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J Therm Anal Calorim (2014) 115:153–160 DOI 10.1007/s10973-013-3259-0

Kinetics of thermal degradation applied to biocomposites with TPS, PCL and sisal fibers by non-isothermal procedures Vitor Brait Carmona • Adriana de Campos Jose´ Manoel Marconcini • Luiz Henrique Capparelli Mattoso



Received: 16 January 2013 / Accepted: 17 May 2013 / Published online: 11 June 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013

Abstract The thermal degradation behavior of the biocomposite with thermoplastic starch (TPS), poly(e-caprolactone) (PCL) and bleached sisal fibers were investigated by thermogravimetry analysis (TG/DTG) under synthetic air atmosphere, differential scanning calorimetry, and their crystal structure by X-ray diffraction. Applying the nonisothermal Ozawa method, the TG/DTG curves average activation energy could be obtained for thermal degradation of the biocomposites with 5, 10, and 20 % of bleached sisal fibers. The apparent activation energy values for the biocomposites decreased when compared with the TPS/ PCL blend, requiring lower energy to recycle this material. However, continuous addition of sisal fibers increased the activation energy of composites. Keywords Thermal degradation  Activation energy  Thermoplastic starch (TPS)  Poly(e-caprolactone) (PCL)  Sisal fibers

Introduction An increase interest in recycling and using of biodegradable materials has stimulated research with biodegradable polymers and natural fibers to obtain composites with

V. B. Carmona Materials Science and Engineering Department (PPG-CEM), Federal University of Sa˜o Carlos, Sa˜o Carlos, SP 13565-905, Brazil V. B. Carmona  A. de Campos  J. M. Marconcini (&)  L. H. C. Mattoso National Nanotechnology Laboratory for Agriculture (LNNA), Embrapa Instrumentation, Sa˜o Carlos, SP 13560-970, Brazil e-mail: [email protected]

useful properties [1]. Starch is a biopolymer present in abundance in nature in a variety of plants including corn, wheat, rice, potatoes, and others. Chemical and physical properties of starch have been widely investigated due to its suitability to be converted into a thermoplastic material (TPS), and then to be used in different applications as a result of its known biodegradability, availability and economical feasibility [2, 3]. Unfortunately, TPS presents poor mechanical properties and is water sensitive. In order to be able to compete with conventional plastics, blending or grafting starch with others materials has been suggested as a suitable route in improving properties [3–6]. Polycaprolactone (PCL) is a biodegradable polyester with a number of potential applications from agricultural usage to biomedical devices [7–9]. Up to now, large-scale application of the PCL has been limited because of their relatively high price, as well as some intrinsic properties. Blending biodegradable polymers with other materials (natural or synthetic) has proved to be an effective and economic method in resolving this problem [10]. Recently, different types of cellulose-based natural fibers (such as sisal, jute, and hemp fibers) have been successfully incorporated into biodegradable polymers to obtain fully biodegradable composites [11–14]. The use of natural fibers as filler improves the thermomechanical properties, decreases water sensitivity and preserves the biodegradability of the material [15]. Adding 20 % of sisal fibers into TPS and PCL matrix increased the tensile strength and Young’s modulus, indicating the good interfacial adhesion of the sisal fibers with the matrix [16]. The thermal degradation of biodegradable polymers is important in understanding their thermal stability for processing, application, and thermal recycling. PCL degradation exhibited distinctly different behaviors under nonisothermal and isothermal heating. Under non-isothermal

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conditions their results suggested that the governing mechanism changes during degradation, and this fact was explained by two parallel mechanisms: random chain and specific chain-end scission. Apparent activation energy changed from low to high temperatures during degradation, which was an indication of changes in the mechanism during non-isothermal heating [17]. Thermal decomposition process of ten types of unmodified natural fibers commonly used in the polymer composite industry [18] and banana trunk fibers chemically treated with different methods [19] were investigated using dynamic thermogravimetry (TG) analysis. For most natural fibers approximately 60 % of the thermal decomposition occurred within a temperature range between 215 and 390 °C. The results also showed that an apparent activation energy of 155–186 kJ mol-1 was obtained for most of the selected fibers throughout the polymer processing temperature range. In composite materials, such as starch blends/ sisal cellulose derivative, it was reported that the increase of fiber content did not affect significantly the first step of the thermal degradation of these matrix [15]. There is a lack of information about thermal degradation of TPS/PCL blends with sisal fibers and to the best of our knowledge, the activation energy of these materials was determined in the present work. Thus, the aim of this study is to determine kinetics of thermal degradation of biodegradable TPS/PCL matrix with sisal fibers, as well the effect of the sisal fibers content in this matrix. This study is important to evaluate the thermal degradation of the composite, and the energy required for recycling this biodegradable material.

V. B. Carmona et al.

solution (NaOH 5 % m/v). The mixture was pre-heated and maintained at 90 °C and stirred for 60 min, and then cooled to room temperature. The fibers were vacuum filtered followed by a wash with distilled water until the pH becomes neutral. Recovered fibers were oven dried at 50 °C, with air circulation, until constant mass was achieved. In the second step, NaOH-treated fibers were further subjected to bleaching with alkaline peroxide solution. About 5 g of NaOH-treated fibers were suspended in a solution containing hydrogen peroxide (H2O2 16 % v/v) and sodium hydroxide (NaOH 5 % m/v) at 55 °C and stirred for 90 min. Treated fibers were recovered as described in the first-step. Extrusion To prepare the TPS, cornstarch and plasticizer (glycerol, in a proportion of 30 % dry mass) were manually pre-mixed to homogenize the material. The mixtures were prepared in a co-rotating twin-screw extruder ZSK 18 (Coperion Ltda, SP, Brazil) equipped with six heating zones and a ribbon die. The screw rotation speed was 200 rpm and the temperature profile was 140, 140, 150, 150, 160, and 160 °C. The TPS/PCL blend was then prepared by mixing TPS and PCL pellets in the same conditions as the homopolymers, as well as TPS/PCL (80:20) composites containing 5, 10, and 20 % bleached sisal fibers (namelly TPS/PCL/BSF5%, TPS/PCL/BSF10%, and TPS/PCL/BSF20%, respectively), and compounding them in a co-rotating twin-screw extruder. Extruded ribbons were pelletized and further processed in a single-screw extruder (AX Plasticos Ltda, Brazil) operating at 150 rpm and temperatures profile was 120, 125, and 130 °C to obtain 1-mm-thick films.

Experimental Thermogravimetric analysis (TG) Materials Sisal fibers were supplied by Embrapa Algoda˜o, PB, Brazil. The corn starch (70 % amylose and 30 % amylopectin) used was AmidexÒ 3001, supplied by Corn Products Brazil, and the PCA (CAPAÒ 6500) was purchased from Perstorp Quimica do Brasil Ltda. Glycerol and stearic acid were supplied by Synth, Brazil. Preparation of the biocomposites Fiber treatment Fiber treatment was carried out in a two-step process outlined according to Sun et al. [20]. In the first step, about 10 g of sisal fibers were ground, sieved (16 mesh) and suspended in a beaker with 200 mL of sodium hydroxide

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Thermogravimetric and differential thermogravimetry (DTG) of composites with bleached sisal fibers were performed in a TGA Q500 equipment (TA Instruments, USA). TG/DTG curves of polymers and composites were carried out under air atmosphere, platinum pan, sample mass around 7 mg, and heating rates of 5, 10, 15, and 20 °C min-1, from 25 to 600 °C. Activation energies were determined using the Ozawa method for polymers and composites [21]. There are two distinct zones of degradation and the first one is the determinant for the TPS/PCL composite. It was used values from 5 to 50 % of conversion degree (a) of the dry mass to report results of activation energies using descriptive statistics. The theoretical background used to determine activation energy of materials was presented in earlier study [22], and in present study we use the same procedure.

Kinetics of thermal degradation applied to biocomposites

155

Differential scanning calorimetry (DSC)

100

CI ¼

DHm  100 f DHm

80

60

Mass/%

Samples were analysed on DSC Q-100 equipment (TA Instrument, USA) under the following conditions: mass 6.00 ± 0.1 mg; nitrogen flow 60 mL min-1; heating rate 10 °C min-1; temperature range -80 to 150 °C. It was determined the crystallinity index (CI) of PCL in the materials from the melting peak area using the following equation described by Vertuccio et al. [23]:

TPS PCL TPS/PCL TPS/PCL/BSF5 % TPS/PCL/BSF10 % TPS/PCL/BSF20 % BSF

40

20

ð1Þ

0

where DHm is the enthalpy of fusion of the sample, DHm is the heat of fusion for 100 % crystalline PCL (it was used 136 J g-1) [24] and f is the mass fraction of PCL.

0

100

200

300

400

500

600

500

600

Temperature/°C BSF TPS/PCL/BSF20 %

X-ray diffraction (XRD)

Results and discussion TG/DTG curves under air atmosphere were carried out to elucidate the thermal degradation behavior of the sisal fibers, polymers, and composite with 5, 10, and 20 % of sisal fibers (Fig. 1; Table 1). The mass loss up to 220 °C was attributed to the evaporation of water, glycerol, and others volatiles compounds. The thermal degradation of TPS and hemicelluloses occurs between 250 and 300 °C [27], while cellulose thermally degrades between 240 and 350 °C. Pyrolysis of starches up to 300 °C generates CO2, CO, water, acetaldehyde, furan, and 2-methyl furan. For corn, levoglucosan is usually the main constituent of the decomposed products, besides complex gases and water liberated. A carbonaceous residue has been described for thermal degradation of corn at 500 °C [28].

DTG/% °C–1

TPS/PCL/BSF10 %

X-ray diffraction patterns of the polymers and composites containing 5, 10, and 20 % of sisal fibers were measured with an X-ray diffractometer (Lab X-XDR-6000-Shimadzu) operating with Cu Ka radiation (wave˚ ) at 30 kV and 30 mA. Samples were length = 1.5406 A placed in a desiccator with 45 % relative humidity for at least 48 h. Scattered radiation was detected in the angle range of 2h (5–40°) at a scan rate of 2° min-1. The diffractograms were adjusted by placing gaussian shaped peaks after the deconvolution peaks, using specific software. Crystallinity indexes (CI) of sisal fibers were estimated based on the areas under crystalline and amorphous peaks after appropriate baseline correction [25]. CI of TPS was estimated as function of B and Vh crystallinity, according to Hulleman et al. [26].

TPS/PCL/BSF5 % TPS/PCL TPS PCL

= 0.5 % °C–1

0

100

200

300

400

Temperature/°C

Fig. 1 TG and DTG curves of TPS, PCL, TPS/PCL blend, TPS/PCL/ BSF5%, TPS/PCL/BSF10%, TPS/PCL/BSF20%, and BSF at 10 °C min-1 heating rate under synthetic air atmosphere Table 1 Onset temperature of thermal degradation of TPS, PCL, TPS/PCL, TPS/PCL/BSF5%, TPS/PCL/BSF10%, TPS/PCL/BSF20%, and BSF Samples

Tonset/°C

TPS

280

PCL TPS/PCL

365 291

TPS/PCL/BSF5%

295

TPS/PCL/BSF10%

296

TPS/PCL/BSF20%

287

BSF

268

PCL presented the highest thermal stability (365 °C), whereas the neat TPS presented the lowest thermal stability (280 °C) throughout most of the temperature range. Also, the TPS/PCL blend showed similar TG curves to the neat TPS sample due to the high TPS concentrations in those samples. PCL thermal degradation occurs in a two-stage

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123

TPS

α α α α α α α α α α

3.0

lnβ

2.5

=5% = 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

2.0

1.5 0.00165 0.00170 0.00175 0.00180 0.00185 0.00190 0.00195

T –1/K–1

PCL

α α α α α α α α α α

3.0

lnβ

2.5

=5% = 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

2.0

1.5 0.00145

0.00150

0.00155

0.00160

T –1/K–1 α α α α α α α α α

TPS/PCL 3.0

lnβ

2.5

= 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

2.0

1.5 0.00165

0.00170

0.00175

0.00180

0.00185

T –1/K–1

BSF

α α α α α α α α α α

3.0

2.5

lnβ

mechanism with a cis-elimination followed by a depolymerisation with production of the monomer and other oligomers [29]. On the other hand, there are reports that PCL thermal degradation under isothermal conditions is given by evolution of volatile products from PCL via an unzipping mechanism. They also claimed the formation of a stable intermediate for the decomposition with no change in the mechanism for the whole process [30]. Sisal fibers present mass loss in the range of 25–150 °C attributed to loss of volatiles, and 200–370 °C attributed mainly to degradation of hemicellulose and cellulose [31]. Lignin is degraded in the range of 370–500 °C as this provides slow degradation over a wide temperature range, being the most difficult component to be decomposed [32]. However, bleached sisal fibers present higher Tonset value when compared to raw sisal fibers due to the hemicellulose and part of lignin that was removed in the fibers treatment, and this phenomenon of rise in the fiber thermal stability was reported earlier, also for other kind of natural fibers [19, 33]. Blends containing fibers had the Tonset shifted to lower temperatures and the blend containing 20 % of fibers had the lower Tonset value (287 °C) because of the higher fiber concentration. This reduced Tonset phenomenon was also reported in other PCL/sisal studies [34, 35]. It is an important knowledge for processing this composite, and may be economically favorable for recycling and because of the lower energy required producing the complete pyrolysis [34]. TG was performed on all samples at constant heating rates of 5, 10, 15, and 20 °C min-1. Figure 2 shows Flynn– Wall–Ozawa plots constructed from 5 to 50 % of conversion of TPS, PCL, TPS/PCL blend, and bleached sisal fiber, and Fig. 3 shows Flynn–Wall–Ozawa plots of TPS/PCL/ BSF5%, TPS/PCL/BSF10%, and TPS/PCL/BSF20%. The parallel straight-line plots indicate that there is no significant change in the activation energy of thermal degradation process in this range of conversion levels. The average activation energy (Ea) of TPS in the range of 5 \ a \ 50 % was 122 kJ mol-1, which is in accordance with Shuttleworth et al. [36], while to PCL was a little higher, 133 kJ mol-1 (Table 2). Also, when TPS and PCL were blended, there was a reduction in activation energy if compared to neat polymers. Ea values for the first peak do not change significantly with conversion as can be concluded from the slopes in Figs. 2 and 3. This result indicates that pyrolysis of PCL seems to proceed through the cleavage of linkages with similar bond energies as well to composites [34]. Table 2 presents activation energy results of thermal degradation process of TPS, PCL, TPS/PCL blend, composites, and bleached sisal fiber. Bleached sisal fibers presented activation energy with comparable values to

V. B. Carmona et al.

=5% = 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

2.0

1.5 0.00160 0.00165 0.00170 0.00175 0.00180 0.00185 0.00190

T –1/K–1

Fig. 2 Flynn–Wall–Ozawa plot of isoconversion dynamic TG data of TPS, PCL, TPS/PCL, and BSF

Kinetics of thermal degradation applied to biocomposites

157

TPS/PCL/BSF5 % α α α α α α α α α α

3.0

lnβ

2.5

=5% = 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

2.0

1.5 0.0016

0.0018

0.0020

0.0022

0.0024

T –1/K–1

TPS/PCL/BSF10 %

α α α α α α α α α α

3.0

lnβ

2.5

=5% = 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

2.0

1.5 0.0016

0.0018

0.0020

0.0022

0.0024

T –1/K–1

TPS/PCL/BSF20 % α α α α α α α α α α

3.0

lnβ

2.5

2.0

=5% = 10 % = 15 % = 20 % = 25 % = 30 % = 35 % = 40 % = 45 % = 50 %

1.5 0.0016

0.0018

0.0020

0.0022

0.0024

0.0026

T –1/K–1

Fig. 3 Flynn–Wall–Ozawa plot of isoconversion dynamic TG data of TPS/PCL/BSF5%, TPS/PCL/BSF10%, and TPS/PCL/BSF20%

other lignocellulosic materials that were determined using the same mathematical methodology [19, 22, 37, 38]. The addition of only 5 % of bleached sisal fibers into TPS/PCL matrix caused a pronounced decrease in activation energy if compared with the TPS/PCL blend. In addition to it, the activation energy of composites among themselves

increased as the bleached sisal fiber content increased up to 10 and 20 %. The Ea against (a) plots of TPS, PCL, TPS/PCL blend, composites, and bleached fibers are shown in Fig. 4. In general, the activation energy changes with increasing conversion degree of polymers. This variation in Ea values was expected because when more than one degradation mechanism takes place, energy will be not always constant [28]. DSC curves of the materials are presented in Fig. 5, and are shown the heats of fusion (DHm ) and melt temperatures (Tm) for PCL pure and in blend TPS/PCL and composites. The blending of TPS with PCL reduced in 1 °C the Tm of PCL. When 5 % of sisal fibers were added, this reduce was more pronounced, with a posterior increase in Tm as sisal fiber content increases on mixtures. These results are in agreement with activation energies of materials, where the same behavior was found. Crystallinity index of pure PCL and PCL present in the others materials was determined using Eq. 1 and the results are shown in Table 3. Pure PCL presented 35.8 % of crystallinity, and this result is similar to the literature [40]. When PCL was blended into TPS, it present a higher CI (76.8 %), caused by the higher mobility of PCL chains in TPS matrix. In addition to it, when only 5 % of sisal fiber was added, PCL CI decreases to 57.2 %, and increased up to 77.6 % with 20 % of sisal fibers, as sisal fibers content increases. At this point, two phenomena are occurring in parallel: while the presence of fibers stiffens the matrix inhibiting polymer chains mobility, it also acts as an efficient nucleating agent and increases the crystallization rate of the PCL matrix [35, 39, 41–44]. The increase in fiber content tends to increase the PCL crystallinity in the blend, and this kind of phenomenon had already been reported by other authors [45, 46]. Observing the composites with 5, 10, and 20 % of sisal fibers, it is noted an increase of crystallinity of PCL and a decrease of crystallinity of TPS when fibers content is increased (Table 3). Same behavior is observed with the activation energy of the composites, comparing composites with 5–20 % of fiber content. The effect of montmorillonite nanoclay content into polypropylene matrix was studied [47], and it was found that the activation energy was increased as the filler content was increased. The higher the degree of reinforcement, more energy is required to initiate the molecular movement of the segments in the main chain. It is observed that the higher the fiber content, the higher activation energy. This phenomenon occurs due to the restriction imposed by the fiber volumetric fraction and stiffness of the fiber. The X-ray diffractogram of the TPS sample exhibited a characteristic peak around 20° (2h) (Fig. 6). This peak has been attributed to the processing-induced crystallinity of single helical amylose molecule [48]. Amylose and lipids

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V. B. Carmona et al.

Table 2 Activation energies and their respective conversion levels of TPS, PCL, TPS/PCL, TPS/PCL/BSF5%, TPS/PCL/BSF10%, TPS/PCL/ BSF20%, and BSF a/%

PCL/ kJ mol-1

TPS/ kJ mol-1

Blend/ kJ mol-1

TPS/PCL/BSF5%/ kJ mol-1

TPS/PCL/BSF10%/ kJ mol-1

TPS/PCL/BSF20%/ kJ mol-1

BSF/ kJ mol-1

5

13.0

101.5

52.1

41.8

63.4

81.9

71.7

10

23.7

132.7

56.5

38.3

49.8

99.8

144.0

15 20

75.4 126.9

141.7 140.8

58.7 59.2

37.2 45.2

54.1 78.4

88.5 107.0

144.67 144.5

25

150.4

138.9

72.0

60.2

117.2

131.4

147.7

30

156.1

143.4

84.6

73.5

136.8

136.1

156.9

35

150.7

139.2

116.1

85.7

141.1

138.9

158.3

40

153.0

131.2

126.4

95.0

142.1

141.8

168.0

45

154.0

129.7

128.4

101.2

143.6

142.1

163.8

50

155.2

130.9

128.0

104.0

Average

115.8 ± 57.0

133.0 ± 12.5

88.2 ± 28.8

68.2 ± 27.0

146.0

141.0

173.7

107.7 ± 40.9

120.8 ± 23.9

147.3 ± 28.5

Activation energy/kJ mol–1

Table 3 Crystallinity index of PCL, TPS, TPS/PCL, TPS/PCL/ BSF5%, TPS/PCL/BSF10%, TPS/PCL/BSF20%, and BSF 150

Samples

Crystallinity index/% PCLa

B-type (TPS)b

Vh-type (TPS)b

Cellulose Ib

100 TPS PCL TPS/PCL TPS/PCL/BSF5 % TPS/PCL/BSF10 % TPS/PCL/BSF20 % BSF

50

0 0

5

10

15

20

25

30

α /%

35

40

45

50

55

(58.4 °C; 16.9 J g–1)

TPS/PCL/BSF20 %

g–1)

TPS/PCL/BSF10 %

Heat flow/mW mg–1

(57.0 °C; 14.8 J g–1)

TPS/PCL/BSF5 %

(57.7 °C; 20.9 J g–1)

TPS/PCL

(58.7 °C; 48.7 J g–1)

PCL

Exo

0

= 1 mW mg–1

20

40

60

80

100

Temperature/°C

Fig. 5 DSC curves of PCL, TPS/PCL, TPS/PCL/BSF5%, TPS/PCL/ BSF10%, TPS/PCL/BSF20%

123

35.8







TPS



26.5

42.0



TPS/PCL

76.8

24.0

35.6



TPS/PCL/BSF5 %

57.2

17.0

19.8



TPS/PCL/BSF10 %

71.5

11.9

11.2



TPS/PCL/BSF20 %

77.6

4.4

6.2





BSF a

Fig. 4 Dependence of the activation energy on the extent of conversion obtained from isoconversional method regarding the thermal decomposition of polymers, composites, and bleached fibers

(58.2 °C; 17.5 J

PCL



– 85.0

Crystallinity index of PCL was determined using DSC curves [23]

b

Crystallinity indexes of TPS (for both B-type and Vh-type) and BSF were determined using XRD diffractograms [25, 26]

in starch are also known to form V-type crystallite complexes of relatively high melting temperatures that are resistant to enzyme attack [49]. It can be noted that the percentage of V-type crystallite complexes decreases as the amount of fibers are added. Thus, the presence of fibers reduced the chains mobility, partially hindering amylose recrystallization. The X-ray pattern of treated fibers indicated about 85 % crystalline structure (Fig. 6). The diffractogram of PCL shows the characteristic pattern of its crystalline structure with 2h peaks at *21.5°, *21.8°, and *23.7°, attributed to the (110), (111), and (200) plane reflections, respectively [50]. In the TPS/PCL blends, the peak at (200) plane reflection was clearly visible, but peaks at (110) and (111) were shifted to lower values. This might be due to the increased interplanar spacing of both phases, and is also an indication of good interaction between TPS and PCL [50].

lntensity/a.u.

Kinetics of thermal degradation applied to biocomposites

159

PCL TPS TPS/PCL TPS/PCL/BSF5 % TPS/PCL/BSF10 % TPS/PCL/BSF20 % BSF 10

15

20

25

30

35

Diffraction angle 2θ /°

Fig. 6 X-ray diffractogram of TPS, PCL, TPS/PCL, TPS/PCL/ BSF5%, TPS/PCL/BSF10%, TPS/PCL/BSF20%, and BSF

Conclusions TG analysis was used to investigate the thermal decomposition process of TPS/PCL based materials. The study of the thermal properties and the activation energy showed that bleached sisal fibers caused a decrease in melting temperature and activation energy, when only 5 % of bleached sisal fibers were added, without interfering with the thermal stability of the materials. On the other hand, adding 10 and 20 % of bleached sisal fibers increased the activation energy of the composite when compared to the neat polymers and blend. Bleached sisal fiber acted as a nucleating agent to the PCL fraction of materials, increasing PCL crystallinity index as sisal fiber was added. On the other hand, both B-type and Vh-type crystallinity index of TPS decreases as fiber content decreases in matrix due to the higher stiffness of the materials caused by sisal fibers addition. Acknowledgments The authors are grateful for the financial support of the projects granted by FAPESP (2008/08264-9), Capes, CNPq, FINEP, and Embrapa.

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