1. Introduction
Due to future scarcity of petroleum resources, which will affect the
supply of raw materials, cost, and production capacity for traditional
plastic products, as well as those plastics' lack of biodegradability and
increased awareness of the need to preserve the natural environment,
many efforts have been recently made to develop biodegradable plastics
from renewable resources. Among the bioplastics investigated as po-
tential alternative raw materials, thermoplastic starch (TPS) has
attracted a great deal of attention due to its manufacture from starch raw
material that is biodegradable, non-toxic, inexpensive, and widely
available.
Native starch is a semi-crystalline material mainly composed of
amylose and amylopectin polymers, in which their structures consist of
glucose monomer units. Strong intra- and inter-molecular hydrogen
bonds between hydroxyl groups of starch polymers make them decom-
pose instead of melt upon heating (T
m
>
T
d
)
[1]
. The thermoplastic
behavior of starch can be developed by plasticization, which is a process
to transform the semi-crystalline starch granules into a homogeneous
TPS in the presence of an appropriate plasticizer (with a tiny amount of
water) while applying sufficient thermomechanical energy
[1]
. During
plasticization, hydrogen bonds between the starch molecules are
destroyed, while new hydrogen bonds between the plasticizer and starch
molecules are synchronously formed. Plasticization with a combination
of thermal and mechanical inputs can be obtained by extrusion, which is
a cost-effective and common plastic processing technique.
The development of TPS requires the addition of high plasticizer
content (15
–
32%)
[2
–
6]
. Although water is the best natural plasticizer,
it cannot withstand high temperatures and easily evaporates during the
extrusion process. Therefore, it must be partially or fully replaced by
less-volatile plasticizers. The plasticizers commonly used today are hy-
drophilic low-molecular-weight polyols including glycerol
[2,3,5,6]
,
sorbitol
[4,7]
, xylitol
[4,7]
, and maltitol
[4,8]
. The nature and compo-
sition of the plasticizer system greatly affect not only the transformation
of granular starch into a homogeneous thermoplastic phase but also the
glass transition temperature (T
g
) of the TPS and consequently its me-
chanical properties and kinetics of crystallization.
Although studies on the preparation of starch films using different
polyol mixtures as plasticizers and their properties have been reported,
these are largely devoted to the solution casting of gelatinized starch
* Corresponding author at: Kasetsart University, Faculty of Agro-Industry, Department of Packaging and Materials Technology, 50 Ngamwongwan Rd, Ladyao,
Chatuchak, Bangkok 10900, Thailand.
E-mail address:
rangrong.y@ku.ac.th
(R. Yoksan).
Contents lists available at
ScienceDirect
International Journal of Biological Macromolecules
journal homepage:
www.elsevier.com/locate/ijbiomac
https://doi.org/10.1016/j.ijbiomac.2021.08.027
Received 26 May 2021; Received in revised form 31 July 2021; Accepted 2 August 2021
International Journal of Biological Macromolecules 188 (2021) 290–299
291
[7
–
9]
. A few studies on TPS blown film with polyol plasticizers have
been undertaken
[2,3,5,6]
; however, only glycerol was used as a single
polyol plasticizer, and the obtained films possessed a single wall
[5]
,
high surface stickiness
[2,5]
, and insufficient tenacity
[5]
. An evaluation
of TPS blown films with mixed polyol plasticizers and their properties
has not yet been reported. Therefore, this study aims to investigate the
effect of mixed polyol plasticizers and their concentrations on charac-
teristics, extrusion processability, melt flowability, and mechanical,
thermal, water vapor, oxygen barrier, and morphological properties of
TPS.
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