VEGETABLE TANNING IN APOLAR SOLVENT MEDIA

Tannage experiments of New Zealand lambskins were carried out in diffe rent water-immiscible organic solvents: dichloromethane, trichloroetha ne, trichloroethylene and tetrachloroethylene with different vegetable tannins: sweetened chestnut, mimosa and sulphited quebracho (ATS) hav e shown, under the conditions of tannage, that according to the amount of water dispersed in the tanning bath (water coming from the pelts), the nature of tannin and the properties of the organic solvent that t he tanning process may be compared to a gel emulsion phenomenon which prevents tannin from diffusing into skin. This effect is more marked i n presence of quebracho and tetrachloroethylene. However, this emulsio n may be avoided by adding a co-solvent (acetone) to the tanning bath. The experiments carried out in tetrachloroethylene-acetone show that, at an equal pelt moisture content, in the case of tanning with sulphi ted quebracho, the amount of acetone added is higher than in the case of the other tannins. Thus, these results indicate that the necessary amount of co-solvent is related to the hydrophilic and hydrophobic str uctural content of the tannin extract. Alternatively, tannage experime nts were carried out in tetrachloroethylene medium by increasing the v olume of tanning bath and in this case without adding co-solvent. In a ll cases, with tannage in chlorinated hydrocarbon media, tannin exhaus tion is more complete and the shrinkage temperature of leather obtaine d is high and its quality is satisfactory. The feasibility studies of tanning in solvent media were prompted by the industrial scaling-up of a newly developed solvent degreasing process. The process solves the water pollution problem and gives complete recovery of the fats. As th e chlorinated hydrocarbons separate easily from aqueous components the y are recovered by distillation in a closed system which also allows r ecovery of the solvent in the skins by heating to 40-degrees-C for 2 h 30, this also aids tannin fixation.