Additional information: Dyeing of wool

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1. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT


Wool can be dyed with the following dyestuffs:

  • acid (metal-free)
  • chrome
  • 1:1 and 1:2 metal complex
  • reactive


Acid dyes

Acid dyes are typically applied in acidic conditions, but the pH range used varies depending on the type of acid dye. The greater the affinity of the dyestuff for the fibre, the more the hydrophobic interaction must be repressed by applying the dye at higher pH.

Therefore level-dyeing colourants are applied under strongly acidic conditions (1-3% formic acid) in the presence of sulphate ions (5-10% sodium sulphate) to assist migration and levelling. HCOOH and HSO4- compete with the sulphonated dyes acting as levelling agents. As a result, other levelling agents are not generally required and the rate of dye uptake is controlled initially by increasing the temperature of the dye bath slowly (1°C/min) and then extending the time at the boil to allow the dye to migrate from areas of high initial uptake.

Fast acid dyes (also known as half-milling dyes or perspiration-fast dyes) exhibit superior fastness properties to level-dyeing acid dyes, while retaining some of the migration properties. Application is from a moderately acidic dye bath (1-3% acetic acid) in the presence of sodium sulphate (5-10%) and levelling agent to assist migration.

Acid milling dyes (including 1:2 metal-complex dyes mentioned later) have good affinity for the fibre and do not migrate well at the boil. They are therefore applied at more neutral pH (5-7,5 with acetic acid) in the presence of sodium acetate (2g/l) or ammonium sulphate (4%) and levelling agent (1-2%). Sodium sulphate is usually avoided as it has little effect on migration and can promote uneven dye adsorption.

Levelling agents play an important role in acid dyeing. A number of non-ionic, cationic, anionic and amphoteric surfactants belong to this category.


Chrome dyes

A number of techniques have been developed for the application of chrome dyes. The Chrome mordant process relies on chroming the fibre prior to dyeing with a chromable dye; the Metachrome process applies both dye and chromium slat simultaneously. Both processes have been largely superseded by the Afterchrome process in which the dye is applied first and the fibre is then chromed in a separate step, utilising the exhausted dye bath and thus conversing water. Application of the dyestuff takes place from a moderately acid bath (1% acetic acid) often with an addition of formic acid towards the end of the boiling period to promote exhaustion of the dye. The dye bath is then cooled back from the boil to approx. 80°C, the pH is lowered to approx. 3,5 with formic acid and the pre-dissolved chromium slat added. The dye bath is then returned to the boil and boiling continued for 20 – 30 minutes. Chromium is added to the dye bath as either sodium or potassium dichromate. In solution the chroming species present vary according to pH, with the dichromate anion Cr2O72- predominating at pH between 3 and 7. Although it is the chromium (VI) anion which is initially adsorbed by the wool, the dye complex is formed with chromium (III), formed by the action of reducing groups in the fibre itself. Strong acids have an activating effect on this process; organic acids (tartaric, lactic, formic acid) or thiosulphate can also be used to enhance the degree of conversion of Cr VI.


Metal-complex dyes

Application of 1:1 metal-complex dyes is carried out at pH 1,8 – 2,5 with sulphuric acid or at pH 3-4 with formic acid, in the presence of sodium sulphate (5-10%) and other organic levelling agents. Owing to these particular operative conditions, this class of dyestuffs is particularly suitable for piece-dyeing of carbonised wool.

1:2 metal complex dyes form the most important group in this class and may be divided into two sub-groups: weakly polar and strongly polar 1:2 complexes. application is usually carried out in moderately acidic conditions: - pH range 4-7 (acetic acid) in the presence of ammonium acetate, for weakly polar complexes - pH range 5-6 (acetic acid) in the presence of ammonium sulphate, for strongly polar complexes

The use if levelling agents is very common when dyeing with metal-complex dyes. The groups of substances used for acid dyes also apply in this case.


Reactive dyes

Reactive dyes are generally applied at pH values of between 5 and 6, depending on the depth of shade, in the presence of ammonium sulphate and specialised levelling agents (amphoteric substances, which form complexes with the dye at low dye bath temperatures and then break down as the dyeing temperature increases).

The dyeing method may include a temperature hold step at 65-70°C, during which the dye bath is maintained at this temperature for 30 minutes in order to allow the dye to migrate without fibre reaction. When dyeing of the very highest fastness is required, the fibre is rinsed under alkali conditions (pH 8-9 with ammonia) to remove unreacted dyestuff.


Dyestuffs and dyeing techniques.jpg


2. NEW TECHNOLOGIES


a) Changes in the process


1.) Use of liposomes as auxiliaries in wool dyeing

The use of liposomes as auxiliary products in wool dyeing with acid dyestuffs allows good dye bath exhaustion at 80°C and in 40 min. The advantages are:

- lower superficial damage of the wool fibre (due to the lower operating temperature the hand-feel of the fabric is softer)

- energy saving

- no electrolyte use

- lower COD load in the waste water

With wool/polyester mixtures, in order to allow the diffusion of disperse dye into the polyester fibre, it is necessary to operate at higher temperatures (100°C), and to add a low concentration of carriers. Liposomes have the effect of increasing the diffusion of disperse dyestuffs into the wool fibre (see BAT in textile industry, 2003 – Section 2.7.7 – “Polyester-wool blends”). It is therefore important to carry out selection essays of the suitable disperse dyestuffs in order to avoid negative effects on the fastness properties of the dyed product.


Operational data

Wool dyeing with acid dyes in the presence of liposomes is carried out at 80°C for 40 min, in a bath containing:

  • liposome 0.1 – 0.2 % o.f.w.
  • formic acid
  • acid dyestuff


Source: BAT in textile industry, 2003


b) Changes in the heat supply system

No information is available.

c) Changes in the energy distribution system

No information is available.


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