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Abstract: Rice straw (Oryza sativa Linn.) was subjected to steam treatment in an alkali solution to solubilize hemicelluloses and lignin seal surrounding the cellulose bundles. After consecutive steps of physico-chemical treatment, microfibers were fibrillated using high-shear homogenizer yielding cellulose nanofibers with an average diameter of 40 nm. Isolated cellulose fibers contained 87 %w/w and 61 %w/w holo-cellulose and alpha-cellulose, respectively. For biocomposites formation, a set of experiments was performed to investigate the influence of cellulose nanofibers on mechanical and physical properties of biocomposites formed by compression molding technique. It was found that starch biocomposites made from 50 %w/w cassava starch and 6 %w/w glycerol provided a good result on shape stability with relatively high modulus and tensile strength. Adding 30 %w/w cellulose nanofibers increased tensile strength and modulus of biocomposites up to 36%. Since more energy is required to degrade polymeric glucose chains of cellulose compared with starch and glycerol, thermogravimetric analysis (TGA) showed that adding 30 %w/w fibers enhanced the decomposition temperature of biocomposites for approximately 10 ?C. Scanning electron microscope (SEM) images illustrated alignment of cellulose fibers on the surface of biocomposites.
Key words: Starch biocomposite foam, compression molding technique, cellulose nanofibers, SEM, TEM, TGA.
Many attempts have been made on using starch as a matrix for foam formation materials using compression molding techniques [3-10] and extrusion techniques[10-12]. However, low moisture resistance, flexibility, strength and high brittleness of foams made from starch have been reported. The influence of (1) additives e.g., calcium carbonate, magnesium stearate, monosterayl citrate, sunflower proteins, (2) plasticizers e.g., polyvinyl alcohol, natural rubber latex, and (3) reinforcing agents e.g. aspen, jute and flax fibers have also been widely studied for the preparation of starch foam in order to improve its mechanical and physical properties by increasing strength and flexibility of composite foam [6, 7, 13-16]. In this work, we studied the chemical and morphology of nanofibers isolated from rice straw by applying a high shear homogenization technique and its influence on mechanical properties of starch composite foam using thermal compression molding technique.
3.1 Cellulose Nanofibers from Rice Straw
The chemical composition of the treated rice straw fibers at each step is shown in Table 1. It is known that an alkali treatment significantly assists the removal of high molecular weight lignin and hemicelluloses [20, 21]. Moreover, swelling effects in an alkaline environment may reduce the crystalline structure of the cellulose [22], thus the lignin and hemicelluloses content decreases in dilute alkali compared with untreated rice straw, from 28.4% and 31.6% to 25.7% and 26.6 %w/w, respectively. The alpha-cellulose content was significantly higher while the lignin content noticeably decreased as alkaline hydrolysis acts on the ester bonds between the hemicelluloses and lignin and leads to solubilisation of lignin [23]. The subsequent step by mercerization with a strong alkali solution with steam exposure in an autoclave at 121 ?C, with a pressure of 15 lb for 4 h considerably enhanced the removal of lignin and hemicelluloses from the straw fibers, therefore 71.8 %w/w of holocellulose or polysaccharides in the treated fibers was achieved. Acid pretreatment of lignocellulosic materials rendered cleavage of hemicelluloses and amorphous moieties of cellulose [24]. In the present work, acid hydrolysis excited with ultrasonic energy increased the holocellulose and alpha-cellulose content in the fibers to 86.5% and 61.4 %w/w respectively while the lignin content was diminished to 12.5 %w/w. Consequently,
dilute acid treatment assisted in the degradation of hemicelluloses and led to the liberation of cellulose bundles. After multi-step chemical treatments, native cellulose fiber bundles with average size of 25-125 ?m[35] were reduced to 3-10 ?m with average length of 400 to thousands of microns as illustrated by the SEM image in Fig. 2A. Further mechanical treatment by using high shear homogenization helped to defibrillate the cellulose fibers to nanoscale. The TEM image illustrates the size of the nanofibers obtained from the high shear homogenization of the chemically treated microfibers, which varied from 5 to 100 nm in a diameter and several microns in length (Fig. 2B).
3.4 Starch Foam Forming, Reinforced with Cellulose Nanofibers
Cassava starch was used as a matrix for biodegradable biocomposites as it is in the majority of Thailand. However, cassava starch granules have an average diameter of 8-22 ?m, which is bigger than that of rice starch (2-4 ?m) that would probably lead to rather inferior quality and mechanical properties compared to the smaller granular starch [36]. Nevertheless, it was previously reported that biodegradable foam made of high amylopectin content starch (waxy starches), particularly potato starch, had a lower density, normal shape and better foaming characteristic than foam made of cereal starches [7]. Because of its high amylopectin content relative to rice starch, cassava starch foam seemingly showed good characteristics for rigid and rectangular foam. To get better starch foam with high strength, the effect of adding cellulose nanofibers isolated from rice straw was investigated since fibers play a crucial role in composites in supporting all the main load and limit deformations and increase the overall strength, stiffness, toughness as well as decreasing corrosion creep and fatigue [37]. A natural plasticizer, glycerol, was also added, not only to plasticize the starch foam but also to act as a dispersing agent and make it compatible with the starch and thermoplastic interfacial interactions.
Starch foam formation was separated into two steps: First, the starch batter, including starch, glycerol, fibers and water, were mixed together and afterward heated to the gelatinization temperature (90 ?C) until the viscosity of the batter was constant. In this step, water, bonded to the starch matrix, was slightly evaporated and thus the gelatinized starch paste easily filled the mold ready for compression foaming. The second step of the foam formation was the rapid water evaporation using the compression molding technique at a high temperature (210 ?C) where the plasticized starch significantly expanded to form rigid foam after the entrapped water escaped from around the edges of the mold and left a pore distribution inside the baked starch foam. Batter containing a higher proportion of glycerol, relative to starch, was not able to form any shape using the compression molding technique as the mixture was at its least viscous (data not shown).
3.4.1 Young’s Modulus
Young’s modulus is proportion of stress to strain thus indicating the elasticity, in other words the stiffness of the materials. The higher its value, the more resistant the material is to being stretched [37]. It has been found that the Young’s modulus of starch foam substantially decreased when the glycerol content increased from 6 to 50% (Table 2), however these results were obtained only from batters containing 50 %w/w cassava starch. In contrast, when the compression period was 80 s, an increase of starch content from 25 to 50% w/w substantially enhanced modulus from 1.96 to 20.35 MPa and 4.33 to 27.77 MPa (P < 0.05) for starch foam in the absence and presence of fibers, respectively. An increase in the cassava starch content in the ingredients caused a significant increase in the stiffness of the starch foam. Therefore, the most appropriate ratio of starch-to-glycerol yielding high modulus value was 50:6 by weight at 80-s formation period.
Adding 30% fibers at the formation period of 80 s for foam formula containing 50% starch significantly enhanced the modulus of the starch foam from 20.35 to 27.77 MPa (36.5%). However, the modulus decreased by ca. 60% when increasing compression period from 80 to 140 s. Thus, the addition of fibers in the batter assisted in enhancing the elasticity of starch foam, represented as a modulus, when compression molding duration was 80s.
3.4.2 Tensile Strength
Highest tensile strength of biocomposite foam (380 kPa) was achieved from biocomposite made from 50% starch and 6% glycerol at 80s with addition of 30% fibers. For starch composite foam produced with an 80-s foam forming period, tensile strength increased significantly ca. 500-900% when the starch content increased from 25% to 50%. In contrast the tensile strength decreased with an increase of starch content for the 140-s foam formation period (unpublished data). The reason was possibly due to too long thermal compression period that led to the arrangement of
significant different among all specimens. Adding fibers decreased the moisture absorption capacity, in contrast, at 50 %w/w starch content, adding fibers increased the moisture absorption capacity to some extent. The moisture absorption was not significantly influenced by the compression molding period.
The results in Fig. 3A demonstrated that for starch foam made of 25 %w/w cassava starch and 6 %w/w glycerol, when adding 30 %w/w nanofibers the moisture absorption of biocomposite foam was reduced from 14.5 to 12.5% after keeping it at 75% relative humidity for 13 days. Adding 30 %w/w nanofibers into the batter ingredients of starch foam, made of 50 %w/w cassava starch and 6 %w/w glycerol when formation period was 80 s, on the other hand had insignificant effect on moisture absorption. Similar results were reported that a fiber concentration between 10% to 20 %w/w improved the mechanical properties, but increased the water absorption capacity of the material by at least 15% [15]. However, starch foam made of 50 %w/w starch absorbed less moisture compared with that made of 25 %w/w starch for both cases with and without adding fibers when the glycerol content was 6 %w/w and the compression time was 80 s.
The influence of the glycerol content in the foam composition was illustrated in Fig. 3(B). An increase in glycerol from 6% to 50 %w/w enhanced moisture absorption into the starch foam up to 30%, which was obtained when 30 %w/w fibers were added since glycerol contains three hydroxyl groups and an
containing fibers (Fig. 4B). However, the lower density of starch foam obtained after adding cellulose nanofibers was possibly due to the escape of air inside the fiber while making the foam using the compression molding technique [38].
3.4.5 TGA
Starch/glycerol foam had the onset of degradation at 280.4 ?C and the end of degradation at 325 ?C whereas starch/glycerol in the presence of 30 %w/w cellulose fiber had an onset of degradation at 297.5?C and the end of degradation at 380 ?C as illustrated in Fig. 5. The decomposition temperatures of glycerol and cassava starch were 240 ?C and 290 ?C, respectively[39], while cellulose required more energy to break down polymeric glucose chains, thus the degradation temperature ranged from 315 to 400 ?C [40]. The thermogravimetric analysis of isolated fibers showed that the degradation of cellulose had the beginning of degradation at 287 ?C (data not shown), therefore adding cellulose nanofibers into starch composite foam notably broadened the degradation temperature of the biocomposite foam and the temperature at the end of thermal degradation was increased. The degradation temperature was increased for approximately 10 ?C when adding 30% fibers. The reason was mainly due to an accumulation of glycerol on the surface of cellulose nanofibers/amylopectin interracial zone which amend the ability of amylopectin chain to crystalline leading to the formation of possible crystalline zone around the fiber [17]. The residue after heating at 500 ?C was
a baking process, Journal of Food Engineering 85 (2008) 435-443.
[16] N. Soykeabkaew, P. Supaphol, R. Rujiravanit, Preparation and characterization of jute- and flax-reinforced starch-based composite foams, Carbohydrate Polymers 58(2004) 53-63.
[17] A. Kaushik, M. Singh, G. Verma, Green nanocomposites based on thermoplastic starch and steam exploded cellulose nanofibrils from wheat straw, Carbohydrate Polymers 82 (2010) 337-345.
[18] A. Alemdar, M. Sain, Isolation and characterization of nanofibers from agricultural residues, Wheat straw and soy hulls, Bioresource Technology 99 (2008) 1664-1671.
[19] B. Zobel, R. McElvee, Variation of cellulose in loblolly pine, Tappi 49 (1966) 383-387.
[20] B. Xiao, X.F. Sun, R. Sun, Chemical, structural, and thermal characterizations of alkali-soluble lignins and hemicelluloses, and cellulose from maize stems, rye straw, and rice straw, Polymer Degradation and Stability 74(2001) 307-319.
[21] R. Zuluaga, J.L. Putaux, J. Cruz, J. Vélez, I. Mondragon, P. Ga?án, Cellulose microfibrils from banana rachis: Effect of alkaline treatments on structural and morphological features, Carbohydrate Polymers 76 (2009) 51-59.
[22] C. Krongtaew, K. Messner, T. Ters, K. Fackler, Characterization of key parameters for biotechnological lignocellulose conversion assessed by FT-NIR spectroscopy: Part I. Qualitative analysis of pretreated straw BioResources 5 (2010) 2063-2080.
[23] E. Sj?str?m, Carbohydrate degradation products from alkaline treatment of biomass, Biomass and Bioenergy 1(1991) 61-64.
[24] G. Sanchez, L. Pilcher, C. Roslander, T. Modig, M. Galbe, G. Liden, Dilute-acid hydrolysis for fermentation of the Bolivian straw material Paja Brava, Bioresource Technology 93 (2004) 249.
[25] M. Sain, S. Panthapulakkal, Bioprocess preparation of wheat straw fibers and their characterization, Industrial Crops and Products 23 (2006) 1-8.
[26] X.F. Sun, F. Xu, R.C. Sun, P. Fowler, M.S. Baird, Characteristics of degraded cellulose obtained from steam-exploded wheat straw, Carbohydrate Research 340(2005) 97-106.
[27] B.M. Cherian, A.L. Le?o, S.F. de Souza, S. Thomas, L.A. Pothan, M. Kottaisamy, Isolation of nanocellulose from pineapple leaf fibres by steam explosion, Carbohydrate Polymers 81 (2010) 720-725.
[28] K.E.Z. Waleed, M.I. Maha, Synthesis and characterization of cellulose resins, Polymers for Advanced Technologies 14 (2003) 623-631.
[29] Y. He, Y. Pang, Y. Liu, X. Li, K. Wang, Physicochemical characterization of rice straw pretreated with sodium hydroxide in the solid state for enhancing biogas production, Energy and Fuels 22 (2008) 2775-2781.
[30] Y. Kataoka, T. Kondo, FT-IR microscopic analysis of changing cellulose crystalline structure during wood cell wall formation, Macromolecules 31 (1998) 760-764.
[31] R. Bod?rl?u, C.A. Teac?, Fourier transform infrared spectroscopy and thermal analysis of lignocellulose fillers treated with organic anhydrides, Romanian Journal of Physics 54 (2009) 93-104.
[32] P. Ga?án, J. Cruz, S. Garbizu, A. Arbelaiz, I. Mondragon, Stem and bunch banana fibers from cultivation wastes: Effect of treatments on physico-chemical behavior, Journal of Applied Polymer Science 94 (2004) 1489-1495.
[33] R.P. Chandra, R. Bura, W.E. Mabee, A. Berlin, X. Pan, J.N. Saddler, Substrate pretreatment: The key to effective enzymatic hydrolysis of lignocellulosics? Adv. Biochem. Eng. Biotechnol. 108 (2007) 67-93.
[34] C. Krongtaew, K. Messner, T. Ters, K. Fackler, Characterization of key parameters for biotechnological lignocellulose conversion assessed by FT-NIR spectroscopy: Part II. Quantitative analysis by partial least square regression, Bioresources 5 (2010) 2081-2096.
[35] P.R. Hornsby, E. Hinrichsen, K. Tarverdi, Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres: Part I. Fibre characterization, Journal of Materials Science 32 (1997) 443-449.
[36] K. Sriroth, V. Santisopasri, C. Petchalanuwat, K. Kurotjanawong, K. Piyachomkwan, C.G. Oates, Cassava starch granule structure-function properties: influence of time and conditions at harvest on four cultivars of cassava starch, Carbohydrate Polymers 38 (1999) 161-170.
[37] L.H. Sperling, Introduction to Physical Polymer Science, Fourth ed., John Wiley and Sons, Inc., New Jersey, 2006.
[38] M. Funabashi, M. Kunioka, Biodegradable composites of poly(lactic acid) with cellulose fibers polymerized by aluminum triflate, Macromolecular Symposia 224 (2005) 309-321.
[39] N.L. García, L. Ribba, A. Dufresne, M.I. Aranguren, S. Goyanes, Physico-mechanical properties of biodegradable starch nanocomposites, Macromolecular Materials and Engineering 294 (2009) 169-177.
[40] H. Yang, R. Yan, H. Chen, D.H. Lee, C. Zheng, Characteristics of hemicellulose, cellulose and lignin pyrolysis, Fuel 86 (2007) 1781-1788.
Key words: Starch biocomposite foam, compression molding technique, cellulose nanofibers, SEM, TEM, TGA.
Many attempts have been made on using starch as a matrix for foam formation materials using compression molding techniques [3-10] and extrusion techniques[10-12]. However, low moisture resistance, flexibility, strength and high brittleness of foams made from starch have been reported. The influence of (1) additives e.g., calcium carbonate, magnesium stearate, monosterayl citrate, sunflower proteins, (2) plasticizers e.g., polyvinyl alcohol, natural rubber latex, and (3) reinforcing agents e.g. aspen, jute and flax fibers have also been widely studied for the preparation of starch foam in order to improve its mechanical and physical properties by increasing strength and flexibility of composite foam [6, 7, 13-16]. In this work, we studied the chemical and morphology of nanofibers isolated from rice straw by applying a high shear homogenization technique and its influence on mechanical properties of starch composite foam using thermal compression molding technique.
3.1 Cellulose Nanofibers from Rice Straw
The chemical composition of the treated rice straw fibers at each step is shown in Table 1. It is known that an alkali treatment significantly assists the removal of high molecular weight lignin and hemicelluloses [20, 21]. Moreover, swelling effects in an alkaline environment may reduce the crystalline structure of the cellulose [22], thus the lignin and hemicelluloses content decreases in dilute alkali compared with untreated rice straw, from 28.4% and 31.6% to 25.7% and 26.6 %w/w, respectively. The alpha-cellulose content was significantly higher while the lignin content noticeably decreased as alkaline hydrolysis acts on the ester bonds between the hemicelluloses and lignin and leads to solubilisation of lignin [23]. The subsequent step by mercerization with a strong alkali solution with steam exposure in an autoclave at 121 ?C, with a pressure of 15 lb for 4 h considerably enhanced the removal of lignin and hemicelluloses from the straw fibers, therefore 71.8 %w/w of holocellulose or polysaccharides in the treated fibers was achieved. Acid pretreatment of lignocellulosic materials rendered cleavage of hemicelluloses and amorphous moieties of cellulose [24]. In the present work, acid hydrolysis excited with ultrasonic energy increased the holocellulose and alpha-cellulose content in the fibers to 86.5% and 61.4 %w/w respectively while the lignin content was diminished to 12.5 %w/w. Consequently,
dilute acid treatment assisted in the degradation of hemicelluloses and led to the liberation of cellulose bundles. After multi-step chemical treatments, native cellulose fiber bundles with average size of 25-125 ?m[35] were reduced to 3-10 ?m with average length of 400 to thousands of microns as illustrated by the SEM image in Fig. 2A. Further mechanical treatment by using high shear homogenization helped to defibrillate the cellulose fibers to nanoscale. The TEM image illustrates the size of the nanofibers obtained from the high shear homogenization of the chemically treated microfibers, which varied from 5 to 100 nm in a diameter and several microns in length (Fig. 2B).
3.4 Starch Foam Forming, Reinforced with Cellulose Nanofibers
Cassava starch was used as a matrix for biodegradable biocomposites as it is in the majority of Thailand. However, cassava starch granules have an average diameter of 8-22 ?m, which is bigger than that of rice starch (2-4 ?m) that would probably lead to rather inferior quality and mechanical properties compared to the smaller granular starch [36]. Nevertheless, it was previously reported that biodegradable foam made of high amylopectin content starch (waxy starches), particularly potato starch, had a lower density, normal shape and better foaming characteristic than foam made of cereal starches [7]. Because of its high amylopectin content relative to rice starch, cassava starch foam seemingly showed good characteristics for rigid and rectangular foam. To get better starch foam with high strength, the effect of adding cellulose nanofibers isolated from rice straw was investigated since fibers play a crucial role in composites in supporting all the main load and limit deformations and increase the overall strength, stiffness, toughness as well as decreasing corrosion creep and fatigue [37]. A natural plasticizer, glycerol, was also added, not only to plasticize the starch foam but also to act as a dispersing agent and make it compatible with the starch and thermoplastic interfacial interactions.
Starch foam formation was separated into two steps: First, the starch batter, including starch, glycerol, fibers and water, were mixed together and afterward heated to the gelatinization temperature (90 ?C) until the viscosity of the batter was constant. In this step, water, bonded to the starch matrix, was slightly evaporated and thus the gelatinized starch paste easily filled the mold ready for compression foaming. The second step of the foam formation was the rapid water evaporation using the compression molding technique at a high temperature (210 ?C) where the plasticized starch significantly expanded to form rigid foam after the entrapped water escaped from around the edges of the mold and left a pore distribution inside the baked starch foam. Batter containing a higher proportion of glycerol, relative to starch, was not able to form any shape using the compression molding technique as the mixture was at its least viscous (data not shown).
3.4.1 Young’s Modulus
Young’s modulus is proportion of stress to strain thus indicating the elasticity, in other words the stiffness of the materials. The higher its value, the more resistant the material is to being stretched [37]. It has been found that the Young’s modulus of starch foam substantially decreased when the glycerol content increased from 6 to 50% (Table 2), however these results were obtained only from batters containing 50 %w/w cassava starch. In contrast, when the compression period was 80 s, an increase of starch content from 25 to 50% w/w substantially enhanced modulus from 1.96 to 20.35 MPa and 4.33 to 27.77 MPa (P < 0.05) for starch foam in the absence and presence of fibers, respectively. An increase in the cassava starch content in the ingredients caused a significant increase in the stiffness of the starch foam. Therefore, the most appropriate ratio of starch-to-glycerol yielding high modulus value was 50:6 by weight at 80-s formation period.
Adding 30% fibers at the formation period of 80 s for foam formula containing 50% starch significantly enhanced the modulus of the starch foam from 20.35 to 27.77 MPa (36.5%). However, the modulus decreased by ca. 60% when increasing compression period from 80 to 140 s. Thus, the addition of fibers in the batter assisted in enhancing the elasticity of starch foam, represented as a modulus, when compression molding duration was 80s.
3.4.2 Tensile Strength
Highest tensile strength of biocomposite foam (380 kPa) was achieved from biocomposite made from 50% starch and 6% glycerol at 80s with addition of 30% fibers. For starch composite foam produced with an 80-s foam forming period, tensile strength increased significantly ca. 500-900% when the starch content increased from 25% to 50%. In contrast the tensile strength decreased with an increase of starch content for the 140-s foam formation period (unpublished data). The reason was possibly due to too long thermal compression period that led to the arrangement of
significant different among all specimens. Adding fibers decreased the moisture absorption capacity, in contrast, at 50 %w/w starch content, adding fibers increased the moisture absorption capacity to some extent. The moisture absorption was not significantly influenced by the compression molding period.
The results in Fig. 3A demonstrated that for starch foam made of 25 %w/w cassava starch and 6 %w/w glycerol, when adding 30 %w/w nanofibers the moisture absorption of biocomposite foam was reduced from 14.5 to 12.5% after keeping it at 75% relative humidity for 13 days. Adding 30 %w/w nanofibers into the batter ingredients of starch foam, made of 50 %w/w cassava starch and 6 %w/w glycerol when formation period was 80 s, on the other hand had insignificant effect on moisture absorption. Similar results were reported that a fiber concentration between 10% to 20 %w/w improved the mechanical properties, but increased the water absorption capacity of the material by at least 15% [15]. However, starch foam made of 50 %w/w starch absorbed less moisture compared with that made of 25 %w/w starch for both cases with and without adding fibers when the glycerol content was 6 %w/w and the compression time was 80 s.
The influence of the glycerol content in the foam composition was illustrated in Fig. 3(B). An increase in glycerol from 6% to 50 %w/w enhanced moisture absorption into the starch foam up to 30%, which was obtained when 30 %w/w fibers were added since glycerol contains three hydroxyl groups and an
containing fibers (Fig. 4B). However, the lower density of starch foam obtained after adding cellulose nanofibers was possibly due to the escape of air inside the fiber while making the foam using the compression molding technique [38].
3.4.5 TGA
Starch/glycerol foam had the onset of degradation at 280.4 ?C and the end of degradation at 325 ?C whereas starch/glycerol in the presence of 30 %w/w cellulose fiber had an onset of degradation at 297.5?C and the end of degradation at 380 ?C as illustrated in Fig. 5. The decomposition temperatures of glycerol and cassava starch were 240 ?C and 290 ?C, respectively[39], while cellulose required more energy to break down polymeric glucose chains, thus the degradation temperature ranged from 315 to 400 ?C [40]. The thermogravimetric analysis of isolated fibers showed that the degradation of cellulose had the beginning of degradation at 287 ?C (data not shown), therefore adding cellulose nanofibers into starch composite foam notably broadened the degradation temperature of the biocomposite foam and the temperature at the end of thermal degradation was increased. The degradation temperature was increased for approximately 10 ?C when adding 30% fibers. The reason was mainly due to an accumulation of glycerol on the surface of cellulose nanofibers/amylopectin interracial zone which amend the ability of amylopectin chain to crystalline leading to the formation of possible crystalline zone around the fiber [17]. The residue after heating at 500 ?C was
a baking process, Journal of Food Engineering 85 (2008) 435-443.
[16] N. Soykeabkaew, P. Supaphol, R. Rujiravanit, Preparation and characterization of jute- and flax-reinforced starch-based composite foams, Carbohydrate Polymers 58(2004) 53-63.
[17] A. Kaushik, M. Singh, G. Verma, Green nanocomposites based on thermoplastic starch and steam exploded cellulose nanofibrils from wheat straw, Carbohydrate Polymers 82 (2010) 337-345.
[18] A. Alemdar, M. Sain, Isolation and characterization of nanofibers from agricultural residues, Wheat straw and soy hulls, Bioresource Technology 99 (2008) 1664-1671.
[19] B. Zobel, R. McElvee, Variation of cellulose in loblolly pine, Tappi 49 (1966) 383-387.
[20] B. Xiao, X.F. Sun, R. Sun, Chemical, structural, and thermal characterizations of alkali-soluble lignins and hemicelluloses, and cellulose from maize stems, rye straw, and rice straw, Polymer Degradation and Stability 74(2001) 307-319.
[21] R. Zuluaga, J.L. Putaux, J. Cruz, J. Vélez, I. Mondragon, P. Ga?án, Cellulose microfibrils from banana rachis: Effect of alkaline treatments on structural and morphological features, Carbohydrate Polymers 76 (2009) 51-59.
[22] C. Krongtaew, K. Messner, T. Ters, K. Fackler, Characterization of key parameters for biotechnological lignocellulose conversion assessed by FT-NIR spectroscopy: Part I. Qualitative analysis of pretreated straw BioResources 5 (2010) 2063-2080.
[23] E. Sj?str?m, Carbohydrate degradation products from alkaline treatment of biomass, Biomass and Bioenergy 1(1991) 61-64.
[24] G. Sanchez, L. Pilcher, C. Roslander, T. Modig, M. Galbe, G. Liden, Dilute-acid hydrolysis for fermentation of the Bolivian straw material Paja Brava, Bioresource Technology 93 (2004) 249.
[25] M. Sain, S. Panthapulakkal, Bioprocess preparation of wheat straw fibers and their characterization, Industrial Crops and Products 23 (2006) 1-8.
[26] X.F. Sun, F. Xu, R.C. Sun, P. Fowler, M.S. Baird, Characteristics of degraded cellulose obtained from steam-exploded wheat straw, Carbohydrate Research 340(2005) 97-106.
[27] B.M. Cherian, A.L. Le?o, S.F. de Souza, S. Thomas, L.A. Pothan, M. Kottaisamy, Isolation of nanocellulose from pineapple leaf fibres by steam explosion, Carbohydrate Polymers 81 (2010) 720-725.
[28] K.E.Z. Waleed, M.I. Maha, Synthesis and characterization of cellulose resins, Polymers for Advanced Technologies 14 (2003) 623-631.
[29] Y. He, Y. Pang, Y. Liu, X. Li, K. Wang, Physicochemical characterization of rice straw pretreated with sodium hydroxide in the solid state for enhancing biogas production, Energy and Fuels 22 (2008) 2775-2781.
[30] Y. Kataoka, T. Kondo, FT-IR microscopic analysis of changing cellulose crystalline structure during wood cell wall formation, Macromolecules 31 (1998) 760-764.
[31] R. Bod?rl?u, C.A. Teac?, Fourier transform infrared spectroscopy and thermal analysis of lignocellulose fillers treated with organic anhydrides, Romanian Journal of Physics 54 (2009) 93-104.
[32] P. Ga?án, J. Cruz, S. Garbizu, A. Arbelaiz, I. Mondragon, Stem and bunch banana fibers from cultivation wastes: Effect of treatments on physico-chemical behavior, Journal of Applied Polymer Science 94 (2004) 1489-1495.
[33] R.P. Chandra, R. Bura, W.E. Mabee, A. Berlin, X. Pan, J.N. Saddler, Substrate pretreatment: The key to effective enzymatic hydrolysis of lignocellulosics? Adv. Biochem. Eng. Biotechnol. 108 (2007) 67-93.
[34] C. Krongtaew, K. Messner, T. Ters, K. Fackler, Characterization of key parameters for biotechnological lignocellulose conversion assessed by FT-NIR spectroscopy: Part II. Quantitative analysis by partial least square regression, Bioresources 5 (2010) 2081-2096.
[35] P.R. Hornsby, E. Hinrichsen, K. Tarverdi, Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres: Part I. Fibre characterization, Journal of Materials Science 32 (1997) 443-449.
[36] K. Sriroth, V. Santisopasri, C. Petchalanuwat, K. Kurotjanawong, K. Piyachomkwan, C.G. Oates, Cassava starch granule structure-function properties: influence of time and conditions at harvest on four cultivars of cassava starch, Carbohydrate Polymers 38 (1999) 161-170.
[37] L.H. Sperling, Introduction to Physical Polymer Science, Fourth ed., John Wiley and Sons, Inc., New Jersey, 2006.
[38] M. Funabashi, M. Kunioka, Biodegradable composites of poly(lactic acid) with cellulose fibers polymerized by aluminum triflate, Macromolecular Symposia 224 (2005) 309-321.
[39] N.L. García, L. Ribba, A. Dufresne, M.I. Aranguren, S. Goyanes, Physico-mechanical properties of biodegradable starch nanocomposites, Macromolecular Materials and Engineering 294 (2009) 169-177.
[40] H. Yang, R. Yan, H. Chen, D.H. Lee, C. Zheng, Characteristics of hemicellulose, cellulose and lignin pyrolysis, Fuel 86 (2007) 1781-1788.