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Bio-based composites from waste agricultural residues
Waste Management 30 (2010) 680–684
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Bio-based composites from waste agricultural residues Alireza Ashori a,*, Amir Nourbakhsh b a b
Department of Chemical Industries, Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815-3538, Tehran, Iran Department of Wood and Paper Science, Research Institute of Forests and Rangelands (RIFR), Tehran, Iran
a r t i c l e
i n f o
Article history: Accepted 11 August 2009 Available online 8 September 2009
a b s t r a c t The main objective of this research was to study the potential of waste agricultural residues such as sunflower stalk, corn stalk and bagasse fibers as reinforcement for thermoplastics as an alternative to wood fibers. The effects of two grades (Eastman G-3003 and G-3216) of coupling agents on the mechanical properties were also studied. In the sample preparation, one level of fiber loading (30 wt.%) and three levels of coupling agent content (0, 1.5 and 2.5 wt.%) were used. For overall trend, with addition of both grades of the coupling agents, tensile, flexural and impact properties of the composites significantly improved, as compared with untreated samples. In addition, morphological study revealed that the positive effect of coupling agent on interfacial bonding. The composites treated with G-3216 gave better results in comparison with G-3003. This could be caused by the high melt viscosity of G-3003. In general, bagasse fiber showed superior mechanical properties due to its chemical characteristics. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Man has a long history of using lignocellulosic fibers for producing composites. Its beginning can be traced back to the ancient times when the Egyptians added straw to mud to make bricks which have proved to be very strong and durable. Today, bio-based composites are becoming attractive in both commercial and noncommercial applications (Ndazi et al., 2006). Due to the global demand for fibrous materials, worldwide shortage of trees in many areas, and environmental awareness, research on the development of composites prepared using various waste materials is being actively pursued. Among the possible alternatives, the development of composites using agricultural residuals (including stalks of most cereal crops, rice husks, coconut fibers, bagasse, maize cobs, peanut shells, and other wastes) is currently at the center of attention (Gorokhovsky et al., 2005; Yao et al., 2008; Ashori and Nourbakhsh, 2009a; Wang et al., 2009). Agricultural residues are excellent alternative waste materials to substitute wood because they are plentiful, widespread, and easily available. Aside from their abundance and renewability, utilization of agricultural residues has advantages for economy, environment, and technology (Çöpür et al., 2007). Although there are some useful studies in the literature on agricultural fibers in composites (Yang et al., 2004, 2007; Sain and Panthapulakkal, 2006; Kim et al., 2006; Nourbakhsh and Ashori, 2009), there are still many gaps in information and knowledge of composites from agriculture residues, which must be closed in or-
* Corresponding author. Tel./fax: +98 21 88838337. E-mail address: [email protected] (A. Ashori). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.08.003
der to stimulate commercial production of these novel materials. The scope of the present work is to process sunflower stalk, corn stalk and bagasse fibers in order to evaluate and compare their suitability as reinforcing agents for composite applications. In addition, the effects of two different grades of coupling agents on the mechanical properties were studied. 2. Experimental 2.1. Materials Three types of agricultural residuals were investigated in this study: sunflower stalk, corn stalk and bagasse fibers. The important chemical components and fiber morphology of lignocellulosic materials are given in Table 1. These parameters are important as they may influence the resulting mechanical properties of the composites. The fibers were produced by refiner mechanical pulping process. The chips were steamed for 15 min at 7 MPa and 175 °C, disc refined and then dried in a laboratory-made hot air tube dryer. Fibers were oven dried at 105 °C for 24 h to adjust the moisture content to 1–3% and then stored in sealed polyethylene bags before compounding. The thermoplastic polymer polypropylene (PP) was supplied by Bandar Imam Petrochemical Commercial Co., Iran, in the form of pellets with a melt flow index of 7–10 g/10 min (190 °C/2160 g) and density of 0.95 g/cm3. The important mechanical characteristics of the polymer matrix used are: tensile strength 28.5 MPa, tensile modulus 1250 MPa, flexural strength 38.5 MPa and flexural modulus 1150 MPa. Maleic anhydride grafted polypropylene (MAPP) was supplied by Eastman Chemical Products Inc. Two grades of MAPP for
681
A. Ashori, A. Nourbakhsh / Waste Management 30 (2010) 680–684 Table 1 Chemical components and fiber morphology of used agricultural residuals. Sunflower stalk
Corn stalk
Bagasse
Cellulose (%) Lignin (%) Ash (%)
38.1 17.3 7.0
36.6 16.7 6.3
52.7 20.6 1.3
Fiber morphology Length (mm) Diameter (lm) Aspect ratio (L/D)
1.18 21.5 55
1.22 24.3 50
1.24 22.9 54
Sunflower stalk Tensile strength (MPa)
Chemical components
40 Corn stalk
Bagasse
35 30 25 20 15 0
1.5
2.5
0
1.5
G-3216
composites were used: Eastman G-3003 and G-3216. Table 2 shows the typical physical properties of Eastman G polymers.
2.5
G-3003 Coupling agent content (wt %)
Fig. 1. Comparison of tensile strength of composites as function of the coupling agent content and grade.
Composites were produced in a two-stage process. In the first stage, lignocellulosic fibers and PP were compounded with and without coupling agent (depending on formulations) using a corotating twin-screw extruder. The mass ratio of the fiber to polymer was 30:70 (w/w) and the weight of coupling agent in the formulation was 0, 1.5 and 2.5 wt.%. The melt temperature at the die was 180 °C and the rotation speed was 60 rpm. In the second stage, the extrudate in the form of strands were allowed to cool to room temperature and then granulated using a CW Brabender Granulator. The resulting granules were dried at 105 °C for 24 h before being compression molded. Molding conditions were: press temperature 170 °C, pressure during heating 3 MPa.
2500 Sunflower stalk Tensile modulus (MPa)
2.2. Sample preparation
Corn stalk
Bagasse
2300 2100 1900 1700 1500 0
1.5
2.5
0
G-3216
1.5
2.5
G-3003
Coupling agent content (wt %)
2.3. Mechanical testing Compression molded specimens were tested following ASTM standard D638 for tensile properties, ASTM D790 for flexural properties and D256 for notched Izod impact strength (ASTM Standards, 1999). Tensile and bending tests were conducted using an Instron Universal Testing Machine (model 1186) at a speed of 1.5 and 2 mm/min, respectively. Impact test was performed with a pendulum apparatus (Zwick model 1446) using conventional V notched specimens. Six replicates were tested for each property under each formulation. 2.4. Morphological study Studies on the morphology of the composites were carried out using a TESCAN model WEGA-II scanning electron microscope (SEM). The fracture surfaces of the specimens after tensile test were sputter-coated with gold before analysis in order to eliminate electron charging. 3. Results and discussion 3.1. Tensile properties Figs. 1 and 2 illustrate the tensile strength and modulus, respectively, of fiber/PP composites made with various fiber types. Maximum tensile modulus of elasticity ranges from 2150 to 2190 MPa
Fig. 2. Comparison of tensile modulus of composites as function of the coupling agent content and grade.
for bagasse composites, while maximum tensile strength is approximately 33 MPa. In other words, modulus of elasticity of pure PP is enhanced at least 1.7 times when bagasse is added. The increment in tensile properties at the presence of cellulosic fibers is expected as the mechanical properties of the composites are determined by several factors, such as nature of the reinforcement fiber, fiber aspect ratio, fiber–matrix interfacial adhesion, and also the fiber orientation in the composites (Sameni et al., 2002). One of the most important parameters controlling the mechanical properties of short fibers composite is the fiber length or more precisely its aspect ratio (length/width). A high aspect ratio is very important in fiber reinforced composites, as it indicates potential strength properties. As can be seen from Table 1, bagasse fiber has high fiber length and aspect ratio compared to the other cellulosic materials. To improve the bonding strength between the lignocellulosic filler and the matrix polymer, coupling agent was used. With addition of the coupling agent, tensile properties of the composites significantly improved up to the same level of pure PP (Figs. 1 and 2). In general, composites with high coupling agent content exhibited better tensile strength and modulus than the untreated ones. The increase in tensile strength is due to the improved chemical bond between the fibers and PP polymer chains. Results demonstrate
Table 2 Typical physical properties of coupling agents. Coupling agent
Acid number (mg KOH/g)
Melting point (°C)
Viscosity, Brookfield @ 190 °C, cP
Molecular weight (Mw)
G-3003 G-3216
9 16
158 142
60,000 18,000
52,000 60,000
A. Ashori, A. Nourbakhsh / Waste Management 30 (2010) 680–684
that modifying the fiber surface enhances the compatibility of hydrophobic polymer and hydrophilic cellulose fiber. The tensile strengths of the bagasse composites at different coupling agent contents are shown in Fig. 1. Each composite made with G-3216 and G-3003 exhibited almost the same tensile properties. The maximum tensile strength value was observed to be 33 MPa for bagasse specimens. Most effective content of coupling agent is 2.5 wt.% as shown in Fig. 1. The tendency of the test results in relation to the coupling agent content is the same as our previous study (Ashori and Nourbakhsh, 2009b). Fig. 2 shows the variations of modulus of composite versus coupling agent content. It is clear that, to improve the reinforcing effect of fiber, the presence of coupling agent is vital. The enhancement in modulus is easily understood because filler in fibrous form can carry more tensile load with increasing fiber content (Thwe and Liao, 2002). In addition, fiber is said to be much stiffer than polymer matrix and as a result it adds stiffness to the composites (Salemane and Luyt, 2006). In bagasse samples, with increasing of coupling agent from 0 to 2.5 wt.% the increment of tensile modulus was about 10% and 12% for G-3216 and G-3003, respectively. This improvement could be related to better adhesion between the fiber and the matrix by chemical interactions. Better adhesion results into more restriction to deformation capacity of the matrix in the elastic zone and increasing modulus. Similar observations were reported for other lignocellulosic fibers based PP composites (Felix and Gatenholm, 1991; Demir et al., 2006; Ashori, 2008). All the compositions showed a tensile modulus higher than the pure PP. 3.2. Flexural properties The flexural strength and modulus of the composites are shown in Figs. 3 and 4 as function of the coupling agent content and grade. The flexural proprieties of the composites vary significantly with fiber type and coupling agent grade. Composites made with bagasse fibers show the highest strength and modulus of flexural, whereas corn stalk and sunflower stalk composites exhibit the lowest properties. The maximum flexural strengths were shown in bagasse and sunflower stalk treated with G-3216 (79.1 and 75.5 MPa, respectively). It was found that composites with 2.5 wt.% coupling agent provided significantly higher flexural strength and modulus, compared with untreated samples. Moreover, bagasse fiber/PP composites showed the highest flexural properties in both grades of MAPP. This indicates good interaction between the bagasse fiber and PP. The flexural properties of bagasse fiber/PP composites are higher than sunflower stalk and corn stalk filled composites. The strength of fiber reinforced composites depends on the properties of constituents and the interface inter-
3150 Sunflower stalk Flexural modulus (MPa)
682
Bagasse
2450 2100 1750 1400 0
1.5
2.5
0
G-3216
1.5
2.5
G-3003
Coupling agent content (wt %) Fig. 4. Comparison of flexural modulus of composites as function of the coupling agent content and grade.
action. However, when considering the flexural properties, homogeneity of the overall composite needs to be taken into account. This is mainly because in bending, the convex side of the specimen is extended and the concave side is compressed (Balasuriya et al., 2001). Generally, all the compositions showed a flexural strength and modulus higher than the pure PP. Several attempts have been made to correlate lignocellulosic fiber properties to wood plastic composites properties (English and Falkm, 1995; Bledzki et al., 1998; Stokke and Gardner, 2003; Stark and Rowlands, 2003; Neagu et al., 2006). Maldas et al. (1989) investigated the effect of wood species on the mechanical properties of wood/thermoplastic composites. They reported that differences in morphology, density, and aspect ratios across wood species account for varying reinforcement properties in thermoplastic composites. 3.3. Impact strength The Izod impact tests were conducted at room temperature. The notched specimens were tested and Fig. 5 shows the Izod impact strengths of the composites made with the different coupling agent content. Like the tensile and flexural properties, the Izod impact strength of composites increased with the increase in the coupling agent content. The Izod impact strengths of the composites made with the two different grade of MAPP are almost the same. In the composites made with G-3003, the impact strength at 1.5 wt.% of coupling agent content is almost the same level as 2.5 wt.%. Lignocellulosic fiber is a kind of stiff organic filler, so adding fiber could decrease the impact strength of composite. Composites treated with coupling agent exhibited better impact strength than
105
40 Sunflower stalk
Corn stalk
Bagasse
Sunflower stalk
90
Corn stalk
Bagasse
36 Impact (J/m)
Flexural strength (MPa)
Corn stalk
2800
75 60 45
32 28 24
30
20 0
1.5 G-3216
2.5
0
1.5
2.5
G-3003 Coupling agent content (wt %)
Fig. 3. Comparison of flexural strength of composites as function of the coupling agent content and grade.
0
1.5
2.5
0
G-3216
1.5
2.5
G-3003 Coupling agent content (wt %)
Fig. 5. Comparison of notched Izod impact strength of composites as function of the coupling agent content and grade.
A. Ashori, A. Nourbakhsh / Waste Management 30 (2010) 680–684
683
Fig. 6. SEM micrographs of the tensile fracture surfaces at 2.5 wt.% MAPP grade G-3216 with magnification of 500. (a) Sunflower stalk; (b) corn stalk and (c) bagasse.
the untreated ones since the debonding behavior between the interface of fiber and PP matrix absorb larger impact energy in modified composites than the unmodified ones. 3.4. Morphology characteristics SEM is an effective media for the morphological investigations of the composites. Through SEM study, the distribution and compatibility between the fiber and the matrix could be observed. The tensile fracture surfaces of the composites treated with 2.5 wt.% MAPP grade G-3216 are shown in Fig. 6. In the case of the composite made with the bagasse, the filler particles are well dispersed in the matrix polymer, as compared with the composite made with the sunflower stalk and corn stalk. There are some voids where the fibers have been pulled-out. The presence of these voids means that the interfacial bonding between the fiber and the matrix polymer is weak. All of the micrographs of the samples treated with G-3216 show less voids, as compared with those treated with the G-3003. This result could be related to poor mixing of the fibers and matrix due to high melt viscosity of the later at 190 °C (Table 2). 4. Conclusions Based on the results of this study the following conclusions can be drawn. (1) Mechanical properties of the composites treated with both grades of coupling agents were significantly superior to those of untreated ones, due to the stronger interfacial bonding between the fiber and the matrix polymer. (2) The tensile strength and modulus of the composite improved with increasing coupling agent content, due to the improvement of interface bond between the fiber and matrix. (3) The Izod impact strengths of the MAPP incorporated composites are slightly higher following the incorporation of the coupling agent. (4) SEM micrographs of the fracture surfaces of the composites treated with G-3216 indicate well dispersed fillers, as compared with those made with G-3003. It could be related to the high melt viscosity of G-3003. (5) SEM study also shows that interfacial adhesion between the fiber and the matrix is improved with the addition of both grades of MAPP. (6) In general, bagasse fiber shows superior mechanical properties due to its chemical characteristics.
References Annual Book of ASTM Standards, 1999. American Society for Testing and Materials, 100 Barr Harbor Dr., West Conshohocken, PA 19428, United States. Ashori, A., 2008. Municipal solid waste as a source of lignocellulosic fiber and plastics for composite industries. Polymer-Plastics Technology and Engineering 47 (8), 741–744. Ashori, A., Nourbakhsh, A., 2009a. Characteristics of wood plastic composites made of recycled materials. Waste Management 29 (4), 1291–1295. Ashori, A., Nourbakhsh, A., 2009b. Mechanical behavior of agro-residue reinforced polyethylene composites. Applied Polymer Science 111 (5), 2616–2620. Balasuriya, P.W., Ye, L., Mai, Y.-W., 2001. Mechanical properties of wood flake– polyethylene composites. Part I: effects of processing methods and matrix melt flow behavior. Composites Part A 32 (5), 619–629. Bledzki, A.K., Reihmane, S., Gassan, J., 1998. Thermoplastics reinforced with wood fillers: a literature review. Polymer-Plastics Technology and Engineering 37 (4), 451–468. Çöpür, Y., Güler, C., Akgül, M., Tasßçıog˘lu, C., 2007. Some chemical properties of hazelnut husk and its suitability for particleboard production. Building and Environment 42 (2007), 2568–2572. Demir, H., Atikler, U., Balkose, D., Tihminlioglu, F., 2006. The effect of fiber treatments on the tensile and water sorption properties of polypropylene–Luffa fiber composites. Composites Part A 37 (3), 447–456. English, B.W., Falkm, R.H., 1995. In: Proceedings of the Wood Fiber–Plastic Composites Conference. Madison, Wisconsin, p. 183. Felix, J.M., Gatenholm, P., 1991. Nature of adhesion in composites of modified cellulose fibers and polypropylene. Applied Polymer Science 42 (3), 609–620. Gorokhovsky, A.V., Escalante-Garcia, J.I., Gashnikova, G.Yu., Nikulina, L.P., Artemenko, S.E., 2005. Composite materials based on wastes of flat glass processing. Waste Management 25 (7), 733–736. Kim, H.-S., Kim, H.-J., Lee, J.-W., Choi, I.-G., 2006. Biodegradability of bio-flour filled biodegradable poly(butylene succinate) bio-composites in natural and compost soil. Polymer Degradation and Stability 91 (5), 1117–1127. Maldas, D., Kokta, B.V., Daneault, C., 1989. Thermoplastic composites of polystyrene: effect of different wood species on mechanical properties. Applied Polymer Science 38 (3), 413–439. Ndazi, B., Tesha, J.V., Bisanda, E.T.N., 2006. Some opportunities and challenges of producing bio-composites from non-wood residues. Materials Science 41 (2), 6984–6990. Neagu, R.C., Gamstedt, E.K., Berthold, F., 2006. Stiffness contribution of various wood fibers to composite materials. Composite Materials 40 (8), 663–699. Nourbakhsh, A., Ashori, A., 2009. Preparation and properties of wood plastic composites made of recycled HDPE. Composite Materials 43 (8), 877–883. Sain, M., Panthapulakkal, S., 2006. Bioprocess preparation of wheat straw fibers and their characterization. Industrial Crops and Products 23 (1), 1–8. Salemane, M.G., Luyt, A.S., 2006. Thermal and mechanical properties of polypropylene–wood powder composites. Applied Polymer Science 100 (5), 4173–4180. Sameni, J.K., Ahmad, S.H., Zakaria, Z., 2002. Effects of processing parameters and graft-copoly(propylene/maleic anhydride) on mechanical properties of thermoplastic natural rubber composites reinforced with wood fibers. Plastics Rubber Composites 31 (4), 162–166. Stark, N.M., Rowlands, R.E., 2003. Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites. Wood Fiber Science 35 (2), 167–174. Stokke, D.D., Gardner, D.J., 2003. Fundamental aspects of wood as a component of thermoplastic composites. Journal of Vinyl and Additive Technology 9 (2), 96– 104. Thwe, M.M., Liao, K., 2002. Effects of environmental aging on the mechanical properties of bamboo–glass fiber reinforced polymer matrix hybrid composites. Composites Part A 33 (1), 43–52.
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Wang, Z., Wang, E., Zhang, S., Wang, Z., Ren, Y., 2009. Effects of cross-linking on mechanical and physical properties of agricultural residues/recycled thermoplastics composites. Industrial Crops and Products 29 (1), 133–138. Yang, H.-S., Kim, H.-J., Park, H.-J., Lee, B.-J., Hwang, T.-S., 2007. Effect of compatibilizing agents on rice-husk flour reinforced polypropylene composites. Composite Structures 77 (1), 45–55.
Yang, H.-S., Kim, H.-J., Son, J., Park, H.-J., Lee, B.-J., Hwang, T.-S., 2004. Rice-husk flour filled polypropylene composites; mechanical and morphological study. Composite Structures 63 (3–4), 305–312. Yao, F., Wu, Q., Lei, Y., Xu, Y., 2008. Rice straw fiber-reinforced high-density polyethylene composite: effect of fiber type and loading. Industrial Crops and Products 28 (1), 63–72.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Bio-based composites from waste agricultural residues Alireza Ashori a,*, Amir Nourbakhsh b a b
Department of Chemical Industries, Iranian Research Organization for Science and Technology (IROST), P.O. Box 15815-3538, Tehran, Iran Department of Wood and Paper Science, Research Institute of Forests and Rangelands (RIFR), Tehran, Iran
a r t i c l e
i n f o
Article history: Accepted 11 August 2009 Available online 8 September 2009
a b s t r a c t The main objective of this research was to study the potential of waste agricultural residues such as sunflower stalk, corn stalk and bagasse fibers as reinforcement for thermoplastics as an alternative to wood fibers. The effects of two grades (Eastman G-3003 and G-3216) of coupling agents on the mechanical properties were also studied. In the sample preparation, one level of fiber loading (30 wt.%) and three levels of coupling agent content (0, 1.5 and 2.5 wt.%) were used. For overall trend, with addition of both grades of the coupling agents, tensile, flexural and impact properties of the composites significantly improved, as compared with untreated samples. In addition, morphological study revealed that the positive effect of coupling agent on interfacial bonding. The composites treated with G-3216 gave better results in comparison with G-3003. This could be caused by the high melt viscosity of G-3003. In general, bagasse fiber showed superior mechanical properties due to its chemical characteristics. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Man has a long history of using lignocellulosic fibers for producing composites. Its beginning can be traced back to the ancient times when the Egyptians added straw to mud to make bricks which have proved to be very strong and durable. Today, bio-based composites are becoming attractive in both commercial and noncommercial applications (Ndazi et al., 2006). Due to the global demand for fibrous materials, worldwide shortage of trees in many areas, and environmental awareness, research on the development of composites prepared using various waste materials is being actively pursued. Among the possible alternatives, the development of composites using agricultural residuals (including stalks of most cereal crops, rice husks, coconut fibers, bagasse, maize cobs, peanut shells, and other wastes) is currently at the center of attention (Gorokhovsky et al., 2005; Yao et al., 2008; Ashori and Nourbakhsh, 2009a; Wang et al., 2009). Agricultural residues are excellent alternative waste materials to substitute wood because they are plentiful, widespread, and easily available. Aside from their abundance and renewability, utilization of agricultural residues has advantages for economy, environment, and technology (Çöpür et al., 2007). Although there are some useful studies in the literature on agricultural fibers in composites (Yang et al., 2004, 2007; Sain and Panthapulakkal, 2006; Kim et al., 2006; Nourbakhsh and Ashori, 2009), there are still many gaps in information and knowledge of composites from agriculture residues, which must be closed in or-
* Corresponding author. Tel./fax: +98 21 88838337. E-mail address: [email protected] (A. Ashori). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.08.003
der to stimulate commercial production of these novel materials. The scope of the present work is to process sunflower stalk, corn stalk and bagasse fibers in order to evaluate and compare their suitability as reinforcing agents for composite applications. In addition, the effects of two different grades of coupling agents on the mechanical properties were studied. 2. Experimental 2.1. Materials Three types of agricultural residuals were investigated in this study: sunflower stalk, corn stalk and bagasse fibers. The important chemical components and fiber morphology of lignocellulosic materials are given in Table 1. These parameters are important as they may influence the resulting mechanical properties of the composites. The fibers were produced by refiner mechanical pulping process. The chips were steamed for 15 min at 7 MPa and 175 °C, disc refined and then dried in a laboratory-made hot air tube dryer. Fibers were oven dried at 105 °C for 24 h to adjust the moisture content to 1–3% and then stored in sealed polyethylene bags before compounding. The thermoplastic polymer polypropylene (PP) was supplied by Bandar Imam Petrochemical Commercial Co., Iran, in the form of pellets with a melt flow index of 7–10 g/10 min (190 °C/2160 g) and density of 0.95 g/cm3. The important mechanical characteristics of the polymer matrix used are: tensile strength 28.5 MPa, tensile modulus 1250 MPa, flexural strength 38.5 MPa and flexural modulus 1150 MPa. Maleic anhydride grafted polypropylene (MAPP) was supplied by Eastman Chemical Products Inc. Two grades of MAPP for
681
A. Ashori, A. Nourbakhsh / Waste Management 30 (2010) 680–684 Table 1 Chemical components and fiber morphology of used agricultural residuals. Sunflower stalk
Corn stalk
Bagasse
Cellulose (%) Lignin (%) Ash (%)
38.1 17.3 7.0
36.6 16.7 6.3
52.7 20.6 1.3
Fiber morphology Length (mm) Diameter (lm) Aspect ratio (L/D)
1.18 21.5 55
1.22 24.3 50
1.24 22.9 54
Sunflower stalk Tensile strength (MPa)
Chemical components
40 Corn stalk
Bagasse
35 30 25 20 15 0
1.5
2.5
0
1.5
G-3216
composites were used: Eastman G-3003 and G-3216. Table 2 shows the typical physical properties of Eastman G polymers.
2.5
G-3003 Coupling agent content (wt %)
Fig. 1. Comparison of tensile strength of composites as function of the coupling agent content and grade.
Composites were produced in a two-stage process. In the first stage, lignocellulosic fibers and PP were compounded with and without coupling agent (depending on formulations) using a corotating twin-screw extruder. The mass ratio of the fiber to polymer was 30:70 (w/w) and the weight of coupling agent in the formulation was 0, 1.5 and 2.5 wt.%. The melt temperature at the die was 180 °C and the rotation speed was 60 rpm. In the second stage, the extrudate in the form of strands were allowed to cool to room temperature and then granulated using a CW Brabender Granulator. The resulting granules were dried at 105 °C for 24 h before being compression molded. Molding conditions were: press temperature 170 °C, pressure during heating 3 MPa.
2500 Sunflower stalk Tensile modulus (MPa)
2.2. Sample preparation
Corn stalk
Bagasse
2300 2100 1900 1700 1500 0
1.5
2.5
0
G-3216
1.5
2.5
G-3003
Coupling agent content (wt %)
2.3. Mechanical testing Compression molded specimens were tested following ASTM standard D638 for tensile properties, ASTM D790 for flexural properties and D256 for notched Izod impact strength (ASTM Standards, 1999). Tensile and bending tests were conducted using an Instron Universal Testing Machine (model 1186) at a speed of 1.5 and 2 mm/min, respectively. Impact test was performed with a pendulum apparatus (Zwick model 1446) using conventional V notched specimens. Six replicates were tested for each property under each formulation. 2.4. Morphological study Studies on the morphology of the composites were carried out using a TESCAN model WEGA-II scanning electron microscope (SEM). The fracture surfaces of the specimens after tensile test were sputter-coated with gold before analysis in order to eliminate electron charging. 3. Results and discussion 3.1. Tensile properties Figs. 1 and 2 illustrate the tensile strength and modulus, respectively, of fiber/PP composites made with various fiber types. Maximum tensile modulus of elasticity ranges from 2150 to 2190 MPa
Fig. 2. Comparison of tensile modulus of composites as function of the coupling agent content and grade.
for bagasse composites, while maximum tensile strength is approximately 33 MPa. In other words, modulus of elasticity of pure PP is enhanced at least 1.7 times when bagasse is added. The increment in tensile properties at the presence of cellulosic fibers is expected as the mechanical properties of the composites are determined by several factors, such as nature of the reinforcement fiber, fiber aspect ratio, fiber–matrix interfacial adhesion, and also the fiber orientation in the composites (Sameni et al., 2002). One of the most important parameters controlling the mechanical properties of short fibers composite is the fiber length or more precisely its aspect ratio (length/width). A high aspect ratio is very important in fiber reinforced composites, as it indicates potential strength properties. As can be seen from Table 1, bagasse fiber has high fiber length and aspect ratio compared to the other cellulosic materials. To improve the bonding strength between the lignocellulosic filler and the matrix polymer, coupling agent was used. With addition of the coupling agent, tensile properties of the composites significantly improved up to the same level of pure PP (Figs. 1 and 2). In general, composites with high coupling agent content exhibited better tensile strength and modulus than the untreated ones. The increase in tensile strength is due to the improved chemical bond between the fibers and PP polymer chains. Results demonstrate
Table 2 Typical physical properties of coupling agents. Coupling agent
Acid number (mg KOH/g)
Melting point (°C)
Viscosity, Brookfield @ 190 °C, cP
Molecular weight (Mw)
G-3003 G-3216
9 16
158 142
60,000 18,000
52,000 60,000
A. Ashori, A. Nourbakhsh / Waste Management 30 (2010) 680–684
that modifying the fiber surface enhances the compatibility of hydrophobic polymer and hydrophilic cellulose fiber. The tensile strengths of the bagasse composites at different coupling agent contents are shown in Fig. 1. Each composite made with G-3216 and G-3003 exhibited almost the same tensile properties. The maximum tensile strength value was observed to be 33 MPa for bagasse specimens. Most effective content of coupling agent is 2.5 wt.% as shown in Fig. 1. The tendency of the test results in relation to the coupling agent content is the same as our previous study (Ashori and Nourbakhsh, 2009b). Fig. 2 shows the variations of modulus of composite versus coupling agent content. It is clear that, to improve the reinforcing effect of fiber, the presence of coupling agent is vital. The enhancement in modulus is easily understood because filler in fibrous form can carry more tensile load with increasing fiber content (Thwe and Liao, 2002). In addition, fiber is said to be much stiffer than polymer matrix and as a result it adds stiffness to the composites (Salemane and Luyt, 2006). In bagasse samples, with increasing of coupling agent from 0 to 2.5 wt.% the increment of tensile modulus was about 10% and 12% for G-3216 and G-3003, respectively. This improvement could be related to better adhesion between the fiber and the matrix by chemical interactions. Better adhesion results into more restriction to deformation capacity of the matrix in the elastic zone and increasing modulus. Similar observations were reported for other lignocellulosic fibers based PP composites (Felix and Gatenholm, 1991; Demir et al., 2006; Ashori, 2008). All the compositions showed a tensile modulus higher than the pure PP. 3.2. Flexural properties The flexural strength and modulus of the composites are shown in Figs. 3 and 4 as function of the coupling agent content and grade. The flexural proprieties of the composites vary significantly with fiber type and coupling agent grade. Composites made with bagasse fibers show the highest strength and modulus of flexural, whereas corn stalk and sunflower stalk composites exhibit the lowest properties. The maximum flexural strengths were shown in bagasse and sunflower stalk treated with G-3216 (79.1 and 75.5 MPa, respectively). It was found that composites with 2.5 wt.% coupling agent provided significantly higher flexural strength and modulus, compared with untreated samples. Moreover, bagasse fiber/PP composites showed the highest flexural properties in both grades of MAPP. This indicates good interaction between the bagasse fiber and PP. The flexural properties of bagasse fiber/PP composites are higher than sunflower stalk and corn stalk filled composites. The strength of fiber reinforced composites depends on the properties of constituents and the interface inter-
3150 Sunflower stalk Flexural modulus (MPa)
682
Bagasse
2450 2100 1750 1400 0
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0
G-3216
1.5
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Coupling agent content (wt %) Fig. 4. Comparison of flexural modulus of composites as function of the coupling agent content and grade.
action. However, when considering the flexural properties, homogeneity of the overall composite needs to be taken into account. This is mainly because in bending, the convex side of the specimen is extended and the concave side is compressed (Balasuriya et al., 2001). Generally, all the compositions showed a flexural strength and modulus higher than the pure PP. Several attempts have been made to correlate lignocellulosic fiber properties to wood plastic composites properties (English and Falkm, 1995; Bledzki et al., 1998; Stokke and Gardner, 2003; Stark and Rowlands, 2003; Neagu et al., 2006). Maldas et al. (1989) investigated the effect of wood species on the mechanical properties of wood/thermoplastic composites. They reported that differences in morphology, density, and aspect ratios across wood species account for varying reinforcement properties in thermoplastic composites. 3.3. Impact strength The Izod impact tests were conducted at room temperature. The notched specimens were tested and Fig. 5 shows the Izod impact strengths of the composites made with the different coupling agent content. Like the tensile and flexural properties, the Izod impact strength of composites increased with the increase in the coupling agent content. The Izod impact strengths of the composites made with the two different grade of MAPP are almost the same. In the composites made with G-3003, the impact strength at 1.5 wt.% of coupling agent content is almost the same level as 2.5 wt.%. Lignocellulosic fiber is a kind of stiff organic filler, so adding fiber could decrease the impact strength of composite. Composites treated with coupling agent exhibited better impact strength than
105
40 Sunflower stalk
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36 Impact (J/m)
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2800
75 60 45
32 28 24
30
20 0
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0
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Fig. 3. Comparison of flexural strength of composites as function of the coupling agent content and grade.
0
1.5
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G-3216
1.5
2.5
G-3003 Coupling agent content (wt %)
Fig. 5. Comparison of notched Izod impact strength of composites as function of the coupling agent content and grade.
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Fig. 6. SEM micrographs of the tensile fracture surfaces at 2.5 wt.% MAPP grade G-3216 with magnification of 500. (a) Sunflower stalk; (b) corn stalk and (c) bagasse.
the untreated ones since the debonding behavior between the interface of fiber and PP matrix absorb larger impact energy in modified composites than the unmodified ones. 3.4. Morphology characteristics SEM is an effective media for the morphological investigations of the composites. Through SEM study, the distribution and compatibility between the fiber and the matrix could be observed. The tensile fracture surfaces of the composites treated with 2.5 wt.% MAPP grade G-3216 are shown in Fig. 6. In the case of the composite made with the bagasse, the filler particles are well dispersed in the matrix polymer, as compared with the composite made with the sunflower stalk and corn stalk. There are some voids where the fibers have been pulled-out. The presence of these voids means that the interfacial bonding between the fiber and the matrix polymer is weak. All of the micrographs of the samples treated with G-3216 show less voids, as compared with those treated with the G-3003. This result could be related to poor mixing of the fibers and matrix due to high melt viscosity of the later at 190 °C (Table 2). 4. Conclusions Based on the results of this study the following conclusions can be drawn. (1) Mechanical properties of the composites treated with both grades of coupling agents were significantly superior to those of untreated ones, due to the stronger interfacial bonding between the fiber and the matrix polymer. (2) The tensile strength and modulus of the composite improved with increasing coupling agent content, due to the improvement of interface bond between the fiber and matrix. (3) The Izod impact strengths of the MAPP incorporated composites are slightly higher following the incorporation of the coupling agent. (4) SEM micrographs of the fracture surfaces of the composites treated with G-3216 indicate well dispersed fillers, as compared with those made with G-3003. It could be related to the high melt viscosity of G-3003. (5) SEM study also shows that interfacial adhesion between the fiber and the matrix is improved with the addition of both grades of MAPP. (6) In general, bagasse fiber shows superior mechanical properties due to its chemical characteristics.
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