Transmission Efficiency of plastic Films Part 1

4. ANTI-STATIC ADDITIVES
TRANSMISSION EFFICIENCY OF PLASTIC FILMS
Plastics films are non-conductors of electricity and tend to accumulate electrical
charges. Such charges may be developed by friction between different layers of
particles of the plastics itself between the plastics and processing equipment or by
electro-ionization from dust or radiation. These electrostatic charges can cause
problems during processing or during end use.
The two main problems in processing are: (a) adhesion, either between individual
particles or layers of the plastic itself, or between the plastics and processing
equipment; and (b) discharging to earth by sparking. This can lead to electric shocks
to operators, pin holes in film and local overheating. In extreme cases it can cause a
fire hazard. End use problems include dust pick-up and adhesion of thin films,
coupled with sparking when they are separated.
The two main properties which an effective anti-static agent must have are a degree
of electrical conductivity and a marked tendency to migrate to the surface of the film
(where lies the main concentration of the electrostatic charge). A wide range of
materials has been investigated for anti-static effectiveness, but the most common in
actual use are either glycol derivatives or quaternary ammonium compounds. The
latter are ionized to some extent in the anhydrous state and hence act as conductors
in their own right. Glycol derivatives do have some slight conductivity but their main
effect is the sorption of moisture (because of their hygroscopic nature) and it is the
moisture layer that acts as the main path to earth for the electrostatic charge.
The presence of a layer of anti-static material on the surface of the film could
interfere with the receptiveness of the film to printing inks. However, migration to the
surface is a slow process, normally several days, so trouble can usually be avoided
by printing the film soon after extrusion. In many instances, printing is carried out inline
with film extrusion.
A rather different problem is that of packaging certain electronic devices and
components. A feature of an increasing number of these is their sensitivity to
electrostatic discharge (ESD) and electrical overstress (OES). Ordinary anti-static
additives are not always effective in producing ‘static safe’ films and carbon blackbased
films are often used.
5. BLOWING AGENTS
Reducing the density of a plastics film by giving it a cellular structure allows the
production of a film having greater stiffness for a given weight of polymer. In the
manufacture of cellular PE and PP films, for example, a master-batch containing a
blowing agent is added to the granules which are then extruded as usual. The
blowing agent is normally a compound that breaks down at the temperature of
extrusion to give nitrogen that then expands the extruded molten mass. One nitrogen
based blowing agent is azobisformamide (ABFA) while another is azodicarbonamide.
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