Innovative Biofibers from Renewable Resources

For centuries, mankind has been clothed using natural cellulose and protein fibers that have been almost entirely derived from dedicated sources. Cultivation of fiber crops and rearing of silkworms and sheep have been the traditional methods of obtaining cellulose and protein fibers, respectively. However, fiber crops were not just sources for clothing, but the by-products generated were major sources for food and means for substantial income. For instance, cotton seeds have been used as a source for oil and also as animal feed. Among the different types of fibers, natural cellulose fibers, mainly cotton, have been the most common source for fibers. Recently, the cultivation of cotton and other natural fibers has been declining due to the difficulties in growing cotton, better profits from biofuel crops such as corn and soybeans, and limited technological improvements in processing and using cotton-based textiles. Similarly, the supply of petroleum resources required for synthetic fibers at affordable prices could be questionable in the near future. At any given time, it can be expected that fuel needs would predominate the use of petroleum resources for textile fibers. In addition, increasing consumption, especially in the developing countries, constraints on the natural resources required to produce fibers, and inability to increase the supply proportionate to the demand are expected to make most of the current fibers either too expensive or unavailable for commodity applications. This scenario is neither unrealistic nor unforeseeable. The production of natural fibers such as cotton is declining due to cotton farmers shifting to more profitable biofuel crops such as corn and soybeans. These biofuel crops are also less demanding in terms of resources required for cultivation, harvesting, and processing into final products. The decrease in cotton production could escalate further due to the demand for biofuels.

Corn or maize is the second largest agricultural crop grown in the world, second only to sugarcane with 875 million tons produced in the world in 2012. Cultivation of corn generates stover (stalk, leaves, and husk) as by-product that has been considered for a variety of uses. In developed countries such as the United States, the recent efforts on producing cellulosic biofuels from biomass have led to the use of corn stover as feedstock for cellulosic ethanol. However, substantial quantities of corn stover are still left unused and are available for industrial use at low cost. Currently, a ton of corn stover baled and ready to be shipped is estimated to cost about $40–$50, making stover one of the cheapest lignocellulosic sources. Corn stover typically consists of about 50 % stalk, 23 % leaves, 15 % cobs, and 14 % husk. The stalks consist of an inner pith and outer rind which is the source for fibers. Cornhusks (ears, shucks) are fibrous structures that can be up to 20 cm in length and have been traditionally used for decoration, food wrapping, and other applications.

Wheat is the fourth most popular crop in the world with a production of 675 million tons in 2012. About 1–1.2 tons of straw are generated per acre and wheat straw accounts for about 50 % by weight of the cereal produced. Straw is mainly used as animal fodder and bedding, for thatching, and for artistic works, and in many countries, wheat straw is burnt to prevent soilborne diseases. Extensive studies have been done to understand the potential of using wheat straw for pulp and paper production. However, wheat straw has a waxy covering on the surface and a unique morphological structure that makes it difficult for alkali to penetrate into the straw and separate fiber bundles with the length, fineness, and tensile properties required for textile and other high-value fibrous applications. As seen in Fig.

3.1

, the individual cells or ultimate fibers in wheat straw have serrated edges that get interlocked with each other. It was found that a pretreatment with detergent and mechanical separation with steel balls were necessary before the alkaline treatment to obtain fiber bundles from wheat straw [07Red]. Fiber bundles obtained from wheat straw had tensile properties similar to kenaf as seen in Table

3.1

. About 20 % fibers were obtained, but the fiber bundles obtained were considerably coarser than cotton and linen.

Unlike corn stover where only the husks and stalks have been used for fiber production, fibers have been produced from both the leaves and stalks of sorghum plants [07Red]. As seen in Table 4.1, fibers obtained from sorghum stalks and leaves have similar properties. Tensile properties of the sorghum fibers were similar to that of jute, but the elongation was lower than that of linen or cotton fibers. About 20 % fibers were obtained from both the stems and leaves, and the fibers had relatively shorter lengths compared to fibers obtained from cornhusks.

The most prominent and oldest known natural cellulose fiber, cotton, has been grown and used for textiles since time immemorial. Cotton was grown in about 34.2 million hectares, and about 26 million tons of cotton was produced worldwide in 2012. In addition to the seed from which cotton fibers are harvested, cotton plants consist of stalks and leaves that are left as by-products, equivalent to 3–5 times the weight of the cotton fiber produced. Cotton stalks consist of an outer bark (20 % by weight of the stalk) and inner pith. The outer bark is fibrous and could be utilized as a source for fibers similar to the bast fibers produced from jute or flax plants. Treating the outer bark of cotton stalks with 2 N NaOH at boil for 1 h resulted in fibers with fineness of about 50 denier. These fibers had strength similar to cotton but lower elongation. When used as reinforcement for polypropylene composites, cotton stalk fibers provided similar tensile and flexural properties compared to jute fibers. Cotton stalks were treated at 150 °C in a mixture of 20 % sodium sulfide, 2 % anthraquinone, 2 % sodium silicate, and different concentrations of sodium hydroxide for 30 min. Concentration of sodium hydroxide considerably influenced the composition and properties of the fibers as seen in Table 5.1 [12Zho]. Substantially finer fibers (0.9 tex) have been produced by the high-temperature treatment reported by Zhou et al. compared to those produced by Reddy and Yang [09Red]. So far, no reports have been available on the processing of cotton stalk fibers into textiles or on the bleaching and dyeing of the cotton stalk fibers.

Palm trees are grown for oil in about 15 million hectares, and about 11 tons of dry mass are produced per hectare of palm grown. Cultivation of palm trees generates by-products called fronds (leaves) shown in Fig. 6.1, and about 164 million tons of fronds are estimated to be produced every year in the world [00Dah, 00Lin, 08Kha]. In addition to the fronds, the palm plants provide two additional sources of fibers. After harvesting the seeds, the fibrous empty fruit bunches (oil palm empty fruit bunch) (OPEFB) have been studied as potential sources for fibers. A kilogram of fruit bunch produces approximately 22 g of palm oil but results in about a kilogram of OPEFB [09Gun, 13Kit]. Similarly, the mesocarp left in the seed after squeezing for oil is also considered a source for fibers. On an average, about 400 g of fibers can be obtained from each OPEFB by natural retting [97Sre]. OPEFB fibers typically are composed of about 63 % cellulose, 18 % hemicellulose, and 18 % lignin. Natural cellulose fibers have been extracted from various types and parts of the palm tree. Native to upper Africa,

Hyphaene thebaica

(doum palm) was used as a source to extract fibers from the folioles and leaf stalks. For mechanical extraction, the plant parts were separated into fibers by beating and grating to liberate the fibers that were later dried in air [09Sgh]. After mechanical extraction, fibers were further treated with NaOH (3 N) for 2 h at 90 °C and later with sodium hypochlorite at room temperature [09Sgh]. As seen in Table 6.1, considerable variations in fiber properties can be seen between the foliole fibers and the leaf stalk fibers. Similarly, alkali treatment resulted in fibers with considerably finer diameter and higher strength, elongation, and modulus for the fibers obtained from the folioles. Morphologically, the fibers contain pores on the surface that were more evident after alkali treatment as seen in Fig. 6.2 [09Sgh].

One of the most ubiquitous fruits, banana is widely grown across the world. About 120–150 million tons of bananas are grown annually in the world, and it is the fourth most important food product in the world. However, the banana fruit only represents about 12 % of the weight of the plant and the stem; leaves and other parts are not generally edible. Therefore, efforts have been made to use banana leaves and stems for various nonfood applications including fiber production. Fibers are obtained from the pseudo-stem of the plant mostly by mechanical means. Full-fledged banana fiber production has been reported to be operational in several countries. Some of the products developed from banana fibers include textiles, paper, floor mats, and composites. In terms of properties, banana fibers have the typical composition of fibers obtained from lignocellulosic by-products and contain about 50 % cellulose, 17 % lignin, and 4 % ash [09Gui]. However, the composition of the banana fibers reported varies widely, and fibers with lignin content as high as 17 % have been reported [08Hab]. In addition to the stem, fibers have also been obtained from the leaf and rachis of the banana plant. Considerable variations in the tensile properties were observed for the fiber bundles obtained from the different parts and also depending on the method of extraction as seen in Table 7.1 [08Gan]. Tensile properties of the fibers obtained from the banana stems are similar to those of common lignocellulosic fibers such as jute, but the elongation is considerably lower than that of the coconut and palm (

Borassus flabellifer

) fibers. Low elongation of the banana fibers should mainly be due to the lower microfibrillar angle (11°) and relatively high % crystallinity [08Muk]. Banana fibers also appear to have a hollow center similar to that found in a few other natural cellulose fibers. Considerable variation in the tensile properties, especially elongation, was observed for fibers with various diameters (50–250 μm) as seen in Tables 7.1, 7.2, and 7.3 [10Ven]. In addition to the stems, fibers have also been obtained from the leaves of the banana plant. Typically, banana plants produce about 30 leaves as long as 2 m and 30–60 cm wide [07Bil]. Fibers obtained from banana leaves had about 26 % cellulose, 17 % hemicellulose, and 25 % lignin, but the fiber properties are not reported [07Bil]. A Switzerland-based company (Swicofil) advertises that it had developed fabrics from ring- and rotor-spun banana fibers. Ring-spun yarns in counts ranging from Ne 8/1 to 40/1 and rotor-spun yarns with counts (Ne) ranging from 8/1 to 30/1 were reported to be available in 100 % form and also as blends with cotton, modal, Tencel, and soy protein fibers. Banana fibers are reported to be available on the market for about US$0.43–0.81/kg compared to $0.15–0.60 for hemp and $0.15–$0.21/kg for flax.

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 generated as coproducts [08Lee]. Bagasse is a lignocellulosic material consisting of 45–55 % cellulose, 20–25 % hemicellulose, 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/m

3

) and consists of coarse fibers and many large cavities compared to the rind with a density of 550 kg/m

3

. 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].

About 62 million tons of coconuts are grown in about 92 countries across the world. Coconut trees or palms and the husks of the coconut fruit have extensively been used as sources for fibers. Fibers obtained from the husks (Fig. 9.1) of coconuts are generally termed “coir fibers” and are used for a variety of applications. Each coconut or copra yields about 80–90 g of husk fibers in Asia, whereas coconuts grown in the Caribbean contain thick husks and could yield up to 150 g of fiber. Each husk is composed of about 70 % pith and 30 % fiber and consists of 60 % long (150–350 mm), 30 % medium, and 10 % short fibers (<50 mm). About 5–6 million tons per year of brown coir and 125,000 tons of white fiber, mostly in India, are produced every year [13Van]. Fibers are obtained from the husks using conventional retting and chemical and biological means. In a conventional process, the husks are retted in brackish water for 3–6 months or in saltwater for 10–12 months to soften the fibers. Later, the fibers are separated by decorticating and beating and hackled and washed. This traditional processing yields the finest and whitest fibers. Alternative to traditional retting, mechanical processes to defibrillate or decorticate the husk have been developed. These machines and processes can process husks that have been treated for 5 days in water, but the quality of the fibers is heavily dependent on the processing conditions and severity of treatments. Recently, enzymatic processes have also been developed that are cleaner and milder and produce fibers with better quality.

Pineapple (

Ananas comosus

) belongs to the Bromeliaceae family and is a short tropical plant that grows to 1–2 m in height and width. About 21.9 million tons of pineapples were produced in the world in 2011 [14FAO]. As seen from Fig. 10.1, pineapple plants consist of a rosette of 20–30 leaves that are generally 6 cm wide and up to 1 m long. About 96–100 tons of fresh leaves are generated per hectare. Fresh pineapple leaves yield about 2.5–3.5 % fibers that are white, creamy, and silk like with a soft texture and also absorb high amounts of moisture [12Nad]. Fibers obtained from pineapple leaves are composed of relatively higher amounts of cellulose (74 %), lignin (10.4 %), and ash (4.7 %) [12Nad]. Pineapple leaf fibers (PALF) fibers are reported to have a microfibrillar angle of 14° that results in lower elongation. Traditionally, fibers are manually extracted from pineapple leaves by scrapping the outer layers. During the last decade, decorticating machines have been developed that can process about 35 kg of fibers per 8 h shift.

Switchgrass is a high-yielding, low-input biomass crop that is considered to be the most suitable crop for cellulosic ethanol production. Although not a by-product, switchgrass can be a source for fibers requiring fewer inputs to grow and could be economically more viable than traditional fiber crops such as jute and flax. In addition, about 25–30 % of switchgrass can be obtained as long fibers for high-value applications, and the remaining 20–25 % of short fibers and hemicellulose could still be used for ethanol production. Switchgrass consists of outer leaves (45 % of total plant weight) and inner sheath (stem) (55 % of total plant weight). Both the leaves and stems were used for fiber production [07Red]. As seen in Table 11.1, fibers obtained from switchgrass have very unique and distinct properties not seen in any other fiber obtained from lignocellulosic by-products. Fibers obtained from the leaves had high strength but low elongation similar to that of linen, whereas fibers from the stems of switchgrass had lower strength but high elongation, similar to that of cotton. A single plant producing two types of fibers with such distinct characteristics is unique. The relatively low fineness of fibers obtained from switchgrass leaves implies that the fibers could be processed on textile machinery. Low costs to grow, high fiber yield (20–25 %), and distinct fiber properties make switchgrass a crop with high potential for fiber production.

Belonging to the same family (

Cannabaceae

) and genus

Cannabis

as hemp, hop (

Humulus lupulus

L.) is a plant grown for its flower, an ingredient used in most beer. After harvesting the flower, the hop plants are cut and considered as waste. Hop stems contain an outer bark and inner pith, typical of any bast fiber plant. The fibrous outer bark has been used to produce long-length fibers (10–15 cm) with tensile properties comparable to that of hemp [09Red]. Hop stem fibers also had cellulose crystal structure similar to that of hemp as seen in Table 12.1. Untreated hop stems and fibers obtained from hop stems have been used as reinforcement for composites [10Zou].

Considerable attention has been drawn towards the generally termed “bamboo fibers” in the last decade mainly because bamboo is a fast-growing (a meter or higher per day) biomass crop that needs minimum inputs and is renewable. However, most reports or articles on bamboo fibers refer to regenerated cellulose fibers that are obtained using bamboo as a source and not the natural fibers extracted from bamboo stems/stalks. Nevertheless, natural fibers have been extracted from bamboo, and some of the literature has been covered here despite bamboo not being a by-product and has to be grown independently. Companies are extracting natural cellulose fibers from bamboo stems and are selling them commercially. Litrax, a France-based company, is marketing “L1 natural bamboo bast fiber” that has been enzymatically extracted from bamboo stems. The extracted fibers have a linear density of about 5.2 denier and are supplied in various staple lengths up to 90 mm.

In addition to the by-products from the major food crops, several other nontraditional lignocellulosic sources have been studied as sources for fibers. Examples of such plants used for fiber production include bamboo [07Rao],

Wrightia tinctoria

[05Sub], piassava [06Alm], blue agave [13Kes], stinging nettle [08Bod], sponge gourd [09Gui],

Luffa cylindrica

[10Siq], and others. Most of these sources are available in small quantities or need to be exclusively grown (bamboo) and do not have highly distinguishable properties. We have therefore not covered these fibers in this chapter.

The production of regenerated cellulose fibers as early as the 1930s resulted in the generation of a new class of fibers. For several decades, the production of regenerated cellulose fibers such as viscose rayon and cuprammonium rayon was extensively done, and these fibers were considered to be ideal substitutes for the natural cellulose fibers. Traditionally, regenerated cellulose fibers were produced using wood as a source for cellulose. Regenerated cellulose fibers generally termed “rayon” were produced in various configurations and properties. Figure 15.1 depicts the cross section and Table 15.1 provides the properties of the different types of conventional regenerated cellulose fibers. As seen in the table, considerable variations in properties are observed depending on the cross section and the type (specifically degree of polymerization) of the cellulose used for fiber production. A rather distinguishing feature of the fibers which is also a major limitation of the regenerated cellulose fibers is their considerably lower wet strength compared to their dry strength, whereas the most common natural cellulose fiber cotton becomes stronger when wet. This unique behavior has been demonstrated to be mainly due to the poor crystallinity (30–35 %) of regular viscose fibers.

A simple approach to producing regenerated cellulose fibers was to dissolve cellulose pulp using alkali. The principle and mechanism of dissolving cellulose in alkali solutions are depicted in Figs. 16.1 and 16.2. It has been proven that the solubility of cellulose in alkali solutions is mainly governed by the degree of breakdown of the intramolecular hydrogen bonding and also by the degree of polymerization [92Kam, 98Iso]. The presence of lignin was found to lower dissolution, whereas the extent of hemicellulose did not affect the solubility [98Iso]. Several authors have used alkali solutions to produce regenerated cellulose films and fibers using cellulose from different sources [92Kam]. Alkali-soluble cellulose was prepared by exploding softwood pulp (DP of 331) with steam, and later, the pulp was dissolved in 9.1 % of NaOH precooled to 4 °C and used to extrude fibers. Fibers were produced with a fineness of 53–84 denier and had % crystallinity between 65 and 67 %. The tensile strength of the fibers varied from 1.5 to 1.8 g per denier, and the elongation was between 4.3 and 7.3 % depending on the conditions used during coagulation [92Yam]. Similar to the NaOH/urea system, the alkali system of dissolving cellulose was also limited by the degree of polymerization. Cellulose with relatively high DP (850) had limited solubility (26–37 %) in the alkali solutions [98Iso, 08Wan]. Contrarily, rayon which has a lower DP, poorly ordered crystalline region, and weak hydrogen bonding completely dissolved in alkali solutions [90Yok].

An extension of the alkali system of dissolving cellulose and the most recent development in the production of regenerated cellulose fibers has been the dissolution of cellulose using NaOH/urea or NaOH/thiourea systems [04Rua, 06Che]. In one such approach, cotton linter pulp (DP ~ 550) of 4–5 wt% was dissolved using NaOH (9.5 %) and thiourea (4.5 %) solution that was precooled to −8 to −10 °C [01Zha, 10Zha]. After dissolution, the solution was filtered, degassed, and extruded through a spinneret into a coagulation bath. Various chemicals (mainly acids or salts) were added into the coagulation bath, and it was found that aqueous solutions of sulfuric acid, hydrochloric acids, acetic acid, or ammonium salts were best for fiber formation. Fibers were produced using a laboratory wet spinning system at a pressure of 0.15 MPa and with a spinneret diameter of 0.12 mm [04Cai, 06Che]. Morphologically, the fibers obtained had a circular cross section contrary to traditionally produced viscose fibers that have a distinguishing irregular cross section. Unlike the conventional viscose process where complete dissolution of cellulose occurs, the new solvent system is considered to be a physical sol–gel process that helps to retain the circular shape of the fibers [04Rua]. Some of the properties of the fibers obtained using the NaOH/urea systems are compared to the traditional viscose fibers obtained from the NMMO system in Table 17.1.

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.

Room temperature ionic liquids are considered green solvents and typically have low vapor pressure and good dissolution power and are easily recyclable [10Mak]. Ionic liquids used to dissolve cellulose should have low melting points, should not decompose cellulose, and should be stable and easily recoverable and relatively inexpensive. Considerable research has been done on dissolving cellulose using ionic liquids. Some of the ionic liquids that have been used to dissolve cellulose with concentrations of 10 % or above are listed in Table 19.1 [10Mak]. Dissolution of cellulose by ionic liquids is mainly related to the combined properties of the cations and anions and the basicity of the hydrogen bonds. Smaller cations were found to be more efficient in dissolving cellulose, and cations containing hydroxyl end groups had lower solubility [05Zha]. This is because the hydroxyl groups in the cations react with acetate of chloride anions and compete with cellulose to form hydrogen bonds. Ionic liquids with high hydrogen bond basicity were also found to have better solubility for cellulose. Ionic liquids are considered to be non-derivatizing solvents for cellulose, and therefore changes to the structure of cellulose are not expected. However, depolymerization of cellulose has been observed at high temperature when ionic liquids such as 1-allyl-3-methylimidazolium chloride [AMIM][Cl] were used [05Zha], whereas [BMIM][Cl] did not depolymerize cellulose.

Conventional approach of producing regenerated cellulose through ionic liquids is based on using pulp (>90 % cellulose) obtained from various sources. Recently, attempts have been made to directly use the biomass containing cellulose, hemicellulose, and lignin and produce composite fibers. Such an approach would avoid the need to produce pulp and substantially reduce the cost of the fiber and the use of chemicals. Biomass from oak, bagasse, and pine was used with and without pretreatment to produce fibers using 1-ethyl-3-methylimidazolium acetate as the solvent. Type and composition of biomass, conditions used for pretreatment, and dissolution and composition and properties of the fibers obtained and compared in Table 20.1. Fiber production conditions varied depending on the type of pulp, and it was found that fibers containing higher amounts of cellulose had higher strength and elongation [11Sun]. Also, pretreatment resulted in higher cellulose content and therefore better properties. Bagasse could be dissolved and made into fibers using low- or high-temperature dissolution, whereas wood cellulose required the use of high temperatures but shorter time. The ability to directly dissolve lignocellulosic sources and produce fibers could lead to novel fibers and also significant cost reductions. However, the viability of commercial-scale production of this process and the properties of the fibers that can be obtained is not known. In addition, the presence of lignin and hemicellulose could affect further processing (dyeing, etc.) of the fibers, and the properties of the fibers could be adversely affected.

Apart from using conventional sources of cellulose such as wood, nonconventional resources such as sugarcane bagasse, bamboo, and bacteria have also been used to produce regenerated cellulose fibers. Viscose fibers with a linear density of 17 tex and length between 1.7 and 38 mm obtained from eucalyptus wood were modified with alkali solutions to improve the adhesive and surface properties [13Roj]. After treating with various concentrations of alkali, about 10–12 % decrease in % crystallinity was observed. Increase in diameter of fibers and decrease in contact angle after alkali treatment were also seen [13Roj].

The NaOH/urea/thiourea systems have been further modified to enable dissolution of wood cellulose and high DP cellulose and obtain stable spinning solutions [13Zha]. Up to 87 % solubility was obtained for wood cellulose with a DP of 648 using NaOH/acetamide/tetraethylammonium chloride [13Zha]. Morphology and thermal stability of the fibers produced using the multicomponent system were studied, but the tensile properties were not reported.

Chitin is a polymer made from units of

N

-acetyl glucosamine as shown in Fig.

23.1

. Chitin is the structural unit that provides strength to most invertebrates and is one of the most common biopolymers found in nature. Unlike most other polysaccharides, chitin contains about 6.9 % nitrogen which makes it useful as a chelating agent and also for various applications in the pharmaceutical, biomedical, paper, textile, photographic, and other applications. Chitin is also found in bacteria and fungi. In its native form, chitin is insoluble in common solvents and therefore has limited applications. Typically, chitin exists with an average molecular weight of 1.036 × 10

6

to 2.5 × 10

6

Da. Generally, chitin is deacetylated and obtained as chitosan which is soluble in aqueous acetic acid. Fibers have been obtained from chitin, chitosan, and several other chitin derivatives.

Several attempts were made in the 1920s and the 1930s to dissolve chitin using ionic salts and produce fibers. For instance, chitin was dissolved in lithium thiocyanate and made into fibers. Instead of using native chitin, chitin xanthates were made by steeping chitin in 28–50 % NaOH solutions at room temperature for 2 h. Later the chitin was exposed to carbon disulfide to obtain the chitin xanthate. Fibers obtained were drawn to 250 % using hot glycerin. Dry strength of the fibers ranged from 1 to 1.2 g per denier, and the breaking elongation was 30 %. Using similar methods, other researchers had reported obtaining fibers with strength ranging from 0.9 to 1.5 g per denier with fineness being 3.08–18 deniers. Attempts were also made to combine chitosan xanthates with cellulose xanthates and produce fibers with better quality than chitin and viscose rayon fibers [97Agb]. However, fibers obtained using these approaches were considerably weak when wet and practically not useful. Some attempts had been made to cross-link the fibers with formaldehyde to improve wet stability, but the elongation and modulus had to be sacrificed [77Nog]. Some of the approaches used to obtain chitin fibers and the properties of the fibers obtained are listed in Table 24.1. Although good tensile strength was obtained using both the halogenated and amide–lithium chloride systems, the wet strengths were only about 0.2–0.5 g/denier, and the fibers were therefore not practically useful. Tokura et al. developed chitin fibers by suspending chitin in 99 % acetic acid to form a gel and later by dispersing the gel in dichloroacetic acid and isopropyl alcohol [79Tok]. Fibers obtained had fineness ranging from 2 to 25 denier, dry strength was between 0.7 and 1.6 g per denier, and elongation ranged from 2.7 to 3.4 %. However, the wet strength of the fibers was only between 0.1 and 0.3 g per denier.

Chitosan has been extensively studied for the production of fibers, and the fibers developed have been thoroughly characterized for their structure, properties, and potential applications. One of the major advantages of using chitosan for fiber production is the solubility of chitosan in common solvents that are relatively inexpensive and environmentally friendly. Table 25.1 lists the most common solvents that have been studied for dissolving chitosan. In addition to the solvents, several other parameters have also been reported to influence the properties of chitosan fibers produced. El-Tahlawy and Hudson studied the effect of various spinning parameters on the production and properties of chitosan fibers [06El]. They reported that viscosity of the solution was critical for fiber production and that adding salt such as sodium acetate assisted in controlling the viscosity, draw ratio, and therefore fiber properties. Similarly, it was reported that the process used to dry the fibers after coagulation also influenced fiber properties. Drying in a methanol coagulation bath provided fibers that could easily separate from each other and have a smooth surface and higher mechanical properties than direct, radiant, or forced air heating [98Kna]. The effect of demineralization time and temperature on the properties and biodegradation of chitosan fibers was investigated by Judawisastra et al. [12Jud]. It was reported that demineralization caused degradation of the polymers and led to an increase in the diameter of the fibers, reduced tenacity by 52 %, and increased elongation (136 %). Biodegradation of the fibers in a phosphate-buffered solution containing 2 % lysozyme increased by 17 %. Similarly, ripening of chitosan dissolved in acetic acid was found to substantially affect fiber properties [03Lee]. Increasing ripening time continually decreased tenacity and modulus but increased elongation. Thermal analysis showed that the peak temperature and thermal degradation temperature decreased with an increase in ripening time.

Chitosan fibers were used to load silver particles by adding sodium hydrogen zirconium phosphate into the spinning solution [07Qin]. The silver particles were reported to be uniformly divided in the fibers and did not affect the color of the fibers. SEM image in Fig. 26.1 shows the presence of the nanoparticles on the surface of the fibers. Silver ions were released when the fibers were placed in water or aqueous protein solution. Incorporation of silver significantly increased the antibacterial activity of the fibers with greater than 98 % reduction for common bacteria. The ability of chitosan fibers to absorb silver and zinc ions that are delivered through wound dressings was investigated [06Qin]. Chitosan fibers were treated with silver nitrate and zinc chloride solutions, and the release of these ions in saline was studied. It was found that the silver-containing fibers had good antimicrobial properties, whereas the zinc-containing fibers could be used to deliver zinc ions for wound care applications. Figure

26.2

shows that the chitosan- and silver-containing fibers had a clear

Escherichia coli

inhibition compared to the viscose fiber-containing solution suggesting that the chitosan fibers inhibited bacterial growth.

Hollow chitosan fibers (Fig. 27.1) were fabricated by removing unprecipitated chitosan through air and water flow [01Vin, 08Ara]. These hollow fibers have been used for various applications. For instance, hollow chitosan fibers were used to extract Cr(VI) with aliquot 336 by assembling the hollow fibers into a module and circulating the metal ion solution and extract inside the hollow lumen. It was observed that Cr(VI) ions were sorbed on the fiber and also by solvent which flowed through the fiber. Reacetylation of the fiber maintained the efficiency of extraction and also increased the mechanical and chemical resistance [01Vin]. Hollow chitosan fibers supported with palladium were also used to degrade nitrophenol found in industrial waste waters [04Vin]. A sodium formate system and a hydrogen system were used, and the former was found to be more efficient. Experimental parameters such as residence time, recycling, and concentration of the chemicals were reported to determine the efficiency of degradation. Similarly, palladium-supported chitosan fibers were also used as a catalytic system for hydrogenation of nitrotoluene [08Blo]. The diffusion of biological agents such as tryptophan, chloramphenicol, amoxicillin, and vitamin B12 through hollow chitosan fibers was investigated to understand the potential of using the fibers as nerve guide channels [08Pei]. pH of the permeant was found to have the most significant impact on permeability with the permeability coefficient decreasing with the molecular weight of the permeant. These fibers were considered suitable for catalysis and support for biological molecules or enzymes or for controlled drug release and enzyme immobilization [08Pei]. Hollow chitosan/cellulose acetate fibers were produced by wet spinning for use as absorptive membranes for affinity-based separations [05Liu]. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) studies showed interactions between cellulose acetate and chitosan. Blend fibers had good tensile properties and showed high surface absorption for copper ions and bovine serum albumin [05Liu]. Absorption of copper up to 30 mg/g of chitosan and 8 mg/g of bovine serum albumin was obtained.

Alginates are polysaccharides found as the cell wall constituents in brown algae (Phaeophyceae) which is considered a seaweed. Although some quantities of alginate are found in most species of brown algae, certain species (

Laminaria

,

Lessonia

,

Macrocystis

,

Sargassum

) contain 30–45 % alginate by dry weight, and these species are used for extraction. In China, about 2 million tons of seaweeds are artificially cultured for alginate production. In 2009, about 27,000 tons of alginate with an estimated value of $318 million were reportedly produced. Alginates are extracted from raw seaweeds by treating with sodium hydroxide when the alginates in salt form are converted into the water-soluble sodium alginate [08Qin]. Chemically, alginates are linear polymers composed of 1,4,-β-

d

-mannuronic acid (M) and α-

l

-guluronic acid (G) residues. The amount of M and G residues and the proportion of the blocks of M and G residues vary between different species and are responsible for the variations in properties between the different alginates [08Qin]. Table

28.1

lists the percentage of M and G residues and the block structures found in commercially available alginates extracted from different types of Brown seaweeds.

Combining the inherent ability of alginate to form gels and sorb moisture with the antimicrobial activity of chitosan and other unique properties of the two polymers would provide ideal materials for various medical applications. Therefore, polyion complex fibers were prepared by combining the advantages of alginate and chitosan polymers for potential use as cartilage tissue engineering scaffolds. Since the anionic nature of alginate was not conducive for the attachment of chondrocytes, it was hypothesized that adding chitosan, a cationic polysaccharide with excellent cell adhesive properties, would improve chondrocyte adhesion [04Iwa]. Sodium alginate (6 %, Mw = 600,000) was mixed with chitosan (0.035 and 0.05 % on weight of alginate) and extruded into a CaCl

2

bath to form fibers. The influence of the addition of chitosan on the tensile properties of the alginate fibers is shown in Table 29.1. Although the addition of chitosan did not change the tensile properties of the fibers, it was found that the composite fibers had enhanced cell attachment and proliferation, in vitro [04Iwa]. SEM image of chondrocytes shown in Fig.

29.1

on the alginate fibers containing 0.05 % chitosan showed the characteristic round morphology and dense collagen II fiber formation indicating good biocompatibility.

Calcium alginate fibers intended for wound dressing applications were mixed with another polysaccharide branan ferulate which is a recognized polymer for wound dressing and treating ulcers and sores [03Mir]. The influence of the alginates supplied by different companies and the addition of ferulate on the mechanical properties were studied. Up to 75 % ferulate could be added to selected types of alginates without sacrificing the tensile properties. Dry tenacities of the fibers varied from 0.2 to 1.6 g/den, and elongation was between 10 and 40 %. In a similar research, calcium alginate fibers were blended with konjac glucomannan (KGM) and later treated with silver nitrate to impart antimicrobial activity, and the properties of the blend fibers were studied. The addition of KGM increased the dry strength but decreased the wet strength. It was suggested that KGM and alginate had good compatibility, and the addition of silver imparted good antimicrobial activity [07Fan]. In this research, dry tenacity of the pure alginate fiber was 1.2 g/den, and elongation was 18 %. The addition of KGM increased the strength up to 1.6 g/den and elongation up to 34 %. Substantial increase in water retention was seen with the retention value being 1,000 % with 70 % KGM compared to 91 % without KGM. Fibers treated with silver had higher than 99.99 % bacterial reduction to

S. aureus

. Wet strength of the fibers varied between 0.04 and 0.3 g/den, considerably lower than the dry strength.

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 Ca

2+

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.

The antifungal activity and cytotoxicity of zinc, calcium, and copper alginate fibers were studied to evaluate the feasibility for tissue engineering and medical applications [12Gon]. Antifungal activity of the fibers was measured against

Candida albicans

, and the cytotoxicity was measured using human fibroblast and human embryonic kidney cells. Figure 32.1 shows the zone of inhibition of the calcium (a), copper (b), and zinc alginate (c) fibers against

C. albicans

. As seen from the figure, zinc alginate fibers had higher inhibitory zone and rates (80 %) compared to copper (60 %) and calcium alginate (40 %) fibers. In addition, zinc alginate fibers did not show any cytotoxicity but promoted cell growth indicating the suitability of the fibers as scaffold for tissue engineering. In a similar research, copper alginate fibers with tenacity up to 2.4 g/den were developed and were reported to have good antibacterial activity [05Mik].

A novel microfluidic spinning method was used to develop flat alginate fibers with grooves for cell scaffolding [12Kan] instead of using the traditional approach of microelectromechanical systems (MEMS) for topological construction of tissue engineering scaffolds. As seen in Fig. 33.1, thin flat fibers with diameters less than 10 μm were continuously fabricated by passing the alginate solution through channels containing calcium chloride. Fibers with various diameters and widths were obtained by changing the flow rate, and the fibers formed were wound continuously onto spools. SEM images of the smooth and grooved flat fibers are shown in Fig. 33.2. Figure 33.2c shows the fibers with 5 and 7 grooves obtained by changing the pattern on the sample channel. Fibers with different number of grooves on each side were also produced as seen in Fig. 33.2g. This approach of fiber formation allowed precise control of dimensions and enabled fabrication of scaffolds that could regulate cellular morphogenesis [12Kan]. The fibers developed were used to culture neuron cells, and the cell attachment, proliferation, and alignment were studied. The cells migrated to the sides of the smooth fibers and along the ridges of the grooved fibers as seen in Fig. 33.3i. As seen in the fluorescent and SEM images, cluster of cells were seen growing on the ridges of the fibers, and the cells were connected by neurites along the length of the grooves unlike the cells on the smooth fibers where the neurites formed a random network. Similar accumulation and alignment of cells in the grooves were also found for myoblast cells. The ability to guide the morphogenesis of cells and achieve topographic control over cell alignment was perceived to be crucial to reconnect muscle tissues and for other tissue engineering applications. In a similar approach, a microfluidic device was used for continuous (on the fly) production of calcium alginate fibers [07Shi]. Basically, a poly(dimethylsiloxane) (PDMS) microfluidic device embedded with a glass capillary pipet was used for fiber production. Sodium alginate solution was introduced in the sample flow, and calcium chloride solution was introduced as the sheath liquid. Sufficient time is allowed for the fibers to precipitate by changing the length of the outlet pipet. Human–mouse fibroblasts and bovine serum albumin–fluorescein isothiocyanate were loaded into the fiber during fiber production to evaluate the suitability of the fiber production method for medical applications. Cells loaded onto the fibers survived the production process and were embedded inside and had about 80 % viability after 24 h suggesting that the process could be useful to load therapeutic materials and for delivery of drugs.

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 (25–30 % 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.

Structurally, five types (coiled coil, β-strand, cross β-sheet, collagen triple helix, and polyglycine) of silk have been identified that vary in the amount of crystalline and amorphous regions and the arrangement (sequence and orientation) of the amino acids along the axis of the fiber. Each type of structure has a specific sequence of amino acids. For instance, the coiled-coil structure has seven amino acid residues, and the protein chains form a right-handed α-helix with 3.2 amino acids per turn. In the coiled-coil silks produced by some insect species such as honeybees and weaver ants, each fibroin contains 210 amino acid residues in the coiled-coil region with alanine-rich cores [07Sut]. A structural model for the coiled-coil silk is shown in Fig. 35.1. Coiled-coil silks were also found to contain unusually high levels of alanine and large hydrophobic residues. The high levels of alanine were required to stabilize the helices and facilitate coiled-coil formation [07Sut]. In a β-strand structure, alternating amino acid side chains form opposite faces of the sheet and in a cross-β sheet, the protein chains form β-strands of uniform length and alternating turns at which the direction of the protein chain reverses. In a collagen triple helix, three 32 helices intertwine and form a superhelix, and finally in a polyglycine structure, the protein chains form a right-handed helix with three amino acids per turn. Figure 35.2 illustrates the five different types of structures discussed here.

Wild or non-mulberry silks are produced from various species of insects. Most popular non-mulberry silks that are commercially available are tasar (

Antheraea mylitta

), eri (

Samia cynthia ricini

), and muga (

Antheraea assamensis

). Properties of these three common types of wild silks are compared to

Bombyx mori

silk in Table 36.1. A typical life cycle of a wild silkworm (

Antheraea mylitta

) is shown in Fig. 36.1 [12Kun]. During production of the wild silk fibers, in addition to the cocoons, some sericin proteins are extruded external to the cocoons and are called peduncles. These peduncles (Fig. 36.2) act as reservoir for sericin and are seen only in the non-mulberry silks. Silk produced in these peduncles was found to be similar to the sericin in the cocoons [06Das] with proteins having molecular weight of 200 kDa and mainly composed of glycine and serine with 36.7 % β-sheets, 52.7 % random coils, and 10.6 % turns with no helices. Other researchers have suggested that

Antheraea mylitta

contains polyalanine repeat sequences, and fibroin extracted from the silk gland of

Antheraea mylitta

had a molecular mass of 395 kDa with monomers of approximately 197 kDa [09Ach]. To determine the structural differences using nuclear magnetic resonance (NMR),

13

C and

15

N labeling, select amino acids were orally fed to the fifth instar larvae. Silk obtained contained 75 % alanine and 65 % glycine residues, the alanine content being much greater than that found in

B. mori

silk [99Asa, 04Asa]. In the solid state, the glycine-rich regions stretched up to 10 times indicating that β-sheets were predominant.

Although silks are characterized by their bright color and luster, there are only limited colors in which cocoons are produced. Recently, attempts have been made to develop a new class of colored silks by feeding mulberry leaves mixed with fluorescent dyes to

Bombyx mori

insects. The dyes were predominantly taken up by the fibroin proteins, and the color was persistent even after degumming. These unique colored silks were found to have similar crystalline structure and tensile properties and also supported the attachment and growth of human colon fibroblasts [11Tan]. The presence of the fluorescent dye provided luminescent fibers (Fig. 37.1) that could enable the detection of cell attachment and spreading more easily. Such colored silk fibers would eliminate the need for dyeing and lead to substantial savings in energy, water, and other resources and also provide unique fibers for medical and other applications.

To obtain fibers with better properties, an artificial method of biospinning silk fiber matrices was adopted, and the fibers and matrices were used as substrates for tissue engineering. Wild silk worms (

Antheraea mylitta

) in their fifth instar were collected, and fibers were manually (forcefully) drawn from the silkworm onto glass slides as shown in Fig. 38.1a. Fibers obtained were aligned in various fashions to develop matrices for tissue engineering. Alternatively, the silk worms were allowed to naturally spin silk onto Teflon-coated glass plates, and the matrices formed (Fig. 38.1b) were collected. Fibers and matrices were degummed and later characterized for their properties, and the potential of using the fibers as substrates for tissue engineering was studied [10Man]. Fibers obtained by forceful extrusion and drawing were circular and had diameters of 12–15 μm compared to 30–35 μm for naturally extruded silk. Similarly, the biospun fibers had tensile strength of 4.1 ± 1.4 g/den, much higher than that of

Bombyx mori

or the natural fibers obtained from

A. mylitta

. The fibers and matrices developed had enhanced stability to degradation by proteases and found to have good compatibility and supported the attachment and proliferation of fibroblasts [10Man].

Weaver ants (Fig. 39.1) belonging to the

Oecophylla smaragdina

family produce natural silks in the form of nanofibers that are connected together to form webs that resemble a piece of fabric as seen in Fig. 39.2. Fibers in the webs were hollow and had average diameters of 450 nm and had a unique architecture. As seen in Fig. 39.3, ants stick the fibers to form a web, and the connecting places were considerably stronger and resist alkali treatment even at boiling temperature. Although properties of individual fibers produced by the ant were not tested, webs produced by the ants were considerably stronger than electrospun protein nanofiber webs with substantially higher elongation (32 %) as seen in Table 39.1. It would be considerably challenging to produce nanofiber webs, especially with hollow nanofibers in the laboratory. Since ants are social insects unlike spiders, it would be possible to produce unique nanofibers webs by rearing the ants. It was found that the webs could be used as substrates for tissue engineering and could also load high amounts of drugs due to the presence of hollow fibers [11Red]. Other researchers have also reported that fibers in the weaver ant webs have diameters between 266 nm and 3 μm and that the proteins are mostly in the form of random coils and β-sheets [10Sir].

Lacewings are insects that lay their eggs on the tips of silken threads called egg stalks as seen from Fig. 40.1 [09Wei]. Unlike most silk-producing insects, green lacewing (

Mallada signata

,

N

europtera) produces two distinct types of silks depending on the life cycle of the insect [08Wei]. Silks produced by lacewing in the larval stage and during the final instar of cocoon production were found to be different. Primary structure of the lacewing silk is composed of motifs containing 16 amino acids with cysteine residues [12Bau]. The cocoon silk is composed of 49 kDa proteins, with >40 % alanine, and contains α-helical secondary structure, considerably smaller than the proteins (>200 kDa) seen in the classic β-sheet silks. In terms of secondary structure, lacewing silk was mainly composed of unique and distinct cross β-sheets that run perpendicular to the fiber axis unlike the silk produced by other insects. A model suggesting the arrangement of the cross β-sheets in lacewing silk is shown in Fig. 40.2 [13Lin]. Atomic force measurements and calculations have shown that the lacewing silk has a bending modulus three times higher than that of silkworm fibers [09Wei]. Tensile properties of the silk were found to be highly dependent on the water content (relative humidity) with modulus decreasing from 50 g/den to 11 g/den when the relative humidity was increased from 30 to 100 % and the corresponding change in breaking stress was from 2.0 to 0.6 g/den. This substantial change in properties due to change in humidity was supposed to be due to the transition of the cross β-sheets to parallel β-sheets caused by the weakening of the hydrogen bonds at high humidity [12Bau]. At low RH, the total strength of the hydrogen bonds in one layer of the stalk is higher than that of the disulfide bonds causing the fibers to absorb low energies. When the RH is high, the hydrogen bonds are weakened, and the disulfide bonds are now stronger than the sum of the hydrogen bonds in one layer causing the hydrogen bonds to break. Such breakage of the hydrogen bonds allows the rearrangement of the β-sheets [12Bau]. SEM image (Fig. 40.3) showed thinning of the fibers after stretching which was not reversible, again indicating the transformation of the β-sheets. The simple process by which lacewing secretes silks is considered to be more suitable for producing recombinant proteins [12Bau].

Marine animals such as mussels produce fibrous attachments generally called byssus as shown in Fig. 41.1. Each thread in a byssus is about 2–3 cm long and about 100–200 μm in diameter [13Lin]. Byssal threads were reportedly woven into fabric in Greece to produce fine clothing [07Ald]. These byssal threads have extraordinary structural arrangement and properties not seen in other protein fibers. The thread consists of two regions, the distal portion (threads) which is rigid and stiff and the proximal region (threads) that is approximately 50-fold less stiff than the distal threads due to the unique composition and structure of the proteins in the threads [01Vac].

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 elliptical-shaped 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 μm 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 (α) and two minor (β, γ) that have similar molecular weights but different isoelectric values of 7.56, 5.67, and 5.31 for the α, β, and γ, 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 α–β 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 %.

Spider silks are recognized for their extraordinary properties due to their composition and structure. Enormous literature is available on the structure and properties of spider silks, on the mechanisms of silk production, and on reproducing the properties of the spider silks through biotechnology. Considerable variation in tensile properties is seen among the fibers produced from different spiders as seen in Table 43.1 and also between the fibers produced from different glands in the spiders. Figure 43.1 depicts the major components in spider and the four most common silk-producing glands. Figure 43.2 shows a schematic of the process of production of spider silk [12Eis]. Dragline silk produced by the major ampullate gland is the most common type of silk fiber studied. Recently, the structure and composition of this gland have been studied. As seen in Fig. 43.3, the gland consists of a tail, a sac, and an elongated duct. The sac can be divided into three distinct epithelial regions (A, B, and C), and it was found that sections A and B produce spidroins, but spidroins were lacking in the C region. Spidroins are proteins that have about 3,500 amino acid residues and consist of N-terminal (NT) and C-terminal (CT) domains which are considered to be responsible for fiber formation [99Hay, 13And]. A two-layered silk fibers consisting of a core and skin produced by zones A and B, respectively, were proposed. It was found that the nonterminal spidroin was homogenously distributed and was also discovered in the skin region.

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 μm in width and 3–40 μm 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 α-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 (

AmelSA1 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.

Poultry feathers are one of the most widely available, low-cost protein by-products. Unlike other protein sources, feathers have a unique hierarchical structure and low density that make them preferable for various applications. Figure 45.1 shows an image of the major parts of a feather. The central rachis or quill is a tough composite-like structure that extends throughout the length of the feather. Barbs are fibers that have lengths up to 4.5 cm in the case of chicken feathers. Barbules that have lengths of few mm are connected to the barbs similar to that of the barbs connecting to the quill as seen from the SEM image in Fig. 45.2 (left). A cross section of the feather quill and rachis reveals a unique honeycomb structure that is hollow as seen in Fig. 45.2 (right). This hollow structure is responsible for the lightweight and therefore low density (0.9 g/cm

3

) of feathers. In terms of physical structure, feather rachis and barbs were found to have typical diffraction pattern of α-keratin, but the orientation of the crystals in the rachis and barbs was found to be different [07Red]. Tensile properties of the chicken feather barbs are compared to those of turkey barbs and wool in Table 45.1. As seen from the table, the strength of the barbs is similar to that of wool but with lower elongation. It was reported that the chicken feather barbs could be hand twisted into yarns when blended with cotton fibers [07Red].

Although most animals contain hairs on their skin, limited studies have been conducted on understanding the structure and properties of animal hairs except for wool from different types of sheep. Zhang et al. had studied the structural characteristics of rabbit hair and found that the hair fibers had average diameters which varied between 10 and 20 μm and the fibers had scales similar to those seen on wool and the cross section of the fibers revealed a hollow center similar to that seen in feathers [11Zha]. Since there is limited literature available on animal hair fibers and an innumerable number of animals with hair exist, this topic has not been reviewed here.

Natural silk exhibits extraordinary properties and is useful for various applications. However, silk is produced in limited quantities and is also not easy to be dissolved, modified, or manipulated for specific applications. With a goal to find an alternative to natural silk, attempts have been made to dissolve proteins and regenerate the proteins into fibers using various approaches. Regenerated protein fibers generally called “azlons” were commercially produced from the proteins in corn, soybean, peanuts, and milk and also poultry feathers during the early 1930s. The poor quality of the protein fibers produced, the use of toxic chemicals during fiber production, and the introduction of inexpensive regenerated cellulose and synthetic fibers led to the decline and eventual elimination of the azlons. Although currently there is very limited or no commercial-scale production of regenerated protein fibers, recent advances in biotechnology, increase in the availability of low-cost biofuel coproducts that contain proteins, environmental awareness on using nondegradable fibers, and distinct properties of protein fibers have renewed interests in regenerated protein fibers. Reproducing proteins through biotechnology, developing novel methods to dissolve proteins and improving the properties of fibers, and biomimicking are some of the approaches that are being considered to develop regenerated protein fibers. This chapter provides an overview of such approaches, properties of the fibers developed, and potential applications of the fibers.

Bombyx mori

silk fibroin was regenerated into fibers, and the structural differences between the native and regenerated fibers were investigated [98Tra]. To produce fibers, degummed natural silk fibers were dissolved (17 %) in 9.3 M LiBr and dialyzed for 72 h. The aqueous fibroin solution obtained was cast into films. Later, the films were dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and spun into fibers using a methanol coagulation bath. During the coagulation of the fibers in methanol, the predominant α-helix form found in fibroin converts to the insoluble crystalline β-sheet. Fibers obtained after drawing had an average diameter of 88 μm and were composed of 56 % β-sheet, 13 % α-helix, 23 % β-turn, and 11 % undefined component similar to that seen in natural silk [98Tra]. Table 48.1 provides a comparison of the secondary structure in the natural and drawn and undrawn regenerated fibers.

Regenerated protein fibers were produced from the protein fibers (threads) found in hagfish slime [12Neg]. Proteins were solubilized in 98 % formic acid to obtain solutions (5, 7.5 %) that were spun into fibers and coagulated into an ethanol, methanol, or electrolyte buffer. However, fibers obtained were too weak and brittle. As an alternative approach, the protein solution was cast into films, and fibers were drawn from the films as shown in Fig. 49.1. Average fiber diameters obtained were between 46 and 137 μm, and the length of the fibers was about 3 mm. Table 49.1 shows the tensile properties of the fibers obtained under various conditions. Tensile properties of the regenerated fibers were considerably lower compared to the properties of the natural slime threads but similar to that of the regenerated fibers produced from spider silks as seen in Table 49.2. Structural analysis using X-ray diffraction and Raman spectroscopy showed that the fibers were composed of about 67 % α-helix and 26 % β-sheet content. Drawing of the fibers was found to increase orientation but not the crystallinity of the fibers.

Natural protein fibers such as spider silks have extraordinary properties, but it is difficult and impractical to obtain quantities of spider silk required for applications through the natural spinning process. To overcome this limitation, extensive efforts have been made to produce regenerated spider silk proteins using biotechnological approaches. Several heterologous host systems such as bacteria, yeast, mammalian cells, and transgenic plants, animals, and insects have been used to produce spider silk proteins as seen in Table 50.1 [12Chu]. Tokareva et al. provide a thorough review of the approaches used to produce recombinant spider silks and the limitations of the approaches [13Tok]. One of the most common and easiest approaches to obtain artificial spider silk is through bacterial production [07Ven]. Several researchers have expressed spider silk genes in

Escherichia coli

and have studied the structure, properties, and functions of protein fibers. Although bacterial production of proteins is possible on an industrial scale, several limitations have been expressed for this approach. The size of the expressible gene in

E. coli

is considerably smaller than the native gene found in spiders, and the bacteria use a distinct codon different than that in spiders. In addition, bacteria often remove repetitive sequences that are necessary to obtain the properties seen in spider silk fibers. To overcome these limitations, engineered genes that include the bacterial codon have been developed and expressed in

E. coli

. In one approach, artificial genes that encode the analogs of the proteins (spidroins 1 and 2) found in

Nephila clavipes

dragline silk were expressed in

E. coli

[97Fah1]. Proteins with purity of up to 99 % were obtained, and both the spidroins had mostly random structures.

Unlike

Bombyx mori

or spider silks that consist of large repetitive sequences and β-sheets, honeybees secrete four different types of small coiled-coil proteins in nearly equal proportions with a molecular weight of about 30 kDa [08Shi, 10Wei]. These proteins are non-repetitive and are rich in alanine residues, and the proteins were found to be stable in water [08Shi]. Due to their lower molecular weights and unique structure, it was supposed that honeybee silks could easily produce recombinant proteins. To examine this, four proteins (ABS 1–4 with 315, 289, 317, and 321 residues) from the Asiatic honeybee were expressed in

Escherichia coli

, and the structures of the proteins were studied. The yield of proteins in

E. coli

was 30, 30, 10, and 60 mg/mL for ABS-1–4, respectively. Corresponding molecular weights obtained for the proteins were 55, 32, 38, and 50 kDa, respectively. Proteins generated were found to have about 65 % coiled-coil sequences but with low (9–27 %) α-helix content and high% (45–56 %) of random coils. In addition, about 26–35 % of β-sheets were also discovered [08Shi]. Some of the properties of the recombinant honeybee silk proteins expressed in

E. coli

are listed in Table 51.1. Overall, it is seen that the recombinant production of honeybee silks was unable to generate the secondary and tertiary structure seen in native honeybee silk [10Wei]. In the native honeybee silks, the four isolated proteins are found as a complex but have weak interactions between them. In solution, α-sheets, β-sheets, and random coils coexist depending on the pH of the solution. The presence of high amounts of alanine that provides limited hydrophobic interactions was suggested to be the reason for the inability of the silks to maintain higher levels of α- or β-helices [08Shi]. In a similar study, recombinant proteins with yields between 0.5 and 2.5 g/L were obtained using honeybees (

Apis mellifera

), and the proteins were formed into fibers [10Wei]. Four of the distinct honeybee proteins were expressed in

E. coli

(Rosetta 2 DE3), and proteins were collected. Proteins containing all four components were concentrated to get the required viscosity, and the solution was manually drawn into fibers between the prongs of tweezers. Fibers were coagulated in 90 % methanol and 10 % water bath and drawn to about 2× the length and air-dried. Tensile properties of the fibers obtained are listed in Table 51.2.

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.

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 μm. Figure

53.1

shows the image of the fibers obtained. It was found that the fibers consisted of ordered β-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 β-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

.

Regenerated protein fibers were produced from cereal grains such as soy and peanuts in the 1950s. Regenerated fibers from peanut under the trade name

Ardil

and proteins from corn zein marketed as

Vicara

and even from soybean were produced on a commercial scale and used for industrial applications [09Poo]. Some of the properties of the fibers regenerated from plant and other protein sources are shown in Table 54.1. As seen from the table, protein fibers regenerated from cereal proteins have considerably lower strength than the weakest protein fiber, wool. More importantly, the regenerated protein fibers have substantially lower wet strength which makes them unusable for practical applications. Various approaches have been used to improve the properties of the regenerated fibers.

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 β-sheet conformation with 96 amino acids having 7 cysteine residues as terminals [09Poo]. However, the central portion of keratin is also reported to have α-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 %.

Amyloid proteins (lysozymes) found in egg white were regenerated into macro- and nanofibers using a wet-spinning approach [03Tsu, 11Mei]. Lysozyme was dissolved in 10 mM HCl and allowed to form nanofibers with diameters of 2.6 ± 0.7 nm and lengths in excess of 10 μm. To form macrofibers, the nanofibers were cross-linked with anionic polyelectrolyte gellan gum through interfibrillar interactions. Figure 56.1 shows images of the nanofibers and macrofibers obtained, and Table 56.1 provides a comparison of the properties of the fibers obtained after cross-linking to various extents. Cross-linking and increasing the concentration of the protein solution improved tensile properties as seen in Table 56.1. The tensile strength of the lysozyme fibers is considerably higher than that of the regenerated fibers produced from plant proteins but lower than that of natural

Bombyx mori

silk. When used for controlled release applications, a pH-triggered release of riboflavin molecules was obtained with 75 % of loaded drug released within 10 min at pH 7 compared to less than 5 % of the drug released at pH 2 suggesting that the fibers could be used for controlled drug release.

Electrospinning is a process where polymeric solutions are extruded through a charged electrical field consisting of + vely and – vely charged source/collector. Fibers in the nano- to micrometer scale are produced by controlling the distance between needle and collector and voltage and other parameters during electrospinning. Electrospun fibers are preferred for a variety of applications due to their high surface area, ability to develop fibrous matrices with desired porosity and pore size, and comparatively easy biodegradability. Due to these advantages, electrospun fibers have been considered suitable as tissue engineering scaffolds and other medical applications, reinforcement for composites, filters for biotechnological applications, protective clothing and smart textiles, and in energy and electronic applications such as batteries/cell and capacitors, sensors, and catalysts [14Bra]. Due to their wide acceptability and unique properties, attempts have been made to develop electrospun fibers from almost every possible raw material. Reports are available on producing electrospun fibers from polysaccharides such as cellulose and chitosan, proteins such as silk fibroin and gelatin, synthetic polymers such as polypropylene and poly(lactic acid), and even from metals such as TiO

2

[08For]. This part provides an overview of the biopolymers including polysaccharides, proteins, and synthetic polymers that have been used to develop electrospun fibers. Since there is a massive amount of literature in developing electrospun fibers, especially from synthetic polymers, our focus in this part is to only cover electrospun fibers produced from polysaccharides and proteins and synthetic biopolymers such as poly(lactic acid) and poly(ethylene glycol) that are derived from renewable resources.

Although chitin has limited solubility in common solvents, chitin and chitin derivatives have been electrospun into fibers for various applications. To produce electrospun fibers, chitin was first depolymerized using irradiation and then dissolved using 1,1,1,3,3,3-hexafluoro 2-propanol and fibers with average diameters of 110 nm were obtained [04Min]. After electrospinning, the chitin mats were deacetylated using 40 % aqueous NaOH solution at 60–100 °C to achieve about 85 % deacetylation and form chitosan fibers. SEM images and some of the properties of the chitin and chitosan fibers obtained after deacetylation of the spun chitin membranes are shown in Fig. 58.1. Minimal changes were observed in the diameters of the fibers before and after deacetylation.

Due to the distinct advantage of silk for various applications, considerable attempts have been made to reproduce silk in the laboratory with specific properties for targeted applications. For instance, to exploit the advantages of protein-based biomaterials and nanostructures for medical applications, silk fibroin was electrospun into fibers [10Zha]. To form the fibers, silk (

Bombyx mori

) was first degummed to remove sericin. Later, the silk fibers were dissolved in 9.3 M lithium bromide solution at 60 °C, and the dissolved solution was dialyzed against a 2,000 molecular weight membrane to obtain a 3–7.2 % protein solution [10Zha]. In addition, lyophilized silk fibroin was also dissolved using HFIP at room temperature. Silk solutions were blended with polyethylene oxide (PEO) to improve spinnability and enable fiber formation. Fibers with relatively larger diameters, between 700 and 880 nm, were obtained. Electrospun mats obtained were treated with methanol to induce crystallization in silk and transform the silk into β-sheet configuration. Methanol treatment removed PEO and increased the surface roughness of the fibers [02Jin]. In another study, silk fibroin has been electrospun and the potential of using the silk nanofibers for various applications has been studied [08Kaw, 05Kim]. Silk nanofibers with diameters from 8 to 2,500 nm have been produced and used for tissue engineering [09Zha].

Synthetic biopolymers such as PLA, PEG, and PHBV that are considered to be suitable for medical applications have been made into electrospun structures. Unique crimped and bicomponent nanofibers were produced from high shrinkage polyethylene terephthalate (PET) and polytrimethylene terephthalate (PTT). The polymers were separately dissolved and electrospun into the same collector from different sources and with opposite charges. Such an arrangement led to the attraction between the oppositely charged polymers and formation of twisted fibers. Such twisted fibers were also produced from polyurethane and polyacrylonitrile and termed as artificial wool [12Li]. Figure 60.1a–d shows SEM images of the twisted fibers obtained with an average diameter of about 800 nm [12Li].

The production of cellulose by

Acetobacter xylinum

was reported by A.J. Brown as early as 1886. From that time, bacterial cellulose (BC) has been used for biomedical, environment, agriculture, electronic, food, and industrial applications [98Las, 14Moh]. Unlike most other sources of cellulose, BC does not contain lignin or hemicelluloses, making it ideally suited for various applications. In terms of structure, BC is composed of fibrils that have a width of about 1.5 nm and these fibrils are crystallized into microfibrils. BC has a relatively high level of crystallinity (60 %) and the degree of polymerization that can be as high as 16,000–20,000. Young’s modulus of a bacterial cellulose fibril has been reported to be in the range of 15–35 GPa and tensile strength between 200 and 300 MPa. However, other researchers have reported the modulus of a single bacterial cellulose fibril to be as high as 114 GPa, compared to a theoretical cellulose crystal modulus of 160 GPa. In addition to these features, BC has a water holding capacity of up to 100 times it weight and a linear thermal coefficient of expansion of only 0.1 × 10

−6

k

−1

. Typical uses of bacterial cellulose have been as wound dressing. Bioprocess, Xcell, and Biofill are some of the products made from bacterial cellulose that are currently available on the market for wound healing [06Cza, 90Fon]. Other commercial scale applications of bacterial cellulose are in cosmetics, food, and electronics to some extent. The remarkably high wet tensile strength, biocompatibility, high porosity, and ability to be easily formed into various structures are considered to be some of the advantages of using bacterial cellulose for medical applications.

Production of natural fibers such as cotton requires substantial use of land, water, and other natural resources. Processing of the fibers into textiles also needs additional water, energy, chemicals, and other resources. Textile processes such as dyeing are energy intensive and also release considerable amounts of wastewater containing dyes that cause environmental pollution. Although improvements in machinery and processes and increase in environmental awareness and regulations have made some textile processing environmentally friendly and sustainable, the majority of the textile processings, especially in the developing countries, are a cause for environmental concern. Considerable efforts have been made to reduce the waste generated during textile processing and/or to use sustainable and green materials. One such attempt has been to develop colored cottons that could eliminate the need for using dyestuffs, water, and energy required for dyeing.

Biothermoplastics are considered to be those developed using polymers that are derived from renewable resources. Figure 63.1 lists some of the biopolymers obtained from bioresources, their structure and routes used to synthesize the biopolymers. In some cases such as poly(lactic acid), the entire polymer is derived from renewable resource whereas in the case of poly(trimethylene terephthalate), only one of the monomer is from an renewable resource [12Che]. As seen in Fig. 63.1, traditional synthetic polymers such as polypropylene (PP) have also been derived using biopolymers but have not been commercialized due to high cost and limitations in processing and properties. Properties of a few selected biopolymers are listed in Tables 63.1 and 63.2.

PTT is one of the more recently manufactured synthetic fiber that is derived from renewable resource. Companies such as Shell chemical company and Dupont are manufacturing PTT on a commercial scale and are selling the fibers under the trade names of Corterra and Sorona, respectively [03Duh]. PTT is said to have excellent resiliency and softness and also chemical stability and stain resistance which makes them particularly suitable for carpet applications. PTT is produced in a two-step process, similar to the common polyester (polyethylene terephthalate). In the first step, terephthalic acid (TPA) is esterified using 1,3-propanediol or transesterified using dimethyl terephthalate. The second step involves polycondensation of the esterified or transesterified product to remove the polycondensation byproducts until the desired molecular weight is reached. It is the use of 1,3-propanediol that is derived from an renewable resource that makes PTT fibers eco-friendly. Two distinguishing features of producing PTT compared to PET are the use of a titanium catalyst instead of the antimony catalyst and a considerably lower polycondensation temperature. Due to the use of low polycondensation temperatures, the cost of producing PTT is considerably higher than that of PET. In addition, PTT has a melting temperature 20–30 °C lower than that of PET and a low initial modulus that provides high flexibility to the fibers [01Lyo]. The high extensibility of PTT fibers is attributed to the arrangement and orientation of the polymers in the chain. As seen in Fig. 64.1, PTT fibers have –O-(CH

2

)

3

-O bond conformation with a concentration of the repeating units and opposite inclination of successive phenylene groups along the chain which force the molecular chain to assume a extended zigzag configuration. This helical structure of PTT with an angle of 60 ° provides an opportunity to extend the PTT chain by drawing during fiber production (zone-drawing) and improve the tensile properties of the fibers [01Lyo].

Polyhydroxyalkonates are a diverse family of biopolyester produced by bacteria as energy and carbon storage materials. Poly(3-hydroxybutyrate) (PHB) is the most common type of PHA used commercially. PHB is an thermoplastic material with a melting temperature of about 180 °C and glass temperature that is below room temperature. Structure and properties of PHB are highly dependent on the conditions prevailing during fiber production. For instance, slow cooling from the melt produced large spherulites and rapid cooling results in amorphous state [01Yam]. It was suggested that PHB assumed orthorhombic or α-form or the β-zigzag form depending on the annealing conditions. PHB crystallized into orthorhombic form when annealed under high tension and into β-zigzag form when annealed under high tension [01Yam]. Based on X-ray diffraction patterns, it was found that the amorphous molecules transformed into orthorhombic crystal when annealed without tension and when annealed under tension, the amorphous regions were stretched and crystallized into the β-form [01Yam].

Poly(lactic acid) commonly known as PLA is produced by condensation polymerization from lactic acid which is derived by fermentation of sugars from carbohydrate sources including corn and sugar cane. Commercial production of PLA is through the conversion of the lactide to PLA through ring opening polymerization catalyzed by a Sn(II)-based catalyst [10Gro]. The processing, properties, and potential applications of PLA are mainly dependent on the ratio of the

l

- and

d

-isomers of lactic acid. Among the different forms of PLA that can be derived, stereo-complex type polylactides that consist of both enantiomeric poly(lactic acid) and poly(

d

-lactic acid) are high performance polymers with melting temperature of 230 °C, higher (50 °C) than that of PLLA or PDLA. Some of the properties of the stereo-complex PLA and PLLA are provided in Table 66.1 in comparison to poly(glycolic acid) (PGA) and poly(3-hydroxybutyrate) (PHB) [10Hir]. PLA and its isomers have been blended with various other synthetic and natural biopolymers to produce blends. PLA can be solution spun or melt spun into fibers, but generally, the latter is more economical and environmentally friendly and also produces fibers with better properties [10Aga]. However, melt spinning of PLA can cause significant hydrolytic degradation and, therefore, solution spinning of PLA is used to obtain fibers with high performance properties. Some of the fiber production conditions and the properties of the fibers obtained are listed in Table 66.2.

The term “biocomposites” has been widely used to denote composites that are made using either the matrix or reinforcement or both from renewable resources that are biodegradable. Conventionally, biocomposites were developed using natural cellulose fibers such as jute and flax as reinforcement to replace glass fibers with polypropylene, polyethylene, epoxy, and other synthetic polymer-based matrices. The advent of biopolyesters such as poly(lactic acid) and poly(hydroxy alkoanates) led to a quantum jump in the research on developing biocomposites using both the matrix and reinforcement from renewable resources. In addition, efforts were made to utilize agricultural by-products such as corn stover, wheat straw, and coir fibers as reinforcement resulting in inexpensive and renewable composites. However, biopolyesters such as poly(lactic acid) are considerably more expensive and also do not have the performance properties comparable to that of the traditional synthetic polymers such as polypropylene and polyethylene. Therefore, resins/matrices have also been developed from agricultural byproducts. For instance, soy proteins and wheat gluten have been used as matrix in their native form and also after various chemical modifications.

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

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

The biopolyester poly(3-hydroxybutyrate-

co

-3-hydroxyhexanoate) was used as matrix, and flax fibers were used as reinforcement to develop composites. Since P(3HB-

co

-3HHx) copolymer is hydrophobic and flax fibers are hydrophilic, the fibers were either acetylated or grafted with polyester to improve adhesion between the fibers and the matrix. In addition, longer length flax fibers were used in matrix form, and short flax fibers were mechanically mixed into the matrix and compression molded into composites. Figure 70.1 shows the strength and modulus of the composites obtained using various configurations of the fibers. As seen from the figure, the addition of the fibers, especially the long fibers, substantially increased the strength and modulus of the composites. However, the strength of the short fiber-reinforced composites (black bars) does not show a major change with the modification of the fibers, but the modulus of the composites obtained using acetylated fibers (A) was considerably higher than the composites without modification (U) and those grafted with polyester (P) [07Zin]. In addition to flax fibers, several other fibers such as abaca and jute have been used as reinforcement with PHBV as the matrix. Some of the properties of the composites developed using PHBV as matrix and natural fibers as reinforcement are provided in Table 70.1. PHBV matrix was also reinforced with 30 or 40 % bamboo fiber having a length of 5 cm and diameter between 10 and 100 μm [08Sin]. No significant increase in properties was observed when the fiber content was increased from 30 to 40 %, but the tensile modulus had increased by 175 % after incorporating the fibers compared to the neat polymer. The improvement in tensile modulus was close to the theoretical possible increase that was calculated using Christensen’s equations. A number of voids and clusters of fibers were observed in the fractured surfaces that were responsible for the relatively low impact and tensile strength [08Sin]. SEM images in Fig. 70.2 show that at 40 % fiber loading, there are excessive fiber and considerable low levels of matrix that lead to poor binding and therefore relatively poor properties. Table 70.2 lists some of the properties of the bamboo–PHBV composites at the two different loading levels studied.

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].

Calcium alginate has also been studied as potential reinforcement for polypropylene composites [10Kha]. Calcium alginate fibers with a diameter of 30 ± 10 μm were silane treated and added (10 %) into PP that was pre-pressed into sheets. Layers of alginate fibers were placed between PP sheets, and the sandwich structure was compression molded into composites at 180 °C. Some of the properties of the PP composites containing alginate fibers with and without the silane treatment are given in Table 72.1. As seen from the table, the addition of the alginate fibers had increased the strength, more than doubled the modulus and decreased the elongation by several magnitudes. Bending properties also showed similar increase with the addition of the alginates. Weight loss ranging from 0.5 to 2.2 % was observed when the samples were buried in soil from 2 to 16 weeks. In a similar approach, calcium alginate fibers (120 MPa strength, 4.3 GPa modulus, and 75 % elongation) were mixed with poly(vinyl alcohol) and compression molded into composites [11Dey]. Some of the properties of the alginate reinforced PVA composites are given in Table 72.2. As seen from the table, incorporation of the alginate fibers leads to increase in strength from 10 to 16 MPa and also increase in modulus, but the elongation decreases drastically. Bending properties of the composites doubled. Degradation tests showed that the composites had lost about 50 % of their strength after being buried in soil for 2 months [11Dey].

Recyclable green catalyst supports were prepared using catalytically active hybrid cellulose fibers in nanochitin hydrogels [12Das]. Hydrogels containing chitin nanofibrils of 9 nm diameter and several micrometers in length were wet spun into macrofibers by extrusion. Figure 73.1 shows SEM images of the surface of the fibers. The extruded microfibers had a large plastic region of 12 % and work to fracture of 10 MJ/m

3

. In addition, Nobel metal nanoparticles were added onto the surface of the chitin macrofibers via the amine functional groups. Developed organic–inorganic supports were considered to be suitable for fast catalytic reductions of model compounds.

Chitosan fibers prepared through the wet-spinning approach were cross-linked with glutaraldehyde and later modified using polyalanine and multiwalled carbon nanotubes for potential use as electrode material for electrical double-layer capacitors [14Dor]. SEM images in Fig. 74.1 show the chitosan fibers modified using polyalanine and with MWCNT. The addition of polyalanine and CNTs onto chitosan fibers resulted in a porous structure shown in Fig. 74.1b. The conductivity of the chitosan/polyalanine/MWCNT fibers was 5.34 × 10

−2

S cm

−1

compared to 7.2 × 10

−2

S cm

−1

for the chitosan/polyalanine fibers. The nanocomposite fibers had a specific capacitance of 14.5 F cm

−2

at a current density of 10 mA cm

−2

suggesting that the fibers would be suitable as electrode materials.

Keratin biofibers separated from chicken feathers were used to prepare polyurethane–keratin membranes for the removal of hexavalent chromium [11Sau]. Table 75.1 shows the chromium removal efficiency of the polyurethane–keratin membranes. As seen from the table, up to 38 % removal could be achieved depending on the type of modification done for the keratin fibers.