Exploration of the natural dye alizarin for supercritical carbon dioxide dyeing of polyester, polyamide and cellulose acetate fibres

Abstract Conventional textile dyeing utilises large amounts of water and generates significant amounts of wastewater, highlighting the need to utilise more sustainable dyeing methods. Supercritical carbon dioxide (scCO 2 ) dyeing offers a zero‐water, zero‐wastewater alternative with reduced energy requirements and carbon dioxide recyclability. While both conventional and scCO 2 dyeing commonly rely on synthetic (petroleum‐based) dyes, incorporating natural dyes could enhance the sustainability of scCO 2 dyeing by reducing dependence on non‐renewable resources. Thus, this study evaluated alizarin, an aglycone anthraquinone dye from Rubia tinctorum L. (madder plant), in scCO 2 without any mordant or auxiliaries. Alizarin was selected for its historical significance, colour intensity and colour stability. Its performance was compared with two additional madder‐derived compounds, the glycosides ruberythric acid (RA) and lucidin primeveroside (LP). Dyes (0.5%, 1% owf) were applied to polyethylene terephthalate (PET) at 120°C and 34 MPa for 60 minutes. Findings indicated that only deglycosylated dye (aglycone) exhibited high uptake (≥65%), while RA/LP exhibited a lower uptake (27.1% ± 2.7%) on PET due to their hydrophilicity. Indeed, alizarin exhibited higher uptake on both PET and recycled PET (r‐PET) fibres in scCO 2 , with a maximum uptake of 95.7% ± 4.3% exhibited on r‐PET. Furthermore, alizarin uptake on PET using scCO 2 as the dyeing medium was comparable to or higher than that achieved with water. Dye uptake, colour and fastness properties were also comparable to those of C.I. Disperse Yellow 54, a synthetic dye.