Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
sacks from a central storage point. The original polyethylene sacks used 250µm thick
film and were dearer than the existing paper sacks. Nevertheless, work went ahead
and a market now exists for OCR 80000 tones/annum of low density polyethylene for
sacks. The fertilizer sacks in particular are now cheaper than their paper counterparts
and are 200 µm in thickness.
There were many difficulties encountered during the development of this now very
successful market and it is interesting to look at some of these. One of the first
difficulties was that of handling. When the open mouth sack was filled and sealed it
assumed a pillow-like shape which was difficult to stack. Stack instability was often
thought to be due to a lower coefficient of friction for low density polyethylene, but in
fact polyethylene on polyethylene has a higher coefficient than has paper on paper
and it was soon recognized that the main trouble was the small area of contact with
the pillow-shaped polyethylene sacks, against that given by the block-shaped paper
sacks. One early solution that was proposed was to place the empty sack in a block
shaped jig, fill, and seal all but a small proportion of the seal. A vacuum was drawn in
the sack and the seal completed. This gave a brick-like package but the taut film was
easily punctured and the sack was then more floppy than the normally filled one.
A more acceptable solution was found to be the provision of a few micro-perforations
along the top of the bag. These holes allowed the air inside the sack to escape and
the contents of the bags were consolidated under the influence of impacts during
handling and movements on conveyor belts, etc. The holes were small enough not to
let in moisture except in heavy rainfall. This meant that bags could not be stored
under completely unprotected conditions, but a light tarpaulin or sheet of
polyethylene film over a stack is sufficient for normal protection. This is still better
than paper sacks which lose strength even under conditions where liquid water is
absent but where the relative humidity is high. There are also many situations where
filling equipment demands valve sacks so that these had to be developed for
polyethylene sacks as well as for paper. Today, sacks are available with fitted valves
(consisting of thin polyethylene flaps covering a slit aperture) and block bottom
construction so that neither filling nor stacking is a problem.
Advantages of the plastic sacks, apart from outdoor storage and lower cost, include
higher resistance to tear propagation and visibility of contents (where necessary).
There are now many other uses for low density polyethylene sacks in addition to the
packaging of fertilizers. Peat is packed in such sacks, so is coal. Another large
market is the packaging of plastics granules themselves. The market was slow to
develop because of difficulties in stacking, palletisation and transport but the advent
of block bottom, valve sacks has solved many of these problems.
2. SHRINK WRAPPING:
Shrink wrapping is another large outlet for low density polyethylene film and one that
is still growing. Shrink wrapping is based on the fact that a plastics film which has
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