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Its the old chicken and egg story-who is responsible-the resin
supplier or the processor?
Processors frequently exprience optical defects when processing
polyolefins on blown, cast, quench or oriented film producing machinery.
The processor invariably blames the resin suppliers and the resin
supplier passes the buck to the unclean and deficient processing
equipment.
The prominent optical defect is GEL which appears in clear film
upto 250 microns. GELS are primarily visual defects caused by small
bits of higher molecular weight (HMW) material. The GELs originate
from HMW tails in resins, cross-linked material caused by overheating,
additives with poor thermal stability, fines from regrind, catalyst
residue and other organic or inorganic contamination.
Because of high shear forces in extrusion, GELs appear as elongated
elipses. Those that are caused by contamination have a 'dot' or
'fish-eye' in the centre. GELs caused by HMW material do not. GELs
formed during polymerisation are referred to as 'P-GELs'. They arise
in the reactor in stagnating regions when resin overheats. GELs
can also be caused during extrusion. These are referred to as 'E-GELs'
and form due to high temperatures or long residence time. E-GELs
are created by dead spots in the extruder- stagnation regions in
the screw, mixing zone or broken filter pack.
It is suggested that incoming material be checked against 'P-GELS'
and if experienced, this should be reported to the resin supplier.
Testing for 'P-GELS' requires producing a thin plaque of material.
The number of gels per unit area in the plaque can be counted using
an overhead projector or polarized light to project an image on
a screen.The GEL text procedure must be standardized by the processor
and pass/fail limits be determined and agreed with the resin supplier.
To prevent 'E-GELS' during extrusion, the screw/die used must be
stream-lined design and the die surfaces free from grooves/scratches
that could collect melt and cause degradation. Besides the above
conditions to be met, processors meet also ensure that improper
hopper loaders or blenders if used, do not cause fines and streamers
to originate. Another way to reduce 'E-GELS' is to start the extruder
with a highly stabilised resin to coat critical surface with a degradation-resistant
layer. The use of a polymer processing aid masterbatch could also
minimize 'E-GEL' apperance. Sometimes, the additional slip and anti-block
additive masterbatch could also be responsible if the carrier resin
used in its production is unclean or not adequately stabilised.
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