Viscoelastic characterization of leathers properties: a novel nondestructive method using micro-indentation technique

Collagen, the main constituent of natural skins and tanned leathers, is responsible for the viscoelastic nature exhibited by these materials. Currently, the most used analytical technique for studying these properties is Dynamic Mechanical Analysis (DMA). However, the instrumentation required for this type of analysis is not only very expensive but also needs the preparation of standardized test samples, which must be appropriately cut from a whole leather. As a result, this characterization methodology is destructive, rendering the initial leather matrix unusable and leading to material waste in an industry that already has an inherent environmental impact. For these reasons, having access to an alternative analytical technique capable of assessing viscoelastic behavior, its temporal variations, and any modifications due to thermal or mechanical treatments should be highly advantageous. In this context, an innovative device designed by VESevo, startup at Federico II University of Naples comes into play. Originating from the world of Formula 1, the device has been designed for non-destructive evaluation of the behavior of high-performance vehicle tires using micro-indentation principles. Due to its non-invasive nature, this technology is particularly well-suited for applications where assessing the viscoelastic properties of the finished product is highly complex or, in some cases, even impossible. One such application is in the leather industry, for the characterization of the change in the viscoelastic properties of leathers changing chemicals or after thermal or mechanical treatments. Thanks to a high-performance displacement sensor, the device defines the mechanical behavior of leather by analyzing rebound curves obtained by dropping a metal rod with a semi-spherical indenter onto the surface of the sample under analysis. A sophisticated processing algorithm takes these curves as input and outputs the viscoelastic properties in terms of Storage Modulus and Loss Factor. Additionally, a thermal sensor allows for the evaluation of these properties at different temperatures, providing a more detailed understanding of the temperature-dependent behavior of leather. In the present study a test method for the assessment of the viscoelastic behaviour of leather is proposed by means of micro-indentation principles.