The effect of ages on the tensile mechanical properties of elementary fibers extracted from two sympodial bamboo species

Industrial Crops and Products 62 (2014) 94–99

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The effect of ages on the tensile mechanical properties of elementary fibers extracted from two sympodial bamboo species Dan Ren a,b , Zixuan Yu a,b , Wanju Li a,b , Hankun Wang a,b , Yan Yu a,b,∗ a b

Department of Biomaterials, International Center for Bamboo and Rattan, Beijing 100102, PR China SFA and Beijing Co-built Key Laboratory of Bamboo and Rattan Science & Technology, State Forestry Administration, Beijing 100102, PR China

a r t i c l e

i n f o

Article history: Received 18 May 2014 Received in revised form 4 August 2014 Accepted 9 August 2014 Keywords: Bamboo fiber Microtension Mechanical properties Variation Bamboo ages

a b s t r a c t Bamboo fibers are known for their outstanding mechanical properties and could be a potential replacement for synthetic fibers used in fiber-reinforced composites. In this paper, mechanical variation related to age for elementary fibers of two important sympodial bamboo species (Dendrocalamopsis oldhami and Dendrocalamus latiflorus Munro) was analyzed using a microtension technique. From the investigation of elementary fibers ranging from 1 to 6 years in age, our results showed the average tensile modulus of the two types of bamboo fibers ranged from 42.84 GPa to 44.29 GPa and 33.51 GPa to 37.35 GPa, whereas the tensile strength ranged from 1.50 GPa to 1.70 GPa and 1.34 GPa to 1.52 GPa, respectively. These values are significantly higher than equivalent properties found in most natural plant fibers. Furthermore, bamboo fibers were found to have nearly reached their optimal mechanical properties after just 1 year, with subsequent variations in older fibers proving statistically insignificant. This highlights the suitability of using young bamboo fibers as the reinforcing phase in polymer composites. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Bamboo is one of the fastest growing plants in the world and can be used for multiple-purposes. The extraordinary mechanical properties of bamboo mainly derive from its fiber components (Amada and Untao, 2001; Lo et al., 2004). Bamboo fibers can compete with synthetic fibers, such as glass fibers, due to their relatively low cost, renewability, biodegradability, accessibility, high aspect ratio and carbon dioxide neutrality (Dittenber and Gangarao, 2012; Faruk et al., 2012). Furthermore, it is much safer to deal with bamboo fibers than glass fibers during the manufacturing process, because the latter produces tiny glass particles that are harmful to human health (Liang et al., 2013). Therefore, bamboo fibers have great potential to replace synthetic fibers in the production of fiberreinforced composites, as has been demonstrated by the increasing number of research papers published in this area in the last decade (Takahashi et al., 2011; Gamon et al., 2013; Kim et al., 2013). The mechanical properties of bamboo fiber-reinforced composites depend largely on the fiber quality (Han et al., 2008). Thus, improving available engineering data on the mechanical properties of bamboo fibers is highly important for ensuring the manufacture

∗ Corresponding author at: No.8, Futong Dong Dajie, Wangjing Area, Chaoyang District, Beijing, China. Tel.: +86 01084789812. E-mail address: [email protected] (Y. Yu). http://dx.doi.org/10.1016/j.indcrop.2014.08.014 0926-6690/© 2014 Elsevier B.V. All rights reserved.

of composites with desired performance grades. Typically, elementary bamboo fibers, which are smaller than most other natural fibers, with lengths ranging from 0.5 to 3 mm and diameters from 5 to 25 ␮m, are technical challenging to test for their mechanical properties. The main challenges include the lack of adequate devices to properly grip the fibers, aligning the fibers parallel to the tensile direction and the measurement of the cell wall area of single broken fibers (Yu et al., 2011a). Therefore, most of the mechanical characterizations of bamboo fibers are performed at the fiber bundle level (Amada et al., 1997; Shao et al., 2010), while only a few measurements have been taken for elementary fibers (Wang et al., 2011a). Recently, Yu et al. (2011b) developed an improved microtensile system to measure the tensile mechanical properties of individual short plant fibers, with the reliability and practicality of this system demonstrated through a case study on elementary bamboo fibers. The mechanical properties of elementary bamboo fibers may be influenced by many factors, including the type of bamboo species, locations of the fiber within the culm and the ages of the bamboo. Bamboo is classified into two main categories, namely sympodial bamboos, primarily found in sub-tropical and tropical regions, and monopodial bamboos, found in temperate regions. In previously published studies, sympodial bamboos appear to have considerable potential in terms of their mechanical properties. For example, Wang et al. (2014) found that the average tensile modulus of Ma bamboo (Dendrocalamus latiflorus Munro) fibers at

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4 years of age were 45.8 GPa, significantly higher than the 34.6 GPa recorded for Moso Bamboo (Phyllostachys edulis (Carr.) H.de Lehaie) of the same age (Yu et al., 2011a). The tensile strength of these two types of bamboo fibers was comparable. It was also observed that the in-tree variation of mechanical properties of Ma bamboo fibers was rather small both in radial and vertical directions. However, despite sympodial bamboo fibers’ seemingly favorable mechanical properties, most previous studies investigating the effect of age on fiber performance have focused on monopodial bamboo, especially Moso bamboo, the most commercially important bamboo species in China. In Moso bamboo, it has been found that the tensile modulus and strength of fibers varies very little across samples ranging from 0.5 to 4 years in age, thereby indicating that age has little effect on mechanical properties at the cell wall level after the first 6 months (Yu et al., 2011a). Huang et al. (2012) also studied the effect of bamboo age (0.5–8.5 years) on the mechanical properties of Moso bamboo fibers, with their results also indicating that there is no significant variation in average tensile modulus and fracture strain. However, according to statistical analysis, although the absolute values change minimally with age, the effect of bamboo age on tensile strength is significant. Although findings for monopodial bamboo appear to be fairly conclusive, there has been little research on how bamboo age affects the mechanical properties of sympodial bamboo fibers. Since the mechanical properties of sympodial bamboo fibers may well be superior to monopodial ones, identifying whether these properties reach a stable, near optimal state at a young age could be potentially important for broadening commercially application of these types of fibers. Therefore, the objective of this research is to evaluate the effect of age on the fiber tensile mechanical properties of two commercially important sympodial bamboo species in China, namely Lv bamboo (Dendrocalamopsis oldhami) and Ma bamboo, which will provide insights on how to improve their selection and utilization. This information, which will optimize bamboo fiber-reinforced composites production, will be highly beneficial to the manufacturing industry. 2. Experimental 2.1. Sample preparation Lv bamboo and Ma bamboo of 1, 2, 4, and 6 years of age were taken from a bamboo plantation in Fujian Province, China. All the blocks were cut out from bamboo culms at 2 m in height and then split into sticks measuring approximately 1 (radial) × 1 (tangential) × 15 (longitudinal) mm3 . The sticks were macerated at 60 ◦ C in a soft solution consisting of one part 30% hydrogen peroxide, five parts glacial acetic acid, and four parts distilled water for 24 h. Subsequently, fibers were washed at least eight times in water and dried on glass slides at room temperature (Fig. 1a). 2.2. Single-fiber tensile measurement For microtensile measurements, elementary bamboo fibers with the length about 2 mm and diameter 8–20 ␮m were selected for the testing (Fig. 1b). We used the widely accepted “ball and socket” fiber gripping methodology using a custom-built fiber gripping system, which has been specially developed and combined to a small commercial high resolution mechanical tester (Instron Microtester 5848, USA, Fig. 1c). Using fine tweezers, two thermoset epoxy droplets of about 200 ␮m in diameter were placed in the center portion of each fiber with spacing of about 0.8–1.0 mm. The thermoset epoxy was solidified in an oven with a temperature of 60 ◦ C for 24 h, followed by an additional balance at room conditions for

(a)

95

(b)

0.8 mm

0.5 mm

(c)

(d)

1 mm Fig. 1. The experimental setup for elementary bamboo fiber tensile testing. (a) The chemically isolated bamboo fibers. (b) Epoxy droplets placed at the ends of bamboo fibers. (c) Instron 5848 microtester combined with a special developed fiber-gripping system for fiber mechanical testing. (d) Patented fiber grips.

24 h. More details for sample preparation can be found from Groom et al. (2002a). The initial span between the two epoxy droplets of each fiber can be measured directly with a vertical digital microscope built into the microtensile system (Fig. 1d). More details about the testing device are given in Yu et al. (2011b). The capacity of the load cell used was 5 N. Elongation was recorded from the crosshead movement with a displacement resolution of 0.02 ␮m and a constant speed of 48 ␮m min−1 . In total, more than 50 fibers were tested for each age of the two species to ensure more than 30 reliable measurements were recorded. All the tensile testing was carried out under an environment of 23 ◦ C and at 15–35% RH. In order to calculate the tensile strength and modulus of bamboo fibers, the cell wall area of each broken fiber was measured with a confocal scanning laser microscope (Meta 510 CSLM, Zeiss). The broken fibers were first immersed in 0.1% acridine orange solution for 20 s and then rinsed in pure fined water 2–4 times (Groom et al., 2002a). The fibers were then imaged with a 63× immersion oil objective lens. The cell wall area of each fiber was then surveyed with the image analysis software, Image Pro 6 (Fig. 2). Based on the value of the initial fiber span and the cell wall area, the load–elongation curves were then converted into stress–strain curves to obtain the tensile strength and modulus. Finally, a certain number of the broken bamboo fibers were coated with conductive platinum film and observed with a filed-emission scanning electron microscope (XL30-FEG-SEM, FEI, USA).

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12 Lv bamboo Ma bamboo

11 10

MFA /

9 8

Fig. 2. Cross-sectional areas of single bamboo fibers determined by confocal scanning laser microscope.

7 6

2.3. Microfibrilar angle test

2.4. ANOVA analysis To determine the significance of ages on the fiber tensile modulus and strength in tested single bamboo fibers, SPSS 16.0 software was used to perform a One-way analysis of variance (ANOVA). 3. Results and discussion 3.1. The MFA variation of bamboo fibers Mechanical properties of wood fibers are largely related to the orientation of the microfibrils, and the relationships between them have attracted tremendous interest during recent years. Many experiments have shown that wood fibers with smaller MFA have higher tensile strength and modulus, but also have smaller extensibility (Cave, 1997; Page et al., 1971; Groom et al., 2002b; Burgert et al., 2002). However, compared to wood fibers, bamboo fibers demonstrate little variation for MFA. For example, Yu et al. (2007) pointed out that the MFA of Moso bamboo fibers only ranged from 8◦ to 10◦ and varied little with changes of culm location and age. In this paper, we also found that the MFA variation for different ages for Lv and Ma bamboo is very small, varying from 9.15◦ to 10.73◦ for the former and 8.51◦ to 8.68◦ for the latter (Fig. 3). The variability in MFA is also tiny with a coefficient of variation ranging from 4.77% to 11.34% for Lv bamboo fibers and 3.08% to 4.72% for Ma bamboo fibers of different ages. Therefore, it is reasonable to expect that the tensile strength, tensile modulus and elongation at break of these two types of bamboo fibers will not change with increasing bamboo age. 3.2. Typical stress–strain curves of single bamboo fibers The typical stress–strain curves of single Lv bamboo and Ma bamboo fibers in tension are presented in Fig. 4. It is clear that

2

1

4

6

1

2

4

6

Bamboo age / year Fig. 3. The MFA of Lv bamboo and Ma bamboo fibers aged from 1 to 6 years old.

all of the tested bamboo fibers demonstrated a linear stress–strain behavior to failure. It has been extensively shown that the stress–strain curves of plant fibers are mainly determined by their MFAs (Page and El-Hosseiny, 1983; Baley, 2002; Lefeuvre et al., 2014). In fibers where MFA is large than 20◦ , stress–strain curves exhibited a curvilinear behavior (Groom et al., 2002b). In contrast, for Moso bamboo fiber, where MFA only ranges from 8◦ to 10◦ , stress–strain curves are entirely linear up to failure (Yu et al., 2011a,b; Wang et al., 2011b; Huang et al., 2012). In this paper, Lv bamboo and Ma bamboo fibers normally have a MFA approximately 10◦ , which explains why for all tested fibers across all ages also produce linear stress–strain curves. Furthermore, the shape of stress–strain curves can also be used to determine whether slipping between fiber and epoxy resin droplet occurs during tension. Our results indicate the method of fiber gripping adopted here causes negligible slipping. In order to further make sure no or negligible slipping occurred during normal conditions of tension, some broken fibers were also carefully observed under a high-resolution SEM. Fig. 5 shows the micrographs of a single broken bamboo fiber at different magnification. The images suggest that there is no epoxy resin runs occurring onto the testing length of the fiber, which could potentially strengthen the recorded mechanical properties of tested fibers. Moreover, no visible “pulling out” of fiber could be observed at the proximity interface between fiber and resin. However, in the present study, the strain was measured directly with cross head displacement, which ignored the additional strain caused by the epoxy

2400 2000

Stress/MPa

The microfibrilar angle (MFA) is defined as the angle between the fiber axis and the cellulose orientation in the dominant S2 layer, which has significant effects on most of the physical and mechanical performance of plant fibers. An X-ray diffractometer (X’pert Pro, Panalytical Company, USA) was used to determine the MFA of the bamboo fibers of different ages to figure out how MFA affects the tensile properties of bamboo fibers. Bamboo sections with the dimensions of 1.5 (radial) × 10 (tangential) × 30 (longitudinal) mm3 were prepared from the above mentioned bamboo blocks. A point-focused X-ray beam was applied to the tangential section with a scanning angle range of 0–360◦ and a scanning step of 2◦ s−1 . The radiation source was CuKa ( = 0.154 nm). The tube voltage was 40 kV, and the current was 40 mA. From the obtained diffraction intensity curves, MFA of bamboo fibers with different ages were determined based on the well-established 0.6 T method (Cave, 1997; Yu et al., 2007). 20 sections from each age were measured and the average MFA was obtained.

5

1600 1200 800 400 0

0

1

2

3

4

5

6

Strain/% Fig. 4. Typical stress–strain curves of elementary bamboo fibers under tension in the axial direction.

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97

Fig. 5. Fracture surfaces of elementary broken bamboo fibers at different magnifications.

resin droplets and the machine frame. Therefore, the elastic modulus tested might be theoretically less than the real value. Another approaches, such as non-contact optical measurement could be attempted to further increase the accuracy of the measurement.

Tensile modulus / GPa

(a)

Lv bamboo Ma bamboo

50 40 30 20

2.5

(b)

Tensile strength / GPa

The mechanical variations in tensile modulus, strength and elongation at break as a function of age of Lv and Ma bamboo fibers are shown in Fig. 6. As a general rule, it seems that age has little effect on the tensile mechanical properties of bamboo fibers. For Lv bamboo fibers, their average tensile modulus and strength for different ages only ranged from 42.84 to 44.29 GPa and 1.50 to 1.70 GPa respectively. Ma bamboo fiber also showed minimal variation across the different ages ranging from 34.97 to 37.14 GPa for tensile modulus and 1.35 to 1.48 GPa for tensile strength. Therefore, bamboo fibers from both Lv and Ma were found to reach their optimal cell wall mechanical performance within the first year, with subsequent variation due to age being rather small. Itoh (1990) proposed that the cell wall of Phyllostachys heterocycla Mitf. bamboo fibers was fully developed in less than one year as lignification could be completed at the end of the first growing season (approximately 6 months). As previously stated, the tensile mechanical properties of Moso bamboo fibers also display similar age dependence (Yu et al., 2011a; Huang et al., 2012). However, despite these findings, in the Chinese bamboo sector it is common practice to wait until bamboos are at least 4 years in age before harvesting. The improved macroscopic mechanical properties of bamboo which are observed at older ages are, therefore, mainly due to the increase of specific density caused by cell wall thickening rather than because of any mechanical improvement of the fiber cell wall itself. Fig. 7 shows the effect of ages on the cell wall area of both Lv and Ma bamboo fibers. It can be found the cell wall area of both fibers increased significantly with ages until 6 years. In order to further verify the effect of age on the tensile modulus and strength of Lv bamboo and Ma bamboo fibers, a detailed statistical analysis was also carried out, with the results shown in Table 1. The results show that there is no significant variation in tensile modulus and strength with age for both Lv and Ma bamboo fibers (P ≤ 0.05). Thus, we conclude that the tensile modulus and strength of these two types of bamboo fibers are predominantly unaffected by age. Furthermore, many previous studies have already demonstrated that the tensile modulus and strength of wood fibers are highly dependent on MFA (Cave, 1968, 1969; Page et al., 1977). Since all the bamboo fibers tested in this study, as shown in Fig. 3, have similar MFA, their tensile modulus and strength should also be similar. Therefore, in terms of reinforcement effect, one-yearold bamboo fibers of both Lv and Ma bamboo could be used to

2.0 1.5 1.0 0.5 0.0 6

Elongation at break / %

3.3. The effect of ages on the mechanical properties of single bamboo fibers

60

(c)

5 4 3 2 1 0 1

2

4

6

1

2

4

6

Bamboo age / year Fig. 6. Tensile modulus (a), tensile strength (b) and elongation at breaking (c) of Lv bamboo and Ma bamboo fibers aged 1–6 years old.

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Table 1 One-way ANOVA test on the effects of the age of Lv bamboo and Ma bamboo fibers on their tensile modulus (TM) and tensile strength (TS). Index

Source of variation

St a

dft

Se b

dfe

Fc

F0.05 (dft , dfe )

Pvalue

Significant

Lv bamboo

Age VS TM Age VS TS Age VS TM Age VS TS

11.20 0.22 30.11 0.08

3 3 3 3

65.73 0.11 24.63 0.06

102 102 111 111

0.17 2.00 1.22 1.33

2.70 2.70 2.69 2.69

0.92 0.10 0.31 0.29

No No No No

Ma bamboo a b c

Mean square of between groups. Mean square of within groups. F equal to St /Se .

Table 2 Statistical comparison of mechanical properties among bamboo fibers, wood fibers and some bast fibers. Index

Tensile modulus (GPa)

Tensile strength (GPa)

Elongation at break (%)

Lv bamboo (1–6 years old) Ma bamboo (1–6 years old) Moso bamboo (0.5–8.5 years old)a Loblolly pineb Fir latewoodc Kenafc Ramiec Hempd

43.67 ± 0.62 35.98 ± 1.06 32–36 6.55–27.5 14 ± 6.70 20 ± 7.60 11 ± 1.90 35.5 ± 17.30

1.61 ± 0.09 1.40 ± 0.06 1.2–1.7 0.41–1.42 0.91 ± 0.42 0.98 ± 0.19 1.00 ± 0.15 0.89 ± 0.47

4.05 ± 0.36 4.26 ± 0.43 3.8–5.8 – 8.20 ± 2.50 5.40 ± 1.70 8.90 ± 1.60 2.6 ± 2.20

a b c d

From Huang et al. (2012). From Groom et al. (2002a,b). From Wang et al. (2011b). From Marrot et al. (2013).

replace other natural plant to fabricate fiber-reinforced composites. Aside from MFA, the procedure of fiber extraction will also affect the performances of elementary bamboo fibers. Chen et al. (2011) compared the mechanical properties of elementary Ci bamboo (Neosinocalamus affinis) fibers macerated with four chemical solutions and found the solution of hydrogen peroxide and glacial acetic acid produced the highest tensile strength and moderate elastic modulus. The average values taken across all ages for Lv bamboo fibers were 43.67 GPa for tensile modulus, 1.61 GPa for tensile strength, and 4.05% for elongation at break, while Ma bamboo fibers average respective values were 35.98 GPa, 1.40 GPa and 4.26%. In order to provide context for the level of performance of these two types of bamboo fibers, a comparison with other plant fibers, including wood, hemp, kenaf, and ramie is shown in Table 2. Both the tensile modulus and strength of Lv and Ma bamboo fibers are significantly higher than those of wood, kenaf and ramie fibers. While hemp fibers have comparable tensile stiffness to bamboo fibers, their tensile strength is much lower. In Yu et al. (2011a), it is suggested that the superior mechanical properties of bamboo fibers are due to their smaller MFA, scarcity, small size of pits and high

220

Cell wall area/ µm

From the results of this experimental investigation by means of microtension testing of single fiber from Lv bamboo and Ma bamboo, we can draw the following main conclusions: (1) Lv bamboo and Ma bamboo fibers reach their optimal tensile strength after 1 year, which means the improved macroscopic mechanical performances of older bamboos are mainly due to cell wall thickening, rather than any enhancement of the mechanical performances of the cell wall itself. (2) Bamboo age has little effect on tensile modulus and tensile strength in both Lv bamboo and Ma bamboo fibers. (3) These two types of bamboo fibers are much stronger and stiffer than many other natural plant fibers, indicating that more attention should be paid to the efficient utilization of bamboo fibers from sympodial bamboo species especially for producing fiber-reinforced composites.

We would like to thank the 12th Five Years Key Technology R&D Program of China (2012BAD54G01) and the National Science Foundation of China (31070491) for their financial support for this research. We also thank Mr. Oliver Frith of the International Network for Bamboo and Rattan (INBAR) for his revision of this manuscript.

2

180

4. Conclusion

Acknowledgments

Lv bamboo Ma bamboo

200

damage tolerance, which in turn originates from its nearly solid cell wall structure (Yu et al., 2011a). Therefore, in terms of their mechanical properties, bamboo fibers appear to be an ideal choice for fabrication of fiber-reinforced composites.

160 140 120 100 80

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60 40 1

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6

Bamboo age / year Fig. 7. Average cell wall areas of tested elementary bamboo fibers aged from 1 to 6 years.

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