Though Lyocell fibres have some key characteristics over other cellulosic fibres such as high dry and wet tenacity, and high wet modulus, these also have a few disadvantageous formation of fibrillation under wet condition, reveal Aravin Prince Periyasamy and Dr Bhaarathi Dhurai.

Pilling is a phenomenon exhibited by fabrics formed from spun yarns ⁽¹⁾. Pills are masses of tangled fibres that appear on fabric surfaces during wear or laundering. Fabrics with pills have an unsightly appearance and unpleasant handle ⁽²⁾. Loose fibres are pulled from yarns and are formed into spherical balls by the frictional forces of abrasion. These balls of tangled fibres are held to the fabric surface by longer fibres called anchor fibres. When fabrics are made from polyester, nylon, Lyocell spun yarns, however, the stronger anchor fibres are not easily broken and the pills that are formed are not released quickly from the fabric ⁽³⁾.

In the swollen state Lyocell has an extensive fibrillation tendency owing to linear high crystalline fibrillar morphology ⁽⁴⁾. The fibrillation tendency of Lyocell enables this fibre to be used in specific finishing effects such as peach skin, silk touch and soft denim. On the other hand, the fibrillations induce, eg, rope marking defect in hank finishing, graying of dyed fabrics and a change of handle of clothes that spoils garment features. Many reports on the morphological structure of man-made cellulosic fibres and their treatment with crosslinking agents have been published ⁽⁵⁾.

Fibrillation mostly leads to pilling and therefore spoils fabric appearance and touch ⁽¹¹⁾. Pill formation is a common problem mainly in knitted fabrics made not only from synthetic fibres but also from natural fibres, man-made cellulosic's and their blends because no consumers accept the undesirably pilled garment. There have been many studies about pilling mechanism for knitted fabrics, which described influences of selected fibre properties, eg, tensile strength, elongation, bending rigidity, fibre count, shape of fibre cross-section and friction on the pilling phenomenon.

Those models were, however, established for dry conditions but not for processes including wet condition, eg, laundry. Man-made cellulosic fibres are hygroscopic materials and their structures of fibre, yarn and fabric dramatically change by swelling with polar solvent such as water ^{(6, 7)}. In the present study, a pilling mechanism including fibrillation and fuzz formation in dry and wet states is discussed and concepts to achieve high durable Lyocell textiles against fibrillation and pilling are suggested ⁽⁸⁾.

Fibrillation

It is one of the important properties of Lyocell. Due to the unique highly crystalline structure of Lyocell, and weaker lateral links between the crystallites, the fibres undergo localised separation of fibrous elements at the surface known as fibrillation, mainly under conditions of wet abrasion. Fibrillation is the longitudinal splitting of a single fibre into microfibres of typically less than 1- 4 µm in diameter ⁽⁹⁾.

This fibrillation behaviour restricts the applications of Lyocell; in particular, fabric dyed in dark hues, as medium/heavy depths can develop a 'frosty' (greying) appearance caused by very fine fibrils that are visually transparent. Basically fibrillated Lyocell shows lower visual colour yield in comparison with non-fibrillated Lyocell (Lyocell LF), independent of exhaustion and fixation ⁽¹⁰⁾. Processing of Lyocell is more technically challenging in fabric and garment form compared to other regenerated cellulosic fibres due to fibrillation ⁽¹¹⁾, and for overall success in the textile industry, it is important to understand the dyeing behaviour of fibrillated Lyocell thoroughly to overcome this problem.

The fibrils formed can be so fine that they become virtually transparent and give a frosty appearance to the finished fabric ⁽¹²⁾. The sample Figure 1 shows an example of a non-fibrillated (a) and a fibrillated (b) Lyocell fibre. If fibrillation is not controlled, these microfibres become entangled giving a serious problem of 'pilling'. It also weakens the mother fibre; also appearance of fabric is become totally unacceptable.

There are two forms of fibrillation - primary and secondary. The first one consists of long and irregular fibrils, which can get entangled, leading to an extremely matted appearance. The secondary form, produced deliberately, is responsible for the fabric's attributes. These fibrils are short and even, and cannot cause pilling. Secondary fibrillation produces change in hand as well as appearance of the fabric.

Reason for fibrillation
Fibrillation can be generated due to more oriented crystalline regions smaller and more oriented amorphous regions are higher in the fibre structure ⁽¹³⁾. This structure is responsible for the high fibre tenacity but low lateral cohesion, especially when subjected to mechanical stress in the swelled state. Other than the mechanical effect, the factors that increase fibrillation are low yarn twist, open structure, high temperature, alkaline pH, low liquor ratio ⁽¹⁴⁾, etc. On the other hand, the factors that decrease fibrillation include reduced mechanical action, use of crease mark reducing agents, singeing before or after dyeing, cellulose enzymatic treatment and finishing with resins.

Pilling mechanism
Firstly a fibre end comes out from the inside of a yarn by a mechanical abrasion during Drying treatment, which induces the fuzz (1). The fuzz fibre is swollen with Washing treatment and gets softer as shown in (2). The swollen soft fibre is easily fibrillated by mechanical abrasion during Washing and Drying treatments (3) and then tangled each other, which develops pilling (4). The fibrillation hardly occurs in dry state (9). Some swollen fuzz would lead to pilling without fibrillation as indicated in (5). The inducement of pill formation from fuzz is significantly hindered without wetting (6). Less degree of fuzz is formed when the fibre is swollen in wet state as shown in (7). After a certain times of washing & drying treatments, the fibre/fibre friction in dry gets higher, which results suppress of fuzz formation in dry state. Increase in fibre/fibre friction in dry state, decrease in degree of swelling might lower tendency of pill formation as well as fibrillation. As shown in Figure 2, the fibrillation plays an important role in pill formation that is significantly affected by fibre swelling ⁽¹⁵⁾.

Defibrillation and methods

Basically fibrillation can be controlled by changing the various spinning parameters ⁽¹⁶⁾ such as spinneret size, temperature, draw ratio, air gap conditions and after treatment of the fibre. Commercially, it was found any one of the following methods used for control the degree of fibrillability ⁽¹³⁾. Removal of the fibrils is absolutely imperative and this is done by using any one of the following methods ⁽¹⁷⁾:

1. Treating of Lyocell with various alkalis
2. Dyeing with poly functional reactive dyes
3. Treating with Cross linking agents
4. Enzymatic treatments

Alkali treatments
It pre-treatment is most important stage in chemical processing of different cellulose fibres and blends is response of these fibres to treatments involving alkali at different concentrations. During scouring, mercerization and dyeing with reactive dyes, sodium hydroxide or sodium carbonate is normally used. It is well-known that the fibrillation tendency of Lyocell fibres is related to swelling state ⁽¹⁸⁾. In view of this, it is necessary to examine the effect of different types of alkali (Sodium hydroxide (NaOH) ⁽¹⁹⁾, Lithium hydroxide (LiOH), Potassium hydroxide (KOH), Tetra methyl- ammonium hydroxide (TMAH) at room temperature on Lyocell fibres.

The effect of sodium hydroxide treatment ⁽¹⁸⁾ on crystallinity in the cellulose II of Lyocell, modal and viscose, and followed this with a separate study comparing Lyocell, acid-hydrolyzed Lyocell, and standard crystalline cellulose II. Recently ⁽¹⁹⁾, Goswami et al 2009 have observed that sodium hydroxide treatment causes the density, orientation and crystallinity of Lyocell fibre to decrease with increasing sodium hydroxide concentration, and that the greatest change in fibre properties occurs between 3.0 and 5.0 mol dm^-3 NaOH. This was attributed to the onset of formation of sodium (Na)-cellulose II at 3.0 mol dm^-3 NaOH; a fully formed Na-cellulose II structure was observed above 6.8 mol dm^-3 NaOH.

The work concluded that formation of Na-cellulose II causes plasticisation of the Lyocell fibres as both inter- and intra-molecular hydrogen bonds are broken by these higher sodium hydroxide concentrations. During the plasticisation state of Lyocell can be caused to reduce the fibrillation tendency up to 40%. Lyocell is subjected to treat with Tetra methyl- ammonium hydroxide (TMAH) will reduce 60% of fibrillation tendency compare to other alkalis.

The critical degree of swelling for Lyocell fibre with no fibrillation was 0.45 cm³/g in ethanol/water mixture. The fibrillation was retarded with alkali treatment in aqueous NaOH and KOH solutions at concentrations between 3.0 and 7.0 mol/l, and minimized at 5.0 mol/l where the uniform reorganisation of macrofibrils was observed with scanning electron microscope. The fibril number of Lyocell fibre treated in trimethylammonium hydroxide was enhanced with increasing concentration and weight loss. The fibrillation was retarded by crosslinking with 1,3-dimethylol-4,5-dihydroxyethylene urea and by treatment with amino functional polysiloxane accompanying decrease in water retention capacity.

Dyeing with poly functional reactive dyes
As Lyocell is a cellulosic fibre, it can be dyed with colours normally used on cotton. Compared to unmercerized cotton & Lyocell except with a few reactive, vat, and direct dyes to produce heavier depth by exhaust methods. But mostly reactive dyes are used for achieving dual functions such as dyeing and reducing degree of fibrillation ⁽¹⁴⁾.

Reactive dyes were introduced 40-years ago, today modern dyestuffs with several reactive groups, which were originally developed for higher wet fastness and better bath exhaustion. Fundamentally bilateral reaction provides the opportunity of cross linking cellulose molecules. The excepted crosslinking in cellulose molecules will be developed. This effect is of particular interest of fibres with high fibrillation. In the case where ploy functional reactive dyes can cause more crosslinking reaction with cellulose and followed by reduce the degree of fibrillation, it stimulates to increase the wet abrasion resistance when compared to bi-functional or tri functional reactive dyes.

Generally, additional reactive groups do offer the important benefit of potentially increasing the fixation of a dye. This parameter is obviously central in determining the practical colour value of a particular product ⁽²⁰⁾. Covalent bond formation and hydrolysis take place concurrently during the dyeing of cellulose with a reactive dye ⁽²¹⁾. Clearly if the dye has only one reactive group, hydrolyzed product can no longer take part in the dyeing. However, if the dye is Poly-functional, ie, contains more than four reactive groups, a further opportunity exists for fixation ⁽²¹⁾.

Specific multifunctional reactive dyes are reported to have favorable effect on fibrillation behaviour of Lyocell fibre ⁽¹⁴⁾. The cross linking of reactive groups of these dyes with adjacent cellulose chains provides an opportunity to reduce fibrillation during wet processing. Certain reactive dyes, which have at least two reactive groups, can form a covalent bond with two adjacent cellulose molecules ⁽²⁰⁾. It is also believed that the presence of several reactive groups is not alone sufficient to produce this effect, but very specific molecular constitution and properties are also required to get the said effect ⁽¹⁰⁾.

Treating with cross linking agents
Resins, which can crosslink with the fabric are frequently used after dyeing. This embrittles the fibrils and enables any fibrillation occurring during the dyeing process to be easily removed ⁽¹¹⁾. This process is particularly suited to woven fabrics as these are prepared and dyed open width and so are free of fibrillation before dyeing.

The cross-linking of the cellulose chains in the fibre in a never dried state result in specific morphological properties of this fibre is then chemically cross-linked in an additional finishing step ⁽²³⁾. This means that the tendency towards fibrillation is prevented by the introduction of additional linking agents between the cellulose molecules. Lyocell fabrics produced by this route have clean bright colouration, a full, soft aesthetic and an excellent performance in use ⁽²⁴⁾. They have proved very comfortable to wear, are durable and retained their 'as new' aesthetic. In blend with polyester they are proving to be excellent for industrial applications, work wear, and career wear.

Enzymatic treatment
Fibrillation can be removed by using of specific cellulase enzymes. These need to be carefully controlled, but are very effective at polishing the fabric surface to remove any unacceptable fibrillation ⁽²³⁾. Enzymes will not prevent the recurrence of fibrillation of fibres but, in conjunction with optimum processing procedures.

Conclusion
The mechanism of pill formation including fibrillation process was proposed taking into account the effects of consecutive wash and dry treatments. Normally fibres came out from yarn by mechanical abrasion due to low fibre/fibre friction; they are fibrillated and tangled owing to r/the softness and high fibre/fibre friction in wet condition.

The pilling was greatly accelerated by a combination of fuzz formation caused in dry state and fibrillation in wet state. The fibrillation tendency is directly related to degree of swelling of fibres, so defibrillation of Lyocell is a very important process. The defibrillation process can be achieved many methods but various alkali treatment and dyeing with poly functional reactive dyes maximum minimise the degree of fibrillation tendency.

The fibrillation is inhibited by not only prevention of fibril separation but also modification of fibre surface, resulting in decrease in surface friction and water accessibility.

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Aravin Prince Periyasamy, M.Tech
SSM Polytechnic College,
Valayakaranoor, B Komarapalayam (Post),
Namakkal (Dist), Tamil Nadu 638 183.
Email: aravinprince@gmail.com

Dr Bhaarathi Dhurai,
Associate Professor,
Dept of Textile Technology,
Kumaraguru College of Technology,
Coimbatore 641 006.