Main Factors Affecting Yellowing of Light-Colored Electrophoretic Coatings
Yellowing of electrophoretic coatings after baking is a common quality issue in the electrophoretic coating process, which is particularly prominent on light-colored or white coating films. The yellowing of electrophoretic films is not caused by a single factor but results from the combined effects of materials, processes, environments and other multiple factors.
The main factors leading to yellowing during the baking of electrophoretic coatings are summarized as follows:
1. Quality and Formula Selection of Electrophoretic Paint
Cathodic electrophoretic paint is mainly based on epoxy resin containing aromatic ether bonds. The cured film is prone to molecular chain degradation and breakage under ultraviolet radiation. In contrast, anodic electrophoretic paint adopts acrylic resin as the main base material. Generally, epoxy coatings are more susceptible to yellowing during curing. Therefore, anodic acrylic electrophoretic products are usually selected to meet high anti-yellowing performance requirements.
2. Improper Control of Electrophoretic Paint Curing Process
Over-baking is a core factor triggering electrophoretic coating yellowing. Standard electrophoretic paint curing conditions are typically 180°C for 20 minutes. Uneven temperature distribution caused by rapid heating in the baking oven, poor ventilation and heat dissipation, or low-precision temperature control systems will make the actual local temperature of workpiece surfaces significantly higher than the standard curing temperature, resulting in thermal oxidation reaction and film yellowing. In addition, excessively long baking time will aggravate the yellowing problem.
3. Thickness Variation of Electrophoretic Coating Film
Excessively high electrophoretic voltage or prolonged electrodeposition time will lead to overly thick coating films and increased internal stress, which impairs curing uniformity and indirectly induces yellowing. Moreover, excessive environmental humidity or incomplete film drying before baking will adversely affect the curing process and further cause yellowing defects.
4. Poor Pretreatment and Pre-electrophoresis Rinsing
Strict control of workpiece blank quality is required for light-colored electrophoretic coating to ensure rust-free surfaces. Phosphating or thin-film pretreatment is commonly adopted in the process. High-quality pretreatment is the key to qualified electrophoretic films. Poor pretreatment will cause workpiece re-rusting, which contaminates the electrophoretic bath solution and eventually leads to coating yellowing.
Judgment Methods for Yellowing Resistance of Electrophoretic Coatings
1. Visual Inspection Method
Judge the occurrence of coating yellowing by observing the surface color of the cured film.
2. Instrument Testing Method
Measure the Lab color values of the coating film with a color difference meter. A larger Δb value indicates more severe coating yellowing.
Summary
Based on the above analysis, the yellowing resistance of electrophoretic coatings can be improved through the following targeted control measures:
1. Select electrophoretic paint products with excellent anti-yellowing performance and optimize raw material selection to guarantee fundamental coating film quality.
2. Stabilize the process parameters of electrophoretic coating to avoid yellowing caused by excessive film thickness.
3. Optimize the baking process by precisely controlling baking time and temperature to prevent film yellowing due to over-baking.
4. Regularly inspect and clean the electrophoretic bath solution to ensure a clean and contamination-free working environment for stable coating performance.
5. Strengthen the water washing process before electrophoresis, ensure clean workpiece surfaces, stabilize pretreatment quality, avoid bath solution contamination by pretreatment impurities, and maintain stable composition and concentration of electrophoretic paint to achieve consistent coating film quality.
