Color fastness is a critical parameter used to evaluate the quality of printed and dyed textiles. Different types of dyes exhibit varying levels of color fastness due to their distinct molecular structures, chemical properties, and bonding mechanisms with textile fibers. Some dyes form covalent bonds with fibers, while others are fixed through physical interactions. As a result, the color fastness of each dye type differs significantly. Since it's often impossible to distinguish between different dyes visually, accurate identification requires chemical analysis. In practice, inspectors rely on information provided by manufacturers or applicants, combined with their experience and knowledge of production processes. However, this approach can lead to errors if the dye type is not correctly identified beforehand, potentially resulting in non-conforming products being mistakenly approved.
To address this issue, several chemical methods exist for dye identification, but they tend to be complex and time-consuming. This article introduces a simple and practical method for identifying dye types on cellulose fibers, which is especially useful in quality control settings.
1. **Principle of the Simple Identification Method**
Dyeing behavior varies depending on the fiber type. For example, cationic dyes are typically used for rayon, acid dyes for nylon and protein fibers, disperse dyes for polyester and other synthetic fibers, and direct, reactive, vat, naphthol, coating, and anthraquinone dyes for cellulose fibers. In blended fabrics, the appropriate dye is selected based on the component. For instance, in a polyester-cotton blend, disperse dyes are used for polyester, while direct or reactive dyes are applied to cotton. Since the color fastness of cellulose fibers largely depends on the type of dye used, accurately identifying the dye type is essential.
2. **Identification of Dye Types on Cellulose Fibers**
2.1 **Sampling and Pre-Treatment**
Proper sampling and preparation are crucial for reliable results. A representative sample should be taken from the same colored area. If multiple colors are present, each should be sampled separately. Before testing, fiber type should be confirmed using FZ/T 01057 standards. If impurities like grease or sizing are present, the sample should be treated with hot detergent at 60–70°C for 15 minutes, followed by washing and drying. For resin-treated samples, specific pre-treatment steps apply:
- Urine formaldehyde resin: Soak in 1% solution at 70–80°C for 15 minutes.
- Acrylic resin: Soak in 50–100 times the volume of water for 2–3 hours.
- Silicone resin: Wash with 5g/L soap and 5g/L sodium carbonate at 90°C for 15 minutes.
2.2 **Identification of Direct Dyes**
Take 1 mL of concentrated solution and mix with 5–10 mL of boiling water. Add 10–30 mg of white cotton fabric and 5–50 mg of sodium chloride. Boil for 40–80 seconds, cool, and rinse. If the cotton fabric takes on a color similar to the original sample, it indicates a direct dye.
2.3 **Identification of Vat Dyes**
Place 100–300 mg of sample in a test tube, add 2–3 mL water and 1–2 mL of 10% sodium carbonate solution. Heat and boil for 1–2 minutes. Then add 10–50 mg of white cotton fabric and 10–20 mg of sodium chloride. Boil again for 1–2 minutes, then allow to oxidize on filter paper. If the color matches the original, it’s likely a vat dye.
2.4 **Identification of Vat Dyes (Alternative Method)**
Add 2–3 mL of water and 0.5–1 mL of 10% sodium hydroxide solution to the sample, heat, and add 10–20 mg of thiourea. Boil for 0.5–1 minute, then add white cotton fabric and continue boiling. After oxidation, if the color matches the original, it confirms the presence of a vat dye.
2.5 **Identification of Naphthol Dyes**
Boil the sample in 1% solution for 3 minutes, then wash and re-boil in 1% solution for 2 minutes. If no significant dye extraction occurs, or if the dye fades after treatment with sodium hydroxide and cannot be restored, further tests are needed. Test 2.5.1 involves boiling in pyridine, and Test 2.5.2 involves treating with sodium hydroxide and ethanol. If the white cotton fabric turns yellow and emits fluorescence under UV light, it’s likely a naphthol dye.
2.6 **Identification of Reactive Dyes**
Reactive dyes form strong chemical bonds with fibers and are difficult to dissolve. A simple test involves soaking the sample in dimethylformamide or dimethyl sulfoxide. If no color appears, it may be a reactive dye.
2.7 **Identification of Coatings (Pigments)**
Coatings, or pigments, have no affinity for fibers and require a binder. To identify them, remove any starch or resin finishes first. Then add ethyl salicylate and observe under a microscope. If particles are visible on the fiber surface, it suggests a resin-bonded pigment.
2.8 **Identification of Anthraquinone Dyes**
Anthraquinone dyes produce a bright green color when exposed to certain chemicals. Additionally, burning the sample in a flame may cause it to emit a green glow, confirming its identity.
3. **Conclusion**
The methods described here provide a quick and effective way to identify dye types on cellulose fibers. By following these procedures, inspectors can avoid relying solely on manufacturer-provided information, ensuring more accurate and reliable assessments. This approach also reduces the need for unnecessary chemical tests, streamlining the overall inspection process.
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