A powder coating and a method for its production

Abstract

This invention discloses a powder coating and its preparation method, relating to the field of powder coating technology. The raw materials for preparing the powder coating include a crystalline polyester resin and a curing agent. The crystalline polyester resin has a melting point of 52–76°C. By mass percentage, the raw materials for preparing the crystalline polyester resin contain 20–35% 1,4-butanediol and 30–50% adipic acid. Because the raw materials for preparing the crystalline polyester resin contain a high content of 1,4-butanediol and adipic acid, the melting point of the crystalline polyester resin (52–76°C) is much lower than the melting temperature of the curing agent and the reaction temperature between the curing agent and the crystalline polyester resin. Therefore, when preparing the powder coating, the two can be melted and mixed under conditions lower than the reaction temperature between the crystalline polyester resin and the curing agent, thereby ensuring sufficient and uniform dispersion. This results in a powder coating with a leveling property of 8 or higher at a lower baking temperature, achieving the level of a liquid coating.

Description

Technical Field

[0001] This invention relates to the field of powder coating technology, and in particular to a powder coating and its preparation method. Background Technology

[0002] Currently, powder coatings on the market typically use polyester resin as the main raw material. During production, polyester resin, curing agent, and pigments are mixed using an industrial screw extruder, then cooled, crushed, and powdered. In use, the powder coating is sprayed onto the workpiece surface and baked to form a protective coating. However, the crystalline or amorphous polyester resins used in existing powder coatings have high melting temperatures and viscosities. Their melting temperatures often exceed their reaction temperature with the curing agent. Therefore, it's impossible to simply melt the polyester resin and then mix it with the curing agent. Even with screw extruder mixing, uneven mixing still occurs, resulting in generally poor coating smoothness. This prevents powder coatings from replacing the leveling advantages of liquid paints.

[0003] It is evident that existing technologies still need improvement and enhancement. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a powder coating and a method for preparing the same, which aims to solve the defect of poor leveling properties of powder coatings in the prior art.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A powder coating, wherein the raw materials for preparing the powder coating include a crystalline polyester resin and a curing agent, wherein the crystalline polyester resin has a melting point of 52-76°C; and the raw materials for preparing the crystalline polyester resin contain 20-35% 1,4-butanediol and 30-50% adipic acid by mass percentage.

[0007] In the powder coating, the raw materials for preparing the crystalline polyester resin also include diethylene glycol, terephthalic acid, isophthalic acid, catalyst, and antioxidant.

[0008] In the powder coating, the acid value of the crystalline polyester resin is 32.0 to 36.0 mgKOH / g.

[0009] A method for preparing a powder coating, wherein the method is used to prepare the powder coating as described above, and the method includes the following steps:

[0010] Step S1: Take 1,4-butanediol, diethylene glycol, and catalyst according to the formula, add them to the reaction vessel, stir and heat to 120°C to melt all materials, add terephthalic acid, adipic acid, and some antioxidants, heat to 150-180°C under nitrogen protection to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating to 180°C and keep at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 10-12 mgKOH / g;

[0011] Step S2: Add isophthalic acid and antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 47-49 mgKOH / g;

[0012] Step S3: Cool the material to 240-245℃ and evacuate it to -0.098--0.1MPa. When the acid value of the material is 32-36 mgKOH / g, release the vacuum to obtain molten crystalline polyester resin.

[0013] Step S4: Cool the molten crystalline polyester resin to 90-120°C, add curing agent and pigment, stir and mix well, then transfer it from the reactor. After the material cools, crush and grind it to obtain powder coating.

[0014] In the powder coating preparation method, the curing agent is triglycidyl isocyanurate, and the amount of curing agent added is 6-10% of the weight of the powder coating.

[0015] In the powder coating preparation method, the raw materials for preparing the crystalline polyester resin, by weight, consist of 20-35% 1,4-butanediol, 30-50% adipic acid, 4-7% diethylene glycol, 10-25% terephthalic acid, 8-15% isophthalic acid, 0.1% monobutyltin oxide, and 0.4-0.9% antioxidant.

[0016] Beneficial effects:

[0017] This invention provides a powder coating and its preparation method. The raw materials for preparing the powder coating include crystalline polyester resin and a curing agent. The melting point of the crystalline polyester resin is 52-76°C, which is much lower than the melting temperature of the curing agent and the reaction temperature between the curing agent and the crystalline polyester resin. Therefore, when preparing the powder coating, the two can be melted and mixed under conditions lower than the reaction temperature between the crystalline polyester resin and the curing agent, so that the two can be fully and uniformly dispersed. This allows the obtained powder coating to have a leveling property of 8 or higher at a lower baking temperature, reaching the level of a liquid coating. Attached Figure Description

[0018] Figure 1 A process flow diagram of the powder coating provided by the present invention. Detailed Implementation

[0019] This invention provides a powder coating and its preparation method. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the following embodiments are provided to further illustrate the invention in detail. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0020] While existing powder coatings offer advantages such as solvent-free evaporation and environmental friendliness, their poor leveling properties, resulting from insufficient uniformity in the mixing of polyester resin and curing agent, significantly differ from liquid coatings. To address this, this invention provides a powder coating prepared from crystalline polyester resin and a curing agent. The crystalline polyester resin has a melting point of 52–76°C, far lower than the melting temperature of the curing agent and the reaction temperature between the curing agent and the crystalline polyester resin. Therefore, during powder coating preparation, the two components can be melt-mixed at a temperature below the reaction temperature of the crystalline polyester resin and the curing agent, ensuring thorough and uniform dispersion. This results in a powder coating with a leveling performance of 8 or higher at a lower baking temperature, achieving the level of liquid coatings.

[0021] Since the melting temperature of crystalline polyester resin affects its melting temperature when mixed with a curing agent, generally speaking, the lower the melting point of the crystalline polyester resin, the easier it is to select the melting temperature when mixing it with the curing agent. However, if the melting point of the crystalline polyester resin is too low, the stability of the powder coating will decrease. Experiments have shown that the higher the content of 1,4-butanediol and adipic acid in the raw materials for preparing crystalline polyester resin, the lower the melting point and the better the elasticity of the prepared crystalline polyester resin. In order to make the powder coating have both good leveling properties and good stability, in a preferred embodiment, the raw materials for preparing the crystalline polyester resin include 20-35% 1,4-butanediol and 30-50% adipic acid. This ratio of 1,4-butanediol and adipic acid can prepare a crystalline polyester resin with a melting point of 52-76°C. Crystalline polyester resins with melting points within this range can be fully mixed with curing agents in a molten state at a temperature higher than their melting point but lower than 120°C, so that the prepared powder coating has excellent leveling properties.

[0022] In addition, the raw materials for preparing the crystalline polyester resin also include diethylene glycol, terephthalic acid, isophthalic acid, catalyst, and antioxidant. With these raw materials, a crystalline polyester resin with a low melting temperature can be obtained through esterification reaction, polycondensation reaction, and vacuum polycondensation reaction.

[0023] In order to achieve good cross-linking and curing effects between the crystalline polyester resin and the curing agent, and to control the amount of curing agent added, it is also necessary to control the acid value of the crystalline polyester resin. In a preferred embodiment, the acid value of the crystalline polyester resin is 32.0 to 56.0 mg KOH / g. Within this acid value range, the amount of curing agent used is moderate, the storage stability is good, and the cost is low.

[0024] A second aspect of the present invention also provides a method for preparing a powder coating, the method being used to prepare the powder coating as described above, such as... Figure 1 As shown, the method includes the following steps:

[0025] Step S1: Take 1,4-butanediol, diethylene glycol, and catalyst according to the formula, and put them into the reaction vessel. Stir and heat to 120°C to make the materials melt and mix evenly. Then add terephthalic acid, adipic acid, and some antioxidants. Heat to 150-180°C under nitrogen atmosphere to carry out the esterification reaction. Water produced during the esterification reaction is distilled off through a distillation column, and the temperature at the top of the distillation column is maintained at 100±1°C to prevent water from flowing back into the reaction vessel. Continue heating to 180°C and hold at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C. Continue heating to 230°C until the acid value of the material is 10-12 mgKOH / g.

[0026] Step S2: Add isophthalic acid and the remaining antioxidant, and carry out esterification polycondensation and end-capping reaction at 245°C until the acid value of the reactants reaches 47-49 mgKOH / g;

[0027] Step S3: Cool the material to 240-245℃ and evacuate the reactor to a vacuum level of -0.098--0.1MPa. Continue the reaction to allow the molecular weight to continue to increase. When the acid value of the material is 32-36 mgKOH / g, release the vacuum to obtain molten crystalline polyester resin.

[0028] Step S4: Cool the molten crystalline polyester resin to 90-120°C, add curing agent and pigment, stir and mix well, then transfer it from the reactor. After the material cools, crush and grind it to obtain powder coating.

[0029] In the powder coating preparation method described in this embodiment, a crystalline polyester resin with a low melting temperature can be prepared by using a high ratio of 1,4-butanediol and adipic acid. Before the prepared crystalline polyester resin solidifies, a curing agent and pigment are added and mixed with it. This mixing method not only reduces the screw extrusion mixing step but also utilizes the residual heat of the molten crystalline polyester resin to melt the curing agent, ensuring better mixing of both in a molten state. This results in better dispersion uniformity of the curing agent and pigment in the obtained powder coating, leading to better leveling properties. Therefore, this preparation method is not only simple in steps but also low in energy consumption, more environmentally friendly, and produces powder coatings with better leveling properties.

[0030] In a preferred embodiment, the curing agent is triglycidyl isocyanurate, and the amount of curing agent added is 6-10% of the weight of the powder coating. The specific amount can be adjusted according to the acid value of the crystalline polyester resin. When the acid value of the crystalline polyester resin is high, the amount of curing agent added is greater.

[0031] In a preferred embodiment, the raw materials for preparing the crystalline polyester resin, by weight, consist of 20-35% 1,4-butanediol, 30-50% adipic acid, 4-7% diethylene glycol, 10-25% terephthalic acid, 8-15% isophthalic acid, 0.05-0.08% monobutyltin oxide, and 0.1-0.5% antioxidant. This composition of raw materials can prepare a crystalline polyester resin with a low melting temperature, resulting in better leveling properties in powder coatings.

[0032] To further illustrate the powder coating and its preparation method provided by the present invention, the following embodiments are provided.

[0033] Example 1

[0034] A powder coating comprising 100 parts of crystalline polyester resin, 8 parts of curing agent, and 5 parts of pigment, wherein the powder coating is prepared by the following steps:

[0035] Step S1: Take 30 parts by weight of 1,4-butanediol, 5 parts by weight of diethylene glycol, and 0.1 parts by weight of monobutyltin oxide, add them to a reaction vessel, stir and heat to 120°C to melt and mix the materials, then add 20 parts by weight of terephthalic acid, 34 parts by weight of adipic acid, and 0.5 parts by weight of antioxidant. Under nitrogen protection, heat to 165°C to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating to 180°C and hold at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 11 mgKOH / g.

[0036] Step S2: Add 10 parts isophthalic acid and 0.4 parts antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 48 mgKOH / g;

[0037] Step S3: Cool the material to 243℃ and evacuate it to -0.098 to -0.1MPa. When the acid value of the material is 35mgKOH / g, release the vacuum to obtain molten crystalline polyester resin.

[0038] Step S4: Cool the molten crystalline polyester resin to 100°C, add 8 parts curing agent and 5 parts pigment according to the formula, stir and mix well, transfer it from the reactor, and after the material cools, crush and grind it to obtain powder coating.

[0039] Example 2

[0040] A powder coating comprising 100 parts of crystalline polyester resin, 10 parts of curing agent, and 5 parts of pigment, wherein the powder coating is prepared by the following steps:

[0041] Step S1: Take 20 parts by weight of 1,4-butanediol, 7 parts by weight of diethylene glycol, and 0.1 parts by weight of monobutyltin oxide, add them to a reaction vessel, stir and heat to 120°C to melt and mix the materials, then add 10 parts by weight of terephthalic acid, 50 parts by weight of adipic acid, and 0.5 parts by weight of antioxidant. Under nitrogen protection, heat to 150°C to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating to 180°C and hold at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 12 mgKOH / g.

[0042] Step S2: Add 12.3 parts of isophthalic acid and 0.1 parts of antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 49 mgKOH / g;

[0043] Step S3: Cool the material to 240℃ and evacuate it to -0.098 to -0.1MPa. When the acid value of the material is 36mgKOH / g, release the vacuum to obtain molten crystalline polyester resin.

[0044] Step S4: Cool the molten crystalline polyester resin to 90°C, add 10 parts of curing agent and 5 parts of pigment according to the formula, stir and mix well, transfer it from the reactor, and after the material cools, crush and grind it to obtain powder coating.

[0045] Example 3

[0046] A powder coating comprising 100 parts of crystalline polyester resin, 6 parts of curing agent, and 5 parts of pigment, wherein the powder coating is prepared by the following steps:

[0047] Step S1: Take 35 parts by weight of 1,4-butanediol, 4 parts by weight of diethylene glycol, and 0.1 parts by weight of monobutyltin oxide, add them to a reaction vessel, stir and heat to 120°C to melt and mix the materials, then add 15.5 parts by weight of terephthalic acid, 30 parts by weight of adipic acid, and 0.1 parts by weight of antioxidant. Under nitrogen protection, heat to 180°C to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating and keep at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 10 mgKOH / g.

[0048] Step S2: Add 15 parts isophthalic acid and 0.3 parts antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 47 mgKOH / g;

[0049] Step S3: Cool the material to 245℃ and evacuate it to -0.098 to -0.1MPa. When the acid value of the material is 32mgKOH / g, release the vacuum to obtain molten crystalline polyester resin.

[0050] Step S4: Cool the molten crystalline polyester resin to 120°C, add 6 parts curing agent and 5 parts pigment according to the formula, stir and mix well, transfer it from the reactor, and after the material cools, crush and grind it to obtain powder coating.

[0051] Example 4

[0052] A powder coating comprising 100 parts of crystalline polyester resin, 7 parts of curing agent, and 5 parts of pigment, wherein the powder coating is prepared by the following steps:

[0053] Step S1: Take 22.3 parts by weight of 1,4-butanediol, 6 parts by weight of diethylene glycol, and 0.1 parts by weight of monobutyltin oxide, add them to the reaction vessel, stir and heat to 120°C to melt and mix the materials, then add 25 parts by weight of terephthalic acid, 38 parts by weight of adipic acid, and 0.3 parts by weight of antioxidant, heat to 170°C under nitrogen protection to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating to 180°C and hold at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 11 mgKOH / g.

[0054] Step S2: Add 8 parts isophthalic acid and 0.3 parts antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 48 mgKOH / g;

[0055] Step S3: Cool the material to 242℃ and evacuate it to -0.098 to -0.1MPa. When the acid value of the material is 34mgKOH / g, release the vacuum to obtain molten crystalline polyester resin.

[0056] Step S4: Cool the molten crystalline polyester resin to 110°C, add 7 parts curing agent and 5 parts pigment according to the formula, stir and mix well, transfer it from the reactor, and after the material cools, crush and grind it to obtain powder coating.

[0057] Comparative Example 1

[0058] It is basically the same as Example 1, except that in step S1, the composition of each material is: 5 parts 1,4-butanediol, 30 parts diethylene glycol, 34 parts terephthalic acid, and 20 parts adipic acid.

[0059] Comparative Example 2

[0060] It is basically the same as Example 1, except that: step S4 is to transfer the molten crystalline polyester resin from the reactor, cool and solidify it, crush and grind it, mix it with the curing agent and pigment, then add the mixture to the screw extruder to mix and extrude it, and then crush and grind it to obtain the powder coating.

[0061] The powder coatings prepared in Examples 1-4 and Comparative Examples 1 and 2 were evaluated for leveling and stability. The leveling evaluation method involved spraying the powder coatings prepared in each example and comparative example onto a flat substrate with a thickness of 80 μm, drying them at 160-200℃, and then comparing them with a standard substrate. The stability evaluation involved storing the coatings at room temperature for 6 months and measuring whether their performance remained consistent with that of 6 months prior. Simultaneously, the acid value and melting point of the crystalline polyester resins in Examples 1-4 and Comparative Examples 1 and 2 were measured. The acid value was determined according to T / GDTL 004-2019 standard, and the melting point was determined using DSC. Specific test results are shown in Table 1.

[0062] Table 1. Performance Test Results

[0063]

[0064] As shown in Table 1, the powder coatings described in Examples 1-4 all achieved a level of 8 in leveling performance and exhibited good stability. This is mainly due to the low melting point of the crystalline polyester resin. Furthermore, the mixing of the crystalline polyester resin with the curing agent and pigment was achieved by melting and mixing the curing agent while the crystalline polyester resin was still in a non-solidified state during preparation. Therefore, the materials exhibited good dispersibility and could be completely cross-linked and cured at a lower baking temperature, resulting in good leveling performance. In contrast, Comparative Example 1 had a lower content of 1,4-butanediol and adipic acid in its raw materials, leading to a higher melting point for the resulting polyester resin. Consequently, at a baking temperature of 160°C, it did not completely melt, resulting in incomplete reaction with the curing agent and poor leveling performance. While Comparative Example 2 had the same melting point as the crystalline polyester resin in Example 1, the mixing method between the crystalline polyester resin and the curing agent was a traditional method, resulting in insufficient dispersion and decreased leveling performance and stability.

[0065] Therefore, it is evident that only by using the powder coating preparation method described in this invention can powder coatings with better leveling and stability be prepared.

[0066] It is understood that those skilled in the art can make equivalent substitutions or modifications to the technical solution and inventive concept of the present invention, and all such substitutions or modifications should fall within the protection scope of the appended claims.

Claims

1. A powder coating, characterized in that, The raw materials for preparing the powder coating include crystalline polyester resin and a curing agent, wherein the melting point of the crystalline polyester resin is 52–76°C; by mass percentage, the raw materials for preparing the crystalline polyester resin consist of 20–35% 1,4-butanediol, 30–50% adipic acid, 4–7% diethylene glycol, 10–25% terephthalic acid, 8–15% isophthalic acid, 0.1% monobutyltin oxide, and 0.4–0.9% antioxidant; the preparation method of the powder coating includes the following steps: Step S1: Take 1,4-butanediol, diethylene glycol, and monobutyltin oxide according to the ratio, put them into the reaction vessel, stir and heat to 120°C to melt and mix the materials, add terephthalic acid, adipic acid, and some antioxidants, heat to 150-180°C under nitrogen protection to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating to 180°C and keep at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 10-12 mgkOH / g; Step S2: Add isophthalic acid and the remaining antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 47-49 mgKOH / g; Step S3: Cool the material to 240-245℃ and evacuate it to -0.098--0.1MPa. When the acid value of the material is 32-36 mgKOH / g, release the vacuum to obtain molten crystalline polyester resin. Step S4: Cool the molten crystalline polyester resin to 90-120°C, add curing agent and pigment, stir and mix well, then transfer it from the reactor. After the material cools, crush and grind it to obtain powder coating.

2. A method for preparing the powder coating as described in claim 1, characterized in that, The method includes the following steps: Step S1: Take 1,4-butanediol, diethylene glycol, and monobutyltin oxide according to the ratio, put them into the reaction vessel, stir and heat to 120°C to melt and mix the materials, add terephthalic acid, adipic acid, and some antioxidants, heat to 150-180°C under nitrogen protection to carry out the esterification reaction, and distill off the water produced during the esterification reaction through a distillation column; continue heating to 180°C and keep at 180°C for 3 hours until the temperature at the top of the distillation column drops below 60°C; continue heating to 230°C until the acid value of the material is 10-12 mgkOH / g; Step S2: Add isophthalic acid and the remaining antioxidant, and carry out esterification polycondensation and end-capping reaction at 245℃ until the acid value of the material reaches 47-49 mgKOH / g; Step S3: Cool the material to 240-245℃ and evacuate it to -0.098--0.1MPa. When the acid value of the material is 32-36 mgKOH / g, release the vacuum to obtain molten crystalline polyester resin. Step S4: Cool the molten crystalline polyester resin to 90-120°C, add curing agent and pigment, stir and mix well, then transfer it from the reactor. After the material cools, crush and grind it to obtain powder coating.

3. The method for preparing powder coating according to claim 2, characterized in that, The curing agent is triglycidyl isocyanurate, and the amount of curing agent added is 6 to 10% of the weight of the powder coating.

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