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Process problems in the dyehouse (Part 4)

1st Aug 2012 , Dr Clive Jackson-Moss; ISTT

The previous article discussed problems that can occur during the retanning process in the dyehouse. This article will concentrate on factors that influence the dyeing of leather, which as will be seen in the next article, can lead to process problems during the dyeing stage.

If a dye is added to water it will rapidly dissolve or it will form lumps.Float conditions, the type of dye and the leather raw material will determine how dyeing will take place. The evenness of dyeing is also determined by the fixation and/or penetration of the dyestuff.

Type of Rawstock The type of rawstock is the initial factor that must be considered when dyeing a piece of leather. The type of tannage is one of the factors, as the charge of the tanning agent will alter the iso-electric point. A low iso-electric point means that neutralisation does not need to occur to a high pH.

The type of retannage will also determine the shade of dye. If vegetable tannins have been used then the shade of the dye tends to be paler. Negatively charged dyes cannot fix to the negatively charged vegetable tannins. A full chrome retannage means that there are plenty of positive charges available (particularly at low pHs) where the dyes can fix. The temperature of the retannage and extent of neutralisation of retannage will determine how deeply the retanning agents penetrate. The depth of penetration means the retanning agents are either deep or superficial.

Retanning agents on the surface means the dyes will fix on the surface. Further penetrated retanning agents means the dyes fix further inside the leather (but only if the pH of the dye bath allows this). Surface fixed dye means the shade of dye will be darker.

The fat, oil and grease (FOG) content will also affect the evenness of dyeing. An evenly distributed natural grease content means the tanning agent will be even. An anionic fatliquor added before the dye, will also bind to some of the positive sites that the dye would bind to (the shade will then be paler). Sheepskins and ostrich skins with a high natural oil content are famous for uneven dyeing unless special degreasing steps have been taken.

Evenly tanned rawstock means that the dye will fix evenly. Dyes fix both to the positively charged sites of the chrome and the collagen. The cationic charges of the collagen are rapidly filled. The tanner must always remember that there are only a certain number of binding sites. If those binding sites are localised the dyeing will be uneven. Localised dye spots give rise to uneven dyeing.

Masked binding sites means that the dye will penetrate into the leather rather than bind to the site. The addition of mordant (chemicals that block sites e.g. syntans) or levelling agents will ensure that the dye disperses evenly first. After dispersion the tanner removes the mordants with some skilful techniques or chemicals to allow the dye to fix.

Physical conditions As with any other tanning process the manner in which the chemicals penetrate or fix is dependent on the physical conditions of the bath. Factors which promote astringent conditions for dyes will obviously result in fixation and non-astringent conditions for dyes will result in penetration.

pH will affect the dye penetration the most and this is related to the number of charges on the collagen. If the pH in the leather is at the iso-electric point then the dye will penetrate into the leather. If the pH is below the iso-electric point anionic dyes will fix to the leather. At very low pHs the dye will fix superficially. At pHs above the iso-electric point anionic dyes will penetrate and cationic dyes will fix.

Temperature will also affect the penetration and fixation of dyes. The higher the temperature the more fixation that will take place. The increase in temperature results in an increase in the energy of the dyestuff. If the dyestuff has an increase in energy it will react with functional groups in the skin.

At low temperatures the energy is low and the dye will penetrate. Complete dyeing, tested by a cut from the butt or neck, will be achieved with a full neutralisation and cold dyeings.

Mechanical action results in flexing of the hides/skins. The greater the action the more flexing there is. Short floats will result in more action hence more penetration of dyes as the squeezing action of the pelt will suck float in. Long floats result in superficial dye penetration.

Float length is connected to mechanical action in that the lower the float length the more mechanical action that takes place. Long floats reduce the mechanical action.

Salt concentration refers to the heavy salts like calcium or magnesium. Hard water and salts will affect the solubility of dyestuffs. If the solubility of dyes is reduced then the dye will not dissolve in water and will not penetrate into the leather.

Dye levellers refers to chemicals which are added either to the pasted dye or to the bath prior to the addition of the dye to the drum.

Dye concentration refers to the amount of dye in the bath. According to laws of diffusion, dyestuffs will move into the leather faster if the concentration is high on the outside. The disadvantage of too high a concentration is that there will be a waste of dye in the effluent and the price per bath is high. High concentrations do, however, result in good deep shades.

Metal ions in the dyebath can result in precipitation of the dye as the metals will start binding the dyestuffs together. Uneven dyeing can then result. If dyestuffs are added to the leather, the bath drained and washed, then metal ions e.g. aluminium and chrome can be added after the top dye to help fix the dyes.

Time affects dyeing in that the longer the material is exposed to the dye the longer the time there is for penetration and fixation. Dye linkages take time to occur, hence the longer the time the better the fixation.

The next article in the series will discuss the various problems that arise during the dyeing of leather.




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