In the electrostatic powder coating process, many people encounter the problem of "powder shedding"—freshly sprayed powder falls off with the slightest touch; they also wonder why some powders have strong adhesion while others are always unevenly sprayed. These problems are actually closely related to the powder's electrical properties and stability. Today, we'll break down these professional concepts in layman's terms, from the principles of electrostatic spraying, the causes of powder shedding, stability assessment, and the influence of resistivity and dielectric constant, helping you avoid pitfalls in practical operations.
1. Why does powder shedding occur?
Many construction workers report that even though the powder is sprayed onto the workpiece during electrostatic spraying, it falls off with the slightest vibration. What's going on? The core reason lies in the powder's "dielectric constant"—this seemingly technical indicator directly determines the powder particles' ability to carry and retain charge. Simply put, the dielectric constant is like the powder's "charge storage tank": the lower the dielectric constant, the shallower the "tank," meaning the powder particles easily become charged, but the charge also easily dissipates, like a lidless cup—the water fills quickly and spills quickly. In actual spraying, this translates to weak powder adhesion to the workpiece; even slight vibrations during transport can cause powder to fall off. For powder coatings used in electrostatic spraying, we need a "deep container"—that is, powder with a high dielectric constant. A high dielectric constant allows the powder to firmly "lock in" the charge, greatly enhancing adhesion. Even after being subjected to bumps in the transport mechanism, it can stably adhere to the workpiece, reducing powder loss and waste. However, it's important to note that high dielectric constant powder is relatively difficult to charge, requiring improvements to the powder spray gun, such as using a multi-electrode forced charging structure to help the powder acquire charge more easily.
2. The "Secret of Charging" in Electrostatic Spraying
To understand how powder becomes charged and adheres to the workpiece, we must first understand the core of electrostatic spraying—the principle of corona discharge. From an electrostatic perspective, on the surface of a charged, isolated conductor, the charge distribution is related to the surface radius of curvature: the greater the curvature (such as the sharpest point of the conductor), the higher the charge density and the stronger the surrounding electric field. When the electric field strength is strong enough to ionize the surrounding air, a corona discharge occurs at the tip of a conductor. In powder coating, the powder spray gun is connected to a negative high voltage, and the workpiece is connected to a positive electrode. When the negative high voltage is applied, a corona discharge occurs at the tip of the powder spray gun: electrons leaving the tip are accelerated by the strong electric field and continuously collide with air molecules, ionizing the air molecules into positive ions and electrons—this is like an "electron avalanche," with more and more electrons being released. Because electrons have small mass, they are quickly adsorbed by air molecules after escaping the ionization region, turning the air molecules into negatively charged ions. These negative ions, under the influence of the electric field, move towards the positive electrode (workpiece). When powder passes through the corona discharge region, it is "bumped" by these negative ions heading towards the workpiece, thus becoming negatively charged. Most industrial powder coatings are polymeric insulators with high surface resistivity. Negative ions can only be adsorbed when there are suitable "vacancies" on the powder surface to accept the charge. These "vacancies" could be positively charged impurities in the powder, potential pits in the molecular structure, or simply mechanical gaps. Regardless of the method, the charge is not uniformly distributed on the powder surface because the high surface resistivity prevents the charge from flowing back through conductivity, resulting in slightly different charge states for each powder particle. However, there's no need to worry; the charged powder particles have a "dual driving force" to aid adhesion: when they first leave the spray gun, they are propelled forward by the thrust of compressed air; as they approach the workpiece (positive electrode), they are "pulled" towards the workpiece by the electric field force and firmly adsorbed onto the surface. Typically, a coating thickness of 50–100 μm can be achieved in just a few seconds. However, once the coating reaches a certain thickness, a thick layer of negative charge "shielding layer" forms on the surface. Subsequent negatively charged powder particles are repelled by this shielding layer and cannot adhere—this is actually the powder's "self-protection," automatically controlling the coating thickness to prevent excessive thickness from causing sagging or cracking.
3. Spraying Techniques for Repainted Parts
When dealing with repainted parts, many people find that the workpiece surface already has a layer of old paint film. When spraying new powder, either the powder application efficiency is low, or the coating is very thin. This is actually related to the resistivity of the old paint film. The high resistivity of the old paint film, while beneficial for the new powder to become charged, also makes it difficult for the charge to dissipate. According to the principle of electrostatics, if the electric field strength (E) is too high, an "induced electric field" will be established on the surface of the old paint film, causing a high concentration of negative charge areas to form on the surface of the workpiece before much new powder has been adsorbed—these negative charges strongly repel subsequent negatively charged powder, preventing the new powder from adhering and ultimately resulting in a thin coating. To solve this problem, the key is to appropriately reduce the electric field strength. Lowering the electric field strength reduces the transfer velocity and charge of powder particles, preventing them from being strongly repelled and bouncing back, thus improving powder application efficiency. In practice, the electric field strength suitable for the part being sprayed can be found by adjusting the voltage or distance of the powder spray gun. This allows new powder to adhere smoothly without causing the coating to be too thin due to repulsion.
4. Powder Coating Stability
If you buy powder and find that it has clumped after a period of time, resulting in orange peel texture and reduced gloss on the sprayed film, this indicates a problem with the powder's "stability." The stability of powder coating refers to whether it will experience problems such as clumping, poor leveling, decreased charge retention, or pinholes and bubbles in the coating film during storage or use. For powder manufacturers and contractors, stability is an "invisible threshold": only powder with adequate stability can guarantee consistent results for users. For example, if the stability of powder coatings used in home decoration is poor, the leveling will deteriorate after a few months of storage, resulting in noticeable orange peel texture on the sprayed wall, affecting aesthetics. In factories producing appliance casings, if the powder's charge retention decreases, it will lead to severe powder shedding, increasing production costs. So how do we determine the stability of powder? A common industry method is the "high-temperature treatment test": the powder is placed at a certain temperature (usually simulating the high-temperature environment of long-term storage) for a period of time, and then its leveling properties are tested. This is because the stability of powder is essentially the degree of molecular cross-linking reaction—if the powder molecules undergo vigorous cross-linking during storage, the molecular weight will increase, the viscosity at the curing temperature will increase, and the leveling properties will naturally deteriorate. For example, if a powder with good leveling properties initially produces a coating with a noticeable grainy texture after high-temperature treatment, it indicates insufficient stability and is unsuitable for long-term storage.
5. Resistivity and Dielectric Constant
In electrostatic spraying processes, two indicators must be closely monitored: resistivity and dielectric constant. They directly affect the powder's adsorption capacity, deposition efficiency, and whether the coating can withstand vibration. Let's first discuss resistivity. The resistivity of a powder determines its charging state in an electrostatic field: for example, powder with a resistivity of 10¹³ ohms only requires a 30–50 kV electrostatic voltage to charge well; while powder with a resistivity of 10⁸–10⁹ ohms requires a 100–120 kV high voltage to achieve the same charging effect. More importantly, the powder's ability to automatically limit coating thickness is also closely related to its resistivity—experiments have shown that only powders with high resistivity can form coatings of suitable and uniform thickness. If the resistivity is too low, the powder is prone to leakage after being charged, failing to form a stable shielding layer and resulting in an excessively thick coating. Combined with the previously mentioned dielectric constant: the dielectric constant affects the powder's ability to retain charge, while the resistivity affects the voltage required for charging and the control of coating thickness. These two factors complement each other. For example, powders with high dielectric constant and high resistivity, while slightly more difficult to charge, adhere firmly once charged and can automatically control coating thickness, making them ideal for electrostatic spraying. Conversely, powders with low dielectric constant and low resistivity are prone to powder shedding and may cause uneven coating thickness due to leakage, making them suitable only for specific applications.
6. Summary
In fact, the issues of powder shedding, charging, and stability in powder coatings can be addressed by understanding the core principles of dielectric constant, resistivity, and corona discharge. Simply put: To avoid powder shedding, prioritize powders with high dielectric constants and use an improved multi-electrode powder spraying gun; when handling repainted parts, remember to reduce the electric field strength to minimize charge repulsion; when purchasing powders, pay attention to stability test results to avoid performance degradation after long-term storage; during electrostatic spraying, adjust the voltage according to the powder's resistivity to ensure efficient powder application and control coating thickness. Whether it's mass production in a factory or small-area spraying in home decoration, as long as you master these principles, you can choose the right powder according to actual needs, adjust the spraying parameters, reduce problems such as powder shedding and uneven coating, and give full play to the advantages of powder coating to produce a smooth, firm, and beautiful coating film.
