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Preparation and Characterization of Cassava-based Polypropylene (PP) and Polybutylene

Succinate (PBS) Biocomposites

����� ���;����<���1* Thierry Tran2 � ���� ����������2 Se-na Lee4 ก��� �!"� #���$�1,3 '�( Hyun-Joong Kim4 Rattana Tantatherdtam1*,Thierry Tran2, Sunee Chotineeranat2, Se-na Lee4, Klanarong Sriroth1,3and Hyun-Joong Kim4

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ABSTRACT

Cassava root fiber was used as a bio-reinforcement in polymer composite. Biocomposite of PP and PBS with cassava root fiber was prepared on a twin-screw extrusion. The maleic anhydride-polypropylene (MAPP) compatibilizer was used to improve interfacial strength between fiber and polymer matrix. The effects of untreated fiber content and compatibilizer on the mechanical properties of composite were investigated. It was observed that with an increase in fiber loading, the Younggs modulus and flexural modulus increased, indicating higher stiffness in fiber filled composites, whereas the tensile strength and flexural strength decreased. The addition of MAPP compatibilizer in PP composite with 50% fibers improved the flexural strength to higher level than pure PP, therefore giving rise to a stronger but less flexible material. Scanning electron microscopic studies revealed better interfacial adhesion between fiber and polymer matrix in composite containing MAPP compatibilizer. Thermogravimetric analysis showed that the thermal stability and degradation temperature of PP composites increased with the presence of MAPP, suggesting the enhanced interfacial interaction and compatibility due to the treatment of compatibilizer.

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Key words: Cassava, fibers, polypropylene, polybutylene succinate, MAPP, composites, extrusion. e-mail address: aaprnt@ku.ac.th

________________________________ 1 ���?6�"��"@��'�(=6j��B���B�>�!ก��:กK��'�($ ���9ก��5:กK�� 59�@�>���6�:กK��#����� ก� !:>=k 10900 Kasetsart Agricultural and Agro-Industrial Product Improvement Institute (KAPI), Kasetsart University, Bangkok 10900, Thailand. 2 Cassava and Starch Technology Research Unit, National Center for Genetic Engineering and Biotechnology, Thailand. 3 T�"@���:>"��������@T�= " ($ ���9ก��5:กK�� 59�@�>���6�:กK��#����� ก� !:>=k 10900 Department of Biotechnology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand. 4 Laboratory of Adhesion and Bio-Composites, Seoul National University, Seoul 151-921, South Korea

INTRODUCTION

Natural cellulosic fibers have been gained considerable attention as a reinforcing filler in polymeric materials. Besides their low cost and density, the plant-based natural fibers are renewable and biodegradable. They are also abundantly available as a waste from agricultural production. Thailand generates large quantity of cassava root fiber, approximately 1.5 million tons/year which can be served as potential sources of bio-fiber materials. Recently, the development of fiber reinforced polymer composite has focused on using biodegradable plant-based fibers as reinforcing materials due to their exceptional specific properties (Reddy and Yang, 2005; Mishra et al., 2004). New materials that provide both economic and environmental benefits have been being developed for application in the packaging, building, furniture and automotive industries. The use of polyolefins, e. g., PP and PE as a matrix polymer in composites has been studied extensively due to their excellent mechanical and thermal properties (Beckermann and Pickering, 2008; Kim et al., 2006). Biocomposites containing natural fiber and a biodegradable polymer matrices i.e., aliphatic polyester, for instance polylactic acid (PLA), polycaprolactone (PCL) and polybutylene succinate (PBS) are widely being developed to achieve fully renewable and environmentally friendly materials as substitutes for plastics from petroleum (Arbelaiz et al., 2006; Huda et al., 2008; Kim et al., 2005). The desirable properties of composite depend greatly on the interfacial compatibility between fiber and polymer matrix. Strong interfacial adhesion is crucial to achieve efficient fiber reinforcement. Accordingly, the use of compatibilizer is useful to reduce the interface tension between hydrophilic cellulose fiber and hydrophobic polymer matrix. Maleic anhydride grafted polypropylene, MAPP is demonstrated to be effective for improving the interfacial interaction by the esterification of anhydride group of MAPP and hydroxyl groups of fiber (Yang et al., 2007; Yang et al., 2007).

Availability of inexpensive natural cellulosic cassava root fiber in Thailand provides the opportunity to investigate the possibility of their utilization for the preparation of fiber reinforced composite. This study therefore aimed to use cassava root fiber for the preparation of PP- and PBS-

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based biocomposites and to evaluate the effect of the fiber on the mechanical properties of the resulting materials.

MATERIALS AND METHODS

1. Materials

The cassava root fibers (CS) obtained from cassava starch processing factories were washed, dried, pulverized and were used without chemical treatment. Polypropylene (PP) and polybutylene succinate (PBS Bionolle #1000 series), used as the matrix polymer, were supplied as homopolymer pellets by GS Caltex Corp. (South Korea) and by Showa Highpolymer Co. (Tokyo, Japan). The maleic anhydride-polypropylene (MAPP) compatibilizer was obtained from Crompton Corporation (Chemtura Corporation, Middlebury, CT, USA) in the form of Polybond 3200 (1% weight maleic anhydride).

2. Preparation of composite

The matrix polymers, PP and PBS, were blended with cassava fiber and compatibilizing agents in a laboratory-sized, co-rotating, twin screw extruder (model BA-19, Bau Technology, Uijungbu, Kyungki, South Korea). The temperature of the mixing (central) zone in the barrel was maintained at 185°C (PP) and 145°C (PBS) with a screw speed of 200 rpm. The extruded strand was cooled in a water bath at room temperature and pelletized using a pelletizer (Bau Technology, Uijungbu, Kyungki, South Korea). Extruded pellets were oven dried at 70°C for 1 hour and compounded a second time in the extruder in order to ensure well-blending, before drying again at 70°C for 12 hours, followed by storage in sealed polyethylene bags. Extruded pellets were injection molded into tensile (ASTM D638), Izod impact (ASTM D256), and three-point bend test bars (ASTM D790) using an injection molding machine (Bau Technology, Uijungbu, Kyungki, South Korea) at 185°C (PP composites) and 140°C (PBS composites) with an injection pressure of 1200 psi and a device pressure of 1500 psi.

3. Measurement and characterization

The tensile test for the composites was conducted according to ASTM D638-99 with a Universal Testing Machine (Zwick Co., Ulm, Germany) at a crosshead speed of 100 mm min-1. The Izod pendulum impact resistance was measured on an impact tester (model DYD-103C, DaeYeong Precision Co., Kunpo, Kyungki, South Korea) by ASTM method D256-97. The three-point bend tests of the composites were carried out according to ASTM D790 with a Universal Testing Machine at a crosshead speed of 5 mm.min-1. Five replications were conducted for all measurements. The thermal degradation of the composites was characterized using a thermogravimetric analyzer TGAQ500 (TA Instruments, New Castle, DE, USA) on 10mg samples, over a temperature range from 25 to 700°C at a heating rate of 20°C/min. Composite morphology was observed on tensile fracture surfaces under scanning electron microscopy (SEM; Sirion, FEI, Hillsboro, OR).

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RESULTS AND DISCUSSION

1. Mechanical properties

Effect of fiber loading

It can be seen that the Younggs modulus of composites increases with an increasing amount of cassava fiber, for both PP and PBS (Figure1 & Table 1). The flexural modulus also follow the same trend as can be observed in Figure 2. This suggested that the increased fiber loading in polymer matrix resulted in composites becoming more rigid and stiffer. The trend of an improvement in Younggs and flexural modulus as the level of fiber increase was more significant in the case of PP composites. The decrease in tensile and flexural strength of the PP and PBS composites with different levels of fibers is shown on Figure 3 and 4. For both PP and PBS, the tensile strength decreased with increasing level of fibers whereas the flexural strength shows relatively small change with the fiber loading. Similar results were obtained with pineapple leaf fibers (Hujuri et al., 2007). Figure 5 shows the strain at maximum force of PP and PBS composites with different fiber loading. As polymer matrix was stiffened by fiber, the elongation (the ability of material to stretch before breaking) decreased. The impact strength of the PP and PBS composites with different levels of fibers is shown on Figure 6. It was observed that the impact strength decreased with increasing level of fibers.

Effect of MAPP compatibilizer The MAPP compatibilizer enhanced the tensile strength of PP composites containing 50% fibers,

for which the tensile strength increased slightly from 23.67 to 27.7 MPa with 3% MAPP as illustrated in Figure 7. Upon increasing the content of compatibilizer, the materialgs strength was relatively unchanged. The tensile strength of composites with MAPP remained lower than that of pure PP. Figure 8 shows the effect of MAPP on the flexural strength improvement of PP composites. The addition of MAPP in PP composite with 50% fibers improved the flexural strength to higher level than pure PP, therefore giving rise to a stronger but less flexible material. The increasing level of MAPP had no statistically significant effect on the material properties. The enhancement in fiber-matrix interfacial interactions gives better stress transfer from matrix into fibers leading to higher tensile strength and flexural strength. Hujuri et al. (2007) reported the increase in flexural strength for PP composites with 5-20% pineapple leaf fibers compatibilized with MAPP.

2. Microstructure study by scanning electron microscope (SEM)

Morphology of tensile fracture surfaces of composites was analyzed by SEM to investigate the extent of fiber and polymer matrix adhesion. Figure 9 shows SEM images of the PP-cassava fiber composites in the

non-compatibilized samples compared to those in the compatibilized ones. It can be clearly seen in composites without MAPP that there is an evidence of fiber debonding with large gap between fiber and matrix, as well as a large number of holes in the matrix where the fibers were pulled out (Figure 9(a) and (b)). Contrarily, the tensile fracture surface of composites containing MAPP showed no clear gap in the interfacial region between the polymer matrix and fiber and the short broken fiber ends

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(Figure 9(c) and (d)). This indicated that MAPP promoted efficient fiber and matrix interfacial bond, resulting in the enhanced mechanical properties.

0 10 20 30 40 50 602000

3000

4000

5000

6000

7000

8000

9000

10000

11000

Young's modulus (MPa)

Fiber content (%)

PP

PBS

0 10 20 30 40 50 60

400

600

800

1000

1200

1400

1600

1800

2000

Flexural modulus (MPa)

Fiber content (%)

PP

PBS

Figure 1 Younggs modulus of PP and PBS composites. Figure 2 Flexural modulus of PP and PBS composites.

0 10 20 30 40 50 600

10

20

30

40

Tensile strength (MPa)

Fiber content (%)

PP

PBS

0 10 20 30 40 50 600

10

20

30

40

50

Flexural strength (MPa)

Fiber content (%)

PP

PBS

Figure 3 Tensile strength of PP and PBS composites. Figure 4 Flexural strength of PP and PBS composites.

0 10 20 30 40 50 600

1

2

3

4

Strain at maximum force (%)

Fiber content (%)

PP

PBS

0 20 40 60

0

2

4

6

8

10

Izod impact strength ( (((KJ/mm

2) )))

Fiber content (%)

PP

PBS

Figure 5 Strain of PP and PBS composites. Figure 6 Impact strength of PP and PBS composites.

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0 2 4 6 8 10

0

10

20

30

40

Tensile strength (MPa)

MAPP compatibilizer content (%)

PP

50:50 PP:fiber

0 2 4 6 8 10

30

35

40

45

50

Flexural strength (MPa)

MAPP compatibilizer content (%)

PP

50:50 PP:fiber

Figure 7 Tensile strength of compatibilized sample. Figure 8 Flexural strength of compatibilized sample.

Table 1 Mechanical properties of PP and PBS composites (average value and standard deviation (in parentheses)). Polymer Fiber

content (%)

Younggs modulus (MPa)

Flexural modulus (MPa)

Tensile strength(MPa)

Flexural strength (MPa)

Elongation at break (%)

Impact strength (kJ/mm2)

0 6904 (562) 1368 (33) 36 (0.5) 39 (0.6) 1.17 (0.13) 4.36 (0.29) 30 7515 (457) 1361 (68) 27 (0.4) 35 (0.1) 0.91 (0.02) 3.97(0.17) 40 8660 (527) 1529 (104) 26 (0.7) 37 (0.1) 0.89 (0.03) 3.68 (0.44) 50 10230(466) 1813 (49) 24 (0.5) 36 (0.6) 0.8 (0.02) 3.1 (0.34)

PP

60 8656 (935) 1807 (40) 21 (0.9) 36 (0.9) 0.6 (0.1) 3.93 (0.21) 0 3187 (223) 505 (33) 37 (0.3) 33 (0.7) 3.12 (0.23) 8.04 (0.73) 30 3637 (119) 879 (14) 18 (0.2) 31 (0.4) 0.84 (0.01) 3.59 (0.17) 40 3800 (346) 964 (29) 16 (0.2) 29 (0.4) 0.6 (0.03) 3.39 (0.17) 50 4267 (280) 1067 (11) 14 (0.3) 29 (0.9) 0.46 (0.03) 3.34 (0.21)

PBS

60 4348 (150) 1132 (6) 13 (0.1) 25 (0.6) 0.37 (0.02) 3.3 (0.17)

(a) (b)

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(c) (d) Figure 9 SEM micrographs of the tensile fracture surfaces of ((a) and (b)) PP- cassava fiber composites without MAPP compatibilizer showing the presence of cavities and large gap between fiber and matrix and ((c) and (d)) composites containing MAPP compatibilizer showing better interfacial adhesion with short broken fiber ends.

3. Thermogravimetric analysis

Figure 10 presents the dynamic TGA and the derivatives of the thermogravimetric curves of PP composites with 50% fibers containing 0%, 3% and 7% MAPP respectively. The peaks on the derivative curves, corresponding to the highest weight loss rates, were used to determine the degradation temperatures of the fiber and polymer fractions. The thermal decomposition characteristics of cassava fiber are also represented. The distinct peak appear at 250-400° C corresponded to the thermal degradation of hemicelluloses, cellulose and lignin constituents in natural cassava fiber. In fiber filled composites, two thermal-degradation regions were observed. The first region is a result of thermal depolymerization of constituents in fiber. The main weight loss corresponding to the degradation of the polymer matrix occurred in the range of 460-470°C. It was evident that the thermal stability and degradation temperature of composites increase with the addition of MAPP. This behavior may attribute to the improved fiber-matrix interfacial adhesion with the formation of the covalent linkage at the interface, leading to better compatibility between hydrophilic fiber and hydrophobic PP matrix.

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Weight loss (%)

0

20

40

60

80

100

Temperature (0C)

200 300 400 500 600

Weight loss derivative

0.0

.5

1.0

1.5

2.0

2.5

0 % M A P P

3 % M A P P

7 % M A P P

c a s s a v a f i b e r

Figure 10 Thermogravimetric analysis of PP composites with 0%, 3% and 7% MAPP compatibilizer.

CONCLUSION

It was shown that the incorporation of cassava root fiber to PP and PBS composites has an effect on increasing stiffness or modulus of the materials. The addition of MAPP compatibilizer resulted in an improved composite strength, compensating the loss in mechanical properties caused by the fiber. The tensile fracture surface of PP composites containing MAPP compatibilizer examined by SEM revealed a stronger interfacial bonding between fiber and PP matrix which is evident from the short broken fiber ends without clear gap around fiber. While the presence of cavities, indicating easy fiber pull out and large gap between fiber and matrix was observed in composites without compatibilizer. The improved thermal stability of compatibilized PP composite was presumably due to the enhanced interfacial adhesion between fiber and matrix polymer.

ACKNOWLEDGEMENT

This work was kindly supported by the Brain Korea 21 program from South Koreags Ministry of Education.

LITERATURE CITED

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Beckermann, G. W. and K. L. Pickering. 2008. Engineering and evaluation of hemp fibre reinforced polypropylene composites: fibre treatment and matrix modificaion. Composites Part A39: 979-988.

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Huda, M. S., L. T. Drzal, A. K. Mohanty and M. Misra. 2008. Effect of fiber surface- treatment on the properties of laminated biocomposites from poly(lactic acid) (PLA) and kenaf fibers. Composites Science and Technology 68: 424-432.

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Kim, H.-S., H.-S. Yang and H.-J. Kim. 2005. Biodegradability and mechanical properties of agro-flour-filled polybutylene succinate biocomposites. Journal of Applied Polymer Science 97 (4): 1513-1521.

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