Rayon fibers structure

The number of reports about hemicelluloses that have been covered by this review indicates the significantly increased importance of all types of hemicelluloses as plant constituents and isolated polymers during the last decade. Attention has been paid not only to known hemicelluloses but also to the primary structure, physicochemical, physical, and various functional properties of hemicelluloses isolated from hitherto uninvestigated plants. The efforts to exploit a variety of plant as potential sources of hemicelluloses were pointed out particularly for agricultural crops, wood wastes, as well as for by-products of pulp and rayon fiber technologies. Many studies were devoted to characterize seed-storage hemicelluloses from plants that have been traditionally applied in food and medicine of many underdeveloped countries to find substitutes for imported commercial food giuns. 

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. 

Muller M, Riekel C, Vuong R, and Chanzy H. Skin/core micro-structure in viscose rayon fibers analysed by x-ray microbeam and electron diffraction mapping. Polymer, 2000 41 2627-2632. 

Because acetylation of cellulose proceeds in a heterogeneous system, the reaction rate is controlled by the diffusion of the reagents into the fiber structure. The quality of the cellulose raw material used for acetate rayon is... 

Carbon and graphite fibers are made by the pyrolysis of certain naturally occurring and man-made fibers, such as regenerated cellulose (rayon) fibers. A wide range of physical, mechanical and chemical properties may be obtained dependent on amount of dehydration. This product is one of the most structurally efficient reinforcements. Unlike any other reinforcement, it retains its 2,800 MPa (400,000 psi) tensile strength when tested up to a temperature of 2700 C (4800F). 

A complete definition of the fine structure of cellulose would entail a knowledge of the exact distribution of the size and shape of these ordered and disordered regions, a position which has not yet been achieved. However, a report claims that four types of material which differ in the degree of orderly arrangement of molecular chains can be distinguished in both native and regenerated celluloses, although quantitative measurements have only been made on one viscose-rayon fiber. These four types of material are ... 

Figure 9 Schematic structure models of native cellulose and cupra rayon fibers-...

Fiber, rayon The generic term for fibers, staples, and continuous filament yarns composed of regenerated cellulose but also frequently used to describe fibers obtained from cellulose acetate or cellulose triacetate. Rayon fibers are similar in chemical structure to natural cellulose fibers (cotton) except that the synthetic fiber contains short plastic units. Most rayon is made using the viscose process. 

In the period 1965-1980 a wide variety of new, stronger, and more durable rayon fibers were developed. Rayon variants are now produced which utilize the comfort and aesthetic qualities of cellulose to compliment synthetic fibers in many textile applications. Considerable emphasis has been placed on the economics and ways to meet environmental and safety standards. Special effects, such as crimp or hollow filaments, may be obtained by appropriate viscose formulations, point-of-stretch applications, spin-bath compositions, and modifiers. Flame-retardant (FR), acid-dyeable, and superabsorbent rayons are typical of the properties that can be attained by incorporating various materials in the fiber structure. Rayon is unique in the respect that the fiber can be permanently modified for a wide variety of end uses simply by adding the appropriate material to viscose. 

The classification of viscose rayon fibers into different types is done mostly on the basis of physical and chemical properties. Fibers produced by nonviscose processes are usually identified separately. It has already been described how the fiber can be produced to have almost any desired structure, and it is considered to be the most versatile of all human-made fibers. It is available in various cross-sectional shapes, from multilobed, serrated, and round to flat longitudinally, it may be straight or curled (crimped). It comes in fine deniers... 

Figure 4.17 Structure of cellulose unit cell (Cellulose I). Source. Reprinted with permission from Dyer J, Daul GC, Rayon Fibers, Lewin M and Pearce EM eds., Handbook of Fiber Chemistry, Marcel Dekker, New York, 775, 1998. Copyright 1998, CRC Press, Boca Raton, Florida.

The structure of the cellulose has a marked influence on the subsequent decomposition [73-77] and less crystalline materials decompose more readily and in terms of thermal decomposition can be rated viscose cord rayon > viscose continuous filament > viscose rayon fiber > Fortisan fiber > Cotton > hydrocellulose. However, hydrocellulose does not follow this rule. 

In order to simplify the procedure above, a one-stage technology has been developed together with the fiber producing company Cordenka based on direct feeding of short cut fiber pellets into the extruder [15]. The actual novelty in this respect is presented by the pellets themselves, since pure short cut rayon fibers cannot be fed into the extruder by usual techniques due to their wadding-like structure and thus low bulk density. Therefore, the fibers were coated with a special sizing to provide fiber adherence and pellet formation. 

Rayon-based ACFs are used in the adsorption of many volatile organic compounds including formaldehyde (80), methyl ethyl ketones (81), and benzene (81). ACFs are also finding uses in natural gas storage (82), electrodes for batteries (83), catalyst supports (84), and NO removal (85). Stabilized rayon fibers are carbonized and then activated with air (80), steam (86), or carbon dioxide (87), much as in granular carbon activation. The extent of pyrolysis governs the pore structure, carbon yield, and surface area of the fiber, while activation impacts the presence of functional groups on the pore surface (12). Properties of some commercial ACFs are summarized in Table 6. 

A number of rayon fibers are available, but the most suitable is the highly polymerized viscose rayon. The molecular structure is as follows ... 

The PSDs for the Rayon and the PAN are reproduced in Figure 4.63(a, b) and show very clearly that the two parent fibers activate in quite different ways, the PAN showing a much broader distribution of pores extending to >20 nm whereas the Rayon fiber had almost no porosity beyond 3nm. A model of structure in PAN fiber, according to Johnson (1987) is reproduced in Figure 4.64 and indicates how porosity may be located within the fiber. 

Sulfur reacts with methane to produce carbon disulfide, which is used to manufacture rayon fibers, which are made into tire cord for structural reinforcement. 

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