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Article
A simple procedure for
the preparation of lapo-
nite and thermoplastic
starch nanocomposites:
Structural, mechanical,
and thermal
characterizations
Fauze A Aouada
1,2
, Luiz HC Mattoso
2
and Elson Longo
1
Abstract
The aim of this article is to propose advances for the preparation of hybrid nanocompo-
sites prepared by the combination of intercalation from solution and melt-processing
methods. This research investigates the effect of the laponite RDS content on the ther-
mal, structural, and mechanical properties of thermoplastic starch (TPS). X-ray diffrac-
tion was performed to investigate the dispersion of the laponite RDS layers into the TPS
matrix. The results show good nanodispersion, intercalation, and exfoliation of the clay
platelets, indicating that these composites are true nanocomposites. The presence of
laponite RDS also improves the thermal stability and mechanical properties of the
TPS matrix due to its reinforcement effect which was optimized by the high degree of
exfoliation of the clay. Thus, these results indicate that the exfoliated TPS–laponite
nanocomposites have great potential for industrial applications and, more specifically,
in the packaging field.
Keywords
Clay, mechanical properties, nanocomposites, reinforcement, thermal properties, X-ray
1
Laborato
´rio Interdisciplinar de Eletroquı
´mica e Cera
ˆmica (LIEC), Chemistry Institute, Sa
˜o Paulo State
University, Araraquara, SP, Brazil
2
National Nanotechnology Laboratory for Agriculture – LNNA – Embrapa Instrumentation – CNPDIA,
Sa
˜o Carlos, SP, Brazil
Corresponding author:
Fauze A Aouada, Laborato
´rio Interdisciplinar de Eletroqu
´umica e Cera
ˆmica (LIEC), Chemistry Institute,
Sa
˜o Paulo State University, Araraquara campus, 14801-907, Araraquara, SP, Brazil.
Email: faouada@yahoo.com.br
Journal of Thermoplastic Composite
Materials
26(1) 109–124
!The Author(s) 2011
Reprints and permissions:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/0892705711419697
jtc.sagepub.com
Introduction
The new group of composites, named as nanocomposites, is receiving a great deal
of attention from different researchers in different fields.
1–4
In the nanocomposite
materials at least one dimension of the particles is in the nanometer size
(1–100 nm).
5
Additionally, when the domain size is equivalent to the dimension
of a molecule, the atomic and molecular interactions can have a significant influ-
ence on the macroscopic properties of that material.
6
The investigation of polymer/clay nanocomposites received considerable
scientific and technological attention during the last years due to important clay
properties, such as the high availability; the reinforcement effect even added
into polymeric matrix in small quantities (1–5 wt%); and the huge knowledge
regarding clay–polymer matrix intercalation chemistry.
7,8
For instance, Delhom
et al.
9
developed a novel flame-retardant nanocomposite based on cellulose and
clay materials. The authors observed that the nanocomposites show significant
improvements in thermal properties, when compared with cellulose control
sources; and tensile testing revealed an increase of approximately 80% in the
ultimate stress of the cellulose/clay nanocomposites.
Laponite is a synthetic mineral with structure and composition similar to natural
hectorite, which belongs to smectic group. The basic layered structures are
composed by two external tetrahedral silica sheets and a central octahedral
magnesia sheet.
10
Laponite RDS is a synthetic hectorite with aspect ratio of
20–30
6
and chemical formula – Si
8
Mg
85.45
Li
0.4
H
4
O
24
Na
0.7
+Na
4
P
2
O
7
. Since
Na
4
P
2
O
7
peptizer is mixed to the laponite aiming to increase their stability in
aqueous solution.
11
Starch is a no thermoplastic polysaccharide, but in the presence of plasticizers
such as glycerol
12
at high temperatures and under shear, it can readily melt and
flow, facilitating its use as extruded or injected material, which is similar to most
conventional synthetic thermoplastic polymers.
13
Thermoplastic starch (TPS) is
thus derived from renewable sources. It is a rather inexpensive material compared
to synthetic thermoplastics and can easily be processed with plastic-processing
machines. However, TPS shows a number of shortcomings that could limit or
restrict their industrial application (e.g., packaging)
14
such as moisture sensitivity
and lower mechanical properties. To overcome these shortcomings, inorganic–
organic nanocomposites have been prepared by the addition of clay into the
TPS matrix.
The aim of this article is to propose advances in the preparation of hybrid
nanocomposites prepared through the combination of intercalation from solution
and melt-processing preparation methods to be applied in the packaging field.
The effect of the laponite RDS on the thermal, structural, and mechanical
properties of the nanocomposites was investigated. The combination of the
both methods has not been applied to prepare the TPS–laponite RDS
nanocomposites.
110 Journal of Thermoplastic Composite Materials 26(1)
Experimental
Materials
Regular corn starch containing 28% amylose (Amidex 3001 TM) and laponite
RDS were acquired by Corn Products Brasil Ltd and Southern Clay Products,
Inc., respectively. Glycerol was purchased from Aldrich. All chemicals were used
as received.
Preparation of TPS and laponite RDS nanocomposites
The TPS and laponite RDS nanocomposites were obtained from combination of
intercalation from solution and melt-processing preparation methods. The content
of starch and glycerol was fixed at 70 and 30 wt%, respectively. The content of
laponite RDS was 1, 2, 3, and 5 wt% based on the total starch and glycerol weight.
Corn starch powder was first dried overnight at 70C in a ventilated oven to
remove the free water.
In the first step, a known quantity of laponite RDS was introduced into 200 mL
of distilled water and dispersed in an ultrasonic bath at 25C for 2 h. Then, the corn
starch was dispersed into laponite RDS dispersion under magnetic stirring for
10 min. The glycerol was slowly added into the same solution under stirring.
After the complete addition of glycerol, the mixture was mixed at high speed
(1500 rpm) to obtain a homogeneous dispersion. The mixture was placed in a
ventilated oven at 90C for 24 h, which facilitated vaporization of the bound
water and diffusion of the glycerol into the starch granules.
In the second step, the mixtures were processed in a Haake Rheomix 600 batch
mixer connected to a torque rheometer with roller-like rotors. In this process, some
external parameters could influence the plasticization of the starch such as temper-
ature, rotor speed, and residence time. These parameters were initially studied to
reveal optimal conditions: temperature = 120–160C; rotor speed = 50–200 rpm;
and residence time = 6–20 min. After processing, mechanical properties (tensile
stress, elastic modulus, and elongation at the break) and physical aspects after final
molding (flexibility, rigidity, and homogeneity) of the TPS (without laponite) were
investigated (data are not shown). The optimal condition was then determined
(120C, 50 rpm, and 20 min) and fixed for nanocomposite processing.
Characterization of nanocomposites
Field Emission Scanning Electron Microscopy. The TPS and TPS nanocomposites
surfaces were characterized by high-resolution Field Emission Scanning Electron
Microscopy (FE-SEM; Zeiss SUPRA 35). The samples were fractured under liquid
nitrogen, dried at 60C for 1 day under vacuum, and adhered onto an aluminum
stub covered with a thin silver layer.
Aouada et al. 111
X-Ray diffraction. The X-Ray diffraction (XRD) studies of the laponite RDS,
TPS, and nanocomposites were carried out using a Rigaku D/Max 2500PC
X-ray diffractometer (40 kV, 150 mA) equipped with Cu K
a
radiation (=
0.15406 nm) and a curved graphite crystal monochromator. All experiments were
carried out at ambient temperature with a scanning rate of 0.5/min and a step size
of 0.02in the range of 2y= 3–30.
Thermogravimetric analysis. The thermogravimetric analysis (TGA) was carried
out using TGA Q-500 equipment from TA Instruments (New Castle, United
States) from room temperature to 700C (or 973.15 K) at a heating rate of 10C/
min under nitrogen flow of 60 mL/min. An initial thermal degradation temperature
(T
d
initial) was reported by the onset degradation temperature where the weight
loss started to occur.
15
The maximum thermal degradation temperature (T
d maxi-
mum
) was calculated using maximum values of derivative thermogravimetric (DTG)
curves of the specimens.
Mechanical properties of nanocomposites. Mechanical properties (tensile
strength, Young’s modulus, and elongation at the break) were determined on
nanocomposites previously conditioned for 14 days at 53% RH (relative humidity
percentage) and room temperature using an Universal Testing Machine
(Model EMIC DL 500 MF) according to ASTM standard D638 for tensile
properties: specimen type IV and articulated screw action grips for maximum
capacity of 500N (50 kgf) – code EMIC GR018. Tensile strength was calculated
by dividing the maximum load for breaking film by the original cross-sectional
area of the sample. Elongation at break was calculated by dividing the difference in
the length at the moment of rupture by the original length of the sample or initial
gage length and multiplying by 100. Young’s modulus values were calculated by
the slope of the initial linear range of the stress–strain curve. The measurements
were conducted using an extensometer with a 50 kgf load cell operating at 10 mm/
min crosshead speed. Measurements were performed in replicate to check
reproducibility; error bars indicate the standard deviation (n= 5).
Results and discussion
TPS–laponite RDS nanocomposites formation
The thermoplastic process is well related in the literature.
15
Basically, the process is
composed by transformation of the semicrystalline starch granule into homoge-
nous material applying shear and heat in the presence of plasticizer agent.
The process occurs through of destruction of hydrogen bonds between the starch
molecules with new formation of hydrogen bonds between the plasticizer and
starch.
In this study, we reported the preparation of TPS and TPS–
laponite RDS nanocomposites through the combination of the following
112 Journal of Thermoplastic Composite Materials 26(1)
methods: (a) intercalation from the solution and (b) melt-processing. The illustration
of possible states of dispersion of laponite RDS into TPS matrix is shown in Scheme 1.
Evolution of nanocomposites formation by melt
viscosity using torque curves
Torque, temperature, and energy as a function of time for TPS and TPS nanocom-
posites were monitored during the processing. Figure 1(a) shows a decrease in
the torque values for TPS until they reach a plateau around 2 min and remains
constant until the conclusion of the experiment. The TPS did not present a
thermoplasticization stage, indicating that this stage was reached in the first step
of nanocomposite preparation; that is, intercalation from solution.
In contrast, TPS nanocomposites presented different behaviors. The torque
increased after 2 min and continued to increase for 12–16 min, depending on
the laponite RDS content in the TPS matrix. This result indicates a steady increase
in viscosity for this sample,
16
indicating that the second step of the processing
(melt-processing preparation) is necessary to complete the destruction, plasticiza-
tion, and homogenization of starch structures.
The melt temperature, shown in Figure 1(b), increased over time and reached
final temperature around 115–123C. This range value is very close to the initial
temperature of the mixing chamber fixed at 120C. In addition, in this temperature
range, significant degradations of TPS and laponite RDS molecules were not
expected (see the TGA section). The variation in energy as a function of residence
Scheme 1. Illustration of possible states of dispersion of laponite RDS into thermoplastic starch (TPS)
matrix.
Aouada et al. 113
time (processing time) for the same nanocomposites is shown in Figure 1(c).
The TPS energy increases linearly with processing time; the required energy
for TPS processing after 20 min was 38.4 kJ. This behavior could be related to
the torque changes (viscosity) as previously discussed (see Figure 1(a)). The
required energies for TPS processing were dependent on the laponite RDS content.
The energy values were 42.8, 45.8, and 46.1 kJ for laponite RDS concentrations of
1, 2, and 3 wt%.
Figure 1. Torque (a), temperature of melting (b), and energy (c) curves for thermoplastic
starch (TPS) and TPS–laponite RDS nanocomposites obtained by processing in a Haake
Rheomix at 120C at a speed of 50 rpm for 20 min.
114 Journal of Thermoplastic Composite Materials 26(1)
Unexpectedly, the TPS with 5% laponite RDS presented the lowest energy,
torque, and melt-temperature values. Possibly the high amount of laponite RDS
in the TPS matrix contributes to the dispersion of energy inside laponite galleries,
which could facilitate processing whereby both torque and melt-temperature
values decrease. Another effect that may be corelated is the decrease in the viscosity
of the melting.
Dispersion investigation by XRD
To investigate the dispersion of the laponite RDS layers in the TPS polymer
matrix, XRD analyses were performed on the nanocomposites. The diffraction
pattern for laponite RDS clay powder is shown in Figure 2. The pattern is consis-
tent with a montmorillonoid-type powder pattern showing some disorder in
the clay. In addition, several sharp diffraction peaks due to Na
4
P
2
O
7
are also
observed
17
at 2y= 19.64(basal d-spacing (d) = 0.45 nm); 2y= 20.11(d =
0.44 nm); and 2y= 26.46(d = 0.34 nm). The basal spacing of laponite RDS was
calculated from Bragg’s equation, = 2d sin y. An intensive peak at 2y= 6.40
corresponds to an interlayer basal spacing of 1.38 nm. In all XRD patterns of the
nanocomposites, no diffraction peaks between 2y= 3–12(Figure 2(b)) were
observed, indicating a good nanodispersion and exfoliation of the clay platelets,
that is, separated platelets dispersed individually in the TPS matrix. According to
Delhom et al.,
9
the lack of a diffraction peak for the one specific composite with
clay is a good indication that this composite is a true nanocomposite with the
polymer intercalated with the exfoliated clay nanospecimens.
Figure 1. Continued.
Aouada et al. 115
Morphologic investigation by FE-SEM
Figure 3(a) and (b) show FE-SEM micrographs of TPS and TPS–laponite RDS
nanocomposites containing 2% laponite. A homogeneous surface is observed
for both figures, indicating that the starch granules were completely disrupted and
the laponite was well dispersed in the polymer matrix. All TPS–laponite
Figure 2. (a) X-Ray diffraction (XRD) patterns of the laponite RDS, thermoplastic starch
(TPS), and TPS–laponite RDS nanocomposites and (b) XRD patterns expanded in the
2y= 3–12region showing the nanodispersion/exfoliation of the clay platelets.
116 Journal of Thermoplastic Composite Materials 26(1)
nanocomposites presented similar morphologies so their micrographs are not shown.
Similar behavior was observed in the laponite RD dispersed into biodegradable
starch described by Chung et al.
18
In addition, there was no phase separation between
laponite–TPS specimens, and no clay aggregation can be seen even at higher magni-
fications (see Figure 3(c)), which is a strong indication of good interaction, compat-
ibility, and miscibility between them and confirms well-dispersed nanocomposites.
Figure 3. Scanning electron microscope (SEM) micrographs of the fractured surface: (a) ther-
moplastic starch (TPS) and (b-c) TPS–laponite RDS nanocomposites containing 2 wt% laponite
at two different magnifications.
Aouada et al. 117
Thermal degradation investigation by TGA
Thermal degradation during the processing of starch and starch nanocomposites
is an important issue. The TGA has been the conventional and most popular
technique used to study the thermal stability and decomposition of starches
and their nanocomposites.
19–21
Figure 4 shows TGA and DTG results of TPS
and TPS nanocomposites with 1–5% laponite RDS. Table 1 showed that the
onset temperature (T
d initial
) of TPS degradation increased from 175C (or
448.15 K) to around 200C (or 473.15 K), indicating an increase in the thermal
stability when the laponite was incorporated in the TPS matrix. This improve-
ment has been documented by other researchers. For instance, Zaidi et al.
21
observed similar behaviors in the thermal analysis of cloisite-PLA nanocompo-
sites. The authors attributed the improvement of the thermal stability due to the
strong interaction between the clay and the polymer matrix. Table 1 also shows
that the nanocomposite maximum degradation temperature (T
d maximum
)
decreased slightly as compared to the TPS. In accordance to the Baniasadi
et al.,
20
possible reasons can be related to this effect. Firstly, the stacked silicate
layers could sustain accumulated heat and accelerate the degradation process;
and secondly, the clay itself can also catalyze the degradation of polymer
matrices.
The decomposition activation energies (E
t
) of the TPS and nanocomposites were
calculated from TGA curves by the integral method adapted from Horowitz et al.
22
Figure 3. Continued.
118 Journal of Thermoplastic Composite Materials 26(1)
as shown in Equation (1):
ln½ln ð1Þ1¼ Ety
RT2ini
ð1Þ
where ais the decomposed fraction, T
ini
is the initial decomposition temperature,
yis the difference between T–T
ini
, and Ris the gas constant.
Figure 5 shows the plots of ln [ln (1 – a)
–1
] versus y, and E
t
can be calculated
using the slope. The E
t
values were 10.60; 11.50; 11.03; 11.42; and 9.49 kJ/mol for
300 400 500 600 700 800 900 1000
Laponite RDS
TPS_5% Lap RDS
TPS_3% Lap RDS
TPS_2% Lap RDS
TPS_1% Lap RDS
TPS
Derivative of weight (% / K)
Temperature (K)
(a)
(b)
Figure 4. (a) Thermogravimetric analysis (TGA) and (b) derivative thermogravimetric (DTG)
curves of thermoplastic starch (TPS) and TPS–laponite RDS nanocomposites prepared at
different laponite RDS contents (1–5 wt%).
Aouada et al. 119
nanocomposites prepared with 0; 1; 2; 3; and 5 wt% laponite RDS. The increase in
the laponite RDS content caused an increased in E
t
values, which confirms the
improvement in the thermal stability of the TPS matrix up to 3% laponite. Despite
of their high initial decomposition temperature (Table 1), the TPS 5% lap had E
t
values lower than TPS. Probably, the decrease in E
t
is related to the low depen-
dence between ln [ln(1 – a)
–1
] and y, indicated by the low slope showed in Figure 5.
Mechanical properties
Tensile strength, Young’s modulus and elongation at the break properties
were determined to evaluate the influence of laponite RDS on the mechanical
behavior of TPS nanocomposites. Figure 6 shows the stress–strain curves for
TPS and TPS–laponite RDS nanocomposites prepared at different laponite
–20 –15 –10 –5 0 5 10 15 20
–1.75
–1.70
–1.65
–1.60
–1.55
y = –1.64788 + 0.00519 x
y = –1.76029 + 0.00615 x
y = –1.84688 + 0.00610 x
y = –1.75605 + 0.00613 x
y = –1.63376 + 0.00635 x
TPS_5% Lap RDS
θ (K)
–1.85
–1.80
–1.75
–1.70
–1.65
TPS_3% Lap RDS
–1.95
–1.90
–1.85
–1.80
–1.75
TPS_2% Lap RDS
ln [ ln (1 –
α)–1]
–1.85
–1.80
–1.75
–1.70
–1.65
TPS_1% Lap RDS
–1.75
–1.70
–1.65
–1.60
–1.55
–1.50
TPS
Figure 5. Plots of ln [ln (1 – a)
–1
)] versus yfor the determination of E
t
.
120 Journal of Thermoplastic Composite Materials 26(1)
contents (1–5 wt%). Table 2 showed that mechanical properties were improved by
the addition of laponite RDS. Tensile strength slightly varied from 1.7 0.2 to 2.0
0.1 MPa; but the Young’s modulus varied significantly from 11.3 0.9 to 16.3
0.6 MPa. The improvement in these properties may be due to good dispersion of
the clay platelets into the TPS matrix, which increases the reinforcement degree due
to the high interaction between the clay and the TPS matrix. This tendency was
also observed by Zhao et al.,
23
where the authors showed that mechanical prop-
erties of the polyamide 12/montmorillonite nanocomposites are very sensitive to
the degree of clay dispersion in the polymer matrix.
Although mechanical properties improve with the addition of laponite into the
TPS matrix, the effect on elongation at the break was not significant (average
Figure 6. The stress–strain curves for thermoplastic starch (TPS) and TPS–laponite RDS
nanocomposites prepared at different laponite contents (1–5 wt%).
Table 1. Thermal stability parameters of TPS and TPS–laponite RDS nanocomposites
obtained from TGA technique.
Nanocomposite T
dinitial
* (K) T
dmaximum
(K) T
dfinal
(K)
TPS 448.05 583.15 626.25
TPS_1% lap 474.95 578.55 628.65
TPS_2% lap 466.25 582.85 629.75
TPS_3% lap 472.65 580.05 630.55
TPS_5% lap 468.85 574.95 630.25
TPS: thermoplastic starch, TGA: thermogravimetric analysis.
*T
dinitial
or T
ini
.
Aouada et al. 121
values around 30%). This is an indicative that the TPS and TPS nanocomposites
had practically the same flexibility; a very important key in the industrial field
(mainly in packaging applications). In addition, several authors related the dimin-
ishing of the elongation at the break of the polymeric nanocomposites with the
addition of inorganic clay,
23,24
which may restrict their industrial application.
Conclusions
It was possible to obtain TPS and TPS–laponite RDS nanocomposites through a
simple procedure involving the combination of intercalation from solution and
melt-processing preparation methods. In XRD spectra of the nanocomposites,
no diffraction peaks between 2y= 3–12(corresponding to the laponite RDS
diffraction peak) were observed, indicating a good nanodispersion and intercala-
tion of the clay platelets. In addition, this result is a good indication that the TPS–
laponite RDS composite is a true nanocomposite with the polymer intercalated
with the exfoliated clay nanospecimens.
The presence of laponite RDS improved the thermal stability and mechanical
properties of the TPS matrix due to the reinforcement effect of the laponite max-
imized by a high interaction with the TPS matrix. As a consequence, the Young’s
modulus varied from 11.3 0.9 to 16.3 0.6 MPa, when the laponite RDS amount
was increased from 0 to 5 wt%. These results indicate that TPS–laponite RDS
nanocomposites with a good degree of exfoliation have great potential for indus-
trial applications (more specifically in the packaging field).
Funding
The authors are grateful to Instituto Nacional de Cieˆ ncias dos Materiais em
Nanotecnologia (INCTMN), National Council for Scientific and Technological
Development (CNPq-Brazil), Foundation for Research Support of Sa
˜o Paulo
(FAPESP), FINEP/MCT for their financial support and fellowships.
Table 2. Mechanical properties of TPS and TPS–laponite RDS nanocomposites obtained from
tensile tests.
Nanocomposite
Tensile
strength (MPa)
Young’s
modulus (MPa)
Elongation at
break (%)
TPS 1.7 0.2 11.3 0.9 25.0 3.6
TPS_1% lap 1.7 0.1 12.8 2.9 25.4 2.4
TPS_2% lap 1.8 0.2 16.1 1.4 27.9 4.2
TPS_3% lap 1.8 0.1 15.7 1.7 27.9 4.2
TPS_5% lap 2.0 0.1 16.3 0.6 30.3 4.0
TPS: thermoplastic starch.
122 Journal of Thermoplastic Composite Materials 26(1)
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