been assessed.
PVOH plasticization was achieved via a similar procedure used
for CA. For that purpose, triacetinwas replaced by glycerol and used
as plasticizer (25 wt%). Clear pellets were produced with an
extruder throughput of 4e5 kg/h.
HEC plasticization was conducted following the two-step process
that we reported recently [9]. This process overcomes the
problems faced when HEC is attempted to be melt plasticized
directly in a single step. Indeed, highly degraded materials were
obtained with high content of non-gelatinized HEC granules. HEC
degradation occurred actually as a consequence of the high shear
forces generated during the kneading process due to the lack of
plasticising effect. Consequently, a two-step plasticization process
has been established: first a premixing step of HEC with plasticizer
in presence of water, where the addition of water allowed for
swelling of HEC granules taking advantage of both HEC watersolubility
and plasticizer water-miscibility followed by the melt
processing of the swollen premix. Table 2 summarizes the
composition of the pre-mixtures prepared in this contribution.
Mixtures were prepared in glass beakers and left to swell overnight
at room temperature. The remarkable increment of volume, up to
10 times, of the pre-mixtures when water was added clearly indicated
that the addition of water improved the swelling of the
premix blend, facilitating plasticizer diffusion. Glycerol and two
polyethylene glycol samples of different molar masses (200 and
400) were tested as plasticizers. For each composition, a gel-like
structure was obtained after melt-processed at 110 C for 6 min,
which indicates effective HEC plasticization. Water vapour was
observed escaping from the mixer similarly to what is observed in
the preparation of thermoplastic starch by melt extrusion using
water to help melt plasticization [29]. In this case, water was added
to decrease shear forces and reduce extruder maximum load and
was observed to come out through the extruder die.
Melt plasticization efficiency of HEC was qualitatively evaluated
according to the coloration of the final material which is related to
the thermal degradation. Table 2 shows that the addition of water
not only improved HEC swelling but also decreased blend coloration.
Less coloured compositions were obtained when both water
and glycerol are used in combination. In contrast, the use of PEG as
plasticizer led to brown samples. Such observations are consistent
with the extent of hydrogen bonds that the plasticizer could form
with HEC chains. Indeed, for a same plasticizer weight fraction, the
molar content of hydroxyl groups present in PEG is lower than that
in glycerol. PEG is thus less efficient as plasticizer than glycerol.
At this stage, it is worth recalling that CA is sensitive to thermal
and hydrolytic degradation during melt processing. Thus, for the
preparation of the blends, it was necessary to remove any residual
water from the plasticized HEC. Water removal already started
during HEC melt blending, where vapour generationwas observed.
Water content was further reduced to negligible levels on a