How To Identify The Dye Used on Textiles?

The types of dyes on textiles are difficult to identify with the naked eye, and can only be accurately determined by chemical methods. If we do not identify the types of dyes in advance, it is very likely that unqualified products will be judged as qualified products. Therefore, this article introduces a simple identification method for the types of dyes on cellulose fibers in printed and dyed textiles.

Principles of determining simple identification methods

According to the dyeing principle of dyes on textiles, the types of dyes generally applicable to common textile fabric components are as follows:

Acrylic fiber-cationic dyes

Nylon and protein fiber-acid dyes

Polyester and other chemical fibers-disperse dyes

Cellulose fiber-direct, sulfurized, active, reduced, nafto, coating and phthalocyanine dyes

For blended or interwoven textiles, the types of dyes are used according to their components. For example, for polyester and cotton blends, the polyester component is treated with disperse dyes, while the cotton component is treated with the above corresponding dye types, such as disperse/active, disperse/reduced processes, etc.

Methods

1 Sampling and pretreatment

The key steps to identify the types of dyes on cellulose fibers are sampling and sample pretreatment. When taking samples, the same dye should be taken. If the sample contains several tones, each tone should be taken. If fiber identification is required, the fiber type should be confirmed according to the FZ/TO1057 standard. If the sample is attached with impurities, grease, and slurry that affect the experiment, it must be treated with a detergent in 60-70℃ hot water for 15 minutes, washed and dried. If it is known that the sample has been treated with resin, it is treated with the following methods respectively.

1) Urea-formaldehyde resin is treated with 1% hydrochloric acid at 70-80℃ for 15 minutes, washed and dried.

2) Acrylic resin can be treated with 50-100 times of dichloromethane reflux for 2-3 hours, taken out, washed and dried.

3) Silicone resin can be treated with 5g/L soap and 5g/L sodium carbonate 90cI= for 15 minutes, washed and dried.

2 Identification method of direct dyes

Boil the sample with 5-10 mL of aqueous solution with 1 mL of concentrated ammonia to fully extract the dye.

Take out the extracted sample, put 10-30 mg of white cotton cloth and 5-50 mg of sodium chloride into the extract, boil for 40-80 seconds, let it cool and then wash with water. If the white cotton cloth is dyed with almost the same color as the sample, it can be concluded that the dye used to dye the sample is a direct dye.

3 Identification method of sulphur dyes

Put 100-300 mg of sample in a 35 mL test tube, add 2-3 mL of water, 1-2 mL of 10% sodium carbonate solution and 200-400 mg of sodium sulfide, heat and boil for 1-2 minutes, take out 25-50 mg of white cotton cloth and 10-20 mg of sodium chloride in the test tube. Boil for 1-2 minutes. Take it out and put it on filter paper to allow it to oxidize again. If the color obtained is similar to the original, but only the depth is different, it can be considered as sulfur or sulfur reduction dye.

4 Identification method of vat dye

Put 100-300mg sample in a 35mL test tube, add 2-3mL water and 0.5-1mL 10% sodium hydroxide solution, heat and boil, then add 10-20mg insurance powder, boil for 0.5-1min, take out the sample and put 25-50mg white cotton cloth and 0-20mg sodium chloride, continue to boil for 40-80s, and then cool to room temperature. Take out the cotton cloth and put it on filter paper for oxidation. If the color after oxidation is similar to the original, it means that reduction dye exists.

5 Identification method of Nafto dye

Boil the sample in 100 times the amount of 1% hydrochloric acid solution for 3 minutes, wash it with water, and boil it with 5-10mL of 1% ammonia water for 2 minutes. If the dye cannot be extracted or the amount of extraction is very small, it will change color or fade after being treated with sodium hydroxide and sodium dithionite. Even if it is oxidized in the air, it cannot restore the original color, and it is also impossible to determine the presence of metal. At this time, the following 2 tests can be performed. If the dye can be extracted in the test 1), and the white cotton cloth is dyed yellow and emits fluorescence in the test 2), it can be determined that the dye used in the sample is Nafto dye.

1) Put the sample in a test tube, add 5mL of pyridine and boil it, and observe whether the dye is extracted

2) Put the sample in a test tube, add 2mL of 10% sodium hydroxide solution and 5mL of ethanol, boil it, then add 5mL of water and sodium dithionite, and boil it to reduce it. After cooling, filter, put white cotton cloth and 20-30mg sodium chloride into the filtrate, boil for 1-2 minutes, cool it down, take out the cotton cloth, and observe whether the cotton cloth emits fluorescence when irradiated with ultraviolet light.

6 Identification method of reactive dyes

The characteristic of reactive dyes is that they have a relatively stable chemical bond with the fiber and are difficult to dissolve in water and solvents. At present, there is no particularly clear test method. You can first conduct a coloring test, use a 1:1 aqueous solution of dimethyl methylamine and 100% dimethyl formamide to color the sample, and the uncolored dye is the reactive dye.

7 Identification method of coatings

Coatings, also known as pigments, have no affinity for fibers and need to be fixed to the fibers by adhesives (generally resin adhesives). Microscopy can be used for inspection. First remove the starch or resin finishing agents that may exist on the sample to prevent them from interfering with the identification of the dye. Add 1 drop of ethyl salicylate to the fiber treated as above, cover it with a cover slip and observe it under a microscope. If the fiber surface is granular, it can be confirmed as a resin-bonded pigment (coating).

8. Identification of Phthalocyanine Dyes

When concentrated nitric acid is dropped on a sample, a bright green color is produced, which is a phthalocyanine dye. In addition, if a sample is burned in a flame and turns a distinct green color, it can also be proved to be a phthalocyanine dye.

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