What are leather dyes today? The relationship between the dye structure and its performance properties.

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1 What are leather dyes today? The relationship between the dye structure and its performance properties. by Dr. Campbell Page and Dr. Jens Fennen Introduction The coloration of leather is typically made with azo dyes and it is interesting to take a deeper look at the factors that allow us to understand these dyes and the process of dyeing leather. While a considerable amount on dyes and dyeing properties has been written over time it has often been out-dated due to new legislation or tannery needs in terms of leather performance. Here we will attempt to bring together the ideas regarding leather dyes as used today and how the structural properties influence the performance properties. The theory being that if one needs specific fastness characteristics one can then use the structure that best meets these needs. An overview of the future trends in dyes and the properties they will need to fulfil will also be presented. Classification of dyes, what are azo dyes? Colouring agents are those substances that absorb light in the visible wavelength region of nm. Hence they appear coloured to our eyes. These colouring agents can be either organic or inorganic in nature. We will consider only the organic colouring agents here and from these only those that come under the definition dyes. That is, those organic compounds which are soluble in the medium they are applied to the substrate, in this case leather dyes applied in water. The vast majority of dyes used today for dyeing leather fall into the azo dyes category. Some 70% of all leather (and textile) dyes listed in the literature have the azo chromophore as the reason for their colour. In practice today more than 90% of all dyed leather will be coloured with dye(s) containing the azo chromophore. The azo dyes have become a much talked about subject, since the introduction of legislation in Germany in 1995 banning the use of a small number of azo dyes that could split under reductive conditions to release toxic aromatic amines. Uninformed comments, especially in the media, about the toxic nature of azo dyes in general have in many instances lead to requests from retailers for textile and leather clothes dyed without using azo dyes. Such requests are based on ignorance and are misleading for the tanner and dye supplier, since they are a wrong interpretation of the actual situation. It is worth reinforcing again that the German ban on using certain azo dyes that split to release specific toxic aromatic amines, relates only to a very small number of dyes and that the majority of the azo dyes can be used without any grounds for concern. Any reputable dye supplier does not use such amines in their production.

2 An azo dye is made by coupling an aromatic amine with another aromatic component. A very simple schematic example for the formation of a mono-azo dye is as follows: H + -, N 2 NH 2 N 2 + N So it is clear to see from the above scheme that one must have an aromatic amine as the starting block for an azo dye. Without it the manufacture of an azo dye is not possible. rganic compounds with one azo group: are called mono-azo compounds and the structure of Acid Red 1 below is an example. H HN CH 3 N N H 3 S S 3 H Naturally by making a series of reactions and couplings it is possible to build up more than one azo group into a dye molecule, these structures are called polyazo dyes. This concept is the basis for the manufacture of the main leather colours of brown and black. Using the simple schematic structure, a polyazo dye would look like this: The more azo chromophores, normally the more the colour tends towards brown or black, as the individual red, yellow, blue, etc. chromophores start to work together. Many of the major brown and black dyes for application to leather are polyazo compounds.

3 How can we influence the dyeing behaviour? In the manufacture of polyazo dyes one can influence the dyeing behaviour by altering the chemistry of the components, especially by changing the substituent on the aromatic rings. Examples are as follows: Example 1 S 3 the addition of a sulphonic acid group increases markedly the solubility and the anionic character of the dye molecule, which consequently penetrates into anionic retanned leather much more. Example 2 N 2 addition of a nitro group can alter the shade of the dye. So one can see it is possible to influence the end result on leather by choosing a suitable structure. However, this is a simplified view and many structures prove unsuitable for other reasons such as solubility, poor light fastness, poor shade consistency, sensitivity to acid, metals, hard water, etc. Naturally the very practical aspect of the cost of manufacture (and registration for new structures) is today very important. It should be noted at this point that the typical metal complex dyes used for textile and leather dyeing are mostly in the azo dye category as well. Effectively one complexes mono-azo dyes with a metal salt to form the metal complex dyes. For example, the schematic structure for a 1:2 chromium complex dye is as follows: Cr

4 What are homogeneous dyes? It should be understood that a homogeneous dye in terms of leather azo dyes does not necessarily mean it has only one chemical structure. Especially if it is a polyazo dye that has undergone several diazotising and coupling steps, it can consist of several components and/or isomers formed during the manufacture. However, it should be carefully noted we are not talking here of physical mixtures, which occur after the raw dye component is manufactured. A thin layer chromatogram illustrates the nature of 3 homogeneous leather dyes: However, as it is to be seen in the chromatogram above the dyes can have more than one chemical component, these are dependent on several factors during manufacture. Aspects of the reaction itself such as: temperature; time; ph; volume; can all influence the resultant raw dye. Additionally the parameters within the manufacturing vessels can strongly influence the manufacture of the polyazo dyes, parameters such as: - flow rate between vessels; - geometry of the vessels; - speed of stirring in the vessels. So it is easy to see that one can have quite some difference between dyes of supposedly the same structure.

5 The Colour Index system is a useful tool of classifying each of the dye structures of homogeneous dyes into a specific category appropriate for the application conditions, such as Acid, Direct, Reactive, Pigment, Disperse, etc. But again the manufacturing parameters of dyes, as explained above, means that often for dyes with the same Colour Index classification number they have different properties. Below is a HPLC comparison of 2 polyazo dyes with the same Colour Index number, Acid Brown 75. The dyes are from different manufacturers, and the chromatograms show the difference quite clearly. HPLC Chromatogram: Acid Brown 75 - Manufacturer 1 HPLC Chromatogram: Acid Brown 75 - Manufacturer 2 What influence does this difference have on the dyeing properties? For the 2 dyes shown above with different HPLC chromatograms, the dye from manufacturer 2 has a fastness to migration into PVC of 4 5, compared with a much poorer note of 2 for manufacturer 1. The dye from manufacturer 2 also has a different shade and somewhat different dye build-up, depending on the anionic charge of the leather substrate. This again illustrates the pitfalls of the common assumption that dyes of the same Colour Index classification number are the same. Too often one sees considerable differences in their dyeing behaviour. What are dye performance requirements today? For quality leathers the leather tanning industry has over recent times tended away from focussing almost exclusively on chrome tanned leather and specifying just the leather dyeing properties of light and wet fastness. Today there is a strong demand for other fastness properties such as migration into PVC and migration into a lacquer finish, which now often have priority before wet fastness and light fastness. ther new properties like stability of colour to climate changes and temperature are also gaining in importance, especially for the car interior requirements.

6 How does one prepare dyes for these changing performance requirements? There is a growing trend in leather for automobiles to make the leather tannage without using metal salts, one example is the use of aldehyde tanned leather instead of chrome tanned. For the dye application and fixation this change has a major impact, as the aldehyde tannage uses the amine sites typically required for the ionic bond between the dye and leather substrate. H H NH 2 H 2 N H 2 Cr H 2 H H 2 Cr H H2 N H N H chrome tanning Aldehyde tanning This results in a considerably lower dye wet fastness for the aldehyde tanned and dyed leather, the deep dyed shades are especially affected. This is a continuing challenge to the chemical and dye manufacturers to find ways to improve the fastness of such dyed leather. What is migration of colour into PVC? The migration of colour into an adjacent soft white PVC plate (at 50 C over 16 hours) is a simple but effective method. It checks the tendency that a dye component will move from a polar substrate like leather into non-polar adjacent substrate, such as a synthetic sole of a shoe. This test indicates the polar/non-polar tendencies of the dye(s) and could be likened to an indication of the attraction towards solvents. Those dyes that do not migrate into the PVC generally have good water solubility and have polar structures. Conversely those that migrate into PVC are more non-polar and generally have lower solubility in water. This leads to the observation that the dyes with non-polar character have in general better wet fastness but normally more migration into PVC. Those dyes with a polar character have in general poorer wet fastness but either no or almost no migration into PVC.

7 This overall scheme is represented below: PVC (non-polar) dyed leather polar dye non-polar dye perspiration solution (polar) How does the form of the dye molecule influence the performance? Generally the larger the molecule the poorer the penetration into the leather crosssection, however, increasing the number of sulphonic groups increases the anionic character of the dye and can improve the penetration for anionic leathers. n the other hand this will reduce the depth of the dyeing, i.e. less intense, and can decrease the wet fastness if the dye is not subsequently fixed with cationic agents. Very small dye molecules, such as for example Acid Black 1, are able to penetrate the leather cross-section more easily but afterwards are notoriously difficult to fix successfully. For some leathers this comes down to a decision between either a dark penetrated cut and poor wet fastness or a lighter cross-section and good wet fastness. Attempts to correlate the light fastness performance and the dye structure have involved many skilled chemists and application experts over the years. The clearest result has been the development and use of the metal complex dyes for reaching high light fastness on dyed leather. After having found a good solution to the problem, the tendency in recent years has been to producing metal-free leathers for automotive uses! This means one is now asking for good light fastness but without using metal complex dyes.

8 What about migration of colour into a lacquer finish? The most important point that is almost always overlooked here when discussing migration of colour into a lacquer finish is that the dye by itself does not migrate. The coloured component must be transported. Typical transporting agents are for example the fatliquor or the slow evaporating solvents from the finishing. How can we measure the tendency of a dye or coloured component to migrate into the lacquer finish? For some time the migration into PVC has served as a good guideline for the performance of dyes, most dye suppliers give these values in their colour cards. Dyes with good resistance to migration into PVC and finish lacquers are typically those with a higher polar character. However, there has been a change towards water-based finishing systems. These systems are not solvent-free, as assumed by many, but rather are based on water and water-miscible solvents with high boiling points. Naturally the evaporation rate of these systems is slower giving more time for the finishing system to interact with the dyed leather substrate. Comparisons of the migration into PVC and the migration into lacquer finishes have confirmed the general trend is similar, especially if the migration colour is different to the dye colour. But enough differences occur to make the results somewhat puzzling. In some cases, dyes with no record of migration are found to be involved in some coloration of the finish. Finally some clarity is appearing as to why some finishes have discoloured and others not. The typical polyurethane finishes are prepared in alkaline conditions to allow them to react. Surprisingly though, there is very often a complete lack of ph measurements of these water-based finishes. Finish preparations of up to ph 11 are not unusual and any dye technician knows these are ideal conditions for the extraction of leather drum dyes. The finish formulations are prepared and continually adjusted to give the right physical properties such as flow, coverage, touch, appearance, etc. and not to meet any requirement of the underlying dyed substrate. It is clear to see now that the slower evaporation properties of the high boiling point solvents and the extremely high ph of the finish preparations could well be the real reason for the transport of dyes resulting in finish discoloration problems over the recent years.

9 How do we get the top performance? This presentation has pointed out that there is no perfect solution to the problems of dyeing but as in other areas one has to compromise according the requirements for the leather being made. Good wet fastness often means poor values for the migration into PVC, and vice versa. Good light fastness means preference for metal complex dyes but some automotive suppliers ask for metal-free leathers. Good resistance to migration into finishes is helped by choosing polar dyes but the longer exposure to water-miscible solvents at high ph increases the chance of them being transported to the surface. Good fixation of the dyes requires sufficient sites for the interaction of the dye molecule and the ionic groups of the leather substrate. Using metal-free tannages can reduce the sites available for the dyes making it difficult to get adequate colour build-up for deep shades and also resulting in poor fastness properties. What trends are foreseen? The demand for chrome-free and metal-free leathers especially in the automotive area will increase, as will the need for high fastness to migration, as well as good light fastness and wet fastness. Improvements will be made in both the fixing systems as well as the chemicals used in processing the leather. Knowing the requirements means the dyes can be focussed more clearly to meet these needs. However, in most cases it will continue to be a compromise, the aim is always to improve on specific performance parameters while avoiding the possible negative effect on other parameters. Ecological and environmental matters will continue to increase in importance and additional laws and labelling regulations must be expected.