Preparation process of low-temperature curing high-corrosion-resistance powder coating for aluminum alloy hub production

Abstract

The invention discloses a preparation process of a low-temperature curing high-corrosion-resistance powder coating for aluminum alloy hub production, which solves the problems of poor adhesive force and insufficient corrosion resistance of the traditional low-temperature powder coating through the combined action of interface chemical anchoring and low-stress stepped curing. The process core comprises the following steps: actively migrating an interface chelating composite curing agent containing a beta-diketone structure coordination group to an interface in a melting and leveling stage to form a high-energy coordination bond with an aluminum substrate; and a micro-nano shielding network constructed by nano aluminum oxide and flaky zinc powder is matched, so that permeation of a corrosive medium is greatly blocked. Meanwhile, a three-section step curing procedure of wetting at 60 DEG C, leveling at 100 DEG C and crosslinking at 140 DEG C is adopted, and the heating rate of less than or equal to 5 DEG C / min is matched, so that the thermal deformation of the hub is controlled within 0.1 mm while the densification of the coating is realized. The neutral salt mist resistance time of the prepared coating exceeds 1200 hours, the adhesive force reaches grade 0, and the dynamic balance performance of the hub is effectively guaranteed.

Description

Technical Field

[0001] This invention relates to the field of metal surface coating technology, and in particular to a process for preparing a low-temperature curing, high-corrosion-resistant powder coating for aluminum alloy wheel production. Background Technology

[0002] Aluminum alloy wheels have become a mainstream feature in modern automobile manufacturing due to their lightweight and aesthetic appeal. Aluminum-silicon-magnesium alloys such as A356.2 require T6 heat treatment to achieve optimal mechanical properties, with an aging temperature typically between 155-170℃.

[0003] Existing powder coating processes for aluminum alloy wheels face two major technical bottlenecks. First, traditional powder coatings require curing at 180-200℃ for over 20 minutes, exceeding the aging temperature of aluminum alloys. This leads to coarsening of the reinforcing phase (Mg2Si) and a decrease in yield strength. More seriously, the high temperature releases residual internal stresses from casting and machining, causing minute plastic deformation (typically >0.5mm) in the wheel hub, severely affecting its roundness and dynamic balance, resulting in high rework rates on the production line. Second, to address the thermal deformation problem, the industry has attempted to use low-temperature curing powders below 150℃. However, lower curing temperatures directly reduce the resin crosslinking reaction rate, resulting in insufficient coating density. Simultaneously, the naturally formed oxide film on the aluminum alloy surface is chemically inert, and the high viscosity of the resin melt at low temperatures makes it difficult to fully wet the micropores of the substrate, causing the coating to adhere primarily through weak van der Waals forces. In salt spray or humid environments, water molecules readily displace the physical adsorption points between the coating and the substrate, leading to blistering and peeling of the coating. Currently, the neutral salt spray resistance life of commercially available low-temperature powder coatings is generally only 500-800 hours, which is difficult to meet the stringent standard of more than 1200 hours proposed by high-end OEMs.

[0004] Therefore, developing a preparation process that can achieve rapid curing at ≤140℃ to control deformation and build super strong interfacial bonding through chemical mechanisms to achieve high corrosion resistance is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] In order to overcome the shortcomings of poor coating adhesion, insufficient corrosion resistance and large thermal deformation of wheel hubs in the existing low-temperature curing process, this invention proposes a low-temperature curing high corrosion-resistant powder coating preparation process for aluminum alloy wheel hub production.

[0006] A process for preparing a low-temperature curing, highly corrosion-resistant powder coating for aluminum alloy wheel manufacturing includes the following steps: S1. Surface pretreatment. The aluminum alloy wheel hub substrate is degreased, acid-etched, and washed with water, followed by non-phosphating silanization treatment to form a chemical conversion film containing Si-O-Al bonds on the substrate surface; S2. Electrostatic spraying. Low-temperature curing powder coating is sprayed onto the surface of the chemical conversion film using an electrostatic spray gun to form a powder coating. S3, Step Curing: The sprayed wheel hub is sent into the curing oven for three-stage step heating and curing, and then cooled to room temperature.

[0007] Furthermore, to better realize the present invention, in S2, the low-temperature curing powder coating comprises the following components in parts by weight: 50-70 parts of low-temperature reactive resin matrix, 10-20 parts of interfacial chelating composite curing agent, 10-20 parts of micro-nano synergistic filler system, and 1-3 parts of leveling agent; the interfacial chelating composite curing agent is a polymer resin grafted with β-diketone structure coordinating groups, which migrate to the substrate interface and form coordination bonds with aluminum ions on the aluminum alloy surface in the cured molten state; in S3, the three-stage stepped temperature rise curing includes: the first stage: heating to 60±5℃ at a rate of 2-5℃ / min and holding for 3-6 minutes to wet the substrate with resin; the second stage: heating to 100±5℃ at a rate of 2-5℃ / min and holding for 3-6 minutes for leveling and degassing; the third stage: heating to 140±5℃ at a rate of 2-5℃ / min and holding for 15-25 minutes to initiate a crosslinking reaction.

[0008] Furthermore, in order to better realize the present invention, the micro-nano synergistic filler system is composed of nano-alumina with a particle size of 20-40nm and flake zinc powder with an aspect ratio of ≥60:1; wherein, nano-alumina fills the interlayer voids of flake zinc powder, and the proportion of nano-alumina in the total mass of the coating is 3-5%, and the proportion of flake zinc powder in the total mass of the coating is 8-10%.

[0009] Furthermore, in order to better realize the present invention, the low-temperature reactive resin matrix is ​​a mixture of carboxyl-terminated polyester resin and bisphenol A type epoxy resin, with a glass transition temperature Tg of 65-70℃; the acid value of the carboxyl-terminated polyester resin is 30-35 mgKOH / g.

[0010] Furthermore, in order to better realize the present invention, the interface chelating composite curing agent is prepared by reacting aluminum acetylacetonate and glycidyl methacrylate copolymer in a solvent and then removing the solvent.

[0011] Furthermore, in order to better realize the present invention, in S1, the treatment solution used for the non-phosphating silanization treatment contains an epoxy silane coupling agent and fluorozirconic acid, the pH value of the treatment solution is 4.0-5.0, and the thickness of the chemical conversion film is controlled at 50-80 nm.

[0012] Furthermore, in order to better realize the present invention, in S2, for the deep concave part where the spokes and rim of the wheel hub are connected, the voltage of the electrostatic spray gun is set to 68-72kV, and the powder output is increased by 5-10% compared with the flat part.

[0013] Furthermore, in order to better realize the present invention, in S3, the heating rate of the three-stage stepped heating and curing process is strictly controlled at ≤5℃ / min, and the thermal deformation of the wheel hub after curing is ≤0.1mm.

[0014] The beneficial effects of this invention are as follows: This invention introduces a β-diketone coordination group into the curing agent molecular chain. This group can actively migrate to the interface during the melt leveling stage to form high-energy coordination bonds with aluminum ions on the substrate surface. Simultaneously, in conjunction with the Si-O-Al covalent bonds formed during pretreatment, a dual chemical anchoring system of coordination and covalent bonds is constructed. Even under low-temperature curing conditions, grade 0 adhesion can be achieved, significantly superior to traditional physical adsorption.

[0015] By utilizing the labyrinth effect of flake zinc powder to extend the corrosion path, and simultaneously using the micropore-filling effect of nano-alumina to block the gaps between the flakes, a dense three-dimensional shielding network is formed. Experiments have shown that this system can extend the neutral salt spray resistance life to over 1200 hours.

[0016] The unique three-stage stepped curing process precisely matches the viscosity-temperature characteristics of the resin. The 60℃ wetting stage reduces surface tension, the 100℃ leveling stage is at the lowest viscosity window to facilitate bubble removal and filler rearrangement, and the 140℃ reaction stage rapidly forms a film. Combined with strict control of the heating rate, stress release caused by thermal shock in aluminum wheels is effectively avoided, reducing thermal deformation to below 0.1mm and significantly improving the dynamic balance pass rate of the finished product. Attached Figure Description

[0017] Figure 1 This is a flowchart of the low-temperature curing powder coating preparation process for aluminum alloy wheel hubs according to the present invention. Detailed Implementation

[0018] The technical solutions of 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 a part of the embodiments of the present invention, and not all of them. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0019] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0020] like Figure 1As shown, the process flow of the present invention mainly includes three core steps: surface pretreatment, electrostatic spraying, and stepped curing.

[0021] Preparation of low-temperature curing powder coatings Ingredients shall be prepared in the following proportions by weight: Resin matrix: 40 parts of carboxyl-terminated polyester resin (acid value 33 mgKOH / g, Tg 68℃) and 25 parts of bisphenol A epoxy resin (epoxy equivalent 750 g / eq); Interface chelating composite curing agent: 15 parts. This curing agent is prepared by grafting aluminum acetylacetonate with GMA acrylic resin and contains acetylacetonate coordinating groups; Micro-nano synergistic filler: 4 parts of nano α-Al2O3 (particle size 30nm, modified by KH-560) and 9 parts of flake zinc powder (diameter-to-thickness ratio 75:1); Additives: 1.5 parts fumed silica, 1 part leveling agent GLP588, 0.5 parts benzoin, and 6 parts toughening agent (core-shell rubber particles).

[0022] Preparation method: The above components are premixed in a high-speed mixer for 3 minutes, melt-extruded through a twin-screw extruder (zone 1 temperature 90℃, zone 2 temperature 110℃), pressed into sheets, pulverized, and passed through an 180-mesh sieve to obtain powder coating.

[0023] 2. Aluminum alloy wheel coating process S1. Surface Pretreatment: An A356.2 aluminum alloy wheel hub was degreased, acid-etched, and washed before being immersed in a non-phosphating silane treatment bath. The treatment solution contained 5 g / L of epoxy silane coupling agent and 0.5 g / L of fluorozirconic acid, with the pH adjusted to 4.5. The wheel was immersed at room temperature for 2 minutes. After drying, a chemical conversion film with a thickness of approximately 65 nm was detected on the wheel hub surface, and infrared spectroscopy showed characteristic peaks of Si-O-Al bonds.

[0024] S2. Electrostatic spraying: Spraying is performed using an automated electrostatic spraying line. The electrostatic voltage is set to 68kV, and the powder supply pressure is 0.45MPa. For the spoke root grooves, the system automatically identifies and increases the voltage to 70kV, increasing the powder output by 8%. The coating thickness is controlled at 75μm.

[0025] S3, Stepped curing: The coated wheel hub is placed into the curing oven, and the following procedure is performed: Increase the temperature to 60°C at 3°C / min and hold for 5 minutes (wetting stage); increase the temperature to 100°C at 3°C / min and hold for 5 minutes (leveling and degassing stage); increase the temperature to 140°C at 3°C / min and hold for 20 minutes (crosslinking reaction stage). After curing, cool to room temperature in the oven.

[0026] The aluminum alloy wheel hubs prepared in the examples were tested: The coating is smooth and even, with a gloss level (60°) of 94, no orange peel (PCI 0 grade), and no pinholes. The cross-cut test result is 0 (ISO 2409). After 1350 hours of neutral salt spray testing (NSS), the erosion width on one side of the cross-cut is <2mm, and no blistering is observed. The rim roundness was measured using a coordinate measuring machine, and the maximum deformation before and after heat treatment was only 0.08mm. No peeling was observed in the stone chip resistance test (SAEJ400).

[0027] Comparative Example 1 Ordinary TGIC was used as the curing agent, without aluminum ion chelation structure; the curing process involved directly heating to 140℃ and holding for 30 minutes. The remaining formulation and pretreatment were the same as in the previous example. Test results: adhesion was grade 2; large-area blistering occurred after only 650 hours of salt spray resistance; wheel hub deformation was 0.12mm (due to rapid heating rate); slight orange peel texture was observed on the coating surface.

[0028] Comparative Example 2 Without adding nano-alumina, only flake zinc powder was used. Test results: Salt spray resistance time was 900 hours, gloss decreased to 85, and microscopic observation showed that the coating density was worse than the example, indicating that the filling effect of nanoparticles is crucial for corrosion barrier.

[0029] The comparison shows that the present invention successfully solves the contradiction between the performance of low-temperature curing coatings and substrate protection by using the chemical and physical dual modification of interface chelating agents and micro / nano fillers, combined with a low-stress step curing process.

[0030] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A process for preparing a low-temperature curing, high-corrosion-resistant powder coating for aluminum alloy wheel manufacturing, characterized in that, Includes the following steps: S1. Surface pretreatment: The aluminum alloy wheel hub substrate is degreased, acid-etched and washed with water, and then non-phosphating silanization treatment is performed to form a chemical conversion film containing Si-O-Al bonds on the substrate surface. S2. Electrostatic spraying: Low-temperature curing powder coating is sprayed onto the surface of the chemical conversion film using an electrostatic spray gun to form a powder coating. S3, Step Curing: The sprayed wheel hub is sent into the curing oven for three-stage step heating and curing, and then cooled to room temperature; In S2, the low-temperature curing powder coating comprises the following components in parts by weight: 50-70 parts of low-temperature reactive resin matrix, 10-20 parts of interface chelating composite curing agent, 10-20 parts of micro-nano synergistic filler system, and 1-3 parts of leveling agent. The interface chelating composite curing agent is a polymer resin grafted with β-diketone structure coordination groups. In the cured and molten state, the coordination groups migrate to the substrate interface and form coordination bonds with aluminum ions on the aluminum alloy surface. In step S3, the three-stage stepped temperature curing includes: the first stage: heating to 60±5℃ at a rate of 2-5℃ / min and holding for 3-6 minutes to wet the substrate with resin; Second stage: Heat to 100±5℃ at a rate of 2-5℃ / min, hold for 3-6 minutes, and perform leveling and degassing; Third stage: Heat to 140±5℃ at a rate of 2-5℃ / min, hold for 15-25 minutes, and initiate cross-linking reaction.

2. The low-temperature curing, high corrosion-resistant powder coating preparation process for aluminum alloy wheel production according to claim 1, characterized in that: The micro-nano synergistic filler system consists of nano-alumina with a particle size of 20-40 nm and flake zinc powder with an aspect ratio of ≥60:1; wherein, nano-alumina fills the interlayer voids of flake zinc powder, and the proportion of nano-alumina in the total mass of the coating is 3-5%, and the proportion of flake zinc powder in the total mass of the coating is 8-10%.

3. The low-temperature curing, high corrosion-resistant powder coating preparation process for aluminum alloy wheel production according to claim 1, characterized in that: The low-temperature reactive resin matrix is ​​a mixture of carboxyl-terminated polyester resin and bisphenol A type epoxy resin, with a glass transition temperature (Tg) of 65-70℃; the acid value of the carboxyl-terminated polyester resin is 30-35 mgKOH / g.

4. The low-temperature curing, high corrosion-resistant powder coating preparation process for aluminum alloy wheel production according to claim 1, characterized in that: The interface chelating composite curing agent is prepared by reacting aluminum acetylacetonate with glycidyl methacrylate copolymer in a solvent and then removing the solvent.

5. The low-temperature curing, high corrosion-resistant powder coating preparation process for aluminum alloy wheel production according to claim 1, characterized in that: In S1, the non-phosphating silanization treatment uses a treatment solution containing an epoxy silane coupling agent and fluorozirconic acid, with a pH value of 4.0-5.0, and the thickness of the chemical conversion film is controlled at 50-80 nm.

6. The low-temperature curing, high corrosion-resistant powder coating preparation process for aluminum alloy wheel hub production according to claim 1, characterized in that: In S2, for the deep recessed area where the spokes and rim of the wheel hub connect, the voltage of the electrostatic spray gun is set to 68-72kV, and the powder output is increased by 5-10% compared to the flat area.

7. The low-temperature curing, high corrosion-resistant powder coating preparation process for aluminum alloy wheel production according to claim 1, characterized in that: In S3, the heating rate of the three-stage stepped heating curing process is strictly controlled at ≤5℃ / min, and the thermal deformation of the wheel hub after curing is ≤0.1mm.

8. An aluminum alloy wheel hub, manufactured based on the low-temperature curing high corrosion-resistant powder coating preparation process for aluminum alloy wheel hub production according to any one of claims 1-8, characterized in that: The powder coating has an adhesion grade of 0 to the substrate and passes the neutral salt spray test of ASTM B117 for ≥1200 hours.

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