DYES APPLIED TO DIFFERENT TYPES OF FIBER PART -1

 

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 cellulosics (rayon) dye more readily than the more crystalline natural cellulosic fibers such as cotton. Cellulosics can be dyed or printed using direct, reactive, basic, vat, sulfur, or azoicdyes. Cellulosics can also be colored using pigment-binder systems. Mordanting of direct and basic dyes on cellulosics improves overall fastness. Since the hydroxyl groups in cellulosics 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, dyeings 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 numerousreactive 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.

Under acid conditions amino groups in the protein fibers are protonated to form NH; groups. In this form, they are able to attract dyes containing acid anions including acid, direct, mordant, and reactive dyes. Special premetallized acid dyes of sufficient solubility are used to dye protein fibers to fast colors. The functional groups available in protein fibers are more reactive than hydroxyl groups in cellulosic fibers. Reactive dyes of more 1imited reactivity have been developed especially for protein fibers. Protein fibers complex very readily with multivalent metal cations. Acid dyes and mordant dyes may be rendered very fast by mordanting with metal salts, and chromium salts are especially effective as mordants.

At neutral or sl ightly acid pH, protein fibers may be dyed with cationic or basic dyes; however, the fastness of the dyed fiber is poor without mordanting with tannic acid or other mordants for cationic dyes.Azoic and vat dyes find only limited use on protein fibers due to the damaging effect that basic solutions of these dyes have on protein fibers. Sulfate esters of reduced vat dyes can be used effectively on protein fibers, however. Natural dyes from many sources have good affinity for protein fibers and are used extensively in the crafts area and often in conjunction with mordants.

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 carboxylic 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 mordanted 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 dyeings disperse dyes are preferred. Nylon 6 and 6,6 are produced in modifications that are light, medium, or deep dyeable by acid dyes or specially dyeable 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 comonomers can be dyed at lower temperatures or with acid or basic dyes depending on the nature of the modifying groups.