Oil-Based and Bio-Derived Thermoplastic Polymer Blends and Composites 245Strain (%)0 0051015202530510152025(1)(2)Stress (MPa) Stress (MPa)(a) (b)PotatoCorn
WheatWaxy corn(^030)
1
2
3
4
5
100 200 300 400
Strain (%)
Figure 8.3(a) The influence of plant sources in derived starch products. (b) The different stress and
strain behaviour of pure amylopectin (1) and pure amylose (2). Saiahet al. 2012 [7].Source:http://www.
intechopen.com/books/thermoplastic-elastomers/properties-and-biodegradation-nature-of-
thermoplastic-starch-Used under CC BY 3.0, https://creativecommons.org/licenses/by/3.0/.
In order to be processed, starch needs a plasticizing process reachable with water,
additives (glycerol is one of the widely used), heat and shear. During this process, named
gelatinization, there is the disruption of crystalline phase in the granules to obtain an
amorphous TPS [8]. The plasticizers act by breaking the starch–starch bonds, replacing
them with starch–plasticizer interactions. The main features of a plasticizer for starch
are compatibility with starch macromolecules, polarity, hydrophilicity, smallness and
boiling point high enough to avoid evaporation during the process [9].
Usually, plasticizers used with TPS are glycerol, sorbitol, glycols and urea [10–15].
TPS plasticized with glycerol showed, after a stored period, retrogradation behaviour,
resulting in embrittlement.
Retrogradation, called also ageing, happened because of the non-thermodynamical
stability of amorphous phase. This problem is strictly related to temperature; higher
the difference between storage temperature and glass transition temperature, the higher
the possibility of ageing phenomenon, especially in high humidity conditions or high
amount of plasticizers [9].
Ageing problem has been overcome through plasticizers based on amide groups,
which unfortunately showed healthy problems. Hence, Jiugaoet al. used different
plasticizers, such as citric acid (one of the same plasticizers of PLA) [16]. They found a
positive effect of citric acid in preventing the retrogradation processes. In fact, citric
acid showed stronger interactions with C–O group than glycerol, assisting to avoid the
formation of hydrogen bonds between hydroxyl groups of TPS.
Recently, TPS has been obtained by adding ionic liquid. Sankriet al. [17] analysed the
plasticization of starch by 1-butyl-3-methylimidazolium chloride [BMIM]Cl and water
(30%), compared to glycerol performances and water (30%). Table 8.6 displays the main
results of this study.
[BMIM]Cl performed as a better plasticizer than glycerol by inducing lower water
absorption, lower glass transition temperature, higher electrical conductivity and
higher elongation at break. As a consequence, the Young’s modulus was lower for ionic
liquid-plasticized TPS than for the glycerol-plasticized one.