Silk Fibre Dyeing

Dyeing of silk fiber

 Like wool, silk is a protein fibre and can therefore be dyed using the same methods as for wool dyeing (Õ Dyeing of wool). As silk is considerably more resistant to alkali than wool, dyeing processes which are carried out under weakly alkaline conditions are also used. In this regard, it must also be borne in mind that under alkaline conditions, high temperatures and conditions of high mechanical stress at the same time, individual fibres (fibrils) can split away from the silk filaments giving the material a fluffy and rough appearance. Compared to wool, silk also has a distinctly lower affinity because of its weaker acid binding capacity. Additional factors influencing the affinity of silk are its origin, extent of degumming, the yarn quality (tram silk, organize) and whether the silk has been weighted or not. In general, it can be said that under the same dyeing machine conditions, level dyeing are less problematic to produce on silk than on wool. The dyeing temperature should not exceed a maximum of 80°C since temperatures at the boil have a negative influence on the luster and handle of silk.

Silk is dyed in all its commercial forms but is best dyed in the form of hanks (yarn) and piece goods. In principle, all the well-known types of dyeing machine can be used if they have been suitably modified to take into account the sensitivity of silk to tension and mechanical stress.

The most important types of dyeing machine for silk fibre dyeing include:

Hank dyeing machines: no hank deformation (with double stick systems), no filament splitting, level dyeings obtainable provided the rate of liquor circulation is adequate;

Spray type dyeing machines for hank yarn: good liquor circulation and therefore hardly any leveling problems, hank deformation and filament splitting is possible; particularly suitable for high-twist and folded yarns;

Package dyeing machines for cross-wound packages: short liquor ratio, no levelling problems with adequate liquor circulation, no filament splitting, uniform winding of packages important because of the risk of channeling;

Star frame dyeing machine: suitable for all types of woven silk fabrics, no chafe marks, low productivity, high energy and water consumption (liquor ratio 100–300:1);

Winch dyeing machine: good levelness is possible, high productivity, average liquor ratio, high risk of chafe marks,

Jigger dyeing machine: high productivity possible, extremely short Dyeing of polyester/polyester copolymer liquor ratio, risk of moiré formation, good leveling resp. slow-exhausting dyes are an advantage here;

Jet-dyeing machines: good levelness, high productivity, short to medium liquor ratio, especially suitable for jersey and crêpe fabrics (due to the virtual absence of fabric tension), possibility of chafe marks;

Beam dyeing machines: good to excellent levelness, short liquor ratio 10–20 : 1, no chafe marks, especially suitable for heavy and smooth silk qualities, risk of moiré formation.

Tab.: Fields of application, advantages and disadvantages and fastness properties of various dye classes suitable for dyeing silk.

Practically all classes of dye can be used for colouring silk (see Table). There is hardly any other textile fibre for which this is possible. Nowadays, silk fibre is mainly dyed with direct, acid, metal-complex or reactive dyes.

 Metal-complex and acid dyes for silk: from the dye classes listed in the Table the 1: 2 metal-complex and acid dyes are the most important in practice. Apart from a few exceptions in the acid dye range, they can all be applied from a weakly acidic bath (Fig. 1) and exhibit good levelling properties. By careful selection, all fashion shades, including those specific to silk, can be dyed although, from time to time, the colour fastness has to take second place. Thus, for the brilliant shades, red, blue, turquoise and green, often only moderate light fastness ratings and, in deep shades, only moderate wet fastness ratings are achieved (e.g. colour fastness to water severe, alkaline perspiration and washing at 40°C).

Fig. 1: Method and dyeing recipes for dyeing silk with metal complex and acid dyes (Ciba-Geigy). - - pale shades;  dark shades

Reactive dyes for silk: These are being used to an increasing extent (Fig. 2). They are characterised by their brilliance, high degree of fixation, high light fastness and outstanding wet fastness properties. In general, they are not very substantive although differences exist between the individual types. Consequently, the aim should be to use the shortest possible liquor ratios for these dyes. Dyeing is carried out first of all in a neutral bath with the addition of sodium sulphate at tempera tures of 60–70°C, with the exception of black dyeing which are dyed at 90°C. Only in the last 15–20 min. at the end temperature is an addition of sodium carbonate made to adjust the pH of the dyebath to 8–8.5 for dye fixation. After rinsing, the dyeing must still be soaped in order to remove the unfixed dye which is not covalently bound to the substrate (source: Flensberg and Hammers).

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