Static charges are generated by electron transfer when non-conductive surfaces contact another surface and are then subsequently separated from that surface. The amount of charge generated and held is determined by a number of factors, including intimacy of contact, speed of movement and separation, friction at the surface, and the nature of the materials and their surroundings. There are several variations of the triboelectric series table, which ranks various materials as to their tendency to develop a negative or a positive charge, and the relative ease with which the charge is generated. There are many indications, however, that charge development is not so simple and that charges are uneven in magnitude and may even be positive at one point on the surface, and negative at another. Static charges may also be irregular in occurrence, developing at one time but not at another.
Several approaches to the elimination or reduction of static charge have been used over the years. These include using conductive components in the part or component, extensive use of grounding, ionization of air or generation of ions in the air near the part or component, surface lubrication of the part, use of antistatic additives, and others. Each of these seems to have its place in the industry, helping to a greater or lesser extent, and at a greater or lesser cost and difficulty. For our purposes here, we will discuss only the traditional antistatic additives.
Traditional antistatic additives fall into two classes. One is made up of materials that are added to the polymer during processing and subsequently migrate to the surface where they attract moisture from the air and hold it on the surface. To the extent that they migrate evenly and spread out on the surface, forming continuous pathways, the water can then provide routes for dissipation of the charges, or at least the spreading of the charges to a more diffuse state. Such materials are called internal antistats, and a number of chemical classes are used, depending upon the plastic involved.
The majority of the products used are di-hydroxy species based on derivatives of fats and oils. These include N,N-diethoxy amines, diethanolamide products, and glycerol monoesters. Some non-ionic ethoxylated species have found success in certain polymers, and other polar species such as sulfonic acid salts and quaternary ammonium compounds have been used.
In theory, a species must be incompatible enough with the polymer and low enough in molecular weight to migrate, be polar enough to attract and hold moisture, and yet must form structures that cover the surface instead of concentrating into micelles or other disjointed structures. In practice, most successful antistats seem to rely on the organic crystalline tendencies of fatty structures to achieve the balance of properties needed. These longer carbon chains also produce the limited volatility needed to maintain the surface coverage.
The second class is topical antistats, which are generally applied to the surface as a solution. The effect is generally more temporary. All of the chemical classes used as internal antistats, plus other polar or hygroscopic species, can be used in this way if they are dispersible in water or water/alcohol systems, but the quaternary ammonium compounds seem to be the most frequently used. The performance of topical antistats is essentially independent of the polymer being used since the film of antistat and water is established on the surface by spraying or dipping. Wiping or washing the surface generally removes most of the antistatic properties. When internal antistats are used, there is a reservoir of material within the polymer structure that can migrate to the surface to replace that which is wiped off (to an extent), but this cannot occur with topical antistats.
In either case, the relative humidity of the atmosphere to which the product will be exposed can have a great effect on the performance of the additive. At relative humidities above 50%, all of the hydroxyl-containing species are quite effective, but at levels below 15% relative humidity, the ethoxylated amines are more effective.
Antistatic agents are generally used at relatively low levels, with considerable variation depending on the type of anti stat and the polymer. The required performance is also a major factor in determining the amount that will be required. Use levels for internal antistats generally range upwards from around .05 % at the low end, with concentrations above 1% being relatively uncommon except among hard-to-treat polymers such as polystyrene.
There can be interferences between antistatic agents and other additives, although it has been found that slip agents and antistatic agents usually work together on the surface in relative harmony. Surface resistivities achievable with this type of additive are generally limited to a range of about 109 to 1012.
There are several methods of testing the effectiveness of antistat agents. The most common of these are surface resistivity measurement (where the ability of a voltage to flow across the surface is measured), static decay (wherein a charge is generated on the surface and a rate of reduction upon grounding is measured by a noncontact voltmeter), or some form of a tendency to pick up a dust is estimated (using a standardized dust or soot). The tests vary in terms of the specified voltages, means of generating the charges, the relative humidity of the surrounding atmosphere, conformation of the samples tested, the acceptable level of performance, and other factors. In some cases, test conditions and methods have been set for particular industries and uses, and the user must become familiar with the most current of these before reasonable evaluation of antistatic additives can be done.
Selection of the antistatic agent to use can range from quite easy to very difficult. There are certain "standard" uses, such as glyceryl monostearate for mild requirements in polypropylene food contact articles at higher relative humidities. In cases of polymers that will experience incidental food contact, the allowable sanctions of the FDA may govern which additive is chosen to an even greater extent than the desired performance. Many PVC applications require engineering the entire stability package to provide the necessary stability plus an antistatic effect.
Some polymers are simply difficult to treat, either because of the solubility of the additives in the polymer matrix or the nature of the surface.
Additive manufacturers and expert formulators can provide guidance as to suitable additives for a particular polymer and use of that polymer, but uses that go beyond the standard may require substantial research programs to find the optimum package. The information set forth here, it is hoped, will provide a conceptual basis for beginning such work.
Thomas E. Breuer, former director of production technology, Chemtura, Business Group [email protected]