**3.2 Treatment of fibers**

The treatment of natural fibers is part of the preparation process. It takes place at the level of defibrillation (fiber separation), removal of impurities (fiber cleaning), reduction of absorption capacity (fiber hydrophobicity) and improvement of fiber adhesion to the matrix of a composite material (surface roughness and fiber bonding). Immersion in an alkaline solution (NaOH) reveals well the cleaning of the fibers which can be observed on a bamboo fiber as well as the change in roughness for a coir fiber as shown in **Figure 7**. Fiber treatments with water, boiling water, water with organic solvents or acidic agents such as ethylene diamine tetra acetic acid (EDTA) are the most eco-responsible treatments [8, 10, 14, 20]. Physical treatments are to a lesser degree environmentally acceptable but energy consuming (autoclave treatment,

*Recycling of Tropical Natural Fibers in Building Materials DOI: http://dx.doi.org/10.5772/intechopen.102999*

#### **Figure 7.**

*Aspect of treated fibers: (a) raw bamboo fiber and (b) bamboo fiber treated for 3 days in 1% NaOH solution [23]—(c) raw coir fiber and (d) coir fiber immersed for 30 minutes in a 5% NaOH solution [20].*

steam explosion, plasmas, Corona technique …). But chemical treatments based on Na2S, Na2CO3, NaOH solutions pose the problem of wastewater treatment. Some other treatments include impregnation, coupling, grafting, acetylation, benzoylation, esterification, etherification, liming …. In the most applications, alkaline solutions (**Figure 7**) and coupling techniques are the most applied. A promising technique based on enzymatic transformations would allow a more ecological treatment. These biological treatments are naturally focused on the development of biocomposites.

The benefits of these treatments are hydrophobicity, modification of the external surface of the fibers for better adhesion and improvement of the durability. The geometry of the fibers changes (decrease of diameter), they lose their flexibility but the mechanical characteristics are more or less similar.

#### **3.3 Water absorption of fibers**

Natural fibers have a very high-water absorption capacity, due to their microstructure. This absorbed water poses a problem in the elaboration of fiberreinforced composite materials with a cementitious matrix (mortar, concrete) or raw fiber earth (adobe, cob). For the former, the water in a mixture must participate in the hydration and in the raw earth, the percentage of water is necessary for the kneading and the preparation for optimal compaction. But in the drying phase, whatever the type of material, the water contained in the fibers will be extracted and the fibers will shrink with a risk of loss of adhesion between the fiber and the matrix as shown in **Figure 8**. This amount of water absorbed must be known when making the material-fiber mixtures and the parameter to be determined is the water absorption capacity Wa expressed in %, defined by:

$$\text{M } W\_a \text{ (\%)} = \frac{\text{Mass of saturated fiber} - \text{Mass of dry fiber}}{\text{Mass of dry fiber}} \times 100\% \tag{1}$$

The methods of water absorption determination are not always standardized. They consist of immersion in water and then measuring the mass of the fibers as a function of time. Each time the mass of the fibers is measured as a function of a time step, it is necessary to wipe them out (**Figure 9**). Various procedures are used such as the use of filter paper, fine synthetic fabric as a bag, tweezers or tea balls or manual wiping of the fibers. However, one protocol can be recommended for bio-based materials: RILEM TC 236-BBM (immersion and then dewatering for 15 seconds at 500 rpm, by means of the centrifuge). Some ranges of Wa values are given in **Figure 9**.

**Figure 8.**

*Behavior of a fiber at the interface of a matrix, from the mixing phase to the curing and/or drying: illustration in the case of a cementitious material [24].*

#### **Figure 9.**

*Fabric bags and clips (a) for plant aggregates, tea baskets (b) for plant fibers, fiber bags (c) for centrifugal spinning and Wa ranges for different fibers [20, 25].*


### **3.4 Specific gravity of fibers**

The measurement of the absolute density or the material constituting the fiber is normally done using a helium pycnometer. But some authors still use the water pycnometer. The problem is that water, depending on the structure of the fiber, does not penetrate all the pores (underestimated density) and the absorption capacity of the fibers may overestimate the density. To avoid these phenomena, gas pycnometer (helium) is more realistic. Specific gravities differ from each type of fibers as reported in **Table 2** from the literature.

### **3.5 Geometry of fibers**

The dimensions useful for defining the geometric parameters of a fiber are its length and diameter. The length is measured after the fibers have been cut to length by a manual (laboratory scale) or mechanical (industrial scale) process. It can be measured directly on a number of selected fibers (manual procedure) or in a more representative way, the distribution of the fibers is analyzed from a volume or a large number of fibers. This distribution gives rise to a statistical analysis (histograms, distribution law, median length…). The measurement of the diameter is more problematic because of its microstructure (compressibility, porosity) and morphology (shape). The shape of the fibers

*Recycling of Tropical Natural Fibers in Building Materials DOI: http://dx.doi.org/10.5772/intechopen.102999*

#### **Figure 10.**

*Dimensions measurements: fiber diameters with digital caliper [18] (a), fiber areas with digital optical microscope (b) for a bamboo fiber [26] and a coir fiber [20] (c).*

can be circular, an ellipse, an oblong shape … depending on the type of fiber but also on the mechanical treatment: separated, crushed, shredded fiber, …. The measurement of diameters, of orthogonal axes allows to define the cross-sectional area of a fiber. But if the shape is arbitrary, the most suitable means of measurement is image analysis using a digital optical microscope associated with image processing software. From microscope images, several geometric quantities can be defined: the largest and smallest dimension (flattening coefficient), the perimeter, the area of the fiber. A more advanced exploitation allows to approach the porosity at the level of the cross-section observed. The length and the diameter of the fibers can be measured with the help of a steel rule and digital caliper respectively for diameters of the fibers as shown in **Figure 10a**. With these measurements, the fiber aspect ratio, i.e., ratio of the length to the diameter of the fiber can be determined and it is useful in the implementation of fiber composites materials.

Also, to determine the ultimate tensile stress at failure, it is necessary to know the fiber cross-section. Two methods are proposed depending on the type and shape of fibers. The first consists of taking two measurements using a caliper with a usual accuracy of ±0.01 mm that makes it possible to obtain the dimensions of the two axes of a disk or an ellipse (assumed cross-sections). The second method more representative of the shape of fibers, is essentially based on microscope image observation. Indeed, once the fiber is broken, an optical microscope is used to obtain an image of the cross-section. This image is then processed by computer-aided drawing software to determine the area of the fiber cross-section as shown in **Figures 10b** and **c**. This second method is applied in studies of the distribution and orientation of fibers within fiber-reinforced cementitious matrices or crude earth. Fiber counting in a crosssection of the composite material allows the counting of fibers in the cross-section but also the study of the observed shapes gives the orientation of fibers in the matrix. The measurement of fiber orientation by the image analysis technique requires the preparation of a material sample cross-section depending on the technique used according to Fu et al. [27]. The spatial position of a fiber can be defined by the two Euler angles θ and φ as shown in **Figures 11a** and **b**, where θ is the angle that the fiber makes with the normal direction 1 of a plane on which the fiber orientation will be observed. φ is the angle of the fiber projected in the 2–3 direction plane. θ is given by the inverse cosine of the ratio b/a (ellipse axes) and φ by the orientation of ellipse axis a to the 2-axis.

### **3.6 Tensile strength of fibers**

One of the mechanical properties of interest is the ultimate tensile strength of the fibers. This strength value is useful in the development of composite materials. But

**Figure 11.**

*An example of definitions and determination of the fiber orientation θ and ϕ angles according to Hine [28] and Fu et al. [27].*

**Figure 12.** *Tensile strength test: possible elementary fiber installation [(a) [29], (b) [9], (c) [11]].*

knowing the tensile behavior law of a free fiber (or gauge fiber) is needed for any development of numerical modeling for these materials. This behavior law is often defined by the stress-strain relationship. It is obtained from the tensile force-displacement relationship recorded during a tensile test on a fiber. To carry out the tensile tests, it is necessary to install the fiber on specific support if the fiber flexibility is limited and becomes too brittle. If the fiber is sufficiently flexible, the fiber can be clamped directly in the jaws of the testing machine. The clamping system is mechanical (M) or pneumatic (P) as shown on **Figure 12a**. But usually, for short fibers, a cardboard is used to hold the fiber before testing (**Figures 12b** and **c**). To install fiber on a cardboard frame, squares or rectangles of card stock are cut and prepared with internal dimensions depending on the free length of fiber testing. The test procedure is presented in **Figure 12c** and is as follows: installation of fiber on cardboard, clamping the cardboard on the machine, cutting the cardboard, putting the fiber under tensile loading till failure. The data recorded concerns load versus axial displacement and mainly the ultimate tensile strength as well as the maximum elongation at failure. Usually, tensile tests are carried out on different machines using different sensors. The test is performed at various constant speed rates ranging from 0.5 mm/min to 5 mm/ min. Also, the machines are equipped with different more or less accurate sensors. Tests are conducted in constant room thermo-hygrometric conditions (temperature around 20–25°C). For short fibers (total length ≤ 50 mm), the free length varies from 10 to 20 mm.

Once the test is validated (failure in the part of free fiber) the stress-strain curve is analyzed and another parameter is determined: the modulus of elasticity if the fiber has an elastic or pseudo-elastic behavior. Depending on the behavior of the fiber, a linear part exists or not. It can be defined then an initial tangent modulus Et (**Figure 13a**), or the modulus of deformation can be defined on the linear part just before the failure

*Recycling of Tropical Natural Fibers in Building Materials DOI: http://dx.doi.org/10.5772/intechopen.102999*

#### **Figure 13.**

*(a) Flax fiber behavior under tension cycle [11], (b) determination of coconut fiber cross-section after a tensile strength test [20].*

as shown in the same **Figure 13a** (modulus Ef). And this choice can be justified by the fact that a cyclic test can demonstrate elastic behavior as for a flax fiber, see **Figure 13a**. Furthermore, the determination of the ultimate stress in a fiber under traction requires the knowledge of the cross-section at the moment of rupture, although there is a constriction of the cross-section as shown in **Figure 13b**, for which the determination of the cross-section is made by using microscope image and image analysis software.

#### **3.7 Pull-out resistance of fibers**

The pull-out strength of the fibers in the matrices in which they are incorporated is another mechanical parameter necessary for the formulation of composite materials. In particular the shear stress at the fiber/matrix interface. It plays a major role in the case of short fibers [27]. It is also used to evaluate the critical fiber length. The critical fiber length (Lc) is the minimum length required to effectively strengthen and stiffen the material. It is defined by:

$$\mathbf{L}\_{\varepsilon} = \mathbb{W}\,\sigma\_{\sharp} \,\,\mathbf{D} / \tau \,\tag{2}$$

where σtf is the ultimate tensile strength of the fiber, D is the fiber diameter, and τ is the interfacial shear strength at the fiber/matrix interface, see **Figure 14e**.

The critical fiber length can be estimated using the measured fiber diameter D and the values of σtf and τ issued from experimental tests or literature. The isolated (single) fiber pull out test requires a particular molding of anchored unit fibers of length Lf as shown in **Figure 14e**. The unit fibers are distributed along with a cast matrix (in the case of a cementitious or polyester resin-based material, **Figure 14d**) or crude earth (**Figures 14a** and **b**). The samples thus produced (**Figures 14c** and **d**) are submitted to a tensile test until the fiber is pulled out (Lf < Lc) or the fiber breaks (Lf > Lc). The test machines are the same as those used for the fiber tensile tests (Section 3.6).

#### **3.8 Some properties of natural tropical fibers**

#### *3.8.1 Useful properties of natural tropical fibers in building materials*

Natural fibers from the Tropics for use in building materials are relatively abundant, as the data in **Table 2** show. But among the fibers incorporated in building materials are

#### **Figure 14.**

*(a) Crude earth specific wooden mold for pull out a test of hemp fiber, (b) crude earth sample with different hemp fibers before pull out testing, (c) pull out test of hemp fiber, (d) coir polyester composite specific mold for pull out test [30], and (e) simple mechanism of shear stress and pull-out force in the case of elementary fiber in a matrix.*

coconut, sugarcane, sisal, palm fibers and to a lesser degree banana spine fiber. These are also the most widely investigated fibers in building materials at present. More recent interest has focused on the recycling of natural fibers considered as waste, such as oil palm fibers after the production of oil from the fruit. The characteristics of tropical fibers detailed below have been the focus of studies conducted by the authors. They are the most widely used fibers in building materials and in particular, the use of palm fibers constitutes an innovation in eco-friendly building materials.
