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Unit 25Dyeing of synthetic fibres

CNR – ISMAC Institute for Macromolecular Studies Biella www.bi.ismac.cnr.it

Politecnico of Turin www.polito.it

Course of Textile Fibres and Technology

Claudio Tonin [email protected] 2014/15

1sabato 13 giugno 2015

http://www.bi.ismac.cnr.it/




mailto:[email protected]

mailto:[email protected]

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Dyeing Polyamide fibres

The two important fibres to be considered are Nylon 6 and Nylon 6.6.Because of their similar chemical composition and molecular weights, many of their properties are alike.

They do not differ in basic dyeing mechanism but rather in some details which are of importance to the dyer; for example, nylon 6 dyes faster than 6.6 and has a lower glass transition point (Tg Nylon 6 = 40 °C; Tg Nylon 6.6 = 50 °C).

Differences in the dyeing behaviour are practically due to differences in the physical structures.

Nylon 6.6 is more crystalline and dye migrability (levelling) is more difficult than with Nylon 6.

This should be taken into consideration when selecting the type of dye to be applied:

- small molecules are more suitable for good levelling in dyeing Nylon 6.6;

- big molecules are suitable to obtain high fastness to wet treatments when dyeing Nylon 6;

- there are no differences in fastness to light between the two fibres dyed with the same dyestuff.


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Polyamide fibres contain both acid (-COOH) and basic (-NH2) functional groups.Therefore, both anionic (acid) and cationic (basic) dyes may be used for dyeing Nylon under appropriate process conditions. Moreover, also disperse dyes can be applied to Nylon.

Dye Tone Levelling Wet fastness

Acid (pH 4 - 5) Brilliant Good - Very good Medium - good

Wealky Acid - Neutral(pH 6 - 7) Brilliant Medium - low Good - very good

Metal complexes (premetallised) Dark - deep Medium - low Good - very good

Chrome Dark - deep Medium - low Very good

Reactive Brilliant Medium - low Very good

Disperse low Molecular Weight Brilliant Very good Medium - low

Disperse high Molecular Weight Brilliant Medium - good Medium - good

Direct Brilliant Medium - low Good

Cationic (basic) Medium Medium Medium - good


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Acid dyes are used to a considerable extent for dyeing Nylon, because of their higher wash and light fasness compared with disperse dyes.

Although the end groups are not made up of equivalent acid and basic groups due to the use of chain stoppers in the polymerisation (acetic or benzoic acid), as with wool adsorption of simple acids may be used as a basis for discussing the adsorption of the more complex dye molecules:

NH3+ NH3+

Ny + H3O+ Ny + H2O COO- COOH

Nylons have a limited number of amino terminals (0.05 meq/g for Nylon 6.6) with respect to wool (0.8 - 0.9 meq/g).

In the acid pH range the maximum quantity of acid taken up reaches a plateau which correspond approximately to the number of amino end groups in the polimer.


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Further reduction in pH (<2) leads to additional adsorption of acid, which is attributed to association with amino groups until the cleavage of the amide bonds with degradationof the fibre:


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NH3+ X- NH3+ Col-SO3-

Ny + Col-SO3- Na+ Ny + Na+ X-

COOH COOH

The mechanism of dyeing is similar to that of wool with acid dyes:

In general, the affinities are larger so that the pH conditions required for dyeing Nylon are correspondingly higher.

Acid dyes for Nylon are classified into two groups: 1) level dyeing nylon, applied at pH 4 - 5 with formic acid and 10% Glauber’s salt (sodium sulphate);

2) high affinity dyes (milling and super-milling dyes), applied at pH 6 - 7, exhausted with ammonium acetate.

Dyes are first adsorbed on the fibre surface, then penetrate into the fibre core where they bond with protonated amino groups (ionic bonds) and with secondary links (Van der Waals).

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1


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In general, dyeing is begun at 40 °C, the temperature gradually raised to 90 - 95 °C and dyeing continued at this temperature for 45 min.

For high affinity dyes, the levelling properties are poor so that the initial application must be as uniform as possible: this is usually achieved by control of temperature and acidity,i.e. by using dyeing conditions which favour slow and even exhaustion of dye by the fibre.

One difficulty which detracts from acid dyes is that the irregularities in the Nylon yarns tend to be emphasised, especially with high affinity dyes (larger molecules).

The irregularities may be intrinsic from the yarn itself but may well have been introduced by non-uniform tensions which may occur during weaving or by non-uniform heat setting.

A way to reduce the problem is dyeing irregular Nylons with disperse dyes, which are relatively insensitive to both chemical and physical changes, because of their very small molecular size.

As for wool, Nylon can also be dyed with reactive dyes, especially in blend with woolor cotton.


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Dyeing Acrylic fibres

Fibres made from 100% acrylonitrile are difficult to dye because of their compact structure.

To overcome this and to improve the solubilty in solvents so that the polymer may be wet or dry spun, copolymers are used. A wide range of comonomers are used, which may be vinyl acetate, acrylic acid, allyl-sulfuric acid,....(CH2=CH-CH2-SO3H).

The groups introduced may be basic, e.g. 2-vinyl pyridine, when the fibre may be dyedwith acid dyes under rather acid conditions.

Normally, the polymerisation of acrylonitrile and its copolymers is carried out using redoxinitiators (chain growth polymerisation), the residues of which, namely SO3H or SO4 are attached to the chain end.

The polymer itself therefore carries acidic groups (anionic groups) capable of acting as sites for cationic dyes (basic dyes).


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The capacity for these dyes is enhanced if the comonomer itself carries an acidic group,i.e. acrilic acid: -CH2-CH-

COOH

Because basic dyes are readly adsorbed by acrylic fibres and the dyed material possessesgood light fastness, interest in dyeing acrylic and modacrylic fibres with basic dyes isconsiderable.


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Basic or cationic dyes are colored cationic salts (usually chlorides, but sometimes oxalates) of amine derivatives. They were among the earliest colouring to be manufactured, especially for cotton treated with tannins as mordants.They remain of great interest because of their very bright shades and high light fastness and are the most widely used for acrylic fibres. Basic dyes fall into roughly five chemical groups, as derivatives of:

Azines

Thiazines

Oxazines

AnthraquinoneTriphenyl methane


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Acrylic fibres possess glass transition temperature in the region of 70 - 80 °C. Dyeing below this temperature is extremely slow but, once exceeded, the rate of sorption becomes rapid. The importance of temperature is shown in figure, when the dye uptake shows a marked increase over a relative narrow temperature range.A typical procedure, the dissolved dye is added to the bath at 60 °C set with with acetic acid and sodium acetate to pH 5. The temperature is quickly raised to 80 °C and then slowly brought to the boil, say at 1°C/3-5 min and dyeing continues for 2 h.The liquor is cooled slowly to 50 °C, the goods are then rinsed and added with cationic softeners.

The dye adsorption is influenced alsoby the pH of the liquor because of the dissociations equilibrium:

Acr-H Acr - + H+

Acr-H + Dye+ Acr-Dye + H+

Dyeing is carried out with the addition of acetic acid and sodium acetate orsulphate, to ensure that the pH is slightly acid in order to avoid the possibility of decomposition of the dye.

curve A: pH = 4

curve B: pH = 5.5


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Unlevelness may result from uneven temperature gradients, especially for pale shades when the carefull control of temperature is strongly needed.

In fact, the affinity of basic dyes for acrylic fibre is high and the dye strikes rapidly onto the fibre: levelling cannot therefore be achieved by migration

Bath exhaustion curves for various depths of shade. The figures on the curves indicate the percentage of Basacryl Blue GL.

Note that the absolute amount of dye adsorbed doesn’t depend from the dye concentration in the dye liquor.


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Bath exhaustion curves for 0.8 % Basacryl Blue GL with different amount of retarder Basacryl Salt G

Levelling is also improved by the addition of retarding agents, cationic retarders,which are quaternised long chain amines, that diffuse at speeds at least equal to those of the dyes. By occupying first some of the fibre sites, they effectively reduce the rate of dyeing, the retarder gradually being replaced by the dye.In a similar way can be explained the moderate retarding action of the Na+ ions from the salts (sodium sulphate or acetate), in competition with the cationic dye.


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Because the number of anionic sites of the acrylic fibres is limited to some extent, special attention must be paid to:

- the saturation factors of the fibre;- the saturation power of the dye;- the saturation power of the cationic retarder.

Saturation factors are given for commercial acrylic fibres and related dyes, in order not to exceed the total amount the fibre can uptake.

Dyebath preparation should be done by selecting dyes with comparable “dyeing speed”,in order to avoid changes of hue: dyeing speed of commercial basic dyes is given in an arbitrary scale from 1 to 5 (1= max dyeing speed).

In addition, care must be taken since the material is very easily deformed at the dyeing temperature and stretching and flattening of the cloths may occur.

Acrylic fibres are often dyed also with disperse dyes.


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Dyeing Polyester fibres

The hydrofobic Polyester fibre has a very compact structure, it is highly crystalline and doesn’t contain functional groups available for bonding with dyes.

In addition, the stiffness imparted to the chains by the phenyl residues of the terephthalate group results in a high glass transition temperature.

Hence, in order to dye at a suitable rate, the temperature must be as high as possible commensurate with the equipment available.

The choice of dyes for this fibre is limited, being confined to the disperse dye range.

Polyester can be dyed as loose stock, yarn or in the fabric form.

Pale shades may be obtained at the boil, but for deeper ones dyeing must be carried outat temperatures up to 130 °C.

The need of pressure dye equipments represents a plant complication and an increase of the process costs.

For the actual dyeing operation, prior scouring is desirable.


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Disperse dyes are small coloured molecules belonging mostly to the following chemical systems:- nitrodiphenylamine, providing yellows and oranges (1);- azo, for a wide range of colours (1, 2);- anthraquinone [derivatives of 3-methoxy-benzanthrone (3) and α-amino anthraquinone (4)] providing orange to greenish blue colours.

1 2

3 4

These dyes carry no charged groups although they contain rather polar substituents such as -OH, -CH2CH2OH, NO2, etc. and their molecules are relatively small (~ 300 Dalton) to give them a small solubility.


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The suggestion that the dyes may be “mixed” with or dissolved in the polymer chains leads to the idea that the location of the dye is not specific.

The role played by the dispersing agent is more that to ensure the stability of the dyebath.The dispersing agent is used over the Critical Micellar Concentration; its chemical composition is affine to the dye which dissolves into the micelles. Thus, the dyebath contains dye in three states, namely a small amount of dissolved dye, dye into the micelles and solid dye, a situation which could be written as being an equilibrium:

Dyesolid Dyemicelle Dyedissolved

Removal of the truly dissolved dye by effect of dyeing, will result in more dye being solubilised.The dye in micelle therefore may be considered to act as a reservoir.

The dye comes into contact with the fibre surface and dissolves into the fibre itself, because of the physical affinity and small molecular size (disperse dyes are also called “plasto-soluble”).

The dyebath can be made up with dispersing agents and dyeing commences at 60 - 70 °C raising the temperature to 120 - 130 °C when dyeing is carried out for 30 - 45 min.


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Structural differencies on the fibres may be the main cause or drawback in polyester dyeing.Heat treatments (drawing, setting) modify the microstructure of polyester fibres; in general, continuous-filament woven fabrics are thermo-set at 170 - 190 °C and warp-knitted fabrics at 200 - 230 °C.Exposure to such temperatures causes further crystalisation of the polymer with subsequent modification of the domains accesible to dye.

The dye uptake decreases as the heat-setting temperature increases, reaching a minimum at 180 °C after which the uptake increases, with increasing the amorphous volume.


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When pressure vessels are not available, an alternative to using high temperatures (ca. 130 °C) for speeding up the rate of dyeing is by the addition of carriers which reduce the glass transition temperature of the fibre.

These substances are rapidly adsorbed by the fibre and are likely to form a film onthe surface of the fibre.

Since disperse dyes dissolve in the carrier, the dyeing rate is increased by the high local concentration of dye in this film.

The word carriers was coined because they “carry” the dye molecules from the bath, where the dye is dispersed, into the fibre.

carrier fibredisperse dye film


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For polyester, the most popular carriers are o-phenylphenol, diphenyl or chlorinated benzenes; among the range of compounds suggested, some are water soluble (phenol or benzoic acid) whereas others are insoluble (diphenyl) and are used as emulsions.

Different carriers augment the rate of uptaketo different extents, the water soluble onesrequiring to be used in larger concentration.

Carriers in general suffer from the disavantage that they are toxic (suspected carcinogenic) and may remain in the fabric even after washing. Being comparative small molecules, residual carrier may volatilise during drying.

Their use is subjected to concern for the environment and thus progressively declining for the more environment friendly “pressure dyeing”.


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Finally, research is in progress on the application of supercritical CO2 as a solvent for general purposes (dyeing or dry cleaning) in the textile field because of its strong solvation properties. Experiments with polyester have shown applicability on industrial scale.

Continuos dyeing polyester with disperse dyes is also carried out with the “Thermosol” process that involves sublimation of the dye under heat and partial vacuum into fibres.Polyester containing solid disperse dye applied to the fibre surface is heated near 200 °Cunder partial vacuum for a short period of time.

At this temperature the molecular motion within the polyester is high, permitting the dye vapor to penetrate into the fibre and the dye to be dissolved.On cooling, the disperse dye is permanently trapped within the fibre, with good levellig andperformances on fastness.

Thermosol is recommended for delicate textiles.


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