Evaluation of combination tanning as a sustainable alternative to chrome tanning for shoe upper leather production

• Development of an eco-friendly method alternative to conventional chrome tanning. • The combination tanning method produced leather with superior mechanical properties. • SEM showed a smoother surface and better fiber alignment in combination-tanned leathers. • Combination tanning resulted in reduced BOD, COD, TDS, and TSS in retanning wastewater. Chrome tanning, though efficient, contributes significantly to environmental pollution through toxic chromium discharge, threatening water, soil, and ecosystems, thereby highlighting the need for sustainable tanning alternatives. Vegetable tanning is a natural option, but it often produces leather with lower mechanical strength and thermal resistance, limiting its use in demanding applications. This study explores an eco-friendly combination tanning approach using vegetable tannin as the primary tanning agent, followed by retanning with aldehyde, aluminum salts, vegetable tannin, and titanium salts to produce high-quality shoe upper leather with minimal environmental impact. The combination-tanned leathers demonstrated superior mechanical properties compared to full vegetable-tanned leather, with tensile strength of 260 – 275 kg/cm², elongation of 41 – 47 %, stitch tear strength of 119 – 155 kg/cm, tongue tear strength up to 55 kg/cm, bond strength of 590 –630 g/cm, grain crack load up to 29 kg, and distension up to 8.8 mm. Shrinkage temperature was also improved, confirming enhanced thermal stability. Slight reductions were observed in color rub fastness and water vapor permeability, attributed to the compact collagen structure and reduced dye fixation from inorganic retanning agents. SEM analysis revealed progressive improvements in fiber alignment, density, and surface smoothness with aldehyde, aluminum, and titanium salts. Environmental assessment of retanning wastewater showed lower BOD, COD, TDS, and TSS compared to vegetable tanning alone, confirming improved retanning agent efficiency and reduced organic load. Overall, the combination tanning method produced shoe upper leather with desirable strength, thermal stability, and environmental compatibility, offering a sustainable alternative to chrome tanning.