Regenerated Protein Fibers

Keywords

Milk protein • Casein • Solubility • Alkaline solution • Stability • Cross-linking • Aldehyde • Carboxylic acid • Allergy • Cytotoxicity

The milk protein casein was made into fibers on an industrial scale as early as the 1950s and was available in the commercial names such as Lanita produced from Snia and Fibrolane produced from Courtalds [07Hea]. Trade names of casein fibers also varied by the country where the fibers were produced. For example, casein fibers were marketed as Aralac and Caslen (USA), Lactofil (Holland), Cargan (Belgium), Tiolan (Germany), Silkool (Japan), and Fibrolane (England) [51Tra]. Traditionally, casein fibers were produced by dissolving casein in alkaline solutions, extruding and coagulating using sulfuric acid and sodium sulfate and later cross-linked with aluminum sulfate and formaldehyde, and finally treated with metal salts such as zinc [69Sal]. Although most reports do not provide the properties of the fibers, it has been suggested that casein fibers had dry tenacity of 0.8-1.0 g/den, wet tenacity of 0.4-0.5 g/den, and elongation of 30-50 % [69Sal]. However, the fibers were soluble to weak alkali and to enzymes and therefore not practically useable. In addition, yellowing of the fibers was observed when fibers were treated with alkali at 70 °C for 40 min, but the fibers were stable under acidic conditions. Casein fibers were reported to have good uniformity, less impurity, and superior spinnability, but the fibers had poor cohesion and frictional resistance necessitating a pretreatment before the fibers could be made into yarns of 136 tex. The protein fibers were dyed with reactive dyes and found to have uniform dyeability.

In a more recent study, casein was mixed with soy proteins and made into fibers. Proteins were dissolved using urea and sodium sulfite, mixed in various ratios, and extruded into an acidic coagulation bath saturated with sodium sulfate [11Sud]. It was found that casein contents higher than 50 % were necessary to obtain fibers with improved morphology and thermal stability. However, tensile properties of the
fibers were not studied. In another recent study, casein fibers were dissolved in aqueous alkali and wet spun into a coagulation bath [12Yan]. Fibers obtained at 30 % casein concentration had strength of about 1 g/den, elongation of 13 %, and modulus of about 87 g/den. However, the fibers were unstable in water and therefore cross-linked with citric acid to improve stability. After cross-linking, the fibers retained about 50 % of their strength after treating in PBS at 37 °C for 12 days. Fibers also had good strength retention in acidic and weak alkaline solutions. However, casein fibers were found to be cytotoxic, probably related to the allergenic nature of milk proteins. It has been recently reported that casein fibers are produced on a commercial scale (http://www. reuters. com/article/2011/10/06/ us-germany-fashion-milk-idUSTRE7953MG20111006). However, the fibers are blends with other polymers and amount of casein in the fibers, the use of cross­linkers, if any, or the stability of the fibers under various conditions is not known.

References

[51Tra] Traill, D.: J. Soc. Dyer. Color. 67(7), 257 (1951)

[69Sal] Salzberg, H. K.: Encycl. Polym. Sci. Technol. 11, 688 (1969)

[07Hea] Hearle, J. W.J.: Mater. Sci. 42(19), 8010 (2007)

[11Sud] Sudha, T. B., Thanikaivelan, P., Ashok Kumar, M., Chandrasekaran, B.: Appl.

Biochem. Biotechnol. 163(2), 247 (2011)

[12Yan] Yang, Y., Reddy, N.: Int. J. Biol. Macromol. 51(1-2), 37 (2012)

Biocomposites from Renewable Resources

Keywords

Agricultural residue • Lignocellulose • Synthetic polymer matrix • Compatibilizer • Palm • Banana • Sugarcane bagasse • Curaua fiber • Reinforcement

A simple method of developing biocomposites is to use agricultural residues in their native form as reinforcement. In one such attempt, agricultural residues such as sunflower stalks, cornstalks, and sugarcane bagasse have been used as reinforce­ment for polypropylene composites. Fibers were obtained from the agricultural residues by mechanical pulping by steaming for 15 min at 175 °C under 7 MPa pressure. Some of the properties of the residues and fibers obtained from the residues are listed in Table 68.1. In addition to neat polypropylene (PP), two different types of maleic acid-grafted PP were also studied as matrices. The fibers and matrices were melt compounded in a twin screw extruder with various levels of compatibilizers. Extrudates obtained were compression molded into composites at 170 °C and 3 MPa of pressure. Tensile, flexural, and impact resistance properties of the composites are compared in Table 68.2. The inclusion of the reinforcements improved the properties of the composites, and the addition of compatibilizer further increased the tensile, flexural, and impact resistance properties due to better binding between the reinforcement and matrix [10Ash]. In a similar approach, cornhusks were mechanically split into various lengths and used to reinforce lightweight polypropylene (PP) composites [09Hud1]. As seen in Table 68.3, the cornhusk-reinforced PP composites had similar or better properties than the jute fiber-reinforced composites. The cornhusk-reinforced composites also had better sound absorption than the jute fiber-reinforced composites as seen in Fig. 68.1. A digital image of the split husk-polypropylene composite is shown in Fig. 68.2. In addition to using the cornhusks in their native form, fibers extracted from cornhusks using alkali and enzymes were also used to reinforce PP composites [08Hud1]. The cornhusk fiber-reinforced composites developed had properties similar to that of

© Springer-Verlag Berlin Heidelberg 2015 391

N. Reddy, Y. Yang, Innovative Biofibers from Renewable Resources,

DOI 10.1007/978-3-662-45136-6_68

Table 68.1 Comparison of the properties of the sunflower, cornstalks, and bagasse and fibers obtained these residues [10Ash] Chemical composition Sunflower stalk Cornstalk Bagasse Cellulose (%) 38.1 36.6 52.7 Lignin (%) 17.3 16.7 20.6 Ash (%) 7.0 6.3 1.3 Length (mm) 1.18 1.2 1.2 Diameter (pm) 21.5 24.3 22.9 Aspect ratio (L/D) 55 50 54

jute fiber-reinforced composites under their respective optimized conditions as seen in Table 68.4 [08Hud1].

Agricultural residues such as rice husks, bagasse, and fish waste were mixed with pure and maleated PP in various ratios and injection molded. Later, the injection molded samples in the form of strands were later reinjection molded into composites [14Nou]. Properties of the rice husk and bagasse fibers used as reinforcement are given in Table 68.5. Considerable variations are observed in the properties of the rice husk and bagasse fibers as seen in the table. Rice husk fibers have higher aspect ratio and therefore can be expected to provide better composite properties [14Nou]. In general, addition of fish waste was found to decrease the mechanical properties. Degradation (weight loss) based on soil burial tests showed that the addition of the reinforcement, especially fish waste, promoted the degrada­tion of the composites. As seen in Fig. 68.3, about 8 % degradation for bagasse and 10 % degradation for rice husk fibers were seen in the composites [14Nou].

In another study, bagasse was separated into outer and inner parts manually and separately used as reinforcement with 0, 5, 10, and 15 % by weight of reinforcement in the composites [08Lee]. The addition of 10 % fibers provided the highest flexural strength for both the pith and rind fibers. Tensile stress increased with the addition of the fibers up to about 15 %, and rind fibers provided better strength than the pith fibers. The presence of the fibers in the composites increased the moisture sorption from 0 to 8 % [08Lee]. Bagasse fibers pretreated with 10 % sulfuric acid were also used to reinforce polypropylene in 5, 10, and 20 % by weight [11Cer]. Adding the bagasse fibers resulted in an increase in tensile properties as seen in Table 68.6. About 15 % increase in tensile strength and 50 % increase in modulus were obtained when 20 % bagasse was used in the matrix. Similarly, 35 % increase in flexural strength and 32 % increase in flexural modulus were observed.

Bagasse fibers with average lengths of 9.1 mm were treated with various concentrations of alkali and used to reinforce biodegradable aliphatic polyester (Randy CP-300) up to 65 % by weight [06Cao]. Fibers treated with 1 % alkali provided better properties to the composites than fibers treated with 3 % alkali. Tensile strength was found to decrease with increasing fiber diameter. Tensile, flexural, and impact resistance properties of the bagasse reinforced biocomposites are shown in Table 68.7.

Table 68.2 Tensile, flexural, and impact resistance properties of PP composites reinforced with sunflower, cornstalk, and bagasse fibers at three different levels of compatibilizer [lOAsh] Reinforcing material Tensile properties Flexural properties Impact resistance (J/m) Strength (MPa) Modulus (MPa) Strength (MPa) Modulus (MPa) 0 1.5 2.5 0 1.5 2.5 0 1.5 2.5 0 1.5 2.5 0 1.5 2.5 Sunflower 25 28 28 1,900 1,950 2,500 58 72 75 2,200 2,500 2,700 25 27 75 Cornstalk 22 25 25 1,720 1,875 1,875 42 50 55 1,650 2,200 2,200 23 28 31 Bagasse 27 31 33 1,875 1,900 2,100 48 75 78 2,300 2,500 2,700 23 27 27

Table 68.3 Comparison of mechanical properties of mechanically split husk (MSH)-PP and jute — PP composites [09Hud1] Composite material Conc. of reinforcing fiber (wt%) Flexural strength (MPa) Modulus of elasticity (MPa) Impact resistance (J/m) Tensile strength (MPa) Tensile modulus (MPa) MSH-PP 25 6.2 ± 0.6 506 ± 14 33.8 ± 7.2 7.1 ± 0.4 551 ± 21 35 7.3 ± 0.5 562 ± 17 41.2 ± 8.7 6.4 ± 0.4 516 ± 18 45 7.7 ± 0.4 586 ± 42 51.5 ± 9.1 6.1 ± 0.6 501 ± 26 55 8.2 ± 0.2 642 ± 22 65.5 ± 8.1 5.9 ± 0.7 526 ± 38 65 7.4 ± 0.2 555 ± 22 52.2 ± 9.4 5.9 ± 0.8 473 ± 41 75 5.6 ± 0.2 501 ± 19 43.0 ± 8.4 5.6 ± 1.1 412 ± 74 Jute fiber — PP 35 8.1 ± 1.8 1,195 ± 71 60.0 ± 6.1 10.4 ± 0.9 650 ± 29 45 8.8 ± 0.7 1,360 ± 91 101.1 ± 7.1 10.8 ± 1.0 537 ± 50 55 9.1 ± 0.4 1,635 ± 99 112.3 ± 8.6 8.9 ± 0.8 469 ± 30 65 8.3 ± 0.3 1,742 ± 87 94.4 ± 9.1 5.5 ± 0.3 388 ± 29 75 7.0 ± 0.5 1,588 ± 99 68.7 ± 7.8 3.2 ± 0.2 256 ± 23

Fig. 68.1 Comparison of the sound absorption coefficients of the split cornhusk-reinforced composites compared to jute fiber-reinforced composites [09Hud1]. Reproduced with permission from John Wiley and Sons

Instead of using the traditional matrix polymers, lignocellulosic agricultural residues such as cornstalk, soy stalk, and wheat stalk and the renewable ligno — cellulosic perennial switchgrass were studied as potential reinforcement for the biodegradable polymers PHBV/PBAT [13Nag1, 13Nag2]. The agricultural residues were chopped to short lengths and used as reinforcement. Based on microscopical analysis of fibers obtained after dissolving the matrix, the average length of the fibers was found to be between 2.9 and 7.7 mm for the different

Fig. 68.2 Digital image of the composite developed using split cornhusk as reinforcement and PP as the matrix

Table 68.4 Comparison of flexural properties and noise reduction coefficient (NRC) of cornhusk fiber-polypropylene and jute-polypropylene composites in their respective optimized conditions [08Hud1] Strength (N) Stiffness (N/mm) Toughness (%) NRC Cornhusk fiber-PP 23.3 ± 1.3 2.78 ± 0.21 168 ± 8 0.08 Jute-PP 22.4 ± 0.5 5.16 ± 0.23 115 ± 9 0.07

Properties	Rice husk fiber	Bagasse fiber
Cellulose (%)	48.9	95.3
Lignin (%)	19.1	21.0
Extractive (%)	2.5	2.9
Ash (%)	12.3	1.9
Fiber length (mm)	0.80	0.96
Aspect ratio	89	42

Table 68.5 Selected properties of the rice husk and bagasse fibers used as reinforcement in polypropylene composites [08Lee]

Table 68.6 Properties of polypropylene composites reinforced with 5-20 % of bagasse fibers [11Cer] Tensile properties Flexural properties Fiber Strength (MPa) Modulus (MPa) Strength (MPa) Modulus (MPa) Impact resistance (J/m2) Polypropylene 19.3 ± 1.1 955 ± 93 27.5 ± 0.9 906 ± 35.8 36.1 ± 1.1 PP/5 % bagasse 22.9 ± 1.4 1,105 ± 23 34.8 ± 2.9 1,047 ± 235 32.7 ± 6.0 PP/10 % bagasse 23.0 ± 0.6 1,027 ± 83 35.5 ± 3.6 961±139 45.0 ± 0.1 PP/20 % bagasse 22.3 ± 0.8 1,443 ± 69 37.2 ± 2.1 1,201 ± 113 52.5 ± 0.6

Table 68.7 Tensile and flexural properties of biocomposites reinforced with various levels of untreated and alkali-treated bagasse fibers [06Cao] Fiber weight (%) Treatment Tensile strength (MPa) Flexural strength (MPa) Flexural modulus (MPa) Impact strength (kJ/m2) 20 Untreated 16.5 ± 0.8 31.2 ± 2.2 1,137 ± 115 4.1 ± 0.5 20 Alkali treated 18.6 ± 0.7 34.7 ± 2.7 1,322 ± 57 6.1 ± 0.8 35 Untreated 18.6 ± 0.8 38.4 ± 3.0 1,452 ± 158 6.8 ± 0.4 35 Alkali treated 21.4 ± 0.7 44.0 ± 3.0 1,622 ± 186 8.3 ± 1.0 50 Untreated 21.1 ± 0.9 40.2 ± 2.4 1,841 ± 75 8.2 ± 0.2 50 Alkali treated 23.1 ± 0.5 46.1 ± 2.6 2,032 ± 155 9.5 ± 0.3 65 Untreated 23.5 ± 0.7 43.9 ± 3.8 2,292 ± 175 8.8 ± 0.6 65 Alkali treated 26.8 ± 0.8 50.9 ± 3.8 2,674 ± 165 11.3 ± 0.9

reinforcing materials used. Table 68.8 provides a comparison of the properties of the reinforcing materials used, and Table 68.9 provides a comparison of the properties of the composites obtained using the different lignocellulosic materials. Fiber lengths and widths for the different materials were similar, but the tensile and flexural properties obtained with these reinforcing materials were considerably different as seen in Table 68.7. Reinforcing the matrix with the fibers resulted in marginal increase in strength, but the modulus increases considerably, and elonga­tion nearly decreases by a factor of 9-10. Flexural strength and modulus also showed improvements with the addition of the fibers. However, no major differences in properties were observed between the various reinforcing materials used. Soy stalk fibers had better thermal stability, and the soy stalk-reinforced composites showed the highest thermal resistance. Although composites were developed using various agricultural residues, no pretreatment or fiber extraction was done. It can be expected that the properties of the composites can be improved

Agricultural residue	Fiber length (mm)	Fiber width (mm)	Cellulose (%)	Hemicellulose (%)	Lignin (%)
Switchgrass	1.6 ± 0.4	0.46 ± 0.26	36.1	28.2	10.2
Miscanthus	1.5 ± 0.5	0.45 ± 0.17	50.2	20.5	15.1
Soy stalk	1.5 ± 0.5	0.46 ± 0.27	45.0	11.4	13.9
Corn stover	1.5 ± 0.5	0.43 ± 0.19	44.8	22.7	10.6
Wheat straw	1.1 ± 0.2	0.47 ± 0.17	47.9	24.0	11.0

Table 68.8 Comparison of the properties of the different lignocellulosic raw materials consid­ered as reinforcement for PHBV/PBAT [13Nag1]

by chemical modification of the raw materials, using extracted fibers with high cellulose content or by the addition of compatibilizing agents. However, the presence of lignin, hemicelluloses, and other impurities in the reinforcing materials may affect the properties and/or performance and the utility of the composites. For instance, under simulated biodegradation conditions, soy and wheat straw added into PLA had higher than 70 % degradation within 45 days of aerobic degradation. As seen from Fig. 68.4, the degradation of the composites was higher than that of neat PLA demonstrating that addition of the lignocellulosic components could increase the degradability of the matrix [10Pra].

PHBV was reinforced with corn straw (CS), wheat straw (WS), and soy stalk (SS) cut into 0.25 mm length individually or in a combination, and the influence of the reinforcements on the mechanical, thermal, and dynamic mechanical properties was investigated [11Aha]. Table 68.10 lists the tensile properties of the composites obtained using various ratios of the different reinforcing agricultural residues. Adding the residues had varying effects depending on the type of reinforcement and the treatment done and the ratio in the PHBV matrix. Overall, alkali-treated biomass provided higher increase in flexural strength and modulus due to the

Table 68.9 Properties of PHBV/PBAT composites reinforced (30 %) with various agricultural residues [13Nagl] Reinforcement Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa) Elongation at break {%) “ Impact strength (J/m) Pure matrix 21.6± 0.3 1.2 ±0.01 37.7 ±0.2 1.3 ±0.1 107 ±52 370 ± 10 Switchgrass 20.6 ±0.7 1.9 ±0.06 37.0 ±0.7 2.0 ±0.2 3.2 ±0.3 35 ±5 Miscanthus 23.3 ±0.3 2.4 ±0.03 44.0 ± 0.3 2.7 ±0.1 3.1 ±0.08 36 ±4 Soy stalk 22.4 ±0.3 2.0 ±0.04 45.2 ±0.6 2.5 ± 0.04 3.1 ±0.3 46 ±5 Cornstalk 21.5 ±0.4 2.3 ±0.04 44.7 ± 0.7 2.7 ±0.1 2.8 ±0.2 40 ±9 Wheat straw 21.5 ±0.3 2.0 ±0.08 44.3 ±0.8 2.6 ±0.1 2.8 ±0.5 36 ±2

Table 68.10 Comparison of the properties of PHBV composites reinforced with various agricultural residues at different concentrations Tensile properties Flexural properties Components Strength (MPa) Modulus (GPa) Elongation (%) “ Strength (MPa) Modulus (GPa) Impact strength HDT (°С) PHBV 23 ±0.2 1.1 ±0.03 5.5 ±0.4 29.8 ±0.9 1.22 ±0.33 49.5 ± 1.5 107.8 PHBV/CS 70/30 23 ±0.6 3.7 ±0.07 1.1 ± 0.1 42.7 ± 0.9 3.9 ±0.07 24 ±0.7 119.7 PHBV/SS 70/30 20.2 ± 0.7 2.5 ±0.07 2.1 ±0.2 32.5 ± 1 2.5 ±0.07 30.6 ±0.9 117 PHBV/WS 70/30 24.9 ± 0.6 3.8 ±0.05 1.5 ±0.1 45.2 ±0.9 4.2 ±0.08 23 ±0.7 128 PHBV WS 70/30a 25.6 ±0.4 3.8 ±0.06 2.0 ±0.2 44.0 ±0.6 3.9 ±0.05 31.1 ±1.8 130 PHBV/CS 55/WS 70/10/10/ 10 22.8 ±0.8 2.9 ±0.09 1.8 ±0.2 37.9 ±0.4 3.1 ±0.05 26.2 ±1.1 121 PHBV/CS 60/40 19 ±0.4 4.2 ±0.1 1.1 ± 0.1 23.2 ± 1.2 4.4 ±0.9 18.1 ±1.9 132 PHBV/SS 60/40 18 ±0.5 2.7 ±0.1 1.5 ±0.2 25.3 ±1.1 3.0 ±0.9 24.6 ±1.0 126 PHBV/WS 60/40 19.9 ±0.4 4.3 ±0.08 1.3 ± 0.1 24.3 ±0.4 4.4 ±0.08 19.2 ±0.8 133 aAlkali-treated wheat straw [llAha] . HDT is heat deflection temperature

removal of noncellulosic substances and increase in surface roughness that promotes better adhesion [11Aha]. In addition to the tensile and flexural properties, the heat deflection temperature (HDT) also increases from 108 up to 133 °C as seen from the table. Increase in HDT suggests compatibility between the reinforcement and the matrix. Although composites with good mechanical properties have been obtained from PHBV and the agricultural residues, the performance of the composites in high humidities or in water was not investigated.

Similar to using agricultural residues obtained after harvesting cereal grains, the residues or by-products obtained after processing oil crops have also been used for composite applications. For example, fibers obtained from date palms have been used as reinforcements for biocomposites, and the influence of various fiber properties and composite formation conditions on properties of the composites was compared and critiqued [14AlO]. It was reported that date palm fibers have similar aspect ratio compared to coir and hemp, higher elongation than hemp and sisal, and available at a cost $0.02 per kg, 15 times cheaper than coir and 60 times cheaper than hemp. Therefore, attempts have been made to utilize palm fibers as reinforcements for biocomposites [14AlO]. Starch-based composites were devel­oped by reinforcing with date palm and/or flax fibers. Fruit-bearing branches of date palms were retted in water and later treated with NaOH at 90 °C for 3 h between 80 and 90 °C. After several such treatments and neutralization, the fibers obtained were manually cut into lengths of 15-30 mm [14Ibr]. Starch was treated with glycerin (30 %) and water (20 %) and processed between 60 and 80 °C to obtain thermoplastic starch. Various ratios of the fibers were mixed with the thermoplastic starch and compression molded at 160 ± 3 °C for 30 min at 5 MPa pressure. Morphological analysis showed that the fibers and matrix had good adhesion, but the presence of voids was observed when the ratio of fibers in the matrix was high due to the lack to sufficient matrices seen in Fig. 68.5 [14Ibr]. The strength of the composites increased from 5 to 30 MPa when the concentration of the fibers was increased from 0 to 50 %, and a similar increasing trend (5-17 MPa) was observed for the modulus. However, both the strength and modulus decreased when the fiber content was above 50 % [14Ibr].

Similar to date palms, oil palms have also been extensively studied as potential reinforcements for composites with various matrices [11Shi]. Oil palm fibers have been combined with polyester, polyurethane, natural rubber, phenol formaldehyde, and others. Composites with properties suitable for various applications have been obtained, and several different pretreatments and addition of plasticizers and compatibilizers have been done to improve the properties of the composites [11Shi]. However, limited attempts have been made to develop completely biode­gradable composites using oil palm fiber and to understand the biodegradability of the composites developed [11Shi].

In addition to date and oil palms, fibers from another variety of palm, sugar palms (Arenga pinnata), have also been used as reinforcement for composites. Several reports are available on developing polyester composites reinforced with sugar palm fibers. In fact, sugar palm fibers have been used with glass fibers to reinforce unsaturated polyester and made into boat shown in Fig. 68.6 [13Ish]. Some

Fig. 68.5 (a) SEM images of date palm fiber-reinforced starch-based composites reveal areas with voids and (b) places with good adhesion [14Ibr]. Reproduced with permission from Elsevier

Fig. 68.6 A boat developed using sugar palm fibers and glass fibers as reinforcement [13Ish]. Reproduced with permission from Elsevier

Table 68.11 Mechanical and physical properties of sugar palm fiber-reinforced unsaturated polyester composites [13Ish] Property Palm frond Palm bunch Palm trunk Ijuk Tensile strength (MPa) 15.18 12.81 9.82 11.47 Tensile modulus (GPa) 0.39 0.43 0.56 0.47 Elongation at break (%) 8.07 5.04 3.19 4.45 Flexural strength (MPa) 38.91 35.17 41.90 33.74 Flexural modulus (GPa) 3.00 2.75 3.36 2.42 Impact strength (kJ/m2) 8.09 6.58 3.92 4.57 Water absorption (%) 1.57 1.35 0.39 0.65 Thickness swelling (%) 1.56 1.11 0.50 0.76 Ijuk refers to a natural fabric-like structure found wrapped onto the palm trunk

of the properties of sugar palm fiber-reinforced unsaturated polyester composites are given in Table 68.11, and properties of alkali-treated sugar palm fibers used as reinforcement for epoxy composites are shown in Table 68.12.

Agave fibers were manually extracted by retting and processed to obtain fibers with a diameter of about 0.3 mm and length of 12 cm and used as reinforcement for epoxy. Further, the fibers were also treated with 5 % alkali for 30 min to remove extractables and increase the surface roughness and promote adhesion to the matrix [11Myl]. Stress-strain curves did not show a major difference in properties between

Table 68.12 Comparison of the properties of alkali-treated sugar palm fiber-reinforced epoxy composites after treating with two different concentrations of alkali at various treatment times [13Ish] Property Untreated 0.25 M 0.5 M 1 4 8 1 4 8 Tensile strength (MPa) 42.85 49.88 37.89 41.41 30.64 37.56 41.86 Tensile modulus (GPa) 3.33 3.78 3.87 3.75 3.66 3.85 3.77 Elongation at break (%) 1.32 1.32 1.00 1.17 0.8 1.01 1.11 Flexural strength (MPa) 77.73 96.69 64.42 72.63 85.3 58.17 90.68 Flexural modulus (GPa) 2.81 3.51 2.21 2.55 5.03 6.95 4.67 Impact strength (kJ/m2) 46.72 35.2 37.8 50.02 40.74 49.28 60.08

A — PHBV/PBAT; B — PHBV/PBAT + 20% SG; C — PHBV/PBAT + 25% SG; B — PHBV/PBAT+ 30% SG; E — PHBV/PBAT+ 40 % SG Fig. 68.7 Tensile properties of PHBV/PBAT composites reinforced with various amounts of switchgrass [13Nag2]. Reproduced with permission from the American Chemical Society

the untreated and treated composites, but considerable differences were observed in flexural and impact resistance. The alkali-treated agave fibers provided better flex­ural properties and thermal resistance than the untreated fibers [11Myl]. Similar to agave, sisal fibers have also been extensively studied as possible reinforcement for various types of matrices [00Li]. However, sisal is an exclusive fiber crop, and therefore sisal-based composites have not been reviewed here.

Switchgrass, a renewable biomass crop considered as a potential feedstock for cellulosic ethanol production, has been studied as possible reinforcement for PHBV-based biocomposites [13Nag2]. Switchgrass fibers (average length of 4— 8 mm) were mixed with a commercially available (ECOFLEX) 45 % PHBV and 55 % PBAT matrix in various ratios and processed through a twin screw com­pounder to obtain the composites. A compatibilizer pMDI was included to improve the compatibility between the fibers and the matrix. The amount of switchgrass used in the composites was 25, 30, or 40 %. Figure 68.7 shows the tensile and

Fig. 68.8 SEM images showing the improved adhesion between the switchgrass fibers and matrix before (a) and after (b) addition of the compatibilizer [13Nag2]. Reproduced with permission from the American Chemical Society

flexural properties of the composites developed. As seen from the figure, the properties of the composites decreased when the amount of fibers increased above 30 %. The addition of the compatibilizer increased the tensile and impact resistance properties due to improved adhesion between the fibers and matrix as seen in Fig. 68.8.

Pineapple leaves were processed to obtain whole ground pineapple leaves (WGL), pineapple leaf fibers (PALF), and non-fibrous material (NFM), and these components were used as filler or reinforcement for polypropylene composites [14Ken]. The composition of the three components obtained from the pineapple leaves is shown in Table 68.13. The components extracted from the pineapple leaves were added at various extents into polypropylene and compression molded into composites. Figure 68.9 shows the flexural and tensile properties of the composites developed. As seen from the figure, the addition of the PALF composites substantially improved the flexural and tensile properties except for the decrease in tensile strength observed at high ratios of NFM. Among the three pineapple leaf components studied, PALF provided substantially higher improve­ment in properties because of the higher aspect ratio and also probably due to the higher cellulose and lower content of impurities [14Ken]. It was suggested that the properties of the composites could be controlled by varying the type and amount of reinforcement.

Pineapple leaf fibers were made into nanofibers through steam explosion and used to develop bionanocomposites for medical and other applications

Table 68.13 Composition of the three types of materials extracted from pineapple leaves [14Ken] Component Dry matter Lignin Hemicellulose Cellulose WGL 90.38 ± 0.11 4.57 ± 0.13 34.38 ± 0.66 46.65 ± 0.83 PALF 92.43 ± 0.10 1.98 ± 0.08 19.80 ± 0.21 70.98 ± 0.93 NFM 89.37 ± 0.48 7.70 ± 0.16 36.53 ± 0.23 43.69 ± 0.78

Fig. 68.9 Flexural modulus (a), flexural strength (b) and tensile strength (c) of PP composites reinforced with various amounts of components extracted from pineapple leaves [14Ken]. Reproduced with permission from Elsevier

[11Che]. PALF fibers were steam exploded in an autoclave at 138 kPa pressure in the presence of 2 % NaOH. Steam-exploded fibers were further bleached and steam exploded eight times to obtain nanofibrils (69 % yield). Fibers (5-15 nm diameter) were made into films and nonwoven mats for potential use as reinforcement for composites. Biodegradable polyurethane was developed and used as the matrix. Composites were developed by placing layers of the nonwoven mats between polyurethane films and compression molding between 150 and 200 °C. The composites were molded into heart valves and implants shown in Fig. 68.10. Tensile properties of the pure polyurethane and cellulose reinforced composites are shown in Table 68.14. As seen from the table, the addition of the nanocellulose leads to more than threefold increase in strength and tenfold increase in modulus. The composites developed were considered to be suitable to manufacture wound dressing, glove, surgical drapes, and medical disposables [11Che].

PALF fibers of two lengths, long fibers >300 mm, and having a unidirectional arrangement and short fibers of 40 mm that had a random arrangement were used as reinforcement for low-density polyethylene and polypropylene composites [ 11Cho]. Incorporating the fibers in the matrix leads to considerable decrease in crystallinity and heat of fusion. Tensile properties of the composites are shown in Table 68.15.

Table 68.14 Mechanical properties of composites made from pure polyurethane embedded with cellulose [11Che] Tensile strength (MPa) Modulus (MPa) Polyurethane 17.5 ± 0.4 37.5 ± 0.5 Polyurethane—25 % nanocellulose 28.2 ± 1.2 94 ± 1.6 Polyurethane—5 % nanocellulose 52.6 ± 0.7 992 ± 1.9 Polyurethane—10 % nanocellulose 51.3 ± 0.1 787 ± 1.4

As seen from the table, increasing the amount of fibers in the matrix increased the strength continually for both the LDPE and PP matrix. At similar levels of reinforce­ment, the long fibers provided better strength than the short fibers. However, the type of matrix did not make a significant difference in terms of strength of the composites. A study was conducted to understand the influence of fiber content on the thermal and water resistance of polyester composites reinforced with pineapple leaf fibers [04Uma]. The resistance of the composites to water was measured by immersing the samples in room temperature water for several weeks and also by boiling in water for 7 h at atmospheric pressure. Thermal resistance studies were conducted by heating the samples at 100 °C for 48 h [04Uma]. Changes in the properties of the composites before and after aging are reported in Table 68.16. As seen from the table, boiling in water resulted in about 35 % decrease in strength and 18 % decrease in modulus. Heating causes considerably lower decrease in strength of about 6 %, but the modulus decreased by as high as 52 %. Higher loss of modulus after aging was suggested to be due to the degradation of the fiber [04Uma].

Banana fibers were modified using various chemical treatments and added into PLA to improve the properties of the composites [13Jan]. Fiber bundles were cut into lengths of 13-15 cm and washed with detergent to remove impurities. Later, the fibers were mercerized using NaOH and also silane treated using [3-amino propyltriethoxysilane (APS) or bis-(3-triethoxysilylpropyl)tetrasulfane (Si69)].

Table 68.15 Properties of LDPE and PP composites reinforced with long and short fibers [11Cho] Matrix, % of fibers Long fibers Short fibers Tensile strength (MPa) Elongation (%) Tensile strength (MPa) Elongation (%) LDPE matrix 0 12.73 ± 1.36 390 ± 39 12.73 ± 1.36 390 ± 39 5 22.73 ± 3.72 4.6 ± 0.8 21.69 ± 2.52 3.4 ± 0.3 10 33.16 ± 4.96 3.2 ± 0.4 35.15 ± 5.17 3.2 ± 0.2 15 51.27 ± 8.95 4.0 ± 0.8 40.43 ± 7.21 3.1 ± 0.2 PP matrix 0 35.17 ± 1.99 11.6 ± 5.2 35.17 ± 1.99 11.6 ± 5.2 5 35.86 ± 3.34 6.7 ± 2.1 37.09 ± 6.58 3.9 ± 1.1 10 39.26 ± 5.78 4.8 ± 1.5 47.11 ± 7.58 3.6 ± 0.9 15 53.14 ± 7.60 3.5 ± 1.3 54.96 ± 5.29 3.4 ± 0.2

Table 68.16 Properties of polyester composites reinforced with 30 % PALF fibers before and after water and thermal aging [04Uma] Treatment Tensile strength (MPa) Elongation at break (%) Modulus (MPa) Untreated 52.9 4 2,290 Boiling water for 7 h 34.5 3 1,900 Heated at 100 °C for 48 h 49.8 4 1,110

These chemically modified fibers were cut into lengths of 2-3 mm before using as reinforcement. Properties of composites reinforced with the various banana fibers are shown in Table 68.17. The addition of the banana fibers progressively increased the tensile strength and modulus but decreased the elongation and impact strength as observed for other composites. Fibers treated with the silane coupling agents, especially with bis-(3-triethoxysilylpropyl)tetrasulfane (Si69), showed substantial increase (more than doubled) in tensile strength and modulus, and an increase in elongation was also observed [13Jan]. In addition, organically modified nanoclay cloisite 30B was included to improve the processability and properties of the composites which were made into plastic cutlery shown in Fig. 68.11.

In an effort to develop completely biodegradable composites, curaua fibers milled to 10-30 mm in length were mixed with cellulose acetate as the matrix and injection molded into tensile bars using dioctyl phthalate (DOP), triethyl citrate (TEC), or glycerol triacetate (GTA) as the plasticizing agent [12Gut]. In addition, fibers were also treated with acetone or mercerized to improve adhesion and compatibility with the matrix. Properties of the composites obtained are displayed in Table 68.18. As seen from the table, the addition of the plasticizer increased the tensile properties of the fibers, and the inclusion of the fibers decreased the thermal coefficient [12Gut]. Chemical treatments caused fibrillation and improved the bonding between the fibers and the matrix leading to better properties. In further continuation of their work, the effect of plasticizer and fiber content on the

Material	Tensile strength (MPa)	Tensile modulus (MPa)	Elongation at break (%)	Impact strength (J/m)
PLA	38 ± 3.3	3,546 ± 37	2.9 ± 1.1	24.7 ± 2.5
PLA/BF (90/10)	10.6 ± 1.2	3,963 ± 45	1.5 ± 0.2	14.7 ± 1.7
PLA/BF (80/20)	13 ± 1.6	4,210 ± 56	1.1 ± 0.6	15.2 ± 1.5
PLA/BF (70/30)	14.6 ± 1.4	4,631 ± 38	1.1 ± 0.3	19.1 ± 1.4
PLA/BF (60/40)	7.8 ± 1.8	4,705 ± 34	0.9 ± 0.5	13.5 ± 2.4
PLA/NaBF (70/30)	16.0 ± 1.5	4,636 ± 56	0.7 ± 0.1	19.7 ± 2.4
PLA/APS-BF (70/30)	17.5 ± 2.4	4,703 ± 23	0.9 ± 0.2	25.0 ± 2.4
PLA/Si69BF (70/30)	34.4 ± 2.6	4,815 ± 29	2.3 ± 0.5	30.0 ± 2.2

Table 68.17 Tensile properties and impact strength of PLA composites reinforced with various ratios of banana fibers with and without chemical treatments [13Jan]

Fig. 68.11 Plastic cutlery manufactured from banana fiber-reinforced PLA and nanoclay [13Jan]. Reproduced with permission from Elsevier

mechanical, morphological, and thermal performance of curaua fiber-reinforced composites was studied [14Gut]. Influence of plasticizer content and type of fiber on the tensile properties is shown in Table 68.17. When the concentration of the plasticizer is increased from 20 to 30 %, the non-reinforced fibers show a 100 % decrease in modulus and 120 % increase in elongation as seen in Table 68.19. Chemical treatment was found to increase modulus but decrease impact strength. However, the type of treatment (acetone/NaOH) did not make a significant differ­ence in tensile properties [14Gut].

Curaua fibers were also used as reinforcement for PHBV composites in ratios up to 30 %. The blend was first processed in a single screw extruder between 150 and 160 °C and later in a twin screw extruder at 170 °C [13Ros]. FTIR spectra of the neat PHBV and fiber-reinforced composites did not show any major difference suggesting that the fabrication process did not alter the chemical structure of PHBV. Figure 68.12 shows that the addition of the fibers increased the tensile

Table 68.18 Influence of the type of plasticizer and chemical modification of curaua fibers on the modulus, impact strength, and thermal properties of cellulose acetate-based composites [14Gut] Plasticizer Curaua fiber (10 %) Modulus (MPa) Impact strength (J/m) Thermal expansion coefficient (pm/mm/ K) T < Tg Tg < T DOP — 494 ± 17 327 ± 7 219 ± 12 333 ± 17 DOP Pristine 540 ± 10 73 ± 2 254 ± 14 699 ± 20 DOP Acetone 550 ± 13 58 ± 2 239 ± 13 870 ± 18 DOP Mercerization 531 ± 10 57 ± 3 179 ± 10 812 ± 17 TEC — 532 ± 18 232 ± 5 166 ± 9 324 ± 15 TEC Pristine 534 ± 18 60 ± 4 188 ± 11 599 ± 21 TEC Acetone 589 ± 11 61 ± 1 184 ± 10 623 ± 14 TEC Mercerization 559 ± 19 59 ± 2 173 ± 10 717 ± 16 GTA — 430 ± 19 222 ± 9 174 ± 10 690 ± 35 GTA Pristine 504 ± 18 94 ± 6 267 ± 14 763 ± 39 GTA Acetone 459 ± 18 74 ± 4 296 ± 16 755 ± 38 GTA Mercerization 545 ± 11 41 ± 2 237 ± 13 744 ± 38

and flexural strength and modulus up to a reinforcing fiber content up to 30 %. Tensile modulus increased from 3 to 5 MPa at a fiber content of 30 %. Contrarily, the addition of the fibers decreased the impact strength due to the reduction of the mobility of the polymer chains.

Lignocellulose fibers obtained from the borassus fruits (borassus fruit fibers) (BFF) were used as reinforcement for PP composites. BFF fibers were used as reinforcement without any treatment and also after treating with sodium hydroxide. To improve the compatibility between the matrix and reinforcing fibers, PP was also maleated [13Sud]. The mixture of PP and the fibers was pelletized and later injection molded into specimens for tensile testing. FTIR spectra suggested strong interaction between the matrix and the borassus fruit fiber. However, fiber agglo­meration and pullouts were seen under scanning electron microscope at higher levels of fiber loading. Some of the properties of the composites developed are given in Table 68.20. Figure 68.13 provides a comparison of the properties of the PP-BFF composites with coir, jute, and sisal fibers. Increasing the amount of BFF in the composites up to 15 % continually increased the tensile and flexural strength, but the strength slightly decreased at 20 % BFF. However, tensile and flexural modulus was found to increase even at 20 % BFF.

Jatropha, a plant that is widely promoted as a promising biodiesel crop, produces considerable amounts of by-products that have been used for biocomposite applications [13Abd]. The oil cake obtained after processing the jatropha seed for oil has been used to develop composites. However, so far, the jatropha oil cake has been used as reinforcement with styrene butadiene and glass-epoxy as matrix/ reinforcement. In addition to the oil cakes, other parts of the jatropha plant such

Table 68.19 Influence of plasticizer content and fiber structure and morphology on the cellulose acetate composite properties [14Gut] % Weight of plasticizer % Curaua fiber Strength (MPa) Strain at break {%) Modulus (GPa) Impact strength (J/M) Milled Milled + acetone Milled + mercerized 20 0 0 0 46 ±1 8.0 ±0.5 2.8 ±0.3 137 ±5 20 10 0 0 53 ±0.5 5 ±0.5 3.8 ±0.5 68 ±3 20 20 0 0 58 ±1 5 ±0.5 4.2 ± 0.5 67 ±3 30 0 0 0 21 ±1 20 ± 0.5 1.4 ±0.2 297 ±5 30 10 0 0 25 ± 0.5 12 ±1 2.2 ±0.2 161 ±5 30 0 10 0 41 ±1 7 ±0.5 3.5 ±0.3 88 ±4 30 0 0 10 40 ±1 6 ±0.5 3.5 ±0.4 88 ±4 30 20 0 0 29 ±1 5 ±0.5 2.8 ±0.3 152 ±5

0 10 20 30 0 10 20 30 Fiber cement (%) Fiber content (%) Fig. 68.12 Influence of fiber content on the tensile and flexural strength (a) and tensile and flexural modulus (b) properties of curaua fiber-reinforced PHBV composites [13Ros]. Reproduced with permission from John Wiley and sons

Table 68.20 Tensile, flexural, and impact resistance properties of borassus fruit fiber (BFF)- reinforced PP composites at various levels of fiber content [13Sud] Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa) Impact strength (J/m) PP 22.5 1.25 35.1 1.08 22.8 5 % BFF/PP 23.5 1.51 42.4 1.18 23.2 10 BFF/PP 25.3 2.07 47.2 1.22 26.1 15 BFF/PP 29.3 2.58 45.3 1.46 28.6 20 BFF/PP 28.6 3.01 44.6 1.80 24.6

as protein isolate, stem of the plant, and shell of the jatropha oil seed, can also been used for reinforcement [13Abd].

Bamboo has been one of the most preferred and widely used biomass to develop biocomposites. Various approaches have been used to process bamboo and make it suitable for use as reinforcement with several different kinds of polymers as matrix [12Kha]. Bamboo-based composite materials have been used to develop several products, and many such products are commercially available. Figure 68.14 shows some of the products developed using bamboo composites. Properties of some of the bamboo-based composites are listed in Table 68.21. A wide variation in properties is observed across the table due to the differences in the amount of reinforcement used, the type and amount of matrix, and the products developed and testing conditions employed. Several researchers have also suggested that bamboo can replace up to 25 % of glass fibers without any decrease in composite properties.

Micro — and nanofibrils obtained from bamboo were combined with modified soy protein concentrate to develop completely biodegradable composites. A dispersion of the bamboo fibrils (10-40 %) was added into a dispersion of soy protein

Tensile Strength

ВйШІ Flexural Strength

50 20 10

PP 5BFF 10BFF 15BFF 20BFF Coir Jute Sisal

Tensile Modulus

ШоЗ Flexural Modulus

PP 5BFF 10BFF 15BFF 20BFF Co r Jute S sa

Fig. 68.13 Influence of percentage of borassus fruit fibers on the tensile and flexural strength (a) and tensile and flexural modulus (b) of PP composites [13Sud]. Reproduced with permission from Elsevier concentrate containing glycerol, and the mixture was made into sheets. The sheets obtained were then compression molded at 120 °C for 25 min under a pressure of 8 MPa [09Hua]. To further improve the properties of the composites, the soy

Fig. 68.14 Examples of products that have been developed using bamboo composites [12Kha]. Reproduced with permission from Elsevier

protein concentrate was chemically modified using (3-isocyanatopropyl) triethoxysilane (ITES) as the cross-linking agent. Properties of the composites developed using various ratios of the bamboo microfibrils and glycerol are shown in Table 68.22. The influence of chemical modification of soy protein concentrate on the mechanical properties is also provided. As seen in Table 68.23, increasing the ratio of glycerol decreased the stress and modulus but increased the strain and moisture content. Cross-linking the soy protein results in a marginal increase (6 %) in the tensile strength and a 7 % increase in modulus. However, the toughness of the composites was increased to 4.6 MPa compared to 2.1 MPa without using the cross­linking agent [09Hua].

Bamboo fibers were used to reinforce poly(butylene succinate) (PBS) before and after grafting poly(ethylene glycol) methyl ether methacrylate [11Bao]. Bamboo

Table 68.21 Comparison of the properties of composites developed using bamboo as reinforce­ment [12Kha] Matrix Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (MPa) Polypropylene 25.8 ± 60.4 1.4 ± 0.9 45.5 ± 0.9 2,077 ± 4 Maleic anhydride — grafted PP 37.4 ± 0.5 1.4 ± 0.6 56.7 ± 0.5 2,929 ± 5 Maleic anhydride PP 35.1 ± 2.4 4.7 ± 0.6 — — Epoxy 86.6 — 120 11,901 Epoxy+ NaOH 135 — 149 9,500 Polyethylene 126 2.5 128.5 3,700 High-density polyethylene 25.5 2.7 27.9 2,912 HDPE-maleic anhydride 28.4 2.9 53.8 4,313 Isophthalate unsaturated polyester resin — — 38.7 ± 4.8 4,005 Polyethylene 74 — 107 4,373 Epoxy 86 — 119 11,901 Epoxy + polyethylene 135 — 149 9,500 Polypropylene 44 1,240 45.5 1,921 Polypropylene-maleic anhydride 47 1,426 52.3 2,097 Polypropylene — — 19.2 3.14 Epoxy 87 — 107 11,901 Epoxy-NaOH 135 — 154 9,500

Table 68.22 Comparison of the properties of the bamboo fibril-reinforced soy protein-based composites at various ratios of glycerol [09Hua] Soy/bamboo/ glycerol Stress (MPa) Strain (%) Modulus (MPa) Toughness (MPa) Moisture content (%) 100/30/15 59.3 13.2 1,816 6.0 13.9 100/30/10 65.7 8.5 2,095 4.1 13.1 100/30/5 78.3 6.5 2,596 3.0 12.6 100/30/2 75.7 4.9 3,019 2.1 11.9

fibers were cut into length of 3-5 mm, extruded with PBS in a twin screw extruder, and later compression molded into composites. SEM images of the fracture surface showed that composites containing unmodified bamboo fibers had smooth surfaces and long length fiber pullouts indicating poor compatibility. On the contrary, the grafted bamboo fibers showed rough fracture surfaces and holes due to fiber pullouts indicating good compatibility. Table 68.24 shows the properties of the PBS composites reinforced with various levels of unmodified and modified bamboo fibers. As seen from the table, the addition of the grafted bamboo fibers increased

Table 68.23 Comparison of the properties of the bamboo fibril-reinforced soy protein-based composites after cross-linking with various levels of the cross-linking agent [09Hua] Cross-linker (%) Stress (MPa) Strain (%) Modulus (MPa) Toughness (MPa) Moisture content (%) 0 75.7 4.9 3,019 2.1 11.9 2 75.7 5.9 3,218 3.3 12.0 5 73.6 6.1 3,271 3.5 11.6 7 75.5 6.8 3,020 3.9 11.4 10 81.6 7.0 3,184 4.3 11.0 12 80.0 7.3 3,221 4.6 11.0

The composition of the composites was 100 % soy protein concentrate, 30 % bamboo fiber, and 2 % glycerol

Table 68.24 Properties of composites reinforced with unmodified and grafted bamboo fibers [11Bao] Sample Fiber content (%) Strength (MPa) Elongation (%) Modulus (MPa) PBS 0 11.3 ± 0.4 5.0 ± 0.4 210 ± 9 PBS/BF 1 9.3 ± 0.6 4.9 ± 0.4 215 ± 10 PBS/BF 5 11.5 ± 0.3 5.1 ± 0.2 227 ± 12 PBS/BF 10 12.0 ± 0.6 4.8 ± 0.3 232 ± 13 PBS/BF 15 9.7 ± 0.5 4.6 ± 0.5 242 ± 20 PBS/BF-g- PPEGMA 1 13.1 ± 0.4 5.2 ± 0.3 232 ± 17 PBS/BF-g- PPEGMA 5 15.2 ± 0.7 5.5 ± 0.2 272 ± 19 PBS/BF-g- PPEGMA 10 17.5 ± 0.9 6.1 ± 0.3 293 ± 12 PBS/BF-g- PPEGMA 15 19.9 ± 0.8 6.5 ± 0.3 319 ± 10

the tensile strength, elongation, and modulus due to the improved interfacial adhesion [11Bao].

Coir fibers were treated with basic chromium sulfate and sodium bicarbonate salt in acidic media to improve biocompatibility, and various levels (10, 15, and 20 %) of the modified fibers were used to reinforce polypropylene composites [13Mir]. Tensile strength of the composites containing raw coir fibers was lower than that of the chemically modified fibers. Increasing the content of the raw or the chemically treated fibers decreased the tensile strength as seen in Fig. 68.15. However, flexural strength and impact resistance were found to increase with increasing fiber content. Lower moisture absorption and better mechanical properties were obtained for the CrSO4-treated samples followed by the NaHCO3-treated fibers. A coir fiber content of 20 % was found to provide the most optimum properties for the polypropylene composites [13Mir].

Fig. 68.15 Changes in the tensile and flexural strength and flexural modulus with increasing content of coir fibers [13Mir]. Reproduced with permission from Elsevier

Apart from the lignocellulosic fibers or agricultural residues that have been used as reinforcement for composites, poultry feathers that are available in large quantities at almost no cost have also been extensively studied for composite applications. In addition to their low cost and large availability, feathers have unique hierarchical structure and low density that make them ideal for developing lightweight composites for automotive and other applications. Composites have been developed using whole feather, feather fiber, and powdered quill as reinforce­ment for composites. Table 68.25 presents the properties of the feather-reinforced composites in comparison to jute-reinforced composites. As seen from the table, feather-reinforced composites provide similar tensile and flexural properties to the composites compared to the synthetic polymer-based composites. In addition, the noise reduction coefficient of the feather-reinforced composites is considerably higher than that of the traditional fiber-reinforced composites.

Table 68.25 Comparison of the properties of feather fiber-reinforced HDPE/PP or PP composites with jute fiber-reinforced composites at their respective optimized conditions [08Hud2, 09Hud2] Composite Concentration of reinforcing fiber (%) Thickness (mm) Strength (N) Stiffness (N/mm) Toughness (%) Noise reduction coefficient Feather fiber — HDPE/PP 35 4.2 16.6 ±0.7 2.42 ± 0.20 133 ±4 0.17 Quill-PP 35 3.2 20.1 ±3.0 2.19 ±0.27 170 ±2 0.11 Jute-PP 60 3.2 22.4 ± 0.5 5.16 ±0.23 115 ±9 0.07

References

[00Li] Li, Y., Mai, Y. W., Ye, L.: Compos. Sci. Technol. 60, 2037 (2000)

[04Uma] Uma Devi, L., Joseph, K., Nair, K. C.M., Thomas, S.: J. Appl. Polym. Sci. 94,

503 (2004)

[06Cao] Cao, Y., Shibata, S., Fukumoto, I.: Compos. Part A 37, 423 (2006)

[08Hud1] Huda, S., Yang, Y.: Macromol. Mater. Eng. 293, 235 (2008)

[08Hud2] Huda, S., Yang, Y.: Compos. Sci. Technol. 68(3-4), 790 (2008)

[08Lee] Lee, S. C., Mariatti, M.: Mater. Lett. 62, 2253 (2008)

[09Hua] Huang, X., Netravali, A.: Compos. Sci. Technol. 69, 1009 (2009)

[09Hud1] Huda, S., Yang, Y.: Ind. Crops Prod. 30, 17 (2009)

[09Hud2] Huda, S., Yang, Y.: J. Polym. Environ. 17, 131 (2009)

[10Ash] Ashori, A., Nourbakhsh, A.: Waste Manag. 30, 680 (2010)

[10Pra] Pradhan, R., Misra, M., Erickson, L., Mohanty, A.: Bioresour. Technol. 101, 8489 (2010)

[11Aha] Ahankari, S. S., Mohanty, A. K., Misra, M.: Compos. Sci. Technol. 71, 653-657 (2011)

[11Bao] Bao, L., Chen, Y., Zhou, W., Wu, Y., Huang, Y.: J. Appl. Polym. Sci. 122, 2456 (2011)

[11Cer] Cerqueira, E. F., Baptista, C. A.R. P., Mulinari, D. R.: Procedia Eng. 10, 2046 (2011)

[11Che] Cherian, B. M., Leao, A. L., de Souza, S. F., Costa, L. M.M., de Olyveira, G. M.,

Kottaisamy, M., Nagarajan, E. R., Thomas, S.: Carbohydr. Polym. 86, 1790 (2011) [11Cho] Chollakup, R., Tantatherdtam, R., Ujjin, S., Sriroth, K.: J. Appl. Polym. Sci. 119, 1952 (2011)

[11Myl] Mylsamy, K., Rajendran, I.: Mater. Des. 32, 3076 (2011)

[11Shi] Shinoj, S., Visvanathan, R., Panigrahi, S., Kochubabu, M.: Ind. Crops Prod. 33,

7 (2011)

[12Gut] Gutierrez, M. C., Paoli, M. D., Felisberti, M. I.: Compos. Part A 43, 1338 (2012)

[12Kha] Khalil, H. P.S. A., Bhat, I. U.H., Jawaid, M., Zaidon, A., Hermawan, D., Hadi, Y. S.:

Mater. Des. 42, 353 (2012)

[13Abd] Abdul Khalil, H. P.S., Sri Aprilia, N. A., Bhat, A. H., Jawaid, M., Paridah, M. T., Rudi, D. A.: Renew. Sustain. Energy Rev. 22, 667 (2013)

[13Ish] Ishak, M. R., Sapuan, S. M., Leman, Z., Rahman, M. Z.A., Anwar, U. M.K., Siregar, J. P.: Carbohydr. Polym. 91, 699 (2013)

[13Jan] Jandas, P. J., Mohanty, S., Nayak, S. K.: J. Clean. Prod. 52, 392 (2013)

[13Mir] Mir, S. S., Nafsin, N., Hasan, M., Hasan, N., Hassan, A.: Mater. Des. 52, 251 (2013) [13Nag1] Nagarajan, V., Misra, M., Mohanty, A. K.: Ind. Crops Prod. 42, 461 (2013)

[13Nag2] Nagarajan, V., Mohanty, A. K., Misra, M.: ACS Sustain. Chem. Eng. 1, 325 (2013)

[13Ros] Rossa, L. V., Scienza, L. C., Zattera, A. J.: Polym. Compos. 34, 450 (2013)

[13Sud] Sudhakara, P., Jagadeesh, D., Wang, Y., Prasad, C. V., Devi, A. P., Balakrishnan, G.,

Kim, B. S., Song, J. I.: Carbohydr. Polym. 98, 1002 (2013)

[14AlO] Al-Oqla, F. M., Sapuan, S. M.: J. Clean. Prod. 66, 347 (2014)

[14Gut] Gutierrez, M. C., Paoli, M. D., Felisberti, M. I.: Ind. Crops Prod. 52, 363 (2014) [14Ibr] Ibrahim, H., Farag, M., Megahed, H., Mehanny, S.: Carbohydr. Polym. 101, 11 (2014)

[14Ken] Kengkhetkit, N., Amornsakchai, T.: Mater. Des. 55, 292 (2014)

[14Nou] Nourbakhsh, A., Ashori, A., Tabrizi, A. K.: Compos. Part B 56, 279 (2014)

Natural Cellulose Fibers from Renewable Resources

Keywords

Sugarcane • Bagasse • Rind • Pith

Sugarcane is the world’s largest crop grown in about 23.8 million hectares with a total harvest of about 1.69 billion tons in 2010. After squeezing the canes for sugar, the remaining materials, generally called bagasse, are obtained as coproducts. About 30-32 % by weight of the cane is produced as coproducts [08Lee]. Bagasse is a lignocellulosic material consisting of 45-55 % cellulose, 20-25 % hemicellu — lose, and 18-24 % lignin. Sugarcane stems consist of three major parts: the pith (5 %), fibers (73 %), and the rind (22 %). Both the pith and the outer rind have been studied as sources for fibers. The pith has a considerably lower density (220 kg/m3) and consists of coarse fibers and many large cavities compared to the rind with a density of 550 kg/m3. In Brazil, the average price for a ton of bagasse is between $3.5 and $11.8, making it one of the cheapest lignocellulosic agricultural by-products [04Fil]. Unlike the fibers obtained from the oil plants, bagasse fibers are reported to have considerably low elongation (1.1 %) and moderate strength of about 222 MPa (1.7 g/den) and modulus of 27 GPa (208 g/den) [04Tri, 09Gui]. Compared to the lignocellulosic fibers obtained from other agricultural by-products, relatively fewer studies have been conducted to understand the poten­tial and properties of obtaining fibers from sugarcane bagasse. Fibers obtained from sugarcane bagasse were reported to have a fineness of 6.5-14 tex and length from

2.5 to 20 cm. In another research, fibers with strength of 290 MPa (2.2 g/den) and modulus of 17 GPa (13.1 g/den) were obtained from sugarcane stems [12Far].

References

[04Fil] Filho, P. A., Badr, O.: Appl. Energy 77(1), 51 (2004)

[04Tri] Trindade, W. G., Ilce, W. H., Razera, A. T., Ruggiero, R., Frollini, E., Castellan, A.:

Macromol. Mater. Eng. 289, 728 (2004)

[08Lee] Lee, S. C., Mariatti, M.: Mater. Lett. 62, 2253 (2008)

[09Gui] Guimaraes, J. L., Frollini, E., Silva, C. G., Wypych, F., Satyanarayana, K. G.: Ind. Crop. Prod. 30, 407 (2009)

[12Far] Faruk, O., Bledzki, A. K., Fink, H., Sain, M.: Prog. Polym. Sci. 37, 1552 (2012)

Chitin, Chitosan, and Alginate Fibers

Keywords

Alginate fiber • Tensile properties • Cellulose • Nanocrystal • Montmorillonite • Carbon nanotube • Cross-linking

In view of the relatively low mechanical properties of alginate fibers, efforts have been made to improve the tensile properties and stability by adding various additives and also by cross-linking. In one such attempt, cellulose nanocrystals (CNC) were isolated and used as fillers to improve the properties of alginate fibers with the expectation that cellulose and alginate would have good compatibility and that the negatively charged sulfate groups on cellulose crystals would have electro­static interaction with the Ca2+ ions in the coagulation bath [10Ure]. Various levels of the nanocrystals (0-10 %) were added into the fibers, and the fibers were extruded at different jet-stretch ratios. It was found that increasing the level of CNC in the fibers decreased, whereas increasing the jet stretch increased the strength and elongation. The tenacity (0.1-0.22 g/den) of the fibers was very low even with the addition of CNC and extrusion at the highest jet speed possible [10Ure]. However, the fibers had about 38 % increase in tenacity and 123 % increase in modulus due to the addition of the filler and optimization of jet speed [10Ure]. Further investigation on the arrangements of the CNC in the alginate fibers showed that the degree or orientation decreased with increasing load of CNC. The interaction of the nanoparticles with the polymer introduced twists opposite to the direction of drawing, and at high concentrations, the crystallites oriented them­selves in a spiral manner in the alginate matrix, similar to the arrangement of fibrils in native cellulose [11Ure]. Such spiral arrangement decreased the strength and modulus of the fibers. However, it was reported that fibers with improved toughness could be obtained by controlling the processing conditions without sacrificing the strength of the fibers [11Ure]. Table 31.1 presents some of the properties of the fibers at two different jet-stretch ratios and different levels of nanoparticle loading.

Table 31.1 Properties of alginate fibers containing various levels of cellulose nanocrystals CNC load Spiral angle Width Tenacity Modulus Toughness (%) (deg) (deg) (x102 g/den) (g/den) (x102 g/den)

2.4 jet stretch 0 0 — 14.9 ± 0.4 3.1 ± 0.1 3.3 ± 0.2 2 3.6 ± 0.8 21 ± 5 15.2 ± 0.5 2.6 ± 0.1 3.9 ± 0.1 5 3.8 ± 0.5 15 ± 2 14.4 ± 0.3 2.8 ± 0.1 3.7 ± 0.1 10 — 16 ± 2 12.9 ± 0.3 2.5 ± 0.1 3.0 ± 0.1 20 3.6 ± 0.2 18 ± 0.3 17.8 ± 0.4 6.8 ± 0.2 2.4 ± 0.1

Max jet stretch 0 — — 14.9 ± 0.4 3.1 ± 0.1 3.3 ± 0.2 2 0 17 ± 3 18.0 ± 0.7 3.8 ± 0.2 3.6 ± 0.2 5 0 18 ± 1 18.7 ± 0.5 5.5 ± 0.1 2.4 ± 0.1 10 0 18 ± 0.6 20.6 ± 0.7 6.9 ± 0.2 2.5 ± 0.1 25 3.0 ± 0.2 17 ± 0.5 16.4 ± 0.6 6.2 ± 0.2 1.6 ± 0.1 20 3.2 ± 0.2 19 ± 0.4 16.4 ± 0.5 6.9 ± 0.2 1.8 ± 0.1 From Urena-Benavides et al. [10Ure]

Two types of nanoadditives p-tricalcium phosphate (TCP) (110 nm) or montmo — rillonite (MMT) (20-1,000 nm) were added (3 % on weight of the polymer) into calcium alginate fibers, and the influence of the additives on the fiber formation process and the properties of fibers were exhaustively investigated [10Bog]. The addition of the nanoadditives did not affect the viscosity and also did not alter the % crystallinity or crystal structure. Some of the properties of the fibers containing TCP or MMT and obtained at different draw ratios are compared in Table 31.2. The addition of the nanoparticles changed the water retention and the strength and elongation of the fibers. Increasing the draw ratio increased the strength, but the elongation showed varying trend probably because of the presence of the nanoadditives and also due to the varying calcium content in the fibers. Similarly, tricalcium phosphate (TCP) and silica in nanoform were added into zinc alginate fibers to improve the functional properties of the fibers. The addition of TCP was found to decrease the strength, whereas silica increased the strength of the fibers [09Mik, 10Mik]. Tensile strength of up to 3.3 g/den was obtained when silica was added compared to 2.6-2.7 g/den for fibers without the nanoadditive. The strength of the fibers with TCP was between 2.3 and 2.8 g/den compared to 2.8-3.3 g/den for fibers without TCP.

Silver nanoparticles were loaded onto alginate hydrogel fibers intended for wound healing [12Nei]. Alginate was spun into calcium chloride bath and formed into fibers. Later, the fibers were immersed in water-acetone mixture containing glutaraldehyde to cross-link the fibers. Silver nanoparticles were loaded onto the fibers via ion exchange by immersing in silver nitrate solution, and the excess of silver nitrate was removed. Then, the fibers were introduced into sodium borohy — dride solution to convert the silver ions into metallic silver. After this step, the fibers were washed three times in water and air-dried. The fibers obtained had

Table 31.2 Properties of alginate fibers containing various levels of tricalcium phosphate (TCP) and montmorillonite (MTT) (from Bogun et al. [10Bog] Fiber type Draw ratio Total pore volume (cm3/g) Water retention (%) Crystallinity (%) Tenacity (g/den) Elongation (%) Alginate + TCP 50 0.196 97.6 27 1.8 6.4 Alginate + TCP 70 0.078 93.0 27 2.8 10.4 Alginate + TCP 90 0.244 85.6 28 2.5 9.1 Alginate + TCP 110 0.214 86.3 26 2.2 7.4 Alginate + TCP 120 0.137 102.2 27 2.1 7.4 Alginate + MMT 50 0.127 107.5 26 2.5 9.9 Alginate + MMT 70 0.091 98.2 27 2.9 10.6 Alginate + MMT 90 0.064 101.3 26 2.8 10.0 Alginate + MMT 110 0.140 96.5 26 2.5 9.3 Alginate + MMT 120 0.172 118.7 29 2.2 10.4

superabsorbent properties and absorbed more than ten times their own weight of water because of the formation of ionizable COOH groups when carboxyl groups in alginate reacted with the catalyst HCl used during cross-linking. It was also reported that the chemically cross-linked fibers had a 20-fold swelling compared to about threefold for the ionically cross-linked fibers. The inclusion of silver nitrate did not affect the cytocompatibility of the fibers, and silver nanoparticles were also considered to be nontoxic within the range studied in this research. The presence of silver nanoparticles increased wound healing and also lowered inflammatory response. Figure 31.1 shows that the presence of silver nanoparticles led to higher epidermal thickness on wounds incised on mice suggesting better growth of fibroblasts.

A natural hydroxyapatite with average specific surface of 73.6 m2/g and particle size from 30 to 500 nm was used as an additive in calcium alginate fibers to improve tensile properties and porosity and make the fibers suitable for bone tissue engi­neering [09Bog]. To prepare the fibers, sodium alginate (7.4 %) was dissolved in distilled water with 3 % additive based on the weight of the alginate. After extrusion, the fibers were passed into a coagulation bath containing CaCl2 and HCl to substitute sodium with calcium and form calcium alginate fibers. Fibers were drawn to various extents in a two-step process. Some of the properties of the

Poitopttttive d«y 10 Fig. 31.1 Growth of epidermal tissue (after 10 days) for the alginate fibers containing various levels of silver nanoparticles from Neibert et al. [12Nei]. Reproduced with permission from Elsevier

fibers obtained with and without hydroxyapatite are listed in Table 31.3. Although the addition to hydroxyapatite did not show a major change in tensile properties, significant changes were observed in terms of % crystallinity and pore volume. The changes were thought to be due to the higher amounts of calcium when hydroxyap­atite was present. It was suggested that the fibers obtained were suitable for use in biocomposites intended for bone tissue regeneration [09Bog]. A unique aspect of the fibers was the relatively high moisture sorption (24-25 %) compared to the common cellulose and protein fibers.

Carbon nanotubes (23 %) were added into alginate fibers by electrostatic assem­bly, and the blend fibers were expected to be suitable for use in supercapacitors, artificial muscles, biomedical sensors, and other applications [11Sa]. Single-walled carbon nanotubes were coated with ionic surfactant sodium dodecyl sulfate (SDS) and then added into the sodium alginate solution. The solution was aged overnight and later wet spun using a syringe into an aqueous solution containing calcium chloride for the fibers to precipitate. A digital picture of the alginate-nanotube fiber formation is shown in Fig. 31.2. The addition of the nanotubes substantially increased the modulus and strength. Morphologically, fibers were found to fibrillate (Fig. 31.3) with increase in fibrillation as the concentration of the nanotubes increased [11Sa].

Nanosilica (25 nm diameter) was added into sodium alginate solution (5 %) and extruded into a calcium chloride bath. The addition of nanosilica increased the tenacity from 5.8 to about 8.2 g/den, and the elongation also increased from about 13 to 16 %. However, both the strength and elongation decreased at high levels of

Table 31.3 Influence of draw ratio on the properties of alginate and alginate hydroxyapatite blend fibers Fiber type Draw ratio Tenacity (g/den) Elongation (%) Crystallinity (%) Calcium content (%) Moisture sorption3 (%) Alginate + TCP 50 2.7 ±0.08 8.8 ±0.5 27 ±0.8 8.7 24.6 ±0.7 Alginate + TCP 70 3.2 ±0.08 10 ±0.4 31 ± 0.9 8.9 24.4 ±0.7 Alginate + TCP 90 2.7 ± 0.09 8.6 ±0.3 31 ± 0.9 9.0 24.5 ±0.7 Alginate + TCP 110 2.7 ±0.1 8.4 ±0.5 30 ±0.9 8.7 24.4 ±1.0 Alginate + TCP 120 2.5 ±0.1 7.7 ±0.4 32 ± 1.0 9.1 24.4 ±1.0 Alginate + hydroxyapatite 50 2.8 ±0.2 9.3 ±0.6 26 ±0.8 9.0 25.4 ±1.0 Alginate + hydroxyapatite 70 2.9 ±0.1 9.8 ±0.4 25 ±0.8 9.5 24.2 ± 0.7 Alginate + hydroxyapatite 90 2.7 ±0.1 9.4 ± 0.5 26 ±0.8 9.1 24.4 ±1.0 Alginate + hydroxyapatite 110 2.3 ±0.1 7.8 ±0.5 28 ±0.8 9.8 24.1 ±0.7 Alginate + hydroxyapatite 120 2.5 ±0.08 9.7 ±0.5 29 ±0.7 9.8 24.5 ±1.0 a65 % Humidity Reproduced from Bogun et al. [09Bog]

Fig. 31.2 Process of producing alginate-nanocomposite fibers. From Sa and Kornev [11Sa]. Reproduced with permission from Elsevier

Fig. 31.3 Confocal laser scanning image shows the striations formed in the alginate-nanotube composite fibers after addition of 23 % nanotubes (right) compared to fibers without any nanotubes (left). Reproduced with permission from Elsevier [11Sa]

silica (8 % or higher). Moisture absorption of the fibers did not change significantly, whereas water retention decreased from 72 to 54 % (9 % silica).

Porous calcium alginate fibers were loaded with TiO2 photocatalysts for mem­brane filtration process. It was expected that the high porosity in the alginate fiber could help to embed the catalysts inside the fiber and help to protect the catalysts from destabilization [12Pap]. Photodegradation of pollutants such as methyl orange was much higher in the alginate fibers containing TiO2 compared to the powder TiO2 due to the high surface area and excellent dispersion and stability of the catalysts in the fiber matrix [12Pap].

Although alginates have been widely used for medical applications, alginates lack specific cellular interactions that limit their use for regenerative applications [05Hou]. Chemical modifications have been used to make alginate more suitable for medical applications, but such modifications are often detrimental to cells and to sensitive biological agents. To avoid chemical modifications, a physical entrapment process was developed to incorporate bioactive molecules within alginate fibers. Fibers were first pre-swollen in sodium chloride/calcium chloride solutions and then immersed in a solution containing various concentrations of rhodamine-tagged polyethylene glycol. Later, the fibers were cross-linked by immersing in barium chloride (5 % w/v) for 15 min. A poly(L-lysine) (PLL) was coupled to GRCDS peptide and entrapped in the alginate fibers. Cell adhesion and proliferation studies on the fibers were done using mouse fibroblast (3T3) cells. It was reported that entrapping PLL-GRDS in the fibers promoted cell proliferation and growth.

Sodium alginate (3 %) was formed into alginic acid gel fibers and later dehydrated and cross-linked with glutaraldehyde to form superabsorbent filament fibers [00Kim]. Cross-linking was done at 50 °C for 4 h by immersing the fibers in various concentrations of glutaraldehyde and 0.1 % HCl. An absorbency of about 80 g/g of fibers was obtained for saline, whereas the absorbency was about 50 % for synthetic urine. Increasing the extent of cross-linking decreased the absorbency considerably. Uncross-linked fibers had tenacity of 1.2 g/den and elongation of

27.1 %. Cross-linking slightly decreased the breaking tenacity, but the elongation decreased by about 35 % at high levels of cross-linking.

References

[00Kim] Kim, Y., Yoon, K., Ko, S.: J. Appl. Polym. Sci. 78, 1797 (2000)

[05Hou] Hou, Q., Freeman, R., Buttery, L. D.K., Shakesheff, K. M.: Biomacromolecules 6, 734 (2005)

[09Bog] Bogun, M., Mikolajczyk, T., Rabiej, S.: J. Appl. Polym. Sci. 114, 70 (2009)

[09Mik] Mikolajczyk, T., Bogun, M., Kurzak, A., Szparaga, G.: Fibers Text East. Eur. 17(2), 12-18 (2009)

[10Bog] Bogun, M., Mikolajczyk, T., Rabiej, S.: Polym. Comp. 31, 1321 (2010)

[10Mik] Mikolajczyk, T., Bogun, M., Rabiej, S., Krol, P.: Fibers Text East. Eur. 18(6), 39-44

(2010)

[10Ure] Urena-Benavides, E. E., Brown, P. J., Kitchens, C. L.: Langmuir 26(17), 14263 (2010) [11Sa] Sa, V., Kornev, K. G.: Carbon 49, 1859 (2011)

[11Ure] Urena-Benavides, E. E., Kitchens, C. L.: Macromolecules 44, 3478 (2011)

[12Nei] Neibert, K., Gopishetty, V., Grigoryev, A., Tokarev, I., Al-Hajaj, N., Vorstenbosch, J., Philip, A., Minko, S., Maysinger, D.: Adv. Healthcare Mater. 1, 621 (2012) [12Pap] Papageorgiou, S. K., Katsaros, F. K., Favvas, E. P., Romanos, G. E., Athanasekou, C. P., Beltsios, K. G., Tzialla, O. I., Falaras, P.: Water Res. 46, 1858 (2012)

To improve the solubility and obtain fibers with good properties, chitosan with Mw as high as 1,400 kDa and 5-25 % PEO was dissolved in a mixture of aqueous acetic and hydrochloric acids and electrospun into fibers [08Des]. Fibers with average diameters of 80 ± 35 nm were obtained with a 95:5 chitosan:PEO blend. Chromium binding as high as 18 mg/g of chitosan was obtained when high molecular weight chitosan and PEO were used, much higher than films with similar ratios of chitosan/PEO.

Bicomponent electrospun fibers were obtained using high molecular weight (1,600 kDa, 82.5 % degree of deacetylation) chitosan and PVA. Addition of PVA was thought to increase the molecular entanglement and lead to fiber formation with chitosan content as high as 50 %. Fibers with diameters ranging from 20 to 100 nm were obtained. It was found that hydrolysis of chitosan into lower molecu­lar weights substantially improved fiber formation. PVA component in the bicom­ponent fibers was removed by treating the electrospun structures with 1 M aqueous NaOH for 12 h. Removal of PVA resulted in porous chitosan fibers with pore diameters ranging from 10 to 100 nm [06Li]. A TEM image of the porous chitosan fiber is shown in Fig. 58.4.

Chitosan has been conventionally blended with PVA or PEO in weight ratios up to 50 % to improve spinnability and obtain fibers for medical and other applications. However attempts to reduce PEO/PVA component and obtain chitosan fibers had limited success. By using ultrahigh molecular weight PEO (UHMWPEO), Zhang et al. reported the production of chitosan fibers with PEO content as low as 5 % and ability to produce fibers with diameters from 100 nm to several micrometers. Aligned nanofibers that could be bundled and made into yarns shown in Fig. 58.5 were obtained [08Zha1].

In a similar study, electrospun fibers were developed from a blend of chitosan, collagen, and PEO and the membranes were cross-linked using glutaraldehyde. Fibers with diameters of 134 ± 42 nm were obtained. Cross-linking led to increase in fiber diameters and modulus, but substantial decrease in elongation, strength, and water absorption was observed [08Che2]. Matrices were found biocompatible to 3T3 fibroblasts in vitro and the in vivo studies indicated that the matrices were better than gauze and commercial collagen sponge wound dressing for wound healing. Addition of chitosan into collagen was found to improve cell attachment and proliferation and provide stable membranes after cross-linking [10Che]. The chitosan blend membranes developed had chitosan contents ranging from 0 to 100 % and strength from 1 to 10 MPa with elongations between 5 and 40 %.

The chelating properties of chitosan were used to study the effect of metal ions on the morphology and integrity of electrospun chitosan structures using a blend of chitosan/PEO solutions [11Su]. The influence of monovalent, bivalent, and triva­lent ions on electrospinnability and morphology of fibers was investigated. Calcium and iron ions reduced fiber diameters and number of beads in the fibers whereas sodium and potassium chloride ions recrystallized and were distributed homo­geneously in the fibers due to the inter — and intramolecular interactions between metal ions and the protonated chitosan [11Su].

Aligned or randomly oriented chitosan nanofibers were produced using chitosan: PEO in 9:1 ratio with acetic acid as the solvent. To improve biocompatibility, the chitosan surfaces were grafted with RGD containing surfaces using bifunctional polyethylene glycol (PEG) chains as the cross-linking agent. RGD containing scaffolds were found to have significantly higher cell compatibility. Although orientation of the nanofibers did not show much effect on cell proliferation, the cell morphology and guidance were influenced by the orientation of the fibers [10Wan].

In another study, carboxymethyl cellulose (CMC) was blended with PVA and made into nanofibers for tissue engineering applications. CMC and PVA were separately dissolved in water and mixed in various proportions and electrospun into fibers. Membranes containing CMC/PVA in 20/80 ratio were cross-linked using glutaraldehyde vapors and heat. Uncross-linked fibers dissolved in water after immersion for 1 h, whereas cross-linked membranes did not dissolve after being in water for 48 h. The membranes obtained could be mineralized using calcium phosphate and were also found to be suitable for culturing mesenchymal stem cells [09Sha]. A 50/50 blend of chitosan and PEO (600 kDa) was electrospun into fibers and cross-linked to various extents using glutaraldehyde [08Von]. Increasing cross-linking time from 10 min to 20 h increased tensile elastic modulus from 0.1 to 2.6 MPa, but the stability and morphological changes after exposure to water were not investigated. Instead of using NaOH in the coagulation bath, it was shown that using saturated sodium carbonate would enable the forma­tion of fibers that were stable in PBS or distilled water for up to 12 weeks [06San].

Antibacterial chitosan nanofibers were obtained by blending chitosan/PVA and adding silver nitrate and titanium dioxide. Fibers with diameters ranging from 270 to 360 nm and ability to inhibit 98-99 % of Escherichia coli and Staphylococ­cus aureus were obtained [09Son].

Biomimetic nanocomposites were prepared using chitosan and hydroxyapatite nanoparticles and then electrospinning the mixture into fibers with the addition of 10 % of ultrahigh molecular weight PEO [08Zha2]. Electrospun matrices containing 70/30 chitosan/hydroxyapatite were seeded with human fetal osteoblast and the ability of the matrices to support the attachment, growth and mineralization were studied. Hydroxyapatite nanoparticles that were synthesized had lengths of 100 nm and a diameter of 30 nm and the electrospun fibers had an average diameter of 214 ± 25 nm. TEM images of the electrospun nanofibers in Fig. 58.6 show that the spindle-shaped hydroxyapatite nanoparticles were distributed across the length of fiber with regions of aggregation. After 10-15 days of culture, the nanofibrous scaffolds were completely covered with layers of cells, secreted extracellular matrix, and mineral deposits. The extents of mineral deposits were found to be much higher on the hydroxyapatite/chitosan scaffolds compared to pure chitosan. Figure 58.7 shows an SEM image of the formation of the extracellular matrices and cluster of mineral deposits on the composite fibers [08Zha2].

Core-shell fibers with chitosan as core and PEO as sheath were developed with fiber diameters of about 250 nm using 3 % chitosan and 4 % PEO in water. Later, PEO was removed by washing with deionized water to obtain chitosan nanofibers

of approximately 100 nm in diameter [08Ojh]. It was also reported that bicompo­nent fibers with chitosan as sheath could be developed using a similar approach.

Blends of chitosan with PEO were made into nanofibers with the assistance of triton X-100 (0.3 %) and DMSO as a co-solvent. Fibers with diameters as fine as 40 nm were obtained and nanofibers developed from 90/10 blend of chitosan/PEO were stable in water and had good cell compatibility [05Bha]. Osteoblasts and chondrocytes cultured on the chitosan/PEO blend matrices showed excellent attachment, growth, and proliferation. Figure 58.8 shows an SEM image of the cells on the scaffolds after 5 days of culture. Scaffolds developed were considered to be suitable for tissue engineering applications.

Blends of collagen, chitosan, and thermoplastic polyurethane (TPU) (60/15/ 25 %) were made into random and aligned nanofibrous scaffolds. The scaffolds developed were cross-linked with glutaraldehyde vapors and characterized for structure and properties and evaluated for potential use as tubular grafts and nerve conduits [11Hua]. It was proposed that collagen and chitosan could mimic the protein and polysaccharide parts in extracellular matrices. Electrospun fibers had diameters in the range of 256-360 nm and the matrices were rolled into tubes and sutured for eventual use as nerve grafts. Figure 58.9 shows pictures of the vascular graft and nerve conduit developed. Addition of TPU increased the mechanical properties as seen from Table 58.6. Cell proliferation and orientation on the blend scaffolds was found to be considerably higher than that on TPU suggesting that the scaffolds could be used for tissue engineering [11Hua].

Chitosan was quaternized, blended with PVA, and electrospun into fibers with diameters ranging from 60 to 200 nm [06Ign]. The membranes were exposed to UV irradiation and cross-linked with TEGDA as the cross-linking agent to improve water stability. Membranes developed exhibited antimicrobial activity to both S. aureus and E. coli.

Fig. 58.7 SEM images showing the deposition of minerals on the nanofibrous chitosan scaffolds after 10 and 15 days (a and c), chitosan-hydroxyapatite scaffolds after 10 and 15 days (b and d, respectively). e and f are higher magnification images showing the minerals and collagen bundles. From Zhang et al. [08Zha2]. Reproduced with permission from Elsevier

Fig. 58.8 SEM (left) and confocal (right) images depicting the growth of chondrocytes on the chitosan/PEO (90/10) scaffolds 5 days after seeding [05Bha]. Reproduced with permission from Elsevier

Fig. 58.9 Macrographic image of small diameter electrospun vascular graft and nerve conduit [11Hua]. Reproduced with permission from Elsevier

Table 58.6 Properties of the chitosan-collagen-TPU scaffold before and after cross-linking with glutaraldehyde Type of scaffold Thickness (mm) Tensile strength (MPa) Elongation (%) Randomly oriented Non-cross- linked 0.086 ± 0.008 4.6 ± 0.2 61.3 ± 3.9 Cross-linked 0.082 ± 0.005 9.4 ± 1.0 9.9 ± 1.8 Aligned, parallel Non-cross- linked 0.080 ± 0.006 10.3 ± 1.7 30.1 ± 5.3 Cross-linked 0.079 ± 0.006 14.9 ± 0.6 58.9 ± 15.5 Aligned, perpendicular Non-cross- linked 0.084 ± 0.009 2.1 ± 0.1 69.9 ± 8.7 Cross-linked 0.081 ± 0.004 5.0 ± 1.0 8.2 ± 0.8 Reproduced from Huang et al. [11Hua]

Formic acid/acetone solvent mixture was used to produce chitosan/poly (caprolactone) (PCL) nanofibers. Amount of chitosan in the solution was 1 % and PCL was 8 % to obtain fibers with diameters of about 116 nm. However, the stability of the fibers in aqueous systems was not studied [10Sha].

Blends of chitosan and poly(lactic acid) were prepared using trifluoroacetic acid as a co-solvent [09Xu1]. Fibers with diameters ranging from 300 to 1,100 nm were obtained and the diameter of the fibers increased almost linearly with increasing ratio of PLA in the blend. Weak interactions between PLA and chitosan were observed using FTIR.

Electrospun fibers for tissue regeneration were prepared using a blend of poly (lactide-co-glycolide) (PLGA), chitosan, and PVA [06Dua]. Chitosan used had a degree of deacetylation of 90 % and a molecular weight of 165 kDa and PVA had a degree of polymerization of 1,750. Fibers were electrospun from PLGA and from a mixture of PVA/Chitosan or PLGA/PVA and chitosan. PLGA was dissolved using tetrahydrofuran (THF) and N, N-dimethylformamide and PVA/chitosan was dissolved using aqueous acetic acid. One syringe containing PLGA and another with PVA/chitosan solution were co-electrospun onto a collection drum and later cross-linked using glutaraldehyde vapors. Table 58.7 shows properties of the electrospun membranes obtained and Fig. 58.10 shows the changes in the dimensions of the membranes before and after incubation in PBS. As seen in the table, the blend membranes had lower strength and modulus but higher elongation than PLGA. Also, cross-linking substantially decreased the swelling of the membranes. Electrospun chitosan/PVA membrane shrunk to 25 % of its original size, whereas the PLGA-chitosan-PVA composite membrane had a shrinkage rate of 47.4 % before cross-linking and 3.2 % after cross-linking [06Dua]. Fibroblasts cultured on the composite membranes showed good attachment and proliferation indicating that the fibers would be suitable for tissue engineering applications [06Dua].

Similarly, chitosan nanofibers implanted subcutaneously in mice did not show any significant changes in morphology after 7 days, but inflammatory cells such as

Table 58.7 Tensile properties and shrinkage of the various chitosan blends [06Dua] Tensile properties Electrospun fibers Strength (MPa) Elongation (%) Modulus (MPa) Shrinkage in PBS (%) PLGA 7.3 ± 1.5 2.9 ± 0.5 419 ± 67 2.1 ± 1.2 PLGA-chitosan/PVA 2.6 ± 0.3 5.6 ± 0.9 88 ± 11 47 ± 1.3 PLGA-chitosan/PVA, cross-linked 3.8 ± 0.4 7.2 ± 1.3 106 ± 33 3.2 ± 0.3 Chitosan/PVA 4.3 ± 0.4 4.3 ± 0.6 176 ± 27 75 ± 3.5 Chitosan/PVA cross-linked 3.1 ± 1.0 2.2 ± 0.9 195 ± 26 45 ± 6

Fig. 58.10 Images showing the dimensional changes of the PLGA-chitosan-PVA scaffolds before (left) and after (right) incubating in PBS at 37 °C for 24 h. Samples are PLGA (a), PLGA-chitosan-PVA (b), cross-linked PLGA-chitosan-PVA (c), and cross-linked chitosan — PVA (d)

macrophages were observed on the surface of the fibers [06Noh]. After 28 days, degradation of connective tissue into short fragments was observed suggesting that the membranes were biocompatible.

Regenerated Cellulose Fibers

Keywords

Cellulose • Green solvent • Toxicity • Fibrillation • High temperature

The NMMO process is considered to be the most environmentally friendly method of producing regenerated cellulose fibers on a commercial scale. Regenerated cellulose fibers generally called “lyocell” (Lenzing) and also available in trade names such as “New Cell” (Akzo Nobel) and “Tencel” (Courtaulds) are regenerated cellulose fibers that are commercially available and are claimed to have considerable advantages over the traditional regenerated cellulose fibers produced through the viscose or cuprammonium process. Schematics of the steps involved in the dissolution, production, and regeneration of the fibers are shown in Figs. 18.1 and 18.2. It has been well documented that the properties of the fibers produced using the NMMO process can be varied to a large extent by controlling the spinning parameters such as type of solvent, extrusion speed, air gap distance, coagulation conditions, etc. [00Dre, 01Fin]. Similarly, post-fiber treatments such as solvent exchange during precipitation from methanol to water or posttreatment with hot water and aqueous NaOH changes the crystallinity, fibrillar structure, and therefore fiber properties [01Fin]. Changes in the tensile properties and fibrillation of the fibers with varying air gap distance and conditions in the air gap are given in Table 18.1. As seen in the table, elongation and fibrillation index are affected by the spinning conditions to a greater extent than the tenacity or modulus because of the changes in the orientation and crystallinity of the fibers. Similar changes in fiber properties were observed when the concentration of cellulose or % water in the solution was changed as seen in Table 18.2. Lower concentration of cellulose will allow the fibers to relax leading to lower tensile properties but less fibrillation [96Mor1]. Morphologically, fibers obtained through the NMMO process have a circular cross section compared to the irregular cross section seen in conventional viscose-type fibers.

© Springer-Verlag Berlin Heidelberg 2015

N. Reddy, Y. Yang, Innovative Biofibers from Renewable Resources, DOI 10.1007/978-3-662-45136-6_18

spinning pump with

filter and spinneret

spinning bath

Fig. 18.1 The NMMO process for producing regenerated cellulose fibers (reproduced with permission from Elsevier)

water

Fiber

Fig. 18.2 Depiction of the steps in producing regenerated cellulose fibers from wood using the lyocell process

The draw ratios of the fibers were also found to significantly affect the properties of the regenerated fibers including those produced using NMMO as the solvent [96Mor2, 96Mor3]. As seen in Table 18.3, increasing the draw ratio substantially increased (more than three times) the strength and modulus and decreased the

Table 18.1 Influence of air gap conditions on the tensile properties and fibrillation index of regenerated fibers produced using the NMMO system [96Mor2, 96Mor3] Air gap [mm] Air gap conditioning Tenacity [g/den] Elongation [%] Modulus [g/den] Fibrillation index 250 No conditioning 0.5 ± 0.02 11 ± 1 154 ± 15 2 ± 0.5 250 2 °C, 0 % RH 0.5 ± 0.02 9 ± 1 169 ± 23 5 ± 1 250 33 °C, 100 % RH 0.3 ± 0.02 6 ± 1 162 ± 15 0.9 ± 0.1 250 58 °C, 0 % RH 0.4 ± 0.01 10 ± 1 146 ± 15 2 ± 0.7 20 No conditioning 0.4 ± 0.02 9 ± 1 154 ± 15 15 ± 2 20 2 °C, 0 % RH 0.4 ± 0.01 7 ± 1 154 ± 15 18 ± 2 20 33 °C, 100 % RH 0.5 ± 0.02 10 ± 1 162 ± 15 6 ± 1 20 58 °C, 0 % RH 0.4 ± 0.01 10 ± 1 131 ± 15 16 ± 2

Table 18.2 Influence of solution concentrations on the tensile properties and fibrillation index of regenerated fibers produced using the NMMO system [96Mor2, 96Mor3] Tenacity [g/den] Elongation [%] Modulus [g/den] Fibrillation index 10 % cellulose 0.4 ± 0.02 11 ± 1 154 ± 15 1 ± 0.05 12.5 % cellulose 0.4 ± 0.02 7 ± 1 146 ± 15 11 ± 2 15 % cellulose 0.4 ± 0.0 2 9 ± 1 162 ± 15 19 ± 2 7.8 % water 0.5 ± 0.01 7.2 ± 0.5 162 ± 15 24 ± 2 12.3 % water 0.4 ± 0.01 7.7 ± 0.5 154 ± 15 13 ± 1

Table 18.3 Influence of draw ratio on the tensile and crystalline properties of regenerated cellulose fibers produced using the NMMO process [96Mor2, 96Mor3] Draw ratio Tenacity [g/den] Elongation [%] Modulus [g/den] Crystalline index Crystalline orientation factor 1.0 1.9 ± 0.22 80 ± 8 48 ± 3.8 43 ± 2 0.67 2.9 2.8 ± 0.2 17.5 ± 2.0 119 ± 7.7 45 ± 2 0.87 4.0 3.2 ± 0.3 13.0 ± 1.0 154 ± 7.7 46 ± 2 0.91 6.5 4.1 ± 0.3 10.9 ± 1.0 157 ± 7.7 43 ± 2 0.91 10.4 4.6 ± 0.4 11.3 ± 1.0 154 ± 7.7 46 ± 2 0.94

elongation. This was mainly due to the better orientation of the fibrils in the fibers as indicated by the increasing orientation factor.

Fibers obtained from the traditional xanthate and the new NMMO process (lyocell fibers) show considerable variations in tensile, mechanical, and perfor­mance properties [04Car]. The lateral order index (LOI) (1,420/893 cm-1) and total crystallinity index (TCI) (1,375/2,900 cm-1) calculated using FTIR spectrums showed that lyocell fibers had higher % crystallinity than viscose fibers which was also confirmed using X-ray diffraction studies as given in Table 18.4. Lyocell fibers were also more thermally stable which was related to the higher crystallinity

Table 18.4 Comparison of the properties of regenerated cellulose fibers produced using different dissolution methods Property Lyocell Hydrolyzed lyocell Modal Viscose References Lateral order index 0.35 0.24 0.52 0.54 [04Car] Total crystallinity index 0.76 0.87 0.71 0.64 [04Car] % Crystallinity 55 — 47 35 [94Len] Decomposition temperature [°C] 305 — 296 282 [04Car] Max heat flow/mass [mW/mg] 7.337 — 7.92 8.76 [04Car] Max decomposition temp [°C] 375 — 351 350 [04Car]

and better crystal orientation in the fibers [04Car]. Similar results were also reported by Xu et al. who compared the structure and thermal properties of Tencel (cellulose produced using the NMMO system) and bamboo viscose and conven­tional viscose fibers. Tencel was found to have higher % crystallinity (69 %) and thermal stability than the viscose fibers [07Xu]. In a theoretical study using viscoelastic models, it was determined that regenerated cellulose obtained using the lyocell process had higher tensile properties in the dry and wet conditions attributable to the higher molecular weights and crystallinity [13Zha]. Microwave heating has been used to decrease the dissolution time and energy consumption during NMMO process of fiber production [09Dog].

Natural Protein Fibers

Keywords

Hagfish • Slime • Silk thread • Tensile properties • Seawater

Hagfishes are marine craniates (animals that contain hard bone or cartilage skull) that produce large amounts of slime [84Dow]. The slime is composed of cells that are made up of threads similar to fibers seen in a silkworm cocoon. These elliptical­shaped cells are produced by highly specialized slime glands. When these gland cells are released into water, they release strands or threads that uncoil and increase the viscosity of the mucus [81Dow]. A typical cell in the hagfish slime is shown in Fig. 42.1. Each cell has threads that are 1-3 pm in diameter and may have lengths up to 60 cm [84Dow, 12Neg]. SDS-PAGE of the threads revealed that the proteins have a molecular weight of about 63,500 Da. Further analyses of the proteins have demonstrated the presence of three components, one major (a) and two minor (P, y) that have similar molecular weights but different isoelectric values of 7.56, 5.67, and 5.31 for the a, p, and y, respectively [84Spi]. The amino acid composition of the three components is shown in Table 42.1. The amino acid composition in the hagfish threads were similar to the keratin polypeptides found in humans and rats [84Spi]. Based on X-ray diffraction studies, it was suggested that the hagfish threads could undergo irreversible a-p transition, under large strains as observed in wool keratins [03Fud]. Using a glass microbeam force transducer apparatus, the tensile properties of the hagfish threads in seawater were determined. It was found that the threads had a low initial stiffness of 6.4 MPa (0.06 g/den) but considerably high strength (180 MPa) (1.6 g/den) and low elongation of 2.2 %.

At a molecular level, the slime threads were found to consist of 10 nm protein nanofibers that are made from non-repetitive genes. Therefore, it was envisaged that it was easier to replicate slime thread fiber properties through biotechnology compared to spider silk [10Fud]. More interestingly, the slime proteins self — assemble into the 10 nm fibers in aqueous buffers. Since it was observed that the proteins in slime undergo a~p transition under high strains, it was hypothesized that the stretching of the fibers could lead to improved mechanical properties. Hagfish slime threads were drawn in water and later dehydrated in ethanol and tested in dry air. Drawn threads showed considerable increase in strength that approached the strength of spider silks. Some of the properties of the slime threads before and after stretching, annealing, and cross-linking are shown in Table 42.2.

Table 42.1 Comparison of the amino acid residues in the three protein components in hagfish slime threads Residues per 100 residues Amino acid a-Component P-Component y-Component Asx 7.71 7.29 7.09 Thr 9.29 8.27 7.76 Ser 9.86 10.86 10.84 Glx 9.41 12.61 12.84 Pro 3.71 1.98 1.64 Gly 15.88 15.14 15.29 Ala 7.72 7.94 7.96 Val 7.00 6.46 6.48 Met 1.70 3.29 3.86 Ile 4.83 4.56 3.90 Leu 6.18 8.45 8.34 Tyr 3.40 2.65 2.46 Phe 2.16 1.99 2.09 His 2.05 0.76 0.70 Lys 3.30 3.10 3.25 Arg 4.89 4.37 4.34 Trp 0.67 0 0 Cys/2 0.22 0.35 0.3

Table 42.2 Properties of hagfish slime threads before and after drawing and cross-linking with 8 % glutaraldehyde Fiber Diameter (pm) Stiffness (g/den) Breaking stress (g/den) Breaking strain (%) Undrawn 1.27 77.4 4.1 1.2 Drawn 1.07 69.5 6.1 0.4 Uncross-linked— undrawn — 31.3 7.0 0.5 Cross-linked— drawn — 86.9 10.4 0.3 Reproduced from Fudge et al. [10Fud]

References

[81Dow] Downing, S. W., Spitzer, R. H., Salo, W. L., Downing, J. S., Saidel, L. J., Koch, E. A.: Science 212, 326 (1981)

[84Dow] Downing, S. W., Spitzer, R. H., Koch, E. A., Salo, W. L.: J. Cell Biol. 98(2), 653 (1984) [84Spi] Spitzer, R. H., Downing, S. W., Koch, E. A., Salo, W. L., Saidel, L. J.: J. Cell Biol. 98, 670 (1984)

[03Fud] Fudge, D. S., Gardner, K. H., Forsyth, T. V., Riekel, C., Gosline, J. M.: Biophys. J. 85, 2015 (2003)

[10Fud] Fudge, D. S., Hillis, S., Levy, N., Gosline, J. M.: Bioinspir. Biomim. 5, 1 (2010)

To improve the properties of bacterial cellulose, blends of bacterial cellulose have been prepared for various applications. In one such effort, bacterial cellulose and hydroxypropyl chitosan were dissolved in N-methylmorpholine-N-Oxide and regenerated cellulose fibers were produced via wet spinning [13Lu]. Cellulose in the fibers was of type II form and the % crystallinity was between 48 and 49 %. The pure regenerated bacterial cellulose fibers had tenacity of 0.9 g/den, elongation of

12.7 %, and modulus of 26 g/den compared to tenacity of 1.2 g/den, elongation of 2.8 %, and modulus of 46 g/den for the blend fibers. Morphologically, the fibers had a rough surface with pores in the middle that were responsible for the poor tensile strength. Bacterial cellulose had unique properties such as high porosity, high purity and crystallinity, good mechanical properties and high water holding capac­ity, excellent biodegradability, and biocompatibility [12Wan] which make it
preferable for applications in batteries, sensors, electrical devices, and antistatic coating. To utilize these advantages, bacterial cellulose in nanofiber form was blended with poly(aniline) and made into composites with flake shaped morphol­ogy with high electrical conductivity. The blend composites had a high surface area of 34 m2/g and outstanding electrical conductivity of 5.1 S/cm and good thermal stability. These attributes were suggested to make the composites perfectly suited for applications in various electronic devices [12Wan]. Strong intermolecular interactions were observed between bacterial cellulose and alginate that led to fibers with good mechanical properties. To form the fibers, the cellulose and alginate were dissolved using the lithium hydroxide (LiOH) and thiourea approach [11Zha]. The BC/alginate blend fibers had tenacity of 1.8 g/den and elongation of

10.8 % in the dry state and 1.0 g/den and 14.4 % elongation in the wet state, considerably higher than the properties of the fibers developed from the individual polymers. Instead of using NMMO, it has been shown that BC with DP less than 40 could be dissolved up to 8.5 % using aqueous NaOH at —5 °C and in urea/NaOH solution up to a DP of 560.

Ionic liquids were used to prepare high-strength chitosan fibers by Li et al. Chitosan with a degree of acetylation of 86 % and molecular weight of 1.5 x 106 was dissolved using glycine chloride. Influence of dissolution and fiber-forming conditions on the properties of the fibers was studied [12Li1]. Filaments extruded from the spinneret were coagulated with dilute Na2SO4/C2H5OH and later freeze — dried. Table 25.2 provides a comparison of the properties of the chitosan fibers produced using the ionic solvent in comparison to the traditional approach of using acetic acid as the solvent. As seen from the table, the ionic solvent produced fibers with more than three times higher strength and more than 12 times higher modulus than the fibers obtained using acetic acid as the solvent. It was suggested that the higher strength of the fibers from the ionic solvents was due to the retention of the type I structure of chitosan which had stronger molecular forces compared to the type II amorphous structure formed when chitosan is dissolved using acetic acid. It was also proposed that glycine chloride could enter the chitosan network more easily, stretch the molecules to a more linear form by increasing the repulsion between the chitosan cations, and increase the strength of the fibers. Morphologi­cally, fibers obtained from glycine chloride were circular and smooth, whereas the fibers from the acetic acid solvent had a rough and uneven cross section.

In a recent study, binary ionic liquids composed of glycine hydrochloride and 1-butyl-3-methylimidazolium chloride (Gly-HCl-Bmimcl) were used to prepare high-strength chitosan fibers. Chitosan was dissolved in the ionic liquid by heating to 80 ° C for 1 h and later extruding the solution into a coagulation bath consisting of separate ethanol and 5 % sodium hydroxide troughs [13Ma]. Further separation of

Table 25.2 Comparison of the properties of chitosan fibers obtained using ionic solvents with the acetic acid dissolution method [12Li1] Solvent(s) Tenacity [g/den] Elongation [%] Modulus [g/den] Glycine chloride 4.3 1.9 2.6 Acetic acid 1.0 1.6 0.2

Fig. 25.2 Longitudinal and cross-sectional views of chitosan fibers produced using the wet and dry-wet spinning approaches [13 Ma]. Reproduced with permission from Elsevier

the fibers into dry and dry-wet spun was done by an additional washing and air-drying step. Fibers produced by the wet spinning approach had striated surface and a circular cross section compared to the smooth surface and irregular cross section seen in the dry-wet spun fibers as seen in Fig. 25.2. Tenacity (2.4 g/den) and elongation (11.9 %) of the dry-wet spun fibers were considerably higher than the fibers obtained from the wet spinning (1.7 g/den, 8.1 %). Fibers obtained from both the dry and dry-wet spinning had good wet strength, measured after immersing the fibers in water for 5 min at room temperature. It was claimed that the fibers produced in this research had better tensile properties than any previous method of producing chitosan fibers.

To prevent hydrolysis of chitosan when acids were used as solvents, a combina­tion of LiOH and urea were used to dissolve chitosan and extrude fibers into a sulfuric acid and ethanol aqueous solutions [12Li2]. Fibers produced from the

LiOH-urea system had smooth and circular cross section, and the strength and elongation of the fibers were 1.3 g/den and 12 %, respectively, higher than the fibers produced using the conventional approach.

Regenerated Protein Fibers

Keywords

Bovine serum albumin • Solubility • Cross-linking • Aligned fiber • Globular core • Reproducing BSA • Recombinant B. mori

Bovine serum albumin was dissolved in water using dithiothreitol as a reducing agent at a pH of 4.7, and the solution obtained was poured onto glass plates. Proteins were dehydrated at 30 °C, and 30 % humidity and fibers were formed by pulling air over the solution at a constant flow rate leading to fibrillation [13Wu]. Fibers obtained were cross-linked with formaldehyde dissolved in metha­nol and additionally cross-linked again with 0.1 % glutaraldehyde or with EDC. Average length of the fibers obtained was 35 cm, and the diameter of the fibers was between 10 and 20 pm. Figure 53.1 shows the image of the fibers obtained. It was found that the fibers consisted of ordered p-sheets at the ends and with globular regions at the center as seen from the SEM image in Fig. 53.2. However, the structure and properties of the fibers were dependent on protein concentration, pH, degree of cross-linking, and other fiber-forming conditions. Fibers without cross-linking dissolved in water or 50 % methanol but reassembled into original fibers when the solvent was removed. Table 53.1 provides a comparison of the tensile properties of the albumin fibers with Bombyx mori silk. As can be inferred from the table, the albumin fibers have strength similar to that of silk, higher modulus, and similar elongation. Higher amounts of tightly packed p-sheets were suggested to provide good tensile properties to the fibers after cross-linking. Fibers were also dyed using acid dyes and spun into yarns. Pictures of the dyed fibers and yarns spun from the fibers are shown in Fig. 53.3.

Recombinant human serum albumin (rHSA) proteins were obtained using trans­genic silkworms with structure and properties similar to that of the native albumin [07Oga]. The DNA from HSA was introduced into silk glands through PiggyBac- based transformation vector, and the glands were transplanted into larvae and reared to produce silk fibers. To obtain HSA that could be easily collected without

Fig. 53.1 Digital image of albumin fibers produced via dehydration and cross-linking [13Wu]. Reproduced with permission from Wiley

Table 53.1 Properties of albumin fibers obtained under different pH and cross-linked with various cross-linkers in comparison to silkworm silk [13Wu] Fiber type and fiber-forming conditions Strength (g/den) Elongation (%) Modulus (g/den) BSA, pH 6, glutaraldehyde cross-linked 0.5 ± 0.1 3.6 ± 1.4 23.5 ± 5.2 BSA, pH 4.7, formaldehyde cross — linked 1.1 ± 0.3 3.9 ± 1.6 46.1 ± 17.4 BSA, pH 4.7, glutaraldehyde cross — linked 1.3 ± 0.03 >30 49.6 ± 2.6 BSA, pH 4.7, EDC cross-linked 1.9 ± 0.8 >30 72.2 ± 37.4 B. mori silk 2.0 ± 0.8 >30 41.7 ± 17.4

Fig. 53.3 Digital picture of the yarns made from the albumin fibers (a) is 35 cm long yarn made using 0.2 grams of fibers having 180 turns per inch and (b) is yarn made from 0.5 grams of fibers having 220 turns per inch. [13Wu]. Reproduced with permission from Wiley

using harsh solvents or contamination from other proteins, the authors attempted to express the HSA genes in the sericin layer of the fibers since sericin dissolves in aqueous solvents. As seen from Fig. 53.4, the authors successfully produced BSA in the outer sericin layers. BSA up to 83 % in the sericin with a purity of up to 99 % was extracted from the cocoons by immersing the cocoons in PBS at 4 °C for 24 h and later precipitating the BSA using ammonium sulfate or blue-sepharose binding. The yield of protein ranged from 2.8 to 5.5 %. In terms of structure, the recombinant

BSA was found to have antiparallel p-sheets, and the primary and secondary structures were similar to that of native BSA [07Oga].

References

[07Oga] Ogawa, S., Tomita, M., Shimizu, K., Yoshizato, K.: J. Biotechnol. 128, 531 (2007) [13Wu] Wu, Y., Wang, K., Bushcle-Diller, G., Liles, M. R.: J. Appl. Polym. Sci. 129, 3591 (2013)