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Testing Lyocell Fiber Properites

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

Wet Processing UpdateATI Special Report Testing Lyocell Fiber Properties Part Two in a two-part series on innovative approaches to dyeing and finishing Lyocell fibersVarious properties of Lenzings Lyocell® in cold pad-batch dyeing were tested against CV and in some cases CO (bleached, non-mercerized). Lyocell Vs. ViscoseColor Strength: Dyeing of 1/3 S.D. was performed on two Lyocell fabrics (Tencel® and Tencel A100) and on viscose and bleached cotton using the silicate 50 method.Figure 1 shows the color strength of various chromophores dyed as self-shades on these substrates.The Tencel fabrics were taken as l00 percent. Comparable color strength was observed on Tencel A100 for all three blue dyes tested; with Levafix® Scarlet E-2GA, it was slightly lower (90 percent), and with Remazol® Turquoise Blue G 133 percent, considerably higher (135 percent).By contrast, the color strength was far lower on viscose. It is a well-known fact that the color strength of virtually all chromophores tends to be lower on cotton (2 to 21 percent) than on viscose. The phthalocyanine dye used here is the exception. It gave a 5-percent darker shade on CO than on CV. The color strength of these dyeing on cotton was between 44 and 60 percent of the level registered on the Tencel fabric.Color strengths of combination dyeing: The color strength of two combinations dyeing on the same substrates was also compared. Both shades gave slightly darker shades on Tencel A100 than on Tencel, which was again used as the 100-percent benchmark (navy 107 percent, green 115 percent). On viscose, the navy dyeing was 15 percent 1ighter, and the green dyeing was 29 percent lighter. On cotton, the color strength was lower than on viscose. The deepest shades were thus obtained on Tencel A100, and far deeper shades were obtained on Tencel than on CV and CO.Relative substantively is measured to give a relative comparison between different dyes or substrates. The aim is to show the likelihood of substantively related problems such as tailing. The following test method was used: lx padding, lx squeezing; 3x padding, 3x squeezing; 5x padding, 5x squeezing; followed by fixation as usual.The dyeing that was padded and squeezed only once was taken as 100 percent. The color strength of the other two dyeings was measured against the 100-percent dyeing. The darker the dyeing padded 3 and 5 times, the higher their relative substantively and the more difficulties are likely in combination dyeing if dyes with very different substantively are used.Remazol Red RB gran 133 percent and Levafix Brilliant Red E-4BA gran were tested on Tencel, Tencel A100, CV and CO at a depth of 20 grams per liter (g/l). Since no specific difficulties are normally encountered with CO at this depth of shade, the results obtained for CO can be regarded as the standard. The laboratory tests indicate that no specific tailing problems are likely in cold pad-batch dyeing on Tencel. This is borne out by one of the biggest dyers of Tencel by the cold pad-batch process. Bulk trials are still required to establish whether the more pronounced substantively on Tencel A100 could cause tailing problems.  

 Tailing: Another test used to establish the likelihood of tailing with specific dyes, combinations and fabrics is the tailing test. This test reflects more extreme bulk conditions. A 20-meter-long piece of fabric is padded, and the trough is allowed to run empty. The dye squeezed back into the trough is thus not mixed with fresh liquor so the substantively phenomenon can be demonstrated more extremely than in bulk conditions.Four shades dyed with Remazol combinations were tested on CV and Lyocell using the silicate 95 method.The start (0 meter) was taken as 100 percent and the center (10 meters) and end (20 meters) were measured against this using color measurement. Some deepening of shade was observed for all four shades on both fabrics. The dyestuff combinations tested for navy, green and gray all behaved similarly. Stronger tailing was observed only for brown.The curves for CV and Lyocell were very close for all the shades tested here. This indicates that there is no significant difference in the tailing behavior of these two types of fabric. Accordingly, the dyes have a greater influence than the differences between CV and Lyocell.Another tailing test was carried out on Tencel, Tencel A100 and CV for a particularly sensitive beige shade dyed with Levafix dyes using the soda ash method.In this test, the color strength increased by 7 percent on both CV and Tencel between the start and end of dyeing. On Tencel A100, the color strength remained constant over the full length of the fabric according to the results of the color measurement. However, a visual examination reveals a slight deepening of shade of roughly the same amount as on the other two fabrics. This shows that despite the apparent higher relative affinity, good results can be obtained on Tencel A100 providing the dyes and fixation method are suitable.The fixation yield in cold pad-batch dyeing was measured for two dyes on Tencel, Tencel A100, CV and CO. Almost identical results were obtained for both dyes on Tencel, Tencel A100 and CV, while the fixation yield on cotton was somewhat lower.The washing-off behavior of dyeing on Tencel, Tencel A100, CV and CO was tested on cold pad-batch dyeing with 20 g/l Remazol Red RB gran 133 percent (silicate 50 method) and 20 g/l Levafix Scarlet E-2GA gran (silicate method).The dyeings were washed off as follows (liquor ratio 50:1): 2 minutes cold; 2 minutes cold; 3 minutes at 60°C + 1 g/l Calgon® T new; 2 minutes at 60°C + 3 ml/l acetic acid 30 percent to pH 6; 2 minutes at 90°C; and 2 minutes at 40°C.Washing-off properties are partly dependent on fixation yield: the higher the fixation yield, the less hydrolysate has to be washed off. They also depend on the substantively of the hydrolysate. Washing-off behavior was evaluated using fastness to water, severe (ISO 105-EO1).While virtually no staining of the adjacent fabric was observed for Remazol Red RB on Tencel and Tencel A100, slight staining (grade 4 to 5 on gray scale) was observed for CV and CO. Similarly for Levafix Scarlet E-2GA, virtually no staining of the adjacent fabric was observed in the water fastness test on Tencel and CV. However, the results for Tencel A100 and CO showed slight staining of the CV/CO adjacent fabric. Since the fixation yield on CO is slightly lower than on CV and Lyocell, more hydrolysate has to be washed off. This may be the reason for the slightly stronger staining of the adjacent fabric in the water fastness tests on the two dyes.By contrast, similar fixation yields were obtained on CV, Tencel and Tencel A100. The substantively of dye hydrolysate from Remazol Red RB dyeing on CV and Levafix Scarlet E-2GA dyeing on Tencel A100 thus seems to be somewhat higher, leading to slightly higher staining of the adjacent fabric in these cases.Having looked at the factors that have to be borne in mind in cold pad-batch dyeing of Lyocell fibers and the results of the tests on the dyeing properties of this material, we will now outline suitable dyeing methods. 
 Soda Ash MethodCold pad-batch dyeing methods for Levafix dyes include the soda ash, soda ash/caustic soda and silicate methods. Soda ash is used on its own in the soda ash method, so alkalinity is relatively low.Low amounts of alkali result in high pad liquor stability of this method. As long as certain dyes are used, it is possible to work without a dosing pump. This method allows reliable dyeing even in tropical conditions with pad liquor temperatures of 30° to 35°C. When dyeing pale shades, 1 to 2 g/l sodium bicarbonate should be added to improve liquor stability (<=10 g/l dye).This method is particularly suitable for Lyocell because the weaker alkali allows a longer migration phase and slower fixation. Depending on the temperature, dyes and shade, fixation periods of up to 24 hours are required. This gives better dye/fiber penetration.The use of soda ash on its own reduces the swelling of Lyocell fibers. These two properties benefit the appearance of the fabrics, i.e., there is less risk of crushing and moirffects. Silicate MethodBecause of the chemical properties of Remazol dyes (including relatively low substantively), the normal alkali recommendations can be used. The silicate 50 method (See Figure 2) requires far less silicate and thus fewer electrolytes (about half) than the silicate 95 method. Since the reduced amount of electrolyte means that substantively factors are less pronounced, the silicate 50 method is preferable for Lyocell fibers. Recommended DyesAs a general rule, all Levafix and Remazol dyes can be used on Lyocell. One exception is Remazol Red BS, which has limited build-up on regenerated cellulosic fibers and is therefore not recommended for these fibers. As mentioned above, some bi- and multifunctional dyes can cause cross-linking at high concentrations or prevent fibrillation. This problem has been examined by various research institutes.In the cross-linking, properties of Lyocell fibers were investigated through wet abrasion tests and repeated washing. These tests showed that no general statements can be made because the cross-linking of Lyocell fibers by multifunctional dyes is highly selective and depends on the chromophore, anchor system and depth of shade: the deeper the dyeing, the stronger the cross-linking triggered by certain chromophores. However, the effect is not as pronounced as with a resin finish and cannot be regarded as a substitute for this. Far more important than the influence of the dyes is the choice of process, which depends on the desired end product.The risk of tailing, listing and poor reproducibility is highest for pale shades. Selecting suitable trichromatic combinations is thus particularly important.

June 2000