of polymerization.

[0013] E. When the polymer acid value drops to below 15-20 mg KOH / g, stop the vacuum system, cool down to 200-210℃, then add the prescribed amount of 2-hydroxyethylbenzimidazole, and keep warm to carry out the benzimidazole grafting and end-capping reaction.

[0014] F. Sampling and detection by liquid chromatography. When the conversion of free 2-hydroxyethylbenzimidazole reaches more than 98%, it indicates that the 2-hydroxybenzimidazole has been grafted and capped. At this time, 3,3',4,4'-benzophenone tetracarboxylic dianhydride is added, and the temperature is raised to 220-225℃ to carry out the anhydride capping reaction.

[0015] G. Sampling and testing: Stop the reaction when the acid value of the reactants is 19-24 mgKOH / g. Discharge the material while it is still hot and cool the polyester resin. Then crush and granulate to obtain the polyester resin.

[0016] Preferably, in step B, the temperature is gradually increased to 210-220℃ at a heating rate of 12-15℃ / h; in step C, the temperature is gradually increased to 225-230℃ at a heating rate of 5-7℃ / h; in step D, the vacuum degree is maintained between -0.095Mpa and -0.097Mpa; and in step G, the polyester resin is cooled with a steel belt containing condensate.

[0017] The resulting product is a pale yellow transparent granule with an acid value of 19-24 mgKOH / g and a softening point of 103-112℃.

[0018] This invention also relates to the application of the polyester resin for low-gloss, low-temperature curing powder coatings described above, or a polyester resin for low-gloss, low-temperature curing powder coatings prepared by the aforementioned preparation method, in TGIC powder coatings.

[0019] The TGIC powder coating formulation may include, in addition to polyester resin and TGIC curing agent, titanium dioxide, barium sulfate, leveling agent, gloss enhancer, benzoin, etc. The coating preparation method is as follows: All materials are mixed according to the powder coating formulation, extruded, pressed, and crushed using a twin-screw extruder, and then the sheets are pulverized and sieved to produce powder coating. The powder coating is then sprayed onto a surface-treated tinplate substrate using an electrostatic spray gun, and cured to obtain the coating layer.

[0020] Beneficial effects:

[0021] This invention starts with the design of the chain segment structure and the end group structure for TGIC curing, and develops a low-gloss, low-temperature curing polyester resin for TGIC powder coating. It mainly uses terephthalic acid, butylmalonic acid, neopentyl glycol, dipropylene glycol, tartaric acid, 2-hydroxyethylbenzimidazole, 3,3',4,4'-benzophenone tetracarboxylic dianhydride and other raw materials for polymerization. The resulting polyester resin has a relatively soft main chain structure, which lowers the softening point of the polyester resin and helps increase its high-temperature fluidity, resulting in better leveling properties of the coating film under low-temperature curing. The appropriate amount of tartaric acid chain extender can improve the functionality of the polyester resin and enhance the water resistance of the cured coating film. 2-hydroxyethylbenzimidazole and 3,3',4,4'-benzophenone tetracarboxylic dianhydride serve as special end-capping agents, giving the polyester resin end groups highly active imidazole and anhydride curing groups. This not only allows for full curing with the epoxy groups in the TGIC curing agent at low temperatures (e.g., 150℃), but also the imidazole group has high initial curing activity, allowing it to begin curing with the epoxy groups in TGIC at 125℃. In contrast, the curing temperature of anhydrides generally needs to be above 140℃ to begin curing. This difference in curing temperature between the imidazole group and the anhydride results in differences in the surface curing of the coating film. The final prepared coating film has a low gloss, generally between 15-30%, achieving a matte finish under low-temperature curing. Other conventional properties meet the application requirements of TGIC powder coatings.

[0022] The present invention has been described in detail above; however, the above embodiments are merely exemplary in nature and are not intended to limit the invention. Furthermore, this document is not limited to the foregoing prior art or the invention itself, or to any theory described in the following embodiments. Detailed Implementation

[0023] The present invention will be further described below with reference to the embodiments. It should be noted that the following embodiments are provided for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0024] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0025] Preparation Example 1

[0026] A low-gloss, low-temperature curing powder coating polyester resin comprises raw materials with the following molar composition:

[0027]

[0028] The catalyst is tetrabutyl titanate, and the molar amount of the catalyst is 0.1% of the molar amount of terephthalic acid.

[0029] The antioxidant is antioxidant 1076, and the molar amount of the antioxidant is 0.05% of the molar amount of terephthalic acid.

[0030] The preparation method of the low-gloss, low-temperature curing powder coating polyester resin includes the following steps:

[0031] A. Add the prescribed amounts of neopentyl glycol, dipropylene glycol, and catalyst to the reaction vessel, stir, and heat to 130°C to mix thoroughly.

[0032] B. Add the prescribed amounts of terephthalic acid and butylmalonic acid to the above mixture, and gradually increase the temperature to 220°C at a rate of 15°C / h under nitrogen protection, and then carry out a heat-preserving esterification polymerization reaction.

[0033] C. Take samples to test the acid value of the reacting polymer. When the acid value is 38-45 mg KOH / g, add the chain extender tartaric acid in the formula amount, and then gradually increase the temperature to 230℃ at a heating rate of 7℃ / h, and continue the heat preservation polymerization reaction.

[0034] D. When the acid value of the polymer reaches 30-35 mg KOH / g, add the prescribed amount of antioxidant, start the vacuum system, maintain the vacuum degree at -0.097 MPa, and carry out vacuum polycondensation reaction at 230℃ to obtain a product with a higher degree of polymerization.

[0035] E. When the polymer acid value drops to below 15-20 mg KOH / g, stop the vacuum system, cool down to 210℃, then add the prescribed amount of 2-hydroxyethylbenzimidazole, and keep warm to carry out the benzimidazole grafting and end-capping reaction.

[0036] F. Sampling and detection by liquid chromatography. When the conversion of free 2-hydroxyethylbenzimidazole reaches more than 98%, it indicates that the 2-hydroxybenzimidazole has been grafted and capped. At this time, 3,3',4,4'-benzophenone tetracarboxylic dianhydride is added and the temperature is raised to 225℃ to carry out the anhydride capping reaction.

[0037] G. Sampling and testing: Stop the reaction when the acid value of the reactants is 19-24 mgKOH / g. Discharge the material while it is still hot and cool the polyester resin with a steel belt with condensate. Then crush and granulate to obtain the polyester resin.

[0038] The resulting product is a pale yellow transparent granule with an acid value of 21 mg KOH / g and a softening point of 106℃.

[0039] Preparation Example 2

[0040] A low-gloss, low-temperature curing powder coating polyester resin comprises raw materials with the following molar composition:

[0041]

[0042] The catalyst is tetrabutyl titanate, and the amount of catalyst used is 0.1% of the molar amount of terephthalic acid;

[0043] The antioxidant is antioxidant 1076, and the amount of antioxidant used is 0.05% of the molar amount of terephthalic acid.

[0044] The preparation method is the same as in Example 1.

[0045] The resulting product is a pale yellow transparent granule with an acid value of 24 mg KOH / g and a softening point of 109℃.

[0046] Preparation Example 3

[0047] A low-gloss, low-temperature curing powder coating polyester resin comprises raw materials with the following molar composition:

[0048]

[0049]

[0050] The catalyst is tetrabutyl titanate, and the amount of catalyst used is 0.1% of the molar amount of terephthalic acid;

[0051] The antioxidant is antioxidant 1076, and the amount of antioxidant used is 0.03% of the molar amount of terephthalic acid.

[0052] The preparation method is the same as in Example 1.

[0053] The resulting product is a pale yellow transparent granule with an acid value of 23 mg KOH / g and a softening point of 111℃.

[0054] Preparation Example 4

[0055] A low-gloss, low-temperature curing powder coating polyester resin comprises raw materials with the following molar composition:

[0056]

[0057] The catalyst is tetrabutyl titanate, and the amount of catalyst used is 0.08% of the molar amount of terephthalic acid;

[0058] The antioxidant is antioxidant 1076, and the amount of antioxidant used is 0.05% of the molar amount of terephthalic acid.

[0059] The preparation method is the same as in Example 1.

[0060] The resulting product is a pale yellow transparent granule with an acid value of 24 mg KOH / g and a softening point of 105℃.

[0061] Comparative Preparation Example 1

[0062] Commercially available TGIC polyester resin, acid value 34mgKOH / g, softening point 115℃, model GH-2240; Zhejiang Guanghua Technology Co., Ltd.

[0063] Comparative Preparation Example 2

[0064] The polyester resin was the same as Comparative Example 1, except that the curing conditions were changed to 200℃ / 15min.

[0065] Examples 1-4 and Comparative Examples 1 and 2 (polyester resins were obtained through the corresponding preparation examples or comparative preparation examples described above)

[0066] The formulation based on TGIC powder coating is as follows, by weight:

[0067]

[0068] Coating preparation: All materials are mixed according to the TGIC powder coating formula, extruded, pressed, and crushed using a twin-screw extruder. The sheets are then pulverized and sieved (160-180 mesh) to produce powder coating. The powder coating is then sprayed onto a surface-treated tinplate substrate using an electrostatic spray gun and cured at 150℃ for 15 minutes to obtain the coating layer.

[0069] Performance Comparison

[0070] Coating performance testing is based on GB / T 21776-2008, "Guidelines for Testing Powder Coatings and Their Coatings". Leveling performance is assessed using the PCI leveling rating system, ranging from 1 to 10, with higher numbers indicating better leveling.

[0071] The results of testing the coating performance of the polyester resins prepared according to the coating formulations provided by the present invention are shown in Table 1 below.

[0072] Table 1. Coating performance of powder coatings

[0073]

[0074]

[0075] As shown in Table 1, the polyester resin prepared by this invention, using a special raw material formulation and process technology, exhibits high end-group activity and significant curing differences. The polyester resin prepared using this technology can achieve full curing of TGIC powder coatings at 150℃ for 15 minutes. The final cured coating film is not only smooth and even with excellent impact resistance, but also has a high leveling grade, reaching level 6. Effective matting is achieved through the curing differences between the terminal imidazole and acid anhydride groups, with the coating gloss generally between 15-30%, and the boiling water resistance is also quite outstanding.

[0076] In contrast, the commercially available polyester resin product in Comparative Example 1 could not be fully cured at 150℃ / 15min, resulting in poor coating appearance, impact resistance, and flowability. Comparative Example 2 improved its curing conditions to 200℃ / 15min, under which it achieved full curing, a higher flowability, and met the requirements for impact resistance and boiling water resistance, but could not achieve a matte finish, with a gloss level of over 94%.

[0077] As can be seen, the polyester resin obtained by the present invention can be fully cured at low temperature when used in TGIC powder coating. The final cured coating film is not only smooth and flat with excellent impact resistance, but also has a high leveling grade, excellent boiling water resistance, and low gloss, achieving a matte effect under low temperature curing. Other conventional properties meet the application requirements of TGIC powder coating.

[0078] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and substance defined by the claims of the present invention; and such modifications or substitutions are still within the scope defined by the claims of the present invention.

Claims

1. A polyester resin for low-gloss, low-temperature curing powder coatings, characterized in that, Raw materials comprising the following molar components: 20-26 parts of terephthalic acid; 12-16 parts of butylmalonic acid; 16-21 parts of neopentyl glycol; 19-24 parts of dipropylene glycol monohydrate; Tartaric acid 1-2 parts; 7-9 parts of 2-hydroxyethylbenzimidazole; 4-5 parts of 3,3',4,4'-benzophenone tetracarboxylic dianhydride; The raw materials also include a catalyst and an antioxidant; the catalyst is tetrabutyl titanate, used at 0.05-0.1% of the molar amount of terephthalic acid; the antioxidant is antioxidant 1076, used at 0.03-0.05% of the molar amount of terephthalic acid. The preparation method of the polyester resin for low-gloss, low-temperature curing powder coating includes the following steps: A. Add the prescribed amounts of neopentyl glycol, dipropylene glycol, and catalyst to the reaction vessel, stir, and heat to 125-130℃ to mix thoroughly. B. Add the prescribed amounts of terephthalic acid and butylmalonic acid to the above mixture, and gradually raise the temperature to 210-220℃ under nitrogen protection, and then carry out the heat-preserving esterification polymerization reaction. C. Take samples to test the acid value of the reacting polymer. When the acid value is 38-45 mg KOH / g, add the chain extender tartaric acid in the formula amount, and then gradually raise the temperature to 225-230℃ to continue the heat-preserving polymerization reaction. D. When the acid value of the polymer reaches 30-35 mg KOH / g, add the prescribed amount of antioxidant, start the vacuum system, and carry out a vacuum polycondensation reaction at 225-230℃ to obtain a product with a higher degree of polymerization. E. When the polymer acid value drops to below 15-20 mg KOH / g, stop the vacuum system, cool down to 200-210℃, then add the prescribed amount of 2-hydroxyethylbenzimidazole, and keep warm to carry out the benzimidazole grafting and end-capping reaction. F. Sampling and detection by liquid chromatography. When the conversion of free 2-hydroxyethylbenzimidazole reaches more than 98%, it indicates that the 2-hydroxybenzimidazole has been grafted and capped. At this time, 3,3',4,4'-benzophenone tetracarboxylic dianhydride is added, and the temperature is raised to 220-225℃ to carry out the anhydride capping reaction. G. Sampling and testing: Stop the reaction when the acid value of the reactants is 19-24 mgKOH / g. Discharge the material while it is still hot and cool the polyester resin. Then crush and granulate to obtain the polyester resin.

2. A method for preparing the polyester resin for low-gloss, low-temperature curing powder coating as described in claim 1, characterized in that, Includes the following steps: A. Add the prescribed amounts of neopentyl glycol, dipropylene glycol, and catalyst to the reaction vessel, stir, and heat to 125-130℃ to mix thoroughly. B. Add the prescribed amounts of terephthalic acid and butylmalonic acid to the above mixture, and gradually raise the temperature to 210-220℃ under nitrogen protection, and then carry out the heat-preserving esterification polymerization reaction. C. Take samples to test the acid value of the reacting polymer. When the acid value is 38-45 mg KOH / g, add the chain extender tartaric acid in the formula amount, and then gradually raise the temperature to 225-230℃ to continue the heat-preserving polymerization reaction. D. When the acid value of the polymer reaches 30-35 mg KOH / g, add the prescribed amount of antioxidant, start the vacuum system, and carry out a vacuum polycondensation reaction at 225-230℃ to obtain a product with a higher degree of polymerization. E. When the polymer acid value drops to below 15-20 mg KOH / g, stop the vacuum system, cool down to 200-210℃, then add the prescribed amount of 2-hydroxyethylbenzimidazole, and keep warm to carry out the benzimidazole grafting and end-capping reaction. F. Sampling and detection by liquid chromatography. When the conversion of free 2-hydroxyethylbenzimidazole reaches more than 98%, it indicates that the 2-hydroxybenzimidazole has been grafted and capped. At this time, 3,3',4,4'-benzophenone tetracarboxylic dianhydride is added, and the temperature is raised to 220-225℃ to carry out the anhydride capping reaction. G. Sampling and testing: Stop the reaction when the acid value of the reactants is 19-24 mgKOH / g. Discharge the material while it is still hot and cool the polyester resin. Then crush and granulate to obtain the polyester resin.

3. The method for preparing low-gloss, low-temperature curing powder coating polyester resin as described in claim 2 is further characterized in that, in step B, the temperature is gradually increased to 210-220℃ at a heating rate of 12-15℃ / h.

4. The method for preparing polyester resin for low-gloss, low-temperature curing powder coating as described in claim 2 is further characterized in that, in step C, the temperature is gradually increased to 225-230℃ at a heating rate of 5-7℃ / h.

5. The method for preparing the polyester resin for low-gloss, low-temperature curing powder coating as described in claim 2, further characterized in that, in step D, the vacuum degree is maintained at -0.095 MPa to -0.097 MPa.

6. The method for preparing low-gloss, low-temperature curing powder coating polyester resin as described in claim 2 is further characterized in that, in step G, the polyester resin is cooled with a steel belt containing condensate.

7. The application of the polyester resin for low-gloss, low-temperature curing powder coatings as described in claim 1, or the polyester resin for low-gloss, low-temperature curing powder coatings prepared by the preparation method described in any one of claims 2-6, in TGIC powder coatings.

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