Effect of organic biocolourants on lightfastness and mechanical properties of polylactic acid

Most modern colourants are acquired from petroleum-based sources or mining, which often causes environmental degradation. Consequently, alternative colouring solutions are sought, such as using biocolourants, which are organic colourants from bio-based sources. Although biocolourant research is prevalent in the textile dyeing field, biocoloration of thermoplastics remains relatively unexplored. Thus, our study presents lightfastness and mechanical properties of polylactic acid (PLA) biocoloured with biochar, kraft lignin, indigo from Isatis tinctoria, madder extract from Rubia tinctorum, and weld extract from Reseda luteola. Indigo, madder extract, and weld extract are common bio-based textile dyes, whereas biochar and lignin are more known as easily accessible filler additives than as colourants. Stemming from promising results with bio-based indigo in our previous studies, this study broadens the scope with new colourants and analysis methods. In this study, mixtures of biocolourant powder and PLA granules were heated to a viscous state, blended inside an injection screw, and injection moulded into plastic specimens. The PLA-biocolourant specimens were inspected with scanning electron microscopy (SEM) and subjected to ASTM D638 tensile test. Furthermore, the specimens were exposed to ISO 105-B02 lightfastness test under xenon arc light. Before and after xenon light exposure, the specimens were analysed with visual inspection, reflectance spectrophotometry, hyperspectral imaging, and Fourier-transform infrared (FTIR) spectroscopy. The results showed that most of the biocolourants produced an even colour with adequate lightfastness. Moreover, lignin, biochar, and madder extract substantially improved photodegradation resistance and tensile properties of PLA. Thus, further development of biocolourants, especially lignin, as functional PLA additives is recommended.