Thermal stability of natural dye lakes from Canadian Goldenrod and onion peel as sustainable pigments

Abstract

The transformation of petrol based polymers towards more sustainable and biodegradable materials includes the development of bio-based pigments for mass coloration. Dye lakes prepared from aqueous plant sources could allow a substitution of synthetic pigments with bio-based material. The shaping of thermoplastic polymers includes a melt extrusion step at higher temperature e.g. 245 °C for polyamide 6, thus suitable natural dye lakes must exhibit sufficient thermal stability. Thermogravimetric analysis and differential scanning calorimetry experiments with aluminium based dye lakes from canadian goldenrod and onion peel demonstrated thermal stability of the lakes up to 245 °C. In isothermal treatment experiments the onset temperature for colour change was found near 220 °C, where only minor colour changes CIELab colour coordinates were observed. Thermally treated dye lakes also were tested in miniaturised dyeing experiments using wool as substrate. Remarkably the colour of the test dyeings on wool samples remained constant up to a treatment temperature of 245 °C. The thermal stability of the studied natural dye lakes was found sufficient to withstand temperatures applied in melt extrusion of polylactic acid or polyamide 6. Thus substitution of synthetic pigments by selected dye lakes from renewable resources could be a promising step towards greener polymers.