There is an industrial interest in biodegradable films, especially
for packaging. Starch is an abundant and low-cost polymer that comes
from renewable resources, and over the last three decades, starch has
been studied as a component in films and plastics (1-4).
The majority of studies on biodegradable films are focused on
casting method (5-10), but the blown extrusion is a more cost effective
processing method, since it is faster, requires less space, and demands
a smaller number of production stages compared with casting. Extrusion
is also the method utilized in large-scale production of commercial
films. However, the production of extruded films with only starch is not
viable, since starches have low mechanical resistance and high water
vapor permeability (WVP). As such, starches do not have suitable
characteristics for commercial production and applications (3), (11-13).
One solution to this problem would be to work with blends of starch and
biodegradable polymers such as poly (butylene adipate-co-terephthalate)
(PBAT), which has mechanical characteristics similar to the
characteristics of conventional polymers such as polyethylene and
polypropylene (11), (14), (15). Fatty acids can also be incorporated to
decrease WVP (16).
For film formation through blown film extrusion, it was necessary
to have a blend that supported both the longitudinal tension exerted by
the spooler as well as the force exerted by the injected air, which
expands the matrix in the transversal direction. If the polymer blend
does not possess adequate viscoelastic properties during the extrusion
process, no film will form. For production of films by extrusion using
polymer blends, the components of the blend were first extruded in an
initial step for the production of cylindrical strands, after which the
strands were cut to form pellets (see Fig. 1).
[FIGURE 1 OMITTED]
The objective of this study was to determine the mechanical and
viscoelastic properties of extruded cylinder strands composed of polymer
blends and to verify the correlation between the properties of the
blends and film formation through blown extrusion. Therefore, the
development of biodegradable films from polymer blends would be
accelerated, since only films with adequate characteristics in the
extruded strands would form. The polymer blends were produced by
extrusion and contained starch, glycerol, fatty acids (caproic, lauric,
and stearic), and PBAT.
MATERIALS AND METHODS
Starch from cassava (Manihot esculenta) was provided by Indemil
(Diadema SP, Brazil) under the brand name AmidoMani. The biodegradable
polymer PBAT was provided by BASF under the commercial name Ecoflex
[R]-F. Commercial glycerol (Dinamica, Brazil) was utilized as a
plasticizer, and the fatty acids that were used were caproic acid (99%,
Vetec, Brazil), lauric acid (98%, Vetec, Brazil), and stearic acid (PA,
Production of the Extruded Strands Composed of Starch, Glycerol,
Fatty Acids, and Films
The extruded cylinders were produced in a BGM brand mono-screw
pilot extruder (model EL-25, Brazil) equipped with a screw measuring 250
mm in diameter. The screw velocity was 35 rpm with a temperature of 120
[degrees] C in the three cannon zones and at the pellet forming matrix.
The formulations for the extruded material are shown in Table 1.
The fatty acid was mixed with glycerol with a mixer for
approximately 1 min. These glycerol + fatty acid mixtures were
incorporated with the starch, and the blend was extruded to produce
cylinders composed of thermoplastic starch (TPS) and fatty acids.
The films were produced by blow extrusion using a pilot extruder
BGM (model EL-25, Brazil) with a blowing system made up of a 250-mm
diameter screw, four heating zones, and an external cooling air ring
with a 300-350-mm diameter for film formation. The screw speed was
maintained at 35 rpm, and the temperature was kept at 120[degrees] C.
The diameter of the extruded cylinders was determined using a
digital caliper (Mitutoyo, Brazil) with a resolution of 0.001 mm.
Mechanical Tension Tests
Tension tests of the strands were performed in a texturometer
(Stable Micro Systems, model TA TX2i, England) in accordance with a
modified method from the American Society for Testing and Materials
(ASTM D-882-00, 2001). The specimens that were 60 mm in length were
adjusted to the grips of the equipment, which had an initial distance of
30 mm and a velocity of 0.8 mm/s. The properties determined in the
tension tests were maximum tensile resistance (MPa), alignment at
rupture (%), and Young's modulus (MPa).
A total of 10 tests were conducted for each formulation. Prior to
testing, the samples were conditioned under 53% relative humidity (RH)
for 48 h.
Mechanical Shear Force Tests
The shear force tests of the strands were performed using a
texturometer (Stable Micro Systems, model TA TX2i, England) with the
probe HDP/BSK BLADE SET WITH KNIFE. The pre-test, test and post-test
velocity was 5 mm/s and the trigger force was 0.05 N. The shear force
resistance (F) was determined for samples measuring 30 mm in length.
A total of 10 tests were conducted for each formulation. Prior to
testing, the samples were conditioned under 53% RH for 48 h.
Mechanical Relaxation Tests
Relaxation tests on the cylinders were conducted using a
texturometer (Stable Micro Systems, model TA TX2i, England) in
accordance with a method provided by the ASTM D-882-91, 1996. The
samples (130 mm in length) were adjusted to the grips of the equipment,
which had initial distance of 100 mm and a velocity of 0.8 mm/s until 1%
of the rupture elongation was reached (which was determined previously
in the tension tests), and the decrease of the force was measured at
0.04-s intervals for 1 min. Adjustments to the relaxation data were
calculated in accordance with Eq. 1.
%relaxation = [- (F (60s)/[F.sup.0])] x 100
The relaxation data were normalized and analyzed with an empirical
model proposed by Peleg (17), in which the force (F) was normalized
through a parameter of decline F(t) as shown in Eq. 2 (8).
F(t)/ [F.sup.0] =1 - [C.sup.1] * t/[C.sup.2] + t
Parameters [C.sup.1] and [C.sup.2]of Eq. 2 were calculated by
non-linear regression using STATISTIC A 5.0 software (Statsoft, 1995).
Analysis of the Data
From the tension, shear, and relaxation tests, parameters were
obtained to compare samples from different blends. The differences
between sample properties were determined by an analysis of variance
(ANOVA) and Tukey tests with 5% significance. The tests were conducted
in STATISTICA 5.0 software (Statsoft, 1995).
RESULTS AND DISCUSSION
The PPBAT formulation containing 50% PBAT in its composition (Table
1), along with starch and glycerol, was used as a standard to compare to
the other formulations, since this formulation was studied by Costa (13)
and presented a good capacity for producing high quality film via blown
In a subjective evaluation, the extruded cylinder strands contained
12.8% glycerol (C128, [C.sub.a] l28, [L.sub.a]l28, and E128), which was
rigid and fragile, likely due to the low concentration of glycerol and
did not permit adequate plasticization. The strands containing fatty
acids ([C.sub.a], [L.sub.a], and E) presented a lower rigidity than the
control (C), but the strands were still quite rigid. Therefore, these
strands could not be analyzed in terms of maximum mechanical force or
tested with relaxation tests.
Considering that the incorporation of PBAT increases the mechanical
properties of the film, an amount of 30% of PBAT was added to the
formulation to verify if the extruded strand properties were correlated
with the film formation by blown extrusion. A subjective evaluation of
the films was performed to verify the homogeneity, handling, and
tendency to tear, and the results are presented in Table 2.
After the analysis of the extruded strands, PBAT were incorporate
to the formulations. All the formulations were film forming, but that
ones with stearic acid presented the poorest subjective characteristics
with low homogeneity and high tendency to tear.
Diameters of the extruded cylinder strands varied from 247 to 371
[micro] m, and the C25 strand had the highest value followed by C20.
These two formulations contained only starch and glycerol. After exiting
the extruder, these formulations experience greater expansion than the
others. The diameter of the extruded cylinders did not correlate with
the glycerol concentration.
The extruded strands that were similar to the standard PBAT
formulation were the formulations the contained caproic acid, likely due
to the better plasticization caused by caproic acid when compared with
the other fatty acids. Formulation El28 was also similar to the
standard, but its extruded strands were more rigid and fragile, which
has been previously reported.
Mechanical Tension Tests
The control formulation with 12.8% glycerol, CI28, did not support
the mechanical test conditions, since the samples were very rigid and
fragile. The tension tests were performed on the [C.sub.a] l28,
[L.sub.a] l28, and E128 formulations, since these formulations possessed
characteristics that were different compared with the formulations that
contained 20% or 25% glycerol. As such, these formulations were compared
separately. Table 3 shows the results of the mechanical tests in terms
of maximum tension (T), elongation at rupture ([epsilon]), and
Young's modulus (Y) for the extruded cylinder strands.
The formulations [C.sub.a] l28, [L.sub.a] 128, and El28 presented
maximum tensile resistance values nearly 10 times greater than the
values presented by the formulations with 25% glycerol. These samples
also presented elongations at rupture that ranged from 2% to 10%, which
was 30 times lower than the other formulations. The values of the
Young's modulus were between 288 and 537 MPa, which was roughly 25
times greater than the greatest value of the other formulations.
The strand extruded with caproic acid ([C.sub.a] l28) presented the
greatest resistance to tension and greatest elongation in relation to
the other formulations. The strands [L.sub.a] 128 and El28 presented a
lower elongation, probably due to the greater compatibility of starch
with the shorter chain fatty acid ([C.sub.a]). This compatibility
decreased with the increase in the carbonic chain of the fatty acid due
to its non-polar characteristics, which were reflected in the mechanical
properties of the extruded strands. This incompatibility between starch
and the long chain fatty acids may generate weaker blends with poorer
The maximum tension of the extruded strands varied from 0.7 MPa to
3.2 MPa. Tension is linked to plasticization of starch; a TPS with
greater plasticization presents lower tension due to its lower rigidity
and greater chain mobility in the structure. An increase in the
concentration of glycerol from 20% to 25% caused a decrease in the
resistance to tension by approximately 7% for the control formulation
and 60% for the formulations with fatty acids. The formulation
presenting the greatest reduction was the one that contained lauric
acid, where there was a reduction of 65% from formulation [L.sub.a] 20
to [L.sub.a] 25.
Plasticization modified the mechanical properties of the materials
by reducing the maximum force and increasing elongation. Therefore, the
plasticization effect of the fatty acids was not demonstrated by the
mechanical properties of the extruded strands. This result may be due to
the low interactions between starch and the studied fatty acids, which
could have generated mechanically weaker materials with lower
A possible explanation for the trends in maximum force and
elongation of the extruded strands may be related to hydroxyl groups
(OH) and fatty acid molar mass. The hydroxyl groups directly influence
plasticization. Therefore, the formulations containing caproic acid had
one OH group per mole of acid (116.16 g). The formulations with lauric
acid had one OH per 200.32 g, and the formulation with stearic acid had
one OH group per 284.48 g. For the same mass of fatty acid, the
formulations containing caproic acid presented the greatest quantity of
hydroxyl groups per mass and thus had greater plasticization.
Only the control presented an increase in the value of elongation
at ruptures with an increase in the glycerol concentration from 20% to
25%. The strands extruded with fatty acids presented a reduction of
approximately 35% in elongation when the glycerol concentration
increased from 20% to 25%, which apparently contradicted the affirmation
that greater glycerol concentrations facilitated movement of starch
chains, thereby increasing film flexibility (1). However, the
Young's modulus decreased with an increase in the glycerol content
from 20% to 25%, indicating a reduction in rigidity by roughly 64% of
the extruded strands containing fatty acids. This occurred do to an
increase in glycerol concentration, which reduced the maximum tension by
roughly 60%, whereas the reduction in the control (C) was only 7%. The
increase in glycerol concentration drastically reduced the mechanical
resistance and the elongation at break of the strands containing fatty
acids, probably due to an incompatibility among the polar components and
the fatty acids that decreased the interactions between the starch
chains and weakened the polymer matrix.
Because the mechanical properties of the extruded strands differed
significantly from the PBAT formulation, which served as a reference for
film formation, it was not possible, based on the studied formulations,
to predict from these properties if a film would form. Despite the
differences between the formulations containing fatty acids and the
standard, all the formulations were capable of film forming (Table 2).
Mechanical Shear Stress Tests
The maximum values of resistance to shear stress (F) for the
extruded cylinder strands are presented in Table 3. The maximum
resistance to shear stress varied from 7.6 to 36.5 MPa. The strands with
12.8% glycerol presented the greatest resistance to shear force and were
the most rigid, as described previously. These strands also did not
present the necessary characteristics to perform all of the tests. There
was a trend in the reduction of the resistance to shear force with an
increase in the glycerol concentration of the extrusions, which may be
associated with the plasticization effect of glycerol that occurs when
it is added to starch (18), (19).
The addition of fatty acids resulted in a decrease in the
resistance to shear force, characterizing the plasticizing effect of the
fatty acids. However, the size of the carbonic chain in the fatty acids
did not influence this parameter. The maximum shear stress remained
within 7.6 MPa for the addition of six carbon fatty acids (caproic acid)
and 12.8 MPa for the addition of stearic acid (18C).
The strand of the PBAT presented a greater resistance to shear
force than the other formulations, since PBAT provided resistance to the
extruded strand as well as the film formed during the extrusion process.
The incorporation of this polymer into the extruded strands containing
fatty acids would probably result in an increase in resistance to shear
Maximum Strain and Relaxation Percentage
The maximum strain was selected as 1% of the length of the sample
due to the linear behavior of the force in the function of elongation in
this range. In this region, the specimen suffered deformation with the
applied strain, and when the force was removed, the specimen returned to
its initial state.
Figure 2 shows the relaxation curves for the strands containing 20%
and 25% glycerol in the formulation as well as the standard (PPBAT). All
of the strands were conditioned at 53% RH. Initially, an ascending curve
obtained by the imposition of a constant deformation rate was noticed;
subsequently, it was observed that the force necessary to maintain
deformation declined with time. This behavior is characteristic of
viscoelastic materials (17), (20),
[FIGURE 2 OMITTED]
Table 4 presents the results of maximum force (F0) and the
deformation percentages for the formulations with 20% and 25% glycerol.
The relaxation force is associated not only with the intrinsic
characteristics of the material but also with the conditions during the
test, which makes a comparison with other results found in the
literature difficult. Viscoelastic characteristics were also observed in
biofilms by Muller et al. (8), who studied the incorporation of
cellulose fibers in starch films produced through casting. Similarly,
Cuq et al. (21) and Chandra and Sobral (20) studied myofibrillar protein
films. However, no data was encountered in the literature for the
viscoelastic strands of extruded strands.
For the extrusions containing 20% glycerol in the formulation, the
encountered maximum force ([F.sub.0]) values varied from 3.7 to 9.5 N.
The forces of extruded strands [C.sub.a] 20 and E20 were nearly equal.
However, formulation La20 presented a lesser force among the analyzed
formulations that was nearly three times less than the force presented
by the control formulation (C20).
The maximum force was related to the plasticizing effect of the
different fatty acids, and therefore it may be inferred that lauric acid
has a greater plasticizing effect than the stearic and caproic acids.
The same behavior may be observed for the formulations with 25%
glycerol. The maximum force of the [L.sub.a] 25 strands differed from
the other formulations. These formulations had values less than half the
force presented by formulations [C.sub.a] 25 and E25, and roughly 10
times less than the force values for the control (C25).
In general, the formulations with 25% glycerol presented a lower
maximum force, a behavior that may be associated with the increase in
plasticization of the extruded strands that paralleled the increase in
glycerol content. The results of the relaxation percentage indicated
that the PBAT sample presented a lower decrease in force relative to the
initial force compared with the other studied formulations. This
behavior was indicative of the more pronounced elastic characteristics
in this sample.
According to Peleg , the value (1 - [C.sub.1]) may be
considered a degree of solidity, whereas ([C.sub.1]/ [C.sub.2]) is the
initial relaxation rate. When (1 - [C.sub.1]) approaches one, the
material behaves as an elastic solid. When this value approaches zero,
the material behaves as a viscous liquid. Values of the constants for
the model are presented in Table 5.
With the exception of formulation [C.sub.a] 25, the addition of
1.5% fatty acids did not appear to influence the degree of solidity (1 -
[C.sub.1]). In other words, the fatty acids did not alter the
viscoelastic character of the extruded strands. Strand [C.sub.a] 25 was
the formulation that presented the best elastic characteristic (0.35)
within the proposed formulation. This behavior may be due to the
interaction between the smaller carbonic chain and starch.
The PBAT strand was more elastic with a greater value of (1 -
[C.sub.1]), which was 0.67 according to the results of the relaxation
percentage. This high value may be an indicator of the characteristics
extruded strands should have for the formation of films through blown
The initial relaxation rate, characterized by the value of
([C.sub.1]/ [C.sub.2]), was influenced by three factors: glycerol
concentration, presence of fatty acids, and the size of the fatty acids
that were present. When this value approached zero, the material
presented a lower relaxation, further resisting the force to which it
was submitted. The ([C.sub.1]/ [C.sub.2]) values of the extruded strands
decreased by approximately 18% when the glycerol concentration increased
from 20% to 25%, which may be due to the plasticizing effects of
glycerol (18), (19).
Among the formulations containing fatty acids, those with caproic
acid presented lower initial relaxation rates, likely due to the greater
interaction between caproic acid and starch. The formulations containing
lauric and stearic acid presented greater initial relaxation rates, and
this result may be related to the greater hydrophobic characteristics of
these components as well as the minimal interaction between these
components and starch, which makes up the polymer matrix.
The PBAT extruded strand presented the lowest value for the initial
relaxation rate (0.29) and presented greater elasticity. Therefore, the
PBAT strand is better able to withstand the applied force. Similar to
the degree of solidity, the value of the initial relaxation rate may be
an indicator of film formation.
It was not possible to correlate the properties of the extruded
strands from the mechanical tests of maximum tension, elongation at
break, and maximum resistance to film formation via blown extrusion. The
viscoelastic characteristics (initial rate and percentage of relaxation
and degree of solidity) are determinant in the processability of polymer
blends studied with the goal of film formation by blown extrusion.
The authors acknowledge CAPES, CNPq, and the Araucaria Foundation
for their financial support as well as the Postdoctoral and Productivity
in Research scholarship program.
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adipalo co-tereftalato) produzidos por extrusao, Londrina Tese de
doutorado (Doulorado em Ciencia de Alimentos), Univcrsidade Estadual de
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Ciencia e Tecnologia de Alimentos), Universidade Estadual de Londrina,
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Collares-Queiroz, Starch Starke, 61, 528 (2009).
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Marcelo Medre Nobrega, (1) Evandro Bona, (2) Carmen Maria Olivera
Muller, (1) Fabio Yamashita (1)
(1) Department of Food Science and Technology, State University of
Londrina, P.O. Box 6001, C.E.P. 86051-990, Londrina, PR, Brazil
(2) Post-Graduation Program of Food Technology (PPGTA), Federal
University of Technology - Parana (UTFPR), P.O. Box 271, C.E.P.
87301-006, Campo Mourao, PR, Brazil
Correspondence to: Marcelo Medre Nobrega; e-mail: marcelo.medre@
TABLE 1. Formulations of the extruded strands.
Formulation Fatty Fatty Starch Glycerol PBAT
acid acid (%) (%) (%)
PPBAT -- -- 37.5 12.5 50
C128 -- 86.9 13.1 --
C20 -- -- 80 20 --
C25 -- -- 75 25 --
[C.sub.a] Caproic 1.5 85.7 12.8 --
[C.sub.a] Caproic 1.5 78.8 19.7 --
[C.sub.a] Caproic 1.5 73.9 24.6 --
[L.sub.a] Laurie 1.5 85.7 12.8 --
[L.sub.a] Laurie 1.5 78.8 19.7 --
[L.sub.a] Laurie 1.5 73.9 24.6 --
E128 Stearic 1.5 85.7 12.8 --
E20 Stearic 1.5 78.8 19.7 --
E25 Stearic 1.5 73.9 24.6 --
TABLE 2. Subjective evaluation of films made of starch,
glycerol, and PBAT with the addition of fatty acids.
Film Film forming Homogeneity Handling Tendency
PPBAT Yes XXX XXX No
C20 Yes XX XXX No
C25 Yes XXX XXX No
[C.sub.a] 20 Yes XXX XXX No
[C.sub.a] 25 Yes XXX XXX No
[L.sub.a] 20 Yes XXX XXX No
[L.sub.a] 25 Yes XXX XXX No
E20 Yes X XXX Yes
E25 Yes X XXX Yes
PPBAT - extruded with 50% PBAT and 25% glycerol in
relation to 50% starch. C20 and C25 - films with 30%
of PBAT, 20% and 25% of glycerol compared with 70% of
thermoplastic starch, respectively.[C.sub.a] 20,
[C.sub.a] 25, [L.sub.a] 20, [L.sub.a] 25, E20, and
E25 - films with 30% of PBAT, 20$ and 25% of glycerol
compared with TPS and 1.5% of caproic, lauric, and
stearic acid, respectively, x - deficient, xx - good,
and xxx - excellent.
TABLE 3. Maximum tension (7), elongation at rupture
(member of). Young's modulus (10, and Maximum shear
stress (F) of the extruded strands composed of starch
and glycerol with the addition of fatty acids.
Formulation T [(MPa). Y F
PPBAT 4.4 [+ or-] 189 [+ or -] 20[+ or -] 19.4[+ or-]
[0.1.sup.a] [25.sup.a] [1.sup.a] [1.2.sup.c]
C20 1.5 [+ or 106 [+ or -] 5 [+ or -] 12.0[+ or-]
[0.0.sup.e] [6.sup.b,c] [0.sup.e] [0.4.sup.d]
C25 1.4[+ or-] 119 [+ or -] 4[+ or -] 11.6[+ or-]
[0.0.sup.e] [4.sup.b] [0.sup.e] [0.3.sup.d]
[C.sub.a] 1.9 [+ or-] 108 [+ or -] 7[+ or -] 7.6[+ or-]
[0.0.sup.d] [8.sup.b,c] [0.sup.d] [0.2.sup.f]
[C.sub.a] 0.7[+ or-] 74 [+ or -] 2[+ or -] 8.7[+ or-]
[0.0.sup.g] [2.sup.c,d] [0.sup.f] 0.3.sup.e,f
[L.sub.a] 3.2 [+ or-] 87 [+ or -] 14[+ or -] 14.1[+ or-]
[0.0.sup.b] [2.sup.b,c,d] [0.sup.b] [0.3.sup.d]
[L.sub.a] 1.1[+ or-] 59 [+ or -] 4[+ or -] 8.3[+ or-]
[0.0.sup.f] [2.sup.d] [0.sup.e,f] [0.1.sup.f]
E20 2.4 [+ or-] 90 [+ or -] 8[+ or -] 12.8[+ or-]
[0.0.sup.c] [2.sup.b,c,d] [0.sup.c] [0.3.sup.d]
E25 1.0 [+or -] 58 [+ or -] 4[+ or -] 8.3[+ or-]
[0.1.sup.f] [3.sup.d] [0.sup.e,f[ [0.3.sup.f]
[C.sub.a] 14.3 [+ or-] 10 [+ or -] 362[+ or-] 24.4[+ or-]
0.2 16 [0.5.sup.b]
[L.sub.a] 13.9 [+ or-] 2 [+ or -] 537[+ or-] 36.5[+ or-]
1.2 0 44 [2.7.sup.a]
E128 8.4[+ or-] 3[+ or -] 288[+ or-] 26.9[+or-]
1.1 0 42 [0.9.sup.b]
PPBAT - extruded with 50% PBAT and 25% glycerol in relation
to 50% starch. C20 and C25 - extruded with 20% and 25%
glycerol in relation to starch, respectively. [C.sub.a]
20, [C.sub.a] 25, [L.sub.a ]20, [L.sub.a] 25, E20, and
E25 -extruded with 20% and 25% glycerol in relation to
starch and 1.5% caproic, lauric, and stearic acid,
respectively. Cal28, La 128, and El28 -extruded with
12.8% glycerol in relation to starch and 1.5% caproic,
lauric, and stearic acid, respectively.
(a) Averages with the same letters in the same column
did not present significant differences in the Tukey
test (p < 0.05).
TABLE 4. Maximum force (F0) and relaxation percentage
of strands extruded from cassava starch and fatty acids.
Formulation % [F.sub.0] %
glycerol [(N).sup.a] [relaxation.sup.a]
C 20 9.5 [+ or -] 81 [+ or -]
25 9.7 [+ or -] 73 [+ or -]
[C.sub.a] 20 6.4 [+ or -] 70 [+ or -]
25 2.2 [+ or -] 65 [+ or -]
[L.sub.a] 20 3.7 [+ or -] 86 [+ or -]
25 0.9 [+ or -] 78 [+ or -]
E 20 5.5 [+ or -] 80 [+ or -]
25 2.2 [+ or -] 70 [+ or -]
PPBAT 25 4.8 [+ or 32 [+ or -]
C - starch extrusions with 20% and 25% glycerol. [C.sub.a],
[L.sub.a], and E -starch extrusions with 20% and 25%
glycerol in relation to TPS and 1.5% caproic, lauric,
and stearic acid, respectively. PPBAT - starch extrusion
with 50% PBAT and 25% glycerol in relation to TPS.
(a) Averages with the same letters in the same column
did not present significant differences in the Tukey
test (p < 0.05).
TABLE 5. Parameters of the Peleg model for relaxation
tests of the extruded starch strands with addition of
fatty acids, conditioned at 53% relative humidity.
Formulation % glycerol [(1 - [C.sub.1]) [([C.sub.1]/
.sup.a] [C.sub.2]).sup.a] (s)
C 20 0.18 [+ or -] 0.96 [+ or -]
25 0.27 [+ or -] 0.83 [+ or -]
[C.sub.a] 20 0.29 [+ or -] 0.85 [+ or -]
25 0.35 [+ or -] 0.72 [+ or -]
[L.sub.a] 20 0.14 [+ or -] 1.19 [+ or -]
25 0.22 [+ or -] 0.96 [+ or -]
E 20 0.19 [+ or -] 0.98 [+ or -]
25 0.29 [+ or -] 0.73 [+ or -]
PPBAT 25 0.67 [+ or -] 0.29 [+ or -]
C - starch extrusions with 20% and 25% glycerol. [C.sub.a],
[L.sub.a], and E -starch extrusions with 20% and 25% glycerol
in relation to TPS and 1.5% caproic, lauric, and stearic acid,
respectively. PPBAT - starch extrusion with 50% PBAT and 25%
glycerol in relation to TPS.
(a) Averages with the same letters in the same column did not
present signification differences in the Tukey test (p < 0.05).