Environmentally Sustainable Fibers from ... - ACS Publications

Nov 26, 2008 - ... weight of 10-50. kDa will produce good fibers.22 Experience from synthetic fibers ... 100% to less than 10% with 16% nanoclay addit...
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January 2009

Published by the American Chemical Society

Volume 10, Number 1

 Copyright 2009 by the American Chemical Society

Reviews Environmentally Sustainable Fibers from Regenerated Protein Andrew J. Poole,* Jeffrey S. Church, and Mickey G. Huson CSIRO Materials Science and Engineering, P.O. Box 21, Belmont, Victoria, 3216, Australia Received September 21, 2008; Revised Manuscript Received October 17, 2008

Concerns for the environment and consumer demand are driving research into environmentally friendly fibers as replacements for part of the 38 million tonnes of synthetic fiber produced annually. While much current research focuses on cellulosic fibers, we highlight that protein fibers regenerated from waste or byproduct sources should also be considered. Feather keratin and wheat gluten may both be suitable. They are annually renewable, commercially abundant, of consistent quality, and have guaranteed supply. They contain useful amino acids for fiber making, with interchain cross-linking possible via cysteine residues or through the metal-catalyzed photocrosslinking of tyrosine residues. Previous commercially produced fibers suffered from poor wet strength. Contemporary nanoparticle and cross-linking technology has the potential to overcome this, allowing commercial production to resume. This would bring together two existing large production and processing pipelines, agricultural protein production and textile processing, to divert potential waste streams into useful products.

Introduction Concern for the environment, rising oil prices, and the finite nature of oil reserves is driving research into ways to replace petrochemical products with biobased materials. Targets include bioplastics, films, packaging, building materials, and a range of other products including fibers.1-3 Global fiber production in 2005 was 70.6 million tonnes, of which 38 million tonnes was synthetic, mainly polyester, nylon, and olefin fiber.4 Developing biobased alternatives for even a portion of this offers the potential of significant environmental benefits. A further driver comes from consumer demand, with growth of the “eco-friendly” and “organic” markets in textiles (as well as food and other areas), reflecting the increased interest and power of consumers. Surveys show environmental compatibility is increasing as a sales argument,5 as demonstrated by organic cotton fetching a premium price over the nonorganic fiber, even though they are physically indistinguishable.6 However, surveys also warn that consumers will not compromise product performance to have an eco-friendly product.6,7 While there is no international standard to describe ecofriendly, a fiber made from renewable raw materials, using an environmentally friendly and commercially viable process, and having triggered biodegradability (i.e., is biodegradable in * To whom correspondence should be addressed. Phone: +61-3-5246 4000. Fax: +61-3-5246 4057. E-mail: [email protected]. 10.1021/bm8010648

composting situations after disposal) or recycling capability can be considered eco-friendly (Figure 1). The desire for such products has led to a renaissance in fibers such as hemp and the adoption of nontraditional fibers, such as bamboo, for use in apparel.8 Attempts are being made to use lignocellulosic agricultural byproduct such as cornhusks, cornstalks, and pineapple leaves as alternative sources of cellulosic fibers,9 and at least one regenerated cellulosic product is in commercial production, Lenzing Modal, which is produced from beech wood in a process described as being in accordance with the principal of sustainability.10 Another biomaterial that has attracted less attention but is worthy of consideration is agriculturally derived protein. Fibers of regenerated protein were produced commercially in the 1930-50s and by today’s standards they would be considered natural, sustainable, renewable, and biodegradable. Casein from milk was used by Courtaulds Ltd. to make Fibrolane and by Snia to make Lanital; groundnut (peanut) protein was used by ICI to make Ardil; Vicara was made by the Virginia-Carolina Chemical Corporation from zein (corn protein); and soybean protein fiber was developed by the Ford Motor Company.11,12 The regenerated fibers had several qualities typical of the main protein fibers, wool and silk; they were soft with excellent drape and high moisture absorbency. They could be processed on conventional textile machinery and colored with conventional

Published 2009 by the American Chemical Society Published on Web 11/26/2008

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Figure 1. Concept of an eco-friendly fiber.

dyes. Superior to wool in some regards, they did not prickle, pill or shrink. They could be produced as staple or filament, crimped or straight, with control over diameter, and dope-dyed if required.13-15 Their drawback was poor mechanical strength when wet. Dry fiber strength was acceptable due to interchain hydrogen bonding between protein macromolecules.11 In the wet state, however, the fiber became weak as hydrogen bonding occurred preferentially with water molecules and the density of interchain covalent cross-links was insufficient to impart strength16 (Table 1). These technical issues, rising raw material and production costs and the ascent of the petrochemical synthetic fibers, with their constant and consistent supply of materials and superior performance, caused production of regenerated protein fibers to stop in the late 1950s. To make a protein fiber for today’s market would require the wet-strength problem to be solved. To this end, the advances made since the 1950s in cross-linking technology and use of nanoparticle reinforcing agents have not been applied to regenerated protein fibers, though they offer the potential of improving tensile strength. Further, since the 1950s new protein sources have become available as agricultural byproducts, with concomitant infrastructure for large-scale production. Keratin from feathers and gluten from wheat are of particular interest. In this paper we discuss whether with new technical advances and new protein sources, regenerated protein fibers are likely to re-emerge as ecologically friendly fibers. Requirements of Textile Fibers. It is rare that one individual property will determine the value of a fiber. Rather, a combination of properties will govern technical and commercial success. For conventional textile applications these relate to20 (1) acceptable tensile strength of around 5 g/denier (574 MPa); (2) acceptable elongation at break (above 10%); (3) reversible elongation in the range up to 5% strain; (4) modulus of elasticity between 30 and 60 g/denier (3443-6887 MPa) conditioned and not dropping too much in the wet; (5) moisture absorption of 2-5%; (6) dyeability, comfort, easy care, and abrasion resistance (though this equally relates to fabric construction and is not always important); (7) resistance to dissolution and strong swelling in water and moderately strong acids, alkalis and basic solvents up to temperatures of 100 °C; and (8) without a tendency to catch fire or support combustion. For the fiber to be utilized by industry, the quality and quantity must be consistent over time.6 This requires a reliable supply chain to be in place with both the quality and quantity of raw material to be consistent over time. There must also be sufficient profit available throughout the production pipeline. Fiber Structure. Fibers are composed of oriented assemblies of linear macromolecules.21 Properties of the assembly can be

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improved through choice of polymer, cross-linking and crystallinity, and the incorporation of reinforcing fillers or nanoparticles.20 Polymer. In general, a polymer molecular weight of 10-50 kDa will produce good fibers.22 Experience from synthetic fibers shows the optimum varies depending on the polymer. For example, the closely related compounds polyhexamethylene fumaride and polyhexamethylene succinamide exhibit good fibrous properties at 12 and 25 kDa, respectively.23 For proteins, increasing molecular weight is thought to increase the area of contact between chains,16 but going beyond the optimum molecular weight does little to improve fiber properties.23 At very high molecular weight the protein chains can loop back and forth, which limits fiber strength.16 Uniformity of chain length is thought to be a potential advantage.21 Molecules should be linear and consist of residues without bulky side groups as these can prevent the close packing of chains and reduce crystallinity.16 Close packing is also desirable to give shorter covalent cross-link distance. Amino acids capable of forming interchain cross-links are desirable. Cysteine residues are particularly useful as they can spontaneously combine to form cystine through formation of a covalent disulphide cross-link. Other desirable residues include tyrosine, glutamic and aspartic acid, arginine, lysine, and serine.24,25 Cross-Linking and Crystallinity. Cross-linking and crystallinity affect the protein fiber’s tensile strength and other properties. Their degree and character are greatly affected by the amino acid composition of the protein and the processing conditions. Before fiber spinning, the dissolved protein chains must be put into an extended, unraveled form (noncovalent interchain bonds disrupted). The spun fiber is drawn (elongated) to maximize chain alignment, give close packing of chains and to allow regions of crystallinity to develop. Elongation occurs above the glass transition temperature (Tg) as below this temperature mobility is suppressed.26 The chain alignment leads to considerable increases in mechanical strength. Spontaneous interchain cross-linking can occur between neighboring chains via suitable residues, particularly cysteine. Additional covalent links can be formed between a range of amino acid residues using chemical cross-linking agents. Metalcatalyzed photo cross-linking forms interchain dityrosine residues.24 Transglutaminase is capable of cross-linking wool fibers and is used in commercial wool finishing; it forms links between lysine and glutamine residues.27 Glutaraldehyde is used in food applications and forms covalent links between lysine and tyrosine residues.28,29 An important factor is the distance that cross-linking agents are able to bridge, and chains must be packed tightly enough that at least two reactive sites are available at the required distance for cross-link formation.25 Cross-linking is also limited by the location of suitable amino acid residues along the chain. Fillers and Nanoparticles. The inclusion of nanoparticles in polymer systems to produce so-called nanocomposite materials has been shown to give remarkable increases in tensile properties at low nanoparticle addition levels.30 The Toyota group were one of the first to demonstrate excellent mechanical properties in a polymer nanocomposite by reinforcing nylon with clay.31 Layered silicates, such as montmorillonite and hectorite, have strong interaction between silicate layers and the matrix via hydrogen bonding in nylon 6.32 These results have been extended to other polar polymers, (e.g., epoxies), though their use is not always straightforward and the change in properties depends on many factors such as nanoparticle size, aspect ratio,

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Table 1. Physical Properties of Commercial Regenerated Protein Fibers Compared To Natural and Synthetic Fibers dry fiber

tenacity (g/denier)

initial modulus (g/denier)

wet breaking extension (%)

tenacity (g/denier)

initial modulus (g/denier)

breaking extension (%)

Fibrolane (casein) 1.1 40 63 0.35 2 60 Ardil (peanut) 0.8-1.0 30 10-110 0.3 0.5 90 Vicara (zein) 1.0 50 28 0.6 15 28 soybean (Drackett Co.) 0.6 40 40 0.12 4 40 wool, merino 1.6 25 43 1.1 10 57 cotton 3.6 30 9 4.0 10 10 silk (Bombyx mori) 3.7 120 16 3.4 30 26 polyester (Terylene 45/24) 5.3 120 15 5.3 15 120 nylon 6 (Grilon 30/7) 5.4 19 31 4.7 19 26 polypropylene (Ulstron) 7.4 80 17 7.4 80 17 Where tenacity is the specific force necessary to break the fiber in units of g/denier, where denier is the mass in grams of 9000 meters of fiber, measurements being standardized on fiber mass rather than diameter.17 Data are from Farrow,18 except polypropylene from Ford.19

surface area, and polymer/filler compatibility.32,33 Other properties can also be modified, for instance, the Tg can either increase or decrease depending on the polymer, nanoparticle, and weight fraction used.34 Biodegradability can be increased as shown by composting experiments with polymers such as polylactic acid.32 Clays have high affinity for protein and so it is not surprising that good results have been obtained with nanoclay fillers in regenerated protein. Chen and Zang35 adding montmorillonite nanoclay at the 20% level in soy protein sheets increased the Young’s modulus from 180 to 587 MPa and tensile strength from 8.8 to 15.4 MPa compared to an unmodified control. The nanoclay was shown to become highly exfoliated with surface positive charges on the globulins anchoring into the negatively charged montmorillonite galleries and good nanoclay dispersion occurred due to electrostatic interactions and hydrogen bonding. Yu et al.36 increased the tensile strength and Young’s modulus of soy protein sheets using rectorite nanoclay. Maximum increase in tensile strength (from 6.8 to 12.9 MPa) was at 12% nanoclay addition while maximum increase in Young’s modulus was at 16% addition (3.6 times increase to 621 MPa). Simultaneously, the elongation at break decreased from over 100% to less than 10% with 16% nanoclay addition. Huang and Netravali37 increased the tensile properties of soy protein using nanoclay and flax fibers and cross-linking with glutaraldehyde. Ai et al.38 enhanced soy polymer sheets with 4% nano-SiO2, while Chen et al.39 used lignin at 6% with glutaraldehyde to increase Young’s modulus from 8.4 to 23.1 MPa and the Tg from 62.5 to 70.4 °C. Carbon nanotubes offer considerable potential for composite reinforcement due to their remarkable mechanical strength.40 Multiwall CNTs of varying sizes have reinforced soy protein sheets. The protein chains wrapped and penetrated the CNTs, interacting with both internal and external surfaces. The strength imparted by the CNTs depended on the protein/matrix interface transferring stress to the CNT. CNTs of 10-15 nm diameter increased Young’s modulus from less than 120 MPa to approximately 250 MPa at 0.25% addition41 with tensile strength increasing from less than 8 to about 12 MPa. It can be assumed that CNTs with functionalized surfaces may interact more strongly with the protein and give a greater effect. Nonetheless, CNTs are likely to remain too expensive to use as commodity fillers for some time. Cellulosic whiskers can be used as reinforcing microfibrils to create nanocomposites of outstanding properties.42 The lignocellulosic fibers are derived from annually renewable resources and provide environmental benefits with respect to disposability and renewability. The increase in mechanical properties is governed by the aspect ratio of the nanofiber, its

degree of crystallinity, processing methods and matrix structure. The modulus of the native cellulose perfect crystal is estimated to be 150 GPa,43 while measurements of the highly crystalline bacterial cellulose nanofibers show a Young’s modulus of 78 ( 17 GPa.44 Cellulose nanofibers have relatively reactive surfaces,42 making them amenable to covalent bonding. Protein Sources. The protein used for regenerated fiber production must have both the correct polymer characteristics and the necessary eco-friendly characteristics discussed earlier. Two proteins which appear to meet these criteria are keratin from chicken feathers and gluten from wheat. Feather Keratin. Chicken feathers are probably the most abundant keratinous material in nature.45 An estimated 5 million tonnes are produced annually as a waste stream from the production of chicken meat, of which over 65 million tonnes was produced worldwide in 2007 (calculated from refs 46 and 47). Meat processing occurs on a year-round basis in centralized locations, and the collected feathers have minimal value. Their disposal represents a significant problem to the poultry farming industry, with some used in low-grade animal feeds and the remainder going to landfill, thus making transport the main cost of the raw material. Feather keratins are small proteins, uniform in size, with a molecular weight around 10 kDa. They are rich in cysteine and hydrophobic residues and have a β-sheet conformation.43,45,46 Arai et al.48 sequenced chicken feather keratin and found it to be 96 residues long containing seven cysteine residues. These were restricted to the terminal regions: six in the N-terminal and one in the C-terminal region. These regions were almost devoid of the β-structure. The central portion of the molecule was rich in R-structure and contained a high proportion of hydrophobic residues. Overall, the molecule was poor in charged amino acids. Whole feathers and feather fibers (barbs) are widely studied for potential biomaterial applications due to their inherent properties of strength and chemical resistance.2,3,49,50 Feather fiber strength and modulus are 1.4 and 35.6 g/denier, respectively (161 and 4086 MPa, respectively), similar to wool,9 leading to their use in composite extruded fibers made with LDPE, HDPE, and polypropylene,51-53 in composite materials with wood-MDF,2 with poly(methyl methacrylate),54 in compression-molded HDPE,55 and in biobased composite materials.56 Solubilized feather keratin is becoming more widely studied as a source of biopolymer for films,3,57,58 including edible films or coatings and50 compostable packaging46 and for inclusion in composite materials.52

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Biodegradable and breathable food packaging films have been produced from solubilized keratin59 and keratin fiber has been produced by extruding at low temperature (120 °C) after mixing with glycerol, water, and sodium sulfite.60 Regenerated keratin fibers were produced by wet-spinning protein-detergent complexes in the laboratory during the 1940s.61 Dry strength of greater than 3 g/denier (344 MPa) was achieved; however, wet strength was poor, being 60% lower. Commercial production was not attempted, partly due to the disorganized supply routes available in the 1930-50s. In a parallel development, regenerated keratin fibers have been produced from wool62 and wool/casein blends63 at laboratory scale. Wool is similar to feather in some regards, both keratins being highly cross-linked, although wool proteins are heterogeneous with a generally higher molecular weight (10-55 kDa64), higher cysteine content (10.5 mol % for whole merino fiber65 with some sulfur-rich components containing 12-41 mol %66), and a predominantly helical configuration.66 The pure wool regenerated fibers were stronger than the protein blend fibers and had a tensile strength of 1 g/denier (115 MPa) and 33% extension at break for conditioned samples, with the R-keratin configuration of the native wool being converted to a β-conformation in the regenerated fiber.62,67 The authors suggested this made the fibers a usable textile material and that their process could form the basis of a commercial process, with the caveat that quality wool fiber was too expensive to use as a starting material. To overcome this, they suggested using waste wool or other keratin sources such as horns, hooves, or nails as protein sources. Commercial production was not attempted, possibly in part because there was only a 35% conversion efficiency of native wool to regenerated fiber.62 Current research interest in regenerated feather keratin materials has not spread to fibers, though the presence of crosslinking sites, hydrophobic residues, and uniform molecular weight suggest this material should form robust fiber. The main drawback is the difficult solubilization route and low molecular weight, which may make fiber production more difficult. Wheat Gluten. Gluten is the principal protein fraction isolated from wheat (and some other grains). It is biodegradable, abundant, and renewable.68 Its availability is likely to increase as industrial use of wheat increases, such as for biofuel production in the European Union and Canada.69 The world production of wheat was 625 million tonnes in 2005/06.47,69 The grain contains about 12% proteins, with 75-85% of this being gluten proteins. Gluten is defined as the material that remains when wheat dough is washed to remove starch granules, though in practice the term refers to the proteins.70 Other constituents in industrially prepared gluten are lipids (3.5-6.8%), minerals (0.5-0.9%), and carbohydrates (7-16%).71 The gluten proteins contain hundreds of components over a wide range of molecular weights, with primary protein chains ranging from 20 to 90 kDa.70 They are subdivided into the gliadins and the glutenins; two groups of roughly equal proportions that can be separated according to their solubility in alcohol-water solutions (e.g., 60% ethanol): the soluble gliadins and the insoluble glutenins.70,72 The gliadins are protein molecules in which disulphide bonding is solely intramolecular.72 They have a molecular weight of 30-80 kDa, are rich in proline and glutamine and have a low level of amino acids with charged side groups. They act as a plasticizer in dough formation, associating with other chains through hydrophobic interactions and hydrogen bonding.71

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The glutenins are proteins with primary protein chains linked together via intermolecular disulphide bonds, although intrachain bonds are also present. Molecular weight varies from 500 to more than 10000 kDa for the cross-linked molecules making them among the largest in nature. The primary protein chains are grouped into the high-molecular-weight glutenin subunits (HMW-GS; 70-90 kDa; 7-13% of total gluten protein) and the low-molecular-weight glutenin subunits (LMW-GS; 20-45 kDa; 19-25% of total gluten proteins, making them predominant).70,72 The reduced subunits have similar solubility in ethanol-water as the gliadins.70 HMW-GS contain cysteine residues that allow end-to-end, probably head-to-tail, linkages. This forms a backbone from which the LMW-GS branch. The LMW-GS contain eight cysteine residues, two for interchain bonding, and six for intrachain bonds.70 During bread making, additional intermolecular cross-links are formed between tyrosine residues.73 Dityrosine links are naturally occurring in a range of proteins, including the elastic ligaments of insects (resilin), elastin, and collagen.74 Transglutaminase is capable of cross-linking globulins and HMWGS.28,29 Gluten has been studied for producing food packaging films due to its ready availability, good film forming properties, potential to make edible packaging, and environmental credentials.1,75 Its main drawback has been its high water sensitivity; water acts as a plasticizer leading to poor wet strength and reduced barrier properties. Gluten films have been found to have a Tg around 38 °C and so are brittle at room temperature.76 Glycerol is often added as a plasticizer, making the films soft and pliable.1 Glycerolplasticized gluten has also been thermo-molded into biodegradable plastics.77 Montmorillonite nanoclays added as fillers have been found to improve tensile properties of gluten films. When added at 5% level to films plasticized with glycerol, the nanoclay increased Young’s modulus from 3.7 to 10.6 MPa.76 Gluten has a structure that is consistent with being able to form highly cross-linked nanocomposite fibers. Residues that self-cross-link are present as well as residues that cross-link with glutaraldehyde, transglutaminase, or metal-catalyzed photo cross-linking, and gluten interacts with montmorillonite nanoclay. However, the first report of gluten fibers produced by a wet-spinning technique appeared only recently.78,79 Gluten solution was extruded from a syringe into a coagulating bath of 10% (w/w) sodium sulfate and 10% (w/w) sulfuric acid. The fiber produced had a Young’s modulus of 5 GPa in the conditioned state (21 °C, 65% relative humidity), though the wet results were not reported. The fibers were drawn but did not develop crystalline regions. No reports have been found of chemically cross-linked nanocomposite gluten fibers. A summary of the desired properties for forming protein fiber is compared to feather keratin and wheat gluten in Table 2. The proteins can be blended to modify their properties or to maximize their utilization, by diluting one protein with another. When the proteins being blended contain cysteine residues, they would also be expected to form interchain disulfide cross-links between the different proteins. Barone et al.80 found there was intimate interaction and cross-linking between blends of feather keratin and gluten and feather keratin and the cysteine-containing protein lactalbumin. The authors stated that blending different types of proteins was a convenient method for altering protein properties. The “toughness” of thermally processed keratin films

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Table 2. Desired Protein Properties for Forming Fibers Compared to the Properties of Feather Keratin and Wheat Gluten property molecular weight molecular weight range crystallinity

optimum for fiber production 10-50 kDa narrow

feather keratin 10 kDa narrow

wheat gluten 20-90 kDa broad

desired

native material noncrystalline has crystallites cross-linking sites desired yes: cysteine yes: cysteine, tyrosine linear molecule desired yes often not hydrophobic groups desired yes often not raw material reliable supply, 5 million potentially availability reliable quality tonnes annually large environmental eco-friendly byproduct; byproduct; credentials low-value food for use or landfill human consumption biocompatibility nontoxic nontoxic nontoxic

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R-CH2-S-S-CH2-R + R′S- h R-CH2-S-S-R′ + R-CH2-SR-CH2-S-S-R′ + R′S- h R′S-S-R′ + R-CH2-Swhere R represents the peptide backbone. The net reaction results in the formation of two free thiol groups attached to the protein chains that can be reformed into disulfide cross-links. Oxidation of the disulfide bond can be carried out using peroxide as shown below: [O]

R-CH2-S-S-CH2-R + O22- y\z 2R-CH2-SO3This reaction forms cysteic acid and is not reversible making it less attractive for protein dissolution. Sulfitolysis describes the cleavage of the disulfide bond by sulfite:

R-CH2-S-S-CH2-R + SO32- h R-CH2-S-SO3- + R-CH2-SIt has been suggested that bisulfite is the active species.83 In any case, the reaction is reversible and gives an S-sulfonate anion and a thiol. Oxidative sulfitolysis converts the disulfide into two Ssulfonate anions: [O]

Figure 2. Wet spinning process. Protein dope is pumped through a spinneret into a coagulation bath to form fiber. The fiber is drawn, usually between rollers of different speeds, and cross-linked prior to drying and winding up.

was increased by blending the keratin with gluten in equal proportions, though at the expense of decreasing the film’s strength and stiffness. Similarly, the properties of keratin and lactalbumin blend films were found to be dependent on the proportion of each protein in the blend, and the authors concluded the film mechanical properties were a compromise between those of the individual proteins. In contrast, Wormell63 showed little correlation between the mechanical properties of regenerated protein fibers and the composition of the protein blend used in their production. Wool keratin and casein were the proteins used. The tenacity of the fibers appeared to be more affected by fiber denier than the protein ratios.63 This highlights that while blending may be a convenient way of modifying protein properties, and perhaps of maximizing protein utilization, the optimum blend of proteins will need to be found empirically, taking into account fiber processing conditions. Fiber Production. The first step in fiber production is dissolution of the protein. As the 1930-50s regenerated protein fibers used globular proteins, dilute alkali solutions were sufficient to swell and dissolve the proteins. Fiber could then be produced by extruding into an acid solution.81 The situation is more complex for feather keratin and wheat gluten as they contain intermolecular covalent bonds (disulphide cross-links between cysteine residues) that must be cleaved while preserving the covalent bonding of the primary protein chain, as short chain segments will adversely affect fiber strength.15 Cleaving the disulphide bonds can be achieved by reduction, oxidation, sulfitolysis or oxidative sulfitolysis.82 Reduction reactions are commonly carried out using thiols (R′-SH). A large excess of the thiol is required.83 The reaction is reversible and proceeds by two nucleophilic displacement reactions.

R-CH2-S-S-CH2-R + 2SO32- + H2O 98 2R-CH2-S-SO3- + 2OHThis reaction is not reversible. Gluten appears easier to dissolve than feather keratin. Reddy and Yang78 readily dissolved it using 1% sodium sulfite as reducing agent and 8 M urea as swelling agent. Keratin is often dissolved using 2-mercaptoethanol and urea, with the reaction cleaving cross-links without damage to the protein backbone. However, the cost of these reagents would prohibit their use for commercial fiber production. An alternative described by Jones and Mecham84 uses the inexpensive and relatively abundant sodium sulfide. The reaction of sodium sulfide with keratin is very complex. Sodium sulfide dissociates in water to form a thiol and a hydroxide anion as follows:85

Na2S + H2O f 2Na+ + HS- + OHThe hydrosulfide anion reduces the disulphide bond according to the reaction:

R-CH2-S-S-CH2-R + HS- f R-CH2-SH + R-CH2-S-SIn reality, a mixture of the protonated and deprotonated forms of both products, cysteine thiol and perthiocysteine, would be present. Both of these products are highly reactive. The high pH not only disrupts hydrogen bonding, disaggregating the protein, but the hydroxide ions can also react with the disulfide bonds forming dehydroalanine by β-elimination (Scheme 1). The dehydroalanine residues readily form cross-links by reacting with the amino acid side chains of cysteine and lysine to form lanthionine (Scheme 2) and lysinoalanine (Scheme 3). These reactions have the potential to provide a mechanism for cross-linking the proteins after fiber formation, thus improving their physical properties. However, the strong reducing conditions generated by sodium sulfide have the potential to damage the protein backbone,62 although Jones and Meecham84

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Scheme 1

Figure 3. Regenerated keratin fiber production would connect two existing production pipelines.

Scheme 2

Scheme 3

supplied some evidence that they managed to cleave the crosslinks without causing substantial damage to the protein chain. Once the protein chains have been dissolved, disaggregated and elongated, they are spun into fiber using a wet-spinning technique (Figure 2). Protein in an efficient solvent (referred to as protein dope) is pumped through a fine spinneret into a coagulation bath that contains a poor solvent, generally a

concentrated salt solution that dehydrates the protein, causing it to come out of solution and form a filament. The filament is then stretched to increase chain alignment and crystallinity and decrease the distance between potential cross-linking sites. Potential for Fiber Manufacture. In the 1930-50s, the choice of protein was based on economic abundance rather than structure. Today a similar compromise between environmental acceptability, availability and fiber forming properties is likely. Feather keratin, wheat gluten, and other protein sources (e.g., zein from corn used for bioethanol production) are eco-friendly insofar as they are annually renewable byproducts that are being under-utilized as a resource, are produced in substantial quantities, and will continue to be produced in the future. Producing regenerated protein fiber will connect the existing production pipelines for protein production and the textile production, marketing, and distribution pipeline (Figure 3), as the protein fibers will feed into existing textile processes including the cotton system.14 Regenerated fibers have not been produced that contain nanoclays, cellulose nanofibers or other nanoparticles as reinforcing fillers, even though these offer improvements in mechanical strength. Nanoparticles can also improve biodegradability and, in the case of cellulose nanofibers at least, are themselves renewable. Combined with cross-linking technologies, these offer the potential to produce fibers of acceptable strength. If experimental results show the combination of nanoparticle fillers and cross-linking techniques produce fibers of acceptable strength, the regenerated protein fibers will have all the environmental and technical attributes required for success. An economic analysis will then be the final determinant as to whether they are commercially viable. Market Opportunities and Competitive Advantage. Demand from consumers for eco-friendly products is growing stronger.5 While organic fibers are meeting part of this need, they are unlikely to be produced in sufficient quantities to meet all the demand for eco-friendly fiber.6 Organic fibers can have additional problems, such as organic cotton returning a yield approximately 50% lower than conventional cotton and so requiring more land and water for the same fiber production. Weed control is often by tillage, which can be detrimental to soil conservation.6 Hemp, organic wool, and recycled petrochemical synthetic fibers are marketed as eco-friendly.5,86 Consumers who buy these materials are generally environmentally conscious.5 Regenerated protein fibers should compete well with organic and other eco-friendly fibers based on their environmental credentials. The processing route should allow them to be certified as organic provided organic practices were used in the

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production of the protein (i.e., organic chicken or wheat). They could similarly be declared free of genetic modification. They will be able to be processed on conventional textile machinery and dyed with conventional textile dyes. Potential product markets where they may have competitive advantage are in ecofriendly apparel as well as technical and industrial applications.

Conclusion Regenerated protein fibers are potentially environmentally sustainable, renewable and biodegradable. Two protein sources, feather keratin and wheat gluten, have been considered for their suitability to make an eco-friendly regenerated fiber. Both appear to be viable, although low wet strength is likely to be problematic. The inclusion of nanoparticles and use of crosslinking technologies offer the potential to improve mechanical strength to make them fit for use in apparel or technical textile applications. All elements of a supply chain are in place for their production: there is a guaranteed supply of material from centralized locations and these materials are inexpensive and consistent in quality. Once produced, fiber can be processed on conventional textile equipment and use conventional dyes, thus moving into the existing textile distribution chain.

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