Unilateral NMR and thermal microscopy studies of vegetable tanned leather exposed to dehydrothermal treatment and light irradiation

Abstract

Unilateral nuclear magnetic resonance (NMR) and imaging thermal microscopy (imageMHT) were performed on newly obtained and artificially aged vegetable tanned leathers. Calf and sheep leathers tanned with vegetal extracts of mimosa bark, quebracho and chestnut wood were compared. Ageing was simulated by exposing the leather samples to heating at 70°C in controlled atmosphere at 30% relative humidity and irradiating them with 4000lx in the visible light region for 8, 16, 32 and 64days. Proton spin-lattice relaxation times T1 and effective spin-spin relaxation times T2eff were measured at room temperature and their trend variation compared with the corresponding variations of shrinkage temperature Ts and total length of shrinkage interval measured by imageMHT. Newly obtained leather displayed different proton relaxation times and shrinkage temperatures depending on both the collagen origin and tannin type. Effective spin-spin relaxation values shown to be discriminative for collagen origin and sensitive to the cross-linking degree, whereas spin-lattice relaxation values were more sensitive to the tannin type. Both NMR relaxation times were sensitive to the changes in the water dynamics upon ageing due to the formation of collagen damaged intermediate states and shown a sudden change when the tannin matrix was depleted (e.g. de-tanning).