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Friday, August 6, 2010

Lyocell fiber process

Preparing the wood pulp Hardwood trees are harvested and trucked to the mill where they are cut to 20 ft (6.1m) lengths and debarked by high-pressure water jets. Next, the logs are chipped by a machine into squares about the size of postage stamps. The chips are digested chemically to soften them enough to be mechanically milled to a wet pulp. This pulp is washed with water, and 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).

Dissolving the cellulose At the Lyocell mill, several rolls of pulp are broken into one-inch squares and mixed with amine oxide in a heated, pressurized vessel.; the cellulose fibers begin to dissolve.

Filtering When the cellulose solution has become clear, it is pumped through filters to insure that all the chips are dissolved and to remove any remaining foreign material that would otherwise clog the spinners.
Spinning The 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.
Drying and finishing 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, making the following steps of carding and spinning into yarn easier.
Final steps 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 which 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 given a variety of finishes, from soft and suede-like to silky.
Recovery of the solvent The amine oxide used to dissolve the cellulose and set the fiber after spinning is recovered and re-used: the dilute solution is evaporated to remove the water, leaving amine oxide which is used for dissolving the next batch of cellulose. 98% of the amine oxide is typically recovered.

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