Extraction, Applications and Characterization of Plant Fibers

Abstract

During the second half of the twentieth century, industrial and scientific interests in plant fibers (PFs) have resulted in their resounding comeback as engineering materials. This chapter is concerned with the characterization of PF materials. Good knowledge of the properties of these materials is essential for safe design of the related structures. Bast fibers that are collected from the phloem surrounding the stem of certain dicotyledonous plants, for instance, are among the most used, owing to their higher tensile strength. However, for an optimum utilization of PFs, a relevant assessment of their physico-chemical and mechanical properties is very crucial. As it is now well established, PFs’ properties are largely influenced by their hierarchic composite microstructure and their viscoelastic behavior. This book chapter focuses on the presentation of various experimental approaches used to characterize the elastic and viscoelastic behaviors of plant fibers. Consideration of their blending in sheet form and relevant mechanical properties will also be of interest.

1. Introduction

In the 1830s, the use of plant resources such as flax, hemp and others was widespread as their fibers were in high demand by the textile, the paper and sailing industries. These plants were grown over large areas for exploitation. However, with the progress of science and technology (loom, steam engine, development of cotton harvesting and processing technique and others), materials such as metals, ceramics, glass, polymers, stones and concrete were preferred to plant resources. In 1900, fiber plants experienced their lowest implantation in terms of surface area. Indeed, the rise of new materials has greatly contributed to the improvement of human living conditions through the construction of more robust and sophisticated habitats, the development of the automotive, railway, aeronautics, textile industries, etc. Subsequently, in a concern for economy, lightness and performance, the development of composite materials was born during the 1930s.

The industrial use of plant fibers began in the early twentieth century with the manufacturing of aircraft seats, fuel tanks or other electronic boxes in plant fibers reinforced polymer materials. The need for securing constructions or structures that are made up of these materials inevitably arose. From an engineering viewpoint, this is taken into consideration during the design, due to a good knowledge of the material characteristics. Plant fibers have specific properties that make them good candidate reinforcing materials for high-performance composites and other applications [1]. However, the mechanical properties of PFs vary considerably both within the same species and from one species to another. Humidity variation, for example, leads to shrinkage or swelling that changes mechanical properties [2]. Similarly, their thermal properties are by far very different from those of synthetic fibers.

Various studies also indicate that plant fibers exhibit, for example, a very complex anisotropic behavior [3, 4, 5]. This anisotropy must be accounted for if a reliable design is to be achieved. Close collaboration between scientific disciplines such as botany, chemistry, biochemistry, molecular structural biology, plant genetics, physics and mechanics allows each of them to make a constructive and complementary contribution. PFs must withstand stresses of all kinds when they are associated with their deriving plants. They are loaded when it comes to supporting the weight of the plant or when it comes to resisting the winds, storms and hurricanes so common in their environment. PFs are diverse, and can all be studied for their use as engineering materials, in order to take benefit of the particular advantages offered by each of them. Meanwhile, their mechanical, physical and chemical characterization can differ between members of the same species and from one species to another. They are most often in the form of bundles (technical fibers) comprising one to twenty elementary fibers. They have a complex hierarchical structure inducing anisotropy and, have great geometric and mechanical variability. Humidity variation, for example, leads to shrinkage or swelling and changes in mechanical properties.

The characterization of material generally involves so-called monotonous tests (tensile, compression, torsion, bending or a combination) according to the load’s direction (uniaxial or multiaxial), cyclic tests, hardness and resilience tests. Tensile test is undoubtedly the most common test applied to PFs [6, 7, 8, 9, 10, 11, 12] because it allows obtaining Young’s modulus, strength and elongation at break. Recent works show that PFs exhibit a delayed behavior over time and temperature [13, 14] highlighting their viscoelastic nature. A tensile test alone is therefore not sufficient to characterize these materials.

This chapter is structured in four sections. Following this introduction, Section 2 will give an overview of some essential applications, the supply chain and the techniques of separating fibers from their plant. In Section 3, we will describe the experimental characterization methods generally used to derive their structure morphology and their elastic, viscoelastic and thermomechanical properties. Section 4 is the conclusion.

2. Plant fibers

2.1 Common applications

The natural fiber derived from wood, sisal, hemp, coconut, cotton, kenaf, flax, jute, abaca, banana leaf fibers, bamboo, wheat straw or other fibrous material and the matrix can be a polymeric material. The key advantage of natural fibers and their composites over traditional materials is their biological and environmental durability as well as their superior biodegradability. Natural PFs are increasingly used in several fields of engineering applications because of their interesting properties [15]. Diverse abundance of natural fiber, shapes and forms is caused by their occurrence in different climatic zones, hence stimulating the interest and opportunities to conduct comprehensive studies for identifying new applications for the fibers in industry. Notably, they are gaining popularity due to their optimal use in reinforcement of bio-composite structures. These fibers are biodegradable, structurally sound and environmental friendly. However, a sound theoretical basis for modeling their structure and mechanical behavior has yet to be established. Thus, it will be a priority field of study that will challenge the scientists and researchers.

2.2 Fibers supply chain

The emerging trends and opportunities for natural fibers are broadening due to desirable attributes such as biodegradability, eco-friendly, sustainability and energy efficiency. Sustainability supply chain of natural fibers is assessed and rated based on the following criteria: water usage, CO₂ emissions, cost, availability and any other impacts [16]. Moreover, in the fashion industry, businesses tend to identify the impacts of fibers on brands that contribute to the most impressive reduction in their impact on environmental footprint. Some of the preferred fibers include Linen, Tencel, Bamboo, Recycled Polyester, Recycled Wool, Cork, Organic Cotton and Hemp.

Perhaps the most important factor is the understanding of the entirety of the supply chain of natural fibers and the stages that contribute to having the biggest impacts. Consequently, a map of biodiversity quantitative impact indicators that help the companies determine where to focus their efforts in supply chain management to alleviate natural fiber environmental footprint was developed.

Nowadays, only 23% of companies take into account their environmental footprint when choosing their suppliers and between 40 and 60% of a company’s environmental footprint actually comes from its supply chain. Hence, in developing the natural fiber supply strategy, it is critical to understand the role of supply chain management and the associated impacts of environmental footprint. Network analysis, optimization of transhipment costs and decision analysis on optimal solutions to minimize both the supply chain cost and environmental footprint are essential toolkits in the advancement and promotion of natural fibers industry.

Moreover, over the last two decades, the trends in production of plant fibers have been declining due to popularity of synthetic fibers as well as adverse drought conditions. The fiber production plants spread across all continents of the globe. Table 1 illustrates the trends of different sources of fibers, production capacities and where they are produced.

Fiber source	World production (103 tonnes)	Origin	Country
Abaca	70	Leaf	Malaysia, Uganda, Philippines, Bolivia, Brazil
Bambou	10,000	Stem	Africa, India, Brazil
Banana	200	Stem	Africa, India, Brazil
Broom	Abundant	Stem
Coir	100	Fruit	India, Sri Lanka, Philippines, Malaysia, Brazil
Cotton Lint	18,500	Stem	India, Europe, USA
Elephant Grass	Abundant	Stem	India, Africa
Flax	810	Stem	Europe
Hemp	215	Stem	Yugoslavia, China
Jute	2500	Stem	India, Egypt, Guyana, Jamaica, Ghana, Malawi, Sudan, Tanzania, Brazil
Kenaf	770	Stem	Iraq, Tanzania, Jamaica
Linseed	Abundant	Fruit	USA
Nettles	Abundant	Stem	Europe
Oil Palm Fruit	Abundant	Fruit	Malaysia, India, Brazil, Indonesia, Philippines
Palmyrah	Abundant	Stem	India
Ramie	100	Stem	Honduras, Mauritius
Roselli	250	Stem	Borneo, Guyana, Malaysia, Sri Lanka, Togo, Indonesia, Tanzania
Rice Husk	Abundant	Fruit/grain	India, Japan, Brazil, Others
Rice Straw	Abundant	Stem	India, Japan, Brazil, Others
Sisal	380	Leaf	East Africa, Bahamas, Antigua, Kenya, Tanzania, India, Brazil
Sun Hemp	70	Stem	Nigeria, Guyana, Siera Leone, India
Wheat Straw	Abundant	Stem	USA, Brazil, India, Canada
Wood	1,750,000	Stem	All Countries

Table 1.

Fiber sources, country and annual production of plant fibers.

In 2018, world production of all apparel and textile fibers reached 110 million tons, with natural fiber production estimated at 32 million metric tons. Natural fibers accounted for 29% of the total world fiber production capacity, with most of annual yield variation linked to dry weather conditions. Moreover, the decline in the amounts of natural fibers in total fiber production in the last decade is due to the exponential growth in polyester production, whose demands were triggered by the fast-fashion apparel industry.

2.3 Plant fibers’ extraction methods

2.3.1 Methods commonly used

Cellulosic fibers originated from plants and trees such as cotton, flax, hemp, jute, ramie, kapok, coir and bamboo are termed natural PFs. Such fibers are derived from various parts of plants including leaves, stems (bast fibers), fruits and seeds. Because all natural PFs are made up of mainly cellulose, they are categorized as ‘natural cellulosic fibres’, which may consist of one plant cell or an aggregate of cells bounded together by non-cellulose materials. Major commercially used PFs include: seed fibers (cotton, coir, kapok), bast fibers (flax, hemp, ramie, bamboo, banana), leaf fibers (sisal, kenaf, pineapple, abaca). To date, bast fibers are produced and utilized to manufacture a wide array of traditional and novel products including ropes, nets, carpets, mats, brushes, mattresses, paper and board materials. Generally, PFs are classified into two groups, namely soft fibers and hard fibers. Soft fibers are obtained through labour-intensive processes. It involves the following steps: selection of plant and harvesting the plant, partial drying, pounding with stone mallet, scraped with devices similar to comb to clean the fibers, wash the fibers, dry in the sun and finally comb the fibers. Subsequently, the fibers are ready to be spun or twisted into thread or cord. Soft fibers are often used to make ropes, string, nets, bags, and hammocks.

Hard fibers are processed through successive phases of cutting, drying, cleaning, and soaking before they can be woven. They are strong and naturally flexible fibers, thus suitable and utilized to make furniture, birdcages, toys, baskets, and mats.

Figure 1(a) and (b) shows the matured flax plants grown under a controlled greenhouse environment and a setup of bench-scale trouph for water retting of flax stems [17].

Figure 1.

Greenhouse controlled experiments for flax plants [17]. (a) Matured flax plants in Phytotron and (b) matured flax stems undergoing water retting.

Historically, most plant fibers were extracted manually, supplemented by natural retting. Evidently, this process is tedious, time-consuming and the extracted quality of fibers depends on the skill of the labourer. Nowadays, these fibers are extracted by chemical, mechanical or biological methods.

Akubueze et al. [18], reviewed the chemical techniques employed to extract fibers from natural plants, which include alkali, acid and other reagents. The typical mechanical extraction methods involve the use of stripping the plant stem (typically known as Bacnis and Leonit processes). The latest mechanical extraction methods utilize the decortication process, whereby the plant stems are crushed between two drum rollers to obtain the fibers after removing the pulp. The use of decorticators increase fiber production by 20–25 times compared with the manual process. With biological processes, both consortium of microorganisms and enzymes are utilized to efficiently extract fibers from plant stems.

Overall, the mechanical extraction is incapable to remove the natural binding material (pectin) from the interspaces of the fibers within fiber bundle, chemical extraction is capable to remove the pectin within the fiber bundle but causes significant environmental pollution, whereas the biological extraction method provides increased fiber yield, with minimum detrimental effects to the environment.

According to the Centre for Learning and Teaching in Art and Design (CLTAD), bast fibers, for example, are generally obtained from the phloem, an inner skin of a plant. These fibers support the cells of the phloem to provide strength to the stem. During processing, the fibers need to be separated from both the interior (xylem) and exterior (epidermis) which is the outermost layer of cells. The processes for separating these fibers from plant stalks are known as retting and decortication. Bast fiber bundles are typically several feet long, composed of overlapping cellulose fibers and a cohesive gum (or pectin), which strengthens the stem of the plant. The processes with which the bast fibers are separated significantly influence the quality of fibers as there are many stages involved. Kumar et al. [19], reported that the processing of sustainable fiber starts with fiber extraction and yarn production followed by bleaching, dyeing, softening, printing and drying.

Moreover, the process that separates the fibers into smaller bundles and elementary fibers is known as retting. Fiber retting is a key process and is an important criterion that most industries value because it determines the ultimate properties of the fibers produced. Traditional retting methods include dew and water retting. Dew retting depends on ambient weather conditions, typically takes several weeks and hence the quality of fibers produced varies considerably. Similarly, water retting has been a primary method for low-cost production of bast fibers. The process involves submerging bast straws into water and then the decomposition of the pectic is effected by the activity of anaerobic microorganisms. The quality of retting is assessed by the weight, degumming rate and the fiber properties. The faster rate of weight loss is preferred, the degumming rate is evaluated as the percentage change in pectin content of phloem regions in the raw plant to those in water-retted plant, whereas the desired fiber properties include color, linear density and tensile strength. Ruan et al. [20], reported that water retting improved both whiteness and fineness as well as the mechanical properties of fibers.

Although water retting is capable to produce good quality fibers, the inherent long duration of 7–14 days and associated odor has made it less attractive. The retting period can be reduced to 100 h by using warm water (35°C), but high water consumption and unpleasant odor limit its use to some developing countries. Retting is the process by which pectin gets dissolved or softened from the fiber bundles and separates the fibers from stems through microbial activity. As such, a group of Clostridium microorganism is commonly known to play a significant role in the process by hydrolysing the pectin as it produces pectinase enzyme. These enzymes initially attack the cambium layer and then the other thin-walled cells in the cortex. This phenomenon takes place in most plant bast fibers as they have similar long filament structures, except those from cotton fibers which are single plant cells. As an example, for the retting process conducted in a bench-scale trouph under no-flow process water conditions, there were distinct features on how the fibers separate from bundles. Figure 2(a) and (b) show the scanning electron microscopy of the unretted and retted fibers of flax.

Figure 2.

A SEM shows the microstructure of flax fibers (a) before retting and (b) after the retting process [17].

Figure 3(a) shows that cellulosic fiber production accounted for 6% of the total in 2018, synthetic filament accounted for 45% and synthetic staple 20%. Similarly, Figure 3(b) depicts that cotton accounted for 81% of natural fiber production by weight in 2018, jute accounted for 7%, while coir and wool each accounted for 3%.

Figure 3.

World total fiber production and natural fiber production [21].

The synthetic fibers are dominated by polyester, which accounts for nearly 90% of world filament production and 70% of world synthetic staple production. The remaining synthetic fibers are composed mostly of nylon, acrylic and polypropylene.

Perhaps a key factor is to consider the role and contribution of human capital and household social economics. Employment statistics in natural fiber industries is difficult to estimate because households do not engage in consistent annual production. In Ref. [21] it is estimated that about 60 million households worldwide are engaged in natural fiber production, and hence the total employment, reflecting both full-time year-round employment and part-time or seasonal employment, is around 300 million, which represents about 4% of the world’s population.

Natural fibers possess superior advantages over synthetic fibers including widespread availability, low cost, low density, moderate strength modulus to weight ratio, high acoustic damping, low manufacturing energy consumption, low carbon footprint and biodegradability. Consequently, there are emerging concerted research initiatives that explore and promote the understanding of the characteristics of natural fibers [15, 22].

2.3.2 Other methods

As discussed in Section 2.3.1 above, dew and water retting are the most common processes for fiber retting. Plant fibers can also be extracted using chemical and enzymatic retting, which provide better control than dew and water retting. Unfortunately, chemical retting while effective in extraction of fibers, causes significant pollution challenges due to higher amount of chemicals utilized. For the chemical extraction methods, alkali and selected reagents have been employed. Alkali treatments promote the fibrillation, whereby the composite fiber bundle is degraded into smaller fibers. Sodium hydroxide (NaOH) is popularly used to reduce the fiber roughness, but also produces good quality fiber. Reagents such as sulfuric acid, hydrogen peroxide, protease and sodium citrate can also be used for chemical extraction [23].

Similarly, enzymatic retting is relatively expensive despite its shorter retting time, yet it produces acceptable fiber quality and is advantageous over other retting processes. In the enzymatic method, the selection of enzymes depends on the type of substrate, composition, size and lignin content. The most common enzymes utilized are cellulases and pectinases. Cellulase enzymes enhance the fiber smoothness by removing fibrils from the outer layer. As such, this results in reduction in the mechanical properties due to the damage caused in the fibers. Pectinases remove the inter-lamellar pectin, which is a natural adhesive compound between fibers.

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.

Figure 4.

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

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].

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°.

Fibers	Crystallinity index (CrI)	Microfibril angle (°)	Cross-section area (mm^2)	Length of the cell (mm)	Aspect ratio l/d
Coco		45.0	1.20	3.3	35
Flax		10.0	0.12	2.	1687
Hemp		6.2	0.06	23	960
Jute		8.0	0.12	2.3	110
Ramie		7.5	0.03	154	3500
RC [32]		42	0.05–0.962		>2000
Sisal	56.6–66.2	20.0	1.10	2.2	100

Table 2.

Structural parameters of some plant fibers [29, 30, 31].

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].

Figure 6.

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

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 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.

Fiber	Chemical content (%)
	Cellulose	Hemicellulose	Lignin	Pectin
Abaca	63.2	19.6	5.1	—
Bamboo	48	23	19	—
Cotton	83	5	—	—
Flax	65–70	10–16	2.9	2–4
Hemp	67	16.1	4	—
Jute	55–64	12–18	12–33	0.2
Kenaf	55–59	18–20	6.8–8	4.5–5
Ramie	68.6	13.1	0.6	—
RC	68.2	16	15.6	—
Sisal	54–66	12	7.3	0.8
TJ	62.7	14.5	4.1	7.6
Wood	83	5	19–26	0

Table 3.

Chemical contents of some fibers.

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. Pull-out 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.

Figure 7.

Cellulose molecule.

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 regions, or randomly in amorphous regions. Mechanical properties of natural fibers depend on their type of cellulose, as each type has its own cellular geometry. If cellulose is a prime structural constituent for the vast majority of plant cell walls, then hemicellulose with lignin acts as binding materials. Properties depend on the fiber cell geometry of each type of cellulose and its degree of polymerization.

Hemicelluloses represent the second most abundant constituent of plant fiber. Hemicelluloses are polysaccharides found in lignocelluloses alongside cellulose and pectin. Hemicelluloses, unlike cellulose, are composed of several sugars that form short chains with ramifications. The sugars present can be divided into different groups: pentoses (xylose, arabinose), hexoses (glucose, mannose, galactose), hexo-uronic acids (glucuronic acid and methyl-glucuronic acid) and l-deoxyhexoses (rhamnose and fucose). Hemicelluloses are, by definition, water-soluble polysaccharides that can be extracted from the plant cell walls using alkaline solutions. They are the most hydrophilic biopolymers in the cell wall that promote moisture absorption. In their natural state, they have a degree of polymerization that varies from 200 to 300, and their structure depends on the plant species. The best-studied class of hemicelluloses are xyloglucans. They have a bridging role between cellulose microfibrils in order to strengthen the cell wall by interaction with cellulose and, in some cell walls, with lignin. They consist of a glucose chain and short side chains of xylose, galactose and fructose.

Lignin together with cellulose and hemicelluloses is part of the wood industry. Its proportion in wood varies between 15 and 30% [43]. Lignin or ‘lignins’ are three-dimensional polymers from the radical polymerization of three phenylpropenoic alcohols: coniferryl alcohol, sinapyl alcohol and p-coumaryl alcohol [44]. Lignin contributes to the rigidity of cell walls, and thus to the erect port of terrestrial higher plants. Lignin also offers a protective barrier against the microbial attack of plants. Indeed, due to its chemical nature, lignin is very resistant to various chemical agents and biological degradation. To sum up, lignin polymers make the cell wall rigid and impermeable, allowing the transport of water and nutrients through the vascular system by protecting plants from microbial invasion. Lignin is totally amorphous and hydrophobic. It is not hydrolysed by acids, but hot soluble in soda, easily oxidized and also condensable with phenol.

Pectins are polymers of acidic polysaccharides, composed of a main chain of uronic acid bound in 1–4. Regularly, rhamnose molecules are interspersed between these monomers by bonds 1–2 and 1–4. Some of these rhamnose units carry side chains composed of neutral oses among which galactose and arabinose are the most abundant. The type of bond between uronic acid and rhamnose molecules forms elbows. The pectin macromolecule appears like a zigzag. This arrangement contributes to its special properties and provides some flexibility to plants. Pectins are extracted from the fiber by a chemical method either by boiling water or by ethylene diamine tetraacetic acid.

3.3 Density

Different methods can be used including solid pycnometers or gas pycnometers [45, 46, 47]. The choice of gases (helium for example) or immersion liquids such as toluene, ethanol and xylene is decisive for quality results [46, 47]. Fibers must be dried for at least 72 h in a desiccator containing silica (previously regenerated). Fibers are then cut into lengths of 5–15 mm and then introduced into the pycnometer which is eventually placed in the desiccator for at least 24 h. Before carrying out the hydrostatic weighing with the immersion liquids, the vortex agitation of fibers to evacuate the microbubbles between needs to be done. Significant degassing could occur at this stage and provides information on the porosity rate of the fibers [28].

3.4 Mechanical characterization

3.4.1 General considerations

In general, PFs are suitable for reinforcing plastics (thermosets and thermoplastics) and textiles manufacturing thanks to their relatively high strength and low density. The tensile strength and the modulus of elasticity of PFs are very important characteristics for the use of fibers as reinforcements in composite and textile materials. However, the tensile test data for most fibers in service have yet to be studied, as the data found in the literature are scattered and often unreliable. In fact, methods used for the characterization are not identical. Table 4 shows the tensile mechanical properties of some plant fibers compared to synthetic fibers [48]. The properties of the fibers and their structure depend on several factors such as the origin, variety, conditions of growth and harvesting of fibers associated with the treatments, the location in the stem, the presence or absence of a lumen, measurement techniques that vary greatly from one research team to another. These factors can make a difference for the same type of fiber and influence test results.

Fiber	Density (g/cm^3)	Diametre (μm)	Length (mm)	Tensile strength (MPa)	Young modulus (GPa)	Elongation at break (%)	Moisture content (%)
Abaca	1.5	10–30 (20)	4.6–5.2 (4.9)	430–813 (621.5)	31.1–33.6 (32.35)	2.9	14
Bamboo	0.6–1.1 (0.85)	25–88 (56.5)	1.5–4 (2.75)	270–862 (566)	17–89 (53)	1.3–8 (4.65)	11–17 (14)
Banana	1.35	12–30 (21)	0.4–0.9 (0.65)	529–914 (721.5)	27–32 (29.5)	5–6 (5.5)	10–11 (10.5)
Coir	1.2	7–30 (18.5)	0.3–3 (1.65)	175	6	15–25 (20)	10
Cotton	1.21	12–35 (23.5)	15–56 (35.5)	287–597 (442)	6–10 (8)	2–10 (6)	33–34 (33.5)
Flax	1.38	5–38 (21.5)	10–65 (37.5)	343–1035 (689)	50–70 (60)	1.2–3 (2.1)	7
Hemp	1.47	10–51 (30.5)	5–55 (30)	580–1110 (845)	30–60 (45)	1.6–4.5 (3.05)	8
Jute	1.23	5–25 (15)	0.8–6 (3.4)	187–773 (480)	20–55 (37.5)	1.5–3.1 (2.3)	12
Kenaf	1.2	12–36 (24)	1.4–11 (6.2)	295–930 (612.5)	22–60 (41)	2.7–6.9 (4.8)	) 6.2–12 (9.1)
Pineapple	1.5	8–41 (24.5)	3–8 (5.5)	170–1627 (898.5)	60–82 (71)	1–3 (2)	14
Ramie	1.44	18–80 (49)	40–250 (145)	400–938 (669)	61.4–128 (94.7)	2–4 (3)	12–17 (14.5)
RC	0.94	70–350 (120)	—	450–1500 (557.1)	5.8 (±3.5)	27.5	—
Sisal	1.2	7–47 (27)	0.8–8 (4.4)	507–855 (681)	9–22 (15.5)	1.9–3 (2.45)	11
TJ	(1.398)	40–90 ()		(404.0)	(32.3)	(1.8)

Table 4.

Mechanical properties of some selected plant fibers versus synthetic fibers [48].

Selection of plant fiber implies a prior study of its mechanical properties, chemical resistance, dimensional stability, separation process, etc. It is worth recalling that linear cellulosic macromolecules are linked by hydrogen bonds and are closely associated with hemicelluloses and lignin, which confer stiffness to fiber. One of the issues of natural fibers is the scattered information and the differences in mechanical properties reported. Likewise, the lack of standards for both producers and users of these materials regarding methods to collect, process, post-process and characterize plant fibers underlines the complexity in the selection.

3.4.2 Quasi-static tensile test

Quasi-static tensile test is the method commonly used in the literature for the characterization of the mechanical properties of plant fibers in the longitudinal direction. This type of characterization presents challenges linked to the assembly and to the single nature of the fiber. In addition, the geometry of the plant fiber makes it often difficult to conduct the tests. Therefore, evaluation of the mean diameter along the fiber using a microscope is necessary for the performance of the test. The single fiber is mounted on a paper frame and a drop of glue is used to stick the fibers. The role of this paper frame is to facilitate the handling and alignment of the fiber on the jaws of the experimental device as shown in Figure 8 [32].

Figure 8.

Tensile test and gripping tab specimens for plant fibers.

The large dispersion of the mechanical properties of the plant fibers observed (Figure 9) is mostly related to the test conditions. The research work by Ntenga et al. [14] focused on the choice of the stress speed and the gage length, in order to keep the deformation in the elastic domain and reduce this dispersion during the tests. The machine cross-head speed of 1 mm/min and the gage length of 10 mm were found to cause less dispersion of the mechanical properties in a tensile test.

Figure 9.

Tensile stress/strain curves for the four cross-head speeds of gage length 10 mm [14].

3.4.3 Nano-indentation test

Nanoindentation is a technique used to characterize the longitudinal and transverse mechanical properties of fibers at the cell wall scale. Commonly measured properties are Young’s modulus and material hardness. In the literature, nanoindentation tests have been carried out to access both transverse and longitudinal mechanical properties on wood fibers [31, 49] and recently on flax fibers [50]. According to Cisse [51], nanoindentation only gives access to local behavior of the fiber, and the identification of mechanical properties requires knowledge and use of a behavior model. The testing technique consists of applying a force to the indenter and taking the area of the indentation, in order to determine the Young’s modulus and the hardness of the material (Figure 10(a) and (b)).

Figure 10.

(a) Nano indentation experimental device and (b) indentor impression Berkovich [52].

A typical set of nanoindentation tests results [53] is shown in Figure 11.

Figure 11.

Transverse modulus of plant fibers obtained in nano indentation.

Differences in transverse and longitudinal modulus noted between the fibers can be explained not only by the differences in micro-fibrillary angles but also by the rate of cellulose that varies between fibers. Hemp and sisal in particular have a cellulose content of around 60%, while that of flax is over 75%; however, the mechanical properties of cellulose are much superior to those of lignin, hemicelluloses and pectins, other constituents of natural fibers [50].

3.4.4 Dynamic mechanical analysis

A large amount of work exists in the field of vibration-based non-destructive testing (NDT) including an extensive survey of over 300 papers by Kong et al. [54]. Indeed, the vibration-based technique has been a very active area of research for many years, however, has always dealt with rigid bodies. As an extension of the use of this technique, the purpose of this section is to present the applicability of the low-frequency vibration-based technique towards estimation of dynamic Young’s modulus of natural fiber-based materials, initially having no bending stiffness. This technique enhances the applicability of non-contact acoustic non-destructive testing to the estimation of dynamic characteristics of thin materials, where the current standard method [55] is not applicable.

Let us consider a thin rectangular specimen having a length b, a width a, a thickness h and a density ρ. Figure 12 shows the specimen configuration in a Cartesian coordinate system at equilibrium (i) and vibrating in flexural mode (ii) and (iii).

Figure 12.

Specimen configuration (i): undeformed, and vibrating at (ii): fundamental frequency, (iii): second frequency in flexural mode.

The specimen, considered as a membrane, initially has no bending stiffness. It is then slightly stretched in the y-direction, in order to make it possible to vibrate transversally (i.e. in the z-direction). The tensile force F is assumed to remain constant during small vibrations in the y-z plane.

In general, for a specimen having intrinsic elasticity, the equation of motion is expressed as follows:

$$\frac{{\partial }^2\xi }{\partial t^2}-c^2\left(\frac{{\partial }^2\xi }{\partial x^2}+\frac{{\partial }^2\xi }{\partial {\mathit{ty}}^2}\right)+d^2{\left(\frac{{\partial }^2\xi }{\partial x^2}+\frac{{\partial }^2\xi }{\partial {\mathit{ty}}^2}\right)}^2\xi =\frac{p\left(x,y,t\right)}{\mathit{\rho h}}$$E1

where $\xi$ is the displacement normal to the plane (x,y) which coincides with the equilibrium position of the membrane, c is the velocity of propagation of bending waves, p is the external pressure on the surface of the membrane. c and d are defined as follow:

$$c=\sqrt{\frac{T}{\mathit{\rho h}}}\text{and}{d}^2=\frac{E{h}^2}{12\rho \left(1-{\nu }^2\right)}$$E2

where E, T and ν are the elastic modulus, tensile force per unit length of the edge, and Poisson’s ratio.

The frequency equation with the fixed-fixed boundary condition shown in Figure 12 above was derived in Mfoumou et al. [56] to obtain the frequency of vibration ${\omega }_{\mathit{mn}}$ of each bending mode as follows:

$${\omega }_{\mathit{mn}}=c\sqrt{{\left(\frac{\mathit{\pi m}}{a}\right)}^2+{\left(\frac{\mathit{\pi n}}{b}\right)}^2}\left\{1+\frac{d^2}{2c^2}\left[{\left(\frac{\mathit{\pi m}}{a}\right)}^2+{\left(\frac{\mathit{\pi n}}{b}\right)}^2\right]\right\},m,n=0,1,2,\dots$$E3

where m and n are the mode numbers.

For a plant fiber-based material considered as a membrane; therefore, no account of intrinsic elasticity is taken so that Eq. (3) is simplified, and the normal frequencies equation is expressed as:

$${\omega }_{\mathit{mn}}=c\sqrt{{\left(\frac{\mathit{\pi m}}{a}\right)}^2+{\left(\frac{\mathit{\pi n}}{b}\right)}^2},m,n=0,1,2,\dots$$E4

The Young’s modulus can therefore be determined using the flexural resonance method by monitoring normal modes of vibration. These modes for an oscillating system are special solutions where all the parts of the system are oscillating with the same frequency. At these modes, considering only bending modes in the length direction (m = 0), the relationship between the frequency in hertz and the state of strain was established as follows [56]:

$$f_{0n}^2=\frac{E.n^2}{4.\rho .b^2}.\epsilon$$E5

thus, enabling extraction of the constant E from experimentally measured normal mode frequencies and corresponding strains.

3.4.5 Creep test

Both creep experiment and relaxation experiment are two techniques commonly used to characterize the delayed behavior of ‘conventional’ materials. A creep test consists of imposing an almost instantaneous stress load on the plant fiber and maintaining it constantly over time and then proceeding to a discharge. The resulting deformation under the action of the load is creep, and that under the action of discharge is recovery. In general, the creep responses can be broken down into three stages depending on the strain rate as shown in the following Figure 13. The first stage in which creep occurs at a decreasing rate is called primary creep; the second step, commonly called secondary creep, is carried out at a relatively constant speed; and the third stage, tertiary creep, occurs at an increasing rate and terminates with material fracture.

Figure 13.

Creep/recovery test of an elementary hemp fiber under a constant environment [53].

The creep test was successfully carried out on an elementary hemp fiber and the results allowed it possible to highlight the viscoelastic nature of the plant fiber [51]. Figure 13 shows the creep test results obtained.

3.4.6 Relaxation test

3.4.6.1 The context

When a constant strain is applied to a material for a long period, cross-links or the primary bonds that form between molecules start breaking with time and spontaneously lose their bonding capability. High level of strain or long period is the main reason for intermolecular bond breakage, thus creating stress decay over time, called stress relaxation. The rate of bond breakage influences the rate of stress relaxation. Other factors control the rate of bond breakdown, such as stress on the bond, chemical interference, molecular chain mobility which allows molecular chains to move out from their position. The behavior of stress relaxation in plant fibers is also influenced by temperature, humidity, and strain levels. The stress relaxation tests are therefore mainly performed with different ranges of temperature, humidity and strain levels. The time taken to reach the end of relaxation is called relaxation time. From other studies, it is reported that at higher temperature relaxation time becomes shorter, while at lower temperature it becomes longer but the shape of relaxation does not change with temperature [57]; moreover, the variation of strain level affects the stress relaxation [58]. The literature also reports the sensitivity of this class of material to loading-directionality, and ductile and brittle phenomena [59].

3.4.6.2 Stress relaxation measurement

During structural design, the properties of the material must be considered. Elastic Modulus is one of the most important material properties describing the stiffness of the material. When a force is applied to an object, modulus of elasticity or elastic modulus gives the mathematical description of the object’s tendency to be deformed elastically.

In orthotropic materials such as wood-based natural fibers, the strain quickly increases linearly with the stress, then exhibit a nonlinear behavior when the strain exceeds the proportional limits. When the stress relaxation tests are conducted for a very small deformation, the viscoelasticity of the material can be considered linear. During stress relaxation test, the material relieves stress over time as well as the elastic modulus of material $E\left(t\right)$ also decreases with time at a constant temperature. According to linear viscoelastic material [60], the elastic modulus relaxation can then be defined as:

$$E\left(t\right)=\frac{a\left(t\right)}{s_o}$$E6

where, $s_o$ is the constant strain and $a\left(t\right)$ is stress of material as a function of time. Indeed, elastic modulus relaxation is the relaxation of modulus of elasticity of material.

3.4.6.3 Sample, experiments, and results

A rectangular strip of specimen is placed between the clamps of the tensile test machine (see Figure 8), and it is slightly loaded within its elastic region. The specimen is tested in uniaxial stress-state at a strain rate of 1 mm/mm with 0.4% strain changes. The elongation is kept constant at 0.4% strain level (1 mm extension) for 5400 s and time, stress, and strain are recorded.

Experiments were carried out for paperboard (PPR) without crack and PPR with crack. Five specimens were tested for each case and each experiment continued for 5400 s (1.5 h) with 1 mm extension. The reason for taking 1 mm extension was to keep the deformation within the elastic region.

The stress relaxation of each specimen was monitored and analyzed at constant.

elongation. The load, stress and time data for constant strain were obtained from the experiments. From the testing of five specimens in each case, we have plotted stress versus time curves. The plotted stress relaxation of PPR without and with the presence of a side crack is presented in Figure 14.

Figure 14.

Stress relaxation of paperboard with and without crack.

Figure 15 show the stress relaxation behavior of PPR at different strain levels (two different extension levels, 1 mm and 0.5 mm).

Figure 15.

Stress relaxation of paperboard for 1 mm and 0.5 mm extension.

3.4.6.4 Formulation of relaxation

The data obtained from the stress relaxation experiments are decreasing type of data with function of time and this type of data can be fitted to the poly-exponential function of the following form:

$$y\left(t\right)=\sum _{i=1}^N{a}_i{e}^{-b_{i}t}$$E7

where, $a_i$ and $b_i$ are the unknown parameters. Several methods have been developed to estimate these parameters. The most available methods are graphical method, regression-difference equation method, method of partial sums, Fourier Transform method, Foss’s method [61] for a sum of two exponentials. However, their uses are limited. For example, the graphical method is not suited where there are consistent fluctuations, regression-difference method and method of partial sums are only appropriate for equally spaced data and Fourier Transform method is suitable for exponentially spaced observation. On the other hand, the use of Foss method is broader than any other method, and even not equally and exponentially spaced data can be treated using this method [61].

3.4.6.5 Model parameters extraction using experimental data

The parameters of a set of mechanical models can be calculated from experimental data. MATLAB, for example, can be used to extract the parameters from the data. To analyze the suitability of the mechanical model with the experimental stress relaxation, Maxwell Model, Two-unit Maxwell Model, Modified Two-unit Maxwell Model, Standard linear solid model are constructed and then compared with the experimental relaxation. Analytical description of these models is given in [62].

In Ref. [56] we have chosen Foss method to develop curve fitting for all models and then compared with the experimental relaxation. Whereas in Ref. [15] we used the Zapas-Phillips method. The best-fitted model with the experimental data was then selected to analysis all experimental data and mathematically stress relaxation equations were derived.

To predict the stress relaxation behavior of natural fibers, we derived the mathematical equations for PPR with and without presence of crack. These equations were derived by the Modified Two-unit Maxwell model which suits best with the experimental result. Though we carried out our experimental tests with five specimens for each kind of test and among them three specimen-data were taken into consideration, but here we will construct the stress relaxation equation for only one specimen for each case.

Below the comparison, diagrams between experimental relaxation data and the Modified Two-unit Maxwell are shown in Figures 16 and 17. The stress relaxation equation for each case is derived using Modified Two-unit Maxwell model.

Figure 16.

Stress relaxation of paperboard—curve fitting.

Figure 17.

Stress relaxation of paperboard with crack–curve fitting.

3.4.7 Inverse characterization

Suitability of materials inverse characterization, destructive or non-destructive, is widely investigated [52, 63, 64]. Furtado et al. [65] used an ultrasound shear wave viscoelastography method to determine the viscoelastic complex shear modulus of macroscopically homogeneous tissues. Ilczyszyn et al. [66] performed the mechanical characterization of flax fibers using an inverse optimization simplex method.

The aim here is to use macro-micro approaches to achieve an efficient estimation of the fiber properties. In fact, homogenization laws of the micromechanics of the elastic/viscoelastic behavior of composite materials provide relationships of the properties of these materials in terms of their constituents’ properties. For an orthotropic material, the knowledge of its off-axes elastic modules in a set of θ directions leads to the calculation of the fibers’ elastic constants for instance. Analytical relationships of elastic constants that account for the orientation of the fibers can be found in the literature. A presentation of an inverse method based on the composite cylinder assembly proposed by Hashin [67] to characterize the anisotropy of plant fibers was discussed in Ref. [64] for a transversely isotropic 2-phase composite. In this case, the matrix and fiber phases are assumed isotropic and transversely isotropic respectively. Using a stress field in cylindrical coordinates and applying Hooke’s law gives the following results:

$\left(x,y,z\right)$ stand for off-axis Cartesian coordinates (reference coordinate system).

$\left(1,2,3\right)$ represent material axes of a unidirectional composite.

For a tensile test in the x-direction, the Young modulus is expressed as:

$$\frac{1}{E_{\mathit{xx}}}=\frac{1}{E_{11}}{cos}^4\theta +\left(\frac{1}{4G_{23}}+\frac{1}{4K_{23}}+\frac{{\nu }_{12}^2}{E_{11}}\right){sin}^4\theta +\left(\frac{1}{G_{12}}-2\frac{{\nu }_{12}}{E_{11}}\right){sin}^2\theta {cos}^2\theta$$E8

There are five independent properties to be determined $\left(E_{11},G_{23},G_{12},K_{23},{\nu }_{12.}\right)$

Analytical expressions of the five properties in terms of fiber and matrix phase properties and the volume fractions are given by:

$$E_{11}=E_z^f{\varphi }^f+E_z^m{\varphi }^m-\frac{{\nu }_{\mathit{z\theta }}^f-{\nu }^m}{\mathrm{\Delta }}\left[E_z^f\left(\frac{{\nu }_{\mathit{rz}}^f}{E_r^f}+\frac{{\nu }_{\mathit{z\theta }}^f}{E_z^f}\right)-2{\nu }^m\right]{\varphi }^m$$E9

$$\overline{{\nu }_{12}}=-2\frac{1-{\left({\nu }^m\right)}^2}{{\mathrm{E}}^m}\frac{{\nu }_{\mathit{z\theta }}^f-{\nu }^m}{\mathrm{\Delta }}+{\nu }^m$$E10

$$\begin{array}{rcl}{k}_{23}& =& \frac{E^m{\varphi }^f\left(I_1{I}_4-I_2{I}_3\right)}{2\left[-\left(1-{\nu }^m\right){\varphi }^f\left(I_4{L}_1-I_2{L}_2\right)+\left(1+{\nu }^m\right){\varphi }^f\left(I_4{L}_1-I_2{L}_2-I_4{I}_1+I_2{I}_3\right)-{\nu }^m{\varphi }^m\left(I_1{L}_2-I_3{L}_1\right)\right]}\end{array}$$E11

$$\frac{\overline{G_{12}}}{G^m}=\frac{G^m{\varphi }^m+G_{\mathit{rz}}^f\left({\varphi }^f+1\right)}{G^m\left({\varphi }^f+1\right)+G_{\mathit{rz}}^f{\varphi }^m}$$E12

$$\frac{\overline{G_{23}}}{G^m}=1+\frac{{\varphi }^f}{G^m\left(G_{\mathit{r\theta }}^f-G^m\right)+\left(k^m+\frac{7}{3}{G}^m\right){\frac{}{\left(2k^m+\frac{8}{3}{G}^m\right)G}}_{\mathit{rz}}^f{\varphi }^m}$$E13

with

$$G_{\mathit{r\theta }}^f=\frac{E_r^f}{2\left(1+{\nu }_{\mathit{r\theta }}^f\right)}{G}^m=\frac{E^m}{2\left(1+{\nu }^m\right)}{k}^m=\frac{E^m}{3\left(1-2{\nu }^m\right)}$$E14

$$\mathrm{\Delta }=\left[\frac{{\nu }_{\mathit{r\theta }}^f}{E_r^f}-\frac{1}{E_{\theta }^f}+{\nu }_{\mathit{z\theta }}^f\left(\frac{{\nu }_{\mathit{rz}}^f}{E_r^f}+\frac{{\nu }_{\mathit{z\theta }}^f}{E_z^f}\right)\right]\frac{{\varphi }^m}{{\varphi }^f}-\frac{{\nu }^m}{E^m}\frac{{\varphi }^m}{{\varphi }^f}-\frac{1}{E^m}\frac{{\varphi }^f+1}{{\varphi }^f}+2\frac{{\left({\nu }^m\right)}^2}{E^m};$$E15

$$\begin{array}{l}{I}_1={\varphi }^m\left(\frac{{\nu }_{\mathit{rz}}^f}{E_r^f}+\frac{{\nu }_{\mathit{z\theta }}^f}{E_z^f}\right)+\frac{2{\nu }^m}{E_m}{\varphi }^f;I_2=-\left(\frac{{\varphi }^m}{E_z^f}+\frac{{\nu }_{\mathit{z\theta }}^f}{E_m}\right)L_1=-\left(\frac{{\nu }_{\mathit{rz}}^f}{E_r^f}+\frac{{\nu }_{\mathit{z\theta }}^f}{E_z^f}\right);\end{array}$$E16

$$\begin{array}{l}{I}_3=\left(\frac{{\nu }_{\mathit{r\theta }}^f}{E_r^f}-\frac{{\nu }_{\mathit{z\theta }}^f}{E_{\theta }^f}\right){\varphi }^m-\frac{1}{E_m}\left({\nu }^m-{\varphi }^f-1\right);\end{array}$$E17

$$\begin{array}{l}{I}_4=\frac{{\nu }_{\mathit{z\theta }}^f}{E_z^f}{\varphi }^m+\frac{{\nu }^m}{E_m}{\varphi }^f{L}_2=-\left[\frac{{\nu }_{\mathit{r\theta }}^f}{E_r^f}-\frac{1}{E_{\theta }^f}+\frac{1}{E_m}\left({\nu }^m-1\right)\right]\end{array}$$E18

Eqs. (8)–(13) are then solved for $E_r^f,E_z^f,{\nu }_{\mathit{rz}}^f,{\nu }_{\mathit{z\theta }}^f,G_{\mathit{r\theta }}^f\mathit{\text{and}}{G}_{\mathit{rz}}^f$ using a multiparameter optimisation algorithm.

4. Conclusion

There are evolving global challenges on the utilization of non-renewable resources in the manufacturing industry and increasingly stringent environmental legislation. Both consumers and regulatory agencies are thriving for products that reduce dependency on fossil fuels and thus, are more environmentally friendly. As such, this paves for an opportunity to embrace the use of natural fibers in products and composites leading to significant growth of biobased economy, which the present chapter intends to stimulate.

The field of study of plant fibers that can be industrially exploited remains open. In this chapter, a particular emphasis has been put on their production, in particular on the methods that are generally used to separate them from their originating plants. To date, the question of improving the quality of the extracted fiber has been satisfactorily answered, particularly as regards the possibility of combining several methods when necessary. Some other questions still require research. These include, among others, growing conditions for seed multiplication and fiber production, harvesting methods, optimisation of fiber separation, the molecular basis for improving fiber decortication and performance. The knowledge gained from this work could be used to design new varieties of fibers, tailored for specific industrial applications. Similarly, the recourse to proteomics [68, 69], to isolate genes involved in the biosynthesis of cell wall lignin and hemicellulose in tobacco. Variations in these constituents can affect the fiber quality and cellulose availability. This could then lead to a new orientation on molecular selection research as well as genetic modifications studies to improve the quality of plant fibers.

Morphology and surface behavior of plant fibers are studied using various techniques such as XRD, FTIR, SEM, AFM, TEM and thermogravimetric analysis that helps in understanding the nature of natural fibers.

In terms of the mechanical behavior of plant fibers, important milestones have been achieved to highlight the influence of the chemical composition and structural parameters of the plant wall on their tensile properties. The microstructure of plant fibers is very complex, precisely when it comes to defining generalizable geometric and analytical models that describe it. As mentioned above, improving the mechanical properties of fibers may require the introduction of new types of fibers. And we could mention in this regard the ongoing research on spinning with solvents [70, 71], to obtain fibers of greater strength and low scattered properties. Understanding how fiber morphology affects the properties of composite materials is essential. More precisely, it is important for the selection of new fibers and for the cultivation of fibrous plants genetically selected. This would help to predict their potential for reinforcement in other materials to achieve desired properties.

Investigation of the viscoelastic properties of plant fibers has also been outlined. A variety of dynamic modulus measurement methods exists including ultrasonic wave propagation and the flexural resonance method presented here, for which normal modes of vibration are monitored. Stress relaxation tests are to be carried out to retrieve stress over time as well as the elastic modulus of the fiber material. A mathematical method for extracting the relaxation modulus from relaxation experimental data has to be proposed to this end. Proper selection of the testing vibrational mode and machine cross-head speed (during relaxation) appear important in the suggested methods in order to avoid dispersive results. The Young’s modulus that is obtained from the dynamic behavior of the specimen should, therefore, reflects the frequency dependence of the material.

Acknowledgments

The authors wish to acknowledge the Director of the University Institute of Technology of the University Ngaoundéré, Prof. Mohammadou Bouba Adji, for providing research facilities within the department of mechanical engineering.