The changes in modern leather processing to meet environmental regulations and higher demands in the performance properties of leather result in a need for better processes and products used in leather manufacture. One area in need of improved performance is the poor wetfastness of deep colours, such as jet-blacks. We all know the staining of socks in leather shoes with dark linings and the poor washfastness of deep black clothing leathers.

Through an innovative new system, the research and development team at TFL have achieved exceptionally high wet-fastness levels previously unattainable on deep black shades. The new Sella HiFA system opens a whole new way of processing leather that was previously not thought possible and has been found to offer, along with the outstanding wetfastness, a chance for the tanner to make innovative leather articles.

A patent application has been filed for this process and the resulting leather manufactured by this process.

High wetfastness for deep shades

The basis of the new Sella HiFA process is to change the nature of the bond between the dye and the leather. To explain it clearly, we go back to examine the typical everyday situation for drum- dyed leathers.

The colouration of leather in the drum application is normally made with anionic dyes and the dye is fixed in the leather matrix through an ionic attraction between the anionic dye and the positively charged leather in an acid medium. This attractive force is often satisfactory but has extreme difficulty in reaching very high fastness values since this ionic bond is reversible. For example, the ionic affinity is easily broken when the pH is raised, as in the case during perspiration fastness testing.

No more stained socks!

An example we all know is the staining of socks in leather shoes, where the dyed leather under the influence of slightly alkaline perspiration reduces the ionic attraction and the dye is free to migrate to adjacent substrates, eg your socks!

In the case of textiles, high wetfastness is often achieved by the use of reactive dyes, which contain a reactive component within the dye molecule. This group can under certain application conditions react to form a stable covalent bond between the dye and the textile fibre. However, the textile application conditions to achieve an optimal reaction such as high temperatures, pH and salt concentrations are typically much too extreme to consider for use with leather.

It’s possible with Sella HiFA

At TFL, they have solved this problem of forming a strong bond between the dye and leather in a completely new way. Rather than using reactive dyes, they investigated the possibility of making the other component, namely the leather substrate, reactive. So when special dyes are added, they can react with the prepared reactive leather to form a strong covalent bond with the leather. Some very high performing new leather articles with deep black colours and very high wetfastness have resulted.

On deep black drum-dyed leathers, perspiration fastness values at or above grey scale 4 have been reached for HiFA treated leathers compared with values often as low as 1 or 2 on traditional leathers with a similar depth of colour. It is easy to understand the reason for the normal poor fastness values when you consider that in the normal procedure, leather is dyed in several steps. Further, to achieve deep shades, often at least one top-up dyeing is made at the end. This leaves a lot of weakly fixed dye at the surface of the leather where it is responsible for poor wetfastness and low rubfastness values.

Technology with an impact

The Sella HiFA process fits readily into the normal tannery procedures and the leather article can be completed in similar or even shorter times compared with traditional processes. Since the dye is chemically bonded to the leather, the dyeing process can take place directly on wet-blue and the typical retanning and fatliquoring steps can follow. This will be a completely new philosophy of working for most tanners but it is assured that the intensity of the dye is retained throughout, such that no additional dyeing steps are needed. That is, the whole dyeing process occurs at the beginning directly on the wet-blue. The high dye wetfastness brings additional benefits, for example in the handling of the wet dyed leather, there is no transfer of dye to machines or movement of the dye when horsed-up.

Interestingly, other benefits from changes in the leather character offer additional possibilities for extending the range of new articles that can be prepared. It is still too early to have evaluated all options but one clear benefit – the firm grain – offers considerable chances to make articles that previously have not been possible. Additional upgrading possibilities expand the range of hides and skins that can be chosen.

The whole Sella HiFA process operates in a way that previously has been rejected as impossible by many tanning experts. The TFL team of researchers, along with their experienced technical colleagues, have challenged the traditional theories of dyeing wet-blue leather and are able to see there is a clear way to achieve what many have wanted in terms of exceptionally high wetfastness.

Meeting customer demands

The performance properties requirement for leather will keep changing according to market needs (of tanners and final consumers). New ideas such as the Sella HiFA system for high wetfastness make it possible to meet these demands and can help to improve the position of leather as an important substrate with many valuable uses also against competing materials.