This is a guest post by Harry Mercer. His brief bio is given below the post.
Dyeing of denim yarns and fabrics with sulfur black canpresent a number of problems that affect fabric profit margins as well as the quality and performance of black jeans. Problems include :
Dye waste (normally 50% or more in washing after dyeing
Color variation after garment laundering
Lower weaving efficiencies with black yarns
Contact dermatitis etc. All of these problems were solved in the past, unfortunately the technical expertise in using sulfur black has greatly diminished in recent decades.
One of the most easily correctable problems is related to acid-damage to black yarns and fabrics, which results from the generation of a sulfur-based acid, possibly sulfuric acid. This results when the pH of the black-dyed cotton is too low to buffer this acid before they can attack the cellulose chain of cotton fibers. Sulfur black-dyed materials are unusual in that they should have a pH of around 11, after dyeing and before the cotton is dried. If significantly lower, the acid generated will result in lower fabric strength or higher yarn-breakage rates during weaving.
Damage Resulting from Chemical Oxidation of Sulfur Black
Sulfur dyes belong to the class of dyes known as “vats”. Vat dyes are insoluble in water and cannot be carried by water into fibers until made water-soluble. Solubility of vat dyes requires that they be first chemically reduced. The reduced dye enters the fiber where it must be oxidized to form the originally insoluble dye. Once made insoluble again, the dye is mechanically trapped inside the fiber.
The chemical oxidation of most sulfur colors can be carried out with agents such as hydrogen peroxide or sodium bromate. Chemical oxidation of these dyes must be conducted at a low pH(4.5-5.5), which requires that an acid be incorporated. For sulfur colors other than black , including greys, browns, blues, violets, greens, turquoises etc., acidic chemical oxidation is necessary to produce bright, consistent and colorfast shades.
Sulfur blacks are an important exception to this. As a rule, sulfur blacks should not be chemically oxidized. There are 2 reasons for this:
First, lowering the pH of a sulfur black with acid will result in the liberation of a strong, sulfuric-type acid that will attack the cotton cellulose.
Secondly, if sulfur black dyes directly contact acids directly, there will be a release of dangerous hydrogen sulfide gas.
This occurs often on continuous yarn or fabric dyeing machines used for denim. Water in the the wash boxes after dyeing become heavily contaminated with sulfur dye, which is often carried over into the acidic oxidation box causing the reaction that releases the gas.
There is a rule-of-thumb regarding how easily a vat dye can be oxidized: if a dye is easy to reduce, it is difficult to oxidize; conversely, if difficult to reduce, easy to oxidize. Sulfur blacks require high temperatures (85-90˚C) for reduction, while all other sulfur colors can be successfully reduced and applied at temperatures as low as 30˚C.
Since they are difficult to reduce, sulfur blacks can be readily oxidized by atmospheric oxygen, in the same manner as Indigo, i.e., by passing the yarn or fabric through air. If the time between the dye box and the 1st washing is adequate to allow the cotton temperature to cool to 40˚C, oxidation will be complete. The first washing should be conducted with cold or warm water since hot washing will promote re-reduction of the dye, resulting in unnecessary dye loss and inconsistent color.
Oxidation of Sulfur Blacks in Batch Equipment
In sulfur black dyeing in batch processes, air oxidation can be conducted on sulfur blacks after dropping the dye bath and circulating the fabric through air before washing. In package dyeing equipment, a compressed air line can be installed that is used to force air from the inside of the yarn package to the outside. In garment machines, uniform air oxidation is difficult and chemical oxidation of sulfur black may be the only option. However, hydrogen peroxide should not be used for sulfur blacks, this would result in a loss of colorfastness. This is probably because the huge sulfur black molecule is broken down into shorter units by peroxides which have less resistance to washing. If chemical oxidation is necessary, then a milder oxidizer such as sodium bromate should be used. After oxidation, the sulfur black dyed material should be buffered to a pH of 11.
A sulfur black-dyed sample of fabric or yarn is placed in a chamber where it is exposed to heat and humidity and dried. The material is allowed to condition and is tested in order to determine strength loss after ageing. The method can be demonstrated simply by placing a sample in a forced-draft oven with about 500 cc’s of water and removing it about 30 minutes after all the water has evaporated.
This is a guest post by Harry Mercer.Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies. Also, as a result of being the laboratory manager of the American Association of Textile Colorists and Chemists (1986-1989, he is an expert colorist for measurement and color matching as well as textile testing.
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