Balancing Color Stability and Mechanical Performance in Sole Leather: A Sustainable Materials Approach Using Vegetable Tannin Formulations

Abstract This study systematically investigates the effects of various vegetable tannin combinations on the physical and color properties of sole leathers. Mimosa, quebracho and chestnut tannins were applied as single (40%), binary (mimosa–quebracho at varying ratios) and ternary systems (mimosa–quebracho with 5% fixed chestnut) to evaluate their influence on mechanical strength, softness, hardness, hydrothermal stability and color stability. FTIR analyses confirmed the successful fixation of vegetable tannins within the collagen matrix and revealed characteristic spectral differences associated with condensed and hydrolyzable tannin structures, while preserving the integrity of the collagen backbone. SEM observations further demonstrated that tannin composition significantly influenced collagen fiber organization, with optimized formulations producing more compact and homogeneous fibrous networks. The results indicated that quebracho-based leathers exhibited higher tensile and tear strength, while mimosa-based leathers provided lighter shades and greater softness. The binary systems demonstrated that increasing the mimosa content improved brightness but reduced mechanical performance. However, certain ratios specifically 15% mimosa with 25% quebracho and 25% mimosa with 15% quebracho struck a balance between color homogeneity and acceptable strength. In ternary systems, the inclusion of 5% chestnut slightly affected color but enhanced shrinkage temperature and overall mechanical stability. UV-aging test results revealed that mimosa-rich leathers showed higher color change (ΔE), while quebracho improved color stability. Chestnut contributed a modest enhancement against UV degradation. Collectively, these findings suggest that optimized tannin combinations can produce vegetable-tanned leathers with lighter tones, enhanced repeatability and balanced mechanical as well as color properties. This research supports the transition to chromium-free tanning systems and provides scalable insights for sustainable material design, highlighting the potential of biobased tanning agents to meet both ecological and industrial needs.