By virtue of their insulating nature, polymers of all types allow static charge to build-up on their surfaces, particularly in the case of films and fibers, which have large surface area to volume ratio. Such static charge build-up leads to several undesirable consequences in the final product. For instance, built-up static charge can attract dust onto a food package, which is undesirable aesthetically. At times, static charge can damage circuit boards in electronics packaging and may cause hazards of fire or explosion in the vicinity of flammable materials.
The static charge may also cause several processing problems such as difficulty in winding of films or melt-spun fibers, agglomeration of powders during transport, adhesion of films during processing, etc. Such undesirable effects of static charge build-up can be avoided by using masterbatches containing antistatic agents. The most commonly used are those compounded internally with the polymer and those applied topically.
The traditional internal antistats are migratory in nature. Typically these migratory antistatic agents have a hydrophobic organic end and a hydrophilic end. The strongly polar hydrophilic end adsorbs water molecules which eliminate static charges by ionic conduction. The long hydrocarbon chain length constitutes the hydrophobic group and controls the rate of diffusion (or migration) of the antistat to the surface of the polymer product.
There are three major classes of internal, migratory antistatic agents: esters, amines and amides. Glycerol mono-stearate is commonly used as an antistatic agent for PE and PP but is useful for only short term antistatic performance of about 1-2 months. The amines and amides are usually ethoxylated products and are more useful for long term antistatic performance. The amine type antistats tend to react with polycarbonate, which is commonly used in making electronic circuit boards. Hence a plastic packaging containing amine antistat is detrimental to electronic circuit boards. The amine antistats have limited FDA approval. However, the amines are extremely effective as antistatic agents. The amides also have limited FDA approval but broader than the amine antistats. The esters, on the other hand, are FDA approved at any level of concentration.
The performance of the migratory antistats depends on various factors such as:
1. Concentration: The higher the concentration of the antistat, the antistat migration is faster and usually the performance is better.
2. Relative humidity: All migratory antistats depend on water for functioning. Hence the antistatic performance is always better at higher humidity.
3. Conditioning time: The migratory antistats diffuse through the polymer matrix and have to come to the polymer surface in sufficient concentration to be effective. It requires time for the antistats to achieve a monolayer surface coverage and to attain equilibrium between the surface and the bulk concentration. Hence the antistatic performance usually improves with conditioning time. In the case of LDPE & LLDPE, it is considered that two days of conditioning time is adequate to get a monolayer of surface coverage for all migratory antistats.
5. Antistat chemistry: The size and the shape of an antistat molecule itself dictates its rate of diffusion through a given polymer matrix. Hence the rate of diffusion of the antistat is dependent on its chemistry. In addition, certain antistat chemistries tend to crystallize once they are on the film surface in sufficient concentration. It is believed that such crystallization of the antistat leads to loss of its antistatic properties. This process depends on the unique mixture of chainlengths in a given antistatic additive. Hence certain antistatic additives lose their antistatic properties more quickly than others and are useful only as “short term antistats”. All of the GMS chemistries are susceptible to such a process.
6. Type of polymer: The polymer used for making films (or other extruded parts) has a major influence on the diffusion of the antistatic additives. The crystallinity and the polarity of the polymer are the key properties that influence the behavior of the antistatic additives. A polymer with high crystallinity (such as HDPE or PP) creates a tortuous path for the diffusion of the antistat, thus retarding the rate of diffusion. In general, all migratory antistats require a very long time to diffuse out of HDPE or PP homopolymer. Similarly, a polymer with polar groups (such as EVA, EMA, Surlyn, Nylon, PET) has chemical interactions (mainly hydrogen bonding) with the polar groups of the antistat molecule. Such interactions also decrease the rate of diffusion of the antistat.
7. Presence of other additives: There are three types of influences from other additives on the behavior of antistats. (a) If the other additives are also migratory, they compete with the antistats for diffusion through the polymer matrix and also compete with the antistats for surface coverage. Slips are a common example of this type of additive, which may exert adverse influence on antistatic properties. (b) Some additives tend to have chemical interactions with certain types of antistat chemistries. Amine and amide type antistats are basic (alkaline) in nature, which may react with some acidic flame retardants, which can result in reduction of antistatic properties. (c) Some antiblocks such as synthetic silica tend to adsorb antistatic additives on their surface, due to their high surface area. Such adsorption will also retard or prevent the migration of the antistats to the surface of a film.
8. Corona treatment: The treatment of a film surface with corona discharge results in oxidation or ‘burning’ of the surface layer of the film. Such corona treatment usually accelerates the migration of the antistats to that side, by increasing the concentration gradient and also by making that surface more polar.
9. Lamination: It is very common in the polyolefins industry to laminate antistatic films by means of adhesive lamination. The most commonly used adhesives are polar chemicals, which attract antistatic additives and interact with them. Once the antistatic additives migrate from the sealant PE side to the adhesive layers, they usually do not come out and antistatic properties on the PE side are lost forever. In some cases, extrusion lamination is carried out by high temp. processing and cross-linking of the PE layer to a polar substrate such as nylon or PET. These polar substrates also attract antistatic additives which results ultimately in loss of antistatic performance in the PE layer.
10. Winding tension: The rate of diffusion of an antistat in a film wound on a roll is much slower than an unwound film. The unwound film offers a large surface for the antistat to diffuse and such surface area is not available on a tightly wound roll. Hence the higher the winding tension, the slower the antistat diffusion.
11. Gauge of film: Higher gauge or thicker films will require more time to achieve equilibrium between the surface and the bulk concentration of the antistat. But thicker films also have more mass of an antistat than thinner films. Therefore, thicker films usually require slightly lower quantity of an antistatic additive than thinner films, to achieve the same antistatic performance. However, among all the factors listed above, the gauge of a film seems to exert the least influence on the antistatic properties.
Recently Ampacet has developed unique, nontraditional antistatic products based on a nonmigratory antistat which have been designed for use only in skin layers of multilayer films. These are clear products based on polymeric antistatic chemistry, which does not depend on atmospheric humidity for functioning. The antistatic additive forms an interconnecting or percolating network (similar to conductive carbon black) and dissipation of the static.
Charge occurs by an ionic conduction mechanism. As a result, much higher loadings of this product are required, as compared to the traditional migratory antistats, to achieve good antistatic performance. Hence these products are recommended to be used only in skin layers of multilayer films.
The major benefits of non-migratory antistatic masterbatches are as follows:
- No conditioning required .. the antistatic properties obtained immediately off-line
- Useful in multilayer films, which require no migration to the other side.
- No migration into adhesive layer or nylon/PET layer in case of laminated films.
- No adverse effect on heat sealing and printing (in contrast with migratory antistats)
- Antistat properties last theoretically for the life of the film.
- Used only in a skin layer for a multilayer film (not needed in the core, like migratory additives).
- Clear product • Able to meet NFPA-99 and Mil-B-81705C criteria.
- High thermal stability hence can be used in blown and cast films
Measurement of Antistatic Properties
Antistatic performance of a product is measured by means of two properties-static decay time and surface resistivity, which are defined as follows:
Static decay time (or charge decay time):
It is the time required to dissipate a certain fraction of an applied 5 kV static charge and is measured in seconds. Most commonly, static decay times are measured for 90% dissipation or 99% dissipation of the applied 5 kV charge. These measurements are referred to as 10% cutoff (90% dissipation, from 5 kV to 0.5 kV) and 0% cutoff (99% dissipation, from 5 kV to 0.05 kV). If a sample does not pick up the applied 5 kV static charge then it has no antistatic performance. In case of migratory antistats, the static decay times are not dependent on the direction of testing. However, in the case of non-migratory antistats, it has been found that the static decay times are dependent on the direction of testing. In the case of blown films, the non-migratory antistats yield much lower static decay times in machine direction (MD) than transverse direction (TD).
It is simply the resistance of a product at its surface and is measured in Ohms. The measurement of surface resistivity is somewhat related to the geometry of the probe. If a product has surface resistivity in the range of 109 to 1013 Ohms, it is considered antistatic. A sample having surface resistivity > 1014 Ohms is considered insulative. Typically, PE and PP have surface resistivities in the range of 1015 to 1016 Ohms. The surface resistivity is not direction dependent for migratory as well as non-migratory antistats. In films containing non-migratory antistats, we have observed that the surface resistivity values at very low humidity such as 12% RH are very similar to those at 50% RH.
Specifications: The antistatic performance of a product is usually compared to a standard specification, set up by some agency. These specifications dictate the test conditions for measuring antistatic properties and set upper limits on the acceptable values of static decay time and surface resistivity. The most commonly used specifications are “NFPA-99” and “Mil-PRF-81705D” (commonly known as “Mil spec.”), which are described below:
Test conditions: 50% RH, 730F Static decay time for 10% cutoff (90% dissipation of 5 kV) < 0.5 sec. OR Surface resistivity < 1011 Ohms
Test conditions: 12% RH, 730F Static decay time for 0% cutoff (99% dissipation of 5 kV) < 2.0 sec. AND Surface resistivity < 1012 Ohms