Introduction
Over the years, traditional dyestuff manufacturers have based their R&D on the production of new molecules that will answer their customer needs.
These efforts resulted in patents and new molecules that have been launched onto the global market.
This R&D has been significant in the improvement of the quality of articles produced in factories and application laboratories.
The need to register these new molecules in developed countries and the cost that this represents (a minimum of €200,000 per molecule) has reduced the number of laboratories and companies working on new dyestuff products.
Market analysis and research shows that the number of companies applying for patents for new dyes specifically designed for the leather industry is significantly low.
Although Clariant still have on-going projects for the research of new molecules, this feature focuses on some of the company’s results they have obtained in the development of new products based on existing molecules.
The objectives of these kinds of projects are, mainly, to improve the properties of existing dyes in order to fulfil our customers’ requirements: better dyeing behaviour, economic and ecological benefits, upgrading of leather, process simplification, better fastness properties…
The results of these projects are normally new products that Clariant believe differentiate them from their competitors.
Derma Supra Range
Production of normal dyes
Figure 1 shows the general process used for the production of azo dyestuffs.
From this, the different sources of salt can be determined:
* raw material: they are normally not pure and can contain certain levels of salt, up to 20-30%
* byproduct from synthesis: azo chemistry is based on the diazotation in an acid medium followed by condensation in a basic medium. When both media are mixed, salt is formed as a byproduct in an equimolar amount
* precipitation of the dye: when the synthesis is finished, it has to be separated from water before drying. The traditional way to do it is by adding sodium chloride in high amounts (from 5% to 25% of the reaction mass)
* cutting agent: in order to standardise the commercial strength of the dye, different cutting agents are used, among them inorganic salts
From these four salt inputs, the last two are the really important ones.
Furthermore, the purification of raw material would increase the cost too much and the chemical reactions cannot be changed, so all the effort has to be focused in the two last inputs.
Production of highly concentrated Derma Supra dyes
The approach used is described in Figure 2.
The following differences can be compared with the normal process:
* instead of precipitation of the dye (with the addition of salt), ultrafiltration is used (with removal of salt)
* there is no salt input during the drying, as the concentration is adjusted during ultrafiltration
What is ultrafiltration? It is a physical process (Figure 3) where, by using pressure and a semi-permeable membrane, small molecules (salt) are removed from a water solution.
This is the same process that is used for the production of salt-free, water-based liquid dyes.
A similar process is used for the desalination of water: reverse osmosis. In this case, the membrane has a very small porous size that only allows the migration of water, therefore obtaining salt-free water.
Once the reaction mass is desalinated and concentrated to the desired degree, the dye is directly dried without the addition of cutting agents.
Advantages of highly concentrated Derma Supra dyes
The following advantages can be described:
Intensity: salts have, in general, a certain tanning effect, decreasing the affinity of the dye to the leather.
This has already been described when comparing the intensity obtained by using liquid dyes and the equivalent amount of powder dyes. In this way, concentrated dyes can be considered as ‘solid liquid dyes’.
* Intensity differences are normally between 10-30%. It has to be understood that the dyeings are done with the same amount of active substance, ie if 4% of a normal dye is used at 100%, then 2% of the equivalent concentrated dye is used (see Figure 4).
* Solubility: as has already been described before, salt is used to precipitate the dyes from the reaction mass.
This means that salt has a reducing effect on the solubility of the dyes and, therefore, the absence of salt will have the opposite effect and will increase it. Figure 5 contains typical solubility values.
This increased solubility allows a better microscopic degradation of the molecules and better levelness of the dyeings.
* Ecology: electrolytes in wastewater are a contamination parameter that has to be taken into consideration more and more every day.
As salts are the main factor responsible for conductivity, any action that tends to reduce its presence in the wastewater will be welcome.
Although dyeing is not the main tanning process for the input of salt, it is always a good help to have dyes with a low salt content. Typical conductivity values are shown in Figure 6.
Reduction of volume and weight of stocked products
Tanneries tend to reduce their stocked products not only for cost reasons but also for space limitations.
The high concentration of these new dyes will reduce by a factor of two the weight of stocked products in comparison with normal powder dyes and by a factor of four in comparison with liquid dyes (see Figure 7).
Reduction of the dyestuffs handling operations in the tanneries
The handling of dyestuffs for mass consumption (ie blacks and browns) in the tanneries can be repetitive and heavy on time and manpower. With these high concentrate dyes, these steps can be reduced by half, therefore having an important economical impact on the tannery results.
The second part of this feature will focus on Clariant’s Derma Soft and Derma Flor ranges and will appear in the October issue of Leather International.
