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Have you heard of PS blown film? Most likely not!
This, because it has been used only for some specific application
segments. In fact, PS blown films have been in existence for almost
the last 40 years but their volumes have been quite meager.
PS can produce stiff clear and easily printable
film. It also has excellent dead fold properties. What is remarkable
that it has good gas transmission properties. These inherent properties
make PS film suitable for packaging of fresh food products. However,
PS film has poor impact properties. To enhance the flexibility or
impact strength, addition of HIPS or clear SB copolymers like K
resin are always required. The increased dosage level of these polymers
into PS enhances the impact but increases gas transmission. Typically
5-10% levels are used to attain impact strength while higher level
up to 30% is used to make the film breathable by increased gas transmission.
Of late even Ethylene -Styrene inter polymers developed by Dow have
been used to blend with GPPS.
PS film is produced on blown film equipment that
is used for Polyolefins. No major modifications are apparently required.
PS film tends to cool much faster compared to PE and therefore has
smaller bubbles. The Blow-Up-Ratio (BUR) required for PS film is
much higher compared to PE. Generally BUR of about 6, a ratio that
provides higher orientation compared to the cast film process is
being used. PS has relatively quite narrow processing window and
also runs slower compared to PE film. Both, monolayer film or multilayer
films can be produced.
Generally PS film is coextruded as a core layer
with PE exterior layers. The processing ease is better in coextrusion.
The ungelled particle can cause the rupture of PS bubble quite easily
compared to PE film. It is therefore necessary to use finer screens
to filter out the impurity and generate higher backpressure to melt
the higher molecular weight material.
Dow was possibly the first manufacturer of PS film.
There are not many producers who are manufacturing PS blown film.
However, some more activities have been seen in the recent past
on account of PS film's niche market of MAP (Modified Atmosphere
Packaging) films, particularly for packaging of fresh food products.
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