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Development of high strength biodegradable composites using Manila hemp fiber and starch-based biodegradable resin
Composites: Part A 37 (2006) 1879–1883 www.elsevier.com/locate/compositesa
Development of high strength biodegradable composites using Manila hemp fiber and starch-based biodegradable resin Shinji Ochi
*
Department of Mechanical Engineering, Miyagi National College of Technology, Medeshima Siote aza Nodayama 48, Natori, Miyagi 981-1239, Japan Received 23 March 2005; received in revised form 15 November 2005; accepted 30 December 2005
Abstract This paper describes the development of high strength biodegradable ‘‘green’’ composites. The unidirectional biodegradable composites were made from Manila hemp fiber bundles and a starch-based emulsion-type biodegradable resin. The tensile and flexural strengths of the composites increased with increasing fiber content up to 70%. The composites possessed extremely high tensile and flexural strengths of 365 MPa and 223 MPa, respectively. The fabrication with emulsion-type biodegradable resin contributed to reduction in voids and fiber contacts in the composites. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Fibres; A. Polymer–matrix composites (PMCs); B. Strength
1. Introduction Plastic materials are indispensable in our lives as they are used extensively in many diverse fields including, but not limited to, stationery goods, electronic products and sports goods. However, a great majority of these products are disposed into landfills after usage. Clearly, this contributes to a high environmental load. In order to reduce the environmental load generated from the disposal of used plastic products, significant attention has been placed on biodegradable plastics. These plastics can be completely resolved into water and carbon dioxide by the action of the microorganism, when disposed of in the soil. Moreover, there are no emissions of toxic gases during incineration. Recently, biodegradable plastics have been used in commercial products such as ball-point pens, toothbrushes, garbage bags, fishing lines, tennis racket strings, wrapping paper and many others. The appli-
*
Tel.: +81 22 381 0346; fax: +81 22 381 0336. E-mail address: [email protected]
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.12.019
cation of biodegradable plastics has been restricted due to their relatively lower strength compared to conventional plastics such as polypropylene and polyethylene. Over the past few years a considerable number of studies [1–9] have been performed on biodegradable composites containing biodegradable plastics with reinforcements of biodegradable natural fibers. The natural fibers such as flax [1–3], ramie [1], jute [1,4], bamboo [5], pineapple [6], kenaf [7], henequen [8] and hemp [9] were used for reinforcements in these studies. Many of the previous natural fiber reinforced composites had a fiber volume fraction of less than 50% and were molded at high temperatures of 180 C and 210 C. Further, these composites had tensile and flexural strengths lower than 100 MPa. The aim of this study is to find a novel technique to obtain high strength biodegradable composites with excellent mechanical properties comparable to those of GFRP (glass fiber reinforced plastics). In this study, Manila hemp fibers were selected as a reinforcement of the biodegradable composites due to their high strength and excellent thermal stability. In order to increase the fraction of the fibers, emulsion-type
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biodegradable resin was used as the matrix. The unidirectional fiber reinforced composites were fabricated by hot pressing.
200 70
10
2. Experimental procedures Aluminum
In order to produce biodegradable composites that have high fiber content, a starch-based emulsion-type biodegradable resin (Miyoshi Oil & Fat Co., Ltd.; CP-300) was used. This resin contains fine particles of approximately 4.6 lm in diameter suspended in aqueous solution with a mass content of 40%. Basic properties of the resin obtained in this research are listed in Table 1. Manila hemp fiber (Fujiseishi Co., Ltd.), ramie (Tosco Co., Ltd.), bamboo fiber (Ban Co., Ltd.), Banana fiber (Mishima Paper Co., Ltd.), cotton fiber (Toyobo Co., Ltd.) and jute fiber (Maruhachi Co., Ltd.) were prepared. Fiber bundles with a diameter of 100–200 lm were used in this study. 2.2. Tensile test of fibers and heat treatment Fig. 1 shows a paperboard prepared to prevent damage to the fiber during its handling. The cross-sectional area of fibers was calculated from their diameters measured using an optical microscope. A fiber was glued to the paperboard, which was then carefully gripped by the testing machine, and cut with a thin heated metal wire along the cutting line indicated in Fig. 1. The JIS R 761 method was followed to determine the tensile strength. The tensile test was performed at a strain rate of 0.04 per min. In the
Table 1 Properties of biodegradable resin used as matrix Density (g/cm3) Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa)
1.18 11.7 0.41 13.7 0.46
45 25
15
10
10
Adhesive
1
2.1. Materials [Unit : mm]
Fig. 2. Shape and dimensions of tensile specimen.
current study, ten specimens of natural fibers were prepared and analyzed. A 95% confidence interval was calculated by statistics analysis. To establish the best molding conditions, the natural fibers were heated in air using an electric drying furnace at 160 C, 180 C and 200 C for 15 min, 30 min, 60 min and 120 min, respectively. 2.3. Molding method of biodegradable composites First, preliminary composites were produced by putting the biodegradable resin on the surface of Manila hemp fibers and drying at 105 C for 120 min in an oven. Next, biodegradable composite specimens were fabricated by hot pressing using a pressing machine. In this process, the preliminary composites were set in a metallic mold and heated to 130 C with a flexible heater rolled around the metallic mold. The metallic mold was held at 130 C for 5 min and specimens were hot-pressed at 10 MPa for 10 min. The dimensions of the biodegradable composite specimens were 10 mm · 200 mm · 1 mm for tensile testing and 15 mm · 100 mm · 1 mm for flexural testing. The volume fraction of Manila hemp fiber in the specimens was varied from 30% to 70%. 2.4. Mechanical testing Three point flexural tests and tensile tests were carried out by using an Instron testing machine (Model 5567). Flexural tests were performed at a crosshead speed of 1 mm/min and a span length of 50 mm. The dimensions of most flexural specimens followed ISO 178, but they did not completely fit standard. Tensile tests were performed according to ISO 3268 at a strain rate of 0.0143 per min and a gauge length of 70 mm. Fig. 2 shows the shape and the dimensions of a tensile specimen. 3. Results and discussion
Paperboard Fiber
Cutting line [Unit : mm]
Fig. 1. Tensile specimens for Manila hemp fibers.
3.1. Mechanical properties of natural fibers and heat-treated Manila hemp fibers The reinforcing fibers used for high strength environment-friendly biodegradable composites must have high
S. Ochi / Composites: Part A 37 (2006) 1879–1883
3.2. Microstructure of biodegradable composites
Tensile strength (MPa)
1000 800
600 400
200
0
Manila hemp
Ramie Bamboo Banana Cotton
Jute
Fig. 3. Tensile strength of several natural fibers.
strength and a short generation time. First, tensile tests for some natural fibers were performed in order to select a high strength natural fiber. The tensile strengths measured for Manila hemp, ramie, bamboo, banana, cotton and jute fibers are shown in Fig. 3. Tensile strength decreases in the order of Manila hemp, ramie, bamboo, banana, cotton and jute. Further, the growth cycle of Manila hemp fibers is from one to two years. Thus, this natural fiber is considered environment-friendly and it will not lead to deforestation. In order to establish the most suitable molding conditions, mechanical properties of heat-treated Manila hemp fibers were examined. Fig. 4 shows the effect of heat treatment on the tensile strength of Manila hemp fibers. The average tensile strength of Manila hemp fibers without heat treatment is 702 MPa. The tensile strength of Manila hemp fibers decreases with increasing heating time at 200 C. The tensile strength of Manila hemp fibers heat-treated at 180 C for 30 min is similar to that of non-heat-treated fibers. At 160 C, the tensile strength of heat-treated Manila hemp fibers does not decrease even with longer heating times. Based on these results, the processing temperature for fabricating Manila hemp fiber reinforced composites should be kept below 160 C in order to prevent strength reduction due to thermal degradation.
The transverse section and top view of the biodegradable composites with 70% of fibers are shown in Figs. 5 and 6, respectively. From these figures, it can be observed that the distribution of fibers is parallel, and that there are no voids or fiber contacts that would cause a decrease in strength. In previous studies, the composites were fabricated by sandwiching layers of fibers between several resin films. However, with this method it was not possible to fabricate high fiber volume fraction composites as the increase of fiber content led to increased fiber contact. The novel technique presented herein, which uses an emulsion-type biodegradable resin, provides a suitable internal environment for achieving high fiber volume, in which voids and fiber contacts are reduced in the composites.
Fig. 5. Optical micrograph of transverse section of biodegradable composites with 70% of fibers.
Norm. tensile strength (%)
120 100 80 60 40
Heating temperature 160 ºC 180 ºC 200 ºC
20 0
1881
0
20
40
60
80
100
120
Heating time (min) Fig. 4. Relationship between tensile strength of Manila hemp fibers and heating time.
Fig. 6. Photograph of top view of biodegradable composites.
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3.3. Tensile and flexural strengths of biodegradable composites Figs. 7 and 8 show the relationships between fiber content and tensile and flexural strengths, respectively. From these figures, it can be seen that tensile and flexural strengths increase linearly with increasing fiber content. The tensile and flexural strengths were 365.4 MPa and 222.7 MPa, respectively, in the samples with a fiber fraction of 70%. Besides, the flexural strength was lower than tensile strength. In tensile mode, the fracture of specimens was caused by fracture of fibers. However, in flexural mode, the fracture of specimens was caused by separation between fibers and resin. Thus the lower flexural strength compared to tensile strength could be attributed to the difference in the fracture mechanism. When fiber content is more than 70%, some voids and fiber contacts caused by insufficient amount of resin are observed in the specimen. Therefore, the fiber content used for fabricating Manila hemp fiber reinforced composites should be kept below 70%. The tensile and flexural strengths of several previous natural fiber reinforced composites are shown in Table 2. Most of these composites indicated tensile and flexural
Tensile strength (MPa)
500 400 300 200 100 0
0
20
40
60
80
100
Volume fraction of Manila hemp fiber (%)
Table 2 Tensile and flexural strengths of several natural fiber reinforced composites [1–9]
Flax Ramie Jute Flax Flax Jute Bamboo Pineapple Kenaf Henequen Hemp
Tensile strength (MPa)
Flexural strength (MPa)
References
50.5 50 34.5 53 33.2 100.5 88.5 55.8 60 71.8 80
– – – – 49.3 – 76.5 86 – 95.9 –
[1] [1] [1] [2] [3] [4] [5] [6] [7] [8] [9]
Table 3 Comparison of theoretical and experimental value of tensile strength Volume fraction of fibers (%)
Theoretical strength (MPa)
Experimental strength (MPa)
Experiment/theory (%)
30 50 70
218.5 356.7 494.8
162.0 254.2 365.4
74.1 71.3 73.8
strengths less than 100 MPa. Thus, the strength of biodegradable composites fabricated in the current study is much higher than those of any biodegradable composites previously reported. Table 3 shows the theoretical tensile strength values calculated from the rule of mixtures [10], and experimental values. From this table, experimental strength values are 70% of the theoretical strength. As shown in Fig. 3, there is a large variation in Manila hemp fiber strength and lower experimental strength values could be attributed to fracture of low strength fibers. 3.4. Elastic modulus of biodegradable composites
Fig. 7. Relationship between tensile strength of unidirectional biodegradable composites and Manila hemp fiber content.
Fig. 9 shows the relationship between elastic moduli in both tensile and flexural modes, and fiber content. From
300
200
Elastic modulus (GPa)
Flexural strength (MPa)
40
100
0
0
20
40
60
80
Tensile modulus Flexural modulus
30
20
10
100
Volume fraction of Manila hemp fiber (%) Fig. 8. Relationship between flexural strength of unidirectional biodegradable composites and Manila hemp fiber content.
0
0
20 40 60 80 Volume fraction of Manila hemp fiber (%)
100
Fig. 9. Relationship between elastic moduli and fiber content.
S. Ochi / Composites: Part A 37 (2006) 1879–1883
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2. Unidirectional biodegradable composites fabricated using an emulsion-type biodegradable resin and Manila hemp fiber bundles with a fiber content of 70% have high tensile and flexural strengths of 365 MPa and 223 MPa, respectively. 3. The tensile and flexural strengths and tensile and flexural moduli increased linearly with increasing fiber content up to 70%. Thus excellent mechanical properties are achieved for composites fabricated by the novel technique proposed in this study in which the composites are fabricated with an emulsion-type biodegradable resin.
Fig. 10. Fracture behavior of biodegradable composites reinforced by Manila hemp fibers after tensile test.
this figure, both tensile and flexural moduli increase linearly with increasing fiber content. Luo and Netravali [6] investigated the relationship between elastic modulus of unidirectional pineapple fiber reinforced composites and fiber content and reported that elastic modulus of composites decreased with fiber content. They explained that this was due to internal voids and low shearing stress between fiber and resin. A photograph of the fracture surface after tensile testing is shown in Fig. 10. There are no cracks observed between fiber and resin, indicating there is good interfacial bonding between fiber and resin in the biodegradable composites produced in this work. However, to understand more about the interfacial characteristics, it may be necessary to make thorough examination of the fracture surfaces in the future. From these results, it is clear that there are no cracks in the composites. Further, bonding between the cellulose fibers and resin is considered to be chemically high due to use of a starch-based resin. 4. Conclusions High strength biodegradable composites were made using an emulsion-type biodegradable resin as the matrix and Manila hemp fiber bundles as the reinforcement. The results obtained are as follows: 1. The tensile strength of Manila hemp fibers heat-treated at 160 C for 120 min did not decrease. Thus, 160 C are the highest fabrication temperatures that do not effect the strength.
Acknowledgements The authors would like to express special thanks to Mr. Yoichi Fujimori (Fujiseishi Co., Ltd.) and Mr. Hideyuki Tanaka (Japan Cornstarch Co., Ltd.) for supplying the experimental materials. References [1] Wollerdorfer M, Bader H. Influence of natural fibers on the mechanical properties of biodegradable polymers. Ind Crops Prod 1998;8:105–12. [2] Oksman K, Skrifvars M, Selin J-F. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos Sci Technol 2003; 63:1317–24. [3] Keener TJ, Stuart RK, Brown TK. Maleated coupling agents for natural fibre composites. Compos A: Appl Sci Manuf 2004; 35:357–62. [4] Plackett D, Andersen TL, Pedersen WB, Nielsen L. Biodegradable composites based on polylactide and jute fibres. Compos Sci Technol 2003;63:1287–96. [5] Okubo K, Fujii T. Eco-composites using bamboo and other natural fibers and their mechanical properties. In: Proceedings of the international workshop on ‘‘Green’’ composites, 2002. p. 17–21. [6] Luo S, Netravali AN. Interfacial and mechanical properties of environment-friendly green composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin. J Mater Sci 1999; 34:3709–19. [7] Nishino T, Hirao K, Kotera M, Nakamae K, Inagaki H. Kenaf reinforced biodegradable composite. Compos Sci Technol 2003; 63:1281–6. [8] Herrera-Franco PJ, Valadez-Gonza´lez A. Mechanical properties of continuous natural fibre-reinforced polymer composites. Compos A: Appl Sci Manuf 2004;35:339–45. [9] Mohanty AK, Wibowo A, Misra M, Drzal LT. Effect of process engineering on the performance of natural fiber reinforced cellulose acetate biocomposites. Compos A: Appl Sci Manuf 2004;35:363–70. [10] Hull D, Clyne TW. An introduction to composite materials. 2nd ed. Cambridge: Cambridge University Press; 1996.
Development of high strength biodegradable composites using Manila hemp fiber and starch-based biodegradable resin Shinji Ochi
*
Department of Mechanical Engineering, Miyagi National College of Technology, Medeshima Siote aza Nodayama 48, Natori, Miyagi 981-1239, Japan Received 23 March 2005; received in revised form 15 November 2005; accepted 30 December 2005
Abstract This paper describes the development of high strength biodegradable ‘‘green’’ composites. The unidirectional biodegradable composites were made from Manila hemp fiber bundles and a starch-based emulsion-type biodegradable resin. The tensile and flexural strengths of the composites increased with increasing fiber content up to 70%. The composites possessed extremely high tensile and flexural strengths of 365 MPa and 223 MPa, respectively. The fabrication with emulsion-type biodegradable resin contributed to reduction in voids and fiber contacts in the composites. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Fibres; A. Polymer–matrix composites (PMCs); B. Strength
1. Introduction Plastic materials are indispensable in our lives as they are used extensively in many diverse fields including, but not limited to, stationery goods, electronic products and sports goods. However, a great majority of these products are disposed into landfills after usage. Clearly, this contributes to a high environmental load. In order to reduce the environmental load generated from the disposal of used plastic products, significant attention has been placed on biodegradable plastics. These plastics can be completely resolved into water and carbon dioxide by the action of the microorganism, when disposed of in the soil. Moreover, there are no emissions of toxic gases during incineration. Recently, biodegradable plastics have been used in commercial products such as ball-point pens, toothbrushes, garbage bags, fishing lines, tennis racket strings, wrapping paper and many others. The appli-
*
Tel.: +81 22 381 0346; fax: +81 22 381 0336. E-mail address: [email protected]
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.12.019
cation of biodegradable plastics has been restricted due to their relatively lower strength compared to conventional plastics such as polypropylene and polyethylene. Over the past few years a considerable number of studies [1–9] have been performed on biodegradable composites containing biodegradable plastics with reinforcements of biodegradable natural fibers. The natural fibers such as flax [1–3], ramie [1], jute [1,4], bamboo [5], pineapple [6], kenaf [7], henequen [8] and hemp [9] were used for reinforcements in these studies. Many of the previous natural fiber reinforced composites had a fiber volume fraction of less than 50% and were molded at high temperatures of 180 C and 210 C. Further, these composites had tensile and flexural strengths lower than 100 MPa. The aim of this study is to find a novel technique to obtain high strength biodegradable composites with excellent mechanical properties comparable to those of GFRP (glass fiber reinforced plastics). In this study, Manila hemp fibers were selected as a reinforcement of the biodegradable composites due to their high strength and excellent thermal stability. In order to increase the fraction of the fibers, emulsion-type
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S. Ochi / Composites: Part A 37 (2006) 1879–1883
biodegradable resin was used as the matrix. The unidirectional fiber reinforced composites were fabricated by hot pressing.
200 70
10
2. Experimental procedures Aluminum
In order to produce biodegradable composites that have high fiber content, a starch-based emulsion-type biodegradable resin (Miyoshi Oil & Fat Co., Ltd.; CP-300) was used. This resin contains fine particles of approximately 4.6 lm in diameter suspended in aqueous solution with a mass content of 40%. Basic properties of the resin obtained in this research are listed in Table 1. Manila hemp fiber (Fujiseishi Co., Ltd.), ramie (Tosco Co., Ltd.), bamboo fiber (Ban Co., Ltd.), Banana fiber (Mishima Paper Co., Ltd.), cotton fiber (Toyobo Co., Ltd.) and jute fiber (Maruhachi Co., Ltd.) were prepared. Fiber bundles with a diameter of 100–200 lm were used in this study. 2.2. Tensile test of fibers and heat treatment Fig. 1 shows a paperboard prepared to prevent damage to the fiber during its handling. The cross-sectional area of fibers was calculated from their diameters measured using an optical microscope. A fiber was glued to the paperboard, which was then carefully gripped by the testing machine, and cut with a thin heated metal wire along the cutting line indicated in Fig. 1. The JIS R 761 method was followed to determine the tensile strength. The tensile test was performed at a strain rate of 0.04 per min. In the
Table 1 Properties of biodegradable resin used as matrix Density (g/cm3) Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa)
1.18 11.7 0.41 13.7 0.46
45 25
15
10
10
Adhesive
1
2.1. Materials [Unit : mm]
Fig. 2. Shape and dimensions of tensile specimen.
current study, ten specimens of natural fibers were prepared and analyzed. A 95% confidence interval was calculated by statistics analysis. To establish the best molding conditions, the natural fibers were heated in air using an electric drying furnace at 160 C, 180 C and 200 C for 15 min, 30 min, 60 min and 120 min, respectively. 2.3. Molding method of biodegradable composites First, preliminary composites were produced by putting the biodegradable resin on the surface of Manila hemp fibers and drying at 105 C for 120 min in an oven. Next, biodegradable composite specimens were fabricated by hot pressing using a pressing machine. In this process, the preliminary composites were set in a metallic mold and heated to 130 C with a flexible heater rolled around the metallic mold. The metallic mold was held at 130 C for 5 min and specimens were hot-pressed at 10 MPa for 10 min. The dimensions of the biodegradable composite specimens were 10 mm · 200 mm · 1 mm for tensile testing and 15 mm · 100 mm · 1 mm for flexural testing. The volume fraction of Manila hemp fiber in the specimens was varied from 30% to 70%. 2.4. Mechanical testing Three point flexural tests and tensile tests were carried out by using an Instron testing machine (Model 5567). Flexural tests were performed at a crosshead speed of 1 mm/min and a span length of 50 mm. The dimensions of most flexural specimens followed ISO 178, but they did not completely fit standard. Tensile tests were performed according to ISO 3268 at a strain rate of 0.0143 per min and a gauge length of 70 mm. Fig. 2 shows the shape and the dimensions of a tensile specimen. 3. Results and discussion
Paperboard Fiber
Cutting line [Unit : mm]
Fig. 1. Tensile specimens for Manila hemp fibers.
3.1. Mechanical properties of natural fibers and heat-treated Manila hemp fibers The reinforcing fibers used for high strength environment-friendly biodegradable composites must have high
S. Ochi / Composites: Part A 37 (2006) 1879–1883
3.2. Microstructure of biodegradable composites
Tensile strength (MPa)
1000 800
600 400
200
0
Manila hemp
Ramie Bamboo Banana Cotton
Jute
Fig. 3. Tensile strength of several natural fibers.
strength and a short generation time. First, tensile tests for some natural fibers were performed in order to select a high strength natural fiber. The tensile strengths measured for Manila hemp, ramie, bamboo, banana, cotton and jute fibers are shown in Fig. 3. Tensile strength decreases in the order of Manila hemp, ramie, bamboo, banana, cotton and jute. Further, the growth cycle of Manila hemp fibers is from one to two years. Thus, this natural fiber is considered environment-friendly and it will not lead to deforestation. In order to establish the most suitable molding conditions, mechanical properties of heat-treated Manila hemp fibers were examined. Fig. 4 shows the effect of heat treatment on the tensile strength of Manila hemp fibers. The average tensile strength of Manila hemp fibers without heat treatment is 702 MPa. The tensile strength of Manila hemp fibers decreases with increasing heating time at 200 C. The tensile strength of Manila hemp fibers heat-treated at 180 C for 30 min is similar to that of non-heat-treated fibers. At 160 C, the tensile strength of heat-treated Manila hemp fibers does not decrease even with longer heating times. Based on these results, the processing temperature for fabricating Manila hemp fiber reinforced composites should be kept below 160 C in order to prevent strength reduction due to thermal degradation.
The transverse section and top view of the biodegradable composites with 70% of fibers are shown in Figs. 5 and 6, respectively. From these figures, it can be observed that the distribution of fibers is parallel, and that there are no voids or fiber contacts that would cause a decrease in strength. In previous studies, the composites were fabricated by sandwiching layers of fibers between several resin films. However, with this method it was not possible to fabricate high fiber volume fraction composites as the increase of fiber content led to increased fiber contact. The novel technique presented herein, which uses an emulsion-type biodegradable resin, provides a suitable internal environment for achieving high fiber volume, in which voids and fiber contacts are reduced in the composites.
Fig. 5. Optical micrograph of transverse section of biodegradable composites with 70% of fibers.
Norm. tensile strength (%)
120 100 80 60 40
Heating temperature 160 ºC 180 ºC 200 ºC
20 0
1881
0
20
40
60
80
100
120
Heating time (min) Fig. 4. Relationship between tensile strength of Manila hemp fibers and heating time.
Fig. 6. Photograph of top view of biodegradable composites.
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3.3. Tensile and flexural strengths of biodegradable composites Figs. 7 and 8 show the relationships between fiber content and tensile and flexural strengths, respectively. From these figures, it can be seen that tensile and flexural strengths increase linearly with increasing fiber content. The tensile and flexural strengths were 365.4 MPa and 222.7 MPa, respectively, in the samples with a fiber fraction of 70%. Besides, the flexural strength was lower than tensile strength. In tensile mode, the fracture of specimens was caused by fracture of fibers. However, in flexural mode, the fracture of specimens was caused by separation between fibers and resin. Thus the lower flexural strength compared to tensile strength could be attributed to the difference in the fracture mechanism. When fiber content is more than 70%, some voids and fiber contacts caused by insufficient amount of resin are observed in the specimen. Therefore, the fiber content used for fabricating Manila hemp fiber reinforced composites should be kept below 70%. The tensile and flexural strengths of several previous natural fiber reinforced composites are shown in Table 2. Most of these composites indicated tensile and flexural
Tensile strength (MPa)
500 400 300 200 100 0
0
20
40
60
80
100
Volume fraction of Manila hemp fiber (%)
Table 2 Tensile and flexural strengths of several natural fiber reinforced composites [1–9]
Flax Ramie Jute Flax Flax Jute Bamboo Pineapple Kenaf Henequen Hemp
Tensile strength (MPa)
Flexural strength (MPa)
References
50.5 50 34.5 53 33.2 100.5 88.5 55.8 60 71.8 80
– – – – 49.3 – 76.5 86 – 95.9 –
[1] [1] [1] [2] [3] [4] [5] [6] [7] [8] [9]
Table 3 Comparison of theoretical and experimental value of tensile strength Volume fraction of fibers (%)
Theoretical strength (MPa)
Experimental strength (MPa)
Experiment/theory (%)
30 50 70
218.5 356.7 494.8
162.0 254.2 365.4
74.1 71.3 73.8
strengths less than 100 MPa. Thus, the strength of biodegradable composites fabricated in the current study is much higher than those of any biodegradable composites previously reported. Table 3 shows the theoretical tensile strength values calculated from the rule of mixtures [10], and experimental values. From this table, experimental strength values are 70% of the theoretical strength. As shown in Fig. 3, there is a large variation in Manila hemp fiber strength and lower experimental strength values could be attributed to fracture of low strength fibers. 3.4. Elastic modulus of biodegradable composites
Fig. 7. Relationship between tensile strength of unidirectional biodegradable composites and Manila hemp fiber content.
Fig. 9 shows the relationship between elastic moduli in both tensile and flexural modes, and fiber content. From
300
200
Elastic modulus (GPa)
Flexural strength (MPa)
40
100
0
0
20
40
60
80
Tensile modulus Flexural modulus
30
20
10
100
Volume fraction of Manila hemp fiber (%) Fig. 8. Relationship between flexural strength of unidirectional biodegradable composites and Manila hemp fiber content.
0
0
20 40 60 80 Volume fraction of Manila hemp fiber (%)
100
Fig. 9. Relationship between elastic moduli and fiber content.
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2. Unidirectional biodegradable composites fabricated using an emulsion-type biodegradable resin and Manila hemp fiber bundles with a fiber content of 70% have high tensile and flexural strengths of 365 MPa and 223 MPa, respectively. 3. The tensile and flexural strengths and tensile and flexural moduli increased linearly with increasing fiber content up to 70%. Thus excellent mechanical properties are achieved for composites fabricated by the novel technique proposed in this study in which the composites are fabricated with an emulsion-type biodegradable resin.
Fig. 10. Fracture behavior of biodegradable composites reinforced by Manila hemp fibers after tensile test.
this figure, both tensile and flexural moduli increase linearly with increasing fiber content. Luo and Netravali [6] investigated the relationship between elastic modulus of unidirectional pineapple fiber reinforced composites and fiber content and reported that elastic modulus of composites decreased with fiber content. They explained that this was due to internal voids and low shearing stress between fiber and resin. A photograph of the fracture surface after tensile testing is shown in Fig. 10. There are no cracks observed between fiber and resin, indicating there is good interfacial bonding between fiber and resin in the biodegradable composites produced in this work. However, to understand more about the interfacial characteristics, it may be necessary to make thorough examination of the fracture surfaces in the future. From these results, it is clear that there are no cracks in the composites. Further, bonding between the cellulose fibers and resin is considered to be chemically high due to use of a starch-based resin. 4. Conclusions High strength biodegradable composites were made using an emulsion-type biodegradable resin as the matrix and Manila hemp fiber bundles as the reinforcement. The results obtained are as follows: 1. The tensile strength of Manila hemp fibers heat-treated at 160 C for 120 min did not decrease. Thus, 160 C are the highest fabrication temperatures that do not effect the strength.
Acknowledgements The authors would like to express special thanks to Mr. Yoichi Fujimori (Fujiseishi Co., Ltd.) and Mr. Hideyuki Tanaka (Japan Cornstarch Co., Ltd.) for supplying the experimental materials. References [1] Wollerdorfer M, Bader H. Influence of natural fibers on the mechanical properties of biodegradable polymers. Ind Crops Prod 1998;8:105–12. [2] Oksman K, Skrifvars M, Selin J-F. Natural fibres as reinforcement in polylactic acid (PLA) composites. Compos Sci Technol 2003; 63:1317–24. [3] Keener TJ, Stuart RK, Brown TK. Maleated coupling agents for natural fibre composites. Compos A: Appl Sci Manuf 2004; 35:357–62. [4] Plackett D, Andersen TL, Pedersen WB, Nielsen L. Biodegradable composites based on polylactide and jute fibres. Compos Sci Technol 2003;63:1287–96. [5] Okubo K, Fujii T. Eco-composites using bamboo and other natural fibers and their mechanical properties. In: Proceedings of the international workshop on ‘‘Green’’ composites, 2002. p. 17–21. [6] Luo S, Netravali AN. Interfacial and mechanical properties of environment-friendly green composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin. J Mater Sci 1999; 34:3709–19. [7] Nishino T, Hirao K, Kotera M, Nakamae K, Inagaki H. Kenaf reinforced biodegradable composite. Compos Sci Technol 2003; 63:1281–6. [8] Herrera-Franco PJ, Valadez-Gonza´lez A. Mechanical properties of continuous natural fibre-reinforced polymer composites. Compos A: Appl Sci Manuf 2004;35:339–45. [9] Mohanty AK, Wibowo A, Misra M, Drzal LT. Effect of process engineering on the performance of natural fiber reinforced cellulose acetate biocomposites. Compos A: Appl Sci Manuf 2004;35:363–70. [10] Hull D, Clyne TW. An introduction to composite materials. 2nd ed. Cambridge: Cambridge University Press; 1996.