Natural fibers structure

Keywords Lignocellulosic natural fibers, structure, processing md applications... 

Lignocellulosic Natural Fibers Structure, Chemical Composition and Properties... 

Naturally, fibers and whiskers are of little use unless they are bonded together to take the form of a structural element that can carry loads. The binder material is usually called a matrix (not to be confused with the mathematical concept of a matrix). The purpose of the matrix is manifold support of the fibers or whiskers, protection of the fibers or whiskers, stress transfer between broken fibers or whiskers, etc. Typically, the matrix is of considerably lower density, stiffness, and strength than the fibers or whiskers. However, the combination of fibers or whiskers and a matrix can have very high strength and stiffness, yet still have low density. Matrix materials can be polymers, metals, ceramics, or carbon. The cost of each matrix escalates in that order as does the temperature resistance. 

Climatic conditions, age, and the digestion process influence not only the structure of fibers but also the chemical composition. Mean values of components of plant fibers are shown in Table 4. With the exception of cotton, the components of natural fibers are cellulose, hemi-cellu-lose, lignin, pectin, waxes, and water-soluble substances. 

Extrusion press processing (express processing) was developed for the production of flax fiber-reinforced PP at the research center of Daimler Benz (Ulm, Germany) [62]. In this processing, natural fiber nonwovens and thermoplastic melt-films are alternatively deposited in a tempered molding tool and molded afterwards. The thermoplastic melt-films are laid on by a mobile extruder. If thi.s process is optimally adapted to the element, a single passage by the extruder suffices. The structural order consists of three layers two layers of... 

The mechanical and physical properties of natural fibers vary considerably, as it is with all natural products. These properties are determined by the chemical and structural composition, which depend on the fiber type and growth circumstances. With this cellulose, the main component of all natural fibers varies from fiber to fiber. 

If the most-probable orientation of the structural entities is no longer parallel to the fiber axis, we may observe a clearly inclined streak. Such orientations are frequently observed in herbal and animal natural fibers [45,260,261], If the split nature of the orientation distribution is clearly detected, the streak method can be applied or adapted. 

The Fitting Problem. In many studies in particular of natural fibers, orientation distributions are picked from spherical arcs in scattering patterns and then fitted by Gaussians or Lorentzians. The result is the finding of an isotropic background. At least part of this background is not related to structure, but to a fundamental misunderstanding. 

In the course of his studies of the dyeing process, he became deeply interested in the structure of natural fibers, and most of his efforts were directed toward this new field of research, with the help of able associates, among them R. Brill, M. Dunkel, G. von Susich, and E. Valkd. His investigation of various aspects of the problem utilized physical means (for example, x-ray diffraction, optical properties, and viscosity) and the purely chemical approach. A young scientist, H. Mark, who later became an authority in the field of high polymers, was appointed head of the physical chemistry laboratory. 

Papanikolopoulou, K., Schoehn, G., Forge, V., Forsyth, V. T., Riekel, C., Hernandez, J.-F., Ruigrok, W. H., and Mitraki, A. (2005). Amyloid fibril formation from sequences of a natural jS-structured fibrous protein, the adenovirus fiber. J. Biol. Chem. 280, 2481-2490. 

Paper products (newsprint, tissue, packaging, etc.) are made from pulps that consist of natural fibers derived from vascular plants such as trees, sugar cane, bamboo, and grass. The vascular fiber walls are composed of bundles of cellulose polymeric filaments. This long, linear glucose polymer is what paper is made from. The polymer has the structure shown in Scheme 8.18. 

The enhanced strength of whiskers and natural fibers, by comparison to the strength of materials of the same composition in another morphology, could be a coincidental in these crystalline synthetic and mineral fibers, a particular crystal direction is parallel to the direction of the applied stress. However, the inverse diameter-strength relationship indicates that factors other than crystal structure contribute to the mechanical strength of fibrous materials. 

Fibers exist as natural, or synthetic, hydrophilic, hydrophobic, nonionic, and ionic. Natural fibers hnvc complex chemical structures with a multitude of possible points of attraction for a dyestuff and are difficult io characterize because of the structure being strongly influenced by regional, climatic variations and the species of plant or animal. Dyeing of natural fibers is therefore much more complex than dyeing synthetic fibers where structures can be characterized and the availability of points of attraction can be deliberately engineered into the fiher s molecular chain. The various types of liher arc summarized in Tahle I. The fiber type dictates the type of dye needed. 

Dyeing Mechanism. Nylon i.s. similar in its general chemical structure to the natural fiber wool, and therefore all the previously described processes lor wool are applicable to dyeing nylon with acid, metallized, and other dyes. There are. however, significant differences. Nylon is synthetic, it has delined chemical structure depending on the manufacturing process, and it is hydrophobic. 

Fig. 12.7. Scanning electron microscopy reveals details of hair fibers. Normal hairs from an adult C57BL/6J examined as a whole mount (A) illustrates density of mouse hairs and the nature of the normal skin surface. Manually plucked hairs illustrate the structural differences between some of the hair fiber types (B). Higher magnification of boxed area in B reveals the regular cuticular scale patterns on these hair fibers (C). These approaches illustrate details of hair fiber structure and density (80).

Cellophane film Is prepared from regenerated cellulose and Is similar to rayon fiber In that It has a lower molecular weight than cotton and contains a small amount of hemlcellulose, as does linen. Cellophane film, therefore, although not a duplicate of any natural fiber, Is similar enough In chemical structure and morphology to make It useful as a model system. Moreover, Its transparency and the precision of Its manufacture make It quite useful for this type of study. 

The two most common natural textile fibers encountered in modern fabrics have contrasting responses to soil burial. Under most soil burial conditions cellulose will degrade rapidly whereas wool will decay at a slower rate. These phenomena are demonstrated by the degradation of textile fibers from the Experimental Earthworks Project (Janaway 1996a). Figures 7.9 and 7.10 compare wool and linen buried in the chalk environments at Overton Down for 32 years. The linen is denatured to the point that there is little surviving morphology, whereas the wool retained some fiber structure. 

Among the natural fibers are cellulose, the primary structural component of plants and bacterial cell walls animal fibers such as wool and silk and biochemical fibers. Plant fibers are composed of cellulose (see Figure 1), lignin (see Figure 2), or similar compounds animal fibers are composed of protein (see Figure 3). 

Electrospinning of natural fibers offers unique capabilities for producing novel natural nanofibers and fabrics with controllable pore structure. Current research effort has focused in understanding the electrospinning of natural fibers in which the influence of different governing parameters are discussed. 

In much the same way, natural polymeric fibers like wool, cotton, silk, etc., are often touted as superior to anything that is man-made or synthetic. But is this fair There is no doubt that natural fibers have a unique set of properties that have withstood the test of time (e.g., it is difficult, but not impossible, to match silk s feel or cotton s ability to breathe ). On the other hand, consider Lycra , a completely synthetic fiber produced by DuPont (Figure 1-12) that has a truly amazing set of properties and is the major component of Spandex (a material that keeps string bikinis on ). Or consider the wrinkle-free polyester fibers used in clothing and the stain proof nylon and polyacrylonitrile polymers used in carpets. The point here is that polymers, be they natural" or synthetic, are all macromolecules but with different chemical structures. The challenge is to design polymers that have specific properties that can benefit mankind. 

---