Viscose Rayon

By: M. Arslan Sohail
(TS1-A)
To: Sir Imran Raza Malik
VISCOSE RAYON

Viscose Rayon
Introduction:
Viscose Rayon is the oldest commercial man-made fiber. Viscose
rayon is the naturally regenerated cellulosic fiber that can be made
from naturally occurring cellulosic based material. (such as cotton
linters, wood pulp etc).
It can be found in cotton-like end uses as well as silk-like end uses.
Due to its fine silk-like properties, it is also known as
“Artificial Silk”.

Viscose Rayon
Chemical Structure:
Chemical structure of viscose is same as that of cotton i.e.
Cellulose, But the polymer chains are much shorter. The
degree of polymerization value of rayon is 400 – 700 while
that of cotton is 5000.

Manufacturing
The process of manufacturing viscose rayon consists of the
following steps mentioned, in the order that they are carried out:
 Steeping,
 Shredding,
 Aging,
 Xanthation,
 Dissolving,
 Ripening,
 Filtering,
 Degassing,
 Spinning,
 Drawing.

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
2. 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.
3. Ageing:
The alkali cellulose is aged under controlled conditions of time and temperature
(between 18 and 30° C) in order to depolymerize the cellulose to the desired
degree of polymerization. In this step the average molecular weight of the

4. 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
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. Dissolving:
The yellow crumbs of cellulose xanthate is dissolved in aqueous caustic
solution. Because the cellulose xanthate solution (or more accurately,
suspension) has a very high viscosity, it has been termed “viscose”.

6. 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
7. Filtering:
The viscose is filtered to remove undissolved materials that might disrupt the
spinning process or cause defects in the rayon filament.

8. 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.
Spinning – (Wet Spinning):
Production of Viscose Rayon Filament: The viscose solution is metered
through a spinneret 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.

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 fibre
formation.
(C6H9O4O-SC-SNa)n + (n/2)H2SO4 –> (C6H10O5)n + nCS2 + (n/2)Na2SO4
In standard viscose of 30-50 poise viscosity made with 32% CS2 is spun into an
aqueous acid salt spin bath of the following type at a temperature of 40-50 0c .
H2SO4 = 8-10%
Na2SO4 = 16-24%
ZnSO4 = 1-2%

Modified Viscose Rayons
1. Tyre yarn:
A viscose solution of viscosity 100 poise containing modifiers 1-3% by weight of
cellulose and with a CS2 content of 40% is spun underripe (salt index-6-15) into a
aqueous spinning bath containing –
H2SO4 8-10%
Na2SO4 16-24%
ZnSO4 6%
The spin bath temperature is kept around 550c and the spinning speed is between
40 and 60 m/min. The stretch applied is 75-125%
2. High wet-modulus yarns (HWM):
In this process the ageing of alkali cellulose is eliminated and dissolving cellulose
xanthate in water rather than in caustic soda also eliminate the ripening of
cellulose xanthate. Lower the concentration of acid used in spinning bath with
little salt. These are known high wet modulus which means to have greater
resistance to deformation when wet.

3. Polynosic fibre
Polynosic is similar rayon fibre but difference in process of manufacturing than
viscose rayon. Since the manufacturing process is different so their
morphological structure also different. Generally polynosic fibre has high
crystallinity and high orientation. This give high mechanical strength and
chemical resistant, high wet modulus and more dimension stable.
NOTE- In polynosic process we eliminated the Ageing stage, Ripening stage
,Diluted acid concentration & zinc sulphate.
4. Super high wet modulus rayon:
By adding 1% formaldehyde to spin bath or to the viscose substantially
increases the toughness and plasticity of viscose gel. We can get the stretch
of 500-600% .Disadvantage of this compound is that it is very toxic.

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.
1. Acids like H2SO4 HCL breaks the cellulose to hydrocellulose.
2. Oxidising agents like Na(OCl)2, Bleaching powder, K2Cr2O7, KMnO4- form
oxycellulose.
3. Cold acid solutions for a short time do not attack viscose rayon.
Chemical Properties
4. Action of Solvents: Textile solvents can be used on Viscose rayon without any
deteriorating effect. Viscose rayon dissolves in cuprammonium hydroxide
solution.
5. 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.

6. 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.
7. 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.
Biological Properties:
1. Rayon resist all insects except silverfish.
2. Regular rayon fibers are subject to harm by rot-producing bacteria
3. High wet modulus fibers are fairly resistant to bacteria.

Physical Properties
Shape:
Colour:
Luster:
Tenacity:
• Dry
• Wet
Elongation:
Elastic recovery:
Density:
Moisture Content:
Dimensional Stability:
Resistance against:
• Alkali
• Acid
• Sunlight
Thermal:
• Heat
• Flame
Controllable
Controllable
Controllable
1.5 – 3.0
1.1 – 1.5
20%
Good
1.5 g/c cm
10 – 15 %
Good
Average
Poor
Average
Not above 150°C
Burn

Identification
Microscopic View:
Longitudinal: Uniform
diameter with striation
running parallel to the
fiber.
Cross-sectional: highly
servated (nearly round for
polynosic)

BURN TEST:
Burns slowly without flame with slight melting and smell of burnt paper, leaves
soft black ash.
Solubility test:
Soluble:
80% Sulphuric acid (m/m) at room temperature for 15 min.
Insoluble:
•Nitric acid (conc.) at room temperature for 15 min.
•Acetone at room temp.
•Chlorine bleach 5%.
•90% Phenol or m-cresol at room temp.
•Formic acid 90%.

USES
Blends:
Regular rayon and high wet modulus usually blend with many fibers like
polyester, acrylic, nylon, acetate, cotton, flax, wool and ramie.
With polyester, nylon and acrylic:
Rayon contributes absorbency, comfort, and softness when blended with
them.
With cotton:
It alters appearance to create soft luster.
With wool:
It will decrease the cost.
With flax and ramie:
It can help produce appearance that are typically associated with linen like
fabrics.

Specialty rayon:
1. Flame retardant fibers:
Flame retardance is achieved by the adhesion of the correct flame- retardant
chemical to viscose. Examples of additives are alkyl, aryl and halogenated alkyl
or aryl phosphates, phosphazenes, phosphonates and polyphosphonates. Flame
retardant rayon have the additives distributed uniformly through the interior of
the fiber and this property is advantageous over flame retardant cotton fibers
where the flame retardant concentrates at the surface of the fiber.
2. Super absorbent rayons:
This is being produced in order to obtain higher water retention capacity
(although regular rayon retains as much as 100 % of its weight). These
fibers are used in surgical nonwovens.

3. Micro denier fibers:
Rayon fibers with deniers below 1.0 are now being developed and introduced into
the market. These can be used to substantially improve fabric strength and
absorbent properties.
4. Cross section modification:
Modification in cross sectional shape of viscose rayon can be used to dramatically
change the fibers aesthetic and technical properties. For example:
Viloft: A flat cross sectional fiber sold in Europe, which gives a unique soft
handle, pleasing drape and handle.
Fiber ML(multi limbed): Another modified cross section fiber that has a very
well defined trilobal shape. Fabrics made of these fiber have considerably
enhanced absorbency, bulk, cover and wet rigidity all of which are suitable for
usage as nonwovens.

Union Fabrics:
Viscose is also used in constructing a union fabric with other yarns. One
great example is Viscose Rayon with Eri silk.
Viscose Rayon as warp with Eri silk of three different counts viz., 2/40s,
2/60s and 2/80s as weft were interwoven on a semi-automatic power loom
to produce union fabrics. Its is also used with cotton fibers to make union
fabric.
Special colour effect of Viscose Union fabrics (stain test):
When viscose is used in union fabric with other fiber it gives special
contrast when dyed with same dye due its different stain test. Lets take
example of different dye mixtures (shirlastains):
Shirlastain D
Fiber
Cotton
Viscose
Colour
Bright Blue
Bright green

Shirlastain A
Fiber
Cotton
Viscose
Diacetate
Triacetate
Bombyx Silk
Tussah silk
Regenerated protein
Wool
Nylon
Etc.
Colour
Pale purple
Bright pink
Greenish yellow
Off white
Dark brown
Chestnut brown
Yellow – Orange
Bright yellow
Cream to yellow

Viscose Rayon

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Viscose Rayon