Antistats: Types, Attributes & Test Standards
Antistats (A/S) are divided into two general categories—external and internal. External antistats are topical agents applied to the surface. For topical antistats, washing the surface completely removes the external antistat, thus requiring reapplication.
All the traditional internal chemical antistats are migratory additives, which are further divided into anionic, cationic (or just “ionic”) and nonionic types. Ionic antistats are recommended for polar resin systems such as PVC. Generally, ionic antistats are not recommended for polyethylene, due to inherently low heat stability. Nonionic antistats are organic compounds composed of both a hydrophilic and a hydrophobic portion. The compound migrates to the substrate surface and, via hydrogen bonding with atmospheric water, creates a microscopic layer of water on the surface. Chemical antistats are therefore dependent upon atmospheric moisture for their mechanism to dissipate static electricity.
Internal antistats can be inert, conductive fillers (e.g. conductive carbon black, metallized fillers and carbon fibers). Recently, new clear polymeric antistats have been developed which are non-migratory and provide antistatic properties independent of atmospheric humidity. The non-migratory antistats form a percolating network similar to conductive carbon black, and therefore fairly high loadings of these nontraditional antistats are required to ensure good antistatic properties in polyolefin films. The function of certain non-migratory antistats is humidity independent. These products may be recommended for use in multilayer films for cost-effectiveness.
Types of Antistats
Three general types of antistats are used in polyethylene and polypropylene: Glycerol monostearate (GMS); ethoxylated fatty acid amines; and diethanolamides.
The type of antistat used is determined by resin system, test specifications, FDA restrictions and end use application.
Influences On Antistatic Behavior
After migrating to the surface, antistats interact with atmospheric moisture, forming a microscopic layer of water on the substrate surface. This layer of water is held in place by hydrogen bonds. As the relative humidity changes, so does the water layer on the substrate surface. At low humidity, less moisture is available to form hydrogen bonds with the antistat than is available at higher humidity.
Since the water layer provides the conductive path for static dissipation, conditioning is essential for accurate comparisons of antistatic performance. Although two antistatic agents might perform well at 50% relative humidity, their performances can change drastically at 12% relative humidity. The dependent nature of relative humidity and antistatic performance mandates test procedures specifying a conditioning period, to ensure samples reach equilibrium before testing. Conditioning improves reproducibility of results between test facilities.
Just like slips, antistats also achieve an equilibrium level on the substrate surface. The remaining antistat below the surface acts as a reservoir. When surface antistat is removed, it is replaced by antistat from within this reservoir. Repeated washing eventually depletes the antistat within the substrate.
Since antistats are surface active, they compete with other surface active additives such as slips in film applications. These additives do not react chemically with each other, but rather, can compete with each other for occupation of the substrate surface.
Amine and amide antistats are basic in nature. They can react with acidic additives. Halogenated flame retardants are particularly reactive with antistats. Acidic blowing agents can also react with antistats and might require modifications to standard formulations.
Resin crystallinity also influences antistatic performance. Crystallinity appears to affect the ability of an antistat to migrate through the resin matrix. The higher the crystallinity, the more difficult the migration.
- LDPE and LLDPE behave similarly with respect to antistats. Diethanolamides typically provide the best performance in LDPE and LLDPE.
- Amine antistats appear to outperform GMS and diethanolamide antistats in HDPE.
- The presence of the polar functional groups in resins, such as EVA & EMA, appear to improve the compatibility of the polar antistats in these resins, limiting their migration and antistatic performance. The antistatic levels may need to be increased with such polar resins, or when they are blended with polyolefins.
Several factors influence antistatic performance. The end application dictates the level of performance the substrate must provide. Resins, slips and other additives influence antistat behavior. Test standards allow antistatic performance to be accurately compared and communicated between different laboratories.