Scots pine and caucasian fir needles as functional fillers of polylactide-based (PLA) composites

Abstract Conifer needles constitute a significant amount of biomass in the forest ecosystem, simultaneously generating large amounts of undeveloped organic matter with great potential. Pine (PN) and fir (FN) needles rich in essential oils were considered as potential active fillers of biodegradable composites based on polylactide (PLA). Ground and fractionated raw plant-based materials were introduced into PLA in 1–20 wt%. Composites produced by injection molding were additionally subjected to an annealing procedure to obtain highly crystalline materials. Multi-criteria analysis of the materials allowed for determining the potential for further development and limitations of the new composites. The morphological and chemical structure of the filler permitted the different effects on the polymer matrix, related to both the surface properties (creation of transcrystalline phase PLA/PN) as well as the complex changes in the mobility of PLA macromolecules caused by the migration of extractives contained in the needles, simultaneous plasticization and nucleation. It was shown that in addition to the structural changes resulting in a change in the crystallinity different reinforcing efficiency, the use of PN and FN allowed to obtain distinct stabilizing effects thanks to antioxidants contained in the needles, increasing the oxidation onset temperature (OOT) compared to pure PLA by 67 and 60 °C, for PN and FN, respectively. Graphical Abstract