A Circular Approach to Finished Tanned Leather: Regeneration by Cryogenic Technology

Finished tanned leather is usually covered by a thin polymeric layer. This layer has the scope to change the morphological aspect of the last leather layer as well as improve the impermeabilization properties. Often, the finished product is refused by the final client, and tanneries must restore significant quantities of materials. Therefore, it is very important to remove this finished polymeric layer, recover the underneath tanned leather, and predispose it to a new finishing. The bonding between the polymeric film and leather is so strong that, today, only a blade shaving process can perform this separation at the expense of also removing a layer of tanned leather and consequently reducing the leather thickness. Here, a novel separation method was developed based on the significant difference in the dilation properties between the tanned hide and the polymeric film at low temperatures. The use of cryogenic fluids, in particular the direct application of liquid nitrogen, can freeze the polymeric layer below the glass transition temperature, inducing brittle behavior. The result is an easy separation without any alteration of the tanned leather layer; for a demonstration of that, some techniques were used, such as FTIR, SEM, Tensile strength evaluation, DSC, and TGA. By this last analysis, it is possible to check how a decrease of weight to 90% happened for the polymeric layer at about 400 °C against the complete blank at about 600 °C. A similar great distance of results exists in the case of tensile strength, where an average value of 34.5% is the deformation stress for blank samples, against 34.8% for processed samples. Thus, the process here developed allows the reuse of the tanned leather towards a new life in respect of the principles of the circular economy.