Centrifugal fiber spinning to fabricate polyhydroxyalkanoate/zinc oxide nanocomposite films: structure-property analysis

Abstract

Polyhydroxyalkanoates (PHAs) show great potential as packaging materials due to their bio-based and biodegradable nature. The copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) is very promising because of its flexibility and intermediate barrier properties. However, novel processing techniques and development of PHBHHx as active packaging material remain relatively unexplored. Adding zinc oxide (ZnO) nanoparticles (NPs) to the polymer matrix is a known approach to introduce functional properties, such as UV barrier and antimicrobial activity, but optimal dispersion of the nanoparticles remains precarious. Therefore, we aim to incorporate ZnO NPs into PHBHHx fibers via centrifugal fiber spinning (CFS) and subsequently deposit the fiber mats on top of PHBHHx film substrates. First, centrifugal fiber spinning of PHBHHx fibers was optimized before ZnO NPs were added to the chloroform dispersions under controlled conditions (4000 rpm, 0.6 mm nozzle diameter). The average PHBHHx fiber diameter (ø=0.50-4.62 µm) and morphology (from beads-on-a-string to continuous) are mainly dependent on the concentration and rheology of the respective solutions (4-12 wt. % PHBHHx). Based on thorough physical, thermal and chemical characterization, 10 wt. % PHBHHx (ø=3.58 µm) was chosen to fabricate nanocomposite fibers. The PHBHHx/ZnO fibers show a predominantly continuous morphology with decreasing diameter (ø=3.50-2.99 µm) and some bead formation upon addition of ZnO (1-10 wt.%), though no chemical interactions were observed. Subsequently, fiber mats were deposited as 10-20 µm continuous layers onto 160 µm-thick PHBHHx substrates by post-process annealing at 160°C in a hot press. The films show good ZnO dispersion quality, they effectively block UVC, UVB, and a major part of the UVA wavelength region, and show suitable hydrophobicity for packaging of food products (water contact angles above 95°). We conclude that this novel incorporation method of ZnO NPs in PHBHHx is a promising approach for the development of packaging films with an active top layer.