Dynamic Mechanical and Thermal Properties of Red ...

Macromolecular Research, Vol. 18, No. 5, pp 00-00 (2010) DOI 10.1007/s13233-009-

www.springer.com/13233

Dynamic Mechanical and Thermal Properties of Red Algae Fiber Reinforced Poly(lactic acid) Biocomposites Kyoung Ja Sim and Seong Ok Han* Nano Materials Research Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseonggu, Daejeon 305-343, Korea

Yung Bum Seo Department of Forest Products, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Korea Received September 29, 2009; Revised December 7, 2009; Accepted December 17, 2009 Abstract: The objective of this study is to compare the dynamic mechanical and thermal properties of biocomposites reinforced with algae fiber; polypropylene (PP) and poly lactic acid (PLA) biocomposites. Biocomposites were manufactured with bleached red algae fiber (BRAF) loading from 30 wt% to 60 wt%. Thermal properties of BRAF, PLA and PP are determined by DSC and TGA. The dynamic mechanical and thermomechanical properties of the PLA matrix and biocomposites are analyzed by DMA and TMA. The dynamic mechanical and thermomechanical properties of both BRAF/PLA and BRAF/PP biocomposites showed improvement with increasing the BRAF loadings. The fibers are well bonded with the PLA matrix, indicating the strong adhesion as observed by SEM of fractured surface. Also, the higher improvement of storage modulus is obtained with BRAF/PLA biocomposites than BRAF/PP biocomposites. As a result, we conclude that BRAF/PLA biocomposite can be used for an alternative to BRAF/PP biocomposite as a completely biodegradable biocomposite. Key words: red algae fiber, poly(lactic acid) biocomposites, dynamic mechanical and thermal properties.

giving rise to a multitude of ecological and environmental concerns. Hence, recently the researches on biodegradable polymers such as poly(lactic acid)(PLA), poly(butylene succinate) (PBS), polyhydroxyalkanoates(PHA), starch etc. stimulated interest for replacing the traditional polymers. The biodegradable polymers are in the limelight of the environmental material, but these have lower physical properties and higher cost than traditional polymers.3-8 Poly(lactic acid), derived from corn, is transparent and crystalline polymer with relatively high melting temperature and has brittle properties, i.e. high strength and low elongation at break. It is, however, possible to overcome the brittleness and poor processability of stiff through combination with other materials.9-12 Biocomposites, generally reinforced with natural fibers like kenaf,7 jute,8 bamboo,13 wood,14 flax,15 hemp,16 silk,17 miscanthus18 have been reported earlier. Recently, the marine plant natural fibre such as red algae fiber has been used as a reinforcing material for biocomposite.19 Especially, characterization of red algae fiber showed that the length and diameter of fiber are ten times smaller than natural fibers. The bleached red algae fibre showed higher thermal stability than that of the crystalline cellulose. The shape of red algae fiber is much uniform,20 therefore, utilizing the red algae fiber as a reinforcing material is effective for obtain-

Introduction The history of fiber reinforced composites began with cellulose fiber in phenolics in 1908. The traditional composites usually made out of glass, carbon or aramid fibers with epoxy, unsaturated polyester resins, polyurethanes or phenolics. Recently, according to serious environmental pollution problem the interest to develop and use natural resources material has rapidly grown for preservation of finite petroleum resources and reduction of carbon dioxide emission. Consequently, researches and developments on environmental friendly materials, namely, biocomposites made a rapid progress. If the biocomposites contain either a biodegradable polymer or natural fibre among binary component parts, the biocomposites have partially or complete biodegradable ability in the environment.1-6 Traditional polymers such as, polypropylene(PP), polyethylene(PE), polystyrene(PS), poly vinyl chloride(PVC), epoxy have advantages of low cost, significant amount of accumulated information, simple industrial processes and high physical properties comparing to the biodegradable polymers. However, once these materials are discarded, they persist in the environment without being degraded thus *Corresponding Author. E-mail: [email protected] The Polymer Society of Korea

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K. J. Sim et al.

ing the uniform properties of biocomposites. The fabrication of biocomposite using the red algae fiber as a reinforcing material and investigated the effect of reinforcement loadings on the properties of PBS and PP based biocomposites are reported.19,21,22 Lee et al. also examined a new manufacturing process to disperse of the red algae fiber in the biopolymer effectively using high temperature grinding method.23 The mechanical, dynamic mechanical and thermomechanical properties of the red algae fiber reinforced biocomposites were improved with increasing fiber loadings. These results support that the algae fiber can be used as an excellent reinforcement of biocomposites. The objectives of this study are to fabricate PLA biocomposites reinforced with the red algae fiber and to investigate the effect of reinforcement loadings on the dynamic mechanical and thermal properties of the PLA biocomposites. The dynamic mechanical and thermal properties of PLA biocomposites are compared to those of PP biocomposites in order to examine the possibility of replacing of PP based biocomposites with PLA based biocomposites as a completely biodegradable biocomposites.

Experimental Materials. Poly(Lactic Acid) (PLA) fiber was supplied from Huvis Co., Korea. The specific gravity and the melting point were 1.2-1.3 and 130-175 oC, respectively. The tenacity and elongation of the PLA fibers were 3.6 g/denier and 67%, respectively. Propylene (PP) fiber was supplied from Kolon Glotech Co., Ltd., Korea. The specific gravity and the melting point were 0.91 and 160-165 oC, respectively. The tenacity and elongation of the PP fibers were 1.5-4.5 g/denier and 50-350%, respectively. The length of PP fiber was 76 mm in average and the fiber diameter was about 20-50 μm. The bleached red algae fiber (BRAF) was prepared by extracting and bleaching the red algae (Morocco, Gelidium corneum) according to the method in the previous paper.19 The average length and diameter of red algae fiber were 0.5-1.0 mm and 2-4 μm, respectively. Fabrication of the BRAF/PLA and BRAF/PP Biocomposites. The fiberization of BRAF for biocomposite was performed using a high-speed powdering machine (Ultra centrifugal mill, Germany) with 6000 rpm for 40-60 sec. The length of pulverized BRAF was below 80 μm. Both PLA and BRAF were dried in vacuum oven 80 oC for 24 h before use and PP was used as received. The biocomposites were manufactured with different BRAF loading ranging from 30 wt% to 60 wt%. Before biocomposite fabrication by compression molding, PLA and PP fiber were randomly chopped, mixed with the pulverized BRAF for 30 sec using a kitchen mixer. The mixture was placed in a stainless steel mold and melted with heating for 40 min, holding at 175 oC (165 oC for BRAF/PP biocomposites) for 20 min, applying 6.89 MPa for 10 min. The biocomposite was cooled down 2

to 15 oC by circulating cold water around the mold with applying a pressure of 6.89 MPa. The specimen dimension is 50 mm×50 mm×1.7 mm. Differential Scanning Calorimeter Analysis (DSC). The melting and crystallization behavior of the pure PLA fiber and PP fiber were studied using a differential scanning calorimeter (DSC Q 100, TA Instruments) equipped with a cooling system. The sample of ~10 mg was heated from 30 to 200 oC and cooled to 20 oC at the rate of 5 oC/min under a nitrogen flow of 50 mL/min. The sample was sealed into aluminum pan and an empty reference aluminum pan which has the same weight as the sample pan was used as a reference. The heat flow and energy changes in and out of the samples in the sealed aluminum pan were recorded with reference of an empty aluminum pan. Based on the endothermic and exothermic peaks the glass transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm), crystallization enthalpy (ΔHc), and melting enthalpy (ΔHm) were determined. Thermogravimetric Analysis (TGA). The thermal stability analysis was carried out to 500 oC using a thermogravimetirc analyzer (TGA Q 500, TA Instruments) at a heating rate of 10 oC/min and nitrogen gas flow rate of 100 mL/min. Approximately 15 mg of each sample was loaded for each measurement. Changes in weight percentage and the decomposition temperatures of the samples were recorded. Thermomechanical Analysis (TMA). The coefficient of thermal expansion (CTE) was measured by heating the specimen from 30 oC to 100 oC at a heating rate of 5 oC/min under the nitrogen atmosphere with flow rate of 100 mL/min. The prove was applied 0.05 N loading and it measured strain and temperature of specimens. The coefficient of thermal expansion was taken as the linear slope of the dimensional stability-temperature curve with a thermomechanical analyzer (TMA Q 400, TA Instruments). The specimen dimension was 7 mm×7 mm×1.7 mm. Dynamic Mechanical Analysis (DMA). The storage modulus, loss modulus, tan delta (tan δ), and glass transition temperature of each specimen were measured as a function of temperature by a dynamic mechanical analyzer (DMA Q 800, TA Instruments). The single cantilever mode at a frequency of 1Hz was used under the nitrogen atmosphere and a heating rate of 5 oC/min. The dynamic mechanical analysis was performed from 20 to 150 oC for BRAF/PLA biocomposites and from -30 to 100 oC for BRAF/PP biocomposites, respectively. The instrument was calibrated to have the correct clamp position and compliance before each measurement. The specimen dimension was 3.5 mm×1.2 mm× 1.7 mm. Scanning Electron Microscopic Observation (SEM). The surface morphology of red algae fiber and the fractured surfaces of both PLA and biocomposites were characterized by Scanning Electron Microscope (SEM, S-4700, Hitachi, Japan). Prior to the measurement, the specimens were coated Macromol. Res., Vol. 18, No. 5, 2010

Dynamic Mechanical Properties of BRAF/PLA Biocomposites

with Au in order to prevent electrical discharge. The acceleration voltage used was 15-25 kV.

Results and Discussion DSC Analysis. The thermal properties of PLA and PP fibers such as Tg, Tc, Tm, ΔHc and ΔHm are shown in Figure 1 and Table I. All values are obtained from the 2nd run DSC scans and melting enthalpies were determined using constant integration limits. The degree of crystallinity (X%) was estimated using equation: X(%) = ΔHm/ΔHm0 × 100, where ΔHm is the measured melting enthalpy and ΔH0m is the melting enthalpy of a pure crystalline matrix, which is 93 J/g and 188.9 J/g for PLA and PP, respectively.24,25 For the PLA fiber the glass transition peak temperature is observed at 56.2 oC and the peak for the heat release due to recrystallization was observed at 103.1 oC. The crystalliza-

tion of PLA fiber during the heating gave a crystalline phase which melt providing two endotherm peaks at 160.5 and 167.2 oC. It has been reported that the double melting behavior in PLA can be linked to the formation of different crystal structures: the α-form (pseudo-orthorhombic, pseudo-hexagonal or orthorhombic) and the β-form (orthorhombic or trigonal) that melts in correspondence of the endotherm at higher temperature and lower temperature, respectively.10 For the PP fiber the melting and crystallization temperatures are at 163.3 and 123.5, respectively. TGA Analysis. The thermal stability of BRAF, PLA, PP and the BRAF/PLA and BRAF/PP composites with different fiber loadings are shown in Figure 2 and Figure 3. The decomposition characteristics of each specimen such as the 5, 25, 50 and 75% weight loss temperatures (T5, T25, T50 and T75) and residue are summarized in Table II. The BRAF shows the main decomposition peak between 315 oC and 390 oC and the highest decomposition temperature at 356.6 oC, which is similar to the cellulose decomposition temperature.19 For both PLA and PP the single maximum decomposition

Figure 1. DSC curves of (a) PLA fiber and (b) PP fiber. Table I. Thermal Properties of PLA Fiber and PP Fiber Tg (oC)

Tc (oC)

Tm (oC) ΔHc (J/g) ΔHm (J/g) X%

PLA

56.2

103.1

167.2

41.2

52.1

56.0

PP

-

123.5

163.3

98.4

96.9

51.3

Macromol. Res., Vol. 18, No. 5, 2010

Figure 2. (a) TGA curves and (b) DTG of BRAF, PLA matrix and its biocomposites with different fiber loadings. 3

K. J. Sim et al.

that of PP and similar to that of BRAF. The maximum decomposition peak of biocomposites for 50 wt%BRAF/PLA was 347.3 oC and for 40 wt%BRAF/PP was 333.6 oC (1st peak) and 459.2 oC (2nd peak). Weight losses of 84.9, 84.3, 81.1 and 16.8% were observed for the BRAF/PLA biocomposites with 30, 40, 50 and 60 wt% fiber loadings at 360 oC, respectively, which indicates that the thermal stability of the BRAF/PLA biocomposite can be enhanced by adding the BRAF reinforcement. Also, the residue of specimen after heating up to 500 oC was observed higher with the higher loading of the BRAF reinforcements. The weight percentages of residue are 0.9 and 4.9, 5.5, 6.7 and 7.4% for the neat PLA and 30, 40, 50 and 60 wt% BRAF/ PLA biocomposites, respectively. The increase of residue could be explained as a higher thermal stability of biocomposites by adhesion between the BRAF reinforcements and the matrix. TMA Analysis. In general, the dimensional change corresponding to the CTE of fiber reinforced polymer composites decreases with increasing fiber loadings. Figure 4(a) represents the thermal expansion behavior of PLA and the

Figure 3. (a) TGA curves and (b) DTG of PP matrix and its biocomposites with different fiber loadings. Table II. The Thermal Decomposition Temperatures for BRAF, PLA, PP, BRAF/PLA and BRAF/PP Biocomposites at 5, 25, 50 and 75% of Total Weight Losses T5 (oC)

T25 (oC)

T50 (oC)

T75 (oC)

Residue (%)

BRAF

84.2

332.1

353.7

369.5

11.7

PLA Matrix

303.7

335.8

350.9

361.6

0.9

30 wt%BRAF/PLA 299.2

332.4

343.9

354.3

4.9


333.7

334.4

354.4

5.5


331.7

345.0

355.9

6.7


330.5

343.7

354.2

7.4

PP matrix

391.6

431.9

445.2

454.3

0

30 wt%BRAF/PP

295.8

375.5

443.6

459.3

4.2

40 wt%BRAF/PP

283.7

337.3

437.1

460.2

5.0

50 wt%BRAF/PP

270.7

327.5

405.3

450.9

6.5

60 wt%BRAF/PP

266.8

322.0

354.4

441.0

7.1

peak was observed at 360.2 oC and 451.2 oC, respectively. The decomposition temperature of PLA is much lower than 4

Figure 4. (a) Dimension change of BRAF/PLA biocomposites with different fiber loadings and (b) comparison of the coefficient of thermal expansion (CTE) between BRAF/PLA and BRAF/PP biocomposites with different fiber loadings. Macromol. Res., Vol. 18, No. 5, 2010


composites reinforced by BRAF loading from 30 wt% to 60 wt%. The PLA matrix shows a large dimensional change with increasing temperature from 30 oC to 100 oC. When the PLA was reinforced with BRAF, the thermal expansion was decreased appreciably. This is attributed to the thermal expansion restraint of PLA matrix by the BRAF in biocomposite. Thermal expansion reduction is attributed to adhesion between BRAF and PLA as a strong interaction restricts the mobility of the polymer chain adhering to the fiber surface.26 Also, the CTE values of both PLA matrix and BRAF/PLA biocomposites began to draft increase ~57 oC as the glassy phase of PLA turn into rubbery phase which is consistent with the results obtained by the DSC analysis. The coefficient of thermal expansion (CTE) was calculated from the slope of the curve of dimension change vs. temperature. The determination of CTE is useful for understanding dimensional changes as well as thermal stresses caused by thermal variation.6 Figure 4(b) represents CTE of PLA matrix and BRAF/PLA biocomposites compare with those of PP matrix and BRAF/PP biocomposites. The CTE values of BRAF/PLA biocomposites gradually decreased with increasing BRAF loadings, whereas CTE value of BRAF/ PP biocomposites increased for the biocomposites with 30 wt% fiber loading compared to PP matrix and gradually decreased with higher fiber loading than 40 wt%. In case of the polymer matrix, CTE value of PP matrix is lower than PLA matrix, whereas CTE value of BRAF/PP biocomposites is higher than those of PBRAF/PLA biocomposites up to 50 wt% fiber loadings. It seems that PLA based biocomposites apparently have higher dimensional stability than PP based biocomposites and also have the better adhesion between fiber and matrix. DMA Analysis. Figure 5 shows the storage modulus and tan delta of the PLA matrix and BRAF/PLA biocomposites with different fiber loadings as a function of temperature. As shown in Figure 5(a) and Table III, the storage modulus

Figure 5. (a) Storage modulus and (b) tan delta of BRAF, PLA matrix and BRAF/PLA biocomposites with different fiber loadings.

of the BRAF/PLA biocomposites at 30 oC largely increased with addition of BRAF up to 50 wt% loading, from 3.1 GPa (PLA matrix) to 5.4 GPa (50 wt% BRAF/PLA). As shown in Table III, the BRAF/PP biocomposites showed the same

Table 3. The Storage Modulus of BRAF/PLA and BRAF/PP Biocomposites Storage Modulus (GPa) 30 C

60 oC

90 oC

Improvement Ratio in Compared to PLA or PP Matrix at 30 oC (%)

89.9

3.1

3.0

0.4

-

30 wt% BRAF/PLA

96.6

4.5

4.4

2.9

45

40 wt% BRAF/PLA

96.9

5.0

4.9

3.6

61

50 wt% BRAF/PLA

97.6

5.4

5.2

3.8

74

60 wt% BRAF/PLA

98.0

5.1

4.8

3.5

63

Fibers/Polymer

Tg (oC)

PLA matrix

o

PP matrix

-1

2.3

1.3

0.9

-

30 wt% BRAF/PP

-7

2.6

2.6

1.6

11

40 wt% BRAF/ PP

-8

3.1

2.7

2.0

33

50 wt% BRAF/ PP

-7

3.2

2.8

2.1

37

60 wt% BRAF/ PP

-4

3.2

2.7

2.0

37


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K. J. Sim et al.

tendency as the storage modulus increases from 2.3 GPa (PP matrix) to 3.2 GPa (50 wt% BRAF/PP) at 30 oC. This is attributed to the reinforcement effect imparted by the natural fibers that stress can be transferred from the PLA or PP to the BRAF.6,12,14,15 However, storage modulus shows 2% decrease at 60 wt% BRAF/PLA and same value at 60 wt% BRAL/PP biocomposites, respectively. It seems to be due to the insufficient filling of PLA or PP into the BRAF during composite processing. Also, it is noticeable that the storage modulus for BRAF/PLA biocomposites improved much higher than that for BRAF/PP according to the fiber loadings, showing the improvement rate of 74% for 50 wt% BRAF/PLA comparing to pure PLA matrix. On the contrary, the storage modulus of 50 wt% BRAF/PP shows the 37% improvement comparing to pure PP matrix. Figure 5(b) shows tan δ of the PLA matrix and BRAF/ PLA biocomposites as a function of temperature. Tan delta is the ratio of the storage modulus to the loss modulus or the ratio of the energy dissipated to the energy stored during a dynamic loading cycle. The magnitude of the tan delta peak is associated with the damping property of a composites material.6 The introduction of BRAF to PLA decreases the tan δ peak reflecting a lower damping property, indicating that the BRAF reinforced biocomposites have good structural damping property, especially at fiber loading of 50 wt%.21 This result indicates that the molecular mobility of the composites materials decreased with increasing of BRAF loadings and the mechanical loss to overcome inter-friction between molecular chains is reduced.19 However, the peak glass transition temperature is not significantly affected by the fiber loading. The similar results for BRAF/PP biocomposites have been reported by Lee et al.; the peak becomes broader and tan delta decreases upon addition of BRAF. This is because the fibers carry stress to a greater extent and allow only a small part of it to strain the interface. Therefore, energy dissipation occurs mostly in the polymer matrix and at the interface with a stronger interface characterized by less energy dissipation.21 Such interaction reflects the interfacial bonding between BRAF and the PLA or PP matrix. Figure 6 shows the comparison of the dynamic storage modulus for the BRAF/PLA and BRAF/PP biocomposites with increasing BRAF loadings at 30 oC. The storage modulus of both BRAF/PLA and BRAF/PP biocomposites gradually increased up to 50wt% of BRAF contents. PLA based biocomposites shows remarkably higher improvement of storage modulus with increase of fiber loading than PP based biocomposites, indicating that the incorporation of BRAF into PLA is much effective resulting in better adhesion between fiber and polymer matrix than PP. SEM Observation. Figure 7 shows the morphology of the BRAF and the fractured surfaces of BRAF/PLA and BRAF/PP biocomposites. In Figure 7(a) and (b) most of the BRAF show uniform diameter. Figure 7(c, d, e and f) shows the micrograph of the BRAF/PLA and BRAF/PP biocom6

Figure 6. Comparison of storage modulus between BRAF/PLA biocomposites and BRAF/PP biocomposites with different fiber loadings at 30 oC.

Figure 7. Scanning electron micrographs of the Bleached red algae fibers ((a) ×2000, (b) ×5000), 40 wt% BRAF/PLA biocomposite ((c) ×2000, (d) ×5000), and 40 wt% BRAF/PP biocomposite ((e) ×2000, (f) ×5000).

posites with different magnification as same 40 wt%BRAF loadings. Also, it can be observed that the network formed inside the BRAF/PLA and BRAF/PP biocomposite, which is contributed to the uniform properties of biocomposites and good interfacial adhesion between the BRAF and polymer matrix. In case of the BRAF/PP biocomposites, most BRAF was pulled out in the PLA matrix, whereas, the most BRAF of BRAF/PLA biocomposites was cut in the PLA matrix. It seems that PLA based biocomposites have the Macromol. Res., Vol. 18, No. 5, 2010


better interfacial adhesion between the BRAF and PLA matrix than that of PP based biocomposites, and that is consistent with the results obtained by the TMA and DMA analysis.

Conclusions In this study, BRAF reinforced PLA biocomposites have been successfully fabricated with different fiber loadings and their thermal and dynamic mechanical properties have been investigated and compared to BRAF reinforced PP biocomposites. The dynamic mechanical and thermomechanical properties of both BRAF/PLA and BRAF/PP biocomposites showed noticeable improvement with increasing the BRAF loadings. The storage modulus of both BRAF/PLA and BRAF/PP biocomposites showed the highest performance at BRAF loading of 50 wt%. Also, the higher improvement of storage modulus is obtained with BRAF/PLA biocomposites than BRAF/PP biocomposites with the BRAF loadings. The storage modulus of the BRAF/PLA biocomposites are showed 74% increase rate which is much higher improvement of compared to 37% increase of 50 wt% BRAF/ PP biocomposites at 30 oC. The greater the BRAF loadings added, the lower the CTE value also is. CTE value of BRAF/ PLA biocomposites is also lower than that of BRAF/PP biocomposites, indicating the higher dimensional stability of BRAF/PLA biocomposites. These results support that the BRAF can be used as an excellent reinforcement in biocomposites and the BRAF/PLA biocomposites, can be alternatives of BRAF/PP biocomposites resulting with higher dynamic and thermo mechanical properties as biodegradable composites or complete green composites. Acknowledgements. This work was financially supported by the Carbon Dioxide Reduction and Sequestration (CDRS) R&D Center (the 21st Century Frontier R & D Program) funded by the Ministry of Education, Science and Technology, Korea. The authors of this paper would like to thank to Huvis Co., Korea for kindly supplying of PLA fiber and Dr. kyriaki kalaitzidou for the review of this paper.

References (1) D. Cho, S. G. Lee, W. H. Park, and S. O. Han, Polym. Sci. Tech., 13, 460 (2002). (2) S. O. Han, S. M. Lee, W. H. Park, and D. Cho, J. Appl. Polym.


Sci., 100, 4972 (2006). (3) K. Oksman, M. Skrifvars, and J. F. Selin, Polym. Sci. Tech., 63, 1317 (2003). (4) A. K. Mohanty, M. Misra, and G. Hinrichsen, Macromol Mater Eng., 276/277, 1 (2000). (5) A. K. Mohanty, M. Misra, and L. T. Drzal, Comp Interfaces., 8, 313 (2001). (6) S. M. Lee, D. Cho, W. H. Park, S. G. Lee, S. O. Han, and L. T. Drzal, Comp Sci Tech., 65, 647 (2005). (7) M. S. Huda, L. T. Drzal, A. K. Mohanty, and M. Misra, Comp Sci Tech., 68, 424 (2008). (8) D. Plackett, T. L. Andersen, W. B. Pedersen, and L. Nielsen, Comp Sci Tech., 63, 1287 (2003). (9) S. Sinha Ray and M. Bousmina, Prog Mater Sci., 50, 962 (2005). (10) K. Fukushima, D. Tabuani, and G. Camino, Mater Sci Eng C., 29, 1433 (2009). (11) A. Iwatake, M. Nogi, and H. Yano, Comp Sci Tech., 68, 2103 (2008). (12) M. S. Huda, L. T. Drzal, A. K. Mohanty, and M. Misra, Comp Sci Tech., 66, 1813 (2006). (13) S. H. Lee and S. Wang, Comp part A., 37, 80 (2006). (14) M. S. Huda, L. T. Drzal, M. Misra, and A. K. Mohanty, Appl Polym Sci., 102, 4856 (2006). (15) B. Bax and J. Müssig, Comp Sci Tech., 68, 1601 (2008). (16) A. Keller, Comp Sci Tech., 63, 1307 (2003). (17) H. Y. Cheung, K. T. Lau, S. M. Tao, and D. Hui, Comp part B., 39, 1026 (2008). (18) A. Bourmaud and S. Pimbert, Comp part A., 39, 1444 (2008). (19) M. W. Lee, S. O. Han, and Y. B. Seo, Comp Sci Tech., 68, 1266 (2008). (20) Y. B. Seo, Y. W. Lee, C. H. Lee, H. C. Yu, and S. M. Boo, Advances in Pulp & Paper Science and Technologies: 2006 Pan Pacific Conference proceedings, Seoul, Korea: KTAPPI, 153 (2006). (21) M. W. Lee, Y. B. Seo, S. O. Han, Proceeding of Spring Conference of the Korea Technical Association of the Pulp and Paper Industry, 166 (2008). (22) M. W. Lee and Y. B. Seo, Proceeding of Autumn Conference of the Korea Technical Association of the Pulp and Paper Industry, 187 (2008). (23) S. O. Han, H. S. Kim, Y. J. You, Y. B. Seo, and M. W. Lee, Korean Patent; 10-0867424(2008). (24) A. P. Mathew, K. Oksman, and M. Sain, Appl Polym Sci., 101, 300 (2006). (25) M. Pracella, D. Chionna, and I. Anguillesi, Comp Sci Tech., 66, 2218 (2006). (26) A. N. Nakagaito and H. Yano, Cellulose., 15, 555 (2008).

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