Regenerated casein-nanocellulose composite fibers via wet spinning

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Regenerated casein-nanocellulose composite fibers via wet spinning Oleksandr Nechyporchuk, and Tobias Köhnke ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05136 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Sustainable Chemistry & Engineering

Regenerated Casein-Nanocellulose Composite Fibers via Wet Spinning Oleksandr Nechyporchuk,*† and Tobias Köhnke† †

RISE Research Institutes of Sweden, Materials and Production, PO Box 104, SE-431 22 Mölndal,

Sweden *e-mail: [email protected] Keywords: wet spinning, composite fibers, proteins, casein, nanocellulose, cellulose nanocrystals. Abstract Development of sustainable bio-based fibers is required to displace their fossil-based counterparts, e.g. in textile, non-woven or composite applications. Regenerated protein fibers have a potential in this regard if their mechanical properties are improved. Herein, we study for the first time the use of nanocellulose as reinforcement in regenerated protein fibers produced using wet spinning. The influence of cellulose nanocrystals (CNC) incorporated into regenerated casein fibers is examined in terms of mechanical and morphological properties. The influence of different conditions for fiber chemical cross-linking is also investigated. Incorporation of CNC (up to 37.5 wt%) into spin dopes results in continuous increase of fiber Young’s modulus (up to two-fold) in dry state. Both maximum and breaking tenacity of dry fibers are enhanced by CNC, with a maximum at 7.0–10.5 wt% of CNC. When testing after being wetted, both breaking tenacity and Young’s modulus of the composite fibers decrease, likely due to weakening of hydrogen bonds between CNC in the presence of water. We also demonstrate that the presence of salt during chemical cross-linking is crucial to produce intact and separated fibers in the yarn.

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Introduction Today, our society is largely dependent on synthetic fossil-based fibers (63%),1-2 which have a detrimental effect on the environment, e.g., climate change and microplastic pollution of marine ecosystems, and should be replaced by more attractive options.3 Proteins are abundant, renewable and biodegradable materials with a long history of use in different fibrous products.4 Silk and wool are composed of keratin and are the most known representatives of natural protein fibers. However, manmade protein fibers can be made from other sources, e.g. casein, soy protein, peanut protein and zein. Those fibers have been commercially produced in 1930-50s,5-6 but have been discontinued and almost forgotten due to emergence of inexpensive synthetic fibers. Today, proteins become of interest again for fiber development due to their renewable character, abundance and the possibility to valorize them from wastes and by-products of food industries. Approximately 1.3 billion tons of edible food waste is created yearly, which is equivalent to one-third of the whole food produced globally for human consumption.7-8 Wastes from dairy production account for 20%,8 which is a valuable source of proteins, such as casein. Moreover, casein is considered as one of the major allergens in human nutrition present in milk.9 Therefore, attempts have been made to extract casein from milk and to use it in various applications other than food. Regenerated casein fibers for use in textiles were introduced in the early twentieth century by Todtenhaupt,10 positioned as “artificial silk”. Those fibers did not find considerable interest until 1930s when Ferretti developed a special process for casein extraction from milk,11 which rendered fibers soft, detached from each other in the yarn and thus more appealing for textile application. Since then, manmade casein fibers have been produced under trademarks Lanital and Merinova in Italy, Fibrolane in England, Aralac in USA and others.6 However, one of the major issues with regenerated casein fibers was their low strength, especially in wet state. For instance, for Fibrolane dry breaking tenacity of 9.7 cN tex−1 (breaking strength of 121 MPa, assuming that the density of casein is 1.25 g cm−3)12 at 2


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elongation at break of 63% and dry stiffness of 354 cN tex−1 have been reported, whereas wet breaking tenacity was 3.1 cN tex−1 (breaking strength of 39 MPa) at elongation at break of 60% and wet stiffness reached only 17.7 cN tex−1.6 In order to improve strength of regenerated protein fibers, cross-linking with formaldehyde has been a straightforward post-treatment for a long time.4-5 However, the provided strength was still low compared to competing bio-based textile fibers on the market, such as viscose. The inclusion of nanoparticles has been shown to reinforce the regenerated proteins.13-15 However, those composites have been mostly developed as films. The properties of composite fibers are expected to differ, since uniaxial alignment of both protein macromolecules and reinforcing nanoparticles usually occurs during fiber spinning and drying. Nanocellulose, namely cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC), have been proposed lately as promising bio-based reinforcement in composites.16-18 CNF represent flexible highaspect ratio nano-objects with a diameter of 5–20 nm and a length of 1–5 µm that consist of alternate crystalline and amorphous regions, whereas CNC are highly crystalline stiff rod-like particles with a diameter of 10–20 nm and a length of 100–400 µm.19-20 They can be extracted from cell walls of different plants, especially wood. Generally, nanocellulose has a low carbon footprint, is sustainable, renewable, biodegradable and nontoxic material;17 therefore, it has a huge potential to be used in regenerated protein fibers. Both CNF and CNC have been explored as reinforcement for films and aerogels of proteins, e.g., silk fibroin, soy protein, resilin and collagen.21-28 Pereda et al.23 proposed the use of CNC to reinforce regenerated casein films. In that work, introduction of 3 wt% of CNC increased a tensile strength from ca. 3.25 MPa to ca. 5.5 MPa. The rather low mechanical properties were obtained since no chemical cross-linking was performed. When applied to fabrication of nanocellulose-reinforced protein fibers, only one study has been found by Cudjoe et al.,29 who used CNC to reinforce collagen fibers produced by so-called coelectrocompaction and reported an increase 3



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of the tensile strength from ca. 30 to ca. 40 MPa by adding 5 wt% CNC. Studies on solution spinning methods, e.g., wet spinning or dry-jet-wet spinning, to produce nanocellulose-reinforced protein fibers remain unknown for the authors of this paper. On the other hand, nanocellulose has been recently proposed as a material for the development of man-made fibers.30-33 In this work, we aim at investigating the effect of CNC introduction into regenerated casein fibers, as well as different cross-linking conditions, on the fiber mechanical properties. Rheological properties of casein solutions with and without CNC are first investigated to design the wet spinning process. Then, the fibers are fabricated and studied in terms of their morphology and mechanical properties both in dry and wet state.

Experimental Section Materials. Casein from bovine milk (96 wt% dry powder, lactic acid precipitated New Zealand casein extracted with ethyl alcohol), formaldehyde solution (ACS reagent, 37 wt. % in H2O, containing 10-15% methanol as stabilizer), sodium sulfate (ACS reagent, ≥99.0%), sulfuric acid (ACS reagent, 95.0–98.0%) and polyethylene glycol sorbitan monostearate (TWEEN® 60) were purchased from Merck AB, Sweden. CNC (98 wt% dry powder, freeze dried, CNC particle width of 5–20 nm, a length of 150–200 nm, 1 wt% sulfur on dry CNC sodium form) was obtained from US Forest Service’s Cellulose Nanomaterials Pilot Plant at the Forest Products Laboratory (FPL), Madison, Wisconsin, USA. Dope formulation. Casein solutions with and without CNC were prepared as specified in Table 1. Nanocellulose suspension was prepared first by suspending a given amount of dry CNC in 70 g of water and sonicating for 2 min using an Ultrasonic Homogenizer 4710 (Cole-Parmer Instruments Co., USA) at a duty cycle of 50% and an output energy of 10 under magnetic stirring. After each 20 s of 4


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homogenization the suspension was manually shaken in a bottle to facilitate the dispersion. 70 g of water or nanocellulose suspension was premixed under mechanical stirring with 20 g of casein for 30 min. Then 10 g of 10 wt% NaOH was added and the dissolution was carried out under mechanical stirring for 3 h at 20 °C, except 25.0%CNC and 37.5%CNC samples, which were heated at 60 °C in order to reduce viscosity. All the samples were stored overnight at the same temperature as used for dissolution to let air bubbles diffuse in the solution. Table 1. Recipes of dope formulations based on casein and CNC.

Sample Composition Casein, g NaOH, g H2O, g CNC, g Total, g

0%CNC

3.5%CNC

7.0%CNC

20.0 1.0 79.0 0 100.0

20.0 1.0 78.3 0.7 100.0

20.0 1.0 77.6 1.4 100.0

10.5%CNC 14.0%CNC 25.0%CNC 37.5%CNC 20.0 1.0 76.9 2.1 100.0

20.0 1.0 76.2 2.8 100.0

20.0 1.0 74.0 5.0 100.0

20.0 1.0 71.5 7.5 100.0

Rheological measurements. Samples were studied using a stress-controlled rheometer Nova (Rheologica Instruments AB, Sweden), equipped with a concentric cylinder geometry with a diameter of inner and outer cylinders of 25 mm and 27 mm, respectively. Samples were equilibrated for 10 min after loading to minimize the shear history. In order to determine the linear viscoelastic regimes, stress sweeps where performed in the range from 10−4 to 102 Pa at a frequency of 1 Hz. Then, frequency sweeps in the range from 10−2 to 102 Hz were executed in the linear viscoelastic regimes in order to determine a complex viscosity and a phase angle. To study the influence of temperature on the rheological properties, the samples were analyzed under oscillatory frequency of 1 Hz in the linear viscoelastic region in the range of 20–60 °C with a step of 2°C and an equilibration time of 1 min at each temperature. Two heating-cooling cycles were performed for each sample. Three independent measurements were carried out for each sample to ensure reproducibility, and one representative data was selected and plotted. 5



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Wet Spinning. The fibers were produced using wet-spinning equipment consisting of a spin pump, coagulation bath and take-up rollers (Orbit Controls Ltd., Southampton, UK), see Figure 1. The dopes were passed through a filter of 15 µm to remove any possible microscopic residues, extruded through a multifilament spinneret (178 holes, a diameter of 60 µm, l/d ratio of 1, Sossna GmbH, Germany) and coagulated in an aqueous bath containing 10 wt% sulfuric acid and 15 wt% sodium sulfate at room temperature. The dopes containing 0–14.0 wt% CNC based on the amount of casein were spun at 20 °C. The dopes at 25.0–37.5 wt% of CNC were spun at 60 °C in order to reduce the viscosity, which was previously ascertained by rheology measurements. The fibers were recovered on take-up rollers with the same velocity as extrusion (draw ratio 1). If other not mentioned in the text, the rollers were then transferred into a hardening bath of 500 mL containing 5 wt% formaldehyde solution prepared by dilution of the stock 37 wt% solution by deionized water. The hardening bath contained sodium sulfate at 15 wt% if other not mentioned. After 1 h the rollers were removed and immersed into a washing bath containing 1.5 wt% aqueous solution of fabric softener (Neutral®, Unilever, Denmark) for another 1 h. The fibers were then dried on the rollers at ambient conditions.

Figure 1. Schematic illustration of the process for fabrication of regenerated casein-nanocellulose composite fibers via wet spinning.

Linear density and mechanical testing. Linear mass density and mechanical properties of the fibers were measured using Vibroskop and Vibrodyn devices (Lenzing Instruments GmbH & Co. KG, 6


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Austria). For measuring fiber properties in dry state, the fibers were kept overnight at a temperature of 20 °C and a relative humidity of 65 %. For the measurements after being wetted, the fibers were immersed into 0.1 wt% aqueous solution of nonionic surfactant (TWEEN® 60) for 2 min. The mechanical properties were measured at a rate of elongation of 20 mm/min and a gauge length of 20 mm. Vibrodyn was used in a manual regime allowing to continue the measurements after reaching a maximum tenacity in order to measure a breaking tenacity. 9 separate measurements were performed for each sample. Scanning Electron Microscopy (SEM). Morphology of the spun fibers was analyzed using lowvacuum scanning electron microscope JSM-6610LV (JEOL, Tokyo, Japan) at an acceleration voltage of 5 kV and a pressure of 30 Pa. To prepare the samples, the fiber yarns were promptly cut with a fresh razor blade hit by a hammer. Optical Microscopy. Nikon Eclipse Ci-POL optical microscope (Nikon Instruments Co., Ltd., Tokyo, Japan) equipped with a Nikon TV-lens (C-0.38x) digital camera was used. The samples were measured under bright-field illumination and cross-polarized light (polarizers located at 90° to each other) at an angle of 45° to the fiber axis. The degree of orientation in the spun fibers was evaluated by comparing the fiber birefringence, measured using the microscope equipped with a Berek compensator (No. 11055, Nichika Inc., Japan). The birefringence (Δn) was calculated by dividing the polarized light retardation (Γ) by the fiber thickness (t):

∆n =

Γ t

(1)

7



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Results and Discussion In order to produce regenerated casein fibers appropriate for textile application, a special grade of casein is required, which is produced from milk by precipitation using an excess of acid.11 Such extraction process endows the spun fibers specific strength and softness.5 A number of works on optimization of the fiber spinning process suggest different ways to improve the fiber strength, e.g., by using a first hardening bath with low concentration of formaldehyde, a stretching bath, and a second hardening bath with higher concentration of formaldehyde.34 Different additives in the coagulation bath, e.g. aluminum salts,34 or sodium chloride at a very low pH,35 have been proposed, as well as increasing the temperature of the coagulation bath to 50 °C.11 Additionally, heat treatment of fibers hardened in formaldehyde further enhances the fiber strength by forming more stable bonds.36 In this proof of concept study, we aim at verifying the reinforcing potential of nanocellulose in regenerated casein fibers. Therefore, a simplified fiber spinning process was employed and regular grades of casein were used. In this work, fibers were fabricated by dissolution of casein in sodium hydroxide solutions with and without incorporated CNC, wet spinning into acid and salt, followed by one stage hardening in formaldehyde solution in the presence of salt, washing in a softener and drying fastened under constant length. Rheology. To design the fiber spinning process, rheological properties of the dopes with and without incorporated CNC were first studied. Figure 2a and b show an evolution of complex viscosity and a phase angle, respectively, with an increasing content of CNC. Pure casein solution has the lowest complex viscosity among all the samples. Slight decrease of the complex viscosity is observed in the given frequency range, which is in agreement with previous studies reporting dynamic moduli, from which the complex viscosity can be calculated.37-38 The high values of a phase angle reflect that the 8


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solution has predominantly viscous behavior. When CNC is introduced into the system, the complex viscosity increases in all the frequency range, accompanied by the reduction of the phase angle. This illustrates that elastic contribution in the complex viscosity of the dope increases. Apparently, the elasticity originates from the CNC network39-41 formed in the casein solution. At high CNC content, the complex viscosity decreases more dramatically with an increase of the frequency , whereas the phase angle increases. This illustrates the breakdown of CNC network at high frequencies. Thus, the dope of protein solution with incorporated CNC that remains undissolved is designed. At high concentrations of CNC, the complex viscosity becomes extremely high and not favorable for wet spinning; therefore, methods to decrease it were searched.

Figure 2. Rheological properties of casein solutions: (a) complex viscosity depending on incorporation of CNC; (b) phase angle depending on incorporation of CNC; (c, d) complex viscosity as a function of temperature.

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Figure 2c shows that increasing temperature from 20 °C to 60 °C can reduce the complex viscosity of casein solution. Similar observations have been reported before.42 Two sequential heating-cooling profiles were performed to check that the dopes do not undergo any thermal degradation, at least in the time scale of few hours that was used for the experiment. Figure 2c demonstrates that no considerable variations upon cooling-heating-cooling runs are reflected on the complex viscosity. Similar behavior is observed for all dopes with incorporated CNC, and the one with 7 wt% CNC is shown in Figure 2c. However, the decrease of the complex viscosity becomes not as dramatic as for the pure casein solution. Dependence of the complex viscosity on the first cooling run for all the samples is shown in Figure 2d. When higher concentrations of CNC are used in the dope, lower decrease of complex viscosity is observed compared to the pure casein solution, likely due to the formed nanocellulose network that is not affected by temperature variations. It is known that viscoelastic properties of pure nanocellulose suspensions remain unchanged in this range of temperature.43-44 Based on the rheology data wet spinning process has been designed. The dopes with 0–14.0 wt% of CNC were spun at 20 °C, whereas the dopes with 25.0–37.5 wt% of CNC were extruded at a temperature of 60 °C (see Figure 1) into the coagulation bath at 20 °C. To avoid dope cooling in the tubing, the dopes with 25.0–37.5 wt% of CNC were extruded from top to bottom through a spinneret that was touching the surface of the coagulation bath. Fiber morphology. Figure 3a–c shows the morphology of cross sections of pure casein fibers produced using different hardening conditions. The fibers produced involving hardening stage in aqueous solution of 5 wt% formaldehyde and 15 wt% sodium sulfate are demonstrated in Figure 3a. The fibers are distinct and have uniform round cross sections. It is seen that one part of the fiber cross section has a clear smooth surface, likely resulting from cutting by a razor blade, whereas the other part is rough and likely originates from the subsequent fracturing that follows the cutting. Figure 3b shows that the fibers lose their uniformity of the cross section and become more sticky to each other 10


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when hardening in formaldehyde without salt. Hardening with formaldehyde is well known in the field of fiber spinning of regenerated proteins,5 providing covalent intermolecular cross-links necessary for fiber wet and dry strength. It has been previously reported that the presence of aluminum salts enhance fiber strength due to ionic cross-linking of carboxyl groups, e.g., due to the presence of high amount of glutamic acid in casein.36 Herein, we show that the presence of sodium sulfate, that yield monovalent (Na+) cations, is crucial on fabrication of distinct fibers with uniform round cross sections. It is believed that the salt creates osmotic pressure that facilitates fiber dehydration and thus results in better fiber forming properties. Figure 3c shows that the fibers become totally distorted when no hardening stage is used, resulting in a bulky material of merged fibers. Thus, all the subsequent samples were produced using hardening in formaldehyde and salt.

Figure 3. SEM images of pure casein fiber yarns: (a) hardened in a formaldehyde solution with salt; (b) hardened in a formaldehyde solution without salt; and (c) fabricated without hardening bath. SEM images of casein-CNC composite fiber yarns: (d) with incorporated 14 wt% CNC and (e) 37.5 wt% CNC with respect to casein. The composite fibers were hardened in a formaldehyde solution with salt.

SEM images of composite fibers containing 14 wt% and 37.5 wt% of CNC with a respect to casein are shown in Figure 3d and Figure 3e, respectively. It can be seen that compared to the pure fibers, the composite ones with 14 wt% of CNC have larger diameter and slightly less uniform cross sections. 11



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The variation of fiber diameter within the yarn is also slightly higher. This variation becomes even larger for the fibers spun with 37.5 wt% of CNC. This is also reflected on fiber titer and the standard deviation (see Table 2), which both increase progressively with increasing CNC concentration. It is assumed that the fiber becomes irregular in diameter due to viscosity-induced difficulties in CNC redispersion from dry powder and its homogeneous distribution within casein solution. Mechanical properties. The fibers were characterized in terms of mechanical properties both in dry and wet state. Figure 4 shows the corresponding difference in tenacity for pure casein fibers. When measuring in dry state, the fibers exhibit a maximum tenacity (4.85 ± 0.13 cN tex−1) at low elongation (4.58 ± 0.49 %) followed by a wide plateau before the failure finally occurs (at 86.1 ± 6.8%). It should be highlighted that the tenacity-elongation profile for these fibers differs rather significantly from those of common textile fibers that exhibit continuous increase of tenacity, reaching a breaking point (maximum), which sometimes is followed by small decrease that corresponds to rupture (failure), as described in ISO 5079:1995.45 Respectively, the terms of a breaking tenacity and a tenacity at rupture are used in that standard. In our case, regenerated casein fibers have features typical for some thermoplastics, exhibiting pronounced maximum (yield) point at low elongation. Therefore, in this work we refer to this value as a maximum tenacity, whereas a tenacity before the failure to as a breaking tenacity, see Figure 4. Introduction of such a terminology in this paper is also motivated by the fact that in the available literature for regenerated casein fibers, a single value of tenacity is often reported without the corresponding tenacity-elongation curves.46-47 Therefore, it is not straightforward to interpret the data. Figure 4 also shows the mechanical properties of regenerated casein fibers measured in wet state, or to be more precise, after being wetted. The fibers exhibit continuous strain hardening in a large range of elongation, which may partially occur due to progressive drying of the fibers. We note that these measurements have been performed according to a standard procedure described in ISO 5079:1995. 12


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When comparing the maximum and breaking tenacity with other reported values, they are in the range of 4–5 cN tex−1, which is in agreement with some other studies that are likely to use regular grades of csein.46-47 When special grades of casein are used, as well as a complex process for fiber spinning, involving fiber drawing and optimized hardening operation, a breaking tenacity of up to 10 cN tex−1 can be achieved.6 Table 2. Properties of casein fibers depending on incorporation of CNC.

Sample Properties Titer, dtex Maximum tenacity (dry), cN tex−1 Elongation at max. tenacity (dry), % Breaking tenacity (dry), cN tex−1 Elongation at break (dry), % Young’s modulus (dry), cN tex−1 Breaking tenacity (wet), cN tex−1 Elongation at break (wet), % Young’s modulus (wet), cN tex−1

0%CNC

3.5%CNC

7.0%CNC

10.5%CNC

14.0%CNC

25.0%CNC

37.5%CNC

7.70 ± 0.14

7.83 ± 0.15

8.07 ± 0.21

8.30 ± 0.49

9.78 ± 0.44

13.96 ± 4.00 18.70 ± 5.59

5.04 ± 0.16

5.72 ± 0.14

5.72 ± 0.10

5.84 ± 0.16

5.69 ± 0.08

5.05 ± 0.49

4.11 ± 0.43

4.58 ± 0.49

4.39 ± 0.34

3.64 ± 0.26

3.56 ± 0.44

3.36 ± 0.52

2.23 ± 0.44

1.81 ± 0.46

3.96 ± 0.26

4.76 ± 0.93

5.02 ± 0.69

4.96 ± 1.23

4.87 ± 1.47

4.30 ± 1.18

3.66 ± 0.74

86.1 ± 6.8

5.24 ± 1.31

4.49 ± 1.10

3.93 ± 0.58

3.70 ± 0.7

2.44 ± 0.54

2.28 ± 0.97

226.6 ±10.2 260.5 ± 17.9 274.0 ± 18.4 304.2 ± 14.8 322.1 ± 37.6 365.2 ± 43.0 406.1 ± 86.6 4.43 ± 0.25

4.46 ± 0.41

3.74 ± 0.65

3.69 ± 0.72

3.59 ± 0.92

2.21 ± 0.43

1.42± 0.69

95.3 ± 10.4

54.5 ± 17.1

45.6 ± 18.2

29.3 ± 16.2

24.6 ± 13.2

17.2 ± 4.7

11.4 ± 2.4

88.0 ± 19.9

79.6 ± 23.6

72.7 ± 24.3

60.7 ± 18.4

47.9 ± 16.6

15.9 ± 8.9

13.5 ± 8.5

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Figure 4. Tensile curves of pure casein fibers in dry and wetted state.

Mechanical properties of casein-CNC composite fibers in dry state are demonstrated in Figure 5a, and are summarized in Table 2. Incorporation of CNC results in continuous increase of the Young’s modulus. Enhancement of both maximum and breaking tenacity till some critical point is also observed, that is accompanied by the reduction of the corresponding values of elongation. The maximum tenacity of 5.84 ± 0.16 cN tex−1 at 10.5% of CNC content was achieved. The reinforcement is believed to be related to the high interparticle hydrogen bonding and homogeneous distribution of CNC in the matrix.48 Based on an average fiber diameter of 30.2 µm that was measured using SEM, a tensile strength of 67.7 MPa can be calculated. It is interesting to compare this value with the mechanical properties of pure CNC films and fibers in order to understand weather the full potential for the reinforcement is achieved. It is known that pure films of CNC have Young’s modulus of 15– 50 GPa and a tensile strength of 40–70 MPa, depending on the cellulose source and the production conditions.49-50 It has been also reported that by enhancing the CNC orientation in films in axial direction to the tensile testing, which is likely to occur in spun fibers, a Young’s modulus could be increased, whereas no distinct conclusion about the change of tensile strength could be made.50 14


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Therefore, a tensile strength of 67.7 MPa achieved in this work is considered as rather high. It has been also demonstrated that pure films of CNC and silk fibroin exhibited a tensile strength of ca. 40 MPa and 30 MPa, respectively. When combined in a composite, an increase to ca. 160 MPa has been observed when using 80 wt% of CNC, thus exhibiting a synergistic effect.21 However, in that work an increase of the mechanical properties was attributed to the conformational change of fibroin from a random coil to a β structure induced by CNC, and not due to the reinforcing behavior of the CNC in the matrix. Previous studies indicated that after reaching some critical level of CNC concentration in the composite, both strength and stiffness decrease.25, 27, 29 This phenomenon was explained by higher viscosity of the composite suspensions, as well as higher excluded volume, at higher content of CNC, which impeded rotation and alignment of the nanoparticles in the suspension.29, 51 It was also suggested that at high content of CNC some disturbances in the microstructure of composites may result in lower mechanical performace.25

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Figure 5. Tensile curves of casein fibers with various amount of incorporated CNC: (a) in dry state and (b) after wetting.

Figure 5b shows mechanical properties of the fibers after being wetted. Young’s modulus for the composite fibers decreases progressively with increasing concentration of CNC on the contrary to the behavior in the dry state. This can be explained by loosened hydrogen bonding between CNC network in wet state and, therefore, lower composite stiffness. It is known that formaldehyde acts as a crosslinker also for cellulose, additionally to casein. However, cross-linking efficiency for cellulose is too low or negligible if no curing at elevated temperature has been performed.52 Therefore, no covalent bonds are anticipated between the CNC. With increased CNC content, both breaking tenacity and elongation at break decrease, as expected. Fiber microstructure and degree of orientation. Optical microscopy of the fibers was performed to have some insight about the fiber microstructure. Figure 6a–c show that a neat casein fiber has more even structure compared to its composite counterparts. The fiber with 37.5 wt% of CNC has sequential 16


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brighter and darker domains observed under crossed polarizers, indicating different level of alignment along the fiber axis. It is assumed that such fiber inhomogeneities may occur due to inferior dispersion of CNC in the dope due to high viscosity issues at such high CNC content. We also note that the dopes with 25 wt% and 37.5 wt% of CNC were conditioned and wet spun at higher temperature compared to the other samples, which may also result in differences of fiber microstructure. The fiber with 14 wt% of CNC does not have such evident inhomogeneities, which is in line with the high strength of these fibers.

Figure 6. Optical microscopy images (a–c) captured under bright-field illumination (left) and cross-polarized light (right) for: (a) pure casein fibers; (b) composite fibers with incorporated 14.0 wt% of CNC and (c) composite fibers with incorporated 37.5 wt% of CNC with respect to casein. Cross-polarized optical microscopy images of: (d) pure casein solution, and the solutions with incorporated (e) 14.0 wt% of CNC and (f) 37.5 wt% of CNC.

The dopes before being processed into fibers were analyzed using polarized optical microscopy in order to verify their possible influence on the fiber spinning and their ensuing microstructure. Figure 6d and e show no aggregation or inhomogeneities of the pure casein solution and the solution with incorporated 14.0 wt% of CNC, respectively. However, Figure 6f demonstrates that with 37.5 wt% of

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incorporated CNC, the dope is heterogeneous and contains some aggregates, indicated by bright regions in the figure. The presence of these inhomogeneities is in line with those observed in the spun fibers, which was also reflected by lower mechanical properties of these fibers. Mechanical properties of fibers are usually associated with their degree of orientation, i.e., order parameter. In this case, the values of fiber birefringence derived from polarized optical microscopy can be used to compare the degree of orientation. Table 3 indicates the progressive increase of the birefringence upon addition of CNC. The maximum values of birefringence are achieved for the composite fibers with the highest amount of CNC used, even though some inhomogeneities in the fiber are present, as shown before. This increase is likely to occur due to incorporation of highly crystalline CNC that becomes aligned during fiber extrusion through the spinneret and upon fiber drying. Table 3. Birefringence of pure casein and casein-CNC composite fibers.

Sample 0%CNC Birefringence, nm µm−1 2.08 ± 0.02

3.5%CNC 3.56 ± 0.02

14.0%CNC 5.19 ± 0.03

37.5%CNC 6.78 ± 0.06

Conclusion This work investigates for the first time the use of nanocellulose as mechanical reinforcement of regenerated protein fibers produced using wet spinning process. Dopes for fiber wet spinning should be rather high in solids content to achieve good fiber forming. Therefore, they cannot be diluted to allow good dispersibility of CNC that are known to form viscous gels at very low solids content. Hereby, we show that controlled incorporation of CNC into casein solutions can yield proper dopes where CNC remains non-dissolved. The dopes can be subsequently processed into fibers assembled in a yarn. We show that the presence of salt during cross-linking operation is important to produce intact separated fibers in the yarn. By introducing CNC, the stiffness of the composite fibers is enhanced in dry state. The fiber strength is also increased up to some critical level of CNC concentration. Thus, the 18


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potential of using such an approach for fiber reinforcement is confirmed. It is anticipated, that the further increase of fiber mechanical properties may be achieved by drawing and thermal curing of spun fibers. For instance, thermal curing may enhance the wet strength of the fibers presumably due to cross-linking of the CNC. Optimization of the fiber spinning process should be targeted by further studies.

Author information Corresponding Author *e-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgements This work has been funded by RISE, Division Materials and Production (former Swerea IVF AB). The authors gratefully acknowledge Melina da Silva from RISE for the support with SEM measurements.

References (1) The price of fast fashion. Nature Climate Change 2018, 8 (1), 1, DOI: 10.1038/s41558-017-00589. (2) The Fiber Year 2018. World Survey on Textiles & Nonwovens; 2018. (3) Geyer, R.; Jambeck, J. R.; Law, K. L. Production, use, and fate of all plastics ever made. Science Advances 2017, 3 (7), e1700782, DOI: 10.1126/sciadv.1700782. (4) Hearle, J. W. S. Protein fibers: structural mechanics and future opportunities. Journal of Materials Science 2007, 42 (19), 8010-8019, DOI: 10.1007/s10853-006-1280-4. (5) Brooks, M. M. Regenerated protein fibres: a preliminary review. In Handbook of Textile Fibre Structure; Woodhead Publishing: 2009; pp 234-265. (6) Poole, A. J.; Church, J. S.; Huson, M. G. Environmentally Sustainable Fibers from Regenerated Protein. Biomacromolecules 2009, 10 (1), 1-8, DOI: 10.1021/bm8010648. (7) Yu, L. J.; Brooks, M. S.-L. Food Industry Protein By-Products and Their Applications. In Protein Byproducts; Dhillon, G. S., Ed.; Academic Press: 2016; pp 120-132. (8) Key facts on food loss and waste you should know! http://www.fao.org/savefood/resources/keyfindings/en/ 2018. 19



Page 20 of 23

(9) Haug, A.; Høstmark, A. T.; Harstad, O. M. Bovine milk in human nutrition – a review. Lipids in Health and Disease 2007, 6, 25, DOI: 10.1186/1476-511X-6-25. (10) Todtenhaupt, F. Production of artificial silk and artificial hair from casein. US836788A, 1906-1127, 1906. (11) Ferretti, A. Improvements in or relating to the manufacture of artificial textile fibres. GB483731 (A), 1938-04-21, 1938. (12) Ellis, B.; Smith, R. Polymers: A Property Database, Second Edition, CRC Press: 2008; p 1210. (13) Chen, P.; Zhang, L. Interaction and Properties of Highly Exfoliated Soy Protein/Montmorillonite Nanocomposites. Biomacromolecules 2006, 7 (6), 1700-1706, DOI: 10.1021/bm050924k. (14) Yu, J.; Cui, G.; Wei, M.; Huang, J. Facile exfoliation of rectorite nanoplatelets in soy protein matrix and reinforced bionanocomposites thereof. Journal of Applied Polymer Science 2007, 104 (5), 3367-3377, DOI: 10.1002/app.25969. (15) Zheng, H.; Ai, F.; Wei, M.; Huang, J.; Chang, P. R. Thermoplastic Soy Protein Nanocomposites Reinforced by Carbon Nanotubes. Macromolecular Materials and Engineering 2007, 292 (6), 780-788, DOI: 10.1002/mame.200700052. (16) Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A New Family of Nature-Based Materials. Angewandte Chemie International Edition 2011, 50 (24), 5438–5466, DOI: 10.1002/anie.201001273. (17) Dufresne, A. Cellulose nanomaterial reinforced polymer nanocomposites. Current Opinion in Colloid & Interface Science 2017, 29, 1-8, DOI: 10.1016/j.cocis.2017.01.004. (18) Nechyporchuk, O.; Pignon, F.; Botelho Do Rego, A. M.; Belgacem, M. N. Influence of ionic interactions between nanofibrillated cellulose and latex on the ensuing composite properties. Composites Part B: Engineering 2016, 85, 188-195, DOI: 10.1016/j.compositesb.2015.09.030. (19) Nechyporchuk, O.; Belgacem, M. N.; Bras, J. Production of cellulose nanofibrils: A review of recent advances. Industrial Crops and Products 2016, 93, 2-25, DOI: 10.1016/j.indcrop.2016.02.016. (20) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose nanocrystals: chemistry, self-assembly, and applications. Chemical reviews 2010, 110 (6), 3479-3500. (21) Noishiki, Y.; Nishiyama, Y.; Wada, M.; Kuga, S.; Magoshi, J. Mechanical properties of silk fibroin–microcrystalline cellulose composite films. Journal of Applied Polymer Science 2002, 86 (13), 3425-3429, DOI: 10.1002/app.11370. (22) Wang, Y.; Cao, X.; Zhang, L. Effects of Cellulose Whiskers on Properties of Soy Protein Thermoplastics. Macromolecular Bioscience 2006, 6 (7), 524-531, DOI: 10.1002/mabi.200600034. (23) Pereda, M.; Amica, G.; Rácz, I.; Marcovich, N. E. Structure and properties of nanocomposite films based on sodium caseinate and nanocellulose fibers. Journal of Food Engineering 2011, 103 (1), 76-83, DOI: 10.1016/j.jfoodeng.2010.10.001. (24) Arboleda, J. C.; Hughes, M.; Lucia, L. A.; Laine, J.; Ekman, K.; Rojas, O. J. Soy protein– nanocellulose composite aerogels. Cellulose 2013, 20 (5), 2417-2426, DOI: 10.1007/s10570-0139993-4. (25) Li, W.; Guo, R.; Lan, Y.; Zhang, Y.; Xue, W.; Zhang, Y. Preparation and properties of cellulose nanocrystals reinforced collagen composite films. Journal of Biomedical Materials Research Part A 2014, 102 (4), 1131-1139, DOI: 10.1002/jbm.a.34792. (26) Rivkin, A.; Abitbol, T.; Nevo, Y.; Verker, R.; Lapidot, S.; Komarov, A.; Veldhuis, S. C.; Zilberman, G.; Reches, M.; Cranston, E. D.; Shoseyov, O. Bionanocomposite Films from ResilinCBD Bound to Cellulose Nanocrystals. Industrial Biotechnology 2015, 11 (1), 44-58, DOI: 10.1089/ind.2014.0026.

20


Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Qazanfarzadeh, Z.; Kadivar, M. Properties of whey protein isolate nanocomposite films reinforced with nanocellulose isolated from oat husk. International Journal of Biological Macromolecules 2016, 91, 1134-1140, DOI: 10.1016/j.ijbiomac.2016.06.077. (28) Weishaupt, R.; Siqueira, G.; Schubert, M.; Kämpf, M. M.; Zimmermann, T.; Maniura-Weber, K.; Faccio, G. A Protein-Nanocellulose Paper for Sensing Copper Ions at the Nano- to Micromolar Level. Advanced Functional Materials 2017, 27 (4). (29) Cudjoe, E.; Younesi, M.; Cudjoe, E.; Akkus, O.; Rowan, S. J. Synthesis and Fabrication of Nanocomposite Fibers of Collagen-Cellulose Nanocrystals by Coelectrocompaction. Biomacromolecules 2017, 18 (4), 1259-1267, DOI: 10.1021/acs.biomac.7b00005. (30) Clemons, C. Nanocellulose in Spun Continuous Fibers: A Review and Future Outlook. Journal of Renewable Materials 2016, 4 (5), 327-339, DOI: 10.7569/JRM.2016.634112. (31) Lundahl, M. J.; Klar, V.; Wang, L.; Ago, M.; Rojas, O. J. Spinning of cellulose nanofibrils into filaments: a review. Industrial & Engineering Chemistry Research 2017, 56 (1), 8-19, DOI: 10.1021/acs.iecr.6b04010. (32) Nechyporchuk, O.; Bordes, R.; Köhnke, T. Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by Interfacial Complexation of Cellulose Nanofibrils with Silica Nanoparticles. ACS Applied Materials & Interfaces 2017, 9 (44), 39069–39077, DOI: 10.1021/acsami.7b13466. (33) Nechyporchuk, O.; Köhnke, T. Continuous Assembly of Cellulose Nanofibrils and Nanocrystals into Strong Macrofibers through Microfluidic Spinning. Advanced Materials Technologies 2018, DOI: 10.1002/admt.201800557. (34) Wormell, R. L. Improvements in the manufacture and production of artificial filaments, threads and the like. GB502710 (A), 1939-03-23, 1939. (35) Kendall Thompson, R. H.; Traill, D. Improvements in or relating to the production of artificial insolubilised wet-spun protein filaments. GB580434 (A), 1946-09-06, 1946. (36) Diamond, C.; Wormmell, R. L. The Manufacture and Properties of “Casein Fibre”. Journal of the Textile Institute Proceedings 1939, 30 (7), 224-228, DOI: 10.1080/19447013908687492. (37) Lucey, J. A. Formation and Physical Properties of Milk Protein Gels. Journal of Dairy Science 2002, 85 (2), 281-294, DOI: 10.3168/jds.S0022-0302(02)74078-2. (38) Madadlou, A.; Emam-Djomeh, Z.; Mousavi, M. E.; Mohamadifar, M.; Ehsani, M. Acid-induced gelation behavior of sonicated casein solutions. Ultrasonics Sonochemistry 2010, 17 (1), 153-158, DOI: 10.1016/j.ultsonch.2009.06.009. (39) Ureña-Benavides, E. E.; Ao, G.; Davis, V. A.; Kitchens, C. L. Rheology and Phase Behavior of Lyotropic Cellulose Nanocrystal Suspensions. Macromolecules 2011, 44 (22), 8990-8998, DOI: 10.1021/ma201649f. (40) Lu, A.; Hemraz, U.; Khalili, Z.; Boluk, Y. Unique viscoelastic behaviors of colloidal nanocrystalline cellulose aqueous suspensions. Cellulose 2014, 21 (3), 1239-1250, DOI: 10.1007/s10570-014-0173-y. (41) Li, M.-C.; Wu, Q.; Song, K.; Lee, S.; Qing, Y.; Wu, Y. Cellulose Nanoparticles: Structure– Morphology–Rheology Relationships. ACS Sustainable Chemistry & Engineering 2015, 3 (5), 821-832, DOI: 10.1021/acssuschemeng.5b00144. (42) Barreto, P. L. M.; Roeder, J.; Crespo, J. S.; Maciel, G. R.; Terenzi, H.; Pires, A. T. N.; Soldi, V. Effect of concentration, temperature and plasticizer content on rheological properties of sodium caseinate and sodium caseinate/sorbitol solutions and glass transition of their films. Food Chemistry 2003, 82 (3), 425-431, DOI: 10.1016/s0308-8146(03)00006-2. (43) Nechyporchuk, O.; Belgacem, M. N.; Pignon, F. Current progress in rheology of cellulose nanofibril suspensions. Biomacromolecules 2016, 17 (7), 2311-2320, DOI: 10.1021/acs.biomac.6b00668.

21



Page 22 of 23

(44) Qiao, C.; Chen, G.; Zhang, J.; Yao, J. Structure and rheological properties of cellulose nanocrystals suspension. Food Hydrocolloids 2016, 55, 19-25, DOI: 10.1016/j.foodhyd.2015.11.005. (45) ISO 5079:1995 - Textile fibres -- Determination of breaking force and elongation at break of individual fibres. https://www.iso.org/standard/11101.html. (46) Yang, Y.; Reddy, N. Properties and potential medical applications of regenerated casein fibers crosslinked with citric acid. International Journal of Biological Macromolecules 2012, 51 (1), 3744, DOI: 10.1016/j.ijbiomac.2012.04.027. (47) Cui, L.; Reddy, N.; Xu, H.; Fan, X.; Yang, Y. Enzyme-modified casein fibers and their potential application in drug delivery. Fibers and Polymers 2017, 18 (5), 900-906, DOI: 10.1007/s12221017-1225-3. (48) Miao, C.; Hamad, W. Y. Cellulose reinforced polymer composites and nanocomposites: a critical review. Cellulose 2013, 20 (5), 2221-2262, DOI: 10.1007/s10570-013-0007-3. (49) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nanocomposites. Chemical Society Reviews 2011, 40 (7), 3941-3994, DOI: 10.1039/C0CS00108B. (50) Reising, A. B.; Moon, R. J.; Youngblood, J. P. Effect of particle alignment on mechanical properties of neat cellulose nanocrystal films. Journal of Science & Technology for Forest Products and Processes: Volume 2, Number 6, 2012; pp. 32-41. 2012, 2 (6), 32-41. (51) Pullawan, T.; Wilkinson, A. N.; Eichhorn, S. J. Influence of magnetic field alignment of cellulose whiskers on the mechanics of all-cellulose nanocomposites. Biomacromolecules 2012, 13 (8), 2528-36, DOI: 10.1021/bm300746r. (52) Rojas, J.; Azevedo, E. Functionalization and crosslinking of microcrystalline cellulose in aqueous media: A safe and economic approach. International Journal of Pharmaceutical Sciences Review and Research 2011, 8 (1), 28-36.

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TOC/Abstract graphic:

Synopsis: This work demonstrates the use of cellulose nanocrystals as reinforcement in regenerated casein fibers produced using wet spinning process.

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Regenerated casein-nanocellulose composite fibers via wet spinning

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