Courtesy: Updated: April, 2004 - Raghavendra R. Hegde, Atul
Dahiya, M. G. Kamath
(Praveen Kumar Jangala,
Haoming Rong)INTRODUCTION
Rayon is
the oldest commercial manmade fiber. The U. S. Trade Commission defines rayon
as "manmade textile fibers and filaments composed of regenerated
cellulose". The process of making viscose was discovered by C.F.Cross and
E.J.Bevan in 1891. The process used to make viscose can either be a continuous
or batch process. The batch process is flexible in producing a wide variety of
rayons, with broad versatility. Rayon's versatility is the result of the fiber
being chemically and structurally engineered by making use of the properties of
cellulose from which it is made. However, it is somewhat difficult to control
uniformity between batches and it also requires high labor involvement. The
continuous process is the main method for producing rayon. Three methods of
production lead to distinctly different rayon fibers: viscose rayon,
cuprammonium rayon and saponified cellulose acetate. Of the methods mentioned,
the viscose method is relatively inexpensive and of particular significance in
the production of nonwoven fabrics. According to the latest data from the fiber
Economics Bureau, domestic producers shipments of rayon staple to nonwoven roll
goods are shown in table 1.
Table
1: Shipments of Rayon Staple to Nonwoven roll goods [11]
|
Years
|
Millions of pounds
|
|
1989
|
98
|
|
1990
|
72
|
|
1991
|
70
|
|
1992
|
70
|
|
1993
|
70
|
|
1994
|
64
|
|
1995
|
60
|
|
1996
|
57
|
|
1997
|
58
|
|
1998
|
60
|
2.
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 shown in Fig. 1, and
clarified below.
Figure 1: Process of manufacture of viscose rayon fiber
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)
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 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.
(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.
(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].
3. 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].
Fig. 2: Cupro flow chart
4.
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]
Fig.
3: Acetate flow chart
5.
STRUCTURE OF RAYON
The unit cell of cellulose is shown in
Fig. 4.
Fig 4. Structure of unit cell of cellulose
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: how cellulose molecules are arranged and held together; the
average size and size distribution of the molecules.
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.
Fig.
5: Cellulose structure
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.
6.
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
tenacies 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.
7. 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].
8. 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].
9.
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].
10.
MARKET POTENTIAL:
The market
share of rayon in the nonwovens area dropped 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.
REFERENCES
1. Handbook of Fiber Science and Technology:
Fiber Chemistry, Vol. IV
2. Vaughn, Ed. A. The Technical Needs:
Nonwovens for Medical/Surgical and Consumer Uses, Chapter 5,pp.61-66, TAPPI
Press.
3. Winter School
Notes on Man-made Fibers, IIT Delhi,
Vol.II.
4. Lunenschloss, J. and Albrecht , W.;
Nonwoven Bonded Fabrics, 1985.
5. Needles, Howard. L;Textiles Fibers ,Dyes
and Finishes.
6. Drelich , Arthur; Nonwoven Fabrics Survey
,Encyclopedia of Polymer Science and Engineering, Vol. 10,pp 204-226, John
Wiley & Sons, Inc.
7. Nonwovens Factbook 1991 pp 76-77.
8. Turbak, Albin F; Nonwovens: Theory,
Process, Performance, and Testing,.
9. Spunlace Technology Today-An Overview of
Raw Materials, Processes, Products, Markets and Emerging End Uses. pp 61-62.
10. Hardy, Craig; The Rayon Fiber Process and
Fiber Characteristics; Principles of Nonwovens; INDA.
11. David Harrason, Shipments of Fibers to
Nonwovens Reported for 1998, Nonwovens Industry, No.6, 1999, pp-52
12. Lyocell fibers: http://www.nonwovens.com/facts/technology/fibers/rayon.htm
14. J. Gordon Cook, Handbook of Textile
fibers, II Manmade Fibers, pp-82
15. Gordon Cook, Handbook of Textile fibers,
II Manmade Fibers, pp-100
