Other Natural Cellulosic Fibers

On treatment with NaOH solution of mercerizing strength, the cellulose I pattern of purified flax completely transforms to the cellulose II pattern. In other natural cellulosic fibers (except ramie), this transformation is only partial [124]. The degree of crystallinity of flax is estimated to be 70% [113]. 

In addition to cotton fibers, they are mar r other types of natural cellulose fibers. One major difference between cotton and other natural cellulose fibers is the cellulose content. Cotton fibers are the purest and other natural cellulose fibers have significantly lower content of cellulose. For example, both cotton and kapok are seed fibers, but cotton has the highest cellulose content and kapok has the lowest (13 wt%). The cellulose contents of other natural cellulose fibers are in the range of 40%-90%. Table 4.2 shows the cellulose contents of several natural cellulose fibers. 

What is the major difference between the chemical compositions of cotton and other natural cellulose fibers ... 

Another major difference among different natural cellulose fibers is their morphology. Kapok fibers are obtained from the seed pods of the tropical kapok tree. Like cotton, kapok fibers are unicellular fibers. However, kapok fibers do not collapse and twist after dried (Figure 5.6). Dried kapok fibers have circitlar, hollow (lumen) cross-sectional stmcture with total wall thicknesses of aroimd 2 /rm and fiber diameters ranging from 15 to 35 //m. As a result, kapok fibers have lower densities (0.31-0.38 g/cm ) than most other natural cellulose fibers. 

Many other natural cellulose fibers form bundles. For example, short flax fibers (27-36 mm) overlap each other and are held together by a mixtirre of non-cellulosic polymers, including hemicellulose, lignin, and pectins (Figure 5.7). The resultant fiber bundles have irregular shapes and contain multiple lumens. Individual flax fibers have an average diameter of around 20 / m, but the bundle... 

The basic raw material is cellulose, a major constituent of sawdust, straw, cardboard or paper wastes, wood chips, or other natural plant fibers. Any of these materials should be chopped or shredded, but never so finely as to eliminate their inherent structural qualities. This cellulosic base comprises approximately 80% of the total substrate mixture. 

Composites of natural cellulosic fibers may have the best option to the array because the nanofibers can interact with other natural materials to form highly ordered structures. The cellulose microlibrils have hydroxyl groups (OH) on their surface, which can form covalent bonds with the matrix. In literature, three alternative routes for the preparation of composites of cellulose are known [40]. The first route refers to the incorporation of hydrophobic libers in matrices such as polyethylene, polypropylene and polystyrene. In this case, it is necessary to a chemical or physical treatment, so that the surfaces of the matrix and the libers... 

Biocomposites are formed through the combination of natural cellulose fibers with other resources such as biopolymers, resins, or binders based on renewable raw materials. The goal is to combine the materials in such a way that a S5mergism between the components results in a new material that is much better than the individual components. 

Composites from natural cellulose fibers or other fibrous components are containing conventionally polyolefin or poly (vinyl chloride) fibers. Molded composite structures may contain up to 50% of binder polymer. One of the limitations of conventional articles is their lack of biodegradability when composted, which is due to the nature of the binder polymer used (32). 

Except cotton and kapok, most of the natural cellulosic fibers are multicellular. They are usually used as groups of individual cells or as bundles of fiber in industrial applications. As a term, a fiber , or a technical fiber [14], refers to a bundle of individual cells bound together by hemicellulose, lignin and other non-cellulosic materials [12]. However, the individual fiber cell is drastically stronger than the fiber bundle [56]. For example, the individual fiber of flax is as stiff as aramid [65]. The individual fiber cell has a lumen inside which imparts a hollow structure to the fiber as seen in Figure 11.7. As an example, okra fibers have a void content of 18-32% [15]. The interface between two cells is called middle lamella [14]. 

However, there are thousands of tons of agricultural wastes produced without proper utilization which found to be useful to prepare polymer composite for commercial purposes for example EFB, sisal fiber, wheat straw, and others (Abdul Khalil et al. 2012b). Numbers of natural cellulose fibers have been used to reinforce polymer composites. In fact, various types of natural fibers were investigated for incorporation in plastics. Basically, natural fibers are diverted into two categories which are wood and non-wood. Table 1 shows varieties of natural fibers that available on current research. 

Bast fiber crops are a group of plants that can produce natural cellulose fibers from plant stem skin. In history, cultivation of bast fiber crops is the oldest method to produce natural fibers for meeting clothing needs and other daily necessaries. The most important bast fiber crops are ramie, flax, hemp, jute, and kenaf, based on their production capacity and consumption quantity. To date, the production of bast fiber crops still primarily aims at the textile market. Facing fierce competition from synthetic fibers that have increased productivity and a steadily expanded end-use market, the world capacity of bast fiber production continues to decline. Currently, the production volume of the major bast fibers in the world is about 4.8 million metric tons, equivalent to 14% of the global production of manufactured fibers (Fibersource, 2002). 

Cellulose materials contain structural defects, pores and capillaries, which affect the physical adsorption of various substances absorption of liquids diffusion and permeability of gases, vapors, and liquids mechanical and some other properties. Investigations have shown that pores between elementary fibrils have diameter about 1.5-2 nm (loelovich et al., 1988]. Diameter of pores and capillaries between microfibrillar bundles can be in the range from 2 to 20 nm (Papkov et al., 1976]. Cell wall of natural cellulose fibers has mesopores with diameter from 40 to 100 nm (Segeeva, 1972]. Samples of paper and paperboard contain also macropores with diameter above 100 nm (loelovich et al., 1988]. 

The image in Fig. 7.26 resembles the supermolecular structure of cellulose of cotton or bast fibers. For other natural celluloses, the ratio between crystallites and non-crystalline domains, as well as sizes of these constituents, can vary in a wide range. The proposed model permits explaining various physico-mechanical, physicochemical, chemical, and biological properties of natural cellulose. 

Natural cellulose fibers also are called plant fibers or vegetable fibers. Natural cellulose fibers include cotton together with flax, jute, jute, ramie and other fibers produced by plants. Natural cellulose fibers can be classified to seed, bast, leaf, and fruit fibers. Figure 4.1 shows the classification of natnral cellnlose fibers. 

NATURAL CELLULOSE FIBERS (HARD FIBERS AND OTHERS) Alfalfa... 

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