Color stability of polylactic acid pigmented with natural indigo of Isatis tinctoria in artificial weathering

Abstract

Our study presents lightfastness properties of polylactic acid colored with plantbased natural indigo. Indigo is a blue colorant which is widely used in textile industry as a vat dye in coloration of jeans. Problematically, it is most commonly synthesized from petroleum-based sources. However, indigo can also be extracted from various plants, for example woad (Isatis tinctoria), thus offering a natural alternative for synthetic indigo. Woad is a biperennial plant, which produces extractable precursors of indigo in its leaves. It grows especially well in the harsh Northern European climate, making it a desirable plant-based source of indigo in Finland. Even though woad indigo is well known in the textile coloration field, little research has been done to determine lightfastness performance of woad indigo in non-textile applications. Thus, for our lightfastness study, natural indigo from woad was obtained and added to plastic matrices via extrusion. The extruded plastic filaments were injection molded into sample rods suitable for artificial weathering. After that, all the indigo-colored plastics and uncolored reference plastics were subjected to the artificial weathering test (ISO 4892-3:2016). Appearance and color fastness of the sample rods were assessed with visual inspection, reflectance spectrophotometry and hyperspectral imaging before and after the artificial weathering test. Hitherto, our results have showed that the natural indigo pigment produces a uniform color comparable to that obtained with synthetic indigo pigment, but the color fastness properties of the natural indigo samples are substantially worse than those of synthetic indigo samples.