First U.S. Commercial Rayon Fiber Production:
1910, Avtex Fibers Inc.
(Formerly FMC Corporation and American Viscose)
Current U.S Rayon Fiber Producers:
None currently
Federal Trade Commission Definition for Rayon Fiber:
A manufactured fiber composed of regenerated
cellulose,
in which substituents have replaced not more than 15% of the
hydrogens of the hydroxyl groups.
(Complete FTC Fiber Rules
here.)
Basic Principles of Rayon Fiber Production
— In the production of rayon, purified
cellulose
is chemically converted into a soluble compound. A solution of
this compound is passed through the
spinneret
to form soft filaments that are then converted or “regenerated”
into almost pure cellulose. Because of the reconversion of the
soluble compound to cellulose, rayon is referred to as a regenerated
cellulose fiber.
There are several types of rayon fibers in
commercial use today, named according to the process by which
the cellulose is converted to the soluble form and then regenerated.
Rayon fibers are
wet spun,
which means that the filaments emerging from the spinneret pass
directly into chemical baths for solidifying or regeneration.
Viscose rayon is made by converting purified
cellulose to xanthate, dissolving the xanthate in dilute caustic
soda and then regenerating the cellulose from the product as
it emerges from the spinneret. Most rayon is made by the viscose
process.
Viscose Process
Most commercial rayon manufacturing
today utilizes the viscose process. This process dates to the
early 1900s, with most of the growth in production occurring
between 1925 and 1955. In the early period, production was mainly
textile filament, although the first staple was produced in 1916.
High performance rayons, such as tire cord, did not appear until
the late 1930s, with the advent of hot-stretching and addition
of larger amounts of zinc to the spin bath. Invention of modifiers
in 1947 brought on super tire cords and marked the beginning
of the high-performance rayon fibers.
All of the early viscose production involved
batch processing. In more recent times, processes have been modified
to allow some semi-continuous production. For easier understanding,
the viscose process is a batch operation. Click on each process
step for a brief explanation.
Cellulose
Purified
cellulose
for rayon production usually comes from specially processed wood
pulp. It is sometimes referred to as “dissolving cellulose”
or “dissolving pulp” to distinguish it from lower grade
pulps used for papermaking and other purposes. Dissolving cellulose
is characterized by a high a -cellulose content, i.e., it is composed of
long-chain molecules, relatively free from lignin and hemicelluloses,
or other short-chain carbohydrates.
Steeping
The cellulose sheets are saturated
with a solution of caustic soda (or sodium hydroxide) and allowed
to steep for enough time for the caustic solution to penetrate
the cellulose and convert some of it into “soda cellulose”,
the sodium salt of cellulose. This is necessary to facilitate
controlled oxidation of the cellulose chains and the ensuing
reaction to form cellulose xanthate.
Pressing
The soda cellulose is squeezed mechanically
to remove excess caustic soda solution.
Shredding
The soda cellulose is mechanically
shredded to increase surface area and make the cellulose easier
to process. This shredded cellulose is often referred to as “white
crumb”.
Aging
The white crumb is allowed to stand
in contact with the oxygen of the ambient air. Because of the
high alkalinity of white crumb, the cellulose is partially oxidized
and degraded to lower molecular weights. This degradation must
be carefully controlled to produce chain lengths short enough
to give manageable viscosities in the spinning solution, but
still long enough to impart good physical properties to the fiber
product.
Xanthation
The properly aged white crumb is placed into a churn, or other mixing
vessel, and treated with gaseous carbon disulfide. The soda cellulose
reacts with the CS2
to form xanthate ester groups. The carbon disulfide also reacts
with the alkaline medium to form inorganic impurities which give
the cellulose mixture a characteristic yellow color – and
this material is referred to as “yellow crumb”. Because
accessibility to the CS2
is greatly restricted in the crystalline regions
of the soda cellulose, the yellow crumb is essentially a block
copolymer of cellulose and cellulose xanthate.
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.
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)
The viscose is forced through a
spinneret,
a device resembling a shower head with many small holes. Each
hole produces a fine filament of viscose. As the viscose exits
the spinneret, it comes in contact with a solution of sulfuric
acid, sodium sulfate and, usually, Zn++
ions. Several processes occur at this point
which cause the cellulose to be regenerated and precipitate from
solution. Water diffuses out from the extruded viscose to increase
the concentration in the filament beyond the limit of solubility.
The xanthate groups form complexes with the Zn++
which draw the cellulose
chains together. The acidic spin bath converts the xanthate functions
into unstable xantheic acid groups, which spontaneously lose
CS2 and regenerate the free hydroxyls of cellulose.
(This is similar to the well-known reaction of carbonate salts with acid to form
unstable carbonic acid, which loses CO2).
The result is the formation of fine filaments of cellulose, or 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.
Other forms of regenerated cellulose fibers
that are classified by the Commission as rayon without separate,
distinctive names include high wet modulus rayon, cuprammonium
rayon and saponified rayon.
High wet modulus rayon is highly modified
viscose rayon that has greater dimensional stability in washing.
Cuprammonium rayon is made by converting the
cellulose into a soluble compound by combining it with copper
and ammonia. The solution of this material in caustic soda is
passed through the spinneret and the cellulose is regenerated
in the hardening baths that remove the copper and ammonia and
neutralize the caustic soda. Cuprammonium rayon is usually made
in fine filaments that are used in lightweight summer dresses
and blouses, sometimes in Combination with cotton to make textured
fabrics with clubbed, uneven surfaces.
When extruded filaments of cellulose acetate
are reconverted to cellulose, they are described as saponified
rayon, which dyes like rayon instead of acetate.
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-thirds 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.
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
General Rayon Fiber Care Tips
— Most rayon fabrics should be dry-cleaned,
but some types of fabric and garment construction are such that
they can be hand or machine washed. For washable items, use the
following as a guide:
- Fabrics containing rayon can be bleached;
some finishes, however, are sensitive to chlorine bleach.
- Use mild lukewarm or cool suds. Gently squeeze
suds through fabric and rinse in lukewarm water. Do not wring
or twist the article.
- Smooth or shake out article and place on
a non-rust hanger to dry. Rayon sweaters should be dried flat.
- Press the article while damp on the wrong
side with the iron at a moderate setting. If finishing on the
right side is required, a press cloth should be used.
- Between wearings, rayon articles may be pressed with a cool iron.
(For specific instructions, refer to garment's sewn-in care label.)
