Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
With more critical performance requirements came developments in blends of waxes
and polymers, and there is now a wide range of modified waxes available. In general,
the addition of polymers to waxes produces tough films, good moisture vapor barrier
properties and better adhesion characteristics. Typical modifiers include
ethylene/vinyl acetate copolymers and polyisobutylene. Some hot melts are based on
polymers without waxes. Polyethylene belongs to this classification, of course, but
there is also a wide range based on polyamides and polyesters.
6.3 Additives for Plastics Films
Most plastics films contain additives of one sort or another. Some are included to
overcome problems encountered during processing. These may arise, for example,
because of the sensitivity to oxidation shown by some plastics, particularly at the high
temperatures encountered during film extrusion. Sometimes materials are added in
order to overcome end-use problems such as blocking or low impact strength. In
many instances, more than one problem may be encountered and the selection of
the correct blend of additives becomes more difficult because of the possibility of
synergistic effects or interference with one additive by another. Some of the additives
used are described below, together with an account of their functions.
1. ANTI-BLOCKING AGENTS
Thin films tend to stick together, especially if they have a smooth surface without
irregularities. This ‘blocking’ can cause difficulties in handling and processing the film.
It is particularly annoying where packs of film are used for wrapping. Additives to
overcome blocking are usually based on very fine powders of low solubility in the
plastic. These then migrate to the surface of the film where they produce surface
irregularities. The most common antilock additive of this type is finely divided silica of
between 1 and 7 microns in diameter. Concentrations range from 1 to 6 parts per
1000. The tendency to block can also be reduced.
2. ANTI-FOGGING AGENTS
Moisture tends to condense as droplets on packaging films, particularly when
wrapping fresh produce such as potatoes and tomatoes. This then obstructs the
customer’s view of the contents. Such droplets can be dispersed by the addition of
non-ionic ethoxylates or hydrophilic fatty acid esters, such as glycerol stearate.
These act by promoting the deposition of continuous films of moisture, thus improving
visibility.
3. ANTIOXIDANTS
Although plastics, as a class, are relatively stable compounds, many are susceptible
to attack by oxygen, particularly at high temperatures. This is because high
temperatures accelerate the oxidation reactions. Hazards may be long-term ones, as
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