Process problems in the dyehouse – part 5
Possible process problems that can occur during the dyeing stage
The previous article discussed problems that can occur during the retanning process in the dyehouse. This article will concentrate on possible process problems that can occur during the dyeing stage in the dyehouse and what the impact of these will be on the final leather quality.
There are a large number of possible problems that could occur during the dyeing process. Some of these will be outlined below.
Poor penetration of the dyestuff : This is mainly due to wet blue that has not been neutralised properly. The wet blue is still too acidic in the center for the dyestuff to penetrate. As soon as the anionic dye comes into contact with the strong cationic charges in the acidic region, it rapidly binds to them and there is no further penetration. Poor penetration can also be due to too little dyestuff being added or the running time being too short and not allowing time for the dye to penetrate.
Lightly stained areas: This can be due to lime blast from the beamhouse or due to grease that has been rubbed onto the leather surface by machines.
Dark, uneven stains: This can be due to too much natural fat that is left in the skin and was not removed properly during the beamhouse processes. It can also be due to dyestuff that is left in contact with the skin while the drum is not turning i.e. the powder is put directly onto the skin and left for a while before the drum turns. Power failures that cause the drum to stand also lead to these stains.
Very pale dyeings: This can be due to too little dyestuff that has been added but is more likely due to too much bleaching syntan or anionic retanning agents that have been used during retanning. If the pH in the dyebath is too high, this can also lead to pale dyeing.
Differences in the dye shade from one batch to the next: This can be due to a number of factors such as different quantities of dyestuff added, different moisture contents of the wet blue before retanning, temperature variations during dyeing, different fixing times and different chrome concentrations in the wet blue.
Dark specks: This can be caused by dyestuffs that have not been dissolved properly and small lumps of dyestuff accumulate in an area leaving these dark specks. It can also be caused by sedimentation in liquid dyes.
Paler shade on grain side and darker shade on the flesh side: If leather is dyed with a mixture of dyes, these different dyes may have different absorption behaviour i.e. some may penetrate easier into the flesh side than into the grain layer, resulting in a difference in the grain and flesh side colours. Poor neutralization where the flesh side may be more neutralized than the grain layer, as well as excessive amounts of electrolytes in the dye bath can also lead to this problem.
Uneven dyeing due to folds in the wet blue: If wet blue is sammed and then folded and placed on pallets, it is easy for the folded edges to dry out completely. After dyeing these folded edges can often be seen as there is a differential uptake of the dye in this area. It is best to ensure that these edges do no dry out completely and remain moist.
Poor colour matching : This is mainly caused by drum operator error as the batch is not matched properly against the master sample. However, colour does change slightly after fatliquoring, and also after drying and milling and this needs to be taken into account when colour matching full drum loads. If two or three dyes are being used to match a colour, each may also have different degrees of penetration into the leather, resulting in the grain layer being correctly matched, but the flesh side being a very different shade, or vice versa.
In the next article, process problems that occur during the fatliquoring process stage will be discussed.
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