How to Identify Dyes Used on Textiles?
Identifying dye types on textiles by visual inspection alone is challenging. Accurate determination typically requires chemical analysis methods. Currently, the common practice relies on information provided by manufacturers or inspection requestors, combined with inspectors' experience and knowledge of production facilities. However, this approach carries risks, as substandard products may be mistakenly approved without proper dye identification. While numerous chemical methods exist for dye identification, they are often complex and time-consuming. This article introduces a simplified process for identifying dye types on cellulosic fibers in printed and dyed textiles.
Principle
The principle behind this simplified identification method is based on the dyeing mechanisms of various dyes on different textile fibers. Common fiber-dye relationships include:
For blended or union fabrics, dyeing processes are tailored to each fiber component. For example, polyester/cotton blends may use disperse/reactive or disperse/vat dyeing techniques.
Methods:
1. Sampling and Pretreatment
Sampling is crucial for accurate dye identification. Samples should be taken from areas dyed with the same dye type. For multi-colored fabrics, samples of each color should be collected. If fiber identification is required, follow the FZ/T01057 standard. Pretreat samples to remove impurities, sizing agents, or oils by washing with a detergent in 60-70°C water for 15 minutes, followed by rinsing and drying. For resin-treated fabrics, use the following methods:
1) Urea-formaldehyde resin: Treat with 1% hydrochloric acid at 70-80°C for 15 minutes.
2) Acrylic resin: Reflux with 50-100 times the volume of dimethylformamide for 2-3 hours.
3) Silicone resin: Treat with 5g/L soap and 5g/L sodium carbonate at 90°C for 15 minutes.
2. Direct Dye Identification
Boil the sample in 5-10mL of water containing 1mL concentrated ammonia to extract the dye. Remove the sample and add 10-30mg of white cotton fabric and 5-50mg sodium chloride to the extract. Boil for 40-80 seconds, cool, and rinse. If the white cotton is dyed a similar shade to the original sample, the dye is likely a direct dye.
3. Sulfur Dye Identification
Place 100-300mg of the sample in a test tube with 2-3mL water, 1-2mL of 10% sodium carbonate solution, and 200-400mg of sodium sulfide. Boil for 1-2 minutes. Remove the sample and add 25-50mg white cotton and 10-20mg sodium chloride. Boil for 1-2 minutes. Allow to air-oxidize on filter paper. If the resulting color is similar to the original sample (may vary in depth), it indicates a sulfur or sulfur vat dye.
4. Vat Dye Identification
Place 100-300mg of sample in a test tube with 2-3mL water and 0.5-1mL of 10% sodium hydroxide solution. Boil, then add 10-20mg sodium dithionite. Boil for 0.5-1 minute. Remove the sample and add 25-50mg white cotton and 10-20mg sodium chloride. Boil for 40-80 seconds, then cool to room temperature. Allow the cotton to air-oxidize on filter paper. If the oxidized color is similar to the original sample, it indicates a vat dye.
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5. Naphthol Dye Identification
Boil the sample in 100 times its volume of 1% hydrochloric acid for 3 minutes. Rinse thoroughly, then boil in 5-10mL of 1% ammonia for 2 minutes. If little or no dye is extracted, and the sample changes color or decolorizes when treated with sodium hydroxide and sodium hydrosulfite without reverting to the original color upon air oxidation, proceed with the following tests:
1) Place the sample in a test tube with 5mL pyridine and boil. Observe if the dye is extracted.
2) Place the sample in a test tube with 2mL of 10% sodium hydroxide solution and 5mL ethanol. Boil, then add 5mL water and sodium hydrosulfite. Boil to reduce. Cool and filter. Add white cotton and 20-30mg sodium chloride to the filtrate and boil for 1-2 minutes. Cool and remove the cotton. Observe if the cotton fluoresces under UV light.
If dye extraction occurs in step 1 and the white cotton in step 2 is dyed yellow with fluorescence, it indicates a naphthol dye.
6. Reactive Dye Identification
Reactive dyes form stable chemical bonds with fibers and are difficult to dissolve in water and solvents. Conduct solubility tests using 1:1 dimethylamine aqueous solution and 100% dimethylformamide. Dyes that do not dissolve are likely reactive dyes.
7. Pigment Identification
Use microscopic examination. Remove any starch or resin finishes from the sample. Add a drop of ethyl salicylate to the treated fiber and cover with a coverslip. Observe under a microscope. A particulate appearance on the fiber surface indicates a resin-bonded pigment.
8. Phthalocyanine Dye Identification
Apply a drop of concentrated nitric acid to the sample. A bright green color indicates a phthalocyanine dye. Alternatively, burn the sample in a flame; a distinct green color also suggests a phthalocyanine dye.
Conclusion:
These rapid identification methods are primarily designed for dyes on cellulosic fibers. By employing these targeted, simplified techniques:
1. The risk of misclassification based solely on provided information is reduced, enhancing inspection accuracy.
2. Unnecessary complex identification procedures are minimized, improving overall efficiency.
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