Glass transition temperature of thermoplastic starches
ABSTRACT Thermoplastic starch was produced by mixing potato starch and glycerol in a single screw extruder. The glass transition temperatures of the materials obtained were measured by differential scanning calorimetry (DSC). Both the influence of extruder parameters and material parameters, such as moisture and glycerol content and amyloses/amylopectine ratio were investigated. Repeated extrusion cycles affect the glass transition temperature only to a very small extent.
A b s t r a c t. Thermoplastic starch was produced by mixing
potato starch and glycerol in a single screw extruder. The glass
differential scanning calorimetry (DSC). Both the influence of
glycerol content and amyloses/amylopectine ratio were investi-
gated. Repeated extrusion cycles affect the glass transition tempe-
rature only to a very small extent.
K e y w o r d s: thermoplastic starch, glass transition
temperature, differential scanning calorimetry, extrusion
new easily biodegradable materials. Several scientific
centers performed research on a special group of natural
materials – starch thermoplastics (De Graaf et al., 2003;
Myllärinen et al., 2002; Shamekh et al., 2002). To obtain
quantity of plasticizer to make the material flow at
temperatures below the decomposition temperature. In this
way it is possible to obtain a product in which the
polysaccharides form a continuous, polymeric entangled
phase. This form of starch is called thermoplastic starch
(TPS) (Van Soest, 1996).
Using the polymer technology designed for synthetic
polymers, the starch plastics can be manufactured as a sup-
plement to the existing synthetic products. However,
thermoplastic starch is not widely used as a commercial
product because of some drawbacks. One of the major
problems connected with starchy material is its brittleness.
This results from a relatively high glass transition
temperature (Tg) (De Graaf et al., 2003). The Tgis a very
important parameter for determining the mechanical
For dry starch the Tgreaches 227°C, whereas with 13%
water content a Tgdecrease to 56°C is recorded. The glass
moisture approximates the ambient temperature (Myllä-
rinen et al., 2002).
The effect of starch plasticized with water addition has
been studied frequently just like the comparison of various
techniques for glass transition temperature measurement.
The method of differential scanning calorimetry (DSC),
which is most often used, showed a Tgthat is 10 to 30°C
higher than that measured by the nuclear magnetic
resonance (NMR) (Myllärinen et al., 2002). The analysis of
water influence on the Tgof amylose and amylopectin
transition temperature than linear amylose.
water are brittle under the condition of natural surroundings
(Bizot et al., 1997).
The three component systems obtained when starch is
plasticized with water and glycerol behave in a more
complex way. In research on the influence of glycerol and
glycerol plasticizes starch in conformity with Couchman`s
model that is valid for polymer-solvent combinations
(Lourdin et al., 1997). Studies on barley starch plasticized
with water and glycerol showed that phase separation is
possible and two calorimetric glass transition temperatures
can be obtained (Forssell et al., 1997). Moreover, basing on
the dielectric-mechanical analysis, the researches where
an increase of the phase separation process together with
a decrease of glycerol content below 25%. Only recently, in
Int. Agrophysics, 2005, 19, 237-241
Glass transition temperature of thermoplastic starches
Food Process Engineering Department, University of Agriculture, Doœwiadczalna 44, 20-236 Lublin, Poland
Received February 18, 2005; accepted May 4, 2005
© 2005 Institute of Agrophysics, Polish Academy of Sciences
Corresponding author’s e-mail: email@example.com
I I IN N NT T TE E ER R RN N NA A AT T TI I IO O ON N NA A AL L L
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amylose-glycerol, it was found that these systems are
composed of phases rich in amylose and phases abounding
with glycerol (Moates et al., 2001).
Determination of the glass transition temperature of
thermoplastic starch using DSC showed so called ‘higher’
and ‘lower’ Tgvalues or both simultaneously. The higher
and lower Tg’s are generally found in mixtures containing
less than 30% glycerol. This behaviour is likely to be
higher and lower transitions seem to be characteristics for
starch plasticized with glycerol and are independent of the
processing method (Forssell et al., 1997).
The tests show that both amylose and amylopectin had
a higher Tg`s in the absence of glycerol. The estimates
demonstrated that the Tgof dry amylose and amylopectin is
332°C. What is more, to lower the Tgof potato starch closer
to the ambient temperature, 0.21 g of water should be used
Myllärinen et al. (2002) confirmed that the Tgof
amylose and amylopectin can be equal to the ambient
temperature when the water content is 21%, however at the
water. On the basis of computations they claim that in order
to lower the Tgvalue to the ambient temperature, 35%
glycerol should be applied.
MATERIALS AND METHODS
Potato starch (Superior Standard) was purchased at the
producer, ie Food Industry Company ‘PEPEES’Stock
Glycerol (98.5% purity) originated from the Chemical
Plant ‘Odczynniki’ Ltd in Lublin.
Determination of amylose in starch
Potentiometric titration with a Pt-electrode and a calo-
mel reference electrode was used. A starch sample with a
solution was pipetted to an Erlenmeyer flask, 2 drops of
methyl red indicator solution were added and neutralized
solution was titrated with KIO3(0.0050 M = t). 1 ml KIO3
solution corresponds to 0.635 mg I2. The potential drop,
measured by the potentiometric system (volume V),
determined the equivalence point and the amylose content
can be calculated from (Bates et al., 1943):
W (amylose) = (V t 127?4?5?1000) m-1(mg g-1).
Starch and glycerol blends were prepared with a ribbon
blender type MPP-100, produced by FMR RogóŸno. The
glycerol content varied between 15 and 30 wt %. First, the
20% based on dry mass. The mixtures were stored in plastic
bags for 24 h to intensify glycerin penetration into starch
granules. Immediately before the extrusion the blends were
remixed (Mitrus, 2004; Mitrus, 2006).
of Process Engineering at the University of Agriculture,
Lublin, in a modified single-screw extruder with a screw
diameter of 45 mm and a length to diameter ratio of 16/1
Two dies were used, ie one with a single opening of 3 mm
diameter and one with a triple opening of 1.5 mm diameter.
The extrudate was chopped to granulate of around 5 mm
length with a high-speed cutter. From this material films
from 75 to 140°C, and the screw rotations from 60 to 100
r.p.m. (Mitrus, 2006).
238 M. MITRUS
Fig. 1. Single screw extruder TS – 45.
Differential scanning calorimetry
The measurements of the glass transition temperatures
were performed using a Perkin Elmer DSC 7 (Fig. 2) at the
Department of Chemical Engineering at the Groningen
University in the Netherlands. The thermoplastic starch
to be finally reheated to 180°C.
To confirm the obtained results the tests were repeated
min-1and then cooled at the same rate to 0°C.
Figure 4 shows the influence of the blend moisture on
the Tgof potato starch placticized with 20 and 25% of
glycerol. The results show that, in the measured range,
decrease of Tgwas observed with an increase of moisture.
This is consistent with the scientific reports published in
literature. In the case of 25% glycerol level a reverse
tendency was noticeable. Together with blend moisture
cases, Tgvalue changes were very small and the blend
Tgchanges (Mitrus, 2004).
GLASS TRANSITION TEMPERATURE OF THERMOPLASTIC STARCHES239
Fig. 2. Perkin Elmer DSC 7 apparatus.
Fig. 3. Apparatus DSC 2920 modulated DSC TA Instruments.
1214 16 1820 22
25% glycerol20% glycerol
Fig. 4. Influence of blend moisture on the Tgof thermoplastic
Figure 5 shows the changes of the glass transition
temperature with changing glycerol content. The highest Tg
was 132.7°C for 15% of glycerol, and it decreased almost
linearly to 18.1°C at a glycerol level of 30%. The moisture
content of all the mixtures was 15% (Mitrus, 2004).
Figure 6 illustrates the influence of repeated extrusion
on the Tgof thermoplastic starch for the mixtures with 25%
of glycerol. A slight drop of a Tgcan be noted when re-
processing thermoplastic starch. The maximum decrease of
the glass transition temperature did not exceed 0.15°C.
However, it should be mentioned that to avoid thermal
destructurization the repeated extrusion experiments were
performed at slightly lower temperatures (by around 10°C)
Figure 7 illustrates the influence of amylose content on
the Tgof thermoplastic starch for the mixtures with 20 % of
glycerol. A slight drop of Tgwith amylose content increase
was noticed. It is possible that the differences in amylose
significant differences in Tg of thermoplastic starch
1. The changes in the glass transition temperature of
thermoplastic starch were only minimally affected by the
moisture content of the mixture. It was found that, in the
measured range, the blend moisture had no significant
influence on Tgchanges.
the glass transition temperature decreased almost linearly
from 132 to 18°C at moisture content of 15%.
3. Multiple extrusion affected the Tgof potato starch
was recorded after multiple re-extrusion, but this did not
Bates F.L., French D., and Rundle R.E., 1943. Amylose and
amylopectin content of starches determined by their
iodine complex formation. J. Amer. Chem. Soc., 65,
Bizot H., Le Bail P., Leroux B., Davy J., Roger P., and Buleon
A., 1997. Calorimetric evaluation of the glass transition in
compounds. Carbohydrate Polymers, 32, 33-50.
De Graaf R.A., Karman A.P., and Janssen L.P.B.M., 2003.
Material properties and glass transition temperatures of
processing. Starch, 55, 80-86.
Phase and glass transition behaviour of concentrated barley
starch-glycerol-water mixtures, a model for thermoplastic
starch. Carbohydrate Polymers, 34, 275-282.
Lourdin D., Coignard L., Bizot H., and Colonna P., 1997.
Influence of equilibrium relative humidity and plasticizer
concentration on the water content and glass transition of
starch materials. Polymer, 21, 5401-5406.
Mitrus M., 2004. Influence of barothermal treatment on physical
PhD. Thesis, University of Agriculture, Lublin.
Mitrus M., 2006. Microstructure of the thermoplastic starch
polymers. Int. Agrophysics, in press.
10 15 202530 35
Glycerol content (%)
Fig. 6. Influence of multiple extrusions on the Tgof thermoplastic
Amylose content (%)
Fig. 7. Influence of amylose content on the Tgfor samples with
20% of glycerol.
mic mechanical and dielectric characterisation of amylose
glycerol films. Carbohydrate Polymers, 44, 247-253.
Myllärinen P., Partanen R., Seppälä J., and Forsella P., 2002.
films. Carbohydrate Polymers, 50, 355-361.
Shamekh S., Myllärinen P., Poutanen K., and Forsell P., 2002.
Film formation properties of potato starch hydrolisates.
Starch, 54, 20-24.
Van Soest J.J.G., 1996. Starch plastic: structure - property
relationships. PhD. Thesis, University of Utrecht, the
GLASS TRANSITION TEMPERATURE OF THERMOPLASTIC STARCHES241