Showing posts with label Rayon. Show all posts
Showing posts with label Rayon. Show all posts
26 October 2012
Characteristics, Manufacturing, End Uses of Rayon
Rayon
is the oldest commercial manmade fiber. It is a manufactured fiber
composed of regenerated cellulose, as well as manufactured fibers
composed of regenerated cellulose in which substituents have replaced
not more than 15% of the hydrogens of the hydroxyl groups. Rayon fibers
include yarns and fibers made by the viscose process, the cuprammonium
process, and the now obsolete nitrocellulose and saponified acetate
processes. Generally, in the manufacture of rayon, cellulose derived
from wood pulp, cotton linters, or other vegetable matter is dissolved
into a viscose spinning solution. The solution is extruded into an
acid-salt coagulating bath and drawn into continuous filaments. Groups
of these filaments may be made in the form of yarns or cut into staple.
Characteristics of Rayon Fiber :
Characteristics of Rayon Fiber :
- Highly absorbent
- Soft and comfortable
- Easy to dye
- Drapes well
Types of Rayons
Rayon fibers are engineered to possess a range of properties to meet the demands for a wide variety of end uses. Types of rayon fiber are given below:
- High wet modulus rayon
- Polynosic rayon
- Specialty rayons
- Super absorbent rayons
- Tencel rayon
- Lyocell
While there are many variations in the manufacturing process that exploit the versatility of the fiber, the following is a description of the procedure that is used in making regular or viscose rayon.
Regardless of whether wood pulp or cotton linters are used, the basic raw material for making rayon must be processed in order to extract and purify the cellulose. The resulting sheets of white, purified cellulose are then treated to form regenerated cellulose filaments. In turn, these filaments are spun into yarns and eventually made into the desired fabric.
The process of manufacturing viscose rayon consists of the following steps mentioned, in the order that they are carried out: (1) Steeping, (2) Pressing, (3) Shredding, (4) Aging, (5) Xanthation, (6) Dissolving, (7)Ripening, (8) Filtering, (9) Degassing, (10) Spinning, (11) Drawing, (12) Washing, (13) Cutting. The various steps involved in the process of manufacturing viscose are explained below.
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| Figure : Process of manufacture of viscose rayon fiber |
1. Steeping:
Cellulose
pulp is immersed in 17-20% aqueous sodium hydroxide (NaOH) at a
temperature in the range of 18 to 25°C in order to swell the cellulose
fibers and to convert cellulose to alkali cellulose.
(C6H10O5)n + nNaOH ---> (C6H9O4ONa)n + nH2O
(C6H10O5)n + nNaOH ---> (C6H9O4ONa)n + nH2O
2. Pressing:
The
swollen alkali cellulose mass is pressed to a wet weight equivalent of
2.5 to 3.0 times the original pulp weight to obtain an accurate ratio of
alkali to cellulose.
3. Shredding:
The
pressed alkali cellulose is shredded mechanically to yield finely
divided, fluffy particles called "crumbs". This step provides increased
surface area of the alkali cellulose, thereby increasing its ability to
react in the steps that follow.
4. Aging:
The
alkali cellulose is aged under controlled conditions of time C) in
order to depolymerize the°and temperature (between 18 and 30 cellulose
to the desired degree of polymerization. In this step the average
molecular weight of the original pulp is reduced by a factor of two to
three. Reduction of the cellulose is done to get a viscose solution of
right viscosity and cellulose concentration.
5. Xanthation:
In
this step the aged alkali cellulose crumbs are placed in vats and are
allowed to react with carbon disulphide under controlled temperature (20
to 30°C) to form cellulose xanthate.
(C6H9O4ONa)n + nCS2 ----> (C6H9O4O-SC-SNa)n
Side reactions that occur along with the conversion of alkali cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and also the resulting viscose solution. The orange cellulose xanthate crumb is dissolved in dilute sodium hydroxide at 15 to 20 °C under high-shear mixing conditions to obtain a viscous orange colored solution called "viscose", which is the basis for the manufacturing process. The viscose solution is then filtered (to get out the insoluble fiber material) and is deaerated.
(C6H9O4ONa)n + nCS2 ----> (C6H9O4O-SC-SNa)n
Side reactions that occur along with the conversion of alkali cellulose to cellulose xanthate are responsible for the orange color of the xanthate crumb and also the resulting viscose solution. The orange cellulose xanthate crumb is dissolved in dilute sodium hydroxide at 15 to 20 °C under high-shear mixing conditions to obtain a viscous orange colored solution called "viscose", which is the basis for the manufacturing process. The viscose solution is then filtered (to get out the insoluble fiber material) and is deaerated.
6. Dissolving:
The
yellow crumb is dissolved in aqueous caustic solution. The large
xanthate substituents on the cellulose force the chains apart, reducing
the interchain hydrogen bonds and allowing water molecules to solvate
and separate the chains, leading to solution of the otherwise insoluble
cellulose. Because of the blocks of un-xanthated cellulose in the
crystalline regions, the yellow crumb is not completely soluble at this
stage. Because the cellulose xanthate solution (or more accurately,
suspension) has a very high viscosity, it has been termed "viscose".
7. Ripening:
The
viscose is allowed to stand for a period of time to "ripen". Two
important process occur during ripening: Redistribution and loss of
xanthate groups. The reversible xanthation reaction allows some of the
xanthate groups to revert to cellulosic hydroxyls and free CS2. This
free CS2 can then escape or react with other hydroxyl on other portions
of the cellulose chain. In this way, the ordered, or crystalline,
regions are gradually broken down and more complete solution is
achieved. The CS2 that is lost reduces the solubility of the cellulose
and facilitates regeneration of the cellulose after it is formed into a
filament.
(C6H9O4O-SC-SNa)n + nH2O ---> (C6H10O5)n + nCS2 + nNaOH
(C6H9O4O-SC-SNa)n + nH2O ---> (C6H10O5)n + nCS2 + nNaOH
8. Filtering:
The
viscose is filtered to remove undissolved materials that might disrupt
the spinning process or cause defects in the rayon filament.
9. Degassing:
Bubbles
of air entrapped in the viscose must be removed prior to extrusion or
they would cause voids, or weak spots, in the fine rayon filaments.
10. Spinning - (Wet Spinning):
Production
of Viscose Rayon Filament: The viscose solution is metered through a
spinnerette into a spin bath containing sulphuric acid (necessary to
acidify the sodium cellulose xanthate), sodium sulphate (necessary to
impart a high salt content to the bath which is useful in rapid
coagulation of viscose), and zinc sulphate (exchange with sodium
xanthate to form zinc xanthate, to cross link the cellulose molecules).
Once the cellulose xanthate is neutralized and acidified, rapid
coagulation of the rayon filaments occurs which is followed by
simultaneous stretching and decomposition of cellulose xanthate to
regenerated cellulose. Stretching and decomposition are vital for
getting the desired tenacity and other properties of rayon. Slow
regeneration of cellulose and stretching of rayon will lead to greater
areas of crystallinity within the fiber, as is done with high-tenacity
rayons.
The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation.
(C6H9O4O-SC-SNa)n + (n/2)H2SO4 --> (C6H10O5)n + nCS2 + (n/2)Na2SO4
Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
The dilute sulphuric acid decomposes the xanthate and regenerates cellulose by the process of wet spinning. The outer portion of the xanthate is decomposed in the acid bath, forming a cellulose skin on the fiber. Sodium and zinc sulphates control the rate of decomposition (of cellulose xanthate to cellulose) and fiber formation.
(C6H9O4O-SC-SNa)n + (n/2)H2SO4 --> (C6H10O5)n + nCS2 + (n/2)Na2SO4
Elongation-at-break is seen to decrease with an increase in the degree of crystallinity and orientation of rayon.
11. Drawing:
The
rayon filaments are stretched while the cellulose chains are still
relatively mobile. This causes the chains to stretch out and orient
along the fiber axis. As the chains become more parallel, interchain
hydrogen bonds form, giving the filaments the properties necessary for
use as textile fibers.
12. Washing:
The
freshly regenerated rayon contains many salts and other water soluble
impurities which need to be removed. Several different washing
techniques may be used.
13. Cutting:
If
the rayon is to be used as staple (i.e., discreet lengths of fiber),
the group of filaments (termed "tow") is passed through a rotary cutter
to provide a fiber which can be processed in much the same way as cotton
.
Major End Uses of Rayon Fiber :
1. Apparel: Accessories, blouses, dresses, jackets, lingerie, linings, millinery, slacks, sportshirts, sportswear, suits, ties, work clothes
2. Home Furnishings: Bedspreads, blankets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery
3. Industrial Uses: Industrial products, medical surgical products, nonwoven products, tire cord
29 July 2012
Properties of Rayon
Rayon Fiber
Rayon is a manufactured fiber composed of regenerated cellulose, as well as manufactured fibers composed of regenerated cellulose in which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups.
Rayon is a manufactured fiber composed of regenerated cellulose, as well as manufactured fibers composed of regenerated cellulose in which substituents have replaced not more than 15% of the hydrogens of the hydroxyl groups.
 |
| Rayon fiber |
Physical Properties of Viscose Rayon
Moisture Absorption
It absorbs more moisture than cotton. Moisture Content of Coton is 6% at 70 deg F and 65% RH, and for Viscose Rayon it is 13% under the same conditions.
Tensile Strength
The Tensile Strength of the fibre is less when the fibre is wet than when dry. 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.
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, whule high tenacity rayon has only 9-17% elongation at break.
Density
The density of Viscose rayon is 1.53 g/cc. Rayon filaments are available in three densities: 1.5, 3.0 and 4.5
Action of Heat and Light
At 300 deg F or more, VR loses its strength and begins to decompose at 350-400 deg F. Prolonged exposure to sunlight also weakens the fibre due to moisture and ultraviolet light of the sunlight.
Chemical Properties of Viscose Rayon
Viscose rayon consists of cellulose of lower DP than cotton cellulose. Also amorphous region of Viscose rayon is present to a greater extent, therefore, Viscose rayon reacts faster than cotton with chemicals. Acids like H2SO4 HCL breaks the cellulose to hydrocellulose. Oxidising agents like Na(OCl)2, Bleaching powder, K2Cr2O7, KMnO4- form oxycellulose. Cold acid solutions for a short time do not attack viscose rayon.
It absorbs more moisture than cotton. Moisture Content of Coton is 6% at 70 deg F and 65% RH, and for Viscose Rayon it is 13% under the same conditions.
Tensile Strength
The Tensile Strength of the fibre is less when the fibre is wet than when dry. 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.
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, whule high tenacity rayon has only 9-17% elongation at break.
Density
The density of Viscose rayon is 1.53 g/cc. Rayon filaments are available in three densities: 1.5, 3.0 and 4.5
Action of Heat and Light
At 300 deg F or more, VR loses its strength and begins to decompose at 350-400 deg F. Prolonged exposure to sunlight also weakens the fibre due to moisture and ultraviolet light of the sunlight.
Chemical Properties of Viscose Rayon
Viscose rayon consists of cellulose of lower DP than cotton cellulose. Also amorphous region of Viscose rayon is present to a greater extent, therefore, Viscose rayon reacts faster than cotton with chemicals. Acids like H2SO4 HCL breaks the cellulose to hydrocellulose. Oxidising agents like Na(OCl)2, Bleaching powder, K2Cr2O7, KMnO4- form oxycellulose. Cold acid solutions for a short time do not attack viscose rayon.
Action of Acids:
The resistance of regenerated cellulose rayon’s to acids is generally less than that of cotton to the same concentrations of the same acids. Therefore , acid treatments must not be too drastic with respect to concentration ,temperature and time .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. Oxalic acid for removal of iron stains is not recommended except at temperatures lower than 150°F.At high temperatures and concentrations all acid will destroy or carbonize regenerated rayon’s. No harmful action will result if applied at .5 to 3 percent solution at room temperature.
Action of Soaps:
Ordinary soaps in usual textile concentration have no direct effect on regenerated cellulose materials. Improper use of soap or use of poorly made soap results in rancidity and odor in rayon fabrics or yarns. When soaps alone is used ,there is a tendency for the ionized fatty acid from the soap to adhere tenaciously to the individual rayon filaments. During the drying filament of such materials and subsequent storage .The free fatty acid radical is very likely to turn rancid & to give the goods & objectionable odor. This phenomenon is specially Prevalent on oil-delustered rayons, because the fatty acid radical of the soap adheres Tenaciously to the minute oil globules in the structure of the yarn.If given time enough the fatty acid radical
Action of Dry Heat:
Most regenerated celluloses, under the influence of heat as well as light ,show rapid loss in strength, this change being accompanied by a increase in copper number and alkali solubility. In a study of effect of drying conditions of textile yearns, Wiegerink in 1940 showed that the quality index of cellulose fibers decreases either as the temperature is increased or as the moisture content of the surrounding atmosphere is increased .Both the breaking strength & fluidity of viscose rayon appear to be functions of the relative humidity to which the samples are exposed. Degradation of cellulose is lower in the absence of oxygen. Continued heating , however , in the absence of oxygen leads to deterioration of the cellulose but little is known about the course of the reactions. Short heating at high temperatures ,such as 140°c is less harmful than long heating at lower temperatures. A decrease of tenacity & eventually a yellow to brown discoloration occurs on aging.
Action of Solvents
Textile solvents can be used on Viscose rayon without any deteriorating effect. 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 ( moulds, mildew, fungus, bacteria) affect the colour, strength, dyeing properties and lustre of rayon. Clean and dry viscose rayon is rarely attacked by moulds and mildew.
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