**3. Characterization of plant fibers**

#### **3.1 Fiber ultrastructure and morphology**

The ultrastructure is about dimensions between the atomic and molecular domains. These are accessed using microscopes. Morphology and quantitative chemistry investigations on plant fibers can be achieved following various analytical techniques such as Fourier transform infrared spectroscopy (FTIR), high-performance liquid chromatography (HPLC) and thermogravimetric analysis (TGA), surface electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) [7, 22]. TEM, which uses the principle of electron diffraction leads to very high magnifications of about 5,000,000. Recent progress in instrumentation has made Raman microscopy an extraordinary analytical tool in biological and plant research [24]. The main advantage of confocal Raman microscopy (CRM) is its lateral spatial resolution and the fact that it provides not only chemical composition information but also structural information.

A plant fiber is a nanostructured, renewable, sustainable and biodegradable composite material (**Figure 4**) [25]. Its cell wall can be likened to a composite lamina, consisting of a few plies reinforced with fibrils. Each individual fiber is composed of a primary wall P and a secondary wall S, itself consisting of three layers S1, S2, S3. In the centre, there may be a cavity called lumen if the cell has not filled up completely during its development. Individual cells are interphased with the middle lamellae (ML) as presented in **Figure 4**. The S2 layer of the secondary wall represents about 80% of the section and governs the mechanical behavior of the fiber [26]. The middle lamella is a wall 0.5–2 μm thick that surrounds the fiber; it plays the role of matrix that maintains the cohesion of the fibers. It is mainly composed of hemicelluloses, pectin and lignin (about 70%) [27]. **Figure 5(a)**–**(d)** show micrographs of the RC fiber [28] obtained on a Hitachi H-7650 TEM.

The microfibrillar angle is defined as the angle that the microfibrils form with the longitudinal axis of the cell. These two parameters explain partially the difference in mechanical properties between different types of cortical fibers (**Table 2**) [15]. The microfibrillar angle has a major influence on the elastic properties of plant fibers. The weaker is this angle, the better are the properties for plant fibers to behave as a composite material, which presents better mechanical properties in the reinforcement direction [22, 33]. Xu and Liu [34] predicted that the cell wall elastic modulus of wood varies by a factor of 3 when microfibril angle changes from 40° to 10°.

**Figure 4.** *Simplified structure of the wood cell wall as seen by Coté [25].*

The cellulose fibrils are oriented in a helix at an angle called micro-fibril angle, as shown in **Figure 4**. The microfibril angle in the S1 and S3 layers is greater than that of the S2 layer. It means that the fibrils in S1 and S3 layers are almost transversely oriented with respect to the fiber axis. According to the small microfibril angle in the S2 layer, its fibrils are oriented more parallel to the axis of the fiber [35]. In addition, for a given percentage of cellulose, the lower the microfibril angle, the higher the stiffness and strength of the fiber. The greater the microfibril angle, the greater the elongation at break [26]. Each microfibril can be considered as chains of cellulose crystals bound by amorphous zones [36].

The microfibril angle partly explains the elastic deformation of the plant fiber and therefore its elongation at break. Under relatively low tensile forces, a plant fiber undergoes a reversible deformation due to the progressive alignment of cellulose microfibrils with the fiber axis and an elasto-visco-plastic deformation of amorphous polymers. If the stress of the fiber is stronger, the deformation of it enters an irreversible phase that can continue until the rupture. A high microfibril angle implies a greater elastic deformation for a low tensile fiber stress. In addition, there is a negative correlation between the microfibril angle and the corresponding Young's modulus (**Figure 6**) [37].

In order to estimate suitability of different fibers to engineering and other applications, it is necessary, among other things, to determine their mechanical properties in the longitudinal and transverse directions as well as the origin of the viscoelastic properties. Thus, we will present in the following paragraphs a state of the art on the main methods used to evaluate the elastic and viscoelastic properties of PFs. Various methods have been used to measure the angle of microfibrils in the S2 layer, which is generally considered a Z-helix. Nevertheless, some studies using cross-field pit punctuations such as those of Pysznski and Hejnowicz [38] on the tracheids of Norwegian Spruce show that in about 80% of the trees studied, the Z-shaped microfibrils have an angle of 10°–40° while in the remaining 20%, the angle is lower with variations in orientation. A complete list of the different microfibril angle measurement techniques

#### **Figure 5.**

*TEM micrographs of the RC fiber (a) consecutive layers (16,400), (b) layer stacking (16,400), (c) warty sub-layer (7660) and (d) reinforcement by a small cell (10,900) [4].*


#### **Table 2.**

*Structural parameters of some plant fibers [29–31].*

*Extraction, Applications and Characterization of Plant Fibers DOI: http://dx.doi.org/10.5772/intechopen.103093*

**Figure 6.** *Variation of the young modulus with the microfibril angle of a unit cell.*

with their advantages and disadvantages is given by Huang et al. [39]. Among these techniques, X-ray diffraction is fast, but it is impossible to measure the angle of a single fiber, because of the bundle, only an average of the angle on the X-rays affected cells can be determined. The results obtained by different methods are often contradictory. For example, the work of Herman et al. [40] on individual tracheids shows large variations in the microfibril angle within annual dark circles with a sharp decrease from spring cells to summer cells. While other studies by Lichtenegger et al. [41] using the SAXS (small-angle X-ray scattering) method, on the same cell type shows a higher microfibril angle in summer tracheids than in spring tracheids. Currently, it is necessary to understand where the differences in results obtained by the available measurement methods originate from and to find a method that gives safe and reproductive results. A technique was developed by Jang [42] which uses polarization confocal microscopy based on dichromic cell wall fluorescence when stained with specific fluorochromes showing a high affinity with cellulose. In this technique, sample preparation still needs to be addressed. In fact, very thin samples, only allow observation of fluorescence intensity in the S2 layer without interference with the other layers. A quick but reliable estimate of the Rhectophyllum Camerunense (RC) fiber [28] microfibrils angle was obtained on the SEM (following a microtome longitudinal section of the fiber coinciding with the S2 layer) and fluorescence micrographs.

### **3.2 Chemical constituents**

The chemical composition of plant fibers depends largely on the particular needs of their stemming plant. However, cellulose, hemicellulose and lignin are the main constituents, and their content depends on the age, origin and extraction conditions of the fibers. Cellulose is the chemical constituent that contributes the most to the strength and stability of the plant cell wall and therefore of the fibers. The cellulose content of the fiber largely influences mechanical properties, the economic aspect and the production of the fiber. Fibers with a high cellulose content would be preferable for use in textiles, paper, composites and other fields of activity while those with a high hemicellulose content would be suitable for the production of ethanol and other fermentation products because hemicellulose is easy to hydrolyse in fermentable sugars. Thus, the value of plant fiber and its potential applications depends largely on

its cellulose content. Let us say, however, that the value of a plant depends mainly on the quality of its fibers and their end-use and not on the cellulose content itself. As with all-natural products, mechanical and physical properties of natural fibers vary greatly. These properties depend on the chemical and structural composition which depends on the origin of extraction (from leaves, seeds or stems), the local environment where the plants grow, the age of the plants and the climate. The chemical composition, structure, defects and dimensions of the fiber cells are the main parameters that condition all properties of the fibers including mechanical properties [12]. With the exception of cotton, the constituents of plant fibers are cellulose, hemicellulose, lignin, pectin, waxes and water-soluble substances. The average chemical composition of some plant fibers is shown in **Table 3**.

The chemical bonds of the fibers can be determined with FTIR. Crystallographic properties can be analyzed with XRD. TGA, DTA and DSC are used to understand the thermal degradation behavior, the maximum degradation temperature of fibers. Pullout tests applied to both raw and NaOH treated fibers aim for evaluation of the surface interaction of fibers with polymer matrices for composite materials applications.

In 1838, Anselm Payen proposed that cell walls of many plant cells be made of the same substance to which he gave the name cellulose. Cellulose is a natural polymer whose molecule, formed by long chains, consists of units of D-anhydroglucopyranoses (formula: (C6 H10 O5)n) linked by β-(1,4)-glycosidic bonds in position C1 and C4 (**Figure 7**). It represents the most abundant biological molecule on our planet. It is present in plants, algae, bacteria and some animals.

Cellulose is the major constituent of wood and is the major constituent of cotton and other textile fibers such as flax, hemp, jute and ramie. Its degree of polymerization varies according to the plant species. It can be 14,000 for native cellulose, but the insulation and purification procedures reduce it very sharply by about 2500. Cellulose contributes to the strength and rigidity of the fiber thanks to its strongly oriented chains. These macromolecular chains can be arranged, either regularly, in crystalline

