How to prevent static electricity in the coating production process (below)

2. In coating production, in addition to the large amount of static electricity generated by the rubber roller, which endangers the safety of personal equipment, the processed paper or plastic film also generates and accumulates a considerable amount of static electricity in the manufacturing and adding of the 3232 art. If it cannot be effectively eliminated, static electricity is also generated. The possibility of sparks is one of the reasons that jeopardizes safe production. According to the momentary situation of coating production, the following analysis was performed:

1) If there is a small amount of static charge g1 somewhere in the PET coating at a certain moment, the distance r from a metal guide roller is still far. Assuming 0.4m, at this time, the electric field strength E1 at the guide roller is still low, and the electric equivalent formed by the guide roller through the ground line and the earth is limited by the electric field, and the induced charge cannot be overcome by the ground resistance. The guide roller surface is still No induced charge, no spark will be produced.

2) When the next moment, this small amount of static charge z1 from the guide roller distance r is very small, such as 0.04m, by formula (2) we can see, the electric field strength E2 sharply increased, because the distance is greatly reduced, r22/r22 = 100, That is E2 II 100E1. At this time, due to the significant function of the electric field, the same amount of inductive charge is transferred from the earth to the guide roller surface through the ground resistance, but the static charge quantity g1 is low, and the attractive force between the positive and negative charges is limited, and the positive and negative charges cannot be generated. And phenomenon, there is still no spark.

However, if the amount of electric charge g1 accumulated in the PET film is high, the electric field intensity is greatly increased and the amount of induced electric charges is also increased by the same amount. According to Coulomb's law, the large attraction between the positive and negative charges overcomes the resistance of the medium (air), the charge is neutralized, a large amount of heat energy is released, and the electric spark is generated.

It can be seen that when the PET film is electrostatically charged during coating production, there are three factors that determine whether the spark is generated. One is the high static charge, which is the decisive factor. The second is the distance of the static charge from the guide roller, that is, the distance between the positive and negative charges, that is, the r value in the electric field strength. Thirdly, in the coating production, PET rapidly passes through many guide rollers, and the minimum r value appears at each moment. The problem is that when the minimum r value is reached, the lubricant film of the guide roller is broken, is there an equal number of signs? Induced charge generation. If the lubricant film is intact, the resistance is high, an effective grounding equivalent cannot be formed, and no inductive charge is present, no spark will be generated even if the PET film has a large amount of static charge. Only when the lubricant film breaks, there is an electric spark generated by the induced charge. This also shows that the generation of electric sparks in coating production is both inevitable and accidental. Of course, if the electrostatic charge is high, the breakdown of the lubricant film's impedance under strong electric field will also be extremely strong. EDM generated.

Elimination of electrostatic charge accumulation methods and conditions in coating production

Similar to (b), when the PET film with a small amount of electrostatic charge is brought into contact with the guide roller, the pitch is small and the base opening r is almost zero, and the roller surface has an equal amount of induced charge. From Coulom's law r→0, 1/r tends to infinity, attraction between positive and negative charges is sufficient to generate positive and negative charge neutralization, static electricity is eliminated, and the released energy is in a dispersed state, far reaching the ignited detonating combustible gas. The lowest energy, this is a safe and neutral way.

From the microscopic view, the neutralization of a small amount of static charge in a dispersed state is a negative charge (electron) returning to a positively charged molecule (cation) orbital, making the charged material particles (molecules) electrically neutral. The energy produced by this neutralization is so small that after the negative charges (electrons) are returned to the molecular orbitals, the remaining energy released is minimal and dispersed. Therefore, this neutral-neutral charge neutralization process is safe.

The conditions for eliminating the accumulation of static charge are: the static charge is very low, and the carrier (paper, plastic film or rubber layer of the pressure roller) is in close contact with the inductive charge carrier (conductor such as guide roller or static electricity roller, static brush, etc.). That is, under the condition of r→0, the induced charge carrier can generate an induced charge under the action of an electric field with a small amount of electrostatic charge (a low electric field intensity), which requires a very low and extremely stable grounding resistance.

Technical measures to eliminate static electricity accumulation and eliminate static sparks

Under normal circumstances, that is, when the plastic film of PET is not added with an antistatic agent during the manufacturing process, static electricity generation in coating production is unavoidable.

To ensure safety, static electricity must be eliminated in a timely manner with a minimum amount of positive and negative charge neutralization. To this end, there must be a very good grounding facility to form an effective electrical equivalent with each part of the body. As long as there is a very small amount of static electricity, it can immediately transfer the induced charge from the earth. Under the condition that the positive and negative charge carriers are in close contact, that is, the value of r tends to zero, a positive and negative charge neutralization phenomenon occurs, so that it is possible to prevent the accumulation of static electricity and thereby prevent the generation of electric spark.

Based on the foregoing discussion, the following simple, easy, and effective technical measures are proposed:

1. According to the analysis of electrostatic spark generation process in coating production and the author's working experience, a special grounding wire of “strong electrolyte-infiltrated copper ground electrode” can be used, and the grounding resistance is very low and the grounding condition is very stable. This measure has been corroborated by relevant data.

2. Each guide roll surface, especially the coating head area and the guide roller surface in the oven, must be directly contacted with a dedicated ground wire to ensure that the grounding condition is very stable.

3. Rubber roller remove static electricity measures:

1) The surface of the rubber pressure roller must be in close contact with the static elimination roller for metal. The roller surface is in direct contact with the dedicated grounding wire.

2) Electrostatic carbon conductive brush can be used instead of static elimination roller, but conductive brush should be cleaned frequently to ensure extremely low and stable grounding resistance;

3) According to Coulomb's law and electric field strength, as long as the process allows, as much as possible to increase the pressure of the rubber layer step, can effectively increase the E = g / 4r2 r value, reduce the electric field strength and the positive and negative charge between the attraction Force helps to reduce the possibility of electrostatic sparks.

(author/ Ren Shengshu)