Effect of the amount of lignin on tensile properties of single wood fi bers

Shuang-Yan ZHANG, Ben-Hua FEI, Yan YU, Hai-Tao CHENG, Chuan-Gui WANG

1 School of Forestry and Landscape Architecture, Anhui Agricultural University, Hefei 230036, China

2 State Forestry Administration Key Laboratory on Bamboo and Rattan Science and Technology, Beijing 100102, China

3 International Center for Bamboo and Rattan, Beijing 100102, China

Effect of the amount of lignin on tensile properties of single wood fi bers

Shuang-Yan ZHANG1,2, Ben-Hua FEI2,3,?, Yan YU2,3, Hai-Tao CHENG2,3, Chuan-Gui WANG1

1School of Forestry and Landscape Architecture, Anhui Agricultural University, Hefei 230036, China

2State Forestry Administration Key Laboratory on Bamboo and Rattan Science and Technology, Beijing 100102, China

3International Center for Bamboo and Rattan, Beijing 100102, China

Chemical components are the main factors affecting the mechanical properties of wood fi bers. Lignin is one of the main components of wood cell walls and has a critical effect on the mechanical properties of paper pulp and wood fi ber based composites. In this study, we carried out tensile tests on single mature latewood tracheids of Chinese fi r (Cunninghamia lanciolata(Lamb.) Hook.), using three different delignif i ed treatment methods to obtain different amounts of lignin. We applied single fi ber tests to study the effect of the amount of lignin on mechanical tensile properties of single wood fi bers at the cellular level. The results show that in their dry state, the modulus of elasticity of single fi -bers decreased with the reduction in the amount of lignin; even their absolute values were not high. The amount of lignin affects the tensile strength and elongation of single fi bers considerably. Tensile strength and elongation of single fi bers increase with a reduction in the amount of lignin.

single fi ber, lignin, modulus of elasticity (MOE), tensile strength, elongation

?Author for correspondence (Ben-Hua FEI)

E-mail: feibenhua@icbr.ac.cn

Introduction

Wood fibers are the primary constituents of wood-based composites. The mechanical properties of wood fi bers have a strong effect on the potential use of fi bers and most end-use properties of fiber-based composite products. Furthermore,the data concerning properties and mechanics of wood fibers might be of interest in pulp and paper manufacture, e.g. the design of the pulping process and that of thermo-mechanical pulping equipment (Bergander and Salmén, 2000; Yu et al., 2003). Therefore, the micro-mechanics of single fi bers to wood science and pulp and paper manufacture is of great importance.

Cellulose, hemicelluloses and lignin are the main components of wood fiber cell walls. The relative density of lignin is only slightly less than that of cellulose. In pulp and paper manufacture,paper strength depends on the amount of lignin and cellulose in raw plant material (Xiao, 2008).Lignin is an undesirable polymer and its removal during pulping requires high amounts of energy and chemicals. At present, very little fundamental information exists on the effect of the amount of lignin on the mechanical properties of individual wood fi bers.

In our study, we used single-f i ber test technology to investigate the effects of the amount of lignin on mechanical properties of single fi bers. The techniques for isolating fi bers mechanically (Burget et al., 2002) and chemically are introduced. A further aim was to provide a scientif i c basis for a substantial improvement in the quality of wood and paper products.

Materials and methods

Materials

Material was taken from the adult wood of a 42-year-old Chinese fir (Cunninghamia lanciolata(Lamb.) Hook.) grown in Anhui Province, China.Tangential slices, 100-μm-thick, were cut with a microtome from the original, i.e., never dried,adult latewood. Some tangential slices were fi xed under a light microscope and single fibers were isolated using very fine tweezers. Others were treated with extraction methods using NaClO2for delignif i cation, consisting of: i) an aqueous solution of 0.3% NaClO2buffered with glacial acetic acid at pH 4.4 for 4 h at 80 ° C (treatment A), ii) an aqueous solution of 0.3% NaClO2buffered with glacial acetic acid at pH 4.82 for 8 h at 80 °C (treatment B) and iii) an aqueous solution (pH 2.69) of 150 mL distilled water, 1.0 g NaClO2and 2.0 mL glacial acetic acid for 8 h at 80°C (treatment C).

Quantif i cation of lignin

The amounts of lignin of native and chemically altered fi bers were determined by an acid-insoluble lignin according to GB/T 2677.8-94 (National Technical Committee 141 on Paper Industry of Standardization Administration of China, 1994).

Microtensile tests

Tensile tests on single fi bers were performed with a custom-built microtension tester (SF-Microtester I, China) . Ball and socket type fi ber gripping was used for microtensile testing (Fig. 1; Cao et al.,2010). Two resin droplets (cold-curing adhesive,HY-914, China), approximately 200 μm in diameter, were placed in the center portion of each fi ber with a fi ne tweezer. The capacity of the load cell used was 5 N. The tensile speed was 0.8 μm·s–1.Tensile testing of the dried fi bers was carried out in an environment of 20°C ± 5°C and 20% ± 5%RH. The cell wall cross-section of every broken fi ber was measured with a confocal scanning laser microscope (Meta 510 CSLM, Zeiss, Germany)(Fig. 2; Groom et al., 2002a, 2002b). In total, 30 mechanically isolated fi bers and 30 fi bers for each treatment were analyzed.

Results and discussion

Amounts of lignin after chemical treatments

The lignin contents of different chemical treatments are shown in Table 1. Compared to native samples, the amount of lignin from different chemical treatments decreased to different degrees.

Stress-strain curves

Fig. 1 Ball and socket system

Fig. 2 Measurement of cross-section area of single fi ber (I is the perimeter, A the area)

Table 1 Amount of lignin

Typical tensile load-displacement curves and stress-strain curves of single fi bers, after the different treatments, are presented in Figs. 3 and 4.All the fibers tested show a linear stress-strain behavior to failure, suggesting that changes in chemical components did not affect the tensile behavior of single fi bers. Groom et al. (2002a, 2002b)and Burger et al. (2002) found that the shape of the stress-strain curves of softwood fibers depended on their microfibrillar angle (MFA) and that individual fibers with MFAs less than 20°appeared to be fully linear during the test. In this study, the MFAs of wood fi bers were around 10°,explaining why all the wood fibers displayed a linear stress-strain behavior.

Effect of the amount of lignin on mechanical properties of single fi bers

Tensile modulus

The average tensile modulus of single fi bers, determined from microtension, as shown in Fig. 5,indicated a decreasing trend of stiffness in the fibers for the different treatments, and was correlated with a decrease in the amount of lignin.The tensile modulus was reduced by 0.87%, 4.43%and 7.13% with the amount of lignin decreasing by 11%, 25% and 99%, respectively (Fig. 5).

In the cell walls of softwood tracheids, cellulose is the main structural component working as a framework substance. Since the stiffness of cellulose (167.5 GPa) is more than 80 times that of lignin (2.0 GPa), even a two-fold increase in the amount of lignin should not noticeably alter the stiffness of a composite cell wall. Our results are consistent with the measurements made by Duchesne et al. (2001) on kraft pulp fibers by FE-SEM and CP/MAS13C-NMR.

Tensile strength

Figure 6 shows the effect of lignin on the tensile strength of single fibers. The average tensile strength of single fibers shows an increasing trend with a decrease in the amount of lignin (Fig.6). That increased by 42.61%, 67.47% and 69.06%when delignification was reduced by 11%, 25%and 99%, respectively. This finding agrees well with the earlier views of Boyd (1982) concerning a lenticular microf i bril arrangement. Such a structure with alternating close proximities or true aggregations of cellulose fi brils may well explain the fact that the removal of some of the components of the matrix may cause increased cellulose aggregation (Fig. 7). However, the result that tensile strength of a single fiber increased considerably when the amount of lignin decreased was beyond our expectations. The single fi ber samples of our indigenous species were isolated by using a mechanical isolation method that was carried out using very fi ne tweezers to peel out fi bers. During this peeling process, the cell wall layers of single fibers may become damaged reducing the true strength of single fi bers.

Fig. 3 Typical tensile load-displacement curves of single fi bers

Fig. 4 Typical tensile stress-strain curves of single fi bers

Fig. 5 Relationship between the amount of lignin and the modulus of elasticity (MOE) of fi bers

Fig. 6 Relationship between the amount of lignin and tensile strength of fi bers

Elongation

The effect of the amount of lignin on elongation of single fi bers is presented in Fig. 8. The elongation increased by 11.27%, 21.83% and 21.48% while the amount of lignin decreased respectively by 11%,25% and 99% (Fig. 8). This decrease in the amount of lignin increased the elongation of single fi bers,suggesting a slipping of microf i brils.

Correlation analysis

Table 2 shows that signif i cant differences exist in the MOE, tensile strength and elongation. There is a positive correlation between the amount of lignin and the MOE, while its correlation with both tensile strength and elongation at the 0.05 level of signif i cance is decidedly negative.

Conclusions

Fig. 7 Schematic diagram of the change of a cellulose microf i bril structure

Fig. 8 Relationship between the amount of lignin and the elongation of fi bers

Table 2 Correlation analysis between mechanical properties of fi bers and the amount of lignin

The changes in the amount of lignin, one of the main components in the matrix of wood cell walls,could account fully for the changes observed in tensile properties of single fi bers. In its dry state,lignin had little impact on the longitudinal tensile modulus of elasticity, while the amount of lignin affected the tensile strength and elongation considerably (0.05 level of signif i cance). The rate of loss in the longitudinal tensile MOE was about 7.13% after treatment for delignification.The tensile strength and elongation of cell walls increased with the reduction in the amount of lignin. Compared to native samples, the tensile strength increased by 42.61%, 67.47% and 69.06%and elongation increased by 11.27%, 21.83% and 21.48% when delignif i cation was reduced by 11%,25% and 99%, respectively.

Acknowledgement

This study was supported by the Key Program of the National Natural Science Foundation of China(Grant No. 30730076).

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23 August 2012; accepted 12 October 2012