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Research Online This is the author’s final peer reviewed version of the item published as:

Wang, H.M. and Wang, Xungai 2005-03, Surface morphologies and internal fine

structures of bast fibers, Fibers and polymers, vol. 6, no. 1, pp. 6-12.

Copyright : 2005, Springer

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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

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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

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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 cross-

sectional and longitudinal directions into the sections of 2m-8m 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.

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Table 1. Characteristics of the fiber width of the degummed hemp, ramie and flax

Bast Fibers

Fiber Width

Mean m)

CV )

Degummed Australian hemp 16.9 52.7

Degummed Chinese hemp 16.7 45.5

Degummed Chinese ramie 27.8 41.0

Scutched Chinese flax 31.1 79.1

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 10.31 m,

with a width of 0.28m, while in Figure 4, the marked striation has a length of 5.11m with

the maximum width of 2.6m. Moreover, in Figure 5, the marked fissure in a degummed

Chinese ramie fiber has a maximum width of 9.23m and a depth of 2.42m.

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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 0.39m, with the minimum of 0.19m and the

maximum of 0.85m. By contrast, the fissures in a typical ramie fiber in Figure 7(a) have a

larger width of 0.67m 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).

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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 stress-

relaxation 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).

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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.

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

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

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Figure 3. A transverse fissure in the degummed Chinese hemp

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Figure 4. A transverse striation in the degummed Chinese hemp

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Figure 5. A transverse fissure in the degummed Chinese ramie

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Figure 6. Longitudinal fissures in the degummed Chinese hemp

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(a) Longitudinal fissures (b) Cross-sectional View

Figure 7. Longitudinal and cross-sectional views of the degummed Chinese ramie

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

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

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

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

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

hemp

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

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(a) Folds of cellular layers (b) Spirals of cellular layers

Figure 14. Folds and spirals of the cellular layers insider the degummed Australian hemp

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

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Figure 16. Dislocated cellular layers inside the degummed Chinese ramie

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

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(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

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

ramie

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Figure 20. A lumen channel disrupted by the inner cellular layers in the degummed

Australian hemp