Lyocell fibres

P WHITE, M HAYHURST, J TAYLOR and A SLATER, Lenzing Fibers Ltd, Derby, UK 

Lyocell is the first in a new generation of cellulosic fibres made by a solvent spinning process. A major driving force to its development was the demand for a process that was environmentally responsible and utilised renewable resources as their raw materials. The first samples were produced in 1984 and 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 make lyocell ideal for nonwoven fabrics and papers. 

Lyocell is a cellulosic fibre derived from wood pulp produced from sustainable managed forests. The wood pulp is dissolved in a solution of an amine oxide (usually A-methylmorpholine-A-oxide (5.1)). The solution is spun into fibres and the solvent extracted as the fibres pass through a washing process. The manufacturing process recovers 99.5% of the solvent. The solvent itself is non-toxic and all the effluent produced is non-hazardous. 

It is the direct dissolution of the cellulose in an organic solvent without the formation of an intermediate compound that genetically differentiates lyocell from other cellulosic fibres such as viscose. 

Lyocell has all the benefits of being a cellulosic fibre, in that it is fully biodegradable and absorbent. It has high strength in both the wet and dry state. It blends well with fibres such as cotton, linen and wool. In common with other highly oriented cellulosic fibres, lyocell fibrillates when the fibre 

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Later, other cellulose solvents were developed, like the N-methylmorpholine-N -oxide (NMMO) in water, systems of lithium chloride with an aprotic polar solvent, dimethyl acetamide, dimethyl formamide and dimethyl sulfoxide, but again, only the former has been successful and led to the LYOCELL fibre. 

Demanding and sophisticated customers as well as fashion designers are looking for new effects. Some of them have been discussed in the chapter on novel finishes, for example anti-odour and protective finishes, wellness finishes that release fragrances or cosmetics and medical finishes that deliver drugs. Additionally, cellulase finishing, especially on microviscose or lyocell fibres, enables novel... 

For synthetic and regenerated fibres, the fibre production (primary spinning) is mainly made by melt spinning (polyester, nylon etc.), dry spinning (elastane etc.) or wet spinning (viscose, lyocell etc.). Lenzing AG has produced carbon footprints of viscose and lyocell fibres and compared these with other fibres (Shen and Patel, 2008 Shen et al., 2010 Terinte et al., 2014 Van der Velden et al., 2014). The production of natural fibres via agriculture or forestry has been studied elsewhere (e.g. Sandin et al., 2013). 

Degrees of crystallinity for cellulose II products are in the range from 25%, which is typical for viscose fibres, to 45% for Lyocell fibres [16]. 

As for viscose fibres, these are mostly produced as staple fibres for textile and nonwoven applications. In 2011, world production was 3.246 million tons [49] while filament yam for textile and technical applications reached 332 000 tons in 2011 [49] with a share of technical yarns of 56 000 tons. Technical viscose fibres, also called rayon or viscose rayon, are used mainly as carcass reinforcing fibres in fast-running and run-flat tyres. Lyocell fibres are produced only as staple and virtually exclusively by Lenzing AG, Austria, with a production capacity of 140 000 tons in 2011 [50]. 

In the course of the recent revival of bio-based polymers [58], cellulose esters have gained attention as matrix materials both for macroscopic and nano composites. In a series of papers, Seavey, Glasser, et al. have investigated continuous cellulose fibre reinforced cellulose ester composites of which the last is dealing with commercial matrix and fibre options [59]. A sort of hand lay-up with acetone solutions was used as the manufacturing method. For various commercial CABs and Lyocell fibres moduli between 15 GPa and 21 GPa were obtained in unidirectional (UD) composites while for cross-ply (CP) architectures, the values were between 10 and 15 GPa. Strengths go up to 310 MPa for UD composites and to 210 MPa for CP materials. 

We already mentioned in Section 3.5 [70] the partial oxypropylation of cellulose fibres and the interest of the ensuing composite materials in which the unmodified fibre cores represent the reinforcing elements and their thermoplastic sleeves the source of a matrix. Other interesting approaches have been recently put forward to prepare composite materials in which cellulose or one of its derivatives prepared in situ are the only component. Glasser was the first to tackle this problem through the combination of cellulose esters and fibres by two distinct approaches, viz. (i) the incorporation of lyocell fibres into a cellulose acetate matrix [92] and (ii) the partial esterification of wood pulp fibres with -hexanoic anhydride in an organic medium [93] that produced thermally deformable materials in which the thermoplastic cellulose ester constituted the matrix and the unmodified fibres the reinforcing elements. 

N. Graupner, and J. Mussig, A comparison of the mechanical characteristics of kenaf and lyocell fibre reinforced poly (lactic acid) (PLA) and poly (3-hydroxybutyrate) (PHB) composites. Compos. A Appl. Sci. Manuf. 42(12), 2010-2019 (2011). 

Lenzing of Austria commenced production of their lyocell fibre in 1997. This rapid expansion led to a temporary overcapacity and no further production occurred until Lenzing expanded their capacity by 20,000 tonnes per year in 2004. Development of a continuous filament spinning process by AKZO/ Tencel continued throughout the 1990s but has not yet been commercialised because of the decline in the market for filament rayon and the high investment cost required. 

The following is a simplified representation of the life cycle of lyocell fibres, from creation through to ultimate disposal and biodegradation (see also Fig. 5.3). 

Lyocell fibre is manufactured from cellulose wood pulp, which is produced from trees grown in managed forests. 

Lyocell fibres are converted to a very wide range of textile and industrial products. In many areas, the particular properties of lyocell lead to environmental benefits for customers during product manufacture and use. 

The inherent properties of lyocell fibres make them particularly suitable for less intensive laundering. 

Composting. Lyocell fibres have been found to degrade completely after six weeks in a static aerated compost pile. Synthetic fibres tested... 

Sewage treatment. Lyocell fibres degrade completely within 8 days in a typical sewage farm anaerobic digester, where the residence cycle is about 20 days. Synthetic fibres showed slight reductions in strength after 12 weeks. 

Alternatively, if lyocell fibre were destroyed by incineration, it would bum readily, to carbon dioxide and water, with a heat of combustion of 15 kJ g. In a commercial incineration plant, this waste energy can be put to good use. 

Lyocell is an excellent example of a sustainable fibre today. Building on this, lyocell manufacturers operate a policy of continuous improvement in partnership with key raw material suppliers, customers and retailers. Future developments are likely to further minimise the environmental impact of lyocell fibres in areas such as ... 

It should be appreciated that lyocell fibres are stable in length when wetted and dried (i.e. they do not shrink). Fabric shrinkage arises only from the fact that the fibres and yams swell in diameter swelling in diameter forces yam crimp to increase so that the fabric will contract (shrink) and become thicker. Thickness is due to an increase in the yam crimp amplitude. 

Staple fibre grades are produced to suit carded dry laid, air laid and wet laid processes. The attributes of lyocell fibre are discussed with reference to each of these conversion technologies below. 

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