How to prevent static electricity in coating production process (top)

In the coating production of the printing and packaging materials industry, the processed paper or plastic film materials and various types of guide rollers of the coating machine generate a large amount of static electricity, and many production processes require the use of low-boiling, volatile flammable and explosive materials. Organic solvents, such as toluene, solvent gasoline and various ketones and ester solvents, produce a large amount of flammable and explosive flux gases. If the static elimination measures are improper, static electric charges will continue to accumulate, which will lead to extremely high local electric field strength, dielectric (air) breakdown, positive and negative charge neutralization, and spark generation, igniting or detonating organic solvent gases, resulting in serious personal injury. Equipment safety accident. Such safety accidents occur in this industry from time to time.

Coating production processes are mostly insulating paper and all kinds of plastic films, such as PE, PET, etc. During the production process, many metal and rubber guide rollers that quickly pass through the coating machine are continuously and repeatedly Pressure contact, separation, friction, generate a lot of static electricity, especially the rubber roller to generate static electricity far stronger than the metal guide a. According to the electrostatic series, the positive charge on the paper, and the negative charge on the PET, the roller surface produces the same amount of static charge. When measured with an electrostatic tester, the measured static voltage reaches several thousand volts.

Current Electrostatic Technology Overview in Coating Production

Although various manufacturers in the coating production using a variety of ways to eliminate static electricity, such as ion current static elimination device, the body. The grounding of the power supply line is simple and the static electricity measures are eliminated. These measures have different functions of eliminating static electricity, but none of them can ensure the safety of production.

1. Ion flow electrostatic remover is only suitable for eliminating the static charge accumulated on the surface of the workpiece, and can not eliminate the static electricity generated through the subsequent guide roller; in particular, there are many space guide rollers near the coating head, space is limited, and it is inconvenient. Use this type of static eliminator to remove static electricity.

2. Use simple body with high grounding resistance. Grounding of power grounds to eliminate static electricity is theoretically limited to the idea of ​​Ohm's Law: V=IR. Ohm's law is the relationship between voltage, current and carrier resistance under the condition that the carrier is a conductor and the charge is evenly and orderly laid on the surface of the conductor. The electrostatic charge carrier in the coating production is insulating paper, plastic film, and rubber. The static charge on the surface is scattered on the surface of the insulator. Therefore, the theoretical concept of Ohm's law cannot solve the problem of static electricity accumulation. Although the voltage value measured by an electrostatic tester in a production site is several thousand volts, it is a reflection of the total energy (voltage) of the electrostatic charges scattered on the surface of paper, plastic film, and each guide roller. The voltage of the static charge dispersed on the surface of the material with which the roller is in contact is limited. Every electrostatic charge dispersed on the insulator cannot flow through the ground to the ground. Especially when the static charge is a positive charge, it is a chargeable point (cation) after the electrons in the charged material lose electrons. It is still one of the charge carrier materials. Obviously, based on the theory of Ohm's law, positive It is impossible for the charge to flow along the earth ground.

The above shows that using Ohm's law as a theoretical basis for eliminating static electricity is difficult to achieve the intended purpose. And using Ohm's law can not explain the process of static spark generation:

Electrostatic spark generation process in coating production

Based on the aforesaid and coating production practices, the measures to eliminate static sparks must be based on electrostatic theory related to Coulomb's law (Equation 1) and electric field strength (Equation 2), ie:

F=g1g2/4Ω2(1)
F=g2/4Ω2(2)

1. Great attention should be paid to the serious damage to the safety caused by the electrostatic spark generated by the rubber roller. In the production, the rubber pressure roller surface and the paper or plastic film (for example, PET) generate the same amount of static charges, the pressure roller surface has a positive charge, and the PET surface has a negative charge. Repeated contact and separation of the rubber pressure roller and the PET, the static charge gradually accumulates, and the electrostatic charge generated by the separation of the PET and the rubber roller once is disorderly distributed on the entire coil material, and the charge density is much lower than the rubber pressure roller surface. As the production progresses, the electrostatic charge of the rubber roller gradually increases. From Equation (2), the electric field intensity gradually increases.

Because of the inside of the rubber pressure roller, it is a ferrous pipe with a diameter of 10cm to 15cm. The outside is covered with a 3-4cm thick rubber under the action of an electric field. The ferrous tubular body bearing is connected to the ground through the body and the grounding line. The grounding resistance is very high. At a small instant, the inductive charge is transmitted from the ground through the grounding line, causing the iron tubular body to be charged with a static electric charge. Because the iron tubular body is an electric conductor, the induced charge on it is repulsive and is close to both ends of the tubular body. With the accumulation of electrostatic charge on the rubber roller surface, the amount of induced charge is also increased gradually, the repulsive force is strengthened, and finally it is concentrated on both ends of the iron pipe to form a strong electric field. The direction of the electric field is the same as that of the original static electric field. The space between the positive and negative charges and the electric field intensity are superimposed. By Coulomb's law (Formula 1), in the case of a very limited r value, the great attraction between positive and negative charges overcomes the obstacles of the medium (air) to produce positive and negative charge neutralization and medium (air) breakdown, releasing a considerable amount Thermal energy - EDM generated. The heat energy released is enough to ignite and detonate the solvent gas, causing personal and equipment safety accidents.

The insulating lubricant in both ends of the rubber roller and the metal guide roller makes the grounding resistance very unstable. Under normal circumstances, the lubricant film inside the bearing is completely applied to all the bearing surfaces, making it a sound insulator. It does not produce induced charges. However, in the long-term operation, there will always be some moments where the lubricant film breaks inside the bearing. The metal surface of the bearing is in direct contact with the grounded body, and the resistance is greatly reduced. The earth is transferred to the iron core under the action of the electric field. Inductive charge. At this time, if the electrostatic charge is high, sparks will be generated and safety accidents will occur. This shows that, due to the influence of the lubricant, the phenomenon that the iron core of the press roll is charged with a different sign due to the extremely unstable grounding resistance is not only accidental but also inevitable. Contingency is difficult to predict the rupture of the lubricant film in a certain moment; the inevitability is that when the bearing rotates rapidly, the phenomenon of instantaneous rupture of the lubricant film always occurs. It also determines that if static electricity measures are not properly applied in coating production, the generation of electrostatic sparks is both accidental and inevitability.

It must be pointed out that the iron core (including the following metal guide rollers) inside the rubber pressure roller forms an electrical equivalent with the earth through the grounding line, which means that under the action of the electric field, the earth can transmit the same number to the iron core. Inductive charge electrical system.