- Original article
- Open Access
Changes in bamboo fiber subjected to different chemical treatments and freeze-drying as measured by nanoindentation
© The Japan Wood Research Society 2016
- Received: 16 May 2016
- Accepted: 22 September 2016
- Published: 18 October 2016
The effects of chemical treatments (H2O2 + CH3COOH, acidified NaClO2, and NaOH) and freeze-drying on bamboo fibers were studied at a submicron level, to characterize chemical and mechanical changes to the secondary cell wall. Specifically, a field emission environmental scanning electron microscope (FE-ESEM) and imaging fourier transform infrared spectroscopy (FTIR) were used to demonstrate degradation in morphology and molecular structure, and nanoindentation was used to track changes in micromechanical properties. The results showed that cellular structures after chemical treatments clearly displayed wrinkles, pores, and microfibrils. The decreased bands at 1508 cm-1 and 1426 cm−1 showed that lignin was degraded on treatment of H2O2 + CH3COOH and acidified NaClO2, which directly resulted in a decrease in hardness (H) in the secondary cell wall for treated fibers. In addition, a diminishing peak at 1733 cm−1 caused by NaOH solution indicated that hemicellulose was seriously degraded. It resulted in a decreased modulus (E r) by 13.71 % in bamboo fibers, while no obvious reduction was observed in the first two steps.
- Bamboo fiber
- Chemical treatment
Bamboo fiber is ideal for use as a reinforcement material in composites, given its relatively low density and energy consumption, high strength and flexibility, biodegradability and relatively reactive surface [1–4]. The natural fiber is mainly consisted of cellulose microfibrils embedded in matrices composed of lignin and hemicellulose . Among these fiber constituents, cellulose is the primary load-carrying component, while hemicellulose (xylan and glucomannan) and lignin play essential roles as binder to hold cellulose to the cell wall. Therefore, there is an increasing need to better understand the influence of hemicellulose and lignin on the mechanical properties of bamboo fibers at the microstructural level.
Nanoindentation has been demonstrated to be an effective method of measuring the mechanical properties of biomass, including wood and bamboo, at the submicron level [6–10]. It is mainly conducted on the embedded cross section of fibers. Nanoindentation studies have been conducted to characterize fiber-reinforced composites , wood adhesive bonds and adhesives , and the effects of delignification , while only a few studies have mentioned raw bamboo fibers . In addition, the mechanical functions of matrix components on the fiber’s cell wall have not been properly characterized with nanoindentation, given the amorphous nature of fibers.
Therefore, it is common practice to chemically alter matrix components and conduct mechanical tests for discriminating treated fiber from raw fiber. In previous studies, it was found that at least 35 % of cell wall materials were removed if chemically treated . Tensile tests on samples with lignin and hemicellulose extracted have also been conducted to analyze the influence of individual matrix components [16–19]. Using nanoindentation, Gindl et al.  suggested that lignification had an obvious effect on the mechanical properties of the cell wall. Later studies [7, 21] on hydrothermally or chemically modified wood supported the argument that material hardness depended on cell wall matrix properties. All of these results demonstrate that the behavior of and modification to the matrix can significantly impact the micromechanical properties. However, studies on the effects of chemical treatment on matrix components and the micromechanical properties of bamboo cell walls, at the submicron level, have been limited.
This study analyzes the influence of combining freeze-drying and chemical treatments [hydrogen peroxide and glacial acetic acid (H2O2 + CH3COOH), acidified sodium chlorite (acidified NaClO2) and sodium hydroxide (NaOH)] on the changes in the chemical and mechanical properties of moso bamboo fibers. Fourier transform infrared spectroscopy (FTIR) and a field emission environmental scanning electron microscope (FE-ESEM) were used to characterize the degradation of chemical components and morphological changes in treated fibers. In addition, nanoindentation was used to evaluate the modulus (E r) and hardness (H) of the secondary cell wall.
Four-year-old moso bamboo (Phyllostachys heterocycla (Carr.) Mitford cv. Pubescens Mazel ex H.de leh.), collected from a public welfare forest located in Huangshan District, Anhui Province, China, was used in this study. Bamboo strips were cut from disks to final dimensions of 20 mm (longitudinal) and 2 × 2 mm (cross section). Samples were obtained from bamboo at 2 m above the ground.
Sample F-A was composed of small strips of bamboo treated with hydrogen peroxide and glacial acetic acid (H2O2 + CH3COOH), mainly for removing lignin. The bamboo strips were immersed in solution for 18 h and heated at 60 °C. Then, they were washed in distilled water until neutral.
Sample F-B was prepared by an additional treatment to a portion of Sample F-A, where an acidified sodium chlorite (NaClO2) solution of 1 wt% concentration was used to further remove lignin and heated at 75 °C for 4 h in a water bath. Two droplets of CH3COOH were added to the NaClO2-impregnated strips, per hour, to maintain an acidic environment. The strips were then washed with distilled water until neutral.
Sample F-C was prepared by an additional treatment to a portion of Sample F-B, where the samples were sequentially immersed in a sodium hydroxide (NaOH) solution of 5, 10 and 25 % concentration for further removing hemicellulose and heated at 75 °C for 2 h in a water bath. The strips were also washed with distilled water until neutral.
Finally, a control sample of raw fiber bundles was prepared mechanically from bamboo blocks.
Characterization by FTIR
The four groups of fibers (samples F-A, F-B, F-C and control) were examined using pressed potassium bromide (KBr) pellets containing 5 % of samples, using FTIR (Nexus 670, Thermo Nicolet, USA) in the scanning range of 4000–650 cm−1.
Nanoindentation samples were prepared by placing chemically treated fiber bundles (F-A, F-B and F-C) and control raw bundles in a vacuum desiccator for 30 min and then cured in an oven at 70 °C for 8 h. This sealing procedure made it easier to mount the fibers inside the embedding mold of the nanoindentation apparatus parallel to the longitudinal axis of the cell wall. The cured samples were then mounted onto an ultramicrotome fitting (UC7, Leica, Germany) using a diamond knife in the middle of the cross section. Finally, the four specimens were smoothed down to a roughness of less than 0.5 nm to ensure good contact.
To investigate the relationship between the morphology of fibers and its mechanical properties, images were obtained by visual examination and micro-morphology analysis. A visual examination of bamboo strips (Fig. 1) clearly demonstrates that the natural pale yellow of the strips gradually turns white throughout chemical treatment, indicating a significant removal of lignin. The change in color directly reflects changes in the constituents of bamboo fibers, in which a light brown color generally indicates the presence of lignin while white fibers generally indicate holocellulose. The additional treatment of acidified NaClO2 and NaOH combined with the freeze-drying method also resulted in air voids inside the cell wall because of the removal of hemicellulose and lignin.
FTIR spectrum analysis
FTIR spectroscopy is an effective technique for establishing the differences in bamboo fiber structure and chemical composition under various chemical treatments, which are important factors to achieve an in-depth understanding of the mechanical performance of cell walls.
Nanoindentation hardness and modulus
As can be seen in Fig. 7b, the values of H in chemically treated fibers (F-A, F-B and F-C) are lower than those of the control fibers, indicating that the removal of the matrix components impairs the mechanical behavior of fibers. In comparison with the average H of the control fibers at 550 MPa, the average H of F-A decreases by 19.27 % and F-B by 17.64 %. These declines show that a large removal of lignin significantly reduces the ability of fibers to resist deformation, while at the same time the removal of different proportions of matrix components can influence fiber mechanical properties in different ways. After NaOH treatment to F-B fibers, H increases by 9.22 % compared to F-A fibers, but it is certainly lower than that of the control fibers. This enhancement can be attributed to the fact that alkali treatments result in fiber fibrillation and microfibril marginal aggregation during freeze-drying, which increases the hardness. This is also indirectly supported by the smaller maximum and residual depths than the other two treated samples. However, there is little difference observed in H among any chemically treated fibers, which strongly suggests that H2O2 + CH3COOH plays a major role in removing the majority of lignin, while the additional treatments of acidified NaClO2 (which removed a small part of the remaining lignin) and NaOH (which removed hemicellulose) play relatively minor roles.
It was demonstrated that bamboo fiber matrix components are one of the major factors that affect bamboo mechanical properties at the microstructural level. Images of chemically treated and denatured fiber surfaces clearly revealed that wrinkles, pores and microfibrils resulted from the dissolution of matrix materials. From the FTIR spectra, the disappearing peak at 1733 cm−1 and decreased bands at 1508 cm-1 and 1426 cm−1 also proved the degradation of xylan and loss of C=O groups linked to the aromatic skeleton in lignin. Compared to the controlled raw fiber (E r at 21.86 GPa), there were little differences observed for treated fibers with H2O2 + CH3COOH and acidified NaClO2 solutions, while a pronounced decrease of 13.71 % was found for those treated with NaOH solution. The fibril E r largely decreased after the raw fiber was first treated with H2O2 + CH3COOH, and the subsequent modulus had minor change. All of the above results indicate that the loss of hemicellulose has greater effect on modulus than hardness, while lignin degradation has more influence on hardness at the submicron level of bamboo fibers.
We are grateful to the Fundamental Research Funds provided by the International Centre for Bamboo and Rattan (Grant No. 1632013003) for financially supporting this project.
- Scurlocka JMO, Dayton DC, Hames B (2000) Bamboo: an overlooked biomass resource? Biomass Bioenergy 19:229–244View ArticleGoogle Scholar
- Long LC, Wang ZK, Chen K (2015) Analysis of the hollow structure with functionally gradient materials of moso bamboo. J Wood Sci 61:569–577View ArticleGoogle Scholar
- Lee SH, Wang S (2006) Biodegradable polymers/bamboo fiber biocomposite with biobased coupling agent. Compos Part A Appl Sci Manuf 37(1):80–91View ArticleGoogle Scholar
- Osorio L, Trujillo E, Van Vuure AW, Verpoest I (2011) Morphological aspects and mechanical properties of single bamboo fibres and flexural characterisation of bamboo/epoxy composites. J Reinf Plast Compos 30(5):396–408View ArticleGoogle Scholar
- Fengel D, Wegener G (1984) Wood: chemistry, ultrastructure, reactions. Holz als Roh und Werkstoff 42(8):314View ArticleGoogle Scholar
- Wimmer R, Lucas BN (1997) Comparing mechanical properties of secondary wall and cell corner middle lamella in spruce wood. IAWA J 18:77–88View ArticleGoogle Scholar
- Gindl W, Schöberl T (2004) The significance of the elastic modulus of wood cell walls obtained from nanoindentation measurements. Compos Part A 35(11):1345–1349View ArticleGoogle Scholar
- Jiang ZH, Yu Y, Qin DC, Wang G, Zhang B, Fu YJ (2006) Pilot investigation of the mechanical properties of wood flooring paint films by in situ imaging nanoindentation. Holzforschung 60:698–701Google Scholar
- Zou L, Jin H, Lu WY, Li X (2009) Nanoscale structural and mechanical characterization of the cell wall of bamboo fibers. Mater Sci Eng C 29(4):1375–1379View ArticleGoogle Scholar
- Jäger A, Hofstetter K, Buksnowitz Ch, Gindl-Altmutter W, Konnerth J (2011) Identification of stiffness tensor components of wood cell walls by means of nanoindentation. Compos Part A Appl Sci Manuf 42(12):2101–2109View ArticleGoogle Scholar
- Lee SH, Wang S, Endo T, Kim NH (2009) Visualization of interfacial zones in lyocell fiber-reinforced polypropylene composite by AFM contrast imaging based on phase and thermal conductivity measurements. Holzforschung 63:240–247View ArticleGoogle Scholar
- Follrich J, Stöckel F, Konnerth J (2010) Macro- and micromechanical characterization of wood-adhesive bonds exposed to alternating climate conditions. Holzforschung 64:705–711View ArticleGoogle Scholar
- Lehringer C, Koch G, Adusumalli RB, Mook WM, Richter K, Militz H (2011) Effect of physisporinus vitreus on wood properties of Norway spruce. Part 1: aspects of delignification and surface hardness. Holzforschung 65:711–719Google Scholar
- Yu Y, Tian G, Wang H, Fei B, Wang G (2011) Mechanical characterization of single bamboo fibers with nanoindentation and microtensile technique. Holzforschung 65(1):113–119Google Scholar
- Burgert I, Gierlinger N, Zimmermann T (2005) Properties of chemically and mechanically isolated fibres of spruce (Picea abies [L.] Karst.). Part 1: structural and chemical characterisation. Holzforschung 59(2):240–246Google Scholar
- Molin U, Teder A (2002) Importance of cellulose/hemicellulose-ratio for pulp strength. Pulp Pap Res J Nord 17:14–28View ArticleGoogle Scholar
- Chen H, Cheng HT, Wang G, Yu ZX, Shi QS (2015) Tensile properties of bamboo in different sizes. J Wood Sci 61:552–561View ArticleGoogle Scholar
- Konnerth J, Eiser M, Jäger A, Bader TK, Hofstetter K, Follrich J, Ters T, Hansmann C, Wimmer R (2010) Macro- and micro-mechanical properties of red oak wood (Quercus Rubra L.) treated with hemicellulases. Holzforschung 64(4):447–453View ArticleGoogle Scholar
- Chen H, Wang G, Chen HT (2011) Properties of single bamboo fibers isolated by different chemical methods. Wood Fiber Sci 43(2):111–120Google Scholar
- Gindl W, Gupta HS (2002) Lignification of spruce tracheid secondary cell wall related to longitudinal hardness and modulus of elasticity using nano-indentation. Can J Bot 80:1029–1033View ArticleGoogle Scholar
- Stanzl-Tschegg S, Beikircher W, Loidl D (2009) Comparison of mechanical properties of thermally modified wood at growth ring and cell wall level by means of instrumented indentation tests. Holzforschung 63(4):443–448View ArticleGoogle Scholar
- Burgert I, Frühmann K, Keckes J, Fratzl P, Tschegg S (2005) Properties of chemically and mechanically isolated fibers of spruce (Picea abies [L.] Karst.). Part 2: twisting phenomena. Holzforschung 59(2):247–251Google Scholar
- Oliver WC, Pharr GM (1992) An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 7:1564–1583View ArticleGoogle Scholar
- Guo J, Song K, Salmén L, Yin Y (2015) A changes of wood cell walls in response to hygro-mechanical steam treatment. Carbohydr Polym 115:207–214View ArticlePubMedGoogle Scholar
- Stevanic JS, Salmén L (2009) Orientation of the wood polymers in the cell wall of spruce wood fibres. Holzforschung 63:497–503View ArticleGoogle Scholar
- Salmén L (2004) Micromechanical understanding of the cell-wall structure. CR Biol 327(9–10):873–880View ArticleGoogle Scholar
- Alemdar A, Sain M (2008) Isolation and characterization of nanofibers from agricultural residues–wheat straw and soy hulls. Bioresour Technol 99(6):1664–1671View ArticlePubMedGoogle Scholar
- Das M, Chakraborty D (2006) Influence of alkali treatment on the fine structure and morphology of bamboo fibers. J Appl Polym Sci 102:5050–5056View ArticleGoogle Scholar