Research Article pubs.acs.org/journal/ascecg
Transparent Woody Film Made by Dissolution of Finely Divided Japanese Beech in Formic Acid at Room Temperature Yuri Nishiwaki-Akine,*,† Sui Kanazawa,‡ Takashi Uneyama,§ Koh-hei Nitta,§ Ryoko Yamamoto-Ikemoto,∥ and Takashi Watanabe¶ †
Career Design Laboratory for Gender Equality, ‡Division of Environmental Design, Graduate School of Natural Science and Technology, §Faculty of Natural System, and ∥Faculty of Environmental Design, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan ¶ Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Kyoto 611-0011, Japan ABSTRACT: A new transparent flat woody film was developed by direct dissolution of finely divided Japanese beech wood in formic acid at room temperature for 4−7 days and subsequent slow evaporation of the solvent on a substrate. The obtained woody film was bendable and foldable without breaking (it could be used for origami) and had a relatively high Young’s modulus and tensile strength. The thermal analyses showed that the woody film was mechanically and thermally stable even at relatively high temperatures and can be utilized up to 180 °C without softening. The film absorbed almost the same amount of water as that absorbed by the cellulose film and had very high biodegradability, which was comparable with that of cellulose. The film was prepared without removing any constituents from wood and using a very simple processing method. We think that this work opens the way for the production of biodegradable and sustainable “molded wood” materials. KEYWORDS: Cellulose, Hemicellulose, Lignin, Formic acid, Film, Japanese beech, Tensile strength, Viscoelasticity
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INTRODUCTION Owing to the depletion of fossil resources, the importance of biomass as a sustainable material has increased in recent years. Among various biomass materials, wood has been extensively studied as a source of biofuels such as bioethanol1 and chemical products such as nanocelluloses.2,3 Wood comprises cellulose, hemicellulose, and lignin. As is well-known, dissolution of wood in common solvents (such as water and organic solvents) is difficult. This is because the cellulose interacts itself very strongly via hydrogen bonds between hydroxyl groups and has low affinity toward solvent molecules. Various solvents have been reported to dissolve cellulose, for example, NaOH/CS2, cupric ammonia, N-methylmorpholineN-oxide (MMNO), aqueous solutions of NaSCN or Ca(SCN)2, NaOH or alkali/urea (low temperature),4 formic acid/ LiCl5or CaCl2,6 and ionic liquids.7,8 Once the solutions are prepared, various regenerated cellulose fibers (e.g., viscose, cupra, and lyocell) and films (e.g., cellophane) can be produced for various applications. In these solutions, the hydroxyl groups of cellulose are derivatized, a complex is formed with the solvent, or solvation occurs, whereby intermolecular or intramolecular hydrogen bonding is hindered and cellulose is dissolved. In the case of wood, dissolution is more difficult due to the presence of hemicellulose and lignin, and there are limited © 2017 American Chemical Society
solvents available for wood. A previous study reported that ballmilled wood can be dissolved in dimethyl sulfoxide (DMSO)/ LiCl,9 DMSO/pyridine,10 urea and NaOH-boric acid,11 and various ionic liquids.12 There are numerous studies that use a wood solution to analyze wood or fractionate constituents, but there are almost no studies on woody films fabricated from the wood solution. Recently, films containing both cellulose and lignin have been prepared.13−15 However, in these studies, lignocellulosic fibers (hemicellulose and soluble lignin were removed from the wood; more than half of the contained lignin was insoluble)13 and pulp14 were used instead of wood as raw materials, or lignin was covalently bonded to dissolved cellulose to produce a material with UV-blocking properties.15 As such, the prepared films cannot be considered to be “woody films” which are prepared without removing any constituents from wood. Recently, we have reported that all constituents of ball-milled wood (Eucalyptus globulus) can be successfully dissolved in formic acid at room temperature.16 We consider that this is due to esterification and/or acetalization. Formic acid has a higher vapor pressure than DMSO or ionic liquids; therefore, it can be easily evaporated from the solution. By evaporating the formic Received: August 17, 2017 Revised: September 26, 2017 Published: November 2, 2017 11536
DOI: 10.1021/acssuschemeng.7b02839 ACS Sustainable Chem. Eng. 2017, 5, 11536−11542
Research Article
ACS Sustainable Chemistry & Engineering
Mechanical Properties (Temperature Dependence of Linear Viscoelasticity and Tensile Test). The temperature dependence of the films was measured using a rheometer (DVE-V4 FT, Rheology Co., Ltd.). Specimen strips (5 mm × 40 mm) cut from the films were utilized. The linear viscoelasticity of the samples was measured in the tensile mode at 10 Hz at temperatures ranging from room temperature (25 °C) to 200 or 250 °C. The temperature was increased at a rate of 2 °C/min. The film tensile tests were performed using a miniature tensile testing machine (TC 05-010, Abe Seisakusho). Dumbbell-shaped specimens (gage size 4 mm × 10 mm) were cut from the films. The tensile tests were performed at room temperature and at an elongation of 0.17 mm/s. We obtained the nominal stress−strain curves for samples and determined the Young’s moduli, tensile strengths (the maximum stresses), and fracture strains. Differential Scanning Calorimetry (DSC). DSC analysis was performed using a PerkinElmer Diamond DSC at a heating rate of 20 °C min−1 under nitrogen flow. The baselines were subtracted from the heat flow data; therefore, only the heat flows of the samples are shown. Density, Water Absorbency, and Water Resistance. We measured the density, water absorbency, and water resistance of the films from sample weights. We cut films into square-shaped specimens (25 mm × 25 mm for the density and long-term water resistance measurements, or 10 mm × 10 mm for the water absorbency and short-term water resistance measurements) and vacuum-dried them for 30 min. The weights of the vacuum-dried specimens were determined and used as the initial weights w0. To determine the film density, we measured the thickness of the films. We calculated density from the thickness t and weight w0 as ρ = w0/tl2, with l = 25 mm being the size of the specimen. To improve statistical accuracy, we measured the density of six different samples and calculated the average. For the water absorbency measurements, the specimens were immersed in water at room temperature for 24 h. After wiping excess water from the surface of the film, we measured the weight of the swollen specimen w1. The specimens were then vacuum-dried for 30 min to remove water. We measured the weight of the immersed-thendried specimen w2. From these weights, we calculated the water absorbency as (w1 − w0)/w0 and the water resistance ratio as (w2 − w0)/w0. For both quantities, we measured three samples and calculated the average. Long-term water resistance was measured in a similar way: the specimens were immersed in water for 6 weeks before removal and vacuum-dried for 30 min. The water resistance ratio was calculated in the same manner as for the short-term measurement. Biodegradability. We measured biodegradability using two methods. For both methods, we utilized 25 mm × 25 mm specimens cut from the films. The specimens were vacuum-dried for 30 min before the biodegradation tests, and the initial weights were measured. The first method measured degradation in soil in the field. We buried specimens in the soil at the Kakuma Campus, Kanazawa University. The specimens were buried from September 2016 (maximum temperature was approximately 25 °C) to December 2016 (maximum temperature was approximately 15 °C). We consider that temperature did not qualitatively affect the results. The specimens were removed from the ground, washed well to remove the soil, and vacuum-dried for 30 min. We measured the weights of the specimens and the change in weight was calculated. We took the relative weight change with respect to the initial weight as the degree of biodegradation. The second method evaluated degradation in soil under laboratory conditions. We took soil from the same location used in the first method and placed it into a transparent container in the laboratory. The specimens were then buried in the soil in the container. The films were placed at the edge of the transparent container so that the state of degradation could be observed. The temperature of the laboratory was kept constant (approximately 20 °C). Since the container was not sealed and water in the soil gradually evaporated, we supplied water by spraying every 2 or 3 weeks. After 6 weeks, the films were removed, washed well, and vacuum-dried for 30 min, as in the first method. We measured the initial and final weights and calculated the degree of
acid from ball-milled wood solutions, we can prepare woody films without removing any constituents from original wood. The obtained films can be considered to be woody film or “molded wood” and can be used as a new biomass-based material. However, in the previous work, we concentrated on the dissolution and separation of the constituents from wood solution, and thus the properties of the woody films have not been evaluated. In this work, we studied (i) the dissolution of Fagus crenata (Japanese beech) into formic acid and (ii) the properties of the films obtained by evaporation of formic acid from the solution. To compare the woody film with a cellulose film in detail, we also prepared a cellulose film by a similar method as the woody film. Japanese beech wood commonly contains more than 40% hemicellulose and lignin in total, which make the properties of the film different from those of cellulose films. Also, we determined whether this new transparent material, which can be molded like plastic, retains the biodegradable characteristics of wood.
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EXPERIMENTAL SECTION
Materials. Fagus crenata (Japanese beech) wood was ball-milled by a ball-milling machine (MB-1, Chuo Kakohki Co., Ltd.) under a nitrogen atmosphere and water cooling for 48 h (unless stated otherwise). The obtained ball-milled wood contained 8.2 ± 0.1% water (the water fraction was estimated from the weight change before and after drying at 105 °C for 24 h). Cellulose (034-22221, with a particle size
