China Aluminum Industry Network – According to the causes of electrophoretic aluminum yellowing, several key factors have been identified: issues with the electrophoretic painting process itself, poor oxidation conductivity, inadequate rinsing before electrophoresis, excessive curing, and contamination of the oxidation bath solution by nitric acid. Understanding these causes is essential for preventing discoloration and ensuring high-quality surface finishes.
1. Yellowing Caused by Poor Conductivity During Oxidation
Poor electrical contact between the aluminum profile and the conductive rod can significantly increase resistance at the contact point. This leads to localized heating, which causes the oxide layer to form too quickly and may result in burning or even cracking of the film. The resulting oxide layer appears cloudy and yellow. If electrophoretic coating is applied afterward, this yellowing becomes very noticeable. Typically, this issue affects only a few sections of the profile, usually near the end. To prevent this, it's crucial to ensure consistent and reliable contact between the profile and the conductive rod during the oxidation process.2. Yellowing Due to Incomplete Washing Before Electrophoresis
The porous structure of the oxide film allows sulfuric acid residues to remain trapped within its pores. If the washing process before electrophoresis is not thorough, these acid residues can react with the electrophoretic paint, leading to yellowing. However, it’s important to note that the yellowing occurs in the oxide film, not the paint itself. A clear and transparent oxide film can become hazy and lose clarity when exposed to residual sulfate at high temperatures. When combined with the high transparency of the electrophoretic paint, this defect becomes more pronounced, resulting in visible yellowing. Therefore, multiple rinses are necessary, along with proper water quality, temperature, and washing time to eliminate any residual acids.3. Yellowing Caused by Nitric Acid Contamination in the Oxidation Bath
While nitric acid is sometimes added to neutralization tanks to improve cleaning efficiency, improper rinsing after neutralization can allow nitric acid to enter the oxidation tank. Once nitrates accumulate in the oxidation bath, they can affect the oxide film formation, leading to yellowing. Nitrates can etch the oxide film, weaken the barrier layer, and deepen the pores, altering the film's structure. This weakens adhesion and makes it harder to remove residual sulfate. To prevent this, it's recommended to adjust the pH of the rinse water to 8–9.5 using a neutralizer like amine radicals before electrophoretic coating. This helps neutralize the sulfate and reduce the risk of yellowing.4. Yellowing Due to Over-Curing
Most electrophoretic paints are cured at around 180°C for 30 minutes, and under normal conditions, the paint film does not yellow significantly. However, if the curing oven has uneven temperature distribution or a faulty control system, some areas may be over-cured, leading to discoloration. Low-quality electrophoretic paints are especially prone to this issue, as they lack the stability to withstand extreme conditions. High-quality paints, on the other hand, are more resistant to yellowing even at higher temperatures. To avoid this problem, it's essential to maintain uniform furnace temperatures and use reliable, high-performance electrophoretic coatings.5. Yellowing Caused by the Paint Itself
Anodic electrophoretic paint typically consists of acrylic resin and amine-based resins. During baking, these components cross-link to form a smooth, transparent coating. However, if the production process is not mature or low-quality raw materials are used, the curing range may be too narrow. Under-curing can lead to a soft, insufficiently hardened film, while over-curing can cause yellowing. This variability poses challenges for production management. To minimize such issues, it's advisable to select a paint supplier known for stable product quality and strong industry reputation.Spotlight Background,Light Spotlight,Black Spotlights,Ceiling spotlight
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