THE 3
rd
INTERNATIONAL SCIENTIFIC CONFERENCES OF STUDENTS AND YOUNG RESEARCHERS
dedicated to the 99
th
anniversary of the National Leader of Azerbaijan Heydar Aliyev
389
Native starch is not considered as thermoplastic material as intermolecular
forces and hydrogen bonds in starch prohibit being pliable and/or moldable
at high specific temperature and hardening after cooling. Because of this
reason, plasticizers which are liquid having mostly
high boiling point and
molecular weight between 300-600 are utilized to eliminate its undesirable
properties and make it more deformable. A plasticizer decreases tensile
strength, glass transition temperature and enhances flexibility of polymer
chain, toughness, fracture resistance and processability in the starch (B.
Khan, 2016). Here, glycerol with the presence of water which is the rich with
hydroxyl group is used as plasticizer due to its abundancy, great boiling point
and minimum cost.
The reason why water is not used alone in plasticization
process is that it reduces the viscosity, leading to more volatile material. As
a consequence, steam generation
in material cause to foaming, bubbles,
which are not favored. Since glycerol has low molecular weight and size is
small, it dominates intermolecular spaces between polymer chains and
interactions among them are eliminated. Being less forces leads to reduction
in energy content needed for movement of molecule and in generation of
hydrogen bond. The result is that addition of plasticizer boost up free volume
of polymer. TPS illustrates viscoelastic and physico-chemical properties similar
to synthetic polymers. Molecular weight, amylose /amylopectin proportion,
processing condition, plasticizer concentration and chemical origin all influence
the final features of TPS. The elongation at breakage is reduced when quantity
of
glycerol was increased, however generally increase in amount of
plasticizer rises the elongation at rupture. The starch plasticized with glycerol
also has certain faults, such as low gas permeation, insufficient water barrier
property, hygroscopicity nature. The main problem is that even though
thermoplastic starch is form of amorphous, crystallization of material breaks
forming intense hydrogen bonds in progress of time which results brittleness
and damage of mechanical strength. Furthermore, glass transition temperature
(T
g
) of starch is not precise, but approximately between -75 °C and 10°C and
it is intuitively obvious that this interval is not suitable for production as TPS
swells irreversibly above its T
g
(Robbert A). TPS is also hydrophilic, so it is
vulnerable to moisture, and this characteristic cause the damage to dimensional
stability and mechanical properties of material. As it was mentioned before,
the obtained TPS is still sensible to water because of its strong hydrophilic
nature. Cross-linking method can be followed
for reducing hydrophilic
behavior of starch, as well as increasing thermal stability and decreasing
swelling power (SP) and solubility which in turn enhances its utilization in
food packaging applications. The most common cross-linking agents for
starch are
glutaraldehyde, epichlorohydrin, citric acid, boric acid, and sodium
hexametaphosphate (SHMP). Glutaraldehyde and epichlorohydrin are not
considered safe due to their toxic and irritant nature, which can entail hazardous
accidents during the experiments. On the other hand, organic acids such as