Wang, H.M. and Wang, Xungai , Surface morphologies and internal fine structures of bast fibers, Fibers and polymers, vol. 6, no. 1, pp

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1 Deakin Research Online Deakin University s institutional research repository DDeakin Research Online Research Online This is the author s final peer reviewed version of the item published as: Wang, H.M. and Wang, Xungai , Surface morphologies and internal fine structures of bast fibers, Fibers and polymers, vol. 6, no. 1, pp Copyright : 2005, Springer

2 Surface Morphologies and Internal Fine Structures of Bast Fibers H. M. WANG AND X. WANG School of Engineering and Technology, Deakin University, Geelong VIC 3217, Australia Abstract Fiber surface morphologies and associated internal structures are closely related to its properties. Unlike other fibers including cotton, bast fibers possess transverse nodes and fissures in cross-sectional and longitudinal directions. Their morphologies and associated internal structures were anatomically examined under the scanning electron microscope. The results showed that the morphologies of the nodes and the fissures of bast fibers varied depending on the construction of the inner fibril cellular layers. The transverse nodes and fissures were formed by the folding and spiralling of the cellular layers during plant growth. The dimensions of nodes and fissures were determined by the dislocations of the cellular layers. There were also many longitudinal fissures in bast fibers. Some deep longitudinal fissures even opened the fiber lumen for a short way along the fiber. In addition, the lumen channel of the bast fibers could be disturbed or disrupted by the nodes and the spirals of the internal cellular layers. The existence of the transverse nodes and fissures in the bast fibers could degrade the fiber mechanical properties; whereas the longitudinal fissures may contribute to the very rapid moisture absorption and desorption. Keywords: Bast Fibers; Internal Structure; Cellular Layers; Nodes and Fissures; Fiber Properties 1

3 Introduction Fiber surface morphologies and associated internal fine structures are closely related to its properties [1, 2]. Bast fibers, such as flax, hemp, ramie and jute, possess special surface morphological features which play important roles in applications [3, 4]. These fibers are usually found in the natural plants as the aggregates of multiple cells which are held together by so called gums such as lignin and pectin. The single fibers and fine fiber bundles can be extracted by totally or partially removing the gums, depending on the treatment conditions [5-9]. The properties of degummed bast fibers have been extensively investigated, including the surface morphologies, physical and chemical properties [10-17]. The very large variations in the fiber mechanical properties have also been reported [10]. In particular, there have been many studies on the existence of transverse nodes and fissures in bast fibers. Under certain external forces, the nodes on flax can disappear and then can reappear when the fiber is relaxed. The explanation for this behaviour is based on the concept that the nodes on bast fibers represent disorder and low density regions and the cellulose molecules are flexible enough to change this conformation under external force [3, 18]. Furthermore, this concept also explains how the nodes with low density could contribute to the ease of bending and creasing as well as to the poor abrasion [3]. The changes of the nodes and the fissures were reported for flax treated by low temperature plasma, and for ramie after mercerization, respectively [2, 12]. However, there has been little research into the connection between the internal structures of bast fibers and the nodes and fissures on the fiber surface. In addition, the moisture absorption and desorption have been investigated mainly in relation to the 2

4 hydroxyl groups, contents of lignin and hemicelluloses, and the degrees of the crystallinity and orientation. The very rapid absorption and desorption of water is believed to be related to the nodes [3]. Nevertheless, the capillary effect of external and internal fine fissures in bast fibers has received little attention. In this paper, the surface morphologies and associated internal fine structures of several typical bast fibers were examined under the scanning electron microscope (SEM). In particular, the internal fine structures of the nodes and fissures in transverse and longitudinal sections were anatomically revealed. Moreover, the fiber longitudinal lumen and the aggregation of the multicellular layers were also described in connection with the transverse nodes. Experimental Samples of degummed ramie, hemp and flax were used for the SEM observation. The ramie and hemp were degummed by chemical processing and the flax was retted by warm-water. The ramie was produced in Hunan province, China; the flax in Heilongjiang and one variety of the hemp in Gansu. Another variety of the hemp was produced in Victoria, Australia. The fiber width was measured with an Optical Fiber Diameter Analyser (OFDA 2000) [19, 20], and the results are listed in Table 1. In the preparation of each fiber sample, the fiber was embedded in the Technovit 7100 resin (Heraeus Kulzer, Germany) and then cut in both crosssectional and longitudinal directions into the sections of 2 m-8 m in thickness. A LEO 1530 scanning electron microscope was employed for imaging. The Image-Pro Plus software was used to calculate the dimensions of the fine structures. 3

5 Table 1. Characteristics of the fiber width of the degummed hemp, ramie and flax Bast Fibers Mean m) Fiber Width CV ) Degummed Australian hemp Degummed Chinese hemp Degummed Chinese ramie Scutched Chinese flax Results and Discussion Surface morphological features of bast fibers Under the SEM, the degummed bast fibers exhibit transverse nodes or striations. Figure 1 shows a typical region with the transverse nodes along a chemically degummed Australian hemp fiber. Two thick nodes marked as a and f on the photo are about 1.20 times thicker than average, respectively. Between them, several thin nodes, b, c, d, and e, are also visible. There is no evidence showing that these nodes were constructed regularly during the plant growth. However, in the thicker node regions, the fiber can be deformed easily (Figures 1 and 2). Meanwhile, incomplete ring-shaped fissures or striations can also be found along bast fibers. The morphologies of these fissures are variable and the sizes also vary largely. For instance, in Figure 3, the marked fissure in a degummed Chinese hemp fiber has a length of m, with a width of 0.28 m, while in Figure 4, the marked striation has a length of 5.11 m with the maximum width of 2.6 m. Moreover, in Figure 5, the marked fissure in a degummed Chinese ramie fiber has a maximum width of 9.23 m and a depth of 2.42 m. 4

6 Besides the transverse fissures, there are longitudinal fissures randomly and intermittently distributed along the bast fibers. As a typical example, the average width of the fissures in a Chinese hemp fiber in Figure 6 is about 0.39 m, with the minimum of 0.19 m and the maximum of 0.85 m. By contrast, the fissures in a typical ramie fiber in Figure 7(a) have a larger width of 0.67 m in average. Such fissures as in Figure 7(a) may cut into the fiber lumen, resulting in the opening of the lumen as shown in Figure 7(b). These fine size longitudinal fissures will lead to the bast fibers very rapid moisture absorption and desorption. Internal fine structures of the nodes and the transverse fissures Figures 8 to 11 anatomically reveal the internal structures of the nodes in different bast fibers. Clearly, the nodes were usually constructed by the folding of some internal cellular layers causing dislocation for a short range during the plant growth, resulting in a thick region on the fibers and fissures inside the nodes. Occasionally, some cellular layers near the fiber tip seem to converge, creating gaps inside the node (Figure 11). The transverse fissures in bast fibers existed where the cellular layers were dislocated over a very short distance or where the external cellular layers were folded inward and/or spiralled. In the first instance, the fissures would be created across the whole fiber section, building a weak region inside the fibers (Figure 12). In the second instance, however, as in Figure 13, the fissure will not be ring-shaped around the fiber. Nevertheless, when many cellular layers were involved in the folding and the spiralling, the ring-shaped fissures would be created and many inner pores would appear in the fissure region (Figure 14). 5

7 The existence of these transverse fissures and nodes will result in flaws that affect fiber properties. In particular, when the fibers are stretched, the regions with nodes and fissures will become the weak points. Frequently, the breakage points would coincide with these weak regions (Figure 15). On the other hand, the low density and the many inner fissures or gaps can also provide certain freedom for the extension and the recovery, when a stressrelaxation examination is applied. Internal longitudinal fissures of bast fibers Many longitudinal fissures are also clearly present inside the bast fibers. When the cellular layers were dislocated, the longitudinal fissures or fine gaps were also usually present in this region. In particular, the degummed ramie has many fine fissures or gaps between the cellular layers, even in the regions without nodes or transverse fissures. As a result, the cellular layers of the degummed ramie can be easily dislocated when cutting the fibers longitudinally (Figure 16) so that strong mechanical processing can result in the fibrillation of the degummed ramie (Figure 17). Morphologies of the fiber lumen Usually, the lumen in flax and hemp are smaller than in ramie, depending on the maturity of the fibers. However, the lumen volume and orientation are also disturbed by the construction of the nodes and the transverse fissures. The dislocation of the cellular layers can also be viewed from inside the lumen (Figure 18). Moreover, the fiber lumen channel can be disrupted by the spirals of some inner cellular layers across the lumen during growth (Figures 19 and 20). 6

8 Conclusion The morphologies of the nodes and the fissures of the bast fibers varied considerably, depending on the fibril cell growth. The transverse nodes and fissures of bast fibers were constructed by the way that the inner cellular layers were folded and dislocated. The dimensions of the nodes depended upon the dislocations of the inner cellular layers. There were many longitudinal fissures in the bast fibers. Some longitudinal fissures may cut into the lumen and open the lumen for a short way along the fibers. Furthermore, the lumen channel of the bast fibers could be disturbed and disrupted by the construction of nodes and the spirals of the internal cellular layers. The existence of the nodes and the fissures in the bast fibers could degrade the fiber mechanical properties. However, the fine longitudinal fissures and gaps may contribute to the bast fibers very rapid moisture absorption and desorption. References 1. W. R. Goynes, Modern Textile Characterization Methods (M. Raheel Ed.), pp , Marcel Dekker INC, New York, K. K. Wong, X. M. Tao, C. W. M. Yuen, and K. W. Yeung, Textile Res. J., 70(10), (2000). 3. S. K. Batra, Handbook of Fiber Chemistry (M. Lewin and E.M. Pearce Eds), pp , Marcel Dekker INC, New York,

9 4. K. V. D. Velde and P. Kiekens, Journal of Thermoplastic Composite Materials, Vol. 15, (2002). 5. D. E. Akin, R. B. Dodd, W. Perkins, G. Henriksson, and K. E. Eriksson, Textile Res. J., 70(6), (2000). 6. C. Garcia-Jaldon, D. Dupeyre, and M.R. Vignon, Biomass & Bioenergy, 14(3), (1998). 7. R. W. Kessler, U. Becher, B. Goth, and R. Kohler, Biomass and Bioenergy, Vol. 14, (1998). 8. R. W. Kessler and R. Kohler, Chemtech, 26(12), (1996). 9. H. M. Wang, R. Postle, R. W. Kessler, and W. Kessler, Textile Res. J., 73(8), (2003). 10. H. M. Wang and X. Wang, in Proceedings of The Textile Institute 83rd World Conference - Quality Textile and Quality Life, Shanghai, China, pp , May 23-27, R. Postle and H. M. Wang, Natural Fibers, Vol. 2 (special edition) - Proceedings of the International Conference-Bast Fibrous Plants on the Turn of Second and Third Millennium, Shenyang, China, L. Cheek and L. Roussel, Textile Res. Inst., August, (1989). 13. Q. Liu, H. Wang, and J. Wang, J. China Textile University, Vol.17 No. 1, (1991). 14. A. Mukherjee, P. K. Ganguly, and D. Sur, J. Textile. Inst., 84 No. 3, (1993). 15. T. K. Guha Roy, A.K. Mukhopadhyay, and A. K. Mukherjee, Textile Res. Inst, December, (1984). 16. S. C. Bag, P. K. Ray, B. K. Das, and A. K. Mukerjee, Textile Res. Inst, October, (1987). 8

10 17. R. Kohler and M. Wedler, Techtextil-symposium, Vortrags-Nr. 331, R. R. Mukherjee and T. Radhakrishnan, Textile Progress, Vol.4 No.4, 1-75, (1972). 19. R. Beltran, C. J. Hurren, A. Kaynak, and X. Wang, Fibers and Polymers, 3(4), (2002). 20. H. W. Wang and X. Wang, Fibers and Polymers, Vol 5 (3), (2004). 9

11 Figure 1. A typical region with transverse nodes along a degummed Australian hemp 10

12 Figure 2. Fiber deformation in the node region of scutched Chinese flax 11

13 Figure 3. A transverse fissure in the degummed Chinese hemp 12

14 Figure 4. A transverse striation in the degummed Chinese hemp 13

15 Figure 5. A transverse fissure in the degummed Chinese ramie 14

16 Figure 6. Longitudinal fissures in the degummed Chinese hemp 15

17 (a) Longitudinal fissures (b) Cross-sectional View Figure 7. Longitudinal and cross-sectional views of the degummed Chinese ramie 16

18 Figure 8. The internal structure of a node in the degummed Australian hemp 17

19 Figure 9. The internal structure of a node in the degummed Chinese ramie 18

20 Figure 10. Fibril dislocations inside a node of the degummed Australian hemp 19

21 Figure 11. Converging cellular layers inside a node of the degummed Australian hemp 20

22 Figure 12. Dislocations of fibrils inside a transverse fissure of the degummed Australian hemp 21

23 Figure 13. A non-ringshaped transverse fissure of the degummed Chinese ramie 22

24 (a) Folds of cellular layers (b) Spirals of cellular layers Figure 14. Folds and spirals of the cellular layers insider the degummed Australian hemp 23

25 Figure 15. Breakage at the transverse fissures and nodes of the degummed Australian hemp 24

26 Figure 16. Dislocated cellular layers inside the degummed Chinese ramie 25

27 Figure 17. Fibrillation of the cellular layers of the degummed Chinese ramie 26

28 (a) Lumen channel in a node region (b) Lumen channel in a transverse fissure region Figure 18. Lumen channels in the node and transverse regions of the degummed Chinese ramie 27

29 Figure 19. A lumen channel disrupted by the inner cellular layers in the degummed Chinese ramie 28

30 Figure 20. A lumen channel disrupted by the inner cellular layers in the degummed Australian hemp 29