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Dec 6, 2014 - determine the influence of thermal treatment on the mechanical properties of bamboo cell walls. Abstract: Bamboo was thermally treated at ...

Holzforschung 2015; 69(7): 909–914

Yanjun Li, Liping Yin, Chengjian Huang, Yujie Meng, Feng Fu, Siqun Wang* and Qiang Wu*

Quasi-static and dynamic nanoindentation to determine the influence of thermal treatment on the mechanical properties of bamboo cell walls Abstract: Bamboo was thermally treated at 180°C and 200°C, and the micromechanical properties of its cell walls were investigated by means of quasi-static and dynamic nanoindentation experiments. With increasing treatment temperatures, the average dry density and mass of the bamboo decreased, whereas the already reduced elastic modulus at 180°C of the fiber cell walls did not change, but the hardness showed increasing tendencies. Dynamic nanoindentation revealed reduced storage mod) for the thermotreated ulus ( E′r ) and loss modulus ( E′′ r bamboo cell walls compared with the untreated bamboo fibers in all frequency regions. Moreover, Er′ , Er′′, and loss tangent (tan δ) of treated bamboo decreased with increasing treatment temperature. Keywords: bamboo, dynamic nanoindentation, heat treatment, quasi-static nanoindentation DOI 10.1515/hf-2014-0112 Received April 9, 2014; accepted November 4, 2014; previously published online December 6, 2014

Introduction Thermal treatment has been widely adopted in wood production and application; its main purpose is to change the chemical composition and structure of wood materials *Corresponding authors: Siqun Wang, Center for Renewable Carbons, University of Tennessee, Knoxville, TN, USA, e-mail: [email protected]; and Qiang Wu, School of Engineering, Zhejiang Agriculture and Forestry University, Linan, Zhejiang, China; and Key Laboratory of Wood Science and Technology, Linan, Zhejiang, China, e-mail: [email protected] Yanjun Li, Liping Yin and Chengjian Huang: School of Engineering, Zhejiang Agriculture and Forestry University, Linan, Zhejiang, China; and Key Laboratory of Wood Science and Technology, Linan, Zhejiang, China Yujie Meng: Center for Renewable Carbons, University of Tennessee, Knoxville, TN, USA Feng Fu: Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing, China

(Sivonen et al. 2002; Schwanninger et al. 2004), to deepen the wood color for aesthetic purposes, and to improve the dimensional stability (Tjeerdsma et  al. 1998; Yildiz and Gumuskaya 2007). However, thermal treatment simultaneously reduces the mechanical properties such as strength and toughness. Aiming at the same quality improvements, the thermal treatment of bamboo products was also investigated. The mechanical properties of bamboo are important in the nanometer and micrometer range for its utilization. The purpose of the present paper was the investigation of the physical changes of the bamboo cell wall after heat treatment by means of quasi-static and dynamic indentation methods. These techniques are well established to detect the structural variations in biomaterials such as wood and bamboo (Yu 2003; Yu et  al. 2007; Yin et al. 2011). Quasi-static indentation characterizes the stiffness and hardness of materials, which are detected from the load-displacement data involved in the nanoindentation (Oliver and Pharr 1992). Quasi-static indentation is also successful for testing the longitudinal hardness and modulus of elasticity of individual wood cell walls (Wimmer et al. 1997; Gindl et al. 2004; Wang et al. 2006; Tze et al. 2007; Yu et al. 2011a; Lehringer et al. 2011; Wang et al. 2014), regenerated cellulose fibers (Gindl et al. 2006; Lee et al. 2007a; Ganser et al. 2014), bamboo fiber (Zou et al. 2009; Yu and Tian 2011; Yu et al. 2011b), and the interphase mechanical properties between cellulose fibers and thermoplastic polymers (Lee et al. 2007b; Clauss et al. 2011; Obersriebnig et  al. 2012). Bamboo products always show significant viscoelastic effects under oscillation stress or stain condition, and these effects are frequently in focus (Molenaar and Dijkstra 1999; Obataya et al. 2000; Guang and Zhang 2006; Huang and Jiang 2008; Jiang et al. 2009). Dynamic nanoindentation can reveal the viscoelastic properties of materials at the small scale (Odegard et al. 2005; White et al. 2005; Chakravartula and Komvopoulos 2006; Herbert et al. 2008) and the viscoelastic properties of wood cell walls (Zhang et  al. 2012). For this purpose, various instrumental setups can be used, and corresponding data can be analyzed to obtain the frequency domain measurements at ambient temperatures. Moreover, combining dynamic and quasi-static indentation methods can

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910      Y. Li et al.: Nanoindentation of bamboo cell walls provide more information about the elastic and viscoelastic properties of bamboo cell walls. The specific goal of this study was to investigate the micromechanical properties of the thermotreated bamboo by means of these techniques and to find the interrelationships between the micromechanical properties of bamboo cell walls and the heat treatment conditions. The expectation was that a deeper knowledge of the cell wall properties could provide a theoretical support for developing a high-performance advanced design for the thermal modification of bamboo.

Materials and methods Six-year-old bamboo [Phyllostachys edulis (Carr.) H. De Lehaie] was obtained from a bamboo plantation located in Linan District, Hangzhou, China. The samples were taken at the internode 2  m from the roots and were cut from one log of bamboo with dimensions of 240 (L) × 11 × 1.5 mm3. For all indentation experiments, the samples were taken from the same layer (i.e., 1 mm from the outer bamboo culm). The mass and volume of 30 bamboo specimens were evaluated to obtain their oven-dried densities. The average oven-dried density was 0.684 g cm-3. The specimens were laid in different aluminum foil boxes, and the boxes were preheated until the target temperature was achieved. The treatment was continued at 180°C and 200°C for 3 and 6 h. These heat-treated bamboo samples were designated as htB180 and htB200, respectively. The masses and volumes of the specimens were measured immediately after the heat treatment. The specimens were enclosed in sealed plastic bags and stored in a silica gel desiccator. The dimension of the samples for nanoindentation was 8 × 8 × 1 mm3, and the slicing was done according to Meng et al. (2013). The small pieces were sealed with a polymer film in FoodSaver vacuum sealing. Each sample was placed between two pieces of film, hot pressed with an electric iron set at 160°C, and embedded in epoxy resin. The samples were mounted onto the metal sample holder of the ultramicrotome and cut into a pyramid shape at the apex with a diamond knife. Nanoindentation instrument: Triboindenter (Hysitron, Minneapolis, MN) equipped with a three-sided pyramid diamond Berkovich tip of nominal radius of curvature equal to ∼100 nm. Indentation was conducted in a load-controlled mode in three stages: first loading in 2 s to a peak force of 150 μN, holding the maximum force for 20 s, and finally unloading in 3 s. The final data represent an average of at least 70 indents on cell walls, containing five or six adjacent cells for each point. After the experiments, the samples were placed in the scanning probe microscope of the nanoindentation apparatus to observe the location and quality of the indentations. The hardness (H) and the elastic modulus (E) were estimated from the load-displacement data based on the procedure of Oliver and Pharr (1992).



H = Pmax / A, 

(1)

where Pmax is the load measured at a maximum depth of penetration (h) in an indentation cycle and A is the projected contact area between the indenter and the sample at Pmax. The reduced elastic modulus (Er), which is the related elastic modulus of both the tested sample and the indenter, is calculated by

π S , 2 A

Er =



(2)



where S = dP/dh (stiffness) is equal to the slope of the upper portion of the unloading curve in the load-displacement plot and A is the projected area of the elastic contact. The nanoDMA tests were taken by the same Berkovich tip and Triboindenter equipped with a nanoDMA model transducer. The test was operated in a ramping dynamic frequency mode. The quasistatic load was equal to 100 μN and the dynamic load was 10 μN. The harmonic frequencies were varied from 10 to 200 Hz with 100 cycles at each frequency. Data were recorded at 20 points for each indentation. At least 30 indentations were taken in five or six adjacent cells for each point. The analysis is based on the dynamic model developed by Asif et al. (1999). For a sinusoidal driving force of amplitude F0 and alternating angular frequency ω, the amplitude of the tip displacement X0 is



X0 =

F0 ( K -mω2 ) 2 + [( Cs +Ci ) ω ] 2

,

(3)



The phase shift between force and displacement is f given as φ = tan−1



( Cs +Ci ) ω K -mω2

,

(4)



where m stands for the indenter mass, Ci is the damping coefficient of the air gap in the capacitive displacement sensor, and Cs is the damping coefficient of the specimen. The combined stiffness (K) is calculated by the following equation: K = Ki + K s ,



(5)



where Ks is the contact stiffness and Ki is the spring constant of the leaf springs holding the indenter shaft. Ci and Ki are provided by the constant calibration of the equipment at air before testing. The displacement amplitude X0 and the phase shift ∅ are evaluated from the lock-in amplifier. The reduced ), and loss tangent storage modulus ( E′r ), reduced loss modulus ( E′′ r (tan δ) were calculated as follows:



Er′ =

Ks π 2 A

Er′′=

ωC s π 2 A

tan δ=

Cs ω , ks

(6)



where A is the projected area of the elastic contact and is a geometric factor arising from the tip shape. The selected areas and the test process are presented in Figure 1. To reduce the influence of the samples’ heterogeneity, each area was tested at least 40 times. At the end of the indentation test, indentations without defects were selected for data analysis based on the indentation morphology and indentation location.

Results and discussion Density and mass after heat treatment As presented in Table 1, the average dry density and mass of the samples decreased with increasing treatment

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a

b

c

d

Figure 1 Process of nanoindentation testing: (a) selection of the test area, (b and c) selection of test points, and (d) indentation after the unloading.

temperature and duration. Thus, these results confirmed those of the literature (Wålinder and Johansson 2001; Deng 2004; Gündüz et al. 2008; Bao 2009; Zhang et al. 2013) reporting on heat treatments above 160°C. It is a general agreement that the degradation of hemicelluloses and lignin and the loss of volatile components from extractives are the reason for these and similar observations (Kubojima et al. 2001). Table 1 Changes in the density and mass of bamboo under heat treatment. Modification Type Control 1 2 3

Temperature (°C) Time (h) – 180 200 200

– 6 3 6

Density (SD) (g cm-3) Mass (SD) (g) 0.684 (0.004) 0.653 (0.008) 0.632 (0.014) 0.618 (0.014)

0.429 (0.023) 0.391 (0.015) 0.369 (0.018) 0.351 (0.017)

Elastic modulus and hardness of cell walls The elastic moduli (Er) and hardnesses of the bamboo cell walls are compiled in Table 2. The difference between the Er of the htB180 and htB200 was not too high compared with that of untreated bamboo. ANOVA also revealed the statistical insignificance of the results after 20°C temperature increment (at the level of 0.05). Similar observations were Table 2 Elastic modulus and hardness values of cell walls before and after heat treatment as determined by quasi-static indentation. Treatment  

Modulus (GPa) 

Hardness (GPa)

Reference   180°C–6 h   200°C–3 h   200°C–6 h  

23.5 (1.44)  23.2 (1.78)  23.2 (1.59)  23.1 (1.42) 

0.592 (0.050) 0.666 (0.054) 0.682 (0.052) 0.692 (0.033)

SD in parenthesis.

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912      Y. Li et al.: Nanoindentation of bamboo cell walls made after the steam treatment on spruce wood (Yin et al. 2011). The hardness of the htB180 and htB200 was slightly higher than that of the untreated bamboo, which was significant at the 0.05 and 0.01 levels, respectively. The hardness increment was in accordance with that described by Stanzl-Tschegg et al. (2009).

Dynamic mechanical properties of cell walls The results of the dynamic indentation of bamboo cell walls for harmonic frequencies are presented in Figure 2(a) and (b). Figure 2(a) shows the Er′ , Er′′, and tan δ as a function of frequency at 180°C and 200°C. Accordingly, the Er′ , Er′′, and tan δ of heat-treated bamboo were much lower than those of untreated bamboo cell walls and decreased with increasing temperature. This was due to hemicellulose degradation and melting (Stanzl-Tschegg

a Reference 180°C-6 h 200°C-6 h

et al. 2009; Zhang et al. 2013). Moreover, the E′r of heattreated bamboo increased steadily with increasing frequency, whereas the E′′ and tan δ decreased significantly r in this direction. Zhang et  al. (2013) interpreted this by the different modes of interaction of molecular chains at different frequencies. At lower frequency, the smaller E′r was assigned to the relatively flexible molecular chains; at higher frequency, the main chain movements were probably frozen and small-scale movements were dominant, resulting in a stiffer material (expressed by higher reduced E′r ). The gradual decrease of tan δ with increasing harmonic frequency was attributed to the stiffening of the material due to the short time available for molecular chain rearrangement (Chakravartula and Komvopoulos 2006; Zhou and Komvopoulos 2007). Based on the viscoelastic theory (tan δ = E″/E′), the variation of tan  δ can indicate that the elastic response is dominant at high frequency, whereas the viscous response increases with a decreasing frequency (Zhang et al. 2009). Similar responses have been reported for tan δ in the dynamic indentation of some glassy polymer films (Zhou and Komvopoulos 2007). Figure 2(b) gives the influence of Er′ , Er′′, and tan δ on the frequency of bamboo with different treatment times of 3 and 6 h at 200°C. Compared with the untreated bamboo, these parameters of the thermotreated bamboo decreased. Moreover, the treatment time did not affect the dynamic mechanical properties anymore.

Conclusions The average density and mass of the thermotreated (180°C and 200°C) bamboo decreased. The already reduced modulus (Er) of heat-treated bamboo fiber cell walls did not change at 20°C higher treatment temperature, whereas the hardness was slightly elevated. Dynamic indentation revealed a reduced Er′ , Er′′, and tan δ upon heat treatment. Moreover, the E′r of heat-treated bamboo increased steadily, whereas the E′′ and tan δ of heat-treated bamboo r dropped remarkably as a function of frequency increment.

b Reference 200°C-3 h 200°C-6 h

Figure 2 Dynamic indentation results of bamboo cell walls: (a) at different thermotreated temperatures and (b) with different treatment times.

Acknowledgments: The authors are grateful for the support of the Foundation of Zhejiang Provincial Natural Science Foundation of China (No. LZ13C160003), the Foundation of Zhejiang Key Level 1 Discipline of Forestry Engineering, the Project of Science and Technology Department of Zhejiang Province (No. 2012R10023-05), and the Project of Graduate Student Research Innovation of Zhejiang Agriculture and Forestry University (No. 3122013240253).

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