Electrostatic theory is the fundamental support for powder coating technology. It is based on the principle of electrostatic action that powder can efficiently and uniformly adhere to the substrate surface, forming a high-quality coating. Currently, according to the different methods of static electricity generation, powder coating spray guns are mainly divided into two categories: corona guns and friction guns. Among them, corona guns are the most widely used, and their core basis is the corona charging theory. However, there are still some controversies within the industry regarding the theory of electrostatic spraying.
1. The Core Principle of Corona Guns
Corona charging is the core theoretical basis of corona spray gun coating. Its working mechanism is clear and has been verified through long-term industrial practice. The core logic of this theory is to ionize the air through electrodes, causing powder particles to acquire a charge, and then adsorb onto the substrate under the action of an electric field. The specific workflow is as follows: First, an electrode is specially installed at the end of the spray gun barrel. During operation, a high-voltage electric field is applied to this electrode. The key function of the electrode is to ionize the air molecules near the nozzle, forming a charged ion cloud (mainly a negative ion cloud). At the same time, the powder coating undergoes fluidization treatment in the powder feeder, fully mixing with air to form a uniform air-powder mixture, which is then sent to the spray gun through a delivery hose. When the air-powder mixture is sprayed out of the spray gun, as it passes through the ion cloud near the nozzle, the powder particles quickly absorb negative ions, thus acquiring a uniform negative charge. At this point, a stable electric field and electric field lines are formed between the spray gun and the grounded workpiece. Under the action of Coulomb force (the attraction between positive and negative charges), the negatively charged powder particles are firmly attracted by the positive charge on the substrate surface (formed due to grounding). Finally, under the combined action of the thrust of compressed air and Coulomb force, the powder particles accurately fly to the workpiece surface and adhere firmly, laying the foundation for subsequent baking and curing.
2. Controversial Theories
While the corona charging theory is widely accepted and applied, some scholars in the industry have proposed a different view: powder particles are not charged at the spray gun tip, but rather polarized to form dipoles, thus achieving adsorption. The core claim of this theory is that powder particles do not directly acquire charge when passing through the electric field of the spray gun; instead, they are polarized under the influence of the electric field, becoming dipoles with one positive and one negative end. Driven by the electric field, these dipoles move towards the workpiece, and upon reaching the workpiece surface, they densely arrange themselves in a "head-to-head" arrangement, adsorbing onto the substrate through the interaction forces between the dipoles. Scholars supporting this theory provide a key experimental example: collecting the powder sprayed from the spray gun in an insulating container and testing its charge level using specialized equipment revealed that the powder particles carried almost no significant charge, contradicting the core claim of the corona charging theory that "the powder is negatively charged." However, this controversial theory has a significant logical flaw—it cannot reasonably explain the industry consensus that "the workpiece must be grounded." In actual spraying operations, workpiece grounding is a necessary condition for ensuring spraying effect. According to polarization theory, dipole adsorption is not directly related to whether the workpiece is grounded. However, in practice, ungrounded workpieces will experience problems such as poor powder adsorption and uneven spraying, which makes this theory difficult to be widely accepted.
3. Conclusion
Although there are some academic controversies surrounding electrostatic spraying theory, it is undeniable that electrostatic spraying technology has been used in the powder coating industry for decades and is currently the most widely used and mature spraying method globally. The corona charging theory, due to its clear working mechanism and high degree of consistency with actual operation, has become the mainstream understanding in the industry and provides core guidance for equipment development and process optimization. Regarding the controversial theory of polarized dipole adsorption, although it is supported by experimental data, it fails to explain the key role of workpiece grounding, and therefore has not yet formed a mainstream consensus. With the continuous advancement of industry technology and the deepening of research, people's understanding of electrostatic spraying will continue to improve. It is believed that in the future, these controversies will be gradually resolved reasonably, and electrostatic theory will become more mature, providing a more solid theoretical support for the upgrading of powder coating technology.
