Process-structure-property relationships in functionalized polyhydroxyalkanoate/ZnO nanocomposites for active packaging applications

Abstract

The environmental challenges regarding conventional plastic packaging materials have drawn significant attention over the past years. In this regard, bioplastics have been proposed as more sustainable alternatives to conventional petroleum based plastics. Current drivers of the bioplastic market are the biobased and biodegradable polyhydroxyalkanoates (PHAs) which show potential for food packaging applications due to their wide range of functional properties. From the PHA family, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) has shown great potential due to its flexibility and increased thermal stability. However, having medium oxygen barrier, relatively low strength, and no natural antibacterial properties, PHAs (including PHBHHx) need to be functionalized to broaden their application in packaging areas. The incorporation of nanoparticles (NPs) into the polymer matrix has shown potential to alter and enhance a wide range of properties. More specific, different studies have identified ZnO NPs as promising agents for improving the mechanical, thermal, antibacterial, UV and gas barrier properties of PHAs. ZnO NPs have been selected in this thesis for their relatively low cost in combination with their wide availability, relative safe nature and current use in a several applications. Despite the potential of ZnO NPs to enhance the properties of PHAs, there is no clear consensus in the literature regarding on how the processing, dispersion and NP characteristics exactly influence the functional properties of PHAs. Therefore, this thesis aimed to develop PHBHHx/ZnO nanocomposite films with good dispersion quality and added functional properties for food packaging applications. PHAs and their ZnO nanocomposites are processed using different techniques – including extrusion, injection molding, miniemulsion, ultrasonic spray coating (USSC), and centrifugal fiber spinning (CFS) – to distribute ZnO NPs in various locations within the material, such as in the bulk of the matrix or concentrated at the surface as coatings. In this way, this experimental work investigates on how the incorporation of ZnO NPs into PHBHHx influences the processing-structureproperty relationships, and specifically focusing on how the ZnO NP distribution, NP characteristics, and processing methods affect the combination of functional properties, such as crystallization, antibacterial activity, mechanical properties, and gas barrier performance. Firstly, this research demonstrates that the extrusion and injection molding parameters significantly affect the microstructure and mechanical properties of PHAs. By adjusting the melt-processing parameters such as mold and melt temperatures, it is possible to influence polymer chain orientation, crystallinity, and overall morphology (depending on the PHA type), which in turn can improve the mechanical properties like strength, flexibility, and rigidity. The selection of the appropriate PHA type in combination with achieving an equilibrium between the mechanical performance and efficient cycle times are essential for maximizing both functionality and productivity, which is highly relevant for industries looking to adopt PHAs as sustainable alternatives. One of the key findings of this research is that (i) the distribution of ZnO NPs within the PHBHHx/ZnO nanocomposites and (ii) the specific ZnO characteristics play a crucial role in the antibacterial and gas barrier properties compared to the ZnO NP concentration or dispersion quality. Coatings with higher surface concentrations of ZnO NPs, especially when applied using a combination of miniemulsion and USSC, outperform bulk melt-processed films regarding functional properties. USSC proves to be an effective method for depositing ZnO NPs at the surface of PHBHHx films, allowing for improved nanoparticle dispersion. The use of miniemulsion particles can potentially minimize NP migration, as ZnO NPs remain encapsulated within the polymer matrix after processes like annealing and washing. The combination of USSC with an appropriate choice of the ZnO NP type shows great potential for active packaging applications, offering antibacterial, gas, and UV barrier properties while using lower ZnO concentrations compared to bulk melt-processing. USSC offers superior functional performance with roll-toroll capabilities, though it could present challenges in achieving uniform coatings over large areas. On the other hand, we showed that CFS allows fabrication of micro-to-nanofibers of PHBHHx and PHBHHx/ZnO, with the fiber morphology highly depending on the solution viscosity. The CFS fibers can be deposited as top layers after annealing. However, the CFS technique has limitations, particularly with respect to controlling the layer thickness and achieving a uniform coverage of the substrate. Despite these challenges, CFS shows potential for incorporating active ingredients such as NPs into nanofibers, making it promising for other applications beyond food packaging, such as in the biomedical field. In fact, we demonstrate the successful incorporation of hydrophilic compounds into hydrophobic PHBHHx nanofibers using a dual solvent system, highlighting the potential of CFS for drug delivery in wound healing applications. In conclusion, this thesis demonstrates that the method of incorporating ZnO NPs into PHBHHx, whether through melt processing, USSC, or CFS, has a profound impact on the final functional properties. Melt processing remains advantageous for its industrial scalability, but USSC offers enhanced surface properties ideal for packaging applications, where antibacterial and gas barrier properties are critical. Although CFS is less suited for large-scale production at this stage, it holds significant promise for specialized applications such as drug delivery. Ultimately, this work provides valuable insights into the design and optimization of PHBHHx/ZnO nanocomposites, emphasizing the importance of processing techniques and ZnO NP distribution in achieving tailored properties for various applications.