Hao Li Sen Application Engineering Center Zhou Zhou
To maintain uniform mixing of the electroplating solution and prevent pigment sedimentation within the tank or on the coated surface, the electroplating solution must be in a continuous mixing and circulation state for 24 hours. The duration of mixing stoppage due to malfunctions should not exceed 2-4 hours. A uniformly dispersed electroplating solution is a prerequisite for ensuring the quality of the electroplating coating.
During the actual operation of an electrodeposition coating production line, the occurrence of sediment in the electrodeposition bath is often caused by the combined effects of factors such as raw material quality, process design, equipment status, and control measures. This not only makes it difficult to ensure the quality of the electrodeposition coating, but also significantly reduces the utilization rate of the coating. A key factor in the deposition of electrodeposition bath is the inherent deposition properties of the electrodeposition product. This article will briefly describe the testing methods for deposition properties, analyze the factors that cause sediment in the electrodeposition bath, and discuss the preventive and control measures that can be taken.
Precipitation test method
1. Scope and Description
Sedimentation testing involves measuring the amount of precipitate formed in an electrophoresis tank after allowing the liquid to stand for a specific period. This can be done using volumetric measurement or solid content measurement methods.
2. Referenced Documents
HG/T3334-2012 "General test methods for electrophoretic coating".
3. Materials and Equipment
Pending testing electrophoresis tank liquid, transparent tape, aluminum tray, temperature-controlled drying box, measuring cylinders (250 mL, 100 mL), analytical balance (sensitivity 0.0001g), steel ruler (resolution 0.5 mm).
4. Testing Methods
(1) Volumetric Measurement Method: Add 250 mL of liquid into a 250 mL (or 22 cm high) container with a flat bottom (other suitable containers can also be used). Seal the container with a plastic wrap and place it under (23 ± 2) °C conditions. Use a ruler to measure the height of the sediment after 2h, 4h, 8h, 24h, and 48h of settling.
- Generally, the settling height after 24 hours should be ≤ 2mm..
(2) Solid Content Measurement Method: Using a 100mL graduated cylinder, take 100mL of the solution. Seal with a plastic wrap and place in (23±2)°C conditions. Allow to stand for 2h, 4h, 8h, 24h, and 48h, respectively. At each time point, withdraw 4mL of the liquid from the top and test the solid content (after shaking). Calculate the settling rate (X) based on the calculated formula.
In the formula, A represents the solid content of the original liquid, and B represents the solid content after settling.
- Generally, a setting time of ≤24 hours is required.
Factors leading to the precipitation of electrolyte in the electrolytic tank
1. Contamination of the electrophoresis tank solution
Pre-treatment with alkaline degreasing agents, phosphatizing solutions, and phosphate residues can cause contamination of the electroplating tank, leading to an abnormal increase in the pH value of the tank liquid, triggering chemical reactions that destabilize the liquid and produce flocculation or precipitation.
2. The quality of the ultrapure water produced by electrophoresis does not meet the required standards.
Fresh, pure water with a pH value exceeding 7.0 and a high conductivity containing impurities such as sodium ions and chloride ions, when introduced into the tank, reacts with the charged resin, forming complexes or precipitates, which destabilizes the paint.
3. The temperature of the electroplating solution is too high.
High tank liquid temperatures can increase solvent evaporation, accelerate tank liquid aging, and lead to coagulation and sedimentation.
4. Poor circulation of circulating fluid
The circulation pipe in the electroplating tank has dead ends, and the circulation pipes and nozzles are blocked, leading to poor circulation and localized sedimentation of the coating liquid. The high speed of the circulation pump (e.g., using a 2900 RPM motor) generates excessive shear forces, which damage the ions in the electroplating coating, resulting in aging and sedimentation. During production, it is necessary to ensure that the circulation pump operates normally, and that the tank liquid is circulated and mixed properly to avoid the formation of stagnant areas, thereby preventing sedimentation.
5. Low solvent content
- Low solvent content: Under normal production conditions, organic solvents are continuously consumed. However, if production is disrupted or the temperature is too high, leading to excessive solvent consumption without adequate replenishment, the solvent content can drop below the minimum level, causing the resin in the slurry to coalesce or precipitate.
6. Abnormal solid content
Solid content variations can also lead to sedimentation: When the solid content is too high, the density becomes too high, and the pigment components are prone to sedimentation. When the solid content is too low, the dispersion uniformity deteriorates, which can also cause sedimentation.
7. Abnormal color ratio
Incorrect material ratio can lead to an imbalance in the color and base components, which can result in precipitation.
8. Improper storage or expired original paint
Improper storage conditions for the original paint (such as high temperatures and direct sunlight) can cause the pigment to aggregate or the emulsion to precipitate. After adding to the electroplating tank, a sedimentation phenomenon occurs.
9. Slow liquid circulation and bacterial growth
- The tank is too large, resulting in low production volume. Slow tank liquid turnover and the accumulation of aged and bacterial-laden liquid can lead to precipitation and particle issues in the electroplating tank.
Control measures to prevent settling of electroplating bath solution.
1. Optimize pre-processing procedures and pure water monitoring.
Strengthen the control of pre-treatment processes to minimize the introduction of foreign matter from adjacent processes into the electroplating tank through proper hanging methods. Strictly control the impurity content of fresh and pre-plating pure water, ensuring regular testing and replacement.
2. Strengthen temperature management
Establish a temperature alarm system to prevent drastic fluctuations in the tank liquid temperature. Maintain the tank liquid temperature within an appropriate range, and especially avoid prolonged high-temperature operation.
3. Improve the circulation system
- Regularly inspect the operation status of the circulating pump and piping system to ensure thorough mixing. Effective circulation of dead corners can be achieved by increasing the circulation volume and adjusting the nozzle direction.
4. Monitor tank parameters
Regularly monitor and control solid content, pH value, conductivity, solvent content, color ratio, and bacterial growth parameters to maintain optimal tank liquid conditions.
5. What are the specific requirements for storage and management?
Pigments and emulsions should be stored in a stable environment between 5-35°C. They should not be exposed to high temperatures, direct sunlight, or freezing temperatures. Use according to the "first in, first out" principle, and use within the expiration date.
6. Choose electrocoating with good anti-settling properties.
[1] Refer to testing methods as specified in HG/T3334-2012《General Test Methods for Electrodeposition Coatings》.