Performance and Durability of Biopolymer Blends Containing Modified Metal Oxide Particles

This study applies circular and sustainable principles to the formulation of biopolymer-based materials using naturally occurring additives. To improve the affinity between the host matrix and additives such as metal oxides, the work involves adding stearic acid-modified zinc oxide (<i>f</i>-ZnO) and sonicated titanium dioxide (<i>s</i>-TiO₂) to a polylactic acid and bio-derived polyamide 11 (PLA/PA11 = 70/30 <i>w</i>/<i>w</i> biopolymer blend via melt mixing. To evaluate the impact of the functionalization and sonication on metal oxides (i.e., <i>f</i>-ZnO and <i>s</i>-TiO₂) introduced into the PLA/PA11 blend, composites containing unmodified ZnO and TiO₂ prepared under the same processing conditions were compared with the modified ones. All of the composites were characterised in terms of their solid-state properties, morphology, melt behaviour, and photo-oxidation resistance. The addition of both <i>f</i>-ZnO and <i>s</i>-TiO₂ appears to exert a plasticising effect on the rheological behaviour, in contrast to unmodified ZnO and TiO₂. The presence of stearic acid tails on ZnO has been estimated at approximately 4%, whereas sonication reduces the diameter of TiO₂ particles by half. In the solid state, both unmodified and modified particles can reinforce the biopolymer matrix, enhancing the Young's (elastic) modulus. Calorimetry analysis suggests that unmodified and modified metal oxide particles do not influence the glass transition of the PLA phase but affect the melt temperatures of both biopolymeric phases by reducing macromolecular mobility. Morphology analysis shows that the presence of both f-ZnO and s-TiO₂ particles does not reduce the size of the PA11 droplets. The f-ZnO particles, which have long stearic tails and are more compatible with the less-polar phase (PLA), are probably located at the interface between the two biopolymeric phases or in the PLA phase. Furthermore, <i>s</i>-TiO₂ particles, like TiO₂, do not reduce the dimensions of PA11 droplets, suggesting that there is no preferential location of the particles. Due to the presence of both <i>f</i>-ZnO and <i>s</i>-TiO₂, an increase in the hydrophobicity of the PLA/PA11 blend has been detected, suggesting enhanced water resistance. The photo-oxidation resistance of the PLA/PA11 blend is significantly reduced by the presence of unmodified metal oxides and even more so by the presence of modified metal oxides. This suggests that metal oxides could be considered photo-sensitive degradant agents for biopolymer blends.