Powder coating for workpiece post-processing and preparation method thereof

Abstract

The invention provides a powder coating for post-processing of workpieces and a preparation method of the powder coating. The powder coating is prepared from the following raw materials: 45 to 60 percent of polyester resin A, 5 to 10 percent of crystalline resin B, 3 to 8 percent of curing agent, 0.8 to 1.2 percent of flatting agent, 0.7 to 28 percent of pigment, 10 to 30 percent of inorganic filler, 0.1 to 1 percent of nucleating agent and 0.1 to 1.0 percent of additive, the acid value of the polyester resin A is 25-55 mgKOH / g, the viscosity is 3000-7000 mPa.S, and the Tg is 57-63 DEG C; the acid value of the crystalline resin B is 20 to 40 mgKOH / g, the viscosity is 500 to 2000 mPa.S, and the crystal melting temperature is 100 to 120 DEG C. Compared with the prior art, the powder coating disclosed by the invention can obviously resist the reduction of mechanical properties of a coating in a delayed storage process and at low temperature, and realizes that the flexibility is not attenuated in delayed storage and low-temperature environments.

Description

Technical Field

[0001] The invention belongs to the technical field of powder coatings, and in particular relates to a powder coating for post-processing of workpieces and a preparation method thereof. Background Art

[0002] Paint refers to materials that can form a thin film under certain conditions when applied to the surface of an object to provide protection, decoration or other special functions (insulation, rust prevention, mildew prevention, heat resistance, etc.). Most powder coatings are usually sprayed after the substrate is prepared into a workpiece. Since the surface shape of the material is different after molding, there will be problems such as coverage defects at corners, depressions, etc. when powder spraying is performed, which affects the service life of parts. The method of spraying powder coating on the substrate first and then processing it into a workpiece has relatively simple requirements for the quality of powder coating, spraying process, etc. Therefore, in the field of more complex profiles, the molding method of pre-coating the substrate with powder coating and then stamping it into shape (post-processing technology) has become an increasingly common way for manufacturers to use powder coatings.

[0003] At present, the common methods of preparing substrates into workpieces include cold stamping, bending, etc., and post-forming processes such as cold stamping and bending have external forces, so they put forward higher requirements on the flexibility of powder coatings. At the same time, due to the time delay, metal substrates are often placed for part of the time before processing after spraying powder coatings, or processed in areas with different temperatures. Technical personnel in the industry are aware of the problem that the mechanical properties of powder coatings decrease with the increase of storage time and the decrease of ambient temperature. For example, the impact performance and bending performance decrease significantly after being placed for 7 days or at low temperatures. Therefore, the post-processing process has very high requirements for the flexibility of powder coatings.

[0004] The patent with publication number CN103739830A published on April 23, 2014, discloses a polyester resin for TGIC-cured high-toughness powder coating and its preparation method. The powder coating prepared with its polyester resin has excellent flexibility and excellent mechanical properties at room temperature and low temperature conditions of about -10°C, and does not involve delayed flexibility. The patent with publication number CN113980253A published on January 28, 2022 discloses a polyester resin for low-curing agent dosage powder coating that is resistant to bending, easy to matte, and has a preparation method. The matte powder coating prepared with its polyester resin has excellent mechanical properties and meets the requirements of post-forming processing that is resistant to bending. However, the application field is matte and does not involve delayed flexibility. The patent with publication number CN112194956A published on January 8, 2021 discloses a powder coating with anti-gravel impact performance, its preparation method and application. By selecting a polyester resin with a low glass transition temperature to react with an epoxy resin, the coating has better anti-gravel impact performance at low temperatures, and the low-temperature flexibility of the coating is further increased by adding PMMA and polyvinyl butyral. The low glass transition temperature represents low storage performance and does not utilize the product's marketing.

[0005] Therefore, it is necessary to provide a powder coating with high time-delay flexibility. Summary of the invention

[0006] The purpose of the present invention is to provide a powder coating for post-processing of workpieces and a preparation method thereof, wherein the raw materials include a polyester resin with excellent flexibility, a crystalline resin and a nucleating agent. The polyester resin with excellent flexibility gives the powder coating good processing performance and storage performance at room temperature; the addition of the crystalline resin further increases the flexibility of the powder coating in low temperature and delayed environments; the combined effect of the nucleating agent and the crystalline resin causes the coating to undergo cold crystallization in low temperature and delayed environments, significantly resisting the decrease in mechanical properties of the coating during the delayed process, and achieving no attenuation of flexibility in low temperature and delayed environments.

[0007] The specific technical solutions of the present invention are as follows:

[0008] A powder coating for post-processing of workpieces, comprising the following raw materials in percentage by mass:

[0009] Polyester resin A: 45-60%, crystalline resin B: 5-10%, curing agent: 3-8%, leveling agent: 0.8-1.2%, pigment: 0.7-28%, inorganic filler: 10-30%, nucleating agent: 0.1-1%, auxiliary agent 0.1-1.0%; the above raw materials total 100%.

[0010] The polyester resin A has excellent flexibility and is a carboxyl-terminated polyester resin. The acid value of the polyester resin A is 25-55 mgKOH / g, the viscosity is 3000-7000 mPa.S, and the Tg is 57-63°C at 200°C.

[0011] The acid value of the crystalline resin B is 20-40 mgKOH / g, the viscosity is 500-2000 mPa.S, and the crystal melting temperature is 100-120°C at 200°C.

[0012] The mass ratio of the polyester resin A to the crystalline resin B is 20:1 to 5:1.

[0013] The curing agent is TGIC triglycidyl isocyanurate.

[0014] The leveling agent is a GLP588 series polyacrylate leveling agent adsorbed by silica.

[0015] The pigment is an inorganic material, selected from any one or more of titanium dioxide, iron yellow, carbon black, and iron black.

[0016] The inorganic filler is any one or more of barium sulfate, mica and calcium carbonate.

[0017] The nucleating agent is one or both of a sorbitol benzyl derivative and a high melting point polymer nucleating agent. The sorbitol benzyl derivative is dibenzylidene sorbitol (DBS) or di(p-methylbenzylidene) sorbitol (PM-DBS); the high melting point polymer nucleating agent is polyvinyl cyclohexane, polyvinyl pentane or ethylene / acrylate copolymer, and the above nucleating agents are all commercial nucleating agents.

[0018] The auxiliary agent is any one or more of benzoin, wetting agent 701B, coupling agent or charge enhancer; preferably benzoin and wetting agent 701B.

[0019] Preferably, the polyester resin A comprises the following raw materials in mass percentage: 30-45% of polyol, 0-1% of branching agent, 40-65% of aromatic polyacid, 5-15% of acidolysis agent, 0.09-1.4% of esterification catalyst, 0.01-1% of curing accelerator, and 0.1-1% of antioxidant, and the total mass percentage of the above raw materials is 100%.

[0020] In the polyester resin A raw material: the polyol is any one or more of neopentyl glycol (NPG), ethylene glycol (EG), methyl propylene glycol (MPO), ethyl butyl propylene glycol (BEPD), and the content of neopentyl glycol is not less than 60%.

[0021] In the polyester resin A raw material: the branching agent is one of trimethylolpropane (TMP) and trimethylolethane (TME) or a combination thereof.

[0022] In the polyester resin A raw material: the aromatic polyacid is at least one of terephthalic acid (PTA) and isophthalic acid (PIA).

[0023] In the polyester resin A raw material: the acidolysis agent is any one or more of isophthalic acid (PIA), adipic acid (ADA), 1,4-cyclohexanedicarboxylic acid (CHDA), and fumaric acid (FCC).

[0024] In the polyester resin A raw material: the esterification catalyst is an organic tin catalyst or a titanium catalyst tin catalyst, preferably one or more of dibutyltin oxide, tributyltin oxide, dihydroxybutyltin chloride, stannous oxalate or monobutyltin oxide;

[0025] In the polyester resin A raw material: the curing accelerator is one or more of tetraethylammonium bromide, tetramethylammonium chloride, tetramethylammonium bromide, benzyltriethylammonium chloride, triphenylethylphosphonium bromide, and dibutyltin dilaurate.

[0026] In the polyester resin A raw material: the antioxidant is a compound of a phosphite antioxidant and a hindered phenol antioxidant in a mass ratio of 1:3 to 3:1; the phosphite antioxidant is selected from tris[2,4-di-tert-butylphenyl]phosphite (antioxidant 168) and bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (antioxidant 626); the hindered phenol antioxidant is tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]pentaerythritol ester (antioxidant 1010) and β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate n-octadecyl alcohol ester (antioxidant 1076).

[0027] The preparation method of the polyester resin A comprises the following steps:

[0028] 1) Add the formulated amount of polyol, branching agent, aromatic polyacid and esterification catalyst into a reaction kettle at the same time, raise the temperature to 235-255°C at a rate of 1-2°C / min in a nitrogen atmosphere and maintain it, and after the system is clarified, take a sample to detect the acid value (AV) to reach the set value of 5-12 mg KOH / g;

[0029] 2) Then, the temperature is lowered to 215-235°C at a rate of 3-5°C / min, and an acidolysis agent is added to the reactor, and the temperature is raised to 235-250°C at a rate of 1-2°C / min and maintained for acidolysis and end-capping until the acid value of the polyester reaches 38-70 mg KOH / g;

[0030] 3) Cool down to 230-240°C and perform polycondensation under -0.1MPa vacuum condition. After the acid value reaches 25-55mg KOH / g, discharge the mixture and cool down to 180-230°C. Add the formulated amount of antioxidant and curing accelerator and maintain for 5-30min before discharging the mixture.

[0031] Preferably, the crystalline resin B comprises the following raw materials in mass percentage: 28-45% polyol; 0.2-1.5% branching agent; 45-60% polyacid; 4-12% acidolysis agent; 0.05-0.2% esterification catalyst; the total mass percentage of the above raw materials is 100%.

[0032] In the crystalline resin B: the polyol is a polyol with a symmetrical structure, comprising any one or more of 1,4-cyclohexanedimethanol (CHDM), ethylene glycol EG, 1,4-butanediol (BDO) and 1,6-hexanediol (HDO), and the content of 1,6-hexanediol is not less than 70%.

[0033] In the crystalline resin B: the branching agent is one or more of trimethylolpropane, trimethylolethane, and 2-hydroxypropionic acid.

[0034] In the crystalline resin B, the polyacid is a polyacid with a symmetrical structure, and includes one or two of terephthalic acid (PTA), 1,4-succinic acid and adipic acid (ADA).

[0035] In the crystalline resin B: the acidolysis agent is any one or more of isophthalic acid PIA, adipic acid (ADA), and 1,4-cyclohexanedicarboxylic acid CHDA.

[0036] In the crystalline resin B: the esterification catalyst is an organic tin catalyst or a titanium catalyst, preferably one or more of dibutyltin oxide, tributyltin oxide, dihydroxybutyltin chloride, stannous oxalate or monobutyltin oxide;

[0037] The method for preparing the crystalline resin B comprises the following steps:

[0038] S1. Add the formulated amount of polyol, branching agent, polyacid and esterification catalyst into a reaction kettle at the same time, heat to 235-255°C at a rate of 1-2°C / min in a nitrogen atmosphere and maintain, and after the system is clarified, take a sample to detect the acid value (AV) to reach 5-15 mg KOH / g;

[0039] S1, then cool down to 215-235°C at a rate of 3-5°C / min and add an acidolysis agent into the reactor, heat up to 235-250°C at a rate of 1-2°C / min and maintain acidolysis and end-capping until the acid value of the polyester reaches 30-52 mg KOH / g;

[0040] S3, cool down to 230-240℃ and carry out polycondensation under -0.1MPa vacuum condition, and discharge the material after the acid value reaches 20-40mg KOH / g.

[0041] The invention provides a method for preparing a powder coating for post-processing of workpieces, comprising the following process flow: fully premixing the raw materials in a mixing tank according to the formula → co-extruding through an extruder → tableting → crushing → grinding → sieving → packaging the finished product.

[0042] The powder coating for post-processing of workpieces provided by the present invention adopts polyester resin A with excellent flexibility, crystalline resin B and nucleating agent as raw materials. The polyester resin A with excellent flexibility gives the powder coating good processing performance and storage performance; the addition of crystalline resin B further increases the low-temperature flexibility and delayed flexibility of the powder coating; the nucleating agent further promotes the crystallization behavior of the crystalline resin at low temperature and improves the flexibility (referred to as cold crystallization behavior) during the delayed storage process, significantly resisting the decrease in the mechanical properties of the coating during the delayed storage process and at low temperatures, and achieving the flexibility performance not decaying during delayed storage and low temperature environment. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0045] If no specific techniques or conditions are specified in the examples, they can be carried out according to the techniques or conditions described in the literature in the art or according to the product instructions.

[0046] Table 1 below shows the raw materials and usage ratios of the polyester resin A used in the example and the polyester resin C used in the comparative example, as well as the parameters of the products. According to Table 1, the preparation methods of the polyester resin A and the polyester resin C are as follows:

[0047] 1) Add the formulated amount of polyol, branching agent, aromatic polyacid and esterification catalyst into a reaction kettle at the same time, raise the temperature to 235-255°C at a rate of 1-2°C / min in a nitrogen atmosphere and maintain it, and take a sample to detect the acid value (AV) after the system is clarified to reach the set value (primary acid value);

[0048] 2) Then, the temperature is lowered to 215-235°C at a rate of 3-5°C / min, and an acidolysis agent is added to the reactor, and the temperature is raised to 235-250°C at a rate of 1-2°C / min and maintained for acidolysis and end-capping until the acid value of the polyester reaches the set value (secondary acid value);

[0049] 3) Cool down to 230-240°C and perform polycondensation under -0.1MPa vacuum conditions. After the acid value reaches the set value, discharge the mixture and cool down to 180-230°C, add the formulated amount of antioxidant and curing accelerator and maintain for 5-30 minutes before discharging.

[0050] Steps 1), 2) and 3) require the addition of industrial high-purity nitrogen (with a purity greater than 99.99%) for protection.

[0051] Table 1 Raw material dosage and finished product parameters of ester resin A and polyester resin C

[0052]

[0053]

[0054] The raw materials and amounts of the crystalline resin B used in the following examples are shown in Table 2, and the preparation method is as follows:

[0055] S1. Add the formulated amount of polyol, branching agent, polyacid and esterification catalyst into a reaction kettle at the same time, raise the temperature to 235-255°C at a rate of 1-2°C / min in a nitrogen atmosphere and maintain it, and take a sample to detect the acid value (AV) after the system is clarified to reach the set value (primary acid value);

[0056] S2, then cool down to 215-235°C at a rate of 3-5°C / min and add an acidolysis agent into the reactor, heat up to 235-250°C at a heating rate of 1-2°C / min and maintain acidolysis and end-capping until the acid value of the polyester reaches the set value (secondary acid value);

[0057] S3, cool down to 230-240℃ and perform polycondensation under -0.1MPa vacuum condition, and vent and discharge the material after the acid value reaches the set value.

[0058] Table 2 Parameters of the crystalline resin B formula and finished resin used in the examples

[0059]

[0060]

[0061] Example 1-Example 3

[0062] A powder coating for post-processing of workpieces includes raw materials: polyester resin A, crystalline resin B, curing agent (triglycidyl isocyanurate (TGIC)), leveling agent, pigment, inorganic filler, nucleating agent and additives to prepare the post-processing powder coating, and the specific amounts are shown in Table 3 below.

[0063] Comparative Example 1-Comparative Example 3

[0064] A powder coating for post-processing of workpieces includes raw materials: polyester resin A, polyester resin C, crystalline resin B, curing agent (triglycidyl isocyanurate (TGIC)), leveling agent, pigment, inorganic filler, nucleating agent and additives to prepare the post-processing powder coating. The specific amounts are shown in Table 3 below.

[0065] Table 3 Powder coating formulations of various embodiments and comparative examples

[0066]

[0067] The preparation method of the powder coatings of the above embodiments and comparative examples is: weigh different raw materials and fully premix them in a mixing tank according to the formula in Table 2 → extrude them together through an extruder → tablet press → crush → grind → sieve → package the finished product. Then, the powder coating is sprayed on the surface of a 0.5 mm thick iron plate by electrostatic spraying, and the different properties of the coating are tested after baking at 200°C @ 10 min. The results are shown in Table 4.

[0068] Table 4 Performance of powder coatings in various embodiments and comparative examples

[0069]

[0070]

[0071] Note: The PCI leveling grade from 1 to 10 represents the improvement of leveling performance, where PCI ~ 10 means the leveling performance is the same as a mirror. Currently, there are commercial standard plates for evaluating PCI grades.

[0072] It can be seen from Table 4 that the powder coating with excellent flexibility, which is composed of the polyester resin A with excellent flexibility, the crystalline resin B and the nucleating agent provided by the present invention, has significantly better delayed impact, bending performance and impact and bending performance at low temperature than the comparative example. Under the premise that the powder coating has good storage conditions, the powder coating of the embodiment can achieve that the flexibility of the coating does not decay during delayed storage and in a low temperature environment.

[0073] Comparative Example 1 uses polyester resin C1, and Comparative Example 2 uses polyester resin C2, neither of which uses the polyester resin A and crystalline resin B of the present invention, and neither uses a nucleating agent, resulting in poor delayed impact, bending performance, and impact and bending performance at low temperatures. Comparative Example 3 uses a powder coating composed of polyester resin A and crystalline resin B with excellent flexibility, but does not use a nucleating agent. Its delayed impact, bending performance, and impact and bending performance at low temperatures are significantly better than Comparative Examples 1 and 2, but weaker than Examples 1, 2, and 3.

[0074] The description of the above embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A powder coating for post-processing of workpieces, characterized in that: The powder coating for post-processing of workpieces comprises the following raw materials in percentage by mass: Polyester resin A: 45-60%, crystalline resin B: 5-10%, curing agent: 3-8%, leveling agent: 0.8-1.2%, pigment: 0.7-28%, inorganic filler: 10-30%, nucleating agent: 0.1-1%, auxiliary agent 0.1-1.0%; the above raw materials total 100%; The polyester resin A has excellent flexibility and is a carboxyl-terminated polyester resin. The acid value of the polyester resin A is 25-55 mgKOH / g, the viscosity is 3000-7000 mPa.S, and the Tg is 57-63°C at 200°C. The acid value of the crystalline resin B is 20-40 mgKOH / g, the viscosity is 500-2000 mPa.S, and the crystal melting temperature is 100-120°C at 200°C.

2. The powder coating for post-processing of workpieces according to claim 1, characterized in that: The mass ratio of the polyester resin A to the crystalline resin B is 20:1 to 5:

1.

3. The powder coating for post-processing of workpieces according to claim 1 or 2, characterized in that: The nucleating agent is one or both of a sorbitol benzylidene derivative and a high melting point polymer nucleating agent.

4. The powder coating for post-processing of workpieces according to claim 1 or 2, characterized in that: The polyester resin A comprises the following raw materials in percentage by mass: 30-45% of polyol, 0-1% of branching agent, 40-65% of aromatic polyacid, 5-15% of acidolysis agent, 0.09-1.4% of esterification catalyst, 0.01-1% of curing accelerator and 0.1-1% of antioxidant, and the total percentage by mass of the above raw materials is 100%.

5. The powder coating for post-processing of workpieces according to claim 4, characterized in that: In the polyester resin A raw material: the polyol is any one or more of neopentyl glycol, ethylene glycol, methyl propylene glycol, ethyl butyl propylene glycol, and the content of neopentyl glycol is not less than 60%.

6. The powder coating for post-processing of workpieces according to claim 4 or 5, characterized in that: The preparation method of the polyester resin A comprises the following steps: 1) Add the formulated amount of polyol, branching agent, aromatic polyacid and esterification catalyst into a reaction kettle at the same time, raise the temperature to 235-255°C at a rate of 1-2°C / min in a nitrogen atmosphere and maintain it, and after the system is clarified, take a sample to detect the acid value (AV) to reach the set value of 5-12 mg KOH / g; 2) Then, the temperature is lowered to 215-235°C at a rate of 3-5°C / min, and an acidolysis agent is added to the reactor, and the temperature is raised to 235-250°C at a rate of 1-2°C / min and maintained for acidolysis and end-capping until the acid value of the polyester reaches 38-70 mg KOH / g; 3) Cool down to 230-240°C and perform polycondensation under -0.1MPa vacuum condition. After the acid value reaches 25-55mg KOH / g, discharge the mixture and cool down to 180-230°C. Add the formulated amount of antioxidant and curing accelerator and maintain for 5-30min before discharging the mixture.

7. The powder coating for post-processing of workpieces according to claim 1 or 2, characterized in that: The crystalline resin B comprises the following raw materials in mass percentage: 28-45% of polyol; 0.2-1.5% of branching agent; 45-60% of polyacid; 4-12% of acidolysis agent; 0.05-0.2% of esterification catalyst; the total mass percentage of the above raw materials is 100%.

8. The powder coating for post-processing of workpieces according to claim 7, characterized in that: The polyol is a polyol with a symmetrical structure, comprising any one or more of 1,4-cyclohexanedimethanol, ethylene glycol EG, 1,4-butanediol and 1,6-hexanediol, and the content of 1,6-hexanediol is not less than 70%.

9. The powder coating for post-processing of workpieces according to claim 7 or 8, characterized in that: The method for preparing the crystalline resin B comprises the following steps: S1. Add the formulated amount of polyol, branching agent, polyacid and esterification catalyst into a reaction kettle at the same time, heat to 235-255°C at a rate of 1-2°C / min in a nitrogen atmosphere and maintain, and after the system is clarified, take a sample to detect the acid value (AV) to reach 5-15 mgKOH / g; S1, then cool down to 215-235°C at a rate of 3-5°C / min and add an acidolysis agent into the reactor, heat up to 235-250°C at a rate of 1-2°C / min and maintain acidolysis and end-capping until the acid value of the polyester reaches 30-52 mg KOH / g; S3, cool down to 230-240℃ and carry out polycondensation under -0.1MPa vacuum condition, and discharge the material after the acid value reaches 20-40mg KOH / g.

10. A method for preparing a powder coating for post-processing a workpiece according to any one of claims 1 to 9, characterized in that: The preparation method comprises the following process flow: fully premixing the raw materials in a mixing tank according to the formula → co-extruding through an extruder → tableting → crushing → grinding → sieving → packaging the finished product.

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