Wool - Chemical & dyeing properties

 

CHEMICAL REACTIVITY OF WOOL
Wool, in common with many other proteins, will react with a large range of chemicals. Wool contains three main types of reactive group: peptide bonds, the side-chains of amino acid residues and disulphide crosslinks. The chemical reactions involving these groups have been studied extensively and discussed
in various textbooks.

The highly reactive nature of wool has enabled many industrial treatments to be developed, particularly in the areas of shrinkproofing, dyeing, bleaching, flame-resistance treatment and finishing.

ROLE OF FIBRE STRUCTURE IN WOOL DYEING
Mechanism of wool dyeing
When a textile substrate is dyed by an exhaustion method, the dyeing operation proceeds in three stages

1 diffusion of dye through the aqueous dyebath to the fibre surface
2 transfer of dye across the fibre surface
3 diffusion of dye from the surface throughout the whole fibre.

The fibre surface as a barrier to dyeing
In order to obtain satisfactory shade development and fastness properties, complete penetration of dye into the fibre interior is essential. The rate at which this occurs is controlled by the rate of dye diffusion across the fibre surface and then throughout the whole interior.


The above finding supports the view that the cuticle, probably the highly crosslinked A-layer of the exocuticle , is a barrier to dye penetration, in that dyes are directed to the gaps between the scales in order to reach the cortex. It appears, however, that lipids present at the intercellular junctions are also a barrier to the diffusion of dyes into the nonkeratinous regions of the cell membrane complex.

The intercellular mode of dye penetration applies to unmodified wool. Different dyeing behaviour may be shown by fibres that have been substantially chemically or physically altered, for example by reduction of the A-layer of the exocuticle, severe surface abrasion or complete removal of the cuticle.

Diffusion of dye in the cortex
After initial penetration into wool fibres, dyes must diffuse throughout the entire cross-section in order to obtain optimum colour yield and fastness properties. Several workers have suggested that the continuous network of the cell membrane complex provides a pathway for the diffusion of reagents into wool. The vapours of organic solvents, the salts of zirconium and titanium and of chromium, and also phosphotungstic acid, all appear to penetrate the fibre by this route. It has been found that the cell membrane complex swells in formic acid to a much greater extent than does the whole fibre. They suggested that this disproportionately
high swelling is the reason why dye is taken up very rapidly from concentrated formic acid.

As the dyeing cycle proceeds, dye progressively transfers from the nonkeratinous regions into the sulphur-rich proteins of the matrix that surrounds the microfibrils within each cortical cell. Dye also transfers from
the endocuticle into the exocuticle, particularly the A-layer. It appears that the hydrophobic proteins located in these regions have a higher affinity for wool dyes than the nonkeratinous regions. At the end of the dyeing process the nonkeratinous regions, which were important in the early stages of the dyeing cycle, are virtually devoid of dye.

For nonreactive dyes, thermodynamic equilibrium with wool is not established until the process of dye
transfer into the keratinous regions is complete. This stage, which is not usually achieved until some time after the dyebath is exhausted, is the reason why a prolonged time at an elevated temperature is required to produce satisfactorily dyed wool. If dye remains largely in the nonkeratinous regions, rapid diffusion out of the fibre can occur and, hence, poor wet fastness properties are obtained.

Reactive dyes, however, may show a somewhat different equilibrium distribution between the nonkeratinous and keratinous regions of wool. Reactive dyes are capable of covalent bond formation with the proteins of the nonkeratinous regions, and therefore at equilibrium these dyes may be present in the cell membrane complex and endocuticle to a greater extent than their nonreactive analogues.