Dyeing auxiliaries for drum applications6 February 2001
Dyeing auxiliaries, as the name implies, are products that influence the dyeing result in an intentionally positive way by improving one or more of the following factors: * penetration of the dye into the leather * levelness of the dyeing * shade brightness * colour intensity * fixation of the dye The target for dyeing leather is the achievement of the desired shade or colour, which should appear level and uniform over the whole surface, as well as between hides/skins in the same batch. Leather dyeing Leather is a difficult substrate to dye; it has structural differences as well as other imperfections. To achieve the target of a level and uniform dyeing the leather dyer needs to be experienced and to have a thorough understanding of the dyeing properties of the dyes and auxiliaries used. Uniformly tanned leather and a suitable selection of dyes are essential for an even shade. It has been shown that to combine dyes of quite different structures, eg, the usual acid with direct dyes or 1:2-metal complex dyes, can lead to problems in achieving levelness and shade consistency. However, the appropriate auxiliaries can minimise the differences in dyeing behaviour between the dyes and the leather. The major reason for the different dyeing behaviour of dyes in leather is their varying affinity for the leather substrate and the variations between the dyes themselves. The behaviour of leather dyes is primarily determined by the charge of both the dye and the leather to be dyed. Differences in exhaustion rate or bath exhaustion and in build-up of the dye on specific leathers are the chief problems. In all these cases a balance has to be struck between the dyeing conditions and the auxiliaries. Practical considerations Here we will concentrate on the effect of specially suited auxiliaries which influence dyeing behaviour in compensating ways. Practical and technical hints are given in order to clarify various points. The differences in chemistry, application and performance characteristics of the various dyeing auxiliaries for leather that are currently in the TFL core range are described. Many of the dyeing auxiliaries in the TFL range are well known products that have been on the market for some time, but in terms of performance they are still amongst the best products available. New developments are on the way and are included in novel leather recipes. Anionic auxiliaries Strongly anionic auxiliaries ensure high affinity for the leather, and markedly decrease the affinity of anionic dyes for the surface of the leather. They are mainly neutral salt mixtures of aromatic sulfonic acids. Naphthalenesulfonic acid-formaldehyde condensation products, and their derivatives are best known for this purpose. Application practice has shown that this type of condensed products are very stable, but depending on the synthesis, some starting material such as formaldehyde and/or naphthalenesulfonic acid can be found. TFL have now optimised the manufacturing process to create a new product1 with an extremely low free formaldehyde content. No free formaldehyde can be detected using the analytical Merck test method, Aquamerck 1.08028.0001. This auxiliary competes with anionic dyes and produces two effects by reaction with the cationic chrome tanned leather. First, it decreases the cationic charge of the leather fibres, allowing better penetration of the anionic dyes and, second, it lowers the isoelectric point of the leather fibres, permitting through dyeing at a lower pH. It promotes the penetration of dyes into the leather without affecting grain smoothness and fineness. It also has a dissolving effect on slow soluble dyes, which markedly increases the levelness of dyeing and is particularly important for preparing pastel shades. For the best results, table 1, these products should be added to the dye bath at the rate of 1 - 4% at least 10 minutes before the addition of the dye. Nonionic auxiliaries Dyeings with outstanding penetration and levelling but reduced bleaching effects are achieved with auxiliaries, which neutralise the ionic charge of the dyes. These chemicals are mostly polyoxyethylene derivatives of fatty alcohols, which are much more effective for penetration and levelling than salt or disaggregating agents. The typical chemical characteristics are based on amphoteric behaviour with an affinity for anionic dyes. The auxiliary2 forms a loose bond with the dye, which in the course of dyeing is broken as the dye fixes itself to the leather. Polyoxyethylenes are considered nonionic, even though they possess a partial cationic charge, due to their polar ether linkages. Their behaviour in the dyebath is based on their ability to combine with anionic dyes. As a result, different rates of dye uptake become more uniform because the variations in reactivity of different dyes are reduced. A retarded effect is obtained, preventing the dyes from being fixed too rapidly. These types of auxiliaries promote improved uniformity of dyeing on the leather surface, as well as enhanced penetration on thinner leathers. On thicker leathers, the diffusion may be somewhat slowed because once inside the leather the dyes lose their attraction to the weak cationic auxiliaries. Diffusion is then controlled by interaction between the dyes and the substrate. It should be noted that the attraction between dye and leather is stronger than the link between dye and polyoxyethylene. The released auxiliary can be washed out later. Benefits include significantly enhanced compatibility of dye combinations. This ensures good levelness by promoting even exhaustion and improves the homogeneous penetration of the dyes into the leather, which in turn improves the buffability of nubuck and suede. It is also best suited for standard shoe uppers. For the best results, table 2, these auxiliaries must be added together with the dye. 0.25 - 1.0% is often sufficient as higher amounts can have often a negative effect on levelness because the dye-auxiliaries complex may remain unbroken and the dye, therefore, remains in the float. Cationic auxiliaries Cationic auxiliaries are mostly used for shade deepening for brilliant and intense dyeings. The amount of product and the timing of the addition should be in accord with the affinity of the substrate. Cationic resins, polyurethane, cationic fatliquors and retanning with chromium or aluminium salts have been used for this purpose. However, too strongly cationic auxiliaries cause dye precipitation, and form a layer on the leather surface which can weaken the adhesion of finishes and result in poor rubfastness. Often polyoxyethylene fatty amine derivatives or polyquaternary amines are used. As the affinity of the substrate and the intensity of the shade are very closely interlinked, the method of application is crucially important to the final result. An improvement in shade intensity is only feasible if properties such as rubfastness or colour brightness are not weakened. It has to be kept in mind that cationic auxiliaries hinder penetration and, therefore, the dye tends to be fixed on the leather surface. A balanced cationic product with a nonionic auxiliary has been found to overcome this disadvantage. This type of weakly cationic auxiliary3 has an affinity for leathers which, in a sandwich dyeing technique, significantly enhances the exhaustion and intensity of anionic dyes on the surface of retanned chrome or vegetable tanned leathers. These auxiliaries are applied after a first dyeing for subsequent penetration and fixation with an organic acid. After the cationic treatment, the second dyeing follows in a fresh bath, avoiding precipitation with the dyes. They work most effectively when used in a fresh bath at 50°C and a pH of approximately 3.5 and added at least 15 minutes before the addition of the top dye. 1.0 - 2.0% auxiliary is usually sufficient and has the benefit of significantly boosting colour intensity on the leather surface without impairing levelness. Wetfastness improvers Strongly cationic fixing agents with outstanding properties are used as an after treatment to improve the wetfastness properties of dyeings on leather. The chemistry involved is completely different from that of the shade intensifying products. The cationic equivalence is much greater with a wetfastness improver, which has the function of neutralising the anionic dye with a large cation. The dye has to lose its solubility to be better fixed. Mostly a quaternary amine is combined with a cationic amino resin, eg melamine formaldehyde compounds Benefits include enhanced fixation of anionic dyes on leather, thereby significantly improving water and perspiration fastness properties. These auxiliaries4 are normally applied as the last wet-end treatment in a clean fresh bath at 25 - 35°C and pH ca. 3.5 for at least 30 minutes. 0.5 - 3.0% is usually sufficient. Note that these are strongly cationic chemicals and will, therefore, precipitate anionic products in the same bath. In any case, an excessive amount of cationic auxiliaries should be avoided as it could cause stripping instead of increasing fixation. Many valuable cationic fixing agents contain some formaldehyde condensation products which under occasional adverse conditions can release very small amounts of formaldehyde (ppm). Chemical suppliers are, therefore, striving to launch fixing agents in which formaldehyde has completely and irreversibly reacted or which have been synthesized without formaldehyde. These new developments5 are now on the market. Typical examples are Guanidine derivatives, which are polymerised with amines to form quaternary ammonium compounds. These new chemicals can be applied in a similar way to the classical fixing agents at the end of the wet end processes. Conclusion Dyeing auxiliaries are used to control the dyeing process. Depending on their chemical composition, type of charge and concentration used, very special effects can be achieved. Given the parameters involved in dyeing with auxiliaries, careful selection and pre-testing are highly recommended. This applies to dyestuffs6, retanning agents7 as well as fatliquors8. The recipe has to be controlled as an entirety. A good application and testing laboratory makes the leather fabrication processes much more consistent by spotting at an early stage if the wrong chemical, quantity or pH is being used. Therefore, close co-operation with the chemical supplier is necessary if quick and impartial advice is to be obtained. Acknowledgements This paper is based on several in-house reports and application tests carried out by TFL colleagues worldwide. Grateful thanks to M Ghezzi for the dyeing recipes. All recipes shown are based on bulk production procedures but will have to be adjusted to the prevailing process circumstances in other tanneries. For further information: [http://www.tfl.com].