Electrostatic discharge and arc discharge

Most molecules in the air don't have any net charge. If you ignore the moisture, air will be a very ideal insulation object. However, in a relatively large volume of space, the air usually contains some charged molecules (ie, ions). This is because the molecules in the atmosphere are affected by cosmic rays, ground radiation, airborne pollutants, thermal energy, or other factors, and their electrons are knocked out so that the remaining molecules are electrically charged and form ions. Under strong electric field, these ions are accelerated and collide with other molecules, resulting in more ions. If there is a strong electric field in the vicinity of a charged object (such as thin wire, nails), an electron cloud will form at the tip. When a charged object approaches the electric field, it attracts ions that can neutralize its own charge, so we also use this to eliminate the charge on the web. If there is a strong electric field between two charged objects, an avalanche of ions will form between these two objects. If these two objects are good conductors, the charge will quickly flow to the place where the charge is released. At this location, the area is small, all the charge will be concentrated here, and finally the tip discharge will occur. The tip discharges the air on the path at the same time. The high temperature in turn causes the thermal ionization of the air and the formation of additional ions. These ions make air a good conductor. As a result, a highly dense, fast, and high-temperature discharge arc is formed which is sufficiently hot to ignite the mixture of solvent vapor and air. We know that in addition to the resistance of dry skin, people have a good conductor. Therefore, all metal parts that come in contact with humans must be grounded. Workers must wear necessary protective shoes to prevent the accumulation of electric charge. All insulating materials on the floor of the workplace must also be removed. Discharges between poor conductors or discharges between good conductors (such as printer metal parts) and poor conductors seldom produce thermal arcs. The object resistance determines how much charge can flow to the discharge area, how fast the charge can be released, and how large the arc discharge can generate heat. Necessary note: The insulating material is not conductive, but its surface will be conductive due to deposits or moisture. For composite materials, especially composites in which conductive particles are incorporated in the insulating material, they can become conductive objects under strong voltage. Some carbon-containing rubber compounds, some dry black inks, and metallic inks belong to this category. Burning Limits and Ignition Energy Burning Points and Lower Explosion Limits In the air above the solvent liquid or liquid ink, the concentration of solvent vapors increases with increasing temperature. We usually use the percentage (volume), parts per million (ppm) or steam pressure to represent the concentration of volatile gases in the air. The ignition point of the solvent is defined as the temperature at which the concentration of the volatile gas in the solvent liquid in the air rises to a certain extent and the mixture of the volatile gas and air can be ignited. Now we will discuss the air pressure and temperature of the printing room and the problem of solvent burning under normal conditions. When the temperature of the solvent liquid or ink in contact with air-solvent vapor reaches or exceeds the flash point, the mixture of the volatile gas and air of the ink or solvent may be ignited. In air-solvent vapor mixtures, solvent volatile components can ignite after reaching a certain concentration. This range is the upper and lower limits of the flammable object. This upper and lower limit of combustion is also often referred to as the upper explosion limit (UEL) and the lower explosion limit (LEL). When the solvent vapor concentration exceeds the upper explosive limit (UEL), the solvent vapor content is too high to burn; however, when the solvent vapor concentration is lower than the lower explosive limit (LEL), it is difficult to burn because the solvent concentration in the mixture is too low. This means that after the amount of air in the mixture exceeds that required for normal combustion, excess air needs to absorb heat and make the mixture difficult to burn. Although the special odor of some solvents can be smelled in the air, this does not mean that the air is near to burning. Most gravure printing solvents, although strong in terms of taste, are far below the OSHA-established PEL. The PEL is far below the Lower Explosive Limit (LEL). During the operation of the printing press, the dryer exhaust unit maintains the concentration of the solvent vapor in the dry air at a safe level, and the concentration is only a fraction of the lower explosion limit. During the reel and cleaning process, if you do not open the exhaust fan, do not use a dryer to dry the web containing ink or solvent. In printing, the flammable air-solvent vapor mixture usually accumulates on top of the solvent or ink, and the ink fountain, the ink tank, the ink cylinder, and the web near the web are particularly careful. Solvent-containing saturated air is generally heavier than normal air. When the ink fountain lid is opened, saturated air will overflow and accumulate on the floor of the printing shop. Afterwards, the saturated solvent vapor is diluted by a large amount of normal air between printings. The concentration is reduced and the specific gravity becomes slightly heavier than that of clean air. The solvent vapors then diffused throughout the print shop, causing the shop to have a strong solvent odor that could exceed OSHA's human tolerance limits. Therefore, in order to avoid this dangerous situation, the ink fountain and ink tank cover should be closed. Ignition Energy When the concentration of air-solvent vapor mixtures falls between the lower and upper explosive limits, they become combustible gases. The type of solvent vapor and the concentration of solvent vapor in the air determine the minimum electrostatic discharge energy to ignite them. The relationship between the ignition energy of heptane and the concentration of solvent vapor in air can be obtained through experiments. Igniting solvents with higher auto-ignition temperatures, such as ethyl alcohol, high alcohol, acetate, etc., or polar solvents with lower heat of combustion requires a lot of ignition energy. It should be noted that many ignition experiments are carried out under critical conditions, ie at this concentration the gases are most easily ignited. Its electrostatic discharge is also at its peak, so the arc is very hot. In addition, in standard tests, thermal diffusion due to air movement or large metallic heat sinks, such as gravure cylinders, is often not considered. Ink or solvent mist, even when the temperature is lower than the ignition point, can burn, but they ignite energy far higher than the ignition energy of their flammable air-steam mixture.