Lyocell Fibres

 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:

N -methylmorpholine –N-oxide /water (NMMO/H2O) Lithium chloride/dimethyl acetamide(LiCl/DMAc) Trifluroacetic acid/dichloroethane (TFA/CH2Cl2) Calcium thiocyanate/water(Ca(SCN)2/H2O) Ammonia/ammonium thiocyanate(NH3/NH4SCN) Zinc chloride/water(ZnCl2/H2O) Sodium hydroxide/water(NaOH/H2O)

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.

Figure 1. Structure of N-methyl morpholine oxide (NMMO)

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.

Figure 2. Polarity of NMMO

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:

Preparation of the dope of the starting cellulose (dissolving pulp) in an NMMO- water mixture. Extrusion of the highly viscous dope at elevated temp through an air gap into a precipitation bath (dry-jet wet spinning) Coagulation of the cellulose fibre in the precipitation bath Washing, drying and post-treatment of the cellulose fibre Recovery of the NMMO from the precipitation and washing baths.

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.

Figure 4. 3-Phase diagram showing the composition for Dope formation

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:

The exchange of solvent against non-solvent (e.g. water) and desolvation of cellulose molecules. Diminished interaction between the cellulose and solvent. Increased interaction between NMMO and water Oriented cellulose interact with each other Phase separation(fast process) Desolvated polymer chains laterally connected with needle like voids, filled with NMMO-water With increasing water content in the cellulose-rich phase the laterally connected lamellae start to crystallize building up a fibrillar network.

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 ).

Figure 7. Cross-section of (a) Lyocell Fibre and (b) Viscose Fibre

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 Viscose HMW Polynosic Cotton Average DP ~600 ~300 ~400 ~500 1600-2000 Water retention, % 65 90-100 75 55-70 50 Tenacity, 65 % RH and 20 ºC, (cNtex-1) 36 25 45 38 20-24 Tenacity, wet(cNtex-1) 29 13 30 30 26-28 Elongation, 65 % RH and 20 ºC,% 14 20 12 8 7-9 Elongation, wet,% 16 23 15 9 12-14

Lyocell Fibre vs Regenerated Viscose

The peculiarities of Lyocell fibres like:

high crystallinity long crystallites, high degree of orientation, well oriented amorphous regions

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:

Adhesive Substrates Battery Separators Electrical Filtration Media Food Casings Flushable Papers Glass Fibre Binder Insulation Papers Medical Papers Napkins, tablecloths, tissues Reinforcement papers Security / Banknote Papers Tea Bags