In addition to the effects of the molecular structure of the dye and the specific fibre type as individual elements, the mechanisms of interaction between these two factors are fundamental in ascertaining the resistance of dyed or printed textile colour to different external agents. Resistance to change or loss of colour is a consequence of the particular relationship formed between the dye and the fibre during the process of dyeing or printing. In order to analyse the effect of different fibre–dye interactions (Cegarra et al., 1992; Peters, 1975) on the colour fastness properties of the resultant coloured textiles, the key groups of fibres and dyes have been considered and the ways in which they can be manipulated to regulate the colour fastness properties of textiles are presented in Table 5.3.
5.3.1 Cellulosic fibres and their dyes
Cellulosic fibres are hydrophilic and possess hydroxyl groups (–OH). They are therefore capable of creating interactions or bonds with specific families of dyes, including direct, azoic, sulphur, vat and reactive dyes.
Table 5.3Dye–fibre interactions and mechanism related to colour fastness properties FibreDyeDye/fibre mechanismDye/fibre InteractionsFastness properties CellulosicDirectDiffuse sorptionVan der Waals and hydrogenLimited fastness to wet treatment bonds Azoic, sulphurDiffuse sorption and subsequentDye particles insoluble in fibreVery good fastness to washing and vatinsolubilization in fibreInsoluble dye adheredLimited fastness to rubbing superficially ReactiveDiffuse sorption and subsequentCovalent bondsGood fastness to washing and reaction of dye with celluloserubbing Limited fastness to bleaching Protein andAcid andIonization of the amino and/orIonic bonds between NH3+ (fibreFastness to wet treatments polyamidepre-metallizedacid groups and sorption of dyeat acid pH) and SO3– (dye)depends on solubility of the acid ions at specific sitesdye AcrylicCationicIonic exchange mechanism atIonic bondsGood fastness to washing and temperature above glassgood light fastness below glass transitiontransition temperature of the fibre PolyesterDisperseSolution of solid in solidNon-polar Van der Waals andGood fastness properties to hydrogen bondswashing and light Moderate fastness to atmospheric contaminants Good protection by the fibre below glass transition temperature
Colour fastness 89 (i) Cellulosic fibres and direct dyes: The dyeing of cellulosic fibres with water soluble direct dyes is controlled by a diffuse sorption mechanism, with electrical effects and the substantivity of the dye toward the fibre playing an essential role in the dye absorption. The interactions between cellulose and direct dyes are mainly based on non-polar Van der Waals forces and hydro- gen bonds. As a result of these interactions, direct dyes on cellulosic fibres show poor fastness in opposition to wet agents (washing or perspiration, for example). It is difficult to obtain dyed textiles with wash fastness ratings above 4 in the Grey Scale (Section 5.4.3). To overcome this issue, subsequent chemical treatments, based mainly on the use of metal salts or cationic active agents, are applied to improve fastness (Sharif et al., 2007).
(ii) Cellulosic fibres and insolubilized dyes (azoic, vat, and sulphur): This class of dyes are insoluble in water. For this reason, the dyes should be initially absorbed by the fibre in a solubilized state, following a mechanism similar to substantive dyes. After this process, the dyes are insolubilized on the fibre by a coupling reaction (for azoic dyes) or oxidation (for vat and sulphur dyes).
Azoic dyes usually have very good to excellent fastness to washing.
However, their rubbing fastness is limited due to the residual presence of insoluble azoic dye, in the form of pigment adhering to the surface. The rubbing fastness can be significantly improved by elimination of the excess of naftol, and by soaping the textiles after the coupling reaction step. In contrast, vat dyes are usually used for deep and dark colours, giving very good fastness to washing. Their fastness to both scouring under pressure and bleach are extremely useful properties. Regarding their fastness to rubbing, vat and sulphur dyes behave similarly to azoic dyes, showing limited ratings due to the presence of insoluble dye on the surface of the fibres.
(iii) Cellulosic fibres and reactive dyes: Reactive dyes are fixed to cellulose chemically by covalent bonds, and are both anionic and soluble in water.
Consequently their sorption by the fibre is based on the same mechanism as that proposed for direct dyes with little affinity. After the sorption step is completed, the subsequent reaction between the dye and the cellulose hy- droxyl groups starts under alkaline conditions. Chemical covalent bonds formed between fibre and reactive dyes are strong, and as a consequence good washing fastness is observed even after boiling. However, whilst these textiles also exhibit adequate rubbing fastness, their resistance to bleaching treatments is poor as the limited stability of the bonds leads to the production of acid and alkali hydrolysis.
5.3.2 Protein fibres, polyamide fibres, and acid and pre- metallized dyes
Protein fibres, such as wool and silk, have an amphoteric nature due to the presence of amine (–NH2) and carboxylic (–COOH) groups. Thus, when the fibre is
90 Understanding and improving the durability of textiles
subjected to acidic conditions, the –NH2 is converted to –NH3+, increasing the possibility of a reaction with the sulphonic (–SO3–) or carboxylic (–COO–) groups present in anionic acid dyes.
(i) Wool fibres and acid dyes: The classification of wool coloured using acid dyes is based on the levelling properties of the dye (levelling, milling and super-milling), the pH application methods (using sulphuric or formic acids, acetic acid, or ammonium acetate, respectively) and the wet fastness after dyeing. Wet fastness properties are related to the solubility and affinity of the different dye elements. For example, greater solubility (levelling dyes with low affinity) corresponds to greater levelling but poorer fastness to wet treatments, and vice-versa.
(ii) Silk fibres and acid dyes: In contrast, dyeing silk with acid dyes results in good resistance to light and washing. Fastness properties of such silk are usually much better than those exhibited by acid dyed wool.
(iii) Polyamide, and acid and pre-metallized dyes: In this case two large groups of dyes can be considered, separated according to the dyeing experience and fastness properties they produce. The first group include dyes with good affinity in weak acid media (pH 4.5–5.5), which give satisfactory wet fastness for pale shades. The second group includes acid and/or pre-metallized dyes with good affinity in neutral liquor (pH 6–7) which produce better wet fastness than that exhibited by dyes of the first group. In addition, the light fastness of non-metallized acid dyes is usually fairly good.
5.3.3 Acrylic fibres and cationic dyes
The presence of sulphate and sulphonate anionic end groups, resulting from the use of initiators or co-monomers during polymerization, permits the dyeing of acrylic fibres with cationic dyes. The dyeing process occurs by means of an ionic exchange mechanism, where the fibre fixes the dye cations by replacing the hydrogen and sodium cations of the sulphate and/or carboxylic groups. This exchange mechanism is only possible by plasticization of the fibre, which occurs at temperatures above the glass transition level. Above this level, good fastnesses to washing and fairly good light fastness properties are usually produced. The protection mechanism of the acrylic fibre for treatments below glass transition temperature adds to the improved chromophoric structure of the new cationic dyes, as seen when they are compared with old basic (natural in most cases) dyes with electron resonant structures (e.g. triphenyl methane derivates).
5.3.4 Polyester fibres and disperse dyes
Hydrophobic fibres (including polyester, cellulose diacetate and triacetate, polya- mide and acrylic) can be dyed with non ionic disperse dyes, through a mechanism
Colour fastness 91 based on a solution of a solid in a solid. Polyester in particular has a compact crystalline structure, meaning that to dye it requires temperatures above 100 ºC (above glass transition temperature). Alternatively, auxiliary carriers can be used to produce pore dilatation or fibre plasticization, allowing the dye to penetrate into the fibre. Disperse dyes on polyester fibres usually exhibit good resistance to washing and light, and moderate fastness to atmospheric contaminants (nitrogen oxides or ozone, for example). As was highlighted in the discussion of acrylic fibres, good protection by the synthetic fibre is observed in treatments performed below the glass transition temperature.