Elementary fibers

The secondary wall found in wood cells is composed of two or three layers, known as SI, S2, and S3, respectively. In each of these layers, the cellulose microfibrils are "spirally-wound" at a different angle to the major axis of the tracheid. This variation in microfibril angle imparts strength to the fiber structure in a variety of directions. Within the bast or schlerenchyma cells found in flax, hemp, jute, and kenaf, the secondary wall is less thick than that of wood, but contains layers of similarly spirally-wound microfibrils embedded in a hemicellulose and pectin-rich matrix. This "composite structure" imparts potentially high strength to regions of the cell wall. Figures 9.1 and 9.2 show a schematic representation of flax fiber and a section of an elementary fiber with its fibrillar structure in its secondary cell wall [31]. 

Figure 9.2 Schematic representation of a section of the elementary fiber with its fibrillar structure in the secondary cell wall [9].

Zylinski (1964) reported younger and finer fibers at the top of the plant, with thicker fibers near the bottom of the stem. Soluble components in flax fiber impact the hygroscopic moisture handling characteristics (Table 3.2). Foulk et al. (2003) reports that late harvest stems are less easily retted than those harvested before full maturity while estimating the percent fiber contributed to total plant weight by the pedicel to be 5% (Foulk et al. 2004). The plant stem has a diameter of 2-3 mm, which produces bast technical fibers with a diameter of 50-100 pm composed of elementary fibers (single plant cells) with a diameter... 

All parts of fibrous plant stem must be removed for proper fiber separation. Bast fiber extraction is usually conducted using special processes such as water and dew retting, degumming, or decortication, in which the separation of bundles craitaining elementary fibers joined together by pectin and calcium ions is also performed. Fiber content in the stem of fibrous plants is shown in Table 4.1. 

The structure of bast fibers is very similar to all types of fiber. Elementary fiber consists of cellulose fibrils held together by a lignin and hemicellulose. Typical phenomenon observed in bast fibers structure is occurrence of lumen with different... 

Properties related to bast fibers shape are shown in Table 4.4. Ramie, nettle, and flax are characterized by the highest density, but kenaf and isora show the lowest density. The longest technical fiber you can find is hemp, but ramie elementary fiber is the longest and the thickest. 

Chemical decomposition of hemicellulose and lignin, causing embrittlement and decreasing reinforce capacity of natural fibers because break the link between elementary fibers [31,40,41]. Chemical decomposition of these compounds, is caused because they are solubilized in the calcium hydroxide and in other alkalis produced during hydration reaction. 

Textural, adsorption, and other characteristics of crude and bleached cottonized flax fibers prepared from the side product of flax scutching (Makarov Lenzavod, Kiev region, Ukraine), i.e., short fibers were carded in order to divide the technical fibers still linked by their tricellular junctions into elementary fibers, were compared with those of commercial (Fisher) carded cotton fibers (Mikhalovska et al. 2012, Mikhalovsky et al. 2012). Here crude and bleached flax fibers are designated as flax and bleached flax, respectively. 

Examination of fibrous materials by electron microscopy showed that the fibrous material uncovered by the current-conducting layer decomposes in 1-2 min, when exposed by electron beam. Examination of polymer orientation in fibers by using birefringence showed that elementary fibers in non-woven fabric are well oriented along fiber direction. However, it is impossible to determine the degree of orientation quantitatively because the fibers are packed randomly. 

The stress-strain curves of individual kenaf and sisal fiber samples are presented in Figure 14.6. Each curve displays a different initial curve, especially at small strain (below 1%), which is attributed to the orientation of the fibrils along the axis of the fiber under tension [82]. WG-treated kenaf fiber displayed a slight decrease of approximately 1.6% in the failure strain, when compared to the untreated fibers. For the sisal fibers, the enhanced strain could have occurred because axial spUtting was promoted and transverse cracking was delayed as a result of the WG treatment [75, 78]. This enhances the tearing type failure of the elementary fibers, which is... 

The maximum attainable values for the strength of elementary fibers from LC solutions of polyamides are 400 cN/Tex [40], and the maximum modulus of elasticity is 238 GPa [41]. (In the last case, the fiber was fabricated from a copolyamide containing an equimolar number of benzidine units together with PPTA units.)... 

The strength properties of Kevlar and other aramid fib are strongly dependent on Ae structure of the filament and in particular on the twist. Figure 10.12 shows that the maximum tensile strength is observed for a twist of up to 100 turns per 1 m. The resistance of Kevlar to flexural deformations is exponentially dependent on the load and the thickness of the elementary fiber [90]. The surface of the fiber cracks during such deformations (Fig. 10.13). 

Fineness (dtex) 1-4 technical fiber 10-40 elementary fiber 1-7 2-50 18-60 jm 1-4... 

Fiber length (mm) 10-60 technical fiber 450-800 elementary fiber 10-40 fine wool 55-75 coarse wool 150-300 reel silk 300-1000 m schappe silk 50-250 m... 

Compared to natural and chemical fibers, asbestos fibers are extremely fine. The diameter of the elementary fibers varies between 0.02 and 0.2 pm. Further advantages of asbestos are its high-temperature stability and low cost. For these reasons, large amounts of asbestos have been mined and processed worldwide. In 1987, the world production was approximately 4 million tons, but this amount has declined in recent years. 

Table 11.2 shows the mechanical properties of natural (plant) and synthetic fibers [18]. Natural fibers are non abrasive towards mixing and molding equipment. This can contribute to significant equipment maintenance cost reductions. The elementary fibers are bound together by a pectin interphase. This interphase is much stronger than the interphase between the technical fibers [39]. Natural fibers are safe to handle and the working conditions are better when compared to synthetic reinforces, such as glass fibers. 

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