Color fastness is a crucial parameter in the evaluation of printed and dyed textiles. Different types of dyes have varying color fastness standards, which depend on their molecular structure, chemical properties, and how they interact with textile fibers. Some dyes form chemical bonds with the fibers, while Others are fixed through physical adhesion. As a result, their performance in terms of color retention can vary significantly. Identifying the type of dye used is essential for accurate assessment, yet it is not always straightforward. The naked eye cannot distinguish between different dyes, so chemical analysis is often required.
In practice, many laboratories rely on information provided by factories or applicants, combined with the experience of inspectors and knowledge of production processes. However, this approach can lead to errors if the dye type is misidentified, potentially allowing non-compliant products to pass as compliant. To address this, several chemical methods exist for dye identification, but they are often time-consuming and complex. This article presents a simple, practical method for identifying dye types on cellulose fibers, making the process more efficient and reliable.
1. **Principle of the Simple Identification Method**
The identification of dyes on textiles is based on the dyeing mechanism and fiber compatibility. For example:
- Cellulose fibers typically use direct, reactive, reducing, naphthol, coating, or anthraquinone dyes.
- Polyester fibers usually require disperse dyes.
- Protein and nylon fibers often use acid dyes.
- Cationic dyes are commonly used for rayon.
For blended fabrics, the dye type depends on the fiber composition. For instance, a polyester-cotton blend would use disperse dyes for polyester and direct or reactive dyes for cotton. Since color fastness largely depends on the dye type, correctly identifying the dye on cellulose fibers is critical.
2. **Dye Type Identification on Cellulose Fibers**
2.1 **Sampling and Pre-Treatment**
Proper sampling is the first step in any dye identification process. A representative sample should be taken from the same colored area. If multiple colors are present, each should be tested separately. If fiber type identification is needed, follow FZ/T 01057 standards. Impurities like grease or slurry must be removed by washing in hot water (60–70°C) for 15 minutes before testing. Resin-treated samples may need additional pre-treatment:
- Urine formaldehyde resin: Treat with 1% solution at 70–80°C for 15 minutes.
- Acrylic resin: Reflux with dioxin for 2–3 hours.
- Silicone resin: Wash with soap and sodium carbonate at 90°C for 15 minutes.
2.2 **Direct Dye Identification**
Place 100–300 mg of the sample in a test tube, add 5–10 mL of concentrated aqueous solution, and boil for 1–2 minutes to extract the dye. Then add 10–30 mg of white cotton cloth and 5–50 mg of sodium chloride. Boil for 40–80 seconds, cool, and wash. If the cotton cloth takes on the same color as the original sample, the dye is likely a direct dye.
2.3 **Reactive and Vat Dye Identification**
For reactive or vat dyes, place 100–300 mg of the sample in a test tube with 2–3 mL water, 1–2 mL of 10% sodium carbonate, and 200–400 mg of sodium hydrosulfite. Boil for 1–2 minutes, then add 25–50 mg of white cotton cloth and 10–20 mg of sodium chloride. Boil again for 1–2 minutes, filter, and allow to oxidize. If the resulting color matches the original, it could be a reactive or vat dye.
2.4 **Vat Dye Test**
Boil 100–300 mg of the sample in 2–3 mL water and 0.5–1 mL of 10% sodium hydroxide. Add 10–20 mg of thiourea and boil for 0.5–1 minute. Transfer to a new tube with 25–50 mg of white cotton cloth and 0–20 mg of sodium chloride. Boil for 40–80 seconds, cool, and oxidize on filter paper. If the color matches the original, the dye is a vat dye.
2.5 **Naphthol Dye Identification**
Soak the sample in 1% solution for 3 minutes, then wash and treat with 1% solution again. If the dye does not dissolve or change color even after exposure to air or oxidation, proceed with the following tests:
- Test 1: Boil the sample in pyridine to see if the dye is extracted.
- Test 2: Soak in 10% sodium hydroxide and ethanol, then add water and dioxin. After cooling, add white cotton cloth and sodium chloride. Boil and observe under UV light. If the cloth fluoresces, it confirms the presence of a naphthol dye.
2.6 **Reactive Dye Test**
Reactive dyes form strong chemical bonds with fibers and are difficult to dissolve. A coloring test can be performed using dimethyloxime and dimethylformamide. If the dye does not stain the fabric, it is likely a reactive dye.
2.7 **Coating (Pigment) Dye Identification**
Coatings, or pigments, do not bond directly to fibers and rely on binders. To identify them, remove any starch or resin finishes using ethyl salicylate. Under a microscope, if the fiber surface appears granular, it suggests the presence of a resin-bonded pigment.
2.8 **Anthraquinone Dye Identification**
If the sample emits a bright green color when exposed to light, it may be an anthraquinone dye. Burning the sample in a flame and observing a greenish hue can also confirm its presence.
3. **Conclusion**
This simple and rapid method allows for quick and accurate identification of dye types on cellulose fibers. By minimizing reliance on external information and reducing unnecessary tests, it enhances the efficiency and reliability of textile quality control. This approach helps ensure that color fastness assessments are both precise and consistent.
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