Abstract

There are a large number of grain defects of natural origin, associated with bovine leathers, many of which do not become fully apparent until after dyeing. Some of these defects, highlighted by dyeing, tend to dye darker than the grain, whereas others dye lighter. These defects are a major cause of downgrading of the leather, resulting in considerable financial losses to the tanner due to re-working and the loss of the premium paid for the top quality, aniline grade leathers. There is, therefore, a need to develop and optimise processes with respect to coverage in order that downgrading can be minimised.

Experiments to investigate the effect of some retanning and dyeing process variables on the coverage of different types of artificially produced grain damage have been carried out. Factors investigated include dye selection, retannage type, process order and float length, temperature and drum speed during dyeing. It has been found that light damage resembling natural light spot tended to dye lighter than the intact grain, whereas heavier damage, such as scratches, tended to dye darker than the grain.

Results have shown that there is no panacea to this problem; process factors that tend to improve the coverage of one type of grain damage tend to highlight other types of damage. As a general rule, however. it can be said that those factors which promote coverage of light spot include long dyeing float, high dyeing temperature, fast drum speed and no anionic retannage, Factors that promote the coverage of heavier damage are in general the opposite to these; short dyeing float, low dyeing temperature, slow drum speed and use bleaching retanning agent before dyeing.