Quick Identification Methods for Common Dyes on Textiles

Identifying dyes on textiles by the naked eye is difficult, so chemical methods are required. The following are simple and rapid identification methods for cellulosic fibers, suitable for routine testing of dyed textiles.

1. Sampling and Pretreatment

• Samples should be taken from areas with the same dye; if multiple shades exist, each shade should be sampled.

• If the sample contains oils, sizing, or other impurities, treat it in 60–70°C hot water with a neutral detergent for 15 minutes, then wash and dry.

• For resin-treated samples, use the following methods:

1. Urea-formaldehyde resin: Treat with 1% HCl at 70–80°C for 15 minutes.

2. Acrylic resin: Reflux in 50–100 times water for 2–3 hours.

3. Silicone resin: Treat with 5 g/L soap + 5 g/L sodium carbonate at 90°C for 15 minutes.

• Wash and dry the sample after treatment.

2. Direct Dyes

• Boil the sample in 5–10 mL water with 1 mL concentrated ammonia to extract the dye.

• Add 10–30 mg white cotton and 5–50 mg NaCl to the extract, boil 40–80 seconds, then cool and wash.

• If the cotton is dyed the same color as the sample, the dye is identified as a direct dye.

3. Sulfur Dyes

• Place 100–300 mg sample in 35 mL water with 1–2 mL 10% sodium carbonate and 200–400 mg sodium sulfide, boil 1–2 minutes.

• Add 25–50 mg white cotton and 10–20 mg NaCl, boil 1–2 minutes, then place on filter paper to oxidize.

• If the color is similar to the original, it is a sulfur or sulfur-reduced dye.

4. Vat/Reduced Dyes

• Place 100–300 mg sample in 2–3 mL water with 0.5–1 mL 10% NaOH, boil, then add 10–20 mg sodium dithionite, boil 0.5–1 minute.

• Add 25–50 mg white cotton and 0–20 mg NaCl, boil 40–80 seconds, then cool and oxidize.

• If the oxidized cotton color is close to the original, a reduced dye is present.

5. Naphthol (Naphthol) Dyes

• Boil the sample in 1% HCl for 3 minutes, wash, then boil in 1% ammonia 5–10 mL for 2 minutes.

• If little or no dye is extracted, further treat with NaOH + sodium dithionite, then perform:

1. Boil the sample in 5 mL pyridine and observe if the dye is extracted.

2. Boil the sample in 2 mL 10% NaOH + 5 mL ethanol, then add 5 mL water + sodium dithionite, cool and filter. Add white cotton and 20–30 mg NaCl, boil 1–2 minutes, cool, and observe under UV light.

• Cotton appearing yellow and fluorescent indicates a naphthol dye.

6. Reactive Dyes

• Reactive dyes form stable chemical bonds with fibers and are difficult to dissolve in water or solvents.

• Test with 1:1 aqueous dimethylamine or 100% dimethylformamide. Non-coloring indicates a reactive dye.

7. Pigments/Coatings

• Pigments do not have fiber affinity and are fixed with binders (typically resin).

• Examine under a microscope; granular surface on fibers confirms a resin-bound pigment.

8. Phthalocyanine Dyes

• Drop concentrated nitric acid on the sample; bright green indicates a phthalocyanine dye.

• Burning in flame producing green also confirms phthalocyanine dye.

Summary

• These methods allow rapid identification of common dyes on cellulosic fibers.

• They are targeted, simple, and efficient, reducing unnecessary procedures while ensuring accurate identification.