Plastic Product
Manufacturing Technology
VISCOSE RAYON
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
illustrated and clarified below.
figure 1 (delete this line when
figure is transferred from old paper)
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
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.
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.
Aging: 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 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.
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.
(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"[13].
(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
(8) Filtering: The viscose
is filtered to remove undissolved materials that might disrupt the spinning
process or cause defects in the rayon filament[13].
(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[13].
(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.
(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[13].
(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].
(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[13].
CUPRAMMONIUM RAYON
It is produced by a solution of
cellulosic material in cuprammonium hydroxide solution at low temperature in a
nitrogen atmosphere, followed by extruding through a spinnerette into a
sulphuric acid solution necessary to decompose cuprammonium complex to
cellulose. This is a more expensive process than that of viscose rayon. Its
fiber cross- section is almost round[14].
Cupro Flow Chart
SAPONIFIED CELLULOSE ACETATE
Rayon can be produced from
cellulose acetate yarns by saponification. Purified cotton is steeped in glacial
acetic acid to make it more reactive. It is then acetylated with excess of
glacial acetic acid and acetic anhydride, with sulphuric acid to promote the
reaction. The cellulose triacetate formed by acetylation is hydrolysed to
convert triacetate to diacetate. The resultant mixture is poured into water
which precipitates the cellulose acetate. For spinning it is dissolved in
acetone, filtered, deaerated and extruded into hot air which evaporates the
solvent. A high degree of orientation can be given to the fiber by drawing
because of the fact that cellulose acetate is more plastic in nature. Its fiber
cross section is nearly round, but lobed[15]
Acetate Flow Chart
STRUCTURE OF RAYON
In regenerated celluloses, the
unit cell structure is an allotropic modification of cellulose I, designated as
cellulose II (other allotropic modifications are also known as cellulose III and
cellulose IV). The structure of cellulose derivatives could be represented by a
continuous range of states of local molecular order rather than definite
polymorphic forms of cellulose which depend on the conditions by which the fiber
is made. Rayon fiber properties will depend on:
Many models describe ways in
which the cellulose molecules may be arranged to form fiber fine structure. The
most popular models of fiber fine structure are the fringed micelle and fringed
fibrillar structures. Essentially, they all entail the formation of crystallites
or ordered regions.
The skin-core effect is very
prominent in rayon fibers. Mass transfer in wet spinning is a slow process
(which accounts for the skin-core effect) compared to the heat transfer in melt
spinning. The skin contains numerous small crystallites and the core has fewer
but larger crystallites. The skin is stronger and less extensible, compared to
the core. It also swells less than the core; hence, water retention is lower in
the skin than in the core although moisture regain is higher in the skin. This
is explained by an increased number of hydroxyl groups available for bonding
with water as a result of a larger total surface area of the numerous small
crystallites.
When rayon fibers are worked in
the wet state,the filament structure can be made to disintegrate into a
fibrillar texture. The extent to which this occurs reflects the order that
exists in the fiber structure, as a consequence of the way in which the
cellulose molecules are brought together in spinning. Another important
structural feature of rayon fiber is its cross-sectional shape. Various shapes
include round, irregular, Y-shaped, E-shaped, U-shaped, T-shaped and flat.
PROPERTIES OF RAYON
Variations during spinning of
viscose or during drawing of filaments provide a wide variety of fibers with a
wide variety of properties. These include:
-
Fibers with thickness of 1.7 to
5.0dtex, particularly those between 1.7 and 3.3 dtex, dominate large scale
production.
-
Tenacity ranges between 2.0 to
2.6 g/den when dry and 1.0 to 1.5 g/den when wet.
-
Wet strength of the fiber is of
importance during its manufacturing and also in subsequent usage.
Modifications in the production process have led to the problem of low wet
strength being overcome.
-
Dry and wet tenacities extend
over a range depending on the degree of polymerization and crystallinity. The
higher the crystallinity and orientation of rayon, the lower is the drop in
tenacity upon wetting.
-
Percentage elongation-at-break
seems to vary from 10 to 30 % dry and 15 to 40 % wet. Elongation-at-break is
seen to decrease with an increase in the degree of crystallinity and
orientation of rayon.
-
Thermal properties: Viscose
rayon loses strength above 149� C; chars and decomposes at 177 to 204� C. It
does not melt or stick at elevated temperatures.
-
Chemical properties: Hot dilute
acids attack rayon, whereas bases do not seem to significantly attack rayon.
Rayon is attacked by bleaches at very high concentrations and by mildew under
severe hot and moist conditions. Prolonged exposure to sunlight causes loss of
strength because of degradation of cellulose chains.
-
Abrasion resistance is fair and
rayon resists pill formation. Rayon has both poor crease recovery and crease
retention.
Rayon Fiber Characteristics
-
Highly absorbent
-
Soft and comfortable
-
Easy to dye
-
Drapes well
The drawing process applied in
spinning may be adjusted to produce rayon fibers of extra strength and reduced
elongation. Such fibers are designated as high tenacity rayons, which have about
twice the strength and two-third of the stretch of regular rayon. An
intermediate grade, known as medium tenacity rayon, is also made. Its strength
and stretch characteristics fall midway between those of high tenacity and
regular rayon[13].
Some Major Rayon Fiber Uses
-
Apparel: Accessories, blouses,
dresses, jackets, lingerie, linings, millinery, slacks, sportshirts,
sportswear, suits, ties, work clothes
-
Home Furnishings: Bedspreads,
blankets, curtains, draperies, sheets, slipcovers, tablecloths, upholstery
-
Industrial Uses: Industrial
products, medical surgical products, nonwoven products, tire cord
-
Other Uses: Feminine hygiene
products[13].
DIFFERENT TYPES OF RAYONS
Rayon fibers are engineered to
possess a range of properties to meet the demands for a wide variety of end
uses. Some of the important types of fibers are briefly described.
High wet modulus rayon: These
fibers have exceptionally high wet modulus of about 1 g/den and are used as
parachute cords and other industrial uses. Fortisan fibers made by Celanese (saponified
acetate) has also been used for the same purpose.
Polynosic rayon: These
fibers have a very high degree of orientation, achieved as a result of very high
stretching (up to 300 %) during processing. They have a unique fibrillar
structure, high dry and wet strength, low elongation (8 to 11 %), relatively low
water retention and very high wet modulus.
Specialty rayons:
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
rayons 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.
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. These fibers are obtained by including water- holding
polymers (such as sodium polyacrylate or sodium carboxy methyl cellulose) in the
viscose prior to spinning, to get a water retention capacity in the range of 150
to 200 % of its weight.
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.
Cross-section modification:
Modification in cross-sectional shape of viscose rayon can be used to
dramatically change the fibers' aesthetic and technical properties. One such
product is Viloft, a flat cross sectional fiber sold in Europe, which gives a
unique soft handle, pleasing drape and handle. Another modified cross section
fiber called Fibre ML(multi limbed) has a very well defined trilobal shape.
Fabrics made of these fibers have considerably enhanced absorbency, bulk, cover
and wet rigidity all of which are suitable for usage as nonwovens [10].
Tencel� rayon: Unlike viscose
rayon, Tencel is produced by a straight solvation process. Wood pulp is
dissolved in an amine oxide, which does not lead to undue degradation of the
cellulose chains. The clear viscous solution is filtered and extruded into an
aqueous bath, which precipitates the cellulose as fibers. This process does not
involve any direct chemical reaction and the diluted amine oxide is purified and
reused. This makes for a completely contained process fully compatible with all
environmental regulations.
Lyocell: A new form of cellulosic
fiber, Lyocell is starting to find uses in the nonwovens industry. Lyocell is
manufactured using a solvent spinning process, and is produced by only two
companies -- Acordis and Lenzing AG. To produce Lyocell, wood cellulose is
dissolved directly in n-methyl morpholine n-oxide at high temperature and
pressure. The cellulose precipitates in fiber form as the solvent is diluted,
and can then be purified and dried. The solvent is recovered and reused. Lyocell
has all the advantages of rayon, and in many respects is superior. It has high
strength in both dry and wet states, high absorbency, and can fibrillate under
certain conditions. In addition, the closed-loop manufacturing process is far
more environmentally friendly than that used to manufacture rayon, although it
is also more costly[12].
MARKET POTENTIAL:
The market share of rayon in the
nonwovens area has decreased since 1987, but has gradually picked up since 1990.
Rayon was a predominant fiber used in the nonwovens industry until 1985. After
1985[3] the production of rayon decreased considerably in the US and Western
Europe because of the increasing cost of the fiber.
Wipes represent the largest
nonwovens market for rayon. Fabric softeners represent the second largest,
despite rayon's loss of market share to PET. Rayon is the fiber of choice in
many medical applications such as surgical packs, drapes and gowns where hand,
absorbency and sterilizability are important[7]. Cellulose acetate is a soft,
supple fiber of low modulus and low sticking point of 180oF and thus, can be
used as a binder fiber in the manufacture of nonwovens[8].
The development and expansion of
hydroentanglement coupled with growing importance of disposability is now
beginning to turn rayon properties into powerful advantages. The
biodegradability and compatibility with both septic tank and main sewage systems
enables them to be used in the manufacture of disposables. Recent trials have
shown that in the sludge digestion plant where sludge is held for about 3 weeks
for cleanup and stabilization prior to disposal, the rayons biodegrade totally
within a week.[9]
Rayon with its unique
characteristics has the potential to become the leading fiber used in the
nowovens industry, if the inherent pollution in the manufacturing process can be
corrected.
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