Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) with zinc oxide nanoparticles for food packaging

Poly(3-hydroxybutyrate) (PHB) is a biodegradable biopolyester synthesized via bacterial fermentation from renewable resources that has received great attention over the last years for medicine and food packaging applications. However, it shows several shortcomings such as narrow processing window and a high crystallinity (>50%), which leads to brittleness and low impact resistance. To solve these drawbacks, the hydroxybutyrate units can be copolymerized with other monomers like 3-hydroxyhexanoate to yield poly(3-hydroxybutyrate-co-3-hydroxy-hexanoate) (PHBHHx), a copolymer with higher ductility, improved impact strength, and lower melting point. However, PHBHHx displays lower crystallinity than PHB, hence reduced stiffness, and poorer barrier properties against moisture and gases, which is a disadvantage for its use as food packaging material. In this regard, (PHBHHx)-based bionanocomposites incorporating 0.5, 1.0, 2.0, and 5.0 wt% zinc oxide (ZnO) nanoparticles were synthesized via solution casting technique. Their morphology, mechanical, thermal, barrier, and antibacterial properties were investigated via transmission electron microscopy, tensile and impact strength tests, thermogravimetric analysis, water vapor, and oxygen permeability measurements as well as antibacterial tests. In terms of gas permeability, the optimum performance was attained at 5.0 wt% ZnO loading, with reductions in the water vapor and oxygen permeability of 45 and 33%, respectively. Conversely, the impact strength decreased with increasing ZnO concentration, leading to 25% reduction for the highest ZnO loading tested. The bionanocomposite with 5.0 wt% ZnO is a promising alternative to synthetic plastics, with an optimal balance of toughness, barrier, and antibacterial properties to be used in packaging and disposable applications such as beverage and food containers. Practical applications This study analyzes the morphology, thermal, mechanical, barrier, and antibacterial properties of biodegradable nanocomposites based on PHBHHx copolymer and ZnO nanoparticles. These safe and environmentally friendly nanomaterials can be used as a sustainable alternative to synthetic plastics for food and beverage containers.