Lyocell

The label of a coat with Tencel

Lyocell is a regenerated cellulose fiber made from dissolving pulp (bleached wood pulp). It was developed and first manufactured as Tencel in the 1980s by Courtaulds Fibres UK at their pilot plant S25. As of 2010 it is manufactured by Lenzing AG of Lenzing, Austria, under the brand name "Lyocell by Lenzing", and under the brand name Tencel by the Tencel group, now owned by Lenzing AG.

The US Federal Trade Commission defines Lyocell as "a cellulose fabric that is obtained by an organic solvent spinning process". It classifies the fibre as a sub-category of rayon. The fiber is used to make textiles for clothing and other purposes.

Major properties

Lyocell first went on public sale as a type of rayon in 1991. It shares many properties with other cellulosic fibers such as cotton, linen, ramie and rayon. Some main characteristics of lyocell fibers are that they are soft, absorbent, very strong when wet or dry, and resistant to wrinkles; lyocell fabric can be machine- or hand-washed or drycleaned, it drapes well, and it can be dyed many colors, and can simulate a variety of textures such as suede, leather, and silk.

The manufacturing process

Hardwood logs are chipped into squares about the size of postage stamps. The chips are digested chemically, to remove the lignin and to soften them enough to be mechanically milled to a wet pulp. This pulp may be bleached. Then it is dried into a continuous sheet and rolled onto spools. At this stage, it has the consistency of thick posterboard paper. The roll of cellulose weighs some 500 lb (227 kg). The waste liquor may be reworked to produce tall oil, used to make alkyd resins.

At the Lyocell mill, rolls of pulp are broken into one-inch squares and dissolved in N-methylmorpholine N-oxide, giving a solution called "dope." The filtered cellulose solution is then pumped through spinnerets, devices used with a variety of manmade fibers. The spinneret is pierced with small holes rather like a showerhead; when the solution is forced through it, long strands of fiber come out. The fibers are then immersed in another solution of amine oxide, diluted this time, which sets the fiber strands. Then they are washed with de-mineralized water. The Lyocell fiber next passes to a drying area, where the water is evaporated from it. The strands then pass to a finishing area, where a lubricant, which may be a soap or silicone or other agent depending on the future use of the fiber, is applied. This step is basically a detangler, prior to carding and spinning into yarn.

The dried, finished fibers are at this stage in a form called tow, a large untwisted bundle of continuous lengths of filament. The bundles of tow are taken to a crimper, a machine that compresses the fiber, giving it texture and bulk. The crimped fiber is carded by mechanical carders, which perform an action like combing, to separate and order the strands. The carded strands are cut and baled for shipment to a fabric mill. The entire manufacturing process, from unrolling the raw cellulose to baling the fiber, takes about two hours. After this, the Lyocell may be processed in many ways. It may be spun with another fiber, such as cotton or wool. The resulting yarn can be woven or knitted like any other fabric, and may be given a variety of finishes, from soft and suede-like to silky.

The amine oxide used to dissolve the cellulose and set the fiber after spinning is recycled. 98% of the amine oxide is typically recovered. Since there is little waste product, this process is relatively eco-friendly. However, it uses a substantial amount of energy, and uses an organic solvent of petrochemical origin.

Fabric and garments from Lyocell fiber

After the fiber is created it is provided to manufacturers for weaving into fabric, then the fabric is used to create garments. Manufacturers may use environmentally unfriendly or chemical treatments to overcome the natural reluctance of the fiber to take dye and to overcome its natural pilling tendency. Although the closed-loop manufacturing process makes Lyocell inherently the most eco-friendly of the naturally regenerating fibers, different fabric and garment manufacturers vary in this respect.

Uses

As of 2010 Lyocell is more expensive to produce than cotton or rayon. It is used in many everyday fabrics. Staple fibres are used in clothes such as denim, chino, underwear, casual wear, and towels. Filament fibers are used in items that have a silkier appearance such as women’s clothing and men’s dress shirts. Lyocell can be blended with a variety of other fibers such as silk, cotton, rayon, polyester, linen, nylon, and wool. Lyocell is also used in conveyor belts, specialty papers and medical dressings. (Textiles, Kadolph & Langford). Tencel is also used for making some brands of baby diaper wipes.

Producers

Lyocell is manufactured by Lenzing AG in Mobile, Alabama, USA, in Grimsby, England, and in Heiligenkreuz, Burgenland, Austria.

Production of Lyocell involves chemically dissolving cellulose then filtering and wet-spinning the resulting dope into fibers. However, dissolving the cellulose at a fast enough rate to make it commercially profitable requires the cellulose dope to be in a highly viscous and concentrated state. Once the viscous mass reaches the spinnerets, it is difficult to force through the spinneret nozzles. A multi-stage solution known as the MasterConti process was developed. This process separates the highly viscous dissolution phase from the downstream phases that require a less viscous state in order to be most productive. The MasterConti process provides a continuous masterbatch process, to overcome these limitations by separating the dissolution process from the downstream processes. The MasterConti process features a robust kneader reactor, able to easily handle the highly viscous cellulose dope. The dope then enters a mixer-diluter that reduces the viscosity to a consistency that is optimal for spinning. By keeping the two processes separate, the MasterConti process enables manufacturers to maintain the best environment for each phase. As a result, producers are able to:

• improve product quality
• achieve higher shear for better homogenization
• realize greater process intensification with multi-step processing

See also

For a comprehensive review of lyocell and other rayon production methods and markets see "Regenerated Cellulose Fibres", (book - Edited by C R Woodings) 348 pages, 234x156mm Hardback 2001, ISBN 1 85573 459 1, Woodhead Publishing Ltd.

• Modal (textile)

External links

• Information about the fiber from fibersource.com
• Lyocell: the environmental aspects
• Lenzing's page on lyocell
• Federal Trade Commission webpage on textiles
• A site advocating the use of lyocell as an eco-friendly textile
• OSU fact sheet
• Uniform Reuse have a long pdf report on fabric properties and suppliers including lyocell
• Dissolving of Cellulosics
• The Development of lyocell and Tencel by Courtaulds Research