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

Table 54.1 Comparison of the properties of regenerated plant proteins and milk casein with wool [09Poo] Protein source Dry Wet Tenacity (g/den) Elongation (%) Modulus (g/den) Tenacity (g/den) Elongation (%) Modulus (g/den) Fibrolane (casein) 1.1 63 40 0.4 60 2 Ardil (peanut) 0.8-1.0 10-110 30 0.3 90 0.5 Vicara (zein) 1.0 28 50 0.6 28 15 Soybean 0.6 40 40 0.1 40 4 Wool 1.6 12-16 25 1.1 16-20 10

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

Table 54.2 Tensile properties of PVA/soy protein blend fibers after heat treatment at various temperatures [03Zha] Sample composition Breaking strength Breaking elongation Modulus (PVA/soy) (g/den) (%) (g/den) 90/10 no heating 1.2 ± 0.1 16 ± 1 25.2 ± 2.6 90/10 heated at 110 °C 1.7 ± 0.04 15 ± 1 27.8 ± 3.5 90/10 heated at 150 °C 2.1 ± 0.04 12 ± 1 37.4 ± 0.9 90/10 heated at 190 °C 2.3 ± 0.1 11 ± 1 46.1 ± 2.6 80/20 untreated 0.3 ± 0.04 73 ± 7 21.7 ± 1.7 80/20 heated at 110 °C 0.4 ± 0.06 64 ± 10 20 ± 1.7 80/20 heated at 150 °C 0.5 ± 0.05 65 ± 4 22.6 ± 2.6 80/20 heated at 190 °C 0.5 ± 0.04 57 ± 3 26.1 ± 2.6

Table 54.3 Properties of regenerated protein fibers obtained using various plant proteins Fiber Strength (g/den) Elongation (%) Modulus (g/den) Soy protein 1.3 ± 0.09 8 ± 2 56.5 ± 14.8 Wheat gluten 1.0 ± 0.06 23 ± 2.7 43.5 ± 1.7 Gliadin 1.0 ± 0.09 25 ± 3.2 36.5 ± 0.3 Zein 0.3-0.5 1.8-5.0 — Wool 1.5-2.3 30-40 37.4-56.5

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

Fig. 54.1 SEM images of the soy protein-nylon 6 fibers developed as core and shell, respectively. The core of the fibers is seen distinctly and marked in (a). The fibers obtained are porous as seen in (b). From [11Sin]. Reproduced with permission from American Chemical Society

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.

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