Tencel - a unique cellulosic fibre

Jim Taylor gives an introduction to the

production and properties of this unique fibre.

Lyocell is the first new generic fibre classification for thirty years and is defined as ‘a cellulose fibre obtained by an organic solvent spinning process.’ Courtaulds plc was the first company to provide a commercial product within this category and gave its product the brand name Tencel.

Product ion Lyocell is produced from wood pulp via a solvent spinning process. The pulp is dissolved in an amine oxide and the resulting viscous solution is extruded into a water bath through fine jets. As the solvent is washed out, the fibre forms

Figure 1 Production process for Tencel fibre

into fine filaments that are collected as a tow, from which the staple fibre is produced. More than 99% of the solvent is recycled within the process, making the fibre production extremely environ- mentally responsible.

The standard fibre produced is 1.4 dtex, 38 mm, but it can be produced in a range of linear densities and staple lengths. The fibre has a smooth surface and a round cross section, providing high lustre in the raw state (Figure 1).

These factors, together with the unique orientation of the crystalline regions, cause significant differences in fibre properties.

The more highly aligned structure of lyocell is obtained as the cellulose quick- ly precipitates when it comes into contact with the spin bath liquor, forming crystall- ine units that are separated by liquid filled amorphous regions. When the fibre is dried the water is removed. This leaves spaces in the structure across which the

‘Lyocell has many interesting properties that result fYom the unique crystalline arrangement of its cellulose units

Structure Lyocell has many interesting properties that result from the unique crystalline arrangement of its cellulose units. The degrees of polymerisation and crystallinity are higher for lyocell than for viscose.

crystalline units are linked by hydrogen bonding. This unique fibre structure pro- duces a number of interesting fibre properties (Table 1).

Lyocell fibre has high strength both wet and dry. The dry tenacity is higher than that of other cellulosics and approaches that of polyester. The fibre also has a high modulus leading to low shrinkage in water.

Fibrillation The unique crystalline structure of lyocell causes fibrillation behaviour. Fibrillation occurs when the fibre is wet and is sub- jected to mechanical action. Swelling of the porous regions of the fibre breaks the hydrogen bonds linking the crystalline units and forces them apart. Mechanical action causes the outer crystalline re- gions to break and peel away from the main fibre. These peelings are referred to as fibrils. The fibrillation effect can be used to advantage in creating fabrics of attractive appearance and appealing handle.

Fibrillation will occur only when the

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Table 1 Fibre properties

Courtaulds Regular Cupro lyocell viscose Polynosic Modal fibre Cotton Polyester

Decitex 1.7 1.7 1.7 1.7 1.7 1.7 Dry tenacity (cN/tex) 40-44 20-24 35-40 34-36 15-20 20-24 50-55 Dry extension (YO) 14-1 6 20-25 10-15 13-1 5 7-23 7-9 25-30

(cN/tex) 34-38 10-15 27-30 19-21 9-12 25-30 50-55 Wet tenacity (YO) 16-1 8 25-30 10-15 13-1 5 16-43 12-14 25-30

Wet tenacity

Water inhibition (YO) 65-70 90-100 55-70 75-80 100 45-55

Figure 2 Effect of pH and temperature on the fibrillation of lyocell

fibre is wet and is subjected to mech- anical aggression. The rate can be increased by raising the temperatures and pH values (Figure 2).

The ‘peach skin’ finish The process required to produce the finish involves three stages: primary fib- rillation, enzyme cleaning and secondary fibrillation. The stages are best described by reference to the effect on a lyocell fabric (Figure 3).

Because of the stiffness of the fibre the yarns made from lyocell are hairy. Singeing of the fabric will help to reduce

this, but a significant number will still be present. The fabric is subjected to water and mechanical action in the primary fibrillation process. The hairs are most accessible to the action and therefore fibrillation occurs predominantly on these surface fibres. They can become entangled, giving a characteristic pilled appearance.

Cellulase enzymes remove the fibrillated fibres and pills from the fabric surface. The previous primary fibrillation process has created weak spots in the fibres allowing the enzyme to depill the fabric effectively. Further wet processing causes more fibrillation (or secondary fibrillation). Now there are no longer any hairs so no pilling occurs, just fibrillation on the fabric highpoints. Final softening and tumbling allow the surface fibrillation to lift, giving a frosted appearance from the light scattering behaviour of the fibrils and the characteristic hand.

Processing routes for woven Tencel Lyocell fibres swell dramatically when wet (by 35% of their diameter), causing fabrics to become stiff. The stiffness can lead to the formation of creases, especially in the preparation stages. Open-width preparation is therefore recommended using a continuous range, pad, beam or jig.

Fabrics can be damaged in wet processing if unsuitable equipment or procedures are used. Scanning electron microscopy (SEM) shows that most typi- cal damage marks are due to localised fibrillation. Because of the optical effects of fibrillation, these areas appear lighter than the bulk of the fabric. This localised fibrillation is caused by repetitive mech- anical action on the same area of the fabric, so the selection of the right pro-

‘The fibrillation effect can be used to advantage in creating fabrics of attractive appearance and appealing handle’

192 JSDC VOLW 114 JULY/AUGUST 1998

Figure 3 Fibre damaged by the primary fibrillation process

cessing machinery is of paramount importance.

In fabric piece processing it is impera- tive that the fabric rope is opened and reoriented as it passes through the machine. Many liquor jets can show poor fabric opening and thus have a high potential for fabric damage. Air jets are more successful at producing damage free lyocell fabrics. When the fabric leaves the jet nozzle the air velocity blows the fabric open, leading to excellent reorient- ation and a lowered potential for localised fibrillation. After the jet processing stages a tumbling treatment allows the full bulky appearance to be generated.

Resin finishing is not normally needed to confer adequate washability, but a light resin finish can be beneficial in producing enhanced crease perform- ance. The concentrations of resin must be kept to a minimum, as too high a level can result in embrittlement of the fine fibrils.

No special types of machinery are needed for processing of fabrics for lyocell garments, but care must be taken to achieve correct loadings, water levels and speeds to ensure adequate reorient- ation and to avoid creasing. Because of the high mechanical action present in garment processing machines, times can be significantly shorter than those utilised in fabric piece process.

'Peach skin' finish on knitted Tencel Production of the peach finish on knitted fabrics can be more difficult due to the more open nature of the knitted struc-

ture. When processing woven fabrics, removal of the surface hairs renders the fabric washable as all the remaining fibres are held in the structure of the fabric.

further wet processing avoided. Fabrics produced by either of these methods can be dry cleaned without further treatment. In order to make them machine wash- able, resins need to be applied.

'Fibrillation can be prevented by avoiding mechanical action whilst wet'

In knitted fabrics, it is often the case that removal of the surface fibres is followed by further movement of fibres to the fabric surface. These hairs will in turn fibrillate and produce a pilled appear- ance. For this reason knitted structures must be carefully selected to hold fibres within the structure by the use of tighter constructions and short stitch lengths, for example. It is often necessary to in- crease processing times for knitted structures and in some cases the use of a light resin finish will be needed to prevent fibre work out and further fibrillation.

Classical finish For a more formal and cleaner appear- ance, fibrillation is not utilised. To pro- duce a classic fabric look the fibrillation effect must be avoided or removed.

Fibrillation can be prevented by avoiding mechanical action whilst wet. Processing of the fabric in an open-width form using for example, cold pad-batch, jigs, beam or continuous methods will provide this mechanical action free condition. If fibrillation does occur it should be removed by enzyme and then

Non-f ibrillating fibre Courtaulds Fibres is currently developing a new fibre variant for the lyocell portfolio. The fibre has been treated with a fibrillation-preventing chemical during production. The fabrics produced from this new variant show wonderful drape, excellent washability and physical performance. The introduction of this new product in the autumn will further increase the options in dyeing and finishing and extend the aesthetic range possible from this exciting new fibre.

The author would like to thank Alix Harnden from Tencel USA for her help in the writing of this paper. Jim Taylor is research manager, dyeing and finishing technology with the Tencel Technology group of Courtaulds Fibres.

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