- Email: [email protected]
Hygrothermal effect on henequen or silk fiber reinforced poly(butylene succinate) biocomposites
Composites: Part B 41 (2010) 491–497
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Hygrothermal effect on henequen or silk fiber reinforced poly(butylene succinate) biocomposites Seong Ok Han a,*, Ho Jung Ahn a, Donghwan Cho b a b
Reaction and Separation Materials Research Center, Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong-gu, 305-343 Daejeon, Republic of Korea Department of Polymer Science and Engineering, Kumoh National Institute of Technology, Gumi, Gyungbuk 730-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 January 2010 Accepted 7 May 2010 Available online 9 May 2010 Keywords: A. Polymer–matrix composites (PMCs) B. Thermal properties D. Fractography E. Compression moulding
a b s t r a c t The hygrothermal effect of poly(butylene succinate) biocomposites reinforced with silk or henequen fibers was investigated. The biocomposites were fabricated using a compression molding method. The biocomposites were maintained in the chamber at 60 °C and 85% RH for 1000 h. The results show that the degradation of PBS matrix is the main cause for the deterioration of biocomposites performance and the biocomposites degraded much slowly than PBS matrix did. The storage modulus for the biocomposites exposed at 60 °C and 85% RH for 1000 h was decreased 20% and 50% with the silk and henequen fibers comparing to the original specimens, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biocomposites made of polymer matrices and natural fiber reinforcements have been actively investigating and applying to automobile industries. Development of biocomposites is laying weight because of their advantages such as environmentally friendliness, light weight and carbon dioxide reduction, etc. [1,2]. Also, the utilization of biodegradable polymer as a matrix of biocomposites has been seriously considered for the development of more environmentally friendly biocomposites [3]. The main applications of these biocomposites are interior parts of automobiles because of the hydrophilic character of natural fiber [4]. Recently, studies on biocomposites have also been performed in order to apply biocomposites to outdoor environments [5]. Natural fiber can be largely divided into two categories such as lignocellulose-based fibers and animal-based fibers. These natural fibers have several advantages such as high specific strength and stiffness due to low density of the fiber over the glass fiber reinforcement in composite applications. Especially, the hollow tubular structure of ligno-cellulose natural fibers enhances acoustic and thermal insulating performances and also reduces their bulk density, leading to energy saving as well as lightweight in automobile applications [6]. Also, ligno-cellulose natural fibers contribute to the CO2 reduction [7]. Ligno-cellulose fibers like kenaf, flax, and henequen fibers have been frequently used as natural fiber reinforcements. Of them, henequen fibers are attracting more attention
* Corresponding author. Tel.: +82 42 860 3149; fax: +82 42 860 3133. E-mail address: [email protected] (S.O. Han). 1359-8368/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2010.05.003
for manufacturing low-cost materials [8]. As an animal-based fiber, silk fibers (Bombyx mori) are consisted of fibroin in the inner layer and sericin in the outer layer, all protein-based. Silk fibers are biodegradable and has high crystalline with well-aligned and continuous structure, higher tensile strength, high toughness, good elasticity, and excellent resilience than glass fiber or lignocellulose-based natural fibers [9,10]. Silk fiber is stable up to 140 °C and decomposes over 150 °C. Poly(butylene succinate) (PBS) is a biodegradable, thermoplastic and aliphatic polyester. PBS has excellent processibility with relatively low melt temperature and comparable mechanical properties with general-purpose thermoplastics like polyethylene and polypropylene [11]. PBS is also completely biodegradable in soil, lake, sea and compost, and has thermal and chemical resistance, and recyclability [12]. Biocomposites based of PBS with henequen or silk fiber reinforcements have been studied and proved that they have comparable specific mechanical properties to glass fiber reinforced polymer composites [13,14]. However, the hydrophilic natural fibers absorb the water molecules, resulting in altering the physical and mechanical properties of the biocomposites. The physical changes include a decrease of the mechanical properties, microcracks, chain scission, and degradation of interfacial bonding between the fibers and the matrix [15]. Till now, the long-term stability of the biocomposites based on the biodegradable polymer matrix, PBS, in wet environment has not been reported yet. In this research, we investigated the hygrothermal effect of PBS matrix and two different kinds of biocomposites reinforced with either silk fibers or henequen fibers. We characterized the changes of thermal stability, storage modulus and topography for the biocomposites specimens maintained in the
492
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
in Jinju, Korea) were used as a ligno-cellulose and animal based reinforcements, respectively. Henequen (Agave fourcroydes) fibers from Yucatan, Mexico, with filaments having lengths in the range of 60–70 cm, were used in the investigation. The average density was about 1.45 g/cm3 and the filaments’ diameter was generally in the range of 150–200 lm. The waste silk or silk scrap used in the present study was kindly supplied from the Korea Silk Research Institute manufacturing silk fabrics in Jinju, Korea. The as-received waste silk used here consists of the fibroin inner layer only, because the sericin outer layer was removed from the raw silk fibers. The fibres were chopped to be short fibres of 10 mm in average length. PBS (Ire Chemical Co., Korea, Enpol G-4500) has a melting point of 115 °C, the specific gravity of 1.22, and the melt index (g/10 min at 190 °C) of 29.
2.2. Biocomposites fabrication
Fig. 1. Weight increase of biocomposites and PBS matrix.
environment of 60 °C and 85% relative humidity (RH) with duration time.
2. Experimental 2.1. Materials Henequen fibers (HQ) (originated from Mexico) and the silk fibers (the Korea Silk Research Institute manufacturing silk fabrics
The chopped natural fibers and PBS pellets were dried at 100 °C for 2 h in a conventional oven and at 80 °C for 5 h in a vacuum oven before use, respectively. Prior to composite fabrication, the pulverized PBS powder was mechanically mixed with chopped natural fiber using a kitchen mixer. After mechanically mixing the natural fiber and the PBS powder pulverized with a blender, the molding compounds obtained were melted at 130 °C for 15 min, holding at 1000 psi for 15 min using a hot-press (Carver 2518), then was retained until the mold was naturally cooled down to ambient temperature. During processing, the PBS powder was melted enough to flow into the natural fibers. A stainless steel mold with cavity dimensions of 50 mm 50 mm was used. The fiber content and the chopped fiber length of biocomposites were 30% in weight and 10 mm in average, respectively. PBS matrix specimen was also prepared as a control. The specimens of both PBS matrix and bio-
Fig. 2. A: Scanning electron micrographs of (a) the silk/PBS biocomposite control, (b) the degraded silk/PBS biocomposite for 1000 h (3000), B: scanning electron micrographs of (a) the HQ/PBS biocomposite control, (b) the degraded HQ/PBS biocomposite for 1000 h (200).
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
composites for durability test were prepared with the dimensions of 1.2 mm 18 mm 35 mm, respectively. 2.3. Water absorption The biocomposites and the PBS matrix were tested in a chamber controlled at 60 °C and 85% RH. The specimens were weighed; set in a chamber, taken out from a chamber at predetermined times, gently wiped dry using lint-free tissue paper and allowed to stand free at ambient environment for 5 min to ensure the removal of excessive surface water. Every procedure was performed by following ASTM D570-98: standard test methods for water absorption of plastics [11]. The weight gain percentage by water absorption, W%, was determined from the equation:
493
2.4.2. Dynamic mechanical analysis The storage modulus of biocomposites and the PBS matrix was measured from 50 °C to 250 °C with a heating rate of 5 °C/min by dynamic mechanical analysis (DMA Q800, TA Instruments). A fixed frequency and the oscillation amplitude were 1 Hz and 0.2 mm, respectively. 2.4.3. Scanning electron microscopic observation The fractured surfaces of PBS matrix and biocomposites were characterized by a scanning electron microscope (SEM, KL30, Philips, The Netherlands). All the specimens were coated with gold and the acceleration voltage used was 15–25 kV. 3. Results and discussion
W% ¼ ðW W d Þ=W d 100 3.1. Water absorption profiles W, the weight of the water absorbed specimen and Wd, the initial weight of the dry specimen. 2.4. Characterization 2.4.1. Thermal stability analysis The thermal stability of the biocomposites, PBS matrix and natural fibers was analyzed under a nitrogen atmosphere using a thermogravimetric analyzer (TGA Q500, TA Instruments) with a heating rate of 10 °C/min.
The weight increase of the biocomposites reinforced with different natural fibers is compared to that of the PBS matrix in Fig. 1. The biocomposite reinforced with silk fiber shows the highest weight increase and the biocomposites exhibit a saturation curve after 50 h in the plots of weight increase as a function of duration time; whereas the weight of PBS matrix is almost linearly increased with the duration time due to continuous water absorption. It can be explained that both silk and henequen fibers absorb the maximum amount of water within 50 h and show the saturation curve beyond the time. The weight increase of both biocom-
A PBS inside: Microcracks observed after 2000 hours duration
(a) ×2000
(b) ×2000
(c) ×2000
B PBS outer surface: Fragmentation and peeling-off from the surfaces observed after 1000 hours duration
(a) ×3000
(b) ×1000
(c) ×1000
Fig. 3. A: Scanning electron micrographs of inner parts for (a) the PBS matrix, (b) the degraded PBS matrix for 1000 h and (c) the degraded PBS matrix for 2000 h, B: scanning electron micrographs of the outer surfaces for (a) the degraded PBS matrix for 1000 h, (b) and (c) the degraded PBS matrix for 2000 h.
494
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
posites after duration of 700 h can be explained from water absorption of degraded PBS matrix of biocomposites. 3.2. Topography analysis Fig. 2A compares the fractured surfaces of the control and the silk fiber reinforced biocomposites exposed at 60 °C and 85% RH for 1000 h. The control biocomposite (a) shows the neat PBS matrix and the good interfacial adhesion between the silk fibers and the
matrix. On the other hand, for the PBS matrix exposed for 1000 h the matrix was fragmented into pieces and showed the poor adhesion between the fibers and the matrix. However, the silk fiber reinforcement shows a little change comparing to the PBS matrix. Fig. 2B shows the fractured surfaces of the control and the henequen fiber reinforced biocomposites exposed at 60 °C and 85% RH for 1000 h. The control biocomposite also shows the neat PBS matrix and the good adhesion between the henequen fibers and the matrix. Also, the henequen fibers maintain the pore structures
Fig. 4. TG and DTG profiles for (a) PBS matrix, (b) silk/PBS biocomposites, and (c) HQ/PBS biocomposites with the duration time.
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
in the biocomposite. On the contrary, it was observed that there was the severe degradation of the henequen fiber reinforcement of the biocomposite after maintaining in the chamber for 1000 h; henequen fibers were fragmented into pieces and the pore structure was almost destructed. It is noticeable that the degradation of PBS matrix was progressed less comparing to the henequen reinforcement. Also, the degradation of the PBS matrix in the henequen fiber reinforced biocomposite undergoes much less than that of the PBS matrix in the silk fiber reinforced biocomposite. It can be interpreted that the higher amount of absorbed water by silk fibers than henequen fibers induced the severer degradation of the PBS matrix in the silk fiber-reinforced biocomposites. The degradation of PBS matrix in the condition of 60 °C and 85% RH was compared to identify the major reason for the degradation of biocomposites. Figs. 3A and B show the inner parts and outer surfaces of the control and the degraded PBS matrix for 1000 h and 2000 h, respectively. It was found that there are some the inner micro cracks (Fig. 3A(c)) and peeling off (Fig. 3B(b) and (c)) of the fragmented parts from the outer surface of the PBS matrix resulting from the PBS degradation when the PBS matrix was exposed at 60 °C and 85% RH for 2000 h. As a result, it can be concluded that the degradation of the PBS matrix and the biocomposites may be initiated at the interfaces between the water molecules and the PBS and propagates toward the adjacent region leading to the micro crack and fragmentation of PBS matrix. It is also noticeable that the silk fibers show much higher stability than henequen fibers in the condition of 60 °C and 85% RH. 3.3. Thermogravimetric analysis Fig. 4 shows the TG and DTG profiles for the PBS matrix and biocomposites reinforced with silk or henequen fiber reinforcements at different duration times, respectively. The control PBS matrix (Fig. 4a) shows the maximum decomposition peak at 389 °C. This maximum peak was sifted to 391 °C and 393 °C after the exposure for 500 h and 1000 h, respectively, and the decomposed amount in the range of 280 °C and 330 °C was increased. It means that the PBS matrix experienced both the chain scission and chain bonding during the durability test. The silk fiber reinforced biocomposites (Fig. 5b) show the two main peaks which represent the PBS matrix (389 °C) and silk fiber (318 °C). The decomposition pattern for PBS matrix shows the same profiles of the degraded PBS matrix itself (Fig. 4a) and there is no peak change for the silk fiber as well. However, the TG and DTG profiles for the henequen fiber reinforced biocomposites (Fig. 4c) were changed with the duration time. The control henequen fiber reinforced biocomposite shows three main peaks which were correspond to the henequen fiber (295 °C for hemicellulose and 360 °C for cellulose) and the PBS matrix (389 °C). Especially, the decomposed amount in the region of 180– 280 °C was increased for the specimen placed in the chamber for 1000 h, resulting from the degradation of henequen fiber and PBS matrix.
495
trol biocomposite. The degradation of both PBS matrix and henequen fiber reinforcement contributes to the decrease of storage modulus of the biocomposites. However, the decrease of storage modulus is less than 5% for the biocomposite reinforced with silk fiber when it is exposed at 60 °C and 85% RH for 1000 h. It means that although the PBS matrix degrades, the dynamic mechanical property can be maintained by the silk fiber. It is noticeable that though the degradation of PBS matrix in the silk fiber reinforced biocomposites is severer than that of PBS matrix for the henequen fiber-reinforced biocomposites, the storage modulus of the silk fiber reinforced biocomposites can be maintained by the hygrothermally stable silk fibers. Fig. 6 shows the change of the storage modulus and tan d for the PBS matrix (a) and the biocomposites reinforced with silk fibers (b) or henequen fibers (c) with the duration time, respectively. For the PBS matrix there is no change of storage modulus and tan d pattern for the specimens exposed at 60 °C and 85% RH for less than 500 h, however, the PBS matrix exposed for more than 500 h was degraded very severely that the dynamic mechanical analysis could not be performed successfully. The tan d of biocomposites reinforced with silk fiber did not show the transition to the higher temperature till 1000 h and the broadening of the tan d peak is not significant. However, the degraded biocomposite for 2000 h shows the disappearing and broadening of the tan d peak. It means that degradation for 2000 h induced the fragmentation of silk fiber and this fragmented silk fiber reinforcement affected the segmental motions of the PBS matrix [16]. For the biocomposites reinforced with henequen fiber the storage modulus is gradually decreased with the duration time resulting from the degradation of both PBS matrix and henequen fibers. The tan d peak has been shifted to higher temperatures comparing to that for the control biocomposite reinforced with henequen fiber. The intensity of the tan d for the degraded biocomposites reinforced with henequen fibers was decreased compared to the control biocomposite, indicating that less polymer chains are participating in this transition. With increasing duration time the tan d peak was broadened reflecting that the temperature span needed for the transition was increased. This can be explained by fragmented henequen fiber hindering the PBS chains to different extents. It has been noted earlier that the tan d peak broadens and shifts to higher temperature for the degraded biocomposites reinforced with henequen fiber
3.4. Dynamic mechanical analysis Fig. 5 shows the storage modulus change of the PBS matrix and biocomposites reinforced with silk fibers or henequen fibers as a function of duration time, respectively. The initial storage modulus of the biocomposites is 5–10% higher than that of the PBS matrix itself because of the reinforcing effect. For the degraded samples the PBS matrix turns very brittle resulting from the fragmentation, it was not possible to measure the storage modulus for the PBS matrix after exposure with duration time of more than 500 h. For the henequen reinforced biocomposite the storage modulus of the control (4 GPa) was decreased with the duration time resulting in the half value (2 GPa) of the storage modulus comparing to that of the con-
Fig. 5. Storage modulus changes at 100 °C of PBS matrix, silk/PBS biocomposites, and HQ/PBS biocomposites with the duration time.
496
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
Fig. 6. Changes of storage modulus and tan d for (a) PBS matrix, (b) silk/PBS biocomposites, and (c) HQ/PBS biocomposites with the duration time.
indicating a reduction in the segmental motions of the polymer matrix [16]. 4. Conclusion The hygrothermal effect of biocomposites with poly(butylene succinate) biodegradable polymer matrix in the atmosphere of 60 °C and 85% RH has been examined. The weight increase occurred in the biocomposites exposed less than 50 h was resulted from the water absorption of natural fibers, showing the constant variation after that. Silk fibers showed higher water absorption
than henequen fibers, leading to greater degradation of PBS matrix in the biocomposites. The degradation of PBS matrix is the main cause for the deterioration of biocomposite performance and the biocomposites degrade much slower than PBS matrix itself. Silk fiber reinforcement did not degrade and biocomposite reinforced with silk fiber maintained the initial storage modulus of biocomposite after 1000 h duration, however, henequen fiber degraded severely resulting in lowering the performance. Natural fibers gave important effects on the biocomposite degradation for the long term durability. Therefore, natural fibers should be critically selected, depending on biocomposite applications.
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
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. References [1] Han SO, Lee SM, Park WH, Cho D. Mechanical and thermal properties of waste silk fiber-reinforced poly(butylene succinate) biocomposites. Appl Polym Sci 2006;100(6):4972–80. [2] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000;276(/277):1–24. [3] Netravali AN, Chabba S. Composites get greener. MaterToday 2003:22–9. [4] Huda MS, Drzal LT, Mohanty AK, Misra M. Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: a comparative study. Comp Sci Technol 2006;63(11/12):1813–24. [5] Holbery J, Houston D. Natural-fiber-reinforced polymer composites in automotive applications. J Miner, Met Mater Soc 2006;58(11):80–6. [6] Wambua P, Ivens J, Verpoest I. Natural fibers: can they replace glass in fiber reinforced plastics. Comp Sci Technol 2003;63:1259–64.
497
[7] Pervaiz M, Sain MM. Carbon storage potential in natural fiber composites. Resour Conserv Recycl 2003;39(4):325–40. [8] Han SO, Cho D, Park WH, Drzal LT. Henequen/poly(butylene succinate) biocomposites. Comp Interf 2006;12(2/3):231–47. [9] Perez-Rigueiro J, Viney C, Llorea J, Elices M. Silk worm silk as an engineering material. J Appl Polym Sci 1998;70:2439–47. [10] Zhang H, Magoshi J, Becker M, Chen JY, Matsunaga R. Thermal Properties of Bombyx mori Silk Fibers. J Appl Polym Sci 2002;86:1817–20. [11]. [12] Hirotsu T, Tsujisaka T, Masuda T, Nakayama K. Plasma surface treatments and biodegradation of poly(butylelen succinate) sheets. J Appl Polym Sci 2000;78:1121–9. [13] Lee SM, Cho D, Park WH, Lee SG, Han SO, Drzal LT. Novel silk/poly(butylene succinate) biocomposites: the effect of short fibre content on their mechanical and thermal properties. Comp Sci Technol 2005;65:647–57. [14] Han SO, Lee SM, Park WH, Cho D. Mechanical and thermal properties of waste silk fiber-reinforced poly(butylene succinate) biocomposites. J Appl Polym Sci 2006;100:4972–80. [15] Han SO, Drzal LT. Water absorption effects on hydrophilic polymer matrix of carboxyl functionalized glucose resin and epoxy resin. Eur Polym J 2003;39:1791–9. [16] Petersson L, Oksman K. Biopolymer based nanocomposites: comparing layered silicates and microcrystalline cellulose as nanoreinforcements. Comp Sci Technol 2006;66:2187–96.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Hygrothermal effect on henequen or silk fiber reinforced poly(butylene succinate) biocomposites Seong Ok Han a,*, Ho Jung Ahn a, Donghwan Cho b a b
Reaction and Separation Materials Research Center, Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong-gu, 305-343 Daejeon, Republic of Korea Department of Polymer Science and Engineering, Kumoh National Institute of Technology, Gumi, Gyungbuk 730-701, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 January 2010 Accepted 7 May 2010 Available online 9 May 2010 Keywords: A. Polymer–matrix composites (PMCs) B. Thermal properties D. Fractography E. Compression moulding
a b s t r a c t The hygrothermal effect of poly(butylene succinate) biocomposites reinforced with silk or henequen fibers was investigated. The biocomposites were fabricated using a compression molding method. The biocomposites were maintained in the chamber at 60 °C and 85% RH for 1000 h. The results show that the degradation of PBS matrix is the main cause for the deterioration of biocomposites performance and the biocomposites degraded much slowly than PBS matrix did. The storage modulus for the biocomposites exposed at 60 °C and 85% RH for 1000 h was decreased 20% and 50% with the silk and henequen fibers comparing to the original specimens, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Biocomposites made of polymer matrices and natural fiber reinforcements have been actively investigating and applying to automobile industries. Development of biocomposites is laying weight because of their advantages such as environmentally friendliness, light weight and carbon dioxide reduction, etc. [1,2]. Also, the utilization of biodegradable polymer as a matrix of biocomposites has been seriously considered for the development of more environmentally friendly biocomposites [3]. The main applications of these biocomposites are interior parts of automobiles because of the hydrophilic character of natural fiber [4]. Recently, studies on biocomposites have also been performed in order to apply biocomposites to outdoor environments [5]. Natural fiber can be largely divided into two categories such as lignocellulose-based fibers and animal-based fibers. These natural fibers have several advantages such as high specific strength and stiffness due to low density of the fiber over the glass fiber reinforcement in composite applications. Especially, the hollow tubular structure of ligno-cellulose natural fibers enhances acoustic and thermal insulating performances and also reduces their bulk density, leading to energy saving as well as lightweight in automobile applications [6]. Also, ligno-cellulose natural fibers contribute to the CO2 reduction [7]. Ligno-cellulose fibers like kenaf, flax, and henequen fibers have been frequently used as natural fiber reinforcements. Of them, henequen fibers are attracting more attention
* Corresponding author. Tel.: +82 42 860 3149; fax: +82 42 860 3133. E-mail address: [email protected] (S.O. Han). 1359-8368/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2010.05.003
for manufacturing low-cost materials [8]. As an animal-based fiber, silk fibers (Bombyx mori) are consisted of fibroin in the inner layer and sericin in the outer layer, all protein-based. Silk fibers are biodegradable and has high crystalline with well-aligned and continuous structure, higher tensile strength, high toughness, good elasticity, and excellent resilience than glass fiber or lignocellulose-based natural fibers [9,10]. Silk fiber is stable up to 140 °C and decomposes over 150 °C. Poly(butylene succinate) (PBS) is a biodegradable, thermoplastic and aliphatic polyester. PBS has excellent processibility with relatively low melt temperature and comparable mechanical properties with general-purpose thermoplastics like polyethylene and polypropylene [11]. PBS is also completely biodegradable in soil, lake, sea and compost, and has thermal and chemical resistance, and recyclability [12]. Biocomposites based of PBS with henequen or silk fiber reinforcements have been studied and proved that they have comparable specific mechanical properties to glass fiber reinforced polymer composites [13,14]. However, the hydrophilic natural fibers absorb the water molecules, resulting in altering the physical and mechanical properties of the biocomposites. The physical changes include a decrease of the mechanical properties, microcracks, chain scission, and degradation of interfacial bonding between the fibers and the matrix [15]. Till now, the long-term stability of the biocomposites based on the biodegradable polymer matrix, PBS, in wet environment has not been reported yet. In this research, we investigated the hygrothermal effect of PBS matrix and two different kinds of biocomposites reinforced with either silk fibers or henequen fibers. We characterized the changes of thermal stability, storage modulus and topography for the biocomposites specimens maintained in the
492
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
in Jinju, Korea) were used as a ligno-cellulose and animal based reinforcements, respectively. Henequen (Agave fourcroydes) fibers from Yucatan, Mexico, with filaments having lengths in the range of 60–70 cm, were used in the investigation. The average density was about 1.45 g/cm3 and the filaments’ diameter was generally in the range of 150–200 lm. The waste silk or silk scrap used in the present study was kindly supplied from the Korea Silk Research Institute manufacturing silk fabrics in Jinju, Korea. The as-received waste silk used here consists of the fibroin inner layer only, because the sericin outer layer was removed from the raw silk fibers. The fibres were chopped to be short fibres of 10 mm in average length. PBS (Ire Chemical Co., Korea, Enpol G-4500) has a melting point of 115 °C, the specific gravity of 1.22, and the melt index (g/10 min at 190 °C) of 29.
2.2. Biocomposites fabrication
Fig. 1. Weight increase of biocomposites and PBS matrix.
environment of 60 °C and 85% relative humidity (RH) with duration time.
2. Experimental 2.1. Materials Henequen fibers (HQ) (originated from Mexico) and the silk fibers (the Korea Silk Research Institute manufacturing silk fabrics
The chopped natural fibers and PBS pellets were dried at 100 °C for 2 h in a conventional oven and at 80 °C for 5 h in a vacuum oven before use, respectively. Prior to composite fabrication, the pulverized PBS powder was mechanically mixed with chopped natural fiber using a kitchen mixer. After mechanically mixing the natural fiber and the PBS powder pulverized with a blender, the molding compounds obtained were melted at 130 °C for 15 min, holding at 1000 psi for 15 min using a hot-press (Carver 2518), then was retained until the mold was naturally cooled down to ambient temperature. During processing, the PBS powder was melted enough to flow into the natural fibers. A stainless steel mold with cavity dimensions of 50 mm 50 mm was used. The fiber content and the chopped fiber length of biocomposites were 30% in weight and 10 mm in average, respectively. PBS matrix specimen was also prepared as a control. The specimens of both PBS matrix and bio-
Fig. 2. A: Scanning electron micrographs of (a) the silk/PBS biocomposite control, (b) the degraded silk/PBS biocomposite for 1000 h (3000), B: scanning electron micrographs of (a) the HQ/PBS biocomposite control, (b) the degraded HQ/PBS biocomposite for 1000 h (200).
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
composites for durability test were prepared with the dimensions of 1.2 mm 18 mm 35 mm, respectively. 2.3. Water absorption The biocomposites and the PBS matrix were tested in a chamber controlled at 60 °C and 85% RH. The specimens were weighed; set in a chamber, taken out from a chamber at predetermined times, gently wiped dry using lint-free tissue paper and allowed to stand free at ambient environment for 5 min to ensure the removal of excessive surface water. Every procedure was performed by following ASTM D570-98: standard test methods for water absorption of plastics [11]. The weight gain percentage by water absorption, W%, was determined from the equation:
493
2.4.2. Dynamic mechanical analysis The storage modulus of biocomposites and the PBS matrix was measured from 50 °C to 250 °C with a heating rate of 5 °C/min by dynamic mechanical analysis (DMA Q800, TA Instruments). A fixed frequency and the oscillation amplitude were 1 Hz and 0.2 mm, respectively. 2.4.3. Scanning electron microscopic observation The fractured surfaces of PBS matrix and biocomposites were characterized by a scanning electron microscope (SEM, KL30, Philips, The Netherlands). All the specimens were coated with gold and the acceleration voltage used was 15–25 kV. 3. Results and discussion
W% ¼ ðW W d Þ=W d 100 3.1. Water absorption profiles W, the weight of the water absorbed specimen and Wd, the initial weight of the dry specimen. 2.4. Characterization 2.4.1. Thermal stability analysis The thermal stability of the biocomposites, PBS matrix and natural fibers was analyzed under a nitrogen atmosphere using a thermogravimetric analyzer (TGA Q500, TA Instruments) with a heating rate of 10 °C/min.
The weight increase of the biocomposites reinforced with different natural fibers is compared to that of the PBS matrix in Fig. 1. The biocomposite reinforced with silk fiber shows the highest weight increase and the biocomposites exhibit a saturation curve after 50 h in the plots of weight increase as a function of duration time; whereas the weight of PBS matrix is almost linearly increased with the duration time due to continuous water absorption. It can be explained that both silk and henequen fibers absorb the maximum amount of water within 50 h and show the saturation curve beyond the time. The weight increase of both biocom-
A PBS inside: Microcracks observed after 2000 hours duration
(a) ×2000
(b) ×2000
(c) ×2000
B PBS outer surface: Fragmentation and peeling-off from the surfaces observed after 1000 hours duration
(a) ×3000
(b) ×1000
(c) ×1000
Fig. 3. A: Scanning electron micrographs of inner parts for (a) the PBS matrix, (b) the degraded PBS matrix for 1000 h and (c) the degraded PBS matrix for 2000 h, B: scanning electron micrographs of the outer surfaces for (a) the degraded PBS matrix for 1000 h, (b) and (c) the degraded PBS matrix for 2000 h.
494
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
posites after duration of 700 h can be explained from water absorption of degraded PBS matrix of biocomposites. 3.2. Topography analysis Fig. 2A compares the fractured surfaces of the control and the silk fiber reinforced biocomposites exposed at 60 °C and 85% RH for 1000 h. The control biocomposite (a) shows the neat PBS matrix and the good interfacial adhesion between the silk fibers and the
matrix. On the other hand, for the PBS matrix exposed for 1000 h the matrix was fragmented into pieces and showed the poor adhesion between the fibers and the matrix. However, the silk fiber reinforcement shows a little change comparing to the PBS matrix. Fig. 2B shows the fractured surfaces of the control and the henequen fiber reinforced biocomposites exposed at 60 °C and 85% RH for 1000 h. The control biocomposite also shows the neat PBS matrix and the good adhesion between the henequen fibers and the matrix. Also, the henequen fibers maintain the pore structures
Fig. 4. TG and DTG profiles for (a) PBS matrix, (b) silk/PBS biocomposites, and (c) HQ/PBS biocomposites with the duration time.
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
in the biocomposite. On the contrary, it was observed that there was the severe degradation of the henequen fiber reinforcement of the biocomposite after maintaining in the chamber for 1000 h; henequen fibers were fragmented into pieces and the pore structure was almost destructed. It is noticeable that the degradation of PBS matrix was progressed less comparing to the henequen reinforcement. Also, the degradation of the PBS matrix in the henequen fiber reinforced biocomposite undergoes much less than that of the PBS matrix in the silk fiber reinforced biocomposite. It can be interpreted that the higher amount of absorbed water by silk fibers than henequen fibers induced the severer degradation of the PBS matrix in the silk fiber-reinforced biocomposites. The degradation of PBS matrix in the condition of 60 °C and 85% RH was compared to identify the major reason for the degradation of biocomposites. Figs. 3A and B show the inner parts and outer surfaces of the control and the degraded PBS matrix for 1000 h and 2000 h, respectively. It was found that there are some the inner micro cracks (Fig. 3A(c)) and peeling off (Fig. 3B(b) and (c)) of the fragmented parts from the outer surface of the PBS matrix resulting from the PBS degradation when the PBS matrix was exposed at 60 °C and 85% RH for 2000 h. As a result, it can be concluded that the degradation of the PBS matrix and the biocomposites may be initiated at the interfaces between the water molecules and the PBS and propagates toward the adjacent region leading to the micro crack and fragmentation of PBS matrix. It is also noticeable that the silk fibers show much higher stability than henequen fibers in the condition of 60 °C and 85% RH. 3.3. Thermogravimetric analysis Fig. 4 shows the TG and DTG profiles for the PBS matrix and biocomposites reinforced with silk or henequen fiber reinforcements at different duration times, respectively. The control PBS matrix (Fig. 4a) shows the maximum decomposition peak at 389 °C. This maximum peak was sifted to 391 °C and 393 °C after the exposure for 500 h and 1000 h, respectively, and the decomposed amount in the range of 280 °C and 330 °C was increased. It means that the PBS matrix experienced both the chain scission and chain bonding during the durability test. The silk fiber reinforced biocomposites (Fig. 5b) show the two main peaks which represent the PBS matrix (389 °C) and silk fiber (318 °C). The decomposition pattern for PBS matrix shows the same profiles of the degraded PBS matrix itself (Fig. 4a) and there is no peak change for the silk fiber as well. However, the TG and DTG profiles for the henequen fiber reinforced biocomposites (Fig. 4c) were changed with the duration time. The control henequen fiber reinforced biocomposite shows three main peaks which were correspond to the henequen fiber (295 °C for hemicellulose and 360 °C for cellulose) and the PBS matrix (389 °C). Especially, the decomposed amount in the region of 180– 280 °C was increased for the specimen placed in the chamber for 1000 h, resulting from the degradation of henequen fiber and PBS matrix.
495
trol biocomposite. The degradation of both PBS matrix and henequen fiber reinforcement contributes to the decrease of storage modulus of the biocomposites. However, the decrease of storage modulus is less than 5% for the biocomposite reinforced with silk fiber when it is exposed at 60 °C and 85% RH for 1000 h. It means that although the PBS matrix degrades, the dynamic mechanical property can be maintained by the silk fiber. It is noticeable that though the degradation of PBS matrix in the silk fiber reinforced biocomposites is severer than that of PBS matrix for the henequen fiber-reinforced biocomposites, the storage modulus of the silk fiber reinforced biocomposites can be maintained by the hygrothermally stable silk fibers. Fig. 6 shows the change of the storage modulus and tan d for the PBS matrix (a) and the biocomposites reinforced with silk fibers (b) or henequen fibers (c) with the duration time, respectively. For the PBS matrix there is no change of storage modulus and tan d pattern for the specimens exposed at 60 °C and 85% RH for less than 500 h, however, the PBS matrix exposed for more than 500 h was degraded very severely that the dynamic mechanical analysis could not be performed successfully. The tan d of biocomposites reinforced with silk fiber did not show the transition to the higher temperature till 1000 h and the broadening of the tan d peak is not significant. However, the degraded biocomposite for 2000 h shows the disappearing and broadening of the tan d peak. It means that degradation for 2000 h induced the fragmentation of silk fiber and this fragmented silk fiber reinforcement affected the segmental motions of the PBS matrix [16]. For the biocomposites reinforced with henequen fiber the storage modulus is gradually decreased with the duration time resulting from the degradation of both PBS matrix and henequen fibers. The tan d peak has been shifted to higher temperatures comparing to that for the control biocomposite reinforced with henequen fiber. The intensity of the tan d for the degraded biocomposites reinforced with henequen fibers was decreased compared to the control biocomposite, indicating that less polymer chains are participating in this transition. With increasing duration time the tan d peak was broadened reflecting that the temperature span needed for the transition was increased. This can be explained by fragmented henequen fiber hindering the PBS chains to different extents. It has been noted earlier that the tan d peak broadens and shifts to higher temperature for the degraded biocomposites reinforced with henequen fiber
3.4. Dynamic mechanical analysis Fig. 5 shows the storage modulus change of the PBS matrix and biocomposites reinforced with silk fibers or henequen fibers as a function of duration time, respectively. The initial storage modulus of the biocomposites is 5–10% higher than that of the PBS matrix itself because of the reinforcing effect. For the degraded samples the PBS matrix turns very brittle resulting from the fragmentation, it was not possible to measure the storage modulus for the PBS matrix after exposure with duration time of more than 500 h. For the henequen reinforced biocomposite the storage modulus of the control (4 GPa) was decreased with the duration time resulting in the half value (2 GPa) of the storage modulus comparing to that of the con-
Fig. 5. Storage modulus changes at 100 °C of PBS matrix, silk/PBS biocomposites, and HQ/PBS biocomposites with the duration time.
496
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
Fig. 6. Changes of storage modulus and tan d for (a) PBS matrix, (b) silk/PBS biocomposites, and (c) HQ/PBS biocomposites with the duration time.
indicating a reduction in the segmental motions of the polymer matrix [16]. 4. Conclusion The hygrothermal effect of biocomposites with poly(butylene succinate) biodegradable polymer matrix in the atmosphere of 60 °C and 85% RH has been examined. The weight increase occurred in the biocomposites exposed less than 50 h was resulted from the water absorption of natural fibers, showing the constant variation after that. Silk fibers showed higher water absorption
than henequen fibers, leading to greater degradation of PBS matrix in the biocomposites. The degradation of PBS matrix is the main cause for the deterioration of biocomposite performance and the biocomposites degrade much slower than PBS matrix itself. Silk fiber reinforcement did not degrade and biocomposite reinforced with silk fiber maintained the initial storage modulus of biocomposite after 1000 h duration, however, henequen fiber degraded severely resulting in lowering the performance. Natural fibers gave important effects on the biocomposite degradation for the long term durability. Therefore, natural fibers should be critically selected, depending on biocomposite applications.
S.O. Han et al. / Composites: Part B 41 (2010) 491–497
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. References [1] Han SO, Lee SM, Park WH, Cho D. Mechanical and thermal properties of waste silk fiber-reinforced poly(butylene succinate) biocomposites. Appl Polym Sci 2006;100(6):4972–80. [2] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000;276(/277):1–24. [3] Netravali AN, Chabba S. Composites get greener. MaterToday 2003:22–9. [4] Huda MS, Drzal LT, Mohanty AK, Misra M. Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: a comparative study. Comp Sci Technol 2006;63(11/12):1813–24. [5] Holbery J, Houston D. Natural-fiber-reinforced polymer composites in automotive applications. J Miner, Met Mater Soc 2006;58(11):80–6. [6] Wambua P, Ivens J, Verpoest I. Natural fibers: can they replace glass in fiber reinforced plastics. Comp Sci Technol 2003;63:1259–64.
497
[7] Pervaiz M, Sain MM. Carbon storage potential in natural fiber composites. Resour Conserv Recycl 2003;39(4):325–40. [8] Han SO, Cho D, Park WH, Drzal LT. Henequen/poly(butylene succinate) biocomposites. Comp Interf 2006;12(2/3):231–47. [9] Perez-Rigueiro J, Viney C, Llorea J, Elices M. Silk worm silk as an engineering material. J Appl Polym Sci 1998;70:2439–47. [10] Zhang H, Magoshi J, Becker M, Chen JY, Matsunaga R. Thermal Properties of Bombyx mori Silk Fibers. J Appl Polym Sci 2002;86:1817–20. [11]