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.

Full text

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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
transitiontemperaturesofthematerialsobtainedweremeasuredby
differential scanning calorimetry (DSC). Both the influence of
extruderparametersandmaterialparameters,suchasmoistureand
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
INTRODUCTION
Overthelastdecadetheincreasingamountofrefuseand
thereducedspaceforlandfill havegivenrisetoaninterestin
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
theseplastics,starchshouldbeperfectlymixedwithaproper
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
properties.
For dry starch the Tgreaches 227°C, whereas with 13%
water content a Tgdecrease to 56°C is recorded. The glass
transitiontemperatureofgelatinizedwheatstarchwith22%
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
provedthatbranchedamylopectinhadaslightlylowerglass
transition temperature than linear amylose.
Onthebasisoftheresearchespublishedandofpractical
observationsitcanconcludedthatstarchplasticscontaining
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
otherplasticizersontheTgofpotatostarchitwasfoundthat
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
amyloseandmaltosewereplasticizedwithglycerolshowed
an increase of the phase separation process together with
a decrease of glycerol content below 25%. Only recently, in
astudyonthebehaviourofsuchtwo-componentsystemsas
Int. Agrophysics, 2005, 19, 237-241
Glass transition temperature of thermoplastic starches
M. Mitrus
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: mmitrus@ar.lublin.pl
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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
connectedwiththeformationofanarearichinstarchandan
arearichinglycerolthatcausespartialphaseseparation.The
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
227°C,whileBizotetal.(1997)assessedthedrystarchTgas
332°C. What is more, to lower the Tgof potato starch closer
to the ambient temperature, 0.21 g of water should be used
for1gofstarch(Bizotetal.,1997;Myllärinenetal.,2002).
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
sameglycerolleveltheTgcanbestillashighas93°C.Itcan
beconcludedthatglycerolisalesseffectiveplasticizerthan
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
Materials
Potato starch (Superior Standard) was purchased at the
producer, ie Food Industry Company ‘PEPEES’Stock
Companyin£om¿a(Poland),PotatoCompany‘I³awaS.A.’
inI³awa(Poland)andAVEBECompany(theNetherlands).
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
massm=200mgwasdissolvedin10mlNaOH(5M).The
solutionwasdilutedwithwaterto100mlexact.25mlofthe
solution was pipetted to an Erlenmeyer flask, 2 drops of
methyl red indicator solution were added and neutralized
withHCl(1M)tillitwasjustred.1mlHCl(1M),1mlKI(1
M)and28mlNaCl(0.5M)wereaddedtothissolution.The
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).
Blend preparation
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
blendsweredampeduptothemaximummoisturecontentof
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).
Granulate extrusion
TheextrusionprocesswasconductedattheDepartment
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
(TS-45producedbyZMChMetalcheminGliwice)(Fig.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
werepressedfortesting.Theextrusiontemperaturesranged
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.

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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
specimensof7-10mgmasswereheatedfrom25to180°Cat
arateof10°Cmin-1andthencooledatthesamerateto25°C
to be finally reheated to 180°C.
To confirm the obtained results the tests were repeated
inaDSC2920modulatedDSCTAInstruments(Fig.3).The
sampleswereheatedfrom0°Cupto150°Cattherateof 1°C
min-1and then cooled at the same rate to 0°C.
RESULTS
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,
moistureofthemixturehadonlyaminimalinfluenceonthe
Tgofthermoplasticstarch.At20%glycerolcontent,aslight
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
growth,theTgvaluewentupaswell.However,inthosetwo
cases, Tgvalue changes were very small and the blend
moisture,inmeasuredrange,hadnosignificantinfluenceon
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.
20
30
40
50
60
70
80
90
100
1214 16 1820 22
Blendmoisture (%)
Tg (°C)
25% glycerol20% glycerol
Fig. 4. Influence of blend moisture on the Tgof thermoplastic
starch.

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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)
(Mitrus, 2004).
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
contentintheinvestigatedstarchesweretoolowtoobserve
significant differences in Tg of thermoplastic starch
(Mitrus, 2004).
CONCLUSIONS
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.
2.Whentheglycerolcontentincreasedfrom15to30%,
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
onlytoaverysmalldegree.AslightdownwardtrendofaTg
was recorded after multiple re-extrusion, but this did not
exceed 0.15°C.
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ForssellP.M.,MikkiläJ.M.,MoatesG.K.,andParkerR.,1997.
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branchedpolyanhydroglucose
starchesafter extrusion
240M. MITRUS
0
20
40
60
80
100
120
140
10 15 202530 35
Glycerol content (%)
Tg (°C)
Fig.5.Influence oftheglycerolcontentontheTgofthermoplastic
starch.
0
5
10
15
20
25
30
35
40
45
123
Extrusionrepeatation
Extrusion repetition
Tg (°C)
Fig. 6. Influence of multiple extrusions on the Tgof thermoplastic
starch.
0
20
40
60
80
100
2222,522,6
Amylose content (%)
Tg(°C)
Fig. 7. Influence of amylose content on the Tgfor samples with
20% of glycerol.

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GLASS TRANSITION TEMPERATURE OF THERMOPLASTIC STARCHES241