5.Requirements to be met by leather dyes

Alkali-fastness

The dissolved dye should be fast to diluted alkalies such as soda or ammonia and should not show any change of colour. Build-up properties Dyeing is determined by the chemical composition of the dyestuff and by the properties of the leather to be dyed.

The build-up properties Dye are established by dyeing in different concentrations and defined in a build-up graph. The saturation limit of the dye has been reached if the intensity of colour does not increase. Excessive dye remains in the liquor, becomes loosely deposited in the substrate or penetrates deeper into the inner zone. The build-up graphs will then show that dyeing beyond the saturation limit is uneconomic. At the moment there is no standardized official method for determining the build-up properties for chrome leathers or retanned leathers.

Absorption properties The absorption properties of a dye are determined by dyeings, and characterize how much dye (%) is absorbed by the leather substrate in a time unit (min.). Besides the chemical structure of the dye the rate of absorption is considerably influenced by the type of tannage, by the type and amount of retanning agents used, by the pH value and by the dyeing temperature. The absorption properties provide information about compatibility with other dyes.

Homogeneity of dyes. According to the recommendations of the German Fastness Commission up to 5% of a shading dye can be added to industrial products, which may contain only one main dyeing component, in order to still classify them as homogeneous. The homogeneity of a dye can be quickly tested by means of the blow-up test or the so-called capillary method.

Intensity of dye This is an important quality factor for the practician and is determined by different methods. A reference analysis of the dye solution is performed without dyeing on leather by a dip test using filter paper or by a comparative measurement in a colorimeter. Another method taken over from the textile branch is dyeing according to penetration standards. They are divided into several stages such as 1/25, 1/12, 1/6, 1/3, 1/1 and 2/1, the weakest concentration being 1/25 and the strongest concentration 2/1. Different amounts of dye are required for a specific penetration standard depending on the type of dye and on the tannage and retannage method.

Stability to hard water The dissolved dye should not show flocculation when diluted with hard water. Dyes that are not resistant to hardness of water often result in increased staining of the flesh side, unevenness and shifts of shade. Therefore such dyes have been removed from the range of products of most suppliers.

Solubility Solubility of a dye is important for dyeing at low temperatures, dyeing by the powder method and dyeing without liquor. Poorly soluble dyes can produce staining in some sections of the leather and smudging on the grain side and on the flesh side. Combinations of dyes may result in shifts of shade. Due to their hydrophilicity dyes which are extremely soluble may result in poor exhaustion of the bath and over-dyeing of the surface after acidification. Solubility is tested by dissolving the dye in distilled water at 20 °C and at 60 °C. The amount of dye which remains dissolved after dissolution by bringing to the boil and cooling to the specified temperatures is then determined. The amount is indicated in grams per litre.

Complex stability Some metal complex dyes, in particular iron complexes, can be displaced from their bonds by highly complexing metals such as copper and thus cause shifts of shade. Therefore the leather to be dyed should not be brought into contact with metals, e.g. copper nails, copper covers or copper pipes.

Metamerism behavior Metamerism is the behaviour of dyes which, after dyeing in dye mixtures, show different shades in daylight or under artificial light sources such as a light bulb, neon light or also twilight. In the latter case the term dusk colour is also used. In order to avoid complaints about the shade, the luminance factor of the dyes to be used should be measured and adjusted such that it is identical with the original colour or very near to it. Content of extenders in dyes. As the manufacture of dyes comprises many chemical processes and the most different processing steps, each batch should be adjusted to a constant colour intensity for the user. In the case of powder dyes this is done by adding inorganic salts such as common salt, sodium sulphate or organic substances such as dextrine, dyeing auxiliaries which are called extenders. Liquid dyes have the advantage of being free of extenders.

Stability to acids The dissolved dye should be resistant to diluted acids such as formic acid or sulphuric acid solutions and must not flocculate.

Fastness to acids The dissolved dye must not result in a change of colour on contact with diluted acids.

Levelness Every dyer endeavors to obtain uniform, level dyeing results on the grain and flesh side in order to conceal defects of the grain as far as possible and to achieve every required fastness. Many factors have to be taken into account to meet these requirements in practice. Besides the condition of the raw hide, correct preliminary operations in the beamhouse and uniform tanning and deacidification processes, all parameters of the dyeing process itself have to be considered, which requires every experience of the practical specialist. There are no official methods for testing the levelness of dyes. Many dye manufacturers test it by means of reference colourings on standard test leather and judge the levelness by a visual inspection.

Lightfastness Lightfastness is important for high-quality leathers such as garment and furniture leather and for the unfinished suéde and nubuk leathers. The dyed leather samples are tested using official methods in daylight or by means of the xenon lamp and the blue scale (fadiometer) and judged in grades of lightfastness of 1 - 8, where 1 is the worst grade and 8 the best. A lightfastness of 4 is adequate for most leathers. In particular, the dyeing substrate itself determines the degree of lightfastness. If it is low, it cannot be improved even by means of dyes which are extremely lightfast. In order to achieve colouring results which are fast to light, lightfast dyes should be selected from the sample cards of the manufacturers, and fading tanning and retanning agents, dyeing auxiliaries or yellowing fatliquoring agents should not be used.

Fatliquor fastness. This provides information on the bonding strength between the dyes and the leather substrate. It is tested on standard chrome leather by means of standard fatliquoring and subsequent wet storage between filter paper. The intensity of dyeing of the filter paper is evaluated.

Solvent fastness Any solvent of interest can be used for the test. Sections of dyed leather are immersed in the solvent, and after a certain time bleeding of the dye into the solvent is evaluated.

Fastness to migration into crêpe and PVC The diffusion of the dye from leather into PVC foil or green rubber crêpe is tested and the dyeing of these materials is subsequently evaluated. Good fastness depends on the selected dye. Dyes having a low content or no content of free sulpho groups such as cationic dyes, 1:2 metal complex dyes and some liquid dyes are unfavourable. Spray and pressure dyeing, a high amount of dyestuff and dyes having a high covering power result in increased migration. Fastness to cleaning Many garment and glove leathers are dry-cleaned. The dyeing of these leathers must be fast both to the solvents used, such as perchloroethylene or the milder trifluorotrichloro-ethane, and to the cleaning intensifiers and should dissolve out only small amounts of dye. This means that solventresisting dyes should be used in these cases.

Fastness to buffing After buffing or roughening, the dyeings of suéde and also nubuk leathers should show little change of shade or brilliance. This is extremely difficult to achieve because the outer zones always bind more dye than the cross-section. The choice of suitable dyes and uniform through-dyeing are absolutely necessary.

Fastness to perspiration This is of importance for all types of leather which are in contact with human perspiration during use, i.e. unlined upper leathers, furniture and car upholstery leathers, garment and glove leathers. Leathers of inadequate fastness to perspiration may produce changes of colour on textiles and ugly, dark stains on upholstered products. This is caused by bacterial decomposition of perspiration in the transition from a weakly acid to a slightly alkaline reaction. For perspiration-resistant colourations it is therefore important that tannage be performed using mineral tanning agents which are fast to perspiration and have a high content of acids. Secondary treatment with glutaraldehyde, cationic fixing agents and a mild deacidification are also favourable. Furthermore, dyes which are fast to perspiration should be chosen from the suppliers′ sample cards. Fastness to perspiration is tested according to a standardized method. A multi-fibre accompanying fabric is soaked in an artificial perspiration solution and placed for some time on the section of leather to be tested under load at 37 °C. After drying the change of colour of the section of leather and the dyeing of the accompanying fabric are evaluated. Fastness to dry and wet rubbing The leather is tested by rubbing it several times by hand with a dry or wet cotton cloth or in a rub fastness tester using dry or wet felt. This test is applied especially in the case of uncovered sorts of leather such as suéde and nubuk leathers and indicates the fastness of the dyes bonded to the fibre. In many cases buffing dust is the cause of inadequate fastness to rubbing. In order to remove the dust it is necessary to wash the leather again with water. Intensive fatliquoring of the surface and a high content of neutral oils also reduce fastness to rubbing.

Fastness to over spraying Often, many coating finishes still have a high content of solvents which, when applied, dissolve the dyes which are not fast to solvents or migration and cause migration to the surface. This applies especially to aniline spray dyeing or aniline dyeing by means of the multima, roll coater or curtain coater as there is an increased concentration of dye on the grain surface compared to drum-dyeing. For good fastness to overspraying it is therefore absolutely necessary to use dyes containing sulpho groups.

Fastness to washing. This is required for garment leathers which are worn directly on the skin such as shirt or glove leathers. It is not only a matter of using dyes that are fast to washing, but of using an appropriate procedure because it is also necessary to ensure softness and stability of shape. The test is performed by washing with a lauryl sulphate solution and accompanying fabric, followed by evaluating the change of colour of the section of leather and the dyeing of the accompanying fabric.

Fastness to water. The behaviour of leather under the action of moisture is tested. The test is identical to that for fastness to perspiration, but distilled water is used instead of the artificial perspiration solution. Changes of colour and dyeing of the accompanying fabric are evaluated.

Fastness to water spotting. The surface behaviour of the leather is tested by application of water drops. The test is particularly important for aniline leathers, suéde and nubuk leathers. Whereas there are no problems in the case of drum dyeing if carried out correctly, increased spotting with formation of edge marks occurs if the dyeing is of inadequate fastness. Metal complex dyes have much more favourable properties than simple penetrating acid dyestuffs.