How is lyocell made?
Lyocell being a derivative of rayon have a similar chemical structure but what sets both fibers apart are the production processes. The manufacturing process of lyocell does not have much environmental impact and facilitates the use of solvent spinning technique and thus cellulose structure remains unchanged. Let us now understand the process more deeply.
• Extraction and Preparation of raw material: The raw material that forms the basis of the lyocell fabric is cellulose which is extracted from the pulp of hardwood trees such as oak and birch. The pulp is a blend of various trees selected on the basis of their cellulose composition and generally cultivated on land which cannot be employed for agricultural practices.
• After cultivation the trees are harvested and taken to mill. The outer bark of the tree is then removed using a water jet machine that releases pressurised water. The bark is then loaded into the chipper machine which crushes the bark into small square chips. The chips are then transferred into a container wherein they are treated with certain chemicals that soften the chip into pulp.
• Washing the Pulp: The pulp obtained is then washed with water and bleached if required. The pulp is then dried and made into sheets. The sheet is quite thick which is then rolled onto the spool. These huge cellulose spools weigh about 500lb.
• Dissolving the cellulose: The sheets are retrieved again by opening the spools and further broken into squares and fed into a heated and pressured vat containing amine oxide.
• Filtering: The chips are soaked in the solvent (amine oxide) for a short span of time so that the cellulose dissolves into a clear liquid. This clear liquid is then filtered to ensure that no residue is left.
• Spinning: The solution is then converted to long fiber strands by extruding it through a spinneret (sieve-like device having small holes at the base). The resultant fiber strands are set by soaking them in amine oxide solution followed by rinsing with demineralized water.
• Drying and Finishing: The fiber strands are then dried and lubricated using soap, silicone or some other substance based on the end-use.
• Carding: The dried and lubricated fibers are obtained in an untwisted form called tow. This step involves imparting crimp wherein the fibers are compressed that adds to the texture and provides bulk. The fibers are then separated and straightened out by passing them through between the carding rollers and then optionally combing may be done. Combing is done in order to produce smooth, fine and even fiber strands.
• The well-arranged and carded fiber strands are then cut depending on the end-use. The yarns may be woven or knitted or blended with other fibers as per the product requirement.
The last step involves recovering the used amine oxide. For this water is removed from the solution and the amine oxide is then transported back pressurised vat. Almost about 99% of amine oxide can be retrieved in the production process thus making lyocell fabric less harmful than conventional rayon.