Despite the numerous advantages claimed by the NMMO process compared to the conventional cuprammonium and viscose processes, several concerns have been expressed on large-scale adoption of the NMMO process. The solvent (NMMO) itself is toxic and explosive by-products are generated during the process of dissolution. In addition, higher temperatures (>90 °C) and antioxidants are

The Lyocell process — Potential negative effects of side reactions

Fig. 18.6 Depiction of the possible side reactions of the NMMO process. Reproduced from [01Ros] with permission from Elsevier necessary for dissolution that could degrade cellulose. It has also been reported that it is difficult to recover the solvent after fiber production [01Ros]. In addition, the dissolution of cellulose in NMMO causes side reactions and also leads to the formation of considerable amounts of by-products that cause various unwanted properties in the fibers. Some of the side reactions of the NMMO system and their potential impacts are listed in Fig. 18.6. High temperatures (100-120 °C) required for dissolution also cause degradation of cellulose.

References

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[01Ros] Rosenau, T., Potthast, A., Sixta, H., Kosma, P.: Prog. Polym. Sci. 26, 1763 (2001) [02Udo] Udomkichdecha, W., Chiarakorn, S., Potiyaraj, P.: Text. Res. J. 72(11), 939 (2002)

[04Car] Carrillo, F., Colom, X., Sunol, J. J., Saurina, J.: Eur. Polym. J. 40, 2229 (2004)

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[10Han] Han, S., Lee, M., Kim, B. K.: J. Appl. Polym. Sci. 117, 682 (2010)

[13Zha] Zhang, S., Wang, W., Li, F., Yu, J.: Cellul. Chem. Technol. 47(5-6), 353 (2013)

Natural Protein Fibers

Keywords

Honeybee • Fiber cross section • Nanofibril • Amino acid

Proteins produced by honeybees have distinct structure and properties compared to Bombyx mori or spider silks. Unlike the silkworm or spider silks that are composed of two filaments (brins) connected to each other, honeybee silk is formed by a single filament with a circular cross section and finer and smoother texture [10Zha]. Honeybee silks are formed by the assembly of 4-4.5 nm wide fibrils that consist of fine filaments of 2-2.5 nm in width similar to B. mori silks. These fibrils further formed tactoids that are 1-3 pm in width and 3-40 pm in length [11Sut]. To study the structure and properties of natural honeybee silk fibers, Italian honeybee (Apis mellifera) larvae were placed on glass plates and allowed to spin fibers at room temperature. Fibers formed were collected for analysis. Figure 44.1 shows a three-dimensional scanning probe microscope image of the honeybee and B. mori silk fibers. As seen from the SEM images in Fig. 44.2, honeybee silk has a circular and smooth cross section and did not show the presence of nanofibrils (dots in Fig. 44.2a) as opposed to the typical triangular cross section and nanofibrils seen in silkworm silk. The presence of a single filament in honeybee silk is evident from the cross section. X-ray diffraction studies have shown that honeybee silks predomi­nantly contain a-helices in a coiled-coil form [06Sut]. In terms of primary structure, honeybee silks primarily contain high levels of alanine, serine, and aspartic and glutamic acid and considerably lower levels of glycine compared to regular silks. Six genes encoding silk proteins were identified in A. mellifera larvae that were named AmelFibroin 1-4. In addition, two genes (AmelSAl and 2) that are associated with silk were also identified [06Sut]. Table 44.1 lists the major differences between the four genes identified in the honeybee silks and silkworm (B. mori) silks. Tensile tests of the honeybee silk also showed substantial

differences. Honeybee silk fibers had a nearly linear stress-strain curve until the fibers were broken. Breaking strength of the honeybee silk fibers was 1.4 g/den, elongation was 3.8 %, and modulus was 56 g/den. It was suggested that the considerably lower strength and elongation of the honeybee silk compared to silkworm or spider silks should be due to the functional differences of the silks. Honeybee silk is mostly secreted to act as reinforcement for the honeycombs and is not required to support heavy loads or strains.

References

[06Sut] Sutherland, T. D., Campbell, P. M., Weisman, S., Trueman, H. E., Sriskantha, A., Wanjura, W. J., Haritos, V. S.: Genome Res. 16, 1414 (2006)

[10Zha] Zhang, K., Si, F. W., Duan, H. L., Wang, J.: Acta Biomater. 6, 2165 (2010)

[11Sut] Sutherland, T. D., Church, J. S., Hu, X., Huson, M. G., Kaplan, D. L., Weisman, S.: PLoS One 6(2), 1 (2011)

Regenerated cellulose films were developed by dissolving bacterial cellulose in NMMO solutions [12Sha]. Bacterial cellulose (5 %) was added into aqueous NMMO solution and stirred at 100 °C for 2 h and the solution obtained was cast into films with the addition of 30 % glycerol as plasticizer. During the dissolution, bacterial cellulose was transformed from cellulose I structure to cellulose II struc­ture and the % crystallinity decreased substantially from 79 to about 38 %. In a similar approach, bacterial cellulose was dissolved using NMMO solution to develop regenerated cellulose fibers [11Gao]. Fibers obtained had a striated surface,

physical form of cellulose II, and a degree of crystallinity of 61 %. Tenacity of the fibers at 0.6-1.7 g/den and elongation at 3-8 % was lower than that of other regenerated cellulose fibers. Bacterial cellulose films were used as reinforcement to protect vulnerable silk fabrics and the effect of BC on the light aging behavior was investigated [12Wu]. It was found that BC restored silk fabrics had 213 % increase in strength and improvements in crystallinity and thermal stability were also observed. BC could be removed from the silk fabrics without any degradation to the silk fiber properties. Presence of abundant hydroxyl groups on the surface of BC promotes good interfacial adhesion and therefore BC was capable of preserving the properties of the silk fabric [12Wu].

Although bacterial cellulose exhibits excellent moisture sorption and forms a hydrogel with good strength, bacterial cellulose films cannot be swollen after drying due to the formation of strong hydrogen bonds between the nanofibrils. Bacterial cellulose was hydroxypropylated using two modifier systems: sodium hydroxide/propylene oxide or sodium hydroxide/urea/propylene oxide. The equi­librium swelling ratio of the modified cellulose films could be controlled between 280 and 7,000 % by adjusting the NaOH concentration in the modifying system. Hydrogels systems suitable for medical applications were successfully developed from dried bacterial cellulose in this research [13Wan2]. Since the biomedical applications of bacterial cellulose are directly dependent on the water holding capacity and water release rate, attempts were made to increase the pore size, pore volume, and surface area using in situ and ex situ modifications [12Ul]. In situ modifications were done using a single sugar а-linked glucuronic acid-based oligosaccharide, and ex situ modifications were done using chitosan and montmo — rillonite. Table 61.7 provides some of the properties of the bacterial cellulose samples modified using different approaches.

Biohybrid fiber yarns as unique materials for tissue engineering were developed using microfibrils extracted from bacterial cellulose and used as reinforcement for poly(methyl methacrylate) (PMMA) by electrospinning [10Ols]. Cellulose mats obtained after 7 days of growth were harvested, boiled twice in 10 % NaOH for 20 min, and later with 50 % sulfuric acid. Finally, the cellulose sheets obtained were dispersed in dimethylformamide/tetrahydrofuran solution and mixed with PMMA in various ratios up to 20 %. Microfibrils developed had a diameter of 15-20 nm and

15 rpm/min

Fig. 61.19 Electrospinning setup used to produce the biohybrid fiber yams [10Ols]. Reproduced with permission from American Chemical Society

length varied between 0.3 and 8 ^m. Using the electrospinning setup shown in Fig. 61.19, the electrospun fibers were aligned by extruding the fibers onto a water surface and subsequently winding the fibers onto spools to form hybrid yarns. Figure 61.20 shows an SEM image of the multifilament-PMMA hybrid fibers obtained using 7 % cellulose microfibrils as reinforcement [10Ols].

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Sustaining the demand for raw materials to meet the needs of future generations will be one of the most challenging tasks for human kind. In addition, complying with the increasing stringent international protocols related to climate change and exploiting of natural resources will also increase the burden on the supply of raw materials and production of commodities. Future conflicts within and between nations are more likely to be on owning or sharing of natural resources rather than ideological differences or technological prowess. Meeting the three basic necessities, food, clothing, and shelter, will be a challenge to the future leaders, especially in the overpopulated and developing countries. Rapid urbanization and consequential decrease in the availability of land and other resources required for agricultural production will put tremendous burden on the availability of food, fuel, fibers, and other basic commodities. The price and availability of fibers produced from petroleum resources that currently are predominant over natural fibers will also be in question due to depleting sources of fossil fuels. Since fuel needs have priority over fiber production, it will be imperative to find alternative sources of raw materials for fiber production.

Maximizing the use of natural resources, reducing consumption, and recycling are some of the possible approaches to meet the future demands for fibers, textiles, and other commodities. Agricultural production inevitably generates by-products (residues) such as stovers and straws that currently have limited applications. Similarly, processing of cereal grains generates coproducts containing proteins and carbohydrates that are mostly used for low-value applications such as animal feed. These agricultural by-products and coproducts could be used to produce fibers, thereby eliminating the need for dedicated fiber crops that require land, water, and other natural resources. These agricultural by-products and coproducts are renewable and biodegradable. Materials developed using these resources will therefore be more environmentally friendly than growing natural fibers or manufacturing fibers from petroleum. Similar to developing fibers from agricultural by-products and coproducts, other sources such as nontraditional silk worms, microorganisms, and bio — and nanotechnology could be used to develop fibers and reduce/eliminate the need for dedicated fiber crops or petroleum resources.

This book is an effort to present the potential, structure, properties, and applications of fibers that are derived from unconventional sources. The innovative biofibers described in this book are not only derived from renewable and sustain­able resources but also do not need exclusive land, water, or other natural resources. Although it would be quite futuristic to think of replacing natural cellulosic fibers such as cotton, protein fibers such as silk, and synthetic fibers such as polyester, this book provides insights into potential addition to these fibers. We hope that textile professionals and academics will find this book useful and attempt to develop and use the innovative fibers in the near future.

Narendra Reddy expresses his sincere thanks to the University of Nebraska — Lincoln and the Center for Emerging Technologies at Jain University. Yiqi Yang thanks Yiqi Yang thanks the University of Nebraska-Lincoln and its agricul­tural research division, and the United States Department of Agriculture for their support to complete this work.

Bangalore, India Narendra Reddy

Lincoln, Nebraska, USA Yiqi Yang

July, 2014

Chitosan fibers lack adequate stability under aqueous environments, and attempts have therefore been made to modify or cross-link the fibers using various approaches. Glyoxal was used to cross-link chitosan fibers, and the effect of cross-linking conditions on the structure and properties of the fibers were studied [05Yan1, 05Yan2]. Uncross-linked fibers had a tenacity of 1.2 g/den, and the tenacity improved to about 2.4 g/den after cross-linking. pH during cross-linking was found to affect the tenacity to a greater extent compared to concentration of glyoxal or cross-linking time. A pH of 3.8 provided the highest tenacity to the fibers. A relationship (S = 79.24 — 5.05C + 0.24C2) was developed to predict the swelling of the fiber (S) and the concentration of glyoxal (C) used. Cross-linking decreased the % crystallinity of the fibers to 27.2 % compared to 34.7 % and changed the crystal structure from a to the p form.

Epichlorohydrin was used to cross-link chitosan fibers, and the effect of the concentration of the cross-linking agent on the properties of the fibers was studied by Lee et. al. [04Lee]. A cross-linking agent was added into the coagulation bath, and the fibers were later drawn in a water bath at 99 °C. Increasing the concentra­tion of the cross-linking agent up to 0.05 M decreased water sorption and therefore swelling. Dry strength of the fibers decreased from 1.6 g/den to about 1.3 g/den when the concentration of epichlorohydrin was increased indicating over or excess cross-linking. However, the wet strength of the fibers showed an increasing trend from 0.9 to 1.2 g/den. Dry elongation decreased substantially from about 16 to 13 %, whereas wet elongation did not show any appreciable change. X-ray studies did not reveal any major changes in the crystal structure, and differential scanning calorimetry curves for the uncross-linked and cross-linked fibers were similar [04Lee]. In another study, the effect of the concentration of coagulation bath retardant sodium acetate on the cross-linking of chitosan fibers with epichlorohy — drin was studied [07Lee]. Both dry and wet tenacities improved with increasing concentration of sodium acetate. Dry tenacity of the fibers obtained was about

1.4 g/den, and wet tenacity was 1.3 g/den when the concentration of sodium acetate was 20 %, whereas the elongation did not show a significant difference. Thermal stability of the fibers increased substantially to 713 J/g compared to 359 J/g before cross-linking. A major advantage of this process was that the coagulation and cross-linking occurred in one step, and the wet stability of the fibers was considerably high.

Glutaraldehyde was also used as a cross-linking agent to cross-link chitosan fibers intended to be used for controlled drug release [00Den]. Chitosan dissolved using acetic acid was extruded into an ethanol coagulation bath containing glutar — aldehyde. The drug 5-fluorouracil (5-FU) was mixed with the chitosan and extruded with the fibers. Cross-linking substantially decreased the swelling ratio from 60 % to about 10 %. Drug loading on the fibers ranged from 0.2 to 7.6 mg/g, and there was an initial burst release and then a stable and uniform release. Fibers cross — linked to higher extents showed lower release because the fibers could not swell [00Den]. The effect of cross-linking conditions on the properties of chitosan fibers cross-linked with glyoxal and glutaraldehyde was studied by Knaul et al. [99Kna1]. Considerable changes in the tensile properties of the fibers were observed by varying the concentration of the cross-linking agent, time, and temperature of cross-linking. Fibers (18.6 den) with tenacities ranging from 1.4 to 1.8 g/den and elongation from 9 to 37 % were obtained by cross-linking with glyoxal. Glutaraldehyde provided higher strength (1.5-2.3) g/den but similar elongation (4.7-37 %) compared to cross-linking with glyoxal. Conventionally, the cross-linking between glutaralde — hyde and chitosan in acidic conditions was considered to be a Schiff’s base reaction. However, the reaction is also proposed when chitosan reacts with glutaraldehyde or glyoxal in neutral aqueous solutions. Knaul et al. confirmed that a Schiff’s base reaction occurs between glutaraldehyde and chitosan but suggested that the reaction is between the chitosan free amine and a hemiacetal [99Kna2]. The reaction between glyoxal and chitosan was considered to be between two polymer acetals and not between two carbon dialdehydes. Although the improvement in dry strength after glyoxal and glutaraldehyde cross-linking was evident, the changes in the wet strength or stability of the fibers in aqueous environments were not investigated.

Phosphate groups were incorporated onto chitosan fibers through ionotropic gelation to provide amphoteric characteristics and improve protein adhesion [11Pat]. Chitosan-tripolyphosphate fibers with varying phosphate content were prepared by extruding chitosan into coagulation bath containing 5 % sodium tripolyphosphate at different pHs. The TPP ions acted as coagulants and cross­linking agents, and a degree of cross-linking of up to 85 % was obtained which was highly dependent on the pH of the solution [11Pat]. Cross-linking with TPP was found to decrease crystallinity and thermal stability of the fibers due to freezing of the polymer network.

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Regenerated Protein Fibers

Keywords

Poultry feather • Keratin • Keratin extraction • Dissolution • Surfactant • Alkali dissolution • Fiber properties

Feathers are natural protein fibers with a unique hierarchical structure [07Red]. Keratin, the major (>90 %) protein in feathers, is a relatively small protein with molecular weight of 10 kDa and contains high levels of cysteine which provides extensive disulfide cross-linking making feather keratins strong and tough. Keratins have a p-sheet conformation with 96 amino acids having 7 cysteine residues as terminals [09Poo]. However, the central portion of keratin is also reported to have а-structures. Several attempts have been made to develop regenerated protein fibers from feather keratin. Regenerated keratin fibers were obtained using alkali and surfactants [47Har, 49Wor]. In another research, ionic solvents were used to dissolve keratin and obtain fibers. However, the tensile strength of the fibers was only 0.2 g/den, much lower than the strength of the natural protein fibers such as wool. Recently, controlled disentanglement and alignment of keratin molecules were achieved by using a surfactant sodium dodecyl sulfate (SDS). Figure 55.1 shows the digital picture of the actual regenerated keratin fibers. The mechanical properties of the fibers are shown in Table 55.1. As seen from the table, the properties of the fibers were affected by the type of coagulation bath used. Fibers obtained had tensile strength of up to 0.7 g/den and had low dry elongation but good wet elongation of up to 28 %.

Fig. 55.1 Digital image of the fibers regenerated from feather keratin (left) compared to wool fibers (right)

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[09Poo] Poole, A. J., Church, J. S., Huson, M. G.: Biomacromolecules 10(1), 1 (2009)

Biocomposites from Renewable Resources

Keywords

Starch • Glycerol • Thermoplastic starch • Nanocellulose • Reinforcement • Biodegradation

Starch is inherently non-thermoplastic but is made thermoplastic using plasticizers and/or chemical modifications, and the modified starch has been used as matrix for composites. In one such study, starch was reinforced with bacterial cellulose, and the tensile properties, resistance to biodegradation, and moisture absorption were studied [09Wan]. Starch was plasticized with 30 % glycerol and made into 10-20 % solutions. Bacterial cellulose sheets cultured from Acetobacter xylinum X-2 were added into the solution and made into composite sheets with an average thickness of 0.5 mm. The amounts of fibers in the starch were 7.8, 15.1, and 22 wt%. Tensile properties of the BC-reinforced starch fiber composites are shown in Table 71.1 [09Wan]. Morphological analysis of the fractured surface of a starch composite containing 22 % bacterial cellulose showed that the BC fibers were present in a layered fashion as seen in Fig. 71.1. Such a layered structure was typical of bacterial cellulose. Pullout length of fibers from the matrix was low suggesting good fiber — matrix interaction [09Wan]. The presence of bacterial cellulose also increased the resistance of the fibers to moisture absorption. Degradation by soil burial tests showed that the weight loss of the composites was similar to that of unreinforced starch, and about 30 % weight loss had occurred after 30 days of burial. However, the bacterial cellulose-reinforced composites had slightly higher strength retention than the starch films. In a similar study, bacterial cellulose containing nanofibrils with diameters between 10 and 100 nm was mixed (1 or 5 %) with starch containing 30 % glycerol. Later, the mixture was heated at 120 °C for 20-30 min and later injection molded into composites in the form of tensile bars [09Mar]. More than six times increase in strength and modulus were obtained for composites containing 5 % nanocellulose compared to the thermoplastic starch [09Mar].

Green coir fibers were milled into lengths of about 10 mm and mixed with starch plasticized with 30 % glycerol. Composites were developed by injection molding and later heated (annealed) at 60 °C for 12 h to improve properties. The addition of the coir fibers increased the tensile strength to 10-11 MPa compared to 3 MPa without the reinforcement. Similarly, Young’s modulus increased to 374 MPa from 176 MPa due to the presence of the coir fibers [11Ram]. Coir-reinforced composites had substantially lower moisture absorption and water take-up than the thermoplas­tic starch matrix. Although composites were successfully developed from starch and coir fibers, the stability of the composites and changes in tensile properties at high humidities or in aqueous environments were not reported. Due to the hydro­philic nature of starch and coir fibers, it is very likely that the composites will have poor performance properties at high humidities or under aqueous environments and therefore have limited applications.

Fibrous materials derived from various sources and in different configurations were used to reinforce thermoplastic starch [04Ave]. Increasing the amount and length of the fibers in the matrix was found to increase the transition temperatures due to improved interfacial bonding and strong hydrogen bond interactions. Ligno — cellulose fibers were found to provide higher degradation temperature than cellu­lose fibers, and the addition of biodegradable synthetic polyesters did not vary the properties of the composites [04Ave].

Curaua fibers in stretched sliver form and those treated with concentrated alkali were used as reinforcement for commercially available cornstarch-based biode­gradable resin containing polycaprolactone [07Gom]. Fibers used in the study had tensile strength of 913 MPa, fracture strain of 3.9 %, and modulus of 30 GPa. Three methods (direct, prepreg sheet, and preforming) were used to fabricate the composites. Tensile properties of the composites obtained using the three methods are listed in Table 71.2. As seen from the table, the prepreg method of developing composites provided the highest tensile properties. Further, alkali treatment enhances the fracture strain without considerably changing the tensile strength. Composites obtained using the direct method after alkali treatment showed sub­stantial increase (nearly twice) in modulus as seen in Table 71.3.

References

[04Ave] Averous, L., Boquillon, N.: Carbohydr. Polym. 56, 111 (2004)

[07Gom] Gomes, A., Matsuo, T., Goda, K., Ohgi, J.: Compos. Part A 38, 1811 (2007)

[09Mar] Martins, I. M.G., Magina, S. P., Oliveira, L., Freire, C. S.R., Silvestre, A. J.D., Neto, C.

P., Gandini, A.: Compos. Sci. Technol. 69, 2163 (2009)

[09Wan] Wan, Y. Z., Luo, H., He, H., Liang, Y., Huang, Y., Li, X. L.: Compos. Sci. Technol. 69, 1212 (2009)

[11Ram] Ramirez, M. G.L., Satyanarayana, K. G., Iwakiri, S., Muniz, G. B., Tanobe, V., Flores — Sahagun, T. S.: Carbohydr. Polym. 86, 1712 (2011)

Among the fibers obtained from various lignocellulosic agricultural by-products, PALF are the most widely used to develop yarns, fabrics, and other textiles. Yarns and fabrics have been made using 100 % PALF on the cotton, jute, and wool spinning systems. Raw and bleached PALF were processed on standard jute and flax spinning systems [88Gho]. Table 10.5 shows some of the properties of the yarns produced from the PALF. Bleaching resulted in improvement in fineness by about 5 % but resulted in yarns with lower tensile strength. Yarns produced were of 84 tex (7s Ne) and were therefore coarser. The PALF fibers used in this study were probably too coarse to be processed on the cotton spinning system.

PALF with staple length of 20 cm, fineness of 1.5 tex, and tenacity of 26 g/tex were blended with cotton, jute, and wool and processed on various spinning systems. To process the fibers on the jute spinning machinery, about 1 % oil was added and blended with jute in ratios up to 25 % PALF to produce yarns of 66 tex. Chemically treated PALF fibers were blended with cotton and processed on the cotton spinning system. Table 10.6 shows some of the properties of the PALF blended yarns compared to similar count cotton yarns. As seen in the table, PALF blended yarns (14s Ne) have similar properties to that of 14s Ne 100 % cotton yarns except that the PALF blends are more uneven (higher U%). When processed into finer (22s Ne) count, the PALF fibers had lower strength and higher unevenness than the cotton yarns. PALF are much coarser than cotton that leads to more unevenness in the blended yarns. Compared to processing on the cotton system, 100 % PALF yarns were easily produced on a semi-worsted spinning system along with blends with wool in various ratios.

Fibers (25-30 denier) obtained by decorticating pineapple leaves were degummed with sodium hydroxide and later softened using lubricants and used to spin 100 % pineapple leaf yarns on a modified cotton spinning system. Yarns (4s and 6s count) were made by directly feeding the draw frame sliver onto the

ring spinning machine. Tensile properties of the 100 % pineapple fiber yarns were similar to similar count 100 % cotton yarns made using short staple cotton but about 22 % lower compared to 100 % cotton yarns made from medium staple cotton [91Dor].

References

[09Spi] Spinace, M. A.S., Fermoseli, K. K.G., De Paoli, M. A.: J. Appl. Polym. Sci. 112, 3686 (2009)

[88Gho] Ghosh, S. K., Day, A., Dey, S. K.: Ind. J. Text. Res. 13, 17 (1988)

[91Dor] Doraiswamy, I., Chellamani, K. P.: Text. Trends 34(6), 41 (1991)

[10Moh] Mohamed, A. R., Sapuan, S. M., Shahjahan, M., Khalina, A.: Polym. Plast. Technol. Eng. 49(10), 972-978 (2010)

[11Li] Li, Z., Liu, G., Yu, C.: Adv. Mater. Res. 306-307, 1516 (2011)

[11Sip] Sipiao, B. L.S.,Paiva, R. L.M., Goulart, S. A.S., Mulinari, D. R.: ProcediaEng. 10, 2028 (2011)

[12Ken] Kengkhetkit, N., Amornsakchai, T.: Ind. Crop. Prod. 40, 55 (2012)

[12Man] Maniruzzaman, M., Rahman, M. A., Gafur, M. A., Fabritius, H., Raabe, D.: J. Compos. Mater. 46, 79 (2012)

[12Nad] Nadirah, W. O.W., Jawaid, M., Masri, A. A.A., Khalil, H. P.S., Suhaily, S. S., Mohamed, A. R.: J. Polym. Environ. 20(2), 404 (2012)

[13Net] Neto, A. R.S., Araujo, M. A.M., Souza, F. V.D., Mattoso, L. H.C.: Ind. Crop. Prod. 43, 529 (2013)

[14FAO] www. faostat. fao. org

Natural Protein Fibers

Keywords

Silk • Protein fiber • Insect • Secretion • Molecular weight • Bombyx mori

Silk is one of the most ancient fibers known to mankind and has been extensively used for various applications. Silk refers to the proteins secreted by insects in fiber form. Interestingly, silk fibers are made by the insects from proteins in an aqueous solution, but the proteins become semicrystalline and insoluble when formed into fibers [10Sut]. To produce fibers, insects accumulate proteins (25­30 % proteins) in their glands to obtain a viscosity nearly 3.5 million times that of water. Such high viscosity allows the insects to extrude continuous fibers. Fibers are formed by expelling a droplet of the protein onto a substrate and then pulling and drawing the solution away from the substrate. Typically, silk fibers are composed of two filaments containing the main protein fibroin that are glued together by the protein sericin. Fibroin found in fibers is classified as heavy fibroin (200-350 kDa), light fibroin (25-30 kDa), and glycoprotein P25 (25 kDa). The heavy chain fibroin is connected to the light chain fibroin through disulfide bonds and to P25 through hydrophobic interactions in a 6:6:1 ratio [13Lin]. Most silks contain high levels of the nonessential amino acids glycine, alanine, and serine avoiding the use of these proteins as diet by the insects.

Based on the type of silk gland, molecular structure, and phylogenetic relation­ship, silk producing insects have been classified into 23 different groups. Silk produced by the insect Bombyx mori is the most common silk available on the market and is commonly referred to as mulberry silk. B. mori silk is predominantly obtained from univoltine insects that produce only one cocoon during their life cycle in contrast to multivoltine insects that produce multiple cocoons. Currently, about 1.5 million tons of silk are produced across the world every year. Although it is reported that silk is produced in nearly 60 countries, China and India account for more than 90 % of the total world silk production. Table 34.1 lists the annual © Springer-Verlag Berlin Heidelberg 2015

N. Reddy, Y. Yang, Innovative Biofibers from Renewable Resources, DOI 10.1007/978-3-662-45136-6_34

production of silk by various countries. Countries such as Brazil, Thailand, and Uzbekistan produce limited quantities but exotic silks that are used for unique and distinct applications.

References

[10Sut] Sutherland, T. D., Young, J. H., Weisman, S., Hayashi, C. Y., Merritt, D. J.: Annu. Rev. Entomol. 55, 171 (2010)

[13Lin] Lintz, E. S., Scheibel, T. R.: Adv. Funct. Mater. 23, 4467 (2013)

Pure starch was electrospun after dissolving in 95 % aqueous dimethylsulfoxide solution. The starch solution was extruded into an “electro-wetspinning” setup consisting of an ethanol bath. After electrospinning, the fibers were heat treated to increase crystallinity and later cross-linked using glutaraldehyde vapors to improve water stability [14Kon]. In another study, researchers have suggested that the forma­tion of electrospun starch fibers in DMSO solutions was dependent on entanglement concentration and that an entanglement concentration of 1.2-2.7 times was necessary to obtain fibers. Similarly, starch with amylose content higher than 35 % was required for fiber formation. Extent of molecular entanglements, molecular conformations, and shear viscosity were other parameters that also had an influence on fiber formation [12Kon]. Fibers obtained had average diameters of 2.6 pm and the cross — linked fibers were stable and did not disperse after immersion in water. Electrospun starch fibers were proposed to be useful for food, textile, and biomedical applications. Hierarchical starch-based fibrous scaffolds for bone tissue applications were devel­oped by rapid prototyping and electrospinning approaches [09Mar].

Starch-polycaprolactone (30/70) blends were made into hierarchical nanofiber meshes and 3D scaffold structures shown in Fig. 58.16 and used as substrates to grow human osteoblast cells. It was found that the nanofiber meshes had topology similar to that of the extracellular matrix and the 3D fibrous structure provided mechanical stability.

Starch-polycaprolactone (30/70) blend was dissolved in acetic acid or chloro­form and electrospun into fibers with diameters between 130 and 180 nm when the solution concentration was between 5 and 15 %. Particles, supposed to be starch and with diameters between 4 and 66 nm, were found embedded in the fibers. The fibers developed were considered to be useful for bone, skin, and cartilage tissue engi­neering [08Juk].

Potato starch and PVA were blended and made into nanofibers through electrospinning after the addition of about 5 % of ethanol. However, the properties or the stability of the fibers in water was not reported [10Suk]. In another research, oxidized starch was blended with PVA in various ratios and the solution was extruded into fibers. Oxidized starch performed as a polyelectrolyte and improved electrospinnability. Increasing the ratio of starch decreased fiber diameter with average fiber diameters decreasing from 460 to 147 nm [11Wan].

Hydroxypropyl starch was blended with poly(ethylene oxide) to develop fibrous scaffolds for tissue engineering [13Sil]. Various ratios of starch and PEO (from 30 to 90 %) were used with electrospinning voltage varying from 11 to 14 kV.

Average fiber diameters obtained varied from 143 to 334 nm. However, an experi­mental study on the relationship between electrospinning conditions and fiber diameters found that starch concentration had higher impact than electrospinning voltage and distance to determine fiber diameter [13Kon]. To improve stability, fibers were coated with polymethyl methacrylate, leading to increase in fiber diameters. In vitro degradation studies showed that fibers containing higher amounts of PEO degraded faster and the weight loss after 700 h in PBS at 37 °C varied from 30 to 50 % [13Sil].

Electrospun fibers from starch acetate were prepared for potential use as drug carriers. Starch acetate with a degree of acetylation of 1.1 and 2.3 was electrospun using formic acid/water or formic acid/ethanol to assist fiber formation [09Xu3]. Effects of degree of acetylation and concentration of the polymer in the solution on properties of the electrospun structures were studied. Fiber matrices with strength ranging from 5 to 18 MPa in the dry state and 5-6 MPa in the wet state were obtained. It was also found that increasing the concentration of starch acetate from 12 to 20 % increased the tenacity from 5.9 to 16 MPa. However, the tenacity of the matrices decreased marginally from 18 to 16 MPa when the degree of substitution (DS) of starch was increased from 1.1 to 2.3 due to decreasing solubil­ity. The fiber matrices retained about 60 % of their strength after being in 90 % humidity for 32 days. Fibers made from higher DS starch acetate had low initial burst and a more sustained release of diclofenac used a model drug [09Xu3].

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