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(Fig. B)
Images of Good Dispersion (Left) and Poor Dispersion (Right) with TiO
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(Source: E. I. Dupont)
DistributiveMixing
Distributive mixing is defined as the uniform spatial
rearrangement of fibers or other dispersed materials
in the base polymer matrix. In most applications,
especially the ones with fibers, some form of
dispersion and forced wetting of fibers brought about
by kneading elements precedes distribution of fibers.
If necessary, a distributive mixing zone with
appropriate elements are configured towards the end
of the Extruder Processing Zone
(EPZ)
and in some
case outside the extruder using a static mixer. The
most common distributive mixing requirement is in
the uniform distribution of different length fibers in the
melt since fibers may be full dispersed and wet
without uniform spatial distribution. Otherwise,
distributive mixing is commonly confused with
Kneading.
SEM Images of Fibers not uniformly spatially distributed.
(Source: Sascha Englich, Master's Thesis U. Chemnitz,
Research carried out at STEER Engineering Bangalore)
THE
HEART OF A
TWIN SCREW EXTRUDER IS
The MIXING ZONE Cont’d
-7-
THE EXTRUDER TIMES / ISSUE 05 / JANUARY-MARCH 2008
DispersiveMixing
Stresses of shear and elongational origin brings about dispersion of the massive agglomerated particle in the melt polymer matrix by separating
individual units or grains or crystals. The lowering of the cohesive strength (strength of bond between the same type of particles) of the
agglomerate (usually a pigment) is a factor resulting in better dispersion. Mixing at the time of melting improves dispersion as result of high
shear stresses during the high viscosity phase of the material. However, with certain material such as TiO and Carbon black, this approach can
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result in a severe issue called re-agglomeration. During the time of melting in an extruder, the material is subjected to high levels of pressure.
The particles that have not been completely surrounded by the wetting agent or melt have a chance to bond together and “sinter”. One of the
SEM pictures show TiO particles of 200 nm
(Fig. B)
fully dispersed in the polymer matrix. In the other picture, agglomerated particle of bigger
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than 10 microns are seen although such particles were not present prior to the extrusion.