PVOH plasticization was achieved via a similar procedure used
for CA. For that purpose, triacetin was 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.