E-coating: Definition, Equipment, and Applications

Electrophoretic coating is a common coating process used in the automotive industry. In this article, we will explore what E-coating is and how it works.

What is Electrophoretic Coating?

Electrophoretic coating is a coating process in which the workpiece to be coated is immersed in a water-soluble paint and serves as one electrode, either the cathode or the anode. A corresponding electrode is provided, and direct current (DC) is applied between the two poles. Under the influence of the electric field, charged paint particles undergo directional migration toward the surface of the workpiece, where they discharge and deposit to form a coating film.

Electrophoretic coating methods are typically classified based on the characteristics of the paint and the polarity of the workpiece being coated.

The process is termed anodic electrophoretic coating when the workpiece acts as the anode and uses anionic paint. Conversely, when the workpiece acts as the cathode and utilizes cationic paint, it is a cathodic electrophoretic coating. Currently, more than 90% of automotive body coatings worldwide utilize cathodic electrophoretic coating.

While anodic electrophoretic coatings are less expensive, their corrosion resistance is inferior to that of cathodic electrophoretic coatings. Consequently, almost all manufacturers currently adopt cathodic electrophoretic coating. Electrophoretic coating produces a uniform surface with excellent adhesion and is suitable for coating workpieces with complex shapes, it can even deposit films on internal cavity surfaces. However, the equipment is complex, requires a large investment, and the variety of suitable coatings is limited, currently restricted to water-soluble and water-emulsified paints.

Process of E-coating

Electrophoretic coating is a highly complex electrochemical reaction. Whether cathodic or anodic, the process involves four simultaneous typical mechanisms: electrophoresis, electrodeposition, electroendosmosis, and electrolysis. In this process, ionized resin within the water-dispersed coating dissociates into charged colloidal particles. Through the combined effects of these four mechanisms in a DC electric field, these particles migrate to the workpiece surface and discharge to form an insulating coating film.

Electrophoresis

Electrophoresis is the process in which charged colloidal particles move toward the electrode with the opposite polarity under the influence of electric field force. During electrophoresis, uncharged pigments and extender pigment particles may also be adsorbed onto the charged colloidal particles and move along with them. Generally, lower solids content and lower viscosity in the coating solution result in less resistance to electrophoresis.

Electrodeposition

Electrodeposition refers to the process where charged colloidal particles reach the electrode (the workpiece) of opposite polarity and discharge to form an insoluble, insulating coating film. Unlike electroplating, the conductivity of the coating film in electrophoresis changes, turning it into an insulator. Electrodeposition first occurs at locations on the workpiece surface where electric field lines are concentrated. As the thickness and area of the insulating film increase, the electrical resistance also rises significantly.

Electroendosmosis

Electroendosmosis is the movement of the dispersion medium in the opposite direction to the charged particles. During the electrophoresis process, the newly formed coating film is a semi-permeable membrane with high water content. Under the influence of electric field force and internal osmotic force, the water and ions within the film undergo directional electroendosmosis and anti-electrophoretic movement to enter the solution. Typically, the dehydration effect of electroendosmosis can reduce the water content of the coating film by 5% to 15%.

Electrolysis

Water undergoes an electrolytic reaction in a DC electric field, releasing hydrogen at the cathode and oxygen at the anode. This causes the pH value at the cathode to rise and the pH at the anode to fall. Higher conductivity in the solution leads to a more intense electrolytic reaction; greater pH changes at the electrodes and more bubbles result in more pinholes and a rougher coating film. Therefore, while electrodeposition is the primary reaction of the process, electrolysis is a critical factor affecting film quality. Although anodic electrophoresis requires less equipment investment and lower paint costs, it suffers from poor corrosion resistance and lower voltage. Compared to anodic films, cathodic electrophoretic coatings provide 3 to 4 times the protective capability, double the Coulombic effect, and 1.2 to 1.4 times the throwing power. Consequently, cathodic electrophoresis is widely used, while anodic electrophoresis is mostly reserved for applications with lower requirements.

Advantages of Electrophoretic Coating

• Easy realization of automation and mechanization: Electrophoretic coating features a high degree of automation, fast coating speeds, and high production efficiency. For example, the efficiency of automotive primer electrophoretic coating can be 450% higher than that of traditional dip-coating processes.
• High uniformity of film thickness: Even on irregular areas such as corners, slots, holes, and weld protrusions, a uniform coating thickness can be achieved by adjusting voltage and controlling the electrodeposition process.
• High coating quality: The resulting film is flat, smooth, and free of sagging. It exhibits excellent leveling during drying, providing high aesthetic quality that saves on sanding time and reduces production costs. The film also possesses good water resistance and adhesion.
• High paint utilization rate: Due to the low concentration and low viscosity of the paint, very little is carried out by the workpiece, allowing for a utilization rate of over 95%.
• Good environmental and working conditions: The coating contains few auxiliary solvents, producing minimal paint mist during volatilization and reducing environmental pollution.

Disadvantages of E-coating

• Large investment: Equipment investment is high, and operational management requirements are strict.
• Numerous process limitations: Electrophoretic films are limited to single colors. Multiple types of metal workpieces cannot be coated simultaneously. Non-conductors like plastics and wood cannot be electrophoresed. Furthermore, a topcoat cannot be electrophoresed onto a primer surface, and the weather resistance of the primer itself is relatively poor.
• Heavy cleaning workload: To ensure conductivity upon contact with the workpiece, hangers must be cleaned thoroughly and frequently, leading to a high workload and labor intensity.

Equipment for E-coating

The equipment for electrophoretic coating consists of an electrophoresis tank, backup tank, circulation and filtration system, ultrafiltration system, electrode devices, paint temperature adjustment device, paint replenishment device, ventilation system, DC power supply, control cabinet, post-electrophoresis rinsing device, and paint storage device.

Electrophoresis Tank

The electrophoresis tank is the core equipment and the container where the process takes place; its dimensions are determined by the maximum external size of the workpieces. It includes the main tank, backup tank, and overflow tank. Electrophoretic coating is generally divided into continuous through-type and intermittent step-type systems. The former is suitable for mass production and uses ship-shaped tanks, while the latter is suited for medium-sized batches and often uses rectangular tanks. Overflow tanks are usually installed on one or both sides to control the liquid level and remove surface foam. The backup tank is used to temporarily store the paint solution during cleaning or maintenance of the main tank.

Circulation and Filtration System

This system includes circulation pumps, piping, and spray nozzles, typically utilizing a combination of filtration circulation, heat exchange, and ultrafiltration. Its purpose is to ensure the stability of the paint composition and concentration, maintain good dispersibility, and prevent pigment precipitation. To avoid sedimentation, a specific flow rate and number of cycles must be maintained.

Electrode Device

This device consists of electrode plates, diaphragm covers, auxiliary electrodes, and a deionized water system. For anodic electrophoresis, copper or stainless steel plates are used as anodes, with a cathode-to-workpiece area ratio of generally 1:1. Cathodic electrophoresis requires stainless steel or titanium alloy plates for the cathode, with a cathode-to-workpiece area ratio of typically 1:4. Diaphragm covers are bag-like structures made of semi-permeable membranes filled with deionized water, they house the electrode plates and prevent electrolytic acids or bases from diffusing into the paint, thereby stabilizing the pH. Auxiliary electrodes are used to ensure uniform film formation on complex internal walls and cavities.

Paint Replenishment Device

Replenishment is performed in a mixing tank where original paint is diluted with working liquid, stirred, and mixed before being added to the electrophoresis tank. Alternatively, a mixer can be integrated directly into the circulation piping for continuous replenishment.

Post-Electrophoresis Rinsing Device

After film formation, workpieces must be immediately and repeatedly rinsed using circulating ultrafiltrate, fresh ultrafiltrate, and deionized water. This removes adhered paint liquid from the film surface and prevents the film from redissolving or thinning.

Materials Suitable for E-coating

• Steel and its alloys: Including automotive bodies, chassis components, engine parts, hardware, metal tools, agricultural machinery, and structural steel for construction.
• Aluminum alloys: Including automotive parts like hoods, wheel hubs, radiators, brackets, aluminum profiles for doors and windows, consumer electronics casings, and handicrafts.
• Zinc and zinc alloys: Such as galvanized steel sheets (cathodic electrophoresis on galvanized layers is very common) and die-cast alloy parts.
• Copper and copper alloys: Special attention is required as the presence of copper ions may affect the stability of the tank solution.

Factors Influencing Electrophoretic Coating

The process flow for electrophoretic coating is:

Degreasing → Cold water wash → Hot water wash → Surface conditioning → Phosphating → Cold water wash → Passivation → Deionized water wash → Drying → Electrophoretic coating → Ultrafiltrate wash → Deionized water wash → Baking → Cooling. Phosphating the workpiece before electrophoresis is done to enhance the protective performance of the coating film.

Solids Content

The solids content of the paint liquid is a critical factor affecting stability, throwing power, film thickness, and appearance. If the solids content is too low, pigment sedimentation becomes severe, stability and throwing power decrease, and the resulting film is thin, rough, porous, and lacks protective capability. If the solids content is too high, the film thickens, electroendosmosis becomes difficult, and the surface becomes rough with an orange-peel texture; high solids also increase the load on filtration and rinsing. Generally, solid content is controlled at 10% to 15% for anodic electrophoresis and 20% ± 0.5% for cathodic electrophoresis.

Temperature

Increasing the temperature of the paint solution facilitates electrodeposition and increases film thickness. However, excessively high temperatures accelerate solvent evaporation, decrease stability, and cause roughness or sagging. Conversely, low temperatures reduce water solubility and increase deposition resistance, leading to thin films or a failure to form a film in recessed areas. Low temperatures also increase viscosity, which traps bubbles and results in a rough, dull surface. Thus, temperatures for anodic and cathodic electrophoresis should be controlled between 20–25°C and 28–30°C, respectively.

pH Value

The pH value directly affects stability and conductivity and must be strictly controlled within a fluctuation of ±0.1. Generally, the pH should be 7.5–8.5 for anodic electrophoresis and 5.8–6.7 for cathodic electrophoresis. High pH in anodic systems causes resin decomposition and instability. In cathodic systems, low pH reduces coulombic force and throwing power while increasing pipe corrosion, while high pH reduces liquid stability.

Conductivity

Conductivity increases with the concentration of impurities. High conductivity intensifies electrolysis, making the film rough and porous, and reduces the voltage, throwing power, and stability of the liquid. Generally, cathodic paint conductivity is 1000–2000μS/cm, while anodic paint conductivity is relatively higher.

Electrode Distance

A greater distance between the electrode and the workpiece increases the resistance of the solution. If the distance is too small, uneven thickness occurs on the protrusions and recesses of the workpiece. If the distance is too large, deposition efficiency drops, and the film becomes very thin or fails to form. Therefore, the electrode distance is typically maintained between 150 and 800 mm.

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