Abstract
Lyocell is a new generic name given to a cellulosic fiber
which is produced under an environmentally friendly process by dissolving
cellulose in the tertiary amine oxide N-methylmorpholine-N-oxide (NMMO).
Lyocell fiber shows some key advantageous characteristics over other cellulosic
fibers; for instance, a high dry and wet tenacity and high wet modulus. For the
majority of the last century, commercial routes to regenerated cellulose fibers
have scoped with the difficulties of making a good cellulose solution by using
an easy to dissolve derivative (e.g. xanthane in the case of viscose rayon) or
complex (e.g. cuprammonium rayon). For the purpose advanced cellulosic fibers
are defined as those made from a process involving direct dissolution of
cellulose. The first examples of such fibers have now been generically designed
as lyocell fibers to distinguish them from rayon’s, and the first commercial
lyocell fiber is Courtaulds’ Tencel.
This paper will consider
all developments of fiber from manufacturing to chemical processing; also
consider the key applications of the lyocell fiber, concentrating on its
application in textile, apparel and nonwoven fabrics.
Introduction
Lyocell is the first in a
new generation of cellulosic fibers made by a solvent spinning process. A major
driving force to its development was the demand for a process that was
environmentally responsible and utilized renewable resources as their raw materials.
The first samples were produced in 1984and commercial production started
in 1988. A wide range of attractive textile fabrics can be made from lyocell
that are comfortable to wear and have good physical performance. This physical
performance combined with its absorbency also makes lyocell ideal for nonwoven
fabrics and papers. The cellulose fibers produced by direct dissolution have
the generic name of Lyocell. Cellulose is one of the most
abundant natural resources on earth, and there has been extensive research on
the films, plastics, and fibers from this material. The history of cellulose
fibers dates back to the 1860s, when the first rayon fibers were commercialized
by Courtaulds. But the so-called rayon process includes toxic chemical
treatments to block hydroxyl groups of cellulose to prepare a spinnable
solution, mostly it causing an ecological problem. Many attempts have been made
to invent new solvents to directly dissolve cellulose, and some successful
results have been reported. Among these, N-methylmorpholine- N-oxide (NMMO)
hydrate turned out to be the best solvent, leading to the commercial success of
cellulose fibers under the trade name of Tencel by Courtaulds in 1994.
Figure:
1, Raw Materials for lyocell Fiber (Oak logs)
Other
lyocell process includes Lenzing Lyocell. These processes are advantageous
because they are environmentally benign, using nontoxic NMMO hydrates instead
of toxic carbon disulfide, which can be almost totally recycled. The lyocell
fiber has a highly crystalline structure in which crystalline domains are
continuously dispersed along the fiber axis. This offers good wet strength as
well as excellent dry strength, which makes lyocell water-washable. Further, it
shrinks less when wetted by water and dried than other cellulose fibers such as
cotton and viscose rayon. Recently, a new lyocell process, which has some characteristic,
features similar to the Tencel process. The new process dissolves finely
powdered cellulose in molten NMMO hydrate within 5 minutes by
means of a pasting stage, which causes much less decomposition of cellulose.
Further, this process can use NMMO hydrates with a hydration number (n) greater
than 1 because it adopts a plasticating extruder. The value of n plays a
significant role in the phase behavior of cellulose solutions. It also affects
the physical properties of the fibers spun from the solution; Fig.2. show the Lyocell processes consume lower amounts of
water, but a similar magnitude of energy.
Figure: 2 Ternary diagram showing the effect of
temperature on the dissolution Cellulose in NMMO
The Properties of Lyocell
Comparisons of lyocell
with viscose and other cellulosic in both laboratory and test markets proved
that the fibers were sufficiently different to deserve separate marketing strategies.
Table.1. shows various physical properties of lyocell with other fibers
Lyocell is:
- Stronger than any other cellulosic fibers,
especially when wet
- Easy to process into yarns and fabrics alone or in
blends
- Easy to blend (unique fiber presentation)
- Easy to spin to fine count yarns
- Very stable in washing and drying
- Thermally stable
- Easy to dye to deep vibrant colors
- Capable of taking the latest finishing techniques to
give unique drape
- Comfortable to wear
Table 1: The comparison properties of lyocell with different
cellulosic fibers
Property
|
Lyocell
|
Viscose
|
Cotton
|
Polyester
(PET)
|
Dry Tenacity
(cN/Tex)
|
38-42
|
22-26
|
20-24
|
55-60
|
Wet Tenacity
(cN/Tex)
|
34-38
|
10-15
|
26-30
|
54-58
|
Dry
Elongation (%)
|
14-16
|
20-25
|
7-9
|
25-30
|
Wet
Elongation (%)
|
16-18
|
25-30
|
12-14
|
25-30
|
The Courtaulds’ Lyocell Process
The Courtaulds’
semi-commercial production system is illustrated in Fig.3. Dissolving
grade wood pulp is mixed into a paste with NMMO and passes through a
high-temperature dissolving unit to yield a clear viscous solution. This is
filtered and spun into dilute NMMO, whereupon the cellulose fibers precipitate.
These are washed and dried, and finally baled as staple or tow products as
required by the market. The spin-bath and wash liquors are passed to solvent
recovery systems which concentrate the NMMO to the level required for re-use
in dissolution.
Figure:
3 Courtaulds’ Lyocell Process
Lyocell Conversion
Lyocell
is similar in strength to polyester and stronger than cotton and all other
man-made staple fiber cellulosic’s. It also has very high dry and wet modulus
for cellulosic fiber in both the dry and wet states, the properties of lyocell
fiber is shown in bellow the table. These properties allow customers great
scope for making strong yarns in blend with virtually all the other
commercially available staple fibers. They also lead to excellent efficiencies
in converting these yarns to woven add knitted fabrics. All man-made cellulosic’s
lost strength and modulus when wetted, but lyocell reduces by much less than
others. This is important in determining how properties of the fabric are
developed during dyeing and finishing.
Yarn Manufacturing
Once lyocell fiber has
been produced, either as cut staple fiber or continuous tow, it will be
converted to yarns and fabrics by a range of conventional textile processes.
The most common way of using lyocell fiber is as cut staple, with 1.4
and 1.7 dtex fibers cut to 38 mm and converted into a spun yarn using
machinery developed over many years for handling cotton fibers that are similar
in dtex and length to lyocell. Lenzing lyocell is made by a wet-cut route and
has different processing characteristics. The fiber can be processed on
conventional machinery, usually requiring a few setting changes in order to optimize
processing performance.
Thus lyocell will open
very easily with little nep formation. In sliver and roving, the fibers pack
together, giving high cohesion and therefore requiring high drafting forces.
Lyocell yields very regular yarns with high tensile strength and few
imperfections. Lyocell blends well with other fibers, including cotton,
viscose, linen, wool, silk, nylon and polyester. Lyocell adds strength to
the yarn as well as enhancing the performance and aesthetic properties of the
final fabrics. Minimal carding power is required, as the fiber is very
open. In drawing, sliver detectors may need to be re-set to adjust for the low
bulk of the lyocell. In roving, the twist should be low to avoid too high a
cohesion. Optimization is very important at this stage of the process.
Yarn steaming should be avoided wherever
possible. Steaming cellulosic fibers, amongst other things affects fiber dye
affinity, twist liveliness and splice strength. The dye affinity for cellulosic
fibers reduces with increasing steam temperature and the influence on lyocell fiber
is greater than for other cellulosic, such as cotton and viscose. Therefore
steaming should be avoided unless this can be extremely well controlled. Twist liveliness can be reduced in other ways,
such as by storing yarn on ring tube for 16–24 hours in a high humidity
environment prior to winding.
Fabric manufacture
Weaving of lyocell fabrics
can be successfully carried out on most conventional looms and in a wide range
of constructions. The construction needs to be carefully engineered with the
dyeing/finishing route to develop the best performance and aesthetics. Very
tight constructions can give problems in dyeing and tend to give fabrics with
poorer easy-care performance.
Dyeing and finishing of lyocell
The dyeing and finishing
of lyocell fabrics is the key to their success. There are three characteristics
of the fibers that can be manipulated to give fabrics with attractive and differentiated
aesthetics- the ease of fibrillation, the high nodules and the wet swelling characteristics.
Fibrillation can yield the characteristic ‘peach skin’ effect surface touch of
fabrics made from this fiber, but un wanted and uncontrolled fibrillation can
also impair the fabric quality, much of the dyeing and finishing development
has been focus on this aspect.
A lyocell is a cellulosic fiber; it can be dyed with
colors normally used on cotton. Compared with unmercerized cotton, lyocell,
except with a few reactive and vat and a number of direct dyes (pale shades),
dyes to a heavier depth by exhaust techniques and therefore many shades can be
attained at lower cost, particularly with reactive dyes. The dyeing mechanism
for most classes of reactive dyes is similar.
First the reactive dye is exhausted on to the cellulose fiber using
salt. In the second stage of dyeing, alkali is added to fix the dye; dyeing
behavior of lyocell is shown in bar chart 1
Many of modern reactive
dyestuffs contain two or three reactive groups. A key discovery, made early in
the development of lyocell, was that these multifunctional dyestuffs can
crosslink the fiber and there by prevent or inhibit the fibrillation of the
fiber. Since manipulation of this fibrillation is critical for the development
of the fabric aesthetics.
Bar chart: 1 Dyeing Behavior of Lyocell with Other
Cellulosic Fibers
.
Easy-care lyocell
As with any fabric,
chemical finishing is an important aspect of the process and this is especially
true when considering the finishing of open-width processed lyocell fabrics. In
such processing, resin treating is the method of controlling fibrillation. If
too little resin is fixed then fabrics will fibrillate on subsequent washing,
too much and physical performance deteriorates. It is also important to
include appropriate softeners and auxiliary products into the chemical finish
so that performance and handle are appropriate to the customer’s requirements. The
application of ~2–3% omf (on mass of fiber) fixed resin
appears to be optimal for easy-care properties, dependant on the fabric
construction and weight. Application levels of 2% omf are needed
to stop fibrillation on domestic washing. In addition to the resin, the choice
of softener can have a large effect on the easy-care performance of fabrics,
and it is important to consider the whole formulation and build it up to give
the required performance. Silicone micro-emulsions penetrate yarns more than
the macro-emulsions. Polyethylene dispersions aid sewing and build the handle
of the fabric, whilst some soft acrylic-based chemicals can increase the
abrasion resistance. It is also worth remembering that caustic soda or liquid
ammonia treatment in preparation will help to increase the easy-care rating of
lyocell fabrics.
Fibrillation:
This
can be defined as the longitudinal splitting of a single fiber filament into
micro fibers. The splitting occurs as a result of wet abrasion, particularly
against metal. The fibrils formed can be so fine that they become virtually
transparent and give a frosty appearance to the finished fabric. The samples
fig 4 shows an example of a non-fibrillated (a) and a fibrillated (b) lyocell
fabric. The fibrillated fabric gives frosty appearances. In case of extreme
fibrillation, the loose fibers on the surface of the fabric fibrillate and then
tangle together to form very light colored pills. The appearance of the fabric
becomes totally unacceptable.
(a) (b)
Fig 4 shows an example of a non-fibrillated (a) and a fibrillated
(b) lyocell fabric.
Applications
Lyocell feels like silk, and drapes luxuriously.
Compared to cotton, Lyocell wrinkles
less, is softer, more absorbent, and much more resistant to ripping. In
material physical properties, Lyocell
is
more like cotton than rayon. Like other cellulosic fibers, it is breathable,
absorbent, and very comfortable to wear. In fact, Lyocell is more absorbent than cotton or
silk, but slightly less absorbent than wool, linen, or rayon.
Lyocell has good resiliency: it does not wrinkle as badly
as rayon, cotton, or linen, and some wrinkles will fall out if the garment is
hung in a warm moist area, such as a bathroom after a hot shower. A light
pressing will renew the appearance, if needed. Also, slight shrinkage is typical
in Lyocell garments.
Lyocell is a stable a fiber better than cotton or linen. Lyocell is more
expensive to produce than cotton or rayon, but is included in many everyday
items. Staple fiber is used in apparel items such as denim, chino, underwear,
other casual wear clothing & towels. Filament fibers are used in items that
have a silkier appearance such as women’s clothing and men’s dress shirts.
Lyocell can be blended with a variety of other fibers such as silk, cotton,
rayon, polyester, linen, nylon, and wool.
Lyocell – a versatile, high performance fiber for
nonwovens
The early stages of the commercialization
of lyocell were focused towards the fashion textile apparel sector. However,
this has changed during the first years of the twenty-first century so that
lyocell is now targeted equally into the industrial sector, with particular
emphasis on the key nonwovens markets of wipes, filters and feminine hygiene
products. The key difference between traditional textile production and
nonwovens production is the omission of the yarn stage from the production
process. In nonwovens manufacture, the fibers are formed into a web and a fiber
bonding or entangling process is used to impart integrity and control the
function, hand and appearance of the resulting nonwovens’ substrate. Staple fiber
grades are produced to suit carded dry laid, air laid and wet laid processes.
Lyocell is also used in
· Conveyer Belt
· Specialty Paper
· Medical Dressing
· Surgical swabs,
drapes, gowns
· Floppy disc
liners, filtration cloth
· Lining materials
Conclusion
Cellulose is one of the
most abundant natural resources on earth, and there has been extensive research
on the films, plastics, and fibers from this material. This century modified
cellulose were investigated with eco friendly route, lyocell is one among
these. Lyocell is not only environmental friendly fiber; it offers more
desirable properties like highly
crystalline structure in which crystalline domains are continuously dispersed
along the fiber axis, good wet strength as well as excellent dry strength,
which makes lyocell water-washable. Further, it dye uptake more, shrinks less
when wetted by water and dried than other cellulose fibers such as cotton and
viscose rayon.
Reference:
1.
Bates
I, Mauchru E, Phillips DAS, Renfrew AHM, Su Y, Xu J (2004) Cross-linking agents
for the protection of lyocell against fibrillation: synthesis, application and
technical assessment of 2,4-diacrylamidobenzenesulphonic acid. Color Technology
120:293–300. doi:10.1111/j.1478-4408.2004.tb00233.x
2.
Chae
DW, Chae HG, Kim BC, Oh YS, Jo SM, Lee WS (2002) Physical properties of lyocell
fibers spun from isotropic cellulose dope in NMMO monohydrate. Text Res J
72:335–340. doi:10.1177/004051750207200410
3.
Chavan
RB, Patra AK
4.
Colom
X, Carrillo F (2002) Crystallinity changes in lyocell and viscose-type fibers
by caustic treatment. Europe Polymer Journal
38:2225–2230. doi:10.1016/S0014-3057(02)00132-5
5.
Goswami
P, Blackburn RS, Taylor J, Westland S, White P (2007) Dyeing behavior of lyocell
fabric effect fibrillation. Color Technology 123:387–393
6.
Goswami
P, Blackburn RS, El-Dessouky HM, Taylor J, White P (2009) Effects of sodium
hydroxide pre-treatment on the optical and structural properties of lyocell. Europe Polymère Journal 45:455–465.
doi:10.1016/j.eurpolymj.2008.10.030
7.
Jakob B. and E.
Agster , “Pretreatment and Finishing of Lyocell Woven Fabrics”,
International Textile Bulletin, No. 3, page 18-26 (1998).
8.
J. M. Taylor, M. J. Bradbury and S.
Moorhouse, “Dyeing Tencel and Tencel A100 with Poly-Functional Reactive Dyes”,
AATCC Review, No. 10 page 21-24 (2001).
9.
J. M. Taylor and A. L. Harnden, “An
Introduction to Tencel Processing”, International Dyer, August 1997, page 14.
10. K.
Gandhi et. al., “A Novel Route
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