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3. Results and discussion
3.1. Extrusion processability and physical characteristics
TPS was successfully prepared by mixing cassava starch with three
different types of plasticizer, i.e. glycerol (G), a glycerol/xylitol mixture
of equal amounts (GX), and a glycerol/sorbitol mixture of equal amounts
(GS), at various concentrations of 38, 40, and 42 phs in a twin-screw
extruder.
Table 1
shows that the induced pressure and torque during
compounding extrusion of TPS tended to increase in the order of G, GX,
and GS. The result suggests that G exhibited better extrusion process-
ability and might have a lower viscosity than GX and GS, respectively.
This may be explained by the different plasticizer efficiency, which is the
ability of a plasticizer to make the polymer softer
[10]
. The size or
molecular weight of the plasticizer is one of the most important factors
for determining the rate of diffusion in the polymer matrix, and even-
tually affects plasticization
[10,11]
. Of the three types of plasticizer in
this study, glycerol is the smallest (92.09 g/mol), while xylitol (152.15
g/mol) and sorbitol (182.17 g/mol) possess larger molecular sizes,
having molecular weights approximately 1.5 and 2 times greater,
respectively than that of glycerol. Therefore, xylitol and sorbitol have a
lower diffusion rate in the starch matrix than glycerol, because of lower
plasticizer efficiency. In general, good diffusion of plasticizer facilitates
an increased free volume of the polymer matrices and enhances mo-
lecular mobility and velocity of polymers by disrupting the intermo-
lecular hydrogen bond interaction between polymer chains
[4]
.
Table 1
also demonstrates that induced pressure and torque during
the compounding extrusion of TPS decreased as a function of plasticizer
content. This indicates that the extrusion processability of TPS improves
when more plasticizer is introduced, most likely due to the enhanced
plasticizing effect. However, if an excessive amount of plasticizer is
used, the blooming of the plasticizer may cause surface stickiness and
poorer heat sealability in the final TPS-based product. Although the
intensive compounding extrusion parameters (i.e. high temperature and
fast screw speed) can be employed to acquire higher energy and greater
shear force to destroy the starch crystalline structure and compensate
the deficient plasticizer, the mechanical properties and color of the final
TPS-based products might not be accepted. Therefore, the balance be-
tween processing parameters and plasticizer type and content should be
considered to optimize the performances of TPS-based materials. In
addition, the throughput rate of TPS tended to rise with increasing
molecular size of plasticizer and with decreasing plasticizer content
(
Table 1
), corresponding to the increased induced pressure and torque.
Although most TPS materials were blown into films without diffi-
culty using a blown film extrusion line, GS38 could not be blown, while
GS40 required higher temperatures for the blown film process (
Table 1
).
This result reflects that blown film extrusion of TPS containing a small
amount of plasticizer with a large molecular size was limited, most likely
due to too high viscosity and rigidity resulting from lower plasticization
efficiency. For other TPS, during blown film extrusion, a decrease in
induced pressure and torque was observed when a higher content of
plasticizer with a smaller molecular size was introduced. The trend re-
flects that found in compound extrusion.
It should be noted that the surface stickiness of TPS films decreased
when xylitol or sorbitol was used to replace glycerol, and the obtained
films exhibited a single-layered wall because the flattened tubular wall
obtained after passing the nip rolls did not stick together. This was
markedly different from the films plasticized with glycerol only, in
which a double wall, or a two-layered wall, resulted soon after passing
the nip rolls. Several studies have also reported that TPS blown films
plasticized with glycerol were difficult to blow, had a surface stickiness
[2,5]
, and possessed a double wall
[5]
. However, in the case of xylitol,
single-wall-layered films were obtained for GX38 and GX40, while the
highest concentration of xylitol/glycerol mixture still gave the film
(GX42) a sticky surface, less severe than all glycerol-plasticized films.
For TPS plasticized with glycerol/sorbitol mixture, films with a single-
wall layer were created even at higher concentration of this mixed
plasticizer. However, the blown bubble construction was unsuccessful
for the TPS plasticized with the lowest concentration of the glycerol/
sorbitol mixture (38 phs). Although the molecular weight and structure
of sorbitol and xylitol were not much different, their effects on the blown
film extrusion processability were observed. The appearance of all TPS
films is presented in Fig. S1. The films plasticized with glycerol showed
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