Factors which influence leather dyeing.

Factors which influence leather dyeing.

Choice of dyestuff. The choice of dyestuff depends primarily on the demanded fastness for the type of leather to be produced, on the desired shade and on the respective main tanning method or the retanning agents used. Perfect combinability of the dyes is a further criterion. This depends on the build-up properties, on the absorption rate and chemical structure of the dyes. In principle, the dyer should adjust the desired shade by means of the smallest possible number of shading dyestuffs in order to restrict the possibility of introducing additional defects. Price is a further factor which influences the choice of dyestuff. However, when using lowprice dyestuffs their fastness and intensity should be examined thoroughly. Often they have a high content of extenders or extremely different mixed dyes. At any rate the specifications of the suppliers′ sample cards should be considered when choosing dyes according to fastness. Solubility of dyes. Anionic, readily soluble dyes are dissolved in water at 70 - 80 °C in a ratio of 1:10 to 1:20. Poorly soluble dyes should be brought to the boil briefly and if they are intended for high-quality aniline leathers filtered through a fine-meshed cloth to be on the safe side. Cationic dyes are made into a paste by means of acetic acid with about half the weight of the dye, especially if hard water is used, and dissolved in water at 80 - 90 °C. Boiling should be avoided. Anionic and cationic dyes should not be dissolved together as otherwise precipitation or colour lake may occur. Liquid dyes have the advantage of being dust-free and better to handle because they are easily diluted. Quantity of dyes In the case of wet leathers the quantity is calculated in relation to the shaved weight of the leathers, in the case of intermediately dried leathers according to the dry weight. However, it would be more favourable to relate the calculation to the leather surface because the thinner a leather, the greater is its surface per kilogram. In this case better constancy of shade without significant variations is achieved for the subsequent lots. Addition of dye. For high-quality aniline leathers the addition of dyes in dissolved form and in several portions is always advantageous for an even and levelled absorption. Liquid dyes should therefore also be added in diluted form. Powder dyes in short float processes mostly result in reduced depth and brilliance of colour. With this form of application it is absolutely necessary to ensure that all dye components are evenly and readily soluble. Float ratio. A large quantity of dye-liquor promotes the distribution of the dyes and auxiliary agents used. Furthermore, it prevents an excessive rate of absorption on the surface or reverse side. This should be considered especially when dyeing pastel shades which should therefore be dyed in larger quantities of dye-liquor. The quantities of dye-liquor commonly used are 100 - 250 %, and up to 400 % for pastel shades. The "dyeing without float" or "short-float dyeing methods" with floats of 0 - 30 % have to be performed at low temperatures in order to avoid unlevelled dyeing. Short floats should not be used for very thin leathers to avoid tearing or tangling of the leather material. These processes should be performed more gently in the automatic chamber or sector machine. Dyeing temperature. The most commonly used dyeing method is dyeing in a hot float. The temperature of application is 50 - 70 °C for chrome leathers and 35 - 45 °C for vegetably/synthetically tanned leathers. In the short float dyeing methods penetration dyeing is done at a temperature of 20 - 25 °C and then the temperature is increased to 60 - 70 °C for fixation of the dyes. It is a general rule that high dyeing temperatures enhance the depth of shade due to increased affinity and faster rate of absorption while low temperatures reduce the fullness of shade due to quicker penetration of the dyes and even distribution over the entire cross-section. Mechanical movement. Increased mechanical movement in the drum promotes the distribution and penetration of the dyes, thus reducing the duration of the dyeing process. Conventional rotating speeds are between 10 - 15 rpm. In order to avoid tearing of thin leathers dyeing should always be performed at low speed and with an increased amount of liquor or in the automatic chamber or sector machine. Affinity of the leather to be dyed. Purely chrome-tanned leathers have a highly positive charge which is more or less reduced by vegetable/synthetic retanning or by a treatment with resin tanning agents, glutaraldehyde or other organic aftertreatment agents. Thus it is possible to control the different affinity of dyes. On the other hand, leathers retanned by means of vegetable/synthetic agents have a negative charge. When dyeing with acid dyes the charge should first be reversed by means of cationic agents in order to achieve fixation of the dyes. Duration of dyeing. For most types of leather a dyeing time of 45 - 60 minutes is adequate. It depends on mechanical movement, on the size of the load and for penetration dyeing on the thickness of the leather. Exhaustion of dyebath. very dyeing process should aim at maximum exhaustion of the dyebath. However, excessive deposition of dyes on the surface of the leather should be avoided. Such depositions may result in bleeding due to migration into the finish layer or in the case of suéde and nubuk leathers inadequate buffing properties or unsatisfactory fastness to wet and dry rubbing. Besides the correct choice of dyes, exhaustion may be influenced by several factors such as temperature pH value, retanning agents, dyeing auxiliaries, float length and mechanical movement. pH value. Chrome leather has an isoelectric point of about 6.5 or 4.5 depending on whether tannage is cationic or anionic (masking) whereas vegetably tanned leathers have an IP of 4 - 3.2. The IP is increased or reduced by retanning, depending on the type and intensity of the process. If the pH of the dye liquor exceeds the isoelectric point the charge of the material to be dyed is largely negative and if the pH is below the IP the charge is mainly positive. This shows that by changing the pH during the dyeing process the absorption behaviour, penetration and fixation of the dyes can be influenced. In practice this is done by means of diluted organic acid or ammonia solutions. Inorganic acids such as sulphuric or hydrochloric acid should not be used as they are non-volatile during drying and cause high differential values in the leather. Deacidification. Correct neutralization is essential for dyeing. An even deacidification over the entire cross-section is very important in order to avoid acid zones. Neutralization has a decisive influence on levelness, depth of shade, dyeing and penetration of dyes. (See also Deacidification).Retanning. Due to the wide number of retanning agents available on the market and their varying chemical compositions there are many possibilities of influencing the dyeing process with regard to depth of shade, absorption behaviour and fixation. To compare the influence of retanning agents on the dyeing process a standard chrome-tanned leather is chosen and dyed with a dye content of 1 %. The achieved depth of shade is defined as 100 %. The retanned dyed leather is then compared against the standard leather and checked for depth of shade, change of colour, exhaustion of dye, fixation, brilliance, levelness, dyeing of grain defects and fastness properties. Dyeing auxiliaries. These auxiliary agents are used to control the dyeing process, depending on their chemical composition and type of charge, in order to achieve very special effects on the substrate leather or also in the dye or the dyeing process. The auxiliaries are conveniently classified as anionic, cationic, nonionic, amphoteric or special products. Anionic products: These are mainly neutral salt mixtures of aromatic sulphonic acids. In most cases they are used for their levelling effect on anionic dyes. They have a fixing effect on cationic dyes. Cationic products: These are mostly condensation products of urea with formaldehyde or dicyandiamide, ethoxylized fatty amino derivatives or conversion products of polyurethanes or protein hydrolizates. They have a fixing effect and increase the depth of shade of anionic dyes, and a levelling effect on cationic dyes. If used in anionic dyeing excessive amounts should be avoided at all costs because this could cause stripping of the dye instead of a deepening or fixation of the shade. These cationic auxiliaries should be added only to a completely exhausted dyebath in order to avoid precipitation with the dye. For safety reasons it is recommendable to use a separate intermediate bath and keep the pH value constantly below 4.0. Nonionic products: These products are mainly ethoxylizing products on the basis of fatty alcohols or nonyl phenols. They promote penetration and levelness of dyes. They should not be used in larger quantities because of their surface-active properties which would give the leather excessive hydrophilicity or wettability. Amphoteric products: These products have both anionic and cationic groups in the molecule. The cationic group is active with low pH values, the anionic group is active with high pH values. Thus it is possible to influence depth of shade, absorption rate, penetration, levelness and exhaustion of the bath by regulation of the pH value. Special dyeing auxiliaries: These products include penetrators used for spray staining, foaminhibiting agents, resisting agents for drum or effect dyeing on one side, thickening agents for dyeing by curtain coater, tray staining or brush staining as well as polyamides to improve lightfastness and fastness to migration of metal complex dyes. Fatliquoring. In view of the large number of available fatliquoring agents and their widely varying compositions the dyeing result can be influenced considerably. This is the case especially if large amounts of fatliquors are used in the dye-bath. The levelness of dye can be impaired as a consequence of an uneven deposition of fatliquors and dyes, caused by unstable mixtures of fatliquors or excessive amounts of neutral oils. An excessive content of emulsifying components in the fat liquor may result in shifts of shade, reduced absorption properties, poor fixation of dye and thus reduced fastness properties. Overfatliquoring should always be avoided. If large amounts of fatliquors are used the mixture of fatliquors should be added in several portions as preliminary, main and subsequent fat liquoring. Surface dyeing. High-molecular dyes should be chosen from the range of available products for pure surface dyeing. Simple acid dyes are less suitable for surface dyeing. Liquid dyes on the basis of metal complexes are very suitable. Deacidification should not exceed a pH value of 4.5. Light, cationic preliminary or intermediate treatments increase the depth of shade. The use of wetting agents and higher sulphited or higher sulphochlorinated fat liquors as well as highly sulphited retanning agents should be avoided. Penetration dyeing. Products that are particularly suitable for penetration dyeing are indicated in the suppliers′ sample cards. These penetrative dyeing agents should be chosen to avoid tedious additional processing. In general, simple acid dyes have a good dyeing capacity, however they are not recommended for high-quality leathers because of their low fastness properties. The following influencing factors should be taken into consideration to achieve good penetration dyeing: Neutralization should be carried out intensively and without the formation of zones, if possible at the isoelectric point of the material to be dyed. Furthermore, attention should be paid to good mechanical movement, sufficient duration (depending on the thickness of leather) and adequate amount of dyestuff. The addition of dyeing auxiliaries having an affinity for dyes and dyeing with short floats are favorable for penetration dyeing. In particularly difficult cases penetration dyeing is achieved by means of intermediate drying. Pigment dyeing. In order to increase the percentage of aniline leathers in leather production fine-particle pigments are being increasingly used for drum dyeing. Good covering of defects in damaged sections of leather is achieved. Since it is necessary to match the shades a smooth dyeing of the surface and thus improved quality is obtained. Perfect dyeing results depend on the chemical composition of the pigments, the particle size, the dispersing agents and auxiliaries, on the charge of the leather and the adjustment of exactly identical shades of the aniline and pigment dyeing. Shading. It is in the adjustment of shades according to samples or in the subsequent shading of large lots where the "good" practical dyer proves his capabilities. A trained eye and an exact knowledge of the dyes to be used are prerequisites for quick and successful dyeing. A detailed knowledge of the shade-intensifying or shade-weakening effect of the auxiliary agents, fat liquors and retaining agents used is also very important. It is always advisable to achieve the desired shade with the smallest possible number of dyes. The dyes used should be fairly similar in terms of rate of absorption, build-up and fastness properties. Dyeing series, chromatic triangles and the sample cards of the dye suppliers can be used for reference. In larger leather factories increasing attempts have been made in recent years to calculate shade adjustments by computer.6 However, due to the many influencing parameters in the leather dyeing process the results obtained are often still incorrec