Article pubs.acs.org/IECR
Facile Preparation of Soy Protein/Poly(vinyl alcohol) Blend Fibers with High Mechanical Performance by Wet-Spinning Dagang Liu,*,† Changqing Zhu,† Kai Peng,† Yi Guo,‡ Peter R. Chang,§ and Xiaodong Cao*,∥ †
Department of Chemistry, Nanjing University of Information Science & Technology, Nanjing, 210044, China Department of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan, 430073, China § Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, Saskatchewan, S7N 0X2, Canada ∥ School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China ‡
ABSTRACT: Using formic acid as a cosolvent and saturated sodium sulfate as a coagulation bath, soy protein/poly(vinyl alcohol) (PVA) blend fibers were prepared using wet-spinning approaches. The structure and mechanical, thermal, and wateruptake properties of the spun fibers were investigated. Morphological analysis with polarized optical microscopy (POM) and scanning electron microscopy (SEM) showed that spun PVA or blend fibers were composed of nanoparticles and exhibited a porous morphology. Blend fibers exhibited only one glass transition temperature in differential scanning calorimetry (DSC) thermograms due to high compatibility between the two components. The best mechanical strength and thermal stability were achieved when 70% PVA was composited with soy protein. This was thought to be due to the effects of cross-linking and hydrogen bonding between functional groups of soy protein and PVA hydroxyl groups.
1. INTRODUCTION Soy protein is a natural resource obtained from soybeans through processes of dehulling, defatting, extraction, centrifugation, and precipitation,1 and is mainly composed of 7S and 11S fractions, according to their sedimentation constants.2,3 It has received considerable attention because of its advantages of being abundant, cost-effective, and environmentally friendly. Many studies have been carried out on the processing and characterization of soy protein based products such as plastic, fibers, adhesives, and films.4−6 The possibility of manufacturing textile fibers from soy protein was investigated by Kajita and Inoue and Boyer in the 1940s.7,8 After World War II, the introduction of petroleumbased fibers marginalized the commercial outlook of fibers spun from plant and animal proteins. However, in recent decades, environmental concerns and the increased price of petroleum have rekindled interest in soy fibers.9 In the earlier research, fibers spun from soy protein showed a lower tensile strength in comparison to natural protein fibers such as wool and silk, especially in the wet state. To improve tensile strength and decrease shrinkage in boiling water, a synthetic polymer with a high modulus, such as poly(vinyl alcohol) (PVA), was blended into soy protein fibers. Unfortunately, different fibers were disassociated from each other in the soy protein/PVA composites due to different swelling properties in water.9 When these components were spun into sheath−core fibers, in which the sheath was PVA and the core was soy protein, they were brittle because the core could not be drawn. Zhang et al.10 reported the results of processing and characterization of thermally denatured soy protein/PVA fibers. Much effort has been made to combine soy protein and PVA in aqueous solvent to produce soy protein isolate/PVA blend fibers;11 however, fibers made with these methods exhibit low tensile strength and some concerns also have been raised regarding these environ© 2013 American Chemical Society
mentally unfriendly processes. Li succeeded in producing hightenacity blend with soy protein (5−23%) and PVA (77−95%) to make the first industrially produced soy protein textiles in the world. In his patented work, soy protein was treated with an auxiliary agent and biological enzymes to modify the structure of globular proteins.12 The properties of soy protein fibers include a tenacity of 3.8−4.0 cN/dtex dry and 2.5−3.0 cN/dtex wet, elongation at break of 18−21% dry, and moisture regain of 8.6%. Despite its merits, the process is indeed complicated and extensive spinning is required, which leaves some room for further improvement. Conventionally, it has been thought that water is a common solvent for soy protein and PVA blend spinning instead of formic acid. Unfortunately, the wet-spin fiber, with sheath−core structure, directly from soy protein (SP)/PVA aqueous solution failed to be applied in industry. In this study, we try to modify the swell or dissolution properties of SP/PVA with a substituted solvent: formic acid. As far as we know, there have been no reported experiments carried out on dissolving soy protein/PVA in formic acid for spinning until now. In this work, we explored using formic acid as the cosolvent and adopted a facile nontoxic process for fabric preparation. The mechanical and thermal properties were then investigated to complement this novel exploration.
2. MATERIALS AND METHODS 2.1. Materials. Commercial soy protein isolate (noted as SP) was purchased from Dupont Yunmeng Protein Technology Co. Ltd. (Yunmeng, China) and used as pure protein. The weight-average molecular weight (Mw) of soy protein was Received: Revised: Accepted: Published: 6177
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Figure 1. (a−d) POM photographs and (e−l) SEM micrographs of the cross sections of PVA/soy protein blend fiber and PVA fibers. (a), (e), and (i) represent pure PVA; (b), (f), and (j) represent PVASP30; (e), (g), and (k) represent PVASP50; and (d), (h), and (l) represent PVASP80.
determined to be 2.05 × 105 g/mol using a multiangle laser light scattering instrument (MALLS, DAWNDSP, Wyatt Technology Co., USA) equipped with a He−Ne laser (λ = 632.8 nm). Poly(vinyl alcohol) (PVA) with a molecular weight range of 125 000−186 000 g/mol was obtained from National Chemical Reagent Co. Other chemical reagents of analytical grade, such as formic acid and sodium sulfate, were purchased from Lingfeng Chemical Reagent Co. Ltd. (Shanghai, China). 2.2. Wet-Spinning from SP/PVA Blend Solutions. The desired amount of SP powder was dissolved in formic acid by vigorous mechanical stirring at 60 °C for 24 h to prepare a 10% (w/v) solution. The solution was then centrifuged at 12 000 rpm for 10 min to remove insoluble slurries in a Sorvall RC-5B high-speed centrifuge (DuPont Instruments). Accordingly, a 6% (w/v) PVA solution was prepared by pouring PVA into formic acid and stirring for 8 h at ambient temperature. The obtained SP and PVA in formic acid solutions were mixed as desired weight ratios and homogenized under magnetic stirring
for 10 min. The blend solution was then transferred to a recirculating oven maintained at 40 °C, in which formic acid was volatilized and recycled to obtain a spinning dope of optimum concentration varying from 10 to 20% mg/mL for further fiber spinning. After vacuum defoaming, the spinning dope was loaded in a 10 mL plastic syringe and injected into the salt coagulation bath containing saturated sodium sulfate. The spun fibers were washed twice in distilled water, vacuum dried at 50 °C to a constant weight, and then conditioned in desiccators with silica gel desiccant before testing. The spun fibers were coded as PVASP90, PVASP70, PVASP50, PVASP30, PVASP10, and PVA, according to the weight ratio of soy protein (90, 70, 50, 30, 10, and 0, respectively). 2.3. Characterization. Fourier transform infrared spectra of the samples were obtained using a Nicolet 5700 FTIR spectrometer (Thermo Electron Co., USA) in the range 4000− 400 cm−1 with a resolution of 4 cm−1 using the liquid film method. Fiber morphology was investigated on an S-570 6178
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appeared on the cross section of pure PVA or blend fibers. As formic acid was not a good solvent for PVA, nanosize particles tended to aggregate in the form of graininess as seen in Figure 1i. When the soy protein content reached 30%, a fibrous cross-linked structure was observed (Figure 1j), indicating strong interactions between polar soy protein and PVA. It is interesting to note that appropriate soy protein contents played an important role in cross-linking because its active acidic groups could react with the hydroxyl groups of PVA to form hydrogen bonding.16 The sea-island morphology could be observed in the sample of PVASP50. When the SP content increased to 80% (Figure 1l), noncontinuous particles aggregated with severe phase separation because PVA cannot act as a plasticizer to lubricate soy protein. It is worth noting that, as the ratio of soy protein in the blend increased, the pore size was enlarged, resulting in the high fragility of the fiber as observed from the cross section of PVASP80 (Figure 1h). 3.2. Mechanical Properties. Figure 2 shows the mechanical properties of soy protein/PVA blend fibers. Pure
scanning electron microscope (SEM; Hitachi, Japan). Fiber samples were frozen in liquid nitrogen and snapped immediately. The cross section and surface were sputter-coated with gold for SEM observations with an accelerating voltage of 20 kV. The appearance of the fiber was observed using a polarized optical microscope (POM; MDA502AA E400). Fibers that were 10 cm long were used for the water uptake (WU) test. The samples were dried in a 100 °C oven for 24 h, and then conditioned for at least 50 h in a desiccator containing saturated NaCl solution with 75% relative humidity (RH) to reach equilibrium at room temperature. The fibers were taken from the desiccator at specific intervals and weighed on a fivedecimal-place analytical balance. The WU value was calculated using eq 1: WU (%) =
Mt − M0 · 100 M0
(1)
where Mt and M0 are the weight of the sample after exposure to 75% RH at time t and the initial weight of the sample at 0% RH, respectively. The tensile strength (σb) and elongation at break (εb) measurements on single fibers were carried out on a tensile tester (LLY-06, Laizhou Electronic Instrument Co., Ltd., China) according to ISO 6939-1988. The fibers were first preconditioned for 24 h at 21 °C and 65% RH. The fiber test length was 20 mm, and a speed of 20 mm/min was used. The σb and εb values represented the averages of 10 measurements which were the strongest in 50 measurements. The pre-tension was 0.3 cN, and the initial length of the fibers was 10 cm. Differential scanning calorimetry (DSC) was performed using a Diamond DSC apparatus (Perkin-Elmer Co., USA). Powder cut from fibers (about 10 mg) was placed in pressure-tight aluminum cells under nitrogen gas flow and conditioned at 100 °C for 5 min to eliminate residual moisture in the samples. The sample was then heated at a heating rate of 10 °C/min from −100 to 250 °C under nitrogen atmosphere. Thermogravimetric analysis (TGA) was carried out on a Pyris TGA linked to a Pyris diamond TA lab system (Perkin-Elmer Co. USA) at a heating rate of 10 °C/min from 25 to 500 °C under nitrogen atmosphere with each sample of 2−5 mg.
Figure 2. Tensile strength (▲) and elongation at break (■) of PVA/ soy protein blend fibers.
soy protein and its blend fiber with high SP content were brittle because of porous and noncontinuous structure as shown in Figure 1h,i. PVASP10 and PVASP20 had relatively low values of tensile strength at about 1−1.5 cN/dTex and a relatively high elongation at break of about 400−500%. When the soy protein content increased to 30 and 40%, the tenacity increased markedly to 3.47 and 3.40 cN/dTex because of the crosslinking and compact structure as seen in Figure 1j. The tensile stress and strain then dropped rapidly with further increase of soy protein. Overall, because of the cooperation of PVA, blend fibers exhibited good elasticity; e.g., the elongation at break of PVASP70 held at 207% which was much higher than that of reported soy protein fibers (SPF).10−12 In comparison to commercial SPF, our fabricated fibers had a comparable tenacity and 10 times higher elongation, indicating excellent mechanical properties.16 3.3. Thermal Properties. Figure 3 shows the DSC thermograms of spun fibers. Pure PVA fiber showed a relatively lower Tg (52.4 °C) than that of normally reported PVA materials. All the blend fibers exhibited only one glass transition temperature (Tg) from 52.4 to 65.1 °C with increasing content of soy protein, which was attributed to the effects of effective cross-linking or hydrogen bonding interactions between PVA
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The absorption band in Fourier transform infrared spectra (not shown) at 2918−3565 cm−1 attributed to the stretching O−H bonds resulting mainly from −OH groups on PVA carbon chains.13 The absorption bands at 1636−1680, 1533−1559, and 1241−1472 cm−1 were ascribed to the amide I, amide II, and N−H bending (amide III) vibrations in peptide bonds forming the primary backbone of soy proteins.13,14 This result indicated that the SP and PVA in the polar solvent (formic acid) had a good compatibility for homogeneous blending. Figure 1 displays POM photographs of the longitudinal views of PVA and PVA/SP blend fibers. Clearly, soy protein blend fibers with more PVA were less opaque and presented a more obvious view of stretching orientation. Pure PVA fibers presented a very smooth surface (Figure 1a,e). It is worth mentioning that soy protein was dissolved in formic acid initially due to its complete denaturation, whereas after the removal of the solvent in the coagulation bath, the fibers formed had a porous structure, as shown in Figure 1i−l. It was believed that garments made of this porous material would possess good breathability.15 Simultaneously, nanoparticles 6179
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point temperature of blend fibers when the soy protein content increased. This is very useful for extending the application of soy protein fibers with higher softening point temperatures. Figure 4 displays the TGA and differential thermal gravimetric (DTG) curves of fibrous samples. The initial weight loss of each sample up to 175 °C was attributed to the volatilization of moisture and solvent from the specimen. Pure PVA showed the onset of degradation under nitrogen atmosphere at about 180 °C, whereas the degradation temperatures of blend fibers shifted to about 30 °C higher. The temperature at the peak (Tmax) of the DTG curves corresponded to the maximum decomposition rate of fibers. All the blend fibers exhibited a higher Tmax (320−350 °C) than PVA did (around 270 °C), and the amount of weight loss at Tmax shifted from 40−50% of the former to about 65% of the latter. Among blend fibers, PVASP30 exhibited a relatively slow rate of weight loss and a higher Tmax, which corresponded to results of mechanical and morphological observation, meaning that loading 30% protein produced a stable and compact structure between hydroxyl PVA and protein biomolecules. 3.4. Water Uptake. The water uptake of spun fiber with varying blend proportions is shown in Figure 5. PVA exhibited a common characteristic of high hygroscopicity up to 6.3%, whereas the soy protein blend fiber had a better water
Figure 3. DSC thermograms of PVA/soy protein fibers. The arrow marks the location of the glass transition temperatures. Tg’s of PVA, PVASP20, PVASP30, PVASP40, PVASP50, PVASP60, PVASP80 were 52.4, 56.5, 58.8, 60.5, 65.0, and 65.1 °C, respectively.
and soy protein. In other words, flexible chains of PVA were tied up by soy protein polypeptide chains and could not move freely as before, thus leading to the increased Tg or softening
Figure 4. TGA and DTG of PVA/soy protein fibers under nitrogen atmosphere at a heating rate of 10 °C/min. 6180
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REFERENCES
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Figure 5. Equilibrium water uptake of fibers as a function of content of soy protein at relative humidity of 75%.
resistance.17 The WU of blend fiber shows a decreasing tendency with increasing soy protein content, probably caused by the cross-linking effects of multifunctional groups located on soy protein globulins. However, when the soy protein content was higher than 70%, the WU of the blend fiber was elevated again, which may mean that the hygroscopic nature of soy protein and the porous structure of fibers played important roles in the moisture sorption of blend fibers.
4. CONCLUSIONS Blend fibers could be wet-spun from soy protein/PVA blends dissolved by formic acid, and the spun fibers produced exhibited a surface oriented morphology and porous structure. The mechanical properties of soy protein fiber could be enhanced by introducing PVA to improve the flexibility and elasticity. Meanwhile, the glass transition temperature and water resistance of blend fibers decreased with an increase in PVA because of the hygroscopicity and flexible nature of the macromolecular PVA chain. The ratio of SP/PVA 30/70 was believed to be the optimum concentration because PVASP30 exhibited the best thermal stability and tensile strength, and had good water resistance and a proper softening point, which are excellent characteristics for fabrics. Therefore, the blend fibers developed in the present work hold great potential in various applications of textile materials.
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AUTHOR INFORMATION
Corresponding Author
*Tel./fax: +86 2558731090 (D.L.). E-mail: dagangliu@gmail. com (D.L.);
[email protected] (X.C.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Nos. 51103073 and 21277073), the National Basic Research Program of China (2012CB619100), the Natural Science Foundation of Jiangsu Province (No. BK2011828), and the Qing Lan Project of Jiangsu Province for financial support. 6181
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