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Lyocell Establishes Presence

Part One in a two-part series on innovative approaches to dyeing and finishing Lyocell fibers.

ATI Special Report Lyocell Establishes Presence Part One in a two-part series on innovativeapproaches to dyeing and finishing Lyocell fibersAs fabrics with new types of hand and appearance characteristics have grown in demand in the last few years, Lyocell fibers have become more established in the man-made marketplace.Now the focus is on optimizing finishing processes. That involves a search for processes that cut down the overall finishing time. It involves research into products, processes and equipment that improve the appearance of these goods even further. Peach-Skin Woven FabricsAfter singeing and desizing in open-width, the fabric is treated open-width with alkali. The fiber swells substantially at this stage. Even when the alkali has been washed off, the fiber remains slightly swollen. This has a positive effect on subsequent wet-finishing processes because it makes the goods more flexible, improving running properties. The risk of permanent crease marks is lower than if initial swelling occurs during treatment in rope form.During primary fibrillation, any fibrils that are not fully bonded are brought to the surface of the fabric. Primary fibrillation should preferably take place in an aerodynamic jet, as this ensures uniform fibrillation. Since Lyocell fabrics are prone to wet rigidity, temperatures less than 60°C should generally be avoided, and a suitable crease inhibitor should be added to all baths. This also minimizes the risk of crease marks. If fibrillation is not uniform, there is a risk of streaks that cannot be leveled out at subsequent treatment stages.The linty fibrils loosened during primary fibrillation are then decomposed by cellulase during subsequent enzymatic defibrillation. Raising the temperature and/or adding alkali stops the enzyme activity.Exhaust dyeing should preferably be carried out on aerodynamic jets. Secondary fibrillation occurs at this stage, creating a pile comprising short, extremely fine fibrils. Migration processes improve levelness and trigger secondary fibrillation because elevated temperatures are used.Alternatively, the fabric can be dyed open-width using the cold pad-batch process. In this case, secondary fibrillation cannot take place during dyeing because the fabric is not exposed to any mechanical forces. To create peach-skin effects, secondary fibrillation can be carried out either before or after cold pad-batch dyeing.High concentrations of some dyestuff chromophores with bi- or multifunctional reactive groups (e.g. Reactive Black 5) may impair fibrillation or cause cross-linking. In such cases, it is advisable to carry out secondary fibrillation before dyeing. Tumble-drying raises the fibrils and creates the desired peach-skin effect.To prevent continued fibrillation, frosting and the formation of crease marks in domestic laundering, a resin finish should be applied. This ensures that the handle and appearance of the goods is retained after repeated domestic laundering cycles.For A Clean SurfaceOne way to create a clean fabric surface is to treat the fabric in rope form. After pretreatment, which comprises scouring and possibly bleaching, the fabric is dyed. Preferably, an aerodynamic jet should be used for dyeing. These steps cause fibrillation. The fabric is then enzymatically defibrillated to restore its clean hand. To ensure that the surface of the fabric remains clear, resin finish is applied.In another process, all steps are performed open-width. As a result, no fibrillation occurs either during pretreatment, or during dyeing (e.g. cold pad-batch process). Consequently, enzymatic defibrillation is not required. A resin finish is particularly important to prevent fibrillation during domestic laundering.An antifibrillation agent (e.g. Axis®, Acordis) is applied to the goods, preferably open-width, at the start of the process. The antifibrillation agent is a colorless, multifunctional auxiliary that reacts with the hydroxyl groups in the cellulose chain and causes them to cross-link. Another option is to use a non-fibrillating fiber type, e.g. Tencel® A100 (Acordis) or Lyocell LF (Lenzing). An antifibrillation agent is added to these fiber types in the spinning mass. This ensures little or no fibrillation. These fiber types can then be pretreated and dyed either open-width or in rope form. To prevent crease marks from forming during domestic laundering, it is advisable to apply a light resin finish.Cold Pad-Batch Dyeing

Lyocell fibers can be dyed by the exhaust or cold pad-batch method. Continuous processes, such as the pad-steam and pad-air/steam process, are also suitable. In volume terms, however, they do not play a significant role in the dyeing of this type of fiber.Of course, productivity depends on batch size: the longer the fabric to be dyed in a particular shade, the fewer batch changes are necessary and thus the higher the productivity.Comparing the productivity of the cold pad-batch process with exhaust dyeing on a 300-kg jet, approximately 0.9 tons per day (three batches) can be pretreated and dyed on the jet, while a far higher batch volume can be bleached and dyed in the cold pad-batch process. The exact amount depends of course on the padding speed, batch size and downtimes between batches.In our comparison of total costs, no distinction is made between the dyestuff costs in the exhaust and cold pad-batch processes, although the fixation yield is generally somewhat higher in cold pad-batch dyeing than in exhaust dyeing. The comparison is based on chemical, water and energy requirements and wage and machinery costs for bleaching, dyeing and drying. The lower energy and water requirements make the cold pad-batch process particularly attractive on ecological grounds. For example, exhaust dyeing at a liquor ratio of 10:1 requires about l00:1 water or even more per kg fabric, compared with only about 60/1/kg in the cold pad-batch method.Apart from the lower water and energy consumption, virtually all suitable cold pad-batch processes are salt-free. Since salt is included in the waste water parameter tdS (total dissolved solids) and will no doubt become more important in the future, this represents a further advantage of the cold pad-batch process compared with the exhaust method, especially when dyeing deep shades (high-salt load in effluent).Additional AdvantagesWhen producing clean (non-fibrillated) goods, it is best to dye the goods directly open-width using the cold pad-batch process in order to minimize fibrillation.Crease marks are often a serious problem in exhaust dyeing, even if a crease inhibitor is used. This problem can largely be avoided by cold pad-batch dyeing of the fabric in open-width. Secondary fibrillation is important when producing peach-skin effects. In the exhaust process, this occurs during dyeing in an alkaline medium at 60° to 80°C, accompanied by mechanical action on the fabric. This is not possible in the cold pad-batch process. However, the step required to deactivate enzymes (alkali and temperature rise) can be used for secondary fibrillation before dyeing.The difference in the fixation time for reactive dyes, which depends on the dye and fixation method, is sometimes seen as a drawback of cold pad-batch dyeing because the results cannot be evaluated immediately after dyeing as in the exhaust process. The batch time is a matter of organization and is not normally a problem.Correcting faulty shades are relatively difficult in the cold pad-batch process, whereas it is comparatively easy to make shading additions in the exhaust process. This greatly increases overall dyeing costs, but is still cheaper than correcting faulty cold pad-batch dyeing. It is therefore particularly important to ensure good reproducibility in the cold pad-batch process by observing all relevant parameters.
Since the liquor level in the trough obviously must not drop at the end of the batch, the amount of liquor must be calculated to ensure at least one additional full trough at the end of each batch. Since this is not fully used in dyeing, it is wasted. However, given the trend towards smaller troughs, the losses are declining.As with other regenerated cellulose fibers dyed by this process, a moirffect may be observed on Lyocell fibers. However, this depends on the type of fabric and is not normally a problem. Action can be taken to minimize the drawbacks of this process and the advantages over exhaust dyeing are obvious.The basic conditions are the same as for other cellulosic fibers. The fabric must have good hydrophilicity to ensure uniform uptake of the dye and should be free of residues to prevent tailing and listing. Moreover, the residual moisture content and temperature of the fabric prior to dyeing should be uniform across the entire length and breadth.Further, a small trough is favorable because it ensures rapid liquor exchange. This is particularly important when applying dyes with low liquor stability. Optimum padding conditions are obtained if complete liquor exchange takes a maximum of three minutes. This reduces the risk of tailing because substantively equilibrium is achieved faster and premature hydrolysis is prevented.The temperature of the dye and alkali liquors and the trough temperature should be monitored continuously as bath stability and fixation yields depend on this. When dyeing regenerated cellulosics, it is advisable that the padding speed should be as high as possible, resulting in a relatively short immersion time of approximately one second.The higher swelling capacity of Lyocell (like CV fibers) means a higher liquor uptake than CO. Depending on the fabric quality, a liquor pick-up of 85 to 100 percent is recommended. Because of its structure, Lyocell has a pronounced tendency to swell. To minimize the pressure caused by swelling on the batch roll, an approximate 30- to 60-second air passage should be incorporated before batching up. Residual swelling can then take place during batching. It is advisable to use relatively high-diameter, foam-covered batch rolls. This leaves room for the fiber to swell and reduces pressure, which can impair the appearance of the fabric (moir Heavy rollers should not be used. To ensure constant winding tension, a center wind should be used.Fixation recipes with longer batch times prolong the migration phase and ensure slow fixation. This is good for the appearance of the goods. Every dye has its own relative substantively in the nip. If the dyes used have different levels of substantively, this can cause a change of shade between the start and end of dyeing (tailing). It is therefore advisable to use trichromatic combinations with balanced substantively. May 2000



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