Introduction. Keywords: bamboo, dynamic nanoindentation, heat treatment, quasi-static nanoindentation

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1 Holzfoschung 2015; 69(7): Yanjun Li, Liping Yin, Chengjian Huang, Yujie Meng, Feng Fu, Siqun Wang* and Qiang Wu* Quasi-static and dynamic nanoindentation to detemine the influence of themal teatment on the mechanical popeties of bamboo cell walls Abstact: Bamboo was themally teated at 180 C and 200 C, and the micomechanical popeties of its cell walls wee investigated by means of quasi-static and dynamic nanoindentation expeiments. With inceasing teatment tempeatues, the aveage dy density and mass of the bamboo deceased, wheeas the aleady educed elastic modulus at 180 C of the fibe cell walls did not change, but the hadness showed inceasing tendencies. Dynamic nanoindentation evealed educed stoage modulus ( E ) and loss modulus ( E ) fo the themoteated bamboo cell walls compaed with the unteated bamboo fibes in all fequency egions. Moeove, E, E, and loss tangent (tan δ) of teated bamboo deceased with inceasing teatment tempeatue. Keywods: bamboo, dynamic nanoindentation, heat teatment, quasi-static nanoindentation DOI /hf Received Apil 9, 2014; accepted Novembe 4, 2014; peviously published online Decembe 6, 2014 Intoduction Themal teatment has been widely adopted in wood poduction and application; its main pupose is to change the chemical composition and stuctue of wood mateials *Coesponding authos: Siqun Wang, Cente fo Renewable Cabons, Univesity of Tennessee, Knoxville, TN, USA, swang@utk.edu; and Qiang Wu, School of Engineeing, Zhejiang Agicultue and Foesty Univesity, Linan, Zhejiang, China; and Key Laboatoy of Wood Science and Technology, Linan, Zhejiang, China, wuqiang@zafu.edu.cn Yanjun Li, Liping Yin and Chengjian Huang: School of Engineeing, Zhejiang Agicultue and Foesty Univesity, Linan, Zhejiang, China; and Key Laboatoy of Wood Science and Technology, Linan, Zhejiang, China Yujie Meng: Cente fo Renewable Cabons, Univesity of Tennessee, Knoxville, TN, USA Feng Fu: Reseach Institute of Wood Industy, Chinese Academy of Foesty, Beijing, China (Sivonen et al. 2002; Schwanninge et al. 2004), to deepen the wood colo fo aesthetic puposes, and to impove the dimensional stability (Tjeedsma et al. 1998; Yildiz and Gumuskaya 2007). Howeve, themal teatment simultaneously educes the mechanical popeties such as stength and toughness. Aiming at the same quality impovements, the themal teatment of bamboo poducts was also investigated. The mechanical popeties of bamboo ae impotant in the nanomete and micomete ange fo its utilization. The pupose of the pesent pape was the investigation of the physical changes of the bamboo cell wall afte heat teatment by means of quasi-static and dynamic indentation methods. These techniques ae well established to detect the stuctual vaiations in biomateials such as wood and bamboo (Yu 2003; Yu et al. 2007; Yin et al. 2011). Quasi-static indentation chaacteizes the stiffness and hadness of mateials, which ae detected fom the load-displacement data involved in the nanoindentation (Olive and Pha 1992). Quasi-static indentation is also successful fo testing the longitudinal hadness and modulus of elasticity of individual wood cell walls (Wimme et al. 1997; Gindl et al. 2004; Wang et al. 2006; Tze et al. 2007; Yu et al. 2011a; Lehinge et al. 2011; Wang et al. 2014), egeneated cellulose fibes (Gindl et al. 2006; Lee et al. 2007a; Ganse et al. 2014), bamboo fibe (Zou et al. 2009; Yu and Tian 2011; Yu et al. 2011b), and the intephase mechanical popeties between cellulose fibes and themoplastic polymes (Lee et al. 2007b; Clauss et al. 2011; Obesiebnig et al. 2012). Bamboo poducts always show significant viscoelastic effects unde oscillation stess o stain condition, and these effects ae fequently in focus (Molenaa and Dijksta 1999; Obataya et al. 2000; Guang and Zhang 2006; Huang and Jiang 2008; Jiang et al. 2009). Dynamic nanoindentation can eveal the viscoelastic popeties of mateials at the small scale (Odegad et al. 2005; White et al. 2005; Chakavatula and Komvopoulos 2006; Hebet et al. 2008) and the viscoelastic popeties of wood cell walls (Zhang et al. 2012). Fo this pupose, vaious instumental setups can be used, and coesponding data can be analyzed to obtain the fequency domain measuements at ambient tempeatues. Moeove, combining dynamic and quasi-static indentation methods can Beeitgestellt von De Guyte / TCS Heuntegeladen am :05

2 910 Y. Li et al.: Nanoindentation of bamboo cell walls povide moe infomation about the elastic and viscoelastic popeties of bamboo cell walls. The specific goal of this study was to investigate the micomechanical popeties of the themoteated bamboo by means of these techniques and to find the inteelationships between the micomechanical popeties of bamboo cell walls and the heat teatment conditions. The expectation was that a deepe knowledge of the cell wall popeties could povide a theoetical suppot fo developing a high-pefomance advanced design fo the themal modification of bamboo. Mateials and methods Six-yea-old bamboo [Phyllostachys edulis (Ca.) H. De Lehaie] was obtained fom a bamboo plantation located in Linan Distict, Hangzhou, China. The samples wee taken at the intenode 2 m fom the oots and wee cut fom one log of bamboo with dimensions of 240 (L) mm 3. Fo all indentation expeiments, the samples wee taken fom the same laye (i.e., 1 mm fom the oute bamboo culm). The mass and volume of 30 bamboo specimens wee evaluated to obtain thei oven-died densities. The aveage oven-died density was g cm -3. The specimens wee laid in diffeent aluminum foil boxes, and the boxes wee peheated until the taget tempeatue was achieved. The teatment was continued at 180 C and 200 C fo 3 and 6 h. These heat-teated bamboo samples wee designated as htb 180 and htb 200, espectively. The masses and volumes of the specimens wee measued immediately afte the heat teatment. The specimens wee enclosed in sealed plastic bags and stoed in a silica gel desiccato. The dimension of the samples fo nanoindentation was mm 3, and the slicing was done accoding to Meng et al. (2013). The small pieces wee sealed with a polyme film in FoodSave vacuum sealing. Each sample was placed between two pieces of film, hot pessed with an electic ion set at 160 C, and embedded in epoxy esin. The samples wee mounted onto the metal sample holde of the ultamicotome and cut into a pyamid shape at the apex with a diamond knife. Nanoindentation instument: Tiboindente (Hysiton, Minneapolis, MN) equipped with a thee-sided pyamid diamond Bekovich tip of nominal adius of cuvatue equal to 100 nm. Indentation was conducted in a load-contolled mode in thee stages: fist loading in 2 s to a peak foce of 150 μn, holding the maximum foce fo 20 s, and finally unloading in 3 s. The final data epesent an aveage of at least 70 indents on cell walls, containing five o six adjacent cells fo each point. Afte the expeiments, the samples wee placed in the scanning pobe micoscope of the nanoindentation appaatus to obseve the location and quality of the indentations. The hadness (H) and the elastic modulus (E) wee estimated fom the load-displacement data based on the pocedue of Olive and Pha (1992). H = P / A, (1) max whee P max is the load measued at a maximum depth of penetation (h) in an indentation cycle and A is the pojected contact aea between the indente and the sample at P max. The educed elastic modulus (E ), which is the elated elastic modulus of both the tested sample and the indente, is calculated by π S E =, (2) 2 A whee S = dp/dh (stiffness) is equal to the slope of the uppe potion of the unloading cuve in the load-displacement plot and A is the pojected aea of the elastic contact. The nanodma tests wee taken by the same Bekovich tip and Tiboindente equipped with a nanodma model tansduce. The test was opeated in a amping dynamic fequency mode. The quasistatic load was equal to 100 μn and the dynamic load was 10 μn. The hamonic fequencies wee vaied fom 10 to 200 Hz with 100 cycles at each fequency. Data wee ecoded at 20 points fo each indentation. At least 30 indentations wee taken in five o six adjacent cells fo each point. The analysis is based on the dynamic model developed by Asif et al. (1999). Fo a sinusoidal diving foce of amplitude F 0 and altenating angula fequency ω, the amplitude of the tip displacement X 0 is X = 0 F ( Km - ω ) [( C C ) ] ω s i The phase shift between foce and displacement is f given as 1 ( C + C ) ω s i tan, 2, (3) φ= (4) Km - ω whee m stands fo the indente mass, C i is the damping coefficient of the ai gap in the capacitive displacement senso, and C s is the damping coefficient of the specimen. The combined stiffness (K) is calculated by the following equation: K = K + K, (5) i s whee K s is the contact stiffness and K i is the sping constant of the leaf spings holding the indente shaft. C i and K i ae povided by the constant calibation of the equipment at ai befoe testing. The displacement amplitude X 0 and the phase shift ae evaluated fom the lock-in amplifie. The educed stoage modulus ( E ), educed loss modulus ( E ), and loss tangent (tan δ) wee calculated as follows: K π ωc π C ω s s s E = E = tan δ=, (6) 2 A 2 A k whee A is the pojected aea of the elastic contact and is a geometic facto aising fom the tip shape. The selected aeas and the test pocess ae pesented in Figue 1. To educe the influence of the samples heteogeneity, each aea was tested at least 40 times. At the end of the indentation test, indentations without defects wee selected fo data analysis based on the indentation mophology and indentation location. Results and discussion Density and mass afte heat teatment As pesented in Table 1, the aveage dy density and mass of the samples deceased with inceasing teatment s Beeitgestellt von De Guyte / TCS Heuntegeladen am :05

3 Y. Li et al.: Nanoindentation of bamboo cell walls 911 a b c d Figue 1 Pocess of nanoindentation testing: (a) selection of the test aea, (b and c) selection of test points, and (d) indentation afte the unloading. tempeatue and duation. Thus, these esults confimed those of the liteatue (Wålinde and Johansson 2001; Deng 2004; Gündüz et al. 2008; Bao 2009; Zhang et al. 2013) epoting on heat teatments above 160 C. It is a geneal ageement that the degadation of hemicelluloses and lignin and the loss of volatile components fom extactives ae the eason fo these and simila obsevations (Kubojima et al. 2001). Elastic modulus and hadness of cell walls The elastic moduli (E ) and hadnesses of the bamboo cell walls ae compiled in Table 2. The diffeence between the E of the htb 180 and htb 200 was not too high compaed with that of unteated bamboo. ANOVA also evealed the statistical insignificance of the esults afte 20 C tempeatue incement (at the level of 0.05). Simila obsevations wee Table 1 Changes in the density and mass of bamboo unde heat teatment. Type Tempeatue ( C) Modification Time (h) Density (SD) (g cm -3 ) Mass (SD) (g) Contol (0.004) (0.023) (0.008) (0.015) (0.014) (0.018) (0.014) (0.017) Table 2 Elastic modulus and hadness values of cell walls befoe and afte heat teatment as detemined by quasi-static indentation. Teatment Modulus (GPa) Hadness (GPa) Refeence 23.5 (1.44) (0.050) 180 C 6 h 23.2 (1.78) (0.054) 200 C 3 h 23.2 (1.59) (0.052) 200 C 6 h 23.1 (1.42) (0.033) SD in paenthesis. Beeitgestellt von De Guyte / TCS Heuntegeladen am :05

4 912 Y. Li et al.: Nanoindentation of bamboo cell walls made afte the steam teatment on spuce wood (Yin et al. 2011). The hadness of the htb 180 and htb 200 was slightly highe than that of the unteated bamboo, which was significant at the 0.05 and 0.01 levels, espectively. The hadness incement was in accodance with that descibed by Stanzl-Tschegg et al. (2009). Dynamic mechanical popeties of cell walls The esults of the dynamic indentation of bamboo cell walls fo hamonic fequencies ae pesented in Figue 2(a) and (b). Figue 2(a) shows the E, E, and tan δ as a function of fequency at 180 C and 200 C. Accodingly, the E, E, and tan δ of heat-teated bamboo wee much lowe than those of unteated bamboo cell walls and deceased with inceasing tempeatue. This was due to hemicellulose degadation and melting (Stanzl-Tschegg a Refeence 180 C-6 h 200 C-6 h et al. 2009; Zhang et al. 2013). Moeove, the E of heatteated bamboo inceased steadily with inceasing fe- quency, wheeas the E and tan δ deceased significantly in this diection. Zhang et al. (2013) intepeted this by the diffeent modes of inteaction of molecula chains at diffeent fequencies. At lowe fequency, the smalle E was assigned to the elatively flexible molecula chains; at highe fequency, the main chain movements wee pobably fozen and small-scale movements wee dominant, esulting in a stiffe mateial (expessed by highe educed E ). The gadual decease of tan δ with inceasing hamonic fequency was attibuted to the stiffening of the mateial due to the shot time available fo molecula chain eaangement (Chakavatula and Komvopoulos 2006; Zhou and Komvopoulos 2007). Based on the viscoelastic theoy (tan δ = E /E ), the vaiation of tan δ can indicate that the elastic esponse is dominant at high fequency, wheeas the viscous esponse inceases with a deceasing fequency (Zhang et al. 2009). Simila esponses have been epoted fo tan δ in the dynamic indentation of some glassy polyme films (Zhou and Komvopoulos 2007). Figue 2(b) gives the influence of E, E, and tan δ on the fequency of bamboo with diffeent teatment times of 3 and 6 h at 200 C. Compaed with the unteated bamboo, these paametes of the themoteated bamboo deceased. Moeove, the teatment time did not affect the dynamic mechanical popeties anymoe. Conclusions b Refeence 200 C-3 h 200 C-6 h The aveage density and mass of the themoteated (180 C and 200 C) bamboo deceased. The aleady educed modulus (E ) of heat-teated bamboo fibe cell walls did not change at 20 C highe teatment tempeatue, wheeas the hadness was slightly elevated. Dynamic indentation evealed a educed E, E, and tan δ upon heat teatment. Moeove, the E of heat-teated bamboo inceased steadily, wheeas the E and tan δ of heat-teated bamboo dopped emakably as a function of fequency incement. Figue 2 Dynamic indentation esults of bamboo cell walls: (a) at diffeent themoteated tempeatues and (b) with diffeent teatment times. Acknowledgments: The authos ae gateful fo the suppot of the Foundation of Zhejiang Povincial Natual Science Foundation of China (No. LZ13C160003), the Foundation of Zhejiang Key Level 1 Discipline of Foesty Engineeing, the Poject of Science and Technology Depatment of Zhejiang Povince (No. 2012R ), and the Poject of Gaduate Student Reseach Innovation of Zhejiang Agicultue and Foesty Univesity (No ). Beeitgestellt von De Guyte / TCS Heuntegeladen am :05

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