Dyes are defined as highly colored substances that can be applied to a substrate to impart color with some degree of permanence.
Color is described by several attributes:
- Hue refers to the basic color and cast
- Value refers to the lightness or darkness
- Chroma refers to the strength or amount of hue
Dyeing a substrate involves several basic stages or steps. First, a substrate is placed in a medium, most often water, to which dyestuffs and auxiliary chemicals have been added. Dyes are then adsorbed to the surface of the substrate and slowly diffuse into the fiber. Once inside the fiber, dyes migrate (“level out”) and are then fixed to the fiber.
Fiber-reactive dyes form strong covalent bonds with cellulosic fibers, which result in good wash fastness. Reactive dyes are subject to hydrolysis, which is the chemical reaction of the dye with the water. Hydrolysis must be controlled for dye optimization. A wide range of reactive dye chemistries is available, resulting in a range of performance attributes.
Direct dyes also are commonly used for cotton. Direct dyes are easy to apply, so dye cycles tend to be short and economical, and a wide range of colors can be applied. However, direct dyes have limited brightness and poor chlorine fastness, and they require after-treatments to achieve adequate wash fastness. Because direct dyes are attached to cotton fiber by weak forces, they are very sensitive to temperature during the dyeing process. Direct dyes are especially useful for pastel shades and lightfastness.
Vat dyes have no initial affinity for cotton and need to be changed chemically (“reduced”) to make them soluble and able to adsorb to and diffuse into the fiber. Once inside the fiber, vat dyes are oxidized back into an insoluble form. In contrast to direct dyes, vat dyes produce good wash fastness.
Sulfur dyes are similar to vat dyes in that they have to be made soluble before they can be applied. Compared with vat dyes, they are easier to reduce but harder to oxidize. Sulfur dyes tend to be inexpensive, but are sensitive to chlorine and are subject to color loss with abrasion, which leads to poor crock fastness. Lightfastness is poor for the light shades but good for the darker shades. Sulfur dyes are widely used for navy blue and dull green shades and are the industry standard for black.
Pigments are totally insoluble in water, show no affinity to any fiber, and do not penetrate the fiber. They are used to apply color to the surface of a fabric or garment, typically by printing. Pigments must be applied with a binder; however, too much binder can ruin the feel or hand of the fabric. Pigments allow for the production of intense fluorescent colors. Fabrics colored with pigments have good lightfastness and are not sensitive to chlorine.