DYES APPLIED TO FIBER CLASSES

Dyes for Cellulosic Fibers

Cellulosic fibers can be dyed readily using a number of dye classes. The less crystalline regenerated cellulosic (rayon) dye more readily than the more crystalline natural cellulosic fibers such as cotton. Cellulosic can be dyed or printed using direct, reactive, basic, vat, sulfur, or azoic dyes.

Cellulosic can also be colored using pigment-binder systems. Mordanting of direct and basic dyes on cellulosic improves overall fastness. Since the hydroxyl groups in cellulosic are less reactive than many functional groups found in protein fibers, reactive dyes such as the dichlorotriazine (Procion M) or dichloroquinoxaline (Levefix) dyes are often used on cellulosic fibers due to their higher reactivity.

The general stability of cellulosic fibers to basic solutions permits them to be effectively dyed by vat, sulfur, and azoic dyes without significant damage.

Dyes for Cellulose Ester Fibers

Acetate and triacetate fibers can be effectively dyed using disperse dyes.

The rate of dyeing is more rapid with the more hydrophobic triacetate fibers than with acetate. Under special conditions, azoic and vat dyes may be used to dye these fibers. Acetate fibers also have affinity for selected acid and direct dyes. Since acetate loses its luster above 85°C, dyeing’s must be carried out at or below this temperature.

Addition of pigments or solvent-soluble dyes to the acetate or triacetate spinning "dope" prior to fiber spinning leads to colored fibers possessing excellent colorfastness, although the colors available are limited.

Dyes for Protein Fibers

Protein fibers are the most readily dyed fibers due to the numerous reactive functional groups present. They can be dyed with a wide range of dyes under acid, neutral, or slightly basic conditions. Since the keratin fibers are less crystalline and oriented than secreted fibers such as silk, they tend to dye more rapidly and more readily to deeper shades.

Dyes for Polyamide Fibers

With the exception of the aramid fibers, the polyamides dye readily with a wide variety of dyes. Since the polyamides contain both acid carboxyl ic and basic amino end groups and have a reasonably high moisture regain, the fibers tend to dye like protein fibers such as wool and silk. Since the molecular structure is somewhat more hydrophobic, more regular, and more densely packed in the polyamides than in protein fibers, they also exhibit to some degree the dyeing characteristics of other synthetic fibers such as polyesters and acrylics. Due to their highly regular molecular structure and dense chain packing, the aramid fibers resemble polyester and are dyed only by small dye molecules such as disperse dyes. Polyamides such as nylon 6, 6,6, and Qiana can be readily dyed with dyes containing anionic groups, such as acid, metallized acid, mordant dyes, and reactive dyes and with dyes containing cationic groups such as basic dyes. Acid dyes on nylon can be mordant effectively for additional fastness; however, the colorfastness of basic dyes is poorer and more difficult to stabilize by mordanting. Vat and azoic dyes can be applied to nylons by modified techniques, and polyamides can be readily dyed by disperse dyes at temperatures above 80°C. Aramids can only be dyed effectively with disperse dyes under rigorous dyeing conditions. The biconstituent fiber of nylon and polyester can be effectively dyed by several dye types due to the nylon component, but for deep dyeing’s disperse dyes are preferred. Nylon 6 and 6,6 are produced in modifications that are light, medium, or deep dye able by acid dyes or specially dye able by cationic dyes.

Dyes for Polyester Fibers

Owing to their high crystallinity and hydrophobicity, the polyester fibers are extremely difficult to dye by normal dyeing techniques unless the fiber has been modified, as in the case of modified terephthalate polyesters. A limited amount of polyester is solution dyed through incorporation of dye or pigment into the polymer melt prior to spinning. It is more common to use this technique to incorporate fluorescent brightening agents into polyester. Only smaller, relatively nonpolar dye molecules can effectively penetrate polyester; therefore disperse dyes have been the dye class of choice for the fiber.

Azoic dyes and pigment-binder systems have also found limited use on polyesters. Polyester modified with appropriate commoners can be dyed at lower temperatures or with acid or basic dyes depending on the nature of the modifying groups.

Dyes for Acrylic Fibers

The nature and distribution of acrylonitrile and commoner or monomers in the acrylic fibers affect the overall dye ability and the classes of dyes that may be used in dyeing these fibers. Both acrylic and mod acrylic fibers can be dyed using disperse dyes, with the more hydrophobic and less crystalline mod acrylic being more dye able with this dye class. The polar cyanide groups in the acrylonitrile unit of these fibers have some affinity for acid dyes and particularly mordant systems containing copper or chromium ions. Addition of an acid or basic commoner such as acrylic acid or vinyl pyridine as commoner imparts improved dye ability with basic and acid dyes, respectively, for these fibers. Vat dyes can be used on acrylic fibers to a limited extent.

Dyes for Polyolefin Fibers

Polyolefin fibers are hydrophobic, and the molecular chains within the fiber are tightly packed. Therefore it is extremely difficult to dye polyolefin fibers or to increase their affinity to dyes. Colored inorganic salts or stable organometallic pigments have been added to the polymer melt prior to fiber spinning to color the fibers. Also, nonvolatile acids or bases or materials such as polyethylene oxides or metal salts have been added to the polymer prior to fiber formation to increase the affinity of the fiber for disperse, cationic, acid, or mordant dyes. Polyolefin fibers can be chemically grafted with appropriate monomers after fiber formation to improve their dye ability.

Dyes for Vinyl Fibers

The vinyl fibers, with the exception of vinal and vinyon-vinal matrix fibers, are extremely hydrophobic and difficult to dye, and consequently they can be dyed only through pigmentation of the polymer melt before fiber formation or through dyeing with disperse dyes. Vinal and vinyon-vinal matrix fibers dye readily with dyes used on cellulosics including direct, mordant, reactive, vat, and sulfur dyes.

Dyes for Elastomeric Fibers

Since the elastomeric fibers are often a component in the core of blended yarns, coloration is not important in all applications. Rubber fibers cannot be dyed readily and are colored through mixing of pigments into the rubber prior to extrusion into fibers. Spandex fibers are more dye able and can be dyed with acid, reactive, basic, or vat dyes. Anidex can be dyed with disperse or basic dyes. The nylon component of spandex nylon fibers can readily be dyed with acid, basic, disperse, or vat dyes.

Dyes for Mineral and Metallic Fibers

The mineral and metallic fibers are essentially undyed able, and special techniques must be used to impart color to the fibers. Thermally stable ceramic pigments can be added to molten glass prior to fiber formation, or pigment-binder systems may be applied to the surface of the mineral and metallic fibers. Glass fibers can also be sized with a protein which then

can be insolubilized and dyed with conventional protein dyes. Glass fibers are colored by carbonizing, which involves preheating of the glass substrate to high temperatures to remove all organic materials followed by coloration with a pigment-binder system. The metallic fibers may also be colored through anodizing the metal (often aluminum) filament present or through pigmentation- of the plastic layer coating the metal. The nature of the metal in the organometallic fibers determines their ultimate color.