Influencing factors of the dyeing operation

 The factors  which  have a bearing upon distribution of the  dye and its fixation are:

1. Pretanning and tanning operation
2. Temperature, Time, pH values of the dye bath and the leather
3. Float variations
4. Electrolytes present in the dye bath and 
5. Dyeing assistants

1. Pretanning and tanning operation

Pretanning and tanning operations have considerable influence on the quality of dyeing with special reference  to intensity, shade and levelling properties. Proper  liming produces pelt with uniform surface fat distribution which improves the dye levelling properties. Intensity and shade development depend mainly on the tannage. Mineral tannage produces rich and  bright dyeing, unequal distribution of  chrome from side, also within the side and also on  the surface of grain have great influence on the dyeing characteristics of leather. Most of the dyes used for dyeing f leather contain sulphonic acid groups  which react with chrome complex. Consequently in areas where there is more chrome, the colour of the base will be bluer-greener and theretofore, a dye will react in that areas to a greater extent. This because of the larger number of active sites and top of it more fatliquor will  also react if  sulphonated  oils are used with the result that the  shade will be a darker patch or streak than  the surrounding areas, in which there is less chrome in the surface. Improper basification also allows for unlevel dyeing.

2. Temperature, Time, pH values of the dye bath and the leather

The most commonly used dyeing method is dyeing in a hot float. The temperature of application is 50-70^o C for chrome leathers and 35-45^o C for vegetally/synthetically tanned leathers. In the short float dyeing methods penetration dyeing is done at  temperature of 20-25^o C and then temperature is increased to 60-70^o C for fixation of dye. For most type off  leather  a dyeing time of 45-60 minutes is adequate. It depends on mechanical movement, o  the size of  thee load and for penetration dyeing on the thickness of the leather. The pH values of the leather and thee  bath  exert a powerful influence on the rate of dyeing. Dye of high affinity build up almost quantitatively on pure chrome leather when applied in the pH region around thee isoelectric point. so a subsequent decrease in  the bath pH has only negligible effect on the rate off dyeing or the build up of shade. Chrome tanned leather IP  of about 6.5 or 4.5 whereas vegetally tanned leathers have an IP 4-3.2. Depending on the type and intensity of the process, the IP is increased or reduced by retanning. If the pH of the dye liquor exceeds the IP the charges of the material to be dyed is largely negative and if the pH is below the IP the  charges is mainly positive. The shows that by changing the pH during the dyeing process  the absorption behavior, penetration ans fixation off the dyes can be influenced. In practice this is done by means  of diluted  organic acid or ammonia solutions. Inorganic acid such ass sulphuric  or hydrochloride acid  should not be used as they are non-volatile during drying and cause high differential values in  the leather.

3. Float variations

When desired to get full shade on grain with some dye penetration  the dyer may  opt for a low float (50-100%) in  dye bath at lower temperature (40^o C) adding dyesuff in  two feeds; when  desired penetration is obtained the bath  is diluted with hot water (65^o C - 70^o C) (about 250-300%) and after  about ten minutes slow acidification is commenced. Full shades with moderate penetration is obtained by using medium to long float (200-250%) at 45-55^o C and  addition of one-half to two third dyestuff , drummed for thirty minutes followed by 30-50% required acid during 15 minutes. The remaining one-half to one-third dyestuff is now added, run  20 minutes, followed by addition of balance added in two or three feeds during 20 minutes. For complete penetration through the cut, the  pH in the initial stage may be a little  higher (chrome leather  5.5-6.0); vegetable tanned leather 4.5-5.0). The float should be lower (50-100%, at  40^o C - 55^o C). If penetration proceeds slowly in chrome tanned leather (This rarely happens in vegetable tanned leather) an addition of 0.5-1% ammonia ( or  some dye penetrating auxiliary) helps. The dyestuff is added in two or three feeds, after complete penetration even in the thickest area slow acidification is commenced.

4. Electrolytes presents in the dye bath

If the rinsing treatment is too brief or if dyeing is  carried out at a short liquor ratio, electrolytes will tend to accumulate in the wash baths. They occur iin the form of sodium chloride, sodium sulphate and occasionally chromium sulphate. Even minute amounts of  chromium compounds may act as precipitation on dyes invariably damaging the levelness of the dyeing. Sodium chloride and  sodium sulphate retard the rate of dyeing of anionic  dyes to a degree which is specific to the particular dye but independent of its affinity. Direct dyes are on the whole more sensitive to electrolytes than metal complex dyes.