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Linear Polyolefins are known to pose processing
problem due to their short relaxation behaviour and narrow molecular
weight distribution. Among the Linear Polyolefins, LLDPE and MDPE
are found to be more difficult because of higher extrusion speeds.
Generally HDPE/HMHDPE are produced at lower extrusion speeds and
in thinner guages. They pose lesser problems because they are processed
at lower shear rates.
To eliminate problems of shark skin (melt fracture),
die deposits and poorer surface finish, Polymer Processing Aids
(PPA) are required. These PPA essentially coat the surface of the
die to prevent sticking of melt. These PPAs are incorporated into
the grades. During processing, PPAs migrate to die surface and coat
it. PPA belong to Fluroelastomer family of polymers. Recently rheologically
modified fluroelastomer grades have been developed. These grades
spread easily and therefore coat die surfaced more effectively.
These modified grades contain either Polyethylene Glycol or Polycaprolactone
as interfacial agents to provide better thermal stability. They
improve efficiency of fluroelastomers. There is also an interesting
PPA grades based on Boron Nitride. It is claimed that Boron Nitride
is a very effective processing aid for Metallocene LLDPE. Some of
HDPE film grades also contain Fluroelastomer for better performance.
All LLDPE / HDPE film producers cannot ignore PPA since they provide
better performance and therefore better profitability.
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