Primarily Industrialized Trial of Novel Fibers Spun from Cellulose

Oct 5, 2010 - Preparation and Properties of Cellulose/Tamarind Nut Powder Green Composites. Journal of Natural Fibers 2018, 15 (1) , 11-20...
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Primarily Industrialized Trial of Novel Fibers Spun from Cellulose Dope in NaOH/Urea Aqueous Solution Ran Li,† Chunyu Chang,† Jinping Zhou,† Lina Zhang,*,† Wenqing Gu,‡ Chuntao Li,‡ Shilin Liu,†,§ and Shigenori Kuga§ Department of Chemistry, Wuhan UniVersity, Wuhan 430072, People’s Republic of China, Jiangsu Longma Green Chemical Fibre Company Ltd., Haian, Jiangsu 226600, China, Graduate School of Agriculture and Life Science, The UniVersity of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

Faced with the serious pollution caused by the viscose method (use of CS2), a primarily industrialized trial via a “green” wet-spinning process based on dissolution of cellulose in NaOH/urea aqueous solution precooled to -12.5 °C was performed. In a dissolution tank of 1000 L capacity, the cellulose could be dissolved completely in the NaOH/urea system within 5 min, and cellulose fibers were spun successfully from the transparent dope. A 15:10 H2SO4/Na2SO4 aqueous solution was adopted as the first coagulants in the bath, and a 5 wt % H2SO4 aqueous solution was used as the second coagulant to fabricate new regenerated cellulose fibers. There was no evaporation of any chemical agent during dissolution and regeneration. The structure of the fibers was characterized with scanning electron microscope, wide-angle X-ray diffraction, 13C NMR, and tensile testing. The cellulose fibers exhibited a bright surface and a circular section, and their tensile strength reached 1.63-1.97 cN/dtex, which is close to commercial viscose fiber, although the drawing orientation in the production process was not performed. Therefore, we created a novel and simple approach combining a nonpolluting, low-cost, and quick process for the production of cellulose fibers, which was important for environmental conservation. 1. Introduction Cellulose as a renewable resource has attracted much attentions due to the oil crisis. However, the utilization of cellulose is limited because it is insoluble in most solvents and also does not melt until thermal degradation because of the strong intraand intermolecular hydrogen bonding.1 Recently, studies on natural fibers,2 regenerated cellulose,3 and its ramification4 by green technology have been reported. However, the commercial routes for producing regenerated cellulose fibers, films, and nonwoven fabrics are mainly based on the viscose method (xanthate in the case of viscose rayon) and complex method (cuprammonium rayon), which cause serious pollution.5 The viscose route generates hazardous byproducts (CS2, H2S, and heavy metals) that pollute soil, water, and the atmosphere.6 Nowadays, in developed countries factories using the viscose route have been closed, but viscose rayon and cellophane are in great demanded in the market. Thus, in developing countries such as China and India, viscose production is increasing year by year. In 2008, it has been reported that the world production of viscose filament is about 3 million tons, half of which are from China. Therefore, there is a growing urgency to develop a novel nonpolluting technology to produce the man-made cellulose fibers for environmental conservation. Recently, an environmentally more friendly process of cellulose fiber spinning using N-methylmorpholine-N-oxide (NMMO) has been developed, leading to a new class of manmade cellulose fibers with the generic name of Lyocell, which has a circular cross-section, with high tensile strength, high humidity modulus, and good dimensional stability.7–10 However, the NMMO system is easy to break down under high temper* To whom correspondence should be addressed. Tel.: +86-2787219274. Fax: +86-27-68754067. E-mail: [email protected]; [email protected]. † Wuhan University. ‡ Jiangsu Longma Green Chemical Fibre Co. Ltd. § The University of Tokyo.

ature, its byproduct generated in a dissolution process pollutes the fiber, and dissolved exothermic reaction can cause a potential explosion that threatens safety.7,8,11 So this technology needs to be improved further. In our laboratory, a novel solvent system for cellulose has been developed. Namely, a NaOH/urea aqueous solution precooled to -12.5 °C, in which the dissolution of cellulose could be achieved rapidly at ambient temperature (below 20 °C). The cellulose dissolution at low temperature arises as a result of a fast, dynamic, self-assembly process among solvent small molecules (NaOH, urea, and water) and the cellulose macromolecules, because the inclusion complex formed through hydrogen bonds is relatively stable at low temperature.12 The cellulose dope could remain in a liquid state for a long period at 0-5 °C.13 This is an environmentally preferable solvent, because of the nontoxic and safety approach, ease of recycle, and the nonevaporation of the chemical agents during production. Moreover, regenerated cellulose multifilament fibers and membranes have been prepared successfully from this system.14–17 Though we have done it successfully in our laboratory and preliminary pilot scale (less than 6 kg), a large-scale dissolution of cellulose in an industrial tank is essential for the successful industrialization of the new regenerated cellulose fibers. More recently, we have realized rapid dissolution (5 min) of cellulose in the solution under low temperature in an industrial-scale trial using custom-made equipment.18 In the present work, an industrial trial for producing novel regenerated cellulose fibers based on cellulose dissolution in NaOH/urea aqueous solution at low temperature was performed. We hope to provide a “green” industrial process to produce the regenerated cellulose fibers, leading to a great impact on the traditional viscose route. 2. Experimental Section 2.1. Materials. The cellulose raw material (cotton linter pulp, DP ) 350) with an R-cellulose content of more than 95% was

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Figure 1. Schematic diagram of the pilot-scale spinning apparatus: (a) dissolution tank of 1000 L capacity; (b) filter; (c) pump; (d) spinneret and the first coagulation bath; (e) the second coagulation bath; (f) roller; (g) washing bath; (h) cellulose fibers.

provided by the Jiujiang Chemical Fiber Co. Ltd. (Jiujiang, China). The weight-average molecular weight (Mw) of cellulose samples was determined to be 5.6 × 104 g · mol-1. Na2SO4, H2SO4, NaOH, and urea were of industrial grade and were used without further purification. 2.2. Spinning Process. A mixture solution with NaOH/urea/ H2O of 7:12:81 by weight was precooled to -12.5 °C by a low-temperature-cycle cooling system and then pumped into a dissolution tank of 1000 L capacity, in which the cellulose raw material with the desired amount was added. Under vigorous stirring for 5-15 min at ambient temperature, we obtain a transparent cellulose dope with concentrations of 5.2 wt %. In fact, a transparent cellulose solution had appeared within 5 min in the large tank. The resulting cellulose dope was filtered through 700, 500, 300, 200, and 100 meshes to remove the impurities and microgels and then degassed under vacuum at 10 °C. The wet-spinning process was carried out on an industrial-scale apparatus that was constructed by Jiangsu Longma Green Chemical Fiber Co. Ltd. (Haian, China). The spinneret cylinder was immersed directly into the first coagulation bath with a 15:10 H2SO4/Na2SO4 aqueous solution at 20 °C, and the resulting gelation fibers solidified in the first coagulation bath were taken up on the first roller and then drawn to the second roller. Subsequently, the cellulose fibers were dipped into the second coagulation bath with 5 wt % H2SO4. To wash out the residual salts and acid, the resultant fibers were put through a water bath at 40 °C until the pH value of the fibers was about pH 7. We used two kinds of spinneret with the apertures of 0.08 and 0.1 mm to obtain two cellulose fibers that were coded as D-I and D-II. 2.3. Characterization. The viscosity of the cellulose dilute solution in cadoxen was measured at 25 ( 0.1 °C with an Ubbelodhe viscometer. The Mw value was calculated from [η] according to the following equation:19 [η] ) 3.85 × 10-2Mw0.76 (mL · g-1)

(1)

The viscosity of the cellulose dope was measured by rotational viscosimeter (NDJ-5S, Shanghai Nirun Technology Co.). The surface and fracture section of the fibers were observed by using a scanning electron microscope (SEM, Hitachi, S-570). Wetted fibers were frozen directly in liquid nitrogen, immediately snapped, and then freeze-dried by using lyophilizer (CHRIST Alpha 1-2). The surface and the fracture section of fibers were sputtered with gold under vacuum and then observed and photographed. Solid-state 13C NMR spectra of the cellulose were recorded on a BRUKER spectrometer operated at a 13C frequency of 100 MHz using the combined technique of proton dipolar decoupling, magic angle spinning (MAS), and crosspolarization (CP). The spinning speed was set at 12 MHz for all samples. The contact time was 2 ms, the acquisition time 30 ms, and the recycle delay 4 s. A typical number of 7500 scans was acquired for each spectrum. Fourier-transform infrared

spectroscopy (FT-IR) of the cellulose fibers was conducted on an FT-IR spectroscope (model 1600, Perkin-Elmer Co.). The samples were prepared by the KBr-disk method. X-ray diffraction (XRD) measurement was carried out on an XRD diffractometer (D8-Advance, Bruker). The patterns with Cu KR radiation (λ ) 0.15406 nm) at 40 kV and 30 mA were recorded in the region of 2θ from 5 to 45°. Samples were ground into powders and dried in a vacuum oven at 60 °C for 48 h. Wideangle X-ray diffraction (WAXD) of D-I and D-II was measured using nickel-filtered Cu KR radiation from a Philips X-ray generator at 30 kV and 20 mA. Diffraction patterns were recorded on Fuji imaging plates, which were subsequently read using a Fuji BAS 1800 II Phosphoimager. Radial intensity profiles were obtained by integrating the intensity over a fanshaped area, and azimuthal intensity profiles were obtained for selected diffraction by measuring the intensity on a circle using a program made on purpose. The radial intensity was fitted with a pseudovoigt peak function and polynomial background to evaluate the peak width. The azimuthal profile was fitted with a Lorentz IV type function to calculate the orientation parameter. Morphology of the cross-section of the cellulose fibers was observed on an optical polarizing microscope (Leica DMLP). The tensile strength (σb) and the elongation at break (εb) of the dried fibers were measured on a universal tensile tester (XQ-1, Shanghai Textile University) according to GB/T 19975-2005. The σb and εb values represented averages of 10 measurements. 3. Results and Discussion 3.1. Dissolution of Cellulose and Spinning. The industrialized trial of producing cellulose fibers in the NaOH/urea system at low temperature was realized, for the first time, in Jiangsu Longma Green Chemical Fiber Co. Ltd. The custom-made equipment for the industrialized trial consisted of a tank of the cellulose dissolution, a filter, and degasification and spinning machines. A schematic diagram of the production route and the equipment of the spinning and washing process is shown in Figure 1. The cellulose was dissolved completely in a 1000 L dissolving tank within 5 min and then stirred another 10 min to homogenize. The dissolution of cellulose represented the most rapid dissolution in the polymer industry. It also proved that cellulose dissolution at low temperature is a quick self-assembly between the solvent and macromolecules, quite different from normal polymers, which are a slow diffusion based on the interchangeability of solvent and polymer and need a long time (heating may accelerate the process).12 This was the first set of dissolution tank equipment with 1000 L capacity for the cellulose dissolution at low-temperature in the world. The dissolubility of cellulose in the solvent precooled to -12.5 °C depends on the raw material molecular weight, sample size, and speed of stirring, etc. The cellulose pieces should be crushed below the size of 1 cm × 1 cm, and the cellulose molecular weight was lower than 1.0 × 105. Figure 2 shows the

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Figure 2. Photographs of the cellulose dissolution (a) and spinning process (b-d). Table 1. Data of Temperature, Time, and Viscosity of the Cellulose Solution during the Industrial Process time (min)

viscosity (mPa · s)

process

temperature (°C)

dissolution filtration 1 2 3 4 5 degasification filature

-11.9

5

3850

2.7 3.4 4 4 4.2 4.8 2.8

30 29 32 31 24 200

3700 3650 3700 3600 3800 4100 7000

photographs of the cellulose dissolution in the large tank and spinning process. From the dissolution tank, a transparent cellulose solution was obtained. The cellulose fibers were spun from the cellulose dope in NaOH/ urea aqueous solvent system. A 15:10 H2SO4/Na2SO4 aqueous solution was adopted as the first coagulant in the bath, and a 5 wt % H2SO4 aqueous solution was used as the second coagulant. For spinning, the viscosity of the cellulose solution is very important. The data of temperature, time, and viscosity of the cellulose solution are summarized in Table 1. During the whole production process, the viscosity of the liquid of the cellulose dope remained basically in the range from 3600 to 4100 mPa · s. It was demonstrated that the cellulose dope was stable on a large-scale production within 2 days. The cellulose solution could form a gel in storage at high or low temperature. However, the cellulose solution could be stored at 10-15 °C for relatively long time. In our findings, it was indicated that the viscose range from 3500 to 5000 mPa · s was more appropriate in this cellulose solution system at about 10 °C. To spin, the cellulose dope needs to be filtrated and degassed. We designed five pieces of filtrating equipment, whose apertures were 125, 75, 50, 30, and 20 µm respectively, to filtrate impurity and a few small gels in the dope within 2.5 h. Subsequently, the resulting dope was degassed under vacuum for 3.5 h. The cellulose was spun from the degassed solution to obtain fibers with a bright surface on the industrial scale as shown in Figure 2. As a result of the rapid dissolution of cellulose, the whole production period, including solvent precooling, cellulose dissolution, filtration, degassing, and spinning, could take about 10 h. It was much shorter than that of the viscose process (a week). Compared with the viscose method, our new method was a simple, low-cost, and no-pollution process. The byproducts, mainly NaSO4 and urea resulting from the coagulation bath, could be easily separated and recycled to be reutilized by

Figure 3. Photographs of primary fiber products (up) and microphotographs of the cross-section (bottom) of novel cellulose fibers (a, c) and commercial viscose rayon in Hubei Chemical Fiber Co. Ltd. (b, d).

crystallization and flash evaporation. Furthermore, the cellulose dissolution could maintain clean air in the production environment, because of no evaporation of any chemical agents in the aqueous system at low temperature. Therefore, this was a real “green” process, and the solvent was safe and low cost. It is worth nothing that faced with the serious pollution caused by the viscose route (use of CS2), the green comprehensive utilization of cellulose has attracted much attention in the world. In 2005, R. D. Rogers won the U.S. Presidential Green Chemistry Challenge Awards for his great contributions to cellulose dissolution in ionic liquids. In his findings, there is no evaporation of any chemical agents in an ionic liquids system at room temperature. The novel cellulose fibers obtained through our aqueous solvent system via a green process would have a great impact on the cellulose chemical industries for a sustainable development. 3.2. Structure and Properties of New Cellulose Fibers. Figure 3 shows the photographs of the novel cellulose fiber from the NaOH/urea aqueous system via industrial-scale machine and microphotographs of the cross-section. The SEM images of the novel cellulose fibers and cross-section are shown in Figure 4. The fibers exhibited a circular section, similar to lyocell, and were different from viscose fibers which have the lobulate skincore structure. This confirmed that the cellulose fibers were regenerated directly from the dope in a quasi-gel state formed mainly by physical cross-linking and residual hydrogen bonds.16 As shown in Figure 4c, the fibers at wet state displayed homogeneous mesh structure with mean mesh size of about 300 nm, which disappeared in the dry fibers. Therefore, with this simple, noncovalent approach, we could readily prepare manmade cellulose fibers. Figure 5 shows CP/MAS 13C NMR spectra of the novel cellulose fibers. There were four main peaks at 105.8, 88.0, 73.2, and (75.0, 72.5) ppm in the spectrum of the novel cellulose fibers, assigned to the C1, C4, C5, and (C3, C2), as well as the C6 peak at 63.3 ppm. Compared with a native cellulose sample, the C6 signal of the cellulose fibers obviously shifted from 65.6 to 63.3 ppm as a single peak, suggesting that the “t-g” conformation (ca.66 ppm for C6) of the C6-OH group for the crystalline parts of cellulose I had changed into a “g-t” conformation (ca. 61-63 ppm for C6) of cellulose II.20 The results strongly indicated that the native cellulose could be dissolved completely in the present solvent system. Moreover, it was confirmed that the cellulose dope did transform into regenerated cellulose II when the cellulose gel was regenerated

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Figure 4. SEM images of the novel cellulose fibers (a, b) and cross-section (c).

Figure 6. Powder X-ray diffraction patterns of the novel regenerated cellulose fibers (b) and the cellulose cotton pulp (a). Figure 5. CP/MAS 13C NMR spectra of cellulose I (cotton linter pulp) (a) and the cellulose fibers (b).

in the coagulation bath. The C4 signal at around 88 ppm and its shoulder peak at 84.0 ppm depended on the status of carbons located in either the crystalline or amorphous regions, respectively.21 For the regenerated cellulose fibers, the C4 peaks located at 88.0 ppm shifted to higher magnetic fields than that of the native cellulose (89.3 ppm), and the intensity was significantly lower, suggesting a decrease in crystallinity. The shoulder peak of C4 for the fibers could be related to the degree of anisotropy.22 The C4 shoulder became higher for the cellulose fibers, reflecting an increase in the degree of anisotropy, i.e., an enhancement in chain orientation. However, we did not perform the drawing orientation in the production process. It was not hard to imagine that orientation was contributed to mainly by the wormlike chain conformation of cellulose in the solution, which has been confirmed in our previous work.12 Figure 6 shows the X-ray diffraction patterns of the novel cellulose fibers and the cellulose cotton pulp. The cellulose fibers exhibited three peaks at 2θ ) 12.4, 20.2, and 22.2°, assigned to the (110), (1ıj0), and (200) planes of crystalline cellulose II. The cellulose fibers were quite different from that in the native cellulose (cotton linter pulp), the three peaks at 2θ ) 14.9, 16.5, and 22.7°, assigned to the (110), (1ıj0), and (200) planes of crystalline cellulose I. This indicated that such a large quantity of cellulose can be dissolved completely in the NaOH/urea aqueous solution through the process of physics, without the chemical reaction. Figure 7 shows 2D wide-angle X-ray diffraction (WAXD) patterns of the cellulose fibers of D-II (a) and D-I (b). Total crystallinity (χc, %) and Hermans’ order parameters are summarized in Table 2. As mentioned above, there was a lack of the drawing progress during spinning. However, the results of XRD have shown the existence of the

Figure 7. WAXD patterns of novel cellulose fibers of D-II (a) and D-I (b). Table 2. Tensile Strength (σb), Breaking Elongation (εb), Total Crystallinity (χc), and Hermans’ Order Parameter of the Novel Cellulose Fibers Spun from NaOH/Urea Aqueous System and Other Commercial Fibers

sample

σba (cN/dtex)

D-I D-II viscose rayon Lyocell fiber cuprammonium rayon

1.8 ( 0.17 1.2 ( 0.11 2.0-2.4 4.0-4.4 1.5-2.0

b b

ε

(%)

19.0 ( 1.55 19.3 ( 3.01 20-25 14-16 7-23

χ (%)

Hermans’ order parameter

54 52 49

0.35 0.22 0.68

c c

a σb ) tensile strength; cN: 0.01 Newton; dtex: weight of 10000 meters fibers. b εb ) elongation. c χc ) total crystallinity.

orientation of the cellulose fibers. Interestingly, the Hermans’ order parameters of the D-II and D-I fibers were 0.22 and 0.35, respectively. Namely, the orientation degree of D-I was much higher than that of D-II, indicating that with a decrease of the diameter of the spinneret the orientation of the cellulose fibers increased. This could be explained as follows: through a relatively small hole, the wormlike cellulose chains could be

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National Supporting Project for Science and Technology (Grant 2006BAF02A09), the National High Technology Research and Development Program of China (863 Program, Grants 2003AA333040 and 2006AA02Z102), major grants of the National Natural Science Foundation of China (Grants 30530850 and 59933070), and the National Natural Science Foundation of China (Grants 20474048 and 20874079). Literature Cited

Figure 8. Photograph of the cellulose fibers in wet state.

more easily oriented and closely packed. Moreover, the wet fibers exhibited unique light, as shown in Figure 8, as a result of anisotropic gel fibers. This could be explained as follows: the wormlike chain conformation of the inclusion complex associated with cellulose, NaOH, urea, and water in the solution12 led to automatic orientation of the cellulose macromolecules in the fluent process through a spinneret with a relatively smaller hole. Comparison of the tensile strength (σb) and breaking elongation (εb) of the novel cellulose fibers spun from NaOH/urea aqueous system and other commercial fibers is summarized in Table 2. The D-I cellulose fibers possessed relatively high tensile strength (1.63-1.97 cN/dtex), close to the commercial viscose rayon. Moreover, the tensile strength of the D-I cellulose fibers was much higher than that of D-II, indicating that with an increase of the orientation degree the mechanical properties of the cellulose fibers increased, in good agreement with the conclusion from XRD. The orientation of the fibers will be enhanced significantly after further optimization of the spinning process with attention to the details of the spinneret design and the conditions for the draw process to achieve drawing orientation. Therefore the mechanical properties of the novel cellulose fibers could be improved significantly in a formal industrialization process. It was noted that the lower cost and much less toxic nature of the NaOH/urea solvent system, and the relative ease for wet spinning, suggested a promising potential for the development of a more economical and environmentally friendly process for cellulose-fiber production. 4. Conclusion Rapid dissolution of cellulose in 7 wt % NaOH and 12 wt % urea (industrial grades) aqueous solution precooled to -12.5 °C was realized successfully in a 1000 L dissolving tank. New cellulose fibers were spun, for the first time, from the cellulose dope via an industrial machine. The NaOH/urea aqueous solvent is nontoxic, and can be recycled easily, so it is a real “green” process. The cellulose crystals were completely transformed from cellulose I (cotton linter pulp) to II for the cellulose fibers during this process. The cellulose fibers exhibited a circular cross-sectional shape and a smooth surface as well as good mechanical properties. Interestingly the cellulose macromolecules in the solution could be orientated automatically when they through the spinneret, and the orientation degree increased with a decrease of the pore size of the spinneret, supported by the wormlike chain conformation of the cellulose in the solution.12 Acknowledgment This work was supported by National Basic Research Program of China (973 Program, Grant 2010CB732203),

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ReceiVed for reView May 24, 2010 ReVised manuscript receiVed September 5, 2010 Accepted September 12, 2010 IE101144H