The tanning sector faces two complex challenges: increasing sustainability and improving leather quality. Shifting to natural dyes and mordants from synthetic and metallic ones eliminates harmful substances and produces 100% organic leather. Natural colouring systems offer additional benefits like antimicrobial and antioxidant properties, contributing to leather preservation. This shift aligns with global regulations, addressing environmental and health concerns by ensuring safer and non-toxic manufacturing processes.
This thesis presents a comprehensive study on the characterization and evaluation of natural dyes and mordants for leather dyeing. In the initial phase, undyed wet-white leather samples and seven natural dyes were supplied from industry partners (Curtumes Aveneda and AQUITEX, respectively). The selected dyes and their extracts were: AQUINAT BioIndigo - Indigofera tinctoria plant; AQUINAT Kareel – Myrobalan (Terminalia chebula) fruit; AQUINAT Leaf Green - Mulberry (Morus alba) plant; AQUINAT Mallow - Pomegranate (Punica granatum) fruit rind; AQUINAT Nimbus – Secretion of Kerria lacca insect; AQUINAT Rubia – Rubia cordifolia root; AQUINAT Yeliona: Marigold (Tagetes erecta), Tesu (Butea monosperma) and Kamala (Mallotus philippensis) plant. Three types of natural mordants were selected: chitosan 85%, mimosa extract and tannic acid, and one metal mordant, aluminium potassium sulphate dodecahydrate (KAl(SO4)2).
These dyes were characterised by employing FTIR to confirm the chemical structure. Additionally, the evaluation of dye colour at different pH was conducted through UV-Vis analysis. The AQUINAT Yeliona dye was the most stable when varying the pH, with AQUINAT Nimbus and Rubia showing good colour variability against this process parameter. Subsequently, mordanting and dyeing tests were performed. The mordanting technique adopted was pre-mordanting. Three leather piece samples were utilized, with a mordant concentration of 6% (OLW), temperature of 40°C, stirring at 120 rpm, and time of 60 minutes. Subsequently,
the leather dyeing step was also applied with a dye concentration of 10% (OLW), a temperature of 40°C, stirred at 120 rpm and dyed for 30, 90, and 180 minutes for each sample. The best dyeing time in all the tests was 180 minutes due to the achieved higher colour intensity. After the dyeing process, the leather colour was monitored over time through the ΔE parameter, which revealed no significant variation in ΔE values over time, showing good colour fastness of the dyes on the leather. Dye exhaustion was quantified, even though results should be evaluated with caution due to solubility problems for some of the analysed dyes. In the final step, performance parameters such as colour fastness to light, perspiration and rubbing were performed to validate the efficacy of the dyes. The experiments revealed varying results for each dye-mordant combination. The AQUINAT BioIndigo dye, despite its potential, failed to penetrate and fix uniformly to the leather, resulting in a stained appearance. In contrast, the AQUINAT Kareel dye demonstrated promising results, particularly when combined with chitosan and aluminium sulphate, yielding a uniform and intense beige colouration. Similarly, the AQUINAT Leaf Green dye exhibited satisfactory results, with tannic acid proving to be the most effective mordant, resulting in a vibrant and uniform green hue. AQUINAT Mallow dye produced a beige colour, with aluminium sulphate and chitosan offering the most intense and uniform result. The AQUINAT Nimbus dye displayed versatility across various mordants, with aluminium sulphate yielding a distinct purple hue, while chitosan resulted in a darker beige tone. AQUINAT Rubia dye produced a
beige colouration, with aluminium sulphate proving to be the most effective mordant in terms of uniformity. Lastly, AQUINAT Yeliona dye showcased a strong yellow colouration across all experiments, with chitosan emerging as the most effective mordant, showing promising potential for applications in leather dyeing.
Overall, this study highlights the importance of mordant selection in enhancing the dyeing process and achieving desired colour outcomes in leather dyeing with natural dyes. The findings contribute to the ongoing efforts towards sustainable practices in the leather industry while offering insights for further research and practical applications. The most promising dyeing procedures have been successfully identified and will be tested in the future on a larger scale, under real conditions, for further evaluation and validation.
