Enhancement of acid dyestuff salt-free fixation by a cationizing sol-gel based coating for cotton fabric

In the last years, several technologies have been investigated to introduce cationic sites onto cotton fibres to enhance anionic dyestuff uptake properties. However, several limitations of proposed technologies, such as fabric yellowing, high cost, inadequate reactivity, and toxicity of used chemicals, have still restricted the development of cationizing functional coatings. Herein, a new non-conventional sol-gel treatment of cellulose-based fabrics was investigated to introduce positive electrical charges as useful sites for binding acid dyestuffs, characterized by a featuring extensive colour range and bright shades. As proved by zeta potential and surface charge measurements as well as Fourier-Transform Infrared spectroscopy, the two-step procedure achieved using tetraethylorthosilicate (TEOS) and (3-Aminopropyl)triethoxysilane (APTES) sol-gel precursors promoted the functionalization of cotton fibres introducing amino groups, able to counteract the negative charge of cellulose during the dyeing procedure under acidic conditions. The dyestuff affinity for treated samples, compared to untreated cotton and polyamide references, was measured by competitive fixation of an acid dyestuff, namely Acid Yellow 114, from a non-electrolyte aqueous solution. The results showed that protonated amino groups, obtained at the highest APTES concentration, can reverse fibres charge enhancing anionic dye uptake properties without the addition of electrolytes. Compared to untreated cotton, the colour strength of cationized cotton was improved by nearly 90 %, depending on the APTES concentration, reaching performances obtained using a polyamide reference. The findings make sol-gel a reliable and promising method for cationizing cotton fibres for eco-friendly dyeing processes of cotton using acid dyestuffs.