An adjuvant for low temperature curing powder coatings and a method for its preparation

Abstract

The present application relates to the technical field of powder coating, and discloses an additive for low-temperature curing powder coating and a preparation method thereof, wherein the additive comprises the following components: a controlled-release curing promotion phase powder and a rheological modification matting masterbatch powder; the controlled-release curing promotion phase powder is made of the following raw materials: a carrier polyester resin, a quaternary ammonium salt or quaternary phosphonium salt catalyst, and the rheological modification matting masterbatch powder comprises the following raw materials: a matting resin phase micro powder, a fatty amide micro powder wax, and benzoin; the matting resin phase micro powder is made of the following raw materials: a carboxyl acrylic resin, 2-mercaptobenzothiazole zinc salt, and polyethylene wax. The present application adopts a controlled-release design of physical coating of the quaternary ammonium salt or quaternary phosphonium salt catalyst by the polyester resin, and in the initial stage of baking, the catalyst is limited in the resin matrix, which delays the premature formation of the crosslinking network of the system and prolongs the curing induction period.

Description

Technical Field

[0001] This invention relates to the field of powder coating technology, specifically to an additive for low-temperature curing powder coatings and its preparation method. Background Technology

[0002] With increasingly stringent environmental regulations, powder coatings have gained widespread adoption in building materials and furniture coatings due to their solvent-free operation. Traditional powder coatings require high-temperature baking to form a film. To reduce industrial energy consumption and adapt to coating scenarios on heat-sensitive substrates such as wood and plastics, the industry's demand for low-temperature curing powder coatings is gradually expanding. In many practical applications, the market not only requires coatings to possess basic physical protective properties but also sets specific standards for the smoothness of the coating surface and a low-gloss matte finish.

[0003] To achieve low-temperature film formation, existing powder coating technologies typically introduce highly reactive accelerators directly into the formulation. These catalysts can rapidly initiate cross-linking of resin groups, significantly lowering the curing threshold temperature of the coating and shortening the overall baking time. To address the need for matte coatings, existing technologies generally incorporate a large proportion of inorganic fillers such as silica or barium sulfate. These fillers increase the surface roughness of the coating, quickly reducing gloss. Small-molecule degassing agents and conventional lubricating wax powders are often dry-mixed into the formulation; these components melt rapidly upon heating, providing basic lubrication and degassing assistance to the system.

[0004] Existing formulation logic struggles to balance reaction rate and coating leveling. Highly reactive accelerators often cause premature cross-linking networks to form in the initial heating phase. Premature gelation severely compresses the powder melting and leveling window, preventing the coating from fully leveling and resulting in orange peel defects. Relying on a high proportion of inorganic fillers to achieve low gloss leads to a surge in the overall melt viscosity of the system. High filler loading weakens the mechanical strength of the coating, easily producing a macroscopic grainy texture. This high viscosity hinders the upward escape path of underlying air bubbles. Conventional dry-mix degassing agents are too dispersed in the system, making it difficult to concentrate and cover the matte micro-areas where viscosity abruptly occurs. Degassing lags behind rapid local cross-linking, trapping gas within the coating and ultimately causing pinhole defects. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an additive for low-temperature curing powder coatings and its preparation method, thereby solving the defects of existing low-temperature curing matte powder coatings during heat-curing film formation, such as orange peel effect caused by excessively rapid curing reaction compressing the leveling window, and pinholes easily forming on the surface due to delayed degassing in the high-viscosity system containing matte resin phase.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0007] In a first aspect, the present invention provides an additive for low-temperature curing powder coatings, employing the following technical solution:

[0008] An additive for a low-temperature curing powder coating comprises the following components in parts by weight: 30-60 parts of controlled-release curing accelerator powder and 40-70 parts of rheology-modified matte masterbatch powder.

[0009] The controlled-release curing accelerator powder is made from raw materials comprising the following weight percentages: 80.0-90.0 wt% carrier polyester resin; 10.0-20.0 wt% quaternary ammonium salt or quaternary phosphine salt catalyst.

[0010] The rheology-modified matting masterbatch powder comprises the following raw materials in parts by weight: 30.0-60.0 parts of matting resin phase micro powder; 5.0-9.0 parts of aliphatic amide micro powder wax; and 1.0-5.0 parts of benzoin.

[0011] The matting resin phase powder is made from raw materials comprising the following weight percentages: 60.0-70.0 wt% carboxylated acrylic resin; 20.0-25.0 wt% 2-mercaptobenzothiazole zinc salt; and 10.0-15.0 wt% polyethylene wax.

[0012] During the melting and curing process of the powder coating, the matting resin phase micropowder can form localized matting micro-regions with higher viscosity, thereby creating differences in curing rate and shrinkage behavior compared to the surrounding main coating.

[0013] By employing the above technical solution, and through the design of using a multiphase physical blend to separately coat the catalyst and the matting component, the effects of extending the curing induction period and reducing the melt viscosity during the leveling stage are achieved. The specific mechanism is as follows:

[0014] Pre-dispersing quaternary ammonium or quaternary phosphine salt catalysts within a carrier polyester resin creates a physically encapsulated state, establishing a delayed catalytic mechanism for controlled release. Specifically, during the room-temperature storage period and the initial melting stage of the powder coating during baking, the catalyst is confined within the resin matrix because the carrier polyester resin has not yet reached a fully flowable state. This typically prevents premature ring-opening reactions between the catalyst and the epoxy groups in the main resin system of the powder coating, thus delaying the rapid formation of the cross-linked network and providing sufficient leveling window for the coating film. As the temperature gradually rises, the carrier polyester resin melts and releases the catalyst, subsequently initiating the cross-linking and curing of the main resin.

[0015] Meanwhile, the aliphatic amide micro-powder wax-benzoin low-melting-point eutectic phase plays a role in reducing viscosity and venting. The aliphatic amide micro-powder wax and benzoin constitute the aliphatic amide micro-powder wax-benzoin low-melting-point eutectic phase, forming a composite structure where the low-melting-point eutectic phase coats the matting resin phase micro-powder. During the initial baking stage, this aliphatic amide micro-powder wax-benzoin low-melting-point eutectic phase melts first, forming a low-viscosity liquid phase layer in the microscopic region. The intermolecular lubrication effect of the aliphatic amide micro-powder wax facilitates the dispersion of the matting resin phase micro-powder, thereby reducing the overall melt viscosity of the powder coating during the leveling stage. The benzoin dissolves in this liquid phase layer, promoting the migration and rupture of air bubbles trapped within the coating to the surface, which helps reduce pinhole defects.

[0016] At the microscopic matting level, when the system enters the curing stage, the carboxyacrylate resin in the matting resin phase micropowder crosslinks with the main resin. Zinc 2-mercaptobenzothiazole salt regulates and promotes the local curing reaction within the micro-region, increasing the viscosity of this region and creating a difference in curing rate and shrinkage compared to the surrounding substrate coating. This non-uniform shrinkage in the micro-region tends to create a microscopic rough structure on the coating surface, achieving diffuse reflection matting. Polyethylene wax migrates to the coating surface in the later stages of curing, helping to maintain this microscopic rough structure.

[0017] Preferably, the additive comprises 45.0 parts of controlled-release curing accelerator powder and 55.0 parts of rheology-modified matte masterbatch powder; the controlled-release curing accelerator powder is made of 85.0 wt% of the carrier polyester resin and 15.0 wt% of the quaternary ammonium salt or quaternary phosphine salt catalyst; the rheology-modified matte masterbatch powder comprises 45.0 parts of the matte resin phase micro powder, 7.0 parts of the aliphatic amide micro powder wax and 3.0 parts of benzoin; the matte resin phase micro powder is made of 65.0 wt% of the carboxyacrylate resin, 22.0 wt% of the 2-mercaptobenzothiazole zinc salt and 13.0 wt% of the polyethylene wax.

[0018] By adopting the above technical solution, the ratio of curing accelerator phase to matting masterbatch powder is precisely set. By adjusting the release concentration of catalyst and the distribution density of local high viscosity areas, it is beneficial for the coating film to obtain a relatively smooth leveling state while maintaining low gloss.

[0019] Preferably, the carrier polyester resin has an acid value of 26-30 mg KOH / g and a softening point of 100-120°C; the carrier polyester resin is polymerized from monomers comprising the following weight percentages: neopentyl glycol 40.0-45.0 wt%, terephthalic acid 46.9-55.0 wt%, trimellitic anhydride 3.0-8.0 wt%, and monobutyltin oxide 0.1 wt%.

[0020] By adopting the above technical solution, the softening point of the carrier polyester resin is controlled within the range of 100 to 120°C, ensuring that the catalyst release temperature matches the leveling temperature of the low-temperature curing powder coating. The acid value is set within the range of 26 to 30 mg KOH / g, promoting the participation of the carrier polyester resin in the cross-linking network of the powder coating system after melting, thus reducing the risk of forming free precipitates.

[0021] Preferably, the carboxylated acrylic resin has an acid value of 33-45 mgKOH / g, a glass transition temperature of 65-70°C, and a softening point of 105-112°C; the carboxylated acrylic resin is polymerized from raw materials comprising the following weight percentages: 40.0-43.0 wt% methyl methacrylate, 33.0-36.0 wt% styrene, 5.0-7.0 wt% methacrylic acid, 15.0-16.0 wt% butyl acrylate, and 2.0-4.0 wt% azobisisobutyronitrile.

[0022] By adopting the above technical solution, the carboxylated acrylic resin with the specific acid value and glass transition temperature provides moderate reactivity when heated. It works synergistically with 2-mercaptobenzothiazole zinc salt to facilitate the formation of a cross-linked network of a specific density in the micro-region, resulting in a viscosity mutation to form an matte microstructure. At the same time, it reduces the probability of macroscopic particle appearance on the coating surface due to excessively fast cross-linking reaction.

[0023] Preferably, the quaternary ammonium salt or quaternary phosphine salt catalyst is selected from tetrabutylammonium bromide or ethyltriphenylphosphine bromide; the aliphatic amide micronized wax is N,N'-ethylenebisstearamide, and the median particle size D of the aliphatic amide micronized wax is... v The particle size is 2.0–3.0 μm, the melting temperature range is 135–145 °C, and the acid value is not greater than 5 mg KOH / g; the repeating unit structure of the polyethylene wax is -[CH2-CH2]. n - The weight-average molecular weight of the polyethylene wax ranges from 2000 to 3000.

[0024] By employing the above technical solution, tetrabutylammonium bromide or ethyltriphenylphosphine bromide, which exhibit steric hindrance, are selected to enhance the catalyst's latency at low temperatures. The particle size and melting temperature of N,N'-ethylene bis-stearamide are limited to facilitate the formation of a uniformly melting coating layer in the presence of benzoin. The molecular weight range of polyethylene wax is limited to regulate its migration rate from the interior of the coating to the surface, thus aiding in surface modification during the later stages of curing.

[0025] Preferably, the average particle size of the matting resin phase micro powder is 30-50 μm; the controlled release curing accelerator powder and the rheology-modified matting masterbatch powder are pulverized particles that have passed through a 120-mesh sieve.

[0026] By adopting the above technical solution and limiting the physical size of each powder, it is helpful to control the dispersion state of fine powders during the dry mixing process. The average particle size of the matting resin phase powder is controlled in the range of 30 to 50 μm. It can usually be distributed as matting nodes inside the coating after the aliphatic amide powder wax-benzoin low-melting-point eutectic phase melts. This reduces the decrease in matting efficiency caused by excessively small particle size, as well as the macroscopic surface roughness caused by excessively large particle size.

[0027] Secondly, the present invention provides a method for preparing an additive for low-temperature curing powder coatings, employing the following technical solution:

[0028] A method for preparing an additive for low-temperature curing powder coatings includes the following steps:

[0029] S1. The carrier polyester resin is mixed evenly with a quaternary ammonium salt or quaternary phosphine salt catalyst, and then melt-extruded, tableted, cooled, and pulverized through a first twin-screw extruder to obtain a controlled-release curing accelerator powder.

[0030] S2. Carboxylated acrylic resin, 2-mercaptobenzothiazole zinc salt and polyethylene wax are mixed evenly, and then melted, cooled and pulverized by a second twin-screw extruder to obtain matte resin phase micro powder;

[0031] S3. After premixing aliphatic amide micro powder wax with benzoin, it is fed into the main feed port of a third twin-screw extruder with a side feeding system at a constant speed; the matting resin phase micro powder obtained in step S2 is fed into the side feed port of the rear temperature zone of the third twin-screw extruder at a constant speed; after co-extrusion, the material is quenched by cooling roller pressing, crushed and sieved to obtain rheology modified matting masterbatch powder.

[0032] S4. The controlled-release curing accelerator powder obtained in step S1 and the rheology-modified matte masterbatch powder obtained in step S3 are added into a three-dimensional motion mixer for continuous dry mixing to obtain the finished additive.

[0033] By employing the above technical solution, a physical isolation system for different active components within the additive was constructed. The specific mechanism is as follows:

[0034] A latent isolation structure for the catalyst was established during the co-extrusion process. In step S1, the quaternary ammonium salt or quaternary phosphine salt catalyst was co-extruded with the support polyester resin, which dispersed the catalyst and physically encapsulated it in the polyester resin matrix, forming a physical barrier state. This limited the catalyst's contact with epoxy groups during the powder coating extrusion processing and storage stages, reducing the possibility of initial crosslinking reactions.

[0035] In order to construct matte resin phase micropowder, in step S2, carboxylated acrylic resin, 2-mercaptobenzothiazole zinc salt and polyethylene wax are pre-mixed and granulated, which constitutes the internal resin micro-regions with concentrated reactivity.

[0036] Subsequently, a side-feed co-extrusion coating process is used to achieve the composite microstructure, forming a composite structure in which a low-melting-point eutectic phase coats the matte resin phase micropowder. In step S3, aliphatic amide micropowder wax and benzoin enter the extruder through the main feed port and melt upon heating, forming a low-melting-point eutectic phase of aliphatic amide micropowder wax and benzoin. The matte resin phase micropowder enters the extruder through the side feed port in the rear heating zone. The short residence time of the matte resin phase micropowder in the extruder reduces the risk of premature formation of cross-linked networks within the micropowder; simultaneously, the aliphatic amide micropowder wax-benzoin low-melting-point eutectic phase formed at the front end coats the surface of the matte resin phase micropowder fed in from the rear side, promoting the formation of a composite structure in which a low-melting-point eutectic phase coats the matte resin phase micropowder.

[0037] In the final physical compounding stage, in step S4, the two coated powders are physically dry-mixed in three dimensions. This non-heated mixing method maintains the physical coating state of each powder.

[0038] Preferably, in step S1, the extrusion temperature of the first twin-screw extruder is set to 100-120°C and the main extruder speed is set to 200-250 rpm; in step S2, the extrusion temperature of the second twin-screw extruder is set to 110-140°C.

[0039] By adopting the above technical solution, the temperature of the first twin-screw extruder is controlled between 100 and 120°C, which ensures that the carrier polyester resin reaches a molten state while reducing the risk of catalyst thermal degradation or volatilization loss due to excessive temperature. Controlling the temperature of the second twin-screw extruder between 110 and 140°C helps to ensure uniform mixing of the carboxyacrylate resin and 2-mercaptobenzothiazole zinc salt, and reduces the risk of localized overheating leading to cross-linking and curing of the resin system, thus causing material agglomeration inside the screw.

[0040] Preferably, the specific process control parameters for step S3 are as follows: the temperature of the first to third temperature zones of the third twin-screw extruder is set to 125-130°C; the temperature of the fourth to sixth temperature zones is set to 115-125°C; the die head temperature is set to 115-120°C; and the main extruder speed is set to 300-450 rpm.

[0041] By adopting the above technical solution, setting the temperature of the front section of the extruder to 125-130℃ allows the aliphatic amide micronized wax and benzoin to melt into a fluid state. Setting the rear section and die head temperatures to decrease to 115-125℃ and 115-120℃ respectively, the material is placed in a cooling environment after entering the extruder from the side feed port, reducing the overall shear heat of the system. This temperature gradient promotes the coating of the low-melting-point liquid phase on the surface of the matte resin particles while preventing the overall softening and adhesion of the matte resin particles.

[0042] Preferably, in step S4, the ambient temperature for dry mixing is controlled at 15-35°C, the set speed of the three-dimensional motion mixer is 15-30 rpm, and the continuous dry mixing time is 20-30 minutes.

[0043] By adopting the above technical solution, the mixing environment is controlled at a normal temperature of 15 to 35°C, and the operating speed is kept low at 15 to 30 rpm. This restricts the mechanical frictional heat generation of particles within the mixing container and suppresses the tendency of low-melting-point substances on the powder surface to soften and cause powder agglomeration. This allows the controlled-release curing accelerator powder and the rheology-modified matte masterbatch powder to maintain their respective independent physical structures, thereby achieving a uniform mixing state.

[0044] This invention provides an additive for low-temperature curing powder coatings and its preparation method. It has the following beneficial effects:

[0045] 1. This invention employs a controlled release design that physically encapsulates quaternary ammonium salt or quaternary phosphine salt catalysts with polyester resin. During the initial baking stage of the coating, the catalyst is confined within the resin matrix, delaying the premature formation of the cross-linked network and extending the curing induction period. Compared to existing technologies that directly incorporate highly active catalysts into the system, this invention solves the problem of orange peel-like surface defects in low-temperature curing coatings due to the limited leveling window.

[0046] 2. This invention utilizes a side-feed co-extrusion process to construct a composite structure in which a low-melting-point eutectic phase composed of aliphatic amide micro-powder wax and benzoin coats a matting resin phase micro-powder. During the baking and heating stage, the aliphatic amide micro-powder wax-benzoin low-melting-point eutectic phase melts first and transforms into a low-viscosity liquid phase layer formed by the melting of the low-melting-point eutectic phase. The micro-powder wax provides microscopic lubrication, while the benzoin promotes the migration and rupture of internal microbubbles. Compared to the conventional physical dry mixing of degassing agents and lubricants in existing technologies, this composite structure solves the problem of pinholes in the coating caused by the high viscosity system containing the matting resin phase hindering the expulsion of bubbles.

[0047] 3. This invention prepares a matting resin phase micropowder system comprising carboxylated acrylic resin and 2-mercaptobenzothiazole zinc salt. During the curing period, the micropowder undergoes rapid local cross-linking, and the sudden increase in viscosity in the micro-regions leads to a difference in curing shrinkage between the micro-region and the main coating, thereby generating a micro-rough surface structure to achieve diffuse reflection matting. Compared with existing technologies that rely on a large proportion of inorganic matting fillers, this mechanism solves the shortcomings of high filler load leading to the degradation of coating mechanical properties and the uncontrolled surface macro-roughness. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the side-feed co-extrusion coating process of the present invention;

[0049] Figure 2 The following is a graph showing the results of thermal analysis and rheological property testing of Test Example 1 of the present invention. (a) is a comparison graph of the DSC endothermic peak temperature of aliphatic amide micronized wax alone, a physical mixture of benzoin and aliphatic amide micronized wax, benzoin alone, Comparative Example 1, Example 3 and Example 1. (b) is a comparison graph of the complex viscosity and the minimum melt viscosity during the heating process of the additive systems containing Example 1, Example 3 and Comparative Example 1, respectively.

[0050] Figure 3 The figures shown are the test results of the coating surface leveling and pinhole defects of Test Example 3 of the present invention. (a) is a comparison of the 60° gloss of the coating samples corresponding to Examples 1-4, Comparative Example 1, and Comparative Example 3; (b) is a comparison of the PCI leveling rating of the coating samples corresponding to Examples 1-4, Comparative Example 1, and Comparative Example 3; and (c) shows the coating samples corresponding to Examples 1-4, Comparative Example 1, and Comparative Example 3 at 100cm. 2 A comparison chart of the average number of pinholes in the region. Detailed Implementation

[0051] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] The main raw materials and reagents used in the following examples and comparative examples are sourced and specified as follows. Unless otherwise stated, all reagents are commercially available analytical grade or higher grade products.

[0053] Neopentyl glycol (CAS No. 126-30-7), terephthalic acid (CAS No. 100-21-0), trimellitic anhydride (CAS No. 552-30-7), monobutyltin oxide (CAS No. 2273-43-0), methyl methacrylate (CAS No. 80-62-6), styrene (CAS No. 100-42-5), methacrylic acid (CAS No. 79-41-4), butyl acrylate (CAS No. 141-32-2), azobisisobutyronitrile (CAS No. 78-67-1), and xylene (CAS No. 1330-20-7) are all commercially available industrial-grade or analytical-grade chemical raw materials.

[0054] The purity of tetrabutylammonium bromide (CAS No. 1643-19-2), ethyltriphenylphosphine bromide (CAS No. 1530-32-7), and benzoin (chemical name: 2-hydroxy-1,2-diphenyl ethyl ketone, CAS No. 119-53-9) is not less than 99.0%. The purity of 2-mercaptobenzothiazole zinc salt (CAS No. 155-04-4) is not less than 98.0%.

[0055] Polyethylene wax (CAS No. 9002-88-4) is a homopolymer of ethylene, with the repeating unit structural formula -[CH2-CH2]. n - The weight-average molecular weight ranges from 2000 to 3000. The aliphatic amide micronized wax uses N,N'-ethylene bis-stearamide (CAS No. 110-30-5), with a median volumetric particle size D. v 50 has a thickness of 2.0–3.0 μm, a melting temperature range of 135–145 °C, and an acid value not exceeding 5 mg KOH / g.

[0056] Preparation Example 1:

[0057] This preparation example provides a method for preparing a carrier polyester resin, comprising the following steps:

[0058] (1) Weigh out 40.0 wt% neopentyl glycol, 53.9 wt% terephthalic acid, 6.0 wt% trimellitic anhydride and 0.1 wt% monobutyltin oxide based on a total mass of 100 wt%.

[0059] (2) Neopentyl glycol and terephthalic acid are added to a reaction vessel equipped with a fractionating column and a condenser. Under nitrogen protection, the temperature is gradually raised to 230°C for esterification. When the top temperature of the reaction system drops below 100°C and the water output reaches 95% of the theoretical value, the first stage of esterification is ended.

[0060] (3) Cool the reactor to 200°C, add trimellitic anhydride for end-capping reaction, and keep warm for 2.5 hours.

[0061] (4) Then, the vacuum was drawn to a gauge pressure of -0.095MPa and the polycondensation reaction was carried out for 1.5 hours. When the resin acid value reached 28mgKOH / g and the softening point reached 110℃, the vacuum was released, the material was discharged and cooled to room temperature to obtain the carrier polyester resin.

[0062] Preparation Example 2:

[0063] This preparation example provides a method for preparing a carrier polyester resin, comprising the following steps:

[0064] (1) Weigh out 45.0 wt% neopentyl glycol, 46.9 wt% terephthalic acid, 8.0 wt% trimellitic anhydride and 0.1 wt% monobutyltin oxide based on a total mass of 100 wt%.

[0065] (2) Neopentyl glycol and terephthalic acid are added to a reaction vessel equipped with a fractionating column and a condenser. Under nitrogen protection, the temperature is gradually raised to 240°C for esterification. When the top temperature of the reaction system drops below 100°C and the water output reaches 95% of the theoretical value, the first stage of esterification is ended.

[0066] (3) Cool the reactor to 200°C, add trimellitic anhydride for end-capping reaction, and keep warm for 3 hours.

[0067] (4) Then, the vacuum was drawn to a gauge pressure of -0.09MPa and the polycondensation reaction was carried out for 1 hour. When the resin acid value reached 30mgKOH / g and the softening point reached 120℃, the vacuum was released, the material was discharged and cooled to room temperature to obtain the carrier polyester resin.

[0068] Preparation Example 3:

[0069] This preparation example provides a method for preparing a carrier polyester resin, comprising the following steps:

[0070] (1) Weigh 41.9wt% neopentyl glycol, 55.0wt% terephthalic acid, 3.0wt% trimellitic anhydride and 0.1wt% monobutyltin oxide based on a total mass of 100wt%.

[0071] (2) Neopentyl glycol and terephthalic acid are added to a reaction vessel equipped with a fractionating column and a condenser. Under nitrogen protection, the temperature is gradually raised to 220°C for esterification. When the top temperature of the reaction system drops below 100°C and the water output reaches 95% of the theoretical value, the first stage of esterification is ended.

[0072] (3) Cool the reactor to 200°C, add trimellitic anhydride for end-capping reaction, and keep warm for 2 hours.

[0073] (4) Then, the vacuum was drawn to a gauge pressure of -0.098MPa and the polycondensation reaction was carried out for 2 hours. When the resin acid value reached 26mgKOH / g and the softening point reached 100℃, the vacuum was released, the material was discharged and cooled to room temperature to obtain the carrier polyester resin.

[0074] Preparation Example 4:

[0075] This preparation example provides a method for preparing a carboxylated acrylic resin, comprising the following steps:

[0076] (1) Weigh out 43.0 wt% methyl methacrylate, 33.0 wt% styrene, 6.0 wt% methacrylic acid, 15.0 wt% butyl acrylate and 3.0 wt% azobisisobutyronitrile based on a total mass of 100 wt%, mix the above monomers with the initiator evenly to obtain the dropping solution.

[0077] (2) Add xylene solvent accounting for 50% of the total mass of monomers to the reactor, heat to 115°C and keep it in reflux.

[0078] (3) The liquid is added to the reaction vessel at a uniform rate over 3.5 hours using a dropping funnel.

[0079] (4) After the addition is complete, keep the reaction at 115°C for 2 hours. Then add 0.5 wt% of azobisisobutyronitrile (AIBN) according to the total mass of the monomer. The initiator added afterward is not included in the above 100 wt% ratio. Continue to keep the reaction at 1 hour.

[0080] (5) Transfer the reaction system to vacuum distillation to remove the solvent, control the gauge pressure to -0.098MPa, and raise the temperature to 155℃ until the solid content of the system reaches more than 99.5%.

[0081] (6) The resin acid value was tested to be 39 mg KOH / g, the glass transition temperature was 68℃, and the softening point was 108℃. The resin was discharged and cooled to room temperature to obtain carboxylated acrylic resin.

[0082] Preparation Example 5:

[0083] This preparation example provides a method for preparing a carboxylated acrylic resin, comprising the following steps:

[0084] (1) Weigh out 40.0 wt% methyl methacrylate, 35.0 wt% styrene, 5.0 wt% methacrylic acid, 16.0 wt% butyl acrylate and 4.0 wt% azobisisobutyronitrile based on a total mass of 100 wt%, mix the above monomers with the initiator evenly to obtain the dropping solution.

[0085] (2) Add xylene solvent accounting for 50% of the total mass of monomers to the reactor, heat to 120°C and keep it in reflux.

[0086] (3) The liquid is added to the reaction vessel at a uniform rate over 4 hours using a dropping funnel.

[0087] (4) After the addition is complete, keep the reaction at 120°C for 2 hours. Then add 0.5 wt% of azobisisobutyronitrile (AIBN) according to the total mass of the monomer. The initiator added later is not included in the above 100 wt% ratio. Keep the reaction at 1 hour.

[0088] (5) Transfer the reaction system to vacuum distillation to remove the solvent, control the gauge pressure to -0.095MPa, and raise the temperature to 160℃ until the solid content of the system reaches more than 99.5%.

[0089] (6) The resin acid value was 33 mg KOH / g, the glass transition temperature was 65℃, and the softening point was 105℃. The resin was discharged and cooled to room temperature to obtain carboxylated acrylic resin.

[0090] Preparation Example 6:

[0091] This preparation example provides a method for preparing a carboxylated acrylic resin, comprising the following steps:

[0092] (1) Weigh 40.0 wt% methyl methacrylate, 36.0 wt% styrene, 7.0 wt% methacrylic acid, 15.0 wt% butyl acrylate and 2.0 wt% azobisisobutyronitrile based on a total mass of 100 wt%, mix the above monomers with the initiator evenly to obtain the dropping solution.

[0093] (2) Add xylene solvent accounting for 50% of the total mass of monomers to the reactor, heat to 110°C and keep it in reflux.

[0094] (3) The liquid is added to the reaction vessel at a uniform rate over 3 hours using a dropping funnel.

[0095] (4) After the addition is complete, keep the reaction at 110°C for 2 hours. Then add 0.5 wt% of azobisisobutyronitrile (AIBN) according to the total mass of the monomer. The initiator added afterward is not included in the above 100 wt% ratio. Continue to keep the reaction at 1 hour.

[0096] (5) Transfer the reaction system to vacuum distillation to remove the solvent, control the gauge pressure to -0.099MPa, and raise the temperature to 150℃ until the solid content of the system reaches more than 99.5%.

[0097] (6) The resin acid value was 45 mg KOH / g, the glass transition temperature was 70℃, and the softening point was 112℃. The resin was discharged and cooled to room temperature to obtain carboxylated acrylic resin.

[0098] In the following embodiments, the rheology-modified matte masterbatch powder was prepared using the following method: Figure 1 The side-feed co-extrusion coating process is shown. Aliphatic amide micronized wax and benzoin enter the front temperature zone of the third twin-screw extruder through the main feed port and form a low-melting-point eutectic phase. The matte resin phase micronized powder enters the rear temperature zone through the side feed port, so that the low-melting-point eutectic phase coats the surface of the matte resin phase micronized powder. Then, after extrusion, cooling roller pressing and quenching, and pulverization and sieving, rheology-modified matte masterbatch powder is obtained.

[0099] Example 1:

[0100] This embodiment provides a method for preparing a composite additive for low-temperature curing powder coatings, including the following steps:

[0101] S1. The carrier polyester resin obtained in Preparation Example 1 with a mass fraction of 85.0 wt% is mixed with 15.0 wt% tetrabutylammonium bromide in a high-speed mixer until homogeneous. Then, the mixture is fed into the first twin-screw extruder, and the extrusion temperature is set to 110°C and the main machine speed is set to 220 rpm. After the extruded liquid is pressed into tablets and cooled, it is pulverized by a classifying pulverizer and passed through a 120-mesh sieve to obtain controlled release type curing accelerator powder for later use.

[0102] S2. 65.0 wt% of the carboxylated acrylic resin obtained in Preparation Example 4, 22.0 wt% of 2-mercaptobenzothiazole zinc salt and 13.0 wt% of polyethylene wax were mixed evenly and then fed into a second twin-screw extruder. The extrusion temperature was set to 125°C for melt mixing. After the material was cooled, it was pulverized to an average particle size of 40 μm to obtain matte resin phase micro powder for later use.

[0103] S3. Weigh out aliphatic amide micronized wax (D v 7.0 parts of 50 (2.5μm) and 3.0 parts of benzoin were premixed and fed into the main feed port of a third twin-screw extruder with a side feeding system at a constant speed. The temperature of the first to third temperature zones of the extruder was set to 128℃. 45.0 parts of the matte resin phase micro powder obtained by S2 were weighed and fed into the third twin-screw extruder through the side feed port at the fourth temperature zone at a constant speed. The temperature of the fourth to sixth temperature zones was set to 120℃. The die head temperature was set to 118℃ and the main machine speed was set to 380rpm. After the material was extruded, it was quenched to room temperature by cooling roller pressing, mechanically crushed and passed through a 120-mesh sieve to obtain rheology-modified matte masterbatch powder.

[0104] S4. Weigh 45.0 parts of the controlled-release curing accelerator powder obtained in S1 and add it to 55.0 parts of the rheology-modified matte masterbatch powder obtained in S3. Put them into a three-dimensional motion mixer and dry mix continuously for 25 minutes at an ambient temperature of 25℃ and a rotation speed of 20 rpm to obtain the composite additive product.

[0105] Example 2:

[0106] This embodiment provides a method for preparing a composite additive for low-temperature curing powder coatings, including the following steps:

[0107] S1. The carrier polyester resin obtained in Preparation Example 2 with a mass fraction of 90.0 wt% is mixed with 10.0 wt% tetrabutylammonium bromide in a high-speed mixer until homogeneous. Then, the mixture is fed into the first twin-screw extruder, and the extrusion temperature is set to 120°C and the main machine speed is 250 rpm. After the extruded liquid is pressed into tablets and cooled, it is pulverized by a classifying pulverizer and passed through a 120-mesh sieve to obtain controlled release type curing accelerator powder for later use.

[0108] S2. 70.0 wt% of the carboxylated acrylic resin obtained in Preparation Example 5, 20.0 wt% of 2-mercaptobenzothiazole zinc salt and 10.0 wt% of polyethylene wax were mixed evenly and then fed into a second twin-screw extruder. The extrusion temperature was set to 140°C for melt mixing. After the material was cooled, it was crushed to an average particle size of 50 μm to obtain matte resin phase micro powder for later use.

[0109] S3. Weigh out aliphatic amide micronized wax (D v 5.0 parts of 3.0μm (50) and 5.0 parts of benzoin were premixed and fed into the main feed port of the third twin-screw extruder at a constant speed. The temperature of the first to third temperature zones of the extruder was set to 130℃. 30.0 parts of the matte resin phase micro powder obtained by S2 were weighed and fed into the side feed port at a constant speed. The temperature of the fourth to sixth temperature zones was set to 125℃, the die head temperature was set to 120℃, and the main machine speed was set to 450rpm. After the material was extruded, it was quenched to room temperature by cooling roller pressing, mechanically crushed, and passed through a 120-mesh sieve to obtain rheology-modified matte masterbatch powder.

[0110] S4. Weigh 60.0 parts of the controlled-release curing accelerator powder prepared in S1 and add it to 40.0 parts of the rheology-modified matte masterbatch powder prepared in S3 into a three-dimensional motion mixer. Under the condition of an ambient temperature of 35℃, set the rotation speed to 30 rpm and continuously dry mix for 30 minutes to obtain the composite additive product.

[0111] Example 3:

[0112] This embodiment provides a method for preparing a composite additive for low-temperature curing powder coatings, including the following steps:

[0113] S1. The carrier polyester resin obtained in Preparation Example 3 with a mass fraction of 80.0 wt% was mixed with 20.0 wt% tetrabutylammonium bromide in a high-speed mixer until homogeneous. Then, the mixture was fed into the first twin-screw extruder, and the extrusion temperature was set to 100°C and the main machine speed was set to 200 rpm. After the extruded liquid was pressed into tablets and cooled, it was pulverized by a classifier and passed through a 120-mesh sieve to obtain controlled release type curing accelerator powder for later use.

[0114] S2. 60.0 wt% of the carboxylated acrylic resin obtained in Preparation Example 6, 25.0 wt% of 2-mercaptobenzothiazole zinc salt and 15.0 wt% of polyethylene wax were mixed evenly and then fed into a second twin-screw extruder. The extrusion temperature was set to 110°C for melt mixing. After the material was cooled, it was pulverized to an average particle size of 30 μm to obtain matte resin phase micro powder for later use.

[0115] S3. Weigh out aliphatic amide micronized wax (D v9.0 parts of 50 (2.0μm) and 1.0 part of benzoin were premixed and fed into the main feed port of the third twin-screw extruder at a constant speed. The temperature of the first to third temperature zones of the extruder was set to 125℃. 60.0 parts of the matte resin phase micro powder obtained by S2 were weighed and fed into the side feed port at a constant speed. The temperature of the fourth to sixth temperature zones was set to 115℃, the die head temperature was set to 115℃, and the main machine speed was set to 300rpm. After the material was extruded, it was quenched to room temperature by cooling roller pressing, mechanically crushed, and passed through a 120-mesh sieve to obtain rheology-modified matte masterbatch powder.

[0116] S4. Weigh 30.0 parts of the controlled release curing accelerator powder obtained in S1 and add it to 70.0 parts of the rheology-modified matte masterbatch powder obtained in S3. Put them into a three-dimensional motion mixer and dry mix continuously for 20 minutes at an ambient temperature of 15℃ and a rotation speed of 15 rpm to obtain the composite additive product.

[0117] Example 4:

[0118] This embodiment provides a method for preparing a composite additive for low-temperature curing powder coatings. Compared to Example 1, the only difference is that in S1, 15.0 wt% tetrabutylammonium bromide is replaced with 15.0 wt% ethyltriphenylphosphine bromide. The remaining raw material ratios, and the operating steps and parameters in S2 to S4 are consistent with Example 1, ultimately yielding the finished composite additive.

[0119] Comparative Example 1:

[0120] Compared with Example 1, the difference is that the side-feed co-extrusion coating process was not used. Instead, 45.0 parts of the matte resin phase micro powder, 7.0 parts of aliphatic amide micro powder wax and 3.0 parts of benzoin obtained in S2 were weighed and directly mixed, and then fed into the main feed port of the third twin-screw extruder. Conventional co-extrusion granulation and pulverization were carried out under the same temperature and speed conditions as in S3 of Example 1. Then, it was dry-mixed with 45.0 parts of controlled release curing accelerator powder. The remaining steps and proportions were the same as in Example 1.

[0121] Comparative Example 2:

[0122] Compared with Example 1, the difference is that: instead of pre-preparing the controlled release type curing accelerator powder in S1, 6.75 parts of free tetrabutylammonium bromide with the same mass as the catalytic active component in Example 1 were directly mixed with the rheology-modified matting masterbatch powder in the S4 stage; in order to keep the total mass of the formulation unchanged, 38.25 parts of the carrier polyester resin of Preparation Example 1 were added to the dry mixing step in S4, and the remaining steps and proportions were the same as in Example 1.

[0123] Comparative Example 3:

[0124] Compared with Example 1, the difference is that benzoin is not added to the raw material formula of the composite additive, and the missing 3.0 parts of benzoin are made up with an equal amount of aliphatic amide micro powder wax, that is, 10.0 parts of aliphatic amide micro powder wax are directly added in S3, and the remaining steps and proportions are the same as in Example 1.

[0125] Comparative Example 4:

[0126] Compared with Example 1, the difference is that: when preparing the matte resin phase micro powder in S2, 2-mercaptobenzothiazole zinc salt is not added, and the missing mass percentage is made up with an equal amount of carboxylated acrylic resin from Preparation Example 4. That is, the amount of carboxylated acrylic resin in S2 is increased to 87.0 wt%, the amount of polyethylene wax is kept at 13.0 wt%, and the remaining steps and total feed amount are the same as in Example 1.

[0127] Test Example 1:

[0128] Test objective: To verify the formation of the low-melting-point eutectic phase of aliphatic amide micronized wax-benzoin and its effect on the temperature-induced rheological behavior of resin systems containing matting resin phases.

[0129] Test steps:

[0130] 1. Weigh equal amounts of pure benzoin sample, pure aliphatic amide micronized wax sample, and a room-temperature physical mixture sample of benzoin and aliphatic amide micronized wax, as well as extruded mixture samples taken from the end of the third temperature zone of the third twin-screw extruder under the process conditions of Examples 1 and 3. Weigh the fully blended co-extruded material sample obtained in Comparative Example 1. Perform thermodynamic tests using a differential scanning calorimeter under a nitrogen atmosphere. Set the heating rate to 10℃ / min and the scanning temperature range to 50℃ to 180℃. Record the endothermic peak temperature T of each sample. m .

[0131] 2. The composite additives prepared in Examples 1, 3 and Comparative Example 1 were uniformly added to commercially available conventional pure polyester powder coating base material at a ratio of 5 wt% of the total mass. The base coating test samples were obtained by twin-screw extrusion, crushing and sieving.

[0132] 3. Dynamic thermomechanical analysis of the coating test specimens prepared above was performed using a rotational rheometer. The test fixture was a parallel plate with a diameter of 25 mm. The normal force was set to 0.1 N, the test frequency was 1 Hz, and the strain was controlled within the linear viscoelastic region of 0.5%.

[0133] 4. In the rheological test, the test system was heated from 100℃ to 160℃ at a heating rate of 5℃ / min. The changes in the complex viscosity of the system at different temperatures were recorded, and the real-time complex viscosity at 125℃ and the lowest melt viscosity during the heating process were extracted.

[0134] Test data are shown in Table 1 and Figure 2 As shown.

[0135] Table 1. Melting phase transition temperature and test data of the system's heating rheological properties:

[0136] Table 1 shows that the peak endothermic temperatures of pure benzoin alone and aliphatic amide micronized wax alone are 133.8℃ and 142.1℃, respectively, while the peak endothermic temperature of their physical mixture at room temperature is 136.2℃. In comparison, the peak endothermic temperature of the material in the third temperature zone of S3 in Example 1 is 123.5℃, and the peak endothermic temperature of the material in the third temperature zone of S3 in Example 3 is 128.4℃, both lower than the corresponding peak temperatures of the raw materials alone and the physical mixture. This result indicates that after shearing and heat treatment in the front section of the third twin-screw extruder, benzoin and aliphatic amide micronized wax form a low-melting-point eutectic phase of aliphatic amide micronized wax-benzoin with a low phase transformation temperature.

[0137] The peak endothermic temperature of the fully blended co-extrusion material in Comparative Example 1 was 131.7℃, which is higher than the peak endothermic temperature of the materials in the third temperature zone of S3 in Examples 1 and 3. This result indicates that first forming a low-melting-point eutectic phase of aliphatic amide micronized wax-benzoin in the front section of the third twin-screw extruder with benzoin, and then introducing a high-viscosity matting resin phase through side feeding, is beneficial for retaining the phase transformation characteristics of the low-melting-point phase; while the direct fully blended co-extrusion process is not conducive to the formation and retention of this low-melting-point phase.

[0138] The results of the temperature-induced rheological tests showed that the coating system containing the additive from Example 1 had a complex viscosity of 321.6 Pa·s at 125°C and a minimum melt viscosity of 194.2 Pa·s during the temperature rise process; the coating system containing the additive from Example 3 had a complex viscosity of 398.3 Pa·s at 125°C and a minimum melt viscosity of 227.5 Pa·s during the temperature rise process; and the coating system containing the additive from Comparative Example 1 had a complex viscosity of 674.2 Pa·s at 125°C and a minimum melt viscosity of 435.8 Pa·s during the temperature rise process. Therefore, compared with the conventional fully blended co-extrusion process, the composite additives obtained by the side-feed co-extrusion coating process can reduce the complex viscosity of the powder coating system during the temperature rise and leveling stage.

[0139] Combining the DSC and temperature-induced rheological test results, it can be seen that the aliphatic amide-based micronized wax-benzoin low-melting-point eutectic phase formed in Examples 1 and 3 can soften in advance and participate in the interface regulation of the molten system when the powder coating is heated to the leveling window stage, thereby reducing the influence of high-viscosity matte resin on the system's fluidity. This rheological change provides a more favorable melt flow environment for the leveling and venting of the coating film under low-temperature curing conditions of 160℃.

[0140] Test Example 2:

[0141] Test objective: To investigate the effects of composite additives on the gel time and non-isothermal curing exothermic behavior of powder coating systems at 160℃, and to evaluate the regulatory effects of carrier coating structure and 2-mercaptobenzothiazole zinc salt on the curing induction period.

[0142] Test steps:

[0143] 1. Weigh equal amounts of powder coating test samples prepared in Examples 1, 2, 2, and 4 as test samples. Comparative Example 2 is a system with directly added equivalent free catalyst, and Comparative Example 4 is a system without 2-mercaptobenzothiazole zinc salt.

[0144] 2. The gel time of the samples was tested using a gel time meter. The temperature of the hot plate was preheated and stabilized at 160℃. Approximately 0.5g of the sample to be tested was placed in the center of the hot plate test chamber. After the sample melted, the timer was started, and the molten material was continuously stirred at a constant frequency using a stirring needle. The timer was stopped when the picked-up resin filaments lost their fluidity and could not retract after breaking. The time from the start of melting to this state was recorded. Each group of samples was tested three times, and the average value was taken.

[0145] 3. Differential scanning calorimetry (DSC) was used to perform non-isothermal curing kinetics tests on the same batch of samples. Under a nitrogen protective atmosphere, with a gas flow rate of 50 mL / min, approximately 6.5 mg of the sample was weighed, placed in an aluminum crucible, pressed, and sealed. The temperature was increased from 25 °C to 220 °C at a rate of 10 °C / min, and the heat flow rate change curve was recorded during the scanning process. The onset temperature T of the curing exothermic peak was extracted using the tangent method. onset and peak temperature T peak .

[0146] The test data is shown in Table 2.

[0147] Table 2. Curing kinetics and gel time test data at 160℃:

[0148] Table 2 shows that the coating system containing the additive from Comparative Example 2 had a gel time of 123 s at 160 °C, a DSC exothermic peak onset temperature of 121.5 °C, and a peak temperature of 148.4 °C. The coating system containing the additive from Example 1 had a gel time of 214 s at 160 °C, a DSC exothermic peak onset temperature of 142.3 °C, and a peak temperature of 158.6 °C. Compared to the system with directly added free catalyst, the gel time of the Example 1 system was prolonged, and both the curing exothermic onset temperature and peak temperature shifted towards higher temperatures. This result indicates that after the carrier polyester resin physically isolates the catalytically active component, the rate at which the catalytically active component enters the polyester / TGIC melt system in the initial stage of heating is reduced, thereby prolonging the curing induction period.

[0149] The coating system containing the additives of Comparative Example 4 had a gel time of 178 s at 160 °C, a DSC exothermic peak onset temperature of 132.8 °C, and a peak temperature of 153.1 °C. Compared with Comparative Example 4, the gel time of the system in Example 1 was further extended by 36 s, and the DSC exothermic peak onset temperature increased by 9.5 °C and the peak temperature increased by 5.5 °C. These results indicate that, based on the polyester resin carrier coating, the addition of 2-mercaptobenzothiazole zinc salt helps to regulate the action process of the catalytically active component in the molten system, allowing the system to maintain a longer induction phase in the lower temperature range.

[0150] The gel time of the system in Example 2 was 226 s, the onset temperature of the DSC exothermic peak was 144.1 °C, and the peak temperature was 159.2 °C, showing only minor changes compared to Example 1. This result indicates that, under controlled conditions of a high proportion of release-type curing promoter, the composite additive can still maintain a relatively long gel time and a curing exothermic peak temperature close to 160 °C, demonstrating that the system still possesses the ability to complete the crosslinking reaction under low-temperature curing conditions.

[0151] The combined gel time and DSC test results show that the composite additives used in Examples 1 and 2 enable the powder coating system to maintain a relatively long melt flow stage in the early stage of heating, while entering the main curing reaction stage at close to 160°C. This curing behavior helps to distinguish the leveling and degassing processes of the coating film from the rapid crosslinking process in time, thus providing a basis for improving the appearance of the coating film under low-temperature curing conditions.

[0152] Test Example 3:

[0153] Test objective: To verify the comprehensive impact of specific preparation processes and component ratios on the leveling state and venting process of powder coatings during the film formation stage.

[0154] Test steps:

[0155] 1. Select a base pure polyester powder coating with composite additives prepared in Examples 1 to 4, Comparative Example 1 and Comparative Example 3 respectively added at a ratio of 5wt%. Use an electrostatic spray gun to evenly spray it onto the surface of a cold-rolled steel plate that has undergone degreasing and phosphating treatment. Then place it in a forced-air constant temperature oven at 160℃ for 15 minutes to cure. After cooling to room temperature, a coating test sample with a film thickness of 65±5μm is obtained.

[0156] 2. A portable multi-angle gloss meter was used, with the measurement geometry angle set to 60°, to measure the gloss of the surface of each group of coating samples. Five different locations were randomly selected from each sample for measurement, and the arithmetic mean was recorded.

[0157] 3. In a standard D65 light source lighting box, visually compare the above-prepared coating test samples with the standard smoothness samples of grades 1 to 10 provided by the Powder Coatings Institute (PCI) to determine the leveling grade of each coating. Grade 1 represents a severe orange peel state, and grade 10 represents a smooth surface without ripples.

[0158] 4. Randomly place an open template with an inner frame size of 10cm×10cm on the sample surface. Use a 10x handheld magnifying glass with a built-in light source to observe and count the micro-pits and air bubbles penetrating the template area. Test 3 samples for each group of samples and record the average number of pinholes.

[0159] Test data are shown in Table 3 and Figure 3 As shown.

[0160] Table 3. Test data on apparent leveling and pinhole defects of coating:

[0161] According to the data in Table 3, the samples containing the additives of Examples 1 to 4 all had a 60° gloss level in the range of 11.8–14.1 GU, indicating that each example could maintain a low gloss level under low-temperature curing conditions. The sample containing the additive of Comparative Example 1 had a 60° gloss level of 13.9 GU, which was similar to that of the examples, but its PCI leveling rating was only 3.5, lower than the 7.0–8.0 ratings of Examples 1 to 4. This result shows that, in terms of gloss index alone, Comparative Example 1 still has a certain matting effect, but its coating surface smoothness is significantly lower than that of the example samples prepared by the side-feed co-extrusion coating process.

[0162] The main difference between Comparative Example 1 and Example 1 lies in the preparation method of the rheology-modified matte masterbatch. In Comparative Example 1, matte resin phase micropowder, aliphatic amide micropowder wax, and benzoin were directly co-extruded. In Example 1, the aliphatic amide micropowder wax and benzoin were first fed into the third twin-screw extruder through the main feed port, and then the matte resin phase micropowder was introduced through a side feed method. Based on the differences in leveling ratings in Table 3, it can be seen that the side-feed co-extrusion coating process is beneficial for improving the spreading state of the high-viscosity system containing the matte resin phase during the film-forming stage, thereby reducing orange peel-like surface defects.

[0163] In Example 3, a higher amount of high-viscosity matte resin phase was used, resulting in a 60° gloss level of 11.8 GU, a PCI leveling rating of 8.0, and a 100cm gloss level. 2 The average number of pinholes in the region was 3. This result indicates that, under controlled conditions of a low proportion of release-type curing promoter and a high amount of aliphatic amide micronized wax, the system can still achieve good leveling properties while maintaining low gloss. This data corresponds to the trend of lower melt viscosity in Test Example 1, indicating that the flow behavior of the low-melting-point, relatively high-viscosity matting resin phase formed by aliphatic amide micronized wax and benzoin has a regulating effect.

[0164] Comparative Example 3 did not contain benzoin and was supplemented with an equal amount of aliphatic amide micronized wax. This sample had a 60° gloss of 12.4 GU and a PCI leveling rating of 6.5, but at 100cm... 2 The average number of pinholes in the region reached 54, significantly higher than the 1-3 pinholes in Examples 1 to 4. This result indicates that, under the same low-temperature curing conditions, the addition of benzoin plays an important role in reducing pinhole defects. Aliphatic amide micronized waxes mainly improve the flow and interfacial state during the melting stage, while benzoin mainly participates in the degassing process. When used together, they are more effective in reducing residual bubbles and pinhole defects in the coating film.

[0165] Based on the combined data of gloss, leveling performance rating, and pinhole count, it can be seen that Examples 1 to 4, while maintaining low gloss, all exhibited high leveling performance and low pinhole count. Compared with Comparative Example 1, the Example samples showed better leveling performance; compared with Comparative Example 3, the Example samples had a lower pinhole count. These results indicate that the combination of side-feed co-extrusion coating process, aliphatic amide micronized wax, and benzoin is beneficial for achieving a balance between matte finish, leveling performance, and degassing properties under low-temperature curing conditions of 160℃.

[0166] Test Example 4:

[0167] Test objective: To verify the effect of curing delay technology on the final degree of curing and mechanical and physical properties at 160℃.

[0168] Test steps:

[0169] 1. The sample was sprayed with a base powder coating containing the composite additives prepared in Examples 1 to 4 and Comparative Example 2 according to the above method, and then baked and cured at 160°C for 15 minutes to form a film as the test object.

[0170] 2. Soak a cotton ball in methyl ethyl ketone solvent and use a test probe with a fixed load of 1000g to rub the coating surface back and forth in a straight line. One push-pull back and forth is counted as one stroke. Continue rubbing and observe the coating condition. Record the number of rubbing strokes when the coating surface is scratched and the underlying metal substrate is exposed.

[0171] 3. Place the test sample on the anvil of the paint film impactor and perform both forward and reverse impact tests. Adjust the drop height of the 1kg hammer and let it fall freely. Check whether the coating in the impact deformation area develops cracks or peels. Record the product of the maximum height without damage and the mass of the hammer, in kg·cm.

[0172] 4. Clamp the test sample in the cupping tester with the coated side facing outward. Press a spherical punch of fixed diameter into the metal side of the back of the sample at a constant speed. Use a magnifying glass to observe the deformation displacement when the coating surface of the embossed area begins to show tiny cracks. Record the corresponding indentation depth in mm.

[0173] The test data is shown in Table 4.

[0174] Table 4. Test data on the degree of crosslinking and mechanical and physical properties of the coating under low-temperature curing:

[0175] According to the data in Table 4, the samples containing the additives of Examples 1 to 4 underwent 129–141 MEK bidirectional wiping cycles after curing at 160°C for 15 minutes; the sample containing the additive of Comparative Example 2 underwent 138 MEK bidirectional wiping cycles. The MEK bidirectional wiping cycles of the various examples and Comparative Example 2 are similar, indicating that after using carrier coating and composite adjustment structure, the system can still achieve good solvent resistance and curing integrity under low-temperature curing conditions of 160°C.

[0176] Regarding mechanical and physical properties, the samples containing the additives of Examples 1 and 2 both achieved a forward and reverse impact resistance of 50 kg·cm, with cupping test depths of 7.8 mm and 7.6 mm, respectively; the samples containing the additive of Example 3 both achieved a forward and reverse impact resistance of 45 kg·cm, with a cupping test depth of 8.1 mm; and the samples containing the additive of Example 4 achieved a forward impact resistance of 50 kg·cm, a reverse impact resistance of 45 kg·cm, and a cupping test depth of 7.5 mm. These data indicate that each example, while maintaining solvent resistance, still possesses good impact resistance and deformation adaptability.

[0177] In comparison, the sample containing the additive from Comparative Example 2 showed a forward impact resistance of 35 kg·cm, a reverse impact resistance of 30 kg·cm, and a cupping test depth of 5.4 mm, all lower than those of the examples. Comparative Example 2, using a direct addition of free catalyst, achieved similar MEK bidirectional wiping cycles to the examples, but its impact and cupping properties decreased significantly. These results indicate that simply increasing catalytic activity cannot simultaneously guarantee the mechanical properties of the low-temperature curing coating; the release method of the catalytically active component affects the coating quality.

[0178] Based on the gel time and curing exothermic peak data in Test Example 2, it can be seen that Examples 1 to 4, by encapsulating the catalytically active component with a carrier polyester resin and combining it with other functional components in the composite additives, allow the system to retain a relatively long melt flow phase in the early stage of heating, while completing the main curing process at close to 160°C. This curing behavior facilitates the coating film to complete spreading and degassing before crosslinking, thereby improving impact resistance and cupping properties while maintaining similar solvent resistance.

[0179] As can be seen from Table 4, the composite additive of the present invention did not reduce the solvent resistance after curing at 160℃ due to the extension of the curing induction period. At the same time, it improved the shortcomings of the free catalyst system in terms of mechanical properties, indicating that the composite additive is suitable for powder coating systems that take into account both the integrity of low-temperature curing and the mechanical properties of the coating film.

[0180] Test Example 5:

[0181] Test objective: To evaluate the effect of 2-mercaptobenzothiazole zinc salt on the b-value and color difference variation Δb of powder coatings under low-temperature curing and over-baking conditions.

[0182] Test steps:

[0183] 1. Weigh out equal amounts of the base powder coatings containing the additives corresponding to Examples 1, 3, 2, and 4, wherein an appropriate amount of titanium dioxide pigment is premixed in the base powder coatings for colorimetric testing. Apply each group of powder coatings uniformly to the surface of a cold-rolled steel sheet using the same process conditions.

[0184] 2. Divide the sprayed samples into two groups. Place the first group of samples in a 160℃ forced-air oven for 15 minutes to cure at a constant temperature, as the samples in the normal baking state; place the second group of samples in a 180℃ forced-air oven for 30 minutes to cure at a constant temperature, as the samples in the over-baked state.

[0185] 3. After the samples cooled to room temperature, the surface color parameters of each group of samples were tested using an integrating sphere spectrophotometer. The test light source was D65, the observation angle was 10°, and the b-value reflecting the color change along the yellow-blue axis was recorded. A positive b-value and a larger value indicate that the coating is more yellow. This paper uses the b-value and the color difference variation Δb as evaluation indicators of the degree of yellowing of the coating.

[0186] 4. Randomly select 5 different locations on each sample for measurement and take the arithmetic mean. Calculate the difference in b-value between the coating at 180℃ under over-baking conditions and at 160℃ under normal baking conditions, and record it as the color difference variation Δb.

[0187] The test data is shown in Table 5.

[0188] Table 5. Test data of b-value and color difference variation Δb under normal baking and over-baking conditions of coating film:

[0189] As shown in Table 5, the sample containing the additive from Example 1 had a b-value of 0.43 after normal baking at 160°C and a b-value of 1.11 after baking at 180°C, with a Δb of 0.68. The sample containing the additive from Example 3 had a b-value of 0.51 after normal baking at 160°C and a b-value of 1.33 after over-baking at 180°C, with a Δb of 0.82. These results indicate that Examples 1 and 3 maintained low b-value changes under both normal and over-baking conditions, resulting in low yellowing of the coating film.

[0190] The sample containing the additive from Comparative Example 2 had a b-value of 1.76 after normal baking at 160°C and a b-value of 5.82 after baking at 180°C, with a Δb of 4.06. Compared to Examples 1 and 3, Comparative Example 2 exhibited higher b-values ​​under both normal and over-baking conditions, and the color difference variation was significantly increased after over-baking. This result indicates that the system with directly added free tetrabutylammonium bromide is more prone to yellowing under heating conditions.

[0191] The sample containing the additive of Comparative Example 4 had a b-value of 1.15 after normal baking at 160°C and a b-value of 3.88 after baking at 180°C, with a Δb of 2.73. Compared with Examples 1 and 3, Comparative Example 4 showed higher b-values ​​and Δb. This result indicates that adding 2-mercaptobenzothiazole zinc salt to the high-viscosity matte resin phase is beneficial in reducing the yellowing of the coating film under over-baking conditions.

[0192] Based on the gel time and curing exothermic behavior data in Test Example 2, it can be seen that the composite additives in Examples 1 and 3 can prolong the curing induction period and concentrate the main curing process in a temperature range close to 160°C. This curing behavior is beneficial for reducing the degree of local rapid crosslinking in the early stage of heating and improving color stability under over-baking conditions.

[0193] It should be noted that in Comparative Example 4, the missing mass of 2-mercaptobenzothiazole zinc salt was supplemented with carboxylated acrylic resin. The test results are used to illustrate that the presence of 2-mercaptobenzothiazole zinc salt has an improving effect on over-baking yellowing, and do not limit this improving effect to be produced by a single reaction mechanism.

[0194] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An additive for low-temperature curing powder coatings, characterized in that, The components include the following parts by weight: 30-60 parts of controlled release curing accelerator powder and 40-70 parts of rheology modified matte masterbatch powder; The controlled-release curing accelerator powder is made from raw materials comprising the following weight percentages: Carrier polyester resin: 80.0~90.0wt%; Quaternary ammonium salt or quaternary phosphine salt catalysts: 10.0–20.0 wt%; The rheology-modified matte masterbatch powder comprises the following raw materials in parts by weight: Matting resin phase micro powder: 30.0–60.0 parts; Aliphatic amide micronized wax: 5.0–9.0 parts; Benzoin: 1.0–5.0 parts; The matting resin phase powder is made from raw materials comprising the following weight percentages: Carboxylated acrylic resin: 60.0–70.0 wt%; 2-Mercaptobenzothiazole zinc salt: 20.0–25.0 wt%; Polyethylene wax: 10.0~15.0wt%.

2. The additive for low-temperature curing powder coating according to claim 1, characterized in that, The additives include 45.0 parts of controlled-release curing accelerator powder and 55.0 parts of rheology-modified matting masterbatch powder; The controlled-release curing accelerator powder is made of 85.0 wt% of the carrier polyester resin and 15.0 wt% of the quaternary ammonium salt or quaternary phosphine salt catalyst; The rheology-modified matting masterbatch powder comprises 45.0 parts of the matting resin phase micro powder, 7.0 parts of the aliphatic amide micro powder wax, and 3.0 parts of the benzoin. The matte resin phase powder is made of 65.0 wt% of the carboxylated acrylic resin, 22.0 wt% of the 2-mercaptobenzothiazole zinc salt and 13.0 wt% of the polyethylene wax.

3. The additive for low-temperature curing powder coating according to claim 1, characterized in that, The carrier polyester resin has an acid value of 26-30 mgKOH / g and a softening point of 100-120℃. The carrier polyester resin is made from raw materials comprising the following weight percentages: neopentyl glycol 40.0–45.0 wt%, terephthalic acid 46.9–55.0 wt%, trimellitic anhydride 3.0–8.0 wt%, and monobutyltin oxide 0.1 wt%.

4. The additive for low-temperature curing powder coating according to claim 1, characterized in that, The carboxylated acrylic resin has an acid value of 33-45 mgKOH / g, a glass transition temperature of 65-70℃, and a softening point of 105-112℃. The carboxylated acrylic resin is made from raw materials comprising the following weight percentages: 40.0-43.0 wt% methyl methacrylate, 33.0-36.0 wt% styrene, 5.0-7.0 wt% methacrylic acid, 15.0-16.0 wt% butyl acrylate, and 2.0-4.0 wt% azobisisobutyronitrile.

5. The additive for low-temperature curing powder coating according to claim 1, characterized in that, The quaternary ammonium salt catalyst includes tetrabutylammonium bromide, and the quaternary phosphine salt catalyst includes ethyltriphenylphosphine bromide; The aliphatic amide micronized wax is N,N'-ethylene bis-stearamide, and the median particle size D of the aliphatic amide micronized wax is... v The particle size of 50 is 2.0–3.0 μm, the melting temperature range is 135–145 °C, and the acid value is not greater than 5 mg KOH / g; The repeating unit structure of the polyethylene wax is -[CH2-CH2]. n - The weight-average molecular weight of the polyethylene wax ranges from 2000 to 3000.

6. The additive for low-temperature curing powder coating according to claim 1, characterized in that, The average particle size of the matting resin phase micro powder is 30-50 μm; the controlled release curing accelerator powder and the rheology-modified matting masterbatch powder are pulverized particles that have passed through a 120-mesh sieve.

7. A method for preparing an additive for a low-temperature curing powder coating as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. The carrier polyester resin is mixed evenly with a quaternary ammonium salt or quaternary phosphine salt catalyst, and then melt-extruded, tableted, cooled, and pulverized through a first twin-screw extruder to obtain a controlled-release curing accelerator powder. S2. Carboxylated acrylic resin, 2-mercaptobenzothiazole zinc salt and polyethylene wax are mixed evenly, and then melted, cooled and pulverized by a second twin-screw extruder to obtain matte resin phase micro powder; S3. After premixing aliphatic amide micro powder wax with benzoin, it is fed into the main feed port of a third twin-screw extruder with a side feeding system at a constant speed; the matting resin phase micro powder obtained in step S2 is fed into the side feed port of the rear temperature zone of the third twin-screw extruder at a constant speed; after co-extrusion, the material is quenched by cooling roller pressing, crushed and sieved to obtain rheology modified matting masterbatch powder. S4. The controlled-release curing accelerator powder obtained in step S1 and the rheology-modified matte masterbatch powder obtained in step S3 are continuously dry-mixed in a three-dimensional motion mixer in proportion to obtain the additive for the low-temperature curing powder coating.

8. The method for preparing the additive for low-temperature curing powder coating according to claim 7, characterized in that, In step S1, the extrusion temperature of the first twin-screw extruder is set to 100-120°C, and the main machine speed is set to 200-250 rpm; In step S2, the extrusion temperature of the second twin-screw extruder is set to 110–140°C.

9. The method for preparing the additive for low-temperature curing powder coating according to claim 7, characterized in that, The specific process control parameters for step S3 are as follows: The temperatures of the first to third temperature zones of the third twin-screw extruder are set to 125–130°C. The temperatures for zones 4 to 6 are set at 115–125°C. Set the head temperature to 115-120℃ and the main unit speed to 300-450rpm.

10. The method for preparing the additives for low-temperature curing powder coatings according to claim 7, characterized in that, In step S4, the ambient temperature for dry mixing is controlled at 15-35°C, the set rotation speed of the three-dimensional motion mixer is 15-30 rpm, and the continuous dry mixing time is 20-30 minutes.

Images