EFFECT OF FATLIQUORING AND FINISHING ON MOISTURE ABSORPTION-DESORPTION OF LEATHER

Abstract

The purpose of the paper is to investigate the moisture absorption-desorption kinetics of leather, a natural collagen fibrous structure, by the application of Vickerstaff¿s method that enables to determine the diffusion coefficient of dyes in fibres, to the diffusion coefficient of moisture in leather. Leather has been fatliquored and finished in order to evaluate the effect of these treatments on moisture diffusion. Samples were subjected to ascending and descending steps of relative humidity, to cause absorption/desorption of moisture in leather. The amount of moisture absorbed/desorbed is measured as regain (% odw). At each step the final regain (at equilibrium) X, the half absorption/desorption time t¿, the ratio X¿/t¿ being X¿=X/2 that corresponds to the diffusion rate were calculated. Following Vickerstaff¿s method the maximum slope of the normalized absorption plot X(t)/X vs. ¿t enabled us to derive the square root of the apparent coefficient of diffusion DA which is based on the % of moisture absorbed/desorbed by a unit of sample mass per unit of time. Hysteresis depends on the water activity (RH%/100) of the environment and points out the energy at which the moisture absorbed at this humidity level is linked to the substrate. The higher the relative humidity, the lower the bonding energy of moisture. At lower water activities the absorbed moisture is linked to collagen as primary water. The comparison of the regain in desorption Xd vs. regain in sorption Xa at different humidity levels yields the hysteresis at each humidity level: Hyst (%) = 100 * (Xd-Xa)/Xa. Fatliquors mainly affect the maximum sorption capacity of leather, decrease the size of the monolayer although makes to ascend the bonding energy. Significant differences in the apparent diffusion coefficients between fatliquors have been observed. The highest coefficients were those of sulphited triglycerides of colza oil and sulphited fish oil, while the lowest were those given by the non fatliquored leather and that fatliquored by the combination of sulphited triglycerides of colza oil and fatty polymer. As regards the effect of finishing according to the relative humidity, finishing enhances the diffusion coefficient in the intermediate range of humidities between30 and 65% while differences decrease at the extreme values.