Antistatic Agents

Abstract

Electrification is the process of producing an electric charge on an object. If the charge is confined to the object it is said to be electrostatic. The term static electricity refers to accumulated, immobile electrical charges, in contrast to charges in rapid flow, which is the subject of electrodynamics. Static is a surface phenomenon. When two surfaces are brought into contact, electrons pass across the interface in both directions. This exchange is not symmetrical, and even with two identical bodies one of the two acquires an excess of electrons at the expense of the other. When the surfaces are separated, part of the charge is discharged into air, and the remainder is left on the material. One of the two bodies has an excess of electrons and the other has a shortage of electrons. The magnitude of the charge depends on the rate of separation of the two bodies. The electrons in a conductor move freely; thus, the excess of electrons is eliminated by backflow. However, the electrons in an insulator are not mobile, and the phenomenon of static electricity thus becomes noticeable. The amount of the electrostatic charge developed on a material represents a balance between the rate of generation and the rate of dissipation. Good conductors disperse a static charge quickly; however, textiles and plastics have a high surface resistivity and charge decay can occur only at a low rate. It is not possible to completely eliminate the generation of a static charge, so measures must be taken to increase the rate of charge dissipation. Increasing the electrical conductivity of the material by increasing its electrolytic conductivity, whether through increasing the moisture content of the surrounding atmosphere (humidification), by application of internal or external antistatic agents, or by chemically modifying the material, is a means of controlling static charges. Materials that have the ability to dissipate a charge formed by any means including tribocharging and induction on the material are referred to as static dissipative. The term antistatic agent is generally used to describe a substance that is added to a material to make that material static dissipative. Textiles and plastics are the two biggest markets for antistatic agents. Antistatic finishes are defined as antistatic agents in combination with water, mineral oil, composite finishes applied by producers during fiber manufacture, oils used to facilitate throwing or coning, textile softeners or lubricants, and hand building compounds. The terms antistatic agents and antistatic finishes have often been used interchangeably. Antistatic agents can function either by reducing the generation of charge, by increasing the rate of charge dissipation, or by both mechanisms. Most antistatic agents operate by increasing the rate of charge dissipation. In general, most antistatic agents belong to one of the following classes: nitrogen compounds such as long‐chain amines, amides, and quaternary ammonium salts; esters of fatty acids and their derivatives; sulfonic acids and alkyl aryl sulfonates; polyoxyethylene derivatives; polyglycols and their derivatives; polyhydric alcohols and their derivatives; phosphoric acid derivatives; solutions of electrolytes in liquids with high dielectric constants; molten salts; metals; carbon black; semiconductors; and liquids with high dielectric constant, such as water, which are usually volatile and have a temporary effect only. Most surface‐active agents reduce static properties of materials to which they are applied and are classified as nondurable antistatic agents. They appear to work on the basis of moisture absorption, and their effectiveness decreases as the humidity and temperature in the atmosphere are reduced. The difficulty with nondurable finishes, as far as the consumer is concerned, is that they are water‐soluble and thus easily removed by washing. An effective antistatic finish must be durable and capable of withstanding repeated laundering and dry‐cleaning cycles. Only a small number of durable antistatic agents are available for textiles. Commercial durable antistatic finishes include Aerotex Antistatic D, Aston 123, a polyamine resin, Permalose T, a nonionic agent, and Stanax 1166, a polyamine resin. Plastics are excellent insulators and have a great tendency to generate and retain static charges. Antistatic agents may be applied to the surface of the finished article or incorporated in the bulk of the polymer during processing. They function by decreasing the rate of charge generation, by increasing the rate of charge dissipation, or by both mechanisms. Types of antistats include inherently conducting polymers, metal‐containing polymers, carbon blacks, surfactant‐type antistats, and surface‐applied surfactants. Antistat agents can be applied directly to the surface of a plastic part. Usually the antistat is diluted in water or in a solvent. The antistatic protection provided by surface treatment is excellent while it lasts. However, surface treatments provide only temporary protection. Antistatic agents are widely used in the plastic industry, where their economic significance is greater than in the textile industry. Food and drug packaging accounts for well over half the market, and FDA‐acceptable products such as polyethylene, poly(vinyl chloride), and polypropylene film and bottles are predominant. The use of antistatic agents in medical and surgical applications of flexible poly(vinyl chloride) and polyethylene film and sheet has increased, providing higher safety in places where a buildup of static charges can trigger explosions. The use of antistatic agents in plastics used for the electronics industry has grown to the point that virtually all of these plastic products contain some type of antistatic agent.