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Influence of fiber surface treatment on properties of Indian grass fiber reinforced soy protein based biocomposites
Polymer 45 (2004) 7589–7596 www.elsevier.com/locate/polymer
Influence of fiber surface treatment on properties of Indian grass fiber reinforced soy protein based biocomposites Wanjun Liua, Amar K. Mohantyb, Per Askelanda, Lawrence T. Drzala, Manjusri Misraa,* a
Composite Materials and Structures Center, 2100 Engineering Building, Michigan State University, East Lansing, MI 48824, USA b The School of Packaging, Michigan State University, 130 Packaging Building, East Lansing, MI 48824, USA Received 28 June 2004; received in revised form 3 September 2004; accepted 3 September 2004
Abstract The influence of fiber treatment on the properties of biocomposites derived from grass fiber and soy based bioplastic was investigated with environmental scanning electron microscopy, thermal and mechanical properties measurements. Grass fibers were treated with alkali solution that reduced the inter-fibrillar region of the fiber by removing hemicellulose and lignin, which reduce the cementing force between fibrils. This led to a more homogenous dispersion of the biofiber in the matrix as well as increase in the aspect ratio of the fiber in the composite, resulting in an improvement in fiber reinforcement efficiency. This led to enhancement in mechanical properties including tensile and flexural properties as well as impact strength. Additionally, the alkali solution treatment increased the concentration of hydroxyl groups on the surface, which led to a better interaction between the fibers and the matrix. q 2004 Published by Elsevier Ltd. Keywords: Biocomposites; Grass fiber; Soy protein plastic
1. Introduction Recent studies have examined the development of natural fibers such as kenaf, flax, jute, hemp, and sisal, as green alternatives to conventional reinforcement materials [1]. These natural fibers have advantages of low cost, low density, acceptable specific strength properties, ease of separation, carbon dioxide sequestration and biodegradability [2]. Agricultural byproducts such as corn stalk, rice stalk and grass are emerging as new and attractive materials with commercial viability and environmental acceptability. These materials are an abundant potential resource for natural fibers since they are quite inexpensive, eco-friendly, sustainable, recyclable, and biodegradable. Generally, grasses are only used as feed for livestock, or used to modify contaminated soil [3]. Grasses when used as biofiber reinforcements for composites have the potential to replace traditional fibers. Elephant grass-based biocomposites are being * Corresponding author. Tel.: C1 517 353 5466; fax: C1 517 432 1634. E-mail address: [email protected] (M. Misra). 0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymer.2004.09.009
investigated in Europe for automotive application. Very little research about switch grass reinforced composites has been published in the area of composite materials [4]. Most native grasses such as big bluestem, little bluestem and Indian grass do not appear as additives for polymer based composites. Indian grass, belongs to the Poaceae family, is a native grass of USA and grows throughout most of North America. It is a perennial plant, and grows during the summer months. Alkali treatment is a common method to clean and modify the fiber surface to lower surface tension and enhance interfacial adhesion between a natural fiber and a polymeric matrix [5]. Several publications have appeared about the effects of alkali treatment on structure and properties of natural fibers such as kenaf, flax, jute, hemp, and sisal [6,7]. However, little research has been reported about effects of alkali solution treatment on morphology and properties of grass fiber reinforced composites. Polymers derived from biomaterials are attractive because using biomass as a source for polymer will reduce the dependence on petroleum and save energy. In particular, soy oil and soy protein can be used in non-food application
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such as the production of plastics. Chemically, soy protein is an amino acid polymer or polypeptide and soy oil is a triglyceride. Through extrusion blending, soy protein polymers can be converted to biodegradable plastics [8], whereas by functionalization of soy oil [9], thermosetting resin can be produced. Several studies on natural fiber reinforced soy based biocomposites are in the literature [10– 12], but few of them on the use of Indian grass fiber as reinforcement for composites have been made. In this paper, the influence of alkali treatment on structure morphology and properties of Indian grass fiber and its reinforced soy based biocomposites are reported.
2. Experimental 2.1. Materials Soy flour was obtained from Archer Daniels Midland (Decatur, IL). Glycerol and sodium hydroxide were supplied by J.T. Baker (Phillipsburg, NJ). Polyester amide was supplied by Bayer Corp. (Pittsburgh, PA). Indian grass was used ‘as received’ from Smith, Adams & Associates LLC (Okemos, MI). 2.2. Alkali treatment of grass fiber Chopped grass stems 20 mm in length were treated in 5 and 10% sodium hydroxide solution in water. After the appropriate treatment time, the fiber was rinsed with
distilled water until the rinse solution reached a pH of 7. After drying at room temperature for 4 days, the alkali treated and raw fibers were dried under vacuum at 80 8C for 16 h. 2.3. Sample preparation 2.3.1. Extrusion Soy flour, fiber and biodegradable polymer were dried at 80 8C under vacuum for 16 h before processing. After drying, soy flour was blended with glycerol at a ratio of 70:30. This mixture was fed into a ZSK-30 Werner and Pfleider Twin-screw Extruder (L/DZ30) with six controllable zones, at zone temperatures of 95, 105, 115, 125, 130, 130 8C and a screw speed of 100 RPM. Plasticized soy flour was re-extruded with polyester amide at a 50:50 ratio with zone temperatures of 130 8C and screw speed of 100 RPM. The pelletized soy flour based bioplastic was extruded with grass fiber under the same conditions. The feeding speeds of soy based bioplastic and grass fiber were 30 and 13 g minK1, respectively. 2.3.2. Injection molding A Cincinnati Milacron Injection Molder with an 85-ton capacity was used to make dumb-bell shaped coupons for mechanical property measurements. Soy flour based bioplastic and biocomposites were injection molded with a barrel temperature of 130 8C and a mold temperature of 20 8C.
Fig. 1. The digital pictures of raw and alkali treated fibers for (a) raw fiber, (b) grass fiber treated with 5% alkali solution for 2 h, (c) grass fiber treated with 10% alkali solution for 2 h and (d) grass fiber treated with 10% alkali solution for 4 h.
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2.3.3. Heat deflection temperature A dynamic mechanical analyzer (2980 DMA, TA instruments, USA) was used to measure the heat deflection temperature (HDT) of the biocomposites with a load of 66 psi as specified by ASTM D648. 2.3.4. Mechanical properties The tensile and flexural properties of injection molded specimens were measured with a United Testing System SFM-20 according to ASTM D638 and ASTM D790, respectively. System control and data analysis were performed using Datum software. The notch impact strength was measured with a Testing Machines Inc. 43-02-01 Monitor/Impact machine according to ASTM D256. Five specimens were measured for each sample for all mechanical property measurements. 2.3.5. FTIR Raw and alkali treated Indian grass fibers were cut to powder, then mixed them with KBr powder, compressed into a thin flake. A Perkin Elmer system 2000 Fourier transform infrared (FTIR) spectrum was used to analysis the structure of these fibers. 2.3.6. XPS A X-ray photoelectron spectroscopy spectrum (XPS, physical Electronics 5400 ESCA) was used to analyze surface composition of raw fiber, alkali solution treated grass fiber and soy plastic. 2.3.7. Morphology The morphology of raw and alkali treated fiber and their reinforced soy based biocomposites were observed with an environmental scanning electron microscopy (ESEM) Phillips Electroscan 2020 with an accelerating voltage of 20 kV.
Fig. 2. The oxygen to carbon ratio of grass fiber and soy protein plastic of (A) untreated grass fiber, (B) 5% alkali solution treated for 2 h grass fiber, (C) 10% alkali solution treated for 2 h grass fiber, (D) 10% alkali solution treated for 4 h grass fiber, (E) 10% alkali solution treated for 8 h grass fiber, and (F) soy protein plastic.
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3. Result and discussion 3.1. Structure and morphology of raw and alkali treated Indian grass fiber The digital image of raw and alkali treated fibers are shown in Fig. 1. Compared with raw grass fibers (Fig. 1(a)), the alkali solution treated grass fibers (Fig. 1(b)–(d)) appear to be finer, smaller in size and more entangled. The extent of shrinkage and entanglement increased with increasing concentration of alkali solution and treatment time. Natural fibers consist of cellulose, hemicellulose and lignin. The removal of the hemicellulose and lignin by alkali treatment causes fiber bundles to separate and shrink, which plays an important role in preparing grass fiber reinforced composites which we have explained latter in this paper. XPS spectra were taken of the grass fibers following each treatment. The surface of the fibers was examined separately. Survey scans for each material revealed carbon, oxygen, nitrogen, and calcium. Changes in oxygen:carbon atomic ratio as a function of alkali solution treatment are shown in Fig. 2. When glass fiber undergone a 2 h treatment with a 5 or 10% alkali solution, the oxygen:carbon atomic ratio in fiber surface increased significantly. This reflects that the surface structure of grass fiber was changed after alkali treatment. It is well known that natural fiber mainly consists of cellulose, hemi-cellulose and lignin [1]. Hemicellulose, is composed of a mixture of different sugars and other various substituents which are water or base soluble because it is easy to hydrolysis. Lignin has low oxygen to carbon ratio and the structure of lignin is similar to a highly unsaturated or aromatic polymer. Part of lignin is soluble in alkali solution. Therefore, it is possible that a part of hemicellulose and lignin was dissolved during alkali
Fig. 3. FTIR spectra of (a) raw fiber, (b) grass fibers treated with 5% alkali solution treated for 1 h and (c) grass fibers treated with 10% alkali solution treated for 4 h.
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solution treatment so that the content of hemicellulose and lignin on the surface of fiber decreased, leading to an increase in oxygen:carbon atomic ratio. Fig. 2 at least shows the information that the lignin and hemicellulose content on fiber surface reduced after alkali solution treatment. Changes in structure of grass fiber derived from alkali treatment were characterized by FTIR and are shown in Fig. 3. Careful examination of the spectra revealed that the structure of the fiber indeed changed after alkali treatment. First, the vibration peak at 1737 cmK1, assigned to a CaO stretching vibration of carboxylic acid or ester, disappeared due to the removal of hemicellulose. Second, the vibration peak at 1515 cmK1, assigned to the benzene ring vibration of lignin, reduced as it was removed. Third, the vibration peak at 1254 cmK1, which belongs to a C–O stretching vibration of the acetyl group in lignin component, was reduced. Additionally, a vibration peak at 2918 cmK1, belonging to the C–H stretching vibration in cellulose and hemicellulose, decreased after alkali solution treatment indicating that part of the hemicellulose was removed. These results all indicate that alkali treatment leads to the
removal of lignin and hemicellulose. This is consistent with the changes in O:C ratios on fiber surface observed by XPS. Micrographs of the raw and alkali treated Indian grass fiber are shown in Fig. 4. It was found that the macro grass fibers are composed of smaller individual single fibers held together by material in the inter-fibrillar region. After alkali solution treatment, the materials in the inter-fibrillar region reduced and decreased with increasing the concentration of alkali solution and treatment time. Based on the above XPS and FTIR results and the structure of natural fiber, the interfibrillar material should consist of hemicellulose or lignin. It is well known that hemicellulose, a branched amorphous polymer with a low degree of polymerization, is always associated with lignin through covalent bonds and interacts with cellulose by hydrogen bonding [13,14]. Therefore, the inter-fibrillar material should be the mixture of lignin and hemicellulose. Indeed, the macro grass fiber itself can be thought of as a composite material in which the matrix is a mixture of hemicellulose and lignin, and the individual micro single fiber is the reinforcement component. ESEM results show evidence for a change in the morphology of the
Fig. 4. ESEM micrographs (550!) of grass fibers with scale bar of 100 mm of raw and alkali treated Indian grass fibers for (a) raw fiber, (b) grass fiber treated with 5% alkali solution for 2 h, (c) grass fiber treated with 10% alkali solution for 2 h and (d) grass fiber treated with 10% alkali solution for 4 h.
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grass composite, which is sensitive to alkali solution because of its structure. With increasing concentration of alkali solution and treatment time, the inter-fibrillar region was more distinct and the fiber surface became smooth. When grass fibers are treated with 10% alkali solution for 4 h, the content of lignin and hemicellulose significantly decreases. This result demonstrates that after removal of hemicellulose and lignin, the interaction between fibrils became weak and the single fibers can be easily separated. 3.2. Dispersion of fiber in matrix The dispersion of grass fibers in the matrix is shown in Fig. 5. It was found that numerous fibers bunched together in raw grass fiber composite, resulting in poor fiber dispersion in the matrix. Since, the surface of the fibers was altered by the alkali treatment and the fiber was separated into micro fibers, the dispersion of the alkali treated fibers in the matrix was much better. This has the added benefit of increasing the contacting area between fiber
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and matrix and increasing the aspect ratio of the fiber so as to increase its effectiveness as reinforcement in the matrix. Generally, the dispersion of the reinforcing fiber in the matrix is determined by their chemical similarity. Though the soy based bioplastic and the cellulose backbone of the grass fiber are both quite polar, the untreated grass fiber is coated with hemicellulose/lignin mixture, which will reduce the interaction between fiber and matrix. The oxygen to carbon atom ratio of grass and soy protein plastic could be used to characterize the relative polarity of them. As shown in Fig. 2, raw grass fiber had very low O/C ratio, compared to soy protein plastic. However, alkali treated grass fiber had similar O/C ratio, compared to soy protein plastic. This indicates that alkali treated grass fiber had similar relative polarity with soy plastic, compared with raw grass. This further indicates that alkali treatment improved the compatibility between grass fiber and soy plastic. Thus, the dispersion of grass fiber in the matrix is decided by the compatibility between fiber and soy plastic. In another words, the dispersion of fiber in matrix is decided by the content of hemicellulose and lignin. After alkali treatment,
Fig. 5. ESEM micrographs of fracture surface in liquid nitrogen of (a) raw fiber (260! with scale bar of 200 mm), (b) grass fiber treated with 5% alkali solution for 2 h (260! with scale bar of 200 mm), (c) grass fiber treated with 10% alkali solution for 2 h (260! with scale bar of 200 mm) and (d) grass fiber treated with 10% alkali solution for 4 h (260! with scale bar of 150 mm) Indian grass fiber reinforced soy based composites.
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most hemicellulose and lignin were removed from grass fibers and hence the relative polarity of grass fiber was changed. Therefore, the cementing force between microfibrils is overcome during processing by the polar interaction between fiber and matrix. This resulted that the dispersion of fiber in matrix increased with increasing fiber treatment time and concentration of alkali solution. The best dispersion formed during extrusion of soy based bioplastic with fibers treated in 10% alkali solution for 4 h. 3.3. Mechanical properties of composites The value of fiber-reinforced composites mainly comes from the significant improvement in strength and modulus, which create opportunities for composite applications. The tensile properties of grass fiber reinforced soy based biocomposites are shown in Fig. 6. It was found that the tensile strength and modulus of grass fiber reinforced composites improved after fiber alkali treatment. Compared to that of the raw fiber, the strength and modulus of alkali
treated fiber should be increased [15]. This is because the content of hemicellulose and lignin decreased in this system after alkali treatment and thereby increase the effectiveness of orientated cellulose fibers. In addition, as discussed above alkali treatment improved the dispersion of fiber in matrix, which resulted increase in fiber aspect ratio. This increases the fiber’s reinforcement effectiveness and hence increases the strength of composites. Since, the size of fiber in the matrix becomes smaller, the extent of stress concentration derived from the fiber will decrease, which leads to increase in the strength. On the other hand, contacting area of fiber with matrix will increase, resulting increase in interaction between fiber and matrix. The flexural properties of grass fiber reinforced soy based biocomposites are shown in Fig. 6. It was found that bending strength and modulus of grass fiber reinforced composites increased with increasing alkali treatment time and concentration of alkali solution. When grass fiber was treated with 10% alkali for 4 h, the bending strength improved 40%. This indicates that flexural properties of
Fig. 6. Tensile and flexural properties of grass fiber reinforced soy based biocomposites of (A), soy plastic and (B) raw fiber reinforced soy plastic composites, (C) 5% alkali solution treated 2 h fiber reinforced composites, (D) 10% alkali solution treated 2 h fiber reinforced composites and (E) 10% alkali solution treated 4 h fiber reinforced composites.
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grass fiber reinforced soy based biocomposites had the same trends as the tensile properties of the composite. Impact strength of a material is a measure of the energy to break the specimen. The value of impact strength reflects the ability of a material to resist high-speed fracture. The notch Izod impact strength of grass fiber reinforced soy based biocomposites is shown in Fig. 7. It was found that impact strength of grass fiber reinforced composites improved after fiber alkali treatment though the values did not change with increasing grass fiber treatment time and alkali solution concentration. This result indicates that alkali treatment is also a good method to improve the impact strength of grass fiber reinforced soy based biocomposites. The enhancement in impact strength of the composites after fiber alkali solution treatment is mainly due to the improved dispersion of fiber in matrix that will reduce the fiber size and hence reduce the extent of stress concentration as well as reduce the crack propagation rate during the impact test. 3.4. HDT behavior HDT is denoted as the maximum temperature at which polymer can be used as a rigid material. HDT is defined as the temperature at which the deflection of the sample reaches 250 mm under an applied load of 66 psi according to
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ASTM D648. HDT behavior of grass fiber reinforced soy based biocomposites is shown in Fig. 7. It was found that HDT temperature increased more than 30% after adding 30% raw grass fiber. With addition of alkali treated grass fiber, HDT temperature still improved, compared with raw fiber reinforced soy based biocomposites. This improvement is mainly derived from the increases in modulus as well as the interaction between fiber and matrix. 3.5. Morphology of tensile fracture surface The morphology of the tensile fracture surface of raw and alkali solution treated Indian grass fibers reinforced biocomposites are shown in Fig. 8. Raw grass fiber is not well dispersed; the fiber is bunched together, with almost no matrix attached to the fiber surface. Alkali treated fiber shows a large amount of soy plastic matrix adhering to the fiber surface, indicative of good adhesion with matrix. The reason for better adhesion between fiber and matrix is that the relative content of hydroxyl group on the surface of fiber increased after alkali treatment because of the removal of hemicellulose and lignin. The increase in oxygen to carbon atom ratio of fiber surface as shown in Fig. 2 is a relative scale for increase in hydroxyl group content, which increased the interaction between fiber and matrix. The interaction includes hydrogen bonding and esterification reaction between carboxyl group or hydroxyl group in matrix and fiber, which lowers the interfacial tension between the fiber and matrix and improves the dispersion of the grass fibers in the matrix so as to improve interfacial adhesion between them and hence mechanical properties. This would be expected to have a positive effect on the impact strength and tensile strength as well.
4. Conclusion
Fig. 7. Impact strength and HDT behavior of grass fiber reinforced soy based biocomposites of (A) soy plastic and (B) raw fiber reinforced soy plastic composites, (C) 5% alkali solution treated 2 h fiber reinforced composites, (D) 10% alkali solution treated 2 h fiber reinforced composites and (E) 10% alkali solution treated 4 h fiber reinforced composites.
Alkali solution treatment of grass biofiber removes hemicellulose and lignin and allows the separation of the grass fiber into finer micro-fibers. This results in better dispersion of the grass fibers in the matrix and also reduces the fiber size. The dispersion of grass fiber in matrix improved with increasing treatment time and concentration of the alkali solution. The impact strength of alkali treated grass fiber reinforced soy based biocomposites was enhanced about 40%, compared with raw fiber reinforced soy based biocomposites. The tensile and flexural properties of alkali treated grass fiber reinforced soy based biocomposites were improved gradually with increasing alkali solution concentration and treatment time. This is because alkali treatment improved the dispersion of fiber in matrix and hence increased the fibers reinforcement efficiency in matrix. Additionally, more hydroxyl groups are present on the grass fiber surface after alkali treatment, which leads to an increased interaction between fiber and matrix.
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Fig. 8. Morphology of tensile fracture surface of (a) (120!) with scale bar of 400 mm and (b) (550!) with scale bar of 100 mm, raw fiber reinforced biocomposites, and (c) (150!) with scale bar of 300 mm and (d) (550!) with scale bar of 100 mm, grass fiber treated with 10% alkali solution for 4 h reinforced biocomposites.
Acknowledgements The partial financial support from USDA-NRI (Grant No. 2001-35504-10734) is gratefully acknowledged for this research. Authors are grateful to GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) 2002 Award No. GR02-066 for partial financial support. Authors also thanks to ADM (Decatur, IL), Bayer Corp. (Pittsburgh, PA), and Smith, Adams & Associates LLC (Okemos, MI) for their supplying soy flour, polyester amide and grass samples, respectively for this research.
References [1] Mohanty AK, Misra M, Hinrichsen G. Biofibres. Macromol Mater Eng 2000;276/277:1–24. [2] Mohanty AK, Misra M, Drzal LT. J Polym Environ 2002;10(1/2): 19–26.
[3] Phillips TA, Belden JB, Henderson KLD, Coats JR. Phytoremediation of pesticide-contaminated soil using a mixture and individual prairie grasses. In: 224th ACS National Meeting. Boston; August 2002. AGRO-068. [4] Stokke DD, Kuo M, Curry DG, Gieselman HH. Grassland flour/polyethylene composites. In: Sixth international conference on woodfiber–plastic composites, Madison; 2001. p. 43–53. [5] Bledzki AK, Gassan J. Prog Polym Sci 1999;24(2):221–74. [6] Sreenivasan S, Bahama IP, Krishna IKR. J Mater Sci 1996;31(3): 721–6. [7] Gassan J, Bledzki AK. J Appl Polym Sci 1999;71(4):623–9. [8] Drzal LT, Mohanty AK, Tummala P, Misra M. Polym Mater Sci Eng 2002;87:117–8. [9] Williams GI, Wool RP. Appl Compos Mater 2000;7(5–6):421–32. [10] Drzal LT, Mohanty AK, Misra M. Polym Prep 2001;42(2):31–2. [11] Mohanty A, Drzal LT, Hokens D, Misra M. Polym Mater Sci Eng 2001;85:594. [12] Mohanty AK, Misra M, Drzal LT. Compos Interf 2001;8(5):313–43. [13] Sun R, Lawther JM, Banks WB. J Appl Polym Sci 1996;62(9): 1473–81. [14] Migne C, Prensier G, Cornu A, Grenet E. Biol Cell 1996;88(3): 137–44. [15] Ray D, Sarkar BK. J Appl Polym Sci 2001;80(7):1013–20.
Influence of fiber surface treatment on properties of Indian grass fiber reinforced soy protein based biocomposites Wanjun Liua, Amar K. Mohantyb, Per Askelanda, Lawrence T. Drzala, Manjusri Misraa,* a
Composite Materials and Structures Center, 2100 Engineering Building, Michigan State University, East Lansing, MI 48824, USA b The School of Packaging, Michigan State University, 130 Packaging Building, East Lansing, MI 48824, USA Received 28 June 2004; received in revised form 3 September 2004; accepted 3 September 2004
Abstract The influence of fiber treatment on the properties of biocomposites derived from grass fiber and soy based bioplastic was investigated with environmental scanning electron microscopy, thermal and mechanical properties measurements. Grass fibers were treated with alkali solution that reduced the inter-fibrillar region of the fiber by removing hemicellulose and lignin, which reduce the cementing force between fibrils. This led to a more homogenous dispersion of the biofiber in the matrix as well as increase in the aspect ratio of the fiber in the composite, resulting in an improvement in fiber reinforcement efficiency. This led to enhancement in mechanical properties including tensile and flexural properties as well as impact strength. Additionally, the alkali solution treatment increased the concentration of hydroxyl groups on the surface, which led to a better interaction between the fibers and the matrix. q 2004 Published by Elsevier Ltd. Keywords: Biocomposites; Grass fiber; Soy protein plastic
1. Introduction Recent studies have examined the development of natural fibers such as kenaf, flax, jute, hemp, and sisal, as green alternatives to conventional reinforcement materials [1]. These natural fibers have advantages of low cost, low density, acceptable specific strength properties, ease of separation, carbon dioxide sequestration and biodegradability [2]. Agricultural byproducts such as corn stalk, rice stalk and grass are emerging as new and attractive materials with commercial viability and environmental acceptability. These materials are an abundant potential resource for natural fibers since they are quite inexpensive, eco-friendly, sustainable, recyclable, and biodegradable. Generally, grasses are only used as feed for livestock, or used to modify contaminated soil [3]. Grasses when used as biofiber reinforcements for composites have the potential to replace traditional fibers. Elephant grass-based biocomposites are being * Corresponding author. Tel.: C1 517 353 5466; fax: C1 517 432 1634. E-mail address: [email protected] (M. Misra). 0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymer.2004.09.009
investigated in Europe for automotive application. Very little research about switch grass reinforced composites has been published in the area of composite materials [4]. Most native grasses such as big bluestem, little bluestem and Indian grass do not appear as additives for polymer based composites. Indian grass, belongs to the Poaceae family, is a native grass of USA and grows throughout most of North America. It is a perennial plant, and grows during the summer months. Alkali treatment is a common method to clean and modify the fiber surface to lower surface tension and enhance interfacial adhesion between a natural fiber and a polymeric matrix [5]. Several publications have appeared about the effects of alkali treatment on structure and properties of natural fibers such as kenaf, flax, jute, hemp, and sisal [6,7]. However, little research has been reported about effects of alkali solution treatment on morphology and properties of grass fiber reinforced composites. Polymers derived from biomaterials are attractive because using biomass as a source for polymer will reduce the dependence on petroleum and save energy. In particular, soy oil and soy protein can be used in non-food application
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such as the production of plastics. Chemically, soy protein is an amino acid polymer or polypeptide and soy oil is a triglyceride. Through extrusion blending, soy protein polymers can be converted to biodegradable plastics [8], whereas by functionalization of soy oil [9], thermosetting resin can be produced. Several studies on natural fiber reinforced soy based biocomposites are in the literature [10– 12], but few of them on the use of Indian grass fiber as reinforcement for composites have been made. In this paper, the influence of alkali treatment on structure morphology and properties of Indian grass fiber and its reinforced soy based biocomposites are reported.
2. Experimental 2.1. Materials Soy flour was obtained from Archer Daniels Midland (Decatur, IL). Glycerol and sodium hydroxide were supplied by J.T. Baker (Phillipsburg, NJ). Polyester amide was supplied by Bayer Corp. (Pittsburgh, PA). Indian grass was used ‘as received’ from Smith, Adams & Associates LLC (Okemos, MI). 2.2. Alkali treatment of grass fiber Chopped grass stems 20 mm in length were treated in 5 and 10% sodium hydroxide solution in water. After the appropriate treatment time, the fiber was rinsed with
distilled water until the rinse solution reached a pH of 7. After drying at room temperature for 4 days, the alkali treated and raw fibers were dried under vacuum at 80 8C for 16 h. 2.3. Sample preparation 2.3.1. Extrusion Soy flour, fiber and biodegradable polymer were dried at 80 8C under vacuum for 16 h before processing. After drying, soy flour was blended with glycerol at a ratio of 70:30. This mixture was fed into a ZSK-30 Werner and Pfleider Twin-screw Extruder (L/DZ30) with six controllable zones, at zone temperatures of 95, 105, 115, 125, 130, 130 8C and a screw speed of 100 RPM. Plasticized soy flour was re-extruded with polyester amide at a 50:50 ratio with zone temperatures of 130 8C and screw speed of 100 RPM. The pelletized soy flour based bioplastic was extruded with grass fiber under the same conditions. The feeding speeds of soy based bioplastic and grass fiber were 30 and 13 g minK1, respectively. 2.3.2. Injection molding A Cincinnati Milacron Injection Molder with an 85-ton capacity was used to make dumb-bell shaped coupons for mechanical property measurements. Soy flour based bioplastic and biocomposites were injection molded with a barrel temperature of 130 8C and a mold temperature of 20 8C.
Fig. 1. The digital pictures of raw and alkali treated fibers for (a) raw fiber, (b) grass fiber treated with 5% alkali solution for 2 h, (c) grass fiber treated with 10% alkali solution for 2 h and (d) grass fiber treated with 10% alkali solution for 4 h.
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2.3.3. Heat deflection temperature A dynamic mechanical analyzer (2980 DMA, TA instruments, USA) was used to measure the heat deflection temperature (HDT) of the biocomposites with a load of 66 psi as specified by ASTM D648. 2.3.4. Mechanical properties The tensile and flexural properties of injection molded specimens were measured with a United Testing System SFM-20 according to ASTM D638 and ASTM D790, respectively. System control and data analysis were performed using Datum software. The notch impact strength was measured with a Testing Machines Inc. 43-02-01 Monitor/Impact machine according to ASTM D256. Five specimens were measured for each sample for all mechanical property measurements. 2.3.5. FTIR Raw and alkali treated Indian grass fibers were cut to powder, then mixed them with KBr powder, compressed into a thin flake. A Perkin Elmer system 2000 Fourier transform infrared (FTIR) spectrum was used to analysis the structure of these fibers. 2.3.6. XPS A X-ray photoelectron spectroscopy spectrum (XPS, physical Electronics 5400 ESCA) was used to analyze surface composition of raw fiber, alkali solution treated grass fiber and soy plastic. 2.3.7. Morphology The morphology of raw and alkali treated fiber and their reinforced soy based biocomposites were observed with an environmental scanning electron microscopy (ESEM) Phillips Electroscan 2020 with an accelerating voltage of 20 kV.
Fig. 2. The oxygen to carbon ratio of grass fiber and soy protein plastic of (A) untreated grass fiber, (B) 5% alkali solution treated for 2 h grass fiber, (C) 10% alkali solution treated for 2 h grass fiber, (D) 10% alkali solution treated for 4 h grass fiber, (E) 10% alkali solution treated for 8 h grass fiber, and (F) soy protein plastic.
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3. Result and discussion 3.1. Structure and morphology of raw and alkali treated Indian grass fiber The digital image of raw and alkali treated fibers are shown in Fig. 1. Compared with raw grass fibers (Fig. 1(a)), the alkali solution treated grass fibers (Fig. 1(b)–(d)) appear to be finer, smaller in size and more entangled. The extent of shrinkage and entanglement increased with increasing concentration of alkali solution and treatment time. Natural fibers consist of cellulose, hemicellulose and lignin. The removal of the hemicellulose and lignin by alkali treatment causes fiber bundles to separate and shrink, which plays an important role in preparing grass fiber reinforced composites which we have explained latter in this paper. XPS spectra were taken of the grass fibers following each treatment. The surface of the fibers was examined separately. Survey scans for each material revealed carbon, oxygen, nitrogen, and calcium. Changes in oxygen:carbon atomic ratio as a function of alkali solution treatment are shown in Fig. 2. When glass fiber undergone a 2 h treatment with a 5 or 10% alkali solution, the oxygen:carbon atomic ratio in fiber surface increased significantly. This reflects that the surface structure of grass fiber was changed after alkali treatment. It is well known that natural fiber mainly consists of cellulose, hemi-cellulose and lignin [1]. Hemicellulose, is composed of a mixture of different sugars and other various substituents which are water or base soluble because it is easy to hydrolysis. Lignin has low oxygen to carbon ratio and the structure of lignin is similar to a highly unsaturated or aromatic polymer. Part of lignin is soluble in alkali solution. Therefore, it is possible that a part of hemicellulose and lignin was dissolved during alkali
Fig. 3. FTIR spectra of (a) raw fiber, (b) grass fibers treated with 5% alkali solution treated for 1 h and (c) grass fibers treated with 10% alkali solution treated for 4 h.
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solution treatment so that the content of hemicellulose and lignin on the surface of fiber decreased, leading to an increase in oxygen:carbon atomic ratio. Fig. 2 at least shows the information that the lignin and hemicellulose content on fiber surface reduced after alkali solution treatment. Changes in structure of grass fiber derived from alkali treatment were characterized by FTIR and are shown in Fig. 3. Careful examination of the spectra revealed that the structure of the fiber indeed changed after alkali treatment. First, the vibration peak at 1737 cmK1, assigned to a CaO stretching vibration of carboxylic acid or ester, disappeared due to the removal of hemicellulose. Second, the vibration peak at 1515 cmK1, assigned to the benzene ring vibration of lignin, reduced as it was removed. Third, the vibration peak at 1254 cmK1, which belongs to a C–O stretching vibration of the acetyl group in lignin component, was reduced. Additionally, a vibration peak at 2918 cmK1, belonging to the C–H stretching vibration in cellulose and hemicellulose, decreased after alkali solution treatment indicating that part of the hemicellulose was removed. These results all indicate that alkali treatment leads to the
removal of lignin and hemicellulose. This is consistent with the changes in O:C ratios on fiber surface observed by XPS. Micrographs of the raw and alkali treated Indian grass fiber are shown in Fig. 4. It was found that the macro grass fibers are composed of smaller individual single fibers held together by material in the inter-fibrillar region. After alkali solution treatment, the materials in the inter-fibrillar region reduced and decreased with increasing the concentration of alkali solution and treatment time. Based on the above XPS and FTIR results and the structure of natural fiber, the interfibrillar material should consist of hemicellulose or lignin. It is well known that hemicellulose, a branched amorphous polymer with a low degree of polymerization, is always associated with lignin through covalent bonds and interacts with cellulose by hydrogen bonding [13,14]. Therefore, the inter-fibrillar material should be the mixture of lignin and hemicellulose. Indeed, the macro grass fiber itself can be thought of as a composite material in which the matrix is a mixture of hemicellulose and lignin, and the individual micro single fiber is the reinforcement component. ESEM results show evidence for a change in the morphology of the
Fig. 4. ESEM micrographs (550!) of grass fibers with scale bar of 100 mm of raw and alkali treated Indian grass fibers for (a) raw fiber, (b) grass fiber treated with 5% alkali solution for 2 h, (c) grass fiber treated with 10% alkali solution for 2 h and (d) grass fiber treated with 10% alkali solution for 4 h.
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grass composite, which is sensitive to alkali solution because of its structure. With increasing concentration of alkali solution and treatment time, the inter-fibrillar region was more distinct and the fiber surface became smooth. When grass fibers are treated with 10% alkali solution for 4 h, the content of lignin and hemicellulose significantly decreases. This result demonstrates that after removal of hemicellulose and lignin, the interaction between fibrils became weak and the single fibers can be easily separated. 3.2. Dispersion of fiber in matrix The dispersion of grass fibers in the matrix is shown in Fig. 5. It was found that numerous fibers bunched together in raw grass fiber composite, resulting in poor fiber dispersion in the matrix. Since, the surface of the fibers was altered by the alkali treatment and the fiber was separated into micro fibers, the dispersion of the alkali treated fibers in the matrix was much better. This has the added benefit of increasing the contacting area between fiber
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and matrix and increasing the aspect ratio of the fiber so as to increase its effectiveness as reinforcement in the matrix. Generally, the dispersion of the reinforcing fiber in the matrix is determined by their chemical similarity. Though the soy based bioplastic and the cellulose backbone of the grass fiber are both quite polar, the untreated grass fiber is coated with hemicellulose/lignin mixture, which will reduce the interaction between fiber and matrix. The oxygen to carbon atom ratio of grass and soy protein plastic could be used to characterize the relative polarity of them. As shown in Fig. 2, raw grass fiber had very low O/C ratio, compared to soy protein plastic. However, alkali treated grass fiber had similar O/C ratio, compared to soy protein plastic. This indicates that alkali treated grass fiber had similar relative polarity with soy plastic, compared with raw grass. This further indicates that alkali treatment improved the compatibility between grass fiber and soy plastic. Thus, the dispersion of grass fiber in the matrix is decided by the compatibility between fiber and soy plastic. In another words, the dispersion of fiber in matrix is decided by the content of hemicellulose and lignin. After alkali treatment,
Fig. 5. ESEM micrographs of fracture surface in liquid nitrogen of (a) raw fiber (260! with scale bar of 200 mm), (b) grass fiber treated with 5% alkali solution for 2 h (260! with scale bar of 200 mm), (c) grass fiber treated with 10% alkali solution for 2 h (260! with scale bar of 200 mm) and (d) grass fiber treated with 10% alkali solution for 4 h (260! with scale bar of 150 mm) Indian grass fiber reinforced soy based composites.
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most hemicellulose and lignin were removed from grass fibers and hence the relative polarity of grass fiber was changed. Therefore, the cementing force between microfibrils is overcome during processing by the polar interaction between fiber and matrix. This resulted that the dispersion of fiber in matrix increased with increasing fiber treatment time and concentration of alkali solution. The best dispersion formed during extrusion of soy based bioplastic with fibers treated in 10% alkali solution for 4 h. 3.3. Mechanical properties of composites The value of fiber-reinforced composites mainly comes from the significant improvement in strength and modulus, which create opportunities for composite applications. The tensile properties of grass fiber reinforced soy based biocomposites are shown in Fig. 6. It was found that the tensile strength and modulus of grass fiber reinforced composites improved after fiber alkali treatment. Compared to that of the raw fiber, the strength and modulus of alkali
treated fiber should be increased [15]. This is because the content of hemicellulose and lignin decreased in this system after alkali treatment and thereby increase the effectiveness of orientated cellulose fibers. In addition, as discussed above alkali treatment improved the dispersion of fiber in matrix, which resulted increase in fiber aspect ratio. This increases the fiber’s reinforcement effectiveness and hence increases the strength of composites. Since, the size of fiber in the matrix becomes smaller, the extent of stress concentration derived from the fiber will decrease, which leads to increase in the strength. On the other hand, contacting area of fiber with matrix will increase, resulting increase in interaction between fiber and matrix. The flexural properties of grass fiber reinforced soy based biocomposites are shown in Fig. 6. It was found that bending strength and modulus of grass fiber reinforced composites increased with increasing alkali treatment time and concentration of alkali solution. When grass fiber was treated with 10% alkali for 4 h, the bending strength improved 40%. This indicates that flexural properties of
Fig. 6. Tensile and flexural properties of grass fiber reinforced soy based biocomposites of (A), soy plastic and (B) raw fiber reinforced soy plastic composites, (C) 5% alkali solution treated 2 h fiber reinforced composites, (D) 10% alkali solution treated 2 h fiber reinforced composites and (E) 10% alkali solution treated 4 h fiber reinforced composites.
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grass fiber reinforced soy based biocomposites had the same trends as the tensile properties of the composite. Impact strength of a material is a measure of the energy to break the specimen. The value of impact strength reflects the ability of a material to resist high-speed fracture. The notch Izod impact strength of grass fiber reinforced soy based biocomposites is shown in Fig. 7. It was found that impact strength of grass fiber reinforced composites improved after fiber alkali treatment though the values did not change with increasing grass fiber treatment time and alkali solution concentration. This result indicates that alkali treatment is also a good method to improve the impact strength of grass fiber reinforced soy based biocomposites. The enhancement in impact strength of the composites after fiber alkali solution treatment is mainly due to the improved dispersion of fiber in matrix that will reduce the fiber size and hence reduce the extent of stress concentration as well as reduce the crack propagation rate during the impact test. 3.4. HDT behavior HDT is denoted as the maximum temperature at which polymer can be used as a rigid material. HDT is defined as the temperature at which the deflection of the sample reaches 250 mm under an applied load of 66 psi according to
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ASTM D648. HDT behavior of grass fiber reinforced soy based biocomposites is shown in Fig. 7. It was found that HDT temperature increased more than 30% after adding 30% raw grass fiber. With addition of alkali treated grass fiber, HDT temperature still improved, compared with raw fiber reinforced soy based biocomposites. This improvement is mainly derived from the increases in modulus as well as the interaction between fiber and matrix. 3.5. Morphology of tensile fracture surface The morphology of the tensile fracture surface of raw and alkali solution treated Indian grass fibers reinforced biocomposites are shown in Fig. 8. Raw grass fiber is not well dispersed; the fiber is bunched together, with almost no matrix attached to the fiber surface. Alkali treated fiber shows a large amount of soy plastic matrix adhering to the fiber surface, indicative of good adhesion with matrix. The reason for better adhesion between fiber and matrix is that the relative content of hydroxyl group on the surface of fiber increased after alkali treatment because of the removal of hemicellulose and lignin. The increase in oxygen to carbon atom ratio of fiber surface as shown in Fig. 2 is a relative scale for increase in hydroxyl group content, which increased the interaction between fiber and matrix. The interaction includes hydrogen bonding and esterification reaction between carboxyl group or hydroxyl group in matrix and fiber, which lowers the interfacial tension between the fiber and matrix and improves the dispersion of the grass fibers in the matrix so as to improve interfacial adhesion between them and hence mechanical properties. This would be expected to have a positive effect on the impact strength and tensile strength as well.
4. Conclusion
Fig. 7. Impact strength and HDT behavior of grass fiber reinforced soy based biocomposites of (A) soy plastic and (B) raw fiber reinforced soy plastic composites, (C) 5% alkali solution treated 2 h fiber reinforced composites, (D) 10% alkali solution treated 2 h fiber reinforced composites and (E) 10% alkali solution treated 4 h fiber reinforced composites.
Alkali solution treatment of grass biofiber removes hemicellulose and lignin and allows the separation of the grass fiber into finer micro-fibers. This results in better dispersion of the grass fibers in the matrix and also reduces the fiber size. The dispersion of grass fiber in matrix improved with increasing treatment time and concentration of the alkali solution. The impact strength of alkali treated grass fiber reinforced soy based biocomposites was enhanced about 40%, compared with raw fiber reinforced soy based biocomposites. The tensile and flexural properties of alkali treated grass fiber reinforced soy based biocomposites were improved gradually with increasing alkali solution concentration and treatment time. This is because alkali treatment improved the dispersion of fiber in matrix and hence increased the fibers reinforcement efficiency in matrix. Additionally, more hydroxyl groups are present on the grass fiber surface after alkali treatment, which leads to an increased interaction between fiber and matrix.
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Fig. 8. Morphology of tensile fracture surface of (a) (120!) with scale bar of 400 mm and (b) (550!) with scale bar of 100 mm, raw fiber reinforced biocomposites, and (c) (150!) with scale bar of 300 mm and (d) (550!) with scale bar of 100 mm, grass fiber treated with 10% alkali solution for 4 h reinforced biocomposites.
Acknowledgements The partial financial support from USDA-NRI (Grant No. 2001-35504-10734) is gratefully acknowledged for this research. Authors are grateful to GREEEN (Generating Research and Extension to meet Economic and Environmental Needs) 2002 Award No. GR02-066 for partial financial support. Authors also thanks to ADM (Decatur, IL), Bayer Corp. (Pittsburgh, PA), and Smith, Adams & Associates LLC (Okemos, MI) for their supplying soy flour, polyester amide and grass samples, respectively for this research.
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