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Structure and Properties of Viscose Rayon Fibers

The structure of rayon fiber is generally that of smooth, inelastic filaments like glass rods. However, different processes, additives and finishes can vary the physical appearance and structure of viscose rayon.

Structure and Properties of Viscose Rayon Fibers

Figure 1: Microscopic view of Viscose Rayon Fiber

Chemical formula of viscose rayon obtained by reacting cellulose with alkali and carbon disulphide CS2 is as follows:

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Figure 2: Chemical Formula of Viscose Rayon.

Different kind of fibers can be identified by their microscopic appearance better, than their physical appearance. When viscose rayon is viewed under microscopic lens:

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Figure 3: Longitudinal and Cross-Sectional view of Viscose Rayon Fibers.

The longitudinal appearance of regular viscose rayon exhibits uniform diameter and interior parallel lines called striations.

In cross section view, viscose fiber shows highly irregular or serrated edges.

Properties of Viscose Rayon

Rayon as a fabric is soft and comfortable. It drapes well which is why it is highly sought after as an apparel fabric. Most characteristics are variable depending on processing, additives and other finishing treatments and fabric construction.

Physical Properties

Moisture Absorption

It absorbs more moisture than cotton. Moisture Content of Cotton is 6% at 70F whereas for Viscose Rayon, it is 13% under the same conditions.

Tensile Strength

The Tensile Strength of the fiber is greater when the fiber is dry as compared to its wet state. Because of this, it stretches and shrinks more than cotton. It is 1.5-2.4 gpd in the dry state and 0.7-1.2 gpd in the wet state. For high tenacity variety the values are 3-4.6 gpd and 1.9 to 3.0 gpd respectively.

Elasticity

The elasticity of Viscose Rayon is less than 2-3%. This is very important in handling viscose yarns during weaving, stentering etc. when sudden tensions are applied.

Elongation at Break

Ordinary Viscose rayon has 15-30% elongation at break, while high tenacity rayon has only 9-17% elongation at break.

Density

The density of Viscose rayon is 1.53 g/cm3. Rayon filaments are available in three densities: 1.5, 3.0 and 4.5.

Action of Heat and Light

At 300o F or more, Viscose Rayon loses its strength and begins to decompose at 350-400o F. Prolonged exposure to sunlight also weakens the fiber due to moisture and ultraviolet light of the sunlight.

Absorbency

Rayon is the most absorbent of all cellulosic fibers, even more so than cotton or linen. Rayon absorbs perspiration and allows it to evaporate away from skin which makes it an excellent summer fabric. Its high absorbency equally applies to dyes as well giving them deep and thick colors.

Abrasion Resistance

Its abrasion resistance is very low thanks to the inelasticity of rayon. It is easily damaged by scraping.

Tenacity

Tenacity of viscose rayon is 2.4-3.2 gpd.

Moisture Regain

Moisture regain is 11-13%.

Chemical Properties

Action of Acids

The resistance of regenerated cellulose rayon's to acids is generally less than that of cotton. Organic acids can be safely used in 1 to 2 percent concentration without injury to the fiber. Inorganic acids such as hydrochloric & nitric can be used in surprisingly strong concentrations provided the temperatures are not too high and the treatment is brief. At high temperatures and concentrations all acid will destroy or carbonize regenerated rayons. In short, it is damaged by strong acids, but it is moderate with weak acids.

Action of Bases

Viscose rayon has a good resistance to weak alkalis but strong alkalis can harm theses fibers thoroughly.

Action of Dry Heat

Viscose Rayon, under the influence of heat as well as light, shows rapid loss in strength. Degradation of cellulose is lower in the absence of oxygen. Continued heating in the absence of oxygen leads to deterioration of the cellulose.

Action of Solvents

Textile solvents can be used on Viscose rayon without any deteriorating effect. For example, Viscose rayon dissolves in cuprammonium hydroxide solution.

Effect of Iron

Contact with iron in the form of ferrous hydroxide weakens viscose rayon yarns. Therefore, staining, marking or touching of rayon to iron or iron surface should be avoided.

Action of Microorganisms

Microorganisms (molds, mildew, fungus, bacteria) affect the color, strength, dyeing properties and luster of rayon. Clean and dry viscose rayon is rarely attacked by mildew.

Relationship between Structure and Properties of Viscose Rayon

In general, the fiber structures of various viscose fibers depend on the properties such as conditions of viscose, acid bath, stretching, regeneration and so forth. In the case of ordinary rayon, in which an acid bath offering high coagulative and regenerative powers is used, the major portion of fiber structure is determined in acid bath. For viscose fibers, however, in which a weak acid bath is employed, the regeneration and crystallization of fibers in acid bath are only carried out to a small extent and the fiber structure is not only dependent on acid bath conditions but also on the stretching and regeneration conditions. Consequently, a wide choice of fiber structures and properties is available by combining such factors such as viscose, acid bath, stretching and regeneration.

Under different stretching and regeneration conditions, the structure of viscose fiber in the same viscose and acid bath solution will vary clearly.
Following figure shows stained cross section of fibers from the same viscose and same acid bath under different stretching and regeneration conditions.

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In above figure, all specimens are of viscose rayon fibers. As evident from specimen No. 1, fiber gives a thick outer layer by the core staining. In specimen No. 2 & 3, this layer gets thinner and in No. 4, it disappears showing total skin staining. Hence, the relationship between structure and properties was verified. It is evident that fiber structure as well as fiber properties greatly vary with stretching and regeneration conditions. Mechanical properties of these specimens were changed as structures were changed when a change in stretching and regeneration of fibers took place.

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There is a strong relationship between wet fibrillation properties and fibrillar structures of viscose fibers. The fibers having a thicker core-stainable layer in the outer part show little fibrillation while those having skin-stainable layer in the outer part show considerable fibrillation.

The following figure confirms the fact that those viscose fibers which have core-stainable layer in the outer part show little fibrillation while fibers having skin-stainable layer in the outer part show considerable fibrillation.

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It is clear from figure that fibrillation (property of fiber) affects the structure of viscose rayon fibers.

Muhammad Rehan Ashraf

I am a Textile Engineer, founder and editor of "Textile Trendz". Currently working in an export-oriented textile organization. I love to share my knowledge about textiles.