How Do I Identify Which Dye We Use For Hotel Linen?

The types of dyes on fabrics are difficult to identify with the naked eye and must be accurately determined through chemical methods. If we do not identify the types of dyes in advance, we will likely judge unqualified products as qualified products. There are undoubtedly great disadvantages. Users may experience health risks.

• Sampling and pre-processing

The key steps in identifying the type of dye in cellulose fibers are sampling and sample pretreatment. When taking a sample, parts of the same dye should be taken. If the sample contains several tones, each color should be taken. If fiber identification is required, the fiber type should be confirmed according to the FZ/TO1057 standard. If there are impurities, grease, and slurry on the sample that will affect the experiment, it must be treated with detergent in hot water at 60–70 °C for 15 minutes, washed, and dried.

• Identification methods of direct dyes

Boil the sample with 5 to 10 mL of an aqueous solution containing 1 mL of concentrated ammonia water 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 extraction solution, boil for 40–80 seconds, leave to cool, and then wash with water. If we dye the white cotton cloth to almost the same hue as the sample, we can conclude that the dye used to dye the sample is a direct dye.

• How to identify sulfur dyes

Place a 100-300mg sample into a 35-mL test tube, add 2-3 mL of water, 1-2 mL of a 10% sodium carbonate solution, and 200–400 mL of sodium sulfide. Heat and boil for 1-2 minutes. Take out 25–50 mg of white cotton cloth and 10–20 mg of sample sodium chloride in a test tube. Boil for 1-2 minutes. Take it out and place it on filter paper to allow it to re-oxidize. If the resulting color light is similar to the original color and only differs in shade, we can consider it sulfide or sulfide vat dye.

• How to identify vat dyes

Place a 100-300mg sample into a 35-mL test tube, add 2-3 mL of water and 0.5–1 mL of 10% sodium hydroxide solution, heat and boil, then add 10–20 mg of insurance powder, boil for 0.5–1 minute, take out the sample, and put it into a 25–10% sodium hydroxide solution. 50mg white cotton cloth and 0–20 mg sodium chloride; continue to boil for 40–80 seconds; then cool to room temperature. Take out the cotton cloth and place it on the filter paper for oxidation. If the color after oxidation is similar to the original color, it indicates the presence of vat dye.

• How to identify reactive dyes

The characteristic of reactive dyes is that they have relatively stable chemical bonds with fibers and are difficult to dissolve in water and solvents. At present, there is no particularly simple testing method. We can carry out a coloring test first, using a 1:1 aqueous solution of dimethyl methylamine and 100% dimethyl formamide to color the sample. The dye that does not color is the reactive dye.

• How to identify paint

Coatings, also known as pigments, have no affinity for fibers and need to be fixed on the fibers through an adhesive (usually a resin adhesive). We can use microscopy for inspection. First, remove any starch or resin finishing agents that may be present on the sample to prevent them from interfering with the identification of the dye. Add 1 drop of ethyl salicylate to the fiber treated above, cover it with a coverslip, and observe it under a microscope. If the fiber surface appears granular, we can identify it as a resin-bonded pigment (paint).