Preparation and Characterization of Biodegradable Agar/Poly(butylene adipate- co-terephatalate) Composites
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Preparation and Characterization of
Biodegradable Agar/Poly(butylene adipate-
co-terephatalate) Composites
T.J. Madera-Santana,1 M. Misra,2 L.T. Drzal,3 D. Robledo,1 Y. Freile-Pelegrin1
1
Department of Marine Resources, Cinvestav, Unidad Mérida, Carretera antigua a Progreso Km, 6. A.P.73
Cordemex, Mérida, Yucatán, México
2
School of Engineering and the Department of Plant Agriculture, Thornbrough Building,
University of Guelph, Ontario, N1G 2W1, Canada
3
Composite Materials and Structures Center, Michigan State University, 2100 Engineering Bldg.,
East Lansing, Michigan 48824-1226
A series of biocomposites were developed by reinforc- INTRODUCTION
ing agar particles from red marine plant Gelidium
robustum into poly(butylene adipate-co-terephatalate) Biodegradable polymers blending with low-price or-
(PBAT) using extrusion and injection molding tech- ganic fillers to produce composites are an alternative to
nique. The effect of different content of agar (0, 10, 20, solve pollution problems and have attracted considerable
30, and 40 wt%) on the physico-mechanical properties
of the biocomposite was evaluated. The dynamic me- attention as green materials. Among commercially avail-
chanical behavior of the composites was studied to able biodegradable polymers, biopolyesters have shown
determine the storage and loss modulus. The incorpo- similar properties of nonbiodegradable polymers. Some
ration of agar particles into PBAT enhanced the tensile aliphatic and aromatic polyesters are one of the most
strength and modulus with a reduced percentage of important classes of the synthetic biodegradable polymers
elongation at break. A reduction in the mechanical loss
factor (tan d) was noticed with the addition of agar par- available in a variety of types. For instance, polycaprolac-
ticles into PBAT. A reverse trend was noticed for stor- tone (PCL), polyhydroxybutyrate (PHB), poly(3-hydroxy-
age and loss modulus. The thermogravimetric analysis butyrate-co-3-hydroxyvalerate) Biopol1, poly(lactic acid)
revealed that the degradation temperature of PBAT- or poly(L-lactide) (PLA), poly(butylene succinate) (PBS),
agar composites lies in between that of their individual poly(butylene succinate-co-adipate) Bionolle1 from
components (agar and PBAT). An increase in melting
(Tm) and crystallization (Tc) temperature of the bio- Showa Highpolymers, the copolyester based on 1,4 buta-
composites were noticed as agar particle content nediol adipic acid and terephatalic acid (EASTAR-BIO1
increased. The rheological study carried out by and ECOFLEX1) and poly(ester amide)s named BAK
dynamic frequency experiments demonstrated that vis- 10951 from BAYER. The main advantage of these poly-
cosity is increased with the presence of agar particles. mers is the wide diversity of mechanical and physical
The morphology of the biocomposites was analyzed
using scanning electron microscope. POLYM. ENG. SCI., properties that are comparable to polymers such as low
49:1117–1126, 2009. ª 2009 Society of Plastics Engineers and high density polyethylene (LDPE, HDPE) and poly-
propylene (PP) [1–4]. Poly(butylene adipate-co-terephata-
late) (PBAT) is a biodegradable polyester supplied by
BASF and by Eastman Chemical Company. It is ali-
phatic–aromatic polyester of butylene glycol and adipic
and terephthalic acids and its high percentage elongation
(flexible nature) makes it suitable for food packaging and
agricultural film application [5].
Correspondence to: Y. Freile-Pelegrin; e-mail: freile@mda.cinvestav.mx Nowadays, biocomposites research is focused in cost
Contract grant sponsor: SAGARPA-CONACYT; contract grant number:
reduction of biodegradable plastics. On this regard, the
2002-C01-1057.
DOI 10.1002/pen.21389
blending of low-cost fillers into the biodegradable poly-
Published online in Wiley InterScience (www.interscience.wiley.com). mer has become an alternative solution and, over the past
V
C 2009 Society of Plastics Engineers two decades, terrestrial plants fibers, mainly starch, have
POLYMER ENGINEERING AND SCIENCE—-2009been receiving considerable attention. However, because TABLE 1. Processing conditions used for PBAT-agar compositions
plant fibers are derived from ligno-cellulose, which con- studied.
tains polarized hydroxyl groups, major limitations of Mixing Mixing Force values Injector
using them include poor interfacial adhesion and difficul- time temperature at steady temperature
ties in mixing due to poor wetting of the fiber with the Sample (min) (8C) state (N) (8C)
matrix, reducing greatly their potential to be used as fill-
ers. In addition, the high energy prices concerns about PBAT 100% 10 135 1150 (45) 135
PBAT 90% þ Agar 10% 10 135 1230 (63) 140
petroleum supplies and the greater recognition of the PBAT 80% þ Agar 20% 10 135 1470 (77) 143
environmental consequences of fossil fuels have driven PBAT 70% þ Agar 30% 10 135 1580 (69) 148
interest in the bioenergy [6]. Under the expectative of PBAT 60% þ Agar 40% 10 135 1920 (92) 155
high consume of sources of terrestrial biomass for this
target, it should look forward to other sources of biode- Note: Standard deviations are given in parenthesis.
gradable fillers that could replace or compete with plant
fibers. An attractive alternative are biopolymers from ma- dium robustum and it was used as-received. Gelidium
rine environment, such as agar, that are extracted from robustum is exploited commercially in the Baja California
marine plants (seaweeds). peninsula, and it is the main agar source in Mexico [14].
Among the industrially attractive biopolymers from The particle size distribution of the biofiller was deter-
marine environment, agar is known to widely use in dif- mined using light scattering instrument (Mastersizer 2000,
ferent industrial fields (i.e., food and pharmaceutical). Malvern Instruments, Southborough, MA) according to
Agar polymers synthesized by species of red seaweeds the procedure described by Le Digabel and Avérous [15].
belonging to the genus Gracilaria, Gelidium, and Ptero- An aliphatic–aromatic polyester resin of butylene glycol
cladia constitute a complex mixture of molecules, con- and adipic and terephatalic acids (PBAT) made by East-
taining several extremes in their structure. Sulphate hemi- man Chemical Corp., under trade name EASTAR BIO
esters, methyl ethers, and pyruvic ketals can alter in a was used as a polymer matrix.
number of ways the structural regularity of agar based on
strictly 1,3-linked b-D-galactopyranose and 1,4-linked 3,6 Biocomposite Preparation
anhydro-a-L-galactopyranose residues. It belongs to the
class of gel-forming polymer, and its microstructural, me- The agar was dried at 608C for 24 h under vacuum
chanical, and rheological properties of agar gels can be and stored over desiccant in sealed bottle before process-
described by a ‘‘crosslinked network’’ model [7, 8]. In ing. The PBAT polyester matrix was predried at 608C for
this model, a homogeneous aqueous sol is gradually 8 h before process. The biofiller content on the compo-
changing to an elastic and turbid gel network during cool- sites were 0, 10, 20, 30, and 40 wt%. The PBAT-agar
ing. This transformation is reversible and this cycle could composites were extruded at 100 rpm with a micro 15 cc
be repeated several times without compromising gel me- compounding system (DSM Research, Geleen, The Neth-
chanical properties [9]. These colloids have been explored erlands) at 1358C for 10 min. The characteristics of the
as biodegradable films [10], casting archeological pieces, extruder are: a screw length of 150 mm, a L/D of 18, and
dental, and sculpture moulds [11], as well as composites a neat capacity of 15 cm3. To obtain the tensile, bar, and
[12, 13]. A thorough literature study revealed that no disc specimens for mechanical, thermal, and rheological
much work has been done on the agar incorporated into analysis, the molten composite samples were transferred
polymeric matrices. Hence, the present research concen- after extrusion through a preheated cylinder to a mini-
trated on the development of agar particle reinforced injection molder. The initial injection temperature was
PBAT biocomposites. The composites were developed 1358C and cooling system of the mold was 258C (room
using extrusion and injection molding technique. The temperature); however, modification on the injection tem-
PBAT-agar biocomposites were characterized for physico- perature was conducted with the agar content (see Table
mechanical, thermal, and morphological analysis. The 1). The injection molded samples were placed in sealed
influence of the agar particles on the filler-matrix compat- polyethylene bags, to prevent the moisture absorption.
ibility was investigated. The experiments have focused to
demonstrate the feasibility to prepare biocomposites using
Biocomposites Characterization
agar as filler.
The static tensile properties were determined using an
universal tensile testing machine (United Calibration
EXPERIMENTAL Corp. model SFM 20) following the ASTM D-638 stand-
ard method. The crosshead speed was set at 50 mm/min,
at least eight specimens of each formulation were tested,
Materials
and the average results were reported. The samples were
A commercial grade of agar powder was supplied by conditioned at room temperature for 48 h before testing.
AGARMEX (Mexico). The agar was extracted from Geli- The dynamic mechanical properties such as storage modu-
1118 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/penlus (E0 ), loss modulus (E00 ), and loss factor (tan d) of SPOT from Diagnostic Instrument was used to record the
PBAT-agar composites was measured using TA 2980 images. The maintained thickness of the composite for
DMA equipment with a constant heating rate of 38C/min the optical microscope study was around 0.2 lm. The
and frequency of 1 Hz. The dimensions of the sample tensile and cryogenically fractured composite specimens
used were 50 3 12 3 2 mm3. were analyzed for surface morphology using Philips Elec-
Thermal stability of the composites was evaluated troscan microscope (ESEM) with an accelerative voltage
using thermogravimetric analyzer (TGA). The measure- of 15–20 kV. The samples were observed perpendicular
ments were performed in a TA Instruments TGA model to the fractured surface.
2950. The samples were scanned from 25 to 6008C at a
heating rate of 108C/min under inert atmosphere of nitro- RESULTS AND DISCUSSIONS
gen with 20 ml/min flow rate. Degradation temperatures
were determined from the DTG thermograms as the peak
maximum. The melting and crystallization behavior of the Biocomposites Preparation
polyester matrix and the composites were studied using a The characterization of agar particles and PBAT-agar
modulated differential scanning calorimeter (MDSC) of biocomposites is described below. As can be reviewed in
TA instruments (Model 2920). The data were collected the literature, composites from marine resources have not
heating the composite from 0 to 2008C at a constant heat- been widely reported; in this section, we report the results
ing rate of 58C/min with a sample weight of 10 mg. of the characterization carried out on this novel biocom-
The results were analyzed using Thermal Advantage posites. Processing conditions (mixing speed, residence
Analysis Software version 1.1.A developed by TA Instru- time, and temperature-pressure of injection) used for
ments. The crystallization peak was measured from the PBAT-agar biocomposite formulation is shown in Table
thermogram during the cooling process at a cooling rate 1. Mixing time and temperature of each formulation was
of 58C/min. held at 10 min and 1358C, respectively. The force values
The heating and cooling cycle was repeated to all the reported are the average of five readings and represent the
samples, and from the second heating, the melting tem- force at steady state during mixing process. As shown in
perature (Tm) and by integration of the heat of fusion Table 1, the agar particles increased the force values dur-
(DHf) were both determined. From second cooling scan, ing mixing process and these values are also related with
the crystallization temperature (Tc) and by integration of the increase in viscosity of the mixture. The injector tem-
the crystallization peak, the heat of crystallization (DHc) perature was increased in order to fill out the mold and it
was also determined. For both temperatures measured, the was carried out in 8 s.
measurement of the Tm is at the onset of the endothermic
peak and for Tc is at the onset of the exothermic peak.
The values calculated for both heats are in J/g units and Mechanical Properties
the correction for diluent effect linked to the biofiller The stress–strain behavior of PBAT-agar biocompo-
incorporation into the PBAT matrix is shown in Eqs. 1 sites is shown in Fig. 1. A ductile behavior with strain
and 2, where / is the biofiller fraction. hardening of neat PBAT can be observed [17] (see insert
of Fig. 1). The incorporation of agar particles into PBAT
DHc
DHc0 ¼ (1) affects the shape of the stress–strain curve. The composite
1f
0 DHm
DHm ¼ (2)
1f
The percentage of crystallinity can be determined by
the Eq. 3 and the value of 114 J/g for 100% crystallized
PBAT was reported by Herrera et al. [16].
0
DHm
wð%Þ ¼ 100 (3)
DH100%
The rheological properties of PBAT-agar composites
were studied using Advanced Rheometric Expansion Sys-
tem. The injection molded sample of thickness 2 mm was
used for this study.
The morphology of PBAT-agar composites were stud-
ied using an optical microscope (Olympus, Model BH2) FIG. 1. Stress–strain curves of PBAT-agar biocomposites. The insert
with objectives 53, 203, and 503. A camera from RT shows the stress–stain curve of pure PBAT matrix.
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1119TABLE 2. Tensile properties of PBAT-agar biocomposites. brittle fracture. These characteristics agree with the
behavior followed by the PBAT-agar biocomposites.
Agar
content Tensile Tensile Elongation
The storage modulus (E0 ), loss modulus (E00 ), and tan d
(wt%) strength (MPa) modulus (MPa) at break (%) of the PBAT-agar biocomposites, as a function of the
temperature are shown in Fig. 2. As can be seen in Fig.
0 14.1 (0.9) 74.5 (8.6) 1214.7 (56.6) 2a, the storage modulus of all the biocomposites
10 70.3 (4.0) 764.5 (157.2) 358.2 (26.8) decreased with increasing the temperature. This behavior
20 43.9 (5.2) 1214.6 (311.0) 76.8 (10.2)
30 49.1 (1.6) 1961.6 (250.8) 9.14 (2.9)
may be due to the softening effect of the PBAT matrix at
40 45.8 (3.9) 2856.8 (354.7) 2.62 (0.6) high temperatures due to polymer chain mobility [20]. In
Table 3, the storage modulus values of PBAT-agar com-
Note: Standard deviations are given in parenthesis. posites at different temperatures (230, 215, 0, 258C) are
higher than that of PBAT matrix. It may be due to the
fact that the agar particles are stiffer than the PBAT. The
with 10% of agar displayed a distinct maximum stress. In increased storage modulus with increase in agar quantity
contrast, biocomposites with 30 and 40% agar showed in the composite could be due to the physicochemical
minor values of yield and at very low strain, suggesting
that the PBAT matrix yield before of agar debonding in
the biocomposites. The small necking observed in these
samples reveled that the plastic deformation is restricted
to the area close to the fracture plane. The failure of com-
posites with 40% agar occurred at 2% of strain, and
almost immediately after the stress reaches the maximum
value (45.8 MPa).
The tensile strength, tensile modulus, and elongation at
break values are given in Table 2. The mechanical per-
formance of filler polymer composites depends on the
strength and modulus of the filler [18]. When compared
with neat PBAT, a significant increase in the tensile
strength by 400, 211, 248, and 245% was noticed for
composites comprising 10, 20, 30, and 40% agar, respec-
tively. The increase in the tensile strength can be attrib-
uted to the good dispersion of agar particles in PBAT,
adhesion at interface due to the formation of hydrogen
bonding, and mechanical anchoring at the interface. The
tensile modulus and elongation at break of the PBAT-agar
biocomposites have showed opposite behavior between
them. The modulus showed significant increments with
the percentage of agar, increasing 16–38 times for compo-
sites reinforcement with agar at 20, 30, and 40 wt%. The
increase of composite stiffness is function to the percent-
age of reinforcement and its good dispersion into the ma-
trix, but is not function to the good quality of the inter-
face [17]. Therefore, the intrinsic mechanical properties
of the reinforcement itself and the degree of dispersion
into the polymeric matrix are a key point for agar par-
ticles to increase the elastic modulus of the composites.
As expected, according to the elastic modulus increase,
the elongation at break of composites decreased dramati-
cally with respect to the agar content, being for agar over
20% above 93% lower in comparison with the pure ma-
trix. The decrease in the elongation of two or three orders
of magnitude of the composites with 30 and 40% agar is
attributed to critical filler volume fraction (uc) [19].
Below this critical value, the composites show, during de-
formation process, a necking formation. Above this value,
the material has low deformation and usually almost neg- FIG. 2. Temperature dependence of (a) storage modulus, (b) loss mod-
ligible shrinkage of the cross-sectional area displaying a ulus, and (c) tan d of PBAT-agar composites.
1120 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/penTABLE 3. Dynamic mechanical properties of PBAT-agar
biocomposites.
Percentage
Agar Storage modulus E0 (MPa) of reinforcement
content of agar at 258C
(wt%) Tga (8C) (2308C) (2158C) (08C) (258C) in modulus (%)
0 231 1041 317 198 98 —
10 231 1258 455 298 150 53.2
20 233 1580 589 365 204 107.6
30 232 1941 775 514 310 216.6
40 229 2232 1053 722 460 369.3
a
Calculated from loss modulus curves.
interaction, intramolecular bonds, and a crystalline struc-
ture of the composite, which improves the reinforcement FIG. 3. TG curves of PBAT, agar and PBAT-agar composites. The
imparted by the agar particles that allowed stress transfer insert shows the DTG of the samples before mentioned.
from the matrix to the agar particles [21]. The glass transi-
tion (Tg) also called alpha transition (Ta) can be determined units. Afterwards, the agar sample showed a progressive
by DMA tests, these are the onset of the E0 drop, the onset decrement in weight and it has been attributed to the fur-
or peak of E00 , and the onset or peak of the tan d curve [22]. ther breakage of the composition products of the second
In our analysis, the Tg values were determined from the step. At 6508C, the remaining material (ash content) was
peak of loss modulus curves. As seen in Fig. 2b and Table around 20.8 wt%. The thermal stability exhibited by the
3, because of the agar particles in the PBAT matrix, the Tg PBAT-agar composites has shown an intermediate behav-
of PBAT-based biocomposites with 10, 20, and 30% agar ior to those of their pure component, exhibiting two deg-
are shifted slightly to lower temperature. In contrast with radation peaks between 268–2758C and 373–3768C. Table
40%, the Tg is shifted to higher temperature. The shift to Tg 4 lists the degradation temperatures (peak values) deter-
to higher temperatures can be associated with the decreased mined for all of the samples studied. The intermediate
mobility of the matrix chains due to addition of agar par- values of degradation temperature suggest over again
ticles. Furthermore, the stress field surrounding the particles some kind of interaction between the agar and PBAT ma-
induces the shift in Tg. Because the loss modulus is a mea- trix. The thermal stability of PBAT-agar composite follow
sure of energy dissipated or lost as heat per cycle of sinusoi- a sequence: 10% [ 20% [ 30% [ 40% (Table 4 and
dal deformation when different system are compared at the Fig. 3).
same strain amplitude, the loss factor are very sensitive to The normalized DSC scans in the melting region of
molecular motions [22]. Figure 2c shows that the height of the pure matrix (PBAT) and blends with different agar
the tan d peak decreased with the presence of agar particles. contents is shown in Fig. 4. Table 5 summarizes the main
It is due to that there is no restriction to the chain motion in thermal characteristics of PBAT-agar biocomposites. The
the case of pure PBAT matrix; meanwhile, the presence of DSC thermograms showed single fusion endotherm which
agar particles hinders the chain mobility which produces a corresponds to PBAT matrix fusion. This peak is shorter
reduction of sharpness and height of the tan d peak. and slightly broader than the other thermograms. The
melting temperature tends to increase slightly (þ58C)
with increasing the agar content. This could be due to
Thermal Properties some intermolecular interactions between the agar par-
The thermal stability of neat PBAT, agar, and PBAT-
agar biocomposites are shown in Fig. 3. The TGA curves
and the insert show the derivative of TGA curves. The TABLE 4. TGA characterization of pure PBAT, PBAT-agar biocom-
corresponding temperature of degradation for 5, 25, 50, posites, and pure agar.
and 75% weight loss is given in Table 4. The main
Temperature at
weight loss of pure PBAT matrix occurs in a single step Agar decomposition peak Temperature of weight loss (8C)
at 3838C. The decomposition temperature of the compo- content
sites is shifted towards agar decomposition temperature as (wt%) 1st (8C) 2nd (8C) T5% T25% T50% T75%
the agar content is increased or the PBAT decreased. The 0 383 — 384 385 385 386
pristine agar tested as-received has a first weight loss 10 275 374 280 371 375 383
attributed to moisture evaporation upon heating and it cor- 20 272 373 276 371 374 382
responds to a weight loss of 14–16 wt%. The second step 30 271 375 270 371 377 387
has a temperature range of decomposition from 260 to 40 269 376 262 366 376 389
100 265 — 62 267 297 616
2758C, probably related to the degradation of galactose
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1121FIG. 5. Comparison of complex viscosity of PBAT-agar composites.
FIG. 4. Second heating thermograms for series of PBAT-agar compo-
sites, and the range of the temperature shows the melting endotherm.
carbonyl groups of the polyester and OH (hydroxyl),
sulfate and glycosidic groups of agar. Such interactions
ticles and PBAT chains, which may produce thinner la- occur in the amorphous state, where they have suppressed
mellar crystals. Based on the measured enthalpy from the
thermograms, the corrected enthalpy (Eqs. 1 and 2) and
percentage crystallinity was determined using the Eq. 3.
For these calculations, we have assumed that the heat of
fusion of completely crystalline (DH100%) PBAT has a
value of 114 J/g as reported by Herrera et al. [16]. In this
sense, the corrected values of heat of fusion (DH0m ) and
crystallization (DH0c ) are higher in comparison with the
values obtained from thermograms (DHm, DHc), because
the Eq. 3 take into account that the diluent effect linked
to the agar particles incorporation into the PBAT matrix.
As can be seen in Table 5, the corrected values are equiv-
alent and both are into the range of 10.1–15.7 J/g. The
second heating scan has shown small crystallization effect
on the PBAT-agar composite. In this sense, the incorpora-
tion of agar particles induces a significant increase (p
0.05) in (Tc) and it is due to the reduction of the polymer
mobility and to a nucleation effect linked to the agar sur-
face. The increase of Tc is much more significant (p \
0.05) in samples with 40% agar. The percentage of crys-
tallinity (vc) of PBAT has shown a decrease tendency
with increasing agar content. This behavior has not
reported before for PBAT-agar biocomposites. However,
other similar systems of polyester-polysaccharides have
shown similar tendency [18, 23]. The decrease of vc is
attributed to hydrogen bond interaction between the
TABLE 5. DSC results of PBAT-agar biocomposites.
Agar
content Tm DHm DH0m Tc DHc DH0c vc
(wt%) (8C) (J/g) (J/g) (8C) (J/g) (J/g) (%)
0 109 20.5 20.5 65 17.4 17.4 18.0
10 113 14.1 15.7 75 10.9 12.1 13.8
20 115 10.1 12.6 78 11.8 14.8 11.1
30 114 10.0 14.2 78 11.5 16.4 14.4
FIG. 6. Frequency of (a) storage modulus, (b) loss modulus, and (c)
40 116 8.7 14.6 80 12.5 20.9 12.8
tan d of PBAT-agar composites.
1122 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/penstress is measured and compared of with each PBAT-agar
biocomposite. The complex melt viscosity of each formu-
lation decreases as frequency is increased; it is due to
shear thinning effect. In addition, agar filled composites
exhibited higher viscosity than neat PBAT, due to the
increased in contacting area between agar particle surface
and polymer. It could support the idea that the enhanced
viscosity is due to increased viscous dissipation in the
matrix [24]. Figure 6 shows the storage modulus (a), loss
modulus (b), and tan d (c) with frequency for comparison
purpose of PBAT-based composites. At low frequencies,
the qualitative behavior of storage modulus increase for
the presence of agar particles, whereas at high frequencies
([6 rad/s), the qualitative behavior of storage modulus of
PBAT-agar biocomposites tend to behave as the neat
PBAT. The loss (viscous) modulus of PBAT-agar compo-
sites followed a linear behavior within the range of fre-
FIG. 7. Morphology of agar particles by scanning electron microscopy quencies studied. Each curve showed an increase in mod-
at 3250 of magnification.
ulus as the agar content is increased. Finally in Fig. 6c, it
can observe that the height and amplitude of the tan d
the effect on the extent of crystallization with respect to decreased due to the presence of agar particles, indicating
PBAT domains. an increase in melt elasticity. When the agar particles
reach 30 and 40% the peak is shifted to higher frequency
without significant change in height. Fillers seem to
Rheological Properties
enhance substantially the viscosity at low shear rate (Fig.
Figure 5 shows the complex melt viscosity of the 6b). In general, we can observe that at high shear rates,
PBAT-agar biocomposites. It was measured by dynamic rheological properties approaches as an unreinforced ma-
frequency experiments, where strain is controlled and terial. It is seen that resins with high viscosity may not
FIG. 8. Optical microphotographs (1) and polarized optical microphotographs (2) of neat PBAT matrix with
10% (a) and 40% (b) agar.
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1123wet out completely for composite with bigger particles,
because the viscosity gearshift the speed of wet-out (satu-
ration). Because the viscosity varies inversely with the
particle size [25], is probably that the agar particles travel
within the polymer matrix within a given velocity field;
while, simultaneously, the layer of melt polymer surround
the agar particles. At this point, the rheological behavior
is determined entirely by the constitution of the polymeric
matrix in the interfacial regions.
Morphology Observations
The granular size and shape of the agar used as filler
in PBAT composites was analyzed using SEM image.
Agar particles had a variety of irregular shapes and sizes
(Fig. 7). Figure 8a and b shows the microphotographs that
reveal the arrangement of agar particles into the PBAT
matrix at two different contents (10 and 40 wt%)
observed under optical microscope (OM) (1) and polar-
ized optical microscope (POM) (2). It can be seen that
the distribution of agar particles is dispersed in the poly-
ester matrix at 10% (Fig. 8a1). Large particles did not
promote agglomeration of small particles of agar. Mean-
while, blends containing a higher amount of agar, such as
40% (Fig. 8b1), the agar particles are dispersed in the
polymeric matrix but some of these are forming agglom-
erates. In contrast to other polysaccharides do, for
instance starch [26] agar particles did not melt. When the
thin films are observed under polarizing microscope,
numerous birefringent entities exhibiting distorted whiten-
ing areas are observed as show in Fig. 8a2–b2, which
reveals a crystalline structure of agar. The size of clear
white zones of agar determined by POM agrees well with
FIG. 9. Scanning electron micrograph of neat PBAT matrix with 10%
the theory that the agar particles have crystalline zone but
(a) and 40% (b) agar. Tensile fractured samples.
in different planes [27], therefore, the positive birefringent
implies that the main axes of the polymer chain are ori-
ented in the radial direction. However, the agar particles shown in Fig. 9. A fibrillar structure in the fracture zone
have irregular shape and the crystalline plane would be in can be observed in Fig. 9a. Being PBAT a ductile poly-
different angles producing diffusion of the polarized light. mer it is obvious to expect a great amount of plastic de-
Comparing with starch granules, these generally exhibit formation with a fibrous structure on the fractured surface
positive birefringent, in contrast to agar, because the re- due to stretching of the surrounding polymer threads and
fractive index is the largest along the chain axis [28]. subsequent failure of the matrix. However, the increase of
They also mentioned that variations in granule size, the agar content did not reduce this characteristic of the
shape, and composition depend on the botanical origin of fractured surface in all the formulations of PBAT-agar
starch. In the case of the agar, different chemical structure composites. Although the increase of agar content pro-
of this polysaccharide has been found from different algal duce a decrease in the elongation at break of the compos-
species mainly related to sulphate content [29]. According ite (Fig. 1 and Table 2), the plastic deformation of the
with the results of OM, the distribution of agar particles matrix remains and less fine fibrils of micron dimensions
is homogeneously dispersed in the polymer matrix at 10% in the core layers of the blends are observed. It would
agar content in the biocomposites (Fig. 8a1–a2). Also this point out that there is localized micronecking and fibrilla-
formulation showed larger tensile strength in comparison tion even in blends containing as 40% by weight of agar
with 20, 30, and 40% agar content. It indicates that the particles. The incorporation and increase of agar particles
dispersion of agar in its blends into PBAT matrix plays produce that the PBAT-agar fractured surface decrease
important role in the mechanical properties showed by the the fine fibrils and the surface becomes smoother and it
resulting composite material. clearly indicates a decrease in ductility of the composites.
ESEM microphotographs of tensile fractured samples ESEM microphotographs of cryogenically-fractured
with 10 and 40 wt% agar, at ambient conditions are surface examinations with 10 and 40% agar, are shown in
1124 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/penbetween their pure components. The effect of agar par-
ticles on the biocomposites showed a reduced percentage
crystallinity and increased Tm and Tc as evidenced from
DSC studies. The dynamic rheology of PBAT-agar bio-
composites suggested that the agar particles travel within
the polymeric matrix for a given velocity field. The mor-
phological analysis by OM and SEM demonstrated that
the agar particles are dispersed in the PBAT matrix. Con-
trary to other polysaccharides such as starch or chitosan,
agar particles do not agglomerate at low concentrations.
The distribution of agar particles observed in the blends
is consistent with the range of particle size observed for
pure agar under SEM. The morphology of cryogenically
fractured surfaces showed a smooth surface and agar par-
ticles remained into the PBAT matrix. It could be due to
the degree of adhesion between agar particles and PBAT.
ACKNOWLEDGMENTS
Madera-Santana thanks CONACYT for the scholarship
support to carry out this research work at CMSC-MSU.
REFERENCES
1. L. Averous and C. Fringant, Polym. Eng. Sci., 41, 727
(2001).
2. S.M. Lai, C.K. Huang, and H.F. Shen, J. Appl. Polym. Sci.,
97, 257 (2005).
3. J.L. Willet and F.C. Felker, Polymer, 46, 3035 (2005).
4. R. Mani and M. Bhattacharya, Eur. Polym. J., 37, 515
FIG. 10. Scanning electron microphotograph of neat PBAT matrix with (2001).
10% (a) and 40% (b) agar. Cryogenically fractured samples.
5. K. Twarowska-Schimidt and M. Ratajska, Fib. & Text East-
ern Europe, 13, 71 (2005).
Fig. 10a and b, respectively. The micrographs provide 6. L.O. Schmidt and P.J. Dauenhauer, Nat. Rep., 3, 44 (2007).
value information about dispersion and adhesion of fillers 7. I.C.M. Dea, A.A. McKinnon, and D.A. Rees, J. Mol. Biol.,
and fibers in blends or composites. The cryogenically- 68, 153 (1972).
fractured surfaces are smoother than those subjected to 8. S. Arnott, A. Fulmer, W.E. Scott, I.C.M. Dea, R. Moor-
tensile fracture (Fig. 9a and b). The agar particles are dis- house, and D.A Rees, J. Mol. Biol., 90, 269 (1974).
persed into the PBAT matrix (figure not shown) but after 9. N.F. Stanley, ‘‘Agar,’’ in Food Polysaccharides and Their
the fracture process the agar particles remain into the ma- Applications, A.M. Stephen, Ed., Marcel Dekker, New York,
trix (Fig. 10a and b) showing sharp edges and irregular 187 (1995).
shape. However, agar particles are debonded from the ma- 10. Y. Freile-Pelegrı́n, T. Madera-Santana, D. Robledo, L.
trix, indicating that the adhesion is attributed to mechani- Veleva, P. Quintana, and J.A. Azamar, Polym. Degrad.
cal or physical irregularities of the agar particles. Stab., 92, 244 (2007).
11. R. Armisén and F. Galatas, ‘‘Production, Properties and
Uses of Agar,’’ in Production and Utilization of Products
CONCLUSIONS from Commercial Seaweeds, D.J. McHugh, Ed., FAO Fish
Tech Papers, Rome, Italy, 288, 1 (1987).
From this study, it can be concluded that the incorpo- 12. J.P. Lee, K.H. Lee, and H.K. Song, J. Mater. Sci., 32, 5825
ration of agar particles into PBAT increased the tensile (1997).
strength and elastic modulus. The DMA studies revealed 13. A.L. Rakow and L.A. Belfiore, J. Appl. Polym. Sci., 57, 139
that the incorporation of agar particles enhanced the stor- (1995).
age modulus (stiffness). However, the Tg calculated using 14. D. Robledo, ‘‘The Seaweed Resources of Mexico,’’ in Sea-
the loss modulus peak of DMA spectra showed a slight weed Resources of the World, A.T. Critchley and M. Ohno,
variation. This may be due to hindered mobility of poly- Eds., JICA Japan, 331 (1998).
mer chains at the interface. The TGA studies showed that 15. F. Le Digabel and L. Avérous, Carbohydr. Polym., 66, 537
the thermal stability of PBAT-agar composites lies in (2006).
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 112516. R. Herrera, L. Franco, A. Rodriguez-Galan, and J. Puiggali, 23. T. Honma, T. Senda, and Y. Inoue, Polym. Int., 52, 1839
J. Polym. Sci. Part A: Polym. Chem., 40, 4141 (2002). (2003).
17. L. Mascia, Thermoplastics: Materials Engineering, 2nd ed., 24. M.S. Huda, L.T. Drzal, A.K. Mohanty, and M. Misra,
Elsevier Science Publishers LTD, UK (1989). Compos. Sci. Technol., 66, 1813 (2006).
18. V.M. Correlo, L.F. Boesel, M. Bhattacharya, J.F. Mano, N.M. 25. M.S. Huda, L.T. Drzal, A.K. Mohanty, and M. Misra,
Neves, and R.L. Reis, Mater. Sci. Eng. A, 403, 57 (2005). Compos. Part B, 38, 367 (2007).
19. I.L. Dubnikova, V.G. Oshmya, and A.Y. Greenberg, 26. D.S. Rosa, C.G.F. Guedes, and F. Casarin, Polym. Bull., 54,
J. Mater. Sci., 32, 1613 (1997). 321 (2005).
20. S. St. Lawrence, P.S. Walia, F. Felker, and J.L. Willet, 27. M. Nizsizawa and T. Hirano, J. Appl. Polym. Sci., 39, 2173
Polym. Eng. Sci., 48, 1250 (2004). (1990).
21. Y.S. Lipatov, Polymer Reinforcement, Chem Tech Publish- 28. A. Boléon, P. Colunna, V. Planchot, and S. Ball, Int. J. Biol.
ing, Toronto (1995). Macromol., 23, 85 (1998).
22. K.P. Menard, Dynamical Mechanical Analysis: A Practical 29. Y. Freile-Pelegrı́n and R. Murano, Biores. Technol., 96, 295
Introduction, CRC Press, New York (1999). (2005).
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