Raw Materials
- Regardless of the design or manufacturing process, the basic
raw material for making rayon is cellulose. The major sources for
natural cellulose are wood pulp-usually from pine, spruce, or hemlock
trees-and
- To make rayon, sheets of purified cellulose
are steeped in caustic soda, dried, shredded into crumbs, and then aged
in metal containers for 2 to 3 days. The temperature and humidity in the
metal containers are carefully controlled. After ageing, the crumbs are
combined and churned with liquid carbon disulfide, which turns the mix
into orange-colored crumbs known as sodium cellulose xanthate. The
cellulose xanthate is bathed in caustic soda, resulting in a viscose
solution that looks and feels much like honey.cotton linters. Cotton
linters are residue fibers which cling to cotton seed after the ginning
process.
- Strictly defined, rayon is a
manufactured fiber composed of regenerated cellulose. The legal
definition also includes manufactured fibers in which substitutes have
not replaced more than 15 percent of the hydrogens.
While the basic manufacturing process for all rayon is similar, this
fabric can be engineered to perform a wide range of functions. Various
factors in the manufacturing process can be altered to produce an array
of designs. Differences in the raw material, the processing chemicals,
fiber diameter, post treatments and blend ratios can be manipulated to
produce a fiber that is customized for a specific application.
Regular or viscose rayon is the most prevalent, versatile and
successful type of rayon. It can be blended with man-made or natural
fibers and made into fabrics of varying weight and texture. It is also
highly absorbent, economical and comfortable to wear.
The Manufacturing Process
- 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.
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.
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".
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
-
Filtering
- The viscose is filtered to remove undissolved materials that
might disrupt the spinning process or cause defects in the rayon
filament.
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 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.
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
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
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.