Plastics are inherently insulative (typical surface resistivities in the range of 1012 to 1014 om/square) and cannot readily dissipate a static charge. The primary role of an antistatic agent or antistat is to prevent the buildup of static electrical charge resulting from the transfer of electrons to the surface.
This static electricity can be generated during processing, transportation, handling, or in final use. Friction between two or more objects (for example, the passage of copy paper over a roller) is usually the cause of static electricity. Typical electrostatic voltages can range from 6000 to 35,000 V.
When the unprotected plastic is brought into contact with another material, loosely bound electrons pass across the interface. When these materials are then separated, one surface has an excess charge, while the other has a deficiency of electrons. In most plastics the excess charge will linger or discharge, causing the following problems:
- Fire and explosion hazards
- Poor mold release
- Damage to electrical components
- Attraction of dust
Antistats function to either dissipate or promote the decay of static electricity. Secondary benefits of antistat incorporation into polymer systems include improved processability and mold release, as well as better internal and external lubrication.
Therefore, in certain applications, antistatic agents can also function as lubricants, slip agents, and mold release agents.
Today, we will focus on chemical antistats as it is an important part in antistatic additives.
Chemical antistatic additives can be categorized by their method of application (external and internal) and their chemistry. Most antistats are hygroscopic materials and function primarily by attracting water to the surface. This process allows the charge to dissipate rapidly.
Therefore, the ambient humidity level plays a vital role in this mechanism. With an increase in humidity, the surface conductivity of the treated polymer is increased, resulting in a rapid flow of charge and better antistatic properties. Conversely, in dry ambient conditions, antistats which rely on humidity to be effective may offer erratic performance.
External, or topical, antistats are applied to the surface of the finished plastic part through techniques such as spraying, wiping, or dipping. Since they are not subjected to the temperatures and stresses of plastic compounding, a broad range of chemistries is possible.
The most common external antistatic additives are quaternary ammonium salts, or “quats,” applied from a water or alcohol solution.
Because of low-temperature stability and potential resin degradation, quats are not normally used as internal antistats. However, when topically applied, quats can achieve low surface resistivities and are widely used in such short-term applications as the prevention of dust accumulation on plastic display parts.
More durable applications are not generally feasible because of the ease with which the quat antistat coating can be removed from the plastic during handling, cleaning, or other processes.
For longer-term protection, internal antistats are used.
Internal antistats are compounded into the plastic matrix during processing. The two types of internal antistats are migratory, which is the most common, and permanent.
Migratory antistats (MAS)
Migratory antistats have chemical structures that are composed of hydrophilic and hydrophobic components. These materials have limited compatibility with the host plastic and migrate or bloom to the surface of the molded product.
The hydrophobic portion provides compatibility within the polymer and the hydrophilic portion functions to bind water molecules onto the surface of the molded part.
If the surface of the part is wiped, the MAS is temporarily removed, reducing the antistat characteristics at the surface. Additional material then migrates to the surface until the additive is depleted. These surface-active antistatic additives can be cationic, anionic, and non-ionic compounds.
Cationic antistatic are generally long-chain alkyl quaternary ammonium, phosphonium, or sulfonium salts with, for example, chloride counterions. They perform best in polar substrates, such as rigid PVC and styrenics, but normally have an adverse effect on the resin’s thermal stability. These antistat products are usually not approved for use in food-contact applications.
Furthermore, antistatic effects comparable to those obtained from other internal antistats such as ethoxylated amines are only achieved with significantly higher levels, typically, five to tenfold.
Anionic antistats are generally alkali salts of alkyl sulfonic, phosphonic, or dithiocarbamate acids. They are also mainly used in PVC and styrenics. Their performance in polyolefins is comparable to cationic antistats. Among the anionic antistats, sodium alkyl sulfonates have found the widest applications in styrenics, PVC, polyethylene terephthalate, and polycarbonate.
Nonionic antistats, such as ethoxylated fatty alkylamines, represent by far the largest class of migratory antistatic additives. These additives are widely used in PE, PP, ABS, and other styrene polymers. Several types of ethoxylated alkyl amines that differ in alkyl chain length and level of unsaturation are available.
Ethoxylated alkylamines are very effective antistatic agents, even at low levels of relative humidity, and remain active over prolonged periods. These antistatic additives have wide FDA approval for indirect food contact applications.
Other nonionic antistats of commercial importance are ethoxylated alkylamines such as ethoxylated lauramide and glycerol monostearate (GMS). Ethoxylated lauramide is recommended for use in PE and PP where immediate and sustained antistatic action is needed in a low-humidity environment.
GMS-based antistats are intended only for static protection during processing. Even though GMS migrates rapidly to the polymer surface, it does not give the sus- tained antistatic performance that is obtainable from ethoxylated alkyl amines or ethoxylated alkylamides.
The optimum choice and addition level for MAS additives depends upon the nature of the polymer, the type of processing, the processing conditions, the presence of other additives, the relative humidity, and the end-use of the polymer.
The time needed to obtain a sufficient level of antistatic performance varies. The rate of buildup and the duration of the antistatic protection can be increased by raising the concentration of the additive.
Excessive use of antistats can, however, lead to greasy surfaces on the end products and adversely affect printability or adhesive applications. Untreated inorganic fillers and pigments like TiO2 can absorb antistat molecules to their surface, and thus lower their efficiency. This can normally be compensated for by increasing the level of the antistat. The levels of antistat for food- contact applications are regulated by the U.S. Food and Drug Administration (FDA).
Permanent antistats. The introduction of permanent antistats is one of the most significant developments in the antistat market. These are polymeric materials which are compounded into the plastic matrix. They do not rely on migration to the surface and subsequent attraction of water to be effective. The primary advantages of these materials are
- Insensitivity to humidity s Long-term performance
- Minimal opportunity for surface contamination
- Low off-gassing
- Color and transparency capability
There are two generic types of permanent antistats: hydrophilic polymers and inherently conductive polymers. Hydrophilic polymers are currently the dominant permanent antistats in the market.
The most common resins are ABS and high-impact poly- styrene (HIPS).
Another approach to achieving permanent antistatic properties is through the use of inherently conductive polymers (ICP). This technology is still in the early development stages.
The potential advantages of ICP include achieving higher conductivity in the host resin at lower additive loading levels than can be achieved with hydrophilic poly- mers. The principal ICP technology to date is polyaniline from Zipperling-Kessler and Neste.
This material is a conjugated polymer composed of oxidatively coupled aniline monomers converted to a cationic salt with an organic acid and is frequently described as an organic metal.
Other approaches to ICPs include neoalkoxy zirconates from Kenrich Petrochemical and polythiophenes from Bayer. The issues to be resolved in achieving commercial success with these materials include improved stability at elevated temperatures and reduction in their relatively high cost. ICPs are not expected to compete with other chemical additives but primarily with carbon black or other conductive fillers.
Permanent antistatic properties can be readily obtained with such particulate materials as carbon black. However, these materials are inappropriate for applications where color and/or transparency capability is important. Also, particulate additives can negatively affect the physical properties of the final part and contribute to contamination in electronic applications also known as sloughing.