Biocomposites from Renewable Resources

Keywords

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

References

[05Liu] Liu, W., Misra, M., Askeland, P., Drzal, L. T., Mohanty, A. K.: Polymer 46(8), 2710 (2005)

[05Lod] Lodha, P., Netravali, A. N.: Ind. Crops Prod. 21, 49 (2005)

[06Tra] Tran, P., Graiver, D., Narayan, R.: J. Appl. Polym. Sci. 102, 69 (2006)

[08Kun] Kunanopparat, T., Menut, P., Morel, M. H., Guilbert, S.: Compos. Part A 39, 777 (2008)

[08Son] Song, Y., Zheng, Q., Liu, C.: Ind. Crops Prod. 28, 56 (2008)

[10Pfi] Pfister, D. P., Larock, R. C.: Bioresour. Technol. 101, 6200 (2010)

[11Mue] Muensri, P., Kunanopparat, T., Menut, P., Siriwattanayotin, S.: Compos. Part A 42, 173 (2011)

[11Red1] Reddy, N., Yang, Y.: Polym. Int. 60(4), 711 (2011)

[11Red2] Reddy, N., Yang, Y.: Ind. Crops. Prod. 33, 35 (2011)

[11Red3] Reddy, N., Yang, Y.: Biomass Bioenergy 35, 3496 (2011)

[12Hem] Hemsri, S., Grieco, K., Asandei, A. D., Parnas, R. S.: Compos. Part A 43, 1160 (2012)

[12Ret] Retegi, A., Algar, I., Martin, L., Altuna, F., Stefani, P., Zuluaga, R., Ganan, P.,

Mondragon, I.: Cellulose 19, 103 (2012)

[13Mon] Montano-Leyva, B., Silva, G. G.D., Gastaldi, E., Torres-Chavez, P., Gontard, N., Angellier-Coussy, H.: Ind. Crops Prod. 43, 545 (2013)

[14Dia] Diao, C., Dowding, T., Hemsri, S., Parnas, R. S.: Compos. Part A 58, 90 (2014)

Natural Cellulose Fibers from Renewable Resources

Keywords

Coconut husk • Retting • Decorticating • Leaf • Fiber properties • Microfibrillar angle • High elongation

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.

Similar to the fibers obtained from palm trees and fruits, coir fibers are relatively coarser and have low tensile strength but high elongation as seen in Table 9.1. Also, properties of the coir fibers vary considerably with the changes in gauge length and strain rate as seen in Tables 9.2 and 9.3. Increasing gauge length decreased strength and elongation due to the increase in the higher number of weak spots that make the

Fig. 9.1 Digital image of a sliced coconut showing the outer husk (brown) that is used for fiber extraction and the inner (white) edible part

fibers more susceptible to breakage. At lower strain rates, the applied load is mostly shared by the amorphous regions, and therefore, the fibers have lower strength at lower strain rates [07Tom]. It was also reported that coir fibers had a crystallinity index of 57 and a microfibrillar angle of 51°, higher than other reports on coir fibers [07Tom].

In Sri Lanka, one of the major producers of coconuts, three major varieties of coconut trees are grown, and fibers obtained from them are graded by length as long (>150 mm), medium (100-150 mm), short (<100 mm), and very short (<50 mm). Although the chemical composition of the different grades and varieties of fibers was similar, considerable variations in tensile properties were observed. Tensile strength was found to vary from about 0.9 to 1.3 g/den and elongation varied between 20 to 28 % [05Nan].

In addition to the husk, other parts of the coconut tree such as the petiole shown in Fig. 9.2 have also been used for fiber production. However, considerable variations in properties have been observed for the fibers from the various parts as seen in Table 9.4 [82Sat].

Up to six times variation in diameter, 50 % difference in density, and 30° variation in microfibrillar angle are seen between fibers obtained from different parts of the coconut plant. A distinguishing feature of all the fibers obtained from the coconut trees is the considerably high microfibrillar angle that is responsible for the high elongation, compared to the fibers obtained from plant basts or stems.

Fibers obtained from the petiole exhibited the highest strength and modulus but considerably low elongation which should be related to the function of the different parts. Since the petiole forms the base of the leaves and is attached to the stem of the coconut tree, it is necessary for the petiole to be strong to withstand the forces of nature. Therefore, fibers obtained from the petiole are considerably stronger.

References

[82Sat] Satyanarayana, K. G., Pillai, C. K.S., Sukumaran, K., Pillai, S. G.K., Rohatgi, P. K., Vijayan, K.: J. Mater. Sci. 17, 2453 (1982)

[05Nan] Nanayakkara, N. H.A. S.Y., Ismail, M. G.M., Wijesundara, R. L.C.: J. Nat. Fibers 2, 69 (2005)

[07Tom] Tomczak, F., Sydenstricker, T. H.D., Satyanarayana, K. G.: Comp. Part A 38, 1710 (2007)

[09Kal] Kalia, S., Kaith, B. S., Kaur, I.: Polym. Eng. Sci 49, 1253 (2009)

[13Bua] Buana, S. A.S., Pasbaskhsh, P., Goh, K. L., Bateni, F., Haris, M. R.H. M.: Fiber Polym. 14(4), 623 (2013)

[13Suj] Sujaritjun, W., Uawongsuwan, P., Pivsa-art, W., Hamada, H.: Energy Procedia 34, 664 (2013)

[13Van] Van Dam, J. E.G., Common fund for commodities. Technical paper no. 6. ftp://ftp. fao. org/docrep/fao/004/y3612e/y3612e00.pdf. Accessed October 2013.

Chitin, Chitosan, and Alginate Fibers

Keywords

Antifungal • Antimicrobial • Inhibition • Biocompatibility

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].

In addition to their medical application, alginates have also been reported to have good inherent flame retardancy. The influence of zinc ions on the thermal degradation and the flame resistance of alginate fibers were investigated [13Tia]. Alginic acid fibers prepared by washing calcium alginate fibers with hydrochloric acid were used as control. Later, the alginic acid fibers were treated with solutions containing zinc sulfate at various concentrations (4 %, 8 %, 10 %, and 25 %) at 60 °C for 2 h. Parameters that can evaluate flame-retardant behavior of the fibers with and without zinc are shown in Table 32.1. The limiting oxygen index (LOI), a simple and direct measure of flammability, is higher for fibers containing zinc compared to the pure alginic acid fibers. Materials with LOI above 27 are considered to be flame retardant which indicates that adding even 4 % of zinc was sufficient to make the fibers flame retardant [13Tia]. Other flame-retardant properties also showed that the addition of zinc makes the fibers flame retardant. In terms of thermal degradation, the addition of zinc increased the maximum

Table 32.1 Antiflammable parameters of alginic acid fibers containing various levels of zinc [13Tia]

degradation temperature to 250 °C compared to 210 °C for the fibers without any zinc.

In another research, calcium alginate fibers were claimed to be inherently flame retardant, and the thermal degradation mechanism and pyrolysis products were studied [11Zha]. Calcium alginate fibers had an LOI of 48 compared to 20 for viscose making the fibers inherently flame retardant. Upon pyrolysis, calcium alginate fibers formed thick residues that inhibited heat transfer. Also, crusts formed after burning by calcium alginate were thicker and more consistent than the crust from viscose fibers demonstrating a condensed phase activity that could inhibit smoke release. Alginate fibers produced much less smoke and formed about 32 % residue. Some of the parameters to evaluate the flame-retardant properties of calcium alginate fibers are given in Table 32.2 in comparison to viscose fibers.

References

[05Mik] Mikolajczyk, T., Wolowska-Czapnik, D.: Fibers Text East. Eur. 13(3), 35-38 (2005) [11Zha] Zhang, J., Ji, Q., Shen, X., Xia, Y., Tan, L., Kong, Q.: Polym. Degrad. Stabil. 96, 936 (2011)

[12Gon] Gong, Y., Han, G., Zhang, Y., Pan, Y., Li, X., Xia, Y., Wu, Y.: Biol. Trace Elem. Res. 148, 415 (2012)

[13Tia] Tian, G., Ji, Q., Xu, D., Tan, L., Quan, F., Xia, Y.: Fibers Polym. 14(5), 767 (2013)

Hyaluronic acid (HA) is a polysaccharide located in the extracellular matrix of soft tissues. Extensive studies have been done to develop hyaluronic acid-based biomaterials for tissue engineering and other applications. Electrospun hyaluronic acid fibers were produced by dissolving hyaluronic acid in aqueous ammonia and dimethyl formamide solutions [12Bre]. Fibers with diameters of 39 ± 12 nm were
obtained. In another study, pure hyaluronic acid was dissolved using a combination of deionized water, formic acid, and dimethylformamide (25/50/25) and nanofibers with average diameters of 100 nm were obtained [11 Liu]. Addition of formic acid increased chain entanglements and viscosity and allowed the formation of nanofibers. However, the membranes obtained were unstable and dissolved in aqueous media. To overcome this deficiency, electrospun hyaluronic acid membranes were cross-linked with ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) [09Xu2]. Although the cross-linked materials showed signifi­cant improvement in stability when treated in water, they were not stable in PBS. To further improve the stability of the membranes in PBS, gelatin was blended with hyaluronic acid with the aim to increase the amine groups and therefore obtain better cross-linking. Membranes containing HA and gelatin were cross-linked with EDC/NHS and found to be stable in PBS at 37 °C for up to 28 days. Unlike previous reports that suggested that HA was cytotoxic, membranes produced from HA/gelatin were cytocompatible and the degradation of the membranes could be controlled from 1 to 30 days. Three-dimensional HA nanofibrous scaffolds were developed using thiolated HA derivative 3,3′-dithiobis(propanoicdihydrazide) (DTPH) with the addition of PEO to assist fiber formation [06Ji]. Scaffolds were further cross-linked with the addition of poly(ethylene glycol)-diacrylate (PEGDA) conjugate. Later, PEO was extracted from the scaffolds using water to obtain HA-DTPH nanofibrous scaffolds. Fibroblasts were found to attach and spread on the scaffolds suggesting that the scaffolds could be useful for cell encapsulation and tissue regeneration [06Ji]. Figure 58.13 depicts the growth of cells on the scaffolds.

Macro- and nanofibrous hyaluronic acid/collagen blend fibers were made into scaffolds using electrospinning and leaching technique [08Kim]. Sodium hydroxide and N, N-dimethyl formamide were used as a solvent mixture to electrospun the fibers in the form of 3D nanofibrous scaffolds shown in Fig. 58.14. Fibers with average diameters between 226 and 357 nm were developed and the average tensile strength of the scaffolds varied between 267 and 432 kPa. Salt leaching resulted in the formation of macroporous and nanoporous (Fig. 58.15) HA scaffolds that were suggested to be suitable for tissue engineering.

18.1.1 Fibrillation of Lyocell Fibers

NMMO fibers have higher orientation in the amorphous regions and consist of long and thin crystallites that prevent fringing of the fibers and lead to fibrillation when wet and under abrasive conditions such as those found during dyeing. Figure 18.3 shows the SEM images of fabrics fibrillated after dyeing with various dyes. Figure 18.4 shows optical images of an extensively defibrillated lyocell fiber. SEM images in Fig. 18.5 show a comparison of the cross sections of fibers produced using the lyocell and viscose processes. A considerably higher level of fibrillation can be seen in the lyocell fibers compared to the fibers produced using the viscose process. The lateral links between crystallites in fibers produced by the NMMO process are also weaker that contributes to the fibrillation. Cross section of the fracture surface of an NMMO fiber reveals considerable fibrillation compared to the viscose-type fibers. The degree of fibrillation is expressed quantitatively in terms of “fibrillation index” which is the sum of the fibril lengths (£i) divided by the fiber length (l) [02Udo]. It has been reported that the fibrillation index is directly proportional to the % crystallinity. A highly crystalline fiber has high degree of

orientation and delaminates to a greater extent and therefore has higher extent of delamination.

Several approaches have been proposed to control the extent of fibrillation. Researchers have changed the type of pulp, drying and pretreatment conditions, and solvents used for precipitation to reduce fibrillation mainly by decreasing the % crystallinity and orientation. Fibrillated fibers have been defibrillated using enzymes and other techniques to improve the properties of the fibers [07Gos]. In addition, cross-linking with triazine derivatives, polycarboxylic acid derivative, and aminofunctional polysiloxane polymers and binding and curing with conven­tional resins have also been done to reduce or eliminate fibrillation [10Han]. Partially oxidized lyocell fibers cross-linked with multifunctional amines led to increased amorphous regions, decreased swelling and water sorption, and therefore low degree of defibrillation [10Han].

Natural Protein Fibers

Keywords

Spider silk • Extraordinary tensile strength • Tensile strength • Silk gland • Fiber production • Major ampullate gland • Spidroin

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.

Silks produced by the dragline glands are typically composed of two major proteins called spidroins. However, the composition and structure of the spidroins in the fibers vary from species to species. For example, in Nephila clavipes, the proteins are classified as major ampullate spidroins MaSp1 and major ampullate spidroin MaSp2 [10Hei], whereas fibers from Araneus diadematus contain fibroins 3 and 4 that are referred to as ADF-3 and ADF-4. MaSp1 and MaSp2 show similar

outer egqsac

Flagelliforme Aggregate tylmdncale

Minor Ampullate

acini form,
trapping *i
and

packing silk

Fig. 43.1 Major components of spider and the four most common silk-producing glands (Major ampullate, minor ampullate, piriforme, aciniforme). From Vollrath [00Vol], reproduced with permission from Elsevier primary structure with highly repetitive core domain containing iterated repeats of alanine — and glycine-rich domains and non-repetitive terminal regions. In addition, MaSp1 is said to be homogeneously distributed throughout the core, whereas the MaSP2 is present as clusters in the core of the fibers [10Hei]. A schematic of the proposed structure of spider silks is shown in Fig. 43.4. The MaSp2 component is composed of about 15 % of proline residues and shown to form the matrix and is mainly responsible for the elongation of the fibers, whereas the MaSp1 is proline free and forms the crystalline regions. The crystalline regions are composed of crystalline p-sheets and provide strength to the fibers. In addition, glycine-rich motifs such as GGX or GPGXX that have flexible helical structures connect crystalline regions and also provide elasticity to the fibers.

Several theories have been proposed on the arrangement, structure, properties, and role of the different components in spider silks. Although recent evidence

provides a clearer understanding of the structure, there are contradictory results presented by different studies. One such study suggests that there is a bimodal distribution of crystalline regions in the fibers with two distinct sizes of crystallites.

Crystallites that are about 2-3 nm long and contain highly ordered and tightly packed p-sheets of polyalanine are found to be inter-dispersed with less ordered crystallites measuring 70-500 nm and consist of different silk motifs. However, using atomic force microscopy (AFM), it was found that fibers produced from the black widow spider Latrodectus hesperus had both unordered and highly ordered region composed of two fibers with diameters of 300 nm that were oriented parallel to the fiber axis and fibers that measured 10-100 nm that were oriented across the fiber axis [99Gou].

Yet another study has reported that spider silks contain highly crystalline regions composed of pleated p-sheets of polyalanine that provide strength and amorphous regions that are rich in glycine and are responsible for the elasticity of the fibers [04Hue1]. Spiders store freshly secreted silk as liquid crystalline spinning dope in concentrations up to 50 %. The silk solution in the spider’s gland is water soluble but becomes water insoluble after extrusion into fibers, a phenomenon also observed in B. mori silks.

At a molecular level, the assembly of the proteins differs considerably even though the amino acid sequences are similar [04Hue2]. Two major proteins ADF3 and ADF4 from the garden spider A. diadematus were used to study the assembly of proteins and their influence of fiber properties. ADF3 and ADF4 have similar amino acid sequences but have remarkably different properties. For example, ADF3 is soluble at high concentrations, but ADF4 is insoluble and self-assembles into filaments under specific conditions. To investigate the structure further, different repetitive and non-repetitive units in ADF3 and ADF4 were constructed by cloning. It was found that acidification and increase in phosphate concentration promoted self assembly but decreased solubility, and this effect was more pronounced in the non-repetitive regions [04Hue2]. Such an effect was attributed to the hydrophobicity of the two regions.

References

[99Gou] Gould, S. A.C., Tran, K. T., Spagna, J. C., Moore, A. M.F., Shulman, J. B.: Int. J. Biol. Macromol. 24, 151 (1999)

[99Hay] Hayashi, C. Y., Shipley, N. H., Lewis, R. V.: Int. J. Biol. Macromol. 24, 271 (1999) [00Vol] Vollrath, F.: J. Biotechnol. 74, 67 (2000)

[04Hue1] Huemmerich, D., Helsen, C. W., Quedzuweit, S., Oschmann, J., Rudolph, R., Scheibel, T.: Biochemistry 43, 13604 (2004)

[04Hue2] Huemmerich, D., Scheibel, T., Vollrath, F., Cohen, S., Gat, U., Ittah, S.: Curr. Biol. 14, 2070 (2004)

[09Fu1] Fu, C., Porter, D., Shao, Z.: Macromolecules 42, 7877 (2009)

[09Fu2] Fu, C., Shao, Z., Fritz, V.: Chem. Commun. 42, 6515 (2009)

[10Hei] Heim, M., Romer, L., Scheibel, T.: Chem. Soc. Rev. 39, 156 (2010)

[12Eis] Eisoldt, L., Thamm, C., Scheibel, T.: Biopolymers 97(6), 355 (2012)

[13And] Andersson, M., Holm, L., Ridderstrale, Y., Johansson, J., Rising, A.: Biomacro­molecules 14, 2945 (2013)

Bacterial cellulose nanofiber membranes have been used as support for polyaniline (PANI) nanocomposites used as supercapacitor electrodes. Initial efforts on pro­ducing BC/PANI nanocomposites through in situ polymerization had limited suc­cess and a relatively low conductivity of 1.6 x 10—4 to 1.3 S/cm was obtained. Further studies by Wang et al. led to the development of nanocomposites having conductivity as high as 5.1 S/cm [12Wan]. A high supercapacitance of 273 F/g was obtained at 0.2 A g—1. The process of developing the nanocomposite is shown in Fig. 61.11. The surface area of the composites developed was about 33.9 m2/g, considerably higher than similar composites developed previously. It was reported that the properties of the composites could be easily controlled by varying the reaction conditions.

Similarly, BC nanofibers were implanted with oriented titanium dioxide (TiO2) nanoparticle arrays and the hybrid nanofiber arrays (Fig. 61.12) showed high photocatalytic activity exceeding that of the commercially available photocatalytic agent Degussa P25. Treating the hybrid nanofibers with nitrogen further increased the photocatalytic activity [10Sun]. Bacterial nanofibers were made into carbon nanofibers (annealing at 1,000 °C for 2 h in nitrogen) and treated with MnO2 to build a super capacitor with high energy and power density. In addition, the carbon fibers derived were also doped with nitrogen to improve capacitance. The 3D BC nanofibers coated with MnO2 were used as the positive electrode and the BC nanofibers doped with nitrogen were used as the negative electrode. The device was able to be reversible charged and recharged at 2 V to reach an energy density of

32.9 W h/kg and maximum power density of 284.6 kW/kg and also had good cycling durability with 95 % specific capacitance retained after 2,000 cycles [13Che2]. Figure 61.13 shows an image of the bacterial cellulose membrane,

Freeze-dried

PANI/BC nanocomposites

Fig. 61.11 Process of development of the bacterial cellulose nanocomposites [12Wan]. Reproduced with permission from American Chemical Society components of the supercapacitor, and a diode made using the supercapacitor [13Che2]. Other researchers have also shown that pyrolyzed bacterial cellulose can be used to develop porous 3D electrodes for high performance lithium ion batteries with the addition of tin oxide (SnO2) and/or germanium (Ge) nanoparticles [13Wan1]. The bacterial cellulose-germanium electrode had a very high specific capacity of 967 mAh g-1 and stable capacity of 230 mAh g-1 attributed to the homogenous distribution of active nanoparticles within the conducting cellulose nanofibrils that provide efficient electron conduction pathways and the interconnected voids facilitated the diffusion of lithium ions.

Bionanocomposites were manufactured by combining bacterial cellulose and starch and the properties of the composites developed were studied [10Woe]. Bacterial cellulose was treated (hydrolyzed) with enzymes (Trichoderma reesei) to improve dispersibility and properties of the thermoplastic blend. For the enzyme treatment, the bacterial cellulose fibers were hydrolyzed using 10 % enzyme, pH 4.8, citrate buffer at 45 °C for 20 to 240 min. Hydrolysis resulted in considerable changes to the morphology and the properties of the fibers. Degree of polymerization of the bacterial cellulose decreased considerably from 2,314 to

430 when the hydrolysis was done for 240 min. In terms of morphology, the cellulose fiber bundles were found to be aggregated in 2-10 цш width before treatment and were rendered into short fibers and randomly distributed after the enzymatic treatment [10Woe]. Figure 61.14 shows AFM image of the bacterial cellulose before and after treatment. Reinforcing starch with the modified and unmodified bacterial cellulose led to substantial improvement in the properties of the composites. Elastic modulus increased from 4.3 to 141 MPa and strength

increased from 1.01 to 4.15 MPa when starch was reinforced with untreated bacterial cellulose. Further increase in strength up to 8.45 MPa and increase in modulus up to 576 MPa were observed when the starch was reinforced with hydrolyzed bacterial cellulose [10Woe]. Hierarchical nanocomposites were devel­oped by depositing bacterial cellulose onto natural fibers and improving the inter­facial adhesion [08Pom]. Sisal and hemp fibers were immersed in culture medium and used as substrate to grow cellulose from the strain Acetobacter xylinum. A weight gain of 5-6 % was observed on the fibers due to the growth of the cellulose. Figure 61.14 shows the digital images of the surface of the sisal fibers before and after bacterial growth. Natural sisal fibers had strength of 2.6 g/den and did not show any appreciable decrease in tensile properties whereas a drastic decrease in strength and modulus was observed for the hemp fibers after the growth of the bacterial cellulose [08Pom]. When used as reinforcement, the bacterial cellulose treated fibers showed substantially increased interfacial adhesion for poly(lactic acid) and cellulose acetate butyrate (CAB) matrices.

Bacterial cellulose was used as a binder for short sisal fibers to obtain preforms for composite with tensile strength of 13.1 kN/m. The BC treated biofiber performs were then mixed with acrylated epoxidized soybean oil and made into composites via resin transfer molding [12Lee]. Figure 61.15 shows SEM images of sisal fibers with bacterial cellulose as binder at different magnifications. It was estimated that the bacterial cellulose sheets had a high tensile strength of about 300 MPa. Properties of the composites obtained without and with the BC as reinforcement are shown in Table 61.5. As seen from the table, addition of the modified sisal fibers substantially increased both the tensile and flexural strength and modulus. Similar improvements were also observed in the viscoelastic properties.

To overcome the lack of antimicrobial activity in bacterial cellulose fibers, silver nanoparticles were in situ synthesized for potential wound dressing application [14Wu]. Commercially available bacterial cellulose membranes were purified and soaked in various concentrations of silver ammonia solution for 24 h. SEM image of the bacterial cellulose nanofiber membranes showed that the membranes had a

pore size in the 100 s nanometer range and were 3D. Such 3D structure allowed the diffusion of the nanoparticles into the inner spaces of the scaffolds. It was also found that silver nanoparticles were extensively adhered onto the surface (Fig. 61.16a) of the nanofibers indicating strong affinity between the silver and cellulose. A linear release rate was observed for the nanoparticles when the scaffolds were immersed in PBS solution but the total release of the nanoparticles was only about 16.5 % after 72 h. The developed BC membranes, especially the silver containing bacterial cellulose membrane, had excellent antimicrobial activity as seen from Fig. 61.16b.

Multiwalled carbon nanotubes were added into bacterial cellulose dissolved in an ionic solvent (1-allyl-3-methyl-imidazolium chloride) and electrospun into fibers. It was observed that the MWCNTs were well embedded into the cellulose and were aligned along the fiber axis [10Che]. A transformation of the cellulose from cellulose I to cellulose II was observed and the addition of the nanotubes led to

Blood vessels

Fig. 61.17 Depiction of the potential applications of bacterial cellulose fiber membranes [13Fu]. Reproduced with permission from Elsevier

an increase in strength and modulus by 290 and 280 %, respectively, and an improvement in the thermal stability and electrical conductivity was also observed.

Bacterial cellulose is considered to be one of the most suitable substrates for tissue engineering since it is biocompatible and contains functional groups required to modify the material or carry various substances for delivery in the body [13Fu]. Some of the potential medical applications for bacterial cellulose-based materials are shown in Fig. 61.17 and Table 61.6. In addition to the other unique features, BC has excellent conformability and is well suited to be applied on to

various parts of the body. Figure 61.18 shows BC dressing applied onto the face and torso demonstrating the excellent flexibility and conformability of the films [13Fu].

Several researchers have also attempted to improve the properties of chitosan fibers by blending or modifying chitosan. N-acyl chitosan fibers were prepared by posttreating chitosan fibers with a series of carboxylic anhydrides (N-acetyl, N-propionyl, N-butyryl, N-hexanoyl). Increasing the length of the acyl chain increased the elongation but decreased the strength of the fibers due to the destruc­tion of the hydrogen bonding. However, N-hexanoyl chitosan fibers had higher strength than the N-acyl chitosan fibers due to the higher hydrophobicity of the fibers [07Cho]. Before fiber production, chitosan was mixed with vanillin, and the N-(4′-hydroxy-3′-methoxybenzylidene) product was collected. Later, the powder was extruded into a coagulation bath containing various chemicals. Fibers obtained were drawn 1.2-1.4 times in 2 % aqueous NaOH-ethylene glycol solution, and the filaments obtained were later cut into staple fibers. Further treatment of the fibers was done using NaOH and methanol to obtain cotton-like chitosan fibers [99Hir1]. Properties of the fibers produced when different coagulation baths were used are given in Table 25.3. As a general trend, it was found that increasing the degree of substitution or treating chitosan with vanillin decreased the strength and elongation of the fibers. N-acyl chitosan fibers were produced by wet spinning using aqueous solution of sodium N-acyl and N-propionylchitosan salts in aqueous 14 % NaOH. Fibers with N-acyl chitosan % ranging from 28 to 95 % were prepared with tenacity from 0.5 to 0.9 g/den and elongation from 19 to 30 %. This method of preparing chitosan fibers would enable mixing the chitosan solution with cellulose xanthate solutions to produce N-acyl chitosan-cellulose fibers [98Hir]. Similarly, chitosan butyrate was blended with cellulose acetate and made into fibers with tensile strength between 0.7 and 0.9 g/den and elongation between 4 and 11 % [07El].

O-Hydroxyethyl chitosan xanthate prepared by esterification was added into cellulose xanthate to produce blend fibers with 3.1, 4.5, and 6.2 wt% of chitosan [10Xu]. Properties of the blend fibers in comparison to pure viscose rayon are given in Table 25.4. As seen from the table, blending did not significantly modify the dry or wet strength, but the elongation of the fibers increased. Thermal decomposition temperature increased, and the rate of decomposition decreased by adding chitosan. In a similar approach, N, O-carboxymethylated chitosan and chitosan emulsion were blended with viscose rayon and wet spun into fibers [02Li]. Addition of chitosan was found to decrease the tensile properties but improved the antibacterial properties.

Chitosan fibers were wet spun using acetic acid as the solvent, and the fibers were later acetylated using acetic anhydride [93Eas]. It was reported that acetylation improved the thermal stability and tensile properties. Fibers with tenacities ranging from 1.8 to 2.0 g/den and elongation varying from 4.9 to 10 % were obtained. Similarly, carboxymethylation of chitosan fibers was done using chloroacetic acid to improve chelating properties and the absorption of Cu(II) ions [06Qin]. Fibers were carboxymethylated up to 41 %. Cu(II) removal ranged from 51.7 to 99.3 % with absorption capacity from 16 to 148 mg Cu(II)/g fiber depending on the degree of carboxymethylation. Absorption was considered to be rapid, and the process could occur at room temperature over a wide range of acid and alkali conditions.

Microcrystalline chitosan was blended with cellulose xanthate alkaline solution, and the effect of aqueous microcrystalline chitosan cellulose gel concentration and additives such as sodium alginate on the spinnability and properties of the fibers was studied by Nousiainen et al. [00Nou]. Fibers obtained appeared normal but had slightly lower tenacity and increased water retention, fineness, and elongation compared to standard viscose fibers. Fineness of the fibers produced was between 3.0 and 5.2 dtex, tenacity was between 1.4 and 1.5 g/den, and elongation was between 15 and 19 %. Silk fibroin and cellulose xanthate were combined with chitosan and extruded into fibers using acyl chitosan in aqueous NaOH [02Hir]. Fibers (4.9-9.9 den) containing less than 10 % fibroin had tensile strength between 1.08 and 1.2 g/den and elongation between 30 and 35 %. Combination of chitosan — fibroin and cellulose acetate with 43 % fibroin produced 3.9-5.0 den fibers with

tenacity between 0.7 and 0.9 g/den and elongation between 21 and 29 %. Morpho­logically, the surface of the fibers was extensively striated due to the coagulation process.

A wet spinning approach was adopted to develop poly(e-caprolacton) (PCL)/ chitosan blend fibers with various diameters. The blend polymers were dissolved using 70/30 formic acid/acetone mixture and extruded into a methanol coagulation bath [10Mal]. Fiber diameters in the dry state varied between 112 and 139 qm and between 135 and 372 qm in the wet state depending on the ratio of chitosan and PCL in the blend. It was suggested that a phase separation between the polymers occurred only at the microlevel (<10 qm), and the fibers had relatively low modulus between 7.7 and 23 g/den. Surface roughness of the blend fibers was considered to be suitable for tissue engineering.

Poly(vinyl alcohol) was blended with chitosan with an aim to improve the wet stability of the fibers, and the fibers were produced by extruding into a NaOH and ethanol bath [01Zhe]. PVA contents in the fibers varied from 10 to 50 %, and fibers were stretched to 29 % at 35 °C and air dried to obtain white fibers which were later cross-linked with glutaraldehyde. Some of the properties of the pure chitosan and chitosan-PVA blend fibers are listed in Table 25.5. As seen from the table, the addition of PVA into chitosan marginally increased strength and elongation but water retention more than doubled due to the hydrophilicity of the PVA. It should be noted that the wet strength of the fibers was about 50 % of their dry strength, whereas wet elongation was approximately twice that of the dry elongation. Cross­linking with glutaraldehyde increased the dry strength of the fibers to 2.6 g/den, elongation decreased to about 15 %, and wet strength nearly doubled to about

1.6 g/den without major change in the elongation.

A blend of chitosan and collagen fibers were developed by Hirano et al., and the fibers were N-modified using carboxylic anhydrides and aldehydes [99Hir2]. To prepare the fibers, tropocollagen or lyophilized collagen was dissolved using 2 % acetic acid solution, and powdered chitosan was added into the solution and allowed to age overnight at room temperature. Solution was extruded into a bath containing 5 % aqueous ammonia solution and 40-43 % ammonium sulfate. Later, the fibers were stretched 1.2-1.3 times in an ethylene glycol solution containing 2 % NaOH. To N-modify the fibers, the fibers were suspended in methanol to which acetic anhydride, n-propionic anhydride, n-butyric anhydride, n-hexanoic anhy­dride, succinic anhydride, benzaldehyde, and vanillin were added. Compatibility of the fibers with blood was also evaluated. Some of the properties of the fibers

obtained are listed in Tables 25.6 and 25.7. It was found that tropocollagen-coated chitosan fibers had no blood coagulation on the surface, whereas some coagulation was seen on the N-acyl chitosan and chitosan-tropocollagen. As can be seen from Table 25.6, fiber fineness and tensile properties significantly varied with change in fiber preparation conditions. No major differences were observed in the strength and elongation of the chitosan fibers coated with collagen or tropocollagen. How­ever, N-substituted fibers show considerable variations in strength and elongation depending on the type of anhydride or aldehyde used as seen in Table 25.7.

Regenerated Protein Fibers

Keywords

Cereal/plant protein • Cereal grain processing • Processing coproduct • Zein • Soy protein • Wheat gluten • Gliadin • Cytotoxicity

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.

The corn protein zein has been widely studied for fiber production because zein dissolves in aqueous ethanol solutions and has excellent spinnability. Zein fibers were prepared by using alcohol as a solvent with approximately 13 to 16.5 % solids, in the pH range of 11.3-12.7. Chemicals such as urea were added to denature the proteins under heat. The fibers formed were coagulated using acids and salts, and in some cases formaldehyde was also used. The properties of the fibers formed have not been reported [45Cro]. In another attempt on developing zein fibers, environ­mentally friendly and low cost cross-linking agents such as BTCA and citric acid were used for cross-linking the zein fibers. The drawn and cross-linked fibers obtained in this research had strength of about 1.0 g/den and an elongation of 25 % [96Yan]. In addition to using ethanol as solvent, various other methods such as using alkaline solutions have also been reported in the literature to obtain zein fibers. Although it is relatively easy to produce fibers from zein, currently, there are no reports on producing zein fibers on a commercial scale. High cost of zein ($18­© Springer-Verlag Berlin Heidelberg 2015 245

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

DOI 10.1007/978-3-662-45136-6_54

30/lb) and relatively poor fiber properties, especially poor water stability, are some of the reasons that restrict the use of zein for fibrous applications.

Soy isolates obtained from processing soybeans have also been used to produce fibers. The Ford motor company was probably the first to develop protein fibers from soy isolates as early as 1935 [40Boy]. In their process, soy isolates were dissolved in a solvent, and after aging for certain time, the solution was extruded into an acid precipitating bath containing sulfuric acid, formaldehyde, and salt such as sodium sulfate. Fibers with diameters ranging from 1.5 to 5 deniers and with tensile properties similar to that of wool were produced in this process. However, the poor economics of producing soy protein fibers (SPF), competition from the low cost synthetic fibers, concerns on using formaldehyde, and limitations in the properties of the fibers made this process unfavorable for commercial development. More recently, researchers at the Center for Crops Utilization Research at the Iowa State University have reported the properties of 100 % SPF produced by extrusion and also by wet spinning [95Hua]. However, the process developed at the Iowa State University uses sodium hydroxide for dissolving the protein. Using an alkali such as sodium hydroxide hydrolyzes the proteins and reduces the degree of polymerization (DP) of the proteins. The hydrolyzed and lower DP proteins cannot produce fibers with good strength and elongation required for high quality fibrous applications. Attempts were also made at the Georgia Institute of Technology, Atlanta, Georgia, to produce 100 % SPF. However, the fibers produced were brittle and not useful for textile applications. To improve the strength and water stability of the soy fibers, a bicomponent fiber was produced by mixing soy protein with polyvinyl alcohol (PVA) [99Zha, 03Zha].

Due to the difficulties in dissolving soy proteins and obtaining solutions with viscosities suitable for spinning, blends of soy protein and poly(vinyl alcohol) were produced [03Zha]. Fibers with PVA contents ranging from 0 to 80 % were produced by extruding fibers at 70 °C and drawing up to a ratio of 5.8. Cross-linking of the fibers was done using aldehydes, and the fibers were also subject to heat treatments. Table 54.2 presents the properties of the fibers obtained. As seen from the table, it was only possible to obtain fibers with good quality when the proportion of soy

proteins was 20 % or less. Breaking strength of the fibers with 20 % soy proteins at about 0.5 g/den is considerably lower compared to wool, the weakest natural protein fiber. In addition, the stability of the fibers under high humidities and temperatures was not studied [03Zha].

Pure (100 %) soy protein fibers were developed and used for tissue engineering and controlled drug release applications [09Red, 09Xu]. Soy proteins (26 %) were dissolved in 8M urea and 1 % sodium sulfite, and the solution was aged up to 96 h. After aging, the solution was extruded into a coagulation bath consisting of 10 % acid and 10 % sodium sulfate using a syringe and needle. Fibers obtained were drawn and annealed to improve tensile properties. Table 54.3 provides a compari­son of the properties of soy proteins fibers with fibers obtained from other cereal proteins. As seen from the table, soy protein fibers have the highest strength among all the regenerated protein fibers produced without cross-linking. Higher molecular weight of soy proteins should be the major reason for the high strength of soy protein fibers. The fibers obtained were found to be suitable as scaffolds for tissue engineering and also for controlled drug release. Drugs such as 5-fluorouracil, diclofenac, and metformin showed high loading and sustained release in artificial gastric juice [09Xu].

Recently, protein fibers produced from soy isolates generally called SPF are reportedly available on the market (www. swicofil. com). However, the SPFs cur­rently available on the market are not 100 % SPF but are a blend of about 45 % soy proteins and another synthetic polymer. In addition, the SPFs available on the

market most likely use formaldehyde for cross-linking to improve the properties of the fibers. Formaldehyde is a known carcinogenic and therefore not appropriate for use, especially in textiles.

Shell-core and monolithic blend fibers were produced from soy proteins and nylon 6 using blow molding. Soy protein and nylon 6 (40/60) were dissolved in acetic acid, and the solutions were blown into fibers. Monolithic fibers had an average diameter of 330 nm, and core-shell fibers had a diameter of 910 nm. Morphologically, it was found that the soy protein was distributed as the core inside the fiber as intended, and the evaporation of acetic acid created pores on the surface of nylon on the outside shell of the fibers as seen from the SEM images in Fig. 54.1.

Similar to soy proteins, proteins in wheat (gluten and gliadin) have also been made into fibers and used for tissue engineering and controlled release applications [08Red2, 07Red, 08Red1]. Wheat gluten is a mixture of proteins and consists of the high molecular weight glutenin and low molecular weight gliadin which is soluble in aqueous ethanol. Although gliadin is soluble in aqueous ethanol, it was found that gliadin fibers obtained using ethanol as solvent were unstable in water and dissolved immediately. Alternatively, gliadin was dissolved using urea and sodium sulfite, and fibers obtained were found to have excellent stability in water [08Red2]. It was reported that gliadin proteins could self-cross-link through inter­molecular bonding, form higher molecular weight proteins when treated in high concentration urea solutions and form water-stable structures [08Red2]. Fibers with strength of 1.0 g/den, elongation of 25 %, and modulus of 36.5 g/den were obtained. Also, the fibers obtained retained 100 % of their strength even after being in water for 40 days but lost 5 % of their strength when heated in pH 11 water at 90 °C for 1 h. Chemical cross-linking of the gliadin fibers using glutaraldehyde or carboxylic acids such as citric acid further improved the tensile properties of the fibers [08Li]. Similar to gliadin, wheat gluten was also made into fibers using the urea and reducing agent approach [08Red2, 07Red]. Fibers obtained had strength of 1.0 g/den and elongation of 23 % as seen in Table 54.3. Cross-linking with glutaraldehyde was also found to improve the strength and water stability of the fibers.

References

[40Boy] Boyer, R. A.: Ind. Eng. Chem. 32(12), 1549 (1940)

[45Cro] Croston, C. B.: Ind. Eng. Chem. 37(12), 1194 (1945)

[95Hua] Huang, H. C., Hammond, E. G., Reitmeier, C. A., Myers, D. J.: JOACS 72(12), 1453 (1995)

[96Yan] Yang, Y., Wang, L., Li, S.: J. Appl. Polym. Sci. 59, 433 (1996)

[99Zha] Zhang, Y., Ghasemzadeh, S., Kotliar, A. M., Kumar, S., Presnell, S., Williams, L. D.:

J. Appl. Polym. Sci. 71, 11 (1999)

[03Zha] Zhang, X., Min, B. G., Kumar, S.: J. Appl. Polym. Sci. 90, 716 (2003)

[07Red] Reddy, N., Yang, Y.: Biomacromolecules 8, 638 (2007)

[08Li] Li, Y., Reddy, N., Yang, Y.: Polym. Int. 57, 1174 (2008)

[08Red1] Reddy, N., Yang, Y.: J. Mater. Sci. Mater. Med. 19, 2055 (2008)

[08Red2] Reddy, N., Tan, Y., Li, Y., Yang, Y.: Macromol. Mater. Eng. 293, 614 (2008)

[09Poo] Poole, A. J., Church, J. S., Huson, M. G.: Biomacromolecules 10(1), 1-8 (2009)

[09Red] Reddy, N., Yang, Y.: Biotechnol. Prog. 25(6), 1796 (2009)

[09Xu] Xu, W., Yang, Y.: J. Mater. Sci. Mater. Med. 20, 2477 (2009)

[11Sin] Sinha-Ray, S., Zhang, Y., Yarin, A. L., Davis, S. C., Pourdeyhimi, B.: Biomacro­molecules 12, 2357 (2011)

Biocomposites from Renewable Resources

Keywords

Biopolyester • Polylactic acid • Poly(3-hydroxybutyrate-co-3-

hydroxyhexanoate) • Copolyester • Interfacial adhesion • Compatibilizer

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 pm [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

Fig. 70.1 Properties of biocomposites developed using PHBV as matrix and various biofibers as reinforcement [12Far]. Reproduced with permission from Elsevier

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.

Bledzki and Jaszkiewicz have compared the properties of PLA and PHBV composites reinforced with various natural fibers [10Ble]. Natural fibers such as abaca and flax and man-made cellulose (rayon) were used as reinforcement (30 %) with various synthetic polymers as the matrix. The PHBV matrix was blended with 27 % poly(butylene adipate-co-butylene terephthalate) to improve processability. PP matrix used for comparison was grafted with 5 wt.% of maleic anhydride. Table 70.3 provides a comparison of the properties of the composites developed using various matrix and reinforcements. As seen from the table, the addition of the reinforcing fibers considerably increases the modulus and strength but decreases the elongation. Among the different fibers studied, jute provides higher improvement in tensile properties than abaca or cellulose. SEM analysis showed that PHBV had poor interaction with the fibers and therefore had relatively inferior properties.

PHBV and PLA fibers were combined with various natural fibers (abaca, jute, and flax) in a 70:30 ratio and used to develop composites in comparison to similar composites made using traditional PP, and the composite properties were analyzed using dynamic mechanical analysis [13Ada]. Table 70.4 provides some of the properties of the composites developed. The addition of the fibers increases the tensile and storage modulus for all the composites studied. Jute fibers seem to provide better properties, and PHBV composites had better tensile and storage

Fig. 70.2 SEM images of bamboo fiber (40 %)-reinforced PHBV composites show low levels of matrix due to the high fiber content and therefore poor properties [08Sin]. Reproduced with permission from Elsevier. (a) shows the distribution of the fibers in the matrix and the higher — magnification image in (b) shows the voids between the fibers and matrix indicating poor compatibility

properties than PLA. In another study, unmodified PHBV and PHBV treated with maleic anhydride were reinforced with kenaf fibers, and composites developed through compression molding at 170 °C. It was reported that kenaf fibers increased the strength from about 12 to 18 MPa. However, poor interaction and binding were

observed between the kenaf and PHBV, but the addition of the compatibilizer increased adhesion and provided good tensile properties to the composites [07Ave].

Abaca fibers were also used to reinforce poly(butylene succinate) (PBS), polyestercarbonate (PEC), or poly(lactic acid) (PLA) as matrix. Fibers used had diameters of 0.2 mm and lengths of 5 mm and were chemically modified through esterification, alkali treatment, and cyanoethylation [03Shi]. An increase in modu­lus was observed after the addition of abaca fibers irrespective of the treatment of

the fibers or the type of matrix used. The addition of fibers into matrices such as PLA which had higher strength did not show a major increase in properties of the composites [03Shi].

PLA has also been reinforced with a natural fabric Sterculia urens and the properties of the reinforced composites were studied [13Jay]. Figure 70.3 shows the image of a natural Sterculia urens fabric (thickness of 0.16 mm), alkali-treated fabric, and a fabric coupled with a silane coupling agent. The fabrics were blended (20 %) with PLA and compression molded into films at 180 °C. Tensile properties of the unreinforced and PLA reinforced composites with various fabrics are shown in Table 70.5. From the table, it can be inferred that the addition of the fabric increases the tensile properties of the composites, and further, alkali-treated and silane-coupled fabrics provide better properties to the composites due to improved compatibility [13Jay]. Hybrid PLA biocomposites were prepared using kenaf fibers and cornhusk flour as the reinforcement [14Kwo]. The ability to predict the influence of aspect ratio on the properties of the composites developed using injection molding was studied. It was found that the actual and predicted value of composite properties did not have good correlation. Initial values of aspect ratio determined before extrusion were suggested to provide a better estimate of the properties [14Kwo].

Bamboo fiber pulp with cellulose content of 96 % was treated with alkali or silane coupled with commercial coupling agent KH560 and used to reinforce virgin or maleated PLA [14Lu]. FTIR studies confirmed coupling of the silane groups, but no major changes in crystallinity or crystal structure were observed. Table 70.6

shows the changes in the properties of PLA composites reinforced with 2 % of untreated and chemically modified bamboo fibers [14Lu]. As seen from the table, marginal increase in the tensile properties was observed after reinforcing with the raw bamboo fibers. However, considerable increase in the toughness was observed when maleated PLLA or alkali-treated bamboo was used. The silane-coupled bamboo fibers provide considerable increase in strength, but the other properties did not show major changes [14Lu]. Figure 70.4 shows the possible mechanism for the improvement in composite properties after treatment of the bamboo fibers [14Lu].

In most studies, biopolymers such as PLA and PHBV have been reinforced with cellulose fibers. In a unique deviation from this approach, PLA was reinforced with silk fibers [10Zha]. Bombyx mori silk fibers were processed into length of about 5 mm and melt compounded with PLA in weight ratios ranging from 1 to 7 %. Compounded samples were later injection molded to form composites. Dynamic mechanical analysis showed that storage modulus increased with the addition of the fibers. Good interfacial bonding was observed between PLA and the silk fibers which were supposed to have a plasticizing effect [10Zha]. Degradation of the composites using enzymes showed that the PLA matrix degraded much faster than silk fibers and that weight loss of the composites increased with increasing amounts of silk due to degradation of the sericin peptides [10Zha]. Instead of using virgin silk fibers, silk waste fibers were used to reinforce wheat gluten plasticized with 10 % glycerol [12Sek]. Before using as reinforcement, the silk fibers were treated with 1 % NaOH for 1 h to remove sericin, and the fibers were later cut into lengths of about 1 mm. The treated silk fibers and wheat gluten were mixed together and cast to form sheets which were later compression molded at 120 °C, 2 MPa for 20 min to form the composites [12Sek]. Glutaraldehyde was also added as a cross­linking agent to improve the properties of the composites. Tensile strength of the composites increased from 17 to 28 MPa with the addition of 5 % of silk fibers. Similarly, modulus increased from 811 to 1,605 MPa, whereas elongation decreased from 13.9 to 3.4 %. SEM images (Fig. 70.5) showed narrow interaction between the fibers and matrix at different loadings.

In another study, waste silk fibers processed into various lengths were used as reinforcement for poly(butylene) succinate (PBS) composites. Silk waste obtained after processing was cut into 25.4,12.7, 6.4, and 3.2 mm and reinforced into PBS in 20, 30, 40, and 50 % by weight. Composites were fabricated by compression molding at 135 °C for 10 min at a pressure of 6.9 MPa [06Han]. Tensile strength and modulus of the composites were found to increase from 35 to 42 MPa and 0.5 to 1.3 GPa, respectively, when the proportion of the fibers was increased from 0 to 40 % [06Han]. Further increase in the fiber content to 50 % resulted in a decrease in tensile properties. Similar phenomenon was observed when the length of the fibers was decreased. However, flexural strength and modulus did not show a significant

change when the length of the fibers was changed [06Han]. Dynamic thermal analysis also showed that the storage modulus and tan S peak height were affected to a larger extent by the amount of fibers rather than the length of the fibers.

Although several researchers claim to have developed biodegradable composites using reinforcement and matrix from renewable resources, actual biodegradability tests or the degradation of the composites in various environments has rarely been reported. In one study, the biodegradation of poly(lactic acid) biocomposites reinforced with coir and starch was tested. The biodegradation test was done according to ISO standard 14855. Samples were placed in a bioreactor and the carbon dioxide evolved during the biodegradation was measured. Figure 70.6 shows the % degradation of the individual components and the composites devel­oped after various incubation times [08Lov]. After 90 days of incubation, the thermoplastic starch has a much higher level of degradation. The composites, however, did not show major differences in degradation. During biodegradation, a biofilm is formed on the surface, and the bacteria and fungi on this biofilm accelerate the degradation of the composites. Significant erosion of the surface of the composites (Figs. 70.7 and 70.8) was observed after 21 days, and complete erosion of the matrix was seen after 70 days of incubation [08Lov].

Nanofiber composites were also developed using eucalyptus kraft pulp and PHBV. Nanofibrous cellulose was obtained by chemical treatment, and fibers had diameters of 5-10 nm in aqueous gels, but the fibers were found to agglomerate and had typical diameters of 1 pm and were used as reinforcement in the composites. PHBV dispersed in distilled water was mixed with nanofibrillated cellulose (2.5­10 %) and injection molded into specimens at 180 °C [13Sri]. The addition of NFC into the PHBV matrix increased the tensile strength as seen in Fig. 70.9. However, ultimate tensile strength did not show a major difference, but modulus of the

composites increased and elongation decreased substantially as seen in Table 70.7. Further, DMA also showed an increased modulus with increasing PHBV content. Crystallization and glass transition temperatures were also found to increase, but the inclusion of PHBV decreased the thermal resistance.

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