Biocomposites from Renewable Resources

Keywords

Protein • Matrix • Plasticizer • Chemical modification • Soybean oil • Soy flour • Soy meal • Wheat gluten • Moisture absorption • High humidity • Soil degradation

Unlike the familiar approach of using natural fibers or agricultural residues as reinforcement and synthetic polymers as matrix, attempts have been made to use matrix from renewable resources with various types of reinforcement [13Mon]. In one such attempt, wheat gluten was used as the matrix and wheat straw ground to various lengths was used as reinforcement. Wheat straw lengths obtained were 2 mm, 0.2 mm, and less than 0.2 pm. The ground wheat straw was mixed with wheat gluten with the addition of glycerol (30 %) as plasticizer, and the mixture was compression molded at 120 °C for 5 min. Some of the properties of the composites obtained are given in Table 69.1. As seen from the table, increasing the fiber content increased the strength and modulus but decreased the elongation. Similarly, impact — and ball-milled fibers provided better tensile properties than the cut-milled fibers at similar levels of fiber loading [13Mon]. Although composites made using wheat gluten and wheat straws have good tensile properties, the wheat gluten and plasti­cizer are highly hydrophilic and absorb considerable amounts of water. Such composites are expected to have poor stability at high humidities or under aqueous environments and therefore not useful for practical applications.

Wheat gluten was reinforced with coir fibers that were treated with sodium chlorite to reduce the lignin content from 42 to 21 %, and the influence of lignin on the surface morphology, fiber-matrix interaction, and properties of the composites were investigated [11Mue]. Composites were prepared by mixing the coir fibers with wheat gluten containing 35 % glycerol and compression molding at 130 °C for 15 min. Some of the properties of the fibers with various levels of lignin are shown in Tables 69.2 and 69.3. Removal of lignin decreased strength and elongation but did not affect the modulus. Gluten composites containing different

Table 69.1 Properties of biocomposites obtained using wheat gluten as matrix and wheat straw in different forms as reinforcement [13Mon] Gluten matrix Fiber content (%) Strength (MPa) Elongation (%) Modulus (MPa) 0.0 34.2 ± 2.8 2.4 ± 0.3 16.4 ± 0.7 Cut milled 1.2 33.7 ± 2.0 2.9 ± 0.1 14.4 ± 1.3 5.2 29.8 ± 3.7 1.4 ± 0.2 19.8 ± 1.1 11.1 40.9 ± 2.3 1.8 ± 0.2 25.5 ± 0.9 Impact milling 1.2 31.5 ± 2.9 1.9 ± 0.2 18.8 ± 0.6 5.1 32.0 ± 0.7 1.8 ± 0.1 20.1 ± 0.4 11.1 40.8 ± 3.1 2.0 ± 0.2 23.2 ± 0.4 Ball milled 1.2 41.7 ± 3.4 2.6 ± 0.3 18.4 ± 2.3 5.2 39.9 ± 2.6 2.3 ± 0.1 19.9 ± 0.8 11.1 35.9 ± 4.5 1.6 ± 0.2 23.7 ± 1.3

amounts of lignin did not show any appreciable change in tensile properties. It was suggested that the lowest amount of lignin (21 %) in this study was already excessive to have any noticeable difference in composite tensile properties. How­ever, a decrease in moisture sorption of the composites from 75 to 67 % was observed. Although composites were successfully developed from coir fiber- reinforced wheat gluten, the inherent hydrophilicity of wheat gluten and the pres­ence of 35 % glycerol would make the composites highly sensitive to moisture and not useful for many applications. In addition, it has been reported that at high fiber contents, the mechanical properties of wheat gluten-based composites are influenced not only due to reinforcement of fibers but also due to deplasticization of the matrix. A competition exists between the matrix and fibers for plasticizer absorption because both of them are hydrophilic that causes deplasticization and leads to poor processability of the material. It was also suggested that the level of lignin in the reinforcing fibers also influenced composite properties [08Kun].

In a similar research, coir fibers were chemically modified with sodium hydrox­ide and/or silane treated to improve interfacial adhesion and used as reinforcement for wheat gluten composites [12Hem]. Composites were obtained by compression molding at 150 °C with 15 % by weight of the coir fibers. X-ray photon spectros­copy analysis confirmed the presence of silane on the surface of the fibers, and it was also found that silane more than doubled after the alkali treatment [12Hem]. Tensile properties of the wheat gluten fibers reinforced with coir fibers with and without various treatments are shown in Table 69.4. The addition of the coir fibers showed marginal increase in the tensile properties except for the alkali — and silane-treated coir fibers which provided 27 % increase in tensile stress to failure. To further improve properties of wheat gluten-coir fiber composites, wheat gluten was toughened using thiolated poly(vinyl alcohol) [14Dia], and the tough­ened wheat gluten was used as matrix with alkali-treated and silane-coupled coir fibers. Properties of the various composites developed using toughened wheat gluten and treated coir fibers are given in Table 69.5. By comparing Tables 69.4 and 69.5, it can be seen that toughened wheat gluten provided much better properties than the unmodified wheat gluten. Morphological analysis using SEM

Table 69.2 Influence of lignin content on the composition and tensile properties of coir fiber-reinforced wheat gluten composites [llMue] Lignin (%) Cellulose (%) Hemicellulose (%) Strength (MPa) Elongation (%) Modulus (GPa) % Crystallinity 42 32.7 22.6 123 ±25 33 ±7 2.3 ±0.5 52.0 31 37.7 24.6 97 ±37 22 ±9 2.6 ±0.6 52.6 21 43.8 24.8 113 ±48 28 ±12 2.4 ± 0.6 49.1

Table 69.3 Properties of wheat gluten composites reinforced with 10 % coir fibers containing various levels of lignin [11Mue] Lignin (%) Strength (MPa) Elongation (%) Modulus (MPa) Gluten 1.7 ± 0.1 163 ± 25 5.5 ± 0.1 42 1.9 ± 0.1 33 ± 6 18 ± 2 31 1.9 ± 0.1 23 ± 6 23 ± 3 21 1.8 ± 0.1 30 ± 3 19 ± 1

Table 69.4 Comparison of the properties of wheat gluten composites reinforced with coir fibers with and without any treatments Sample % NaOH Failure stress (MPa) Elongation (%) Modulus (GPa) WG 0 46.2 ± 1.6 1.61 ± 0.05 2.8 ± 0.2 WG + CCF 0 48.4 ± 2.3 1.69 ± 0.06 3.0 ± 0.2 WG + SCCF 0 50.0 ± 2.6 1.74 ± 0.16 3.0 ± 0.2 WG + ACCF 2.5 47.9 ± 1.4 1.65 ± 0.03 3.0 ± 0.1 WG + ACCF 5 51.7 ± 3.7 1.61 ± 0.10 3.1 ± 0.2 WG + ACCF 10 45.6 ± 1.4 1.60 ± 0.04 2.9 ± 0.1 WG + ASCCF 5 58.6 ± 2.2 1.89 ± 0.10 3.3 ± 0.2

CCF indicates coir fibers without treatment, SCCF is silane-treated coir fibers, ACCF is alkali — treated coir fibers, and ASCCF is alkali — and silane-treated coir fibers [12Hem]

Table 69.5 Comparison of the properties of composites developed using toughened wheat gluten and alkali-treated and silane-coupled coir fibers [14Dia] Sample Max stress (MPa) Max elongation (%) Modulus (GPa) WG 45.8 ± 5.5 1.1 ± 0.1 4.4 ± 0.3 WG + TPVA 88.9 ± 3.0 2.7 ± 0.1 4.0 ± 0.1 WG + ASCCF 85.8 ± 4.8 3.1 ± 0.6 4.9 ± 0.1 WG + TPVA + ASCCF 106.0 ± 6.6 2.3 ± 0.3 5.4 ± 0.3

has shown longer fiber pullouts for the WG + TPVA +ASCCF composites suggesting poor interfacial adhesion. However, strains at first failure and ultimate stress have shown that the alkali and silane treatment provides better compatibility and adhesion and therefore composites with higher properties [14Dia].

In another study, wheat gluten containing 30 % glycerol was reinforced with hydroxyethyl cellulose (HEC) in ratios up to 25 % [08Son] and compression molded at 120 °C for 5-30 min at 10 MPa. The addition of HEC decreased the moisture absorption. However, the tensile strength and modulus were found to increase with increasing content of HEC [08Son]. Instead of using glycerol or other chemicals as plasticizers, it has been shown that water can plasticize wheat gluten and provide composites with properties similar to that of using polypropylene as the matrix [11Red1]. However, the water-plasticized wheat gluten composites lost up to 50 % of the tensile and flexural properties when conditioned at 90 % humidity for 24 h as seen in Table 69.6.

Table 69.6 Comparison of the properties of the wheat gluten and polypropylene composites reinforced with jute fibers at two different humidities [11Red1] Matrix Flexural strength (MPa) Stiffness (N/mm) Modulus of elasticity (MPa) Tensile strength (MPa) Tensile modulus (GPa) Gluten, 21 °C 65 % RH 20.2 ± 2.0 8.3 ± 2.4 2,928 ± 259 68.6 ± 9.1 7.7 ± 1.2 Gluten, 21 °C 90 % RH 10.4 ± 2.2 1.2 ± 0.6 434 ± 77 28.6 ± 3.8 3.7 ± 0.6 Polypropylene 21 °C 65 % RH 10.6 ± 1.1 1.4 ± 0.2 500 ± 79 35.2 ± 5.5 3.2 ± 0.6 Polypropylene 21 °C 90 % RH 10.6 ± 1.1 1.4 ± 0.3 495± 116 31.3 ± 8.1 2.4 ± 0.5

Fig. 69.1 Effect of addition of PALF fibers on the tensile strength and modulus of the soy-based composites [05Liu]. (A) soy based bioplastic. (B) 15 wt% PALF reinforced soy composites. (C) 30 wt% PALF reinforced biocomposites. (D) 30 wt% PALF with 5 % PEA-g-GMA reinforced biocomposites. Reproduced with permission from Elsevier

Similar to wheat gluten, researchers have also studied the possibility of devel­oping composites using soy protein or soy flour as the matrix. Commercially available soy flour was mixed with a plasticizer (30 % glycerol) and extruded in the form of ribbons. The plasticized soy protein was mixed 1:1 ratio with polyester amide and extruded at 130 °C to form the soy protein thermoplastic which was then mixed with various ratios of pineapple leaf fibers. In addition, polyester amide-grafted glycidyl methacrylate (PEA-g-GMA) was used as compatibilizer [05Liu]. Samples were injection molded at 130 °C using a twin screw extruder. The addition of the PALF fibers increased the tensile strength and modulus of the composites substantially as seen in Fig. 69.1. The presence of the compatibilizer further increased the strength and modulus. Similar behavior was also observed for the flexural strength and modulus. A fiber content of 30 % and the addition of the compatibilizer provided the best tensile and flexural properties to the composites [05Liu]. In another such

Fig. 69.2 Films formed from ESO (left), bacterial cellulose (middle), and a composite of 25 % ESO and 75 % bacterial cellulose (right) [12Ret]. Reproduced with permission from Springer

Table 69.7 Properties of the bacterial cellulose, epoxidized soybean oil (ESO), and bacterial cellulose-reinforced ESO composites [12Ret] Sample Elastic modulus Tensile strength (MPa) Strain at break (%) ESO 0.45 ± 0.1 5.5 ± 0.4 3.6 ± 0.5 ESO + 25 % bacterial cellulose 2.8 ± 0.4 25 ± 0.4 2.1 ± 0.5 ESO+ 75 % bacterial cellulose 5.9 ± 0.5 81 ± 0.7 2.1 ± 0.4 Bacterial cellulose 9.2 ± 0.7 135 ± 8.0 1.8 ± 0.1

study, soybean oil was epoxidized and used as matrix, and bacterial cellulose fibers were the reinforcement [12Ret]. Bacterial cellulose was obtained in nanofibrillated film form and was also acetylated to improve compatibility with the matrix. The epoxidized soybean oil (ESO) and bacterial cellulose films were made into composites using the resin transfer molding approach. The ESO matrix was highly transparent (Fig. 69.2, left), whereas the bacterial cellulose films were semitrans­parent (Fig. 69.2, middle). However, composites containing 25 % ESO and 75 % bacterial cellulose were transparent as seen in Fig. 69.2, right. Tensile properties of the composites are shown in Table 69.7. As seen from the table, the addition of bacterial cellulose increased the strength and modulus and decreased the elongation compared to the ESO matrix. The three-dimensional network of bacterial cellulose that penetrates into the voids was suggested to lead to higher tensile properties [12Ret].

Soybean oil was conjugated with divinylbenzene and n-butyl methacrylate and used as resin with corn stover as reinforcement. Composites were developed by resin transfer molding after mechanically processing (cutting) the cornhusks to 0.5, 1, and 2 mm lengths. The amount of corn stover in the composites was increased from 20 to 80 %, and it was observed that the tensile properties increased with increasing ratio of corn stover up to 70 % [10Pfi]. Longer length corn stover produced composites with inferior properties compared to shorter length stover. It

Fig. 69.3 Changes in the tensile strength of soy protein composites reinforced with various natural fibers [06Tra]. Reproduced with permission from John Wiley and Sons

was suggested that composites containing up to 70 % of degradable material can be developed for application in the construction, furniture, and other industries [10Pfi].

Soy oil was functionalized with epoxy (ESO) or succinic anhydride (MSO) using hexamethylenediamine or 4-dimethylaminopyridine as the catalyst, and the modified soy oil was used as resin to develop composites [06Tra]. The modified soy oil was then reinforced with various fibrous materials and made into composites using resin transfer and compression molding approaches. Figure 69.3 shows the tensile properties of the compression-molded soy-based composites reinforced with various fibrous materials in the presence or absence of catalyst. As seen from the figure, soy oil modified in the presence of hexamethylenediamine provided better tensile strength to the composites. Similarly, composites reinforced with kenaf fibers had higher strength than the other reinforcements used [06Tra].

Instead of using plasticizers such as glycerol that causes substantial decrease in water stability and mechanical properties or chemical modifications of the matrix that increased cost and decreased biodegradability, Reddy and Yang have shown that soy protein composites reinforced with jute fibers can be developed using water without any chemicals as the plasticizer [11Red2]. Tensile and flexural properties of the soy protein composites were similar to that of composites developed using polypropylene as the matrix under the same composite fabrication and testing conditions. However, the soy protein composites lost nearly 50 % of their tensile and flexural properties after conditioning at 90 % humidity and 21 °C for 24 h as seen in Table 69.8. [11Red2]. To overcome the poor water stability of soy protein — based composites, soy protein isolates were modified using stearic acid. Inclusion of the stearic acid up to 25 % increased the Young’s modulus and the resistance of the composites to moisture [05Lod].

Table 69.8 Comparison of the properties of the soy protein and polypropylene composites reinforced with jute fibers at two different humidities [11Red2] Matrix Flexural strength (MPa) Stiffness (N/mm) Modulus of elasticity (MPa) Tensile strength (MPa) Tensile modulus (GPa) Soy protein, 21 °C 65 % RH 24.1 ± 4.4 11.5 ± 1.8 4,074 ± 648 64.0 ± 5.7 6.1 ± 0.8 Soy protein, 21 °C 90 % RH 11.3 ± 2.5 1.6 ± 0.4 558±144 49.0 ± 7.5 4.2 ± 0.4 Polypropylene 21 °C 65 % RH 10.6 ± 1.1 1.4 ± 0.2 500 ± 79 35.2 ± 5.5 3.2 ± 0.6 Polypropylene 21 °C 90 % RH 10.6 ± 1.1 1.4 ± 0.3 495±116 31.3 ± 8.1 2.4 ± 0.5

Table 69.9 Comparison of the flexural and tensile properties of plant protein-based composites reinforced with jute fibers compared to PP composites reinforced with jute fibers [11Red1, 11Red2, 11Red3] Flexural Modulus of Tensile Tensile Matrix strength Stiffness elasticity strength modulus material (MPa) (N/mm) (MPa) (MPa) (GPa)

21 °C, 65 % RH Zein 21.5 ± 3.2 6.9 ± 2.3 2,458 ± 824 64.0 ± 6.1 6.1 ± 0.8 Wheat gluten 20.2 ± 2.0 8.3 ± 2.4 2,928 ± 259 68.6 ± 9.1 7.7 ± 1.2 Soy protein 24.1 ± 4.4 11.5 ± 1.8 4,074 ± 648 64.0 ± 5.7 6.1 ± 0.8 Polypropylene 10.6 ± 1.1 1.4 ± 0.2 500 ± 79 35.2 ± 5.5 3.2 ± 0.6

21 °C, 90 % RH Zein 16.0 ± 2.7 2.0 ± 0.4 710±150 49.4 ± 4.3 5.4 ± 0.3 Wheat gluten 10.4 ± 2.2 1.2 ± 0.6 434 ± 77 28.6 ± 3.8 3.7 ± 0.6 Soy protein 11.3 ± 2.5 1.6 ± 0.4 558 ±144 49.0 ± 7.5 4.2 ± 0.4 Polypropylene 10.6 ± 1.1 1.4 ± 0.3 495±116 31.3 ± 8.1 2.4 ± 0.5

Similar to soy protein and wheat gluten, the corn protein zein was also used as matrix and reinforced with jute fibers. Water without any chemicals was used as plasticizer, and the properties of the composites were studied at two different temperatures and humidities. Table 69.9 provides a comparison of the properties of the zein composites along with soy protein and wheat gluten composites fabricated under similar conditions [11Red3]. As seen from the table, zein composites have nearly doubled the flexural and tensile strength and tensile modu­lus than the PP composites. However, the properties of the zein composite decrease considerably at 90 % relative humidity. Despite the sharp decrease in properties, the zein composites had properties similar or better than that of the PP composites at 90 % RH. Among the three plant proteins, zein composites had better flexural properties, but no major difference was observed in the tensile properties.

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