Lightfastness of polypropylene pigmented with natural indigo of Isatis Tinctoria

Purpose The purpose of this study is to purify natural indigo powder with hydrochloric acid (HCl) and study how purification affects the light fastness of the pigment, both in a dimethyl sulfoxide (DMSO) solution and inside a polypropylene matrix. Design/methodology/approach Natural indigo powder was purified by keeping it in hydrochloric acid for 24 h, after which the solid pigment was separated, dried and ground into powder again. Next, HCl-purified natural indigo, unpurified natural indigo and synthetic indigo were dissolved in DMSO, and their absorbance properties were analyzed using a UV-Vis spectrophotometer to determine the effect of purification. Thermogravimetric analysis and Fourier-transform infrared spectroscopy analyses were performed to compare amount of impurities found in each sample. To test light fastness of the pigments, each pigment was mixed with polypropylene powder and extruded into a monofilament. The filaments were 3D-printed into sample rods and subjected to an indoor lightfastness test. Color properties of the samples were evaluated using a reflectance spectrophotometer before and after the lightfastness test. Findings According to the results, HCl purification of natural indigo pigment substantially decreased the amount of impurities in natural indigo. Furthermore, HCl purification increased the lightfastness of natural indigo in DMSO solution and inside 3D-printed polypropylene samples. Originality/value This study proposes that the lightfastness performance of natural indigo pigment can be considerably enhanced with HCl purification, which warrants further research on the use of indigo pigment in non-textile applications.