Abstract
Corncob cellulose from residue of saccharified agricultural waste corncob was used as a new resource of cellulose to dissolve in 1-allyl-3-methylimidazolium chloride (AmimCl) and then to regenerate in three different coagulation baths: water, 60 wt% AmimCl aqueous solution, and anhydrous ethanol. The effects of the different coagulation baths on the properties of corncob cellulose aerogels and regenerated films were studied. The results showed that the aerogels had porous network structures, and the regenerated films were relatively transparent with high strengths and good thermal stabilities. When 60 wt% AmimCl was used as the coagulation bath, the network of the obtained aerogel was dense and uniform, and the regenerated film had good thermal stability and a tensile strength superior to the films from the other regeneration baths. The films might have uses in packaging or other fields and aid in comprehensive utilization of agricultural wastes.
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Preparation and Study of Regenerated Aerogels and Films from Corncob Cellulose
Xingya Kang,a Baoying Wang,a Yan Zhang,a Yong Huang,b Jinzhu Zhang,c An Zhang,c and Min Wu b,*
Corncob cellulose from residue of saccharified agricultural waste corncob was used as a new resource of cellulose to dissolve in 1-allyl-3-methylimidazolium chloride (AmimCl) and then to regenerate in three different coagulation baths: water, 60 wt% AmimCl aqueous solution, and anhydrous ethanol. The effects of the different coagulation baths on the properties of corncob cellulose aerogels and regenerated films were studied. The results showed that the aerogels had porous network structures, and the regenerated films were relatively transparent with high strengths and good thermal stabilities. When 60 wt% AmimCl was used as the coagulation bath, the network of the obtained aerogel was dense and uniform, and the regenerated film had good thermal stability and a tensile strength superior to the films from the other regeneration baths. The films might have uses in packaging or other fields and aid in comprehensive utilization of agricultural wastes.
Keywords: Corncob; Cellulose; Ionic liquid; Regenerate
Contact information: a: School of Packaging and Printing Engineering, Henan University of Animal Husbandry and Economy, Zhengzhou 450046, People’s Republic of China; b: Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China; c: Jinan Shengquan Group Share Holding Co., Ltd., Jinan 250204, People’s Republic of China;
* Corresponding author: wumin@mail.ipc.ac.cn
INTRODUCTION
China, as a large agricultural country, produces great quantities of agricultural wastes. Corncob is a relatively common agricultural waste. Generally it has been incinerated, with negative environmental effects. Recently, in the context of sustainable development and environmental protection, the development and utilization of biomass resources, especially agricultural waste, has received increasing attention, and corncob is gradually being developed and utilized (Liu et al. 2010; Zhang et al. 2010). Saccharification of corncob and extraction of xylose are common development and utilization methods (Procentese et al. 2015), but approximately 30% remains as residue. Therefore, this study used the residue of corncob saccharification as a raw material to study its utilization value by dissolution and regeneration.
Ionic liquid (IL), an ionic compound composed of organic cations and inorganic anions, is a large class of green solvents of cellulose. After dissolving, cellulose can be regenerated by adding a non-solvent of cellulose to the solution, namely by employing a coagulation bath. The regeneration process is determined by the inter-diffusion between the solvent and the coagulation bath. Therefore, the composition, concentration, temperature, time, and additives of the coagulation bath will affect the regeneration of the cellulose, which in turn affects the properties of the regenerated hydrogel, aerogel, and film (Sescousse and Budtova 2009; Isobe et al. 2011). For cellulose solutions of IL, the common coagulation baths are water, ethanol, and acetone, which is volatile and has good compatibility with IL (Östlund et al. 2013). Among them, water is the most commonly used coagulation bath due to its miscibility with IL and recycling convenience. Recently, Mi et al. (2016) used a new coagulation bath, which was a high concentration of IL aqueous solution, to regulate the regeneration process of cellulose to obtain a transparent and flexible cellulose aerogel with high porosity.
In this study, IL was selected as the solvent to study the solubility of corncob cellulose. Then, three different coagulation baths (water, 60 wt% IL aqueous solution, and anhydrous ethanol) were used to study their effects on cellulose regeneration. Finally, the properties of the regenerated corncob cellulose aerogel and film were examined to assess the utilization value of corncob cellulose dissolution and regeneration.
EXPERIMENTAL
Materials
Corncob cellulose was provided by Jinan Shengquan Group Share Holding Co., Ltd. (Jinan, China). Microcrystalline cellulose was purchased from Sigma-Aldrich (St. Louis, MO, USA), and 1-allyl-3-methylimidazolium chloride (AmimCl) was provided by Zhang Jun, researcher of the Institute of Chemistry, Chinese Academy of Sciences (Beijing, China). Anhydrous ethanol and tert-butyl alcohol, analytically pure, were purchased from Beijing Chemical Works (Beijing, China).
Dissolution of Corncob Cellulose
A 2 wt% corncob cellulose solution in AmimCl was prepared by mechanical stirring at 80 °C for 2 h to obtain a transparent solution. Then, the transparent solution was cooled to room temperature, and air bubbles were removed by standing still.
Regeneration of Corncob Cellulose
The corncob cellulose solution was poured onto a clean glass plate and scraped by a four-sided preparation machine with a fixed thickness, and then the plate was immersed into the coagulation bath to regenerate and gel at room temperature. After gelation, the hydrogels were washed several times by using the same solvent with the coagulation bath until no chloride ions were detected by AgNO3.
Preparation of Regenerated Corncob Cellulose Aerogel and Film
The water in the cleaned hydrogel was replaced with tert-butanol to obtain a cellulose tert-butanol gel, which was frozen in liquid nitrogen and then placed on a lyophilizer (FDU-2110, Eyela, Tokyo, Japan) to prepare the corncob cellulose aerogel by freeze drying. The regenerated films were obtained by fixing the cleaned hydrogel on the glass plate and drying at room temperature overnight. The samples obtained from water, 60 wt% AmimCl aqueous solution, and anhydrous ethanol were named Re-W, Re-IL, and Re-E, respectively. The preparation process is shown in Fig. 1.
Characterization
The Fourier-transform infrared (FTIR) spectra were scanned by an FTIR spectrometer (Varian 3100, Palo Alto, CA, USA) in transmission mode in the range of 400 cm-1 to 4000 cm-1 with a resolution of 2 cm-1 and accumulation of 32 scans. The samples were ground with KBr in a weight ratio of 1:100.
Fig. 1. Preparation processes of regenerated corncob cellulose aerogel and film
X-ray diffraction (XRD) was measured with an X’Pert PRO X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). The scanning range was from 5° to 60°, and the scanning speed was 2.4°/min, with λ = 0.154184 nm.
Nitrogen adsorption and desorption isotherms were measured by a Quadrasorb SI-MP system (Quantachrome, Boynton Beach, FL, USA). The pore volume and pore size distributions were measured according to the Barrett-Joyner-Halenda (BJH) method. The density of the aerogel (ρa) was calculated by dividing its weight by its volume. The porosity of the aerogel (P) was calculated by Eq. 1, where ρc was the density of bulk cellulose (1.528 g/cm3).
(1)
The surface and cross-sectional morphologies of the aerogels and films were observed using a scanning electron microscope (SEM, Hitachi S-4800, Tokyo, Japan). The cross-sections were first cryofractured in liquid nitrogen.
The mechanical properties of the regenerated films were tested using a universal testing machine (MTS Sintech-1, Eden Prairie, MN, USA). The samples were cut into 5-mm-wide and 30-mm-long strips for tensile testing. The gauge of the strips was 20 mm, and the strain rate was 4 mm/min. The samples were conditioned in a desiccator at 20 °C ± 5 °C for 48 h before testing. Measurements were made for more than five test pieces.
Thermogravimetry of the regenerated films was performed by a thermogravimetric analyzer (Q 50, TA Instruments, New Castle, DE, USA) in nitrogen atmosphere from room temperature to 600 °C with a heating rate of 10 °C/min.
RESULTS AND DISCUSSION
Morphologies of Aerogels
The coagulation bath affects the regeneration and gelation of the cellulose, which affects the intertwining of cellulose molecules during the regeneration process, resulting in different internal structures.
Figure 2 shows the surface morphologies of the aerogels. The 20 K magnification SEM images showed that the surface of aerogel Re-IL was relatively dense, and almost no network was visible. Meanwhile, aerogels Re-W and Re-E displayed clear network structures. The 70 K magnification SEM images showed that aerogel Re-IL had a compact network, while those of Aerogel Re-E and Re-W were loose. Aerogel Re-E’s surface was more uniform than that of aerogel Re-W, and the pores were larger.
Fig. 2. Surface morphologies of aerogels
Fig. 3. Cross-sectional morphologies of aerogels
Figure 3 shows the cross-sectional morphologies of the aerogels. The 30 K magnification SEM images indicated that the cross-sections of the aerogels all had clear network structures. The 80 K magnification SEM images indicated that those of aerogels Re-IL and Re-E were relatively uniform, but that of aerogel Re-W was not uniform. This result was related to the speed of regeneration and gelation.
When water was used as the coagulation bath, the regeneration process and entanglement of the cellulose molecular chains were fast, encouraging the formation of a dense network structure. However, when the speed was too fast, a non-uniform network structure was obtained. The water content in 60 wt% AmimCl was low, so the regeneration rate was regulated, making the regeneration and entanglement of the cellulose molecular chains relatively uniform.
When the non-aqueous solvent ethanol was used as the coagulation bath, the compatibility and diffusion speed between ethanol and AmimCl were low. Therefore, the regeneration speed was decreased, and there was sufficient time for the cellulose molecular chains to regenerate and entangle, resulting in a loose and uniform network structure (Hauru et al. 2016; Meenatchi et al. 2017).
Specific Surface Areas and Pore Size Distributions of Aerogels
The density and porosity of an aerogel can also reflect the compactness of its structure. As shown in Table 1, the specific surface areas of the three aerogels ranged from 226 m2/g to 270 m2/g, indicating that the three aerogels were of porous structure. The density of aerogel Re-E was the lowest (0.035 g/cm3), and its porosity was the greatest (97.8%), further indicating that the aerogel structure obtained from the anhydrous ethanol was the loosest. Those of Re-W and Re-IL were more compact.
Table 1. Physical Properties of Regenerated Cellulose Aerogels Obtained from Different Coagulation Baths
SBET – Brunauer-Emmett-Teller surface area
The nitrogen adsorption and desorption curves of the aerogels (Fig. 4(a) to 4(c)) were type IV with sharp capillary condensation at high relative pressures and obvious hysteresis loops, indicating that the aerogels were of mesoporous structure (Ma et al. 2014; Mi et al. 2016). Figure 4(d) shows most of the pore sizes were in the range of 3 nm to 100 nm, and the average pore sizes was in the range of 10 nm to 20 nm and pore volumes was up to 1.3 cm3/g. (Table 1).
FTIR Spectra of Regenerated Films
The FTIR spectra of the regenerated films (Fig. 5) showed no new peaks, indicating that the corncob cellulose was directly dissolved in the AmimCl with no derivatization reaction. After dissolution and regeneration, the hydroxyl stretching vibration peak of cellulose at 3329 cm-1 was blue-shifted to 3359 cm-1, indicating that the hydrogen bonding interactions among the cellulose were weakened after regeneration (Cai et al. 2008). After regeneration, the vibration peak at 1430 cm-1, which was related to the crystal form of cellulose I, was weakened; and the vibration peak at 898 cm-1, which was related to the amorphous region, was enhanced, indicating that the crystal form of the corncob cellulose was changed after regeneration (Zhang et al. 2005; Yan and Gao 2008).