Case Study: Main Factors Affecting Yellowing of Light-Colored Electrophoretic Coatings
Case Study: 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, and it is particularly prominent on light-colored or white films. Yellowing is not caused by a single factor but results from the combined effects of materials, processes and environment.
1. Quality and Formula Selection of Electrophoretic Paint
Cathodic electrophoretic paint is mainly based on epoxy resin containing aromatic ether bonds, and the cured film is prone to molecular chain degradation under ultraviolet radiation. In contrast, anodic electrophoretic paint uses acrylic resin as the main base material. Generally, epoxy coatings are more susceptible to yellowing during curing, so anodic acrylic electrophoretic products are usually selected to meet high anti-yellowing performance requirements.
2. Improper Control of the Curing Process
Over-baking is a core factor triggering electrophoretic coating yellowing. Standard curing conditions are typically 180 °C for 20 minutes. Uneven temperature distribution caused by rapid heating, poor ventilation and heat dissipation, or low-precision temperature control systems makes the actual local workpiece temperature significantly higher than the standard curing temperature, resulting in thermal oxidation and film yellowing. Excessively long baking time also aggravates the problem.
3. Thickness Variation of the Coating Film
Excessively high electrophoretic voltage or prolonged electrodeposition time leads to overly thick films and increased internal stress, impairing curing uniformity and indirectly inducing yellowing. Moreover, excessive environmental humidity or incomplete drying before baking adversely affects curing and further causes 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. High-quality pretreatment is the key to qualified films; poor pretreatment causes workpiece re-rusting, which contaminates the bath and eventually leads to coating yellowing.
Judgment Methods for Yellowing Resistance
- Visual Inspection: judge the occurrence of yellowing by observing the surface color of the cured film.
- Instrument Testing: measure the Lab color values with a color difference meter; a larger Δb value indicates more severe yellowing.
Summary and Targeted Control Measures
- Select electrophoretic paint with excellent anti-yellowing performance and optimize raw material selection.
- Stabilize process parameters to avoid yellowing caused by excessive film thickness.
- Optimize the baking process by precisely controlling time and temperature to prevent over-baking.
- Regularly inspect and clean the bath solution to ensure a clean, contamination-free environment.
- Strengthen pre-electrophoresis water washing, stabilize pretreatment quality, and maintain stable paint composition and concentration for consistent film quality.
