Influence of Bulk-Mixed and Surface-Coated Morphologies on Functionality and Migration in Nano Ag/PHBHHx Packaging Films

Abstract

Silver nanoparticles (Ag NPs) in (bio)plastic (packaging) materials are known to introduce antimicrobial activity. However, the morphology of the nanocomposite must enable effective interactions between Ag and microorganisms, while keeping Ag migration within regulatory safety limits. Therefore, this study examines how bulk-mixed versus surface-coated morphologies affect functional packaging properties and Ag migration in Ag/poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) nanocomposite films. For the bulk-mixed model, Ag NPs were melt-mixed into PHBHHx (0.2 and 2.0 wt.%) via extrusion and compression moulded into films. For the surface-coated model, stable aqueous dispersions of PHBHHx-encapsulated Ag NPs (10 and 40 wt.%, < 300 nm), prepared via mini-emulsion and solvent evaporation, were ultrasonically spray-coated (50 layers) onto film-extruded PHBHHx substrates. Commercial spherical Ag NPs (50 nm) with and without PVP-coating were investigated. Optical and SEM images show a homogeneous NP distribution in both models, albeit with some agglomeration. Due to the higher absolute Ag content, the bulk-mixed films show a significant difference in colour, opacity and UV-Vis transmittance compared to the surface-coated films. While the coating layer is insufficient to affect the oxygen barrier, the oxygen permeability coefficient of the bulk-mixed films is reduced by 15%-31%. Despite their lower Ag content, surface-coated films effectively inhibit the growth of S. aureus and E. coli, while bulk-mixed films show no clear antibacterial properties. The rationale for the antibacterial activity is further supported by a detailed migration study using single particle inductively coupled plasma mass spectrometry across different food simulants (10 v/v% ethanol and 3 w/v% acetic acid), which elucidated distinct migration mechanisms for the surface-coated (fast desorption and continuous dissolution) and bulk-mixed models (slow diffusion). Overall, these findings highlight the critical influence of nanocomposite morphology on film functionality and safety. Further optimising the two approaches may enhance processing flexibility, leading to safe active packaging applications.