Use of Switchable Carbamate Surfactants for Design of Reversible Colouring of Cotton

Doctoral thesis, 2019

Circularity of materials is a global need. However, recycling of cotton often requires many steps involving chemical or mechanical treatments. According to the waste hierarchy, reuse is a more environmentally benign approach than recycling, giving extended lifecycles with only limited processing of materials. To facilitate recycling and reuse of cotton, in particular reuse, an important step to meet is the possibility of re-colouring of cotton fabrics.  In this thesis, a new colouring method, with an embedded function for removal of the colour, has been designed. The colouring method is based on pigment nanoparticles, stabilized by a switchable surfactant that enables control of the pigment adsorption at the surface of the fibres through electrostatic interactions. The pigment desorbs from the surface upon exposure to an external trigger, resulting in de-colouring. The switchable surfactant is based on a Y-shaped diamine (denoted Y12-amine), which, by reaction with CO2 at a pH above the both pKa values of the Y12-ammonium (protonated Y12-amine), forms an anionic Y12-carbamate. The anionic Y12-carbamate reverts to the Y12-amine upon heating, imparted by the cleavable and switchable character of the surfactant.  The work presented in this thesis is divided into three parts. In the first part, the physicochemical properties of the Y12-amine and its carbamates are determined. The formation and stability of the anionic Y12-carbamate were shown to be dependent on pH and temperature. The anionic Y12-carbamate exhibited a higher CMC and less surface activity than the Y12-amine at pH 12. Furthermore, the ion-pair Y12-ammonium-Y12-carbamate, which was formed at neutral pH, exhibited the lowest CMC and the highest surface activity. In the second part of this work, the anionic sodium Y12-carbamate surfactant was used to disperse and stabilize nanopigments. The effect of pH on the colloidal stability with the anionic Y12carbamate as surfactant was investigated by measuring particle size and electrophoretic mobility. It was found that the pH value strongly affected the stability of the nanopigments. Moreover, by exposing the pigment dispersion to N2/heat or CO2 at pH 12, flocculation and re-dispersion cycles could be achieved without significantly affecting the particle size.  Finally, a hydrophobic pigment stabilized by the anionic Y12-carbamate was used to colour cationised cotton through adsorption. Desorption was triggered by acid hydrolysis of the carbamate groups, generating Y12-ammonium. The mechanical properties of the de-coloured cationised cotton were determined and the maximum tensile stress was found to increase by ~15% and the strain decrease by ~16% compared to cationised cotton. The de-coloured cationised cotton could also be re-coloured. The reversible colouring method on cotton, which is presented in this thesis, demonstrates that pigment-dyeing triggered by the Y12-carbamate surfactant enables reversible colouring of cotton fabric, which can benefit the reuse of cotton