LYOCELL
Lyocell is a form of rayon. It consists of cellulose fibre, made from dissolving
pulp and then reconstituting it by dry jet-wet spinning. The fibre is used to make
textiles for clothing and other purposes.[1] Unlike rayon made by the viscose process,
lyocell production does not use harmful carbon sulfide,[2][3] which is toxic to workers and
the environment.[4][5][2] As of 2018, the lyocell process is more expensive than the more
common viscose process for making rayon.[2]
"Lyocell" has become a genericized trademark, used to refer to the lyocell process for
making cellulose fibers.[3][6] The U.S. Federal Trade Commission defines lyocell as a
fibre "composed of cellulose precipitated from an organic solution in which no
substitution of the hydroxyl groups takes place and no chemical intermediates are
formed". It classifies the fibre as a sub-category of rayon.
Uses[edit]
Shirt made from lyocell
As of 2010, lyocell is more expensive to produce than cotton or viscose rayon. It is used
in many everyday fabrics. Staple fibres are used in clothes such as denim, chino,
underwear, casual wear, and towels. Filament fibres, which are generally longer and
smoother than staple fibres,[10] are used in items that have a silkier appearance such as
women's clothing and men's dress shirts. Lyocell may be blended with a variety of other
fibres such as silk, cotton, rayon, polyester, linen, nylon, and wool. Lyocell also is used
in conveyor belts, specialty papers, and medical dressings.
Properties[edit]
Lyocell shares many properties with other fibres such
as cotton, linen, silk, ramie, hemp and viscose rayon, (to which it is very closely
related chemically). Lyocell fiber can absorb around 20% of its own weight in moisture.
[citation needed]
The vapour and sweat absorption rate, also known as wicking ability, is better
than cotton or hemp, (8%), but not as good as wool (30%). Wicking is an important
property for blankets and outdoor active wear.[citation needed]
Compared to cotton, consumers often say Lyocell fibres feel softer, and "airier", due to its
better ability to wick moisture. Industry claims of higher resistance to wrinkling are as yet
unsupported. Lyocell fabric may be machine washed or drycleaned. It drapes well, and
may be dyed many colors, needing slightly less dye than cotton to achieve the same
depth of colour.
Manufacturing process[edit]
The lyocell process uses a direct solvent rather than indirect dissolution such as the
xanthation-regeneration route in the viscose process. Lyocell fiber is produced
from dissolving pulp, which contains cellulose in high purity with little hemicellulose and
no lignin. Hardwood logs (such as oak and birch[13]) are chipped into squares about the
size of postage stamps. The chips are digested chemically, either with the prehydrolysis-
kraft process or with sulfite process, to remove the lignin and hemicellulose. The pulp is
bleached to remove the remaining traces of lignin, dried into a continuous sheet and
rolled onto spools. The pulp has the consistency of thick posterboard paper and is
delivered in rolls weighing some 500 lb (230 kg).
N-Methylmorpholine N-oxide is a key solvent in the lyocell Process
At the lyocell mill, rolls of pulp are broken into one-inch squares and dissolved in N-
methylmorpholine N-oxide (NMMO[2]), giving a solution called "dope". The filtered
cellulose solution is then pumped through spinnerets, devices used with a variety of
synthetic fibres. The spinneret is pierced with small holes rather like a showerhead; when
the solution is forced through it, continuous strands of filament come out. The fibres are
drawn in air to align the cellulose molecules, giving the lyocell fibres its characteristic
high strength. The fibres are then immersed into a water bath, where desolvation of the
cellulose sets the fibre strands. The bath contains some dilute amine oxide in a steady
state concentration. Then the fibres are washed with de-mineralised water. Next, the
lyocell fibre passes to a drying area, where the water is evaporated from it.
Manufacture then follows the same route as with other kinds of fibers such as viscose.
The strands pass to a finishing area, where a lubricant, which may be a soap or silicone or
other agent, depending on the future use of the fibre, is applied. This step is basically a
detangler, prior to carding and spinning into yarn. At this stage, the dried, finished fibres
are in a form called tow, a large untwisted bundle of continuous lengths of filament. The
bundles of tow are taken to a crimper, a machine that compresses the fibre, giving it
texture and bulk. The crimped fibre is carded by mechanical carders, which perform an
action like combing, to separate and order the strands. The carded strands are cut and
baled for shipment to a fabric mill. The entire manufacturing process, from unrolling the
raw cellulose to baling the fibre, takes about two hours. After this, the lyocell may be
processed in many ways. It may be spun with another fibre, such as cotton or wool. The
resulting yarn can be woven or knitted like any other fabric, and may be given a variety of
finishes, from soft and suede-like to silky.[11]
The amine oxide used to dissolve the cellulose and set the fibre after spinning (NMMO) is
recycled. Typically,[citation needed] 99 percent of the amine oxide is recovered.[9] NMMO
biodegrades without producing harmful products.[2] Since there is little waste product,
this process is relatively eco-friendly, though it is energy-intensive.
