Lyocell Fibres
The regenerated cellulose fibre is produced by derivatizing cellulose in CS2
or in cupraammonium and by subsequent spinning and coagulation. In both
these processes environmentally hazardous by-products are formed (CS2, H2S
etc.). However, if the cellulose can be dissolved without
derivatization, the problems associated with derivatization and
regeneration can be avoided, resulting in reduced environmental
pollution and chemical waste generation. Alternate processes using
direct dissolution of cellulose have been studied. Some of the suitable
solvent systems are:
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
The most promising of these solvent systems, which has been commercially exploited, is the amine oxide /water system (NMMO/H2O).
|
||||||||||||||||||||||||||||||||||||||||||
Cellulosic
fibres produced using this ‘organic solvent spinning process’ are known
as lyocell fibres. These are also known as Tencel.
|
||||||||||||||||||||||||||||||||||||||||||
Lyocell Process
|
||||||||||||||||||||||||||||||||||||||||||
The use of N -methylmorpholine –N-oxide
(NMMO) as a solvent in lyocell process is advantageous as
derivatization or xanthation is not required. Also, the process requires
very few chemicals (NMMO and water), which are completely recyclable.
The main advantage of NMMO solvent is that it is biodegradable and is
non-toxic. Hence the process is environment friendly.
|
||||||||||||||||||||||||||||||||||||||||||
Mechanism of dissolution in N-methyl morpholine oxide( NMMO)
|
||||||||||||||||||||||||||||||||||||||||||
N-methyl morpholine oxide (NMMO) is a cyclic, aliphatic, tertiary amine oxide.
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
NMMO is a highly polar organic amine with N-O bond having a dipole moment, as shown in the Figure 2.
Due to its highly polar nature it can form hydrogen bonds with the
hydroxyl groups of cellulose and has extremely high solubility in water.
A competing reaction takes place between water and cellulose for NMMO
molecules and water being a smaller molecule is preferred. These
properties are the basis for its use as cellulose solvent.
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
NMMO is a
strong oxidizer and a very corrosive solvent. Pure NMMO melts at 170 °C.
At temperatures higher than 150 °C, NMMO can undergo highly exothermic
decomposition reactions. Therefore, pure NMMO melt cannot be used as a
solvent for cellulose.
|
||||||||||||||||||||||||||||||||||||||||||
NMMO mono and dihydrate melt at 74 °C and ~35
°C respectively. Therefore, water content and temperature of NMMO play
an important role in dissolution of cellulose. NMMO hydrates of required
composition or NMMO/water can be used as a possible solvent and
relatively homogeneous cellulose solutions can be prepared only with
relatively, minor amounts of water.
|
||||||||||||||||||||||||||||||||||||||||||
Manufacturing of lyocell fibres
|
||||||||||||||||||||||||||||||||||||||||||
The process
for manufacture of lyocell is much shorter than that for viscose, where
the need for various ageing stages extends the process time to more
than 40 hours. The main steps for lyocell fibres manufacture are shown
in Figure 3. The process involves:
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
Figure 3. Flow chart showing key stages in the manufacturing of lyocell fibres
|
||||||||||||||||||||||||||||||||||||||||||
Dope Preparation | ||||||||||||||||||||||||||||||||||||||||||
For
preparation of spinning dope, a 50–60% aqueous NMMO is used for making a
slurry of cellulose pulp. In a typical process, the slurry is produced
from cellulose pulp and an aqueous NMMO solution. Typical compositions
used for this are 50–60 %NMMO, 20–30% water, and 10–15% pulp. Addition
of 0.01–0.10% antioxidant n-propyl gallate (PG) is desirable for
preventing degradation of cellulose.
|
||||||||||||||||||||||||||||||||||||||||||
Subsequently, excess water is removed by evaporation under reduced
pressure and at temperatures lower than 150 °C till the cellulose is
dissolved and a homogeneous solution is formed.
|
||||||||||||||||||||||||||||||||||||||||||
A typical isotropic spinning dope composition contains 14% cellulose, 10% water, and 76% NMMO. Temperatures between 90-120 °C are used.
|
||||||||||||||||||||||||||||||||||||||||||
As shown in the Figure 4, the solubility of cellulose in the aqueous NMMO solution depends on water content.
|
||||||||||||||||||||||||||||||||||||||||||
Usually
aqueous NMMO solutions containing more than 15–17 wt% water do not
dissolve cellulose. At such high concentrations (>17 wt%) of water,
NMMO forms hydrogen bonds with water and is not available for
interaction with cellulose hydroxyl groups. This prevents the
dissolution of cellulose.
|
||||||||||||||||||||||||||||||||||||||||||
At lower
concentrations of water, oxygen of N-O can form hydrogen bonds with
cellulose hydroxyl groups and the dissolution can occur. At water
concentration lower than 4 wt%, the dissolution temperatures are very
high (close to degradation temperature of cellulose). Therefore 4 wt% is
considered as the lower dissolution limit.
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
Typical,
safe processing temperatures are in the range of 80–130 °C. Cellulose
dissolution increases with increase in temperature and input of
mechanical energy, while it decreases with increase in water content,
concentration and DP of cellulose.
|
||||||||||||||||||||||||||||||||||||||||||
In case of
lyocell, no ripening step is involved, therefore, the pulps used for
NMMO processing usually have slightly lower DP (molecular weight) than
that of viscose process.
|
||||||||||||||||||||||||||||||||||||||||||
Anisotropic cellulose solutions can be
obtained if the water content is below 11% thus indicating that some
water from hydrated NMMO molecules must be released. High-modulus,
high-strength fibres from concentrated anisotropic cellulose dopes with a
molar ratio of NMMO to water of less than one can be obtained.
Obviously, the viscosity of cellulose anisotropic solutions is strongly
dependent on the concentration and DP of cellulose pulp.
|
||||||||||||||||||||||||||||||||||||||||||
Fibre Formation
|
||||||||||||||||||||||||||||||||||||||||||
Figure 5. Schematic of Fibre formation process
|
||||||||||||||||||||||||||||||||||||||||||
The
spinning of ternary solutions of cellulose-NMMO-water is carried out at
elevated temperatures ranging from 90-120 °C. The fibres are produced by
using a dry-jet wet-spinning process in an NMMO–water solution. Air
gaps vary from 20 to 250 mm. Although, it is claimed that NMMO is
non-toxic and biodegradable, it is very expensive, and a closed –loop
process has been developed and nearly 99.5% of the NMMO is recovered and
can be reused.
|
||||||||||||||||||||||||||||||||||||||||||
The molar ratio of NMMO to water is close to
1:1. The spinning speeds of about 100 m/min. are used. A higher speed
tends to improve final fibre orientation and depends on the air-gap
length. The final properties of Lyocell fibres depend on a number of
variables that are grouped in Figure 6.
As shown in the figure, the final fibre strength will depend on the
properties of spinning dope, spinning conditions, coagulation condition,
and post-treatment conditions.
|
||||||||||||||||||||||||||||||||||||||||||
Figure 6. Process variables in lyocell process
|
||||||||||||||||||||||||||||||||||||||||||
The Structure Formation
|
||||||||||||||||||||||||||||||||||||||||||
During
spinning process, the structure formation is determined by concurrent
orientation, coagulation and crystallization processes. The
crystallization is affected by solution characteristics, the
precipitation conditions, and drying / post treatment conditions. All
these processes are interdependent.
|
||||||||||||||||||||||||||||||||||||||||||
During the
shear deformation in the nozzle and in the air gap, it is proposed that
orientation of the polymer molecules is similar to liquid crystal
polymers occur. The relatively long relaxation times are expected to
help in preserving the oriented state in the air gap.
|
||||||||||||||||||||||||||||||||||||||||||
In the coagulation bath series of steps are reported to occur:
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
So, one can
conclude that the voids originate from the precipitation process and
not from an incomplete densification of the fibrillar elements during
drying.
|
||||||||||||||||||||||||||||||||||||||||||
Properties and Applications of Lyocell Fibres
|
||||||||||||||||||||||||||||||||||||||||||
Lyocell
fibres have very different structural properties than that of viscose.
The DP of cellulose in Lyocell fibres is ~ 600, which is about twice
that of viscose fibres.
|
||||||||||||||||||||||||||||||||||||||||||
Lyocell
fibres are more crystalline and more oriented than viscose fibres. NMMO
-type fibres have a circular cross-section which is markedly different
from the lobulated shape of textile viscose fibres. They have an oval or
round shape ( Figure 7a ) with smooth
surface and tend to be highly fibrillar on the other hand, viscose
fibres exhibit skin–core morphology and are more porous (see Figure 7b ).
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
Due to these structural features, lyocell fibres have a greater tenacity and greater wet strength than viscose fibres.
|
||||||||||||||||||||||||||||||||||||||||||
The main properties of Lyocell fibres in comparison to other cellulosic fibres are summarized below in Table 1 | ||||||||||||||||||||||||||||||||||||||||||
Table 1. Main properties of Lyocell fibre and other cellulosic fibres
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
Lyocell Fibre vs Regenerated Viscose
|
||||||||||||||||||||||||||||||||||||||||||
The peculiarities of Lyocell fibres like:
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
result in a
very high dry and wet tensile strength, a high wet modulus and high loop
tenacity. The high amorphous orientation prevents a sufficient lateral
cohesion and thus results in fibrillation effect.
|
||||||||||||||||||||||||||||||||||||||||||
The
fibrillation is useful for manufacture of non-woven technical products
and composites.Since fibrillation is not always desirable property,
Lyocell fibre is cross-linked using multifunctional crosslinkers.
|
||||||||||||||||||||||||||||||||||||||||||
Modified lyocell fibres can be produced by incorporating chitosan and/or nanomaterials such as silver, ZnO or TiO2 in the spinning dope to impart various functionalities, such as odour-reducing and antibacterial properties.
|
||||||||||||||||||||||||||||||||||||||||||
The main applications of lyocell fibres are in:
|
||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||
No comments:
Post a Comment