Eur. J. Wood Prod. (2013) 71:61–67 DOI 10.1007/s00107-012-0643-6
ORIGINALS ORIGINALARBEITEN
Effect of thermal treatment on the physical and mechanical properties of phyllostachys pubescen bamboo Ya Mei Zhang • Yang Lun Yu • Wen Ji Yu
Received: 24 July 2012 / Published online: 15 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Thermal treatment of Phyllostachys pubescen bamboo was performed in a dry oven at seven temperature levels (100–220 °C) for four duration times (1–4 h). The results showed that mass loss increased with increasing temperature and duration, and the maximum reduction reached 29.0 %. The color of heat-treated bamboo was darkened and all three color parameters (L*a*b*) were significantly changed. Modulus of elasticity (MOE) was affected slightly when samples were heat-treated below 200 °C, even a slight increase compared with control samples; but it decreased quickly when samples were treated above 200 °C and the maximum reduction was 20.2 %. However, the Modulus of rupture (MOR) and the contents of holocellulose and a-cellulose reduced significantly with increasing temperature and duration when samples were heat-treated above 160 °C, they both strongly correlated with mass loss. It could be confirmed that thermal treatment on bamboo shows an interesting potential to improve bamboo quality. Einfluss der thermischen Behandlung auf die physikalischen und mechanischen Eigenschaften von Phyllostachys pubescen Bambus Zusammenfassung Phyllostachys pubescen Bambus wurde in einem Trocknungsofen bei sieben Temperaturstufen (100 bis 220 °C) und vier unterschiedlichen Behandlungszeiten (1–4 h) thermisch behandelt. Die Ergebnisse zeigten, dass mit zunehmender Temperatur und
Y. M. Zhang Y. L. Yu W. J. Yu (&) Chinese Academy of Forestry - Research Institute of Wood Industry, After the summer palace, Haidian District, Beijing, Beijing 100091, China e-mail:
[email protected]
Behandlungsdauer der Masseverlust zunahm. Der ho¨chste Masseverlust betrug 29 %. Wa¨rmebehandelter Bambus wurde dunkler und alle drei Farbparameter (L*a*b*) vera¨nderten sich signifikant. Eine Wa¨rmebehandlung unter 200 °C wirkte sich nur gering auf den Elastizita¨tsmodul aus; er nahm im Vergleich zu den Kontrollproben teilweise sogar leicht zu. Jedoch nahm er bei einer Behandlung u¨ber 200 °C schnell bis zu 20, 2 % ab. Bei einer Wa¨rmebehandlung u¨ber 160 °C nahmen die Biegefestigkeit (MOR) sowie der Holocellulose- und a-Cellulosegehalt mit steigender Temperatur und Behandlungsdauer signifikant ab. Dies korrelierte stark mit dem Masseverlust. Es konnte besta¨tigt werden, dass eine Wa¨rmebehandlung von Bambus eine interessante Mo¨glichkeit fu¨r die Verbesserung der Bambusqualita¨t bietet.
1 Introduction Recently, environmental concerns and scarcity of high quality wood encourage the search for new methods of preservation. Thermal treatment has been investigated since the middle of last century for the purpose of avoiding toxic effects of chemical treatment which involves chemical substances. Now, thermal treatment has been widely used in wood industry. Thermal treatment decreases hydrophilic behavior of wood, increases its dimensional stability, mainly due to degradation of hemicelluloses (Tuong and Li 2011). Lignin also shows significant thermal alteration after thermal treatment (Windeisen et al. 2007). More crystallization is found and it is due to other components in wood which are involved in the increase of crystallinity accompanying wood cellulose during thermal treatment (Bhuiyan et al. 2000). Thermal treatment is also a suitable method of color homogenization and colorization
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(Varga and Van Der Zee 2008). However, it reduces the wood’s strength properties (Campean et al. 2007) and lowers its surface wettability (Esteves et al. 2007). Bamboo has been developing rapidly in China since the 1990s as an important forest resource alternative for wood. Along with the development of bamboo-based panel industry, bamboo glued-laminated timber and bamboo scrimber have elicited considerable attention as sustainable and renewable materials for furniture and flooring. In China, flooring and furniture made from bamboo can be divided into natural and carbonized colors based on the surface color. Generally, bamboo slabs are placed at 0.4–0.5 MPa steam pressure and heated for 1–2 h in the process to obtain darker color (commonly called carbonization). Lots of heat-treated bamboo products have been produced. Compared with wood thermal treatment, bamboo has small size strip and is not easily cracked, so the treatment duration is much shorter than that of wood. The Phyllostachys pubescen bamboo is prominent among other types and has been widely used in manufacturing furniture, constructing houses, and craft decorative products. However, due to its hygroscopic nature, the species has some undesirable properties such as swelling and shrinkage caused by water absorption and desorption, which limit applications. Thermal treatment of bamboo at high temperature is one of the bamboo modification methods for improving the dimensional stability and bio durability of bamboo. Qin (2010) had studied the effect of thermal treatment on reconstituted bamboo lumber of Neosinocalamus affinis at temperatures of 160, 180 and 200 °C, heat-treated for 2, 3 and 4 h, and found that the thickness swelling and decay resistance of the material has improved after thermal treatment, However, its strength properties reduced. Shao et al. (2003) observed a similar result for Phyllostachys pubescen bamboo, when it was heated under pressure at 0.4–0.5 MPa for 1–2 h. Hou et al. (2010, 2011) found that thermal treatment in steam made the color of Phyllostachys pubescen bamboo darker and reduced its wettability. This paper focuses on the influence of thermal treatment on the properties of Phyllostachys pubescen. Mass loss, color changes, mechanical properties and chemical components contents were measured.
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Fig. 1 Sketch of the samples processing Abb. 1 Darstellung der Pru¨fko¨rperausformung
The bamboo specimens at 8 % moisture content were thermally treated in a dry oven set at temperatures of 100–220 °C for 1–4 h. 2.2 Mass loss measurement The tested samples were kept in an oven at 20 °C and 65 % relative humidity for equilibrium. The mass loss of specimens after thermal treatment is presented on oven-dry basis. Then the mass loss after thermal treatment was estimated according to Eq. (1), where w1 represents initial mass of the dried sample, and w2 represents mass of the dried sample after heat treatment. Eight test specimens per series were measured. w1 w2 Mass loss ð%Þ ¼ 100 % ð1Þ w1 2.3 Statistical analysis Analysis of variance was performed for mass loss data by using SAS software (Version 8.0, SAS Institute, Cary, NC, USA). 2.4 Measurement of mechanical properties
2 Experimental
For the determination of thermal treatment effect on mechanical properties, Modulus of elasticity (MOE) and Modulus of rupture (MOR) were determined with 80 9 10 9 3 mm3 samples and a three point bending device. Fifteen bamboo samples were tested for each set of parameters. The vertical speed of the mobile head was 3 mm/min for MOE and a velocity of 10 mm/min for MOR. MOE and MOR were tested near the outer bamboo culms and calculated according to Eqs. (2) and (3), respectively:
2.1 Material and heat treatment
MOE N=mm2 ¼
Phyllostachys pubescen bamboo aged 5 years was obtained from Zhejiang province. The bamboo was kept in a room for air drying, and then cut to a size of 80 9 10 9 3 mm. A sketch of the sample processing is shown in Fig. 1.
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l3 DP 4b h3 Ds 3p l MOR ðMPaÞ ¼ 2b h2
ð2Þ ð3Þ
where P is the load on rupture in N, Dp Ds is the slope of the elastic zone in N/mm, l is the span, it was 16 times of the
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thickness, h represents height and b represents the width, all expressed in millimeters. 2.5 Color measurement The bamboo color was measured with a spectrophotometer (Mercury 2000) under a D65 light source and an observer angle of 10°. The sensor head of the spectrophotometer was 8 mm in diameter. Color was expressed according to the CIE (L*a*b*) system. The difference in DL*, Da* and Db* and the total color change (DE*) were calculated with the following formulas: DL ¼ L L0 Da ¼ a a0 Db ¼ b b0
ð4Þ
DE ¼ ½ðDL Þ2 þ ðDa Þ2 þ ðDb Þ2 1=2 where L* represents lightness with values varying from 0 (black) to 100 (white). The parameters a* and b* describe the chromatic coordinates on green–red (a*) and blue– yellow (b*) axes. L0 , a0 and b0 are the reference values obtained as the average of 20 untreated specimens. Color was measured for each group at 20 points near the outer bamboo culms. The results were then averaged. 2.6 Measurement of chemical components contents 2.6.1 Bamboo flour preparation For chemical analysis, the bamboo for each treatment was sawn into small pieces and milled. Powdered samples were sieved into three fractions. The middle fraction (0.3–0.4 mm) was used for chemical determinations. 2.6.2 Holocellulose content measurement Firstly, 2 g (accurate to 0.0001 g) of powders was bound with cotton after being wrapped by qualitative filter paper and extracted by benzene-ethanol. The treated bamboo powders were then placed in a 250-mL conical flask containing 65 mL distilled water, and then heated at 75 °C; acetic acid (0.5 mL) and sodium chlorite (0.6 g) were added into the solution each hour until the powders turned white. The mixture was shaken constantly. The mixture was then filtered and washed with water until the filtrate was no longer acidic, washed with acetone three times, and dried at 105 °C to a constant mass. Two sets for each sample were prepared, and the error rate of calculation of the two measurements was less than 0.4 %. The result was the average of two sets. Bamboo is no wood material; the content of ash specification should thus be subtracted in the calculation. However, for relative comparison, this was not followed.
2.6.3 a-cellulose content measurement The holocellulose (separated according to the description above) was placed in a 150-mL breaker and infused with 17.5 % NaOH (30 mL). The beaker was then placed in a water bath (20 °C) for 45 min. A total of 30 mL distilled water was then added. Next, the mixture was filtrated and washed with 9.5 % NaOH (25 mL) three times, and washed again with distilled water (400 mL). Acetic acid was then added to the residue and was hold for 5 min. The residue was washed with water until the filtrate solution was no longer acidic, and dried at 105 °C to a constant mass. Two sets for each sample were prepared, and the error rate of calculation of the two measurements was less than 0.4 %. The result was the average of two sets. Bamboo is no wood material; the content of ash specification should thus be subtracted in the calculation. However, for relative comparison, this was not followed.
3 Results and discussion 3.1 Mass loss Table 1 shows the mass loss of Phyllostachys pubescen bamboo after thermal treatment. Mass loss was less than 1.5 % when samples were heat-treated at 100–140 °C. This indicates that bamboo remains stationary when treatment temperature was below 160 °C, the mass loss is mainly attributed to water evaporation in bamboo. Mass loss increased significantly with increasing treatment temperature and duration, and reached the maximum reduction of 29.0 % when samples were heat-treated above 160 °C for 4 h. Hakkou et al. (2005) had studied the effect of thermal treatment on two hardwood species (poplar and beech) and two softwood species (pine and spruce), and found that when the wood was heat-treated at 20 and 180 °C, the mass loss remained stationary. The mass loss became significant when treatment temperature was above 200 °C. The result was similar to this research here because the organic constituents of bamboo are similar to wood (Yin 1996). Esteves et al. (2007) compared the mass loss between hardwood (eucalypt) and softwood (pine) after thermal treatment. The mass loss of eucalypt wood was much higher than that of pine wood at the same temperature and treatment time, and the reason for that is not only because of the difference in the composition of hemicelluloses, but also because the hardwood xylans have a higher susceptibility to thermal degradation than the ones of softwood. No information is available in literature with regard to the thermal degradation of Phyllostachys pubescen bamboo; the mass loss obtained in the study was higher than that of eucalypt. More research needs to be done for that reason.
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Table 1 Effect of treatment temperature and duration on mass loss of the specimens Tab. 1 Einfluss der Behandlungstemperatur und Behandlungsdauer auf den Masseverlust der Pru¨fko¨rper Mass loss (%) Treatment duration
1h
Treatment temperature (°C)
Mean value
2h *CV (%)
Mean value
3h *CV (%)
Mean value
4h *CV (%)
Mean value
*CV (%)
100
0.4
10.5
0.4
9.3
0.4
14.0
0.4
7.3
120
0.5
14.7
0.5
15.6
0.5
12.3
0.7
14.2
140
1.4
16.2
1.4
15.9
1.4
16.5
1.4
16.0
160
2.1
17.0
2.6
16.0
2.6
10.4
2.9
13.6
180
4.1
16.2
6.3
9.4
6.3
9.0
7.4
5.4
200
13.3
6.7
15.6
6.7
15.6
6.2
16.9
5.0
220
22.1
2.4
28.4
3.0
28.4
3.4
29.0
8.3
* CV ¼
SD M
100
Where CV is coefficient of variation, SD is standard deviation, M is the mean
3.2 Mechanical properties Figure 2a and b present the effect of thermal treatment on Modulus of rupture (MOR) and Modulus of elasticity (MOE). According to the obtained results, MOR increased first and then decreased when samples were heated at 100–220 °C for 1–4 h. The maximum value was found when samples were heat-treated at 120 °C. When samples were heat-treated at 100 °C for 1–4 h, MOR changed slightly, but it increased by 12.7 % compared with control bamboo. Relative increase of 1.2 % was found when samples were heat-treated at 120 °C compared with heat treatment at 100 °C. Campean et al. (2007) studied beech wood heat-treated at five temperatures (20, 80, 90, 100 and 115 °C) in the same relative air humidity (50 %) for 10 day, and found a clear increase in all the bending properties, which is in agreement with the result in this paper. The result shows that cellulose and lignin change little when samples are heat-treated below 120 °C. There is a slight degradation of hemicelluloses, but the effect on MOR is negligible. The increase in MOR is due to the water evaporation. MOR increases with decreasing moisture content within the fiber saturation point (Yin 1996). A slight decrease of 3.52 % in MOR was observed when samples were heat-treated at 140 °C compared with heat treatment at 120 °C, but compared with control samples the increase was 10.0 %. The increase in MOR may be due to the reduced moisture content within the fiber saturation point. However, the decomposition of chemical constituents led to a relative reduction. When samples were heat-treated at 160 °C for 1 h MOR increased by 5.7 % compared with control samples, but it decreased by 6.3 % for 4 h. The result indicates that chemical components degradation begins at 160 °C for
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longer duration. Hemicelluloses are the substrate material of bamboo and have an effect on bonding; they degrade first because of their low molecular weight and branching structures (Yin 1996). MOR decreased with increasing treatment temperature and duration. A decrease of 12.3 % in MOR was found when samples were heat-treated at 180 °C for 4 h and significant reduction was found when treatment temperature was above 200 °C. As shown in Fig. 2b, the effect of thermal treatment on MOE was less significant than that on MOR. The maximum value appeared at 140 °C and the inflection point was at 200 °C. There was a significant decrease in MOE when samples were heat-treated above 200 °C. Similar results were observed in wood and bamboo (Poncsa`k et al. 2006; Qin 2010). There was a slight increase in MOE when specimens were heat-treated at 100–140 °C, an increase of 3.8–8.8 % was observed compared with control samples. The increase in MOE can be due to the reduction of moisture content within fiber saturation point, which makes the rigidity enhance and leads to increase in MOE. This also confirms the fact that high temperature has a positive effect upon the plasticity of bamboo. A slight change was observed in MOE when samples were heat-treated at 160–180 °C. The effect of treatment duration on MOE was not obvious at the temperature range. An increase of 5.0 % was observed when samples were heat-treated at 180 °C compared with samples heattreated at 160 °C. As reported by Windeisen et al. (2007) and Kubojima et al. (2000), part of amorphous cellulose in wood partially crystallized, which increased the wood stiffness. When samples were heat-treated above 200 °C, MOE significantly decreased with increasing temperature and duration, and reached the maximum reduction of 20.1 %. The result suggests that the bamboo becomes brittle after
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Fig. 2 Effect of thermal treatment on MOR and MOE Abb. 2 Einfluss der thermischen Behandlung auf den E-Modul und die Biegefestigkeit
Fig. 3 Relation between mass loss and MOR Abb. 3 Beziehung zwischen Masseverlust und Biegefestigkeit
thermal treatment, which was due to the chemical components degradation. As shown in Fig. 3, MOR strongly correlates with mass loss. MOR decreased with increased mass loss. The decrease in MOR for Phyllostachys pubescen bamboo was about 12.3 % for 7.4 % mass loss and reached 34.0 % for 16.9 % mass loss. Esteves et al. (2007) obtained a similar reduction of 35.0 % for a mass loss of approximately 6.0 % for eucalypt. The reduction in MOR is the result of chemical changes induced by thermal treatment. 3.3 Color measurement Color affects the aesthetic properties of the material surface. The specimens became visibly darker after thermal treatment. The lightness decreased as treatment temperature increased. So, by proper thermal treatment, materials can copy the color of famous species, thereby enhancing the product value of bamboo. The lightness (L*) was affected by thermal treatment. When treatment temperature was below 140 °C, L* did not change much, however, it decreased significantly when samples were heat-treated above 140 °C with increasing temperature and duration. A decrease in the L* values was
associated with darker color. According to the research of Tjeerdsma et al. (1998), deepening of wood color is mainly due to the formation of benzoquinone by oxidation. Windeisen et al. (2007) studied the thermal treatment of beech for elemental analysis, and gave the conclusion that carbon content increased with increasing temperature. The increase in the content of carbon showed the loss of functional groups containing oxygen (such as carboxyl, hydroxyl acetyl, etc.). Explanations for this phenomenon are lacking in bamboo. Yellow hue (b* co-ordinate) had a similar trend to L*, significant changes were found at treatment temperature of 140 °C. Lower values were detected with increasing treatment temperature and duration. This indicates that the surface color closes to the center axis direction of the yellow–blue axis. The changes in the red color represented by a* coordinate followed another pattern. A slight change was observed when samples were heat-treated below 120 °C. When samples were heat-treated at 120–200 °C, a* values increased first and then decreased, the maximum value was found at 160 °C. The coordinate a* shifts positively indicating the color of the bamboo becomes more red. When samples were heat-treated above 200 °C, a* values decreased compared with control samples, showing a loss of red color. As L*, a*, b* were affected by thermal treatment, the final color change was significant compared with control samples. As shown in Fig. 4, the DE* values increased with increasing temperature and duration. According to the results obtained by Varga and Van Der Zee (2008), the main factors of color change were the dissolution and oxidation of wood components and the decomposition of extractives. Sundqvist et al. (2006) found that birch heattreated at 160–220 °C can produce acetic acid; the heat absorption coefficient of heated lignin increased in acidic condition; thereby it increased the degree of wood discoloration. The lignin heat-treated in acidic conditions increased more phenolic hydroxyl groups, and new
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Fig. 4 Effect of thermal treatment on color changes Abb. 4 Einfluss der thermischen Behandlung auf die Farba¨nderung
phenolic hydroxyl groups were formed, so the color of the wood was deepened (Gao et al. 2008). For the bamboo color change reason, more research needs to be done. 3.4 Contents of chemical components In thermal treatment process, the chemical components of bamboo underwent chemical reactions, which made the contents of holocellulose and a-cellulose change significantly, as shown in Fig. 5. As shown in Fig. 5, the contents of holocellulose and a-cellulose changed slightly when treatment temperature was below 160 °C, which was less than 3.0 % compared with control samples. The maximum values were observed when samples were heat-treated at 120 °C for 4 h. This is mainly due to the evaporation of its internal moisture and partial loss of volatile substances in the thermal treatment process. The contents of holocellulose and a-cellulose decreased significantly when samples were heat-treated above 160 °C,
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the maximum reductions were 26.3 and 52.9 %, respectively. This indicates that, holocellulose and a-cellulose degraded, and degraded violently with increasing temperature and duration. Windeisen et al. (2007) found that the polysaccharide had lower thermal stability due to the branched structures and amorphous tissues. They easier pyrolyze at high temperature than other components. The amount of polysaccharide decreased as treatment temperature increased. Thermal treatment made acetyl reduce; most of acetic acid was mainly from the degradation of acetyl which was in the hemicelluloses component. The contents of holocellulose and a-cellulose strongly correlate with mass loss. As shown in Fig. 6, the former decreased when the latter increased. When samples were heat-treated below 160 °C, there were slight changes in the mass loss and chemical components contents. However, the contents of holocellulose and a-cellulose decreased with increasing mass loss significantly when samples were heattreated above 160 °C. This indicates that the degradation of hemicelluloses and cellulose results in more mass loss when samples are heat-treated at higher temperature.
4 Conclusion Thermal treatment of Phyllostachys pubescen bamboo in a dry oven at temperatures between 100 and 220 °C for 1–4 h can change the mechanical and chemical properties of bamboo. It also affects the surface color. Mass loss increased significantly with increasing treatment temperature and duration. MOR strongly correlates with mass loss and decreased with increasing mass loss when samples were heat-treated above 160 °C. The effect of thermal treatment on MOE was less significant than that on MOR. There was a significant decrease in MOE when samples were heat-treated above 200 °C.
Fig. 5 Effect of thermal treatment on the contents of holocellulose and a-cellulose Abb. 5 Einfluss der thermischen Behandlung auf den Holocellulose- und a-Cellulosegehalt
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Fig. 6 Relation between mass loss and chemical components contents Abb. 6 Beziehung zwischen Masseverlust und Holocellulose- und a-Cellulosegehalt
Thermal treatment has greatly changed the color of the bamboo. The DE* values increased with increasing treatment temperature and duration. The chemical components contents also changed. The contents of holocellulose and a-cellulose decreased significantly with increasing temperature and duration when samples were heat-treated above 160 °C, and they also strongly correlate with mass loss. It can be concluded that the mass loss in samples heattreated at higher temperature is mainly due to the degradation of hemicelluloses and cellulose, and the decrease in MOR is also related to the chemical change.
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