**3.1.2 Properties of polyester fibres**

#### **Physical properties**

Polyester fibres are very hydrophobic, therefore, they absorb only a very small amount of water and there is no significant change in their tensile properties when they are wetted. The moisture regain of polyester fibre is approximately 0.4% at 65% relative humidity and 20C (Moncriff, 1970). Polyester materials dry quickly because of their very low water absorption.

The tensile properties of polyester fibres vary with temperature; at 180C, the fibre retains approximately half the tenacity it possesses at room temperature and its extensibility increases (Nunn, 1979). Medium-tenacity yarns shrink by approximately 6% in boiling water but only by 3% in hot air at the same temperature; similar differences are found at higher temperatures. Polyester fibres will, however, take a permanent-set when distorted at high temperature (Moncriff, 1970)

Polyester fibres exhibit high initial moduli of elasticity, high resistance to blending deformations and good recovery from them, negligible creep under the low extensions to which the fibres are most commonly subjected in use and high resistance to abrasion [65].

Polyester can be exposed to sun light for 600 hours and the fibre strength will still be around 60-70% of its original strength (Moncriff, 1970).

#### **Chemical properties**

Polyester fibres show outstanding resistance to damage by most common chemicals under ordinary conditions of exposure and a wide range of substances have little or no effect on their strength.

Their resistance to oxidizing and reducing agents is excellent and, as a consequence, bleaching treatments using sodium chlorite, sodium hypochlorite or hydrogen peroxide may be employed. Concentrated formic acid, acetic and oxalic acids produce strength losses

dye attachment as well as the use of a copolymer that lowers the compact structure of polyester (Moncriff, 1970). To increase the aesthetic quality of textured yarns, high refractive index inorganic particles have been incorporated into fibres and silk-like polyester fibre have been developed (Brunnschweiler & Hearle, 1993). However, although these modifications enhanced the lustre of polyester, softness in handle was lacking. During the 1970's and 1980's, the 'touch' of polyester fibre was enhanced; alkali deweighting treatments were used to make the fibre more delicate and a wrinkle finish imparted it to an appearance similar to that of silk. In addition, there have been many other activities in fibre development, such as polymer modification, fibre blending, surface treatment, the mixture of various fibre cross sections, special spinneret design and fine denier spinning for

Polyester fibres are very hydrophobic, therefore, they absorb only a very small amount of water and there is no significant change in their tensile properties when they are wetted. The moisture regain of polyester fibre is approximately 0.4% at 65% relative humidity and 20C (Moncriff, 1970). Polyester materials dry quickly because of their very low water

The tensile properties of polyester fibres vary with temperature; at 180C, the fibre retains approximately half the tenacity it possesses at room temperature and its extensibility increases (Nunn, 1979). Medium-tenacity yarns shrink by approximately 6% in boiling water but only by 3% in hot air at the same temperature; similar differences are found at higher temperatures. Polyester fibres will, however, take a permanent-set when distorted at high

Polyester fibres exhibit high initial moduli of elasticity, high resistance to blending deformations and good recovery from them, negligible creep under the low extensions to which the fibres are most commonly subjected in use and high resistance to abrasion [65]. Polyester can be exposed to sun light for 600 hours and the fibre strength will still be around

Polyester fibres show outstanding resistance to damage by most common chemicals under ordinary conditions of exposure and a wide range of substances have little or no effect on

Their resistance to oxidizing and reducing agents is excellent and, as a consequence, bleaching treatments using sodium chlorite, sodium hypochlorite or hydrogen peroxide may be employed. Concentrated formic acid, acetic and oxalic acids produce strength losses

polyester and silk-like fibre developments (Brunnschweiler & Hearle, 1993).

Fig. 13. Modified polyester dyeable with cationic dyes.

**3.1.2 Properties of polyester fibres** 

**Physical properties** 

temperature (Moncriff, 1970)

**Chemical properties** 

their strength.

60-70% of its original strength (Moncriff, 1970).

absorption.

of 15%, 6% and 8%, respectively, after treatment at 80C for 72 hours, but dilute solutions of mineral acids are resisted, even at 100C.

Polyester fibres can be treated with dilute alkalis at temperatures up to 100C and can withstand the strongly alkaline conditions used in vat-dyeing and or in mercerizing. However, solutions of caustic alkalis do, in fact, attack and hydrolyze the polymer, but at temperatures up to the boil, such attack is confined to the surface of the fibre; this particular characteristic has been utilized in the production of silk-like polyester.

Polyester polymers display the typical reactions of ester and can be hydrolyzed in the presence of dilute alkali or acid or by water alone. No serious change can be expected to be observable in the textile-processing properties of fibres and yarns dyed for one to two hours at 130C, so long as the pH of the bath has been maintained close to 7. However, in an acidic bath of pH substantially less than 4 or in an alkaline bath, more rapid attack will occur Above pH 8, high-temperature dyebaths can induce serious degradation of polyester fibres if treatment is prolonged (Nunn, 1979).

#### **3.2 Acetate fibres**

Cellulose acetates are esters of cellulose in which a large fraction or even all the hydroxyl groups have been esterified using acetic anhydride. The two major types of cellulose acetate have about 55 and 62% of combined acetic acid. These values correspond to cellulose with degree of substitution of 2.48 and 3.00, respectively. The latter is called cellulose triacetate, and the former is called cellulose diacetate (Fig. 14) (Broadbent, 2001).

Fig. 14. Chemical structures of acetate fibres

Acetate fibres belong to the class of man-made cellulosic fibres and are manufactured by treating cellulose in the form of pure wood pulp or, less frequently, cotton linters, with a mixture of glacial acetic acid and acetic anhydride at low temperature in the presence of an activation catalyst such as sulphuric acid, perchloric acid, zinc chloride or similar salts (Rouette, 2000). Cooling prevents an increase in temperature of the mixture that will promote excessive hydrolysis of the cellulose. This initial product is cellulose triacetate (primary cellulose acetate) and cellulose diacetate (secondary cellulose acetate) is obtained directly from the triacetate by partial hydrolysis (Broadbent, 2001).

Cellulose diacetate, once widely known by its producer's company name, Celanese, can be written and drawn in a similar manner to cellulose, except that between 77-80% of the hydroxyl groups have been acetylated by reaction with acetic acid, to give cellulose acetate esters (Trotman, 1984).

#### **3.2.1 Historical background**

Acetate was the first hydrophobic man-made fibre, and when it appeared on the market, knowledge about the mechanism of dyeing and of molecular structure of fibres was limited. Because acetylation makes the fibre hydrophobic, resistant to swelling, and endows it with a

Dyeing with Disperse Dyes 209

dispersing agent molecules are inside the micelle which, as a consequence, is able to solubilise the disperse dye molecules, so conferring a higher apparent solubility on the dye. The dye transfer to the fibre from the micelles. As micelles empty their dye, they re-from

Much of the evidence that is available on the subject suggests that in dyed polyester fibres the disperse dyes are present chiefly in the monomolecular state [Schroeder & Boyd, 1957; Hoffman et al, 1968]. At the end of the dyeing process, the dye that has been absorbed by the fibre is in a state of dynamic equilibrium with the dye that remains in the bath, and the fraction of the latter that is in aqueous solution must be present in the same state of aggregation as the dye in the fibre. It is reasonable to infer that the transfer of the dye to the fibre takes place from a monomolecular aqueous solution, the concentration of which is maintained during the first phase of the dyeing process by the progressive dissolution of solid dye from the particles in dispersion in the bath. In the presence of dispersing agents

and dissolve more dye from the solid particles (Ingamells, 1993)

the following equilibrium situation is set up (Fig. 15) (Johnson, 1989).

The four stages of the process mechanism are as follows (Murray & Mortimer, 1971):

c. The solution in the dyebath is replenished by the dissolution of more solid material

The process of transfer from the aqueous solution to the fibre is comparable with the extraction of a solute from one solvent by a second, immiscible solvent and similar laws of partition are applicable. Distribution coefficients that are related to the solubilities of the dyes in the aqueous and fibre phases can be determined for different processing temperatures, although they may be affected by the simultaneous equilibrium between the aqueous and solid phases of the dye. The rates of the first and second stages of the process

It was established that the disperse dyeing system was truly reversible and that the results conformed to a rectilinear isotherm. Typical results show linear relationship in distribution of dye between polyester and water. It is well established that dyeing with disperse dyes is

b. Molecules of dye are transferred from solution to the surface of the fibre.

Fig. 15. Disperse dyeing mechanism

from the dispersion.

a. Some of the dyes dissolve in the water of the dyebath.

mechanism are governed by these solubilities.

d. The adsorbed dye diffuses monomolecularly into the fibre.

greater electronegative surface charge in water, there is no response to direct dyes. The absence of basic groups affords no sites of attachment for acid dyes, but the yarn does show some substantivity for basic dyes. In the early days, limited ranges of water-soluble dyes, selected from a variety of sources by trial and error, were placed on the market. In many cases both their fatness and exhaustion left much to be desired (Trotman, 1984)

Cellulose di- or triacetates have no ionic groups. They are quite hydrophobic fibres. When introduced in the 1920s, cellulose diacetate was initially difficult to dye satisfactorily with existing ionic dyes. Fine dispersion of simple, non-ionic azo and anthraquinone compounds, of limited water solubility, however, efficiently dyed this fibre. These so-called disperse dyes are slightly soluble in water and are extracted from the aqueous solution by the solid fibre in which the dyes are quite soluble (Broadbent, 2001).
